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Calcite and carbocernaite exsolution and cotectic textures in a Sr,REE-rich carbonatite dyke from Rajasthan, India

Published online by Cambridge University Press:  05 July 2018

F. Wall
Affiliation:
Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, U.K.
M. J. Le Bas
Affiliation:
Department of Geology, The University, Leicester LE1 7RH, U.K.
R. K. Srivastava
Affiliation:
Department of Geology, M.L. Sukhadia University, Udaipur 313001, India

Abstract

A carbonatite dyke from the Sarnu-Dandali alkaline complex, Rajasthan, India, contains a remarkable suite of rare earth, strontium-rich minerals with spectacular primary textures.

Sr, Mn-rich calcite in the outer 5 mm of the dyke contains exsolved lamellae of carbocernaite, (Ca,Na)(Sr,Ce,Ba)(CO3)2, orientated parallel to its twin and cleavage planes. The amount of exsolved carbocernaite increases away from the dyke margin as the Sr content of the calcite increases to a maximum 13 wt.%. Sr levels as high as this in calcite have previously been recorded only in experimental work. The carbocernaite exsolution suggests that Sr-rich calcium carbonate can be a host for major amounts of REE in carbonatite magma.

Separated by a sharp internal boundary, is a complex possibly cotectic intergrowth of carbocernaite and Sr-rich calcite with late Ca-rich strontianite (19 wt.% CaO). Other minerals in the dyke include baryte, pyrrhotite, alabandite, sphalerite and occasional bastnäsite-(La) and thorite. Bands of late britholite-(Ce) traverse the dyke.

The host rock for the dyke is fenitized melanephelinite which is itself traversed by narrow, <1 mm, carbonatite veins beleived to predate the carbonatite dyke. Allanite, britholite-(Ce) and rare monazite-(Ce), developed at the boundary between the carbonatite dyke and the fenite, may have been produced by a reaction between the dyke and the wall rock, or may be related to the later britholite mineralisation.

The textures and mineral compositions indicate primary crystallisation. They are unique amongst rare earth-rich carbonatites which are usually late-stage phenomena with signs of secondary alteration.

Comparison with experimental data available for the calcite-strontianite system suggests conditions of 500°C and 2 kbar for coexisting Sr-rich calcite and Ca-rich strontianite. A smaller scale intergrowth of calcite containing only 2.9 wt.% SrO and coexisting Ca-strontianite may correspond to a further unmixing at 350°C and 2 kbar. Since no experimental data are available for a calcite-carbocernaitestrontianite system, mineral chemistries and the interpreted sequence of crystallisation have been used to construct a hypothetical phase diagram.

