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1.
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   

2.
The two major Early to Middle Palaeozoic tectonic/metamorphic events in the northern Appalachians were the Taconian (Middle to Late Ordovician) in central to western areas and the Acadian (Late Silurian to early Middle Devonian) in eastern to west-central areas. This paper presents a model for the Acadian orogenic event which separates the Acadian metamorphic realm into eastern and western belts based on distinctively different styles. We propose that the Acadian metamorphism in the east was the delayed consequence of Taconian back-arc lithospheric modification. East of the Taconian island arc, thick accumulations of Late Ordovician and Silurian sediments, coupled with plutons rising along a magmatic arc, produced crustal thermal conditions appropriate for anomalously high-T, low-P metamorphism accompanied by major crustal anatexis. In this zone, upward melt migration was coupled with subsequent E-W crustal shortening (possibly due to outboard collision with the Avalon terrane) to produce mechanical conditions that favoured formation of fold and thrust nappes and resultant tectonic thickening to the west (and probably to the east as well). The basis for the distinction between the Eastern and Western Acadian events lies in the contrasting styles of metamorphism accompanying each. Evidence for contrasting metamorphic styles consists of (1) estimated metamorphic field gradients (MFGs) based on thermobarometric studies, and (2) petrological evidence for contrasting P–T trajectories. West of the Acadian metamorphic front, the Taconian zone has an MFG in which peak temperatures of 400-600° C were reached at pressures of about 4–6 kbar, with both P and T increasing to the east. Near its western edge, the Western Acadian metamorphic overprint has a similar MFG to the Taconian, and is mainly discriminated by 40Ar/39Ar dating and microtextural evidence. East of this narrow zone, the Western Acadian overprint is characterized by progressively higher temperatures (600–725° C) and pressures (6.5–10 kbar, or more) to the east, yielding an overall MFG that lies along, or slightly above, the kyanite–sillimanite boundary on a P–T diagram. There is little or no plutonism accompanying Western Acadian metamorphism. In contrast, thermobarometry in the Eastern Acadian, east of the Bronson Hill Belt, yields high-T, intermediate-P conditions for the highest grade rocks known in New England: T= 650–750° C, P= 4.5–6.5 kbar for granulite facies assemblages which apparently formed along an ‘anticlockwise’P–T path. The Bronson Hill Belt lies geographically between the Eastern and Western Acadian zones and shows transitional petrological behaviour: anomalously high temperatures at intermediate pressures, but a ‘clockwise’ path with decompression cooling. Radiometric dating indicates peak Taconian conditions may have been achieved as early as 475 Ma in the Taconian hinterland and as late as 445 Ma in the Taconian foreland (including the Taconic allochthons). Eastern Acadian magmatism may have started as early as 425 Ma, and most nappe-stage deformation and metamorphism in the Eastern Acadian zone appears to have ended by about 410 Ma. Tectonic thickening in the Western Acadian (including the western counterparts of the nappe-stage deformation documented in the Eastern Acadian) must pre-date attainment of peak metamorphic conditions dated at 395–385 Ma. Dome-stage deformation clearly post-dates peak metamorphism and deforms metamorphic isograds. The end of Western Acadian deformation is well constrained by 370-375 Ma radiometric ages of late pegmatites and granitoids which cross-cut all structures.  相似文献   

3.
《Precambrian Research》1999,93(2-3):201-213
New palaeomagnetic results are presented from the recently dated Palaeoproterozoic ultramafic Konchozero sill, and associated basalts (three sites, 38 oriented samples). Three stable components of remanence have been isolated during thermal and alternating field demagnetisation. The component I, with a mean direction of D=103°, I=40°, k=18, α95=11° (N=11 samples), pole position of 14°S, 282°E, has been obtained from the unaltered deeper part of the sill and from baked schists. The study of the baked contact confirms the conclusion that component I is supposed to be primary and corresponds to the Sm–Nd age of the sill of 1974±27 Ma. The palaeopole of component I is not consistent with the accepted Fennoscandian apparent polar wander path (APWP) for the period 2120–1880 Ma, and for that part the Fennoscandian APWP should be revised. Two other components (component II: D=349°, I=39°, k=35, α95=6°, N=19 samples, pole position 49°N, 231°E; and component III: D=17°, I=41°, k=44, α95=5°, N=19 samples, pole position 50°N, 190°E) fit the APWP well, with palaeomagnetically estimated ages of ca. 1860 and 1760 Ma respectively.  相似文献   

