METAMORPHIC CONDITIONS lN THE HIGH-GRADE TERRAIN OF SOUTH lNDIA: DEDUCED FROM SAPPHIRINE-BEARING GRANULlTES FRüM KIRANUR, KARUR (TAMIL NADU)

D. ACKERMAND, R. K. LAL, P. RAASE, M. RAITH

Resumo


Sapphirine-cordierite-gedrite-sillimanite (± corundum, spinel, phlogopite, plagioclase)-bearing granulites occur as enclaves in the Peninsular gneisses of the granulite fades belt of the South Indian Shield. These enclaves at Kiranur (10°47' N, 78°17' E) belong to the Satymangalam Group which is considered to be homotaxial with theSargur Group (2.9 to 3.5 b.y.) representing the oldest Precambrian supracrustal unit exposed in the Shield. Gedrite and sillimanite, although they both occur in the same thin section, are texturally incompatible and are separated by a reaction zone consisting of cordierite or cordierite + sapphirine ± spinel intergrowth (sometimes with plagioclase). Corundum occurs as idioblasts with cordierite in this reaction zone. The textural relations and the plots of the microprobe data of the coexisting minerais in the system FeO-MgO-Al203-Si02-(H20) suggest reaction sequences: (1) Gedrite + Sillimanite + Quartz → Cordierite, (5) Gedrite + Sillimanite → Cordierite + Sapphirine + Spinel, (8) Sapphirine + Sillimanite → Cordierite + Corundum + Spinel. A petrogenetic P-T grid based on the data of the pertaining mineral equilibria which were determined experimentally and calculated thermodynamically is proposed for the system mentioned above. It is proposed that the reaction sequences given above occurred isothermally (7500 ± 50°C) with fall in pressure (from 9 to 7 kb). The upper pressure limit proposed on the basis of reaction (1) arid the temperature estimated are consistent with lhe evaluated P-T conditions for the peak of granulite fades metamorphism in the South Indian Shield (730-800°C/6.6-9.5 Kb). The sapphrrine-bearing granulites have presumably been formed according to reaction (5) at P-T conditions estimated to be 750 ± 50°C / 7.5 ± 1 kb. The coexisting sapphirine-sillimanite-cordierite-bearing assemblage within the stability field of gedrite is favoured by Mg ⇋ Fe substitution, reduced activity of H2O (xH20 ~0.5) and ubiquitous presence of Na2O in these rocks. The latter also explains the absence of kornerupine in these rocks.


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