Tectonophysics 500 (2011) 20–33
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Tectonophysics
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o
Multiscale magmatic cyclicity, duration of pluton construction, and the paradoxical
relationship between tectonism and plutonism in continental arcs
Michel de Saint Blanquat a,⁎, Eric Horsman b,1, Guillaume Habert a,2, Sven Morgan c, Olivier Vanderhaeghe d,
Richard Law e, Basil Tikoff b
a
CNRS – University of Toulouse, LMTG, 14 Avenue Edouard-Belin 31400 Toulouse, France
Dept. of Geology and Geophysics, Univ. of Wisconsin – Madison, 1215 W Dayton St., 53706, Madison, WI, USA
Department of Geology, Central Michigan University, Mount Pleasant, MI, 48859, USA
d
G2R, Géologie et Gestion des Ressources Minérales et Energétiques, BP 239, 54506 Vandoeuvre-les-Nancy, France
e
Dept. of Geological Sciences, Virginia Tech. Institute, Virginia, 24061 Blacksburg, USA
b
c
a r t i c l e
i n f o
Article history:
Received 31 January 2009
Received in revised form 7 December 2009
Accepted 20 December 2009
Available online 4 January 2010
Keywords:
Pluton
Magma
Pulse
Tectonics
Fabric
Pluton emplacement
a b s t r a c t
The close relationship between crustal magmatism, an expression of heat dissipation, and tectonics, an
expression of stress dissipation, leads to the question of their mutual relationships. Indeed, the low viscosity of
magmas and the large viscosity contrast between magmas and surrounding rocks favor strain localization in
magmas, and then possible “magmatic” initiation of structures at a wide range of scales. However, new data about
3-d pluton shape and duration of pluton construction perturb this simple geological image, and indicate some
independence between magmatism and tectonics. In some cases we observe a direct genetic link and strong
arguments for physical interactions between magmas and tectonics. In other cases, we observe an absence of
these interactions and it is unclear how magma transfer and emplacement are related to lithospheric-plate
dynamics. A simple explanation of this complexity follows directly from the pulsed, incremental assembly of
plutons and its spatial and temporal characteristics. The size of each pluton is related to a magmatic pulsation at a
particular time scale, and each of these coupled time/space scales is related to a specific process: in small plutons,
we can observe the incremental process, the building block of plutons; in larger plutons, the incremental process
is lost, and the pulsation, which consists of a cycle of injections at different timescales, must be related to the
composition and thermal regime of the source region, itself driving magmatic processes (melting, segregation,
and transfer) that interact with tectonic boundary conditions. The dynamics of pulsed magmatism observed in
plutonic systems is then a proxy for deep lithospheric and magmatic processes. From our data and a review of
published work, we find a positive corelation between volume and duration of pluton construction. The larger a
pluton, the longer its construction time. Large/fast or small/slow plutons have not been identified to date. One
consequence of this observation is that plutonic magmatic fluxes seem to be comparable from one geodynamic
setting to another and also over various geologic time spans. A second consequence of this correlation is that small
plutons, which are constructed in a geologically short length of time, commonly record little about tectonic
conditions, and result only from the interference between magma dynamics and the local geologic setting.
The fast rate of magma transfer in the crust (on the order of cm/s) relative to tectonic rates (on the order of cm/yr)
explain why the incremental process of pluton construction is independent of – but not insensitive to – the
tectonic setting. However, in large plutonic bodies, which correspond to longer duration magmatic events,
regional deformation has time to interact with the growing pluton and can be recorded within the pluton-wall
rock structure. Magma transfer operates at a very short timescale (comparable to volcanic timescales), which can
be sustained over variable periods, depending on the fertility of the magma source region and its ability to feed the
system. The fast operation of magmatic processes relative to crustal tectonic processes ensures that the former
control the system from below.
© 2010 Elsevier B.V. All rights reserved.
⁎ Corresponding author.
E-mail addresses: [email protected] (M. de Saint Blanquat), [email protected] (E. Horsman), [email protected] (G. Habert), [email protected] (S. Morgan),
[email protected] (O. Vanderhaeghe), [email protected] (R. Law), [email protected] (B. Tikoff).
1
Now at: Dept. of Geological Sciences, East Carolina University, 101 Graham Building, Greenville, NC, 27858, USA.
2
Now at: Laboratoire Central des Ponts et Chaussées, 58Bd Lefebvre, 75732 Paris Cedex 15, France.
0040-1951/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2009.12.009
M. de Saint Blanquat et al. / Tectonophysics 500 (2011) 20–33
1. Introduction
At the plate scale, magmatism (i.e. all processes associated with the
formation, evolution, and transfer of magma) is directly controlled by
lithospheric-plate dynamics, in the sense that plate divergence and
convergence are responsible for the modification of the P, T, and X
conditions leading to partial melting. This relationship is exemplified
by the production of oceanic crust associated with basaltic tholeiitic
magmatism at divergent plate boundaries and by the production and
differentiation of continental crust linked to calc-alkaline and
peraluminous magmatism at convergent plate boundaries. At the
regional scale, numerous studies on plutons of various ages have
shown that their location and internal structure are correlated with
surrounding regional structures, and consequently seem to be
controlled by tectonics (i.e. regional-scale deformation in response
to lithospheric plate motion). Consequently, granitic plutons are
commonly used in tectonic studies to reconstruct the geodynamic
evolution of continental crust. The principal source of information is
the structure of pluton–wallrock systems, whose evolution is
controlled by the regional stress regime, the local geologic setting
(geometry and thermal structure), and the dynamics of magma
infilling. The structure of pluton–wallrock systems then allows us to
study the interactions and the individual characteristics of both
tectonic and magmatic processes during emplacement.
This close relationship between crustal magmatism, an expression
of heat dissipation, and tectonics, an expression of stress dissipation,
leads to the question of their mutual relationships. Indeed, the low
viscosity of magmas, and consequently the sharp viscosity contrast
between magmas and surrounding rocks, favor strain localization in
magmas, and thus the possible “magmatic” initiation of structures at a
wide range of scales (Tikoff and Saint Blanquat, 1997; Bons et al.,
2008).
However, some new data and ideas challenge this simple geological
image, and indicate some independence between magmatism and
tectonics. For example, analysis of the 3-d shape of plutons from
various tectonic settings has shown that the majority are tabular to
funnel-shaped and that their shape evolves according to a power–law
relationship, typical of systems exhibiting scale-invariant behaviour
(McCaffrey and Petford, 1997; Cruden, 1998; Petford et al., 2000). A
consequence is that the construction of plutons is controlled by a self
organization process of magmatic origin, irrespective of the tectonic
context. Another example is provided by recent data on duration and
rate of pluton construction, which show that some plutons are
constructed in less than 100,000 years (Saint Blanquat et al., 2001;
Saint Blanquat et al., 2006; Michel et al., 2008), a duration which
precludes any significant intervention and recording of syn-plutonic
regional deformation during pluton construction. Thus, on the one
hand we observe a direct genetic link, and some strong arguments for
physical interactions between magmas and tectonics. Yet, on the other
hand, we also observe an absence of these interactions, as it is not clear
how magma transfer and emplacement are related to lithosphericplate dynamics. In this paper, we describe how the recently recognized
pulsed, or incremental, growth of plutons will help us resolve this
apparent paradox.
