Abstract

The Butedale pluton, a ca. 100 km long compositionally zoned batholith, is part of the Coast Plutonic Complex that extends the length of the Canadian Pacific coast. Its age and thermal history are relevant to the Butedale pluton’s role as a test case of the Baja–BC hypothesis, as paleomagnetic evidence suggests that it may have formed thousands of kilometres to the south and moved northward along what is now the Coast Shear Zone. High-resolution U–Pb (chemical abrasion – thermal ionization mass spectrometry (CA–TIMS), zircon) analysis of rocks across the width of the Butedale pluton indicates that it is actually made up of at least two distinct magmatic events that formed the West Butedale pluton (ca. 95 Ma) and the East Butedale pluton (ca. 85 Ma). The East Butedale pluton was reheated by the emplacement of a younger adjacent pluton, which may have caused partial Pb loss and resulting excess scatter of 206Pb/238U zircon ages within individual samples. The West Butedale pluton may be the same age as, and part of, the nearby Ecstall pluton, thereby doubling the length of the Ecstall pluton to ca. 200 km. Single-grain, high-precision U–Pb analysis of zircon reveals previously unknown complexity and detail of emplacement and thermal history in the Butedale plutons.

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