U-Th-Pb geochronology of the Coast Mountains batholith in north-coastal British Columbia: Constraints on age and tectonic evolution

Previously published and new U-Pb geochronologic analyses provide 313 zircon and 59 titanite ages that constrain the igneous and cooling history of the Coast Mountains batholith in north-coastal British Columbia. First-order findings are as follows: (1) This segment of the batholith consists of three portions: a western magmatic belt (emplaced into the outboard Alexander and Wrangellia terranes) that was active 177-162 Ma, 157-142 Ma, and 118-100 Ma; an eastern belt (emplaced into the inboard Stikine and Yukon-Tanana terranes) that was active ca. 180-110 Ma; and a 100-50 Ma belt that was emplaced across much of the orogen during and following mid-Cretaceous juxtaposition of outboard and inboard terranes. (2) Magmatism migrated eastward from 120 to 80 (or 60) Ma at a rate of 2.0-2.7 km/Ma, a rate similar to that recorded by the Sierra Nevada batholith. (3) Magmatic flux was quite variable through time, with high (> 35-50 km(3)/Ma per km strike length) flux at 160-140 Ma, 120-78 Ma, and 55-48 Ma, and magmatic lulls at 140-120 Ma and 78-55 Ma. (4) High U/Th values record widespread growth (and/or recrystallization) of metamorphic zircon at 88-76 Ma and 62-52 Ma. (5) U-Pb ages of titanite record rapid cooling of axial portions of the batholith at ca. 55-48 Ma in response to east-side-down motion on regional extensional structures. (6) The magmatic history of this portion of the Coast Mountains batholith is consistent with a tectonic model involving formation of a Late Jurassic-earliest Cretaceous magmatic arc along the northern Cordilleran margin; duplication of this arc system in Early Cretaceous time by > 800 km (perhaps 1000-1200 km) of sinistral motion (bringing the northern portion outboard of the southern portion); high-flux magmatism prior to and during orthogonal mid-Cretaceous terrane accretion; low-flux magmatism during Late Cretaceous-Paleocene dextral transpressional motion; and high-flux Eocene magmatism during rapid exhumation in a regime of regional crustal extension.