Paleomagnetism of the Glen Mountains Layered Complex: Dipolar field behavior at ~530 Ma

Results from the Sept-Îles intrusive suite suggest that the Earth’s magnetic field reached ultra-low intensities at 565 Ma (~3 μT, Bono et al., 2019) during the Ediacaran Period. Additional evidence from this study suggests abnormally high paleosecular variation (S= ~26˚). Other studies of Ediacaran rocks indicate very high reversal frequencies. Based on this abnormal geomagnetic field behavior, Bono et al. (2019) suggested inner core nucleation at about 565 Ma. Therefore, we conducted a detailed paleomagnetic study of the early Cambrian (²⁰⁶Pb/²³⁸U age of 532.49±0.12 Ma, Wall et al., 2020) Glen Mountains Layered Complex (GMLC) of southwestern Oklahoma to assess behavior of the geomagnetic field 30 myr later. One hundred ninety independently oriented cores were drilled from 17 sites collected from the anorthosites of the GMLC. Primary paleomagnetic directions were isolated by thermal and alternating field demagnetization. Magnetic susceptibility versus temperature and isothermal remanence acquisition experiments both indicate that low-Ti magnetite is the dominant remanence carrier, although minor pyrrhotite is seen in some samples. Nearly antipodal directions, collected from different sites, pass a simple reversals test, confirming that geomagnetic field polarity reversals are recorded (Roggenthen et al., 1981) and that the GMLC anorthosites carry a primary remanence. Paleosecular variation recorded by the GMLC (S=10.9˚) for a paleolatitude of 10.3˚ is in agreement with Smirnov et al.’s (2011) tabulation of paleosecular variation for 1.0-2.2 Ga intrusive and extrusive igneous rocks. A reversed polarity paleopole for the complex is located at 26.1°E, 25.3 °N (A₉₅ = 7.4°). Our results suggest that the geomagnetic field returned to stable, dipole-dominated behavior by about 530 Ma.

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