Geochemical characteristics of ultramafic rocks in Sumdo area, Tibet and its enlightenment for the evolution of the Sumdo Paleo-Tethys Ocean

The proposal of Tangjia-Sumdo Paleo-Tethys suture zone has changed the understanding of Paleo-Tethys evolution pattern in the Qinghai Tibet Plateau.The petrological and geochemistry of ultramafic rocks in Sumdo area are studied to manifest the genesis and type of the ophiolite in the Tangjia-Sumdo Paleo-Tethys suture zone.The results show the ultramafic rocks have higher MgO and TFe₂O₃ but lower Al₂O₃and TiO₂ contents compared with those of the primitive mantle.The total rare-earth element(REE)contents of the samples are 4.04×10^-6~9.31×10^-6, which are lower than those of the primitive mantle.The chondrite-normalized REE patterns display a U-type, and they are enrichment in LREE.The primitive mantle-normalized spider diagrams show an abyssal peridotites affinity, there are obvious negative anomalies of Th and Nb.Most of the samples show slight negative Zr and Hf anomalies.Through quantitative modeling, we conclude that the ultramafic rocks are refractory residual formed by more than 25% batch melting of spinel lherzolite mantle, and the inheritance of some garnet signatures was observed in the samples to a certain extent.They are not modified by later rock-melt reaction, the protolith of the ultramafic rocks may be depleted abyssal peridotite.Combining with regional geology, we suggest that the ultramafic rocks in Sumdo area may have been formed in the mid ocean ridge(MOR)setting and are one of the typical end members of mid ocean ridge basalt(MORB)type ophiolites.