Shear Zone-hosted Migmatites (Eastern India): the Role of Dynamic Melting in the Generation of REE-depleted Felsic Melts, and Implications for Disequilibrium Melting

In the Ranmal migmatite complex, non-anatectic foliated graniteprotoliths can be traced to polyphase migmatites. Structural–microtexturalrelations and thermobarometry indicate that syn-deformationalsegregation–crystallization of in situ stromatic and diatexiteleucosomes occurred at 800°C and 8 kbar. The protolith,the neosome, and the mesosome comprise quartz, K-feldspar, plagioclase,hornblende, biotite, sphene, apatite, zircon, and ilmenite,but the modal mineralogy differs widely. The protolith compositionis straddled by element abundances in the leucosome and themesosome. The leucosomes are characterized by lower CaO, FeO+MgO,mg-number, TiO₂ , P₂O₅ , Rb, Zr and total rare earth elements(REE), and higher SiO₂ , K₂O, Ba and Sr than the protolith andthe mesosome, whereas Na₂O and Al₂O₃ abundances are similar.The protolith and the mesosome have negative Eu anomalies, butprotolith-normalized abundances of REE-depleted leucosomes showpositive Eu anomalies. The congruent melting reaction for leucosomeproduction is inferred to be 0·325 quartz+0·288K-feldspar+0·32 plagioclase+0·05 biotite+0·014hornblende+0·001 apatite+0·001 zircon+0·002sphene=melt. Based on the reaction, large ion lithophile element,REE and Zr abundances in model melts computed using dynamicmelting approached the measured element abundances in leucosomesfor >0·5 mass fraction of unsegregated melts withinthe mesosome. Disequilibrium-accommodated dynamic melting andequilibrium crystallization of melts led to uniform plagioclasecomposition in migmatites and REE depletion in leucosome. KEY WORDS: migmatite; REE; trace element; partial melting; P–T conditions     

Dehydration Melting of Metabasalt at 8-32 kbar: Implications for Continental Growth and Crust-Mantle Recycling

We report the results of partial melting experiments between8 and 32 kbar, on four natural amphibolites representative ofmetamorphosed Archean tholeiite (greenstone), high-alumina basalt,low-potassium tholeiite and alkali-rich basalt. For each rock,we monitor changes in the relative proportions and compositionof partial melt and coexisting residual (crystalline) phasesfrom 1000 to 1150C, within and beyond the amphibole dehydrationreaction interval. Low percentage melts coexisting with an amphiboliteor garnet amphibolite residue at 1000–1025C and 8–16kbar are highly silicic (high-K₂O granitic at 5%; melting, low-Al₂O₃trondhjemitic at 5–10%). Greater than 20% melting is onlyachieved beyond the amphibole-out phase boundary. Silicic tointermediate composition liquids (high-Al₂O₃ trondhjemitic-tonalitic,granodioritic, quartz dioritic, dioritic) result from 20–40%melting between 1050 and 1100C, leaving a granulite (plagioclase+ clinopyroxene orthopyroxene olivine) residue at 8 kbarand garnet granulite to eclogite (garnet + clinopyroxene) residuesat 12–32 kbar. Still higher degrees of melting ( 40–60%)result in mafic liquids corresponding to low-MgO, high-Al₂O₃basaltic and basaltic andesite compositions, which coexist withgranulitic residues at 8 kbar and edogitic or garnet granulitic(garnet + clinopyroxene + plagioclase orthopyroxene) residuesat higher pressures (12–28 kbar). As much as 40% by volumehigh-Al₂O₃ trondhjemitic-tonalitic liquid coexists with an eclogiticresidue at 1100–1150C and 32 kbar. The experimental datasuggest that the Archean tonalite-trondhjemite-granodiorite(TTG) suite of rocks, and their Phanerozoic equivalents, thetonalite-trondhjemite-dacite suite (including ‘adakites’and other Na-rich granitoids), can be generated by 10–40%melting of partially hydrated metabasalt at pressures abovethe garnet-in phase boundary (12 kbar) and temperatures between1000 and 1100C. Anomalously hot and/or thick metabasaltic crustis implied. Although a rare occurrence along modern convergentplate margins, subductionrelated melting of young, hot oceaniccrust (e.g. ocean ridges) may have been an important (essential)element in the growth of the continental crust in the Archean,if plate tectonic processes were operative. Coupled silicicmelt generation-segregation and mafic restite disposal may alsooccur at the base of continental or primitive (sub-arc?) crust,where crustal overthickening is a consequence of underplatingand overaccretion of mafic magmas. In either setting, net growthof continental crust and crustmantle recycling may be facilitatedby relatively high degrees of melting and extreme density contrastsbetween trondhjemitictonalitic liquids and garnet-rich residues.Continuous chemical trends are apparent between the experimentalcrystalline residues, and mafic migmatites and garnet granulitexenoliths from the lower crust, although lower-crustal xenolithsin general record lower temperatures (600–900C) and pressures(5–13 kbar) than corresponding residual assemblages fromthe experiments. However, geo-thermobarometry on eclogite xenolithsin kimberlites from the subcontinental mantle indicates conditionsappropriate for melting through and beyond the amphibole reactioninterval and the granulite-eclogite transition. If these samplesrepresent ancient (eclogitized) remnants of subducted or otherwisefoundered basaltic crust, then the intervening history of theirprotoliths may in some cases include partial melting. KEY WORDS: dehydration melting; metabasalt; continental growth; crust–mantle recycling ^*Corresponding author. Present address: Mineral Physics Institute and Center for High Pressure Research, Department of Earth and Space Sciences, State University of New York at Stony Brook, Stony Brook, NY 11794, USA     

