Petrology and geochemistry of trondhjemites from the Waziristan Ophiolite, NW Pakistan
Abstract
This paper presents the field relationships, petrography and geochemistry of the trondhjemites from Waziristan Ophiolite. The ophiolite occurs in the suture zone between the Indian plate to the east and Afghan block to the west. From east to west, the Waziristan Ophiolite is divisible into three thrust sheets or nappes: the Vezhda Sar nappe, entirely comprised of pillow basalts; the Boya nappe, made up of ophiolitic melange with an intact ophiolite section in the basal pan; and the Datta Khel nappe, consisting of sheeted dykes with variable proportions of ultramafic, gabbroic and volcanic rocks. Trondhjemites have so far been recorded in the later two nappes only, where they form lensoid bodies, mostly intruding the ultramafic and rarely gabbroic rocks.
Petrographic studies reveal that the trondhjemites are metamorphosed in greenschist facies. They show high Na2O, low TiO2, Rb, Nb, Y and very low K2O contents. The various parameters used in specifying the tectonic setting of granitic rocks suggest that most of the rocks have subduction related/ induced chemical characteristics, but a few show normal ocean-ridge granite affinity. None of the tectonomagmatic discriminants suggests a within plate or syn-collisional setting for these rocks. The transitional chemistry of these rocks between volcanic-arc granite, subduction-related ocean ridge granite and normal ocean ridge granite suggests a supra-subduction zone (back-arc basin) origin, as proposed also for the mafic members of the ophiolite.
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