Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Amit Kumar Sen ¹

Affiliations
1 Department of Earth Sciences, University of Roorkee, Roorkee-247667, U.P., India, IN

Abstract

The Sung Valley carbonatite complex is situated in the Shillong Plateau, Meghalaya and includes an array of silicate rocks along with sovitic carbonatite. The silicate rocks are represented by serpentinized peridotite, pyroxenite, melilitolite, ijolite and nepheline syenite and occupy more than 99% of the total outcrop. Variation in major element composition divides the silicate rocks into two distinct groups: i) alumina-alkali-poor, represented by peridotite, pyroxenite and melilitolite and ii) alumina-alkali-rich consisting of ijolite and nepheline syenite. Perceptible variation in chemical composition can also be noted among the individual group members. The carbonatite is dominantly CaCO₃-rich and poor in alkalies. Present study suggests that the parent magma in the Sung Valley complex was of melilitic composition. In the initial stages this magma produced peridotite and pyroxenite by fractional crystallization. With progressive crystallization the parent magma gradually became enriched in Ca and generated carbonatitic melt by liquid immiscibility. The residual silicate melt, after separation of immiscible carbonate melt, produced ijolite and nepheline syenite by fractional crystallization.

Keywords

Petrogenesis, Carbonatite, Alkaline Rocks, Geochemistry, Meghalaya.

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Aniket Chakrabarty ¹, Kamal L. Pruseth ², Amit Kumar Sen ¹

Affiliations
1 Department of Earth Sciences, IIT Roorkee, Roorkee - 247 667, IN
2 Institute Instrumentation Centre, IIT Roorkee, Roorkee - 247 667, IN

Abstract

Eudialyte is a group of hydrated silicate minerals essentially consisting of Na and Zr with a very complex crystal structure, and generally associated with alkaline rocks. The complexity of the eudialyte structure can be understood from the fact that Na alone exist in five distinct sites and extensive solid solubility can occur in almost all cation sites, sometimes one element occupying multiple sites to the extent of exclusion of other elements. Structurally, eudialyte can be represented as Na₁₅[M₁]₆[M₂]₃Zr₃[M₃](Si₂₅O₇₃)(O,OH,H₂O)₃X₂ (Johnsen and Grice, 1999), where M₁ and M₂ sites are occupied by Ca, Mn and Fe, M₃ by Nb and X by OH, Cl and F. In addition, cations like Al, Hf, W, Ta, Sr, Ba and various REEs get incorporated into the eudialyte structure by substitution, and additional site vacancies even may develop in order to maintain electrical neutrality. Eudialyte, approximately of the composition Na₉Ca₈Mn₃Nb(Zr,Ce)₃Si₂₅O₇₃(OH)₂, has hydrothermally replaced albite in the nepheline syenite gneiss exposed south of the Sushina hill of Purulia district, West Bengal. The eudialyte contains ≈2.25 atom% Zr and 0.75 atom% Nb. In addition to eudialyte in nepheline syenite, an unknown Na-Zr silicate (NZS) has also replaced the albite crystals. The NZS contains ≈ 7 atom% Zr with a possible empirical formula of Na₁₂Zr₁₁Si₃₆O₉₅(OH)₁₀. Surface exposures of these rocks are limited at Sushina hill, yet a detailed and systematic investigation on this enigmatic rock is warranted for they may turn out to be a resource for Zr.

Keywords

Eudialyte, Na-Zr Silicate (NZS), Agpaitic Nepheline Syenite, Sushina Hill.

