Eocene high Ba−Sr magmatism in northern Tibet links ultra-enriched mantle to relamination of Songpan−Ganzi sediments

Beyond its andesite-like bulk composition, the continental crust is also markedly enriched in large-ion lithophile elements such as Ba and Sr. The relative importance of the two mechanisms proposed to explain the enrichment of these elements, mantle metasomatism versus magmatic differentiation in the crust, is debated. Newly identified Eocene high Ba−Sr volcanic rocks from the Bandaohu area of northern Qiangtang, central Tibet, provide a rare opportunity to shed light on this debate. These rocks span from primitive shoshonitic lavas to evolved trachytes, yet all display extreme Ba (2148−5998 ppm) and Sr (2182−3625 ppm) concentrations, enriched Sr−Nd isotopic signatures [(87Sr/86Sr)i = 0.70843−0.70886; εNd(t) = −8.5 to −7.3], and elevated zircon δ18O values (6.8‰−8.6‰). Trace-element ratios (high Th/Yb, low Ba/Th) and isotopic modeling indicate derivation from lithospheric mantle metasomatized by Triassic sediments of the subducted Songpan−Ganzi terrane. Fractional crystallization of ∼30%−35% clinopyroxene further magnified Ba−Sr enrichments, while the absence of plagioclase crystallization preserved them. These results establish a hierarchy of controls, with source enrichment as the primary driver and differentiation as a secondary amplifier. We propose a two-stage geodynamic model in which intracontinental subduction relaminates Songpan−Ganzi sediments at the base of the Qiangtang lithosphere and subsequent lithospheric thinning triggers partial melting of the resulting ultra-enriched mantle domains, producing high Ba−Sr magmas. This mechanism links sediment influx, mantle enrichment, and magmatic differentiation, offering new insights into crust-mantle interaction and continental growth during the India-Asia collision.