Type
Mineralogy
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1993

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References

Appleton, J. D., Bland, D. J., Nancarrow, P. H. A., Styles, M. T., Mambwe, S. H., and Zambezi, P. (1992) The occurrence of daqingshanite-(Ce) in the Nkombwa Hill carbonatite. Zambia. Mineral. Mag., 56. 419-22.Google Scholar
Bryan, W. B., Finger, L. W., and Chayes, F. (1969) Estimating proportions in petrographic mixing equations by least-squares approximation. Science. 163. 926-7.Google Scholar
Bulakh, A. G., Kondrat'eva, V. V., and Baranova, E. N. (1961) Carbocernaite. a new rare earth carbonate. [Zapisiki Vses. mineral. Obsch., 90. 42-9 (in Russian)]; Abstr. In New Mineral Names. Am. Mineral., 46. 1202.Google Scholar
Carlson, W. D. (1980) The calcite-aragonite equilib-rium: effects of Sr substitution and anion orlenta-tional disorder. Am. Mineral., 65, 1252–62.Google Scholar
Chandrasekaran, V., Srivastava, R. K., and Chawade, M. P. (1990) Geochemistry of the alkaline rocks of Sarnu-Dandali area. district Barmer. Rajasthan, India. J. Geol. Soc., India., 36, 365–82.Google Scholar
Chang, L. L. Y. and Briee, W. R. (1972) Subsolidus phase relations in aragomte-type carbonatites: II. The systems CaCO3-SrCO3-PbC.O3 and CaCO3-BaCO3-PbCO3. Am. Mineral., 57. 155-68.Google Scholar
Clark, A. M. (1984) Mineralogy of the Rare Earth Elements. In Rare Earth Element Geochemistry, (Henderson, P., ed), Elsevier, Amsterdam, 3361.Google Scholar
Clarke, L. B., Le Bas, M. J., and Spiro, B. (1992) Rare earth, trace element and stable isotope fractionation of carbonatites at Krudfontein, Transvaal, S. Africa. International Kimberlite Conference, 5 Araxd 1991. Proceedings. CPRM Special publication. CPRM, Brasflia (in press).Google Scholar
Exley, R. A. (1980) Microprobe studies of REE-rich accessory minerals: implications for Skye granite petrogenesis and REE mobility in hydrothermal systems. Earth Planet. Sci. Lett., 48, 97110.Google Scholar
Jones, A. P. and Wyllie, P. J. (1986) Solubility of rare earth elements in carbonatite magmas, indicated by the liquidus surface in CaCO3-Ca(OH)2-La(OH)3 at 1 kbar pressure. Appl. Geochem., 1, 95102.Google Scholar
Le Bas, M. J. and Srivastava, R. K. (1989) The mineralogy and geochemistry of the Mundwara carbonatite dykes, Sirothi District, Rajasthan, India. NeuesJahrb. Mineral., Abh., 160, 207–27.Google Scholar
Le Maitre, R. W., Bateman, P., Dudek, A., Keller, J., Lameyre, J., Le Bas, M. J., Sabine, P., Schmid, R., Scrensen, H., Streckeisen, A., Woolley, A. R., and Zanettin, B. (1989) A Classification of Igneous Rocks and Glossary of Terms. Blackwell, London, pp. 193.Google Scholar
Mariano, A. N. (1989) Nature of economic mineralisation in carbonatites and related rocks. In Carbona-tites: genesis and evolution (Bell, K., ed.). Unwin Hyman, London, 149-76.Google Scholar
Nash, W. P. (1972) Apatite chemistry and phosphorous fugacity in a differentiated igneous intrusion. Am. Mineral., 57, 877–86.Google Scholar
Olsen, J. C., Shawe, D. R., Pray, L. C., and Sharp, W. N. (1954) Rare-earth mineral deposits of the Mountain Pass district, San Bernadino County, California. U.S. Geological Survey Professional paper, 261, 75 pp.Google Scholar
Srivastava, R. K. (1977) A comprehensive atomic absorption and spectrophotometric scheme for the determination of major and trace elements in rocks and minerals. Neues Jahrb. Mineral., Abh., 9, 425–32.Google Scholar
Wakita, H., Rey, P., and Schmitt, R. A. (1917) Abundances of the 14 rare earth elements and 12 other elements in Apollo 12 samples: five igneous and one breccia rocks and four soils. Proc. 2nd Lunar Sci. Conf., Geochim. Cosomochim. Acta, Suppl., 2, 1319–29.Google Scholar
Wall, F. (1991) Comparison of element distribution in rare earth rich rocks from the Kangankunde and Nkornbwa Carbonatite Complexes. International Kimberlite Conference, 5 Araxá 1991. Extended abstracts. CPRM Special publication, 2/91. CPRM, Brasflia, 446–7.Google Scholar
Woolley, A. R. and Kempe, D. R. C. (1989) Carbonatires: nomenclature, average chemical compositions, and element distribution. In Carbonatites: genesis and evolution (Bell, K., ed.). Unwin Hyman, London, 114.Google Scholar
Wood, D. A., Joron, J. L., and Treuil, M. (1979) A reappraisal of the use of trace elements to classify and discriminate between magma series erupted in different tectonic settings. Earth Planet. Sci. Lett., 45, 326–36.Google Scholar
Wyltie, P. J. (1989) Origin of carbonatites: evidence from phase equilibria studies. In Carbonatites: genesis and evolution (Bell, K., ed). Unwin Hyman, London, 500-45.Google Scholar