Synmetamorphic High-Density Carbonic Fluids in the Lower Crust: Evidence from the Nilgiri Granulites, Southern India

High-density CO² inclusions occur abundantly in granulite fadesrocks (age of metamorphism 2–5b.y.) of the Nilgiri massif,southern India. The chronology of carbonic inclusions in thewidespread enderbitic granulites studied in relation to thedevelopment of micro-textures and mineral assemblages indicatesthat randomly oriented, negative-crystal-shaped CO² inclusions(4–20 µim) in garnet and quartz grains (qtz I) armouredby garnet entrap syn-peak-metamorphic pore fluids. The moreabundant trail-bound CO₂ inclusions in the deformed, polygonized,and partially recrystallized matrix quartz grains (qtz II andIII) and plagioclase grains were formed in connection with astage of compressional deformation and subsequent annealingrelated to the development of the late-Proterozoic Bhavani shearzone. These inclusions resulted from local re-equilibrationof the former peak-granulitic carbonic inclusions and re-entrapmentof released fluids. The presence of pure CO₂ in all the inclusionsis confirmed by microthermometric data and laser-excited Ramanmicrospectrome-try. Temperatures of homogenization (liquid phase)are in the range of 50 to +20C, and the corresponding CO₂ densitiesare between 1.154 and 0–807 g/cm3. Mineralogical thermobarometry on the enderbitic granulites documentsa continuous gradient of near-peak metamorphic conditions from750C, 9–10 kb in the northern part to 73OC, 7 kb inthe southwestern part of the Nilgiri massif. Uniform P, Testimates(600–650 C, 6–7 kb) for late coronitic garnet +quartz assemblages in enderbites and metadolerites indicatethat differential uplift of the massif to mid-crustal levelswas accomplished before late compressional deformation. In conformity,carbonic inclusions in quartz II and III are characterized byuniformly high density (1.154–1.08 g/cm3). In contrast,early carbonic inclusions in garnet and quartz I preserve thedensity contrast reflecting the regional P, T gradient duringnear-peak metamorphic fluid entrapment. The fluid inclusionsys-tematics indicate ‘near-isochoric’ uplift ofthe northern high-P domain, but near-isobaric cooling of thesouthwestern low-P domain. The carbonic fluids are thought tohave been derived either from internal sources during dehydration-meltingprocesses or from freezing synmetamorphic intrusives into thelower crust.     

Contrasting Episodes of Regional Granulite-Facies Metamorphism in Enclaves and Host Gneisses from the Aravalli Delhi Mobile Belt, NW India

The Aravalli–Delhi Mobile Belt in the northwestern partof India demonstrates how granulite enclaves and their hostgneisses can be utilized to unravel multistage metamorphic historiesof orogenic belts, using three suites of metamorphic rocks:(1) an enclave of pelitic migmatite gneiss–leptynite gneiss;(2) metamorphosed megacrystic granitoids, intrusive into theenclave; (3) host tonalite–trondhjemite–granodiorite(TTG) gneisses associated with an interlayered sequence of garnetiferousmetabasite and psammo-pelitic schist, locally migmatitic. Basedon integrated structural, petrographic, mineral compositional,geothermobarometric studies and P–T pseudosection modellingin the systems NCKFMASH and NCFMASH, we record three distincttectonothermal events: an older, medium-pressure granulite-faciesmetamorphic event (M₁) in the sillimanite stability field, whichis registered only in the enclave, a younger, kyanite-gradehigh-pressure granulite-facies event (M₂), common to all thethree litho-associations, and a terminal amphibolite-faciesmetamorphic overprint (M₃). The high-P granulite facies eventhas a clockwise P–T loop with a well-constrained prograde,peak (M₂, P 12–15 kbar, T 815°C) and retrograde (M_2R,6·1 kbar, T 625°C) metamorphic history. M₃ is recordedparticularly in late shear zones. When collated with availablegeochronological data, the metamorphic P–T conditionsprovide the first constraint of crustal thickening in this belt,leading to the amalgamation of two crustal blocks during a collisionalorogeny of possible Early Mesoproterozoic age. M₃ reactivationis inferred to be of Grenvillian age. KEY WORDS: Northwestern India; polycyclic granulite enclave; pseudosection; high-pressure metamorphism; P–T path     

Melting of Biotite + Plagioclase + Quartz Gneisses: the Role of H2O in the Stability of Amphibole

