GLOBAL MAFIC MAGMATISM AND CONTINENTAL BREAKUP AT 2.2 GA: EVIDENCE FROM THE DHARWAR CRATON, INDIA

It is now well established that the Early Paleoproterozoic (2.5-2.0 Ga) was a very transitional time in Earth history, and one principal feature currently being recognized is the occurrence of punctuated “flare-ups” of catastrophic mafic magmatic activity, largely recorded by the widespread emplacement of continental dyke swarms at specific times in multiple Archean cratons worldwide. The first unequivocal global mafic magmatic episode of this type occurred at ~2.45 Ga, and was probably associated with the breakup of one or more Late Archean Supercontinents. Fundamental questions remain as to the degree to which now scattered dyke swarm fragments from such events can be aligned to reconstruct Archean/Paleoproterozoic supercontinents, and determining the full extent of these events still awaits precise and accurate dating of a large number of dyke swarms. Here we present a chemical (Electron Microprobe U-Th-total Pb) and isotopic (U-Pb IDTIMS) dating study of mafic dyke swarms from the Dharwar craton (India), incorporating field observations and high resolution Landsat7 imagery. Chemical and isotopic ages of baddeleyite and zircon from 11 mafic dykes from a large expanse of the Dharwar are bracketed between 2.4 and 2.0 Ga. In one study, three IDTIMS analyses of baddeleyite from a ~100m wide E-W trending gabbro dyke near Harohalli yield an upper-intercept age of 2365.5 +/- 1.1 Ma. Because this dyke has been the subject of two independent paleomagnetic studies (Dawson and Hargraves 1994, Precambrian Res 69; Prasad et al., 1999, JGSI 54) yielding similar paleomagnetic poles that are thought to be primary, the data provide key time constraints for paleocontinental reconstructions involving the Dharwar at 2366 Ma. U-Pb baddeleyite and zircon dating of two dolerite dykes sampled ~500 km apart, including a NW-SE trending dyke from the western edge of the Shimoga supracrustal belt and an E-W trending dyke from the NE Dharwar yield identical ages of 2.18 Ga. These dykes may be an expression of a protracted, global episode of mafic magmatic activity and supercontinent breakup at ~2.2 Ga (c.f.: ~2.45 Ga event) that has already been identified in several other cratons worldwide including the Superior, Slave, Churchill, and Nain provinces of North America, the Baltic shield (Finland/Russia), and the Vestfold craton (Antarctica).