Lithos 44 Ž1998. 53–82
Interpreting magmatic fabric patterns in plutons
Scott R. Paterson a,) , T. Kenneth Fowler Jr. a , Keegan L. Schmidt a ,
Aaron S. Yoshinobu a , E. Semele Yuan a , Robert B. Miller b
a
Department of Earth Sciences, UniÕersity of Southern California, Los Angeles, CA 90089-0740, USA
b
Department of Geology, San Jose State UniÕersity, San Jose, CA 95192, USA
Received 12 August 1997; accepted 30 April 1998
Abstract
Most plutons have widespread magmatic fabrics, the interpretation of which remains controversial. We propose a method
to constrain likely causes of fabric patterns, the application of which indicates the following: Ž1. preserved fabric patterns
often form after chamber construction and only rarely provide information about ascent or emplacement; Ž2. fabrics are poor
recorders of total strain and are easily reset, preserving only the last increment of strain during crystallization; Ž3. in
magmatic systems mechanically decoupled from host rocks, patterns may result from strain during internally driven flow,
filter pressing or porous flow in relatively static chambers, or by final increments of strain during emplacement; Ž4. with
greater emplacement depths, fabric patterns increasingly reflect strain caused by regional deformation; and Ž5. given that
magmatic fabrics are easily reset and reflect only the last increment of strain of comparatively weak materials, they may
provide a relatively direct record of paleostress in orogenic belts. q 1998 Elsevier Science B.V. All rights reserved.
Keywords: Magmatic fabric; Fabric pattern; Pluton
1. Introduction
The pioneering work of Hans Cloos ŽCloos, 1925.,
popularized in the United States by Robert Balk
ŽBalk, 1937., led to widespread acceptance of the
importance of magmatic fabrics Žfoliations and lineations. for understanding processes within magma
chambers. These structures have since been interpreted to reflect a variety of features: Ž1. magmatic
flow planes and directions ŽBalk, 1937; Philpotts and
Asher, 1994. that form during Ža. mechanically
driven flow within chambers ŽAbbott, 1989; Tobisch
and Cruden, 1995., Žb. thermal or compositionally
)
Corresponding author. Tel.: q1-213-740-6103; Fax: q1-213740-8801; E-mail: paterson@usc.edu
driven convection ŽBarriere, 1981., Žc. magma surges
Že.g., Murray, 1979; Huppert et al., 1986., Žd. the in
situ expansion of magma chambers ŽHolder, 1979;
Ramsay, 1989., or during Že. passive fault-controlled
dilation of chambers ŽGuineberteau et al., 1987;
Hutton, 1988.; Ž2. the shapes of intrusions, with the
assumption that fabrics are parallel to the walls of
the magma chambers ŽDavis, 1963; Vigneresse,
1990.; Ž3. strain orientations in deforming magmas
ŽBlumenfeld and Bouchez, 1988; Benn and Allard,
1989.; and Ž4. fabrics formed during regional deformation Že.g., Hutton, 1982; Archango et al., 1994..
The interpretation of fabric patterns has also
played a prominent role in establishing timing relationships of structures in and around plutons ŽPitcher
and Berger, 1972; Paterson and Tobisch, 1988.. Since
0024-4937r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 4 - 4 9 3 7 Ž 9 8 . 0 0 0 2 2 - X
54
S.R. Paterson et al.r Lithos 44 (1998) 53–82
so much has been inferred from these structures, and
because many of these inferences are contradictory
or poorly constrained, it is important to reassess how
and when magmatic fabrics form and thus what
information can be inferred from fabric patterns.
Although the contributions of Cloos and Balk
represent milestones in geologic thought, certain aspects of their terminology impose a conceptual bias
that hampers progress. Their term ‘primary flow
fabric’ blends three separate concepts into a single
phrase: grain-scale deformation mechanisms, relationship of fabric geometry to particle displacement
paths, and driving forces of fabric formation. According to Balk Ž1937., Ž1. a primary flow fabric
results from the hydrodynamic alignment of mineral
grains suspended in melt, Ž2. planar and linear mineral fabrics define flow planes and lines, respectively, and Ž3. map-scale fabric patterns form exclusively by internal magma chamber processes. Magmatic foliations and lineations continue to be interpreted as flow planes and lines Že.g., Philpotts and
Asher, 1994; Tobisch and Cruden, 1995.. Furthermore, confusion still exists in the literature as to
whether magmatic fabrics are formed exclusively by
internally driven processes. We use the term ‘magmatic fabric’ to refer to foliations, mineral lineations,
and associated microstructures formed during crystal
alignment in the presence of a melt without any
inference about the forces that formed these structures.
Some workers have challenged the assumptions of
Cloos and Balk. Berger and Pitcher Ž1970. concluded that preferred orientations of magmatic crystals represent strain of a crystal–melt mush, and
emphasized that regional deformation may play an
important role in forming magmatic fabrics Žsee also
Hutton, 1988; Bouchez et al., 1990.. Many recent
studies agree that alignment of igneous minerals
records strain during flow and crystallization of a
rheologically complex mush Že.g., Bergantz and
Dawes, 1994; Ildefonse et al., 1997.. Others have
summarized criteria for the recognition of melt-present deformation Že.g., Hutton, 1988; Paterson et al.,
1989; Bouchez et al., 1992.. Magma rheology is
partially constrained by experimental studies on crystalrmelt systems ŽIldefonse et al., 1992; Means and
Park, 1994; Rushmer, 1995.. Numerical and analogue experiments have helped to establish the rela-
tionships between fabric geometry, strain, and variable flow regimes for both magmatic and subsolidus
fabrics Že.g., Hanmer and Passchier, 1991; Ildefonse
et al., 1992; Tikoff and Teyssier, 1994..
In contrast, an understanding of the driving forces
responsible for magmatic fabrics remains elusive.
Previous studies have variously attributed the cause
of fabric formation to magma ascent, emplacement,
tectonic strains, or internal magma chamber processes, usually with insufficient justification for distinguishing between the possibilities. For example,
ballooning emplacement models assume that magmatic fabrics record in situ magma chamber expansion ŽHolder, 1979; Ramsay, 1989., whereas Cruden
Ž1990. showed that these same fabric patterns and
intensities could be formed by internal convection
without any chamber expansion. Fault-assisted emplacement models assume that magmatic lineations
record the direction of ‘tectonic opening’ ŽOlivier
and Archanjo, 1994., whereas others have assumed
that similar magmatic lineations reflect regional strain
of a preexisting magma chamber Že.g., Ferre et al.,
1995; Miller and Paterson, 1995..
Following Balk’s example, some of the ambiguity
inherent in these interpretations can be illustrated by
the patterns of foam on the surface of a stream. In
Fig. 1, foam bands in a pool record the deformation
field at the water’s surface and delineate a pattern
similar to fabric patterns seen in some plutons. The
figure also shows particle displacement paths as
determined by tracking individual bubbles through
four photographed time steps, information no longer
available in studies of magma flow in plutons. The
relationship between strain and particle displacements in this example is complex; flow lines lie at
every angle to local finite extension directions. Furthermore, the vorticity of flow is not apparent from
the final strain field; what looks like a growing
plume is actually a migrating whirlpool. Many workers have attributed similar fabric patterns in plutons
to inflation of a magma chamber. However, the
patterns in the foam do not represent expansion of
the ‘chamber’ Ži.e., the pool. but merely record the
circulation of a low viscosity material Žwater. inside
a chamber with rigid walls Žrock.. Additionally, the
foam patterns entirely postdate creation of the pool
itself and thus contain no information about how the
‘chamber’ was formed. Although the viscosity con-
S.R. Paterson et al.r Lithos 44 (1998) 53–82
55
Fig. 1. Line drawing, from four time-steps captured in a series of photographs, of strain of foam on water flowing into a pool. Hatched
pattern shows rocks enclosing the pool, white shows water. Thin dashed and solid lines represent bands of foam on the pool surface. Arrows
show displacement paths of individual bubbles in the foam tracked through the four photographs. Due to the large strain magnitudes, foam
bands lie subparallel to the local direction of greatest finite extension. Note that flow lines Žbubble displacement paths. lie at all angles to
local extension directions Žlengths of foam bands.. Also note ‘shear bands’ in foam at upper left of figure. Photos obtained and line drawing
completed by Ken Fowler.
trast between granitic magma and host rock is smaller
than that for water and rock, it is probable that a
similar decoupling of flow can occur in magma
chambers. Furthermore a mechanical contrast at the
pluton-host rock contact will induce contact-parallel
magmatic structures, regardless of the cause of magmatic strain ŽPaterson and Tobisch, 1988.. Thus, we
believe it is no simple matter to infer large-scale
kinematics or causes of magmatic fabric formation
from map patterns alone.
Below we incorporate data from our recent studies of fabric patterns with recent work of others into
a discussion about the challenges faced when interpreting magmatic fabric patterns. We then suggest an
approach to evaluate fabric patterns, and finish by
reexamining the validity of the inferences made from
fabric patterns in granitoids.
