40Ar/39Ar Spectrum Ages on Logan Intrusions, a Lower Keweenawan Flow, and Mafic Dikes in Northeastern Minnesota—Northwestern Ontario

1975 ◽  
Vol 12 (5) ◽  
pp. 821-835 ◽  
Author(s):  
Gilbert N. Hanson
Keyword(s):  
Lake Superior ◽  
Igneous Rocks ◽  
Basalt Flow ◽  
Mafic Rocks ◽  
Stepwise Heating ◽  
Northwestern Ontario ◽  
Igneous Activity ◽  
Higher Temperature

The 40Ar/39Ar method using stepwise heating has been applied to 11 Precambrian mafic rocks in Minnesota and Ontario to test the technique on samples varying from extremely altered to essentially unaltered and to date some of the Lower and Middle Keweenawan igneous rocks. Four essentially unaltered samples give plateau ages which appear to be reliable and may date the time of intrusion. One strongly metamorphosed sample gives a plateau age which may date the time of metamorphism. The altered mafic rocks give patterns which are inconsistent, difficult to interpret, and have higher ages for the higher temperature fractions. Essentially unaltered samples may also give inconsistent patterns. For the Keweenawan igneous activity along the northwest shore of Lake Superior near the Minnesota–Ontario border and a Logan sill near Lake Nipigon, the plateau ages suggest that three Lower Keweenawan Logan intrusions occurred 1150 to 1170 m. y. ago, although it is possible that the sills near the Minnesota–Ontario border are older; that the Keweenawan igneous activity ended about 1135 m.y. ago, based on one late dike analysis; and that a basal Keweenawan basalt flow was metamorphosed and cooled enough to retain argon by 1095 m.y. ago.

Possible extension of the Midcontinent Rift in west Texas and eastern New Mexico

1994 ◽  
Vol 31 (4) ◽  
pp. 709-720 ◽  
Author(s):  
Donald C. Adams ◽  
G. Randy Keller
Keyword(s):  
New Mexico ◽  
Lake Superior ◽  
Igneous Rocks ◽  
Rift System ◽  
Central Basin ◽  
Midcontinent Rift ◽  
Sacramento Mountains ◽  
Midcontinent Rift System ◽  
Southern Oklahoma ◽  
Igneous Activity

The Midcontinent Rift System forms one of the most prominent gravity features in North America. The recognized geophysical anomaly extends in an arc from southern Oklahoma to Lake Superior and then into southern Michigan. The Midcontinent Rift System was active between 1185–1010 Ma, as indicated in the Lake Superior region by age determinations on intrusive igneous rocks. We suggest that the period of formation of the Midcontinent Rift was also a time of extensive igneous activity in Texas and New Mexico. This activity is represented by intrusions beneath the Central basin platform (Texas and New Mexico), intrusions which crop out at the Pajarito Mountain in the Sacramento Mountains (New Mexico), a basaltic debris flow in the Franklin Mountains (Texas), basalt flows at Van Horn (Texas), and the Crosbyton geophysical anomaly (east of Lubbock, Texas). These bodies and other bodies located by geophysical anomalies and wells drilled into mafic Precambrian rocks may be related to the Midcontinent Rift System. Alternatively this magmatism could be related to Grenville age tectonics in Texas. The mafic igneous rocks in this area form a 530 km diameter Middle Proterozoic igneous province, which formed between 1070 and 1220 Ma. Comparison of the Midcontinent Rift System and its extensions proposed here with the Mesozoic and Cenozoic African rift systems indicates that these features are of comparable scale and complexity.

