5:15 PM - 6:45 PM
[SEM13-P05] Paleomagnetism of the Mull dyke swarm in the British Tertiary Igneous Province
Keywords:Mull dyke swarm, dyke group, paleomagnetic direction, paleomagnetic field, paleomagnetic polarity
Within the North Atlantic Igneous Province (NAIP), there have been many paleomagnetic studies of volcanic rocks in the British Tertiary Igneous Province (BTIP) (e.g., Wilson et al., 1982; Dagley et al., 1984; Riisager et al., 2002).Paleomagnetic studies of lava and dykes erupted from the Mull volcano, which is part of the BTIP, have also been conducted; Ade-Hall et al. (1972) studied the paleomagnetic directions of dykes that intruded from Mull volcano, Hall et al. (1977) -studied the paleomagnetic directions of lava flows of the Mull volcano, and Ganerod et al. (2008) studied the paleomagnetic directions of lava flows and dykes of the Mull volcano. However, long-distance dykes fed by Mull volcano have not been comprehensively studied paleo-magnetically for dykes distant from the source. Ishizuka et al. (2017) analyzed the chemical composition of long-range dykes fed by Mull volcano to determine the direction and timing of magma transport. This study aims to conduct paleomagnetic studies of these long-distance dykes and to contribute to the study of the formation process of the dykes and paleomagnetic field variations during this period (ca. 60 Ma). In this study, we measured the paleomagnetic directions of 77 sites in the Mull dyke swarm. Based on the obtained paleomagnetic data, we combined Ar-Ar dating and magnetostratigraphy to estimate the age of igneous activity in the Mull dyke swarm. The paleomagnetic data obtained in this study are combined with existing data to discuss the characteristics of the paleomagnetic field at that time.
For the paleomagnetic direction measurements, we first demagnetized one sample from each site up to 110 mT and identified the demagnetization step at which characteristic remanent magnetization could be recognized. Other samples from the same site were demagnetized to the same demagnetization step. The results of the paleomagnetic direction measurements yielded characteristic remanent magnetization directions from 555 samples, of which 513 samples were used to obtain the average direction for each site. Paleomagnetic directions were obtained from all 77 sites measured. Fifteen sites showed the normal magnetic polarity, 58 sites showed the reverse polarity, and 4 sites showed the intermediate polarity. The average directions of the normal and the reverse pole sites passed the reversal test. Thus, the obtained mean paleomagnetic direction is the time average based on data of sufficient duration, which is considered to be the direction of the geocentric axial dipole field. The obtained paleomagnetic pole is consistent with the reported Mull volcano data and with the overall BTIP data. The angular standard deviation, which indicates the distribution of the virtual geomagnetic poles, is 22.9 degrees. This value is about 5 degrees greater than the modeled angular standard deviation for the past 5 million years. New Ar-Ar ages of about 55-60 Ma were obtained for the 10 sites. Geomagnetic polarity timescales were referenced to the Ar-Ar ages of these 10 sites and are consistent with the paleomagnetic polarity obtained in this study except for one site.
For the paleomagnetic direction measurements, we first demagnetized one sample from each site up to 110 mT and identified the demagnetization step at which characteristic remanent magnetization could be recognized. Other samples from the same site were demagnetized to the same demagnetization step. The results of the paleomagnetic direction measurements yielded characteristic remanent magnetization directions from 555 samples, of which 513 samples were used to obtain the average direction for each site. Paleomagnetic directions were obtained from all 77 sites measured. Fifteen sites showed the normal magnetic polarity, 58 sites showed the reverse polarity, and 4 sites showed the intermediate polarity. The average directions of the normal and the reverse pole sites passed the reversal test. Thus, the obtained mean paleomagnetic direction is the time average based on data of sufficient duration, which is considered to be the direction of the geocentric axial dipole field. The obtained paleomagnetic pole is consistent with the reported Mull volcano data and with the overall BTIP data. The angular standard deviation, which indicates the distribution of the virtual geomagnetic poles, is 22.9 degrees. This value is about 5 degrees greater than the modeled angular standard deviation for the past 5 million years. New Ar-Ar ages of about 55-60 Ma were obtained for the 10 sites. Geomagnetic polarity timescales were referenced to the Ar-Ar ages of these 10 sites and are consistent with the paleomagnetic polarity obtained in this study except for one site.