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Magmatism during the maturation phase of Archean greenstone belts produced voluminous tonalite--trondhjemite--granodiorite (TTG) suites, as well as a lesser amount of tonalite--trondhjemite--diorite (TTD) suites. Such TTD suites have recently been recognized in the Archean Abitibi greenstone belt, on the southern flank of the Superior Craton, Canada, but their source(s), differentiation processes, and depths of emplacement remain poorly constrained. The Neoarchean Eau Jaune Complex lies in the northeastern corner of the Abitibi greenstone belt and represents one of the most voluminous tonalite-dominated and diorite-bearing intrusive suites of the Chibougamau region. This TTD suite comprises six intrusive phases with distinct petrology and chemistry. All units were emplaced as laccolith-like intrusions injected along discontinuities within the volcanic succession at ca. 2724 Ma (U--Pb zircon dating), during the synvolcanic interval (i.e., construction and maturation phase), at a depth of approximately 7-8 km. The most heavy rare-earth element (HREE)-depleted phases (granodiorite, tonalite, and trondhjemite) correspond to magmas that fractionated amphibole and were likely produced by partial melting of a garnet- and titanate-bearing amphibolite, akin to TTG magmas. The least HREE-depleted phases are dioritic in composition and correspond to mantle-derived magmas that may have interacted with TTG melts. This indicates interaction between coeval mantle-derived and crustal melts during the maturation phase of the Abitibi greenstone belt. Models formulated to address the geodynamic evolution of greenstone belts must account for the coeval production of basalt-derived (TTG suites) and mantle-derived (tholeiitic magmatism) melts occasionally interacting to form TTD suites. Key words: Abitibi greenstone belt, Chibougamau, Eau Jaune Complex, igneous petrology, Neoarchean magmatism, structural geology, TTD suite, TTG suite, tonalite. Durant la phase de maturation des ceintures de roches vertes archéennes, l'activité magmatique a produit de volumineuses suites de type tonalite-trondhjémite-granodiorite (TTG) ainsi que de moindres quantités de suites de type tonalite-trondhjémite--diorite (TTD). De telles suites TTD ont récemment été découvertes dans la ceinture de roches vertes archéenne de l'Abitibi, située dans la partie sud du craton du Supérieur (Canada), mais leurs sources, leurs processus de différenciation et les profondeurs auxquelles elles ont été mises en place demeurent mal établis. Le complexe néoarchéen d'Eau Jaune est situé à l'extrémité nord-est de la ceinture de roches vertes de l'Abitibi et constitue l'une des plus volumineuses intrusions tonalitiques à composante dioritique de la région de Chibougamau. Cette suite TTD est constituée de six phases intrusives distinctes des points de vue pétrologique et géochimique. Toutes les unités ont été injectées sont forme de laccolithes le long de discontinuités dans la séquence volcanique vers 2724 Ma (datation U-Pb sur zircons), soit durant l'intervalle synvolcanique (correspondant aux phases de construction et de maturation), et à une profondeur de 7 à 8 km. Les phases les plus appauvries en terres rares lourdes (granodiorite, tonalite et trondhjémite) correspondent à des magmas apparentés aux magmas de type TTG, caractérisés par la cristallisation fractionnée d'amphiboles et qui sont probablement le produit de la fusion partielle d'une amphibolite à grenat et titanate. Les phases les moins appauvries en terres rares lourdes sont de composition dioritique et correspondent à des magmas dérivés du manteau qui pourraient avoir interagi avec des magmas de type TTG. Cela indique l'interaction de magmas dérivés du manteau et de la croûte durant la phase de maturation de la ceinture de roches vertes de l'Abitibi. Les modèles géodynamiques expliquant l'évolution des ceintures de roches vertes doivent donc inclure la production contemporaine de magmas dérivés de basaltes (suites de type TTG) et du manteau (magmatisme tholéiitique), pouvant occasionnellement interagir pour former des suites de type TTD. [Traduit par la Rédaction] Mots-clés : ceinture de roches vertes de l'Abitibi, Chibougamau, Complexe d'Eau Jaune, géologie structurale, magmatisme néoarchéen, pétrologieignée, suitede type TTD, suite de type TTG, tonalite., Introduction Tonalite--trondhjemite--granodiorite (TTG) suites are the main component of intrusive Archean rocks and can represent up to 50-70 vol.% of a craton (Condie 1993; Moyen and Stevens 2006; Hoffmann et [...]

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 [...]

A large dyke of quartz tholeiitic gabbronorite has been mapped for 59 km in southern New Brunswick, Canada, between Lepreau River in the northeast and Indian Island in the southwest. Scattered outcrops occur along a positive aeromagnetic lineament, providing a dyke strike of N42[degrees]E overall (segments N30[degrees]E to N72[degrees]E), dips of 80[degrees] to 90[degrees]NNW, and widths of 4 to 30 m. A [new.sup.40]Ar/[.sup.39]Ar plagioclase age of 201.67 6 0.35 Ma for the Lepreau River Dyke is similar to dates for the massive North Mountain Basalt in the Fundy Basin to the east. The dyke is associated with the Ministers Island and Christmas Cove dykes, which are indistinguishable in chemistry, petrology, and probable age, and we regard them as segments of the same co-magmatic dyke system. In addition, their petrology is similar to that of the basalts of the adjacent Early Mesozoic Fundy and Grand Manan basins. We propose that the Lepreau River and associated dykes were sources for the regional basin basalts, which in turn are part of the Central Atlantic Magmatic Province (CAMP) that overlaps the Triassic-Jurassic boundary and associated mass extinction event. Key words: Mesozoic dykes, Central Atlantic Magmatic Province (CAMP), flood basalt, Fundy Basin. Un grand dyke de gabbronorite tholeitique quartzifere a ete cartographie sur une distance de 59 km dans le sud du Nouveau-Brunswick (Canada) entre la riviere Lepreau, au nord-est, et l'ile Indian, au sud-ouest. Des affleurements disperses sont presents le long d'un lineament aeromagnetique positif, qui donnent une orientation globale de N[42.sup.o]E pour le dyke (les differents troncons allant de N30[degrees]E a N72[degrees]E), des pendages de 80[degrees] a 90[degrees] vers le NNO et des largeurs de 4 m a 30 m. Un nouvel [age.sup.40]Ar/[.sup.39]Ar sur plagioclase de 201,67 6 0,35 Ma pour le dyke de la riviere Lepreau est semblable a des ages obtenus pour le basalte massif de North Mountain, dans le bassin de Fundy plus a l'est. Le dyke est associe aux dykes de l'ile Minister et de Christmas Cove, dont la chimie, la petrologie et l'age probable sont essentiellement identiques, et nous estimons qu'ils constituent des troncons d'un meme systeme de dykes comagmatiques. Leur petrologie est en outre semblable a celle des basaltes des bassins de Fundy et de Gand Manan attenants, d'age mesozoique precoce. Nous proposons que le dyke de la riviere Lepreau et les dykes associes constituaient des sources pour les basaltes de bassin regionaux qui, eux, font partie de la province magmatique centre atlantique (PMCA) qui chevauche la limite Trias-Jurassique et l'episode d'extinction massive associe. [Traduit par la Redaction] Mots-cles : dykes mesozoiques, province magmatique centre atlantique (PMCA), basalte de plateau, bassin de Fundy., Introduction Flood basalts in large igneous provinces are produced by fissure eruptions, which can be exceptionally large (tens to hundreds of kilometres long), and with huge eruption volumes of tens [...]

