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Lithos
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Hot and deep: Rock record of subduction initiation and exhumation of
high-temperature, high-pressure metamorphic rocks, Feather River
ultramafic belt, California
Christopher M. Smart, John Wakabayashi ⁎
California State University, Fresno, Department of Earth and Environmental Sciences, 2576 E. San Ramon Avenue, Mail Stop ST-24, Fresno, CA 93740, USA
a r t i c l e
i n f o
Article history:
Received 23 July 2008
Accepted 6 June 2009
Available online xxxx
Keywords:
Subduction initiation
Metamorphic soles
Ophiolites
High-pressure rock exhumation
a b s t r a c t
Studies of a 10 to 300-m-thick unit of high-grade metamorphic rock (“external schists”) that crops out along
the western border of the Feather River ultramafic belt (FRB), northern California, yield new insights into
subduction initiation and ophiolite emplacement processes. The high-temperature (T) foliation of the
external schists dip moderately to steeply eastward beneath the ultramafic rocks of the FRB, a 150-km-long
slab of suboceanic upper mantle and the high-T fabric shows a tops-to-the-west (FRB-side-up) sense of
shear. The structurally highest external schists record peak metamorphic conditions of 650–760 °C at 1.3–
2.2 GPa. In contrast, sheeted dikes of the Devil's Gate ophiolite that overlie the ultramafic rocks yield
metamorphic conditions of 710–730 °C at about 0.3–0.7 GPa. A km-scale lens of amphibolite within
ultramafic rocks yields somewhat lower pressures than the structurally highest external schist, as do the
structurally lower rocks within the external schists. Significant exhumation of the external schists relative to
the structurally overlying ophiolitic rocks occurred along at least two major zones and the most significant
exhumation was accommodated at least 1.5 km structurally above the ultramafic-external schist contact.
Based on available geochronology, intraoceanic subduction may have initiated at approximately 240 Ma, and
exposure of the external schist occurred prior to the deposition of rocks in the structurally highest part of the
Calaveras Complex (minimum 177 Ma), a subduction complex that structurally underlies the external schists.
High-T metamorphism of the Devil's Gate ophiolite may have resulted from partial (failed) ridge subduction.
© 2009 Published by Elsevier B.V.
1. Introduction
Mechanisms of subduction initiation are the subject of considerable debate, but most authors agree subduction initiation exploits preexisting weaknesses and material contrasts in the oceanic lithosphere
(Casey and Dewey, 1984; Mueller and Phillips, 1991; Stern and
Bloomer, 1992; Wakabayashi and Dilek, 2003). Ophiolites are on-land
remnants of oceanic crust and many of these ophiolites structurally
overlie the position of former subduction zones (e.g., Moores, 1970).
Structurally beneath many ophiolites are thin (b500 m) units of highgrade metamorphic rocks called metamorphic or dynamothermal
soles. These soles are thought to have formed during subduction
initiation beneath young oceanic lithosphere (hot subduction initiation), based primarily on the high temperature of metamorphism
recorded in them (peak temperatures in the 700–900 °C range), their
lithologies (primarily metabasite with meta-pelagic sediments), and
their structural and chronologic relationships with the ophiolite that
directly overlies them (Williams and Smyth, 1973; Spray, 1984;
Jamieson, 1986; Hacker, 1990). Metamorphic soles commonly have
⁎ Corresponding author.
E-mail address: jwakabayashi@csufresno.edu (J. Wakabayashi).
inverted metamorphic gradients due to tectonic underplating during
subduction of progressively older (and colder) oceanic lithosphere
(Peacock, 1987; 1988; Hacker, 1990; 1994; Gnos, 1998), and show
anticlockwise pressure–temperature–time (P–T–t) paths (P on positive y-axis) (Wakabayashi, 1990; Dilek and Whitney, 1997; Önen and
Hall, 2000; Guilmette et al., 2008). Because metamorphic soles
apparently formed during inception of subduction, their geology,
and the geology of adjacent rocks, provide insight into the setting and
mechanisms associated with hot subduction initiation (i.e. Jamieson,
1986; Hacker, 1990; Guilmette et al., 2008).
Although many studies have been conducted on metamorphic
soles, some critical aspects of metamorphic sole development have
received little attention. For example, metamorphic soles were once
assumed to have been “welded” to the base of ophiolites after they
were underplated (scraped off the downgoing plate) as subduction
began beneath the ophiolite (Williams and Smyth, 1973; Malpas,
1979; Searle and Malpas, 1980). Such a model assumed that no
exhumation of the sole relative to the ophiolite occurred after
metamorphism. As new geobarometric methods became available,
studies showed metamorphic pressures for soles that vastly exceeded
that which could be explained by the structural thickness of the
ophiolite above the sole, indicating significant exhumation of the sole
0024-4937/$ – see front matter © 2009 Published by Elsevier B.V.
doi:10.1016/j.lithos.2009.06.012
Please cite this article as: Smart, C.M., Wakabayashi, J., Hot and deep: Rock record of subduction initiation and exhumation of hightemperature, high-pressure metamorphic rocks, Feather River ultramafic belt, California, Lithos (2009), doi:10.1016/j.lithos.2009.06.012
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amphibolite beneath ophiolites that are too thick (kilometers) to
have been generated by conductive heating beneath a hot mantle
hanging wall (Harper et al., 1996; Barrow and Metcalf, 2006), highgrade rocks that may have been derived from the base of a magmatic
arc instead of from the top of the downgoing plate (Grove et al., 2008),
and high-grade metamorphic rocks that structurally overlie, rather
than underlie an ophiolite (Dilek et al., 2008).
This paper presents structural, lithologic, and metamorphic P–T
estimates for an amphibolite unit bordering the Feather River
ultramafic belt in northern California. We will review the regional
framework of these rocks, then present new field, petrographic, and
metamorphic petrologic data that bear on the origin and evolution of
these rocks. We will show that structural, lithologic, and petrologic
evidence supports a metamorphic sole model for these rocks and the
specific field and petrologic relationships give new insight into the
exhumation of such rocks and the importance of such exhumation in
models of subduction initiation and ophiolite emplacement.
2. Regional setting
Fig. 1. Location map. Modified from Edelman and Sharp (1989). Abbreviations are
CC: Calaveras Complex, DGO: Devil's Gate ophiolite, RAS: Red Ant schist, SFU: Shoo Fly
Complex and other rocks bordering the east side of the Feather River ultramafic belt,
WU: Undifferentiated Mesozoic (primarily) and Paleozoic metamorphic and plutonic
rocks.
relative to the ophiolite (summarized in Wakabayashi and Dilek
(2000, 2003). For example, metamorphic pressures estimates range
from about 0.95 GPa to 1.8 GPa for different parts of the sole beneath
the Semail ophiolite of Oman (Gnos, 1998; Searle and Cox, 2002; Gray
and Gregory, 2003), probably the world's most thoroughly studied
metamorphic sole. The thickness of the overlying ophiolite can only
account for burial pressures of about 0.5 to 0.6 GPa (e.g., Searle and
Malpas, 1980). Many ophiolites are much thinner than the Semail
ophiolite, and the disparity between pressure estimates associated
with metamorphic sole pressures (of about 0.5 to 1.5 GPa) and the
potential burial pressure associated with the ophiolite thickness
(about 0.1 to 0.4 GPa) may be much greater (e.g., Jamieson, 1986;
Guilmette et al., 2008). In addition, studies have identified inverted P
gradients (structurally high parts with pressure estimates of about 1.0
to 1.8 GPa to structurally low parts of about 0.3 to 0.4 GPa) within
metamorphic soles, indicating major internal imbrication within the
sole (Jamieson, 1980; Jamieson, 1986; Gnos, 1998). These metamorphic pressure contrasts have not been addressed in detail in
models of subduction initiation (and metamorphic sole development)
and ophiolite emplacement (Wakabayashi and Dilek, 2003). Another
problem that has introduced complexity into the study of metamorphic soles and subduction initiation processes has been the
identification of high-grade metamorphic rocks that are spatially
associated with ophiolite belts but do not appear to be classic
metamorphic soles as defined above. These include units of
The 150-km-long by 1–8 km wide Feather River ultramafic belt
(FRB) of the northern Sierran Nevada, California (Fig. 1), comprises
variably serpentinized ultramafic rocks, with lesser amounts of
metagabbro, metadiabase, and metabasalt; collectively these rocks
have been considered an ophiolite (Ehrenberg, 1975; Sharp, 1988;
Edelman et al., 1989, Saleeby et al., 1989; Edelman and Sharp, 1989).
All rocks of the FRB appear to have undergone peak metamorphism at
amphibolite grade, with locally variable retrogression, although there
are significant internal differences in peak metamorphic conditions as
we will show. The FRB has yielded a rather large range in igneous (two
dates of 385 ± 10 and 314 + 10/−8 Ma, U/Pb zircon; Saleeby et al.,
1989) and metamorphic ages (about 234 to 387 Ma, Ar/Ar and K/Ar
hornblende; Weisenberg and Avé Lallemant, 1977; Standlee, 1978;
Hietanen, 1981; Böhlke and McKee, 1984) and it has been called a
polygenetic ophiolite (Saleeby et al., 1989) (geochronology summarized in Table 1).
