PALEOMAGNETIC STUDY OF THE GREENSTONE LAVA FLOW EXPOSED IN THE KEWEENAW PENINSULA

We conducted a detailed paleomagnetic and rock-magnetic investigation on thirteen sites of the Greenstone Lava Flow (GSF). The GSF is a very unique and one of a kind lava flow due to its size and extending cooling time that has yet to have a detailed paleomagnetic study. The GSF belongs to the Portage Lake Volcanics (PLV) of the Mid-Continent Rift System that formed 1.1 billion years ago. The volume and thickness of the GSF together promoted an extremely long cooling time during which three major distinct rock facies differentiated from the lava: amygdaloid at the top, ophite in the lower and upper parts, and pegmatite in the middle of the flow allowing for a secular variation investigation. These facies are exposed by tectonic folding and faulting in the Keweenaw Peninsula and Isle Royale. We sampled from three units of the GSF: the upper ophite, lower ophite, and pegmatite center. The magnetic properties of all collected samples are dominated by ferromagnetic minerals (low-Ti titanomagnetite and magnetite) with some contribution from paramagnetic minerals. All measured specimens manifested pseudo-single domain behavior. The paleomagnetic directions obtained from the three GSF units are consistent and do not show any systematic differences but do have an unusually large scatter compared to other Portage Lake Volcanics (PLV). When grouped by GSF unit the scatter clearly shows secular variation but suggests other questions and further investigation when compared to the PLV. The paleomagnetic group mean direction calculated before structural correction has a declination of 255.9° and inclination of 53.4° with an uncertainty circle (a95) of 8.3° and after structural correction has a declination of 226.2° and inclination of 47.6° with an a95 of 9.3°. The group mean direction obtained from the GSF is significantly different from the group mean previously obtained from the PLV flows exposed along the northeastern coast of the Keweenaw Peninsula. This difference may reflect extreme secular variation of the Earth’s magnetic field, or a different structural history of the GSF, such as rotation during the Grenville Orogeny, with respect to the other smaller PLV flows