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The Caledonian and Ellesmerian orogenies were followed by extension and development of intracontinental rift basins since the late Paleozoic, as represented by the Sverdrup Basin along the Canadian Arctic Islands. The initial rifting was accompanied by pulses of volcanic activity during the Carboniferous and Permian. A new occurrence of mafic volcanic rocks, the 'Taconite volcanics' (informal name), was discovered on northern Ellesmere Island between the head of Ayles Fiord and M'Clintock Inlet. The mainly alkali-picritic lavas are exposed within the central part of the Pearya Terrane as an outcrop in faulted contact with upper Carboniferous red beds of the Canyon Fiord Formation. The contact to the Ordovician island-arc volcanic rocks of the Pearya Terrane is unclear. The outcrop is characterized by a small circular magnetic anomaly. Ar-Ar whole-rock geochronology on the volcanic rocks yielded an age of 290 [+ or -] 19 Ma suggesting emplacement of the lavas during the early Permian. Whole rock geochemical analyses for eight samples revealed a geochemical affinity to ocean island basalt (OIB) and indicate variable mixing of low-degree melts in the fields of garnet and spinel peridotite (~80-90 km depth). The involvement of a metasomatized subcontinental lithospheric mantle is indicated by high Pb, Nb, and Ta concentrations. Geochemical differences (as enrichments in Ti, Nb, Zr, and the light rare earth elements (REE)) to the known Carboniferous and Permian spilitic altered basalt occurrences of northwestern Ellesmere and northern Axel Heiberg islands are probably based on differences in the mantle source. The Sr isotope ratios of the Taconite volcanics are primitive ([([.sup.87]Sr/[.sup.86]Sr).sub.t]: 0.7037-0.7042) and its Nd isotope ratios are moderately depleted ([epsilon][Nd.sub.(t)]: +2.20 to +2.85) in contrast to the enriched s[Nd.sub.(t)] values of the Permian Esayoo formation. Key words: Taconite volcanics, rifting, volcanism, Ellesmere Island, Canadian High Arctic, Sverdrup Basin, 1. Introduction The target area of the expedition CASE 11 (Circum-Arctic Structural Events), organized and led by the German Federal Institute for Geosciences and Natural Resources (BGR), in the summer [...]

Geochemical and geochronological data from the Pinguicula Group and unit PR1 of the lower Fifteenmile Group (Yukon, Canada) provide information on sediment provenance and timing of break-up of supercontinent Columbia and seaway development on Laurentia's northwestern margin. The older unit PR1, in the Coal Creek inlier, has a near-unimodal detrital zircon population with an age of 1499 [+ or -] 3 Ma. The Pinguicula Group detrital zircon data, in the Wernecke and Hart River inliers, display a polymodal detrital zircon population with a maximum age of Key words: Mesoproterozoic, Pinguicula, Columbia, supercontinents, geochronology, carbon isotopes, Introduction The evolution of Mesoproterozoic basins on the western margin of Laurentia has become increasingly well understood through the addition of provenance studies to stratigraphy and field relations. Provenance has [...]

The Mesoproterozoic Husky Creek Formation is an interlayered redbed and basalt package that overlies a ca. 2.5 km thick, regionally extensive basaltic plateau linked to the 1.27 Ga Mackenzie Large Igneous Province. This succession provides an opportunity to study the interaction between contemporaneous fluvial sedimentation and flood basalt volcanism. The Husky Creek Formation is approximately 1900 m thick, consisting predominantly of fluvial channel and subordinate floodplain and eolian strata dominated by lithic detritus intercalated with basalt flows. U-Pb dating of detrital zircon grains collected from four stratigraphic levels in the Husky Creek Formation reveals two main age groupings: (1) a 1270 Ma peak attributed to the Mackenzie Large Igneous Province; and (2) Pre-1.6 Ga detrital zircon ages, which reflect sources from a broad region of northwestern Laurentia and are interpreted to be recycled from underlying sedimentary rocks of the Hornby Bay Basin. An upsection decrease in the proportion of ca. 1270 Ma zircon grains, coupled with increasing sandstone compositional maturity, reflects the expansion of the drainage basin during burial of a volcanic pile, with recycling becoming more dominant as the Husky Creek Formation accumulated. The Husky Creek Formation was deposited in the waning stages of the Mackenzie Igneous Event by west-flowing rivers into a geographically restricted basin above an extensive mafic volcanic plateau. This paper provides insight into fluvial basin development during the aftermath of one of Earth's largest igneous events. Key words: fluvial sedimentology, Mackenzie LIP, Husky Creek Formation, Mesoproterozoic, detrital zircon geochronology, Introduction Large igneous provinces (LIPs), and particularlycontinental flood basalts, have been a subject of substantial research targeting their relationship to mantle plumes and continental breakup (Ernst and Buchan 1997). While [...]

The Nemiscau subprovince is a metasedimentary rocks-dominated sequence of the Archean eastern Superior Province. It is bounded by the gneissic and tonalite-trondhjemite-granodiorite (TTG) rocks-dominated La Grande and Opatica subprovinces. The Nemiscau consists of variably migmatized metasedimentary rocks and felsic to intermediate gneisses and plutonic suites. Maflc-to-ultramaflc metavolcanic rocks occur along its northern and southern boundaries. Previous structural and metamorphic studies suggested that it was the result of subduction-related, accretionary and collisional tectonics with adjacent plutonic terranes during the Kenorean orogeny. This study integrates various sets of structural, metamorphic, and U-Pb geochronological data suggesting a long-lasting tectonometamorphic evolution between ca. 2843 and 2598 Ma. Four tectonometamorphic events have been recognized. The first event ([D.sub.1]) is only locally preserved and occurred shortly after the deposition of the oldest volcanic sequences of the Nemiscau (ca. 2756-2736 Ma) under amphibolite facies metamorphic conditions ([M.sub.1]). The kinematic analysis of shear zones bounding the supracrustal sequences of both the Nemiscau and La Grande subprovinces suggests their relative crustal sinking as compared to TTG of the La Grande and Opatica subprovinces, shortly after deposition (at [less than or equal to] 2724 and [less than or equal to] 2706 Ma, respectively). Sedimentation was followed by regional dip-slip-dominated ([D.sub.2]) deformation between ca. 2704 and 2671 Ma, coeval with extensive high-grade granulite facies metamorphism ([M.sub.2]) and anatexis in the Nemiscau subprovince from ca. 2697 to 2685 Ma, followed by exhumation between ca. 2677 and 2671 Ma. This [D.sub.2] event was followed by regional-scale dextral strike-slip shearing ([D.sub.3]) from ca. 2658 until 2621 Ma at amphibolite facies metamorphism ([M.sub.3]). The youngest deformation event, [D.sub.4], was accompanied by strain localization along brittle-to-ductile conjugated shear zones and waning crustal cooling from amphibolite to greenschist facies conditions at ca. 2598 Ma and younger. It is suggested that the Nemiscau, La Grande, and Opatica subprovinces represent a single composite terrane, and that their mutual boundaries do not correspond to 'collisional sutures' between different crustal blocks or microcontinents. The Nemiscau subprovince is interpreted as a sedimentary sequence unconformably overlying a ca. 2760-2756 Ma (and older) basement made up of volcanic and crosscutting TTG. In terms of tectonic evolution, the overall structural architecture and isotopic ages of the Nemiscau, La Grande, and Opatica subprovinces support a tectonic model in which the vertical transfer of crustal material occurred during the early stages ([D.sub.2]) of regional deformation, which evolved into a predominant lateral crustal flow of the ductile crust during later stages ([D.sub.3]-[D.sub.4]). A non-uniformitarian tectonic model for the Archean more adequately accounts for synchronous vertical and horizontal tectonism as preserved in the Nemiscau basin. Key words: Nemiscau subprovince, structures, U-Pb geochronology, tectonometamorphic evolution, Archean tectonism, 1. Introduction The Superior Province is the largest Archean craton in the world (Calvert and Ludden 1999). The southern part of the craton is characterized by an E-W linear arrangement [...]

Sedimentary rocks of the Ocoee Supergroup crop out in the Appalachian Blue Ridge of western North Carolina and eastern Tennessee. This ~12 km thick sequence of strata was mapped as nonconformably overlying Mesoproterozoic crystalline basement (about 1.20-1.02 Ga) and beneath the lower Cambrian Chilhowee Group, leading to the traditional interpretation that the Ocoee Supergroup was deposited between about 1.02 and 0.54 Ga. This interpretation was challenged by Unrug and Unrug (1990, Geology, 18: 1041-1045) and Unrug et al. (2000, Geological Society of America Bulletin. Vol. 112, pp. 982-996), who claimed that Paleozoic fossils were recovered from rocks of the Walden Creek Group (upper Ocoee), and thereby created a new tectonic model for the origin of Laurentia. To resolve the age controversy concerning the time of deposition of the Ocoee Supergroup, diagenetic xenotime overgrowths on detrital zircon and diagenetic monazite on detrital monazite were dated by the U-Pb method. Sensitive High Resolution Ion Microprobe ages of diagenetic xenotime from three samples of sandstone (Thunderhead Sandstone, Cades Sandstone, and Shields Formation) indicate that the Ocoee Supergroup was deposited in the interval of about 580-550 Ma. A sample of Sandsuck Formation (uppermost Ocoee) yielded xenotime overgrowths of about 410 Ma, probably related to post-depositional low-grade metamorphism. Various origins of xenotime overgrowths may be distinguished by trace element distributions. Ages and trace element concentrations of monazite overgrowths support the xenotime age results, although concordia systematics are complicated by high concentrations of common Pb and inheritance of ages (~1.2 and 0.6 Ga). These age data support the observed field relations for a late Neoproterozoic depositional age of the Ocoee Supergroup. Key words: U-Pb geochronology, xenotime, monazite, Ocoee Supergroup, trace element geochemistry, diagenesis, Introduction Sedimentary rocks of the Ocoee Supergroup unconformably overlie Mesoproterozoic crystalline basement in the French Broad massif of the southern Appalachian Blue Ridge. They are part of the belt of [...]

The Bras d'Or terrane of central Cape Breton Island, Nova Scotia, Canada, contains a well-preserved record of the Ediacaran to early Cambrian evolution of Ganderia, a Gondwana-derived terrane in the northern Appalachian orogen. A complex assemblage of low- to high-grade metasedimentary rocks has varied detrital zircon signatures from laser ablation inductively coupled plasma mass spectrometry U-Pb zircon dating, but combining three or more samples yielded representative age spectra that support correlation of the low- and high-grade metasedimentary rocks throughout the Bras d'Or terrane and the corresponding Ganderian Brookville terrane of southern New Brunswick. In quartzite samples from the McMillan Flowage Formation in the northwestern Bras d'Or terrane, the youngest detrital zircons have ages >900 Ma, in contrast to previously studied psammitic and semipelitic samples from correlative units in the eastern Bras d'Or terrane in which youngest detrital ages are 620-600 Ma. Both quartzite and semipelitic samples from the McMillan Flowage Formation contain Neoproterozoic dates from zircon rims, which reflect metamorphic overgrowths during peak metamorphism at ca. 550 Ma, providing a robust age for peak metamorphism in the Bras d'Or terrane that supports similar, albeit sparse, ages reported previously from monazite and titanite samples. This metamorphism is coeval with the emplacement of voluminous dioritic to granitic plutons that occur throughout the Bras d'Or terrane and form in an Andean-type continental margin subduction zone. New U-Pb zircon ages presented here from plutons in the northern Bras d'Or terrane, combined with previously published ages, are consistent with subduction-related magmatism and associated metamorphism between ca. 575 and 540 Ma. Key words: U-Pb geochronology, Appalachian metamorphism, Bras d'Or terrane, Cape Breton Island, McMillan Flowage Formation, detrital zircon, plutonic ages, Introduction The Bras d'Or terrane of central Cape Breton Island, Nova Scotia (Fig. 1), consists of mainly Ediacaran and Cambrian metamorphic and plutonic rocks overlain unconformably by Carboniferous sedimentary rocks [...]

The Wager shear zone is an ~450-km long zone of high strain hosted within Proterozoic and Archean rocks of the Rae domain in northwestern Hudson Bay, Nunavut. New field mapping and microstructural analyzes, combined with titanite and apatite geochronology, define the style, kinematics, and timing of Proterozoic ductile deformation. The results indicate that the Wager shear zone accommodated high-temperature deformation between ca. 1.75 and 1.74 Ga with post-kinematic cooling through apatite U-Pb closure (425-530 [degrees]C) at ca. 1705 Ma. These new data show that the Wager shear zone was last active later than inferred in previous work, at a time when the western Churchill Province was undergoing lithospheric delamination and exhumation following the terminal collisional events in the western portion of the Trans-Hudson Orogen. Key words: shear zone, titanite, apatite, U-Pb geochronology, Zr-in-titanite geothermometry, Trans-Hudson Orogen, 1. Introduction The Trans-Hudson Orogen is a large and foundational orogen of the North American plate that formed during the amalgamation of the Nuna (Columbia) supercontinent, which subsequently broke up [...]

