GeologySpiritMountainBatholith Walker2007

See discussions, stats, and author profiles for this publication at: https://www.researchgate.
net/publication/222521204
Geology and geochronology of the Spirit Mountain batholith, southern

Nevada: Implications for timescales and physical processes of batholith
construction
Article in Journal of Volcanology and Geothermal Research · November 2007

DOI: 10.1016/j.jvolgeores.2006.12.008
CITATIONS READS
169 826
5 authors, including:
Barry A. Walker Calvin F. Miller

Portland State University Vanderbilt University
25 PUBLICATIONS 1,007 CITATIONS 241 PUBLICATIONS 11,222 CITATIONS
SEE PROFILE SEE PROFILE
Lily Claiborne Jonathan S. Miller

Vanderbilt University San Jose State University
53 PUBLICATIONS 1,274 CITATIONS 92 PUBLICATIONS 2,922 CITATIONS
SEE PROFILE SEE PROFILE
Some of the authors of this publication are also working on these related projects:
Geochemistry and petrology of mafic lavas preceding the 18.8 Ma Peach Spring Tuff supereruption, southern Black Mountains, AZ View project
Review paper on magma chamber dynamics View project
All content following this page was uploaded by Barry A. Walker on 22 December 2019.
The user has requested enhancement of the downloaded file.

Available online at www.sciencedirect.com
Journal of Volcanology and Geothermal Research 167 (2007) 239 – 262

www.elsevier.com/locate/jvolgeores
Geology and geochronology of the Spirit Mountain batholith,

southern Nevada: Implications for timescales and physical
processes of batholith construction
B.A. Walker Jr. a,⁎, C.F. Miller a , L. Lowery Claiborne a , J.L. Wooden b , J.S. Miller c
a
Department of Earth and Environmental Sciences, Vanderbilt University, Nashville, TN 37235, USA
b
US Geological Survey, Stanford-USGS Micro-analytical Center, School of Earth Sciences, Stanford University,
Green Earth Sciences Building, Stanford, CA 94305-2220, USA
c
Department of Geology, San Jose State University, San Jose, CA 95192-0102, USA
Received 22 November 2005; received in revised form 12 September 2006; accepted 14 December 2006
Available online 9 January 2007
Abstract
The Spirit Mountain batholith (SMB) is a ∼250 km2 composite silicic intrusion located within the Colorado River Extensional
Corridor in southernmost Nevada. Westward tilting of 40–50° has exposed a cross-section from the roof through deep levels of the
batholith. Piecemeal construction is indicated by zircon geochronology, field relations, and elemental geochemistry. Zircon U/Pb
data (SHRIMP) demonstrates a ∼ 2 million year (17.4–15.3 Ma) history for the SMB. Individual samples contain zircons with ages
that span the lifetime of the batholith, suggesting recycling of extant zircon into new magma pulses. Field relations reveal several
distinct intrusive episodes and suggest a common injection geometry of stacked horizontal sheets.
The largest unit of the SMB is a gradational section (from roof downward) of high-silica leucogranite through coarse granite
into foliated quartz monzonite. Solidification of this unit spans most of the history of the batholith. The 25 km × 2 km leucogranite
was emplaced incrementally as subhorizontal sheets over most or all of the history of this section, suggesting repeated fractional
crystallization and melt segregation events. The quartz monzonite and coarse granite are interpreted to be cumulate residuum of this
fractionation. Age data from throughout this gradational unit show multiple zircon populations within individual samples.
Subsequent distinct intrusions that cut this large unit, which include minor populations of zircons that record the ages of earlier
events in construction of the batholith, preserve a sheeted, sill-on-sill geometry.
We envision the SMB to have been a patchwork of melt-rich, melt-poor, and entirely solid zones throughout its active life.
Preservation of intrusion geometries and contacts depended on the consistency of the host rock. Zircons recycled into new pulses of
magma document remobilization of previously emplaced crystal mush, suggesting the mechanisms by which evidence for initial
construction of the batholith became blurred.
© 2007 Published by Elsevier B.V.
Keywords: magma chamber; batholith construction; zircon geochronology; granite; Colorado River Extensional Corridor
1. Introduction
Prevailing views of the nature of magma chambers

