Stratigraphy of the Steep Rock Group, northwest Ontario: a major Archaean unconformity and Archaean stromatolites

1988 ◽  
Vol 25 (3) ◽  
pp. 370-391 ◽  
Author(s):  
M. E. Wilks ◽  
E. G. Nisbet
Keyword(s):  
Structural Break ◽  
Fault Zones ◽  
Thin Layers ◽  
Older Age ◽  
Lava Flows ◽  
Pyroclastic Rock ◽  
Superior Province ◽  
Rock Group ◽  
Northwest Ontario ◽  
The Witch

The Archaean Steep Rock Group of northwest Ontario, situated in the Wabigoon Subprovince of the Superior Province, Canada, comprises five formations: Wagita Formation (clastics), Mosher Carbonate, Jolliffe Ore Zone, Dismal Ashrock, and Witch Bay Formation (metavolcanics). Reinvestigation of the geology of the group has shown that the basal clastics of the Wagita Formation (0–150 m) unconformably overlie the Marmion Complex (a massive tonalite – tonalite gneiss terrane, 3 Ga old). Overlying the basal elastics is the Mosher Carbonate (0–500 m), containing diverse stromatolite morphologies. Extensive zones of carbonate breccia occur adjacent to fault zones and mafic dykes. Stratigraphically above the Mosher Carbonate is the Jolliffe Ore Zone (100–400 m), which is divided into a lower Manganiferous Paint Rock Member and an upper Goethite Member. Within the Jolliffe Ore Zone thin layers of "Buckshot Ore" occur. These are horizons of haematitic pisolites and fragments, set in a lighter ferruginous matrix of kaolinite and gibbsite. Overlying the Jolliffe Ore Zone is the Dismal Ashrock, a dominantly high-Mg pyroclastic rock (22% MgO) with minor interbedded lava flows (15% MgO). In contact with the Dismal Ashrock are the metavolcanics of the Witch Bay Formation. This juxtaposition is not exposed in the Steep Rock mine section, and the Witch Bay Formation may be separated from the Dismal Ashrock by a structural break. The Witch Bay Formation is only provisionally included in the Steep Rock Group.The group is interpreted as a sequence deposited in an extensional or rifting environment. The unconformity has regional significance, and it may be possible to define an extensive cratonic nucleus of 3 Ga or older age in northwest Ontario.

Orogen parallel and transverse shearing in the Opatica belt, Quebec: implications for the structure of the Abitibi Subprovince

1992 ◽  
Vol 29 (11) ◽  
pp. 2429-2444 ◽  
Author(s):  
Keith Benn ◽  
Edward W. Sawyer ◽  
Jean-Luc Bouchez
Keyword(s):  
Greenstone Belt ◽  
Regional Scale ◽  
Structural Evolution ◽  
Fault Zones ◽  
Crustal Thickening ◽  
Superior Province ◽  
Northern Margin ◽  
Abitibi Greenstone Belt ◽  
Ductile Shearing ◽  
Abitibi Subprovince

The late Archean Opatica granitoid-gneiss belt is situated within the northern Abitibi Subprovince, along the northern margin of the Abitibi greenstone belt. Approximately 200 km of structural section was mapped along three traverses within the previously unstudied Opatica belt. The earliest preserved structures are penetrative foliations and stretching and mineral lineations recording regional ductile shearing (D1). Late-D1 deformation was concentrated into kilometre-scale ductile fault zones, typically with L > S tectonite fabrics. Two families of lineations are associated with D1, indicating shearing both parallel and transverse to the east-northeast trend of the belt. Lineations trending east-northeast or northwest–southeast tend to be dominant within domains separated by major fault zones. In light of the abundant evidence for early north–south compression documented throughout southern Superior Province, including the Abitibi greenstone belt, D1 is interpreted in terms of mid-crustal thrusting, probably resulting in considerable crustal thickening. Movement-sense indicators suggest that thrusting was dominantly southward vergent. D2 deformation resulted in the development of vertical, regional-scale dextral and sinistral transcurrent fault zones and open to tight upright horizontal folds of D1 fabrics. In the context of late Archean orogenesis in southern Superior Province, the tectonic histories of the Abitibi and Opatica belts should not be considered separately. The Opatica belt may correlate with the present-day mid-crustal levels of the Abitibi greenstone belt, and to crystalline complexes within the Abitibi belt. It is suggested that the Abitibi Subprovince should be viewed, at the regional scale, as a dominantly southward-vergent orogenic belt. This work demonstrates that structural study of granitoid-gneiss belts adjacent to greenstone belts can shed considerable light on the regional structure and structural evolution of late Archean terranes.

