Fluid–metasedimentary rock interactions in subduction-zone me´lange: Implications for the chemical composition of arc magmas

Christopher M. Breeding*  Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven,  Jay J. Ague*  Connecticut 06520-8109, USA Michael BroÈcker Institut fu¨r Mineralogie, Universita¨t Mu¨nster, Corrensstrasse 24, 48149 Mu¨nster, Germany

ABSTRACT ments liberates both U and Th (Johnson and Elevated concentrations of certain large ion lithophile elements (LILE; e.g., Ba, K, Rb, Plank, 1999), and thus cannot account for the Cs, Ca, Sr), U, and Pb in arc magmas relative to high ®eld strength elements (HFSE; e.g., U-Th isotope disequilibrium or the LILE en- Ti, Th, Hf, Nb, Zr) are considered key indicators of ¯uid addition to arc magma source richments relative to Th observed in arc lavas regions worldwide, but the ¯uid sources and processes of mass transfer are controversial. from many subduction settings, including Dehydration of downgoing slabs releases ¯uids that can ¯ow through and react with meta- Tonga-Kermadec, Mariana, and Sunda (Turner morphosed ultrama®c-ma®c rock packages in meÂlange zones near slab-mantle interfaces. et al., 2001). New geochemical data from Syros, Greece, reveal that these ¯uids preferentially leach Given these dif®culties, we propose an al- LILEs, U, and Pb when they in®ltrate and react with subducted metasedimentary rocks. ternative model for element mass transfer Transfer of these LILE-, U-, and Pb-enriched ¯uids to the mantle wedge at subarc depths based on geochemical study of ¯uid-rock in- could directly trigger partial melting and generate magmas with elevated Ba/Th, Sr/Th, teractions that affected subducted metasedi- Pb/Th, and U/Th, as well as radiogenic Sr. Alternatively, if ¯uid transfer occurs at shal- ments on the island of Syros, Greece (Fig. 1; lower depths (e.g., Syros), the metasomatized mantle could be carried deeper by wedge Data Repository Figs. DR1, DR21), the type corner ¯ow to ultimately undergo partial melting in subarc regions. locality for glaucophane. This study was pos- sible because we could sample the outcrops of Keywords: dehydration, metamorphism, subduction zones, arcs, magmas, chemical evolution. fresh rock present on Syros and avoid the lim- ited exposure and deep weathering of many INTRODUCTION sedimentary phengite (Melzer and Wunder, exhumed subduction terranes. Peak metamor- Fluid-driven processes in subduction zones 2000), but their concentrations are small, phism reached ϳ500±550 ЊC and ϳ18±20 are of importance because they can trigger ma- and devolatilization ¯uid ¯uxes within high- kbar (blueschist-eclogite facies), correspond- jor earthquakes and generate explosive arc vol- pressure, low-temperature and ultrahigh-pressure ing to depths of ϳ60 km or more within the canism (e.g., Peacock, 1990; Stolper and New- metasediments are often insuf®cient to mobi- Cretaceous±Eocene subduction zone (Trotet et man, 1994; Noll et al., 1996; Hawkesworth et lize these elements (Spandler et al., 2003, al., 2001). In northwestern Syros, large-scale al., 1997; Davies, 1999; Kerrick and Connolly, 2004). Busigny et al. (2003) detected little el- faulting and folding during subduction com- 2001). The geochemistry of arc magmas world- ement mobility during subduction of the plexly juxtaposed metasedimentary rocks and wide is generally characterized by elevated Schists LustreÂs nappe, although retrograde an ophiolitic meÂlange zone (Dixon and Ridley, 1987; Okrusch and BroÈcker, 1990; BroÈcker concentrations of large ion lithophile elements overprinting may complicate interpretations and Enders, 2001) (Fig. 1). The meta-igneous (LILE; e.g., Ba, K, Rb, Cs, Ca, Sr), U, and Pb there (Bebout et al., 2004). When mobilization meÂlange blocks, reaction rinds, and meta- relative to high ®eld strength elements (HFSE; occurs, the suite of elements contrasts with ultrama®c matrix of the meÂlange are texturally e.g., Ti, Th, Hf, Nb, Zr). Although this geo- that required for arc metasomatism, and much similar to those of meÂlange zones elsewhere chemical ®ngerprint is