Large-scale fluid infiltration, metasomatism and re-equilibration of Archaean basement granulites during Palaeoproterozoic thrust belt construction, Ungava Orogen, Canada

Abstract The metamorphic history of the Archaean Superior Province crystalline basement in the Palaeoproterozoic Ungava Orogen attests to the importance of structural and geohydrological controls on a retrograde amphibolite-granulite transition. Two distinct metamorphic suites, separated in age by nearly one billion years, are recognized in extensively exposed tonalitic to dioritic metaplutonic gneisses. The older suite comprises c. 2.7-Ga granulite facies assemblages (orthopyroxene-clinopyroxene-hornblende-plagioclase-ilmenite ± biotite ± quartz) that record moderate pressures (±5 kbar) and high temperatures (±800° C). A younger, c. 1.8-Ga suite resulted from amphibolitization of the granulites and is characterized by regionally extensive amphibolite facies mineral zones that broadly parallel the basal décollement of the overlying Proterozoic Cape Smith Thrust Belt. Deformation/mineral growth relationships in the amphibolitized basement indicate that extensive hydration and re-equilibration of the Archaean granulites occurred during thrust belt deformation. The transition from granulite facies to amphibolite facies assemblages is characterized by the growth of garnet-hornblende-quartz ° Cummingtonite coronas between plagioclase and orthopyroxene-clinopyroxene, as well as titanite coronas on ilmenite. Multi-equilibrium thermobarometry on the coronitic assemblages documents re-equilibration of the granulitic gneiss to 7.7 kbar at 644° C in the south and 9.8 kbar at 700° C in the north. The variably deformed, amphibolite facies domain sandwiched between the coronitic garnet zone and the basal décollement is marked by significant metasomatic changes in major element concentrations within tonalite. These changes are compatible with equilibrium flow of an aqueous-chloride fluid down a temperature gradient. The source of fluids for basement hydration/metasomatism is interpreted to be dehydrating clastic rocks in the overlying thrust belt, with fluid flow probably focused along the basal décollement.     

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     

柴西缘花土沟超高温变质地体中的镁铝麻粒岩岩石成因及其与熔体行为的关系

镁铝麻粒岩泛指一类全岩化学成分富镁、铝的麻粒岩相变质岩,是研究超高温变质作用的峰期变质条件和变质演化历史的重要对象,但目前对它的原岩属性和岩石成因的认识仍十分有限。本文以柴达木地块西缘的花土沟超高温变质地体为例,在野外调查基础上,对镁铝麻粒岩和泥质片麻岩进行了岩相学和全岩地球化学分析,发现镁铝麻粒岩与含浅色体的泥质片麻岩的SiO₂、TiO₂、P₂O₅ 含量相似,^TFe₂O₃、Al₂O₃、MnO、CaO、Na₂O含量虽有差异但变化范围存在交集。此外,两类岩石具有相似的微量和稀土元素配分曲线,结合两者的矿物组合也存在相似性,提出花土沟镁铝麻粒岩的原岩可能是与泥质片麻岩类似的泥质沉积岩。从低角闪岩相变泥质岩到含浅色体的泥质片麻岩,再到镁铝麻粒岩,其全岩化学成分向着贫铝、钙、钾、钠,富铁、镁的趋势变化。其中,高X_Mg值（0.51~0.69）是镁铝麻粒岩与其他泥质片麻岩（X_Mg=0.34~0.43）的最大差别。通过对变泥质岩的相平衡模拟和理论计算,发现部分熔融和熔体丢失能解释大部分的变化趋势,但基本不影响全岩X_Mg值;只有在进变质升温过程中丢失含石榴子石的熔体才能造成变泥质岩的镁铝麻粒岩化。此外,富石榴子石的泥质残留体相比附近的含浅色体泥质片麻岩,贫硅、钠、钾,富集铝、铁、镁、锰、钙,重稀土元素含量显著高于后者,上述地球化学特征对应石榴子石熔体的加入而后长英质熔体的丢失,支持野外观察到的熔体携带石榴子石迁移的现象。最后,对镁铝麻粒岩只呈透镜体产出的原因做出了推测,即熔体很难带着石榴子石完成长距离迁移,只有被长英质正片麻岩包围的泥质沉积物,其进变质加热阶段形成的熔体才能携带石榴子石完全迁出原岩,促成变泥质岩透镜体的镁铝麻粒岩化,目前仍需更多的相关研究来验证这一推测。在世界其他高温-超高温变质岩区,石榴子石熔体的迁出和泥质岩的镁铝麻粒岩化可能也不同程度有所保留和记录。     

