THE GEOCHEMICAL EVOLUTION OF GREATER THAN 100 MILLION YEARS OF SUBDUCTION-RELATED MAGMATISM, COAST PLUTONIC COMPLEX, WEST-CENTRAL BRITISH COLUMBIA

Prolonged magmatic activity characterizes most Mesozoic-Cenozoic batholiths of the North American Cordillera including the Coast Mountains Batholith of west-central British Columbia, Canada. Typically such extended magmatic activity includes arc-normal migrations that are associated with systematic changes in petrology and geochemistry. In most systems these petrologic and geochemical changes are related to crustal transitions (i.e. from juvenile oceanic to mature continental). While the Coast Mountains batholith exhibits arc-normal migrations throughout much of it's >100 m.y. magmatic evolution crustal ages and compositions of source regions are broadly similar over ~150 km of arc-normal distance. Approximately equal source compositions for the entire arc of west-central British Columbia provides an unique opportunity to observe environmental changes of the source during crustal anatexis.

Major and trace elemental, and various isotopic compositions indicate that the Coast Mountains batholith is comprised of relatively primitive calc-alkaline intrusives that are similar to the western or outboard portions of other batholithic segments of the North American Cordillera. Elevated oxygen isotopic compositions, however, indicate a period of near surface residence for the rocks comprising the source(s) of the batholith. The depth of melting, as indicated by rare earth elemental compositions, suggest relatively shallow levels (i.e. ≤ 40 km) during the Jurassic and Early Cretaceous and east of the Coast Shear zone only during the Late Cretaceous. Melts generated at crustal depths in excess of 40 km dominate for Late Cretaceous intrusives west of the Coast Shear zone and during the Paleocene and Eocene east of this structure. Periods of melt generation from deep crustal sources correlate well with known periods of early Late Cretaceous and latest Late Cretaceous-Paleocene contraction. Dehydration melting appears most prevalent for Jurassic through Late Cretaceous plutonic rocks west of the Coast Shear zone, but may also be responsible for Late Cretaceous and Paleocene melts to the east.