Petrogenesis of the Main Range and Eastern Province granites in eastern Myanmar: New insights from zircon U–Pb ages and Sr–Nd isotopes

Abstract

The Main Range and Eastern Provinces are two major granite belts in Southeast Asia. These granite belts extend southward from the southeastern Tibetan Plateau to Myanmar, and through Thailand into Peninsular Malaysia. They are interpreted to represent the magmatic expression of the closure of the Paleo-Tethys from the Permian to the Triassic. Myanmar lies in the heart of these granite belts. The Kyaing Tong and Tachileik granites in the far east of Myanmar are important components of the granite belts of Southeast Asia; however, due to the lack of reliable geochronology within eastern Myanmar, delineation of the Main Range and Eastern Province belts in this region is very poorly constrained. Here we present new zircon U–Pb age, whole-rock composition, and Sr–Nd isotope data from the Kyaing Tong and Tachileik granites from eastern Myanmar to address this geological problem. Measured ages of 207–216 Ma from the Kyaing Tong granites imply that they are a northern extension of the Main Range Province, whereas the Tachileik granites yield ages of 246–250 Ma, which suggests that they are the northern extension of the Eastern Province granite belt. Both belts in eastern Myanmar comprise biotite monzogranites and granodiorites and show similar geochemical features, such as having a high aluminum saturation index and an unfractionated composition. The granites from both provinces show enrichment in light rare earth elements (REE) and negative Eu anomalies. All samples demonstrate characteristic negative Ba, Nb, Ta, Sr and Ti anomalies, and a positive Pb anomaly, when plotted on spidergrams. The Kyaing Tong granites have high and variable initial 87Sr/86Sr ratios (0.717735–0.731271), negative εNd (t = 215 Ma) values (−14.2 to −10.4), and old TDM2 ages. Similarly, the Tachileik granites have high and variable initial 87Sr/86Sr ratios (0.715336–0.722712), negative εNd (t = 250 Ma) values (−12.4 to −11.3), and old TDM2 ages. Sr–Nd isotope values show that these granites may be generated by mixing of two end-member lithologies: amphibolite and schist of the Lancang Group, which represents the lower crust of the Indochina block. We consider that both the Kyaing Tong and Tachileik granites are of I-type affinity. They were derived from partial melting of the amphibolite and underwent assimilation of schist. Our petrogenetic and zircon U–Pb age data support models that relate the Eastern Province granites to continental arc during the Permian and syn-collisional magmatism in the Early Triassic, and the Main Range Province granites to post-collisional magmatism during the Middle to Late Triassic.