Slab-derived adakites and subslab asthenosphere-derived OIB-type rocks at 156 ± 2 Ma from the north of Gerze, central Tibet: Records of the Bangong–Nujiang oceanic ridge subduction during the Late Jurassic

Abstract

This paper reports zircon U–Pb age and Hf isotope, whole-rock major and trace element, and whole-rock Sr–Nd–Hf isotope data of the dacites from Rena Tso and mafic rocks (diabases and basalts) from Duobuza, north of Gerze, central Tibet. These data reveal the presence of a distinct rock association of slab-derived adakites (154±1Ma) and subslab asthenosphere-derived OIB-type (oceanic island basalt) mafic rocks (157.6±1.4Ma). The medium-K calc-alkaline dacites (SiO2=66–69wt.%) from Rena Tso are enriched in Sr (520–1083ppm) and depleted in heavy rare earth elements (HREEs) and Y (9.8–10.8ppm), resembling adakites. These adakitic dacites have low whole-rock initial 87Sr/86Sr ratios of 0.7043–0.7046, positive εNd(t) (+1.0 to +3.4), εHf(t) (+6.4 to +7.0), and zircon εHf(t) (+1.9 to +7.6) values, indicating an oceanic slab origin (crust and sediment). Considering the low Mg# (32–53) and (La/Yb)N (19–23), the adakitic dacites are most likely derived from the partial melting of the subducting slab at shallow depths and the subsequent interaction with peridotite in a thin mantle wedge during magma ascent. The diabases and basalts (SiO2=49–53wt.%) from Duobuza show an alkali signature with enrichment of high field strength elements (HFSEs) (e.g., Zr=213–285ppm) and exhibit positive Nb–Ta–Ti anomalies that are geochemically comparable to those of OIB. These samples show positive whole-rock εNd(t) values of +3.3 to +3.7, εHf(t) values of +4.7 to +5.7, and negative to positive zircon εHf(t) values of −1.5 to +5.2. These OIB-type mafic samples are interpreted as the products of low-degree decompression melting of the upwelling subslab asthenosphere with a minor contribution from the sub-continental lithospheric mantle (SCLM). Our new data indicate the presence of a distinct rock association of coeval slab-derived adakites and subslab asthenosphere-derived OIB-type rocks. Such an association along with the normal arc rocks further to the north of the Bangong-Nujiang suture zones (BNSZ) records the northward ridge subduction of the Tethyan Bangong-Nujiang Ocean at ca. 156±2Ma.