New geochemical data demonstrate the similarity between Neoarchaean TTGs found in the middle crust of the Zimbabwe Craton and the lower crust of the Northern Marginal Zone of the Limpopo Belt. Hence the Neoarchaean segment of the Zimbabwe Craton provides a complete crustal section-from lower crustal granulites to the supracrustal rocks of the greenstone belts. Mantle activity recorded in the basalts of the greenstone belts is interpreted as plume volcanism, setting a context for the process of crustal generation. However, published isotopic data and new trace element data reported here show that magma genesis in Zimbabwe Craton in the Neoarchaean incorporated up to 20% of older crust, indicating that not all the granitoids are juvenile. This paper seeks to identify the trace element composition of the parental melts to the TTGs of the Zimbabwe Craton, define the petrological processes by which they were generated and set limits on possible petrological models for the process of crust generation. The least contaminated TTGs have low SiO.sub.2 and moderately fractionated trace element patterns. Calculations indicate that they were derived by the partial melting of a garnet amphibolite source at ca 10 kb depth. Rare intrusive diorites represent a low fraction melt of an enriched mantle source derived from refertilised depleted mantle. These observations lead to a model in which the source material of the TTGs was basaltic and the thermal energy to melt the basalts came from continued basaltic and komatiitic magmatism. Partial melts of old felsic crust were incorporated into the TTG magmas. The resulting 'enriched' TTGs provided a fertile lower crust from which an extensive suite of late tectonic granites could be generated. These data do not support a modern-style plate tectonic model for the growth of the Zimbabwe Craton in the Neoarchaean., Author(s): Hugh Rollinson [sup.1] Author Affiliations: (1) grid.57686.3a, 0000 0001 2232 4004, School of Built and Natural Environment, University of Derby, , Kedleston Road, DE22 1GB, Derby, UK Introduction The [...]

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 Earn Group of central Yukon records the transition from a passive to an active margin along western Laurentia in the Late Devonian. Fine-grained clastic rocks and chert of the lower Earn Group contain late Early to Middle Devonian fossils and were deposited in an offshelf environment. The upper Earn Group comprises a mixture of sandstone and conglomerate, fine-grained siliciclastic rocks, and widespread crystal lithic tuff. Zircons from this succession are precisely dated using chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) methods on igneous (ca. 363 Ma) and detrital (ca. 378-363 Ma) grains and the ages were confirmed by Frasnian to Famennian fossils. Abrupt, along-strike facies changes within the upper Earn Group of the Glenlyon-Tay River area occur across mapped faults that are inferred to have originated as syndepositional extension faults in the Late Devonian. Occurrences of ca. 363 Ma tuff horizons within all facies of the upper Earn Group provide a temporal correlation across the area. Diorite plutons intrude lower Paleozoic rocks in the area and have U-Pb zircon crystallization dates of ca. 364 Ma. The diorite has calc-alkaline composition consistent with arc magmatism or crustal contamination. The Late Devonian magmatism in the Earn Group is coincident with onset of arc magmatism in the allochthonous Yukon-Tanana terrane, and extension related to rifting and opening of the Slide Mountain ocean in a back-arc setting. Magmatic rocks in the Earn Group of central Yukon thus represent part of a remnant continental arc and back-arc stranded behind the Slide Mountain ocean in the Mississippian. Key words: Late Devonian, magmatism, Earn Group, extension, western Laurentia, biostratigraphy. Le Groupe d'Earn dans le centre du Yukon preserve des preuves d'une transition d'une marge passive a une marge active le long de la Laurentie occidentale au Devonien tardif. Des roches clastiques a grains fins et deschertsdu Groupe d'Earn inferieur renferment des fossiles d'age devonien precoce tardif a devonien moyen et ont ete deposes dans un milieu oceanique. Le Groupe d'Earn superieur comprend un melange de gres et de conglomerats, de roches silicoclastiques a grains fins et de tuf cristallolithique ubiquiste. Des zircons provenant de cette succession sont dates de maniere precise par des methodes de spectrometrie de masse d'ionisation thermique d'ionisation d'isotope d'abrasion chimique (CA-ID-TIMS) appliquees a des grains ignes (~363 Ma) et detritiques (~378-363 Ma), ces ages etant confirmes par des fossiles frasniens et famenniens. Des changements de facies abrupts paralleles a la direction des strates dans le Groupe d'Earn superieur de la region de Glenlyon-riviere Tay sont observes de part et d'autre de failles cartographiees interpretees comme etant, a leur origine au Devonien precoce, des failles d'extension synsedimentaires. La presence d'horizons de tuf de ~363 Ma dans tous lesfaciesduGrouped'Earn superieur fournit une correlation temporelle sur l'ensemble de la region. Des plutons de diorite recoupent des roches du Paleozo'ique inferieur dans la region et donnent des ages U-Pb de cristallisation des zircons de ~364 Ma. La diorite presente une composition calco-alcaline que pourraient expliquer un magmatisme d'arc ou une contamination crustale. Le magmatisme d'age devonien tardif dans le Groupe d'Earn coincide avec le debut du magmatisme d'arc dans le terrane allochtone de Yukon-Tanana et avec l'extension associee au rifting et a l'ouverture de l'ocean de Slide Mountain dans un milieu d'arriere-arc. Les roches magmatiques dans le Groupe d'Earn du centre du Yukon representent donc une partie des vestiges d'un arc continental et d'un arriere-arc echoues derriere l'ocean de Slide Mountain au Mississipien. [Traduit par la Redaction] Mots-cles: Devonien tardif, magmatisme, Groupe d'Earn, extension, Laurentie occidentale, biostratigraphie., Introduction The western edge of North America (or Laurentia in the Paleozoic) was a passive continental margin from early Cambrian to Middle Devonian (Bond et al. 1983; Bond and Kominz [...]

Devonian granitoid plulons comprise a major part of the bedrock of northwestern Maine representing the magmatic expression of the Acadian orogeny in this part of the northern Appalachian orogen. They are petrographically diverse with minerals characteristic of both I- and S-type granites, in some cases within the same intrusion, and some are compositionally zoned. New LA-ICP-MS ages presented here elucidate the timing and duration of this magmatism. The earliest phase of granitoid magmatism began around 410-405 Ma with the emplacement of the Flagstaff Lake Igneous Complex, and the presence of contemporaneous mafic rocks suggests that mantle-derived magmas were also produced at this time. Late Devonian ages, ca. 365 Ma, for many intrusions, such as the Chain of Ponds and Songo plutons, reveal that magmatism continued for 45 million years during which compositionally diverse I- and S-type magmas were produced. In addition, there is evidence that intrusive activity was prolonged within some plutons, for example the Rome-Norridgewock pluton and the Mooselookmeguntic Igneous Complex, with 10-15 myr between intrusive units. The new ages suggest a break in magmatism between 400 Ma and 390 Ma apparently separating Acadian magmatism into early and late pulses. The production of lower crustal I-type magmas appears to have been concentrated later, ca. 380-365 Ma, although several S-type granitoids were also emplaced during this period. These Late Devonian plulons display abundant zircon inheritance with ages around 385 Ma, which suggests that the crust was experiencing enhanced thermal perturbations during this extended timeframe. The new data for granitoid plutons in northwestern Maine are consistent with tectonic models for other parts of Ganderia which propose initial flat slab subduction followed by slab breakoff and delamination. Les plutons granitoides du Devonien constituent une tranche importante du substrat rocheux du nord-ouest du Maine representant l'expression magmatique de l'orogenese acadienne dans cette partie de l'orogene appalachien septentrional. Ils constituent une succession petrographiquement diversifiee, composee de mineraux caracteristiques tant des granites de type I que de type S, dans certains cas a l'interieur de la meme intrusion, alors que la composition des aulres plutons regionaux est zonee. Les nouvelles datations par ablation laser et speclrometrie de masse a plasma induclif presentees ici elucidenl le moment ou est survenu ce magmatisme et sa duree. L'episode le plus ancien du magmatisme granitoide a debute vers 410-405 Ma avec la mise en place du complexe de roches eruptives du lac Flagstaff, meme si la presence de roches mafiques contemporaines laisse supposer que des magmas d'origine mantellique ont egalement ete produits a ce moment. Les datations du Devonien tardif faisant remonter a environ 365 Ma de nombreuses intrusions, comme les plutons Chain of Ponds et Songo, revelent que ce magmatisme s'est dans l'ensemble poursuivi durant 45 millions d'annees au cours desquelles des magmas de type I et S de compositions diverses ont ete produits. II existe de plus des preuves que l'activite intrusive s'est prolongee a l'interieur de certains plutons, par exemple le pluton Rome-Norridgewock et le complexe de roches eruptives de Mooselookmeguntic, qui accusent un ecart de 10 a 15 Ma entre les unites intrusives. Les nouvelles datations permettent de supposer une interruption du magmatisme entre 400 et 390 Ma., qui semble separer le magmatisme acadien en impulsions precoce et tardive. La production de magmas de type I crustaux inferieurs semble s'etre concentree a un moment ulterieur, vers 380 a 365 Ma, bien que plusieurs roches granitoides de type S se soient egalement mises en place durant celte periode. Ces plutons du Devonien tardil presentent un heritage abondanl de zircons remontant a peu pres a 385 Ma, ce qui laisse entendre que la croute terrestre a subi des perturbations thermiques de plus en plus prononcees au cours de cet intervalle prolonge. Les nouvelles donnees sur les plutons granitoides du nord-ouest du Maine correspondent aux modeles tectoniques d'autres parties du terrane de Gander, qui supposent une subduction initiale de plaques plates suivie par une rupture des plaques et une delamination. [Traduit par la redaction], INTRODUCTION Granitic magmas produced during collisional tectonic events are important repositories of information regarding (i) the duration of orogenesis, (ii) the role of mantle inputs, both physical and thermal, and [...]