The iron-rich suite from the Amîtsoq gneisses of southern West Greenland: early Archaean plutonic rocks of mixed crustal and mantle origin

A distinctive group of augen gneisses and ferrodiorites (termed the iron-rich suite) is a component of the early Archaean Amîtsoq gneisses of southern West Greenland. The iron-rich suite outcrops south of the mouth of Ameralik fjord in an area that underwent granulite facies metamorphism in the early Archaean. The iron-rich suite forms approximately 30% of the Amîtsoq gneiss of this area and occurs as sheets and lenses up to 500 m thick. The rest of the Amîtsoq gneisses are predominantly tonalitic-granodioritic, banded grey gneisses. Despite intense deformation and polymetamorphism, there is local field evidence that the iron-rich suite was intruded into the grey gneisses after they had been affected by tectonism and metamorphism. The banded grey gneisses are interpreted as 3,700 to 3,800 Ma old; U-Pb zircon ages from the iron-rich suite give concordia intercepts at circa 3,600 Ma.Coarse grained augen gneisses with microcline mega-crysts are the dominant lithology of the iron-rich suite. They are mostly granodioritic, grading locally into granite and diorite, and are generally rather massive, but locally have well-preserved layering or are markedly heterogeneous. Mafic components are commonly concentrated into clots rich in hornblende and biotite and containing apatite, ilmenite, sphene and zircon. Variation in the proportion of these clots is the main reason for the compositional variation of the augen gneisses. The ferrodiorites of the suite occur as lenses in the augen gneisses. Leucocratic granitoid sheets locally cut the iron-rich suite. The augen gneisses and ferrodiorites have geochemical characteristics in common, such as high Fe/Mg values and high contents of FeOt, TiO₂, P₂O₅, Zr, Y and total REE (rare earth elements).The iron-rich suite probably formed as follows:Heating of the lower crust adjacent to mantle-derived basic intrusions caused melting of the lower crust, giving rise to granodioritic magmas. Disruption of partially crystallised basic intrusions caused mixing of the crustal melts and the fractionated mantle melts to produce the augen gneisses with their high FeOt, TiO₂, P₂O₅, Zr, Y and total REE enrichment. Fragmented, crystallised parts of the basic intrusions gave rise to the ferrodiorite inclusions. These heterogeneous plutons rose to higher crustal levels where they crystallised as sheets and possibly were responsible for the local granulite facies metamorphism. The granitoid sheets that cut the iron-rich suite are interpreted as crustal melts of local origin.The iron-rich suite resembles Proterozoic rapakivi granite-ferrodiorite-norite (anorthosite) associations which form characteristic suites in late- to post-tectonic environments in recently thickened sial. The occurrence of this type of magmatism in the early Archaean is evidence of the complex, polygenetic nature of the oldest known continental crust.     

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.     

苏鲁造山带东北端石榴辉石麻粒岩的Sm-Nd同位素年代学及其大地构造含义

 大量含石榴石的基性麻粒岩透镜体出露于苏鲁变质带的北部及邻近地区,它们可能是再变质的高压变质岩石。在详细的岩相学研究的基础上,确定采自莱西和文登的样品WD01、WD04、ML06 是由高压麻粒岩经中-高压麻粒岩相再变质形成的,而采自威海的样品WH1 是由柯石英榴辉岩经中-高压麻粒岩相再变质形成的。Sm-Nd 同位素年代学研究也证实了二者的重大差别。3 个高压麻粒岩样品的矿物-全岩内部等时线年龄分别是1 846+ /-76Ma,1 743+ /-79Ma 和1 752+ /-30Ma,TDM 模式年龄是3.3Ga,3.0Ga 和2.8Ga.上述数据说明原岩形成在太古宙,而1 800Ma 是麻粒岩相降压变质事件的记录,这与华北克拉通前寒武纪高压麻粒岩的年代学一致。威海样品的Sm-Nd 同位素特征则完全不同。矿物和全岩形不成等时线,表现出它们之间的同位素不平衡。ε_Nd(0)值高达+ 127,TDM 模式年龄是1.3Ga.这与Jahn(1994,1996)对威海同类样品的测定结果相同。可以推测威海样品的原岩是元古宙岩石,在后来复杂的变质过程中,在水岩相互作用和岩浆及重熔作用的影响下,同位素系统发生重大变化。同位素年代学为苏鲁变质带和华北克拉通的界限是昆嵛山岩浆-变质杂岩带提供了依据。     

