International Geology Review, Vol. 49, 2007, p. 873–906.
Copyright © 2007 by V. H. Winston & Son, Inc. All rights reserved.
40Ar/39Ar
Ages from Coherent, High-Pressure Metamorphic
Rocks of the Franciscan Complex, California:
Revisiting the Timing of Metamorphism of the
World’s Type Subduction Complex
JOHN WAKABAYASHI1
Department of Earth and Environmental Sciences, California State University, 2576 E. San Ramon Ave.,
Fresno, California 93740
AND TREVOR
A. DUMITRU
Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305
Abstract
In the Franciscan subduction complex of California, high-pressure metamorphic rocks have
generally been divided into two groups, “high-grade blocks” and “coherent metamorphic rocks.”
High-grade blocks, typically discrete blocks centimeters to tens of meters in diameter, exhibit
coarse metamorphic minerals and relatively high metamorphic grade, including blueschist, eclogite,
and amphibolite assemblages, and are typically found encased within mélange matrix of distinctly
lower grade. Coherent metamorphic rocks are generally of epidote blueschist and lower grade and
consist of large, more or less intact thrust sheets of metasedimentary and metavolcanic rocks.
Considerable age dating work has been completed on the high-grade blocks, but dating the metamorphism of the coherent units has proven very difficult because of the very fine grain size of their
metamorphic minerals, which has precluded the preparation of pure mineral separates for isotopic
analysis. In this paper, we report new white mica and amphibole 40Ar/39Ar step heating ages from
mineral separates from several coherent units. These new ages include: (1) white mica ages of 120.5
Ma from the South Fork Mountain Schist; (2) white mica ages of approximately 132 Ma from the
Skaggs Springs schist; (3) a white mica age of approximately 154 Ma from the coherent schists of
Ward Creek; and (4) white mica ages of 146.5 Ma (eclogite) and 134 Ma (epidote blueschist) from
the Willow Spring slab, a newly recognized 2.1 by 1.2 km (map view) coherent metabasite slab in the
Panoche Pass area comprising rocks ranging from lawsonite blueschist to eclogite. We also obtained
hornblende ages of about 156 and 159 Ma, respectively, from high-grade amphibolite blocks from
McLaren Park and the Hunters Point Shear Zone in San Francisco. A review of the new and previous
ages indicates that the ages of the high-grade blocks and the coherent metamorphic rocks partially
overlap, strongly suggesting they formed in the same subduction zone. Moreover, our field mapping
near Panoche Pass has identified several fault-bounded sheets of coarse-grained, coherent metamorphic rocks similar to the Willow Spring slab that display mineralogy identical to that of many highgrade blocks.
Unlike many of the world’s subduction-related metamorphic terranes, the Franciscan exhibits
evidence for high-pressure metamorphism that spanned at least 80 m.y. in time, the product of subduction, accretion, and metamorphism of different packets of rock at different times. Although the
Franciscan may be an exception among subduction complexes, attempts to define a single age of
metamorphism in other high-pressure or ultrahigh-pressure terranes may need reconsideration.
Introduction
SINCE THE ADVENT of plate tectonic theory, many
have regarded the Franciscan Complex of coastal
1Corresponding
author; email: jwakabayashi@csufresno.edu
0020-6814/07/956/873-34 $25.00
California as the type subduction complex and as a
benchmark locality for high-pressure/low-temperature (HP) metamorphic rocks, the metamorphic
signature of the subduction process (e.g., Hamilton,
1969; Ernst, 1970). Although many geochronologic
studies have been conducted on Franciscan rocks,
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WAKABAYASHI AND DUMITRU
significant questions remain regarding the timing of
metamorphism of much of the Franciscan. These
questions leave uncertain the relationship between
tectonic events in the Franciscan and other parts of
the arc-trench system, and affect the accuracy of
forearc evolution and HP rock exhumation models
derived from this well-known region.
Within the Franciscan, only blueschist and
higher-grade rocks contain newly formed metamorphic minerals that can potentially be used to date
metamorphic recrystallization (e.g., Blake et al.,
1988). Rocks of such grade make up more than onefourth of the exposed Franciscan (e.g., Blake et al.
1988; Terabayashi and Maruyama, 1998), but in
almost all cases the metamorphic minerals are so
fine grained that it is impossible to separate and
prepare pure mineral concentrates for isotopic analysis. However, a very tiny fraction of Franciscan
rocks do indeed have sufficiently coarse metamorphic minerals for analyses to be attempted (Fig. 1).
The highest-grade metamorphic rocks are coarsegrained blueschists, eclogites, and amphibolites
that occur almost entirely as exotic tectonic blocks
encased within lower-grade shale- or serpentinitematrix mélanges, and are commonly referred to as
“high-grade blocks.” Collectively, such high-grade
blocks comprise <<1% of Franciscan metamorphic
rocks (Coleman and Lanphere, 1971). The remainder of Franciscan metamorphic rocks consist of
intact thrust sheets and mélange matrix, primarily of
blueschist grade. Such rocks exhibit fine-grained
metamorphic minerals and, excluding mélange
matrix, are commonly referred to as “coherent”
metamorphic rocks, to distinguish them from the
high-grade blocks.
Overall, various isotopic dating techniques (KAr, Rb/Sr, 40Ar/39Ar, U-Pb, Sm-Nd, and Lu-Hf) have
yielded a range of model ages of metamorphic
recrystallization from about 90 to about 170 Ma,
with most of the older ages coming from the highgrade blocks. Unlike many HP and ultrahighpressure (UHP) terranes, where metamorphism is
interpreted to have been of limited duration (e.g.,
Maruyama et al., 1996), metamorphic and detrital
age data from the Franciscan indicate an extended
duration of Franciscan HP metamorphism (e.g.,
Wakabayashi, 1992, 1999a).
The geochronologic methods used on high-grade
blocks have included K-Ar on actinolite, hornblende, and white mica (phengite) (Coleman and
Lanphere, 1971; Suppe and Armstrong, 1972); UPb isochron (Mattinson, 1986); 40Ar/39Ar step heat-
ing of hornblende, glaucophane, and white mica
(Ross and Sharp, 1986, 1988); 40Ar/39Ar laser step
heating of white mica (Wakabayashi and Deino,
1989); 40 Ar/ 39 Ar laser spot ages of white mica
(Catlos and Sorenson, 2003); and U-Pb, Sm-Nd, and
Lu-Hf isochron analysis of garnet-bearing assemblages (Mattinson, 1986; Anczkiewicz et al., 2004).
With few exceptions, exceedingly fine metamorphic
grain size has thus far precluded application of
these methods to coherent metamorphic rocks.
Nearly all ages obtained from coherent metamorphic
rocks have been K/Ar or total fusion 40Ar/39Ar ages
of whole-rock samples, where most of the K resides
within very fine grained white mica (Suppe and
Armstrong, 1972; Lanphere et al., 1978; McDowell
et al., 1984). The few exceptions include two K-Ar
ages and one laser step-heating 40Ar/39Ar age from
white mica separated from the coherent blueschists
of Ward Creek (Lee et al., 1964; Wakabayashi and
Deino, 1989), and a U-Pb isochron dating blueschist metamorphism of a mafic intrusion into
graywacke in the Diablo Range (Mattinson and
Echeverria, 1980). We also consider several ages
from earlier studies to be from coherent rocks,
rather than high-grade blocks as originally interpreted, based on new field evidence and/or interpretations presented in this paper.
The whole-rock ages from the coherent units
have generated two divergent interpretations that
reflect fundamentally different views of Franciscan
geochronology. One view regards the temporal clustering of isotopic ages to best represent the metamorphic crystallization age of a unit (e.g., Lanphere
et al., 1978). In this interpretation, the younger
tail of the age distribution reflects partial loss of
radiogenic daughter products, whereas the older tail
may result from uptake of unsupported daughter
products (excess argon) or radiogenic argon (premetamorphic) from detrital minerals. The second
view regards all ages as products of post-metamorphic cooling, with the densest cluster of ages representing the time when most of the unit cooled
through the effective closure temperature (Suppe
and Armstrong, 1972; McDowell et al., 1984).
Because the second interpretation regards measured
ages as post-metamorphic cooling ages, it implies a
much older timing of coherent metamorphism than
the first view.
These two interpretations have spawned significantly different opinions concerning the relationship between high-grade block and coherent
metamorphism in the Franciscan: (1) Metamorphism
FRANCISCAN COMPLEX, CALIFORNIA
875
FIG. 1. Distribution of Franciscan and other basement rocks of central and northern California, showing Franciscan
rocks of different metamorphic grades and sample localities discussed in this paper. Map modified from Wakabayashi
(1999).
recorded in the high-grade blocks occurred roughly
20 million years or more before the oldest coherent
blueschist metamorphism and was unrelated to
Franciscan subduction (e.g., Coleman and Lanphere, 1971; Blake et al., 1988). (2) The timing of
metamorphism in the high-grade and coherent
metamorphic terranes overlap, suggesting that both
formed during the same subduction event (Suppe
and Armstrong, 1972; Platt, 1975; Cloos, 1985,
1986).
It is clear that Franciscan geochronology has
been hindered by the paucity of isotopic ages from
mineral separates from coherent metamorphic
rocks, as well as the difficulties inherent in compar-
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WAKABAYASHI AND DUMITRU
ing isotopic ages produced using a variety of different methods with different underlying assumptions,
closure temperatures, and accuracies. This paper
presents new resistance furnace 40Ar/39Ar incremental heating data from 11 samples of coherent
metamorphic rocks, plus 5 other Franciscan samples. Following the presentation and discussion of
the data, we will discuss how the new results suggest
a partial overlap in metamorphic age between the
high-grade blocks and the coherent terranes, and
thus favor metamorphism of both in the same evolving subduction zone. We will conclude by discussing Franciscan metamorphic geochronology in terms
of general HP and UHP metamorphic processes and
geochronology.
