Petrogenesis of Cr-rich calc-silicate rocks from the bandihalli supracrustal belt, Archean Dharwar Craton, India

Abstract

Chromium-rich calc-silicate rocks in the Archaean Bandihalli supracrustal belt in the Dharwar Craton, India, show a characteristic mineral assemblage that includes euhedral and anhedral (resorbed) green garnet associated with chromite, in addition to diopside, quartz, clinozoisite and calcite. Two distinct types of green garnets and a brown variety were identified in the calc-silicate rocks. These garnets show complex chemical compositional variations that range between the endmembers of uvarovite, grossular, and spessartine. Chromite grains in the matrix are euhedral or subhedral, whereas inclusions in garnet are rounded or amoeboid in shape. The two textural varieties of chromite from Bandihalli are separated by a compositional gap. Furthermore, chromites from the study area have high Cr# (0.80-0.97) and low Mg# (0.01-0.04), which is similar to the chromite in serpentinites and are in contrast with chromite compositions reported from igneous rocks. Therefore, the chromite in the calc-silicate rocks may have been derived from serpentinized ultramafic rocks. Textural and compositional data presented here provide evidence for chemical changes of the primary chromite with increasing metamorphic grade, presumably associated with primary hydrothermal alteration that resulted in serpentinization and carbonation of the ultramafic protolith. Therefore, Cr-rich calc-silicate rocks can be considered as products of metamorphosed carbonated and serpentinized mafic or ultramafic igneous rocks.