In the headwaters of the South Fork Feather River, and Slate Creek,
pillow basalts, sheeted dikes, and gabbros crop out structurally above
ultramafic rocks of the FRB. These mafic igneous rocks and the
subjacent ultramafic rocks have been called the Devil's Gate ophiolite
(Edelman et al., 1989), so the Devil's Gate ophiolite may be considered
a subunit of the FRB (Fig. 1). Metamorphic age dates obtained from the
Devil's Gate ophiolite are 276 ± 6 Ma (Ar/Ar hornblende; Standlee,
1978) and 248 Ma (K/Ar hornblende, Hietanen, 1981).
The FRB is faulted along both eastern and western boundaries
against rocks of dramatically different age and lithology (Sharp, 1988;
Saleeby et al., 1989). East of the FRB is the Shoo Fly complex which
consists of Ordivician to Devonian continentally-derived metasandstone and chert deposited that are structurally overlain by a tectonic
mélange (Varga and Moores, 1981; Hannah and Moores, 1986). A
Devonian to Permian volcanic sequence overlies the Shoo Fly complex
at an angular unconformity (Durrell and d'Allura, 1977; Harwood,
1983; Hannah and Moores, 1986). The Shoo Fly Complex has
Table 1
Feather River ultramafic belt geochronology.
Location
Age (Ma)
Method
Reference
Alleghany schist
Oriental Mine granite (intrudes Alleghany schist)
Metagabbro intruding FRB in Yuba River area
Devil's Gate ophiolite
Gabbro dike intruding FRB north of Devil's Gate
Red Ant schist
Metaplagiogranite interlayered with “internal schist”
“External schist”
322 ± 27, 345 ± 9; 343.7 ± .6
388 ± 22/−12
285 ± 8
248; 272 ± 6;
387 ± 7
N174
306–324
236 ± 4
K/Ar, hornblende; Ar/Ar, hornblende
U/Pb, zircon
K/Ar, hornblende
K/Ar, hornblende; Ar/Ar, hornblende
Ar/Ar hornblende
K/Ar, muscovite
U/Pb, zircon
Ar/Ar hornblende
Böhlke and McKee (1984); Hacker (1993)
Saleeby et al. (1989)
Hietanen (1981)
Hietanen (1981); Standlee (1978)
Standlee (1978)
Schweickert et al. (1980)
Saleeby et al. (1989)
Weisenberg and Avé Lallemant (1977)
Please cite this article as: Smart, C.M., Wakabayashi, J., Hot and deep: Rock record of subduction initiation and exhumation of hightemperature, high-pressure metamorphic rocks, Feather River ultramafic belt, California, Lithos (2009), doi:10.1016/j.lithos.2009.06.012
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undergone pumpellyite–actinolite grade metamorphism in the
regions flanking the FRB, (Day et al., 1988; Hacker, 1993).
The FRB is faulted against the Calaveras Complex on its west and,
locally, the Red Ant schist. The Calaveras Complex is considered a
subduction complex composed mainly of phyllite and metachert with
blocks of volcanic rocks (Hietanen, 1981; Sharp, 1988; Edelman et al.,
1989). In-situ conodonts and fusulinids found in the metasediments
indicate that they were deposited at least as late as the Permian and
that the unit youngs westward (Hietanen, 1981; Bateman et al., 1985).
The subduction–accretion or assembly of the Calaveras Complex was
underway by 177 Ma, based on the U/Pb zircon age of a pluton that
cross cuts some of the earlier structures within the Calaveras Complex
(Sharp, 1988). In the Calaveras Complex is mainly of pumpellyite–
actinolite grade in regions adjacent to the FRB (Day et al., 1988;
Hacker, 1993).
3
The Red Ant schist consists of quartz-rich schists (metachert and
metaclastic rocks) and metavolcanic rocks that underwent blueschist
facies metamorphism (Schweickert et al., 1980; Hietanen, 1981;
Edelman et al., 1989). The Red Ant schist crops out structurally
beneath and west of the Devil's Gate ophiolite, whereas 10 km to the
south it occurs east of and structurally beneath the FRB in the North
Yuba River area (Edelman et al., 1989). In the North Yuba River area an
amphibolite-grade unit, known as the Alleghany schist, is found
structurally beneath FRB ultramafic rocks and structurally above Red
Ant schist. Radiometric dates on the Alleghany schist are 322 ± 27 and
345 ± 9 Ma (K/Ar, hornblende; Böhlke and McKee, 1984) and 343.7 ±
0.5 Ma (Ar/Ar, hornblende; Hacker, 1993). A K/Ar age on a white mica
taken from the Red Ant schist indicates that the metamorphic age of
the Red Ant schist is at least 174 Ma (Schweickert et al., 1980). The
actual metamorphic age of the Red Ant schist is difficult to interpret
Fig. 2. Geologic map of the western border of the Feather River ultramafic belt.
Please cite this article as: Smart, C.M., Wakabayashi, J., Hot and deep: Rock record of subduction initiation and exhumation of hightemperature, high-pressure metamorphic rocks, Feather River ultramafic belt, California, Lithos (2009), doi:10.1016/j.lithos.2009.06.012
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because the closure temperature for the white mica may be close to
(or have been exceeded by) the temperature of the ubiquitous
pumpellyite–actinolite overprint of all of the metamorphic terranes in
this part of the northern Sierra Nevada, and because excess argon
cannot be directly interpreted in conventional K/Ar results (Hacker,
1993).
A thin unit (10 to 300 m thick) of amphibolite, locally garnetbearing, crops out along the west border of the FRB in the North Fork
Feather River area (Fig. 2). Williams and Smyth (1973) and Ehrenberg
(1975) proposed that these high-grade metamorphic rocks may be a
metamorphic sole. Our study focuses on this unit and its structural
and metamorphic relationships to adjacent units.
3. Field relationships and structural geology
3.1. Geologic units and lithologies
The study area covers ~15 km2 of area near the confluence of the
North Fork Feather River and the East Branch North Fork Feather River
(Fig. 2) and spans the western border of the FRB, where it is faulted
against the eastern Calaveras Complex. The Calaveras Complex and
FRB units in this area generally strike northwesterly and dip steeply to
the east and are intruded by gabbroic and dioritic bodies that cross cut
the major foliation in both the FRB and Calaveras Complex (one of the
latter is shown on the northwestern part of Fig. 2).
The variably serpentinized ultramafic rocks of the Feather River
ultramafic body make up the easternmost and structurally highest
unit in the area. In this region, the ultramafic belt is 3–5 km wide.
These ultramafic rocks are metamorphosed in amphibolite facies
conditions with characteristic minerals such as tremolite, talc, and
locally anthophyllite, with antigorite (Ehrenberg, 1975, this study).
The tremolite reaches a centimeter in length and is commonly several
mm long. These rocks have a metamorphic foliation defined by planar
layering of amphibole long-axes. Although foliated, many of these
rocks form massive outcrops with relatively sparse fractures. In
contrast some outcrops exhibit closely spaced (cm scale or less)
fractures or a brittle foliation, and some of these fractured rocks tend
to have been retrograded to lower grade serpentinite mineralogy
(lizardite-dominated). In this area, crustal rocks are rare within the
ultramafic body and this is generally representative of the FRB as a
whole (Ehrenberg, 1975). Crustal rocks within the ultramafic body
consist of small (less than a hundred meters in long dimension) postmetamorphic dioritic or gabbroic intrusions, and lenses up to 3 km in
the long dimension of amphibolite that have been called the internal
schists (Ehrenberg, 1975). Saleeby et al. (1989) obtained a U/Pb date
of 306–324 Ma for a plagiogranite (tonalite) that intrudes internal
schist and Weisenberg and Avé Lallemant (1977) report an Ar/Ar
hornblende age of 236 ± 4 Ma from the same unit.
The western contact of the ultramafic body, the Rich Bar fault, dips
moderately to steeply eastward and appears to steepen in dip or
become overturned in the southernmost part of the field area (Fig. 2).
