Peninsular Gneiss - Geology of The Lalbagh Hill Monument

Geology of The Lalbagh Hill Monument

The Lalbagh hill, which has been declared as a Geological Monument (plaque pictured) to represent the Peninsular Gneiss, has dark biolite gneiss of granitic to granodioritic composition containing streaks of biolite. Remnants of older rocks are seen in the form of enclaves. The research information of the geological record of the gneisses, collated and reported in the publication “Geological Monuments of India” published by the Geological Survey of India, is quoted below to provide an undiluted version.

A common enclave is a dark grey to black rock called amphibolite containing the minerals plagioclase feldspar and hornblende. Due to interactions with granite fluids, these enclaves have developed a border rich in biotite mica. Later magmatic activity represented by grey poryphiritic granite followed by pink massive granite is also seen at several places. The gneisses and granites have been profusely invaded by still younger veins of coarse grained pegmatite which have cut up the gneisses into several irregular and lenticular shapes.

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The earliest rock was a dark coloured amphibolite which was converted into grey biotite gneiss during migmatization. The grey biotite gneiss during migmatization was first intruded by grey porphyritic granite and later by pink granites. Pegamatities of several generations have traversed all these rocks.