Southward subduction of the Bangong–Nujiang oceanic lithosphere triggered back-arc spreading: Evidence from the Late Jurassic Kongnongla magnesian andesites in central Tibet

Abstract

We report zircon UPb age and Hf isotope and whole-rock major and trace element and SrNd isotope data for magnesian andesites from the Kongnongla area, central Tibet. These Kongnongla magnesian andesites are directly in contact with high-Nb basalts and are part of the Baila–Lanong ophiolitic mélanges. Zircon UPb dating shows that the magnesian andesites crystallized at ca. 158 Ma. The samples are characterized by moderate SiO2 (59.3–61.7 wt%), high MgO (3.74–4.69 wt%), Fe2OT3 (4.95–5.30 wt%), Ba (570–1320 ppm), and Th (6.5–7.0 ppm) contents, and low (87Sr/86Sr)i (0.704491–0.704857), and Sr/Y (17.5–24.0) ratios. The samples also exhibit enrichments of compatible elements, such as Cr (110–150 ppm) and Ni (24.7–36.5 ppm), and display fractionated rare earth element (REE) patterns with light REE enrichment relative to heavy REEs. Negative zircon εHf(t) values (−10.8 to −15.7) and whole-rock εNd(t) values (−3.85 to −6.12) of the Kongnongla magnesian andesites are similar to those of DaruTso magnesian andesites, Hohxili high magnesian andesites, and sediments of the Mugagangri Group. We propose that the Kongnongla magnesian andesites were formed by the interaction between slab fluids, subducting sediments, and overlying mantle under high-temperature–low-pressure conditions. Based on previously published data and the regional geology, we also suggest that these magnesian andesites were generated in a back-arc setting caused by the southward subduction of the Bangong–Nujiang oceanic lithosphere during the Late Jurassic.