FINGERPRINTING DISCRETE MAGMA BATCHES UNDERNEATH MAKUSHIN VOLCANO, UNALASKA ISLAND: EVIDENCE FROM TEST-WELL ST-1 DRILL CORE

The Makushin volcano, its associated satellite vents, and adjacent plutonic structures collectively form Unalaska Island. The Captain’s Bay pluton has been our only source to investigate episodic injections (e.g., aplitic intrusions) and the mixing between mafic and silicic magmas (e.g., oscillatory zoning and resorbed minerals). As opposed to the plutonic sequences exposed at the surface, an ~590 m continuous drill core beneath Makushin offers an in situ plutonic record of melting, assimilation, storage, and homogenization (MASH). Studies such as this are rare in the Aleutian arc and will lead to a more coherent model for the origin of the Aleutian crust.

Major-element data classifies Makushin samples as gabbro to gabbrodiorite, whereas adjacent plutons (i.e., Captain’s Bay, Sedanka, Shaler) are increasingly felsic (granodiorite to granite). Compositional heterogeneity predominates across the drill core; SiO₂ ranges between 52.1-68.9 wt%, MgO from 0.50-5.38 wt%, and CaO/Al₂O₃ ranges from 0.12-0.58. These values match the spread in composition to other Unalaska plutons. Makushin displays an SiO₂-K₂O-Na₂O enrichment and MgO-Fe₂O₃ depletion up-section. However, at ~307 m, a dyke with high SiO₂ (68.96 wt%) is consistent with the occurrence of aplite dykes found in the Captain’s Bay pluton. Potentially mafic dykes have been identified in hand sample. NMORB-normalized trace-element data displays negative REE profiles (e.g., La/Lu ranges from 23-58), enriched LILE (10-17x NMORB), and Nb-Ta exhibit negative anomalies characteristic of island arcs. Wider ranges in Sr and Ba are reflective of hydrothermal mobilization. However, Eu/Eu* reveals potentially 3 distinct magma batches; batch 1 = 0.33 (bottom), batch 2 = 0.28 (middle), and batch 3 = 0.25 (top). These results are consistent with primitive mantle-derived magma undergoing calc-alkaline evolution.

The primary mechanism suggested for the petrogenesis of the igneous rocks of Unalaska Island has been the fractional crystallization of a basaltic magma, however, our samples do not fit conventional crystallization models. We suggest that a shallow “crystal mush” undergoes fractional crystallization but is recharged with primitive high-Mg basaltic melts (e.g., Lerner et al. (2018) JVGR, 357), along with interaction of felsic fractionates.