detrital chromian spinels from beach placers of andaman islands, india: a perspective view of...

Research ArticleDetrital chromian spinels from beach placers of Andaman Islands,

India: A perspective view of petrological characteristics and variationsof the Andaman ophiolite

BISWAJIT GHOSH,1,* TOMOAKI MORISHITA2 AND KOYEL BHATTA1

1Department of Geology, University of Calcutta, Kolkata, West Bengal, India (email: [email protected]);2School of Natural System, College of Science and Engineering, Kanazawa University, Kakuma,

Kanazawa, Japan

Abstract Along the east coast of the Andaman Islands, abundant detrital chromian spinelsfrequently occur in black sands at the confluence of streams meeting the Andaman Sea.The mineral chemistry of these detrital chromian spinels has been used in reconstructingthe evolutionary history of the Andaman ophiolite. The chromian spinels show wide varia-tion in compositional parameters such as Cr# [= Cr/(Cr + A1) atomic ratio] (0.13–0.91),Mg# [= Mg/(Mg + Fe2+) atomic ratio] (0.23–0.76), and TiO2 (<0.05–3.9 wt%). The YFe3+

[= 100Fe3+/(Cr + A1 + Fe3+) atomic ratio] is remarkably low (usually <10 except for southAndaman). The ranges of chemical composition of chromian spinels are different in eachlocality. The spinel compositions show very depleted signatures over the entire island,which suggests that all massifs in the Andaman ophiolite were affected under island-arcconditions. Although the degree of depletion varies in different parts of the island, adirectional change in composition of the detrital chromian spinels from south to north isevident. Towards the north the detrital chromian spinels point to less-depleted sourcerocks in contrast to relatively more depleted towards the south. The possibilities to explainthis directional change are critically discussed in the context of the evolution of Andamanophiolite.

Key words: Andaman ophiolite, arc-dominant area, detrital chromian spinel, MOR-dominant area, paleogeodynamic setting.

INTRODUCTION

Since the chemical compositions of chromianspinels in mafic and ultramafic rocks are influencedby geological factors such as magma compositions,crystallization sequence, oxygen fugacity, andpressure–temperature conditions (Irvine 1965,1967, 1975; Jackson 1969; Evans & Frost 1975; Arai1980, 1992, 1994a,b; Ozawa 1983; Sack & Ghiorso1991; Arai et al. 2006, 2011; Gahlan & Arai 2007),they can be correlated with different tectonicsettings (Dick & Bullen 1984; Arai 1992, 1994a,b;

Zhou & Robinson 1997; Barnes & Roeder 2001;Kamenetsky et al. 2001; Arai et al. 2011). Chromianspinels, due to their mechanical and chemical resis-tivity, are commonly found in beach placers wheremafic–ultramafic rocks exist in the close proximity.Chromian spinels in sediments and sedimentaryrocks have immense geological implications asprovenance markers of their source protolith(Press 1986; Arai & Okada 1991; Hisada & Arai1993; Cookenboo et al. 1997; Kadoshima & Arai1999, 2001; Lee 1999; Lenaz et al. 2000; Arai et al.2006; Faupl et al. 2006).

The Andaman ophiolite, India consists of a thin,upper crustal section of cumulate rocks along withvolcanic rocks and a lower, thicker mantle sectionof tectonized residual peridotites, which containabundant chromian spinel with varying chemicalcompositions and modes of occurrence (Ghosh

*Correspondence: Department of Geology, Calcutta University, 35 Bally-gunge Circular Road, Kolkata, West Bengal 700019, India (email:[email protected]).

Received 15 December 2011; accepted for publication 19 March 2012.

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Island Arc (2012)

© 2012 Blackwell Publishing Asia Pty Ltd doi:10.1111/j.1440-1738.2012.00812.x

et al. 2009). The lower mantle section of the ophio-lite (i.e. peridotites) shows wide variations inpetrology and mineral chemistry (Pal et al. 2003;Ghosh et al. 2009; Pal 2011). The relationshipsbetween these peridotites are the key to under-standing the tectonic evolution of the Andamanophiolite. Concentrated black sands with detritalchromian spinel as a major phase occur preferen-tially at places along the eastern coast of theAndaman Islands where the rock types in the closevicinity consist of ophiolite, mainly various types ofperidotites and volcanic rocks. In order to estimatelithological distributions and variations of thesource rocks over a wide area more effectively,examining the detrital chromian spinels is the bestway to grasp the petrological and chemical char-acteristics of various rock types of ophiolites, par-ticularly peridotites with significant lithologicalvariations (Arai et al. 2006). Moreover, due to theirhigh mechanical and chemical resistivity the detri-tal chromian spinels can represent in situ spinelsin the mantle rocks of the catchment areas to someextent (Kadoshima & Arai 2001).