This contact is offset by late brittle faults (Fig. 2). Directly west of the
contact is a b300 meter thick unit of amphibolite facies metamorphic
rocks, the external schist of Ehrenberg (1975), the rocks that have
been proposed as a possible metamorphic sole (Williams and Smyth,
1973; Ehrenberg, 1975). The external schist consist primarily of
amphibolite, with lesser amounts of metachert (nearly pure quartz
with very small amounts of garnet, white mica, and other phases), and
somewhat intermediate rock, that we call “quartz-bearing amphibolite” that may have been mafic rock with cm-scale (or less) chert
interlayers or lenses. The structurally highest part of the external
schist includes garnet amphibolites (Fig. 2). Plagioclase amphibolite
or epidote amphibolite make up most of the structurally lower parts of
the external schist. The metamorphic grain sizes of most minerals
range from tenths of a mm, to about 2 mm for most these rocks. The
external schist shows abundant partial melting textures (Fig. 3),
indicative of peak metamorphic temperatures above the wet basalt
solidus. Some of the external schist in the Rich Bar area (southern part
of Fig. 2 along East Branch Feather River) appears bluish in outcrop,
but microprobe analyses (see below) show that these rocks lack sodic
amphibole or other blueschist facies minerals.
West of, and structurally beneath, the external schist crop out
slates/phyllites, cherts, and minor metavolcanic rocks of the Calaveras
Complex. This unit has the aspect of a melange with a slate/phyllite
matrix and chert blocks up to tens of meters or so in long dimension.
The structurally highest part of Calaveras Complex rocks exposed near
the Beldon Siphon (Fig. 2) appear to have a coarser metamorphic grain
size (metamorphic white mica and actinolite to several tenths of a
mm) than the very fine grained (hundredths of mm metamorphic
grain sizes) rocks that characterize the remainder of the Calaveras
Complex in this area. The Beldon Siphon exposures also include what
appear to be metamorphosed breccias with clasts of amphibolite and
mafic volcanic rocks set in a phyllite or fine white mica quartz schist
matrix.
A hornblende gabbro dike or small pluton intrudes the Calaveras
Complex and cuts the northern section of the external schist in Yellow
Creek canyon (“gb” in the northwestern part of Fig. 2). This dike lacks
the foliation seen in the Calaveras Complex and external schist and
lacks high-grade metamorphism.
Additional samples were collected from the Devil's Gate ophiolite,
about 40 km southeast of the study area (Fig. 1), for comparison with
samples from the field area. Here sheeted dikes and pillow basalts are
recognizable despite having been metamorphosed at amphibolite
grade (Edelman et al., 1989).
3.2. Structural geology
Fig. 3. Photo of melt segregations in external schist. The black arrows point to areas
where zones that appear to have been melt rich (felsic material between small pieces of
amphibolite restite) restite) feed leucosomes. This location about 15 m north of sample
location FR6.
The ultramafic rocks, internal schist and external schist exhibit a
foliation defined by the planar alignment high-temperature metamorphic minerals that strikes northwest and dips northeast (Fig. 2).
Foliations in the southeast portion of the area tend to dip vertically or
to the west. The steepening is apparent in the map patterns exhibited
by the external and internal schist contacts. Throughout the area the
foliations are subparallel to the contact between the external schist
and the ultramafic unit. The external schist has a stretching lineation,
most easily recognized by the alignment of the long axes of
amphiboles. This lineation orientation shows much scatter. Lineations
Please cite this article as: Smart, C.M., Wakabayashi, J., Hot and deep: Rock record of subduction initiation and exhumation of hightemperature, high-pressure metamorphic rocks, Feather River ultramafic belt, California, Lithos (2009), doi:10.1016/j.lithos.2009.06.012
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5
Fig. 4. Photomicrograph of garnet amphibolite sample 92-2. Plane polarized light.
Abbreviations: grt: garnet, hbl: hornblende; rt: rutile.
Fig. 5. Photomicrograph of sample FR6, a quartz-rich garnet–clinopyroxene amphibolite. Plane polarized light. Abbreviations as for Fig. 4 and: cpx: clinopyroxene, qtz:
quartz, ttn: titanite, phen: phengite.
plunge direction is tied to the foliation dip, so east-plunging lineations
are associated with east-dipping foliation. C and s surfaces, shear
bands, and asymmetric porphryclast tails, and asymmetric small-scale
(generally centimeter to meter scale) folds in the foliation, consistently ultramafic-side-up sense-of-shear, which is tops-to-the-west
for east-dipping foliation. Isoclinal folds are common in the external
schist, although these are most easily observed only in outcrops that
show pronounced compositional layering. Amplitudes of these folds
vary from centimeter scale to at least tens of meters. Axes of these
folds appear to be subparallel to the foliation strike. The consistent
shear sense orientation in the external shear sense indicates that this
sense of shear predates the folds, rather than being associated with
high-temperature passive flow or flexural slip folding which would
result in opposite senses of shear on opposing limbs of folds. The early
isoclinal folds are themselves folded by at least one generation of more
open folds at the scale of meters or larger, and these later generations
of folds are responsible for the scatter in the foliation and lineation
orientations.
Foliation within the Calaveras Complex is subparallel to the
foliation in the external schist and ultramafic rocks. Calaveras complex
folding, and shear sense within the Calaveras Complex were not
evaluated in this study.
been heavily retrogressed or altered, with a fine-grained growth of
white mica, and Ca-silicate minerals. Epidote tends to show a higher
pistachite content (higher birefringence and deeper yellow pleochroism) in epidote amphibolite than the garnet-amphibolite. Rutile is
commonly rimmed and in some cases nearly entirely replaced by
titanit. Garnet is rare in the metabasite and most are badly retrograde
and fractured, and consist of fragments generally less than 0.1 mm, but
some reach 4 mm. The grain size of most minerals in amphibolite range
from 0.1 to 5 mm with the tendency for somewhat finer grain sizes in
epidote amphibolite. Hornblendes ranges in size from 1 to 5 mm.
Hornblendes exhibit variable retrogression and commonly have
patchy brownish regions with surrounding areas of brownish green,
green and bluish green amphibole. Epidote found in the garnet
amphibolite is usually 1–2 mm while those found in the epidote
amphibolite ~ 0.1 mm in size. Pale green clinopyroxene up to 0.5 mm in
size occurs in some plagioclase amphibolite. We did not find
clinopyroxene in mafic garnet amphibolite (Fig. 4), whereas clinopyroxene does occur in the quartz-bearing garnet amphibolite.
Hornblende shows a strong preferred orientation with the long
axes lying in the foliation planes. Quartz shows evidence of plastic
4. Petrography
Mineral abbreviations in the following sections are from Kretz
(1983).
4.1. External schist
We have divided the external schist into three main rock types,
metabasites, rocks that appear to reflect fine interlayering of
metacherts and metabasite in varying proportions, and metacherts.
The metabasites of the external schist can be divided into garnet
amphibolite (Hbl + Grt ± Ep ± Ab ± Qtz + Rt), plagioclase amphibolite
(Hbl + Pl (entirely or nearly entirely replaced by Ab) ± Cpx ± Qtz + Rt
or Ttn) or epidote amphibolite (Hbl + Ab + Ep ± Qtz ± Chl with either
Rt or Ttn). Some amphibolite from the Rich Bar area also have rare
bluish rims on green or green-brown hornblende. Apparent blue
amphibole rims on hornblende from amphibolite associated with the
FRB have been previously identified by Ferguson and Gannett (1932)
from the Alleghany district in the Yuba River region, about 60 km
southeast of the field area. We did not find fresh plagioclase (excluding
albitic plagioclase) in the external schist. The (inferred) plagioclase has
Fig. 6. Back scattered electron (BSE) image of FR6. garnet (grt), phengite (phen),
clinopyroxene (cpx) with albite (ab), and quartz (qtz). The rims of the phengites are
very slightly brighter than the cores, probably reflecting higher Fe concentration and
correspondingly higher Si substitution; this subtle difference is best viewed on the
upper of the two phengites in the view.
Please cite this article as: Smart, C.M., Wakabayashi, J., Hot and deep: Rock record of subduction initiation and exhumation of hightemperature, high-pressure metamorphic rocks, Feather River ultramafic belt, California, Lithos (2009), doi:10.1016/j.lithos.2009.06.012
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schist (Fig. 2). Rutile is widely distributed at all structural levels of the
external schists in both the amphibolites and quartz-rich amphibolites. Titanite commonly rims rutile (Fig. 7) and in some samples
nearly completely replaces it. Some of the amphibolites do not contain
rutile, but contain titanite only; ilmenite appears to rim titanite in
some samples.
The metachert consists mainly of quartz (generally 90% or more)
with layers of phengite and sparse garnets. The quartz grains are 0.1–
0.5 mm in size. Their textures show significant plastic strain with
subgrain development, ribbon grains, and c and s surface development. Garnets in the metachert are small (0.5 mm) and broken up.
Bluish (hand specimen) rocks that resemble blueschist from Rich Bar
area are quartz rich (possibly metacherts) with pale green amphiboles
with dark blue rims, stilpnomelane, and rare garnet.
4.2. The internal schist
Fig. 7. Photomicrograph showing titanite (ttn) rimming relics of rutile (ru) in sample
FR6. Plane polarized light. Other abbreviations same as Figs. 4 and 5.
strain with ribbon grains and subgrain development. Asymmetric
shear fabrics with shear bands and c and s surfaces are common.
