Paleomagnetic Study and Chemistry of Newer Dolerites from Singhbhum, Bihar, India

1974 ◽  
Vol 11 (8) ◽  
pp. 1043-1054 ◽  
Author(s):  
R. K. Verma ◽  
S. N. Prasad
Keyword(s):  
Free Space ◽  
Regional Metamorphism ◽  
Magnetic Mineral ◽  
Polished Section ◽  
Thermal Demagnetization ◽  
Paleomagnetic Study ◽  
Younger Age ◽  
Igneous Activity

Results of paleomagnetic studies on seventeen dikes sampled from the Newer Dolerites of Singhbhum, between latitude 22°0′N to 22°45′N and longitude 85°48′E to 86°25′E are reported. The dikes are of Precambrian age, probably older than 1000 my. Stable directions of magnetization were obtained for eleven sites by thermal demagnetization in a field free space in temperatures ranging from 250° to 400 °C. These studies showed that the dikes had been reheated by igneous activity of neighboring dikes of a younger age or by regional metamorphism of a younger period. Polished section studies carried out on these dikes showed that the magnetic mineral had been significantly altered as a result of these processes. Reliable virtual geomagnetic poles were obtained for ten different sites. These pole positions can be collected into three different groups which may represent three episodes of igneous activity of the Newer Dolerites. A very tentative polar wandering curve for the Precambrian in India is given.

Étude paléomagnétique des roches protérozoïques de la formation de Sakami, région de la Grande Rivière, Territoire du Nouveau-Québec, Canada

1981 ◽  
Vol 18 (12) ◽  
pp. 1893-1899
Author(s):  
Maurice K. Seguin ◽  
Kamal N. M. Sharma ◽  
Gérard Woussen
Keyword(s):  
Magnetic Memory ◽  
Thermal Demagnetization ◽  
Paleomagnetic Study ◽  
Geological Formation ◽  
The Mean

We describe a paleomagnetic study of Mid-Proterozoic sandstone of the Sakami Formation. The thermal demagnetization is more effective than by alternating field; this indicates that part of the magnetic memory is located in hematite. The mean direction of magnetization after thermal and alternating field treatments is 031, −25 α95 = 7.1°, K = 73 and the corresponding paleopole position is 103°W, 18°S (dm = 11°, dp = 8°). These results are important because a part of this geological formation will be covered with water after completion of the development works of La Grande Rivière. [Journal Translation]

Paleomagnetism of the Lac St-Jean anorthosite and related rocks, Grenville Province, Quebec

1983 ◽  
Vol 20 (2) ◽  
pp. 246-258 ◽  
Author(s):  
K. L. Buchan ◽  
W. F. Fahrig ◽  
G. N. Freda ◽  
R. A. Frith
Keyword(s):  
North American ◽  
Regional Metamorphism ◽  
The Other ◽  
Central Portion ◽  
Grenville Province ◽  
Polar Wander ◽  
Thermal Demagnetization ◽  
The North ◽  
Apparent Polar Wander Path ◽  
Normal Polarity

Alternating field and thermal demagnetization study of the Lac St-Jean anorthosite and related rock units in the central portion of the exposed Grenville Province reveals two components of magnetization, one of reversed and the other of normal polarity. Both components are thought to have been acquired during the last regional metamorphism, which was sufficiently intense in this area (mostly amphibolite grade) to reset any earlier magnetization. Corresponding paleopoles at 193°W, 8°S (dm = 7.3°, dp = 4.6°) and 213°W, 19°S (dm = 10.5°, dp = 8.5°) lie along the 950–900 Ma segment of the recently calibrated Grenville track of the North American apparent polar wander path, a track that has thus far been defined largely by results from rock units of the western Grenville.

