Magma and fluid percolation in arc to forearc mantle: Evidence from Sapat (Kohistan, Northern Pakistan) Pierre Bouilhol a, ⁎, Jean-Pierre Burg b , Jean-Louis Bodinier c , Max W. Schmidt b , Hamid Dawood d , Shahid Hussain d a Department of Earth Sciences, ETH Zurich, 8092 Zurich, Switzerland b Department of Earth Sciences, ETH and University Zurich, 8092 Zurich, Switzerland c Géosciences Montpellier, Université de Montpellier 2 and CNRS, 34095 Montpellier, France d Pakistan Museum of Natural History, Garden Avenue-Shakarparian, 44 000 Islamabad, Pakistan abstract article info Article history: Received 13 October 2007 Accepted 6 July 2008 Available online 18 July 2008 Keywords: Melt–rock reaction Primitive arc-magma Trace elements Metasomatism Fluids The Sapat peridotite in Kohistan (NW Pakistan) is a 12×1 km sized body of meta-harzburgites, dunites, pyroxenites and meta-gabbros in the hanging wall of the Indus Suture Zone. Field relations, micro-textures, bulk and mineral chemistry document melt–rock reactions producing dunites at the expense of meta- harzburgites, and a consecutive olivine-clinopyroxenite formation from the same melt type. The association of meta-gabbros, dunites, and pyroxenites is interpreted as a crust–mantle transition zone produced by the percolation of highly depleted primitive arc melt. The meta-harzburgites represent large portions of refractory mantle that have suffered melt extraction and metasomatism prior to their interaction with the dunite-forming primitive arc melt and the establishment of the crust–mantle transition zone. Uncommon vein assemblages in the dunites that include Cr-clinochlore, calcite and olivine document a late percolation of slab-related, volatile-rich fluids. Taking into account the highly refractory composition of the Sapat mantle, its structural position, the evidence for interaction with highly depleted arc melts and the sub-solidus interaction with volatile-rich arc fluids, the Sapat ultramafic rocks are interpreted as being derived from the frontal Kohistan arc, representing a crust–mantle transition formed by arc magmatism. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Studies of the production and evolution of magmas, of their migration and of their petrological and geochemical characteristics allow an understanding of the origin and growth of continental and oceanic lithosphere. Arc-processes are the main actors of continental crust formation (Rudnick, 1995), and lead to the chemical character- istics of the continental crust (depletion of Nb, Ti, enrichment in LREE and Pb; e.g. Kelemen et al., 2003). Early differentiation, thought to take place at the base of the arc crust, is an important stage in the evolution of arc magmas. However, the magmatic processes involved remain poorly documented because of the inaccessibility of present-day arc roots and the scarcity of exhumed ones. Two places on Earth offer the opportunity to study deep arc processes, namely the Talkeetna Arc in South Alaska (e.g. Debari and Coleman, 1989; Greene et al., 2006) and the Kohistan Arc in North Pakistan (e.g. Bard, 1983; Burg et al., 1998). Studying arc roots, especially arc mantle lithosphere, is a prerequisite to specifying the first differentiation of primary arc melt compositions, the mode of transfer of primitive melts, and to recognizing the relationships between transferred melt and the host mantle. For this purpose, it is necessary to distinguish ultramafic rocks formed as cumulates from ultramafic rocks formed by melt–rock reactions or through precipitation from a cooling and depressurizing magma as arc melts percolated through the mantle. In this study, we investigate the Sapat Complex at the front of the Kohistan paleo-island-arc through detailed mapping, and by char- acterizing its mineral and bulk rock chemistry. Our goals are (1) to elucidate the origin of these ultramafic rocks, (2) to evaluate the imprint of melts passing through the ultramafics and (3) to characterize the melts involved. Our investigation shows that the Sapat ultramafic rocks mostly consist of meta-harzburgites represent- ing ultra-depleted mantle that further reacted with primitive melts and volatile-rich fluids, leading to intra-mantle fractionation pro- cesses. We consider that the evolution of the Sapat ultramafic rocks reflect subcrustal processes in front of the island arc. 2. Geological setting The Kohistan Complex (North Pakistan) represents an intra- oceanic island arc formed during northward subduction of the Tethys Lithos 107 (2009) 17–37 ⁎ Corresponding author. Structural Geology and Tectonics, ETH-Zurich, Leonhard- strasse 19 LEB, 8092 Zurich, Switzerland. Tel.: +41 44 632 82 13; fax: +41 44 632 60 27. E-mail addresses: pierre.bouilhol@erdw.ethz.ch (P. Bouilhol), jean-pierre.burg@erdw.ethz.ch (J.-P. Burg), jean-louis.bodinier@gm.univ-montp2.fr (J.-L. Bodinier), max.schmidt@erdw.ethz.ch (M.W. Schmidt), hdakbar@yahoo.com (H. Dawood). 0024-4937/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2008.07.004 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos