Pelitlc Hornfels Xenoliths in the Garnetiferous Granulites and Granitlc Rocks of the Kondavidu Area, Guntur District, Andhra Pradesh

The geological setting, petrography and petrochemistry of the pelitic hornfels xenoliths of the Kondavidu area and the mineral transformations observed in them form the subject of this account. Based on mineralogy, the hornfelses have been classified into six types, and the minerals found in these types are garnet, andalusite, sillimanite, cordierite, corundum, anthophyllite and spinel. The instability of the almandine garnet of the composition Alm₆₉₊₅ Py₂₅₊₉ Sp₂₊₃ And₂₊₃ found in the pelitic hornfels is a dominant feature and it is attributed to the superimposition of thermal metamorpnism over regionally metamorphosed pelitic rocks in which the garnet is a stable mineral.

The mineralogical transformations, especially, the breakdown of almandine garnet to aggregates of cordierite, corundum, spinel and magnetite, and the formation of pseudomorphs of cordierite after garnet are attributed to the Oxidation of the garnet. The formation of fibrolite from cordierite, garnet, anthophyllite, corundum and even from potash felspar is due to metasomatism of alumina-rich solutions associated with the later injection metamorphism.

The chemical data indicate that the hornfelses are pelitic in composition and they belong to the pyroxene hornfels facies of contact metamorphism. The pelitic hornfelses owe their origin to the thermal metamorphism of the regionally metamorphosed pejites by the parent magma of the intermediate charnockites.