Petrology of the Obducted Mafic and Ultramafic Metamorphites from the Southern part of the Kohistan Island Arc Sequence
Abstract
The ~ 36,000 sq. km Kohistan region is a well-defined tectonic zone with unusual lithology when compared to the neighboring regions. It is occupied mostly by plutonic and volcanic igneous rocks ranging from ultramafic to silicic. A few authors have suggested that some or all of the Kohistan zone represents a fossil island arc. Based on over 250 rock and minerals analyses, mainly from Swat, a petrological account of the mafic and ultramafic rocks of Kohistan is presented below, together with comments on their tectonic environments, metamorphism, and the duration of the presumed island arc. The basic rocks occur in two NE-trending belts of amphibolite and pyroxene granulite stretching from Nanga Parbat to eastern Afghanistan, and in the wedge-like tectonically-emplaced Jijal Complex f 200 sq. km) which comprises garnet granulites and alpine ultramafics. It is suggested here that most of the southern amphibolites (and the overlying metasediments of the Kalam group) represent the Tethyan oceanic crust, and the alpine ultramafics the underlying upper mantle. The pyroxene granulites, some amphibolites and, probably, Jijal garnet granulites have been derived from a high alumina tholeiite (calc-alkaline) magma that was intruded into the oceanic crust during the early phases of arc-building.
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