Abstract
The Purulia carbonatite of West Bengal, India is a dyke occurring within the 100 km long South Purulia Shear Zone (SPSZ), which marks the boundary between the Singhbhum Group of Rocks and the Chhotanagpur Granite Gneissic Complex (CGGC). It is composed essentially of calcite with apatite, amphibole, phlogopite, biotite, magnetite and ilmenite as common accessories. Based on optical properties and mineral chemistry two varieties of the amphibole are recognized: magnesiokatophorite and richterite. The latter is characterized by a relatively high content of Si and Na, while the former is enriched in Al and Ca. Such a composition of the amphibole is characteristic for the intermediate to the late stage carbonatite development. These two co-existing amphiboles reflect a sudden variation in total pressure within the magma chamber during the intrusion of the carbonatite dyke. It is inferred that the magnesiokatophorite started crystallizing first along with calcite and apatite. Subsequently, the ascent of carbonatitic magma to a more shallow depth (hypabyssal) resulted in the formation of the richterite. The difference in amphibole composition reflects a variation in the total pressure within the magma chamber that took place during the formation of the Purulia carbonatite. However, an alternative explanation, such as wallrock contamination, or liquid immiscibility, followed by carbonate magma segregation or magma mixing, may also be possible. There is, however, no evidence corroborating such an interpretation.
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Acknowledgements
We express sincere thanks to CSIR, New Delhi for funding the project (no. 24(0286)/05/EMR-II). The help rendered by Mr. Sandeep Banerjee (Durgapur Govt. College) during the fieldwork, Head, Earth Sciences and IIC, IIT Roorkee for providing laboratory facilities and Prof. O. P. Varma, and Prof. R. P. Gupta for improving the manuscript is gratefully acknowledged. The manuscript has been substantially improved by the suggestions of the reviewers Dr. Keith Bell and Dr. Frances Wall. We sincerely thank both of them. We also express our sincere gratitude to Prof. L.G. Gwalani for his excellent editorial handling.
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Editorial handling: L.G. Gwalani
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Chakrabarty, A., Sen, A.K. & Ghosh, T.K. Amphibole—a key indicator mineral for petrogenesis of the Purulia carbonatite, West Bengal, India. Miner Petrol 95, 105–112 (2009). https://doi.org/10.1007/s00710-008-0024-3
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DOI: https://doi.org/10.1007/s00710-008-0024-3