Biotite + plagioclase + quartz (BPQ) is a common assemblagein gneisses, metasediments and metamorphosed granitic to granodioriticintrusions. Melting experiments on an assemblage consistingof 24 vol. % quartz, 25 vol. % biotite (XMg = 0·38–0·40),42 vol. % plagioclase (An_26–29), 9 vol. % alkali feldsparand minor apatite, titanite and epidote were conducted at 10,15 and 20 kbar between 800 and 900°C under fluid-absentconditions and with small amounts (2 and 4 wt %) of water addedto the system. At 10 kbar when 4 wt % of water was added tothe system the biotite melting reaction occurred below 800°Cand produced garnet + amphibole + melt. At 15 kbar the meltingreaction produced garnet + amphibole + melt with 2 wt % addedwater. At 20 kbar the amphibole occurred only at high temperature(900°C) and with 4 wt % added water. In this last case themelting reaction produced amphibole + clinopyroxene ±garnet + melt. Under fluid-absent conditions the melting reactionproduced garnet + plagioclase II + melt and left behind a plagioclaseI ± quartz residuum, with an increase in the modal amountof garnet with increasing pressure. The results show that itis not possible to generate hornblende in such compositionswithout the addition of at least 2–4 wt % H₂O. This reflectsthe fact that conditions of low aH₂O may prevent hornblendefrom being produced with peraluminous granitic liquids fromthe melting of biotite gneiss. Thus growth of hornblende inanatectic BPQ gneisses is an indication of addition of externalH₂O-rich fluids during the partial melting event. KEY WORDS: biotite; dehydration; gneisses; hornblende; melt     

Fluids in granulites of the Southern Marginal Zone of the Limpopo Belt, South Africa

The Southern Marginal Zone of the Limpopo Belt in South Africa is characterised by a granulite and retrograde hydrated granulite terrane. The Southern Marginal Zone is, therefore, perfectly suitable to study fluids during and after granulite facies metamorphism by means of fluid inclusions and equilibrium calculations. Isolated and clustered high-salinity aqueous and CO₂(-CH₄) fluid inclusions within quartz inclusions in garnet in metapelites demonstrate that these immiscible low H₂O activity fluids were present under peak metamorphic conditions (800-850 °C, 7.5-8.5 kbar). The absence of widespread high-temperature metasomatic alteration indicates that the brine fluid was probably only locally present in small quantities. Thermocalc calculations demonstrate that the peak metamorphic mineral assemblage in mafic granulites was in equilibrium with a fluid with a low H₂O activity (0.2-0.3). The absence of water in CO₂-rich fluid inclusions is due to either observation difficulties or selective water leakage. The density of CO₂ inclusions in trails suggests a retrograde P-T path dominated by decompression at T<600 °C. Re-evaluation of previously published data demonstrates that retrograde hydration of the granulites at 600 °C occurred in the presence of H₂O and CO₂-rich fluids under P-T conditions of 5-6 kbar and ~600 °C. The different compositions of the hydrating fluid suggest more than one fluid source.     

U–Pb zircon dating of the Gruf Complex: disclosing the late Variscan granulitic lower crust of Europe stranded in the Central Alps

Permian granulites associated with noritic intrusions and websterites are a common feature of the post-Variscan European crust. Such granulites are common in the Southern Alps (e.g. Ivrea Zone), but occur only in the Gruf Complex in the Central Alps. To understand the geotectonic significance of these granulites, in particular in the context of Alpine migmatisation, zircons from 15 high-grade samples have been U–Pb dated by SHRIMP II analysis. Oscillatory zoned zircons from charnockite sheets, interpreted as melts generated through granulite facies fluid-absent biotite melting at 920–940°C, yield ages of 282–260 Ma. Some of these zircons contain inclusions of opx, unequivocally attributable to the granulite facies, thus confirming a Permian age for the charnockites and associated granulites. Two samples from an enclave-rich orthogneiss sheet yield Cambrian and Ordovician zircon cores. Two deformed leucogranites and six ortho- and augengneisses, which compose two-thirds of the Gruf Complex, give zircon ages of 290–260 Ma. Most zircons have milky rims with ages of 34–29 Ma. These rims date the Alpine amphibolite facies migmatisation, an interpretation confirmed by directly dating a leucosome pocket from upper amphibolite facies metapelites. The Gruf charnockites associated with metre-scale schlieren and boudins of opx–sapphirine–garnet–granulites, websterites and gabbronorites can thus be identified as part of the post-Variscan European lower crust. A geotectonic reconstruction reveals that this piece of lower crust stranded in the (European) North upon rifting of the Neotethys, such contrasting the widespread granulite units in the Southern Alps. Emplacement of the Gruf lower crust into its present-day position occurred during migmatisation and formation of the Bergell Pluton in the aftermath of the breakoff of the European slab.     

Pleistocene Underplating and Metasomatism of the Lower Continental Crust: a Xenolith Study

Xenoliths from Engeln–Kempenich in the East Eifel volcanicfield (Germany) comprise gabbroic to ultramafic cumulates, andmeta-igneous and meta-sedimentary granulite- to amphibolite-facieslithologies. They provide evidence for Pleistocene heating andmetasomatism of the lower continental crust by mafic magmas.The metamorphic xenoliths were divided into three types: (1)primitive type P, which are little affected by metasomatic replacementstructures; (2) enriched type E1 defined by metasomatic replacementof primary pyroxene and garnet by pargasitic amphibole and biotite;(3) enriched type E2 defined by breakdown of hydrous phases.Type E rocks are geochemically related to type P and cumulatexenoliths by compositional trends. During modal metasomatism,type E rocks were oxidized. Type E1 rocks were typically enrichedin Rb, Th, U, Nb, K, light rare earth elements (LREE) and Zr,and E2 enriched in Rb, Th, U, Nb, K, REE, Zr, Ti and Y, relativeto type P rocks. Formation of the hydrous, chlorine-bearingphases amphibole and scapolite containing glass and fluid inclusionsin the E1 rocks provides evidence for a water and Cl-bearingfluid phase coexisting with silicate melt. Accordingly, we calculated10 mol % H₂O back into the CO₂-dominated fluid inclusions, inagreement with experimental data on the composition of a fluidphase coexisting with mafic alkaline melts at elevated pressure.Primary CO₂-dominated fluid inclusions coexisting with glassinclusions in metamorphic corona phases and neoblasts, and incumulate xenoliths, have overlapping densities. Fluid inclusionbarometry using the corrected densities indicates that bothcumulates and metamorphic xenoliths originated from the samedepth at 22–25 km (650 ± 50 MPa). This is interpretedas being a main magma reservoir level within the upper partof the lower crust close to the Conrad discontinuity, wherethe xenoliths represent wall-rocks. The Conrad discontinuityseparates an upper-crustal layer, consisting of preferentiallyductile granodioritic and tonalitic gneisses, and more brittlelower-crustal mafic granulites. The brittle–ductile transitionappears to be a preferred level of magma stagnation. KEY WORDS: continental lower crust; fluids; magma chamber; metasomatism; xenoliths     

