Jurassic Gabbro-Granite-Syenite Suites from Southern Jiangxi Province, SE China: Age, Origin, and Tectonic Significance

Abstract

SHRIMP U-Pb zircon and hornblende 39Ar/40Ar ages, major and trace elements, and Sr-Nd isotopes are reported for the Chebu gabbro, the Zhaibei granite, and the Quannan and Tabei syenites, southern Jiangxi, Southeast China. Two major episodes of Jurassic magmatism were identified, i.e., the ~172 Ma bimodal gabbro-granite and the ~165 Ma syenite suites. The Chebu gabbros show geochemical affinities of intraplate transitional basaltic rocks with restricted Sr and Nd isotopic variations (I Sr = 0.7065 to 0.7067, εNd(T) = +0.55 to -0.82). The Zhaibei granites have variable εNd(T) values (-0.78 to -6.55) and I Sr > 0.710. They share geochemical characteristics with aluminous A2-type granites, and probably were generated by shallow dehydration melting of hornblende-bearing granitoid source rocks heated by contemporaneous intrusion of mantle-derived basaltic magmas. The Quannan and Tabei syenites are high in K2O, with K2O/Na2O ratios of 0.9-1.3, typical of shoshonitic affinities. They have a relatively large range of Sr and Nd isotopic compositions (I Sr = 0.704-0.710, εNd(T) = -3.54 to +3.44). Among them, the least crust-contaminated syenites display OIB-type incompatible trace-element patterns with Nb/La >1 and Nb/Th >14. The geochemical and isotopic data of the gabbros and syenites imply that their mantle sources might have been metasomatized by OIB-type melts in an intraplate regime, contrasting with the subduction-related metasomatism occurring in the mantle wedge overlying a subduction zone. The 172-165 Ma gabbro-A-type granite-syenite suites in southern Jiangxi are coeval with the 175-160 Ma alkaline basalts and syenites in southern Hunan-southeastern Guangxi, suggesting that an extension-to-rifting regime was dominant over much of Southeast China during the Middle to Late Jurassic. This rifted continental margin seems appropriate to account for the widespread Middle to Late Jurassic magmatism in Southeast China.