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1.
A paleomagnetic survey (34 sites, 200 core samples) of an Archean greenstone belt just north of the Grenville Front is described. Sixty per cent of the samples have blocking temperatures less than 400° C and remanent coercive forces predominantly below 100 Oe, and they provide no information about the Precambrian geomagnetic field. The other samples contain many magnetizations of which two (CS and CH) can be clearly recognized. The CS magnetization occurs in basic sills, and has high blocking temperature (550 to 650°C) and remanent coercive force (often exceeding 1000 Oe) and a direction (186°,?38°) that is not significantly different from that in Matachewan diabase dikes (2690 m.y.). CS is thought to have been caused by uplift following the Kenoran orogeny and its age is probably about 2600 m.y. No older magnetization has been found. The CH magnetization occurs in several different bodies and has a direction (119°, +51°) similar to that observed in rocks from other places close to the Grenville Front. CH has more variable blocking temperatures (200 to 650°C) and remanent coercive forces (200 to > 1000 Oe) and is considered to have been acquired by heating during uplift of the highly metamorphose Grenville Province to the south. Uplift occurred about 1000 m.y. ago, and the CH magnetization is considered to be of this age. Evidence is presented to support the suggestion that there was an extensive magnetization episode both within and adjacent to the Grenville Province at that time. 相似文献
2.
The Devonian Winnepegosis and Duperow Formations were examined in well 4-27-11-22W1, located in at the eastern edge of the Williston Basin in Manitoba. The variation in characteristic remanent magnetization (ChRM) direction and magnetic mineral carrier is obvious: the older Winnepegosis Formation has a primary or early post-depositional magnetization held in magnetite or pyrrhotite (n = 15; D = 324.1°, I = − 27.3°, α95 = 10.4°, k = 15.7), whereas the younger Duperow Formation magnetizations are carried by hematite and could be as late as Early Jurassic. The variability may be attributable to the intervening Prairie Evaporite acting as an aquitard to fluid migration. 相似文献
3.
A total of 81 samples (244 specimens) from Upper Cretaceous Indus Molasse and Middle to Upper Cretaceous Dras Flyschoids of the Indus-Tsangpo suture zone in Ladakh (northwest Himalaya) has been studied by thermal demagnetization methods.Both formations showed a characteristic magnetization component indicative for equatorial to low northern palaeolatitudes of acquisition. Similar palaeolatitudes have been obtained before from secondary magnetization components of Early Tertiary age in the Ladakh Intrusives and in the Tibetan Sedimentary Series of central Nepal. The present characteristic components are interpreted likewise as secondary magnetizations which stabilized between 50 and 60 m.y. ago, during Greater India's collision with Asia's southern margin.The Dras Flyschoids show another magnetic component which, in case of primary origin, indicates acquisition at a low southern palaeolatitude. If correct, this interpretation supports recent suggestions for Late Cretaceous obduction of an island arc on Greater India's northern margin. 相似文献
4.
Paleomagnetic study of dykes and intrusions remanent in the central part of the Kola Peninsula has been carried out; the Devonian
age of these objects has been confirmed by isotopic-geochronological studies. The component analysis of the magnetization
vector in the samples has shown that there are two magnetization components in most samples. The paleomagnetic pole corresponding
to the direction of a more stable component is located in the close vicinity of the Middle Devonian segment of the apparent
polar wander path (APWP) for the East European Craton, so this enables us to estimate its age to be as old as the Devonian.
The second magnetization component was found in Devonian dykes of both northern and southern parts of the Kola Peninsula;
the paleomagnetic pole corresponding to this component is located close to the Mesozoic (Early Jurassic) part of the APWP
for the East European Craton. It is suggested that the extensive remagnetization of Devonian intrusions in the Kola Peninsula
was caused by the thermal effect of the Barents-Amerasian superplume and by the appearance of an extensive area with trap
magmatism within the modern Arctic Basin region. Discovery of a significant thermal event that covered the Fennoscandian northeast
allows us to explain the geochronological problem concerning the Mesozoic ages of particular singular zircon grains from Precambrian
rocks of the shield derived via the SHRIMP method. 相似文献
5.
