Discovery and geological significance of the Indosinian granulites in Hainan Island: new evidence from U-Pb dating and trace element composition of zircons

Abstract

The Qiongzhong mafic granulite is one of the most important components of high-grade metamorphic complex in Hainan Island. A further study of the Qiongzhong mafic granulites will provide new constraints on fundamental geological problems which include the objective restoration of the Late Paleozoic to Early Mesozoic tectonic evolution and potential dynamics of Hainan Island and the determination of the eastward extension of the Paleo-Tethys Ocean. Based on TIMA analyses of mineral inclusions, cathodoluminescence (CL) images, LA-ICP-MS U-Pb dates, and in-situ rare earth element (REE) compositions of zircons in the Qiongzhong mafic granulites, the Triassic zircons can be further divided into two groups (magmatic and metamorphic zircons). Thirty magmatic zircons yielded the consistent 206Pb/238U ages ranging from 245 Ma to 230 Ma, with a corresponding weighted mean age of 238±2 Ma (MSWD=0.38), which is interpreted as the protolithic timing of the Qiongzhong mafic granulites. Twenty-four metamorphic zircons also yielded consistent 206Pb/238U ages ranging from 243 Ma to 231 Ma, with a corresponding weighted mean age of 237±2 Ma (MSWD=1.70), which is the metamorphic timing of the Qiongzhong mafic granulites. Combined with the published and related data in Hainan Island, the Qiongzhong mafic granulites were generated in Triassic, not Mesoproterozoic, and may have been formed in a post-collisional extensional setting during the collision orogenesis between South China and the Indochina Blocks after the closure of the Paleo-Tethys Ocean.