OKANAGAN GNEISSES: INSIGHTS INTO PROTOLITH, GNEISS FORMATION, AND EOCENE EXTENSION

The Okanagan Valley Shear Zone (OVSZ) delineates the western margin of a large Eocene metamorphic core complex which juxtaposes sillimanite-grade and non-metamorphosed rocks across the Okanagan Valley. The OVSZ is characterized by a series of strongly prolate, transposed gneisses (the Okanagan gneisses) and associated intermediate and felsic intrusions in the footwall. Most regional-scale stratigraphic and isotopic studies have concluded that the Okanagan gneisses are correlative with gneisses in the Monashee complex, which represent Ancestral North American crust. If this is correct, then the Okanagan gneisses may represent the most westerly (outboard) segment of exposed North America basement in the southern Cordillera, or the remnants of a basin filled with sediment derived from it.

Very little is known in detail about the Okanagan gneisses, for example: (1) the proportion of para- to orthogneiss; (2) the age of its protolith(s); (3) the age of gneiss formation; and (4) how these factors relate to the formation of the adjacent OVSZ. Using U-Pb zircon ages (LA-ICP-MS and SHRIMP) from a suite of samples that include gneissic “basement” and variably deformed and cross-cutting felsic intrusions, we provide new insight into the timing of gneiss formation and duration of extension. By comparison, age constraints for the Monashee basement orthogneisses include ca. 2.08 – 1.86 Ga igneous zircon; in contrast, the Okanagan gneisses include magmatic zircon as young as 160 Ma, with 55-50 Ma rims that may be derived from anatectic melts. Few zircon grains in the gneiss and associated intrusions consist of >1.6 Ga inherited cores. The main pulses of zircon growth in the orthogneisses appear to be 165-155, 110-105, 95-90 and 60-50 Ma. Our preferred working model is that the Okanagan gneisses are dominantly paragneisses (meta-pelites and meta-psammites) with multiple amphibolite sheets (probably originally basic intrusions). The age of the paragneisses is unresolved, but they may represent Lower Paleozoic miogeoclinal and pericratonic rocks that include the Paleozoic basement to Quesnel terrane. Intrusion of basic sills at 160 Ma preceded metamorphism and gneiss formation during Mesozoic contraction associated with the Cordilleran orogeny; followed by exhumation and anatectic melting in the Eocene.