Sample Descriptions
Almost all Franciscan HP coherent rocks are too
fine grained to allow preparation of mineral separates of requisite purity for isotopic dating. White
mica of phengitic composition, the key K-bearing
metamorphic phase, is common in the coherent
units, but nearly in all cases as grains smaller than
0.03 mm. At such sizes, separating white mica from
the chlorite present in essentially all white micabearing rocks is virtually impossible owing to
similar magnetic properties and densities of the two
mineral species and the extreme difficulty of hand
picking such small grains.
Based on review of previous studies and our own
field experience, several Franciscan coherent blueschist-facies units are probably totally devoid of
rocks of sufficient metamorphic grain size to permit
the preparation of mineral separates. These units
include the Yolla Bolly, Eylar Mountain, and Burnt
Hills terranes. However, we were able to identify
and collect analyzable samples from several of the
most strongly recrystallized and structurally highest
coherent Franciscan units. These units included the
South Fork Mountain Schist of the Pickett Peak
terrane, the Skaggs Springs schist, the Ward Creek
coherent schist, and the Willow Spring slab near
Panoche Pass (Fig. 1). The coherent rocks at Willow
Spring are particularly interesting, inasmuch as they
are mineralogically equivalent to many high-grade
blocks and even include eclogite assemblages.
In addition to sampling coherent rocks, we have
also obtained data from a garnet amphibolite highgrade block from the Hunters Point shear zone, a
gabbro from the Hunters Point shear zone, an
epidote amphibolite slab of ambiguous field rela-
tions from McLaren Park in San Francisco, a garnet
blueschist from Glaucophane Ridge near Panoche
Pass, and the Pilarcitos gabbro in San Mateo
County.
The petrography of the samples and their field
relations are briefly described below. Because the
Willow Spring and McLaren Park localities have not
been previously described, we provide additional
explanation of their field relations. Grain sizes given
below refer to the maximum dimension of the largest
metamorphic mineral grains observed.
South Fork Mountain Schist
The South Fork Mountain Schist samples were
collected along the road to Tomhead Mountain from
the structurally highest of the thrust sheets of the
unit, according to the detailed mapping of Worrall
(1981) (Figs. 1 and 2). The metamorphic grade of
this unit is epidote blueschist (Brown and Ghent,
1983; Blake et al., 1988). We selected our samples
from extremely rare mm-scale lenses or selvages
where mica or amphibole grain size is atypically
large. Presumably, local compositional and/or fluid
anomalies at these sites allowed growth of somewhat
larger grains.
Samples SFM-4c and SFM-13 consist of white
mica (to 0.1 mm and 0.2 mm, respectively) separated from very quartz-rich schists that also contain
chlorite and hematite (Fig. 3). These rocks correspond to the “cgs” map unit of Worrall (1981), as
shown on Figure 2, and are almost certainly metacherts. Sample SFM-3 consists of sodic amphibole
(to 0.5 mm) separated from a manganiferous
metachert that consists primarily of quartz, with
sodic amphibole, stilpnomelane, hematite, and
minor white mica (to 0.05 mm) (Fig. 3). This rock
corresponds to the “bgs” map unit of Worrall (1981).
Sample SFM-1x consists of white mica (to 0.15 mm)
from a mica-rich selvage in a metavolcanic rock
whose other metamorphic minerals include actinolite, sodic amphibole, epidote, albite, acmitic clinopyroxene, and chlorite (Fig. 3).
Skaggs Springs schist
The Skaggs Springs schist consists of completely
recrystallized metagraywacke. The schist crops out
in the Diablo Range and northwestern Coast Ranges
and occupies a structurally high position within the
Franciscan (Fig. 4) (Wakabayashi, 1992, 1999b).
This unit generally features coarser metamorphic
minerals than equivalent lithologies in the South
Fork Mountain Schist, although the occurrence
FRANCISCAN COMPLEX, CALIFORNIA
877
FIG. 2. Geologic map of the South Fork Mountain Schist and adjacent units in the Tomhead Mountain area, simplified from Worrall (1981). Sample localities indicated by stars. Abbreviations: Non-Franciscan: Jcro = Coast Range
ophiolite; Jkgv = Great Valley Group; Q = Quaternary deposits, undifferentiated. Franciscan: sfs = South Fork Mountain
schist (SFM) metaclastic rocks; mv = SFM metavolcanics, structurally highest; mvl = as above, except structurally lower;
cgs = SFM chlorite-bearing metachert; bgs = SFM blue amphibole-bearing metachert; vs = Valentine Springs Formation.
Geology simplified from Worrall (1981).
of white mica of separable grain size is rare. The
largest exposure of the Skaggs Springs schist is a 70
km long belt in the western Coast Ranges (Figs. 1
and 4). Two mineralogically similar samples (FR-1
and NS-10) were collected from this belt (Fig. 4).
NS-10 has some of the coarsest metamorphic minerals of the Skaggs Springs schist, but the white mica
grain size is comparable to that found in the South
Fork Mountain Schist (to 0.2 mm). Its mineralogy
includes sodic amphibole (to 2 mm), lawsonite (to 0.
5 mm), quartz (to 1 mm), albite (to 1 mm), and white
mica (Fig. 5). Sample FR-1 is similar but with somewhat finer metamorphic grain sizes, except for the
white mica (to 0.2 mm). Jadeitic clinopyroxene is
found in a number of localities within the Skaggs
Springs schist, but not these specific samples.
Ward Creek
The Ward Creek locality comprises a well-known
coherent schist exposed in Ward Creek near Cazadero (e.g., Coleman and Lee, 1963; Maruyama and
Liou, 1988). This coherent unit appears to be structurally high and is faulted against or imbricated with
Skaggs Springs schist and other units. The field
relations in the Ward Creek area are more complex
than the simplified structure shown in Figure 4, and
may include metabasite slabs that structurally overlie and underlie the Skaggs Spring schist (Erickson,
1991). Our sample WC-1 is a metachert containing
abundant quartz, sodic amphibole (to 1 mm), white
mica (to 0.7 mm; many grains in the 0.5 mm range),
and spessartine garnet (Fig. 5).
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WAKABAYASHI AND DUMITRU
FIG. 3. Photomicrographs of South Fork Mountain Schist samples from the Tomhead Mountain area. Plane-polarized
light. Abbreviations: am = sodic amphibole; q = quartz; wm = white mica.
Willow Spring slab near Panoche Pass
Four samples were collected from different
structural levels of a fault-bounded sheet of coherent schist in the vicinity of Willow Spring in the
Panoche Pass area (Figs. 1 and 6). To distinguish
this unit from other localities in the Panoche Pass
area, we will refer to it as the “Willow Spring slab.”
The mineralogy and grain size of the rocks of this
slab are similar to those found in many high-grade
blocks. Numerous artificial exposures afforded by
dirt roads and their roadcuts, as well as good natural
exposures on steep slopes, enable one to ascertain
that the outcrops observed comprise part of a coherent sheet rather than blocks-in-mélange.
The Willow Spring slab was originally mapped as
the largest of several coherent metabasite units in
the Panoche Pass region by Ernst (1965), although
that mapping, conducted over a much larger area
and without the aid of the numerous recent artificial
exposures, did not reveal the highest-grade assem-
blages of the slab. The fact that formal recognition of
the mélange concept in the Franciscan (e.g., Hsü,
1968) followed three years after the publication of
Ernst’s paper may have led some to believe that the
field relations depicted by Ernst (1965) were
mélange rather than coherent. The metabasite sheet
has a map dimension of 2.1 km by 1.2 km and an
estimated structural thickness of 200 to 300 m (Fig.
6). Metabasites comprise the bulk of the sheet, but
minor metacherts and meta-ultramafic rocks also
occur. The sheet has an apparent internal inverted
metamorphic gradient, with lawsonite blueschist
structurally low, passing upward through epidote
blueschist, garnet blueschist with local fine-grained
eclogite (grain size tenths of mm), to coarse eclogite
(grain size several mm) at the structurally highest
level (Fig. 6). It is likely that this internal metamorphic gradient is a consequence of internal faulting
separating thin sheets of different metamorphic
FRANCISCAN COMPLEX, CALIFORNIA
879
FIG. 4. Geologic map of the Skaggs Springs schist and Ward Creek areas, with sample localities (stars). Abbreviations: Franciscan: fcb = Coastal Belt; fg = metabasalt, blueschist facies, non-schistose; fml = mélange; fr = Rio Nido
terrane. Other: ogv = Great Valley Group and Coast Range ophiolite. Modified from Wakabayashi (1992).
grade, but the exposures are not sufficient to identify discrete faults separating internal subunits.
The mafic slab appears to rest on a stack of faultbounded sheets of metagraywacke (Fig. 6). The
structurally highest metagraywacke unit is
completely recrystallized and mineralogically and
texturally identical to the Skaggs Spring schist
described above. It overlies a foliated metagraywacke that retains clastic texture (commonly
called textural zone TZ2; e.g., Blake et al., 1988)
and whose metamorphic minerals include jadeitic
clinopyroxene, lawsonite, and sodic amphibole.
This metagraywacke overlies a graywacke that lacks
a penetrative fabric and jadeitic clinopyroxene but
has lawsonite. This structurally lowest and lowest
grade unit appears be the most areally extensive
unit in the area surrounding the metabasite slab.