Provenance studies of sedimentary rocks aim todecipher the composition and geological evolutionof the sediment source areas and to constrain thetectonic setting of the depositional basin (Zimmer-mann & Bahlburg 2003). In the present study,detrital heavy minerals were collected from each

part of the island (north middle and southAndaman) as well as from Rutland Island. Thesampling points are proximate to the OphioliteGroup of rocks at the confluence of streamsmeeting the sea (Andaman Sea) along the eastcoast. Noteworthy here is the effect of wave ener-gies and longshore currents on the eastern coastof the Andaman Islands that meets the AndamanSea. A restricted back-arc basin is minimum incontrast to the western coast, which is the open-ocean direction. Here we report the mineral chem-istry of the detrital chromian spinels with specialemphasis on their use in deciphering the evolution-ary history of the Andaman ophiolite.

GEOLOGICAL SETTING

The ophiolitic Neotethyan suture zone at thenorthern boundary of the Indian Plate extendssouth from the eastern Himalayan syntaxis to theIndonesia–Sumatra–Java arc system (Fig. 1). TheAndaman Islands represent the northern segmentof Sunda–Java subduction complex lying seawardof the Andaman Sea (Curray 2005). According toRay et al. (1988), the islands are situated on theouter-arc ridge of the Indonesian Arc system(Fig. 2). The oceanic part of the Indian Plate issubducting obliquely beneath the Burma Subplate,

Fig. 1 Map showing the distributionof ophiolites along the NeotethyanSuture Zone (Dilek & Furnes 2011) andthe location of the Andaman Islands in it.

2 B. Ghosh et al.

© 2012 Blackwell Publishing Asia Pty Ltd

AN

DA

MA

N S

EA

MIDDLEANDAMAN

NORTHANDAMAN

SOUTHANDAMAN

RUTLANDISLAND

Havelok

BA

YO

F B

EN

GA

L

ArchipelagoGroup

Andaman FlyschGroup

Mithakhari Group

Ophiolite Group

12 12

13 13

92 15’

0 30 Km

Mio-Pliocene

Oligocene

Eocene

Cretaceous

(b)N93 00’

Drainage pattern

S10S11

S12S13

30 90 100

CHINA

TR

IASSIC

SUT

UR

E

NIC

OB

ER

AN

DA

MA

N

BU

RM

AT

RE

NC

H

An

Ni

AN

DA

MA

N

SEA

SUMATRA

MA

LAY

SIA

THAILAND

20

1010

20

100

BAY

OF

BENGAL

SF

AY

CH

IB

R

KF

KGR

SST

An - Andaman Is.AY - Arakan YomaCH - Chin HillsIBR - Indoburman RangesKF - Khlong Marui Fault

KGR - Katha -Gangaw RangeNi - Nicober Is.SF - Sagaing FaultSST - Shan Scarps Thrusts

Albian to Pliocene sediments

Shan PlateauMogok BeltEarly Tertiary tin granite beltNorth Thailand extensional zone

(a)

R2

M6

N7

Fig. 2 (a) Regional tectonic framework of southeast Asia (redrawn after Mitchell 1985). (b) Geological map of the Andaman Islands (after Pal et al. 2003)showing the distribution of the Ophiolite Group along with other lithostratigraphic units. Sample locations marked as � with sample numbers.

Detrital Cr-spinels of Andaman Islands 3


a part of the much larger Eurasian Plate (Luhr &Haldar 2006). The tectonic elements of the islandchain of Andaman are: (i) an outer-arc comprisedof ophiolitic oceanic crust and trench deposits; and(ii) a forearc represented by siliciclastic to carbon-ate turbidites (Pal et al. 2003). A number of north–south trending dismembered ophiolite slices orthrust sheets occur at different structural levelswith trench-slope sediments uplifted and emplacedby a series of east-dipping thrusts.

Stratigraphically, the island consists of the Cre-taceous Ophiolite Group at the base followed bysedimentary rocks of the Eocene MithakhariGroup, Oligocene Andaman Flysch Group, andMio–Pliocene Archipelago Group successively(Fig. 2). The Mithakhari Group represents thetrench deposits whereas the Andaman FlyschGroup together with the Archipelago Group con-stitutes forearc deposits (Pal et al. 2003). Theophiolite sequence of the Andaman Islands is sub-divided into a lower mantle section represented byperidotite tectonites overlain by a crustal sectionconsisting of ultramafic–mafic cumulate rocks anda plagiogranite–diorite–andesite suite that repre-sents high-level intrusive rocks and basic to inter-mediate volcanic rocks intercalated with deep-seapelagic sediments (Pal et al. 2003; Ghosh et al.2009).

The residual peridotites in the AndamanIslands, which are the residues after the extractionof the melt that formed the oceanic crust, mostlybelong to variably depleted mantle that consistsof lherzolite, clinopyroxene-bearing harzburgite,and harzburgite with development of olivine-richdunitic pods. Chromian spinel occurs both asmassive chromitite as well as accessory residualanhedral grains showing features of partialmelting, uniformly distributed in the host peridot-ite. Massive-type chromitite includes pods varying8–30 cm in diameter enclosed in a serpentinizeddunitic envelope within depleted mantle tectoniteand also as millimeter-scale chromitite bands inultramafic cumulate rocks.

