Olivine basalt and trachyandesite peperites formed at the subsurface/surface interface of a semi-arid lake: An example from the Early Miocene Bigadiç basin, western Turkey

Abstract

Miocene successions in western Turkey are dominated by lacustrine, fluvial and evaporitic sedimentary deposits. These deposits include considerable amounts of volcaniclastic detritus derived from numerous NE-trending volcanic centres in western Turkey as well as in the Bigadiç region. Early Miocene syn-depositional NE-trending olivine basalt and trachyandesite bodies that formed as intrusions and lava flows occur within the Bigadiç borate basin. Olivine basalts occur as partly emergent intrusions, and trachyandesite dykes fed extensive lava flows emplaced in a semi-arid lacustrine environment. Peperites associated with the olivine basalt and trachyandesites appear to display contrasting textural features, although all the localities include a large variety of clast morphologies from blocky to fluidal. Fluidal clasts, mainly globular, ameboidal and pillow-like varieties, are widespread in the peperite domains associated with olivine basalts, apparently due to large-volume sediment fluidisation. In contrast, fluidal clasts related to trachyandesites are restricted to narrow zones near the margins of the intrusions and have commonly elongate and polyhedral shapes with digitate margins, rather than globular and equant varieties. Blocky and fluidal clasts in the olivine basalt peperite display progressive disintegration, suggesting decreasing temperature and increasing viscosity during fragmentation. Abundance of blocky clasts with respect to fluidal clasts in the trachyandesite peperite indicates that the fluidal emplacement and low-volume sediment fluidisation in the early stages were immediately followed by quench fragmentation due to the high viscosity of the magma. Size, texture and abundance of the blocky and fluidal clasts in the olivine basalt and trachyandesite peperites were mainly controlled by sediment fluidisation, pulsatory magma injection and magma properties such as composition, viscosity, vesicularity, and size, abundance and orientation of phenocrysts. Variously combining these contrasting features to varying degrees may form diverse juvenile clast shapes in peperitic domains.