Two komatiitic pyroclastic units, Superior Province, northwestern Ontario: their geology, petrography, and correlation

1991 ◽  
Vol 28 (9) ◽  
pp. 1455-1470 ◽  
Author(s):  
Stephen J. Schaefer ◽  
Penelope Morton
Keyword(s):  
Explosive Volcanism ◽  
Pyroclastic Rock ◽  
Superior Province ◽  
Metasedimentary Rocks ◽  
The North ◽  
Northwestern Ontario ◽  
Volcanic Breccia ◽  
Metavolcanic Rocks ◽  
Volcaniclastic Rocks ◽  
Lapilli Tuff

Two Archean komatiitic pyroclastic rock units occur on opposite sides of the Quetico Fault in northwestern Ontario. The eastern unit, the Dismal Ashrock, is located 3 km north of Atikokan, Ontario, on the north side of the Quetico Fault within the Wabigoon Subprovince of the Superior Province. It is part of a suprascrustal sequence, the Steep Rock Group. The Grassy Portage Bay ultramafic pyroclastic rock unit (GUP) is located 100 km to the west, on the south side of the Quetico Fault, and is part of an overturned succession comprising mafic metavolcanic rocks, GUP, and metasedimentary rocks. The Dismal Ashrock dips steeply, is little deformed, has undergone greenschist metamorphism, and is divided into komatiitic lapilli tuff, komatiitic volcanic breccia, komatiitic volcaniclastic rocks, and a mafic pillowed flow. GUP outcrops form an arcuate fold interference pattern, are strongly deformed, and have undergone amphibolite metamorphism. GUP is divided into komatiitic lapilli tuff and komatiitic volcanic breccia. Both pyroclastic units contain cored and composite lapilli, evidence for explosive volcanism. Locally, some of the lapilli fragments are highly vesicular (up to 30% by volume), greater than reported for any other komatiites. Other fragments show no vesicularity. The low vesicularity of some of the pyroclasts and, in the case of the Dismal Ashrock, their association with pinowed lava flows may indicate explosive hydrovolcanic activity. The Dismal Ashrock and GUP are high in MgO, Cr, and Ni and are unusually enriched in Fe, Ti, Zr, Mn, P, Ba, Nb, Rb, and Sr compared with other komatiites. These unique geochemical compositions are not understood at this time.

Constraints from 40Ar/39Ar geochronology on the tectonothermal history of the Kapuskasing uplift in the Canadian Superior Province

1994 ◽  
Vol 31 (7) ◽  
pp. 1146-1171 ◽  
Author(s):  
J. A. Hanes ◽  
D. A. Archibald ◽  
M. Queen ◽  
E. Farrar
Keyword(s):  
Fault Zone ◽  
Age Groups ◽  
Fault Zones ◽  
Fault Reactivation ◽  
Superior Province ◽  
Lower Grade ◽  
Upper Limit ◽  
Step Heating ◽  
History Of ◽  
Structural Levels

The Kapuskasing uplift (KU) in the Superior Province of the Canadian Shield has been interpreted as an oblique cross section through the Archean mid-crust. However, the time of juxtaposition of the granulites of the KU against the lower grade rocks of the Abitibi greenstone belt (AGB) along the Ivanhoe Lake fault zone is problematic. To constrain the postmetamorphic tectonothermal history of the KU, we have conducted 57 40Ar/39Ar step-heating analyses on mineral and rock samples collected in a transect across the southern KU and adjacent AGB. The age spectra record a complex thermal history. Amphiboles from the AGB in the footwall of the Ivanhoe Lake fault zone have ca. 2.66 Ga dates, similar to closure ages for amphiboles from farther east in the AGB. Amphibole dates of 2.46–2.52 Ga from the deepest structural levels of the KU place an upper limit on the time of major uplift of the granulites and their juxtaposition with the AGB. Biotite and muscovite dates from the transect cluster into three age groups. The presence in the deepest structural levels of the KU of biotite with 2.40–2.45 Ga dates indicates that significant uplift (15–20 km or more) of the granulites had occurred by this time. Micas with dates in the 2.25–2.30 Ga range are close to fault zones; these dates may indicate a ca. 2.30 Ga episode of fault reactivation. Feldspar, fault-related whole rocks, and some micas record events post 2.1 Ga. These correspond to the emplacement of mafic and lamprophyric dykes and fault reactivation.

Introduction

1985 ◽  
Vol 122 (5) ◽  
pp. 415-417
Author(s):  
G. Malcolm Brown
Keyword(s):  
Sea Level ◽  
Thin Layers ◽  
Lava Flows ◽  
Small Island ◽  
Volcanic Centre ◽  
Calcic Plagioclase ◽  
Plagioclase Feldspar ◽  
Initial Assumption ◽  
Full Explanation ◽  
Horizontal Layers

The Tertiary volcanic centre that dominates the constitution of Rhum, one of the Scottish Inner Hebridean group of islands, is immediately provocative to the visiting geologist. The higher mountains are entirely ultrabasic in composition and their near-horizontal, terraced layers remain nearly constant in composition to about 1 km above sea level. Any initial assumption of geological simplicity is quickly dispelled by those and more detailed features. Horizontal layers of ultrabasic rocks with coarsely crystalline texture do not fit happily on a Tertiary landscape, towering high above the adjacent Hebridean lava flows of similar age. And what of the rocks themselves? Thin layers entirely of calcic plagioclase feldspar compete for attention with thick olivine-rich layers, while any day of traversing the Hallival or Askival mountain slopes will reveal a wealth of superbly exposed question marks in the form of spinel–sulphide layers, slump structures, finger structures, ‘harrisitic olivine’ formations, undulatory ‘trough-like’ structures, ultrabasic veins and currently lesser features. Search as one may (and little is hidden from view), there is no marginal gabbroic envelope between the ultrabasics and the country rocks that would lead to a Skaergaard analogy, and no overlying gabbroic layered rocks that would elicit guidelines from the Bushveld or Stillwater intrusions. This is Rhum, and much of what is on that small island is unique and, indeed, still defies full explanation. It is the achievement of this issue, through its several contributors whose voices succeed mine, that the uniqueness of Rhum is welcomed, many of the problems challenged, and many of the secrets unravelled.

