Use of trace element abundances in augite and hornblende to determine the
size, connectivity, timing, and evolution of magma batches in a tilted batholith
N. Coint, C.G. Barnes, A.S. Yoshinobu, M.A. Barnes, and S. Buck
Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA
ABSTRACT
The tilted Wooley Creek batholith (Klamath Mountains, California, USA) consists of
three main zones. Field and textural relationships in the older lower zone suggest batchwise emplacement. However, compositions of
augite from individual samples plot along individually distinct fractionation trends, confirming emplacement as magma batches that did
not interact extensively. The younger upper
zone is upwardly zoned from tonalite to granite. Major and trace element compositions
of hornblende show similar variations from
sample to sample, indicating growth from a
single magma batch that was homogenized
by convection and then evolved via upward
percolation of interstitial melt. Highly porphyritic dacitic roof dikes, the hornblende compositions of which match those of upper zone
rocks, demonstrate that the upper zone mush
was eruptible. The central zone contains rocks
of both lower and upper zone age, although
in most samples hornblende compositions
match those of the upper zone. The zone is
rich in synplutonic dikes and mafic magmatic enclaves. These features indicate that
the central zone was a broad transition zone
between upper and lower parts of the batholith and preserves part of the feeder system to
the upper zone. Homogenization of the upper
zone was probably triggered by the arrival of
mafic magma in the central zone. Continued
emplacement of mafic magmas may have provided heat that permitted differentiation of the
upper zone magma by upward melt percolation. This study illustrates the potential for use
of trace element compositions and variation in
rock-forming minerals to identify individual
magma batches, assess interactions between
them, and characterize magmatic processes.
INTRODUCTION
The ways in which plutons are assembled
are currently the subject of vigorous debate.
The occurrence of large volumes of volcanic
rocks ejected during single eruptive events (e.g.,
Ritchey, 1980; Bacon and Druitt, 1988; Lipman
et al., 1997; Bachmann et al., 2002, 2005; Christiansen, 2005; Hildreth, 2004; Christiansen and
McCurry, 2008) suggests that large volumes of
magma are stored in the middle to upper crust.
Recent high-precision U-Pb dating of zircon
indicates that it is not uncommon for intrusive
systems to grow over several millions of years
(e.g., Glazner et al., 2004; Matzel et al., 2006;
Grunder et al., 2008; Memeti et al., 2010). Thermal modeling, however, indicates that even large
batches of magma should not remain above their
solidus for >1 m.y. (Glazner et al., 2004; Paterson
et al., 2011). These modeling results have led to
emplacement models in which plutons form in
increments that do not interact extensively with
one another, and have also been interpreted to
indicate that plutons are not necessarily related to
volcanic rocks (Coleman et al., 2004; Mills and
Coleman, 2010). These emplacement models also
imply that important processes in the chemical
evolution of magmatic suites such as fractional
crystallization, assimilation, and magma mixing
(e.g., De Paolo, 1981; Bohrson and Spera, 2007;
Ohba et al., 2007; Claiborne et al., 2010; McLeod
et al., 2010; du Bray et al., 2011) are restricted to
the lower part of the crust (Annen, 2009; Mills
and Coleman, 2010; Tappa et al., 2011). Incremental emplacement of batholith-scale intrusions
has been demonstrated through high-precision
geochronology (Glazner et al., 2004; Matzel
et al., 2006; Walker et al., 2007; Grunder et al.,
2008; Memeti et al., 2010; Miller et al., 2011) in
cases where emplacement spanned hundreds of
thousands to millions of years. However, some
batholith-scale plutons may be emplaced in time
scales of <1 m.y. (Miller et al., 2011; Coint et al.,
2013). In such cases, alternative methods must be
used to identify magma batches and assess their
size and evolution.
High-temperature mafic minerals crystallize
early in magmatic systems and are capable of
incorporating trace elements in abundances large
enough to be measured in situ. These minerals
record the evolution of the melt composition in
magmatic systems and thus have great potential to
provide information that allows reconstruction of
the history of intrusive and extrusive complexes.
In the case of calc-alkaline magmas, augite and
hornblende are of particular interest because they
can crystallize at or close to the liquidus, depending on the amount of water present in the system
(Piwinskii, 1968; Eggler and Burnham, 1973),
and because they occur throughout a wide range
of compositions, from basaltic andesite to rhyolite. By studying the compositions of these minerals, we can also track the chemical evolution of
the melt, which is generally not accessible when
working with bulk-rock data on intrusive rocks
because many plutonic rocks are partial cumulates (e.g., Deering and Bachmann, 2010).
In this study we utilize major and trace element abundances in augite and hornblende to
reconstruct the assembly of a tilted calc-alkaline pluton, the Wooley Creek batholith (WCb;
Barnes et al., 1986b; Barnes, 1987). Tilting and
subsequent erosion have exposed ~9 km of
structural relief through the intrusion, making it
a good candidate for study of the organization
of intrusive bodies. Furthermore, the presence of
roof dikes in the structurally highest part of the
system provides samples of magma that escaped
the system (Barnes et al., 1986a), enabling us to
address the problem of the connection between
volcanic and plutonic rocks.
GEOLOGICAL SETTING
Klamath Mountains
The WCb is situated in northern California,
USA, in the Klamath Mountains geologic province. The province consists of a series of northsouth–oriented accreted terranes bounded by
regional east-dipping thrust faults, resulting in
preservation of a record of more than 400 m.y.
of active subduction along the western North
American margin (Snoke and Barnes, 2006).
The WCb is one of a series of plutons (Wooley
Creek suite) emplaced from Middle to Late
Geosphere; December 2013; v. 9; no. 6; p. 1747–1765; doi:10.1130/GES00931.1; 9 figures; 5 supplemental files.
Received 13 March 2013 ♦ Revision received 17 July 2013 ♦ Accepted 29 August 2013 ♦ Published online 23 October 2013
For permission to copy, contact editing@geosociety.org
© 2013 Geological Society of America
1747
Coint et al.
Legend
Ji
123°20’00”W
Late Jurassic
plutons
Western
Klamath terrane
c. 152 Ma
Orleans thrust
Western/eastern
wHt
eHt Hayfork terranes
RCt
Lower zone
351
Rattlesnake Creek
terrane
Ukonom
Lake
Central zone
6209
397
Ten
Bear Cuddihy
Mtn. Lakes basin
208
6309
5709b
133
6809
128a 777b
Deadman
Lake
317
4809
4909
Z5
833
394
687
WCb
30
41°30’30” N
7109
594
Pigeon
Roost
0
10
09
52 32
1
Jurassic time (167–156 Ma) into rocks of the
western Paleozoic and Triassic belt, central
metamorphic belt, and eastern Klamath belt
(Allen and Barnes, 2006). The Slinkard pluton, which crops out northeast of the WCb,
is related to WCb magmatism (Barnes et al.,
1986a). However, extensive subsolidus recrystallization prevented us from including the
Slinkard pluton in this study. At the time of the
plutonic event, the area was undergoing extension associated with formation of the backarc
Josephine ophiolite ca. 159–164 Ma (Harper
et al., 1994; Hacker et al., 1995). The Wooley
Creek suite was emplaced east (modern coordinates) of the Josephine ophiolite. At the same
time, subduction-related magmatism was active
west of the Josephine ophiolite and is now represented by the Chetco plutonic complex and
Rogue Formation. The Chetco-Rogue arc, the
Josephine ophiolite, and its cover sequence, the
Galice Formation, form the western Jurassic
belt, which was thrust underneath the western
Paleozoic and Triassic belt along the Orleans
thrust (Harper et al., 1994) during the Nevadan orogeny (153–150 Ma; Allen and Barnes,
2006). This thrusting truncated the base of at
least some Wooley Creek suite plutons (Barnes,
1982, 1983; Jachens et al., 1986). Exhumation
of high-pressure rocks of the Condrey Mountain
Schist through a structural window tilted the
WCb ~15°–30° toward the southwest (Barnes et
al., 1986b), and subsequent erosion has resulted
in ~9 km of structural relief through the pluton.
Medicine
Mtn.
Upper zone
The WCb (Fig. 1) was emplaced between
159 and 155 Ma (Coint et al., 2013). The batholith intrudes three host terranes of the western
Paleozoic and Triassic belt; from structurally
higher to lower, these are (1) the eastern Hayfork terrane, a chert-argillite mélange, (2) the
western Hayfork terrane, a volcaniclastic sandstone and argillite unit, and (3) the Rattlesnake
Creek terrane, a serpentine matrix to block-onblock ophiolitic mélange overlain by a coherent cover sequence (Wright and Wyld, 1994).
The WCb can be divided into three zones, all of
which display gradational contacts (Fig. 1). The
lower zone is composed primarily of biotite ±
hornblende two-pyroxene diorite and/or gabbro
and biotite ± hornblende two-pyroxene tonalite.
Two U-Pb (zircon, chemical abrasion–thermal ionization mass spectrometry, CA-TIMS)
ages from this unit are identical with analytical
uncertainty at 158.99 ± 0.17 Ma and 159.22 ±
0.10 Ma (Coint et al., 2013). Lower zone rocks
are heterogeneous at the scale of the outcrop and
locally display sheet-like organization with variable amounts of pyroxene. Numerous pyroxene-
Figure 1. Simplified geologic map of the Wooley Creek batholith displaying the location of samples analyzed for this study (modified from
Barnes, 1987).
1748
Geosphere, December 2013
Gradational
compositional
boundary
Orleans thrust fault
(teeth on HW)
Thrust fault
(teeth on HW)
Field area
590
N
471
CA
377b
551
548
164b
Ji
Klamath
Mts.
region of abundant
“Roof zone” dikes
236a
2408
257a
0
5 km
Pluton assembly using trace elements in augite and hornblende
rich blocks and dikes (pyroxenite and melagabbro), showing variably angular and gradational
contacts with the host magmatic rock, as well as
sparse mafic magmatic enclaves (MME), mafic
dikes, and appinite dikes and pods, are present in the lower part of the intrusion. Foliation
is primarily magmatic, is oriented north-south,
and dips steeply to the east or west (Coint et al.,
2013) except along the northeastern contact,
where tonalitic rocks display a protoclastic foliation that dips northeast, subparallel to the contact. Because deformation of these latter rocks
was subsolidus, it is unlikely that the minerals
preserve their igneous composition; therefore,
these rocks were excluded from this study.
The central zone (Fig. 1) is mainly composed
of biotite hornblende quartz diorite and tonalite;
some samples contain scraps of pyroxene surrounded by hornblende. Detailed mapping in
the Cuddihy Lakes basin (Fig. 1) shows that the
zone consists of multiple decimeter- to meterscale sheets of tonalite and quartz diorite (with
or without MME) along with variably deformed
and disrupted mafic synplutonic dikes, including appinites (Barnes et al., 1986a; Leopold and
Yoshinobu, 2010; fig. 3F in Coint et al., 2013).
