Late Cretaceous to Paleocene evolution of the Blanchard River assemblage, southwest Yukon; implications for Mesozoic accretionary processes in the northwestern Cordillera

Bedrock mapping, petrography and U-Pb in-situ monazite and detrital zircon geochronology elucidates the geological evolution of the Blanchard River assemblage, southwest Yukon. The Blanchard River assemblage belongs to a series of Middle Jurassic to early-Late Cretaceous pull-apart basinal assemblages deposited between the Insular and Intermontane terranes. Detrital zircon maximum depositional ages from 130-120 Ma suggest Early Cretaceous deposition of the Blanchard River assemblage along the western Laurentian margin, detritus was sourced primarily from inboard Intermontane terranes with minor input from outboard Insular terranes. Collapse of the Blanchard River basin is recorded by 83-76 Ma deformation and associated amphibolite-facies metamorphism, interpreted as the timing of terminal Insular terrane accretion to the western Laurentian margin. A second phase of Blanchard River assemblage metamorphism indicates ca. 70 Ma retrograde metamorphism associated with exhumation. Intrusion of the Ruby Range suite induced contact metamorphism within the Blanchard River assemblage from 63-61 Ma.