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Geosphere
Determining relative magma and host rock xenolith rheology during magmatic
fabric formation in plutons: Examples from the middle and upper crust
Aaron S. Yoshinobu, Jeannette M. Wolak, Scott R. Paterson, Geoffrey S. Pignotta and Heather S.
Anderson
Geosphere 2009;5;270-285
doi: 10.1130/GES00191.1
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Notes
© 2009 Geological Society of America
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Determining relative magma and host rock xenolith rheology
during magmatic fabric formation in plutons:
Examples from the middle and upper crust
Aaron S. Yoshinobu*
Jeannette M. Wolak*†
Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA
Scott R. Paterson*
Geoffrey S. Pignotta*§
Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740, USA
Heather S. Anderson*
Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA
ABSTRACT
Field observations, structural analysis, and
analytical calculations are utilized to evaluate
the strength of intermediate magmas during
crystallization in a regional strain field. Two
plutons are examined, the subvolcanic 98 Ma
old Jackass Lakes pluton, central Sierra
Nevada, California, and the voluminous middle crustal 442 Ma old Andalshatten pluton,
central Norway. The Andalshatten example
contains millimeter- to kilometer-scale xenoliths that display evidence for synmagmatic
deformation, including fold reactivation
and boudinage, after being isolated in the
magma. Fabrics within the pluton adjacent
to the xenoliths are usually magmatic, with
only local, discontinuous zones of crystalplastic deformation <1 m from the xenolith
contact. Examination of particularly well
exposed mafic metavolcanic xenoliths in the
Jackass Lakes pluton indicates that all were
strained prior to incorporation and then
separated from the remaining host rock by
brittle cracking. Once isolated from the host
rocks, some of these xenoliths were intruded
by veins fed by the in situ draining of melt and
magma from the surrounding crystal mush
zone. The xenoliths continued to deform duc-
tilely at presumably fast strain rates. Axialplanar magmatic foliations within folded
granodioritic dikes within xenoliths are parallel to magmatic foliations throughout the
Jackass Lakes pluton and metamorphic foliations within the host rocks, indicating that
the xenolith deformation occurred within the
regional 98 Ma old strain field that affected
the pluton.
The behavior of these xenoliths suggests
that late in the crystallization history, magmas in both middle crustal and subvolcanic
settings behaved as a high-strength crystalmelt mush capable of transmitting deviatoric
stresses, which drove both elastic and plastic
deformation in the enclosed xenoliths. Simultaneously, intercrystalline melt, and in some
cases magma, was drained from the host
intrusions into the xenoliths. Rheological
modeling based on geochemical data yields
an effective viscosity of a crystal-free melt of
~104 Pa s and increased to ~107 Pa s as cooling proceeded to 758 °C and crystal content
approached 40% for the Jackass Lakes pluton. Such viscosities are too low to impart or
transmit deformation into the xenoliths. The
preservation of xenoliths in both plutons is
compatible with higher crystallinities and/
or magma yield strengths as an explanation
to arrest the xenoliths in their final position
and allow deformation. Estimated effective
viscosities considering magma yield strength
and measured density variables (melt and
solid) are ~1013 Pa s.
INTRODUCTION
The strength of magmas as they crystallize
remains poorly constrained in natural environments. However, the rheological transitions that
occur during crystallization (and melting) must
play a pivotal role in the mechanical evolution
of the lithosphere, the segregation and migration of magma, and the eventual solidification
and thus emplacement of igneous bodies. In this
article we explore how magma-xenolith relations in plutons may provide information on the
viscosity of partially molten systems and the
strength of magmas as they crystallize. Many
experimental and theoretical investigations have
demonstrated the complexity and difficulty in
quantifying the rheology of magmas because of
their multiphase nature across a range of temperatures, pressures, ambient deviatoric stresses,
and fluid compositions and/or concentrations
(e.g., Arzi, 1978; Van der Molen and Paterson,
1979; McBirney, 1993; Lejeune and Richet,
1995; Rutter and Neumann, 1995; Barboza
*Emails: Yoshinobu: aaron.yoshinobu@ttu.edu; Wolak: jmwolak@montana.edu; Paterson: paterson@usc.edu; Pignotta: PIGNOTGS@uwec.edu; Anderson:
hs.anderson@ttu.edu.
†
Present address: Department of Earth Sciences, Montana State University, Bozeman, Montana 59717, USA.
§
Present address: Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, Wisconsin 54702, USA.
Geosphere; June 2009; v. 5; no. 3; p. 270–285; doi: 10.1130/GES00191.1; 12 figures.
270
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© 2009 Geological Society of America
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Relative magma and host rock xenolith rheology during fabric formation
and Bergantz, 1998; Renner et al., 2000; Rosenberg, 2001; Petford, 2003). Our approach is to
utilize field relations, structural analysis, and
geochemistry to place constraints on the rheology of magmas during xenolith incorporation
and deformation in shallow crustal and middle
crustal environments. Our results may bear
on the efficacy of magma emplacement processes such as diking and stoping, as well as
provide constraints on magma viscosity at high
crystal fractions.
In this paper we use the term xenolith to
describe any body of rock that is foreign to and
entirely surrounded by the host igneous rock
in contact with the xenolith. This may include
stoped blocks from the roof, walls, or floor of the
magma chamber, cognate xenoliths (autoliths)
derived from earlier crystallized portions of the
magma system, and screens (kilometric-scale
xenoliths, sometimes referred to as pendants),
or raft trains of xenoliths (e.g., Pitcher, 1970).
We differentiate xenoliths from mafic magmatic
enclaves or microgranitoid enclaves (e.g., Didier
and Barbarin, 1991). Xenocrysts are defined as
mineral fragments that are encapsulated in the
host igneous rock and have no direct chemical
relationship with the host magma. The study of
such xenoliths may provide information about
magma compositional changes due to assimilation (e.g., Barnes et al., 2004), the paleohorizontal at the time of final chamber construction,
timing of formation of structures in plutons,
kinematics of magma flow, and the rheology
of magmas at the time of xenolith capture (e.g.,
Paterson and Miller, 1998). Xenoliths also may
preserve examples of the complex interactions
that occur along chamber margins, including the
interplay of processes such as host rock melting
(McLeod et al., 1998), thermal cracking (Clarke
et al., 1998), high-temperature creep (Langdon,
1985), the initiation of dikes (Rubin, 1993a,
1993b), and magma fractionation and crystallization along boundaries (Mahood and Cornejo,
1992; McBirney et al., 1987; McBirney, 1993).
Two pluton–host rock systems are examined
that preserve abundant well-exposed xenolithmagma interactions. The 98 Ma old Jackass
Lakes pluton, Sierra Nevada, is a subvolcanic
pluton that intruded its own volcanic ejecta
and minor metasedimentary rocks and contains
numerous metavolcanic and metasedimentary
xenoliths (see Peck, 1980; McNulty et al., 1996).