P–T conditions of the cratonic rocks and Eastern Ghats granulites along the Eastern Ghats Frontal Thrust,Jeypore (Orissa), India

The Eastern Ghats Frontal Thrust (EGFT) demarcates the boundary between the Archaean/Paleoproterozoic cratonic rocks to the west, and the Meso/Neoproterozoic granulites of the Eastern Ghats Mobile Belt (EGMB) to the east. At Jeypore (Orissa, India), mafic schists and granites of the cratonic domain document a spatial increase in the metamorphic grade from greenschist facies (garnet, clinozoisite – absent varieties) in the foreland to amphibolite facies (clinozoisite- and garnet-bearing variants) progressively closer to the EGFT. Across the EGFT, the enderbite–charnockite gneisses and mafic granulites of EGMB preserves a high-grade granulite facies history; amphibolite facies overprinting in the enderbite–charnockite gneisses at the cratonic fringe is restricted to multi-layered growth of progressively Al, Ti – poor hornblende at the expense of pyroxene and plagioclase. In associated mafic granulites, the granulite facies gneissic layering is truncated by sub-centimeter wide shear bands defined by synkinematic hornblende + quartz intergrowth, with post-kinematic garnet stabilized at the expense of hornblende and plagioclase. Proximal to the contact, these granulites of the Eastern Ghats rocks are intruded by dolerite dykes. In the metadolerites, the igneous assemblage of pyroxene–plagioclase is replaced by intergrown hornblende + quartz ± calcite that define the thrust-related fabric and are in turn mantled by coronal garnet overgrowth, while scapolite is stabilized at the expense of recrystallized plagioclase and calcite. Petrogenetic grid considerations and thermobarometry of the metamorphic assemblages in metadolerites intrusive into granulites and mafic schists within the craton confirm that the rocks across the EGFT experienced prograde heating (T_max value ∼650–700 °C at P ∼ 6–8 kbar) along the prograde arm of a seemingly clockwise P–T path. Since the dolerites were emplaced post-dating the granulite facies metamorphism, the prograde heating is correlated with renewed metamorphism of the granulites proximal to the EGFT. A review of available age data from rocks neighboring the EGFT suggests that the prograde heating of the cratonic granites and the re-heating of the Eastern Ghats granulites are Pan – African in age. The re-heating may relate to an Early Paleozoic Pan-Gondwanic crustal amalgamation of older terrains or reactivation along an old suture.     

Widespread 18O depletion in some precambrian granulites of Australia

In the Strangways Range a broad tract of lower Proterozoic mafic and silicic granulites with δ ¹⁸O = 0.1 to 7.3% is depleted in ¹⁸O on average by 2–47% compared with high-grade gneisses and granitoids of Canada and other shields. The Fraser Range mafic granulites (δ¹⁸O ~ 7.2%) are enriched with respect to unaltered sea-floor basalts (~5.7%).In some rocks depletion in ¹⁸O could be related to dehydration during granulite-facies metamorphism and removal of the resultant products of partial melting. In other rocks pre-granulite reaction between heated seawater and hot basic intrusives seems to be a plausible mechanism of depletion in ¹⁸O. A direct correlation between depletion in ¹⁸O and the abundance of brown granulite hornblende suggests that ¹⁸O-depleted water was present in certain mafic rocks before the onset of granulite metamorphism, whereas in others brown hornblende was introduced during a phase of the granulite facies metamorphism itself.     