Quartz-bearing amphibolite, which may have been derived from
variable proportions of finely interleaved metachert and metabasite,
contains 10–50% quartz. Those rocks with the highest quartz contents
may be impure (perhaps tuffaceous) metacherts or siliceous metatuffs, whereas those with the lowest quartz contents may represent
metabasites with limited intercalated metachert. The primary (highgrade) assemblage is Hbl + Qtz + Cpx + Grt + Phen ± Pl ± Chl ± Rt ±
Zo. Biotite has been reported from these rocks (Ehrenberg, 1975;
Hacker and Peacock, 1990) and we observed grains in some samples
that may have been biotite, but have been altered to chlorite and clay
minerals. Most garnet is 0.5 mm in size, but some reach 2 mm. Garnet
is commonly rather heavily altered or retrogressed with ragged rims
and common replacement by chlorite. The garnet tends to be heavily
fractured with considerable alteration along the fractures, both of
garnets and the abundant inclusions (Fig. 5). Very pale green
clinopyroxene is typically 0.5–1 mm forms heavily fractured grains
with ragged margins (Fig. 6). The pyroxene appears to be intergrown
with albite, the textural affinity of which is not clear, and fine-grained
alteration minerals are present along fractures and margins of grains.
Hornblende is brownish with occasional green rims and locally pale
green actinolitic outermost rims. The hornblende commonly is less
than 1 mm, but some reach 3 mm in size. Hornblendes appear to
reflect variable retrogression with irregular brownish patches in the
interior of the grains surrounded by greenish or brownish green
amphibole, with actinolite representing the texturally latest amphibole forming the rims or along fractures. Phengite forms grains of
0.5 mm or smaller (average about 0.1 mm). It appears to have grown
in multiple textural generations with an earliest generation in
apparent textural equilibrium with the garnet, clinopyroxene and
brown amphibole (Fig. 6) defining an early foliation. The texturally
early phengite grains tend to be the larger ones, and many of them are
bent with undulatory extinction. Later strain-free phengites are found
in the matrix, and also cross cutting the early, larger grains. Zoisite
occurs as elongate grains up to 2 mm in length and is colorless with
low birefringence, and anomalous colors on parallel extinction; it
appears in textural equilibrium with garnet, clinopyroxene, and
brown hornblende. Secondary pumpellyite is present, as limited
overgrowths and as comparatively rare vein filling. Foliation and fabric
in the quartz-bearing amphibolite resembles that of the mafic
amphibolite.
Garnet in both quartz-bearing amphibolites and the metabasites
appears to be restricted to the upper structural levels of the external
The internal schist consists mainly of plagioclase amphibolite (Hbl +
Pl (replaced by Ab)± Qtz± Rt± Ttn) that is interleaved with hornblendite (Hbl ± Rt ± Ttn). No garnets were found by us or Ehrenberg
(1975) in the internal schist. In some samples the hornblende is optically
homogeneous and olive green whereas in others the amphibole is zoned
from a pale, apparently actinolitic core to an olive green rim (Fig. 8).
Plagioclase locally appears to be fresh, although we were unable to
obtain to find plagioclase during our electron microprobe analysis (see
below), whereas in many other samples, it is riddled with later alteration
products that include fine-grained white mica and other minerals. Kfeldspar occurs in felsic segregations in the amphibolites. Euhedral
titanite is common in the plagioclase amphibolites. Late veins of
prehnite are common. Grain sizes are 1–3 mm for the plagioclase
amphibolite and 3–5 mm for the hornblendite. The planar orientation of
amphiboles and amphibole-rich and feldspar-rich layers define the
high-temperature foliation in these rocks. A preferred elongation
direction in amphiboles defines a mineral lineation that appears to be
present in some samples of the internal schist. Ehrenberg (1975)
interpreted these rocks as metagabbros, but we did not find any samples
that exhibited textures that suggest a gabbroic, rather than basaltic
protolith. The lack of associated metasediments (such as metacherts or
the quartz-bearing amphibolites) in the internal schists may suggest
gabbroic, rather than basaltic protolith, however.
Fig. 8. Photomicrograph of internal schist sample FR16. Plane polarized light. Most of
this view shows amphiboles that are zoned from pale actinolitic cores to darker
hornblende rims. Rutile grain is shown. Most of the rutile in this sample has been
replaced by titanite.
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7
rocks found at Belden Siphon (Fig. 2) are somewhat coarser grained
with metamorphic grain sizes up to 0.3 mm. A breccia, noted above
contains a matrix of quartz-phengite schist with some actinolite, and
clasts up to several cm in size of amphibolite. These amphibolite clasts
appear similar to the plagioclase amphibolites of the external schist.
Many of the amphibolite clasts contain rutile and some have bluish
rims on the hornblendes.
5. Electron microprobe analysis of selected phases
Fig. 9. Photomicrograph of sample from sheeted dike unit of the Devil's Gate ophiolite.
Plane polarized light. Abbreviations: act: actinolite, alt-px: probable altered pyroxene
(completely replaced by fine-grained intergrowth of minerals), hbl: hornblende, ilm:
ilmenite.
4.3. Devil's gate ophiolite
In order to assess metamorphic contrasts within the FRB, we
examined samples from the Devil's Gate ophiolite, 40 km southeast of
the study area. In contrast to the external schists, the Devil's Gate
ophiolite appears to represent the remnants of the upper plate of
subduction system in contrast to the apparent metamorphic sole rocks,
the external schists. The Devil's Gate ophiolite samples appeared to
have had upper oceanic crustal protoliths: dikes and pillow basalts. We
examined them in order to assess the contrast between the
metamorphism of the upper plate of a subduction system and the
apparent metamorphic sole. The upper crustal lithologies were chosen
because they were reported in the literature (e.g. Edelman et al., 1989;
Hacker, 1990) to have been metamorphosed at amphibolite grade, and
because amphibolite grade metamorphism in the dikes and/or basalt
levels of an ophiolite is higher than expected for sea floor metamorphism (e.g., Alt and Teagle, 2000; Schiffman and Smith, 1988).
A sample from a sheeted dike outcrop has hornblende to 2 mm in
size that is zoned from a pale green actinolitic core to a greenishbrown rim (Fig. 9). Plagioclase appears to have once been part of the
metamorphic assemblage, but it is largely replaced by albite and dense
mats of fine-grained white mica and other minerals. Brownish clots of
minerals appear to replace former blocky mineral forms; these may
have been igneous or metamorphic pyroxene. Some clinopyroxene to
0.5 mm remains in this rock but there is no direct textural connection
between this clinopyroxene and the brownish mineral clots. Because
this clinopyroxene is locally concentrically rimmed by actinolite and
hornblende outward, it is likely igneous clinopyroxene. Ilmenite
occurs as irregular opaque grains to 0.7 mm in size. Texturally late
prehnite is common. Although most of the hornblende exhibits a
somewhat static fabric without notable preferred orientation, shear
zones cut the rock and these shear zones have plastically-deformed
quartz and albite, and green to brown green amphibole.
4.4. Calaveras complex
Calaveras Complex rocks in this area are commonly extremely fine
grained and many of them exhibit few metamorphic minerals that are
readily identifiable in thin section. Most metamorphic minerals have
grain sizes of 0.1 mm or less. The slate samples have little visible
mineralogy other than quartz, albite, and fine white mica. Cherts tend
to be nearly all quartz with some white mica. Metavolcanic rocks
contain quartz, albite, white mica, chlorite, epidote, and actinolite. The
In order to evaluate the P–T conditions of metamorphism as well as
identify certain minerals, mineral compositions were determined for
minerals from six samples, four from the external schist, one from the
internal schist, and one from the sheeted dike unit of the Devil's Gate
ophiolite. The mineral chemistry was determined using a CAMECA SX100 Electron Microprobe at the University of California, Davis. The
accelerating potential was 15 kV and the beam current was10 nA, with
counting times of 10 s for peaks, and 5 s for background. Amphiboles,
pyroxenes, garnets, and epidote minerals were analyzed with a beam
diameter of 1 micron, whereas feldspars and phengites were analyzed
with a beam diameter of 10 μm. Mineral formulae were calculated from
data on the following basis: Amphiboles: For some site assignments
discussed in the text: 13 total cations excluding Ca, Na, and K, although
for Table 4 amphiboles formulae are simply charge balanced to 23
oxygens. Clinopyroxene: 6 oxygens and 4 cations. Garnet: 12 oxygens.
Phengite: 22 oxygens. Representative results for garnet, clinopyroxene,
phengite, and amphibole are shown in Tables 2 to 5, respectively.