Immunoferritin localization of intracellular antigens in positively stained frozen sections

1978 ◽  
Vol 36 (2) ◽  
pp. 168-169
Author(s):  
K. T. Tokuyasu
Keyword(s):  
Surface Tension ◽  
Water Surface ◽  
Thin Layers ◽  
The Other ◽  
Positive Staining ◽  
Frozen Sections ◽  
The Past ◽  
Ultrathin Frozen Sections ◽  
Definition Of

During the past investigations of immunoferritin localization of intracellular antigens in ultrathin frozen sections, we found that the degree of negative staining required to delineate u1trastructural details was often too dense for the recognition of ferritin particles. The quality of positive staining of ultrathin frozen sections, on the other hand, has generally been far inferior to that attainable in conventional plastic embedded sections, particularly in the definition of membranes. As we discussed before, a main cause of this difficulty seemed to be the vulnerability of frozen sections to the damaging effects of air-water surface tension at the time of drying of the sections.Indeed, we found that the quality of positive staining is greatly improved when positively stained frozen sections are protected against the effects of surface tension by embedding them in thin layers of mechanically stable materials at the time of drying (unpublished).

EELS Measurement of the Local Electronic Structure of Copper Atoms Segregated to Aluminum Grain Boundaries

1996 ◽  
Vol 54 ◽  
pp. 342-343
Author(s):  
S.J. Splinter ◽  
J. Bruley ◽  
P.E. Batson ◽  
D.A. Smith ◽  
R. Rosenberg
Keyword(s):  
Electronic Structure ◽  
Grain Boundaries ◽  
Boundary Segregation ◽  
Thin Layers ◽  
Nominal Composition ◽  
Spatially Resolved ◽  
Service Lifetime ◽  
Heat Treated ◽  
Probe Size ◽  
Local Electronic Structure

It has long been known that the addition of Cu to Al interconnects improves the resistance to electromigration failure. It is generally accepted that this improvement is the result of Cu segregation to Al grain boundaries. The exact mechanism by which segregated Cu increases service lifetime is not understood, although it has been suggested that the formation of thin layers of θ-CuA12 (or some metastable substoichiometric precursor, θ’ or θ”) at the boundaries may be necessary. This paper reports measurements of the local electronic structure of Cu atoms segregated to Al grain boundaries using spatially resolved EELS in a UHV STEM. It is shown that segregated Cu exists in a chemical environment similar to that of Cu atoms in bulk θ-phase precipitates.Films of 100 nm thickness and nominal composition Al-2.5wt%Cu were deposited by sputtering from alloy targets onto NaCl substrates. The samples were solution heat treated at 748K for 30 min and aged at 523K for 4 h to promote equilibrium grain boundary segregation. EELS measurements were made using a Gatan 666 PEELS spectrometer interfaced to a VG HB501 STEM operating at 100 keV. The probe size was estimated to be 1 nm FWHM. Grain boundaries with the narrowest projected width were chosen for analysis. EDX measurements of Cu segregation were made using a VG HB603 STEM.

Ulcerative Colitis in Older Age Patients

1960 ◽  
Vol 39 (1) ◽  
pp. 28-33 ◽  
Author(s):  
Z.T. Bercovitz
Keyword(s):  
Ulcerative Colitis ◽  
Older Age

The Biological Clock and Sleep in the Elderly

2006 ◽  
Vol 19 (1) ◽  
pp. 45-51 ◽  
Author(s):  
Myriam Juda ◽  
Mirjam Münch ◽  
Anna Wirz-Justice ◽  
Martha Merrow ◽  
Till Roenneberg
Keyword(s):  
Circadian Rhythms ◽  
Biological Clock ◽  
The Elderly ◽  
Early Morning ◽  
Behavioral Changes ◽  
Older Age ◽  
Age Related ◽  
Theoretical Considerations ◽  
Sleep Difficulties ◽  
Circadian Biology

Abstract: Among many other changes, older age is characterized by advanced sleep-wake cycles, changes in the amplitude of various circadian rhythms, as well as reduced entrainment to zeitgebers. These features reveal themselves through early morning awakenings, sleep difficulties at night, and a re-emergence of daytime napping. This review summarizes the observations concerning the biological clock and sleep in the elderly and discusses the documented and theoretical considerations behind these age-related behavioral changes, especially with respect to circadian biology.

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