Journal of Petrology | Volume 43 | Number 10 | Pages 1979-1983 | 2002
© Oxford University Press 2002
The Plagioclase–Magma Density Paradox Re-examined and the Crystallization of Proterozoic Anorthosites: a Reply
DEPARTMENT OF EARTH AND OCEAN SCIENCES, UNIVERSITY OF BRITISH COLUMBIA, 6339 STORES ROAD, VANCOUVER, B.C., V6T 1Z4, CANADA
Received December 30, 2001; Revised typescript accepted March 27, 2002
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INTRODUCTION |
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The primary goals of the work presented in Scoates (2000) were to evaluate the potential density contrasts between the mafic blocks and their hosting anorthositic cumulates in the 1·43 Ga Poe Mountain anorthosite and to place constraints on the crystallization histories of Proterozoic anorthosites where intermediate-composition plagioclase is the dominant liquidus mineral. After demonstrating that all of the blocks are more mafic than their hosting anorthositic cumulates, that their plagioclase compositions are more calcic, that each block is in strong Sr isotopic disequilibrium with its host, and that an asymmetric set of structures in the hosting anorthositic cumulates consistently envelops many of the blocks, I quantitatively evaluated block–magma density variations taking into account possible variations in temperature, pressure, oxygen fugacity, phosphorus contents, and reasonable estimates of H2O and CO2 contents in proposed parental and residual magma compositions. It was shown that all blocks, except one where the sample may not have been representative of the entire block, were dense enough to have settled through the host magma and strike the chamber floor. However, the intermediate-composition plagioclase in the anorthositic cumulates was always less dense than the possible parental and residual melt compositions and was thus either transported to the floor in relatively dense packets of cooled liquid plus crystals or crystallized in situ. I further proposed that the dense residual liquid produced within the plagioclase-rich crystal pile must have escaped downslope as a result of the presence of an inclined floor in order for large areas of anorthositic cumulates to form and that this inclined floor may have resulted through slow, diapiric ascent at the level of emplacement of the plagioclase-rich core anorthosites. The precise mechanism of transport of the anorthositic magmas and their origin by partial melting of a mantle source were not specifically addressed, although a general overview of how Proterozoic anorthosites are considered to have formed was given in the Introduction. In the Discussion of Mukherjee & Das (2002), those workers attempt to reinterpret the observed field relations and conclusions presented in Scoates (2000) to fit a model whereby all Proterozoic anorthosites must form by diapiric transport from depth.
Despite their relatively simple mineralogy, Proterozoic anorthosites display a remarkable variety of structures (e.g. diapirs, layered intrusions, massive bodies; Wiebe 1992). These structural variations can be used to infer the mode of emplacement and history of internal differentiation for individual anorthositic plutons. Diapirism has been considered as a probable mode of anorthosite emplacement for plutons that show the appropriate characteristics (Longhi & Ashwal, 1985; Wiebe, 1992; Barnichon et al., 1999). Specifically, diapiric bodies are sub-circular bodies of low density surrounded by denser rocks with parallel foliations within the diapir and host rocks and, importantly, an increase of the strain intensity towards the margin of the diapir [see Barnichon et al. (1999) for an excellent overview of this subject]. Based on these criteria, some, but certainly not all, anorthosite plutons can be interpreted to have been emplaced by diapiric transport; the best documented examples include the Egersund–Ogna massif, southern Norway (Michot & Michot, 1969; Maquil & Duchesne, 1984; Barnichon et al., 1999), the Lower Leuconorite of Paul Island, Nain Plutonic Suite (Wiebe, 1990) and foliated marginal zones to large anorthositic intrusions in the Nain Plutonic Suite (Mt. Lister and Pearly Gates intrusions; Ryan, 1991, 1992, 1993). As described in Scoates (2000) and in earlier works (Frost et al., 1993; Scoates & Chamberlain, 1995; Scoates & Frost, 1996), the structures and stratigraphy present within the Poe Mountain anorthosite of the Laramie anorthosite complex are not consistent with those formed by diapiric ascent and emplacement alone. The arguments that Mukherjee & Das present for such an origin unfortunately result mainly from a major misinterpretation of the presented field relations. Below, I discuss each of their arguments in turn and note the potential pitfalls and oversights of pursuing their reasoning.
