Geochemistry of the Mamainse Point volcanics, Ontario, and implications for the Keweenawan paleomagnetic record

1990 ◽  
Vol 27 (9) ◽  
pp. 1194-1199 ◽  
Author(s):  
Kenneth W. Klewin ◽  
Jonathan H. Berg
Keyword(s):  
North America ◽  
Lake Superior ◽  
Lava Flows ◽  
Geochemical Data ◽  
Basaltic Lava ◽  
Trace Element Data ◽  
Eastern Shore ◽  
Midcontinent Rift ◽  
History Of ◽  
Entire Sequence

The Keweenawan (1100 Ma) Mamainse Point volcanics, located along the eastern shore of Lake Superior in Ontario, formed in the Midcontinent Rift of North America. They are a 5250 m thick sequence of over 350 predominantly basaltic lava flows. The Mamainse Point section is the most continuous Keweenawan volcanic sequence and spans nearly the entire igneous history of the rift. The lower part of the section consists of high-MgO picrites and basalts, but the upper part of the section is composed of olivine tholeiites intercalated with numerous conglomerate layers. Major- and trace-element data reveal that the section consists of numerous stratigraphically constrained, geochemically distinct groups of lava flows. The comprehensive geochemical data on the entire sequence indicate that the section has no repetition due to faulting, as has been suggested by other workers on the basis of paleomagnetic studies. Evidently, the three paleomagnetic reversals previously found in the Mamainse Point section are real, and therefore there were multiple paleomagnetic reversals during Keweenawan time.

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.

Lithospheric roots beneath western Laurentia: the geochemical signal in mantle garnets

2003 ◽  
Vol 40 (8) ◽  
pp. 1027-1051 ◽  
Author(s):  
D Canil ◽  
D J Schulze ◽  
D Hall ◽  
B C Hearn Jr. ◽  
S M Milliken
Keyword(s):  
Rare Earth ◽  
North America ◽  
Trace Element ◽  
Earth Element ◽  
Geochemical Data ◽  
Trace Element Data ◽  
Mantle Lithosphere ◽  
Western North America ◽  
Wyoming Province ◽  
Thick Mantle

This study presents major and trace element data for 243 mantle garnet xenocrysts from six kimberlites in parts of western North America. The geochemical data for the garnet xenocrysts are used to infer the composition, thickness, and tectonothermal affinity of the mantle lithosphere beneath western Laurentia at the time of kimberlite eruption. The garnets record temperatures between 800 and 1450°C using Ni-in-garnet thermometry and represent mainly lherzolitic mantle lithosphere sampled over an interval from about 110–260 km depth. Garnets with sinuous rare-earth element patterns, high Sr, and high Sc/V occur mainly at shallow depths and occur almost exclusively in kimberlites interpreted to have sampled Archean mantle lithosphere beneath the Wyoming Province in Laurentia, and are notably absent in garnets from kimberlites erupting through the Proterozoic Yavapai Mazatzal and Trans-Hudson provinces. The similarities in depths of equilibration, but differing geochemical patterns in garnets from the Cross kimberlite (southeastern British Columbia) compared to kimberlites in the Wyoming Province argue for post-Archean replacement and (or) modification of mantle beneath the Archean Hearne Province. Convective removal of mantle lithosphere beneath the Archean Hearne Province in a "tectonic vise" during the Proterozoic terminal collisions that formed Laurentia either did not occur, or was followed by replacement of thick mantle lithosphere that was sampled by kimberlite in the Triassic, and is still observed there seismically today.

The sedimentology, palaeoecology and preservation of the Lower Carboniferous plant deposits at Pettycur, Fife, Scotland

1987 ◽  
Vol 124 (1) ◽  
pp. 43-66 ◽  
Author(s):  
Gillian M. Rex ◽  
Andrew C. Scott
Keyword(s):  
Lava Flows ◽  
Sediment Type ◽  
Basaltic Lava ◽  
Lower Carboniferous ◽  
Plant Fossils ◽  
History Of ◽  
Volcanic Terrain ◽  
Original Ecology ◽  
Plant Assemblages

AbstractThe Lower Carboniferous (Asbian) sediments and volcanics of the Pettycur region in Fife, Scotland, yield several important anatomically preserved floras including that from the famous ‘Pettycur Limestone’. The plant fossils are preserved as calcareous permineralizations and fusain within limestone blocks which occur at the base of basaltic lava flows or within pyroclastic sequences. The geology and sedimentology of these plant deposits have been investigated, and it is demonstrated that a number of plant-bearing facies can be recognized which reflect different modes of transport, deposition and fossilization. Of these facies the ‘Pettycur Limestone’ is the most well known. The lithology is composed of a distinct assemblage dominated by lycopods and the pteridosperm, Heterangium. Other assemblages include a limestone dominated by zygopterid ferns which are frequently fusainized and the Kingswood Limestone which contains a completely different flora to that at Pettycur, being dominated by pteridosperms, other gymnosperms and the lycopod Oxroadia. Each sediment type is characterized by a distinct mineralization history of the plants reflecting different sites of fossilization.A hypothesis concerning the original ecology of the plant assemblages within the basaltic volcanic terrain is proposed. It is suggested that the Pettycur Limestone represents an established original peat within which the plants were permineralized. The zygopterid ferns occupied sites which were susceptible to wildfire and did not establish long-lived peat-forming communities. The Kingswood flora was established in a region where plants were prone to fire and then subsequently transported into an area of limestone deposition along with unburnt plant fragments. This flora was separated by space and/or time from the Pettycur floras. Lakes developed on lava surfaces and provided sites of fossilization for plant fragments as compressions.

