Tectonostratigaphic and geochemical evaluation of ophiolites-melanges in Kurram and north Waziristan, NW Pakistan: implications for India Afghan suturing

Abstract

The Kahi mélange complex along the India–Afghan suture in the northwestern Pakistan is characterized by the association of dismembered ophiolite, localized tectonic mélange and syn orogenic sedimentary deposits. The tectonostratigraphic framework of various components in the Kahi mélange complex were studied using structural, biostratigraphic and geochemical evidences to constrain the tectonic evolution of the northwestern margin of the Indian plate. The ophiolite package is structurally lying above the syn orogenic and forearc basin (trench–accretionary system). The sedimentary deposits are deformed (folded, faulted, sheared, weathered), show blocks in matrix fabric at places, and form a chaotic mass representing trenches or accretionary remains. Radiolarians and planktons of the Late Cretaceous in the chaotic sediments structurally underlain by ophiolite indicate initial ophiolite obduction around Late Cretaceous. Trace elements and REEs concentration in the mafic and ultramafic rocks were determined by ICPMS for petrogenetic implications. The lower Nb/La ratio (<0.85), negative Nb and Ti anomalies, flat pattern of HFS-elements (Zr, Y) in the NMORB normalized spider diagram, enrichment of LIL-elements (Rb, Ba, Th, Sr) and depletion in Nb (<0.8 ppm) suggest addition of a subduction component to the depleted mantle source for gabbroic and basaltic magmas. Tectonic discrimination diagrams i.e., Th/Y–Ta/Y diagram, Ti/Yb–Nb/Yb diagram, Ti/100–Zr–3*Y and Ti/100–Zr–Sr/2 diagrams show the transitional character of gabbroic and basaltic rocks between Island Arc Tholeiite (IAT) and depleted mantle Island Arc basalt (NMORB) which is possible in a low–pressure extensional environment directly above a subduction or a supra subduction zone. These chaotic deposits juxtapose along an unconfirmable contact with the Palaeocene rocks of the Indian plate, which is indicative of the closure of the trench–accretionary system in the Palaeocene. These data show that the India–Afghan suturing was completed in two tectonic events; i: ophiolite obduction in the Late Cretaceous on the syn orogenic basin and ii: final emplacement of ophiolite and syn orogenic deposits on the Indian margin around the Palaeocene. It further implies that the Kahi mélange complex is a polygenetic mélange formed as a result of the mixing and reworking of mafic and ultramafic rocks and chaotic sediments in a subduction–accretion setting during the episodic collision of the Indian and Afghan plates.