Podiform chromitites in lherzolitic mantle rocks (Andaman ophiolite, India): the role of magma/rock...

Podiform chromitites in lherzolitic mantle rocks (Andaman ophiolite, India):the role of magma/rock interaction and parental melt composition

BISWAJIT GHOSH1* and KOYEL BHATTA1

Key-words. – Podiform chromitite, Andaman ophiolite, Parental melt, East coast volcanics

Abstract. – In this study, Alpine-type (ophiolite hosted) podiform chromitites from the lherzolite dominant mantle se-quence of the Andaman ophiolite, India are described. The mantle host rocks for the chromitite pods show features ofmelt-mantle interaction. The chromites of the podiform chromitites have high-Cr# (= Cr/[Cr + Al] atomic ratio) rangingbetween 0.69 and 0.75 and high-Mg# (= Mg/[Mg + Fe2+] atomic ratio) ranging between 0.58 and 0.72. Geochemically,the chromites of these high-Cr chromitites have an arc-spinel trend and suggest a supra-subduction zone setting. Con-trarily, the low-Cr chromitites (reported elsewhere) show MORB-spinel trend. Parent melt composition is assessed onthe basis of liquidus chromite compositions using various methods. The deduced composition shows that the melt paren-tal to the high-Cr chromitite of the Andaman ophiolite had 12-13 wt.% Al2O3 and 0.2-0.5 wt.% TiO2 whereas the samefor the low-Cr chromitites had ~ 16 wt.% Al2O3 and 0.5-0.9 wt.% TiO2. A correlation of the deduced parent melt com-positions with the volcanic rocks of the Andaman ophiolite suggests a close affinity with the East Coast volcanics(south-Andaman) for the high-Cr chromitites. Contrarily, a similar correlation for the low-Cr chromitites is not so cer-tain.

Chromitites podiformes dans les roches manteliques lherzolitiques (ophiolite d’Andaman,Inde) : le rôle de l’interaction magma/roche et de la composition du liquide parental

Mots-clés. – Chromite podiforme, Ophiolite d’Andaman, Liquide parental, volcanites de la Côte Est.

Résumé. – Dans cette étude, des corps de chromite podiforme de type alpin (ophiolite hébergé), affleurant dans la sé-quence mantelique à dominante lherzolitique de l’ophiolite d’Andaman (Inde), sont décrits. Les péridotites encaissantles corps de chromite présentent des traces d’interaction liquide/roche. Les spinelles chromifères de ces chromites podi-formes présentent des Cr# (=Cr[Cr + Al], rapport atomique) et des Mg# (=Mg/[Mg + Fe2+], rapport atomique) élevés al-lant de 0.69 à 0.75 et de 0.58 à 0.72, respectivement. Géochimiquement, les spinelles chromifères de ces chromites àCr# élevé décrivent une tendance charactéristique des spinelles de type “arc” suggérant un contexte géotectonique desubduction. Inversement, ceux des chromites à faible Cr# (signalés ailleurs) témoignent d’une affinité avec les MORBs.La composition du liquide parental est déduite sur la base de celles des spinelles chromifères au liquidus, en utilisantdifférentes méthodes. Celle-ci révèle que le liquide parental des chromites à Cr# élevé de l’ophiolite d’Andaman conte-nait 12-13 wt.% Al2O3 et 0.2-0.5 wt.% TiO2 , alors que celui des chromites à faible Cr# contenait ~16 wt.% Al2O3 et0.5-0.9 wt.% TiO2. Une corrélation entre les compositions du liquide parental, ainsi déduites, et les volcanites del’ophiolite d’Andaman suggèrent une étroite affinité entre les chromites à Cr# élevé et les volcanites sur la côte Est(sud-Andaman). Cependant, une pareille corrélation avec les chromites à faible Cr# semble uncertaine.

