Permo-Triassic granitoids in the northern part of the Truong Son belt, NW Vietnam: Geochronology, geochemistry and tectonic implications

Abstract

The northern segment of the Truong Son belt of northwestern Vietnam is composed of three granitoid complexes (Chieng Khuong, Dien Bien, and Song Ma, respectively). Our new LA-ICP-MS U–Pb dating results of magmatic zircons from five granitoid samples in the Dien Bien and the Chieng Khuong complexes reveal the existence of three age populations of magma emplacement and crystallization from 276 to 202Ma. The new data show that the quartz diorite of the Chieng Khuong complex is dated to be 271Ma. However, the previously named Dien Bien complex, exposed at different localities, has various ages (from 281 to 202Ma) of magmatic crystallization. The Chieng Khuong quartz diorite exhibits high Mg-value (60.7) and enriched Sr–Nd isotopic features (ISr=0.713822, εNd (t)=−11.34), with subduction-related geochemical features (e.g., depletion in Nb, Ta and Ti and enrichment in Rb and La). The Dien Bien gabbroic diorite at Muong Lay is characterized by relatively depleted isotopic data (ISr=0.705050, εNd (t)=+0.09) and extensive depletion in Nb and Ta. By contrast, the Dien Bien granodiorite and quartz monzonite at Dien Bien are high-K calc-alkaline to calc-alkaline, with high Mg-values (45.5 to 54.4), and they display enriched Sr–Nd isotopic data (ISr=0.714912 to 0.716430, εNd (t)=−9.58 to −8.85) and post-collisional geochemical characteristics (e.g., enrichment in total alkali, LILE and LREE and depletion in Eu, Sr, P, Ti and Nb).Under the framework of regional tectonic evolution, the Permo-Triassic magmatism along the northern Truong Son belt is thus grouped into an early stage from ca. 280 to 250Ma and a late stage from 229 to 202Ma. The early Permian-early Triassic arc magmatism (280–270Ma and 250–245Ma), which is widely distributed in the Truong Son belt, provides reliable information on the continuous subduction of the Song Ma oceanic plate beneath the Indochina block. Furthermore, the magmatism of the Dien Bien complex at Dien Bien was formed during post-collisional extension at ca. 229 to 202Ma. Diachronous suturing of the eastern Tethyan evolution is characterized by the differences in the ages of subduction/post-collisional evolution along the Ailaoshan and the Song Ma belts.