No abstract

The Bangong-Nujiang Suture Zone (BNSZ) in central Tibet hosts a series of dismembered Jurassic ophiolites that are widely considered as remnants of the vanished Meso-Tethys Ocean. In this study we present new compositional, isotopic, and geochronological data from anorthosites and gabbros of the Dongco and Lanong ophiolites in order to test several hypotheses about the nature of subduction in the Bangong-Nujiang Tethys Ocean (BNTO) during the Mesozoic era. Uranium–Pb dating of magmatic zircons separated from the Dongco anorthosites yielded an (average) age of 169.0 ± 3.7 Ma. Zircons separated from the Lanong anorthosites and gabbros yielded U–Pb ages of 166.8 ± 0.9 Ma and 167.3 ± 1.1 Ma, respectively. Zircons separated from the Dongco and Lanong anorthosites have positive εHf(t) values (5.62–15.94 and 10.37–14.95, respectively). The Dongco anorthosites have moderate initial 87Sr/86Sr (0.703477–0.704144) and high εNd(t) (+6.50 to +7.91). The Lanong anorthosites have high (87Sr/86Sr)i (0.706058–0.712952) and εNd(t) in the range of −1.56 to +2.02. Furthermore, the Lanong gabbros have high (87Sr/86Sr)i (0.705826–0.706613) and εNd(t) in the range of −0.79 to +4.20. Most gabbros from Dongco and a few gabbros from Lanong show normal mid-ocean ridge basalt (N-MORB)-like primitive mantle (PM)-normalized multi-element patterns. In contrast, most gabbros from Lanong show U-shaped chondrite-normalized rare earth element (REE) profiles. The investigated gabbros are characterized by wide ranges of δEu {(Eu)N/[(Sm)N*(Gd)N]1/2} values (0.83–2.53), indicating that some of them are cumulative rocks. The trace element contents of all anorthosite samples imply that their composition was controlled by cumulative processes. The geochemical and isotopic compositions of the non-cumulative gabbros from Dongco (δEu: 0.95–1.04) and Lanong (δEu: 0.83–1.03) indicate that their parental melts were derived from melting of heterogeneously depleted, juvenile mantle reservoirs. These rocks have arc-related affinities, indicating that their mantle sources were influenced by minor inputs of subducted lithospheric components. Our preferred hypothesis for the origin of the non-cumulative gabbros from Dongco is that they were formed in a transient back-arc basin (BAB) setting in the middle-western segment of the BNTO, whereas our preferred scenario about the origin of the non-cumulative gabbros from Lanong is that they were generated in a forearc setting in the middle part of the BNTO. We conclude that both geotectonic settings were developed in response to the northward subduction of the BNTO during the Middle Jurassic.

The middle of the Bangong-Nujiang Suture (BNS) in the central Tibetan Plateau hosts a series of dismembered ophiolitic fragments that document the evolution of part of the Tethys Ocean. However, the origin of these ophiolitic fragments in the Zangbei lakes region remains debated. Using new and existing field observations and petrographic, geochronologic, isotopic, and whole-rock chemical data from ophiolitic rocks in the Zangbei lakes region, we evaluate their origins and constrain the tectonic evolution of the Bangong-Nujiang Tethys Ocean (BNTO). The Lanong peridotites have low rare-earth element (REE) concentrations and typically exhibit U-shaped REE patterns that are similar to those of forearc peridotites from South Sandwich and Xigaze. Lanong basalts and others mafic rocks from the Zangbei lakes region show enrichment in large-ion lithophile elements and depletion in high-fieldstrength elements, and they have clear forearc and boninitic affinities in various tectonic discrimination diagrams. In addition, the Lanong basalts have initial 143 Nd/ 144 Nd ratios of 0.512307 to 0.512773, and ε Nd (t) values of −2.7 to +6.3. Considering the regional geology, previous geochronologic data from the ophiolitic fragments (147.6 ± 2.3 Ma to 189.8 ± 3.3 Ma) and the Darutso high-Mg andesites (161.5 ± 0.9 Ma to 164.2 ± 1.4 Ma), and the lack of Jurassic arc-related rocks in the northern Lhasa terrane, we conclude that the Jurassic ophiolitic fragments of the Zangbei lakes region were derived from a depleted mantle source and formed in a forearc basin in response to north-directed subduction of the BNTO.