PLAGIOCLASE ZONING AS AN INDICATOR OF MAGMA PROCESSES AT OLLAGÜE VOLCANO, ANDEAN CENTRAL VOLCANIC ZONE, CHILE

The effects of thick continental crust on the genesis of intermediate composition magmas at continental arcs are subject of debate in igneous petrology. Continental crust is estimated to be silicic andesites to dacite in composition, but origin of intermediate composition magmas is difficult to constrain. Textural and chemical analysis of plagioclase can be used to constrain source, depth and processes from magma generation to eruption/crystallization. Here, we assess backscatter electron images and major element geochemistry of plagioclase phenocrysts from polished thin sections representing the eruptive history. Four distinct textures were observed in all thin sections including: euhedral zoned crystal faces, dissolution surfaces, sieved zones and spongy cores and/or zones. Euhedral zoned crystal faces were observed in all four populations of crystals. Dissolution surfaces are observed in the Vinta Loma and Santa Cecilia crystal populations. Dissolution surfaces in both crystal populations are predominantly seen at or near the cores. However, it should be noted there are crystals that display dissolution surfaces toward the mantle rims in both populations. Sieved zones are observed in the Vinta Loma and El Azufre populations, select phenocrysts in the El Azufre population exhibit sieving near their rims. Sieved cores are exhibited in the cores of phenocrysts within the Vinta Loma populations. Crystal populations from the Vinta Loma display prominent spongy surfaces in both the core of certain phenocrysts, and towards the rim of others. These textures suggest continuous temperature changes within the chamber as demonstrated by the chemical zoning of the phenocrysts showing variations in the anorthite content. It is possible that multiple pressure-controlled events occurred during the Vinta Loma as noted by the prominent spongy surfaces in the cores of some phenocrysts and on the rim of others. Textures in plagioclase phenocrysts preserve records of the chemical and physical dynamics of magma chambers. The textures observed in this study indicate the sub-volcanic system of Ollagüe has shown disequilibria both in the system and in the crystals. However, to further constrain the dynamics of the system, a detailed chemical analysis of the disequilibria phases would allow a clearer understanding.