Fluid inclusions in granulites, granulitized eclogites and garnet clinopyroxenites from the Dabie–Sulu terranes, eastern China

Minor granulites (believed to be pre-Triassic), surrounded by abundant amphibolite-facies orthogneiss, occur in the same region as the well-documented Triassic high- and ultrahigh-pressure (HP and UHP) eclogites in the Dabie–Sulu terranes, eastern China. Moreover, some eclogites and garnet clinopyroxenites have been metamorphosed at granulite- to amphibolite-facies conditions during exhumation. Granulitized HP eclogites/garnet clinopyroxenites at Huangweihe and Baizhangyan record estimated eclogite-facies metamorphic conditions of 775–805 °C and ≥15 kbar, followed by granulite- to amphibolite-facies overprint of ca. 750–800 °C and 6–11 kbar. The presence of (Na, Ca, Ba, Sr)-feldspars in garnet and omphacite corresponds to amphibolite-facies conditions. Metamorphic mineral assemblages and P–T estimates for felsic granulite at Huangtuling and mafic granulite at Huilanshan indicate peak conditions of 850 °C and 12 kbar for the granulite-facies metamorphism and 700 °C and 6 kbar for amphibolite-facies retrograde metamorphism. Cordierite–orthopyroxene and ferropargasite–plagioclase coronas and symplectites around garnet record a strong, rapid decompression, possibly contemporaneous with the uplift of neighbouring HP/UHP eclogites.

Carbonic fluid (CO₂-rich) inclusions are predominant in both HP granulites and granulitized HP/UHP eclogites/garnet clinopyroxenites. They have low densities, having been reset during decompression. Minor amounts of CH₄ and/or N₂ as well as carbonate are present. In the granulitized HP/UHP eclogites/garnet clinopyroxenites, early fluids are high-salinity brines with minor N₂, whereas low-salinity fluids formed during retrogression. Syn-granulite-facies carbonic fluid inclusions occur either in quartz rods in clinopyroxene (granulitized HP garnet clinopyxeronite) or in quartz blebs in garnet and quartz matrices (UHP eclogite). For HP granulites, a limited number of primary CO₂ and mixed H₂O–CO₂(liquid) inclusions have also been observed in undeformed quartz inclusions within garnet, orthopyroxene, and plagioclase which contain abundant, low-density CO₂±carbonate inclusions. It is suggested that the primary fluid in the HP granulites was high-density CO₂, mixed with a significant quantity of water. The water was consumed by retrograde metamorphic mineral reactions and may also have been responsible for metasomatic reactions (“giant myrmekites”) occurring at quartz–feldspar boundaries. Compared with the UHP eclogites in this region, the granulites were exhumed in the presence of massive, externally derived carbonic fluids and subsequently limited low-salinity aqueous fluids, probably derived from the surrounding gneisses.     

Very high-density carbonic fluid inclusions in sapphirine-bearing granulites from Tonagh Island in the Archean Napier Complex, East Antarctica: implications for CO2 infiltration during ultrahigh-temperature (T>1,100 °C) metamorphism

