JURASSIC TO EOCENE MAGMATIC HISTORY OF THE COAST MOUNTAINS BATHOLITH IN NORTH-COASTAL BRITISH COLUMBIA

Over 200 U-Pb ages have been generated in and adjacent to the Coast Mountains of central British Columbia as part of the ACCRETE and BATHOLITHS projects and a variety of GSC and BCGS projects. U-Pb ages of zircons demonstrate that this portion of the Coast Mountains batholith has a long (>150 m.y.) and complex history, and comprises four different tectonic entities: (1) A western magmatic arc, active 160-140 Ma and 120-90 Ma, was built on the Alexander and Wrangellia terranes. As recognized by van der Heyden (1992), this magmatism migrated progressively eastward. Most intrusions are quartz diorite to granodiorite in composition. (2) An eastern magmatic arc, active nearly continuously from 200 Ma to 110 Ma, was built on the Yukon-Tanana and western Stikine terranes. There is no sign of a 140-120 Ma magmatic gap, or migration of magmatism, and plutons range in composition from diorite to granite. (3) A younger magmatic arc, active 90 to 60 Ma, was built across both of the older arc systems as well as the mid-Cretaceous thrust belt and high-grade metamorphic belt that separates the two arc systems. Very high (10-200) U/Th values of zircons record the presence of abundant metamorphic fluids during magma generation/emplacement/crystallization between 90 and 80 Ma. (4) The youngest phase of widespread magmatism, 60 to 48 Ma, was restricted to the eastern portion of the batholith. Titanite ages of 58-48 Ma demonstrate that rapid exhumation and cooling of axial portions of the batholith occurred during this same period. This magmatism and exhumation/cooling occurred synchronous with extensional motion on the Coast shear zone and Shames/Exchamsiks shear zones. There is no evidence of magmatism of this age west of the Coast shear zone, but plutons of Eocene age occur far to the east of the Coast Mountains within the Stikine terrane. High U/Th values in 62-55 Ma zircons record the generation of abundant metamorphic fluids during this phase of regional extension and uplift.

These magmatic patterns are consistent with previous models in which the western arc terrane (and Alexander and Wrangellia terranes) were juxtaposed against the inboard arc system (and Stikine and Yukon-Tanana terranes) beginning in mid-Cretaceous time. The lack of a collapsed convergent margin assemblage between the two Jurassic-Early Cretaceous arc systems remains a fundamental problem in northern Cordilleran geology.