40Ar–39Ar dating of multicomponent magnetizations in the Archean Shelley Lake granite, northwestern Ontario

Abstract

The Shelley Lake granite yields two paleopoles which could represent either of two scenarios: both poles were acquired early, SL2 at 2.7 Ga and SL1 during metamorphic overprinting near 2.6 Ga; pole SL1 was acquired first on cooling at 2.6 Ga while the SL2 antipole represents a late Proterozoic remagnetization. We chose biotite and K-feldspars from the available cores — biotite to record high temperature events and K-feldspars to detect any weak late Proterozoic reheating event. The biotite 40Ar/39Ar age spectrum is flat for 80% of the released Ar, indicating closure to Ar loss at 300 °C 2.60 Ga ago. The two feldspars show coincident minima in their age spectra at ca. 1.1 Ga, indicating a low-level heating event near that time with temperatures in the interval 150–300 °C and a duration less than ca. 1 Ma. Diffusion calculations show that the biotite could have been totally outgassed during the Kenoran orogeny and so we are unable to decide unambiguously between the two paleomagnetic scenarios. Nevertheless, we have detected a weak late Proterozoic thermal event that would otherwise have escaped notice and have thereby demonstrated the power of this dating technique when applied to K-feldspars from polymetamorphic terranes.