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1.
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.  相似文献   

2.
Two stages of granitic magmatism occurred during the Pan-African evolution of the Kerala Khondalite Belt (KKB) in southern India. Granitic gneisses were derived from porphyritic granites, which intruded prior to the main stage of deformation and peak-metamorphism. Subsequently, leucogranites and leucotonalites formed during fluid-absent melting and intruded the gneiss sequences. Monazites from granitic gneisses, leucogranites and a leucotonalite were investigated by conventional U-Pb and electron microprobe dating in order to distinguish the different stages of magma emplacement. U-Pb monazite dating yielded a wide range of ages between 590–520 Ma which are interpreted to date high-grade metamorphism rather than magma emplacement. The results of this study indicate that the KKB experienced protracted heating (>50 Ma) at temperatures above 750–800 °C during the Pan-African orogeny. The tectonometamorphic evolution of the study area is comparable to southern Madagascar which underwent a similar sequence of events earlier than the KKB. The results of this study further substantiate previous assertions that the timing of high-grade metamorphism in East Gondwana shifted from west to east during the Late Proterozoic.  相似文献   

3.
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4.
An assemblage of predominantly metasedimentary rocks in the Eastern Ghats Province, India, underwent granulite facies metamorphism and deformation in early Neoproterozoic times, and was subsequently intruded by the Koraput alkaline complex. The intrusion was earlier believed to be syn- to late tectonic. The gabbroic core of the complex hosts nepheline-bearing syenitic dykes and veins. Following emplacement, magmatic amphibole within the syenites, and early orthopyroxene in feldspathic gneisses within the country rocks were retrogressed to biotite during pervasive solid-state deformation. Subsequent prograde metamorphism resulted in the formation of anhydrous assemblages at the expense of relict magmatic amphibole within the syenites, and metamorphic biotite in both the complex and the country rocks. Reactions reconstructed from textural observations indicate breakdown of biotite and amphibole to garnet + clinopyroxene ± orthopyroxene-bearing assemblages. Schreinemakers’ analysis on the relevant mineral associations suggests that heating was followed by loading of the region. This indicates thermal rejuvenation of the complex and the host granulites during an intracrustal orogeny that post-dates emplacement and cooling of the pluton. Available ages suggest that this event occurred in the mid-Neoproterozoic, and is probably unrelated to the amalgamation of the granulite belt with the Archaean Bastar/Dharwar craton.  相似文献   

5.
The digital image of airborne radiometric data across SouthAfrica reveals that the largest anomaly, 100 nGy/h, is causedby the granulite-facies rocks of the Namaquan metamorphic complex,whereas most of the country is <60 nGy/h. This observationis consistent with geochemical data that show that the 1900± 100 Ma greenschist-facies Richtersveld Terrane nearNamibia (max. U = 3·4 ppm; Th = 20·1 ppm) andthe adjacent, 1100 ± 100 Ma, amphibolite-facies Aggeneys/SteinkopfTerranes (max. U 10 ppm; Th 52 ppm) are the least enrichedin U, Th and K. In contrast, the lower-T granulite-facies OkiepTerrane near Springbok hosts more enriched granites (max. U 17 ppm; Th 66 ppm) and noritic intrusions (max. U = 14 ppm;Th = 83 ppm). The most enriched rocks are found in the 1030Ma higher-T granulite-facies core of the Namaquan belt and includequartzo-feldspathic gneisses (max. U = 46 ppm; Th = 90 ppm)and charnockites (max. U = 52 ppm; Th = 400 ppm). Our findingscontradict the notion that granulite-facies terrains are characteristicallydepleted in U and Th. In this study we modeled the heat productionin the core of the Namaquan complex, where the granulites havehad a very unusual metamorphic history, and show that ultra-high-T(1000°C, P 10 kbar) metamorphic conditions could have beenachieved by radiogenic heating without invoking external heatsources. However, monazite-rich veins of charnockite and patchesof granulites mark the passage of CO2-dominated melts and fluidsderived from fractionated noritic intrusions. KEY WORDS: charnockite; granulite; Namaqualand; thorium; uranium; radioactive heating; metamorphism  相似文献   

