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Guochun Zhao - One of the best experts on this subject based on the ideXlab platform.
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u pb and hf isotopic study of zircons of the helanshan complex constrains on the evolution of the khondalite belt in the western block of the north china craton
Lithos, 2011Co-Authors: Guochun Zhao, X. ZhouAbstract:Abstract The Helanshan Complex is located at the westernmost segment of the Khondalite Belt, a continent–continent collisional belt along which the Yinshan Block in the north collided with the Ordos Block in the south to form the Western Block of the North China Craton. The complex consists of S-type granites and high-grade pelitic granulite/gneiss, felsic paragneiss, quartzite, Calc-Silicate Rock and marble, together called the Khondalite Series Rocks. LA-Q-ICP-MS U–Pb geochronology and LA-MC-ICP-MS Lu–Hf isotopic data of zircons, combined with cathodoluminescence (CL) imaging, enable the resolution of magmatic and metamorphic events that can be directed towards understanding the formation and evolution of the Khondalite Belt in the Western Block of the North China Craton. CL images reveal the coexistence of magmatic-type detrital zircons and metamorphic zircons in most of the Khondalite Series Rocks, of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating magmatic-type detrital zircon cores. LA-Q-ICP-MS U–Pb analyses on magmatic-type detrital zircons reveal two distinct age populations, with one in the Archaean (2.85–2.53 Ga) and the other in the Palaeoproterozoic (2.2–2.0 Ga), suggesting that the sedimentary protoliths of the Khondalite Series Rocks in the Helanshan Complex must have been deposited at some time after 2.0 Ga. The Hf analyses show that the Archaean detrital zircons possess negative eHf(t) values from − 7.64 to − 0.15 with depleted mantle model ages ranging from 3.34 to 3.10 Ga. This implies the existence of a Paleo-Mesoarchaean (3.34–3.10 Ga) crust in the Western Block, which underwent a recycling event in the period 2.85–2.53 Ga. Nearly all Palaeoproterozoic (2.2–2.0 Ga) detrital zircons possess positive eHf(t) values (0.86–9.38) with depleted mantle model ages of 2.45–2.15 Ga, suggesting a significant crustal growth event in the Western Block in the Palaeoproterozoic. Metamorphic zircons yield two groups of age, with one at ~ 1.95 Ga and the other at ~ 1.87 Ga, of which the former is considered as the timing of the collision between the Yinshan and Ordos Blocks to form the Western Block, whereas the latter is consistent with the age of ~ 1.86 Ga from two S-type granites which are considered to have formed by the partial melting of pelitic granulites/gneisses at the stage of the exhumation of the Helanshan Complex.
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single zircon grains record two paleoproterozoic collisional events in the north china craton
Precambrian Research, 2010Co-Authors: Guochun Zhao, Simon A Wilde, Peter A Cawood, Xuping LiAbstract:Abstract The Western Block of the North China Craton consists of the Yinshan Block in the north and the Ordos Block in the south which were amalgamated along the east-west trending Khondalite Belt at ∼1.95 Ga. The Western Block then collided with the Eastern Block to form the coherent basement of the North China Craton along the north-south trending Trans-North China Orogen at ∼1.85 Ga. The Huaian Complex, a high-grade terrrane located at the conjunction of the Khondalite Belt and Trans-North China Orogen, records metamorphic events associated with both collisions. The complex consists of lithologies from both the Khondalite Belt and Trans-North China Orogen, of which the former consist of graphite–garnet–sillimanite gneiss, garnet quartzite, felsic paragneiss, Calc-Silicate Rock and marble, together called the Khondalite series. Zircons in the graphite–garnet–sillimanite gneiss can be divided into three types: (1) spherical grains without internal structures, (2) grains with a core-and-rim structure; and (3) grains with a dark core surrounded by double rims. Except for the dark core in type 3, all other types of zircon domains are structureless and highly luminescent, with very low Th/U ratios, typical of a metamorphic origin. Analyses on the cores of type 2 and the inner rims of type 3 from two samples yield upper intercept ages of 1946 ± 26 and 1947 ± 22 Ma, similar to previously determined metamorphic ages from the Khondalite Belt and thus interpreted as the time of collision between the Yinshan and Ordos Blocks. Analyses on type 1 zircons, rims of type 2 and the outer rims of type 3 from the same two samples give ages of 1850 ± 15 and 1857 ± 16 Ma, interpreted as the time of collision between the Eastern and Western Blocks. Thus, zircons in the graphite–garnet–sillimanite gneiss of the Huaian Complex record both of the Paleoproterozoic collisional events in the North China Craton.
