Eocambrian granite clasts in southern British Columbia shed light on Cordilleran hinterland crust

Abstract

The Spa Creek assemblage is a distinctive thin pericratonic succession that crosses the Okanagan Valley in the hinterland of the southern Cordilleran Orogen in Canada. The succession was ductilely deformed and metamorphosed before deposition of overlying Triassic dark metaclastic strata. A metaconglomerate within the succession, locally composed of more than 90% biotite granite clasts, yielded five fractions of euhedral zircon that define a precise U–Pb upper intercept of 555.6 ± 2.5 Ma, inferred to be the age of a nearby pluton. Other clasts in the metaconglomerate are generally more abundant, consisting of quartzite, amphibole schist, chlorite schist, sericite schist, biotite schist, and quartz–feldspar porphyry. They are likely host rocks of the pluton and, if so, are Late Proterozoic or older. The granite is interpreted as a terminal product of the Eocambrian rifting that preceded Paleozoic miogeoclinal sedimentation farther inboard. The continuity of pericratonic rocks west of the miogeocline and the occurrence of Proterozoic cratonic rocks at the surface west of the Okanagan Valley show that the ancient continental margin extends into a region where most of the crustal lithosphere was until now thought to consist of accreted Phanerozoic arc and accretionary complexes.