Geology and geochemistry of the Palaeozoic plutonic bodies of the Trans-Altay Gobi, SW Mongolia: Implications for magmatic processes in an accreted volcanic-arc system

Abstract

Three geological domains (Gobi Tien Shan, Shargyn Gobi and Trans-Altay) have been distinguished in the Trans-Altay Gobi (SW Mongolia) consolidated during the Carboniferous and intruded by numerous plutons of the Devonian, Carboniferous and Permian ages. According to relationship to the Variscan orogenic cycle, pre-orogenic, syn-orogenic and post-orogenic intrusive bodies have been recognized. A pre-orogenic phase is represented by the Gurvan Khar Massif composed of geochemically primitive granodiorites that are geologically related to the relics of the Silurian-Devonian oceanic crust in the Zoolen Terrane (Shargyn Gobi Domain). Calc-alkaline intrusive rocks of I-type in the Gobi Tien Shan and Naran Sevest intrusive complexes exposed in the Gobi Tien Shan Domain are syn-orogenic to post-orogenic. Laser ablation U-Pb ICP-MS dating on zircon yielded an Early Carboniferous age for the Zamyn Belgekh Pluton, representing the largest plutonic body in this domain. Other plutons (Bayan Ayrag, Trans-Altay and Ikh Bayan) are of high-K chemistry, have Late Carboniferous to Permian age and are clearly post-orogenic. Minor gabbro massifs and small oval bodies of granites of the Ikh Bayan Massif in the Baytag Terrane (Shargyn Gobi Domain) are spatially related to the Trans-Altay shear zone of Late Carboniferous to Permian age. This zone accommodated dextral lateral movements between the Shargyn Gobi and Trans-Altay domains.