Chromite–silicate chemistry of the Neoarchean Sittampundi Complex, southern India: Implications for subduction-related arc magmatism

Precambrian Research 227 (2013) 259–275

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Chromite–silicate chemistry of the Neoarchean Sittampundi Complex, southern India: Implications for subduction-related arc magmatism C.V. Dharma Rao a,∗ , M. Santosh b , K. Sajeev c , B.F. Windley d a

National Disaster Management Authority, Government of India, A-1 Safdarjung Enclave, New Delhi, India Department of Interdisciplinary Science, Faculty of Science, Kochi University, Akebono-cho, Kochi 780-8520, Japan Centre for Earth Sciences, Indian Institute of Sciences, Bangalore 560012, India d Department of Geology, University of Leicester, Leicester LE1 7RH, UK b c

a r t i c l e

i n f o

Article history: Received 25 July 2011 Received in revised form 14 November 2011 Accepted 19 November 2011 Available online 8 December 2011 Keywords: Suprasubduction zone Arc magmatism Geochemistry Tectonics Sittampundi Southern India

a b s t r a c t The Neoarchean layered anorthositic complex at Sittampundi in southern India is known for its chromitite layers that are mostly associated with anorthosite (An90–100 ). The chromitites contain FeAlrich chromites concentrated in layers between amphibole-rich layers with a dominant mineralogy of amphibole–spinel–plagiocase ± sapphirine. The chromite-rich layers contain only amphibole and plagioclase. Mineral compositions illustrated by X-ray composition maps and profiles show subtle chemical differences. The chrome spinels are of refractory grade with Cr2 O3 and Al2 O3 contents varying between 34–40 wt.% and 23–28 wt.%. The chromite compositions are noticeably different from those in layered igneous intrusions of the Bushveld-Stillwater type. The existence of original highly calcic plagioclase, FeAl-rich chromite, and magmatic amphibole is consistent with derivation from a parental magma of hydrous tholeiitic composition that was most likely generated in a supra-subduction zone arc setting. In terms of mineralogy and field relations, the Sittampundi chromitites are remarkably similar to anorthosite-hosted chromitites in the Neoarchean Fiskenæsset anorthositic complex, Greenland. We propose that the Sittampundi chromitites formed by partial melting of unusually aluminous harzburgite in a hydrated mantle wedge above a subduction zone. This melting process produced hydrous, aluminous basalt, which fractionated at depth to give rise to a variety of high-alumina basalt compositions from which the anorthositic complex with its cumulate chromite-rich and amphibole-rich layers formed within the magma chamber of a supra-subduction zone arc. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Chromite is an accessory mineral in a wide variety of mafic, ultramafic and anorthositic rocks of Cenozoic to Archean age, and has long been known as a critical petrogenetic indicator (Irvine, 1965, 1967; Thayer, 1970; Stowe, 1994). Although its abundance on earth is generally low (∼1 vol.%), chromite commonly accumulates to form chromitite layers in layered igneous intrusions and some ophiolites. The composition of chromite is highly sensitive to parental melt composition, which is generated in different tectonic settings, and thus it can provide a useful indication of the tectonic processes in oceanic and continental crust and mantle (Barnes and Roeder, 2001). However, in spite of the many publications on chromite and its chemistry in recent decades, it has been increasingly re-evaluated (e.g. Rollinson et al., 2002), especially because of advances in understanding of supra-subduction ophiolites (Dilek

∗ Corresponding author. E-mail address: dharma [email protected] (C.V. Dharma Rao). 0301-9268/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2011.11.012

and Polat, 2008), and the magma chambers of island arcs (e.g. Polat et al., 2009, 2010, 2011a,b; Windley and Garde, 2009; Stern, 2010; Dilek and Furnes, 2011; Dharma Rao and Santosh, 2011; Dharma Rao et al., 2010, 2011a,b; Hébert et al., 2011; Shi et al., 2011). In Archean layered igneous complexes, chromites with diagnostic FeAl-rich compositions in calcic anorthosites contain bytownite–anorthite (An80–100 ) plagioclase, and associated gabbronorites and gabbros (Subramaniam, 1956; Windley et al., 1981; Rollinson et al., 2010; Dutta et al., in press). Although it has been thought that these Archean anorthosites are more calcic than their younger counterparts (Ashwal, 1993), it is now realized (e.g. Windley and Garde, 2009) that the modern equivalents are calcic anorthosites in Phanerozoic island arcs such as the Black Giants anorthosite in Fiordland, New Zealand (Gibson and Ireland, 1999), and the xenoliths of cumulate anorthosites, gabbros and gabbronorites derived from the magma chambers of modern island arcs (Beard, 1986), as in the Monserrat volcano (Arculus and Wills, 1980; Kiddle et al., 2010). Moreover, comparable chromitelayered anorthosite complexes are prominent in the Proterozoic Eastern Ghats belt of southern India as at Kondapalle (Dharma

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Rao and Santosh, 2011; Dharma Rao et al., 2011a) and Chimalpahad (Dharma Rao et al., 2010). Archean layered anorthositic complexes, often chromite-bearing, include the Fiskenæsset Complex, West Greenland (Windley et al., 1973; Myers, 1985); the Messina Complex, Limpopo belt, South Africa (Hor et al., 1975; Barton et al., 1979; Barton, 1996); the Mponono Intrusive Suite in Swaziland (Jackson, 1984); the Bad Vermilion Lake Complex in Ontario, Canada (Ashwal et al., 1983); the Windimurra Complex in the Yilgarn Block, Australia (Ahmat and de Laeter, 1982; Myers, 1988); the Shawmere anorthosite in Kapuskasing, Canada (Percival and West, 1994), the Lagoa da Vaca Complex in Brazil (Paixão and Oliveira, 1998), and the chromite-layered Sittampundi Complex in southern India (Subramaniam, 1956). However, few of these Archean complexes and/or their chromites have yet been described in detail, making many aspects of their petrogenesis uncertain, even though they provide key constraints on the evolution of the continental crust (Phinney et al., 1988). In order to help redress this imbalance, we present in this paper new geochemical data on the chemistry of the chromites in the Sittampundi Complex from southern India with the aim of predicting the composition of the mantle-derived melts. Our results have important implications for the evolution of a subductiongenerated, hydrous island arc, and its bearing on crustal growth in the Archean. 2. Regional geological framework The southern Indian shield comprises several crustal blocks of Archean to Paleoproterozoic age separated by major shear zones (Fig. 1a) (Chetty, 1996; Meißner et al., 2002). South of the Dharwar craton, dominated by Neoarchean greenstone belts, is an amphibolite to granulite facies region that has a wide variety of isotopic ages ranging from Mesoarchean to end-Precambrian (Kröner et al., 2011; Collins et al., 2007; Clark et al., 2009; Santosh et al., 2006, 2009). The Palghat–Cauvery Shear Zone (PCSZ) is considered to mark a major collisional suture welding the Archean craton to the north with the Proterozoic crustal blocks to the south (e.g. Santosh et al., 2006, 2009, 2011; Chetty and Bhaskar Rao, 2006; Collins et al., 2007; Clark et al., 2009; Naganjaneyulu and Santosh, 2010), although different opinions also exist (e.g. Mukhopadhyay et al., 2003; Chardon et al., 2008). Geophysical data have been used to support the idea that shear zones of the PCSZ mark a suture between two contrasting tectonic blocks (Harinarayana et al., 2006; Rao and Prasad, 2006; Naganjaneyulu and Santosh, 2010; Behera, in press). Recent zircon U–Pb age data from suprasubduction zone ophiolites and arc magmatic suites within the PCSZ suggest that this region marks the zone of at least two major ocean closures during two critical periods in earth history: Neoarchean–early Paleoproterozoic and Precambrian–Cambrian (Mohan et al., 2011; Teale et al., 2011; Santosh et al., 2011; Yellappa et al., 2011). Within the PCSZ, amphibolite to granulite facies orthogneisses contain many narrow layers of metasediment such as banded iron formation, marble, quartzite, and schist, as well as layered igneous complexes such as Sittampundi (Subramaniam, 1956), and Kanjamala (Mukhopadhyay and Bose, 1994; Saitoh et al., 2011). 3. The Sittampundi layered complex The Neoarchean Sittampundi (11◦ 14 N:77◦ 54 E) Complex (up to 36 km × 2 km), located about 80 km SSW of the city of Salem, is a metamorphosed anorthositic complex occurring within the orthogneisses of the PCSZ (Fig. 1a and b) (Subramaniam, 1956; Janardhanan and Leake, 1975; Windley and Selvan, 1975; Windley et al., 1981). In spite of the high-grade metamorphic overprint, the complex retains its original

