MAGMATIC EVALUATION OF THE KOH E SULTAN VOLCANO CHAGAI DISTRIC, BALOCHISTAN, PAKISTAN

The Koh-e-Sultan volcano occurs in the western part of Chagai magmatic arc, situated in north-western part of Balochistan, Pakistan. The Koh-e-Sultan volcano stretches in a 770 km2 area and is mainly represented by andesitic to dacitic lava flows and volcanoclastic sediments including agglomerate, tuffs, pumice, volcanic conglomerate, breccia and pyroclastic flows. The last explosive activity of this volcano is represented by pumice deposits of 0.09 million years old, mostly found towards west and southwest of Koh-e-Sultan volcano. The petrological study of various samples collected reveals that the lava flows are mainly represented by three main types of porphyritic andesites including; (a) hypersthene-andesite, (b) hypersthene-hornblende-andesite and (c) hornblende-andesite. The dacitic lava flows are also porphyritic and represented by only one variety of hornblende dacite. The pumice has rhyolitic composition and contains fragments of plagioclase, quartz, volcanic glass and minor biotite. The petrographic studies show a systematic decrease of anorthite contents as being younger, in hypersthene andesite range decreasing from An28-50 to An27-49, in hornblende-hypersthene andesite from An26-47 to An21-40 in hornblende andesite from An20-38 to An17-35 and in dacite the range is An12-30. The mafic mineral exhibits change from hypersthene through hornblende to biotite in rhyolitic pumice. The composition and proportion of mafic minerals in younger lava flows also changes. The concentration of opaque minerals generally decreases towards younger lava flows. The phenocryst and groundmass ratios increase towards younger volcanics from 30:70 to 60:40. These studies clearly indicate the progressive magmatic evolution of Koh-e-Sultan volcano from older to younger volcanics during Pliocene to Late Pleistocene. Keywords: Magmatic evolution; Koh-e-Sultan Volcano; Chagai magmatic arc.