well known, the ¯uid of the mass transfer appears to occur at shal- (Sorensen and Grossman, 1989; Bebout and sources and processes of mass transport during low depths in the forearc (Bebout et al., 1999). subduction, metamorphism, and magma gene- Barton, 1989, 1993; Manning, 1997). Reac- Moreover, investigations of hydrous mineral sis are actively debated. The main potential tion rinds containing omphacite, glaucophane, breakdown reactions in subducting slabs sug- sources for LILE-enriched ¯uids are dehydra- and/or chlorite are common around the blocks gest independent ¯uid and trace element tion of oceanic crust, dehydration of subducted and document the presence of ¯uids in the meÂ- sources (Spandler et al., 2003, 2004). Land- sediments, and partial melting of sediments (cf. lange (cf. Dixon and Ridley, 1987; BroÈcker mark studies of exhumed subduction complexes Stolper and Newman, 1994; Noll et al., 1996; and Enders, 2001). Altered, glaucophane-rich demonstrate that ¯uid ¯ow and element mass Hawkesworth et al., 1997; Johnson and Plank, zones as thick as 2.5 m found within the meta- transfer occur at slab-mantle interfaces (Soren- 1999; Becker et al., 2000; Turner et al., 2001; sediments along contacts with meta-ultrama®c Spandler et al., 2003, 2004). Recent studies in- sen and Grossman, 1989, 1993; Bebout and Bar- meÂlange matrix also indicate substantial ¯uid- dicate that dehydration of the oceanic crust and/ ton, 1989, 1993; Manning, 1997). Fluid-driven or sediments alone appears to be unable to lib- geochemical alteration of ma®c and ultrama®c 1GSA Data Repository item 2004165, location erate the necessary LILEs, U, and Pb observed blocks in such meÂlange has been clearly docu- map (Fig. DR1), photo of an ϳ2.5-m-thick alter- ation zone (Fig. DR2), detailed sample description in arc lavas (Chalot-Prat et al., 2003; Spandler mented (cf. Dixon and Ridley, 1987; Sorensen and Grossman, 1989; Bebout and Barton, (Fig. DR3), photomicrographs of the sample (Fig. et al., 2003, 2004). For example, K, Rb, and DR4), brief descriptions of mineral analysis proce- Cs are present in ¯uid coexisting with meta- 1989, 1993; King et al., 2003), but ®eld-based dures, thermobarometric calculations, and estimated assessment of the chemical alteration of me- Ba extraction from metasediments (Appendix DR1), tasediments is complicated by variable proto- and rock chemical data (Table DR1), is available *Present address: BreedingÐGemological Insti- lith compositions and strong deformation online at www.geosociety.org/pubs/ft2004.htm, or tute of America, 5345 Armada Drive, MS33, Carls- on request from [email protected] or Docu- bad, California 92008, USA. E-mail: AgueÐ (e.g., Catalina Schist, USA; Bebout and Bar- ments Secretary, GSA, P.O. Box 9140, CO 80301- [email protected]. ton, 1993). Partial melting of subducted sedi- 9140, USA.

᭧ 2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; December 2004; v. 32; no. 12; p. 1041±1044; doi: 10.1130/G20877.1; 3 ®gures; Data Repository item 2004165. 1041 Figure 1. Sample locality DISCUSSION AND CONCLUSIONS at edge of meÂlange zone The LILEs (K, Ba, Rb, Cs, Ca, Sr), U, and in northern Syros, Greece. Ophiolitic meÂlange con- Pb extracted from the metasedimentary layer sists mostly of meta- by meÂlange ¯uids are also elements that are igneous blocks with re- commonly enriched in arc lavas, whereas action rinds in matrix HFSE elements like Hf, Th, Nb, Ti, Zr, and dominated by meta- ultrama®c rock. Meta- REEs that are commonly depleted in arcs were sedimentary sample col- not strongly mobilized by the ¯uids (Fig. 2A). lected from Kampos The metasomatism we envision begins with Schists at contact with ¯uid generation by dehydration of metama®c meÂlange matrix pre- serves primary interlay- (and possibly meta-ultrama®c) rocks in down- ering of metapsammite going slabs (Fig. 3). If our study area is anal- (nonpatterned layers) ogous to other high-pressure meÂlange zones, and metapelite (dotted layers, except metapelite layer sampled for geochemical analysis then, at some time, the Syros meÂlange resided shown