Early Mesozoic deep-crust reworking beneath the central Lhasa terrane (South Tibet): evidence from intermediate gneiss xenoliths in granites

Xiang Zhou, Jian-Ping Zheng, Qing Xiong, Jing-Sui Yang, Yaun-Bao Wu, Jun-Hong Zhao, William L. Griffin, Hong-Kun Dai

Research output: Contribution to journalArticleResearchpeer-review

Abstract

Understanding the rheological behavior of the Tibetan Plateau and its response to geodynamic processes requires a clear knowledge of the composition, evolution and lithological properties of the deep crust. Here we present U–Pb–Hf isotopes of zircons, bulk-rock geochemistry and mineral compositions for seven intermediate gneiss xenoliths and their host Early Mesozoic granites (205 ± 6 Ma) in the central Lhasa terrane to probe the deep crust beneath Southern Tibet. The xenoliths contain plagioclase, amphibole, titanite, allanite, quartz, biotite and muscovite, with accessory Fe–Ti oxides, apatite and zircon. Bulk-rock and mineral geochemistry suggests that these xenoliths have a magmatic origin and experienced deformation and amphibolite-facies metamorphism (equilibration at pressures of 0.46–0.83 GPa and temperatures of ~ 650 °C), before they were captured by the host granite at ~ 205 Ma. Zircons in these xenoliths show complex microstructures, including inherited cores, magmatic or metamorphic bands, and high U–Th hydrothermal rims. Inherited zircon cores record U–Pb ages from 2277 Ma to 517 Ma. Igneous zircons show a range of concordant U–Pb ages, suggesting a protracted magmatism from 236 Ma to 203 Ma. Metamorphic zircon zones record the timing of amphibolite-facies metamorphism from 224 to 192 Ma, while the high U–Th hydrothermal rims show a subsequent fluid activity until ~ 150 Ma. Unradiogenic Hf isotopic compositions of both xenoliths and host granites [xenolith εHf(t) = − 11.2 to 0; host granite εHf(t) = − 17.3 to − 3.3] indicate that the Early Mesozoic deep crust in the central Lhasa terrane originated mainly from ancient (i.e., Proterozoic) crust, with little or no interaction with juvenile magmas. This study suggests a possible continental differentiation mechanism during crustal reworking; progressive melting may initiate from the lower mafic crust (at ca. 236 Ma) and gradually migrate into the sediment-rich upper crust (until ca. 203 Ma). The reworking results in the transition from small fluxes of intermediate magmas to voluminous peraluminous S-type granite in a convergent depth.

LanguageEnglish
Pages225-239
Number of pages15
JournalLithos
Volume274-275
DOIs
Publication statusPublished - 1 Mar 2017

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reworking
gneiss
terrane
zircon
crust
Geochemistry
granite
amphibolite facies
Minerals
metamorphism
Amphibole Asbestos
geochemistry
Chemical analysis
Rocks
Geodynamics
Apatites
allanite
Quartz
xenolith
Accessories

Keywords

  • gneiss xenolith
  • deep crustal reworking
  • early Mesozoic
  • central Lhasa terrane
  • South Tibet

Cite this

@article{132b5f800dc143c78d8a2a927829ab82,
title = "Early Mesozoic deep-crust reworking beneath the central Lhasa terrane (South Tibet): evidence from intermediate gneiss xenoliths in granites",
abstract = "Understanding the rheological behavior of the Tibetan Plateau and its response to geodynamic processes requires a clear knowledge of the composition, evolution and lithological properties of the deep crust. Here we present U–Pb–Hf isotopes of zircons, bulk-rock geochemistry and mineral compositions for seven intermediate gneiss xenoliths and their host Early Mesozoic granites (205 ± 6 Ma) in the central Lhasa terrane to probe the deep crust beneath Southern Tibet. The xenoliths contain plagioclase, amphibole, titanite, allanite, quartz, biotite and muscovite, with accessory Fe–Ti oxides, apatite and zircon. Bulk-rock and mineral geochemistry suggests that these xenoliths have a magmatic origin and experienced deformation and amphibolite-facies metamorphism (equilibration at pressures of 0.46–0.83 GPa and temperatures of ~ 650 °C), before they were captured by the host granite at ~ 205 Ma. Zircons in these xenoliths show complex microstructures, including inherited cores, magmatic or metamorphic bands, and high U–Th hydrothermal rims. Inherited zircon cores record U–Pb ages from 2277 Ma to 517 Ma. Igneous zircons show a range of concordant U–Pb ages, suggesting a protracted magmatism from 236 Ma to 203 Ma. Metamorphic zircon zones record the timing of amphibolite-facies metamorphism from 224 to 192 Ma, while the high U–Th hydrothermal rims show a subsequent fluid activity until ~ 150 Ma. Unradiogenic Hf isotopic compositions of both xenoliths and host granites [xenolith εHf(t) = − 11.2 to 0; host granite εHf(t) = − 17.3 to − 3.3] indicate that the Early Mesozoic deep crust in the central Lhasa terrane originated mainly from ancient (i.e., Proterozoic) crust, with little or no interaction with juvenile magmas. This study suggests a possible continental differentiation mechanism during crustal reworking; progressive melting may initiate from the lower mafic crust (at ca. 236 Ma) and gradually migrate into the sediment-rich upper crust (until ca. 203 Ma). The reworking results in the transition from small fluxes of intermediate magmas to voluminous peraluminous S-type granite in a convergent depth.",
keywords = "gneiss xenolith, deep crustal reworking, early Mesozoic, central Lhasa terrane, South Tibet",
author = "Xiang Zhou and Jian-Ping Zheng and Qing Xiong and Jing-Sui Yang and Yaun-Bao Wu and Jun-Hong Zhao and Griffin, {William L.} and Hong-Kun Dai",
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language = "English",
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}

Early Mesozoic deep-crust reworking beneath the central Lhasa terrane (South Tibet) : evidence from intermediate gneiss xenoliths in granites. / Zhou, Xiang; Zheng, Jian-Ping; Xiong, Qing; Yang, Jing-Sui; Wu, Yaun-Bao; Zhao, Jun-Hong; Griffin, William L.; Dai, Hong-Kun.