New paleomagnetic data are obtained for Middle Devonian rocks of Central Tuva. The rocks contain one-, two-, or three-component magnetization. The low-temperature (LT) components of magnetization are close to the directions of the present-day or Cenozoic magnetic field in Tuva. Based on the directions of the high-temperature (HT) components of magnetization, which were distinguished in the magnetite spectrum of blocking temperatures of up to 580оС, we revealed a prefolding magnetization of different polarity. The time when Middle Devonian rocks acquired the prefolding HT component of magnetization almost does not differ from the time of rock formation. Middle Devonian sequences were formed at low latitudes (19°–25° N). We calculated the Middle Devonian paleomagnetic pole (Φ =–13°, Λ = 106°, A95 = 7), which can be used to describe the movement of the Caledonian block in Central Asia, and probably Siberia, if these blocks had been tectonically coupled by the Devonian. 相似文献
6.
From alternating-field and thermal demagnetization studies on two dolerite “Traps” in the Gwalior Series (Central India), dated at 1830 ±200 m.y., three different palaeomagnetic directions could be distinguished. The characteristic magnetization component, which is considered as the primary magnetization, has a mean direction: D=78°, I=+34.5°, α95=5°, k=369, N=4 (Pole): 155.5°E19°N, dp=3°, dm=5.5°.A comparison of the presented data with other Precambrian and Phanerozoic data from the Indian subcontinent might suggest that the Indian subcontinent underwent a continuous anticlockwise rotational movement during the last 1800 m.y. 相似文献
7.
Paleomagnetic directions of 35 sites of Cambro-Ordovician carbonates from 10 anticlines were analyzed to test models of curvature along the Pennsylvania Salient of the Appalachians and to constrain the relative timing of magnetization acquisition. The sites yield directions of magnetization that are almost all reversed with near-horizontal inclinations upon appropriate structural correction. The common, Late Paleozoic (Kiaman-aged) direction and incremental fold tests show that these directions represent remagnetizations carried by authigenic magnetite, acquired just before or during the earlier phases of folding. No convincing indications were found of primary magnetizations. Mean declinations from the northeastern and southwestern limbs of the salient differ by a few degrees, indicating negligible, if any, rotation between the limbs. The results are similar to prior studies of overlying Siluro-Devonian carbonates, showing coherent behavior of the entire Paleozoic cratonic cover. We conclude that the statistically negligible difference in declination indicates that (previously demonstrated) oroclinal bending occurred before carbonates of the Paleozoic stratigraphic cover were remagnetized during the Permian and before regional folding was completed. 相似文献
8.
Isotopic age determinations on granitic rocks from Tasmania 总被引：1，自引：1，他引：0
Potassium‐argon and rubidium‐strontium isotopic age measurements show that emplacement of granitic rocks in Tasmania occurred during the Late Devonian and Early Carboniferous and in pre‐Devonian times, possibly in the Cambrian. In addition, a Precambrian granite, dated at about 750 m.y., has been recognized on the west coast of King Island. The granitic bodies of pre‐Devonian age include the Murchison River Granite, the Dove River Granite and its correlatives, and the adamellite on the southwest coast of Tasmania at Elliott Bay. These rocks were deformed during the Devonian Tabberabberan Orogeny with the result that leakage of radiogenic daughter products has occurred from minerals. Hence the indicated ages are younger than the true ages. Possibly these granitic rocks were emplaced during the Jukesian Movement of the Tyennan Orogeny, in the Late Cambrian, although a Precambrian age cannot be excluded for some of the bodies. As recognized by earlier workers the most important period of emplacement of granitic rocks in Tasmania was in the Middle Palaeozoic. The measured dates for this group of rocks range from 375 to 335 m.y., and indicate that intrusion occurred over an extended period from the Late Devonian to the Early or possibly Middle Carboniferous. There are distinct concentrations of measured ages at about 370 and 340 m.y. The granitic bodies of northeast Tasmania mainly yield the older age, whereas those of northwest Tasmania give the younger age. As the granites are post‐tectonic bodies the older age of about 375 m.y. provides a younger limit to the time of completion of the main folding in the Tabberabberan Orogeny, and this is consistent with the stratigraphic evidence. The evidence suggests that generation of granitic magma was initiated during the main folding associated with the Tabberabberan Orogeny, but that emplacement of the granites into the upper crust continued over a long period subsequently to the main folding phase. Alternatively, the younger granitic bodies, dated at about 340 m.y., may indicate that these rocks are related to the Early Carboniferous Kanimblan Orogeny recognized in Victoria and New South Wales; however, there is no field evidence to support such a proposition. 相似文献
9.
