Synchronous Emplacement of the Anorthosite Xenolith‐Bearing Beaver River Diabase and One of the Largest Lava Flows on Earth

Abstract

The North American Midcontinent Rift (MCR) is a ca. 1.1 Ga large igneous province for which there is excellent exposure of both the intrusive and extrusive components in the Lake Superior region (Figure 1). An exceptional feature within the MCR is the occurrence of large anorthosite xenoliths within a diabase sill and dike network known as the Beaver River diabase that outcrops in northeastern Minnesota, USA, as part of the Beaver Bay Complex (Figure 1). The anorthosite xenoliths range in size from centimeter-scale megacrysts to meter-scale, decimeter-scale and even  E 150-m-scale blocks (Figure 2; Grout & Schwartz, 1939;Morrison et al., 1983). A particularly large anorthosite xenolith is exposed at Carlton Peak in the eastern Beaver Bay Complex with minimum dimensions of 180  E 240 m (Figures 1 and 2; Boerboom et al., 2006). In the southern Beaver Bay Complex, a large anorthosite xenolith near Corundum Point has dimensions of 180  E 230 m while one exposed at Split Rock Point has dimensions of 180  E 260 m (Boerboom , 2004). To be able to accommodate such large xenoliths during magma ascent, the Beaver River diabase conduits must have been of at least the width of the anorthosite short-axis diameters. Such wide conduits in these near-surface intrusions suggest high magma flux rates and make it likely that the magma extruded to the surface-feeding voluminous lava flows. Miller and Chandler (1997) emphasized the composite nature of the Beaver River diabase network and Silver Bay intrusions (Figure 1), which are locally marked by abrupt transitions to progressively more evolved lithologies. Furthermore, Miller and Chandler (1997) documented geochronologic, geochemical, and structural evidence to support the notion that the diabase network may have served as principal feeder conduits to lava flows including parts of the Portage Lake Volcanics (PLV) on the Keweenaw Peninsula and Isle Royale of Michigan (Figure 1). To more directly test this inferred intrusive-extrusive correlation, Doyle (2016) compared the mineralogical, textural, and geochemical attributes and the composite lithologic nature of the Beaver River diabase against those of the Greenstone Flow, the largest lava flow within the MCR and one of the largest lava flows on Earth (Figure 3). Doyle (2016) documented remarkable similarities in petrography, mineral chemistry, whole rock geochemistry, and interpreted lithologic zonation between the Beaver River

Quantifying Inclination Shallowing and Representing Flattening Uncertainty in Sedimentary Paleomagnetic Poles

“…Its age serves as a maximum age constraint for the deposition of Cut Face Creek Sandstone and is likely close to the absolute age. The minimum depositional age of the sandstone is constrained by the 1,091.7 ± 0.2 Ma Beaver River diabase of the Beaver Bay Complex, which crosscuts the NSVG (Zhang et al., 2021). Paleomagnetic data from 28 lava flows of the upper Northeast sequence of the NSVG (blue diamonds in Figure 2; Books, 1972; Tauxe & Kodama, 2009) result in a paleomagnetic pole at 181.7°E, 31.1°N ( A 95 = 4.2°; Swanson‐Hysell, Ramezani, et al., 2019).…”

“…We compare the detrital remanence directions of the Cut Face Creek specimens to the expected directions from the volcanics to determine the amount of 2001)). CA-ID-TIMS 206 Pb/ 238 U dates constrain the Cut Face Creek Sandstone to be younger than the 1,093.52 ± 0.43 Ma Grand Marais Rhyolite (purple cross; Swanson-Hysell, Ramezani, et al, 2019) and older than the 1,091.7 Ma ± 0.2 Ma cross-cutting Beaver River diabase of the Beaver Bay Complex (green unit, Zhang et al, 2021). (c) The Cut Face Creek Sandstone overlies the Good Harbor Bay andesites (purple) while the Terrace Point basalt (tan orange) erupted atop the sandstone.…”