Extreme oxygen isotope zoning in garnet and zircon from a metachert block in mélange reveals metasomatism at the peak of subduction metamorphism

Abstract

A tectonic block of garnet quartzite in the amphibolite-facies mélange of the Catalina Schist (Santa Catalina Island, California, USA) records the metasomatic pre-treatment of high-δ18O sediments as they enter the subduction zone. The block is primarily quartz, but contains two generations of garnet that record extreme oxygen isotope disequilibrium and inverse fractionations between garnet cores and matrix quartz. Rare millimeter-scale garnet crystals record prograde cation zoning patterns, whereas more abundant ~200-μm-diameter crystals have the same composition as rims on the larger garnets. Garnets of both generations have high-δ18O cores (20.8‰–26.3‰, Vienna standard mean ocean water) that require an unusually high-δ18O protolith and lower-δ18O, less variable rims (10.0‰–11.2‰). Matrix quartz values are homogeneous (13.6‰). Zircon crystals contain detrital cores (δ18O = 4.7‰–8.5‰, 124.6 +1.4/−2.9 Ma) with a characteristic igneous trace element composition likely sourced from arc volcanics, surrounded by zircon with metamorphic age (115.1 ± 2.5 Ma) and trace element compositions that suggest growth in the presence of garnet. Metamorphic zircon decreases in δ18O from near-core (24.1‰) to rim (12.4‰), in equilibrium with zoned garnets. Collectively, the data document the subduction of a mixed high-δ18O siliceous ooze and/or volcanic ash protolith reaching temperatures of 550–625 °C prior to the nucleation of small garnets without influence from external fluids. Metasomatism was recorded in rims of both garnet and zircon populations as large volumes of broadly homogeneous subduction fluids stripped matrix quartz of its extremely high oxygen isotope signature. Thus, zoned garnet and zircon in high-δ18O subducted sediments offer a detailed window into subduction fluids. INTRODUCTION The nature and timing of mass transfer between the subducting plate and the sub-arc mantle is critical to our understanding of crustal formation at convergent margins and its geochemical signatures. Chemical and mechanical hybridization within subduction mélange plays an important role in these processes (e.g., Bebout and Penniston-Dorland, 2016), giving rise to models suggesting that partial melting of diapirs of hybridized mélange rocks is responsible for the classic trace element signature of arc rocks (Marschall and Schumacher, 2012) and the diversity of magma series found at convergent margins (Cruz-Uribe et al., 2018). Adding to these complications is the recent discovery that some sediments have entered the mantle and melted without mixing or hybridization, preserving extreme oxygen isotope signatures of surface weathering in their neoformed igneous zircon (Spencer et al., 2017). If subducted sediment can regularly carry its characteristically enriched oxygen isotope signature (δ18O = ~7‰–42‰, Vienna standard mean ocean water [VSMOW]; Kolodny and Epstein, 1976; Eiler, 2001; Payne et al., 2015) into the mantle (δOzrc = 5.3‰ ± 0.6‰, 2SD [standard deviation]; Valley et al., 1998; Page et al. 2007a), it is surprising that oxygen isotope variability within the sub-arc mantle is so subtle and challenging to measure (Eiler et al., 1998). A solution to this discrepancy may be found in the fluid metasomatism of subducted sediments. The first and perhaps most dramatic illustrations of a high degree of fluid flow within subduction mélange were studies of the oxygen isotope ratios of quartz and carbonate in veins within the Catalina Schist subduction complex (Southern California, USA) suggesting kilometer-scale oxygen isotope homogenization driven by large fluid fluxes (Bebout and Barton, 1989; Bebout, 1991). Over the last quarter century, the Catalina Schist has served as a laboratory for the study of subduction mélange, with numerous studies detailing fluid metasomatism and mechanical mixing processes in the subduction channel by means of stable isotopes (e.g., Bebout, 1991; PennistonDorland et al., 2012), major and trace elements (e.g., Sorensen and Barton, 1987; Hickmott et al., 1992; Penniston-Dorland et al., 2014) and radiogenic isotopes (King et al., 2006). The in situ analysis of oxygen isotopes in garnet is a powerful tool with which to decipher complex or extremely subtle fluid histories and tie them to the metamorphic record. In rocks that have experienced significant metasomatism, the extremely slow intragranular diffusion of oxygen in garnet allows it to preserve a robust geochemical record through all but the hottest and longest of metamorphic events (Vielzeuf et al., 2005). Oxygen isotope variability in garnets from eclogite has illustrated signals of infiltration by mantle (Russell et al., 2013) and supra crustal (e.g., Page et al., 2014; Martin et al., 2014; Rubatto and Angiboust, 2015) fluids that were previously undetectable or not clearly distinguishable using bulk methods. Chert and siliceous schist are high-δ18O lithologies (Eiler, 2001) that are found within the CITATION: Page, F.Z., et al., 2019, Extreme oxygen isotope zoning in garnet and zircon from a metachert block in mélange reveals metasomatism at the peak of subduction metamorphism: Geology, v. 47, p. 655–658, https:// doi .org /10 .1130 /G46135.1 *Current Address: Kochi Institute for Core Sample Research, JAMSTEC, 200 Monobe-otsu, Nankoku, Kochi 783-8502, Japan Manuscript received 8 February 2019 Revised manuscript received 28 March 2019 Manuscript accepted 17 April 2019 https://doi.org/10.1130/G46135.1 © 2019 Geological Society of America. For permission to copy, contact [email protected]. Published online 13 May 2019 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/7/655/4775227/655.pdf by University of Wisconsin Madison user on 18 October 2019 656 www.gsapubs.org | Volume 47 | Number 7 | GEOLOGY | Geological Society of America amphibolite-facies Catalina Schist mélange (Platt, 1975). In this contribution, we explore the metasomatism of a high-δ18O garnetand zircon-bearing metachert from a classic subduction mélange, in order to better understand the timing and metamorphic conditions of subduction fluid metasomatism, and to gain a more complete picture of how fluids mitigate the influence of high-δ18O subduction inputs. CATALINA GARNET QUARTZITE Although much less abundant than the betterstudied garnet-hornblende lithology, tectonic blocks of garnet quartzite are also found within the amphibolite-facies metasedimentary mélange of the Catalina Schist (Santa Catalina Island, California), as well as in more coherent, fault-bounded sheets (Platt, 1975; Bebout, 1991). In this study, we report on one exceptional sample of garnet quartzite collected from a meter-scale tectonic block hosted in a shalematrix mélange from upper Cottonwood Canyon (33°23′46.20′′N, 118°24′52.80′′W; Fig. 1A). The quartzite is composed primarily of quartz (93%), garnet (6%), and chlorite (<0.5%), with trace rutile, apatite, amphibole, and zircon (Fig. 1B). Garnet is present in two populations: copious fine-grained (<200-μm-diameter) crystals dispersed throughout the sample, and a smaller number of larger garnets (1–3 mm diameter; Fig. 1B). The larger crystals have abundant inclusions, which are primarily quartz and apatite. Xray mapping and major element traverses show that the larger garnets display classic prograde cation-zoning profiles with decreasing Mn and increasing Mg# from core to rim, and with rim compositions similar to the compositions of the smaller, more homogenous (in cations) garnets in the same rock (Figs. 2A and 2B). Oxygen Isotopes of Quartz and Garnet Ion microprobe analysis of garnets (Page et al., 2010; see the GSA Data Repository1 for full data [Tables DR1 and DR2] and methods) shows extreme oxygen isotope zoning; values of δ18O are 20.8‰–26.3‰ in garnet cores and 10.0‰–11.2‰ in garnet rims (Fig. 2A). Both large and small garnets in this sample show a similar range in δ18O, despite the difference in cation zoning and crystal size. Zoning in oxygen isotopes is sharp, with as much as a 7‰ drop in δ18O over a few micrometers, whereas cation zonation is much more gradual, with slightly increased Ca and Mg in the rims of larger garnets (Fig. 2). Smaller garnets are nearly homogeneous, with a slight increase in Mg# from core to rim. Matrix quartz has no systematic zoning in cathodoluminescence (CL) imaging and is homogeneous in δ18O, with ion microprobe analyses (13.5‰) identical (within uncertainty) to bulk (~2 mg) analysis by laser fluorination (13.6‰). Garnet-core and quartz pairs yield reversed fractionations (δOgrt > δOqz), indicating profound disequilibrium. Eleven analyses of quartz inclusions in large garnet cores yield δ18O = 13.8‰–16.2‰, higher than in matrix quartz, but not in equilibrium with host garnet. Inclusions were generally >50 μm, and commonly along cracks and so are unlikely to preserve their original values. Oxygen Isotopes in Zircon Zircons were separated from the sample and mounted in epoxy for analysis (see the Data Repository). CL imaging (Fig. 3A) reveals oscillatory-zoned cores, commonly as fragments of crystals, containing inclusions of quartz, K-feldspar, and biotite. These detrital cores are surrounded by annuli of variable-CL-intensity, somewhat mottled zircon, containing inclusions of quartz, biotite, sphene, and rutile. Outside of this mottled zone, zircons typically have darkerCL-intensity oscillatory-zoned rims, with rare crystals containing a brighter outer rim with faint oscillatory zoning. Zircons were analyzed for their oxygen isotope ratios by ion microprobe using both a ~15-μmand a sub–1-μm-diameter beam ( Tables DR4, DR5). Highly precise and accurate oxygen isotope ratios from the larger analy sis pits are correlated with CL zonation and inclusion population. Zircon cores (n = 7) have δ18O from 4.7‰ to 8.4‰ (Figs. 3A and 3B). Zircon with mottled CL immediately outside of detrital cores (n = 17) has an extremely high δ18O of 22.6‰ ± 3.3‰ (2 SD, of 17 analyses in this zone) if one anomalously low