The area northeast of Sudbury, Ontario, is well known for hosting one of the largest unexplained geophysical anomalies in the Canadian Shield, the Temagami Anomaly. In search of a geological explanation for this anomaly, low-grade metamorphic ultrabasic dykes have been discovered in the overlying Huronian Supergroup sedimentary rocks, both in outcrop and in a deep drill core. Here, we report on the first geochemical and geochronological data obtained on these dykes and compare these data with known magmatic units in and around the 1850 Ma impact-generated Sudbury Igneous Complex (SIC). The NW-striking dykes, which cut across sedimentary rocks of the ca. 2.3 Ga Cobalt Group, and which are, in turn, crosscut by pseudotachylitic breccia, are characterized by distinctively high concentrations of Ti, P, Nb, and Zr, highly fractionated rare earth element patterns (La/[Yb.sub.N] 7.6-15.5), and a lack of crustal contamination (Nb/Th > 10). Such features are typical of modern ocean island basalts but very different from Palaeoproterozoic rocks previously documented in the wider region. Multigrain U-Pb laser ablation inductively coupled plasma mass spectrometry analyses performed on magmatic titanite and apatite with high Th/U ratios yielded 1876.0 [+ or -] 8.7 and 1880.9 [+ or -] 8.3 Ma, respectively, which we interpret as the intrusion age of the dykes. This interpretation is supported by similar whole-rock Sm-Nd model ages of 1890-2000 Ma (initial [[epsilon].sub.Nd] +2.5). This magmatic event in the footwall of the SIC shortly before the impact was coeval with, and likely genetically related to, the 1.88-1.87 Ga Circum-Superior Large Igneous Province. Key words: mafic dykes, titanite dating, apatite dating, Circum-Superior LIP, Cobalt Embayment, Temagami Anomaly, Introduction The giant impact that hit the boundary between the Archaean Superior Province and the Palaeoproterozoic Southern Province at 1850 Ma left behind not only the Sudbury Impact Structure with [...]

The Ellsworth belt is one of several fault-bounded blocks exposed along the southeastern coast of Maine that formed within Ganderia. New ID-TIMS U--Pb geochronological data integrated with field relationships provide additional insights into the timing of magmatism and deformation in the Ellsworth belt. The deformed Lamoine Granite was selected for U--Pb zircon analysis to: (i) establish the protolith age; (ii) provide direct temporal constraints on regional low-grade metamorphism and deformation; and (iii) elucidate relationships between the Ellsworth belt and coeval rocks elsewhere in the Appalachian orogen. The Lamoine Granite was emplaced within the Ellsworth Schist at 492 [+ or -] 1.7 Ma; this is the first unequivocal evidence for a Furongian magmatic event in the Ellsworth belt. The schistosity in the Lamoine Granite is parallel to the main fabric of the host Ellsworth Schist and provides a maximum estimate for timing of the regional metamorphic overprint. Widespread deformation in the Ellsworth belt where kinematic indicators indicate a top-to-northwest sense of shear is attributed to thrusting during which progressive horizontal shortening, caused crustal thickening and peak greenschist facies metamorphism. The Cambrian U--Pb age permits correlation of the Lamoine Granite with the Cameron Road Granite in the Annidale belt of New Brunswick where subduction-related magmas intruded the Penobscot arc-back-arc and were subsequently deformed during the Penobscot Orogeny. Key words: Appalachians, Ganderia, geochronology, granites, Maine, Cambrian. La ceinture d'Ellsworth figure parmi plusieurs blocs faillés exposés le long du littoral sud-est du Maine qui se sont formés au sein de la Gandérie. Des nouvelles données de géochronologie U--PbparID-TIMS jumelées à des relations de terrain jettent un nouvel éclairage sur la chronologie du magmatisme et de la déformation dans la ceinture d'Ellsworth. Le granite déformé de Lamoine a été sélectionné pour l'analyse U--Pb sur zircons afin (i) d'établir l'âge du protolite, (ii) d'obtenir de l'information temporelle directe sur le métamorphisme de faible degré et la déformation à l'échelle régionale et (iii) d'élucider les relations entre la ceinture d'Ellsworth et des roches de même âge ailleurs dans l'orogène des Appalaches. Le granite de Lamoinea étémis en placedans leschiste d'Ellsworth à 492 [+ or -] 1,7 Ma; il s'agit de la première preuve sans équivoque d'un épisode magmatique furongien dans la ceinture d'Ellsworth. La schistosité dans le granite de Lamoine est parallèle à la fabrique principale dans le schiste d'Ellsworth qu'il recoupe et fournit une limite supérieure estimée pour l'âge du métamorphisme régional surimposé. Une déformation ubiquiste dans la ceinture d'Ellsworth dont les indicateurs cinématiques témoignent d'un cisaillement avec déplacement du bloc supérieur vers le nord-ouest est attribuée à un chevauchement durant lequel un raccourcissement horizontal graduel s'est traduit par un épaississement crustal et un pic de métamorphisme au faciès des schistes verts. L'âge U--Pb cambrien permet de corréler le granite de Lamoine au granite de Cameron Road dans la ceinture d'Annidale du Nouveau-Brunswick, où des magmas associés à la subduction se sont mis en place dans l'arc-arrière-arc de Penobscot et ont ensuite été déformés durant l'orogenèse de Penobscot. [Traduit par la Rédaction] Mots-clés : Appalaches, Gandérie, géochronologie, granites, Maine, Cambrien., Preamble The Cambrian Ellsworth belt occupies the eastern portion of the Penobscot Bay inlier, coastal Maine (Fig. 1; Reusch et al. 2018). It remains an inadequately documented yet highly significant [...]

We report herein on new zircon U-Pb ages, the major and trace elements of whole-rock, and the Sr-Nd-Hf isotope composition for adakitic intrusives collected from the West Ujimqin district in the Southeast region of the Central Asian Orogenic Belt (CAOB). These data provide important constraints on the petrogenetic evolution and geodynamic setting of late Permian magmatism in the Southeast CAOB. The U-Pb dating of zircon shows that the ages of Seerbeng pluton and Nuhetingshala pluton in West Ujimqin are 255.3 [+ or -] 0.71 and 254.4 [+ or -] 1.2 Ma, respectively, which signifies that these are products of magmatic activity in the late Permian. The adakitic intrusives are characterized by high levels of Sr (Sr > 741 ppm), low Y, low Yb, high Sr:Y ratios, and strongly fractionated rare earth elements (10.3 < [La.sub.N]/[Yb.sub.N] < 22.5), which is similar to the features of the adakite. The magmatic zircons exhibit positive Hf values (+8.1 to +13.3), and young two-stage model ages vary from 430 to 760 Ma. The high eNd(t) and low ([.sup.87]Sr: Key words: adakite, late Permian, U-Pb age of zircon, slab break-off Nous rendons compte de nouvelles dates U-Pb sur zircons et donnees sur les elements majeurs et en traces sur roche totale et de la composition des isotopes de Sr-Nd-Hf pour des roches intrusives adakitiques du district d'Ujimqin occidental dans le secteur sud-est de la ceinture orogenique dAsie centrale (COAC). Ces donnees fournissent d'importantes contraintes concernant l' evolution petrogenetique et le milieu geodynamique du magmatisme permien tardif dans le sudest de la COAC. La datation U-Pb sur zircons montre que les ages des plutons de Seerbeng et de Nuhetingshala dans l'Ujimqin occidental sont de 255,3 [+ or -] 0,71 et 254,4 [+ or -] 1,2 Ma, respectivement, ce qui signifie qu'ils sont des produits d'une activite magmatique au Permien tardif. Les roches intrusives adakitiques sont caracterisees par de fortes concentrations de Sr (Sr > 741 ppm), de faibles concentrations de Y et Yb, ainsi que des rapports Sr : Y eleves et des terres rares fortement fractionnees (10,3 < [La.sub.N]/[Yb.sub.N] < 22,5), ce qui s'apparente aux caracteristiques des adakites. Les zircons magmatiques presentent des valeurs de Hf positives (+8,1 a +13,3), et les ages de modeles en deux etapes jeunes vont de 430 a 760 Ma. Les valeurs eNd(t) elevees et de ([.sup.87]Sr : Mots-cles: adakite, Permien tardif, age U-Pb sur zircons, rupture de la plaque., Introduction The Central Asian Orogenic Belt (CAOB) is the largest accretion orogenic belt and the most prominent area of continental crust growth in the world. The geotectonic division of the [...]

The Silurian Croagh Patrick succession, which crops out just south of a fundamental Caledonian structural zone near Clew Bay, western Ireland, is a series of psammites and pelites with a strong penetrative cleavage. These rocks are intruded by the Corvock granite. A suite of minor intrusions associated with the granite contains the regional cleavage whereas the Corvock granite is undeformed. New U-Pb dates are 413 +7 / -4 Ma for a strongly cleaved sill and 410 [+ or -] 4 Ma for the main granite, and closely constrain the age of crystallization of the granite and coeval cleavage formation as Lower Devonian (Lochkovian or Pragian), implying syn- to late-kinematic granite emplacement. These data are consistent with evidence for strong sinistral shear shown by the Ox Mountains granodiorite just to the northeast dated at 412.3 [+ or -] 0.8 Ma. This Devonian cleavage is superimposed on Ordovician rocks of the South Mayo Trough. The localisation of the strong deformation is interpreted as being due to its position at a restraining bend during regional sinistral motion on a segment of the Fair Head - Clew Bay Line to the north. Contemporaneous deformation in the syn-kinematic Donegal batholith suggests a transfer of sinistral motion to this intra-Grampian structure rather than simple along-strike linkage to the Highland Boundary Fault in Scotland. Our new data indicate diachronous deformation during the late Silurian and early Devonian history of the Irish and Scottish Caledonides and also support previous interpretations of diachronous deformation between these areas and the Appalachian orogens. Key words: Caledonides, geochronology, granites, cleavage formation, Ireland, Devonian. La succession silurienne de Croagh Patrick, qui affleure immediatement au sud d'une zone structurale Caledonienne fondamentale pres de Clew Bay, dans l'ouest de l' Irlande, consiste en une serie de psammites et de pelites presentant un clivage penetratif marque. Ces roches sont recoupees par le granite de Corvock. Une suite d'intrusions mineures associees au granite contient le clivage regional, alors que le granite de Corvock n'est pas deforme. Des nouveaux ages U-Pb de 413 +7 / -4 Ma pour un filon-couche au clivage fortement developpe et de 410 [+ or -] 4 Ma pour le granite principal delimitent etroitement l' age de cristallisation du granite et de la formation contemporaine du clivage comme etant Devonien inferieur (Lochkovien ou Praguien), ce qui sous-entend une mise en place syn- a tardicinematique du granite. Ces donnees concordent avec des preuves d'un cisaillement senestre intense revelees par la granodiorite des Ox Mountains immediatement au nordest, qui a produit un age de 412,3 [+ or -] 0,8 Ma. Ce clivage Devonien est superpose sur des roches ordoviciennes de la fosse de South Mayo. La localisation de la deformation intense est interpretee comme etant le resultat de son emplacement dans un crochon restrictif durant le mouvement senestre regional sur un segment de la ligne de Fair Head - Clew Bay au nord. Une deformation contemporaine dans le batholite syncinematique de Donegal indiquerait un transfert du mouvement senestre a cette structure intra-grampienne, plutot qu'une simple connexion parallelement a sa direction a la Highland Boundary Fault en Ecosse. Nos nouvelles donnees indiquent une deformation diachronique durant l 'evolution au Silurien tardif et au Devonien precoce des Caledonides irlandaises et ecossaises et appuient egalement des interpretations anterieures du diachronisme de la deformation entre ces secteurs et l'orogene des Appalaches. [Traduit par la Redaction] Mots-cles: Caledonides, geochronologie, granites, formation de clivage, Irlande, Devonien., Introduction and regional geology The geology of South Mayo in western Ireland is an important area in understanding the history of the Laurentian margin. It is the only location where [...]

