The Grand Forks complex (GFC) is a metamorphic core complex within the composite Shuswap complex in the southern Omineca belt of the Canadian Cordillera. It is juxtaposed against the surrounding lo...

Thermochronological constraints on the Eocene exhumation of the Grand Forks complex, British Columbia, based on 40Ar/39Ar and apatite fission track geochronology

Medford (1975) presents the interesting suggestion that cooling of the Shuswap gneisses, just east of Okanagan Lake, British Columbia, occurred at a rate in excess of 25 "C1m.y. The argument for such a high cooling rate is based on the differences observed between the mean of K-Ar ages on biotite, hornblende, and whole rock samples and the means of fission track ages on sphene and apatite samples from the same area. Before this cooling rate can be accepted, a closer look at some of the assumptions involved and at the statistical significance of the age differences is warranted. The first assumption that we question is that the closure temperature of the K-Ar clock is about 150 "C. Even if this figure were correct for one mineral, it would be considerably different for most other minerals. In fact, according to Damon (1968), in order to have the same fractional argon loss as a biotite heated to 155 "C for a given time, a hornblende would have to be heated to 325 "C for the same period of time. Thus the author's contention, that the average of K-Ar ages from biotites, hornblendes, and whole rocks is the time at which the rocks cooled to below 150 "C, is incorrect. The second assumption that we question is that "track retention (for a period of lo5 to lo6 y) is essentially complete in apatite at temperatures below about 75 "C." In fact, Naeser and Faul(1969) give a figure of 50 "C over a period of lo6 y for essentially complete track retention. Obviously, considerable uncertainty exists since any estimate of track retention temperature requires extrapolation over seven orders of magnitude! Our most serious criticism is that the author has not attempted to assess the statistical significance of the differences between the means used in his cooling rate calculations. For example, the mean of the 14 fission track ages

K–Ar and fission track geochronometry of an Eocene thermal event in the Kettle River (west half) map area, southern British Columbia: Discussion

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K–Ar and fission track geochronometry of an Eocene thermal event in the Kettle River (west half) map area, southern British Columbia: Discussion