The Jackass Lakes pluton was emplaced during
regional transpressive deformation (Krueger,
2005). The 442 Ma old Andalshatten pluton,
central Norway, was constructed in the middle
crust during regional contractional deformation and preserves numerous xenoliths at scales
ranging from millimeters to kilometers (Nordgulen et al., 1992; Anderson et al., 2007). Exam-
ination of xenoliths in both plutons indicates
that they were all pervasively deformed during
magmatic fabric formation. While this ductile
deformation was ongoing, the xenoliths were
brittlely cracked, sometimes injected by melts
and magmas from the surrounding magma, and
in some cases these veins were boudinaged and/
or folded at relatively fast strain rates.
The above observations are interpreted to
indicate that late in the crystallization history
of both plutons, the magma behaved as a highstrength crystal-melt mush capable of transmitting deviatoric stresses, which drove both
elastic and plastic deformation in the xenoliths,
while intercrystalline melt drained from the host
magma into the xenoliths. Furthermore, comparison of structures in the host pluton and in
the xenoliths indicates that the strength of the
crystal-melt mush during magmatic fabric formation was equal to or greater than that of the
metavolcanic xenoliths in the Jackass Lakes
pluton. The magmatic foliations and lineations
in both plutons are interpreted to reflect regional
strain. Thus, both plutons preserve evidence for
the orientation of the regional strain field during
magma emplacement.
XENOLITHS IN THE JACKASS LAKES
PLUTON, CENTRAL SIERRA NEVADA,
CALIFORNIA
The Jackass Lakes pluton (Fig. 1) in map view
is an ~13 × 17 km rectangular body that intruded
slightly older metavolcanic and plutonic pendants and screens (Peck, 1980; McNulty et
al., 1996). McNulty et al. (1996) analyzed two
samples of the granodiorite for U-Pb zircon geochronology, and reported a concordant age of
98.5 ± 0.3 Ma old from a population of zircons
from the northeast portion of the pluton and a
discordant age of 97.1 ± 0.7 Ma old from the
southwest portion. The pluton is truncated to
the south by the ca. 90 Ma old Mount Givens
pluton (McNulty et al., 2000) and to the north
by the 95 ± 2 Ma old Red Devil Lake pluton
(Tobisch et al., 1995). Aluminum-in-hornblende
geobarometric calculations yield anomalously
high (~400 MPa) pressures of crystallization
(i.e., 13–15 km depths; Ague and Brimhall,
1988). Geologic reconstructions of the overlying ca. 98–101 Ma old Minarets caldera (Fiske
and Tobisch, 1978, 1994), the presence of miarolitic cavities, and the observation that the pluton
intruded its own volcanic ejecta and subvolcanic
plutonic rocks point to a subvolcanic emplacement depth for the pluton (Peck, 1980; Stern et
al., 1981; Fiske and Tobisch, 1978; 1994; Coyne
et al., 2004; Wolak, 2004; Krueger, 2005).
The composition of the Jackass Lakes pluton
can be variable down to the meter scale, and
Geosphere, June 2009
includes diorite, quartz diorite, biotite ± hornblende granodiorite, granite, leucogranite, and
additional hybrid phases (McNulty et al., 1996;
Coyne et al., 2004). Generally these various
phases form north-northwest–trending, steeply
dipping sheet-like bodies ranging in width from
meters to kilometers with variable contacts
ranging from gradational to locally sharp. However, the central and western portions of the pluton contain distinct compositional units several
kilometers in width with quite variable shapes,
although generally elongate in the northwest direction. Mafic microgranitoid enclave
swarms of dioritic composition as wide as 50 m
occur in the more felsic units and are often
northwest trending. Textures vary considerably,
from aphanitic to coarse grained, and from equigranular to porphyritic.
Numerous metavolcanic and metasedimentary xenoliths occur throughout the Jackass
Lakes pluton and vary from kilometer-scale
screens to millimeter-scale xenoliths (Figs. 1
and 2). The largest screens resemble lithologies
exposed in the Minarets caldera sequence to
the east (Fig. 1) and include both metavolcanic
and metasedimentary rocks. Some pendants are
also intruded by a porphyritic leucogranite with
miarolitic cavities called the Post Peak phase,
which probably is a slightly earlier, subvolcanic
leucogranitic phase of the Jackass Lakes pluton
(Peck, 1980). Individual xenoliths and/or raft
trains of xenoliths are particularly common near
the large pendants, but occur throughout the pluton with variable sizes (meters to several hundred meters) and rock types (metavolcanic, leucogranitic, and metasedimentary; Wolak, 2004).
Magmatic foliations in the Jackass Lakes pluton strike north to northwest, dip steeply, and are
subparallel to elongate host-rock screens within
the pluton (Figs. 1, and 2). Magmatic lineations typically plunge steeply to shallowly north
within the foliation plane. Magmatic lineations
in the western quarter of the pluton are shallowly north plunging (Krueger, 2005). McNulty
et al. (1996) interpreted the magmatic fabric in
the Jackass Lakes pluton to reflect kinematics
of magma flow during chamber construction.
Wiebe (1999, 2000) suggested that Jackass Lakes
pluton formed by episodic input of mafic and felsic sheets that accumulated on a subhorizontal
(10°–30°) chamber floor and were subsequently
tilted to their present steep dips. Magmatic fabrics in this model reflect compaction in cumulate
piles below layers and local flow during convection above mafic sheets. However, magmatic
foliations cut across compositional boundaries
and are subparallel to regional structural trends
in the xenoliths and host rocks (see following; Peck, 1980; McNulty et al., 1996). Therefore, in contrast to the previous interpretations
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Yoshinobu et al.
noted above (e.g., Wiebe, 1999, 2000), we
suggest that this fabric reflects regional strain
imprinted on the still hypersolidus magma
97–98 Ma ago (see also Krueger, 2005).
In the following sections, different xenolith
suites are characterized in terms of the magmatic structures surrounding the xenoliths and
internal structures within the xenoliths.
Descriptions of Xenoliths
Black Beauty Xenolith
The metavolcanic xenolith described here,
informally named the Black Beauty xenolith, is
andesitic in composition and is located ~100 m
from the margin of the nearest exposure of
the pluton roof called the Post Peak pendant
Sierra
Nevada
batholith
K
ca. 95-85 Ma plutons
85
Field
area
Khd
60
80
Fig. 2
Post K
Peak
Pendant
K
50
85
ca. 107-99 Ma plutons
70
K
85
80
K
85
85
75
85
80
K
75
Fig. 7
K
N
60
Jr
Sing
Peak
Pendant
70
K
50
0
1
2
km
37°30′
ca. 90 Ma Mount Givens pluton
98 Ma Jackass Lakes pluton
Jr
K
K
K
K
MCs
K
Jurassic (Jr) metasedimentary and
Cretaceous (K) metavolcanic rocks of
the Minarets Caldera sequence (MCs).