The ultrahigh-temperature (UHT) metamorphism of the Napier Complex is characterized by the presence of dry mineral assemblages, the stability of which requires anhydrous conditions. Typically, the presence of the index mineral orthopyroxene in more than one lithology indicates that H₂O activities were substantially low. In this study, we investigate a suite of UHT rocks comprising quartzo-feldspathic garnet gneiss, sapphirine granulite, garnet-orthopyroxene gneiss, and magnetite-quartz gneiss from Tonagh Island. High Al contents in orthopyroxene from sapphirine granulite, the presence of an equilibrium sapphirine-quartz assemblage, mesoperthite in quartzo-feldspathic garnet gneiss, and an inverted pigeonite-augite assemblage in magnetite-quartz gneiss indicate that the peak temperature conditions were higher than 1,000 °C. Petrology, mineral phase equilibria, and pressure-temperature computations presented in this study indicate that the Tonagh Island granulites experienced maximum P-T conditions of up to 9 kbar and 1,100 °C, which are comparable with previous P-T estimates for Tonagh and East Tonagh Islands. The textures and mineral reactions preserved by these UHT rocks are consistent with an isobaric cooling (IBC) history probably following an counterclockwise P-T path. We document the occurrence of very high-density CO₂-rich fluid inclusions in the UHT rocks from Tonagh Island and characterize their nature, composition, and density from systematic petrographic and microthermometric studies. Our study shows the common presence of carbonic fluid inclusions entrapped within sapphirine, quartz, garnet and orthopyroxene. Analysed fluid inclusions in sapphirine, and some in garnet and quartz, were trapped during mineral growth at UHT conditions as 'primary' inclusions. The melting temperatures of fluids in most cases lie in the range of -56.3 to -57.2 °C, close to the triple point for pure CO₂ (-56.6 °C). The only exceptions are fluid inclusions in magnetite-quartz gneiss, which show slight depression in their melting temperatures (-56.7 to -57.8 °C) suggesting traces of additional fluid species such as N₂ in the dominantly CO₂-rich fluid. Homogenization of pure CO₂ inclusions in the quartzo-feldspathic garnet gneiss, sapphirine granulite, and garnet-orthopyroxene gneiss occurs into the liquid phase at temperatures in the range of -34.9 to +4.2 °C. This translates into very high CO₂ densities in the range of 0.95-1.07 g/cm³. In the garnet-orthopyroxene gneiss, the composition and density of inclusions in the different minerals show systematic variation, with highest homogenization temperatures (lowest density) yielded by inclusions in garnet, as against inclusions with lowest homogenization (high density) in quartz. This could be a reflection of continued recrystallization of quartz with entrapment of late fluids along the IBC path. Very high-density CO₂ inclusions in sapphirine associated with quartz in the Tonagh Island rocks provide potential evidence for the involvement of CO₂-rich fluids during extreme crustal temperatures associated with UHT metamorphism. The estimated CO₂ isochores for sapphirine granulite intersect the counterclockwise P-T trajectory of Tonagh Island rocks at around 6-9 kbar at 1,100 °C, which corresponds to the peak metamorphic conditions of this terrane derived from mineral phase equilibria, and the stability field of sapphirine + quartz. Therefore, we infer that CO₂ was the dominant fluid species present during the peak metamorphism in Tonagh Island, and interpret that the fluid inclusions preserve traces of the synmetamorphic fluid from the UHT event. The stability of anhydrous minerals, such as orthopyroxene, in the study area might have been achieved by the lowering of H₂O activity through the influx of CO₂ at peak metamorphic conditions (>1,100 °C). Our microthermometric data support a counterclockwise P-T path for the Napier Complex.     

Early metamorphic evolution and exhumation of felsic high-pressure granulites from the north-western Bohemian Massif

Summary Several granulite terrains are exposed in the Bohemian Massif of Central Europe. These were metamorphosed at pressures close to 12 kbar and temperatures of more than 800 °C c. 340 Ma ago. The corresponding penetrative deformation almost totally erased the record of the preceding metamorphic evolution. Nevertheless, rare relics such as mineral inclusions in large garnet grains are witness of this earlier evolution, which was previously related to significantly higher pressures and, thus, to a subduction-related event. The exemplary investigation of such mineral relics in a felsic granulite from the Granulitgebirge rather points to pressures of 13–14 kbar only at relatively low temperatures of 620 °C and, thus, to considerable, nearly isobaric heating before the exhumation of the granulites started at 800 °C or somewhat higher temperature. The inferred P–T evolution is compatible with a geodynamic model of lithospheric delamination, with crustal material having been involved. The delamination at c. 340 Ma ago followed long-lasting, continuous collision of Gondwana and Laurussia forming the Variscan orogen. Within the thickened continental crust, the delamination concerned mainly the dense basic material in the lower crust. This event also caused upwelling of the mantle asthenosphere. Both processes resulted in heating of the more felsic lower portion of the continental crust, thinner than before delamination. Heating by 200 °C or more caused prograde mineral reactions and created buoyancy forces, as the overlying crust became denser than the underlying hot and felsic granulites. As a consequence, considerable volumes of felsic granulite could have reached shallow crustal levels (corresponding to 3 to 4 kbar), conditions documented in granulite bodies in the north-western Bohemian Massif.     

太古宙TTG的Nb/Ta变化特征:对“Nb-Ta悖论”的启示

英云闪长岩-奥长花岗岩-花岗闪长岩(TTG)是地球早期大陆地壳最重要的组成部分。TTG的Nb/Ta比值变化不仅与它的成因相关,而且与早期构造环境和地壳分异过程关系紧密。本文选择阴山地块出露的TTG片麻岩及下地壳斜长角闪岩/麻粒岩包体作为研究对象,开展了寄主花岗闪长岩和同源镁铁质包体中的角闪石和黑云母的原位微区矿物的微量元素分析工作,以及TTG与非同源斜长角闪岩包体的全岩主微量元素分析工作。矿物化学研究结果表明,花岗闪长岩和同源镁铁质包体的角闪石具有相似的Mg^#值,但是两者具有明显不同的Nb/Ta比值。镁铁质包体中的角闪石更富Cr、Ta,Nb/Ta比值为30～50; TTG岩石中的角闪石Cr和Ta含量偏低,但具有更高的Nb/Ta比值(38～70)。TTG和镁铁质包体中的角闪石Cr含量与Nb/Ta具有较好的负相关关系。全岩地球化学分析结果揭示,TTG片麻岩的具有高Nb/Ta比值(13～65,平均值31),斜长角闪岩和麻粒岩包体具有变化的Nb/Ta比值(10～56)。太古宙绿岩带中玄武质岩石的Nb/Ta平均值为～15,阴山地块斜长角闪岩和麻粒岩包体具有高的Nb/Ta比值...     

