Petrogenesis of Early Carboniferous Ultramafic–Mafic Volcanic Rocks in the Southern Changning–Menglian Belt, Southeastern Tibetan Plateau: Implications for the Evolution of the Paleo-Tethyan Ocean

The Changning–Menglian Belt represents the main Paleo-Tethyan Suture in the southeastern Tibetan Plateau,which divides Gondwana-and Eurasia-derived continental fragments from each other. The belt contains ultramafic–mafic volcanic rocks that provide evidence of the tectonic processes which operated during the evolution of the Paleo-Tethyan Ocean. New geochemical data for Early Carboniferous volcanics in the southern Changning–Menglian Belt show that wehrlites have cumulate and poikilitic textures, and contain low-Fo（84.2–87.2） olivine, clinopyroxene with low Mg# values（79.4–85.6）, and spinel with high Cr# values（67.8–72.4）. Estimated equilibrium temperatures obtained using olivine-spinel Fe-Mg exchange geothermometry range from 978°C to 1373°C（mean = 1205°C; n = 3）. These observations combined with a lack of reaction or melt impregnation textures indicate that these units represent magmatic cumulates rather than having formed as a result of mantle-melt reactions. Both wehrlites and basalts in the belt have subparallel rare earth element（REE）-and primitive-mantle-normalized multi-element patterns with slightly positive Nb-Ta anomalies, but negligible Eu and ZrHf anomalies. The volcanics have similar Sr-Nd-Pb isotopic compositions with εNd（t） values of 4.2–4.5（mean = 4.3; n = 3）and 4.0–4.4（mean 4.2; n = 4）, respectively, and also have similar immobile element ratios, such as Nb/La, Nb/U, Th/La, Zr/Nb, Th/Ta, La/Yb, Nb/Th, Nb/Y, and Zr/Y. These characteristics indicate both units have ocean island basalt（OIB）-like geochemical affinities, consistent with the fact that the clinopyroxene in the wehrlites is compositionally similar to OIBrelated cumulus clinopyroxene. This suggests that both the wehrlites and basalts were derived from similar parental magmas that underwent generally closed-system magmatic differentiation dominated by fractionation of the olivine and clinopyroxene.This parental magma was likely generated in an oceanic seamount setting from an OIB-type mantle source（i.e.,asthenospheric mantle） containing garnet-spinel lherzolite material. Combing this new data with that from oceanic seamount volcano-sedimentary suites derived from previous research enables the identification of a mature late Paleozoic ocean basin between the passive northeastern Gondwanan margin and the northward-migrating microcontinent of Lanping–Simao.