⁎ Corresponding author. and pluton construction have been challenged in recent
E-mail address: walkerb@geo.oregonstate.edu (B.A. Walker). years. Recent geochronological studies of volcanic and
0377-0273/$ - see front matter © 2007 Published by Elsevier B.V.
doi:10.1016/j.jvolgeores.2006.12.008
240 B.A. Walker Jr. et al. / Journal of Volcanology and Geothermal Research 167 (2007) 239–262
plutonic rocks suggest that magmatic systems may be Mountains at the southern tip of Nevada. He briefly
long-lived, on the order of N 105 to N 106 years (Halliday described the structure and petrology of a large Tertiary
et al., 1989; Mahood, 1990; Davies et al., 1994; Brown pluton within the block. Subsequent studies (Faulds
and Fletcher, 1999; Schmitt et al., 2002; Vazquez and et al., 1992; Hopson et al., 1994; Howard et al., 1994;
Reid, 2002; Cates et al., 2003; Hawkins and Wiebe, Haapala et al., 2005) established the Spirit Mountain
2003; Coleman et al., 2004; Miller and Wooden, 2004; pluton and the adjacent Mirage pluton as tilted granite
Hildreth, 2004; Grunder and Klemetti, 2005; Charlier complexes. These two plutons have since been con-
et al., 2005; Cruden et al., 2005; Simon and Reid, 2005; sidered separate intrusions, but based on chemistry,
Walker et al., 2005)—longer than the anticipated geochronology, and field relations presented here, we
lifespan of a large magma body as indicated by thermal refer to them and other smaller intrusive units exposed
modelling (Glazner et al., 2004). A growing body of in the area collectively as the Spirit Mountain batholith
evidence suggests that many exposed plutons are (SMB). Preliminary geochronological studies of the
composite, formed by multiple replenishments of both Spirit Mountain pluton yielded ages of 17 Ma (U–Pb
monotonously similar and highly contrasting magma in sphene; Howard et al., 1996) and 20 Ma (Rb–Sr
(Wiebe, 1994; Paterson and Miller, 1998; Miller and whole rock; Rämö et al., 1999). The Mirage pluton
Miller, 2002; Hawkins and Wiebe, 2003; Glazner et al., yielded an age of 15 Ma (U–Pb in zircon; Howard
2004), and similar processes are inferred for chambers et al., 1996).
that feed volcanoes (Hildreth, 1981; Bacon and Metz, The SMB is located within the northern Colorado
1984; Eichelberger et al., 2000; Koyaguchi and Kaneko, River Extensional Corridor (Fig. 1), which experi-
2000; Hildreth, 2004). The importance, or even the enced crustal thinning from ∼ 16 to 11 Ma; magma-
existence, of an identifiable magma chamber in the tism outlasted extension, beginning ∼ 18 Ma and
construction of plutons has been questioned (Glazner terminating at ∼ 8 Ma (Faulds et al., 1995; Howard
et al., 2004), based upon (1) the inability of geophysical et al., 1996; Gans and Bohrson, 1998). After em-
investigations to identify sizable zones with high melt- placement, the SMB was tilted 40–50° westward, as
rich fraction in the Earth's crust (e.g. Iyer et al., 1990; indicated by paleomagnetic data (Faulds et al., 1992)
Lees, 2005), and (2) field and geochronological evidence and west–east progression from miarolitic leucogra-
for piecemeal accumulation over protracted periods. nite to coarse-grained, foliated quartz monzonite
However, giant eruptions (N 500 km3) provide indisput- (Hopson et al., 1994). This tilting affords a cross-
able evidence that large reservoirs of melt-rich, felsic section of the batholith in map view, with a westward
magma do reside, at least from time to time, beneath the paleo-up direction.
Earth's surface (Hildreth, 1981; Christiansen, 1984; The SMB intruded three extant units: a 1.7 Ga gneiss
Chesner et al., 1991; Bachmann et al., 2002). complex, a 1.4 Ga megacrystic granite, and the Late
The purpose of this paper is two-fold. The first Cretaceous White Rock Wash pluton. Four other
objective is to describe and interpret the history of the Miocene plutons of similar age, all dominantly granitic
Spirit Mountain batholith, a large composite intrusion that with small to large mafic components, lie within the
is well-exposed in cross-section in southernmost Nevada. northern Colorado River Extensional Corridor in the
The second objective is to discuss how the magmatic vicinity of SMB: Aztec Wash pluton (15.6–15.8 Ma),
history of this batholith may yield insight regarding the Searchlight pluton (17.7–15.8 Ma), Mt. Perkins pluton
timescales and physical processes of accumulation of (15.8–16.0 Ma), and Nelson pluton (∼ 16.5) (Falkner
granitic rock in the upper crust. A combination of field et al., 1995; Metcalf et al., 1995; Faulds et al., 1995; Lee
relations, zircon U–Pb (SHRIMP) data, and elemental et al., 1995; Bachl et al., 2001; Cates et al., 2003; Miller
geochemistry of the batholith reveals a protracted history et al., 2004). The Searchlight pluton, located 30 km to
of repeated replenishment, remobilization, and segrega- the north, is similar to the SMB in age and architecture,
tion of fractionated melt. The patchwork character of this with a thick sequence of quartz monzonite cumulate
intrusion may mark a common process of batholith underlying a zone of granite (Bachl et al., 2001). The
construction that, in part, reconciles conflicting evidence final major intrusive episode in the region was marked
regarding the existence of large, felsic magma chambers. by ∼ 15.3–15.5 Ma dike swarms centered on the plutons
(Faulds et al., 1992; Falkner et al., 1995; Faulds et al.,
2. Geologic background 2001; Bachl et al., 2001; Steinwinder et al., 2004;
G. Rhodes, unpub. data). The Newberry dike swarm
Volborth (1973) first mapped and described in recon- (George et al., 2005), discussed below, is typical of these
naissance the “Spirit Mountain block” in the Newberry swarms.
B.A. Walker Jr. et al. / Journal of Volcanology and Geothermal Research 167 (2007) 239–262 241
3. Lithologies and field relations ization of this poorly exposed northeastern part of the
SMB and of the Mirage granite therefore remains less
The SMB is exposed over an area of about ∼ 250 km2 complete than that of the western part of the batholith.
(Fig. 2). Exposure of fresh rock in most of the western The entire SMB, including later intrusions and more
(upper) ∼ 7 km of the SMB is almost continuous. poorly exposed sections, consists mostly of granitic rock
Narrow canyons provide good exposures in the eastern with identical mineral assemblages. Dioritic to basaltic
portion of the batholith, but outside the canyons, dikes, sills and pods are present, but they are volumet-
exposure is limited and rocks become increasingly rically minor in comparison to the granites; the sills and
altered toward the Newberry Mountains detachment. pods are restricted to the lower 1/3 of the exposed
Zones of mylonitization, fracturing, and alteration lead batholith, and the dikes are part of the late Newberry
us to infer two large ∼ N–S-striking faults in this gen- swarm that marks termination of the intrusive history.
erally poorly exposed area (see Fig. 2), but their con- The granitoids all contain varying proportions of
tinuity and magnitude of displacement remain uncertain. the assemblage plagioclase + alkali feldspar + quartz +
Lithologies permit the interpretation that normal biotite + accessories (sphene [titanite], apatite, allanite
displacement along these faults has resulted in the [and/or chevkinite], zircon, and opaque minerals,
repetition of a portion of the batholith. Our character- fluorite in some leucogranites, and schorl tourmaline
in some pegmatites). Accessory minerals are generally
euhedral and are enclosed in all other phases. Biotite,
plagioclase, and in most cases alkali feldspar are
subhedral to euhedral, suggesting that all were early
liquidus phases. Quartz is interstitial in less felsic rocks
and forms prominent subhedral crystals in more felsic
rocks, indicating that it joined the other minerals late in
general but was on the liquidus throughout crystalliza-
tion of the most silicic magmas.
Much of the well-exposed portion of the SMB appears
locally homogeneous, but close examination of field
relations reveals that it varies subtly throughout, and as a
whole it preserves a variety of textures and compositions.
With the cross-sectional view afforded by tilting, it is
apparent that most of this area has a fairly consistent
textural and chemical succession that varies from top to
bottom (cf. Hopson et al., 1994). This package is
interrupted in several places by younger intrusions.
We divide the SMB into six units that are distinct in
texture, field relations, and in some cases compositions.
These include a small exposure of the moderately felsic
roof unit at one corner of the western margin the batholith;
the Spirit Mountain granite, which is by far the most
extensive unit; Mirage granite, which forms a discrete
pluton; diorite sheets that are locally abundant; fine-
grained granite that cuts most other units as thin to thick
sheets; and the mafic to felsic Newberry Mountains dike
swarm, the latest intrusions into the batholithic system.
Fig. 1. Southernmost Nevada and adjacent California and Arizona with
the Spirit Mountain batholith and other plutons, the northern Colorado 3.1. Roof unit
River Extensional Corridor, and the Mojave–Arizona terrane bound-
ary (Bennet and DePaolo, 1987; Wooden and Miller, 1990). Miocene The roof unit is a small granitic section exposed over
plutons: 1—Boulder City; 2—Nelson; 3—Aztec Wash; 4—Search- an area of ∼ 3 km2 at the northwest corner of the
light; 6—Spirit Mountain batholith; 7—Sacramento; 8—Mt. Perkins.
5 is the Cretaceous White Rock Wash pluton. LVVSZ—Las Vegas
batholith, in contact with gneiss that forms the roof. It
Valley shear zone; LMSZ—Lake Meade shear zone. Towns shown for is a fine- to medium-grained, pink granite with typically
reference. ∼ 40–50% alkali feldspar (anhedral), 30% quartz
(anhedral), up to 15–20% plagioclase (anhedral), b1% altered northeast. In the western area, it constitutes a
biotite (sub-euhedral), and ∼ 1% opaques (a majority of sequence that ranges from west to east, for the most part
which replace sphene and biotite). Plagioclase, alkali gradationally, from high-silica leucogranite to foliated
feldspar, and round quartz phenocrysts are present quartz monzonite.
locally, none exceeding 5 mm in diameter. Myrmekitic The western, or upper, margin of the Spirit Mountain
intergrowths of quartz and plagioclase are very granite (and thus of the batholith, except where the roof unit
abundant in this unit. is exposed) is a ∼25 km × 2 km zone of high-silica
leucogranite. This zone comprises sheets of aplite,
3.2. Spirit Mountain granite porphyry, and fine- to medium-grained, equigranular
granite, with contacts that are sharp to barely perceptible.
By far the largest unit of the SMB will be referred to Most sheets were initially subhorizontal, but dikes
as the Spirit Mountain granite. It dominates both the (subvertical) are also common. We interpret these relations
