Formation of green-core clinopyroxene in continental basalts through magmatic differentiation and crustal assimilation: Insights from in-situ trace element and Pb isotopic compositions

Abstract

Green-core clinopyroxene (GCPX) widely occurs in diverse magmas and various tectonic settings. Due to its special textual and compositional zonation, it is usually considered as solid evidence for involvement of exotic felsic melt and some unique geological processes (e.g., magma mixing, mantle metasomatism and recycling of foundered lower continental crust (LCC)). However, this premise is challenged by its more complicated and enigmatic origins than previously thought. To explore potential origins of GCPXs in continental basalts, here we investigate in-situ trace element and Pb isotopic compositions of the GCPXs in the Feixian and Sihetun primitive basalts from the eastern North China Craton (NCC). The results show that the Feixian GCPXs can be subdivided into three types in addition to the Sihetun GCPXs. Regardless of the types of these GCPXs, their surrounding rims are regarded as overgrowths that crystallized from the basaltic host magmas. The wide range of Mg# (67–86), trace element patterns similar to those of clinopyroxene equilibrated with the basaltic host magmas and host lava-like Pb isotopic ratios of the Feixian type-1 GCPXs suggest that they have crystallized from differentiated melts genetically related to the basaltic host magmas. Besides unradiogenic Pb isotopic ratios, the Feixian type-2, type-3 and Sihetun GCPXs are characterized trace element signatures that are identical or similar to those of clinopyroxenes in pyroxene-rich granulite, garnet-bearing granulite and feldspar-rich granulite xenoliths from the NCC, respectively, indicating their derivation from lower crustal granulite wall rocks during the ascending of the basaltic host magmas. Our work suggests that GCPXs can be simply formed through the two common magmatic processes of continental basaltic magmas (i.e., crystallization differentiation and crustal assimilation) and highlights that exotic felsic melt and unique geological processes are not additionally required despite being possible. Before using GCPXs as an indicator of specific magmatic processes, comprehensive data (e.g., trace element and isotopic compositions) should be carefully investigated for their exact origins, especially for those in continental magmas. Additionally, GCPXs may provide valuable information about the structure and composition of the lower crust and a window to unravel crustal assimilation and digestion during magma extraction and their influence on compositions of basaltic magmas.