REE distributions in a high-grade Archaean gneiss complex in Scotland: Implications for the genesis of ancient sialic crust

Seventeen rocks from the Lewisian Gneiss of the Inner Hebrides of Scotland, which represent three distinct lithological types at granulite to greenschist facies of metamorphism show rare-earth element patterns which seem not to have been disturbed by their complex metamorphic history. Some indication of their origin can be obtained by simple geochemical models.Three tonalitic pyroxene gneisses are characterized by: (1) light REE enrichment and heavy REE depletion; (2) low total REE contents; (3) moderate Eu enrichment. Their REE chemistry can be approximated by a model involving 10% partial melting of various garnet-bearing basaltic source materials. Alternatively, they may be some form of crystal cumulate, preserving their original anhydrous mineralogy, representing 30% crystallization of a parent tonalitic magma.Three tonalitic to granodioritic hornblende-biotite gneisses are characterized by: (1) light REE enrichment and heavy REE depletion; (2) significantly higher total REE contents than the pyroxene gneisses; (3) moderate Eu depletion. Their REE patterns can be approximated by a residual silicic melt in a model involving 30% fractional crystallization of solids with the modal mineralogy of the pyroxene gneisses or 40% removal of pure anorthosite from a parent dacitic magma.Two strongly metasomatised diopside-actinolite gneisses and one highly sheared epidote-chlorite gneiss have REE patterns which are not significantly different from the hornblende-biotite gneisses which were their precursors before metasomatism and late greenschist-facies shearing. This suggests that strong alteration has not enciphered the REE systematics of the gneisses.Basic gneisses of quartz tholeiite composition occurring as early dykes, which shared the same metamorphic history as the tonalitic to granodioritic gneisses, are characterised by: (1) slight enrichment in light REE relative to heavy REE; (2) variable total REE contents; (3) little difference between granulite and amphibolite facies types. Their REE patterns can be matched by models involving 5–15% partial melting of ultrabasic mantle with 3 times chondritic REE abundances, leaving a residue of olivine and orthopyroxene.     

沂水青龙峪超镁铁质岩石和基性麻粒岩的锆石SHRIMP U-Pb定年

本文主要对沂水青龙峪出露的超镁铁质岩石和基性麻粒岩进行了锆石SHRIMP U-Pb定年研究。超镁铁质岩石以捕掳体形式存在于沂水杂岩中,不发育鬣刺结构,氧化物组成具有超镁铁质科马提岩的高MgO、富CaO、低SiO2、TiO2、K2O和Na2O含量特征;矿物组合以单斜辉石+橄榄石±斜方辉石+铬铁矿为主;变质矿物以角闪石+蛇纹石化为特征;该岩石以稀土元素总含量(∑REE)低、LREE/HREE=3.35～4.40及Ce和Eu负异常为特征。微量元素组成以Ba、Nb、Zr负异常和Nd、Sm正异常为特征。根据锆石SHRIMP U-Pb定年法对该超镁铁质岩石中捕获的早期岩浆结晶锆石和新生的变质锆石进行的研究,年龄值分别为2657～2702Ma和2551～2585Ma,表明该超镁铁质岩石形成年龄为2585～2657Ma。基性麻粒岩的氧化物组成特征表明其属高Mg的洋岛拉斑玄武岩,麻粒岩相——高角闪岩相变质作用与新太古代的深熔和岩浆侵入作用有关,矿物组合以紫苏辉石+单斜辉石±角闪石+斜长石±石榴子石为特征;晚期蚀变作用与辉长岩墙、辉绿岩脉及石英闪长岩买的侵入有关,矿物组合以滑石化+绢云母化+绿泥石化为特征;稀土元素组成以轻重稀土元素无分异和无Eu异常为特征;微量元素组成以Nb、Zr、P、Ti负异常和Sr、K正异常为特征;锆石SHRIMP U-Pb定年结果表明麻粒岩相——角闪岩相变质作用年龄为2498.4±7.6Ma,导致麻粒岩相——角闪岩相变质的深熔和岩浆结晶年龄为2551±24Ma,晚期蚀变作用的年龄分别为2231～2235Ma和1850±19Ma。     

Quantitative P–T–X constraints on orthopyroxene-bearing high-pressure granulites in felsic–metapelitic rocks: evidence from the Huangtuling granulite, Dabieshan Orogen

