Magmatic evolution of ediacaran alkali rhyolites from the Acampamento Velho volcanism in the Tupanci area, southern Brazil: A study based on mineral chemistry, LA-ICP-MS Ti-in-quartz and zircon saturation geothermometry

Abstract

Post-eruptive, metamorphic, and weathering processes are issues that affect not only the identification of geochemical signature but also mineral paragenesis on ancient volcanic successions. Still, the estimate of pressure, temperature and oxygen fugacity conditions prevalent during magma crystallization can be achieved for volcanic systems of any age by combining experimental, thermodynamic and natural data using geothermobarometers. In this regard, highly resistant minerals such as zircon and quartz can provide key information on the origin and evolution of silicic magmatic chambers. For example, the substitution of Ti4+ for Si4+ in quartz is a function of the activity of TiO2 in the magma and in the crystal, temperature and pressure. Therefore, the aim of this study is to apply Ti-in-quartz (TitaniQ) coupled to Zircon saturation geothermometry to provide a better insight into the igneous processes that originated post-collisional Ediacaran sodic-alkaline rhyolites from the Acampamento Velho in southern Brazil. Zircon saturation geothermometry estimates range from ca. 825 °C–1000 °C (within an error of ~25 °C), in agreement with previously reported zircon crystallization temperatures for the Acampamento Velho volcanism in the Ramada and Taquarembó plateaus. Temperature estimates using the Ti in quartz thermometer range from 700 to 710 °C for the intrusive rocks and from 710 to 810 °C for the lava flows that show quartz zonation. The Ti zonation pattern in quartz was probably generated by partial phenocryst dissolution in response to a magma chamber recharge and increase in the pre-eruption temperature. This type of process could be similar to the injection of mafic melts rich in volatiles that originated giant magma bodies, indicating that the Acampamento Velho volcanism was likely formed in a supereruption event with associated caldera collapse, as previously evidenced by detailed petrography studies in other occurrences in the Sul-Rio-Grandense shield.