Disequilibrium phenocryst textures in an Andean volcanic complex: mixing or rapid decompression?

MAUREEN FEINEMAN; PATRICIA SRUOGA; DANA DREW; TIMOTHY MURRAY

San Francisco

Congreso; 2010 Fall Meeting AGU; 2010

American Geophysical Union

Mantle-derived magmas of the Andean Central Volcanic Zone and northern Southern Volcanic Zone pass through >50 km of continental crust prior to eruption.  Geochemical and geophysical evidence suggests that these magmas pause in both the lower and upper crust prior to eruption.  As upper crustal magma chambers must be periodically replenished from deep-rooted magma sources, it is reasonable to expect some degree of cyclical magma mixing over the life span of a volcano.  Lavas erupted from Maipo Volcano in the northern Southern Volcanic Zone display cyclical variability in silica content from 54% to 68% over its 86 Ka eruptive history to date.  Disequilibrium phenocryst textures, including sieve-textured plagioclase, embayed and anhedral phenocrysts, oxidized horblende rims, and olivine phenocrysts with orthopyroxene overgrowths are ubiquitous.  While these disequilibrium features are characteristic of magma mixing, many of these could also be brought on by rapid magma ascent, which changes the position of the liquidus relative to the crystallizing assemblage. Some of the most primitive lavas found at Maipo are the 28 (±17) Ka basaltic andesites (SiO2 = 57-61%; 87Sr/86Sr = 0.7049), constituting the fourth of seven major eruptive events.  These lavas appear to emanate from the base of the eastern flank and are found only on the caldera floor, suggesting a lateral or fissure eruption.  The location and relatively primitive nature of the Event IV lavas may be interpreted as indicating that they have bypassed extensive interaction (i.e., mixing) with magmas in the main upper crustal chamber.  However, basaltic andesites of comparable composition (SiO2 = 53-59%, 87Sr/86Sr = 0.7048) are also associated with the ultimate (potentially historic) Event VII, consisting of scoria bombs sampled within and emanating from the central crater.  While it is unlikely that these lavas could have bypassed the shallow magma chamber, it is conceivable that these flows tap a deep, relatively primitive region of a zoned magma chamber with limited mixing.  In contrast, the bulk of the volcanic edifice is composed of 45(±14) Ka dacites (SiO2 = 61-66%; 87Sr/86Sr = 0.7051-0.7054) associated with Eruptive Event III. Multiple generations of plagioclase phenocrysts in these lavas exhibit sieve-textured cores, zones, and rims consistent with multiple episodes of magma mixing.  We propose that the disequilibrium textures observed in the more primitive basaltic andesites reflect rapid ascent, while those in the dacites record recharge and mixing in a shallow magma chamber.  The multiple generations of zoned phenocrysts, lack of mingling features in the matrix, and relatively long pauses (~40 Ka) between major eruptions from the central cone suggest that magma recharge does not necessarily trigger eruption at this volcanic center.