2. Challenges faced during interpretation of fabric patterns
2.1. Inferring displacement from strain patterns and
particle behaÕior
To interpret displacement paths of magma from
the final configuration of magmatic fabrics, relationships between flow, strain, and particle behavior
must be established. Flow of magma Žmelt " crystals.
can be described using a displacement or velocity
56
S.R. Paterson et al.r Lithos 44 (1998) 53–82
vector field. We define the flow direction as the
direction of particle paths, and the flow plane as the
plane containing the flow direction and perpendicular to any velocity gradient. Three end-member types
of flow, uniform, nonuniform, and turbulent, may
occur in magma chambers. In uniform flow, the
velocity vectors within the magma all have the same
magnitude and orientation, and any markers Že.g.,
crystals or xenoliths. behave in a passive manner.
Therefore, uniform flow is rarely if ever achieved
Fig. 2. Summary diagram showing convergent ŽA., divergent ŽB., and non-coaxial ŽC. flows after Mackin Ž1947., and ŽD. internal flow
pattern during convection in a non-expanding magma chamber after Schmeling et al. Ž1988.. In D paths with arrows are flow directions and
ellipses are resulting strain ellipses. Also shown are locations of types of simple non-uniform flows ŽA, B, C.. In A, B, C, ellipses are shapes
of enclaves, thin parallel lines are orientation of tabular crystals, and arrows are flow directions. Note that during simple convection ŽD., it
takes unusual circumstances to obtain flow paths that are parallel to ellipse axes.
S.R. Paterson et al.r Lithos 44 (1998) 53–82
since magmas have complex variations in composition, physical properties and dynamics and, in addition, contain many nonpassive markers.
Mackin Ž1947. described three types of simple
nonuniform flow, which he called acceleration, deceleration, and velocity-gradient flow ŽFig. 2a., and
evaluated the resulting particle behavior and strain.
Acceleration or convergent flow occurs where magma
is transported from a wide region into a narrow
channel, thus requiring an increase in velocity and a
convergence of flow lines. Resulting strains are constrictional and crystals tend to align their long axes
with the X axis Ž X ) Y ) Z . of the magma strain
ellipsoid. Thus, at high strains, a linear prolate fabric
forms parallel to the flow direction.
Deceleration or divergent flow occurs where
channelized magma spreads out into a broader region
and slows down ŽFig. 2b., resulting in the divergence
of flow lines. This results in flattening, or oblate,
strain with the XY plane of the strain ellipsoid at
high angles to flow directions. Particles tend to align
their largest crystal faces with the XY plane of the
57
strain ellipsoid and thus form a foliation at high
angles to flow planes and flow directions.
Progressive noncoaxial flow occurs whenever
there is drag along a boundary surface, for example
where a viscosity contrast exists between two parts
of the flow ŽFig. 2c.. It may be common in sheet-like
magma bodies, along the margins of chambers, and
at crystallization fronts within chambers because of
the rheological gradients in these settings. Assuming
simple shear along planar surfaces, flow lines remain
parallel and triaxial plane strain results with the XY
plane and X axis of the strain ellipsoid initially at
458 to flow planes and flow directions, respectively.
Therefore, in magmas that undergo only a small
amount of strain in noncoaxial flows, foliations and
lineations form at angles to the flow planes and
directions, but rotate towards parallelism with increasing strain.
Examples of these three types of nonuniform flow
are displayed in the studies of Schmeling et al.
Ž1988. and Cruden Ž1990. of convection in a spherical diapir ŽFig. 2d.. In their models, convergent flow
Fig. 3. Chart summarizing the types of fabrics that can form from various combinations of uniform, non-uniform, and turbulent flows and
passive and non-passive marker behavior.
58
S.R. Paterson et al.r Lithos 44 (1998) 53–82
occurs in the central portion of the diapir, divergent
flow occurs where magma reaches the upper parts of
the chamber and spreads out towards the chamber
margins, and progressive noncoaxial flow occurs
along the chamber walls.
Laminar flows more complicated than those discussed by Mackin Ž1947. may occur in magmas in
which final strain, and thus fabric, is a function of
vorticity and three perpendicular principal stretching
rates Že.g., Means, 1994.. Under these circumstances
the idea of flow planes and directions is less useful
and Passchier Ž1997. has proposed the concept of
planar or linear ‘fabric attractors’ towards which all
material lines rotate. Even more complex or chaotic
laminar flows may form because velocity gradients
and fabric attractors may be temporally variable
andror spatially nonplanar. With even greater complexity of particle paths, complex laminar flow may
grade into turbulent flow. Tritton Ž1988. states that
‘‘turbulence is a state of continuous instability. Each
time a flow changes as a result of an instability,
one’s ability to predict the details of the flow are
reduced. When successive instabilities have reduced
the level of predictability so much that it is appropriate to describe flow statistically, rather than in every
detail, then one says that the flow is turbulent.’’ For
our purposes, we simply note that during turbulent
flow the displacement vectors are spatially and temporally highly variable, and thus no simple geometrical or temporal relationship exists between flow and
fabrics Že.g., Martin and Nokes, 1988.. Below we
argue that preserved fabrics largely form near magma
solidi, conditions under which turbulent flow will
usually not occur Že.g., Brandeis and Marsh, 1989..
Thus laminar flow is the most likely type of flow
during fabric formation in granitoids.
Magmatic fabric development is further complicated because of differences in behavior of passive
and nonpassive markers ŽFig. 3.. Nonpassive mark-
ers in melts undergoing uniform flow follow Stokes
law of settling Žif particle interactions can be ignored. and tend to align their long axes parallel to
the flow direction and their long and intermediate
axes in the flow plane ŽShaw, 1965; Folkes and
Russell, 1980; Martin and Nokes, 1988.. However,
the presence of nonpassive markers in a uniformly
flowing material potentially leads to eddies and instabilities in the flow immediately surrounding the
particle ŽTritton, 1988; Ildefonse et al., 1992..
In the case of nonuniform flow, nonpassive markers, and rare particle–particle interactions, marker
behavior is a function of Ž1. velocity gradients, Ž2.
coupling between the matrix flow and markers, and
Ž3. aspect ratios and initial orientations of the markers Že.g., Fernandez, 1987; Hanmer and Passchier,
1991.. Markers with smaller axial ratios tend to
rotate faster and more than those with larger axial
ratios. Thus, at small strains Žg - 5. they tend to
show a greater degree of parallelism with the shear
plane but at higher strains are more likely to rotate
past the shear direction or flow plane and thus are
less likely to approach parallelism with the shear
direction ŽNicolas, 1992.. At larger strains Žg ) 5.
markers with larger axial ratios are more likely to be
parallel with the flow direction ŽFernandez and Laporte, 1991.. Markers with smaller axial ratios develop statistical maxima around these positions
ŽFernandez, 1987; Ildefonse et al., 1992.. Examples
of such particle alignment have been observed in
many material science experiments Že.g., Folkes and
Russell, 1980; Ildefonse et al., 1992.. However, Park
and Means Ž1996. and Ildefonse et al. Ž1997. note
that when significant particle–particle interactions
occur, particle behavior becomes even more complex, and the preferred orientation of particles does
not have a simple relationship to strain. In crystallizing magmas, mineral alignment will also depend on
an increasing frequency of crystal–crystal interac-
Fig. 4. Ža. Regional pattern of fabrics in and around the Mount Stuart batholith. Foliations in the batholith are largely magmatic. Foliations
in host rock represent average orientations of most pervasively developed fabric at each location. Foliation dips indicated as follows: 08 to
298 filled squares, 308 to 598 filled triangles, and 608 to 908 no dip symbol. Note that magmatic fabric overprints internal compositional
variations and that the host rock fabric pattern is only deflected within a short distance from pluton. D s dunite and P s metapsammite
stoped blocks shown in block diagram insets. In these insets lines represent magmatic foliation patterns around stoped block. Žb. Regional
pattern of mineral lineations in and around the Mount Stuart batholith. Arrows are mineral alignment lineations with plunges of 08 to 198 Žno
arrowhead., 208 to 498 Žfilled arrowhead., and ) 508 Ždots..
S.R. Paterson et al.r Lithos 44 (1998) 53–82
59
60
S.R. Paterson et al.r Lithos 44 (1998) 53–82
tions, increased coupling at the crystal–melt interface, continued addition of new crystals, and growth
of existing crystals, all of which will complicate the
development of preferred orientations in marker populations Že.g., Kerr and Lister, 1991; Ildefonse et al.,
1997..
The above discussion indicates that one of the
first challenges in interpreting magmatic fabric patterns in plutons is to find methods of using the
resulting strain pattern to determine the types of
displacement paths Ži.e., probably various types of
nonuniform flow. that occurred during formation of
the foliation and lineation. This may be difficult
when the final strain pattern is a function of the
variable behavior of different populations of markers
undergoing complex interactions ŽFig. 3.. Without
knowledge about marker behavior and flow kinematics, it is difficult to uniquely infer displacement paths
in chambers from final fabric geometries.
