Tectonic and metasomatic mixing in a high-T, subduction-zone mélange—insights into the geochemical evolution of the slab–mantle interface

Abstract

The Catalina Schist (California) contains an amphibolite-grade (0.8–1.1 GPa; 640–750 °C) mélange unit consisting of mafic and ultramafic blocks in high-Mg, schistose mélange matrix with varying modal proportions of talc, chlorite, anthophyllite, calcic-amphibole, enstatite, and minor phases including zircon, rutile, apatite, spinel, and Fe–Ni sulfides. This mélange unit is interpreted as a kilometer-scale zone of tectonic and metasomatic mixing formed within a juvenile subduction zone, the study of which may yield insight into chemical mixing processes at greater depths in subduction zones.

Relationships among the major and trace element compositions of the mafic and ultramafic blocks in the mélange, the rinds developed at the margins of these blocks, and the surrounding mélange matrix are compatible with the evolution of the mélange matrix through a complex combination of infiltrative and diffusional metasomatism and a process resembling mechanical mixing. Simple, linear mixing models are compatible with the development of the mélange matrix primarily through simple mixture of the ultramafic and mafic rocks, with Cr/Al ratios serving as indicators of the approximate proportions of the two lithologies. This conclusion regarding mafic–ultramafic mixing is consistent with the field observations and chemical trends indicating strong resemblance of large parts of the mélange matrix with rinds developed at the margins of mafic and ultramafic blocks. The overall process involved development of metasomatic assemblages through complex fluid-mediated mixing of the blocks and matrix concurrent with deformation of these relatively weak rind materials, which are rich in layer silicates and amphibole. This deformation was sufficiently intense to transpose fabrics, progressively disaggregate more rigid, block-derived materials in weaker chorite- and talc-rich mélange, and in some particularly weak lithologies (e.g., chlorite-, talc-, and amphibole-rich materials), intimately juxtapose adjacent lithologies at the (sub-)cm scale (approaching grain scale) sampled by the whole-rock geochemical analyses.

Chemical systematics of various elements in the mélange matrix can be delineated based on the Cr/Al-based mixing model. Simple mixing relationships exhibited by Al, Cr, Mg, Ni, Fe, and Zr provide a geochemical reference frame for considerations of mass and volume loss and gain within the mélange matrix. The compositional patterns of many other elements are explained by either redistribution (local stripping or enrichment) at varying scales within the mélange (Ca, Na, K, Ba, and Sr) or massive addition from external sources (Si and H₂O), the latter probably in infiltrating H₂O-rich fluids that produced the dramatic O and H isotopic shifts in the mélange.

Mélange formation, resulting in the production of high-variance ultramafic assemblages with high volatile contents, may aid retention of volatiles (in this case, H₂O) to greater depths in subduction zones than in original subducted mafic and sedimentary materials. The presence of such assemblages (i.e., containing minerals such as talc, chlorite, and Mg-rich amphiboles) would impact the rheology of the slab–mantle interface and perhaps contribute to the low-velocity seismic structure observed at/near the slab–mantle interface in some subduction zones. If operative along the slab–mantle interface, complex mixing processes such as these, involving the interplay between fluid-mediated metasomatism and deformation, also could impact slab incompatible trace element and isotopic signatures ultimately observed in arc magmas, producing “fluids” with geochemical signatures inherited from interactions with hybridized rock compositions.

Introduction

Although geochemical studies of arc magmatism commonly call upon metasomatic “fluids” (hydrous fluids or silicate melts) as agents for transporting “slab signatures” to sites of arc magmatism (e.g., Gill, 1981, Pearce and Peate, 1995), little is known about the structural and geochemical processes that operate along the slab–mantle interface. At shallow levels (<10 km), large volumes of dominantly sedimentary material accumulate in accreted wedges, occurring in many complexes as mélange-like material between the slab and the hanging wall (see Shreve and Cloos, 1986). The scarcity of appropriate exposures has made it difficult to infer the nature of the slab–mantle interface zone at greater depths. Recent geophysical study of the slab–mantle interface has identified zones of low seismic velocity at or near the top of the subducting oceanic lithosphere (e.g., Fukao et al., 1983, Hori et al., 1985, Matsuzawa et al., 1986, Helffrich et al., 1989, Helffrich, 1996, Helffrich and Abers, 1997). These low-velocity domains have generally been interpreted as containing extensively hydrated assemblages in the subducting oceanic lithosphere; however, Abers et al. (1999; also see discussion by Abers, 2000) have speculated that they might also represent hydrated hanging-wall materials. The generation of zones rich in layered hydrous minerals, perhaps in part a mechanical mixing zone, could also promote aseismic behavior at great depths in subduction zones (Peacock and Hyndman, 1999). The Catalina Schist, exposed on Santa Catalina Island (California), contains a kilometer-scale amphibolite-grade, dominantly ultramafic, mélange (Fig. 1). Petrological considerations indicate that the mélange formed at 0.8–1.1 GPa and 640–750 °C Sorensen and Barton, 1987, Sorensen, 1988. The mélange contains mafic and ultramafic blocks showing varying metasomatic alteration and, in the case of some mafic blocks, migmatization (Sorensen, 1988). The unit is thought to represent a zone of tectonic and metasomatic mixing near the slab–mantle interface (Bebout and Barton, 1989), and may, thus, yield insights into the complex structural and metasomatic processes operating at depth in subduction zones.

The amphibolite mélange unit was first mapped by Bailey (1941), who recognized that it consists predominantly of schists containing assemblages of talc±anthophyllite±chlorite±actinolite±enstatite±quartz. Sorensen (1988) and Sorensen and Grossman (1989), in their detailed petrologic and geochemical studies, documented metasomatic exchange between mafic blocks and the surrounding mélange matrix to produce “rinds” on these blocks and suggested that infiltrating aqueous fluids facilitated this exchange. Sorensen and Barton (1987) and Sorensen (1988) suggested that this high-T infiltration event also resulted in varying degrees of hydration and metasomatism leading to migmatization of some mafic blocks floating in the mélange. Detailed isotopic studies of the mélange Barton et al., 1987, Bebout, 1991 led to an infiltration model whereby large amounts of aqueous fluid, previously equilibrated with metasedimentary rocks, entered the mélange, leading to large-scale stable isotope homogenization and producing abundant metasomatic features within the mélange unit. In this paper, we combine petrological, field (including mapping), petrographic, and geochemical (major and trace element; isotopic) evidence relevant for consideration of the larger-scale development of this kilometer-scale mélange unit. We argue that the rind-forming process demonstrated by Sorensen (1988) and Sorensen and Grossman (1989) for mafic blocks, and illustrated in our study for ultramafic blocks, is representative of the initial mixing processes involving combinations of diffusive and infiltrative metasomatism and mechanical (tectonic) mixing, which ultimately resulted in the production of the far more voluminous mélange matrix in this mélange unit. Finally, we discuss some implications of these mixing processes for rheology and geochemical hybridization along the forearc and subarc slab–mantle interface.