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Tectonic models for development of the Svalbard Caledonides depend on reliable assessment of the metamorphic evolution of the various basement provinces involved. The Mosselhalvoya Group (MG) and the Atomfjella Complex (AC) have previously been assigned to the Nordaustlandet and West Ny-Friesland terranes, respectively. New analytical data and petrographic observations indicate that both units experienced two-stage metamorphism under similar pressure-temperature (P-T) conditions. Two stages of amphibolite facies metamorphism (Ml and M2) are clearly recorded by garnet and staurolite porphyroblast textures. The results of thermodynamic phase equilibrium modeling indicate that peak M2 metamorphism occurred at ~ 7-7.5 kbar and 590-600 [degrees]C in both units. Zirconium-in-rutile trace element thermometry confirms the temperature estimates for Ml and M2 stages of metamorphism. Monazite chemical Th-U-Pb dates from the MG resolve a two-stage garnet growth at 444 [+ or -] 7 Ma (Ml) and 423 [+ or -] 6 Ma (M2). In contrast, monazite dated in the AC defines a single age of 420 [+ or -] 4 Ma interpreted as M2 growth. We suggest M2 was coeval with early strike-slip motion along the Billefjorden Fault Zone, whereas Ml reflects initial tectonic burial of the studied units. The similarity in metamorphic history between the both units suggests that the boundary between them is a subordinate thrust fault within the Atomfjella thrust stack rather than a major boundary separating the Nordaustlandet and West Ny-Friesland terranes. The MG should be included within the West Ny-Friesland terrane and the tectonic boundary with the Nordaustlandet terrane is likely the Eolussletta Shear Zone. Key words: Mosselhalvoya Group, Atomfjella Complex, West Ny-Friesland terrane, Nordaustlandet terrane, thermodynamic phase equilibrium modelling, monazite, 1. Introduction The Svalbard Caledonides are traditionally divided into three main basement blocks, terranes, or provinces with differing structural history, sedimentary record, and tectono-thermal evolution (Harland 1997). The Northeastern, Northwestern, [...]

In intraplate areas where regional tectonic strain is accommodated by the reactivation of pre-existing structures, the level of seismic hazard associated with faults maybe underestimated due to the poor surface expression of faults, scattered earthquake distribution, and long earthquake recurrence intervals. The cause(s) of seismicity in eastern Canada remains unresolved. This is partially because surface expressions of faults have been eroded during glacial and deglacial periods and in the Ottawa-Bonnechere Graben (OBG) are undetectable until a seismic event. Morphotectonic analysis has been widely applied to assess relative tectonic activity in various geological settings. To establish whether active uplift is occurring and to investigate the spatial distribution of relative uplift rates, 131 bedrock drainage basins in OBG were analysed. The aim of this was to (1) test the applicability of geomorphic indices for quantifying active deformation, (2) quantify the spatiotemporal distribution of relative uplift rates, and (3) explore the implications for faulting mechanisms, deformation styles, and ultimately regional seismic hazard. We measured valley floor width-to-height ratio (Vf), basin elongation ratio (Re), basin hypsometric integral, and normalized channel steepness index ([k.sub.sn]). The results show that high relative uplift rates exist in all the six bedrock escarpment sections investigated, suggesting that they are presumably related with regional broad wavelength uplift (epeirogeny) caused by a complex interaction between far-field tectonic stress and glacial isostatic adjustment (GIA). Our analysis showed that [V.sub.f], Re, and [k.sub.sn] reveals no considerable spatial diferences in high relative uplift rates, consistent with the Canadian Base Network GPS uplift rates. Highlights * Geomorphic indices are applied in a low relief and low-medium seismicity area. * Geomorphic indices reveal a broad wavelength regional uplift and show relative tectonic activity in the OBG. * Relatively high geomorphic indices values provide longer term deformation history compared to global positioning system (GPS) records. * Landscape formation and river incision is driven by a complex interaction between GIA and far-field tectonic stresses. * Relative uplift rate results show similar patterns to published GPS uplift rates. * Possible link between relative tectonic activity and current seismicity. Keywords: Ottawa-Bonnechere Graben, western Quebec seismic zone, morphotectonic analysis, intraplate seismicity, regional uplift, Canada, 1. Introduction Eastern Canada is characterized as a stable continental region (SCR) which has a relatively lower seismicity rate and longer earthquake recurrence intervals compared to active plate tectonic zones/boundaries [...]

Whole-rock geochemical and U-Pb-Hf isotopic data were used to investigate the age, petrogenesis, and tectonic setting of a suite of gabbro-diorite-monzonite-syenite rock exposed in Qinsai area, Hainan Island. Zircon U-Pb dating shows that the gabbro and monzonite, respectively, crystallized at 179.5 [+ or -] 2.7 and 179.0 [+ or -] 2.3 Ma. The suite consists of shoshonitic alkaline intrusive rocks with [SiO.sub.2] content ranging from 41.7 to 63.4 wt%, with high U, K, and Pb and low Nb, Ta, and Ti. The suite is enriched in light rare earth elements relative to heavy rare earth elements. The gabbro has no obvious Eu anomaly but a positive Eu anomaly is evident in intermediate rocks. Zircon [[epsilon].sub.Hf](t) values are positive and have a wide variation (gabbro [[epsilon].sub.Hf](t) = +5.2 [+ or -] 8.2, monzonite [[epsilon].sub.Hf](t) = +4.7 [+ or -] 7.1). Gabbro and syenite are both likely derived from an enriched lithospheric mantle that was metasomatized by subduction fluids. Syenite evolved from the differentiation of gabbro, monzonite, and diorite. Collectively these intrusive rocks formed in an extensional regime within an arc formed by subduction of the ancient Pacific plate beneath the Asian margin, which was accompanied by extensional thinning of the lithosphere. Key words: Hainan, Middle Jurassic, intrusive rocks, U-Pb age, geochemistry, petrogenesis, Introduction The South China plate is located to the south of the Chinese mainland. Since the Late Paleozoic, the southeastern margin of the South China plate has been affected by [...]

Fifty-five new Nd isotope analyses are presented for plutonic orthogneisses from the Grimsthorpe domain in the marble-rich segment of the Grenvillian Central Metasedimentary Belt (CMB) to test the back-arc aulacogen model for its origin. Nd isotope analyses from the Weslemkoon batholith, Elzevir batholith, Lingham Lake complex, and Canniff tonalite are used to probe the crustal formation age of their source rocks. Despite its concentric foliation, the Weslemkoon batholith displays a complex geochemical pattern consisting of several northeast-trending domains with older [T.sub.DM] ages, surrounded by juvenile crustal material. The new Nd isotope results, coupled with geochemistry for the Weslemkoon and Elzevir batholiths, depict the fragmentation of a block of old crust that formed a screen between en echelon segments of a mid-Mesoproterozoic back-arc rift zone. The isotope boundaries identified within the Weslemkoon batholith delineate magma pulses sampling two distinct sources, interpreted as Laurentian basement and juvenile basaltic underplate. Under-plating could be attributed to slab rollback under the pre-Grenvillian continental margin arc. The intensification of rift-related magmatism in the CMB is demonstrated by its bimodal petrological character. A modern analogue for the tectonic context of the CMB is the Gulf of California, where subduction-related magmatism has transitioned to rift-related magmatism. However, the Gulf of California exhibits more transcurrent motion than is evidenced by the geometry of the CMB rift. A geometrical analogue for the break-up of the Elzevir block between two rift segments is provided by the Danakil block of the Red Sea, which is currently undergoing similar tectonic fragmentation. Key words: Weslemkoon batholith, marble-rich domain, back-arc rift, geochemistry. Cinquante-cinq nouvelles analyses d'isotopes du Nd sont présentées pour des orthogneiss plutoniques du domaine de Grimsthorpe dans le segment riche en marbre de la ceinture métasédimentaire centrale (CMC) grenvillienne, afin de valider le modèle d'aulacogène d'arrière-arc proposé pour expliquer son origine. Des analyses d'isotopes du Nd des batholites de Weslemkoon et d'Elzevir, du complexe du lac Lingham et de la tonalite de Canniff sont utilisées pour examiner l'âge de formation crustale de leurs roches mères. Malgré sa foliation concentrique, le batholite de Weslemkoon présente un motif géochimique complexe qui comprend plusieurs domaines d'orientation nord-est caractérisés par des âges [T.sub.DM] plus vieux, entourés de matériau crustal juvénile. Ces nouvelles données sur les isotopes du Nd, jumelées à la géochimie des batholites de Weslemkoon et d'Elzevir, témoignent de la fragmentation d'un bloc de croûte ancienne qui a formé un écran entre des tronçons en échelon d'une zone de rift d'arrière-arc d'âge mésoprotérozoïque moyen. Les limites isotopiques cernées dans le batholite de Weslemkoon délimitent des apports pulsés de magmas qui échantillonnent deux sources distinctes interprétées comme étant le socle laurentien et un sous-placage basaltique juvénile. Ce sous-placage pourrait être attribuable au recul d'une plaque sous l'arc de marge continentale pré-grenvillien. L'intensification du magmatisme relié au rift dans la CMC est démontrée par son caractère pétrologique bimodal. Le golfe de Californie, où un magmatisme relié à la subduction est passé à un magmatisme relié au rifting, constitue un analogue moderne du milieu tectonique de la CMC. Le golfe de Californie présente toutefois un plus grand déplacement en coulissage que celui dont témoigne la géométrie du rift de la CMC. Le bloc de Danakil de la mer Rouge, qui fait actuellement l'objet d'une fragmentation tectonique semblable, constitue un analogue géométrique de la dislocation du bloc d'Elzevir entre deux tronçons de rift. [Traduit par la Rédaction] Mots-clés : batholite de Weslemkoon, domaine riche en marbre, rift d'arrière-arc, géochimie., Introduction The Grenville Province of the Canadian Shield is an exhumed Mesoproterozoic orogenic belt, largely consisting of high-grade gneiss terranes of various ages that were tectonized during the late-Mesoproterozoic Grenville [...]