INTRODUCTION

Boudier and Nicolas [1985] divided ophiolites into (a) thelherzolite ophiolite type (LOT) and (b) the harzburgiteophiolite type (HOT). Although the LOT model for theTrinity ophiolite was based on the argument that the low-crustal thickness is related to the low degree of partial melt-ing of the associated mantle, Ceuleneer and le Sueur [2008]have shown that the crustal section of the Trinity ophioliteis actually made of discrete plutonic bodies from boniniticmelt that are not petrogenetically related to the mantlelherzolite they intrude. Nevertheless, Ophiolite hostedpodiform chromitites (Alpine-type) are mostly found withindepleted harzburgite or in the overlying transition-zone dunite

[Boudier and Nicolas, 1985; Noller and Carter, 1986;Leblanc and Temagoult, 1989; Gervilla and Leblanc, 1990;Nicolas and Al Azri, 1991; Arai, 1997]. Earlier works sug-gested that a moderately refractory harzburgite is the mostsuitable host for podiform chromitites, because the lherzolitewall is too low in Cr# to concentrate chromite throughmelt-rock interactions [Arai and Abe, 1995; Arai, 1997]. Inthe mantle section of the lherzolite-dominated ophiolites,they are extremely rare but however, not absent [Zhouet al., 1996; Andal et al., 2005; Morishita et al., 2006].Chromites formed under different tectonic settings showcompositional differences [Arai, 1992; Kamenetsky et al.,2001; Arai et al., 2011] and understanding the genesis ofthis mineral and its accumulation is significantly important

Bull. Soc. géol. France, 2014, t. 185, no 2, pp. 123-130

Bull. Soc. géol. Fr., 2014, no 2

1. Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata 700019, India*Author for correspondence, E-mail: [email protected] deposited on April 5, 2013; accepted on November 30, 2013.

because it gives us an insight into the nature of the parentalmelt composition from which it has crystallized.

The Neotethyan suture zone at the northern boundary ofthe Indian plate extends south from the eastern Himalayansyntaxis to the Indonesia-Sumatra-Java arc system. The Anda-man group of islands represents the northern segment of thissubduction complex lying seaward of the Andaman sea[Curray, 2005]. The Indian plate is subducting obliquely be-neath the Burma sub-plate, a part of the much larger Eura-sian plate [Luhr and Haldar, 2006]. Being one of the mostimportant and presently active geodynamic setting in theIndian subcontinent the Andaman islands received the at-tention of workers since long and were extensively studiedcovering various geological aspects. Dismembered bodiesof Cretaceous ophiolite slices and Tertiary sediments, toge-ther representing a trench-slope fore-arc setting are exposedin the Andaman islands from south to north. The Mithakha-ri Group represents the trench sediments whereas the Anda-man Flysch Group together with the Archipelago Groupconstitutes the fore-arc sediments (see next section). Thereis a recent resurgence of petrological interest specially theophiolitic rocks exposed in the Andaman islands [Ghosh etal., 2009, 2012; Ghosh and Morishita, 2011; Pal, 2011].The lherzolite dominated mantle section of this ophiolite inmiddle- and north-Andaman hosts scarcely developed podi-form chromitite [Ghosh et al., 2013]. The occurrence ofchromitite in lherzolitic mantle peridotite is very limited,being different from ordinary podiform chromitites, whichare associated with harzburgitic peridotites [Arai, 1997].Here, we infer the composition of the parental melt fromwhich the podiform chromitite of the Andaman island crys-tallized. An attempt has been taken to correlate the compo-sition of the parent melt, deduced from chromite with thatof the volcanics present in the ophiolite stratigraphy of theisland.

