Miocene adakitic intrusions in the Zhongba terrane: Implications for the origin and geochemical variations of post-collisional adakitic rocks in southern Tibet

Abstract

Post-collisional adakitic rocks (26–9Ma) have been recognized in southern Tibet for almost 20years; however, their origins and geodynamics remain highly controversial. This paper reports geochronological and geochemical data for the Lasa intrusion exposed in the Zhongba terrane and proposes implications for the origins of the Miocene adakitic rocks in southern Tibet. The Lasa intrusion consists of granite and granodiorite porphyries with zircon U–Pb ages of ca. 16Ma. The porphyries have high SiO2 (65.14–67.61wt.%), Al2O3 (15.13–16.10wt.%), K2O (2.87–3.54wt.%), and Sr (727–1046ppm) contents and low Y (9–13ppm) and Yb (0.82–1.19ppm) contents. They are enriched in light rare earth elements (LREEs) and depleted in heavy rare earth elements (HREEs). The porphyries display relatively low MgO contents (1.39–1.98wt.%), high Sr/Y (65–98) and (La/Yb)N (18.8–25.5) ratios, initial (87Sr/86Sr)i ratios of 0.70912–0.70805, εNd(t) values of –6.4 to –7.9, (206Pb/204Pb)t ratios of 18.7108–18.7665, (207Pb/204Pb)t ratios of 15.6837–15.7118, and (208Pb/204Pb)t ratios of 39.1600–39.3034. These signatures indicate that the Lasa porphyries are adakitic rocks derived from the partial melting of a thickened lower crust. The Miocene adakitic rocks in the Zhongba terrane have similar Sr–Nd isotopic compositions to those of western Lhasa and the Himalaya terrane, which differ from coetaneous adakitic rocks of the eastern Lhasa terrane in terms of Sr–Nd isotopic compositions. Based on the spatial distributions and isotopic features of the post-collisional adakitic rocks in southern Tibet, we suggest that all of the adakitic rocks were derived from the partial melting of thickened lower crust and that their geochemical variations are mainly caused by the heterogeneity of lower crust.