Age Of Remanence Research Articles

Constraining the Intracontinental Tectonics of the SW Central Asian Orogenic Belt by the Early Permian Paleomagnetic Pole for the Turfan‐Hami Block

AbstractIn order to better understand the kinematics and geodynamics of the late Paleozoic to early Mesozoic transition from accretionary to intracontinental tectonics in the southwest Central Asian Orogenic Belt, we performed a paleomagnetic study on Lower Permian strata in the Turpan‐Hami Basin, NW China. Magnetite and hematite are shown to be the principal magnetic remanence carriers. A typical sedimentary fabric that has not been modified by postdepositional deformation is recognized via magnetic fabric study. Stable characteristic remanent magnetizations, all of reverse magnetic polarity, are revealed by step demagnetizations. Remanence age is determined to be of Early Permian affinity, based on a fold test, remanence behavior of minerals, and geochronologic data. With inclination shallowing corrections, the first Early Permian paleomagnetic pole for the Turpan‐Hami Block is acquired as λ = 75.7°N, φ = 276.3°E, A95 = 5.7°, and N = 10 sites. Comparisons of this new pole with published ones from neighboring cratonic blocks provide kinematic constraints on the tectonic transition and subsequent intracontinental tectonics of this region. The Turpan‐Hami and Yili blocks have kept their relative positions since the Early Permian. Significant latitudinal movement of 14.7° ± 7.2° occurred between the Turpan‐Hami and South Junggar blocks and could be mainly accommodated by late Paleozoic crustal shortening of at least ~530 km across an intra‐arc basin in the Bogda belt. The Turpan‐Hami Block has experienced large‐magnitude relative rotations of −41.1° ± 7.1° and 21.2° ± 9.4° with respect to Tarim and South Junggar since the Early Permian, respectively, corresponding to dominant intracontinental tectonics characterized by large‐magnitude strike‐slip displacements along megashear zones.

New paleomagnetic results for Ordovician and Silurian rocks of the Tarim Block, Northwest China and their paleogeographic implications

The kinematics of the Tarim Block is poorly understood prior to the late Paleozoic and this study aims to clarify motions during Ordovician and Silurian times. We report the paleomagnetic record from 12 sites in Ordovician marine sediments and 21 sites in Silurian fine-grained sandstones and mudstones from the Aksu and Kalpin areas of northwestern Tarim. Together with our previously reported preliminary results, the Ordovician sediments isolate a sporadically-preserved pre-Cenozoic characteristic remanence with dual polarity. Although a positive fold test can only define the remanence as pre-Cenozoic, the corresponding paleomagnetic pole (33.7°S, 185.0°E, dp/dm = 2.7°/4.0°) is removed from post-Silurian paleopoles from Tarim and combined with evidence from magnetic fabrics indicating a quasi-primary origin. A characteristic remanence isolated at 17 sites in Silurian sediments from the Aksu and Kalpin areas includes 7 sites with suspected contamination from regional Permian igneous activity. The 10 remaining sites, all from Kalpin, yield a positive fold test and a paleopole at 19.1°N, 172.9°E (A95 = 5.5°) significantly removed from younger paleopoles from Tarim indicating a Late Llandovery to Wenlock remanence age. The new Mid-Late Ordovician paleopole places the Tarim Block at ~32°S, probably in the vicinity of the Arabian margin of Gondwana. Rapid APW between ~460 and ~433 Ma records large differential dextral motions of Tarim together with other continental blocks along the Paleo-Asian Ocean margin which brought the China continental blocks into proximity with Australia by mid-Silurian times in low (~9°N) northerly paleolatitudes.

First Early Permian Paleomagnetic Pole for the Yili Block and its Implications for Late Paleozoic Postorogenic Kinematic Evolution of the SW Central Asian Orogenic Belt