The currently active Andaman–Java subductionzone began only in the late Miocene (Curray 2005;Acharyya 2007) and the Andaman ophiolites werebelieved to have been accreted in Eocene times(Sengupta et al. 1990). Recent sensitive high-resolution ion microprobe (SHRIMP) results fromthe subduction-related plagiogranite provide acrystallization age of about 95 Ma (Pedersen et al.2010; Sarma et al. 2010). The plagiogranites, anintegral part of Andaman ophiolite, show an intru-sive relationship with the host gabbro and volcanic

rocks of the Andaman ophiolite. In terms of Rb,Yb, Nb, Ta, and Y abundances, these plagiogranitesshow geochemical similarities characteristic ofvolcanic-arc granites rather than the ocean-ridgegranites and are inferred to have formed within asubduction zone setting (Jafri et al. 1995). Becausethe Andaman ophiolitic rocks predate this plagiog-ranite, the Miocene subduction could not haveemplaced this ophiolite. Thus, arc magmatism wasinitiated much earlier than the Eocene accretionof the Andaman ophiolites and possibly repre-sents extension of an earlier phase of Early–Mid-Cretaceous ophiolite accretion.

REVIEW OF COMPOSITIONS OF IN-SITU CHROMIANSPINELS FROM MID-OCEANIC RIDGE ANDARC SETTINGS

The primary sources of chromian spinel in ophi-olitic settings cover volcanic rocks as well asmafic–ultramafic plutonic rocks, which are eithercumulate rocks or residual mantle peridotites. Thevolcanic rocks include mid-oceanic ridge basalts(MORBs) and arc lavas like boninites and arc-tholeiites whereas the plutonic rocks take accountof lherzolites, harzburgites, dunites, wehrlites,troctolites, and olivine gabbros. We here summa-rize chemical characteristics of chromian spinels(hereafter spinels) in these rocks with special ref-erence to the tectonic setting of the ophiolite(MOR vs arc). In the case of volcanic spinels, thosefrom arc magmas have a wide spread of Cr# [= Cr/(Cr + A1) atomic ratio]. The TiO2–Cr# relation-ship indicates that the field for MORB isoverlapped to a great extent with that of island-arcbasalts (Fig. 3). However, spinels from boninitesare easily discriminated from other volcanicspinels by their higher Cr and lower Ti contents.YFe3+ [= 100Fe3+/(Cr + A1 + Fe3+) atomic ratio] ofspinel, which is strongly dependent on the degreeof differentiation of the host magma, is character-istically lower for MORB spinels than in othervolcanic rocks (Arai 1992) (Fig. 4). In the case ofplutonic rocks, the Cr# of spinels is highly diverse.The relationships are almost parallel with those involcanic rocks, but the Ti content is slightly lowerin residual rocks than in volcanic rocks at a giventectonic environment. Spinels in cumulate rocks ofMOR setting are typically high in TiO2 and havemuch more extended YFe3+ than in other plutonicrocks (Fig. 4). Summing up, the chemistry ofspinels in volcanic and plutonic rocks from eachof the MOR and arc settings is to some extent

4 B. Ghosh et al.


overlapping in related variation diagrams; how-ever, one can appreciably detect arc- and MOR-dominant areas and can discriminate betweenthese two. With reference to ophiolitic terrain, con-sidering volcanic and plutonic spinels as a whole,the TiO2–Cr# systematics can discriminate theMOR-dominant area, characterized mainly bycumulate rocks from the arc-dominant area whichis characterized by volcanic rocks as well as plu-tonic rocks of both types, cumulate rocks andmantle residues (Fig. 3). The TiO2–YFe3+ system-atic can detect an arc-dominated area, character-

ized by volcanic rocks and cumulate rocks (Fig. 4).Likewise, the Cr#–Mg# [= Mg/(Mg + Fe2+) atomicratio] relationship for plutonic rocks can clearlydiscriminate an arc-dominated area which is char-acterized mainly by residual peridotites and cumu-late rocks (Fig. 5).

SAMPLING AND ANALYTICAL METHODS

The detrital spinel-rich sands were collected alongthe eastern coast of the Andaman Islands (Fig. 6)

Fig. 3 TiO2 vs Cr# systematics showing fields of chromian spinels in(a) mid-oceanic ridge, and (b) island-arc settings (data from Arai 1992and Arai et al. 2011). (c) Discrimination diagram. Circles (large andsmall) filled with different gray tones represent corresponding fields.

Fig. 4 Compositional fields of chromian spinels in various lithologiesfrom (a) mid-oceanic ridge, and (b) island-arc settings in a TiO2 vs YFe3+

variation diagram (data from Arai 1992 and Arai et al. 2011). (c) Discrimi-nation diagram. Circles and rectangle filled with different gray tonesrepresent corresponding fields.



from four locations (S10–S13) between Brichgunjand Chidiyatapu in south Andaman, Betapur inmiddle Andaman, south of Kalipur Beach in northAndaman, and north of Chain nala in Rutland

Island. As mentioned earlier, concentrated blacksands occur preferentially in close proximity to theOphiolite Group rocks, at the confluence ofstreams meeting the Andaman Sea. This unique

Fig. 5 Cr#–Mg# systematics showing fields of chromian spinels in plutonic rocks from (a) mid-oceanic ridge, and (b) island-arc settings (data from Arai1992 and Arai et al. 2011). (c) Discrimination diagram.

