Ore Geology Reviews 56 (2014) 199–208

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Grain-scale plastic deformation of chromite from podiform chromitite ofthe Naga-Manipur ophiolite belt, India: Implication to mantle dynamics

Biswajit Ghosh a,b,⁎, Jyotisankar Ray a, Tomoaki Morishita b

a Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata, Indiab School of Natural System, College of Science and Engineering, Kanazawa University, Japan

⁎ Corresponding author at: Department of GeologBallygunge Circular Road, Kolkata, India. Tel.: +91 33 246

E-mail address: bghosh_geol@hotmail.com (B. Ghosh)

0169-1368/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.oregeorev.2013.09.001

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 April 2013Received in revised form 15 August 2013Accepted 2 September 2013Available online 6 September 2013

Keywords:ChromititeCrystal plastic deformationSubgrainNodularNaga-Manipur ophiolite belt

The mantle peridotites of the Naga-Manipur ophiolite belt, India host concordant podiform chromitite bodiesthat preserve variousmagmatic and deformation structures. Magmatic structures preserve evidence of a gradualtransition from nodular to massive varieties. Nodular types are least affected by deformation. Deformation struc-tures, restricted mainly in massive types include intracrystalline features of brittle as well as crystal plastic pro-cesses within chromite. Brittle deformation is preserved as grain fracturing and intense microbrecciation alongnarrow zones forming protocataclasite. Crystal plastic deformation has generated chromite subgrains and newgrains in response to recovery and recrystallization processes. Subgrains are polygonal in shape and the bound-aries between themare commonly straight forming amosaicwithmostmeeting at triple junctions. Subgrains arecommonly internally zoned and the mosaics formed by them correlate well with the compositional maps.Subgrains show high Al at their cores and high Fe at their boundaries in response to differential, diffusive chem-ical re-equilibration. We discuss how this compositional resetting related to deformation is significant in variousgeologic studies related to tectonic setting and thermobarometric evolution of an area. Deformationmechanismsare likely to have started with crystal plastic processes and later switched over to the brittle regime. The preser-vation of all grain-scale deformation structures in the chromitite bodies of the Naga-Manipur ophiolite belt issuggestive of mantle dynamics, vertical accretion and horizontal movement in particular.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Chromite is a significantmineral phase in considering the chromiumreservoir of the bulk earth, the mantle in particular. It is an essentialmineral in peridotites and chromitites of the upper mantle (e.g., Shiet al., 2012) and also in some ophiolites and other supra-subductionzone complexes (e.g., Hebert et al., 2012; Pasava et al., 2011; Penget al., 2012; RamMohan et al., 2013). Grain-scale microscopic observa-tions especially the behaviour and geometry of grain boundaries andother internal structures,which generally provide information on defor-mationmechanisms, are difficult to undertake on chromite because it isisotropic and predominantly opaque in transmitted light microscopy.Cataclasis has long been regarded as the only deformational behaviourin chromite (Doukhan et al., 1979; Ramdohr, 1969); however, the rhe-ological conditions to address this failure in high-temperature mantleenvironment were constrained much later by numerical experiments(Holtzman, 2000). Studies on pole-figures of deformed ophiolitic chro-mites obtained utilising X-ray goniometer also could not reveal anysubgrains in the respective samples (Christiansen, 1985). As a result, dis-location creep was not thought to be a dominant mechanism; instead,

y, University of Calcutta, 351 4891; fax: +91 33 2461 4849..

ghts reserved.

diffusional creep which normally does not result in a crystallograph-ic fabric was considered as the principal mechanism in the deforma-tion of chromites (Ozawa, 1989). However, back scatter electron(BSE) images, obtained using scanning electron microscope (SEM)(Christiansen, 1986) as well as electron probe micro analyser (EPMA)(Ghosh et al., 2013) identified the substructures in ophiolitic chromites,and this suggests that dislocation processes are active in this mineraldeforming at high-temperature duringmantle flow. Recent observationsfrom BSE images on chromite deformed at eclogite facies conditions(near to crust–mantle boundary) demonstrate that both dislocation anddiffusion creep can operate at varying temperature conditions (Ghoshand Konar, 2011, 2012).

