Is stoping a volumetrically significant pluton emplacement process?:
Discussion
Aaron S. Yoshinobu†
Calvin G. Barnes*
Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA
Glazner and Bartley (2006) cite and interpret
five lines of evidence to indicate that magmatic
stoping is unlikely to be a volumetrically significant process during pluton emplacement.
Because of the iconoclastic tone of the paper, it
is useful to examine each line of evidence cited
by Glazner and Bartley and to test their conclusions against previous work. Although we appreciate the alternative hypotheses that Glazner and
Bartley (2006) present, we argue that two of the
five lines of evidence are model-driven and not
consistent with natural observations, and that
the remaining lines of evidence are either not
supported by the published literature or based
on untested assumptions. While we remain as
troubled as Glazner and Bartley by some of the
implications of stoping to magma emplacement
and magma compositional evolution, we do not
think that their arguments support their premise.
Therefore, the process of stoping as a potential
mechanism for magma-rock interaction must
continue to be evaluated.
We begin by defining terms and adopting a
modified version of Daly’s (1903a, 1903b) definition for stoping, which includes the incorporation of host-rock xenoliths during intrusion
and migration of magma. While Daly (1903b)
hypothesized that extreme thermal stresses facilitate stoping, we adopt a more general causal
explanation to include both thermal and mechanical stresses associated with dike emplacement
and fluid migration and/or expulsion. Although
Glazner and Bartley (2006) did not specifically
define stoping, we infer from the article that the
authors use the term to describe the incorporation of host rocks (xenoliths) into an invading
magma (by any mechanism), rotation of the
xenoliths, and downward transport of the xenoliths (e.g., third column, p. 1185, and throughout
the article). In contrast, our definition does not
require blocks to rotate or be translated downward, but they should be displaced relative to a
fixed host-rock reference frame.
†
E-mail: aaron.yoshinobu@ttu.edu.
*E-mail: cal.barnes@ttu.edu.
Glazner and Bartley (2006) cited three related
lines of field evidence to substantiate their argument that stoping is not a significant process:
(1) Few observed xenoliths in plutons indicate
that few xenoliths ever make it into plutons. (2)
Large-volume magmatic breccias do not exist,
and therefore stoping must not have occurred.
(3) Xenolith fragmentation results in a fractal
size distribution. All three lines of evidence rely
on two assumptions or assertions. The first is
that xenolith size distributions are well documented in plutons. The second is that the only
process that may affect xenoliths after incorporation is thermal fragmentation.
With regard to xenolith distributions in plutons, seemingly contradictory observations are
presented. On page 1185, the authors state, “the
absence of small xenoliths at pluton contacts…
argues against stoping.” However, on page 1186,
they state, “the concentration of xenoliths in a
pluton locally may be high (tens of percent),
especially near pluton margins.” No references to
published work or to new quantitative data that
demonstrate the size distribution of xenoliths in
plutons are presented. Rather, the reader is asked
to accept these qualitative field observations. In
this regard, we note that any understanding of
stoping will require intensive field, petrological,
geochemical, and theoretical study of the process
zone between the magma and its host rocks.
Recent work by our research group (Wolak et
al., 2004, 2005; Marko et al., 2005) indicates that
xenolith distributions do not follow fractal sizefrequency distributions, as suggested by Glazner
and Bartley (2006). Xenolith frequency-size distributions diverge from a power-law relationship
at sizes <~150 m2 to several square meters and
smaller (Marko et al., 2005). We presently
interpret these results to indicate that xenolith
size distributions reflect a number of competing processes, including thermal fragmentation
(e.g., stoping or “diking”), but also dissolution
and melting (particularly at finer block sizes),
and inherited anisotropy/shape from the host
rocks (e.g., Clarke et al., 1998). Therefore, we
are skeptical whether or not their “exploratory”
GSA Bulletin; July/August 2008; v. 120; no. 7/8; p. 1080–1081 doi: 10.1130/B26141.1.
1080
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thermal fracturing experiment (p. 1187) and
its resulting fractal distribution of fragments
actually characterize natural disaggregation
processes in magmas. Given the current state
of knowledge, there are insufficient field data
to warrant the dismissal of the stoping process
based on xenolith size-frequency or distribution
studies in natural plutons.
The next line of evidence cited by Glazner
and Bartley (2006) is that “geochemical data
from many plutons clearly rule out significant
bulk assimilation of local wall rocks.” Glazner
and Bartley (2006) cite three examples of plutons that show no evidence for assimilation of
local host rocks. Although the literature is replete
with examples in which authors interpret compositional variation of magmas to result from
in situ assimilation (e.g., Clarke et al., 1988;
Barnes et al., 1990; Tegtmeyer and Farmer,
1990; Landoll and Foland, 1996; Coogan et al.,
2002; Dumond et al., 2005, among many), a
thorough recitation of individual examples, pro
or con, is pointless here. Instead, we prefer to
focus on the mechanical and thermal conditions
that relate to host-rock assimilation and stoping:
For example, does efficient assimilation require
partial melting of the contaminant (e.g., Spera
and Bohrson, 2001), or can it be achieved by
dissolution (discussion by McBirney, 1979), or
by incongruent reactions (Beard et al., 2005)?
