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The White Mountain Granitoid Suite: Isotopic constraints on source reservoirs for Cretaceous magmatism within the Wrangellia Terrane

By
Darin C. Snyder
Darin C. Snyder
1
Department of Geology, 114 Shideler Hall, Miami University, Oxford, Ohio 45056, USA
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William K. Hart
William K. Hart
2
Department of Geology, 114 Shideler Hall, Miami University, Oxford, Ohio 45056, USA
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Published:
January 01, 2007

The White Mountain granitoid suite represents an isolated window into Cretaceous age magma intruded into Wrangellia terrane basement. Although the total area of exposed granitoid at White Mountain is relatively small (∼1 km2), substantial textural, chemical, and isotopic complexities exist. The granitoid suite consists of six surficially isolated bodies, all of which are calc-alkaline and metaluminous, ranging in composition from hornblendebiotite quartz diorite to biotite granodiorite. Three 40Ar/39Ar analyses provide cooling ages between 113.3 ± 1.3 and 117.38 ± 0.54 Ma, suggesting at least two pulses of magmatism are represented in the granitoid suite. Two of the bodies, comprising ∼20% of the total exposed granitoid, are enclave-bearing, with the hosts representing the most chemically evolved material at White Mountain and the enclaves among the least evolved. The enclaves typically are <15 cm in size and circular to oval in shape, are dominated by plagioclase and amphibole, and are intermediate in composition (∼54 wt% SiO2). Enclave rare-earth element patterns and isotopic characteristics, and the lack of petrographic evidence for quenched margins, suggest that they are cumulates from liquids chemically similar to but isotopically distinct from their host materials. One granitoid hand specimen exhibits textural and geochemical evidence for mixing at the low MgO end of the compositional spectrum. Although the granitoid suite exhibits a narrow range in whole rock isotopic compositions (εNd(115 Ma) 7.2–9.1 and 87Sr/86Sr(i) 0.7032–0.7043) further suggesting open system differentiation, these compositions do not require an appreciable role for ancient, evolved continental lithosphere in the White Mountain magmatic system(s). Rather, the dominant source reservoir was depleted mantle. This conclusion provides evidence that mid-Cretaceous magmatism in this region was generated in either an intraoceanic island arc or an “immature” (proto-) continental arc tectonic setting.

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GSA Special Papers

Tectonic Growth of a Collisional Continental Margin: Crustal Evolution of Southern Alaska

Kenneth D. Ridgway
Kenneth D. Ridgway
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Jeffrey M. Trop
Jeffrey M. Trop
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Jonathan M.G. Glen
Jonathan M.G. Glen
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J. Michael O'Neill
J. Michael O'Neill
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Geological Society of America
Volume
431
ISBN print:
9780813724317
Publication date:
January 01, 2007

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