5.1. Garnets
Garnets from a quartz-rich external schist (sample FR6) were pyrope
poor and rich in almadine and grossular. The compositional range of the
garnets is Py8–10Alm42–47Sp4–5Gr35–43And2–9Uva0–2, and they have an
average composition of Py9Alm43Sp5Gr38And6Uva1. Because of the
heavy alteration and fragmentation of the garnets in this rock, we
were unable to identify zoning. The garnet analyses we obtained show
relatively small compositional variation and no systematic spatial
variation. Whereas our analysis did not identify, neither could we
demonstrate that the original garnet was unzoned, owing to its poor
preservation. In addition, our analyses may be biased toward the core
regions of the garnet relics owing to greater degree of alteration and
fracturing of the rim regions. Garnet was analyzed from a metachert
Table 2
Garnet compositions (weight percent).
Sample analysis
MgO
CaO
MnO
FeO
Al2O3
Cr2O3
SiO2
TiO2
Total
Formula based on 24 O
Mg
Ca
Mn
Fe2+
Fe3+
Al
Cr
Si
Ti
Total
FR6
FR6
FR6
FR6
YR32
grt-1
grt-2
grt-3
grt-4
grt-1
2.27
15.00
2.24
20.38
22.88
0.00
37.19
0.16
100.12
2.17
15.22
2.12
20.37
22.16
0.04
38.60
0.10
100.79
2.26
15.31
1.96
20.35
22.02
0.02
37.26
0.17
99.36
2.26
15.04
2.02
21.19
21.96
0.01
37.88
0.11
100.46
0.82
4.96
25.71
9.92
21.45
0.01
36.97
0.42
100.26
0.53
2.50
0.30
2.45
0.21
4.20
0.00
5.80
0.02
16.01
0.50
2.53
0.28
2.66
0.00
4.05
0.01
5.98
0.01
16.02
0.53
2.58
0.26
2.46
0.21
4.08
0.00
5.85
0.02
15.99
0.52
2.51
0.27
2.59
0.17
4.03
0.00
5.90
0.01
16.00
0.20
0.86
3.51
1.35
0.00
4.08
0.00
5.96
0.05
15.96
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C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx
Table 3
Clinopyroxene compositions (weight percent).
Sample analysis
Na2O
MgO
Al2O3
SiO2
K2O
CaO
TiO2
FeO
MnO
Cr2O3
Total
Formula based on 6 O
Na
Mg
Al
Si
K
Ca
Ti
Fe2+
Fe3+
Mn
Cr
Total
Table 5
Amphibole compositions.
FR6
FR6
FR6
cpx-1
cpx-2
cpx-3
0.28
12.00
0.75
52.96
0.01
22.75
0.00
10.46
0.14
0.02
99.37
0.33
12.08
0.94
52.83
0.01
23.75
0.02
9.90
0.18
0.03
100.08
0.35
12.51
1.11
52.72
− 0.01
23.57
0.05
9.29
0.18
0.01
99.80
0.02
0.68
0.03
2.01
0.00
0.92
0.00
0.36
0.00
0.00
0.00
4.02
0.02
0.68
0.04
1.98
0.00
0.96
0.00
0.30
0.01
0.01
0.00
3.98
0.03
0.70
0.05
1.98
0.00
0.95
0.00
0.27
0.02
0.01
0.00
4.01
(YR32) that showed zoning in backscatter electron (BSE) imaging, but
only two usable analyses were obtained. This garnet was spessartine rich
with a composition of Py4–8Alm19–30 Sp42–62Gr8–14And6–7Uva0.
5.2. Amphibole
Amphiboles were analyzed from the quartz-bearing external schist
(FR6), plagioclase (former) amphibolite from the external schist (YR45),
a quartz-bearing schist or metachert from the external schist that
appeared to have blue amphibole rims on green amphibole (RB52), and
Table 4
Phengite compositions (weight percent).
Sample analysis
FR6
FR6
FR6
FR6
FR6
wm-1
wm-2
wm-2 (rim)
wm-3
wm-3 (rim)
4.40
0.18
10.43
2.88
0.04
0.01
3.40
NA
28.28
0.01
48.60
0.04
98.25
4.44
0.11
10.13
2.82
0.15
0.06
4.12
0.49
28.27
0.02
49.04
0.15
99.80
4.49
0.22
10.21
3.57
0.01
0.03
4.70
0.09
25.18
0.00
52.13
0.02
100.66
4.49
0.22
10.38
2.59
0.00
0.06
3.85
0.77
31.10
0.05
47.56
0.21
101.29
4.44
0.02
9.97
4.33
0.03
0.05
6.01
0.01
21.85
0.00
53.11
0.00
99.81
Formula based on 22 O
Na
0.04
K
1.78
Mg
0.55
Ca
0.00
Mn
0.00
Fe
0.37
Ba
NA
Aliv
3.25
Alvi
1.48
Cr
0.00
Si
6.52
Ti
0.01
Total
18.00
0.03
1.75
0.57
0.02
0.01
0.47
0.03
3.12
1.38
0.00
6.62
0.02
18.00
0.06
1.74
0.71
0.00
0.00
0.52
0.01
2.94
1.03
0.00
6.96
0.00
17.96
0.06
1.77
0.52
0.00
0.01
0.43
0.04
3.19
1.68
0.01
6.35
0.02
18.09
0.00
1.72
0.87
0.00
0.01
0.68
0.00
2.69
0.80
0.00
7.18
0.00
17.94
H2O
Na2O
K2O
MgO
CaO
MnO
FeO
BaO
Al2O3
Cr2O3
SiO2
TiO2
Total
Sample analysis
H2O
Na2O
K2O
CaO
MgO
MnO
FeO
Al2O3
Cr2O3
SiO2
TiO2
Total
FR6
FR6
YR45
RB52
FR16
FR16
DGO1
am-2
am-4
am-3
am-3
am-4
am-5
am-2
2.01
1.37
1.60
11.71
9.08
0.27
16.48
15.32
0.01
41.55
0.86
100.26
2.03
1.29
1.63
11.65
9.86
0.17
15.55
15.37
0.04
42.35
0.88
100.83
2.00
2.22
0.36
10.51
9.45
0.30
19.67
13.13
0.01
42.53
0.64
100.82
2.07
0.43
0.03
11.64
15.16
0.38
13.15
1.75
0.01
54.44
0.06
99.14
2.00
1.89
0.49
10.96
9.75
0.39
18.52
12.84
0.03
42.60
0.82
100.29
2.00
1.54
0.43
11.06
10.38
0.37
17.62
10.82
0.02
44.27
0.81
99.30
2.07
0.93
0.33
11.43
14.93
0.21
12.69
8.13
0.08
47.90
1.20
99.90
0.63
0.07
1.64
2.05
0.04
1.15
1.25
1.74
0.52
0.00
6.19
0.07
15.33
0.12
0.02
1.82
3.22
0.05
1.36
0.28
0.15
0.08
0.00
7.85
0.00
14.96
0.54
0.09
1.72
2.13
0.05
1.22
1.05
1.67
0.55
0.00
6.24
0.09
15.35
0.44
0.08
1.75
2.28
0.05
1.30
0.87
1.38
0.50
0.00
6.53
0.09
15.27
0.26
0.06
3.16
1.74
0.03
0.54
0.97
1.07
0.30
0.01
6.81
0.13
15.06
Formula proportion based on 23 oxygens
Na
0.39
0.37
K
0.30
0.30
Ca
1.86
1.82
Mg
2.00
2.15
Mn
0.03
0.02
1.62
1.43
Fe2+
0.42
0.47
Fe3+
Aliv
1.75
1.72
Alvi
0.92
0.93
Cr
0.00
0.00
Si
6.15
6.18
Ti
0.10
0.10
Total
15.55
15.49
plagioclase amphibolite from the internal schist (FR16). We also
analyzed amphibole from a sheeted dike sample from the Devil's Gate
ophiolite. The amphiboles are edenites and pargasites (Yavuz, 2007)
(Fig. 10). Toward more aluminous amphiboles in Fig. 10 is a bit deceiving
in that actinolite was intentionally avoided (not entirely with success)
with the exception of RB52 where it is the primary calcic amphibole;
actinolite appears as a late overprint in most of the external schist. In
Fig. 11, aluminum and titanium contents of amphibole are shown, as
these concentrations are pressure and temperature dependent, respectively in high-temperature calcic amphiboles (Ernst and Liu, 1998). Note
that Fig. 11 excludes the more actinolite-rich analyses, but is comprised
of variably retrogressed amphiboles. The variable retrogression appears
to be reflected by the positive correlation between Al and Ti contents of
amphibole in each sample. The spread of data shows the greatest scatter
or spread for FR6 consistent with the greater degree of retrogression of
amphiboles seen in thin sections that sample.
In RB52 the blue amphibole rims do not appear to be sodic
amphibole. Although most of these rims are small enough so that they
are difficult to analyze, a few patches were large enough so that we
believe the microprobe analysis consists entirely of one of these
patches instead of a combination between the patch and the main
core amphibole. These analyses show the bluish rims and patches to
be richer in magnesioriebeckite component than the core amphibole,
but the NaB occupancy is no higher than about 0.4. These results are
similar to microprobe results obtained by the second author on
apparent blue amphibole rims on garnet amphibolite of the external
schist from the Rich Bar area in 1987.
