In-situ trace element and Sr isotope in mantle xenoliths from the West Qinling orogen: Evidence for carbonate metasomatism

Abstract

Subduction process plays an important role in triggering chemical metasomatism of the lithospheric mantle beneath orogenic belts. The lithospheric mantle beneath the West Qinling orogen is extensively heterogeneous, but the lithospheric modification process and related metasomatic agents remain poorly constrained. In this paper, we report in-situ elemental and isotopic compositions of minerals in mantle xenoliths from the West Qinling orogen. The mantle xenoliths are lherzolites and can be categorized into two types. Lherzolite with refractory compositions represents the residue of lithospheric mantle that have undergone high degrees of melt extraction. In contrast, lherzolites with fertile compositions contain clinopyroxenes variably enriched in incompatible elements and radiogenic Sr isotopes. These fertile lherzolites represent lithospheric mantle modified by metasomatic process in different degrees. High (La/Yb)N, Ca/Al, Nb/Ta, Nd/Yb, 87Sr/86Sr ratios and low Ti/Eu ratios of the clinopyroxenes suggest the metasomatic agent was dominated by carbonate-rich melts. This inference is supported by the identification of carbonate inclusion, reaction structures, and clinopyroxene chemical zoning in the lherzolites. Considering the tectonic background of the West Qinling orogen, the orogenic lithospheric mantle would have been metasomatized by carbonate-rich melts derived from partial melting of subducted carbonate-bearing Paleo-Tethyan oceanic slab. In this regard, petrological observations, along with in-situ elemental and Sr isotopic compositions of clinopyroxenes in mantle xenoliths can be powerful means to identify carbonate metasomatism and constrain the evolution of the lithospheric mantle beneath orogenic belts.