well-exposed western area and the less exposed, more to indicate repeated emplacement of the leucogranites—
Fig. 2. Geologic map of the Spirit Mountain batholith. Up direction prior to tilting indicated by arrrow. Highway 163 shown for reference. Newberry
Mountains dike swarm cuts through the central portion of the batholith, but is absent to avoid visual complication.
some sheets intruding a hot, melt-bearing mush, and some this fabric is interpreted to be dominantly magmatic
intruding solid rock. Vesicles, or miarolitic cavities, are and probably related to compaction of a crystal mush
widespread and most common toward the west (top). (cf. Bachmann and Bergantz, 2004).
Pegmatite pods and dikes dominated by coarse quartz and A distinct intrusion marks a break in the gradation
alkali feldspar are present locally. Typical leucogranites within part of the Spirit Mountain granite. This
have ∼40–50% alkali feldspar (subhedral), 30–40% intrusion also grades from a high-silica leucogranite cap
quartz (anhedral in groundmass, but phenocrysts are (1–100 m thick) downward into coarser, less felsic granite.
subhedral), ∼10% plagioclase (sub-euhedral), and ∼1% The contact between this unit and the overlying granite
biotite (euhedral). Porphyritic variants contain ∼0.5 cm ranges from straight to very sinuous. In places, large
phenocrysts of quartz and alkali feldspar. vesicles (Fig. 3a) and pegmatites are common below the
The base of the leucogranite grades into coarser, less contact. Locally, small (bm) blocks of the overlying
felsic granite over a distance of about 10–20 m. Alkali granite are also present just below the contact. We have
feldspars become pink, biotite becomes more abundant, been unable to locate a basal contact of this sequence. A
and quartz decreases in abundance. This coarse granite, network of leucogranite dikes and sills (Fig. 3c) emanates
which extends downward for ∼3 km, averages ∼20–35% from the top of this intrusive sequence into the overlying
plagioclase (euhedral laths), 30–40% alkali feldspar (sub- granite. Large pods of leucogranite up to 500 m long by
euhedral, ∼1.5 cm), 15–30% quartz (interstitial and ∼150 m thick, elongated in the paleohorizontal direction
anhedral to ∼1 cm subhedral), 3–8% biotite (euhedral), and bounded by sharp contacts on all sides, are present
and ∼1% sphene (euhedral). Quartz forms prominent, within the Spirit Mountain granite about 1 km above the
discrete, round grains to the west, but diminishes in size top of this internal intrusion.
and abundance to the east (deeper levels). Rapakivi rims
are evident on some alkali feldspar grains and become 3.3. Mirage granite
more abundant with depth. Locally, small (b 2 cm),
plagioclase + fine-grained biotite-rich clusters are present The Mirage granite is separated from the Spirit
within the granite. Mountain granite by a thin (∼ 5 to 50 m) septum of
Fine-grained dioritic enclaves are present throughout Proterozoic granite along part of the contact. Where a
the lower ∼1/2 of the coarse granite. They range in contact with SM granite is exposed, the Mirage granite
maximum dimension from ∼5–30 cm and are typically cuts the Spirit Mountain granite foliation. Likewise,
ellipsoidal, with irregular margins that are penetrated by felsic dikes that appear to emanate from the Mirage
crystals of the host granite, suggesting liquid/crystal mush granite intrude the Spirit Mountain granite. The Mirage
(or mush/mush) contact. The enclaves are composed granite ranges in texture from very fine- to medium-
mainly of plagioclase and biotite, with minor hornblende grained and is characterized by abundant quartz that is
and clinopyroxene. Some contain large alkali feldspars, usually visible in hand sample to the unaided eye
suggesting crystal incorporation from the host granite. (b5 mm). The upper rocks near the contact with the
These enclaves are the only manifestation of mafic input Spirit Mountain granite are medium-grained and have
during solidification of the Spirit Mountain granite unit. abundant quartz and a low biotite content (1–2%).
Widespread schlieren show no apparent preferred Similar felsic granite is also present in sheets and pods
orientation. Abundant pegmatite pods are commonly (1–5 m thick) elsewhere in the Mirage granite. The
bounded by schlieren at their paleo-upper surfaces. interior of the Mirage granite is medium-grained, has
The coarse-grained granite grades downward into less quartz, more biotite, and is characterized by ∼ 2 cm
magmatically foliated quartz monzonite that is poorer in alkali feldspar phenocrysts. The granite has a well
quartz and richer in biotite. The quartz monzonite is developed fabric in places, with polycrystalline
coarse-grained, with 40–50% alkali feldspar (euhedral), stretched quartz indicating subsolidus deformation.
30–35% plagioclase (euhedral), 10–15% biotite (euhe- Fine-grained basaltic to medium-grained dioritic pods
dral), and 5–15% quartz (interstitial, anhedral). Folia- and dikes cut and locally mingle with the eastern (lower)
tion, defined by aligned biotite, alkali feldspar, and part of the granite. These mafic intrusions may be
plagioclase, is parallel to paleohorizontal and gradually associated with the dioritic rocks described below.
becomes stronger downward. Dioritic enclaves are
abundant, very large (up to 3 m), pancake–shaped, and 3.4. Diorite sheets
oriented parallel to the rock's ∼N–S-striking, W-dipping
fabric (Fig. 3a). Based on euhedral to subhedral feldspar Relatively mafic rocks, typically dioritic, are exposed
crystal shapes and weakly to unstrained interstitial quartz, as initially subhorizontal sheets to pod-like intrusions up
Fig. 3. (a) Large, pancake–shaped mafic enclaves within the SM quartz monzonite. Enclaves are aligned parallel to the host's foliation. 5′ 7″ Ben
George for scale. (b) Large quartzofeldspathic cavities within the SM leucogranite. (c) A network of leucogranite dikes and sills within the SM
granite. Paleo–up direction indicated.
to ∼100 m thick that cut the deeper and more southerly The diorite is fine- to medium-grained, and typically
parts of the Spirit Mountain granite. Locally, these contains close to 50% hornblende, ≥ 50% plagioclase,
sheets pinch out and then reappear along strike. In b5% quartz, and up to 5% biotite. Sphene, apatite,
places, the diorite is also present as pillows in the fine- opaque minerals, and minor zircon are present as well.
grained granite unit (see below). Though visually
striking, the diorite is, volumetrically, a very minor 3.5. Fine-grained granite
unit of the SMB. Blocks of Spirit Mountain granite are
present but sparse within the diorite bodies. Whereas Like the diorite, the fine-grained granite (FGG) is
some granite blocks are angular and have sharp margins, observed primarily as subhorizontal sheets to pod-like
others have irregular, poorly defined margins and the intrusions within the middle to deeper parts of the
host mafic rock is contaminated by feldspar crystals, southern Spirit Mountain granite (cf. Hopson et al.,
suggesting partial disaggregation of the granite 1994). Sharp cross-cutting relationships clearly show
(Fig. 4a). At one well-exposed outcrop, many xenoliths that FGG intruded the Spirit Mountain granite. Blocks
can be seen in what appears to be a spectrum of dis- of the Spirit Mountain granite (in some cases perhaps
aggregation, from angular blocks to loose collections of disaggregated or in situ screens), ranging in size from
feldspars within contaminated diorite (Fig. 4b). At the cm's to N100 m, are common within the FGG.
same outcrop, a xenolith with a comet-like “tail” of Individual FGG sheets, where discernible, are cm's to
contaminated rock may document ascent and partial 50 m thick, though in many places it is difficult to
disaggregation of a block of granite within denser mafic identify separate sheets. In some areas, internal contacts
magma, with the “tail” representing a wake of con- separate FGG phases that are distinguishable by the
tamination and feldspar dissemination (Fig. 4c). presence or absence of 1–2 cm alkali feldspar
megacrysts. It is unclear whether these megacrysts were irregular and inclusion-rich boundaries, perhaps indicat-
derived from the Spirit Mountain granite wall rock, or if ing minor resorption followed by regrowth. At many
they grew from the FGG magma. locations, biotite is aligned and the quartz is strongly
In places, blobby pillows of diorite within the FGG strained with conspicuous subgrain development, sug-
(Fig. 4d) indicate that intrusion of the two magmas gesting minor to moderate subsolidus deformation of the
coincided, but angular enclaves of diorite in FGG and FGG unit.
sharp contacts of FGG dikes into diorite indicate that at
least some of the FGG intruded after diorite became 3.6. Dike swarm
rigid. This, along with the internal contacts, suggests
that there were multiple pulses of FGG emplaced into The Newberry Dike swarm strikes ∼N–S and cuts all
the SMB. Small dikes of the FGG cross-cut portions of other units in the SMB. Paleomagnetic data show that
the Mirage granite, although a clear contact between the these dikes intruded roughly vertical and were tilted to
two units has not been observed. ∼ 40–65° E dips (Faulds et al., 1992; George et al.,
FGG is uniformly fine-grained and equigranular, 2005). The dikes are aphanitic to fine-grained phaneritic
except for the local alkali feldspar megacrysts. It typically and range in composition from granitic to basaltic. The
has 30–35% plagioclase (small, euhedral laths), 25–30% felsic dikes are ∼ 3–20 m thick, whereas most of the
quartz (anhedral), 30–35% alkali feldspar (blebby, mafic dikes are ∼ 1–5 m thick (one unusual mafic dike
anhedral), and 5–10% biotite (euhedral). Myrmekitic is 15 m thick). The felsic dikes are porphyritic and
intergrowths of quartz and alkali feldspar are common. contain up to 10% each of quartz, alkali feldspar, and
Megacrysts of alkali feldspar where present have slightly plagioclase phenocrysts. Rounded quartz phenocrysts,
Fig. 4. (a) A xenolith of Spirit Mountain granite within the diorite. Poorly defined margins of the xenolith and large feldspar crystals in the diorite
suggest this xenolith was partially disaggregated. (b) A loose collection of feldspar crystals within a contaminated diorite perhaps documenting the
final stages of xenolith disaggregation. (c) A Spirit Mountain granite xenolith with a comet-like tail of feldpsar-bearing, contaminated diorite,
possibly documenting the frozen process of xenolith flotation (note up direction) in the more dense diorite. (d) Contaminated dioritic pillows within a
contaminated fine-grained granite (FGG). Note also the small, white xenoliths of the Spirit Mountain granite.
246
Table 1
Whole rock elemental data for the Spirit Mountain batholith