Geochemistry of North Shore Hypabyssal dikes and sills in the Midcontinent Rift of Minnesota: an example — the 47th Avenue sill

2004 ◽  
Vol 41 (7) ◽  
pp. 829-842 ◽  
Author(s):  
Karl E Seifert ◽  
James F Olmsted
Keyword(s):  
Lake Superior ◽  
Geochemical Data ◽  
Rift System ◽  
Midcontinent Rift ◽  
North Shore ◽  
Volcanic Group ◽  
The North ◽  
Midcontinent Rift System ◽  
Basalt Composition

This study presents geochemical data for several of the numerous small to large dikes and sills, including the 47th Avenue sill, exposed along the shore of Lake Superior in and north of Duluth, Minnesota. These intrusions are late magmatic features of the Proterozoic Midcontinent Rift System and together form the North Shore Hypabyssal Group. The dikes are geochemically distinct from the sills, and, when the two are exposed together, the younger dike intrudes the older sill. Dikes are primitive with Mg# up to 68, have positive εNd values, and are oriented approximately north–south with steep westerly or near vertical dips. The older sills are more evolved, usually have εNd values near or below 0, and have the same gentle easterly dip as the thick sequence of North Shore Volcanic Group flows they intrude. Dike compositions correlate best with a mixture of widespread basalt compositions types 4 and 5, with primitive geochemistry and positive εNd values, whereas sill compositions are similar to widespread basalt composition type 4 typical of most North Shore Volcanic Group flows. The 47th Avenue sill in Duluth is an evolved single intrusion North Shore Hypabyssal Group diabase sill with trough banding, sharp lower and upper contacts, and a spectacular fractured and undulating roof zone containing blocks of the overlying ferroandesite flow.

Paleointensity study of late Miocene igneous rocks from British Columbia, Canada

1970 ◽  
Vol 7 (1) ◽  
pp. 176-181 ◽  
Author(s):  
D. T. A. Symons ◽  
E. J. Schwarz
Keyword(s):  
Magnetic Field ◽  
British Columbia ◽  
North America ◽  
Late Miocene ◽  
Igneous Rocks ◽  
Lava Flows ◽  
Basaltic Lava ◽  
Remanent Magnetism ◽  
South Central ◽  
Demagnetizing Fields

Sixty-nine specimens representing 49 late Miocene (10–15 m.y. ago) basaltic lava flows and 4 associated gabbroic intrusive plugs were studied in an attempt to estimate the paleointensity of the earth's magnetic field in south-central British Columbia. The paleointensity determination was based on the comparison of the decay of natural remanent magnetism intensity with that of an artificial thermoremanent magnetism (H = 0.35 Oe) in progressively higher alternating demagnetizing fields (peak: 800 Oe). Only 22 of the 69 specimens were considered to yield reliable paleointensity determinations which give an estimated average equatorial intensity for the late Miocene earth's field of 0.18 Oe ± 0.11. This result agrees reasonably well with those from contemporaneous rocks from North America, Japan, and Iceland. Several low determinations with consistent, normal, or reversed remanence directions suggest that the intensity of the non-dipole components of the late Miocene earth's field must have been very small in the sampled area.

Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: evaluating the earliest phases of rift development

2007 ◽  
Vol 44 (8) ◽  
pp. 1087-1110 ◽  
Author(s):  
Thomas Hart ◽  
Adam Richardson ◽  
Carole Anne MacDonald ◽  
Pete Hollings
Keyword(s):  
Continental Crust ◽  
Geochemical Characteristics ◽  
Geochemical Data ◽  
Olivine Gabbro ◽  
Midcontinent Rift ◽  
Northwestern Ontario ◽  
Intrusive Rocks ◽  
History Of

The intrusive rocks of the Nipigon Embayment comprise a series of four mafic to ultramafic intrusions and a number of laterally extensive diabase sills that are among the oldest expression of the ~1.1 Ga Mesoproterozoic Mid-continent Rift. New geochemical data indicate that the sills can be subdivided into five distinct groups: three mafic sills (Nipigon, Inspiration, and McIntyre diabase sills), with the Nipigon sills forming the bulk of the outcrop, and two spatially restricted ultramafic to mafic sills (Jackfish and Shillabeer sills). The latter mafic sills are typically massive, medium-grained, intergranular textured gabbros ranging in thickness from a few metres to more than 250 m. Two of the ultramafic intrusions included in this study (Disraeli and Hele) consist of a core of pyroxene peridotite with olivine gabbro along the margins. The geochemical characteristics of the ultramafic intrusions and diabase sills are consistent with plume-derived melts that have undergone subsequent fractionation and been contaminated by continental crust, likely at depth, but a few samples from the Hele and Disraeli intrusions have the characteristics of primary, uncontaminated melts that have been rapidly transported through the lithosphere with little interaction with wall rocks. The field and geochemical characteristics of the intrusions and sills are consistent with the ultramafic intrusions having been emplaced before the diabase sills and indicate that the history of the Midcontinent Rift is more complex and protracted than previously recognized.

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