Abstract
Remanent magnetization in dyke contact zones in the Hoting area of the Fennoscandian Shield in the central part of Sweden has been studied in order to establish the ambient temperature of the host rock and the depth of burial of the present erosion surface at the time of intrusion. A positive baked contact test for two Subjotnian dykes demonstrates the primary nature of the dyke magnetization. From the magnetic properties and the palaeomagnetic data, it can be concluded that the overprinting in the hybrid zone of one of the basic dykes is a partial thermoremanent magnetization. Reliable results were also obtained from a palaeointensity study of samples from the hybrid zone in the baked host rock. The study was performed in the laboratories at Luleå and at Scripps and a mean intensity of the Earth's magnetic field of 5.8 ± 1.9 μT was determined. The maximum temperature due to the dyke intrusion in the hybrid zone has been defined and from that an ambient temperature of 375 °C in the host rock at the time of intrusion has been calculated. This calculated temperature is not contradicted by the 40Ar/39Ar data. A palaeothermal gradient in the crust at ca. 1.6 Ga is calculated at ca. 34 °C km−1 and yields a depth of burial of the present erosion surface at ca. 10.4 km. This implies an uplift rate of 0.65 km (100 Ma)−1. A slow cooling of the gabbroic host rock (ca. 3.5 °C Ma−1) has been calculated from the difference in the U-Pb age of zircon (1.786 ± 0.010 Ga) and the 40Ar/39Ar biotite ages (1.648 ± 0.012 Ga; 1614 ± 0.024 Ga) of this study. This slow cooling resulted in a palaeomagnetic age of ca. 1.7 Ga for the gabbro, which is also the age of the determined palaeointensity. The calculated ca. 1.614 Ga palaeomagnetic pole from the basic dykes fulfils most of the criteria for a reliable pole and may be regarded as a new key-pole for Fennoscandia.
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