High‐pressure granulites are generally characterized by the absence of orthopyroxene. However, orthopyroxene is reported in a few high‐pressure, felsic–metapelitic granulites, such as the Huangtuling felsic high‐pressure granulite in the North Dabie metamorphic core complex in east‐central China, which rarely preserves the high‐pressure granulite facies assemblage of garnet + orthopyroxene + biotite + plagioclase + K‐feldspar + quartz. To investigate the effects of bulk‐rock composition on the stability of orthopyroxene‐bearing, high‐pressure granulite facies assemblages in the NCKFMASHTO (Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃) system, a series of P–T–X pseudosections based on the melt‐reintegrated composition of the Huangtuling felsic high‐pressure granulite were constructed. Calculations demonstrate that the orthopyroxene‐bearing, high‐pressure granulite facies assemblages are restricted to low X_Al [Al₂O₃/(Na₂O + CaO + K₂O + FeO + MgO + Al₂O₃) < 0.35, mole proportion] or high X_Mg [MgO/(MgO + FeO) > 0.85] felsic–metapelitic rock types. This study also reveals that the X_Al values in the residual felsic–metapelitic, high‐pressure granulites could be significantly reduced by a high proportion of melt loss. We suggest that orthopyroxene‐bearing, high‐pressure granulites occur in residual overthickened crustal basement under continental subduction–collision zones and arc–continent collision belts.     

大青山太古宙高级变质岩近水平顺层伸展变形——地球上早期的伸展构造

内蒙古大青山—乌拉山位于华北克拉通北缘,是我国高级区发育较好的区域之一。区内高级变质杂岩总体上呈近东西向延伸,经历多期变质变形作用改造。近水平顺层伸展变形是高级变质杂岩中最早的塑性流动变形作用,发生在下地壳麻粒岩相环境下。变形作用伴随麻粒岩相变质作用、深熔作用,形成了顺层滑动韧性变形带、穹隆构造、层内底辟褶皱和L构造岩等构造形迹组合。这期变形作用不仅导致区内的孔兹岩系和麻粒岩系以近水平构造面接触,也造成孔兹岩系中各个地层单位具有以透镜状岩片堆叠的地层结构,并且还伴随有不同类型的深熔片麻岩形成。近水平顺层伸展变形作用的确立,对研究早前寒武纪高级区地壳形成演化和高级变质地层构造格架的建立具有重要的理论意义。     

Premetamorphic lateritisation in Proterozoic metabasites of Rogaland,SW Norway

Heterogeneous layers of granulite facies metamorphic basites of the Proterozoic supracrustal Faurefjell Formation in Rogaland, S.W. Norway, display an extreme chemical variation. Within a single layer the bulk chemical composition gradually changes from approximately basaltic in basic granulites to alumina-iron-rich in granofelses. Component-ratios and composition-volume relations indicate open-system chemical reactions mainly involving the extraction of silica. Apparent enrichment in Fe, Ti, P, Al, Zr, Ni, Co, Zn, Y, Nb, Hf and REE and variations in resulting metamorphic mineral assemblages are related to premetamorphic progressive lateritisation of a basaltic protolith. The weathering generated a continuous chemical suite from SiO₂=48 wt%, Fe₂O₃=10 wt% and Al₂O₃=19 wt% in the basic granulites to 14 wt%, 40 wt% and 25 wt% in the Fe-Al granofelses. Metasomatism during diagenesis and during (very) high-grade metamorphism (1200–900 Ma) further perturbed the concentrations of relatively mobile elements Ca, Mg, K, Rb, Sr, Ba, Na and Li in the laterites without affecting the transition metal ratios. In particular, the REE did not fractionate differentially during the supracrustal and metasomatic alteration.     

中国与蒙古之地质

东昆仑中部缝合带清水泉一带发育石榴斜长紫苏麻粒岩、紫苏辉石黑云母石榴子石麻粒岩、石榴二辉斜长麻粒岩和石榴单斜辉石麻粒岩,它们与混合岩化黑云母石榴子石变粒岩、黑云母辉石变粒岩、石墨大理岩、含透辉石透闪石大理岩、透辉石大理岩、黑云斜长角闪岩和片麻岩等高级变质岩系以及纯橄岩、辉橄岩、橄长岩、辉长岩、辉绿岩和玄武岩等共同构成蛇绿混杂岩。麻粒岩相变质作用的温压条件为T=760~880℃,p=830~1200MPa,为高温中高压麻粒岩相变质作用,估算其形成深度为40~45km。麻粒岩相变质作用的SHRIMP锆石U-Pb年龄为(507·7±8·3)Ma。清水泉地区蛇绿岩形成于~520Ma,到~508Ma时俯冲至地下40~45km深处而发生中高压麻粒岩相变质作用,然后发生构造折返而剥露至地表。证实了清水泉高级变质岩和基性—超基性岩片是形成于早—中寒武世的蛇绿混杂岩,标志一个古生代早期的非常重要的板块汇聚边界,这对于进一步研究东昆仑造山带构造演化、乃至中国西部大地构造格局具有非常重要的意义。     