AbstractWe conducted a paleomagnetic study on the Early Permian volcanic and sedimentary rocks, and the Neoproterozoic mafic dikes in the Yili Block, NW China. Magnetite and hematite were proven to be the principal magnetic remanence carriers. Demagnetizations revealed stable characteristic remanence magnetizations with a sole reversed magnetic polarity. The magnetic remanence of only the Early Permian strata turned out to be primary based on positive fold tests; meanwhile, the magnetic remanence age of the mafic dikes is ambiguous. Accordingly, the first Early Permian paleomagnetic pole for the Yili Block is calculated at λ = 81.5°N, φ = 256.5°E, N = 11, and A95 = 10.9°. Comparisons of this new pole with published ones from the Yili, Tarim, and South Junggar blocks provide new quantitative constraints on late Paleozoic kinematic evolution of the SW Central Asian Orogenic Belt: (1) Between the Yili and Tarim blocks, significant relative movement took place along major strike‐slip faults during the Late Carboniferous to Early Permian (580 ± 290 km) and the Early to Late Permian (585 ± 340 km), and the displacement rate increased from the Late Carboniferous to Early Permian (~19.3 ± 9.7 mm/yr) to the Early to Late Permian (~29.3 ± 17.0 mm/yr); (2) a significant relative rotation of 28.3° ± 18.3° in the Late Permian, and a lateral displacement of 630 ± 295 km after the Late Permian occurred between the Yili and South Junggar blocks. The significant strike‐slip movements played an important role in the formation of the Central Asian Orogenic Belt and should be considered with great attention in tectonic and paleogeographic reconstructions.

Paleointensity during periods of rapid reversal: A case study from the Middle Jurassic Shamrock batholith, western Nevada

Paleointensity is one of the least deter­ mined parameters in geomagnetism, yet this information has the potential to address many fundamental geophysical problems that bear on the evolution of the Earth’s core and lower mantle. We test two important hy­ potheses that affect our understanding of how paleointensity has changed in Earth’s past: (1) the Mesozoic Dipole Low (MDL) hypoth­ esis and (2) the inverse relationship between geomagnetic polarity reversal rate and paleo­ intensity. We report paleointensity results determined using both the Coe modified Thellier method and the IZZI protocol on the medium­grained hornblende­biotite granite of the Middle Jurassic Shamrock batholith in the southern Singatse Range, western Nevada (USA). Previous zircon U­Pb dating gives an age of batholith emplacement at ca. 165.8 ± 0.4 Ma. This age estimate is coincident with a period of unusually high reversal frequency (~10 reversals/Ma) in the Middle Jurassic. Remanence in the Shamrock batholith is held primarily by exsolved submicron magnetite inclusions in plagioclase. The labora tory un­ blocking temperature range for the well­de­ fined thermal remanent magnetization is nar ­ row, between ~540 and 575 °C, and median destructive fields are between 40 and 50 mT, thus we infer that the age of rema nence ac­ quisition is approximated by the U­Pb age estimate for pluton emplacement. Two sets of quality criteria were used to assess the paleo­ intensity results. The first set yielded an 87% success rate while the second set using more stringent criteria resulted in only a 20% suc­ cess rate. Mean paleointensity values for the Shamrock batholith using the “loose” and “strict” quality criteria are 34.0 ± 6.4 µT (N = 13) and 33.6 ± 9.6 µT (N = 3) respectively, with errors reported at one standard deviation. After correction for both remanence anisot­ ropy and cooling rate, the mean values that pass “loose” and “strict” criteria are respec­ tively 17.9 ± 2.7 µT and 17.5 ± 2.6 µT. The uncorrected and corrected paleointensity values for the “strict” estimates yield respec­ tive mean virtual dipole moments (VDMs) of 62.7 ± 18 ZAm 2 (Z = 10 21 ) and 32.7 ± 4.9 ZAm 2 . The uncorrected value is within error of the VDMs reported for plagioclase grains isolated from ca. 160 Ma basalts from Ocean Drilling Program Site 801, however these values have an absolute difference ranging from ~6 to 20 ZAm 2 , while the corrected VDM value is more consistent with the esti­ mated average Jurassic field strength of ~29 ZAm 2 . Using a recent estimate for the long­ term stable field strength of ~42 ZAm 2 , our corrected values add further credence to both the MDL and the inverse relationship between geomagnetic polarity reversal rate and dipole moment hypotheses.

Rock magnetism of tiny exsolved magnetite in plagioclase from a Paleoarchean granitoid in the Pilbara craton

