Palaeomagnetism of granitic intrusives from the Precambrian basement under eastern Kansas: orienting drill cores using secondary magnetization components |
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| Authors: | Kenneth P Kodama |
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| Institution: | Department of Geological Sciences, Williams Hall #31, Lehigh University, Bethlehem, PA 18015, USA |
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| Abstract: | Summary. The Precambrian basement under east-central Kansas was drilled at two circular aeromagnetic positives, one at Osawattamie and one at Big Springs. The core retrieved from these sites is a coarse to medium grained granite which has been dated by U-Pb to be 1350 Ma old. The palaeomagnetism of these azimuthally unoriented cores was studied to see if a technique which uses low-coercivity, low-temperature magnetization components to orient the cores would allow an independent confirmation of the core's mid-Proterozoic age. Orthogonal projection plots of the alternating field (af) and thermal demagnetization data show that the magnetization of these cores is relatively simple, having only two components: a low-temperature, low-coercivity magnetization with steep positive inclinations and a shallow, negative inclination characteristic magnetization for the Osawattamie core or a positive, moderate inclination characteristic magnetization for the Big Springs core. If the declination of the low-temperature, low-coercivity component is aligned parallel to the present field declination, the characteristic directions may be azimuthally oriented. This allows the calculation of palaeomagnetic poles for the Big Springs core (lat. = 4.5°S, long. = 29.9°E) and the Osawattamie core (lat.= 20.2°N, long. = 39.3°E) which are consistent with Irving's apparent polar wander path for Laurentia at about 1300–1400 Ma. Comparison of anhysteretic remanent magnetization (ARM), viscous remanent magnetization (VRM), and isothermal remanent magnetization af demagnetization curves with a natural remanent magnetization (NRM) demagnetization curve suggests that the Osawattamie core probably acquired a piezoremanent magnetization (PRM) parallel to the core axis during drilling. |
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