Podiform chromitite bodies are usually irregular, discontinuousconcentrations of chromite commonly hosted within discordant orsemi-concordant dunite of the shallow mantle sequence of the oce-anic lithosphere. It is generally accepted that these bodies actuallyrepresent channels of focused melt flow (Arai and Abe, 1995; Arai andYurimoto, 1994; Zhou et al., 1994, 1996). Favourable tectonic settingsfor podiform chromitite bodies include island arcs, back-arc basins, andfast spreading ridges (Arai, 1997; Johnston, 1936; Leblanc and Nicolas,1992; Robinson et al., 1997; Thayer, 1964). In ophiolites they are signif-icantly important because they can record subduction initiation in an arcsetting (Rollinson andAdetunji, 2013). These bodies are characterised bydistinctivemagmatic and deformation structures. They form in channels

200 B. Ghosh et al. / Ore Geology Reviews 56 (2014) 199–208

of highmelt flow beneath the ridge axis and deform in the outward flowof peridotite (Cassard et al., 1981; Ceuleneer and Nicolas, 1985).Magmatic structures dominantly nodular, orbicular and massiveare interpreted to result from fast flowing magmatic systems. In con-trast, deformation structures characterised by pull-apart, foliatedand mylonitic banded fabrics are thought to have resulted fromstrong high-temperature (N1000–1200 °C) plastic flow (Ceuleneerand Nicolas, 1985; Huang et al., 2004; Leblanc and Ceuleneer, 1992;Li et al., 2002).

Plastic deformation of chromite by dislocation creep mechanism hasbeen speculated from mantle chromitite pods (Holtzman, 2000). In thisstudy we document various grain-scale deformation structures in chro-mite from podiform chromitite bodies of the Naga-Manipur ophiolitebelt, India. This study describes the intracrystalline deformation that gen-erated subgrains and new grains in response to recovery/recrystallizationprocesses as a result of mantle dynamics related to its vertical accretionor horizontal flow, induced by temperature gradients. We emphasizethe importance of this study in understanding the grain-scale composi-tional re-equilibration of deformedmineralswhichmay havemajor im-plications in establishing the geotectonic environment as well as ingeothermobarometric studies. This work shows how a detailed micro-structural work is a prerequisite before finding the right places for

Shiro

Ukhrul

Phaisat

M

A

N

I

P

U

R

094 20’

Fig 1. Generalised geological map of the NMO belt. The green asterisk around the village Shiroicolour in this figure legend, the reader is referred to the web version of this article.)

carrying out chemical analyses of chromites to use the analytical datafor several purposes as mentioned before.

2. Brief geological overview

The Naga-Manipur ophiolite (NMO) belt forms a part of the Tethyanophiolite belt and is generally interpreted as representing the easternsuture of the Indian Plate (Acharyya, 2007; Gansser, 1980; Mitchell,1981) (Fig. 1). The NMO belt was accreted just prior to the mid-Eocene as a result of the collision of the Indian plate with the Burmaplate and is best preserved as a narrow belt along the eastern marginof the NNE–SSW trending Indo-Burma Range (IBR), northeast India.The ophiolitic rocks occurmainly as rootless subhorizontal bodies over-lying Eocene–Oligocene flyschoid sediments and are represented bydismembered ultramafic (and rarely mafic) rocks closely associatedwith subordinate volcanics and oceanic pelagic sediments. The maficcrustal units, gabbroic rocks in particular are poorly developed in theNMO belt. Lherzolite (with ~10 modal% of clinopyroxene, recalculatedin serpentine-free basis), transitional to harzburgite is the most domi-nant among the mantle peridotites. Podiform chromitites in the NMObelt occur mainly as concordant bodies of variable dimensions up to

Akhen

i

Gamnom

N A G A L A N D

Disang Group(Sedimentaries)Ophiolites and(Sedimentary enclaves)

Metamorphics

0 10 km

02500’

02510’

094 30’

Study area

indicates the study area (N25°08′57″:E94°28′20″). (For interpretation of the references to

201B. Ghosh et al. / Ore Geology Reviews 56 (2014) 199–208

20 m length associatedwith strongly serpentinised and tectonised peri-dotites (Singh et al., 2013).

