Abstract
AbstractNew geochronologic and paleomagnetic data from the North American Midcontinent Rift (MCR) reveal the synchronous emplacement of the Beaver River diabase, the anorthosite xenoliths within it, and the Greenstone Flow—one of the largest lava flows on Earth. A U‐Pb zircon date of 1091.83 0.21 Ma (2) from one of the anorthosite xenoliths is consistent with the anorthosite cumulate forming as part of the MCR and provides a maximum age constraint for the Beaver River diabase. Paired with the minimum age constraint of a cross‐cutting Silver Bay intrusion (1091.61 0.14 Ma; 2), these data tightly bracket the age of the Beaver River diabase to be 1091.7 0.2 Ma (95% CI), coeval with the eruption of the Greenstone Flow (1091.59 0.27 Ma; 2)—which is further supported by indistinguishable tilt‐corrected paleomagnetic pole positions. Geochronological, paleomagnetic, mineralogical and geochemical data are consistent with a hypothesis that the Beaver River diabase was the feeder system for the Greenstone Flow. The large areal extent of the intrusives and large estimated volume of the volcanics suggest that they represent a rapid and voluminous ca. 1,092 Ma magmatic pulse near the end of the main stage of MCR magmatism.
Highlights
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
By integrating the geochronologic and paleomagnetic perspectives with previous lithologic and geochemical analyses (Doyle, 2016; Miller & Chandler, 1997), we show that these data are consistent with the Beaver River diabase network acting as the feeder system for the Greenstone Flow of the Portage Lake Volcanics (PLV)
A sample of an anorthosite xenolith within the Beaver River diabase was collected for U-Pb geochronology along Hwy 61 across from the Silver Bay taconite plant (MS99033; 91.26358°W 47.28888°N; Figure 1)
Summary
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. 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). Geochemistry, Geophysics, Geosystems diabase intrusions in northern Minnesota and the Greenstone Flow on both Isle Royale and Keweenaw Peninsula. Based on the interpreted feeder system being in northern Minnesota, Doyle (2016) estimated the full areal extent of the Greenstone FlowEto be 2E0,000 km and its volume to be between 2,000 and 6,000 E km (Figure 3)
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