K-Ar dating, geochemical and Sr-Nd-Pb isotopic systematics of Paleocene mafic rocks in Central Jiangxi, SE China: Evidence for lithosphere replacement

Abstract

Geochemical and isotopic studies were carried out on Paleocene mafic rocks in the Taihe Basin, central Jiangxi, Southeast China, in order better to understand their magma sources and tectonic implications. K-Ar dating results show that these mafic rocks intruded during the Paleocene (50-65 Ma). These Paleocene mafic rocks are porphyritic diabase and have similar geochemical features, such as strong enrichment of large ion lithophile element (LILE, e.g., Rb, Ba, Sr), slight enrichment of light rare earth element (LREE) and high field strength element (HFSE, e.g., Nb, Ta, Ti, P). The mafic rocks are also characterized by high rare earth element (REE) and minor Eu anomalies (δEu = 0.88-1.2) and have uniform initial ⁸⁷Sr/⁸⁶Sr (0.7041-0.7064) and Pb isotopic composition (²⁰⁶Pb/²⁰⁴Pb = 18.338-18.677) as well as a relatively wide range of initial Nd isotopic ratios (ε_Nd(T)) varying from +0.8 to +6.2. These geochemical characteristics are different from those of subduction-related basalts, but similar to oceanic island basalts (OIB). Geochemical and isotopic evidence suggests that mafic rocks in the Taihe Basin were not significantly affected by crustal contamination and originated from asthenospheric mantle with a minor involvement of EM 2 mantle component. It is noted that Cretaceous basaltic rocks in Southeast China show enriched geochemical and isotopic compositions and were derived from a highly heterogeneous lithospheric mantle with minor asthenosphere components. On the other hand, Late Cenozoic basalts in Southeast China, which display OIB-like geochemical characteristics, were probably generated by melting of depleted asthenospheric mantle (DMM or MORB) with an involvement of EM 2 lithospheric mantle and a paucity of EM 1 lithospheric mantle. The Paleocene mafic rocks of the Taihe Basin exhibit transitional geochemical features between Cretaceous and Late Cenozoic basaltic magmatism, indicating that the involvement of asthenospheric mantle beneath Southeast China increased with time from Late Mesozoic to Cenozoic. It is suggested that the lithosphere replacement beneath Southeast China was associated with lithosphere extension and thinning, with decompression melting of the upwelling asthenosphere having taken place during the Paleocene period. Lithosphere replacement might be related to subduction roll-back or/and steepening of Paleo-Pacific plate due to injection of asthenospheric material into the mantle wedge beneath Southeast China.