Architecture and Evolution of the Crust during Continental Arc Magmatism: A Transect through the Coast Mountains Batholith, British Columbia

Abstract

The Coast Mountains batholith (CMB) is one of the largest continental margin batholiths in the world. It is nearly continuously exposed for >1700 km along the west coast of North America in British Columbia through southeastern Alaska into southwestern Yukon Territory. This guide, prepared for the GSA Thompson Field Forum held in August 2018, describes the geology along the readily accessible Skeena River transect of the CMB in British Columbia. At this latitude, the CMB is bounded on the east by generally low-grade stratified rocks and subordinate Jurassic to Eocene plutons. These rocks are bounded on the west by a Paleogene, low-angle, top-to-the northeast detachment (the Eastside detachment). West of the detachment, the Central Gneiss Complex (CGC), which forms the lower plate of the detachment, consists of amphibolite to granulite-facies schist, gneiss, and orthogneiss, intruded by Late Cretaceous to Paleogene plutons. The CGC is characterized by regionally consistent Eocene 40Ar/39Ar and K-Ar cooling dates. This core belt is bounded on the west by the Paleogene Coast shear zone, a steep crustal-scale structure. Paleogene plutons do not occur west of this belt. West of the Coast shear zone, schists of the Western metamorphic belt show evidence for southwest-verging thrusting, and form an inverted metamorphic sequence with grade dramatically decreasing to the west. These rocks are intruded by Jurassic to Late Cretaceous plutons. We use this transect as a basis to examine the growth of the CMB as a whole, emphasizing commonalities and variations with the batholith and how these traits may reflect magmatic processes that create this and other convergent-margin batholiths. We conclude by highlighting a few of the many open questions regarding the evolution of this complex batholith.