A WORLD IN A GRAIN OF SAND - REGIONAL METAMORPHIC HISTORY FROM AR-40 AR-39 LASER PROBE ANALYSES OF PROTEROZOIC SEDIMENTS FROM THE CANADIAN SHIELD/

A reconnaissance examination, using the Ar-40/Ar-39 laser probe for st ep-heating analysis of millimeter-size whole-rock chips, representing a variety of Huronian sediments from northern Ontario, has demonstrate d several novel and surprising features. Most of the samples show one to three linear segments on the Ar-36/Ar-40 versus Ar-39/Ar-40 correla tion diagram, but about half of these have a positive slope. These lin ear segments represent mixing of argon from distinct components, one o f which is always a virtually pure radiogenic component. Mixing of thi s radiogenic component with unradiogenic argon produces lines with the normal negative slope (isochrons), whereas mixing with a relatively y oung, high-K component is required to generate lines with a positive s lope. Such positive slopes are rare in the literature, and they are te rmed here as 'ambichrons'. All of the x-intercepts of these linear seg ments indicate significantly younger ages than the deposition age of t he sediments (2200-2450 Ma). Intercept ages are recorded: (a) at 1800- 1850 Ma, corresponding to the peak of the Penokean orogeny and subsequ ent cooling; (b) at 1700-1760 Ma, probably representing a previously p ostulated period of rapid post-Penokean uplift; and (c) at 1480-1570 M a, representing either further uplift and cooling, or heating in the K illarney orogeny. Despite the relatively mild chemical and textural ef fects of metamorphism, all pre-depositional (indeed, pre-Penokean) his tory has been eradicated from the K/Ar systematics. In addition to the isochrons and ambichrons, two of the samples show very significant re coil effects, displayed as horizontal line segments in the correlation diagrams. Even this preliminary study clearly demonstrates that impor tant gee-history is recorded by the K/Ar system in Precambrian sedimen tary rocks, and that the laser step-heating approach obviously has muc h potential for revealing this history. (C) 1998 Elsevier Science B.V.