PLAGIOCLASE ZONATION STYLES IN HORNBLENDE GABBRO INCLUSIONS FROM LITTLE GLASS MOUNTAIN, MEDICINE LAKE VOLCANO, CALIFORNIA - IMPLICATIONS FOR FRACTIONATION MECHANISMS AND THE FORMATION OF COMPOSITION GAPS

The rhyolite of Little Glass Mountain (73-74% SiO2) is a single erupti ve unit that contains inclusions of quenched andesite liquid (54-61% S iO2) and partially crystalline cumulate hornblende gabbro (53-55% SiO2 ). Based on previous studies, the quenched an desire inclusions and ho st rhyolite lava are related to one another through fractional crystal lization and represent an example of a fractionation-generated composi tion gap. The hornblende gabbros represent the cumulate residue associ ated with the rhyolite-producing and composition gap-forming fractiona tion event. This study combines textural (Nomarski Differential Interf erence Contrast, NDIC, imaging), major element (An content) and trace element (Mg, Fe, Sr, K, Ti, Ba) data on the style of zonation of plagi oclase crystals from representative andesite and gabbro inclusions, to assess the physical environment in which the fractionation event and composition gap formation took place. The andesite inclusions (54-61% SiO2) are sparsely phyric with phenocrysts of plagioclase, augite and Fe-oxide+/-olivine, +/-orthopyroxene, +/-hornblende set within a glass y to crystalline matrix. The gabbro cumulates (53-55% SiO2) consist of an interconnected framework of plagioclase, augite, olivine, orthopyr oxene, hornblende and Fe-oxide along with highly vesicular interstitia l glass (70-74% SiO2). The gabbros record a two-stage crystallization history of plagioclase+olivine+augite (Stage I) followed by plagioclas e+orthopyroxene+hornblende+Fe-oxide (Stage II). Texturally, the plagio clase crystals in the andesite inclusions are characterized by complex , fine-scale oscillatory zonation and abundant dissolution surfaces. C ompositionally (An content) the crystals are essentially unzoned from core-to-rim. These features indicate growth within a dynamic (convecti ng?), reservoir of an desite magma. In contrast, the plagioclase cryst als in the gabbros are texturally smooth and featureless with strong n ormal zonation from An,, at the core to around An(30). K, and Ba abund ances increase and Mg abundances decrease steadily towards the rim. Ti , Fe, and Sr abundances increase and then decrease towards the rim. Th e trace element variations are fully consistent with the two-stage cry stallization sequence inferred from the gabbro mineralogy. These resul ts indicate progressive closed-system in situ crystallization in a qui escent magmatic boundary layer environment located along the margins o f the andesite magma body. The fractional crystallization that generat ed the host rhyolite lava is one of inward solidification of a crystal lizing boundary layer followed by melt extraction and accumulation of highly evolved interstitial liquid. This mechanism explains the format ion of the composition gap between parental andesite and rhyolite magm a compositions.