摘要:

New U–Pb ages for late Precambrian volcanic and associated plutonic units in the Mira (Avalon) terrane of southeastern Cape Breton Island indicate that volcanic suites were erupted over a span of at least 100 Ma. The oldest dated rock is a quartz–feldspar rhyodacitic porphyry from the unit that hosts the Mindamar Zn–Pb–Cu–Ag–Au deposit in the Stirling belt, which has an age of. The most widespread volcanism and plutonism occurred at ca. 620 Ma in the East Bay Hills and Coxheath Hills belts, and probably the Sporting Mountain belt, as indicated by U–Pb ages and U–Pb maximum ages for rhyolite flows and U–Pb and Ar–Ar ages of crosscutting plutons, as well as stratigraphic constraints. Younger volcanic rocks occur in the Coastal belt, from which a rhyodacitic crystal tuff is dated atand a pluton is dated at 574 ± 3 Ma. A rhyolite flow from the contiguous Main-à-Dieu sequence yields a maximum age of ca. 563 Ma, and a minimum age for this sequence is indicated by overlying latest Precambrian to Cambrian fossiliferous sedimentary rocks. Middle Devonian plutonism in the Mira terrane is confirmed by an age offrom the Lower St. Esprit granodiorite in the Coastal belt. The range of ages of volcanic and plutonic rocks in Mira terrane is similar to that in other parts of Avalon terrane in eastern Newfoundland and southern New Brunswick. Many of the dated rocks contain xenocrystic zircons of Middle Proterozoic ages which suggest a South American source.

ISSN:0008-4077    

DOI:10.1139/e93-001

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

U–Pb zircon, titanite, and baddeleyite ages are reported for 20 volcanic and plutonic rock units in the Chibougamau, Matagami, and Joutel areas of the northern Abitibi belt. Pretectonic magmatism occurred in three main pulses. Volcanic rocks of the Obatagamau Formation in the Chibougamau area were erupted at 2759 Ma. Following a possible hiatus in igneous activity, volcanism, accompanied by the emplacement of layered mafic intrusions, occurred in the Chibougamau, Matagami and Joutel areas in the period of 2730–2722 Ma. In the Chibougamau area, a slightly younger period of plutonism between 2720 and 2712 Ma is thought to be comagmatic with felsic volcanic rocks in the uppermost part of the Roy Group. Syntectonic intrusive rocks in the northern Abitibi belt range in age from 2701 to 2693 Ma. These ages are broadly comparable to ages obtained for syntectonic plutons elsewhere in the Abitibi belt, suggesting that tectonism associated with the Kenoran orogeny occurred at approximately the same time throughout the belt.

ISSN:0008-4077    

DOI:10.1139/e93-002

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

U–Pb zircon ages for 15 volcanic and plutonic units in the Noranda and Kirkland Lake areas help constrain the history of volcanism, plutonism, sedimentation, and deformation in the south-central part of the Abitibi belt. Volcanism occurred over an interval of at least 50 Ma, beginning with the deposition of the volcanic and volcaniclastic units within the Pacaud Structural Complex at 2747 Ma. Following a period of apparent quiescence, magmatism resumed at 2730–2725 Ma with the eruption of volcanic rocks in the Normétal and Lac Abitibi area. From 2715 until about 2698 Ma, volcanism occurred sporadically throughout much of the area, culminating in the eruption of the Blake River Group from 2703 to 2698 Ma. Several large intrusive bodies yield ages that indicate that they are plutonic equivalents of the Blake River Group. Plutons that are considered to have been emplaced during the Kenoran orogeny give ages that are only slightly younger than the youngest volcanic units of the Blake River Group, emphasizing the very rapid onset of Kenoran deformation following the cessation of volcanic activity.The Cléricy syenite, dated at 2682 ± 3 Ma, postdates the main period of Kenoran deformation in this area and intrudes sedimentary rocks of the Kewagama Group which contain detrital zircons as young as 2687 Ma. These data suggest that the Kewagama Group is the same age as late sedimentary sequences such as the Timiskaming Group and may have been deposited in a similar tectonic setting.

