ZRICON U-Pb AGE OF GRANITIC ROCKS FROM THE BASEMENT BENEATH THE SHI ISLAND, XISHA ISLANDS AND ITS GEOLOGICAL SIGNIFICANCE

The petrological evidence of the Mesozoic tectonic-magmatic activity in the Xisha block, one of micro-blocks dispersed in the South China Sea, remains deficient. In 2012-2013, the Zhanjiang Branch of CNOOC successfully drilled the Well XK-1 with a core length of 1 268.02 m in the Shi Island, Xisha Islands, and met the crystalline basement composed of metamorphic and intrusive rocks at core depth of 1 257.52 m. In this paper, zircon U-Pb ages of granitic basement rocks from the Well XK-1 were measured with LA-ICP-MS technique. These granitic rock samples, identified as alkali-feldspar granite and monzogranite in lithology, belong to I-type granites and are formed under the geotectonic setting of active continental margin, same as those from the Nansha block. The geochemical characteristics of zircon trace elements indicate that the magma source is possibly influenced to some extent by mantle-derived components. The granitic rocks in the Xisha block with zircon U-Pb age ranging from 144 to 158 Ma are the product of the Late Jurassic tectonic-magmatic activity and comparable to the Yanshanian granites in the South China, which reveals that the Yanshanian magmatism occurred widely in both South China and the South China Sea. The petrological data show that both the Xisha-Zhongsha block and Nansha block are transformed by the Yanshanian multi-stage magmatic activities and belong to the extension of basement structure of the South China Continent.