Zircon SHRIMP U-Pb dating of the Neogene coral reefs,Xisha Islands,South China Sea:implications for tectonic evolution

摘要

The Xisha Block is a minor one in the South China Sea and an important tectonic unit in the northwestern part of the region.Zircon SHRIMP U-Pb ages for three volcanic intrusive core samples from Xike-1,an exploratory well penetrating the bioherms of the Xisha Islands.The core samples are from the Miocene reef carbonate bedrock and are recognized as dark-gray biotite-hornblende gabbro,gray fne-grained biotite diorite,and gray fine-grained granite,respectively.Zircon cathodoluminescence (CL)images and trace Th,U and Pb compositions of the zircons show that these rocks are of volcanic intrusive origin.Zircon SHRIMP U-Pb dating yielded six groups of ages,ranging from 2451-1 857 Ma to early Cretaceous, which indicate that the formation and evolution of the Xisha Block was affected by the evolution and closure of Neotethys Ocean,probably within its eastern extension into South China Sea.Both old,deepsourced material,including fragments from Rodina supercontinent,and recent mantle-derived magma products contributed to the emergence and formation of the Xisha block.The SHRIMP U-Pb results also proved that this process differed from that of the Kontum massif,the Hainan Block,and the South China Block,but is similar to that of the Nansha and Zhongsha blocks.The process was associated with the effects of Yanshanian magmatism induced by subduction mechanisms of the Paleo-Pacific Plate or the reworking of the multiple magmatisms since the Early to mid-Yanshanian,possibly jointly experienced by the Xisha-Zhongsha-Nansha Block.