4.
Four distinct components of natural remanent magnetization were isolated from a single site in welded tuffs in the Upper Cretaceous Kisin Group of the Sikhote Alin mountain range, Russia. In order to contribute toward a basis for an interpretation of multicomponent magnetization, rock magnetic experiments were performed on the welded tuffs. All four magnetization components essentially reside in magnetite. The lowest-temperature component up to 300 °C (component A: D=349.3°, I=60.9°, α95=7.3°, N=7) is a present day viscous magnetization. The third-removed component (component C: D=41.4°, I=51.8°, α95=3.5°, N=8), isolated over the temperature range of 450–560 °C, is a primary remanence. The second- and fourth-demagnetized components (component B: D=174.7°, I=−53.1°, α95=21.2°, N=3 and component D: D=188.1°, I=−64.5°, α95=4.0°, N=8, respectively) are secondary magnetizations related to a thermal event in Sikhote Alin between 66 and 51 Ma. Components B and D were acquired through different remagnetization processes. Component B is ascribed to a thermoviscous remanent magnetization carried by single-domain magnetite, and component D is a chemical remanent magnetization.  相似文献   

5.
An Early Devonian age for the continental, red‐bed succession of the Peel Sandstone Group can be defined on the basis of: (1) a derived marine fauna of late Wenlock (Homerian) age, (2) a Scoyenia ichnofacies assemblage (including Beaconites and Diplichnites) characteristic of latest Silurian to Early Devonian (Lower Old Red Sandstone magnafacies) sediments in the British Isles, (3) a microflora of late Lochkovian to Pragian age, (4) a detrital palaeomagnetic remanence that pre‐dates local, Acadian palaeomagnetic directions and coincides with a prominent, southerly, Late Silurian to Early Devonian excursion in the local apparent polar wander path, and (5) a mid‐Devonian palaeomagnetic remanence that overprints (?)Acadian, thrust‐related folding. Data presented in this study confirm previous suggestions (Allen and Crowley 1983) that the Peel Sandstone Group represents a rare example of Early Devonian sedimentation preserved on the northern margin of the former Eastern Avalonia microcontinent. Potential correlations and linkages with other Lower Old Red Sandstone successions exposed in the Anglo‐Welsh Basin are developed and discussed. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

6.
A paleomagnetic study of subsurface core samples from dolomitized carbonates of two producing reservoirs in the Upper Ordovician Trenton Formation, collected from four wells in southwestern Ontario yielded a paleomagnetic direction of D = 152.3°, I = − 12.3° (N = 49, α95 = 8.7). This characteristic remanent magnetization (ChRM) direction was azimuth-corrected by aligning the viscous remanence magnetization (VRM) with the present Earth's magnetic field direction. A drilling-induced magnetization (VRMdi) was present in less than half the specimens sampled in this study. In addition, where the VRM correction could not be made, a paleolatitudinal arc calculated from the inclination-only mean of I = − 9.0° (N = 34, α95 = 3.0°) intersected the apparent polar wander path in the Late Permian–Early Triassic. These paleodirections are similar to the paleomagnetic directions observed in Ordovician Trenton carbonates from the Michigan Basin and New York State, U.S.A., suggesting a related regional late Paleozoic remagnetization.  相似文献   

7.
华南早三叠世的古地磁学与大地构造   总被引：50，自引：0，他引：50       下载免费PDF全文
从华南几个具代表性地点的下三叠统灰岩中,获得了有意义的古地磁学初步结果。这些结果支持华南存在分离板块的设想。根据新近获得的地质与古地磁学证据,推断华南存在4个岩石图板块。它们是扬子(D=232.4°,I=-11.8°,K=28.2,α95=5.5°),湘桂(D=198.0°,I=32.2°,α95=12.9°),华夏(D=88.0°,I=13.0°,K=20.1,α95=9.1°)和海南(上二叠统D=338.2°,I=15.9°,K=10.2,α95=14.3°)板块。4个板块的古地磁极位分散,纬度差明显,并存在碰撞后的局部与区域性的旋转作用,尤其是顺时针旋转。华南早中生代的大地构造可理解为4个来自古特提斯和冈瓦纳的岩石圈碎块与欧亚大陆聚合作用的产物。  相似文献   

