Late Early Cretaceous magmatic constraints on the timing of closure of the Bangong–Nujiang Tethyan Ocean, Central Tibet

Abstract

As one of the most important parts of the eastern Tethys, the evolution of the Bangong–Nujiang Tethyan Ocean (BNTO) has important geological implications for paleogeographic reconstructions of the Tethyan Realm. However, the closure time of the BNTO remains highly debated. Here, we report geochronological, petrological and geochemical data from three newly identified undeformed stitching plutons that intrude a north–south-oriented section of ophiolitic mélanges in the middle–eastern parts of the Bangong–Nujiang suture zone (BNSZ), central Tibet. The stitching plutons comprise the Chaqu diorite–granite association, the Jiang Co granite, and the Konglong diorite porphyry. Secondary ion mass spectroscopy (SMIS) zircon UPb dating suggests that these three plutons were emplaced at ca. 116–112 Ma. The stitching plutons show heterogeneous Sr–Nd–Hf–O isotopic compositions. The Chaqu diorites have initial 87Sr/86Sr ratios varying from 0.7062 to 0.7064 and εNd(t) values from −2.0 to −1.6, and slightly high zircon δ18O (6.2–6.8‰) and positive zircon εHf(t) (+2.6 to +7.4) values. The Chaqu granites show high initial 87Sr/86Sr (0.7064–0.7065) ratios and low εNd(t) (−2.3 to −2.1) values, and elevated zircon δ18O (6.2–7.3‰) values and variable zircon εHf(t) (−1.7 to +4.9) values, similar to those of the associated Chaqu diorites. The Jiang Co granites exhibit higher initial 87Sr/86Sr (0.7085–0.7094) ratios and negative εNd(t) (−8.33 to −8.27) values, and variable zircon δ18O (4.8‰ to 6.0‰) and εHf(t) (−6.9 to −2.0) values. The Konglong diorite porphyries have initial 87Sr/86Sr isotopic ratios ranging from 0.7069 to 0.7075 and negative εNd(t) values ranging from −4.2 to −4.1, and high zircon δ18O (9.0‰ to 9.8‰) and variable zircon εHf(t) (−7.3 to +0.7) values. Geochemical evidence indicates that they were derived from different magma sources that included varying proportions of mantle and crustal components. The Chaqu diorites were derived by partial melting of an ancient metasomatized mantle source, and the Chaqu granites were formed by fractional crystallization of the Chaqu dioritic magmas. The Jiang Co granites were generated by partial melting of low δ18O high-temperature hydrothermally altered ancient continental crust. The Konglong diorite porphyries were likely generated by partial melting of high-δ18O enriched lithospheric mantle. In combination with data from adjacent ophiolites, magmatic rocks, and Cretaceous sedimentary rocks, we suggest that the stitching plutons were generated in a post-collisional setting induced by upwelling of asthenospheric mantle due to slab breakoff beneath the BNSZ. Our data provide an important new upper age limit for the time of collision between the Lhasa and Qiangtang terranes, i.e., the latest closure time for the BNTO.