Positive Contact Test Research Articles

Paleomagnetic study of 1765 Ma dyke swarm from the Singhbhum Craton: Implications to the paleogeography of India

We report the first key paleopole as a result of paleomagnetic study on a precisely dated 1765.3±1.0Ma WNW-ESE trending dyke swarm from Singhbhum Craton. This pole has been used in this study to propose the paleogeographic reconstruction of India with Baltica Craton and North China Craton. Incremental alternating field (AF) and thermal demagnetization, isolated high coercivity components with north to north-westerly declination and shallow negative inclination from 9 sampling sites which are representing different individual dykes. The primary origin of the ChRM is supported by the positive baked contact test. The WNW-ESE trending dykes yield a mean paleomagnetic direction with a declination=329.2° and an inclination=−22.8° (k=31.6; α95=9.3°). The positive bake contact test proves the primary nature of remanence. The pole position of Singhbhum Craton at 1765Ma is 45°N, 311°E (dp=5.2 and dm=9.9). Paleogeographic reconstruction at ca. 1770Ma, supported by geological, tectonic and metallogenic evidences indicate that the Baltica Craton and India linkage can be stable for at least ∼370Ma (∼1770–1400Ma). There is also reasonable evidence in support of India-North China Craton spatial proximity at ∼1770Ma.

Tectonic implications of the 1419 Ma Nova Guarita mafic intrusives paleomagnetic pole (Amazonian Craton) on the longevity of Nuna

The Nuna supercontinent was probably assembled in the Paleoproterozoic, but its paleogeography and the timing for its demise are still a matter of debate. A paleomagnetic and geochronological study carried out on the Mesoproterozoic Nova Guarita dyke swarm (northern Mato Grosso State, SW Amazonian Craton) provides additional constraints on the duration of this supercontinent. Paleomagnetic AF and thermal treatment revealed south/southwest (northeast) magnetic directions with downward (upward) inclinations for 19 analyzed sites. These directions are carried by PSD magnetite with high unblocking temperatures as indicated by additional magnetic tests, including thermomagnetic curves, hysteresis loops and the progressive acquisition of isothermal remanence in selected samples. A positive contact test with the host granite in one of the studied dykes further attests to the primary origin of the characteristic magnetic component. A mean site direction was calculated at Dm=220.5°, Im=45.9° (α95=6.5°, K=27.7), which yielded a paleomagnetic pole located at 245.9°E, 47.9°S (A95=7.0°). 40Ar/39Ar dating carried out on biotites from four analyzed dykes yielded well-defined plateau ages with a mean of 1418.5±3.5Ma. The Nova Guarita pole precludes a long-lived Nuna configuration in which Laurentia, Baltica, North China, and Amazonia formed a long and continuous block as previously proposed for the Paleoproterozoic. It is nevertheless fully compatible with a SAMBA (Amazonia-Baltica) link at Mesoproterozoic times.

A new 200 Ma paleomagnetic pole for Africa, and paleo-secular variation scatter from Central Atlantic Magmatic Province (CAMP) intrusives in Morocco (Ighrem and Foum Zguid dykes)

