M. Fukuoka

Mafic Granulites from the Eastern Ghats, India: Further Evidence for Extremely High Temperature Crustal Metamorphism

Mafic granulites occurring as conformable lenses within khondalite, leptynite, and calc-silicate rocks from the Anantagiri-Araku areas of the Eastern Ghats belt contain orthopyroxene, clinopyroxene, garnet, quartz, ilmenite, and plagioclase as major constituents. Orthopyroxene contains (100) lamellae of clinopyroxene, and clinopyroxene contains (100) orthopyroxene lamellae and (001) spindles of pigeonite. Reaction textures indicate formation of garnet through coupling of two reactions, viz. orthopyroxene + anorthite = garnet + clinopyroxene + quartz and anorthite + ilmenite -

Geochemistry of braunite and associated phases in metamorphosed non-calcareous manganese ores of India

hematite_{ss}

Atypical ferromanganese nodules from pelagic areas of the Central Indian Basin, equatorial Indian Ocean

+ quartz = garnet + ilmenite +

Oxidation gradients in metamorphosed non-carbonatic manganese formations

O_{2}

Petrology of granulites from Anakapalle-evidence for Proterozoic decompression in the Eastern Ghats, India

. Thermobarometry indicates exsolution of pigeonite at ~950°C, of (100) clinopyroxene in orthopyroxene and vice versa at 750-820°C and formation of garnet at around 700-725°C, 7 kb, all in response to cooling of the rock. This shows a nearly isobaric cooling path subsequent to peak metamorphic conditions. Subsequently, the rock suffered nearly isothermal decompression to 5 kb. Hydration and K-metasomatism of the assemblage occurred at lower temperatures (~500°C) in the final phases. The structural characteristics and thermal history of the mafic granulites suggest that the mafic rocks were emplaced during the culmination of prograde metamorphism accompanying a compressive orogeny. Mafic magmas could have contributed significantly to enhance the thermal budget for granulite facies metamorphism. This study, therefore, attests to the very high metamorphic temperatures (as high as 1000°C regionally) attained by the Eastern Ghats granulites.

Mineral chemistry and reaction textures in metabasites from the Eastern Ghats belt, India and their implications

ABSTRACT Chemical characteristics of glauconite and ferric illite derived from alteration of K-feldspar in three Proterozoic formations of India are studied. The phases show widely varying octahedral and tetrahedral cation contents, but K-content is nearly constant. Co-existence of glauconite and ferric illite is interpreted as due to influx of fresh water in a marine-dominated sequence. A continuum in composition between illitic and glauconitic minerals at high alkali content is established. No correlation between composition of glauconite and environment of deposition or degree of diagenesis is deduced. Ferric illite in fluvial environment contains less Mg than that in a marine-dominated one. Direct pseudomorphism of K-feldspars by glauconite and ferric illite in their respective chemical en ironments controlled largely by the availability of Fe and Mg is indicated in this study. Chemical characteristics of glauconite and ferric illite in an environment with high aK+ induced by dissolution of K-feldspar are markedly different from those described earlier.