Age Constraints for Emplacement of the Northern Cache Creek Terrane and Implications of Blueschist Metamorphism

Ophiolitic and accretionary units and associated blueschists of the Cache Creek terrane constitute the most complete evidence of a coherent subduction zone within the Canadian Cordillera. The rocks occupy a central position in the Intermontane Superterrane, a complex of arcs that includes most of the accreted crust in British Columbia (Figure 1). The Cache Creek terrane contains exotic fossil fauna interpreted to have originated in the equatorial Tethyan realm. In contrast, coeval rocks in adjacent terranes contain fossils endemic to ancestral North America. How this arrangement of terranes can be explained has been the focus of much speculation (see Mihalynuk et al., 1994). It is clear that the Cache Creek terrane plays a central role. Data constraining its history bear on the geodynamic evolution of the Cordillera as a whole. One of the most complete crustal sections is preserved in the part of the northern Cache Creek terrane known as the Atlin complex (Figure 1; Mihalynuk, 1999). Current understanding of the regional stratigraphy and fossil age range derives from the work of Monger (1969, 1975). Northern parts of the Atlin complex have been mapped by Hart and Radloff (1990), Hart (1997), and Gordey and Stevens (1994). Studies with a topical focus include those on the origin of enclosed ultramafic rocks (Terry, 1977), their geochemical character, and their recognition as mantle tectonites (Ash, 1994). Isotopic characterization and detailed mapping in the northern Atlin complex by Jackson (1992) revealed that sandstones were derived from the Stikine terrane. Microfossil biostratigraphic studies by Cordey et al. (1991, and F. Cordey unpublished data, 1990) showed that the radiolarian-bearing strata range in age from Permian to Early Jurassic. In contrast, the southern portion of the Atlin complex has been less studied. Recent studies include a regional geological compilation by Gabrielse (1998) and detailed mapping of volcanic rocks in the French Ranges near Dease Lake by Mihalynuk and Cordey (1997). Data reported here complement work by Mihalynuk and Cordey (1997) in the French Ranges and Teslin Lake area, and by Mihalynuk et al. (1995a,b) in the Tulsequah area.