Available apparent polar wander (APW) paths for the 200 Ma configuration of Pangea, just prior to the opening of the Central Atlantic Ocean, differ as much as 10o in arc length. Here, we add new data from northwest Africa for this time, obtained from the northeasttrending Foum-Zguid and Ighrem dykes (ca. 200 Ma). These dykes form part of the northern domain of the Central Atlantic Magmatic Province (CAMP), and crosscut the Anti-Atlas Ranges in Morocco, and compositionally correspond to quartz-normative tholeiites intruded in continental lithosphere shortly before the opening of the Central Atlantic Ocean. The Foum-Zguid dyke has been intensively studied, whereas the Ighrem dyke has received less scientific focus. We sampled both dykes for paleomagnetic investigation along 100 km of each dyke (12 sites for Foum-Zguid and 11 for Ighrem, 188 samples included in the final analyses). Rock magnetic experiments indicate a mixture of multidomain and single-domain magnetite and/or low-Ti titanomagnetite particles as the principal remanence carriers. In both dykes, the primary nature of the characteristic remanent magnetization is supported by positive contact tests, related to Fe-metasomatism or baked overprints of the corresponding sedimentary country rocks. The directions of the characteristic magnetization exhibit exclusively normal polarity. Site-mean virtual geomagnetic poles are differently grouped in each dyke, suggesting distinct geomagnetic secular variation records. The Foum-Zguid paleomagnetic pole (N = 12, PLat = 67.9◦N, PLon = 247.9◦E, κ = 125, A95 = 3.9◦) plots close to that of Ighrem (N = 11, PLat = 78.4◦N, PLon = 238.2◦E, κ = 47, A95 = 6.7◦), confirming those mineralogical and geochemical evidences supporting that they represent dissimilar magmatic stages. Virtual geomagnetic poles dispersion from both dykes (S = 10.5◦13.0◦ 8.1◦ ) is in line with those obtained from recent studies of a CAMP-related dyke in Iberia and results from CAMP lavas in the Argana basin. These three new estimates of paleosecular variation at low latitudes around the Triassic–Jurassic boundary are concordant with a recently proposed dispersion curve for the Jurassic but suggest a slightly lower geomagnetic scatter than considered so far. After combining results from both dykes, the resulting paleomagnetic pole (PLat = 73.0◦N, PLon = 244.7◦E, N = 23, κ = 55, A95 = 4.1◦) is statistically compared with existing and coeval African paleopoles, and with global synthetic 200 Ma running mean poles in northwest Africa coordinates.

Paleomagnetic study of NeoArchean–Paleoproterozoic dykes in the Kaapvaal Craton

431 oriented samples were collected from 27 dolerite dykes at 17 sites, belonging to 2.95, 2.65, and 1.90 Ga swarms, that trend SE, E and NE, respectively from the Bushveld Igneous Complex into the eastern Kaapvaal Craton (ages determined by Olsson et al., 2010; Olsson in Söderlund et al., 2010). Samples were analyzed for paleomagnetism and also anisotropy of magnetic susceptibility (AMS). For the 2.95 Ga SE-trending dykes high temperature/coercivity ‘P’ component has unblocking temperatures up to 590 °C and coercivity 40–90 mT and demonstrate SSW declination and intermediate positive inclination. Based on positive contact and conglomerate tests we argue for a primary origin of this component. The paleopole (BAD), calculated from ‘ P’ component, does not correspond to any of the previously obtained Archean–Paleoproterozoic paleopoles for the Kaapvaal Craton, and represents a new key pole for 2.95 Ga. The high-coercivity ‘ H’ component for the 2.65 Ga-old E-trending dykes has a SSW declination and steep positive inclination. Paleomagnetic pole (RYK), recalculated from this component, is close to the paleopoles, obtained by Wingate (1998) and Strik et al. (2007) for 2.78 Ga Ventersdorp volcanics. The third group, NE-trending dykes of the 1.90 Ga Black Hill swarm demonstrate an ‘ M’ component with dual polarity high-coercivity component with SSE-declination and negative intermediate inclination. The paleopole (BHD), calculated from this component is close to the 1.87 Ga pole of the Kaapvaal Craton obtained by Hanson et al. (2004). Overprint directions include a very well developed thermo-chemical overprint (Dec = 329° Inc = −36°), which is believed to be associated with a ∼0.18 Ga regional ‘Karoo’ thermal event.

The Egersund dykes (SW Norway): a robust Early Ediacaran (Vendian) palaeomagnetic pole from Baltica

