Cal-alkaline Magmatism at Destructive Plate Margin in Kohistan, Northern Pakistan
Abstract
Four episodes of calc-alkaline igneous activity have been identified in the crust of a fossil island arc, ‘the Kohistan Sequence’, in the Kalam area, Swat. The sequence which includes a thick complex of pyroxene granulite, amphibolite, various types of diorites along with metasediments and associated volcanics is 10-15 kilometers thick and is presumed to have developed, as a result of subduction of the primary Tethyan oceanic crust beneath an arc during the suturing of Indo-Pakistan and Eurasia. The calc-alkaline rocks within the sequence are divisible into two distinct associations. One of these, characterized by iron- and alkali-rich differentiation on variation diagrams, is volcanic, (the Utror volcanics), consisting of andesite, dacite, rhyodacite and rhyolite. The second association includes rocks varying from hornblende biotite diorite to pyroxene-biotite diorite. Analyses of these rocks, unlike the volcanics, display an iron- and alkali-poor and Mg-rich variation trend. Chemically these rocks have been distinguished into lowsilica diorite, normal diorite, high-K diorite and granodiorite (Majid, 1979). The former two types contain lesser GO than the later two. Field evidences suggest that the volcanic phase follows plutonic activity within the province. Most of the first-order features of the studied suite are identical with the calc-alkaline rocks of the circum Pacific type. The petrochemical indices, which identify the Kohistan calc-alkaline rocks, reflect a pronounced episodicity in their evolution. The macro episode of major significance defined in the proposed genetic scheme is synchronous with first major tectonic event which is referred to as the subduction of the Tethyan ocean crust in the plate tectonic model of Kohistan.
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