赣南中侏罗世正长岩-辉长岩的起源及其地质意义

It is well known that the Syenite-gabbro association may originate from either the evolution of a single magma or two independent magmas which are usually related to intraplate extensional tectonics. In this study, we focus on the geochronology and petrochemistry of Middle Jurassic Huangbu syenite and Chebu gabbro from southern Jiangxi Province. LA-ICP-MS U-Pb ages of both Huangbu syenite and Chebu gabbro are very consistent (≈ 178Ma) within error indicate that they were the products of contemporary magma activities. The major and trace element compositions and Nd isotope geochemical characteristics reveal that they were derived from different sources instead of differentiation from a comagmatic sources. It is suggested that they may be generated from lithospheric mantle which was metasomatized and enriched by asthenosphere originated melts, and the melting occurred at the spinel-garnet transition zone of upper mantle. They had undergone low extent of contamination by crustal material along with fractional crystallization. However, Huangbu syenite contains more incompatible elements and has higher Ce/Yb, La/Yb, Sm/Yb ratios and higher ε _Nd, (t) value compared to Chebu gabbro, which implies that the magma source of Huangbu syenite was relatively deeper and thus underwent higher degree of metasomatism. Therefore, the Huangbu syenite and Chebu gabbro provide insight into lithosphere-asthenosphere interaction under intraplate extensional tectonic setting.