Petrography and mineralogy of the Ahingaro serpentinites, Kabal (Swat), NW Pakistan: Implications for their tectonic setting and timing of emplacement

Abstract

A small (^-6 x 1 km), lens-shaped body predominantly consisting of ultramafic rocks, occurs at the Ahingaro Banda Kandao near Kabal, Swat. This body is enclosed within the Kamila amphibolites that constitute the basal part of the intra-oceanic Kohistan island arc. The ultramafic rocks are highly fractured and almost completely seppentinized. They consist of abundant fine-grained serpentine, varying amounts of a variety of bastites after orthopyroxene, veins and stringers of magnetite, invariably altered disseminated grains of chrome spinel, trace amounts of relict clinopyroxene and traces of rather sparsely disseminated granules of sulphide (pentlandite). Some of the samples also contain thin veins of calcite.

The Ahingaro serpentinites display both pseudomorphic and non-pseudomorphic textures. The serpentine predominantly consists of the lizardite and chrysotile varieties, whereas the antigorite occurs rarely. The chrysotile is mostly distributed as veins. A detailed petrogrphic and mineralogical comparison with the serpentinized rocks of the Indus suture melange zone suggests that the Ahingaro serpentinites most probably represent the original Tethyan lithosphere that served as a substratum or platform for the construction of the KIA. These rocks were emplaced together with the enclosing amphibolites along the Kohistan fault during post-Early Oligocene.