The origins of ideas developed in our paper include: (1) pioneering
work demonstrating that magma intrusion and tectonic rates do
not operate at the same time scales (e.g., Paterson and Tobisch,
1992; Nyman et al., 1995); (2) the observation that some plutons
may have an internal record which is only interpretable in terms of
emplacement-related processes injection and magma chamber
processes) and has nothing to do with external regional deformation
during construction (Sylvester et al., 1978; Cruden et al., 1999;
McNulty et al., 2000; Saint Blanquat et al., 2001; Harper et al., 2004;
Barbey et al., 2008); and (3) work showing that pluton shapes seem to
be controlled by internal processes which are independent of
chemical composition and crustal tectonic regime (McCaffrey and
21
Petford, 1997; Cruden, 1998; Petford et al., 2000; Cruden and
McCaffrey, 2001).
In this paper, we first briefly summarize and then compare a series
of petrostructural studies we have conducted on plutons of various
sizes, constructed in various tectonic settings, but all related to the
same geodynamic setting of arc magmatism. These intrusions include:
(1) the Black Mesa pluton in the Henry Mountains, Utah, of Oligocene
age and with no associated regional deformation; (2) the Mono Creek
and Papoose Flat plutons, and Tuolumne intrusive suite in the Sierra
Nevada and White-Inyos Mtns of California of late Cretaceous age and
transpressional setting; and (3) the Tinos pluton in the Cycladic
islands, Greece, of Miocene age and extensional setting. Based on
these comparisons, we discuss the relations between the nature of the
structural record and the tectonic setting of the studied plutons. We
conclude that the incremental process of pluton construction is the
same for all these plutons, with similar characteristics irrespective of
their tectonic setting, age, and composition. We also show that the
first parameter to check before interpreting the nature of the plutonic
record is the total duration of pluton construction, which is itself
directly related to the final pluton volume.
2. The plutonic structural record, a definition
The structural development of plutons is classically described as
being recorded by the internal layering and fabrics. Layering can be
defined as “the combination at any scale of layers differing by
composition or texture” (Barbey, 2009). Fabric is formed by the
shape preferred orientation (SPO) of minerals, and is defined by its
orientation, shape, and intensity. Following Barbey (2009), three
processes are involved in the formation and evolution of the structural
record within a growing pluton: (1) injection processes (incremental
growth, magma channelized flow, mingling, mixing, etc…); (2)
magma chamber processes (hydrodynamic processes, crystal settling,
etc…); and (3) tectonic processes (forces applied to the boundaries of
the magmatic bodies inducing a deformation). In other words, any
structural plutonic record can be interpreted as: (1) a record of
deformation, either related to emplacement dynamics (injection) or to
regional deformation, and/or (2) a magmatic record related to magma
differentiation during cooling of magma pulses.
3. The construction of plutons, historical perspective
The classical view of pluton construction consists of a dichotomy
between “forceful” or “active” type, including doming, diapirism and
ballooning, versus “passive” or “permitted” type, including stoping,
cauldron subsidence and associated sheeting. Permitted types were
considered to characterize magma emplacement in the upper crust, in
contrast with forceful types that were considered to work at greater
depths (Pitcher, 1979). In addition, although the presence of a
structural control on magma transfer and emplacement has long
been recognized, this was primarily based on observed relationships
between pre-existing structures and magma ascent, rather than on
interaction between evolving tectonic structures and magma transfer.
Hutton (1988) first questioned the interaction between magmatic and
tectonic forces by hypothesizing that ‘active’ emplacement occurs
when the magma infilling rate is greater than the rate of tectonic
opening, and ‘passive’ emplacement when it is less, and examining all
types of tectonic setting, including transcurrent, extensional, and
contractional. Hutton proposed that the combination of these two
processes could generate the variety of emplacement mechanisms
observed in different plutons. A consequence is that most plutons
could be considered syntectonic. This idea was addressed in a
discussion on the so-called syntectonic paradigm by Karlstrom
(1989), who stated that all granitoids are syntectonic in a broad
sense, as they are emplaced into crust experiencing regional
deformation, and that many and perhaps most of the granitoids are
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M. de Saint Blanquat et al. / Tectonophysics 500 (2011) 20–33
syntectonic in a strict sense, because there is melt present during
regional deformation. Paterson (1989) challenged this view with the
suggestion that only a small percentage of plutons are syntectonic in
the strictest sense, citing as evidence the contrasting rates of pluton
emplacement, cooling and regional deformation, and arguing for a
more rigorous use of the term syntectonic.
Since the 1980–1990s, with the help of the development of new
methods of fabric quantification (Ellwood and Whitney, 1980;
Bouchez, 1997; Launeau and Robin, 2005; Gaillot et al., 2006), and
after the theoretical and experimental validation of the dike model for
magma transport (Lagarde et al., 1990; Clemens and Mawer, 1992;
Petford et al., 1993; Clemens, 1998; Menand, 2011), a large number
of publications have studied at various scales the nature of the link
between pluton construction and tectonics. These studies consider
magmatism–tectonic links ranging from the plate scale (e.g., Glazner,
1991; Grocott et al., 1994; Brown and Solar, 1998; Saint Blanquat et al.,
1998 and Benn et al., 2001 among many others), through the regional
scale(e.g. Brun and Pons, 1981; Bussel and Pitcher, 1985; Castro, 1987;
Brun et al., 1990; Hutton and Reavy, 1992; D'Lemos et al., 1992;
Vigneresse, 1995; Druguet and Hutton, 1998; Gleizes et al., 1997;
Tikoff and Saint Blanquat, 1997; Archanjo et al., 1999; Brown and Solar,
1999; Vigneresse et al., 1999 and Grocott and Taylor, 2002 among
many others), to the pluton scale (Hutton, 1988; Paterson and Tobisch,
1988; Gapais, 1989; Hutton et al., 1990; Hutton and Ingram, 1992;
Tikoff and Teyssier, 1992; Karlstrom et al., 1993; Cruden, 1998;Barros
et al, 2001; Weinberg et al., 2004, and Grocott et al., 2009 among many
others). One important consequence of these studies was the
recognition that magmatism and pluton construction can strongly
influence the rheological behaviour of the lithosphere (see for
example Hollister and Crawford, 1986; Davidson et al., 1992, 1994;
Klepeis et al., 2003). The question of the ultimate control on the
tectono-magmatic systems then arrived with work on plutons and
shear zones in northeast Brazil (shear zone-controlled magma
emplacement or magma-assisted nucleation of shear zones; Neves
et al., 1996), and in the Mesozoic Sierra Nevada Batholith of California
(see below, and Saint Blanquat et al., 1998). The “room problem” of
how space is made in the crust for large volumes of intruding magma
was also largely resolved by rethinking old ideas about emplacement
(magmas pushing their wallrocks, laterally, upward —roof uplift, or
downward—floor depression; Cruden, 1998; Acocella, 2000; Cruden
and McCaffrey, 2001), and taking into account all the components of
the regional and local displacement/deformation fields, including
translation, which is not recorded by rock internal strain (Tikoff et al.,
1999). The analysis of 3-d pluton shape by McCaffrey and Petford
(1997) and Petford et al. (2000) has shown that the observed tabular
shape of many plutons is best explained by a self organization process,
irrespective of the tectonic context or composition of magmas
involved. This period ended with the recognition of the essential role
of deformation at all stages of magmatism (melting, segregation,
transport, and emplacement), and the promotion of a new image of
granite magmatism as a rapid and dynamic process that can operate at
timescales of less than 105 years, irrespective of tectonic setting
(Petford et al., 2000).