Mantle heterogeneity is caused by recycling of crustal materials into the mantle, and mafic arc magmatism provides important imformation on the nature of crust-mantle interaction. Here we undertake an integrated study of whole rock major-trace elements and Sr-Nd-Hf isotopes, in-situ zircon U-Pb ages and Hf-O isotopes as well as mineral major-trace elements and clinopyroxene Sr isotopes for mafic intrusives in the Qilian orogen. The studied Jinyuan and Jishishan mafic igneous rocks show consistent zircon U-Pb ages of ca. 448-450 Ma. They all display arc-like trace element distribution patterns, relatively depleted Sr-Nd isotope compositions, and variably high zircon O isotopes. In comparison, the Jinyuan mafic igneous rocks have less depleted Sr-Nd-Hf and higher O isotope compositions than the Jishishan mafic rocks. These geochemicial features indicate they originated from a heterogeneous orogenic lithospheric mantle source, which resulted from the incorporation of variable crustal materials into the mantle. Model calculations confirm that the heterogeneous mantle source would be generated by reaction of the mantle wedge peridotite with different proportions of the subducting Proto-Tethyan oceanic crust-derived aqueous solutions and sediment-derived hydrous melts. Partial melting of the heterogeneous mantle source would generate different batches of mafic melts, which led to magma mixing and recharge processes prior to emplacement as evidenced by mineral compositional zonings. Therefore, in addition to heterogeneous crust-mantle interaction during oceanic subduction, magmatic processes such as magma recharge and mixing may also contribute to the petrogenesis of arc igneous rocks above the oceanic subduction zone., Author(s): Qi Chen [sup.1], Li-Qun Dai [sup.1] [sup.2] [sup.3], Zi-Fu Zhao [sup.1] [sup.2] [sup.3], Yong-Fei Zheng [sup.1] [sup.2], Yao Zhou [sup.1] Author Affiliations: (1) grid.59053.3a, 0000000121679639, CAS Key Laboratory of [...]

Genetic models for the formation of K-rich magmas in subduction-related settings range from partial melting of subduction-affected mantle sources to melting of crustal rocks depending on the local tectonic framework. The Miocene high-K calc-alkaline to shoshonitic rocks of Limnos Island reflect the magmatic activity in the northern Aegean, which migrated southwards in response to trench retreat and the collision of continental terranes in the Hellenic subduction system. New whole rock and mineral data of basaltic andesites, dacites and monzonites from Limnos indicate that the magmas underwent fractional crystallization of olivine, clinopyroxene, amphibole, apatite, and Fe-Ti oxides at 1100 to 700 °C and 0.5 to 0.1 GPa without significant assimilation of crustal rocks during the magma evolution. The strong enrichment of large ion lithophile elements and light rare-earth elements relative to depleted heavy rare earth and high-field strength elements points towards a mantle source that has been extensively hybridized by subducted sedimentary material. New Sr-Nd-Pb isotope data reveal a distinct isotopic composition of the Limnos rocks with high .sup.207Pb/.sup.204Pb at low .sup.206Pb/.sup.204Pb and .sup.143Nd/.sup.144Nd ratios that is likely related to the subduction of the continental crustal succession of the Apulian block which was subducted prior to the onset of magmatism on Limnos. Partial melting models assuming a hybridized mantle source suggest that the primary melts of Limnos formed by melting of a phlogopite pyroxenite at melting degrees of 5 to 10%. Compositional differences between high-K calc-alkaline and shoshonitic magmas are explained by variable melting degrees and varying amounts of sediment supply to the mantle. The magmatic and tectonic evolution of Limnos island is typical for the Oligocene and Miocene volcanic centres of the migrating western Aegean arc front., Author(s): Leon Gläser [sup.1], Anna Grosche [sup.1], Panagiotis C. Voudouris [sup.2], Karsten M. Haase [sup.1] Author Affiliations: (1) grid.5330.5, 0000 0001 2107 3311, GeoZentrum Nordbayern, Friedrich-Alexander-Universität (FAU) Erlangen-Nürnberg, , Schlossgarten [...]

Volcanoes with ocean island basalt (OIB)-like characteristics occur in all settings. Their origin is particularly enigmatic when they occur in close spatial and temporal association with subduction-related volcanism (e.g., back-arc basin basalts; BABB). A set of cognate enclaves and a late-intruding dike from Dokdo Island (Republic of Korea) were used to probe an intraplate mafic alkaline plumbing system developed in a back-arc basin. Detailed petrography, mineral chemistry, and whole-rock major- and trace-element geochemistry were combined with geothermobaro-hygrometric and whole-rock geochemical models. The cognate samples are defined as basalt-trachybasalt and gabbro, while a late-intruding dike has basaltic trachyandesite composition. In general, plagioclase crystals in the samples are classified as labradorite-bytownite, with cores richer in anorthite component compared to rims. Clinopyroxene crystals are sector-zoned diopside characterized by [2.sup.IVSi.sup.4+ + .sup.VIMg.sup.2+].sub.basal â [2.sup.IVAl.sup.3+ + .sup.VITi.sup.4+].sub.prism. The prism sector-based thermobarometric models suggest that the cognate enclaves may represent multi-stage magmatic conditions from near-source (> 1250 °C and 1.5 GPa) through underplating (~ 1220 °C at ~ 640 MPa) to intermediate (~ 1180 °C at ~ 400 MPa) stages. Shallow crustal stagnant conditions (~ 1100 °C at ~ 120 MPa) are estimated from the late-intruding dike. Clinopyroxene- and plagioclase-based hygrometric models give a constant H.sub.2O abundance of ~ 1.5-2 wt% during the Dokdo basaltic magma evolution and ascent. According to our trace-element models, deep near-source parental melt may result from ~ 35% partial melting of hornblendite + spinel-peridotite and the associated mixing with clinopyroxene mush-derived 0.5% partial melt by a ratio of 0.6:0.4. From the parent, crystallization of up to 50% with the fractionating proportions of 0.7 plagioclase + 0.3 clinopyroxene can produce the whole compositional range of the Dokdo mafic alkaline magma. Rare or absent olivine in the early formed enclave may suggest that clinopyroxene and olivine are, respectively, crystallized at high and low pressure due to depressed olivine liquidus via melt compositional effect. We suggest that, during back-arc basin opening, BABB-related melt percolation into the extensional cracks in the lithospheric mantle and accompanying metasomatism provide hornblenditic source for the subsequent OIB magmatism., Author(s): Jongkyu Park [sup.1] [sup.2], Hoseong Lim [sup.3], Bora Myeong [sup.4], Yun-Deuk Jang [sup.1] [sup.2], Marco Brenna [sup.5] Author Affiliations: (1) grid.258803.4, 0000 0001 0661 1556, Department of Geology, Kyungpook [...]