igneous stratigraphy and therefore the prefix meta is dropped for these largely isochemically recrystallized igneous rocks. From the base upwards, the complex consists of magnesite-veined dunite, chromite-layered clinopyroxenite, chromite-layered anorthosite, and gabbro that contains clinopyroxenite layers (Dutta et al., in press); banded iron formations and basaltic amphibolites situated above the gabbros are, in our opinion, bordering metasupracrustal rocks, not to be included in the stratigraphy of the layered igneous complex (Dutta et al., in press). Retrogressed eclogites, strictly termed high-pressure granulites, occur in anorthosite as layers and lenses up to 80 m wide that indicate peak P–T conditions of ∼20 kbar and 1020 ◦ C (Sajeev et al., 2009). Chromitite layers are up to 6 m thick in anorthosite and a few tens of centimeters in clinopyroxenite (Subramaniam, 1956). The chromite-bearing clinopyroxenites are in places altered to amphibole-chlorite schists. The Sittampundi Complex is folded into an isoclinal antiform, and weakly refolded on a N–S axial trace (Ramadurai et al., 1975). Although this paper is only concerned with the magmatic chromites of the Sittampundi Complex, some chromitites are associated with metamophic assemblages such as corundum with plagioclase rims (Koshimoto et al., 2004), and cmthick schistose layers that contain sapphirine, spinel, corundum, clinozoisite, phlogopite, gedrite and calcic plagioclase; these metamorphic assemblages will not be discussed further.

4. Geochronology There has been considerable ambiguity about the age of the Sittampundi Complex and its relationship with the major regional tectonic history, which has arisen largely from random and imprecise geochronological data. Several recent studies applying precise U–Pb zircon geochronology reveal Neoarchean–Paleoproterozoic ages from the Sittampundi Complex and other magmatic units in the proximity within the northern domain of the PCSZ. Mohan et al. (2011) reported LA-ICPMS U–Pb and Hf isotope data on zircons from the Sittampundi anorthosites, which suggest an emplacement age of 2487 ± 18 Ma. The positive εHf values and the disposition of 176 Hf/177 Hf initial ratios between CHUR and depleted mantle obtained from Sittampundi suggest that these rocks were derived from a mantle source component within a subduction system. From a newly discovered dismembered ophiolitic complex from Devanur within the PCSZ, Yellappa et al. (2011) reported petrological and geochemical data from mafic and ultramafic intrusive rocks and zircon U–Pb SHRIMP data from a trondhjemite in this complex that yield 238 U/206 Pb ages of 2528 ± 61 and 2545 ± 56 Ma. From a charnockite and a charnockite-hosted leucosome in the Salem Block (Fig. 1a) on the southern margin of the Dharwar craton, immediately to the north of the PCSZ Clark et al. (2009) reported that the oscillatory-zoned cores of zircons have high Th/U ratios and weighted mean SHRIMP zircon 207 Pb/206 Pb ages of 2538 ± 6 Ma and 2529 ± 7 Ma, which they interpreted to reflect the time of crystallization of the original magmatic charnockite protolith, and that zircon overgrowths and grains with low Th/U ratios have weighted mean 207 Pb/206 Pb ages of 2473 ± 8 Ma and 2482 ± 15 Ma, interpreted to constrain the time of post-crystallization, high-grade metamorphism and partial melting of the magmatic rocks. Clark et al. (2009) correlated these ages with accretionary processes on the margin of the Dharwar craton during Neoarchean convergent tectonics along the southern margin of the craton. At Kanjamalai (Fig. 1b) north of Sittampundi a charnockite and a quartzo-feldspathic garnet gneiss have SHRIMP zircon 207 Pb/206 Pb magmatic ages of 2536.1 ± 1.4 Ma and 2532.4 ± 3.7 Ma, and 2477.6 ± 1.8 Ma and 2483.9 ± 2.5 Ma metamorphic overgrowth ages and syntectonic granites have a zircon U–Pb emplacement age of 2647 ± 11 Ma and a metamorphic age of 2443 ± 20 Ma (Sato et al.,

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Fig. 1. (a) Simplified geological map of southern India (after Santosh and Sajeev, 2006). (b) Geological map of the Sittampundi region showing various lithologies and the location of the Sittampundi study area. (c) Detailed geological map showing the location of the studied chromitite seam at Paramati in the Sittampundi Complex (modified after Subramaniam, 1956; Ramadurai et al., 1975).

2011a). Satyanarayanan (cited in Mohan et al., 2011) reports an unpublished U–Pb badelleyite age of 2401 Ma from the Bhavani layered complex northwest of Sittampundi. Mid-Neoproterozoic ages have been described from several other parts of the PCSZ. A suprasubduction zone ophiolitic complex at Manamedu in the southern domain of the PCSZ has SHRIMP zircon 206 Pb/238 U magmatic crystallization ages of 737 ± 23–782 ± 24 Ma from plagiogranites and 744 ± 11–786 ± 7.1 Ma from gabbros (Santosh et al., 2011), plagiogranites from Manamedu have a LA-ICPMS zircon U–Pb age of 800 ± 14 Ma (Sato et al., 2011b), and zircons in a quartzite and metamorphosed banded iron formation incorporated in the subduction complex have a younger intercept age 759 ± 41 Ma (Sato et al., 2011b). Teale et al. (2011) reported zircon LA-ICPMS 207 Pb/206 Pb ages of 825 ± 17 Ma from the Kadavur Dome, a gabbro–anorthosite complex in the northern part of Madurai Block immediately south of the PCSZ, and U–Pb ages of 843 ± 23 Ma from metamorphic rims on zircons from the surrounding quartzites, which compare closely with the ages of Sato et al. (2011b). Oscillatory-zoned euhedral magmatic zircons from the felsic gneisses at Kadavur have an LAICP-MS zircon age of 766 ± 8 Ma (Teale et al., 2011). Sato et al. (in press) reported a 206 Pb/238 U age of 819 ± 26 Ma from arc-related rapakivi granites at Tangalamvaripatti within the southern domain of the PCSZ. All these data suggest there was a prominent midNeoproterozoic subduction system along the southern margin of the PCSZ (Santosh et al., 2011), and they confirm the plate tectonic model for the Neoproterozoic–Cambrian history of southern India associated with Gondwana assembly as proposed by Santosh et al. (2009). Several recent studies from the PCSZ have reported precise late Neoproterozoic–Cambrian metamorphic and magmatic ages of zircon and monazite (e.g. Collins et al., 2007; Santosh et al.,

2006, 2009). Ultrahigh-temperature granulites in the PCSZ have a U–Pb zircon metamorphic age of 530 ± 4.9 Ma, and monazite dates range from ca. 525 to 537 Ma with inheritance ages in the range of 2400–2600 Ma (Collins et al., 2007). And ultrahigh-temperature granulites have U–Th–Pb microprobe monazite ages in the range 550–520 Ma (Santosh et al., 2006). It is thus becoming increasingly clear that the PCSZ incorporates a vast mélange zone with remnants of at least two major subduction–accretion systems widely separated in time (Santosh et al., 2011). The northern domain of the PCSZ including the Salem Block largely forms part of a Neoarchean–early Paleoproterozoic subduction–accretion belt, whereas the southern domain of the PCSZ and the northern part of the Madurai Block belong to a midNeoproterozoic subduction system, which culminated in Cambrian collisional orogeny during the birth of the Gondwana supercontinent. 5. Petrography The Sittampundi anorthosite consists of >70% highly calcic plagioclase (up to An100 ) plus clinopyroxene and accesory chromite. Samples (Fig. 2a and b) for this study were collected from Paramati village (10◦ 09 N:77◦ 09 E) (Fig. 1c), approximately 35 km NNW of Karur. Sample PMT-B-5C-3a contains an amphibolerich and a chromite-rich layer (Fig. 2b). The former has the mineral assemblage: sapphirine–spinel–corundum–gedrite– plagioclase–amphibole–chromite (Fig. 3a), whereas the latter consists of chromite–plagioclase–amphibole. Fig. 3b is a photomicrograph of the contact between the two layers. In their mutual contact zone, there are millimeter-thick layers of chromite between plagioclases (Fig. 3b). In the chromite-rich layers chromite

