Early Cretaceous back-arc basin basalt-type gabbros in the southeastern Tibetan Plateau: Implications for Neo-Tethyan oceanic slab subduction

Although progress has been made in defining and explaining the temporal and spatial distributions of Neo-Tethyan magmatism in the Tibet-Himalaya region, the correlation of magmatic suites in the SE Tibetan Plateau remains poorly constrained. This paper reports zircon U-Pb dating, geochemistry, and Sr-Nd isotope compositions of a concealed gabbroic unit (coverage area of >6.0 km(2)) in the southern Lancangjiang tectonic zone, western Yunnan, SW China. Zircon Pb-206/U-238 ages of 143-133 Ma indicate that the unit formed during the Early Cretaceous (ca. 139.7 Ma). The gabbro samples have variable MgO (7.88-16.1 wt%; Mg-# = 58-74) and relatively low TiO2 (0.76-1.24 wt%) and total-alkali (K2O + Na2O = 2.61-4.06 wt%) contents. The samples exhibit weakly fractionated rare earth element (REE) patterns with slight depletion in light REEs and negligible Eu anomalies. In a primitive-mantle-normalized element-variation diagram, the gabbros, display variable enrichment in large-ion lithophile elements and relatively flat patterns of high-field-strength elements. Initial Sr-87/Sr-86 ratios of 0.7079 to 0.7142 and relatively depleted epsilon(Nd)(t) values of +3.96 to +4.84 are similar to those of the mantle source of back-arc basin basalts (BABBs) in the central Lhasa Block and mid-ocean ridge basalt in the Indus-Tsangpo Suture Zone. These various characteristics suggest that the gabbroic unit was derived from a shallow and depleted mantle source by relatively high-degree partial melting in the spinel stability field. The parental magma underwent slight crustal contamination, as well as fractional crystallization of clinopyroxene. The studied gabbros have distinct BABB-type geochemical affinities similar to those of the Okinawa BABB. Combining our data with the coeval occurrence of a continental rift basin (the Simao-Lanping Basin) leads us to suggest the existence of a back-arc rifting setting in the SE Tibetan Plateau during the Jurassic-Cretaceous. This setting can also be geochronologically and genetically correlated to the back-arc basin developed within the Lhasa Block. These settings and constituent characteristics formed as a result of Neo-Tethyan oceanic slab subduction during the late Mesozoic.