Much recent work has focused on examining the discontinuous and
episodic growth of plutons. While this subject is not a new idea (see for
example Pitcher and Berger, 1972; Hardee, 1982; Wiebe, 1988; Wiebe
and Collins, 1998), recent field and theoretical work has appreciably
improved our understanding of these processes by documenting the
spatial and temporal scales of the incremental assembly of batholiths
and individual plutons (Cruden and McCaffrey, 2001; Saint Blanquat
et al., 2001; Saint Blanquat et al., 2006; de Silva and Gosnold, 2007;
Fig. 1. Graphic representation of the incremental process of pluton construction; see
text for explanation. Top: the incremental mechanism of pluton construction. Bottom:
different intrusion histories for the same pluton thickness, showing that one pluton
could correspond to very contrasting intrusion histories, and that one averaged rate of
construction could correspond to many and very different intrusion histories.
Lipman, 2007; Menand, 2008; Bartley et al., 2008; Vigneresse, 2008;
Horsman et al., 2009; Miller et al., 2011). Combined with considerable
improvement in knowledge about the timescales of magmatic
processes due to radiogenic isotope techniques and chemical diffusion
methods (Coleman et al., 2004; Matzel et al., 2006; Miller et al., 2007;
Walker et al., 2007; Turner and Costa, 2007; Michel et al., 2008; Costa,
2008), the new data initiated vigorous debate on upper crustal
plutonic systems, the relation between magma chamber and pluton
dynamics, and the relation between plutonism and volcanism
(Glazner et al., 2004; Bachmann et al., 2007; Annen, 2009; Annen,
2011). Additionally, important questions were raised about the
entire system at the crustal scale, particularly the mechanisms and rate
of crustal growth (Ducea, 2001; Kemp et al., 2006; Hawkesworth and
Kemp, 2006; Kemp et al., 2007; DeCelles et al., 2009).
In Fig. 1, we present a model of how many plutons are constructed,
with – to recall the active/forceful versus passive/permitted terminology – an ‘active’ part of emplacement (magma pulse intrusion) and a
‘passive’ part during which the previously intruded magma pulses may
be deformed at geologically ‘normal’ strain rates during cooling. When
considering rates of pluton construction, it is essential to recognize the
difference between the instantaneous construction rate, that is the rate
of magma infilling during the injection of one pulse, and the averaged
construction rate, which is the simple ratio between pluton volume and
total duration of pluton construction. This averaged rate takes into
account all the repose times between injections. Major challenges now
in the study of the plutonic processes are: (1) to identify and
characterize the geologic marker(s) allowing reconstruction of the
growth history of a pluton, and (2) to better quantify the rates of
pluton construction.
Fig. 2. Synthesis of previously obtained data; explanation and references are in the text. All stereograms are equal area, lower hemisphere, Kamb contour (2σ). (A) Real relative size
of the plutons. (B) Maps: mineralogical zonation at pluton scale; stereos: poles of magmatic layering. (C) Map of microstructures; transition from one type to another are sharp on
the maps for clarity but gradual on the field. (D) Maps of the AMS lineation trajectories; stereos: AMS lineation. (E) Maps of the AMS foliation trajectories; stereos: AMS foliation
poles.
M. de Saint Blanquat et al. / Tectonophysics 500 (2011) 20–33
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M. de Saint Blanquat et al. / Tectonophysics 500 (2011) 20–33
With the help of previously completed petrostructural studies of
plutons of different sizes located in various tectonic settings, the aims of
this paper is to present relevant data and ideas about these challenges,
and to explore the geological implications of the observed episodic
growth of many plutons.
4. The dataset
We present below summaries of some of our previous work, sorted
by increasing pluton volume. These petro-structural studies are
primarily based on fieldwork, anisotropy of magnetic susceptibility
(AMS) analysis, microstructural observations, and preliminary petrological and geochemical analyses. The detailed geology of the studied
plutons can be found in the cited references, and a synthesis of
structural and microstructural data is given in Fig. 2.
Black Mesa pluton (Habert and Saint Blanquat, 2004; de Saint
Blanquat et al., 2006; Horsman et al., 2009): the small Black Mesa
pluton is a satellite of the larger Mount Hillers intrusion in the Henry
Mtns (Utah, USA) on the Colorado Plateau. No regional deformation
was active during construction of this Oligocene pluton at 3 km
depth. The pluton is a porphyritic microdiorite and has a volume of
approximately 0.6 km3. The intrusion is broadly cylindrical in shape,
but important details provide evidence of its construction history. The
western margin of the intrusion is laccolithic, with upward rotated
bedding. The eastern margin, where the horizontal wall rocks are
displaced upward ∼200 m by a syn-magmatic vertical fault, is a
bysmalith, or fault-bounded, piston-like inflated laccolith. The internal
structure of the Black Mesa pluton is characterized by sub-horizontal
magmatic layering, defined by sharp variation in the diorite texture.
Internal contacts are sub-horizontal, with compositional layering
mainly recognized in vertical magnetic susceptibility profiles, a weak
vertical zonation in the content of some minor and trace elements, and
sub-horizontal internal structures. The combination of these features
suggests that the internal contacts are primary, and due to discontinuous construction of the pluton, and not due to secondary in-situ
processes. Consequently, we infer that the pluton was constructed by
the amalgamation of sequentially injected horizontal sheet-like
magma pulses. The foliation is sub-horizontal except along the
margins where it is concordant to the steeply dipping pluton contacts.
The controlling factor for the foliation orientation is then the evolving
3-d geometry of the pluton. The mineral lineations at the upper contact
are orthogonal to lineations throughout the exposed interior of the
intrusion. We suggest that the fabric at the contacts is a record of the
strain due to the relative displacement between the intruding magma
and wallrock, while the fabric away from the contacts is a record of the
strain within the flowing magma during infilling. Both in the field and
in thin section, magmatic textures are observed everywhere in the
pluton, except in the outermost few centimetres of the diorite at the
pluton-host rock contact, where cataclastic deformation is observed.
All the structural, textural and petrologic data (1) argue for a short
duration between intrusion of the earliest and latest magma batches,
and (2) a short duration between the initial intrusion and the full
crystallisation of the pluton, but (3) preclude the presence of any
magma chamber process such as crystal settling at the level of
emplacement. One-dimensional thermal modeling of the pluton,
constrained by texture observations, suggests that emplacement of
the pluton was a very rapid event, with a maximum duration on the
order of 100 years, which requires a minimum upward vertical
displacement rate of the wall rocks immediately above the pluton on
the order of 1 m per year.
Tinos pluton (St Blanquat et al. submitted): located in the Aegean
sea in Greece, the Tinos massif is a composite granodioritic pluton
emplaced in the Cycladic blueschist unit of Tinos island during the
mid-Miocene (14–15 Ma; Brichau et al, 2007) at 10–12 km depth
during protracted regional NE–SW extension. The pluton is a semielliptical body, with its long axis parallel to the NE–SW regional
stretching direction. The surface exposure is probably only the SW
half of a larger pluton, its NE part being located beneath the Aegean
Sea due to late activity on NW–SE striking, NE–verging normal
faults. Thus, it is possible to examine a vertical cross section of the
massif from its SW/bottom part to its NE/upper part. The total
minimum vertical exposure through the pluton is 530 m. The total
pluton thickness is not directly observable, but scaled cross sections
suggest a total thickness of approximately 1–2 km. The total pluton
volume is then approximately 10–40 km3 (or 20–80 km3 including
the submarine part). Microstructural analysis indicates the upper
part of the pluton was affected by ductile deformation, while the
lower part remains “undeformed” and exhibits magmatic texture.