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Fig. 2. (a) Field photograph showing chromite-rich layers alternating with hornblende-rich layers from the Sittampundi Complex; pencil for scale. (b) Photograph and schematic sketch of a hand-specimen showing the minerals in chromite-rich and amphibole-rich layers. Discussed in detail in the text.

occurs in disseminated layers (Fig. 3c) and as spongy aggregates surrounded by truncated coronae of plagioclase and amphibole (Fig. 3d). 6. Mineral chemistry and its implications Electron microprobe analyses of the minerals in the amphibolerich and chromite-rich layers were carried out using a JEOL JXA-8900R microprobe at Kochi University, operating with an

accelerating voltage of 15 kV, and a beam current of 12 nA. Natural and synthetic silicates and oxides were used for calibration. The data were reduced using ZAF correction procedures. Representative mineral analyses are given in Tables 1–4. Fig. 4a and b is a micrograph showing the EMPA spot analyses of chromite and associated silicate minerals from the chromite-rich and amphibole-rich layers. X-ray elemental maps of selected minerals in Fig. 3c and d showing tonal variations in Al, Cr, Fe, Mg and Ca are presented in Fig. 5a and b.

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Fig. 3. Photomicrographs showing textural relations of minerals in the amphibole-rich and chromite-rich layers. Mineral abbreviations after Kretz (1983). (a) Plagioclase megacryst sieved with mineral inclusions and a large spinel in an amphibole-rich layer. Plane polarized light. (b) Photomicrograph of the mineral assemblage of spinel–amphibole–plagioclase–gedrite–chromite in the contact zone between the amphibole-rich and chromite-rich layers. Cross-polarized light. (c) Chromite with silicate inclusions in a chromite-rich layer bordered by tschermakitic amphibole (Ts), corundum (Crn), sapphirine (Spr), spinel (Spl), gedrite (Ged) and plagioclase (Pl). Plane polarized light. (d) Chromite surrounded by plagioclase and tschermakitic amphibole in a chromite-rich layer. In plane polarized light.

Table 1 Representative electron microprobe analysis of chromite in chromite-bearing layer from Paramathi. Mineral

Chromite

Grain no.

1

Point no.

2

SiO2 TiO2 Al2 O3 Cr2 O3 FeO MnO MgO CaO Na2 O K2 O P2 O5 NiO ZnO Total O Si Ti Al Cr Fe3+ Fe2+ Mn Mg Ca Na K P Ni Zn Total Cr# Fe# Mg#

33 3

26

34 29

1

35 3

0.00 0.01 0.01 0.01 0.04 0.17 0.02 0.03 0.01 0.06 0.05 0.04 33.78 36.21 34.70 34.89 39.39 39.09 31.09 27.06 28.25 28.34 24.80 25.59 26.85 28.15 29.83 29.39 26.64 27.19 0.16 0.25 0.30 0.22 0.20 0.17 7.44 8.19 7.29 7.32 8.78 7.75 0.02 0.02 0.00 0.00 0.11 0.03 0.01 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.10 0.13 0.10 0.15 0.21 0.17 0.14 0.19 0.17 0.10 0.13 0.18 99.62 100.23 100.66 100.46 100.35 100.39 4 4 4 4 4 4 0.000 0.000 0.000 0.000 0.001 0.005 0.001 0.001 0.000 0.001 0.001 0.001 1.215 1.276 1.233 1.240 1.365 1.366 0.750 0.639 0.673 0.676 0.577 0.600 0.034 0.084 0.090 0.082 0.052 0.020 0.651 0.619 0.662 0.660 0.603 0.654 0.004 0.006 0.008 0.006 0.005 0.004 0.338 0.365 0.328 0.329 0.385 0.342 0.001 0.001 0.000 0.000 0.003 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.002 0.003 0.002 0.004 0.005 0.004 0.003 0.004 0.004 0.002 0.003 0.004 3.000 2.999 3.001 3.000 3.001 3.001 0.382 0.017 0.342

0.334 0.042 0.371

0.353 0.045 0.331

0.353 0.041 0.333

0.297 0.026 0.390

0.305 0.010 0.344

3 0.22 0.04 36.58 26.97 26.96 0.24 8.17 0.01 0.00 0.00 0.00 0.22 0.22 99.61 4 0.007 0.001 1.294 0.640 0.054 0.622 0.006 0.365 0.000 0.000 0.000 0.000 0.005 0.005 2.999 0.331 0.027 0.370

11 5

9

4 11

0.01 0.02 0.01 0.05 0.06 0.06 35.28 33.62 32.40 27.72 29.78 30.69 28.42 28.87 29.22 0.23 0.27 0.23 7.74 7.30 7.09 0.01 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.16 0.11 0.12 0.27 0.16 0.17 99.89 100.20 100.00 4 4 4 0.000 0.001 0.000 0.001 0.001 0.001 1.256 1.205 1.170 0.662 0.716 0.743 0.079 0.073 0.082 0.639 0.661 0.666 0.006 0.007 0.006 0.348 0.331 0.324 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.004 0.003 0.003 0.006 0.004 0.004 3.000 3.001 3.001 0.345 0.040 0.353

0.373 0.037 0.334

0.388 0.041 0.327

6

12

20

0.00 0.02 36.83 26.72 26.89 0.24 8.38 0.00 0.00 0.00 0.00 0.20 0.12 99.40 4 0.000 0.000 1.303 0.634 0.061 0.614 0.006 0.375 0.000 0.000 0.000 0.000 0.005 0.003 3.001

0.01 0.07 0.11 0.03 37.52 35.02 26.17 28.62 26.37 28.03 0.14 0.27 8.71 7.88 0.02 0.01 0.00 0.02 0.00 0.00 0.00 0.00 0.25 0.26 0.17 0.06 99.47 100.26 4 4 0.000 0.002 0.003 0.001 1.320 1.243 0.617 0.681 0.059 0.071 0.599 0.635 0.004 0.007 0.387 0.353 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.006 0.006 0.004 0.001 2.999 3.001

0.327 0.030 0.379

0.319 0.030 0.393

5

0.354 0.035 0.358

37 24

20

22

0.03 0.07 37.03 25.99 25.06 0.21 8.38 0.00 0.00 0.00 0.00 0.09 0.13 96.99 4 0.001 0.002 1.335 0.628 0.032 0.609 0.005 0.382 0.000 0.000 0.000 0.000 0.002 0.003 3.000

0.01 0.05 37.77 25.44 27.25 0.27 8.61 0.00 0.00 0.00 0.00 0.15 0.23 99.78 4 0.000 0.001 1.325 0.598 0.075 0.603 0.007 0.382 0.000 0.000 0.000 0.000 0.004 0.005 3.000

0.01 0.03 37.82 25.52 27.09 0.16 8.73 0.02 0.00 0.00 0.02 0.19 0.26 99.83 4 0.000 0.001 1.325 0.599 0.074 0.599 0.004 0.386 0.001 0.000 0.000 0.001 0.004 0.006 2.999

0.320 0.016 0.385

0.311 0.037 0.387

0.312 0.037 0.392

264

Table 2 Representative analysis of amphibole from chromite-bearing layer. 3

4

5

Point no.