We extracted white mica from sample Diab-107,
a coarse eclogite from the structurally highest part of
the metabasite slab (Fig. 6). This coarse-grained (to
3 mm in metamorphic grain size) sample consists of
omphacite, garnet, barroisite, epidote, white mica
(to 2 mm), and rutile with some overprinting growth
of sodic amphibole and titanite rimming rutile (Fig.
7). Near the Diab-107 eclogite outcrop, a coarsegrained actinolite–chlorite–white mica rock crops
out, from which we separated actinolite. This rock
appears to be identical in mineralogy to metasomatic reaction rinds found around many high-grade
blocks, and it may have formed during metasomatic
reactions between the metabasite and intercalated
selvages of ultramafic rock. Alternatively, the structurally highest part of the metabasite slab, including
the coarsest eclogites, may be a serpentinite matrix
mélange. We believe, however, that the progressive
increase in metamorphic grade with structural
level within the metabasite slab (Fig. 6), and the
occurrence of similar actinolite-bearing schists
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WAKABAYASHI AND DUMITRU
FIG. 5. Photomicrographs of Skaggs Springs schist sample NS10 and Ward Creek sample WC-1. Plane-polarized
light. Abbreviations: am = sodic amphibole; gt = garnet; law = lawsonite; q = quartz; wm = white mica.
associated with mappable, but volumetrically minor,
sheets of meta-ultramafic rocks at structurally lower
levels in the metabasite slab, supports the former
scenario.
Sample Diab-33 is actinolite separated from a
rock composed of actinolite (to 2 mm), chlorite, and
white mica that may be part of a reaction zone
between metabasite and a narrow (probably a few
meters thick) zone of meta-ultramafic rocks that
crops out at structurally lower level than the metasomatic rock sample Diab-107, described above. The
metabasites in this part of the slab are mainly garnet
blueschist, with some rocks composed primarily of
fine (up to 0.5 mm) omphacite, epidote, chlorite, and
rare garnet (fine-grained eclogites) (Fig. 6).
Sample Diab-41fn is white mica (to 0.5 mm in
size) from an epidote blueschist, which includes
sodic amphibole, lawsonite, epidote, and chlorite.
As is typical of the epidote blueschists of the Willow
Spring slab, lawsonite occurs as texturally late
poikiloblasts overgrowing earlier-formed minerals,
including epidote. Similar poikiloblastic lawsonite
is common in high-grade blocks throughout the
Franciscan.
Glaucophane Ridge
Glaucophane Ridge is east of Panoche Pass,
some 14 km ENE of the Willow Spring area. Glaucophane Ridge includes one of several rock bodies
in the southern Diablo Range that the Jennings
(1977) state geologic map depicts as coherent
schists. In this region, we investigated several of
the schist bodies indicated on the state map; these
coarse blueschists appeared to be coherent slabs
rather than blocks-in-mélange. The Willow Spring
slab described above is one such unit. Unlike the
FRANCISCAN COMPLEX, CALIFORNIA
881
FIG. 6. Geologic map of the Willow Spring slab, Panoche Pass area, with sample localities. Mapping based on this
study except where noted.
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WAKABAYASHI AND DUMITRU
FIG. 7. Photomicrograph of eclogite sample Diab-107 from the Willow Spring slab. Plane-polarized light. Abbreviations: bar = barroisite; chl = chlorite; ep = epidote; gl = glaucophane; gt = garnet; om = omphacite; rt = rutile; wm =
white mica.
demonstrably coherent relations of the Willow
Spring slab, our limited reconnaissance of Glaucophane Ridge indicates that our garnet blueschist
sample Diab-4 may come from either a mélange
block or a coherent slab. If the latter, the slab is less
than 500 m in long dimension, with the alternative
interpretation being much smaller blocks set in
shale matrix. The blueschists on the ridge occur
along the contact between Franciscan rocks on the
west and Great Valley Group rocks on the east, and
exposures of shale matrix mélange occur within a
hundred meters of where we collected Diab-4. We
separated white mica (to 0.5 mm) from sample
Diab-4. Suppe and Armstrong (1972) dated two
samples from Glaucophane Ridge, but insufficient
location information is provided in their paper to
determine whether they sampled the same outcrop
as Diab-4.
Epidote amphibolite of McLaren Park
A sheet of epidote amphibolite crops out in the
McLaren Park area of San Francisco (Wakabayashi,
1990, 1992) (Fig. 8). This sheet appears to be at
least 1 km long by 0.3 km wide in map view. It is
bordered by a shear/mélange zone on its southern
side, whereas the northern contacts are not exposed.
The southern mélange zone is comparatively small
(up to 100 m wide in map view) compared to the size
of the slab. Graywacke, shale, chert, and basalt of
the coherent, prehnite-pumpellyite facies Marin
Headlands terrane crop out beyond the immediate
borders of the slab. The nearest regional-scale
mélange zone is the City College fault zone, a shalematrix mélange with a map width that exceeds a
kilometer locally. The northern border of the City
College fault zone passes as close as 250 m to the
southern limit of the mélange that bounds the southern edge of the epidote amphibolite slab (Fig. 8).
Thus, the general field relations of the epidote
amphibolite do not appear to be that of a typical
block in mélange. It is possible the slab may represent a tectonic sliver or large block emplaced along
the northern margin of the City College fault zone, a
window into the City College fault zone, or a klippe
of the structurally lowest part of the Hunters Point
shear zone. The largest dimension of high-grade
blocks found in either of the two regional shear
zones above are less than a tenth of the size of the
epidote amphibolite.
The epidote amphibolite body consists mainly of
strongly foliated metabasite (epidote amphibolite)
with some layers of metachert and rare meta-ultramafic rocks. The epidote amphibolite mineralogy
includes hornblende (to 1 mm), epidote, chlorite,
albite, quartz, iron oxides, and apatite. The metachert consists of quartz, hornblende, stilpnomelange, hematite, and apatite (Fig. 9). Rare
riebeckitic amphibole rims hornblende in one of 10
thin sections examined of the metachert. No petrographic evidence of a blueschist-facies overprint
FRANCISCAN COMPLEX, CALIFORNIA
883
FIG. 8. Geologic map of part of San Francisco, with sample localities (stars). Geology from Schlocker (1974), Bonilla
(1971), Wakabayashi (2004), and this study.
was observed in any other thin section of either
metabasite or metachert (collectively about 20 thin
sections). Some lawsonite-bearing graywacke blocks
are associated with the shear zone bounding the
southern edge of the epidote-amphibolite body,
along with a few blocks of fine-grained lawsonite
blueschist. The hornblende separate MP-1 was
taken from an epidote-amphibolite sample.
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WAKABAYASHI AND DUMITRU
FIG. 9. Photomicrographs of amphibolite samples MP-1 and HPSZ (meta). Plane-polarized light. Abbreviations:
ab = albite; am = hornblende; ep = epidote; gt = garnet.
High-grade block from Hunters Point shear zone
The Hunters Point shear zone consists of a shale
matrix mélange structurally overlain by a comparatively coherent sheet of serpentinite (Wakabayashi,
1990, 2004) (Fig. 8). Garnet-amphibolite highgrade blocks with blueschist facies overprinting
occur within shale matrix mélange, or along the
border between the shale matrix mélange and
serpentinite at Baker Beach (Fig. 8). Sample HPSZmeta consists of tschermakitic hornblende separated from a garnet amphibolite whose metamorphic
minerals include calcic amphibole (to 3 mm),
garnet, inferred plagioclase (replaced by albite, lawsonite, and other fine-grained minerals), and titanite
(Fig. 9). The overprinting minerals include barroisitic hornblende, sodic amphibole, lawsonite, and
chlorite. Similar to the McLaren Park metabasite
body, the presence of metachert interlayers suggests
a basalt (rather than mafic plutonic rock) protolith
for the garnet amphibolite.
Hunters Point shear zone gabbro
and Pilarcitos gabbro
These two samples differ from any of those
above, in that neither exhibits HP subductionrelated metamorphism, and because of their
gabbroic protoliths. The Hunters Point shear zone
gabbro sample is taken from a gabbro lens (about 30
m in the long dimension) within the serpentinite
sheet of the shear zone (Schlocker, 1974). This is the
largest of several such gabbro lenses that occur
within the serpentinite sheet in the Potrero Hill area
of San Francisco. None of the gabbros exhibit blueschist-facies or other HP metamorphic assemblages.
The gabbros have a hornblende + plagioclase
assemblage defining a weak metamorphic(?) foliation, with variable retrogression of the plagioclase,
FRANCISCAN COMPLEX, CALIFORNIA
and local texturally late growth of fine pumpellyite
needles. Sample HPSZ-gabbro is greenish-brown
hornblende (to 1.5 mm) separated from a rock that
consists mostly of hornblende and plagioclase, with
some relict clinopyroxene, as well as variably retrogressed plagioclase, and fine-grained pumpellyite.
The Pilarcitos gabbro was collected from directly
east of Pilarcitos Dam in northern San Mateo
County, along the tectonic contact between the
Franciscan and the Salinian block (Wakabayashi,
1999b). Directly west of the gabbro are low-temperature mylonites derived from Paleocene sedimentary rocks of the Salinian block, whereas east of the
gabbro are prehnite-pumpellyite–grade metabasalts, clastic sedimentary rocks, and limestone of
the Franciscan Permanente terrane. The gabbro has
a “bleached” appearance in outcrop, probably
owing to the severe alteration of plagioclase, and the
fact that the mafic minerals (predominantly actinolite) are comparatively pale. Petrographically, the
rock consists primarily of severely altered plagioclase and pale green actinolite (to 1 mm), with some
relict clinopyroxene. The rock lacks a penetrative
fabric. All alternative exposures of this gabbro we
examined show various degrees of alteration. The
sample we dated consists of actinolite separated
from the gabbro.
