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 40 Ar/ 39 Ar 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.

Originally prepared for the GSA Thompson Field Forum that ran from Terrace to Prince Rupert, British Columbia, this guide describes the geology along the Skeena River transect of the Coast Mountains batholith, the largest Cordilleran batholith of western North America and one of the largest continental-margin batholiths in the world. The last guide to this area was published in 1983 and this new volume is the only modern overview of the last decades of work. The authors use the transect as a basis to examine the growth of the Coast Mountains batholith as a whole, emphasizing commonalities and variations with the batholith and how these traits may reflect magmatic processes that create convergent-margin batholiths.

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