Four intrusive complexes from southern Maine, containing rocks ranging in composition from olivine gabbro to granodiorite, were sampled to determine an Early Cretaceous reference pole for the North American plate. Sampling included the Lebanon diorite, the Tatnic Complex, Alfred Complex, and the Cape Neddick Complex dated radiometrically by the 40Ar/39Ar or K-Ar method on biotite between 125 and 120 Ma. The majority of samples have unblocking temperatures between 550¿C and 580¿C and Curie temperatures between 550¿C and 570¿C, indicating that the dominant remanence carrier is titanomagnetite, which crystallized from the magma or was produced by deuteric oxidation alteration during post-magmatic cooling. Titanomagnetite occurs as discrete C1-C3 grains, as exsolution or oxidation-exsolution lamellae in the silicates, as a product of magmatic reactions forming magnetite-ilmenite symplectites with pyroxene or olivine, or as ultrafine dust. Hemo-ilmenite is present as a minor component in the Lebanon Diorite, and, in this case, the remanence direction carried by the hemo-ilmenite is indistinguishable from the titanomagnetite. Detailed petrographic observations and rockmagnetic experiments demonstrate that, despite the coarse average grain size of the oxides in these rocks, which might lead one to expect unstable magnetic remanence, the magnetic remanence in most samples is extremely stable and caused by single-domain or pseudo-single-domain behavior in magnetite containing fine oxidation-exsolution lamellae of ilmenite. The Tatnic and Alfred intrusions are of normal polarity (D=333¿, I=62¿, pole 70.5¿N, 210.3¿E, A95=3.7¿, N=24 sites); the Cape Neddick and Lebanon intrusions are of reversed polarity (D=161¿, I=-57¿, pole 73.7¿N, 185.7¿E, A95=5.0¿, N=17 sites). Together these intrusions yield a mean paleomagnetic pole at 72.2¿N, 198.9¿E, A95=3.3¿, N=41 sites. The well-characterized reversed polarity recorded in the precisely dated Cape Neddick Complex may represent the best age for the magnetic polarity chron M0r, the last reversal before the onset of the Cretaceous normal superchron at the Barremian-Aptian stage boundary. ¿ American Geophysical Union 1996 |