MAGMATIC INCLUSIONS IN THE MT. HELEN DOME, LASSEN VOLCANIC CENTER, SOUTHERNMOST CASCADES: CONSTRAINTS ON UPPER-CRUSTAL MAGMATIC AND ERUPTIVE PROCESSES

Mt. Helen is a dacitic lava dome (~249 ka.) in the Lassen volcanic center; a long-lived (~600ka. - present), silicic volcanic center superimposed on regional mafic magmatism in the southernmost Cascade volcanic arc. Mafic to intermediate composition magmatic inclusions are a common feature of intermediate to silicic composition, continental arc volcanic rocks. Their presence as texturally and compositionally distinct entities in otherwise homogeneous host rocks is widely accepted to reflect (1) mixing of two magmas upon injection of a mafic magma into a relatively silicic magma chamber, (2) formation of some type of mixed (hybrid) layer of magma beneath the bulk of the silicic magma, and (3) subsequent breakup of the hybrid layer of magma to form inclusions of the hybrid magma in the silicic magma. The mechanism of magmatic inclusion formation and the magmatic and eruptive processes that disperse and disaggregate inclusions into the host-magma are constrained in this study by textural, compositional, and spatial inclusion-distribution data.

Textural and compositional variation of inclusions suggest (1) most inclusions formed during a binary mixing process over some protracted period of time, (2) inclusions were predominantly liquid when entrapped in the silicic host-magma, (3) inclusions were derived from variably mixed and fractionated magmas, probably generated from non-uniform degrees of mantle partial melting, (3) inclusions disaggregated at or near the time of eruption, probably during ascent in the conduit, to contribute to host-rock compositional diversity on a local scale, and (4) the extent of inclusion disaggregation was uniform (~25 %). Spatial distribution of inclusions varies consistently from dome margin to core and indicates that inclusions may have concentrated near the top of the magma chamber prior to eruption. The compositional and textural diversity of inclusions in this single eruptive unit require a complex history that challenges overly-simplistic models for larger systems.