In outcrop some sheets display characteristics of
upper zone rocks and others of lower zone
rocks. These distinctions are reinforced by U-Pb
(zircon) ages of two central zone samples (Coint
et al., 2013). The age of one sample (sample Z5;
159.01 ± 0.20 Ma) is identical within uncertainty
to lower zone samples, whereas the age of the
other (sample WCB-4909; 158.30 ± 0.16 Ma)
is identical to samples from the upper zone (see
following). The presence of samples with both
upper and lower zone characteristics is reflected
in the boundaries of the central zone with the
two adjacent zones. Ridge traverses indicate
that these boundaries are gradational and diffuse
over distances of at least 500 m. Foliation in the
central zone is magmatic and has variable strike
and dip (Coint et al., 2013).
The upper zone ranges from structurally
lower biotite hornblende quartz diorite and/
or tonalite to structurally highest biotite hornblende granite (Fig. 1), and three samples have
U-Pb zircon ages ranging from 158.21 ± 0.17
Ma to 158.25 ± 0.46 Ma (Coint et al., 2013).
Outcrops in the upper zone are relatively homogeneous and repeated traverses along ridges and
in cirque basins over a 20-yr period have failed
to identify internal intrusive contacts. Swarms
of MME are locally present and are most common near the contact with the central zone and
along the southwestern intrusive contact. Magmatic foliation is typically weak and forms a
broadly concentric pattern that is subparallel
to intrusive contacts except east of Ten Bear
Mountain, where the approximate east-west
strike of foliation is nearly perpendicular to the
intrusive contact (Coint et al., 2013). The southern part of the upper zone is intruded by a small
(2 km × 0.7 km) body of hornblende biotite
granite (Fig. 1) with a U-Pb (zircon) age of ca.
156 Ma (Coint et al., 2013). The majority of this
unit is finer grained than the surrounding upper
zone rocks, making the sharp intrusive contacts
readily apparent in the field.
Selvages of two-pyroxene ± hornblende diorite and quartz diorite occur locally along the
western and southern contacts of the upper zone
(Fig. 1), and have U-Pb (zircon) ages of 159.28
± 0.17 Ma and 158.32 ± 0.32 Ma, respectively
(Coint et al., 2013). The contact between the
southern mafic selvage and the granodiorite is
lobate (fig. 7 in Coint et al., 2013). No sharp
contact between the western mafic selvages and
the granitic to granodioritic rocks of the upper
WCb was observed.
Along the western and southwestern contact
zone, a series of porphyritic dikes intrudes the
host rocks and the structurally highest parts of
the upper zone. Andesitic dikes generally contain phenocrysts of augite, enstatite (herein
orthopyroxene), and plagioclase, but some contain augite and hornblende. Dacitic dikes have
phenocrysts of plagioclase and hornblende ±
biotite ± quartz. Crosscutting relationships suggest that some andesitic dikes were emplaced
before the emplacement of the upper WCb
magma, because the dikes are cut by the upper
granodiorite, whereas some andesitic dikes
intrude the upper WCb (Barnes et al., 1986a).
In contrast, dacitic dikes that cut the upper zone
have not been observed. In Barnes (1987) and
Barnes et al. (1990), the andesitic dikes were
interpreted to be derived from the lower zone,
whereas the dacitic dikes were interpreted to be
from the upper zone. These interpretations are
tested (see following) on the basis of mineral
compositions.
PETROGRAPHY
Lower Zone
Other than the pyroxenites and melagabbros,
rocks from the lower zone range from biotite
two-pyroxene diorite and/or gabbro to biotite
hornblende quartz diorite and tonalite. They
display hypidiomorphic granular texture and are
more or less porphyritic, with subhedral augite
phenocrysts (Fig. 2A). Augite, orthopyroxene,
and plagioclase are euhedral and define a weak
magmatic foliation. Plagioclase compositions
vary between An78 and An33, with normal to
oscillatory normal zoning (Barnes, 1987; this
paper). Biotite is subhedral and commonly
shows a reaction relationship with pyroxene.
Geosphere, December 2013
Hornblende is present in some samples as a late
magmatic phase, growing in continuity with
pyroxenes (Fig. 2A). Quartz, where present,
is interstitial. Accessory minerals are apatite,
zircon, allanite, and rarely tourmaline. Actinolite, chlorite, epidote, and sericite are secondary minerals, which are variable in abundance.
Where epidote displays sharp contacts with
hornblende, it is interpreted as being magmatic,
whereas where present as an anhedral phase
associated with sericitized plagioclase and chloritized biotite, it is secondary.
Pyroxenite and melagabbro blocks and dikelike sheets are coarse grained and have an orthocumulate texture (Fig. 2B). These rocks consist
of subhedral exsolved augite and orthopyroxene, with or without olivine or olivine pseudomorphs. Pale green to brown amphibole is present as poikilitic crystals and in some samples is
optically continuous with pyroxene. Hercynite
is present as an accessory mineral. Talc, serpentine, and green actinolite are secondary minerals. Plagioclase (An77 to An50) and quartz are
interstitial and occur in veins that brecciate the
pyroxenite blocks.
Central Zone
Rocks in the central zone range from biotite
hornblende quartz diorite to biotite hornblende
tonalite. Although these samples contain similar mineral assemblages, they display a range
of grain size and magmatic foliation intensity.
Pyroxenes are rarely preserved as scrappy inclusions in hornblende (Fig. 2D) and some relict
grains are evident as intergrown actinolitic hornblende and quartz blebs surrounded by magmatic hornblende. Hornblende habits vary from
euhedral prismatic to poikilitic (Fig. 2C). The
hornblende crystals are optically zoned, with
brown cores and green rims. Plagioclase (An60
to An35) is subhedral to euhedral and displays
oscillatory normal and oscillatory reverse zoning. Subhedral biotite, plagioclase, and amphibole define a weak magmatic foliation. Quartz
and K-feldspar are interstitial. Chlorite, epidote,
sericite, minor sphene, and actinolite are secondary minerals and can locally be abundant.
The association of epidote with hornblende suggests that some epidote is magmatic.
Upper Zone
Tonalite through granite rocks of the upper
zone are hypidiomorphic granular (Fig. 2E).
Euhedral to subhedral hornblende and plagioclase define a weak magmatic foliation. Plagioclase is weakly normally zoned in samples
collected near the transition with the central
1749
Coint et al.
A
Lower zone quartz diorite
MMB-351
MMB-351
MMB-100
B
Pyroxene-rich blocks
Cpx
Hbl
Bt
Pl
Act
Pl
Srp
Cpx
Px
Hbl
Pl
Act
Pl
Pl
Cpx
Px
Px
Biot
D
Hbl
1 mm
WCB-6209
Hbl
Cpx
Act
Central zone tonalite
E
Upper zone granite
WCB-2308
Hbl
0.5mm
Qz
WCB-7109
Hbl
Opx
Px
2 mm
Hbl
C
Cpx
K-spar
Bt
Pl
Hbl
Bt
Hbl
Pl
Hbl
Pl
Qz
Pl
F
Qz
2mm
Andesite roof dike
MMB-553
Pl
2 mm
Pl
G
Act
Dacite roof dike
MMB-548
Pl
Hbl
Pl
Opx
Cpx
Cpx
Pl
Hbl
Qz
Hbl
Cpx
Bt
2mm
Pl
H
MME
Plag
Granite
Hbl
Qtz
Pl
Bt
Hbl
Pl
Bt
K-spar
Qz
Pl
Hbl
Hbl
Pl
2 mm
1750
1 mm
Figure 2. Thin section scans and photomicrographs. Abbreviations: Cpx—clinopyroxene, Opx—orthopyroxene, Bt—
biotite, Srp—serpentine, Pl—plagioclase, Hbl—hornblende,
Act—actinolite, Qz—quartz, K-spar—K-feldspar, Px—
pyroxene. (A) Biotite hornblende two-pyroxene quartz diorite
from the lower zone. White dash lines outline pyroxene crystals. (B) Pyroxene-rich block. (C) Biotite hornblende tonalite. (D) Pyroxene surrounded by actinolite and green hornblende. (E) Biotite hornblende granodiorite from the upper
zone. (F) Andesitic roof-zone dike containing phenocrysts of
orthopyroxene, augite, and plagioclase. (G) Dacitic roof dike
with quartz, hornblende, plagioclase, and biotite phenocrysts.
(H) Contact between a mafic magmatic enclave (MME) and
the host granodiorite. The groundmass in the enclave is mainly
composed of poikilitic K-feldspar, optically continuous across
the boundary between the granite and the enclave.
Geosphere, December 2013
Pluton assembly using trace elements in augite and hornblende
zone (An56 to An42) and becomes strongly
normally zoned (An50 to An19) in samples collected from the structurally higher (southwestern) part of the unit (Barnes, 1987). Subhedral
biotite crystals are randomly oriented. Quartz
is interstitial in tonalite but has euhedral faces
against K-feldspar in granodiorite and granite.
Orthoclase is interstitial to poikilitic and is variably perthitic. Zircon, apatite, and allanite are
accessory phases. Pyroxene is rarely preserved
as remnant cores in hornblende. However, the
presence of rounded quartz inclusions in hornblende suggests that pyroxene was present
as a high-temperature phase. Whether these
crystals grew from the upper zone magma or
were inherited is not possible to determine.
The upper zone is petrographically distinct
from both the central and lower zones in having euhedral hornblende phenocrysts as much
as 1 cm long, a seriate distribution of euhedral
to subhedral hornblende, and a general lack
of evidence for augite to hornblende reaction.
Chlorite, epidote, actinolite, sericite, and minor
sphene are secondary minerals.
Late Granite
The late granite that intrudes the southern part
of the upper zone (Fig. 1) is texturally distinct,
with euhedral, strongly zoned plagioclase (An31
to An11), acicular hornblende, and acicular to
prismatic biotite. Perthitic orthoclase is present
as poikilitic crystals that reach 1 cm in diameter.
Quartz is subhedral to interstitial and displays
slight undulose extinction. Euhedral sphene is
a primary mineral and is commonly associated
with magnetite. The late granite is the only part
of the WCb in which sphene is a primary phase.
Apatite and zircon are accessory minerals.
Mafic Selvages
Two main mafic selvages rim the intrusion
along the southern and western contacts. The
textural complexity observed in the southern
mafic selvage suggests a complex history. For
example, samples MMB-236a and WCB-2408
are slightly porphyritic, with optically zoned
euhedral augite microphenocrysts. Relict olivine is surrounded by orthopyroxene and secondary oxides. The groundmass consists of
euhedral to subhedral plagioclase (An72 to An46)
defining a weak magmatic foliation. Apatite is
present as acicular needles included in plagioclase. In contrast, MMB-257a is distinct from
the other pyroxene-bearing rocks in the selvage.
Pyroxenes occur as aggregates associated with a
relatively large proportion of opaque minerals,
and the pyroxene-oxide clusters are surrounded
by a groundmass of subhedral, aligned plagio-
clase (An91 to An81); the foliation defined by the
latter wraps around the pyroxene glomerocrysts.
Sample MMB-594 from the western selvage
(Fig. 1) is a quartz-bearing biotite two-pyroxene
diorite. Plagioclase is euhedral and zoned (An79
to An33) and augite and orthopyroxene are euhedral, whereas biotite is poikilitic to interstitial.
