Mineralogy and pressure–temperature–time path of Cretaceous granulite gneisses, south-eastern San Gabriel Mountains, southern California

Mid-Cretaceous granulite gneisses crop out in a narrow belt in the Cucamonga region of the south-eastern foothills of the San Gabriel Mountains, southern California. Interlayered mafic granulites and pelitic, carbonate, calc-silicate and quartzofeldspathic metasediments record hornblende granulite subfacies metamorphism at approximately 8 kbar and 700–800°C. Regional deformation and formation of banded gneisses ceased by c. 108 Ma. although mafic-intermediate magmatism and high-grade metamorphism continued locally as late as c. 88 Ma. Garnet zoning in metapelitic gneisses suggests that peak metamorphism was followed locally by a period of near-isobaric cooling, but this interpretation requires diachronous cooling of the granulite belt which cannot be demonstrated without detailed thermo-chronological data. It is more likely that the entire terrane remained at granulite facies P–T conditions until 88 Ma, followed by rapid uplift associated with juxtaposition against adjacent middle and upper crustal arc terranes. Uplift occurred between c. 88 and 78 Ma at rates of approximately 1–2 km Ma^-1. The geotectonic evolution of the Cucamonga granulites is similar to mid-Cretaceous high- P granulites in the Sierra Nevada and Salinian block of central California. Late Cretaceous uplift common to these granulites may provide an important tectonic link between dismembered Mesozoic batholithic terranes in the California Cordillera.     

Partial Melting of Aluminous Metagreywackes in the Northern Sierra de Comechingones, Central Argentina

We describe a suite of metamorphic and migmatitic rocks fromthe Sierra de Comechingones (Sierras Pampeanas of Central Argentina)that include unmelted gneisses, migmatites and refractory granulites.The gneisses are aluminous greywackes metamorphosed in the amphibolitegrade and are likely to have been the protoliths for the higher-grademigmatites and granulites. Mineralogical characteristics andmajor and trace element compositions show that metatexite migmatites,diatexite migmatites and granulites are all melt-depleted rocks.The migmatites (both metatexites and diatexites) have undergoneH₂O-fluxed melting and lost     

Metaterrigenous rocks of the Irkut granulite-gneiss Block as indicators of the evolution of the Early Precambrian crust

Major, trace element, and Sm-Nd isotope data are presented for the garnet-biotite and cordierite-garnet-biotite gneisses from the Early Precambrian granulite complex of the Irkut Block (Sharyzhalgai Uplift, Siberian Craton). The garnet-biotite and cordierite-bearing gneisses of the Irkut Block were formed owing to the granulite metamorphism of metaterrigenous rocks. The chemical index of weathering and the content of clayey (pelitic) components in the normative mineral composition increase from the garnet-biotite gneisses to the cordierite-bearing gneisses, thus reflecting the maturation degree of initial sediments. Protoliths of the studied paragneisses correspond to a rock series ranging from the graywacke siltstones to clayey rocks. The trace and rare-earth element distribution indicates that the terrigenous material of the paragneisses was derived from felsic and mafic provenance. Increase in contents of Fe, Ti, Cr, Ni, and Sc and the Cr/Th ratio and decrease in the La/Sc ratio from the garnet-biotite to the cordierite-bearing gneisses reflect growth of the abundance of mafic rocks in the provenance. Potential sources of the detrital material were intermediate-felsic and mafic volcanic rocks (orthogneisses and basic crystalline schists) of the Irkut Block. The paragneisses show a distinct negative Eu anomaly (Eu/Eu* = 0.38–0.85), which suggests the input of crustal melting products, such as the potassium granites. A wide range of model Nd age (T_Nd(DM) = 2.4–3.1 Ga) of the paragneisses indicates the Archean to Early Paleoproterozoic age of their protoliths. The complex of isotopic, geochemical, and geochronological data, as well as the character of association of metaterrigenous rocks (mature pelites and carbonate rocks included), implies that sedimentation was separated in time from volcanism. The sedimentation was preceded by metamorphism, granite formation, and tectonic stabilization of the Irkut Block crust.     