AbstractGranitoids are widespread in Precambrian terranes as well as the Phanerozoic orogenic belts, but they have garnered little attention in paleomagnetic studies, because granitoids often contain abundant coarse‐grained, magnetically unstable oxides. In this study, the first example of tiny, needle‐shaped, exsolved oxides in plagioclase in a Paleoarchean granitoid is reported. The magnetic properties of single plagioclase crystals with the exsolved oxide inclusions have been studied to determine their paleomagnetic recording fidelity. Demagnetization experiments and hysteresis parameters indicate that the oxide inclusions are near stoichiometric magnetite and magnetically very stable. First‐order reversal curve (FORC) diagrams reveal negligible magnetostatic interactions. Minimal interactions are also reflected by very efficient acquisition of anhysteretic remanent magnetization. Single plagioclase crystals exhibit strong magnetic remanence anisotropies, which require corrections to their paleodirectional and paleointensity data. Nonetheless, quantitative consideration of anisotropy tensors of the single plagioclase crystals indicates that the bias can be mitigated by properly averaging data from a few tens of single crystals. From the nonlinear thermoremanence acquisition of the plagioclase crystals, we estimate that the plagioclase crystals can reconstruct paleointensity up to 50 μT. Local metamorphic condition suggests that those magnetite may carry remanence of ∼3.2 to 3.3 Ga. We suggest that exsolved magnetite in granitoids is potentially a suitable target for the study of the early history of the geomagnetic field, and prompt detailed microscopic investigations as well as paleomagnetic tests to constrain the age of remanence.

Age constraints on the India–Asia collision derived from secondary remanences of Tethyan Himalayan sediments from the Tingri area

We report paleomagnetic results from the Zhepure Shan Formation and the Youxia Formation of the Tingri section (28°42′N/86°43′E) and the Longjiang section (28°27′N/86°40′E) in southern Tibet. A lower coercive component demagnetized below 30mT and a higher coercive component demagnetized between 30mT and 100mT were separated. Rockmagnetic analyses reveal magnetite as the remanence carrier. The differences in coercivity are probably due to a wider grain size spectrum and therefore different domain states. Remanence directions of the low-coercive component are highly scattered and were not considered for further analysis. Directions of the high coercive component of Members A–C of the Zhepure Shan Fm are fairly well grouped within sites (k>8 for most sites) whereas those of Member D and the overlying Youxia Fm are highly dispersed. Applied fold tests indicate a postfolding remanence for Members A–C. The latest possible time for remanence acquisition of Members A–C is constrained to a mean of 48Ma by comparing the determined paleolatitudes to the curve of expected paleolatitudes versus age, calculated from the APWP of India. Deformation of the rocks is likely related to India–Asia collision and therefore the remanence age yields a constraint on the latest possible age of the India–Asia collision. In consideration of an extended pre-collisional northern Indian margin (Greater India) the determined values of 48Ma (mean) and 37Ma (youngest possible age within 95% confidence limits) underestimate the age of the continental collision.

Paleomagnetism and magnetic mineralogy of Grenville metamorphic and igneous rocks, Adirondack Highlands, USA

Paleomagnetic samples were collected from an east–west traverse across the Highlands region of the Adirondack Mountains, northern New York, which forms part of the southern most exposure of the Grenville Province of North America. Granulite facies metamorphism (T>650°C) at ∼1050Ma completely reset pre-existing magnetic directions in sampled microcline gneisses and metamorphosed anorthosites. Fourteen sites of microcline gneiss yield a mean direction of I=−62.8°, D=289.2° with α95=7.6° and a corresponding pole at 18.4°S and 151.1°E. Metamorphic anorthosites and associated rocks (N=14) show a direction of I=−67.3°, D=283.9° with α95=7.7° and a corresponding pole at 25.1°S and 149.0°E. Post-metamorphic fayalite granites possess a statistically different direction of I=−75.8° and D=297.0° (α95=3.9°, N=8 sites) and a pole of 28.4°S and 132.7°E. Both normal and reverse polarities are recorded in all units, with reverse sites occurring on the eastern and western ends of the traverse, and normal polarities restricted to the central part of the massif. The remanence is carried by ilmeno-hematite in the microcline gneisses. In the metamorphosed anorthosites, and the unmetamorphosed fayalite granites, hemo-ilmenite and magnetite occur, though magnetite is the predominant oxide. Using cooling curves established for the Highlands, and blocking temperatures determined for ilmeno-hematite, hemo-ilmenite and magnetite, the age of remanence is determined to be ∼990Ma for the magnetite-bearing Wanakena granite, ∼970Ma for the metamorphosed anorthosite and related rocks, and ∼960Ma for the ilmeno-hematite rich microcline gneiss. The pole data from the Adirondacks, as well as selected studies from other areas of the Grenville Province on units with similar mineralogy and some age control, helps define the southerly part of the Grenville loop of the apparent polar wander path. The three units from the Adirondacks indicate counter-clockwise motion of the APWP between 990 and 960Ma.