3. Petrography

There are a variety of chromitite bodies present in the study area.These are predominantly massive (with more than 90 modal% of chro-mite). Primary silicates are virtually absent in the matrix of the massivechromitite bodies with chlorite preserved as the dominant secondarymineral. Nodular chromitite bodies are not as common in the field andwhen present always show a gradual transition to massive chromitites(Fig. 2). Individual nodules are mostly spherical to sub-ellipsoidal innodular ores (Fig. 2a). In the transitional semi-massive variety nodulesare generally ellipsoidal, flattened and coalesce with each other, set in

a

b

c

Fig. 2. Photographs of various textures of podiform chromitite from the NMO. Note thegradual transition from (a) nodular type to (b) semi-massive type to (c) massive type. In-dividual nodules with elliptical cross sections are still discernible in massive to semi-massive types. In some cases a crude layering is noticedwith the long axes of the ellipsoidsparallel to the layering (marked with white line).

a serpentinised matrix. A crude layering is locally present in this varietywith the long axes of the ellipsoids parallel to layering (Fig. 2c),interpreted to have formed soon after crystallization (Li et al., 2002).Nodules consist of aggregates of euhedral to subhedral chromite grainswith interstitial silicates. The proportion of silicates sharply decreasesthrough transitional varieties to the massive ores. The flattening of thenodules suggests that they settled while in a melt. These features areinterpreted to be a result of rapid deposition of chromite noduleswhile they were still plastic (Li et al., 2002).

3.1. Substructures

Chromite in most of the chromitite varieties is heavily fractured andcataclastic microstructures are common in most samples. Massivechromitites are in some cases traversed by discrete or anastomosingcataclastic zones of varying thickness (50–800 μm) producing crushedmicrobreccias or protocataclasites (Figs. 3a, b, 4 and 5). The matrix ofthese protocataclasite zones is filled with chlorite (Fig. 4). Chromite,in massive and semi-massive varieties, displays substructures (herein“subgrains”) as evidence of recovery processes in only a few samples.In these samples the strained, recovered grains are preserved primarilywithin or very close to the protocataclasite zones (Fig. 5). However, notall of the protocataclasite zones contain subgrains (Fig. 4); instead theseappear very limited in abundance. Subgrain boundaries are defined bynetworks of dislocation arrays, seen as bright lines in high contrastBSE images (Fig. 3c). Interestingly, these subgrains were first identifiedunder reflected light, using a high power objective lens (50×) duringmicroscopic studies (Fig. 3d). Noteworthy to mention here is thatthese samples were not treated by any etching techniques to revealthe substructures. Subgrains are generally equidimensional and polygo-nal in shape showing considerable variation in size (Fig. 6a, b). Subgrainboundaries are commonly straight and form a mosaic with most meet-ing at triple junctions (Fig. 6c).

BSE images reveal that the formation of subgrains is not always uni-form throughout porphyroclasts, instead they occur in domains (Fig. 6a,b). These domains where subgrains concentrate are relatively brighterin comparison to that part of the host porphyroclast devoid of subgrains.Furthermore, the subgrains are in some cases internally zoned with adarker core and comparatively lighter rim (Fig. 6). Recovered grains,preserving abundant subgrains, do not show any manifestation of thesubgrain boundary on surface topographic images (Fig. 7a). In contrast,recrystallized grains show development of new grain boundaries(Fig. 7c). In such grains remains of the old porphyroclast at the corewith intragranular strain features are surrounded by moats of newrecrystallized grains giving rise to typical core–mantle structures(Fig. 8). Evidence for bulging of the high-angle grain boundariesis not observed in the samples studied.

4. Mineral chemistry

Chromite analyses were carried out at the Central PetrologicalLaboratory, Geological Survey of India, Kolkata on polished thin sectionsof chromitites with a CAMECA SX100 EPMA machine. The analyses(Table 1) were performed at an accelerating voltage of 15 kV and abeam current of 12 nA using 1 μmdiameter beam. Calibration of the in-strument was performed using mineral standards, supplied by BRGM,France and results were corrected with a PAP matrix correction pro-gramme. The Fe3+ in chromites was calculated according to the chargebalance equation of Droop (1978). Compositionalmapping of the grainswas carried out using a JEOL JXA-8800 Superprobe at Kanazawa Univer-sity, Japanusing an accelerating voltage of 15–20 kVand a beamcurrentof 15–20 nA with a 3 μm diameter beam.

