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Origin of the island arc Moho transition zone via melt-rock reaction and its implications for intracrustal differentiation of island arcs: Evidence from the Jijal complex (Kohistan complex, northern Pakistan) Carlos J. Garrido* Jean-Louis Bodinier Bruno Dhuime Delphine Bosch Ingrid Chanefo Olivier Bruguier Shahid S. Hussain Hamid Dawood Jean-Pierre Burg Departamento de Mineralogía y Petrología & Instituto Andaluz de Ciencias de la Tierra, Facultad de Ciencias, Universidad de Granada & CSIC, 18002 Granada, Spain Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Pakistan Museum of Natural History, Garden Avenue, Shakarparian, 44000 Islamabad, Pakistan Structural Geology and Tectonics, ETH Zürich & Universität Zürich, Geologisches Institut, Leonhardstrasse, 19/LEB, CH-8092 Zürich, Switzerland ABSTRACT If the net flux to the island arc crust is primitive arc basalt, the evolved composition of most arc magmas entails the formation of complementary thick ultramafic keels at the root of the island arc crust. Dunite, wehrlite, and Cr-rich pyroxenite from the Jijal complex, constituting the Moho transition zone of the Kohistan paleo–island arc (northern Pakistan), are often mentioned as an example of high-pressure cumulates formed by intracrustal fractionation of mantle-derived melts, which were later extracted to form the overlying mafic crust. Here we show that calculated liquids for Jijal pyroxenites-wehrlites are strongly rare earth element (REE) depleted and display flat or convex-upward REE patterns. These patterns are typical of boninites and are therefore unlike those of the overlying mafic crust that have higher REE concentrations and are derived from light rare earth element (LREE)–enriched melts similar to island arc basalt. This observation, along with the lower 208 Pb/ 204 Pb and 206 Pb/ 204 Pb ratios of Jijal pyroxenites-wehrlites relative to gabbros, rejects the hypothesis that gabbros and ultramafic rocks derive from a common melt via crystal fractionation. In the 208 Pb/ 204 Pb versus 206 Pb/ 204 Pb diagram, ultramafic rocks and gabbros lie on the same positive correlation, suggesting that their sources share a common enriched mantle 2 (EM2) signature but with a major depleted component contribution for the ultramafic rocks. These data are consistent with a scenario whereby the Jijal ultramafic section represents a Moho transition zone formed via melt-rock reaction between subarc mantle and incoming melt isotopically akin to Jijal gabbroic rocks. The lack in the Kohistan arc of cogenetic ultramafic cumulates complementary to the evolved mafic plutonic rocks implies either (1) that a substantial volume of such ultramafic cumulates was delaminated or torn out by subcrustal mantle flow from the base of the arc crust in extraordinarily short time scales (0.10–0.35 cm/yr), or (2) that the net flux to the Kohistan arc crust was more evolved than primitive arc basalt. Keywords: island arcs, Kohistan, Jijal, Cr-rich pyroxenite, wehrlite, lower crust, Moho transition zone, boninite. *E-mail: carlosg@ugr.es INTRODUCTION Oceanic island arcs exhibit intense igneous activity characterized by volcanic and plutonic rocks that are more evolved than their midoceanic -ridge counterparts. The current paradigm is that parental arc magma is primitive basalt with Mg# > 60 [Mg# = 100 × MgO/(MgO + FeO) molar ratio] and major element chemistry similar to that of mid-oceanic-ridge olivine tholeiitic basalt (e.g., Stern, 2002; Kelemen et al., 2003). Experimental work and mass balance calculations indicate that primitive arc basalts undergo extensive intracrustal fractionation of ultramafic cumulates to generate arc products with evolved composition, leaving a complementary ultramafic keel at the island arc root (Kay and Kay, 1985; DeBari et al., 1987; Muntener et al., 2001). Direct testing of the crystal-fractionation linkage of deep mafic and ultramafic rocks in nature has proven elusive, as terrains exposing the ultramafic roots of island arcs are scarce. The Jijal mafic-ultramafic complex (northern Pakistan) constitutes the deepest levels of the Kohistan paleo–island arc complex (Fig. 1A) (Jan and Howie, 1981; Bard, 1983). The abrupt appearance of gabbroic rocks overlying a thick ultramafic section in the Jijal complex constitutes the Moho transition zone of the Kohistan paleo–island arc (Fig. 1B). This zone is interpreted either as the mantle-crust transition between the island arc plutonic crust and modified, subarc residual mantle (Bard, 1983; Burg et al., 1998), or as the transition between “crustal” ultramafic and mafic cumulates formed by high-pressure crystal fractionation from a common primitive arc basalt (Muntener et al., 2001; Kelemen et al., 2003). Here we present geochemical evidence that indicates that the Jijal Moho transition zone is a mantle-crust transition where residual subarc lithospheric peridotites reacted extensively with incoming arc melts. THE ULTRAMAFIC ROOTS OF THE KOHISTAN ISLAND ARC The Kohistan complex (Fig. 1) is an exhumed section of a Cretaceous island arc formed during subduction of the Neo-Tethys Ocean beneath the Karakoram plate (Bard, 1983; Khan et al., 1993; Treloar et al., 1996). The Jijal