Blueschists and Blue Amphibole Schists-Gneisses Associated with the Nan River Mafic – Ultramafic Belt, North Thailand

Abstract

The discovery of an areally limited outcrop of blueschists in Huai Sak, near Ban Huai Lao, SE Nan Province in the early 1980s has not been followed up until now. This is despite these blueschists being used in many regional tectonic compilations as unequivocal evidence for the presence of a former subduction zone along the trend of the Nan river. We report additional structural information and mineral analyses for the Huai Sak blueschists and the discovery nearby, in Huai Phi Rong, of structurally similar schists containing the Na-Ca amphibole barroisite. In addition, we report the discovery in Huai Phi Rong of float samples, believed to be derived from a thrust sheet of serpentinite matrix melange, consisting of coarse schists and gneisses that contain a variety of blue amphiboles associated with garnet. EPMA analysis confirms these blue amphiboles to be variously riebeckite, glaucophane, winchite and barroisite.