Theageand natureof the Neoproterozoic--early Paleozoic rift-drift transition has been interpreted differently along the length of the North American Cordillera. The Ediacaran 'upper' group (herein elevated to the Rackla Group) of the Coal Creek inlier, Yukon, Canada, represents a key succession to reconstruct the sedimentation history of northwestern Laurentia across the Precambrian-Cambrian boundary and elucidate the timing of active tectonism during the protracted breakup of the supercontinent Rodinia. These previously undifferentiated late Neoproterozoic--early Paleozoic map units in the Coal Creek inlier are herein formally defined as the Lone, Cliff Creek, Mount Ina, Last Chance, Shade, and Shell Creek formations. New sedimentological and stratigraphic data from these units is used to reconstruct the depositional setting. In the Last Chance Formation, chemostratigraphic observations indicate a ca. 5[per thousand] [[delta].sup.13][C.sub.carb] gradient coincident with the globally recognized ca. 574-567 Ma Shuram carbon isotope excursion. Map and stratigraphic relationships in the overlying Shell Creek Formation provide evidence for latest Ediacaran--middle Cambrian tilting and rift-related sedimentation. This provides evidence for active extension through the Cambrian Miaolingian Series in northwestern Canada, supporting arguments for a multiphase and protracted breakup of Rodinia. Key words: Rodinia, Yukon, Shuram carbon isotope excursion, Rackla Group, Coal Creek inlier. Les interpretations different quant a l'age et la nature de la transition de rifting a derive, du Neoproterozoique au Paleozoique precoce, le long dela cordillere nord-americaine. Le groupe d'Ediacara [much less than] superieur [much greater than] (eleve dans le present article au Groupe de Rackla) de l'enclave de Coal Creek (Yukon, Canada) represente une succession cle pour reconstituer l'histoire sedimentaire du nord-ouest de la Laurentie de part et d'autredela limite Precambrien-Cambrien et elucider la chronologie de l'activite tectonique durant la longue dislocation du supercontinent Rodinia. Ces unites cartographiques d'age neoproterozoique tardif a paleozoique precoce dans l'enclave de Coal Creek, qui n'etaient pas differenciees aupara-vant, sont formellement definies comme etant les Formations de Lone, de Cliff Creek, de Mount Ina, de Last Chance, de Shade et de Shell Creek. De nouvelles donnees sedimentologiques et stratigraphiques sur ces unites sont utilisees pour reconstituer le contexte sedimentaire. Dans la Formation de Last Chance, des observations chimiostratigraphiques indi-quent un gradient du [[delta].sup.13][C.sub.carb] d'environ 5 [per thousand] coincidant avec l'excursion des isotopes de carbone de Shuram vers 574-567 Ma, reconnue a l'echelle planetaire. Les relations cartographiques et stratigraphiques dans la Formation de Shell Creek sus-jacente fournissent des preuves d'un basculement et d'une sedimentation reliee au rifting de la fin de l'Ediacara au Cambrien moyen. Cela indiquerait une extension active pendant la serie miaolingienne du Cambrien dans le nord-ouest du Canada, ce qui argue en faveur d'une dislocation de Rodinia en plusieurs phases et de longue duree. [Traduit par la Redaction] Mots-cles: Rodinia, Yukon, excursion des isotopes de carbone de Shuram, Groupe de Rackla, enclave de Coal Creek., Introduction Neoproterozoic and lower Paleozoic strata of the western United States and Canada generally record a transition from continental rifting to passive margin sedimentation during the breakup of the supercontinent [...]

The oldest rocks in the Avalonian Antigonish Highlands of northern mainland Nova Scotia, Canada, are late Neoproterozoic (>618 Ma) volcanic and sedimentary rocks of the Georgeville Group intruded by gabbroic/dioritic to granitic plutons. New U-Pb zircon ages presented here for five samples from plutons intruded into the James River and (or) Keppoch formations of the Georgeville Group have ages ranging from ca. 615 to 604 Ma. They have petrological characteristics of expanded calc-alkalic 'Andean-type' suites but are compositionally biased toward evolved high-silica compositions and are interpreted as evolved I-type granites. They were emplaced at shallow depths and some were likely comagmatic with felsic volcanic components of their host rocks. These plutons are younger and show less varied Sm-Nd isotopic compositions than most plutonic rocks formed in the early Ediacaran 'main arc phase' elsewhere in Avalonian terranes in the northern Appalachian orogen, although they are similar in age to plutons in southeastern New England and in the Bass River and Jeffers blocks of the Cobequid Highlands, Nova Scotia. The Jeffers block of the Cobequid Highlands appears to be most similar to the Antigonish Highlands but both areas record a Neoproterozoic history less protracted than in other parts of Avalonia. Key words: Avalonia, Appalachian orogen, Ediacaran, granitoid, U-Pb zircon age, chemistry. Les roches les plus anciennes des hautes terres avaloniennes d'Antigonish du nord de la Nouvelle-Ecosse continentale (Canada) sont des roches volcaniques et sedimentaires d'age neoproterozoique tardif (>618 Ma) du Groupe de Georgeville recoupees par des plutons gabbro'iques/dioritiques a granitiques. De nouveaux ages U-Pb sur zircons pour cinq echantillons provenant de plutons qui recoupent les Formations de James River et (ou) de Keppoch du Groupe de Georgeville vont d'environ 615 Ma a 604 Ma. Ces roches presentent des caracteristiques petrologiques de suites calcoalcalines de type andin elargies, mais leurs compositions sont biaisees vers des compositions evoluees riches en silice, et elles sont interpretees comme etant des granites de type I evolues. Ces plutons ont ete mis en place a faible profondeur et certains d'entre eux etaient probablement comagmatiques de composants volcaniques felsiques de leurs encaissantes. Les plutons sont plus jeunes et presentent des concentrations d'isotopes de Sm-Nd moins variables que la plupart des roches plutoniques formees durant la[much less than] phase d'arc principale [much greater than] d'age ediacarien precoce ailleurs dans des terranes avaloniens de l'orogene du nord des Appalaches, bien qu'ils soient d'age semblable a ceux de plutons dans le sud-est de la Nouvelle-Angleterre et dans les blocs de Bass River et de Jeffers des hautes terres de Cobequid (Nouvelle-Ecosse). Le bloc de Jeffers semble presenter la plus grande similitude avec les hautes terres d'Antigonish, mais les deux regions temoignent d'une evolution neoproterozoique moins longue que dans d'autres parties de l'Avalonie. [Traduit par la Redaction] Mots-cles: Avalonie, orogene des Appalaches, Ediacarien, granito'ide, age U-Pb sur zircons, chimie., Introduction Since the pioneering work of Williams (1978, 1979), the Antigonish Highlands of northern mainland Nova Scotia have been included in the Avalon zone in the northern Appalachian orogen, now [...]

