A reconnaissance examination, using the 40Ar/ 39Ar laser probe for step-heating analysis of millimeter-size whole-rock chips, representing a variety of Huronian sediments from northern Ontario, has demonstrated several novel and surprising features. Most of the samples show one to three linear segments on the 36Ar/ 40Ar versus 39Ar/ 40Ar correlation diagram, but about half of these have a positive slope. These linear segments represent mixing of argon from distinct components, one of which is always a virtually pure radiogenic component. Mixing of this radiogenic component with unradiogenic argon produces lines with the normal negative slope (isochrons), whereas mixing with a relatively young, high-K component is required to generate lines with a positive slope. Such positive slopes are rare in the literature, and they are termed here as `ambichrons'. All of the x-intercepts of these linear segments indicate significantly younger ages than the deposition age of the sediments (2200–2450 Ma). Intercept ages are recorded: (a) at 1800–1850 Ma, corresponding to the peak of the Penokean orogeny and subsequent cooling; (b) at 1700–1760 Ma, probably representing a previously postulated period of rapid post-Penokean uplift; and (c) at 1480–1570 Ma, representing either further uplift and cooling, or heating in the Killarney orogeny. Despite the relatively mild chemical and textural effects of metamorphism, all pre-depositional (indeed, pre-Penokean) history has been eradicated from the K/Ar systematics. In addition to the isochrons and ambichrons, two of the samples show very significant recoil effects, displayed as horizontal line segments in the correlation diagrams. Even this preliminary study clearly demonstrates that important geo-history is recorded by the K/Ar system in Precambrian sedimentary rocks, and that the laser step-heating approach obviously has much potential for revealing this history.