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S. W. Nicholson

Publications and source records attributed to S. W. Nicholson.

15 recordsLinked to original sources
USGS2012

Midcontinent rift volcanism in the Lake Superior region: Sr, Nd, and Pb isotopic evidence for a mantle plume origin

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 low-TiO 2 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 87 Sr/ 86 Sr i ≈ 0.7038, ε Nd (1095 Ma) ≈ 0±2, and μ 1 ≈ 8.2. Model ages with respect to a depleted mantle source ( T DM ) average about 1950–2100 Ma. Portage Lake rhyolites fall into two groups. Type I rhyolites have Nd and Pb isotopic characteristics (ε Nd (1095 Ma) ≈ 0 to −4.7; μ 1 ≈ 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 (ε Nd (1095 Ma) ≈ −13 to −16; μ 1 ≈ 7.6–7.7) which are consistent with partial melting of Archean crust. Early Proterozoic crust was not a major contaminant of MRS rocks in the Lake Superior region. Most reported Nd and Pb isotopic compositions of MRS tholeiites from the main stage of volcanism in the Lake Superior region and of the Duluth Complex are comparable to the Nd and Pb isotopic data for Portage Lake tholeiites. The isotopic enrichment of the MRS source compared to depleted mantle is striking and must have occurred at least 700 m.y. before 1100 Ma. There are two likely sources for such enriched MRS tholeiitic magmatism: subcontinental lithospheric mantle enriched during the early Proterozoic or enriched mantle derived from an upwelling plume. Subcontinental lithospheric mantle alone as a source cannot be ruled out strictly on isotopic grounds, but melting of enriched lithosphere potentially ranging in age from 3.6 to 1.8 Ga would have been less likely to generate either the homogeneity or the volume of MRS tholeiites. Decompression melting of an upwelling enriched mantle plume in a region of lithosphere thinned by extension could have successfully generated the enormous volume (850×10 3 km 3 ) of relatively homogeneous magma in a restricted time interval.

Journal of Geophysical Research Solid Earth↗
USGS2004

Bedrock geologic map of the Port Wing, Solon Springs, and parts of the Duluth and Sandstone 30' X 60' quadrangles, Wisconsin

This Open-File Report provides digital data (shapefiles and .e00 files) for the bedrock geology in the Port Wing, Solon Springs, and parts of the Duluth and Sandstone quadrangles in Wisconsin. A Miscellaneous Investigations Series map (I map) is currently in review with analogous data in paper format. This map portrays the geology of part of the Midcontinent rift system (MRS) along the southern extension of the Lake Superior syncline in northern Wisconsin. The map area contains the St. Croix horst, a rift graben filled with Mesoproterozoic rocks of the Keweenawan Supergroup that was subsequently inverted. The horst exposes about 15 - 20 km of strata that record the opening of the Midcontinent rift, its subsequent transition to a thermal subsidence basin, and eventual inversion. About 3 km of underlying Mesoproterozoic strata, including the Gogebic iron range, and about 10 km of Neoarchean rocks, exposed in the southernmost part of the map area lie to the southeast of the horst. The nearly flat-lying continental red beds of the Oronto and Bayfield Groups, the youngest strata of the Keweenawan Supergroup, overlie the volcanic rocks. A wealth of geologic data exists for the area as a result of many individual studies over the last hundred years, but much has remained unpublished in theses, dissertations, and other reports of limited availability. This map has incorporated most of that data (see list of data sources) and includes results of our investigations conducted from 1992 to 2000. Our studies were designed to fill gaps in existing data and reconcile conflicting interpretations on some aspects of the geology of the region. The purpose of this map is to complete digital coverage of quadrangles with significant exposure of rocks of the Midcontinent rift in Wisconsin and Michigan at a scale of 1:100,000. Other maps in this series include the bedrock geologic maps of the Ontonagon-Wakefield, the Ashland-Ironwood, and the Keweenaw Peninsula areas (USGS Open-File Reports 01-088 , 99-546 , and 99-149 , respectively).