There continues to be debate regarding the timing of the collision between the Indian and Eurasian plates and the uplift of the Tibetan Plateau. This study presents zircon U-Pb geochronology, whole-rock geochemistry, and Lu-Hf isotopic data for the Saiduopugangri granite of the Qiangtang Terrane, located within the core of the Tibetan Plateau. These data provide the basis for the geodynamic setting, petrogenesis, and characteristics of its magma source. Zircons from the Saiduopugangri granite yield a weighted-mean [ (.sup.206] [)Pb/.sup.238]Uage of 62.72 [+ or -] 0.06 Ma, indicating that these rocks formed during the early Palaeocene. The rocks are members of the highly calc-alkaline to shoshonitic series, with weak peraluminous characteristics. Trace elements are characterised by high Sr (483-616 ppm), and low Y (6-10 ppm) and Yb (1 ppm) content, typical of a high Sr and low Yb granite. The [[epsilon].sub.Hf(t)] of zircon ranges from -2.14 to 2.35, with two-stage Hf model ages ([T.sub.DM2]) ranging from 1182 to 895 Ma. These data suggest that the Saiduopugangri granite magma was derived from the melting of lower-crustal clastic meta-sedimentary rocks and mantle-derived basalts. The high Sr and low Yb granite characteristics and experimental results indicate that melting occurred at >1.2 GPa and >750 [degrees]C, consistent with a crustal thickness greater than 50 km. Magmatism occurred from the Late Cretaceous to the early Palaeogene and is broadly synchronous with the collision timing between the Indian and Eurasian plates. The Saiduopugangri granite provides evidence of crustal thickening of the Tibetan Plateau and its age and petrogenesis constrain the timing of the initial uplift. Key words: Tibetan Plateau, Qiangtang Terrane, thickening and uplifting of crust, granite, petrochemistry, U-Pb chronology. Un debat persiste quant au moment de la collision des plaques indienne et eurasienne et du soulevement du plateau tibetain. Nouspresentonsdes donneesde geochronologie U-Pb sur zircon, de geochimiesur rochetotaleet dechimie desisotopesde Lu-Hf pour le granite de Saiduopugangri du terrane de Qiangtang, situe dans la partie centrale du plateau tibetain. Ces donnees permettent d'asseoir le contexte geodynamique, la petrogenese et les caracteristiques de la source du magma. Des zircons du granite de Saiduopugangri donnent un age [ (.sup.206] [)Pb/.sup.238]U moyen pondere de 62,72 [+ or -] 0,06 Ma, ce qui indique que ces roches se sont formees durant le Paleocene precoce. Les roches sont des membres de la serie tres calcoalcaline a shoshonitique, presentant des caracteristiques faiblement hyperalumineuses. Les elements en traces sont caracterises par de fortes concentrations de Sr (483-616 ppm) et de faibles concentrations de Y (6-10 ppm) et Yb (1 ppm), typiques desgranites richesen Sret pauvres en Yb. Les valeursde [[epsilon].sub.Hf(t)] des zircons vont de -2,14 a 2, 35, avec des ages de Hf selon un modele a deux etapes ([T.sub.DM2]) allant de 1182 Ma a 895 Ma. Ces donnees donnent a penser que le magma du granite de Saiduopugangri est derive de la fusion de roches metasedimentaires clastiques de la croute inferieure et de basaltes derives du manteau. Les caracteristiques d'un granite riche en Sr et pauvre en Yb et des resultats experimentaux indiquent que la fusion s'est produite a >1,2 GPa et >750 [degrees]C, ce qui concorde avec une croute de plus de 50 km d'epaisseur. Le magmatisme s'est produit du Cretace tardif au Paleogene precoce, a peu pres en meme temps que la collision des plaques indienne et eurasienne. Le granite de Saiduopugangri presente des preuves d'un epaississement crustal du plateau tibetain, et son age et sa petrogenese permettent de delimiter le moment du soulevement initial. [Traduit par la Redaction] Mots-cles: plateau tibetain, terrane de Qiangtang, epaississement et soulevement de la croute, granite, petrochimie, chronologie U-Pb., Introduction The Tibetan Plateau is the highest and largest plateau on Earth, forming part of an orogenic belt undergoing continent-continent collision. Numerous studies have investigated the timing and mechanisms of [...]

The east-northeast-striking Thelon and Judge Sissons faults of south-central Nunavut are well preserved and record long-lived dextral transcurrent movement with complex reactivation and fluid flow histories. The faults cut across Archean gneisses, Paleoproterozoic plutons, and a Mesoproterozoic sedimentary basin in the Rae domain of the western Churchill Province. They formed and were reactivated during multiple deformation events beginning with an initial faulting event at 1830-1760 Ma, followed by an epithermal faulting event at 1760-1750 Ma and late reactivation events at 1600-1300 Ma. The initial faulting event produced the core-damage zone architecture of the faults. Damage zones are characterized by multiple fracture sets, quartz veins and hydrothermal crackle breccias, surrounding core zones defined by multiple mosaic to chaotic breccias and cataclasites with dextral slip indicators. The epithermal faulting event is expressed by the presence of crosscutting comb, crustiform-cockade and lattice-bladed quartz [+ or -] hematite [+ or -] carbonate veins, and is likely associated with a magmatic event of similar age. The late reactivation events resulted in the formation of irregular, non-cohesive crackle to mosaic breccias and gouges, which became the primary pathways for uranium-bearing hydrothermal fluids and the formation of unconformity-type uranium deposits. The Thelon and Judge Sissons faults are similar to other major continental faults in the Rae domain (e.g., McDonald fault, Wager Bay shear zone), which formed during the Paleoproterozoic Taltson-Thelon and Trans-Hudson orogenies and to modern analogues, such as the Karakorum, Altyn Tagh, and Hunan-Jiangxi faults, which formed during the Himalayan-Tibetan orogeny and experienced prolonged hydrothermal and even hot spring activity. Key words: Aberdeen Lake, strike-slip fault, U-Pb zircon geochronology, Rae domain, Thelon fault, Judge Sissons fault. Les failles d'orientation est-nord-est de Thelon et de Judge Sissons, dans le centre-sud du Nunavut, sont bien preservees et temoignent d'un deplacement en decrochement dextre de longue duree associe a des episodes complexes de reactivation et d'ecoulement de fluides. Les failles recoupent des gneiss archeens, des plutons paleoproterozo'iques et un bassin sedimentaire mesoproterozoique dans le domaine de Rae de la province de Churchill occidentale. Elles se sont formees et ont ete reactivees au fil de multiples episodes de deformation, en commencant par un episode initial de deformation par faille a 1830-1760 Ma, suivi d'un episode de deformation par faille en contexte epithermal a 1760-1750 Ma et des episodes de reactivation tardive a 1600-1300 Ma. L'episode de deformation par faille initial a produit l'architecture de zone d'endommagement au cceur des failles. Les zones d'endommagement sont caracterisees par de multiples ensembles de fractures, veines de quartz et breches de dislocation hydrothermales entourant des zones de cceur definies parde multiples brechesen mosa'ique ou chaotiqueset des cataclasites presentant des indicateurs de deplacement dextres. L'episode de deformation par faille en contexte hydrothermal s'exprime par la presence de veines de quartz [+ or -] hematite [+ or -] carbonate en peigne, en cocardes crustiformes et en treillis et est vraisemblablement associe a un episode magmatique d'age semblable. Les episodes de reactivation tardive se sont traduits par la formation de breches de dislocation et en mosa'ique non cohesives et d'argiles de faille, qui sont devenues les principaux conduits pour des fluides hydrothermaux contenant de l'uranium, et la formation de gisements d'uranium de type discordance. Les failles de Thelon et de Judge Sissons sont semblables a d'autres grandes failles continentales dans le domaine de Rae (p. ex. faille de McDonald, zone de cisaillement de Wager Bay), formees durant les orogeneses paleoproterozo'iques transhudsonienne et de Taltson-Thelon, et a des analogues modernes comme les failles du Karakorum, de l'Altyn Tagh, et de Hunan-Jiangxi, formees durant l'orogenese himalayenne-tibetaine, et qui ont connu une longue activite hydrothermale et meme geothermale. [Traduit par la Redaction] Mots-cles: lac Aberdeen, faille de decrochement, geochronologie U-Pb sur zircons, domaine de Rae, faille de Thelon, faille de Judge Sissons., Introduction Studies on the architecture, kinematics, and alteration history of long-lived, strike-slip faults and their related fracture networks provide important insights into the structural and hydrothermal evolution of fault zones. [...]

The Seve Nappe Complex (SNC) comprises continental rocks of Baltica that were subducted and exhumed during the Caledonian orogeny prior to collision with Laurentia. The tectonic history of the central SNC is investigated by applying in-situ zircon and monazite (Th-)U-Pb geochronology and trace element analysis to (ultra-)high pressure (UHP) paragneisses in the Avardo and Marsfjället gneisses. Zircons in the Avardo Gneiss exposed at Sippmikk creek exhibit xenocrystic cores with metamorphic rims. Cores show typical igneous REE profiles and were affected by partial Pb-loss. The rims have flat HREE profiles and are interpreted to have crystallized at 482.5 ± 3.7 Ma during biotite-dehydration melting and peritectic garnet growth. Monazites in the paragneiss are chemically homogeneous and record metamorphism at 420.6 ± 2.0 Ma. In the Marsfjället Gneiss exposed near Kittelfjäll, monazites exhibit complex zoning with cores enveloped by mantles and rims. The cores are interpreted to have crystallized at 481.6 ± 2.1 Ma, possibly during garnet resorption. The mantles and rims provide a dispersion of dates and are interpreted to have formed by melt-driven dissolution-reprecipitation of pre-existing monazites until 463.1 ± 1.8 Ma. Depletion of Y, HREE, and U in the mantles and rims compared to the cores record peritectic garnet and zircon growth. Altogether, the Avardo and Marsfjället gneisses show evidence of late Cambrian/early Ordovician partial melting (possibly in (U)HP conditions), Middle Ordovician (U)HP metamorphism, and late Silurian tectonism. These results indicate that the SNC underwent south-to-north oblique subduction in late Cambrian time, followed by progressive north-to-south exhumation to crustal levels prior to late Silurian continental collision., Author(s): Christopher J. Barnes [sup.1], Michal Bukala [sup.1], Riccardo Callegari [sup.2] [sup.3], Katarzyna Walczak [sup.2], Ellen Kooijman [sup.4], Melanie Kielman-Schmitt [sup.4], Jaroslaw Majka [sup.2] [sup.3] Author Affiliations: (1) grid.413454.3, 0000 [...]

The Machangqing Cu-Mo (Au) deposit is located in the central part of the Jinshajiang - Red River belt in the Sanjiang orogen, which lies across the Qiangtang terrane and western Yangtze craton, southwestern China. Zircon U-Pb dating constrains that the granite porphyry and porphyritic granite emplacements occurred at 35.92 [+ or -] 0.31 Ma and 34.92 [+ or -] 0.31 Ma, respectively. The Re-Os model ages of molybdenite are 34.94 [+ or -] 0.38 Ma. The new ages presented here, along with previously published data in the region, define a short duration of potassic magmatism and mineralization from 37 Ma to 34 Ma in the Jinshajiang - Red River belt. Zircon [Ce.sup.4+]/[Ce.sup.3+] values of the porphyritic granite and granite porphyry vary from 50.32 to 1579.20 (averaging 481.01) and 33.18 to 1511.80 (averaging 452.98), respectively, and the log(f[o.sub.2]) values vary from -6.66 to -23.86 and -9.88 to -25.18, respectively, which plot within the range of the fayalite-magnetite-quartz buffer curve to the magnetite-hematite buffer curve, indicating an oxidized magma source, which may have facilitated the Cu-Au enrichment. Zircons from granitoids show [epsilon]Hf(t) values ranging from -0.75 to +2.33 and crustal model ages between 0.9 and 1.1 Ga. The features of Lu-Hf isotopes and wide range of Mg#, Cr, and Ni contents imply that the magmas of the Machangqing granitoids were probably derived from partial melting of juvenile lower crust and mixed with some mantle melts. Combined with the features of the Machangqing granitoids, the following evolution process are concluded. During the Cenozoic, the India-Asia continental collision triggered upwelling of hot asthenosphere and underplating of the thickened juvenile lower crust, which caused the formation of mafic and felsic magmas. Those magmas ascended, mixed, crystallized, and formed Machangqing ore-bearing granitoids in an intracontinental extension setting. Key words: LA-ICP-MS zircon U-Pb dating, molybdenite Re-Os dating, oxidation magma, Machangqing Cu-Mo (Au) deposit, Jinshajiang - Red River belt, eastern Tibetan plateau. Le gisement de Cu-Mo (Au) de Machangqing est situe dans la partie centrale de la ceinture de failles de Jinshajiang - riviere Rouge de l'orogene de Sanjiang, qui enjambe le terrane de Qiangtang et le craton du Yangtze (sud-ouest de la Chine). La mineralisation à Cu-Mo (Au) est genetiquement associee à des stocks de granitoïdes comprenant un granite porphyrique et un porphyre granitique. La datation U-Pb sur zircons par LA-ICP-MS permet d'etablir que le porphyre granitique et le granite porphyrique ont ete mis en place à 35,92 [+ or -] 0,31 Ma et 34,92 [+ or -] 0,31 Ma, respectivement. L'âge modele Re-Os sur la molybdenite est de 34,94 [+ or -] 0,38 Ma, ce qui indiquerait un episode de magmatisme et de mineralisation paleogene à Machangqing. Les nouveaux âges presentes ici, combines à des âges dejà publies pour la region definissent un court intervalle de magmatisme potassique et de mineralisation de 37 Ma à 34 Ma dans la ceinture de Jinshajiang - riviere Rouge. Les valeurs de [Ce.sup.4+]/[Ce.sup.3+] des zircons du granite porphyrique et du porphyre granitique vont de 50,32 à 1579,20 (moyenne de 481,01) et de 33,18 à 1511,80 (moyenne de 452,98), respectivement, et les valeurs de log(f[o.sub.2]) vont de -6,66 à -23,86 et de -9,88 à -25,18, respectivement, du champ tampon redox fayalite-magnetite-quartz au champ tampon redox magnetite-hematite, indiquant une source de magma oxydee qui pourrait avoir facilite l'enrichissement en Cu-Au. Les zircons des granitoïdes presentent des valeurs de [epsilon]Hf(t) allant de -0,75 à +2,33 et des âges modeles crustaux entre 0,9 Ga et 1,1 Ga. Les caracteristiques des isotopes de Lu-Hf et les grandes fourchettes definies par les valeurs de Mg#, Cr et Ni indiqueraient que les magmas des granitoïdes de Machangqing sont probablement derives de la fusion partielle d'une croûte inferieure juvenile, melanges avec des produits de fusion du manteau. Ces donnees, combinees à l'histoire geologique, la geochimie et les donnees isotopiques à l'echelle regionale, permettent de conclure au processus d'evolution suivant: durant le Cenozoïque, la collision continentale de l'Inde et de l'Asie a declenche la remontee d'asthenosphere chaude et son plaquage sous la croûte inferieure juvenile epaissie, ce qui a entraîne la formation de magmas mafiques et felsiques. Ces magmas ont remonte, se sont melanges, ont cristallise et ont forme les granitoïdes mineralises de Machangqing dans un milieu d'extension intracontinentale dans l'est du plateau tibetain. [Traduit par la Redaction] Mots-cles: datation U-Pb sur zircons par LA-ICP-MS, datation par Re-Os sur molybdenite, magma oxydant, gisement de Cu-Mo (Au) de Machangqing, ceinture de Jinshajiang - riviere Rouge, est du plateau tibetain., Introduction Porphyry Cu-(Mo-Au) deposits are among the most important types of economic deposits that account for a significant proportion of global Cu production (e.g., Cooke et al. 2005; Sillitoe 2010). [...]