Relict olivine has been replaced by orthopyroxene and opaque minerals. Zircon and apatite are
accessory minerals. This mineral assemblage is
identical to that common in the lower zone, and
the bulk compositions of samples from the western selvage are similar to lower zone compositions (Coint et al., 2013).
Roof Dikes
Andesitic roof dikes are porphyritic and contain 28%–41% phenocrysts. Optically zoned
plagioclase, augite, and orthopyroxene occur as
phenocrysts and glomerocrysts (Fig. 2F). Plagioclase displays oscillatory normal to oscillatory reverse zoning with compositions varying
between An77 and An50 (Barnes, 1987). Olivine
is thought to have been present in a few samples
in which vermicular opaque minerals are surrounded by orthopyroxene. Orthopyroxene cores
surrounded by augite rims are present in a several samples (e.g., MMB-557 and MMB-164a).
Pyroxene is more or less altered to actinolitic
amphibole, and plagioclase is partially albitized.
Dacitic roof dikes are porphyritic (Fig. 2G),
with 23%–54% phenocrysts of euhedral hornblende, plagioclase ± biotite, and quartz. Relict
pyroxenes are rare as cores in amphibole phenocrysts and as inclusions in plagioclase phenocrysts. Plagioclase is oscillatory normally
zoned with compositions varying between An53
to An29 (Barnes, 1987). Glomerocrysts are composed of large zoned plagioclase and euhedral
hornblende, with interstitial quartz and minor
K-feldspar and biotite. Accessory minerals are
zircon, apatite, and allanite. Chlorite, actinolite,
epidote, and sphene are secondary minerals and
partially replace hornblende and biotite.
Mafic Magmatic Enclaves and Synplutonic
Dikes
The synplutonic dikes that are characteristic
of the central zone are slightly porphyritic to
aphanitic. Phenocrysts and groundmass minerals are mainly hornblende and plagioclase
with lesser subhedral to anhedral biotite. The
MME are porphyritic, with plagioclase and
hornblende phenocrysts (Barnes, 1987; Buck et
al., 2010). The groundmass of the MME consists of 200–300 µm subhedral plagioclase and
hornblende. In the upper zone, poikilitic quartz
and/or K-feldspar are optically continuous with
Geosphere, December 2013
crystals in the host granodiorite to granite and
contain numerous inclusions of hornblende, plagioclase, and biotite (Fig. 2H). Mafic magmatic
enclaves from the upper zone are porphyritic,
with phenocrysts of zoned plagioclase, hornblende, and subhedral biotite. Hornblende clots
are common in MME; some display actinolitic
cores with small opaque mineral inclusions. Zircon and apatite are accessory minerals.
ANALYTICAL METHODS
Major and Trace Element Analyses
Methods used to analyze bulk-rock major
and trace element compositions were presented
in Coint et al. (2013). Major element data for
minerals come from a variety of sources. Some
hornblende data are available in the literature
(Barnes, 1982, 1983, 1987). New microprobe
data collected for this project are presented
in Supplemental File 11. Some of the data
published here were collected on the JEOL
JXA8900 electron microprobe at the University of Wyoming, with operating conditions
of 15 kV accelerating voltage, 20 nA current,
and a beam diameter of 1–2 µm when focused.
Matrix-matched natural and synthetic standards were used for calibration. The precision
for individual major component was <0.1 wt%
(Li, 2008). The remaining data, mainly obtained
on rocks from the central zone, were collected
at the University of Oklahoma using a Cameca
SX50 electron microprobe equipped with five
asynchronous wavelength-dispersive spectrometers and PGT PRISM 200 energy-dispersive
X-ray analyzer. Analysis was done by wavelength-dispersive spectrometry using 20 kV
acceleration, 20 nA beam current (measured at
the Faraday cup), and 2 µm spot size. The PAP
algorithm (Pouchou and Pichoir, 1985) was
used to correct from matrix effects, with oxygen
content calculated by stoichiometry.
Hornblende, augite, and orthopyroxene trace
element data were collected in situ, in polished
sections, by laser ablation (LA) ICP-MS analysis using a NewWave 213 nm solid-state laser
and Agilent 7500CS ICP-MS at Texas Tech University. Spot diameter was 40 µm with a laser
pulse rate of 5 Hz and fluence of 11–12 J cm–2.
For each analysis, 25 s of background (laser
off) and 60 s of signal were recorded. NIST 612
glass was used as the standard and was analyzed
every 5–7 unknown mineral analyses. Precision
1
Supplemental File 1. New microprobe data of
hornblende and pyroxene. If you are viewing the PDF
of this paper or reading it offline, please visit http://
dx.doi.org/10.1130/GES00931.S1 or the full-text article on www.gsapubs.org to view Supplemental File 1.
1751
Coint et al.
and accuracy of the LA-ICP-MS system were
determined by repeated analysis of basaltic
glass BHVO-2 and are given in Supplemental
File 22. The contents of SiO2 or CaO for hornblende and orthopyroxene and CaO for augite,
as determined by electron microprobe, were
used as internal standards to correct for variability in ablation efficiency. Reduction of the hornblende data using either CaO or SiO2 as internal
standards gave results within the uncertainty of
the counting statistics. The LA-ICP-MS data set
is presented in Supplemental File 33.
Mg/(Mg+Fe)
1752
B
600
0.7
lower zone tonalite
lower zone
tonalite
400
0.5
0.3
2
Supplemental File 2. BHVO statistics obtained
on the laser ablation system at Texas Tech University. If you are viewing the PDF of this paper or reading it offline, please visit http://dx.doi.org/10.1130
/GES00931.S2 or the full-text article on www.gsapubs
.org to view Supplemental File 2.
3
Supplemental File 3. LA-ICP-MS data of augite
and hornblende. If you are viewing the PDF of this
paper or reading it offline, please visit http://dx.doi.org
/10.1130/GES00931.S3 or the full-text article on
www.gsapubs.org to view Supplemental File 3.
800
synplutonic dikes
andesitic roof-zone dikes
dacitic roof-zone dikes
Mg-rich roof-zone dikes
A
Geochemistry
A discussion of the bulk-rock major and trace
element data was presented in Coint (2012) and
Coint et al. (2013), in which we show that the
lower zone has scattered compositions that do
not define a clear-cut compositional trend. The
data also show that mafic selvages along the
western contact are compositionally identical to
the lower zone. In contrast, rocks of the central
and upper zone were interpreted to define a compositional array with inflections in the trends of
a number of trace and minor elements that are
indicative of fractional crystallization. The data
also support a cogenetic relationship between
rocks of the upper zone and dacitic roof dikes.
These relationships are illustrated and
expanded upon in Figures 3A and 3B, in which
the following is evident. (1) The lower zone is
clearly distinct from the central and upper zones
in terms of Mg/(Mg + Fe) and Sr contents (Figs.
3A, 3B). (2) The compositions of lower zone
tonalites do not plot on the crude trend defined
by more mafic lower zone rocks (Fig. 3A); this
indicates that fractional crystallization is not
an adequate explanation for the compositional
variation in the lower zone. (3) The central and
upper zones define an array of compositions
that overlaps those of dacitic roof-zone dikes.
(4) Andesitic roof zone dike compositions tend
to cluster in the low-SiO2 part of the central and
upper zone trends. (5) The late-stage hornblende
biotite granite is compositionally distinct from
the rest of the batholith.
Rare earth element (REE) patterns of most
WCb samples are broadly similar across rock
Sr (ppm)
0.9
0.1
45
200
lower zone
l.z. pyroxenite/melagabbro
central zone
upper zone
late-stage upper zone granite
50
55
60
65
SiO2
0
70
75
80
5
10
15
20
25
MgO
100
central zone
C
D
upper zone
increasing SiO 2
in central zone samples
low
low
er z
er z
one
one
increasing SiO2
10
lower zone
pyroxenite &
melagabbro
1
La Ce Pr Nd
SmEu Gd Tb Dy Ho Er TmYb Lu
100
La Ce Pr Nd
E
SmEu Gd Tb Dy Ho Er TmYb Lu
F
synplutonic dikes
roof-zone dikes
2-p
y
rox
en
ea
nd
esi
te
increasing SiO2
in dacitic dikes
10
1
La Ce Pr Nd
SmEu Gd Tb Dy Ho Er TmYb Lu
La Ce Pr Nd
SmEu GdTb Dy Ho Er TmYb Lu
Figure 3. Bulk-rock major element diagrams. (A) Mg/(Mg + Fe) versus SiO2 (l.z.—lower
zone). (B) Sr versus MgO. (C) Rare earth element (REE) patterns normalized to chondrite (Boynton, 1984) of the central and lower zone rocks. (D) REE patterns normalized to
chondrite (Boynton, 1984) of upper zone rocks. (E) REE patterns normalized to chondrite
(Boynton, 1984) of the roof-zone dikes; both dacite and andesite. (F) REE patterns normalized to chondrite (Boynton, 1984) of synplutonic dikes.
Geosphere, December 2013
Pluton assembly using trace elements in augite and hornblende
types, with moderate negative slopes and small
or no Eu anomalies (Figs. 3C–3E). Typical gabbroic through tonalitic lower zone rocks are
shown as a field in Figure 3C because these
samples have parallel slopes. As shown in Coint
et al. (2013), the REE abundances of these rocks
are positively correlated with SiO2 contents. In
contrast, pyroxenite and melagabbro blocks and
intrusive sheets in the lower zone have lower
REE abundances and flatter patterns, and display both positive and negative Eu anomalies
(Fig. 3C). Although samples from the central
and upper zones have total REE (REETOT)abundances broadly similar to lower zone samples
(Figs. 3C, 3D), with increasing SiO2 the abundances of middle and heavy REEs decrease and
the REE patterns show more pronounced concave-upward shapes (Fig. 3C; Barnes, 1983).
The REE patterns of andesitic and dacitic roof
zone dikes are shown in Figure 3E. The andesitic dike patterns are parallel to those of the least
evolved (lowest SiO2) dacite dikes. As with
upper zone samples, the REE abundances of the
dacitic dikes decrease with increasing SiO2 and
the REE patterns show greater upward concavity (Fig. 3E).
Mafic synplutonic dikes, which characterize the central zone, show wide compositional
scatter, but essentially overlap compositions of
lower zone samples (Figs. 3A, 3B). The REE
patterns of these dikes are similar to those of
lower zone rocks, but show a greater range of
REE abundances (Fig. 3F).
Mineral Chemistry
The major element compositions of minerals from the WCb are summarized here from
previously published work (Barnes, 1987; for
additional analyses, see Supplemental File 1
[see footnote 1]; for LA- ICP-MS analyses for
augite, orthopyroxene, and hornblende, see Supplemental File 3 [see footnote 3]). Attributions
of core, intermediate, or rim labels for the locations of the laser spots are based on geometrical criteria, because most augite crystals in the
batholith lack optical zoning. In contrast, augite
in the roof dikes displays sharp growth zoning.
