MAGNETIZATION, REMAGNETIZATION, AND COMPLICATION IN SINGHBHUM CRATON, INDIA

The Singhbhum Craton in northeastern India is among the eldest of the Archean cratons that core Peninsular India. A spectacular array of mafic dykes cuts Archean basement rocks of the Singhbhum, presenting a tempting opportunity for paleomagnetic insights covering a broad swath of geologic time. However, paleomagnetic data from Singhbhum Craton have been severely lacking until recently. Three precisely dated generations of intrusive activity took place in the Singhbhum – ~2760 Ma, ~2250 Ma, and ~1765 Ma – although it is likely that other phases of intrusion will be delineated in future work. Recent work has presented paleomagnetic data from two of the intrusive phases: Kumar et al. (2017) on the ca. 2760 Ma dykes and Shankar et al. (2017) on the ca. 1765 Ma dykes. These studies reported these data to be primary magnetizations. However, our detailed work on mafic dykes of the Singhbhum (including resampling dykes of the aforementioned studies), supported by multiple negative baked contact tests, clearly demonstrates that the published 2765 Ma pole is not primary and that there are issues associated with the younger 1765 Ma pole.

All is not lost vis-à-vis paleomagnetic data from the Singhbhum Craton. We report two primary paleomagnetic pole positions from dykes of the Singhbhum Craton. The first is from 1765 Ma dykes and falls at 38° N, 194° E (A95 =11°; 9 dykes), supported by a baked contact test. The second is from a much more limited dataset; only 5 dykes, however, a baked contact test does suggest it is primary at 9° S, 203° E (A95 =20°). This provides a pole position for the Singhbhum craton prior to 1765 Ma, although the exact age of this magnetization is the subject of ongoing research. Magnetizations from the Singhbhum Craton are complex; multiple demagnetization methods and rock magnetic factors must be taken into account to fully tease out accurate data.