PLUTONIC ROCKS DRILLED BENEATH MAKUSHIN VOLCANO, UNALASKA ISLAND: A MASHED VIEW OF THE ALEUTIAN ISLANDS

The Makushin drill core, located on the flanks of the Makushin Volcano, Unalaska Island in the Aleutian chain, recovered ~2000 ft of igneous rocks that provide unprecedented access to the rarely observed shallow subvolcanic plumbing system that presumably stored and fed magmas erupted from this active volcano. Core samples can be divided into three stratigraphic units: 1) the upper, unaltered volcanic unit; 2) the middle, volcanogenic sedimentary rocks from the Unalaska Formation; and 3) the lower gabbroic complex,primarily comprised of gabbro (some highly brecciated), gabbronorite, and diorite. Contacts between coarse- and fine-grained gabbro (possibly mineral layering) and felsic veining are common throughout the core.

Major-element compositions determined via pXRF broadly overlaps with the Captains Bay and Shaler plutons, and Unalaska Formation volcanics (e.g., Perfit et al., 1980; Gust and Perfit, 1987; Trim, 2017); e.g., CaO/Al₂O₃= 0.08-0.97 vs. 0.05-0.88, respectively. However, the drillcore plutonics are, in general, less evolved in SiO₂, e.g., 44-65 vs. 44-78 wt%, respectively, and Al₂O₃, 8.6-17 vs. 11-23 wt%, respectively. Several depth trends are observed with major- and trace-element concentration; 1) a general decrease in Al₂O₃wt% and increase in TiO₂wt% up-section; 2) Cr and Rb decreases and Y increases up-section; and 3) from 800-1000 ft, a subset of samples have high SiO₂contents (55-65 wt%), which are identified as intrusive veins.

The petrogenesis of igneous rocks on Unalaska Island has been modeled as fractional crystallization of a basaltic magma with varying amounts of crustal partial melting (Perfit et al., 1980; Trim, 2017). Our focus is not recalculating those parent melts, but rather an understanding of the intra-pluton processes of melts derived from the source and their interaction with (or assimilation of) pre-existing magma or crystal mushes. Contacts between gabbro and andesite (or basalt) with granophyric textures observed throughout the core are indicative of melt-rock interaction. Additional textures include oscillatory-zoned plagioclase, resorbed/replaced clinopyroxene, and oxide symplectites. Our goal is to construct a more coherent model for the MASH (melting, assimilation, storage, homogenization) processes that may be common in Aleutian magmatism.