Neogene alkaline volcanism of the Afyon-Isparta area, Turkey: petrogenesis and geodynamic implications

Abstract

In the Kirka-Afyon and Isparta areas, potassic and ultrapotassic alkaline magmatism followed Upper Eocene – Middle Miocene calc-alkaline volcanism, which was related to the northward subduction of the African Plate beneath Eurasia. The alkaline volcanism lasted from 14.8 Ma (in the north) to 4 Ma (in the south) and was associated with a presently still active extensional tectonic regime. It is mainly located along a N-S fault, which marks the limit between the Aegean Arc and the Cyprus Arc (Isparta Angle). In the Kirka-Afyon area the potassic to ultrapotassic rocks range from silica saturated (trachybasalts to trachytes) to highly undersaturated (phonolitic leucitites). In the Isparta area ultrapotassic rocks occur together with rhyolitic lavas. In both areas, the ultrapotassic magmatism is transitional between lamproitic and Roman type, with a more lamproitic character for the Isparta rocks. The potassic suites of both the Kirka-Afyon and Isparta areas are characterised by rocks with Sr isotope ratios increasing from mafic to silicic rocks. The rocks from Kirka-Afyon area have an orogenic affinity, whereas those from Isparta area exhibit orogenic (the potassic suite) and within-plate (the ultrapotassic suite) affinities. Extreme enrichments in LIL elements characterise the Afyon phonolitic leucitites, whereas enrichments in Sr and REE are observed in Isparta potassic rocks. A residual, probably lithospheric mantle metasomatised by fluids and/or melts of different origin (subduction-related for the Kirka-Afyon rocks; deep astenospheric origin for the Isparta rocks), is considered to be the source of the ultrapotassic magmas. A more “fertile” and probably deeper mantle, enriched by subduction-related components, is believed to be the source of the potassic rocks. Lower degrees of partial melting, in the mantle, at higher pressure, and probably at higher XCO2, may account for the genesis of the phonolitic leucitite parental magmas. Partial melting of the mantle took place simultaneously in both sources, probably due to post-collision extensional tectonics, which was also responsible for the uprise of deep astenospheric mantle.