Hornblende is optically zoned, with olive-brown
cores and green rims. Intermediate zones (mantles) in hornblende are typically brown. Subsolidus amphibole, mainly actinolitic hornblende
and actinolite, is not considered in this study.
Augite
In the following section, pyroxenes are classified following the official nomenclature (Morimoto et al., 1988). Both augite and diopside are
present; however, for simplicity clinopyroxene
is referred to as augite. Nonquadrilateral com-
ponents vary between 2.3–11.1 mol%, with Al
being the most abundant nonquadrilateral cation
(Barnes, 1987). Trace element abundances are
compared to Zr contents in Figure 4 because
Zr is incompatible in pyroxene and behaved
incompatibly in rocks of the lower zone (e.g.,
Barnes, 1983). Therefore, increasing Zr in a
single pyroxene grain is reflective of increasing
Zr in the melts from which the pyroxene grew.
Augite from the lower zone varies between
wollastonite, Wo 0.42–0.47, enstatite, En 0.40–
0.47, ferrosilite, Fs 0.14–0.18. In terms of trace
elements, augite crystals from the lower zone
have compositions that vary widely and can be
divided in two groups based on their Cr content, with one group containing >3700 ppm Cr
and the other containing <2700 ppm Cr. With
increasing Zr, Cr decreases exponentially from
5995 to 167 ppm (Fig. 4A). REETOT contents
range from 5 to 156 ppm. Individual crystals
tend to show increasing REETOT from core to
rim (Fig. 4B). Strontium contents are nearly
constant with increasing Zr and vary between
12.2 and 25.6 ppm (Fig. 4D). The augite crystals displaying the highest Sr content are part of
the high Cr group.
Augite crystals in pyroxene-rich blocks have
compositions slightly more magnesian (Wo
0.44–0.49, En 0.42–0.46, Fs 0.07–0.18) than the
augite from the common rock types in the lower
zone. Augite from the pyroxene-rich blocks
plots in two distinct groups in the Zr versus Cr
and REETOT diagrams (Figs. 4A, 4B). The first
group, mainly composed of cores and intermediate zones, overlaps with compositions of
augite from the common rock types in the lower
zone, with high Cr and low REETOT. The second
group, composed of intermediate zones, rims
and one core, has REETOT values higher than
typical augite from the lower zone.
In the upper zone, relict augite was analyzed
in only one sample, MMB-397. The few augite
crystals preserved have low Cr (92–471 ppm),
intermediate Zr (14.8–19.3 ppm; see Supplemental File 3 [see footnote 3]; Figs. 4E, 4F), and
Sr concentrations that overlap with augite from
the lower zone.
Compositions of augite in the western selvage overlap those from the lower zone and the
pyroxene-rich blocks (Figs. 4E, 4F). In contrast,
augite from the southern selvage is distinct in
having a broader range of Mg# (0.62–0.78 versus 0.65–0.68 from the western selvage augite;
Barnes, 1987). In terms of trace element abundances, augite from the western selvage has Cr
and Sr contents similar to those from the lower
zone (Figs. 4E, 4F), but has higher concentrations of REEs. Augite from the southern mafic
selvage has higher Sr (>22 ppm) and lower Cr
compared to the augite from the western sel-
Geosphere, December 2013
vage. The Zr and REETOT contents of augite
from the southern mafic selvage are similar
to augite from the two-pyroxene andesite roof
dikes (Figs. 4E, 4F).
Augite from the two-pyroxene andesitic roof
dikes has higher TiO2 and Al2O3 and lower CaO
contents than augite from the lower part of the
intrusion (Barnes, 1987). Chromium contents
are relatively constant and low (<1000 ppm)
compared to augite from the lower WCb (Fig.
4C). A few spots analyzed in the intermediate
parts of the crystals display higher Cr contents,
especially in augite from samples WCB-164b
and MMB-557, where Cr concentrations reach
3060 ppm and 2700 ppm, respectively. The
augite from the roof dikes has higher Sr (>22
ppm) than augite from the lower zone, except
in sample MMB-590, in which augite Sr concentrations overlap those of lower zone augite
(Fig. 4D).
Hornblende
In the following we use the amphibole
nomenclature of Leake et al. (1997). We refer
to brown-green amphibole as hornblende and to
pale green amphibole as actinolite. Actinolitic
hornblende is the transition between the two.
Amphibole compositions in the lower zone
range from magnesiohornblende to actinolite
(fig. 6 in Barnes, 1987). Chromium concentrations vary from 2023 to 139 ppm, with most
spots below 1200 ppm (Fig. 5A); Cr is not correlated with Ti contents. Hornblende contains
variable amounts of REEs (Fig. 5C); the light
(L) REEs vary from 10× to 100× chondritic
values, whereas heavy (H) REEs vary from
6× to 50× chondrites. Hornblende REE patterns are not necessarily parallel to each other,
especially for the LREEs, where the shape of
the patterns is quite variable (Fig. 5C). The
size of the negative Eu anomaly increases with
increasing REE abundance.
Amphiboles in the pyroxene-rich blocks are
magnesiohornblende and these amphiboles
have higher Mg# (0.8–0.9) than does hornblende in other lower zone samples. In contrast,
Cr contents overlap with hornblende from the
lower zone (Fig. 5A). Titanium and Zr are present in relatively low concentrations (<5000 ppm
and <50 ppm, respectively; Fig. 5B). The REE
concentrations in magnesiohornblende in the
pyroxenites are lower than those from the other
rocks of the lower zone, with LREE concentrations between 7× and 50× chondrites, and
HREE concentrations between 2.5× and 12×
chondrites. Both slightly positive and slightly
negative Eu anomalies are present.
Amphiboles in the central zone are magnesiohornblende. In these amphiboles, Ti, P,
and Zr contents decrease from core to rim in
1753
Coint et al.
Two-pyroxene andesitic roof dikes
Lower zone
6000
Cr
#
A
Int
Rim
5209
133
30
6809
128
833
5000
augite from
pyroxenite and
melagabbro
blocks
4000
Core
6000
3000
Cr
C
#
164b
704
590
555
2000
F
1000
Rim
553
All lower zone
augite
but one
3000
2000
Int
557
5000
4000
Core
1000
0
0
REETOT
150
B
Sr
D
35
30
100
augite from
pyroxenite and
melagabbro
blocks
50
25
20
All lower zone
augite but one
15
F
10
0
0
10
20
30
40
50
0
Zr
10
20
Zr
30
40
50
Mafic selvages
E
Sr Pyroxenite and melagabbro blocks augite
two-pyroxene andesitic
roof dike augite but 3
35
Mafic selvage augite
core int rim
236a
30
594
25
257a
20
Upper zone augite
397
15
10
Lower zone augite
MMB-590
200
REETOT
Lower zone augite
F
Figure 4. Diagrams displaying trace element concentrations in augite crystals. Int—intermediate parts of the
augite. (A, B) Trace element composition of augite from
the lower zone and from the pyroxenite-melagabbro
blocks. Expected fractionation trends (F) are displayed
in a schematic diagram in the lower right corners. The
gray arrows in B display variable fractionation trends
recorded by the augite in different samples. REE tot—
total rare earth elements. (C, D) Trace element composition of augite from the roof-zone dikes. (E, F) Trace
element composition of augite from the mafic selvages
and upper zone.
150
100
50
two-pyroxene andesitic
roof dike augite but 3
0
0
10
20
30
40
50
Zr
1754
Geosphere, December 2013
Pluton assembly using trace elements in augite and hornblende
2550 ppm
Lower zone
Cr
2000
1600
5209
6809
128a
351
Figure 5. (A, B) Trace element plots of hornblende from
the lower zone and pyroxenite-melagabbro blocks. The
striped pattern shows the range of composition found
in the upper zone hornblende. In A, the black arrows
indicate analysis of hornblende close to the pyroxene
core, where the reaction is potentially incomplete and
Cr is inherited from the augite (see the text for more
explanations). (C) Rare earth element patterns of hornblende (Hbl) from the lower zone, normalized to chondrite (Sun and McDonough, 1989).
833
133
Pyroxene-rich blocks
132
100
1200
800
400
A
B
0
Zr
120
100
Hbl/Chondrites
upper zone Hornblende
C
100
80
60
40
10
20
0
0
5000
10000
15000
Ti
every sample (Fig. 6A). Chromium concentrations are between 42 and 484 ppm, but only
samples from the Cuddihy basin and Deadman Lake (Fig. 1) contain hornblende with
Cr > 200 ppm (Fig. 6A). Hornblende crystals
from both Granite Lakes and Ukonom Lake
(Fig. 1) have lower Cr contents than hornblende
from the other samples collected in the central
zone (black dashed line in Fig. 6A). The hornblende in the central zone has the tendency to
be reversely zoned, with higher REE concentrations in the cores associated with larger negative
Eu anomalies, and lower REE concentrations in
the rim with slightly negative to no Eu anomalies (Figs. 6C–6H). LREE concentrations vary
between 35× and 100× chondrites, and HREE
concentrations vary between 12× and 60× chondrites. Two crystals, in samples WCB-7109 and
WCB-6209, display lower REE concentrations
(10× chondrites for LREEs and HREEs; Figs.
6F, 6G). Hornblende from sample WCB-5709b
(Fig. 1) has the lowest REE abundances of all
samples from the central zone and displays small
negative to positive Eu anomalies (Fig. 6H).
Hornblende from MME and synplutonic
dikes is magnesiohornblende (Barnes, 1987).
Phenocryst cores contain higher concentrations
of Zr and Ti than in the rims or in late hornblende
20000
25000
La
Ce
Pr
Nd
found in the groundmass. Cr contents are anticorrelated to Zr and Ti and increase in concentration from core to rim (Fig. 7A). Small, euhedral
groundmass hornblende interpreted as growing
late in the crystallization history of the enclaves
has Cr concentrations that are highly variable,
from 60 to 800 ppm (Fig. 7A). REE concentrations are correlated with Ti and Zr concentrations
and decrease from core to rim. The negative Eu
anomalies in the cores of hornblende phenocrysts are larger than in rims and in groundmass
hornblende (Fig. 7D). Hornblende crystals in
synplutonic dikes contain higher amounts of Cr,
Zr, and Ti than are found in MME.
Hornblende from the upper zone is magnesiohornblende to ferrohornblende in which Ti
and AlIV decrease from core to rim as Mg# (figs.
6A and 7A in Barnes, 1987) and Sc abundances
(this work) remain approximately constant. The
trace elements P, Hf, Y, Sr, V, and Zr all decrease
toward the rims (not shown). The REE abundances are between 40× and 100× chondrites for
the LREEs and 20–60× chondrites for HREEs
(Figs. 8D–8F). Most of the REE patterns are
subparallel to each other. Hornblende cores
display medium-sized negative Eu anomalies,
which decrease in the intermediate part of the
crystal and increase toward rims (Fig. 8C).
Geosphere, December 2013
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hornblende phenocrysts in the dacitic roof
dikes are magnesiohornblende to actinolitic
magnesiohornblende (Barnes et al., 1987).
