Tectonic evolution of the northwestern margin of the South China Sea: Insights from geochronology, geochemistry, and Sr-Nd-Pb isotopes of the newly discovered latest Permian granite in the Xisha Islands

We present zircon U-Pb ages, whole-rock geochemistry, and Sr-Nd-Pb isotopes for the newly discovered granites on North Island, Xisha Islands, at the northwestern margin of the South China Sea. These peraluminous granites yield a crystallization age of ca. 254 Ma, and are characterized by high SiO2 contents (71.28-78.36 wt%), K2O/ Na2O ratios (0.9-1.8, average = 1.3), and Fe numbers (0.73-0.96, average = 0.87). The granites are enriched in light rare earth and large-ion lithophile elements, depleted in high-field strength elements, and have marked negative Eu anomalies. These features and high (87Sr/86Sr)i ratios of 0.7090-0.7113 and negative epsilon Nd(t) values of-6.5 to-2.1 indicate that the granitic magmas were derived by partial melting of crustal rocks with some contributions from mantle-derived melts. The magmas underwent crustal assimilation and fractional crystalli-zation during magma ascent. These new and literature data suggest the Southeast Asian margin was strongly influenced by the closure of the eastern Paleo-Tethyan Ocean and subsequent post-orogenic extension during the latest Permian-Early Triassic. This inference is consistent with a diachronous, clockwise collision between the Indochina and South China blocks.