http://www.sciencedirect.com/science/article/pii/S1674987115000948

The formation of continents involves a combination of magmatic and metamorphic processes. These processes become indistinguishable at the crust-mantle interface, where the pressure-temperature(P-T)conditions of(ultra) high-temperature granulites and magmatic rocks are similar. Continents grow laterally, by magmatic activity above oceanic subduction zones(high-pressure metamorphic setting), and vertically by accumulation of mantle-derived magmas at the base of the crust(high-temperature metamorphic setting). Both events are separated from each other in time; the vertical accretion postdating lateral growth by several tens of millions of years. Fluid inclusion data indicate that during the high-temperature metamorphic episode the granulite lower crust is invaded by large amounts of low H_2O-activity fluids including high-density CO_2 and concentrated saline solutions(brines). These fluids are expelled from the lower crust to higher crustal levels at the end of the high-grade metamorphic event. The final amalgamation of supercontinents corresponds to episodes of ultra-high temperature metamorphism involving large-scale accumulation of these low-water activity fluids in the lower crust.This accumulation causes tectonic instability, which together with the heat input from the subcontinental lithospheric mantle, leads to the disruption of supercontinents. Thus, the fragmentation of a supercontinent is already programmed at the time of its amalgamation.     

http://www.sciencedirect.com/science/article/pii/S1674987112000564

High-pressure（HP） granulites widely occur as enclaves within tonalite-trondhjemitegranodiorite （TTG） gneisses of the Early Precambrian metamorphic basement in the Shandong Peninsula, southeast part of the North China Craton（NCC）.Based on cathodoluminescence（CL）,laser Raman spectroscopy and in-situ U-Pb dating,we characterize the zircons from the HP granulites and group them into three main types:inherited（magmatic） zircon,HP metamorphic zircon and retrograde zircon.The inherited zircons with clear or weakly defined magmatic zoning contain inclusions of apatites,and ²⁰⁷Pb/²⁰⁶Pb ages of 2915—2890 Ma and 2763—2510 Ma,correlating with two magmatic events in the Archaean basement. The homogeneous HP metamorphic zircons contain index minerals of high-pressure metamorphism including garnet,clinopyroxene.plagioclase,quartz,rutile and apatite,and yield ²⁰⁷Pb/²⁰⁶Pb ages between 1900 and 1850 Ma,marking the timing of peak HP granulite fades metamorphism.The retrograde zircons contain inclusions of orthopyroxene.plagioclase.quartz,apatite and amphibole.and yield the youngest ²⁰⁷Pb/²⁰⁶Pb ages of 1840—1820 Ma among the three groups,which we correlate to the medium to low-pressure granulite fades retrograde metamorphism.The data presented in this study suggest subduction of Meso- and Neoarchean magmatic protoliths to lower crust depths where they were subjected to HP granulite facies metamorphism during Palaeoproterozoic（1900—1850 Ma）.Subsequently, the HP granulites were exhumated to upper crust levels,and were overprinted by medium to low-pressure granulite and amphibolite facies retrograde event at ca.1840—820 Ma.     

U–Pb and Hf-isotope analysis of zircons in mafic xenoliths from Fuxian kimberlites: evolution of the lower crust beneath the North China craton

