Baked Contact Test Research Articles

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.

The palaeomagnetism of some Upper Cretaceous kimberlite occurrences in South Africa

Palaeomagnetic results are reported from the De Beers, DuToitspan and Wesselton kimberlite pipes ( 86 ± 3Ma) at Kimberley and from the Finsch and Koffyfontein pipes. The latter are both less than 150 km from Kimberley and although undated are geologically correlated with the Kimberley pipes. Eighty blocks, oriented without recourse to magnetic methods, have been collected from the five pipes. The samples comprise kimberlite, accidental inclusions of widely varying lithologies and wall rock thus enabling inclusion and baked contact tests to be performed. Extensive alternating field and thermal demagnetization experiments show that the magnetization of the kimberlite is primary. Secular variation is not averaged out in any one pipe (with the possible exception of Finsch) and it is suggested the three Kimberley pipes were emplaced simultaneously. The mean pole position of the five pipes (57.2°E, 58.2°S with K = 25.8 and A 95 = 15.3° ) is believed to be a good estimate of the palaeomagnetic pole 86 ± 3Ma ago. It is now possible to state that the African Mesozoic palaeomagnetic pole remained essentially in the same position until at least 86 ± 3Ma ago.

Palaeomagnetic results from the Hart dolerite of the Kimberley Block, Australia

A routine palaeomagnetic study of the 1800 Ma old Hart dolerite shows that sample directions of magnetism give good within-site grouping at 12 sites after magnetic cleaning. However, the site mean directions are scattered over one-quarter of the sphere, a feature that apparently does not arise from poor structural control. A baked contact test confirms that the magnetization is original. Comparison of bulk NRM, TRM, ARM and saturation IRM properties has been made with the Modipe gabbro of southern Africa from which a palaeointensity has previously been estimated. These measurements are consistent with the hypothesis that the main dipole field intensity temporarily was much reduced with respect to the non-dipole field 1800 m.y. ago resulting in increased directional scatter from secular variation. The average of the site VGPs at 29N 46E with A 95 = 24° thus is still a viable estimate of the palaeomagnetic pole at that time bearing in mind the larger than usual statistical error associated with it.

Dual-polarity IC regulators aid design and packaging: R. A. Mammano. Electronics, 15 February (1973), p. 108

Several new paleomagnetic and geological studies focused on the reconstruction of the North China Craton (NCC) within the Paleo-Mesoproterozoic Columbia supercontinent. In spite of these new data, there are still widely divergent opinions regarding supercontinental reconstructions. In addition to qualitative correlations of orogenic belts, rift basin ages, stratigraphy and distribution of igneous provinces, paleomagnetic data can provide key constraints on the positioning of individual cratons on the globe. In this paper, we report a detailed paleomagnetic study on the coeval ∼1780 Ma mafic dyke swarm and Xiong’er volcanic province, which extended for more than one thousand kilometers in the central NCC. Rock magnetic studies, including thermomagnetic curves, hysteresis loops and the progressive acquisition of isothermal remanence conducted in selected samples, indicate that the dominant magnetic carriers are PSD magnetite. Stepwise thermal demagnetization isolated higher-temperature components directed to NNE/SSW with shallow inclinations from 37 sampling sites (16 sites in Yinshan area, 13 sites in Taihang area and 8 sites in Xiaoshan area). A baked contact test conducted on two Yinshan dykes intruded by a younger dyke demonstrates the magnetization in the Yinshan dykes pre-dates 1320 Ma. The existence of dual-polarity magnetizations in both Taihang and Xiong’er areas support our contention that the ChRM was acquired during the cooling of the magma. The primary origin of the ChRM is also supported by a positive fold test on the Xiong’er data, and a coherent regional test between the results from the Taihang and Xiong’er areas. Two different site-mean directions were compiled from these new results along with previous publications. The first direction, from the Taihang and Xiong’er areas, yields Declination (D)/Inclination (I) = 12.4°/−3.7° (κ = 20.5, α95 = 4.3°, N = 57 sites). The second, from the Yinshan area is at (D) 36.7°/(I)−12.4° (κ = 86.8, α95 = 2.7°, N = 32 sites). We argue that the difference is due to Mesozoic and/or Cenozoic vertical-axis rotation of the Taihang and Xiong’er areas with respect to the fixed Yinshan-Ordos basin. The corresponding paleopoles for the Yinshan dykes falls at 245.2°E/35.5°N (A95 = 2.4°). A comparison between the NCC, Laurentia, Siberia and Baltica is consistent with possible links between these four blocks in a perhaps, even larger, continent. The proximity of Australia and India to the NCC is also evaluated.

Magnetization of dikes: A study of the paleomagnetism of the Widgiemooltha Dike Suite, western Australia

Oriented hand samples have been collected at twelve sites in five members of the Widgiemooltha dike suite of Western Australia. These dikes form a major tectonic lineament striking N 80°E, and they have a Rb : Sr isochron age of 2420 ± 30 m.y. At eleven sites, initially scattered directions became well grouped after magnetic cleaning, with a mean direction of D = 242°, I = −67°. Further magnetic stability is demonstrated by baked-contact tests, and the cleaned results yield a paleomagnetic pole situated in the southwest Pacific Ocean at 157°E, 8½°S with A95 = 8°. The mean direction lies close to the dike planes. A previous suggestion that by a suitable physical mechanism dikes tend to be magnetized in their plane is investigated. If it is assumed that dikes record the direction of the geomagnetic field at the time of intrusion, a simple geometrical analysis shows that for vertical dikes there is a 50% chance that this direction will lie within 22° of the dike plane. A compilation of seventy-six examples from the literature shows good agreement with the expectations of the analysis. It is suggested, therefore, that the data available at present provide no reason for supposing that dikes do not become magnetized parallel to the ambient field.