Geologic and geochemical attributes of the Beaver River Diabase and Greenstone Flow: Testing a possible intrusive-volcanic link in the 1.1 Ga Midcontinent Rift

Abstract

Over the last century, numerous geological studies have reasonably resolved the overall tectonomagmatic evolution of the 1.1 Ga Midcontinent Rift (MCR). The structural complexity, duration of magmatic activity, and relative scarcity of continuous exposure, however, have thus far hindered efforts to correlate the numerous MCR flood basalts with their intrusive feeder systems. Providing such correlations is essential to furthering our understanding of the magmatic processes that operated during rifting. This study provides field, petrographic, and geochemical evidence of one such intrusive-volcanic correlation between the Beaver River diabase (BRD) and Greenstone Flow (GSF) – two of the largest igneous systems associated with the MCR. The BRD is an extensive, multiply-intrusive, composite dike and sill network and the most extensive intrusive phase of the Beaver Bay Complex (BBC) in northeastern Minnesota. The GSF is an enormous (> 2,000 km3), similarly composite flood basalt exposed on Isle Royale and the Keweenaw Peninsula in northern Michigan. The notion that these two systems might be related was first suggested by Miller and Chandler (1997) based on the recognition that very large (≤ 400m) inclusions of anorthosite, interpreted to be xenoliths derived from the lower crust, occur in BRD dikes and sills at hypabyssal depths. Because vertical conduits wide enough to accommodate such large xenoliths traversed the crust to shallow depths, these intrusions likely reached the Earth’s surface to erupt extraordinarily large lava flows. Likewise, the GSF may possibly be the product of this venting, as supported by several lines of circumstantial evidence: 1) overlapping U-Pb ages - 1094.0 ± 1.5 Ma for the GSF (Davis and Paces, 1990) and 1095.8 ± 1.2 Ma for the BRD (Paces and Miller, 1993); 2) similar ranges in lithologies – ophitic olivine diabase/basalt to ferromonzodiorite; and 3) similar enormous sizes. This study seeks to more rigorously evaluate a possible comagmatic link between these two systems by carefully comparing their field, petrographic, and geochemical attributes. The BRD is composed of a network of ophitic olivine diabase dikes and sills that intrude the medial section of the North Shore Volcanic Group and host numerous, smaller composite intrusions of variably fractionated lithologies. In the southern BBC near Silver Bay, MN, these composite intrusions occur as a series of singularly emplaced, concentrically-zoned bodies of vari-textured ferromonzodiorite cored by foliated ferrogabbro/diorite, which are collectively termed the Silver Bay Intrusions (SBI). In the northern BBC, composite intrusions in the BRD range from intergranular gabbro to ferrodiorite and have been interpreted by Miller and Chandler (1997) to have been emplaced in at least two intrusive pulses into BRD diabase dikes. The GSF is composed predominantly of ophitic to subophitic, olivine tholeiitic basalt which can be divided into upper and lower zones. The core of the GSF, here termed the Heterolithic Zone, separates the ophitic zones and contains a heterogeneous suite of evolved rocks ranging from intergranular gabbro to prismatic ferromonzodiorite. Field observations and petrographic data suggest that the core of the initial tholeiitic basalt flow was intruded, inflated, and partially displaced by one or possibly two intrusive pulses of evolved magma. Previous workers (e.g. Cornwall, 1951) have suggested that the occurrence of evolved lithologies in the core of the GSF resulted from in situ differentiation. However, evidence for composite emplacement of evolved magmas within the GSF, presented here, is given by: 1) abrupt changes in mineralogy, texture, mineral chemistry, and lithogeochemistry over centimeter to meter scale; 2) inclusion relationships between evolved and ophitic GSF lithologies; and 3) the occurrence of remnant blocks of initially crystallized GSF ophitic basalt interlayered with evolved lithologies within the Heterolithic Zone. Petrographic observations show that the comparable rock types in the BRD and GSF are nearly indistinguishable in terms of modal mineralogy and texture. Most notably are the similar occurrences of distinctive clustering of plagioclase, and the occurrence of coarse-grained plagioclase megacrysts in both the BRD ophitic diabase and GSF ophitic basalt. The presence of plagioclase megacrysts in the GSF ophitic basalt with similar anorthite contents (up to 81 mol%) as reported for the anorthosite inclusions in the BRD (An54-80; Morrison et al., 1983) strongly supports the interpretation that these crystals are xenoliths derived from a similar source as those in the BRD ophitic diabase. Geochemical data are also consistent with the interpretation that the various rock types in the BRD and GSF crystallized from chemically similar parental magmas. SEM-EDS analyses of mafic mineral compositions show considerable overlap in the mean and range of En’ contents in augite and Fo contents in olivine between comparable rock types in the BRD and GSF systems. Compositional overlap is also observed in whole rock analyses of trace element abundances and ratios between comparable rock types in each system. Concluding from the field, petrographic, and geochemical data that the BRD dike and sill network acted as the intrusive feeder system for the GSF lava flow implies that previous estimates of the volume of the GSF (White, 1960; Long, 1984) are grossly underestimated. Projecting the GSF westward under Lake Superior from exposures on Isle Royale and the Keweenaw Peninsula to the proposed feeder system on the Minnesota shore implies that the GSF has an areal extent of roughly 20,000 km2 and a total volume of at least 2,000 km3. These estimates indicate that the GSF is possibly the largest single lava flow on Earth.