Ultramafic xenoliths from aillikites in the Tarim large igneous province: Implications for Alaskan-type affinity and role of subduction

Abstract

The Tarim large igneous province (TLIP) located in northwestern China is characterized by a relatively long duration of magmatism and complex lithology as compared to other global LIPs. Previous studies correlated these features to the interaction between mantle plume and subducted slab. However, no direct subduction-related magmatic records were so far recognized to support this interpretation. Here we report the petrological, mineralogical, and geochemical characteristics of a suite of ultramafic xenoliths occurring within aillikites from the northwestern margin of the TLIP. The xenoliths include dunite, wehrlite, olivine clinopyroxenite, clinopyroxenite, amphibole clinopyroxenite, clinopyroxene hornblendite, and hornblendite. These rocks show broadly similar mineralogy and are mainly composed of olivine, clinopyroxene and amphibole with rare phlogopite, chromite, and magnetite, in the absence of orthopyroxene and plagioclase. We evaluate the petrogenesis of these rocks based on whole-rock major and trace element compositions and mineral chemistry. The rocks exhibit enrichment of large ion lithophile elements (LILEs) relative to high field strength elements (HFSE) with negative Nb, Ta, and Ti anomalies, similar to the features of arc-like lavas. Their (87Sr/86Sr)t (0.7035–0.7041) and εNd(t) (1.17–5.67) values are consistent with depleted mantle source. The Mg# [molar Mg/(Mg + Fe2+)] and Fe3+# [molar Fe3+/(Fe3++Al + Cr)] values of spinel in the ultramafic xenoliths are consistent with those of the Alaskan-type mafic-ultramafic intrusions. The Al2O3 contents in clinopyroxene increase with decreasing Mg#, which is comparable with the Al2O3 variation trend of clinopyroxene in Alaskan-type and arc-related magma, in contrast to those of the LIPs. Our data establish that the ultramafic xenoliths in the aillikites are comparable to Alaskan-type ultramafic complexes, and correspond to magmatism associated with the southward subduction of oceanic plate beneath the Tarim craton during Neoproterozoic or Early Paleozoic. Our study provides direct evidence for the interaction between mantle plume and subducted slab in generating the TLIP.