The pluton displays internal contacts and layering marked by the
juxtaposition of granitic layers with different grain size and/or
mineralogy, or by biotite-enriched layers. These features can be
traced across large outcrops. Magmatic layering is typically sub
horizontal and has a shallower dip than the foliation. The pluton was
mainly emplaced by pushing its wallrocks upward, as exemplified by
wallrock deflection at the exposed margin and by concentric internal
foliations. The lineation defines a magmatic NW–SE trending pattern
in the lower/SW magmatic part of the pluton, and a NE–SW trending
pattern in its upper/NE ductilely deformed part. Our observations
suggest complex interactions between magma intrusion and the
regional extensional deformation, and document the vertical
migration of the brittle/ductile transition during regional extension.
However, some parts of the structural record, such as the NW–SE
magmatic lineation at the lowest exposed structural level of the
pluton, are not compatible with the regional deformation and may
magma infilling. The duration of construction of the Tinos pluton is
not constrained to date.
Papoose Flat pluton (Law et al., 1992; Morgan et al., 1998; Saint
Blanquat et al., 2001): located in the White-Inyo Range of California,
east of the Sierra Nevada batholith, this pluton is commonly cited as a
classic example of a ‘forcibly’ emplaced pluton (Sylvester et al., 1978),
although the relative importance attributed to magmatic versus
tectonic processes in controlling the structural evolution of the pluton
has been controversial (Paterson et al., 1991; Law et al., 1992). Reexamination of this Late Cretaceous (83 Ma) pluton has shown that
it is an inclined and internally zoned tabular structure, assembled by
the vertical stacking of successive magma sheets at a crustal depth of
about 12 km. The pluton is exposed over a 16 km by 8 km area, and
characterized by more than 1700 m of topographic relief and vertical
exposure. In map view the pluton is roughly elliptical in shape, with
the long axis trending WNW–ESE. A narrow ‘tail’ or ‘apophysis’ of
granite protrudes at the eastern end, and is topographically and
structurally located below the main body of the pluton. The total
volume is estimated to be between 100 and 200 km3. The Papoose Flat
pluton displays a compositional range from granite in the eastern part
of the pluton to granodiorite in the western part. Systematic analysis of
thin sections reveals a ring-shaped distribution pattern of mineralogy
and microstructures centered on the core of the pluton. The
preservation of compositional zoning within minerals, together with
the presence of internal magmatic contacts within the pluton,
indicates that these mineral and microstructural distribution patterns
are syn-emplacement, and are not the result of in-situ sub-solidus
reequilibration associated with metasomatism, and can therefore be
interpreted as evidence of incremental pluton assembly. A preliminary
examination of mineral chemistry by microprobe analysis has shown
only slight variations in mineral composition across the pluton,
possibly indicating a limited range in whole-rock chemistry, in
agreement with petrographic field observations. The foliation defines
an elongate WNW–ESE dome-shaped pattern trending sub-parallel
to the pluton's long axis and which, in map view, is not always
concordant with pluton margins. Lineations are sub-horizontal to
gently plunging and trend mainly NE–SW in the center of the pluton,
and NNW–SSE in the westernmost third of the pluton. The
M. de Saint Blanquat et al. / Tectonophysics 500 (2011) 20–33
microstructures define an elongate WNW–ESE trending concentric
map distribution with a domainal transition from magmatic microstructures in the centre of the pluton to high temperature solid-state
deformation features everywhere else, except near the pluton's
sidewalls and roof where a thin rind of intense gneissification is
recorded. The center of the pluton with its magmatic microstructures
locally contains either a NE–SW or less commonly NNW–SSE trending
lineation that must be synchronous with magma emplacement. In the
western third of the pluton the NNW–SSE trending lineation is
ubiquitously recorded in all three microstructural domains, suggesting
that lineations in the magmatic and solid-state domains are also
synchronous. This lineation is parallel to the grain-shape stretching
lineation in the surrounding aureole rocks. These data indicate that
solid-state fabrics near the pluton's sidewalls and roof (previously
suggested to be associated with regional deformation; Paterson et al.,
1991) cannot be chronologically separated from fabrics in the centre of
the pluton which appear to be related to magma flow during
emplacement. Initial pluton formation involved magma ascent in a
vertical WNW-striking feeder dike, which was arrested at a stratigraphically controlled mechanical discontinuity in the overlying
Cambrian metasedimentary rocks, leading to formation of a SWdipping sill. Subsequent sill inflation, accompanied by horizontal
infilling from the feeder dike at the base of the sill, resulted in
deformation and vertical translation of earlier magma pulses, and local
raising of the sill roof, facilitated by thermal weakening as the wall rock
temperatures progressively rose during emplacement of successive
magma pulses. Cooling from the roof of the pluton downward resulted
in cessation of vertical inflation on the W side of the pluton, and
promoted lateral expansion toward the NE and floor depression below
the eastern part of the pluton. We have been unable to document any
regional scale structures (e.g. equivalent to similar age syn-plutonic
strike-slip shear zones in the Sierra Nevada Batholith to the W, see
below) that may have controlled emplacement of the Papoose Flat
pluton. However, this does not preclude the likelihood that the country
rocks were subjected to a regional deformation at this time, as shown
by evidence of Late Cretaceous deformation on the White Mountain
shear zone and Santa Rita shear system, which constitutes the western
border of the White-Inyo range (Bartley et al., 2007; Sullivan and Law,
2007). Simple thermal modeling, constrained by microstructural and
thermobarometric data (Nyman et al., 1995; Saint Blanquat et al.,
2001), indicates that the total duration time for emplacement of the
pluton was on the order of 100,000 years.
Mono Creek pluton (Saint Blanquat and Tikoff, 1997; Tikoff and
Saint Blanquat, 1997; Saint Blanquat et al., 1998; Tikoff et al., 1999):
the late Cretaceous (86 Ma; Coleman and Glazner, 1997) Mono Creek
pluton is one of the youngest plutons within the Sierra Nevada
batholith. This porphyritic granodiorite / monzogranite covers an
area of ∼ 600 km2 and at least 1800 m of vertical relief, providing a
minimum volume estimate of ∼ 1000 km3. The depth of emplacement
is to date unconstrained, but hypo-volcanic textures found in the
northern part of the pluton suggest an upper crustal depth (1–
5 km?), confirmed by preliminary Al-in-hornblende geobarometry.
