Abstract

A paleomagnetic study was made to elucidate the ambiguities of the radiometric ages of Mont Megantic intrusions in relation to field observations and to determine the magnetochronology of the intruded rocks. Some 179 samples (550 specimens) were collected over 58 sites, and their magnetization was cleaned by thermal and (or) alternating field treatment. The paleopoles obtained for the Compton Formation metasediments indicate an Early to Middle Devonian age and for the overlying Frontenac Formation metavolcanics indicate an age definitely different from that for the intrusive rocks. The baked contact test on the hornfels forming the contact metamorphic aureole is positive, and the magnetization component was acquired in the Early to Late Cretaceous interval. Syenite contains two components: one with normal polarity, the other with reversed polarity; their ages are Juro-Cretaceous. The gabbro contains only one magnetization component (reversed), which was acquired in the Early Cretaceous, whereas the granite bears one component with a mostly normal polarity; its intrusive age is Late Cretaceous.The paleomagnetic ages for the intrusive rocks support the multiple intrusion interpretation; it appears that the emplacement of the intrusive bodies is Juro-Late Cretaceous. [Journal Translation]

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