Extensive Early Cretaceous arc-like magmatic rocks in central Tibet manifest subduction rollback of the Neo-Tethyan ocean

Abstract Spatiotemporal distribution of magmatism in continental arcs is generally accompanied by compositional change of igneous rocks. However, it remains unclear whether and how the variation footprint and geochemical affinities of magmatism in fossil magmatic arcs are effective for reconstructing subduction polarity. In this study, new geochronological, mineralogical, geochemical, and Sr-Nd-Hf isotope data are presented to characterize the Early Cretaceous (ca. 130–110 Ma) bimodal volcanic rocks of the Zenong Group in central Lhasa, central Tibet. Our data show that the Zenong Group volcanic rocks are dominated by rhyolite and dacite, subordinate basalt, and local andesite. The basalts have enriched Sr-Nd-Hf isotopic compositions and arc-like trace elements, suggesting a lithospheric mantle source in the spinel stability field with minor asthenospheric mantle contributions. The andesites have similar Mg# values and isotopic compositions, indicating a fractionation origin from the basaltic magmas. The coeval dacites and rhyolites display relatively low Mg# values and variable isotopic compositions, pointing to an ancient lower crust source with minor mantle contribution. The bimodal compositional characteristics and contrasting magma sources of the volcanic rocks95 indicate an extensional setting. We propose that the northward migration of the Early Cretaceous (ca. 130–110 Ma) magmatism in central Lhasa is the result of backarc rifting associated with slab rollback of the subducting Neo-Tethyan oceanic plate. The backarc rifting model helps to reconcile ca. 130–110 Ma sedimentary-magmatic evolution records in a broader region, including the Xigaze forearc spreading, magmatic lull in the Gangdese arc, and volcanic–sedimentary rocks in the central Lhasa subterrane.