Several Triassic and earliest Jurassic sedimentary units from the Colorado Plateau region have distributions of virtual geomagnetic poles (VGPs) that are highly elongate along the path of apparent polar wander (APW). This suggests that the remanent magnetizations measured in these units were acquired over an extended period of time, possibly approaching 35 m.y., and are not precisely coeval with the stratigraphic age of the rock. Comparison with other paleomagnetic studies shows that the observed elongation is not a general attribute of the age of the rock, nor is it related to paleolatitude. The rocks that yield elongate VGP distributions are dominantly red to brown mudstones, and it is possible that their remanence is dominated by a slowly acquired chemical remanent magnetization, as suggested by Larson et al. [J. Geophys. Res. 87 (1982) 1081] and other authors. However, several superficially similar units from the Colorado Plateau have nearly circular VGP distributions. The process by which remanence is acquired in clastic sedimentary rocks merits further study. 相似文献
10.
Fission‐track ages have been determined on sphene and apatite from 28 granitic intrusions across the western half of Victoria. The sphene ages compare closely with independent K‐Ar biotite ages for the same intrusions, where these are available, and are invariably older than apatite ages by 35 to 135 m.y. This is in accord with the effective geological track annealing temperatures for these two minerals which are estimated to be 260 ± 20°C and 80 ± 10°C respectively. Both sphene and apatite ages decrease from west to east across western Victoria, the sphenes ranging from 470 ± 28 to 355 ± 19 m.y. The Wando Vale granodiorite and Dergholm granite from the Dundas Tableland of far‐western Victoria have sphene ages of 470 ± 28 m.y. and 452 ±16 m.y. respectively, clearly suggesting a relationship to the Ordo‐vician granitic rocks of southeastern South Australia. Fission‐track ages from the numerous post‐tectonic granites in the Ballarat Trough fall into two distinct groups. Rocks from the western area have sphene ages in the relatively narrow range 393 ± 14 m.y. suggesting emplacement in the Early Devonian time whereas those in the east have sphene ages of 362 ± 7 m.y. (near the Devonian‐Carboniferous boundary). Over the temperature interval recorded by sphene‐apatite pairs, cooling of the granitic rocks was very slow ranging from 0.8 to 5.3°C/m.y. Cooling in this range was probably controlled by uplift and erosion of overburden during a long period of post‐tectonic relaxation. Corresponding uplift rates are estimated to be 0.03 to 0.18 km/ m.y. assuming a normal continental geothermal gradient of 30°C/km. Below 80°C average cooling and uplift rates were probably about l°C/m.y. and 0.03 km/m.y. respectively so that cooling was essentially complete within about 80 m.y. of the apatite ages. 相似文献
11.
