The Group III Rocks are distributed in the Mae Korn area and show welldeveloped foliation. These rocks are fine-grained, and consist principally of amphibole, with subordinate or small amounts of quartz, plagioclase,zoisite/clinozoisite,titanite/leucoxene and opaque minerals, and small amounts or very small amounts of muscovite/prehnite. Chlorite,biotite and tourmaline are present in very small amount in some samples The foliation is produced by the preferred alignments of nematoblastic amphibole, which shows pale yellowish green or yellowish brown pleochroic colors. Their colors are chaeacteristic of tremoliteactinolite series or hornblende. A few of ?glaucophane-riebeckite series amphibole have been observed in sample no. MK6-1. The Group III Rock have Zr/TioO2 and Nb/Y ratios in the ranges of 0.005-0.009 and 0.063-0.356, respectively, and are tholeiitic andesite/basalt. The tholeiitic affinity is well supported by their positions in variation and tectonic discrimination diagrams,and REE pattern. The N-MORB normalized pattern for the representative rock shows a negative niobium anomaly. The REE pattern and N-MORB normalized pattern are very similar to those of immature backarc basin basalt from Hokkaido,Japan.