Individual subgrains could not be analysed quantitatively because oftheir small size. However, the bulk composition in the subgrain domainindicates that this part is relatively chromium-rich in comparison to therest part of the porphyroclast (Fig. 9). From the BSE tonal difference for

50 µm

PC zonePC zone

a b

c d

Fig. 3. (a) Reflected photomicrograph showing the protocataclasite (PC) zones; magnified central part of this (red rectangle) is shown in (b); (c) BSE image showing the mosaic ofsubgrains in part of Fig. 3b marked with red rectangle. Note the disrupted fragment (red arrow) which also shows subgrains inside; (d) same with the earlier under reflected light.(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

202 B. Ghosh et al. / Ore Geology Reviews 56 (2014) 199–208

individual internally zoned subgrains it appears that themajority have acore which concentrates aluminium (Fig. 6b, c). This feature becomesmore evident during compositional mapping (discussed next). TheCr# [=Cr / (Cr + A1) atomic ratio] in the subgrain domain rangesbetween 0.81 and 0.82 whereas that outside the domain, in the rest ofthe porphyroclast ranges between 0.77 and 0.78 (Table 1). The compo-sition within and outside the protocataclasite zone does not show anysignificant difference, matching closely with the composition of theundeformed grains of the nodular chromitites (Fig. 9). Thus, all the

Fig. 4. A collage of BSE images showing the microbrecciation within th

chromite compositionsdefine a cluster except the subgrain rich domain,the only exception which shows higher Cr#. This correlation stronglyspeaks about deformation induced chemical re-equilibration.

4.1. Correlation between substructures and chemistry

In order to assess whether the subgrain network enhanced the ele-ment mobility and thereby caused a compositional change (relaxation)we undertook detailed compositional mapping of the same chromite

e discrete protocataclasite zone with the matrix filled by chlorite.

1

2

3

4

5

5 4

12

3

100 µm

Fig. 5.BSE image showing the distribution of the recovered grains in relation to the anastomosing protocataclasite zones. Individual rectanglesmarkedwith yellow (numbered) have beenzoomed in (red rectangles). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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grains. The compositional maps for Al and Fe match strongly and corre-late with the mosaic pattern of the subgrain boundaries visible in theBSE image (Figs. 10 and 11). The same correlation with Mg is feeble,and that with Cr is not apparent. Subgrain boundaries are marked byhigh Fe content whereas the cores of subgrains are marked by high Aland Mg contents.

5. Discussion

Podiform chromitites of the NMO belt appear to be sufficiently de-formed and since there has been no major tectonic event in this studyarea since the emplacement of the ophiolite we discuss the preserveddeformation structures as a result of mantle processes.

There is one important factor that controls the intensity of defor-mation in mantle chromitites, the chromite–silicate ratio. Nodularchromitites with lower proportions of chromite in comparison to mas-sive types are less deformed and the chance of the preservation of mag-matic structures is greater. During deformation most of the stress inchromitites is absorbed by the silicate phases, protecting the chromite.The massive and semi-massive varieties preserve evidence for bothbrittle and crystal plastic deformation in the same samples. Consideringthe various aspects of deformation (i.e. strain rate, temperature andothers) these twoprocessesmay not necessarily have operated simulta-neously. Crystal plastic deformation could be independent of cataclasticdeformation and could represent a temporally separate event. The rela-tive timing between these two processes is difficult to establish howev-er, Nyman et al. (1992) described subgrain formation in amphiboles as aresult of cataclastic rather than crystal plastic deformation. Because the

chromite in most of the protocataclasite zones in our samples do notshow evidence for recovery (Fig. 4), the possibility of crystal plastic de-formation being later than the brittle event is precluded. Whether thebrittle deformation is due to flow of the mantle in a region of highstrain-rate, facilitated with relatively low-temperature, or simply re-lated to emplacement, with no link to mantle processes, is difficult toascertain.

However, since subgrain formation is confined to certainprotocataclasite zones within our samples we interpret the cataclasticand ductile processes in terms of progressive deformation. We proposethat deformation was initially crystal plastic with the development ofsubgrains. In the later stages deformation switched over to the brittleregime because of a change in environmental condition, mainly strainrate and/or temperature. Therefore, the zones which underwent crystalplastic deformation initially may have become locales of further strain,facilitating later brittle deformation. This speculation is supported bythe presence of disrupted chromite fragments at the peripheral part ofrelatively large chromite porphyroclasts where both the fragments andthe porphyroclasts display features of subgrain formation (Figs. 3c, 6a).This implies that these unsupported fragments were originally part ofthe large porphyroclasts. New protocataclasite zones which are devoidof any subgrains might have been produced during this period.