2.2. Determining the timing of fabric formation
The timing of fabric formation is central to understanding whether fabrics are formed during ascent,
emplacement, post-emplacement internal processes,
or regional tectonics. Several authors have noted
that, during cooling and crystallization of magma
bodies, a ‘crystal-mush zone’ forms at the edges of
the body and migrates inwards with time Že.g., Marsh,
1989; Tait and Jaupart, 1990; Bergantz, 1991.. These
crystal-mush zones represent the transition from low
viscosity Newtonian melts with lower crystal contents to higher viscosity Bingham magmas near their
solidi, a transition during which we believe magmatic fabrics are ‘frozen in’ and preserved
ŽYoshinobu et al., in press.. With increasing size or
emplacement depth of plutons, or in regions with
high geothermal gradients, the crystal-mush zone
may take hundreds of thousands to millions of years
to migrate through a pluton Že.g., Hanson and
Glazner, 1995; Yoshinobu et al., in press.. Thus, the
timing of magmatic fabric preservation relative to
other processes, such as magma ascent or chamber
construction, is less certain.
Four lines of evidence indicate that magmatic
fabrics generally form during the final stages of, or
following magma chamber construction in granitoids, and very late in the physical evolution of
magmas: Ž1. fabric relationships around stoped
blocks, Ž2. relationships between fabrics and compositional boundaries in shallow to mid-crustal plutons,
Ž3. relationships between fabrics and intrusive sheet
margins in mid- to deep crustal plutons, and Ž4. the
presence of magmatic folds in plutons.
We have recently published descriptions of stoped
blocks in two plutons, which help constrain the
timing of fabric formation ŽFowler and Paterson,
1997; Paterson and Miller, in press.. In the Sierra
Nevada, stoped blocks Ž10 m to ) 100 m in length.
of the 115 Ma Lodgepole granite occur approximately 360 m below the roof of the 98 Ma Castle
Creek granodiorite ŽFowler, 1994.. Magmatic fabrics
developed throughout the Castle Creek granodiorite
are deflected only within 1 m and locally only within
a few centimeters of the stoped blocks. Enclave
ratios and qualitative fabric intensities in this pluton
do not deviate from chamber wide values even short
distances from the blocks and thus do not begin to
approach the orientation or magnitudes of strain
predicted around sinking objects ŽFowler and Paterson, 1997..
Stoped blocks near the roof of the 93 Ma Mount
Stuart batholith, Washington and ; 1 km below the
roof ŽFig. 4. show fabric relationships identical to
those of the Castle Creek example. Magmatic foliation and locally lineation are well developed in this
batholith and have complex patterns that will be
discussed later. Meter to decameter scale stoped
blocks of host rock occur meters to hundreds of
meters below the roof of this batholith and have
internal metamorphic fabrics indicating that some of
the blocks have rotated with respect to the roof. As
in the Castle Creek granodiorite, only within 2 m or
less do magmatic fabrics show any deflection near
the blocks ŽFig. 4.. Instead fabrics maintain
regional-scale orientations and mimic structural patterns in the surrounding host rock. We have also
examined fabric patterns around several stoped blocks
preserved ; 1 km below the roof. Again these fabric
patterns contrast with those predicted for block settling but are compatible with the deflection from
regional trends around stationary blocks ŽPaterson
and Miller, in press..
In both of the above plutons, the stoped blocks
and discordant, stepped roof contacts indicate that
stoping occurred during the final process of chamber
S.R. Paterson et al.r Lithos 44 (1998) 53–82
construction. Since the blocks are now trapped in the
chamber and, in some cases, have not moved far
from their points of origination, they are the last
blocks to have formed. Furthermore, magma viscosities andror yield strengths must have been high
enough to prevent further sinking of these large
blocks, indicating that the magmas were crystal-rich
Že.g., Sparks et al., 1977; Paterson and Miller, in
press.. Since magmatic fabrics follow regional patterns up to the blocks, with local variations consistent with deflections around a previously existing
object, we argue that the fabrics do not record strain
formed during magma ascent, early emplacement, or
block sinking. Instead they record strain that occurred after the crystal-rich magmas trapped the
blocks during or after final chamber construction.
Elliptical Žin map view., normally zoned plutons
are common at mid-crustal depths in magmatic arcs
Že.g., Buddington, 1959; Bateman, 1992.. In some of
these bodies, magmatic foliations are concentric and
parallel to the host-rock contacts, but in others the
patterns are more complex. The most striking aspect
61
of these patterns in both types of plutons is that
fabrics commonly cut across internal contacts ŽFig.
5. and overprint gradational compositional changes
Že.g., Buddington, 1959; Bateman, 1992; Paterson
and Vernon, 1995.. In some plutons, the magmatic
fabrics refract across internal contacts Že.g., Compton, 1955. or are locally parallel to internal contacts.
However, it is also common for the magmatic fabrics
to follow smooth trend lines across the contacts.
These observations imply the following: Ž1. although
probably time-transgressive, magmatic fabrics postdate the juxtaposition of compositional phases at any
single locality; Ž2. fabrics were not formed by flow
during vigorous pluton-wide convection, since convection would alter or destroy the compositional
heterogeneities; Ž3. large viscosity contrasts did not
exist across the internal contacts when the fabrics
were ‘frozen in’ Žfor an exception see Compton,
1955.; and Ž4. magmatic fabrics must have formed
over a relatively short time interval, specifically after
ascent and juxtaposition of separate magma pulses,
but before final crystallization.
Fig. 5. Summary map of the Early Cretaceous White Creek Batholith and enclosing host rocks, southeast British Columbia, after Reesor
Ž1958. and Brandon and Lambert Ž1994.. Bold lines in pluton with diamond symbols represent trend lines of magmatic foliations. Thin lines
in host rocks show lithological contacts and bedding trends, bold lines show pre-emplacement faults.
62
S.R. Paterson et al.r Lithos 44 (1998) 53–82
S.R. Paterson et al.r Lithos 44 (1998) 53–82
We have recently examined several mid- to deepcrustal plutons, including the Entiat and Cardinal
Peak plutons in the Cascades Mountains ŽWashington, USA. and the Main Donegal granite ŽIreland..
These plutons display spectacular layering, ranging
from millimeter-scale, compositionally and texturally
distinct bands, to kilometer-scale sheets ŽPitcher and
Berger, 1972; Yuan and Paterson, 1993b; Miller and
Paterson, 1995.. Magmatic fabrics in the Entiat and
Cardinal Peak plutons, are sometimes parallel to and
sometimes crosscut internal layering ŽFig. 6.. Timing
relationships indicate that the host rock fabric is
continuous with the magmatic fabric ŽMiller and
Paterson, 1995.. In the Main Donegal pluton, magmatic fabrics are generally subparallel to sheets but,
in detail, there are many examples of fabrics cutting
sheets ŽPitcher and Berger, 1972; Yuan and Paterson,
1993b.. We interpret the above relationships in all
three plutons to indicate that Ž1. the host rock fabrics
are typically continuous with the magmatic fabrics,
Ž2. the magmatic fabrics postdate formation of internal sheeting, and Ž3. the magmatic fabrics formed
after the magma chambers were constructed, but
before the plutons completely crystallized ŽMiller
and Paterson, 1995..
Magmatic folds Ži.e., hypersolidus folding of
magmatic fabrics, layering, or enclaves. occur in all
three of these plutons ŽPitcher and Berger, 1972;
Yuan and Paterson, 1993b., sometimes with subsequent magmatic fabrics parallel to their axial planes.
There is evidence that these magmatic folds formed
at the same time as folding in the host rock ŽPaterson
et al., 1994b, Miller and Paterson, 1995.. Fig. 7
shows a photograph Ža. and line drawing Žb. of a
magmatic fold in the Entiat pluton in which most of
the rock consists of aligned igneous plagioclase and
hornblende now defining an upright, tight fold. Some
late, poikilitic, igneous hornblende occur parallel to
the axial plane of the fold. Only minor syn-folding
subsolidus deformation is observed and there are no
optical differences between folded and axial-planar
63
hornblende crystals. The large number of igneous
grains that define the folded fabric and the poikilitic
texture of the axial-planar hornblendes suggest that
the fabric formed and was then folded in the presence of only a small amount of melt.
In summary, we have described magmatic fabrics
that Ž1. postdate the settling of stoped blocks near
and far from pluton roofs, Ž2. overprint internal
contacts in shallow to mid-crustal plutons, Ž3. overprint internal sheets or compositional layering and, in
places, chamber margins in mid- to deep-crustal
plutons, and Ž4. define magmatic folds. These observations indicate that the magmatic fabrics formed
late in the crystallization histories of these intrusive
bodies, and largely after the chambers were constructed. They therefore preserve information about
neither ascent nor most of the emplacement histories.
Although we suspect this is a widespread phenomenon, we believe that this conclusion must be
evaluated for each pluton. Therefore, a further challenge in interpreting magmatic fabric patterns is to
establish the timing of fabrics relative to other igneous processes Že.g., formation of internal contacts.,
to chamber construction processes Že.g., stoping and
expansion., and to regional deformation.
