IBM

1
IBM type Arc Protoliths for the High Grade
Blocks of the Franciscan Complex,
California Arundhuti Ghatak, University of
Rochester, Rochester, NY-14627 (arun_at_earth.rochest
er.edu) Asish R. Basu, University of Rochester,
Rochester, NY-14627 (abasu_at_earth.rochester.edu) (T
his 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 Additional unpublished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.
Initial 208Pb/204Pb
Initial 207Pb/204Pb
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.
Initial 206Pb/204Pb
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 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.mpch-mainz.gwdg.de/Ent
ry. 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 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.
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 high-grade 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.
Initial eNd
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.
Initial 87Sr/86Sr
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
36253-262 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 125237-261 Jakes P,
Gill J (1970) Rare Earth Elements and the Island
Arc Tholeiitic Series. Earth and Planetary
Science Letters 917-28 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. 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
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Izu-Bonin 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.
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.
This work is supported by a NSF-EAR grant