SIMS ZIRCON U-PB GEOCHRONOLOGY AND REE GEOCHEMISTRY OF THE LATE JURASSIC SAINT ELIAS INTRUSIVE SUITE, NORTHERN COAST PLUTONIC COMPLEX, YUKON, CANADA

The Coast Plutonic Complex (CPC) is a 1900 km-long belt of Jurassic to Eocene intrusive rocks that from north to south underlies the Saint Elias Mountains of SW Yukon and NW British Columbia, the Coast Mountains of SE Alaska and coastal British Columbia, and the Cascade Mountains of northern Washington. The oldest CPC rocks in SW Yukon were emplaced into the Alexander terrane and comprise diorite to granodiorite batholiths and discrete plutons of the Saint Elias intrusive suite that have previously yielded K-Ar biotite and hornblende ages of 130-160 Ma. To further investigate the timing and petrogenesis of CPC magmatism in SW Yukon, we conducted reconnaissance field studies in the Saint Elias Mountains and analyzed seven rock samples of the Saint Elias intrusive suite for zircon U-Pb geochronology and rare earth element (REE) geochemistry using ion-microprobe (SIMS) methods. Intrusive rocks of the Mt. Maxwell, Alsek, Snowshoe (2 samples), Lowell Glacier (2 samples), and Ham plutons yield 207-corrected, weighted mean 206Pb/238U ages of 147 ± 2 Ma, 148 ± 2 Ma, 149 ± 2 Ma and 152 ± 3 Ma, 150 ± 2 Ma and 155 ± 2 Ma, and 155 ± 2 Ma, respectively. A few of the rock samples show evidence of 155-161 Ma inheritance. In general, Late Jurassic intrusive rocks become slightly more felsic and decrease in emplacement age from west to east across the Saint Elias Mountains. Zircon REE geochemical signatures are broadly consistent with oxidized magma conditions in a suprasubduction zone environment. In combination with published geological constraints, the new results from the Saint Elias intrusive suite support a plate tectonic model that involves: (1) the formation of a Late Jurassic arc along the Cordilleran margin after the Middle Jurassic accretion of the Alexander terrane; and (2) the subsequent Late Jurassic-Early Cretaceous cessation of arc magmatism because of changing plate motions and sinistral transpression along the Coast Belt. The new U-Pb ages further imply that the Saint Elias intrusive suite was a source region for c. 150 Ma detrital zircons in adjacent Jurassic-Cretaceous sedimentary successions of Alaska and NW Canada, including backarc strata of the Gravina-Dezadeash-Nutzotin belt and mélange units of the Chugach accretionary complex.