The 186 Ma Dashibalbar alkaline granitoid pluton in the north-Gobi Rift of central Mongolia: Evidence for melting of Neoproterozoic basement above a plume

Abstract

The Jurassic Dashibalbar granitoid pluton (300 km 2 ) crops out in the Triassic North-Gobi rift of central Mongolia, just south of the 230 to 195 Ma Khentei batholith. The granitoids are shallow-seated dominantly, amphibole-bearing alkali feldspar granite that contain quartz-syenite/syenite enclaves. They are all composed of megacrystic mesoperthite, quartz, Ca-Na amphibole altered to biotite and rarely with pyroxene cores, magnetite and ilmenite. The pluton yielded a concordant U-Pb zircon age of 186 1 Ma, which is similar to a published 189 3M a 40 Ar/ 39 Ar amphibole age, and indicates rapid cooling through ca. 550 °C. This age is ca. 10 my younger than the 196 4 Ma age of the bimodal volcanic complex intruded by the pluton. The volcanic complex is composed of augite-phyric transitional basalt and rhyolite/ comendite. Both basalts and rhyolites/comendites are evolved within-plate varieties with positive Nd(t) ( 2.5) values. The granitoids are evolved alkaline, A-type granites and quartz-syenites/syenites that are enriched in light REE's, but show a distinct depletion in Eu, Sr and Ba, indicative of feldspar fractionation. The data are consistent with derivation of the granites from the syenites by an assimilation- fractional crystallization process involving a silicic crustal contaminant. The granitic rocks have Nd(t) values of 0.8 to 1.2, which are slightly lower than Nd(t) values 1.3 to 1.6 in the syenites, although both have similar TDM model ages (800-970 Ma). The 800 Ma model ages of the basalt and rhyolite/comendite are comparable to those of the intrusion and enclaves. The compositions of all these rocks, including Nd(t) and T DM , are within the range of A-type granites and volcanic complexes of the Early Mesozoic Mongolian-Transbaikalian igneous province. The results suggest deriva- tion of a parent magma of the granitoids and felsic volcanic rocks from underplated, enriched, Neoproterozoic mantle-derived basaltic rocks in the lower crust, whereas the Dashibalbar basalts were derived from Neoproterozoic subcontinental lithospheric mantle; Neoproterozoic megablocks crop out in adjacent parts of the Central Asian Orogenic Belt. Melting of lower crust and subcontinental lithospheric mantle implies a rising heat source. Although such a heat source is consistent with both rifting and passage over the Mongolian mantle plume, only the latter explains the west-to-east migration of the magmatism and rifting.