Paper No. 8
Presentation Time: 10:00 AM

GEOCHRONOLOGIC CONSTRAINTS ON THE EARLY PALEOZOIC EVOLUTION OF THE SOUTHERN ALEXANDER TERRANE, NORTHWESTERN COASTAL BRITISH COLUMBIA


MAHONEY, J. Brian1, NELSON, JoAnne2, GEHRELS, G.E.3, PECHA, Mark3, DIAKOW, Larry2, KARL, Susan M.4, VAN STAAL, C.R.5, ANGEN, Joel6 and TOCHILIN, Clare7, (1)Department. of Geology, University of Wisconsin Eau Claire, Eau Claire, WI 54702, (2)B.C. Geological Survey Branch, Box 9333, Stn Prov Govt, Victoria, BC V8W 9N3, Canada, (3)Department of Geosciences, University of Arizona, Tucson, AZ 85721, (4)USGS, 4210 University Dr, Anchorage, AK 99508-4626, (5)Geological Survey of Canada, 625 Robson Street, Vancouver, BC V6B 5J3, Canada, (6)University of Waterloo, Waterloo, ON N2L 3G1, Canada, (7)Department of Geosciences, University of Arizona, Tucson, AZ 85719, MAHONEJ@uwec.edu

The Alexander terrane (AT) is one of the few truly exotic Cordilleran terranes, with no known linkage to Laurentia until Jurassic time. Recent bedrock geologic mapping in coastal northwestern British Columbia, including stratigraphic, structural, geochemical, and most importantly, geochronologic studies, has extended and enhanced understanding of the Paleozoic geologic evolution of the far southern AT. The oldest rocks recognized in the AT in BC are calc-alkaline andesitic to rhyolitic volcanic rocks, hypabyssal intrusions and associated volcaniclastic sediments of the Ordovician Descon Formation (ca. 460-520 Ma), which represent a juvenile volcanic arc system. The western edge of the Descon arc system was deformed during the Late Silurian-Early Devonian Klakas orogeny, which involved deep burial and metamorphism of Descon-age strata (protolith age ca. 490 Ma) and plutonic bodies as young as ca. 413 Ma. Deformed strata are cut by post-deformational Early Devonian plutons (ca. 410 Ma).

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.