John C. Green

Geologic and Geochemical Evidence for the Nature and Development of the Middle Proterozoic (Keweenawan) Midcontinent Rift of North America

Abstract General models of active continental rifting must take into account the large plateau basalt provinces, many of which have been precursors of or associated with continental breakup. The Keweenawan (Middle Proterozoic) Midcontinent Rift (MCR) of North America, 2300 km long, is one such major continental rift, but it aborted before significant crustal separation was achieved. These plateau-basalt rifts, including the MCR, differ from the East African type in being dominated by broad subsidence rather than rift valleys, tholeiitic flood basalts rather than alkalic central volcanoes, and large positive gravity anomalies. Geologic and geophysical evidence is reviewed which contradicts some recent models which have been proposed for the development of the MCR. The basic causes of the pre-rift subsidence, the U-shaped nature of the structure, and the abrupt termination of its activity are obscure. A triple junction in the usual sense does not appear to have been formed.

Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition

Abstract The ≈8-km-thick sequence of plateau lavas (North Shore Volcanic Group) in northeastern Minnesota that was erupted during the formation of the 1.1 Ga Midcontinent Rift System includes an unusually large proportion (10–25%) of felsic flows (rhyolites and icelandites). Individual flows are now recognized with estimated volumes up to 600 km 3 and lateral extents up to 40 km; some were probably originally much broader. Many are anomalously large compared to most silicic lava flows. Primary structures and textures of these essentially unmetamorphosed rocks indicate eruptive style for most of these large flows. In particular, the features of the basal meter or so and uppermost several meters show that some flows were emplaced as fluid lavas, and others as strongly rheomorphic tuffs. It may be essential in interpreting the origin of felsic flows elsewhere to examine petrographically the lowermost few decimeters and the top, where the only evidence (poorly to strongly welded shards) may exist of the pyroclastic origin of a large, otherwise lava-like flow. Interior textures of these crystalline flows provide evidence of high-temperature eruption, especially in the form of abundant paramorphs of magmatic tridymite in the groundmass, commonly arranged in a pilotaxitic flow structure. Based on structures, textures, field relations, and geochemistry, We conclude that (a) all the icelandites and some rhyolites were emplaced as large, moderately to very fluid lava flows that spread widely while crystallizing tridymite; and that (b) some large rhyolites, which are only sparsely phyric, were emplaced as pyroclastic flows subsequently remobilized as rheoignimbrites. The mode of emplacement of one of the largest aphyric flows remains enigmatic. Vent areas or characteristics are unknown. The low apparent viscosity of the erupted magmas is probably attributable to compositional factors (F content, high Fe 2+ ) as well as to high temperatures and eruption rates. Estimated temperatures of about 1000–1100 C are associated with the low water content and perhaps great depth of origin of the magmas at the time of massive influx of hot matic melts from the mantle during continental rifting.

The Finland Tectono-Magmatic Discontinuity— A Growth Fault Marking the Western Margin of the Portage Lake Volcanic Basin of the Midcontinent Rift System

Seismic reflection, aeromagnetic, and gravity data along the 1.1 Ga Midcontinent Rift System reveal that the greatest thicknesses (15–22 km) of Keweenawan lavas are contained within asymmetric grabens bounded by what are now listric reverse faults, but which must have acted as normal growth faults during much of the volcanism (Chandler et al. 1989; Cannon et al. 1989; Cannon 1992; Thomas and Teskey 1994). This is particularly true of the Portage Lake Volcanics (PLV) and their equivalents which form the upper part of the Keweenawan volcanic sequence in the Lake Superior region. Although incomplete sections of what have historically been termed the PLV are exposed only on the Keweenaw Peninsula and Isle Royale, recent geophysical studies have shown the PLV sequence to underlie most of Lake Superior and attain a thickness as great as 15 km. These geophysical studies also show that the PLV basin terminates in the western Lake Superior region in a structurally complex manner Although geologic interpretations of some seismic profiles (Grant-Norpac lines 57 and 53, Fig. 1) by us and others (Thompson et al. 1990; Sexton and Henson 1994) suggest that the PLV sequence projects onshore in northeastern Minnesota, the geologic and structural manifestation of that projection has yet to be characterized. Toward that end, we present geologic, geophysical, petrologic, and structural evidence that suggests that (1) the Schroeder-Lutsen basalts, which occupy the uppermost part of the North Shore Volcanic Group (NSVG), are correlative with the base of the PLV section, and (2) the southwestern margin of the PLV basin is marked by a growth fault that is recognized as an extensive composite diabase dike system within the Beaver Bay Complex, which we term the Finland tectono-magmatic discontinuity (FTMD, Fig. 1).

Geology of the Connecticut Lakes-Parmachenee Area, New Hampshire and Maine

The Connecticut Lakes-Parmachenee area, in northernmost New Hampshire and northwesternmost Maine, is underlain principally by Lower to Middle Paleozoic clastic sediments, and felsic and mafic volcanic rocks, all of low metamorphic grade. The area includes a section of the entire northwest flank of the Boundary Mountains anticlinorium (the southeast flank of the Connecticut Valley-Gaspe synclinorium) and part of the axial region of the latter synclinorium. Approximately 23,000 feet of Ordovician, and perhaps some Cambrian, strata are represented. These include, from oldest to youngest, the Aziscohos, Albee, and Dixville formations, the last containing the coarse, clastic Magalloway Member in the Maine part of the area. These rocks were folded during the Taconic orogeny. Along the Taconic unconformity are very local, thin Upper Silurian limestones, and to the northwest lie about 20,000 feet of Devonian strata, constituting the Seboomook and overlying Frontenac formations. The Compton Formation, believed to be equivalent to the Seboomook across a synclinal axis, may appear at the northwest corner of the area. The stratified rocks are intruded by a granodiorite stock of probable Taconic age, and by two granitic stocks and many microgranite lenses, a gabbro and a tonalite stock, many gabbro sills, and a few small serpentinites and peridotites, all of Acadian age. Study of the minor structures shows that the Cambro-Ordovician rocks were deformed and strengthened by recrystallization at low temperatures during the Taconic orogeny, but the principal metamorphism and regional deformation were Acadian.

