Petrology of the Younger Andesites and Dacites of Iztacc huatl Volcano, Mexico: II. Chemical Stratigraphy, Magma Mixing, and the Composition of Basaltic Magma Influx

Abstract

The Younger Andesites and Dacites of Iztaccihuatl volcano, Mexico, constitute a medium-K calcalkaline rock suite (58-66 wt. per cent SiO2) characterized by high Afg-numbers (100Mg/(Mg + O85 IFe) = 55-66) and relatively high abundances of MgO (2-5-6-6 wt per cent), Ni (17-158 p.p.m.), and Cr (42-224 p.p.m.). Chemical stratigraphy plots of eruptive sequences indicate the existence of a plexus of long-lived dacite magma chambers periodically replenished by influxes of basaltic magma ascending from depth. Short-term geochemical evolution after batch influx was dictated by magma mixing and eventual dilution of the basaltic component by 'quasi-steady state' hornblende dacite magma. The chemical data support textural and mineralogical evidence for rapid homogenization of originally diverse magmas by convective blending of residual liquids accompanied by dynamic fractional crystallization (Nixon, 1988). Internally-consistent mixing calculations used to derive the composition of basaltic magma influx incorporate analytical uncertainties and the observed range of salic end-member compositions. Mafic end-members are basalts to basaltic andesites (52-54 wt. per cent SiO2) with M^-numbers (73-76), MgO (9-11 wt. per cent), Ni (250 p.p.m.), and Cr (340-510 p.p.m.) concentrations, and liquidus olivine compositions (Fo90_88), appropriate for unfractionated partial melts of mantle peridotite. The majority of model compositions are Ol-Hy-normative, similar to those of primitive basaJtic lavas on the flanks of Iztaccihuatl and in the Valley of Mexico. However, calculated magma batches range from weakly Qz-normative to strongly Ne-normative. Both calculated and analyzed basaltic compositions are distinguished by highly variable abundances of alkalies and incompatible trace elements, notably Rb, Ba, Sr, P, Zr, and Y. Initial Sr/Sr ratios for Iztaccihuatl lavas (0-7040-0-7046; n = 24) are comparable to those for primitive basaltic rocks (0-7037-O-7O45; n = 4) and indicate that (1) mantle source regions are isotopically heterogeneous; and (2) contamination of Iztaccihuatl magma chambers by radiogenic crustal rocks was not a significant factor in the evolution of calc-alkaline andesites and dacites. The replenishment of Iztaccihuatl dacite reservoirs by Ne-normative magmas late in the history of cone growth precludes exhaustion of mantle source regions by progressive partial melting. Trie waning-Stages of volcanic activity at Iztaccihuatl appear to reflect the inability of dense basaltic influxes to successfully penetrate a large high-level chamber of low density hornblende dacite magmai