2.3. Poor strain memory of magmatic fabrics
Intuitively, magmatic fabrics are poor recorders of
total strain, since fabrics are commonly weakly developed in melts that have ascended long distances
by nonuniform flow. Several lines of evidence indicate that preserved fabrics only record the youngest
increments of strain during magma crystallization.
We have described examples in which strain caused
by the sinking of stoped blocks is not recorded by
fabrics surrounding the blocks but instead record
younger increments of strain. It is also informative to
compare axial ratios of microgranitoid enclaves in
plutons to adjacent fabric ellipsoids calculated from
crystal shapes and preferred orientations ŽTable 1..
Fig. 6. Simplified structural map and cross section of the Entiat and Cardinal Peak plutons, Washington, showing location of mappable
sheets and more irregular masses. These plutons are internally complex and particularly in the NW third of the Entiat pluton and along
margins of both plutons consist of 100s of meter-scale sheets. Note that foliation patterns, largely magmatic and even more complex than
displayed, define large and small-scale folds Že.g., Fig. 7., sometimes crosscut internal sheeting, and are typically continuous with fabrics in
the host rock.
64
S.R. Paterson et al.r Lithos 44 (1998) 53–82
Fig. 7. Photograph Ža. and line drawing Žb. of a magmatic fold from the Entiat pluton showing folded magmatic foliation defined by aligned
hornblende and some plagioclase overprinted by aligned magmatic, poikilitic hornblende parallel to axial plane of fold. We suggest that the
percent of minerals which define the folded foliation and presence of igneous hornblende parallel to the axial plane of the fold indicate that
the fabric formed in a crystal-rich magma during grain supported flow.
Such comparisons are not straightforward, since data
sets are obtained using different techniques Že.g.,
Rfru , Fry, and averages of ratios. and different sizes
of populations Ža single enclave vs. multiple minerals., and since data are collected at different scales
Žthin sections to large outcrops.. In spite of these
difficulties, some general trends are recognizable.
For example, near the roof of the Mount Stuart
batholith, consistently oriented enclaves have constrictional shapes with XrYrZ ratios ranging from
3r1.25r1 to 23r7r1. Nearby fabric ellipsoids typically have ratios of 2r1 or less ŽTable 1.. In the Inyo
Batholith, California, our initial studies indicate that
crystal fabric ellipsoids typically have XrZ ratios of
less than 2.5 to 1, even where adjacent to enclaves
with XrZ ratios as high as 25r1. In the Ardara
pluton, Ireland, enclaves near the northern margin
have XrZ ratios in horizontal and vertical exposures
as large as 14r1, whereas nearby crystal fabric
ellipsoids have ratios of 1.3r1 for plagioclase and
biotite in both horizontal and vertical surfaces. In
part, these results may reflect limitations of the
techniques used, particularly since maximum ratios
for magmatic crystals are limited by crystal size
S.R. Paterson et al.r Lithos 44 (1998) 53–82
65
Table 1
Comparison of magmatic fabric and enclave ratios for several plutons
Pluton
Fabric ratios
X
Gelles Žaverage.
Ardara
plagioclase
biotite
Y
6.5
1.3
1.2
Enclave ratios
X
Y
Z
1.0
2.2
14.0
12.0
1.0
1.0
Fernandez and Laporte Ž1991.
Holder Ž1979.
3.0
23.0
1.3
7.0
1.0
1.0
Paterson Žunpublished.
1.0
Paterson Žunpublished.
1.0
1.0
Paterson Žunpublished.
1.0
1.0
Mount Stuart
plagioclase
Beer Creek
plagioclase
biotite
quartz
Joshua Flat
1.0
plagioclase
biotite
quartz
1.1
1.3
1.2
Reference
Z
1.8
1.0
10.0
1.1
1.3
1.3
1.0
1.0
1.0
22.0
40.0
12.0
3.0
1.0
1.0
1.0
Žthus affecting Rfru ratios. and crystal interactions
and final packing may alter center-to-center distances Žthus affecting Fry results.. However, inspection of thin sections for all the above examples
shows that statistically significant numbers of crystals have their long axes at high angles to fabric
directions. This supports the conclusion that grain
preferred orientations in these plutons are not particularly intense.
Paterson and Vernon Ž1995. summarized evidence
that enclaves are not good recorders of the total
strain experienced by the magma. The above enclave
ratios support this notion, given that all these magmas probably traveled tens of kilometers, but have
enclaves with ratios of 25r1 or less. Adjacent crystal
fabrics typically record an order of magnitude less
strain than enclaves or enclave populations. In fact,
we are impressed with just how weak crystal alignments typically are, commonly defined by fabric
ellipsoids with ratios of less than 3r1, even in
plutons with prominent magmatic foliations andror
lineations visible in the field.
These data support the following conclusions: Ž1.
it only takes a small amount of strain to form
magmatic fabrics and thus they can be easily reset;
Ž2. preserved fabrics record the final increments of
strain before magmas lock up near their solidi; and
Ž3. because fabrics record only a small increment of
strain, they have a greater chance of not being
parallel to flow planes and directions during noncoaxial flow. Thus an additional challenge in interpreting fabric patterns is to determine which increment of strain is preserved and whether different
increments are preserved in different parts of the
pluton.
2.4. Interpreting multiple magmatic fabrics in a single body
The examples above indicate that different objects
in magma may record different amounts of strain,
even though they are subparallel. It is also increasingly recognized that multiple magmatic fabrics with
different orientations may form in a single pluton.
Examples have been reported by Bouchez et al.
Ž1981. in the Guerande pluton ŽFrance., Blumenfeld
and Bouchez Ž1988. in sheared migmatites and orthogneisses, Bilodeau and Nelson Ž1993. in the Sage
Hen Flat pluton ŽCalifornia., and Schulmann et al.
Ž1997. in the deep borehole EPS-1 near Soultzsous-Foret ŽFrance.. We have examined several examples including: Ž1. a magmatic foliation defined
by plagioclase and hornblende alignment at high
angles to aligned enclaves in the Dirty Face pluton,
66
S.R. Paterson et al.r Lithos 44 (1998) 53–82
S.R. Paterson et al.r Lithos 44 (1998) 53–82
Washington, Ž2. two magmatic foliations in the Main
Donegal pluton, Ireland, one parallel to and one
oblique to layering ŽYuan and Paterson, 1993a., and
Ž3. folded and cross-cutting axial-planar magmatic
foliations in the Entiat and Mount Stuart plutons,
Washington. Explanations for multiple magmatic
fabrics include: differential rotation of minerals with
different shapes or axial ratios during noncoaxial
strain ŽBlumenfeld and Bouchez, 1988.; formation of
metastable orthogonal linear fabrics under a combination of pure and simple shear ŽWillis, 1977.; and
formation of orthogonal fabrics where minerals with
different axial ratios are aligned parallel to unequal
elongation components during coaxial flow ŽJezek et
al., 1994; Schulmann et al., 1997..
Thus another challenge when interpreting fabric
patterns is to determine whether multiple magmatic
fabrics exist and, if so, to evaluate their relationships
with the overall fabric pattern in the pluton. This is
particularly problematic for fabric patterns determined from anisotropy of magnetic susceptibility
analyses Že.g., Bouchez, 1997. because this technique only provides integral information of magnetic
minerals and assumes that fabric populations have
orthorhombic symmetry.
2.5. Deformation mechanisms and magma rheology
during fabric formation
Inferences of the timing and causes of magmatic
fabric formation could be placed on firmer ground if
the deformation mechanisms controlling the rheological behavior during fabric formation were established. The late formation of fabrics indicates that it
is particularly important to understand magma behavior immediately above the solidus.
Figs. 8 and 9 provide a preliminary summary of
information about relationships between viscosity,
temperature, crystal percentages, and deformation
mechanisms for basaltic and granitic magmas. Fig. 8
67
emphasises the well-known fact that magma rheology changes dramatically during the transition from
hypersolidus to solidus conditions, as crystal percentages increase from ; 50% to 100% and magma
viscosities increase tenfold due to decreasing temperature ŽWickham, 1987; Cruden, 1990., and thus increasing melt polymerization ŽLofgren, 1980. and
increasing grain–grain interactions. Methods for calculating melt viscosities exist and now can be corrected for crystal interactions ŽWildemuth and
Williams, 1984; Lejeune and Richet, 1995..
Many recent studies indicate that, as magmas
crystallize and change from suspension to grain supported flow, their behavior changes from Newtonian
Ž- 35% crystals., to Bingham andror power law
behavior at high crystal contents Že.g., Webb and
Dingwell, 1990; Fernandez and Gasguet, 1994.. The
relationship between viscosity and crystal content is
complex and depends on composition Že.g., Ryan
and Blevins, 1987., crystal shape ŽKerr and Lister,
1991., pressure and temperature Že.g., Kushiro, 1980;
Scaillet et al., 1997., volatile content Že.g., Scaillet et
al., 1997., and strain rate ŽWebb and Dingwell,
1990; Weinberg and Podladchikov, 1994.. Thus the
transitions from Newtonian to Bingham andror
power law behavior remain poorly defined Žbut see
excellent summary by Dingwell et al., 1993..