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 [...]

Voluminous moderate- to high-magnesian [Mg# = molar Mg/(Mg + Fe.sup.2+) = 44-64] andesitic and dacitic rocks with high silica (mostly 61-66 wt%) adakitic affinity (Y = 13-22, Yb = 1.3-2.1, Sr/Y = 18-44, La/Yb = 10-25) and common mafic magmatic enclaves (MMEs) are first reported in the Keçiboyduran stratovolcano (KSV) from the Cappadocia volcanic province (CVP), Central Anatolia, Turkey. We present comprehensive whole-rock geochemistry and Sr-Nd-Pb isotope data, mineral chemical compositions and .sup.40Ar-.sup.39Ar ages for KSV samples. Based on the volcanostratigraphy and .sup.40Ar-.sup.39Ar dating results, two successive eruption ages of 2.2-1.6 Ma (stage I: amphibole-rich) and 1.6-1.2 Ma (stage II: pyroxene-rich) were established for the KSV, corresponding to the Gelasian and Calabrian stages of Early Pleistocene, respectively. Textural and geochemical evidence indicates that the KSV magnesian andesites-dacites are products of a hybrid magma formed by mixing between mantle-derived mafic and crust-derived felsic magmas with further fractionation and minor contamination during magma storage and ascent. Our new data, combined with previous geological and geophysical results suggest that parental magnesian mafic melts of the KSV rocks originated from a heterogenous mantle source generated through the metasomatism of mantle wedge material by subducted sediment-derived melts, and then partially melted through asthenospheric upwelling in response to slab break-off. The mafic magma underplated the overlying lower crust, resulting in its partial melting to generate crustal felsic magma. Both magmas mixed at lower crustal levels creating MME-rich hybrid magmas. Subsequently, the hybrid magmas were emplaced at different depths of the crust (c. 4-11 and 11-15 km for the stage I and II, respectively), where they crystallized at moderate temperatures (c. 1180-840 °C) and under relatively high oxygen fugacity (LogÆO.sub.2 = - 11.4 to - 9.2), water-rich (H.sub.2O.sub.melt = 5.6-3.6 wt%) and polybaric (~ 1.2 to 5.1 kbars) conditions, and underwent fractionation of primarily amphibole ± pyroxene causing adakitic affinity. We propose a new petrogenetic model for the early Quaternary magnesian/adakitic andesites/dacites of the CVP in a post-subduction tectonic setting. Our results provide robust evidence for slab break-off of the eastern Cyprus oceanic lithosphere and put further constraints on the tectonic evolution of the eastern Mediterranean collision zone during the Early Quaternary., Author(s): Faruk Aydin [sup.1], Mustafa Sönmez [sup.2], Wolfgang Siebel [sup.3], Orhan Karsli [sup.1], Abdurrahman Lermi [sup.2] Author Affiliations: (1) grid.31564.35, 0000 0001 2186 0630, Department of Geological Engineering, Karadeniz Technical [...]

The Margaree pluton extends for >40 km along the axis of the Ganderian Aspy terrane of northern Cape Breton Island, Nova Scotia. The pluton consists mainly of coarse-grained megacrystic syenogranite, intruded by small bodies of medium-grained equigranular syenogranite and microgranite porphyry, all locally displaying rapakivi texture. The three rock types have similar U-Pb (zircon) ages of 363 [+ or -] 1.6, 364.8 [+ or -] 1.6, and 365.5 [+ or -] 3.3 Ma, respectively, consistent with field and penological evidence that they are coeval and comagmatic. The rare earth elements display parallel trends characterized by enrichment in the light rare earth elements, flat heavy rare earth elements, moderate negative Eu anomalies, and, in some cases, positive Ce anomalies. The megacrystic and rapakivi textures are attributed to thermal perturbation in the magma chamber caused by the mixing of mafic and felsic magma, even though direct evidence of the mafic magma is mainly lacking at the current level of exposure. Magma evolution was controlled by fractionation of quartz, K-feldspar, and Na-rich plagioclase in molar proportions of 0.75:0.12:0.13. The chemical and isotopic (Sm-Nd) signature of the Margaree pluton is consistent with the melting of preexisting continental crust that was enriched in heat-producing elements, likely assisted by intrusion of mantle-derived mafic magma during Late Devonian regional extension. The proposed model involving magma mixing at shallow crustal levels in a cryptic silicic-mafic magma chamber during post-Acadian extension is consistent with models for other, better exposed occurrences of rapakivi granite in the northern Appalachian orogen. Key words: rapakivi, megacrysts, U-Pb zircon, Devonian, crystal fractionation, PER diagrams. Le pluton de Margaree s'etale sur plus de 40 km le long de l'axe du terrane ganderien d'Aspy du nord de l'île du Cap-Breton (Nouvelle-Ecosse). Le pluton est principalement constitue de syenogranite megacristique à grains grossiers, recoupe par de petits corps de syenogranite equigranulaire à grains moyens et de porphyre microgranitique, qui presentent tous une texture rapakivi. Les trois types de roche donnent des âges U-Pb sur zircons semblables de 363 [+ or -] 1,6, 364,8 [+ or -]1,6 et 365,5 [+ or -] 3,3 Ma, respectivement, ce qui concorde avec des observations petrologiques et de terrain indiquant qu'ils sont contemporains et comagmatiques. Les terres rares presentent des spectres paralleles caracterises par un enrichissement des terres rares legeres, des tronçons des terres rares lourdes plats, des anomalies negatives moderees d'Eu et, dans certains cas, des anomalies de Ce positives. Les textures megacristique et rapakivi sont attribuees à une perturbation thermique dans la chambre magmatique causee par le melange de magmas mafique et felsique, bien que des preuves directes de la presence d'un magma mafique soient generalement absentes au niveau d'exposition actuel. L'evolution du magma etait controlee par la cristallisation fractionnee du quartz, du feldspath potassique et du plagioclase riche en Na dans des proportions molaires de 0,75:0,12:0,13. Les signatures chimique et isotopique (Sm-Nd) du pluton de Margaree concordent avec la fusion de croûte continentale preexistante qui etait enrichie en elements thermogenes, vraisemblablement aidee par l'intrusion de magma mafique derive du manteau durant l'extension regionale au Devonien tardif. Le modele propose qui comprend le melange de magmas à des niveaux crustaux peu profonds dans une chambre magmatique siliceuse-mafique cryptique durant l'extension post-acadienne concorde avec les modeles proposes pour d'autres cas mieux exposes de granite rapakivi dans le nord de l'orogene des Appalaches. [Traduit par la Redaction] Mots-cles: rapakivi, megacristaux, U-Pb sur zircons, Devonien, cristallisation fractionnee, diagramme de Pearce., Introduction Rapakivi granite sensu stricto is characterized by the presence of plagioclase-mantled K-feldspar, independent of K-feldspar morphology or origin (Ramo and Haapala 1995). The debate over the formation of rapakivi [...]