Mafic xenoliths from the Paleozoic Fuxian kimberlites in the North China craton include garnet granulite, and minor pyroxene amphibolite, metagabbro, anorthosite and pyroxenite. The formation conditions of the amphibolites are estimated at 745–820 °C and 7.6–8.8 Kb (25–30 km); the granulites probably are derived from greater depths in the lower crust. LAM-ICPMS U–Pb dating of zircons from four granulites reveals multiple age populations, recording episodes of magmatic intrusion and metamorphic recrystallisation. Concordant ages and upper intercept ages, interpreted as minimum estimates for the time of magmatic crystallisation, range from 2,620 to 2,430 Ma in three granulites, two amphibolites and two metagabbros. Lower intercept ages, represented by near-concordant zircons, are interpreted as reflecting metamorphic recrystallisation, and range from 1,927 to 1,852 Ma. One granulite contains two metamorphic zircon populations, dated at 1,927±55 Ma and 600–700 Ma. Separated minerals from one granulite and one amphibolite yield Sm–Nd isochron ages of 1,619±48 Ma (¹⁴³Nd/¹⁴⁴Nd)i=0.51078), and 1,716±120 Ma (¹⁴³Nd/¹⁴⁴Nd)i=0.51006), respectively. These ages are interpreted as recording cooling following metamorphic resetting; model ages for both samples are in the range 2.40–2.66 Ga. LAM-MC-ICPMS analyses of zircon show a range in ¹⁷⁶Hf/¹⁷⁷Hf from 0.28116 to 0.28214, corresponding to a range of _Hf from –34 to +12. The relationships between ²⁰⁷Pb/²⁰⁶Pb age and _Hf show that: (1) the granulites, amphibolites and metagabbro were derived from a depleted mantle source at 2.6–2.75 Ga; (2) zircons in most samples underwent recrystallisation and Pb loss for 100–200 Ma after magmatic crystallisation, consistent with a residence in the lower crust; (3) metamorphic zircons in several samples represent new zircon growth, incorporating Hf liberated from breakdown of silicates with high Lu/Hf; (4) in other samples metamorphic and magmatic zircons have identical ¹⁷⁶Hf/¹⁷⁷Hf, and the younger ages reflect complete resetting of U–Pb systems in older zircons. The Fuxian mafic xenoliths are interpreted as the products of basaltic underplating, derived from a depleted mantle source in Neoarchean time, an important period of continental growth in the North China craton. Paleoproterozoic metamorphic ages indicate an important tectonic thermal event in the lower crust at 1.8–1.9 Ga, corresponding to the timing of collision between the Eastern and Western Blocks that led to the final assembly of the North China craton. The growth of metamorphic zircon at 600–700 Ma may record an asthenospheric upwelling in Neoproterozoic time, related to uplift and a regional disconformity in the North China craton.     

The lower crust of the Gangdese magmatic arc,southern Tibet,implication for the growth of continental crust

Magmatic arcs are thought to be the primary sites of modern-day continental crustal growth, and arc crustal sections provide an exceptional opportunity to directly observe the geological processes that occur there, yet few deeply exposed arc sections are available for direct study. The Gangdese magmatic arc, southern Tibet, formed during the Mesozoic subduction of Neo-Tethyan oceanic lithosphere and Cenozoic collision between the Indian and Asian continents, and represent juvenile continental crust. However, the petrological components and compositions of the lower crust of the Gangdese arc remain unknown. Based on detailed geological mapping, we conducted a systemic geochemical, geochronological and zircon Hf isotopic study of well-exposed high-grade metamorphic and migmatitic rocks from the lower crust of the eastern Gangdese arc. The results obtained show that Late Cretaceous garnet amphibolites, dioritic and granitic gneisses, and Paleocene–Eocene garnet amphibolites and granitic gneisses are the main components of the Gangdese lower arc crust. These meta-intrusive rocks witnessed a long period of magmatic, and metamorphic and anatectic processes from the Middle Jurassic to the Late Eocene, and have chemical compositions that range from ultramafic to felsic, with an average SiO₂ content of 57.61 wt% and Mg# value of 0.49. These new data indicate firstly that the Gangdese lower arc crust has an overall intermediate composition and typical feature of juvenile crusts, and therefore supports the recent proposition that continental lower crusts are relatively felsic in composition, instead of mafic. We consider that the downward transport of felsic intrusives and associated sedimentary rocks into the deep crustal levels and subsequent partial melting resulted in componential and compositional changes of the Gangdese arc lower crust over time. This is a potential key mechanism in transforming primary lower arc crust to mature continental lower crust for the magmatic arcs with a complete growth history.     

Ultrahigh-pressure eclogite transformed from mafic granulite in the Dabie orogen, east-central China