SUMMARY Palaeomagnetic data for Baltica in the Late Precambrian are highly ambiguous, and have therefore, given rise to different interpretations concerning the need to explain Varangerian glaciations through snowball Earth conditions. We present new palaeomagnetic data from the 616 ± 3 Ma (U-Pb) Egersund dykes in SW Norway, which yield a palaeolatitude of 53 ◦ +16 ◦ /−13 ◦ for the studied location. This would indicate that the Baltica plate spanned latitudes between 50 ◦ and 75 ◦ S in the Early Ediacaran. The pole position (31 ◦ N, 44 ◦ E, dp/dm = 15/17) confirms earlier studies, but the primary nature of the remanence is now supported by two positive contact tests. A new 40 Ar/ 39 Ar biotite age of 600 ± 10 Ma from one of the dykes suggests that remanence acquisition in the dykes took place between 600 and 616 Ma. The Egersund palaeomagnetic data demonstrate that Baltica was located at relatively high latitudes at the time of the Varangerian glaciations. Additional palaeomagnetic sites in the Rogaland Igneous Complex yield a pole position (46 ◦ S, 238 ◦ E, dp/dm = 17/19) that confirms previous studies. The age of this remanence has traditionally been quoted as c. 930 Ma based on U-Pb ages from the complex. However, previous 40 Ar/ 39 Ar hornblende ages of around 870 Ma are now supported by a new 40 Ar/ 39 Ar biotite age of 869 ± 14 Ma obtained from a noritic dyke, and we argue that this represents an uplift/cooling age which better represents the age of the remanence in SW Norway.

Paleomagnetic study of the Great Foum Zguid dyke (southern Morocco): A positive contact test related to metasomatic processes

[1] When a paleomagnetic pole is sought for in an igneous body, the host rocks should be subjected to a contact test to assure that the determined paleopole has the age of the intrusion. If the contact test is positive, it precludes the possibility that the measured magnetization is a later effect. Therefore, we investigated the variations of the remanent magnetization along cross-sections of rocks hosting the Foum Zguid dyke (southern Morocco) and the dyke itself. A positive contact test was obtained, but it is mainly related with Chemical/Crystalline Remanent Magnetization due to metasomatic processes in the host-rocks during magma intrusion and cooling, and not only with Thermo-Remanent Magnetization as ordinarily assumed in standard studies. Paleomagnetic data obtained within the dyke then reflect the Earth magnetic field during emplacement of this well-dated (196.9 ± 1.8 Ma) intrusion.

Palaeomagnetic results from the Palaeozoic basement of the southern Drummond Basin, central Queensland, Australia

SUMMARY Palaeomagnetic results from the Theresa Creek Volcanics and the Middle Devonian (∼380 Ma) Retreat Batholith, located in the southern Drummond Basin, central Queensland, Australia, reveal up to three components of magnetization. Component 1 is characteristically of reversed polarity with a mean palaeopole at 64.0 ◦ S, 79.7 ◦ E( N = 7, A95 = 11.5 ◦ ), possibly reflecting a remagnetization event associated with the Middle Carboniferous Alice Springs Orogeny, is found in both units. A shallow to very shallow roughly north‐south direction, Component 2, was isolated in seven sites, predominantly within the Retreat Batholith, D = 345.8 ◦ , I =− 6.7 ◦ (α 95 = 10.1 ◦ , k = 36.8). The corresponding palaeomagnetic pole lies at 66.0 ◦ S, 290.7 ◦ E( A95 = 9.1 ◦ ). This pole is consistent with the Middle Devonian segment of Australia’s apparent polar wander path (APWP) and is thought to reflect a primary magnetization related to the emplacement and cooling of the batholith. Component 3 is found exclusively within the Theresa Creek Volcanics and is characterized by east‐west directions with moderate to steep inclinations, D = 62.4 ◦ , I = 59.5 ◦ (α 95 = 10.5 ◦ , k = 25.0). A consistent set of site level virtual geomagnetic pole (VGPs) yield a palaeopole at 4.1 ◦ N, 188.9 ◦ E( N = 9, A95 = 14.0 ◦ ). The presumed Middle Devonian age for the volcanics is at odds with this direction, with respect to the Australian Early to Middle Palaeozoic APWP, suggesting the characteristic direction found in the volcanics reflects an Ordovician or possibly Early Silurian magnetization age. A pre-Middle Devonian magnetization age for the Theresa Creek Volcanics is supported by a positive contact test between the volcanics and the hornfelsing Retreat Batholith as both units are characterized by stable magnetizations whose palaeopoles are dissimilar from each other and younger segments of Australia’s APWP.

