Abstract: The Pattison Pluton is a high level Tertiary subaluminous alaskite that is part of a northwest trending chain of igneous intrusions in the Yukon Crystalline Terrane, southwestern Yukon. It is roughly circular in form with a diameter of about 18 km and a funnel-like profile. Crosscutting it are a series of shallowly dipping aplite dykes which intruded into open fractures. Compositionally and mineralogically, the alaskite body and aplite dykes are very uniform but the pluton is texturally and gradationally divided into a fine-grained upper border phase, a medium-grained graphic, strongly miarolitic, weakly porphyritic phase, and a lower coarse-grained phase.

Due to eutectic crystallization, no strong major element trends are found within the pluton and dykes. Trace element chemistry, however, shows an apparent liquid line of descent with the fine grained alaskite the least differentiated and the aplite dykes the most differentiated. The medium and coarse-grained alaskites plot in between these. Rayleigh fractional crystallization of alkali feldspar accounts for these trends.

Textural zonation evolved by the initial chilling of the originally H2O-poor magma at its upper edges. Crystallization of anhydrous phases from the melt had the effect of increasing the volatile pressure within the enclosed chamber. The volatiles migrated to and concentrated in the upper regions where the melting temperature was depressed, causing the pluton to crystallize from the bottom up. This allowed for the lower regions to reach more advanced stages of phenocryst crystallization, hence forming the lower coarse-grained alaskite. The volatile pressure eventually exceeded the confining pressure, resulting in failure of the surrounding rocks and retorgrade boiling, as well as vertical extension and lateral injection of residual magma due to filter pressing to form the aplite dykes. This final series of events also resulted in the formation of the graphic groundmass, straining of the minerals within the pluton, brecciation of some of the surrounding rocks, as well as quartz veining and molybdenum mineralization.

Authors: Lynch, G.V., Pride, C., and Watson, P.

Map Scale: 1 : 0

Citation: Lynch, G.V., Pride, C., and Watson, P., 1983. Petrology and geochemistry of the Pattison Alaskite Pluton, Yukon. In: Yukon Exploration and Geology 1982, Yukon Geological Survey , Indian & Northern Affairs Canada/Department of Indian & Northern Development: Exploration & Geological Services Division.