119°30′
Magmatic foliation inclined; trace
Metamorphic foliation inclined; vertical
Figure 1. Simplified geologic map of the ca. 98 Ma old Jackass Lakes granodiorite
pluton, modified after Peck (1980), McNulty et al. (1996), and Wolack (2004). Only the
kilometer-scale xenoliths and screens are depicted on this map. For larger-scale maps
of xenolith fields contained within the pluton, see Wolak (2004) and Krueger (2005).
272
Geosphere, June 2009
(Figs. 1 and 2). The xenolith is 45 × 25 m with
a long dimension that strikes ~N25°W and is
exposed on a glacially polished subhorizontal
surface. The xenolith is metamorphosed and
contains a well-developed foliation oriented
~N35°W/83°NE defined by aligned phenocrysts
of plagioclase and hornblende and fine-grained
biotite and quartz (Fig. 3A). Metamorphic foliations in the xenolith are subparallel to foliations
in the nearby Post Peak pendant. Around the
xenolith magmatic foliation (N5°W/84°NE)
and lineation (70° at S30°E) occur in the host
granodiorite and are well defined by the alignment of magmatic biotite ± hornblende ± feldspar and flattened mafic microgranitoid enclaves
(Fig. 2D). Little to no deflection of this foliation occurs as the xenolith is approached at the
two ends of the xenolith; i.e., in regions where
the foliation and xenolith margins are at high
angles (Fig. 2). Local deflections at the meter
scale occur along other portions of the xenolith
margins. Both field and microstructural observations (Figs. 3B–3D) indicate that this foliation
is entirely magmatic, using criteria outlined in
Paterson et al. (1998).
Xenolith Margins
At the meter to centimeter scale the margins of the Black Beauty xenolith are defined
by straight to undulating segments connected
by lobate to angular corners where margins
abruptly change directions (Fig. 4). Undulating
and lobate margins tend to have amplitudes and
wavelengths of centimeter to decimeter dimensions. Straight margin segments in both the
larger xenolith and smaller pieces usually have
cutoff angles between metamorphic foliations
in the xenolith and the xenolith contact of 10°
or more, indicating that xenolith internal anisotropy did not control fracture formation.
Exceptions to the development of planar
margins occur at the centimeter scale in locations where the overall xenolith margins strike at
high angles to the magmatic foliation, i.e., at the
ends of elongate north-northwest–trending xenoliths. In the Black Beauty example, the margins
become concave with respect to the granodiorite,
with centimeter scale V-shaped cusps of host rock
at the magma-xenolith interface pointing outward
toward the host granodiorite and subparallel to
the magmatic foliation (Figs. 4A, 4B). Between
these cusps, lobate margins of the granodiorite
are concave toward the xenolith. Such cuspatelobate patterns may form between two materials with different effective viscosities, with the
material forming the cusps having the lower
viscosity (e.g., Fletcher, 1982; Smith, 2000).
The occurrence of these margins only on the
sides of the xenolith at high angles to the granodiorite magmatic foliation and subparallelism
Kj
Kj
C
Fig. 2 C
Km
View looking south
Km
Geosphere, June 2009
Xenolith long axis, N30°W
Metamorphic foliation,
N35°W/83°SW
Magmatic foliation, N5°W/84°SW
Kj
Kr
Kr
84
D
0
40
10
0
80
64
N
65
89
86
55
85
80
Km
72
79
88
86
80
100
83
85
87
meters
84
85
Kj
82
Contours in feet
37.646°
66
67
Km
66
77
68
B
75
75
Km
81
80
82
83
70
84
Kj
82
83
85
70
Metavolcanics
Km
Kj
Km
65
69
80
73
70
Metamorphic lineation; N=10
Metamorphic foliation; N=14
Magmatic lineation; N=12
Magmatic foliation; N=20
88
Fig. 2 C
85
Magmatic
foliation;
lineation
Metamorphic foliation;
lineation
Lithologic
Contact
Jackass Lakes
pluton
Kj
Stereonet Key
54
78 73
88
76
82
90
Km
Figs. 2A & C
83 Perspective of
Kj
10400
-119.315°
10000
View looking south
Relative magma and host rock xenolith rheology during fabric formation
Figure 2. (A) Location of Black Beauty xenolith with respect to host rocks. Kj—Jackass Lakes pluton; Kr—Red Devil Lake pluton (ca. 95 Ma old; McNulty et al., 1996); Km—
Cretaceous metavolcanic rocks. The red contact separates the Jackass Lakes pluton from the metavolcanic rocks of the Post Peak pendant shown in Figure 1. (B) Geologic map of
the xenolith and surrounding rocks. (C) Outcrop photograph displaying variation in orientations of foliations and xenolith long axis. (D) Lower-hemisphere stereonet displaying
orientations of structures in and around the xenolith. Xenolith in A and C is approximately 45 m long.
A
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Yoshinobu et al.
between the cusps and magmatic foliation
suggest that these margins and the magmatic
foliation formed at the same time during eastnortheast–west-southwest contraction.
Dikes and Veins in Xenoliths
Granodioritic dikes and quartzofeldspathic
veins of various sizes intrude virtually all of the
xenoliths observed within the Jackass Lakes
pluton. The Black Beauty xenolith displays
structures associated with these dikes and veins
that bear on magma-xenolith rheology. Millimeter- to centimeter-thick veins and dikes intrude
the xenolith and taper to thin tips (Figs. 4B–4D).
In some cases these veins can be traced continuously across the xenolith-granodiorite contact
and into the granodiorite, where they become
A
indistinguishable from the surrounding groundmass (Figs. 4B–4D). Vein tips taper to <1 mm
in width and typically die out in preserved fractures that extend for short distances in front
of the vein tip. It is not possible to determine
whether these fractures were preexisting or
formed during vein formation. However, fractures with no vein material exist in the xenolith.
In no case are dikes or veins observed to be truncated at the xenolith-granodiorite contact, indicating that dikes and veins must have formed
once the xenolith was stationary and could not
move within the magma.
Veins range from strongly deformed to late,
crosscutting, relatively undeformed examples
and are divided into three sets (Figs. 5 and 6).
For the deformed veins, the type of deformation
1 cm
is largely a function of the vein orientation relative to the xenolith foliation. Those striking at
low angles to the foliation (set 1) display boudinage (Fig. 5B), whereas those striking at high
angles are folded (sets 2 and 3; Fig. 5C). Field
observations (Figs. 3–5) and microstructural
observations (Fig. 3D) indicate that the veins
underwent little to no subsolidus crystal-plastic
deformation (beyond minor dislocation glide in
quartz and feldspar).