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 [...]

Our understanding of the nature of crustal formation in the Eoarchean is limited by the scarcity and poor preservation of the oldest rocks and variable and imperfect preservation of protolith magmatic signatures. These limitations hamper our ability to place quantitative constraints on thermomechanical models for early crustal genesis and hence on the operative geodynamic regimes at that time. The recently discovered ca. 3.75 Ga Ukaliq supracrustal enclave (northern Québec) is mainly composed of variably deformed and compositionally diverse serpentinized ultramafic rocks and amphibolitized mafic schists whose metamorphic peak, inferred from phase equilibria modeling, was below 720 °C. Inferred protoliths to the Ukaliq ultramafic rocks include cumulative dunites, pyroxenites, and gabbros, whereas the mafic rocks were probably picrites, basalts, and basaltic andesites. The bulk-rock and mineral chemistry documents the partial preservation of cumulative pyroxenes and probably amphiboles and demonstrates the occurrence of a clinopyroxene-dominated, tholeiitic suite and an orthopyroxene-dominated, boninite-like suite. Together with the presence of negative [mu].sup.142Nd anomalies in the boninitic basalts, two liquid lines of descent are inferred: (i) a damp tholeiitic sequence resulting from the fractionation of a basaltic liquid produced by mantle decompression; and (ii) a boninitic suite documenting the evolution of an initially primitive basaltic andesite liquid produced by flux melting. Petrographic observations, thermodynamic modeling, bulk-rock and mineral chemistry, and .sup.142Nd isotopic compositions identify the Ukaliq supracrustal belt as the remnant of an Eoarchean arc crust produced by the recycling of Hadean crust in a similar way as modern-style subduction., Author(s): Thomas Grocolas [sup.1] [sup.2], Pierre Bouilhol [sup.1], Guillaume Caro [sup.1], Stephen J. Mojzsis [sup.3] [sup.4] [sup.5] Author Affiliations: (1) grid.29172.3f, 0000 0001 2194 6418, Université de Lorraine, CNRS, CRPG, [...]

Our understanding of magmatism in the New Zealand North Island subduction system is limited by sparse knowledge of the structure and composition of the deep arc crust, particularly in the northern Taupo Volcanic Zone (NTVZ), where there are multiple major tectono-magmatic transitions impacting the crust. Herein, we use detailed textural and micro-chemical analyses from multiple crystalline phases and melt inclusions to probe the magmatic processes occurring in the lower crust that ultimately control the early evolution of magmas beneath Putauaki (Mt. Edgecumbe) in the NTVZ. Here, two-pyroxene-plagioclase glomerocrysts in dacite lavas have textures, compositions, and phase associations consistent with a lower crustal cognate cumulate origin. Importantly, the glomerocrysts record a multi-stage evolution in the lower crust. Elevated .sup.87Sr/.sup.86Sr isotope ratios (0.7058-0.7062) in high-An (up to An.sub.92) plagioclase, along with relatively low Mg/Fe ratios in olivine and pyroxene within the glomerocrysts, record the deep assimilation of isotopically enriched lower crust and the significant fractionation of Mg-rich phases, near the base of the crust. The evolved melts appear to have then ascended into the lower crust and subsequently crystallized the gabbroic mineral assemblage preserved in the xenoliths. Two pyroxene thermobarometry indicates that crystallization/equilibration occurred at temperature between 926 and 1001 °C and pressure between 4 and 7 kbars, consistent with crustal depths of 15-26 km. Melt inclusions hosted within pyroxenes in glomerocrysts are rhyodacitic to rhyolitic (68-74 wt% SiO.sub.2) indicating that fractionation of the glomerocryst assemblage resulted in the production of melts. We propose that these represent melts that ascend into the upper crust and fractionate to produce rhyolites that erupt from the large calderas of the central TVZ (CTVZ). Our findings also indicate that crustal architecture and magma pressure-temperature-time pathways beneath the NTVZ are similar to magmatic systems in the STVZ, but distinct from those beneath the large rhyolitic systems in the central TVZ. These differences may arise from variations in mantle flux and rates of extension along the arc., Author(s): Dale H. Burns [sup.1], Shanaka L. de Silva [sup.2], Phil Shane [sup.3], Matthew A. Coble [sup.1] [sup.4] Author Affiliations: (1) grid.168010.e, 0000000419368956, Department of Geological Sciences, Stanford University, , [...]

New data and interpretations on geological relationships of igneous rocks at Mokka Fiord, Axel Heiberg Island, Nunavut, provide insight into the timing and nature of magmatism associated with the Sverdrup Basin and High Arctic Large Igneous Province (HALIP). Field relationships indicate that the igneous rocks, previously interpreted to be volcanic flows, are most likely an intrusive unit discordant to regional bedding. An intrusive origin helps resolve chronostratigraphic inconsistencies in previous work. The host rocks are palynologically constrained to be late Barremian to late Aptian in age and are interpreted to be Paterson Island or Walker Island member of the Isachsen Formation. If the igneous body is intrusive, it's previously reported Ar--Ar age (102.5 [+ or -] 2.6 Ma) is no longer in conflict with accepted stratigraphic interpretations and probably reflects the emplacement age of the intrusion. Lingering uncertainties in interpreting the normal and reverse magnetic polarities determined in the previous work remain, and both are considered viable. Although this uncertainty precludes definitive conclusions on the significance of paleomagnetic data at Mokka Fiord, examination of the stratigraphic, paleomagnetic, and geochronologic relationships there highlight potential for the study of excursions, or reversed magnetic polarity subchrons, in the Cretaceous Normal Superchron elsewhere in the HALIP.De nouvelles donnees et interpretations sur les relations geologiques de roches ignees dans le fjord Mokka (ile Axel Heiberg, Nunavut) fournissent de nouveaux renseignements sur la chronologie et la nature du magmatisme associe au bassin de Sverdrup et a la grande province ignee de l'Extreme Arctique (HALIP). Les relations de terrain indiquent que les roches ignees, interpretees precedemment comme etant des coulees volcaniques, constituent tres probablement une unite intrusive discordante par rapport au litage regional. Une origine intrusive aide a resoudre des incoherences chronostratigraphiques dans des travaux anterieurs. Il est determine, du point de vue palynologique, que les roches hotes sont d'age barremien tardif a aptien tardif et sont interpretees comme etant le membre de l'ile Paterson ou de l'ile Walker de la Formation d'Isachsen. Si le corps igne s'avere intrusif, son age Ar--Ar (102,5 [+ or -] 2,6 Ma) deja publie n'entre plus en conflit avec les interpretations stratigraphiques acceptees et reflete probablement l'age de mise en place de l'intrusion. Des incertitudes demeurent concernant l'interpretation des polarites magnetiques normales et inverses determinees dans les etudes anterieures, et les deux sont jugees viables. Si ces incertitudes empechent de tirer des conclusions definitives sur la signification de donnees paleomagnetiques au fjord Mokka, l'examen des relations stratigraphiques, paleomagnetiques et geochronologiques font ressortir un potentiel pour l'etude d'excursions, ou de sous-chrons de polarite magnetique inversee, dans le superchron normal du Cretace ailleurs dans l'HALIP. [Traduit par la Redaction], IntroductionCretaceous siliciclastic and igneous rocks of the Sverdrup Basin near the mouth of Mokka Fiord, Axel Heiberg Island (Fig. 1), were examined to clarify conflicting and apparently irreconcilable published data [...]

