A PRESERVED OCEANIC CORE COMPLEX IN THE CACHE CREEK TERRANE, NORTHERN CORDILLERA, CANADA
The Carboniferous-Jurassic Cache Creek terrane of the Canadian Cordillera comprises oceanic mantle rocks, submarine lavas, chert, limestone and rare plutonic rocks. The Cache Creek is bounded by the Quesnel and Stikine arc terranes, which were accreted to North America and deformed by the Middle Jurassic. Large, thrust-bounded ophiolitic massifs expose well-preserved mantle/crust transitions in northern British Columbia (BC) and southern Yukon. In the Atlin area (BC), mantle harzburgite and minor dunite are in structural contact with a crustal sequence comprising slivers (~400 m) of gabbro, and a composite volcano-sedimentary sequence dominated by depleted arc tholeiites, calc-alkaline lavas and dykes, and rare alkali basalts. The SW-dipping mantle-crust contact is a ~100 m wide chrysotile/antigorite serpentinite shear zone that contains cm- to m-scale fragments of mantle and crustal rocks. These are interpreted to be thrust-related structures as asymmetric fragments and C-S structures show reverse motions. Possible syn-oceanic structure are preserved as small structural windows exposing the mantle rocks beneath the crust on Union Mt. An excellent example of an extensional shear contact occurs near Squanga Lake (Yukon), where a shallowly SE-dipping, serpentinite shear zone (SLSZ) separates a dominantly lherzolitic mantle from brecciated crustal rocks, including ultramafic cumulates, massive gabbro, basalt, chert and limestone. Gabbro dykes are injected along the SLSZ, but locally are affected by shearing, suggesting syn-deformation magmatism along the mantle/crust contact. Titanite is associated with rodingite alteration in gabbro. U-Pb dating analysis of zircon (age of crystallisation) and titanite (age of alteration) from gabbro altered along the SLSZ, suggest coeval crystallisation of gabbro and alteration along the SLSZ, at ~249 Ma. The occurrence of extensional low-angle shear-zones between mantle and crust, coeval with magmatism and alteration, as well as the scarcity of lower crustal rocks, the lack of lateral stratigraphic continuity, and the brecciated aspect of many Cache Creek supracrustal sequences, suggest common structural exhumation of the oceanic mantle prior to its continental accretion.