Precambrian muscovite–quartz (agalmatolite) palaeosols from Scotland and Canada

1992 ◽  
Vol 29 (12) ◽  
pp. 2523-2529 ◽  
Author(s):  
Iain Allison ◽  
Lindsay K. Ferguson ◽  
Fidel A. Cardenas ◽  
Barbara I. Kronberg
Keyword(s):  
Lower Cambrian ◽  
Low Grade ◽  
Marine Transgression ◽  
Fine Grained ◽  
Late Precambrian ◽  
Basement Rocks ◽  
The World ◽  
Northwest Ontario ◽  
Rock Lake ◽  
Middle Proterozoic

Weathering, during late Precambrian times, of pink granitic gneisses and pegmatites of the Middle Proterozoic Lewisian Complex in northwest Scotland has produced a characteristic pea-green palaeosaprolite called agalmatolite, which occurs from centimetres to metres beneath the planar unconformity with the overlying quartz sandstones of the Lower Cambrian marine transgression. Agalmatolite consists predominantly of massive, felted, very fine-grained muscovite of the form called pinite together with variable amounts of relict quartz. Pyrophyllite may also occur in the most altered parts, close to the unconformity, and the proportions of precursor minerals, mainly quartz and feldspar, increase as the parent gneisses are approached. Trends in whole-rock elemental variations are consistent with a weathering hypothesis, and the present mineralogy reflects the burial of the saprolite subjected to conditions of anchimetamorphism.Palaeosols developed on both granitic and basaltic basement rocks, with ages ranging up to about 3.0 Ga, have been described from a number of areas throughout the world. In all these palaeosols, muscovite, usually referred to as sericite, is the dominant mineral. In northwest Ontario weathering of the granitic basement beneath the Steep Rock Lake Group has produced a rock almost identical to the Scottish agalmatolite and consisting of muscovite, of the form pinite, relict quartz, and rutile. From published descriptions of other palaeosols, we conclude that the term agalmatolite may be applied to most Precambrian palaeosols, as it conveys more information than terms such as argillite and saprolith that have previously been in use.Compared with the less well-exposed alteration profiles of older palaeosols, the clear field relationships of agalmatolite in northwest Scotland allow more confidence to be placed on interpretations that such alteration profiles have been caused by surficial weathering, with subsequent changes due to burial and even low-grade metamorphism.

40Ar/39Ar whole-rock phyllite ages from late Precambrian rocks of the Avalon composite terrane, New Brunswick: evidence of Silurian–Devonian thermal rejuvenation

1994 ◽  
Vol 31 (5) ◽  
pp. 818-824 ◽  
Author(s):  
R. David Dallmeyer ◽  
R. Damian Nance
Keyword(s):  
New Brunswick ◽  
Low Grade ◽  
Fine Grained ◽  
Metasedimentary Rocks ◽  
Late Precambrian ◽  
Minimum Age ◽  
Tectonic Contact ◽  
Age Constraints ◽  
Gas Fractions ◽  
Thermal Overprint

Several variably deformed and metamorphosed, late Precambrian volcanic–sedimentary successions have been recognized within the Avalon composite terrane exposed in the Caledonian Highlands of southern New Brunswick. Whole-rock samples of metasedimentary phyllite and phyllitic metatuff from the oldest (ca. 600–635 Ma) Avalonian succession display similar, internally discordant 40Ar/39Ar age and apparent K/Ca spectra. Intermediate-temperature gas fractions were experimentally evolved solely from very fine grained, cleavage-aligned white micas. These yield apparent ages between ca. 430 and 410 Ma, and are interpreted to closely date a static Late Silurian – Early Devonian thermal rejuvenation.Evidence for a Silurian – Devonian thermal event has not been previously documented in Avalonian rocks of the Caledonian Highlands (Caledonia assemblage). However, a thermal overprint of similar age (ca. 400 Ma) is recorded by metamorphic muscovite in high-grade gneisses and platformal metasedimentary rocks (Brookville assemblage), which are in tectonic contact with the low-grade Caledonia assemblage. These potentially correlative thermal overprints may provide minimum age constraints on the juxtaposition of these contrasting tectono-stratigraphic assemblages, which are likely to have been palinspastically separate tectonic elements during the earliest Paleozoic.

