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The Canadian Mineralogist Vol. 48, pp. 205-229 (2010) DOI : 10.3749/canmin.48.1.205
MINERAL COMPOSITIONS AND PETROGENETIC EVOLUTION OF THE ULTRAMAFIC-ALKALINE – CARBONATITIC COMPLEX OF SUNG VALLEY, NORTHEASTERN INDIA Leone MELLUSO§ Dipartimento di Scienze della Terra, Università di Napoli Federico II, I–80134 Napoli, Italy
Rajesh K. SRIVASTAVA Department of Geology, Banaras Hindu University, Varanasi 221 005, India
Vincenza GUARINO Dipartimento di Scienze della Terra, Università di Napoli Federico II, I–80134 Napoli, Italy
Alberto ZANETTI Istituto di Geoscienze e Georisorse, CNR, I–27100 Pavia, Italy
Anup K. SINHA Department of Geology, Banaras Hindu University, Varanasi 221 005, India
Abstract The Sung Valley alkaline complex is a relatively small intrusion of Lower Cretaceous age emplaced slightly before or during the India–Antarctica break-up. It consists of ultramafic rocks (dunites, wehrlites, clinopyroxenites, uncompahgrites), mafic rocks (ijolites sensu lato), felsic rocks (nepheline syenites) and carbonatites. The chemical composition of the mafic minerals indicates the expected enrichment in iron toward the felsic rocks. On the other hand, carbonatites feature very Mg-rich minerals, generally Cr-rich, indicating that their genesis is completely unrelated to that of mafic and felsic rocks (ijolites and nepheline syenites). The parageneses indicate that this complex was formed by batches of primitive magmas with a distinct magmatic affinity (olivine melilitites and olivine nephelinites, basanites, and possibly also carbonatites) which evolved independently, generating the observed spectrum of intrusive rocks. Clinopyroxenites have interstitial alkali feldspar and titanite, indicating that they formed from evolved feldspar-normative (phonotephritic, tephriphonolitic) magmas. The sequence perovskite–titanite and titanite–garnet noted in some ijolitic rocks indicates changes in the chemical composition of coexisting silicate melts and, most likely, an increasing f(O2). The trace-element profiles of coexisting phases in interesting associations in a sample of ijolite were documented by means of LA–ICP–MS analyses. Keywords: ultramafic-alkaline rocks, carbonatite, mineral compositions, LA–ICP–MS data, trace elements, garnet, titanite, perovskite, clinopyroxene, Sung Valley complex, India.
Sommaire Le complexe alcalin de Sung Valley, en Inde, d’âge crétacé inférieur, est un massif intrusif relativement petit dont la mise en place a légèrement précédé ou accompagné la rupture du socle Inde–Antarctique. On y trouve des roches ultramafiques (dunites, wehrlites, clinopyroxénites, uncompahgrites), mafiques (ijolites sensu lato), felsiques (syénites néphéliniques) et des carbonatites. La composition des minéraux mafiques témoigne de l’enrichissement en fer typique en direction des roches felsiques. En revanche, les carbonatites contiennent des minéraux fortement enrichis en Mg et, en général, aussi enrichis en Cr, indication que leur filiation serait complètement différente de celle qui a produit les roches mafiques et felsiques (ijolites et syénites néphéliniques). Les paragenèses indiquent que ce complexe a été formé par des venues de magma primitif ayant une affinité distincte,
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produisant un cortège de mélilitites à olivine, des néphélinites à olivine, des basanites, et possiblement aussi des carbonatites, qui auraient évolué de façon indépendante, générant ainsi le spectre observé de roches intrusives. Les clinopyroxénites possèdent un feldspath alcalin interstitiel et la titanite, indication qu’elles ont été formées à partir d’un magma évolué de composition phonotephritique ou tephriphonolitique et à feldspath normatif. La séquence pérovskite–titanite et titanite–garnet présente dans certaines roches ijolitiques indique des changements compositionnels des magmas silicatés coexistants et, tout probablement, une augmentation en f(O2). Les profils d’éléments traces dans des phases coexistantes dans les associations interessantes d’un échantillon d’ijolite ont fait l’object d’analyses par la technique LA–ICP–MS.
(Traduit par la Rédaction)
Mots-clés: roches ultramafiques alcalines, carbonatite, compositions de minéraux, données LA–ICP–MS, éléments traces, grenat, titanite, pérovskite, clinopyroxène, complexe de Sung Valley, Inde.
