Enigmatic association of the carbonatite and alkali-pyroxenite along the Northern Shear Zone, Purulia, West Bengal: A saga of primary magmatic carbonatite |
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| Authors: | Aniket Chakrabarty and Amit Kumar Sen |
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| Institution: | (1) Department of Earth Sciences, IIT Roorkee, Roorkee, 247667, India;(2) Institute Instrumentation Centre, IIT Roorkee, Roorkee, 247667, India;(3) Present address: R&D Division, Tata Steel Ltd, Jamshedpur, India |
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| 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. |
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