16

17

5

20

6

7

5

6

30

31

11

12

11

12

1

2

2

11

22

23

SiO2 TiO2 Al2 O3 Cr2 O3 FeO MnO MgO CaO Na2 O K2 O P2 O5 NiO ZnO Total

42.80 0.11 16.09 0.25 6.93 0.09 15.39 10.73 2.03 0.05 0.00 0.06 0.00 94.53

42.72 0.21 16.31 0.20 6.82 0.11 15.23 10.79 1.79 0.02 0.02 0.05 0.00 94.25

44.03 0.21 15.61 0.10 6.68 0.09 15.83 11.28 1.82 0.00 0.01 0.16 0.08 95.88

43.55 0.19 16.00 0.09 6.96 0.13 15.84 10.84 1.94 0.03 0.00 0.08 0.04 95.69

43.69 0.24 15.62 0.22 6.77 0.10 15.75 10.98 1.88 0.01 0.00 0.06 0.00 95.33

43.86 0.26 15.75 0.15 6.53 0.12 15.55 10.96 1.74 0.02 0.01 0.09 0.04 95.06

43.97 0.22 16.09 0.20 6.71 0.08 15.33 10.95 1.83 0.05 0.00 0.07 0.00 95.50

44.00 0.26 15.55 0.13 6.64 0.11 15.82 11.32 1.79 0.03 0.00 0.08 0.00 95.73

43.94 0.21 15.59 0.16 6.63 0.13 15.84 11.41 1.83 0.01 0.01 0.10 0.00 95.87

43.98 0.23 15.35 0.22 6.73 0.11 15.90 11.35 1.76 0.00 0.03 0.03 0.03 95.73

44.01 0.18 15.62 0.19 7.09 0.13 15.82 11.45 1.74 0.05 0.00 0.10 0.16 96.53

44.42 0.28 15.90 0.25 7.05 0.07 16.11 11.14 1.83 0.05 0.00 0.10 0.12 97.30

45.49 0.25 16.26 0.13 5.75 0.08 15.13 11.65 1.64 0.00 0.02 0.05 0.00 96.46

44.79 0.23 16.15 0.05 5.71 0.07 16.02 11.72 1.86 0.04 0.00 0.09 0.01 96.73

44.59 0.18 15.79 0.23 6.31 0.09 15.68 11.78 1.89 0.00 0.02 0.04 0.00 96.58

45.03 0.17 18.11 0.13 5.87 0.14 13.74 10.90 1.65 0.06 0.01 0.10 0.00 95.92

44.65 0.19 15.67 0.11 5.92 0.09 16.15 11.48 1.83 0.00 0.01 0.02 0.05 96.16

45.17 0.26 15.85 0.18 5.63 0.09 15.72 11.01 1.63 0.04 0.00 0.10 0.00 95.68

44.30 0.15 15.96 0.10 6.86 0.17 15.76 11.01 1.76 0.04 0.00 0.06 0.00 96.17

44.36 0.15 16.00 0.09 6.64 0.12 15.69 10.80 1.67 0.03 0.01 0.05 0.00 95.60

O Si Ti Al Cr Fe Mn Mg Ca Na K P Ni Zn Total

23 6.275 0.013 2.781 0.029 0.850 0.012 3.363 1.686 0.576 0.009 0.000 0.007 0.000 15.600

23 6.270 0.023 2.823 0.023 0.837 0.013 3.331 1.697 0.508 0.004 0.003 0.006 0.000 15.537

23 6.354 0.023 2.655 0.011 0.806 0.011 3.405 1.743 0.509 0.000 0.001 0.018 0.008 15.543

23 6.302 0.020 2.730 0.010 0.842 0.016 3.417 1.680 0.545 0.005 0.000 0.009 0.004 15.582

23 6.340 0.026 2.673 0.025 0.822 0.013 3.406 1.708 0.528 0.002 0.000 0.007 0.000 15.550

23 6.367 0.028 2.696 0.017 0.793 0.014 3.365 1.704 0.491 0.003 0.001 0.010 0.004 15.494

23 6.358 0.023 2.742 0.023 0.811 0.010 3.302 1.697 0.513 0.010 0.000 0.008 0.000 15.498

23 6.356 0.028 2.648 0.015 0.803 0.014 3.407 1.752 0.501 0.006 0.000 0.009 0.000 15.538

23 6.342 0.023 2.654 0.018 0.800 0.015 3.407 1.765 0.513 0.003 0.001 0.012 0.000 15.554

23 6.357 0.025 2.616 0.025 0.814 0.014 3.425 1.758 0.492 0.000 0.003 0.004 0.003 15.537

23 6.329 0.019 2.648 0.021 0.853 0.016 3.391 1.764 0.485 0.010 0.000 0.011 0.016 15.564

23 6.325 0.030 2.669 0.028 0.840 0.008 3.419 1.699 0.504 0.008 0.000 0.012 0.012 15.553

23 6.464 0.027 2.724 0.015 0.683 0.009 3.205 1.774 0.453 0.000 0.003 0.006 0.000 15.362

23 6.370 0.025 2.708 0.005 0.679 0.008 3.396 1.786 0.514 0.007 0.000 0.010 0.001 15.509

23 6.375 0.019 2.662 0.025 0.755 0.011 3.340 1.804 0.524 0.000 0.002 0.004 0.000 15.522

23 6.416 0.018 3.042 0.014 0.700 0.017 2.917 1.664 0.457 0.010 0.001 0.011 0.000 15.269

23 6.392 0.021 2.645 0.012 0.709 0.011 3.445 1.762 0.507 0.000 0.001 0.003 0.006 15.511

23 6.467 0.027 2.675 0.021 0.674 0.011 3.354 1.688 0.453 0.007 0.000 0.011 0.000 15.388

23 6.363 0.017 2.704 0.011 0.824 0.021 3.373 1.695 0.491 0.007 0.000 0.007 0.000 15.512

23 6.391 0.016 2.717 0.010 0.801 0.015 3.369 1.667 0.467 0.005 0.001 0.006 0.000 15.463

0.798

0.799

0.809

0.802

0.806

0.809

0.803

0.809

0.810

0.808

0.799

0.803

0.824

0.833

0.816

0.807

0.829

0.833

0.804

0.808

Mg/(Fetotal + Mg)

7

8

9a

9b

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Grain no.

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Table 3 Representative plagioclase analysis. Amphibole-rich layer Mineral Grain no.

12

Point no.

5

SiO2 TiO2 Al2 O3 Cr2 O3 FeO MnO MgO CaO Na2 O K2 O P2 O5 NiO ZnO Total

Chromite-bearing layer

Plagioclase

44.20 0.00 35.81 0.00 0.09 0.00 0.00 19.12 0.42 0.01 0.00 0.00 0.01 99.66

13

14

11

10

11

44.13 0.03 35.60 0.00 0.15 0.02 0.02 19.18 0.36 0.00 0.00 0.00 0.03 99.51

44.07 0.00 35.54 0.04 0.07 0.00 0.00 19.30 0.45 0.00 0.01 0.04 0.00 99.51

44.22 0.00 35.64 0.02 0.15 0.03 0.01 19.33 0.39 0.00 0.00 0.02 0.04 99.84

2 43.54 0.01 35.92 0.02 0.37 0.00 0.01 19.48 0.37 0.00 0.03 0.05 0.02 99.82

16 3 43.53 0.02 35.74 0.00 0.16 0.03 0.00 19.93 0.31 0.00 0.00 0.00 0.03 99.73

17

16

17

43.02 0.00 35.48 0.00 0.00 0.06 0.00 19.42 0.36 0.00 0.00 0.00 0.00 98.34

43.21 0.02 35.53 0.00 0.06 0.02 0.00 19.63 0.34 0.00 0.02 0.07 0.00 98.90

4 43.23 0.02 35.77 0.00 0.20 0.00 0.00 20.19 0.32 0.00 0.00 0.00 0.00 99.73

22 6 43.37 0.00 35.70 0.00 0.12 0.01 0.00 20.16 0.34 0.00 0.03 0.01 0.01 99.76

4 43.55 0.00 36.81 0.00 0.05 0.01 0.00 18.73 0.30 0.00 0.03 0.07 0.00 99.54

35 7 43.67 0.04 35.67 0.05 0.03 0.00 0.00 19.33 0.33 0.00 0.00 0.00 0.00 99.12

2 43.02 0.00 35.30 0.00 0.13 0.01 0.00 19.17 0.48 0.00 0.00 0.01 0.07 98.19

36 3 43.87 0.00 35.53 0.05 0.00 0.02 0.00 19.29 0.45 0.00 0.01 0.00 0.05 99.27

1 44.28 0.00 35.47 0.05 0.11 0.00 0.02 19.21 0.44 0.02 0.00 0.03 0.01 99.64

45 2 43.88 0.00 35.77 0.05 0.12 0.03 0.00 19.06 0.47 0.02 0.02 0.04 0.00 99.45

1 44.30 0.00 36.13 0.00 0.21 0.00 0.00 20.18 0.53 0.00 0.00 0.00 0.00 101.36

O Si Ti Al Cr Fe Mn Mg Ca Na K P Ni Zn Total

8 2.047 0.000 1.955 0.000 0.004 0.000 0.000 0.949 0.038 0.001 0.000 0.000 0.000 4.994

