Research Paper THEMED ISSUE: Subduction Top to Bottom 2 GEOSPHERE Multiple veining in a paleo–accretionary wedge: The metamorphic rock record of prograde dehydration and transient high pore- GEOSPHERE, v. 16, no. 3 fluid pressures along the subduction interface (Western Series, https://doi.org/10.1130/GES02227.1 11 figures; 2 tables; 1 set of supplemental files central Chile) Jesús Muñoz-Montecinos1,2,*, Samuel Angiboust1,*, Aitor Cambeses3,*, and Antonio García-Casco2,4,* CORRESPONDENCE: 1 [email protected] Institut de Physique du Globe de Paris, Université de Paris, CNRS, F-75005 Paris, France 2Department of Mineralogy and Petrology, Faculty of Sciences, University of Granada, Campus Fuentenueva s/n, 18002 Granada, Spain 3Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Bochum 44801, Germany CITATION: Muñoz-Montecinos, J., Angiboust, S., 4Instituto Andaluz de Ciencias de la Tierra, CSIC–Universidad de Granada, Armilla, Granada 18100, Spain Cambeses, A., and García-Casco, A., 2020, Multiple veining in a paleo–accretionary wedge: The metamor- phic rock record of prograde dehydration and transient high pore-fluid pressures along the subduction inter- ABSTRACT that the formation of interlayered blueschist and fluid-rock interaction events (Zack and John, 2007). face (Western Series, central Chile): Geosphere, v. 16, greenschist layers in Pichilemu metavolcanics is a Textures recorded in veins yield information on no. 3, p. 765–786, https://doi.org/10.1130/GES02227.1. High pressure–low temperature metamorphic consequence of local bulk composition variations, crack aperture as well as crystal growth kinetics rocks from the late Paleozoic accretionary wedge and that greenschists are generally not formed due during each veining event (Cox and Etheridge, 1983; Science Editor: Shanaka de Silva exposed in central Chile (Pichilemu region) are to selective exhumation-related retrogression of Bons, 2001), while vein-filling mineral assemblages Guest Associate Editor: Gray E. Bebout characterized by a greenschist-blueschist litholog- blueschists. The early vein sets are a consequence and fluid inclusions provide a valuable source of Received 23 December 2019 ical association with interbedded metasediments of prograde internal fluid production followed by information on fluid chemistry and pressure-tem- Revision received 13 February 2020 that reached peak burial conditions of ~400 °C sets of hydrofractures formed at near-peak burial perature regimes of fluid migration (e.g., Philippot Accepted 23 March 2020 and 0.8 GPa during late Carboniferous times. We that are interpreted as a record of external fluid and Selverstone, 1991; Scambelluri and Philippot, herein combine new extensive field observations, influx. We postulate that such a fractured sequence 2001; Raimbourg et al., 2018). Published online 29 April 2020 structural measurements, and geochemical and represents a close analogue to the high-Vp/Vs Previous work on metamorphic rocks has petrological data on vein and matrix material from regions documented by seismological studies demonstrated that the vein-filling material can either Pichilemu transitional greenschist-blueschist facies within the base of the seismogenic zone in active be of local origin, resulting from diffusion processes rocks. The studied veins were first filled by albite, fol- subduction settings. between the host and an almost stagnant fluid pro- lowed by quartz and calcite as well as glaucophane duced by local dehydration reactions (e.g., Spandler and winchite. Field, structural, and microscopic and Hermann, 2006), or from external provenance, zoning patterns show that these rocks underwent ■ INTRODUCTION resulting from advective transport over kilome- a protracted sequence of prograde vein-opening ter-scale distances before deposition in the crack events, which have been largely transposed to the Blueschist-facies metamorphic rocks exhumed (Etheridge et al., 1984; Cartwright and Barnicoat, main foliation before and during underplating in from ancient subduction interfaces enable doc- 1999; Bebout and Penniston-Dorland, 2016; Lewer- the basal accretion site near 25–30 km depth. While umentation and understanding of fluid-rock entz et al., 2017; Jaeckel et al., 2018). Fluid-inclusion some of the earliest albite-filled vein sets may have interaction occurring near the downdip end of thermometry has been also used to propose that the formed after prograde breakdown of sub–green- the seismogenic zone (Bebout and Barton, 1993; fluid phase from which the vein-filling material pre- schist facies minerals (<250 °C), our thermodynamic Halama et al., 2014; Angiboust et al., 2015) where cipitates was warmer than the host, thus advocating modeling shows that relatively minor