Lower Proterozoic volcanic rocks and their setting in the southern Lake Superior district

1983 ◽  
pp. 67-85 ◽  
Author(s):  
Jeffrey K. Greenberg ◽  
Bruce A. Brown
Keyword(s):  
Lake Superior ◽  
Volcanic Rocks ◽  
Lower Proterozoic

Geochemistry and petrogenesis of the early Proterozoic Hemlock volcanic rocks and the Kiernan sills, southern Lake Superior region

1988 ◽  
Vol 25 (4) ◽  
pp. 528-546 ◽  
Author(s):  
W. C. Ueng ◽  
T. P. Fox ◽  
D. K. Larue ◽  
J. T. Wilband
Keyword(s):  
Trace Elements ◽  
Rare Earth ◽  
North Atlantic ◽  
North Atlantic Ocean ◽  
Lake Superior ◽  
Volcanic Rocks ◽  
Wall Rock ◽  
Major Elements ◽  
Chemical Contamination ◽  
Early Proterozoic

During the early Proterozoic, the 2 km thick differentiated gabbroic Kiernan sills were emplaced into a thick accumulation of pillow basalt and associated deep-water strata, the Hemlock Formation, in the southern Lake Superior region. On the basis of major elements and trace elements (including rare-earth-element data), the Kiernan sills and the hosting volcanic rocks of the Hemlock Formation were determined to be comagmatic in origin, and both evolved from assimilation – crystal fractionation processes. The major assimilated components in these igneous rocks are identified as terrigenous sedimentary rocks. Assimilation affected the abundance of Nb, Ta, light rare-earth elements, and most likely P, Rb, Th, and K in the magma. The effect of chemical contamination from wall-rock assimilation accumulates with increasing differentiation.With wall-rock contamination carefully evaluated, a series of tectonic discriminating methods utilizing immobile trace elements indicates that the source magma was a high-Ti tholeiitic basalt similar to present-day mid-ocean-ridge basalts (MORB). It is suggested from this study that most of the enriched large-ion lithophile elements and LREE of the magma were not inherited from the mantle but from assimilation of supracrustal rocks. Chemical signatures of these rocks are distinctively different from those of arc-related volcanics. A rifting tectonic regime analogous to the opening of the North Atlantic Ocean and extrusion of North Atlantic Tertiary volcanics best fits the criteria revealed by this study.

The Beginnings of Microscopic Petrography in the United States, 1870-1885

1994 ◽  
Vol 13 (1) ◽  
pp. 35-46 ◽  
Author(s):  
Carl-Henry Geschwind
Keyword(s):  
United States ◽  
Lake Superior ◽  
Volcanic Rocks ◽  
The United States ◽  
Substantial Portion ◽  
The West ◽  
The U.S

In the 1860's and 70's, microscopic petrography flourished in Germany, where descriptions and classifications of rocks were highly valued for their own sake. American geologists, however, were more interested in stratigraphical correlations and had relatively little use for petrographical details. Thus, such Americans as George Hawes and Alexis Julien, who attempted to introduce the microscope for purely petrographical work in the early 1870's, had great difficulties in finding an audience. During the late 1870's, however, a number of American geologists-including federal geologists working amongst the volcanic rocks of the West, state geologists mapping in the Lake Superior region, and mining geologists examining the Comstock Lode and the Leadville district-came to appreciate the aid microscopic petrography could provide for stratigraphical correlations. This growing interest led to the hiring of a number of microscopic petrographers around 1880. These petrographers were trained in Germany, where they had imbibed the German passion for petrography for its own sake, but most of them adapted themselves to the American practice of using petrography for stratigraphy. Unlike many of their German counterparts, these American petrographers spent a substantial portion of their time in the field and combined mapping with microscopic examinations to solve stratigraphical problems. Thus, the different scientific cultures of Germany and the U.S. significantly affected the ways in which the petrographic microscope was used.

