| Two problems of Precambrian paleomagnetism, namely the scarcity of well-dated paleopoles for this geological era, and the aggravating effects of retrograde hydrothermal alteration on the rocks from which the scanty paleomagnetic record available derives, are evaluated by the study of two specific test-cases in the northcentral and northwestern Superior Province.;For the southern lobe of Kapuskasing Structural Zone (KSZ) (Chapleau Block), a steep up direction of remanence (component B) had previously been isolated in the Archean Shawmere anorthosite. As part of a test of the first hypothesis, the same direction of remanence (B1) was isolated in both normal and reversed polarities (B1N: D = 308;Along with B1N and B1R, component B2N (D = 68;Granulometric, rock magnetic and petrographic analyses of the Shawmere anorthosite suggest that these TCRMs originated by growth and crystallization of secondary magnetite as a consequence of extensive hydrothermal alteration. This alteration is hypothesized to have taken place along a carbonatite trend parallel to the Ivanhoe Lake Cataclastic zone (ILCZ) connecting the alkaline complexes in the Chapleau block. These analyses also allow a preliminary mapping of the extent of hydrothermal alteration in the Shawmere anorthosite away from the carbonatite trend. (Abstract shortened by UMI.);For the Red Lake greenstone belt, natural remanent magnetization (NRM) component RLR (D = 191... |