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

Subduction zone processes are challenging to study because of the rarity of good exposures and the complexity of rock relationships within accretionary prisms. We report the results of field mapping and petrographic, geochemical, and geochronological analyses of the McHugh Complex accretionary prism melange in south-central Alaska that was recently exposed due to retreat of the Nelchina Glacier. Our new mapping and analyses of the melange, as well as adjacent Talkeetna arc intrusives, suggests that the previously mapped trace of the Border Ranges fault should shift northward in this location. Detailed petrographic analysis places this melange exposure with the Potter Creek assemblage of the McHugh Complex. Blocks of pillow lavas within the melange have both mid-ocean ridge basalt and intra-plate geochemical affinities, attesting to the complex relations of subduction-zone inputs in an alternating erosive--accretionary margin. A new zircon U-Pb age and geochemical analyses of a set of felsic dikes that cross-cut the accretionary sequence provide constraints on the regional tectonic evolution, including near-trench plutonism associated with the migration of a subducting spreading ridge along the southern Alaska margin during the Paleocene--Eocene. The McHugh section and cross-cutting dikes in this location are pervasively hydrothermally altered, which we attribute to elevated temperatures related to ridge subduction. Late-stage motion along the Border Ranges fault system, which is also recorded in the area, may also have contributed to the widespread alteration. Our data indicate that the Talkeetna volcanic arc and associated accretionary prism sediments were in their current configuration by 55 Ma. Key words: McHugh Complex, melange, Chugach terrane, southern Alaska, Talkeetna arc. Les processus associes aux zones de subduction sont difficiles a etudier en raison de la rarete de bons affleurements et de la complexite des relations entre les roches au sein de prismes d'accretion. Nous faisons etat des resultats de travaux de cartographie de terrain et d'analyses petrographiques, geochimiques et geochronologiques du melange de prisme d'accretion du complexe de McHugh, dans le centre-sud de l'Alaska, recemment expose par le retrait du glacier Nelchina. La nouvelle cartographie et les nouvelles analyses du melange, ainsi que des roches intrusives attenantes de l'arc de Talkeetna, donnent a penser que la trace cartographiee anterieurement de la faille des Border Ranges devrait se deplacer vers le nord en cet endroit. L'analyse petrographique detaillee place cette exposition du melange avec l'assemblage de Potter Creek du complexe de McHugh. Des blocs de lave a coussins dans le melange ont des affinites geochimiques avec du basalte de dorsale medio-oceanique et intraplaque, ce qui temoigne des relations complexes des apports de zone de subduction dans une marge alternant entre l'erosion et l'accretion. Un nouvel age U-Pb sur zircons et des analyses geochimiques d'un ensemble de dykes felsiques recoupant la sequence d'accretion permettent de preciser l'evolution tectonique regionale, dont du plutonisme proximal a une fosse associe a la migration d'une dorsale d'expansion en subduction le long de la marge sud de l'Alaska au Paleocene--Eocene. La coupe de McHugh et les dykes discordants dans cette localite temoignent d'une alteration hydrothermale penetrante que nous attribuons a des temperatures elevees associees a la subduction d'une dorsale. Un deplacement tardif le long du systeme de la faille des Border Ranges, qui est egalement preserve dans la region, pourrait aussi avoir contribue a l'alteration repandue. Nos donnees indiquent que la configuration actuelle de l'arc volcanique de Talkeetna et des sediments de prisme d'accretion associes etait deja etablie a 55 Ma. [Traduit par la Redaction] Mots-cles: complexe de McHugh, melange, terrane de Chugach, sud de l'Alaska, arc de Talkeetna., Introduction Arc--forearc relationships are difficult to study but are key to understanding processes that contribute to continental growth. Active margins are offshore and underwater, potentially buried by sediment. Ancient accretionary [...]

The geochemistry and Sr-Nd isotope, zircon U-Pb, and zircon Hf isotope compositions are reported for monzogranites and granodiorites from the Hazhu area in the northern Beishan orogen, northwestern China. Zircon U-Pb dating yielded ages of 270.1 [+ or -] 1.1 and 277.4 [+ or -] 1.2 Ma for the monzogranites and 263.6 [+ or -] 1.2 and 262.2 [+ or -] 1.1 Ma for the granodiorites. These monzogranites and granodiorites are metaluminous to weakly peraluminous I-type and belong to mid-K calc-alkaline and high-K calc-alkaline series. They exhibit high [Mg.sup.#] values and moderate degrees of differentiation (D.I. = 70.7-88.1). They are enriched in large-ion lithophile elements and light rare earth elements and depleted in high field strength elements. They show high [([.sup.87]Sr/[.sup.86]Sr).sub.i] ratios of 0.6995-0.7070 and high [[epsilon].sub.Nd](t) values of 4.37-5.70 with Nd model ages ([T.sub.DM]) of 522-789 Ma, suggesting a juvenile crustal origin. Furthermore, their [[epsilon].sub.Hf](t) values are all positive, and Hf isotopic crustal model ages ([T.sup.C.sub.DM] = 394-1097 Ma) also indicate a juvenile crustal origin. According to the data obtained in this study and other regional geological data acquired recently, the Hazhu granitoids were derived from common sources of melting from the Neoproterozoic to late Paleozoic juvenile crusts. The younger intrusions (granodiorites) are more basic, likely as a result of more juvenile lower crust being melted along with asthenospheric upwelling, which led to the addition of more basic components. These granitoids formed in a post-collisional setting. The tectonic regime transformed from an arc-related compressional setting to post-collisional extension, likely as a result of lithospheric extension and thinning in response to oceanic lithospheric delamination. These granitoids in the northern Beishan orogen were probably emplaced in a post-collisional extensional setting and suggest vertical continental crustal growth in the southern Central Asian Orogenic Belt. Key words: northern Beishan orogen, Early--Middle Permian, tectonic setting, zircon U-Pb dating, Sr-Nd isotopes, zircon Hf isotopes. Nous faisons etat de donnees geochimiques et des compositions d'isotopes de Sr-Nd, et de U-Pb et Hf dans les zircons pour des monzogranites et granodiorites de la region de Hazhu, dans l'orogene de Beishan septentrional (nord-ouest de la Chine). La datation U-Pb sur zircons donne des ages de 270,1 [+ or -] 1,1 Ma et 277,4 [+ or -] 1,2 Ma pour les monzogranites et 263,6 [+ or -] 1,2 Ma et 262,2 [+ or -] 1,1 Ma pour les granodiorites. Ces monzogranites et granodiorites sont metalumineux a legerement hyperalumineux de type I et font partie de la serie calcoalcaline a teneurs moyennes a elevees en K. Ils presentent des valeurs de [Mg.sup.#] elevees et des degres de differenciation intermediaires (ID = 70,7-88,1). Ils sont enrichis en elements lithophiles a grand rayon ionique et en terres rares legeres et appauvris en elements a forte liaison atomique. Ils presentent des rapports [([.sup.87]Sr/[.sup.86]Sr).sub.i] eleves de 0,6995-0,7070, ainsi que des valeurs de [[epsilon].sub.Nd](t) elevees de 4,37-5,70 avec des ages modeles Nd ([T.sub.DM]) de 522-789 Ma, ce qui indiquerait une origine dans la croute juvenile. Leurs valeurs de [[epsilon].sub.Hf](t) sont en outre toutes positives et les ages modeles de la croute bases sur les isotopes de Hf ([T.sup.C.sub.DM] = 394-1097 Ma) indiquent egalement une origine dans la croute juvenile. Au vu des donnees obtenues dans la presente etude et d'autres donnees geologiques regionales recentes, les granitoides de Hazhu proviennent de sources communes de fusion de croutes juveniles d'age neoproterozoique a paleozoique tardif. Les intrusions plus jeunes (granodiorites) sont plus basiques, probablement en raison de la fusion d'une croute plus juvenile accompagnee d'une remontee asthenospherique, qui a mene a l'ajout de composants plus basiques. Ces granitoides se sont formes dans un milieu post-collision. Le regime tectonique est passe d'un milieu de compression associe a un arc a un milieu d'extension post-collision, probablement en raison de l'extension et de l'amincissement lithospheriques decoulant de la delamination de la lithosphere oceanique. Ces granitoides dans le nord de l'orogene de Beishan se sont probablement mis en place dans un milieu d'extension post-collision et temoigneraient de la croissance verticale de la croute dans le sud de la ceinture orogenique d'Asie centrale. [Traduit par la Redaction] Mots-cles: nord de l'orogene de Beishan, Permien precoce a intermediaire, milieu tectonique, datation U-Pb sur zircons, isotopes de Sr-Nd, isotopes de Hf dans les zircons., Introduction The Central Asian Orogenic Belt (CAOB), located among the Eastern Europe, Siberia, Tarim, and North China plates (Fig. 1a), and which evolved from the late Mesoproterozoic (Kroner et al. [...]

