K-Ar Dating, Geochemical, and Sr-Nd-Pb Isotopic Systematics of Late Mesozoic Mafic Dikes, Southern Jiangxi Province, Southeast China: Petrogenesis and Tectonic Implications

摘要

A geochemical and isotopic study was carried out on late Mesozoic mafic dikes in southern Jiangxi Province in order to clarify their petrogenesis and explore tectonic implications. K-Ar dating results show that these mafic dikes intruded during Jurassic to Late Cretaceous (147-79 Ma) lime. Dominantly sub-alkaline in composition, these dikes share several geochemical features: they are significantly enriched in large-ion lithophile element (LILE, e.g., Rb, Ba, Sr) and slightly enriched in light rare-earth elements (LREE), and are variably depleted in high-field-strength elements (HFSE, e. g., Nb, Ta, Ti). However, mafic dikes intruded in the Late Jurassic to Early Cretaceous (147-139 Ma) are also characterized by weaker Nb and Ta negative anomalies compared wilh dikes intruded in the mid-Late Cretaceous (118-78 Ma). Age-corrected Sr-Nd isotopic ratios show relatively wide ranges for (~(87)Sr/~(86)Sr); from 0.7042 to 0.7130 , and -10.8 to +5.9 for epsilon_(Nd)(T). ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb, and~(208)Pb/~(204)Pb ratios for these dikes range from 17.819 to 18.400,"15.472 to 15.730. and 37.711 to 38.787, respectively. Geological, geochemical, and-isotopic evidence suggests that the mafic dikes in southern Jiangxi Province were not significantly affected by crustal contamination. We consider them to have originated from dominant)}' EM-2 type lithosphefic mantle ± minor asthenospheric mantle (MORB or DMM). The geochemical and Sr-Nd-Pb isotopic characteristics of basalts and mafic dikes, together with geochemical and isotopic data from granitoids, volcanic rocks, and rift basins in Southeast China suggest that this region was dominated by a lithospheric extensional tectonic regime in the late Mesozoic. This study, integrated with published geochemical and isotopic data for upper Mesozoic and Cenozoic basaltic rocks, allows us to postulate lhat lithospheric thinning in Southeast China continued until the end of Late Cretaceous time, and was accompanied by a shift in the mantle sources of basaltic rocks.