ORIGIN OF PHENOCRYSTS AND COMPOSITIONAL DIVERSITY IN PRE-MAZAMA RHYODACITE LAVAS, CRATER LAKE, OREGON

Phenocrysts in porphyritic volcanic rocks may originate in a variety of ways in addition to nucleation and growth in the matrix in which they are found. Porphyritic rhyodacite lavas that underlie the eastern half of Mount Mazama, the High Cascade andesite/dacite volcano that contains Crater Lake caldera, contain evidence that bears on the general problem of phenocryst origin. Phenocrysts in these lavas apparently formed by crystallization near the margins of a magma chamber and were admixed into convecting magma before eruption. About 20 km3 of pre-Mazama rhyodacite magma erupted during a relatively short period between approximately 400 and 500 ka; exposed pre-Mazama dacites are older and less voluminous. The rhyodacites formed as many as 40 lava domes and flows that can be assigned to three eruptive groups on the basis of composition and phenocryst content. Phenocryst abundance decreases (from 32 to 8 vol.%) and SiO2 content increases (from 68 to 73 wt.%) in the apparent order of eruption. Phenocrysts (plagioclase, orthopyroxene, augite, and Fe-Ti oxides) are commonly fragmental or form polycrystalline aggregates with interstitial glass. Discrete phenocrysts with complete euhedral outlines are rare except for small elongated crystals. The abundance of discrete phenocrysts increases with that of aggregates. The grain-size of minerals in the aggregates covers the range of discrete phenocrysts (0.2-4.2 mm). Rim compositions of phenocrysts and the range of chemical zoning are almost uniform among the three rhyodacite groups, regardless of whether crystals are discrete or in aggregates. However, a small fraction of phenocrysts, especially small elongated crystals, have different compositions: plagioclase with Fe-rich cores and augite with Wo-poor cores, both of which are characteristic of crystals in undercooled andesite enclaves in the rhyodacites. The majority of phenocrysts were derived by disintegration of polycrystalline aggregates; rare, small phenocrysts crystallized in andesitic magma similar to that represented by the andesite enclaves. The modal and chemical compositions of the rhyodacites can be explained by different degrees of admixing of crystals, represented by the aggregates, into magma having less-than-or-equal-to 4 vol.% 'true' phenocrysts, mainly plagioclase. The aggregates may be parts of the rind formed by in situ crystallization near the wall and roof of the magma chamber. The rind was disrupted during or just before eruption, and pieces were variably disaggregated and incorporated into erupting magma. The amount of rind incorporated declined during the sequence of eruptions. Owing to vesiculation of interstitial liquid and shearing during flow, crystals in the aggregates were separated and became phenocrysts. Pre-Mazama rhyodacite was erupted dominantly as lava, as opposed to the compositionally similar rhyodacite pumice of the Holocene caldera-forming eruption of Mount Mazama, apparently because its source chamber was crystallizing inward rather than actively growing.