Controversies exist regarding the mechanismofformationofbasins locatedonthe continental marginof South China as well as when they formed. It was ascertained based on clastic petrology, geochemical analysis, and zircon U--Pb dating that the sedimentary provenances in the eastern Guangdong Basin are mainly felsic igneous rocks from the late Early Jurassic to the Middle Jurassic. The late Early Jurassic Qiaoyuan Formation mainly shows major age peaks atapproximately 238 Ma, 259 Ma, and 1858 Ma, and the Middle Jurassic Tangxia Formation shows major age peaks at approximately 169 Ma and 172 Ma. From the late Early Jurassic to the Middle Jurassic in the eastern Guangdong Basin, the source region changes from southwestern South China and southern South China to the eastern Nanling Range. It was determined by comparing the detrital zircon ages of the Qiaoyuan Formation and the Tangxia Formation with thoseofthe late Paleozoictoearly Mesozoic basins, and analyzing both the geochemical data and sedimentation, that the eastern Guangdong Basin changed from the basin-arc foreland basin of the late Early Jurassic to the back-arc extension basin of the Middle Jurassic. The changes in early Mesozoic detrital zircon age peaks indicate that the tectonic regime of the eastern Guangdong Basin ended the transformation from the Tethyan tectonic domain to the paleo-Pacific tectonic domain in the early Middle Jurassic (approximately 172 Ma). Key words: eastern Guangdong Basin, late Early Jurassic to Middle Jurassic, provenances, tectonic regime transformation, zircon U--Pb dating. Des controverses persistent quant au mecanisme de formation de bassins situes sur la marge continentale de la Chine meridionale, ainsi qu'au moment de leur formation. Il a ete etabli, sur la base de la petrologie, l'analyse geochimique et la datation U--Pb sur zircons de depots clastiques que les sources de sediments dans le bassin de Guangdong oriental sont principalement des roches ignees felsiques d'age jurassique precoce tardif a jurassique moyen. La Formation de Qiaoyuan du Jurassique precoce tardif produit principalement de grands pics d'ages vers 238 Ma, 259 Ma et 1858 Ma, et la Formation de Tangxia du Jurassique moyen donne de grands pics d'ages vers 169 Ma et 172 Ma. Du Jurassique precoce tardif au Jurassique moyen dans le bassin de Guangdong oriental, la region source passe du sud-ouest et du sud de la Chine meridionale a la chaine de Nanling orientale. Il est determine, en comparant les ages de zircons detritiques des Formations de Qiaoyuan et de Tangxia a ceux des bassins d'age paleozoique tardif a mesozoique precoce et en analysant les donnees geochimiques et la sedimentation, que le bassin de Guangdong oriental est passe d'un bassin d'avant-pays d'un systeme arc-bassin au Jurassique precoce tardif a un bassin d'extension d'arriere-arc au Jurassique moyen. L'evolution des pics d'ages de zircons detritiques du Mesozoique precoce indique queleregime tectonique danslebassindeGuangdong orientalacloslatransformationdudomaine tectonique tethysien en un domaine tectonique paleopacifique au Jurassique moyen precoce (vers 172 Ma). [Traduit par la Redaction] Mots-cles : bassin de Guangdong oriental, Jurassique precoce tardif--Jurassique moyen, provenances, transformation du regime tectonique, datation U--Pb sur zircons., Introduction The main geotectonic outline of South China and even eastern China is controlled by the Indosinian and Yanshan movements (Wang and Zhou 2006; He et al. 2007; Z.H. Li [...]

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

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

Our paper reports the detrital zircon record from Late Devonian to late Carboniferous foreland basin deposits in the Rheno-Hercynian (RH) Variscides of Germany. Together with a review of petrography and detrital mineral ages from the literature, the data permit to reconstruct accretion and exhumation along the RH active margin. From Frasnian to latest Carboniferous, the main source (now eroded) was a north-Armorican microcontinent (Franconia) with magmatic rocks representing late Neoproterozoic arc or back-arc, Cambro-Ordovician rift and Silurian--Early Devonian subduction of the Rheic ocean and (or) RH rifting. At ca. 380 Ma, detrital magmatic zircons combined with high- to medium-pressure mica and detrital glaucophane suggest the existence of a paired metamorphic belt at the RH tectonic front. From the Visean onwards, zircons reveal younging of granitoid debris from ca. 380-360 Ma in Late Devonian sediments to ca. 320-300 Ma in the Westphalian C-D and Stephanian. Greywackes of the Namurian A record a change from dominant magmatic clasts toward meta-arenites associated with Baltoscandian zircons, which document accretion to and exhumation from the base of the orogenic wedge. Their source must be sought in metamorphosed Devonian sandstones of the type presently encountered in parts of the active margin crystallines (Mid-German Crystalline High), but in eroded higher units. Basal accretion implies heating of the lower plate beyond the brittle--ductile boundary and supports the model of a high-temperature regime before and during Variscan collision. Palinspastic restoration of the estimated volume of recycled material yields >100 km of distal shelf deposits lost in the process, which adds to the known shortening of the RH basin. The Variscan geology of southwestern England and southern Portugal and provenance studies in those areas are compatible with a geodynamic evolution similar to that in Germany. Key words: detrital zircon, U--Pb laser ICP-MS, foreland basin deposits, Rheno-Hercynian belt, Variscides. Nous rendons compte du registre des zircons detritiques de depots de bassin d'avant-pays d'ages devonien tardif a carbonifere tardif dans la zone rheno-hercynienne (RH) de la chame varisque d'Allemagne. Combinees a un examen de la petrographie et des ages de mineraux detritiques publies, les donnees permettent de reconstituer l'accretion et l'exhumation le long de la marge RH active. Du Frasnien a la fin du Carbonifere, la source principale (aujourd'hui erodee) etait un microcontinent nord-armoricain (la Franconie) avec des roches magmatiques representant un arc ou arriere-arc d'age neoproterozoique tardif, un rift cambro-ordovicien et la subduction du Silurien au Devonien precoce de l'ocean Rheique ou un rifting RH. Vers 380 Ma, des zircons magmatiques detritiques combines a des micas de haute a moyenne pression et du glaucophane detritique indiquer-aient l'existence d'une ceinture metamorphique jumelee au front tectonique RH vers 380 Ma. A partir du Viseen, les zircons revelent un rajeunissement des debris de granitoide a partir d'environ 380-360 Ma dans des sediments d'age devonien tardif a environ 320-300 Ma dans le Westphalien C-D et le Stephanien. Des grauwackes du Namurien A enregistrent le passage de clastes principalement magmatiques a des meta-arenites associees a des zircons baltoscandiens, qui documentent l'accretion a la base du biseau orogenique et l'exhumation subsequente. Leur source doit se trouver dans des gres devoniens metamorphises semblables a ceux qui se trouvent actuellement dans les roches cristallines de la marge active (ride cristalline medio-allemande), mais dans des unites plus hautes erodees. L'accretion basale signifie que la plaque inferieure a ete chauffee au-dela de la limite cassant--ductile et appuie le modele d'un regime de haute temperature avant et durant la collision varisque. La reconstitution palinspatique du volume estime de materiau recycle donne >100 km de depots de plateforme distale evacues durant le processus, ce qui vient s'ajouter au raccourcissement connu du bassin RH. La geologie varisque du sud-ouest de l'Angleterre et du sud du Portugal et des etudes de provenance dans ces regions sont compatibles avec une evolution geodynamique semblable a celle observee en AUemagne. Mots-cles : zircon detritique, analyse U--Pb par ICP-MS avec ablation au laser, depots de bassin d'avant-pays, ceinture rhenohercynienne, chame varisque., Introduction Rationale The Rheno-Hercynian (RH) Variscides (Portugal, southwestern England, Germany, Poland, Turkey) were formed from the infill of the narrow RH ocean. Closure of that ocean is documented, especially in [...]

