Spinel peridotite xenoliths from the Tariat Depression, Mongolia. II: Geochemistry and Nd and Sr isotopic composition and their implications for the evolution of the subcontinental lithosphere

Abstract

A suite of spinel peridotite xenoliths from the Shavaryn-Tsaram volcano, Tariat Depression (central Mongolia) represents (for major elements) fertile to moderately depleted subcontinental lithosphere. Part of the variation of moderately incompatible trace elements is ascribed to small-scale mineralogical heterogeneities caused by processes like metamorphic differentiation accompanying partial melting or by mechanical segregation. Several bulk lherzolites show a high relative enrichment of the LREE over HREE which can be traced to a grain boundary phase genetically linked to, but not directly representing, the host basanitoid. In Nd and Sr isotopic composition the anhydrous peridotites cover the field of oceanic basalts (143Nd/144Nd = 0.5128–0.5133, 87Sr/86Sr = 0.7020–0.7039). In contrast, a phlogopite peridotite has a high 87Sr/86Sr and also a less radiogenic 143Nd/144Nd. The majority of “dry” lherzolites have Nd and Sr “bulk earth” model ages around 2 Ga. They may be interpreted as dating a small-degree (< ~5%) melting event which would not have severely affected the major element chemistry of the xenoliths. The ~2 Ga model ages may indicate a genetic relation between the lithospheric mantle and the stabilization of the continental crust in Mongolia at that time. Alternatively, if the peridotites are unrelated to the overlying crust, they may be pieces of a young asthenospheric diapir. Coexisting ortho-and clinopyroxenes are in Nd isotopic equilibrium for Iherzolites having equilibrated at temperatures around 950°C at mantle pressures. Disequilibrium melting models of mantle rocks are not supported by our data because for medium to coarse-grained mantle spinel peridotite the Rb-Sr and Sm-Nd isotopic systems close with respect to diffusional exchange at temperatures around 900°C, as indicated by recently published diffusion experiment results and supported by our data.