Permian volcanic rocks, including picrites, flood basalts and silicic volcanic rocks, crop out in the Song Da district, northern Vietnam. They occur as a ~300-km-long belt emanating from the southernmost part of the Emeishan large igneous province, SW China. The belt represents a detached part from the province and was displaced ~600. km southeastward along the Ailao Shan-Red River shear zone during the mid-Tertiary in response to the India-Eurasia collision. We report here geochemical and Sr-Nd isotopic data on the Song Da volcanic rocks to decipher the petrogenetic processes responsible for their formation. The picrites show evidence of olivine fractionation and formed from high-Mg magmas (MgO = 20-25. wt.%) originated from depleted mantle peridotites from the Emeishan plume head. The plume head is characterized by εNd(t) value of +8. Some picrites and low-Ti basalts have low Nb/La, Nb/U and εNd(t), which are indicative of plume-lithosphere interaction in various degrees. Contamination of continental crustal rocks (<20%) is able to produce the spectra of the observed trace element patterns and isotopic ratios. The silicic volcanic rocks are metaluminous or peraluminous (aluminium saturation index = 0.92-1.24) and are sufficiently different from those present elsewhere in the Emeishan large igneous province that are dominantly peralkaline. The Song Da silicic volcanic rocks have εNd(t) values of -0.1 to +0.6 and show strong depletion in compatible trace elements (e.g. Ni = 0.8-3. ppm, V = 4-55 ppm). They most likely formed by extensive fractional crystallization from the associated high-Ti basalts, possibly with some crustal contamination.
All Science Journal Classification (ASJC) codes
- Earth-Surface Processes