8.
Palaeomagnetic measurements were carried out on low-grade metamorphic carbonates, of Mesozoic age from the Shiar area (85.1°E, 28.6°N) of the Tethyan Himalaya (TH) in north central Nepal. Two characteristic remanence components carried by pyrrhotite (ChRM1) and magnetite (ChRM2) could be identified by their unblocking temperature spectra of 270–340 and 430–580°C, respectively. Fold tests are not significant, due to the uniform bedding of all sites. However, according to results from other areas of the TH, the pyrrhotite component has been probably acquired as a secondary (p)TRM during exhumation and cooling; thus the age of remanence acquisition can be related to the last cooling event (25–17 Ma in the surrounding areas). The inclination of the magnetite component matches the value expected from the Indian APWP. This may the primary origin of the ChRM2.Pyrrhotite site-mean directions show a small-circle distribution, with a best fit parallel to the N–S direction. Backtilting to the expected inclination (Iexp) by intersection of the remanence small-circle with the small-circle of constant Iexp yields a clockwise block rotation of 30–35° with respect to the Indian Plate. Characteristics of the pyrrhotite component (small-circle distribution of site-means, secondary origin, (p)TRM with unblocking temperatures below about 300°C), allow the interpretation of the chronologic order of the thermo-tectonic history: (i) an earlier main folding phase at elevated temperatures; (ii) a later event of cooling through about 300°C coinciding with the acquisition of ChRM1; (iii) clockwise block rotations with respect to the Indian Plate and (iv) long-wave folding as the youngest tectonic event.  相似文献   

9.
Palaeomagnetic, geochemical and geochronological studies have been conducted on a set of dolerite dykes intruding the Peninsular gneisses near Huliyurdurga town, Karnataka, as a reconnaissance survey indicated a Cretaceous age for them. The dykes are mainly tholeiitic in composition with their 87Sr/86Sr ratios tightly clustered around 0·7045. Their palaeomagnetic data (D m =329°,I m =−55°) and the corresponding palaeopole coordinates (λ p = 34°S,L p =108°E) are strikingly close to those of the Deccan Traps to the north. Whole rock K-Ar ages of these dykes ranging between 69 and 84 Ma are also similar to the range of K-Ar ages of the Deccan basalts. The chemical, palaeomagnetic and temporal coherence between the dykes and the Deccan basalts indicate that they may indeed be tectonically related events.  相似文献   

10.
A section of the orogenic middle crust (Orlica‐?nie?nik Dome, Polish/Czech Central Sudetes) was examined to constrain the duration and significance of deformation (D) and intertectonic (I) phases. In the studied metasedimentary synform, three deformation events produced an initial subhorizontal foliation S1 (D1), a subsequent subvertical foliation S2 (D2) and a late subhorizontal axial planar cleavage S3 (D3). The synform was intruded by pre‐, syn‐ and post‐D2 granitoid sheets. Crystallization–deformation relationships in mica schist samples document I1–2 garnet–staurolite growth, syn‐D2 staurolite breakdown to garnet–biotite–sillimanite/andalusite, I2–3 cordierite blastesis and late‐D3 chlorite growth. Garnet porphyroblasts show a linear Mn–Ca decrease from the core to the inner rim, a zone of alternating Ca–Y‐ and P‐rich annuli in the inner rim, and a Ca‐poor outer rim. The Ca–Y‐rich annuli probably reflect the occurrence of the allanite‐to‐monazite transition at conditions of the staurolite isograd, whereas the Ca‐poor outer rim is ascribed to staurolite demise. The reconstructed P–T path, obtained by modelling the stability of parageneses and garnet zoning, documents near‐isobaric heating from ~4 kbar/485 °C to ~4.75 kbar/575 °C during I1–2. This was followed by a progression to 4–5 kbar/580–625 °C and a subsequent pressure decrease to 3–4 kbar during D2. Pressure decrease below 3 kbar is ascribed to I2–3, whereas cooling below ~500 °C occurred during D3. In the dated mica schist sample, garnet rims show strong Lu enrichment, oscillatory Lu zoning and a slight Ca increase. These features are also related to allanite breakdown coeval with staurolite appearance. As Lu‐rich garnet rims dominate the Lu–Hf budget, the 344 ± 3 Ma isochron age is ascribed to garnet crystallization at staurolite grade, near the end of I1–2. For the dated sample of amphibole–biotite granitoid sheet, a Pb–Pb single zircon evaporation age of 353 ± 1 Ma is related to the onset of plutonic activity. The results suggest a possible Devonian age for D1, and a Carboniferous burial‐exhumation cycle in mid‐crustal rocks that is broadly coeval with the exhumation of neighbouring HP rocks during D2. In the light of published ages, a succession of telescoping stages with time spans decreasing from c. 10 to 2–3 Ma is proposed. The initially long period of tectonic quiescence (I1–2 phase, c. 10 Ma) inferred in the middle crust contrasts with contemporaneous deformation at deeper levels and points to decoupled P–T–D histories within the orogenic wedge. An elevated gradient of ~30 °C km?1 and assumed high heating rates of c. 20 °C Ma?1 are explained by the protracted intrusion of granitoid sheets, with or without deformation, whereas fast vertical movements (2–3 Ma, D2 phase) in the crust require the activity of deformation phases.  相似文献   

