Paleomagnetism of Late Archean metavolcanics and metasediments, Abitibi orogen, Canada: tholeiites of the Kinojevis Group

Abstract

Iron-rich and magnesium-rich basaltic tholeiites of the latest Archean Kinojevis Group, central Abitibi Belt, northeastern Ontario, contain a natural remanent magnetization that is generally multivectorial. The components in all units reside in essentially pure magnetite, not the original titanomagnetites (where x for Fe3−xTixO4 probably was 0.60–0.65). Any original TRM was lost by chemical reconstitution of the magnetic oxides in response to primary deuteric conditions, long-term burial, regional metamorphism to prehnite–pumpellyite facies, and possibly intrusion by Matachewan dikes. Data from contact tests with Matachewan dikes indicate that the units are indeed capable of retaining a very latest Archean – earliest Proterozoic field (e.g., D = 194.9°, I = −14.3°, k = 8.1, α95 = 7.9°; n = 45 vectors, 38 samples). High-coercivity, high-blocking-temperature directions from samples from other flows, corrected for nearly penecontemporaneous downwarping, are in only crude agreement with those of Matachewan dikes, possibly suggesting that these components reflect a general Late Archean – Early Proterozoic field for the Superior Province. The paleomagnetic data from Kinojevis tholeiites indicate the emplacement, burial, and tight downwarping of the 10 km or so of Kinojevis stratigraphy were nearly synchronous with Matachewan intrusion. The Kinojevis data by themselves cannot be taken as statistically reliable indicators of the pre- (or immediately post-) Matachewan geomagnetic field nor can temperature–time relations for magnetic viscosity be used to predict the preservation of a statistically reliable TRM in any of these units. Individual magnetization components were blocked over geologically short periods of time, whereas the ensemble of data from discrete flows must record secular variations, field excursions, and possibly long-term polar wander.