Geochemistry of the northern Cache Creek terrane and implications for accretionary processes in the Canadian Cordillera

Abstract

The northern Cache Creek terrane in the Canadian Cordillera includes a subduction complex that records the existence of a late Paleozoic – Mesozoic ocean basin and provides an opportunity to assess accretionary processes that involve the transfer of material from a subducting plate to an upper plate. Lithogeochemical data from basaltic rocks indicate that the northern Cache Creek terrane is dominated by two different petrogenetic components: (1) a dominant suite of subalkaline intrusive and extrusive rocks mostly of arc affinity and (2) a volumetrically less significant suite of alkaline volcanic rocks of within-plate affinity. The subalkaline intrusive and extrusive rocks constitute a section of oceanic lithosphere that is interpreted to have occupied a fore-arc position during the Late Triassic and Early Jurassic before it was accreted during collisional orogenesis in the Middle Jurassic. Alkaline volcanic rocks in the northern Cache Creek terrane are stratigraphically associated with carbonate strata that contain Tethyan fauna that are exotic with respect to the rest of North America; together, they are interpreted as remnants of oceanic seamounts and (or) plateaux. The volcanic rocks are a minor component of the carbonate stratigraphy, and it appears that the majority of the volcanic basement was either subducted completely at the convergent margin or underplated at greater depth in the subduction zone. In summary, accretion in the northern Canadian Cordillera occurred principally by the accretion of island arcs and emplacement of fore-arc ophiolites during collisional orogenesis. The transfer of oceanic sediments and the upper portions of oceanic seamounts from the subducting plate to an accretionary margin accounts for only small volumes of growth of the upper plate.