8 2.048 0.001 1.948 0.000 0.006 0.001 0.001 0.954 0.032 0.000 0.000 0.000 0.001 4.992

8 2.047 0.000 1.946 0.001 0.003 0.000 0.000 0.961 0.041 0.000 0.000 0.001 0.000 4.999

8 2.047 0.000 1.946 0.001 0.006 0.001 0.000 0.959 0.035 0.000 0.000 0.001 0.001 4.997

8 2.021 0.000 1.966 0.001 0.014 0.000 0.000 0.969 0.034 0.000 0.001 0.002 0.001 5.010

8 2.024 0.001 1.958 0.000 0.006 0.001 0.000 0.992 0.028 0.000 0.000 0.000 0.001 5.011

8 2.025 0.000 1.968 0.000 0.000 0.002 0.000 0.979 0.033 0.000 0.000 0.000 0.000 5.008

8 2.024 0.001 1.962 0.000 0.002 0.001 0.000 0.985 0.031 0.000 0.001 0.003 0.000 5.009

8 2.012 0.001 1.963 0.000 0.008 0.000 0.000 1.007 0.029 0.000 0.000 0.000 0.000 5.020

8 2.017 0.000 1.957 0.000 0.005 0.001 0.000 1.005 0.031 0.000 0.001 0.000 0.000 5.017

8 2.017 0.000 2.010 0.000 0.002 0.000 0.000 0.929 0.027 0.000 0.001 0.003 0.000 4.990

8 2.036 0.001 1.961 0.002 0.001 0.000 0.000 0.966 0.030 0.000 0.000 0.000 0.000 4.996

8 2.029 0.000 1.963 0.000 0.005 0.000 0.000 0.968 0.043 0.000 0.000 0.000 0.002 5.012

8 2.043 0.000 1.950 0.002 0.000 0.001 0.000 0.962 0.041 0.000 0.000 0.000 0.002 5.001

8 2.053 0.000 1.939 0.002 0.004 0.000 0.001 0.955 0.040 0.001 0.000 0.001 0.000 4.997

8 2.039 0.000 1.960 0.002 0.005 0.001 0.000 0.949 0.042 0.001 0.001 0.001 0.000 5.001

8 2.027 0.000 1.949 0.000 0.008 0.000 0.000 0.990 0.047 0.000 0.000 0.000 0.000 5.021

An Ab Or

0.961 0.038 0.001

0.967 0.033 0.000

0.960 0.040 0.000

0.964 0.035 0.000

0.967 0.033 0.000

0.973 0.027 0.000

0.967 0.033 0.000

0.970 0.030 0.000

0.972 0.028 0.000

0.970 0.030 0.000

0.972 0.028 0.000

0.970 0.030 0.000

0.957 0.043 0.000

0.959 0.040 0.000

0.959 0.040 0.001

0.956 0.043 0.001

0.955 0.045 0.000

6.1. Compositions of minerals in the chromite-rich and amphibole-rich layers Chromite analyses from published papers together with unpublished data of co-authors are given in Appendix. The entire data-set used for this study is in Appendix (Supplementary data) and only representative analyses of chromite, amphibole, plagioclase and spinel are presented in Tables 1–4.

6.1.1. Chromite The Cr2 O3 and Al2 O3 contents vary in the ranges 24.80–31.09 wt.% and 32.40–39.39 wt.% respectively (Table 1). The Sittampundi chromites meet the criteria of refractory-grade chromite (Al2 O3 > 20 wt.% and Cr2 O3 + Al2 O3 > 60 wt.%). Spot analyses of chromite show uniformly low TiO2 (<0.11 wt.%) and Fe3+ (0.20–0.90 atoms per formula unit; apfu) contents, irrespective of their association and textural type (Table 1). Core and rim compositions in individual cumulate chromite grains in the chromite-rich layers (Appendix) show only minor changes in terms of Mg# and Cr#. Chromite grains show marked positive correlation in Al vs. Mg/(Mg + Fe2+ ) and a strong negative correlation in Cr vs. Mg/(Mg + Fe2+ ) (Fig. 6a and b). All these features confirm that the primary composition of the cumulate chromites, particularly in their cores, did not change significantly during sub-solidus and metamorphic processes. In the Cr#–TiO2 (Fig. 7a) and Fe3+ #–TiO2 (Fig. 7b) diagrams the analyzed chromites plot close to the MORB field.

6.1.2. Amphibole Amphiboles in the amphibole-rich and chromite-rich layers are (Table 2) tschermakites according to the classification scheme of Leake (1978). They show marked compositional variations depending upon their associated phases. Amphiboles from the amphibole-rich layer are less aluminous (13.90–15.92 wt.% Al2 O3 ) compared with those in the chromite-rich layer (14.80–18.11 wt.% Al2 O3 ) (Fig. 8a and b). The tschermakites have slightly lower Cr (0.010–0.029 apfu) in the chromite-rich layer compared with the amphibole-rich layer (0.022–0.190 apfu) (Fig. 8a). TiO2 varies in the range 0.13–0.30 wt.%, whereas Na2 O is in the range 1.64–2.03 wt.% (Table 2). Compositional profiles from core to rim of amphiboles for Mg, Cr and Al (Fig. 8c) are in conformity with the tonal banding in X-ray elemental maps (Fig. 5b).

6.1.3. Plagioclase The Sittampundi plagioclases are highly calcic with An contents upto An97 (Table 3) (Subramaniam, 1956 reported An100 ), probably most calcic plagioclases known in layered anorthosite complexes. Plagioclases from the amphibole-rich layer are calcic and show a very narrow range of compositions; in cores only from An0.96 to An0.97 . This narrow range probably reflects the fact that metamorphic recrystallization has not affected the originally igneous plagioclase megacrysts that characterize this layer. Plagioclases in the chromite-bearing layer are slightly less calcic with compositions not exceeding An96 (Fig. 9).

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Table 4 Representative analysis of spinel. Mineral

Spinel discrete

Grain no.

14

Point no.

2

SiO2 TiO2 Al2 O3 Cr2 O3 FeO MnO MgO CaO Na2 O K2 O P2 O5 NiO ZnO Total

0.04 0.02 64.21 0.04 17.65 0.05 16.31 0.01 0.00 0.00 0.02 0.34 0.12 98.81

Surrounded by sapphirine 25

6 0.04 0.02 65.15 0.06 17.28 0.10 16.53 0.00 0.01 0.00 0.00 0.35 0.14 99.67

16 0.03 0.01 64.69 0.12 17.03 0.10 16.53 0.01 0.01 0.00 0.00 0.37 0.13 99.03

21

25

0.03 0.04 0.01 0.02 65.41 64.19 0.17 0.93 16.60 18.62 0.13 0.05 16.45 15.74 0.02 0.00 0.02 0.00 0.00 0.00 0.01 0.00 0.32 0.43 0.12 0.16 99.29 100.17

16 26

27

28

6

0.03 0.00 0.00 0.00 0.04 0.01 63.92 64.40 64.35 0.99 1.08 1.24 18.57 18.48 18.38 0.10 0.13 0.11 15.58 15.83 15.83 0.02 0.01 0.04 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.50 0.49 0.61 0.18 0.18 0.18 99.89 100.65 100.73

Inclusion in corundum

19 7

17

26 8

11

0.01 0.00 0.03 0.01 0.01 0.02 0.03 0.01 0.00 0.02 65.06 65.39 65.48 65.55 64.63 0.10 0.20 0.14 0.08 0.14 18.79 18.67 17.32 17.08 19.73 0.13 0.15 0.08 0.08 0.09 15.51 15.75 16.70 17.00 15.10 0.00 0.01 0.00 0.00 0.02 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.00 0.00 0.00 0.29 0.28 0.24 0.27 0.28 0.04 0.03 0.04 0.00 0.10 99.98 100.50 100.04 100.07 100.11

20

27

28

1

4

0.03 0.01 0.00 0.01 0.00 0.02 64.56 64.89 64.64 0.22 0.21 0.25 19.11 19.60 19.82 0.07 0.08 0.12 15.69 14.80 14.64 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.03 0.00 0.27 0.67 0.52 0.00 0.10 0.10 99.97 100.39 100.11