Reconnaissance paleomagnetic study of igneous rocks from the eastern sector of the Labrador Trough

1985 ◽  
Vol 22 (11) ◽  
pp. 1561-1570
Author(s):  
Maurice K.-Seguin ◽  
Thomas Clark
Keyword(s):  
North America ◽  
Igneous Rocks ◽  
Polar Wander ◽  
Thermal Demagnetization ◽  
Eastern Sector ◽  
Paleomagnetic Study ◽  
Component C ◽  
The Mean ◽  
Three Components

A paleomagnetic study has been carried out on 75 samples of metagabbro and metaperidotite from 14 sites in the Labrador Trough. The sites were distributed over a distance of 500 km and are situated in sills intruded into formation near the top of the trough stratigraphic column. Four components (A, B, C, and D) were obtained after alternating field (AF) and thermal demagnetization. The mean directions of magnetization are A. 143°, +33°. α95 = 14.4°. k = 15.4, 8 sites: B, 69°, +51°, α95 = 10.3d, k = 22.9, 10 sites: C. 278°. +15°, α95 = 7.7°, k = 28, 14 sites; and D, 200°, +16°. α95 = 13.3°, k = 16.2, 9 sites. All components are secondary. The fold test is negative, and consequently no primary magnetization has been preserved in the isolated components. Component C may have erased all others in zones of highest metamorphism. The paleopoles obtained for components D, A, and C are, respectively, 270°E. 23°S (δP = 10°. δm = 18°); 327°E, 9°S (δp = 11°. δm = 20°); and I9°E, 11°S (antipole) (δp = 10°. δm = 17°). The paleopole positions obtained for these three components are compared with me existing late Aphebian and early Paleohelikian apparent polar wander (APW) path for North America and fall in the 1750–1850 Ma interval.

The Mugford Group Volcanics of Labrador: Age, Geochemistry, and Tectonic Setting

1975 ◽  
Vol 12 (7) ◽  
pp. 1196-1208 ◽  
Author(s):  
Jackson M. Barton Jr.
Keyword(s):  
Tectonic Setting ◽  
Regional Metamorphism ◽  
Volcanic Rocks ◽  
Phosphorus Fractionation ◽  
Flood Basalts ◽  
Continental Flood Basalts ◽  
The North ◽  
Isotopic Analyses ◽  
Igneous Activity ◽  
North Atlantic Craton

The Mugford Group is a sequence of volcanic and sedimentary rocks exposed within the Khaumayät (Kaumajet) Mountains of Labrador. Separated from an intensely deformed and deeply eroded Archean basement complex by an angular unconformity, these rocks are nearly everywhere flat-lying and only locally altered. The volcanic rocks within the Mugford Group are of three types: tholeiitic basalts, komatiitic basalts and greenstones. A phosphorus fractionation diagram indicates that the tholeiitic and komatiitic basalts may be differentiates of a common magma. The greenstones, however, have undergone a separate crystallization history, but plot within the field of tholeiitic basalts on a FMA diagram, suggesting they were originally tholeiites. K–Ar whole-rock ages show that the Mugford volcanics are at least 1490 m.y. old. Rb–Sr whole-rock isotopic analyses of the tholeiitic and komatiitic basalts and the greenstones define an isochron of 2369 ± 55 m.y. with an initial 87Sr/86Sr ratio of 0.7033 ± 0.0002. This age is interpreted as approximating the time of extrusion of the Mugford volcanics. The low initial 87Sr/86Sr ratio indicates that the magmas giving rise to these rocks were not appreciably contaminated with older crustal material.The Mugford volcanics are presently the oldest recognized continental flood basalts. Their extrusion apparently occurred contemporaneously with the intrusion of the Okhakh granite at Okhakh (Okak) Harbour, 25 km to the south. This suggests that while no regional metamorphism accompanied extrusion of these volcanics, some local igneous activity did occur at that time. The Mugford volcanics may represent the extrusive equivalents of numerous basic dikes that were intruded during the final stages of stabilization of the North Atlantic craton.

