Partial melting of thickened continental crust in central Tibet: Evidence from geochemistry and geochronology of Eocene adakitic rhyolites in the northern Qiangtang Terrane
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The composition of the deep crust is a key to understanding the formation of the low-velocity zone in the middle to lower crust of the Tibetan Plateau. The Suyingdi rhyolites exposed in the northern Qiangtang Terrane have high Sr (296–384 ppm) and low Y (5.81–7.93 ppm), with therefore high Sr/Y ratios (42–56), showing geochemical features of adakitic rocks. Zircon U–Pb dating yields an eruption age of 38.2±0.8 Ma38.2±0.8 Ma (MSWD = 0.78). These adakitic rhyolites are high-K calc-alkaline in composition, displaying a weakly peraluminous character. They have low MgO content (0.20–0.70 wt.%) and Mg# values (24–39), as well as low Sc (2.25–2.76 ppm), Cr (8–14 ppm), Co (1.6–3.5 ppm) and Ni (2–3 ppm) concentrations. The rocks are LREE-enriched ((La/Yb)N = 50–62) and display weakly negative Eu anomalies (Eu/Eu* = 0.82–0.95) and pronounced negative Nb and Ta anomalies. They have low initial (87Sr/86Sr)i ratios (0.707860 to 0.708342) and enriched Nd isotopic compositions with εNd(t)εNd(t) values ranging from −8.4 to −5.0, which are indistinguishable from those of Cenozoic potassic and ultra-potassic lavas exposed in northern Tibet. Their much higher SiO2 and lower Fe2O3 contents, yet similar MgO, Cr, Co, Ni, and Mg# values to the potassic and ultra-potassic lavas, however, indicate that the rhyolites are unlikely to have formed by fractional crystallization of these lavas. Because of their low Nb/Ta ratios and similar Sr–Nd isotopic compositions to granulite xenoliths within the Cenozoic potassic rocks, we infer that the Suyingdi adakitic rhyolites were most likely produced by partial melting of a thickened lower crust in the garnet stability field. The magma source is most likely dominated by granulite facies metabasalts and clay-poor metamorphosed sedimentary rocks which indicate that the lower crust of northern Tibet is heterogeneous. In combination with data from previously-reported peraluminous and metaluminous adakitic rocks in the same region, the age and petrogenesis of the Suyingdi adakitic rhyolites suggest that the low-velocity zone in the deep crust of central and northern Tibet was most likely the result of partial melting of thickened crust.
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