Detailed thin section analysis shows that the pluton is mineralogically
zoned, from biotite–hornblende–sphene in the border to biotite–
hornblende to biotite only in the center of the pluton. One of the most
striking characteristics of the pluton is its textural homogeneity;
except along the margins, where sheeted complexes and internal
contacts exist. No enclaves and no internal structures have been
observed in the main central part of the pluton. Nevertheless, rare
internal contacts exist between the three mineralogical facies, and
show that the biotite-only facies (similar to the Johnson Porphyry
facies in the Tuolumne Intrusive suite, see below; Titus et al., 2005)
constitutes the youngest magma pulse, and the biotite–hornblende–
sphene facies the oldest. The observations of the sheeted complex
along the margins and successives mineral facies with sharp
boundaries constitute field evidence for incremental assembly of
25
the Mono Creek pluton. Fabrics define a sigmoidal pattern of foliation
and lineation consistent with syn to late magmatic dextral shear
within the Rosy Finch Shear Zone, part the ∼ 200 km long Late
Cretaceous Sierra Crest Shear Zone system. We have observed a
continuous evolution between two end-member geometries: the
inferred earliest fabrics, located along pluton margins, are characterized by magmatic east–west sub-vertical foliations and sub-vertical
lineations, while the latest fabrics, in the center of the pluton, are
characterized by a solid-state fabric with N–S sub-vertical foliations
and sub-horizontal lineations. The transition between these two endmember fabrics is always progressive. We propose this fabric
evolution characterizes a progressive switch from emplacementdominated strain to regional-deformation-dominated strain. The E–
W foliation orientation of the first batches of magma occurs at a high
angle to the inferred regional finite strain (the dominant NNW–SSE
verticalfoliation suggests a principal shortening axis oriented WSW–
ENE and horizontal); this orientation could be due to over-pressured
magma opening accommodation space for itself. Thus, the first
batches of melt may have initiated the shear zones that parallel the
pluton, rather than being intruded passively between them. This
provides a good example of how magmatically induced strike-slip
partitioning may occur. Subsequently, the eastern bulge of the pluton
forcefully intruded 8 km toward the NE, pushing aside older magmas
and pre-existing wall rocks. This localized forceful intrusion was
apparently caused by an increase in the rate of magmatic infilling
relative to the steady-state rate of regional strike-slip motion. Finally,
the presence and cooling of this NE bulge locked the northern shear
zone and triggered initiation of the Rosy Finch Shear Zone (RFSZ)
coeval with the last pulses of magma emplacement. The RFSZ
connects the two original shear zones located on the sides of the
pluton, facilitating continued dextral strike-slip movement during
the solidification and cooling of the pluton. Magmatic foliation and
lineation are rotated into parallelism with the shear zone boundaries.
The duration of pluton construction is estimated to be on the order of
1–2 m.y., and is constrained with both geochronology and plate
kinematics (Tikoff and Saint Blanquat, 1997).
Tuolumne Intrusive Suite (Habert, 2004; Tikoff et al., 2005; Titus
et al., 2005; unpublished data): located in the central part of the Sierra
Nevada batholith, this spectacularly exposed plutonic complex has
been the subject of extensive debate (e.g. Bateman and Chappell, 1979;
Kistler et al., 1986; Bateman, 1992; Coleman et al, 2004; Glazner et al.,
2004; Zak and Paterson, 2005; Burgess and Miller, 2008; Gray et al.,
2008; Solgadi and Sawyer, 2009; and Johnson and Glazner, 2009,
among others). Individual plutons within the Tuolumne Intrusive
Suite (TIS) are nested and characteristically elongated NNW in map
view, and range in composition from granodioritic older plutons to
leucogranitic younger plutons. Based on Al-in-hornblende barometry,
these plutons were intruded at upper crustal depths (∼ 1–3 kbar; Ague
and Brimhall, 1988). The suite is exposed over an area of about
1200 km2, and its thickness is estimated at a minimum of 2 km
(maximum vertical relief) and at a maximum 10 km (based on gravity
data; Oliver, 1977). The suite's volume is estimated at between 2400
and 12,000 km3. The individual plutons constituting the suite are, from
older to younger, the Kuna Crest granodiorite, the Half Dome
granodiorite, the Cathedral Peak granite, and the Johnson porphyry,
constitute approximately 15%, 30%, 50% and 5% of the total volume
respectively. The internal structure of the TIS is complex. Each pluton
has an internal structure which shares similar characteristics with the
Mono Creek pluton, including internal textural homogeneity, sheeted
margins and rare internal contacts locally crosscut by the fabric, which
is mainly magmatic except along some margins. Our field and AMS
measurements document a vertical WNW–ESE magmatic foliation,
with more NW–SE strike toward pluton contacts and a more E–W
orientation in the pluton center, especially for the Cathedral Peak
pluton. The magmatic lineation is steeply plunging toward the NW in
the western part and toward the SE in the eastern part of the suite. In
26
M. de Saint Blanquat et al. / Tectonophysics 500 (2011) 20–33
the Cathedral Peak pluton, the fabric define a dextral sigmoid, like
in the Mono Creek pluton (see above), but here the shear zones are
located along the borders of the plutons and the tectonically
oriented fabrics are along pluton margins, especially the eastern
margin (see Tikoff et al., 2005), and not in the middle as observed
in the Mono Creek. We propose that there is a transition in the TIS
between fabrics which record an emplacement-dominated strain
(more E–W fabrics) and fabrics that record a regional-deformationdominated strain (more NW–SE fabrics). The total duration of
construction of the TIS is about 8–10 m.y. (Coleman et al., 2004).
Individual plutons within the suite have shorter durations of
construction, with a maximum of about 4 m.y. for the Half Dome
granodiorite. The existence of a repose time without magma
injection is suggested by the presence of mapable gradual or sharp
contacts between individual plutons and also by textures suggesting the presence of remelting in rocks in the old/cold side of some
internal contacts, for example in the Half Dome close to the contact
with the Cathedral Peak (Habert, 2004). But these field-based
criteria for a discontinuous magma injection are also questioned by
recent petrological and geochronological data (Coleman et al.,
2004; Gray et al., 2008; Johnson and Glazner, 2009) suggesting a
petrological continuum without significant time gap between
individual mapped “plutons”.
5. Significance of the plutonic record in the studied examples
We now use these plutons as a basis for generalizations about the
nature of the plutonic record preserved across a wide range of spatial
scales and in a wide range of tectonic settings. This broad dataset allows
us to consider several topics. First, we evaluate whether the plutonic
record effectively preserves evidence of tectonic setting. Second, we
examine relationships between the nature of the plutonic record and
pluton volume. Finally, we use the conclusions of the two previous
analyses to consider the different types of information that can be
reliably inferred from the plutonic record as pluton volume increases.
In the case of plutonism in the absence of regional tectonic
deformation (BM), and the case where timing relationships between
plutonism and regional deformation are uncertain (PFP), the pluton
foliation pattern has a domal shape, and is subparallel to the contact.
Foliations are concordant at the pluton scale but discordant at the
regional scale. The lineation pattern is complex and heterogeneous at
the pluton scale, which we interpret as a record of the history of
internal strain during pluton construction. The map distribution of
microstructures shows an absence of correlation between microstructure and fabric orientation, and a systematic development of
solid-state deformation at the pluton margins that becomes increasingly important with greater depth of emplacement.
When the fabric pattern and the microstructural zonation are
parallel to the pluton margins (BM and PFP), the pluton structural
record is clearly linked to pluton construction and the pluton contains
essentially no record of contemporaneous regional deformation. This
situation may arise from an effective absence of regional deformation
during pluton construction, as found in the Henry Mountains study. An
alternative explanation is necessary when it is obvious from other
geologic arguments that the area is subjected to a regional deformation during pluton growth, as in the case of the Papoose Flat pluton
example. One explanation is that the spatial distribution of tectonic
deformation and magmatism does not “exactly” coincide. Another
explanation is that the duration of the magmatic event is too short, and
the cooling rate of the magma too fast, to allow the recording of a
significant part of the regional deformation within the growing pluton.
An interaction between regional deformation and the construction
of a pluton can be indentified when the fabric pattern and microstructural zonation are parallel or partly parallel to the contemporaneous regional structural trend (TI, MCP, and TIS). However, the fact that
the orientation of the fabric is not always compatible with the regional
syn-magmatic deformation suggests that in many cases, the final pluton
structure is not completely controlled by or transposed into the regional
deformation. In fact, when a record of the syn-plutonic regional deformation is identifiable within the internal structure of a pluton, remnants
of construction processes such as infilling may also be present, as
observed in all studied plutons, including the largest ones.