The Xiarihamu Cu--Ni sulfide deposit is the second-largest Cu--Ni deposit in China, located in the Eastern Kunlun orogenic belt. However, despite previous study, the formation and evolution of this deposit remains a key unknown in the tectonic evolution of the Eastern Kunlun Orogenic Belt. Moreover, the petrogenesis of the ore-bearing rocks and the nature of ore genesis are the subjects of ongoing debate. Here, we present detailed field observations, petrology, zircon U--Pb geochronology, and Re--Os isotopic analyses to constrain the timing and genesis of the Xiarihamu Cu--Ni sulfide deposit. Sulfides from the massive ores yielded a Re--Os isotopic isochron age of 408 [+ or -] 11 Ma. Four samples from the ore-bearing Xiarihamu mafic-ultramafic intrusions yielded zircon U--Pb ages of 423.1 [+ or -] 2 Ma, 422.9 [+ or -] 3.1 Ma, 422.7 [+ or -] 2.3 Ma, and 422.6 [+ or -] 2.7 Ma, respectively, indicating that the ore formed during the Silurian. The calculated initial [.sup.187]Os/[.sup.188]Os ratio and [gamma]Os values ranged from 0.5109 to 0.8499 and from 311 to 584, respectively, indicating that crustal contamination played an extremely important role in triggering sulfide saturation. In combination with previous research, drill core observations and Re--Os data provide robust evidence for multiple pulses of magma replenishment throughout the process of ore genesis. Based on this evidence, the large-scale magmatic Cu--Ni mineralization in Xiarihamu likely occurred in a post-collisional setting related to tectonic slab breakoff. Key words: Xiarihamu Cu--Ni deposit, geochronology, Re--Os isotope, crustal contamination, Eastern Kunlun Orogenic belt. Le gisement sulfure de Cu--Ni de Xiarihamu, dans la ceinture orogenique du Kunlun oriental, est le deuxieme gisement de Cu--Ni en importance en Chine. Malgre certaines etudes anterieures, la formation et l'evolution de ce gisement dans le contexte de l'evolution tectonique de cette ceinture orogenique demeurent d'importantes inconnues. La petrogenese des roches mineralisees et la nature de la genese du minerai font en outre toujours l'objet de debats. Nous presentons des observations de terrain detaillees, ainsi que des donnees d'analyses petrologiques, de geochronologie U--Pb sur zircons et d'isotopes de Re--Os pour mieux definir l'age et la genese du gisement sulfure de Cu--Ni de Xiarihamu. Des sulfures provenant de minerais massifs ont donne un age d'isochrone pour les isotopes de Re--Os de 408 [+ or -] 11 Ma. Quatre echantillons d'intrusions mafiques--ultramafiques mineralisees de Xiarihamu ont donne des ages U--Pb sur zircons de 423,1 [+ or -] 2 Ma, 422,9 [+ or -] 3,1 Ma, 422,7 [+ or -] 2,3 Ma et 422,6 [+ or -] 2,7 Ma, respectivement, indiquant que le minerai s'est forme au Silurien. Les valeurs calculees du rapport [.sup.187]Os/[.sup.188]Os initial et de [gamma]Os vont de 0,5109 a 0,8499 et de 311 a 584, respectivement, indiquant que la contamination crustale a joue un role extremement important dans le declenchement de la saturation en sulfure. Combinees aux resultats de travaux anterieurs, des observations de carottes et des donnees de Re--Os fournissent des preuves robustes de plusieurs episodes d'apport de magma tout au long du processus de genese du minerai. A la lumiere de ces preuves, la mineralisation magmatique en Cu--Ni a grande echelle a Xiarihamu s'est vraisemblablement produite dans un milieu post-collision associe a la rupture d'une plaque tectonique subductee. [Traduit par la Redaction] Mots-cles : gisement de Cu--Ni de Xiarihamu, geochronologie, isotopes de Re--Os, contamination crustale, ceinture orogenique du Kunlun oriental., Introduction The Eastern Kunlun Orogenic Belt (EKOB) is an important metallogenic belt located in the western portion of the Central China Orogenic Belt. Large numbers of skarn, porphyry, and orogenic [...]

U-Pb geochronology was applied to a combination of magmatic and hydrothermal minerals to help constrain the timing of emplacement of three units in the Mount Douglas Granite (MDG) and reveal their association with a complex mineralized hydrothermal system containing endogranitic Sn-W-Mo-Zn-Bi-U-bearing greisen/sheeted veins within the pluton. Magmatic monazite and zircon U-Pb ages obtained by LA-ICP-MS overlap at 368 Ma, recording a Late Devonian crystallization age for the MDG. Although discrimination, outside analytical error, of sequential pulses of magmatism is beyond the resolution of LA-ICP-MS U-Pb geochronology, geochemical variations of monazite accompanied by previous whole-rock geochemical analyses support a progressive fractional crystallization process starting from a parental magma (Dmd1), leading to the generation of Dmd2, and finally Dmd3 as the most fractionated unit. Hydrothermal uraninite, cassiterite, and monazite, collected from endogranitic greisen/sheeted veins, reveal evidence for syn-magmatic-related mineralization and a longer-lived post-magmatic hydrothermal system. The first stage is recorded by concordant uraninite dates at 367 [+ or -]3 Ma and by an inverse isochron lower intercept of 362[+ or -]8Mafor cassiterite. In contrast, hydrothermal monazite crystallized over a wider range of ages from 368 to 344 Ma, demonstrating post-magmatic hydrothermal activity within the MDG. These magmatic and hydrothermal ages combined with the geochemical signature of the MDG are similar to those documented for the nearby Mount Pleasant Sn-W-Mo-Bi-In granite-related deposit, which suggests that the two mineralizing systems occur at different levels of the same magmatic system. Key words: laser ablation ICP-MS U-Pb geochronology, monazite, zircon, uraninite, cassiterite, Sn-W-Mo-Zn-Bi-U-bearing Mount Douglas Granite, New Brunswick. La geochronologie U-Pb a ete appliquee a une combinaison de mineraux magmatiques et hydrothermaux pour tenter d'etablir le moment de la mise en place de trois unites du granite du mont Douglas (GMD) et de determiner leur association a un systeme hydrothermal mineralise complexe contenant des greisen/filons en nappes endogranitiques a Sn-W-Mo-Zn-Bi-U dans le pluton. Les ages U-Pb sur monazite et zircon granitiques obtenus par LA-ICP-MS se chevauchent a 368 Ma, temoignant d'une cristallisation d'age devonien tardif pour le GMD. Bien que la discrimination, en dehors de l'erreur analytique, d'episodes sequentiels de magmatisme depasse la capacite de resolution de la geochronologie U-Pb par LA-ICP-MS, les variations geochimiques de la monazite combinees a des analyses geochimiques sur roche totale anterieures appuient l'interpretation d'un processus de cristallisation fractionnee graduel debutant par un magma parental (Dmd1), menant a la production de Dmd2 et, finalement, de Dmd3, l'unite la plus fractionnee. De l'uraninite, de la cassiterite et de la monazite hydrothermales, prelevees de greisen/filons en nappes endogranitiques, fournissent des preuves d'une mineralisation d'affinite synmagmatique et d'un systeme hydrothermal postmagmatique plus perenne. La premiere etape se reflete dans des ages sur uraninite concordants de 367 [+ or -] 3 Ma et dans le point d'intersection inferieur d'un isochrone inverse a 362 [+ or -]8 Ma pour la cassiterite. En comparaison, la cristallisation de la monazite hydrothermale definit une plus grande fourchette d'ages allant de 368 a 344 Ma, ce qui demontre une activite hydrothermale postmagmatique a l'interieur du GMD. Ces ages magmatiques et hydrothermaux, combines a la signature geochimique du GMD, sont semblables a ceux qui sont documentes pour le gisement de Sn-W-Mo-Bi-In associe a du granite de Mount Pleasant, situe a proximite, donnant a penser que les deux systemes mineralisateurs se situent a differents niveaux d'un meme systeme magmatique. [Traduit par la Redaction] Mots-cles: geochronologie U-Pb par ICP-MS avec ablation au laser, monazite, zircon, uraninite, cassiterite, granite a Sn-W-Mo-Zn-Bi-U du mont Douglas, Nouveau-Brunswick., Introduction Three units of the highly evolved Mount Douglas Granite (MDG) that were previously mapped in detail by McLeod (1990; Figs. 1 and 2) are a cogenetic group derived from [...]

The magmatic evolution of the Early Jurassic Turnagain Alaskan-type intrusion is inextricably linked to terrane accretionary events at the North American continental margin. We have calibrated the multistage assembly of the Turnagain ultramafic-mafic complex using high-precision air and chemical abrasion ID-TIMS U-Pb and [.sup.40]Ar/[.sup.39]Ar geochronology. Our results are the first published geochronological calibration of zoning in an Alaskan-type intrusion. Three of four intrusive phases (Stages 1-4, oldest to youngest) were successfully dated. Cooling ages obtained on Stage 2 dunite-wehrlite include statistically identical [.sup.40]Ar/[.sup.39]Ar plateau dates of 187.4 [+ or -] 1.5 (2[sigma]) Ma on hornblende and 188.6 [+ or -] 1.2 Ma on phlogopite, and a concordant [.sup.206]Pb/[.sup.238]U date of 190.3 [+ or -] 4.6 Ma on titanite. Stage 3 diorite has a [.sup.206]Pb/[.sup.238]U zircon crystallization age of 188.11 [+ or -] 0.13 Ma. Stage 4 wehrlite and leucodiorite yield [.sup.206]Pb/[.sup.238]U zircon crystallization ages of 185.68 [+ or -] 0.19 and 185.30 [+ or -] 0.12 Ma, respectively. The ca. 189 Ma phlogopite date represents a minimum crystallization age for Stage 2, indicating that assembly of the Turnagain intrusion spanned at least ~4 million years from ca. 189 Ma to 185 Ma. Regional progressive contractional deformation in the Early Jurassic is constrained by the geochronological results (>189 to 189 Ma), deformed Stage 1 wehrlite-clinopyroxenite prior to emplacement of Stages 2-4, and subsequently thrust the intrusion and its host rocks onto the ancient North American continental margin ( Key words: Alaskan-type intrusion, geochronology, Early Jurassic, terrane accretion, Yukon-Tanana terrane. L'evolution magmatique de l'intrusion de type alaskien de Turnagain, d'age jurassique precoce, est inextricablement reliee a des episodes d'accretion de terranes a la bordure du continent nord-americain. Nous avons etalonne l'assemblage en plusieurs etapes du complexe ultramafique-mafique de Turnagain en utilisant la geochronologie U-Pb de haute precision par ID-TIMS avec abrasion chimique et a l'air et la geochronologie [.sup.40]Ar/[.sup.39]Ar. Nos resultats constituent le premier etalonnage publie de la zonation geochronologique dans une intrusion de type alaskien. Des ages ont ete obtenus pour trois des quatre phases intrusives (phases 1-4, de la plus vieille a la plus jeune). Les ages de refroidissement de la dunite-wehrlite de la phase 2 comprennent des ages plateaux [.sup.40]Ar/[.sup.39]Ar statistiquement identiques de 187,4 [+ or -] 1,5 (2[sigma]) Ma sur hornblende et 188,6 [+ or -] 1,2 Ma sur phlogopite, et un age [.sup.206]Pb/[.sup.238]U concordant de 190,3 [+ or -] 4,6 Ma sur titanite. La diorite de phase 3 a un age de cristallisation [.sup.206]Pb/[.sup.238]U sur zircon de 188,11 [+ or -] 0,13 Ma. La wehrlite et la leucodiorite de la phase 4 donnent des ages de cristallisation [.sup.206]Pb/[.sup.238]U sur zircon de 185,68 [+ or -] 0,19 et 185,30 [+ or -] 0,12 Ma, respectivement. L'age de ~189 Ma sur phlogopite represente un age minimum de cristallisation pour la phase 2, qui indique que l'assemblage de l'intrusion de Turnagain s'est etale sur au moins ~4 millions d'annees, de ~189 Ma a 185 Ma. Une deformation regionale progressive par contraction au Jurassique precoce delimitee par les resultats geochronologiques (>189 Ma a 189 Ma), la deformation de la wehrlite-clinopyroxenite de phase 1 avant la mise en place des roches des phases 2 a 4 et le chevauchement subsequent de l'intrusion et des roches encaissantes sur l'ancienne bordure continentale nord-americaine ( Mots-cles : intrusion de type alaskien, geochronologie, Jurassique precoce, accretion de terranes, terrane de Yukon-Tanana., Introduction The Early Jurassic Turnagain Alaskan-type intrusion in north-central British Columbia, situated 70 km east of Dease Lake (Fig. 1 inset), belongs to a global class of ultramafic-mafic intrusions emplaced [...]

