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

摘要

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