Causes and effects of geochemical variations in late Cenozoic volcanism of the Foca volcanic centre, NW Anatolia, Turkey

The Foca volcanic centre (FVC) occupies a NNE-SSW-oriented highland between two EW-trending structural grabens in western Anatolia, and includes early-middle Miocene mafic and felsic extrusive suites. Its evolutionary history consists of an older volcano stage (16.6-16.1Ma) and a younger volcano stage (15.2-14.1Ma), which are characterized by different eruption styles and compositional and geochemical features. The older units include high-K calc-alkaline basalt, andesite, trachyandesite, rhyolite, and associated pyroclastic rocks, which formed during ignimbrite eruptions and plinian-subplinian air-fall episodes. The younger sequences are composed of shoshonitic-alkaline basalt lavas and dikes, trachytes, phonolites, and phonolitic ignimbrites that formed strombolian cones. The Foca volcanic rocks display high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7075-0.7082 for the calc-alkaline mafic lavas, 0.7073-0.7064 for calc-alkaline felsic lavas, and 0.7063-0.7075 for the alkaline series) and low ¹⁴³Nd/ ¹⁴⁴Nd (0.5123-0.5125 in both series with ^εNd values varying from -1.3 to -6.0). These FVC geochemical features are consistent with those of other volcanic centres in western Anatolia (i.e. Bodrum, Urla-Cumaovas) and on the Aegean islands (i.e. Samos, Patmos, Chios). The geochemical and Sr-Nd isotopic compositions of the Foca volcanic units suggest that both lithospheric and asthenospheric mantle melts were involved in their evolution; however, the mantle lithosphere fingerprint was diminished by the middle Miocene, as the asthenospheric mantle melt input became dominant. These findings, combined with the bimodal character of post-collisional volcanism in the study area, suggest that geochemical variations in the nature of volcanism from calc-alkaline to alkaline and the changes in tectonic regimes through time may have been caused by successive thermal relaxations associated with possible 'piecemeal' removal of the base of subcontinental lithospheric mantle beneath western Anatolia. This interpretation is more plausible than a catastrophic collapse or wholesale delamination of the entire lithospheric mantle. Asthenospheric upwelling caused by this inferred convective thinning provided underplating of mantle-derived magmas, which interacted with the previously metasomatized lithospheric mantle and the overlying crust, resulting in their partial melting and in production of high-K calc-alkaline to mildly alkaline, incompatible element enriched magmas in separate magma chambers in which fractional crystallization occurred.