Relative Timing of Flows in the North Shore Volcanic Group in northern Minnesota using Paleomagnetic Measurements

Roughly 1.1 billion years ago the interior of the North American craton experienced prolonged extension resulting in the formation of the Midcontinent Rift System (MRS). The MRS failed after 15-22 million years of active extension accompanied by massive volumes of bimodal intrusive and extrusive igneous rocks. The North Shore Volcanic Group (NSVG) represents the extrusive portion of these rocks in northern Minnesota. Many outstanding questions remain regarding the emplacement and failure of the MRS. How long was the rift active? How rapidly were extrusive and intrusive units emplaced? To begin to address such questions at the outcrop scale, we will focus on the relative timing between a basalt flow and a cross-cutting dike using a paleomagnetic baked-contact test. We collected a profile of samples from a NSVG outcrop near Lutsen, MN where a basalt flow is cross-cut by a near-vertical feeder dike. The dike shows hydrothermal alteration and contains xenoliths of brecciated basalt. Oriented samples were collected within the dike and from the adjoining basalt at increasing distances up to 10 m from the dike contact. We plan to compare paleomagnetic directions recorded across the baked contact to determine if the dike was emplaced shortly after the basalt flow erupted (in which case paleodirections would be similar), or if sufficient time had elapsed to resolve secular variation, which would imply that centuries to thousands of years passed between the emplacement of the lava flow and dike. In addition to paleomagnetic directions, we will characterize the magnetic mineralogy in both rock units using high- and low-temperature magnetic measurements, hysteresis loops, and petrographic analysis of thin sections. Determining the relative timing of these two rock units will contribute to our understanding of the relative rate of eruption of the North Shore Volcanic Group, and perhaps the rate of effusive eruptions in the later stages of the MRS.