Early Proterozoic geomagnetic field in western Laurentia: implications for paleolatitudes, local rotations and stratigraphy

Abstract

Abstract The east-west trending, post-orogenic subvertical Cleaver Dykes from the Great Bear Magmatic Arc are dated by the U–Pb (baddeleyite) method to be 1740 +5/−4 Ma. They have a mean direction of magnetization of D, I=136.5 and 57.4°, k=64, α95=4.5°, based on samples from 17 diabase dykes, a mean location 65.7°N, 118.03°W, and a corresponding paleopole at 19.4°N, 83.3°W, A95=6.1°. Studies of baked contacts, country rocks distant from dykes, and later intrusions that cut them, show that the dykes became magnetized soon after intrusion. The paleopole is not significantly different from that obtained from the 1759±1 Ma post-orogenic Jan Lake Granite from the Hudsonian Orogen in southeast Saskatchewan. These paleopoles fall among a cluster previously determined from post-orogenic overprints in a wide variety of rocks and geological settings across Laurentia. Their mean is 20.8°N, 94.5°W, A95=5.2°. Collectively they record a widespread remagnetization episode following the Hudsonian Orogeny, during which the polarity of the field remained constant. This single polarity interval (the Cleaver Superchron) has a mean age of approximately 1747 Ma, and its duration is at least 13 million years but is probably much longer. With further refinement, it could become a global statigraphical marker. There are 18 paleopoles from red beds and igneous rocks which record the paleofield in western Laurentia mainly over an extended interval we estimate to be at least 100 million years between about 1960 and 1830 Ma during the Hudsonian Orogeny. Over this interval, western Laurentia remained at a low and essentially constant latitude. Reversals of the geomagnetic field occurred opposed magnetizations being of approximately equal frequency; there are numerous magnetochrons which could, in future, assist in regional stratigraphic correlations. On the basis of six studies, we estimate that the reference paleopole for this interval is at 1.1°S, 71.7°W, A95=12.1° (N=6). Paleopoles from the other 12 studies are spread over a 100° arc indicating large variable rotations about local vertical axes which probably resulted from transcurrent faulting caused by indentation of the bordering Archean Slave and Superior cratons during the Hudsonian Orogeny. These reference paleopoles show that during the Hudsonian Orogeny, western Laurentia was in paleolatitudes of 15–30°. During post-Hudsonian time (ca. 1750 Ma) paleolatitudes were about 20° higher. Paleowind directions inferred by others, indicate that Laurentia during these intervals was in the northern hemisphere.