The seismotectonics of Western Africa show the occurrence of major earthquakes (e.g., 1636 southwestern Ghana, 1855 offshore Monrovia, 1939 offshore Accra, and 1983 Gaoual-Guinea) and prominent offshore transform faults. However, there is no analysis that links the continental active tectonics with the oceanic fault zones of the Gulf of Guinea. We study the active tectonics by firstly mapping the main transform faults using a combination of bathymetric, gravimetric, and magnetic data. The data analysis associates regional seismicity (historical and instrumental) with focal mechanisms as extracted from the recently published seismotectonic map of Africa. We identify active transform faults, the Chain (CFZ), Romanche (RFZ), Saint Paul (SPFZ), and Arkhangelskiy (AFZ) fault zones. We also calculate strain rates on these faults from Late Cretaceous (-85 Ma) to present time using paleomagnetic data and infer slip rates from the seismic moment data. The strain rates show a first stable trend around 2 cm/year and then accelerate to 4 cm/year in the last 10 million years. The comparison of Late Quaternary strain rates with geodetic strain rates shows an accumulation of seismic energy that could lead to the initiation of an Mw 7-7.5 earthquakes on the Saint Paul transform fault. Our seismotectonic analysis clearly links oceanic and continental tectonics, with about a 20[degrees]-30[degrees] anticlockwise fault trend rotation for CFZ, RFZ, and SPFZ. The potential for the occurrence of large earthquakes in the Gulf of Guinea should be taken into account for a realistic regional seismic and tsunami hazard of the Gulf of Guinea. Key words: West Africa, Gulf of Guinea, transform faults, seismic hazard assessment. La sismotectonique de l'Afrique de l'Ouest comprend de grands seismes (p. ex., ceux de 1636 dans le sud-ouest du Ghana, 1855 au large de Monrovia, 1939 au large d'Accra et 1983 a Gaoual en Guinee) et d'importantes failles transformantes oceaniques. Il n'existe toutefois aucune analyse reliant la tectonique continentale active aux zones de fractures oceaniques dans le golfe de Guinee. Nous etudions la tectonique active en cartographiant d'abord les principales failles transformantes a l'aide des donnees bathymetriques, gravimetriques et magnetiques. L'analyse des donnees associe la sismicite regionale (sismicite historique et instrumentale) a des mecanismes focaux extraits de la carte sismotectonique de l'Afrique publiee recemment. Nous mettons en evidence des failles transformantes actives, les zones de fractures Chain (ZFC), Romanche (ZFR), Saint Paul (ZFSP) et Arkhangelskiy (ZFA). Nous calculons egalement les vitesses de deformation le long de ces failles du Cretace superieur (-85 Ma) au present en utilisant des donnees paleomagnetiques et obtenons les vitesses de glissement a partir de donnees de moment sismique. Les vitesses de deformation montrent une premiere tendance stable a environ 2 cm/an, pour ensuite augmenter a 4 cm/an dans les 10 derniers millions d'annees. La comparaison des vitesses de deformation au Quaternaire superieur et des vitesses de deformation obtenues par la geodesie fait ressortir une accumulation d'energie sismique qui pourrait mener au declenchement d'un seisme de Mw 7-7,5 le long de la faille transformante Saint Paul. Notre analyse sismotectonique etablit un lien clair entre la tectonique oceanique et la tectonique continentale, qui comprend une rotation antihoraire d'environ 20[degrees]-30[degrees] des directions de la ZFC, de la ZFR et de la ZFSP. Le potentiel de grands tremblements de terre dans le golfe de Guinee devrait etre pris en consideration pour etablir une evaluation realiste du risque sismique et de tsunami dans le golfe de Guinee. [Traduit par la Redaction] Mots-cles: Afrique de l'Ouest, Golfe de Guinee, failles transformantes, evaluation de l'alea sismique., Introduction Transform faults are major tectonic structures often characterized by large earthquakes (Mw > 6) with significant coseismic horizontal movements. Also called fracture zones and either continental or oceanic, these [...]

Slip on the Nacimiento fault of the central Coast Ranges of California has been variably interpreted as dextral, sinistral, or reverse. The currently prevailing interpretation is that the Nacimiento fault represents subduction erosion, by which the central to eastern part of the Cretaceous California batholith was thrust over the western part of the batholith and forearc basin, resulting in juxtaposition of the Salinian batholithic block against the Franciscan Complex, concurrently with Laramide flat-slab subduction (75-55 Ma) and underplating of the Pelona-Orocopia-Rand schist. No modern convergent plate margin includes such overthrusting. The closest modern analog to the likely configuration of the Salinian continental margin near the end of the Laramide deformation is southern Mexico, where arc plutons are exposed near the trench. Although commonly considered an example of subduction erosion, this margin is 'missing' parts of the plutonic arc and forearc because they have been displaced to the southeast by sinistral slip. By analogy, the Nacimiento forearc was modified as a trench-trench-transform triple junction migrated southeastward along the continental margin during flat-slab subduction. This model makes testable predictions involving northwest-to-southeast younging of deep-marine deposits on batholithic crust underlain by contemporaneous schist. Correct restoration of later Cenozoic primarily dextral slip and Maastrichtian--Early Eocene primarily sinistral slip must result in realignment of north--south-trending belts of the Sierra Nevada--Salinia--Peninsular Ranges batholith, Great Valley forearc, and Franciscan Complex. These modern and ancient examples suggest that several 'erosional' subduction zones are more plausibly explained by strike-slip truncation of forearcs. Key words: tectonics, sedimentary basins, California paleotectonics, southern Mexico margin, sinistral transforms. Le rejet le long de la faille de Nacimiento, dans les chames cotieres de la Californie, a ete interprets, tour a tour, comme etant dextre, senestre ou inverse. L'interpretation dominante a l'heure actuelle veut que la faille de Nacimiento represente une erosion par subduction, un processus dans lequel la partie centrale a orientale du batholite de Californie d'age cretace a ete charriee sur la partie occidentale du batholite et le bassin d'avant-arc, causant la juxtaposition du bloc batholitique salinien sur le complexe franciscain en meme temps que la subduction de la plaque subhorizontale laramienne (75-55 Ma) et le sous-placage du schiste de Pelona-Orocopia-Rand (POR). Aucune marge convergente de plaque moderne ne montre un tel charriage. L'exemple moderne le plus semblable a la configuration probable de la marge continentale salinienne vers la fin de la deformation laramienne est le sud du Mexique, ou des plutons d'arc sont exposes pres de la fosse. Si cette marge est souvent considered comme un exemple d'erosion par subduction, il lui <> des parties de l'arc plutonique et de l'avant-arc parce qu'elles ont ete deplacees vers le sud-est par deplacement senestre. Par analogie, l'avant-arc de Nacimiento a ete modifie durant la migration vers le sud-est d'une jonction triple fosse-fosse-faille transformante le long de la marge continentale durant la subduction de la plaque subhorizontale. De ce modele decoulent des predictions verifiables quant a la presence de depots marins profonds de plus en plus recents vers le sud-est sur une croute batholitique sous-tendue par des schistes contemporains. Une bonne restauration du glissement principalement dextre plus tard au Cenozoique et du glissement principalement senestre au Maastrichtien-Eocene precoce doit produire le realignement de ceintures d'orientation nord-sud du batholite des Sierra Nevada--Salinia--chames peninsulaires, de l'avant-arc de la grande vallee et du complexe franciscain. Ces exemples modernes et anciens donnent a penser que plusieurs zones <> par subduction s'expliquent plus plausiblement par la troncation par decrochement d'avant-arcs. Mots-cles: tectonique, bassins sedimentaires, paleotectonique de la Californie, marge du sud du Mexique, failles transformantes senestres., Introduction Dedication This speculative contribution honors the decades of insightful paleotectonic analyses by my long-time friend A.M. Celal Sengor. Celal and I came of age professionally during the exciting 1970s, [...]