Pyrrhotite remagnetizations in the Himalaya: a review

Abstract This paper reviews results on the nature and thermo-tectonic interpretation of widespread pyrrhotite remagnetizations in the Himalaya. Throughout the last two decades a large dataset has been acquired, in particular from low-grade metamorphic rocks of the Tethyan Sedimentary Series (TSS). The nature of this magnetization is a thermoremanence when the peak metamorphic temperature T max exceeded the Curie temperature ( T c c. 325 °C) of pyrrhotite; in this case the remanence age can be related to last metamorphic cooling. For T max < T c , the remanence can be of chemical, thermochemical or thermoremanent origin. Cooling ages show a systematic trend of c. 50–20 Ma from the western to the eastern Himalaya. The pyrrhotite remagnetizations post-date main Himalayan folding and record late orogenic long-wavelength rotations and tiltings around vertical and horizontal axes. Remanence directions in the western Himalaya are well matching with large-scale deformations of rotational shortening and oroclinal bending, while in the central and eastern Himalaya they are predominantly controlled by meso-scale effects due to crustal doming. Stable pyrrhotite remanences are especially typical for low-grade marly limestones in the TSS, but were also found in medium-grade rocks of the Lesser Himalaya, highly metamorphic rocks of the Higher Himalayan Crystalline and diorite dykes intruding into the TSS.

Primary Carboniferous and Permian paleomagnetic results from the Yili Block (NW China) and their implications on the geodynamic evolution of Chinese Tianshan Belt

In order to better understand the tectonic role of the Yili Block on the Paleozoic evolution of the Chinese Tianshan Belt, we performed a primary paleomagnetic study on Carboniferous and Permian rocks from different areas in the Yili Block, NW of China. More than 320 sedimentary and volcanic samples were collected from 39 sites. Except for the Ordovician samples showing a weak and unstable magnetic remanence, the majority of this collection presents characteristic remanent magnetization carried by magnetite and hematite. In the study area, though positive fold test has been observed on the Early Carboniferous rocks, a general re-magnetization of these rocks has been identified and attributed to the Late Carboniferous magmatism. Moreover, all Early and Late Carboniferous samples from the interior of the Yili Block yield stable and coherent magnetic directions with exhaustively reverse magnetic polarity. The Late Carboniferous (C 2) is considered as the magnetic remanence age since these rocks are covered or intruded by synchronous magmatic rocks of the Yili arc, which lasted until to ∼ 310 Ma. The C 2 paleomagnetic pole is therefore calculated at 68.6°N, 290.6°E with A 95 = 6.4° and n = 15. The Late Carboniferous rocks located close to a deformation zone present a consistent magnetic inclination but significant different declination with respect to other areas and are suspected to have probably experienced a local rotation. Although no fold test can be performed due to the monoclinal bedding, stable magnetic components are isolated from Late Permian (P 2) red beds in the interior of the Yili Block with also a solo reverse magnetic polarity, the P 2 paleomagnetic pole of the Yili Block has been, therefore, calculated from the characteristic remanent magnetization: 79.7°N, 172.0°E with A 95 = 11.3° and n = 5. Keeping important uncertainties in mind, comparisons of the C 2 and P 2 paleomagnetic poles of the Yili Block with available coeval poles of Junggar, Tarim and Siberia indicate (1) no significant relative motion between the Yili and Junggar blocks since the Late Carboniferous, (2) no significant or weak latitudinal relative motion occurred since the Late Carboniferous among these blocks, but (3) the 46.2° ± 15.1° and the 31.6° ± 15.1° counterclockwise rotations of the Yili–Junggar blocks with respect to Tarim and Siberia took place during C 2 to P 2. These rotations are accommodated by the Permian dextral strike–slip faults along the northern and southern sides of Tianshan Belt and sinistral strike–slip faulting along the Erqishi Fault of Altay Belt, resulting in about 1000 km and 600 km lateral displacements in the Tianshan and Altay belts, respectively.

New paleomagnetic, rock magnetic, and petrographic results from the Valle Group, Baja California, Mexico: Exploring the causes of anomalously shallow paleomagnetic inclinations