Although ultrahigh‐pressure (UHP) metamorphic rocks are present in many collisional orogenic belts, almost all exposed UHP metamorphic rocks are subducted upper or felsic lower continental crust with minor mafic boudins. Eclogites formed by subduction of mafic lower continental crust have not been identified yet. Here an eclogite occurrence that formed during subduction of the mafic lower continental crust in the Dabie orogen, east‐central China is reported. At least four generations of metamorphic mineral assemblages can be discerned: (i) hypersthene + plagioclase ± garnet; (ii) omphacite + garnet + rutile + quartz; (iii) symplectite stage of garnet + diopside + hypersthene + ilmenite + plagioclase; (iv) amphibole + plagioclase + magnetite, which correspond to four metamorphic stages: (a) an early granulite facies, (b) eclogite facies, (c) retrograde metamorphism of high‐pressure granulite facies and (d) retrograde metamorphism of amphibolite facies. Mineral inclusion assemblages and cathodoluminescence images show that zircon is characterized by distinctive domains of core and a thin overgrowth rim. The zircon core domains are classified into two types: the first is igneous with clear oscillatory zonation ± apatite and quartz inclusions; and the second is metamorphic containing a granulite facies mineral assemblage of garnet, hypersthene and plagioclase (andesine). The zircon rims contain garnet, omphacite and rutile inclusions, indicating a metamorphic overgrowth at eclogite facies. The almost identical ages of the two types of core domains (magmatic = 791 ± 9 Ma and granulite facies metamorphic zircon = 794 ± 10 Ma), and the Triassic age (212 ± 10 Ma) of eclogitic facies metamorphic overgrowth zircon rim are interpreted as indicating that the protolith of the eclogite is mafic granulite that originated from underplating of mantle‐derived magma onto the base of continental crust during the Neoproterozoic (c. 800 Ma) and then subducted during the Triassic, experiencing UHP eclogite facies metamorphism at mantle depths. The new finding has two‐fold significance: (i) voluminous mafic lower continental crust can increase the average density of subducted continental lithosphere, thus promoting its deep subduction; (ii) because of the current absence of mafic lower continental crust in the Dabie orogen, delamination or recycling of subducted mafic lower continental crust can be inferred as the geochemical cause for the mantle heterogeneity and the unusually evolved crustal composition.     

Petrological Investigations and Zircon U‐Pb Dating of High Pressure Felsic Granulites from the Yushugou Complex,South Tianshan,China

As a window of insight into the lower crust, high pressure granulite has received much attention since last decade. Yushugou high pressure granulite-peridotite Complex was located in the northeast margin of Southern Tianshan, NW China. Previous ideas agreed that the peridotite unit in Yushugou, combined with the ultramafic rocks in Tonghuashan and Liuhuangshan, represent an ophiolite belt. However, the metamorphic evolution and tectonic mechanism of the Yushugou high pressure(HP) granulite remain controversial. Petrological investigations and phase equilibrium modelling for two representative felsic granulite samples suggest two stages metamorphism of the rocks in Yushugou Complex. Granulite facies metamorphism(Stage Ⅰ) with P-T conditions of 9.8–10.4 kbar at 895–920°C was recorded by the porphyroblastic garnet core; HP granulite facies metamorphism(Stage Ⅱ) shows P-T conditions of 13.2–13.5 kbar at 845–860°C, based on the increasing grossular and decreasing pyrope contents of garnet rims. The Yushugou HP felsic granulites have recorded an anticlockwise P-T path, characterized by the temperature decreasing and pressure increasing simultaneously. The LA-ⅠCP-MS isotopic investigations on zircons from the felsic granulite show that the protolith ages of the granlulites are ～430 Ma, with two age groups of ～390 Ma and 340–350 Ma from the metamorphic rims of zircon, indicating the Stage Ⅰ and Ⅱ metamorphic events, respectively. A tectonic model was proposed to interpret the processes. The investigated felsic granulite was derived from deep rooted hanging wall, with Stage Ⅰ granulite facies metamorphism of ～390 Ma, which may be related to the Devonian arc magmatic intrusion; Stage Ⅱ HP granulite facies metamorphism(340–350 Ma) may due to the involvement of being captured into the subducting slab and experienced the high pressure metamorphism.     

Geochronological constraints on the timing of granitoid magmatism, metamorphism and post-metamorphic cooling in the Hercynian crustal cross-section of Calabria