Testing of Middle Permian--Lower Triassic stratigraphic data from the Antalya Nappes in a half-graben/tilt-block system has revealed the presence of episodic rifting events separated by periods of tectonic quiescence. Following a period of uplift during the Permian (Late Artinskian to Roadian), the basement rocks have been activated by displacement faults and several depocenters in half-graben-like asymmetrical basins began to be filled with Roadian to Wordian continental clastic deposits intercalated with coal and marine rocks. The Early Capitanian time was a period of tectonic quiescence. The second event occurred in Middle to Late Capitanian times and produced basaltic volcanic rocks intercalated in the shallow marine fossiliferous carbonate successions. Following the Lopingian (Wuchiapingian and Changhsingian) and Permian-Triassic boundary interval representing a long tectonic quiescence, the last rifting episode started with an abrupt facies change in the late Griesbachian. Variegated shales, limestones, volcanics, talus breccia, and debris flow deposits were laid down in a half-graben/tilt-block system. As normal faulting has become active, the deposition continued on the subsiding hanging wall side. The stratigraphic gap increased in magnitude as the erosional truncation has incised deeply the footwall side. This initial rifting phase in the Antalya Nappes is prior to the onset of a stronger and more continuous rifting event that occurred in the Anisian-Carnian interval including a variety of deepwater clastic and carbonate deposits, radiolarites containing sometimes blocks and clasts derived from the basin margins, and volcanic rocks carrying intraoceanic setting character. Key words: rifting, stratigraphy, Permian--Early Triassic, Antalya Nappes, Turkey. La validation de donnees stratigraphiques pour le Permien moyen--Trias inferieur des nappes d'Antalya dans un systeme de demi-grabens/blocs inclines revele la presence d'episodes de rifting separes par des periodes de quiescence tecto-nique. A la suite d'une periode de soulevement durant le Permien (Artinskien tardif a Roadien), les roches du socle rocheux ont ete mobilisees par des failles, et plusieurs centres de depot dans des bassins asymetriques apparentes a des demi-grabens ont commence a se remplir de depots clastiques continentaux roadiens a wordiens avec charbon et roches marines interstratifies. Le debut du Capitanien etait une periode de quiescence tectonique. Le deuxieme episode s'est produit au Capitanien moyen a tardif et a produit des roches volcaniques basaltiques interstratifiees dans les sequences de carbonates fossiliferes marins d'eau peu profonde. Apres l'intervalle du Lopingien (Wuchiapingien et Changhsingien) et de la limite permo-triasique representant une longue quiescence tectonique, le dernier episode de rifting a commence par un changement de facies abrupt au Griesbachien tardif. Des shales bigarres, calcaires, roches volcaniques, breches de talus et depots de coulee de debris ont ete mis en place dans un systeme de demi-grabens/blocs inclines. Avec l'activation de failles normales, le depot s'est poursuivi du cote du toit subsident. En creusant profondement dans le mur, l'erosion a accru l'ampleur de la lacune stratigraphique. Cette phase initiale de rifting dans les nappes d'Antalya a precede le debut de l'episode de rifting plus intense et soutenu durant l'intervalle Anisien-Carnien qui a produit divers depots clastiques et carbonates d'eau profonde, des radiolarites contenant parfois des blocs et clastes derives de marges de bassins et des roches volcaniques a d'affinite intraoceanique. [Traduit par la Redaction] Mots-cles: rifting, stratigraphie, Permien--Trias inferieur, nappes d'Antalya, Turquie., Introduction Studies in the last 50 years have shown that lithospheric extension is characterized by a distinctive development of tilt-blocks and half-grabens bounded by major normal faults (Mckenzie 1978; Sengor [...]

The Suluova Basin is a prominent member of the wide transtensional Amasya Shear Zone located at the central part of the North Anatolian Shear Zone. This basin is crucial and provides well-resolved data to understand the evolution of transtensional tectonic zones as well as the morphological and paleoenvironmental changes of North Anatolia during the Quaternary. Analysis of detailed stratigraphical sections, faulting data, and mammal paleontology reveals that the Suluova Basin has started to evolve as a closed half-graben along the NW-SE-trending, SW-dipping basin bounding fault zone with normal slip in the early Quaternary. Initial sedimentation mode of the basin was dominated by alluvial fan facies associations. Progressive basin subsidence resulted in an expansion of a freshwater lake at the basin depocenter as faults propagated westwards. Further extensions in the basin were caused to initiate the E-W-trending southern tectonic boundary. Newly created accommodation space hosted a vast freshwater lake during the Calabrian (~1.8-0.78 Ma) acting as a refugia for a rich faunal assemblage of large and small land mammals. The conditions prior to the onset of Middle Pleistocene (MIS19, ~0.79 Ma) is marked with increasing regional erosion where paleo-Lake Suluova was captured by the regional river system. Synchronously, the next phase of the shear zone formation was introduced with E-W-trending dextral and NE-SW-trending sinistral strike-slip faults, cross-cutting the former basin structure, forming new depocenters. These faults are still active with noticeable seismic activity and comprise future risks for the major cities of the region. Key words: Suluova Basin, North Anatolian Shear Zone, Amasya Shear Zone, Quaternary palaeoenvironment. Le bassin de Suluova est un element important de la large zone de cisaillement en transtension d'Amasya situee dans la partie centrale de la zone de cisaillement nord-anatolienne. Il s'agit d'un bassin d'importance cruciale qui fournit des donnees de bonne resolution pour comprendre l'evolution de zones tectoniques en transtension, ainsi que les changements morphologiques et paleoenvironnementaux de l'Anatolie septentrionale durant le Quaternaire. L'analyse de coupes stratigraphiques detaillees, de donnees sur les failles et de la paleontologie des mammiferes revele que la formation du bassin de Suluova a commence par un demi-graben ferme le long de la faille bordiere du bassin, d'orientation NO-SE, de pendage SO et de rejet normal, au debut du Quaternaire. Le mode de sedimentation initial du bassin etait domine par des associations de facies de cone alluvial. La subsidence graduelle du bassin a entraine l'expansion d'un lac d'eau douce dans le centre de depot du bassin durant la propagation de failles vers l'ouest. La poursuite de l'extension dans le bassin a entraine la formation de sa bordure tectonique sud d'orientation E-O. De l'espace nouvellement cree a accueilli un vaste lac d'eau douce durant le Calabrien(~1,8-0,78 Ma) qui aservi de refuge aunriche assemblage de petits et grands mammiferes terrestres. Les conditions qui precedaient le debut du Pleistocene moyen (MIS19, ~0,79 Ma) etaient caracterisees par une erosion regionale accrue durant laquelle le paleo-lac Suluova a ete capture dans le reseau fluvial regional. En meme temps, la phase suivante de formation de la zone de cisaillement a debute par des failles de decrochement dextres d'orientation E-O et senestres d'orientation NE-SO recoupant la structure anterieure du bassin et formant de nouveaux centres de depot. Ces failles sont toujours actives et sont caracterisees par une activite sismique perceptible qui pose un risque pour les grandes villes de la region. [Traduit par la Redaction] Mots-cles : bassin de Suluova, zone de cisaillement nord-anatolienne, zone de cisaillement d'Amasya, paleoenvironnement quaternaire., Introduction The North Anatolian Fault (NAF) is a 1400 km long continental transform fault characterized by right-lateral strike-slip motion. The NAF forms the northern boundary and accommodates 21 mm/year westward [...]

