Geochemistry and Zircon U-Pb-Hf Isotopes of Early Mesozoic Granites in the Youjiang Basin, SW China: Implications for the Tectonic Evolution of the Eastern Paleo-Tethys Domain

Abstract

The Youjiang Basin is located in the southwestern margin of the South China Block and at the junction between the South China and Indochina Blocks at the eastern end of the Tethyan tectonic domain. The processes responsible for Triassic magmatism in the basin are crucial to understanding the closure of the Paleo-Tethys Ocean. This study presents the geochemistry and zircon U-Pb-Hf isotopes of granites from the Pingxiang area in the southwestern Youjiang Basin. The granites belong to the high-K and peraluminous series with A/CNK ratios of 1.16–1.38. They have high SiO2 and K2O contents and moderate Al2O3 content with moderately fractionated REE patterns (LaN/YbN = 6.35–7.83) and negative Ba, Sr, P and Ti anomalies, consistent with peraluminous A2-type granites. The granites have LA-ICP-MS zircon U-Pb ages of 243–241 Ma, with negative εHf(t) values ranging from −13.22 to −6.52 and TDM2 model ages from 1654 to 2073 Ma. They were formed by partial melting of Paleoproterozoic metasedimentary rocks in an extensional setting. Combined with existing data, we suggest that the 243–241 Ma extensional event is related to the subduction-to-collision transition likely induced by slab breakoff during the closure of the Paleo-Tethys Ocean.