A Garnet^Zircon Oxygen Isotope Record of Subduction and Exhumation Fluids from the Franciscan Complex, California Downloaded From
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JOURNAL OF PETROLOGY VOLUME 55 NUMBER1 PAGES 103 ^ 131 2014 doi:10.1093/petrology/egt062 A Garnet^Zircon Oxygen Isotope Record of Subduction and Exhumation Fluids from the Franciscan Complex, California Downloaded from F. ZEB PAGE1,2,3*, ERIC J. ESSENE2y,SAMUELB.MUKASA2z AND JOHN W. VALLEY3 http://petrology.oxfordjournals.org/ 1DEPARTMENT OF GEOLOGY, OBERLIN COLLEGE, 52 W. LORAIN ST., OBERLIN, OH 44074, USA 2DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, UNIVERSITY OF MICHIGAN, 2534 C. C. LITTLE BUILDING, ANN ARBOR, MI 48109-1007, USA 3WISCSIMS, DEPARTMENT OF GEOSCIENCE, UNIVERSITY OF WISCONSIN, 1215 W. DAYTON ST., MADISON, WI 53706, USA RECEIVED MARCH 1, 2013; ACCEPTED SEPTEMBER 27, 2013 ADVANCE ACCESS PUBLICATION NOVEMBER 13, 2013 at University of Wisconsin - Madison, General Library System on December 20, 2013 Tectonic blocks of high-grade metamorphic rock hosted by the KEY WORDS: subduction; ion microprobe; SIMS; oxygen isotopes; Franciscan me¤lange commonly preserve textural and geochemical evi- eclogite; Franciscan; P^T^time^fluid path dence for metasomatism and blueschist-facies overprinting within a subduction zone. These exhumation events can obscure information on the nature of subduction fluids at depth that might otherwise be INTRODUCTION preserved by these rocks in this tectonic setting. Cation and oxygen The relationships between the record of fluids preserved in isotope zoning in garnets from eclogite and garnet^hornblende rocks high-pressure metamorphic rocks exhumed from subduc- with minimal textural evidence of retrogression reveals separate tion zones, the fluids that are presumed to be responsible prograde and retrograde fluid histories. Garnet cores preserve a homo- for the generation of arc magmas, and the characteristic geneous prograde oxygen isotope history with both high (d18O subduction-zone trace element signature, are areas of 11ø) and low (4ø) values in different samples probably re- great interest in arc petrology (e.g. Bebout, 2007). cording altered ocean-crust protolith compositions. Multiple episodes Dehydration reactions within the down-going slab gener- of garnet resorption and regrowth record a dramatically changing ate substantial fluxes of aqueous fluid that migrate into bulk oxygen isotope ratio resulting in 6^7ø neoformed garnet rims the mantle hanging wall of the subduction zone, dictating with sharp core^rim boundaries in both samples. Matrix omphacite the extent to which mantle melting will occur (e.g. and hornblende are in profound oxygen isotope disequilibrium with Peacock, 1990; Kerrick & Connolly, 2001). The record of garnet cores but are in equilibrium with garnet rims despite the ap- these fluids found in exhumed subduction-zone meta- pearance of overall textural equilibrium in thin section.This cryptic morphic rocks is mixed, however. Whole-rock stable iso- metasomatism found in samples from different locations within the tope and trace element data have been used to establish Franciscan Complex brings into question existing thermobarometry extensive fluid^rock interaction and metasomatism within on non-inclusion phases for this complex, as well as interpretations the subduction environment (Bebout & Barton, 1989, 1993; based on whole-rock geochemical data. Zircon ages and oxygen iso- Sorensen & Grossman, 1989), as well as mechanical tope equilibrium with late-forming garnet are consistent with post- mixing along the slab^mantle interface (King et al., 2006, metasomatic formation of zircon.The ability of garnet and zircon to 2007; Marschall & Schumacher, 2012). Metasomatic rocks record multiple fluid events and tie them to P^T^t^fluid history is such as jadeitites and related lithologies are commonly a powerful tool in gaining an improved understanding of the complex found in serpentinite me¤ langes associated with subduction fluid environments within subduction zones. zones, and preserve evidence of complicated fluid histories *Corresponding author. Present address: Department of Geology, Oberlin College,52 W.Lorain St., Oberlin, OH 44074, USA.Telephone: þ14407756701.Fax:þ14407758038. E-mail: [email protected] yDeceased.zPresent address: College of Engineering and Physical ß The Author 2013. Published by Oxford University Press. All Sciences, University of New Hampshire, Kingsbury Hall RoomW 289, rights reserved. For Permissions, please e-mail: journals.permissions@ 33 AcademicWay, Durham, NH 03824, USA. oup.com JOURNAL OF PETROLOGY VOLUME 55 NUMBER 1 JANUARY 2014 (Johnson & Harlow, 1999; Harlow et al., 2003; Sorensen exhumation phases of metamorphism (Krogh et al., 1974; et al., 2006; Fu et al., 2010). Krogh, 1982; Rubatto et al., 1999; Schmitz & Bowring, Other