A Discussion on global tectonics in Proterozoic times - Early and Middle Proterozoic history of the Great Lakes area, North America

1976 ◽  
Vol 280 (1298) ◽  
pp. 605-628 ◽  
Keyword(s):  
North America ◽  
Plate Tectonics ◽  
Regional Metamorphism ◽  
Volcanic Rocks ◽  
Low Grade ◽  
Tectonic Event ◽  
Early Proterozoic ◽  
Igneous Activity ◽  
Area North ◽  
Orogenic Belts

Two major supracrustal sequences, the Huronian Supergroup in Ontario and the Marquette Range Supergroup and Animikie Group of Michigan and Minnesota, overlie an Archean basement. These sequences are about 2200—2300 Ma and 1900-2000 Ma old respectively. The major Early Proterozoic tectonic event is the ‘Penokean Orogeny’, which occurred about 1850-1900 Ma ago and included deformation, high-grade regional metamorphism, and extrusive and intrusive igneous activity. This was followed by formation of rhyolitic, ignimbritic volcanic rocks and emplacement of associated granites about 1790 Ma ago. The entire region was subsequently subjected to low-grade regional metamorphism 1650-1700 Ma ago, followed by emplacement of anorogenic quartz-monzonite, in part rapakivi, plutons 1500 Ma ago. Late Proterozoic Grenville and Keweenawan events represent the youngest major Precambrian activity in the region. The rocks involved in the Penokean Orogeny lie along the southern margin of the Archean craton of the Superior Province and are interpreted as representing Early Proterozoic cratonic-margin orogenic activity. The distribution of rocks types and structures associated with the Penokean Orogeny and with similar orogenic belts along the margin of the Archean craton of North America suggest that these orogenic belts may have formed as a result of processes similar to modern plate tectonics, although the data are far from conclusive at present.

Geochronology and tectonic implications of magmatism and metamorphism, southern Kootenay Arc and neighbouring regions, southeastern British Columbia. Part I: Jurassic to mid-Cretaceous

1983 ◽  
Vol 20 (12) ◽  
pp. 1891-1913 ◽  
Author(s):  
D. A. Archibald ◽  
J. K. Glover ◽  
R. A. Price ◽  
E. Farrar ◽  
D. M. Carmichael
Keyword(s):  
British Columbia ◽  
North America ◽  
Regional Metamorphism ◽  
Granitic Rocks ◽  
Early Paleozoic ◽  
Small Bodies ◽  
Western Margin ◽  
The North ◽  
Windermere Supergroup ◽  
Igneous Activity

K–Ar dates and U–Pb zircon dates define three periods of igneous activity in the southern Kootenay Arc: (1) emplacement of late-synkinematic to post-kinematic granodioritic plutons in mid-Jurassic time (170–165 Ma) accompanying amphibolite-facies regional metamorphism; (2) emplacement of post-kinematic granitic plutons in mid-Cretaceous time (~100 Ma); and (3) emplacement of small bodies of syenite in Eocene time (~50 Ma) in the western part of the area. Micas from mid-Jurassic plutons that yield the oldest K–Ar dates (158–166 Ma) also yield plateau-shaped 40Ar/39Ar age spectra. Age spectra for biotites younger than these but older than 125 Ma reflect thermal overprinting.In southeastern British Columbia, the Kootenay Arc marks the transition from the North American rocks of the Cordilleran miogeocline to the tectonic collage of allochthonous terranes that have been accreted to it.Deformation, metamorphism, and plutonism recorded in rocks of the southern Kootenay Arc commenced in mid-Jurassic time as a composite allochthonous terrane was accreted to and overlapped the western margin of North America. The geochronology and metamorphic geothermobarometry show that in less than 10 Ma between 166 and 156 Ma: (1) rocks as young as the late Proterozoic Windermere Supergroup and the early Paleozoic Lardeau Group were carried rapidly to depths of 20–24 km while being deformed and intruded by granitic rocks of a hornblende–biotite suite that were also being emplaced at a much shallower level in the overriding allochthonous terrane; and (2) the miogeoclinal rocks of the Windermere Supergroup in the southern Kootenay Arc were then uplifted by more than 7 km at an estimated rate of 2 mm/year, and thrust over the allochthonous terrane prior to being intruded by post-kinematic granitic rocks, many of which belong to the two-mica suite of mid-Cretaceous age..