A Late Cretaceous paleomagnetic pole from Koto rhyolite, southwest Japan: Implications for eastern margin deformation of Asia

A paleomagnetic pole was obtained from Late Cretaceous rhyolitic ash flow tuffs and associated ring dikes, which are collectively called the Koto rhyolite in the eastern part of southwest Japan. Characteristic remanent magnetizations were isolated from 33 sites through thermal demagnetization. Normal and reversed polarity magnetizations were found and are mutually antipodal. Although the magnetization directions become more dispersed after tilt correction, the clearly defined polarity sequence and the positive contact test between ash flow tuffs and ring dikes ensure that the characteristic magnetizations of ash flow tuffs predate the intrusion of associated ring dikes. Since no regional tilting is expected, we regard the in situ magnetization directions to represent the Late Cretaceous (70 Ma) paleomagnetic field (D = 84.6°, I = 57.5°, α95 = 5.5°) in eastern southwest Japan. The paleomagnetic pole is not significantly different from the coeval poles of western southwest Japan. This result gives a further support to a microplate model for the Miocene clockwise rotation of southwest Japan. By canceling out the effect of the Miocene rotation, we were able to obtain a paleomagnetic pole of 70 Ma (219.7°E, 63.0°N, α95 = 12.2°) for southwest Japan when the Japan Arc was incorporated in the eastern margin of Asia. Paleomagnetic poles of Late Cretaceous age from Korea, south China coast, and restored southwest Japan are statistically indistinguishable from each other but significantly differ from that of the continental interior of east Asia. The observed rotation of about 20° probably results from shear deformation exerted by northeast‐trending strike‐slip faults along the eastern continental margin of Asia.

New 42 Ma cratonic North American paleomagnetic pole from the Yukon underscores another Cordilleran paleomagnetism-geology conundrum

Eocene posttectonic plutons of the Beaver River alkalic complex in southeastern Yukon intruded Devonian–Mississippian and Triassic sandstones in the Foothills of the Canadian Cordillera. A paleomagnetic collection of 27 sites from three separate plutons produced 326 specimens that were analyzed using alternating field and thermal step demagnetization methods. The A component characteristic remanent magnetization (ChRM) resides in magnetite with normal polarity in the 42.6 ± 0.8 Ma Beaver River pluton, reversed polarity in the 42.1 ± 0.7 Ma Larson Creek East pluton, and both polarities in the 41.3 ± 0.4 Ma Larson Creek West pluton, corresponding with magnetic polarity chrons 20n, 19r, and the boundary between chron 19r and 18n, respectively. The ChRMs of the plutons are indistinguishable (2σ) with a mean for the 42.0 ± 0.5 Ma complex of D = 158.8°, I = –73.1° (N = 21 sites, α95 = 3.0°, k = 116.8). A positive paleomagnetic contact test shows the A component to be primary, and the poorly isolated B component suggests the host rocks for Larson Creek West are Early to Middle Devonian. The paleopole for the Beaver River complex at 79.2°N, 145.8°E (N = 21, dp = 4.8°, dm = 5.4°; Q = 7) is concordant with interpolated 42 Ma reference poles for the North American craton. In contrast, paleopoles from the accreted Intermontane and eastern Coast Belt terranes record clockwise rotations of 24° ± 10° (Eocene) and 13° ± 5° (Oligocene–Pliocene), indicating that the allochthonous Intermontane terranes have been progressively driven ~240 ± 120 km eastwards up and over pericratonic and cratonic North American lower crust by Pacific plate subduction since the mid-Eocene.