Fig. 6 Field photographs showing (a) intermittent thin layers rich in black sand in a section of beach berm, and (b) concentrated black sands rich indetrital spinels on the beach floor.

6 B. Ghosh et al.


mode of occurrence excludes the possibilities ofany other source for these spinels except the ophi-olitic rocks. If it is argued that the MithakhariGroup could be a potential source, the ultimatesource for these reworked spinels is always theophiolitic rocks because the bulk of the MithakhariGroup sedimentary rocks represent coarse clas-tics, mainly conglomerate and grit, composeddominantly of ophiolite-derived clasts (Pal et al.2003; Allen et al. 2007), and are described as imma-ture ophiolite-wash deposited in trench-slopebasins (Chakraborty et al. 1999). Since the placerspinels show variable degrees of oxidation andrelated alteration as evidenced from the develop-ment of ferrian chromite along grain margins andfractures, we carefully selected the analyticalpoints to avoid these later modifications. Repre-sentative chemical analyses are presented inTable 1. The samples were sun-dried before treat-ing them using a standard bromoform separationtechnique. The heavy mineral fractions with sig-nificant proportion of spinels along with magnetiteand ilmenite were collected, washed, dried, andmounted for preparation of thin-sections.

Major-element compositions of minerals wereanalyzed using an electron probe micro-analyzer(JEOL JXA-8800 Superprobe) at Kanazawa Uni-versity. The analyses were performed under anaccelerating voltage of 15 kV and beam current of20 nA, using a beam diameter of 3 mm. Natural andsynthetic mineral standards were used for all min-erals. We assume that all Ti forms an ulvospinelphase (Fe2TiO4) in chromian spinel. The Fe2+–Fe3+

partitioning in spinel was calculated assumingspinel stoichiometry.

MINERAL CHEMISTRY

The detrital spinels of the Andaman Islands showwide variations in Cr# (0.13–0.91) and Mg# (0.23–0.76) atomic ratios (Table 1). The TiO2 content andYFe3+ are usually low at <0.5 wt% except for onelocality in south Andaman (up to 3.9 wt%), and<10 (up to 40 for some grains), respectively. It isemphasized here that the ranges of chemical com-position of spinels are different in each locality. Insouth Andaman, spinel compositions are relativelypredominant in the Cr# (0.4–0.6) and Mg# (0.4–0.8), whereas several high-Cr# grains with slightlylower Mg# are detected (Figs 7 and 8). Somegrains with intermediate Cr# (0.4–0.6) are high inTiO2 contents (Fig. 7). On the other hand, spinelsfrom middle and north Andaman have a relatively

wide range of Cr# (0.2–0.8). However, most of thegrains have values <0.6. The Cr# of spinels inRutland Island ranges 0.4–0.9; however, theydominantly concentrate around 0.8. The chemicalcharacteristics of Andaman spinels as a whole, con-sidering all four locations in south Andamantogether, are described next with reference tospecific variation diagrams.1. TiO2–Cr# relationship. In north and middle

Andaman most spinels plot in the arc-MORoverlap field with a very few in the arc-dominant field (Fig. 9). In contrast, appreciablenumbers of spinel plot in the arc-dominant fieldin south Andaman, in addition to a few plotsin the MOR-dominant field. In Rutland Island,the plots for spinels predominantly occupy thearc-dominant field.

2. TiO2–YFe3+ relationship. In this diagram theplots for spinels for north and middle Andamanalmost completely occupy the arc-MOR overlapfield (Fig. 10). In south Andaman, appreciablenumbers of points occupy the arc-dominantfield. Few plots occupy the MOR-dominantfield. For Rutland Island, few plots occupy thearc-dominant field.

3. Cr#–Mg# relationship. In north and middleAndaman the plots for spinels predominantlyoccupy the arc-MOR overlap field, with fewplots in the arc-dominant field (Fig. 11). Inter-estingly, several plots for south Andamanoccupy the arc-dominant field. For RutlandIsland the plots for spinels predominantlyoccupy the arc-dominant field.

DISCUSSION

The chemistry of spinels in volcanic rocks is mainlydependent on the chemistry of magma from whichthey precipitate. The melts from which spinelsprecipitate are commonly boninites (includinghigh-Mg andesites) or tholeiites (Malpas & Robin-son 1987; Roberts 1988; Yumul & Balce 1994; Zhouet al. 1996). The Cr# of spinel in arc magmasranges 0.2–0.9. Boninites and high-Mg tholeiiteshave spinels with extremely high Cr# (>0.8).Spinels in genetically related magmas and mantlerestites have similar composition ranges (Dick &Bullen 1984). In currently active volcanic arcs,the degree of partial melting recorded in theperidotites appears to decrease away from theforearc towards the back-arc region. Highlydepleted harzburgites, more depleted than abys-sal harzburgites, occur only in the frontal arc to



Table 1 Representative electron microprobe analyses of detrital chromian spinels of Andaman Islands

Sample SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O NiO Total Mg# Cr# YFe YCr YAl