Chronology of Precambrian events in the Front Range, Colorado

1968 ◽  
Vol 5 (3) ◽  
pp. 749-756 ◽  
Author(s):  
Zell E. Peterman ◽  
Carl E. Hedge
Keyword(s):  
Regional Metamorphism ◽  
Volcanic Rocks ◽  
Time Span ◽  
Tectonic Activity ◽  
Front Range ◽  
Igneous Activity ◽  
Volcanic Pile ◽  
Metamorphic Terrane ◽  
Geologic Record ◽  
Intrusive Activity

The Precambrian rocks of the Front Range have undergone a long and complex history involving multiple periods of metamorphism, deformation, and intrusive activity. Before 1750 m.y. ago sedimentation and volcanism resulted in the accumulation of many thousands of feet of sandstone and shale, and lesser amounts of calcareous and volcanic rocks on a basement terrane that has not yet been recognized.Regional and dynamic metamorphism at 1700 to 1750 m.y. ago converted the sedimentary-volcanic pile to a metamorphic terrane of dominantly medium- to high-grade gneisses and schists. Plutonic rocks typified by Boulder Creek Granite were emplaced during this event. After this orogeny, there is a time span of 250 to 350 m.y. for which there is no obvious geologic record. The interval 1390 to 1450 m.y. was a period of major igneous activity during which the extensive Silver Plume Granite, Sherman Granite, granitic and mafic dikes, and pegmatites were emplaced. Regional heating attendant with this plutonism was effective in resetting most K-Ar and Rb-Sr mineral ages of the older rocks. Tectonic activity along the great shear zone that trends northeast through Idaho Springs probably began during this orogeny and may have continued intermittently until about 1200 m.y. ago.The Pikes Peak batholith and smaller related granites were emplaced at 1040 m.y. ago. This event was not accompanied by regional metamorphism. There is no recognized sedimentary record between the earlier orogeny and this igneous activity. The Pikes Peak igneous activity is the youngest recognized Precambrian event, but it appears to be limited to the Pikes Peak batholith and satellite plutons.

A regional paleomagnetic study of Archean rocks from the Superior Geotraverse area, northwestern Ontario

1979 ◽  
Vol 16 (10) ◽  
pp. 1906-1919 ◽  
Author(s):  
David J. Dunlop
Keyword(s):  
Reliable Determination ◽  
Polar Wander ◽  
Apparent Polar Wander Path ◽  
Northwestern Ontario ◽  
Paleomagnetic Study ◽  
Igneous Activity ◽  
Greenstone Belts ◽  
Mafic Intrusives ◽  
Iron Formations ◽  
East West

Preliminary paleomagnetic data are reported for Archean metavolcanics, felsic and mafic intrusives, gneisses, and iron formations from the Quetico, Shebandowan, and Wabigoon belts in the western Superior Structural Province. Eleven of the 23 formations sampled have been studied in detail using stepwise alternating-field demagnetization and, in some cases, thermal demagnetization. Two characteristic components of magnetization are revealed. One is widespread in occurrence and reasonably well grouped (D = 4.5°, I = + 55.7°, k = 17.9, α95 = 5.4°, N = 40 samples for the Shelley Lake granite for example). It is a regional magnetic overprint due to the Kenoran orogeny, acquired, according to the position of its paleopole (71.7°E, 77.3°N, δp = 5.5°, δm = 8 °for the Shelley Lake granite) on the Laurentian apparent polar wander path and to independent radiometric evidence, about −2600 Ma. The second magnetic component is spotty in occurrence, more prevalent in the Wabigoon belt than elsewhere, and everywhere rather scattered. The only fully reliable determination is D = 66.0°, I = −5.1 °(k = 10.2, α95 = 15°, N = 11 samples) for the Wabigoon gabbro. The corresponding paleopole is either 17.8°E, 13.1°N (δp = 7.5°, δm = 15°) falling at about −2800 Ma on the apparent polar wander path or the antipole of this direction, dating between −1350 and −1250 Ma. The younger date, implying a late ProterQzoic metamorphic event in the region, is more likely than the older age, which would require that the magnetization be primary or a pre-Kenoran overprint. The metamorphism seems to have occurred too early to have been caused by igneous activity ca. 1100 Ma in the nearby Keweenawan basin. About −2600 Ma, the region was in its present orientation, but at a latitude of 20 to 35°N and probably drifting northward. About −1250 Ma, the region was equatorial but rotated 90°, so that presently east–west trending greenstone belts were north–south. It is possible, but un-proven, that a paleoocean existed between the Wabigoon subprovince and the other belts about −1250 Ma, the Wabigoon region having since moved > 500 km westward (present-day direction) to its present location.