11.
This study has investigated magnetic remanence, rock magnetism and anisotropy of magnetic susceptibility (AMS) in granulite and amphibolite grade metamorphic terranes of the Huabei Shield between Inner Mongolia in the west and the Bohai Sea in the east. Rock magnetic studies identify annealed metamorphic magnetite grains with multidomain properties as the remanence carriers; a widely recorded stable remanence was probably fixed by grain shape effects. Granulite facies terranes are typically between one and two orders more strongly magnetised than amphibolite terranes and AMS fabrics correlate mostly with metamorphic mineral fabrics observed in the country rocks. Progressive thermal demagnetisation identifies a range of two and three component structures resident in magnetite. An important component recognised as a partial or complete remagnetisation by Late Mesozoic–Tertiary tectonic/magmatic activity is present in basement at the southern margin of the outcrop (Miyun terrane) and where extensive granite plutonism has occurred (Zhunhua terrane). These components have directions corresponding to remanence in the Yunmeng Shan Granite (119–114 Ma, D/I=33/58°, 39 samples, a95=3.5°, palaeopole at 201°E, 64°N). Most remanence elsewhere was probably acquired during post-tectonic uplift and cooling of the basement between ∼2200 and 1850 Ma because palaeomagnetic directions are removed from the Phanerozoic palaeofield path and they are distinct from the palaeomagnetic record in the overlying Jixian Supergroup deposited at ∼1840–900 Ma. These latter magnetisations are considered reliable indicators of the palaeofield during Late Palaeoproterozoic times because deformation of overlying supracrustal rocks is mostly slight and no prominent deflection of magnetic remanence by magnetic fabrics is observed. Palaeofield directions and poles attributed to the time of uplift-related cooling are: Qian’an Terrane (D/I=215/71°, a95=9°, 17 samples, pole at 99°E, 10°N) and North Qianxi Terrane (D/I=44/−45°, a95=4°, 41 samples, pole at 79°E, 11°S). In addition, a more widely-preserved shallow northerly component correlates with a NW→E swathe of components recorded by uplift-related cooling within the Datong–Huan’an granulite terrane in the west of the shield. A preliminary Palaeo-Mesoproterozoic apparent polar wander path for the Huabei Shield is defined from the Palaeoproterozoic record in the metamorphic basement rocks and the Meso-Neoproterozoic record in the overlying Jixian Supergroup. It incorporates a loop between ∼2200 and 1850 Ma and exhibits a general east to west trend in subsequent times.  相似文献   

