Genesis of the Markam gneiss dome within the Songpan-Ganzi orogenic belt, eastern Tibetan Plateau

The Markam gneiss dome within the Songpan-Ganzi orogenic belt, eastern Tibetan Plateau, is cored by the Keryin-Taiyanghe granitoids and mantled by Upper Triassic, turbiditic, metasedimentary rocks, which have been intruded by numerous pegmatites and veins. The large-scale Markam detachment fault separates the granitoid core from the overlying Upper Triassic metasedimentary rocks. Microstructures and quartz fabrics from the Markam detachment fault show a top-to-the-south shearing sense and point to formation under high temperatures (>650 °C). New zircon UPb geochronological data from the Markam gneiss dome reveal two stages for the granitoid core at ca. 226–212 Ma and ca. 224–218 Ma. In addition, zircon UPb dating of syn-tectonically deformed pegmatites in the Markam detachment fault shows that the detachment fault was initiated at ca. 212–207 Ma. Later-stage, undeformed pegmatites (including spodumene-bearing varieties) that form sill- and dyke-networks have ages of ca. 200–190 Ma. Our data indicate that formation of the Markam gneiss dome was triggered by development of the Markam detachment fault after the emplacement of the granitoid core. And the following diapiric uplift of the granitoids continued the growth of the Markam gneiss dome, resulting in formation of a thermal domal structure, which further uplifted the magma to a more favorable metallogenic environment and controlled the occurrence of the ore veins. Thus, it indicates that the spodumene-bearing pegmatites emplaced under extensional and decompressional conditions during the Songpan-Ganzi exhumation orogeny.