BRIEF GEOLOGICAL OVERVIEW

The structure of the Andaman island has been inferred to bean accretionary prism formed by an imbricate stack ofeast-dipping thrust slices and folds that increasingly youngto the west, linked to a westward-shifting subduction zone[Allen et al., 2007]. The tectonic elements of the islandchain of Andaman are: (a) outer-arc comprised of ophioliticoceanic crust and trench sediments and (b) fore-arc repre-sented by siliciclastic to carbonate turbidites. The geologyof the Andaman island comprises of four major lithostra-tigraphic units namely the Cretaceous Ophiolite Group,Eocene Mithakhari Group, Oligocene Andaman FlyschGroup and Mio-Pliocene Archipelago Group (fig. 1). TheOphiolite Group of rocks lies tectonically above theMithakhari Group with a thrust contact. The details of theindividual lithostratigraphic units including their tectonicsettings have been described vividly in recent literatures[Bandopadhyay, 2012; Ghosh et al., 2013].

The Andaman ophiolite consists of a tectonised, restiticmantle sequence overlain by crustal cumulate sequence andvolcanics [Ghosh et al., 2013]. The restitic peridotite mos-tly belongs to less-depleted, lherzolite dominated mantlethat occasionally grades to clinopyroxene bearing harzbur-gite with development of olivine-rich dunitic pods (fig. 2a).This restitic mantle peridotite occasionally hosts massive

chromitite pods, enveloped by serpentinised dunite(fig. 2b). Very recently Ghosh et al. [2013] documentedcoexistence of both high- and low-Cr chromitite pods in thesame area from north-Andaman.

CHROMITITE PETROGRAPHY

Here, we describe some typical chromitite textures whichare commonly found in many ophiolitic terrains elsewhere.The details of some other textures in chromitites and associ-ated peridotites which are indicative of mantle processeslike vertical accretion and/or horizontal flow are already de-scribed elsewhere [Ghosh et al., 2013]. The size of thechromitite pods, found as strewn boulders in quarry dumpmaterial range from 8-30 cm. Because of the high rheologi-cal contrast between chromitite and extremely serpentinisedhost, these strewn pod samples are seldom found enveloped.Pods are mostly massive with varying chromite-silicateratio (figs. 3a and b). The dominant magmatic texture iscumulus (fig. 3c). In contrast, cataclasis and relatedbrecciation is the major deformational texture (fig. 3d).Whether the brittle deformation is due to horizontal move-ment in connection with the spreading center tectonics[Ghosh et al., 2014] or simply related to emplacement isdifficult to ascertain.


124 GHOSH B. and BHATTA K.

FIG. 1. – Outline geological map showing distribution of different lithos-tratigraphic units along and across the Andaman islands [modified fromBandopadhyay, 2012]. Inset shows major tectonic features of the Indonesia-Sumatra-Java arc.

CHROMITE CHEMISTRY

Electron Probe Micro Analysis (EPMA) was carried outfrom the Geological Survey of India, Kolkata Laboratory onpolished sections with a CAMECA SX100 Electron ProbeMicro Analyzer at 15 kV, 12 nA using 1 µm beam diameter.Instrument calibration was performed using mineral stan-dards supplied by the French Geological Survey (BRGM,Orléans, France) and results were corrected with a PAP ma-trix correction program. The standards were analyzed atregular intervals to check the precision of analysis.

Since the chromites of the chromitite pods are oxidizedand altered, due care was taken for selecting grains for mi-cro analysis. And because of unaltered- to least alteredgrains do not show much compositional variation across thegrain, the chromite analyses presented in table I mostly re-present core compositions. Chromites of the podiform chro-mitites have a restricted range of chemical composition.The Cr2O3 and Al2O3 contents vary between 53-55 wt.%and 12-16 wt.% respectively. The chromites of the podi-form chromitites have high Cr# (= Cr/[Cr + Al] atomic ra-tio) (0.69-0.75) and Mg# (= Mg/[Mg + Fe2+] atomic ratio)(0.58-0.72) and they are comparable with the chromites ofhigh-Cr chromitites reported by Ghosh et al. [2013]. Acomparison between chromites of low-Cr chromitites and