12.
The Connecticut Valley–Gaspé (CVG) trough represents a major, orogen-scale Silurian–Devonian basin of the Northern Appalachians. From Gaspé Peninsula to southern New England, the CVG trough has experienced a contrasting metamorphic and structural evolution during the Acadian orogeny. Along its strike, the CVG trough is characterized by increasing strain and polyphase structures, and by variations in the intensity of regional metamorphism and contrasting abundance of c. 390–370 Ma granitic intrusions. In southern Quebec and northern Vermont, a series of NW–SE transects across the CVG trough have been studied in order to better understand these along-strike variations. Detailed structural analyses, combined with phase equilibria modelling, Raman spectrometry, and muscovite 40Ar/39Ar dating highlight a progressive and incremental deformation involving south–north variation in the timing of metamorphism. Deformation evolves from a D1 crustal thickening event which originates in Vermont and progresses to southern Québec where it peaked at 0.6 GPa/380°C at c. 375 Ma. This was followed by a D2 event associated with continuous burial in Vermont from 378 to 355 Ma, which produced peak metamorphic conditions of 0.85 GPa/380°C and exhumation in Quebec from 368 to 360 Ma. The D3 compressional exhumation event also evolved from south to north from 345 to 335 Ma. D1 to D3 deformation events form part of a continuum with an along-strike propagation rate of ~50 km/Ma During D1, the burial depth varied by more than 15 km between southern Quebec and Vermont, and this can be attributed to the occurrence of a major crustal indenter, the Bronson Hill Arc massif, in the New England segment of the Acadian collision zone.  相似文献   

13.
Detailed palaeomagnetic and rock magnetic analyses provide improved palaeomagnetic results from 23 sites in the Borgmassivet intrusions in the Ahlmannryggen region of Dronning Maud Land, East Antarctica. These intrusions are of similar age to their host, the ca. 1130 Ma Ritscherflya Supergroup (RSG). A mean direction of D=235.4°, I=−7.6° with k=45.9 and α95=4.5° was obtained from this study. When combined with previously reported results from 11 sites in the same region, including sites from the Ritscherflya Supergroup, it gives an overall mean direction for 34 sites from the igneous suite with D=236.5°, I=−3.6°, k=27.9 and α95=4.8°. Isothermal remanent magnetization (IRM) experiments on several specimens suggest magnetite or titanomagnetite as the primary remanence carrier, while high temperature magnetic susceptibility experiments indicate the presence of single domain particles. These observations, together with field evidence and the high coercivities and unblocking temperatures, support a primary origin for the observed characteristic remanence. The Borgmassivet palaeomagnetic pole lies at 54.5°E, 8.3°N with A95=3.3°. If Antarctica is moved to its Gondwanan position adjacent to southeast Africa, the Borgmassivet pole (BM) coincides with that of the African well-established, well-dated (1100 Ma) Umkondo Large Igneous Province pole, supporting the hypothesis that the Grunehogna craton of Dronning Maud Land was part of the Kalahari craton of southern Africa at ca. 1100 Ma.  相似文献   

14.
The South Indian Craton is composed of low-grade and high-grade metamorphic rocks across different tectonic blocks between the Moyar–Bhavani and Palghat–Cauvery shear zones and an elongated belt of eastern margin of the peninsular shield. The Madras Block north of the Moyar–Bhavani shear zone, which evolved throughout the Precambrian period, mainly consists of high-grade metamorphic rocks. In order to constrain the evolution of the charnockitic region of the Pallavaram area in the Madras Block we have undertaken palaeomagnetic investigation at 12 sites. ChRM directions in 61 oriented block samples were investigated by Alternating Field (AF) and Thermal demagnetization. Titanomagnetite in Cation Deficient (CD) and Multi Domain (MD) states is the remanence carrier. The samples exhibit a ChRM with reverse magnetization of Dm = 148.1, Im = + 48.6 (K = 22.2, α95 = 9.0) and a palaeomagnetic pole at 37.5 °N, 295.6 °E (dp/dm = 7.8°/11.8°). This pole plots at a late Archaean location on the Indian Apparent Polar Wander Path (APWP) suggesting an age of magnetization in the Pallavaram charnockites as 2600 Ma. The nearby St. Thomas Mount charnockites indicate a period of emplacement at 1650 Ma (Mesoproterozoic). Thus the results of Madras Block granulites also reveal crustal evolution similar to those in the Eastern Ghats Belt with identical palaeopoles from both the areas.  相似文献   

