Triassic three-stage collision in the Paleo-Tethys: Constraints from magmatism in the Jiangda–Deqen–Weixi continental margin arc, SW China

Abstract

Triassic volcanic rocks in the Jiangda–Deqen–Weixi continental margin arc (DWCA) in the Sanjiang area of SW China formed as a result of the closure of the Paleo-Tethys and associated continental collision, and are therefore important for reconstructing the evolution of the Paleo-Tethys. Representative samples of rhyolite and basalt from the Jijiading–Luchun–Yezhi area yield weighted mean 206Pb/238U ages of 245–249Ma and 245Ma, respectively, and exhibit the features of bimodal volcanic rocks. The mafic rocks are enriched in large ion lithophile elements (e.g., Rb, Ba, U), are depleted in high field strength elements (e.g., Nb, Ta, Ti), and have low εNd(t) values (−3.55 to −3.69) and high initial 87Sr/86Sr values (0.70788–0.70804). The mafic rocks were formed by partial melting of an enriched lithospheric mantle source that had been metasomatized by fluids derived from subducted sediments. The felsic rocks are characterized by high K2O (K2O/Na2O=2.16–71.33) and A/CNK (>1.1), and low Mg# (20–45) and εNd(t) values (−9.61 to −10.66), with zircon εHf(t) values of −8.1 to −16.6. Their old Nd model ages are similar to those of ancient Yangtze basement, indicating that they formed by partial melting of this ancient crust as a result of underplating by basaltic magma. We suggest that the bimodal volcanic rocks in the DWCA formed in an extensional setting as a result of slab break-off during a margin of arc–continent collision. The present results, combined with previous data, reveal three stages of collision in the Paleo-Tethys: early collision convergence (~255–250Ma), syn-collision extension (249–237Ma), and late collision orogenesis (236–212Ma).