Magmatic and tectonic evolution of the Intermontane Superterrane and Coast Plutonic Complex in southern Yukon Territory
Creators: name: Hart, Craig Joseph Ronald
Publisher: name: University of British Columbia
Year: 2009

description: The Intermontane Superterrane and Coast Plutonic Complex in southern Yukon Territory are characterized by four episodes of Mesozoic and Cenozoic magmatism which are defined by geological mapping, geochronometry, and whole rock and Sr isotopic geochemistry. Late Triassic to Early Jurassic Klotassin Episode (220-175 Ma), mid-Cretaceous Whitehorse Episode (115-106 Ma), Late Cretaceous Carmacks Episode (85-68 Ma) and Early Tertiary Skukum Episode (61-54 Ma). There was a pronounced magmatic lull between 172-120 Ma. Twenty-two U-Pb, 25 K-Ar dates and greater than 60 strontium isotopic analyses from plutonic and volcanic rocks across the study area are presented to define the timing and nature of magmatic and tectonic events. U-Pb dates are mostly concordant to mildly discordant with minor amounts of Pb-loss-older inherited components are rare. Each magmatic episode is represented by two or more plutonic suites. The Klotassin Episode comprises the pre-accretionary Stikine and Red Ridge suites, the syn-accretionary Aishihik and Long Lake suites and the post-accretionary Bennett and Fourth of July suites. The Stikine suite is the plutonic equivalent to Lewes River Group volcanism, whereas Long Lake granites are coeval with Nordenskiold dacite. The Whitehorse Episode is composed of the Teslin, Whitehorse and Mount Mclntyre suite. The Carmacks Episode is composed of felsic and mafic phases of the Wheaton River suite as well as the Carcross suite. Skukum Episode magmatism includes Nisling Range plutonic suite as well as high level rhyolite plugs that are associated with Skukum Group volcanism. All plutonic suites have characteristics of calc-alkaline, magnetite-series, l-type subduction-related granitoids except those of the Skukum Episode which contain fluorite and have high Rb/Sr ratios (up to 100) and are akin to A-type magmas. Initial 8 7Sr/8 6Sr ratios of all suites are largely transitional (~ 0.7045) and range from 0.7035 to 0.7066. Elevated values reflect local upper crustal contamination from the pericratonic Nisling Terrane Post-accretionary volcanic successions in southwestern Yukon Territory were deposited as part of a continental margin volcanic arc across the amalgamated terranes during Late Mesozoic time. Isotopic dating indicates that volcanism occurred episodically during mid- to Late Cretaceous time at 106, 98, 84 and 81-78 Ma. The mid-Cretaceous (106 Ma) Carbon Hill volcanic rocks comprise a few small occurrences of intermediate to felsic pyroclastic units around a comagmatic pluton. The Montana Mountain volcanic rocks occur in a fault-bounded complex comprising 98 Ma intermediate flows and pyroclastics overlain by felsic flows that are -13 m.y. younger. Late Cretaceous Wheaton River volcanics (81-78 Ma) consist of an extensive succession of basic to intermediate lava flows cut by 70-62 Ma rhyolite dykes and plugs. The ages of these volcanic successions provide maximum age constraints for the epigenetic precious metal deposits they host, and minimum ages for the underlying coal-bearing strata of the Tantalus Formation. Major element geochemistry indicates that all three suites were formed from medium to high-K, calc-alkaline magmas. Initial strontium ratios vary considerably between the suites (0.7041 to 0.7061). Low ratios in the Wheaton River and Montana Mountain suites (-0.7042) indicate derivation from primitive, mantle-derived magmas. Higher initial strontium ratios in the Carbon Hill suite (-0.7052) suggest contamination from ancient continental material-probably from Nisling Terrane metasedimentary rocks. U-Pb zircon dating of granitic cobbles, and paleocurrents in Lower Jurassic Laberge Group conglomerate of the Mesozoic Whitehorse Trough suggest provenance from a western source containing Late Triassic (ca. 215 to 208 Ma) plutons. Small, isotopically unevolved plutons of Late Triassic to earliest Jurassic age that intrude the Lewes River Group volcanic arc rocks along the western margin of the Whitehorse Trough are the likely source. The age dates, the lack of zircon inheritance, and the primitive initial strontium values of the clasts rule out previous suggestions that the clasts were derived from the Early Jurassic Klotassin suite batholiths which intrude Nisling Terrane rocks. The deposition of very coarse Lower Jurassic boulder conglomerate on top of Late Triassic carbonate fades represents a dramatic change in the depositional style of the Whitehorse Trough. Sudden uplift incised a Lower Jurassic erosional disconformity into arc and arc-flanking shelf deposits along the western margin of the Whitehorse Trough. Episodic uplift resulted in paleotopographic relief in the arc sufficient to prograde coarse-grained debris flows into the basin and expose the plutonic roots of the arc throughout Early Jurassic time.