One granodioritic dike is unusually thick and
shows several features not seen in other veins
within the xenolith. (Figs. 5A and 6). This dike
is no longer planar and shows both fold patterns and lobate margins (Figs. 6A, 6B). It is
by far the thickest dike in the xenolith and has
walls that are subperpendicular to the xenolith
B
0.5 mm
C
D
4 mm
Figure 3. (A) Photomicrograph of metamorphic foliation in the metavolcanic xenolith shown in Figures 2A–2C; also shown are granitic
veins. (B) Magmatic foliation, defined by aligned igneous minerals and enclaves in the Jackass Lakes pluton. This outcrop is a short distance from xenolith in Figure 2. (C) Folded and locally sheared schlieren layers and granodiorite several hundred meters south of examined xenolith in Figure 2. (D) Photomicrograph of magmatic foliation in the Jackass Lakes granodiorite; white line in upper right depicts
average orientation of foliation in rock. The main foliation shown in D is axial planer to folds in C. Ruler in top center of C is 6 cm long.
274
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Relative magma and host rock xenolith rheology during fabric formation
stretching direction. The vein is coarser grained,
and muscovite and biotite locally form comb
layering where crystals nucleated on and grew
perpendicular to the margin (Fig. 6C), indicating that any deformation in this vein occurred
in the magmatic state. This dike has curving
margins that, over several wavelengths, form
cuspate-lobate patterns similar to those seen
along the main xenolith margin (cf. Figs. 4 and
6). Here also the andesitic xenolith forms cusps
perpendicular to the xenolith-vein margin, suggesting that it had a lower effective viscosity
during folding of the vein. Locally foliation in
the xenolith is deflected around fold hinges or
lobate protrusions, indicating that the xenolith
was ductile and flowing during magmatic deformation of the vein.
Snake Xenolith
Mapping throughout the Jackass Lakes pluton
reveals similar xenolith-magma relationships. In
another example, a large (50 m × 55 m) andesitic xenolith crops out along the eastern margin
of the Sing Peak pendant (Figs. 1 and 7). The
xenolith contains a northwest-trending, steeply
east dipping metamorphic foliation that is subparallel to a well-developed magmatic foliation
within the host granodiorite. Contact relations
display similar geometries as the Black Beauty
xenolith: discordant hypersolidus foliations in
the granodioritic host occur at the ends of the
xenolith, whereas concordant fabrics occur
along the sides (Fig. 7A). Large-scale mapping
(1:300) reveals a meter-wide granodioritic dike
that can be traced continually from the southern to northern edge of the xenolith and into
the host granodiorite (Figs. 7A, 7B). This dike
is folded about axial planes that are subparallel to the metamorphic foliation in the xenolith
and magmatic foliation within the granodioritic
host (Figs. 7C–7F). Schlieren layering within
the dike is folded. The dike contains an axial
B
A
1m
C
D
Figure 4. Field photos of xenolith margins. (A) Sharp margin between xenolith (dark color) and granodiorite (light color) showing ~60° jog
in margin. (B) Cuspate-lobate pattern at ends of xenolith. Black and white arrows in A and B point toward concave granodiorite-xenolith
contacts where cusps of the meta-andesite point into the granodiorite. Three veins of late melt extend into the xenolith, are continuous
across the xenolith margin, and have no subsolidus deformation. The largest vein (blue arrow) extends a short distance into the granodiorite
before merging into the granodiorite matrix. (C) Close-up of ~90° step along xenolith margin with both straight and slightly curved margins. More felsic magma (lighter-colored material) accumulated along margin and a thin vein intruded into xenolith at inner step (arrow).
(D) Vein of late melt extending into xenolith. Note that vein is continuous across xenolith margin. Orange ruler in all images is 15 cm long.
Geosphere, June 2009
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Yoshinobu et al.
A
Set 1
Fig. 6A
Set 3
Set 2
Fig. 6B
B
C
Extension = 32%
D
Shortening = -49%
Figure 5. Photos of granodiorite and granite veins in meta-andesitic xenolith. (A) Best-exposed portion of xenolith
and contact with granodiorite showing three vein sets. Set 1 is subparallel to metamorphic foliation in the xenolith;
set 2 is subperpendicular to the foliation; set 3 is at high angles to foliation and typically defined by thicker dikes.
Single veins can abruptly change orientation from one set to the next. Crosscutting relationships are complex, but
typically suggest that set 1 formed before set 2, followed by set 3. (B) Boudinaged vein set 1; note rectangular shapes
and lack of change in thickness of most boudins. (C) Close-up of folded vein (set 2); 5 cm of ruler for scale. (D) Folded
and boudinaged veins showing estimates of extension and shortening in subhorizontal plane, i.e., at high angles to
mineral lineation. Orange scale in A and B is 15 cm long.
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Relative magma and host rock xenolith rheology during fabric formation
planar, magmatic foliation defined by subhedral
plagioclase laths and amphibole crystals that is
subparallel to the metamorphic foliation within
the xenolith and magmatic foliation within the
host granodiorite. In addition, centimeter-scale
elongate xenoliths of andesitic composition
occur within the dike and share the axial planar
orientation (Fig. 7D).
XENOLITHS IN THE ANDALSHATTEN
PLUTON
The Andalshatten pluton, central Norway, is
a large (~630 km2) elongate intrusive complex
with exposed dimensions of 18 × 35 km (Fig. 8).
The pluton consists of a large K-feldspar megacrystic granodiorite unit with lesser tonalite,
diorite, and leucogranite. Nordgulen et al.
(1993) published a U-Pb zircon age of 447
± 8 Ma. More recent chemical abrasion–thermal
ionization mass spectrometry (CA-TIMS) dating on two samples from the granodiorite phase
yields ages of 442.66 ± 0.18 Ma and 442.86
± 0.20 Ma old (Anderson et al., 2007). The pluton was emplaced at pressures of ~670 MPa,
based on Al-in-hornblende barometry using the
Anderson and Smith (1995) calibration (Yoshinobu et al., 2002). This pressure is consistent
with the presence of magmatic epidote + hornblende and with pressure estimates for regional
metamorphism using the GASP (garnetaluminosilicate-plagioclase-quartz) barometer
(Barnes and Prestvik, 2000). Regional meta-
A
morphic temperatures were estimated by Barnes
and Prestvik (2000) to be >500 °C at the time
of emplacement. Along its western margin, the
pluton intrudes amphibolite-grade interbedded
marbles, calc-silicate, and metaarkosic rocks
that contain bedding and other structures that
are broadly concordant with the contact at the
map scale (Fig. 8). Prior to emplacement of the
Andalshatten pluton, these rocks were deformed
during ca. 480–470 Ma ago regional contraction
(Yoshinobu et al., 2002; Barnes et al., 2007).
