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, 187Os/188Os and 187Re/188Os 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 (εNd(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

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