Crustal xenoliths from Tertiary volcanics of the Northern Hessian Depression

A suite of crustal xenoliths from Tertiary basaltic tuffs of the Northern Hessian Depression (NHD) volcanic field comprises abundant meta-igneous pyroxene granulites of mafic, noritic to anorthositic, IAT and tonalitic composition. Less abundant are granitic, tonalitic and leucogranitic gneisses and metasedimentary xenoliths. A total of 49 samples were analyzed for modal compositions, for major and trace elements (including Li, Rb, Sr, Ba, Cs, V, Sc, Cr, Co, Ni, Y, Zr, Nb, Ta, Hf, Th and REE) and oxygen isotopes. Two-pyroxene thermometry yields temperatures between 700 and 900° C for mafic and noritic granulites. Feldspar thermometry indicates temperatures of 660°–710° C for tonalitic granulites and 470°–520° C for granitic and tonalitic gneisses. One highly depleted sillimanite-rich metasediment contains cordierite and garnet which have equilibrated at temperatures of 780° C. The general lack of garnet in the mafic and noritic granulites and the presence of sillimanite in felsic xenoliths indicates that metamorphic pressures have not exceeded 10 kb. Major and trace element data and oxygen isotope compositions of the mafic granulites are compatible with an origin from spilitized enriched-type MORB rocks (enrichment in ¹⁸O to 11 and in Li to 34 ppm at average SiO₂ contents of 44.1 wt%). These low-T spilites were transformed into amphibolites and then pyroxene granulites during subsequent high temperature metamorphic events. Low Si, Al, K, and Rb concentrations along with An contents in plagioclase ranging from near 50 to 98 mole percent suggest that amphibolite facies protoliths have generated tonalitic melts during partial melting at temperatures above 700° C. The mafic granulite xenoliths are interpreted as restites whereas the tonalitic samples probably represent the extracted partial melts derived by 20 to 30 percent degree of melting. Metasedimentary xenoliths strongly depleted in granitic component could represent restites from which granitic S-type partial melts have been extracted. Tonalitic and leucogranitic gneisses including one trondhjemite xenolith have many chemical characteristics (e.g. REE distribution) in common with tonalite-trondhjemite-granodiorite suites of the North Atlantic region but cannot be accounted for a more specific origin. Estimated elastic properties of the main types of NHD xenoliths yield P-wave velocities of 6.0–6.4 km^-1 for granitic, tonalitic and trondhjemite gneisses including tonalitic granulites and 6.5–7.0 for the more mafic xenoliths. When compared with two seismic depths-V_p profiles these data are in accordance with a model where the mafic, andesitic, noritic and tonalitic granulites comprise abundant rock types at depths between 29 km (Moho) and 20 km which mainly consists of old oceanic crust including subduction related volcanic products. The more felsic xenoliths probably represent material from depths between 12 and 20 km.     