6.
Partial melting has been shown to be an important mechanism for intracrustal differentiation and granite petrogenesis. However, a series of compositional differences between granitic melt from experiments and natural granites indicate that the processes of crustal differentiation are complex. To shed light on factors that control the processes of crustal differentiation, and then the compositions of granitic magma, a combined study of petrology and geochemistry was carried out for granites (in the forms of granitic veins and parautochthonous granite) from a granulite terrane in the Tongbai orogen, China. These granites are characterized by high SiO2 (>72 wt%) and low FeO and MgO (<4 wt%) with low Na2O/K2O ratios (<0.7). Minerals in these granites show variable microstructures and compositions. Phase equilibrium modelling using P–T pseudosections shows that neither anatectic melts nor fractionated melts match the compositions of the target granites, challenging the conventional paradigm that granites are the crystallized product of pure granitic melts. Based on the microstructural features of minerals in the granites, and a comparison of their compositions with crystallized minerals from anatectic melts and minerals in granulites, the minerals in these granitoids are considered to have three origins. The first is entrained garnets, which show comparable compositions with those in host granulites. The second is early crystallized mineral from melts, which include large plagioclase and K-feldspar (with high Ca contents) crystals as well as a part of biotite whose compositions can be reproduced by crystallization of the anatectic melts. The compositions of other minerals such as small grained plagioclase, K-feldspar and anorthoclase in the granites with low Ca contents are not well reconstructed, so they are considered as the third origin of crystallized products of fractionated melts. The results of mass balance calculation show that the compositions of these granites can be produced by mixing between different proportions of crystallized minerals and fractionated melts with variable amounts of entrained minerals. However, the calculated modal proportions of different crystallized minerals (plagioclase, K-feldspar, biotite and quartz) in the granites are significantly different from those predicted by melt crystallization modelling. Specifically, some rocks have lower modes of biotite and plagioclase, whereas others show lower K-feldspar modes than those produced by melt crystallization. This indicates that the crystallized minerals would be differentially separated from the primary magmas to form the evolved magmas that produce these granites. Therefore, the crystal entrainment and differential melt-crystal separation make important contributions to the composition of the target granites. Compared with leucogranites worldwide, the target granites show comparable compositions. As such, the leucogranites may form through the crystal fractionation of primary granitic magmas at different extents in addition to variable degrees of partial melting.  相似文献   

7.
Psammitic and pelitic gneisses of the Kerala Khondalite belt in southern India were subjected to granulite-facies metamorphism and migmatisation through fluid-absent biotite dehydration-melting which led to the generation of garnet-bearing leucogranites. Geochemical investigations provide support that the leucogranites were generated from gneisses compositionally similar to those into which they were emplaced.The intrusion of apatite-rich granites subsequent to the garnet-bearing leucogranites is recognized predominantly in leptynitic gneisses. A comparison of the chemical composition of both garnet- and apatite-bearing varieties displays a systematic shift to intermediate compositions (increase in CaO, FeO, LREE, Zr, Y; decrease in SiO2 and δ18O) which coincides with higher modal abundances of apatite, garnet and biotite and decreasing contents of quartz and alkalifeldspar. The observation that apatite occurs in aggregates together with biotite and/or garnet as well as the extreme concentrations of FeO, LREE and Zr in some of the apatite-beraing granites which clearly exceed the maximum solubility values for these elements in granitic melts indicates that dehydration-melting alone, as it has been proposed for the generation of garnet-bearing leucogranites, cannot have been responsable for generation of apatite-rich leucogranites but that additional processes like crystal entrainment and accumulation or liquid removal must also have played a role.  相似文献   