Parts of Figs. 8 and 9 are constrained by experimental results. Dell’Angelo et al. Ž1987. experimentally deformed fine-grained Ž2 to 50 mm. granite
aggregates at 9008C, 15 kbar, fluid present melting
with H 2 O contents of - 1 wt.%, strain rates of 10y6
sy1 , and total strains of 48% to 65%. The dominant
deformation mechanism was dislocation creep for
coarser-grained aggregates with 0% to 3% melt,
whereas melt-enhanced diffusion creep was dominant for aggregates with grain sizes less than 10 mm
and with 3% to 5% melt. This study suggests that
crystal plasticity, and particularly melt-assisted diffusion creep, may occur during the final stages of
crystallization.
Fig. 8. Cartoon illustrating approximate magma viscosity as a function of crystal content and temperatures for basaltic and granitic magmas
crystallizing in a closed system. Sources for constraints on positions and shapes of the two curves are shown on curves in italics. Positions
of curves are further constrained by theoretical calculations of Bergantz and Dawes Ž1994.. Also shown are the approximate locations of
hypersolidus, near-solidus, and subsolidus regions, transitional zone from magmas dominated by suspension versus grain supported flow,
and where some of the deformation mechanisms discussed in the text are particularly important. The probable transition from Newtonian to
Bingham behavior is indicated along the viscosity scale.
68
S.R. Paterson et al.r Lithos 44 (1998) 53–82
Fig. 9. Chart showing at what degrees of crystallinity different deformation mechanisms likely operate in magmas. Some processes Že.g.,
grain boundary sliding. may occur by different mechanisms, the latter of which are listed along the horizontal lines. Boundaries between
regions in which different mechanisms operate are not well constrained.
Rutter and Neumann Ž1995. completed fluid-absent melting of cores of Westerly granite at 3 kbar
and 10y7 sy1 strain rates. The lower pressures and
fluid-absent melting resulted in brittle deformation
during initial melting. Through-going shear zones
developed in samples with less than 10% melt
whereas, in samples with 10% and 40% melt, melt
was squeezed from individual pockets into throughgoing fractures. Above 40% melt, unfractured grains
were passively carried by flowing melt.
Wolf and Wyllie Ž1991. examined microstructures
and viscosities of cores of amphibolite Ž70% hornblende, 30% plagioclase. during fluid-absent melting
at 10 kbar and 8508 to 10008C. The amphibolite
began to melt at 8508C. Melt interconnectivity was
obtained by 8758C with 2% melt, although estimated
viscosities remained high. By 8508 to 9008C low
viscosity layers formed, but not until the samples
contained 30% melt, near 9758C, did the rock mass
become mobile.
Rushmer Ž1995. performed fluid-absent melting
experiments on amphibolites at strain rates of 10y5
sy1 , 18 kbar, and temperatures of 6508 to 10008C.
Melt first appeared at about 8508C, with deformation
mechanisms changing as follows. Ž1. At near-solidus
conditions Ž8008 to 9008C and 1% to 5% melt.,
crystal plastic behavior of quartz and plagioclase
occurred while hornblende showed a transition from
brittle to plastic behavior with increasing temperature. Ž2. At 9358C, as melt increased from 5% to
15%, pervasive fracturing and development of
through-going shear zones were common. Ž3. At
10008C and G 20% melt, a breakdown of the loadsupporting crystal network occurred, resulting in
S.R. Paterson et al.r Lithos 44 (1998) 53–82
grain-boundary sliding ŽGBS., some crystal–crystal
interaction, and no evidence of focused deformation.
Rushmer Ž1995. concluded that embrittlement caused
by the presence of melt may be an important nearsolidus deformation mechanism, and that above 20%
melt, suspension flow occurred, during which total
strain was not recorded by crystal fabrics. ŽSee also
Arzi, 1978; Van der Molen and Paterson, 1979..
A magma analogue, a poly-phase thiocyanate
compound, was used by Means and Park Ž1994. and
Park and Means Ž1996. to examine processes across
the transition from hypersolidus suspension flow,
through near-solidus grain-supported flow, to subsolidus viscous flow. Results from their dynamic
crystallization experiments indicate that GBS aided
by crystal plasticity and dynamic recrystallization
dominated at high strain rates Ž) 10y5 sy1 ., whereas
GBS aided by contact melting dominated at lower
strain rates ŽG 10y6 sy1 .. GBS, aided by contact
69
melting, is thought to occur in gabbros with - 20%
melt ŽNicolas and Ildefonse, 1996.. Park and Means
Ž1996. also noted that crystal plasticity aided the
processes of filter pressing and grain-supported flow.
They suggested that such filter pressing may be
recognizable in natural layered systems where layers
with deformed crystals and one composition Ži.e.,
filter-pressed layers. occur within layers of another
composition that show only igneous microstructures.
Grain-supported flow in these experiments occurred
by melt-enhanced GBS, which accomodated both
distributed and discrete strain. The distributed strain
produced a grain-shape-preferred orientation in all
phases of the rock analogue, and therefore occurred
with little melt present ŽFig. 7.. Discrete strain occurred by melt-enhanced GBS along shear surfaces,
which Park and Means noted may be very difficult to
identify in plutonic rocks ŽFig. 10. if shear surfaces
do not draw in or expel melt.
Fig. 10. Photograph of schlieren offset along magmatic faults in the Dinky Creek pluton, Sierra Nevada. Offset of several centimeters occurs
without any mineral preferred orientation being developed along the fault. Subsolidus microstructures are not localized along the fault and
some igneous crystals grow across the fault suggesting that the offset occurred by melt-assisted grain boundary sliding and stopped before
final crystallization. Note the slight concentration of mafic minerals along the fault.
70
S.R. Paterson et al.r Lithos 44 (1998) 53–82
The Park and Means Ž1996. study, in particular,
suggests that an important fabric-forming process at
low melt percentages may be alignment of crystals
by local porous flow and GBS in response to small
deviatoric stresses, whether or not the magma is
undergoing regional-scale flow. This process is analogous to alignment of clay particles during dewatering of sediments or to alignment of crystals during
filter pressing in magma, a process now thought to
form along both vertical and horizontal cooling fronts
in crystallizing magma chambers ŽStevenson and
Scott, 1987; Marsh, 1989; McBirney, 1993.. McKenzie Ž1984., Kerr and Tait Ž1986., and Stevenson and
Scott Ž1987. have provided theoretical and experimental evidence that porosity in crystal mushes may
be high Ž40% to 10%., that extensive flow of melt
can occur in such mushes, and that the crystal matrix
deforms during flow. A stressed crystal in magma
will tend to rotate such that its largest face is perpendicular to s 1 Že.g., DeVore, 1969.. More importantly, deviatoric stress will cause porous flow of
melt towards the least principal stress in the mush,
aiding in crystal alignment ŽFolkes and Russell,
1980.. Hibbard Ž1987. discussed ‘melt relocation’
microstructures that form during such a process.
Alignment of crystals during porous flow, aided
by melt-enhanced GBS and contact melting, is an
attractive concept to us because it provides a mechanism for the formation of weak to moderate magmatic fabrics without significant deformation of igneous markers that occur at angles to these fabrics
Že.g., layers in the Main Donegal and internal contacts in many zoned plutons.. It also allows for
formation of magmatic fabrics very late in a chamber’s history in an inward migrating crystal-mush
zone. What remains unclear is the nature of stresses
in these crystal-mush zones and the degree to which
these stresses drive fabric formation.
The studies described in this section indicate that
a change from grain-supported to suspension flow
typically occurs in deforming magmas between 20%
and 40% melt ŽFigs. 8 and 9. and that large amounts
of strain may occur in magmas without this strain
being recorded by the final fabric. During suspension
flow, crystals freely rotate and move past one another while strain is accommodated by the melt
phase. At lower melt percentages, perhaps as low as
a few percent, strain may be accommodated by a
variety of mechanisms, such as Ž1. melt-assisted
GBS, Ž2. contact-melting assisted grain boundary
migration, Ž3. strain partitioning into melt-rich zones,
and Ž4. grain rotation during porous flow. The only
indication of strain accomodated is this fashion may
be the formation of a ‘magmatic-looking’ fabric
Že.g., Nicolas and Ildefonse, 1996.. Park and Means
Ž1996. study particularly emphasizes that the operation of some deformation mechanisms may be difficult to infer from preserved microstructures. Furthermore, other near-solidus processes, such as tiling of
minerals, differential grain rotation, grain interference, and grain fracturing or melt-aided recrystallization, may prevent or reduce grain alignment Že.g.,
Ildefonse et al., 1992.. The above studies also indicate that some deformation mechanisms previously
assumed to reflect subsolidus deformation are active
during near-solidus, grain-supported flow. If so,
preservation of the microstructures indicative of processes active during igneous grain alignment may be
destroyed by fracturing, crystal plasticity, and recrystallization even before the magma reaches its solidus.
