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

D. Prakash ¹, M. Arima ², A. Mohan ¹

Affiliations
1 Departmeont of Geology, Banaras Hindu University, Varanasi-221005, IN
2 Geological Institute Yokohama National university, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, JP

Abstract

Orthopyroxene-plagioclase symplecti tes have been observed in garnet-bearing mafic granulites from Panrirnalai, South India. Symplectites are arrested textures from which one can study diffusional processes. We have used colour-coded compositional mapping for reaction textures from mafic granulites to understand the element mobility pattern. The variable.composition of anorthite in plagioclase supports the interpretation that the orthopyroxene-plagioclase symplectites adjoining embayed garnet blast were formed by open-system diffusion - controlled reaction in the NCFMAS model system.

Keywords

Mafic granulites, Symplectites, Diffusion, South India.

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S. B. Dwivedi ¹, A. Mohan ¹, R. K. Lal ¹

Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi-221 005, IN

Abstract

Reactions involving garnet, cordierite, aluminosilicate and quartz in FMASH system are calibrated as geothermobarometers, namely 1/2 Fe-crd + 1/3 Pyrope = 1/2 Mg-crd + 1/3 Almandine (1); 2 Almandine + 4 sillimanite/andalusite + 5 quartz = 3 Fe-crd (2 and 4); and 2 Pyrope + 4 sillimanite/andalusite + 5 quartz = 3 Mg-crd (3 and 5).

T = [6170+O.031(P-1)-400(X_Fe-X_Mg)^Crd-A]/(RlnK_D+2.812+B) (1)

A= 166 X²_Mg-506 X²_Fe+680 X_FeX_Mg+336 (X_Ca+X_Ma) (X_Mg-X_Fe)-

X_Ca - 358 X_Mn and B = 1.5 X_Ca

For Fe and Mg end-member reactions involving sillimanite, the following geobarometric formulation is obtained:

P_Fe=1+[{(24.02-6RlnK_Fe)T-17369}/4.003]                                  (2)

P_Mg=1+[{(7.238-6RlnK_Mg)T+19640}/3.82]                                  (3)

Similarly for reactions involving and alusite, P_Fe and P_Mg expressions are : P_Fe=1+[{(26.04-6RlnK_Fe)T-19471}/3.86]                                    (4)

P_Mg = 1+[{(9.25-6RlnK_Mg)T+17536}/3.68)                                  (5)

Keywords

Geothermobarometry, Metapelites, Metamorphism.

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