Geology and petrology of Proterozoic mafic dikes, north-central Minnesota and western Ontario

1983 ◽  
Vol 20 (4) ◽  
pp. 622-638 ◽  
Author(s):  
D. L. Southwick ◽  
W. C. Day
Keyword(s):  
Hot Spot ◽  
Lake Superior ◽  
Volcanic Rocks ◽  
North Central ◽  
Early Proterozoic ◽  
Iron Range ◽  
Tectonic Model ◽  
Group A ◽  
Archean Crust ◽  
Mesabi Iron Range

Proterozoic diabase and gabbro dikes (~2120 Ma old) form a major northwest-oriented swarm extending about 300 km from the Mesabi iron range in Minnesota to the vicinity of Kenora, Ontario. The dikes were emplaced into Archean crust at about the time that the Early Proterozoic basin of the Lake Superior region was opening by rifting, and the swarm and the basin may therefore be tectonically related. The dikes are overlain unconformably by the Animikie Group, the upper sedimentary sequence in the Proterozoic basin in Minnesota, but may be approximately coeval with mafic volcanic rocks in the pre-Animikie Mille Lacs Group. A two-stage tectonic model involving (1) regional right-lateral crustal shear in the late Archean and (2) hot-spot rifting in the Early Proterozoic is proposed to account for the swarm.The dikes are iron-rich quartz tholeiites that are differentiated toward dioritic compositions. Late alteration to hydrous phases, including blue–green amphibole, chlorite, and sericite, together with lesser amounts of prehnite and epidote, is ubiquitous but variable in intensity, and is regarded as a deuteric phenomenon. The interior portions of some large dikes are compositionally layered parallel to contacts; the layers differ from each other in the proportions of primary hornblende, clinopyroxene, and plagioclase and thus range in composition across the gabbro–diorite boundary.Chilled margins of the dikes contain flow-aligned phenocrysts of plagioclase, clinopyroxene, titanomagnetite, and ilmenite. The clinopyroxenes occur as three morphotypes that have distinct compositions, indicating a complex intratelluric history. The dike magma was emplaced into cool Archean crust at an inferred temperature of about 1085 °C and was quenched in a matter of minutes at the dike walls. Complete solidification at the centers of dikes wider than 100 m appears to have taken more than 40 years.

Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: implications for the tectonic setting of Paleoproterozoic iron formations of the Lake Superior region

2002 ◽  
Vol 39 (6) ◽  
pp. 999-1012 ◽  
Author(s):  
D A Schneider ◽  
M E Bickford ◽  
W F Cannon ◽  
K J Schulz ◽  
M A Hamilton
Keyword(s):  
Lake Superior ◽  
Tectonic Setting ◽  
Volcanic Rocks ◽  
Age Determination ◽  
Iron Formation ◽  
Continental Rifting ◽  
The South ◽  
Zircon Age ◽  
Iron Formations ◽  
Northern Michigan

A rhyolite in the Hemlock Formation, a mostly bimodal submarine volcanic deposit that is laterally correlative with the Negaunee Iron-formation, yields a sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon age of 1874 ± 9 Ma, but also contains inherited Archean zircons as old as 3.8 Ga. This precise age determination for the classic Paleoproterozoic stratigraphic sequence of northern Michigan, the Marquette Range Supergroup (MRS), necessitates modification of previous depositional and tectonic models. Our new data indicate that the Menominee Group, previously ascribed to continental rifting during early, pre-collision phases of the Penokean orogenic cycle, is coeval with arc-related volcanic rocks now preserved as accreted terranes immediately to the south and is more aptly interpreted as a foredeep deposit. We interpret these to be second-order basins created by oblique subduction of the continental margin rather than basins formed on a rifting margin. Along with a recently reported age for the Gunflint Formation in Ontario of 1878 ± 2 Ma, our data suggest that an extensive foredeep in the western Lake Superior region was the locus of iron-formation deposition during arc accretion from the south. Further, we interpret the lower MRS (Chocolay Group), a glaciogenic and shallow-marine succession that lies atop Archean basement, to be equivalent to the upper part of the Huronian Supergroup of Ontario and to represent the original continental rifting and passive-margin phase of the Penokean cycle. The upper MRS (Baraga Group) represents deeper marine basins, dominated by turbidites and lesser volcanic rocks, resulting from increased subsidence and continued collision. A stitching pluton, which cuts correlatives of the Hemlock Formation in a thrust sheet, yielded a U–Pb zircon age of 1833 ± 6 Ma, consistent with other post-tectonic plutons in Wisconsin and northern Michigan, indicating that Penokean convergence lasted no longer than ~40 million years.