studies, principally on eclogite and related rocks, 2001; Page et al., 2003). By analyzing oxygen isotope ratios that report textural evidence of metamorphic fluids in the by ion microprobe in zircon and garnet, we can use the form of fluid inclusions (Scambelluri & Philippot, 2001) robust nature of these minerals to see through the multiple and veins (Spandler & Hermann, 2005) suggest that these episodes of metasomatism that might be expected in the features are the products of internal fluid flow and are un- subduction zone environment. Furthermore, by identifying likely to represent fluid entering or leaving the subducting the oxygen isotope record of fluids in minerals that also Downloaded from slab. A large number of geochemical studies are similarly record pressure^temperature and geochronological infor- suggestive of limited fluid flow in samples of mafic meta- mation, the fluid^rock interaction history may be tied dir- morphic rocks formed in subduction zones (Philippot & ectly to the P^T^t^fluid(s) history for a more complete Selverstone, 1991; Selverstone et al., 1992; Getty & understanding of subduction dynamics. In doing so, for ex- Selverstone, 1994; Barnicoat & Cartwright, 1995; Ganor ample, it may be possible to resolve the complications of et al.,1996; Putlitz et al., 2000, 2001; Fru« h-Green et al., 2001). widespread fluid overprinting during exhumation, and http://petrology.oxfordjournals.org/ A commonly cited reason for these disparate results is gain more insight into the nature and timing of fluid that fluid flow within subducting slabs may be highly chan- fluxes at depth in subduction zones. nelized, and the discrepancy between studies reflects the Two zircon- and garnet-bearing eclogite and hornblen- low permeability of subducted mafic rocks and the compli- dite samples were chosen for this study from the cated me¤ lange structure of the slab^mantle interface Franciscan Complex of central California. The Franciscan (Bebout & Barton, 2002; Breeding et al., 2004; King et al., is among several paleo-subduction zones in which tectonic 2006, 2007; Ague, 2007). blocks of high-pressure metamorphic rocks are hosted by Additionally, the prograde and peak metamorphic geo- a sedimentary^ultramafic me¤ lange. These rocks are re- chemical record of some samples can be obscured by vari- nowned for their textural, chemical, and isotopic evidence at University of Wisconsin - Madison, General Library System on December 20, 2013 able metasomatism during exhumation by channelized of large fluid fluxes and metasomatism. High-grade meta- flow immediately above the subducting slab. Geochemical morphic blocks preserve textural evidence of metasoma- studies of metamorphic rocks that preserve evidence of tism in the form of glaucophane- and actinolite-rich rinds substantial fluid flow and metasomatism often employ (Coleman,1980; Horodyskyj et al., 2009) as well as the pres- ence of euhedral minerals (zoisite, aragonite, titanite, whole-rock or bulk mineral separates, an approach that omphacite), which grew into fluid-filled cavities (Essene & homogenizes zoned minerals and makes discerning be- Fyfe,1967; Cloos,1986; Page et al., 2007a). Geochemical evi- tween prograde and retrograde fluids challenging. Recent dence of metasomatism in the form of zoned minerals detailed work on high-pressure veins from alpine eclogites exists both for elements typically viewed as highly mobile, suggests that these rocks can preserve a chemically distinct such as the large ion lithophile elements (LILE; e.g. record of both internally and externally sourced fluids, re- Sorensen et al., 1997; Catlos & Sorensen, 2003), and those solvable only by preserving the spatial context of geochem- that are generally thought to be less soluble in aqueous so- ical analyses (Spandler et al. , 2011). In a dd it i o n, ga r n et s lution, such as high field strength elements (HFSE) and from orogenic eclogites preserve subtle zoning in oxygen the light rare earth elements (LREE; e.g. Sorensen & isotope ratios that records a previously unrecognized Grossman, 1989). To maximize the potential for recording prograde fluid infiltration (Russell et al., 2013). both an early and late history of subduction fluids, a In this study we interrogate the stable isotopic record of sample was chosen that contains garnets that record a pro- subduction zone metamorphism preserved in garnet and grade subduction history, and a second sample with zircon at a fine (mm) scale. In addition to its wide use as a garnet that records the oldest ages in the region. geochronometer, zircon has become highly valued for its ability to preserve a robust record of a variety of isotopic and trace-element geochemical systems through rather ex- REGIONAL GEOLOGY treme conditions (e.g. Hanchar & Hoskin, 2003; Bowman The high-grade metamorphic rocks hosted by the et al., 2011). Similarly,