8.
Abstract Lewisian grey gneisses from Gruinard Bay, North-west Scotland retain mineralogical and geochemical evidence for Scourian horn-blende-granulite facies metamorphism, and they may be used to assess current models of elemental depletion at granulite grade. Their 'immobile'major and trace element geochemistry is indistinguishable from that of Lewisian amphibolite and pyroxene-granulite facies counterparts. The K, Rb, Th and U contents of the Gruinard Bay gneisses are depleted relative to amphibolite facies gneisses, but generally the abundances of these elements are above those of comparable pyroxene granulites. U and Th have reached an advanced stage of depletion, but allanite appears to be crucial in maintaining significantly higher U and Th abundances at Gruinard Bay than in pyroxene granulites. K and Rb loss is less extreme, and depends on the stability of the rock-forming minerals: K-feldspar; biotite; and, amphibole. Early removal of K and Rb has resulted in a small rise in K/Rb, but further preferential Rb loss would have been required to generate the characteristically high K/Rb ratios of Lewisian pyroxene granulites.
The residence of U and Th in the accessory minerals of granulite facies gneisses, which are often correlated with the residua of intracrustal partial melting, renders unlikely their extreme incompatibility required by such models. Even if such phases are ignored, high mineral-melt partition coefficients for silicic melts argue against partial fusion as an efficient depletion mechanism. On the other hand, the advanced stage of U and Th depletion reached in Gruinard Bay gneisses, which were still partly hydrous, severely restricts the role played by CO2-dominated fluids and a hydrous medium is preferred.  相似文献   

9.
Magmatism in central Anatolia is characterized by petrographically and chemically distinct granitic and syenitic rocks. The granitic magmatism comprises C-type (crustal-derived) and H-type (hybrid) monzogranites and monzonites. Garnet-bearing C-type leucogranites represent the oldest magmatic phase, but younger hornblende ± biotite ± K-feldspar H-type plutons dominate the geology of the Central Anatolian Crystalline Complex (CACC). These typically include mafic microgranular enclaves. The granitic magmatism predates syenitic intrusions, among which quartz-bearing syenites were emplaced prior to feldspathoid-bearing ones.

The nature of magmatism in central Anatolia varies through time from peraluminous to metaluminous to alkaline. These different magma types reflect distinct stages of postcollisional magmatism, in which interaction between crust and mantle varied considerably. The C-type granites of the early stages of postcollisional magmatism were likely derived by partial melting of the lower continental crust induced by mafic magma underplating as a result of lithospheric delamination. The H-type granites and syenites of the mature and advanced stages of postcollisional magmatism indicate a significant contribution from mande-derived magma within a continuous or episodic extensional tectonic regime.  相似文献   

10.
The results of a chemical study of a suite of low-pressure granulite facies rocks in Namaqualand, South Africa, are reported. The area is underlain by augen gneisses and quartzites, which contain interlayered granular quartz-feldspar rocks (termed ‘granulites’) derived by extensive partial melting of the gneiss. The K/Rb ratio of the gneiss increases from 140 to 250 over a melting interval of 70%: the rate of increase being influenced by the presence of biotite. Simultaneously K/Ba and Rb/Sr decrease from 80 to 25 and from 4 to 0.3, respectively. The partial melts (granulites), which reflect, in part, a cumulate character, have similar K/Rb ratios to the parent gneiss (175) but larger K/Ba (238) and Rb/Sr (5) ratios, due to the retention of Ba and Sr in the residue.Three granites intrude the gneisses. One of these was produced by very advanced partial melting of the gneiss. Continuity of chemical composition suggests that the remaining two granites, although spatially separate, are comagmatic, and evolved by feldspar fractionation during ascent. Lower Sr87/Sr86 ratio coupled with enrichment of Ba, Sr and Rb in the parent magma of these granites relative to the country rocks precludes local derivation and indicates a lower crustal source rock of intermediate composition.The progressive increase in cafemic character of the gneisses, which is similar to that observed in world granulite terrains as a whole, coupled with intrusive granite which reflects reworking of the lower crust in the area studied, supports a partial melting model for the development of a lower crust of progressively more cafemic composition.  相似文献   