Thus three further challenges that complicate interpretation of fabrics are that: Ž1. some deformation
mechanisms operating in magmas potentially leave
little imprint on preserved grain alignments and microstructures; Ž2. deformation mechanisms usually
thought to reflect subsolidus processes sometimes
operate above the solidus, complicating issues of
timing and the causes of fabric formation; and Ž3.
considerable uncertainty remains about the rheological state of magmas during fabric formation and thus
the likely degree of mechanical coupling with relatively stiff, and sometimes actively deforming, host
rocks.
3. A proposed method of fabric-pattern interpretation
3.1. Summary of fabric patterns
A review of maps published over the last century
indicates that magmatic fabric patterns can be
grouped into three end-member types ŽFig. 11.. One
of the more common consists of margin-parallel or
‘onion-skin’ patterns ŽFig. 5. in circular to sub-elliptical plutons ŽBuddington, 1959; Murray, 1979;
S.R. Paterson et al.r Lithos 44 (1998) 53–82
71
The geometries of this pattern differ from the previous ones in being internally complex and having
little continuity with host rock fabric patterns. Many
plutons have components of one or more of the
above patterns Že.g., Fig. 4. and thus are the most
challenging to interpret.
3.2. Interpreting fabric patterns
Fig. 11. Three end-member types of magmatic fabric patterns in
plutons showing gradations and important processes in forming
each pattern. See text for full discussion.
Brun et al., 1990.. Typically, foliation intensity increases toward the margins, whereas mineral lineations are weak Že.g., Bateman, 1985; Courrioux,
1987.. Another common pattern involves elongate
plutons Žwith length to width ratios greater than 3.
with magmatic fabrics typically subparallel to the
long dimensions of the intrusions andror to the
regional structural trends in the host rocks ŽFig. 6..
Magmatic structures commonly cut across internal
igneous contacts and sometimes across host rock
contacts Že.g., Pitcher and Berger, 1972; Moore and
Sisson, 1987; Miller and Paterson, 1995.. Magmatic
lineations are typically subparallel to stretching lineations in the host rock. In some of these plutons,
magmatic fabrics are folded, with fold axes and axial
planes subparallel to similar structures in nearby host
rock. A third common pattern varies from complex
fabric geometries that define lobes, folds, and rectilinear patterns over short distances, such as those
described by Abbott Ž1989. for the South Mountain
batholith, to multiple lobate and rectilinear patterns
on a batholith scale, such as those described by
Bateman Ž1992. for large intrusive suites in the
Sierra Nevada. Mineral lineations are typically weak
to absent in outer portions of these plutons, but in
places plunge shallowly to steeply towards one or
more centers within the body ŽVigneresse, 1990..
The variability in fabric patterns and fabric-forming processes may make the interpretation of patterns seem a daunting if not an impossible task. This
may be true for some plutons. However, the common
occurrence and relative simplicity of many patterns
suggest that there are a few first-order controls and
that many of the complications discussed above reflect second order processes. However, fabric patterns are sufficiently unique that they must be fully
evaluated for each pluton. Below we suggest a procedure for doing so. This procedure must be used
with caution. It is difficult to apply to some plutons
and results in ambiguous interpretations in others.
Despite this, for a given pluton, it provides a means
of ruling out many of the potential causes of fabric
formation presented in the introduction.
After careful mapping of magmatic foliations and
lineations, the following steps should be completed.
Ž1. Use whatever means available to determine
the timing of fabric development relative to other
igneous features Že.g., enclaves, layering, and internal contacts., to features formed during chamber
construction Že.g., stoped blocks and faults., and to
features formed during regional deformation. This is
an important and commonly overlooked step since
our results indicate that fabrics formed during ascent
and early emplacement are rarely, if ever, preserved
in plutons.
Ž2. Use structural and microstructural observations Že.g., Figs. 8 and 9, and Table 2a. to determine
the rheological state during fabric formation and
whether fabrics formed during suspension or grainsupported flow. The success of this step depends on
the use of experimental studies that relate preserved
microstructures to deformation mechanisms which
operated during different rheological states.
Ž3. Determine fabric ellipsoid characteristics at a
number of stations, including shape Žprolate, triaxial,
72
Mineral
Magmatic
High-temperature
Low-temperature
K-feldspar
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Euhedral to subhedral
Weak zoning
Growth twins common
Synneusis, exsolution
Orthoclase, high triclinicity
Moulded by matrix of igneous crystals
Little intracrystalline strain
Euhedral inclusions in other igneous crystals
Euhedral to subhedral laths
Zoning Žnormal, patchy.
Growth twins common parallel to long axis
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Synneusis
Appropriate Ca content or high-T albite
Moulded by matrix of igneous crystals
Euhedral inclusions in other igneous crystals
Little intracrystalline strain
Euhedral to subhedral
Lath-shaped to stubby
Single crystals,
Rare twins, synneusis
High-T pleochroism
Moulded by matrix of igneous crystals
Euhedral inclusions in other igneous crystals
Little intracrystalline strain
Ø Creep microstructures
Ø Recovery textures common
Plagioclase
Hornblende
Solid state
Sub- to anhedral
Low triclinicity Žmicrocline.
Albite exsolution in recrystallized grains
Creep microstructures
Ø Anhedral polygonal crystals
Ø An 15 to An 35 typical
Ø Forms clusters or bands
Ø
Ø
Ø
Ø
Ø
Ø
Anhedral polygonal to lath-shaped crystals
Forms clusters or bands
Replaces pyroxene, olivine
High-T pleochroism
Creep microstructures
Recovery textures common
Relict porphyroclasts with subgrains or fractures
Glide twins
Altered to clays, micas
At low T : weaker than Qtz
Pressure solution microstructures
Ø Relict porphyroclasts with subgrains or fractures
Ø An 15 to An 0 typical
Ø Glide twins Žless common. and little relation to
long axis
Ø Altered to clays, micas and epidote
Ø At low T : weaker than Qtz
Ø Pressure solution microstructures
Ø Relict porphyroclasts with subgrains or fractures
Ø Alter to chlorite, biotite, or actinolite–tremolite
Ø Pressure solution microstructures
S.R. Paterson et al.r Lithos 44 (1998) 53–82
Table 2
Microstructures associated with growth and deformation of minerals in granitoids
Ø
Ø
Ø
Ø
Euhedral, single books
Smooth 001 faces
Euhedral inclusions in other igneous crystals
Little intracrystalline strain
Muscovite
Ø
Ø
Ø
Ø
Ø
Ø
Eu- to subhedral,
Single crystals to clusters
Smooth 001 faces
Euhedral inclusions in other igneous crystals
Little intracrystalline strain
Eu- to anhedral crystals
Ø
Ø
Ø
Ø
Spherical pools moulding other igneous crystals
No strong preferred orientation
Inclusions in other igneous crystals
Little intracrystalline strain except minor
undulose extinction
Quartz
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Forms clusters or bands
Variable grain sizes
Creep microstructures
Recovery textures common
Irregular 001 faces
Forms clusters or bands
Variable grain sizes
Creep microstructures
Recovery textures common
Irregular 001 faces
Polygonal recrystallized grains:
triple junctions
Ø Forms Qtz-rich bands
Ø Creep microstructures
Ø Recovery textures common
Ø Lipital C dominates
Forms bands with variable grain sizes
Kinking, fracturing
Irregular 001 faces
Pressure solution microstructures
Recrystallize to chlorite
Forms bands with variable grain sizes
Kinking, fracturing
Irregular 001 faces
Pressure solution microstructures
Ø
Ø
Ø
Ø
Ø Weak preferred orientation
Ø Pressure solution microstructures
Ø Subhedral grains
Creep microstructures
Qtz ribbons, S–C structures
Subgrains, fractures
a-slip dominates
S.R. Paterson et al.r Lithos 44 (1998) 53–82
Biotite
73
74
S.R. Paterson et al.r Lithos 44 (1998) 53–82
or oblate., size Žfabric intensity. and orientation for
elements defining the magmatic fabric. Also determine kinematics associated with the fabrics Že.g.,
Nicolas, 1992; Smith et al., 1993.. It is important to
do the above for different populations of markers
Že.g., enclaves, feldspars, micas, etc.. as it is clear
that different markers behave differently and that
more than one magmatic fabric may form in a single
pluton. Combined fabric ellipsoid shapes and kinematic data provide information about likely magma
displacement paths at individual locations ŽFig. 13..
Ž4. Use data from step 3 to evaluate displacement
gradients in chambers. Some internal processes Že.g.,
convection or magma surges. require large gradients
in the shapes, sizes, or orientations of fabric ellipsoids because of continuity requirements of flow in a
relatively fixed chamber volume ŽFig. 2.. If fabrics
result from regional strain, similar gradients should
occur in both the pluton and the host rock ŽFigs. 6
and 13c.. If fabrics result from late alignment of
crystals during porous flow in a relatively static
chamber, large gradients should not be observed,
although small gradients may occur near rheological
boundaries.