40Ar/39Ar
Age Determinations
40Ar/39Ar
Below, we describe
resistance furnace
step-heating results from hornblende, actinolite,
sodic amphibole, and white mica from the abovedescribed localities in the Franciscan Complex.
Table 1 summarizes the preferred model ages for
each sample. McDougall and Harrison (1999)
provide an overview of the 40Ar/39Ar dating method.
Because of the high bulk closure temperature of
hornblende (525 ± 50°C; Harrison, 1981), hornblende ages from subduction environments are generally expected to closely approximate the time of
metamorphic recrystallization. However, difficulties
related to excess 40Ar and intergrown K-bearing
phases often arise in hornblende analyses. Such
problems may be amplified when K-poor amphiboles such as sodic amphibole and actinolite are
analyzed. Sodic amphiboles in particular commonly
yield problematic 40Ar/39Ar results (e.g., Ross and
Sharp, 1986; Baldwin and Harrison, 1989, 1992).
Consequently, careful examination of the data
systematics (age spectrum, K/Ca spectrum, inverse
isochron, etc.) is necessary to detect problematic
885
behavior and assess which model age (plateau,
total fusion, isochron, etc.) is geologically most
meaningful.
We assume that all of our white mica samples are
phengites, based on compositional data from previous studies of equivalent Franciscan HP rocks (e.g.,
Maruyama and Liou, 1988; Wakabayashi, 1990),
although we did not obtain compositional data from
our samples. The bulk Ar closure temperature of
phengite is not well known, but has been estimated
as ≈350–450°C (e.g., McDougall and Harrison,
1999). It may be lower in relatively fine grained
Franciscan rocks. Typically, phengite 40 Ar/39 Ar
ages are interpreted as recording cooling through
the phengite bulk closure temperature. However, in
the Franciscan Complex, peak metamorphic temperatures of some units may have been similar to or
cooler than closure temperatures, and the phengites
may instead date metamorphic crystallization. As
with amphiboles, phengite analyses may be complicated by excess 40Ar, as well as by multiple generations of mica or by 40Ar retained in incompletely
recrystallized detrital micas. Hence, close examination of data systematics, petrography (especially
textural relationships), and ideally petrology (especially estimates of the metamorphic temperature), is
necessary to assess which model age is geologically
most meaningful.
Laboratory techniques
For this study, analyses were completed in the
laboratory of M. O. McWilliams at Stanford University. The minerals of interest were concentrated
using standard mechanical and magnetic methods.
For most samples of white mica and sodic amphibole, the concentrates were prepared from small
rock specimens selected because they contained
thin layers or selvages, generally a few millimeters
thick, where the target mineral grains were substantially larger than was typical for the target rock unit.
Grains chosen for neutron irradiation were individually inspected and hand selected under the binocular microscope. Total masses of analyzed material
were about 0.6–5 mg for white mica, 10–75 mg for
sodic amphibole, 7–25 mg for actinolite, and 8–15
mg for hornblende. Step heat Ar isotopic analyses
were completed using a Staudacher-type resistance
furnace and a MAP 216 mass spectrometer, following procedures similar to those of Hacker and Wang
(1995). The J irradiation parameter was monitored
using sanidine from the Taylor Creek Rhyolite (nominal age of 27.92 Ma; Duffield and Dalrymple, 1990)
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WAKABAYASHI AND DUMITRU
TABLE 1. Summary of New Franciscan 40Ar/39Ar Ages
Metamorphic
temperature
Mineral
Preferred model
age (Ma, ±1s)
Model type
Confidence
in age
Sample
Rock type
SFM-13
metachert
WM
120.53 ± 0.63
Plateau
High
SFM-1x
metavolcanic
WM
122.5 ± 1.8
Isochron
High
South Fork Mtn Schist—Tomhead Mtn (coherent blueshist)
SFM-4c
Summary age
metachert
(epidote-BS facies)
WM
121.8 ± 3.7
Isochron
High
WM
120.53 ± 0.63
Plateau
Very high
Na-Amph
Not interpretable
None
–
Plateau
Moderate
330–345°C
SFM-3
metachert
WC-1
Metachert
NS-10
Metagraywacke
WM
~132
High-T steps
Moderate
FR-10
Metagraywacke
WM
~132
High-T steps
Moderate
WM
~132
High-T steps
Moderate
Ward Creek—Sonoma County (coherent blueschist)
290-400°C
WM
154.0 ± 1.6
Skaggs Springs Schist--Sonoma County (coherent blueschist)
Summary age
>330°C?
Willow Spring Slab—Panoche Pass (coherent blueschist and ecologite)
95Diab-107 Act
Metasomatic rind
Act
(154.9 ± 5.1)
Isochron
Low
95Diab-107 Mica
Coarse eclogite
WM
146.51 ± 0.36
Plateau
High
95Diab-33
95Diab-41 fn
Reaction zone
Act
(142 ± 16)
Isochron
Very low
Epidote blueschist
WM
134.2 ± 0.4
Total fusion
Moderate
Glaucophane Ridge—Panoche Valley (may be coherent blueschist or high-grade block)
95Diab-4
Garnet blueschist
WM
132.1 ± 0.2
Total fusion
Moderate
Maclaren Park—San Francisco (probably high-grade block)
MP-1
Epidote amphibolite
High T
Hbl
156.2 ± 3.4
Isochron
Moderate
HPSZ Meta
Garnet amphibolite
high T
Hbl
158.91 ± 0.57
Isochron
Moderate
HPSZ Gabbro
Gabbro (not HP)
–
Hbl
(146.5 ± 8.6)
Isochron
Low
Pilarcitos Gabbro
Gabbro (not HP)
Isochron
Very low
Hunters Point Shear Zone—San Francisco (blocks in melange)
Pilarcitos Dam—San Mateo County (low-pressure gabbro)
–
and the IUGS recommended decay constants
(Steiger and Jager, 1977). Data were plotted and
evaluated statistically with the Isoplot 3.0 software
package (Ludwig, 2003). The default criteria in
Isoplot were used to assess the statistical validity of
potential age plateaus. Data are presented and interpreted in the form of standard step-heat age spectrum plots, step-heat K/Ca ratio spectrum plots, and
inverse isochron plots (e.g., McDougall and Harri-
Act
(166 ± 38)
son, 1999; Ludwig, 2003). Unless otherwise indicated, uncertainties are quoted and plotted at the
one-sigma level and include propagated uncertainties in peak heights, blank values, rates of reactorproduced Ar isotopes, duration of irradiation, time
delay between irradiation and analysis, J, monitor
age, and decay rates of 37Ar, 39Ar, and 40K. Copies of
tabular data are available from the authors on
request.
FRANCISCAN COMPLEX, CALIFORNIA
South Fork Mountain Schist
White micas from three samples of South Fork
Mountain Schist were analyzed. Metachert sample
SFM-13 yielded an age spectrum with a welldefined age plateau that included 87% of the total
39Ar yield (Fig. 10A). The plateau age is 120.53 ±
0.63 Ma, which is statistically indistinguishable
from the total fusion age of 120.2 ± 0.9 Ma (both
uncertainties are ±1s and include uncertainty in J).
A K/Ca spectrum shows a fairly consistent K/Ca
ratio averaging about 90 for this sample (Fig. 10B),
a reasonable pattern for white mica. An inverse
isochron plot (Fig. 10C) indicates that the sample
yielded negligible atmospheric Ar (as indicated by
negligible 36Ar). Because there is negligible atmospheric argon, the isochron plot cannot be used
directly to assess whether excess radiogenic 40Ar is
present and has perturbed the ages. However, the
fact that negligible dissolved atmospheric Ar has
been incorporated into white mica from fluids within
this presumably fluid-rich metachert system suggests that incorporation of fluid-transported excess
40 Ar was similarly negligible (see below). We
consider the plateau age of 120.53 ± 0.63 Ma to be
the most appropriate model age for this sample. The
essentially identical total fusion age might be used
instead, but may have been slightly perturbed by the
erratic ages and low K/Ca ratios at the extreme highand low-temperature ends of the release spectra.
White mica from metavolcanic sample SFM-1x
yielded an age spectrum characterized by initially
older ages that decrease slightly with increasing
39Ar release toward a plateau at 125 Ma (Fig. 10D).
In a mica, such a descending pattern could have
several causes, including excess 40Ar. Excess 40Ar
has been observed in white mica from many HP and
UHP terranes, although virtually all of those studies
have been directly at rocks considerably higher in
grade than the South Fork Mountain Schist (e.g., de
Jong et al., 2001; Sherlock and Kelley, 2002). An
inverse isochron plot shows a clear linear trend,
with an initial 36Ar/40Ar ratio somewhat less than
atmospheric, evidence that the sample is moderately
contaminated with excess 40Ar (Fig. 10F). Statistical
tests indicate that the fit of the isochron is good. The
isochron age is 122.5 ± 1.8 Ma and we consider this
the most appropriate model age for this sample. The
K/Ca spectrum shows lower K/Ca ratios than sample
SFM-13, with considerable fluctuation (Fig. 10E).
This suggests this separate is not pure white mica
and may contain Ca-bearing intergrowths, inclusions, or alteration products.
887
White mica from metachert sample SFM-4c
yielded an age spectrum generally similar to SFM1x, although without a well-defined plateau (Fig.
