Impact of volcanism on the evolution of Lake Van I: evolution of explosive volcanism of Nemrut Volcano (eastern Anatolia) during the past >400,000 years

Abstract

The historically active Nemrut Volcano (2,948 m asl) (Eastern Anatolia), rising close to the western shore of huge alkaline Lake Van, has been the source of intense Plinian eruptions for >530,000 years (drilled lake sediments). About 40 widespread, newly recognized trachytic and less common rhyolitic fallout tephras and ca. 12 interbedded ignimbrites, sourced in Nemrut Volcano, are documented in stratigraphic traverses throughout an area of >6,000 km² mostly west of Lake Van. Phenocrysts in the moderately peralkaline trachytes and rarer large-volume comenditic rhyolites comprise anorthoclase, hedenbergite-augite, fayalite and, especially in trachytic units, augite, minor aenigmatite, apatite and quartz, and rare chevkinite and zircon. Dacitic to rhyolitic tephras from nearby calcalkalic Süphan Volcano (4,058 m asl), locally interbedded with Nemrut tephras, are characterized by disequilibrium phenocryst assemblages (biotite, augitic clinopyroxene and hypersthene, minor olivine, common crystal clots and/or, in some deposits, amphibole). The magma volume (DRE) of the largest Nemrut tephra sheet (AP-1) described in detail may exceed 30 km³. Extreme facies and systematic compositional changes are documented in the ca. 30 ka Nemrut Formation (NF) deposits formed from one large and complex eruption (thick rhyolitic fallout overlain by ignimbrite, welded agglutinate, overbank surge deposits, and final more mafic fallout deposits). Common evidence of magma mixing in Nemrut ignimbrites reflects eruption from compositionally zoned magma reservoirs. Several young Çekmece Formation trachytes overlying ca. 30 ka old NF deposits and the late trachytes of the NF deposits show compositional affinities to tephra from Süphan Volcano possibly due to temporary influx of Süphan magmas into the Nemrut system following the evacuation of >10 km³ magma (DRE) during the caldera-forming NF eruption. Axes of large fallout fans are dominantly SW–NE but W–E in the younger sheets resembling the direction of the present dominant wind field. Growth of Nemrut volcanic edifice and its peripheral domes since before 0.5 Ma in the hinge area between the Van and Muş tectonic basins is likely to have been the major factor in isolating Lake Van basin thus initiating the origin and subsequent alkaline evolution of the lake. This alkalinity was later significantly controlled by climate forcing. Internal forcing mechanisms (volcanic and geodynamic) may also have contributed to major lake level changes in addition to climate forcing.

Appendix

Bulk XRF analyses

Selected rock fragments (pumice lapilli) were crushed in a jaw crusher, pulverized in agate mills and dried at 110 °C in an oven for about 12 h to drive off moisture. Fused beads were prepared by mixing 600 mg sample powder with 3,600 mg lithium tetraborate (Li₂B₄O₇). The mixture was melted at about 1,000 °C; the melt was cast into a preheated mold and slowly cooled to avoid shattering due to thermal stress. Major elements and selected trace elements were determined on a fully automated WDS X-ray fluorescence spectrometer equipped with a Rh-tube (Panalytical Magix Pro). International rock standards prepared as described above were used for internal calibration according to the recommended values given by Govindaraju (1994). Several international standards were run as unknowns and the measured values are in good agreement with the recommended values. Detection limits for major elements are <0.01 wt% except for Na₂O and MgO, which are <0.02 wt%. Detection limits for trace elements are generally lower than 10 ppm except for Ba, La, Ce, and Nd which are ca. 20 ppm. LOI (loss-on ignition) was determined gravimetrically at 1,050 °C for 1 h (Lechler and Desilets 1987).

EMP analyses

Major elements and S, Cl, and F of glassy groundmasses and minerals of tephra particles were carried out with a JEOL JXA 8200 electron microprobe (EMP) at the GEOMAR Helmholtz Institute for Ocean Research Kiel. Analytical conditions were 15 kV of accelerating voltage, 6 nA of beam current for felsic glass, 10 nA for sideromelane, feldspar, amphibole, and mica, 20 nA for pyroxene, and 20 s of peak counting time. Analyses were performed with an electron beam rastered to 5 × 5 μm in the scanning mode for felsic glass, sideromelane, feldspar, and 10 × 10 μm for other minerals. Basaltic glasses USNM 111240/52 and USNM 113498/1 (VG-A99), apatite USNM 104021, and microcline USNM 143966 (Jarosewich et al. 1980) were used as standards for calibration.

Analyses of phenocrysts of feldspar, pyroxenes, olivine and glass shard were selected from either grain mount polished sections of sieved tephra samples (generally −1 to −2 phi) or polished sections of solid rock (or pumice) samples. As shown in Tables 2, 3, 4 and 5, several points were analyzed in single grains (number of analyses) and recalculated as average. We here only show analyses of the cores of phenocrysts.