The Petrology of the Tarosero Volcanic Complex: Constraints on the Formation of Extrusive Agpaitic Rocks

Abstract The Quaternary Tarosero volcano is situated in the East African Rift of northern Tanzania and mainly consists trachyte lavas some trachytic tuffs. In addition, there are minor occurrences extrusive basalts, andesites latites, as well peralkaline trachytes, olivine trachytes phonolites. Some phonolites contain interstitial eudialyte, making one few known for agpaitic rocks. This study investigates genetic relationships between various rock types focuses on peculiar formation conditions rocks using a combination whole-rock geochemistry, mineral chemistry, petrography, thermodynamic calculations, major trace element modelling. formed at redox around or below fayalite–magnetite–quartz buffer (FMQ). During multi-level magmatic fractionation depths ?40 km shallow crust, temperature decreased from >1100 °C near-liquidus basalts to ?700 residue. Fractional crystallization models characteristics do not indicate simple relationship other Tarosero. However, primitive intermediate latites can be explained by high-pressure fractional + clinopyroxene magnetite plagioclase apatite. Further these phases addition amphibole ilmenite led evolution towards eudialyte-bearing varieties required additional low-pressure alkali feldspar magnetite, contrast phonolites, instead amphibole, thus indicating magma even lower pressure conditions. They modelled derivation plagioclase, clinopyroxene, olivine. general, magmas evolve under closed-system conditions, which impede escape volatile phases. case Tarosero, early exsolution fluids halogens was prevented low water activity. resulted high concentrations rare earth elements (REE) field strength (HFSE) eudialyte most evolved Within suite, lowest HFSE REE concentrations, consistent with mineralogical evidence relatively lack fractionation, act melt, caused loss water-soluble elements. strong enrichment before finally extruded.