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

Abstract Mantle peridotites and pyroxenites from arc crust root record vital information about petrogenetic processes in supra–subduction environments, which are commonly obscured by subsequent metamorphic and/or metasomatic overprints. The Heigou Complex from the western Central Qilian arc root (NE Tibet, China) includes peridotites and pyroxenites. Petrography, bulk–rock and mineral geochemistry, zircon and titanite U–Pb ages, and bulk–rock Sr–Nd–Hf isotopes are presented to decipher its petrogenetic and metasomatic history. The peridotites display highly refractory compositions of low SiO2, Al2O3, CaO, but high MgO contents. Geochemical modeling indicates that these are the residues after up to ~22% melt extraction. The geochemical and isotopic data (87Sr/86Sr i = 0.7116–0.7179, εNd(t) = −4.2 to −7.1 and εHf(t) = −5.3 to −5.4) suggest the pyroxenitic protoliths were derived from a subduction-metasomatized mantle source. Combining petrographic and fractional crystallization simulation, the pyroxenites formed as cumulates through 50% to 80% differentiation of primitive basaltic magma in the arc root at ca. 523 Ma. Elevated LILEs (Rb, Sr, Pb, etc.) and LREE in bulk-rock and mineral compositions, together with microstructure evidence, suggest that mantle peridotite experienced mantle metasomatism. Subsequent hydrous melt (fluid) metasomatism during exhumation then reacted with the preexisting clinopyroxenite, leading to local Mg-hornblende, titanite and epidote formation. The timing of retrograde melt infiltration is constrained by a titanite U–Pb crystallization age of ca. 485 Ma. The combined evidence reveals a picture of continental arc magmatism and metasomatism at arc crust roots. Dehydration of the Proto-Tethys oceanic slab could induce flux melting in the mantle wedge that had undergone metasomatism, resulting in the generation of hydrous arc magmas. These arc magmas intruded the overlying lithospheric mantle, segregating pyroxenite cumulates. The ultramafic rocks at the arc crust root were subsequently modified by hydrous melt (fluid).