Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the Midcontinent Rift system

Davis, Donald W.; Paces, James B.

doi:10.1016/0012-821x(90)90098-i

Cited by 174 publications

(93 citation statements)

References 23 publications

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“…1). Existing age constraints on the Lake Shore Traps come from a weighted mean 207 Pb/ 206 Pb ID-TIMS date of 1087.2 ± 1.6 Ma obtained by Davis and Paces (1990) based on multigrain zircon fractions from a feldspar-phyric andesite flow within the upper lavas of the middle Lake Shore Traps. Our sample from the same feldspar-phyric andesite flow (sample LST-KP1; Fig.…”

Section: Geological Setting and Geochronologymentioning

confidence: 99%

“…1109 Ma (Davis and Sutcliffe, 1985) and continued to ca. 1087 Ma (Davis and Paces, 1990). Given the longevity and volcanic output of the Midcontinent Rift, the reason why the rift ultimately failed to dismember the ancient North American craton, Laurentia, continues to be a topic of active interest.…”

Section: Introductionmentioning

confidence: 99%

“…Before the advent of the CA-TIMS pretreatment technique, zircons for U-Pb geochronology were customarily subjected to a mechanical (air) abrasion procedure after Krogh (1982). Though highly beneficial, the latter technique was demonstrably less efficient in eliminating Pb loss, which would result in anomalously young (and discordant) Pb or concordia intercept dates from discordant zircon analyses, thereby limiting precision (e.g., Palmer and Davis, 1987;Davis and Paces, 1990;Davis and Green, 1997). It has been shown that even for statistically coherent sets of U-Pb data (i.e., no detectable scatter due to U-Pb open-system behavior), the (Schoene et al, 2006).…”

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confidence: 99%

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The End of Midcontinent Rift Magmatism and the Paleogeography of Laurentia

Fairchild

Swanson‐Hysell

Ramezani

et al. 2016

Geological Society of America Abstracts With Programs

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Paleomagnetism of the North American Midcontinent Rift provides a robust paleogeographic record of Laurentia (cratonic North America) from ca. 1110 to 1070 Ma, revealing rapid equatorward motion of the continent throughout rift magmatism. Existing age and paleomagnetic constraints on the youngest rift volcanic and sedimentary rocks have been interpreted to record a slowdown of this motion as rifting waned. We present new paleomagnetic and geochronologic data from the ca. 1090-1083 Ma "late-stage" rift volcanic rocks exposed as the Lake Shore Traps (Michigan), the Schroeder-Lutsen basalts (Minnesota), and the Michipicoten Island

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Section: Geological Setting and Geochronologymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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confidence: 99%

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The End of Midcontinent Rift Magmatism and the Paleogeography of Laurentia

Fairchild

Swanson‐Hysell

Ramezani

et al. 2016

Geological Society of America Abstracts With Programs

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“…The oldest unit of the Oronto Group, the Copper Harbor Conglomerate, interfingers with the overlying Nonesuch Formation and includes lava flows known as the Lake Shore Traps. A high-precision U-Pb date of 1094.1 ___ 3.6 Ma for the Lake Shore Traps provides the age of deposition for the middle Copper Harbor Conglomerate (Davis and Paces, 1990). The Nonesuch Formation and the Freda Sandstone exhibit a conformable and gradational contact frequently extending over several meters (Daniels, 1982).…”

Section: Geologic Settingmentioning

confidence: 99%

Illite/Smectite Geothermometry of the Proterozoic Oronto Group, Midcontinent Rift System

Price

McDowell

1993

Clays and clay miner.