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 [...]

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. [...]

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 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. [...]

Volcaniclastic sequences drilled during IODP Expedition 330 on top of Burton Guyot preserve a unique record of rejuvenated magmatic activity along the Louisville Seamount trail. Geochemical analysis of clinopyroxenes in primary volcaniclastic deposits of this rejuvenated phase allows the reconstruction of magmatic evolution from the shield to post-erosional phases of a Louisville seamount, and to compare this evolution to that of Hawaiian volcanoes. Our results reveal the occurrence of three main types of clinopyroxenes in the rejuvenated volcaniclastic deposits at Burton Guyot, with a Na (and Al)-poor phenocrystic clinopyroxene and two types of Na-rich clinopyroxenes from disaggregated ultramafic xenoliths. The rejuvenated Na-poor phenocrysts have the same compositional range as clinopyroxenes associated with the shield stage of the volcano, indicating an overlap in shield and rejuvenated magma compositions. The dominant type of Na-rich clinopyroxene (Type 1) is very similar to clinopyroxenes in Hawaiian pyroxenitic xenoliths thought to represent high pressure cumulates. Their relatively low Mg/(Mg + Fe), Cr, and Sc contents, similar trace element abundances and high Al(vi):Al(iv) to Hawaiian cumulates indicates that they too are cumulates. This contrasts with lower Al(vi):Al(iv) of the Na-poor phenocrysts that crystallized between 6-7 kbars and 1150-1200 °C. Type 2 clinopyroxenes are Mg-rich, and have major and trace element compositions very similar to clinopyroxenes in Hawaiian peridotites. These clinopyroxenes are interpreted as fragments of mantle xenoliths. They show intermediate amounts of incompatible element depletion, between more enriched Hawaiian peridotites and strongly depleted abyssal peridotites. Some grains exhibit the effects of mantle metasomatism, having spoon-shaped, chondrite-normalized REE patterns like those of Hawaiian peridotite xenoliths. The occurrence of disaggregated pyroxenitic cumulates and metasomatized mantle xenoliths in rejuvenated magmas of both Burton Guyot and Hawaiian islands suggests that the plumbing system of these volcanic systems share significant similarities. However, consistently with previous geochemical studies of the Louisville seamounts, geochemical consistency of shield and rejuvenated clinopyroxenes at Burton Guyot show that this volcano experienced similar alkaline magmatism from shield to rejuvenated stages. This is an important difference with the evolution of Hawaiian volcanoes that includes a dominantly tholeiitic shield stages and alkaline post-shield and rejuvenated stages, which suggests that the model of Hawaiian island formation may not be fully applicable to Louisville seamounts., Author(s): Michael J. Dorais [sup.1], David M. Buchs [sup.2] Author Affiliations: (1) 0000 0004 1936 9115, grid.253294.b, Department of Geological Sciences, Brigham Young University, , 84602, Provo, UT, USA (2) [...]

Volcanism during the Neogene in the Central Volcanic Zone (CVZ) of the Andes produced (1) stratovolcanoes, (2) rhyodacitic to rhyolitic ignimbrites which reach volumes of generally less than 300 km.sup.3 and (3) large-volume monotonous dacitic ignimbrites of up to several thousand cubic kilometres. We present models for the origin of these magma types using O and Sr isotopes to constrain crust/mantle proportions for the large-volume ignimbrites and explore the relationship to the evolution of the Andean crust. Oxygen isotope ratios were measured on phenocrysts in order to avoid the effects of secondary alteration. Our results show a complete overlap in the Sr-O isotope compositions of lavas from stratovolcanoes and low-volume rhyolitic ignimbrites as well as older (>9 Ma) large-volume dacitic ignimbrites. This suggests that the mass balance of crustal and mantle components are largely similar. By contrast, younger ( Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1152-5) contains supplementary material, which is available to authorized users., Author(s): Heye Freymuth[sup.1] [sup.2] , Melanie Brandmeier[sup.1] [sup.3] , Gerhard Worner[sup.1] Author Affiliations: (1) Geowissenschaftliches Zentrum, Universität Gottingen, Goldschmidtstr. 1, 37077, Gottingen, Germany (2) Bristol Isotope Group, School of Earth [...]

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 [...]

Magmatism on Earth is most abundantly expressed by surface volcanic activity, but all volcanism has roots deep in the crust, lithosphere, and mantle. Intraplate magmatism, in particular, has remained enigmatic as the plate tectonic paradigm cannot easily explain phenomena such as large flood basalt provinces and lithospheric rupture within continental interiors. Here, I explore the role of deep crustal magmatic processes and their connection to continental rift volcanism as recorded in deep crustal xenoliths from northern Tanzania. The xenoliths are interpreted as magmatic cumulates related to Cenozoic rift volcanism, based on their undeformed, cumulate textures and whole-rock compositions distinct from melt-reacted peridotites. The cumulates define linear trends in terms of whole-rock major elements and mineralogically, can be represented as mixtures of olivine + clinopyroxene. AlphaMELTS modeling of geologically plausible parental melts shows that the end-member cumulates, clinopyroxenite and Fe-rich dunite, require fractionation from two distinct melts: a strongly diopside-normative melt and a fractionated picritic melt, respectively. The former can be linked to the earliest, strongly silica-undersaturated rift lavas sourced from melting of metasomatized lithosphere, whereas the latter is linked to the increasing contribution from the upwelling asthenospheric plume beneath East Africa. Thus, deep crustal cumulate systematics reflect temporal and compositional trends in rift volcanism, and show that mixing, required by the geochemistry of many rift lava suites, is also mirrored in the lavas' cumulates., Author(s): Emily J. Chin [sup.1] Author Affiliations: (Aff1) 0000 0001 2107 4242, grid.266100.3, Scripps Institution of Oceanography-University of California San Diego, , La Jolla, United States Introduction How magmatism at [...]

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 [...]

