Bulletin of the Geological Society of Malaysia, Volume 43, Dec. 1999, pp. 113 – 129
Department of Geological Sciences, Chiang Mai University, Chiang Mai, 50200, Thailand
Abstract: Provenance and tectonic setting of deposition of metagreywackes from the Sirikit Dam area in northern Thailand which covers part of the Nan River Suture were reconstructed on the basis of geochemical and petrographic data. The metagreywackes belong to the Pha Som Metamorphic Complex that consists of a coherent unit of multiply-deformed pumpellyite-actinolite facies metasedimentary rocks and tectonic slices of ophiolitic mafic-ultramafic rocks. The lithology, deformation history, and metamorphism of the Pha Som Metamorphic Complex are consistent with those observed in ancient and modern accretionary complexes in many parts of the world.
Geochemically, the metagreywackes are divided into two categories: the PSM-l and PSM-2 groups. The PSM-1 group is characterised by relatively low average concentrations of TiO2, Al2O3, Fe2O3*, and V, a low Zr/Th ratio, relatively high abundances of Pb, Th, light rare-earth elements (La, Ce, and Nd), and high Th/Sc and Ce/V ratios. The PSM-2 group is characterised by relatively high average concentrations of TiO2, Al2O3 and Fe2O3*, a high Zr/Th ratio, relatively low abundances of Pb, Th, light rare-earth elements (La, Ce, and Nd), and low Th/Sc and Ce/V ratios. Immobile trace element characteristics suggest that the PSM-1 metagreywackes were derived from a continental island arc source and were probably deposited in a submarine fan setting at a subduction zone. In contrast, major and trace element characteristics suggest that the PSM-2 metagreywackes represent sediments derived mainly from an oceanic island arc source.
On the basis of modal compositions of the framework grains, the metagreywackes can be divided into quartz-rich greywackes, which are comparable to the sandstone of modern and ancient accretionary complexes, and quartz-poor greywackes, which have a transitional magmatic arc source. This study further supports the existence of an ancient accretionary complex prior to the amalgamation of the Shan-Thai and Indochina terranes in Late Triassic-Early Jurassic time.