ISSN:0008-4077    

DOI:10.1139/e93-003

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

U–Pb ages of 2713–2711 Ma for zircon and titanite from tonalitic gneiss of the Lapparent Massif in the northeastern Abitibi belt are younger than those obtained for surrounding supracrustal rocks. These data indicate that the Lapparent gneisses are not old basement on which the supracrustal rocks were deposited, as has been postulated by some workers, but rather represent a late synvolcanic intrusion that was emplaced at a relatively deep crustal level. The earliest phase of penetrative deformation recognized in the gneiss must have occurred during or immediately after emplacement of the body and represents a deformation event that is not presently recognized in the higher level rocks in this area. The Lapparent gneisses were apparently juxtaposed against adjacent supracrustal rocks along outward-dipping mylonite zones.

ISSN:0008-4077    

DOI:10.1139/e93-004

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Volcanogenic massive sulphide deposits at Anyox in the Tracy Arm terrane of the Canadian Cordillera are associated with a sequence of tholeiitic basalts with minor intercalated basaltic andesite tuffs and siliceous sediments. Sm–Nd and Pb–Pb systematics indicate an Early to Middle Jurassic age. The tholeiites are characterized by normal mid-ocean-ridge basalt to weak island-arc tholeiite trace element signatures with slight enrichment in large-ion lithophile elements and depletion in high-field-strength elements, high207Pb/204Pb, and ϵNd(170 Ma) values of +8.2 to +8.4. The mineralized sequence is conformably overlain by argillaceous sediments and minor limestones. These features, combined with the location of the strata and similarities with the Spider Peak Formation of the Methow terrane, indicate an origin in a narrowing marginal basin that once separated superterranes I and II.

ISSN:0008-4077    

DOI:10.1139/e93-005

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Weathering profiles developed side-by-side on sandstone and a mafic dike provide an unusual opportunity to examine the role of parent rock bulk composition in the chemical evolution of middle Precambrian regoliths. Because the profiles are adjacent to one another, differences in topography can be eliminated in accounting for differences in the chemical evolution of the two profiles. Both profiles show upward increases in Al, Ti, K, and Rb and decreases in Mg, Ca, and Na. In addition, the mafic regolith increases upward in Zr and Nb and decreases in Zn and Ni. Total Fe decreases upward in both profiles, but the sandstone profile retains significantly more of its initial Fe than does the mafic dike. This difference in Fe loss is consistent with weathering in a low-oxygen atmosphere of rock types with very different initial Fe contents and therefore different atmospheric requirements for complete oxidation of the Fe present. The Fe in the sandstone was mostly oxidized and retained within the profile, whereas much of the Fe in the mafic dike was not oxidized and was removed from the profile in the more soluble ferrous state. Petrographic evidence indicates that both sandstone and mafic dike weathering profiles underwent preweathering diagenesis, postweathering K–Rb metasomatism, and very low-grade metamorphism. Mineral chemistry indicates that, in the absence of chlorite, white mica composition closely reflects variation in bulk composition. Where both white mica and chlorite are present, changes in bulk composition are accommodated by variations in the proportions of these two minerals rather than by variations in white mica composition.

