Petrogenesis of Late Carboniferous–Early Permian mafic igneous series in the Baoshan block: Implications to birth of Neo-Tethys and generation of magmatic sulfide deposit

Abstract

The petrogenesis of the rift-related Late Carboniferous–Early Permian mafic igneous rocks (basaltic lavas, dolerite dykes and mafic–ultramafic intrusive rocks) from the Sibumasu block in the eastern end of Cimmeria was still ambiguous. This petrogenesis is related to understand the drive for the formation of Neo-Tethys which was developed along the margin of Cimmeria, and to the generation of magmatic sulfide deposit via partial melting of previously subduction-modified mantle induced by rifting of continental margin. Most of the mafic rock samples belong to the subalkaline series, with lithologies varying from basalt to basaltic andesite. Magmatic zircons from the doleritic dykes yield weighted mean 206Pb/238U ages of 308.7 ± 2.5 Ma. The εHf(t) values of zircon are ranging from 3.2 to 11.8, which are much lower than those of the coeval depleted mantle (εHf(t) ≈ 16 at 305 Ma). The igneous series share similar features, such as elevated light rare earth elements, negative Nb-Ta anomalies and enriched Sr-Nd-Hf isotopes. Based on geochemical analyses of the mafic igneous series in the northern part of Sibumasu, we suggest that the primary melts of the mafic igneous rocks were derived from a fertile subcontinental lithospheric mantle (SCLM) which was previously modified by oceanic subduction, and these rocks were intruded and erupted in a passive plate margin during plate extension. The subsequent melting-induced softening of SCLM triggered the evolution of continental margin from the rift to the open of Neo-Tethys Ocean. The proposed model emphasizes that the melt generated from previously-modified SCLM in a continental rift setting is beneficial for the generation of magmatic sulfide deposit.