造山带中成对出现的高压麻粒岩与榴辉岩及其地球动力学意义

在一些典型碰撞造山带中,高压麻粒岩与榴辉岩在空间和时间上密切相关,它们之间的关系对揭示碰撞造山带的造山过程和造山机制具有重要意义.本文以中国西部的南阿尔金、柴北缘及中部的北秦岭造山带为例,详细陈述了这3个地区榴辉岩和相关的高压麻粒岩的野外关系、变质演化和形成时代,目的是要建立大陆碰撞造山带中榴辉岩和相关高压麻粒岩形成的地球动力学背景模式.南阿尔金榴辉岩呈近东西向分布在江尕勒萨依,玉石矿沟一带,与含夕线石副片麻岩、花岗质片麻岩和少量大理岩构成榴辉岩一片麻岩单元,榴辉岩中含有柯石英假象,其峰期变质条件为P=2.8～3.0GPa,T=730～850℃,并在抬升过程中经历了角闪岩-麻粒岩相的叠加;大量年代学研究显示其峰期变质时代为485～500Ma.南阿尔金高压麻粒岩分布在巴什瓦克地区,包括高压基性麻粒岩和高压长英质麻粒岩,它们与超基性岩构成了一个大约5km宽的构造岩石单元,与周围角闪岩相的片麻岩为韧性剪切带接触.长英质麻粒岩和基性麻粒岩的峰期组合均具有蓝晶石和三元长石(已变成条纹长石),形成的温压条件为T=930～1020℃,P=1.8～2.5GPa,并在退变质过程中经历了中压麻粒岩相变质作用叠加.锆石SHRIMP测定显示巴什瓦克高压麻粒岩的峰期变质时代为493～497Ma.都兰地区的榴辉岩分布柴北缘HP-UHP变质带的东端,在榴辉岩和围岩副片麻岩中均发现有柯石英保存,形成的峰期温压条件为T=670～730℃和P=2.7～3.25GPa,退变质阶段经过了角闪岩相的叠加;榴辉岩相变质时代为420～450Mao都兰地区的高压麻粒岩分布在阿尔茨托山西部,高压麻粒岩包括基性麻粒岩长英质麻粒岩,基性麻粒岩的峰期矿物组合为Grt+Cpx+Pl±Ky±Zo+Rt±Qtz,长英质麻粒岩的峰期矿物组合为：Grt+Kf+Ky+Pl+Qtz.峰期变质条件为T=800～925℃,P=1.4～1.85GPa,退变质阶段经历了角闪岩-绿片岩的改造,高压麻粒岩的变质时代为420～450Ma.北秦岭榴辉岩分布在官坡-双槐树一带,榴辉岩的峰期变质组合为Grt+Omp±Phe+Qtz+Rt,所计算的峰期温压条件为T=680～770℃和P=2.25～2.65GPa,年代学数据显示榴辉岩的变质时代为500Ma左右.北秦岭高压麻粒岩分布在含榴辉岩单元的南侧松树沟一带,包括高压基性麻粒岩和高压长英质麻粒岩,与超基性岩在空间上密切伴生,高压麻粒岩的峰期温压条件为T=850～925℃,P=1.45～1.80GPa,锆石U-Pb年代学研究显示其峰期变质时代为485～507Ma.以上三个实例显示,出现在同一造山带、在空间上伴生的高压麻粒岩和榴辉岩有各自不同的变质演化历史,但榴辉岩中的榴辉岩相变质时代和相邻的高压麻粒岩中的高压麻粒岩相变质作用时代相同或相近,这种成对出现的榴辉岩和高压麻粒岩代表了它们同时形成在造山带中不同的构造环境中,即榴辉岩的形成于大陆俯冲带中,而高压麻粒岩可能形成在俯冲带之上增厚的大陆地壳根部.     

中国显生宙造山带麻粒岩相高级变质岩石的地质特征、变质时代、P-T轨迹及其形成的大地构造背景

本文重点介绍我国显生宙造山带中麻粒岩的地质特征、岩石类型、P-T轨迹、变质时代及其形成的大地构造背景。我国显生宙造山带主要包括阿尔泰造山带、南天山-西南天山造山带、西昆仑造山带、东昆仑造山带、阿尔金-柴北缘造山带、北秦岭造山带、南秦岭勉略造山带、东秦岭-桐柏-大别造山带、班公湖-怒江造山带和喜马拉雅中东段造山带。这些造山带中麻粒岩的围岩有许多为蛇绿岩套或蛇绿混杂岩带,部分为副片麻岩和花岗质片麻岩,并一起经历了麻粒岩相变质改造,造山带中大多出现一种高压麻粒岩,有的与榴辉岩并存,但少数造山带中(例如阿尔泰造山带)多种压力类型麻粒岩并存,既有低-高压泥质麻粒岩、中低压基性麻粒岩、高压基性和长英质麻粒岩,又有高温-超高温泥质麻粒岩。变质时代除个别为新元古代晚期外,变质时间多为加里东、海西、印支、燕山、喜山期。麻粒岩的P-T轨迹除西天山木札尔特河低压麻粒岩具逆时针轨迹,反映大陆弧构造环境外,其它都是具有等温降压(ITC)特点的顺时针轨迹,形成的大地构造环境大部分为洋陆俯冲碰撞环境,少部分为陆-陆碰撞环境。目前显生宙造山带中麻粒岩的研究大多数尚在起步阶段,少数研究较详细,不少造山带中麻粒岩的类型和变质时代以及形成的构造背景还不清楚,有待深入研究,新的麻粒岩产地有待发现。     

Geochemistry and origin of granitic rocks,Scourian Complex,NW Scotland

Concordant granite sheets from the granulite facies Scourian Complex, N.W. Scotland exhibit the following features:

1. a common planar fabric with their host pyroxene granulites;
2. the presence of an exsolved ternary feldspar phase;
3. a low-pressure, water-saturated minimum composition;
4. K/Rb ratios (450–1,350) distinctly higher than most upper crustal granites but similar to the surrounding granulites;
5. low absolute concentrations of the rare earth elements (REEs), light REE enrichment, and large positive Eu anomalies.