These hornblende samples are similar in major
and trace element concentrations (Mg#, Ti,
Cr, Zr, V, Sc, REEs) to hornblende from the
upper zone (Fig. 8H) and from several samples
from the central zone. This similarity includes
decreasing concentrations of Zr, Ti, and Cr from
crystal cores to rims.
DISCUSSION
The influence of orthopyroxene exsolution on
augite composition is discussed in Supplemental
File 44. A simple experiment in which exsolved
cores and rims were analyzed demonstrates that
the REE patterns of augite from which orthopyroxene has exsolved are not affected by the
amount of orthopyroxene exsolution, because
the proportion of the REEs that orthopyroxene can accommodate is small. Therefore, we
4
Supplemental File 4. The role of exsolution on augite REE content. If you are viewing the PDF of this
paper or reading it offline, please visit http://dx.doi
.org/10.1130/GES00931.S4 or the full-text article on
www.gsapubs.org to view Supplemental File 4.
1755
Coint et al.
Deadman Lake
Cuddihy Lakes basin
4809
Core
Rim
Late
777b
Core
Rim
Hbl/Chondrites Cuddihy Lakes basin MMB-777b
D
Cuddihy Lakes basin WCB-4809
E
100
Ukonom Lake
Granite Lake bassin
7109
Core
Rim
Late
5709b
Core
Rim
Late
4909
Core
Rim
Late
6309
Core
Rim
Late
6209
Core
Rim
Late
10
A
Cr
400
WCB-4909
100
300
200
10
Ukonom Lake
WCB-6209
F
Granite Lakes
WCB-6309
WCB-7109
G
100
100
0
B
Zr
120
100
10
80
60
40
100
20
0
0
5000
10000
Ti
15000
20000
25000
Eu*
1
C
10
Rim
Deadman Lake
WCB-5709b
H
100
0.5
Eu
Core
0.1
0
20
40
60
Zr
80
100
120
10
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Figure 6. Trace element plots from the hornblende of the central zone. (A) Cr versus Ti binary diagram. (B) Zr versus Ti diagram.
The dashed line contours the hornblende analysis from the Granite Lakes basin. (C) Eu* versus Zr in hornblende. The back
arrows points out the trend existing from core to rim. (D–H) Hornblende (Hbl) rare earth element (REE) patterns normalized to
chondrite (Sun and McDonough, 1989) shown by location compared to the REE patterns of the hornblende from the upper zone
(striped field) and three late hornblendes from synplutonic dikes (gray field). Sample numbers are indicated in italics.
1756
Geosphere, December 2013
Pluton assembly using trace elements in augite and hornblende
1000
Cr
Dikes
Core
Rim
Late
MME
Core
Rim
Late
A
900
800
700
Hbl/Chondrites
Syn-plutonic dikes
C
100
600
500
400
10
300
MME
200
100
D
100
0
Zr
B
100
1
10
80
60
40
20
0
0
5000
10000
Ti
15000
assume that the compositions obtained by LAICP-MS reflect magmatic augite compositions.
Pluton Assembly Based on the Mineral
Chemistry
Mineral compositions record information
about the composition of the melt from which
the mineral grew. The melt composition is, in
turn, influenced by the minerals that precipitate from it. These minerals need not be physically fractionated (i.e., to be removed from the
system) to induce compositional variations in
newly formed growth zones of existing minerals. Therefore, core to rim compositional variations in minerals can result from a variety of
magmatic processes such as fractional crystallization, mixing, and assimilation–fractional crystallization, assuming that intracrystal diffusion
is slow enough for the zoning patterns to be preserved (see following). Bulk-rock compositions,
unlike mineral ones, will also record processes
such as accumulation. Therefore, both data sets
need to be combined to understand in detail the
effects of these different processes. Here we
refer to fractional crystallization when there is
chemical evidence for separation between the
minerals and the residual melt, to in situ fractionation when chemical variation was induced
by precipitation of minerals from the melt with-
20000
Ce
Nd
Sm
Gd
Dy
Er
Yb
La
Pr
Pm
Eu
Tb
Ho
Tm
Lu
Figure 7. (A, B) Binary diagrams displaying chemical variations in
hornblende from mafic magmatic enclaves (MME; MMB-681b and
MMB-686b) and synplutonic dikes (MMB-775, MMB776, and MMB860) compared to those found in the upper zone (striped pattern). (In
the legend, “Dikes” refers to synplutonic dikes.) (C, D) Hornblende
(Hbl) rare earth element patterns normalized to chondrite (Sun and
McDonough, 1989) from mafic magmatic enclaves and synplutonic
dikes, normalized to chondrite (Sun and McDonough, 1989).
out physical removal of the newly grown phase
from the system, and to fractionation when the
process responsible for the chemical variation is
not identified.
The amount of a trace element incorporated
into a mineral lattice is governed by the partition coefficient between the melt and the mineral, which varies with pressure, temperature,
and magma composition. Inasmuch as both
augite and hornblende were high-temperature
phases in the various parts of the WCb, the two
minerals recorded the evolution of the melt from
which they crystallized, and therefore allow us
to identify and map the extent of the distinct
magma batches in the batholith. Distinct magma
batches should produce minerals with distinct
compositional and zoning patterns and magma
mixing should result in discernible compositional reversals.
In this study, the core to rim variations in
trace element abundances are used to understand the evolution of the melt composition
through time. Because intracrystalline diffusion
of REEs in diopside is very slow under crustal
conditions (Van Orman et al., 2001), the REE
record preserved in both hornblende and augite
is used as a proxy for the melt evolution. Elements such as Ti (Cherniak and Liang, 2012),
Sr, and Cr are more sensitive to intracrystalline
diffusion. However, these element data are used
Geosphere, December 2013
in this study because their intracrystalline variations are broadly correlated with the REE data,
and we therefore conclude that compositional
variation in the minerals is primarily a function
of the magmatic record.
Magmatic Evolution of the Lower WCb
Based on Augite Compositions
High bulk-rock concentrations of MgO and
Sc in the lower zone indicate that some of
these rocks are cumulates, and therefore that
fractional crystallization affected lower zone
magmas. Augite was one of the first minerals
to crystallize, along with plagioclase (Fig. 2A),
and may be used to track the evolution of the
magma compositions. Hornblende, which is
present as a rimming phase around pyroxene,
grew at lower temperature conditions and is a
less valuable indicator of changes in melt composition in this zone (Fig. 2A).
The abrupt core to rim decrease in Cr and the
proportional increase in REETOT and associated
increasing Zr concentrations in augite (Figs.
4A, 4B) are consistent with evolution of magmas in the lower zone by fractional crystallization. However, the extent of fractional crystallization is not the same in every sample. In some
samples, augite displays wide compositional
variations (e.g., WCB-5209), whereas in others
1757
Coint et al.
Zr
100
A
80
Hbl/Chondrites
Qtz monzodiorite
MMB-317
MMB-208
Tonalite
MMB-687
D
100
60
40
10
20
0
Cr
B
300
E
100
200
100
10
0
0
5000
10000
15000
20000
25000
MMB-394
MMB-471
Granodiorite-granite
Ti
F
100
1.1
Eu/Eu*
C
MMB-397
0.9
10
0.7
G
Tonalite MMB-397
0.5
100
0.3
Low LREE Hbl
patterns
0.1
0
50
100
Zr
10
Dacitic/rhyodacitic roof dikes
Roof dike MMB-397 Upper zone
Hbl cores
Hbl intermediate
Hbl rims
100
MMB-548
MMB-551
MMB-693
H
Smaller Hbl
Upper zone Hbl
Late granite Hbl
RZD Hbl
10
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Figure 8. Trace elements in hornblende (Hbl) from the upper zone. (A–C) Binary diagrams displaying chemical variations in hornblende from the upper zone compared to hornblende from the late granite and hornblende from the dacitic roof dikes (RZD). In C,
the red arrow points out the evolution trend for hornblende from the upper zone, from core to rim, whereas the black arrow follows
the trend from core to rim described by hornblende in sample MMB-397. LREE—light rare earth elements. (D–H) Hornblende REE
patterns normalized to chondrite (Sun and McDonough, 1989). Black arrows show the LREE–depleted patterns (see the text for
explanations) interpreted as late rims crystallizing from strongly fractionated residual melt. Qtz—quartz.
1758
Geosphere, December 2013
Pluton assembly using trace elements in augite and hornblende
(e.g., WCB-128) variations are limited (Figs.
4A, 4B). Furthermore, compositional trends for
both augite and hornblende are distinct from one
sample to the next, as is evident from the variable slopes in plots of REETOT versus Zr (Figs.
4A, 4B, 5A, and 5B). These different slopes are
interpreted to indicate that each sample crystallized from a distinct magma batch and that each
batch had a distinct crystallization history. This
variation can in part be explained by differences
in initial magma composition, and can also be
related to differences in the extent of fractional
crystallization. Such differences could result
from variable cooling rates or from the effectiveness of crystal-liquid separation. What is clear
is that compositional variations of individual
magma batches can result in the distinct augite
compositional trends. We conclude that variable
augite compositions in the lower zone reflect
emplacement of multiple magma batches, the
compositions of which ranged from gabbroic to
tonalitic. These compositional variations combined with differences in the extent of fractional
crystallization resulted in the observed range of
augite trace element abundances and patterns.
Lower Zone Hornblende: Magmatic or
Inherited Compositions?
On the basis of textural relationships, hornblende crystals in samples from the lower zone
grew late in the magmatic history (Fig. 2A), and
so are more likely to have recorded processes
that occurred at lower temperatures relative to
those preserved in augite. The Cr contents of
some of the hornblende (Fig. 5A) are anomalously high for crystals that formed from a lowtemperature, evolved melt, and are much higher
than the Cr concentrations found in the upper
zone hornblende. These Cr concentrations
partly overlap with the Cr contents of the lower
zone augite crystals (cf. Figs. 4A and 5A). A
few high-Cr hornblende crystals are also found
in the central zone (Fig. 6A) and in one rock
from the upper zone (Fig. 8B). These high Cr
contents may result from incomplete reaction
between melt and pyroxene to form hornblende
or from direct crystallization of hornblende
from a Cr-rich mafic melt. In the lower zone,
the Cr-rich hornblende forms clusters or is associated with quartz beads. Both of these textural
features result from pyroxene + melt reactions
(Castro and Stephens, 1992; Stephens, 2001),
suggesting that the high Cr content of the hornblende is inherited from the original pyroxene.
The extent of this inheritance is not well constrained. Cr is present in larger concentrations
in augite than in hornblende; therefore if diffusion is slow, or if the reaction takes place rapidly, complete equilibration between the newly
formed hornblende and the melt might not be
reached. Other cations with 3+ and 4+ valences,
such as Al, Ti and REEs, could also be inherited
from preexisting augite. However, Supplemental File 1 (see footnote 1) shows that the abundances of these elements are generally higher
in hornblende than in augite, which indicates
that they are more readily partitioned into hornblende. Therefore, it is probable that the concentrations of these elements in the latter represent
equilibrium between the growing hornblende
and melt, whereas Cr concentrations reflect
inheritance from precursor augite. This example
shows the importance of combining textural
observations with the geochemistry of mineral
phases when interpreting the nature of peritectic
reactions.