Geochemistry and origin of metabasites from the granulite-gneiss complex of the Angara-Kan block,southwestern Siberian craton

In this paper, we present data on major and trace elements in highly metamorphosed mafic rocks from the granulite-gneiss complex of the Angara-Kan block (southwestern Siberian craton), identify igneous protoliths of the metabasites, and assess the mobility of elements during metamorphism. Two types of rocks with different geologic relations and compositions were recognized. Garnet-bearing two-pyroxene granulites (Cpx + Pl + Grt + Opx) occur as sheet- and boudin-like bodies, which were folded and deformed with their host paragneisses. Dikes, which in most cases underwent only brittle deformation, are composed of metabasites characterized by the assemblage Cpx + Hbl + Pl + Grt. The major element compositions of igneous protoliths for the mafic granulites and metabasite dykes correspond to variously differentiated basaltic magmas. The protoliths of the metabasites are depleted in K₂O, LILE, Zr, Nb, and LREE and were derived from a depleted mantle source. The major and trace element compositions of the dike metabasites are similar to those of low-K tholeiitic basalts of oceanic island arcs. Continental intraplate basalts derived from an enriched mantle source are possible igneous protoliths for the mafic granulites enriched in Ba, LREE, Nb, Ta, Zr, and Hf. It is assumed that lower Rb, Th, and U contents in the mafic granulites compared with continental flood basalts, high K/Rb and La/Th, and moderate Th/U ratios reflect the loss of Rb, Th and U during granulite-facies metamorphism.     

Geochemistry of the Neoproterozoic (800–767 Ma) Cerro Bori orthogneisses, Dom Feliciano Belt in Uruguay: tectonic evolution of an ancient continental arc

The Cerro Bori orthogneisses, crystallized between ca. 800 and 767 Ma, are composed of a sequence of mafic gneisses, with dioritic-gabbroic and dioritic composition tectonically interleaved with a sequence of tonalitic and granodioritic gneisses. These rocks intruded the Chafalote paragneisses (metapelites, semipelites, carbonate and mafic rocks) and they were metamorphosed of high P-T conditions at ca. 676–654 Ma. This paper presents the first major and trace geochemical signatures, as well as Sm and Pb isotopic composition for the Cerro Bori orthogneisses, which allow distinguishing three different groups of rocks. Type I rocks are mafic gneisses with tholeiitic affinity, whereas the Type II rocks are tonalitic and granodioritic gneisses with calc-alkaline affinity. The third type is composed of biotite-rich mafic gneisses with potassic and ultrapotassic affinities. All the three types of rocks have negative ?_ND values (between ?2.12 and ?6.67) and old T_DM ages (between 1.2 and 2.0 Ga), indicating that the process of crustal assimilation/contamination was an important process, together with fractional crystallization. An continental arc tectonic setting is suggested to this association of rocks between 800 and 767 Ma. This subduction suggests the existence of an ocean between Rio de La Plata and adjacent cratons during the break up of the Rodinia supercontinent.     

The Granulite-Facies Rocks of the Northern Segment of the Ribeira Belt, Eastern Minas Gerais, SE Brazil

Granulite rocks are exposed in eastern Minas Gerais, Brazil. Its early neoproterozoic evolution is characterised by a history of an active continental margin, including the accretion of suspect terranes. The Manhuaçu Terrane is one of those which is represented by a granitic continental plutonic arc and terrigeneous metasediments reflecting a continental margin. A metasedimentary gneiss belt at this margin with shallow to deep marine clastic lithologies as well as metavolcanic and metaplutonic mafic rocks was interpreted as an extensive tectonic segment with suspect development in a back-arc setting. Fragments of a volcanic arc are identified and interpreted as an evidence for a probable island-arc domain. The granulites occur as massive rocks as well as high-grade gneisses and show lithological, structural and metamorphic attributes consistent with their host belt type. In the western portion granulites derived from sedimentary protoliths, have been deposited, deformed and metamorphosed together with the mafic intrusions and as well as with their crystallization. Regional uplift exposed these rocks probably immediately after the metamorphism. In these belts the metamorphic grade is not uniform, especially where uplifting has exposed oblique cross sections over the granulitic rocks. Geothermobarometric calculations indicate that the granulites has been generated under T conditions between 800 and 990 ± 50 C and from medium (4.8 kb) to relatively high (10.0 kb) pressures.     

