In the Wrangell Volcanic Belt (WVB) a northwesterly increase in volume of calc-alkaline versus transitional (sodic alkaline/calc-alkaline) magmatism is accompanied by a migration in the locus of magmatic activity. The space-time-composition relationships reflect oblique convergence between the North American and Pacific plates over the last 17.3 million years. Compositional- temporal trends are particularly well preserved in the four stages of volcanic stratigraphy in the St. Clare Creek field (17.3-6.5 Ma). Initially, alkaline olivine basalts, hawaiites and mugearites were erupted from small, isolated shield volcanoes in the axis of a continental molasse basin. The alkaline lavas were followed by an early stratovolcano stage of transitional trachybasalts and high-Fe basaltic trachyandesites, succeeded by basaltic trachyandesites, trachyandesites, trachytes, rhyolites and rare basaltic andesites. Widespread basaltic fissure eruptions dominated the third volcanic stage. The late stratovolcano stage consisted of renewed eruption of intermediate and felsic transitional lavas. A systematic temporal-chemical relationship between early alkaline and younger transitional and calc-alkaline lavas in the St. Clare Group is illustrated by a decrease in FeO/MgO, Na+K/Si, NB/Zr/Y, and an increase in Rb/Zr with increasing stratigraphic levels. Primitive basalts are non-primary and show variable degrees of fractionation between large ion lithophile (LILE) and high field strength element abundances. A model is proposed in which the alkaline shield volcano and early stratovolcano stage magmas formed by progressive melting of a rising mantle diapir in response to local extension along the Duke River fault. Early Fe-rich magmas may have undergone clinopyroxene fractionation at high pressures, but most magmas appear to have differentiated in the near surface environment via fractional crystallization and local magma mixing. With the onset of Yakutat subduction, progressively larger amounts of slab-derived, LILE-enriched fluids metasomatised overlying peridotite, which in turn melted to form primitive, late-stratovolcano stage magmas.

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A copy of this thesis is available at the EMR library – QE195.R37 2013. This thesis is available online at https://circle.ubc.ca/handle/2429/44391.

The Wrangellia Terrane along the northwest margin of North America is an extensive accreted oceanic plateau. These volcanic sequences erupted onto an extinct island arc in less than 5 million years at ca. 230 Ma. Triassic Wrangellia basalts and intrusions form a 1 to 10 km-wide linear belt of mafic and ultramafic rocks extending 300 km across southwest Yukon. A total of 85 samples were collected for geochemical and isotopic analysis from 10 widespread areas along the entire length of the linear belt. Field observations during the summer of 2004, and a synthesis of previous research for the Yukon portion of Wrangellia, are part of a larger research project involving Wrangellia basalts extending from Vancouver Island to central Alaska. The Wrangellia volcanic sequences represent one of the finest examples of an accreted oceanic plateau worldwide. They provide an excellent opportunity to gain a better understanding of the mantle source of oceanic plateaus and to assess the role of accretion of oceanic plateaus in continental growth.

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A copy of this thesis is available at the EMR library – QE685 W66.

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We present new field and U-Pb analytical data from the Sonora Gulch Project that demonstrate a protracted history of polymetallic mineralization (Au-Ag-Cu-Zn ± Mo) associated with several pulses of Cretaceous magmatism. Recent exploration on the Sonora Gulch Project has highlighted the presence of two important mineralized zones: the Nightmusic zone, a mesothermal Au-enriched base metal skarn, and the Amadeus zone, an epithermal Au-Ag system. Four U-Pb age dates determined from each of two feldspar porphyry dykes (ca. 74 Ma), a weakly mineralized quartz porphyry stock (ca. 75 Ma) within the Nightmusic zone and the Au-Ag mineralized Amadeus stock (ca. 75 Ma), demonstrate the widespread occurrence of Late Cretaceous magmatism. The age determinations indicate that mineralization occurring within the Sonora Gulch project area are temporally equivalent to the Casino Cu-Au-Mo deposit, located roughly 40 km to the west-northwest. These new data extend the currently known eastern limit of Late Cretaceous magmatism and associated mineralization.

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