Petrography and Geochemistry of the Neoarchaean Greywackes from Central Part of the Dharwar-Shimoga Greenstone Belt, Western Dharwar Craton

Greywackes are the most dominant of litho units in the central part of the Dharwar-Shimoga greenstone belt of the Western Dharwar Craton. They are texturally immature with angular to subangular detrital sand grains with 25 to 60% matrix. Quartz forms the main detrital component and is mostly monocrystalline with distinct undulatory extinction. Amongst feldspars, plagioclase feldspar exceeds K-feldspar. Micas occur as large and small flakes that are oriented. Biotite is the main mica mineral. The phyllite, chert, quartzite and felsic volcanic rock fragments are common. The matrix is generally made up of chlorite, sericite, clay to silt-sized quartz and feldspar. In Q-F-L classification diagram, these rocks plot in the fields of quartz wackes and quartzose wackes. The silica content varies between 60.93 and 68.07 wt% and alkalies between 3.69 and 5.21 (Table 2), and compositionally they are comparable to andesite and dacite. Moderately higher values of Fe₂O₃ (av. 8.16 wt%), MgO (av. 7.89 wt%) and K₂O/Na₂O (av. 1.65) indicate a matrix with an abundance of chlorite, biotite, clays and iron oxides. The petrographic and geochemical evidence suggests that these greywackes were derived from the source which had felsic to intermediate composition, with a moderate relief exposed to arid and dry palaeoclimatic conditions. As compared to greywackes of the northern part of the Dharwar-Shimoga greenstone belt, the greywackes of the present study area (central part) have higher values of MgO, Fe₂O₃+MgO, K₂O/Na₂O, Al₂O₃/Na₂O and lower values of Al₂O₃, Na₂O, and SiO₂/MgO.