Exposed cross‐sections of the continental crust are a unique geological situation for crustal evolution studies, providing the possibility of deciphering the time relationships between magmatic and metamorphic events at all levels of the crust. In the cross‐section of southern and northern Calabria, U–Pb, Rb–Sr and K–Ar mineral ages of granulite facies metapelitic migmatites, peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late‐Hercynian peak metamorphism and granitoid magmatism as well as on the post‐metamorphic cooling. Monazite from upper crustal amphibolite facies paragneisses from southern Calabria yields similar U–Pb ages (295–293±4 Ma) to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust (300±10 Ma). Monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303–302±0.6 Ma. Zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290±2 Ma, which may be interpreted as the minimum age for metamorphism or intrusion. U–Pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern Calabria (Sila) also point to a near‐synchronism of peak metamorphism and intrusion at 304–300±0.4 Ma. At the end of the granulite facies metamorphism, the lower crustal rocks were uplifted into mid‐crustal levels (10–15 km) followed by nearly isobaric slow cooling (c. 3 °C Ma^?1) as indicated by muscovite and biotite K–Ar and Rb–Sr data between 210±4 and 123±1 Ma. The thermal history is therefore similar to that of the lower crust of southern Calabria. In combination with previous petrological studies addressing metamorphic textures and P–T conditions of rocks from all crustal levels, the new geochronological results are used to suggest that the thermal evolution and heat distribution in the Calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late‐Hercynian orogeny.     

U–Pb and Hf isotopic analysis of zircon in lower crustal xenoliths from the Navajo volcanic field: 1.4 Ga mafic magmatism and metamorphism beneath the Colorado Plateau

Zircon from lower crustal xenoliths erupted in the Navajo volcanic field was analyzed for U–Pb and Lu–Hf isotopic compositions to characterize the lower crust beneath the Colorado Plateau and to determine whether it was affected by ∼1.4 Ga granitic magmatism and metamorphism that profoundly affected the exposed middle crust of southwestern Laurentia. Igneous zircon in felsic xenoliths crystallized at 1.73 and 1.65 Ga, and igneous zircon in mafic xenoliths crystallized at 1.43 Ga. Most igneous zircon has unradiogenic initial Hf isotopic compositions (ɛ_Hf=+4.1–+7.8) and 1.7–1.6 Ga depleted mantle model ages, consistent with 1.7–1.6 Ga felsic protoliths being derived from “juvenile” Proterozoic crust and 1.4 Ga mafic protoliths having interacted with older crust. Metamorphic zircon grew in four pulses between 1.42 and 1.36 Ga, at least one of which was at granulite facies. Significant variability within and between xenoliths in metamorphic zircon initial Hf isotopic compositions (ɛ_Hf=−0.7 to +13.6) indicates growth from different aged sources with diverse time-integrated Lu/Hf ratios. These results show a strong link between 1.4 Ga mafic magmatism and granulite facies metamorphism in the lower crust and granitic magmatism and metamorphism in the exposed middle crust.     

南阿尔金高压-超高温麻粒岩变质作用:大陆地壳超深俯冲与折返过程的记录

南阿尔金造山带是目前报道的具有最深俯冲记录的大陆超高压变质带,其内出露有高压-超高温麻粒岩,它们对深入理解大陆地壳岩石超深俯冲与折返过程具有重要意义.介绍了对南阿尔金巴什瓦克地区长英质麻粒岩和基性麻粒岩的岩相学、矿物化学、相平衡模拟及锆石U-Pb年代学研究成果.其中基性麻粒岩主要记录了深俯冲大陆地壳折返过程的变质演化:包括高压榴辉岩相、高压-超高温麻粒岩相、低压-超高温麻粒岩相及随后的近等压降温演化阶段;长英质麻粒岩除了记录与基性麻粒岩相似的折返过程外,还记录了从角闪岩相到超高压榴辉岩相的进变质演化过程.结合已有研究资料,确定超高压榴辉岩阶段峰期条件> 7~9 GPa和>1 000℃,可达到斯石英稳定域.锆石年代学显示两种岩石类型的原岩和变质年龄均分别在900 Ma和500 Ma左右.变质作用与年代学研究表明,南阿尔金大陆地壳岩石在早古生代发生超深俯冲至200~300 km后,折返至加厚地壳底部发生高压-超高温变质作用,随后被快速抬升至地壳浅部发生低压-超高温变质作用并经历迅速冷却.      

