Temporal and spatial evolution of Northern Cascade Arc magmatism revealed by LA–ICP–MS U–Pb zircon dating

Abstract

We present thirty new laser ablation inductively coupled plasma mass spectrometry U–Pb zircon dates for intermediate to silicic plutons of the Northern Cascade Arc with emphasis on the Chilliwack batholith – Mount Baker magmatic focus, located in southwestern British Columbia and northern Washington. Chilliwack magmatism commenced at ∼35 Ma in southwestern British Columbia and the most voluminous plutons define a cluster at ∼32–29 Ma, documenting an early flare-up. During the same interval, the Index, Squire Creek, and Cascade Pass intrusions were emplaced south of the Chilliwack batholith. North of the Chilliwack, maximum pluton ages become progressively younger northward, tracking the northerly migration of the edge of the Farallon–Juan de Fuca–Explorer plate system relative to North America. Chilliwack magmatism continued from ∼29 Ma to 22 Ma at a slightly reduced flux, followed by a lull from 22 to 11 Ma during which magmatism shifted north to the Mount Barr batholith (18 Ma). Chilliwack magmatism resumed by 11 Ma but was intermittent and the intrusive flux was significantly lower. The temporal decrease in intrusive flux displayed by the Chilliwack batholith correlates with the declining convergence rate of the Juan de Fuca plate since arc inception. The 11 Ma-to-present magmatism extends a pattern of southwesterly migration of the magmatic focus previously identified from ∼4 Ma (Hannegan caldera) to the modern Mount Baker volcanic field. Crustal rotation accounts for the rate of the first ∼7 million years of migration. However, the migration rate more than doubled at ∼4 Ma, coinciding with separation of the Explorer plate and initiation of Juan de Fuca plate rollback.