in black). Matrix crosscuts layers at one end. Metasomatic alteration of layers (light at or near the slab-mantle interface and inter- gray band) is visible near contact with meta-ultrama®c meÂlange matrix. acted with such ¯uids (cf. Dixon and Ridley, 1987; Sorensen and Grossman, 1989, 1993; rock interaction (Fig. DR2; see footnote 1); Results Bebout and Barton, 1989, 1993, 2002). contacts extend regionally for linear distances The altered parts of the layer adjacent to the Next, in a key step of the process, ¯uid of more than 2 km. The contact zones likely matrix contact were geochemically compared equilibrated with metamorphosed ultrama®c- measured tens or even hundreds of square ki- to relatively unaltered regions away from the ma®c rock packages in meÂlange in®ltrates and lometers within the subduction zone, such that contact (cf. Ague and van Haren, 1996; Ap- reacts with metasedimentary rocks (Fig. 3). ¯uid-driven alteration would have affected pendix DR1; Table DR1 [see footnote 1]). On Syros, these ¯uids stripped out LILEs, U, large volumes of metasedimentary rock. Near the contact, K, Rb, Ba, Cs, Ca, Sr, U, and Pb (and added Mg, Cu, and P). For ex- and Pb concentrations decrease substantially ample, in®ltration of a K-poor meÂlange ¯uid CHEMICAL ALTERATION OF within the metasedimentary layer, whereas into phengite-bearing, K-rich metapelites will METASEDIMENTARY ROCK Mg, P, and Cu increase (Fig. 2A). Ratios drive chemical reactions that reduce chemical Sample Description among Ti, Zr, Hf, Nb, Th, and rare earth ele- potential gradients for ¯uid species by de- Chemical heterogeneities inherent in layered ment (REE) concentrations do not vary sig- stroying phengite and releasing K (and Rb, clastic sediments often hinder geochemical ni®cantly across the layer, suggesting limited Ba, and Cs) to ¯uids. It is possible that ¯uids in42terpretation (Bebout and Barton, 1993; mobility for these elements. Correspondingly, bearing some of this geochemical signature Ague, 1997), so we performed high spatial res- bulk K/Th, Rb/Th, Ba/Th, Cs/Th, Ca/Th, Sr/ may be produced if metasedimentary rocks olution sampling along a single, continuous, Th, U/Th, and Pb/Th sharply decrease, where- are in®ltrated by ¯uids equilibrated solely metapelitic layer that is in contact with meta- as Mg/Th, P/Th, and Cu/Zr increase with with ma®c crust (cf. Breeding and Ague, ultrama®c meÂlange matrix at one end (Figs. 1 proximity to the meta-ultrama®c matrix con- 2002), or if ma®c crust is in®ltrated by ¯uids and DR3 [see footnote 1]). The sample is rep- tact (Figs. 2B±2F). Zr/Th, Ti/Th, and Hf/Zr equilibrated with meta-ultrama®c rocks. How- resentative of the centimeter- to several-meter- remain nearly constant across the metasedi- ever, reaction of metasedimentary rocks with thick alteration zones found at matrix contacts mentary layer and alteration zone, but are sig- strongly LILE-, U-, and Pb-depleted ¯uids (Fig. DR2; see footnote 1) and offers an ex- ni®cantly different within the meÂlange matrix, from meta-ultrama®c±bearing rock packages ceptionally clear view of a layer that is cut at demonstrating that mechanical mixing of ma- would almost certainly result in the most ef- a high angle by the matrix. Mineralogical al- trix and metasediments did not cause the teration increases toward the matrix contact ®cient element extraction. As an illustration, chemical changes within the altered parts of (Figs. DR3, DR4; see footnote 1). Relatively the losses of the key tracer Ba documented the layer (Fig. 2C). Meta-ultrama®c matrix unaltered metapelite distal to the contact con- herein indicate that alteration of only ϳ7 immediately adjacent to the metasedimentary 3 tains phengite ϩ sodic-pyroxene ϩ epidote ϩ vol% of a 1 km section of metasediments is layer contains low concentrations of LILEs, garnet ϩ glaucophane ϩ quartz ϩ titanite ϩ suf®cient to raise Ba concentrations by 100 U, and Pb, indicating that these elements were zircon ϩ rutile. As the contact is approached, ppm for an eruption the size of the 1980 event not exchanged locally, but instead