Discovery and its tectonic significance of the Proterozoic Miaowan ophiolites in the southern Huangling anticline, western Hubei, China.

Studies of strongly deformed and metamorphosed mafic- ultramafic rocks from the Proterozoic Miaowan Formation in the high-grade metamorphic Kongling Group in the southern Huangling anticline show that the mafic rocks are composed of layered fine-grained amphibolites, metagabbro rockbodies and dykes and diabase dykes, while the ultramafic rocks are mainly composed of serpentinized dunite and harzburgite, cropping out as tectonite slices and blocks within amphibolites. The geochemical features of the fine-grained layered amphibolite rocks including: TiO2=1.14%-1.48%, slightly depleted-flat type REE curve with no obvious Eu anomaly, (La/Yb)N= 0.87-1.12, average La/Nb, Ce/Zr, Zr/Nb, Zr/Y, Ti/Y are respectively 1.04,0.15,18.78,2.53,290.51, and the average of Nb / Th is 9.88. These features show that these amphibolites are N-MORB type tholeiites typical of those formed in mid-ocean ridge tectonic settings. The metagabbro have typical cumulate texture, flat REE distribution and obvious positive Eu anomaly; The REE characteristics of serpentinized dunite show a U-shape of slight loss of middle REE, representing cumulates metasomatized by LREE slightly enriched mantle. All these features indicate that the mafic-ultramafic rocks from the Proterozoic Miaowan Formation within Kongling Group in the southern part of Huangling anticline, are a structurally dismembered ophiolite resting above an ophiolitic mélange, sitting on top of the Proterozoic shelf sequence on the Yangtze Craton. The discovery of the Proterozoic Miaowan ophiolite supplies important evidence for the existence of an Mesoproterozoic oceanic basin, as well as tectonic events of Columbia supercontinent aggregation and break-up.