The chemical evolution of the Canadian Shield

1968 ◽  
Vol 5 (5) ◽  
pp. 1247-1252 ◽  
Author(s):  
W. F. Fahrig ◽  
K. E. Eade
Keyword(s):  
Chemical Evolution ◽  
Sedimentary Basins ◽  
Crystalline Rocks ◽  
Canadian Shield ◽  
Progressive Change ◽  
Chemical Zoning ◽  
Crystalline Crust ◽  
Secondary Result ◽  
Dike Swarms ◽  
Proterozoic Age

Archean surface crystalline rocks of the Canadian Shield differ chemically from those of Proterozoic age. The younger rocks are higher in K2O, TiO2, U, and Th and lower in Na2O, Cr, Ni, and possibly MgO and CaO. This may be a secondary result of vertical chemical zoning of the crust as a result of metamorphism accompanied by anatectic melting. The erosion of a zoned crust would result in the enrichment of elements such as K, Na, Si, U, and Th in younger sedimentary basins. The crystalline crust subsequently evolved from the sediments of these basins would be enriched in these elements relative to older rocks exposed as a result of erosion. The secular chemical evolution of the shield may also reflect a progressive, though irregular, shift in the composition of new sial being added to the crust. Evidence for this progressive change is found in the increase in the abundance of potassium of successive dike swarms that were feeders of new sial to the upper part of the crust.

Flood basalts, dike swarms and sub-crustal flow

1968 ◽  
Vol 5 (1) ◽  
pp. 93-96 ◽  
Author(s):  
Paul M. Clifford
Keyword(s):  
Unsteady Motion ◽  
Canadian Shield ◽  
Published Data ◽  
Flood Basalts ◽  
Crustal Movements ◽  
Geologic Time ◽  
Dike Swarms ◽  
Convective Cells

A review of published data on flood basalts and diabase dike swarms suggests that they are related in origin, typically being developed in zones of crustal tension induced by sub-crustal flow. Perhaps, therefore, these basaltic effusions may be used to map convective cells in the mantle for various periods during geologic time. The inferred pattern of sub-crustal movements, particularly for the southern continents, is seen to be complicated, both temporarily and spatially, and is strongly suggestive of unsteady motion. Dikes of the Canadian Shield permit inferences about the site of convective cells back to 2150 million years.

The oxygen isotope composition of the surface crystalline rocks of the Canadian Shield

1978 ◽  
Vol 15 (11) ◽  
pp. 1773-1782 ◽  
Author(s):  
Yuch-Ning Shieh ◽  
Henry P. Schwarcz
Keyword(s):  
Isotope Composition ◽  
Crystalline Rocks ◽  
Canadian Shield ◽  
Granitic Rocks ◽  
Baffin Island ◽  
Grenville Province ◽  
Metasedimentary Rocks ◽  
Rock Types ◽  
Northern District ◽  
Basic Rocks

The average 18O/16O ratios of the major rock types of the surface crystalline rocks in different parts of the Canadian Precambrian Shield have been determined, using 47 composite samples prepared from 2221 individual rock specimens. The sampling areas include Baffin Island, northern and southwestern Quebec, Battle Harbour – Cartwright, northern District of Keewatin, Fort Enterprise, Snowbird Lake, Kasmere Lake, and Saskatchewan, covering approximately 1 400 000 km2. The granitic rocks from the Superior, Slave, and Churchill Provinces vary only slightly from region to region (δ18O = 6.9–8.4‰) and are significantly lower in 18O than similar rock types from the younger Grenville Province (δ = 9.2–10.0‰). The sedimentary and metasedimentary rocks have δ18O = 9.0–11.7‰ and hence are considerably lower than their Phanerozoic equivalents, possibly reflecting the presence of a high percentage of little-altered igneous rock detritus in the original sediments. The basic rocks in most regions fall within a δ18O range of 6.8–7.6‰, except in northern and southwestern Quebec where the δ-values are abnormally high (8.5–8.9‰). The overall average 18O/16O ratio of the surface crystalline rocks of the Canadian Shield is estimated to be 8.0‰, which represents an enrichment with respect to probable mantle derived starting materials by about 2‰.

scholarly journals Isotopic studies in the East Greenland Caledonides (72°–74°N) – Precambrian and Caledonian ages

1981 ◽  
Vol 104 ◽  
pp. 47-72
Author(s):  
D.C Rex ◽  
A.R Gledhill
Keyword(s):  
Crystalline Rocks ◽  
Reference Line ◽  
Published Data ◽  
East Greenland ◽  
Early Proterozoic ◽  
Late Proterozoic ◽  
Isotopic Studies ◽  
Data Points ◽  
Proterozoic Age

