Petrography and Geochemistry of High Grade Rocks from the Eastern Madurai Domain of the Granulite Terrane of Southern India

Padmaja, J. (2018) Petrography and Geochemistry of High Grade Rocks from the Eastern Madurai Domain of the Granulite Terrane of Southern India. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Andipatti and Usilampatti regions in the western part of Madurai City of Tamil Nadu, which lie in the central part of Eastern Madurai Domain of Granulite Terrane of Southern India, is dominated by metaigneous rocks intercalated with metapelites, of which the tectonometamorphic evolution is still not well studied. The garnet + orthopyroxene + cordierite + biotite + sillimanite + spinel bearing metapelitic rocks in the Andipatti region mark three stages of mineral growth. The garnet + orthopyroxene assemblage preserves the peak metamorphic conditions (T=750-850ºC and P=7-8 kbar) and the garnet + retrograde biotite assemblage preserves the retrograde conditions (T=550-650ºC and P=5-6 kbar). The P-T conditions of different stages and the garnet + retrograde assemblage indicates that these rocks have cooled from a relatively high pressure conditions. Texturally controlled in-situ monazite dating in the metapelites yields two ages of metamorphism. The cores of the grains, which are darker in BSE images and have less Th content, yield an age of 594±8 Ma and the rim of the grains, which are brighter in BSE images and have higher Th content, yield an age of 528±15 Ma. The correlation of these two age clusters with the chemical and physical zones in some of the grains strengthens the distinction between the two clusters. The compositions of the metaigneous rocks in both the Andipatti and Usilampatti range from mafic granulites to enderbites to charnockites SiO₂ content ranging from 49.78 to 71.23 wt.%. They are subalkaline, ferroan, alkali-calcic to calc-alkalic and metaluminous. They have highly enriched light rare earth elements (LREE) and large ion lithophile elements (LILE) with less enriched heavy rare earth elements (HREE) and high field strength elements (HFSE) with consistent depletions of Nb, Ta and Ti. Some of the rocks have high Sr/Y and La/YbCN ratios, which are typical characters of adakites. These characters indicate that these rocks are formed in an arc setting. Therefore, this work suggests that the metapelitic rocks have undergone two metamorphic events at 594 and 528 Ma which coincides with the formation of Gondwana and the protoliths of the metaigneous rocks were the product of calc-alkaline subduction related magmatism. But the age of emplacement of these rocks are still unknown, due to which this arc setting cannot be correlated to any particular subduction event as of now.

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