Magmatism that accompanied the 1.1 Ga Midcontinent rift system (MRS) is attributed to the upwelling and decompression melting of a mantle plume beneath North America. Five distinctive flood-basalt compositions are recognized in the rift-related basalt succession along the south shore of western Lake Superior, based on stratigraphically correlated major element, trace element, and Nd isotopic analyses. These distinctive compositions can be correlated with equivalent basalt types in comparable stratigraphic positions in other MRS localities around western Lake Superior. Four of these compositions are also recognized at Mamainse Point more than 200 km away in eastern Lake Superior. These regionally correlative basalt compositions provide the basis for determining the sequential contribution of various mantle sources to flood-basalt magmatism during rift development, extending a model originally developed for eastern Lake Superior. In this refined model, the earliest basalts were derived from small degrees of partial melting at great depth of an enriched, ocean-island-type plume mantle source (εNd(1100) value of about 0), followed by magmas representing melts from this plume source and interaction with another mantle source, most likely continental lithospheric mantle (εNd(1100) < 0). The relative contribution of this second mantle source diminished with time as larger degree partial melts of the plume became the dominant source for the voluminous younger basalts (εNd(1100) value of about 0). Towards the end of magmatism, mixtures of melts from the plume and a depleted asthenospheric mantle source became dominant (εNd(1100) = 0 to +3).
Between 1091 and 1098 Ma, most of a 15-to 20-km thickness of dominantly tholeiitic basalt erupted in the Midcontinent Rift System of the Lake Superior region, North America. The Portage Lake Volcanics in Michigan, which are the youngest MRS flood basalts, fall into distinctly high-and 1ow-TiO2 types having different liquid lines of descent. Incompatible trace elements in both types of tholeiites are enriched compared to depleted or primitive mantle (La/Yb = 4.3-5.3; Th/Ta = 2.12-2.16; Zr/Y = 4.3-4.4), and both basalt types are isotopically indistinguishable. Sr, Nd, and Pb isotopic compositions of the Portage Lake tholeiites have 87Sr/86Sr/• 0.7038, t•Nd (1095 Us) • 0 +---2, and/x I • 8.2. Model ages with respect to a depleted mantle source (TDM) average about 1950-2100 Ms. Portage Lake rhyolites fall into two groups. Type I rhyolites have Nd and Pb isotopic characteristics (eNd (1095 Ms) • 0 to -4.7;/xl • 8.2-7.8) consistent with contamination of tholeiitic rocks by 5-10% Archean crust. The one type II rhyolite analyzed has Nd and Pb isotopic compositions (eNd (1095 Ms) • --13 to 10 3 km 3) of relatively homogeneous magma in a restricted time interval. 10,851 10,852 NICHOLSON AND SHIREY' MIDCONTINENT RIFT ISOTOPIC SYSTEMATICS Km • -----R L ---1.4-1.3 0, 2.00 SUPERIOR PROVINCE 2.8-2.6 N 1.5-1.4 •'J -j C:__._C__ Fig. 1. The Midcontinent Rift System cuts across terranes ranging in age from 3.6 to 1.65 Ga [after Van $chmus et al., 1987]. The location of the rift south of Lake Superior in Minnesota, Michigan, Wisconsin, Iowa, Nebraska, and Kansas is inferred from gravity and magnetic data. The hatched areas denote volcanic basins, and the stippled areas represent the sedimentary basins. Abbreviations and ages of terranes surrounding Lake Superior are as follows: MRV, Minnesota River Valley (3.6-2.6 Ga); RL, Rainy Lake area of the Wabigoon subprovince (2.7 Ga); V, Vermilion district of the Wawa subprovince (2.7 Ga); N, dikes of Nippissing diabase (2.2 Ga); CM, central Minnesota (2.1-1.8 Ga); H, volcanic rocks of the Hemlock Formation (1.9 Ga); and Q, volcanic rocks of the Quinnesec Formation (1.9 Ga). accumulations up to 15 km thick [Behrendt et al., 1988; Cannon et at., 1989], much of which was extruded within about 3-5 m.y. [Paces and Davis, 1988]. Geomorphological and geophysical similarities between the Precambrian MRS and the Phanerozoic examples of plume magmatism suggest a common origin [Hutchinson et at., this issue]. Sr, Nd, and Pb isotopic compositions are reported for a diverse suite of MRS-related basalts to rhyolites, the Portage Lake Volcanic rocks, which are the youngest flood basalts associated with the MRS. Major element, trace element, and isotope data are used to evaluate the nature of the source of MRS magmatism and to assess the roles of various mantle and crustal components. GEOLOGICAL SETTING The Midcontinent Rift System is a 1.1-Ga continental rift that extends more than 2000 km from Kansas north beneath Lake Superior and then southeast beneath the Michigan basin to the Grenville Front (Figure ...
New single-crystal zircon U–Th–Pb ages for plutonic and rhyolitic Keweenawan Supergroup rocks from the south shore of Lake Superior provide geochronological constraints on magmatic evolution associated with the 1.1 Ga Midcontinent rift. Analyses of a granophyric phase of the Mineral Lake intrusion and the Mellen granite, both parts of the Mellen Intrusive Complex, and a laterally extensive rhyolite from the top of the Kallander Creek Volcanics have weighted average 207Pb/206Pb ages of 1102.0 ± 2.8 Ma (N = 2), 1100.9 ± 1.4 Ma (N = 5), and 1098.8 ± 1.9 Ma (N = 4), respectively. Analyses of a pyroclastic rhyolite flow at the top of the Porcupine Volcanics result in variable 207Pb/206Pb ages that range from 1080 to 1137 Ma. This rhyolite exhibits a continuum between morphologically complex and simpler prismatic zircon crystals, the latter yielding concordant analyses having a weighted average 207Pb/206Pb age of 1093.6 ± 1.8 Ma (N = 2). Four prismatic zircons from an aphyric rhyolite of the Chengwatana Volcanics in the Ashland syncline form a linear array intersecting concordia at 1094.6 ± 2.1 Ma (MSWD = 1.3). Another presumed Chengwatana rhyolite recovered from drill core intersecting the Hudson–Afton horst in southeast Minnesota yielded only ~20 morphologically indistinguishable zircons. Six analyses give 207Pb/206Pb ages ranging from 1112 to 1136 Ma, including one analysis with a virtually concordant age of 1130 Ma. This age, however, is considerably older than that obtained for the Chengwatana Volcanics in the Ashland syncline or any other precisely dated rock from the Midcontinent rift.
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