Origin of CO2-rich fluid inclusions in leucosomes from the Skagit migmatites, North Cascades, Washington, USA

CO₂–CH₄ fluid inclusions are present in anatectic layer-parallel leucosomes from graphite-bearing metasedimentary rocks in the Skagit migmatite complex, North Cascades, Washington. Petrological evidence and additional fluid inclusion observations indicate, however, that the Skagit Gneiss was infiltrated by a water-rich fluid during high-temperature metamorphism and migmatization. CO₂-rich fluid inclusions have not been observed in Skagit metasedimentary mesosomes or melanosomes, meta-igneous migmatites, or unmigmatized rocks, and are absent from subsolidus leucosomes in metasedimentary migmatites. The observation that CO₂-rich inclusions are present only in leucosomes interpreted to be anatectic based on independent mineralogical and chemical criteria suggests that their formation is related to migmatization by partial melting. Although some post-entrapment modification of fluid inclusion composition may have occurred during decompression and deformation, the generation of the CO₂-rich fluid is attributed to water-saturated partial melting of graphitic metasedimentary rocks by a reaction such as biotite + plagioclase + quartz + graphite ± Al₂SiO₅+ water-rich fluid = garnet + melt + CO₂–CH₄. The presence of CO₂-rich fluid inclusions in leucosomes may therefore be an indication that these leucosomes formed by anatexis. Based on the inferences that (1) an influx of fluid triggered partial melting, and (2) some episodes of fluid inclusion trapping are related to migmatization by anatexis, it is concluded that a free fluid was present at some time during high-temperature metamorphism. The infiltrating fluid was a water-rich fluid that may have been derived from nearby crystallizing plutons. Because partial melting took place at pressures of at least 5 kbar, abundant free fluid may have been present in the crust during orogenesis at depths of at least 15 km.     

Partial fusion of basic granulites at 5 to 15 kbar: implications for the origin of TTG magmas

Partial fusion experiments with basic granulites (S6, S37) believed to represent the lower crust beneath the Eifel region (Germany) were performed at pressures from 5 to 15 kbar. Water-undersaturated experiments were carried out in the presence of 1 wt% H₂O plus 2.44 or 0.81 wt% CO₂ equivalent to mole fractions of H₂O/(H₂O + CO₂) of 0.5 and 0.75, respectively, of the volatile components added. At temperatures from 850 to 1100 °C the weight proportions of melt range from 7 to 30 %. Melt compositions change from trondhjemitic over tonalitic to dioritic with increasing degree of partial melting. Crystalline residua are plagioclase/pyroxene dominated at 5 kbar to garnet/pyroxene dominated at 15␣kbar. Dehydration melting was studied in granulite S35 similar in composition to S6. The magmatic precursors of the granulite xenoliths used in this study had geochemical characteristics of cumulate gabbro (metagabbro S37) and evolved melts (metabasalts S6, S35), respectively. Melts from granulite S37 match the major element compositions of natural trondhjemites and tonalites. At 5 kbar, their Al₂O₃ is relatively low, similar to tonalites from ophiolites. At 15 kbar, Al₂O₃ in the melts is high due to the near absence of plagioclase in the crystalline residua. The Al₂O₃ concentrations in 15 kbar melts from S6 (˜20 wt%) are higher than in natural tonalites. Depth constraints on the formation of tonalitic magmas in the continental crust are provided by REE (rare earth element) patterns of the synthetic melts calculated from the known REE abundances in metagabbro S37 and metabasalt S6 assuming batch melting and using partition coefficients from the literature. The REE patterns of tonalites from active continental margins and Archean trondhjemite-tonalite-granodiorite␣associations low in REE with La_N (chondrite normalised) from 10 to 30 and Yb_N from 1 to 2 are reproduced at pressures of 10 and 12.5 kbar from metagabbro S37 which displays a slightly L(light)REE enriched pattern with La_N = 8 and Yb_N = 3. Natural tonalites with La_N from 30 to 100 require a source richer in REE than granulite S37. At 15 kbar, H(heavy)REE_N in melts from granulite S37 are depressed below the level observed in natural tonalites due to the high proportion of garnet (>30 wt%) in the residue. Melts from metabasalt S6 (enriched in REE with La_N = 38 and Yb_N = 16) do not match the REE characteristics of natural tonalites under any conditions. Received: 1 July 1994 / Accepted: 11 September 1996     

Garnetiferous Metabasites from the Sausar Mobile Belt: Petrology, P-T Path and Implications for the Tectonothermal Evolution of the Central Indian Tectonic Zone

