PERMIAN TO TRIASSIC MAGMATISM IN THE MONGOL-OKHOTSK BELT: THE MAGMATIC RECORD OF A CLOSING ACCRETIONARY MARGIN

The Mongol-Okhotsk Belt (MOB) is part of the Phanerozoic Central Asian Orogenic Belt, the largest accretionary belt on Earth. The MOB extends for over 3,000 km from central Mongolia to the Sea of Okhotsk and formed as a result of the closure of the Mongol-Okhotsk Ocean. Magmatism within the MOB records active margin and collisional magmatism and thus provides a window into pre-, syn- and post-collisional processes in accretionary margins. The distribution of magmatism in the MOB reveals an eastward scissor-like closure that ended in the Jurassic, however, the timing of the transition from active margin to collision in the western end remains unconstrained. Permian and Triassic magmatism was abundant, especially in the western extent of the MOB, and sharply decreased by the Jurassic. In this contribution, we used previously published geochemical data (major and trace elements, and isotopes) from Permian to Triassic magmatic rocks to investigate first-order tectono-magmatic processes formed along the paleo accretionary margin, or during or after the ocean closure. Data show that magmatism was generally enriched in fluid mobile elements, LILE, and LREE, and depleted in HFSE and HREE, evidencing a mantle metasomatized by subduction-related fluids regardless of crustal contamination and thus an active or previously active subduction system. Isotopic data show that magmatism assimilated juvenile crustal components and geochemical data analysis supports a higher enrichment in sediment melts near the suture zone in the western extent of the MOB. Despite the evidence of crustal contamination and sediment melt contributions, magmatism in the western and older extent of the MOB did not produce abundant S-type peraluminous magmatism, suggesting contributions from the mantle and crust. Thus, we conclude that magmatism along this sediment-rich accretionary margin was able to recycle and stabilize a juvenile and compositionally evolved crust in the Central Asian Orogenic Belt.