11.
The Imataca Complex in an area near Cerro Bolivar, Venezuela, consists of a conformable, predominantly acid, compositionally-intergradational, sequence of acid, intermediate and mafic granulites, granitic gneisses and amphibolites with minor iron-formation and other metasedimentary interlayers.Major- and trace-element compositions of granulites, granitic gneisses, and amphibolites, and compositional relations in pyroxenes and irontitanium oxides indicate an igneous protolith. Pyroxenes and oxides in the granulites appear to be relict igneous phases, and record TfO2 of original igneous crystallization, despite high-grade metamorphism. Mineral thermometers in granitic gneisses (biotite, FeTi oxides) and in metasedimentary biotite gneisses (cordieritegarnet, FeTi oxides) indicate metamorphic TPT of 625–675°C, 4–6.5 kbar.Major- and trace-element variations in meta-igneous granulites, granitic gneisses, and amphibolites are calc-alkaline in character, and the sequence, particularly in its predominantly acid composition, most closely resembles continental (i.e., “Andean”) calc-alkaline series. Mafic granulites and amphibolites, however, are tholeiitic and relatively iron-rich.The compositional similarity between the Imataca series and other granulite series raises the possibility that acidic calc-alkaline continental volcanism may be more important in the Archean than recent emphasis on greenstones and island-arc oceanic types of volcanism would suggest.  相似文献   

12.
Summary The granulite facies rocks of the Lower Austrian Moldanubian Zone of the Bohemian Massif were formed under conditions of the earth's lower crust. The U contents lie below 0.4 ppm. The rocks are also characterized by low Th contents (<3 ppm), although in some cases Th is higher in biotite-rich layers of banded granulites, which could be explained as a primary feature. Marginal parts of the granulite facies complexes occasionally exhibit retrograded U enrichments. The K contents of the granulites are generally lower than in the surrounding rock types of similar mineralogical composition (Gföhl gneisses).
Uran, Thorium und Kalium in Gesteinen der Granulit-Fazies aus der Böhmischen Masses in Niederösterreich, Österreich
Zusammenfassung Die Granulitfaziesgesteine des niederösterreichischen Moldanubikums wurden unter Bedingungen der tiefen Kruste gebildet. Die U-Gehalte der Gesteine liegen zum größten Teil unterhalb 0,4 ppm. Sie weisen auch geringe Th-Gehalte (<3 ppm) auf. Th ist allerdings in gewissen biotitreichen Partien höher, was auch als primäre Anlage gedeutet werden kann. In randlichen Bereichen der Granulitfacieskomplexe ist gelegentlich U leicht angereichert. Die K-Gehalte der Granulite sind im allgemeinen geringer als in umgebenden Gesteinen mit ähnlicher mineralogischer Zusammensetzung (GfÖhler Gneis).

With 4 Figures

Dedicated to Prof. DDr.H. Wieseneder on the occasion of his 70th birthday.  相似文献   

13.
SINGH  S. 《Journal of Petrology》1966,7(2):171-192
Orthopyroxene-bearing rocks of the South Savanna—Kanukucomplex of British Guiana vary in composition from basic toacid types. They include basic granulites; acid, orthopyroxene-bearingbiotite gneisses; orthopyroxene-bearing acid granulites; orthopyroxene-bearinggranites, and orthopyroxene-bearing xenoliths contained in theSouth Savanna Granite. The field, petrographic, and chemicalcharacters of the rocks bear strong resemblances to charnockitesdescribed from other parts of the world, but the origins formany charnockites are inconsistent with evidence advanced forthe evolution of the South Savanna—Kanuku rocks. Fieldand petrographic criteria indicate that the basic granuliteswere originally intrusive basic rocks and have suffered regionalmetamorphism of Granulite Facies grade, which induced the conversionof clinopyroxene to orthopyroxene. The orthopyroxene-bearing,acid biotite gneisses and acid granulites have resulted fromcontamination by enclosed basic granulites which occur as boudins,bands, and lenses. The orthopyroxene-bearing granites and xenolithsare related to the assimilation of intruded orthopyroxene-bearingcountry rocks by the South Savanna Granite and its apophyses. The chemistry of the rocks appears to support the conclusionsreached from field and petrographic studies by indicating consistentenrichment of Ca, Mg, Fe, and Na in the orthopyroxene-bearingrocks compared with their non-orthopyroxene-bearing equivalents.Variation diagrams also indicate slightly anomalous plots forthe orthopyroxene-bearing rocks of the complex.  相似文献   