Snowflake Texture Not Diagnostic of Devitrified Ash-Flow Tuffs: Discussion

Recent claims as to the distribution of geologically old welded tuffs (Anderson, 1970) are apparently based largely on the use of “snowflake texture” as a criterion diagnostic of such deposits. Late Precambrian vesicular felsite lavas in Minnesota commonly show snowflake texture, and it should not be used as a criterion for recognition of devitrified welded tuffs. Anderson (1970) has recently published a note that implies, if not claims, that very large volumes of altered Cambrian and late Precambrian felsic volcanic rocks in Oklahoma and Missouri are ash-flow tuffs. Before this claim becomes generally accepted, it is desirable to examine the available evidence, and I would like to suggest that the criterion for identification of ash-flow tuffs apparently relied upon by Anderson for many of the rocks is invalid (Anderson, 1969). Although I am not acquainted with the particular rock groups described by Anderson, for several years I have been carrying out detailed mapping of the late Precambrian (Keweenawan) North Shore Volcanic Group in Minnesota (Goldich and others, 1961), with the support of the Minnesota Geological Survey and the National Science Foundation GP-5056 (Green, 1968; in prep.). These rocks are gently dipping and unmetamorphosed, except for local hydrothermal or pneumatolytic effects, or both, and contain many flows of felsic (quartz-latite) composition. They are well exposed along the shore of Lake Superior. All are devitrified, and a few are spherulitic. The rocks display grain sizes which are dense-aphanitic to very fine-grained, and most are porphyritic. The texture referred to as “snowflake” by Snyder (1962) and Anderson (1970) is common, with poikilitic patches of quartz, up to 2 or 3 mm across, enclosing small, blocky or tabular, alkali-feldspar euhedra in a pseudo-ophitic relationship. In contrast o t the suggestion of Anderson (1970, p. 287) that the quartz crystallized last, it would appear more compatible with crystallization from either a magma or a glass if the feldspar and quartz were contemporaneous during most of the crystallization, with fewer nuclei for the quartz. The feldspar may or may not, of course, have started to crystallize first. I find nowhere in Ross and Smith (1961) or Anderson9s other references, however, mention of snowflake texture as a criterion for ash-flow tuff origin, whether welded or not. Although a few flows with flattened lapilli and shards typical of welded tuffs have been reported from the Keweenawan (Foster, 1962; W. S. White, 1969, written commun.), the great bulk of felsic flows that I have examined shows no such fragmental textures either in outcrop or thin section (I have examined Paleozoic devitrified welded tuffs in England, Wales, and Maine, and vitric ones from New Mexico and in Japan). On the other hand, many of the Keweenawan flows are very massive and uniform, but develop a frothy, vesicular zone in the top few feet (Fig. 1A), clearly implying the presence of fluid lava and not a pyroclastic origin. These flows commonly show snowflake texture in their interiors (Fig. 1B). Lofgren9s (1968) experimental work and conclusions, quoted by Anderson (1969, p. 2077–2078) show no essential correlation of snowflake texture with ash-flow origin. It appears that such a texture has little bearing on the eruptive characteristics of a flow, but perhaps is simply a common result of crystallization of any felsic glass.

Volcanic Rocks of the Midcontinent Rift System: A Review

Continental rifting at 1.1 Ga produced approximately 1.3 × 106 km3 of igneous rocks over a rift length of 2200 km (Midcontinent Rift System, MRS). These constitute one of the largest as well as oldest flood basalt provinces. Both geophysical (Hutchinson et al. 1990) and geochemical evidence (Nicholson and Shirey 1990) suggest that magma production resulted from decompression melting of a large, hot mantle plume centered beneath what is now Lake Superior. These rocks are now exposed only around Lake Superior in Ontario, Michigan, Wisconsin, and Minnesota, and in east-central Minnesota and northwestern Wisconsin; the rest of the igneous rift products are known from geophysics and scattered drill holes. The plateau lavas are geochemically bimodal, dominated by olivine tholeiites (many of them high-Al), transitional basalts, and basaltic andesites, with significant volumes of rhyolites, especially in Minnesota (Green 1982). Structural and stratigraphic relationships of these lavas indicate that they were erupted in several distinct plateaus, each of which subsided centrally as rifting intensity shifted in time and place along the MRS (Green 1977; 1983), but age determinations so far show no “unzipping” progression (or hot-spot progression) from Lake Superior to the extremities in Kansas and southeastern Michigan. The maximum known time span of MRS (Keweenawan) magmatism, from recent precise U/Pb zircon work by D.W. Davis and co-workers, is 1109 to 1086 Ma, but the greatest bulk of eruptive and intrusive activity (for exposed rocks) occurred during a magnetically reversed interval around 1108–1105 Ma and especially during a magnetically normal interval around 1097–1094 Ma.