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摘要
The Lhasa terrane records voluminous magmatism related to the subduction of the Neo-Tethyan oceanic lithosphere, the study of which constrains the tectonomagmatic evolution of the region. We report zircon U-Pb ages, whole-rock compositions and Sr-Nd isotopic data from volcanic rocks in the Bangba district within the central Lhasa subterrane to constrain their magmatic source and petrogenesis. Zircon U-Pb dating of two volcanic rock samples yields End Cretaceous ages of 70.0 +/- 0.8 and 74.3 +/- 1.2 Ma. The rocks have high SiO2 (65.41 wt.%-68.45 wt.%), Al2O3 (16.16 wt.%-16.59 wt.%) and K2O (5.00 wt.%-6.73 wt.%) contents, and low TFe2O3 (2.33 wt.%-2.79 wt.%), MgO (0.64 wt.%-1.44 wt.%) and TiO2 (0.61 wt.%-0.65 wt.%) contents, with aluminium saturation indices (A/CNK) of 0.99-1.06. The major- and trace-element compositions of the rocks show they are metaluminous to slightly peraluminous high-K calc-alkaline trachydacite. The relatively high SiO2 and Sr-Nd isotopic compositions ((Sr-87/Sr-86)(i) = 0.722 654, 0.722 038 and 0.725 787 and epsilon(Nd)(t) = -12.27, -12.36 and -6.09, respectively) indicate that the trachydacites formed by partial melting of crustal material. The trachydacites are relatively enriched in light rare earth elements, depleted in heavy rare earth elements, have high (La/Yb)(N) and (Gd/Yb)(N) ratios (>61 and >6, respectively), and low Y (<18 ppm) and Yb (<18 ppm) contents, indicating they most likely formed from partial melting of lower crust in the garnet stability field. Considering the geodynamic setting of the region during this period, partial melting of the ancient Lhasa crust was likely triggered by underplating mafic magmas during rollback of the Neo-Tethyan slab.
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单位桂林理工大学
