Tracking Magmatism and Metasomatism in Continental Arc Crust Root: Significance from Western Central Qilian Belt in NE Tibet, China.

This article focuses on the petrogenesis and metasomatic history of mantle peridotites and pyroxenites from the Heigou Complex, an exhumed arc crust root in the western Central Qilian belt, NE Tibet, China. The study integrates petrography, mineral and bulk-rock geochemistry, U–Pb zircon and titanite geochronology, and Sr–Nd–Hf isotopes to reveal that the peridotites represent residues after up to ~22% melt extraction from a primitive mantle source, while the pyroxenites formed as cumulates through 50% to 80% fractional crystallization of primitive basaltic magma at ca. 523 Ma, derived from a mantle metasomatized by subducted sediments. Subsequent hydrous melt metasomatism during exhumation at ca. 485 Ma altered the pyroxenites, producing amphibole, epidote, and titanite, indicating active modification of the sub-arc mantle–crust boundary by slab-derived fluids. These findings provide insights into magmatic and metasomatic processes operating in continental arc roots during early Paleozoic subduction of the Proto-Tethys Ocean.