14.
The 1.1 Ga Concordia Granite Gneiss (CGG) is part of the late to postorogenic Spektakel Suite in the western Namaqualand Metamorphic Complex, South Africa. It intruded synkinematically, with respect to the main (D2) deformation event, into lower to middle crustal rocks and granite emplacement was more or less coeval with the peak of granulite-facies metamorphism ( > 800°C, 5 kbar). Several genetically related rock types, megacrystic garnet-bearing granite, minor aplitic leucogranites and pegmatites are distinguished. All varieties are SiO2-rich (69–79 wt.%) peraluminous granites and show subalkaline-monzonitic magma characteristics. Geochemical differences in whole-rock chemistry between megacrystic granite and aplitic leucogranites (e.g., lower Al2O3, MgO, CaO, Ba, Zr; higher K2O, Rb, Nb, W, Rb/Sr, Ga/Al) and the decrease of e.g., CaO, MgO, Fe2O3, Ba, Zr, Th/U with increasing SiO2 in the megacrystic granites as well as the variation in Fe/Mn of magmatic garnets are best explained with crystal fractionation processes. Fractional crystallization of plagioclase produced potassium- and silica-rich residual melts characterised by very high Rb/Sr, Rb/Ba, U/Th, Mn/Fe ratios and higher concentration of W, Cu and Zn. Crystal fractionation processes also resulted in a relative LREE depletion and HREE enrichment (megacrystic granite: La/Lu)cn = 8.87−31.67; aplitic leucogranite and pegmatites: La/Lu)cn = 0.71−1.44) and evolution of pronounced negative Eu-anomalies. The crystallization sequence (near-solidus crystallization of biotite prior to alkali feldspar) suggests that the CGG magmas were H2O-undersaturated over a long period of their evolution. Water saturation during late-stage crystallization is, however, indicated by coarse late-stage eutectic mineral textures, pegmatites and WMo-bearing siliceous rocks. Furthermore the economic potential of the CGG is supported by its geochemical signature (e.g., high U, Th contents) which is similar to evolved high heat production (HHP) granites. The granitic magmas are attributed to partial melting of peraluminous crustal source rocks and are tentatively interpreted as fractionated S-type granites. The WMo deposits represent vein-type and pegmatitic deposits genetically related to a deep-seated granitic system.  相似文献   

15.
A suite of metapelites, charnockites, calc-silicate rocks, quartzo-feldspathic gneisses and mafic granulites is exposed at Garbham, a part of the Eastern Ghats granulite belt of India. Reaction textures and mineral compositional data have been used to determine the P–T–X evolutionary history of the granulites. In metapelites and charnockites, dehydration melting reactions involving biotite produced quartzofeldspathic segregations during peak metamorphism. However, migration of melt from the site of generation was limited. Subsequent to peak metamorphism at c . 860° C and 8 kbar, the complex evolved through nearly isothermal decompression to 530–650° C and 4–5 kbar. During this phase, coronal garnet grew in the calc-silicates, while garnet in the presence of quartz broke down in charnockite and mafic granulite. Fluid activities during metamorphism were internally buffered in different lithologies in the presence of a melt phase. The P–T path of the granulites at Garbham contrasts sharply with the other parts of the Eastern Ghats granulite belt where the rocks show dominantly near-isobaric cooling subsequent to peak metamorphism.  相似文献   

16.
A comparison of K, Rb, Th and U concentrations in granulite facies rocks with those of unmetamorphosed common rock types shows that depletion of these elements in granulites is variable. K/Rb ratios for granulites are generally higher than unmetamorphosed rocks, but K/Rb ratios only reach extreme values when K < 1%. The covariation of K/Rb ratio with K concentration suggests that protolith composition, hence mineralogy, is very important in controlling the degree of Rb depletion in granulites. Felsic granulites exhibiting extreme K/Rb ratios are mainly Archean, reflecting the high abundance of low K felsic rocks in Archean terrains. The Scourian granulites of Scotland all have very high K/Rb ratios and cannot be considered to be representative of granulite facies terrains. It is impossible from this data set to state conclusively whether K is depleted in granulites; K/La ratios of granulites show complete overlap with igneous rocks. Th/U ratios in many granulites are greater than 4, indicating U loss relative to Th. Felsic granulites with low Th/U ratios also have high La/Th ratios, indicating that these granulites have been depleted in Th. The low Th/U ratios of these rocks may reflect retention of Th and U in resistant accessory phases.  相似文献   