Rb-Sr isotopic studies on crystalline rocks from different rock units of the East Greenland Caledonides have given a number of isochron and 'errorchron' ages, as well as sets of inconclusive data. All the results are presented here as an illustration of the problems encountered, though previously published data is given in summary form. Eleven collections from Archaean - early Proterozoic rock units have given seven early Proterozoic Rb-Sr whole rock isochrons, while one set of data points plot about a 2450 m.y. reference line. Seven collections from metasediments of presumed middle to late Proterozoic age produced only one 'errorchron' of c. 1245 m.y., but data points from all the collections plot about a 1250 m.y. reference line. Six collections from a suite of 'older' granites gave four isochron ages of 1080, 1000,765 and 650 m.y. Eight collections from Caledonian granites gave four isochron ages, three 'errorchrons' and one mineral-whole rock isochron; the ages fall in the range 377-550 m.y.

scholarly journals Ambient seismic noise tomography of Canada and adjacent regions: Part I. Crustal structures

2021 ◽  
Author(s):  
H Kao ◽  
Y Behr ◽  
C Currie ◽  
R Hyndman ◽  
John Townend ◽  
...  
Keyword(s):  
Ambient Noise ◽  
Large Scale ◽  
Sedimentary Basins ◽  
Shear Velocity ◽  
Canadian Shield ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Waveform Data ◽  
Continental Scale ◽  
Crustal Structures

This paper presents the first continental-scale study of the crust and upper mantle shear velocity (Vs) structure of Canada and adjacent regions using ambient noise tomography. Continuous waveform data recorded between 2003 and 2009 with 788 broadband seismograph stations in Canada and adjacent regions were used in the analysis. The higher primary frequency band of the ambient noise provides better resolution of crustal structures than previous tomographic models based on earthquake waveforms. Prominent low velocity anomalies are observed at shallow depths (<20 km) beneath the Gulf of St. Lawrence in east Canada, the sedimentary basins of west Canada, and the Cordillera. In contrast, the Canadian Shield exhibits high crustal velocities. We characterize the crust-mantle transition in terms of not only its depth and velocity but also its sharpness, defined by its thickness and the amount of velocity increase. Considerable variations in the physical properties of the crust-mantle transition are observed across Canada. Positive correlations between the crustal thickness, Moho velocity, and the thickness of the transition are evident throughout most of the craton except near Hudson Bay where the uppermost mantle Vs is relatively low. Prominent vertical Vs gradients are observed in the midcrust beneath the Cordillera and beneath most of the Canadian Shield. The midcrust velocity contrast beneath the Cordillera may correspond to a detachment zone associated with high temperatures immediately beneath, whereas the large midcrust velocity gradient beneath the Canadian Shield probably represents an ancient rheological boundary between the upper and lower crust. Key Points Unprecedented large-scale and homogeneous analysis Considerable crustal variations across Canada Prominent mid-crust velocity gradients beneath Cordillera and Canadian Shield ©2013. American Geophysical Union. All Rights Reserved.

scholarly journals Ambient seismic noise tomography of Canada and adjacent regions: Part I. Crustal structures

2021 ◽  
Author(s):  
H Kao ◽  
Y Behr ◽  
C Currie ◽  
R Hyndman ◽  
John Townend ◽  
...  
Keyword(s):  
Ambient Noise ◽  
Large Scale ◽  
Sedimentary Basins ◽  
Shear Velocity ◽  
Canadian Shield ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Waveform Data ◽  
Continental Scale ◽  
Crustal Structures

This paper presents the first continental-scale study of the crust and upper mantle shear velocity (Vs) structure of Canada and adjacent regions using ambient noise tomography. Continuous waveform data recorded between 2003 and 2009 with 788 broadband seismograph stations in Canada and adjacent regions were used in the analysis. The higher primary frequency band of the ambient noise provides better resolution of crustal structures than previous tomographic models based on earthquake waveforms. Prominent low velocity anomalies are observed at shallow depths (<20 km) beneath the Gulf of St. Lawrence in east Canada, the sedimentary basins of west Canada, and the Cordillera. In contrast, the Canadian Shield exhibits high crustal velocities. We characterize the crust-mantle transition in terms of not only its depth and velocity but also its sharpness, defined by its thickness and the amount of velocity increase. Considerable variations in the physical properties of the crust-mantle transition are observed across Canada. Positive correlations between the crustal thickness, Moho velocity, and the thickness of the transition are evident throughout most of the craton except near Hudson Bay where the uppermost mantle Vs is relatively low. Prominent vertical Vs gradients are observed in the midcrust beneath the Cordillera and beneath most of the Canadian Shield. The midcrust velocity contrast beneath the Cordillera may correspond to a detachment zone associated with high temperatures immediately beneath, whereas the large midcrust velocity gradient beneath the Canadian Shield probably represents an ancient rheological boundary between the upper and lower crust. Key Points Unprecedented large-scale and homogeneous analysis Considerable crustal variations across Canada Prominent mid-crust velocity gradients beneath Cordillera and Canadian Shield ©2013. American Geophysical Union. All Rights Reserved.

Sulfate in brines in the crystalline rocks of the Canadian shield

1994 ◽  
Vol 58 (1) ◽  
pp. 57-65 ◽  
Author(s):  
P. Fritz ◽  
S.K. Frape ◽  
R.J. Drimmie ◽  
E.C. Appleyard ◽  
K. Hattori
Keyword(s):  
Crystalline Rocks ◽  
Canadian Shield
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