The Andalshatten pluton is rich in xenoliths of
all lithologies observed in the host rocks and at
centimeter to kilometer scales.
Regional mapping (Nordgulen et al., 1992;
Anderson et al., 2007) has demonstrated that
magmatic fabrics in the Andalshatten pluton
cut across compositional boundaries at the map
scale. We interpret this relationship to indicate
that the magmatic fabrics formed late in the
crystallization history of the Andalshatten pluton. Ongoing structural work along the north
and eastern margin of the pluton indicate a pervasive crystal-plastic overprint that is interpreted
to reflect ca. 442 Ma ago regional deformation.
Therefore, we view the magmatic fabrics within
the Andalshatten pluton as the hypersolidus
record of this deformation.
Descriptions of Xenoliths
The following descriptions are from a suite of
xenoliths and screens ranging in size from sev-
B
eral to tens of meters in length exposed near the
western contact of the pluton (Fig. 9). In contrast to the Jackass Lakes pluton, xenoliths and
screens within the Andalshatten pluton show
more evidence for ductile modification of contacts and internal strain.
A spectacular example of synmagmatic
deformation of a screen occurs along shoreline exposures near the southwest margin of
the pluton (Fig. 9). At this locality, termed the
“taffy” screen, an ~100-m-long, elongate screen
of folded calc-silicate rock was intruded by
north-trending sheets of coarse-grained granodiorite and shows evidence for boudinage in the
granodioritic magma. The contacts of the screen
dip steeply to shallowly east and broadly define
an S shape in map view (Fig. 9B). The screen
contains sets of upright, tight to isoclinal folds
defined by transposed layers in calc-silicate
and metaarkosic rocks. A spaced axial planar
cleavage is well developed in the folds. Both
the axial planar cleavage and the limbs of the
folds mimic the S shape of the screen. At the
neck of the boudin, point P in Figure 9A, the
interlimb angles of the folds approach 0° and the
axial planar cleavage becomes penetrative with
lithons that decrease in width to <2 cm.
Xenolith Margins
The contacts of the taffy screen are variable
in terms of the relative concordance and shape
with respect to internal structure within the
screen. At the meter scale, undulating, lobate
C
Figure 6. Field photos of largest dike within the xenolith (shown in Fig. 5A). (A) Folds, cuspate-lobate margins, deflection of host foliation
(near ruler), and fragments of host in vein. (B) Close-up of cuspate-lobate margin occurring at two scales (white arrow). Red line in A
and B is trace of metamorphic foliation in meta-andesite. (C) Large igneous muscovite forming comb layering along lobate margin. No
subsolidus deformation observed. Orange ruler is 15 cm long.
Geosphere, June 2009
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Yoshinobu et al.
B
Km
C
Fig. D
Kj
Km
GPS 4158860 N
D
87
Kj
85
81
74
82
85
Location
71
of
Figs. 7B-D
87
68
Km
86
89
84
79
74
84
76
84
87
83 85
North
Km
65
82
86
Kj
Km
81
76
GPS 292170 E
GPS 292100 E
A
40 m
0
Kj
GPS 4158780 N
70
Magmatic foliation
60
Metamorphic foliation
F
E
Magmatic foliations, n = 154
mean plane N14W/ 83NE
278
Metamorphic foliations, n = 196
mean plane N14W/ 81NE
Geosphere, June 2009
Figure 7. (A) Geologic map of
large metavolcanic xenolith
(Km—Cretaceous metavolcanic) within the Stanford Lakes
xenolith field, located in the
southwestern Jackass Lakes
pluton (Kj; see Fig. 1). Universal Transverse Mercator projection, NAD 1972 Zone 11. Arrow
in A denotes area of folded dike
show in B–D. (B) Photo of xenolith. (C) Drawing of B. Note the
granodiorite dike that intrudes
the xenolith and is folded
about an axial plane parallel
to magmatic fabrics in the Kj.
(D) Hypersolidus foliations in
the dike and metamorphic foliations in the enclosing xenoliths
are subparallel parallel to the
pencil. (E) Lower-hemisphere
stereonet displaying poles to
magmatic foliations. (F) Lowerhemisphere stereonet displaying
poles to metamorphic foliations.
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Relative magma and host rock xenolith rheology during fabric formation
to cutoff angles between strata and faults in brittle fault systems (e.g., Boyer and Elliot, 1982).
At the lobe apex, bedding, although attenuated,
is usually concordant to the contact.
contacts with amplitudes and wavelengths of
as much as tens of centimeters can be followed
that display a transition from obliquely discordant to concordant (Fig. 9). The maximum
length of straight and planar contacts is ~1 m.
Where lobate contacts with granodiorite bow
into the screen, the layering is attenuated at the
apex of the lobe (Fig. 9C). Locally, the layering
is truncated along the inflection points of lobes
(Fig. 9C) forming a bedding cutoff, analogous
Dikes and Sills in Xenoliths
Dikes and sills of granodiorite and leucogranite intrude most of the larger xenoliths and
virtually all of the screens. The taffy screen
contains two examples that bear on the nature
of synmagmatic deformation of metasedimentary xenoliths. In Figure 10 a granodioritic dike
intrudes the eastern margin of the screen at a
very shallow angle with respect to layering.
The dike continues for several meters obliquely
into the screen where it tapers to a pointed end
in the eastern limb of a tight syncline. An en
echelon vein of granodiorite is observed 20 cm
west of this dike that cuts the axial planar cleavage in the core of the syncline (Fig. 10A). The
NORWAY
Map
area
Upper nappe
Atlantic
Ocean
442 Ma
Andalshatten
pluton
Location of
Figs. 9, 10
rd
fjo
12°00'
65°30'
l
Ve
Atlantic
Ocean
Sauren-Torgatten nappe
N
Middle nappe
Lower nappe
Figure 8. Simplified geologic
map of the Andalshatten pluton and Helgeland Nappe
Complex. The Andalshatten
pluton intrudes rocks of all
the major nappes within the
Helgeland Nappe Complex.
Note that screens and xenoliths
within the Andalshatten contain lithologies that are apparently observed along strike in
the host rocks.
10 km
ca. 442-448 Ma ‘Velfjord’ diorite suite
442 Ma Andalshatten pluton
Thrust fault; inferred
Mafic/ultramafic rock
Sauren-Torghatten nappe; calc-silicate
rocks/pelitic schist & intrusive rocks
Detachment fault; inferred
Upper nappe marble
Upper nappe quartzo-feldspathic
gneiss/migmatite & intrusive rocks
Middle nappe
Lower nappe; calc-silicate rocks & intrusive rocks
Geosphere, June 2009
279
Geosphere, June 2009
67
84
80
Bedding
Crystal-plastic foliation
7278000
65
Syncline/synform
Anticline/antiform
Calc-silicate, meta-arkose
Granodiorite
Quarternary
60
63
P
Fig. 9C.