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la icp ms u pb zircon ages of the qianlishan complex constrains on the evolution of the khondalite belt in the western block of the north china craton
Precambrian Research, 2009Co-Authors: Guochun Zhao, X. Zhou, Winghang LeungAbstract:Abstract The Qianlishan Complex is located in the westernmost part of the Khondalite Belt, a continent–continent collisional belt along which the Yinshan Block in the north and the Ordos Block in the south amalgamated to form the Western Block, which then collided with the Eastern Block along the Trans-North China Orogen to form the North China Craton. The complex is dominated by high-grade supracrustal Rocks and minor S-type granites, of which the supracrustals consist of graphite-bearing sillimanite-garnet gneiss, garnet quartzite, felsic paragneiss, Calc-Silicate Rock and marble. CL images reveal the existence of detrital and metamorphic zircons in major Rocks of the Qianlishan Complex. In most cases, detrital zircons occur as either single grains with oscillatory zoning or oscillatory zoning cores, typical of igneous origin, which are surrounded by metamorphic overgrowth rims that are structureless, high bright and low in Th/U ratio. Detrital zircons from the Qianlishan Complex yield nearly concordant 207 Pb/ 206 Pb ages ranging from 2.3 to 2.0 Ga, suggesting that the protoliths of the high-grade supracrustal Rocks in the Qianlishan Complex were deposited at some time after 2.0 Ga. Metamorphic zircons yield two age populations with one at ∼1.95 Ga and another at ∼1.92 Ga, of which the former is interpreted as the timing of the collision between the Yinshan and Ordos Blocks to form the Western Block, whereas the later is considered to be the age of subsequent post-orogenic extensional event. Minor S-type granites were emplaced at ∼1.88 Ga, as a result of partial melting of supracrustals at the stage of the exhumation of the Qianlishan Complex. These new zircon ages, combined with structural and metamorphic considerations, enable resolution of the tectonothermal events involving the collision between the Yinshan and Ordos Blocks to form the Western Block, followed by the post-collisional extension and subsequent exhumation of the Khondalite Belt.
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la icp ms u pb zircon ages of the qianlishan complex constrains on the evolution of the khondalite belt in the western block of the north china craton
Precambrian Research, 2009Co-Authors: Changqing Yin, Guochun Zhao, X. Zhou, Min Sun, Xiaoping Xia, Chunjing Wei, Winghang LeungAbstract:Abstract The Qianlishan Complex is located in the westernmost part of the Khondalite Belt, a continent–continent collisional belt along which the Yinshan Block in the north and the Ordos Block in the south amalgamated to form the Western Block, which then collided with the Eastern Block along the Trans-North China Orogen to form the North China Craton. The complex is dominated by high-grade supracrustal Rocks and minor S-type granites, of which the supracrustals consist of graphite-bearing sillimanite-garnet gneiss, garnet quartzite, felsic paragneiss, Calc-Silicate Rock and marble. CL images reveal the existence of detrital and metamorphic zircons in major Rocks of the Qianlishan Complex. In most cases, detrital zircons occur as either single grains with oscillatory zoning or oscillatory zoning cores, typical of igneous origin, which are surrounded by metamorphic overgrowth rims that are structureless, high bright and low in Th/U ratio. Detrital zircons from the Qianlishan Complex yield nearly concordant 207 Pb/ 206 Pb ages ranging from 2.3 to 2.0 Ga, suggesting that the protoliths of the high-grade supracrustal Rocks in the Qianlishan Complex were deposited at some time after 2.0 Ga. Metamorphic zircons yield two age populations with one at ∼1.95 Ga and another at ∼1.92 Ga, of which the former is interpreted as the timing of the collision between the Yinshan and Ordos Blocks to form the Western Block, whereas the later is considered to be the age of subsequent post-orogenic extensional event. Minor S-type granites were emplaced at ∼1.88 Ga, as a result of partial melting of supracrustals at the stage of the exhumation of the Qianlishan Complex. These new zircon ages, combined with structural and metamorphic considerations, enable resolution of the tectonothermal events involving the collision between the Yinshan and Ordos Blocks to form the Western Block, followed by the post-collisional extension and subsequent exhumation of the Khondalite Belt.
