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Lawsonite Composition and Zoning As an Archive of Metamorphic Processes in Subduction Zones

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Lawsonite Composition and Zoning As an Archive of Metamorphic Processes in Subduction Zones Research Paper THEMED ISSUE: Subduction Top to Bottom 2 GEOSPHERE Lawsonite composition and zoning as an archive of metamorphic processes in subduction zones GEOSPHERE; v. 15, no. 1 Katherine F. Fornash, Donna L. Whitney, and Nicholas C.A. Seaton Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA https://doi.org/10.1130/GES01455.1 ABSTRACT fluid-rock interaction, their compositions and microstructures may provide a 13 figures; 2 tables; 1 set of supplemental materials record of fluid compositions and sources as well as fluid transport pathways in The hydrous, high-pressure mineral lawsonite is important in volatile and the subducted slab. Of these phases, lawsonite [CaAl2Si2O7(OH)2·H2O] is of par- CORRESPONDENCE: forna011@ umn .edu element cycling between the crust and mantle in subduction zones and may ticular importance to fluid processes and element cycling in subduction zones also influence the rheology and deformation behavior of the subducted crust because it is abundant over a wide range of depths (and may be the main CITATION: Fornash, K.F., Whitney, D.L., and Seaton, and associated sediments. However, despite its potential geochemical and hydrous phase at pressures [P ] >2.5 GPa) (e.g., Pawley, 1994; Schmidt and N.C.A., 2019, Lawsonite composition and zoning as an archive of metamorphic processes in subduction geodynamic significance, little is known about the trace element affinity and Poli, 1994), has a high water content (11.5 wt%), and is a significant reservoir for zones: Geosphere, v. 15, no. 1, p. 24–46, https:// the types and origins of zoning patterns in lawsonite. To evaluate the signif- trace elements in high-pressure assemblages, particularly rare earth elements doi .org /10 .1130 /GES01455.1. icance of trace element variations and zoning in lawsonite, we conducted a (REEs), Sr, Pb, Th, and U (Tribuzio et al., 1996; Ueno, 1999; Spandler et al., geochemical and microstructural study of lawsonite in a suite of different rock 2003; Martin et al., 2014; Vitale Brovarone et al., 2014). It can also be used to Science Editor: Shanaka de Silva types from the Sivrihisar Massif, Turkey, one of the few places in the world date subduction metamorphism (Mulcahy et al., 2009, 2014; Vitale Brovarone Guest Associate Editor: Gray Bebout where pristine lawsonite has survived in eclogite during exhumation from and Herwartz, 2013), to document the deformation behavior of subducted oce- Received 7 November 2016 depths of ~75–80 km. Lawsonite in metamafic, metasedimentary (impure anic crust and associated sedimentary rocks (Teyssier et al., 2010; Kim et al., Revision received 17 June 2018 quartzite and quartz-rich schists), and metasomatic chlorite-rich rocks con- 2013, 2015; Cao et al., 2014; Cao and Jung, 2016; Whitney et al., 2014), and to Accepted 23 October 2018 tains Fe, Ti, and/or Cr as major constituents (substituting for Al) and com- interpret seismic properties of subducted slabs (e.g., Abers and Sarker, 1996; Published online 10 January 2018 monly displays zoning in these elements. Intragrain variations (up to two or- Hacker, 1996; Connolly and Kerrick, 2002; Hacker et al., 2003; Fujimoto et al., ders of magnitude) in rare earth elements and other trace elements are also 2010; Chantel et al., 2012; Mookherjee and Bezacier, 2012; Reynard and Bass, common and in some cases correlate with transition-metal zoning. For some 2014). Lawsonite dehydration has also been proposed as a driving force for elements (e.g., Ti), uptake was crystallographically controlled, whereas for some intermediate-depth earthquakes in subduction zones (e.g., Kita et al., others, compositional variations may reflect changes in the local metamor- 2006; Abers et al., 2013). It is therefore important to understand the chemical phic environment, such as the growth or breakdown of other mineral phases and physical properties and behavior of lawsonite. that compete for trace elements (garnet, titanite, epidote-group minerals, In comparison to other hydrous phases in subduction systems, relatively apatite) or shifts in the bulk-rock composition during subduction. Deforma- little is known about compositional variations and zoning of lawsonite because tion may have assisted the mobilization of some elements during and after it has been proposed to exhibit little compositional variation (e.g., Pawley, crystal growth, including relatively immobile elements such as Ti. Intersample 1994) and because it is rarely preserved in subduction-related rocks exhumed variations in lawsonite composition likely reflect variations inherited from the to the Earth’s surface (Zack et al., 2004; Whitney and Davis, 2006). Furthermore, protolith. Lawsonite from Sivrihisar metamafic rocks has high Sr/Pb, whereas many lawsonite-bearing localities preserve lawsonite only in texturally and/or OLD G lawsonite from quartz-rich metasediments yielded lower Sr/Pb, with a few spatially restricted sites, such as inclusions in garnet (Zhang and Meng, 2006; exceptions that may indicate interactions between oceanic crust and sedi- Tsujimori et al., 2006) or in a xenolith (Usui et al., 2006) (Fig. 1). In some cases, ments during metamorphism. This study shows that lawsonite composition, the (former) presence of lawsonite can only be inferred from rectangular or zoning, and microstructure can be used to track processes during subduction prismatic-shaped aggregates of epidote + paragonite + quartz ± albite ± talc OPEN ACCESS metamorphism and deformation and can potentially be used to document (at lower pressures; e.g., Ballèvre et al., 2003) or epidote + kyanite + quartz/ fluid-rock interaction within and between different lithologic layers. coesite ± garnet ± omphacite (at higher pressures), from phase equilibria mod- eling, or from mass-balance calculations (e.g., Guo et al., 2013) (Fig. 1). The INTRODUCTION scarcity of well-preserved lawsonite, particularly in eclogite, has prevented a comprehensive understanding of the compositions, substitution mechanisms, Water is transported into the deep parts of subduction systems via hydrous trace element affinity, and types and origins of zoning patterns in lawsonite. This paper is published under the terms of the phases such as lawsonite, phengite, amphibole, epidote-group minerals, talc, One of the few places in the world with fresh, unaltered lawsonite in CC‑BY‑NC license. chlorite, and serpentine. Because these hydrous phases form as a result of eclogite- and blueschist-facies rocks is the Sivrihisar Massif of the Tavşanlı © 2019 The Authors GEOSPHERE | Volume 15 | Number 1 Fornash et al. | Lawsonite composition and zoning as an archive of metamorphic processes in subduction zones Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/1/24/4619067/24.pdf 24 by guest on 03 October 2021 Research Paper 120°W 0° 120°E Spitsbergen 60°N Kyrgyzstan 60°N Tianshan Sivrihisar North Qilian British Columbia Monviso Central Pontides Corsica Franciscan Dominican Eastern Shikoku Republic Figure 1. Distribution of lawsonite eclogite Colorado Plateau localities in the world, classified by the xenolith Western Dabie Guatemala textural position of the lawsonite (Lws) (matrix, inclusion in garnet [grt], pseudo- 0° 0° morph). Sulawesi New Caledonia Port Macquarie Lawsonite Eclogite Localities 60°S 60°S Lws in matrix Lws inclusions in grt ( ± Lws pseudomorphs) Lws pseudomorph 120°E Zone, Turkey (Davis and Whitney, 2006, 2008) (Fig. 1), which contains a coher- OVERVIEW OF THE TAVŞANLI ZONE ent sequence of metamafic and metasedimentary rocks that were metamor- phosed and deformed at or near the top of a subducting slab (Teyssier et al., The Tavşanlı Zone of western Turkey is a Late Cretaceous paleosubduction 2010; Whitney et al., 2014). Lawsonite is abundant in the Sivrihisar Massif and zone (Okay and Kelley, 1994; Okay, 1998; Sherlock et al., 1999; Seaton et al., occurs in blueschist- and eclogite-facies metamafic, metasedimentary (calc- 2009, 2014; Mulcahy et al., 2014; Fornash et al., 2016) formed during the closure schist, quartzite), and metasomatic rocks, as well as in lawsonite-rich veins and of the Neo-Tethys Ocean (Okay, 1980a, 1984, 1986) and is exposed as a 50-km- layers developed at mafic pod margins (Figs. 2, 3). Petrographic and textural wide and 350-km-long, east-west–trending, high-pressure–low-temperature evidence suggests that lawsonite was stable along the prograde metamorphic (HP-LT) metamorphic belt. It consists of a coherent continental blueschist- path (inclusions in garnet), at the peak (matrix grains), and along the retro- facies sequence overlain by an accretionary complex and large ophiolite slabs grade path. The preservation of lawsonite in a variety of rock types, coupled (Okay, 1998; Okay and Whitney, 2010). Undeformed Eocene granitoids (ca. with its stability over a significant portion of the subduction and exhumation 48–53 Ma; Harris et al., 1994; Okay, 1998; Sherlock et al., 1999) locally intruded cycle in the Sivrihisar Massif, provides an opportunity to document in a sys- the HP-LT sequence and the overlying ophiolite. tematic way the composition and zoning of lawsonite and to interpret these Blueschist-facies rocks from the western part of the belt record pres- features in the context of subduction metamorphism and deformation. sure-temperature (P-T ) conditions of up to 2.4 GPa and 430 °C (Okay and Kel- In this study, we integrate results of major and trace element compositional ley, 1994; Okay, 2002) to 470–550 °C (Plunder et al., 2013), whereas blueschists analyses of lawsonite by electron microprobe (EMP) and laser ablation–induc- from a more southeastern portion of the belt record lower P (0.9–1.1 GPa) and tively coupled mass spectrometry (LA-ICPMS) with microstructural analysis of T (375–450 °C) (Droop et al., 2005). Preserved lawsonite eclogite occurs only in lawsonite by electron backscatter diffraction (EBSD) to characterize different types the Sivrihisar Massif, located where the Tavşanlı Zone changes from an east- of zoning and to document compositional variations in lawsonite from different west to a NE-SW trend, and records maximum P-T conditions of 2.4–2.5 GPa rock types.
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