An investigation of Superior Shoal, central Lake Superior, with a manned submersible

1991 ◽  
Vol 28 (1) ◽  
pp. 145-150 ◽  
Author(s):  
Matthew L. Manson ◽  
Henry C. Halls
Keyword(s):  
Lake Superior ◽  
Complex Structure ◽  
Volcanic Rocks ◽  
Lava Flows ◽  
Basaltic Lava ◽  
Isle Royale ◽  
Midcontinent Rift ◽  
Boundary Fault ◽  
The North ◽  
Manned Submersible

A Johnson-Sea-Link submersible was used to examine the geology of Superior Shoal in central Lake Superior. Here, glacially scoured, vertical cliffs, some more than 100 m high, are formed of 1.1 Ga middle Keweenawan basaltic lava flows displaying ophitic interiors and red amygdaloidal tops. Flat-lying sandstones, lithologically similar to the upper Keweenawan Bayfield–Jacobsville sequences, occur to the north of the volcanic rocks. These are inferred to have been downthrown along an eastward extension of the Isle Royale fault, a major boundary fault of the Midcontinent rift. The volcanic rocks are normally magnetized, supporting lithological evidence that they correlate with the middle Keweenawan sequence on Isle Royale. Paleomagnetic data suggest that the volcanics have a complex structure, possibly involving drag folding along the Isle Royale fault.

Late Precambrian structural evolution of Pigeon Point, Minnesota and relations to the Lake Superior syncline

1976 ◽  
Vol 13 (7) ◽  
pp. 877-888 ◽  
Author(s):  
M. G. Mudrey Jr.
Keyword(s):  
Lake Superior ◽  
Volcanic Rocks ◽  
Structural Evolution ◽  
Cook County ◽  
Tectonic Event ◽  
Late Precambrian ◽  
North Shore ◽  
Volcanic Group ◽  
The North ◽  
Tectonic Events

A sequence of semi-brittle deformational tectonic events in gently dipping middle Precambrian argillite and graywacke, and late Precambrian sandstone and volcanic rocks on the northwest coast of Lake Superior is interpreted from detailed geologic mapping in the Pigeon Point, Cook County, Minnesota area. The earliest tectonic event was broad, open folding in the middle Precambrian Rove Formation along N 35 °E axes and the development of two sets of joints (North and N 70° E). The second event was eastward trending, high-angle faulting in the Rove Formation and the disconformably overlying late Precambrian Puckwunge sandstone and North Shore Volcanic Group; the south sides moved upward relative to the north sides. These faults and associated joints were the loci of emplacement of 'early mafic' dikes of ilmenite-bearing diabase, which probably correlate with the 'Logan intrusions'. Subsequently, east-northeastward trending olivine diabase sills and dikes were emplaced in the Rove Formation and the North Shore Volcanic Group, and they cross-cut the early mafic dikes and sills. The third event occurred after cessation of igneous activity; two sets of regional joints (N 14 °W and N 53 °E) were formed, apparently as a result of fracturing accompanying initial subsidence of the so-called 'Lake Superior syncline' to the southeast. The fourth event was the development of a zone of cataclastic rock trending N 65 °E from the Pigeon Point area northeastward at least 60 km. To the north of this fracture zone, bedding in the Rove Formation dips 15–25 °SE. Copper and silver mineralization was subsequently emplaced within this fracture zone.The first three tectonic events appear to represent reactivation of structures of early Precambrian age, and appear to control the general outcrop pattern. The fourth event is an entirely Keweenawan feature, and marks the beginning of subsidence of the 'Lake Superior syncline'. It may correlate in time with the deposition of the Copper Harbor Conglomerate, and other late middle Keweenawan events.

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