Paleomagnetism of the Middle Proterozoic Mount Nelson Formation: evidence for a regional remagnetization event in the Late Precambrian of the Cordillera

1983 ◽  
Vol 20 (4) ◽  
pp. 561-567 ◽  
Author(s):  
W. A. Morris ◽  
M. E. McMechan
Keyword(s):  
British Columbia ◽  
National Park ◽  
Acid Leaching ◽  
Pole Position ◽  
Glacier National Park ◽  
Geological Process ◽  
Thermal Demagnetization ◽  
Fine Grained ◽  
Late Precambrian ◽  
Middle Proterozoic

Chemical and thermal demagnetization of 92 specimens from the Mount Nelson Formation (uppermost Purcell Supergroup) in the Purcell Mountains of southeastern British Columbia yields two distinct directional groups. MN-A, found by thermal and chemical demagnetization, has a mean direction of D = 274°, I = 19° (α95 = 10°, tilt corrected), which corresponds to a pole position at 156 °E, 10 °N, MN-A resides in fine-grained hematite and is similar to other overprint directions reported from middle and upper Belt–Purcell strata in the Glacier National Park – Clark Range area. Genesis of this overprint is related to some regional geological process, most probably the Goat River Orogeny [Formula: see text]. Direction MN-B, which is found after acid leaching of more than 250 h, has a mean direction of D = 331°, I = 47°(α95 = +4°, tilt corrected), corresponding to a pole at 119 °E, 59 °N. The MN-B pole is significantly different from all poles previously reported from the Belt–Purcell Supergroup (ca. 210 °E, 20 °S). As the age of the MN-B pole is unconstrained the significance of this marked discrepancy is at present uncertain.

TEM/AEM characterization of fine-grained clay minerals in very-low-grade rocks: Evaluation of contamination by empa involving celadonite family minerals

1996 ◽  
Vol 54 ◽  
pp. 694-695
Author(s):  
Gejing Li ◽  
D. R. Peacor ◽  
D. S. Coombs ◽  
Y. Kawachi
Keyword(s):  
Electron Microscopy ◽  
Volcanic Rocks ◽  
Analytical Electron Microscopy ◽  
Metamorphic Rocks ◽  
New Mineral ◽  
Chemical Compositions ◽  
Low Grade ◽  
Fine Grained ◽  
Depth Analysis ◽  
Mineral Aggregates

Recent advances in transmission electron microscopy (TEM) and analytical electron microscopy (AEM) have led to many new insights into the structural and chemical characteristics of very finegrained, optically homogeneous mineral aggregates in sedimentary and very low-grade metamorphic rocks. Chemical compositions obtained by electron microprobe analysis (EMPA) on such materials have been shown by TEM/AEM to result from beam overlap on contaminant phases on a scale below resolution of EMPA, which in turn can lead to errors in interpretation and determination of formation conditions. Here we present an in-depth analysis of the relation between AEM and EMPA data, which leads also to the definition of new mineral phases, and demonstrate the resolution power of AEM relative to EMPA in investigations of very fine-grained mineral aggregates in sedimentary and very low-grade metamorphic rocks.Celadonite, having end-member composition KMgFe3+Si4O10(OH)2, and with minor substitution of Fe2+ for Mg and Al for Fe3+ on octahedral sites, is a fine-grained mica widespread in volcanic rocks and volcaniclastic sediments which have undergone low-temperature alteration in the oceanic crust and in burial metamorphic sequences.

The middle lower Cambrian (Ovetian)Lunagraulosn. gen. from Spain and the oldest trilobite records

2015 ◽  
Vol 152 (6) ◽  
pp. 1123-1136 ◽  
Author(s):  
ELADIO LIÑÁN ◽  
JOSÉ ANTONIO GÁMEZ VINTANED ◽  
RODOLFO GOZALO
Keyword(s):  
Fossil Record ◽  
Lower Cambrian ◽  
New Genus ◽  
Type Locality ◽  
New Taxon ◽  
Northern Spain ◽  
Central Spain ◽  
Additional Material ◽  
The World ◽  
The Village

AbstractThe type material ofAgraulos antiquusSdzuy, 1961 from the La Herrería Formation, northern Spain, is revised together with additional material and included in the new genusLunagraulos. The stratigraphical range ofLunagraulos antiquus(Sdzuy, 1961) – occurring below that of the trilobite species of the generaLunolenus,MetadoxidesandDolerolenusin the type locality of Los Barrios de Luna in the province of León, northern Spain – and the accompanying ichnofossil assemblage demonstrate an Ovetian age (lower part of Cambrian Stage 3, currently being discussed by the International Subcommission on Cambrian Stratigraphy) for this species. Moreover, the trilobiteLunagraulos tamamensisn. gen. n. sp. is found in the Tamames Sandstone near the village of La Rinconada in the province of Salamanca, central Spain. The biostratigraphical position of this new taxon and its accompanying ichnoassemblage is also analysed and assigned to the lowermost Ovetian Stage. The genusLunagraulosis therefore the oldest agraulid found in the fossil record. The exceptional presence ofLunagraulosin a marine coarse siliciclastic succession – a facies rather typical for the ichnofossilsCruzianaandRusophycus, some of the oldest signs of trilobite activity – suggests that first trilobite representatives may have inhabited high- to middle-energy, marine environments. This hypothesis may also explain both the taxonomic and biostratigraphic heterogeneity of the first trilobite genera appearing across the world, due to preservation problems in this type of facies. Comparison of theLunagraulos biostratigraphy with other coeval Spanish fossil assemblages allows us to propose its intercontinental correlation with the oldest records of currently known trilobites.

