Biltan Kurkcuoglu

Geochemical approach to magmatic evolution of Mt. Erciyes stratovolcano Central Anatolia, Turkey

Erciyes stratovolcano, culminating at 3917 m, is located in the Cappadocian region of central Anatolia. During its evolution, this Quaternary volcano produced pyroclastic deposits and lava flows. The great majority of these products are calc-alkaline in character and they constitute Kocdag and Erciyes sequences by repeated activities. Alkaline activity is mainly observed in the first stages of Kocdag and approximately first-middle stages of Erciyes sequences. Generally, Kocdag and Erciyes stages terminate by pyroclastic activities. The composition of lavas ranges from basalt to rhyolite .48.4-70.5 wt.% SiO . Calc-alkaline rocks are represented mostly by andesites and dacites. Some compositional 2 differences between alkaline basaltic, basaltic and andesitic rocks were found; while the composition of dacites remain unchanged. All these volcanics are generally enriched in LIL and HFS elements relative to the orogenic values except Rb, Ba, Nb depleted alkaline basalt. 87 Srr 86 Sr and 143 Ndr 144 Nd isotopic composition of the volcanics range between 0.703344-0.703964, 0.512920-0.512780 for alkaline basalts and change between 0.704322-0.705088, 0.512731-0.512630 for alkaline basaltic rocks whereas calc-alkaline rocks have relatively high Sr and Nd isotopic ratios 0.703434-0.705468, . 0.512942-0.512600 . Low Rb, Ba, Nb content with high ZrrNb, low BarNb, LarYb ratio and low Sr isotopic composition suggest an depleted source component, while high Ba, Rb, Nb content with high LarYb, BarNb, low ZrrNb and low 87 Srr 86 Sr ratios indicate an OIB-like mantle source for the generation of Erciyes alkaline magma. These elemental and ratio variations also indicate that the different mantle sources have undergone different degree of partial melting episodes. The depletion in Ba, Rb, Nb content may be explained by the removal of these elements from the source by slab-derived fluids which were released from pre-collisional subduction, modified the asthenospheric mantle. The chemically different mantle sources interacted with crustal materials to produce calc-alkaline magma. The BarNb increase of calc-alkaline samples indicates the increasing input of crustal components to Erciyes volcanics. Sr and Nd isotopic compositions and elevated LIL and HFS element content imply that calc-alkaline magma may be derived from mixing of an OIB-like mantle melts with a subduction-modified asthenospheric mantle and involvement of crustal materials in intraplate environments. q 1998 Elsevier Science B.V. All rights reserved.

Volcanological evolution of Mount Erciyes stratovolcano and origin of the Valibaba Tepe ignimbrite (Central Anatolia, Turkey)

Abstract Mount Erciyes (3917 m) is the largest stratovolcano of Central Anatolia (Turkey). The volcanological evolution of Mount Erciyes from Pliocene–Quaternary to historical times exhibits two distinct stages: (1) Koc Dag and (2) Erciyes. During the Koc Dag stage, basaltic and andesitic lava flows were emitted from the cinder cones of Kizil Tepe and Topakkaya Tepe. Then, Koc Dag pyroclastics were emplaced, leading to a caldera collapse of 14×18 km in diameter with a volume estimated at 110 km3. Two eruption phases separated by scoria fall and mud flow deposits are recognised associated with the caldera forming event. During Phase 1, plinian fall and pumice flows were emplaced on the eastern part of the volcano. Plinian fall deposits occur more than 50 km away from the source area. The volume of tephra is estimated at 63.3 km3 (16 km3 dense rock equivalent (DRE)), including 62 km3 of plinian fall and 1.3 km3 pumice flow deposits. Phase 2 mainly consists of pyroclastic flow deposits (two pumiceous flow units and Valibaba Tepe Ignimbrite (VTI)). The pumiceous flow deposits (4.2 km3 DRE) extend 30 km from the inferred source area towards the north and northeast. The final pyroclastic flows of Phase 2 occurred 2.8 Ma ago and produced the VTI. The VTI is a low aspect ratio welded ignimbrite and its volume is estimated at 40 km3 DRE. Plinian fall deposits (0.8 km3 DRE) preceded the VTI and are only observed on the eastern part of the volcano, covering an area of 1500 km2. During the Erciyes stage, two eruptive cycles are defined. The first cycle (2.6–0.17 Ma) is characterised by effusive, extrusive and weakly explosive activity with emplacement of andesitic lava flows, dacitic dome flows, basaltic andesite lava flows, and andesitic cones. The second cycle involved dacitic extrusive and explosive activity, located at the summit area, and produced block-and-ash flows derived from rhyodacitic domes. Plinian fall, surge and pumice flow deposits were emplaced prior to rhyodacitic dome extrusions. The last event was the emplacement of debris avalanche deposits related to sector collapse caldera.