Volcanogenic Cu–Pb–Zn bodies in granulites of the central Arunta Block, central Australia

Abstract Small unexploited copper-lead-zinc deposits, characterized by a distinctive wall-rock association of cordierite quartzite, silica-undersaturated rocks, calc-silicate rocks and impure marbles, occur in quartzofeldspathic gneisses and mafic granulites of the Strangways Metamorphic Complex, central Arunta Block, central Australia. Available data support the hypothesis that these are metamorphosed volcanogenic ore bodies. The chemical compositions of the quartzofeldspathic gneisses are comparable with those of less metamorphosed felsic igneous rocks, particularly the felsic igneous rocks emplaced in the North Australian Orogenic Province in the interval 1880–1800 Ma; and the mafic granulites are chemically similar to basalts (olivine-normative tholeiites). The wall-rock suite can be correlated from chemistry and lithological association with the suites of wall rocks found in unmetamorphosed volcanogenic ore deposits. That the protolith of the cordierite quartzites may well have been leached tuff, similar to the illite-chlorite-quartz tuff found in volcanogenic ore deposits, is also shown by retrogression of the granulitefacies assemblage: cordierite-garnet-ortho-pyroxene-biotite-quartz in the cordierite quartzites to cordierite-anthophyllite-bearing assemblages and thence to chlorite-muscovite-quartz assemblages. Lenses of silica-undersaturated rocks with spinel and, less commonly, sapphirine are interpreted as the metamorphosed equivalents of chlorite-rich pods found within leached tuffs in volcanogenic ore deposits. The wall rocks form sheet-like bodies; this suggests that they were deposited in relatively shallow water, thus precluding the formation of massive sulphides.     

Early Precambrian high-grade metamorphosed terrigenous rocks of granulite-gneiss terranes of the Sharyzhalgai uplift (southwestern Siberian craton)

We present results of geochemical and Sm-Nd isotope studies of high-grade metaterrigenous rocks of the Kitoi and northwestern Irkut terranes of the Sharyzhalgai uplift on the Siberian Platform in comparison with paragneisses of the southeastern Irkut terrane. The metasedimentary rocks of the first region are high-alumina garnet-sillimanite-cordierite-bearing paragneisses; their protoliths were mostly mudstones and pelitic mudstones by major-element composition. The low-alumina biotite gneisses of the Kitoi terrane formed, most likely, from magmatic protoliths similar in petrochemical features to intraplate volcanics. The major factor controlling the composition of the studied high-alumina paragneisses is precipitation of most of incompatible trace elements in the clay fraction of sediments, as evidenced from the positive correlation between trace-element and Al₂O3 contents. The Cr and Ni contents, showing a positive correlation with MgO and no correlation with Al₂O3, are an indicator of the contribution of the mafic-source material to the formation of high-alumina rocks. The contribution of a mafic source-derived material to the formation of terrigenous rocks increases in passing from Kitoi to northwestern Irkut terrane. The high-alumina and garnet-biotite paragneisses of the southeastern Irkut terrane are similar in trace-element patterns to the analogous rocks of the Kitoi terrane and northwestern part of the Irkut terrane but show higher Th contents and a distinct negative Eu anomaly related to the change in the composition of the felsic source. The participation of felsic potassic igneous rocks in the formation of the southeastern terrigenous sediments is consistent with their deposition after the Neoarchean collision processes (metamorphism and granite magmatism), whereas sedimentation in the Kitoi and northwestern Irkut terranes preceded them. The Sm-Nd isotope characteristics indicate that the latter sediments formed mostly as a result of the erosion of the Paleo-Mesoarchean crust, whereas the metasediments of the southeastern Irkut terrane formed with the participation of Paleoproterozoic juvenile rocks. Thus, the variations in the trace-element and isotope compositions of the high-grade metamorphosed terrigenous rocks reflect recycling and growth of the continental crust of the Sharyzhalgai uplift during the Neoarchean-Pa- leoproterozoic transition.     

