Geochemical characteristics of the early Neoarchean komatiite from the North China Craton: Evidence for plume–craton interaction

Abstract

Komatiite is characterized by extremely high MgO contents and H2O-poor nature and plays an important role in understanding the early mantle-derived magma processes, crust–mantle differentiation of the Archean earth, but it is sparse in the North China Craton. Although komatiitic rocks have ever been discovered in the Mengyin area, their formation ages and types are still in debate. Here, we firstly report a new komatiite outcrop near Nanbaita village (Jinan city) of the western Shandong Province and present new major and trace element contents, as well as Sr–Nd and Re–Os isotopic compositions. The rocks show spinifex texture with primary olivine and pyroxene skeletal needles have been transformed into secondary minerals (e.g., serpentine, tremolite). The komatiites display extremely high MgO, but low TiO2 and Na2O + K2O. Their chondritic Al2O3/TiO2 and GdN/YbN ratios suggest they are Aluminium-undepleted komatiites which are generated by larger degrees of melting (~50%) in relatively lower pressure (ca. 5–7 GPa) with garnet absence than Aluminium-depleted komatiites. The formation age of these komatiites was no later than 2.69 Ga constrained by the Re depletion age. Flat heavy rare earth element patterns and low 187Re/188Os and Th/Yb ratios show the primary komatiitic magma was not contaminated by substantial crustal material. The γOs value of sample 18LH51 is 0.09, which is adjacent with chondrite and other contemporaneous komatiites in the world, indicating that the Nanbaita komatiites were originated from a chondritic mantle. Furthermore, the widely distribution of ~2.7 Ga ultramafic–mafic rocks in the western Shandong Province might be the reflection of a contemporaneous worldwide superplume event.