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Is stoping a volumetrically significant pluton emplacement process?
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By A. Glazner and J. Bartley
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Discussion by:
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Aaron S. Yoshinobu1 and Calvin G. Barnes2, Department of Geosciences, Texas
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Tech University, Lubbock, TX, 79409-1053, U.S.A.
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Glazner and Bartley (2006, hereafter cited as G&B) cite five lines of
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evidence they interpret to indicate that magmatic stoping is unlikely to be a
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volumetrically significant process during pluton emplacement. Because of the
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iconoclastic tone of the paper, it is useful to examine each line of evidence cited
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by G&B, and to test their conclusions against previous work. Although we
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appreciate the alternative hypotheses that G&B present, we argue that two of the
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five lines of evidence are model-driven and not consistent with natural
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observations, and that the remaining lines of evidence are either not supported
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by the published literature, or based on untested assumptions. While we remain
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as troubled as Glazner and Bartley by some of the implications of stoping to
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magma emplacement and magma compositional evolution, we do not think that
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their arguments support their premise. Therefore the process of stoping as a
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potential mechanism for magma-rock interaction must continue to be evaluated.
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We begin by defining terms. A modified version of Daly’s (1903a,b)
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definition for stoping is adopted; it includes the incorporation of host rock
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xenoliths during intrusion and migration of magma. While Daly (1903b)
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hypothesized that extreme thermal stresses facilitate stoping, we adopt a more
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general causal explanation to include both thermal and mechanical stresses
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associated with dike emplacement and fluid migration and/or expulsion. Although
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G&B did not specifically define stoping, we infer from the article that the authors
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Email: [email protected]; phone: 1-806-742-4025
Email: [email protected]; phone: 1- 806-742-3106
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use the term to describe the incorporation of host rocks (xenoliths) into an
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invading magma (by any mechanism), rotation of the xenoliths, and downward
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transport of the xenoliths (e.g., third column p. 1185, and throughout the article).
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In contrast, our definition does not require blocks to rotate or be translated
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downward, but they should be displaced relative to a fixed host rock reference
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frame.
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G&B cited three related lines of field evidence to substantiate their
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argument that stoping is not a significant process: i) Few observed xenoliths in
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plutons indicates that few xenoliths ever make it into plutons. ii) Large-volume
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magmatic breccias do not exist, therefore stoping did not occur. iii) Xenolith
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fragmentation results in a fractal size distribution. All three lines of evidence rely
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on two assumptions or assertions. The first is that xenolith size distributions are
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well documented in plutons. The second is that the only process that may affect
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xenoliths after incorporation is thermal fragmentation.
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With regard to xenolith distributions in plutons, seemingly contradictory
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observations are presented. On page 1185, the authors state, “the absence of
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small xenoliths at pluton contacts… argues against stoping”. Yet, on page 1186,
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they state, “the concentration of xenoliths in a pluton locally may be high (tens of
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percent), especially near pluton margins”. No references to published work or to
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new quantitative data that demonstrate the size distribution of xenoliths in plutons
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is presented. Rather, the reader is asked to accept these qualitative field
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observations. In this regard we note that any understanding of stoping will require
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intensive field, petrological, geochemical, and theoretical study of the process
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zone between the magma and its host rocks.
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Recent work by our research group (Wolak et al., 2004; 2005; Marko et
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al., 2005) indicates that xenolith distributions do not follow fractal size-frequency
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distributions, as suggested by G&B. Xenolith frequency-size distributions diverge
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from a power-law relationship at sizes less than about 150 m2 to several m2 and
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smaller (Marko et al., 2005). We presently interpret these results to indicate that
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xenolith size distributions reflect a number of competing processes, including
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thermal fragmentation (e.g., stoping or ‘diking’), but also dissolution and melting
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(particularly at finer block sizes), and inherited anisotropy/shape from the host
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rocks (e.g., Clarke et al., 1998). Therefore, we are skeptical whether or not their
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‘exploratory’ thermal fracturing experiment (p. 1187) and its resulting fractal
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distribution of fragments actually characterizes natural disaggregation processes
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in magmas. Given the current state of knowledge, there are insufficient field data
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to warrant the dismissal of the stoping process based on xenolith size-frequency
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or distribution studies in natural plutons.
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The next line of evidence cited by G&B is that “geochemical data from
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many plutons clearly rule out significant bulk assimilation of local wall rocks.”
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G&B cite three examples of plutons that show no evidence for assimilation of
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local host rocks. Although the literature is replete with examples in which authors
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interpret compositional variation of magmas to result from in situ assimilation
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(e.g., Clarke et al., 1988; Barnes et al., 1990; Tegtmeyer and Farmer, 1990;
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Landoll and Foland, 1996; Coogan et al., 2002; Dumond et al., 2005 among
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many), a thorough recitation of individual examples, pro or con, is pointless here.