A detailed paleomagnetic, rock magnetic, and sedimentologic study was conducted on Cenomanian‐age strata of the Valle Group at El Campito, Baja California, to examine the burial compaction effects on the paleomagnetic inclination of marine sedimentary rocks. Alternating field and thermal demagnetization isolated a characteristic remanence at 16 of 18 sites with a mean of Dec = 342.6°, Inc = 19.1°, and α95 = 3.4°. The inclination is 38.7° shallower than that predicted by the reference paleopole for North America. Laboratory experiments indicate that compaction could account for only 7.5° of the inclination shallowing. Fold tests at different scales yield both positive and negative results, making a tight constraint on the remanence's age impossible. A primary postdepositional remanence is supported by bedding‐parallel foliations in the anisotropy of anhysteretic remanence fabric. Although primary anomalously shallow inclinations could indicate significant southerly, then northerly, paleolatitudinal offset, a more likely scenario is a late Cenozoic low‐temperature remagnetization, which is suggested by alteration along the edges of some detrital silicate grains and a bimodal magnetic grain size distribution revealed by isothermal remanence data. The synthesis of the data indicates that the Vizcaino terrane was contiguous with the Cordillera margin during remagnetization and underwent a subsequent 13.5 ± 5.4° clockwise rotation. We tentatively interpret that the remagnetization resulted from burial and/or Miocene volcanism, which caused an acquisition of thermochemical remanent magnetization (TCRM). This study reveals a complicated remanence that resists easy interpretation and suggests caution in studies of similar rocks.

Record of deformation by secondary magnetic remanences and magnetic anisotropy in the Nar/Phu valley (central Himalaya)

Secondary magnetic remanences residing in pyrrhotite and anisotropy of magnetic susceptibility (AMS) were studied in low-grade metamorphic carbonates of the Tethyan Himalaya in Nar/Phu valley (central Nepal) and used for interpretation of tectonic deformations. The characteristic remanence (ChRM) is likely of thermomagnetic origin related to post-peak metamorphic cooling occurring after the Eohimalayan phase (35–32 Ma). The ChRM postdates small-scale folding (main Himalayan folding F1 and F2) as shown by a negative fold test of site mean directions at 99% confidence level, and has been probably acquired between 32 and 25 Ma. Late-orogenic long-wavelength folding associated with the Chako antiform (CA) is recorded by the spatial dispersion of ChRM directions and the distribution of the main axes of the AMS tensor. The mean tilting of the ChRM direction since remanence acquisition (≈20–30°) approximately coincides with the tilting of the CA (31°) at the study area indicating that the pyrrhotite remanence predates the CA (CA formed at <18 Ma according to preliminary U/Pb dating). However, comparison of tilt angles of remanence directions and AMS tensor axes suggests that remanence acquisition was not completed before the onset of the CA formation. This could imply a younger age (Early Miocene or even younger) of the ChRM. Using the distribution of remanence directions along a small-circle as well as the distribution of AMS tensor axes, a clockwise mean rotation of 16° is obtained for a remanence age of ≈30 Ma. An Early Miocene remanence age would not change this result substantially. Compilation of rotations in the Tethyan Himalaya deduced from secondary pyrrhotite remanences reveals an increasing clockwise rotation from the Hidden valley in the W to the Shiar valley in the E (≈150 km distance), incompatible with an oroclinal bending model.

Palaeomagnetic and rock magnetic study of a deeply exposed continental section in the Charnockite Belt of southern India: implications to crustal magnetisation and palaeoproterozoic continental nucleii

Abstract This study has investigated palaeomagnetism and magnetic properties of a tract of continental basement in south India metamorphosed to granulite facies close to the Archaean–Proterozoic boundary (∼2600 Ma) at former depths of 20–35 km. The metamorphism is characterised by extensive development of charnockite produced by flushing of CO2-rich fluids through older crust. Rock magnetic studies (thermomagnetic, high and low temperature susceptibility, hysteresis and IRM) show that very low-Ti metamorphic magnetite is the ubiquitous ferromagnet in all rock types. It has predominantly multidomain characteristics but is of high magnetic stability probably due to exsolution precipitation and microfracture filling producing anisotropic grain shapes. Magnetic susceptibility shows little increase with temperature falling to zero at ∼550 °C implying that induced magnetisation drops to a weak paramagnetic effect near this isotherm. Susceptibilities and intensities of magnetisation increase from amphibolite to granulite facies and by an order of magnitude within charnockites in the lower dehydrated crust. Induced magnetisations are able to explain ∼50% of observed magnetic anomalies of deep origin. Intensities of magnetisation and Qn values are high, especially in charnockite where metasomatism has resulted in new magnetite formation, and it is concluded this facies is able to explain observed magnetic anomalies provided that viscous remnant magnetisation is enhanced at depth to contribute the remaining magnetisation. The magnitude of anisotropy of magnetic susceptibility (AMS) is also increased by charnockite formation, as are degrees of lineation and foliation. However, directions of AMS in these anhydrous rocks are essentially random and reflect the static nature of the charnockite metasomatism. No appreciable Phanerozoic overprinting is evident in this terrane and the dominant feature of the palaeomagnetic record is a west to north shallow swathe of component directions with a minor antiparallel sequence between means summarised by A1 (D/I=280/−8°), A2 (D/I=319/−11°) and A3 (D/I=1/−10°). This swathe is interpreted as an uplift-related cooling acquisition following charnockite formation at ∼2600 Ma and a remanence age of ∼2600–2400 Ma is implied by the absence of most of the swathe in early Palaeoproterozoic and younger dyke swarms cutting the region. A second group of component directions (‘B’, mean D/I=238/−87°) is similar to magnetisations found in dykes linked to an age of ∼2370 Ma and interpreted as somewhat younger than the ‘A’ swathe. We argue that the entire magnetic record was acquired before final basement uplift prior to 2000–1800 Ma. Collective late Archaean–early Proterozoic palaeopoles from India define a low to high latitude movement of the shield at ∼2600–2300 Ma which is replicated in contemporaneous results from the other ancient Gondwana nucleii of southern Africa and Australia. The cratonic nucleii of these three shields have the oldest laterally-extensive supracrustal covers and comprise the core of the protocontinent of ‘Ur’. The 2900–2200 Ma palaeopoles are consistent with close proximities and subsequent movements between them were apparently limited because continental proximities, but not configurations, were similar within the (Palaeozoic) supercontinent of Gondwana.