The Early Carboniferous Magerøy dykes, northern Norway: palaeomagnetism and palaeogeography

Palaeomagnetic data from the 337 Ma Magerøy dykes (northern Norway) are of exceptionally high quality, and a positive contact test along with an existing regional result from the Silurian Honningsvåg Igneous Suite attests to a primary Early Carboniferous magnetic signature. The palaeomagnetic pole (S14.8°, E320.1°, dp/dm=4.4/8.6°) is the first Early Carboniferous pole from Baltica, and implies that northernmost Norway–Greenland, the Barents Sea and Svalbard were located at tropical to low northerly latitudes at this time. Northward drift during Carboniferous times (5–6 cm/yr) as demonstrated from palaeomagnetic data is also reflected in the sedimentary facies in the Barents Sea realm, that is, a change from tropical (Early Carboniferous) to subtropical (20–30° N) carbonates and evaporites in the Late Carboniferous. The Magerøy dykes are continental tholeiites which intruded into a set of NW–SE-trending normal faults parallel to the Trollfjorden–Komagelva Fault Zone and the Magerøysundet Fault immediately to the north and south of Magerøya, respectively. These, and many other NW–SE-trending faults (onshore and offshore), were active during Late Palaeozoic extension, and the dykes were probably contemporaneous with the earliest syn-rift sedimentation in the Barents Sea (for example, the Nordkapp Basin).

Deciphering shallow paleomagnetic inclinations: 2. Implications from Late Cretaceous strata overlapping the Insular/Intermontane Superterrane boundary in the southern Canadian Cordillera

Significant discrepancies exist between paleomagnetic and geologic estimates of the timing and magnitude of terrane displacement in the southern Canadian Cordillera. Lithostratigraphic, palynological, geochronologic, geochemical, and structural data, to which we add paleomagnetic data, demonstrate that Upper Cretaceous strata in Churn Creek are laterally equivalent facies of the Silverquick/Powell Creek (SPC) succession. These rocks therefore comprise a single overlap sequence linking the Insular and Intermontane Superterranes by Late Albian‐Cenomanian time (∼95 Ma), contradicting previous assertions based solely on paleomagnetic interpretation that major (∼2000 km) latitudinal displacement separated the two regions. The Churn Creek succession retains primary magnetic remanence, as demonstrated by positive fold, conglomerate and contact tests. The sediments do not hold the anisotropy signature of compaction‐shallowed inclinations. The mean inclination of all SPC sites, from both Mount Tatlow and Churn Creek, is 55.6° ± 2.3° (N = 36 sites), corresponding to a paleolatitude of 36.1° ± 2.4°. This reflects northward translation of the composite Insular/Intermontane Superterrane of 3050 ± 450 km between 85 and 50 Ma. The Silverquick/Powell Creek succession at Churn Creek unconformably overlies the northern end of the Spences Bridge Group, a late Early Cretaceous volcanic arc assemblage on the Intermontane Superterrane. Paleomagnetic data in the companion paper place it 1050 ± 450 km south of its current position between 100 and 105 Ma. Integration of these new data sets require that the Insular and Intermontane Superterranes formed a single, enormous crustal block that underwent complex, episodic south‐north translation between circa 105 and 50 Ma. The timing, rate, and vectors of translation are problematic in terms of geologic constraints along the continental margin.

A precisely dated Proterozoic palaeomagnetic pole from the North China craton, and its relevance to palaeocontinental reconstruction

A palaeomagnetic pole position, derived from a precisely dated primary remanence, with minimal uncertainties due to secular variation and structural correction, has been obtained for China’s largest dyke swarm, which trends for about 1000 km in a NNW direction across the North China craton. Positive palaeomagnetic contact tests on two dykes signify that the remanent magnetization is primary and formed during initial cooling of the intrusions. The age of one of these dykes, based on U–Pb dating of primary zircon, is 1769.1 ± 2.5 Ma. The mean palaeomagnetic direction for 19 dykes, after structural correction, is D = 36°, I = − 5°, k = 63, α95 = 4°, yielding a palaeomagnetic pole at Plat=36°N, Plong=247°E, dp = 2°, dm = 4° and a palaeolatitude of 2.6°S. Comparison of this pole position with others of similar age from the Canadian Shield allows a continental reconstruction that is compatible with a more or less unchanged configuration of Laurentia, Siberia and the North China craton since about 1800 Ma

Age and palaeomagnetism of the Mundine Well dyke swarm, Western Australia: implications for an Australia–Laurentia connection at 755 Ma