were re- glaucophane increases noticeably in size and at Mount St. Helens, USA (Appendix DR1; moved by ¯uids ¯owing through the meÂlange abundance; chlorite, albite, magnetite, and ap- see footnote 1). There is evidence for frac- atite appear; and phengite, epidote, sodic-py- (Figs. 2B±2E). K, Rb, Ba, and Cs losses result tionation of Cs in ¯uids relative to Rb and K roxene, quartz, and garnet abundances decrease from the destruction of phengite during alter- (Hart and Reid, 1991) for some arcs (e.g., Ma- dramatically. The meta-ultrama®c meÂlange ma- ation. Epidote, garnet, and clinopyroxene rianas) but not all (e.g., Kuriles; cf. Bebout et trix consists mostly of chlorite, talc, serpentine, breakdown released Ca. Mg addition corre- al., 1999); meÂlange-sediment contacts from a and magnetite. Thermobarometry (Holland and lates with increased glaucophane abundance range of depths could be investigated to assess Powell, 1998) using the alteration assemblage and the appearance of chlorite. Apatite hosts this process further. yields pressures of 11.7±12.3 kbar for temper- the added P, and Cu was sequestered into chlo- Once ¯uids obtain major and/or trace ele- atures of 500±550 ЊC, corresponding to depths rite (up to ϳ1700 ppm Cu; Appendix DR1; ment enrichments through reaction with sub- of at least ϳ40 km (Appendix DR1; see foot- see footnote 1). Epidote (containing up to ducted metasedimentary rock, they can ascend note 1). Albite forms pseudomorphs after ϳ1500 ppm Pb and 2600 ppm Sr; Appendix via permeable conduits (e.g., fractures, litho- jadeite-rich pyroxene, so ϳ12 kbar is proba- DR1; see footnote 1) broke down, releasing logic contacts; cf. Davies, 1999; Bebout and bly a minimum recorded during exhumation Pb and Sr. Epidote was a likely host for U (cf. Barton, 1993; Ague, 2003) and impart an and retrogression; we consider 18±20 kbar a Zack et al., 2002), but the small U content of LILE-, U-, and Pb-rich ®ngerprint to the man- more likely peak pressure for blueschist-eclogite the rocks (1±2 ppm) prevented identi®cation tle wedge (Fig. 3). Metasomatic minerals in- facies alteration (cf. Trotet et al., 2001). of the mineral that reacted to release U. cluding phlogopite, amphibole, and pyroxene

1042 GEOLOGY, December 2004 (cf. Chesley et al., 2004) presumably hosted the transported elements in the metasomatized mantle inferred to have existed above the Sy- ros subduction zone. This mantle could have produced arc magmas if wedge corner ¯ow (cf. Thomas et al., 2002) dragged it to subarc depths. Furthermore, the process of LILE, Pb, and U release we document should be a proxy for deeper ¯uid in®ltration that directly trig- gers partial melting in the subarc because the liberated elements reside mainly in phengite and epidote, phases that persist to ultrahigh- pressure conditions (Busigny et al., 2003). We note in this regard that the U-Th isotopic dis- equilibrium discovered in many arcs suggests that some magmas migrate as rapidly as 1000 m per year from their sources to the surface (Turner et al., 2001). Our ®ndings provide a mechanism for the geochemical decoupling of Th from U and Pb necessary to produce this disequilibrium. If ¯uid-rock interactions be- tween metama®c, meta-ultrama®c, and meta- sedimentary reservoirs are limited, metasedi- mentary rocks may have to be subducted to deeper levels to liberate LILEs, U, Pb, and Th by partial melting (e.g., Johnson and Plank, 1999).

ACKNOWLEDGMENTS We thank D.E. Wilbur and E.L. Donald for ®eld assistance; J.O. Eckert for electron microprobe as- sistance; G.E. Bebout, J.A.D. Connolly, and J.M. Ferry for constructive reviews; and the Greece In- stitute of Geology and Mineral Exploration (IGME) for work permits. This work was supported by Na- tional Science Foundation grants EAR-9727134 and EAR-0105927 (Ague), United States Department of Energy grant DE-FG02-01ER15216 (Ague), a Geo- logical Society of America Graduate Research grant (Breeding), and a Yale University Enders Research grant (Breeding).

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1044 GEOLOGY, December 2004