In the case of crystal plastic deformation in ductile environment dis-locations are relatively free to climb (mobile) from one lattice plane toanother and the newly formed dislocations gradually pile up alongcertain narrow low energy zones that defines the subgrain boundaryand eventually initiates climb accommodated dislocation creep (Druryand Urai, 1990; Passchier and Trouw, 2005). In deciphering dynamic

a b

c

Fig. 6. (a) BSE image of a chromite grain showing two domains (red and green rectangles) of subgrain development in it. Individual domains have been enlarged in (b) and (c). Greenarrows in last two figures indicate that these subgrains are internally zoned as reflected by the tonal variation. Red arrow indicates a disrupted chromite fragment with subgrains insideas shown in Fig. 3c.

a b

c d

Fig. 7. Pair of BSE images (left) and corresponding topographic images (right). (a) Recovered grainwith abundant subgrains; (b) subgrain boundaries are notmanifested on the surface ofthe grain (green arrow); (c) recrystallized grain; (d) boundaries of the new grains are visible on the surface image (red arrow).

204 B. Ghosh et al. / Ore Geology Reviews 56 (2014) 199–208

Fig. 8. BSE image showingmoats of newly crystallized grains around an old porphyroclastat the core giving rises to the core–mantle structure.

Table 1Electron microprobe analyses of chromites of podiform chromitites from the NMO belt.

Nature SG HP PCZ

Cr2O3 61.96 61.58 62.63 62.89 61.93 59.91 58.94 60.46 59.67 58.43 59.94 60.09 59.25 59.55Al2O3 9.72 9.48 9.58 10.12 9.08 11.64 11.23 11.26 11.33 11.70 11.57 11.52 12.22 11.63TiO2 0.04 0.06 0.13 0.00 0.06 0.07 0.04 0.09 0.11 0.02 0.14 0.22 0.15 0.15FeO 16.14 15.95 16.19 15.92 15.91 18.28 15.61 18.18 18.62 16.68 14.59 14.04 13.01 11.31Fe2O3 1.71 1.68 1.20 1.29 1.73 0.73 1.93 0.82 0.89 2.08 2.23 3.04 2.08 3.27MgO 11.62 11.54 11.71 11.91 11.39 10.45 11.79 10.58 10.17 11.09 12.96 13.51 13.93 14.94MnO 0.14 0.13 0.08 0.19 0.20 0.07 0.07 0.01 0.05 0.13 0.01 0.04 0.01 0.03NiO 0.00 0.01 0.00 0.11 0.16 0.00 0.03 0.00 0.00 0.29 0.07 0.17 0.00 0.17Total 101.32 100.44 101.52 102.43 100.47 101.15 99.65 101.40 100.84 100.41 101.50 102.63 100.65 101.05

Formula units based on 32 oxygensCr 12.683 12.723 12.796 12.703 12.828 12.274 12.151 12.370 12.299 12.000 12.049 11.927 11.886 11.862Al 2.967 2.921 2.919 3.048 2.805 3.556 3.453 3.435 3.482 3.583 3.468 3.410 3.656 3.455Ti 0.008 0.012 0.025 0.000 0.012 0.014 0.008 0.018 0.022 0.004 0.027 0.042 0.029 0.028Fe3+ 0.332 0.331 0.234 0.247 0.341 0.142 0.379 0.159 0.175 0.406 0.426 0.575 0.398 0.620Fe2+ 3.496 3.489 3.499 3.403 3.489 3.963 3.408 3.936 4.060 3.627 3.105 2.954 2.763 2.389Mg 4.484 4.495 4.510 4.535 4.448 4.036 4.582 4.081 3.952 4.294 4.911 5.055 5.268 5.610Mn 0.031 0.030 0.017 0.041 0.045 0.015 0.016 0.002 0.011 0.028 0.002 0.008 0.003 0.007Ni 0.000 0.002 0.000 0.023 0.034 0.000 0.006 0.000 0.000 0.061 0.014 0.034 0.000 0.034Total 24.002 24.002 24.001 24.001 24.002 24.000 24.002 24.000 24.000 24.003 24.003 24.005 24.002 24.006

Mg# 0.56 0.56 0.56 0.57 0.56 0.50 0.57 0.51 0.49 0.54 0.61 0.63 0.66 0.70Cr# 0.81 0.81 0.81 0.81 0.82 0.78 0.78 0.78 0.78 0.77 0.78 0.78 0.76 0.77