North <0.03 0.09 19.81 45.74 22.47 0.31 11.17 <0.03 <0.03 <0.03 0.09 99.7 0.524 0.608 5.8 36.9 57.2<0.03 0.20 9.62 54.44 24.50 0.35 9.42 <0.03 <0.03 <0.03 0.03 98.6 0.470 0.792 7.9 19.2 72.9<0.03 0.11 33.09 33.26 17.58 0.17 15.20 <0.03 <0.03 <0.03 0.17 99.6 0.660 0.403 4.4 57.1 38.5<0.03 0.48 24.83 37.04 24.91 0.28 11.85 <0.03 <0.03 <0.03 0.13 99.6 0.538 0.500 8.8 45.6 45.6<0.03 0.39 27.56 34.70 22.75 0.34 13.37 <0.03 <0.03 <0.03 0.18 99.3 0.597 0.458 8.5 49.6 41.9<0.03 <0.05 53.41 14.27 11.39 0.09 19.80 <0.03 <0.03 <0.03 0.36 99.3 0.781 0.152 1.7 83.4 14.9<0.03 0.05 51.36 16.23 10.49 0.07 20.00 <0.03 0.05 <0.03 0.34 98.6 0.798 0.175 1.6 81.2 17.2<0.03 0.16 9.87 53.01 28.72 0.42 6.72 <0.03 0.04 <0.03 0.03 99.0 0.341 0.783 8.0 20.0 72.0<0.03 0.19 7.82 56.06 25.19 0.36 8.75 <0.03 0.05 <0.03 0.05 98.5 0.443 0.828 8.1 15.8 76.1<0.03 0.08 42.24 22.58 15.62 0.15 17.42 <0.03 <0.03 <0.03 0.22 98.3 0.728 0.264 4.8 70.1 25.1

Middle <0.03 0.09 39.03 27.48 16.52 0.21 15.86 <0.03 <0.03 <0.03 0.19 99.4 0.672 0.321 3.3 65.7 31.0<0.03 0.41 19.63 38.88 28.98 0.26 9.84 <0.03 <0.03 <0.03 0.12 98.1 0.468 0.570 12.3 37.7 50.0<0.03 0.11 48.75 18.44 12.50 0.12 18.77 <0.03 <0.03 <0.03 0.29 99.0 0.757 0.202 2.0 78.1 19.8<0.03 0.35 28.86 34.75 21.07 0.31 14.14 <0.03 <0.03 <0.03 0.16 99.6 0.624 0.447 7.4 51.2 41.4<0.03 0.27 8.75 56.29 23.86 0.41 8.93 <0.03 <0.03 <0.03 <0.08 98.5 0.449 0.812 6.2 17.6 76.1<0.03 0.10 19.87 47.52 20.38 0.24 11.11 <0.03 <0.03 <0.03 <0.08 99.2 0.524 0.616 3.1 37.2 59.7<0.03 0.16 19.64 42.61 25.10 0.35 9.71 <0.03 <0.03 <0.03 <0.08 97.6 0.468 0.593 7.4 37.7 54.9<0.03 0.05 46.88 20.76 13.14 0.17 18.76 <0.03 <0.03 <0.03 0.30 100.0 0.757 0.229 2.7 75.0 22.3<0.03 0.31 12.27 46.21 30.74 0.38 7.42 <0.03 <0.03 <0.03 <0.08 97.3 0.374 0.716 12.4 24.9 62.8<0.03 0.06 55.49 13.00 11.64 0.17 19.47 <0.03 <0.03 <0.03 0.34 100.2 0.759 0.136 0.7 85.8 13.5

South <0.03 0.29 27.76 41.57 14.24 0.22 15.28 <0.03 <0.03 <0.03 0.19 99.6 0.675 0.501 1.5 49.2 49.4S10 <0.03 0.14 19.50 37.31 31.74 0.38 8.79 <0.03 <0.03 <0.03 0.10 98.0 0.425 0.562 14.4 37.5 48.1

<0.03 0.92 8.38 25.64 57.20 1.00 0.29 <0.03 <0.03 <0.03 0.14 93.6 0.016 0.672 41.3 19.2 39.5<0.03 0.41 29.51 23.99 33.83 0.33 9.38 <0.03 <0.03 <0.03 0.19 97.7 0.431 0.353 15.5 54.7 29.8<0.03 0.23 27.12 33.94 24.14 0.29 12.15 <0.03 <0.03 <0.03 0.09 98.0 0.555 0.456 8.8 49.6 41.6<0.03 0.37 16.73 37.53 34.51 0.45 6.99 0.04 <0.03 <0.03 0.08 96.7 0.348 0.601 15.9 33.6 50.5<0.03 0.30 14.57 43.57 29.81 0.41 8.25 <0.03 <0.03 <0.03 0.13 97.1 0.411 0.667 12.4 29.1 58.5<0.03 0.35 27.41 36.23 19.35 0.27 13.91 <0.03 <0.03 <0.03 0.10 97.7 0.629 0.470 6.1 49.8 44.1<0.03 0.28 5.98 55.76 28.19 0.47 7.59 0.04 <0.03 <0.03 0.09 98.4 0.391 0.862 10.4 12.4 77.2<0.03 0.81 23.71 38.70 21.29 0.23 13.54 <0.03 <0.03 <0.03 0.13 98.5 0.612 0.523 7.9 44.0 48.1