ISSN:0008-4077    

DOI:10.1139/e93-006

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Two-dimensional density, temperature, and rheological models are constructed for a 350 km northeast-trending transect of the southeastern Canadian Cordillera. All models highlight several major physical differences between foreland and hinterland lithosphere. Significant features of the density model are the presence of an anomalously low-density (3.10 × 103 kg∙m−3) layer, with a maximum thickness of 12 km, beneath the Moho in the hinterland; the similar densities of the Monashee Terrane and the cratonic crust of the foreland; and an increase in crustal thickness beneath the Southern Rocky Mountain Trench. The temperature model shows steeper gradients and higher Moho temperatures beneath the hinterland than beneath the foreland. In the rheological model the hinterland is characterized by a thin, brittle, upper crust beneath which the entire lithosphere is hot, weak, and ductile. In contrast, the foreland is composed of a thick, brittle, upper crust, with an additional brittle zone in the upper mantle. The Moho is a large strength discontinuity beneath the foreland, and the total lithospheric strength there is an order of magnitude larger than in the hinterland. The models are constrained and supported by geological mapping and a number of independent geophysical data sets. Palinspastic cross sections, together with paleotemperature and paleopressure information, are used to generate a time series of one-dimensional paleorheological profiles at a number of times during deformation. This sequence of profiles indicates that the foreland and hinterland have been rheologically distinct since pre-Late Cretaceous times. The profiles are used to clarify the geodynamic evolution of the area and to explain why deformation remained thin skinned in the foreland whereas in the hinterland the entire lithosphere was deforme

ISSN:0008-4077    

DOI:10.1139/e93-007

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The regional deposits of the Middle and Upper Mannville subgroups of east-central Alberta are characterized by a series of stacked offshore-paralic successions which represent transgressive–regressive cycles superimposed upon a longer term regressional trend. This trend eventually resulted in the complete withdrawal of the lower Albian Clearwater sea from eastern Alberta and western Saskatchewan. The marine deposits of the Clearwater sea (the Clearwater shales) carry the lower AlbianVerneuilinoides cummingensis–Marginulinopsis collinsiforaminiferal assemblage. The next diagnostic foraminiferal assemblage, the upper AlbianHaplophragmoides gigasfauna, is found in the Joli Fou shales of the Colorado Group which overlie the Mannville Group. Middle Albian strata are absent between the Mannville and the Colorado in eastern Alberta, but the exact placement of the maximum corresponding unconformity has been controversial. This study indicates that the uppermost unit of the Mannville (the Colony unit) should be regarded not as a basal Colorado transgressive sand but as a Mannville unit deposited prior to the maximum break.Sampling of microfauna from successively younger units of the Middle and Upper Mannville indicates a shift away from normal marine environments and towards increasingly brackish depositional conditions. This shift may have resulted from the development of embayment-like conditions during the withdrawal of the Clearwater sea, as well as from increases in fluvial drainage during the late stages of Mannville deposition.

ISSN:0008-4077    

DOI:10.1139/e93-008

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

New radiocarbon ages pertaining to the Holocene sea-level history of the Canadian Beaufort shelf are presented. The ages were obtained on samples of freshwater and tidal-marsh peat beds from offshore boreholes and shallow cores in the coastal zone and on molluscs and a single piece of wood deposited in foraminifera-bearing marine sediments. Although none of the samples record directly the position of relative sea level, the suite of ages constrains the regional curve sufficiently to suggest a faster rate of mid Holocene sea level rise (7–14 mm/a) than previously thought. The rate of relative rise slowed markedly in the last 3000 years, approaching the present at a maximum probable rate of 2.5 mm/a.

ISSN:0008-4077    

DOI:10.1139/e93-009

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The δ18O–δD relationship for ice and water is frequently summarized with a line fitted by least-squares linear regression. This technique assumes that one variable is known exactly and all error can be ascribed to the other. Unfortunately, determinations by mass spectrometry of both δ18O and δD are subject to experimental error. Often a blanket laboratory precision is provided for δ18O and δD, in which case functional analysis, accounting for the relative error in the variables, is appropriate. Properly, however, each sample has an individual analytical error in both variables, defined by the variance in estimates of isotope concentration provided by the mass spectrometer. Where individual errors are known, the least-squares cubic method, which assigns a weight to each sample and generates the summary line by an iterative method, may be used. An algorithm sufficient to determine both the functional fit and the least-squares cubic regression line is presented. Illustrations are provided, one of which demonstrates that if the plot of δ18O versus δD is scattered (r2 < 0.9), both the functional fit and the least-squares cubic regression line may be significantly different from the least-squares linear regression lines.

ISSN:0008-4077    

DOI:10.1139/e93-010

出版商:NRC Research Press    

年代:1993

数据来源: NRC