Triassic olivine-normative diabase from Northumberland Strait, eastern Canada: implications for continental rifting

1986 ◽  
Vol 23 (7) ◽  
pp. 1013-1021 ◽  
Author(s):  
Georgia Pe-Piper ◽  
Lubomir F. Jansa
Keyword(s):  
Early Jurassic ◽  
Igneous Rocks ◽  
Late Triassic ◽  
Geochemical Evolution ◽  
High Alumina ◽  
Continental Rifting ◽  
Eastern Canada ◽  
Mafic Rocks ◽  
Eu Anomaly ◽  
Igneous Activity

Two intervals of mafic igneous rocks were encountered within a Silurian to Carboniferous sequence in an exploratory offshore well located in the Gulf of St. Lawrence, eastern Canada. Geochemical and radiometric analyses show that the lower mafic rocks are Early Silurian continental tholeiite lavas, with their radiogenic clock thermally reset during the Late Devonian. The upper igneous interval consists of several dikes of high-alumina diabase characterized by flat, relatively unenriched REE spectra and a positive Eu anomaly. This diabase resembles olivine tholeiites. Two K/Ar dates suggest a Late Triassic age for these intrusions. The dike composition differs from that of known Late Triassic and Early Jurassic continental tholeiites nearby in Nova Scotia, which are associated with a late stage of continental rifting. The Triassic – Early Jurassic igneous activity shows a pattern of geochemical evolution that we relate to mantle upwelling.

Rb–Sr geochronology of the Coldwell Complex, northwestern Ontario, Canada

1982 ◽  
Vol 19 (9) ◽  
pp. 1796-1801 ◽  
Author(s):  
R. Garth Platt ◽  
Roger H. Mitchell
Keyword(s):  
Upper Mantle ◽  
Lake Superior ◽  
Parental Magma ◽  
Initial Ratio ◽  
North Shore ◽  
The North ◽  
Northwestern Ontario ◽  
Alkaline Intrusion ◽  
Intrusive Rocks ◽  
Igneous Activity

The Coldwell Complex of Northwestern Ontario is North America's largest structurally and petrologically complex alkaline intrusion. Situated on the north shore of Lake Superior, it consists of at least three intrusive centres and is cross-cut by a diverse suite of coeval–cogenetic dikes. The main intrusive rocks range from gabbros to ferroaugite syenites, nepheline syenites, and quartz syenites. The dikes are predominantly lamprophyric. A seventeen point whole rock Rb–Sr isochron (MSWD 2.22) gives an age of 1044.5 ± 6.2 Ma (2σ) and an initial ratio of 0.70354 ± 0.00016 (2σ). The age is late Neohelikian and is younger than the bulk of igneous activity (Keweenawan activity) prevalent in the Lake Superior Basin during the Neohelikian. The low initial ratio indicates an upper mantle origin for the parental magma of the complex.

scholarly journals U–Pb zircon geochronology and phase equilibria modelling of HP-LT rocks in the Ossa-Morena Zone, Portugal

2020 ◽  
Vol 109 (8) ◽  
pp. 2719-2738
Author(s):  
Ismay Vénice Akker ◽  
Lucie Tajčmanová ◽  
Fernando O. Marques ◽  
Jean-Pierre Burg
Keyword(s):  
Phase Equilibria ◽  
Geodynamic Evolution ◽  
Mafic Rocks ◽  
Tholeiitic Magma ◽  
Metamorphic History ◽  
Metamorphic Temperature ◽  
Minimum Age ◽  
Higher Temperature ◽  
Geochemical Analyses ◽  
Orogenic Events

Abstract The Ossa-Morena Zone (OMZ) has a complex geological history including both Cadomian and Variscan orogenic events. Therefore, the OMZ plays an important role in understanding the geodynamic evolution of Iberia. However, the P–T–t evolution of the OMZ is poorly documented. Here, we combine structural and metamorphic analyses with new geochronological data and geochemical analyses of mafic bodies in Ediacaran metasediments (in Iberia known as Série Negra) to constrain the geodynamic evolution of the OMZ. In the studied mafic rocks, two metamorphic stages were obtained by phase equilibria modelling: (1) a high-pressure/low-temperature event of 1.0 ± 0.1 GPa and 470–510 °C, and (2) a medium-pressure/higher-temperature event of 0.6 ± 0.2 GPa and 550–600 °C. The increase in metamorphic temperature is attributed to the intrusion of the Beja Igneous Complex (around 350 Ma) and/or the Évora Massif (around 318 Ma). New U–Pb dating on zircons from the mafic rocks with tholeiitic affinity yields an age between 815 and 790 Ma. If the zircons crystallised from the tholeiitic magma, their age would set a minimum age for the pre-Cadomian basement. The ca. 800 Ma protolith age of HP-LT tholeiitic dykes with a different metamorphic history than the host Série Negra lead us to conclude that: (1) the HP-LT mafic rocks and HP-LT marbles with dykes were included in the Ediacaran metasediments as olistoliths; (2) the blueschist metamorphism is older than 550 Ma (between ca. 790 Ma and ca. 550 Ma, e.g., Cadomian).

