NEW CONSTRAINTS FROM LA-ICP-MS DETRITAL ZIRCON U-PB DATA ON NEOPROTEROZOIC DEPOSITIONAL AGES AND CRETACEOUS METAMORPHISM IN THE CARIBOO ALP IN SOUTHEASTERN BRITISH COLUMBIA

The Thor-Odin dome is a structural and tectonothermal culmination comprising Paleoproterozoic Laurentian “basement” orthogneiss and paragneiss, and Proterozoic to Paleozoic predominantly supracrustal metasedimentary “cover” rocks. The Thor-Odin cover sequence is attenuated and complexly infolded with the basement, and Cariboo Alp, a high strain zone on the southwestern flank of the dome, has traditionally been interpreted as intercalated basement and cover rocks. This study examined detrital zircons, using LA-ICP-MS dating of cores and rims, in three migmatitic paragneisses at Cariboo Alp. Based on the youngest zircon date in each of the samples, the rocks are younger than ca. 1070, 620 and 600 Ma, respectively. All three samples have major detrital zircon age populations between ~1.9 and 1.65 Ga and ~1.3 and 1.1 Ga, and minor populations of > 3.0 Ga and 2.8 – 2.2 Ga consistent with a < 1.1 Ga quartzite previously dated at Cariboo Alp. One sample also contains a major detrital zircon population between ~750 and 700 Ma, and a minor population between 700 Ma and 500 Ma; another sample has a minor population between 700 and 600 Ma. The Cariboo Alp rocks were therefore derived from sediment sources similar to the rift-related strata of the Windermere Supergroup, specifically the Horsethief Creek and Kaza Groups. These data suggest that the highly strained rocks at Cariboo Alp represent intercalated units that have maximum deposition ages that are much younger than the Paleoproterozoic Thor-Odin dome basement rocks. These rocks were juxtaposed over the Thor-Odin basement-cored dome during late stages of the Cordilleran orogen and do not represent basement or cover rocks deposited on Paleoproterozoic basement. Based on the U-Pb ages of metamorphic zircon overgrowths, all three samples have Late Cretaceous age populations between ~80 Ma and 60 Ma, interpreted as a period of protracted metamorphism consistent with structural data and geochronology from this domain. There are minor populations between 120 Ma and 90 Ma reflecting Cretaceous metamorphism, as well as a population of rims at ~250 Ma, evidence for an earlier period of metamorphism during the initiation of the Cordilleran collision on the western margin of Laurentia.