Rock BW40 BW43 BW48 BW49 BCOZ 101Z BC101z BW41SML 120Z BW36 BW32 SML 213 SWZ LGZ BW 24 BW 47 SML 47
type
SMG SMG SMG SMG SMG SMG SMG SMG SMG SMG SM QM SM QM SM QM SM LG SM LG SM LG SM LG
SiO2 74.24 74.85 75.58 68.88 72.33 69.93 70.84 67.55 73.16 73.59 64.38 60.81 63.19 76.80 77.74 77.01 77.31
Al2O3 13.75 13.36 13.07 15.53 14.62 15.18 14.77 16.22 13.65 14.27 18.69 18.60 17.64 12.65 12.14 13.24 12.28
B.A. Walker Jr. et al. / Journal of Volcanology and Geothermal Research 167 (2007) 239–262
Fe2O3 1.59 1.72 1.35 2.94 2.06 2.65 2.37 3.73 1.75 1.35 2.46 4.18 4.10 0.92 0.93 0.52 0.85
MnO 0.05 0.05 0.05 0.06 0.04 0.05 0.06 0.06 0.05 0.04 0.05 0.10 0.09 0.05 0.02 0.03 0.03
MgO 0.36 0.51 0.29 0.85 0.50 0.77 0.69 1.14 0.41 0.32 0.44 1.27 1.35 0.10 0.11 0.04 0.13
CaO 1.22 1.40 0.93 2.18 1.62 1.41 1.71 3.24 1.18 1.10 1.53 3.06 2.73 0.67 0.65 0.72 0.55
Na2O 3.82 3.82 3.97 4.00 3.65 4.42 4.06 4.00 3.31 3.90 4.75 5.53 4.70 3.92 2.86 4.32 3.44
K2O 4.66 3.97 4.46 4.84 4.78 4.91 4.95 3.36 5.24 5.14 7.13 4.37 5.07 4.75 5.35 4.03 5.22
TiO2 0.23 0.25 0.23 0.53 0.29 0.52 0.43 0.53 0.25 0.23 0.49 0.79 0.87 0.13 0.16 0.06 0.14
P2O5 0.08 0.07 0.06 0.18 0.09 0.14 0.13 0.16 0.08 0.06 0.09 0.33 0.27 0.02 0.02 0.03 0.04
Rb 175 123 156 98 159 84 131 97 192 133 96 67 83 236 80 196 159
Sr 224 210 121 356 311 323 272 456 191 188 383 578 551 27 83 11 73
Ba 820 541 341 1323 1212 1085 886 1103 826 699 1932 1811 1850 92 253 17 277
Cs 1.3 1.1 0.9 0.7 0.7 0.2 1.0 2.0 1.6 0.9 0.4 0.4 0.3 0.7 0.4 1.0 1
Ta 1.73 1.33 2.22 1.73 1.37 1.03 1.74 0.98 1.40 1.53 0.59 0.90 1.11 3.33 0.51 0.60 1.5
Nb 19.2 16.9 29.2 22.0 14.1 21.3 24.3 13.7 17.9 18.7 11.1 20.5 19.2 38.3 7.0 10.7 20
Tl 0.95 0.59 0.70 0.51 0.91 0.44 0.82 0.71 0.81 0.87 0.53 0.24 0.38 0.86 0.48 1.34 0.7
Pb 21 18 25 20 28 18 24 16 26 24 25 22 21 28 21 35 24
Hf 4.9 4.4 5.1 7.7 5.0 8.2 5.8 5.6 5.4 3.9 10.4 12.5 12.3 4.1 4.5 2.2 2.8
Zr 160 152 153 312 186 322 229 204 202 125 421 586 557 83 152 47 83
Y 19 19 23 43 16 26 28 20 22 18 19 26 26 18 14 5 20
Sc 3.9 4.1 3.5 5.6 3.5 4.2 3.9 7.5 3.8 2.6 3.5 5.9 7.0 1.8 2.1 2.5 1.05
Cr b 0.5 4.0 5.2 6.7 3.3 2.6 1.7 3.6 b 0.5 b0.6 b0.7 1.9 b 0.7 b 0.5 1.5 1.4 b0.5
Ni 3 3 3 7 2 2 2 2 2 b1 b1 3 4 b1 b1 b1 1
V 16 14 13 37 23 31 29 56 23 11 22 53 57 5 5 b5 7
Cu 4 1 4 6 3 3 4 8 8 2 b1 6 5 1 3 1 6
Th 19.6 14.1 27.2 9.26 12.1 7.72 17.4 9.14 35.10 19.2 12.5 10.1 8.91 19.1 4.41 20.9 10.9
U 3.07 2.54 2.89 1.26 1.43 0.84 1.81 1.41 3.12 2.88 0.55 0.71 1.09 2.67 0.43 3.02 1.58
Ga 19 17 20 20 27 26 28 20 24 18 31 29 22 31 14 18 17
La 68.6 43.9 54.1 73.3 66.5 95.8 106 53.2 55.2 65.1 183 106 103 46.7 28.0 12.3 39.9
Ce 128 82.9 96.9 153 123 181 187 102 104 114 336 181 210 77.7 57.1 22.8 76
Pr 12.6 8.18 9.01 16.7 11.5 17.4 17.8 10.2 11.7 10.1 33.6 20.0 21.7 6.39 5.95 1.88 8.03
Nd 39.9 26.0 27.0 57.0 40.6 62.0 60.0 38.5 36.7 33.0 115 63 79.5 17.9 21.9 5.37 25.1
Sm 6.07 4.17 4.52 10.0 6.04 10.1 9.41 6.61 6.51 5.01 15.8 10.0 12.0 2.76 4.05 0.73 4.42
Eu 0.897 0.720 0.574 1.64 1.12 1.74 1.52 1.46 0.93 0.807 2.45 1.98 2.41 0.235 0.958 0.113 0.581
Gd 4.14 3.12 3.20 7.83 4.02 7.14 7.06 5.07 4.75 3.62 9.76 7.18 7.86 2.16 3.15 0.50 3.14
Tb 0.61 0.51 0.60 1.30 0.63 1.16 1.18 0.83 0.70 0.63 1.23 1.02 1.15 0.43 0.55 0.09 0.62
Dy 3.24 2.96 3.45 7.14 3.16 5.83 5.98 4.03 3.82 3.34 5.14 5.16 5.65 2.60 3.01 0.52 3.49
Ho 0.61 0.59 0.71 1.34 0.59 1.10 1.16 0.77 0.76 0.67 0.87 0.96 1.00 0.58 0.56 0.13 0.69
Er 1.83 1.84 2.34 4.15 1.91 3.35 3.64 2.43 2.27 2.14 2.55 2.75 3.06 2.18 1.74 0.59 2.21
Tm 0.291 0.291 0.398 0.636 0.289 0.457 0.534 0.339 0.361 0.308 0.348 0.380 0.442 0.400 0.257 0.119 0.328
Yb 1.90 1.91 2.62 3.80 1.71 2.61 3.06 2.14 2.37 2.05 2.06 2.41 2.92 2.67 1.60 0.92 1.97
Lu 0.271 0.281 0.370 0.460 0.251 0.342 0.393 0.294 0.360 0.294 0.308 0.350 0.414 0.404 0.233 0.168 0.257
Table 1 (continued )
Rock SML 49Z SML 129Z SML 130 SML 132 SML 133 SML 52 SML 54Z SML 63C SML 67 SML 69 SML 71 SML73 SML74 SML76 SML78 SML59Z MPL53Z MI-1
type
SM LG SM LG SM LG SM LG SM LG SM LG SM LG SM LG SM LG SM LG LG SM LG SM LG SM LG SM LG roof unit MG MG
SiO2 77.37 75.71 76.53 77.04 75.69 77.00 75.57 77.19 78.49 77.96 78.10 77.84 77.55 77.51 78.59 74.41 73.77 73.69
Al2O3 12.46 13.05 12.76 12.36 12.79 12.50 13.09 12.03 12.12 12.22 11.92 12.11 12.30 12.40 11.97 13.82 13.82 13.66
Fe2O3 0.68 0.90 0.61 0.63 0.99 0.99 1.19 1.46 0.67 0.57 0.81 0.77 0.73 0.73 0.59 1.11 1.86 2.19
MnO 0.05 0.08 0.09 0.05 0.06 0.06 0.05 0.07 0.01 0.04 0.05 0.07 0.07 0.09 0.08 0.06 0.04 0.04
MgO 0.09 0.07 0.08 0.08 0.18 0.13 0.28 0.11 0.04 0.07 0.07 0.07 0.07 0.05 0.03 0.15 0.54 0.61
CaO 0.43 0.47 0.40 0.38 0.57 0.48 0.78 0.24 0.38 0.43 0.40 0.33 0.40 0.35 0.20 0.42 1.56 1.85
Na2O 4.08 4.09 4.06 3.81 3.96 4.04 3.67 3.62 3.94 3.94 3.90 3.87 3.94 4.31 4.19 4.34 3.39 3.28
K2O 4.69 5.01 4.80 4.76 4.90 4.62 5.12 5.05 4.23 4.68 4.58 4.79 4.80 4.50 4.26 5.45 4.72 4.25
TiO2 0.12 0.12 0.09 0.10 0.17 0.16 0.20 0.21 0.10 0.09 0.13 0.12 0.12 0.07 0.08 0.21 0.24 0.32
P2O5 0.04 0.03 0.02 0.02 0.04 0.02 0.05 0.02 0.01 0.01 0.03 0.02 0.01 0.00 0.01 0.03 0.07 0.09
Rb 204 266 296 210 273 154 157 184 143 177 189 277 284 321 286 148 150 124
Sr 17 26 7 16 48 42 89 4 5 23 12 15 13 7 5 63 200 261
Ba 37 102 21 38 165 120 326 22 10 59 37 49 48 20 19 252 830 915
Cs 0.7 1.2 2.1 0.8 1.4 0.7 0.8 1.1 0.3 0.7 0.7 1.4 3.1 3.5 1.9 0.9 0.8 0.7
Ta 3.2 4.2 4.1 2.8 3.8 2.3 1.5 4.7 3.7 1.8 3.1 3.6 3.8 3.8 3.7 2 1.2 1.48
Nb 43.8 51.6 53.8 31.1 45.2 30.8 23.4 65.3 49.8 21.9 44.3 60.7 51.9 50.2 55.4 31.2 16.9 14.2
Tl 0.88 1.07 1.29 0.78 0.98 0.79 0.82 0.73 0.63 0.71 0.92 1.5 1.29 1.6 1.49 0.68 0.85 0.71
Pb 31 33 31 30 26 13 30 24 20 28 33 43 28 41 36 19 33 21
Hf 4.2 5.1 4.4 3.9 6.3 4.6 4.2 8.1 5.6 2.9 5.4 6.2 5.2 6.9 5.4 6.6 4.9 5.6
Zr 99 116 92 93 165 137 127 186 114 68 131 128 111 123 96 199 166 203
Y 19 21 20 16 25 27 16 42 20 15 23 22 23 24 18 26 22 23
Sc 1.31 1.95 1.64 1.36 2.45 1.18 1.97 1.84 2.03 0.91 1.18 2.15 1.79 1.79 1.6 1.7 4.22 4.9
Cr b 0.5 b 0.5 b0.5 b 0.5 2.8 b0.5 b 0.5 b 0.5 b0.5 b0.5 b0.5 b0.5 b0.5 b 0.5 b 0.5 b 0.5 6.6 8.6
Ni 1 b1 b1 b1 b1 b1 3 2 b1 b1 b1 2 b1 2 1 b1 3 5
V b5 b5 b5 b5 9 5 11 b5 b5 b5 b5 b5 b5 b5 b5 7 18 25
Cu 11 2 2 2 1 6 7 6 27 18 9 7 8 8 6 6 7 3
Th 20.8 35.5 30.4 25.9 40.3 17 17.1 44.3 38.5 17.5 25.3 67.7 29.8 41.5 33.6 15.3 15.8 18.7
U 2.89 4.19 3.9 3.74 3.92 1.87 1.34 2.99 4.71 2.14 4.01 8.12 3.06 6.27 4.88 1.79 2.33 2.13
Ga 20 25 26 22 25 20 18 22 21 18 18 19 20 24 22 19 17 18
La 36 36 29.7 29.2 45.5 40.6 36.5 60.5 41.8 32.7 43.9 38.9 50.6 44.6 26.7 62 53.5 58.9
Ce 63 60.2 49 48.1 79.3 87.4 73.2 123 64.8 59.9 77.3 65.3 86.1 74.2 49.6 120 99.5 110
Pr 5.77 5.58 4.4 4.47 7.76 8.29 7.81 12 5.48 5.64 7.01 5.6 7.38 5.81 4.41 12.7 10.8 11.2
Nd 15.7 15.2 11.9 12.5 22.4 25.9 23.7 35.1 13.6 15.8 18.8 14.6 18.8 14.1 11.9 37.5 33 36.6
Sm 2.49 2.42 1.9 1.97 3.59 4.56 3.93 5.91 1.84 2.66 2.84 2.2 2.76 1.94 1.79 6.53 5.62 6.20
Eu 0.202 0.19 0.09 0.15 0.32 0.414 0.546 0.197 0.09 0.229 0.203 0.157 0.174 0.103 0.076 0.777 0.913 1.06
Gd 1.65 1.79 1.43 1.46 2.68 3.16 2.52 4.43 1.08 1.69 1.84 1.49 1.59 1.17 1.33 4.37 3.82 4.82
Tb 0.38 0.38 0.31 0.3 0.51 0.64 0.47 0.97 0.28 0.36 0.4 0.34 0.38 0.31 0.28 0.76 0.69 0.75
Dy 2.48 2.51 2.18 2.04 3.25 4.09 2.61 6.3 2.05 2.32 2.72 2.43 2.55 2.32 2.1 4.14 3.57 4.09
Ho 0.58 0.59 0.53 0.48 0.72 0.88 0.55 1.41 0.51 0.51 0.65 0.61 0.64 0.6 0.51 0.84 0.71 0.77
Er 2.15 2.19 2.08 1.73 2.49 2.9 1.81 4.82 2.01 1.74 2.43 2.43 2.4 2.39 1.96 2.69 2.3 2.34
Tm 0.387 0.451 0.42 0.34 0.456 0.466 0.298 0.8 0.404 0.294 0.445 0.455 0.448 0.475 0.37 0.41 0.353 0.351
Yb 2.62 3.29 3.13 2.5 3.35 2.86 1.89 5 3.02 1.89 3.09 3.35 3.18 3.73 2.66 2.52 2.24 2.25
Lu 0.395 0.51 0.5 0.39 0.54 0.384 0.272 0.684 0.493 0.268 0.481 0.544 0.507 0.588 0.421 0.359 0.343 0.312
(continued on next page)
247
248
Table 1 (continued)
Rock MI-2 BW11 BW62 BW63 BGZ BW33 BD3 BD9 BD16 BD18 BD24 BD35 MD3 MD9 BW34 BW61 Dioritez BWEN
type
MG FGG FGG FGG FGG FGG dike dike dike dike dike dike dike dike diorite diorite diorite enclave
SiO2 74.01 72.12 71.36 72.98 73.83 70.93 72.67 72.42 71.50 73.21 69.17 73.12 53.44 58.65 54.99 56.04 53.56 58.15
Al2O3 13.79 14.38 14.83 14.28 14.23 15.00 14.51 14.39 14.36 14.25 15.45 14.20 16.93 16.11 16.40 16.06 17.02 19.54
Fe2O3 1.98 2.39 2.40 1.90 1.56 2.47 2.01 2.08 2.49 1.91 2.95 1.93 8.20 7.13 8.48 7.86 7.80 5.23
MnO 0.04 0.04 0.05 0.04 0.04 0.05 0.04 0.04 0.04 0.04 0.05 0.04 0.12 0.11 0.13 0.13 0.13 0.11
MgO 0.56 0.80 0.69 0.51 0.34 0.63 0.63 0.53 0.86 0.52 0.93 0.48 5.29 4.62 5.54 5.29 6.77 1.66
CaO 1.59 2.10 2.09 1.64 1.48 2.00 1.78 1.68 2.12 1.56 2.80 1.59 7.33 6.61 7.92 7.57 8.76 3.29
Na2O 3.31 3.77 3.65 3.48 3.63 3.59 3.49 3.64 3.62 3.48 3.87 3.57 4.06 3.45 3.57 4.35 3.62 5.87
K2O 4.37 3.97 4.47 4.87 4.62 4.82 4.52 4.79 4.53 4.68 4.18 4.72 2.18 2.03 1.53 1.39 1.22 4.71
TiO2 0.27 0.34 0.35 0.23 0.23 0.38 0.27 0.33 0.36 0.27 0.44 0.27 1.70 1.11 1.22 1.13 0.95 0.98
P2O5 0.09 0.10 0.11 0.08 0.05 0.11 0.09 0.08 0.10 0.08 0.14 0.08 0.74 0.18 0.22 0.18 0.17 0.45
Rb 141 110 148 171 169 125 155 155 125 144 105 147 38 47 42 55 46 84
Sr 215 268 332 240 243 315 287 209 271 225 384 246 1167 351 368 319 386 507
Ba 822 1063 1183 1053 1009 1291 980 956 1099 796 1407 848 1409 568 494 279 337 1805
Cs 0.8 1.1 1.3 1.3 0.8 0.9 1.2 1.4 1.0 0.7 0.6 1.1 0.3 0.4 0.6 0.4 0.4 0.3
Ta 1.21 1.18 1.09 1.16 0.84 1.33 1.05 1.56 0.93 1.17 0.83 1.18 1.32 0.85 0.64 0.65 0.50 0.52
Nb 16.2 13.0 13.7 14.2 15.3 16.1 12.3 17.6 12.0 13.5 11.1 13.3 24.1 11.8 8.8 8.8 8.2 21.8
Tl 0.74 0.58 0.74 0.98 0.63 0.59 1.04 0.93 0.63 1.04 0.47 0.75 0.23 0.28 0.20 0.32 0.30 0.47
Pb 20 20 4 22 30 23 26 19 26 27 23 26 10 8 b3 7 9 19
Hf 4.5 4.4 4.7 4.9 4.4 5.0 4.2 5.4 4.4 4.6 4.5 4.3 8.0 4.1 4.8 4.3 3.4 19.2
Zr 163 167 172 180 150 186 140 192 175 159 163 139 326 149 193 160 137 907
Y 23 20 20 21 19 24 15 21 16 16 17 16 24 23 31 27 21 31
Sc 4.8 6.5 5.1 4.2 3.2 4.1 3.9 4.2 5.7 3.8 6.1 4.1 17.5 21.1 31.7 27.9 27.1 6.6
Cr 5.9 12.1 6.7 7.6 b0.5 2.6 b 0.5 2.0 8.1 b0.6 5.3 8.2 72.6 85.2 139 149 165 b 0.8
Ni 5 7 65 6 b1 2 3 8 8 3 5 3 67 57 62 3 4 1
V 20 29 29 17 15 21 20 21 36 21 44 19 172 134 157 145 137 74
Cu 4 7 36 7 4 5 20 12 8 6 3 6 69 27 45 35 12 7
Th 14.9 12.8 11.7 16.7 14.8 11.7 13.8 19.1 12.4 15.8 9.86 14.9 8.51 6.83 5.63 4.90 4.04 6.77
U 2.03 1.66 1.57 1.68 1.14 0.79 1.91 2.60 1.67 1.88 1.40 2.14 1.57 1.03 0.85 0.82 0.67 0.53
Ga 18 18 20 19 27 17 18 18 25 17 18 18 23 18 18 19 27 35
La 54.0 45.3 50.2 58.6 54.1 58.3 42.2 60.9 44.3 49.3 46.8 42.5 99.3 33.2 33.9 28.7 31.6 162
Ce 99.9 85.9 94.9 108 101 108 77.3 111 80.6 89.6 86.5 79.3 190 65.2 67.1 57.4 61.0 287
Pr 10.3 8.76 9.79 10.7 9.75 10.5 7.54 10.5 7.77 8.56 8.37 7.70 19.6 6.74 7.44 6.38 6.32 27.6
Nd 34.1 28.8 32.2 35.1 34.5 38.7 27.2 36.9 28.1 30.4 31.0 27.5 74.7 26.2 27.6 24.6 25.3 95.2
Sm 5.78 4.97 5.51 5.82 6.07 6.61 4.68 6.06 4.66 5.04 5.26 4.65 11.6 5.12 5.76 5.01 5.09 13.5
Eu 0.896 0.946 1.02 0.916 1.04 1.28 0.871 1.07 0.931 0.851 1.17 0.841 3.04 1.40 1.56 1.47 1.53 2.88
Gd 4.37 3.88 4.20 4.19 4.35 5.11 3.49 4.50 3.40 3.63 3.92 3.50 8.03 4.61 5.50 4.93 4.69 9.61
