STRUCTURAL GEOMETRY IN A PART OF THE SOUTHERN GRANULITE BELT NEAR UKKARPATTI, DINDIGUL DISTRICT, TAMIL NADU, INDIA

The metamorphic rocks of Southern Granulite Terrain exposed in portions of Dindigul district of Tamil Nadu, India provide an excellent opportunity to study the relation between the deformation as expressed by the extreme complexity of structure and metamorphism in granulite facies. The area forms a part of a large folded supracrustal belt south of the Palghat-Cauvery Shear Zone (PCSZ). It shows a large southwesterly closing major fold, which is defined by the contact between supracrustal rock and the surrounding gneiss. A part of the N-S trending western limb of this fold is exposed in the area of study.

The main rock type in this area is marble with calc-silicate bands. West of the supracrustal rocks is the expanse of biotite gneiss and porphyritic granitic gneiss. Various discontinuous bodies of aplite, pegmatite, khondalite and biotite gneiss occur within marble country. These bodies are probably disrupted bands formed due to intense deformation.

The area has undergone polyphase deformation. Due to the first deformational event the original compositional banding is isoclinally folded and is transposed to the secondary gneissic banding along the axial plane. This gneissic banding is folded by later deformational events. In the western limb the early axial planes have become nearly parallel to the NE-SW trending late axial plane of the Alambadi Fold. It is therefore often difficult to distinguish between the two sets of folds. The distribution pattern of poles of axial planes on stereograms shows that they are all nearly subvertical having strike more or less parallel to the general trend of gneissosity in the area of study. All the isoclinal folds having axial planes more or less parallel to the overall trend of the gneissosity are clubbed together as earlier folds and the Alambadi Fold is designated as late fold. Minor folds of both generations are polyclinal in nature. The folds are in general steeply plunging and different generations of fold axes are more or less parallel. These folds are of plane noncylindrical type. Besides these two sets there are a few E-W trending axial planes, possibly related to a phase of deformation later than the Alambadi Folding. East of the study area is the complexly folded hinge zone and the E-W trending eastern limb of the Alambadi Fold, which was beyond the scope of my study.