Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 25-18
Presentation Time: 9:00 AM-6:00 PM


MCLEOD, Jennifer R.1, BOHRSON, Wendy A.1 and WORNER, Gerhard2, (1)Department of Geological Sciences, Central Washington University, 400 E. University Way, Ellensburg, WA 98926, (2)Geoscience Center Gottingen, University of Gottingen, Goldschmidtstrasse 1, Gottingen, Germany

We present new thermodynamic models that help constrain recharge and fractional crystallization (RFC) processes for Parinacota Volcano, northern Chile. Parinacota is a basaltic andesitic to rhyolitic dormant stratovolcano whose eruptive history spans from 163 ka to recent. In this study, the Magma Chamber Simulator (MCS; Bohrson et al., 2014) was used to produce quantitative models that reflect the open system magmatic history of Parinacota. MCS employs mass and enthalpy conservation in a composite system composed of a resident magma body, cumulate reservoir, and a set of recharge reservoirs. Over 200 RFC models were run with varying pressure, fO2 and parent and recharge magma masses, temperatures, and compositions. This study focuses on recreating major element data and mineral assemblages from the Old Cone (OC; 52-20 ka; 56-66% SiO2) and Healing Flows (HF; >8 ka- recent; 58-64%) eruptive stages, which are separated by a sector collapse that occurred ~8.8 ka (Jicha et al., 2015). Following the sector collapse, there was a distinct change in geochemistry from calc alkaline to more tholeiitic, and a correlated increase in eruptive flux. Best fit MCS results from major element runs suggest that a magma storage system resides at relatively shallow depths (~7-11 km) in crust that is ~70 km thick. A Rhyolite Domes (RD; 47-40 ka; 74% SiO2) lava from an episode occurring before the OC stage serves as the parent magma in these model runs. Due to the parent being more silicic than the daughter magma, recharge is required if these stages are using the same magma storage system. Lavas from the youngest eruptive stage, the Ajata Flows (10-3 ka), represent the mafic recharge into the system (Upper Ajata; 53 wt.% SiO2; Lower Ajata; 57 wt.% SiO2). Modeling indicates a large recharge mass to initial magma mass ratio of ~20:1, with little fractionation to reach the most mafic OC compositions. HF compositions are predominantly recreated by fractionation to a more silicic resident body, coupled with punctuated recharge events. These results support previous geochemical studies (e.g. Wörner et al., 1988; Ginibre and Davidson, 2014; Blum-Oeste and Wörner, 2016). MCS modeling of trace elements and Sr isotopes will further refine the magma mixing history and allow estimates of intrusive to extrusive ratios for these stages of magmatism at Parinacota.