Tb 0.71 0.61 0.65 0.66 0.75 0.87 0.57 0.75 0.56 0.58 0.65 0.59 1.10 0.82 0.93 0.86 0.85 1.40
Dy 3.78 3.27 3.47 3.35 3.86 4.60 2.89 3.85 2.85 2.95 3.32 3.05 5.34 4.48 5.28 4.94 4.53 6.82
Ho 0.74 0.62 0.65 0.62 0.78 0.90 0.54 0.72 0.53 0.54 0.65 0.58 0.91 0.89 1.07 0.97 0.89 1.27
Er 2.25 1.87 1.92 1.92 2.46 2.80 1.72 2.36 1.74 1.75 1.99 1.93 2.79 2.88 3.22 2.93 2.85 3.77
Tm 0.338 0.282 0.290 0.286 0.372 0.400 0.255 0.350 0.263 0.280 0.291 0.286 0.373 0.427 0.478 0.433 0.400 0.520
Yb 2.20 1.83 1.81 1.82 2.29 2.51 1.58 2.30 1.65 1.77 1.80 1.85 2.40 2.63 2.99 2.77 2.46 3.20
Lu 0.314 0.268 0.264 0.251 0.337 0.341 0.233 0.328 0.230 0.253 0.251 0.275 0.332 0.370 0.413 0.395 0.334 0.466
Major element oxides concentrations displayed as wt.%, trace elements as ppm.
SMLG-SM leucogranite, SMG-SM granite. SMQM-SM quartz monzonite.
SM LG—leucogranite, MG—Mirage granite, FGG—fine-grained granite.
though smaller than some of the euhedral feldspars monzonite at the extremes. Other felsic units display far
(∼ 3 mm vs. up to 1 cm), are distinctive and prominent. more limited chemical variation (FGG: 71–74 wt.% SiO2;
Felsic dikes contain up to 5% biotite phenocrysts within Mirage granite: 73–74 wt.% SiO2; felsic dikes: one with
a gray groundmass that comprises 60–80% of the rock. 69 wt.%, all 6 other analyses 72–73 wt.% SiO2).
Typically, the dikes are more phenocryst-rich in their Chondrite-normalized rare-earth element (REE) pat-
interior than at the margins. The mafic dikes are rich in terns for all granitoids are, for the most part, broadly
plagioclase and hornblende (some with clinopyroxene uniform (Fig. 6), with enrichment in light REE (40–
cores) and contain variable amounts of biotite; most are 600 × chondrite), negative Eu anomalies, and flat middle
fine-grained phaneritic, and phenocrysts are sparse to to heavy REE. In detail, however, there are striking
absent. In most places, the contacts between the dikes differences for different rock types. Low-silica grani-
and the host are sharp and the dike margins are chilled. toids (b70% SiO2) have the highest light REE and small
At one location, however, the contact between a felsic Eu anomalies, whereas the high-silica granites (N75%
dike and the FGG locally interfingers, suggesting that SiO2) have the lowest light REE, very large negative Eu
the dike sufficiently remobilized the FGG to allow for anomalies, and depressed middle REE.
magma interaction to occur. These dikes represent the Granitoid samples with b∼ 70–72 wt.% SiO2 show
last pulses of magma emplaced into the SMB. textural evidence for accumulation of feldspars, biotite,
and accessory minerals (see above), and they are
4. Methods enriched in elements that would be concentrated by
accumulation (Figs. 5 and 6). They have high Ca, Al,
Fifteen fresh samples were selected for U–Pb Ba, Sr, and Eu (feldspar accumulation), Fe and Mg
geochronology (see Table 2). Zircons were separated (biotite and oxide mineral accumulation), and Ti, P, Zr,
using standard procedures at Vanderbilt University, and light REE (accessory sphene, apatite, zircon,
mounted in epoxy, polished, and imaged by cathodolu- allanite, chevkinite accumulation). In contrast, the
minescence (CL) on the JEOL JSM 5600 scanning most silicic rocks are low in all of these elements and
electron microscope at the USGS/Stanford Ion Micro- extremely depleted in Sr and Ba (to b 10 and b20 ppm,
probe Laboratory. Spots on the zircons ∼30–40 μm in respectively) and Eu. Their relative depletion in middle
diameter were analyzed using the USGS/Stanford REE is attributable to extraction of sphene. Very low Zr/
Sensitive High Resolution Ion Microprobe, Reverse Hf (17–30) in these highly silicic rocks is consistent
Geometry (SHRIMP-RG). Zircon standards R33 with fractionation of zircon (Miller et al., 2005; Lowery
(419 Ma) and CZ3 (550 ppm U, 29.5 ppm Th) were Claiborne et al., 2006). Samples with less than about
used as U–Pb isotopic and U and Th concentration 74 wt.% SiO2 have textures suggesting that quartz was a
standards, respectively. late-crystallizing phase, whereas those with N∼ 74 wt.%
Fifty-four samples were selected for geochemical appear to have had early quartz. Two samples of quartz
analysis. Whole rock powders were prepared from fresh monzonite fall off the trends for K2O and to a lesser
samples using an alumina ceramic shatter box at extent other elements. One of these samples is K2O
Vanderbilt and analyzed for major and trace elements enriched and also has high Ba, whereas the other has
by Activation Laboratories Ltd. (Ontario, Canada), low K2O and Ba, suggesting that they reflect local
using inductively coupled plasma mass spectrometry mechanical concentration or depletion of alkali feldspar.
and instrumental neuron activation analysis. The almost indistinguishable elemental chemistry of
the FGG, Mirage granite, and late felsic Newberry dikes
5. Geochemistry suggests that felsic magma input during the latter stages
of batholith construction was highly uniform. Although
The granitoid rocks of the SMB (excluding the mafic none of the Spirit Mountain granite samples appear to
rocks–dioritic sheets, late mafic dikes, and enclaves) reliably reflect input magma composition (instead, their
exhibit a broad range of chemical compositions (Table 1). textures and compositions indicate crystal accumulation
Silica contents range continuously from 63–79%, and and melt fractionation), the composition of the suite as a
major element Harker diagrams show predictable trends, whole is consistent with derivation from magma very
with those elements compatible with the early mineral similar to the FGG–Mirage–felsic dike compositions.
assemblage decreasing with increasing SiO2 and those The mafic rocks (diorites, late dikes, a single enclave
(few) that are not compatible increasing (Fig. 5). The Spirit analysis) range in silica content from 53–59 wt.% and
Mountain granite encompasses the entire spectrum of have compositional trends that appear to be unrelated to
granitoid compositions, with high-silica granite and quartz the granitoids (Fig. 5). They are relatively enriched in
Fig. 5. Harker diagrams for selected major element oxides and trace elements of SMB rocks.
incompatible elements for mafic rocks, as is typical of the expected for accumulation of early zircon and falling
Colorado River Extensional Corridor (Metcalf, 2004). zircon solubility in lower-T melts. Zircon saturation
Zirconium concentrations range from 50–600 ppm, temperatures (TZr) for granitoids range from 700–880 °C
except for the dioritic enclave, with 900 ppm. They (Watson and Harrison, 1983). The lower values
correlate negatively with SiO2 in the granitoids, as is (b770 °C) are for the high-silica granites and probably
Fig. 6. Chondrite-normalized rare-earth element patterns for the SMB samples, separated as indicated. The FGG, felsic dikes, and Mirage granite field
is shaded for comparison to the Spirit Mountain granite and mafic samples.
reflect melt segregation temperatures; the moderate-SiO2 statistically coherent populations representative of all or
granitoids (including FGG, Newberry dikes, and Mirage most of the data (high MSWD [mean square weighted
granite, as well as some of the Spirit Mountain granites) deviation], N∼ 1.5–2 for the numbers of analyses of
have TZr of 770–810 °C, which may approximate the T samples, suggests that a population is not coherent
of input magmas. Highest TZr's are for rocks that are [Wendt and Carl, 1991]); (2) display the data as
interpreted to have accumulated zircon and therefore are probability density graphs, which reveal dominant and
unrealistic (cf. Miller et al., 2003). secondary age peaks and shoulders that may suggest an
additional population; and (3) discriminate statistically
6. Geochronology meaningful populations from the age spectra using the
UNMIX add-in (for details, see Sambridge and
In interpreting our data (available from C.F. Miller Compston, 1994). UNMIX requires the user to supply
upon request), we assume that Miocene zircons have the number of populations he/she wishes to identify. To
suffered minimal radiation damage and therefore are determine the number of populations to identify for each
essentially concordant. We therefore rely on 207Pb- sample, we considered both the number of apparent
corrected 206Pb/238 U analyses, which is much more probability density curve peaks and the relative misfits
precise than 204Pb correction. The 206 Pb/238U ages were using different numbers of populations (a large decrease
inspected directly for each sample, and we used routines in relative misfit calculated with an added population
in Isoplot 3.0 (Ludwig, 2003) that: (1) seek single, suggests that the population is “real;” Sambridge and
Fig. 7. Cathodoluminescence images of SMB zircons from various samples. Ages are in Ma, and are 206Pb/238U. Errors are 2 sigma. Circles show
approximate spot size and location.
Compston, 1994). This procedure obviously involves Fig. 7 shows typical CL images of SMB zircons with
some subjectivity, but consistency among population analyzed spots. Zoning in zircons is almost invariably
statistics, peaks and shoulders identified on probability euhedral and concentric, consistent with magmatic
density plots, and UNMIX results permits confidence growth; generally subtle truncations suggesting resorption
in our interpretations. and subsequent growth are fairly common. Fig. 8 shows
Fig. 8. Probability density plots of each SMB zircon sample. Vertical lines behind the histogram indicate age populations, each of which is labeled
with a date that was established by UNMIX (after Sambridge and Compston, 1994) in Isoplot. SMG-SM granite. SML-SM leucogranite. FGG-fine-
grained granite. SMQM-SM quartz monzonite.
Fig. 8 (continued).
Table 2
Synthesis of the Spirit Mountain Batholith zircon samples
Sample Location Rock type No. of Inferred Basis Other Comments
Lat/Long analyses “Age” ⁎ ±2σ populations ±2σ
SML59z 114.7269, Granite 20 17.4 ± Dominant peak of the data None
35.2559 (Roof unit) 0.2 Ma set with a ∼Gaussian
distribution.
LGz 114.7461, Porphyritic 23 16.7 ± Dominant peak identified 16.4 ± 0.6 Ma Apparent antecrysts 17–
35.2594 SM 0.2 Ma by UNMIX 17.4 ± 0.3 Ma 18 Ma abundant. Recharge
leucogranite 17.8 ± 0.1 Ma possible at ∼16.4 Ma.
SML49z 114.7869, Med grained 21 16.2 ± Dominant peak of the 15.3 ± 0.6 Ma Older population appears to
35.1364 leucogranite 0.2 Ma data set. 16.8 ± 0.4 Ma be comprised of antecrysts.
17.4 ± 0.4 Ma
SML120z 114.7092, SM 22 17.4 ± Dominant peak of the 15.5 ± 0.4 Ma Apparent antecrysts and
35.1941 leucogranite 0.2 Ma data set. 16.5 ± 0.5 Ma newer zircon populations
18.3 ± 1.0 Ma present
SML129z 114.7438, Leucogranite– 14 17.7 ± Dominant peak of the 16.7 ± 0.3 Ma Age uncertain due to few
35.2092 granite 0.2 Ma (?) data set. 17.2 ± 0.2 Ma analyses and multiple