Introduction The Sung Valley ultramafic-alkaline–carbonatite complex (UACC) is one of the Cretaceous intrusions emplaced within the Shillong Plateau, an uplifted horstlike feature in northeastern India (Chattopadhyay & Hashimi 1984, Krishnamurthy 1985, Ray et al. (2000), Srivastava & Sinha 2004, Srivastava et al. 2005, and references therein). Ray & Pande (2001) dated the carbonatite of this complex by the 40Ar–39Ar method and placed it at 107.2 ± 0.8 Ma. They suggested that the plateau ages represent near-surface crystallization (or emplacement) ages (Ray & Pande 2001). Later, Srivastava et al. (2005) provided a U–Pb perovskite age of 115.1 ± 5.1 Ma for the ijolite SV58 of the Sung Valley complex, as well as bulk-rock Sr–Nd isotope data. The Sung Valley ijolite thus may have a slightly older age of emplacement than the carbonatite, consistent with field relationships. The emplacement of the Shillong Plateau alkaline province is related to the initiation of the Ninety-East Ridge volcanism in the Indian Ocean (e.g., Duncan 2002). In addition, strongly alkaline rocks (melilite-bearing ultramafic lamprophyres) with roughly similar ages as the northeastern Indian intrusions (117–110 Ma) are recorded also in East Antarctica (Foley et al. 2002). These are likely counterparts of the northeast Indian alkaline rocks during the rifting event(s), which split the two continents apart in the Early Cretaceous (Duncan 1992, Storey et al. 1992, Royer & Coffin 1992). The Sung Valley intrusion consists of ultramafic rocks (clinopyroxenite, serpentinized peridotite, and melilitolite), alkaline rocks (ijolite and nepheline syenite) and carbonatites (Krishnamurthy 1985, Srivastava & Sinha 2004, Srivastava et al. 2005, Fig. 1). Clinopyroxenite, serpentinized peridotite and ijolites form most of the complex, whereas the other rock types constitute less than 5% of the exposures. The serpentinized peridotite occupies the central part of the complex, being surrounded by clinopyroxenite. Serpentinized peridotite and clinopyroxenite are among the oldest rocks of the complex. The ijolite body forms a ring structure. Small dykes of melilitolite intrude the peridotite and clinopyroxenite. Nepheline syenite and
carbonatite occur in dykes, veins, stocks, and ovoid bodies, intruding the ultramafic rocks as well as the ijolites. Carbonatite is the youngest member of the complex, as it intrudes all the other units. Mineral compositions and evolutionary trends in the Sung Valley intrusion have been not adequately studied, except by Viladkar et al. (1994). This is surprising, given the level of detail reached by age and isotope determinations and whole-rock geochemistry. In this paper, we aim to fill such a gap by characterizing the mineral composition of the main lithotypes found in this complex. The chemical sequence and composition of the phases are of prime importance in deciphering the petrogenetic history of intrusive complexes. In particular, minerals and parageneses are better able to identify liquid lines of descent, as the composition of intrusive rocks can be determined by cumulus processes and not by closed-system crystallization of magmas. The range of compositions shown by the Sung Valley rocks is similar to those of nepheline–pyroxene-rich intrusive complexes worldwide (Srivastava & Sinha 2004, and see discussion). Yet, there are contrasting hypotheses to explain the petrogenesis of coexisting carbonatites and ijolites (cf. Le Bas 1977, Beccaluva et al. 1992) and, more generally, that of carbonatites, these being either late products of liquid immiscibility (perhaps fractional crystallization) or early products of partial melting in the mantle (Treiman & Essene 1985, Bell et al. 1998, Mitchell 2005).
Analytical Techniques Polished thin sections were prepared for seventeen slabs of the main lithologies of the Sung Valley complex (Fig. 1). More than 400 analyses obtained with energydispersive spectrometry (EDS), and with back-scattered electron (BSE) images, have been performed at CISAG, University of Napoli Federico II, utilizing an Oxford Instruments Microanalysis Unit. The latter is equipped with an INCA X-act detector and a JEOL JSM–5310 microscope operating at 15 kV primary beam voltage, 50–100 mA filament current, a 15–17 mm spot size and a net acquisition-time of 50 s. Measurements were done with an INCA X-stream pulse processor. The
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following standards were used for calibration: diopside (Mg), wollastonite (Ca), albite (Al, Si, Na), rutile (Ti), almandine (Fe), vanadium (V), Cr2O3 (Cr), rhodonite (Mn), nickel (Ni), orthoclase (K), zircon (Zr), apatite (P), barite (Ba), strontianite (Sr), galena (Pb), synthetic Smithsonian phosphates (La, Ce, Nd), and internal standards (Nb, Ta, U, Th). A subset of the samples were analyzed at IGAG–CNR, Rome, using a Cameca SX50 WDS electron microprobe, using techniques already described elsewhere (e.g., Melluso et al. 2005). In situ trace-element analyses of clinopyroxene, perovskite, titanite and garnet from ijolite SV58 have been performed on the same thin section as used for electron-microprobe analysis by means of laser-ablation – inductively coupled plasma – mass spectrometry (LA– ICP–MS) at IGG–CNR, Unit of Pavia (Italy). The laser probe consists of a Q-switched Nd:YAG laser, model Quantel (Brilliant), whose fundamental emission in the near-IR region (1064 nm) was converted to 266 nm wavelength using two harmonic generators. Spot diameter was typically 60 mm. Each spot has been checked in order to assess the homogeneity of the ablated area and the absence of contributions from mineral inclusions and fluid inclusions. The ablated material was analyzed by using an Elan DRC-e quadrupole mass spectrometer.
