Bimodal maar volcanism in a post-collisional extensional regime: A case study of Acıgöl (Nevşehir) volcanic field (central Anatolia, Turkey)

The crustal structure is one of the fundamental factors that affects the type, composition, and spatial distribution of monogenetic volcanoes. The formation of maars, the second-most common type of monogenetic volcanoes, is mainly influenced by crustal lithologies, depth of explosions, and water-magma interactions together with magma rheology and tectonic structures. The Acıgöl caldera, located in the extensional setting of the central Anatolian plateau, contains both felsic and mafic maars. This rare compositional juxtaposition makes it a suitable location to better understand the relationship between magma chemistry and maar architecture. It includes closely spaced yet compositionally different monogenetic complexes (i.e., maars with either lava dome or scoria cone) and provides a fabulous opportunity to elucidate the role of crustal processes in the eruptional dynamics of maars.

Here we present an integrative study with detailed morphological (drone mapping), depositional (componentry, ash morphology), and petrological (whole-rock, glass, and mineral geochemistry) characteristics of rhyolitic (whole-rock; ~76.7 wt.% SiO₂, glass; ~77.2 wt.% SiO₂) İnallı, Kalecitepe, Acıgöl, and Korudağ maars, and mugearitic (~52.7 wt.% SiO₂) İcik maar. Our observations show a wide range of morphological features with spectacular examples of nested and compound craters. Field observations, together with the detailed stratigraphical analysis and literature-based geochronological data, reveal that the formations of maars and the subsequent lava domes or scoria cones are spatially migrating events within the same magmatic episode. We hence relate this to the rejuvenation of conduits, along with the pre-existing structures of the Acıgöl caldera that are almost perpendicular to the local extensional direction (NE-SW).

Non-modal batch melting models reveal that all investigated maars have a similar parental magma source (i.e., the most primitive basalt in central Anatolia with the Mg# of 72.4). This is formed by partial melting of a metasomatized lithospheric mantle with contribution from an OIB-like asthenospheric melt. The uprising magma that also produced the entire Quaternary volcanics in central Anatolia was possibly trapped at different crustal depths beneath the Acıgöl caldera and formed the maars with various degrees of magmatic differentiation processes. We conclude that İcik maar emanated from a relatively deep (lower crustal?) mantle-derived magma source evolved by assimilation and fractional crystallization processes. In contrast, the felsic maars were presumably formed by the short-lived ponding of the same magma source at shallower depths, which was partially assimilated by the basement intrusive rocks and dominantly shaped by the feldspar-driven fractional crystallization. Finally, the well-exposed examples of felsic maars in the study area and their comparison with the mafic counterparts could be a good contribution to the ever-growing literature on maar volcanism.