The Geochronological And Geochemical Studies Of The Early-Yanshanian Igneous Rocks In Southern Jiangxi Province And Their Tectonic Implications

Geochronological,geochemical and Sr-Nd isotopic data are reported for the alkaline igneous rocks (syenites and Zhaibei aluminous A-type granite) and Chebu gabbro in southern Jiangxi province. SHRIMP zircon U-Pb dating results indicate that the Quannan syenite,Zhaibei A-type granite and Chebu gabbro formed during the early Yanshanian period. The Quannan syenite is strongly enriched in K (KO/NaiOcl),LILE,LREE and HFSE (Nb/La 1.4,Nb/Th 15> eNd(t) ?3.4),characteristic of the intraplate,rift-related potassic rocks,but different from the subduction-related potassic rocks. Therefore,the formation of the syenites in southern Jiangxi was not genetically related to the subduction,or low angle subduction,of the Paleo-Pacific plate during Jurassic time. The Chebu gabbro shows geochemical affinities to the transition basalt,whilst the Zhaibei granite shows geochemical and mineral characteristics similar to the aluminous A-type granite. With the same age of-170 Ma,the Chebu gabbro and the Zhaibei aluminous A-type granite constitute a bimodal igneous suite. According to the regional petro-tectonic association,we interpret that the early Yanshanian tectonic enviornment of Southeast China might be similar to the continental rift or "Basin and Range Province". The alkaline basalts in southern Hunan are suggested to have formed by small degrees of decompression melting of the asthenosphere mantle during the initial extension. The transitional (theChebu gabbro) and tholeiitic basaltic magmas were generated by relatively larger degrees of partial melting of the mantle. Partial melting of the enriched lithospheric mantle owing to a raised geotherm caused by lithosphere thinning and following the AFC process resulted in the formation of the Quannan and Tabei syenites in southern Jiangxi.