GEOCHRONOLOGIC CONSTRAINTS ON THE EARLY PALEOZOIC EVOLUTION OF THE SOUTHERN ALEXANDER TERRANE, NORTHWESTERN COASTAL BRITISH COLUMBIA
The arc system and its deformed equivalents (Ogden Channel Complex) are succeeded by an Early Devonian complex basinal assemblage called the Mathieson Channel (MC) unit, which is correlative with the Karheen Formation of SE AK. The MC unit is characterized by shallow water sandstone, siltstone, argillite and lesser conglomerate, extensive thin carbonate beds, and lesser volcanic strata. Its detrital zircon signature is dominated by 400-460 Ma grains, with subordinate grains of Descon age. Most of these grains were derived from Late Ordovician to Silurian plutons, which, with the exception of the areally restricted Ogden Channel Complex, are not exposed in the southern AT. A few samples with significant Precambrian populations and a quartzite cobble conglomerate at Jorkins Point are inferred to represent basement sources in the MC basin. The quartzite conglomerate displays a broad, distinctive Proterozoic detrital spectra (ca 900-2000 Ma).
Stratigraphic, paleontologic, structural and geochronologic similarities between the AT and the Scandanavian Caleondonide system suggests a northern Caledonide origin for the AT and subsequent tectonic transport across a paleo-Arctic basin.