New African Lower Carboniferous paleomagnetic pole from intrusive rocks of the Tin Serririne basin (Southern border of the Hoggar, Algeria) 总被引：2，自引：0，他引：2
M.E.M. Derder B. Henry B. Bayou A. Ouabadi H. Bellon H. Djellit A. Khaldi M. Amenna K. Baziz A. Hemmi M.A. Guemache 《Tectonophysics》2006,418(3-4):189-203
A paleomagnetic study has been conducted on intrusive doleritic rocks cropping out within Devonian horizontal tabular formations of the Saharan craton (Tin Serririne basin, South of Hoggar shield). The 40K/40Ar dating of the dolerites gave an age of 347.6 ± 8.1 Ma, i.e. Tournaisian. The paleomagnetic data present three different directions. The first has a paleomagnetic pole close to the previous African poles of Permian age. This direction is therefore interpreted as a Permian remagnetization. The second direction, which is defined by both linear regression and remagnetization circles analysis, is considered as the primary magnetization. It yields a new African Tournaisian paleomagnetic pole (λ = 18.8° S, = 31.2° E, K = 29, A95 = 7.5°) very close to the Ben Zireg Tounaisian pole [Aifa, T., Feinberg, H., Pozzi, J.P., 1990. Devonian/Carboniferous paleopoles for Africa. Consequences for Hercynian geodynamics. Tectonophysics, 179, 288–304]. The third direction has intermediate orientation between those of the first or second directions and that of the Upper Cenozoic field. It is interpreted as related to a composite magnetization. This new Tin Serririne pole improves the APWP of Gondwana, for this key period of the evolution of the Pangea. This APWP confirms the previous paleogeographic reconstruction which shows that the pre-Hercynian ocean between Gondwana and Laurussia is still not close during the beginning of the Carboniferous. 相似文献
12.
Progressive thermal demagnetization of samples from the Tan y Grisiau granite defines a coherent easterly positive characteristic remanence (D/I = 124.9/60.3°;, 42 samples, R = 40–51, a95 = 4.8°;) residing in magnetite. An ancient reversal of magnetization is recovered in the highest blocking temperature spectrum of a few samples and suggests that a cooling-related dipolar axis is recorded by this pluton. Only facies of the granite which have been reddened, probably by submagmatic streaming, have recorded a stable remanence. Adjustment for tilt yields a very steep remanence (D/I = 193/88°;) incompatible with any known Early Palaeozoic and younger field direction from Britain. The in situ remanence has a similar declination to the primary magnetization in Late Ordovician dolerites from the Welsh Borderlands and yields a comparable palaeolatitude (41.5°;S). It is concluded that the Tan y Grisiau pluton was magnetized in Late Ordovician times after deformation. Folding in this region is therefore interpreted to be substantially of Taconic (Late Ordovician) origin and not Acadian in age. As both in situ and tilt-adjusted remanence directions are incompatible with Silurian and younger palaeofield directions from Britain, the pluton is interpreted as a subvolcanic component of the North Wales igneous province. Large anticlockwise rotation of Avalonia is identified between Late Ordovician and Late Silurian times. 相似文献
13.
Paleomagnetic measurements were made on samples from Silurian, Devonian, Carboniferous and Permian aged rocks from the Omulevka terrane, northeast Russia. The Omulevka terrane is a fragment of continental margin that forms a major component of the Kolyma–Omolon superterrane, a superterrane that constitutes the core of northeast Russia. All samples were collected from sites along the Taskan river, and all showed a very strong secondary overprint and in some cases more than one. With detailed thermal demagnetization, a component of magnetization that decayed towards the origin when viewed as an orthogonal plot was commonly present and showed better clustering of magnetization directions in stratigraphic coordinates. All of the sites sampled were tilted, but none had enough variation in bedding attitude to perform fold tests. Comparing data between localities was used as a pseudo fold test and indicates that in all but one case the characteristic remanent magnetization found represents a magnetization acquired very early in the history of the rock. Interpretation of these data in terms of both virtual pole positions and paleolatitude changes with time shows that the Omulevka terrane and the adjacent Omolon terrane were together at low latitudes in Silurian and Devonian time and until late Devonian their latitude was consistent with them being closely related to the Siberian Craton. Between Devonian and Permian times, the paleolatitudes of the terranes decreased and, when compared with equivalent reference paleolatitudes for the nearby Siberian platform, show a clear latitudinal separation. From Permian to Jurassic time, the Omulevka and Omolon terranes move steadily northwards, reducing the relative latitudinal separation from Siberia. By late Jurassic to early Cretaceous time, the observed and equivalent Siberian paleolatitudes are identical, indicating the accretion of the Kolyma–Omolon superterrane to the eastern Siberian continent. 相似文献
14.