Paleomagnetism of the Middle Proterozoic Laramie anorthosite complex and Sherman Granite, southern Laramie Range, Wyoming and Colorado

We present the results of a combined paleomagnetic and 40Ar/39Ar geochronologic investigation of the Middle Proterozoic Laramie anorthosite complex and Sherman Granite in the southern Laramie Range of Wyoming and Colorado. Anorthosites and monzosyenites of the Laramie anorthosite complex yield a well‐defined characteristic magnetization of northeast declination (D) and moderate negative inclination (I), although antipodal normal and reverse polarity magnetizations are present at three sites. A grand mean direction from 29 of 35 sites in the complex is D = 44.6°, I = −48.7° (k = 77.4, α95 = 3.1°). Alternating field (AF) and thermal demagnetization behavior and rock magnetic experiments indicate that magnetization is carried by low‐Ti titanomagnetite of single or pseudo‐single domain character that occurs as elongate to rod‐shaped inclusions in plagioclase and potassium feldspar. The Sherman Granite contains a dual polarity magnetization that is less well defined than that of the Laramie anorthosite complex but similar in declination and inclination (D = 53.1°, I= −48.1°, k = 46.5, α95 = 7.6°, n = 8/14 sites); rock magnetic data indicate the primary carrier of remanence in Sherman Granite is magnetite. The 40Ar/39Ar geochronologic data from Sherman Granite hornblende, biotite, and microcline indicate that subsolidus cooling was moderate to relatively rapid through the range of temperatures over which magnetization was blocked and that the age of remanence is about 1415 Ma. Microcline data indicate that the Laramie anorthosite complex and Sherman Granite have probably not been thermally remagnetized. Paleomagnetic poles from the Laramie anorthosite complex and Sherman Granite are indistinguishable at the 95% confidence level, and individual virtual geomagnetic poles (VGPs) from both units are combined to provide a mean pole at 215.0°E, 6.7°S (K = 46.9, A95 = 3.5°, N = 37 VGPs). The location of this pole is similar to paleomagnetic poles derived from 1480 Ma to 1450 Ma intrusions elsewhere in North America, but it plots significantly north of those from Middle Proterozoic sedimentary strata of the Belt Supergroup and Sibley Group. In addition to the characteristic magnetization, samples from some sites in Sherman Granite contain a remanence of southeast declination and shallow negative inclination (D = 154.9°, I = −16.0°, k = 90.6, α95 = 9.7°, n = 4 sites). This secondary magnetization was probably acquired during late Paleozoic time.

Proving the age of a paleomagnetic pole: The case of the Ntonya Ring Structure, Malawi