Byline: C. D. Deering (1,4), D. M. Gravley (1), T. A. Vogel (2), J. W. Cole (1), G. S. Leonard (3) Keywords: Taupo Volcanic Zone; Subduction zone; Fluid; Magma distribution; Oxidation state Abstract: Slab-derived aqueous fluid components (Ba, Cl) correlate well with oxygen fugacity, and other well-defined characteristics of silicic magmas in the Taupo Volcanic Zone (TVZ) between a cold-wet-oxidizing magma type [R1: amphibole +- biotite high Sr, low Zr and FeO*/MgO, depleted middle rare-earth elements (MREE)] and a hot-dry-reducing magma type (R2: orthopyroxene +- clinopyroxene low Sr, high Zr, and FeO*/MgO, less depleted MREE). Oxygen fugacity was obtained from analysis of Fe--Ti oxides and ranges between -0.04 and +2.1 log units (IQFM, where QFM = quartz + fayalite + magnetite buffer) and is positively correlated with the bulk-rock Ba/La ratio, indicating that slab-derived fluid is the oxidizing agent in the rhyolites. Chlorine contents in hornblende also correlate with the bulk-rock Ba/La ratio. Hence, high-fluid flux typically correlates with the R1 and low-fluid flux with R2 rhyolite magma types. A geochemical evolution and distribution can be tracked in time and space throughout the central region of the TVZ from 550 ka to present and has revealed two distinct magmatic cycles that vary in length. The first cycle included widespread R1 type magmatism across the central TVZ beginning ca. 550 ka and was directly associated with previously unreported dome-building and ignimbrite-forming volcanism, and led to a voluminous (>3,000 km.sup.3) ignimbrite 'flare-up' between ca. 340 and 240 ka. The second cycle began roughly 180 ka, erupting ca. 800 km.sup.3 of magma, and continues to the present. The duration, rate, and composition of magma production within these cycles appears to be governed by the flux of fluid released from the subducting slab, while the distribution of magmas may be governed more by extension along the central rift axis. Shorter cycles have also been identified and are unrelated to subduction processes, but occur following large, caldera-forming events. Author Affiliation: (1) Department of Geological Sciences, University of Canterbury, PO Box 4800, Christchurch, 8020, New Zealand (4) Department of Earth and Space Sciences, University of Washington, Mailstop, Seattle, 98195-1310, USA (2) Department of Geological Sciences, Michigan State University, 206 Natural Sciences Bldg., East Lansing, MI, 48824-1115, USA (3) Institute of Geological and Nuclear Sciences, PO Box 30368, Lower Hutt, 6315, New Zealand Article History: Registration Date: 28/01/2010 Received Date: 24/08/2009 Accepted Date: 28/01/2010 Online Date: 25/02/2010 Article note: Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-010-0496-0) contains supplementary material, which is available to authorized users.

Byline: Galina P. Bulanova (1), Michael J. Walter (1), Chris B. Smith (1), Simon C. Kohn (1), Lora S. Armstrong (1), Jon Blundy (1), Luiz Gobbo (2) Keywords: Brazil; Collier 4 kimberlite; Sublithospheric diamonds; Inclusions; Carbon isotopes; Subduction; Carbonatite melt Abstract: We report on a suite of diamonds from the Cretaceous Collier 4 kimberlite pipe, Juina, Brazil, that are predominantly nitrogen-free type II crystals showing complex internal growth structures. Syngenetic mineral inclusions comprise calcium- and titanium-rich phases with perovskite stoichiometry, Ca-rich majoritic-garnet, clinopyroxene, olivine, TAPP phase, minerals with stoichiometries of CAS and K-hollandite phases, SiO.sub.2, FeO, native iron, low-Ni sulfides, and Ca--Mg-carbonate. We divide the diamonds into three groups on the basis of the carbon isotope compositions ([delt[a].sup.13]C) of diamond core zones. Group 1 diamonds have heavy, mantle-like [delt[a].sup.13]C (-5 to -10[per thousand]) with mineral inclusions indicating a transition zone origin from mafic protoliths. Group 2 diamonds have intermediate [delt[a].sup.13]C (-12 to -15[per thousand]), with inclusion compositions indicating crystallization from near-primary and differentiated carbonated melts derived from oceanic crust in the deep upper mantle or transition zone. A 206.sup.Pb/ [.sup.238]U age of 101 +- 7 Ma on a CaTiSi-perovskite inclusion (Group 2) is close to the kimberlite emplacement time (93.1 +- 1.5 Ma). Group 3 diamonds have extremely light [delt[a].sup.13]C (-25[per thousand]), and host inclusions have compositions akin to high-pressure--temperature phases expected to be stable in pelagic sediments subducted to transition zone depths. Collectively, the Collier 4 diamonds and their inclusions indicate multi-stage, polybaric growth histories in dynamically changing chemical environments. The young inclusion age, the ubiquitous chemical and isotopic characteristics indicative of subducted materials, and the regional tectonic history, suggest a model in which generation of sublithospheric diamonds and their inclusions, and the proto-kimberlite magmas, are related genetically, temporally and geographically to the interaction of subducted lithosphere and a Cretaceous plume. Author Affiliation: (1) Department of Earth Sciences, University of Bristol, Queen's Rd, Bristol, BS8 1RJ, UK (2) Rio Tinto Desenvolvimentos Minerais Ltda, Brasilia, Brazil Article History: Registration Date: 11/01/2010 Received Date: 04/06/2009 Accepted Date: 11/01/2010 Online Date: 07/02/2010 Article note: Communicated by M. W. Schmidt.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.earscirev.2010.06.004 Byline: Mikhail I. Kuzmin (a), Vladimir V. Yarmolyuk (b), Vadim A. Kravchinsky (c) Keywords: Icelandic hot spot; intraplate magmatism; isotopic composition; paleomagnetism; mantle plume; rare element; reconstructions; Siberia; Siberian traps Abstract: We review intraplate magmatism in Siberia and its folded surroundings from 480Ma to the present. We describe several large igneous provinces (LIPs) and the intervals in which they were continuously formed within the limits of the Siberian continent: the Altay-Sayan Early Paleozoic magmatic area (598-446Ma), the Altay-Sayan LIP (408-393Ma), the Viluy LIP (380-350Ma), the Barguzin-Vitim LIP (310-275Ma), the Late Paleozoic rift system of Central Asia (318-242Ma), the Siberian traps and West Siberian rift system (250-249Ma), the East-Mongolian and West-Trans-Baikalian LIP (228-195Ma), and a number of various aged Late Mesozoic and Cenozoic rift zones and magmatic areas (from 160Ma to the present day). Following Lawver and Muller (1994), Kharin (2000), Lawver et al. (2002) and Chernysheva et al. (2005), we accept the position of the Icelandic hot spot under the Siberian trap area at the Permo-Triassic boundary. That enables us to estimate the geographic coordinates of the Siberian trap location at 250Ma (they are the same as Iceland today). Presently the Icelandic hot spot is situated above the African large low shear velocity province (LLSVP) that indicates a hot mantle plume. We suggest a set of paleogeographic reconstructions of the Siberian continent for which we evaluate the paleolatitude based on the paleomagnetic data and estimate the paleolongitude position by placing Siberia above the African LLSVP. Furthermore, we estimate the geographic coordinates for other ancient hot spots in the framework of the African LLSVP that we consider to be responsible for the intraplate magmatism during different time periods in the Phanerozoic eon. Available rare element and isotopic characteristics of the intraplate magmatic rocks of Siberia enable us to determine three primary sources - moderately depleted mantle (PREMA), enriched mantle (EM-I and EM-II) - of the mantle origin magma. We propose that the model explains the interaction of the hot mantle plume including hot spots (plume tails) with the Siberian intraplate magmatism areas throughout the Phanerozoic eon. Author Affiliation: (a) Institute of Geochemistry, Siberian Branch of Russian Academy of Sciences, Irkutsk, 664033, Russia (b) Institute of Mineral Geology, Petrography, Mineralogy, and Geochemistry of the Russian Academy of Sciences, Moscow, 109017, Russia (c) Physics Department, University of Alberta, Edmonton, Alberta, Canada T6G 2G7 Article History: Received 20 April 2009; Accepted 16 June 2010