Pyroxene-Rich Blocks and Their
Relationships with the Lower Zone
Pyroxene-rich blocks and intrusive bodies
in the lower zone can be either (1) cumulates
resulting from disruption of dikes, or (2) fragments of ultramafic host rocks that reacted with
the host magma, resulting in the crystallization
of pyroxenes. The composition of the olivine
found in sample MMB-100 (forsterite, Fo71) is
not appropriate for mantle rocks (as pointed out
in Barnes, 1983, 1987), leaving the first hypothesis as the most likely explanation. The composition of the augite in the lower zone overlaps
with augite compositions from the pyroxenite
and melagabbro bodies (Figs. 4A, 4B), and this
similarity indicates that augite in these bodies
crystallized from magmas similar to those that
formed the lower zone. The field relationships
and the textural observations suggest that this
overlap is mostly the result of crystal exchange
between the pyroxene-rich bodies and the host
magmas. For example, sample WCB-133 displays a gradational contact with a large block
of pyroxene melagabbro. All analyzed augite
cores and mantles in sample WCB-133 have
high Cr concentrations and are interpreted as
being inherited from the adjacent melagabbro,
whereas rims overlap with augite compositions
typical of lower zone augite (Fig. 4A). These
rims are interpreted to form after augite from
the pyroxene-rich block was transferred to the
magma that hosted the block. The high Cr contents and high Mg# (~0.8 instead of values of
0.65–0.75 typical of most lower zone rocks;
Barnes, 1987) of the augite in pyroxenite and
melagabbro bodies and the high Sr contents of
these augite crystals compared to the rest of the
lower zone augite (Figs. 4A, 4E) suggest that
parent magmas of the pyroxene-rich bodies
were relatively primitive and water rich, consistent with the paucity of olivine in these rocks.
Geosphere, December 2013
Mafic Selvage
The augite crystals found in the western
selvage (e.g., sample MMB-594) display geochemical characteristics similar to those from
the lower zone (Figs. 4E, 4F). This similarity
and the fact that the western selvage was coeval
with the lower zone are interpreted to indicate
that the western selvage formed from magma
batches similar to those that formed the lower
zone of the pluton.
Rocks from the southern mafic selvage show
much more diversity in texture, bulk-rock compositions (Fig. 3), and mineral compositions
(Figs. 4E, 4F). Augite analyzed in samples
MMB-257a and MMB-236a overlap with
augite from the two-pyroxene andesite roof
dikes in terms of Sr concentrations and display
similar trends in REETOT versus Zr (Figs. 4E,
4F). Therefore, we interpret the southern mafic
selvage to be related to the two-pyroxene andesitic roof dikes.
Upper Zone
Bulk-rock compositions from the upper zone
of the WCb display narrow trends (Fig. 3).
Therefore, fractional crystallization and associated crystal accumulation were interpreted
to be the main processes responsible for bulk
compositional variation (Coint et al., 2013).
In the upper zone, allanite, apatite, and zircon
are accessory phases. Zircon and apatite occur
as inclusions in biotite, hornblende, and plagioclase, suggesting that these minerals were
stable at temperatures above the solidus and
were likely to have fractionated during crystallization. Fractionation of hornblende along with
the accessory minerals will result in decreasing
concentrations of the REEs in the residual melt,
and therefore in the minerals that grow from the
progressively fractionated melt. One of the consequences is that incompatible elements whose
enrichment can be used to unequivocally track
the evolution of the system are absent. However,
the amount of Ti incorporated into hornblende
depends on temperature (Otten, 1984; Ernst and
Liu, 1998; Femenias et al., 2006), and therefore
is used as a proxy to track the down-temperature
variations of trace elements (Fig. 8).
Hornblende from the upper zone shows a
limited variation in Mg# (Barnes, 1987), REE
concentrations, shape of the REE patterns
(Figs. 8D–8H), and Cr contents (Fig. 8B).
The abundance of REE elements and the size
of the Eu anomaly tend to decrease from core
to mantle (Fig. 8C), but hornblende rims tend
to display larger Eu anomalies than the mantle. This core to rim decrease in trace element
abundances can be explained in terms of the
1759
Coint et al.
compatible behavior of the REEs, although the
slight decrease in the Eu anomaly from core to
mantle (Figs. 8D–8H) is counterintuitive. In the
upper zone, fractionation of a small amount of
apatite and allanite is thought to counteract the
expected increase in the size of the negative Eu
anomaly. We present simple models to illustrate
this effect in Supplemental File 55. The presence
of large (1 mm diameter) allanite crystals in the
granodioritic and granitic rocks suggests that in
some cases the proportion of allanite that fractionated may be greater than the value used in
the model (see Supplemental File 5 [see footnote 5]). In such a case, allanite fractionation
will deplete the residual melt in LREEs relative
to the HREEs. This depletion effect can explain
the presence of a few hornblende rims that have
lower LREE concentrations than typical (Figs.
8D, 8F). The larger negative Eu anomaly associated with some of the LREE-depleted patterns,
compared to the rest of the hornblende patterns
from the upper zone, cannot be accounted for by
allanite fractionation, but could be the result of
rare instances of in situ K-feldspar fractionation
(e.g., Bachmann et al., 2005). This possibility is
consistent with the fact that most of the hornblende rims with relatively large Eu anomalies
are from granitic or granodioritic samples.
The abundances of elements such as Zr and
Ti in upper zone hornblende are well correlated
and define a single trend (Fig. 8A). By comparison with the lower zone (Fig. 4B), which
represents multiple magma batches with limited
chemical communication, this single trend of
upper zone hornblende compositions indicates
that upper zone hornblende crystallized from (1)
a single batch of magma or (2) multiple magma
batches that were compositionally identical and
had identical crystallization histories. The welldefined bulk-rock trends in Harker diagrams
(figs. 3 and 8 in Coint et al., 2013) coupled
with the decreasing An content of plagioclase
(An46 to An12; Barnes, 1987) with increasing
bulk-rock SiO2 concentrations are consistent
with either interpretation. However, the lack of
intrusive contacts in the upper zone compared
with the abundance of intrusive contacts in the
central and lower zones, and the gradual upward
change from tonalitic to granitic compositions
among upper zone rocks support crystallization
from a large homogeneous magma body.
Hornblende from one upper zone sample,
MMB-397 (Figs. 1 and 8G), displays wider
5
Supplemental File 5. Fractional crystallization
model of apatite and allanite and the consequence on
the size of the Eu anomaly. If you are viewing the PDF
of this paper or reading it offline, please visit http://
dx.doi.org/10.1130/GES00931.S5 or the full-text article on www.gsapubs.org to view Supplemental File 5.
1760
variations in REE compositions than the rest of
the upper zone hornblende (cf. Figs. 8D, 8E,
8F). This rock is also distinct from other analyzed samples because of the presence of relict
pyroxene as inclusions in poikilitic hornblende
and plagioclase with reverse zoning (Barnes,
1987). These features suggest a distinct magmatic history compared to the rest of the upper
zone. The composition of the relict pyroxene
(Figs. 4E, 4F) indicates that the pyroxene
grew from a magma similar to magmas that
formed the lower zone and the western mafic
selvage (e.g., MMB-594; Fig. 1). Hornblende
cores and mantles have compositions similar
to hornblende from the central and the upper
zones, and are interpreted to have grown from
upper zone magma. In contrast, the rims of
these hornblendes display steeper REE patterns with enrichment in LREEs but without
Eu anomalies. The REE patterns of hornblende
rims are interpreted as resulting from crystallization from a hybrid magma that formed by
mixing of upper zone magma with a crystalrich lower zone–type magma batch from the
adjacent mafic selvage. Most of the textural
characteristics of upper zone rocks are present
in this rock: medium grain size (2 mm), poikilitic to interstitial K-feldspar, large (at least
100 µm long), subequant zircons, euhedral
hornblende, and glomerocrysts of zoned plagioclase. All of these features indicate that the
upper zone magma accounted for much of the
mass of this rock. The presence of pyroxene
crystals similar in composition to ones found
in the western selvage (MMB-594; Figs. 4E,
4F) as inclusions in hornblende, plus abundant
hornblende grains that contain actinolite ±
oxide inclusions are interpreted to be the result
of reaction between magma of the upper zone
with augite from the western selvage. If this is
the case, it would suggest that the upper zone
magma invaded mushy magma of the western
selvage and incorporated some of its minerals,
such as augite. This interpretation is consistent with hornblende compositional variations
from core to rim. Hornblende cores grew from
upper zone magma, whereas the rims crystallized from a more mafic, hybrid magma, which
resulted in variations in the LREE pattern
shape and smaller negative Eu anomalies than
found in most upper zone hornblende.
Central Zone
The central zone of the intrusion plays a key
role in understanding the link between the upper
and lower zones because, as indicated above, it
constitutes a transition between the two. Hornblende in the central zone displays major and
trace element concentrations similar to horn-
Geosphere, December 2013
blende from the upper zone, with moderate Cr
concentrations and decreasing Ti and Zr from
core to rim (Figs. 6A, 6B). In samples WCB4809 and WCB-4909, hornblende REE patterns
overlap perfectly with the REE patterns of hornblende from the upper zone (Fig. 6E). These
data suggest that hornblende in WCB-4809 and
WCB−4909 crystallized from magmas similar
in composition to the upper zone magma and
are consistent with the age of zircon from WCB4909, which is identical to upper zone samples.
These two samples were collected from the
structurally lower part of the central zone (Fig.
1) and they are organized in a series of alternating comagmatic sheets (fig. 3F in Coint et al.,
2013). They are, therefore, interpreted be part of
the feeder system to the upper zone.
Five other central zone samples contain hornblende having REE patterns that show more
variability. For example, hornblende cores in
these five samples commonly overlap compositions of hornblende from the upper zone; however, their rim compositions have lower REE
contents and display smaller Eu anomalies than
in upper zone samples (Figs. 6D, 6F–6H). One
sample, MMB-777b, shows a bimodal distribution of the REE patterns (Fig. 6D). The compositional overlap between hornblende cores
from the central zone with hornblende from
the upper zone is strongly suggestive that these
hornblende cores grew from upper zone magmas. However, unlike WCB-4809 and WCB4909 and most of the hornblende in the upper
zone, the rims of central zone hornblende have
lower REE contents and small negative to no Eu
anomalies (Fig. 6C). These zoning features can
be explained in several ways.
Fractional Crystallization
It has already been demonstrated that in
the upper zone REEs were compatible and
concentrations generally decreased with fractional crystallization. In the central zone, minerals such as K-feldspar and allanite, which
were involved in the evolution of upper zone
magma, are scant. Therefore, the variations
in the rims of the hornblende in the central
part cannot be associated with fractionation
of such minerals. Textures of MMB-777b,
WCB-6209, WCB-6309, and WCB-7109 are
also distinct from the upper zone samples. In
the latter samples, hornblende and biotite are
poikilitic and contain numerous inclusions of
plagioclase, hornblende, and apatite, and relict
pyroxene cores are common. These features
are rarely observed in the upper zone rocks,
so it is unlikely that hornblende compositions
from these samples result from simple fractional crystallization.