We present systematic U-Pb age data collected by laser ablation multi-collector inductively coupled plasma mass spectrometry, precise geochemical data, and Nd isotope data for igneous rocks from the southeastern Lesser Xing'an Range (SE LXR). The results indicate that the formation ages as follows: Maojiatun alkaline granite, 207.2 [+ or -] 0.84 Ma and 204.6 [+ or -] 0.93 Ma; Diorite porphyrite, 164.5 [+ or -] 0.97 Ma; and Tieli syenogranite, 186.7 [+ or -] 1.50 Ma. The alkaline granite has high silicon, potassium, alkali, and Fe[O.sub.T] contents; it is enriched in high field strength elements, Zr, Hf, Th, Rb, and U; is depleted in Ba, Sr, Nb, Ta, P, Ti, etc.; and has high ratios of 10000Ga/Al. It shows an [A.sub.2]-type granite affinity. The Tieli alkali-feldspar granite has high total alkali contents and is enriched in high field strength elements and rare earth elements and depleted in Sr, Ba, Ti, and P, and shows varying degrees of alkalinity. Rocks from SE LXR display similar [[epsilon].sub.Nd] (t) values with corresponding to Nd model ages of 1095 to 813 Ma. The igneous rocks from the SE LXR are proposed to be derived from melting of the Neoproterozoic lower crust and potential magma mixing with ancient crystalline basement. The formation of the Maojiatun alkaline granite occurred in response to a postorogenic event following the closure of the Paleo-Asian Ocean. However, the SE LXR exhibited an extensional back-arc tectonic setting in the Early Jurassic. The Middle Jurassic diorite porphyrite could be related to the temporary stagnation of the westward subduction of the Paleo-Pacific plate. Key words: alkaline rocks, Lesser Xing'an Range, A-type granite, lithospheric delamination. Nous presentons des donnees de datation U-Pb systematique obtenues par spectrometrie de masse avec plasma a couplage inductif et collection d'ions multiples jumelee a l'ablation au laser, des donnees geochimiques precises et des donnees sur les isotopes du Nd pour des roches ignees du sud-est de la chaine du Petit Xing'an (SE LXR). Les resultats indiquent les ages de formation suivants: granite alcalin de Maojiatun, 207,2 [+ or -] 0.84 Ma et 204,6 [+ or -] 0,93 Ma; porphyre dioritique, 164,5 [+ or -] 0,97 Ma; et syenogranite de Tieli, 186,7 [+ or -] 1,50 Ma. Le granite alcalin a des concentrations elevees en silice, potassium, alcalis et Fe[O.sub.T], et il est enrichi en elements a forte liaison atomique, Zr, Hf, Th, Rb, et U, appauvri en Ba, Sr, Nb, Ta, P, Ti, et ainsi de suite, et est caracterise par des rapports 10000Ga/Al eleves. Il montre une affinite aux granites de type [A.sub.2]. Le granite a feldspaths alcalins de Tieli a de fortes concentrations totales d'alcalis et est enrichi en elements a forte liaison atomique et en terres rares et appauvri en Sr, Ba, Ti et P, et il presente des degres d'alcalinite variables. Les roches de la SE LXR presentent des valeurs de [[epsilon].sub.Nd] (t) semblables correspondant a des ages Nd modeles de 1095 Ma a 813 Ma. Il est propose que les roches ignees de la SE LXR sont derivees de la fusion de la croute inferieure neoproterozoique et du possible melange du magma avec des roches du substrat cristallin ancien. La formation du granite alcalin de Maojiatun s'est produite en reponse a un episode post-orogenique qui a suivi la fermeture de l'ocean paleo-Asiatique. Le contexte tectonique de la SE LXR au Jurassique precoce etait toutefois celui d'un bassin d'extension d'arriere-arc. Le porphyre dioritique d'age jurassique moyen pourrait etre associe a la stagnation temporaire de la subduction vers l'ouest de la plaque paleo-Pacifique. [Traduit par la Redaction] Mots-cles: roches alcalines, chaine du Petit Xing'an, granite de type A, delamination lithospherique., Introduction The southeastern Lesser Xing'an Range (SE LXR) innortheastern China belongs to the northern portion of the Central Asian Orogenic Belt (Sengor and Natal'in 1996; Jahn et al. 2000a, 2000b; [...]

Late Mesozoic granitic rocks are widely distributed in the Sulu orogenic belt, but the source, tectonic affinity, and associated geodynamic setting that produced the respective magmas remain controversial. To provide insights into these issues, we present field-based petrological, whole-rock major and trace element and Sr-Nd isotope geochemical, zircon U-Pb dating, and Lu-Hf isotope studies on two types of granite porphyry dykes that are newly recognized from the central Sulu belt. U-Pb dating of magmatic zircons from both types yields consistent ages that vary between 124 [+ or -] 2 and 118 [+ or -] 2 Ma, constraining the timing of intrusion as Early Cretaceous. The granitic rocks have high-K calc-alkaline peraluminous compositions with low Mg# values and are characterized by fractionated rare earth element patterns with strong depletion in high field strength elements. Compared with type I of the granite porphyry dykes, type II exhibits higher Si[O.sub.2] but slightly lower [Na.sub.2]O and [K.sub.2]O abundances, contains higher Rb/Sr and [.sup.87]Sr/[.sup.86]Sr ratios, and shows more pronounced negative Eu, Sr, and Ba anomalies. Both types I and II have high initial [.sup.87]Sr/[.sup.86]Sr ratios of 0.709-0.711 and negative [[epsilon].sub.Nd](t) values of-19.8 to -18.4. The magmatic zircons possess negative [[epsilon].sub.Hf](t) values of-29.1 to -20.8, with mostly Neoarchean Hf model ages of 3001-2478 Ma. These features, together with the presence of Neoproterozoic inherited zircons, indicate that the two types of granite porphyries successively crystallized from a joint granite magma that derived from partial melting of the continental crust of the Yangtze Craton. Therefore, an interrelationship between the granite porphyry dykes and massive magmatic granitoids from adjacent regions in the Sulu belt may be documented, recording evidence of a joint ancient crustal reworking and recycling in a fossilized continental subduction zone during the Early Cretaceous. Key words: collisional orogen, paleo-Pacific plate subduction, granite porphyry, U-Pb zircon geochronology, Sulu orogenic belt. Des roches granitiques d'age mesozoique tardif sont largement repandues dans la ceinture orogenique de Sulu, mais la source, l'affinite tectonique et le contexte geodynamique associe qui ont produit leurs magmas respectifs demeurent controverses. Afin de jeter un nouvel eclairage sur ces questions, nous presentons les resultats d'analyses geochimiques des elements majeurs et en traces sur roche totale et des isotopes de Sr-Nd et de Lu-Hf, et de la datation U-Pb sur zircons pour deux types de dykes de granite porphyrique nouvellement decouverts dans la partie centrale de la ceinture de Sulu. La datation U-Pb sur zircons magmatiques des deux types de granite donne des ages coherents allant de 124 [+ or -] 2 a 118 [+ or -] 2 Ma, ce qui fixe le moment de leur intrusion au Cretace precoce. Les roches granitiques ont des compositions hyperalumineuses calco-alcalines riches en K avec de faibles Mg#, et elles sont caracterisees par des spectres de terres rares fractionnes montrant un appauvrissement marque des elements a forte liaison atomique. Comparativement au premier type de dykes de granite porphyrique, le deuxieme type presente de plus fortes teneurs en Si[O.sub.2], mais des concentrations de [Na.sub.2]O et [K.sub.2]O legerement plus faibles, des rapports Rb/Sr et [.sup.87]Sr/[.sup.86]Sr plus eleves et des anomalies negatives plus prononcees de Eu, Sr et Ba. Les deux types presentent des rapports initiaux de [.sup.87]Sr/[.sup.86]Sr eleves de 0,709-0,711 et des valeurs negatives de [[epsilon].sub.Nd](t) allant de -19,8 a -18,4. Les zircons magmatiques ont des [[epsilon].sub.Hf](t) negatifs de -29,1 a -20,8, et des ages modeles Hf principalement neoarcheens de 3001 Ma - 2478 Ma. Ces caracteristiques, combinees a la presence de zircons neoproterozoiques herites, indiquent que les deux types de porphyre granitique ont cristallise successivement d'un meme magma granitique derive de la fusion partielle de la croute continentale du craton du Yangzi. Une interrelation entre les dykes de granite porphyrique et des granitoides magmatiques massifs de regions attenantes dans la ceinture de Sulu pourrait ainsi etre documentee, qui temoigne de leur remaniement crustal et de leur recyclage communs dans une zone de subduction continentale fossilisee durant le Cretace precoce. [Traduit par la Redaction] Mots-cles: orogene de collision, subduction de la plaque paleo-Pacifique, porphyre granitique, geochronologie U-Pb sur zircons, ceinture orogenique de Sulu., Introduction Migration of granitic magmas from their source regions in the deep orogenic crust to sites of accumulation and emplacement at shallower crustal levels is an important process for mass [...]

Pressure--temperature--time (P--T--t) estimates for the Montresor Belt, obtained using phase equilibria and geospeedometry modelling integrated with in situ U--Th--Pb monazite geochronology, shed new light on the tectonometamorphic effects of the Snowbird phase of the Trans-Hudson orogeny. Typical metapelitic assemblages of the lower Montresor group consist of white mica, biotite, plagioclase, quartz, and andalusite, which in some rocks is partly or completely pseudomorphed by white mica. The observed assemblages reflect peak P--T conditions centring at approximately 575 [degrees]C and 3 kbar. Rocks with high bulk Fe/Mg contents contain compositionally zoned garnet, permitting the addition of further constraints on the conditions of metamorphism in the Montresor Belt: Core compositions of earliest-grown garnets indicate initial garnet crystallization at approximately 535 [degrees]C and 2.3 kbar, suggesting a nearly isobaric P--T path of prograde metamorphism with a gradient of approximately 50 [degrees]C x [kbar.sup.-1]. Chemical age-dating of monazite inclusions in garnet yields ages of ca. 1870 [+ or -] 9 to 1837 [+ or -] 9 Ma. Retrograde, pseudomorphic andalusite replacement by white mica at approximately 540 [degrees]C is inferred to have been controlled by variations in bulk rock chemistry. Morphologically corroded and chemically heterogeneous monazite adjacent to white mica pseudomorphs suggests that andalusite replacement took place at ca. 1792 [+ or -] 10 Ma, possibly associated with extension and movement along the detachment fault separating the upper and lower Montresor groups. Simulations of diffusion across chlorite- and biotite-filled cracks in garnet assumed to be coeval with andalusite replacement suggest that the rocks have experienced the retrograde event for at least 20 My. Des estimations des conditions de pression-temperature-temps (P-T-t) pour la ceinture de Montresor, obtenues par modelisation de l'equilibre des phases et de la geospeedometrie integree a la geochronologie U-Th-Pb in situ sur monazite, jettent un nouvel eclairage sur les effets tectonometamorphiques de la phase de Snowbird de l'orogenese transhudsonienne. Les assemblages metapelitiques typiques du groupe de Montresor inferieur comprennent du mica blanc, de la biotite, du plagioclase, du quartz et de l'andalousite qui, dans certaines roches, est partiellement ou completement pseudomorphisee en mica blanc. Les assemblages observes refletent des conditions de P-T maximums autour de 575 [degre]C et 3 kbar. Les roches de rapports Fe/Mg eleves contiennent du grenat dont la composition presente une zonation, ce qui permet de delimiter encore mieux les conditions du metamorphisme dans la ceinture de Montresor : les compositions des noyaux des grenats les plus precoces indiquent que la cristallisation du grenat a commence autour de 535 [degre]C et 2,3 kbar, ce qui indiquerait une trajectoire P-T presque isobare du metamorphisme prograde suivant un gradient d'environ 50 [degre]C*[kbar.sup.-1]. La datation chimique d'inclusions de monazite dans le grenat donne des ages d'environ 1870 [+ ou -] 9 a 1837 [+ ou -] 9 Ma. Il est infere que le remplacement pseudomorphique retrograde de l'andalousite par du mica blanc autour de 540 [degrees]C etait contrôle par des variations de la composition globale de la roche. Des monazites de forme corrodee et de composition heterogene attenantes a des pseudomorphes de mica blanc indiquent que le remplacement de l'andalousite s'est produit vers 1792 [+ ou -] 10 Ma et etait possiblement associe a l'extension et au deplacement le long du decollement separant les groupes de Montresor superieur et inferieur. Des simulations de la diffusion a travers des fissures remplies de chlorite et de biotite dans le grenat presumees contemporaines au remplacement de l'andalousite indiqueraient que le metamorphisme retrograde subi par ces roches a dure au moins 20 millions d'annees. [Traduit par la Redaction], Introduction The Trans-Hudson orogeny was a continent-scale, Paleoproterozoic event characterized by widespread metamorphism, deformation, and plutonism at ca. 1.91-1.75 Ga, part of which is preserved in the Canadian Shield (Corrigan [...]