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Abstract--Characterization of the Nonesuch Formation, middle unit of the Proterozoic Oronto Group, as a potential hydrocarbon source for the Lake Superior basin portion of the Midcontinent Rift system requires an understanding of the thermal maturity of the region and its relationship to the thermal history. Illite/smectite (I/S) expandability data were collected from the Nonesuch Formation and the overlying Freda Sandstone and compared with organic thermal maturity data; both data sets coupled with a thermal and burial history for the White Pine area of Michigan allow regional interpretation of maximum formation temperatures of the Nonesuch Formation and the Freda Sandstone with respect to time. Samples collected from drill holes in northeastern Wisconsin display nearly pure smectite within the lower Freda Sandstone trending abruptly to ordered I/S within the Nonesuch Formation. Regular trends of decreasing expandability with depth occur in four other drill holes to the northeast. Comparison ofI/S expandability between similar stratigraphic intervals reveals a significant trend of increasing thermal maturity to the northeast, with the lowest thermal maturities observed in the Iron River Syncline area just west of White Pine, Michigan.I/S geothermometry suggests maximum temperatures in the Nonesuch Formation of 140~ in Wisconsin, 115~ in the Iron River Syncline area, 160~ at White Pine, and 190~ near the southern portions of the Keweenaw Copper District. The geographic pattern of temperatures determined from I/S geothermometry is identical to that determined from organic thermal maturity indicators in the Nonesuch Formation (Imbus et aL, 1988(Imbus et aL, , 1990Hieshima and Pratt, 1991;Pratt et al., 1991;Mauk and Hieshima, 1992).Regular variations in I/S expandability with depth occur in the Freda Sandstone and the Nonesuch Formation near the southern limits of the Keweenaw Copper District. These variations suggest a fossil geothermal gradient of 55~ and limit the thickness of sediment above the Nonesuch Formation to approximately 3 kin. In comparison, 3.6 km of Freda Sandstone are presently exposed near the Wisconsin border, and numerical modeling suggests a range of 4-6 km of sediment overlying the Nonesuch Formarion. None of the data indicate the presence of the Bayfield Group sediments above the Nonesuch Formation at the time of clay diagenesis. Samples from White Pine suggest a two-stage burial history: 1) clay reaction, possible hydrocarbon maturation, and copper-sulfide mineralization at maximum temperatures above 100~ during the main rifting and burial event, followed by 2) fracturing, reverse faulting, and fluid circulation during a rift-terminating compressional event that may have allowed petroleum migration and native copper mineralization at temperatures below 100~ Abrupt changes in I/S expandability with depth and the presence of poorly crystalline I/S (greater than 80% expandable) and kaolinite in the Freda Sandstone in Wisconsin appear to represent later overprinting of the diagenetic assemblage ...

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“…The principal argument is field evidence that documents native copper was precipitated long after almost all of the rift-related magmatism ended [37]. Subsequent radiometric age dating and bracketing of lithostratigraphic units (e.g., [38][39][40]), and the dating of the native copper mineralization itself [41] demonstrate that more than 10 million years elapsed between the end of significant magmatism and native copper mineralization. Temperature history models predict that peak metamorphic conditions at depth in the rift-filling volcanic strata occurred millions of years after rift-filling magmatism ended [42].…”

Section: Model Of Native Copper Depositionmentioning

confidence: 99%

Copper Isotope Constraints on the Genesis of the Keweenaw Peninsula Native Copper District, Michigan, USA

Bornhorst

Mathur

2017

Minerals

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Abstract:The Keweenaw Peninsula native copper district of Michigan, USA is the largest concentration of native copper in the world. The copper isotopic composition of native copper was measured from stratabound and vein deposits, hosted by multiple rift-filling basalt-dominated stratigraphic horizons over 110 km of strike length. The δ 65 Cu of the native copper has an overall mean of +0.28‰ and a range of −0.32‰ to +0.80‰ (excluding one anomalous value). The data appear to be normally distributed and unimodal with no substantial differences between the native copper isotopic composition from the wide spread of deposits studied here. This suggests a common regional and relatively uniform process of derivation and precipitation of the copper in these deposits. Several published studies indicate that the ore-forming hydrothermal fluids carried copper as Cu 1+ , which is reduced to Cu 0 during the precipitation of native copper. The δ 65 Cu of copper in the ore-forming fluids is thereby constrained to +0.80‰ or higher in order to yield the measured native copper values by reductive precipitation. The currently accepted hypothesis for the genesis of native copper relies on the leaching of copper from the rift-filling basalt-dominated stratigraphic section at a depth below the deposits during burial metamorphism. Oxidative dissolution of copper from magmatic source rocks with magmatic δ 65 Cu of 0‰ ± 0.3‰ is needed to obtain the copper isotopic composition of the metamorphogenic ore-forming hydrothermal fluids. In order to accommodate oxidative dissolution of copper from the rift-filling basalt source rocks, the copper needs to have been sited in native copper. Magmatic native copper in basalt is likely stable when the magma is low in sulfur. Low sulfur is predicted by the lack of sulfide minerals in the ore deposits and in the rift-filling basalt-dominated section, which are source rocks, the same rocks through which the ore fluids moved upwards, and the host rocks for the native copper ores. When combined with geologic evidence and inferences, the copper isotopic composition of native copper helps to further constrain the genetic model for this unique mining district.

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Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the Midcontinent Rift system

Cited by 174 publications

References 23 publications

The End of Midcontinent Rift Magmatism and the Paleogeography of Laurentia

The End of Midcontinent Rift Magmatism and the Paleogeography of Laurentia

Illite/Smectite Geothermometry of the Proterozoic Oronto Group, Midcontinent Rift System

Copper Isotope Constraints on the Genesis of the Keweenaw Peninsula Native Copper District, Michigan, USA

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