THE KNOB HILL COMPLEX - A PALEOZOIC SUPRA-SUBDUCTION ZONE OPHIOLITE IN SE BC?

The Knob Hill Complex (KHC), in the Greenwood area, is a disrupted ophiolite. It is composed of serpentinites, gabbro, sheeted dykes, basalt flows and chert-rich sediments. Stratigraphic relationships have been disrupted by thrusting, however, north of Rock Creek, gabbro and serpentinite pass northwards, and upwards, into greenstones, mixed greenstone and chert and finally into cherts and argillites.

The age of the KHC is still poorly constrained. Zircons recovered from pegmatitic gabbro suggest an age of 380-385 Ma in agreement with a Late Devonian conodont determination from limestone interbedded with volcanics. However, a chert higher in the mixed greenstone-chert section yields good Late Pennsylvanian age radiolaria (~290-295 Ma). 80-100 Ma is too long an age range for such a small piece of oceanic crust and suggests a tectonic history that may involve several spreading events.

Analyses of basalts from the KHC show the presence of at least 3 magma suites:- a) a N-MORB suite; b) an E-MORB suite and c) a significant IAT suite. There is no apparent spatial nor stratigraphic control on the occurrence of the 3 suites. The chemistry suggests a complex history for the Knob Hill, perhaps in a fore-arc basin setting.

Throughout the Boundary District, the KHC is sandwiched by thrusts between more complexly deformed quartzite-schist-greenstone sequences – the Anarchist to the south and Kobau to the north. In the Greenwood area, the Kobau is represented by quartzite and schist of the Eholt Creek area. Despite Tertiary extensional faults disrupting the thrust sheets, the KHC can be traced westwards from Greenwood into the Rock Creek area. Similarly, the Anarchist can be traced into the Bridesville area before passing westwards into the granitic gneisses of the Okanagan batholith.

West of the Okanagan Fault, the Palmer Mountain Greenstone in Washington comprises mafic volcanics, gabbro and serpentinite that can be correlated with the KHC. These are thrust over “Anarchist Group” (including significant Triassic Brooklyn Formation) and, in turn, are structurally overlain by the Kobau.

It is still unclear how the Kobau Group and Anarchist Schist correlate with each other. They may be the same package structurally repeated or represent opposite sides of a basin (the KHC) that is now closed and telescoped.