In: Lithos, Vol. 274-275, 01.03.2017, p. 225-239.

Research output: Contribution to journalArticleResearchpeer-review

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T1 - Early Mesozoic deep-crust reworking beneath the central Lhasa terrane (South Tibet)

T2 - Lithos

AU - Zhou, Xiang

AU - Zheng, Jian-Ping

AU - Xiong, Qing

AU - Yang, Jing-Sui

AU - Wu, Yaun-Bao

AU - Zhao, Jun-Hong

AU - Griffin, William L.

AU - Dai, Hong-Kun

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N2 - Understanding the rheological behavior of the Tibetan Plateau and its response to geodynamic processes requires a clear knowledge of the composition, evolution and lithological properties of the deep crust. Here we present U–Pb–Hf isotopes of zircons, bulk-rock geochemistry and mineral compositions for seven intermediate gneiss xenoliths and their host Early Mesozoic granites (205 ± 6 Ma) in the central Lhasa terrane to probe the deep crust beneath Southern Tibet. The xenoliths contain plagioclase, amphibole, titanite, allanite, quartz, biotite and muscovite, with accessory Fe–Ti oxides, apatite and zircon. Bulk-rock and mineral geochemistry suggests that these xenoliths have a magmatic origin and experienced deformation and amphibolite-facies metamorphism (equilibration at pressures of 0.46–0.83 GPa and temperatures of ~ 650 °C), before they were captured by the host granite at ~ 205 Ma. Zircons in these xenoliths show complex microstructures, including inherited cores, magmatic or metamorphic bands, and high U–Th hydrothermal rims. Inherited zircon cores record U–Pb ages from 2277 Ma to 517 Ma. Igneous zircons show a range of concordant U–Pb ages, suggesting a protracted magmatism from 236 Ma to 203 Ma. Metamorphic zircon zones record the timing of amphibolite-facies metamorphism from 224 to 192 Ma, while the high U–Th hydrothermal rims show a subsequent fluid activity until ~ 150 Ma. Unradiogenic Hf isotopic compositions of both xenoliths and host granites [xenolith εHf(t) = − 11.2 to 0; host granite εHf(t) = − 17.3 to − 3.3] indicate that the Early Mesozoic deep crust in the central Lhasa terrane originated mainly from ancient (i.e., Proterozoic) crust, with little or no interaction with juvenile magmas. This study suggests a possible continental differentiation mechanism during crustal reworking; progressive melting may initiate from the lower mafic crust (at ca. 236 Ma) and gradually migrate into the sediment-rich upper crust (until ca. 203 Ma). The reworking results in the transition from small fluxes of intermediate magmas to voluminous peraluminous S-type granite in a convergent depth.

AB - Understanding the rheological behavior of the Tibetan Plateau and its response to geodynamic processes requires a clear knowledge of the composition, evolution and lithological properties of the deep crust. Here we present U–Pb–Hf isotopes of zircons, bulk-rock geochemistry and mineral compositions for seven intermediate gneiss xenoliths and their host Early Mesozoic granites (205 ± 6 Ma) in the central Lhasa terrane to probe the deep crust beneath Southern Tibet. The xenoliths contain plagioclase, amphibole, titanite, allanite, quartz, biotite and muscovite, with accessory Fe–Ti oxides, apatite and zircon. Bulk-rock and mineral geochemistry suggests that these xenoliths have a magmatic origin and experienced deformation and amphibolite-facies metamorphism (equilibration at pressures of 0.46–0.83 GPa and temperatures of ~ 650 °C), before they were captured by the host granite at ~ 205 Ma. Zircons in these xenoliths show complex microstructures, including inherited cores, magmatic or metamorphic bands, and high U–Th hydrothermal rims. Inherited zircon cores record U–Pb ages from 2277 Ma to 517 Ma. Igneous zircons show a range of concordant U–Pb ages, suggesting a protracted magmatism from 236 Ma to 203 Ma. Metamorphic zircon zones record the timing of amphibolite-facies metamorphism from 224 to 192 Ma, while the high U–Th hydrothermal rims show a subsequent fluid activity until ~ 150 Ma. Unradiogenic Hf isotopic compositions of both xenoliths and host granites [xenolith εHf(t) = − 11.2 to 0; host granite εHf(t) = − 17.3 to − 3.3] indicate that the Early Mesozoic deep crust in the central Lhasa terrane originated mainly from ancient (i.e., Proterozoic) crust, with little or no interaction with juvenile magmas. This study suggests a possible continental differentiation mechanism during crustal reworking; progressive melting may initiate from the lower mafic crust (at ca. 236 Ma) and gradually migrate into the sediment-rich upper crust (until ca. 203 Ma). The reworking results in the transition from small fluxes of intermediate magmas to voluminous peraluminous S-type granite in a convergent depth.

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KW - deep crustal reworking

KW - early Mesozoic

KW - central Lhasa terrane

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