C
y
x
20 cm
4m
Yoshinobu et al.
Figure 9. (A) North-looking perspective of the taffy screen (see text). Note the attenuation of the screen at point P. White arrow shows perspective of view in Figure 10. Black arrows
denote contacts that are impinged by the granodiorite. (B) Geologic map of the taffy screen and other nearby xenoliths. (C) Detail of contact between xenolith and granodiorite.
White arrow denotes concordance, whereas black arrows denote cutoffs between layering and xenolith-granodiorite contact. Marker layers (bedding) x and y display progressive
attenuation before they are truncated at the contact.
Bedding/structure form lines
55
N
Hypersolidus foliation
20 m
68
“Taffy Screen”
80
Contact, dashed where inferred
75
76
55
7277920
0378645
B
+
280
+
A
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Relative magma and host rock xenolith rheology during fabric formation
dike is boudinaged in the screen along a flattening plane that is subparallel with the axial
planar cleavage contained within the screen
(Fig. 10A). Cutoffs between the dike and layering are consistently <15°.
In contrast to the boudinaged dike, Figures 10B and 10C display a composite grano-
A
dioritic and/or leucogranitic sill that intruded
parallel to layering within the calc-silicate
rock. Bedding is folded about the same axes
shown in Figure 9. The sill, therefore, has a
folded geometry but contains no evidence of
strain associated with folding. In contrast to
the examples from the Jackass Lakes pluton
(e.g., Fig. 7), there is no preserved axial planar magmatic foliation within the sill. However, within the sill, panels of calc-silicate
rock 2–3 cm wide and as much as 1 m long
are entrained and in some cases tightly to isoclinally folded about chaotically oriented fold
axes (Fig. 10C).
Area shown in Fig. 10C
2m
sc
al
e
ch
an
ge
s
wi
th
pe
rs
pe
ct
ive
B
C
10 cm
Figure 10. Detail view of internal strain and diking in the taffy screen in Figure 9.
(A) Oblique view to south of the east margin of the taffy screen displaying the
tightly folded syncline, transposed bedding, and boudinaged granodioritic dikes
that emanated from the host magma (bottom and left side of the image). Layeringcontact cutoffs are <10°. Crystal-plastic fabrics within the granodiorite are localized to within a few centimeters to meters of the screen contact. Note how scale
changes with perspective. (B) Folded composite granodioritic-leucogranitic sill that
intruded previously folded calc-silicate rocks. (C) Lower limb of folded granodioritic dike in B displaying panels of calc-silicate rock that are folded about randomly
oriented axes (black arrow). Coin is ~15 mm in diameter.
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Yoshinobu et al.
DISCUSSION
Xenolith Formation, Incorporation, and
Strain
Both the subvolcanic Jackass Lakes pluton and
the middle crustal Andalshatten pluton are littered with pieces of host rock ranging in size from
meter-scale to kilometer-scale xenoliths. In some
cases it is possible to establish that xenoliths have
been detached from screens or the host rocks and
moved through the magma prior to being trapped
at their present location. In other cases, xenoliths
appear to have been translated away from the host
by an invading dike, but do not appear to have
been considerably rotated. Whether most xenoliths or screens have remained in situ cannot be
constrained because magma emplacement must
move either the host rock contact or the screen (or
both) during intrusion.
In both plutons, xenoliths and screens have
variably discordant margins, or cutoffs, defined
by planar fractures that typically cut across any
structure in the xenolith (e.g., Figs. 2, 4, 7, 8,
and 10). Such bedding and/or layering contact
cutoff angles provide information about initiation of xenolith incorporation as well as subsequent xenolith deformation within the magma.
These planar contacts were deformed to various
extents after the xenolith was incorporated into
the magma, resulting in tightened cutoff angles,
folding, and boudinage (e.g., Fig. 9). In the following sections we examine how xenoliths may
be used to evaluate magma-xenolith rheology
and strain fields in plutons.
The folding and boudinage of veins (Fig. 5),
thickening and formation of cuspate-lobate margins of some veins (e.g., Fig. 4), and the deflection of foliations in the xenoliths around vein
margins all indicate that the xenoliths underwent
additional ductile strain during vein formation.
The late crosscutting undeformed veins establish
that this ductile deformation ceased before final
crystallization of the Jackass Lakes pluton. The
presence of a single foliation in all of the xenoliths examined and the orientation of this foliation
parallel to the direction of vein extension and perpendicular to vein shortening (indicated by vein
boudinage and folding, respectively) imply that
this preexisting foliation was reactivated during
the in situ ductile deformation of the xenoliths
and the formation of the magmatic foliation in
the Jackass Lakes pluton. This conclusion is supported by the axial planar, hypersolidus foliation
observed in folded dikes (e.g., Fig. 7).
In the Black Beauty xenolith we have estimated the magnitude of strain associated with
this synmagmatic pulse of ductile deformation
by using the folded and boudinaged veins. These
veins occur on subhorizontal faces that are sub-
282
perpendicular to the steeply plunging lineation
in this region and therefore reflect the YZ plane
of the strain ellipsoid. Because the thickness of
the vein boudins does not vary along veins, and
because they show no signs of necking or other
internal strain, we treat the boudins as solid rectangles formed by brittle cracking and then rigidly
translated during extension. Therefore, we can
use their original and deformed lengths to calculate extension along the vein in the foliation plane
(Fig. 5D; Ferguson, 1981). We have also treated
the folded veins as ptygmatic folds and used their
deformed and undeformed lengths to calculate the
amount of shortening perpendicular to the foliation plane (Fig. 5D). Our measurements indicate
that the most deformed veins record ~31% extension and 49% shortening in the horizontal surface.
We have no quantitative information about strain
in the vertical direction in this xenolith. These values for strain are interpreted to reflect the minimum strain that affected the xenoliths and magma
during folding and boudinage of the granodioritic
veins and dikes prior to final cracking and intrusion of the late crosscutting dikes.
Figure 11 depicts a schematic characterization
of xenolith-magma deformation as observed in
both the Jackass Lakes and Andalshatten examples. Deformation within the xenolith is kinematically compatible with regional deformation
associated with fabric development in the host
magma. At the point of fabric formation in the
system, we envision that the rheology of the
xenolith and magma are similar. Given the apparent fast rate at which dikes in the Jackass Lakes
pluton may have cooled, the xenoliths likely
continued to deform ductilely at presumably fast
strain rates so as to maintain the magmatic foliation within the dikes. Albertz et al. (2005), on
the basis of thermal modeling and field observations in shallow crustal settings, suggested
that strain rates may approach 10−2 s–1 in order
to preserve magmatic fabrics within the axial
planes of folded granodioritic dikes. Regardless of the absolute rate of deformation, axial
planar magmatic fabrics in the folded granodiorite dikes that are parallel to metamorphic
fabrics in the xenoliths imply relatively fast
strain rates in the pluton–host rock system.
