The densities of Precambrian rocks from northern Manitoba

1968 ◽  
Vol 5 (3) ◽  
pp. 433-438 ◽  
Author(s):  
R. A. Gibb
Keyword(s):  
Rock Type ◽  
Gravity Anomalies ◽  
Average Density ◽  
Canadian Shield ◽  
Granitic Rocks ◽  
Precambrian Rock ◽  
Average Composition ◽  
Precambrian Rocks ◽  
Surface Rock ◽  
Greenstone Belts

To assist in the interpretation of gravity anomalies in northern Manitoba and northeastern Saskatchewan, a part of the western Canadian Shield, the average densities of 2004 Precambrian rock samples, grouped according to rock type, are tabulated. Relatively positive anomalies may be expected over greenstone belts (2.85 g/cm3), sedimentary gneisses (2.79 g/cm3), granulites (2.73 g/cm3), and mafic to ultramafic intrusions—diorite (2.77 g/cm3), gabbro (3.00 g/cm3), and peridotite (3.19 g/cm3), whereas negative anomalies may be expected over granitic rocks (2.64 g/cm3). These results are consistent with results from other areas.The average composition of a slab of surface rock in this part of the western Canadian Shield is shown to be granodioritic with an average density of 2.67 g/cm3.

scholarly journals Pre-Nagssugtoqidian crustal evolution in West Greenland: geology, geochemistry and deformation of supracrustal and granitic rocks north-east of Kangaatsiaq

2006 ◽  
Vol 11 ◽  
pp. 33-52
Author(s):  
Jean-François Moyen ◽  
Gordon R. Watt
Keyword(s):  
Sedimentary Basin ◽  
Granulite Facies ◽  
Great Circle ◽  
Granitic Rocks ◽  
Early Event ◽  
Porphyritic Granite ◽  
The Core ◽  
North East ◽  
Greenstone Belts ◽  
Area North

The area north-east of Kangaatsiaq features polyphase grey orthogneisses, supracrustal rocks and Kangaatsiaq granite exposed within a WSW–ENE-trending synform. The supracrustal rocks are comprised of garnet-bearing metapelites, layered amphibolites and layered, likewise grey biotite paragneisses. Their association and geochemical compositions are consistent with a metamorphosed volcano-sedimentary basin (containing both tholeiitic and calc-alkali lavas) and is similar to other Archaean greenstone belts. The Kangaatsiaq granite forms a 15 × 3 km flat, subconcordant body of deformed, pink, porphyritic granite occupying the core of the supracrustal synform, and is demonstrably intrusive into the amphibolites. The granite displays a pronounced linear fabric (L or L > S). The post-granite deformation developed under lower amphibolite facies conditions (400 ± 50°C), and is characterised by a regular, NE–SW-trending subhorizontal lineation and an associated irregular foliation, whose poles define a great circle; together they are indicative of highly constrictional strain. The existence of a pre-granite event is attested by early isoclinal folds and a foliation within the amphibolites that is not present in the granite, and by the fact that the granite cuts earlier structures in the supracrustal rocks. This early event, preserved only in quartz-free lithologies, resulted in high-temperature fabrics being developed under upper amphibolite to granulite facies conditions.

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‰.

The geochronology of the granitic rocks along the Bear–Slave Structural Province boundary, northwest Canadian Shield

1977 ◽  
Vol 14 (6) ◽  
pp. 1356-1373 ◽  
Author(s):  
Rosaline Frith ◽  
R. A. Frith ◽  
R. Doig
Keyword(s):  
Age Groups ◽  
Canadian Shield ◽  
Granitic Rocks ◽  
Thermal Event ◽  
Slave Province ◽  
Fault Block ◽  
Granitic Intrusions ◽  
Main Period ◽  
Archean Crust ◽  
Archean Basement

Archean granitic rocks along the southern Bear–Slave boundary fall into three age groups: the oldest are 3000 Ma old intrusive tonalites and granodiorites that form the basement to the Yellowknife Supergroup; the second are syn-volcanic granitic intrusions of ~ 2700 Ma; and the youngest are ~ 2560 Ma granitic and migmatitic diapirs formed in part from supracrustal and granitic rocks. Two Proterozoic thermal events are recognized within the Slave Province. A ~ 2300 Ma event may be related to early rift breakup of the Archean crust and is recorded in Rb–Sr whole-rock and K–Ar mineral systems. A ~ 1970 Ma event was less intense but may be related to further rifting of the Archean and to fault-block depression of the Indin Lake supracrustal basin, the intrusion of a group of granodioritic stocks, and the formation or granitic pegmatite.Within the Bear Province, evidence of a ~ 2700 Ma intrusive event and a ~ 2300 Ma thermal event are preserved in Rb–Sr whole rock systems. Practically all the granitic rocks of the Bear Province, including the Hepburn batholitic rocks, are thought to have been derived wholly or partly from Archean rocks. The main period of Hudsonian deformation and metamorphism was accompanied by a diapiric remobilization of the Archean basement about 1800 Ma ago. Twelve Rb–Sr isochrons, as well as other published geochronologic data from the region, support these conclusions.