A Discussion on the evolution of the Precambrian crust - Tectonic displacements and thermal activity in two contrasting Proterozoic mobile belts from Greenland

1973 ◽  
Vol 273 (1235) ◽  
pp. 513-533 ◽  
Keyword(s):  
Plate Tectonics ◽  
Regional Metamorphism ◽  
The Other ◽  
Crustal Thickening ◽  
Thermal Activity ◽  
Crustal Shortening ◽  
Granite Intrusion ◽  
Igneous Activity ◽  
History Of ◽  
The One

The Proterozoic Nagssugtoqidian and Ketilidian mobile belts are comparable in scale with those of the Phanerozoic rather than those of the Archaean. These two Proterozoic belts differ from one another both in the tectonic displacements which gave rise to them, and in their thermal activities as expressed by igneous and metamorphic characteristics. Similar differences between modern tectonic belts have been interpreted in terms of plate tectonics. The Nagssugtoqidian is characterized by considerable crustal shortening, very limited igneous activity, and high-pressure regional metamorphism which may be related to crustal thickening resulting from both ductile and brittle overthrusting of the Nagssugtoqidian rocks over the Archaean foreland. Evidence of crustal shortening in the Ketilidian is limited, but vertical and transcurrent movements are important. Widespread igneous activity throughout the active history of the belt resulted in the formation of mainly acid volcanic supracrustal rocks and widespread granite intrusion. The appinite suite is also well represented. Metamorphism is mainly of low-pressure type. A tentative comparison can be made between the Alpine and Nagssugtoqidian belts on the one hand, and Andean and Ketilidian belts on the other.

Early Silurian paleolatitude for central Newfoundland from paleomagnetism of the Wigwam Formation

1992 ◽  
Vol 29 (8) ◽  
pp. 1652-1661 ◽  
Author(s):  
Kenneth L. Buchan ◽  
Joseph P. Hodych
Keyword(s):  
Volcanic Rocks ◽  
Opposite Polarity ◽  
Secondary Component ◽  
Lake Area ◽  
Thermal Demagnetization ◽  
Paleomagnetic Study

Redbeds and minor volcanic rocks of the Early Silurian Wigwam Formation of the Botwood Group were sampled in the Exploits Subzone of the Dunnage Zone of central Newfoundland. At 18 sedimentary and 3 volcanic sites thermal demagnetization isolated a shallow northward (or southward) magnetization (D = 356.0°, I = −16.8°; k = 12.9; α95 = 9.2°; n = 21 site means) after correction for the tilt of bedding. A positive fold test demonstrates that this remanence predates regional Siluro–Devonian folding. A primary age for the remanence is indicated by positive conglomerate tests on volcanic clasts, and reversals correlated with sedimentary stratigraphy. Antiparallel remanence directions at sites of opposite polarity indicate that the primary remanence is not partially overprinted by a secondary component. The Exploits Subzone of the Dunnage Zone is interpreted to have been at low paleolatitudes (8.5°S ± 5°) during the Early Silurian. We reached a similar conclusion for the Notre Dame Subzone of the Dunnage Zone in a previous paleomagnetic study of Early Silurian redbeds and volcanic rocks of the King George IV Lake area. The low paleolatitudes obtained for the Wigwam and King George IV Lake rocks do not differ significantly and, hence, provide no evidence for a large ocean between the two subzones in the Early Silurian. However, the azimuthal orientation of the subzones at that time is still uncertain, and an ocean with its axis oriented in a north–south direction would not be detected by the paleomagnetic method.

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