Open-File Report↗
USGS2001

USGS Mineral Resources Program; national maps and datasets for research and land planning

The U.S. Geological Survey, the Nation’s leader in producing and maintaining earth science data, serves as an advisor to Congress, the Department of the Interior, and many other Federal and State agencies. Nationwide datasets that are easily available and of high quality are critical for addressing a wide range of land-planning, resource, and environmental issues. Four types of digital databases (geological, geophysical, geochemical, and mineral occurrence) are being compiled and upgraded by the Mineral Resources Program on regional and national scales to meet these needs. Where existing data are incomplete, new data are being collected to ensure national coverage. Maps and analyses produced from these databases provide basic information essential for mineral resource assessments and environmental studies, as well as fundamental information for regional and national land-use studies. Maps and analyses produced from the databases are instrumental to ongoing basic research, such as the identification of mineral deposit origins, determination of regional background values of chemical elements with known environmental impact, and study of the relationships between toxic elements or mining practices to human health. As datasets are completed or revised, the information is made available through a variety of media, including the Internet. Much of the available information is the result of cooperative activities with State and other Federal agencies. The upgraded Mineral Resources Program datasets make geologic, geophysical, geochemical, and mineral occurrence information at the state, regional, and national scales available to members of Congress, State and Federal government agencies, researchers in academia, and the general public. The status of the Mineral Resources Program datasets is outlined below.

Fact Sheet↗
USGS1997

Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: Implications for multiple mantle sources during rift development

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 (ε Nd(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 (ε Nd(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 (ε Nd(1100) value of about 0). Towards the end of magmatism, mixtures of melts from the plume and a depleted asthenospheric mantle source became dominant (ε Nd(1100) = 0 to +3).

Michigan, Wisconsin↗
USGS1997

U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior

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 207 Pb/ 206 Pb 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 207 Pb/ 206 Pb 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 207 Pb/ 206 Pb 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 207 Pb/ 206 Pb 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.

Michigan, Wisconsin↗
USGS1992

Metallogeny of the midcontinent rift system of North America

The 1.1 Ga Midcontinent rift system of North America is one of the world's major continental rifts and hosts a variety of mineral deposits. The rocks and mineral deposits of this 2000 km long rift are exposed only in the Lake Superior region. In the Lake Superior region, the rift cuts across Precambrian basement terranes ranging in age from ∼ 1850 Ma to more than 3500 Ma. Where exposed, the rift consists of widespread tholeiitic basalt flows with local interlayered rhyolite and clastic sedimentary rocks. Beneath the center of Lake Superior the volcanic and sedimentary rocks are more than 30 km deep as shown by recent seismic reflection profiles. This region hosts two major classes of mineral deposits, magmatic and hydrothermal. All important mineral production in this region has come from hydrothermal deposits. Rift-related hydrothermal deposits include four main types: (1) native copper deposits in basalts and interflow sediments; (2) sediment-hosted copper sulfide and native copper; (3) copper sulfide veins and lodes hosted by rift-related volcanic and sedimentary rocks; and (4) polymetallic (five-element) veins in the surrounding Archean country rocks. The scarcity of sulfur within the rift rocks resulted in the formation of very large deposits of native metals. Where hydrothermal sulfides occur (i.e., shale-hosted copper sulfides), the source of sulfur was local sedimentary rocks. Magmatic deposits have locally supported exploration and minor production, but most are subeconomic presently. These deposits occur in intrusions exposed near the margins of the rift and include CuNiPGE and TiFe (V) in the Duluth Complex, U-REE-Nb in small carbonatites, and breccia pipes resulting from local hydrothermal activity around small felsic intrusions. Mineralization associated with some magmatic bodies resulted from the concentration of incompatible elements during fractional crystallization. Most of the sulfide deposits in intrusions, however, contain sulfur derived from country rocks; the interaction between magma and country rocks was important in generation of the magmatic CuNi sulfide deposits. A mantle plume origin has been proposed for the formation of the Midcontinent rift. More than 1 million km 3 of mafic magma was erupted in the rift and a comparable volume of mafic intrusions are inferred beneath the rift, providing a ready and structurally confined supply of mafic source rocks that were available for leaching of metals by basinal brines. These brines were heated by a steep geothermal gradient that resulted from the melting and underplating of magma derived from the plume. Hydrothermal deposits were emplaced for at least 30–40 m.y. after rift magmatism and extension ceased. This time lag may reflect either the time required to heat deeply buried rocks and fluids within the rift, or may be due to the timing of post-rift compression that may have provided the driving mechanism for expulsion of hydrothermal fluids from deep portions of the rift.

Precambrian Research↗