What are the energetic limitations to the mass
of material that can be assimilated (e.g., Spera
and Bohrson, 2001)? What is the effect of thermal preconditioning of the host rocks? How can
in situ assimilation (due to stoping or any other
mechanism) be unambiguously distinguished
from partial melting of heterogeneous source(s)
and/or magma mixing in the source, conduit, or
site of emplacement? As outlined by Glazner
and Bartley (2006), if geochemical tests demonstrate that magma compositional evolution is
compatible with some amount of assimilation
of observed host rocks, then one would expect
the host-rock xenoliths to have been physically
and chemically digested by the magma. If such
xenoliths did not enter the magma by stoping
Discussion of the significance of stoping
(either the strict definition [e.g., Daly, 1903a;
Marsh, 1982] or our modified definition), then
alternative mechanisms of in situ assimilation
must be found.
A corollary to the process of in situ assimilation by stoping is that some xenoliths, particularly refractory ones, may be preserved. If
such xenoliths are rotated or translated from
their original orientation, they provide de facto
evidence for stoping. In the Bindal Batholith,
central Norway, we have utilized the types of
geochemical tests that Glazner and Bartley
(2006) prescribe in studies of plutons emplaced
at 700 MPa pressure. In one example (Barnes et
al., 2004; Dumond et al., 2005), major-element,
trace-element, rare earth element, and stable isotopic data indicate that as much as 20% of the
observed magma mass represented by a crystallized pluton is the result of local assimilation of
“stoped” metapelitic host rocks. These results
suggest to us that stoping is a significant process
in facilitating magma compositional evolution,
at least at these pressures.
The final line of evidence against stoping
cited by Glazner and Bartley (2006) involves
their preferred model for pluton construction
in the crust: incremental magma emplacement.
On the basis of U-Pb zircon geochronology,
emplacement of the large Tuolumne intrusive
suite, central Sierra Nevada Batholith, California, has been interpreted to have taken several
million years (Coleman et al., 2004). Because
thermal models suggest that large batholiths
emplaced over millions of years could not have
been entirely molten at any given time, Glazner
and Bartley (2006) infer that these bodies were
not capable of dislodging and engulfing large
trains of host-rock blocks, nor were they able to
hybridize or convect (e.g., Glazner et al., 2004).
We do not fully understand this inference. Presumably, Glazner and Bartley (2006) make
this inference because incrementally emplaced
bodies are smaller than whole plutons and lose
energy through cooling so quickly that they cannot mix, assimilate, or deform their host rocks.
We see no a priori reason why incremental
pluton growth is incompatible with stoping and
xenolith assimilation, or with hybridization and
convection, for that matter. Small intrusions are
capable of host-rock detachment and xenolith
assimilation. For example, Green (1994) demonstrated that assimilation of “wall-rock xenoliths”
in dikes may significantly affect the compositional evolution of continental-arc magmas. In a
well-documented study of lavas from Parícutin,
McBirney et al. (1987) demonstrated how local
assimilation of crustal rocks similar to those
exposed on the surface near the volcano could
explain differentiation to more evolved lavas.
Partially melted felsic xenoliths included in the
lavas formed one component of the differentiation trend (McBirney et al., 1987). While these
authors hypothesized that melting of crustal
rocks along the walls of a stratified magma
chamber facilitated differentiation through
assimilation, the observations that xenoliths are
included in the lavas and that they provide one
component for a mixing trend imply that stoping
and assimilation of these xenoliths occurred during eruption of Parícutin lavas. Lastly, we note
that the margins of some dioritic plutons in the
Bindal Batholith acted as zones of mafic magma
intraplating in which anatexis of metapelitic host
rocks and hybridization of these compositions
occurred in conduits that may have been operative for hundreds of thousands to possibly millions of years (Barnes et al., 2002). While not
significant in terms of map area, these bodies
were fundamental to the compositional evolution of the arc crust. In summary, incremental
magma emplacement is quite compatible with
fundamental physical and chemical processes
such as thermal/mechanical fracturing, anatexis,
assimilation, and hybridization.
The iconoclastic perspective presented by
Glazner and Bartley is not supported by existing
data and is based on untested assumptions. Existing field observations and interpretations either
do not support their contentions regarding xenolith distributions or are inconclusive. The few
existing field studies on the size-frequency distributions of xenoliths in natural pluton–host rock
systems are contrary to Glazner and Bartley’s
theoretical and analog models. While Glazner
and Bartley (2006) documented three plutons
that do not show evidence for local assimilation,
we and others have previously demonstrated that
the compositional evolution of some plutons
may be the result of assimilation of local host
rocks, in one case, 20% of the observed magma
mass. Whether such masses are significant is a
semantic rather than a geologic issue.
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REVISED MANUSCRIPT RECEIVED 10 APRIL 2007
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