On the origin and fluid content of some rare crustal xenoliths and their bearing on the structure and evolution of the crust beneath the Bakony–Balaton Highland Volcanic Field (W-Hungary)

Rarely occurring clinopyroxene-plagioclase bearing, felsic granulite and skarn xenoliths were studied from the mantle and crustal xenolith-bearing alkaline basaltic and pyroclastic localities of the Bakony–Balaton Highland Volcanic Field (W-Hungary). Geobarometry and geothermometry of the xenoliths made it possible to categorise them in three groups according to their depth of formation. The first group formed in the lower crust together with mafic and metasedimentary granulites. The second group represents magmatic intrusions of the middle crust, and the third one comprises contact metamorphic rocks of relatively shallow origin. The calculated pressure difference from the core and rim compositions of plagioclase and clinopyroxene as well as garnet breakdown reactions in some xenoliths show evidence for pressure decrease due to crustal thinning both in lower crustal and middle crustal xenoliths during formation of the Pannonian Basin. Fluid inclusion studies reveal the dominance of the CO₂-rich fluids in the whole crustal section in contrast with fluids found in mafic garnet-bearing xenoliths. Crustal stratigraphy was constructed for the periods prior to the extension and after the extension on the basis of geobarometry and geophysical data. On the basis of mineral stabilities and geothermo-barometry, we estimated that the pre-extensional thickness of the lower and upper crust may have been 27–34 and 26–28?km, respectively. Comparison of pre-extensional and present-day thickness of the lower and upper crust indicate that thinning affected both the lower and the upper portion of the crust but on a different scale. The calculated thinning factors are between 2.25 and 3.4 for the lower crust and 1.3–1.56 for the upper crust.     

Fluid inclusion studies on the Koraput Alkaline Complex,Eastern Ghats Province,India: Implications for mid-Neoproterozoic granulite facies metamorphism and exhumation

Following ultrahigh temperature granulite metamorphism at ∼1 Ga, the Eastern Ghats Province of India was intruded by the Koraput Alkaline Complex, and was subsequently re-metamorphosed in the granulite facies in the mid-Neoproterozoic time. Fluid inclusion studies were conducted on silica undersaturated alkali gabbro and syenites in the complex, and a pre-metamorphic pegmatitic granite dyke that intrudes it. High density (1.02–1.05 g/cc), pseudo-secondary pure CO₂ inclusions are restricted to metamorphic garnets within the gabbro and quartz within the granite, whereas moderate (∼0.92–0.95 g/cc) and low density (∼0.75 g/cc) secondary inclusions occur in garnet, magmatic clinopyroxene, plagioclase, hornblende and quartz. The isochores calculated for high density pseudo-secondary inclusions pass very close to the peak metamorphic window (∼8 kbar, 750 °C), and are interpreted to represent the fluid present during peak metamorphism that was entrapped by the growing garnet. Microscopic round inclusions of undigested, relict calcite in garnet suggest that the CO₂ present during metamorphism of the complex was internally derived through carbonate breakdown. Pure to low salinity (0.00–10.1 wt% NaCl equivalent) aqueous intra-/intergranular inclusions showing unimodal normal distribution of final ice-melting temperature (T_m) and temperature of homogenization (T_h) are present only in quartz within the granite. These represent re-equilibrated inclusions within the quartz host that were entrapped at the metamorphic peak. Rare, chemically precipitated graphite along the walls of carbonic inclusions is interpreted as a post-entrapment reaction product formed during decompression. The fluid inclusion evidence is consistent with rapid exhumation of a thickened lower crust following the mid-Neoproterozoic granulite facies metamorphic event. The study suggests that mantle CO₂, transported by alkaline magma into the crust, was locked up within carbonates and released during granulite metamorphism.     

Granulite and pyroxenite xenoliths from the Deccan Trap: insight into the nature and composition of the lower lithosphere beneath cratonic India

Granulite and pyroxenite xenoliths in lamprophyre dykes intruded during the waning stage of Deccan Trap volcanism are derived from the lower crust beneath the Dharwar craton of Western India. The xenolith suite consists of plagioclase-poor mafic granulites (55% of the total volume of xenoliths), plagioclase-rich felsic granulites (25%), and ultramafic pyroxenites and websterites (20%) with subordinate wehrlites. Rare spinel peridotite xenoliths are also present, representing mantle lithosphere. The high Mg #, low SiO₂/Al₂O₃ and low Nb/La (<1) ratios suggest that the protoliths of the mafic granulites broadly represent cumulates of sub-alkaline magmas. All of the granulites are peraluminous and light rare-earth element-enriched. The felsic granulites may have resulted from anatexis of the mafic lower crustal rocks; thus, the mafic granulites are enriched in Sr whereas the felsic ones are depleted. Composite xenoliths consisting of mafic granulites traversed by veins of pyroxenite indicate intrusion of the granulitic lower crust by younger pyroxenites. Petrography and geochemistry of the latter (e.g. presence of phlogopite) indicate the metasomatised nature of the deep crust in this region.Thermobarometric estimates from phase equilibria indicate equilibration conditions between 650 and 1200 °C, 0.7-1.2 GPa suggestive of lower crustal environments. These estimates provide a spatial context for the sampled lithologies thereby placing constraints on the interpretation of geophysical data. Integration of xenolith-derived P-T results with Deep Seismic Soundings (DSS) data suggests that the pyroxenites and websterites are transitional between the lower crust and the upper mantle. A three-layer model for the crust in western India, derived from the xenoliths, is consistent with DSS data. The mafic nature of this hybrid lower crust contrasts with the felsic lower crustal composition of the south Indian granulite terrain.     