Winghang Leung - One of the best experts on this subject based on the ideXlab platform.
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la icp ms u pb zircon ages of the qianlishan complex constrains on the evolution of the khondalite belt in the western block of the north china craton
Precambrian Research, 2009Co-Authors: Guochun Zhao, X. Zhou, Winghang LeungAbstract:Abstract The Qianlishan Complex is located in the westernmost part of the Khondalite Belt, a continent–continent collisional belt along which the Yinshan Block in the north and the Ordos Block in the south amalgamated to form the Western Block, which then collided with the Eastern Block along the Trans-North China Orogen to form the North China Craton. The complex is dominated by high-grade supracrustal Rocks and minor S-type granites, of which the supracrustals consist of graphite-bearing sillimanite-garnet gneiss, garnet quartzite, felsic paragneiss, Calc-Silicate Rock and marble. CL images reveal the existence of detrital and metamorphic zircons in major Rocks of the Qianlishan Complex. In most cases, detrital zircons occur as either single grains with oscillatory zoning or oscillatory zoning cores, typical of igneous origin, which are surrounded by metamorphic overgrowth rims that are structureless, high bright and low in Th/U ratio. Detrital zircons from the Qianlishan Complex yield nearly concordant 207 Pb/ 206 Pb ages ranging from 2.3 to 2.0 Ga, suggesting that the protoliths of the high-grade supracrustal Rocks in the Qianlishan Complex were deposited at some time after 2.0 Ga. Metamorphic zircons yield two age populations with one at ∼1.95 Ga and another at ∼1.92 Ga, of which the former is interpreted as the timing of the collision between the Yinshan and Ordos Blocks to form the Western Block, whereas the later is considered to be the age of subsequent post-orogenic extensional event. Minor S-type granites were emplaced at ∼1.88 Ga, as a result of partial melting of supracrustals at the stage of the exhumation of the Qianlishan Complex. These new zircon ages, combined with structural and metamorphic considerations, enable resolution of the tectonothermal events involving the collision between the Yinshan and Ordos Blocks to form the Western Block, followed by the post-collisional extension and subsequent exhumation of the Khondalite Belt.
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la icp ms u pb zircon ages of the qianlishan complex constrains on the evolution of the khondalite belt in the western block of the north china craton
Precambrian Research, 2009Co-Authors: Changqing Yin, Guochun Zhao, X. Zhou, Min Sun, Xiaoping Xia, Chunjing Wei, Winghang LeungAbstract:Abstract The Qianlishan Complex is located in the westernmost part of the Khondalite Belt, a continent–continent collisional belt along which the Yinshan Block in the north and the Ordos Block in the south amalgamated to form the Western Block, which then collided with the Eastern Block along the Trans-North China Orogen to form the North China Craton. The complex is dominated by high-grade supracrustal Rocks and minor S-type granites, of which the supracrustals consist of graphite-bearing sillimanite-garnet gneiss, garnet quartzite, felsic paragneiss, Calc-Silicate Rock and marble. CL images reveal the existence of detrital and metamorphic zircons in major Rocks of the Qianlishan Complex. In most cases, detrital zircons occur as either single grains with oscillatory zoning or oscillatory zoning cores, typical of igneous origin, which are surrounded by metamorphic overgrowth rims that are structureless, high bright and low in Th/U ratio. Detrital zircons from the Qianlishan Complex yield nearly concordant 207 Pb/ 206 Pb ages ranging from 2.3 to 2.0 Ga, suggesting that the protoliths of the high-grade supracrustal Rocks in the Qianlishan Complex were deposited at some time after 2.0 Ga. Metamorphic zircons yield two age populations with one at ∼1.95 Ga and another at ∼1.92 Ga, of which the former is interpreted as the timing of the collision between the Yinshan and Ordos Blocks to form the Western Block, whereas the later is considered to be the age of subsequent post-orogenic extensional event. Minor S-type granites were emplaced at ∼1.88 Ga, as a result of partial melting of supracrustals at the stage of the exhumation of the Qianlishan Complex. These new zircon ages, combined with structural and metamorphic considerations, enable resolution of the tectonothermal events involving the collision between the Yinshan and Ordos Blocks to form the Western Block, followed by the post-collisional extension and subsequent exhumation of the Khondalite Belt.
