Petrology and geochemistry of mafic dykes from the Muslim Bagh Ophiolite (Pakistan): implications for petrogenesis and emplacement

Abstract

Two different types of mafic dykes are found in the Muslim Bagh Ophiolite: 1) a sheeted dyke complex and 2) a mafic dyke swarm. Relative to the host plutonic section, the sheeted dykes are poorly developed, implying that they formed in an oceanic setting with a low and intermittent supply of magma, probably because of cyclic accumulation of crystals at the base of the magma chamber. Both the sheeted dykes and the dyke swarms have been metamorphosed to greenschist/amphibolite facies conditions. With the exception of the upper level gabbros and sheeted dykes, the dyke swarms crosscut almost the whole ophiolite suite as well as the metamorphic sole rocks, but are truncated structurally at the contact with the underlying mélange and sediments. Hence, the injection of the dyke swarms postdates the formation of both the main Muslim Bagh Ophiolite and the metamorphic sole rocks, but predates the accretion of the mélange and the final emplacement of the ophiolite onto the Indian plate margin. Both the sheeted dykes and dyke swarms are tholeiitic and have a geochemical signature of either island arc tholeiites (IAT) or are transitional between mid-oceanic ridge basalts and IAT. Oceanic rocks with such characteristics, especially their enrichment in large-ion lithophile elements, are generally thought to have formed by processes involving a subduction zone component in the source region by fluids released from the subducting slab. The Muslim Bagh Ophiolite sheeted dykes originated in the late Cretaceous, in a supra-subduction zone tectonic setting related to the subduction of a narrow branch of the Neo-Tethys Ocean, followed by a subduction rollback due to splitting of the nascent arc in the Tethys Ocean. This intra-oceanic subduction led to the formation of a metamorphic sole, followed by the off-axis intrusion of mafic dykes into the ophiolite through a slab window. The Muslim Bagh Ophiolite was accreted to the Bagh Complex and finally obducted onto the Indian Platform.