29

0.01 0.01 63.31 0.13 21.05 0.12 14.42 0.00 0.01 0.00 0.00 0.37 0.14 99.57

O Si Ti Al Cr Fe3+ Fe2+ Mn Mg Ca Na K P Ni Zn Total

4 0.001 0.000 1.970 0.001 0.027 0.357 0.001 0.632 0.000 0.000 0.000 0.000 0.007 0.002 2.999

4 0.001 0.000 1.978 0.001 0.019 0.354 0.002 0.635 0.000 0.000 0.000 0.000 0.007 0.003 3.000

4 0.001 0.000 1.976 0.003 0.018 0.351 0.002 0.639 0.000 0.001 0.000 0.000 0.008 0.002 3.001

4 0.001 0.000 1.991 0.004 0.004 0.355 0.003 0.633 0.001 0.001 0.000 0.000 0.007 0.002 3.000

4 0.001 0.000 1.958 0.019 0.020 0.383 0.001 0.607 0.000 0.000 0.000 0.000 0.009 0.003 3.000

4 0.001 0.000 1.957 0.020 0.020 0.383 0.002 0.603 0.001 0.000 0.000 0.000 0.010 0.003 3.001

4 0.000 0.001 1.955 0.022 0.020 0.378 0.003 0.607 0.000 0.000 0.000 0.001 0.010 0.003 3.000

4 0.000 0.000 1.952 0.025 0.024 0.372 0.002 0.607 0.001 0.000 0.000 0.000 0.013 0.003 2.999

4 0.000 0.000 1.983 0.002 0.012 0.394 0.003 0.598 0.000 0.000 0.000 0.001 0.006 0.001 3.000

4 0.000 0.001 1.982 0.004 0.012 0.389 0.003 0.603 0.000 0.000 0.000 0.000 0.006 0.000 3.001

4 0.001 0.000 1.980 0.003 0.015 0.357 0.002 0.638 0.000 0.000 0.000 0.000 0.005 0.001 3.001

4 0.000 0.000 1.977 0.002 0.022 0.344 0.002 0.648 0.000 0.000 0.000 0.000 0.006 0.000 3.000

4 0.000 0.000 1.975 0.003 0.021 0.406 0.002 0.583 0.000 0.000 0.000 0.000 0.006 0.002 3.000

4 0.001 0.000 1.969 0.004 0.025 0.388 0.002 0.605 0.000 0.000 0.000 0.000 0.006 0.000 3.000

4 0.000 0.000 1.981 0.004 0.013 0.412 0.002 0.571 0.000 0.000 0.000 0.001 0.014 0.002 3.000

4 0.000 0.000 1.981 0.005 0.013 0.418 0.003 0.567 0.000 0.000 0.000 0.000 0.011 0.002 3.000

4 0.000 0.000 1.958 0.003 0.037 0.425 0.003 0.564 0.000 0.000 0.000 0.000 0.008 0.003 3.001

Cr/(Al + Cl) Fe3+ /(Al + Cr + Fe3+ ) Mg/(Fetotal + Mg)

0.000 0.013 0.622

0.001 0.009 0.630

0.001 0.009 0.634

0.002 0.002 0.638

0.010 0.010 0.601

0.010 0.010 0.599

0.011 0.010 0.604

0.013 0.012 0.605

0.001 0.006 0.595

0.002 0.006 0.600

0.001 0.007 0.632

0.001 0.011 0.639

0.001 0.011 0.577

0.002 0.012 0.594

0.002 0.006 0.574

0.003 0.006 0.568

0.001 0.018 0.550

6.1.4. Spinel Spinels vary in composition according to which mineral they abut. The spinel is essentially a solid solution of hercynite and spinel (sensu stricto) with insignificant Ti, Zn, Ni, and Mn (Table 4). Recalculated Fe3+ is always low (<5 pfu). Values of XMg [Mg/(Mg + Fe2+ )] of spinel are in the range 0.57–0.64, although individual grains are compositionally homogeneous. Values of XFe3+ [Fe3+ /(Al + Cr + Fe3+ )] range from 0.002 to 0.018, though there is no perceptible difference in compositions in terms of XMg and XFe3+ of spinels in the matrix and as inclusions. 6.2. Composition of the parental melt from which the Sittampundi chromites crystallized Chromite in chromitite seams is well regarded as a leastmodified, refractory phase, and hence may reflect the original liquidus composition (see Stowe, 1994; Rollinson, 1995). The only significant factor affecting the composition of Cr-spinel crystallizing at a liquidus temperature is a change in melt composition (Allan et al., 1988). The composition of liquidus chromian spinel closely reflects the composition of its host melt, and is therefore dependent on the differentiation process. Sub-solidus re-equilibration between chromite and silicate in layered intrusions is relatively simple and is characterized by a Cr/Al exchange coupled with an inversely correlated Mg/Fe exchange. Late magmatic enrichment in Fe3+ or Ti is typical of most layered intrusions. Experiments show that Cr/Al falls with temperature and protracted fractionation, as long as plagioclase does not crystallize (Ballhaus and Glikson, 1989). Furthermore, the Cr/Al ratio of chromite is extremely sensitive to the Si/Al ratio of the magma.

The composition of the melt from which the Sittampundi chromites formed is constrained by our detailed mineral–chemical observations, by the calculations of mineral formulae of previous investigations, and by the results of experimental petrology as follows: (1) The mineral–chemical features of the chromite-rich layer show that the melt parental to the chromites contained calcic plagioclase, amphibole and Fe-rich, Ti-poor chromite as a liquidus phase, and this suggests that it was aluminous, calcic, relatively rich in Fe and Cr, but relatively poor in silica, alkalis, Ti and Mg. (2) The chromite compositions in our study are Fe-rich and aluminous (Fe-number ∼ 0.8, Cr-number ∼ 0.46), and they evolve to more Fe-rich and more Cr-rich compositions during crystal fractionation. We note that, unlike chromites in mafic and ultramafic rocks, these anorthositic chromites formed late in the crystallization sequence, after the start of the plagioclase crystallization and simultaneous with amphibole crystallization. Thus the variation in Cr-number of the chromite is related to the variation in the amount of plagioclase crystallizing along with chromite, because plagioclase crystallization will deplete a melt in Al and so increase the Cr-number in the chromite. Similarly, the Fe/Mg ratio of the chromite is governed by the degree of amphibole crystallization, such that the amphibole preference for Mg over Fe enhances the Fe-rich character of the chromite (Rollinson et al., 2010). (3) The Ti and Al contents of chromite reflect the composition of the magma from which it crystallized (Kamenetsky et al., 2001). In the case of the most primitive chromites (Al2 O3 ca. 25 wt.%; TiO2 ca. 0.16 wt.%) this would suggest a low-Ti, aluminous melt.

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267

Fig. 4. Photograph of a thin-section showing the positions of EMPA spot analyses of chromite and associated silicate minerals from the amphibole-rich and chromite-rich layers.

Kamenetsky et al. (2001) also showed that the Ti and Al contents of chromites of mantle-derived melts can be used to indicate the former tectonic setting of the magma. However, these results apply only when chromite is either an early liquidus phase or is in equilibrium with a Ti–Al-free phase such as olivine. In Sittampundi chromite crystallized together with other aluminous phases (plagioclase and amphibole), and not with olivine. (4) Experimental studies indicate that the Al2 O3 content and FeO/MgO ratio of chromites are directly related to the parental melt and this relationship is not affected by crystallization and temperature. The nature of the parental melt is assessed on the basis of the composition of liquidus spinels i.e. cumulus chromites in massive chromitites that occur in layers, which would be little changed by equilibration due to the lever rule (Irvine, 1965, 1967; Dick, 1977). Chromite-melt inclusion data in volcanic rocks indicate that there is a linear relationship between the Al2 O3 - and TiO2 contents of chromites and the Al2 O3 and TiO2 concentrations in the melt (Maurel and Maurel, 1982; Kamenetsky et al., 2001). Experimental data on peridotite melting by Wasylenki et al. (2003) also corroborate these observations. Using the data from Kamenetsky et al. (2001) and Roeder and Reynolds (1991), Rollinson (2008) obtained a power law expression between melt-Al2 O3 and spinel-Al2 O3 for MORB lavas and a logarithmic expression for arc lavas (Fig. 8). However, in the case of melt-TiO2 vs. spinel-TiO2 , the relationship maintains a power law both for MORB and arc lavas. In a recent study Ghosh and Konar (2011) calculated the Al2 O3 contents of the parent melt from which the Sittampundi chromitites were formed, using both the approaches of (a) Maurel and Maurel (1982) (Table 1) and (b) Rollinson (2008), which range between 14 and 16 wt.%, which is consistent with a modern MORB parental melt (Wilson,