15.
《Precambrian Research》2004,128(1-2):167-188
Thirty-nine oriented block samples of iron-formation were collected at 13 sites, including opposite limbs of major folds, from the 1.88-Ga Sokoman Formation (Knob Lake Group) in the Schefferville–Knob Lake area of the central New Québec Orogen, northern Québec. The samples assayed up to 80.24% Fe2O3T (54.08% Fe), implying Fe-enrichment of the iron-formation up to ore grade. Anisotropy of magnetic susceptibility measurements on 245 standard specimens indicate a well preserved bedding-parallel fabric in the iron-formation, suggesting minimal alteration of the magnetic mineralogy since deposition and/or a mimetic secondary magnetic mineralogy. The iron-formation has not been internally deformed since the magnetic mineralogy was established. Analyses by variable-field translation balance and X-ray diffraction showed that the predominant magnetic mineral is hematite but a small amount of magnetite also is present in most samples. Following low-temperature pre-treatment as appropriate, stepwise thermal and alternating-field demagnetization of 218 specimens revealed a low-temperature, post-folding component (maximum Tub≈400 °C, D=27.1°, I=20.1°, α95=10.9°, from seven sites; pole position of 40.6°S, 257.0°E), and components carried by magnetite (maximum Tub≈580 °C, D=35.8°, I=3.9°, α95=9.1°, from 10 sites; pole position of 29.6°S, 250.9°E) and hematite (maximum Tub≈680 °C, D=40.0°, I=1.6°, α95=18.6°, from seven sites; pole position of 26.8°S, 247.0°E). The components carried by magnetite and hematite are pre-, syn- and post-folding depending on the sampling site, indicating that the magnetization was acquired continuously with deformation in the New Québec Orogen at 1.84–1.83 Ga. No evidence was found for acquisition of magnetization during the Mesozoic, when many of the iron oxide orebodies in the Schefferville–Knob Lake area are thought to have formed. Our findings imply that an episode of Fe-enrichment of iron-formation in the Sokoman Formation involved the circulation of hydrothermal fluids related to late Paleoproterozoic orogenesis. Such orogenic circulation of fluids may have contributed to the development of hematitic orebodies in the central New Québec Orogen.  相似文献   

16.
We report a new paleomagnetic pole for the Black Range Dolerite Suite of dykes, Pilbara craton, Western Australia. We replicate previous paleomagnetic results from the Black Range Dyke itself, but find that its magnetic remanence direction lies at the margin of a distribution of nine dyke mean directions. We also report two new minimum ID-TIMS 207Pb/206Pb baddeleyite ages from the swarm, one from the Black Range Dyke itself (>2769 ± 1 Ma) and another from a parallel dyke whose remanence direction lies near the centre of the dataset (>2764 ± 3 Ma). Both ages are slightly younger than a previous combined SHRIMP 207Pb/206Pb baddeleyite weighted mean date from the same swarm, with slight discordance interpreted as being caused by thin metamorphic zircon overgrowths. The updated Black Range suite mean remanence direction (D = 031.5°, I = 78.7°, k = 40, α95 = 8.3°) corresponds to a paleomagnetic pole calculated from the mean of nine virtual geomagnetic poles at 03.8°S, 130.4°E, K = 13 and A95 = 15.0°. The pole's reliability is bolstered by a positive inverse baked-contact test on a younger Round Hummock dyke, a tentatively positive phreatomagmatic conglomerate test, and dissimilarity to all younger paleomagnetic poles from the Pilbara region and contiguous portions of Australia. The Black Range pole is distinct from that of the Mt Roe Basalt (or so-called ‘Package 1’ of the Fortescue Group), which had previously been correlated with the Black Range dykes based on regional stratigraphy and imprecise SHRIMP U–Pb ages. We suggest that the Mt Roe Basalt is penecontemporaneous to the Black Range dykes, but with a slight age difference resolvable by paleomagnetic directions through a time of rapid drift of the Pilbara craton across the Neoarchean polar circle.  相似文献   