S11 <0.03 0.28 8.33 52.39 29.31 0.48 7.27 <0.03 <0.03 <0.03 <0.08 98.2 0.372 0.808 10.8 17.1 72.1<0.03 0.05 32.24 28.90 24.02 0.21 12.55 <0.03 <0.03 <0.03 0.19 98.2 0.562 0.375 8.3 57.3 34.4<0.03 0.21 31.87 32.85 18.94 0.23 14.58 <0.03 <0.03 <0.03 0.11 98.8 0.641 0.409 5.4 55.9 38.7<0.03 0.26 11.67 50.01 28.16 0.40 8.10 <0.03 <0.03 <0.03 <0.08 98.7 0.404 0.742 9.7 23.3 67.0<0.03 <0.05 31.20 33.36 21.28 0.24 13.53 <0.03 <0.03 <0.03 0.15 99.8 0.598 0.418 6.3 54.6 39.1<0.03 <0.05 38.22 26.11 19.86 0.19 14.98 <0.03 <0.03 <0.03 0.16 99.6 0.639 0.314 5.8 64.6 29.6<0.03 0.20 16.10 47.56 23.43 0.36 10.53 <0.03 <0.03 <0.03 0.10 98.3 0.509 0.665 7.3 31.1 61.6<0.03 0.16 2.66 42.22 46.26 0.43 4.36 <0.03 0.04 0.04 0.21 96.4 0.237 0.914 32.8 5.8 61.5<0.03 1.87 12.19 22.63 54.06 0.45 4.82 <0.03 <0.03 0.04 0.32 96.4 0.239 0.555 40.9 26.3 32.8<0.03 0.25 10.08 47.60 32.44 0.44 7.10 <0.03 <0.03 <0.03 0.11 98.0 0.361 0.760 14.5 20.5 65.0

S12 <0.03 0.79 21.81 27.44 38.26 0.43 7.54 <0.03 <0.03 <0.03 0.21 96.6 0.362 0.458 20.5 43.1 36.4<0.03 0.89 20.54 24.54 42.19 0.43 6.44 <0.03 <0.03 <0.03 0.18 95.2 0.316 0.445 24.9 41.7 33.4<0.03 0.67 20.49 39.84 25.51 0.34 11.22 <0.03 <0.03 <0.03 0.16 98.3 0.524 0.566 10.0 39.1 51.0<0.03 0.52 16.92 43.42 27.45 0.36 9.43 <0.03 <0.03 <0.03 0.09 98.2 0.454 0.632 10.0 33.1 56.9<0.03 0.10 19.40 42.01 28.50 0.36 8.81 <0.03 <0.03 <0.03 0.12 99.3 0.422 0.592 9.5 36.9 53.6<0.03 0.30 12.00 44.03 33.15 0.47 6.74 <0.03 <0.03 0.04 0.09 96.8 0.344 0.711 14.9 24.6 60.5<0.03 0.37 13.76 47.77 26.85 0.56 9.04 <0.03 <0.03 <0.03 0.08 98.5 0.444 0.700 9.4 27.2 63.4<0.03 0.72 5.82 14.39 71.40 0.41 1.92 <0.03 <0.03 <0.03 0.13 94.8 0.103 0.624 65.7 12.9 21.4<0.03 0.92 20.10 34.59 32.21 0.40 9.19 <0.03 <0.03 <0.03 0.14 97.6 0.435 0.536 15.2 39.4 45.4<0.03 0.07 20.82 41.02 25.01 0.31 10.82 <0.03 <0.03 0.04 0.18 98.3 0.513 0.569 9.0 39.2 51.8

S13 <0.03 2.66 16.19 36.12 33.76 0.39 8.05 <0.03 <0.03 <0.03 0.14 97.3 0.375 0.599 14.8 34.1 51.1<0.03 0.25 34.55 27.36 23.80 0.21 12.30 <0.03 <0.03 <0.03 0.17 98.7 0.542 0.347 6.7 61.0 32.4<0.03 1.52 23.34 36.90 23.66 0.21 12.20 <0.03 <0.03 <0.03 0.21 98.1 0.550 0.515 8.0 44.7 47.4<0.03 0.06 29.63 38.20 15.39 0.19 15.33 <0.03 <0.03 <0.03 0.11 99.0 0.678 0.464 3.0 52.0 45.0

0.04 0.76 23.89 34.85 29.16 0.43 8.93 <0.03 <0.03 <0.03 0.13 98.2 0.417 0.495 9.4 45.8 44.80.04 3.01 15.26 35.88 36.44 0.61 6.18 <0.03 <0.03 <0.03 0.15 97.6 0.290 0.612 14.7 33.1 52.2

