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Journal of Petrology | Volume 43 | Number 10 | Pages 1975-1978 | 2002
© Oxford University Press 2002
The Plagioclase–Magma Density Paradox Re-examined and the Crystallization of Proterozoic Anorthosites: a Discussion
125/1 SUBODH PARK, BANSDRONI, KOLKATA-700070, INDIA
2DEPARTMENT OF GEOLOGY AND GEOPHYSICS, INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR-721302, INDIA
Received October 23, 2001; Revised typescript accepted December 18, 2001
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INTRODUCTION |
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 TOP INTRODUCTION SHAPE FABRIC OF THE...BLOCK-HOST-ROCK CHEMICAL AND...AN ALTERNATIVE MODELREFERENCES |
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Scoates (2000) presents a wealth of information on the blocks in the Poe Mountain anorthosite, and offers a genetic hypothesis for the anorthositic laminate-block build-up through deposition, both in situ and from above, on the sloping floor of the magma chamber. Furthering yet once again the unending debate on the massif-type anorthosites, let us at once state our preferred view, namely, that diapiric transport from depth, and not growth on the floor of a magma chamber, is the major process responsible for the currently visible, overall architecture of the Poe Mountain anorthosite. Our arguments relate to three areas of contention: (1) preferred orientation of the blocks; (2) petrographic and chemical relationships of the blocks and the host rocks; (3) an alternative, and possibly more acceptable, genetic model.
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SHAPE FABRIC OF THE BLOCKS |
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TOPINTRODUCTIONSHAPE FABRIC OF THE...BLOCK-HOST-ROCK CHEMICAL AND...AN ALTERNATIVE MODELREFERENCES |
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The blocks in the Poe Mountain anorthosite display a shape fabric. Their flat sides seem to be aligned parallel to the layers of the host anorthosites and leucogabbros. Also, in the majority of the illustrations (Scoates, 2000, PM 555, fig. 5b; GR 289, fig. 6; PM 466, fig. 7; fig. 13), 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, especially in figs 5b, 6 and 13. We believe the primary reason for Scoates’s observation (Scoates, 2000, pp. 631-632) that the igneous layering is strongly disrupted below the blocks and not above them is that the host laminate layers have tightly wrapped round the blocks and have mimetically retained the smooth, flat contours of the upper sides and the irregular contours of the lower sides. An important question, therefore, would be: if the blocks indeed represent rock fragments dislodged from the roof of the magma chamber making an impact on the magma floor, then why should most of them assume a horizontal flat-lying position parallel to the host-rock layers, with their flat and smooth faces turned upwards and the irregular faces turned downwards?
Theory and experiments (Allen, 1982, pp. 186–189) indicate that a wide range of rigid, homogeneous solids (cube–octahedron–tetrahedron, oblate and prolate spheroids, circular disc and cylinder, fragments of spherical shell, etc.) fall through still fluids of a low Reynolds number (Re < 0·1) with their initial orientation unchanged. A large number of such particles released from above would have a random orientation on the depositional floor, unless they acquire some preferred orientation right at the point of release or they are rotated by gravity into a preferred orientation after they strike the floor. For ‘drop-stones’, that is, large fragments that sink some way into the mud after reaching the depositional bed and disturbing its laminations, an initial edge-on impact and subsequent toppling over on the bed would leave a characteristic asymmetrical impact mark on the laminations.
Blocks of any shape, falling through a column of basaltic magma (Re 
6 x 10-8, Philpotts, 1990, p. 242), would therefore be expected to retain their initial orientation acquired at the point of dislodgement from the roof. 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. Telltale asymmetric impact signatures should also be preserved. In any case, a preferred broad-side-on and flat-side-up orientation of the blocks would be extremely difficult to explain in this scenario.
An interesting parallelism exists, in both geometrical and dynamic terms, between blocks carried by a largely crystalline, presumably hot, pasty, moving magma body, and clasts of diverse shapes, sizes and materials, embedded in and transported as moraine or till by a moving glacier. Drake (1974), from a large database of glacial till fabric from east central New Hampshire, has shown that the till fabric is controlled by individual clast shapes. For clasts of intermediate Zingg dimensions, that is, A:B:C (length:breadth:thickness) ratio in the neighbourhood of 1:0·67:0·45, a dominantly transverse fabric results, with the broad side of most clasts oriented perpendicular to the direction of ice flow. In a study of boulders, deeply embedded in lodgement tills of some Icelandic glaciers, Boulton (1978, p. 783, fig. 8) has noted that the down-glacier sides of these boulders are abruptly truncated and flat and the up-glacier sides are curved with till piling up against the up-glacier flanks.