The Almacik Block is an approximately 73 km long and 21 km wide tectonic sliver formed by the North Anatolian Fault Zone in northwestern Turkey. Morphologically, it is one of the most pronounced structures along the North Anatolian Fault Zone. All the segments bounding the Almacik Block were ruptured during the second half of the 20th century. The fifty-four apatite (U-Th)/He ages we obtained showed that the region including the Almacik Block was exhumed during the Oligo-Miocene interval and then original exhumation pattern was distorted by the North Anatolian Fault Zone during the Miocene to recent. To interpret this distortion and to reconstruct it to the original state, we modelled 'A'-shaped mountain fronts in the most probable deformation scenarios. The block has been tilted southward about an approximately east-west-trending horizontal (slightly dipping to the east) axis. As a result of this rotation, the northern part of the block has been uplifted about 2800 m, whereas the southern part has subsided about 430 m, likely during the last 2.5 Myr. The exhumation in the studied region started at around 34 Ma and lasted until 16 Ma with a mean exhumation rate of about 60 m/Myr. Key words: NW Turkey, Almacik Block, North Anatolian Fault Zone, apatite (U-Th)/He (data or age), rotation. Le bloc d'Almacik est un copeau tectonique de ~73 km de longueur et ~21 km de largeur forme par la zone de faille nord-anatolienne dans le nord-ouest de la Turquie. Du point de vue morphologique, il s'agit d'une des structures les plus prononcees le long de la zone de faille nord-anatolienne. Tous les segments qui limitent le bloc d'Almacik ont ete le lieu de ruptures durant la deuxieme moitie du 20e siecle. Cinquante-quatre ages (U-Th)/He sur apatite que nous avons obtenus montrent que la region qui comprend le bloc d'Almacik a ete exhumee durant l'intervalle Oligocene-Miocene, puis que le motif d'exhumation initial a ete distordu par la zone de faille nord-anatolienne du Miocene jusqu'a la periode recente. Pour interpreter cette distorsion et reconstituer l'etat initial du bloc, nous avons modelise un front de montagnes en forme de <> dans les scenarios de deformation les plus probables. Le bloc a ete deverse vers le sud autour d'un axe a peu pres horizontal (plongeant legerement vers l'est) d'orientation E--O. Du fait de cette rotation, la partie nord du bloc a ete soulevee sur ~2 800 m, alors que sa partie sud s'est affaissee de ~430 m, probablement durant les dernieres 2,5 millions d'annees. L'exhumation dans la region etudiee a debute autour de 34 Ma et s'est poursuivie jusqu'a 16 Ma, a un taux d'exhumation moyen de ~60 m/million d'annees. [Traduit par la Redaction] Mots-cles: nord-ouest de la Turquie, bloc d'Almacik, ZFNA, (U-Th)/He sur apatite (donnes ou age), rotation., Introduction Apatite (U-Th)/He (AHe) and fission track (AFT) dating are frequently used methods in low-temperature thermochronological studies (e.g., Zattin et al. 2010; Stubner et al. 2018; Cavazza et al. 2018; [...]

The Mae Ping shear zone (MPSZ) is one of the major ductile strike-slip systems associated with the Cenozoic extrusion tectonics in Southeast Asia. However, its sinistral shear lacks a robust temporal constraint. This study attempts to acquire the deformation timing by applying .sup.40Ar/.sup.39Ar thermochronology on a sheared pegmatitic leucogranite showing sinistral S-C fabrics with a thrust component. The contact of the leucogranite sub-paralleling to the major foliation in host gneiss indicates it could be a pre- to syn-shearing intrusion. Most minerals, including garnet, muscovite, K-feldspar, albite, and quartz, exhibit ductile to brittle deformation. Mineral microstructural analysis suggests a retrograde sinistral shear from > 600 to 250 °C. In situ .sup.40Ar/.sup.39Ar dating on muscovite yield ages mainly within 42-38 Ma with calculated closure temperatures of 435-330 °C. Fine-grained muscovite aggregates are slightly older than fish, implying that grain size reduction may not always reset .sup.40Ar/.sup.39Ar ages. The K-feldspar .sup.40Ar/.sup.39Ar step heating age spectrum with two segments of contiguous steps at 24.5 and 35.4 Ma may reflect the coexistence of high-T porphyroclast and low-T K-rich fine-grain recrystallizing at pressure shadows. The reconstructed cooling path and inferred deformation temperatures constrain a shear duration of 42-30 Ma for the MPSZ. The activation of the MPSZ before 42 Ma could be linked to the Eocene metamorphism within a transpressional regime triggering crustal thickening that may further induce the leucogranitic melt. This study also shows leucogranite can be a nice thermal history recorder for a shear zone regarding its petrogenesis and suitable mineral assemblage for thermochronology., Author(s): Yu-Ling Lin [sup.1], Tung-Yi Lee [sup.1], Ching-Hua Lo [sup.2], Sarah C. Sherlock [sup.3], Yoshiyuki Iizuka [sup.4], Tadashi Usuki [sup.2], Long Xiang Quek [sup.1], Punya Charusiri [sup.5] Author Affiliations: (1) [...]

In Late Proterozoic mafic granulites of the Sandmata Complex in Rajasthan, northwestern India, multilayer corona textures were formed along the interface between orthopyroxene and plagioclase. We examined the metamorphic conditions and processes of formation of these coronae, which provide an insight into the interplay between steady state and sequential diffusion-controlled mineral growth mechanisms. The individual corona-symplectite layers consist of clinopyroxene + quartz|garnet + clinopyroxene|garnet + quartz|K-feldspar, from the inner to the outer margins of the coronae. The single-value decomposition models suggest that the multilayered coronae were formed in a locally closed system, via sequential diffusion of Mg, Fe and Ca into the reaction zone, which has acted as the main driving force for the growth of corona textures. The relict orthopyroxene with exsolved clinopyroxene yields primary crystallization conditions of 8.4 ± 1.5 kbar and ~ 1100-1000 °C. Clinopyroxene in the innermost corona layer grew at ~ 9 kbar and 850-800 °C, whereas clinopyroxene + garnet grew outward at ~ 8 kbar and 700-600 °C. Subsequent hydrous retrogression (~ 6 kbar and 600-550 °C) resulted in the development of rimward zoning in garnet and the growth of amphibole. On combining textural relations and the above conventional P-T estimates, a near-isobaric cooling P-T path was reconstructed using phase equilibria modeling. Further, the near-isobaric cooling path is consistent with the magmatic underplating hypothesis in the Sandmata Complex, where the intrusion of magmatic bodies (i.e., Gyangarh-Asind igneous complex and Anjana granite) favored the development of granulite facies assemblage in norite and gabbronorite protoliths., Author(s): Suranjan Ghosh [sup.1], N. Prabhakar [sup.1], Joseph D'Souza [sup.1] [sup.2] Author Affiliations: (1) grid.417971.d, 0000 0001 2198 7527, Department of Earth Sciences, Indian Institute of Technology Bombay, , 400076, [...]

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

Tonalite-trondhjemite-granodiorite (TTG) gneiss forms a major component of Archean continental crust. Resolving the origin of TTG gneisses and the secular change in their compositions is critical for better understanding how the continental crust has evolved and when a global plate tectonic regime began on Earth. Archean TTGs were generated by partial melting of hydrous basalts, although the geodynamic setting in which this occurred is still debated. In this study, an integrated modeling including thermodynamic modeling, accessory mineral solubility modeling, and trace element modeling is conducted on Coucal basalts from Pilbara craton and averaged Archean arc-like basalts along various thermal gradients for both closed and melt-drained systems. The results show the amount and composition of melts would be primarily controlled by source compositions, although geothermal gradient and the ability for melt to leave the system also contribute. Of both bulk compositions, averaged Archean arc-like basalts generate more melts with a medium- to low-pressure signature by fluid-absent melting during intracrustal loading. High-pressure TTGs may be generated by mixing residua and melts derived by fluid-absent melting or just fluid-present melting in a hot subduction zone. Most TTGs produced before the Neoarchean were likely generated at the lower levels of thickened basaltic crust; however, rare Eoarchean and Mesoarchean TTGs with high-pressure signatures suggest subduction occurred on the early Earth in isolated localities. We interpret a secular increase in the proportion of high-pressure TTGs at c. 3.0-2.5 Ga as recording a transition from localized subduction to a global plate tectonic regime., Author(s): Guangyu Huang [sup.1], Richard Palin [sup.2], Dan Wang [sup.3], Jinghui Guo [sup.1] Author Affiliations: (1) grid.9227.e, 0000000119573309, State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese [...]