those present as accessory grains in the host lherzolitic peri-dotites indicates that the later have contrastingly higherAl2O3 (~ 55 wt.%) and lower Cr# (~ 0.1) than the former(table I). Chromites from diverse tectonic setting in generalhave distinctive compositions based on TiO2 vs. Al2O3 wt.%discrimination diagram [Kamenetsky et al., 2001]. Basedon that, chromites of the high-Cr chromitites of the Anda-man ophiolite (present study) occupy exclusively the fieldof supra-subduction zone peridotites (fig. 4). Contrarily, thechromites of the low-Cr chromitites of Ghosh et al. [2013]occupy the overlapping field with MORB type peridotites.Further, chromites of the high-Cr chromitites follow anarc-spinel trend in comparison to the low-Cr chromititesthat have MORB-spinel trend (fig. 4).

The compositions of chromites could be modified du-ring subsolidus reequilibration and/or metamorphism [Irvine,1967; Mitra et al., 1992; Eales and Reynolds, 1986; Barnes2000; Lord et al., 2004]. Massive chromitite occurring inpods, however, minimizes the effects of reequilibration withadjacent silicates during cooling, and thus is likely closestto its magmatic composition [Rollinson, 1995]. Hence, theliquidus chromite compositions for massive chromitite areused for interpreting the petrogenesis of the Andamanophiolite (see next section).


PODIFORM CHROMITITES IN LHERZOLITIC MANTLE ROCKS (INDIA) 125

TABLE I. – Representative electron microprobe analyses of chromites of chromitite and host lherzolite from the Andaman ophiolite. Ch: Chromitite;Lh: Lherzolite

FIG. 2. – (a) Field photograph showing the development of dunitic pods in lherzolitic mantle rocks; (b) Hand specimen photograph of a broken chromititepod enveloped by serpentinised dunite at the upper part.

INFERRED PARENTAL MELT COMPOSITION

Podiform chromitites of the Andaman ophiolite of the pres-ent study shows high Cr#, high Mg#, low Al2O3 and lowTiO2 which is typical of the geochemical character ofchromitites preserved in arc lavas (fig. 4). The parent meltcomposition can be inferred directly from mineral inclu-sions that are, in most chromitite occurrences, minerals thatnormally do not crystallize from MORB (e.g. pargasite,aspidolite, orthopyroxene) and are absent from the matrixof the ore, that in most cases is dominated by crystallizationproducts from MORB like olivine, anorthitic plagioclase,

clinopyroxene [Borisova et al., 2012]. The general lack ofsuch direct evidences in the present study prompted us toinfer the parent melt composition from liquidus chromitecompositions. Experimental studies indicate that Al2O3content of the chromites is directly related to its parentalmelt and this relation is not affected by crystallization andtemperature variations. The nature of parental melt is as-sessed on the basis of the composition of liquidus chromitesi.e. cumulus chromites from massive chromitites occurringas pods which would be little changed by equilibration[Irvine, 1965, 1967; Dick, 1977].

The chromite-melt inclusion data in volcanic rocksshows that there is a linear relationship between the Al2O3and TiO2 content of chromites and the Al2O3 and TiO2concentrations, respectively, in the melt [Maurel and Mau-rel, 1982; Kamenetsky et al., 2001]. Experimental data onperidotite melting by Wasylenki et al. [2003] also corrobo-rate the observations made by the earlier workers. Using thedata from Kamenetsky et al. [2001] and Roeder and Rey-nolds [1991], Rollinson [2008] obtained power law expres-sion between melt-Al2O3 and chromite-Al2O3 for MORBlava and logarithmic expression for arc lava (fig. 5). Howe-ver, in case of melt-TiO2 vs. chromite-TiO2, the relationshipmaintains a power law both for MORB and arc lava. The re-gression expression of Zaccarini et al. [2011] slightlydiffers with that obtained by Rollinson [2008].

