Petrology and Oxygen Isotope Geochemistry of the Pucón Ignimbrite - Southern Andean Volcanic Zone, Chile: Implications for Genesis of Mafic Ignimbrites

Introduction: Although mafic components of dominantly intermediate to silicic ignimbrites are rather common, voluminous, dominantly mafic ignimbrites are rare (e.g., Smith, 1979; cf. Freundt and Schmincke, 1995). Volcán Villarrica, the most active composite volcano in South America, located in the Southern Andean Volcanic Zone (SAVZ, López-Escobar and Moreno, 1994a), has produced two such ignimbrites, respectively the Licán and Pucón Ignimbrites, in the last 14,000 years (Clavero, 1996). The two ignimbrites are low-Si andesite and basaltic-andesite to low-Si andesite, respectively, the former about twice as voluminous as the later (10 and 5 km3). Eruption of the ignimbrites produced calderas respectively 5 and 2 km in diameter (Moreno, 1995; Clavero, 1996). In addition to its mafic bulk composition, the Pucón Ignimbrite (PI) is also distinguished by numerous xenolithic fragments among and also within magmatic pyroclasts. Many of these are fragments of granitoid rocks. Volcán Villarrica has also produced numerous smaller mafic ignimbrites and pyroclastic surge deposits, as well as dominantly basaltic fallout and lava flows (López-Escobar and Moreno, 1994; Moreno, 1995; Clavero, 1996; Hickey-Vargas et al., 1989; Tormey et al., 1991). Reasons for the unusual style of mafic explosive activity at Volcán Villarrica are unclear. Clavero (1996), based upon an exemplary thesis-study of the physical volcanology and petrology of the PI, suggests it formed in response to a sequence of events beginning with injection of a shallow basaltic andesite magma chamber by hotter basaltic magma. In his model mixing and heat transfer between the two magmas initiated a violent Strombolian eruption that destabilized the chamber causing infiltration of large amounts of meteoric-water saturated country rocks. The Pucón Ignimbrite formed in response to subsequent phreatomagmatic interactions. In contrast, López-Escobar and Moreno (1994) infer on geochemical grounds that volatiles leading to the explosive eruptions were originally dissolved within in the pre-PI magma. They document an important repeating pattern between chemically evolved compositions and explosive eruptions at Volcán Villarrica that is particularly prominent in the cases of the two largest ignimbrite-forming eruptions. They infer that the precursor magma chamber became compositionally stratified through time, with the most evolved, volatile rich magma accumulating towards the top of the chambers, and culminating in voluminous mafic ignimbrite eruptions. In this study we characterize petrologic, geochemical and oxygen isotope characteristics of magmatic pyroclasts from the Pucón Ignimbrite, and one accidental granitoid lithic, extending on the previous work by Clavero (1996) and López-Escobar and Moreno (1995). We critically evaluate previous models, and combine ours and previous data to constrain and develop an alternate petrogenetic model of magmatic evolution of the pre-PI magma chamber, focusing on possible sources of volatiles derived from country rocks and from processes operating within the original magma chamber. We also document the occurrence of a previously unidentified, dacitic, magmatic enclave component of the ignimbrite.