10G). Again, the inverse isochron plot shows a clear
linear trend with initial 36Ar/40Ar less than atmospheric, indicating excess 40Ar, and statistical tests
indicate the fit of the isochron is good (Fig. 10I). The
isochron age is 121.8 ± 3.7 Ma, and we consider this
the most appropriate model age for this sample. The
K/Ca spectrum shows features similar to SFM-1x
(Fig. 10H).
The preferred model ages from the three white
mica samples are statistically identical, even though
the samples incorporated varying amounts of excess
40Ar. This increases confidence that the ages are
reliable and geologically meaningful. Because
sample SFM-13 exhibits a well-defined age plateau,
essentially identical plateau and total fusion ages,
low uncertainties on ages, and very low 36Ar/40Ar,
we infer that its plateau age, 120.53 ± 0.63 Ma
(±1s), best represents the 40Ar/39Ar white mica age
of the South Fork Mountain Schist at this locality
(Table 1). For purposes of discussion, we will refer to
this age later as 120.5 Ma.
In many HP metamorphic terranes, white micas
(phengites) have yielded consistent and apparently
reliable 40Ar/39Ar ages, but, in other HP areas,
excess 40Ar has clearly perturbed the system. Judgments as to the reliability of the ages are generally
based on the detailed Ar systematics of individual
samples and the consistency of ages between
samples. Given this, potential complications with
the South Fork Mountain Schist samples should be
considered. First, SFM-13 yielded an excellent age
plateau (Fig. 10A), which might be interpreted as
indicating rapid cooling through the bulk argon
closure temperature. However, the degassing mechanism for a mica in a high-temperature laboratory
furnace is physically different from that in nature, so
such a plateau could alternatively be an experimental artifact caused by Ar homogenization during
laboratory step heating (e.g., Sletten and Onstott,
1998; McDougall and Harrison, 1999). The age
spectra for samples SFM-1x and SFM-4c exhibit
moderate curvature (Fig. 10D and 10G), which we
have attributed to relatively minor excess Ar. Alternatively, curved age spectra in white micas are
sometimes attributed to mixing of argon from two
generations of micas of different ages and compositions (e.g., phengite overgrowth by muscovite or
phengite altered to illite), such as in terranes where
HP rocks have been overprinted by greenschist-
888
WAKABAYASHI AND DUMITRU
FIG. 10. 40Ar/39Ar data from four samples from the South Fork Mountain Schist. Upper plots are age spectra, middle
plots are spectra of K/Ca ratios, and lower plots are inverse isochrons (e.g., McDougall and Harrison, 1999; Ludwig,
2003). Nominal furnace degassing temperatures (°C) are indicated on the age spectra and inverse isochron plots. In
plots of 40Ar/39Ar data, all uncertainties are quoted and plotted at the one-sigma level unless otherwise indicated.
facies conditions (e.g., Wijbrans and McDougall,
1986; de Jong et al., 2001). However, the Franciscan Complex is notable among subduction com-
plexes for its lack of any greenschist-facies
overprint (Ernst, 1988). Although we do not have
compositional data from our white micas, multiple
FRANCISCAN COMPLEX, CALIFORNIA
889
FIG. 10. Continued.
generations of phengites have not previously been
reported in the Franciscan coherent rocks we have
examined (e.g., Maruyama and Liou, 1988) and we
infer that these samples contain only a single generation of white mica of phengitic composition.
A tendency may exist for major excess 40Ar contamination in phengites to be more prevalent in
closed, fluid-absent systems, where fluid-mediated
transport of dissolved argon is very slow (Sherlock
and Kelley, 2002), such as in exhumed UHP
890
WAKABAYASHI AND DUMITRU
predominately a porous marine mudstone metamorphosed to relatively low grade. Metasedimentary
rocks within the South Fork Mountain Schist exhibit
major syndeformational quartz veining (Worrall,
1981), evidence of substantial fluid flow during
metamorphism.
Another potential complication in low-grade
metamorphic terranes is the retention of radiogenic
40 Ar within incompletely recrystallized coarsegrained detrital muscovite grains in metagraywackes (e.g., McDowell et al., 1984). The genesis of our South Fork Mountain Schist samples as
metacherts and metavolcanics presumably lacking
in detrital muscovite makes this complication
unlikely. In addition, these samples were collected
from the highest-grade, most thoroughly recrystallized rocks in the Franciscan Eastern Belt, those
least likely to retain any incompletely recrystallized
detrital muscovite.
In sum, the three white mica samples from this
area have yielded well-behaved data that indicate a
consistent age from sample to sample. We therefore
conclude that 120.5 Ma represents a reliable and
geologically meaningful 40Ar/39Ar age for the South
Fork Mountain Schist at this locality.
Ward Creek
FIG. 11. 40Ar/39Ar data from white mica sample WC-1
from Ward Creek.
terranes where dry conditions are a key requirement
for preservation UHP assemblages (Hacker and
Peacock, 1995). In contrast, our white mica samples
are likely from open, wet systems, given that the
protolith of the South Fork Mountain Schist was
White mica from Ward Creek metachert sample
WC-1 yielded a monotonically ascending “staircase” age spectrum, with its oldest step at 160.4 Ma
(Fig. 11). The center of the spectrum exhibits a
weakly sloping plateau at 154.0 Ma, similar to the
total fusion age of 152.6 Ma. The K/Ca spectrum
shows fluctuating ratios between about 10 and 38,
somewhat low for a simple, pure white mica. The
inverse isochron plot indicates negligible atmospheric Ar and is not useful for assessing excess
40Ar.
The overall staircase shape of the age spectrum
conforms remarkably well to theoretical spectra
expected with partial loss of Ar by simple diffusion
processes (e.g., McDougall and Harrison, 1999),
and similar spectra from white micas have commonly been interpreted in such a manner (e.g.,
Wijbrans and McDougall, 1986; Baldwin and Harrison, 1989, 1992; Grove and Bebout, 1995). However, because release of Ar in a high-temperature
laboratory furnace occurs under conditions that are
well outside of the thermal stability field of hydrous
white mica, it is unclear how such a natural partial
loss situation would generate a staircase spectrum
(e.g., Sletten and Onstott, 1998; McDougall and
FRANCISCAN COMPLEX, CALIFORNIA
Harrison, 1999). If a partial loss interpretation of the
spectrum is accepted, it would suggest crystallization of the mica at about 160 Ma or earlier, followed
by cooling through the mica closure temperature at
about 154 Ma, followed by additional later cooling
accompanied by additional partial loss of Ar.
Another possible explanation of staircase patterns is
that multiple generations of white mica with different ages and different compositions are degassing
at different temperatures (e.g., Wijbrans and
McDougall, 1986). We discount this explanation,
because neither a greenschist-facies overprint nor
multiple generations of white mica growth have been
reported from this well-studied Franciscan locality
(Maruyama and Liou, 1988). A third possible explanation is that minor, micron-scale alteration of the
white mica has generated less retentive zones within
the grains that are responsible for the younger ages
measured during the lower temperature steps (e.g.,
Sletten and Onstott, 1998). Recognizing the ambiguities with this sample, we accept the plateau age
of 153.99 ± 1.62 Ma as the preferred model age for
this sample, mainly because the plateau age
partially filters out the young steps at low release
temperatures, which are difficult to explain geologically. In addition, the plateau and total fusion ages
are essentially identical. Our sample was taken from
the epidote blueschist part of the Ward Creek coherent sequence, estimated to have undergone metamorphism at 290–400°C (Maruyama and Liou,
1988; Oh and Liou, 1990). Accordingly, our model
age may represent either the time of metamorphism
or of cooling through the bulk phengite closure
temperature.
Skaggs Springs schist
White mica from Skaggs Springs schist samples
NS-10 and FR-1 both yielded moderately descending age spectra and fluctuating K/Ca spectra (Fig.
12). Note that the lowest temperature furnace steps
were inadvertently omitted for NS-10, and this probably obscures the young ages commonly observed
over the initial several percent of gas release. Total
fusion ages for the samples are similar, 136.2 ± 1.3
Ma and 132.6 ± 2.4 Ma, respectively. Isochron plots
indicate that minor amounts of atmospheric Ar are
present, but valid isochrons cannot be fit. These
data are difficult to interpret. The decreasing spectra may represent minor to moderate excess Ar. We
interpreted similar spectra and isochron plots for
SFM-1x and SFM-4c as reflecting excess 40Ar (Fig.
10). If excess 40Ar is present, the near-horizontal
891
portions of the spectra between about 60% and 90%
cumulative 39Ar may best approach the correct age,
about 132 Ma in both cases, and we tentatively
propose this as the most plausible model age for
these samples (Table 1). However, for SFM-1x and
SFM-4c, the ages of the corresponding intervals on
the age spectra were about 3 million years older
than the isochron ages we ultimately preferred for
those samples, so a “correct” age for the Skaggs
Springs samples could be somewhat different from
132 Ma.
The fine grain size of the white mica (same size
as the South Fork Mountain Schist samples)
suggests that these ages may reflect cooling through
Ar closure rather than metamorphic crystallization.
Although no estimates of metamorphic temperature
have been obtained from the Skaggs Springs schist,
it is more thoroughly recrystallized and exhibits
coarser metamorphic minerals than the South Fork
Mountain Schist. Unfortunately, the exceedingly
rare occurrences of separable white mica we found
in the former were no coarser than the similarly very
rare separable white micas we sampled from the
latter.