We calculated the Al2O3 contents of the parent meltfrom which the chromites of the high-Cr pods of Andamancrystallized using the approaches as given by (a) Maureland Maurel [1982], by (b) Rollinson [2008] and by Zaccariniet al. [2011] (table II). The same has been done for low-Crchromitite pods using data from Ghosh et al. [2013] forcomparison. In the first case [Maurel and Maurel, 1982], it



FIG. 3. – Photomicrographs showing (a) massive chromitite with very high chromite (Cr)-silicate ratio (reflected light); (b) chromitite with much higherproportion of silicates (Ser: serpentine) (transmitted light); (c) cumulus texture. (d) BSE image showing brecciated and cataclastic texture in chromitite.

FIG. 4. – TiO2 (wt. %) – Al2O3 (wt. %) variations of chromites of Andamanpodiform chromitites (present study) with respect to present day tectonicsetting. Fields are from Kamenetsky et al. [2001].

was determined using the formula as: Al2O3 wt% in chro-mite = 0.035 Al2O3 in parent melt. In the last two cases thesame was determined from the point of intersection of the

regression line with the lines representing the Al2O3 chro-mite composition (fig. 5). For high-Cr chromitites, only theline for the arc lava was considered because the podiformchromitites of the Andaman ophiolite have arc-spinel trend(fig. 4). In the same analogy the line for the MORB lavawas considered for the low-Cr chromitites. Similarly, theTiO2 contents of the parental melt were determined in thesame approach as suggested by Rollinson [2008] and Zac-carini et al. [2011].

COMPARISON BETWEEN LAVA CHEMISTRY ANDPARENTAL MELT COMPOSITION

The volcanic rocks of the Andaman ophiolite are dividedinto (a) pillow lava and (b) East Coast volcanics [Ray et al.,1988], each having their own field and chemical character-istics. Vohra et al. [1989] described these two volcanicgroups as upper lava and lower lava respectively. Pillowlavas are basaltic in composition and have a very restrictedchemical range in contrast to East Coast volcanics whichare non-pillowed and have a wide range of chemical compo-sitions from basaltic to basaltic andesites to more acid dif-ferentiates. In the Al2O3 vs. TiO2 diagram, the formergroup has a negative correlation unlike the latter whichhave a positive correlation.



TABLE II. – Calculated parental melt composition of representative chromi-tes of chromitite from the Andaman ophiolite.

FIG. 5. – Chromite – melt compositional relationship after Rollinson [2008] and Zaccarini et al. [2011]. (a) and (c) Al2O3-chromite vs. Al2O3-melt; (b) and(d) TiO2-chromite vs. TiO2-melt.

For a comparative study, the calculated parent meltcompositions are plotted on Al2O3-TiO2 bivariate diagramwhere the fields for pillow lavas and East Coast volcanics[after Ray et al., 1988] are shown (fig. 6). The diagramshows that the calculated parental melt compositions forboth the high- and low-Cr chromitites fall in the low-TiO2region (< 1 wt.%). Moreover, the parent melt compositionfor the high-Cr chromitites are very akin to East Coast vol-canics, suggesting a genetic relationship between these twowhereas, the same for low-Cr chromitites is not so definite.

DISCUSSION

The well accepted view regarding the genesis of massivechromitite is the melt hybridization between a mafic primi-tive melt and a silica enriched melt [Irvine, 1977]. In thesame line of thinking, the concept of magma mixing wasadopted in case of ophiolitic podiform chromitites. How-ever, the fundamental role of water in chromite concentra-tion process was also demonstrated. Water may have amechanical effect on the transport of chromite grains[Matveev and Balhaus, 2002] or a petrological influence ingenerating exotic, silica-enriched melts [Borisova et al.,2012]. The origin of podiform chromitite in general is ex-plained by the process of melt-mantle interaction [Arai andYurimoto, 1994; Arai and Abe, 1995; Arai, 1997; Zhou etal., 1994, 1996, 2005]. Mantle generated primitive melt, af-ter coming to the shallower level becomes pyroxeneundersaturated because of the inflation of olivine field atlow pressure. Thus the primitive melt being not in equilib-rium with the host peridotite in the new environment startsreacting with the surrounding rocks. In case of lherzolitichost-rock, clinopyroxenes being the low temperature form