5.1. Relation between the plutonic record and tectonic setting
5.2. Relation between the plutonic record and pluton volume
Several patterns emerge from examination of our data in the light
of the regional tectonic setting active during pluton construction.
In the case of transpression (TIS and MCP), the foliations are
vertical, and define a sigmoidal pattern in the horizontal plane, in
general agreement with the transpressional context; but depending
on their location, foliations may be either concordant or discordant
relative to pluton–wallrock contacts and to the regional foliation
trend. The lineations show a progressive change in orientation from
vertical to horizontal, and a close relationship is recorded between
their plunge and their trend. The NNW–SSE lineations are subhorizontal and parallel to the regional syn-magmatic deformation in the
wallrocks. The map distribution of microstructures shows a correlation between microstructure and fabric orientation, with solid-state
lineations parallel to the regional lineation, and a horizontal zonation
of microstructures.
In the extensional case (TI), the foliations are sub-horizontal and
define a flat dome. Because of the involvement of the top of the pluton in
a regional detachment, they define a sigmoid in the vertical plane, and
are either concordant or discordant depending on location. The
lineations are horizontal and show a bimodal orientation distribution
with no clear progressive transition between the NE–SW and NW–SE
end members, although the main NE–SW trend is parallel to regional
synplutonic deformation. Here also, the fabric pattern is only partially
compatible with the regional context. The map distribution of microstructures shows a correlation between microstructure and fabric
orientation, with solid-state lineation parallel to the contemporaneous
regional lineation, and a vertical microstructural zonation.
The previous analysis strongly suggests that one pertinent parameter to examine when considering these data is the final volume of the
pluton. We note that the smallest studied plutons (BM, TI, and PFP)
show a domal shape and are mostly regionally discordant. In contrast,
the largest plutons or suite of plutons (MCP and TIS) are commonly
elongate in map view and more or less regionally concordant. These
observations suggest that as pluton volume increases, the tectonic
history recorded within the pluton increases. In other words, a small
pluton has less possibility than a large pluton to record information on
syn-construction regional tectonic activity. Consequently, the internal
record of pluton construction may be partly or totally erased, depending
on the location and intensity of the tectonic overprint, and also on the
presence and intensity of secondary magma chamber processes.
It could be argued that the concordant nature of the fabric
observed in large plutons is an artifact of sampling, as in small plutons
where we have more samples per unit of surface area. Thus the
relatively low density of data used for fabric analysis in a large pluton
may not record the heterogeneity that would be apparent with more
data. However, this argument is contradicted by studies demonstrating that fabric orientation, from centimetre to kilometre scales, is not
typically scale dependant (see Olivier et al., 1997).
To summarize our observations, the 3-d shapes of the studied
plutons provide a better record of syn-plutonic regional deformation for
large plutons. Accordingly, the spatial variability of the internal
structure increases as pluton volume decreases. In small plutons, the
fabric is heterogeneous, and is due to the history of magma injection. In
M. de Saint Blanquat et al. / Tectonophysics 500 (2011) 20–33
large plutons the fabric pattern is more homogeneous, but shows only a
partial tectonic control. Large plutons commonly have both magmatic
and solid-state microstructures and the solid-state deformation may be
the result of regional deformation or magma injection, depending on the
cooling rate and the evolution of the thermal structure during pluton
construction. Small plutons also show a transition between magmatic
and solid-state microstructures, but the solid-state deformation is
clearly the result of the cooling of the intrusion margins during pluton
growth. Emplacement mechanisms of large plutons contain evidence
for ‘active’ intrusion due to pulse injection, followed by ‘passive’
straining due to regional deformation both between and during pulse
injection. In all plutons, the ‘active’ intrusion can produce a magmatically induced local ‘tectonic-like’ deformation, such as the spectacular
margin deformation on the west side of the PFP. Features like these are
preferentially preserved in small plutons.
5.3. Magmatic episodes during pluton construction
Such studies of the 3-d geometry, composition, internal structure,
and microstructure allow us to propose a model for the history of magma
intrusion within each studied pluton or group of associated plutons.
The Black Mesa pluton is very homogeneous, and we have found
no field evidence for separating the pluton into different intrusion
episodes. The successive magma injections have the same size and the
same composition, and the texture suggests a rapid cooling.
The Tinos pluton is a composite pluton with two main facies, a
central large granodiorite facies constructed by the amalgamation of
petrographically similar magma pulses, that are surrounded by small
marginal leucogranites, which are younger. This may indicate two
magmatic episodes with very different volumes.
Our model for construction of the Papoose Flat pluton involves two
main stages with slightly different chemical composition, one associated
with roof uplift and producing the spectacular deformation along the
western margin of the pluton, and one associated with lateral expansion
and floor depression. This change in emplacement mechanism may
indicate the presence of two main episodes of magma intrusion,
separated by a repose time, in order to account for cooling of the upper
part of the pluton which in turn induced locking of roof uplift.
The structural and compositional homogeneity of the Mono Creek
pluton indicate protracted magma intrusion, in the form of successive
pulses, as indicated by the mineralogical zonation and by sheeted
complexes along pluton margins. But the presence of a bulge on its NE
side indicates that this process was interrupted by a phase of more
intense magmatic activity, either because of an increase in the
injection rate and/or an increase in the injection volume.
For the Tuolumne Intrusive Suite, the debate rests on the question
of the existence of a large and dynamic magma system during its
construction, and on the relationship between mapable magmatic
units and intrusive events (Coleman et al., 2004; Glazner et al., 2004;
Zak and Paterson, 2005; Burgess and Miller, 2008; Gray et al., 2008;
Solgadi and Sawyer, 2009; Johnson and Glazner, 2009). In a recent
study, Gray et al., (2008) found that map units, i.e. the plutons
constituting the suite, record parts of a single petrological continuum
rather than distinct intrusive phases, and that the textural differences
that define the units probably reflects thermal evolution of the system
during cooling rather than distinct intrusive events. As the thermal
evolution of this kind of arc system should be magmatically
controlled, we think the interpretation is sustainable that each pluton
of the TIS corresponds to a period of protracted magmatic activity,
with similar characteristics as the MCP, and separated by a repose
time. Therefore, the contact between two plutons may be either sharp
or gradational, and have different relationship with the internal fabric,
depending on the cooling rate during the repose time, which itself
depends on the local 3-d geometry of all intrusions. However, in the
TIS (and other large plutons), the size and initial 3-d geometry of
individual injections are to date precisely unknown, as well as the
27
duration of repose times between the main injection episodes.
Consequently, the definition of what we call “pluton” is still an open
question, particularly in a batholithic setting.
In each of our case studies, the main space-making mechanism is
wallrock translation, vertically toward the surface (roof uplift, all
studied plutons), or downwards toward the Moho (PFP), but also
laterally (PFP, MCP, TIS). This deformation is mainly due to magma push,
as clearly shown by the BM, TI and PFP cases. But the orientation of the
magmatic fabric in the large ‘syntectonic’ MCP pluton and TIS suite,
which is E–W and vertical and also not compatible with or controlled by
the syn-plutonic regional strain, shows that this mechanism is present
in all plutons whatever the spatial and temporal scales.