X. Zhou - One of the best experts on this subject based on the ideXlab platform.
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u pb and hf isotopic study of zircons of the helanshan complex constrains on the evolution of the khondalite belt in the western block of the north china craton
Lithos, 2011Co-Authors: Guochun Zhao, X. ZhouAbstract:Abstract The Helanshan Complex is located at the westernmost segment of the Khondalite Belt, a continent–continent collisional belt along which the Yinshan Block in the north collided with the Ordos Block in the south to form the Western Block of the North China Craton. The complex consists of S-type granites and high-grade pelitic granulite/gneiss, felsic paragneiss, quartzite, Calc-Silicate Rock and marble, together called the Khondalite Series Rocks. LA-Q-ICP-MS U–Pb geochronology and LA-MC-ICP-MS Lu–Hf isotopic data of zircons, combined with cathodoluminescence (CL) imaging, enable the resolution of magmatic and metamorphic events that can be directed towards understanding the formation and evolution of the Khondalite Belt in the Western Block of the North China Craton. CL images reveal the coexistence of magmatic-type detrital zircons and metamorphic zircons in most of the Khondalite Series Rocks, of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating magmatic-type detrital zircon cores. LA-Q-ICP-MS U–Pb analyses on magmatic-type detrital zircons reveal two distinct age populations, with one in the Archaean (2.85–2.53 Ga) and the other in the Palaeoproterozoic (2.2–2.0 Ga), suggesting that the sedimentary protoliths of the Khondalite Series Rocks in the Helanshan Complex must have been deposited at some time after 2.0 Ga. The Hf analyses show that the Archaean detrital zircons possess negative eHf(t) values from − 7.64 to − 0.15 with depleted mantle model ages ranging from 3.34 to 3.10 Ga. This implies the existence of a Paleo-Mesoarchaean (3.34–3.10 Ga) crust in the Western Block, which underwent a recycling event in the period 2.85–2.53 Ga. Nearly all Palaeoproterozoic (2.2–2.0 Ga) detrital zircons possess positive eHf(t) values (0.86–9.38) with depleted mantle model ages of 2.45–2.15 Ga, suggesting a significant crustal growth event in the Western Block in the Palaeoproterozoic. Metamorphic zircons yield two groups of age, with one at ~ 1.95 Ga and the other at ~ 1.87 Ga, of which the former is considered as the timing of the collision between the Yinshan and Ordos Blocks to form the Western Block, whereas the latter is consistent with the age of ~ 1.86 Ga from two S-type granites which are considered to have formed by the partial melting of pelitic granulites/gneisses at the stage of the exhumation of the Helanshan Complex.
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la icp ms u pb zircon ages of the qianlishan complex constrains on the evolution of the khondalite belt in the western block of the north china craton
Precambrian Research, 2009Co-Authors: Guochun Zhao, X. Zhou, Winghang LeungAbstract:Abstract The Qianlishan Complex is located in the westernmost part of the Khondalite Belt, a continent–continent collisional belt along which the Yinshan Block in the north and the Ordos Block in the south amalgamated to form the Western Block, which then collided with the Eastern Block along the Trans-North China Orogen to form the North China Craton. The complex is dominated by high-grade supracrustal Rocks and minor S-type granites, of which the supracrustals consist of graphite-bearing sillimanite-garnet gneiss, garnet quartzite, felsic paragneiss, Calc-Silicate Rock and marble. CL images reveal the existence of detrital and metamorphic zircons in major Rocks of the Qianlishan Complex. In most cases, detrital zircons occur as either single grains with oscillatory zoning or oscillatory zoning cores, typical of igneous origin, which are surrounded by metamorphic overgrowth rims that are structureless, high bright and low in Th/U ratio. Detrital zircons from the Qianlishan Complex yield nearly concordant 207 Pb/ 206 Pb ages ranging from 2.3 to 2.0 Ga, suggesting that the protoliths of the high-grade supracrustal Rocks in the Qianlishan Complex were deposited at some time after 2.0 Ga. Metamorphic zircons yield two age populations with one at ∼1.95 Ga and another at ∼1.92 Ga, of which the former is interpreted as the timing of the collision between the Yinshan and Ordos Blocks to form the Western Block, whereas the later is considered to be the age of subsequent post-orogenic extensional event. Minor S-type granites were emplaced at ∼1.88 Ga, as a result of partial melting of supracrustals at the stage of the exhumation of the Qianlishan Complex. These new zircon ages, combined with structural and metamorphic considerations, enable resolution of the tectonothermal events involving the collision between the Yinshan and Ordos Blocks to form the Western Block, followed by the post-collisional extension and subsequent exhumation of the Khondalite Belt.