transition 17.9 ± 0.2 Ma plausible peaks.
SML54z 114.7428, SM granite— 22 16.6 ± Dominant peak of the 17.1 ± 0.5 Ma
35.1817 transitional 0.4 Ma data set. 17.5 ± 0.2 Ma
leucogranite/
granite
101z 114.6280, SM granite 23 16.3 ± Dominant peak identified 17.0 ± 0.3 Ma Apparent antecrysts 17–18
35.2269 0.2 Ma by UNMIX 17.7 ± 0.3 Ma Ma abundant
BC101z 114.7139, SM granite 17 16.5 ± Dominant peak identified 15.5 ± 0.2 Ma Younger peak may reflect late
35.1996 0.3 Ma by UNMIX heating by recharge, with
accompanying zircon
dissolution and new growth.
SWz 114.6655, SM quartz 21 15.8 ± Two populations identified 16.6 ± 1.2 Ma Older ages are generally in
35.2400 monzonite 0.2 Ma by UNMIX, Younger is 16.8 ± 0.3 Ma the cores/interior of zircons,
dominant peak, older peaks younger ages in rims and
comprise the prominent interiors. May be part of the
shoulder. younger sequence within the
SM granite and carry
abundant antecrysts, OR it
may be an older part of the
SMg and show evidence for
zircon dissolution and new
growth as consequence of
recharge.
DSCGz 114.6928, SM granite— 20 15.7 ± Dominant peak of the data set. 16.2 ± 0.3 Ma ∼15.7 Ma analyses are
35.2028 within the 0.1 Ma Age consistent with field 16.6 ± 0.2 Ma primarily on rims, though
younger relations showing this sample 17.8 ± 0.1 Ma present in a few interiors.
intrusive as part of later sequence Apparent antecrysts mostly
sequence within SM granite comprising cores/interiors.
MPL53z 114.6926, Mirage granite 15 16.0 ± Dominant peak of the 17.3 ± 0.3 Ma Oldest Miocene grains in the
35.1225 0.2 Ma data set. 18.7 ± 0.8 Ma SMB present in this sample:
18.5 Ma, and 19.1 Ma.
Dioritez 114.6676, Fine-med 16 16.0 ± Mean of analyses—excludes Several apparent antecrysts
35.1788 grained diorite 0.2 Ma 4 antecrysts(?) and a single 16–17 Ma. Most grains lack
anomalously young age. euhedral, oscillatory zoning
∼Gaussian distribution. that characterize other
samples (typical of zircons
crystallized from mafic
magmas)
BGz 114.6788, Fine-grained 19 15.9 ± Largest peak, identified 15.4 ± 0.3 Ma There appear to be multiple
35.1705 biotite granite 0.4 Ma by UNMIX—many older 16.9 ± 0.3 Ma ages of antecrysts, 16–18 Ma,
grains interpreted as 17.6 ± 0.9 Ma and possible reheating due to
antecrysts Newberry dike intrusion
(continued on next page)
Table 2 (continued)
Sample Location Rock type No. of Inferred Basis Other Comments
Lat/Long analyses “Age” ⁎ ±2σ populations ±2σ
BD18z 114.6878, Porphyritic 13 15.4 ± Most plausible of 2 subequal 16.2 ± 0.3 Ma Abundant “antecrysts;” 4
35.2025 felsic dike 0.3 Ma populations from UNMIX— 17.0 ± 1.0 Ma Mesozoic cores (170, 102,
field relations demonstrate 99, 72 Ma)
youth, many zircons derived
from SM granites relations.
BD35z 114.7075, Porphyritic 16 15.2 ± Relatively small population 15.9 ± 0.2 Ma Abundant SM-age
35.1798 felsic dike 0.2 Ma identified by UNMIX—but 17.4 ± 0.9 Ma “antecrysts”
most plausible age, given field
relations.
probability distribution plots of each SMB zircon sample, measurable range. Furthermore, individual samples may
with UNMIX-identified populations indicated where contain zircons that span much of this range. Our U–Pb
applicable. Table 2 presents our interpretation of these data for the SMB further document the potential for
data for each of the 15 samples. In Table 2, the “inferred identifying a crystallization age spectrum for a plutonic
age” for specific samples indicates the time at which we system and mixing of zircons of different ages within
interpret the bulk of the magma that formed the host rock single samples.
to have arrived and solidified at the sample location. Wes Hildreth (presentation at Penrose Conference,
Forty-one core-rim pairs were analyzed for single 2001) coined the term “antecryst” for grains that are
zircons. Cores most commonly yielded ages between older than the solidification age of their host rock, but
zero and 1 million years older than rims (weighted mean apparently represent earlier crystal growth within the
difference 0.5 ± 0.2 m.y., MSWD 2.0). The rather high magmatic system (see also Bacon and Lowenstern,
MSWD is consistent with the expectation that age 2005; Charlier et al., 2005; “crystal memory” in Vasquez
differences would be non-uniform. Core-rim age differ- and Reid, 2002). These grains are presumably entrained
ences ranged from − 1.6 to 2.4 m.y. Where cores yielded by a younger magmatic pulse and then accumulate with
ages younger than rims, the ages were always within 2σ. newly formed/forming grains. This term and the concept
it represents are critical for interpretation of SMB zircon
7. Discussion data.
Excluding a single Proterozoic and six Mesozoic
7.1. Interpretation of age spectra: antecrysts and mixed cores (all from two samples), essentially all of the 303
populations individual analyses fall between 15 and 18 Ma, with
obvious dominance between 15.5 and 17.5 Ma. We
Conventionally, the expectation has been that dating argue that this dominant range of ages is real.
magmatic zircons yields the age of crystallization of an Specifically, we conclude the following (see Table 2):
intrusion (or at least the portion of the intrusion (1) The roof unit sample SML59z yielded an “old”
represented by the analyzed sample). Determination of age, 17.4 ± 0.2 Ma, and a relatively simple age
this age may be hampered by Pb loss or by inheritance of spectrum. This sample appears to represent a
older, unrelated grains, but it has been assumed that if remnant of an early, perhaps initial, intrusion of the
these complexities can be eliminated, the “true” age of a SMB preserved locally at the roof. Several
sample (and the intrusion of which it is a part) can be younger ages in this sample may mark partial
ascertained. With the recent advent of much higher- rejuvenation and zircon growth between 16 and
precision “conventional” (thermal ionization mass 17 Ma.
spectrometry) analysis (e.g. Coleman et al., 2004), (2) All samples except roof unit SML59z appear
high-resolution ion probe dating (e.g. Cates et al., to contain multiple zircon age populations. In most
2003; Miller et al., 2004; Miller and Wooden, 2004), samples, the prominent age peak is younger than the
and U-series disequilibria dating of young volcanic minor peaks, and so the minor populations are
zircons (e.g. Vasquez and Reid, 2002; Charlier et al., probably composed of antecrysts. In some cases,
2005), it has become apparent that zircon ages in plutons though, the prominent peak is older than other peaks.
and the products of volcanic eruptions may span a Accordingly, in these samples, the principal age of
solidification may be represented by an older The nine dated samples of Spirit Mountain granite all
population, with later population(s) representing have at least two age populations of zircons. In most
renewed, essentially in situ growth as a consequence samples, the populations that comprise the minor peaks
of heating that accompanied magma recharge (e.g. consist of older grains (or areas within grains), which we
BC101z). Alternatively, final emplacement of these interpret to be antecrysts. The common presence of
samples may have involved transport of a relatively antecrysts indicates that an appreciable fraction of
zircon-rich slurry to a final resting place where lesser zircons (and presumably other phases from preceding
amounts of new zircon grew from fractionated, Zr- pulses as well) was recycled into or affected by
poor melt (e.g., SML120z). subsequent injections. In extreme cases, smaller,
(3) All samples of Spirit Mountain granite have at young peaks are likely to reflect the time of final
least one major population between 16 and 17 Ma. emplacement of reactivated mush; low-T, zircon-
In detail, it appears that there is a span of ages in saturated melt has considerably lower Zr concentration
this range that represents crystallization of a than the likely zircon-bearing crystalline assemblage.
majority of the zircon in the SMB and, probably, This appears to be the case for one sample (SML120z)
solidification of most of its mass. of the younger internal intrusion within the Spirit
(4) All samples that represent units that are Mountain granite. Elemental zoning within the zircons,
demonstrably young (based on field relations) which documents major fluctuations in temperature and
appear to be 15.3–16.0 Ma (FGG, diorite, Mirage host melt composition, strongly supports this complex
granite, Newberry dikes; samples BGz, Dioritez, history for individual zircons and zircon populations
MPL53z, BD18z, and BD35z, respectively). (fluctuating melt composition and temperature [by Ti-
(5) One or more analyses from almost all samples in-zircon thermometry]; cf. Watson and Harrison, 2005;
yield ages of 17–18 Ma, suggesting wide redistribu- Lowery Claiborne et al., 2006). This recycling suggests
tion of zircons from the earliest stages of batholithic an explanation for the rarity of sharp contacts in a unit
growth (possibly represented by SML59z). whose crystallization age spans two million years.
Intrusion of fresh magma could remobilize an area
7.2. Gradational granite: simple appearance, complex within a stagnant, crystal-rich mush pile. The resulting
history physical interaction might obscure evidence of the
injection and erase earlier contacts as well. Segregation
The SMB is composed of multiple, discrete intrusions of fractionated, interstitial melt may result from mush
and was assembled over at least 2 million years. Field compaction, or it could be a consequence of destabili-
evidence for pulsatory construction is clear in some areas, zation of stagnant mush during recharge. The buoyant
as younger intrusions (Mirage granite, FGG, diorites, high-silica melt would then migrate upward by porous
Newberry dikes) can be distinguished from older units. flow or via dikes through the crystal-rich mush.
Geochronology supports the interpretation of this relatively We envision a large pile of crystal-rich mush that
late-stage growth of the batholith. The Spirit Mountain compacted under its own weight. This compaction
granite, which comprises a majority of the SMB, appears to reduced the pore space and subsequently caused a portion
be a massive unit with a simple, monotonic history of the residual interstitial melt to evacuate these shrinking
(Hopson et al., 1994). Based on its upward gradation from reservoirs (Bachmann and Bergantz, 2004). The resulting
cumulate-textured quartz monzonite through coarse granite cumulate was enriched in the earlier crystallizing phases
into highly evolved, fine-grained leucogranite, it could be (feldspars, biotite, accessories), and poor in quartz, a late-
viewed as a type example of a single-stage compacted crystallizing phase. This process would plausibly have the
cumulate with a segregated cap of high-silica rhyolite most impact on the bottom of the mush pile, where the
