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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.

D105

--1--*0 1•.--11

1 1 IIi >1 LFIrLandslide IDZr<<

area J oA :.::.11•*reIW<Sandstone J f

---XSSSs. 4:1•1•2'Dv•<S>C>>·242tl-j*•\..0:1•T <EA##»'1------t--J-----»2=-\

E/&83Graywacke, shale, andconglomerate

Greenstone

1 1Serpentine..r

Gneissic glaucophane schist( tectonic blocks, type IV)

\ ii-'30" •\X7•0%

1--2--- __ --- --

, myM E-<WHkDOn

00 Lnnzl | ALD - 1---1 | Sample locality< 0 Bedrock metamorphosed to |0 W glaucophane schist facies

(includes type III) •---Contact-I.-0.-I

FaultDallhed where approzimately located

Geology by E. H. Bailey, M. C. Blake, J. 0. Berkland,J. W, Stewart, and C. Wentworth

FIGURE 142.1.-Geologic map of area surrounding Ward Creek, Cazadero, Calif.

0S0

\0 R. 11 W 4

9

\\ 1 - -f-\ --- \ ----4.\\\ 1 --4-A --AL ----E- Creek• 38 ° 32'30" / .=5-ZZ---_.. ---1.-) -- -9 ..--* --/m,-17-- + --_Warsl -- 1.1----·-i »\•

0 14 \----------A-- -- 0. \\\

.3 13 00.4 4=-• 3 33•# faa:16

*T 8 N. %. /. . 4.- -.t- ----1I �25451> --- • --ir..: r \ / '.*...'<\:, *•"It ..1 - -I -------- e .6 1-2•/In/2-I- <azade •--- - -7 --72 3.S=S=- , :2 %-f------ --1-r .-

••=3• -- - 0. 04-- - ----» --6=4•2»-. ---------i,tr

-0..+% 1---- (« W-- 0---Ad---t'---*-- ----th--- - ---0,--- ----- -2,27 -0. rd0-- \-- 1-- ./ t-1

li-4 \

- 0%' §

23\ J

--

«, - 24-

-11.\ (

-20

al C0 1 MILEEXPLANATION

07 15xArea of * I.report O \.4 .\+ '', \1 1

-1

LEE, THOMAS, MARVIN, AND COLEMAN D107

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.)

A____-_-_____ 55-CZ-59--______ Type-IV metabasalt- --_--- -_- 8.81 8.92 0.0805 , 94 0.00902 150(9_---_----_-- 27-CZ-60.-_----- Type-IV metabasalt.__ 9.96 10.09 .0858 86 . 00850 140B__._.-_--___ 50-CZ-60_ Type-IV metabasalt_ - -_____,_ - 9.06 9.18 . 0750 88 . 00817 135A____________ 201-RGC-59I_____ Type-III metashale-__-____-___ 8.41 8.52 .0694 94 . 00815 135A-___-_-____- 26-CZ-59________ Type-III metashale_______ ___ 7.62 7.72 .0596 92 . 00772 130

Map locality Field No. Host rock 2 Rb87 Normal Sr *Sr87 *Sr87 *SrE:/Rb" Age(flg. 142. 1) (Ppm) (Ppm) (ppin) (percent) (in.Y.)

A--_-__ ___-_ 55-CZ-59-_______ Type-IV metabasalt__.....____ 73:73 21.8 0.164 9.7 0.02224 150