6. Synthesis: multiscale magmatic cyclicity
In all settings and in plutons of all sizes we have observed an
internal textural homogeneity associated with a more or less
pronounced petrological zonation and/or internal contacts and
magmatic layering showing spatially rapid textural change, which
sometimes corresponds with associated mineralogical changes. These
observations strongly suggest pluton construction by successive
injections of pulses of magma. The textural homogeneity may be
due to the emplacement of pulse n before the complete solidification
of pulse n-1. However, even if pulse n-1 has the time to completely
crystallize before the injection of pulse n, the texture could be
homogeneous because of remelting or textural aging due to the
numerous heating and cooling cycles induced by this magma pulsing
(Habert, 2004; Johnson and Glazner, 2009; Miller et al., 2011). The
critical control is then the cooling condition of each pulse. When
mappable, these geologic markers can be interpreted as evidence for
contacts between successive pulses of magma, more or less smoothed
and transposed by younger intrusion, deformation events, or magma
chamber processes. These features may not represent true ‘isochrons’,
or ‘emplacement time-lines’, but they are probably not very far from
it. This process of successive magma pulse injection is thus the
fundamental process underlying pluton construction.
Consideration of all the above data concerning pluton structural
record, pluton volume, and duration of pluton construction, and their
interpretation in terms of relationships between the dynamics of
magmatic and tectonic processes, leads to the conclusion that pluton
size is closely tied to the time required for pluton assembly (Fig. 3). This
apparently trivial observation indicates that the incremental process of
pulse injection has roughly the same characteristics (frequency, volume,
and rate) for all plutons in each setting. To date, in arcs, we do not know
any example of large and very rapidly emplaced plutons (more than
∼1000 km3 in less than 100,000 years), or small and very slowly
emplaced plutons (less than ∼100 km3 in more than 100,000 years). It
also appears that each pluton size is related to magmatic pulsation at a
particular time scale independent of the regional tectonic context. Thus,
the observed pulsation has a different significance at each time/space
scale. In small plutons, it is the incremental process, the building block of
plutons. In larger plutons, the incremental process is lost, and the
observed pulsation consists of a cycle of injections at different timescales
(the ‘episodicity’ of Pitcher, 1979). The origin of this pulsation over
longer periods of time must be related to lithology and thermal regime
of the upper mantle–lower crust region, itself driving magmatic
processes (melting, segregation, and transfer) in interactions with
tectonic boundary conditions (e.g. Cruden, 2006).
7. Discussion
7.1. Validity of our database and comparison with other plutons
The plutons described here are not exceptional cases and, in fact,
we view each of them as representative of a certain type of pluton. For
example, the PFP is representative of many other “ballooning” plutons
28
M. de Saint Blanquat et al. / Tectonophysics 500 (2011) 20–33
Fig. 3. Evolution of pluton construction with time, from geochronological data were available (Coleman et al., 2004 for the TIS), and from our petrostructural data and thermal
modelling (see text). This figure illustrates the multiscale cyclicity of plutonic activity, from the incremental process of pulsed injection in the smallest plutons (BM), to the longer
magmatic episodes of the largest plutons and suites of plutons (PFP, MCP and TIS); see text for more explanation. The Tinos pluton is not represented in this figure because of the
absence of constraints on its duration of construction kc Kuna Crest pluton, hde equigranular Half Dome pluton, hdp porphyric Half DOme pluton, cp Cathedral Peak pluton, jp
Johnson Porphyry; hb hornblende, sph sphene, bi biotite.
(Sylvester et al., 1978; Paterson et al., 1991; Law et al., 1992; Nyman
et al., 1995; Morgan et al., 1998; Saint Blanquat et al., 2001), like
Flamanville (Brun et al., 1990), Ardara (Vernon and Paterson, 1993;
Morgan, 1995; Vernon and Paterson, 1995; Molyneux and Hutton,
2000), and Cannibal Creek (Bateman, 1985a, 1985b; Davis, 1993,
1994; Godin, 1994). The question asked by these different authors is
the same for all these plutons: what are the respective contributions
of magmatic and tectonic processes in the plutonic record? The
answers we have arrived at for the PFP may be appropriate for the
other plutons, but the determination of the duration of construction of
these plutons are essential to interpret their plutonic record. Similarly,
the large plutons or intrusive suites (MCP and TIS) share similar
characteristics with many other arc-type large plutons, like other
Cretaceous Sierran plutons (e.g., Dinkey Creek and Mount Givens
plutons; Cruden et al., 1999; McNulty et al, 2000), the Panafrican
transpressive plutons in Brazil (Archanjo et al., 1999) or Africa (Ferré
et al., 1998), or Variscan transpressive plutons in western Europe
(Gleizes et al., 1997). The unique aspect of the data we discuss is
consideration of plutons, such as the Henry Mountains examples,
which were emplaced in the absence of regional deformation. These
intrusions constitute key case studies that allow us to study magmatic
processes without tectonic interference, and document the initial
stages of pluton growth (Horsman et al., 2009).
Exceptions to our assumption of the increasing tectonic nature of
the plutonic record as pluton volume increases certainly exist. For
large granitic plutons in arc settings, these exceptions are commonly
due to the effective absence of regional deformation during pluton
construction or the localization of the pluton in a quiet tectonic area,
as in the case of the Jurassic Eureka–Joshua Flat–Beer Creek (EJB)
pluton in eastern California (Morgan et al., in prep.) or the Mount
Givens granodiorite in the Sierra Nevada batholith (McNulty et al.,
2000). For small plutons showing a significant tectonic control, like
the small plutons in the Dolbel batholith (Pons et al., 1995; Pupier
et al., 2008), we must first ensure this inprint was acquired during
pluton construction. Then, maybe particular emplacement conditions
(higher P and/or T) could explain a slower cooling rate and a longer
“magmatic life” for the pluton.
7.2. The nature of the plutonic structural record
As stated in the beginning of this article, any plutonic record may be
interpreted in two ways: (1) as a record of deformation, either related
to emplacement dynamics (injection) or to the regional deformation,
and/or (2) as a magmatic record related to magma chamber processes
acting during cooling of successive magma pulses.
A basic observation is that depending on the individual pluton, none,
only one, or both types of record may be observed. The prominence of
one or another type of record is strongly dependent on the cooling rate
of the system. Fast cooling rates favor preservation of the record of
injection processes. In contrast, slow cooling rates allow deformation,
transposition, or the complete ereasing of the injection record, and favor
initiation of magma chamber processes and interference between the
M. de Saint Blanquat et al. / Tectonophysics 500 (2011) 20–33
growing “magmatic” pluton and the regional deformation, if present.
The cooling rate itself is linked to the thermal structure of the crust,
which, in magmatic arcs, is directly controlled by the geometry, the
frequency and the volume of magma batches (Barton and Hanson,
1989). Cooling rate is therefore directly controlled by the history and
geometry of magma injection. The dynamics of magma infilling, which is
source-controlled, will then determine the nature of the plutonic record.
This is in agreement with our general assumptions concerning the
importance of constraining the duration of construction in interpreting
the nature of the plutonic record. All things being equal, small/rapidly
constructed plutons generally have faster cooling rates than large/more
slowly constructed plutons.