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PREFERRED ORIENTATION OF THE BLOCKS |
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Mukherjee & Das claim that ‘in the majority of the illustrations, the comparatively flatter and smoother sides of the blocks are turned upwards towards the stratigraphically younger direction, giving them a simplified inverted flatiron-like shape in section’. This is true for only a single block, the flat-topped block shown in fig. 6 of Scoates (2000). This statement is incorrect for all other blocks and thus the main point upon which their entire discussion is based—the presence of shape-oriented blocks—is not valid. Most of the blocks are in fact sub-rounded in shape, as stated on p. 631 of Scoates (2000) (‘The blocks are typically elongate and sub-rounded with the longest dimension ranging from 1 to 50 m’). This is evident in both the photographs in fig. 5 and the maps in figs 6 and 7 (note that fig. 5b is the same block as drawn in fig. 7). It is also important to note that the long axis of most blocks is not strictly parallel to undisturbed layering within each outcrop (not always observable in the photographs and maps presented), but is slightly inclined to layering. Figure 6 of Scoates (2000) also shows several other pieces of blocks, which either have sub-rounded margins (large central block) or an indeterminate shape (block in left corner of the figure). The key feature from fig. 6 that allows for the interpretation that the blocks settled to their final position (i.e. that deformation of the upper portions of the crystal pile occurred by impact and penetration of blocks) is the prominent downwarping of the lamination, defined by the parallel arrangement of tabular plagioclase crystals, stratigraphically under the blocks. This is especially evident between the two large blocks in the left portion of fig. 6 and at the far left corner of the flat-topped block. In both cases, continuous horizons of plagioclase are locally bent through 90° adjacent to blocks and individual tabular plagioclase grains are bent (note that this outcrop was mapped on a 1 m x 1 m grid with mapsheets completed for each m2). These asymmetric structures are similar to those observed in other layered intrusions (Irvine, 1987; Irvine et al., 1998) and not consistent with shear-related fabrics produced during movement and rotation of the blocks within a nearly solid anorthosite envelope as required by a mechanism of diapiric ascent.
The presence of nearly horizontal blocks appears perplexing to Mukherjee & Das. To constrain the orientation of blocks in layered intrusions, which clearly form by magmatic processes, they make extensive reference to processes involved during the formation of glaciers, tills, moraines, hydroclastic deposits and salt diapirs. Unfortunately, the processes that are required to form moraines and basal till units associated with glaciers have little or nothing to do with crystallization at high temperatures in mafic magma chambers. Additionally, for some reason Mukherjee & Das believe that most blocks in magmatic systems should fall and strike the crystal pile edge-on (‘A fair proportion of such blocks should reach the magma floor in edge-on or near edge-on orientation and partly retain it after being embedded in the soft, unconsolidated crystal mush’). Again, there is little geological evidence for this suggestion. A review of the major papers concerning magmatic blocks and layering, especially the well-documented papers of Irvine (1987) and Irvine et al. (1998), as well as numerous photographs in the Origins of Igneous Layering (Parsons, 1987), is instructive, as it reveals that many, perhaps most, blocks in layered intrusions are layer-parallel or nearly layer-parallel and many of them are indeed flat-topped. Based on theories and experiments developed for the settling of particles through water, Mukherjee & Das suggest that ‘blocks of any shape ... would therefore be expected to retain their initial orientation acquired at the point of dislodgement from the roof’ and that ‘a preferred broadside-on and flat-side-up orientation of the blocks would be extremely difficult to explain’. In fact, fluid dynamic experiments support the field observations of layer-parallel blocks because large non-equant blocks should become oriented with their principal cross-sectional area perpendicular to the direction of settling and settle in horizontal orientations (Irvine, 1974; Irvine et al., 1998). Thus, it appears that invoking edge-on orientations has little basis in theory nor observation and the orientations of blocks observed in the Poe Mountain anorthosite are consistent with those found in other layered mafic intrusions.
Within the same discussion, Mukherjee & Das propose that the mafic pegmatoids found beneath some of the blocks should be considered as melt ‘accumulated in pressure shadow zones beneath the irregular stoss sides of the blocks, as the blocks pressed upwards, ... driven by buoyant gravitational forces’, similar to those that form on the stoss side of large particles in thinly bedded hydroclastic deposits with water flowing over them. This analogy is inappropriate for magmatic processes. Diapiric ascent, the only mechanism that Mukherjee & Das consider important in the formation of anorthosites, would imply that the blocks move within a deforming matrix, which is not the same as pebbles stuck in the base of a streambed. As proposed, the pressure shadow zones would form around the blocks as they (the blocks) rise by buoyant forces within a near-solid anorthositic crystal mush. For this mechanism to work, the blocks must be less dense than the cumulates, which of course is not correct—a major conclusion of Scoates (2000) is that all of the blocks are denser (because they contain a significantly higher content of mafic minerals) than the cumulates within which they reside.
