THE SLIDE MOUNTAIN OCEAN FROM INCEPTION TO DESTRUCTION: DETRITAL ZIRCON AND OTHER EVIDENCE FROM THE SYLVESTER ALLOCHTHON

The Devonian-Triassic Slide Mountain terrane, innermost of the allochthonous terranes in Canada, is of key significance to Cordilleran paleogeographic models. In the Sylvester allochthon (northern BC), it consists of an imbricated stack of oceanic supracrustal and lower crust-upper mantle slices. Lower Mississippian supracrustal units comprise chert, argillite, chert-quartz sandstone, and basalt. Overlying late Paleozoic units consist of chert, argillite, basalt and minor siliciclastic material, suggesting an evolution of sediment sources from proximal to distal. A detrital zircon sample of Mississippian grit shows a dominant Paleoproterozoic (ca 1.8 Ga; 61 grains) and lesser Archean (ca 2.6 Ga; 28 grains) peak, with small groups in the ranges 676-699 Ma (4 grains) and 340-354 Ma (3 grains) . The main age peaks match signatures of siliciclastic sequences and basement provinces of northwestern Laurentia. The Neoproterozoic zircons relect igneous activity during rifting of the western Laurentian margin, such as the ca. 690 Ma Gataga volcanics in the northern Kechika trough. The 340-354 grains reflect arc activity in the Yukon-Tanana terrane, a pericratonic terrane that now lies outboard of SMT. The mix of siliclastic debris with MORB basalts, and the combination of northwestern Laurentian and outboard arc sources for detrital zircons, are consistent with models that require early opening of the SM ocean as a back-arc basin to YTT in Early Mississippian time.

Late Paleozoic arc-related Quesnellian strata form a separate set of of thrust imbricates structurally above those of SMT. In them, giant parafusulina and brachiopods are similar to those in the McCloud terranes, which have been interpreted to lie several thousand kilometres west of the Laurentian margin - an estimate for the maximum width of the SM ocean basin. By Middle Triassic time, it had re-closed against northwestern Laurentia, as shown by a regional Triassic overlap assemblage. Quartz clastics of the Triassic Table Mountain succession, which unconformably overlies imbricated SMT units, contain detrital zircon populations with a strong Laurentian signatures but also characteristic signals from YTT, Quesnellia and more exotic, Arctic-derived terranes containing Caledonian and Grenvillian debris.