Abstract
At Clear Lake, California, the episode of volcanism from 0.65 to 0.30 Ma culminated in production of mixed dacite and rhyodacite lavas with bimodal phenocryst populations exhibiting extreme disequilibrium textures. These lavas formed by the interaction of subequal fractions of mafic recharge magma with the crystal-rich remnants of earlier-formed felsic magma bodies. The blending of basaltic andesite and rhyolite magmas dominated the early stages of mafic-felsic magma interaction. Partial quenching and segmentation of the resulting hybrid magma eventually led to the formation of quenched andesitic inclusions. During later stages of interaction, quenched inclusions suffered extensive disaggregation, contributing crystalline debris and residual silicic liquid to mixed dacites. The large range in mineral compositions and textures imply that given sufficient mafic input, pre-eruptive mixing is a remarkably efficient process, even in silicic magmas.
Phenocryst assemblages derived from the end-member magmas in dacite lavas preserve distinct compositions and styles of textural reequilibration. Felsic end-member phenocrysts responded to mixing mainly by dissolution and reaction recorded by (1) a rounded and embayed crystal form, (2) compositional reversals, (3) a partial reaction progressing inward from crystal margins, and (4) mantles or coronas that formed by diffusion-limited reactions in dissolution boundary layers. Sodic plagioclase derived from the felsic end-member shows a variety of features, including simple dissolution, abrupt shifts to more calcic composition, and fritted texture (representing a partial to complete reaction to more calcic compositions). Fritting occurs when sodic plagioclase experiences large changes in both liquid composition and temperature during residence in hybrid andesitic magma, whereas simple dissolution and calcic shifts occur when sodic crystals residing in felsic magma experience dramatic increases in temperature, followed by relatively small changes in liquid composition. Sanidine shows features analogous to plagioclase, including simple dissolution and shifts to more Ba-rich compositions. Some sanidine crystals are also mantled by plagioclase. Mafic end-member magmas (basaltic andesite) responded to mixing by combined liquid blending and undercooled crystallization, resulting in quenched andesitic inclusions and disseminated crystalline debris, with compositions intermediate between the end-member phenocryst populations. Plagioclase microphenocrysts formed during mixing are characterized by dendritic or skeletal growth forms and strong normal zoning. Calcic plagioclase phenocrysts present prior to mixing have abrupt sodic shifts and normal zoning at their margins, with rim compositions that match those of microphenocrysts in inclusions.