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Helium, used as the carrier gas, was mixed with argon downstream of the ablation cell. We used NIST SRM 610 as an external standard, whereas 44Ca was used as an internal standard. Precision and accuracy were assessed from repeated analyses of the BCR-2g standard, usually resulting in a precision better than 10% for concentrations at the ppm level.
Petrography and Whole-Rock Composition of the Sung Valley Rocks Images of the samples of this study are reported in Figure 2 and in the supplementary plates 1 and 2, placed in the Depository of Unpublished Data on the MAC website [document Sung Valley CM48_205]. The paragenesis is reported in Table 1. Peridotites Dunite SV31 is a heavily serpentinized coarsegrained rock, with cumulus olivine, rare interstitial clinopyroxene, amphibole, magnetite and perovskite. Wehrlite SV19 contains totally serpentinized olivine, still fresh diopside, magnetite, perovskite and rare crystals of phlogopite. Both samples have aluminous
Fig. 1. Geological map of the Sung Valley ultramafic-alkaline – carbonatite complex (modified after Srivastava et al. 2005). Nepheline syenite and melilitolite dykes exposed around the villages Sung and Maskut are very small, hence not reported on the map.
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spinel in magnetite (Figs. 2a, b). Sample SV31 also contains ilmenite, found coexisting with magnetite, or as fine lamellae in it. Uncompahgrites Uncompahgrite SV33 (a melilite-rich intrusive rock) is a coarse-grained rock formed by cumulus melilite and clinopyroxene (the latter commonly shows very tiny black oriented inclusions of Fe–Ti oxide) (Fig. 2c). Layers alternatively enriched in clinopyroxene and melilite are observed. Minor minerals are titanian magnetite, a yellowish to greenish phlogopite, and rare interstitial perovskite and sulfides. A modal analysis indicates roughly identical amounts of the two main cumulus minerals (46.3% melilite, 46.8% clinopyroxene, 6.9% phlogopite + opaque oxides + sulfides + perovskite). Clinopyroxenites The clinopyroxenites of the Sung Valley intrusion have very variable grain-size and textures. Sample SV6 is a medium-grained rock made up of zoned clinopyroxene, with minor interstitial alkali feldspar and rare idiomorphic titanite. Sample SV7 is coarse grained, with zoned clinopyroxene and rare interstitial and finely exsolved K-feldspar. Veins rich in alkali feldspar and very rare pyrochlore cut across this lithotype. Alkali feldspar is clearly igneous. Sample SV8 is a very finegrained sample, almost completely made up of green
F ig . 2. Back-scattered electron (BSE) and microscope images of peculiar petrographic features of Sung Valley rocks. a) BSE image of intergrowth of perovskite, amphibole and magnetite, sample SV31. b) BSE image of patches of Al-rich spinel in an otherwise chemically homogeneous titanian magnetite, sample SV31. c) Intergrowths of melilite and clinopyroxene, uncompahgrite SV33, crossed nicols. d) Clinopyroxene and magnetite in the clinopyroxenite SV8, parallel nicols. e) BSE image of the sequence perovskite ! titanite ! garnet in the ijolite SV58 (see text). Note also the idiomorphic clinopyroxene and the late-crystallized nepheline. This is the area analyzed for LA–ICP–MS data. The numbers indicate the spot analyses made, as reported in Table 11. Circles are larger than the actual size of the spot. f) Idiomorphic garnet, green clinopyroxene and titanite, with nepheline and nosean in the ijolite SV83, parallel nicols. g) Early alkali feldspar, subidiomorphic nepheline, aegirine-rich clinopyroxene and mica, defining a tinguaitic texture in nepheline syenite SV22, crossed nicols. h) Zoned phlogopite with a clear, more Mg-rich rim, clinohumite and calcite. Olivine, apatite and oxides are microcrystals, in carbonatite SV49, parallel nicols. i) BSE image of calcite, Fe–Ti oxides and dolomite, carbonatite SV68.