5.3. Clinopyroxene
Clinopyroxene from quartz-bearing amphibolite (FR6) span a
compositional range is Wo47–49En34–36Fs15–17 Jd1–2. Clinopyroxene
exhibits inclusions or intergrowths of albite (Fig. 6). Although the
clinopyroxene is heavily fractured and has fine-grained alteration
products growing along these fractures, the remaining pyroxene does
not appear to have suffered much retrogression or alteration, based on
the narrow range of compositions. It is possible that the albite within
Please cite this article as: Smart, C.M., Wakabayashi, J., Hot and deep: Rock record of subduction initiation and exhumation of hightemperature, high-pressure metamorphic rocks, Feather River ultramafic belt, California, Lithos (2009), doi:10.1016/j.lithos.2009.06.012
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Fig. 10. Amphibole compositions and classification.
the clinopyroxene represents (1) growth in equilibrium with
clinopyroxene, (2) exsolution of a Na-poor clinopyroxene and albite
from an earlier Na-rich clinopyroxene, because there are no other
obvious minerals that replace the clinopyroxene or (3) a later
retrograde product that may have formed in conjunction with other
retrograde minerals along the fractures.
5.4. Phengite
Phengite in FR6 varies in Si content from 3.23 to 3.72 formula units,
based on an 11 oxygen formula. Fe content ranged from .14 to .43, and Mg
content ranged from .22 to .46. We did not analyze all of the phengites
for barium, but those we did showed low Ba concentrations (Table 4).
Phengite analyses can be divided into two groups, those with low Si
contents of about ~3.3 (3.18–3.37) and those with high Si contents
(3.45–3.72). Owing to the small size of the grains (most grains had a
width of no more than four microprobe beam diameters) we could not
quantitatively evaluate zoning in the phengites. The higher Si phengite is
texturally late. This can be seen in BSE imagery, where lighter rims
(higher average atomic number with higher phengite substitution) are
seen on some of the larger phengite grains (Fig. 6).
5.5. Feldspars
We tried to find plagioclase relics in the various amphibolite
samples but could not. Many grains that appeared to be plagioclase in
Fig. 11. Aluminum and titanium contents in amphiboles. Note that this plot includes some partly retrogressed amphiboles, but excludes some of the more actinolitic (clearly late)
amphiboles. It is likely that only the highest aluminum and titanium concentrations reflect the true high-grade amphibole compositions.
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C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx
thin section, proved to be albite and various intergrown alteration
minerals. In FR16 our analyses showed potassium feldspar was
present in the felsic layers in that sample. The other feldspar identified
was nearly pure albite in samples YR45.
6. Thermobarometry
Peak P–T conditions were estimated for several samples from the
external schists, one from the internal schists and one from the
sheeted dike unit of the Devil's Gate ophiolite (results summarized on
Fig. 12). For all samples, we used Ernst and Liu (1998) amphibole
thermometry and barometry. We also noted the presence of partial
melting textures in many rocks that indicated that temperatures
exceeded the wet basalt solidus. The presence or absence of garnet in
mafic rocks and the occurrence of rutile were also employed to
estimate minimum or maximum pressure. One sample, quartz-rich
amphibolite/schist FR-6 contained garnet, clinopyroxene, hornblende,
phengite and rutile, so we were able to apply all of the above
constraints in addition to estimated temperature from the garnet–
clinopyroxene Fe–Mg exchange reaction of Ravna (2001), and
pressure by garnet–clinopyroxene–phengite barometry of Ravna and
Terry (2004). An older calibrations of the grt–cpx Fe–Mg exchange
reaction (Ellis and Green, 1979) was included for comparison.
Similarly the older grt–cpx–phen calibration of Waters and Martin
(1993); with empirical correction published in Wain et al. (2001) was
used for comparison to results obtained from the Ravna and Terry
(2004) calibration. We shall present the thermobarometric estimates
for sample FR-6 first, then review those of the other samples.
Apparent partial melting textures were found on the outcrop
where sample FR6 was collected constrain the minimum temperature
to 650 °C based on wet basalt solidus (Poli, 1993; Peacock et al., 1994)
(Fig. 12). Garnet–clinopyroxene thermometry (Ravna, 2001) gives a
temperature range of garnet–clinopyroxene pairs of about 550–
760 °C. Owing to the degraded nature of garnets, that may have
influenced the range in their composition, we prefer the higher part of
the temperature range. Rutile found in FR6 constrains the minimum
pressure to be about 1.3 GPa at 650 °C (Ernst and Liu, 1998).
The maximum pressure of 2.2 GPa at 660 °C is from the stability of
hornblende in wet basalt or amphibolite (Peacock et al., 1994; Liu
et al., 1996). Application of garnet–clinopyroxene–phengite barometry (Ravna and Terry, 2004) yielded pressures of about 1.4–1.9 GPa at
650–760 °C for phengite cores (of about Si 3.3). Because of the
somewhat ambiguous textural relationships of the phengite we also
did the P–T calculations using what we believed to be texturally late
phengites (high Si) as the phengite in equilibrium with garnet and
clinopyroxene. This produced ultrahigh pressure (UHP) results
(N3 GPa) which we consider unrealistic owing to the lack of UHP
mineral assemblages and presence of hornblende. This supports our
interpretation of the high Si phengites as being late and not in
equilbrium with the high T assemblage. The updated version of the
Waters and Martin (1993) calibration (Wain et al., 2001) applied to
phengite cores yielded pressures of 2.2–2.4 GPa at 650–760 °C; most
of this range is above estimated hornblende stability. The amphibole
thermobarometer of Ernst and Liu (1998) applied to the calcic
amphiboles gave a temperature of about 670–710 °C at 1.7–2.0 GPa.
Owing to the retrograded nature of the amphiboles in all of our rocks,
we selected amphiboles with reasonable stoichiometry and good
totals that had the highest Al and Ti contents. Our selection of the
compositional clusters with the highest Al and Ti contents for each
sample may artificially restrict the natural compositional variation
and uncertainty in the P–T estimate, or it is possible that we are
overestimating the compositional variability (and resultant P–T
estimate range) by including some variably retrogressed amphiboles
in our analysis. It is difficult to assess the true variability of the highgrade amphibole compositions with such widespread retrogression.
For the samples other than FR6, we cannot directly compare the
same methods, except for the presence or absence or rutile, but we can
compare amphibole compositions. P–T conditions for YR45, which is
structurally low in the external schist, based on amphibole thermobarometry are 630–670 °C and 1.55–1.75 GPa. The presence of rutile
places a lower limit of 1.3 Gpa for peak metamorphic conditions.
Amphibole thermobarometry applied to the internal schist yielded
temperatures of 640–680 °C and pressures of 1.4–1.6 GPa (Fig. 12).
Partial melting textures are found in the internal schists suggest that
peak metamorphic temperature exceeded 650 °C. The presence of
rutile in the internal schist appears to suggest a minimum pressure of
about 1.3 GPa at 650 °C. Amphibole thermobarometry applied to the
Devil's Gate ophiolite sheeted dike sample results in estimated P–T
conditions of about 710–730 °C and 0.3–0.4 GPa. The presence of
ilmenite and lack of titanite, along with the lack of garnet, indicates a
maximum pressure of 0.7–0.8 GPa.
7. Discussion
7.1. Discussion of thermobarometric results
Fig. 12. Summary diagram of P–T estimates of metamorphism for various samples. Spots
and polygon are P–T estimates from amphibole thermobarometry of Ernst and Liu
(1998) for sample DGO (Devil's Gate ophiolite, sheeted dike sample), IS (internal
schist), and samples YR45 and FR6 of the external schist. Other thermometers and
barometers applied specifically to sample FR6 are as follows: R2000: Ravna (2001)
garnet–clinopyroxene thermometer (the lower limit for R2000 is well below the wet
basalt solidus, so it is not plotted), RT04: Ravna and Terry (2004) garnet–
clinopyroxene–phengite barometer, EG79: Ellis and Green (1979) garnet–clinopyroxene thermometer, shown for comparison, as well as WM: Waters and Martin (1993)
garnet–clinopyroxene–phengite barometer revised as in (Wain et al., 2001). The
preferred limits on the P–T conditions of metamorphism for sample FR6 are shown in
gray. Other curves shown: hbl out: hornblende breakdown (Liu et al., 1996), garnet-in
(Liu et al., 1996), ilmenite, rutile and titanite stability (Liu et al., 1996), and the wet
basalt solidus from Poli (1993).
In order to compare our P–T estimates to those published from
similar tectonic settings and for us to compare P–T conditions
between different samples we will further discuss the thermobarometry. Although the Waters and Martin (1993); with correction in
Wain et al. (2001) calibration of the grt–cpx–phen barometer
commonly gives lower P estimates in eclogites (Page et al., 2007),
we found it to give higher pressures for FR6. Whether or not the grt–
cpx–phen barometry is applicable to sample FR6 is unclear, owing to
the somewhat unclear textural association of the earliest formed
white mica, and the question as to whether the clinopyroxene
retained the composition it had at peak pressure. Accordingly, our
grt–cpx–phen results should be viewed with some caution.