Pluton assembly using trace elements in augite and hornblende
Mixing between MME, Synplutonic Dikes,
and the Upper Zone Magma
A second possibility is that hornblende rims
in samples MMB-777b, WCB-6209, WCB6309, and WCB-7109 grew from hybrid magma.
Ample evidence of mixing and mingling is present in the central zone, such as disrupted synplutonic dikes and swarms of MME (Barnes et
al., 1986a). The presence of pyroxene cores surrounded by hornblende (Fig. 2D) would suggest
that mixing could have occurred between lower
zone and upper zone magmas.
The extent of the interactions between parent
mafic magmas of MME and synplutonic dikes
(mainly basaltic andesite) and central zone magmas is difficult to constrain for the following
reasons. The cores of hornblende phenocrysts
in the MME have compositions similar to hornblende cores from central and upper zone rocks
(Fig. 7). This similarity is interpreted to indicate that hornblende phenocrysts in MME are
xenocrysts from the surrounding magma that
were mechanically incorporated into the MME
magmas. One of the MME from the upper part
of the pluton has a groundmass of centimeterscale, poikilitic K-feldspar crystals that are optically continuous with K-feldspar crystals in the
host granodiorite (Fig. 2H). This relationship
suggests that many MME are complex hybrids
and that mixing involved not only exchange of
large crystals, but also flow of interstitial melt
between enclave and host magma. For these reasons, the MME are not viewed as appropriate
mixing end members.
In contrast, hornblende in the synplutonic
dikes is more likely to reflect compositions of
the mafic magmas because there was evidently
much less interaction between the dikes and
the surrounding magma, as is indicated by the
sharp contacts between dikes and their host
rocks (fig. 3E in Coint et al., 2013). As is the
case in the MME, the cores of hornblende phenocrysts found in synplutonic dikes have REE
compositions similar to those of hornblende in
the host magma (Fig. 7C), suggesting that the
hornblende phenocrysts in the dikes were inherited from their hosts. However, phenocryst rims
and groundmass hornblende in the synplutonic
dikes have high Cr contents (Fig. 7A) yet lack
evidence for reaction from pyroxene. This lack
indicates that the Cr concentrations observed
in the groundmass hornblende and hornblende
rims are a reflection of the concentration of Cr
in the dike magma. Therefore, we consider the
compositions of hornblende rims and groundmass hornblende to be representative of a possible mixing end member.
If mixing between the central zone magma
and the synplutonic dike magmas was impor-
tant in the central zone, then the resulting hornblende compositions should be between those
of the groundmass hornblende from the synplutonic dikes and the hornblende from the upper
zone (Fig. 7A). However, such mixing does not
explain the low Cr contents of hornblende rims,
the wide range of Cr contents, and the lack of
Ti increase in phenocryst rims and groundmass
hornblende (Fig. 6A). These features indicate
that the potential mixing end member was not
primitive (in the sense of containing high Cr and
Ti contents) and therefore not a synplutonic dike
magma. Instead, the mixing end member was
probably of intermediate composition.
Hornblende in central zone sample WCB5709b is unlikely to be the result of mixing
of upper zone magma with synplutonic dike
magma because hornblende in this sample has
higher LREE and lower middle REE and HREE
concentrations than the hornblende from the
synplutonic dikes (Fig. 6H). The low middle
and heavy REE concentrations (Fig. 6H) and the
small Eu anomaly suggest that the hornblende
in WCB-5709b crystallized from a relative
mafic magma. Relict pyroxene, now completely
transformed into hornblende, are abundant in
the sample, and we interpret this feature to indicate that sample WCB-5709b is a fragment of
the lower zone now surrounded by central zone
rocks, an interpretation consistent with the age
of central zone sample Z5, which is identical to
ages of lower zone samples.
Mixing between the Lower and Upper Zone
Magmas
A second mixing model for the central zone
involves mixing between magmas of the upper
and lower zones. Texturally, the central zone
rocks are possibly hybrids between the lower
zone, which contains pyroxene partially reacted
to actinolitic hornblende or hornblende (Fig.
2D), and the upper zone, which contains coarse
phenocrysts of euhedral zoned plagioclase and
hornblende (Fig. 2E). The gradational contacts
between the different zones of the intrusion and
abundant evidence for mixing in the central zone
indicate that this hypothesis needs to be tested.
Unfortunately, most of the relict pyroxenes are
partially transformed into actinolite and do not
preserve magmatic compositions.
Sample MMB-397 was interpreted to result
from interaction between the mafic magma
that formed the western selvage (compositionally and temporally related to the lower zone;
see Fig. 1) and upper zone magma. Hornblende
from sample MMB-397 has compositions and
zoning patterns similar to hornblende in samples from Ukonom Lake and Granite Lakes
(WCB-6209, WCB-6309, WCB-7109; Figs.
Geosphere, December 2013
8F, 8G). We therefore conclude that the rocks
from the Ukonom Lake and Granite Lakes areas
of the central zone formed by mixing between
a pyroxene-bearing magma and upper zone
magma. The nature of the pyroxene-bearing end
member is difficult to constrain as no obvious
mixing trends are observed in Harker diagrams
(fig. 8 in Coint et al., 2013) or in Figures 6, 7,
and 8. However, these samples were collected
in proximity to the gradational contact with the
lower zone, which is a good candidate for a mixing end member.
In summary, most of the central zone crystallized from upper zone magma, and we interpret
the sheeted part of the central zone preserved in
the Cuddihy basin to be part of the feeding system
to the upper zone (see Coint et al., 2013). Some
samples, particularly ones collected near the
boundary with the lower zone (samples MMB777b, WCB-7109, WCB-6309, and WCB-6209),
are primarily the result of mixing between lower
and upper zone magmas, with local complications due to further mingling and mixing with
magmas parental to the MME and synplutonic
mafic dikes. Among the analyzed samples, WCB4809 and WCB-4909 are the only ones that show
no evidence of input from mafic magmas.
Roof Dikes
Two-Pyroxene Andesitic Roof Dikes
Some of the two-pyroxene andesitic dikes are
composite, with older external two-pyroxene
andesite and inner biotite granodiorite related to
the upper zone (Barnes et al., 1986a; Coint et al.,
2013). Other two-pyroxene andesitic dikes cut
the uppermost part of the upper zone (Coint et al.,
2013). These mutual crosscutting relationships
indicate that the two-pyroxene andesitic magmas were coeval with the magmas of the upper
zone. The previous interpretation of the origin of
the two-pyroxene andesitic dikes (Barnes et al.,
1986a) was that they represented magmas that
were derived from the lower zone. This interpretation was based on the broad overlap of bulk-rock
compositions of the dikes with the lower zone
(Barnes et al., 1986a). However, augite crystals
in the andesitic roof dikes have compositions that
are distinct from augite in the lower zone, with
higher Sr and lower Cr contents (Figs. 4C, 4D).
Augite from the andesitic dikes is also distinct
in showing zoning reversals (e.g., MMB-164b;
Figs. 4C, 4D), whereas most lower zone augite
crystals are normally zoned (Figs. 4A, 4B). The
zoning reversals indicate that many of the twopyroxene andesitic roof dike magmas underwent
a mixing event, a conclusion that is consistent
with the common occurrence of orthopyroxene
phenocrysts rimmed by augite and zoning reversals in plagioclase phenocrysts (Barnes, 1987).
1761
Coint et al.
The absence of high-Al orthopyroxene in these
dikes (Barnes, 1983, 1987) suggests that mixing
was not a deep-crustal event and was more likely
to occur in a shallower reservoir.
Although the compositions and zoning patterns in the andesitic roof dikes rule out an origin from lower zone magmas, it is possible that
the dike magmas came from the central zone,
the only part of the pluton with abundant evidence for mixing and mingling. Pyroxene in the
central zone is rarely preserved, which makes
comparison with pyroxene in the roof dikes difficult. Zoned augite crystals in roof-zone andesite (e.g., MMB-164b) show an abrupt increase
in Cr contents that suggests mixing with relatively primitive mafic magma. No evidence of
such mafic input has been recorded by hornblende in the central part of the intrusion. We
therefore conclude that the source of the twopyroxene andesitic magmas was a part of the
batholith that is not exposed. This zone of the
batholith most probably underlies the southern
and southwestern portion of the upper zone.
Dacitic and Rhyodacitic Roof Dikes
Dacitic and rhyodacitic roof dikes were interpreted as originating from the upper zone magma
(Barnes et al., 1986a; Coint et al., 2013) on the
basis of their bulk-rock compositions, which
overlap compositions of the upper zone rocks
(fig. 8 in Coint et al., 2013). In addition, hornblende phenocrysts in the dacitic and rhyodacitic
roof dikes are compositionally identical to hornblende from the upper zone in terms of major
and trace element abundances, and display the
same zoning relationships (Fig. 8H). The presence of these dikes structurally above the upper
zone indicates that the upper zone magma was
eruptible. Presumably, the compositional range
of dacitic to rhyodacitic roof dikes reflects a
temporal change in composition of differentiated
magmas in the uppermost part of the upper zone.
IMPLICATIONS FOR THE PLUTON
ASSEMBLY AND COMPARISON WITH
OTHER SYSTEMS
Our results indicate that the lower zone
resulted from emplacement of many individual
magma batches of broadly andesitic composition. The CA-TIMS ages cited here (presented
in detail in Coint et al., 2013) show that this
zone was emplaced through a short period of
time, from 159.22 ± 0.10 Ma to 158.99 ± 0.17
Ma. The predominantly north-south strike and
steep dips of internal contacts and foliation in
the lower zone are suggestive of batch-wise
magma emplacement in subvertical sheets
(Fig. 9A; Coint et al., 2013). The range of rock
compositions and texture from sample to sample
1762
(Figs. 4A, 4B) can be explained by variation in
the compositions of individual magma batches,
different cooling rates of the individual magma
batches, and variable segregation of interstitial
melts. Local gradational contacts between texturally distinct rocks indicate that some of the
magma batches were significantly above their
solidus when adjacent batches were emplaced,
and this relationship is exemplified by inheritance of augite from pyroxenitic blocks in adjacent quartz diorite and tonalite.
The andesitic to dacitic upper zone magmas
were more buoyant than the rocks and mushes
in the lower zone and so were emplaced above
the lower zone, into rocks of the eastern Hayfork and western Hayfork terranes (Figs. 9B,
9C). Local mixing between the lower zone and
upper zone magmas occurred, such as along the
western mafic selvage and in the central zone
(Fig. 9C). Mixing created hybrid rocks with
relict pyroxene from the lower zone surrounded
by euhedral hornblende crystals, the compositions of which indicate an affinity with the upper
zone. In the meantime, hot basaltic andesitic was
emplaced into the central zone, where it locally
mingled with the host magma and reheated the
system (Fig. 9C). Ages obtained on the central
zone are in agreement with the geochemical
data presented here, in that they overlap with
ages for both the upper and lower zone (Fig. 9
and Coint et al., 2013).
