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

S. Sinha-Roy ¹, John Mathai ¹, Narayanaswamy ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum, IN

Abstract

Cordierite-bearing gneiss of the Quilon district, Kerala, occurs in a 8-10km wide zone within quartzo-feldspathic gneiss, being bordered on the north by a charnockite massif. These gneisses are deformed and are polymetamorphic in character. The cordierite-forming metamorphism is the second in metamorphic chronology. The reaction producing the bulk of cordierite was: biotite + sillimanite + quartz ⇌ cordierite + garnet + K-fetdspar + H₂O. The K_D (Mg-Fe) values of coexisting garnet-cordierite and garnet-biotite pairs indicate that the PT conditions of metamorphism (∼ 7 kb, ∼ 720°C) were those of granulite facies. Under these conditions cordierite was produced from Fe-Mg enriched melt restite, being trapped at the fringe of the charnockite massif.

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T. Radhakrishna ¹, John Mathai ¹, M. Yoshida ²

Affiliations
1 Centre for Earth Science Studies, P. B. No. 7250, Trivandrum 695031, IN
2 Department of Geosciences, Osaka City University, Osaka 58, JP

Abstract

The Punalur-Achankovil sector is a southwestern segment of the high-grade granulite terrain of South India. It is characterised by charnockite and garnet-biotite. Quartz-feldspar (GBQF) gneiss with minor occurrences of sillimanite and graphitebearing gneiss and basic granulite. Structurally. The terrain has undergone three phases of intense deformation and metamorphism and a late less intense deformation. The first deformation (D₁) has disrupted original depositional layering with development of quartzo-feldspathic bands; the accompanying metamorphism (M₁) is represented by granulite facies sillimanite-bearing gneiss and basic granulite. The second deformation (D₂) resulted in isoclinal folding and the corresponding metamorphism (M₂) produced large neosome now represented by the GBQF gneiss. Lithologies representing the D₃-M₃ event are the charnockite and minor granite bodies developed under high-grade metamorphic conditions; open to tight" asymmetric folds are characteristic of this deformation. The final deformation (D₄) is less penetrative and is characterised by open folds. Structural data from the area do not indicate any major shear zone features nor does the region constitute an intercratonic suture belt as suggested earlier. The topographic expression along Achankovil-Punalur represents only a neotectonic lineament.

Keywords

Structural Geology, Granulites, Punalur-Achankovil, Kerala.

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