IBM – type Arc Protoliths for the High Grade Blocks of the Franciscan Complex, California Arundhuti Ghatak, University of Rochester, Rochester, NY Asish R. Basu, University of Rochester, Rochester, NY (This work is in collaboration with John Wakabayashi, California State University, Fresno) Abstract – Subduction is generally believed to introduce geochemical heterogeneity in the mantle, through incorporation of basaltic oceanic crust, geochemically depleted mantle peridotite and ocean floor sediments. Our principal objective in a recently published study was to identify the protoliths of the Franciscan high-grade blocks by the geochemical signatures of their pre-subduction lithologies and reconstruct the tectono-metamorphic history of these rocks. The preliminary results of this study of blue schist, garnet amphibolite, and eclogite from the San Francisco Bay area strongly indicate a nascent island-arc basalt origin for these rocks, similar to that proposed for the Coast Range Ophiolite and different from that of other Franciscan volcanic rocks that have been studied thus far. Additional unpublished geochemical data on the Franciscan high-grade metamorphic rocks from other distant localities also demonstrated an arc origin for the protoliths of the high-grade metamorphic rocks. These results require unique pre-Franciscan subduction plate configuration off coastal California including the presence of an Izu-Bonin type nascent arc. It is our contention that these arc rocks formed the protoliths of the high-grade Franciscan metamorphic rocks. Figure 1. Distribution of Franciscan and related rocks of central and northern California Modified from Wakabayashi (1999). High-grade block localities are shown by filled red squares. RM is the Ring Mountain locality and samples from here have been analyzed in a previous study by Saha et al. (2005). See legend for the high- grade block locality in the bottom right of figure. Figure 2 (top). Chondrite-normalized REE patterns of high-grade rocks, coherent schists, and metagraywackes (~120 Ma deposition age) and metacherts. The high-grade rocks and coherent schists display REE patterns analogous to South Sandwich Islands, Izu-Bonin and Mariana arc basalts (Hawkesworth et al., 1977; Tatsumi and Eggins, 1995). The shaded region is a summary of Western Pacific arc tholeiite data (Jakes and Gill, 1970). For location of samples plotted in figures 2- 3, see legends in figures 4-6. Figure 3. Multiple trace element concentrations normalized over N- MORB for the Franciscan metamorphic rocks. Elements are arranged according to varying incompatibility (Sun and McDonough, 1989; Tatsumi and Eggins, 1995). Along with low Ce/Pb, Nb/U, generally high Ba/Rb and Ba/Th, and inordinately high Pb- enrichments of the high-grade rocks are noteworthy, which indicate arc protoliths. Initial 207 Pb/ 204 Pb Initial 206 Pb/ 204 Pb Initial 208 Pb/ 204 Pb Figure 4. Initial Pb-isotopic compositions at 169 Ma of the Franciscan rocks, compared with Pb-isotope ratios of three intra-oceanic arcs of the western Pacific. These arc data are from various sources and compiled by the Max Planck Data Sources ( mainz.gwdg.de/Entry. html). The Franciscan Pb-data are most remarkably similar to the frontal-arc lavas of Izu-Bonin (Taylor and Nesbitt, 1998). The open squares are samples from Saha et al., (2005). Initial  Nd Initial 87 Sr/ 86 Sr Figure 5 (left). Initial 87 Sr/ 86 Sr and  Nd at 169 Ma in the Franciscan high-grade rocks and coherent schists, compared with fields for present day MORB, global arc tholeiites, and part of the field for oceanic sediments (Tatsumi and Eggins, 1995). All the high- grade rocks fall within or below the field of arc tholeiite and are distinctly different from the two coherent schists that have higher  Nd and fall in the field of present day MORB. Figure 6 (left). Th/Yb versus Ta/Yb diagram showing the compositional variation of basalts in different tectonic settings. The ‘mantle’ array, and the tholeiitic (TH), calc-alkaline (CA), and shoshonitic (SHO) boundaries for arc basalts are from Pearce (1989). High-grade and coherent rocks from the current study and from a previous study (Saha et al., 2005) have been plotted in this diagram and they show a strong vertical trend from N-MORB to Mariana arc. The Metagraywackes with ~120 Ma depositional ages from the current study fall close to or within the field of Central Andes. Figure 7 (left). Tectonic Model Proposed by Saha et al., 2005; (A) West dipping subduction begins;(B) Nascent arc crust forms causing lithospheric extension; (C) Due to blocked westward dipping subduction, eastward dipping Franciscan subduction is initiated within the infant arc crust; (D) Off scraping of the arc crust to form high grade blocks and later subduction of MORB; (E) Anti-clockwise P-T-t paths of the high-grade blocks. REFERENCES Bloomer, S. H., 1987, Geochemical characteristics of boninite and tholeiite-series volcanic rocks from the Mariana forearc and the role of an incompatible element-enriched fluid in arc petrogenesis: Geological Society of America Special Paper, v. 215, p Hawkesworth C.J., O'Nions R.K., Pankhurst R.J., Hamilton P.J., Evensen N.M. (1977) A geochemical study of island-arc and back-arc tholeiites from the Scotia sea. Earth and Planetary Science Letters 36: Pearce J.A., Thirlwall M.F., Ingram G, Murton B.J., Arculus R.J., van der Laan S.R. (1992) Isotopic evidence for the origin of boninites and related rocks drilled in the Izu-Bonin (Ogasawara) forearc, Leg 125. Proceedings of the Ocean Drilling Program, Bonin Mariana region; covering Leg 125 of the cruises of the Drilling Vessel JOIDES Resolution, Apra Harbor, Guam, to Tokyo, Japan, sites , 15 February April : Jakes P, Gill J (1970) Rare Earth Elements and the Island Arc Tholeiitic Series. Earth and Planetary Science Letters 9:17-28 Saha A, Basu A.R., Wakabayashi J, Wortman G.L. (2005) Geochemical Evidence for Subducted Infant Arc in Franciscan High-grade Tectonic Blocks. Geological Society of America Bulletin 117(9/10): Sun, S., -S., and McDonough, W. F., 1989, Chemical and isotopic systems of ocean basalts: Implications for mantle composition and processes, in Saunders, A. D., and Norry, M. J., eds., Magmatism in Ocean Basalts, Geol. Soc. London Spec. Publ., p Tatsumi Y, Eggins S (1995) Subduction Zone Magmatism, vol. Blackwell Sciences, Oxford, p 211. Taylor, R. N., and Nesbitt, R. W., 1998, Isotopic characteristics of subduction fluids in an intra-oceanic setting, Izu- Bonin Arc, Japan: Earth and Planetary Science Letters, v. 164, p Wakabayashi J (1999) Subduction and the rock record: Concepts developed in the Franciscan Complex, California. In: Sloan D, Moores E.M., Stout D (eds) Classic Cordilleran Concepts: A View From California, vol 338. Geological Society of America Special Publication, pp This work is supported by a NSF-EAR grant