Geochemistry and geochronology of dolerite dykes from the Daba and Dongbo peridotite massifs, SW Tibet: Insights into the style of mantle melting at the onset of Neo-Tethyan subduction

Abstract

This study reports on compositional, whole-rock SrNd isotopic, and zircon UPb geochronological data for dolerite dykes in the Daba and Dongbo ultramafic massifs of the southwest Yarlung–Zangbo Suture Zone (YZSZ), SW Tibet. The 121 ± 2 Ma dolerite dykes from the Daba peridotite exhibit primitive mantle (PM)-normalised multi-element patterns with (La/Yb)N = 0.43–0.72 and negative Nb anomalies. They have high initial 87Sr/86Sr ratios (87Sr/86Sr(i) = 0.707197 to 0.707879) and high εNd(t) values (+7.4 to +7.9). The 125 ± 2 Ma dolerite intrusions within the Dongbo peridotite show PM-normalised trace element profiles [(La/Yb)N = 0.65–0.84] that are characterised by apparent negative Nb anomalies and moderate negative Ti (±Y) anomalies. They also have high 87Sr/86Sr(i) ratios (0.706108–0.706793) and high εNd(t) values (+7.8 to +8.2). Semi-quantitative Sm/Yb vs. La/Sm modelling demonstrates that the parental magmas of the investigated dykes were derived from 10%–20% (cumulative) melting of a (broad) mantle source region that had a spinel-bearing normal mid-ocean ridge basalt (N-MORB)-like lherzolitic composition. The geochemical and isotopic data indicate that the composition of the inferred mantle source was influenced by minor inputs of subducted crustal material. The petrogenesis of the Daba and Dongbo massifs can be linked to the upwelling of an asthenospheric source that caused continental rifting and subsequent seafloor spreading that was followed by the subduction initiation adjacent to a passive margin during the Early Cretaceous (~130–120 Ma). The results provide a more detailed and perhaps more elegant hypothesis for the tectonomagmatic evolution of the southwestern YZSZ “ophiolitic” peridotites after their accretion beneath a Neo-Tethyan marginal basin.