Integrated U–Pb geochronology and palaeomagnetic studies of the Mundine Well dyke swarm (MDS) of the Pilbara Craton, Western Australia, are employed to test the hypothesis that Australia and Laurentia were joined as part of the Rodinia supercontinent. Ion microprobe U–Pb dating of zircon and baddeleyite indicates that the MDS was emplaced at 755±3 Ma (95% confidence interval) and palaeomagnetic results confirm that the swarm is equivalent in age to dykes of the Northampton Inlier. A positive contact test between an MDS dyke and an older dolerite sill indicates that the MDS magnetisation dates from the time of dyke emplacement and cooling at 755 Ma. Combining new palaeomagnetic data for eight MDS dykes with previous results for six Northampton dykes yields a palaeopole for Australia at 135°E, 46°N ( A 95=4°). The MDS palaeopole lies 30° away from the 780 to 740 Ma APW path segment for Laurentia, indicating that, if the Rodinia reconstruction is correct, breakup between Australia and Laurentia occurred prior to 755 Ma. It has been suggested elsewhere that the Sturtian glaciation in Australia preceded or accompanied breakup. If so, the Sturtian rocks should be older than 755 Ma and hence may not be coeval with glaciogenic rocks of the post-755 Ma Rapitan Group in Laurentia.

Palaeomagnetism of Proterozoic dykes from Bornholm, Denmark

SUMMARY The island of Bornholm (Denmark) is a structural horst situated at a kink on the margin of and linking two major fault zones, the Sorgenfrei-Tornquist Zone (STZ) and the Tornquist-Teisseyre Zone (TTZ), the fault zones of which separate the downfaulted Danish-Polish Basin from the Baltic Shield. Thirteen dolerite dykes, out of a swarm of more than 200, all belonging to the supposed Precambrian N-S to NE-SW striking dyke swarm on Bornholm, have been sampled and palaeomagnetically investigated. The dykes vary in thickness between 0.2 and 60m and are intruded in Precambrian gneisses and granites of Gothian and/or Subjotnian (1 650-1280 Ma) age. Principal component analysis of detailed AF and some thermal demagnetization experiments reveal three groups of stable direction clusterings from the dykes investigated. The stable remanent magnetization of the major dyke Keldse2 dyke and the baked gneiss contact corresponds to a palaeomagnetic mean pole (group A). Comparing it with the APW curve for Fennoscandia. an initial cooling age for this dyke of around 1360 Ma (Subjotnian age) is suggested. The result is confirmed by dual polarities and a positive baked contact test. Nine other dykes show a magnetization corresponding to a younger group of poles (group B), which cluster on the APW path around 1200Ma (Jotnian age). This result is also confirmed by a positive contact test. Two poles (group C) fall on the APW path around 950Ma (Sveconorwegian age). Remaining directions display a distribution of poles on the sphere, which suggest the presence of either late Proterozoic-Cambrian overprints or injections of some younger dykes. Based upon a total of 16 Bornholm dykes hitherto studied, the polarity frequency ratio NIR is 719. The remanent magnetizations and equivalent palaeomagnetic poles found in the Bornholm dyke swarm thus appear to indicate at least three different periods of igneous activity with dyke injections in early Proterozoic time for this otherwise homogeneously looking group of dykes from the margin of the Baltic Shield.

Palaeomagnetic and rock magnetic investigations of the Sipoo Subjotnian quartz porphyry and diabase dykes, southern Fennoscandia

