Paleomagnetism of Late Hadrynian Fourchu gabbro, southern Cape Breton Island, Nova Scotia.

Abstract

A paleomagnetic study was carried out on three gabbroic intrusives in the Avalonian terrane of southeastern Cape Breton Island. Thirty five samples (96 specimens) from seven sites were collected for this purpose. Thermal and/or alternating field treatments were performed. Four components of magnetization were detected. One component coincides with the present Earth's field direction. A second component (A) oriented south-southeast with shallow to intermediate inclination (165, +41) is interpreted as representing a Siluro-Devonian overprint. A third component (B) yields a mean direction to the southeast and an intermediate inclination (147, -54). This component is not observed in North American rock units of Devonian age or younger; it is thus bracketed in the Early Devonian-Late Hadrynian interval. Based on magnetic properties, mineralogical and geological grounds, an Ordovician-Cambrian age of acquisition is attributed to this component of magnetization. A fourth component characterized by a low unblocking temperature and median destructive field has a mean direction to the northeast and an intermediate inclination (54, +47). This uncommon direction is inferred to represent an incompletely isolated magnetization. Compaged to low southern paleolatitudes for the Late Hadrynian period of cratonic North America, the paleolatitudes of the Avalonian terrane are slightly higher but this is not really significant. A sea at the most 1000km wide may have separated North America and Avalon at that time. A comparison of the paleolatitude of Avalon and those of Late Hadrynian Atlantic Ocean borderlands indicates that Avalon could have belonged to North Africa at that time. The Avalonian apparent polar wander path in the Devonian-Late Hadrynian interval still remains uncharted. The collision of Avalon with North America occurred most probably at Middle Devonian time.