AGE OF EMPLACEMENT AND BASEMENT CHARACTER OF THE CACHE CREEK TERRANE AS CONSTRAINED BY NEW ISOTOPIC AND GEOCHEMICAL DATA

New U - Pb and Rb - Sr isotopic and major and trace element geochemical data are reported for Late Triassic to Eocene granite bodies that intrude the Cache Creek and Stikine terranes in the Atlin - Bennett area of northwestern British Columbia. The U - Pb isotopic age data help constrain closure of the Cache Creek ocean and obduction of the Cache Creek terrane to before 172 Ma (Middle Jurassic). Low Sr-87/Sr-86 initial ratios (0.7037-0.7046) and lack of evidence for inheritance in zircons suggest that rocks underlying the Cache Creek terrane are largely primitive in nature, with derivation of intrusive rocks predominantly from unevolved sources. A genetic link between Late Triassic granitic plutons and Stuhini arc volcanics of Stikinia is supported by trace and rare-earth element data which indicate generation of magma in a volcanic-arc setting. Geochemical patterns for postaccretion Middle Jurassic and Late Cretaceous to early Tertiary age plutons are similar to those of the Late Triassic, indicating that Middle Jurassic and younger plutons could be derived from the same source area as the Late Triassic plutons. These data do not support recent theories proposing that the Cache Creek and Stikine terranes are klippe overlying a thick section of deformed, Proterozoic and lower Paleozoic continental-margin strata and attenuated cratonal basement. Rather, they are consistent with models in which remnants of Cache Creek ocean basin are placed over the Stikine arc in the Early to Middle Jurassic. Both terranes in turn overlie mainly late Paleozoic to early Mesozoic juvenile crustal material or the upper mantle.