VARIABLE REGIMES IN MAGMA SYSTEMS DOCUMENTED IN PLAGIOCLASE ZONING PATTERNS: EL MISTI STRATOVOLCANO AND ANDAHUA MONOGENETIC CONES

Abstract

Magmas may experience closed- or open-system evolution during their ascent and evolution in the upper crust. Chemically closed-system processes can involve heat transfer from underplating mafic magmas or decompression due to magma ascent. On the other hand, newly injected magmas can mix with the residing magma and hybridize. Plagioclase zonation patterns and electron microprobe major and trace element analysis constrain these contrasting scenarios for andesites from the 80 km³ El Misti stratovolcano (5822 m) and a field of relatively small monogenetic Andahua scoria cones, both in southern Peru. In particular, Fe concentration in plagioclase reveals valuable information within a magmatic system to explore the dominance of closed- vs. open-system processes. Large and correlated Anorthite-Fe contrasts at resorption zones identify compositional mixing related to recharge. Subhorizontal An–Fe variations record thermal or water-induced effects. El Misti volcano, a large but still relatively youthful stratovolcano, is dominated during most of its approx. 120 ka history by closed-system effects during its evolution. Recharge and mixing mostly occurred cryptically as it is not recorded in drastic compositional changes in magmas or their phenocrysts.