The Song Da magmatic suite revisited: A petrologic, geochemical and Sr–Nd isotopic study on picrites, flood basalts and silicic volcanic rocks

Abstract

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.