The origin and implications of undercooled andesitic inclusions in rhyolites, clear lake volcanics, California

Abstract

At Clear Lake, California, undercooled andesitic inclusions are common in rhyolitic to dacitic lava flows and pyroclastic deposits. One group of rhyolite lava flows contains about 1 vol % undercooled andesitic inclusions <2 cm in size. The inclusions are evenly distributed in several flows and do not vary significantly in size or abundance. They consist of plagioclase, orthopyroxene, ilmenite, apatite needles, and vesicular rhyolitic glass ±olivine±clinopyroxene. Microphenocryst size in these small inclusions correlates positively with inclusion size, suggesting that inclusions have not undergone extensive disaggregation since formation. Texturally similar inclusions up to 50 cm in size occur in associated pyroclastic deposits. None of the inclusions represent the mafic end‐member of the system. Rather, they represent intermediate magmas formed by liquid‐crystal fractionation and hybridization of distinct andesitic magmas at or near the base of a silicic magma chamber. The large inclusions are transitional in composition and mineralogy between Clear Lake basaltic andesites and smaller inclusions, which show more evidence of hybridization. These differences suggest similar but distinct sources and mechanisms of formation for the large and small inclusions. The small inclusions probably formed by episodic mingling across an interface or hybrid zone separating rhyolitic and andesitic magmas. We speculate that earthquakes generated along the San Andreas fault system may have triggered mingling events. This occurrence demonstrates that dispersion of undercooled intermediate to mafic inclusions in silicic magma chambers is the dominant mode of mafic‐felsic magma interaction in some systems.