17.
A combined paleomagnetic and geochronological investigation has been performed on Cretaceous rocks in southern Qiangtang terrane (32.5°N, 84.3°E), near Gerze, central Tibetan Plateau. A total of 14 sites of volcanic rocks and 22 sites of red beds have been sampled. Our new U–Pb geochronologic study of zircons dates the volcanic rocks at 103.8 ± 0.46 Ma (Early Cretaceous) while the red beds belong to the Late Cretaceous. Rock magnetic experiments suggest that magnetite and hematite are the main magnetic carriers. After removing a low temperature component of viscous magnetic remanence, stable characteristic remanent magnetization (ChRM) was isolated successfully from all the sites by stepwise thermal demagnetization. The tilt-corrected mean direction from the 14 lava sites is D = 348.0°, I = 47.3°, k = 51.0, α95 = 5.6°, corresponding to a paleopole at 79.3°N, 339.8°E, A95 = 5.7° and yielding a paleolatitude of 29.3° ± 5.7°N for the study area. The ChRM directions isolated from the volcanic rocks pass a fold test at 95% confidence, suggesting a primary origin. The volcanic data appear to have effectively averaged out secular variation as indicated by both geological evidence and results from analyzing the virtual geomagnetic pole (VGP) scatter. The mean inclination from the Late Cretaceous red beds, however, is 13.1° shallower than that of the ~ 100 Ma volcanic rocks. After performing an elongation/inclination analysis on 174 samples of the red beds, a mean inclination of 47.9° with 95% confidence limits between 41.9° and 54.3° is obtained, which is consistent with the mean inclination of the volcanic rocks. The site-mean direction of the Late Cretaceous red beds after tilt-correction and inclination shallowing correction is D = 312.6°, I = 47.7°, k = 109.7, α95 = 3.0°, N = 22 sites, corresponding to a paleopole at 49.2°N, 1.9°E, A95 = 3.2° (yielding a paleolatitude of 28.7° ± 3.2°N for the study area). The ChRM of the red beds also passes a fold test at 99% confidence, indicating a primary origin. Comparing the paleolatitude of the Qiangtang terrane with the stable Asia, there is no significant difference between our sampling location in the southern Qiangtang terrane and the stable Asia during ~ 100 Ma and Late Cretaceous. Our results together with the high quality data previously published suggest that an ~ 550 km N–S convergence between the Qiangtang and Lhasa terranes happened after ~ 100 Ma. Comparison of the mean directions with expected directions from the stable Asia indicates that the Gerze area had experienced a significant counterclockwise rotation after ~ 100 Ma, which is most likely caused by the India–Asia collision.  相似文献   

18.
The Taurides, the southernmost of the three major tectonic domains that constitute present‐day Turkey, were emplaced following consumption of the Tethyan Ocean in Late Mesozoic to mid‐Tertiary times. They are generally assigned an origin at the northern perimeter of Gondwana. To refine palaeogeographic control we have investigated the palaeomagnetism of a range of Jurassic rocks. Forty‐nine samples of Upper Jurassic limestones preserve a dual polarity remanence (D/I=303/−9°, α95=6°) interpreted as a primary magnetization acquired close to the equator and rotated during emplacement of the Taurides. Result from mid‐Jurassic dolerites confirm a low palaeolatitude for the Tauride Platform during Jurassic times at the Afro–Arabian sector of Gondwana. Approximately 4000 km of Tethyan closure subsequently occurred between Late Jurassic and Eocene times. Although related Upper Jurassic limestones and Liassic redbeds preserve a sporadic record of similar remanence, the dominant signature in these latter rocks is an overprint of probable mid‐Miocene age, probably acquired during a single polarity chron and imparted by migration of a fluid front during nappe loading. This is now rotated consistently anticlockwise by c. 30° and conforms to results of previous studies recording bulk Neogene rotation of the Isparta region following Lycian nappe emplacement. The regional distribution of this overprint implies that the Isparta Angle (IA) has been subject to only small additional closure (<10°) since Late Miocene time. A smaller amount (c. 6°) of clockwise rotation within the IA since Early Pliocene times is associated with an ongoing extensional regime and reflects an expanding curvature of the Tauride arc produced by southwestward extrusion of the Anatolian collage as a result of continuing northward motion of Afro–Arabia. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