The Songjianghe deposit is a newly discovered altered gold deposit in the southeastern Jiapigou-Haigou Gold Metallogenic Belt (JHGMB) in southeastern Jilin Province of NE China. The host rocks were considered to be the Mesoproterozoic Seluohe Group, and the metallogenic epoch lacked accurate isotopic constraints. To determine the age and metallogenic setting of the deposit, we describe the geologic characteristics of the deposit and present the results of petrographic and geochronologic analyses of the host rocks and ores. The ore bodies are hosted within a suite of amphibolite facies metamorphic rocks superimposed by greenschist facies indicative of retrograde metamorphism. Zircon U--Pb dating results indicate that the host rocks belong to the Jiapigou Group that formed at the end of the Neoarchean (2543--2527 Ma). Subsequently, the rocks successively underwent metamorphism during the late Neoarchean (2521--2506 Ma), retrograde metamorphism caused by the closure of the Paleo-Asian Ocean during the late Permian to Early Triassic (262--250 Ma), and extension after the closure of the Paleo-Asian Ocean during the Late Triassic (231--210 Ma). Sericite [.sup.40]Ar/[.sup.39]Ar dating results suggest that the Songjianghe deposit formed during the Late Jurassic between 157 Ma and 156 Ma. By combining these new insights with those of previous studies, we propose that the Songjianghe deposit is a mesothermal gold deposit and that mineralization occurred during the extensional period in the intermittent stage that followed the first subduction of the Paleo-Pacific Plate. All the gold deposits in the JHGMB formed from the late Permian to Early Cretaceous by multi-stage mineralization events that corresponded temporally with the tectonic evolution of the Paleo-Asian Ocean and the episodic subduction of the Paleo-Pacific Plate. Key words: geochronology, sericite [.sup.40]Ar/[.sup.39]Ar, mesothermal gold deposit, Songjianghe, Northeast China. Le gisement de Songjianghe est un gisement aurifere altere decouvert recemment dans le sud-est de la ceinture metallogenique aurifere de Jiapigou-Haigou (CMAJH), dans le sud-est de la province de Jilin du nord-est de la Chine. Les roches hôtes etaient considerees comme constituant le Groupe mesoproterozoïque de Seluohe, et l'epoque metallogenique n'etait pas delimitee par des donnees isotopiques exactes. Afin de determiner l'age et le contexte metallogenique du gisement, nous en decrivons les caracteristiques geologiques et presentons les resultats d'analyses petrographiques et geochronologiques des roches hôtes et des minerais. Les corps mineralises se trouvent dans une serie de roches metamorphiques au facies des amphibolites sur lequel est surimpose un metamorphisme retrograde au facies des schistes verts. Les resultats de la datation U-Pb sur zircon indiquent que les roches hôtes appartiennent au Groupe de Jiapigou qui s'est forme a la fin du Neoarcheen (2543--2527 Ma). Par la suite, les roches ont successivement subi un metamorphisme durantle Neoarcheen tardif (2521--2506 Ma), un metamorphisme retrograde cause parlafermeture de l'ocean Paleoasiatique du Permien tardif au Trias precoce (262--250 Ma) et une extension apres la fermeture de l'ocean Paleoasiatique durant le Trias tardif (231-210 Ma). Les resultats de la datation [.sup.40]Ar/[.sup.39]Ar sur sericite donnent a penser que le gisement de Songjianghe s'est forme au Jurassique tardif, entre 157 Ma et 156 Ma. En combinant ces nouveaux renseignements aux resultats d'etudes anterieures, nous proposons que le gisement de Songjianghe est un gisement aurifere mesothermal et que la mineralisation s'est produite durant la periode d'extension dans l'intervalle qui a suivi la premiere subduction de la plaque paleo-Pacifique. Tous les gisements auriferes de la CMAJH se sont formes du Permien tardif au Cretace precoce par des episodes de mineralisation en plusieurs etapes qui correspondaient dans le temps avec l'evolution tectonique de l'ocean Paleoasiatique et la subduction episodique de la plaque paleo-Pacifique. [Traduit par la Redaction] Mots-cles: geochronologie, [.sup.40]Ar/[.sup.39]Ar sur sericite, gisement aurifere mesothermal, Songjianghe, nord-est de la Chine., 1. Introduction The Jiapigou-Haigou Gold Metallogenic Belt (JHGMB) is located in the NW-striking collage zone between the eastern section ofthe northern margin of the North China Craton (NCC) and the [...]

A suite of six Devonian granites and one syenite were emplaced into the upper crust of northern Yukon between 364.8 [+ or -] 2.7 and 371.2 [+ or -] 1.4 Ma. The Bear Mountain syenite and related rhyolite porphyry in adjacent Alaska intruded at 52.3 [+ or -] 0.4 and 53.5 [+ or -] 0.2 Ma, respectively. A felsic volcaniclastic unit and quartz-phyric sill are newly documented adjacent to the Mount Sedgwick granite. The volcaniclastic unit may indicate the presence of a related volcanic edifice. The presence of xenocrystic zircon grains in most of the intrusions suggests initial emplacement of magmas began 10-20 Myr before final emplacement into the upper crust. A Famennian final intrusion age coincides with Late Devonian encroachment of Ellesmerian deformation into the region. Attendant crustal flexure, or evolving foreland structures, may have facilitated upward migration of the magmas. Geochemistry of the intrusions indicates that the Devonian magmatism was largely derived from partial melting of lower and middle crust, implying widespread mafic magmatic underplating in Middle to Late Devonian time. Only Dave Lord syenite retains evidence of an original mantle geochemical signature. Mantle underplating may have played a role in localizing extension, volcanism, and rifting that led to the Late Devonian opening of the Angayucham ocean basin. The Eocene Bear Mountain pluton is inferred to be a northerly example of widespread Cenozoic within-plate magmatism in Alaska. Une serie de six granites et d'une syenite d'ages devoniens ont ete mis en place dans la croûte superieure du nord du Yukon entre 364,8 [+ ou -] 2,7 et 371,2 [+ ou -] 1,4 Ma. La syenite de Bear Mountain et le porphyre rhyolitique associe en Alaska voisin se sont mis en place a 52,3 [+ ou -] 0,4 et 53,5 [+ ou -] 0,2 Ma, respectivement. Une unite volcanoclastique et un filon-couche a phenocristaux de quartz viennent d'être documentes a côte du granite de Mount Sedgwick. L'unite volcanoclastique pourrait indiquer la presence d'un edifice volcanique relie. La presence de xenocristaux de zircon dans la plupart des intrusions laisse croire que la mise en place initiale des magmas a debute quelque 10 a 20 millions d'annees avant leur emplacement final dans la croûte superieure. Un age d'intrusion finale famennien coïncide avec l'empietement au Devonien tardif de la deformation ellesmerienne dans la region. La flexion crustale associee, ou des structures en evolution dans l'avant-pays, pourraient avoir facilite la migration vers le haut des magmas. La geochimie des intrusions indique que le magmatisme devonien est en bonne partie derive de la fusion partielle de la croûte inferieure a intermediaire, ce qui suppose un sous-placage magmatique mafique repandu au Devonien intermediaire a tardif. Seule la syenite de Dave Lord conserve des indices d'une signature geochimie mantellique initiale. Le sous-placage mantellique pourrait avoir contribue a localiser l'extension, le volcanisme et le rifting qui ont mene a l'ouverture au Devonien tardif du bassin oceanique d'Angayucham. Le pluton eocene de Bear Mountain est interprete comme etant un exemple plus septentrional du magmatisme intraplaque cenozoïque repandu en Alaska. [Traduit par la Redaction], Introduction The tectonic evolution of the Arctic region has been a topic of debate and speculation for many decades (see reviews by Lawver and Scotese 1990; Lane 1997; Lane et [...]

The Wuxing Pt-Pd-rich Cu-Ni sulfide deposit in Heilongjiang Province, Northeast China, is located to the northeast of the Dunhua-Mishan fracture of the eastern Central Asian Orogenic Belt. The mafic-ultramafic complex consist of early-period hornblende-olivine pyroxenite, diopsidite, and hornblende pyroxenite and late-period gabbro and diabase units. An early-period hornblende pyroxenite yielded a zircon U-Pb age of 208.2 [+ or -] 2.6 Ma and a late-period diabase yielded a U-Pb age of 205.6 [+ or -] 1.1 Ma, with zircon eHf(t) values of+1.24 to +8.13. The early- and late-period lithofacies are relatively enriched in LILE (Rb, Ba, and Sr) and LREE, and variably depleted in HFSE (Nb, Ta). The whole-rock and single-mineral analyses of the early-period lithofacies yield ([.sup.87]Sr/[.sup.86]Sr).sub.i ratios of 0.7055-0.7083 and [[epsilon].sub.Nd](t) ratios of -7.98-+3.10. These geochemical data suggest that the parental magmas of the Wuxing complex are high-Mg subalkaline basaltic in nature and were derived from an enriched mantle source. The magmas chamber formed after the injection of magma into the crust along with crustal contamination, producing early crystalline minerals and ore-bearing magmas. The rupturing of the magma chamber released evolved magmas, which then ascended and generated Pt-Pd-bearing lithofacies and Cu-Ni sulfide orebodies by fractional crystallization, accumulation, and liquation. During the late period, the residual magma invaded the early lithofacies and Cu-Ni orebodies. The fluids exsolved from the gabbroic magmas concentrated the mineralized metal elements and enhanced the precipitation of Pt-Pd-bearing veinlet-disseminated orebodies and Pt-Pd-Cu-Ni orebodies. Resume : Le gisement de sulfures a Cu-Ni riche en Pt-Pd de Wuxing, dans la province de Heilongjiang du nord-est de la Chine, est situe au nord-est de la fracture de Dunhua-Mishan de la partie orientale de la ceinture orogenique d'Asie centrale. Le complexe mafique-ultramafique consiste en des unites precoces de pyroxenite a hornblende-olivine, de diopsidite et de pyroxenite a hornblende et d'unites tardives de gabbro et de diabase. Une pyroxenite a hornblende precoce a donne un age U-Pb sur zircon de 208,2 [+ or -] 2,6 Ma et un diabase tardif a donne un age U-Pb de 205,6 [+ or -]1,1 Ma, avec des valeurs [[epsilon].sub.Hf](t) pour le zircon allant de +1,24 a +8,13. Les lithofacies precoces et tardifs sont relativement enrichis en LILE (Rb, Ba, et Sr) et en terres rares legeres et appauvris a divers degres en HFSE (Nb, Ta). Les analyses de roche totale et de differents mineraux des lithofacies precoces donnent des rapports ([.sup.87]Sr/[.sup.86]Sr).sub.i de 0,7055-0,7083 et des rapports [[epsilon].sub.Nd](t) de -7,98-+3,10. Ces donnees geochimiques donnent a penser que les magmas parentaux du complexe de Wuxing sont de nature basaltique subalcaline riche en Mg et qu'ils sont derives d'une source mantellique enrichie. La chambre magmatique s'est formee apres l'injection de magma dans la croute, accompagnee d'une contamination crustale, produisant des mineraux cristallins precoces et des magmas mineralises. La rupture de la chambre magmatique a cause la liberation de magmas evolues qui sont montes dans la croute et ont produit des lithofacies a Pt-Pb et des corps mineralises en sulfures a Cu-Ni par cristallisation fractionnee, accumulation et liquation. Durant la periode tardive, le magma residuel s'est introduit dans les lithofacies precoces et les corps mineralises en Cu-Ni. Les fluides exsolves des magmas gabbroiques ont concentre les elements metalliques mineralises et rehausse la precipitation de corps mineralises en veinules a Pt-Pd dissemines et de corps mineralises en Pt-Pd-Cu-Ni., Introduction Magmatic Ni-Cu-(platinum-group elements, PGE) sulfide deposits are important sources of copper, nickel, platinum, and palladium and are related to mafic-ultramafic rocks. Magmatic sulfide deposits can be divided into two [...]

Four voluminous ignimbrites (150-500 km.sup.3) erupted in rapid succession at 27 Ma in the central San Juan caldera cluster, Colorado. To reconstruct the timescales and thermal evolution of these magma reservoirs, we used zircon ID-TIMS U-Pb geochronology, zircon LA-ICP-MS geochemistry, thermal modeling, and zircon age and crystallization modeling. Zircon geochronology reveals dispersed zircon age spectra in all ignimbrites, with decreasing age dispersion through time that we term a 'chimney sweeping' event. Zircon whole-grain age modeling suggests that 2[sigma] zircon age spans represent approximately one-quarter of total zircon crystallization timescales due to the averaging effect of whole-grain, individual zircon ages, resulting in zircon crystallization timescales of 0.8-2.7 m.y. Thermal and zircon crystallization modeling combined with Ti-in-zircon temperatures indicates that magma reservoirs were built over millions of years at relatively low magmatic vertical accretion rates (VARs) of 2-5 x 10.sup.-3 m y.sup.-1 (2-5 x 10.sup.-6 km.sup.3 y.sup.-1 km.sup.-2), and we suggest that such low VARs were characteristic of the assembly of the greater San Juan magmatic body. Though we cannot unequivocally discern between dispersed zircon age spectra caused by inheritance (xenocrystic or antecrystic) versus prolonged crystallization from the same magma reservoir (autocrystic), our findings suggest that long-term magma input at relatively low VARs produced thermally mature upper crustal magma reservoirs resulting in protracted zircon crystallization timescales. Compiling all U-Pb ID-TIMS zircon ages of large ignimbrites, we interpret the longer timescales of subduction-related ignimbrites as a result of longer term, lower flux magmatism, and the shorter timescales of Snake River Plain ignimbrites as a result of shorter term, higher flux magmatism., Author(s): Adam Curry [sup.1] [sup.3], Sean P. Gaynor [sup.1], J. H. F. L. Davies [sup.2], Maria Ovtcharova [sup.1], Guy Simpson [sup.1], Luca Caricchi [sup.1] Author Affiliations: (1) grid.8591.5, 0000 0001 [...]

