AGE OF ‘THE WILLISON BAY PLUTON AND OVERLYING SEDIMENTS: IMPLICATIONS FOR THE CARNIAN STAGE BOUNDARk

A nonconformity between the granodioritic Willison Bay pluton and Triassic sediment ins well exposed near the south end of At:iin Lake, northwestern British Columbia (NTS 104M/8; Ecigues 1 and 2). Earliest comprehensive mapping in the region (Christie, 1957) recognized this nonconformity and attributed a possible pre-Pennian age to the Willison Bay Pluton on the basis of overlying sediments that cemtain clasts of the pluton, which in hxn are capped by a l.imestone layer, believed at the time to be of Permian age. ‘Bulbnan (1979) considered the limestone to be Upper Triassic (Norian or Camian) in age based upon several fossil collections identified by John Wells (Cornell University) and himself. He correlated the limestone with the Sinwa Formation along southwest structural trend in the Tulsequah area. Subsequent mapping in the Atlin Lake (Mihalynuk el al., 1990, 1996) and Tulsequah :regions (Mihalynuk ef al., 1994, 1995, 1996) confmt:d this correlation and led to the recognition of Camian fossils within strata behveen Sinwa Formation and the eroded top of the Willison Bay pluton (Figures Z! and 3). Bultman obtained K-AI age determinations on hornblende from two samples of the Willison Bay pluton which gave 180 +3 and 222 i5 Ma (recalculated from 175 h-3 and 215 *5 Ma, using revised decay constants of Steiger and JBger, 1977). Argon loss during alteration of hornblende in th: less fresh sample, probably accounts for the younger age. The older age is consistent with Norian fossils in the overlying strata based on the Norian age limits of Harland et al (1990, 223.4 to 209.5; Figure 3), but is too young to be overlain by Carnian strata, unless the error limits for Camian stage boundaries are considered (235 *‘s/.,g to 223,4 *‘/., Ma; Harland et al. 1990, Table 5.4). New data presented here provides better control on the age of the pluton. Absolute ages for Late Triassic stage boundaries are relatively unconstrained. They are lnterpolated from bracketing data sets, and as such, have large uncertainties. Imprecise Upper Triassic stage boundaries have been a source of confwion for mineral explorationists working in rocks from this important metallogenic time frame. Conflicting isotopic and biochronologic w determinations are recognized from geological investigations of the Guichon Creek Batholith (McMillan, Figure 1. Location of the study area, southern A tlin Lake