METAMORPHIC ANALYSIS OF THE MARGHAZAR FORMATION WITHIN THE KOTAH DOME, SWAT, PAKISTAN, WESTERN HIMALAYA

The Marghazar formation, located in Swat, Pakistan, forms the base of the Alpurai group within the Kotah and Loe Sar domes. Unconformably above the Swat Gneiss, the Marghazar formation is upper Permian to Triassic in age. The formation consists of amphibolite, quartzo-feldspathic schist, and schistose marble and correlates with the Panjal formation of western India. The rocks form part of the Pakistan metamorphic belt at the northern edge of the Indian plate south of the Indus Suture zone and Kohistan arc complex. Three rock samples from the Marghazar formation were chosen from different locations within the Kotah dome for metamorphic analysis and to determine the approximate pressure and temperature of metamorphism. All three samples are quartzo-feldspathic schists with major minerals of quartz, plagioclase, garnet, muscovite, and biotite. Program GTB: GeoThermoBarometry (Spear, F.S., and Kohn, M.J., version August 2006, http://ees2.geo.rpi.edu/MetaPetaRen/Software/GTB_Prog/GTB.html) was applied to calculate pressure and temperature using average garnet, biotite, muscovite, and plagioclase compositions. Peak temperature and pressure was calculated at approximately 640˚ to 670˚ Celsius and 10.9 to 12.4 kilobar. AFM diagrams were plotted to determine degree of equilibrium in each of the samples. Coexisting garnet and biotite compositions from all samples plotted approximately parallel indicating near equilibrium conditions. Thin section analysis showed sharp biotite-garnet contacts, a further indicator of equilibrium. Calculated temperature and pressure conditions are close to the lower limit of eclogite facies metamorphism which is consistent with field data that suggests metamorphism resulted from subduction beneath the Indus ophiolitic mélange prior to collision with the Kohistan arc complex.