Element Transfer and Redox Conditions in Continental Subduction Zones: New Insights from Peridotites of the Ulten Zone, North Italy

JOURNAL OF PETROLOGY Journal of Petrology, 2019, Vol. 60, No. 2, 231–268 doi: 10.1093/petrology/egy112 Advance Access Publication Date: 19 December 2018 Original Article Element Transfer and Redox Conditions in Continental Subduction Zones: New Insights from Peridotites of the Ulten Zone, North Italy Dominik Gudelius1,2*, Sonja Aulbach1, Roberto Braga3, Heidi E. Höfer1, Alan B. Woodland1 and Axel Gerdes1 1 Institute for Geosciences, Goethe University, Altenhöferallee 1, 60438 Frankfurt, Germany; 2Karlsruhe Institute for Technology, Institute for Applied Geosciences, Adenauerring 20b, 76131 Karlsruhe, Germany; 3Department of Biological, Geological and Environmental Sciences, University of Bologna, Piazza di Porta S. Donato 1, 40126 Bologna, Italy *Corresponding author. Karlsruhe Institute for Technology, Institute for Applied Geosciences, Adenauerring 20b, 76131 Karlsruhe, Germany. Telephone: þ49-721-608-44171. E-mail: Received December 6, 2017; Accepted November 30, 2018 ABSTRACT The orogenic peridotites and pyroxenites of the Ulten Zone (north Italy) record multistage metasomatism by crust-derived melts and fluids within a Late-Variscan mantle wedge. We acquired new major and trace element data as well as garnet and whole-rock iron speciation for a representative suite of samples, with the aim to further constrain element cycling and the redox state attending the development of the major mineralogical and textural rock types that occur within the Ulten Zone. Initially, spinel peridotites were refertilized by mafic melts in the hot and shallow mantle wedge, followed by garnet formation as the peridotites were carried towards a cool, subducting slab of continental crust by corner flow. Upon exhumation, ingress of aqueous, crust-derived fluids provoked amphibole-forming reactions, which caused gradual consumption of garnet and clinopyroxene and transformation from coarse- to fine-grained assemblages. Since Si, Al, Na, Ti, Ca and HREE formerly stored in reactants were not fully accommodated in newly-formed phases, these elements were partially removed from the bulk-rock, generating more depleted compositions resembling residues after partial melting. Unexpectedly, the remaining garnet retains low Fe3þ/RFe (<0046) even after the bulk-rocks became strongly enriched in Fe3þ during metasomatism and retrogression (Fe3þ/RFe ¼ 011–023), which was mostly stored in coexistent amphibole and interstitial serpentine. Low Fe3þ/RFe in garnet is consistent with DlogfO2 ¼ FMQ-17 to FMQ-03 at 2 GPa. Combined with previous studies, this is evidence for garnet growth within a heterogeneously oxidized mantle wedge, reflecting a variable extent of percolation by oxidizing aqueous fluids. During metasomatism, concomitant variable enrichment in LILE, LREE and some HFSE, and significant compositional differences between sampling localities, reflect both variable fluid/rock ratios at small spatial scales but also indicate chromatographic effects that likely relate to different positions relative to the subducting crust releasing fluids into the mantle wedge. Hydration by dilute fluids during retrogression did not result in additional enrichment in fluid-mobile elements, but caused further replacement of garnet and clinopyroxene. This study highlights the control that changing mineralogies, developed in response to interaction with various crustal melts and fluids under variable pressure-temperature and redox conditions in a continental subduction zone, exert on the retention or release of major and trace elements in peridotite. In particular, formation and/or persistence of amphibole and dolomite, as documented in the present study, suggest that the subductionmodified mantle wedge is an efficient trap for volatiles and fluid-mobile elements. C The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: V 231 232 Journal of Petrology, 2019, Vol. 60, No. 2 Key words: crustal fluids; mantle wedge metasomatism; orogenic peridotites; oxidation state; Ulten Zone INTRODUCTION The Ulten Zone, a high-grade basement unit adjacent to the Periadriatic fault system, contains garnet-bearing peridotite bodies that allow us to directly study the interaction between continental crust and mantle (Rampone & Morten, 2001; Scambelluri et al., 2006; Tumiati et al., 2007). These rocks belong to a small group of tectonically emplaced, garnet-bearing peridotites occurring in HP- (high pressure) terranes of orogenic belts that are considered to originate from a mantle wedge in continental collision settings. Due to tectonic forces occurring during subduction, they are incorporated into continental crust which was either dragged down to mantle depths or amalgamated with mantle material at shallow depths in a subduction channel (Brueckner, 1998; Brueckner & Medaris, 2000; Bousquet, 2008; Bodinier & Godard, 2014). Rocks of the continental crust generally contain less water stored in minerals than serpentinized oceanic crust. So, during subduction, less water is released, which prevents the formation of an extensively hydrated mantle wedge necessary for arc magmatism (Zheng, 2012; Zheng & Hermann, 2014). Nevertheless, fluid phases are released from subducting continental crust and lead to moderate hydration of the mantle wedge, which likely was already altered by fluids released from subducting oceanic crust before the onset of continent–continent collision (Chen et al., 2017). These fluids can be, depending on temperature and pressure conditions, low-density aqueous fluids, high-density, solute-rich aqueous fluids, hydrous melts or supercritical fluids, each with significantly different capabilities of element mobilization and transport (Kessel et al., 2005; Hermann et al., 2006; Hermann et al., 2013; Schmidt & Poli, 2014; Zheng & Hermann, 2014). The mobile agents migrate in both pervasive and channelized ways at the slab-mantle boundary, which causes the geochemical modification of both slab crust and mantle wedge peridotite (Zheng & Hermann, 2014). At the contact between crust and mantle, a so-called subduction channel forms, which is a shear zone comprised of a highly reactive mechanical mixture of different lithologies with a thickness of a few to several kilometers (Zheng, 2012). The peridotite-bearing, highgrade rocks of the Ulten Zone can be regarded as part of such a mélange zone, formed atop a subducting continental plate during the Variscan orogeny (Godard et al., 1996). As it can be assumed for the Ulten Zone, coherent slices of continental crust containing entrained peridotites can be detached at considerable depths during subduction (Ulten: <100 km (Nimis & Morten, 2000); down to 200 km in other continental collision zones such as the Western Gneiss Region, Norway (Scambelluri et al., 2010)) and become rapidly exhumed during continued convergence. After slab-breakoff, these peridotites are further exhumed together with the ambient, lo (...truncated)