Orbicular Structure in Bundelkhand Craton
The Bundelkhand Craton is composed of palaeo‐Proterozoic granitic batholiths with
slivers and narrow belts and rafts of older supra‐crustal rocks which range in age from 2.5 to 3.2
Ga. It is composed of various granitic phases ranging in composition from tonalite‐
trondjhemite‐granodiorite (TTG) suite to granite sensu‐stricto. The grain size is also variable
form fine to coarse grained. These phases are emplaced during early, syn, late to post tectonic
periods. The tectonic signatures are preserved in the form of textural characteristics of these
Orbicular structure in India has been reported from several granitic terrains (Pati, 1996;
Prakash, 1996; Srinivasan, 1995, Srikantia, 1994; etc.. The structure is defined by more or less
spherical masses called orbicules which lie in a granitic groundmass. The centre of the orbicule
is not monomineralic as is the case for rapakivi granite but it is composed of a granitic material.
These large sized orbicular structure are within the
Bundelkhand Granitoid Complex (BGC) exposed in Kutawali village in Shivpuri district, M.P. It
occurs as a near circular outcrop of approximately 30m x 40m size within the coarse grained
granite (CPG). The exposure is a composite of two types of orbicules, one is large sized potash
feldspar rich and another is smaller in size and rich in white feldspar than pink feldspars.
Megascopally the rock is granular, pink coloured biotite granite (similar to the main
granitic phases of the Bundelkhand Gneissic Complex). It consists of orthoclase, perthite,
quartz, oligoclase and some biotite as the major constituent minerals.
The presence of a granitic core within the orbicule suggests that the magma was initially
granitic, as is the case in BGC, but with the advent of crystallisation it was replenished by a melt
rich in the plagioclase feldspar. Then by mere differentiation it became highly rich in potassic
phase. It is therefore, a case of magma mingling.