Field and Isotopic Evidence for Fluid Mobility in the Franciscan Complex: Forearc Paleohydrogeology to Depths of 30 Kilometers

Abstract

In the Franciscan Complex exposed in the California Coast and Diablo ranges (representing depths of up to 30 km in a Mesozoic to Early Tertiary accretionary prism), differences in porosity, permeability, and rheology impacted geometries and scales of fluid mobility and the deep entrainment of pore water (chemically evolved seawater). Carbon and O isotope compositions of abundant, texturally diverse CaCO 3 veins (most veins with δ 13 C VPDB = -11.0 to -3.0‰, δ 18 O VSMOW = +12.0 to +18.5‰) are in part consistent with control of fluid isotopic compositions by relatively local-scale exchange with large volumes of metaclastic host-rocks. Although this apparent local-scale equilibration, observed for some veins, complicates assessment of external sources for the fluids that produced these veins, many veins with elevated δ 18 O (relative to calculated rock-buffered values) could reflect up-dip flow of H 2 O released at greater depths and previously equilibrated with similar lithologies at higher temperatures (and sluggish reequilibration of the rocks with these externally derived fluids). In the Coastal Belt, differences in vein δ 13 C in adjacent coherent graywacke and shaley mélange zones may be due to preferential infiltration of the more permeable mélange zones by deeply derived CH 4 -bearing fluids, perhaps in part during exhumation and cooling. A number of variations in vein isotopic composition in individual exposures can be attributed to vein formation over a range of increasing T during underthrusting, then decreasing T during exhumation, and related to varying rheology (affecting permeability) within intercalated highly disrupted shales, sandstone-shale sequences (with interbedding at mm to cm scales), and more massive metagraywacke.Calculated fluid δ 18 O for veins in the Franciscan units peak metamorphosed at lower temperatures (Coastal Belt) spans the range of fluids venting in active accretionary prisms and producing forearc serpentinite seamounts (mostly 0 ± 3‰). In general, higher flux of aqueous fluid from depth in accretionary prisms, preventing reequilibration with rocks along its path, should favor expulsion of fluid with δ 18 O higher than that of seawater that could be traced in fluids along fault structures at shallower levels. Salinities of fluid inclusions in vein quartz are mostly lower than that of seawater, consistent with delivery of "fresher" aqueous fluids toward the surface along structural heterogeneities. Recent work on volatile and trace element contents of the Coast and Diablo ranges exposures of the Franciscan Complex indicates significant loss of structurally bound H 2 O (i.e., not pore water) due to clay-to-mica transitions, but at shallower levels than those for peak metamorphism of all units studied here (i.e., at <5 km). Much of the fluid flux through these rocks involved fluids liberated at greater depths, precipitating calcite and quartz along down-T, down-P paths, and mobilizing trace elements (e.g., B, Cs, perhaps also K, Rb, As, and Sb) liberated at higher temperatures. Ca...

Tracing the effects of high-pressure metasomatic fluids and seawater alteration in blueschist-facies overprinted eclogites: Implications for subduction channel processes

“…These metasomatized rocks may constitute new bulk compositions that can influence the redistribution of major and trace elements (e.g., Bebout, 2007;King et al, 2006;Putnis and John, 2010;Scambelluri et al, 2006), which may be transported over short distances (redistribution on cm scale; e.g., Hermann, 2002;Spandler et al, 2003; or long distances (removal from a system on outcrop scale; e.g., Gao et al, 2007;Zack and John, 2007;John et al, 2008;Spandler et al, 2011). Radiogenic and stable isotope studies in fluid-overprinted metamorphic rocks can provide information about the extent of fluid-rock interactions (e.g., Baker et al, 1997;Yui et al, 1997;Zheng et al, 1999) and the identification of potential fluid sources (e.g., Bebout and Barton, 1993;Halama et al, 2010Halama et al, , 2011King et al, 2006;Scambelluri and Phillipot, 2001;Sadofsky and Bebout, 2004). In principle, fluids in the subduction zone may originate from (I) the partly serpentinised, subducted mantle peridotite underneath the oceanic crust (Hyndman and Peacock, 2003;Ranero et al, 2003), (II) the altered oceanic crust (AOC; e.g.…”

Metasomatism in Subduction Zones of Subducted Oceanic Slabs, Mantle Wedges, and the Slab-Mantle Interface

“…The Monviso example represents a within-slab tract of gabbroic rocks, metamorphosed in a more continental-type subduction setting (the more saline inclusions were also noted by Scambelluri et al 1998Scambelluri et al , 2002Nadeau et al 1993;Scambelluri and Philippot 2001;Fu et al 2002;Franz et al 2001). The other settings reflect higher degrees of fluid flux in deforming mélange zones metamorphosed in a more oceanic setting (lowersalinity inclusions also noted by Sadofsky and Bebout 2004; also see Gao and …”

“…However, in some cases it is difficult to identify isotopic effects of large-scale flow of fluid through relatively uniform reservoirs (e.g., large expanses of sandstone-shale sequences in accretionary complexes) and without large temperature differences along flow paths, given the inherent isotopic heterogeneities in these rocks. In this case, the fluids can be relatively non-reactive, having previously, isotopically equilibrated with similar rocks at similar temperatures (see discussion in Sadofsky and Bebout 2004).…”

“…9.12b; from Bebout and Barton 1989;Bebout 1991a, b). Other isotopic studies of fluid mobility in ancient accretionary complexes include Vrolijk (1987) (also see work on the modern Barbados accretionary prism; Vrolijk et al 1990), Sadofsky and Bebout (2004), and Sakakibara et al (2007). Nelson (1995) documented kilometer-scale mobilization of fluids carrying sedimentary Nd-and Sr-isotope signatures, through analyses of metasomatic rinds on mafic tectonic blocks in the Franciscan Complex mélange.…”

“…9.9c, are believed to have had a greater than 10% addition of SiO 2 due to infiltrating fluids (see Breeding and Ague 2002). Quartz-bearing vein assemblages, commonly monomineralic quartz veins, are a nearly ubiquitous feature of forearc metasedimentary suites (ancient accretionary systems) and likely reflect the massive transfer of dissolved SiO 2 toward the seafloor in H 2 O-rich fluids liberated by a combination of pore water expulsion and metamorphic dehydration (see Sadofsky and Bebout 2004). More unusual occurrences of pegmatites and leucosomes, which develop during subduction-zone metamorphism (see Fig.…”