the Chachahuen and Auca Mahuida volcanoes, while the Tromen volcano is located southwest from Payún Matrú. Lava and magma composition The volcanic field has produced rocks with composition ranging from alkali basalts over basalts, trachyandesite, basaltic trachyandesite, trachybasalt and trachyte to rhyolite. They define a calc-alkaline volcanic suite with some variation between the various volcanic centres; Los Volcanes is formed mainly by calc-alkaline magmas while Payun and Payún Matrú are more potassium-rich and shoshonitic. The volcanic rocks contain variable amounts of phenocrysts, including alkali feldspar, amphibole, apatite, biotite, clinopyroxene, olivine, plagioclase and sanidine, but not all phenocryst phases can be found in every rock formation. Magma temperatures of have been inferred. Volcanic rocks erupted at Payún Matrú resemble ocean island basalt volcanism, implying a deep origin of the magma although a shallow origin cannot be ruled out. Magnetotelluric observations indicate the presence of a "plume"-like structure that rises from depth close to the edge of the Nazca Plate slab to underneath Payún Matrú; it may indicate that magma erupted in the volcanic field originates at such depths which would explain the ocean island basalt-like composition. The magma ejected at Payún Matrú originates during partial melting of enriched mantle; the resulting melts then undergo crystal fractionation, assimilation of crustal material and magma mixing in magma chambers. The magmas eventually reach the surface through deep faults. The edifice of Payún Matrú acts as an obstacle to magmas ascending to the surface; this is why only evolved magmas are erupted in the caldera area of Payún Matrú while basic magmas reached the surface mainly outside of the main edifice. Obsidian from Payún Matrú has been found in archeological sites, although its use was not widespread in the region perhaps owing to its low quality, the difficulty of accessing the volcanic complex and that human activity in Payunia only began comparatively late in the Holocene and mostly from the margins of the region. Further, Payun volcano is notable for large crystals of hematite pseudomorphs which originated in fumaroles. Climate, soils and vegetation The climate at Payún Matrú is cold and dry with strong westerly winds. Annual temperature varies between while the average temperature in the wider region is about and the average annual precipitation amounts to . Generally, the area of Payún Matrú is characterized by a continental climate with hot summers especially at lower elevations and cold winters especially at higher elevations. The