Byline: Nigel Harris (1), Alison Hunt (1), Ian Parkinson (1), Andrew Tindle (1), Magisuren Yondon (2), Samantha Hammond (1) Keywords: Intra-plate volcanism; Xenoliths; Thermobarometry; Mongolian tectonics Abstract: Garnet-bearing mantle xenoliths have been recovered from Quaternary alkali basalts, both within and peripheral to the Hangay dome of central Mongolia. Microfabric analysis and thermobaromery, combining empirical thermobarometers and the self-consistent dataset of THERMOCALC, indicate that garnet websterites from the Shavaryn-Tsaram volcanic centre at the dome core were formed in the spinel-lherzolite upper mantle at pressures of 17--18 kbars and temperatures of 1,070--1,090degC, whereas garnet lherzolites were derived from greater depths (18--20 kbars). Garnet lherzolites from the Baga Togo Uul vents near the dome edge were formed at 18--22 kbars under significantly cooler conditions (960--1,000degC). These xenoliths reveal reaction coronas of (1) orthopyroxene, clinopyroxene, plagioclase and spinel mantling garnets (2) spongy rims of olivine replacing orthopyroxene and (3) low-Na, low-Al clinopyroxene replacing primary clinopyroxene. Trace-element abundances indicate that clinopyroxene from these coronas is in chemical equilibrium with the host magma. The thermobarometric and textural data suggest that lherzolite xenoliths from both sites were derived from depths of 60--70 km and entrained in magma at 1,200--1,300degC. The average rate of ascent, as determined by olivine zoning, lies in the range 0.2--0.3 m s.sup.-1. The contrast in thermal profiles of the upper mantle between the two sites is consistent with a mantle plume beneath the Hangay dome with elevated thermal conditions beneath the core of the dome being comparable to estimates of the Pleistocene geotherm beneath the Baikal rift. Author Affiliation: (1) Department of Earth and Environmental Sciences, The Open University, Milton Keynes, MK7 6AA, UK (2) School of Geology, Mongolian University of Science, and Technology (MUST), P.O. Box 49/418, Ulaanbaatar, Mongolia Article History: Registration Date: 13/11/2009 Received Date: 25/07/2009 Accepted Date: 13/11/2009 Online Date: 01/12/2009 Article note: Communicated by C. Ballhaus.

Byline: Sebastien Pilet (1), Peter Ulmer (2), Samuel Villiger (2) Keywords: Experimental petrology; Basanite; Metasomatism; High-pressure crystallization; Alkaline lavas Abstract: The metasomatism observed in the oceanic and continental lithosphere is generally interpreted to represent a continuous differentiation process forming anhydrous and hydrous veins plus a cryptic enrichment in the surrounding peridotite. In order to constrain the mechanisms of vein formation and potentially clarify the nature and origin of the initial metasomatic agent, we performed a series of high-pressure experiments simulating the liquid line of descent of a basanitic magma differentiating within continental or mature oceanic lithosphere. This series of experiments has been conducted in an end-loaded piston cylinder apparatus starting from an initial hydrous ne-normative basanite at 1.5 GPa and temperature varying between 1,250 and 980degC. Near-pure fractional crystallization process was achieved in a stepwise manner in 30degC temperature steps and starting compositions corresponding to the liquid composition of the previous, higher-temperature glass composition. Liquids evolve progressively from basanite to peralkaline, aluminum-rich compositions without significant SiO.sub.2 variation. The resulting cumulates are characterized by an anhydrous clinopyroxene + olivine assemblage at high temperature (1,250--1,160degC), while at lower temperature (1,130--980degC), hydrous cumulates with dominantly amphibole + minor clinopyroxene, spinel, ilmenite, titanomagnetite and apatite (1,130--980degC) are formed. This new data set supports the interpretation that anhydrous and hydrous metasomatic veins could be produced during continuous differentiation processes of primary, hydrous alkaline magmas at high pressure. However, the comparison between the cumulates generated by the fractional crystallization from an initial ne-normative liquid or from hy-normative initial compositions (hawaiite or picrobasalt) indicates that for all hydrous liquids, the different phases formed upon differentiation are mostly similar even though the proportions of hydrous versus anhydrous minerals could vary significantly. This suggests that the formation of amphibole-bearing metasomatic veins observed in the lithospheric mantle could be linked to the differentiation of initial liquids ranging from ne-normative to hy-normative in composition. The present study does not resolve the question whether the metasomatism observed in lithospheric mantle is a precursor or a consequence of alkaline magmatism however, it confirms that the percolation and differentiation of a liquid produced by a low degree of partial melting of a source similar or slightly more enriched than depleted MORB mantle could generate hydrous metasomatic veins interpreted as a potential source for alkaline magmatism by various authors. Author Affiliation: (1) Institute of Mineralogy and Geochemistry, University of Lausanne, Anthropole, 1015, Lausanne, Switzerland (2) Department of Earth Sciences, ETH-Zurich, Clausiusstrasse 25, 8092, Zurich, Switzerland Article History: Registration Date: 01/09/2009 Received Date: 23/01/2009 Accepted Date: 31/08/2009 Online Date: 21/09/2009 Article note: Communicated by J. Blundy.