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Instead, we prefer to focus on the mechanical and thermal conditions that relate
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to host rock assimilation and stoping: For example, does efficient assimilation
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require partial melting of the contaminant (e.g., Spera and Bohrson, 2001), or can
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it be achieved by dissolution (discussion by McBirney, 1979), or by incongruent
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reactions (Beard et al., 2005)? What are the energetic limitations to the mass of
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material that can be assimilated (e.g., Spera & Bohrson, 2001)? What is the
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effect of thermal pre-conditioning of the host rocks? How can in situ assimilation
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(due to stoping or any other mechanism) be unambiguously distinguished from
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partial melting of heterogeneous source(s) and/or magma mixing in the source,
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conduit, or site of emplacement? As outlined by G&B, if geochemical tests
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demonstrate that magma compositional evolution is compatible with some
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amount of assimilation of observed host rocks, then one would expect that host
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rock xenoliths were physically and chemically digested by the magma. If such
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xenoliths did not enter the magma by stoping (either the strict definition, e.g.,
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Daly, 1903a; Marsh, 1982; or our modified definition) then alternative
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mechanisms of in situ assimilation must be found.
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A corollary to the process of in situ assimilation by stoping is that some
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xenoliths, particularly refractory ones, may be preserved. If such xenoliths are
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rotated or translated from their original orientation, they provide de facto evidence
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for stoping. In the Bindal Batholith, central Norway, we have utilized the types of
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geochemical tests that G&B prescribe in studies of plutons emplaced at 700 MPa
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pressure. In one example, (Barnes et al., 2004; Dumond et al., 2005) major,
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trace, rare earth element and stable isotopic data indicate that as much as 20%
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of the observed magma mass represented by a crystallized pluton was the result
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of local assimilation of ‘stoped’ metapelitic host rocks. These results suggest to
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us that stoping is a significant process in facilitating magma compositional
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evolution at least at these pressures.
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The final line of evidence against stoping cited by G&B involves their
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preferred model for pluton construction in the crust: incremental magma
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emplacement. On the basis of U–Pb zircon geochronology, emplacement of the
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large Tuolumne Intrusive Suite, central Sierra Nevada batholith, California, has
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been interpreted to have taken several million years (Coleman et al., 2004).
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Because thermal models suggest large batholiths emplaced over millions of
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years could not have been entirely molten at any given time, G&B infer that these
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bodies were not capable of dislodging and engulfing large trains of host rock
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blocks, nor were they able to hybridize or convect (e.g., Glazner et al., 2004). We
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do not fully understand this inference. Presumably, G&B make this inference
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because incrementally emplaced bodies are smaller than whole plutons and lose
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energy through cooling so quickly that they cannot mix, assimilate nor deform
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their host rocks.
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We see no a priori reason why incremental pluton growth is incompatible
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with stoping and xenolith assimilation, or with hybridization and convection, for
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that matter. Small intrusions are capable of host rock detachment and xenolith
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assimilation. For example, Green (1994) demonstrated that assimilation of ‘wall
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rock xenoliths’ in dikes may significantly affect the compositional evolution of
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continental arc magmas. In a well-documented study of lavas from Paricutin,
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McBirney et al., (1987) demonstrated how local assimilation of crustal rocks
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similar to those exposed on the surface near the volcano, could explain
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differentiation to more evolved lavas. Partially melted felsic xenoliths included in
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the lavas formed one component of the differentiation trend (McBirney et al.,
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1987). While these authors hypothesized that melting of crustal rocks along the
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walls of a stratified magma chamber facilitated differentiation through
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assimilation, the observation that xenoliths are included in the lavas and provide
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one component for a mixing trend, implies that stoping and assimilation of these
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xenoliths was occurring during eruption of Paricutin lavas. Lastly, we note that
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the margins of some dioritic plutons in the Bindal Batholith acted as zones of
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mafic magma intraplating in which anatexis of metapelitic host rocks and
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hybridization of these compositions occurred in conduits that may have been
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operative for 100’s of thousands to possibly millions of years (Barnes et al.,
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2002). While not significant in terms of map area, these bodies were fundamental
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to the compositional evolution of the arc crust. In summary, incremental magma
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emplacement is quite compatible with fundamental physical and chemical
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processes such as thermal/mechanical fracturing, anatexis, assimilation, and
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hybridization.
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The iconoclastic perspective presented by Glazner and Bartley is not
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supported by existing data and is premised on un-tested assumptions. Existing
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field observations and interpretations either do not support their contentions
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regarding xenolith distributions or are inconclusive. The few existing field studies
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on the size-frequency distributions of xenoliths in natural pluton-host rock
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systems are contrary to Glazner and Bartley’s theoretical and analog models.
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While G&B documented three plutons that do not show evidence for local
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assimilation, we and others have previously demonstrated that the compositional
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evolution of some plutons may be the result of assimilation of local host rocks, in
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one case 20% of the observed magma mass. Whether such masses are
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significant is a semantic rather than a geologic issue.
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Sundvoll, B., 2002, Mafic magma intraplating: Anatexis and hybridization
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