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

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Petrology of a Scapolite-Bearing Rock From Karera, District Bhilwara, Rajasthan


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1 Department of Geology, Banaras Hindu University, Varanasi 221 005, India
     

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The scapolite-garnet-epidote-hornblende-pyroxene-calcite ± plagioclase rock of the Aravalli Group (Early Precambrian) is found as thin bands within the high-grade paragneisses of the underlying Banded Gneissic Complex around Karera in northwest peninsular India. The mineralogy and chemical composition of the rock imply that it is derived from impure calcareous sediments. Significant amounts of chlorine are present in both scapolite and hornblende. In epidote and garnet ferric iron is an important component and its presence together with hematite indicates high fo2 during recrystallization of the rock. The partitioning data of Na/(Na+Ca) between scapolite (Me71.3) and the coexisting plagioclase (An85-90) and its reference to the experimentally based diagram yield recrystallization temperature of 650°-700° C, conforming to those computed from other mineral equilibria of the enclosing paragneisses. The scapolite is considered to develop as a result of extreme diminution of PH20, presumably by incoming of chlorine from an evaporite component.
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  • Petrology of a Scapolite-Bearing Rock From Karera, District Bhilwara, Rajasthan

Abstract Views: 175  |  PDF Views: 2

Authors

Ram S. Sharma
Department of Geology, Banaras Hindu University, Varanasi 221 005, India

Abstract


The scapolite-garnet-epidote-hornblende-pyroxene-calcite ± plagioclase rock of the Aravalli Group (Early Precambrian) is found as thin bands within the high-grade paragneisses of the underlying Banded Gneissic Complex around Karera in northwest peninsular India. The mineralogy and chemical composition of the rock imply that it is derived from impure calcareous sediments. Significant amounts of chlorine are present in both scapolite and hornblende. In epidote and garnet ferric iron is an important component and its presence together with hematite indicates high fo2 during recrystallization of the rock. The partitioning data of Na/(Na+Ca) between scapolite (Me71.3) and the coexisting plagioclase (An85-90) and its reference to the experimentally based diagram yield recrystallization temperature of 650°-700° C, conforming to those computed from other mineral equilibria of the enclosing paragneisses. The scapolite is considered to develop as a result of extreme diminution of PH20, presumably by incoming of chlorine from an evaporite component.