The Sipoo quartz porphyry dyke swarm (UPb zircon age 1.63 Ga) in southern Finland carries an intermediate to high coercivity remanent magnetization A N ( D = 22.2°, I = −1.8°, k = 34.2, α95 = 13.3°, n = 5 dykes) which is regarded as primary. The Sipoo diabase dykes, emplaced slightly before the granite porphyries, yield a highly scattered antiparallel direction A R ( D = 198.0°, I = −6.5°, k = 6.2, α95 = 40.1°, n = 4 dykes) isolated in intermediate to high coercivities. Based on positive contact test results, remanence A R is primary in the diabases. The best preserved diabase dyke, as determined by petrographic and bulk magnetization properties, carries both the A R magnetization and the A N magnetization as a secondary component in some samples. The most altered diabase dyke is almost completely overprinted by A N . The overprinting has taken place as a result of hydrothermal alteration and formation of new magnetite during the emplacement of quartz porphyry dykes and rapakivi granite. The quartz porphyries and diabases carry mixed SD and MD magnetic grain sizes, the large MD grains being responsible for a relatively large Present Earth's Field (PEF) magnetization. In addition, a high coercivity magnetization B ( D = 23.9°, I = 52.2°, k = 215.8, α95 = 6.3°, n = 4 dykes), presumably a chemical overprint, has been isolated in the quartz porphyries. The B magnetization is interpreted to be either an intermediate direction between A N and PEF with no geological significance, or to represent a separate reactivation of the crust at about 1.32 Ga ago.

Age and duration of Mississippi Valley–type ore-mineralizing events

Statistics of a combined paleomagnetic data set from six studies of Mississippi Valley–type deposits and their surrounding host rocks show that the characteristic remanent magnetization of the host rocks predates the magnetization of the mineralization, providing a positive contact test confirming that the ore magnetization is primary. Either multiple fluids or dramatic changes to a single fluid through time are required to account for the separate magnetization ages observed in the host rocks and ores. The dispersion of the characteristic remanent magnetization directions provides an estimator for the duration of the mineralizing process, suggesting a mineralizing event of about 4 m.y. duration.

The Pictou Red Beds' Pennsylvanian Pole: Could Phanerozoic rocks in the interior United States be remagnetized?

The Pennsylvanian red beds of the Pictou Group on Prince Edward Island, Canada, retain a stable remanence in hematite that gives a paleomagnetic pole position at 42°N, 131°E (dp=2°, dm=4°). Positive conglomerate and contact tests along with dual polarities show that the remanence is primary and of Stephanian age (286–306 Ma). For elongated pole distributions through time, Bingham statistics is more suitable than Fisher statistics. Bingham statistics shows that there is a consistent discordance in the Pennsylvanian to Late Triassic poles from the Appalachian orogenic belt and interior craton of North America. This discordance is most readily explained by younger partial remagnetization, or possibly complete remagnetization, in many rock units, especially red beds of the interior craton. Bingham analysis also shows that there is a significant difference between the Colorado Plateau and interior craton that is most readily explained by a Laramide 5.5° ± 2.9° clockwise rotation of the plateau relative to the craton.

The palaeomagnetism of major (middle proterozoic) igneous complexes, South Greenland and the Gardar apparent polar wander track

The Gardar Province comprises a major rift-related igneous suite emplaced in three pulses at ca. 1300–1280 Ma, ca. 1220 Ma and 1160–1125 Ma. This study reports a palaeomagnetic study of the northeastern sector of the present outcrop and incorporates igneous units belonging to the first and last episodes. Early lavas from the basal part of the rift infill record a reversal of magnetisation and define a palaeomagnetic pole at 207°E, 10°N ( 5 sites, dp/dm= 7 14° ). This primary magnetisation record has been partially overprinted in the aureoles of large alkaline complexes. The latter plutons include the North Qoroq intrusion (1291 ± 61 Ma) yielding a pole position at 203°E, 13°N ( 65 samples from 12 sites, dp/dm= 6 12° ), the Motzfeldt intrusion (1291 ± 31 Ma) yielding a pole position at 212°E, 10°N ( 25 samples, dp/dm= 6 12° ), and the Narssarssuaq Stock yielding a pole position at 207°E, 9°N ( 23 samples, dp/dm= 4 7° ). The younger South Qoroq complex (1160 ± 8 Ma) yields a pole position at 216°E, 42°N ( 67 samples from 9 sites, dp/dm= 11 16° ). Positive contact tests indicate primary cooling-related magnetisations. These bodies are predominantly of single polarity equivalent to W + directions of magnetisations although reversals are recorded. The majority of a suite of NE-SW trending dykes emplaced at ca. 1154–1143 Ma are magnetised in a similar direction [ D/I= sol3 51° , pole position at 231°E, 33°N ( 18 dykes, dp/dm= 5 7° )] to the slightly-older South Qoroq complex. Together with earlier results from the Gardar Province, these data define a motion of the region from shallow to equatorial and then back to intermediate palaeolatitudes between 1290 and 1140 Ma. Pole positions trace out an apparent polar wander path comprising a small loop followed by a long track. This track is the outward path of the “Great Logan” palaeomagnetic loop for which data from the Keweenawan Province of North America (ca. 1100–1050 Ma) define the return path. The loop was executed between ca. 1200 and 1050 Ma following a near-static interval between ca. 1290 and 1200 Ma. This long static interval was also characterised by constant field polarity. Asymmetry of geomagnetic field reversals is recognised in early Gardar events; it is not apparent in younger Gardar data although it recurs in Keweenawan times.

