Geochronology of mid-Cretaceous to Eocene magmatism, Babine porphyry copper district, central British Columbia

New U/Pb and Ar-40/Ar-39 isotopic dating in the Babine porphyry copper dist rict of central British Columbia documents three distinct magmatic events a t 107-104, 85-78, and 54-50 Ma. The earliest event involved emplacement of rhyolite domes into submarine volcanic rocks of the Rocky Ridge Formation. The rhyolite domes and related dacitic to basaltic volcanic rocks gave a U- Pb age of 107.9 +/- 0.2 Ma and an Ar-40/Ar-39 age of 104.8 +/- 1.2 Ma. The rhyolites, which were previously mapped as Eocene, are reinterpreted to be part of a previously unrecognized mid-Cretaceous cauldron subsidence comple x. The regionally extensive Late Cretaceous magmatic event is also recogniz ed in the Babine district and is represented by Ar-40/Ar-39 ages of 85.2 +/ - 2.8 and 78.3 +/- 0.8 Ma on two Bulkley intrusions, one of which has assoc iated porphyry copper mineralization. The final magmatic event is the most widespread and involved emplacement of the Babine intrusions and formation of numerous porphyry copper deposits including the Bell and Granisle past p roducers. Twenty-one new Ar-40/Ar-39 isotopic ages for these intrusions and coeval andesites of the Newman Formation have a narrow range from 53.6 +/- 0.9 to 49.9 +/- 0.6 Ma, whereas previous K-Ar isotopic dating had a possib le range of 15 Ma. The mid-Cretaceous, Late Cretaceous, and Eocene magmatic suites in the Babine district are interpreted to be part of a long-lived v olcano-plutonic complex that was the site of periodic magmatism and porphyr y copper mineralization over a 60 Ma time period. This complex may have evo lved within a zone of extension (pull-apart basins) situated between dextra l strike-slip faults that were active during periods of rapid oblique plate convergence.