Nature PCZ Out Nod

Cr2O3 59.24 59.74 60.50 60.64 61.00 60.15 60.53 59.36 60.98 60.53 59.71 60.08 60.64 60.39Al2O3 10.96 11.77 11.60 11.80 11.71 11.91 11.37 11.25 11.27 11.44 11.45 11.21 11.20 11.13TiO2 0.17 0.16 0.20 0.12 0.12 0.23 0.13 0.15 0.17 0.15 0.13 0.10 0.15 0.13FeO 14.47 11.05 14.73 13.41 11.31 12.70 12.26 11.43 12.02 11.30 11.66 10.91 11.72 12.10Fe2O3 2.51 2.80 2.18 2.38 2.02 2.21 2.48 2.98 2.42 3.15 2.57 3.74 2.67 2.18MgO 12.71 15.14 12.90 13.79 14.95 14.29 14.36 14.61 14.63 14.99 14.51 15.17 14.62 14.31MnO 0.06 0.02 0.17 0.04 0.01 0.09 0.02 0.04 0.01 0.06 0.02 0.09 0.04 0.02NiO 0.00 0.06 0.25 0.28 0.28 0.00 0.13 0.13 0.09 0.34 0.18 0.19 0.25 0.00Total 100.12 100.74 102.54 102.46 101.39 101.58 101.28 99.95 101.59 101.95 100.23 101.49 101.30 100.27

Formula units based on 32 oxygensCr 12.103 11.906 12.059 12.016 12.106 11.960 12.090 11.979 12.130 11.972 12.017 11.928 12.099 12.180Al 3.339 3.498 3.448 3.487 3.466 3.531 3.387 3.386 3.343 3.374 3.437 3.319 3.332 3.348Ti 0.033 0.030 0.038 0.023 0.023 0.044 0.025 0.029 0.032 0.028 0.025 0.019 0.028 0.025Fe3+ 0.488 0.531 0.415 0.449 0.381 0.419 0.471 0.572 0.459 0.592 0.492 0.707 0.508 0.419Fe2+ 3.132 2.334 3.110 2.814 2.376 2.674 2.594 2.445 2.533 2.370 2.486 2.300 2.479 2.585Mg 4.895 5.688 4.847 5.151 5.593 5.356 5.407 5.558 5.486 5.589 5.505 5.678 5.499 5.441Mn 0.014 0.004 0.036 0.008 0.001 0.019 0.004 0.009 0.001 0.012 0.005 0.019 0.008 0.004Ni 0.000 0.012 0.051 0.056 0.057 0.000 0.026 0.027 0.018 0.068 0.037 0.038 0.051 0.000Total 24.004 24.004 24.003 24.003 24.002 24.003 24.003 24.005 24.003 24.006 24.004 24.008 24.004 24.003

Mg# 0.61 0.71 0.61 0.65 0.70 0.67 0.68 0.69 0.68 0.70 0.69 0.71 0.69 0.68Cr# 0.78 0.77 0.78 0.78 0.78 0.77 0.78 0.78 0.78 0.78 0.78 0.78 0.78 0.78

SG: subgrain domain; HP: host porphyroclast; PCZ: protocataclasite zone; Out: outside of PCZ; Nod: nodular.

0.76

0.77

0.78

0.79

0.80

0.81

0.82

0.83

0 0.2 0.4 0.6 0.8

subgrain

host grain

cataclasticzone

outside

nodular

Mg#

[= M

g/(M

g +

Fe2+

) at

omic

rat

io]

Cr# [= Cr/(Cr + Al) atomic ratio]

Fig. 9. Compositional variations of chromites of chromitite pods from the NMO onbivariant Mg# vs. Cr# diagram. For detailed abbreviations see Table 1.

205B. Ghosh et al. / Ore Geology Reviews 56 (2014) 199–208

Al

Cr Fe

Mg

Fig. 10. Compositionalmaps (X-ray) of the chromite in Fig. 6a showing the distribution of Al, Cr, Fe andMg.(For interpretation of the references to colour in thisfigure legend, the reader isreferred to the web version of this article.)

206 B. Ghosh et al. / Ore Geology Reviews 56 (2014) 199–208

recrystallization of feldspars, Tullis and Yund (1985) described the sameprocess as recovery accommodated dislocation creep. In the case of chro-mite of the present study the morphology of subgrains is possibly con-trolled by interfacial energy of cubic crystals as evidenced from thegeneration of triple point junctions (Figs. 3c, 6c). Subsequent to the plas-tic deformation at a relatively deeper level these chromitite bodiesmighthavemoved away because of themantle flowwhere the strain rate and/or temperature conditionwas no longer favourable for crystal plastic de-formation. The balance between the rate of migration and rate of forma-tion of dislocations was lost. In this changing scenario the dislocationswere no longer mobile (tangles) and the movement of newly formeddislocations was inhibited. In due course it led to strain-hardening orwork-hardening and thereby facilitated late cataclastic processes.