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la icp ms u pb zircon ages of the qianlishan complex constrains on the evolution of the khondalite belt in the western block of the north china craton
Precambrian Research, 2009Co-Authors: Changqing Yin, Guochun Zhao, X. Zhou, Min Sun, Xiaoping Xia, Chunjing Wei, Winghang LeungAbstract:Abstract The Qianlishan Complex is located in the westernmost part of the Khondalite Belt, a continent–continent collisional belt along which the Yinshan Block in the north and the Ordos Block in the south amalgamated to form the Western Block, which then collided with the Eastern Block along the Trans-North China Orogen to form the North China Craton. The complex is dominated by high-grade supracrustal Rocks and minor S-type granites, of which the supracrustals consist of graphite-bearing sillimanite-garnet gneiss, garnet quartzite, felsic paragneiss, Calc-Silicate Rock and marble. CL images reveal the existence of detrital and metamorphic zircons in major Rocks of the Qianlishan Complex. In most cases, detrital zircons occur as either single grains with oscillatory zoning or oscillatory zoning cores, typical of igneous origin, which are surrounded by metamorphic overgrowth rims that are structureless, high bright and low in Th/U ratio. Detrital zircons from the Qianlishan Complex yield nearly concordant 207 Pb/ 206 Pb ages ranging from 2.3 to 2.0 Ga, suggesting that the protoliths of the high-grade supracrustal Rocks in the Qianlishan Complex were deposited at some time after 2.0 Ga. Metamorphic zircons yield two age populations with one at ∼1.95 Ga and another at ∼1.92 Ga, of which the former is interpreted as the timing of the collision between the Yinshan and Ordos Blocks to form the Western Block, whereas the later is considered to be the age of subsequent post-orogenic extensional event. Minor S-type granites were emplaced at ∼1.88 Ga, as a result of partial melting of supracrustals at the stage of the exhumation of the Qianlishan Complex. These new zircon ages, combined with structural and metamorphic considerations, enable resolution of the tectonothermal events involving the collision between the Yinshan and Ordos Blocks to form the Western Block, followed by the post-collisional extension and subsequent exhumation of the Khondalite Belt.
M Satishkumar - One of the best experts on this subject based on the ideXlab platform.
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contrasting carbon and oxygen isotopic evolution in metacarbonates from the kerala khondalite belt southern india
Gondwana Research, 2001Co-Authors: M Satishkumar, Hideki Wada, M SantoshAbstract:Abstract The Kerala Khondalite belt is a Proterozoic metasupracrustal granulite facies terrain in southern India comprising garnet-biotite gneiss, garnet-sillimanite gneiss and orthopyroxene granulites as major Rock types. Calc-Silicate Rocks and marbles, occurring as minor lithologies in the Kerala Khondalite Belt, show different mineral assemblages and reaction histories of which indicate a metamorphic P-T-fluid history dominated by internal fluid buffering during the peak metamorphism, followed by external fluid influx during decompression. The carbon and oxygen isotopic compositions of Calcite from three representative metacarbonate localities show contrasting evolutionary trends. The Ambasamudram marbles exhibit carbon and oxygen isotope ratios (δ13C ∼ 0‰ and δ18O ∼ 20‰) typical of middle to late Proterozoic marine carbonate sediments with minor variation ascribed to the isotopic exchange due to the devolatilization reactions. The δ13C and δ18O values of ∼ −9‰ and 11‰, respectively, for Calcite from Calc-Silicate Rocks at Nuliyam are considerably low and heterogeneous. The wollastonite formation here, possibly corresponds to an earlier event of fluid infiltration during prograde to peak metamorphism, which resulted in decarbonation and isotope resetting. Further, petrologic evidence supports a model of late carbonic fluid infiltration that has partially affected the Calc-Silicate Rocks, with subsequent isotope resetting, more towards the contact between Calc-Silicate Rock and charnockite. At Korani, only oxygen isotopes have been significantly lowered (δ18O ∼ 13‰) and the process involved might be a combination of metamorphic devolatilization accompanied by an aqueous fluid influx, supported by petrologic evidence. The stable isotope signatures obtained from the individual localities, thus indicate heterogeneous patterns of fluid evolution history within the same crustal segment.