<0.03 0.87 23.09 37.58 25.29 0.25 11.85 <0.03 <0.03 <0.03 0.15 99.1 0.539 0.522 9.6 43.2 47.2<0.03 0.35 13.20 41.02 34.62 0.43 7.64 <0.03 <0.03 <0.03 0.25 97.5 0.382 0.676 18.0 26.6 55.4<0.03 0.14 8.93 55.23 25.02 0.39 8.60 <0.03 <0.03 <0.03 0.09 98.4 0.435 0.806 7.3 18.0 74.7<0.03 3.90 13.27 28.00 44.68 0.38 7.08 <0.03 <0.03 <0.03 0.15 97.5 0.325 0.586 29.0 29.4 41.6

Rutland <0.03 0.13 10.74 51.23 27.22 0.41 8.02 <0.03 <0.03 <0.03 <0.08 97.8 0.406 0.762 9.1 21.7 69.3<0.03 0.22 5.26 56.26 30.64 0.48 5.91 <0.03 <0.03 <0.03 <0.08 98.8 0.308 0.878 10.3 11.0 78.7<0.03 0.16 29.79 33.41 22.16 0.28 13.15 <0.03 <0.03 <0.03 0.16 99.1 0.587 0.429 7.2 53.0 39.8<0.03 0.72 15.97 39.84 33.98 0.41 6.64 <0.03 <0.03 <0.03 <0.08 97.6 0.327 0.626 13.8 32.3 54.0<0.03 0.35 8.43 48.57 29.52 0.41 7.18 <0.03 <0.03 <0.03 <0.08 94.6 0.379 0.794 12.9 17.9 69.2<0.03 0.32 9.04 50.00 32.72 0.47 6.74 <0.03 <0.03 <0.03 0.09 99.4 0.340 0.788 13.6 18.4 68.1<0.03 0.57 20.56 38.45 28.95 0.30 9.29 <0.03 <0.03 <0.03 0.17 98.3 0.440 0.556 10.8 39.6 49.7<0.03 0.33 15.40 43.91 29.87 0.44 7.90 <0.03 <0.03 <0.03 0.08 98.0 0.390 0.657 11.0 30.6 58.4<0.03 0.08 36.12 28.70 18.96 0.22 14.68 <0.03 <0.03 <0.03 0.17 98.9 0.636 0.348 4.8 62.1 33.1<0.03 0.50 5.57 40.73 44.49 0.48 4.34 <0.03 <0.03 <0.03 0.14 96.3 0.231 0.831 28.8 12.1 59.1

S10–S13, sample numbers shown on Figure 2b.

8 B. Ghosh et al.


forearc region. The degree of depletion increasesagain, comparable to that of the most depletedabyssal harzburgites within the back-arc exten-sional region, whether or not a back-arc basin isdeveloped (Arai & Ishimaru 2008). The forearcregion is characterized by high-Cr spinels, whereasthe spreading centers are characterized by low-Crvarieties. The addition of water and other volatilesto the mantle wedge beneath the forearc enhancesmelting, leading to the production of highlydepleted melts from which high-Cr spinels crystal-lize, in contrast to the spreading centers wherelower degrees of partial melting produce tholeiiticmagmas from which low-Cr spinels crystallize(Zhou & Robinson 1994; Robinson et al. 1997).

(a)

(b)

( c)

(d)

0

1

2

3

4

0

1

2

3

4

0

1

2

3

4

0

1

2

3

4

TiO

2w

t%

TiO

2w

t%

TiO

2w

t%

TiO

2w

t%

0 0.2 0.4 0.6 0.8 1.0

0 0.2 0.4 0.6 0.8 1.0

0 0.2 0.4 0.6 0.8 1.0

0 0.2 0.4 0.6 0.8 1.0

Cr/(Cr+Al) atomic ratio

S13

S12

S11

S10

Fig. 7 Plots of detrital chromian spinels of south Andaman on a TiO2

vs Cr# variation diagram. (a–d) Samples S10–S13.

(a) (b)

( c) (d)

0

0.2

0.4

0.6

0.8

1.0

0

0.2

0.4

0.6

0.8

1.0

Cr#

(C

r/(C

r+A

l) a

tom

ic r

atio)

Cr#

(C

r/(C

r+A

l) a

tom

ic r

atio)

0

0.2

0.4

0.6

0.8

1.0

0

0.2

0.4

0.6

0.8

1.0

1 0.8 0.6 0.4 0.2 0 1 0.8 0.6 0.4 0.2 0

1 0.8 0.6 0.4 0.2 0 1 0.8 0.6 0.4 0.2 0

S13 S12

S10 S11

Mg/(Mg+Fe2+) atomic ratio

Fig. 8 Plots of detrital chromian spinels of south-Andaman on Cr#–Mg# variation diagram. (a–d) Samples S10–S13.



Fig. 9 Plots of detrital chromian spinels of (a) north Andaman, (b)middle Andaman, (c) south Andaman, and (d) Rutland Island on TiO2 vsCr# variation diagram discriminating between MOR- and arc-dominantareas.

Fig. 10 Plots of detrital chromian spinels of (a) north Andaman, (b)middle Andaman, (c) south Andaman, and (d) Rutland Island on TiO2 vsYFe3+ variation diagram discriminating between MOR- and arc-dominantareas.

