Tectonics of the Keweenawan basin, western Lake Superior region

White, Walter S.

doi:10.3133/pp524e

Cited by 30 publications

(24 citation statements)

References 8 publications

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“…3b-3d). Gravity data (White, 1966), however, do not support the concept of a collapsed feeder under Lake Superior for the Keweenawan layered intrusions. Plate tectonic concepts have provided a new perspective from which to view the structural relations of the Keweenawan layered intrusions.…”

Section: Layered Gabbroic and Associated Granitic Plutonic Rocksmentioning

confidence: 54%

“…8), Cooper and others (1981) have accumulated evidence for the existence of a northeast-and northwest-trending fault system. This system may be interpreted to represent a series of rift White, 1966). Western part of Mellen Complex mapped by Leighton (1954) is included in the Mineral Lake intrusion.…”

Section: Layered Gabbroic and Associated Granitic Plutonic Rocksmentioning

confidence: 99%

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6: Keweenawan intrusive igneous rocks

Weiblen¹

1982

Geological Society of America Memoirs

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A bstractThe Keweenawan intrusive rocks of the Lake Superior region may be conveniently divided into alkaline intrusions, layered tholeiitic intrusions, and quartz and olivine tholeiite dike and sill swarms. Within each group is a characteristic internal stratigraphic succession of rock types. The alkaline rocks are characterized by successions of fractionated, undersaturated to saturated gabbroic through alkalic differentiates. On the basis of data from the Duluth Complex, the layered tholeiitic intrusions have been subdivided into an early group of cumulates consisting of peridotite, anorthositic rocks and granophyre, and a later group of troctolitic through ferrogabbroic cumulates with minor granophyre. On the basis of crosscutting relations and paleomagnetic data, most quartz tholeiite dikes and sills are older (magnetically reversed) than olivine tholeiites (magnetically normal). However, some magnetically normal dikes and sills are quartz tholeiites. Age data and crosscutting relations do not provide a means for placing the alkaline rocks in a stratigraphic succession with the tholeiitic rocks; however, comparison with modern rift environments suggests the following succession of magma types: alkaline (COi-rich to C02-poor), quartz tholeiite (alkalic, H20-saturated), olivine tholeiite (low alkalies and H2O). The distribution of the intrusive products of these magmas is consistent with a transition in the Keweenawan tectonic regime from northwest-southeast wrench faulting to rifting along northwest-southeast transform faults. IntroductionThe data base on the Keweenawan intrusive igneous rocks consists of the results of field mapping, petrographic descriptions, petrologic and geochemical analyses, and interpretive studies. The extent of the data base decreases in the order listed. In view of this and because field mapping is an iterative process, it is fair to say that knowledge of these rocks has not progressed substantially beyond the preliminary characterization of major exposed rock units. Most of the interpretive aspects of geologic mapping remain to be made or have not been verified. For example, definition of individual intrusive units, correlation of stratigraphic sequences within presumed intrusive units, and pre-and postemplacement structural control on stratigraphic relations of the intrusive igneous rocks are vague and uncertain. If this pessimistic view of the quality and extent of the available data is not overdrawn, it poses the question of what has hampered growth of a substantial data base on a group of interesting rocks that have been studied for over a century. At least two of the possible answers to this question are pertinent to this review. First, the low relief of the region,

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Section: Layered Gabbroic and Associated Granitic Plutonic Rocksmentioning

confidence: 54%

Section: Layered Gabbroic and Associated Granitic Plutonic Rocksmentioning

confidence: 99%

6: Keweenawan intrusive igneous rocks

Weiblen¹

1982

Geological Society of America Memoirs

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“…These intense anomalies are not well understood, although Hinze et al (1982) and Dickas (1986c) suggest that they might be produced by relatively thick sections of low-density Bayfield Group sedimentary rocks, thinning of the rift's dense volcanic sequence, or low-density sources beneath the rift basin. White (1966b) suggested that the southern (Bayfield Peninsula) negative anomaly is produced by a ridge of pre-Keweenawan basement rocks above which the rift's volcanic rocks are relatively thin or absent. Until this study, however, no quantitative models have been proposed.…”

Section: Geophysical Elements and Methodologymentioning

confidence: 98%

“…Reverse faults (Fig. 2) thrust older volcanic rocks and Oronto Group strata over younger Bayfield Group rocks and delineate the margins of uplifted volcanicrooted horsts (Thiel, 1956;White, 1966b;Dickas, 1986b), such as the St. Croix Horst of Minnesota and Wisconsin (SCH, Fig. 2).…”

Section: Geologic Elementsmentioning

confidence: 99%

Integrated geophysical modeling of the North American Midcontinent Rift System: New interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota

Allen¹,

Hinze²,

Dickas³

et al. 1997

Middle Proterozoic to Cambrian Rifting, Central North America

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Integrated geophysical investigations of the North American Midcontinent RiftSystem have resulted in a new understanding of the structure, stratigraphy, and evolution of this 1,100-Ma aborted continental rift. Interpretation of seismic reflection, gravity and magnetic anomaly, seismic refraction, rock physical property, and geologic data has identified a great degree of structural heterogeneity of the rift system in eastern Minnesota, northwestern Wisconsin, and western Lake Superior. In the western Lake Superior region, two ridges of pre-rift basement rocks are identified by pinch out of the rift's volcanic strata and lower portion of the overlying sedimentary sequence. In addition, regional rift reverse faults terminate above these ridges. Three-dimensional gravity modeling, constrained by seismic reflection profiles, suggests that both ridges are underlain by and composed of a belt of granitic rocks within the buried Archean greenstone-granite province beneath the rift basin, suggesting a significant influence of ancestral structures throughout the evolution of the rift system. Magnetic modeling indicates that magmatism did not occur uniformly along the length of the rift. In Minnesota and Wisconsin, the great majority of the rift's volcanic rocks are normally polarized and probably younger than the recorded ca. 1,098-Ma magnetic reversal, in contrast to western Lake Superior, where the lower half of the volcanic sequence is reversely polarized and was probably erupted before ca. 1,098-Ma. Gravity modeling suggests that the mass deficiency associated with crustal thickening along the rift is probably compensated by the positive effect of dense rift intrusions in the lower crust. The volume of magma trapped in the lower crust may be similar to that erupted into the rift basin.

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“…The Keweenawan Supergroup (1.1 Ga) in the Lake Superior Basin (Figure 1) consists of a thick sequence of volcanic and sedimentary rocks which filled a portion of the Mideontinent rift system during Proterozoic time (White 1966, Chase and Gilmer, 1973, Daniels 1982, Cannon et al 1989. The stratigraphic section near White Pine consists of a basal unit of 5-13 km of mafic to intermediate flows of the Portage Lake Volcanics and the Porcupine Volcanics (Cannon and Nicholson 1992).…”

Section: Geological Setting and Units Analyzedmentioning

confidence: 99%

Preservation of Clay Minerals in the Precambrian (1.1 GA) Nonesuch Formation in the Vicinity of the White Pine Copper Mine, Michigan

1995

Clays and clay miner.

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Abstract--The Middle Proterozoic (1.1 Ga) Nonesuch Formation, host of the stratiform copper deposit at White Pine, Michigan, consists of 200 m of principally dark grey clastic sediments which contain detritus obtained dominantly from underlying mafic to intermediate volcanic rocks. Clay minerals from samples collected from the mine area and drill holes up to 100 km away have been studied using SEM, EMPA, TEM and AEM. Two morphologies of phyUosilicates, both including white mica and chlorite, occur in the 'lower' Nonesuch Formation: (1) detrital-shaped and (2) matrix. D~trital-shaped phyllosilicate grains are up to 450 microns long with long axes parallel to bedding. Matrix phyllosilicates occur as packets typically <200 .~ thick and as pore-filling cement.TEM images of detrital-shaped chlorite generally display 14-A periodicity, although 24-A corrensitelike units occur locally. Most dgtdtal-shaped chlorite from the mine area samples have a relatively restricted range of Fe/(Mg+Fe) ratios from 0.52 to 0.58, but the Fe/(Fe+Mg) ratios of detrital-shaped chlorite outside the mine area range from 0.27 to 0.64. TiO2 crystals occur within and surrounding the detrital-shaped chlorite. Matrix chlorite has Fe/(Fe+Mg) ratios of 0.47 to 0.63, indicating that it is relatively homogeneous and enriched in Fe compat~ to detrital-shaped chlorite.Detrital-shaped white mica occurs as a 2M~ polytype and generally has a phengitic composition. Matrix illite-rich I/S occurs as a 1Ma polytype, is K and AI deficient relative to end-member muscovite and contains significant Fe and Mg.The data are consistent with homogenization of detrital-shaped chlorite in the White Pine mine area by hydrothermal fluids during copper mineralization. The TiO2 crystals and corrensite-like units in detritalshaped chlorite imply that it is at least in part derived from alteration of biotite. The presence of immature 1Ma illite-rich I/S and a one layer chlorite polytype with stacking disorder suggests that the matrix clays are in their original, post-smectite state of formation as consistent with an authigenic origin during early burial diagenesis; i.e., they have not undergone subsequent transformation even though sedimentation and ore deposition occurred prior to 1 Ga.

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Tectonics of the Keweenawan basin, western Lake Superior region

Cited by 30 publications

References 8 publications

6: Keweenawan intrusive igneous rocks

6: Keweenawan intrusive igneous rocks

Integrated geophysical modeling of the North American Midcontinent Rift System: New interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota

Preservation of Clay Minerals in the Precambrian (1.1 GA) Nonesuch Formation in the Vicinity of the White Pine Copper Mine, Michigan

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