Forearc tectonic evolution in the middle of the Bangong–Nujiang Tethys Ocean: New geochemical evidence of the Lanong ophiolites from the Zangbei lakes region

Abstract

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.