华北克拉通孔兹岩带中段大青山-乌拉山变质杂岩立甲子基性麻粒岩年代学及地球化学研究

大青山-乌拉山变质杂岩立甲子基性麻粒岩主要由角闪二辉麻粒岩、含榴角闪二辉麻粒岩和黑云角闪二辉麻粒岩所组成,并以变形岩墙和不规则透镜体形式赋存于富铝片麻岩和花岗质片麻岩之中.立甲子基性麻粒岩中变质锆石含有单斜辉石(Cpx)+角闪石(Amp)+斜长石(Pl)+磷灰石(Ap)的包体矿物,与寄主岩石——基性麻粒岩矿物组合及其化学成分十分一致,相应的207 pb/206 Pb表面年龄变化于1933±39Ma ～ 1834±40Ma,加权平均年龄为1892±7Ma(MSWD =0.50,n=46),应代表立甲子基性麻粒岩原岩经历中低压麻粒岩相的变质时代.在变质过程中,以大离子亲石元素(K、Na、Sr、Rb)为代表的活动元素发生了显著的改变;而高场强元素(Nb、Zr、Ti)和稀土元素基本无变化,保持稳定.立甲子基性麻粒岩原岩属于拉斑玄武质岩石系列,其SiO2集中变化于45.58％ ～51.40％,Mg#值集中介于41 ～54之间;在球粒陨石标准化稀土配分图中,立甲子基性麻粒岩具有平坦型的稀土配分曲线特征((La/Yb)cN=1.30～1.51),Eu异常不明显(Eu/Eu*=0.93～1.04).与显生宙岛孤拉斑玄武岩类似,立甲子基性麻粒岩所有样品皆具有Nb、Zr、Ti负异常特征.综合分析认为,立甲子基性麻粒岩原岩形成于2450～1930Ma,并于～1900Ma经历中低压麻粒岩相变质作用的改造,其主量元素和微量元素组成具有岛弧拉斑玄武质岩石的地球化学特征,其原岩可能是板块汇聚动力学背景下,岛弧构造环境中形成的辉长岩或辉绿岩.     

Geothermobarometry of mafic granulites and metapelite from the Palghat Gap, South India: petrological evidence for isothermal uplift and rapid cooling

The Palghat Gap region is located near the centre of the large southern Indian granulite terrane. at the northern edge of the Kodaikanal charnockite massif. The dominant rock types in the region are hornblende-biotite ± orthopyroxene gneisses and charnockites along with minor amounts of intercalated mafic granulite, metapelite and calc-silicate. The P-T estimates from garnetiferous mafic granulites and metapelite samples are generally in the range 9-10 kbar and 800-900 C using both conventional thermobarometric methods and the TWEEQU thermobarometry program. These P-T estimates, which should be taken as minimum values, are among the highest yet reported for South Indian and Sri Lankan granulites. The occurrence of orthopyroxene + plagioclase symplectites around embayed garnet grains in the mafic granulites and cordierite rims around garnet grains in metapelite suggest an isothermal decompression-type path. Similarly, a core-rim P-T trajectory indicates c. 3 and 7 kbar decompression at high temperature in the mafic granulites and metapelite, respectively. In both rock types, the key to the determination of the retrograde P-T path was the recognition of small amounts of second generation plagioclase with a more anorthitic composition than the matrix plagioclase. The preservation of high garnet-pyroxene temperatures in the mafic granulites (despite small garnet grain size) suggests rapid cooling of the terrane. Calculated minimum cooling rates range from 8 to 80 C Ma^-1. Such cooling rates are more rapid than those associated with normal isostatic processes and suggest that the terrane was tectonically exhumed at high temperature.     

Origin and age of ultramafic rocks and gabbros in the southern Puna of Argentina: an alleged Ordovician suture revisited

Ultramafic rocks and gabbros are exposed in the southern Puna (NW Argentina) in tectonic association with continental arc-related Ordovician (volcano) sedimentary successions and granitoids. The origin of this mafic rock suite has been debated for three decades as either representing an Ordovician terrane suture, primitive Ordovician arc-related rocks or relics of the pre-Ordovician basement in tectonic contact with the Ordovician retro-arc basin successions. We present the first U–Pb ages of primary and inherited zircon from gabbros of this mafic–ultramafic assemblage. LA-ICP-MS analyses on cores and rims of these zircon grains yielded a concordia age of 543.4 ± 7.2 Ma for the gabbroic rocks. Other analysed zircons have Mesoproterozoic, and Early Ediacaran core and rim ages indicating that the magmas also assimilated Meso- and Neoproterozoic crustal material prior to final crystallization. The mafic rocks witnessed higher metamorphic grade than associated Ordovician rocks, which are unmetamorphosed or only affected by anchimetamorphism. The gabbros are mostly tholeiitic and enriched in Zr, Th, as well as other incompatible elements and have εNd _t=540Ma ranging from 1.3 to 7.4 with most of the values between 5 and 7. ¹⁴⁷Sm/¹⁴⁴Nd ratios show evidence of weak crustal contamination. The mafic rocks do not reveal any affinity to mid-ocean ridge basalts in their geochemistry but point instead to an emplacement in an active plate margin arc environment. Chromites from ultramafic rocks show typical Ti, Al, Cr#, Fe³⁺ abundances found in magmatic arc rocks. The formation of the gabbros and the associated ultramafic rocks in the southern Argentine Puna is related to the evolution of the margin of the Pampia terrane, including the Puncoviscana basin, during the Late Neoproterozoic and earliest Cambrian. In contrast to previous interpretations, the rocks predate the Ordovician evolution of the Central proto-Andean active margin. Consequently, interpretations assuming these rocks to represent an oceanic terrane suture of Ordovician age have to be dismissed as much as all palaeotectonic models that define Ordovician terranes in the Central Andes based on assumption that the ultramafic rocks and gabbros exposed in the southern Puna mark plate boundaries.     

