HOW DO YOU BUILD THE LARGEST BATHOLITH IN THE PHANEROZOIC? EVIDENCE FOR THREE MAGMATIC FLARE-UPS IN THE COAST PLUTONIC COMPLEX OF BRITISH COLUMBIA, CANADA

Surface areas of intrusive rocks between Bella Coola and Prince Rupert in western British Columbia indicate that significant magmatism in the Coast Plutonic Complex was generally confined to three, nearly equal pulses of activity within the past 150 m.y. Despite the arc being active since at least the mid-Jurassic, magmatic flare-ups in the Late Jurassic-Early Cretaceous (~150-140 Ma), mid-Cretaceous (~100-90 Ma), and Early Tertiary (~60-50 Ma) account for ~75% of exposed intrusive bodies. Assuming an arc thickness of 20 km, peak magmatic addition rates averaged over 10 m.y. intervals are ≥50 km³/km·m.y., compared to island arc magmatic addition rates that are typically only ~20-40 km³/km·m.y. (Reymer and Schubert, 1984). The periodicity of the Coast Mountains Batholith is similar to other Cordilleran arcs. Multiple, equally voluminous arc flare-ups, however, have not been observed elsewhere. This multiple-pulse magmatism likely accounts for the fact that the Coast Plutonic Complex is the largest in the Phanerozoic. Like has been suggested for the California arc to the south (Ducea, 2001), large-scale crustal thickening may be responsible for magmatic flare-ups. Future research is needed, however, to better understand how deformation in the foreland is related to discrete pulses of magmatism.