Geological application of paleomagnetic data generally relies on equating radiometric or biostratigraphic age with that of the characteristic remanence. Yet the standard of proof of paleomagnetic age is often unsatisfactory and is extremely difficult to achieve where contact tests or conglomerate tests are unavailable. In this study of the Ntonya igneous ring complex the thermoremanent (TRM) origin of one component of natural remanent magnetization (NRM) is confirmed by paleointensity determination and its carrier is identified mineralogically, and the age of this TRM is estimated by extrapolation from 40Ar/39Ar mineral ages. The characteristic remanence in the Ntonya Ring Structure, which was determined by Briden (1968) as D = 311° I = +43° α95 = 1.9°, pole 28°N 15°W, is shown to be a TRM in single‐domain/pseudo‐single‐domain magnetite, acquired in a field of approximately half the present geomagnetic field strength. The 40Ar/39Ar mineral ages (biotites 471.5 ± 7 Ma; hornblendes 510 ± 7 Ma) and the high precision of the paleomagnetic directions combine in a model of steady cooling (∼ 4.6° m.y. −1) that leads us to the conclusion that the thermoremanence was acquired over approximately 13 m.y., 522 ± 13 m.y. ago. In passing it is shown that the isothermal (IRM) component that dominates the remanence is carried by multidomain magnetite. The study shows how remanence age may be determined with precision in the absence of geological constraints. It makes the Ntonya pole a key point in the African (and Gondwanan) apparent polar wander path, correlating with Middle Cambrian glacial deposits in the Sahara at about 80°S paleolatitude.

Palaeomagnetism and 40Ar/ 39Ar dating of the Trégor dolerites (Armorican Massif, France)

The Trégor doleritic dike swarm intrudes the Cadomian North Trégor Batholith (Armorican Massif, France). Alternating field (A.F.) and thermal demagnetization performed on 158 samples from 18 dolerite dikes and 37 samples from 4 baked contact sites reveal a multi-component magnetization and a complex magnetic mineralogy which was thoroughly investigated with thermomagnetic experiments and polished thin-section examinations. Three magnetization components were recognized, in addition to a low-temperature component with a viscous origin. The most frequently observed component, named B ( D = 214°, I = 10°), was isolated at 12 sites, independent upon their magnetic mineralogy, and appears to be a remagnetization (negative baked contact test). A Carboniferous age of acquisition is inferred from comparison of the B-component palaeopole (29° S, 318°E) with an Armorican APWP. At some sites, after demagnetization of the B-component, higher-temperature and coercivity components were isolated. Their directions fall into two groups, named A and C. Positive baked contact tests were obtained for both. The C-component ( D = 174°, I = 50°), whose magnetic carrier is magnetite, yields a palaeopole (11° S, 2° E) which is similar to some Ordovician palaeopoles from the British Isles, south of the lapetus suture, or Eocambrian palaeopoles from the Trégor area. The A-component ( D = 191°, I = −33°), whose magnetic carrier is magnetite, partly transformed to leucoxene, yields a palaeopole (58° S, 336° E) and a palaeolatitude (17.9° N) not previously observed for the Armorican Massif in Palaeozoic time. Comparison between this palaeolatitude and those for northern Spain from Ordovician to Carboniferous time suggests a late Silurian remanence age. This interpretation supports the model of apparent polar wander for Gondwana between Ordovician and Devonian time proposed by Van der Voo, and Bachtadse and Briden. 40Ar/ 39Ar dating of mineral separates from the dikes do not allow us to place constraints on the acquisition age of the A- and C-components.

Ordovician magnetostratigraphy: Llanvirn-Caradoc limestones of the Baltic platform

SUMMARY Palaeomagnetic sampling has been performed covering 43 stratigraphic levels within the Baltoscandian Ordovician carbonates. After removing a ubiquitous PermoCarboniferous (287 f 14 Ma) remagnetization between 200 and 500 C, a LlanvirnCaradoc reversal stratigraphy is delineated by components with maximum unblocking temperatures up to 550-580C. Three reversed (SE, down) and three normal (NW, up) antipodal polarity intervals have been recognized. A primary/early diagenetic remanence age is therefore inferred for the stratigraphically linked polarity chrons. Primary magnetizations are resident in detrital/biogenic or early diagenetically formed single- and pseudo-single domain magnetite phases and subordinate early diagenetic.pigmentary haematite. The recognition of a primary remanence within these well-dated Ordovician carbonates has the following important tectonic and magnetostratigraphic consequences. (1) Accurate time-calibration of the Baltic APW path implies that rapid counterclockwise rotation took place in late Tremadoc and Llandeilo times. The ArenigLlanvirn epochs are characterized by a 'still stand'. Baltica occupied intermediate to high Southerly latitudes during the early Ordovician (Tremadoc-Llanvirn). Systematic northward drift is recognized in post-Llanvirn times. (2) A time-calibrated Ordovician reversal stratigraphy is proposed. Presently available data suggest the geomagnetic field was predominantly reversely polarized during Tremadoc and Arenig times. Two normal polarity zones of short duration are identified within mid-Llanvirn and mid-Llandeilo strata. Discontinuities within the succession may mask other short-period events. Late Llandeilo to mid-Caradoc times were then characterized by a normal polarity field.