Ballistically emplaced xenoliths in loose, thinly bedded hydroclastic deposits are known to display a distinctive flatiron shape fabric with the flat face turned leeward, the irregular face turned to the stoss side, and a marked pressure shadow zone generated in the stoss side behind the irregular face (Fisher & Schmincke, 1984, p. 246, figs 9–13).
One of Scoates’s most significant observations (Scoates, 2000, p. 636, caption of fig. 6) is that numerous mafic-rich pegmatoids are found beneath the blocks, but no pegmatoids are found above them (italics ours). We believe the reason for this is that the Fe-rich residual liquid, parental to the pegmatoids, accumulated in the pressure shadow zone beneath the irregular stoss sides of the blocks, as the blocks pressed upwards, broadside-on and flat side turned leeward, driven by buoyant gravitational forces.
The shape fabric of ellipsoidal inclusions embedded in moving glaciers has been interpreted in terms of simple shear (Jeffery, 1922), pure shear (Gay, 1966, 1968), and a combination of the two (Paterson, 1969; Allen, 1982). Long-axis orientation of the inclusions both parallel and transverse to the direction of ice flow is possible. We may expect a predominant transverse orientation, where the ice is in compression and the inclusions after a prolonged transport drift into orbits of minimum energy dissipation.
Ductile metals, glacier ice, salt, and arguably nearly solidified, near solidus temperature, hot magma bodies may deform by creep through a combination of simple and pure shear. We suggest that the blocks in the Poe Mountain anorthosite may have been transported upward and oriented in this process by a diapirically rising magma body. The upward transport may have been a long protracted process. The blocks may represent fragments broken loose from different levels of a deep-lying, layered basic complex that acted as the precursor to the anorthosites and at some stage of its evolution foundered in parts as a result of post-crystallization gravitational instabilities (Mukherjee, 1996; Mukherjee et al., 1999).
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BLOCK–HOST-ROCK CHEMICAL AND MINERALOGICAL RELATIONS |
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TOPINTRODUCTIONSHAPE FABRIC OF THE...BLOCK-HOST-ROCK CHEMICAL AND...AN ALTERNATIVE MODELREFERENCES |
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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. Some of these blocks containing very coarse-grained clinopyroxene (
20 cm across, 6·2 wt % Al2O3 and 510 ppm Cr) and plagioclase (>1 m) cannot have formed in the fast-cooling and rapidly crystallizing roof or border zones of the complex, and a high-pressure, deep-lying, slowly crystallized source is likely. Excepting block SR246, the only block listed from the leucogabbro layered zone (LLZ) and from the highest stratigraphic horizon, all other blocks are less evolved or more primitive than the average anorthosite layered zone (ALZ) and LLZ rocks. Overall indications for this come from weight per cent K2O (0·64–0·92 for blocks against 1·09 and 0·99 for average ALZ and LLZ, respectively), P2O5 (0·04–0·09 for blocks against 0·15 and 0·25 for average ALZ and LLZ), mg-number (0·38–0·52 for blocks against 0·38 and 0·36 for average ALZ and LLZ), ppm Rb (3·41–5·56 for blocks against 6·49 and 8·68 for average ALZ and LLZ), and total rare earth elements (20·1–24·6 for blocks against 33·7 and 72·9 for average ALZ and LLZ) (Scoates, 2000, table 1). In addition, each block has higher normative An than its host rock.
A curious feature of the variation diagrams in Scoates’ fig. 8a and c is a mushroom-like spread of the host-rock normative An and ISr values at the higher stratigraphic levels, affecting in particular the LLZ and upper ALZ rocks (Scoates, 2000). Not only are the blocks out of chemical equilibrium with their host rocks, but there is also a marked disequilibrium among the host rocks themselves at the same stratigraphic level.
Scoates advocates a model in which the blocks have fallen from above on the floor of the magma chamber and the floor has moved upwards through progressive accumulation of mainly plagioclase, crystallizing from the superincumbent melt either in situ on the floor or brought there by two-phase magmatic convection currents.
Host-rock normative An variation from 43 to 50 at about 200 m RSP (relative stratigraphic position) and from 45 to 53 at about 600 m RSP, and ISr variation from 0·7040 to 0·7048 at 600 m RSP and from 0·7048 to 0·7055 between approximately -300 and -600 m RSP (Scoates, 2000, fig. 8) 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. Also, shape-oriented blocks of anorthositic rocks with signatures of an older, primitive, high-pressure, deep-lying source do not fit in this scenario of building up on the floor of a magma chamber by addition of materials from above.
