Geochronology and thermal history of the Coast Plutonic Complex, near Prince Rupert, British Columbia

Abstract

Fission track, K–Ar, and Rb–Sr mineral dates for the Ecstall and Quottoon plutons were determined to resolve the different cooling histories indicated by Rb–Sr whole-rock and mineral isochrons and published K–Ar dates.Comparison of mineral dates with previously assigned closure temperatures for the various isotopic chronometers has allowed temperature–time plots for the thermal history of individual samples and sample suites to be constructed.The Quottoon pluton, emplaced approximately 51 Ma ago, cooled rapidly during initial uplift that ended ~46 Ma ago with the present-day land surface at a depth of ~4 km. A second episode of uplift at an average rate of 0.05 cm year⁻¹ (measured in Kasiks pluton that lies immediately to the east) began in the late Miocene. Cooling of the Ecstall pluton, following emplacement approximately 80 Ma ago, was disrupted by a thermal event approximately 65 Ma ago. Biotite in the Ecstall pluton appears to have incorporated extraneous argon during this event so that the calculated K–Ar dates lie between the time of emplacement and time of cooling through the closure temperature for Ar in biotite.The cooling curves and observed dates yield estimates for closure temperatures, at an intermediate cooling rate, for Ar in plagioclase and K-feldspar of ~260 °C and ~160 °C, respectively. For fission tracks in epidote the closure temperature estimate is ~240 °C.