Although the voluminous granitoids that constitute the upper crust of the Sierra Nevada batholith (California) have been investigated in detail, comparatively few studies focus on the origin of mafic bodies at similar crustal levels. Here, we present field and petrographic observations, geochronology, and geochemistry of the Hidden Lakes mafic complex in the central-eastern Sierra Nevada batholith. Our results show that the complex comprises norites, gabbros, monzondiorites, and monzonites that record fractional crystallization of a hydrous (~ 3 wt% H.sub.2O), non-primitive basalt within the upper crust (0.3 GPa) at c. 95-96 Ma. To quantitatively model the generation of the observed lithologies, we construct a two-stage polybaric crystallization model based on cumulate and melt-like bulk-rock compositions. In the first step, we model fractionation of a primitive, mantle-derived basalt at > 30 km depth, generating dominantly pyroxenite cumulates. The evolution of the derivative melt (67% of melt mass remaining) is then modeled to fractionate at 12 km depth to produce the observed lithologies within the Hidden Lakes mafic complex. Extension of this model to higher-silica melt compositions (> 65 wt% SiO.sub.2) replicates observed granodiorite compositions in the batholith, suggesting that polybaric crystallization could be an important process for the generation of arc granitoid melts. The depth of differentiation in continental arcs is debated, as field observations indicate abundant lower crustal fractionation while experimental data suggest that high-pressure crystallization of hydrous basalts cannot produce the non-peraluminous granitoid compositions observed in continental arc batholiths. Our model supports polybaric differentiation as one potential mechanism to resolve this inconsistency., Author(s): Madeline J. Lewis [sup.1], Claire E. Bucholz [sup.1], Oliver E. Jagoutz [sup.2] Author Affiliations: (1) grid.20861.3d, 0000000107068890, Division of Geological and Planetary Sciences, California Institute of Technology, , Pasadena, [...]

Zircon occasionally crystallizes in evolved melt pockets in mafic large igneous province (LIP) magmas, and in these cases, it is used to provide high-precision age constraints on LIP events. The precision and accuracy of high-precision ages from LIPs are crucially important, because they may be implicated in mass extinctions. However, why zircon crystallizes in these magmas is not clearly understood, since their mafic compositions should limit zircon saturation. Here, we investigate the occurrence of zircon (and baddeleyite) in intrusive and extrusive mafic rocks from Central Atlantic Magmatic Province (CAMP) using petrography, trace-element analysis, Ti temperatures, Hf and oxygen isotopes, and high-precision U-Pb geochronology, along with petrological and thermal modeling. We provide new ages for CAMP sills that intruded into Paleozoic sediments in Brazil, indicating that the high and low Ti magmatism in this area occurred synchronously over 264 ± 57 ka. We show that upper crustal assimilation, especially of shales, during the emplacement of the CAMP likely led to zircon saturation. Assimilation of upper crustal sediments is also supported by high [delta].sup.18O values and some rare negative [epsilon]Hf values in the zircon crystals. The only extrusive sample analyzed was the North Mountain basalt in Nova Scotia, Canada. This sample contains a large age variation in its zircon crystals (up to 4 Ma), and the older crystals have slightly more negative [epsilon]Hf values suggesting the presence of small (micron scale) xenocrystic cores associated with very late-stage sediment assimilation. However, the CAMP dataset as a whole suggests that the presence of xenocrystic cores is rare. Assuming no xenocrystic cores, and considering the zircon undersaturated nature of LIP mafic melts, the oldest zircon age clusters in a population should record the magma emplacement (or time when assimilation occurred), and the younger ages in a population are more likely to reflect Pb loss, especially given the high U concentrations of LIP zircon. Our identification of heterogeneous isotopic and elemental compositions in LIP zircon indicates that zircon in these magmas saturate in isolated minute melt pockets just before the system cools below its solidus., Author(s): J. H. F. L. Davies [sup.1] [sup.2], A. Marzoli [sup.3], H. Bertrand [sup.4], N. Youbi [sup.5] [sup.6], M. Ernesto [sup.7], N. D. Greber [sup.2] [sup.8], M. Ackerson [sup.9], G. [...]

The Khammam Schist Belt (KSB), southeastern India, represents a part of the collision zone that is sandwiched between the Eastern Dharwar Craton to the west and the Eastern Ghats Mobile Belt (EGMB) to the east. Quartzofeldspathic gneisses of the EGMB and garnet-kyanite metapelites of the KSB have been investigated to characterize P-T-t evolution of the Khammam region. Pseudosection modeling reveals that the melt-bearing quartzofeldspathic gneiss experienced peak and post-peak metamorphism at P-T conditions of 7.9-8.1 kbar/790-810 °C and ~ 7.0 kbar/740-750 °C, respectively. In contrast, metapelite witnessed peak metamorphism at 7.4-7.8 kbar/600-640 °C, followed by post-peak retrogression at 6.1-6.6 kbar/590-625 °C. U-Th-(total) Pb monazite ages from the gneiss constrain the peak and retrograde metamorphic episodes at 1.63-1.53 Ga and 1.48-1.38 Ga, while those in the metapelites were determined at 1.25-1.20 Ga and 1.18-1.10 Ga, respectively. These P-T-t estimates indicate that the KSB, Vinjamuru, and Ongole domains evolved distinctly during Late Paleoproterozoic-Late Mesoproterozoic. The younger ages (0.90-0.81 Ga) were ascribed to the formation of the Eastern Indian Tectonic Zone, implying its extension beyond the western margin of the EGMB. Besides, the distinct Neoarchean ages (2.79-2.45 Ga) are related to the Archean protolith of the quartzofeldspathic gneisses, which were likely derived from the Archean crust of the Napier-Rayner Complex. This tectonothermal restoration is new and characterizes the Khammam region as the hot and composite collision zone with protracted geological history. (250 words), Author(s): H. Hrushikesh [sup.1] [sup.2], Praveen C. Singh [sup.1], N. Prabhakar [sup.1], Sachin K. Thakur [sup.1], Bidisha Dey [sup.1] [sup.3] Author Affiliations: (1) grid.417971.d, 0000 0001 2198 7527, Department of [...]

Accessory mineral thermometry and thermodynamic modelling are fundamental tools for constraining petrogenetic models of granite magmatism. U-Pb geochronology on zircon and monazite from S-type granites emplaced within a semi-continuous, whole-crust section in the Georgetown Inlier (GTI), NE Australia, indicates synchronous crystallisation at 1550 Ma. Zircon saturation temperature (T.sub.zr) and titanium-in-zircon thermometry (T.sub.(Ti-zr)) estimate magma temperatures of ~ 795 ± 41 °C (T.sub.zr) and ~ 845 ± 46 °C (T.sub.(Ti-zr)) in the deep crust, ~ 735 ± 30 °C (T.sub.zr) and ~ 785 ± 30 °C (T.sub.(Ti-zr)) in the middle crust, and ~ 796 ± 45 °C (T.sub.zr) and ~ 850 ± 40 °C (T.sub.(Ti-zr)) in the upper crust. The differing averages reflect ambient temperature conditions (T.sub.zr) within the magma chamber, whereas the higher T.sub.(Ti-zr) values represent peak conditions of hotter melt injections. Assuming thermal equilibrium through the crust and adiabatic ascent, shallower magmas contained 4 wt% H.sub.2O, whereas deeper melts contained 7 wt% H.sub.2O. Using these H.sub.2O contents, monazite saturation temperature (T.sub.mz) estimates agree with T.sub.zr values. Thermodynamic modelling indicates that plagioclase, garnet and biotite were restitic phases, and that compositional variation in the GTI suites resulted from entrainment of these minerals in silicic (74-76 wt% SiO.sub.2) melts. At inferred emplacement P-T conditions of 5 kbar and 730 °C, additional H.sub.2O is required to produce sufficient melt with compositions similar to the GTI granites. Drier and hotter magmas required additional heat to raise adiabatically to upper-crustal levels. S-type granites are low-T mushes of melt and residual phases that stall and equilibrate in the middle crust, suggesting that discussions on the unreliability of zircon-based thermometers should be modulated., Author(s): S. Volante [sup.1] [sup.2] [sup.4], W. J. Collins [sup.1], E. Blereau [sup.3], A. Pourteau [sup.1], C. Spencer [sup.1], N. J. Evans [sup.3], V. Barrote [sup.4] [sup.3], A. R. Nordsvan [...]

The Late Silurian to Late Devonian Saint George Batholith in southwestern New Brunswick is a large composite intrusion (2000 [km.sup.2]) emplaced into the continental margin of the peri-Gondwanan microcontinent of Ganderia. The batholith includes: (1) Bocabec Gabbro; (2) equigranular Utopia and Wellington Lake biotite granites; (3) Welsford, Jake Lee Mountain, and Parks Brook peralkaline granites; (4) two-mica John Lee Brook Granite; (6) Jimmy Hill and Magaguadavic megacrystic granites; and (6) rapakivi Mount Douglas Granite. New LA ICP-MS in situ analyses of six samples from the Saint George Batholith are as follows: (1) U-Pb monazite crystallization age of 425.5 [+ or -] 2.1 Ma for the Utopia Granite in the western part of the batholith (2) U-Pb zircon crystallization ages of 420.4 [+ or -] 2.4 Ma and 420.0 [+ or -] 3.5 Ma for two samples of the Utopia Granite from the central part of the batholith; (3) U-Pb zircon crystallization age of 418.0 [+ or -] 2.3 Ma for the Jake Lee Mountain Granite; (4) U-Pb zircon crystallization age of 415.5 [+ or -] 2.1 Ma for the Wellington Lake Granite; and (5) U-Pb monazite crystallization age of 413.3 [+ or -] 2.1 Ma for the John Lee Brook Granite. The new geochronological together with new and existing geochemical data suggest that the protracted magmatic evolution of the Late Silurian to Early Devonian plutonic rocks is related to the transition of the Silurian Kingston arc-Mascarene backarc system from an extensional to compressional tectonic environment during collision of the Avalonian microcontinent with Laurentia followed by slab break-off. Le batholite du Silurien tardif au Devonien tardif de Saint George dans le sud ouest du Nouveau Brunswick est une vaste intrusion composite (2 000 kilometres carres) qui s'est mise en place a l'interieur de la marge continentale du microcontinent perigondwanien de Ganderia. Le batholite englobe : (1) le gabbro de Bocabec; (2) les granites a biotite isogranulaire Utopia et du lac Wellington; (3) les granites hyperalcalins de Welsford, du mont Jake Lee et du ruisseau Parks Brook; (4) le granite a deux micas du ruisseau John Lee; (5) les granites macrocristallins de la colline Jimmy et de Magaguadavic; et (6) le granite rapakivi du mont Douglas. De nouvelles analyses par spectrometrie de masse a plasma induit couplee a l'ablation laser in situ de six echantillons provenant de Saint George ont revele ce qui suit : (1) une cristallisation survenue il y a 425,5 [+ or -] 2,1 Ma d'apres une datation U-Pb sur monazite, dans le cas du granite Utopia dans la partie occidentale du batholite; (2) des ages de cristallisation en vertu d'une datation U-Pb sur zircon de 420,4 [+ or -] 2,4 Ma et de 420,0 [+ or -] 3,5 Ma dans le cas de deux echantillons en provenance du granite Utopia dans le centre du batholite; (3) un age de cristallisation de 418,0 [+ or -] 2,3 Ma dans le cas du granite du mont Jake Lee, d'apres une datation U-Pb sur zircon; (4) un age de cristallisation de 415,5 [+ or -] 2,1 Ma obtenu par datation U-Pb sur zircon dans le cas du granite du lac Wellington; et (5) un age de cristallisation de 413,3 [+ or -] 2,1 Ma d'apres une datation U-Pb sur monazite, dans le cas du granite du ruisseau John Lee. Les nouvelles donnees geochronologiques conjuguees aux donnees geochimiques existantes laissent supposer que l'evolution magmatique prolongee de des roches plutoniques du Silurien tardif au Devonien precoce est apparentee a la transition de l'arc silurien de Kingston-arriere-arc de Mascarene d'un milieu tectonique de distension a un milieu de compression durant la collision du microcontinent avalonien avec Laurentia suivie d'une rupture par tranches., [Traduit par la redaction] INTRODUCTION The Silurian (Ludlovian to Pridolian) to Late Devonian (Famennian) Saint George Batholith in southwestern New Brunswick is a large composite intrusion (2000 [km.sup.2]) that was [...]

