Insights into continental rift-related magma chambers: Cognate nodules from the Kula Volcanic Province, Western Turkey

Abstract

The Quaternary volcanism of Kula, Western Turkey, occurred in response to Miocene, lower lithospheric, N–S, extension, which resulted in the formation of ∼80 monogenetic cinder cones, each with an associated lava flow (basanites, tephrites and phono-tephrites), comprising a total magma volume of ∼2.3 km 3. Many cones form closely spaced, genetically related groups of up to 5 cones. One such group of cones, in the immediate vicinity of Kula, contains large crystal-rich nodules dominated by amphibole and clinopyroxene. The nodule suite is varied, both compositionally and texturally, and is likely to comprise solid fractionates of the Kula lavas, formed at the floor or roof zones of a suite of synchronously active mid- to lower crustal magma lenses or (?sill-like) chambers, and in narrow, tectonically active conduits. The local occurrence of nodules is probably because the plumbing system in this region was used by a series of eruptions which occurred over a short time interval during which solidification of the conduit system was incomplete. The amphibole nodules generally contain more vesicular glass than clinopyroxene-dominated nodules. Comparison of the textures with those of other studies suggests that the highly porous amphibole-dominated nodules were derived from a relatively porous thick mush. The highly crystalline clinopyroxene nodules were most likely excavated from the deeper levels in the mush. At least part of the crystal mush was layered compositionally, with irregular variations in porosity and grain size attesting to the dynamic nature of crystallisation in such a small, short-lived system. The presence of adcumulates together with poikilitic heteradcumulates points to a highly variable degree of liquid exchange with the magma adjacent to the mush zone.