maficultramafic complex is the structurally lower unit and together with the overlying Metaplutonic complex (Fig. 1) represents the plutonic section of the Kohistan island arc formed before 95 Ma (Schaltegger et al., 2002). The Jijal complex consists of an upper gabbroic section over lying a thick ultramafic section (Fig. 1B) (Jan and Howie, 1981). The gabbroic section contains minor hornblendite lenses and is dominated by gabbroic rocks (Fig. 1B) whose igneous textures and mineral compositions were pervasively overprinted by granulite-facies metamorphism (Yamamoto, 1993). Hornblende gabbronorite of the Jijal mafic section and the overlying Sarangar gabbros display melt-like, chondrite-normalized rare earth element (REE) patterns (Fig. 2) consistent with in situ, plutonic crystallization of island arc basalt (Garrido et al., 2006; their Figs. 9 and 17). The ultramafic section is composed of a basal peridotite zone, a pyroxenite zone, and a thin garnet-hornblendite zone (Fig. 1B). The field structure, petrology, and mineral © 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. GEOLOGY, Geology, August August 2007; 2007 v. 35; no. 8; p. 683–686; doi: 10.1130/G23675A.1; 4 figures; Data Repository item 2007176. 683
Lithos 96 (2007) 325–352 www.elsevier.com/locate/lithos Micro-scale element migration during eclogitisation in the Bergen arcs (Norway): A case study on the role of fluids and deformation J. Schneider a,b, ⁎ , D. Bosch a , P. Monié c , O. Bruguier d a Laboratoire de Tectonophysique, CNRS UMR 5568, Université Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 05, France b Institute of Earth Sciences, Academia Sinica, 128 Academia Road, Sec.2, Nangang, Taipei 115 29, Taiwan, R.O.C. c Laboratoire de Dynamique de la Lithosphère, CNRS UMR 5573, Université Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex O5, France d Service ICPMS, ISTEEM, Université Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 05, France Received 19 March 2006; accepted 25 October 2006 Available online 8 December 2006 Abstract Understanding chemical element mobility during high-pressure metamorphism is paramount to the knowledge of the transformations occurring during the course of eclogite-facies metamorphism. In particular, the role of deformation and fluid circulation appear essential. In order to better decipher this role, we investigated two eclogite-facies samples from the Lindås Nappe in the Bergen arcs (Norwegian Caledonides), which are related to a fluid/deformation metamorphic event. This study, based on detailed microtextural investigations, coupled with major and trace element analyses, either in situ EMPA and LA-ICP-MS or by conventional ICP-MS nebulisation on mineral fractions, illustrates the complexity of chemical reactions accompanying the eclogiteand amphibolite-facies metamorphism. A single eclogite-facies mineral species two eclogite-facies (i.e. phengite and epidote) depicts various chemical features depending on its textural location at thin-section scale. The correlation with the mineralogical reaction occurring in each textural site demonstrates that the compositional variations are inherited from the precursor minerals. Thus, the composition of eclogite-facies minerals is locally controlled by the composition of few hundred-micron domains. On the contrary, alteration phases of the amphibolite facies (symplectite and calcic amphibole) display clear enrichment in LILE, Pb, Sr and LREE compared to their precursor minerals (omphacite and garnet). This supports that the transport of elements at hand-sample scale was enhanced during the retrogression and elements were efficiently delocalised from one textural site to another. The contrasting behaviour of element mobility between peak eclogite-facies metamorphism and retrogression can be explained in terms of rate processes and fluid/deformation activity. During eclogitisation, deformation allows the formation of textural and mineralogical microdomains. Contemporaneously the fluid circulation favours the development of heterogeneities by transport of elements in excess towards areas protected from deformation where phengite-bearing quartz lenses crystallise. This, together with the very fast recrystallisation of eclogite-facies minerals, results in a heterogeneous redistribution of elements at sample scale and in a chemical disequilibrium between different microdomains of hundred-micron size. During retrogression in the amphibolite facies, coeval with long-term ductile deformation, the fluids act as an efficient vector to redistribute the elements from a given textural site to another and, thus, allow to reach, at hand-sample scale, a new chemical equilibrium between the different crystallising phases. © 2006 Elsevier B.V. All rights reserved. Keywords: Eclogite; LA-ICP-MS; Microdomains; Element mobility; Amphibolite-facies metamorphism; Fluid; REE ⁎ Corresponding author. Institute of Earth Sciences, Academia Sinica, 128 Academia Road, Sec.2, Nangang, Taipei 115 29, Taiwan, R.O.C. Tel.: +886 2 2783 9910x618; fax: +886 2 2783 9871. E-mail address: juliesch@earth.sinica.edu.tw (J. Schneider). 0024-4937/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2006.10.001
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