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| Titel |
Palaeointensity and palaeodirection determinations of Paleoproterozoic dykes in the Kaapvaal Craton (South Africa) |
| VerfasserIn |
V. V. Shcherbakova, N. V. Lubnina, V. P. Shcherbakov, G. V. Zhidkov |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250065377
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| Zusammenfassung |
| A combined palaeodirectional and palaeointensity study of a representative collection from
the Bushveld Igneous Complex from 27 dolerite dykes from the 2.9, 2.7, and 1.8 Ga age
swarms radiating SE, E and NE, respectively [Olsson et al., 2010] was carried out.
Conventional progressive thermal or AF demagnetization was applied to all specimens. The
palaeomagnetic directions have been calculated after thermal demagnetization. The ChRMs
were isolated over the temperature interval 440-590 C and their intensities amount to 95% of
total NRMs. Paleopole calculate from the primary high-temperature component, separated
in the 2.9 Ga SE-dykes, is close to the paleopoles, obtained by Wingate (1998)
and Strik et al. (2007) for 2.78 Ga volcanics. The paleopole calculated for the 2.7
Ga age E-trending dykes of the eastern region does not correspond to any of the
previously obtained Archean-Paleoproterozoic paleopoles for the Kaapvaal Craton. The
paleopole calculated for some NE-trending dykes of the Black Ridge swarm in the NE
region is close to the 1.87 Ga pole of the Kaapvaal Craton obtained by Hanson et al.
(2004).
Palaeointensity determinations were carried out on rocks from ten dykes of different ages
using Thellier-Coe method with the “check-points” procedure on specimens of 1
cm in edge length cut from either drilled cores or hand samples. Rock magnetic
measurements were made on sister specimens. Curie temperatures and the thermal
stability of magnetic minerals were estimated from thermomagnetic heating-cooling
cycles to incrementally higher temperatures Ti with a Curie balance in an external
magnetic field H = 0.45 T. To assess the magnetic hardness and mineralogy of
samples, measurements of magnetic susceptibility and hysteresis loop parameters were
performed. The domain structure (DS) was estimated also from the thermomagnetic
criterion by evaluating the tails of pTRMs. Wilson’s method of palaeointensity
determination based on comparison of thermodemagnetization curves of NRM(T) and
TRM(T) was used too. Reliable palaeointensity determinations were obtained on
only the site N28 of the age 1.85 Ga (Olsson et al., 2010). The rocks from this site
demonstrated extremely stable magnetic properties to heating, their Curie points are
closed to Tc of the magnetite. The thermodemagnetization curves NRM(T) and
TRM(T) are very similar and the positions of check-points on the Arai-Nagata
diagrams are close to the initial pTRM values. Seven samples (12 sister cubes) showed
very similar intensities of paleofield Hanc lying in the interval 15-23 μT, with the
mean VDM = 2.85 Am^2. This result agrees with the widespread opinion that the
field in the Paleoproterozoic was considerably less than the modern magnetic field. |
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