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| Titel |
Presence of ca. 43 Ma highly fractionated and normal calc-alkaline granites first identified in the Gangdese Batholith, southern Tibet |
| VerfasserIn |
Qing Wang, Di-Cheng Zhu, Peter A. Cawood, Zhi-Dan Zhao, Sheng-Ao Liu, Sun-Lin Chung, Dong Liu, Jin-Gen Dai, Xuan-Xue Mo |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250091763
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| Publikation (Nr.) |
 EGU/EGU2014-6072.pdf |
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| Zusammenfassung |
| The Gangdese Batholith in southern Tibet has long been linked with the subduction of
Neo-Tethyan oceanic lithosphere and subsequent India-Asia collision. However, the specific
processes of magmatic evolution and petrogenesis of the batholith remain poorly constrained
because most existing studies have focused on its geochronological framework. New
integrated whole-rock major and trace element, zircon U-Pb age, and zircon Hf isotopic data
of granites from Dajia in the western Gangdese Batholith, southern Tibet establish the
presence of highly fractionated syenogranite (HFS) and normal calc-alkaline monzogranite
(NCM). One syenogranite sample has been dated by LA-ICP-MS zircon U-Pb method to be
43.9 ± 0.3 Ma and three monzogranite samples yielded ages of 42.6 ± 0.3 Ma, 42.7 ± 0.4
Ma, and 43.6 ± 0.3 Ma, representing the late-phase magmatism of the Gangdese Batholith.
Six NCM samples display SiO2 of 69-72 wt.%, K2O of 4.9-5.5 wt.%, and Na2O of 3.2-3.8
wt.%, with differentiation index (DI) in the range of 84-93. These rocks are enriched
in Rb, Th, U, and LREE and depleted in Ba, Nb, Sr, P, and Ti, with (La/Yb)N =
18.1-26.9 and Eu/Eu* = 0.6-0.83. Eight HFS samples are characterized by high SiO2
(75-78 wt.%) and DI (95-97), and significant negative Eu anomalies (Eu/Eu* =
0.27-0.70), although K2O (4.8-5.3 wt.%) and Na2O (3.2-3.6 wt.%) contents and
(La/Yb)N (12.5-27.2) ratios are comparable to those of the NCM samples. These
HFS samples display marked concave-upward middle rare earth element (MREE;
Gd-Ho) patterns that are not observed in the NCM samples. The NCM and HFS
samples have similar zircon Hf isotopic compositions with zircon ÉHf(t) of -5.6 to
+6.3 and -1.6 to +4.6, respectively. Such isotopic compositions, together with low
heavy REE and Y abundances, indicate that both the NCM and HFS samples were
most likely derived from partial melting of the thickened juvenile crust beneath the
southern Lhasa subterrane with varying contributions from ancient continental
crust material. The absence of fractionation trends between the NCM and HFS
samples (e.g., SiO2 vs. Sc and SiO2 vs. Dy/Yb) suggests that the HFS samples with
concave-upward MREE patterns can be interpreted as resulting from partial melting of
basaltic lower crust leaving an amphibole-rich residuum followed by significant fractional
crystallization of feldspar, plagioclase and apatite. The presence of HFS with low heavy
REE and Y (6.2-15.3 ppm) abundances identified for the first time in the Gangdese
Batholith corroborates that the southern Lhasa crust had already been thickened by ca.
43 Ma. High whole-rock zircon saturation temperatures (815°C-869°C) for the
NCM samples suggest high heat supply likely associated with rising asthenospheric
flow in response to post-collision breakoff of the Neo-Tethyan oceanic lithosphere. |
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