Orbicular Structures from Lingsugur Area, Raichur District, Karnataka

Well developed single shell orbicules and proto-orbicules with hornblende as the nucleus and plagioclase as the shell, are well preserved in a 30 m x 11 m outcrop of diorite at jalibenchi, Lingsugur area, Raichur district. It could also be described as esboitic as the feldspar is oligoclase. The normative anorthite content in the orbicule varies from 26.13% to 33.64%. The solidification indices are low suggesting a slow but steady rate of cooling. The chemical and mineralogical discriminant diagrams show that the orbicular rocks are tonalitic to mafic tonalitic in composition. The two units of the orbicular structure viz., (1) the core made up of hornblende is attributed to have been derived from an enclave of amphibolite which was invaded and shattered by an upwelling hydrous pegmatitic plagioclase vein and gel-like in nature and acted as the cause for the formation of the (2) single shell structure. The monomineralic composition of the shells of orbicules and the unicomponent composition of the plagioclase vein are thus genetically explained.