The large granitic pegmatite Li deposits formed at final collision of a three micro-continent amalgamation in the Central Altun Tagh, Northwest China

Abstract

Micro-continent collision and amalgamation play pivotal roles in continental convergence and contribute significantly to continental crustal growth. However, the understanding of multiple micro-continent amalgamation processes, particularly their influence on the formation of large Li-Be deposits, remains limited. The Altun orogenic system, resulting from the collision and amalgamation of three micro-continents in the Proto-Tethys Ocean, has recently revealed numerous granitic pegmatite-type Li-Be deposits, presenting an ideal opportunity to investigate the mineralization. Our research focused on two large granitic pegmatite Li deposits, South Washixia and Tamuqie in the Central Altun Tagh, Northwest China. Our findings suggest: 1) The structural and photomicrograph characteristics observed in both the Tamuqie and South Washixia granitic pegmatite Li deposits, indicate that the pegmatites emplaced in shear zone within a contractional deformation zone. 2) The age of the large granitic pegmatite Li deposit in South Washixia between 447 ∼ 445 Ma, while the Tamuqie Li pegmatites formed at 448 Ma with Li-barren pegmatites formed at around 418 Ma. It is suggested that the large granitic pegmatite Li deposit in the Central Altun Block, encompassing South Washixia and Tamuqie granitic pegmatite Li deposits, originated during the final collision event of three micro-continent blocks (Qaidam, Altun-Qilian, and Dunhuang-Alex). We suggest that during the final collision and amalgamation event of multiple micro-continents triggering intense compression and thickening of the crust, and the thickened lower crust may undergo shear-driven dehydration melting of biotite in granulite facies to produce large-volume granitic magmas. These magmas crystallize and differentiate into lithium-rich pegmatitic magmas along shear zones, forming large lithium pegmatite deposits. Our research presents a novel mechanism for the formation of granitic pegmatite Li deposits, which emphasizes the role of shear-driven dehydration melting during the final collision and amalgamation events of three micro-continents.