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clinopyroxene, with rare veins rich in garnet, and minor interstitial titanite, magnetite, apatite and garnet (Fig. 2d). Sample SV9 is mainly composed of zoned green clinopyroxene, with small amounts of titanite. Ijolites The nepheline-rich rocks have a variable paragenesis. Sample SV58 is slightly heterogeneous in grain size, and is composed of idiomorphic clinopyroxene and subhedral nepheline. Perovskite with a reaction rim of titanite and garnet, likely itself a reaction rim on titanite (Fig. 2e), complete the paragenesis. No magnetite was observed. Sample SV14 is a pegmatitic ijolite, mainly composed of large crystals of nepheline and zoned clinopyroxene, with lesser amounts of apatite, titanite and magnetite. Poikilitic garnet is an interstitial phase. Sample SV10 is very fine-grained, nepheline and clinopyroxene coexist with small patch-like clusters richer in perovskite, titanite, mica and magnetite. Sample SV83 is medium- to fine-grained and formed by euhedral, zoned crystals of clinopyroxene, with nepheline, zoned garnet, titanite, and interstitial nosean (Fig. 2f). Garnet and titanite are both idiomorphic and do not show any reaction relationships where found in contact. Nepheline syenites Samples SV22 and SV25 have a typical tinguaitic (fluidal) texture and are composed of optically zoned K-feldspar (with a rim of albite), interstitial nepheline (usually corroded by cancrinite), deep green clinopyroxene, titanite, mica and Fe–Ti oxides (Fig. 2g). Sample SV56 is fine grained, and mostly consists of idiomorphic alkali feldspar, subidiomorphic nepheline, green clinopyroxene, magnetite, very rare ilmenite, titanite and interstitial cancrinite. Sample SV52A is a composite rock consisting of a more fine-grained nepheline syenite with alkali feldspar, nepheline and deep green clinopyroxene in contact with a coarser-grained rock, mostly made up of the same minerals with, in addition, Fe–Ti oxides and some mica. Carbonatites Coarse- to fine-grained carbonatitic facies are present at Sung Valley. Sample SV49 is fine-grained with coarser areas. The fine-grained area is made up of calcite, in some cases with the appearance of phenocrystic olivine, mostly altered to serpentine, markedly zoned phlogopite, with a darker (Fe-rich) core and a clear (Mg-rich) rim, clinohumite, apatite, Fe–Ti oxides and rare pyrochlore (Fig. 2h). The coarse-grained areas are almost completely made up of calcite. Sample SV68 is coarse grained, and mainly composed of calcite, with rare crystals of dolomite, clear and idiomorphic phlogopite, ilmenite, magnetite and apatite (Fig. 2i). Sample SV73 is also coarse-grained and consisting of
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calcite, subhedral olivine, sulfides, oxides and apatite. Clinopyroxene is absent in the carbonatites. Bulk compositions Major- and trace-element compositions of Sung Valley rocks are reported in Srivastava & Sinha (2004), and in the supplementary table, placed in the Depository of Unpublished Data on the MAC website [document Sung Valley CM48_205]. One should note that mineralogy, textures, grain size and whole-rock geochemistry make most of these rocks unlikely representatives of liquid compositions. They are the result of accumulation of minerals on the bottom or in the borders of a magma reservoir emplaced in the Earth’s upper crust. Indeed, Srivastava & Sinha (2004) and Srivastava et al. (2005) had difficulty to find reliable indications of magma compositions and liquid lines of descent from the study of bulk-rock geochemical data. To overcome this difficulty, we decided to focus on the chemical variations of the coexisting phases.
