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

Abstract

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 SiO 2 contents (71.28–78.36 wt%), K 2 O/Na 2 O 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 ( 87 Sr/ 86 Sr) i ratios of 0.7090–0.7113 and negative ε 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 crystallization 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. • Geochemistry, Sr-Nd-Pb isotopes and zircon U-Pb ages of Xisha granites presented. • They originally are parts of Late Permian granites in Hainan Islands. • They were derived from a mixing of infracrustal and mantle compositions. • They formed from orogenic collapse related to Indochina-South China collision. • They favor diachronous Indochina-South China collision in the late Permo-Triassic.