The Ren Co MOR-type Ophiolite in the North-central Tibetan Plateau: A Fast-spreading Ridge Segment of the Meso-Tethys Ocean?

The Meso-Tethys Ocean is generally considered to have opened in the late Early Permian as a result of the Cimmerian continental block drifting away from the Gondwana supercontinent. This ocean is also termed the north branch of the Neo-Tethys Ocean, and is now represented by the Bangong–Nujiang suture zone in the north-central Tibetan plateau. However, it is still unknown for the evolutionary history for as such a huge ancient ocean basin. Ophiolites are remnants of oceanic lithosphere and preserve key information in rebuilding the evolutionary history of ancient oceans. In this study, we undertook detailed geological mapping for the Ren Co ophiolite in the middle part of the Bangong–Nujiang suture zone, and a typical Penrose-type ophiolite sequences was newly documented in the Ren Co area. The Ren Co ophiolite comprises serpentinized peridotite, cumulate rock, gabbro/diabase, sheeted dike, massive and pillow lavas, and minor red chert. These rocks exhibit well-preserved mantle and crust rock suites, and show close similarities to oceanic lithospheres at modern fast-spreading ridges. Zircon U–Pb dating for gabbro and plagiogranite yielded ages of 169–147 Ma, which suggest that the Ren Co ophiolites were formed during the Middle to Late Jurassic. Harzburgite in the Ren Co area shows similar features to those of abyssal peridotite indicating the residues of the oceanic mantle. Mafic rocks(basalt, diabase and gabbro) of the Ren Co ophiolite show geochemical features similar to those of typical N-MORB. Moreover, all samples have positive whole-rock εNd(t), and zircon εHf(t) and mantle-like δ18O values. These features suggest that these rocks were derived from a depleted mantle source, thus the Ren Co ophiolite was typical MOR-type ophiolite. We suggest that the Ren Co ophiolite was formed in a fast-spreading mid-ocean-ridge(MOR) setting, and they most likely represented the ridge segment of the BangongNujiang Meso-Tethys Ocean. The Bangong–Nujiang MesoTethys Ocean was a wide paleo-ocean, and experienced continuous oceanic spreading, subduction, accretion before final Lhasa and South Qiangtang continental assembly.

Sub-basin scale inhomogeneity of mantle in the South China Sea revealed by magnesium isotopes

The South China Sea(SCS)is the largest extensional basin in the western Pacific and was formed after rifting of the Euro-Asian continental margin.The nature of its underlying mantle remains enigmatic due to the lack of sampling of the seafloor’s igneous crust.The International Ocean Discovery Program Expedition 349 cored seafloor basalts of the southwestern(Site U1433)and eastern(Site U1431)SCS sub-basins.The recovered basalt samples exhibit different source lithologies and geochemistries.The Mg isotopic compositions of seafloor basalts from these sites were investigated to elucidate the origin of this large-scale mantle inhomogeneity.Results indicate that the Site U1431 basalts have a mantlelike averageδ^(26)Mg value of-0.27‰±0.06‰(2 SD;n=10).Together with inhomogeneous Sr-Nd-Pb-Hf isotopic compositions,the Site U1433 basalts have an averageδ^(26)Mg value(-0.20‰±0.06‰;2 SD;n=8)higher than those of the Site U1431 basalts and normal mantle.Their heavier Mg isotopic compositions and low206 Pb/204 Pb ratios(17.7)indicate that the Site U1433 basalts were affected by the re-melting of detached continental-arc lithosphere in the sub-ridge mantle.The coupling of Mg and Sr-Nd isotopes provides robust evidence that the mantle-likeδ^(26)Mg values of the Site U1431 basalts resulted from mixing between detached continental arc lithosphere and the nearby Hainan plume,with respective supra-and sub-normalδ^(26)Mg values.From the perspective of Mg isotope,the mantles of the southwestern and eastern sub-basins are compositionally inhomogeneous,with their mantle evolutionary histories being distinct.