ALBERT

Description: Thirty-two new single crystal ages document 400 000 years of widespread explosive volcanism of historically active Nemrut Volcano towering over huge alkaline Lake Van (Eastern Anatolia). The dated deposits were selected to monitor the volcanic and compositional evolution of Nemrut Volcano through time and thus to provide a rigorous temporal framework for the tephra record of the PaleoVan Drilling Project. Tephra samples were taken from large-volume deposits or those that occur in medial to distal localities, well-exposed stratigraphic sections or from the initial phase of an eruptive sequence. Mainly fallout deposits were chosen because most ignimbrites show more complex and corroded feldspar populations owing to compositional zoning and magma mixing. Moreover, fallout deposits held the promise to be more clearly identifiable with-and correlatable to-〉300 tephra layers in the PaleoVan drill cores, even though commonly in amounts marginal or insufficient in thickness to allow well-supported single crystal dating. The crystals dated are dominantly anorthoclase, the main phenoctyst phase in the trachytic to rhyolitic, slightly to strongly peralkaline Nemrut magmas. Ages obtained so far range from ca. 400 ka to ca. 30 ka for Nemrut Volcano. The causes of significant changes in the frequency, volume and composition of tephra layers per unit time are discussed in terms of external (erosion, climate changes, geodynamic factors) and internal forcing (changes in magma supply and composition and incubation periods preceding large volume rhyolitic eruptions). For example, the low frequency of tephra layers deposited prior to ca. 200 ka may be due to low explosive activity, severe erosion between MIS 9 and MIS 11, or both. Nevertheless, the overall frequency of explosive eruptions appears to have increased during the past ca. 200 ka. We also recognize a slight peak in explosive eruptions during warm periods (e.g. MIS 5 and MIS 7) and speculate on lithospheric unloading triggering increased partial melting or magma reservoir unloading following massive glacier melting. The ages of 5 dated ignimbrites span ca. 250 000 years suggesting that Nemrut Volcano went through a polycyclic evolution with multiple caldera collapses and major pyroclastic flow eruptions, the oldest dated so far as 265 ka. The widely held view of the impressive Nemrut Caldera now dated to have formed at ca. 30 ka, as the main paroxysmal event during the evolution of the volcano is no longer tenable. Distinct and coherent compositional characteristics, especially in trace element concentrations, characterize several groups of trachytic tephras. We speculate that the growth of Nemrut Volcano caused the isolation of the Lake Van basin. On account of their mineralogical (anorthoclase, hedenbergite, fayalite, aenigmatite) and alkalic chemical compositions and large volume, dated Nemrut fallout tephras are likely to represent excellent markers in lakes and other sites of paleoclimatological or archeological interest in neighboring countries to the northeast of Lake Van as far as the Caspian Sea in what may be called the East Anatolian Tephra Province