A suite of garnetiferous amphibolites and mafic granulites occuras small boudins within layered felsic migmatite gneiss in thenorthern part of the Sausar Mobile Belt (SMB), the latter constitutingthe southern component of the Proterozoic Central Indian TectonicZone (CITZ). Although the two types of metabasites are in variousstages of retrogression, textural, compositional and phase equilibriastudies attest to four distinct metamorphic episodes. The earlyprograde stage (M_o) is represented by an inclusion assemblageof hornblende₁ + ilmenite₁ + plagioclase₁ ± quartz andgrowth zoning preserved in garnet. The peak assemblage (M₁)consists of porphyroblastic garnet + clinopyroxene ±quartz ± rutile ± hornblende in mafic granulitesand garnet + quartz + hornblende in amphibolites and stabilizedat pressure–temperature conditions of 9–10 kbarand 750–800°C and 8 kbar and 675°C, respectively.This was followed by near-isothermal decompression (M₂), andpost-decompression cooling (M₃) events. In mafic granulites,the former resulted in the development of early clinopyroxene_2A–hornblende_2A–plagioclase_2Asymplectites at 8 kbar and 775°C (M_2A stage), synchronouswith D₂ and later anhydrous clinopyroxene_2B–plagioclase_2B–ilmenite_2Bsymplectites and coronal assemblages at 7 kbar, 750°C (M_2Bstage) and post-dating D₂. In amphibolites, ilmenite + plagioclase+ quartz ± hornblende symplectites appeared during M₂at 6·4 kbar and 700°C. During M₃, coronal garnet+ clinopyroxene + quartz ± hornblende-bearing symplectitesin metabasic dykes and hornblende₃–plagioclase₃ symplectitesembaying garnet in mafic granulites were formed. P–T estimatesshow near-isobaric cooling from 7 kbar and 750°C to 6 kbarand 650°C during M₃. It is argued that the decompressionin the mafic granulites is not continuous, being punctuatedby a distinct heating (prograde?) event. The latter is alsocoincident with a period of extension, marked by mafic dykeemplacement. The combined P–T path of evolution has aclockwise sense and provides evidence for a major phase of earlycontinental subduction in parts of the CITZ. This was followedby a later continent–continent collision event duringwhich granulites of the first phase became tectonically interleavedwith younger lithological units. This tectonothermal event,of possibly Grenvillian age, marks the final amalgamation ofthe North and the South Indian Blocks along the CITZ to producethe Indian subcontinent. KEY WORDS: Central Indian Tectonic Zone; clockwise P–T path; continental collision; metabasite     

Rare earth element mobility during prograde granulite facies metamorphism: significance of fluorine

 Mafic gneisses occur as lenses or thin layers in spatial association with tonalitic leucosomes in a granulite zone of the Quetico subprovince of the Superior Province, Ontario, Canada, and exhibit concentric zoning with a biotite-rich margin, orthopyroxene-rich outer zone, clinopyroxene-rich central zone, and, occasionally, patches of relict amphibolites within the clinopyroxene-rich zone. The granulites (biotite-, orthopyroxene- and clinopyroxene-rich zones) in the mafic gneisses are characterized by significant amounts of rare earth element (REE)-bearing fluorapatite (1–10 vol.%) and other REE-rich minerals (allanite, monazite and zircon). Fluorapatite shows an increase in modal abundance from the biotite- and orthopyroxene-rich zones to the clinopyroxene-rich zone, but is rare in the relict amphibolites. Textural evidence and element partitioning indicate that fluorapatite (and other REE-rich minerals) was part of the peak metamorphic assemblages. Whole-rock geochemical analyses confirm that the granulites in the mafic gneisses contain anomalously high contents of REE and high field strength elements (HFSE), whereas the relict amphibolites are geochemically typical of tholeiitic basalts. Mass-balance calculations reveal that REE and HFSE were introduced into the mafic gneisses during the prograde granulite facies metamorphism, pointing to REE mobility under granulite facies metamorphic conditions. The presence of high F contents in the REE-rich minerals and their associated minerals (e.g. biotite and hornblende) suggests that REE and HFSE may have been transported as fluoride complexes during the granulite facies metamorphism. This conclusion is supported by previously published results of hydrothermal experiments on the partitioning of REE between fluorapatite and F-rich fluids at 700°C and 2 kbar. Received: 2 May 1995 / Accepted: 28 September 1995     

Origin of K-poor leucosomes in a metasedimentary migmatite complex by ultrametamorphism, syn-metamorphic magmatism and subsolidus processes

Two types of stromatic leucosomes are identified in metasedimentary rocks from the Skagit migmatite complex, North Cascades, Washington state, U.S.A. Both types are trondhjemitic and appear similar in outcrop, but, although both contain low abundances of REE, one type consists of leucosomes that are relatively REE-enriched compared to the other, and contains (1) small (<0.8 mm), Fe-rich garnets that are compositionally and texturally different from mesosome and melanosome garnet; (2) Ti-rich minerals (rutile, titanite) that are not present in the groundmass of the associated mesosomes or melanosomes and (3) CO₂-rich fluid inclusions in quartz. Leucosomes of the second type are REE-depleted compared to the first type, lack garnet and Ti-minerals, and contain only H₂O-rich fluid inclusions. The first type of leucosome is interpreted to have formed by in situ partial melting accompanied, and perhaps initiated, by an influx of water-rich fluid during upper amphibolite facies metamorphism. These conclusions are based on estimates of metamorphic P-T-X_fluid conditions (9–10 kbar, > 700°C, water-rich fluid present), inferences about the origin of the above-listed mineralogical and fluid inclusion features, and modeling of leucosome trace element abundances. The second type of leucosome is interpreted to have formed entirely by subsolidus processes (e.g., metamorphic differentiation) because these leucosomes lack features consistent with an origin by partial melting.

