Textural and mineralogical diversity of compositionally homogeneous dacites from the summit of Mt. Erciyes, Central Anatolia, Turkey

摘要

We present new compositional data on a suite of dacite units that form the summit area of the Pliocene-Quaternary ML Erciyes stratovolcano in Central Anatolia, Turkey. Whole rock major and trace element analyses demonstrate the remarkable compositional homogeneity of these dacites (67.3 ± 0.3 wt.% SiO2), with only a few elements (Mg, Fe, Ca, Ni, Cu, and As) varying significantly (up to 10%) outside of analytical error. In contrast, the dacites show significant diversity in mineral textures, mineral compositions and phase assemblages, which indicate different magmatic histories for each magma batch erupted in the summit region. All samples contain two distinct plagioclase populations (clear phenocrysts with minor zoning ~An_(33)-An_(49) and cloudy, sieve-textured phenocrysts ~An_(59)-An_(80), both with clear rims of ~An_(42)-An_(65)), demonstrating the hybrid nature of the dacites. Temperatures calculated from magnetite-ilmenite pairs show a broad range from ~680 °C to ~1000 °C, and indicate more oxidized conditions for dacites from the SW ridge (types A1 and A2: △NNO = 1.2 to 1.9 log units) compared to dacites from the SE ridge (types B1 and B2: △NNO = 0 to 0.5 log units; type B3: △NNO = -0.1 to 1.0 log units). The higher-T units (types B1 and B2: 844 to 970 °C) contain fresh to partially-altered amphibole±biotite phenocrysts, while these phases are completely pseudo-morphed in the lower-T units (types A1, A2 and B3: 623 to 842 °C). Amphibole and clinopyroxene geobaro-metry indicate crystallization depths of 5-9 km and >15 km respectively. The dacites were likely formed through mixing between a basaltic andesite magma (source of the high-An sieve-textured plagioclase and orthopyroxene phenocrysts) and a rhyolitic magma (source of the low-An clear plagioclase and biotite phenocrysts). The minor element variability of Fe, Mg, and K found in the different plagioclase populations requires multiple sources for each type, indicating that more than two compositionally distinct end-member components (magmas, crystal mushes) were involved in mixing to form the summit dacites.