Implication of corona formation in a metatroctolite to the granulite facies overprint of HP–UHP rocks in the Moldanubian Zone (Bohemian Massif)

Corona and inclusion textures of a metatroctolite at the contact between felsic granulite and migmatites of the Gföhl Unit from the Moldanubian Zone provide evidence of the magmatic and metamorphic evolution of the rocks. Numerous diopside inclusions (1–10 μm, maximum 20 μm in size) in plagioclase of anorthite composition represent primary magmatic textures. Triple junctions between the plagioclase grains in the matrix are occupied by amphibole, probably pseudomorphs after clinopyroxene. The coronae consist of a core of orthopyroxene, with two or three zones (layers); the innermost is characterized by calcic amphibole with minor spinel and relicts of clinopyroxene, the next zone consists of symplectite of amphibole with spinel, sapphirine and accessory corundum, and the outermost is formed by garnet and amphibole with relicts of spinel. The orthopyroxene forms a monomineralic aggregate that may contain a cluster of serpentine in the core, suggesting its formation after olivine. Based on mineral textures and thermobarometric calculations, the troctolite crystallized in the middle to lower crust and the coronae were formed during three different metamorphic stages. The first stage relates to a subsolidus reaction between olivine and anorthite to form orthopyroxene. The second stage involving amphibole formation suggests the presence of a fluid that resulted in the replacement of igneous orthopyroxene and governed the reaction orthopyroxene + anorthite = amphibole + spinel. The last stage of corona formation with amphibole + spinel + sapphirine indicates granulite facies conditions. Garnet enclosing spinel, and its occurrence along the rim of the coronae in contact with anorthite, suggests that its formation occurred either during cooling or both cooling and compression but still at granulite facies conditions. The zircon U–Pb data indicate Variscan ages for both the troctolite crystallization (c. 360 Ma) and corona formation during granulite facies metamorphism (c. 340 Ma) in the Gföhl Unit. The intrusion of troctolite and other Variscan mafic and ultramafic rocks is interpreted as a potential heat source for amphibolite–granulite facies metamorphism that led to partial re‐equilibration of earlier high‐ to ultrahigh‐P metamorphic rocks in the Moldanubian Zone. These petrological and geochronological data constrain the formation of HP–UHP rocks and arc‐related plutonic complex to westward subduction of the Moldanubian plate during the Variscan orogeny. After exhumation to lower and/or middle crust, the HP–UHP rocks underwent heating due to intrusion of mafic and ultramafic magma that was generated by slab breakoff and mantle upwelling.     

Plume — Lid interactions during the Archean and implications for the generation of early continental terranes

Many Archean terranes are interpreted to have a tectonic and metamorphic evolution that indicates intra-crustal reorganization driven by lithospheric-scale gravitational instabilities. These processes are associated with the production of a significant amount of felsic and mafic crust, and are widely regarded to be a consequence of plume-lithosphere interactions. The juvenile Archean felsic crust is made predominantly of rocks of the tonalite–trondhjemite–granodiorite (TTG) suite, which are the result of partial melting of hydrous metabasalts. The geodynamic processes that have assisted the production of juvenile felsic crust, are still not well understood. Here, we perform 2D and 3D numerical simulations coupled with the state-of-the-art of petrological thermodynamical modelling to study the tectonic evolution of a primitive Archean oceanic plateau with particular regard on the condition of extraction of felsic melts. In our numerical simulations, the continuous emplacement of new, dry mafic intrusions and the extraction of the felsic melts, generate an unstable lower crust which drips into the mantle soon after the plume arrival. The subsequent tectonic evolution depends on the asthenosphere T_P. If the T_P is high enough (≥ 1500 ^∘C) the entire oceanic crust is recycled within 2 Myrs. By contrast at low T_P, the thin oceanic plateau slowly propagates generating plate-boundary like features.     