10 B. Ghosh et al.


CRITICAL COMPARISON BETWEEN RUTLAND ANDOTHER LOCALITIES

The compositions of spinels from the entireAndaman Island (north, middle, and south) indi-cate wide variation. Rutland Island is an exception.Although it is very difficult to discriminatebetween plutonic and volcanic spinels from chemi-cal characteristics because of almost overlappingcompositions, however, our field observationssupport that most of the detrital spinels in middleand north Andaman are derived from residualperidotites. MOR-dominant areas could only bedetected from one location in south Andaman,

characterized mainly by MOR-related cumulaterocks. This observation agrees with the lithologicaldistributions of ophiolite as reported by earlyworkers (Pal et al. 2003; Pal 2011). It is importantto note that our spinel data show depleted signa-tures over the entire island, which suggests thatall massifs in the Andaman ophiolite were affectedunder arc conditions. However, the degree ofdepletion varies in different parts of the island.Moreover, we noticed a directional change in com-position of the detrital spinels from north to south.Towards the south there is an increasing popula-tion of arc-dominant spinels. A critical comparisonof spinel compositions in terms of the Cr#–Mg#systematics between north Andaman and RutlandIsland is presented in Fig. 12. The compositionalfield for spinels from mantle peridotites has onlybeen taken into account in this diagram because

Fig. 11 Plots of detrital chromian spinels of (a) north Andaman, (b)middle Andaman, (c) south Andaman, and (d) Rutland Island on Cr#–Mg# variation diagram discriminating between MOR- and arc-dominantareas.

Fig. 12 Plots of detrital chromian spinels on Cr#–Mg# variationdiagram, comparing between north Andaman and Rutland Island.



these two areas are reportedly dominated by thisrock type (Pal 2011). For north Andaman, the plotseither occupy the MOR-mantle peridotite field orare very close to it (except two), but those forRutland Island are exclusively occupied inthe arc–mantle peridotite field. Interestingly, atthe same Cr# the plots for north Andaman havehigher Mg# than those for Rutland Island. Thisfeature is more likely to be related to effectivecooling by H2O released from the subducted slabin a forearc environment (Okamura et al. 2006). Itis inferred that the mantle section of Andamanophiolite towards the south is depleted arc-mantlein contrast to less depleted MOR-mantle towardsthe north at north Andaman.

IMPLICATIONS FOR EVOLUTION OF THEANDAMAN OPHIOLITE

Based on the discrimination diagrams (Figs 6–7)we observe that the detrital spinels derived fromthe inferred ophiolitic source show variablydepleted mantle and melt signatures. Since it isdifficult to discriminate the chemical signaturesamong the spreading centers, such as MOR andback-arc basin based on spinel composition, therecan be more than one possibility to explain thisspatial change pertinent to Andaman Islands. TheAndaman ophiolite may not necessarily be uni-formly exposed at the present level of erosion.Moreover, this spatial change in spinel composi-tions may reflect a temporal change as well. Insuch a scenario, the possibilities are listed below.1. It is inferred that in the late Mesozoic, a replica

of the present-day geodynamic features with anisland-arc setting existed along the easternperiphery of the Indian subcontinent. A forearcsetting of that paleogeodynamic configurationoccurred towards the south, which might havegradually shifted away from the trench towardsnorth and given rise to the back-arc setting.This behavioral change in subduction kinemat-ics may have a direct link with the rotation ofthe plates in response to oblique subduction inthe Andaman region (Hall 2002).

2. It is highly possible that the upper mantle of theAndaman ophiolite displays a transition fromoceanic mantle to island-arc mantle. This isexplained by a process related to the modifica-tion of the pre-existing oceanic crust above thesubduction zone. Arai et al. (2006) also noted asimilar switching of the tectonic setting from amid-oceanic ridge to an island-arc for the Omanophiolite.

Therefore, this spatial and/or temporal directionalchange in spinel composition may reflect the varia-tions linked to the melting history where the samesliver of oceanic mantle underwent different stylesof melting in different tectonic settings at differenttimes.

CONCLUSION

The chemical compositions of the detrital spinels ofAndaman Islands from south to north suggest adepleted signature in all the localities. It is inferredthat all the peridotite massifs in the Andamanophiolite were affected under island-arc conditions.However, the degree of depletion varies in differentparts of the island. A directional change in compo-sition of the detrital spinels is evident, being rela-tively less depleted towards the north. This spatialand/or temporal directional change in detritalspinel composition may either reflect a Cretaceousforearc–back-arc setting of the Andaman ophioliteor indicates a switching of the tectonic setting frommid-oceanic ridge to island-arc, as documentedfrom other parts of the world.

ACKNOWLEDGEMENTS

Part of the fieldwork was funded from INSA–JSPS Bilateral Exchange Programme for jointcollaborative research. B.G. acknowledges thefinancial support received from the Department ofScience and Technology, India (SR/FTP/ES-65/2009). The authors express their heartiest thanksto U. Zimmermann and S. Arai for their insightfulcomments on an earlier version of the manuscript.We are grateful to M. Satish-Kumar for his criticaland encouraging comments that contributedgreatly in improving this manuscript. The authorsappreciate numerous suggestions from P. Faupl toimprove the presentation and overall technicalquality of the paper.

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