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reaction textures in scapolite wollastonite grossular Calc Silicate Rock from the kerala khondalite belt southern india evidence for high temperature metamorphism and initial cooling
Lithos, 1998Co-Authors: M Satishkumar, Simon L HarleyAbstract:Abstract Scapolite–wollastonite–grossular bearing Calc-Silicate Rocks from the Vellanad area in the Kerala Khondalite Belt (KKB) of Southern India preserve a number of reaction textures which help to deduce their P – T –fluid history. Textures include Calcite+plagioclase±quartz symplectites after scapolite, grossular+quartz coronas between wollastonite and plagioclase, grossular coronas between wollastonite and plagioclase+Calcite that replace former scapolite, and grossular blebs replacing anorthite+Calcite+quartz pseudomorphs of scapolite. Garnet coronas are also observed between clinopyroxene and wollastonite or scapolite or plagioclase. The reactions, apart from those involving clinopyroxene, can be modelled in the simple CaO–Al 2 O 3 –SiO 2 –CO 2 system and interpreted using partial reaction grids constructed for the activities of end-members in the analysed phases. The reaction topologies produced are good approximations for the peak as well as retrograde mineral assemblages and reaction textures. For the compositions of the phases present in this study, the medium pressure Calc-Silicate assemblages are defined by the stable pseudo-invariant points [Qtz], [Mei] and [Grs]. The textural features interpreted using these activity-corrected grids indicate a phase of isobaric cooling from about 835°C to 750°C at 6 kbar in the Vellanad area. This is inconsistent with earlier studies on other lithologies from the KKB, most of which imply a post-peak P – T path involving near-isothermal decompression. However, as the temperatures obtained for the KKB from the Calc-Silicates are higher than those previously deduced from metapelites and garnet–orthopyroxene assemblages, the phase of near-isobaric cooling reported here is inferred to have proceeded prior to the onset of the decompression documented from studies of other Rock types.
M Santosh - One of the best experts on this subject based on the ideXlab platform.
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petrogenesis of incipient charnockite in the ikalamavony sub domain south central madagascar new insights from phase equilibrium modeling
Lithos, 2017Co-Authors: Takahiro Endo, M Santosh, Toshiaki Tsunogae, E Shaji, Roger A RambelosonAbstract:Abstract Incipient charnockites representing granulite formation on a mesoscopic scale occur in the Ambodin Ifandana area of Ikalamavony sub-domain in south-central Madagascar. Here we report new petrological data from these Rocks, and discuss the process of granulite formation on the basis of petrography, mineral equilibrium modeling, and fluid inclusion studies. The incipient charnockites occur as brownish patches, lenses, and layers characterized by an assemblage of biotite + orthopyroxene + K-feldspar + plagioclase + quartz + magnetite + ilmenite within host orthopyroxene-free biotite gneiss with an assemblage of biotite + K-feldspar + plagioclase + quartz + magnetite + ilmenite. Lenses and layers of Calc-Silicate Rock (clinopyroxene + garnet + plagioclase + quartz + titanite + Calcite) are typically associated with the charnockite. Coarse-grained charnockite occurs along the contact between the layered charnockite and Calc-Silicate Rock. The application of mineral equilibrium modeling on the mineral assemblages in charnockite and biotite gneiss employing the NCKFMASHTO system as well as fluid inclusion study on coarse-grained charnockite defines a P–T range of 8.5–10.5 kbar and 880–900 °C, which is nearly consistent with the inferred P–T condition of the Ikalamavony sub-domain (8.0–10.5 kbar and 820–880 °C). The result of T versus H 2 O activity ( a (H 2 O)) modeling demonstrates that orthopyroxene-bearing assemblage in charnockite is stable under relatively low a (H 2 O) condition of 0.42–0.43, which is consistent with the popular models of incipient-charnockite formation related to the lowering of water activity and stabilization of orthopyroxene through dehydration of biotite. The occurrence of Calc-Silicate Rocks adjacent to the charnockite suggests that the CO 2 -bearing fluid that caused dehydration and incipient-charnockite formation might have been derived through decarbonation of Calc-Silicate Rocks during the initial stage of decompression slightly after the peak metamorphism. The Calc-Silicate Rocks might have also behaved as a cap Rock that trapped CO 2 infiltrated from an external source. ‘CO 2 -rich fluid ponds’ formed beneath Calc-Silicate layers could have enhanced dehydration of biotite to orthopyroxene, and produced layers of coarse-grained charnockite adjacent to Calc-Silicate layers.