Origin of Mg-Metatholeiites of the Schirmacher Region,East Antarctica: Constraints from Trace Elements and Nd-Sr Isotopic Systematics

The mafic granulites of Schirmacher region, East Antarctica, the rocks under study, occur more or less as concordant sills or as lenses or as boudinage structures within the felsic rocks, charnockites or metapelites of the region. They show variation from garnet bearing two-pyroxene granulites and garnet free pyroxene granulites to transitional amphibolite-pyroxene granulites. Their major, trace, REE and isotopic chemistry are not distinct from each other and they represent Mg-basalts with MgO >7% and Al₂O₃ <16%.The majority of the analyzed samples plot in the tholeiitic field or show tholeiitic trends, suggesting their metatholeiitic nature as well as general preservation of original composition. The rocks are characterized by enriched large-ion lithophile elemental concentrations than that of mid-oceanic ridge basalts. Their high-field strength elements and heavy rare-earth elemental concentrations, however, are as that of mid-oceanic ridge basalts, a feature which is also reflected in the ratios of their large-ion lithophile elements against high-field strength elements and heavy rare-earth elements, wherein we find these ratios are higher than N-type MORB. Further, the rocks show negative Nb anomaly, high Th/Ta ratio and low La/Nb ratio, which are also characteristics of subduction-related magmatism.The isotopic studies carried out on these samples show that, the Sm-Nd and Rb-Sr dating did not yield much spread, but suggested a Sm-Nd metamorphic age of ∼960 Ma. Rb-Sr dating gave ages ∼886 Ma, suggesting the reworking of the Rb and Sr elements during subsequent tectonothermal overprinting. The Nd model ages (T_DM^Nd) of these rocks show a relatively restricted range of 1120 to 1357 Ma, suggesting mafic magmatism ∼1200 Ma. The positive eNd values (+4.22 to +6.07) shown by these rocks, represent a juvenile crustal fragment derived from melting of mantle precursors, without significant reworking of older crustal material. It is proposed that these rocks were produced by partial melting of a mantle source, characterized by LILE enrichment, related principally to dehydration of subducted oceanic crust.     

Geochemical Characteristics of Archean High-Grade Terrain of Kongling Complex and Early Crustal Evolution of Yangtze Craton

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Geochemistry,origin, and nature of metamorphic rocks of the Batomga granite-greenstone terrane (Aldan Shield)

Two series of volcanic rocks with different petrochemical affinities-calc-alkaline and komatiitetholeiitic series-were identified as protoliths for the Early Proterozoic metamorphic rocks of the Batomga granite-greenstone terrane. The metavolcanic rocks of the calc-alkaline series comprise metabasalts, metaandesites, metadacites, and metarhyolites. The distribution of the trace element abundances in the felsic metavolcanic rocks is similar to that of the Archean grey gneisses from the platform basements, thus suggesting a similar petrological mechanism for the formation of their protoliths. The protoliths for the komatiite-tholeiitic metavolcanic rocks include komatiite and tholeiite basalts. The chemical behavior of the tholeiites tends to support the fractionation of primary high-Mg basaltic magmas in a transient magma chamber at low pressures. The variations in the Nb, Y, and Zr contents of the metatoleiites indicate the derivation of their parental magmas from a plume-related source.     