Interaction of Asthenospheric and Lithospheric Mantle: The Genesis of Calc-alkaline Volcanism at Erciyes Volcano, Central Anatolia, Turkey

The quaternary Erciyes stratovolcano and its associated monogenetic cones exhibit tholeiitic, alkaline, and calc-alkaline suites, with the calc-alkaline series dominating. Calc-alkaline products are andesitic, dacitic, and rhyodacitic, showing moderate to low MgO and high K2O, plus LIL and HFS element contents. The andesites display trace-element patterns similar to those of the Central Mexican Volcanic belt, the Colville igneous complex (northeastern Washington, USA) and the Taos Plateau volcanic field (in north-central New Mexico, associated with northern Rio Grande Rift). Dacites from Erciyes also resemble these igneous complexes and give patterns similar to those of the upper crust. Furthermore, the high Ba/Nb ratio (> 28) is typical of arc volcanism (Fitton et al., 1988) ranging from 27 to 47 for andesites, and 27 to 50 for dacites at Erciyes. Calc-alkaline suites also show high Rb/Nb ratios (4-7.5). Moreover, 87Sr/86Sr and 143Nd/144Nd ratios range from 0.703434 to 0.705143, and 0.51294 to 0.5126 for andesites and dacites, respectively. These incompatible element and isotopic ratios suggest two possible mechanisms for the genesis of calc-alkaline suites: subduction enrichment or crustal involvement. Isotopic similarities between tholeiitic and andesitic rocks, LIL and HFS element compositions, and spider plots all indicate that various source components have been involved in the genesis of these products. The calc-alkaline products of Erciyes volcano were generated by the mixing of partial melts derived from depleted (tholeiitic) and enriched asthenospheric sources. In addition, the calc-alkaline suite was derived from an initially tholeiitic basalt end-member via a 25-30% assimilation of crustal material. Thus, post-collisional extension led to development of the Erciyes volcanism in central Anatolia. Calc-alkaline products were widely generated by the combined effects of mixing of depleted and enriched asthenospheric sources, together with a progressive AFC process that started from an initially tholeiitic component, without direct melting of the pre-subducting slab.

Trace-Element Modeling and Source Constraints for Tholeiitic and Cale-alkaline Basalts from a Depleted Asthenospheric Mantle Source, Mt. Erciyes Stratovolcano, Turkey

The Mt. Erciyes stratovolcano was built up in an intraplate tectonic environment as a consequence of Eurasian and Afro-Arabian continental collision. However, the volcanic products generally exhibit a calc-alkaline character; minor amounts of tholeiitic basalts are also present. Tholeiitic basalts show high Fe2O3, MgO, CaO, low K2O, and depleted Ba, Nb, and especially Rb (2.3-5.97 ppm) contents, low 87Sr/86Sr (0.703344-0.703964), and high 143Nd/144Nd (0.512920-0.512780) isotopic ratios. These compositional features show that they were derived from a depleted asthenospheric mantle source, possibly a MORB-like source component. In contrast, calc-alkaline basaltic rocks exhibit relatively high large-ion-lithophile and high-field-strength elements, high 87Sr/86Sr (0.704591-0.70507) and low 143Nd/144Nd (0.51272-0.512394) isotopic ratios. The bulk-rock chemistry of the tholeiitic basalts reflects the chemical composition of the extracted source component. Furthermore, trace-element concentrations may be calculated from an accepted mantle source component (starting composition) for different degrees of partial melting. These calculations also provide a sensitive approach to the origin of tholeiitic basalts. Modeled trace-element compositions of tholeiitic basalts are calculated from a primitive mantle composition. Calculated trace-element compositions imply that tholeiitic basalts are derived by minor fractional melting (1-1.5 %), in the absence of assimilation or deep-crustal melting. The calc-alkaline basalts were subsequently produced from initially tholeiitic basalts by the way of an AFC (assimilation-fractional crystallization) process, with a crustal assimilation of 10-15 %. The geochemical data, partial melting, and AFC modeling all indicate that basaltic products have a complex evolutionary history involving partial melting from a MORB-like mantle source. The assimilation and fractional crystallization processes are considered as providing an example for the chemical evolution of basaltic products, from tholeiitic to calc-alkaline, in an intraplate environment.