The Andean margin of the South American continent has been magmatically and tectonically active for over 330 million years. It is the type location where 'Cordilleran-type' magmatism and orogenesis are manifest. In Argentina and Chile, between the latitudes of 28° and 40° S, magmatism related to the Gondwanan 'Cordilleran-type' orogeny is reflected in a series of Carboniferous to Triassic intrusions. A comprehensive model exists for the petrogenesis of such magmas in Chile, however there is relatively little understanding of the nature and timing of Permo-Triassic magmatism in the Frontal Cordillera and Precordillera in Argentina. To address this, we present a new dataset of in situ zircon U-Pb, O and Hf isotopes from 15 felsic intrusions from Argentina. Zircon geochronology shows that magmatism in this region commenced at ca. 285 Ma and continued until ca. 250 Ma. Zircon O and Hf isotopes suggest that the oldest Permian magmas were derived from young supracrustal sources, with elevated [delta].sup.18O (~ 8.5 to 7.5â°) and negative [epsilon].sub.Hf values (~ - 1 to - 3 [epsilon].sub.Hf). The emplacement of these magmas was coeval with the formation of mantle-derived magmas characterised by mantle-like [delta].sup.18O (~ 6.0 to 5.5â°) and moderately positive [epsilon].sub.Hf values (~ 4 to 1 [epsilon].sub.Hf). As magmatism continued, transitional isotope signatures became predominant as melts of these disparate sources interacted and hybridised. It is proposed that under a compressional regime, mantle-derived magmas were halted in the lower continental crust, where they exchanged heat and volatiles with an older fertile lithosphere to generate melts from supracrustal sources. A shift in the stress regime at ca. 285 Ma permitted both crustally derived and juvenile mantle-derived magmas to exploit newly formed conduits to rise into the upper crust. A regional compilation of zircon O and Hf isotopes from felsic igneous rocks reveals a coherent secular trend over ~ 100 million years, where the oldest magmatism exhibits a dominant supracrustal component and younger magmas progressively (over 50 Ma) transition towards juvenile mantle-like isotopic compositions. This new dataset from Argentina fills a significant gap in the previous regional models between 285 and 250 Ma and documents the isotopic response of magmas produced in back-arc regions to a transition between compression and extensional/neutral stress regimes. These results give insight into the generation of new, or recycling of, continental crust in a back-arc setting and how the transition from compression to extension is imperative for ore-forming magmas to reach the upper crust., Author(s): Gregory H. Poole [sup.1] [sup.2], Anthony I. S. Kemp [sup.1], Steffen G. Hagemann [sup.1], Marco L. Fiorentini [sup.1] [sup.2], Heejin Jeon [sup.3] [sup.7], Ian S. Williams [sup.4], Eduardo O. [...]

Translating burial and exhumation histories from the petrological and geochronological record of high-pressure assemblages in subduction channels is key to understanding subduction channel processes. Convective return flow, either serpentinite or sediment hosted, has been suggested as a potential mechanism to retrieve rocks from significant depths and exhume them. Numerical modelling predicts that during convective flow, subducted material can be cycled within a serpentinite-filled subduction channel. Geochronological and petrological evidences for such cycling during subduction are preserved in lawsonite eclogite from serpentinite melange in the Southern New England Orogen, eastern Australia. Ar-Ar, Rb-Sr phengite and U-Pb titanite geochronology, supported by phase equilibrium forward modelling and mineral zoning, suggest Cambro-Ordovician eclogite underwent two stages of burial separated by a stage of partial exhumation. The initial subduction of the eclogite at ca. 490 Ma formed porphyroblastic prograde-zoned garnet and lawsonite at approximate P-T conditions of at least 2.9 GPa and 600 °C. Partial exhumation to at least 2.0 GPa and 500 °C is recorded by garnet dissolution. Reburial of the eclogite resulted in growth of new Mg-rich garnet rims, growth of new prograde-zoned phengite and recrystallization of titanite at P-T conditions of approximately 2.7 GPa and 590 °C. U-Pb titanite, and Ar-Ar and Rb-Sr phengite ages constrain the timing of reburial to ca. 450 Ma. This was followed by a second exhumation event at approximately 1.9 GPa and 520 °C. These conditions fall along a cold approximate geotherm of 230 °C/GPa. The inferred changes in pressure suggest the lawsonite eclogite underwent depth cycling within the subduction channel. Geochronological data indicate that partial exhumation and reburial occurred over ca. 50 M y., providing some estimation on the timescales of material convective cycling in the subduction channel., Author(s): R. Tamblyn [sup.1], M. Hand [sup.1], L. Morrissey [sup.2], T. Zack [sup.1] [sup.3], G. Phillips [sup.4], D. Och [sup.5] Author Affiliations: (1) grid.1010.0, 0000 0004 1936 7304, Department of [...]

In situ LA-ICP-MS U-Pb geochronology has been performed on apatite and zircon within thermally recrystallized clast-laden and clast-poor impact melt rocks from the Brent impact structure. A total of 377 laser analyses on 120 impact melt-grown (n = 9) and impact-recrystallized zircon grains (n = 111) were obtained, from which a concordia age of 452.8 ± 2.7 Ma (MSWD 0.57, n = 11), and a weighted average mean .sup.206Pb/.sup.238U age of 453.2 ± 2.9 Ma (MSWD 0.60) (n = 11) are calculated. A total of 300 laser analyses from 100 relict apatite grains were obtained, with an unanchored regression through all data yielding a lower intercept age of 453.2 ± 6.0 Ma (MSWD 5.8, n = 300), that overlaps within error of zircon. .sup.207Pb/.sup.206Pb ratios obtained from feldspar clasts within clast-laden impact melt retain the same initial Pb composition as the target rocks from which they are derived, while feldspars that crystallized from impact melt have .sup.207Pb/.sup.206Pb ratios indicative of isotopic re-equilibration between basement lithologies of two different ages. A similar variability in .sup.207Pb/.sup.206Pb is recorded by apatite. This provides evidence for the involvement of Neoproterozoic Lake Nipissing alkaline suite, as well as Mesoproterozoic Grenville gneisses in the production of impact melt at Brent. Recrystallized apatite grains exhibit enrichments in light rare earth elements (LREEs) along neoblast grain boundaries, indicative of trace element substitution and phase precipitation during impact-induced recrystallization. An age of 452.8 ± 2.7 Ma from zircon and 453.2 ± 6.0 Ma from apatite places the impact event in the Late Ordovician, at or near the Sandbian-Katian boundary, confirming Brent's involvement in the Middle to Late Ordovician crater cluster event-a period of enhanced impactor flux to Earth related to the L-Chondrite parent body disruption., Author(s): Maree McGregor [sup.1] [sup.2], Michael R. Dence [sup.3], Christopher R. M. McFarlane [sup.2], John G. Spray [sup.1] Author Affiliations: (1) grid.266820.8, 0000 0004 0402 6152, Planetary and Space Science [...]

Previous interpretations of a Jurassic subduction in Iran were based on trace element classification diagrams for granitoids, but their reliability is questionable, underscored by modern examples of continental break-up zones such as the Baja California. We present new field observations, bulk rock geochemistry, Sr and Nd isotope analyses and U-Pb zircon geochronology to assess the age and tectonic setting of previously undated intermediate to felsic magmatic rocks cropping out in the Precambrian basement of NW Iranian Azerbaijan. The geochronology revealed an uneven distribution in space and time: Late Jurassic (159-154 Ma) intrusions and dikes are alkaline to calc-alkaline. Their melt source is mantle dominated with a distinct continental contribution disclosed by radiogenic isotopes and abundant inherited zircon cores. Mid-Cretaceous (112-96 Ma) plutonic bodies and associated volcanic rocks occur only to the east of the major Siah Cheshmeh-Khoy Fault. They have geochemical signatures typical of a metasomatized mantle. In consistence with the sedimentation history of the area, our new interpretation attributes the Late Jurassic magmatism to thinning of a continental lithosphere in a rift-related setting. Mid-Cretaceous magmatism was produced by oceanic subduction beneath the Central Iran continent. We interpret the 40-Ma age gap between the two magmatic episodes as the time of opening of the oceanic basin witnessed by the Khoy ophiolite in the study area., Author(s): Anna Lechmann [sup.1] , Jean-Pierre Burg [sup.1] , Peter Ulmer [sup.1] , Ali Mohammadi [sup.1] , Marcel Guillong [sup.1] , Mohammad Faridi [sup.2] Author Affiliations: (Aff1) 0000 0001 2156 [...]

Cherubini et al. (above) question the reliability of identifying annual growth increments in olive trees, and therefore voice caution against the result of the wiggle-match of the four sections of [...]

The subduction system in southern Patagonia provides direct evidence for the variability of the position of an active continental arc with respect to the subducting plate through time, but the consequences on the arc magmatic record are less well studied. Here we present a geochemical and geochronological study on small plutons and dykes from the upper crust of the southern Patagonian Andes at ~ 51°S, which formed as a result of the subduction of the Nazca and Antarctic plates beneath the South American continent. In situ U-Pb geochronology on zircons and bulk rock geochemical data of plutonic and dyke rocks are used to constrain the magmatic evolution of the retro-arc over the last 30 Ma. We demonstrate that these combined U-Pb and geochemical data for magmatic rocks track the temporal and spatial migration of the active arc, and associated retro-arc magmatism. Our dataset indicates that the rear-arc area is characterized by small volumes of alkaline basaltic magmas at 29-30 Ma that are characterized by low La/Nb and Th/Nb ratios with negligible arc signatures. Subsequent progressive eastward migration of the active arc culminated with the emplacement of calc-alkaline plutons and dikes ~ 17-16 Ma with elevated La/Nb and Th/Nb ratios and typical subduction signatures constraining the easternmost position of the southern Patagonian batholith at that time. Geochemical data on the post-16 Ma igneous rocks including the Torres del Paine laccolith indicate an evolution to transitional K-rich calc-alkaline magmatism at 12.5 ± 0.2 Ma. We show that trace element ratios such as Nb/Ta and Dy/Yb systematically decrease with increasing SiO.sub.2, for both the 17-16 Ma calc-alkaline and the 12-13 Ma K-rich transitional magmatism. In contrast, Th/Nb and La/Nb monitor the changes in the source composition of these magmas. We suggest that the transition from the common calc-alkaline to K-rich transitional magmatism involves a change in the source component, while the trace element ratios, such as Nb/Ta and Dy/Yb, of derivative higher silica content liquids are controlled by similar fractionating mineral assemblages. Analysis of a global compilation of Nb/Ta ratios of arc magmatic rocks and simple geochemical models indicate that amphibole and variable amounts of biotite exert a major control on the low Dy/Yb and Nb/Ta of derivative granitic liquids. Lastly, we suggest that the low Nb/Ta ratio of silica-rich magmas is a natural consequence of biotite fractionation and that alternative models such as amphibolite melting in subduction zones and diffusive fractionation are not required to explain the Nb/Ta ratio of the upper continental crust., Author(s): Othmar Müntener [sup.1] , Tanya Ewing [sup.1] [sup.2] , Lukas P. Baumgartner [sup.1] , Mélina Manzini [sup.1] , Thibaud Roux [sup.1] , Pierre Pellaud [sup.1] , Luc Allemann [sup.1] [...]

We present new high-precision CA-ID-TIMS and in situ U-Pb ages together with Hf and O isotopic analyses (analyses performed all on the same grains) from four tuffs from the 15-10 Ma Bruneau-Jarbidge center of the Snake River Plain and from three rhyolitic units from the Kimberly borehole in the neighboring 10-6 Ma Twin Falls volcanic center. We find significant intrasample diversity in zircon ages (ranges of up to 3 Myr) and in [delta].sup.18O (ranges of up to 6â°) and [epsilon].sub.Hf (ranges of up to 24 [epsilon] units) values. Zircon rims are also more homogeneous than the associated cores, and we show that zircon rim growth occurs faster than the resolution of in situ dating techniques. CA-ID-TIMS dating of a subset of zircon grains from the Twin Falls samples reveals complex crystallization histories spanning 10.sup.4-10.sup.6 years prior to some eruptions, suggesting that magma genesis was characterized by the cyclic remelting of buried volcanic rocks and intrusions associated with previous magmatic episodes. Age-dependent trends in zircon isotopic compositions show that rhyolite production in the Yellowstone hotspot track is driven by the mixing of mantle-derived melts (normal [delta].sup.18O and [epsilon].sub.Hf) and a combination of Precambrian basement rock (normal [delta].sup.18O and [epsilon].sub.Hf down to - 60) and shallow Mesozoic and Cenozoic age rocks, some of which are hydrothermally altered (to low [delta].sup.18O values) by earlier stages of Snake River Plain magmatism. These crustal melts hybridize with juvenile basalts and rhyolites to produce the erupted rhyolites. We also observe that the Precambrian basement rock is only an important component in the erupted magmas in the first eruption at each caldera center, suggesting that the accumulation of new intrusions quickly builds an upper crustal intrusive body which is isolated from the Precambrian basement and evolves towards more isotopically juvenile and lower-[delta].sup.18O compositions over time., Author(s): Dylan P. Colón [sup.1] , Ilya N. Bindeman [sup.1] , Jörn-Frederik Wotzlaw [sup.2] , Eric H. Christiansen [sup.3] , Richard A. Stern [sup.4] Author Affiliations: (Aff1) 0000 0004 1936 [...]