Ophiolites are distinctive assemblages of mafic, ultramafic, and felsic igneous rocks that are commonly thought to represent oceanic crust and mantle that has been accreted to a continental margin. The accretion of ophiolite and island arc terranes has been the primary mechanism of continental growth since the Proterozoic.
Geochemical Processes in Forearc Peridotites: Depletion, Enrichment, and Melt Reactions in the Mantle Wedge above Subduction Zones
The question of geochemical flux in the mantle wedge during subduction is critical to our understanding of arc volcanism, and forms an important aspect of the global geochemical flux. This is one of the first order problems identified by the “Geochemical Earth Reference Model” (GERM) initiative, and by the “subduction factory” focus of the NSF “MARGINS” initiative. Quoting from the MARGINS program announcement: “At convergent margins, raw materials (sediments, oceanic crust and upper mantle) are fed into the "subduction factory" where many processes (including dewatering, metamorphism, melting) under changing physical and chemical conditions shape the final products (magma, volatiles, ore deposits, new continental crust, recycled materials) with significant environmental consequences. In practice, it has been difficult to investigate processes and estimate fluxes through the "factory" owing to poor constraints on the volumes of magmas, fluids, and volatiles produced.” The MARGINS program attempts to understand these processes by studying active subduction zones, where the processes may be observed directly. This approach has much to recommend it, but it does suffer from the fundamental problems posed by poor exposure of lithospheric mantle derived from the supra-subduction zone wedge, and by the cover of several thousand feet of seawater.
An alternative approach is to examine outcrops of lithospheric mantle that underlie crust known to have formed by supra-subduction zone (SSZ) magmatism. This lithospheric mantle represents in part the source from which the overlying crust was extracted, and its mineralogy and composition reflect the processes that have affected it through time, including melt extraction, fluid phase enrichment, and subsequent interactions with melt derived from lower in the mantle tectosphere. These processes have been frozen in place by cooling and emplacement of the mantle lithosphere and its overlying crust. This approach offers some advantages over currently active subduction zones, such as on-land exposures, continuous outcrops that can be related structurally and stratigraphically to their overlying crust, and relatively low costs associated with the field studies. A primary advantage, however, is the fact that large tracts of supra-subduction lithosphere are commonly exposed at the base of many SSZ ophiolites, allowing us to examine their petrology and geochemistry on larger length scales than is currently possible in active systems.
The Coast Range ophiolite of California (CRO) is one of the most extensive tracts of oceanic crust in North America. Our recent work at three important CRO localities (Elder Creek, Stonyford, Cuesta Ridge), along with work by other investigators at Del Puerto, Llanada, Sierra Azul, Mount Diablo, Point Sal, has now established that the CRO represents, in large part, formation by fore-arc extension above an east-dipping, proto-Franciscan subduction zone, modified in part by subsequent ridge-trench interactions (Shervais, 1990, 2001; Giaramita et al., 1990; Stern and Bloomer, 1992; Shervais and Hanan, 1989; Shervais et al, 2004a,b,c).
We propose to study geochemical flux in the lithospheric mantle above the proto-Franciscan subduction zone, as represented by partially serpentinized harzburgite and dunite tectonites that underlie the Coast Range ophiolite of California. Our goal is constrain nature and extent of these fluxes, as documented by whole rock major element, trace element, tracer isotope, and stable isotope analyses, and by mineral analyses using electron and ion beam techniques. Major questions we will pose include the cumulative extent of melt extraction and the nature of the melt extracted, the nature, source, and extent of fluid flux to the mantle in the SSZ wedge, and the nature and extent of mantle-melt interactions subsequent to melt extraction (e.g., addition of melt from sublithospheric sources, or reaction of this melt with the previously depleted peridotites).
Shervais, J.W., Kolesar, P., and Andreasen, K., 2005, Field and Chemical Study of Serepentinization – Stonyford, California: Chemical Fluxes and Mass Balance,, in W.G. Ernst, editor, Serpentine and serpentinites: mineralogy, petrology, geochemistry, ecology, geophysics, and tetonics (a tribute to Robert G. Coleman), International Book Series, v. 8, Bellwether Publishing Ltd., Columbia, MD, 452- 474. [reprinted from International Geology Review]
Shervais, J.W., Kolesar, P., and Andreasen, K., 2005, Field and Chemical Study of Serepentinization – Stonyford, California: Chemical Fluxes and Mass Balance, International Geology Review, v. 47, 1-23.
Shervais, Kolesar, Andreasen, and Vetter, Compositional Variation in Serpentine as a Function of Primary Phase Chemistry: Coast Range ophiolite, California, International Geology Review, in preparation.
Shervais and Vetter, Serpentinized Peridotites of the Coast Range ophiolite, Stonyford, California: Melt reactions in a fore-arc mantle wedge; in preparation.