Experimental Study of the Melting Reaction and Genetic Mechanism of Mineral Phase Transformation in Granulite Facies Metamorphism

The high-temperature and high-pressure experiment on natural block rock indicates that dehydration-melting of hydrous biotite (Bi) and partial melting of felsic minerals in garnet-biotite-plagioclase gneiss are mainly controlled by temperature, while mineral phase transformation is not only controlled by temperature-pressure conditions but also genetically associated with hydrous mineral dehydration-melting and partial melting of felsic minerals. According to the characteristics of biotite dehydration-melting and garnet transformation reaction, three stages may be distinguished: (1) when the experimental temperature is 700℃, biotite transforms to ilmenite (Ilm) + magnetite (Mt) + H2O and garnet to magnetite (Mt); (2) when the temperature is 730-760℃, biotite is dehydrated and melted and transformed into K2O-rich melt + Ilm + Mt, and garnet, into hypersthene (Hy) + cordierite (Crd); (3) when the temperature is up to or higher than 790℃, biotite is dehydrated and melted and transformed into melt + Hy +     

The deformation and recrystallization of biotite in the Woodroffe Thrust mylonite zone

The deformation and recrystallization microstructures in biotite from the Woodroffe Thrust mylonites are described and interpreted. The degree of strain causing recrystallization and the nucleation mechanisms differ across the mylonite zone. These differences are associated with the contrast in water content between the granulite and amphibolite facies felsic gneisses on either side of the zone. p]In moderately mylonitized granulite facies felsic gneisses (0.1–0.6% H₂O) subgrains form in intensely deformed host biotite and recrystallization mechanisms involve subgrain rotation both on host grain boundaries and associated with kink band bulge. In the amphibolite facies felsic gneisses (0.9–1.2% H₂O) the biotite recrystallizes by a mechanism involving localized internal kinking of the host and subsequent migration of high angle boundaries generated on the kink limbs. This combined with rotation due to the concurrent deformation generates high angle grain boundaries around the entire original kink limb and thus a new grain.     

Crustal Growth by Magmatic Accretion Constrained by Metamorphic P-T Paths and Thermal Models of the Kohistan Arc, NW Himalayas

Magmatic accretion is potentially an important mechanism inthe growth of the continental crust and the formation of granulites.In this study, the thermal evolution of a magmatic arc in responseto magmatic accretion is modeled using numerical solutions ofthe one-dimensional heat conduction equation. The initial andboundary conditions used in the model are constrained by geologicalobservations made in the Kohistan area, NW Himalayas. Takingconsideration of the preferred intrusion locations for basalticmagmas, we consider two plausible modes of magmatic accretion:the first involves the repeated intrusion of basalt at mid-crustaldepths (‘intraplate model’), and the second evaluatesthe simultaneous intrusion of basalt and picrite at mid-crustaldepths and the base of the crust respectively (‘double-platemodel’). The results of the double-plate model accountfor both the inferred metamorphic P–T paths of the Kohistanmafic granulites and the continental geotherm determined frompeak P–T conditions observed for granulite terranes. Thedouble-plate model may be applicable as a key growth processfor the production of thick mafic lower crust in magmatic arcs. KEY WORDS: thermal model; magmatic underplating; P–T path; granulite; lower crust     

Oxide and sulphide isograds along a Late Archean, deep-crustal profile in Tamil Nadu, south India