1989; Mondal et al., 2006). Another important constraint is the FeO/MgO ratio of the melt. In order to calculate this ratio we require the composition of minerals that have not undergone re-equilibration, which is the case for chromite in the massive ores. Similarly, using the Maurel and Maurel (1982) formula, Ghosh and Konar (2011) calculated the FeO/MgO ratio of the parental melt of Sittampundi chromitites to be >3, which is higher than the limit of modern MORB melts and reflects a highFe content (Mondal et al., 2006). Accordingly, they inferred that the melt parental to the chromitites was an Fe-rich aluminous basaltic melt. (5) Several experimental studies have demonstrated that the An content of plagioclase in mafic magmas is controlled by variations in melt composition, melt water content, temperature and pressure (Beard and Borgia, 1989; Sisson and Grove, 1993; Feig et al., 2006). Takagi et al. (2005) calculated that at a constant composition, in a low-alkali, high-alumina, arc tholeiite (17 wt.% Al2 O3 ), there is a linear relationship between the An content of plagioclase and the water content of the melt. Moreover, high-An plagioclase is the liquidus phase in melts with up to 5% H2 O at a low pressure, and it decreases to about 3% at 5 kbar, which suggests that the optimum pressure condition for the crystallization of high-An plagioclase is 2–3 kbar.

Also, the high An content of plagioclase in a hydrous mafic magma is consistent with the presence of primary amphibole in the Sittampundi anorthosites. The high-An plagioclases (An90 ) reported by Takagi et al. (2005) are comparable with the ∼An90–100 plagioclase of the Sittampundi anorthosite. Hence, we make the robust conclusion that the Sittampundi parental melt was hydrous.

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6.3. Crystallization sequence of Sittampundi chromite-rich layers The Sittampundi Complex is made up of calcic anorthosite (An90–100 ), noritic anorthosite, leucogabbro, gabbro, clinopyroxenite, troctolite and chromitite, and many of the macro-scale structures and textures observed in these rocks are the products of fractionation processes within a layered igneous intrusion (Subramaniam, 1956). From the phase relations outlined earlier in this paper, we have constructed the followed general

crystallization sequence for the Sittampundi Complex. A parental melt having the composition of hydrous tholeitic magma first crystallized a combination of dunite, wehrlite and harzburgite. Such early-crystallized rocks are still preserved in the lowermost section of the Bhavani Complex (Dutta et al., in press), but are missing in the Sittampundi Complex; they may lie beneath the exposed level at Sittampundi, but they may have been removed tectonically. Polat et al. (2011a) calculated that in the Fiskenæsset Complex there was originally a 500-m thick ultramafic unit (dunite,

Fig. 5. X-ray element maps of the minerals and textures seen in the thin-sections of Fig. 3b–d showing the distribution of Al, Cr, Fe, Mg, Ca and Cr. (a) Element mobility during the sapphirine- and spinel-producing reaction during ultrahigh-temperature metamorphism (see also Koshimoto et al., 2004). (b and c) Element distribution in a chromite–tschermakite interface.

C.V. Dharma Rao et al. / Precambrian Research 227 (2013) 259–275

269

Fig. 5. (Continued ).

peridotite, pyroxente, and hornblendite) at the bottom, but now there is less than 50 m, suggesting that more than 90% of the ultramafic rocks were either delaminated or recycled back into the mantle as a residual cumulate, or were destroyed during thrusting and TTG intrusion. Continued crystallization of the evolved Sittampundi melt produced an alternating sequence of amphibole-rich and chromite-rich layers. Because Mg is preferentially partitioned

into amphibole than into a coexisting melt, crystallization of large volumes of amphibole to form amphibole-rich layers enriched the parental melt with Cr, Fe and Al, which triggered the crystallization of FeAl-rich chromite to form chromitite seams as seen today. Crystallization of the FeAl-rich chromite, in turn, enriched the magma in Mg and paved the way for crystallization of amphibole. This process eventually enriched the residual melt with Ca, Al and H2 O from

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Fig. 6. (a) Al vs. Mg/(Mg + Fe2+ ) plot of Sittampundi chromites showing a strong positive correlation similar to the compositions of chromites in modern arc basalts (Polat et al., 2009). (b) Cr vs. Mg/(Mg + Fe2+ ) plot of Sittampundi chromites showing a strong negative correlation. The higher Cr and lower Mg#s of the chromites from Sittampundi are characteristic of those formed in subduction-related environments (Fig. 14A; Jan and Windley, 1990).

which anorthosite with highly calcic plagioclase crystallized. Crystallization of large volume of calcic plagioclase increased the Cr concentration in the melt, which, when reached a critical value, deposited chromite in the anorthosite. High PH2 O in the evolved melt also assisted crystallization of chromite in the anorthosite (Matveev and Ballhaus, 2002). Temperatures during the entire crystallization sequence may have been more than 100 ◦ C in order to suppress crystallization of voluminous magmatic amphibole. 7. Discussion 7.1. Petrogenesis and tectonic setting The petrogenesis of some Archean layered anorthositic complexes has been studied in great detail. Noticeably, the compositions of FeAl-rich chromite, calcic plagioclase and amphibole in the Sittampundi Complex are remarkably similar to those in the Fiskenæsset Complex (Windley and Smith, 1974; Rollinson et al., 2010; Polat et al., 2011a,b). In this section we shall discuss the petrogenesis of these phases and their tectonic implications constrained by relevant experimental data. Chromite: Several field, petrographic and geochemical indices suggest that the Sittampundi Complex was generated from a hydrous basaltic magma in a subduction-generated arc environment. In an Al2 O3 vs. TiO2 diagram (Fig. 10a), chromites from Sittampundi plot in the field of supra-subduction zone peridotites. A wide range in the chemical composition of the Sittampundi chromites is characteristic of many layered, arc-type complexes (see also Krause et al., 2007). The higher Cr#s and lower Mg#s of the Sittampundi chromites are characteristic of those formed in subduction-related environments (Fig. 14A; Jan and Windley, 1990). The low TiO2 content of the Sittampundi chromites (Fig. 10a) also attests to the arc-related environment of the ultramafic cumulates (Fig. 14B; Arai, 1992). As Fig. 11 demonstrates, the Sittampundi

Fig. 7. (a) The Cr#–TiO2 diagram for chromites from different tectonic situations. The Sittampundi chromites mostly plot in the MORB field. Fields for different geochemical affinities and tectonic environments are adopted from Arai (1992). The low TiO2 contents of the Sittampundi chromites are consistent with an arc-related environment (Fig. 14B; Arai, 1992). (b) Fe#–TiO2 diagram for chromites. The Sittampundi chromites plot in the MORB field as in (a). Fields for different geochemical affinities and tectonic environments are adopted from Arai (1992). The low TiO2 Sittampundi chromites point to an arc environment (Fig. 14B; Arai, 1992).

chromites are similar in composition to those of the Fiskenæsset Complex (Rollinson et al., 2010). Plagioclase: Experimental data support the thesis that highly calcic plagioclase in anorthosite and FeAl-rich chromite in the Sittampundi Complex were generated from hydrous melts. Several studies have indicated that hydrous basaltic magmas were generated in extant and accreted arcs from the Archean to present (e.g. Arculus and Wills, 1980; Takagi et al., 2005; Tiepolo and Tribuzio, 2008; Polat et al., 2009; Windley and Garde, 2009). Furthermore, some primitive arc basalts also have chromites with low Mg# (0.6) and low Cr# (0.4–0.6), which overlap with the compositions of chromites (Fig. 10b) from the Sittampundi and Fiskenæsset Complexes (Grove et al., 2003; Righter et al., 2008; Rollinson et al., 2010). Amphibole: Primary igneous amphiboles are typical products of differentiation of arc magmas (Davidson et al., 2007). In consequence, plutonic xenoliths of amphibole-bearing anorthosite, leucogabbro and gabbro are common in modern arcs (Beard,

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Fig. 8. (a) Cr vs. Mg/Mg + Fetotal variation diagram for Sittampundi amphiboles in the tschermakite-rich and chromite-rich layers. (b) Al vs. Mg/Mg + Fetotal variation diagram for Sittampundi amphiboles in the tschermakite-rich and chromite-rich layers. (c) Al, Cr, Mg variations in amphibole composition as a function of distance across a chromitite seam. Note that except for a crest and trough a few microns across, the composition of tschermakite remains virtually constant.

