Petrology, geochemistry and petrogenesis of the dykes from the Zhob Ophiolite, Pakistan

The highly deformed Zhob ophiolite comprises the Ali Khanzai, Naweoba, and Omzha blocks. Each of these blocks contains both felsic and mafic dyke swarms. The felsic dykes occur as patches and pods in gabbroic bodies of crustal sections while mafic dykes crosscut the mantle peridotite of these blocks. Felsic dykes are plagiogranite and are composed of quartz, plagioclase and accessory ferromagnesian minerals whereas mafic dykes are basaltic in composition and contain largely plagioclase, clinopyroxene, and hornblende with minor quartz grains. Major, trace and rare earth elements have been analyzed in the felsic and mafic to assess the tectonomagmatic setting of the Zhob ophiolite. The felsic dykes are calc-alkaline oceanic plagiogranites while mafic dykes are tholeiitic in composition. Chondrite normalized patterns for the felsic dykes are characterized by low values of the REEs and positive Eu anomalies which indicate that they were formed by partial melting of basic rocks under hydrous conditions. The mafic dykes show very slight Nb depletion and enrichment in large ion lithophile elements (LILE) over high field strength elements (HFSE) which suggest that mafic dyke swarms are derived from an undepleted mantle source. Oceanic rocks with such characteristics are generally thought to have formed by processes involving a subduction zone component in the source region by fluids released from the subducting slab. These features suggest a subduction related setting which indicates an island arc, back arc or supra-subduction zone affinity for the formation of both felsic and mafic dykes of the Zhob ophiolite.