Oxide–sulphide–Fe–Mg–silicate and titanite–ilmenite textures as well as their mineral compositions have been studied in felsic and intermediate orthogneisses across an amphibolite (north) to granulite facies (south) traverse of lower Archean crust, Tamil Nadu, south India. Titanite is limited to the amphibolite facies terrane where it rims ilmenite or occurs as independent grains. Pyrite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade. Pyrrhotite is confined to the high‐grade granulites. Ilmenite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade and occurring primarily as hemo‐ilmenite in the high‐grade granulite facies rocks. Magnetite is widespread throughout the traverse and is commonly associated with ilmenite. It decreases in abundance with increasing metamorphic grade. In the granulite facies zone, reaction rims of magnetite + quartz occur along Fe–Mg silicate grain boundaries. Magnetite also commonly rims or is associated with pyrite. Both types of reaction rims represent an oxidation effect resulting from the partial subsolidus reduction of the hematite component in ilmenite to magnetite. This is confirmed by the presence of composite three oxide grains consisting of hematite, magnetite and ilmenite. Magnetite and magnetite–pyrite micro‐veins along silicate grain boundaries formed over a wide range of post‐peak metamorphic temperatures and pressures ranging from high‐grade SO₂ to low‐grade H₂S‐dominated conditions. Oxygen fugacities estimated from the orthopyroxene–magnetite–quartz, orthopyroxene–hematite–quartz, and magnetite–hematite buffers average 2.5 log units above QFM. It is proposed that the trends in mineral assemblages, textures and composition are the result of an external, infiltrating concentrated brine containing an oxidizing component such as CaSO₄ during high‐grade metamorphism later acted upon by prograde and retrograde mineral reactions that do not involve an externally derived fluid phase.     

Fluid-assisted granulite metamorphism: A continental journey

Lower crustal granulites, which constitute the base of all continents, belong to two series: high-pressure granulites generated by crustal thickening (subduction) and (ultra)high-temperature granulites associated with crustal extension. Fluid inclusions and metasomatic features indicate that the latter were metamorphosed in the presence of low-water activity fluids (high-density CO₂ and brines), which have invaded the lower crust at peak metamorphic conditions (fluid-assisted granulite metamorphism). High-pressure and (ultra)high-temperature granulites commonly occur along elongated paired belts. They were formed, from the early Proterozoic onwards, during a small number of active periods lasting a few hundreds of m.y. These periods were separated from each other by longer periods of stability. Each period ended with the formation of a supercontinent whose amalgamation coincided with low- to medium pressure (ultra)high-temperature granulite metamorphism, immediately before continental break-up. It is proposed that large quantities of mantle-derived CO₂ stored in the lower crust at the final stage of supercontinent amalgamation, are released into the hydro- and atmosphere during breakup of the supercontinent. Fluid-assisted granulite metamorphism, therefore, appears to be an important mechanism for transferring deep mantle fluids towards the Earth's surface. Possible consequences were, for example, the sudden end of Proterozoic glaciations, as well as the post-Cambrian explosion of life.     

Granulite facies xenoliths from the Yuhuashan complex,central Jiangxi,South China: constraints on Late Palaeozoic orogeny and middle‐lower crust components

The Cretaceous Yuhuashan igneous complex contains abundant xenoliths of high‐grade metamorphic rocks, with the assemblage garnet ± hypersthene + biotite + plagioclase + K‐feldspar + quartz. The biotite in these samples has high TiO₂ (>3.5%), indicating high‐T metamorphism (623–778 °C). P–T calculations for two felsic granulites indicate that the peak metamorphism took place at 880–887 °C and 0.64–0.70 GPa, in the low pressure/high temperature (LP‐HT) granulite facies. Phase equilibrium modelling gives equilibrium conditions for the peak assemblage of a felsic granulite of >0.6 GPa and >840 °C, consistent with the P–T calculations, and identifies an anticlockwise P–T–t path. LA‐ICPMS U–Pb dating of metamorphic and detrital zircon from one xenolith reveals that the granulite facies metamorphism took place at 273.6 ± 2.2 Ma, and the protolith was a sedimentary rock deposited later than 683 Ma. This represents the first Late Palaeozoic (Variscan) granulite facies event identified in the South China Block (SCB). Coupled with other geological observations, the LP‐HT metamorphic conditions and anticlockwise P–T–t path suggest that Variscan metamorphism probably occurred in a post‐orogenic or intraplate extensional tectonic setting associated with the input of external heat, related to the underplating of mantle‐derived magma. Based on P–T estimates and the comparison of the protolith composition with mid‐ to low‐grade metamorphic rocks in the area, it is suggested that the mid‐lower crust under the Xiangshan–Yuhuashan area consists mainly of these felsic granulites and gneisses, whose protoliths were probably subducted to these depths during the Early Palaeozoic orogeny in the SCB, and underwent two episodes of metamorphism during Early Palaeozoic and Late Palaeozoic time.