Willow Spring slab
White mica from coarse eclogite sample Diab107 (Fig. 13) yielded a well-defined age plateau at
146.51 ± 0.36 Ma (±1σ, including uncertainty in J),
which we accept as its most geologically meaningful
model age. The K/Ca spectrum shows somewhat low
ratios between about 20 and 30, suggestive of an
impure mica. The inverse isochron plot shows negligible atmospheric Ar and is therefore not useful in
assessing excess 40Ar. Because this coarse eclogite
likely crystallized at greater than 450°C based on
thermobarometric studies of similar Franciscan
rocks (e.g., Tsujimori et al., 2006), the results are
best interpreted as recording cooling through the
bulk closure temperature of white mica at about
147 Ma.
Actinolite from a metasomatic reaction zone near
eclogite Diab-107 yielded an ascending age spectrum (Fig. 14). The center of the spectrum exhibits a
poorly developed plateau of 148 ± 10 Ma, whereas
the high-temperature end of the spectrum rises to
about 160 Ma. A K/Ca spectrum shows a nearconstant ratio of 0.01 over 96% of 39Ar release,
consistent with degassing of actinolite. An isochron
on the steps with near-constant K/Ca ratio yields
an age of 154.9 ± 5.9 Ma and an intercept near
atmospheric, revealing no evidence for excess Ar.
892
WAKABAYASHI AND DUMITRU
FIG. 12. 40Ar/39Ar data from two white mica samples from the Skaggs Spring schist. Note that a non-optimal laboratory heating schedule starting at 600°C was used for sample NS-10. This probably obscured the younger ages normally
expected over the initial several percent of 39Ar release.
Statistically, however, the fit of the isochron is poor.
Despite the poor fit, we accept the isochron age at
the most geologically meaningful model age for this
sample. Similar actinolite-rich reaction zones in the
Franciscan are estimated to have formed at about
300–370°C (Moore, 1984), significantly cooler than
the bulk closure temperature of hornblende of about
525 ± 50°C (Harrison, 1981). The data are probably
FRANCISCAN COMPLEX, CALIFORNIA
893
FIG. 13. 40Ar/39Ar data from two white mica samples from the Willow Spring slab.
best interpreted as indicating crystallization of actinolite at about 154.9 ± 5.9 Ma.
Actinolite from sample Diab-33, probably a
metasomatic zone between meta-ultramafic rocks
and garnet blueschist at a structurally lower level in
the slab than Diab-107, yielded an irregular ascend-
ing spectrum that rises to a maximum of roughly 160
Ma (Fig. 14). A K/Ca spectrum shows a near-constant K/Ca ratio of about 0.003 over about 84% of
39Ar release, consistent with degassing of actinolite.
An isochron on the steps with near-constant K/Ca
yields an age of 142 ± 16 Ma and an intercept close
894
WAKABAYASHI AND DUMITRU
FIG. 14. 40Ar/39Ar data from two actinolite samples from the Willow Spring slab.
to atmospheric, revealing no evidence for excess Ar.
However, the fit of the isochron is poor. The isochron
age is probably the best model age for this sample,
but given the poor isochron fit, the high uncertainty
on the isochron age, and the erratic age spectrum, a
useful geologic interpretation of this sample is not
possible.
White mica from epidote blueschist sample
Diab-41fn yielded a relatively flat age spectra, with
most steps between about 132 and 137 Ma (Fig. 13).
FRANCISCAN COMPLEX, CALIFORNIA
895
Although this sample did not yield a plateau, the
total fusion age of 134.2 ± 0.4 describes the data
well, and we accept this as the best model age for
this sample. The K/Ca spectrum shows low and variable K/Ca ratios between about 3 to 8, inconsistent
with a pure white mica. While the inverse isochron
plot does not meet statistical expectations of a
simple two-component mixture, the imprecise
isochron age of 132 Ma is very similar to the total
fusion age. Based on metamorphic temperature estimates for similar epidote blueschists as Ward Creek,
the metamorphic temperature of this sample may
have been about 290–400°C (Maruyama and Liou,
1988; Oh and Liou, 1990). These data are best interpreted as recording metamorphic white mica growth
or cooling through the bulk closure temperature of
white mica at about 134 Ma.
Glaucophane Ridge
White mica from sample Diab-4 at Glaucophane
Ridge yielded a complex age spectrum generally
similar to Diab-41fn from Willow Spring about 14
km away, with most steps yielding ages between
about 122 and 140 Ma (Fig. 15). The K/Ca spectrum
shows K/Ca ratios between about 30 and 40. The
inverse isochron plot shows negligible atmospheric
Ar and is not useful in assessing excess 40Ar. We
tentatively accept the total fusion age of 132 Ma as
the most geologically meaningful model age for this
sample. Based on comparison to similar mineral
assemblages at Ward Creek, the metamorphic temperature for this sample may have been about 350–
400°C (Oh and Liou, 1990). Accordingly, we infer
that the data are best interpreted as recording metamorphic growth of white mica or cooling through the
bulk closure temperature of white mica at about 132
Ma. This interpretation is essentially identical to
that for sample Diab-41fn, which yielded a 134 Ma
total fusion age.
Epidote amphibolite of McLaren Park
Hornblende from the epidote amphibolite sample
MP-1 yielded a somewhat irregular age spectrum,
which is not unusual for hornblende analyses (Fig.
16). K/Ca ratios are consistent over the last ~60% of
39Ar release and are compatible with a relatively
pure hornblende phase. Anomalous ages between
about 58 and 66% cumulative 39Ar are not unexpected and reflect incongruent melting of
hornblende in the laboratory furnace (e.g., Wartho et
al., 1991). The most reliable age for such samples is
generally considered to be obtained from the higher
FIG. 15. 40Ar/39Ar data from white mica sample Diab-4
from Glaucophane Ridge.
temperature steps after the melting anomalies. An
isochron on the five highest-T steps yields an age of
156.2 ± 3.4 Ma. The initial 36Ar/40Ar ratio is near
atmospheric and gives no indication of excess 40Ar.
The quality of the isochron fit is fair. The isochron
896
WAKABAYASHI AND DUMITRU
FIG. 16. 40Ar/39Ar data from two hornblende samples from the city of San Francisco.
age is statistically indistinguishable from the total
fusion age of 153.9 ± 0.6 Ma. We therefore accept
the isochron age as the most geologically meaningful
model age for this sample. The poorly constrained
peak temperature of this unit has been estimated as
about 400–600°C (Wakabayashi, 1990) so metamor-
phic crystallization temperatures may have
exceeded the bulk closure temperature of hornblende. Accordingly, the model age for the epidote
amphibolite of McLaren Park may reflect cooling
through the bulk closure temperature of hornblende
or metamorphic crystallization. Cooling of high-
FRANCISCAN COMPLEX, CALIFORNIA
grade rocks of the Franciscan has been interpreted
to be rapid from the peak temperature of metamorphism to the bulk hornblende closure temperature,
based on evaluation of earlier geochronologic studies, regional tectonic models, and thermal modeling
(Wakabayashi, 1990). The positive correlation
between Lu-Hf of garnet-bearing high-grade Franciscan rocks and corresponding metamorphic
temperatures suggests slow cooling (Anczkiewicz et
al., 2004), however, so the assumption that hornblende 40Ar/39Ar ages in Franciscan high-grade
rocks closely approximate metamorphic crystallization because of rapid cooling may be incorrect.
High-grade block from the Hunters Point
shear zone
Hornblende from garnet amphibolite block
HPSZ-meta from the Hunters Point Shear zone
yielded data generally similar to sample MP-1 (Fig.
16). The isochron age of the six highest-T steps is
158.91 ± 0.57 Ma, with an intercept that is essentially atmospheric. The fit of the isochron is good.
As with MP-1, we accept the isochron age as the
most geologically meaningful model age for this
sample. Wakabayashi (1990) estimated the metamorphic temperature of the HPSZ block as about
690–730°C, significantly higher than the bulk hornblende closure temperature. We therefore consider
the model age for the Hunters Point shear zone
garnet amphibolite block to date cooling through the
bulk hornblende closure temperature.
Gabbro block from the Hunters Point Shear zone
Hornblende from sample HPSZ-gabbro, a gabbro
lens within Hunters Point shear zone serpentinite,
also yielded data generally similar to sample MP-1
(Fig. 17). The isochron age of the eight highest-T
steps is 146.5 ± 8.6 Ma, with an intercept that is
essentially atmospheric. However, the fit of the isochron is poor. Although the fit is poor, we tentatively
accept the isochron age as the most geologically
meaningful model age for this sample and consider
that it dates hornblende crystallization. Note that the
large uncertainty on the isochron age means that
hornblende in this gabbro sample did not necessary
crystallize after the hornblende in HPSZ-meta.
Pilarcitos gabbro
Actinolite from a sample of the Pilarcitos gabbro
yielded an irregular ascending age spectrum (Fig.
17). A K/Ca spectrum shows a fairly uniform K/Ca
ratio of about 0.005. Figure 17B shows an attempt to
897
fit an isochron to the four highest-T steps. This
isochron yields at age of 166 ± 38 Ma and an initial
36Ar/40Ar ratio near atmospheric. However the fit of
the isochron is poor. Alternative isochrons including
different steps are no better (not shown). As mentioned above, this sample showed evidence of significant alteration and this may be a factor in the poor
data quality. These data cannot be interpreted in
detail. We can infer only that actinolite crystallization in this sample was apparently grossly coeval
with hornblende crystallization in MP-1, HPSZmeta, and HPSZ-gabbro.
Sodic amphibole samples
We also attempted to date sodic amphiboles from
five samples. As mentioned, sodic amphiboles
commonly yield problematic 40Ar/39Ar results. Four
of our samples yielded erratic, uninterpretable data,
which are not presented here. These four samples
included sodic amphiboles from samples WC-1 and
NS-10 discussed above, from sample SFM-14, a
metabasite collected near white mica sample SFM13, and from sample AI, a coherent metachert from
Angel Island in San Francisco Bay.