Scoates has proposed a high-Al basaltic magma in the Poe Mountain magma chamber, resident over a sloping floor of cumulus plagioclase, down which the gravitationally unstable dense ferrodioritic residual melt is supposed to have drained off. We believe gravitational instability would have almost equally affected not only the dense liquid but also the unconsolidated, soft, residual liquid-soaked crystalline mush, visualized as resting on the sloping magma floor. Gravitationally triggered down-slope sliding and slumping would surely have affected these top layers and an initial slope, if at all present, should have quickly levelled off, leaving marks of asymmetric, contemporaneous deformation. Small, contemporaneous folds and faults are known to form in soft, unconsolidated sediments as a result of sliding down on slopes as low as 2·5° (Billings, 1960, p. 240). Slope-triggered deformation structures have been reported from a wide range of volcaniclastic deposits (Fisher & Schmincke, 1984). We believe the idea of a sloping floor of the magma chamber, sufficiently stable, extended and repeatedly occurring over geologically significant time and place scales to generate massif-type anorthosites is not realistic.
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AN ALTERNATIVE MODEL |
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TOPINTRODUCTIONSHAPE FABRIC OF THE...BLOCK-HOST-ROCK CHEMICAL AND...AN ALTERNATIVE MODELREFERENCES |
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The Poe Mountain anorthosite, in form, structure, and internal petrographic–chemical constitution, closely resembles a diapir—a word we use here in its most general connotation (Ramberg, 1981), namely, a body that has buoyantly ascended and pierced its overburden in a process of gravitational readjustment of an initially unstable density distribution.
The domical geometry of the Poe Mountain anorthosite, its steep-dipping and strongly developed marginal foliations, and the massive, structureless inner core with marks of high-temperature (1050°C), near-solid-state, syn-emplacement deformation (Lafrance et al., 1996) are all in excellent accord with the model of diapiric emplacement.
Among the analogous features of granitoid diapirs, penetrative planar structures as a rule develop more strongly near the diapir wall and parallel to it (Holder, 1979; Ramsay, 1989; Paterson & Fowler, 1993). Where quantitative strain measurement is available, the marginal foliations can be shown to be oriented perpendicular to the direction of maximum shortening (Ramsay, 1989). Microstructures have shown a rheological transition from a predominantly magmatic, structureless and isotropic inner core of the diapir to prominent solid-state flow near the border (Hippertt, 1994). Experimental and theoretical studies predict strong rheological and temperature gradients for the margin of internally circulating magmatic diapirs, leading to the formation of strong, penetrative foliations at the margin and isotropic fabrics in the interior (Cruden, 1990).
As we have already argued, magmatic co-precipitation and addition of materials from above in a magma chamber cannot explain the petrographic and chemical–isotopic heterogeneities at the same stratigraphic level of the Poe Mountain anorthosite. The only other arguable alternative, in our judgement, is that these heterogeneities are all derivative features, features not created at the site where we see them now, but tectonically derived, transported and juxtaposed through multiple stages of diapiric ascent from a precursor, deep-lying layered basic complex (Mukherjee, 1996; Mukherjee et al., 1999).
Buoyant overturn of the inverted density stratification in a layered system in the gravity field is a fundamental natural process, widely documented in diverse geological settings (Ramberg, 1981). Such a process should operate in a layered magmatic complex also, provided it cools so slowly that the required viscosity contrasts and the rheological ‘softness’ are retained for some time even after the gross layered-system architecture is completed. Buoyant stresses should specially affect the thick anorthositic layers under such conditions, and would tend to flex them into gentle arches, eventually disrupt and disjoint them at places, and initiate a phase of cognate anorthosite diapirism (Mukherjee, 1996; Mukherjee et al., 1999). Subsequent ascent of such gravitational instability domains through many stages of mixing, coalescence and growth may produce a typical massif-type anorthosite diapir with ‘stitched-in’ petrographic and chemical–isotopic heterogeneities, inherited from the parent and precursor layered complex.
Dense blocks and rafts of dolomite, sandstone, granite gneiss and even basic rocks (up to 3–6 km2 in area) are known to have been lifted by salt diapirs (Weinberg, 1993), so possible density differences between the anorthositic blocks and the host anorthositic rocks per se should not be a problem with the diapiric model. Also, small amounts of dense, residual liquid, enriched in Fe and incompatible elements, and derived from the cryptic upper layers of the precursor layered complex, may be filter-pressed into the cracks, fissures and marginal openings of the top layers of the ascending anorthosite diapir (Mukherjee, 1996).
To conclude generally, diapirism seems to be nature’s way to resolve the plagioclase–magma density paradox of layered complexes. The end result of this process, when carried out successfully, is a massif-type anorthosite. Perhaps no other place and time have offered better thermal–tectonic conditions for this success than the mobile belts of the Proterozoic.
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FOOTNOTES |
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*Corresponding author. Telephone: (33) 411-3139. E-mail: amalbikash{at}yahoo.com
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  J. S. SCOATES The Plagioclase-Magma Density Paradox Re-examined and the Crystallization of Proterozoic Anorthosites: a Reply J. Petrology, October 1, 2002; 43(10): 1979 - 1983. [Full Text] [PDF]
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