Early Mesozoic granitoids in southern Vietnam and Cambodia: A continuation of the Eastern Province granitoid belt of Thailand

Abstract

This study examines three granitoid plutons, intruded during the latest Triassic - earliest Jurassic, that are exposed in south-western Vietnam and south-eastern Cambodia. On geochemical and petrological grounds the two plutons in Vietnam (Hon Da Bac and Hon Khoai island) are composed of slightly peraluminous I-type granitoids, with a high-K calc-alkaline affinity. The Cambodian pluton (Tamao) consists of peraluminous S-type granitoids, with high-K calc-alkaline and ferroan affinity. Based on zircon U-Pb geochronology the Hon Da Bac granitoids were intruded between 200.7 ± 2.7 and 196.2 ± 3.0 Ma, those from Hon Khoai between 199.6 ± 2.7 and 197.6 ± 2.7 Ma and the Tamao granitoids at 187.8 ± 3.2 Ma. These ages are consistent with intrusion during and soon after the Indosinian II Event of the Indosinian Orogeny, which was driven by docking of the Sibumasu and Indochina Blocks. Although these granitoids have some geochemical signatures similar to those of arc-related magmas, we conclude that they are more likely derived from syn to post-collisional magmatism. In terms of timing of their emplacement and geochemistry, all three plutons can be convincingly correlated with the Late Triassic-Early Jurassic Eastern Province granitoids in the Loei Fold Belt, which stretches along the western margin of the Indochina Block, extending through eastern Thailand and southern Cambodia. By this correlation we show that in fact, the Eastern Province granitoid belt extends further southeast than previously thought, occurring in south-eastern Cambodia and south-western Vietnam.