Magnesium isotopic behaviors between metamorphic rocks and their associated leucogranites,and implications for Himalayan orogenesis

Magnesium isotopic compositions, along with new Sr–Nd–Pb isotopic data and elemental analyses, are reported for 12 Miocene tourmaline-bearing leucogranites, 15 Eocene two-mica granites and 40 metamorphic rocks to investigate magnesium isotopic behaviors during metamorphic processes and associated magmatism and constrain the tectonic-magmatic-metamorphic evolution of the Himalayan orogeny. The gneisses, granulites and amphibolites represent samples of the Indian lower crust and display large range in δ²⁶Mg from −0.44‰ to −0.09‰ in mafic granulites, −0.44‰ to −0.10‰ in amphibolites, and −0.70‰ to −0.03‰ in granitic gneisses. The average Mg isotopic compositions of the granitic gneisses (−0.19 ± 0.34‰), mafic granulites (−0.22 ± 0.17‰) and amphibolites (−0.25 ± 0.24‰) are similar, indicating the limited Mg isotope fractionation during prograde metamorphism from granitic gneisses to mafic granulites and retrograde metamorphism from mafic granulites to amphibolites. The Eocene two-mica granites and Miocene leucogranites are characterized by large variations in elemental and Sr–Nd–Pb isotopic compositions. The leucogranites and two-mica granites have their corresponding (⁸⁷Sr/⁸⁶Sr)_i varying from 0.7282 to 0.7860 and 0.7163 to 0.7191, (¹⁴³Nd/¹⁴⁴Nd)_i from 0.511888 to 0.512040 and 0.511953 to 0.512076, ²⁰⁷Pb/²⁰⁴Pb from 15.7215 to 15.7891 and 15.7031 to 15.7317, ²⁰⁸Pb/²⁰⁴Pb from 38.8521 to 39.5286 and 39.2710 to 39.4035, and ²⁰⁶Pb/²⁰⁴Pb from 18.4748 to 19.0139 and 18.7834 to 18.9339. However, they have similar Mg isotopic compositions (−0.21‰ to +0.06‰ versus −0.24‰ to +0.09‰), which did not originate from fractional crystallization nor source heterogeneity. Based on hornblende/biotite/muscovite dehydration melting reaction and Mg isotopic variations in two-mica granites and leucogranites with the proceeding metamorphism, along with elemental discrimination diagrams, Eocene two-mica granites and Miocene leucogranites could be related to hornblende dehydration melting and muscovite dehydration melting, respectively. Mg isotopic compositions of Eocene two-mica granites become heavier compared to the source because of residues of isotopically light garnet in the source; while those of Miocene leucogranites become lighter because of entrainment of isotopically light garnet from the source region. Thus, a new model for crustal anatexis and Himalayan orogenesis was proposed based on the Mg isotope fractionation in the leucogranites and metamorphic rocks. This model emphasizes a successive process from Indian continental subduction to rapid exhumation of the Higher Himalayan Crystalline Series (HHCS). The former underwent high-temperature (HT) and high-pressure (HP) granulite-facies prograde metamorphism, which resulted in the hornblende dehydration melting and the formation of Eocene two-mica granites; while the latter experienced amphibolite-facies retrogression and decompression, which resulted in the muscovite dehydration melting and the formation of Miocene leucogranites.     

Tracing crustal contamination of the Cenozoic basalts with OIB-affinity in northern marginal region of North China Craton: An Os perspective

The Cenozoic basalts with OIB-affinity in northern marginal region of the North China Craton are thought to experience minor even no crustal contamination during the magma evolution. The whole-rock Sr-Nd-Pb-Hf isotopes are attributed to a two-component mixing between depleted and enriched mantle sources, while the major element variations are controlled by the fractional crystallization of olivine and clinopyroxene. However, in this study, the new Os isotopic data proposes an opposite model for the Cenozoic basalts in northern marginal region of the North China Craton. In this model, the Jining basalts were contaminated by the Archean mafic rocks during the magma storage and ascent. The crustal contamination process is supported by (1) the highly radiogenic Os isotopic compositions, and (2) the positive correlation between ¹⁸⁷Os/¹⁸⁸Os and 1/Os of the Jining basalts. By modeling the Os isotopic composition of the basalts, an incorporation of < 10% mafic granulites/amphibolites to the parental magma can successfully explain the initial values of highly radiogenic Os. In contrast, the unradiogenic and uniform Os isotopic compositions of the Chifeng basalts suggest negligible crustal contamination. Os isotopic data acts as an indicator of crustal contamination during magma evolution, providing us a novel insight into the evolution of the intra-continental OIB-like basalts worldwide.©2021 China Geology Editorial Office.     

