The process of subduction, wherein an oceanic plate descends into the Earth's mantle, is believed to have contributed to the geochemical heterogeneity of the mantle, and played a major role in the growth of continents and mountain belts. Many regard the Franciscan Complex of California as the world's best example of a subduction complex--an assemblage of materials that is formed of rocks partly subducted and scraped off the downgoing plate. Accordingly, the Franciscan Complex serves as an ideal field laboratory to investigate processes of subduction, including processes associated with the initiation of subduction. In this study of the geochemistry of Franciscan metamorphic rocks, the team will seek to understand the physical and chemical processes associated with the initiation of subduction and continued subduction and the generation of oceanic crust at spreading ridges above subduction zones. In addition to giving insight into fundamental Earth processes, the project may yield information relevant to the development of certain ore deposits, as well as natural concentrations of some metals that pose environmental hazards.
The investigators plan to examine the geochemical characteristics of Franciscan Complex basaltic rocks of different ages and metamorphic grade and use the geochemical data to gain insight into subduction, oceanic crust generation, and associated geochemical processes that spanned over 100 million years. The results will build on a preliminary study that suggest some of the parent rocks of the metamorphosed basalts formed in an island arc or arc-related oceanic spreading center whereas others formed at a mid-ocean ridge (MORB), and that the arc parent rocks may be limited to the oldest and most intensely metamorphosed (high-grade) rocks that are thought to have been metamorphosed at the inception of subduction. None of the samples showed evidence of a continental geochemical signature, indicating the lack of geochemical exchange with fluids derived from continentally-sourced sediments. These results, on a limited number of samples, suggest that a more extensive sampling (over 80 samples from at least 15 localities, spanning 500-km-long section of the paleo subduction zone)may help determine the transition (in time and space) in subduction history between early subduction of mixed arc and mid-ocean ridge generated basalt to subsequent subduction of entirely MORB or ocean island basalt. It is also planned to examine whether younger Franciscan basalts show evidence for a lack of geochemical exchange with fluids derived from subducted continental sediments. This will prove illuminating, since these younger basalts should have been incorporated into the Franciscan at a time when subduction of continental sediments was ongoing. The team will exploit data from recent and ongoing geochronologic studies of the Franciscan Complex in which one of the PIs (JW) is a participant. The new geochronologic data help place the geochemical studies into a tighter temporal framework than is offered by the existing published ages. In addition to the specific issues outlined above, our study will: (1) help illuminate details of the a subduction zone, from subduction inception through 100 million years of subduction history, and the generation of oceanic crust involved in the subduction process; (2) provide insight into geochemical fluxes and arc magma genesis from the geochemistry of deeply subducted volcanic rocks; (3) determine the role of aqueous fluids formed by decomposition of hydrous phases in transporting elements and determining geochemical fluxes within subduction zones. Part of this research will be included in the PhD thesis of a female graduate student at the University of Rochester and an M.S. thesis at Fresno State.