Byline: Fleurice Parat (1), Francois Holtz (2), Milos Rene (3), Renat Almeev (2) Keywords: Experimental study; Ultrapotassic magma; Durbachite; TAebic; Bohemian Massif Abstract: The equilibrium phase relations of a mafic durbachite (53 wt.% SiO.sub.2) from the TAebic pluton, representative of the Variscan ultrapotassic magmatism of the Bohemian Massif (338--335 Ma), have been determined as a function of temperature (900--1,100degC), pressure (100--200 MPa), and [H.sub.2]O activity (1.1--6.1 wt.% [H.sub.2]O in the melt). Two oxygen fugacity ranges were investigated: close to the Ni--NiO (NNO) buffer and 2.6 log unit above NNO buffer (aNNO + 2.6). At 1,100degC, olivine is the liquidus phase and co-crystallized with phlogopite and augite at 1,000degC for the whole range of investigated pressure and water content in the melt. At 900degC, the mineral assemblage consists of augite and phlogopite, whereas olivine is not stable. The stability field of both alkali feldspar and plagioclase is restricted to low pressure (100 MPa) at nearly water-saturated conditions ( Author Affiliation: (1) Institut fur Geowissenschaften, Mineralogie-Geochemie, Universitat Freiburg, Albertstrasse 23b, 79104, Freiburg, Germany (2) Institut fur Mineralogie, Leibniz Universitat Hannover, Callinstrasse 3, 30167, Hannover, Germany (3) Institute of Rock Structure and Mechanics, ASCR V, Holesovickach 41, 182 09, Prague, Czech Republic Article History: Registration Date: 13/07/2009 Received Date: 20/11/2008 Accepted Date: 13/07/2009 Online Date: 04/08/2009 Article note: Communicated by J. Hoefs.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.06.032 Byline: J. Vaucher (a), D. Baratoux (a), N. Mangold (b), P. Pinet (a), K. Kurita (c), M. Gregoire (a) Keywords: Mars; Volcanism Abstract: Central Elysium Planitia (CEP) is located south of Elysium Mons. Back to the era of the Viking orbiters, clues accumulated in favor of recent volcanism in relation with ground water release and the formation of long sub-parallel fissures. Four aqueous flood channel systems emanate from linear fissures. Recent eruptions of low viscosity lavas originate from these fissures and from low shield volcanoes. The objective of this paper is to constrain the volcanic history of this region, and to determine the chronological relationships with fluvial/erosional processes. New observations (e.g., new shield volcanoes and one new fluvial event) are summarized on a context map. Thirty-five surfaces have been dated from the count of about 15,000 impact craters. Ages have been cross-checked with relative stratigraphy when possible. A probabilistic approach has been introduced to compare similar ages and define periods of volcanic activity. Our results confirm that some volcanic features are extremely recent ([approximately equal to]2My). Active periods are found at 2.5-3My, 4.3My, 13.5-16.2My, 19My, 21-32My, 58My, 71My, 85-95My, 134My, 173My and 234My, not excluding the possibility that some of the gaps would be filled with additional crater counts. The volcanic activity thus extended for at least the last 250My. The lava volumes have been estimated from the topographic modeling of the floor of depressions filled up by volcanic products, including the volumes of several large crater cavities buried under lavas (>20% of the total volume). Our new estimation of the total lava volume is 1.5[+ or -]0.2x10.sup.5 km.sup.3. This value corresponds to an average thickness of one hundred meters of lavas for the young volcanic plain. As a consequence, the total eruption rate at CEP, defined as the total volume of lava divided by the time of emplacement 1.4x10.sup.-2-1.8x10.sup.-2 m.sup.3/s is lower than values typically estimated for terrestrial hot spots or large igneous provinces, suggesting longer inactive periods. The concept of mantle plumes responsible for terrestrial flood volcanism may not be applicable to the case of CEP and the mechanism proposed in offers a plausible alternative to explain our observations. Author Affiliation: (a) Laboratoire Dynamique Terrestre et Planetaire, UMR 5562, Observatoire Midi-Pyrenees, Universite de Toulouse and CNRS, Toulouse, France (b) Universite de Nantes, UFR des Sciences et des Techniques, Laboratoire de Planetologie et Geodynamique, 2 rue de la Houssiniere, B.P. 92208, 44322 Nantes Cedex 3, France (c) Earthquake Research Institute, Zip. 113-0032, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, Japan Article History: Received 12 June 2007; Revised 17 May 2009; Accepted 1 June 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.04.035 Byline: A.T. Basilevsky (a)(c), M. Aittola (b), J. Raitala (b), J.W. Head (c) Keywords: Venus; Venus; Surface; Tectonics; Volcanism Abstract: In order to assess the age relations between astra/novae (features with extensive radial fracture-graben systems) and their surroundings, and to determine the duration of their activity, we undertook a photogeologic analysis of Magellan images of 78 astra, 49 dark-parabola craters and 114 clear-halo craters. For seven of these 78 astra it was found that the astrum-forming faulting started before or close to the time of emplacement of regional plains and extended into the second part of post-regional-plains time. Because the mean age of the regional plains is close to the mean surface age of Venus (which is estimated to be [approximately equal to]750m.y), this means that the duration of activity of these seven astra was several hundred millions of years. This is longer than the duration of activity of ongoing mantle plumes on Earth, but shorter than the duration of activity of the plume feeding the martian volcano Olympus Mons. The basic morphologic characteristics of these seven astra, as well as their age relations with other geologic units, are similar to those of the majority of other astra; therefore, such a long duration could also be typical of some other astra. We confirm the two-phase (pre- and post-regional-plains) evolution of astrum-forming faulting suggested in previous studies. For the first phase we show evidence for purely tectonic faulting caused by the diapiric rise of a mantle plume. For the second phase we find evidence supporting the interpretation of other studies that the observed faults resulted from subsurface dike intrusions produced by magmatism associated with the plume. We also found that faulting during the second phase was not instantaneous but distributed over a long period of time. Author Affiliation: (a) Vernadsky Institute of Geochemistry and Analytical Chemistry, RAS, Moscow, Russia (b) Department of Astronomy, Oulu University, Finland (c) Department of Geological Sciences, Brown University, Providence, P.O. Box 1846, RI 02912, USA Article History: Received 28 April 2008; Revised 14 April 2009; Accepted 17 April 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.earscirev.2009.06.005 Byline: Giacomo Corti Keywords: continental rifting; rift evolution; continental break-up; magmatism; deformation; East African Rift; Main Ethiopian Rift; Nubia-Somalia kinematics Abstract: The Main Ethiopian Rift is a key sector of the East African Rift System that connects the Afar depression, at Red Sea-Gulf of Aden junction, with the Turkana depression and Kenya Rift to the South. It is a magmatic rift that records all the different stages of rift evolution from rift initiation to break-up and incipient oceanic spreading: it is thus an ideal place to analyse the evolution of continental extension, the rupture of lithospheric plates and the dynamics by which distributed continental deformation is progressively focused at oceanic spreading centres. The first tectono-magmatic event related to the Tertiary rifting was the eruption of voluminous flood basalts that apparently occurred in a rather short time interval at around 30Ma; strong plateau uplift, which resulted in the development of the Ethiopian and Somalian plateaus now surrounding the rift valley, has been suggested to have initiated contemporaneously or shortly after the extensive flood-basalt volcanism, although its exact timing remains controversial. Voluminous volcanism and uplift started prior to the main rifting phases, suggesting a mantle plume influence on the Tertiary deformation in East Africa. Different plume hypothesis have been suggested, with recent models indicating the existence of deep superplume originating at the core-mantle boundary beneath southern Africa, rising in a north-northeastward direction toward eastern Africa, and feeding multiple plume stems in the upper mantle. However, the existence of this whole-mantle feature and its possible connection with Tertiary rifting are highly debated. The main rifting phases started diachronously along the MER in the Mio-Pliocene; rift propagation was not a smooth process but rather a process with punctuated episodes of extension and relative quiescence. Rift location was most probably controlled by the reactivation of a lithospheric-scale pre-Cambrian weakness; the orientation of this weakness (roughly NE-SW) and the Late Pliocene (post 3.2Ma)-recent extensional stress field generated by relative motion between Nubia and Somalia plates (roughly ESE-WNW) suggest that oblique rifting conditions have controlled rift evolution. However, it is still unclear if these kinematical boundary conditions have remained steady since the initial stages of rifting or the kinematics has changed during the Late Pliocene or at the Pliocene-Pleistocene boundary. Analysis of geological-geophysical data suggests that continental rifting in the MER evolved in two different phases. An early (Mio-Pliocene) continental rifting stage was characterised by displacement along large boundary faults, subsidence of rift depression with local development of deep (up to 5km) asymmetric basins and diffuse magmatic activity. In this initial phase, magmatism encompassed the whole rift, with volcanic activity affecting the rift depression, the major boundary faults and limited portions of the rift shoulders (off-axis volcanism). Progressive extension led to the second (Pleistocene) rifting stage, characterised by a riftward narrowing of the volcano-tectonic activity. In this phase, the main boundary faults were deactivated and extensional deformation was accommodated by dense swarms of faults (Wonji segments) in the thinned rift depression. The progressive thinning of the continental lithosphere under constant, prolonged oblique rifting conditions controlled this migration of deformation, possibly in tandem with the weakening related to magmatic processes and/or a change in rift kinematics. Owing to the oblique rifting conditions, the fault swarms obliquely cut the rift floor and were characterised by a typical right-stepping arrangement. Ascending magmas were focused by the Wonji segments, with eruption of magmas at surface preferentially occurring along the oblique faults. As soon as the volcano-tectonic activity was localised within Wonji segments, a strong feedback between deformation and magmatism developed: the thinned lithosphere was strongly modified by the extensive magma intrusion and extension was facilitated and accommodated by a combination of magmatic intrusion, dyking and faulting. In these conditions, focused melt intrusion allows the rupture of the thick continental lithosphere and the magmatic segments act as incipient slow-spreading mid-ocean spreading centres sandwiched by continental lithosphere. Overall the above-described evolution of the MER (at least in its northernmost sector) documents a transition from fault-dominated rift morphology in the early stages of extension toward magma-assisted rifting during the final stages of continental break-up. A strong increase in coupling between deformation and magmatism with extension is documented, with magma intrusion and dyking playing a larger role than faulting in strain accommodation as rifting progresses to seafloor spreading. Author Affiliation: Consiglio Nazionale delle Ricerche, Istituto di Geoscienze e Georisorse, via G. La Pira, 4, 50121, Firenze, Italy Article History: Received 15 January 2009; Accepted 23 June 2009