Stratigraphic Status of Javanahalli Belt in the Archaean Geology of Karnataka

Javanahalli belt situated at the eastern margin of Chitradurga superbelt is either juxtaposed against the latter or separated from it by linear stripe of granite. The rocks of Chitradurga and Javanahalli belts and the associated enclaves are part of the Chitradurga Group. The primary lithologies of both the belts as well as the enclaves are comparable despite differences in metamorphic grade. These are ortho- and para-amphibolites, calc-silicate rocks, limestones, fuchsite quartzites, mica schists, cordierite-anthophyllite rocks and banded iron formation which is locally manganiferous. The meta-arkose/paragneiss, is mylonitic, potash-rich ortho-gneiss, and constitutes the deformed and partly remobilised basement. Preliminary structural studies have shown that Javanahalli belt is affected by early recumbent folds (F₁) followed by the major second phase of nearly co-axial upright N-S folds (F₂) which control the present map pattern. The ENE trending third phase open folds (F₃) have caused local fold interference patterns. There is a progressive increase in metamorphism from west to east towards the gneissic contact, ranging from greenschist facies in Chitradurga belt to middle amphibolite facies in Javanahalli belt and enclaves, culminating in extensive granitic veining and migmatisation. Javanahalli belt represents a classic case of wide-spread basement reactivation associated with a boundary thrust in an ideal, ensialic, Archaean orogen.