Geochronological and geochemical constraints on the coexistent N-MORB- and SSZ-type ophiolites in Babu area (SW China) and tectonic implications

Abstract

Our detailed geological, geochemical and geochronological investigations indicate the coexistence of late Paleozoic (270–265 Ma) normal middle ocean ridge basalt (N-MORB)-type and suprasubduction-zone (SSZ)-type ophiolites in the Babu area (SW China). Metaperidotite samples from the Babu ophiolites show high Os concentrations and low Re/Os, 187 Os/ 188 Os and 187 Re/ 188 Os ratios, and represent residual upper mantle. Metagabbro and metabasalt samples show light rare earth element (LREE)-depleted REE patterns and N-MORB-like primitive mantle-normalized spider diagrams without Nb–Ta and Zr–Hf anomalies, and depleted Sr–Nd isotopic compositions (ε Nd ( t ) = +6.9 – +8.3). These geochemical and Re–Os–Sr–Nd isotopic data show that the Babu ophiolites are derived from a depleted mantle source and represent ancient N-MORB-like oceanic crust. In addition, peridotites, basalts and boninites with SSZ signatures (e.g. Nb–Ta negative anomalies) were also reported in the Babu ophiolites consistent with their generation in a forearc or arc setting. In combination with the arc-related high-Mg # limburgite, basalt and andesite, these late Paleozoic ultramafic–mafic rocks are genetically related to Paleotethyan subduction, rather than being Emeishan mantle plume related. The coexisting N-MORB- and SSZ-type ophiolites in the Babu area show similar geochemical and geochronological characteristics to the Jinshajiang–Ailaoshan–Song Ma ophiolites. Supplementary material: A description of the geology of the Babu ultramafic–mafic complex, sample descriptions, detailed analytical methods and results, and a summary of previously published age data for late Paleozoic magmatic rocks along the China–Vietnam border are available at https://doi.org/10.6084/m9.figshare.c.4007569