K-poor (tonalitic/trondhjemitic) leucosomes associated with metasedimentary (biotite-bearing) source rocks may form by water-saturated partial melting or by subsolidus processes. Both general leucosome-forming mechanisms may operate at different times during upper amphibolite facies regional metamorphism. Partial melting may be initiated by syn-metamorphic magmatic activity if crystallizing plutons serve as external sources of the water-rich fluid necessary for ultrametamorphism in the middle crust during orogenesis. Large-scale migmatite complexes such as the Skagit migmatites may form at least in part in response to contact effects of plutonism associated with high-grade metamorphism, so, although migmatite complexes are a volumetrically substantial part of many orogenic belts, they may not themselves represent a significant original source of magma for larger-scale igneous bodies.     

Fluids in low-pressure migmatites: a fluid inclusion study of rocks from the Gennargentu Igneous Complex (Sardinia, Italy)

Summary The low-pressure emplacement of a quartz diorite body in the metapelitic rocks of the Gennargentu Igneous Complex (Sardinia, Italy) produced a contact metamorphic aureole and resulted in migmatisation of part of the aureole through partial melting. The leucosome, formed by dehydration melting involving biotite, is characterised by granophyric intergrowth and abundant magnetite crystals. A large portion of the high temperature contact aureole shows petrographic features that are intermediate between quartz diorite and migmatite s.s. (i.e. hybrid rocks). A fluid inclusion study has been performed on quartz crystals from the quartz diorite and related contact aureole rocks, i.e. migmatite sensu stricto (s.s.) and hybrid rocks. Three types of fluid inclusions have been identified: I) monophase V inclusions, II) L + V, either L-rich or V-rich aqueous saline inclusions and III) multiphase V + L + S inclusions. Microthermometric data characterised the trapped fluid as a complex aqueous system varying from H₂O–NaCl–CaCl₂ in the quartz diorite to H₂O–NaCl–CaCl₂–FeCl₂ in the migmatite and hybrid rocks. Fluid salinities range from high saline fluids (50 wt% NaCl eq.) to almost pure aqueous fluid. Liquid-vapour homogenisation temperatures range from 100 to over 400 °C with an average peak around 300 °C. Temperatures of melting of daughter minerals are between 300 and 500 °C. Highly saline liquid- and vapour-rich inclusions coexist with melt inclusions and have been interpreted as brine exsolved from the crystallising magma. Fluid inclusion data indicate the formation of fluid of high iron activity during the low-pressure partial melting and a fluid mixing process in the hybrid rocks.     

The trace element geochemistry of clinopyroxenes from pyroxenites in the Lewisian of NW Scotland: insights into light rare earth element mobility during granulite facies metamorphism

Clinopyroxenes from layered pyroxenites and from pyroxenite pods in felsic gneisses of the Lewisian granulite complex, NW Scotland, have distinctive chemistries suggestive of different origins. Clinopyroxenes in the layered pyroxenites crystallised from mafic melts in a magma chamber located in the middle to shallow crust, whereas clinopyroxenes in pods in the felsic gneisses crystallised from the tonalitic protolith to the felsic gneisses. In detail clinopyroxenes in the layered pyroxenites are variably enriched in the light REE. Inversion modelling shows that this is not a primary feature inherited from their parent magmas. Rather selective light rare earth element enrichment took place through reaction with a felsic melt generated by the localised partial melting of the hornblende pyroxenites during granulite facies metamorphism. Published isotopic evidence suggests that the light REE mobilisation took place at ca 2.7 Ga, about 200 Ma after the time of crust formation. This observation provides an explanation for the scattered pattern of whole-rock isochron ages from the Lewsian granulites.     

Thickness and geothermal gradient of Neoarchean continental crust: Inference from the southeastern North China Craton

The thickness and geothermal gradient of Archean continental crust are critical factors for understanding the geodynamic processes in Earth's early continental crust. Archean tonalite-trondhjemite-granodiorite (TTG) gneisses provide one of the potential indicators of paleo-crustal thickness and geothermal gradient because crust-derived TTG melts are generally thought to originate from partial melting of mafic rocks at the crustal root. In the Western Shandong Province (WSP) of the North China Craton (NCC), two episodes of Neoarchean TTG magmatism are recognized at ~2.70 Ga and ~2.55 Ga which were sourced from partial melting of juvenile crustal components. The ~2.70 Ga TTG gneisses show highly fractionated rare earth element (REE) patterns (average (La/Yb)_N = 39), whereas the ~2.55 Ga TTG gneisses have relatively less fractionated REE patterns (average (La/Yb)_N = 18). Petrogenetic evaluation suggest that the magmatic precursors of the TTG gneisses of both episodes originated from partial melting of juvenile crustal materials at different crustal depths with residual mineral phases of Grt, Cpx, Amp, Pl and Ilm. Together with the garnet proportion in the residue, the P–T pseudosections of equilibrium mineral assemblages, and the temperature calculated from Titanium-in-zircon thermometer, we estimate the Neoarchean crustal thicknesses as 44–51 km with geothermal gradients of 17 to 20 °C/km for the ~2.70 Ga TTG gneisses whereas the ~2.55 Ga TTG gneisses show lesser crustal thicknesses of 35–43 km with geothermal gradients of 19 to 26 °C/km, with an approximately 10 km difference in crustal thickness. Our estimates on the thicknesses and geothermal gradients of the Neoarchean crust are similar to those (~41 km, ~20 °C/km) of the modern average continental crust, indicating that a modern-style plate tectonic regime may have played an important role in the formation and evolution of the Neoarchean continental crust in the NCC.