Iron Formations in Archaean Granulite-Gneiss Belts With Special Reference to Southern India

In Archaean granulite-gneiss belts there are banded iron formations (BIF) less than 20m thick which are weakly banded magnetite quartzites occurring in a supracrustal sequence of quartzites, mica schists, marbles and metavolcanic amphibolites. This sequence forms stratigraphic markers within voluminous intrusive tonalitic to trondbjemitic gneisses. These rocks are highly folded and metamorphosed to high amphibolite or granulite facies at 7-12 kb pressure in the lower Archaean crust. Such iron formations are well represented in Tamil Nadu, Southern Karnataka, Kerala and Andhra Pradesh in South India; in Hebei Province in China; Kola Peninsula, Stanovoy Range, and Ukraine in USSR; Kambui schists of Sierra Leone; Nimba series of Liberia; Limpopo belt of southern Africa; Imataca complex of Venezuela; Uivak gneisses of Labrador; and Amitsoq gneisses of West Greenland. They are obviously different in type and environment from the Proterozoic Superior type of BIF; it is shown here how they are different from the Archaean Algoma type of BIF. They were deposited in a quartzite-pelite-carbonate association and they lack the silicate-carbopate-sulphide facies of the Algoma-type which, in contrast, was deposited with a greywacke-flyschconglomerate- shale association in subsiding basins (Greenstone belts). Because they are dissimilar to the Algoma-type of BIF in both their sedimentary facies and tectonic environment (the granulite-gneiss belts are not highly metamorphosed greenstone belts), they should be given a separate status and we tentatively suggest they be termed the Tamil Nadu type.