19.
Early Palaeozoic volcanic and sedimentary rocks from the Saxothuringian Basin (Franconian Forest, northern Bavaria) have been subjected to detailed radiometric and palaeomagnetic studies in order to determine the tectonic environment and geographic setting in which they were deposited. Two hand samples were collected from the as yet undated pyroclastic flow deposits for 207Pb/206Pb age dating. Radiometric results for these samples, obtained by the single-zircon evaporation technique, are identical within error, and the mean age of all measured grains is 478.2ǃ.8 Ma (n=11). This age is considered to be primary and firmly constrains the eruption of the ignimbrites and formation of the subaqueous pyroclastic flows as having occurred in Early Ordovician (Arenig) times. Palaeomagnetic studies were carried out on these Early Ordovician volcanic rocks, and also on the biostratigraphically dated, Late Ordovician (Ashgillian) Döbra sandstones. The volcanic rocks carry up to three directions of magnetisation. The poorly defined, low and intermediate unblocking temperature directions are thought to represent secondary overprint directions of post-Ordovician age. The high temperature component, however, identified at temperatures of up to 580 °C, is of mixed polarity and passes the fold test with 99% confidence. The overall mean direction after bedding correction is 189°/76°, !95=11.6°, k=44.7 (25 samples, five sites), and is considered to be primary and Early Ordovician in origin. It yields a palaeo-south pole at 24°N and 007°E, which translates into palaeolatitudes of 63°+21.7°/-17.3° S for the Saxothuringian Terrane. Samples from the Late Ordovician Döbra sandstone are generally very weakly magnetised. A high temperature D component of magnetisation can be identified in some samples and yields a mean direction of 030°/-58°, !95=18.5°, k=25.7 (15 samples, four sites) after bedding correction. The Arenig palaeomagnetic results indicate high palaeolatitudes, but separation from northern Gondwana. This is in basic agreement with data from elsewhere in the Armorican Terrane Assemblage, all of which suggest high southerly palaeolatitudes in the Early Ordovician. The geochemical signatures of these rocks indicate emplacement in an extensional environment. These new data, therefore, are interpreted as marking the onset of rifting of Saxothuringia from the north African margin of Gondwana, and the start of the relative northward migration of the Saxothuringian Terrane. Although the Late Ordovician palaeomagnetic results presented here are only poorly constrained, they suggest an intermediate palaeolatitude for Saxothuringia in Ashgillian times, in good agreement with Late Ordovician palaeomagnetic data from the Barrandian.  相似文献   

20.
J.D.A. Piper   《Tectonophysics》2007,432(1-4):133-157
The Southern Uplands terrane is an Ordovician–Silurian back-arc/foreland basin emplaced at the northern margin of the Iapetus Ocean and intruded by granite complexes including Loch Doon (408.3 ± 1.5 Ma) during Early Devonian times. Protracted cooling of this 130 km3 intrusion recorded magnetic remanence comprising a predominant (‘A’) magnetisation linked to initial cooling with dual polarity and mean direction D / I = 237 / 64° (α95 = 4°, palaeopole at 316°E, 21°N). Subsidiary magnetisations include Mesozoic remanence correlating with extensional tectonism in the adjoining Irish Sea Basin (‘B’, D / I = 234/− 59°) and minority populations (‘C’, D / I = 106/− 2° and ‘D’, D / I = 199/1°) recording emplacement of younger ( 395 Ma) granites in adjoining terranes and the Variscan orogenic event. The ‘A’ directions have an arcuate distribution identifying anticlockwise rotation during cooling. A comparable rotation is identified in the Orthotectonic Caledonides to the north and the Paratectonic Caledonides to the south following closure of Iapetus. Continental motion from midsoutherly latitudes ( 40°S) at 408 Ma to equatorial palaeolatitudes by  395 Ma is identified and implies minimum rates of continental movement between 430 and 390 Ma of 30–70 cm/year, more than double maximum rates induced by plate forces and interpreted as a signature of true polar wander. Silurian–Devonian palaeomagnetic data from the British–Scandinavian Caledonides define a 430–385 Ma closed loop comparable to the distributed contemporaneous palaeomagnetic poles from Gondwana. They reconcile pre-430 Ma and post-380 Ma APW from this supercontinent and show that Laurentia–Baltica–Avalonia lay to the west of South America with a relict Rheic Ocean opening to the north which closed to produce Variscan orogeny by a combination of pivotal closure and right lateral transpression.  相似文献   

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