dissolute first and then orthopyroxenes with consequentcrystallization of olivines. As a result, the lherzolitic hostrock gradually changes to a harzburgite and then to a duniteoutward along the zone of mixing. The net result of this in-teraction is the production of a secondary melt enriched inSi and Cr by selective dissolution of pyroxene and crystalli-zation of secondary olivine in its place. Mixing of the Si-and Cr-rich secondary melt with subsequently suppliedprimitive melt, which is saturated with chromite, may makethe mixed melt chromite-oversaturated thereby precipitatingexclusively chromites [Arai and Yurimoto, 1994; Zhou etal., 1994; Morishita et al., 2007]. The main reason for thepaucity of chromitites associated to lherzolites is that Cr ishighly compatible in clinopyroxene (diopside), a mineralabundant in lherzolites and almost absent in harzburgites.Accordingly, the main reservoir of Cr in lherzolites isdiopside, while the same in harzburgites is chromite.

The chromite compositions are sensitive petrogeneticindicators and are used to constrain the parent melt compo-sition [Irvine, 1967; Dick and Bullen, 1984; Barnes, 1986;Auge, 1987; Arai, 1992; Zhou et al., 1996; Melcher et al.,1997; Bedard, 1999; Kamenetsky et al., 2001; Python et al.,2008]. The mantle section of the Andaman ophiolite imme-diately below the Moho hosts persistent developed podi-form chromite. The deduced composition shows that themelt parental to the high-Cr chromitite of the Andamanophiolite had 12-13 wt.% Al2O3 and 0.2-0.5 wt.% TiO2whereas the same for the low-Cr chromitites had ~ 16 wt.%Al2O3 and 0.5-0.9 wt.% TiO2.

The Cr# of chromite is a strong indicator of the tectonicenvironment of the chromitite formation. The high Cr# ofthe high-Cr podiform chromite (> 0.6) is suggestive for anarc related settings [Dick and Bullen, 1984; Arai, 1994].The bivariate Al2O3-TiO2 wt% plots for these chromititesalso show an arc-spinel trend and attest supra-subductionzone setting of the Andaman ophiolite corroborating withthe findings of Ghosh et al. [2013] (fig. 4). A correlation ofthe deduced parent melt compositions with the volcanicrocks of the Andaman ophiolite suggests a close affinitywith the East Coast volcanics for the high-Cr chromitites.Contrarily, the low-Cr chromitites have a MORB-spineltrend (fig. 4) and a similar correlation for them is not socertain. They may either be related with the same volcanicslike the high-Cr chromitites or there may be some linkswith the pillow lavas. In the first case the coexistence ofboth these chromitites in north Andaman reflects a temporalvariation of separate melt intrusions produced through spe-cific melting stages and emplaced in different sub-arcmantle domains [Ghosh et al., 2013]. The second possibili-ty may represent a switch over of the tectonic settings fromMORB to arc [Ghosh et al., 2012].

Acknowledgements. – The authors sincerely thank G. Ceuleneer and ananonymous reviewer for their insightful comments to improve the qualityof the manuscript. O. Lacombe, the Editor-in Chief and R. Augier, Assis-tant Editor of the journal are cordially acknowledged for their graceful atti-tude in handling the manuscript. Khadidja Abbou-Kebir is speciallythanked for translating the abstract to French. The authors acknowledge thefinancial support received from the Department of Science and Technology,India (SR/FTP/ES-65/2009).SGF associate editor: Romain Augier



FIG. 6. – TiO2 vs. Al2O3 plot showing the fields of pillow lavas and Eastcoast volcanics [after Ray et al., 1988] of the Andaman ophiolite. The cal-culated compositions [after Zaccarini et al., 2011] of the parental melts tothe podiform chromitites are also shown here.

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Podiform chromitites in lherzolitic mantle rocks (Andaman ophiolite, India): the role of magma/rock...

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