Origin of the Cretaceous Biluocuo Low-Mg Trachy-Andesites in the Southern Qiangtang Block: Implications for Crust–Mantle Interactions and the Precambrian Basement

Abstract

The Cretaceous andesites were discovered in the Biluocuo area, and provided key records to understand the late Mesozoic geodynamic evolution and crustal basement for the southern Qiangtang block in the central Tibet. In this study, we present a detailed study of zircon U-Pb dating, major and trace elemental composition, and Sr-Nd-Hf isotopes for the Biluocuo trachy-andesites. The trachy-andesites yielded zircon U-Pb ages at ca. 97 Ma, and exhibited SiO₂ contents ranging from 55.92 to 69.04 wt%, low TiO₂ contents (0.37 to 0.75 wt%) and low Mg^# values (18.6 to 53.7), suggesting that they belong to high-K calc-alkaline series. They showed adakitic signatures, such as high Sr/Y ratios (almost 24 to 55) and low Y (<20 ppm), implying they were generated at great depths (>15 kbar). The samples have initial Sr isotopic ratios of 0.70963 to 0.70964, ε_Nd(t) values of –4.7 to –4.6, and zircon ε_Hf(t) values of –1.2 to +1.3 with two-stage Hf model ages of 0.95 to 1.09 Ga. Elemental and Sr-Nd-Hf isotopic signatures suggest that the trachy-andesites were derived from the partial melting of the thickened lower crust with involvement of metasomatized mantle components. Combined with coeval high-Mg andesites, we proposed that underplating of mantle induced melting of the lower crust at ca. 97 Ma in the southern Qiangtang block, following by lithospheric delamination. Inherited/xenocrystic zircons from the trachy-andesites revealed magmatic activities at 2562 Ma, 1850–1804 Ma, 1768–1665 Ma, 1043–935 Ma, 851–736 Ma and 642–540 Ma. There is a Precambrian crustal basement in the southern Qiangtang block, which may have experienced the assembly and break-up of the Columbia, Rodinia, and Gondwana supercontinents.