FABRIC EVOLUTION AND INTRUSION SEQUENCE WITHIN GABBROS OF THE KORAPUT ALKALINE COMPLEX, INDIA

The Koraput alkali igneous complex is located within the Western Khondalite Zone of the Eastern Ghats Belt in India. The gabbro body is strongly foliated in the marginal regions, but the central part generally retains igneous textures. Present study reveals major strain partitioning took place during both deformation events, with the central part of the gabbro being relatively undeformed. In some outcrops, the foliations subtend angles of more than 45°, implying that the two fabrics cannot be regarded as simultaneously developed S- and C- planes. Petrographic study indicates that amphibole needles underwent intracrystalline deformation, with aspect ratios greater than one. Crystal deformation was cataclastic, accomplished by microfaulting on planes parallel to cleavage traces. The gabbros of the alkaline complex are intruded by several phases of dykes. Cross-cutting relationships observed and documented during detailed mapping in three separate zones reveal that the syenites dykes were followed by mafic intrusives, and finally by pegmatitic granitic rocks. Mafic dykes are metamorphosed in the amphibolite facies, like the last phase of dykes in the country rocks. It appears that the alkaline complex intruded after peak granulite metamorphism in the country rocks, but underwent later amphibolite facies metamorphism. The evidence for a major metamorphic event after the intrusion of the gabbro, and the subsequent sequence of intrusions, it appears unlikely that oversaturated granitic melts represented by the pegmatites were comagmatic with, and later derivatives of the initial undersaturated gabbroic magma. They are more likely to represent later, unconnected magmatic activity.