equivalent (cf. Bachl et al., 2001; Bachmann and Bergantz, pressure is the greatest. This could explain the large zone
2004). However, zircon geochronology suggests this unit of quartz monzonite at the bottom of the Spirit Mountain
developed over a span of a million years, longer than the granite, which is: (1) enriched in feldspars, biotite, and
plausible lifetime of a single magma batch, even if it were accessories; (2) depleted in quartz; and (3) strongly
the size of a fully molten SMB (several thousand km3) foliated perpendicular to the up direction.
(Glazner et al., 2004). Furthermore, the distribution of ages The other chemical extreme of the Spirit Mountain
is far from defining the pattern expected for a monotonic granite, the high-silica leucogranite zone, also docu-
history. Under close scrutiny, field relations also contradict ments protracted assembly. Field relations indicate that
this simple scenario and are instead consistent with a multi- many pulses of high-silica granite were emplaced at the
stage intrusive history. roof as sheets (dikes and sill-like bodies). Zircon samples
from this unit (LGz, SML54z, SML49z) yield ages that We extend the reasoning above to propose that the
bracket most of the lifespan of the Spirit Mountain Spirit Mountain granite, the dominant unit of the SMB,
granite (16.1–17.2 Ma). This suggests that segregation is the product of a protracted history of repeated
events occurred throughout the assembly of the Spirit intrusions. Although little evidence for the initial form
Mountain granite. Ascent pathways for these fractionat- of these intrusive pulses was preserved, we consider it
ed melts (i.e. feeder dikes and/or intergranular channels) very likely that they had sheet-like geometry similar to
are only rarely preserved within the underlying granite that of well-preserved FGG. These sheets, in our view,
and quartz monzonite, probably because they were subsequently merged as a consequence of mush
disrupted by subsequent destabilization of the host mush destabilization, producing a gradational, relatively
or because they collapsed after drainage of melt. homogeneous body with, for the most part, only subtle
internal contacts.
7.3. Architecture of construction Episodic melt segregation from the Spirit Mountain
granite mush yielded a thick cap of fractionated
The composite, multi-stage constructional architec- rock. The distinct, younger intrusive sequence within
ture of the SMB hinted at by the Spirit Mountain granite the Spirit Mountain granite is probably a manifesta-
is made clearer by the younger units. The diorites and tion of magma injection into a mechanically sturdier
the FGG are exposed as hundreds of initially subhor- medium, the existing portion of the Spirit Mountain
izontal sheets, essentially exhibiting sill-on-sill geome- granite being poor enough in melt (and therefore
try (Fig. 9). This arrangement is similar to that described rigid enough) to permit preservation of intrusive
by Westerman and others (2004), who used the term contacts. Blurring of Spirit Mountain granite contacts
“Christmas-tree laccolith” to describe a sheeted body on through mush destabilization was probably a conse-
Elba Island, Italy. Hunt et al. (1953) described a quence of larger magma flux, and therefore thermal
complex assemblage of subhorizontal (laccolithic) mass, than that which formed the distinct sheet-like
sheets and subordinate dikes and protrusions in the intrusions. The larger flux would preserve melt longer
Henry Mountains, Utah, as a “cactolith.” Though and thus permit more time to rework initial intrusion
partially in facetious reference to the increasing amount boundaries.
of geological jargon, Hunt was graphically describing a
branching, cholla cactus-shaped intrusion. His descrip- 7.4. Assembly sequence
tion and his term evoke a pattern similar to what we
observe in the SMB. This geometry applies not only to Field relations and zircon geochronology discussed
the FGG and diorite sheets, but also to networks of above suggest the following history for the SMB (Fig. 10):
leucogranite dikes, sills, and laterally extensive pods in (1) ∼ 17.4 Ma: The magma associated with the
the Spirit Mountain granite both within the main mass roof unit was emplaced, followed by a brief pause
and in the upper leucogranite zone. These networks in magmatism. Zircons associated with this initial
presumably reflect accumulation of late, fractionated phase of emplacement were later redistributed
melts. throughout most of the batholith.
Fig. 9. Successively stacked fine-grained granite (FGG) sills intruded into diorite. This sill-on-sill geometry is observed throughout the FGG and
diorite units. For scale, FGG sills are ∼ 5 m thick. Initial up direction is indicated.
Fig. 10. Assembly cartoon for the Spirit Mountain Batholith. See text for explanation. Drawings by Rob Smith.
(2) ∼17–16 Ma: Sporadic injection and crystalli- magma were injected intermittently, becoming
zation of granitic magma formed the bulk of the small pods and/or mafic enclaves.
Spirit Mountain granite. Magma was probably Our U–Pb data cannot confidently establish an order
emplaced as horizontal sheets within a semi-rigid of steps 3–7, as samples have multiple closely
crystal mush. Injections partially remobilized the spaced apparent ages and our interpretations of
surrounding area, entraining zircon antecrysts, and emplacement ages fall within error of each other.
creating (small?) local magma chambers. Within Field relations and zircon data are consistent with the
these local chambers, fractional crystallization following sequence and approximate timing.
occurred, producing high-silica melt which migrat- (3) ∼ 16.0 Ma: The Mirage granite intruded,
ed upward toward the roof. Minor amounts of mafic probably in sheet by sheet fashion. Fractional
crystallization of this magma produced a leucogranite spans an interval of more than a million years,
cratic roof zone, similar to but much smaller than zircon age populations are mixed within samples, and field
that of the Spirit Mountain granite. relations reveal that both the fractionated leucogranite zone
(4) ∼ 15.9 Ma: Injections of basaltic–dioritic and the underlying portion record multiple injections.
magma formed horizontal sheets and pods that Younger, fine-grained granite and diorite are preserved as
cut the Spirit Mountain granite and were in part complex sets of sheets, resulting in “cactolithic” or
coeval with the Mirage granite and FGG. “Christmas tree” geometries. We envision accumula-
(5) ∼15.9 Ma: FGG was emplaced as a series of tion of horizontal sheets in this manner as the dominant
successively stacked, horizontal sheets. Cooling of process in the assembly of the SMB. Presumably de-
these injections was probably relatively rapid, as pending on the consistency and temperature of the host,
suggested by the fine-grained texture. Minor these sheets either locally remobilized a crystal mush,
mafic–dioritic magma emplacement continued or froze relatively quickly to preserve a sharp geometry.
through this time, as well. Field relations are Where remobilization occurred, original geometry was
consistent with FGG being at least in part coeval obscured, resulting in a relatively homogeneous mass.
with the Mirage granite. According to this interpretation, the shape of the
(6) ∼15.8–15.7 Ma: Magma continued to be batholith, with an aspect ratio of ∼ 3, does not reflect
injected into the Spirit Mountain granite which was its building blocks, which had far higher aspect ratios
relatively rigid (low melt fraction) by this time to (cf. McCaffrey and Petford 1997; Glazner et al., 2004).
produce the distinct, younger sequence. Fractional At any given time, the SMB was a patchwork of melt-
crystallization again produced high-silica melt which rich, melt-poor, and probably entirely solid zones. The
accumulated at the roof zone of this intrusion. Some location, size, shape, and melt fraction of the mush
of this fractionated melt debouched from the intra- zones must have varied dramatically over the two
batholith cupola, upward into the overlying granite. million year history of the batholithic system.
A preserved network of leucogranite dikes, sills, and Zircon samples from the SMB have age spectra that
pods document the ascent pathways and ponding span the assembly time of the batholith, with dominant
zones of this material. This new magma entrained peaks in individual samples representing the time at
abundant zircons, and presumably other crystals from which the host rock was largely consolidated. The older
the extant granite (mush?) and apparently provided populations are most likely due to the recycling of
enough heat to dissolve zircons and subsequently zircon antecrysts into new injections of magma. In this
induce new in situ rim growth. way, each new injection might acquire memory of
(7) ∼15.3 Ma: Termination of SMB magmatism previous crystallization events. The patchwork nature
was marked by injection of a series of vertically thus ranges in scale from map view to hand sample and
intruded, felsic to mafic dikes. Emplacement of the single crystal, with evidence for multiple injections
Newberry dike swarm followed final solidification of ranging from contacts to U–Pb ratios in zones within
the remainder of the batholith and may have been individual zircons.
facilitated by the onset of rapid E–W extension
(George et al., 2005), which is suggested to have Acknowledgements
commenced ∼16.0 Ma (Faulds et al., 2001).
Ben George generously contributed time and energy
8. Construction of a “Patchwork Batholith” in the field and lab and shared the results of his se-
nior research. Thanks to Steve Ludington, Bob Wiebe,
Piecemeal construction of the SMB is documented by Heather Bleick, and Jim Faulds for their insights
zircon geochronology, field relations, and elemental and help in the field. Cliff Hopson, Tapani Ramo,
geochemistry. These data demonstrate multiple injections, and Ilmari Haapala shared their accumulated knowl-
repeated melt segregation events, and remobilization and edge of the Spirit Mountain system. Thanks to George
modification of early phases over a protracted period. The Bergantz, Drew Coleman, and Bob Wiebe, who pro-
general pattern of the largest unit, the Spirit Mountain vided extremely helpful reviews to this manuscript.
granite, is deceptively simple: a large roof zone of Special thanks go to Rob Smith for his artistic ef-
fractionated, high-silica leucogranite that grades through forts that contributed greatly to our efforts to por-
coarse-grained granite into a foliated quartz monzonite. tray the assembly of the batholith. Funding for this
Closer examination of this unit, however, reveals its more project came from NSF grants EAR-0409876 and
complex history. Zircon growth in the Spirit Mountain EAR-0107094.
References Extensional Corridor. Journal of Geophysical Research 94,

