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U-PB geochronometry and regional ecology of the southern Okanagan Valley, British Columbia : the western boundary of a metamorphic core complex Parkinson, David Lamon


The Okanagan Valley is the boundary between the Okanagan Metamorphic and Plutonic Complex of the Omenica Belt to the east and the Intermontane Belt to the west. The Okanagan Metamorphic and Plutonic Complex consists of greenschist to amphibolite grade paragneiss and large areas of massive, gneissic, and mylonitic granitic rock. The Intermontane Belt consists of tectonically scrambled late Paleozoic to Triassic eugeosynclinal rocks, intruded by large Jurassic plutons and locally by plutons of mid-Cretaceous age. These are overlain by Eocene non-marine volcanic and sedimentary rocks, capped by fanglomerate breccias and gravity slide megabreccias. The thesis area contains all of these elements. In particular, the mid-Jurassic Oliver pluton is composed of three separate intrusive phases. The oldest phase is a heterogeneous biotite-hornblende diorite, which was intruded by the most extensive phase: a porphyritic biotite granite. The youngest phase is a garnet-muscovite granite. The intrusion of this last phase created the porphyritic biotite granite from an originally more mafic, hornblende bearing granodiorite. The mineralogy of the garnet-muscovite granite suggests that it might be of S-type. Several geochemical plots contradict this and suggests it is a highly evolved I-type magma. Previous geochronometry indicates that the tectonic boundary between the Okanagan Metamorphic and Plutonic Complex and the Intermontane Belt separates: 1) gneisses on the east that consistently yield K-Ar dates of 40-60 Ma, typically 51 Ma for hornblende and 48-50 Ma for biotite, from 2) intrusive rocks on the west that yield Jurassic K-Ar and Rb-Sr dates and Eocene volcanic rocks, erupted largely between 53 and 45 Ma. U-Pb dating of zircons indicates the presence of early Jurassic to mid-Jurassic plutons both east (granite of Anarchist Mtn., 160Ma; gneiss of Osoyoos, 201Ma deformed) and west (Similkameen granodiorite, 170Ma; Olalla Syenite, 18O-190Ma; undeformed) of the Okanagan Valley. East of the Okanagan Valley there are also mylonitic gneisses of Cretaceous age (gneiss of Skaha Lake, 105-120Ma; gneissic sill of Vaseaux Lake, 97Ma), as well as metamorphosed and deformed Eocene intrusives (Rhomb Porphyry, 51Ma). The interpretation is thaL although there are Jurassic plutons and early Mesozoic deformation in both the Okanagan Metamorphic and Plutonic Complex and the Intermontane Belt, there are also Cretaceous and Tertiary intrusive bodies within the Okanagan Metamorphic and Plutonic Complex that have been highly deformed in late Cretaceous to early Tertiary time. Regional geochronometry summarized on time versus blocking temperature graphs emphasizes the large (10 km) and rapid (1-4 mm/yr) unroofing needed to bring the gneisses east of the Okanagan Valley to near surface temperatures in Eocene time. Field evidence for a low angle west dipping detachment fault (Okanagan Valley fault) which juxtaposes brittle disrupted Eocene and older rocks against unannealed mylonitic rocks with Eocene K-Ar dates justifies comparison of the Okanagan Metamorphic and Plutonic Complex with other Cordilleran metamorphic core complexes.

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