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Provenance constraints for Whitehorse Trough conglomerate: U-Pb zircon dates and initial Sr ratios of granitic clasts in Jurassic Laberge Group, Yukon Territory

By
Craig J. R. Hart
Craig J. R. Hart
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John R. Dickie
John R. Dickie
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Dipak K. Ghosh
Dipak K. Ghosh
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Richard L. Armstrong
Richard L. Armstrong
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Published:
January 01, 1995

Geologic and isotopic data suggest a depositional link between granitic plutons of northern Stikinia and the adjacent Jurassic Laberge Group sedimentary rocks of the Whitehorse Trough. U/Pb zircon dating of granitic cobbles in Lower Jurassic Laberge Group conglomerate of the Mesozoic Whitehorse Trough suggests clast derivation from a source terrane containing Late Triassic (ca. 215–208 Ma) granitic plutons. Initial strontium ratios are primitive and paleocurrent data show that detritus comprising Laberge Group conglomerate was westerly derived. A string of small, isotopically unevolved plutons of Late Triassic to earliest Jurassic age intrude the Lewes River volcanic arc rocks along the western margin of the Whitehorse Trough and are interpreted as the probable western source for the clasts. Dates and 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 extremely coarse Early Jurassic boulder conglomerate on top of Late Triassic carbonate facies represents a dramatic change in depositional style. 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 periodically maintained extreme paleotopographic relief in the arc, sufficient to prograde coarse-grained debris flows into the basin and erode the plutonic roots of the arc throughout Early and early Middle Jurassic time.

Initial Sr isotopic ratios of the granitic clasts average 0.7045 and suggest that they were derived from unevolved island-arc magmas. U/Pb systematics do not indicate the presence of inherited zircon xenocrysts. These data suggest that the plutons which acted as the source for the clasts had limited, if any, interactions with isotopically evolved continental crust and likely intruded oceanic or transitional crust. The source of metamorphic clasts in Laberge Group conglomerate is presumed to be the Nisling Terrane, suggesting that Nisling and Stikinia were linked by Middle Jurassic time.

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GSA Special Papers

Jurassic Magmatism and Tectonics of the North American Cordillera

David M. Miller
David M. Miller
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Cathy Busby
Cathy Busby
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Geological Society of America
Volume
299
ISBN print:
9780813722993
Publication date:
January 01, 1995

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