Evolution of the Martian Crust

2017 ◽  
Author(s):  
John C. Bridges
Keyword(s):  
Plate Tectonics ◽  
Chemical Weathering ◽  
Compositional Data ◽  
Crustal Contamination ◽  
Planetary Science ◽  
Mantle Metasomatism ◽  
Igneous Rocks ◽  
Forthcoming Article ◽  
Physical Weathering ◽  
Igneous Activity

This is an advance summary of a forthcoming article in the Oxford Encyclopedia of Planetary Science. Please check back later for the full article.Mars, which has a tenth of the mass of Earth, has cooled as a single lithospheric plate. Current topography gravity maps and magnetic maps do not show signs of the plate tectonics processes that have shaped the Earth’s surface. Instead, Mars has been shaped by the effects of meteorite bombardment, igneous activity, and sedimentary—including aqueous—processes. Mars also contains enormous igneous centers—Tharsis and Elysium, with other shield volcanoes in the ancient highlands. In fact, the planet has been volcanically active for nearly all of its 4.5 Gyr history, and crater counts in the Northern Lowlands suggest that may have extended to within the last tens of millions of years. Our knowledge of the composition of the igneous rocks on Mars is informed by over 100 Martian meteorites and the results from landers and orbiters. These show dominantly tholeiitic basaltic compositions derived by melting of a relatively K, Fe-rich mantle compared to that of the Earth. However, recent meteorite and lander results reveal considerable diversity, including more silica-rich and alkaline igneous activity. These show the importance of a range of processes including crystal fractionation, partial melting, and possibly mantle metasomatism and crustal contamination of magmas. The figures and plots of compositional data from meteorites and landers show the range of compositions with comparisons to other planetary basalts (Earth, Moon, Venus). A notable feature of Martian igneous rocks is the apparent absence of amphibole. This is one of the clues that the Martian mantle had a very low water content when compared to that of Earth.The Martian crust, however, has undergone hydrothermal alteration, with impact as an important heat source. This is shown by SNC analyses of secondary minerals and Near Infra-Red analyses from orbit. The associated water may be endogenous.Our view of the Martian crust has changed since Viking landers touched down on the planet in 1976: from one almost entirely dominated by basaltic flows to one where much of the ancient highlands, particularly in ancient craters, is covered by km deep sedimentary deposits that record changing environmental conditions from ancient to recent Mars. The composition of these sediments—including, notably, the MSL Curiosity Rover results—reveal an ancient Mars where physical weathering of basaltic and fractionated igneous source material has dominated over extensive chemical weathering.

Geochronology in the Lake Superior region

1968 ◽  
Vol 5 (3) ◽  
pp. 715-724 ◽  
Author(s):  
S. S. Goldich
Keyword(s):  
World Wide ◽  
Lake Superior ◽  
Decay Constants ◽  
Minnesota River ◽  
The Past ◽  
Daughter Nuclide ◽  
Igneous Activity ◽  
Radiometric Ages ◽  
Minnesota River Valley