amounts of episodic tremor and slow-slip events nucleate for fast advective flow (Vrolijk et al., 1988). While fluid are produced in the subducted pile by dehy- (Obara, 2002; Audet et al., 2009; Peng and Gomberg, short-distance diffusive transport does not require dration reactions between 250 and 400 °C along 2010). The most obvious marker of fluid transport in large-scale connectivity, transportation over hun- the estimated geothermal gradient. It also confirms metamorphic rocks is vein systems (e.g., Fyfe et al., dreds or thousands of meters implies the presence 1978; Fisher and Brantley, 1992). Their geometry of highly permeable pathways such as brittle faults This paper is published under the terms of the *E-mail: [email protected]; angiboust@ipgp sheds light on the connectivity of fluid pathways (e.g., Sibson, 2013; Angiboust et al., 2015; Dielforder CC-BY-NC license. .fr; [email protected]; [email protected] as well as on the spatial and temporal scale of the et al., 2015). Yet, the genetic link between individual © 2020 The Authors GEOSPHERE | Volume 16 | Number 3 Muñoz-Montecinos et al. | Prograde fluid production in a paleo–accretionary wedge Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/16/3/765/5019483/765.pdf 765 by guest on 01 October 2021 Research Paper veining events, connection of vein networks by retrogressive fluid imprint or as a consequence of to centimeter wide that have not been studied in larger fault systems, and the fate of fluids along centimeter-scale bulk-rock composition variations detail so far. Willner et al. (2005) suggested that the interface over several seismic cycles requires (Hyppolito, 2014; Halama and Konrad-Schmolke, albite (± sodic amphibole) veins probably reflect further investigation (e.g., Husen and Kissling, 2001; 2015). Resolving this issue is critical for the second precipitation from metamorphic fluids under peak Fagereng et al., 2010). The relative timing of vein part of this study, namely the design of a prograde metamorphic conditions. The metavolcanic rocks in formation with respect to matrix deformation during fluid-production model that aims at understanding the Pichilemu region exhibit relict pillow and hyalo- burial and/or exhumation as well as the link with the chronology of vein-formation events, which clastite structures (e.g., Hervé et al., 1984; Willner, paleoseismic events also requires further documen- will ultimately refine our vision of fluid mobility in 2005) and have oceanic island basalt (OIB), enriched tation (e.g., Dielforder et al., 2015). active subduction margins. mid-ocean-ridge basalt (MORB), and normal MORB Another consequence of fluid-rock interaction geochemical signatures, suggesting scraped-off at depth is the variable metasomatic imprint that sections of the subducting upper oceanic crust obliterates the original geochemical signature of ■ GEOLOGICAL SETTING (Hyppolito et al., 2014a). These meta basites form the affected lithologies (e.g., Harlow and Sorensen, meter- to decimeter-sized layers of greenschists and 2005; Vitale Brovarone et al., 2014; Bebout and Pen- In the Chilean Coastal Range, metamorphic blueschists that underwent peak pressure-tempera- niston-Dorland, 2016). Selective retrogression due to rocks formed at the active Paleozoic continental ture (P-T) conditions of 0.70–0.93 GPa at 380–420 °C exhumation-related fluid circulation may enhance, margin, representing deformational records of the and 0.95–1.07 GPa at 350–385 °C, respectively (Will- in specific cases, the formation of blueschists and Gondwanan tectonic cycle, are exposed (Hervé, ner, 2005). Their contrasting P-T conditions led greenschists interlayered at centimeter to meter 1988) (Fig. 1). This tectonic cycle (Late Devonian– Willner (2005) to suggest that greenschists and scale (e.g., Bröcker, 1990; Barrientos and Selver- late Permian) is characterized by the subduction of blueschists may represent different sections of the stone, 1993; Halama and Konrad-Schmolke, 2015). oceanic crust under the continental margin of the zone of basal accretion in the range from 25 to 40 km However, many studies have proposed that bulk-rock southwestern part of Gondwana and formation of depth. At the Infiernillo locality, Hyppolito (2014) and geochemical differences may lead to the equilibra- accretionary complexes (Ramos, 1988; Willner et al., Hyppolito et al. (2014a) classified the greenschist tion of cofacial greenschists and blueschists in the 2005; Hyppolito et al., 2014a, 2014b), which were and blueschist metavolcanics as mostly OIBs that subducted material (Dungan et al., 1983; Owen, 1989; intruded by north-south–trending syn- and post-ac- underwent uniform peak conditions of 0.75–0.80 GPa Baziotis and Mposkos, 2011; Hyppolito,