Mineral Compositions Olivine and clinohumite Olivine occurs in dunites, wehrlites and carbonatites. Olivine relics in the serpentinized dunite SV31 have a narrow range in composition (Fo86–Fo87), with relatively high Ca contents (0.014–0.021 apfu, based on
four atoms of oxygen). Olivine in the carbonatites SV73 and SV49 has also a narrow range of compositions, but at markedly higher forsterite contents (Fo94–Fo96), and with much lower Ca contents (0–0.008 apfu). This appears to be curious for a mineral in equilibrium with such Ca-rich rocks and minerals. Nevertheless, it is not unexpected, given the lack of monticellite– kirschsteinite solid solutions to buffer Ca contents in forsterite–fayalite solid solutions at their highest values (Sharp et al. 1986). Conversely, the MnO contents are very similar in both olivine compositions (Table 2). Olivine in the carbonatites has a core slightly more Fe-rich than the rim, a feature much more evident in the coexisting phlogopite (see below). Clinohumite is a minor, relatively late-crystallized primary phase of carbonatite SV49. It has TiO2 ranging from 2 to 2.57 wt%, which is well within the range of values found in the Jacupiranga carbonatite (Brazil, TiO2 up to 5.96 wt%, Mitchell 1978, Morbidelli et al. 1986, Gaspar 1992). The Mg# of this mineral (95–96) is identical to that of coexisting olivine. Clinopyroxene Clinopyroxene of the Sung Valley intrusion shows the complete range from diopside to aegirine (Fig. 3, Table 3). The clinopyroxene in wehrlite and in clinopyroxenites is diopside, but has marked compositional differences in minor elements such as Ti and Al. In wehrlite, the clinopyroxene is in the range
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Ca50–52Mg40–42Fe6–8, with 1.3–2 wt% TiO2 (0.03–0.05 Ti apfu, based on six atoms of oxygen), 3.6–5.6 wt% Al2O3 (0.15–0.25 Al apfu), and Mg# [atomic Mg*100/ (Mg + Fe) throughout] varying from 84 to 87. This is a typical diopside crystallized from an alkaline melt. The clinopyroxenite SV8 has the most peculiar compositional range. Indeed, clinopyroxene has a variable Ti (0.35–1.91 wt% TiO2, 0.01–0.05 Ti apfu), but high Al (2.6 to 9.8 wt% Al2O3, 0.12–0.44 Al apfu), at Mg# variable from 54 to 63. The other clinopyroxenites, which contain alkali feldspar, have uniformly low-Ti, low-Al clinopyroxene (TiO2 from 0.07 to 0.68 wt%, 0.002–0.02 Ti apfu, and Al2O3 from 0.4 to 1.15 wt%, 0.02–0.05 Al apfu), with Mg# ranging from 62 to 87. In uncompahgrite SV33, the diopside is chemically indistinguishable from that in wehrlite SV19 and has a limited compositional range (TiO2 from 1 to 1.4 wt%, 0.03–0.04 Ti apfu, Al2O3 from 2.6 to 4.4. wt%, 0.12–0.18 Al apfu). The Mg# ranges from 79 to 85.
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Fig. 3. a) Classification of clinopyroxene of Sung Valley rocks. b) and c) Element concentrations in clinopyroxene versus Mg#.
The ijolites contain diopside to increasingly sodic aegirine-augite (Na2O from 1 to 3.24 wt%, 0.07–0.23 Na apfu), with relatively low Al2O3 (1.1 to 3.8 wt%, 0.05–0.17 Al apfu) and variable, but generally low, TiO2 contents (0.14–1.8 wt%, 0.004–0.05 Ti apfu) at
Mg# ranging from 35 to 74. The titanite–garnet-bearing ijolite SV14 has the highest concentrations of Na and Ti; consequently, the data plot in a different field from the other samples of ijolite. The ijolite SV83 has the most Mg-poor clinopyroxene (35 < Mg# < 48), with
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the highest Na and the lowest Ti contents among all the other ijolitic rocks. Clinopyroxene of the nepheline syenites varies from aegirine-augite to aegirine (Na2O from 4 to 13 wt%, 0.35–0.94 Na apfu, CaO from 1 to 16 wt%, 0.05–0.65 Ca apfu, Mg# from 4 to 32). This type of clinopyroxene typically crystallizes from melts that reached peralkaline conditions. The Al2O3 contents are low and almost constant at about 1 wt%, whereas TiO2 is variable (0.2–1.4 wt%, 0.01–0.03 Ti apfu), but does not show any correlation with Na, Mg, Fe or Ca. Spinels and ilmenite Titanian magnetite is the dominant iron oxide of the Sung Valley rocks (Table 4). The compositions are poor in the ulvöspinel component, being not higher
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than 25 mol.%, with the highest values in the peridotitic rocks and the lowest in the nepheline syenites. Spinels of wehrlite and dunite have among the most Mg-rich compositions [MgO from 2.7 to 6.2 wt%, Mg# in the range 19–27, where Mg# is 100Mg/(Mg + Fe2+)]. The lack of Cr-rich compositions in the peridotites (Cr2O3