THE SHAPE AND THICKNESS OF AN ARCHAEAN GREENSTONE BELT BY GRAVITY METHODS

1965 ◽  
Vol 2 (5) ◽  
pp. 418-424 ◽  
Author(s):  
F. S. Grant ◽  
W. H. Gross ◽  
M. A. Chinnery
Keyword(s):  
Greenstone Belt ◽  
Granitic Rocks ◽  
Superior Province ◽  
Precambrian Shield ◽  
Maximum Thickness ◽  
Red Lake ◽  
Gravity Effects ◽  
Greenstone Belts

The Red Lake greenstone belt is Archaean in age (older than 2.5 billion years) and is located in the Superior province of the Canadian Precambrian Shield. It is a fairly typical greenstone belt, being composed of a complex assemblage of lavas, sediments, and intrusives. The belt is completely surrounded, and therefore is isolated from other greenstone belts, by granitic batholiths and acid paragneiss. Generally speaking, greenstones are more dense than the surrounding granitic rocks and they therefore give positive gravity effects, the amplitudes of which give some indication of their shape and overall thickness.At Red Lake, the greenstone belt is approximately 35 mi long by 18 mi wide. Gravity readings taken across the width of the belt indicate that the greenstones taper sharply in depth to a maximum thickness of approximately 25 000 ft. These results appear to confirm, as most geologists feel intuitively, that greenstone belts are basin-shaped and are underlain by granitic batholiths and gneiss.

scholarly journals Instability of the southern Canadian Shield during the late Proterozoic

2018 ◽  
Author(s):  
Kalin T. McDannell ◽  
Peter K. Zeitler ◽  
David A. Schneider
Keyword(s):  
Elevated Temperatures ◽  
Geothermal Gradient ◽  
Canadian Shield ◽  
Granitic Rocks ◽  
Deep Time ◽  
Magmatic Underplating ◽  
Thermal Histories ◽  
Lower Temperature ◽  
The Stability ◽  
Uplift And Erosion

Cratons are generally considered to comprise lithosphere that has remained tectonically quiescent for billions of years. Direct evidence for stability is mainly founded in the Phanerozoic sedimentary record and low-temperature thermochronology, but for extensive parts of Canada, earlier stability has been inferred due to the lack of an extensive rock record in both time and space. We used 40Ar/39Ar multi-diffusion domain (MDD) analysis of K-feldspar to constrain cratonic thermal histories across an intermediate (~150-350°C) temperature range in an attempt to link published high-temperature geochronology that resolves the timing of orogenesis and metamorphism with lower-temperature data suited for upper-crustal burial and unroofing histories. This work is focused on understanding the transition from Archean-Paleoproterozoic crustal growth to later intervals of stability, and how uninterrupted that record is throughout Earth’s Proterozoic “Middle Age.” Intermediate-temperature thermal histories of cratonic rocks at well-constrained localities within the southern Canadian Shield of North America challenge the stability worldview because our data indicate that these rocks were at elevated temperatures in the Proterozoic. Feldspars from granitic rocks collected at the surface cooled at rates of <0.5°C/Ma subsequent to orogenesis, seemingly characteristic of cratonic lithosphere, but modeled thermal histories suggest that at ca. 1.1-1.0 Ga these rocks were still near ~200°C – signaling either reheating, or prolonged residence at mid-crustal depths assuming a normal cratonic geothermal gradient. After 1.0 Ga, the regions we sampled then underwent further cooling such that they were at or near the surface (<< 60°C) in the early Paleozoic. Explaining mid-crustal residence at 1.0 Ga is challenging. A widespread, prolonged reheating history via burial is not supported by stratigraphic information, however assuming a purely monotonic cooling history requires at the very least 5 km of exhumation beginning at ca. 1.0 Ga. A possible explanation may be found in evidence of magmatic underplating that thickened the crust, driving uplift and erosion. The timing of this underplating coincides with Mid-Continent extension, Grenville orogenesis, and assembly of the supercontinent Rodinia. 40Ar/39Ar MDD data demonstrate that this technique can be successfully applied to older rocks and fill in a large observational gap. These data also raise questions about the evolution of cratons during the Proterozoic and the nature of cratonic stability across deep time.

THE CORRELATION BETWEEN PRE‐CAMBRIAN ROCK DENSITIES AND BOUGUER GRAVITY ANOMALIES NEAR PARRY SOUND, ONTARIO

Geophysics ◽  
1954 ◽  
Vol 19 (1) ◽  
pp. 76-88
Author(s):  
C. H. G. Oldham
Keyword(s):  
Gravity Anomaly ◽  
Gravity Anomalies ◽  
Canadian Shield ◽  
Gravity Survey ◽  
Uniform Density ◽  
Bouguer Gravity ◽  
Geological Evidence ◽  
Bouguer Gravity Anomaly ◽  
Bouguer Gravity Anomalies

A gravity survey and a survey of rock densities have been carried out over an area of two thousand square miles near Parry Sound. A closed positive Bouguer gravity anomaly of thirty milligals was delineated, and a considerable variation was found to exist in the densities of pre‐Cambrian gneisses. In most previous interpretations of gravity over the Canadian Shield the gneisses have been assumed to possess a uniform density and anomalies have been attributed to changes in the thickness of horizontal crustal layers. In this paper it is shown that the Parry Sound anomaly can be explained in terms of structures within the crust taking the form of projections downward of the density variations found at the surface. The postulated structure is a nearly circular basin of dense gneisses. The shape is reasonable and agrees with such geological evidence as is available.

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