XXIII.—The Development of Brief Narrative in Modern French Literature: A Statement of the Problem

PMLA ◽  
1917 ◽  
Vol 32 (4) ◽  
pp. 583-597
Author(s):  
Horatio E. Smith
Keyword(s):  
United States ◽  
Henry James ◽  
French Literature ◽  
English Language ◽  
The United States ◽  
Modern Literature ◽  
Low Grade ◽  
The World ◽  
Bret Harte

Brief narrative, at first thought, connotes the abridged fiction of low grade with which American magazines are now saturated; but as soon as the term is used to cover the whole field in modern literature, it calls to mind a genre which, under various names, has risen to a position of dignity in many places in the world and has worthily engaged the attention of literary historians, particularly in America and in Germany.The chief features in the development of the form in the United States and England have been discussed at length, and there is now a definitive record, with abundant bibliographical apparatus, of its evolution. Poe is looked upon as the pioneer, and his perpetually quoted definition (1842) has set a standard for the majority of the practitioners of the art in the English language. The form suggests, for America, such experts as Hawthorne, Bret Harte, and Henry James; in England it does not gain the attention of writers of the first magnitude until near the end of the century, in the persons of Stevenson and Kipling.

Challenges in the Management of COVID-19 Patients with Metabolic Syndrome with Special Emphasis on Gender and Age - A Review

2021 ◽  
Vol 8 (38) ◽  
pp. 3401-3405
Author(s):  
Arunima Chaudhuri ◽  
Suhrita Paul ◽  
Tapas Ghosh
Keyword(s):  
Metabolic Syndrome ◽  
Virus Disease ◽  
Adult Population ◽  
Density Lipoprotein ◽  
Blood Stream ◽  
Low Grade ◽  
Corona Virus ◽  
Gender And Age ◽  
The World ◽  
Low Grade Inflammation

BACKGROUND The corona virus disease-19 (Covid-19) pandemic has put human civilization into a huge challenge, especially in the field of medicine in the management of patients with co-morbidities. Health authorities across the world depend greatly on reliable data to make major decisions and this is especially true during this global pandemic. The present review was conducted to estimate the challenges in the management of Covid-19 patients with metabolic syndrome with special emphasis on gender and age. Patients having pre-existing health conditions e.g., heart disease, diabetes are at higher risk of morbidity and mortality due to COVID-19. According to the WHO newsletter, COVID-19 has tragically claimed more than 1.5 million lives. The burden of obesity across the world has nearly tripled since 1975. In 2016, 1.9 billion adults, were overweight; 650 million were obese; 13 % of the world's adult population (11 % of males and 15 % of females) were obese in 2016. Obesity has been observed to be a high-risk factor for COVID-19 severity. Severe acute respiratory syndrome-corona virus-2 (SARS-CoV-2) targets the angiotensinconverting enzyme 2 (ACE2) for cell entry and ACE2 is highly expressed in adipose tissue. This suggests an important role for the tissue in determining COVID-19 disease severity in obese individuals.1-2 There has been an increase in death from diabetes by 70 % globally between 2000 and 2019, and an 80 % rise in deaths among males has been observed. Metabolic syndrome comprises three or more of the following factors: increased waist circumference; hypertriglyceridemia; elevated blood pressure; reduced high-density lipoprotein cholesterol; hyperglycemia.1-2 Visceral fat is known to produce higher concentrations of proinflammatory cytokines. These are then released in the bloodstream. Release of proinflammatory markers in blood stream may cause auto-amplifying cytokine production (“cytokine storms”) and low-grade inflammation. Cytokine storm and low-grade inflammation can contribute to worsening of COVID-19 patients with obesity. Components of metabolic syndrome such as hypertension, type 2 diabetes mellitus (T2DM), and obesity are highly prevalent among the general population and have been observed to significantly increase the risk of hospitalization and mortality in COVID-19 patients.1-2 KEYWORDS Covid-19 Pandemic, Metabolic syndrome, Aging, Gender