Extension direction
Lobate contacts with
host magma
Maximum
shortening
direction
Magmatic
foliation
Figure 11. Schematic illustration depicting kinematic compatibility in subhorizontal planes between internal and external structures in an idealized xenolithmagma system.
Geosphere, June 2009
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Relative magma and host rock xenolith rheology during fabric formation
Rheology of the Magma–Host Rock–
Xenolith System
Several observations suggest that the granodioritic magma in both the subvolcanic Jackass Lakes chamber and the middle crustal
Andalshatten chamber was as strong as or stronger than the andesitic and calc-silicate xenoliths,
respectively, while the xenoliths were internally
deforming. The ptygmatic folding and boudinage of veins (e.g., Figs. 5 and 7) and xenoliths
(e.g., Fig. 9), the presence of cuspate-lobate
margins (Fig. 4), and the thin V-shaped cusps
all suggest that the magma effective viscosity
approached and in some cases exceeded the
xenolith viscosity in both plutons. One consequence of these observations is that if the xenoliths are internally shortening and elongating
during fabric formation within the magma, then
the magma must have been capable of transmitting deviatoric stresses to the xenoliths. This
implies that the magmas in both cases were nonNewtonian, crystal-rich mushes with significant
strength and that the strength of the xenoliths
must be equal to or less than the magma. We
also note that the magmatic fabrics in the Jackass Lakes pluton are not strongly deflected
around xenoliths, again suggesting that xenoliths were not stronger than the magma.
Structural relations in the Jackass Lakes
pluton–host rock system can be used to constrain relative magma viscosities at the time
of fabric formation. For example, the degree
of deflection of magmatic foliations along the
margins of xenoliths may provide information
about the viscosity ratio between the crystallizing magma and xenoliths (Paterson and Miller,
1998). Undeflected magmatic foliations and
subparallel metamorphic foliations in xenoliths
in the Jackass Lakes pluton suggest a very low
viscosity ratio. That is, where xenoliths and their
internal foliations are parallel to the magmatic
fabric, and where granodiorite dikes in xenoliths
are folded about axial planes that are parallel to
the magmatic fabrics, then the viscosity of the
crystallizing magma was similar to that of host
rock xenoliths. Assuming that individual xenoliths were entirely surrounded by granodioritic
magma, the magma viscosity must have been
high enough to trap xenoliths (i.e., higher than
the most dense block), yet still had sufficient
melt present to allow a magmatic foliation to
form during regional tectonism and after incorporation of some of the xenoliths.
Granodioritic melt viscosities during ascent,
emplacement, and fabric formation are dependent on a number of factors, including composition, pressure, temperature, volatile content,
and crystallinity (e.g., McBirney, 1993; Scaillet et al., 1997; Bachmann and Bergantz, 2004).
To estimate magma viscosities over a range of
conditions in the Jackass Lakes pluton, we analyzed five samples of granodiorite from the Jackass Lakes pluton for major and trace elements
(Wolak, 2004). These data were then used as
input in the programs MELTS (Ghiorso and Sack,
1995) and PELE (Boudreau, 1999) to model
effective magma viscosities. Although originally
developed for simulating melt evolution in basaltic magmas, these programs may also be used
to determine silicic melt parameters at low crystal percentages (<40% crystals). Unfortunately,
the programs cannot account for hydrous mafic
phases, and are thus less reliable as crystallization
proceeds. However, these results provide minimum estimates of the viscosity of the crystal-melt
mush and therefore provide a minimum estimate
of the strength of the magma.
The shallow emplacement depth, temperature (~900–800 °C), and compositions are well
constrained for the pluton (Fiske and Tobisch,
1994; McNulty et al., 1996; Coyne et al., 2004;
Wolak, 2004; Krueger, 2005). Although less well
constrained in our study, volatile contents are
probably between 3 and 8 wt% H2O (Scaillet et
al., 1997; Bachmann and Bergantz, 2003). For
granodioritic melts with 3 wt% H2O, a liquidus of
~983 °C was calculated. The effective viscosity of
a crystal-free melt of this composition was ~104
Pa s and increased to ~107 Pa s as cooling proceeded to 758 °C and crystal content approached
40%. Increasing the H2O content to 6 wt% lowered the effective viscosities to 103–104 Pa s and
yielded a liquidus temperature of 872 °C. This is
compatible with Scaillet et al. (1997) and other
studies, which predict that higher volatile contents
will result in lower magma viscosities. In addition, our values agree with Coyne et al. (2004),
who suggested an estimated solidus of ~670 °C.
Modeled densities of the magma described
above ranged from 2230–2410 kg/m3, much lower
than our average measured density for metavolcanic xenoliths of 2640 kg/m3 (Wolak, 2004).
These data indicate that host rock xenoliths in the
Jackass Lakes pluton were negatively buoyant
and should have sunk steadily through the crystalpoor magma, assuming xenolith incorporation at
crystallinities <40%. Assuming an average block
radius of 1 m, the densities given above, granodioritic magma viscosities of 104–107 Pa s, and using
Stokes’ settling laws for static, Newtonian melts,
settling rates of 4.0 × 10−3 to 1.5 × 10−7 m/s (~5 m
a–1) are predicted for our metavolcanic xenoliths.
To achieve slower settling rates (e.g., <10 cm/ka)
requires significantly higher viscosities in order to
trap xenoliths and allow a magmatic foliation to
form after incorporation of the xenolith into the
magma. For example, a magma viscosity on the
order of 1013 Pa s due to increased crystallinity
yields a settling velocity of ~2.13 cm/ka. There-
Geosphere, June 2009
fore, we suggest that the Jackass Lakes magma
must have been crystal rich (>>50% crystals) at
the time of xenolith capture and thus at temperatures approaching the solidus. This conclusion
agrees with: (1) the conclusions of Scaillet et al.
(1997) and Bachmann and Bergantz (2004), who
suggested that the greatest increase in viscosity,
due to crystal content, occurs at temperatures
nearing the solidus; and (2) previous workers who
have estimated magma viscosities ranging from
1014 to 1016 Pa s in crystal-rich mushes during fabric formation in granodioritic to gabbroic compositions (Nicolas and Ildefonse, 1996; Paterson and
Miller, 1998; Yoshinobu and Hirth, 2002).
