MIDDLE ARCHEAN AGE OF SARGUR GROUP BY SINGLE GRAIN ZIRCON DATING AND GEOCHEMICAL EVIDENCE FOR THE CLASTIC ORIGIN OF METAQUARTZITE FROM JC PURA GREENSTONE-BELT, KARNATAKA

The middle Archaean J. C. Pura greenstone belt of Sargur Group underli es the Dharwar angular unconformity along the Kibbanahalli arm of Chit radurga schist belt. It consists essentially of periodotitic komatiite (serpentinite), basaltic komatiite (dark amphibolite), rare BIF and m inor metaquartzite. In view of the ongoing debate on the origin of Sar gur quartzite as to whether it is a chemogenic, biogenic or detrital-r ich chert or quartz arenite, the study of J. C. Pura metaquartzite is important as it will have a direct bearing on the depositional setting of the metaquartzite in particular, and the Sargur association in gen eral. In the absence of primary sedimentary structures, accessory zirc ons in the metaquartzite provide the main clue to its origin. Euhedral zircons with minor rounding of pyramidal terminations in the metaquar tzite suggest little mechanical corrosion and negligible sedimentary t ransport. These zircons are interpreted as detrital grains and the hos t rock as siliciclastic in origin. Geochemical parameters like Al2O3 v s TiO2, SiO2 and K2O, trace element spider plots, REE patterns and La/ Sc ratio indicate a significant continental contribution, low Fe and M n content, low TiO2-normalised values and variable Co/Zn and Ni/Zn rat ios preclude major sea water hydrothermal activity. The proximal prove nance indicated by euhedral zircons did not apparently have a signific ant felsic volcanic component since such rocks are neither present wit hin the schist belt nor found as enclaves in the surrounding gneisses. Furthermore, the Sr, Zn, Y, Yb values of the metaquartzite are lower than that of felsic volcanics and pyroclastics, thereby ruling out the ir prevalence in the source area. The most likely source would therefo re be the widespread early Archaean continental crust of southern Indi a made up of granitoids. The metaquartzite was probably deposited as q uartz arenite in shallow marine environment close to the continent. Th e associated pillowed komatiite also lends support to this environment al interpretation. Euhedral zircons from J. C. Pura metaquartzite were dated by single grain evaporation technique which yielded a Pb-Pb age of 3230 +/- 5 Ma. This date representing the maximum age of the Sargu r metaquartzite is fully in accord with previous Sm-Nd model ages and SHRIMP U-Pb zircon ages (ca 3200 Ma) of Sargur rocks from nearby areas . The Sargur Group is pervasively intruded by gneisses and granites ar ound 3000 Ma. Still older gneisses seen in the neighbourhood such as t he Gorur gneiss of 3300 Ma age may constitute a possible basement to t he Sargur supracrustals and represent widespread early Archaean granit ic crust of the Dharwar craton.