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Metasomatism During Subduction: Products and Possible Paths in the Catalina Schist, California

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Metasomatism During Subduction: Products and Possible Paths in the Catalina Schist, California ChemicaIGeology, 108 (1993)61-92 61 Elsevier Science Publishers B.V., Amsterdam Metasomatism during subduction: products and possible paths in the Catalina Schist, California Gray E. Bebout a and Mark D. Barton b aDepartment of Earth and Environmental Sciences, 31 14~lliams Drive. Lehigh UniversiO,. Bethlehem, PA 18015. USA bDepartment ofGeosciences, LSm,ersity of Arizona. Tucson, AZ 85287. USA (Received February 15, 1993; revised and accepted April 8, 1993 ABSTRACT On Santa Catalina Island, southern California, lawsonite-albite to amphibolite facies metasedimentary, metamafic, and metaultramaflc rocks show veining and chemical alteration that reflect fluid flow and mass transfer at 15 to 45 km depths in an Early Cretaceous subduction zone. In many exposures, multiple generations of cross-cutting syn- and post-kinematic veins record fluid transport and metasomatism during various stages of prograde metamorphism and uplift. Mineralogy and whole-rock compositions demonstrate chemical redistribution, especially of Si, AI, and alkali elements (Na, K ), but also of many trace elements, particularly B and LILE (Rb, Cs, St, and Ba). Evidence exists for mass transfer, at both local and larger scales, via mechanical mixing, diffusional, and fluid-mediated transfer processes. Highest-grade, amphibolite facies rocks contain feldspar + quartz _+ mica _+ amphibole leucosomes and pegmatites attributed to migmatization; the leu- cosomes and pegmatites reflect high-P/7" mass transfer in felsic silicate liquids. Veining and replacement in blueschist grade rocks comprise three contrasting types of assemblages: ( 1 ) silica-saturated (quartz-rich), ( 2 ) potassic (white-mica _+ quartz-rich ), and ( 3 ) sodic and silica-undersaturated (albite/Na-amphibole- rich, quartz-absent). Evidence for silicification and alkali exchange also occurs in greenschist and amphibolite facies units. In all units, the evidence for metasomatism (e.g., veins; stable isotope homogenization; rinds on blocks) is particularly abundant in melange zones, in which melange matrix compositions resulting from mechanical mixtures of mafic, ultra- mafic, and sedimentary rocks were shifted by metasomatic additions and subtractions during melange formation. Geo- chemical evidence (particularly stable isotope data) indicates that the blueschist, greenschisL, and amphibolite units ex- changed with fluids of similar compositions. The diverse metasomatic features in the Catalina Schist provide evidence regarding fluid sources and paths. Based on the stable isotope data, the H20-rich, low-salinity ( ~ 1 to 2 equivalent wt. % NaCI), C-O-H-S-N fluids are believed to have been derived from low-grade, largely sedimentary parts of the subduction zone (analogs lbr fluid sources are the low- grade units). Metasomatic changes could be driven by flow across boundaries between contrasting lithologies and by variations in pressure and temperature along the fluid flow paths. Simple predictions of mass changes along different P-T paths suggest that both mechanisms could be effective at producing the range of observed features, even though the re- quired equilibrium constants are only poorly estimated at the relevant P-T conditions. Decreasing T and P favors fixing of K, Si, C, and H in rocks, whereas increasing T ( _+ moderately decreasing P) should fix Na but leach most other com- ponents. The Si-rich, K _+ Si-rich, and Na-rich/Si-poor assemblages are thus consistent with differing fluid P-T flow paths. Regular differences are expected in silica precipitation/dissolution, alkali exchange, and hydrogen-alkali exchange reac- tions, among others. Silica_+ carbonate addition, consistent with the majority of veins observed, is likely the consequence of cooling _+ decompression whereas sodic ( _+ silica-undersaturated ) assemblages would be expected for rarer, but geolog- ically plausible up-T fluid flow paths. A composite fluid flow path, first up-grade, then down P and T. is indicated for the silica addition to the largely ultramafic amphibolite-facies melange. Although mass balance and physical constraints appear to preclude pervasive major element metasomatism on large scales, focussing of fluids would likely produce pervasive changes in significant volumes (e.g., up to kin-scale melange zones). Vein mineralogy would record the paths even at small fluxes. Study of the Catalina Schist demonstrates the signif- icance of metasomatism at all scales, but indicates that large-scale changes in vein mineralogy and bulk composition are in some cases attributable to fluid flow over large distances. Comparison with other areas and elementary theoretical consid- erations suggest that these processes may be widely developed and that their petrographic and[ geochemical effects poten- tially give insight into the dynamics of subduction zones. 