Paleomagnetism of the Moreton's Harbour Group, northeastern Newfoundland Appalachians: Evidence for an Early Ordovician Island Arc near the Laurentian Margin of Iapetus

Paleomagnetic results have been obtained from mafic volcanic units in the upper part of the Moreton's Harbour Group, which is part of an accreted island arc terrane that is preserved in the Notre Dame Bay subzone of the Central Mobile Belt of the Newfoundland Appalachians. Detailed thermal and alternating field demagnetization reveals a stable characteristic component of magnetization, carried by magnetite, at a large number of sites in pillow basalts and coeval basaltic dikes. The intrusives contain both polarities, and a primary age for the characteristic magnetization is indicated by a positive contact test for one of the dikes, and by a positive structural test involving a correction for block rotations (strike‐correction). The overall mean direction for the characteristic component after tectonic (tilt and strike) correction (flows and intrusives: D = 171°, I = +22°, k = 22.6, α95 = 6.5°, pole 29°N, 135°E; flows: D = 166°, I = +22°, k = 33.2, α95 = 6.5°) corresponds to an Early Ordovician paleolatitude for the arc of 11°S, which is indistinguishable from the expected paleolatitude of the North American margin. This implies that the arc formed at or near the margin of the craton. In contrast, the Avalon block, which formed the other margin of Iapetus, was widely separated from the arc and the craton at this time. The Early Ordovician paleolatitude of the arc terrane supports a tectonic model in which coeval ophiolitic sequences inboard of the arc were formed in a narrow ocean basin between the arc and the craton. Subsequent convergence between the arc and Laurentia in Middle Ordovician time resulted in closure of this narrow ocean basin and obduction of the back‐arc basin oceanic crust onto the ancient margin of the craton, thus giving rise to the Taconic orogenic pulse in the Newfoundland Appalachians.

Age of the Firesand River carbonatite complex from paleomagnetism

The 2.3 km diameter Firesand River complex intrudes Archean volcanics and granites of the Wawa Subprovince in the Superior Province about 8 km east of Wawa, Ontario. It has given differing Middle Proterozoic K–Ar biotite ages of 1018 ± 50 and 1097 Ma. Alternating-field and thermal step demagnetization of specimens from three calcific carbonatite sites, five ferruginous dolomitic carbonatite sites, and one lamprophyre dike site isolated a stable mean direction of 290°, 33 °(α95 = 12°). Isothermal remanent magnetization tests indicate the remanence is held by single-to pseudosingle-domain magnetite and hematite in the carbonatite. The dike remanence is Keweenawan in age, thereby confirming its genetic relationship to the complex, and it gives a positive partial contact test with its host rock, indicating no postintrusive remagnetization. The blocking-temperature spectra indicate that postintrusive uplift has not exceeded about 4 km. The pole position for the complex is 183°E, 27°N (dp = 8°, dm = 13°). This pole lies directly on the well-dated Keweenawan apparent polar wander path, giving an age of 1090 ± 10 Ma, in agreement with the older K–Ar age. It also confirms geologic and aeromagnetic evidence that the complex has not been tectonically tilted since emplacement.