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 87 Sr/ 86 Sr (0.7041-0.7064) and Pb isotopic composition ( 206 Pb/ 204 Pb = 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.

Formation of the Japan Sea basin: Reassessment from Ar–Ar ages and Nd–Sr isotopic data of basement basalts of the Japan Sea and adjacent regions

“…Basaltic volcanism in the JSN region was subsequently characterized by a depleted isotopic signature, reflecting the influence of the upwelling asthenosphere. Similar upwelling, associated with lithospheric extension and thinning, has also been interpreted from the isotopic systematics of Cenozoic mafic rocks in SE China (Xie et al, 2006).…”

K–Ar age and geochemistry of the SW Japan Paleogene cauldron cluster: Implications for Eocene–Oligocene thermo-tectonic reactivation

“…1; Kawano and Kagami, 1993 and references therein). Paleogene rollback of subducted Pacific slab and resultant upwelling of asthenosphere were invoked by Xie et al (2006) and Guo et al (2007) to explain the origin of Paleocene mafic intrusive rocks in the central Jiangxi area of SE China and Late Paleocene adakitic andesites in the Yanji area of NE China, respectively (Fig. 1).…”

Zircon U‐Pb Age and Geochemistry of Rhyolite in Jianshui, Yunnan and Its Geological Implications

“…Previous studies in eastern and central regions of the Cathaysia block have discussed that the old (i.e., Proterozoic) mantle that existed beneath the block has been largely removed and replaced by younger, hotter and fertile mantle (Xu et al, 2000Liu et al, 2012). This lithospheric replacement could be associated with the lithospheric extension and thinning process Xie et al, 2006). In contrast, the western region of the Cathaysia block was relative stable since Mesozoic, and the study of its lithospheric evolution is relative sparse.…”