Petrogenesis of a high-temperature metamorphic terrane: a new tectonic interpretation for the north Dabieshan, central China

ABSTRACT The northern Dabie terrane consists of a variety of metamorphic rocks with minor mafic-ultramafic blocks, and abundant Jurassic-Cretaceous granitic plutons. The metamorphic rocks include orthogneisses, amphibolite, migmatitic gneiss with minor granulite and metasediments; no eclogite or other high-pressure metamorphic rocks have been found. Granulites of various compositions occur either as lenses, blocks or layers within clinopyroxene-bearing amphibolite or gneiss. The palaeosomes of most migmatitic gneisses contain clinopyroxene; melanosomes and leucosomes are intimately intermingled, tightly folded and may have formed in situ. The granulites formed at about 800–830 °C and 10–14 kbar and display near-isothermal decompression P–T paths that may have resulted from crust thickened by collision. Plagioclase-amphibole coronae around garnets and matrix PI + Hbl assemblages from mafic and ultramafic granulites formed at about 750–800 °C. Partial replacement of clinopyroxene by amphibole in gneiss marks amphibolite facies retrograde metamorphism. Amphibolite facies orthogneisses and interlayered amphibolites formed at 680–750 °C and c. 6 kbar. Formation of oligoclase + orthoclase antiperthite after plagioclase took place in migmatitic gneisses at T ≤ 490°C in response to a final stage of retrograde recrystallization. These P–T estimates indicate that the northern Dabie metamorphic granulite-amphibolite facies terrane formed in a metamorphic field gradient of 20–35 °C km^-1 at intermediate to low pressures, and may represent the Sino-Korean hangingwall during Triassic subduction for formation of the ultrahigh- and high-P units to the south. Post-collisional intrusion of a mafic-ultramafic cumulate complex occurred due to breakoff of the subducting slab.     

A seismogenic zone in the deep crust indicated by pseudotachylytes and ultramylonites in granulite-facies rocks of Calabria (Southern Italy)

Pseudotachylyte veins frequently associated with mylonites and ultramylonites occur within migmatitic paragneisses, metamonzodiorites, as well as felsic and mafic granulites at the base of the section of the Hercynian lower crust exposed in Calabria (Southern Italy). The crustal section is tectonically superposed on lower grade units. Ultramylonites and pseudotachylytes are particularly well developed in migmatitic paragneisses, whereas sparse fault-related pseudotachylytes and thin mylonite/ultramylonite bands occur in granulite-facies rocks. The presence of sillimanite and clinopyroxene in ultramylonites and mylonites indicates that relatively high-temperature conditions preceded the formation of pseudotachylytes. We have analysed pseudotachylytes from different rock types to ascertain their deep crustal origin and to better understand the relationships between brittle and ductile processes during deformation of the deeper crust. Different protoliths were selected to test how lithology controls pseudotachylyte composition and textures. In migmatites and felsic granulites, euhedral or cauliflower-shaped garnets directly crystallized from pseudotachylyte melts of near andesitic composition. This indicates that pseudotachylytes originated at deep crustal conditions (>0.75 GPa). In mafic protoliths, quenched needle-to-feather-shaped high-alumina orthopyroxene occurs in contact with newly crystallized plagioclase. The pyroxene crystallizes in garnet-free and garnet-bearing veins. The simultaneous growth of orthopyroxene and plagioclase as well as almandine, suggests lower crustal origin, with pressures in excess of 0.85 GPa. The existence of melts of different composition in the same vein indicates the stepwise, non-equilibrium conditions of frictional melting. Melt formed and intruded into pre-existing anisotropies. In mafic granulites, brittle faulting is localized in a previously formed thin high-temperature mylonite bands. migmatitic gneisses are deformed into ultramylonite domains characterized by s-c fabric. Small grain size and fluids lowered the effective stress on the c planes favouring a seismic event and the consequent melt generation. Microstructures and ductile deformation of pseudotachylytes suggest continuous ductile flow punctuated by episodes of high-strain rate, leading to seismic events and melting.     