Information on chemical re-equilibration in relation to recrystalliza-tion and recovery is limited. Compositional change of solid solution

minerals associated with deformation is reported for feldspar (Yundand Tullis, 1991), amphibole–pyroxene (Stunitz, 1998), olivine (Hirthand Kohlstedt, 1995) and garnet (Smit et al., 2011; Storey and Prior,2005). It seems reasonable to discuss this aspect of kinetic demixingin terms of element diffusion, induced by deformation. This occurs ei-ther (1) at relatively lower temperatures in the dislocation creep regimeor (2) during diffusion creep which operates at comparatively highertemperatures. In the dislocation creep regime (recrystallization accom-modated) a high-angle grain boundarymay sweep past a given point ina grainmany times thus providing amechanism for elemental diffusion.Thus, Yund and Tullis (1991), on the basis of the experimental work onsynthetic plagioclase feldspar demonstrated that the movement of high-angle grain boundaries through strained grains in the dislocation creepregime provides high diffusivity pathways for the rapid exchange of com-ponents during deformation. This process may produce compositional

Al

Cr Fe

Mg

Fig. 11. Compositionalmaps (X-ray) of the chromite in Fig. 3c showing the distribution of Al, Cr, Fe andMg. (For interpretation of the references to colour in thisfigure legend, the reader isreferred to the web version of this article.)

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zoning in the recrystallized grains. Similar observations from naturalchromites, showing compositional change during recrystallization, weredescribed from the Fiskenaesset deposit of West Greenland (Ghisler,1976) and Sittampundi anorthosite complex of southern India (Ghoshand Konar, 2012). Since there is no evidence for bulge nucleation at thegrain boundaries the recrystallization in our samples is unlikely to be mi-grational. Instead, we presume it to be rotational (climb accommodated);and this excludes the possibility of element diffusion in the dislocationcreep regime.

We explain the compositional zoning in our samples by diffusioncreep at the subgrain scale at extremely low strain rates (Ozawa, 1989;Sammis, 1989). This involves either (1) lattice (volume) interdiffusion(Nabarro-Herring creep), or (2) boundary diffusion (Coble creep). Inthe first case the rate of change may be enhanced significantly owingto the reduction in grain size because it is facilitated by shorter distances.In the case of boundary diffusion subgrain boundaries may provide high

diffusivity paths for elemental re-equilibration. Grain-scale composition-al re-equilibration of minerals during recovery is reported from garnet(Konrad-Schmolke et al., 2007) as well as from chromites (Ghosh andKonar, 2012). Subgrain boundaries in these cases might have enableddiffusive elemental migration. The extent of diffusional re-equilibrationis dependent on element diffusivities (Suzuki et al., 2008). Composition-al mapping in our study showed that the subgrain boundary pattern al-ways coincides with chemical modifications. Fe is strongly influenced bydiffusion along the sub-grain boundarieswhereasAl behaves in a reverseway and Cr is the least influenced. This differential diffusive chemical re-equilibration leads to very complex chemical zonation that cannot beeasily interpreted. This compositional re-equilibration has immense im-plications since this differential resetting may cause difficulty in findingthe right position for mineral analyses. Extreme caution and carefuljudgement should always be taken in dealing mantle chromites becausethis intraphase compositional re-equilibration in deformed chromites

208 B. Ghosh et al. / Ore Geology Reviews 56 (2014) 199–208

may lead to plausible errors in establishing geotectonic environment ofthe area and also in yielding geothermobarometry of the rock.

Acknowledgements

We express sincere gratitude to T.M. Kusky and S. Arai for theirstrongly motivating discussions to improve the quality of the manu-script. We cordially thank M. Santosh for his valuable suggestions andefficient editorial reviewing of the manuscript. This research was sup-ported partly by grants received from the Department of Science andTechnology, Government of India (No. SR/S4/ES-389/2008). The prepa-ration of the manuscript was supported by a Japan Society for the Pro-motion of Science (JSPS) Invitation Fellowship awarded to BG.

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