We present geochronologic and paleomagnetic data from a north-trending quartz diorite intrusion that cuts Archean metasedimentary and metaigneous rocks of the South Pass Greenstone Belt of the Wyoming craton. The quartz diorite was previously thought to be either Archean or Early Proterozoic (?) in age and is cut by north and northeast-trending Proterozoic diabase dikes of uncertain age, for which we also report paleomagnetic data. New U–Pb analyses of baddeleyite and zircon from the quartz diorite yield a concordia upper intercept age of 2170±8 Ma (95% confidence). An 40Ar/39Ar amphibole date from the same sample yields a similar apparent age of about 2124±30 Ma (2σ), thus confirming that the intrusion is Early Proterozoic in age and that it has probably not been thermally disturbed since emplacement. A magmatic event at ca. 2.17 Ga has not previously been documented in the Wyoming craton. The quartz diorite and one of the crosscutting diabase dikes yield essentially identical, well-defined characteristic remanent magnetizations. Results from eight sites in the quartz diorite yield an in situ mean direction of north declination and moderate to steep positive inclination (Dec.=355°, Inc.=65°, k=145, α95=5°) with a paleomagnetic pole at 84°N, 215°E (δm=6°, δp=7°). Data from other diabase dike sites are inconsistent with the quartz diorite results, but the importance of these results is uncertain because the age of the dikes is not well known. Interpretation of the quartz diorite remanent magnetization is problematic. The in situ direction is similar to expected directions for magnetizations of Late Cretaceous/early Tertiary age. However, there is no compelling evidence to suggest that these rocks were remagnetized during the late Mesozoic or Cenozoic. Assuming this magnetization to be primary, then the in situ paleomagnetic pole is strongly discordant with poles of 2167, 2214, and 2217 Ma from the Canadian Shield, and is consistent with proposed separation of the Wyoming Craton and Laurentia prior to about 1.8 Ga. Correcting the quartz diorite pole for the possible effects of Laramide-age tilting of the Wind River Range, based on the attitude of nearby overlying Cambrian Flathead Sandstone (dip=20°, N20°E), gives a tilt corrected pole of 75°N, 58°E (δm=4°, δp=6°), which is also discordant with respect to time-equivalent poles from the Superior Province. Reconstruction of the Superior and Wyoming Province using a rotation similar to that proposed by Roscoe and Card [Can. J. Earth Sci. 46(1993)2475] is problematic, but reconstruction of the Superior and Wyoming Provinces based on restoring them to their correct paleolatitude and orientation using a closest approach fit indicates that the two cratons could have been adjacent at about 2.17 Ga prior to rifting at about 2.15 Ga. The paleomagnetic data presented are consistent with the hypothesis that the Huronian and Snowy Pass Supergroups could have evolved as part of a single epicratonic sedimentary basin during the Early Proterozoic. 相似文献
15.
