Tectono-magmatism evolution in the Gaoligong orogen belt during Neoproterozoic to Paleozoic: Significance for assembly of East Gondwana

Abstract

The precise constraining of Proterozoic to Paleozoic tectono-magmatic processes of the orogen belt is important to understanding the evolution of the Proto-Tethys Ocean and assembly processes of Gondwana. This study presents newly defined Neoproterozoic to Paleozoic granitic gneiss in the Gaoligong orogen belt, western Yunnan, China, by using a combined study of the structure, petrology, zircon U-Pb age and Hf isotopic composition, and whole-rock geochemistry. The magmatic origin zircons yield three main cluster ages of ca. 600–520 Ma, 520–460 Ma, and 460–420 Ma, with decreasing average Th/U ratios. Zircons have εHf(t) values ranging from −8.6 to 2.4, and Hf model ages from 1.23 Ga to 1.78 Ga. The geochemistry of the granitic gneisses shows strongly peraluminous and similar to S-type granites, with high SiO2, Rb/Sr, Rb/Ba and total alkali contents but low Al2O3, MgO, TiO2, FeOt and CaO/Na2O ratios, and are enriched in LILEs (such as Rb, Th, U, K and Pb) and depleted in HFSEs (such as Nb, Sr, P, Eu, and Ti). The geochemical signatures and zircon Hf isotopic results indicate that the granitic gneisses were derived mainly from the anatexis of ancient crustal material during the collision of the Tengchong and Baoshan blocks at 600–520 Ma. Then they together experienced subduction of the Proto-Tethys ocean at 520–460 Ma and the accretion of the micro-continents at 460–420 Ma. Combined with detrital zircon U-Pb ages and the paleogeographic position of the micro-continents that related to East Gondwana. Here, we propose a new evolution model as follows: 1) the Gaoligong orogen belt lies between the Tengchong and Baoshan blocks that collided during the Precambrian, and they together became the passive continental margin of East Gondwana; 2) then the Gaoligong orogen belt and the micro-continents underwent subduction of the Proto-tethys Ocean during the Middle Cambrian to Lower Ordovician; and 3) finally they experienced accretionary orogeny during the Late Paleozoic.