Geochemical characteristics of genesis of the basalts in Zhongba Mélange of southern Tibet

Abstract：Zhongba Mélange in southern Tibet belongs to the accretionary wedge in the western segment of the Yarlung Zangbo suture zone. The basalts from Zhongba Mélange are in coexistence with radiolarian chert, siliceous shale and mudstone, showing a pillow structure. The age of basalts is constrained to the early Cretaceous by radiolarian assemblage in its companion chert. The basalts have relatively high P2O5 (0.34%-0.87%), lower Al2O3 (13.48%-15.38%) and Mg# (15-35). The samples are characterized by high iron (Fe2O3=7.62%-13.16%) and titanium (TiO2=1.76%-3.11%), and poor silicon (SiO2=43.57%-51.99%), and can be classified as Fe-Ti basalts (ferrobasalts). And the basalts are alkaline. The Fe-Ti basalts were produced through a high degree of high iron and poor silicon trend differentiation. The REE patterns are similar to those of OIB, with LREE enrichment ((La/Yb)N=8.48-32.04) and no obvious Ce and Eu anomaly. The basalts have typical geochemical characteristics similar to those of OIB, with the enrichment of large ion lithophile elements (LILE) (Ba, Th) and high field-strength elements (HFSE) (Nb, Ta, Zr, Hf), and no obvious Nb and Ta anomalies. The geochemical characteristics suggest that the basalts were formed in an ocean island within an extensional tectonic setting, and the magma originated from enriched mantle composed mainly of garnet peridotite with 6%-10% partial melting. Geological signatures of whole-rock trace elements indicate little or no appreciable crustal contamination for the samples. The Zhongba basalts might have been derived from the mantle plume, which suggests that there probably existed a hotspot in the Neo-Tethys.