LATE-PALEOZOIC PRE-ACCRETION PALEOGEOGRAPHY OF THE WESTERN MARGIN OF THE ANCESTRAL NORTH AMERICAN CRATON: AN EXAMPLE FROM THE KLINKIT GROUP, NORTHERN BRITISH COLUMBIA AND SOUTHERN YUKON

The study of Late Paleozoic volcano-sedimentary sequences, lying between Ancestral North America to the east and the accreted terranes to the west, provides insights on the pre-accretion paleogeography of the western margin of the North American craton in Late Paleozoic time.

Detailed stratigraphic and geochemical study of the Mississippian-to-Permian volcano-sedimentary Klinkit Group of northern British Columbia and southern Yukon reveal that it is broadly coeval and compositionally similar to other volcano-sedimentary sequences of the Omineca Belt in northern Canadian Cordillera.

The Klinkit Group is characterized in its lowermost part of carbonates of Mississippian to Early Pennsylvanian age, the English Creek and Screw Creek limestones, and by the upper Butsih and Mount McCleary formations, predominantly volcaniclastic rocks with minor clastic intervals. The volcaniclastic rocks are calc-alkaline and moderately fractionated. They indicate a volcanic-arc setting ((La/Yb)_n=2.77-4.73), with no involvement of the crust in their genesis (e_Nd=+6.7 to +7.4). The Mount McCleary Formation also contains a few metres of alkali-basalt flows. Alkali basalts in the Mount McCleary Formation ((La/Yb))_n=12.5-17.8) suggest an intra-arc rifting event.

The volcano-sedimentary Lay Range Assemblage of north-central British Columbia presents very similar stratigraphy and geochemistry than the Klinkit Group, suggesting similar source compositions and tectonic history. It is interpreted as the basement of the Mesozoic Quesnel arc. However, the volcano-sedimentary Little Salmon succession and Boswell and Semenof formations of central Yukon have similar characteristics, and are part of the pericratonic Yukon-Tanana terrane. The similarity suggests that the basement of the Mesozoic arc of the Quesnel terrane is the pericratonic Yukon-Tanana terrane or its southern equivalent, and therefore the Quesnel terrane includes a pericratonic pre-arc history.

The described volcano-sedimentary sequences are probably all pieces of a long-lived, Late Paleozoic arc system that was dismembered prior to its accretion onto ancestral North America.