PLAGIOCLASE GROWTH IN CRYSTAL-RICH VS. CRYSTAL-POOR LAVAS IN THE V. TEQUILA VOLCANIC FIELD, MEXICO

Andesites (58 –62 wt% SiO₂) of similar bulk composition, but with diverse crystalline assemblages and abundances, have erupted in the V. Tequila volcanic field in the western Mexican arc. Lavas forming the ~200 kyr old stratocone of V. Tequila (~31 km3), and its ~90 kyr old flank flows (~14 km³) (Lewis-Kenedi et al., 2004) are phenocryst-rich (15-30 vol%), with plag + opx + tmg ± cpx ± hbl ± il. In contrast, the small volume (~0.04 km³)peripheral scoria cones are older (370-620 ka) and characterized by a crystal-poor assemblage (< 6 vol %) of plag + opx + cpx + tmg ± hbl. The disparate phase assemblages and abundances suggest that the magmas experienced different crystallization histories prior to eruption. Because the Ca-Na equilibrium exchange between plagioclase and liquid is dependent on the H₂O content of the liquid (e.g. Housch and Luhr, 1991), we examined plagioclase compositional profiles to assess the influence of changing water content on the crystallizing assemblages. In the crystal-rich voluminous andesites, there is evidence for magma mingling. The plagioclase core compositions are highly variable (e.g. An₄₂-An₈₀) with K_D^Ca-Na values from 1 to 10. This variation cannot be explained by the degassing of water alone, but is consistent with the existence of a chamber injected by discrete magma batches that have mingled beneath the stratocone during its construction (Wallace and Carmichael, 1994). However, most of these variable cores have a 10-15 micron rim of similar composition (e.g. An_50-59), which suggests a common storage and crystallization history for these magmas immediately prior to eruption. The crystal-poor scoria cones feature little zoning from core to rim in individual crystals, but the variation between plagioclase is significant, ranging from An₄₅-An₇₅. Nonetheless, the corresponding K_D^Ca-Na range (1-3.1) can be explained solely by variable H₂O content. These lavas show no evidence of stalling in the crust for a sufficient duration to grow a common rim, but they do display textures consistent with rapid growth under dramatically changing water content, owing to degassing upon decompression. Many euhedral plagioclase have abundant melt inclusions, parallel to growth planes. Such plagioclase textures, coupled with their flat compositional profiles, are not consistent with magma mingling.