MINERALOGY AND GEOCHEMISTRY OF ULTRAMAFIC ROCKS FROM THE INDUS SUTURE OPHIOLITE IN SWAT, NW PAKISTAN

Genetically diverse lithologies, which include blueschist, greenschist, ultramafic, mafic plutonic, e.g. gabbros, and volcanic, e.g. pillowed basalts, and metasedimentary rocks, occur as a wedge\|shaped zone in the area to the north of the Mingora town in Swat, NW Pakistan. This zone of fault\|bounded rock bodies and blocks is distinguished into three principal groups, each of which is itself a composite of lithologies, or a melange—the blueschist melange, greenschist melange and ophiolitic melange. All these lie along the Main Mantle Thrust (MMT)—the westward extension of the Indus\|Tsangbo suture zone and the locus of collision between the Indo\|Pakistan plate and the Kohistan island arc—and, are therefore, collectively termed as the Main Mantle Thrust Melange Group (MMTMG).The ultramafic rocks, locally containing chromite pods and constituting a part of the ophiolitic member of the MMTMG, occur as small to large lenticular bodies, distributed along the marginal part of the Indo\|Pakistan plate. They are invariably serpentinized to varying degrees and, therefore, consist of abundant, fine grained serpentine which is mostly accompanied by subordinate amounts of completely bastitised, medium to coarse grained orthopyroxene, trace to accessory amounts of commonly altered (zoned with ferritchromite and/or magnetite) chrome spinel, variable proportions of olivine, and accessory amounts of diopside. In other words, the ultramafic rocks are mostly harzburgitic in mineralogy and, therefore, the ophiolite as a whole can be classified as the harzburgite sub\|type..Detailed field and mineralogical studies show that the present petrographic ch aracteristics and mineral\|chemical composition of these dominantly harburgitic rocks are the result of a number of phenomena involving partial melting, serpentinization, metamorphism and talc\|carbonate alteration.