Paper No. 13
Presentation Time: 12:00 PM
PALEOZOIC EVOLUTION OF THE SOUTHERN ALEXANDER TERRANE, NORTHWEST BRITISH COLUMBIA
MAHONEY, J. Brian1, NELSON, JoAnne
2, GEHRELS, George E.
3, KARL, Susan M.
4, DIAKOW, Larry
2 and ANGEN, Joel
5, (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)University of Waterloo, Waterloo, ON N2L 3G1, Canada, MAHONEJ@uwec.edu
Bedrock mapping and geochronology, in coastal NW British Columbia constrain the Paleozoic evolution of the southern Alexander Terrane (AT). The lowest stratigraphic unit is the early to middle Ordovician Descon Formation (Moira Sound unit?) (ca. 460-520 Ma), which consists of andesitic breccia, tuff, felsic volcanic rocks, volcanogenic sediments and associated hypabyssal rocks produced by island arc magmatism. It is overlain by two stratigraphically distinct Devonian clastic successions. The Karheen Formation contains lithic feldspathic arenite and plutonic cobble conglomerate with shallow water sedimentary structures; and pillow basalt. The Mathieson Channel Formation contains lithic feldspathic sandstone, matrix-supported polymict conglomerate, abundant carbonate and basalt. Detrital zircon populations (ca 400-460 Ma), with a peak at 423 Ma, overlap. The two sequences were deposited in different parts of a basin unconformably overlying the Descon Formation. Descon-age zircons within the sequence are minor, and the primary source seems to have been late Ordovician-Silurian plutonic rocks that are not recognized in the region.
The Banks Island assemblage, west of the AT, contains rhythmically interbedded thin bedded quartzite and carbonate with a detrital zircon signature of ca. 410-420 Ma and subordinate Precambrian peaks. It may correlate with thin bedded successions in the Mathieson Channel unit. Enigmatic quartzite cobble conglomerate occurs in one locality along the western edge of the AT, and displays a broad, enigmatic Proterozoic detrital spectra (ca 900-2000 Ma). The detrital zircon spectra of thin bedded strata adjacent to the Jorkins Point conglomerate contains a Descon-age peak that suggests a potential linkage to the AT.
Metamorphic complexes on Porcher Island occur along the western edge of typical Alexander stratified rocks, east of the Banks Island outcrop belt. Evidence for Devonian tectonism is shown by deep burial and metamorphism of both Descon-age strata (protolith age ca. 490 Ma) and plutonic bodies as young as ca 413 Ma, cut by post-deformational plutonism (ca. 410 Ma). This may represent a mid-Paleozoic accretionary boundary between the AT and a pericratonic fragment, while the Karheen, Mathieson Channel and Banks Island strata represent the associated clastic overlaps.