The compositional dependence of mineral compositions and
stability may also impact the P and T estimates. To better evaluate
Please cite this article as: Smart, C.M., Wakabayashi, J., Hot and deep: Rock record of subduction initiation and exhumation of hightemperature, high-pressure metamorphic rocks, Feather River ultramafic belt, California, Lithos (2009), doi:10.1016/j.lithos.2009.06.012
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C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx
the applicability of the equilibria used in our PT estimates we obtained
major element chemical data by X-Ray fluorescence analysis (PANalytical MagiX Pro). With the exception of sample FR6, the external and
internal schists, and the Devil's Gate ophiolite samples appear to have
major and minor element compositions similar to MORB used in the
experiments that the Ernst and Liu (1998) amphibole thermobarometer was calibrated with (Table 6). FR6 is derived from a protolith that
is more felsic than MORB, perhaps MORB with interlayered chert, so
amphibole compositions in that sample may not be directly comparable to those in the other samples we investigated. The minimum
pressure represented by rutile is also compositionally dependent.
Rutile is widely distributed throughout the external and internal
schists in rocks of a wide range in compositions, from quartz-rich
rocks more felsic than FR6 to those of true MORB composition. Thus,
we believe it is reasonable to apply minimum pressure of rutile
occurrence summarized in Ernst and Liu (1998) that was based on
MORB. Similarly garnet stability is strongly compositionally dependent, but garnet is present in true MORB-derived metabasites (sample
92-3 in Table 6, for example) as well as the quartz-bearing FR6.
The highest temperatures obtained were the grt–cpx temperatures
estimated from sample FR6. It is possible that this thermometer did
not capture the peak temperature of metamorphism because: (1) our
analyses of garnet in FR6 may have avoided the more degraded rim
areas where prograde zoning may have been reflected; and (2) the
clinopyroxene may not necessarily reflect its composition at the peak
of metamorphism because of subsequent retrograde metamorphism
or exsolution.
In view of the above we believe the external and internal schists
formed at about 650–760 °C (possibly higher) at pressures greater
than 1.3 GPa (rutile stability MORB composition), but less than
2.2 GPa (hornblende stability in MORB composition), and the grt–
cpx–phen pressure estimate for sample FR6 falls within that broader
pressure range. Comparison of amphibole compositions between the
external and internal schists suggests that the external schists may
have formed at slightly to moderately higher pressures (say 0.1 to
0.7 GPa) than the internal schists and significantly higher pressures
(ca. 1 GPa) than the Devil's Gate ophiolite. The latter conclusion is also
consistent with the occurrence of ilmenite instead of rutile in the
Devil's Gate ophiolite. Although FR6 is not of MORB composition, its
amphibole compositions may indicate an inverted pressure gradient,
or imbrication, within the external schist, but the difference in bulk
composition between the non-MORB FR6 and MORB YR45 renders
this assessment uncertain. Garnet within metabasites (of approximate
MORB composition) is restricted to the structurally higher part of the
Table 6
Whole rock compositional data.
Sample oxide
SiO2
Al2O3
FeO
Fe2O3
Fe2O3b
MgO
MnO
TiO2
CaO
Na2O
K2O
P2O5
Total
FR6
92-3
YR45
FR16
DGO
68.23
10.00
51.33
13.96
51.98
15.11
51.46
13.33
47.60
18.47
6.02
4.20
0.09
0.93
10.43
0.2
0.92
0.224
101.187
15.27
10.17
0.09
1.23
7.55
1.04
0.22
0.091
100.9337
14.31
6.74
0.17
0.97
8.73
2.65
0.08
0.061
100.812
13.41
6.83
0.22
2.17
9.61
2.63
0.23
0.164
100.0643
9.56
10.25
0.17
0.67
13.59
0.59
0.12
0.008
101.0569
MORB
Experimentsa
49.11–52.38
12.74–16.93
7.55–10.72
1.89–3.23
6.58–10.31
0.17–0.22
1.24–2.51
10.05–11.10
1.93–3.76
0.06–0.49
0.15–0.27
99.28–100.75
a
MORB material compositions for amphibole experiments compiled by Ernst and Liu
(1998), including that study, Liou et al. (1974), Apted and Liou (1983), Spear (1981), Poli
(1993), and Helz (1979).
b
All iron as Fe2O3.
11
external schist (where sample FR6 was collected), and this qualitatively supports an inverted pressure gradient within the external
schists, as well as higher pressures of the structurally higher part of
the external schist compared to the internal schist.
The presence of pumpellyite, actinolite, and chlorite, as late
mineral growth in many of the samples is consistent with the
pumpellyite–actinolite overprint that is characteristic of many rocks
of the northern Sierra Nevada and crystallized at conditions of about
150–350 °C and 0.2–0.4 GPa (Hacker, 1993). The high Si content
(3.45–3.72 formula units, 11 oxygen formula) of the late phengites in
FR6 is not compatible with phengites associated with the pumpellyite–actinolite overprint (Hacker, 1993; Wakabayashi, unpub. data).
In contrast, such high-Si phengites in FR6 are similar in composition to
those observed in blueschist facies terranes (e.g. Sorenson, 1986;
Wakabayashi, 1990; El-Shazly et al., 1997; Smith et al., 1999; Tsujimori
and Liou, 2004). The late phengite growth may be evidence of a
blueschist facies overprint on this rock. Other evidence for a blueschist
facies overprint in the external schists is lacking, however. Thus,
external schists may have experienced retrograde blueschist facies
conditions, whereas all of the FRB and associated rocks (internal and
external schists, Devil's Gate ophiolite) underwent late pumpellyite–
actinolite metamorphism.
The PT estimates we obtained from the external schists (650–
760 °C, 1.3–2.2 GPa) are comparable to those obtained for other
metamorphic soles such as beneath the Semail ophiolite of Oman
(700–900 °C, 0.95 to 1.77 GPa (Gnos, 1998; Searle and Cox, 2002)),
beneath the Palawan ophiolite of the Philippines (700–760 °C,
N0.9 GPa (Encarnacion et al., 1995)), and beneath the Yarlung-Tsangpo
ophiolite of Tibet (750–875 °C, 1.3–1.5 GPa (Guilmette et al., 2008)).
7.2. Are the external schists a metamorphic sole?
As noted in the Introduction, high-grade metamorphic rocks have
been found in association with ophiolite belts that are not metamorphic soles, so it is useful to evaluate the field and petrologic data
from the external schists in light of the metamorphic sole hypothesis.
Williams and Smyth (1973) and Ehrenberg (1975) suggested that the
external schists may be a metamorphic sole on the basis of the highgrade metamorphism and contact with the ultramafic rocks. Our field
and structural data indicates that the external schists are a thin (10 to
300 m thick) unit that dips eastward beneath the ultramafic rocks and
that the high-temperature shear fabric in the external schist exhibits a
tops-to-the-west (ultramafic-side-up) sense-of-shear. Such a hightemperature fabric should be expected in metamorphic soles. The
external schists appear to be composed entirely of metabasite with
metachert, similar to most metamorphic soles (e.g., Spray, 1984;
Jamieson, 1986). Inverted temperature and pressure gradients found
within some soles (e.g., Jamieson, 1986; Gnos, 1998) may be present in
the external schists, but the limitations of our thermobarometry do
not allow us to define inverted metamorphic gradients as definitively
as the two studies cited above.
In summary we believe the external schists that field, lithologic,
structural, and metamorphic characteristics of the external schists are
typical of a metamorphic sole. We suggest these rocks formed at the
initiation of intra-oceanic subduction as previously proposed for many
metamorphic soles.
7.3. Tectonic synthesis: subduction initiation and exhumation of a
metamorphic sole
Our structural and petrologic data indicate that the external schists
formed as a metamorphic sole during the inception of eastward
subduction beneath the Feather River ultramafic belt. In addition, the
metamorphic data provides additional insight into the tectonic
evolution of the external schists and related rocks. The pressure of
metamorphism of the external schist (N1.3 Gpa) is well in excess of
Please cite this article as: Smart, C.M., Wakabayashi, J., Hot and deep: Rock record of subduction initiation and exhumation of hightemperature, high-pressure metamorphic rocks, Feather River ultramafic belt, California, Lithos (2009), doi:10.1016/j.lithos.2009.06.012
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C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx
what can be explained by the structural thickness of rocks intervening
between these rocks and the crustal rocks within or at the higher
structural levels of the FRB. The position of the internal schist is
equivalent to 1 to 1.5 km kilometers of cross-strike map distance from
the FR6 internal schist, corresponding to a maximum thickness of 1 to
1.5 km for a vertical average foliation dip. Owing to the uncertainties
in the barometric estimates it is difficult to estimate the absolute P
difference between the internal schist and FR6, but it is likely in the
range of 0.1 to 0.7 GPa, corresponding to a burial difference of 3.3 to
23.0 km, for an average overburden specific gravity of 3.1 (allowing for
some serpentinization of the overlying mantle as well as the
possibility of some eroded oceanic crust). This requires that faults of
an apparent normal sense accommodated exhumation of the external
schists relative to the internal schists. This exhumation is required
even if the peak metamorphism of the internal and external schists
occurred at significantly different times. The internal schists do not
appear to have been deeper than recorded by their peak metamorphic
assemblage. Amphiboles in some of the samples of internal schist
show prograde zoning with actinolite cores and there is no evidence of
high P retrograde metamorphism.