A detailed 40Ar/39Ar age study of an Abitibi dike from the Canadian Superior Province

1979 ◽  
Vol 16 (5) ◽  
pp. 1060-1070 ◽  
Author(s):  
J. A. Hanes ◽  
Derek York
Keyword(s):  
Canadian Shield ◽  
Superior Province ◽  
Low Grade ◽  
Polar Wander ◽  
Mafic Mineral ◽  
Fine Grained ◽  
Apparent Polar Wander Path ◽  
Step Heating ◽  
Apparent Age ◽  
Hydrothermal Event

40Ar/39Ar step-heating analyses were performed on 11 felsic and mafic mineral separates from a 90 m wide Precambrian diabase dike of the Abitibi swarm in the Superior Province of the Canadian Shield. Deuterically altered minerals from the dike interior define a primary age of 2150 ± 25 Ma. Updated ages, obtained from felsic separates within 30, and mafic within 1.5 m of the dike border, are evidence of a previously undetected 'Hudsonian' (1.7–1.8 Ga) hydrothermal event in the area. It is possible to distinguish the deuteric from the later hydrothermal alteration by both dating and petrographic methods. The data from this study demonstrate the successful application of 40Ar/39Ar dating to early Proterozoic dikes which have suffered low grade metamorphism. The ages support a north to south sense of motion of the Track 5 apparent polar wander path (APWP). A monotonic decrease in apparent age of felsic spectra indicates reactor induced recoil effects which are correlated with the fine-grained saussurite in the feldspar.

Early Cenozoic resetting of potassium–argon dates and geothermal history of north Okanagan area, British Columbia

1981 ◽  
Vol 18 (8) ◽  
pp. 1310-1319 ◽  
Author(s):  
Wm. H. Mathews
Keyword(s):  
Vitrinite Reflectance ◽  
Volcanic Rocks ◽  
Blocking Temperature ◽  
Low Grade ◽  
High Grade ◽  
Basement Rocks ◽  
Thermal Changes ◽  
History Of ◽  
The Trinity ◽  
Early Cenozoic

Unmetamorphosed Early Eocene sediments and volcanic rocks of the Trinity Hills and Enderby Cliffs yield K–Ar dates of 42–49 Ma. These overlie high-grade gneisses yielding K–Ar ages on biotites, muscovites, and hornblende ranging from 47 to 60 Ma. The Eocene sediments and volcanics rest nearby on low-grade phyllites, greenstones, and schists yielding dates from 83 to 155 Ma. The gneiss dates are regarded as reset by some Late Cretaceous to earliest Cenozoic thermal event that did not affect, at least to the same degree, the nearby less metamorphosed basement rocks. A thermal history has been constructed to account for the decreasing apparent ages of biotite (assumed blocking temperature of 250 °C) with increasing depth below the sub-Eocene unconformity, for the greater ages of hornblende and muscovite in the same rocks (blocking temperatures of 500 and 350 °C), as well as for thermal changes associated with high vitrinite reflectance from coal at one site in the covering sediments. Very rapid stripping (something like 5 km in 12 Ma) is inferred for the areas of reset gneisses, but not for the schist areas, in early Cenozoic time.

The Gow Lake impact structure, northern Saskatchewan

1977 ◽  
Vol 14 (8) ◽  
pp. 1788-1795 ◽  
Author(s):  
M. D. Thomas ◽  
M. J. S. Innes
Keyword(s):  
Impact Crater ◽  
Fine Grained ◽  
Model Studies ◽  
Deformation Structures ◽  
Basement Rocks ◽  
Clear Glass ◽  
Microscopic Features ◽  
Minimum Age ◽  
Central Island ◽  
Quartz Grains

Gow Lake, in the Precambrian Shield of Saskatchewan, is circular, 4 km in diameter, and has a large central island. Granites and quartzofeldspathic gneisses are exposed around the perimeter of the lake, whereas the island is formed largely of brecciated equivalents. Most of the breccias are composed entirely of clastic material, but at one locality fine-grained felted matrices form a significant component of the breccias, and coronas of clear glass surround quartz grains. The latter breccias also contain microscopic features characteristic of shock metamorphism, among which multiple sets of planar deformation structures in quartz are particularly diagnostic. Similar shock metamorphic features have been widely reported from terrestrial meteorite craters; accordingly, Gow Lake is interpreted as a deeply eroded impact crater and the felted matrices as impact melts.A local negative gravity anomaly with an amplitude of 3 mGal centered on the lake is attributed mainly to highly fractured basement rocks underlying the lake, which model studies indicate may extend to a depth of 900 m. A provisional minimum age of 100 Ma is proposed for the crater.

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