Karl Hans Wedepohl Maryse H. Delevaux Bruce R. Doe 《Contributions to Mineralogy and Petrology》1978,65(3):273-281
New lead isotopic compositions have been measured for Paleozoic bedded and vein ore deposits of Europe by the high precision thermal emission (triple filament) technique. Eleven samples have been analyzed from the Upper Permian Kupferschiefer bed with representatives from Poland to England, three samples from the Middle Devonian Rammelsberg deposit and one from the Middle Devonian Meggen deposit, both of which are conformable ore lenses and are in the Federal Republic of Germany (FRG); and also two vein deposits from the FRG were analyzed, from Ramsbeck in Devonian host rocks and from Grund in Carboniferous host rocks. For Kupferschiefer bed samples from Germany, the mineralization is of variable lead isotopic composition and appears to have been derived about 250 m.y. ago from 1700 m.y. old sources, or detritus of this age, in Paleozoic sedimentary rocks. Samples from England, Holland, and Poland have different isotopic characteristics from the German samples, indicative of significantly different source material (perhaps older). The isotopic variability of the samples from the Kupferschiefer bed in Germany probably favors the lead containing waters coming from shoreward (where poor mixing is to be expected) rather than basinward (where better mixing is likely) directions. The data thus support the interpretation of the metal source already given by Wedepohl in 1964. Data on samples from Rammelsberg and Meggen tend to be slightly less radiogenic than for the Kupferschiefer, about the amount expected if the leads were all derived from the same source material but 100 to 150 m.y. apart in time. The vein galena from Ramsbeck is similar to that from Rammelsberg conformable ore lenses, both in rocks of Devonian age; vein galena from Grund in Upper Carboniferous country rocks is similar to some bedded Kupferschiefer mineralization in Permian rocks, as if the lead composition was formed at about the same time and from similar source material as the bedded deposits. Although heat has played a more significant role in the formation of some of these deposits (veins and Rammelsberg-Meggen) than in others (Kupferschiefer), there is no indication of radically different sources for the lead, all apparently coming from sedimentary source material containing Precambrian detritus. One feldspar lead sample from the Brocken-Oker Granite is not the same in isotopic composition as any of the ores analyzed.Publication authorized by the Director, U.S. Geological Survey 相似文献
16.
Remanent magnetization (RM) of rocks with hematite–ilmenite solid solution (HISS) minerals, at all crustal levels, may be an important contribution to magnetic anomalies measured by ground and satellite altitude surveys. The possibility that lower thermal gradient relatively deep in the crust can result in exsolution of HISS compositions with strong remanent magnetizations (RM) was studied for two bulk compositions within the HISS system. Samples from granulite-terrane around Wilson Lake, Labrador, Canada contains titanohematite with exsolved ferrian ilmenite lamellae. Other samples from the anorthosite-terrane of Allard Lake, Quebec, Canada contain ferrian ilmenite with exsolved titanohematite lamellae. In both cases, the final exsolved titanohematite has similar Ti content and carries dominant magnetic remanence with REM (=NRM/SIRM, where NRM is the natural remanent magnetization and SIRM is the saturation isothermal remanent magnetization) that is comparable to the Ti-free end member. The RM was acquired prior to exsolution and the ilmeno-hematite-rich rock possesses thermal remanent magnetization (TRM), whereas rocks with hemo-ilmenite possess chemical remanent magnetization (CRM). In both cases, we found fairly large homogeneous grains with low demagnetizing energy that acquired intense RM. The magnetism of the ilmeno-hematite solid solution phases is not significantly perturbed by the continuous reaction: ilmeno-hematitetitanohematite solid solution. Hence, the occurrence of HISS in rocks that cooled slowly in a low intensity magnetic field will have an intense magnetic signature characterized by a large REM. 相似文献
17.
