INCLUSIONS IN MOUNT ST-HELENS DACITE ERUPTED FROM 1980 THROUGH 1983

Inclusions of plutonic, metavolcanic and volcanic rocks are abundant i n dacite pumice and lava from the 1980-1986 eruption sequence at Mount St. Helens. Point counts of inclusions exposed in talus blocks from t he dome from 1980 through 1983 show that inclusions form approximately 3.5 vol% of the lava. Eighty-five percent of the inclusions are mediu m-grained gabbros with an average diameter of 6 cm. Additional rock ty pes include quartz diorite, hornfelsic basalt, dacite, andesite and ve in quartz. Disaggregated inclusions are common and define shear planes within the dome. These fragmented inclusions may significantly contam inate analyses of the dacite. The gabbroic inclusions are of four dist inct types, all with mineral assemblages consistent with crystallizati on pressures of less than 9 kb. Textures and major-element composition s indicate that most of the gabbroic inclusions are cumulates. The mos t abundant inclusion type is laminated gabbronorite, which contains up to 9% interstitial glass, derived from partial melting. The presence of quartz veins and hornblende-bearing veins within sheared zones in t he laminated gabbronorite indicates that the source of these inclusion s was holocrystalline rock that had been penetrated by water-rich flui ds. The gabbronorite contained sufficient water to be susceptible to p artial melting when the magma that fed the 1980-1986 eruption sequence was emplaced nearby. Various types of gabbroic inclusions, including the laminated gabbronorite, are common in Mount St. Helens lavas of ap proximately the last 3000 years. This coincides with the interval, in which Mount St. Helens first erupted basalt and basaltic andesite lava s. These observations, together with the fact that the gabbroic inclus ions are compositionally unlike any of the Tertiary intrusive rocks in the Mount St. Helens area, strongly suggest that the inclusions are r elated to the introduction of basalt to the Mount St. Helens magmatic system. The source of the gabbros could be a layered mafic pluton form ed through crystal accumulation from multiple batches of basaltic magm a emplaced at mid- to upper-crustal depths beneath the volcano. The pr evalence of explosive eruptions at Mount St. Helens may play a part in bringing the inclusions to the surface. The eruptive products of the cataclysmic eruption of May 18, 1980 contain notably fewer inclusions than the pyroclastic flows and dome lavas erupted subsequently. This s uggests that the May Is eruption shattered conduit wall rock that was subsequently stoped into the magma and carried to the surface later in the eruption series.