FISSION-TRACK AGES OF DETRITAL ZIRCONS FROM CRETACEOUS STRATA, SOUTHERN BRITISH-COLUMBIA - IMPLICATIONS FOR THE BAJA BC HYPOTHESIS

The Methow/Tyaughton basin contains a thick sequence of mid-Cretaceous (Albian-Cenomanian) marine clastic sediments, which have been interpr eted as synorogenic fill deposited during collision and accretion of t he Insular superterrane at the North American margin. Three distinct, regionally extensive petrofacies have been recognized in this sequence : (1) a west-derived volcanic petrofacies, (2) a cherty petrofacies de rived from a local intrabasinal high, and (3) an east-derived arkosic petrofacies. Grain ages for detital zircon are used here to identify p rovenance. The fission track (FT) method is used to date seven sandsto ne samples, with approximately 45 grains ages per sample. Grain age di stributions show marked differences between petrofacies and close simi larities within a petrofacies. All petrofacies are dominated by a youn g population of grain ages (113 to 89 Ma), statistically indistinguish able from the depositional ages of the samples. This population is att ributed to contemporaneous volcanism in the source region. The age of older populations varies among the petrofacies, indicating that each p etrofacies was derived from different source terrains. Most important, all Arkosic samples have nearly identical grain age distributions, ev en though they come from widely separated parts of the Methow/Tyaughto n basin. This evidence suggests that the Arkosic petrofacies is an ove rlap sequence that ties together disparate basement terranes, includin g the Bridge River, Cadwallader, and Methow terranes, by the Albian (a pproximately 100 Ma). The source of the Arkosic petrofacies provides a n important constraint on the mid-Cretaceous location of these outboar d terranes. This petrofacies increases in thickness and coarseness to the east, suggesting a source inboard of the Methow/Tyaughton basin. T he detrital composition of the sediment indicates that high-grade meta morphic rocks and S-type plutonic rocks were present in the source. Gi ven the rapid onset of arkosic sedimentation in the Albian, we infer t hat this petrofacies were derived from newly uplifted metamorphic and plutonic rocks. We consider two interpretations for this arkosic sourc e. An in situ interpretation would derive these sediments from metamor phic and plutonic rocks of the Omineca Crystalline belt, which present ly lies to the east of the basin. The onset of arkosic sedimentation d uring the Albian is compatible with evidence from the Alberta foreland basin that indicates Albian uplift and denudation of the Omineca Crys talline belt. However, Nd/Sm data from the Arkosic petrofacies (Barfod and Nelson, 1992) indicate a fairly primitive source, which is not co nsistent with the presence of Precambrian rocks in the Omineca belt. T he second interpretation accounts for paleomagnetic data that indicate s the outboard terranes of western British Columbia have been transpor ted 3000 km northward during the latest Cretaceous and early Tertiary. In this case, the arkosic sediments would most likely have been deriv ed from an inboard continental setting near the present-day latitude o f central Mexico. A possible source might have been the western half o f the Peninsular Ranges batholith, which is characterized by plutonic rocks with relatively primitive isotopic characteristics.