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

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Geology and Geochemistry of Anorthosites of Karnataka Craton and their Tectonic Significance


Affiliations
1 17, Raja Mahal Vilas Extension, Bangalore-560006, India
2 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, India
     

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Anorthosites of Karnataka occur in two distinct tectonic milieu, within a major arcuate zone, namely, (i) as minor differentiates of gabbro in the ultramafic-mafic complexes emplaced in Archaean high-grade supracrustal belts and the 'Dharwar type' greenstone belt of Shimoga; and (ii) as layered concordant sheets, and as cumulates in a layered basic pluton occurring within the gneissic complex of Archaean high grade terrain. Their absence in 'Keewatin type' greenstone belts and the other  'Dharwar type' belts of Bababudan and Chittadurga is probably due to their location away from the arcuate zone in Karnataka Craton. This arcuate zone reflects a progressive vertical zonation of the crust to deeper levels from northwest to southeast.

The anorthositic rocks of mafic-ultramafic complexes are nonlayered and extensively deformed, whereas those of the high grade terrain show layering and cumulus textures. In the transitional zone, the anorthositic rocks of mafic-ultramafic complexes show cumulus textures, thereby indicating a genetic connection between the anorthosites of two tectonic milieu.

Whole-rock chemistry of the anorthosites of Karnataka indicates differentiation trend of iron-poor environment comparable to other Archaean anorthosites. They are enriched in CaO and A12O3 Niggli mg against Cr or Ni shows positive correlation, corresponding to an igneous parentage for the original rock. Similarly, the plot of Niggli c against mg, c against (al-alk) and 100 mg-c-(al-alk) diagrams exhibit a well defined igneous trend. A common differentiation trend is shown by anorthosites of Karnataka, even though they occur in two distinct tectonic milieu. The singularised trends support the view that the ultramafic-mafic complexes are possibly derived by the dismembering of the basal parts of the layered complexes. The Archaean anorthosites of Karnataka, although comparable broadly to lunar anorthosites, are not related to the early history of the Earth-Moon system.


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  • Geology and Geochemistry of Anorthosites of Karnataka Craton and their Tectonic Significance

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Authors

M. Ramakrishnan
17, Raja Mahal Vilas Extension, Bangalore-560006, India
M. N. Viswanatha
17, Raja Mahal Vilas Extension, Bangalore-560006, India
N. Chayapathi
17, Raja Mahal Vilas Extension, Bangalore-560006, India
T. R. Narayanan Kutty
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, India

Abstract


Anorthosites of Karnataka occur in two distinct tectonic milieu, within a major arcuate zone, namely, (i) as minor differentiates of gabbro in the ultramafic-mafic complexes emplaced in Archaean high-grade supracrustal belts and the 'Dharwar type' greenstone belt of Shimoga; and (ii) as layered concordant sheets, and as cumulates in a layered basic pluton occurring within the gneissic complex of Archaean high grade terrain. Their absence in 'Keewatin type' greenstone belts and the other  'Dharwar type' belts of Bababudan and Chittadurga is probably due to their location away from the arcuate zone in Karnataka Craton. This arcuate zone reflects a progressive vertical zonation of the crust to deeper levels from northwest to southeast.

The anorthositic rocks of mafic-ultramafic complexes are nonlayered and extensively deformed, whereas those of the high grade terrain show layering and cumulus textures. In the transitional zone, the anorthositic rocks of mafic-ultramafic complexes show cumulus textures, thereby indicating a genetic connection between the anorthosites of two tectonic milieu.

Whole-rock chemistry of the anorthosites of Karnataka indicates differentiation trend of iron-poor environment comparable to other Archaean anorthosites. They are enriched in CaO and A12O3 Niggli mg against Cr or Ni shows positive correlation, corresponding to an igneous parentage for the original rock. Similarly, the plot of Niggli c against mg, c against (al-alk) and 100 mg-c-(al-alk) diagrams exhibit a well defined igneous trend. A common differentiation trend is shown by anorthosites of Karnataka, even though they occur in two distinct tectonic milieu. The singularised trends support the view that the ultramafic-mafic complexes are possibly derived by the dismembering of the basal parts of the layered complexes. The Archaean anorthosites of Karnataka, although comparable broadly to lunar anorthosites, are not related to the early history of the Earth-Moon system.