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Aniket Chakrabarty ¹, Amit Kumar Sen ²

Affiliations
1 Department of Geology, Durgapur Government College, Durgapur – 713 214, IN
2 Department of Earth Sciences, IIT Roorkee, Roorkee - 247 667, IN

Abstract

The Purulia carbonatite, 'carbonatite'-'alkali-pyroxenite'-'apatite-magnetite rock' association, is located at Beldih area of Purulia district, West Bengal and falls within the 100 km long Northern Shear Zone (NSZ). Published literature suggests that the Purulia carbonatite was formed by the process of liquid immiscibility from under-saturated silicate parent magma. However, no silica under-saturated rocks like ijolite, nepheline-syenite etc. is known from the area. The trace element geochemistry (Ba/La, Nb/Th, Nb/Pb and Y/Ce ratios in the present study) also does not support this view. Present study indicates that the Purulia carbonatite is enriched in ΣREE and incompatible elements but the carbonatite is also poorer in Nb, Th and Pb compared to the world average of calicocarbonatites. The lower value of Nb is characteristics of carbo(hydro) thermal carbonatite where carbonatite is associated with alkali-pyroxenite and suggests probable origin of the carbonatite as carbothermal residua evolved from an unknown parentage. However, the field, petrographic and geochemical data indicate the genesis of this carbonatite from a primary carbonatitic magma of mantle decent. The ⁸⁷Sr/⁸⁶Sr ratio of the carbonatite and apatite separated from the carbonatite (∼0.703) implies primary magmatic derivation of the Purulia carbonatite. Close similarity of the apatite of the apatite-magnetite rock with the mantle apatite (of type Apatite B) indicates that they are also of primary magmatic origin. The present work portrays a unique example where primary magmatic carbonatite is associated with the alkali-pyroxenite.

Keywords

Carbonatite, Pyroxenite, Carbothermal Residua, Mantle Apatites, Liquid Immiscibility, Northern Shear Zone.

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Aniket Chakrabarty ¹, Kamal L. Pruseth ², Amit Kumar Sen ³

Affiliations
1 Department of Geology, Durgapur Government College, Durgapur - 713 214, IN
2 Department of Geology and Geophysics, IIT Kharagpur, Kharagpur - 721 302, IN
3 Department of Earth Sciences, IIT Roorkee, Roorkee - 247 667, IN

Abstract

The eudialyte-group of minerals (EGM) is one of the most important index minerals of the peralkaline (agpaitic) nepheline syenites. They crystallize in varied physico-chemical conditions ranging from the early-magmatic (orthomagmatic) to late-magmatic and even in the post-magmatic (hydrothermal) stage. In India, the only agpaitic nepheline syenite gneisses of the Sushina Hill region contain both late-magmatic as well as hydrothermal eudialytes. Compositionally these are Mn-Nb-Ca rich eudialytes and are comparable to the other EGM occurrences such as Ilímaussaq (Greenland), Tamazeght (Morocco), Mont-Saint Hilaire (Canada) and Pilansberg (South Africa). High Mn content (>6.5 wt.%) for both varieties of the Sushina EGM indicates that they are highly evolved in nature. In terms of the calculated site occupancy, particularly the [M(3)] and [M(2)], the Sushina eudialytes mimic some Pilansberg eudialytes. In addition to the eudialyte, the host nepheline syenite gneiss also contains an unknown Na-Zr-silicate (NZS) which is often found to be replacing both types of eudialytes. Compositionally these NZS can be tentatively represented as Na₂Zr₂S₆O₁₇. These NZS are characterized by much higher Zr, but lower Mn and Nb concentrations compared to the associated eudialytes. Two distinct varieties of eudialyte and NZS indicate subtle changes in the alkalinity during their formations. The formation of the late-magmatic as well as hydrothermal eudialyte essentially took place at somewhat elevated pH conditions. The replacement or alteration of eudialytes by NZS indicates a decreasing pH condition. In terms of the chemical composition the late-magmatic eudialytes can be represented as a solid-solution series between the kentbrooksite-taseqite-aqualite while the hydrothermal eudialyte represents solid-solution between kentbrooksitetaseqite-Ce-zirsilite.

Keywords

Eudialyte-Group of Minerals (EGM), Na-Zr Silicates (NZS), Agpaitic Systems, Late-Magmatic, Sushina Hill.

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