For his PhD. thesis, Zijderveld (1975) studied the paleomagnetism of the Permian Esterel rocks (southern France). High-quality thermal and alternating-field demagnetization diagrams were interpreted to determine the direction of the characteristic natural magnetization. For the Esterel volcanics, a mean direction of Dec = 206.5°, Inc = –23°, 95 = 5.7°, k = 112 was found for this magnetization. The dispersion in this mean is remarkably low. Only the declination of the Reyran Rhyolite in the Reyran River quarry clearly deviated from this mean. This deviating direction is not found in our samples, taken at the same site. As many faults occur in this quarry, it is suggested that Zijderveld sampled this rhyolite on a small rotated block. To verify whether the small dispersion in the mean paleomagnetic direction of the Esterel rocks has a geomagnetic or a rock-magnetic origin, two conglomerate tests were carried out. One of these might be interpreted as positive. The results of the other conglomerate test (Agay Formation) are ambiguous: four of the six measured boulders show directions close to the mean paleomagnetic direction of the Esterel rocks. Rock-magnetic measurements show that the remanence is carried by a magnetite and a hematite fraction. The low dispersion in the paleomagnetic directions, the conglomerate tests, and hematite as remanence carrier suggest that the characteristic remanence in the Esterel volcanics was not instantaneously acquired during cooling, but might be affected by remagnetization due to weathering. 相似文献
18.
Laboratory of Isotope Geology Yichang Institute of Geology Mineral Resources the Ministry of Geology China 《中国地球化学学报》1982,1(2):159-174
Fourty-four isotopic ages have been determined by K-Ar and U-Th-Pb methods for Late Palaeozoic granitic rocks in the Nanling Region, South China. All dating values vary within the range of 231–348 m.y. From the obtained dates, further evidence has been found that there do exist Late Palaeozoic granitic rocks, which can be subdivided into Late Devonian and Permian granitic rocks. Within a Late Devonian terrain, there is a granitic pluton, namely granodiorite with a zircon U-Th-Pb age of 348 m.y., while ten granitic plutons have been recognized within a Permian terrain where granites are predominant, yielding biotite K-Ar ages of 236–289 m.y. (λ β =4.72×10?10yr.?1,λ K=5.57×10?11yr.?1) and zircon U-Th-Pb ages ranging from 231 to 280 m.y., respectively. It is obvious from the dates that intrusive activity of granitic magma extensively took place in the Nanling Region during Late Palaeozoic, although no records of orogenie movements have been found, indicating that the faults are the main factor controlling the activity of granitic magma, whereas the orogenic movements are not the only prerequisite for the formation of granitic magma and the intrusive activity. 相似文献
19.
Jean Bernard-Griffiths Jean-Marie Cantagrel Jean-Louis Duthou 《Contributions to Mineralogy and Petrology》1977,61(2):199-212
Systematic Rb-Sr determinations on granites and orthogneisses in the Western part of Massif Central Français (Limousin) permit us to say that metamorphism and associated folding occured in this area during the Devonian (between 400 and 350 m.y.) with anatexis stated at 360 m.y. Two premetamorphic magmatic episodes are located in the upper Cambrian-early Ordovician (520-490 m.y.) and in the Ordovician (460-450 m.y.). This should be compared with very similar radiometric results from Rouergue to South Brittany; thus a South Brittany-Limousin-Rouergue axis may be easily related to the acadian part from Appalachian belt. 相似文献
20.
Rob Sternberg Egon Lass Eric Marion Kaushik Katari Matthew Holbrook 《Geoarchaeology》1999,14(5):415-439
As part of a program to investigate archaeomagnetic secular variation in Israel and implications for archaeomagnetic dating, we have identified “anomalous” results that have yielded information about site formation processes. Stereonets, vector demagnetization diagrams, and sample location maps are most useful in examining stability of magnetization and consistency of archaeomagnetic directions with each other and with what would be expected from normal secular variation. A collection of examples is presented, including: strong and hard remanence due to vitrification, unstable magnetization due to lightning strikes, superimposed thermal magnetizations, chemical remagnetization, magnetization of a hearth fragment before falling, magnetization of a hot brick after falling, and mechanical deformation of an archaeomagnetic feature. © 1999 John Wiley & Sons, Inc. 相似文献