One sodic amphibole sample did yield tantalizing data. This sample, SFM-3, was collected from a
manganiferous metachert within the South Fork
Mountain Schist (Figs. 2 and 3). It exhibits a staircase spectrum, with the oldest step at 135 Ma (Fig.
10J). Sodic amphiboles lack K in their nominal
compositional formulae and Sisson and Onstott
(1986) suggested any K may be hosted within finegrained white mica inclusions. Alternatively, Eide
et al. (1992) reported strongly zoned trace K up to
0.16% within the main sodic amphibole phase. The
K/Ca spectrum from SFM-3 (Fig. 10K) appears to
show a phase with a low and very uniform K/Ca ratio
of 0.03 as the source of Ar over the last 56% of the
39Ar released, and we tentatively infer that this
interval does indeed represent degassing of the main
amphibole phase. An inverse isochron plot indicates
that all steps contain significant atmospheric Ar
(Fig. 10L). However, attempting to project a single
isochron through the data requires an initial 36Ar/
40Ar ratio that is greater than atmospheric, which is
not reasonable physically (e.g., McDougall and
Harrison, 1999), so the isochron age from the plot is
not valid. Thus while the data patterns from sodic
amphibole sample SFM-3 are initially intriguing, a
reliable interpretation is not possible.
898
WAKABAYASHI AND DUMITRU
FIG. 17. 40Ar/39Ar data from two gabbro samples not subjected to HP metamorphism.
Discussion
Preliminary comment on Franciscan Lu-Hf ages
Anczkiewicz et al. (2004) reported five new LuHf garnet ages that are especially important in
understanding the early evolution of the Franciscan
Complex, because they probably directly date metamorphic recrystallization of the high-grade, garnetbearing units. However, it should be noted that
important uncertainties remain in the value of the
176Lu decay constant. Anczkiewicz et al. (2004)
used what is probably the best available estimate
FRANCISCAN COMPLEX, CALIFORNIA
(Scherer et al., 2001) and their samples appear to
yield geologically reasonable ages, especially their
sample from Santa Catalina Island. We therefore
accept their ages in the discussions below. However,
use of alternative values would reduce the Lu-Hf
ages by up to about 5% or increase them by up to
about 10% (Begemann et al., 2001; Norman et al.,
2004), and this unfortunately generates considerably ambiguity when attempting to compare the LuHf ages with 40Ar/39Ar ages.
High-grade blocks and coherent metamorphic
rocks: Unrelated or part of the same subduction
event?
Our new 40Ar/39Ar ages (Table 1), when evaluated in the context of existing ages, show that the
ages of coherent metamorphism partially overlap the
ages of high-grade block metamorphism (Fig. 18).
This strongly suggests that the metamorphism of
both was associated with evolution of the same
subduction zone (e.g., Platt, 1975; Cloos, 1985;
Wakabayashi, 1990), rather than with separate
subduction episodes (e.g., Blake et al., 1988). This
is consistent with the observations that: (1) metamorphic assemblages show an overlap rather than a
sharp discontinuity in conditions between the highest-grade coherent metamorphic rocks and the highgrade blocks; and (2) field relations at the Willow
Spring slab define a coherent sheet of rocks in
which mineral assemblages are identical to those in
many high-grade blocks.
The Willow Spring slab is clearly one of a stack
of Franciscan thrust sheets and not a mélange block.
Anczkiewicz et al. (2004) obtained a Lu-Hf garnet
age of 146.7 ± 0.7 Ma from garnet blueschist of the
slab, within the range of our 40Ar/39Ar ages. Field
relations of the Willow Spring slab are similar to a
smaller coherent body of blueschist-overprinted
amphibolite and garnet amphibolite at Goat Mountain in the northern Coast Ranges. The Goat Mountain rocks exhibit high-grade block mineralogy, but
coherent field relations (Ernst et al., 1970) and have
yielded K/Ar ages that are identical to those
obtained from high-grade blocks using the same
methods (Suppe and Foland, 1978).
During field reconnaissance in the southern
Diablo Range, we examined several other slabs of
coarse-grained metabasite, somewhat smaller (up to
1.5 km in the long dimension) than the Willow
Spring slab, that appear to be thrust sheets overlying
metagraywacke, rather than blocks-in-mélange.
Several of these are denoted as “csch” near Willow
899
Spring on Figure 1 and some are also broken out as
coherent schists on the state geologic map of Jennings (1977). Our field observations suggest that
these slabs occupy the structurally highest levels of
the Franciscan in the southern Diablo Range. The
slabs crop out along or near the regional contact
between the Franciscan and the structurally overlying Coast Range ophiolite and/or Great Valley
Group. One of these apparently coherent sheets is a
garnet amphibolite slab with variable blueschist
overprint located about 2 km east of the Willow
Springs slab. This slab, which is about 700 m in long
dimension and structurally overlies metagraywacke,
was described by Ernst (1965) and by Hermes
(1973), and we refer to it informally here as the
“Antelope Creek slab.” 159-163 Ma Ar-Ar hornblende ages and a 168.7 ± 0.8 Ma Lu-Hf garnet age
have been obtained from this slab (Ross and Sharp,
1988; Anczkiewicz et al., 2004). These observations
suggest that coherent, high-grade rocks in the Franciscan are more common than previously recognized, although they still constitute much less than
1% of Franciscan metamorphic rocks. The coherent
field relations indicate that although much of the
high-grade material in the Franciscan may have
been exhumed as blocks-in-mélange, at least some
of it has been exhumed as coherent, fault-bounded
sheets in a manner generally similar to the larger,
lower-grade coherent units.
In addition to the rocks with demonstrably coherent field relations (Ward Creek, Willow Spring,
Antelope Creek), high-grade block sample HPSZmeta and the epidote amphibolite of McLaren Park
of somewhat ambiguous field relations gave hornblendes 40Ar/39Ar ages similar to those obtained
from high-grade blocks in previous studies (Ross
and Sharp 1986, 1988).
Does the high-grade geochronology of the
Franciscan reflect a slow initiation of subduction?
Models for the thermal histories of subduction
zones generally predict very rapid cooling over the
first few million years following the initiation of
subduction (e.g., Peacock, 1988; Hacker, 1991).
Anczkiewicz et al. (2004) reported a correlation
between the peak metamorphic temperatures and
the Lu-Hf ages of their high-grade, garnet-bearing
samples (n = 5), with older samples recording metamorphism at higher temperatures. Their data
suggested remarkably slow and protracted cooling of
the Franciscan subduction zone over about the first
20 million years following the initiation of subduc-
900
WAKABAYASHI AND DUMITRU
FIG. 18. Summary of age data on Franciscan blueschist and higher grade rocks. 40Ar/39Ar ages from this study are
in bold and larger type, with ±1σ error bars displayed unless uncertainties are smaller than the symbol size. “<<<<<“
symbols with grayed letters denote protolith ages that predate metamorphism (for fossils, the range of “<<<<<“ denotes
the biostratigraphic age range). “>>>>>” symbols represent zircon fission track ages that postdate metamorphism
(range of “>>>>>” represents ±2σ uncertainty range on the fission track ages). Abbreviations: Ar-Ar = 40Ar/39Ar step
heating, except wrtf (whole-rock total fusion); Dzrc = maximum depositional age from detrital zircon; Fss = depositional
age from fossils in clastic rocks; K/Ar: potassium-argon; Lu-Hf = lutetium-hafnium, garnet-bearing assemblages; U/Pb
= uranium-lead, zircon; U/Pbi = uranium lead isochron; ZFT = zircon fission track. Mineral/rock abbreviations: act =
actinolite; bi = biotite; ep-bsch = epidote-blueschist; gt-bsch = garnet-blueschist; hbl = hornblende; WM = white mica;
wr = whole rock; NaAm = sodic amphibole. Sources: Lee et al. (1964), Coleman and Lanphere (1971), Suppe and
Armstrong (1972), Lanphere et al. (1978), Suppe and Foland (1978), Mattinson and Echeverria (1980), McDowell et al.
(1984), Blake et al. (1984, 1988), Mattinson (1986), Ross and Sharp (1986, 1988), Wakabayashi and Deino (1989),
Elder and Miller (1993), Tagami and Dumitru (1996), Mertz et al. (2001), Catlos and Sorensen (2003), Joesten et al.
(2004), Tripathy et al. (2005), Dumitru et al. (2006).
tion, much slower than indicated by the models.
They suggested that the slow cooling might be
evidence for slow subduction rates over that time
period.
Examination of 40Ar/39Ar ages from Franciscan
rocks allow additional insights into this slow cooling
hypothesis in the temperature range around the bulk
closure temperatures of hornblende (525 ± 50°C)
and white mica (400 ± 50°C). Figure 19 is a plot of
age versus closure or metamorphic temperature for
the oldest Franciscan HP rocks. An overall trend of
decreasing temperature with decreasing age is
apparent, but the plot exhibits considerable scatter.
