Miocene Adakitic Intrusive Rocks In The Zhongba Microterrane Of Southern Tibet: Implications For Origin And Tectonics

The Himalayan-Tibetan orogen is the most distinctive landform on our planet. In the evolution of which widespread magmatic activities occurred, including the adakitic magma. Crustal thickening and uplift of the Tibetan plateau are long-standing problems in the geological evolution of the Himalayan–Tibetan orogen. Knowledge of the post-collisional magmatism is crucial to understand the mechanism of the crustal thickening and provide important constraints on both evolution and geodynamics of the Tibetan plateau. Post-collisional adakites(26–9 Ma) have been recognized in southern Tibet for many years, and there are many articles published which reported the adakitic rocks at home and abroad. However, their origins and geodynamics remain highly controversial.This paper reports the new implications for the origins of the Miocene adakites in the southern Tibet by researching the geochronology, Major and trace elements and whole-rock Sr–Nd–Pb isotopes of the adakitic intrusion exposed in the Zhongba microterrane and comparing the contemporaneous adakites in the southern Tibet.The intrusion consists of granite and granodiorite porphyries with the zircon U–Pb ages of ca.16 Ma. The porphyries have high Si O2(65.14–67.61 wt.%), Al2O3(15.13–16.10 wt.%), K2O(2.87–3.54 wt.%), Sr(727–1046 ppm), and low Y(9–13 ppm) and Yb(0.82–1.19 ppm) contents, they are enriched in light rare earth elements(LREE) and depleted in heavy rare earth elements(HREE). The porphyries display relative low Mg O(1.39–1.98 wt.%), high Sr/Y(65–98) and(La/Yb)N(18.8–25.5) ratios, with initial(87Sr/86 Sr)iratios of 0.70877–0.70805, εNd(t) values(–6.4 to –7.9),(206Pb/204Pb)tratios of 18.71–18.77,(207Pb/204Pb)tratios of 15.68–15.71, and(208Pb/204Pb)tratios of 39.16–38.30. These signatures indicate the Lasa porphyries are adakitic rocks derived from partial melting of a thickened lower crust.The Miocene adakitic rocks in the Zhongba microterrane show similar Sr–Nd isotopic compositions to those of western Lhasa and the Himalaya terrane, and differ from coetaneous adakites in the eastern Lhasa terrane. Based on the spatial distributions and isotopic features of the post-colisional adakites in southern Tibet, we suggest that all the adakites were derived from partial melting of thickened lower crust and their geochemical variations were mainly caused by the heterogeneity of lower crust in nature.