It is proposed that the granite sheets have originated by anatexis of gneisses undergoing granulite facies metamorphism — gneisses that were already essentially dry and depleted in incompatible elements. Their unusual trace element chemistry may be explained by either disequilibrium melting and/or sub-solidus reequilibration of the granite sheets with the surrounding gneisses. Isotopic and trace element data suggest that cross-cutting, potash-rich pegmatites represent reworking of the granite sheets during a later amphibolitization.     

Neoarchaean magmatism and metamorphism of the western granulites in the central domain of the Mozambique belt, Tanzania: U–Pb shrimp geochronology and PT estimates

U–Pb sensitive high resolution ion microprobe (SHRIMP) dating of zircons from charnockitic and garnet–biotite gneisses from the central portion of the Mozambique belt, central Tanzania indicate that the protolith granitoids were emplaced in a late Archaean, ca. 2.7 Ga, magmatic event. These ages are similar to other U–Pb and Pb–Pb ages obtained for other gneisses in this part of the belt. Zircon xenocrysts dated between 2.8 and 3.0 Ga indicate the presence of an older basement. Major and trace element geochemistry of these high-grade gneisses suggests that the granitoid protoliths may have formed in an active continental margin environment. Metamorphic zircon rims and multifaceted metamorphic zircons are dated at ca. 2.6 Ga indicating that these rocks were metamorphosed some 50–100 my after their emplacement. Pressure and temperature estimates on the charnockitic and garnet–biotite gneisses were obscured by post-peak metamorphic compositional homogenisation; however, these estimates combined with mineral textures suggest that these rocks underwent isobaric cooling to 800–850 °C at 12–14 kbar. It is considered likely that the granulite facies mineral assemblage developed during the ca. 2.6 Ga event, but it must be considered that it might instead represent a pervasive Neoproterozoic, Pan African, granulite facies overprint, similar to the ubiquitous eastern granulites further to the east.     

High radiogenic heat production in the Kerala Khondalite Block,Southern Granulite Province,India

In situ radioelemental (K, U and Th) analysis and heat production estimates have been made at 59 sites in the Kerala Khondalite Block (KKB) of the Southern Granulite Province (SGP) of India. Together with the in situ analyses on granulites and gneisses previously reported from 28 sites, and heat production estimated from the published geochemical analyses on granites and syenites of the KKB, the new data set allows good characterization of heat production for the major granulite facies rocks and granitoids of the KKB. Garnet biotite gneisses are characterized by high levels of Th and U, with mean values of 60 and 3 ppm, respectively. Khondalites, leptynites and charnockites have slightly lower levels of Th (23, 20 and 22 ppm, respectively) and U (2.9, 2.4 and 0.9 ppm, respectively). The mean K, U, Th abundances for the granites, leucogranites and granitic gneisses ranges from 3.9 to 4.3%, 2.6 to 4.3 ppm, 22 to 50 ppm respectively, and for the syenites 4.8%, 2 ppm and 5.7 ppm. Mean radiogenic heat production values for garnet–biotite gneiss, khondalite, leptynite and charnockite are 5.5, 2.7, 2.4 and 2.2 μW m⁻³, respectively. For the granites, leucogranites, granitic gneisses and syenites it is 2.6, 3.4, 4.6 and 1.4 μW m⁻³, respectively. Heat production of granulite facies rocks, which are the most abundant rocks in KKB, correlate well with Th, but less with U, suggesting that variation is caused by Th and U bearing accessory minerals such as monazite and zircon. The high heat production of the KKB granulites are in contrast to the low heat production of the Late Archaean granulites of the Northern Block (NB) of the SGP which are highly depleted in radioelements and also the granulites of Madurai Block (MB) that have higher radioelemental abundances than in the granulites of the NB. The high heat production of the KKB granulites could be due to the nature of protoliths and/or metasomatism associated with Neoproteroic- to- Pan African alkaline magmatic activity represented by alkali granite and syenite–carbonatite emplacements and emplacement of pegmatites.