0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved. 62 G.E. BEBOUT AND M.D. BARTON I. Introduction sitional changes during: metamorphism may have resulted from fluid infiltration (e.g., Subduction of oceanic lithosphere results in Ernst, 1965; Essene and Fyfe, 1967; Fyfe and the transfer of volatile-rich, hydrothermally- Zardini, 1967; Liegeois and Duchesne, 1981 ; altered mid-ocean ridge basalts and sediments Moore and Liou, 1979; Moore et al., 1981; to the upper mantle (e.g., Ito et al., 1983). In- Moore, 1984; Maruyama and Liou, 1987, terstitial pore fluids make up a large fraction 1988). Stable isotopic data from subduction of the volatiles initially subducted ( > 50 vol- complexes (Taylor and Coleman, 1968; Ma- ume % in some sediments), but much of this garitz and Taylor, 1976; Rumble and Spear, is probably expelled at shallow levels in sub- 1983; Matthews and Schliestedt, 1984; Agri- duction zones and escapes toward the surface nier et al., 1985; Barton et al., 1987; Bebout ( Langseth and Moore, 1990). At greater depths and Barton, 1989a; Bebout, 1991a,b) demon- in subduction zones ( > 15 km), volatiles are strate extensive isotopic exchange and fluid primarily those bound in minerals. These vol- mobility. Radiogenic isotope studies of high- atiles, when released by metamorphic devola- P/T metamorphic complexes (Barton et al., tilization reactions, may play a key role in the 1987; Nelson, 1991 ), together with trace ele- geochemical evolution of the slab-mantle in- ment analyses of veins and other metasomatic terface and overlying mantle wedge. Fluids may features (Gillet and Goffe, 1988; Sorensen and flux partial melting in arc magma source re- Grossman, 1989; Philippot and Selverstone, gions (70 to 150 km) and contribute slab-de- 1991; Bebout et al., 1993a,b), document mo- rived chemical components to such regions bility of Sr (and other large-ion lithophile ele- (Gill, 1981; Wyllie and Sekine, 1982; David- ments, LILE), rare-earth elements (REE), and son, 1987: Ellam and Hawkesworth, 1988; other trace elements in metamorphic fluids Morris et al., 1990). Fluids may also strongly during subduction-zone metamorphism. influence deformation in subduction zones In this paper, we present field and geochem- ( Dumitru, 1991 ). ical evidence for local and large scale fluid Once released from the subducted rocks, by transport in the Catalina Schist Terrane, an either mechanical processes or by devolatili- early Cretaceous subduction complex in zation reactions, volatiles (supercritical fluids) southern California. These results, and those are potential agents of metasomatism and for other subduction-related metamorphic could potentially redistribute mass over large complexes, are used along with theoretical ar- distances. Abundant evidence exists for mass guments to propose that different fluid P-T transfer associated with fluid mobility in shal- flow paths may produce distinctive metaso- low parts of accretionary complexes (see matic events (Barton and Bebout, 1989; Be- Moore and Vrolijk, 1992 ). Although many ob- bout and Barton, 1989a). We first describe the servations imply the presence of abundant field and petrographic evidence for fluid mo- metamorphic fluid during high-pressure/tem- bility and metasomatism in the Catalina Schist perature (P/T) metamorphism at intermedi- (summarized by Bebout and Barton, 1989a), ate depths of subduction zones ( 15 to 70 km), emphasizing veining systematics and altera- few studies have documented the fluid-related tion within rocks of various bulk compositions mass transfer. Subduction-zone metamorphic as functions of metamorphic grade. We then terranes commonly show complex vein sets and present major and trace element data which local evidence for bulk chemical alteration. A document compositional changes of mafic, ul- number of studies have suggested that compo- tramafic, and sedimentary protoliths during METASOMATISM DURING SUBDUCTION: PRODUCTS AND POSSIBLE PATHS IN CATALINA SCHIST, CA 63 high-P/T metamorphism. Also, we summarize Schist contains lithologies that range in grade O, H, and C isotope data (see Bebout and Bar- from lawsonite-albite- and blueschist-facies to ton, 1989a; Bebout, 1991a,b) which demon- amphibolite-facies (Platt, 1975, 1976; Bebout, strate trends toward isotopic homogenization 1989). Three distinct metamorphic units are in rocks; these trends can be explained only by juxtaposed along low-angle faults, resulting in extensive fluid-rock interaction over large dis- an inverted metamorphic gradient, with struc- tances. The stable isotope systematics, partic- turally-lowest blueschist-facies rocks overlain ularly for O, C, and N, indicate that the low- by greenschist-facies rocks,
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