7.3. The timescales of pluton construction
Our studies have shown that the duration of pluton construction
can vary by orders of magnitude during a single tectonic event, and
that the duration of construction is correlated at first order to pluton
volume. This is confirmed by the very precise geochronological data
obtained recently on the Tuolumne Intrusive suite (N2500 km3, 8 m.y.,
29
Coleman et al., 2004), the Mount Stuart (1200 km3, 5.5 m.y.) and
Tenpeak (400 km3, 2.6 m.y.) intrusions (Matzel et al., 2006), and the
Torres del Paine granitic pluton (between 100 and 200 km3,
90,000 years; Michel et al., 2008).
In order to compare our data with the published literature on the
same subject, we have made a compilation of all available data on the
duration of pluton construction and compared these data with
corresponding data on pluton volume when available (Fig. 4). We
have sorted the data based on tectonic setting, and have included
currently growing plutons detected by geophysical methods. Data
comparing ancient with currently growing volcanic systems are also
shown. The primary observations from this compilation are as follows:
(1) We find a positive correlation between volume and duration of
pluton construction. As discussed previously, the larger a pluton,
the longer its construction time. Big/fast or small/slow plutons
have not been identified to date. The direct consequence is that
plutonic magmatic fluxes seem to be comparable from one
context to another and also over various geologic time spans. This
is of fundamental importance and confirms that deep magmatic
processes like melting and segregation, controlled both by mantle
Fig. 4. Compilation of available data on duration and rates of pluton construction; see text for explanation; 1 - Black Mesa (Habert et St-Blanquat, 2004; St-Blanquat et al., 2006), 2 Elba island (Rocchi et al, 2002), 3 - Papoose Flat (St-Blanquat et al, 2001), 4 - Emerald lake (Coulson et al, 2002), 5 - Mono Creek (St-Blanquat et al, 1998), 6- Half Dome (Coleman et
al, 2002), 7 - Tuolumne intrusive suite (Coleman et al, 2002), 8 - Scuzzy (Brown & McClelland, 2000), 9- Tinos (St-Blanquat et al, in prep), 10 - Bald Mtn (Petford et al, 2000) , 11 Dinkey Creek (Petford et al, 2000), 12 - Mnt Givens (Petford et al, 2000) , 13 - Socorro magma body (Fialko et Simons, 2001), 14 - Syowa Sinzan (Minakami et al., 1951), 15 - Lazufre &
Hualca Hualca volcano (Pritchard & Simons, 2002), 16 - Empereur Mnts (Shaw, 1985), 17 - Unzen volcano (Nakada & Motomura, 1999), 18 - Tenpeak pluton (Matzel et al, 2006), 19 Geysers plutonic complex (Schmitt et al, 2003), 20 - Bergell pluton (from Oberli et al, 2004),21 - Salmi complexes (Amelin et al, 1997), 22 - Lastarria (Froger et al, 2007), 23 - Torres
del Paine (Michel et al, 2008), 24 - APVC (de Silva and Gosnold, 2007), 25 - Aleutian Island arc (Jicha et al, 2006), 26 - Stuart Mtn batholith (Walker et al, 2007), 27 - Mnt Stuart
(Matzel et al, 2006), 28 - Tenpeak (Matzel et al, 2006), 29 - Manaslu (Annen et al, 2006), 30 - PX1 pluton (Allibon et al, this vol.), 31 - Southern Rocky Mtn volc. field (Lipman, 2007),
32 - volcanism in a: oceanic arcs; b: continental arcs (White et al, 2006), 33 - Uturuncu volcano; a: long term; b: actual (Sparks et al., 2008).
30
M. de Saint Blanquat et al. / Tectonophysics 500 (2011) 20–33
(2)
(3)
(4)
(5)
(6)
and crust fertility and boundary conditions (e.g. age and rate of the
downgoing oceanic plate in subduction systems), are the controlling processes of pluton formation in the middle and upper crust.
We observe an absence of correlation between duration and
rate of pluton construction and the tectonic context, which
again indicates that the elementary controlling process is
magmatic (pulse intrusion).
We observe a weak tendency for time averaged construction
rates to be faster in small plutons than in large plutons. As
discussed above, this is linked to the existence of a characteristic
cyclicity timescale that depends on pluton size. This cyclicity
may be due to the existence of magmatic episodes, which are
separated by longer repose times in large plutonic systems.
In the available dataset, the range of time averaged pluton
construction rates covers 3 orders of magnitude, from 10− 1 to
10− 4 km3yr− 1 (or approximately 1 to 10−3 m3 s− 1). This
variation does not seem to be explained by the processes
associated with construction of plutons. Further work should
explore the interactions between magmatic and geodynamic
processes located below the level of pluton emplacement, and
particularly the link(s) between the magmatic productivity and
the changes in the boundary condition of the system.
A strong correlation exists between data on ancient and
modern plutonic systems, which validates our results and
interpretations.
Finally, we note a similarity between plutonic and volcanic
rates, in both ancient and currently active systems. This must
be considered in the light of on-going debate concerning the
link between volcanism and plutonism, and whether large
open magma chambers exist or not, both at the present day and
in the geologic past (e.g. Wiebe, 1988; Robinson and Miller,
1999; Barnes et al., 2001; Miller and Miller, 2002; Wiebe et al.,
2002; Metcalf, 2004; Glazner et al., 2004; Bachmann et al.,
2007; Annen, 2009; Annen, 2011).
8. Conclusion
We find a positive correlation between volume and duration of
pluton construction. The larger a pluton, the longer its construction
time. Large/fast or small/slow plutons have not been identified to
date. The direct consequence is that plutonic magmatic fluxes seem to
be comparable from one geodynamic context to another and also over
various geologic time spans. A consequence of this correlation is that
small plutons, which are constructed over short geological time at the
geologic time scales, commonly record little about tectonic conditions
during their construction. In large plutonic bodies, which correspond
to longer duration magmatic events, regional deformation has time to
interact with the growing pluton and can be recorded within the
pluton–wallrock structure. In addition to other important parameters
like cooling rate, the structural record of an individual intrusion
therefore cannot be interpreted without information on the duration
of its construction, which will determine the potential extent of
mechanical interaction between the magma and host rocks.
The pulsed nature of plutonism, that is the incremental assembly
of plutons and its spatial and temporal characteristics, offers a simple
explanation of this complexity. Specifically, the fast rate of magma
transfer in the crust (on the order of cm/s) relative to normal tectonic
rates (on the order of cm/yr) makes pluton construction independent
of – but not insensitive to – the tectonic context. Each pluton size is
related to a magmatic pulsation at a particular time scale, and each of
these coupled time/space scales is related to a specific process: in
small plutons, we can observe the incremental process, the building
block of plutons; in larger plutons, the incremental process is lost, and
the pulsation, which consists of a cycle of injections at different
timescales, must be related to the composition and thermal regime of
the source region, itself driving magmatic processes (melting,
segregation, and transfer) interacting with tectonic boundary conditions. The fast operation of magmatic processes relative to crustal
tectonic processes ensures that magmatic process control the system
from below.
Consequently, the dynamics of the pulsed magmatism observed in
arc plutonic systems is a proxy for deep lithospheric and magmatic
processes.
Acknowledgements
All our works cited in this paper were funded by CNRS/INSU grants
(DBT 1992–1993, IT 2001–2002, DyETI 2003–2005, and 3F 2008–
2009), NSF grants (EAR–9305262, EAR-0003574, EAR-0510893, EAR9506525, and EAR-9018929), and CNRS/NSF grants (12971 and
94N92/0049). Comments on drafts and revised versions of the
manuscript by C. Annen and T. Menand, and critical and constructive
reviews by A.R. Cruden and P. Barbey are gratefully acknowledged.
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