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Fig. 9. Ab–Or–An triangular diagram for Sittampundi plagioclases in the chromiterich and tschermakite-rich layers. Note the overlap of plagioclase compositions in the layers indicating that plagioclase compositions largely remain unchanged.

1986) such as the Aleutians (Bacon et al., 2007; Yogodzinski and Keleman, 2007), the Lesser Antilles (Powell, 1978; Arculus and Wills, 1980; Kiddle et al., 2010), the Marianas (Meijer and Regan, 1981), Indonesia (Morrice et al., 1983), and Kamchatka (Erlich et al., 1979) and in the roots or magma chambers of hydrous arcs of Paleozoic and Precambrian age (Windley and Garde, 2009), and specifically at Lac des Iles in Canada (Brügmann et al., 1997), Victoria Land in Antarctica (Bracciali et al., 2009), Fiskenæsset, W. Greenland (Polat et al., 2009, 2010, 2011a,b), and at Kondapalle (Dharma Rao and Santosh, 2011; Dharma Rao et al., 2011a) and Chimalpahad (Dharma Rao et al., 2010) in the Eastern Ghats belt of India. The following relations apply to interpretation of amphiboles in the Sittampundi Complex, because Ghosh and Konar (2011) proposed that they are igneous in origin, whereas Dutta et al. (in press) argued that they formed during later metamorphic recrystallization. Because clinopyroxene rarely occurs in the cores of inter-cumulus hornblendes in leucogabbros and gabbros of the Fiskenæsset Complex, Myers (1985) favored the idea that the hornblende was a metamorphic derivative of igneous pyroxene created during a metamorphic event millions of years after the formation of the complex. However, in unmetamorphosed anorthosites in Cretaceous layered complexes in the Peruvian Andes, intercumulus hornblende is secondary after clinopyroxene, and was derived from a late volatile-rich residual melt (Mullan and Bussell, 1977). And, unmetamorphosed cumulate nodules in the Pleistocene Dominican volcanic island arc of the Lesser Antilles indicate that some amphibole crystallized as a cumulus phase from the main magma, but some amphibole elsewhere in the arc overgrew and in places replaced clinopyroxene, this amphibole having crystallized from a late, trapped, inter-cumulus liquid (Powell, 1978). In the 143–137 Ma unmetamorphosed Darran Complex in Fiordland in New Zealand, hornblende-bearing gabbronorites formed in the magma chamber of an island arc by melting of the mantle wedge above a subducting slab (Muir et al., 1998). In view of the above examples of hornblende growth in hydrous cumulates of arcs present to past, we have independently concluded from our textural studies of cumulate leucogabbros and gabbros in the Sittampundi, Fiskenæsset, Kondapalle and Chimalpahad Complexes that most of the hornblende originally grew as a primary cumulus or inter-cumulus igneous phase. This conclusion is consistent with experimental data of Claeson and Meurer (2004) and AlonsoPerez et al. (2009) who demonstrated that amphibole is a primary phase in the crystallization of hydrous arc magmas. Moreover, the fact that calcic amphibole is a matrix phase in unmetamorphosed chromitites in the upper arc section of the Oman ophiolite (Schiano

Fig. 10. (a) Al2 O3 vs. TiO2 tectonic discrimination diagram showing that Sittampundi chromites plot in the field of supra-subduction zone peridotites; after Kamenetsky et al. (2001). (b) Fe# vs. Cr# tectonic discrimination diagram showing that Sittampundi chromites plot close to chromites from arc-basalts. The Fiskenæsset chromites are represented by the core compositions and the high-Fe and high-Cr-number samples are indicated by shading (Rollinson et al., 2010). The large, light blue shaded field is the 90 percentile field for arc basalts from the compilation of Barnes and Roeder (2001). The darker shaded field is for harzburgites from the mantle of the Oman ophiolite (Le Mée et al., 2004). Data points for arc-basalt chromites are from Stewart et al. (1996), Heath et al. (1998), Grove et al. (2003) and Righter et al. (2008). The arrows show the possible evolution of melts from aluminous (fertile) mantle in the Archaean (lower arrow) and from less aluminous (depleted) mantle in recent arcs (upper arrow). The diagram shows that Sittampundi chromites plot on a trend close and parallel to that of chromites from the arc-related Fiskaneasset Complex (Rollinson et al., 2010) indicating that Sittampundi chromites were also cumulates derived from an arc basaltic magma.

et al., 1997; Rollinson, 2008) underlines the fact that amphibole can form from a hydrous magma in equilibrium with chromite. Fig. 8 shows that the bulk compositions of some mafic rocks from the Sittampundi and Bhavani Complexes plot in the field of IAB with some data straddling the boundary between MORB and IAB (Dutta et al., in press); mafic rocks of the Fiskenæsset Complex show a similar trend. The compositional plots together with the inferred high PH2 O of the parental liquids are consistent with the parental liquid of the Sittampundi Complex being derived from a Neoarchean (ca. 2.9 Ga) upper mantle that was metasomatized by subduction-derived components during formation of an oceanic arc. However, compared with the composition of chromites in modern arc basalts, chromites from the Neoarchaean layered magmatic complexes of Fiskenæsset and Sittampundi are distinctly aluminous for any similar Mg#. This difference has been attributed to the fact that the Neoarchaean mantle wedge above the subducting

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was most probably a mantle wedge in a subduction setting and the water was derived from the subducted oceanic plate. (2) The main product of the partial melting was a hydrous, aluminous basalt, which evolved by fractionation from peridotite and pyroxenite to a FeAl-rich basaltic melt, parental to the chromitites. (3) The Sittampundi Complex with its chromite horizons formed as a cumulate in the magma chamber of a suprasubduction zone island arc. Acknowledgements

Fig. 11. Fe# vs. Cr# diagram showing Sittampundi chromite compositions from the present study compared with those of other studies, and compared with Fiskenæsset chromites (Rollinson et al., 2010). See text for details.

oceanic plate was more aluminous owing to percolation of Al-rich melts derived from melting of subducting oceanic crust (Polat et al., 2009; Rollinson et al., 2010). 7.2. Comparison with other arc-related anorthositic chromites in Archean high-grade terranes Several Archean high-grade terranes expose comparable midlower continental crust and layered igneous complexes (Windley et al., 1981; Phinney et al., 1988; Ashwal, 1993; Windley and Garde, 2009), the most similar of which are the Sittampundi, Bhavani, Fiskenæsset and Messina Complexes that all contain interlayered calcic anorthosites, chromitites and pyroxenites. The main difference between them is that the first two were first subjected to eclogite facies metamorphism (Sajeev et al., 2009; Saitoh et al., 2011), whereas the last two were only metamorphosed to granulite and amphibolite facies (Windley et al., 1973; Myers, 1985; Barton, 1996; Dymek and Owens, 2001). The Sittampundi and Fiskenæsset chromites have a single Fe-enrichment trend shown in Fig. 10b. A comparable subductionrelated, high-alumina, hydrous basaltic magma was reported by Eyuboglu et al. (2011) from an Alaskan-type mafic–ultramafic complex in eastern Pontides, N. Turkey. A higher geothermal gradient in the Archean probably provided optimal conditions for slab melting that metasomatized the sub-arc mantle wedge by slab-derived melts (Polat et al., 2011a,b) and made it unusually aluminous, and this acted as a source for hydrous aluminous basalts. In summary, chromite-bearing rocks in layered complexes in Archean high-grade terranes share a number of common geological features. Anorthosites with highly calcic plagioclase and FeAl-rich chromite are diagnostic, and are attributed to their hydrous and aluminous parental melt compositions. The chromite analyses from the present study not only overlap with those of previous studies of Sittampundi chromites, but they also overlap with the compositions of arc-related Fiskenæsset chromites. 8. Conclusions Our studies lead to the following conclusions: (1) The partial melting of unusually aluminous mantle harzburgite was initiated by the migration of hydrous fluids; the mantle

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