The Chitradurga Schist Belt and its Adjacent Plutonic Rocks, NW of Thngabhadra, Karnataka: a Duplex in the Late Archaean Convergent Setting of the Dharwar Craton

The Late Archaean Dharwar Supergroup in the Chitradurga schist belt NW of Tungabhadra river is dominated by low-grade metabasalts, polymict conglomerates, ferruginous cherts and greywackes with intercalations of dolomitic limestone and felsic volcanic rocks. The belt was thickened as the result of overturning on large-scale folds and stacking on reverse faults in a hinterland-dipping duplex, herein called the Gadag duplex. Gold mineralisation is confined to, and was controlled by, the duplex structure. In the Late Archaean oblique convergent setting of the Dharwar craton, the duplex dips NE away from the foreland formed by the western part of the craton and into the accretionary complex of granites (Dharwar batholith) and intra-arc schist belts comprising the eastern part.

The Gadag duplex comprises four thrust slices. Its foot wall block comprises metabasalts and greywackes of the Dharwar Supergroup and underlying, variably mylonitised granites s.l. with magmatic-state and mylonite fabrics indicating top-NW shear sense. The supergroup in the foot wall block is arched over domes of these granites to link with the lowest metabasalts and greywackes in the eastern margin of the Ranibennur schist belt. Lit-par-lit injections in the metabasalts suggest the plutonic rocks are younger than the Dharwar Supergroup, i.e. they are probably Late Archaean, not pre-2900 Ma basement to the schist belt. Arching of the foot wall block is attributed to NE-SW shortening broadly contemporaneous with, or soon after, thrust stacking in the Gadag duplex.

The hanging wall block comprises mylonitised granites s.l. with top-SW shear sense indicators. The roof thrust south of Gadag dips gently NE, but steepens further SE. Lit-par-Ut injections in metabasalts adjacent to the roof thrust indicate a tectoni sed intrusive contact like that in the foot wall block. Large-scale gentle folding of the shallow part of the roof thrust is linked to sinistral displacement along the steepened thrust. Plutonic rocks in the hanging and foot wall blocks are tentatively correlated with granites in the western margin of the Dharwar batholith.