GENERATION OF A CRYSTAL-POOR ZONED SILICIC PYROCLASTIC DEPOSIT BY PARTIAL MELTING OF A SUB-SOLIDUS MAGMA CHAMBER BENEATH VOLCAN TEPETILTIC, MEXICO

Volcan Tepetiltic is a predominantly andesite stratovolcano (~42 km³) in the Western Mexican volcanic arc constructed of crystal rich (20-45 vol%) lavas erupted over an interval of ~160 kyrs, between 575 and 416 ka, as determined by ⁴⁰Ar/³⁹Ar dating. Following a >100 kyr hiatus, an explosive caldera-forming eruption deposited ~6-9 km³ of crystal-poor (0-3 vol%), zoned (60-75 wt% SiO₂) pyroclastic material. During this period of explosive activity, ~9 km³ of basaltic andesite erupted peripheral to the main vent. The age of the pyroclastic deposits is constrained by the age of mafic lavas found beneath and on top of the deposits to be 281 ± 8 ka.

Pumice from the pyroclastic deposits is crystal-poor (0-3 vol%), but contains an assemblage of plagioclase + titanomagnetite + ilmenite + apatite ± quartz ± orthopyroxene ± hornblende. Two distinct eruptions, correlating to two visible calderas within the main edifice, can be defined based on bulk geochemistry, plagioclase composition and geographic distribution. The northern pumices are compositionally restricted (71-74% SiO₂) and more enriched in REE and HFSE relative to the southern deposits which show a decrease in silica with increasing stratigraphic position (75-60 wt% SiO₂). Plagioclase phenocrysts from northern pumice clasts are enriched in Ba, K, Fe, Mg, and Ti compared to the southern plagioclase. All plagioclase phenocrysts are unzoned and free of melt-inclusions, but within a single pumice clast span a compositional range of 10-25 mol% An. The trace element abundances in the plagioclase from the pumice are distinctive from the complexly zoned plagioclase from lavas in the older main edifice.

The nearly aphyric pumice and lack of crystal zoning suggests that the pyroclastic material erupted rapidly after melt segregation, and is not genetically related to lavas comprising the main edifice through crystal fractionation and/or magma mingling processes. The magma supply of the explosive eruption was more likely generated as a partial melt from a subsolidus granitoid. Heat flow modeling and the temporal association of the peripheral basaltic andesites and the explosive, zoned silicic eruption, are consistent with the hypothesis that the mafic magma acted as a heat source which partially melted and remobilized the subsolidus graniotoid beneath the central vent.