During the past 10 years many radiometric ages have been determined on minerals and rocks in the Lake Superior region. The oldest known rocks are the granitic gneisses in the Minnesota River Valley, which have been dated at 3300 to 3550 m.y. ago. Both K–Ar and Rb–Sr methods have been applied to samples from a number of the metasedimentary formations in the region. The ages, however, appear to be the time of folding or of metamorphism rather than of deposition for which only limits or ranges can be given from the ages for associated igneous and metamorphic rocks.Although considerable progress has been made, significant uncertainties remain in the decay constants and in the analytical measurements. More serious problems, however, are geologic ones, such as the effects of metamorphism and of weathering on the parent–daughter nuclide ratios. Both analytical and geological considerations must enter into any proposal for a time classification of the Precambrian.A three-fold division of the Precambrian with time boundaries at 2600 and 1800 m.y. serves well for the Lake Superior region. In addition, the Keweenawan igneous activity is well dated at approximately 1100 m.y. ago. Terms such as Keweenawan, Huronian, and others are best used locally, and time units of a Precambrian classification that might have world-wide utility should not be tied closely to geographic localities. A single radiometric method, as for example K–Ar largely on micas, is not a satisfactory basis for a classification.

Deglaciation chronology and revegetation in northwestern Ontario

1985 ◽  
Vol 22 (6) ◽  
pp. 850-871 ◽  
Author(s):  
Svante Björck
Keyword(s):  
Black Spruce ◽  
Lake Superior ◽  
The South ◽  
Migration Routes ◽  
White Pine ◽  
Cool Period ◽  
Radiocarbon Dates ◽  
Northwestern Ontario ◽  
History Of ◽  
Ice Retreat

Along a 420 km transect in northwestern Ontario, Canada, sediments from four lakes were analyzed with respect to lithology, pollen, and macrofossils. Radiocarbon dates show that the region was deglaciated between ca. 11 500 and 8000 years BP, and periods of both rapid ice retreat and readvance influenced the history of Glacial Lake Agassiz. In the south the ice sheet was succeeded by a lengthy interval of park–tundra with stands of spruce, ash, and elm. The ash and elm seem to have disappeared during a suggested cool period (11 100–10 200 years BP). Farther north the park–tundra phase lasted not more than 50–100 years after ca. 10 200 years BP before boreal trees dominated. The climatic change around 10 200 years BP permitted the very rapid migration of spruce, larch, birch, and jack or red pine into northwestern Ontario from northern Minnesota. The migration routes for Pinus strobus (white pine), Alnus rugosa, and A. crispa were divided, however: one from the south (south of Lake Superior) and one from the east-southeast (north of Lake Superior). White pine reached its maximum distribution 6500–6000 years BP, when the limit was probably 150–200 km north of today's. The composition of the boreal forest during the altithermal was only slightly changed, but the influx of presumed prairie pollen reached a peak ca. 8000–7000 years BP. Since then Picea mariana (black spruce) gradually became the dominating tree species.

Early Historic Populations In Northwestern Ontario: Archaeological And Ethnohistorical Interpretations

1975 ◽  
Vol 40 (1) ◽  
pp. 54-63 ◽  
Author(s):  
Charles A. Bishop ◽  
M. Estellie Smith
Keyword(s):  
Seventeenth Century ◽  
Social Organization ◽  
Lake Superior ◽  
Cultural Ecology ◽  
Northern Ontario ◽  
Ethnic Boundaries ◽  
Northwestern Ontario ◽  
International Border ◽  
North And South

There seems to be little doubt that major population shifts occurred in Northern Ontario and Manitoba following the Iroquois wars of the seventeenth century and the advent of the fur trade. These movements have tended to blur pre-contact ethnic boundaries. Although recent archaeological reports have provided much needed data, differing interpretations of that data have only further obscured the issue. For instance, while some scholars have argued that Blackduck Focus sites in western Northern Ontario and northern Minnesota were occupied by Algonkian-speakers (either Ojibwa or Creej, others have interpreted the materials as having an Assiniboin authorship. In support of the latter view, we analyze both the archaeological and early ethnohistorical data. From these sources we conclude that, at contact, the Ojibwa extended no further west than Michipicoten Bay, while the Cree occupied most of Northern Ontario except for a strip of about 50 mi north and south of the present International border west of Lake Superior which was Assiniboin territory. Proper determination of ethnic boundaries is of theoretical importance to studies of cultural ecology and social organization.

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