17.
阴山地区从东南到西北，依次划分为桑干麻粒岩区、集宁副变质岩区、乌拉山－大青山前缘构造区和色尔腾山岛弧过渡区。桑干麻粒岩区代表了中太古微古陆并经新太古强烈岩浆增生的太古地壳；集宁区代表基底之上新太古末期－古元古沉积盖层；乌拉山－大青山区是基底岩石构造剥露的前缘构造区；色尔腾山区主要由深部花岗岩和浅部绿岩构成的太古岛弧区，古元古时期北部古岛弧和南部古大陆碰撞汇聚，继后北东南面向伸展使阴山地区早前寒武纪岩石剥露。  相似文献   

18.
刘平华  邹雷  田忠华  冀磊  施建荣 《地质通报》2019,38(10):1691-1710
乌拉山岩群是狼山地区最重要的前寒武纪变质基底之一,准确测定其原岩成岩与变质时代,对于进一步探讨狼山地区前寒武纪地质演化具有重要的意义。对狼山地区乌拉山岩群角闪黑云斜长片麻岩及其伴生的花岗质浅色脉体进行了岩石学和锆石U-Pb年代学研究。碎屑锆石U-Pb定年和野外地质调查表明,狼山地区乌拉山岩群角闪黑云斜长片麻岩碎屑锆石年龄介于2591～1800Ma之间,其中最小一组碎屑锆石年龄为1873Ma,结合其约270Ma的变质年龄,初步限定乌拉山岩群角闪黑云斜长片麻岩的原岩沉积年龄为1873～270Ma。综合新的研究资料,认为狼山地区乌拉山岩群除存在新太古代—古元古代变质岩外,可能还存在中元古代—晚古生代变沉积岩。锆石阴极发光图像与U-Pb定年结果综合表明,角闪黑云斜长片麻岩中发育大量变质锆石,获得的206Pb/238U年龄加权平均值为269±4Ma,代表狼山地区乌拉山岩群遭受晚古生代末期角闪岩相变质作用的时代,可能与华北板块与西伯利亚板块晚古生代末期碰撞造山作用有关。此外,采用预剥蚀方法,在乌拉山岩群高硅花岗质浅色脉体高U锆石中,获得的~(206)Pb/~(238)U年龄加权平均值为264±3Ma,被解释为乌拉山岩群花岗质浅色脉体的形成时代,代表本区晚古生代造山作用由同碰撞挤压向碰撞后伸展转换的时限。  相似文献   

19.
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 (TNd(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.  相似文献   

20.
The Madurai Block, constituting part of the southern granulite terrain in southern India, has contributed significantly towards understanding the UHT (ultrahigh-temperature) granulites that serve as a window into the mid-lower continental crust. The dominant rock types are charnockites, sapphirine-bearing granulites, garnet cordierite gneisses, and quartzites. Significant textural relations reveal multiphase reactions responsible for the formation of diverse mineral parageneses during prolonged metamorphic history of the area. Prograde reaction is evident from the textural relationship where biotite/sillimanite relics are seen as inclusion in garnet/orthopyroxene, suggesting dehydration reactions. The symplectitic assemblages that formed during isothermal decompression involve a series of cordierite-forming reactions, followed by retrogression and cooling. Variety of mineral assemblages present in the rocks of this area offer a wide spectrum of P–T sensors that provide details on the physical conditions of metamorphism. For the rigorous interpretation of the P–T path in the Perumalmalai area, quantitative phase diagrams (P–T pseudosections) have been constructed and contoured for the compositional as well as modal isopleths of involved mineral phases. The rocks of Perumalmalai area document a clockwise decompression P–T trajectory, consistent with crustal thickening followed by extensional collapse. SHRIMP U–Pb ages from zircon associated with sapphirine-bearing granulite facies rocks of Perumalmalai area suggest a widespread Ediacaran tectonothermal event. The occurrence of Ediacaran UHT metamorphism followed by isothermal decompression in the Madurai Block is consistent with the timing and physical conditions associated with the formation of East African Orogen during the amalgamation of Gondwana.  相似文献   

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