7885–7898.
Faulds, J.E., Geissman, J.W., Shafiqullah, M., 1992. Implications of
Bachl, C.A., Miller, C.F., Miller, J.S., Faulds, J.E., 2001. Construction paleomagnetic data on Miocene extension near a major accom-
of a pluton: evidence from an exposed cross section of the modation zone in the Basin and Range province, northwester
Searchlight pluton, Eldorado Mountains, Nevada. Geological Arizona and southern Nevada. Tectonics 11 (2), 204–227.
Society of America Bulletin 113 (9), 1213–1228. Faulds, J.E., Feuerbach, D.L., Reagan, M.K., Metcalf, R.V., Gans, P.B.,
Bachmann, O., Bergantz, G.W., 2004. On the origin of crystal-poor Walker, J.D., 1995. The Mount Perkins block, northwestern Arizona:
rhyolites: extracted from batholithic crystal mushes. Journal of an exposed cross section of an evolving, preextensional to syexten-
Petrology 45 (8), 1565–1582. sional magmatic system. Journal of Geophysical Research 100 (B8),
Bachmann, O., Dungan, M.A., Lipman, P.W., 2002. The Fish Canyon 15249–15266.
magma body, San Juan volcanic field, Colorado; rejuvenation and Faulds, J.E., Feuerbach, D.L., Miller, C.F., Smith, E.I., 2001. Cenezoic
eruption of an upper-crustal batholith. Journal of Petrology 43 (8), evolution of the northern Colorado River Extensional Corridor,
1469–1503. southern Nevada and northwest Arizona. Utah Geological
Bacon, C.R., Lowenstern, J.B., 2005. Late Pleistocene granodiorite Association Publication 30-American Association of Petroleum
source for recycled zircon and phenocrysts in rhyodacite lava Geologists Publication GB78, pp. 239–271.
at Crater Lake, Oregon. Earth and Planetary Science Letters 233 Gans, P.B., Bohrson, W.A., 1998. Suppression of volcanism during
(34), 277–293. rapid extension in the Basin and Range Province, United States.
Bacon, C.R., Metz, J., 1984. Magmatic inclusions in rhyolites, Nature 279, 66–68.
contaminated basalts, and compositional zonation beneath the George, B.E., Miller, C.F., Walker, B.A., Wooden, J.L., 2005. Newberry
Coso volcanic field, California. Contributions to Mineralogy and Mountains Dike Swarm, southern Nevada: final, extension-related
Petrology 85 (4), 346–365. pulse of the Spirit Mountain batholith. Eos, Transactions of the
Bennet, V.C., DePaolo, D.J., 1987. Proterozoic crustal history of the American Geophysical Union 86 (18) (Jt. Assem. Suppl., Abstract
western United States as determined by neodymium isotopic V13A-01, JA511).
mapping. Geological Society of America Bulletin 99, 674–685. Glazner, A.F., Bartley, J.M., Coleman, D.S., Gray, W.M., Taylor, R.Z.,
Brown, S.J.A., Fletcher, I.R., 1999. SHRIMP U–Pb dating of the pre- 2004. Are plutons assembled over millions of years by
eruption growth history of zircons from the 340 ka Whakamaru amalgamation from small magma chambers? GSA Today 14 (4/5).
ignimbrite, NZ: evidence for N250 k.y. magma residence times. Grunder, A.L., Klemetti, E.W., 2005. Volcanic Record of Batholith
Geology 27, 1035–1038. Assembly: the Aucanquilcha Volcanic Cluster, Northern Chilean
Cates, N.L., Miller, J.S., Miller, C.F., Wooden, J.L., Ericksen, S., Andes. 2005 Annual Meeting, Abstracts and Programs, vol. 56 (7).
Means, M., 2003. Longevity of plutonic systems: SHRIMP Geological Society of America, p. 131.
evidence from Aztec Wash and Searchlight plutons. 2003 Haapala, Ilmari, Ramo, O. Tapani, Frindt, Stephen, 2005. Comparison
Cordilleran Section Annual Meeting, Abstracts with Programs, of Proterozoic and Phanerozoic rift-related basaltic–granitic
vol. 35 (4). Geological Society of America, Nevada. 25–2. magmatism. Lithos 80 (1–4), 1–32.
Charlier, B.L.A., Wilson, C.J.N., Lowenstern, J.B., Blake, S., Van Halliday, A.N., Mahood, G.A., Holden, P., Metz, J.M., Dempster, T.J.,
Calsteren, P.W., Davidson, J.P., 2005. Magma generation at a large, Davidson, J.P., 1989. Evidence for long residence times of rhyolite
hyperactive silicic volcano (Taupo, New Zealand) revealed by U–Th magma in the Long Valley magmatic system: the isotopic record in
and U–Pb systematics in zircons. Journal of Petrology 46 (1), 3–32. the precaldera lavas of Glass Mountain. Earth and Planetary
Chesner, C.A., Rose, W.I., Deino, A., Drake, R., Westgate, J.A., 1991. Science Letters 94, 274–290.
Eruptive history of Earth's largest Quaternary caldera (Toba, Hawkins, D.P., Wiebe, R.A., 2003. High-precision temporal con-
Indonesia) clarified. Geology 19 (3), 200–203. straints on the construction of a periodically replenished,
Christiansen, R.L., 1984. Yellowstone Magmatic Evolution; its subvolcanic magma chamber. The Silurian Vinalhaven Intrusion,
Bearing on Understanding Large-Volume Explosive Volcanism. Maine. The Origin of Granites and Related Rocks, Hutton
National Acadamy Press, pp. 85–95. Symposium V, Abstracts, vol. 91.
Coleman, D.S., Gray, W., Glazner, A.F., 2004. Rethinking the Hildreth, W., 1981. Gradients in silicic magma chambers: implications
emplacement and evolution of zoned plutons: geochronologic for lithospheric magmatism. Journal of Geophysical Research 86,
evidence for incremental assembly of the Tuolumne Intrusive 10153–10192.
Suite, California. Geology 32 (5), 433–436. Hildreth, W., 2004. Volcanological perspectives on Long Valley,
Cruden, A.R., et al., 2005. Timescales of incremental pluton growth: Mammoth Mountain and Mono Craters: several contiguous but
theory and a field-based test. 2005 Annual Meeting, Abstracts and discreet systems. Journal of Volcanology and Geothermal Research
Programs, vol. 56 (9). Geological Society of America, p. 131. 136, 169–198.
Davies, Gareth R., Halliday, Alex N., Mahood, Gail A., Hall, Chris M., Hopson, C.A., Gans, P.B., Baer, E., Blythe, A., Calvert, A., Pinnow, J.,
1994. Isotopic constraints on the production rates, crystallisation 1994. Spirit Mountain Pluton, Southern Nevada: A Progress
histories and residence times of pre-Caldera silicic magmas, Long Report. 1994 Cordilleran Section, Abstracts with Programs,
Valley, California. Earth and Planetary Science Letters 125 (1–4), vol. 60. Geological Society of America.
17–37. Howard, K.A., John, B.E., Davis, G.A., Anderson, J.L., Gans, P.B., 1994.
Eichelberger, J.C., Chertkoff, D.G., Dreher, S.T., Nye, C.J., 2000. A guide to Miocene extension and magmatism in the lower Colorado
Magmas in collision; rethinking chemical zonation in silicic River Region, Nevada, Arizona, and California. A Guide for Field Trip
magmas. Geology 28 (7), 603–606. 3 Eighth International Conference on Geochronology, Cosmochronol-
Falkner, C.M., Miller, C.F., Wooden, J.L., Heizler, M.T., 1995. ogy, and Isotope Geology, May 31–June 4, 1994.
Petrogenesis and tectonic significance of the calc-alkaline, Howard, K.A., Wooden, J.L., Simpson, R.W., 1996. Extension-related
bimodal Aztec Wash pluton, Elorado Mountains, Colorado River Plutonism along the Colorado River Extensional Corridor. 1996
Annual Meeting, Abstracts and Programs, vol. A-450. Geological Paterson, S.R., Miller, R.B., 1998. Mid-crustal magmatic sheets in the
Society of America. Cascades Mountains, Washington; implications for magma ascent.
Hunt, C.B., Averitt, P., Miller, R.L., 1953. Geology and geography of the Journal of Structural Geology 20 (9–10), 1345–1363.
Henry Mountains region, Utah. USGS Professional Paper 228, 151. Rämö, O.T., Haapala, I.J., Volborth, A., 1999. Isotopic and general
Iyer, H.M., Evans, J.R., Dawson, P.B., Stauber, D.A., Achauer, U., geochemical constraints on the origin of Tertiary granitic
1990. Differences in magma storage in different volcanic Plutonism in the Newberry Mountains, Colorado River Exten-
environments as revealed by seismic tomography; silicic volcanic sional Corridor, Nevada. 1999 Cordilleran Section, Abstracts with
centers and subduction-related volcanoes. In: Ryan, M.P. (Ed.), Programs, vol. 31 (6). Geological Society of American, p. A86.
Magma Transport and Storage. John Wiley & Sons, United Sambridge, M.S., Compston, W., 1994. Mixture modeling of multi-
Kingdom, pp. 293–316. component data sets with application to ion-probe zircon ages.
Koyaguchi, T., Kaneko, K., 2000. Thermal evolution of silicic Earth and Planetary Science Letters 128, 373–390.
magma chambers after basalt replenishments. Transactions of Schmitt, A.K., Lindsay, J.M., de Silva, S., Trumbull, R.B., 2002. U–
the Royal Society of Edinburgh. Earth Sciences 91 (1–2), Pb zircon chronostratigraphy of early-Pliocene ignimbrites from L
46–60. Pacana, north Chile: implications for the formation of stratified
Lee, Y.F.S., Miller, C.F., Unkefer, J., Heizler, M.T., Wooden, J.L., magma chambers. Journal of Volcanology and Geothermal
Miller, J.S., 1995. Petrology, emplacement, and tectonic setting of Research 120, 43–53.
the Nelson pluton, Eldorado Mountains, Nevada. Eos, Transac- Simon, J.I., Reid, M.R., 2005. The pace of rhyolite differentiation
tions of the American Geophysical Union 76 (17), S290. and storage in an ‘archetypical’ silicic magma system, Long
Lees, J.M., 2005. Tomography of crustal magma bodies. 2005 Annual Valley, California. Earth and Planetary Science Letters 235,
Meeting, Abstracts and Programs, vol. 56 (8). Geological Society 123–140.
of America, p. 131. Steinwinder, T.R., Miller, C.F., Faulds, J.E., Koteas, G.C., Ericksen, S.M.,
Lowery Claiborne, L., Miller, C.F., Walker, B.A., Wooden, J.L., 2004. Transition from Plutonism to Voluminous Diking in the
Mazdab, F.K., Bea, F., 2006. Tracking magmatic processes in Eldorado Mountains, Northern Colorado River Extensional Corridor.
felsic magmas through Zr/Hf ratios in rocks and Hf and Ti zoning 2004 Cordilleran Meeting, Abstracts with Programs, vol. 36 (4).
in zircons: an example from the Spirit Mountain batholith, Nevada. Geological Society of America, p. 8.
Mineralogical Magazine 70 (5), 499–525. Vazquez, J.A., Reid, M.R., 2002. Time scales of magma storage and
Ludwig, K.R., 2003. Isoplot 3.00: A Geochronological Toolkit for differentiation of voluminous high-silica rhyolites at Yellowstone
Microsoft Excel. Berkeley Chronological Center, Berkeley, CA. caldera, Wyoming. Contributions to Mineralogy and Petrology
Mahood, G.A., 1990. Second reply to comment of R.S.J. Sparks, H.E. 144, 274–285.
Huppert and C.J.N. Wilson on “Evidence for long residence times Volborth, A., 1973. Geology of the granite complex of the Eldorado,
of rhyolite magma in the Long Valley magmatic system: the Newberry, and northern Dead Mountains, Clark County, Nevada.
isotopic record in the precaldera lavas of Glass Mountain”. Earth Nevada Bureau of Mines and Geology, Bulletin 80, 1–40.
and Planetary Science Letters 99, 395–399. Walker, B.A., Miller, C.F., George, B.E., Luddington, S., Wooden, J.L.,
McCaffrey, K.J.W., Petford, N., 1997. Are granitic intrusions scale Bleick, H.A., Miller, J.S., 2005. The Spirit Mountain batholith:
invariant? Journal of the Geological Society 154 (Part 1), 1–4. documenting magma storage in the upper crust one pulse at a time.
Metcalf, R.V., 2004. Volcanic–plutonic links, plutons as magma Eos, Transactions of the American Geophysical Union 86 (18) (Jt.
chambers and crust–mantle interaction: a lithospheric scale Assem. Suppl., Abstract V21A-03, JA516).
view of magma systems. Transactions of the Royal Society of Watson, E.B., Harrison, T.M., 1983. Zircon saturation revisited-
Edinburgh. Earth Sciences 95 (1–2), 357–374. temperature and composition effects in a variety of crustal magma
Metcalf, R.V., Smith, E.I., Walker, J.D., Reed, R.C., Gonzales, D.A., types. Earth and Planetary Science Letters 64 (2), 295–304.
1995. Isotopic disequilibrium among commingled hybrid magmas: Watson, E.B., Harrison, T.M., 2005. Zircon thermometer reveals
evidence of a two-stage magma mixing–commingling process minimum melting conditions on earliest Earth. Science 308 (5723),
in the Mt. Perkins Pluton, Arizona. Journal of Geology 103, 841–844.
509–527. Wendt, I., Carl, C., 1991. The statistical distribution of the mean squared
Miller, C.F., Miller, J.S., 2002. Contrasting stratified plutons exposed weighted deviation. Chemical Geology. Isotope Geoscience Section
in tilt blocks, Eldorado Mountains, Colorado River Rift, Nevada, 86, 275–285.
USA. Lithos 61, 209–224. Westerman, D.S., Dini, A., Innocenti, F., Rocchi, S., 2004. Rise and
Miller, J.S., Wooden, J.L., 2004. Residence, Resorption and recycling fall of a nested Christmas-tree laccolith complex, Elba Island, Italy.
of zircons in Devils Kitchen rhyolite, Coso Volcanic Field, In: Breitkreuz, C., Petford, N. (Eds.), Physical Geology of High-
California. Journal of Petrology 45 (11), 2155–2170. Level Magmatic Systems. Geological Society London, Special
Miller, C.F., McDowell, S.M., Mapes, R.W., 2003. Hot and cold Publication, vol. 234, pp. 195–213.
granites? Implications of zircon saturation temperatures and Wiebe, Robert A., 1994. Silicic magma chambers as traps for basaltic
preservation of inheritance. Geology 31 (6), 529–532. magmas; the Cadillac Mountain intrusive complex, Mount Desert
Miller, J.S., Miller, C.F., Cates, N.L., Wooden, J., Means, M.A., Island, Maine. Journal of Geology 102 (4), 423–437.
Ericksen, S., 2004. Time scales of pulsatory magmatic construction Wooden, J.L., Miller, D.M., 1990. Chronologic and isotopic
and solidification in Miocene subvolcanic systems, Eldorado framework for Early Proterozoic crustal evolution in the eastern
Mountains, Nevada (USA). Eos, Transactions of the American Mojave Desert region, SE California. Journal of Geophysical
Geophysical Union 85, JA 496. Research 95, 20133–20146.
Miller, C.F., Lowery, L.E., Bea, F., 2005. Zircon and Zr/Hf: assessing
magmatic fractionation in the crust. 15th Annual Goldschmidt
Abstract with Programs, p. A10.
View publication stats

GeologySpiritMountainBatholith Walker2007

Uploaded by

Copyright:

Available Formats

GeologySpiritMountainBatholith Walker2007

Uploaded by

Document Information

Original Description:

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

GeologySpiritMountainBatholith Walker2007

Uploaded by

Copyright:

Available Formats

See discussions, stats, and author profiles for this publication at: https://www.researchgate.

Geology and geochronology of the Spirit Mountain batholith, southern

Article in Journal of Volcanology and Geothermal Research · November 2007

Barry A. Walker Calvin F. Miller

SEE PROFILE SEE PROFILE

Lily Claiborne Jonathan S. Miller

SEE PROFILE SEE PROFILE

Review paper on magma chamber dynamics View project

The user has requested enhancement of the downloaded file.

Journal of Volcanology and Geothermal Research 167 (2007) 239 – 262

Geology and geochronology of the Spirit Mountain batholith,

Prevailing views of the nature of magma chambers

(continued on next page)

References Extensional Corridor. Journal of Geophysical Research 94,

View publication stats

You might also like