Crustal make-up of the northern Andes: evidence based on deep crustal xenolith suites, Mercaderes, SW Colombia

Samples of the deep crust and upper mantle in the Northern Andes occur as abundant xenoliths in the Granatífera Tuff, a late Cenozoic vent in the Mercaderes area of SW Colombia. The lower crustal assemblage includes granulites, hornblendites, pyribolites, pyroxenites and gneisses; mafic rocks predominate, but felsic material is also common. P–T conditions for the pyribolite assemblages (i.e. Hbl+Fs/Scp+Grt+Cpx+Qtz±Bt), which are the best constrained, are 720–850 °C and 10–14 kbar, consistent with a deep-to-lower crustal origin. A notable feature of this xenolith suite is that it is dominated by hornblende. However, mineral reactions within the suite show that there is a transition from amphibolite to granulite facies, and there is a probable restite–melt relationship represented within the suite. However, the latter appears to be dominated by hornblende and garnet.The mafic rocks mostly lack the high Cr and Ni that would be expected of cumulates. Neither do they possess the positive Sr and Eu anomalies that would be consistent with resite or cumulate models for the lower crust. They bear greatest similarity to oceanic basalts (s.l.). The Rb contents of the xenoliths, whether mafic or silicic, are very low, and the more silicic members of the suite tend to have small positive Sr and Eu anomalies, which are transitional to adakitic compositions. The Sr isotopic compositions of the xenoliths lie between 0.704 and 0.705; however, the Nd isotopic compositions are much more variable, indicating considerable long-term heterogeneity. Few of the xenoliths can be compositionally recognised as metasedimentary; however, a sedimentary component is evident in the Pb isotopic compositions. Within these constraints, our favoured model is a deep crust formed by basaltic components (subduction–accretion?), and minor sediment, which is subject to an increase in thermal gradient to produce the granulites, any melting being dominated by hornblende-out reactions involving garnet. However, there is no evidence of any pervasive crustal melting, leading to the conclusion that the voluminous Andean magmatism arises from the mantle wedge.     

Superposition of replacements in the mafic granulites of the Jijal complex of the Kohistan arc, northern Pakistan: dehydration and rehydration within deep arc crust

A deep-level crustal section of the Cretaceous Kohistan arc is exposed in the northern part of the Jijal complex. The occurrence of mafic to ultramafic granulite-facies rocks exhibits the nature and metamorphic evolution of the lower crust. Mafic granulites are divided into two rock types: two-pyroxene granulite (orthopyroxene+clinopyroxene+plagioclase±quartz [1]); and garnet–clinopyroxene granulite (garnet+clinopyroxene+plagioclase+quartz [2]). Two-pyroxene granulite occurs in the northeastern part of the Jijal complex as a relict host rock of garnet–clinopyroxene granulite, where the orthopyroxene-rich host is transected by elongated patches and bands of garnet–clinopyroxene granulite. Garnet–clinopyroxene granulite, together with two-pyroxene granulite, has been partly replaced by amphibolite (hornblende±garnet+plagioclase+quartz [3]). The garnet-bearing assemblage [2] is expressed by a compression–dehydration reaction: hornblende+orthopyroxene+plagioclase=garnet+clinopyroxene+quartz+H₂O↑. Subsequent amphibolitization to form the assemblage [3] is expressed by two hydration reactions: garnet+clinopyroxene+plagioclase+H₂O=hornblende+quartz and plagioclase+hornblende+H₂O=zoisite+chlorite+quartz. The mafic granulites include pod- and lens-shaped bodies of ultramafic granulites which consist of garnet hornblendite (garnet+hornblende+clinopyroxene [4]) associated with garnet clinopyroxenite, garnetite, and hornblendite. Field relation and comparisons in modal–chemical compositions between the mafic and ultramafic granulites indicate that the ultramafic granulites were originally intrusive rocks which dissected the protoliths of the mafic granulites and then have been metamorphosed simultaneously with the formation of garnet–clinopyroxene granulite. The results combined with isotopic ages reported elsewhere give the following tectonic constraints: (1) crustal thickening through the development of the Kohistan arc and the subsequent Kohistan–Asia collision caused the high-pressure granulite-facies metamorphism in the Jijal complex; (2) local amphibolitization of the mafic granulites occurred after the collision.