Evolution of a high-level, high-silica magma chamber: the Pattison pluton, Nisling Range alaskites, Yukon

Abstract

The Pattison pluton is a high-level, high-silica Tertiary alaskite intruding rocks of the Yukon Crystalline Terrane. The pluton is roughly circular in form with a diameter of about 18 km. The intrusion is cut by a series of shallowly dipping aplite dikes. Mineralogically, the alaskite body and the aplite dikes are very similar, but the pluton is divided into a series of phases based on texture and "stratigraphic" position: a fine-grained upper border phase, a medium-grained, graphic, strongly miarolitic, porphyritic phase, and a lower, coarser grained phase, contacts being gradational.No strong major element trends are found within the pluton (including the aplite dikes) because of eutectic crystallization. The trace elements, particularly Ba, Sr, Rb, Zr, and the REE, however, do show a liquid line of descent, with the upper border phase as the least differentiated and the aplite dikes as the most differentiated phases. Rayleigh fractional crystallization of quartz, K-feldspar, and plagioclase (and accessory allanite) in eutectic proportions readily accounts for the observed trends.The parental magma was water undersaturated (< 1.2% H2O). Crystallization of anhydrous phases led to water-saturated conditions late in the pluton's history. Volatiles migrated to and concentrated in the upper parts of the chamber where the melting temperature was depressed, causing the pluton to crystallize from the bottom up. The volatile pressure eventually exceeded the confining pressure, resulting in failure of the surrounding rocks, retrograde boiling, vertical extension, and lateral injection of residual magma caused by filter pressing to form the aplite dikes. The escaping volatile phase resulted in quartz veining and Mo mineralization.