Multiphase Late Devonian to Carboniferous volcanic events in the west of Oyu Tolgoi, southeastern Mongolia: New geochronological, geochemical, and isotopic constraints on tectonic history

Abstract

The Late Paleozoic volcanic rocks distributed in southeastern Mongolia are key to reconstructing the tectonic processes along the south margin of the Central Asian Orogenic Belt during this period. In this study, representative volcanic samples were collected in the west of Oyu Tolgoi near the eastern border of the South Gobi and Trans-Altai Zones. The geochronological results reveal intermittent Late Devonian to Early Carboniferous volcanic belts with zircon UPb ages of 360, 320, and 340 Ma for the south, center, and north belts, respectively. Geochemical and isotopic results imply comparable petrogenesis for the samples from different belts. The basic samples are primarily derived from the different degrees of partial melting of a mantle source, and their compositions are slightly affected by the assimilation of crustal materials with different maturities. The intermediate to acidic samples were more likely produced by partial melting of a crust-derived source triggered by underplating of the mantle magma. The low Mg# and apparent Eu anomalies of the extremely high-Si rhyolites sampled from a bimodal-like volcanic sequence in the south belt indicate strong fractional crystallization after derivation from the partial melting of the juvenile crust. In conjunction with the spatiotemporal distribution pattern of magma and the observations recorded in previous studies, we propose that the Late Devonian to Early Carboniferous volcanic rocks from the south and north belts are a continental arc formed from the northward subduction of the Tsagaan Uul terrane under the Mongolian microcontinent and that the Late Carboniferous volcanism in the center belt was produced under a post-collision extensional setting. The subsequent increasingly intense extension was also responsible for the local Late Carboniferous to Early Permian calc-alkaline to alkaline granitic plutons.