Transcript Ghatak_franciscan-07 - MARGINS

IBM – type Arc Protoliths for the High Grade Blocks of the Franciscan Complex, California
Arundhuti Ghatak, University of Rochester, Rochester, NY-14627 ([email protected])
Asish R. Basu, University of Rochester, Rochester, NY-14627 ([email protected])
(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 Additional unpublished geochemical data on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 highgrade block locality in the bottom right of figure.
Initial eNd
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 23, see legends in figures 4-6.
Initial 206Pb/204Pb
Figure 3. Multiple trace element concentrations normalized over NMORB 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 Pbenrichments of the high-grade rocks are noteworthy, which indicate
arc protoliths.
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
(http://georoc.mpchmainz.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).
Figure 5 (left). Initial 87Sr/86Sr and eNd 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 highgrade rocks fall within or below the field of arc tholeiite and are distinctly different from
the two coherent schists that have higher eNd and fall in the field of present day MORB.
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.
87
86
Initial Sr/ Sr
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. 151-164.
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 36:253-262
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 778-786, 15 February 1989-17 April 1989 125:237-261
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):1318-1335.
This work is supported by a NSF-EAR grant
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.
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. 313-345.
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, IzuBonin Arc, Japan: Earth and Planetary Science Letters, v. 164, p. 79-98.
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 123-133.