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petrology and geochronology of the namche barwa complex in the eastern himalayan syntaxis tibet constraints on the origin and evolution of the north eastern margin of the indian craton
Gondwana Research, 2012Co-Authors: Zeming Zhang, M Santosh, Xin Dong, Feng Liu, Wei Wang, Fei Yiu, Kun ShenAbstract:Abstract The Namche Barwa Complex (NBC) in the eastern Himalayan syntaxis, south Tibet, is generally interpreted as the north-eastern extremity of the exposed Greater Himalayan Sequence, comprising Neoproterozoic to early Paleozoic sedimentary strata along the northern margin of the Indian continent. Field and petrological investigations indicate that the NBC consists mainly of orthogneiss, paragneiss, amphibolites and Calc-Silicate Rocks. U–Pb zircon data demonstrate that the protoliths of the orthogneiss formed during late Paleoproterozoic at ca. 1610 Ma and also in early Paleozoic at ca. 490–500 Ma. The amphibolites were derived from mafic magmatic Rocks formed during 1645 to 1590 Ma. Zircons in the paragneisses have highly variable inherited zircon ages ranging from the Neoarchean to early Paleozoic, with four major age populations of 2490 Ma, 1640 Ma, 990 Ma and 480 Ma. The Calc-Silicate Rock has zircons with early Paleozoic metamorphic age of 538 Ma. Almost all the Rocks of the NBC have been metamorphosed during Cenozoic with the metamorphic zircon U–Pb ages ranging from 8 to 30 Ma and a peak at 23 Ma. These, together with previous results suggest that the NBC was originally derived from an Andean-type orogeny following the Columbia supercontinent assembly, and experienced multiple reworking during the Grenvillian, Pan-African and Himalayan orogenies. We conclude that the NBC in the eastern Himalayan syntaxis was derived from different provenance and tectonic setting as compared to those of the Greater Himalayan Sequence which constitutes the high-grade metamorphic core of the western and central Himalayan orogenic belt. We thus infer that the NBC was originally part of the eastern segment of the Central Indian Tectonic Zone.
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contrasting carbon and oxygen isotopic evolution in metacarbonates from the kerala khondalite belt southern india
Gondwana Research, 2001Co-Authors: M Satishkumar, Hideki Wada, M SantoshAbstract:Abstract The Kerala Khondalite belt is a Proterozoic metasupracrustal granulite facies terrain in southern India comprising garnet-biotite gneiss, garnet-sillimanite gneiss and orthopyroxene granulites as major Rock types. Calc-Silicate Rocks and marbles, occurring as minor lithologies in the Kerala Khondalite Belt, show different mineral assemblages and reaction histories of which indicate a metamorphic P-T-fluid history dominated by internal fluid buffering during the peak metamorphism, followed by external fluid influx during decompression. The carbon and oxygen isotopic compositions of Calcite from three representative metacarbonate localities show contrasting evolutionary trends. The Ambasamudram marbles exhibit carbon and oxygen isotope ratios (δ13C ∼ 0‰ and δ18O ∼ 20‰) typical of middle to late Proterozoic marine carbonate sediments with minor variation ascribed to the isotopic exchange due to the devolatilization reactions. The δ13C and δ18O values of ∼ −9‰ and 11‰, respectively, for Calcite from Calc-Silicate Rocks at Nuliyam are considerably low and heterogeneous. The wollastonite formation here, possibly corresponds to an earlier event of fluid infiltration during prograde to peak metamorphism, which resulted in decarbonation and isotope resetting. Further, petrologic evidence supports a model of late carbonic fluid infiltration that has partially affected the Calc-Silicate Rocks, with subsequent isotope resetting, more towards the contact between Calc-Silicate Rock and charnockite. At Korani, only oxygen isotopes have been significantly lowered (δ18O ∼ 13‰) and the process involved might be a combination of metamorphic devolatilization accompanied by an aqueous fluid influx, supported by petrologic evidence. The stable isotope signatures obtained from the individual localities, thus indicate heterogeneous patterns of fluid evolution history within the same crustal segment.