Garnet-bearing basalts: an example from Mt. Hasan, Central Anatolia, Turkey

Summary Mt. Hasan is a complex stratovolcano located in Central Anatolia, Turkey. It exhibits four evolutionary stages: (1) Kecikalesi volcano, (2) Palaeovolcano, (3) Mesovolcano and (4) Neovolcano. Each stage is characterized by differentiated magmas varying from basaltic andesite to rhyolite, but basaltic activities are recorded on the volcano flanks. Garnets occur in rhyodacitic and rhyolitic pumices of the Palaeo-, Meso- and Neovolcanoe, as well as in the basaltic andesite lava flows of Palaeovolcano. In addition, garnets are recognised also in recent basalts which were erupted contemporaneously with Neovolcano during the late Quaternary. Garnets appear as euhedral grains in a basaltic groundmass. Mineralogy and geochemistry of older volcanics (Kecikalesi, Palaeovolcano) exhibit a tholeitic trend, while Mesovolcano and Neovolcano are calc-alkaline in affinity. However, the recent basalts exhibit alkaline chemistry and mineralogy. Garnets of Mt. Hasan volcanic rocks are pyrope- and almandine-rich and display rather uniform composition throughout the volcanic evolution. Trace element data suggest that while subduction components in magma composition decreased from Miocene to late Quaternary, the within plate character increased. We propose that the generation of garnet-bearing volcanics reflects an enriched source inherited from ancient subduction processes. Received November 1, 1999; revised version accepted June 30, 2001