Structural, petrological and geochemical constraints on transfer and evolution of arc magmas in the mafic-ultramafic Sapat Complex (Kohistan; Northern Pakistan)

Kohistan refers to a Himalayan region in NW Pakistan. This geographical region, as big as Switzerland, is known among geologist to host one of the best exposures of PaleoIsland arcs. The Kohistan paleo-island arc formed in the Tethys Ocean above a northdipping subduction during Mesozoic times. The paleo-arc is now delimited by two suture zones, the Indus Suture zone to the south and the Karakoram Suture to the north. The best known section of the Kohistan Paleo-island Arc is along the Karakoram Highway, which displays the arc from its roots, above the Indus Suture, to its sediments, in the footwall of the Karakoram-Kohistan Suture. The Sapat Complex lies directly above the Indus Suture in tributaries of the Kaghan Valley. It is one of these areas that have been little documented because working conditions (remote, high and steep valley) are not favourable. Literature mentioned the existence of ultramafic and mafic rocks but the area is best known for the occurrence of gem olivine. This research work aimed elucidating the geodynamical setting of the Sapat Complex and characterizing the magmatic processes recorded in these rocks. For this purpose, field work, structural and petro-geochemical analyses and geochronological determination have been carried out. Results shed light on the forearc affiliation of the Sapat Complex and bring new insights on how arc-magmas are transferred and evolve in the mantle via porosity waves and in the crust via magmatic pipes. Previously believed to represent a magmatic chamber of the arc, the Sapat Complex actually includes an upper mantle section and its overlying crust. The ultramafic mantle rocks are made of depleted meta-harzburgite showing in some places dunites and pyroxenites-gabbros that formed at the expense of the meta-harzburgites via melt-rock reactions. The formation of the dunite-pyroxenite is best explained by the percolation via porosity waves of primitive arc melt into the meta-harzburgites. The chemical characteristics of the melts and surroundings peridotites suggest that the dunitepyroxenites formed in a forearc region. The presence of gem olivine and associated calcite + cr-clinochlore within the dunites testifies for the percolation of carbonated fluids. The chemical characteristics of these vein minerals show that they are not issued from late Himalayan metamorphism but are of subduction origin. Their peculiar chemistry (low Nb/Ta, low δB) shows that carbonated fluids would influence the behavior of high field strength element and B in the mantle wedge. The crustal part of the Sapat Comlex comprises series of meta-plutonic rocks enclosing pyroxenite bodies. These series represent a cumulative sequence issued from the same parental melt as those forming the dunite-pyroxenite association within the mantle. The isotopic characteristics of the series validate the cogenitic nature of all lithologies and strengthen the interpretation of forearc origin. They also show that the widespread fluid-assisted metasomatism affecting the rocks occurred during subduction. The pyroxenite bodies enclosed in the metaplutonic series represent magmatic pipes in which primitive melts have been transferred toward higher crustal levels and in which differentiation took place. These conduits are analogue to deep magmatic chambers used as melt highways within the crust. The Sapat Complex formed on a forearc region between 105 and 99 Ma, possibly favored by trenchward migration of the melting source region.