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PETROGRAPHIC AND CHEMICAL RELATIONS OF THE BLOCKS AND THE HOST ROCKS |
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In the first paragraph of this section, Mukherjee & Das suggest that ‘characteristic minerals and textures of the roof zone rocks of a layered complex, for example, skeletal magnetite and ilmenite, skeletal and hopper crystals of apatite, sector-zoned augite and hopper zircon crystals (Naslund, 1984), have not been reported from the blocks in the Poe Mountain anorthosite’, which unnecessarily supposes that all roof zones to layered intrusions and anorthosites look like that of the Skaergaard. The Skaergaard intrusion is relatively small (
7–8 km x 10 km) and was emplaced as a single injection of magma at very low pressures (i.e. the upper contact of the intrusion was within 3 km of the surface during solidification; Lindsley et al., 1969; Williams, 1971; Morse et al., 1980), thus cooling rates were relatively high and the formation of skeletal crystals and other signs of dramatic undercooling and supersaturation were favoured (Naslund, 1984). This is not the case for most large layered intrusions and anorthosites emplaced at mid-crustal depths, where cooling rates are significantly slower, and which are characterized by numerous injections of magma over extended periods of time. In Scoates (2000), it was suggested that the blocks in the Poe Mountain anorthosite ‘either represent fragments of a now-eroded roof zone to the same chamber, possibly formed by disruption during magma replenishment, or perhaps a distinct earlier phase of anorthositic magmatism’ and that one of the blocks may have originally crystallized at higher pressures based on the presence of high-Al clinopyroxene megacrysts. None of these possibilities imply that the blocks came from a ‘fast-cooling and rapidly crystallizing roof’ as Mukherjee & Das suggest. Uncontaminated roof zones to major intrusions typically look much like the underlying cumulates and are most commonly distinguished from them by their smaller grain sizes. This is consistent with the observed smaller grain sizes (nearly an order of magnitude) in most of the blocks relative to the hosting anorthositic cumulates. The paragraphs that follow concerning the chemistry of the blocks and their hosting cumulates mostly restate what was written in Scoates (2000). Mukherjee & Das declare that the An and initial 87Sr/86Sr relations ‘cannot be explained by co-precipitation of laminates from the same melt at a particular time and at a particular depositional level of the magma chamber’. Although not entirely clear, their basic point appears to be that all of the cumulates cannot have formed through differentiation of a single pulse of magma. If so, then I agree entirely with this interpretation, having proposed and discussed this very scenario in Scoates & Frost (1996). We proposed that the earliest, most recrystallized anorthosites in the core of the Poe Mountain anorthosite formed by the emplacement of a plagioclase-charged magma, which may have undergone syn-emplacement diapiric ascent. The deviations in An and initial 87Sr/86Sr in the layered series cumulates reflect new injections of anorthositic magmas carrying significantly less suspended plagioclase than the earlier anorthosites in the core. The different isotopic compositions reflect a combination of distinct crustal contamination histories for the different batches of magma and mixing of the newly injected magmas with the resident magmas.
The final paragraph of this section in the discussion by Mukherjee & Das centres around one of the major points of Scoates (2000), which is that sloping floors are probably a requirement for layered mafic intrusions including anorthosites where intermediate-composition plagioclase is the major crystallizing phase. Mukherjee & Das declare that sloping floors cannot exist under any circumstances because they will be gravitationally unstable, and that if there were sloping floors then slumping features should be observed. Indeed, I suggest that the large area of layer disruption shown in fig. 7 of Scoates (2000) is related to slumping in the crystal pile (see caption to fig. 7). However, this is not the critical problem with their proposal. First, they make reference to soft-sediment deformational structures in unconsolidated sedimentary deposits, which probably have little bearing on high-temperature magmatic structures and processes in crystal piles where crystallization is occurring and where significant gradients in temperature and viscosity exist. As proposed in Scoates (2000), plagioclase crystals within the crystal pile may form a relatively rigid, interconnected permeable network, similar to that observed in the experimental studies of Philpotts & Carroll (1996) and Philpotts et al. (1998). Second, a review of the schematic form of mafic–ultramafic magma chambers based on detailed mapping by workers too numerous too mention [see, for example, either of the recent books dedicated to studies of layered intrusions edited by Parsons (1987) and Cawthorn (1996), and the references found within each of the papers in those books] shows a nearly universal requirement for a sloping floor, varying from relatively shallow (<10°) to relatively steep (up to 50–60°). Only the giant stratiform intrusions (i.e. Bushveld) appear to be characterized by extensive truly flat-floored cumulates. In detail, even the floor of the large Stillwater intrusion probably contained numerous local highs and lows with sloping floors between them (see fig. 4 of Naldrett et al., 1987). There are also documented occurrences of layered gabbroic intrusions with plagioclase as the major crystallizing phase where the layering is consistently vertical (e.g. Falcon Lake Intrusive Complex; Mandziuk et al., 1989). Finally, where layering is observed within intrusions from Proterozoic anorthosites (and where post-emplacement deformation is not a factor), it is typically inclined (e.g. see descriptions, maps and photos in Emslie, 1962, 1980; Morse, 1969; Wiebe, 1988, 1992; Wiebe & Snyder, 1993). Thus, the weight of geological evidence seems to point to a rather obvious conclusion—horizontal floors are not a requirement for the formation of cumulates in mafic layered intrusions and anorthosites. And, critically, if the residual liquid is denser than the surrounding crystal network in the slowly consolidating crystal pile (the case when plagioclase is the major liquidus mineral), then this liquid is free to circulate downslope as a result of gravitational forces.
