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NOTICE CONCERNING COPYRIGHT RESTRICTIONS
This document may contain copyrighted materials. These materials have been made available for use in research, teaching, and private study, but may not be used for any commercial purpose. Users may not otherwise copy, reproduce, retransmit, distribute, publish, commercially exploit or otherwise transfer any material.
The copyright law of the United States (Title 17, United States Code) governs the making of photocopies or other reproductions of copyrighted material.
Under certain conditions specified in the law, libraries and archives are authorized to furnish a photocopy or other reproduction. One of these specific conditions is that the photocopy or reproduction is not to be "used for any purpose other than private study, scholarship, or research." If a user makes a request for, or later uses, a photocopy or reproduction for purposes in excess of "fair use," that user may be liable for copyright infringement.
This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law.
Article 142
ISOTOPIC AGES OF GLAUCOPHANE SCHISTS
FROM THE AREA OF CAZADERO, CALIFORNIA
By D. E. LEE, 1 H. H. THOMAS, 2 R. F. MARVIN,2 and R. G. COLEMAN, 1
1 Menlo Park, Calif.,2 Washington, D.C.
Abstract.-Glaucophane-bearing tectonic blocks are strati- Coarsely crystalline gneissic schists Itype IV] that occur as large
graphically out of place with respect to the glaucophane-bearing isolated blocks; these blocks cannot be mapped as in place or as
bedrock terrain on which they rest. Five isotope age de- stratigraphically equivalent to the bedrock terrain on whichterminations ranging from 130 to 150 m.y. indicate that both the they rest. Some of the minerals present along with glaucophane
tectonic blocks and the bedrock schists were recrystallized as and muscovite in these rocks are epidote, rutile, and almandine-
part of a Late Jurassic and Early Cretaceous metamorphic event. grossular garnet. The mineral assemblages present in thesetectonic blocks indicate that they were recrystallized at higher
pressure-temperature conditions than were the bedrock schists.
Near Cazadero these tectonic blocks are concentrated in a bandField study has shown that muscovite-bearing that conforms to some of the major faulting in the area.
glaucophane schists are present in two different struc-
tural positions within the Franciscan Formation in the The K-Ar and Rb-Sr ages of muscovite from
area of Cazadero, Sonoma County, Calif., north of the glaucophane schists near Cazadero (fig. 142.1) are
San Francisco area (fig. 142.1) ( Coleman and Lee, listed in the accompanying table. Potassium was
1963). Glaucophane-bearing tectonic blocks are strati- determined by a flame photometer using lithium as an
graphically out of place with respect to the glaucophane- internal standard. Rubidium and strontium were
bearing bedrock terrain on which they rest. Labora- determined by isotope-dilution techniques described
tory study sliows also that the tectonic blocks can be by Goldich and others ( 1961, p. 8-35). The overall
distinguished mineralogically from the bedrock terrain, analytical error is approximately zE 5 percent of the
eveii tliougli glaucophane and muscovite are present quoted age value.
iii both (Coleman and Lee, 1963; Lee and others, 1963).Although type-III glaucophane schists may be dis-
This article presents isotope-age data determined for tinguished from those of type 1V, both on the basis ofmuscovites from three samples of tectonic blocks
mineralogy and on the basis of field occurrence, the(type-IV rocks of Coleman and.Lee) and two samples of isotope-age results listed in tlie table indicate that both
bedrock schist (type-III rocks).
Franciscan rocks designated types III and IV aretype-III and type-IV rocks were recrystallized tls part
described by Coleman and Lee (1963) as follows:of a metamorphic event that took place during Late
Jurassic and Early Cretaceous time (Holmes, 1960).Bedrock glaucophane schists [type III] as exemplified by a
conformable sequence of metabasalts and metasediments ex-Pending further isotope-age work on these racks, it
posed at the bottom of Ward Creek canyon near Cazadero, appears that some of the glaucophane schists in this
California. Some of the minerals present along with glauco- area represent the metamorphic equivalents of volcianicpliane and muscovite in these rocks are aragonite (Coleman and
Lee, 1962), lawsonite, and almandine-spessartine-grossularand sedimentary rocks that were laid down during pre-
garnet. Cretaceous time.
ART. 112 IN U.S. GEOL. SURVEY PROF. PAPER 475-D, PAGES, D105-D107. 1964.
Analytical data and isolopic ages of muscovite from glaucophane schists in the area of Cazadero, Calif.
[Analysts: Potassium-argonage, H. II. Thomas, R. F. Marvin, Paul Elmore, and Ivan Barlow; rubidium-strontium age, F. G. Walthall and W. D. Long]
Const ants : IOoke- 0.585 X 10-10/yearK'OX# -4.72 X 10-10/yearK40-1.22 X1O-4 gm/gm K
Symbol: * radiogenic.
Potassium-argon age
Rubidium-argon age
Rb'TAB- 1.47 X 10-11/yearRb'7-0.283 gm/gm Itb
1 Near Momtt Ranch Road, SWM' sec. 11, T. 10 N., R. 12 W., Tombs Creek quadrangle, TH-minute series, California.2 TYpe-III rocks are bedrock glaucophane schists; type-IV rocks are tectonic blocks (Coleman and Lee, 1963).
REFERENCES
Coleman, R. G., and Lee, D. E., 1962, Metamorphic aragonite
in the glaucophane schists of Cazadero, California: Am.
Jour. Sci., v. 260, p. 577-595.
-- 1963, Glaucophane-bearing metamorphic rock types of
the Cazadero area, California: Jour. Petrology, v. 4, p.
260-301.
716-026 0-64----8
R
Goldich, S. S., Nier, A. 0., Baadsgaard, H., Hoffman, J. H., and
Krueger, H. W., 1961, The Precambrian geology and
geochronology of Minnesota: Minnesota Geol. Survey
Bull. 41, 193 p.
Holmes, Arthur, 1960, A revised geological time-scale:
Edinburgh Geol. Soc. Trans., v. 17, pt. 3, p. 183-216.
Lee, D. E., Coleman, R. G., and Erd, R. C., 1963, Garnet typesfrom the Cazadero area, California: Jour. Petrology,
v. 4, no. 3, p. 460-492.
Map locality Field No. IIost rock 2 KIO Kdo *Ardo Ario *Ario/Kto Ago( fig. 142.1) (percent) (ppni) (percent) Oil.y.)