The significance of Cenozoic magmatism from the western margin of the eastern syntaxis, southeast Tibet |
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Authors: | Hongfei Zhang Nigel Harris Liang Guo Wangchun Xu |
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Institution: | (1) State Key Laboratory of Geological Processes and Mineral Resources and Faculty of Earth Sciences, China University of Geosciences, 430074 Wuhan, China;(2) Department of Earth and Environmental Sciences, The Open University, Milton Keynes, MK7 6AA, UK |
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Abstract: | The major and trace-element geochemistry, Sr–Nd bulk-rock isotopes, U–Pb zircon chronology and Lu–Hf isotopic compositions
are described for three granitic bodies which intrude the Nyingchi gneisses (Lhasa terrane) along the western margin of the
eastern Himalayan syntaxis. The Bayi two-mica granite and Lunan granite–granodiorite were intruded at 22 ± 1 and 25.4 ± 0.3 Ma,
respectively, whereas the Confluence biotite granite was emplaced at 49.1 ± 0.4 Ma. All share strong depletions in Y and HREE
requiring a garnet-bearing source both during and following the Eocene collision of the Indian plate with the Lhasa terrane.
The isotope geochemistry of these intrusives (ε
Nd(t) = −3 to −5, 87Sr/86Sr(t) = 0.706–0.707) indicates a crustal source within the Lhasa terrane. Sr–Nd systematics of the garnet-bearing Nyingchi gneisses
together with the U–Pb and Lu–Hf isotopic ratios of detrital zircons recovered from this unit identifies it as a potential
melt source. The combined element and isotope geochemistry of the plutons indicate a mixed source; the gneisses provide the
older component whereas the Gangdese batholith provides a younger, siliceous component. The involvement of garnet-bearing
crustal material in melt sources from the Cretaceous (80 Ma) to the Miocene (20 Ma) is consistent with the presence of a thicker
continental crust in the eastern Lhasa terrane, as is the presence of magmatic epidote in several plutons which indicates
a regional deepening level of exposure eastwards. Post-collision crustal melting is synchronous with proposed slab break-off
during the early Miocene, suggesting advective heating by rising asthenospheric melts. |
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