Settling rate estimates for a Newtonian liquid, as described above, are well established for
a wide range of conditions (McBirney, 1993,
and others) but are probably not realistic for
a magma. Sparks et al. (1977) suggested that
magmas may behave as granular materials with
some yield strength, i.e., viscoplastic or Bingham behavior. One effect of an increase in yield
strength would be to capture xenoliths at lower
magma viscosities. Figure 12 is reproduced
from Sparks et al. (1977) and plots the maximum diameter of spheres at excess densities
of 300, 500, and 700 kg/m3 with respect to the
enclosing magmas as functions of yield strength.
Given excess densities of 200–400 kg/m3
for metavolcanic xenoliths (diameter 1 m) in
the Jackass Lakes pluton, we estimate that yield
strengths needed to trap blocks range from ~250
to 500 N/m2. Like Paterson and Miller (1998),
we note that our yield strength estimates are
much higher than those measured by Sparks et
al. (1977), and therefore require a crystal-rich
magma (>50% crystals). Therefore, we contend that higher crystal fractions are required in
order to explain the observations from the Jackass lakes pluton. We note that such Bingham
behavior would reduce melt viscosities needed
to trap blocks by one to two orders of magnitude
(Sparks et al., 1977; Paterson and Miller, 1998).
We summarize the following steps regarding
changing magma viscosities, incorporation of
host rock xenoliths, and formation of magmatic
fabrics in the granodiorite of Jackass Lakes.
(1) Xenoliths removed from the margins of the
growing magma chamber, whether via stoping
or diking, during the final stages of emplacement must have been incorporated into a crystalrich magmatic mush with viscosities ranging
in excess of 1013 Pa s, or possibly 1–2 orders of
magnitude lower if the magma had some yield
strength. (2) The granodioritic mush retained
enough melt to allow reorientation of framework grains after xenolith capture, thereby erasing most evidence for xenolith incorporation.
(3) The viscosities and densities of the magma–
host rock–xenolith system were very similar at the
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Yoshinobu et al.
2. The amount of strain recorded by the
deformed veins in the xenoliths is certainly sufficient enough to form the relatively weak fabric
in the Jackass Lakes pluton, implying that the
preserved Jackass Lakes pluton fabric may have
formed within a short duration and during rapid
strain rates.
3. The overprinting of internal intrusive
contacts by the fabric and local discordance
between the north-south–striking Jackass Lakes
pluton fabric and chamber margin indicate
that the Jackass Lakes pluton fabric formed
after chamber construction, and thus reflects
regional strain of an existing chamber and not
emplacement-related deformation. If the Jackass Lakes pluton is a subcaldera chamber, then
this implies that following caldera collapse (represented by the numerous pendants and xenoliths in the chamber), regional tectonic stresses
dominated over any stresses related to the evolution of the magmatic-volcanic system.
time of fabric formation, so that fabrics in each
record regional tectonism ca. 98 Ma ago, some
time after xenolith removal and incorporation.
Implications for Fabric Formation in the
Jackass Lakes Pluton
One important conclusion is that the preserved magmatic fabric (i.e., foliation and lineation) in the Jackass Lakes pluton (and the
Andalshatten pluton) formed at the same time
as the xenoliths were internally deforming. We
base this conclusion on the following observations: (1) the subparallelism between the magmatic foliation and/or lineation and metamorphic foliations and/or lineation contained in
the xenoliths; (2) subparallelism between these
structures and axial planes defined by magmatic
foliations within folded granodioritic dikes and
veins; (3) lack of significant deflection of the
magmatic foliation around the xenoliths; and
(4) synchronous pluton fabric formation and
vein and/or dike deformation. Given the above,
the general characteristics of the xenoliths and
Jackass Lakes pluton veins (Figs. 6–9), and the
map pattern of the Jackass Lakes pluton fabric
(Fig. 1), we summarize the following.
1. The preserved Jackass Lakes pluton fabric
formed under a deviatoric stress very late in the
crystallization history of the chamber.
CONCLUSIONS
Field observations, structural analysis, and
modeled crystallization histories based on major
element data may be used to place constraints
on the effective viscosity of the magmas in the
presence of xenoliths during fabric formation.
We summarize our conclusions as follows.
3
/m
kg
0
0k
ACKNOWLEDGMENTS
70
70
50
300
Diameter (cm)
80
g/m
kg/
90
3
m3
100
60
50
40
30
20
10
100
200
300
400
500
1. The subvolcanic Jackass Lakes pluton, central Sierra Nevada, California, and the middle
crustal Andalshatten pluton, Norway, contain
numerous centimeter- to kilometer-scale xenoliths that display evidence for postincorporation,
synmagmatic deformation.
2. Late in the cooling history, the crystal-melt
mush of both plutons behaved as high-strength
materials capable of transmitting deviatoric
stresses to the xenoliths, which drove both elastic
and crystal plastic deformation in the xenoliths.
3. During synmagmatic deformation melts
drained into the xenoliths from the surrounding
crystal-rich magma.
4. Estimates of magma viscosity at the time of
xenolith capture are in the range of 1013 Pa s for
the Jackass Lakes pluton, suggesting a crystalrich mush in which realignment of grains to
form a magmatic foliation was still possible.
We conclude that magma-xenolith systems
may share similar viscosities during near solidus
crystallization. In addition, evidence from the
Jackass Lakes pluton supports the notion that
deformation of the magma-xenolith system may
reflect the regional strain field ca. 97–98 Ma
ago. This study documents the potential of
utilizing detailed field observations of xenolith and igneous structures in plutons to better
understand magma rheologies and paleostrain
fields in arcs. An interesting future study would
include examination of fabric geometry in different plutons with different ages in the same arc
system to attempt to track the changing strain
field through time.
600
700
800
900
1000
This research was supported by National Science
Foundation grants EAR-0106557 and EAR-0439750
(Yoshinobu) and EAR-0073943 and EDMAP (Educational Component of the National Cooperative
Geological Mapping Program) grant 02HQAG0089
(Paterson). Yoshinobu also acknowledges support
from the Department of Geosciences, Texas Tech
University, and the Norwegian Geological Survey.
Pignotta and Anderson acknowledge support from
the Geological Society of America and Sigma Xi. We
thank Cal Barnes and Øystein Nordgulen for continued discussions about the interactions of magmas and
host rocks, and Bob Miller and Brendan McNulty
for their thoughtful reviews. Stereonets were made
with Rick Allmendinger’s software. We especially
thank Øystein Nordgulen for guiding us through the
diverse collection of igneous rocks that occur within
the Bindal batholith, Norway, and for his fantastic
regional mapping.
Yield Strength (N/m2)
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Figure 12. The maximum diameter of spheres that can be supported in a Bingham liquid, plotted as a function of yield strength
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400 kg/m3 (shaded area).
284
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