The scatter could reflect a variety of factors, including different cooling, mineral growth, and recrystal-
FRANCISCAN COMPLEX, CALIFORNIA
901
FIG. 19. Plot of age versus temperature for the oldest Franciscan metamorphic rocks. For the 40Ar/39Ar ages, closure
temperatures of 525 ± 50°C (Harrison, 1981) were used for hornblende ages where metamorphic (hornblende crystallization) temperatures are estimated to have been higher. For white mica ages from the Willow Spring slab eclogite, a 400
± 50°C bulk closure temperature for phengite (McDougall and Harrison, 1999) was assigned because the micas were
coarse grained and probably crystallized at temperatures exceeding the bulk closure temperature. The phengite bulk
closure was also applied for texturally early (pre-rind) white micas from Tiburon Peninsula high-grade blocks (Catlos
and Sorensen, 2003), although it is possible that at least some of these phengites grew at temperatures below the bulk
closure temperature. For the Ward Creek, Glaucophane Ridge (Diab-4), and Willow Spring slab epidote blueschist
(Diab-41) white mica samples, where peak metamorphic temperature estimates are lower than phengite bulk closure
temperature, the metamorphic temperature range estimates were used on the plot (see text). For the Skaggs Springs
schist and South Fork Mountain Schist samples, we used a temperature range of 250–345°C, reflecting the reduction in
bulk closure temperature of white mica owing to fine grain size, as well as maximum metamorphic temperature estimate
for the South Fork Mountain Schist (see text). The lack of a plotted uncertainty in age for the Skaggs Spring schist
reflects the difficulty in assigning an uncertainty to this model age (see text) rather than a high level of precision. The
Mount Hamilton white mica ages of Catlos and Sorensen (2003) are plotted in grey tone because of the possibility that
white mica crystallization there reflects late fluid infiltration events at poorly constrained temperatures. We have tried
to make the metamorphic temperature estimates as internally consistent as possible, so that temperature estimates
reflect the application of the same geothermometers. This led to a slight reduction (300–450°C to 290–400°C) in the
assigned temperature of the youngest Anczkiewicz et al. (2004) sample from the Willow Spring slab to match our Willow
Spring slab and Ward Creek temperature estimates for similar mineral assemblages.
lization histories in different areas, as well as errors
in some of the temperatures and age determinations.
From hornblende analyses, Ross and Sharp
(1988) reported a mean age of 161.1 ± 1.7 Ma from
the Antelope Creek garnet amphibolite slab and we
obtained ages of 158.9 ± 0.6 from the garnet
amphibolite in the Hunters Point shear zone and
156.2 ± 3.4 Ma from the epidote amphibolite in
902
WAKABAYASHI AND DUMITRU
McLaren Park (Fig. 19). The ages of these three
units are statistically indistinguishable (mean
159.1 ± 1.1 Ma, MSWD 1.11, prob. 0.33). This is
consistent with the results of Ross and Sharp (1986),
who reported 40Ar/39Ar hornblende plateau ages of
158–162 Ma from widely distributed Franciscan
amphibolite localities. From white mica analyses,
our Ward Creek sample suggests that, by about 154
Ma, at least part of the Franciscan had cooled
to metamorphic temperatures of 290–400°C
(Maruyama and Liou, 1988; Oh and Liou, 1990) and
Catlos and Sorenson (2003) reported essentially
identical mean laser spot ages of 153 ± 2, 154 ± 2,
155 ± 2, and 156 ± 6 Ma from eclogite and garnet
amphibolite high-grade blocks at Tiburon. Comparison of these very limited data suggest somewhat
more rapid cooling from bulk hornblende closure
(525 ± 50°C) to temperatures in the 350 ± 50°C
range than suggested by the Lu-Hf ages. However,
enough uncertainty exists in ages and temperatures
that slow cooling is alternatively permitted. Accordingly, it appears the existing quite limited Franciscan 40Ar/39Ar data set can neither refute nor
confirm the slow cooling hypothesis suggested by
the Lu-Hf ages.
When was the South Fork Mountain Schist
metamorphosed?
Several dozen argon samples have previously
been dated from the South Fork Mountain Schist
(Suppe and Armstrong, 1972; Lanphere et al., 1978;
McDowell et al., 1984). Because of the extremely
fine grain size of these rocks, this work was completed using whole-rock 40Ar/39Ar total fusion methods (not step heating) and whole-rock K-Ar
methods. These ages tend to cluster around 120–
125 Ma, but many ages are substantially older or
substantially younger (Fig. 18). Because of the limitations of the total fusion 40 Ar/ 39 Ar and K-Ar
approaches, it was not possible to assess these samples for excess argon, argon partial loss, or other
complications.
As discussed above, we have obtained 40Ar/39Ar
step heat data from three pure white mica mineral
separates from the Tomhead Mountain area. The
ages of these samples are statistically indistinguishable and yield a preferred age of 120.5 Ma, which
we consider highly reliable. Two of these samples
contain moderate excess 40Ar contamination, suggesting that excess 40Ar could explain some of the
older whole-rock ages reported previously. Very
recently, Dumitru et al. (2006, in prep.) obtained
135 Ma detrital zircon ages from the Tomhead area,
requiring that accretion and metamorphism postdate 135 Ma. Therefore, the 120.5 Ma 40Ar/39Ar age
may date metamorphic recrystallization of the South
Fork Mountain Schist at about 120.5 Ma, or may
date cooling following metamorphism that occurred
sometime between 135 and 121 Ma. In either case,
accretion and metamorphism of the coherent South
Fork Mountain Schist definitely post-dates all dated
Franciscan high-grade block metamorphism and
also the metamorphism of the higher-grade coherent
units at Willow Spring and Ward Creek.
Interpretation of the Hunters Point shear zone
gabbro and Pilarcitos gabbro ages
Neither of these rocks exhibits high-P metamorphism, in contrast to the high-grade blocks and the
coherent, blueschist-facies rocks. The early metamorphism of these blocks and formation of calcic
amphibole, the lack of a strong penetrative fabric,
and their gabbro protoliths are consistent with seafloor hydrothermal metamorphism (Wakabayashi,
2004) in contrast to the high-grade blocks, whose
high-T metamorphism has been ascribed to inception of subduction in young oceanic crust (Platt,
1975; Wakabayashi, 1990). The late growth of
pumpellyite in these gabbro blocks may be related
to their subduction history. Accordingly, we interpret the ages from these gabbros as representing the
ages of their hydrothermal metamorphism at or near
a spreading center. The large uncertainties in the
ages discourage further speculation about their
tectonic significance.
Franciscan metamorphism: Subduction
of different units at different times and comparison
with other HP terranes
The Franciscan Complex is one of the many HP
and UHP terranes of the world and, for the majority
of such terranes, a narrow age range, or a singular
age, of metamorphism is commonly interpreted or
sought (e.g., Maruyama et al., 1996). Discussion of
the HP or the UHP age (emphasis on the singular) of
a given unit implies the expectation that all of the
HP and UHP metamorphism of a given terrane
occurred at the same time. This, in turn, suggests a
tectonic model in which the entire terrane was subducted and then metamorphosed at the same time.
In contrast to such models, the collective data from
this and earlier studies of Franciscan geochronology
strongly indicate that HP metamorphism in the
Franciscan took place at different times as a result
903
FRANCISCAN COMPLEX, CALIFORNIA
of progressive subduction and deep offscraping/
underplating that spanned tens of millions of years.
We do not raise this point to suggest that a “one
metamorphic age” assessment of other HP and UHP
terranes is wrong, but such a model certainly does
not apply to the Franciscan, and this issue may be
worth revisiting in some other HP and UHP
domains.
The span of radiometric metamorphic ages of
Franciscan HP rocks is from 169 Ma to about 90 Ma
(Fig. 18). In addition, some Franciscan HP rocks
lacking radiometric ages have yielded fossil and/or
detrital zircon ages as young as about 85 Ma and
their metamorphism must post-date these ages
(Elder and Miller, 1993; Joesten et al., 2004;
Tripathy et al., 2005; Unruh et al., in press) (Fig.
18). This span of metamorphism clearly reflects
subduction and HP metamorphism over an extended
period of time, rather than a single event and subsequent multiple episodes of fluid infiltration and
mineral growth or variable cooling, because the
protolith ages for the youngest Franciscan HP rocks
are about 85 million years younger than the oldest
HP metamorphic ages. Thus the HP rocks now
exposed in the Franciscan were metamorphosed
over a period of time spanning a minimum of 85
million years and reflect the subduction and HP
metamorphism of different units of the subduction
complex at different times. The age of metamorphism of the coherent units and, by inference, their
incorporation into the subduction complex youngs
structurally downward, suggesting progressive offscraping/underplating of units during an extended
period of subduction (Wakabayashi, 1992, 1999a).
High-grade blocks occupy different structural levels
within the Franciscan, including levels beneath
coherent units metamorphosed up to 85 million
years after high-grade metamorphism. This relationship suggests that tectonic and/or sedimentary
processes have moved some of these blocks from
their original (peak metamorphic) positions within
the subduction complex (Wakabayashi, 1992).
On the basis of the lack of a thermal overprint
and the continuity of U/Pb zircon ages in the Sierran
Batholith, Franciscan subduction is believed to
have been continuous (Ernst, 1984; Dumitru, 1991;
Wakabayashi, 1992). Whether the subduction
accretion process was fairly continuous, or whether
it was episodic with some periods of subduction
erosion (e.g., von Huene and Scholl, 1991), cannot
be discerned from the current metamorphic age
data.
Acknowledgments
This work was partially supported by the Donors
of the Petroleum Research Fund of the American
Chemical Society (grant 25282-AC2). We are grateful to M. O. McWilliams for his hospitality in his
40Ar/39Ar laboratory, the Oregon State Radiation
Center for sample irradiations, A. T. Calvert for
advice on data interpretation, and W. D. Sharp and
M. Grove for their reviews of the paper. We thank
W. G. Ernst for giving us the opportunity to participate in the symposium in honor of J.G. Liou where
this work was presented. JW is especially grateful to
J. G. Liou for his guidance in studies of high-pressure metamorphism that began during his graduate
student days (when Louie served on his thesis
committee) and continues to this day.
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