Early Miocene Kırka-Phrigian Caldera, western Turkey (Eskişehir province), preliminary volcanology, age and geochemistry data

Abstract

Large rhyolitic ignimbrite occurrences are closely connected to the Early Miocene initiation of extension in the central-western Anatolia crossing the Tavşanlı-Afyon zones. Field and laboratory data performed at the apex of the Eskişehir-Afyon-Isparta volcanic area allowed recognition of newly identified caldera structure, named here “Kırka-Phrigian caldera”. Transtensive/distensive tectonic stresses since 25Ma ago resulted in the NNW-SSE elongation of the magma chamber and influenced the roughly elliptical shape of the subsided block (caldera floor). The caldera, which is roughly oval (24km×15km) in shape, formed during a series of collapse events, starting at ~19Ma, by the generation of a huge volume of extra- and intracaldera ignimbrites. Intracaldera post-collapse sedimentation and further volcanism at the northern edge (at ~18.6Ma) were controlled through subsidence-related faults with generation of a series of volcanic structures (domes and lavas) showing a large compositional range. Enriched mantle components within the subcontinental lithospheric mantle began to melt via decompression melting during the initiation of extension. The heat resulting from the fractionation of ascending mantle melts produced the silicic compositions in large mushy crustal reservoirs; interaction of these melts with fertile crustal rocks further caused crustal anataxis and consequently two different compositions: Rhyolite-1 and Rhyolite-2. The eruptions of Kırka-Phrigian caldera-related ignimbrites were probably triggered by basaltic intrusion. Rock volumes and geochemical evidence suggest that silicic volcanic rocks come from a long-lived complex magma chamber system. After caldera generation there was a northern shift to small volume extra- and intra-caldera episodic rhyolitic, basaltic-trachy andesitic, trachytic and lamproitic volcanism, the latter being the youngest (16.2Ma) indicating a more primitive magma input which originated in an enriched mantle lithosphere. The rock succession provides a direct picture of the state of the magmatic system at the time of eruptions that generated caldera and post-caldera structures and offers an excellent example of silicic magma generation and associated potassic and ultrapotassic intermediate-mafic rocks in a post-collisional extensional setting.