Ordovician appinites in the Wugongshan Domain of the Cathaysia Block, South China: Geochronological and geochemical evidence for intrusion into a local extensional zone within an intracontinental regime.

Abstract: Palaeozoic mafic igneous rocks are potentially significant in constraining the tectonic nature and evolution of the Kwangsian Orogeny in the eastern South China Block, yet they have received little attention because of their limited outcrop. Geochemistry and geochronology was carried out on newly identified Ordovician ultramafic–mafic appinites in the Wugongshan Domain of the Cathaysia Block. Seven appinite samples yielded 206Pb/238U crystallisation ages ranging from 452±4Ma to 473±3Ma. Abundant 480–500Ma zircon xenocrysts and/or inherited zircons were found in the appinites, possibly indicating an earlier magmatism episode in the early Palaeozoic period. The Wugongshan appinites are ultramafic to mafic in composition, and the ultramafic rocks display features of cumulates (high concentrations of MgO, Fe2O3t, Cr, Ni, and low concentrations of total alkali and total rare earth elements [REE]). The appinite geochemistry displays: relatively flat chondrite normalised REE patterns with slight enrichment in light REE and weak negative Eu anomalies; enrichment in large-ion lithophile elements (such as Rb, K), and weak depletion in Nb–Ta in primitive mantle normalised trace element patterns. We suggest that the Wugongshan appinites likely originated from an ancient metasomatised mantle, and that crustal assimilation, fractional crystallisation (AFC), magma mingling and hydration were involved in the petrogenetic process, based on the combination of geochemistry, crust-like bulk Sr, Nd and zircon Hf isotopic compositions (εNd (t)=−8.2 to −3.2, initial 87Sr/86Sr ratios of 0.7067–0.7144, zircon εHf (t) values peaking at −9 to −3) and regional geological data. Further considering the alignment and chronology of the appinites, we suggest that the appinitic magmas probably were emplaced along the Jiangshan–Shaoxing Fault in a local extensional zone in an intracontinental regime in the early Palaeozoic. [Copyright &y& Elsevier]