Ž5. Compare deflections of regional preemplacement markers and geometries of syn-emplacement
fabrics in the host rock with magmatic fabric patterns in plutons. Use the degree of structural continuity to evaluate the degree of mechanical coupling
between the host rock and magma chamber during
fabric formation. If structural patterns are completely
discontinuous ŽFig. 13a., indicating a high degree of
mechanical decoupling, fabric patterns probably result from internal magma chamber processes. If magmatic structures are continuous with syn-emplacement host rock structures ŽFig. 13c., a high degree of
Fig. 12. Flow chart showing how information about fabric ellipsoid characteristics and kinematics help constrain flow type at a single
locality.
S.R. Paterson et al.r Lithos 44 (1998) 53–82
mechanical coupling is implied and fabric patterns
probably reflect strain caused by regional deformation.
Ž6. Ideally, complete the above steps in three
dimensions, andror attempt to determine the threedimensional shape of the pluton by looking for preserved roofs or floors of chambers ŽFowler, 1996.,
75
mapping in regions with significant vertical relief or
in tilted plutons ŽPaterson et al., 1996., or by combining surface geology with geophysical data
ŽVigneresse, 1990..
When possible, we have completed diagrams to
exemplify how each of the above data sets can be
used to deduce the probable causes of fabric pattern
Fig. 13. Chart showing how the degree of continuity between host rock structures and structures in magma chambers place constraints on
probably causeŽs. of magmatic fabrics. Cartoons at top show circular magma chambers and simplified structural patterns, best viewed as
end-members on the sides of a triangle. A s complete decoupling, B s partial coupling Ždashed lines in host rock s foliations formed during
emplacement., and C s complete coupling. Although only foliations are shown both foliation and lineation should be used to evaluate
coupling. After the degree of coupling Žfirst row. is confirmed by information about timing of host rock and magmatic structures, it can be
used along with information about whether internal fabrics are magmatic or subsolidus Žsecond row. to determine likely causes of fabric
formation Žthird row.. Box at the bottom shows possible fabric-forming processes grouped into internal, crystallization, emplacement, and
regional processes.
76
S.R. Paterson et al.r Lithos 44 (1998) 53–82
formation Ži.e., Figs. 8 and 9 and 11–13, and Table
2.. The greatest difficulty is that rarely are all of the
above data sets available for a single pluton. For
example, an increasingly popular procedure is to
evaluate magmatic fabric patterns based on the use
of AMS techniques ŽBouchez, 1997.. Although this
is a powerful tool for determining integrated fabric
ellipsoid geometries of magnetic phases, it provides
no information about the kinematics, timing, or magnitude of flow, or even a check on fabric geometries
for separate mineral populations. It is also common
that fabric patterns in plutons are evaluated without
consideration of host rock structures Že.g., Courrioux, 1987; Saint Blanquat and Tikoff, 1997.. Another difficulty is that some of the above observations provide only permissive constraints on processes, rather than proof that a given process occurred Že.g., coaxial, noncoaxial, and more complicated laminar flows can all produce L-S fabrics..
However, we emphasize that the ambiguity in fabric
pattern interpretations increases rapidly if one or
more of the above data sets are not considered. In
Section 4 we use the above steps to illustrate the
constraints and ambiguities that can result from interpretations of magmatic fabric patterns.
4. Examples of fabric pattern interpretation
Magmatic fabric patterns in the Entiat and Cardinal Peak plutons, Washington ŽFig. 6. reflect strongly
coupled systems. Above we described relationships
in and around these plutons which indicate that
preserved magmatic fabrics: Ž1. formed after emplacement since they crosscut internal contacts and
sometimes host rock contacts, Ž2. formed during
grain supported flow in a crystal-rich magma ŽFig.
7., Ž3. have geometries, kinematics, and gradients
that mimic host-rock geometries, kinematics, and
gradients ŽMiller and Paterson, 1995., and Ž4. are
sometimes continuous with host-rock structure. We
argue that the magmatic foliation and lineation in
these plutons cannot represent flow planes and directions because the structures locally occur at high
angles to the pluton margin and cut internal contacts
between sheets ŽFig. 6.. We know of no flow along
margins that would form such a geometry. The
complex variations of fabric orientation over short
distances argue against fabric formation during
chamber expansion or pluton-wide convection. Instead, these data indicate that magmatic fabrics
formed during regional deformation largely after
magma chamber construction and before the pluton
was completely crystallized.
Fabric patterns in the White Creek batholith,
British Columbia ŽFig. 5. offer insight into the complexities of interpreting the ‘onion-skin’ fabric pattern so typical in zoned, subcircular plutons. This
batholith is a roughly elliptical, nested body with a
monzodioritic border facies, followed inward by
hornblende–biotite granodiorite and porphyritic granodiorite, and an inner intrusive pulse of two-mica
granite ŽReesor, 1958; Brandon and Lambert, 1994..
Magmatic foliation forms a roughly concentric pattern subparallel to the batholith margin. A lineation
occurs locally in the foliation, but lacks consistent
orientation. Enclave ratios and mineral alignment
define weak to moderate triaxial to oblate fabric
ellipsoids and show a moderate increase in intensity
toward the host rock contact ŽReesor, 1958.. No
information is available about kinematics and only
indirect information is available on the rheological
state during fabric formation and 3D chamber shape.
Analysis of available data indicates that the fabric
formed after juxtaposition of earlier phases, since it
crosscuts contacts between these phases, but before
the innermost two-mica granite was intruded, since
this phase sharply truncates fabric ŽFig. 5.. Map
patterns also show that internal fabrics are strongly
decoupled from host rock fabrics on the sharply
discordant western, northern, and eastern margins,
but possibly weakly coupled where margin-parallel
fabrics are locally recognized in a narrow contact
aureole ŽReesor, 1958.. We suggest that these data
rule out regional deformation as a significant cause
of fabric formation and instead suggest that fabrics
formed late during emplacement due to internal processes.
One common interpretation of this kind of fabric
pattern is that it formed due to a large amount of
lateral expansion of the magma chamber. If so the
following observations are problematic: Ž1. the magmatic fabric cuts internal contacts, implying that it
formed after juxtaposition of these phases rather than
during intrusion of one phase by another; Ž2. the
fabric is equally well developed along both discor-
S.R. Paterson et al.r Lithos 44 (1998) 53–82
dant and concordant host rock contacts, indicating
that significant deformation of the host rock was not
required to form the fabric; Ž3. the innermost twomica granite discordantly cuts along sharp contacts
the magmatic fabric in outer phases, indicating that
emplacement of at least this phase did not cause
chamber expansion; and Ž4. there is little evidence in
the host rock for large amounts of lateral expansion
ŽPaterson and Vernon, 1995..
A second interpretation of this type of fabric
pattern is that it reflects pluton-wide convection Že.g.,
Schmeling et al., 1988; Cruden, 1990.. This is a
more plausible explanation because it does not require large amounts of chamber expansion but can
still cause the increase in fabric intensity and enclave
ratios towards the margin. However, it is difficult to
accept convection as an explanation for the following reasons: Ž1. rheological studies generally indicate
that chamber wide convection is increasingly unlikely as the solidus is approached, i.e., at the time
the preserved fabrics formed; Ž2. convection should
have significantly altered the contacts between different internal phases, particularly where fabrics are
at moderate angles to these contacts; and Ž3. foliation maintains relatively steep dips well toward the
center of the pluton and steep, intensely developed
lineations are absent in the center—a geometry at
odds with that predicted for chamber-wide convection ŽSchmeling et al., 1988..
We favor a third possibility in which stresses in a
constructed and crystallizing chamber cause alignment of crystals subparallel to chamber margins.
Magma-host rock contacts represent high-viscosity
boundaries surrounding a material Žmagma. that cannot support large deviatoric stresses. Thus, principal
stresses will refract parallel to Žusually s 2 and s 3 .
and perpendicular to Žusually s 1 . these contacts regardless of the cause of these stresses. Thus in a
relatively static chamber these stresses would drive
margin-parallel filter pressing, aided by porous flow
and stress induced grain rotation, in an inward migrating crystal mush zone. Such a process would
explain the lack of visible deformation of internal
contacts, the steep dips of foliation, and the lack of
mineral lineation in the center of the chamber. Alternatively, if the magma chamber is not static, we note
that any late movement of magma in these chambers,
again regardless of the driving forces, would initially
77
result in progressive noncoaxial flow parallel to the
magma-host rock margin and later parallel to the
crystal-mush zone due to the strong viscosity gradient near these features. This scenario is most likely
where a strong thermal and viscosity gradient occurs
across the magma-host rock contact, which may be
one reason why these ‘onion-skin’ patterns are most
commonly preserved in mid- to shallow crustal plutons.
Fabric patterns in the Mount Stuart batholith ŽFig.
4. reflect a more complicated system with components of coupled and decoupled systems. Aligned
mafic microgranitoid enclaves and igneous minerals
Žplagioclase, pyroxene, hornblende, and biotite. define a magmatic foliation and locally a lineation.