Comment on “Palaeomagnetic results from volcanic rocks of the Shelve Inlier, Wales: evidence for a wide Late Ordovician Iapetus Ocean in Britain” by C. McCabe and J.E.T. Channell

In a recent paper, McCabe and Channell [1] report a Late Ordovician palaeomagnetic pole from the Shelve Inlier, Wales, and argue for a Iapetus Ocean of 30 degrees latitudinal width between Laurentia and Southern Britain in Late Ordovician times. They further suggest that Southern Britain lay in high latitudes adjacent to the northern margin of Gondwana at this time. Unlike McCabe and Channell we contend a primary Llanvirn (Mid-Ordovician) remanence age for the Shelve volcanics. Additionally, several important points regarding the palaeomagnetism of Southern Britain may have been overlooked by McCabe and Channell. In particular, re-examination of a previous palaeomagnetic study of the Shelve Inlier [2] reveals a dual polarity remanence, which passes a fold test at 95% confidence, and indicates temperature southerly palaeolatitudes of 32S _+ 8.

An Upper Carboniferous-Lower Permian (280 Ma) palaeomagnetic pole from the western margin of the Bohemian Massif

In the area of the drill site of the German Continental Deep Drilling Project KTB on the western margin of the Bohemian Massif 15 sites of Uppermost Carboniferous to Lowermost Permian rocks (quartz porphyries, granites and a siltstone, geographic mean coordinates: 12°E, 49.8°N) have been studied in order to obtain a reliable pole position for this time interval. The investigated rocks are syngenetic or only slightly younger than the youngest granites of the Fichtelgebirge for which a K/Ar and Rb/Sr age of 280 Ma has been determined by Besang et al. (1976). While four sites (quartz porphyries) showed too weak intensities of the natural remanent magnetization NRM, the results of two sites with otherwise good rock magnetic properties had to be discarded because of uncertain tectonic position in a shear zone. The remaining nine sites have all reversed polarity as expected for a remanence age within the reversed Kiaman interval and a mean pole position at 165.9°E, 42.3°N with N = 9, R = 8.8776, A95 = 6.4°. This is in good agreement with the mean Upper Carboniferous-Lower Permian European pole position of Stable Europe according to Irving & Irving (1982) and of the Bohemian Massif (Krs 1968). Rock magnetic investigations revealed that the ChRM of the siltstone is due to primary magnetite, while the mostly single-component characteristic remanent magnetization ChRM of the magmatic rocks is carried by primary magnetite and by primary haematite produced by syngenetic high-temperature oxidation of primary magnetite.

The earth's tectonosphere

The Trégor doleritic dike swarm intrudes the Cadomian North Trégor Batholith (Armorican Massif, France). Alternating field (A.F.) and thermal demagnetization performed on 158 samples from 18 dolerite dikes and 37 samples from 4 baked contact sites reveal a multi-component magnetization and a complex magnetic mineralogy which was thoroughly investigated with thermomagnetic experiments and polished thin-section examinations. Three magnetization components were recognized, in addition to a low-temperature component with a viscous origin. The most frequently observed component, named B (D = 214°, I = 10°), was isolated at 12 sites, independent upon their magnetic mineralogy, and appears to be a remagnetization (negative baked contact test). A Carboniferous age of acquisition is inferred from comparison of the B-component palaeopole (29° S, 318°E) with an Armorican APWP. At some sites, after demagnetization of the B-component, higher-temperature and coercivity components were isolated. Their directions fall into two groups, named A and C. Positive baked contact tests were obtained for both. The C-component (D = 174°, I = 50°), whose magnetic carrier is magnetite, yields a palaeopole (11° S, 2° E) which is similar to some Ordovician palaeopoles from the British Isles, south of the lapetus suture, or Eocambrian palaeopoles from the Trégor area. The A-component (D = 191°, I = −33°), whose magnetic carrier is magnetite, partly transformed to leucoxene, yields a palaeopole (58° S, 336° E) and a palaeolatitude (17.9° N) not previously observed for the Armorican Massif in Palaeozoic time. Comparison between this palaeolatitude and those for northern Spain from Ordovician to Carboniferous time suggests a late Silurian remanence age. This interpretation supports the model of apparent polar wander for Gondwana between Ordovician and Devonian time proposed by Van der Voo, and Bachtadse and Briden. 40Ar/39Ar dating of mineral separates from the dikes do not allow us to place constraints on the acquisition age of the A- and C-components.