The Devil's Gate ophiolite dike sample, although collected about
40 km to the southeast of the main field area, appears to be
representative of the upper oceanic crust that may have overlain
much of the ultramafic rocks of the FRB. A comparison of the Devil's
Gate ophiolite with the internal and external schists is appropriate
because there is no evidence of an along-strike metamorphic
gradient in either internal or external schist metamorphism between
the study area and the Devil's Gate ophiolite (e.g. Hacker and
Peacock, 1990; Hacker, 1993). The Devil's Gate ophiolite sheeted
dike sample records a much lower metamorphic pressure than the
internal schists (about 0.6 to 1 GPa lower, corresponding to a burial
depth difference of 20 to 33 km), and the intervening ultramafic rock
mass (at most 6 km across strike) is much too thin to account for the
pressure difference. The metamorphic pressure contrast indicates
that the internal schist was exhumed relative to the Devil's Gate
ophiolite by lithospheric-scale normal faults located east of (structurally above) the internal schists within the ultramafic rocks. Such
faults would have to be located structurally beneath the Devil's
Gate ophiolite (Fig. 13). Relative exhumation is demanded for the
internal schist relative to the Devil's Gate ophiolite regardless of the
comparative ages of metamorphism because neither rock has
prograde or retrograde assemblages that record a higher P than the
peak assemblages.
The exhumation of the external schist (metamorphic sole) relative
to the overlying ophiolite crustal section was accommodated by at
least two major zones of exhumation, one of which was at least 1 km
structurally above the ultramafic-external schist interface, east of the
internal schist body, and the other permissibly located anywhere
between the ultramafic-external schist interface and the western
boundary of the internal schist body. More accurate locating of
metamorphic pressure contrasts within the ultramafic rocks themselves is not feasible owing to the insensitivity of various reactions in
metaultramafic rocks to pressure (e.g., Evans, 1977).
If in fact an inverted pressure gradient is preserved in the external
schist, then internal thrust faults are required to have juxtaposed the
rocks of the external schist (Fig. 13). The structurally lowest unit of the
external schist is much higher P than the Calaveras Complex that
structurally underlies it, and this exhumation (apparent thrust fault
sense) may have taken place prior to the deposition of the structurally
highest parts of the Calaveras Complex as noted below.
Fig. 13. Tectonic cartoons showing the evolution of the northern Feather River ultramafic belt with emphasis on the metamorphism and exhumation of the external schists. Note that
the thickness of the external schists is exaggerated so that they are visible on these diagrams. I.S. (internal schist) schematically shows the relative position of the internal schist
sample, whereas YR45 and FR6 track two samples of the external schist.
Please cite this article as: Smart, C.M., Wakabayashi, J., Hot and deep: Rock record of subduction initiation and exhumation of hightemperature, high-pressure metamorphic rocks, Feather River ultramafic belt, California, Lithos (2009), doi:10.1016/j.lithos.2009.06.012
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C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx
Timing of subduction initiation, sole development and sole
exhumation is difficult to ascertain owing to the scarcity of
geochronologic data, but the hornblende Ar/Ar age of 236 ± 4 Ma
(Weisenberg and Avé Lallemant, 1977) reflects cooling of the internal
schist through hornblende closure (~525 ± 50 °C; Harrison, 1981).
The high-pressure, high-temperatures in the external and internal
schists may have been approximately coeval owing to their proximity
and similar PT conditions of metamorphism, so the internal schist age
may have formed in the same subduction initiation event as the
external schist and cooled through hornblende closure for Ar at
approximately the same time. Cooling of the sole is expected to rapid
with continued subduction, so the hornblende age is probably less
than 5 m.y. older than the actual metamorphic age (e.g., Hacker, 1991).
Exhumation of soles relative to the upper crustal parts of overlying
ophiolites appears to be rapid for examples where ophiolites were
emplaced over continental margins. In such cases many ophiolites
were emplaced over continental margins within 10 m.y. after
formation (Dewey, 1976; Dilek et al., 1999), and structural relationships indicate that exhumation of the sole relative to the ophiolite
must have occurred prior to final emplacement (reviewed in
Wakabayashi and Dilek, 2003). For ophiolites structurally above
subduction complexes, as appears to be the case with the FRB,
geochronologic data is much scarcer worldwide.
Subaerial exposure of the external schist appears to have occurred
prior to the accretion of the structurally highest unit of the Calaveras
Complex that has breccia clasts of external schist. The accretion age of
the oldest Calaveras Complex is uncertain, however, except that it
exceeds 177 Ma (Sharp, 1988). Collectively the existing structural,
metamorphic, and geochronologic framework suggests initiation of
subduction sometime slightly before 236 Ma, subduction of the sole to
depths exceeding 43 km (N1.3 GPa) and exhumation and subaerial
exposure of some of these rocks by 177 Ma.
The eastern contact of the FRB against the Shoo Fly Complex and
other rocks is more difficult to evaluate. The pressure of hightemperature metamorphism estimated for the Devil's Gate ophiolite is not significantly different (within uncertainty) than that of
estimates for the low-grade metamorphism of the Shoo Fly
Complex and related rocks (Day et al., 1988; Hacker, 1993). On
this basis the eastern FRB contact does not record large amounts of
differential vertical movement. In contrast the temperature of
metamorphism (~ 700 °C) for the upper crustal parts of the FRB
reflected by the temperature of metamorphism of the sheeted dike
sample of the Devil's Gate ophiolite is vastly higher than any of the
rocks adjacent to the FRB to the east. This metamorphic relationship
(no P contrast but large T contrast) suggests that the eastern
boundary of the FRB may have accommodated significant strike-slip
displacement.
The origin of the metamorphism of the Devil's Gate ophiolite poses
some difficult problems. In many of the classic ophiolites of the world,
the ophiolite structurally overlying the sole exhibits negligible burial
metamorphism (most exhibit sea floor metamorphism) (e.g., Wakabayashi and Dilek, 2000; 2003). The crustal ophiolitic rocks of the FRB,
such as the Devil's Gate ophiolite, and possibly the internal schist, are
clearly different. The high T, low P metamorphism of the Devil's Gate
ophiolite clearly reflects a much higher geothermal gradient than that
recorded in the high T, high P external schist. The metamorphism is not
compatible with sea floor metamorphism because the prograde
amphibole zoning indicates cooling after crystallization, followed by
heating, in contrast to sea floor metamorphism where temperatures
should generally reflect a progressive cooling. In addition, sea floor
amphibolite metamorphism is restricted to the gabbro layer and
beneath, and has not been found associated with sheeted dikes (e.g.
Liou and Ernst, 1979; Evarts and Schiffman, 1983; Schiffman and Smith,
1988, Alt and Teagle, 2000). Ridge subduction has been suggested to be
associated with very high geothermal gradients (e.g., Sisson and Pavlis,
1993; Brown, 1998) and such an event may explain the high-T
13
metamorphism of the crustal ophiolitic parts of the FRB such as the
Devil's Gate ophiolite. One speculative scenario that would explain the
metamorphism of the Devil's Gate ophiolite would be failed subduction of the ridge crest during the same event that led to the inception of
subduction and creation of the external schist (Fig. 13).
Many of the details of the tectonometamorphic evolution of the
FRB require more geochronologic data and related metamorphic
data tied to a specific structural-tectonic setting, for better understanding. We are currently engaged in a geochronologic campaign in
this region and we hope that the results will help refine models for
ophiolite genesis, subduction initiation, and early subduction zone
exhumation.
Acknowledgments
This research was supported by the National Science Foundation
Grant EAR-0635767 to J.W., and awards from the California State
University, Fresno, College of Science and Mathematics to C.M.S. We
thank S. Roeske and S. Mulcahy for their assistance with microprobe
analyses, G. Torrez for XRF analyses, and R. Jamieson, W. Sharp, and V.
Sisson for the constructive reviews that greatly improved the paper.
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Please cite this article as: Smart, C.M., Wakabayashi, J., Hot and deep: Rock record of subduction initiation and exhumation of hightemperature, high-pressure metamorphic rocks, Feather River ultramafic belt, California, Lithos (2009), doi:10.1016/j.lithos.2009.06.012