These magmatic fabrics oÕerprint or are parallel to
all other igneous structures Že.g., layering, schlieren,
etc.. including internal contacts, postdate the settling
of stoped blocks ŽFig. 4., and thus formed after
chamber construction. Vertical relief Ž; 2500 m.
and local roof contacts indicate that the batholith has
a gently dipping roof and steep typically NE-dipping
wall contacts ŽPaterson et al., 1994a..
In the compositionally zoned, southeastern mushroom-shaped region of the batholith, magmatic foliation in general has steep dips and defines a roughly
margin-parallel pattern in map view ŽFig. 4., with
foliation intensity increasing towards the margins.
Lineations are weak and have variable plunges. This
overall ‘onion-skin’ pattern is interrupted in two
regions. A zone of strongly developed, gently dipping foliation occurs near a 2 km2 xenolith of schist
near the southwestern edge of the batholith ŽFig. 4..
About halfway between the eastern margin and pluton center, magmatic foliation defines a marginparallel zone where dip reversals define a fan-like
pattern that laterally changes into a V-shaped synformal pattern. This part of the batholith is largely
discordant to preemplacement markers, although
there is a narrow, discontinuous aureole defined by
margin-parallel fabrics.
We again argue that chamber expansion cannot be
the dominant cause of this ‘onion-skin’ pattern because the fabrics form equally along discordant and
concordant margins, the fabrics cut internal contacts,
and expansion would not form the fan-like or synformal pattern. We therefore argue that this pattern
reflects late internal flow or alignment during porous
78
S.R. Paterson et al.r Lithos 44 (1998) 53–82
flow in a crystal mush zone. We suspect that the
fan-like or synformal pattern reflects differential flow
within the chamber Že.g., magma surges. adjacent to
the inward migrating crystal mush zone. The
flat-lying foliations occur along the projection of a
thrust fault ŽWindy Pass Thrust. into the batholith
and are interpreted to reflect syn-emplacement
movement on this thrust ŽMiller and Paterson, 1994..
In the central sill-like region, magmatic structures
are well developed but vary in orientation both laterally and vertically. A flat-lying, roof-parallel foliation occurs within a few tens of meters of the roof
and steep foliation and horizontal lineation occur
directly below. At the northeastern host rock contact,
a short distance below the roof, magmatic foliation is
margin-parallel and an intense magmatic lineation,
associated with constrictional fabrics, plunges gently.
Large and small magmatic folds with roughly horizontal axes and steep NNE-dipping axial planes are
common ŽMiller and Paterson, 1994.. In the center
of the sill, the margin-parallel foliation bends into
orientations 308 to 608 to the contact, forming an
S-shaped pattern in map view.
In the northwestern, hook-shaped region, magmatic foliation and lineation are only weakly to
moderately developed. The foliation tends to strike
NW and dip steeply, and the lineation trends NW–SE
with variable plunges. However, in detail, these
structures show complicated patterns that include
rapid variations in orientation. Although patchy exposure and the weakness of these structures make
observations difficult, we interpret these rapid
changes in orientation to reflect tens-of-meters to
kilometer-scale magmatic folds, similar in orientation to folds in the sill-like region.
In contrast to the southeastern, mushroom-shaped
part of the batholith, the foliation and lineation patterns and magmatic fold orientations in the sill- and
hook-like regions match those in nearby host rock
ŽMiller and Paterson, 1994. and indicate a strong
degree of coupling between magma and host rock
during regional deformation. This strong coupling,
the outcrop-scale magmatic folds, and the lateness of
fabric formation Ži.e., after settling of late stoped
blocks., indicate that these fabrics reflect strain
caused by regional deformation.
We believe that the Mount Stuart Batholith preserved these very different types of fabric patterns
because of differences in the post-emplacement history of the NW and SE parts of the batholith. The
southeastern, mushroom-shaped part of the batholith
cooled quickly after emplacement, as supported by
KrAr hornblende and biotite cooling ages within 1
Ma of the emplacement age ŽPaterson et al., 1994a..
However, the northeastern, sill-like and hook-shaped
parts of the batholith where immediately buried after
emplacement, and cooling ages are up to 11 Ma
younger in these regions Že.g., references in Paterson
et al., 1994a..
5. Conclusions
Due to the complex processes involved in forming
magmatic fabric patterns, we emphasize the need to
fully evaluate features both in the chamber and in the
host rock before interpreting the significance of fabric patterns. We suggest the following approach: Ž1.
use timing relationships to determine timing of magmatic fabric development relative to other igneous
features and to chamber construction processes; Ž2.
use microstructures and fabric intensities to confirm
the rheological state during fabric formation; Ž3. use
mineral fabric and outcrop-scale structural analyses
to evaluate the type and kinematics of flow; Ž4.
examine the nature of fabric gradients in plutons; Ž5.
use host rock markers and fabric patterns in plutons
to determine the degree of coupling of the fabric
forming processes; and Ž6. when possible, do the
above in three dimensions. Many of our attempts at
interpreting magmatic fabric patterns are poorly constrained because of lack of information about one or
more of the above steps. We recognize that for many
plutons, it may be difficult to obtain all of the above
data for reasons of time, money, or geological constraints. However, in these cases, interpretation of
fabric patterns is commonly ambiguous, and we
suggest it is important to not overstate conclusions
about the significance of these patterns. In this spirit,
we have tried to provide examples of both the successes and ambiguities in using the above approach
for choosing between various causes of fabric formation.
Nevertheless, we believe existing data allow several general conclusions to be drawn. Our evaluation
of many published maps as well as our own work
S.R. Paterson et al.r Lithos 44 (1998) 53–82
suggests that three end-member fabric patterns are
common in large intrusive bodies ŽFig. 11.: Ž1.
complex patterns typically in elongate plutons that
mimic and are continuous with host rock patterns;
Ž2. ‘onion-skin’ or margin-parallel patterns; and Ž3.
multiple lobate to rectilinear patterns that are highly
decoupled from host rock patterns. Evaluation of
these patterns using the above guidelines supports
the following conclusions: Ž1. preserved magmatic
fabric patterns typically form during the final stages
of, or after construction of magma chambers; Ž2.
fabrics preserve only the last increment of strain
before the magma crystallizes; Ž3. complex patterns
in structurally decoupled plutons result from strain
caused by internally driven magma flow in already
existing chambers; Ž4. margin-parallel fabrics in
weakly coupled to decoupled systems possibly form
by late crystal alignment during porous flow in
response to margin parallel and perpendicular principal stresses; and Ž5. with increasing emplacement
depth or chamber size, and thus slower cooling rates,
increased coupling between host rock and magma
occurs and most fabric patterns in these plutons
reflect strain of magma caused by regional deformation.
In the introduction we noted that magmatic fabrics are interpreted to represent a variety of features.
We see no evidence that magmatic fabrics can be
used to infer pluton shapes and have provided examples where they do not. Although fabrics may sometimes be parallel to flow planes and directions, we
have also provided examples where they are not. We
have also concluded that fabrics provide little to no
information about ascent or early emplacement. Instead we have argued that these fabrics typically
record the last increment of strain during magmatic
flow caused by either internal processes or by regional deformation of a crystallizing chamber.
One intriguing line of future enquiry is to evaluate
whether some magmatic fabric patterns provide relatively direct records of paleostress fields in orogenic
belts. Most host rocks are rheologically heterogeneous because of variations in composition, anisotropy, and previous deformational history. Thus, using
structures in these rocks to infer paleostresses is
fraught with difficulty. In contrast, magmas, being
Newtonian or relatively weak Bingham materials
with no previous recorded history, are more homoge-
79
nous and potentially more easily interpretable. If, as
we have suggested, fabrics form late, represent only
the last increment of strain during crystallization, can
be easily and rapidly reset, and may be aligned by
porous flow in a relatively static magmas, these
fabrics may provide a record of whether chamber-related or regional stresses dominated during final
crystallization. If so, they may also provide a record
of the orientation of regional stresses, particularly in
coupled systems undergoing late coaxial flow. For
example, we are intrigued by examples in the Sierra
Nevada, such as the Tuolumne ŽBateman, 1992;
Paterson and Vernon, 1995., John Muir ŽBateman,
1992; Saint Blanquat and Tikoff, 1997., Mitchell
ŽSisson and Moore, 1984; Moore and Sisson, 1987.,
and Whitney Intrusive Suites, which typically have
relatively consistent foliation trends over much of
their extent, in orientations not easily explained by
internal processes. Do these foliation orientations
preserve information about Cretaceous paleostress in
the Sierra Nevada?
Acknowledgements
This research was supported by NSF Grants EAR8916325, EAR-9218741, and EAR-9304058 awarded
to Paterson and EAR-8917343 and EAR-9219536
awarded to Miller. We thank Keith Benn, Sandy
Cruden, Chris Mawer, Tracy Rushmer, and Ron
Vernon for thoughtful reviews of the paper and
continued discussions about magmatic fabrics and
John Clemens for his editorial assistance.
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