Several studies based on numerical modeling
and natural examples have suggested that mixing due to injection of mafic magma into silicic
magma is unlikely to be extensive, leaving the
mafic magma as a source of heat and fluid (Robinson and Miller, 1999; Bachmann et al., 2002;
Huber et al., 2009; Ruprecht et al., 2012). Burgisser and Bergantz (2011) demonstrated that
the heat generated by a small volume of mafic
material emplaced at the base of a crystal-rich
silicic magma can cause rapid remobilization of
the overlying mush (unzipping), in a few days to
several months after the intrusion of the mafic
material. This remobilization results in homogenization of the felsic magma due to convection (Robinson and Miller, 1999). If this type of
mobilization occurred in the upper zone magma,
the scant evidence for resorption in plagioclase,
hornblende, and quartz suggests that heating did
not rejuvenate a locked mush such as in some
large dacitic eruptions (Bachmann et al., 2002),
but instead initiated the homogenization of
magma having crystallinity low enough to permit
free flow (Fig. 9D; Robinson and Miller, 1999).
Hornblende in the central zone, the inferred
feeder system of the upper zone, is compositionally nearly identical to hornblende in the
upper zone, and hornblende in the upper zone is
essentially identical from one sample to the next
Geosphere, December 2013
over a range of rock types and structural levels.
These similarities suggest that the broadly tonalitic magmas initially emplaced in the central and
upper zones were very similar in bulk composition. Convective mixing inferred for the upper
zone did not significantly affect the central zone,
because the heterogeneous nature of the central
zone (e.g., deformed synplutonic dikes, MME
swarms) is preserved. Evidently, these similar
tonalitic magmas in the central and upper zones
gained their compositional features in structurally deeper levels of the magmatic system.
Convection in the upper zone permitted growth
of hornblende and plagioclase phenocrysts from
a broadly homogeneous magma body. However,
once large-scale convection stopped (Fig. 9E),
either due to increased crystallinity or loss of
mafic heat input, upward percolation of residual
melt through the mush resulted in the broad
upward zoning of the upper zone, in a manner
similar to that proposed for monotonous ignimbrites (Christiansen, 2005). This process resulted
in the lower part of the upper zone becoming a
partial cumulate, upward increase in the abundances of quartz and K-feldspar, and development of euhedral quartz. Percolation of residual
melt through a mush can also explain why the
upward zoning is not systematic (fig. 2 in Coint et
al., 2013), because melt percolation would depend
on local conditions of porosity and permeability.
In contrast, upward zoning due to emplacement
of successively more evolved magmas in subhorizontal layers should result in much more systematic zonation (e.g., Bartley et al., 2008). Moreover,
emplacement of variably evolved magma batches
would not be expected to result in crystallization
of hornblende with uniform composition throughout the thickness of the upper zone.
We therefore conclude that the upper zone
was once a large, convecting body of chemically and physically interconnected magma that
crystallized compositionally very similar hornblende and plagioclase (Fig. 9D). Upward zoning occurred after initial crystallization of hornblende and plagioclase phenocrysts by upward
percolation of residual melt (Fig. 9F).
CONSEQUENCES FOR VOLCANIC
SYSTEMS: DID WCb MAGMAS ERUPT?
The abundance of mafic through felsic dikes
in the roof zone (Figs. 2E, 2F) of the batholith
leads to the conclusion that some WCb magmas escaped the system and probably erupted.
The first magmas to erupt (i.e., roof dikes) are
comparable to the lower part of the system, and
were mainly basaltic andesitic in composition.
Few clear-cut examples exist because many
of the oldest mafic roof dikes were intensely
altered by thermal effects of emplacement of the
Pluton assembly using trace elements in augite and hornblende
Depth (km)
P (kbar)
159.32 to 158.82 Ma
A
MMB-590
SSW
NNE
2
10
159.32 Ma
B
SSW
NNE
2
10
4
4
20
20
6
6
30
8
10
40
30
8
10
40
12
andesite
to 158.82 Ma
C
SSW
NNE
Western
selvage
MMB-594
10
2
12
dacite
D
158.71 Ma
SSW
NNE
2
10
4
4
20
20
6
6
30
8
10
40
Andesitic magma
basaltic andesite
8
40
10
andesitic
magma
12
to 158.71 Ma
Fig. 2e
SSW
30
E
NNE
Fig. 2c & d
2
10
Dacite
4
basaltic
andesite
156.71 Ma to 154.79 Ma
domes
eHt
wHt
10
2
RCt
Fig. 2f & g
4
2 mica
granite 6
6
30
8
10
40
andesitic
magma
basaltic
andesite
Basaltic andesite
TwoPyroxene
Andesite
12
G
NNE
SSW
20
20
12
30
RCt
8
10
40
late andesitic
granite magma
basaltic
andesite
12
Figure 9. Emplacement model for the
Wooley Creek system (WCb—Wooley
Creek batholith). Ages reported here are
chemical abrasion–thermal ionization mass
spectrometry (CA-TIMS) U-Pb zircon ages
from Coint et al. (2013). They are presented
as the oldest to youngest ages for each step
of the pluton assembly accounting for the 2σ
error. P—pressure. (A) Emplacement of the
lower zone as individual magma batches.
The black lines between the different
magma batches indicate that the interaction
between these particular batches with the
surrounding ones were limited; however,
they do not indicate the presence of a sharp
contact in the field. (B) Emplacement of the
central zone as individual magma batches.
(C) Emplacement of the upper zone and
interaction between the upper and lower
zone magmas in the western mafic selvage
followed by the arrival of the basaltic andesitic magma in the central zone and emplacement of the two-pyroxene andesitic batches
at depth. (D) Basaltic andesite emplacement in the central zone triggers convection and homogenization in the upper zone.
(E) Development of the upper zone as a large
batch of magma of relatively homogeneous
composition. (F) Evolution of the upper zone
by fractional crystallization with development of a broad upward zoning with more
felsic rocks occurring toward the top of the
intrusion. Eruption of more two-pyroxene
andesitic dikes and emplacement of the late
and two-mica granite occurred later. The
dashed line is the current level of exposure.
RCt—Rattlesnake Creek terrane; eHt—
eastern Hayfork terrane, wHt—western
Hayfork terrane.
Mafic magmatic enclaves
Lower zone and
Slinkard pluton
Andesite
Upper WCb
Dacite
Rhyolite/Granite
Pyroclastic flow
Geosphere, December 2013
1763
Coint et al.
upper zone. However, dike samples such as finegrained gabbro MMB-590 (Barnes et al., 1990)
indicate that lower zone magmas reached high
levels of the system and were capable of erupting. In typical volcanic systems, such magmas
would have formed scoria cones and lava flows;
however, the volume ejected would have been
limited (Figs. 9A, 9B). The lateral extent of the
lower zone suggests that volcanism was unlikely
to be focused at one volcano, but rather formed a
volcanic field with several eruptive vents, releasing magmas of variable compositions.
As the dacitic magma batches that formed the
upper zone arrived in the system, it is possible
that these batches erupted, forming part of the
roof dike system observed today. The emplacement of several batches of dacitic magma would
suggest that eruptions would not necessarily be
focused in a single edifice, but probably at several
vents. At the same time, batches of two-pyroxene
andesite were emplaced underneath the southern
part of the intrusion; these magmas rose into the
uppermost part of the upper zone and the roof
zone to form the two-pyroxene andesite dikes.
Arrival of these porphyritic magmas at the surface could have formed thick lava flows (Fig. 9C).
In this model, emplacement of basaltic andesite in the central WCb resulted in convection
and homogenization of the upper zone magmas
(Figs. 9C–9E). There are numerous examples of
dacitic eruptions triggered by the arrival of mafic
magma at the base of silicic systems (e.g., Bachmann et al., 2002; Pallister et al., 1992). If the
dacitic roof dikes are appropriate examples of
the state of the upper zone at the time of eruption,
then the percentage of crystals, ~40%–50%, is
similar to crystal proportions in homogeneous
dacitic ignimbrite deposits described throughout
the southwestern United States (Hildreth, 1981;
Bachman et al., 2002; Christiansen, 2005). The
size of the WCb upper zone is not comparable to
these large ignimbrite deposits, which represent
1000–5000 km3 of erupted magma, but it could
easily have fed eruptions as large as or larger
than the 7000 ka culminating eruption of Crater
Lake (Bacon and Druitt, 1988) (Figs. 9E, 9F).
As volcanism waned, the upper zone magma
differentiated by fractional crystallization,
developing a felsic cap that could have erupted
as rhyolitic domes, capable of producing minor
pyroclastic flows.
CONCLUSIONS
On the basis of analogies with modern arc
volcanoes, such as in the Cascade Range, the
WCb corresponds to the type of magmatic reservoir expected to be beneath typical arc-related
composite volcanoes. The first magmatism to
occur was mafic to intermediate and represented
1764
emplacement of multiple batches of magma in
the middle crust, magma batches that underwent very limited homogenization. As a result,
individual magma batches can be identified
on the basis of augite compositional variation.
The central zone of the pluton corresponds to
a complex mixing and transition zone, where
magmas from the lower zone interacted with
the upper zone magmas. It is also a part of the
intrusion where replenishment of mafic magmas
occurred. The mafic magmas provided heat (and
possibly fluids) to the upper part of the system,
but interaction of the mafic magmas with the
host dacitic (upper zone) magmas was limited.
The upper zone crystallized from convecting,
relatively homogeneous, dacitic magma. Once
convection stopped, fractionation occurred by
melt percolation through the mush pile, resulting
in a broad, nonsystematic upward zoning of the
unit with more mafic, partly cumulative rocks in
structurally lower parts of the zone and progressively more evolved granodiorite and granite
toward the roof. Roof dikes of dacitic composition indicate that the upper zone magmas were
eruptible and escaped the underlying magma
chamber. Unlike many models for incremental
assembly of plutons, we can demonstrate that
fractional crystallization was a viable process in
the middle to upper parts of the crust and that
evolved volcanic rocks need not acquire all their
chemical characteristics in the lower crust.
This study demonstrates that mineral trace
element chemistry provides an important complement to geochronology in order to better
understand the history and pace of batholith
emplacement. In particular, the mineral compositions provide a tool to map the extent of ancient
magma reservoirs, identify magma batches, and
understand their chemical and physical evolution
and interactions.
ACKNOWLEDGMENTS
We thank Monika Leopold, Samantha Buck, and
Brendan Hargrove for assistance in the field and
George Morgan (University of Oklahoma) and Susan
Swapp (University of Wyoming) for their help with
microprobe analyses. We thank Calvin Miller, an
anonymous reviewer, and associate editor Mike Williams for their helpful comments and advice. This
work was supported by National Science Foundation
grant EAR-0838342 to Yoshinobu and C. Barnes, a
2009 Geological Society of America Penrose grant
to Coint, and the Texas Tech University Department
of Geosciences.
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