The origin of the Pailin Crystalline Complex in western Cambodia, and back-arc basin development in the Paleo-Tethys Ocean

Abstract

The Pailin Crystalline Complex, which consists of amphibolites, metagabbros, and felsic igneous rocks, is located in western Cambodia. We used the geochemistry of the amphibolites and the zircon U[sbnd]Pb ages of felsic igneous rocks to constrain the origin of the Pailin Crystalline Complex and to gain insights into the tectonic framework of Southeast Asia. Geochemical analyses indicate that the amphibolites are normal (N-type) mid-ocean ridge basalts, similar to the Sa Kaeo Back-arc Basin basalts of southeastern Thailand. The zircon U[sbnd]Pb ages are 283 ± 2 Ma (anorthosite dike), 280 ± 2 Ma (aplite dike), and 275 ± 2 Ma (plagiogranite). The geochemistry and detrital zircon U[sbnd]Pb ages of sandstones adjacent to the Pailin Crystalline Complex suggest that they are sediments that filled the Sa Kaeo Back-arc Basin during the Triassic. We infer that the Pailin Crystalline Complex is part of a dismembered ophiolite related to the back-arc basin, and that the igneous rocks formed during the Early Permian. Most of the metamorphism occurred during continental collision in Southeast Asia. However, the amphibolites of the Pailin Crystalline Complex also experienced ocean-floor metamorphism at a back-arc spreading center, in a tectonic setting similar to that of the mafic metamorphic rocks of the Nan Back-arc Basin, northern Thailand. We conclude that the Pailin Crystalline Complex is the southernmost extension of a back-arc basin related to subduction of Paleo-Tethys oceanic crust and the Sukhothai Arc.