SR-ND-PB ISOTOPIC AND AGE CONSTRAINTS ON THE ORIGIN OF LAVAS FROM THE ACIGOEL COMPLEX, NEVSEHIR, CENTRAL TURKEY

The Acigöl Volcanic Complex is a major late Quaternary rhyolitic centre within the Volcanic Province of Cappadocia in central Anatolia where paroxysmal silicic eruptions resulted in the formation of a caldera, c. 6 km in diameter¹. The Late Pleistocene evolution of the Acigöl complex initiated with the eruption of pre-caldera Lower Acigöl Tuff and associated rhyolite lavas (Group I: Boğazköy and Taşkesik), a major obsidian source in Turkey. This stage was followed by the caldera-forming eruption of the voluminous Upper Acigöl Tuff and of the coeval Kocadağ rhyolite dome (Group II). Late post-caldera magmatism comprises five rhyolitic domes (Group III) and maars within the caldera floor. The Acigöl caldera is bordered by Quaternary basaltic and andesitic lava flows and scoria cones that are inferred to post-date the rhyolite domes¹ and, altogether, form a typical bimodal magma suite attributed to partial fusion of the crust and input of melts from the mantle. Limited dating is mainly based on obsidian fission track ages that suggest eruption ages between ~190 ka and ~20 ka².

Here, we present strontium, neodymium, and lead isotopic data for silicic rocks within the Acigöl complex as well as for peripheral basalts. Aphyric rhyolite lava, tephra, and pumice (Group I-III rhyolites) define narrow ranges in ¹⁴³Nd/¹⁴⁴Nd isotope ratios (0.51257-0.51265, or e_Nd(t) from –1.4 to +0.2), and show virtually no difference in Pb isotope composition (²⁰⁶Pb/²⁰⁴Pb: 18.87-18.88, ²⁰⁷Pb/²⁰⁴Pb: 15.65-15.67 and ²⁰⁸Pb/²⁰⁴Pb: 38.94-38.98). In terms of Sr isotopes, Group III rhyolites extend to markedly more radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.7065-0.7091) compared to those of Group I and II (0.7059-0.7065). Isotopic ratios correlate with indices of differentiation such as SiO₂, suggesting that Group I to III rhyolites are genetically closely related. More radiogenic Sr isotopic compositions in Group III rhyolites are intriguing as they could reflect isotopic heterogeneity in the crustal sources. Regional basement (Mesozoic granites) has by far too primitive Sr-Nd isotopic compositions for representing a possible protolith of the Acigöl lavas, and therefore more radiogenic source rock must abound at depth. Amongst the basaltic association, the most mafic sample (basalt lava flow) is characterised by ⁸⁷Sr/⁸⁶Sr = 0.7040, ¹⁴³Nd/¹⁴⁴Nd = 0.51280 (e_Nd(t) +3.6), ²⁰⁶Pb/²⁰⁴Pb = 18.85, ²⁰⁷Pb/²⁰⁴Pb = 15.65, and ²⁰⁸Pb/²⁰⁴Pb = 38.90. These values suggest a moderately depleted signature for the mantle source. Intermediate compositions between basalt and rhyolite (Group I-III) imply mixing between mantle and crustally derived components. We are further investigating the temporal evolution of the Acigöl Volcanic Complex and the genetic relation between Group I-III rhyolites through detailed zircon geochronology using (U-Th)/He and U-Th disequilibrium methods.

¹Druitt et al. (1995): J. geol. Soc. London 152: 655-667

²Bigazzi et al. (1993): Bull. Volcanol. 55: 588-595