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AN ALTERNATIVE, AND POSSIBLY MORE ACCEPTABLE, GENETIC MODEL |
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In their final section, Mukherjee & Das state that the form of the Poe Mountain anorthosite is domical and that it must therefore be a diapir. They support this by making reference to the study of Lafrance et al. (1996), which showed that the inner, massive core of the Poe Mountain anorthosite was deformed (recrystallized) at high temperatures (
1050°C), consistent with deformation related to diapiric ascent. In Lafrance et al. (1996), we indeed argued that the core of the Poe Mountain anorthosite underwent high-temperature deformation related to limited syn-emplacement diapiric ascent as the inner core rocks show no vestige of magmatic structures (layering, tabular plagioclase, lamination) and are extensively recrystallized. The problem with invoking diapiric ascent and deformation for the entire Poe Mountain anorthosite is that the anorthosites and leucogabbros found at higher stratigraphic levels within the layered series (i.e. the outer part of the domical structure) are much less recrystallized than the core, as noted in Scoates (2000). As one moves further upsection, beyond the stratigraphically highest block occurrence, the cumulates are essentially undeformed. The pervasive plagioclase lamination and regular layering observed at these higher stratigraphic levels are not deformational structures (no preferential lineation is observed in the planar-oriented plagioclase crystals nor are penetrative shear structures observed), but primary magmatic structures, similar to those observed in undeformed, unrecrystallized anorthosites and gabbroic layered intrusions the world over. Thus, the observed structural and stratigraphic variation in the Poe Mountain anorthosite contradicts the requirement for strong, penetrative fabrics near the margins of diapirs. |
SUMMARY AND CONCLUSIONS |
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In summary, the original interpretations of the outcrop features described in Scoates (2000) remain robust, allowing for important constraints to be placed on the relative densities of the blocks and enveloping anorthositic magmas during the time of their impact onto an inclined floor within a progressively crystallizing magma chamber and for an extended discussion on the crystallization environments during consolidation of Proterozoic anorthosites. The original conclusions reached in Scoates (2000) remain valid and are in no way modified by the discussion of Mukherjee & Das. Most importantly, those workers misinterpret the relations presented in figs 5, 6 and 7 of Scoates (2000)—there is no systematic shape-orientation of the blocks. They base their evaluation of magmatic processes mainly on concepts from studies of sedimentary successions and glaciers (moraines, tills), which have little or no bearing on processes that occur within high-temperature silicate magmas. Their prediction for the preponderance of edge-on impact of blocks has no apparent basis in theories of magmatic processes nor is this feature typically observed in layered intrusions (layer-parallel blocks are most commonly observed). Their claim that most roof zones to layered intrusions must look like that of the Skaergaard, which underwent extreme supersaturation as a result of an elevated cooling gradient, is misinformed. Their claim that the chemistry of the blocks and cumulates in the Poe Mountain anorthosite is inconsistent with formation from a single parental magma is entirely correct and was originally proposed and discussed at length in Scoates & Frost (1996). Their claim that sloping floors cannot exist in layered intrusions is in conflict with the geometry of reconstructed intrusions as proposed by the majority of workers on layered intrusions. Their proposal that the entire Poe Mountain anorthosite is a dome formed by diapiric ascent is refuted by the observed field relations as described in Scoates (2000), which are the opposite to what is required (the margins should be more deformed than the interior). Thus, diapirism is evidently not nature’s way to resolve the plagioclase–magma density paradox in layered intrusions and anorthosites. Diapirism is merely the mode of transport and emplacement of some anorthosite bodies in Proterozoic anorthosite plutonic suites that meet the required structural criteria—these criteria are not met by the geological field relations preserved in the Poe Mountain anorthosite.
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FOOTNOTES |
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*Telephone: +604-822-3667. Fax: +604-822-6088. E-mail: jscoates{at}eos.ubc.ca
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REFERENCES |
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