A middle Triassic extensional event in the Hainan Island: Geochronologic and geochemical evidence from igneous rocks from Dazhou Island

Abstract

Hainan Island lies at the southern tip of the South China block that was affected by the opening and closure of the paleo-Tethys from Paleozoic to Mesozoic and subsequently by the subduction of the paleo- Pacific plate beneath the Euroasian plate since the late Mesozoic era. This study presents zircon U-Pb ages and geochemistry of granites and basic diabase veins in Dazhou Island located 5 km southeast of Hainan Island. The age for granites is 237.2 Ma (MSWD = 0.14, n = 14), suggesting that they were emplaced in the late middle Triassic and are products of magmatic-tectonic episodes during the closure of paleo-Tethys. The granites belong to the shoshonitic magmatic series and are metaluminous I-type granites. They are systematically depleted in Ba, Nb, Sr, P, and Ti, and have variable ranges of initial 87Sr/86Sr ratios (0.71005 ~ 0.71110), but relatively constant eNd(t) values (–6.4~–6.8). The two-stage depleted mantle Nd model ages (TDM2 = 1531–1562 Ma) for these granites suggest that they may have originated from partial melting of the lower crust with a crustal residence age of early Middle Proterozoic Era. The zircon U-Pb ages for diabase veins can be divided into three groups. The age for Group I is 236.8 Ma (n = 4), which represents the crystallization age of the veins and somewhat younger than the accompanying granites. The ages for Group II and III are 1767 Ma (n = 6) and 2432 Ma (n = 3), respectively. These suggest that the veins have inherited some old zircons that possibly represent old geological events in the Hainan Islands. The diabase also belongs to the shoshonitic magmatic series and has arc-like trace element characteristics. The veins have high 87Sr/86Sr and low 143Nd/144Nd ratios, which suggest that they may have originated from an enriched mantle II type source or a mantle source metasomatized by aqueous fluids released from previously subducted slab. The new ages obtained for the Dazhou igneous rocks in the present study are similar to those for the Qiongzhong batholith in the Hainan Island, which implies that the Hainan Island had also regionally experienced a middle Triassic extension event after the Indosinian Orogeny.