Magmatic and tectonic evolution of the Yuntdag magmatic complex (Western Anatolia)

ABSTRACT In the Aliağa-Foça region, three main rock associations crop out. These are, 1. The Yuntdağ volcanics, 2. The Foça volcanic complex and 3. Aliağa limestone. The Yuntdag volcanics form the oldest unit in the area and consist of a thick and widespread andesitic- trachyandesitic lava sequence and accompanying coarse grained volcaniclastic rocks. In the Yuntdağ volcanics four main rock units are distinguished which are 1. The Helvacı andeshes consisting of pink to gray, coherent lava flows of andesite composition, 2. The Çaltılı andeshes which are formed by the black, aphanitic andesite lava flows, 3. The Haykıran blocky pyroclastics which are made up of the blocky andesHes in fine grained tuffeceous matrix and 4. The Dumanlitepe dykes consisting of dykes and subvolcanic stocks of these coherent lavas in central parts of the main vents. In the Yuntdağ volcanic complex, 6 different volcanic fades are distinguished which are 1. Flow banded lavas, 2. Subvolcanic andesites, 3. Columnar jointed andesite, 4. Flow breccias, 5. Block and ash flow deposits and 6, Grain supported, andesite lava-scoria fades. Geochemically, the coherent fades of the Yuntdağ volcanics are andesite to dacite in composition and calc-alkaline in nature. The chemical composition of various rocks in die succession indicate that the Yuntdağ volcanic assocciation is made up of genetically related units and has evolved in continental margin. The Yuntdağ volcanics are overlain by the rhyolitic Foça volcanic complex. In this sequence, three different subunits are distinguished which are 1. The Foça rhyolites, 2. The Foça volcaniclastics and 3. Foça alkaline volcanics. The Foça rhyolites are made up of massive rhyolite lava flows and domes. The Foça rhyolites are surrounded by the Foça volcaniclastic sequnce in which three different subunits are distinguished which are 1. IVPyroclastic flows, 2. Hyaloclastic breccias, 3. Perthes. Close to the domes, coarse grained hyaloclastites and perlites cover the massive rhyolites and succeded laterally by fine grained, intensity to slightly welded ignimbrites. The lithological and sedimentological features of this unit show that the rhyolite domes intruded phreatically into a shallow lacustrine environment and the accompanying pyroclastic sequence formed as blankets surrounding the domes. Since the shallow lakes filled by the rapid deposition, in upper parts of the hyaloclastites subaerial pyroclastic rocks were formed characterized by channel-fill deposits, in the upper parts of the rhyolitic volcaniclastic sequence, there are alkaline lava flows interdigitating with the volcaniclastics and lacustrine sedimentary rocks. In the Foça volcanic complex 13 different facies are distinguished depending on their conditions of formation. These are 1. Massive rhyolite, 2. Massive perlites, 3. In-situ hyaloclastic breccias, 4. Reassembled hyaloclastic breccias, 5. Laminated mudstone, 6. Fine grained rhyolite pumice-lithic fragment facies, 7. Coarse grained rhyolite pumice- lithic fragment facies, 8. Lithic fragment-rich channel-fill facies, 9. Subaqueous welded ignimbrite facies, 10. Brecciated perlite, 11. Subaqueous ash fallout deposits, 12. Coherent alkaline lavas and 13. Alkaline dykes. Geochemically, the Foça rhyolites are calc-alkaline in nature. They resembles to a volcanic association of a post orogenic tectonic setting. The alkaline lava flows show chemical characteristics of within plate basalts or rift-related volcanics. The Foça volcanic complex laterally and vertically passes into the Neogene Aliağa limestone consisting of white to yellow, gastropoda-bearing limestones and clayey limestones. The stratigraphic relations and geochemical affinities of die Yuntdağ volcanics, the Foça volcanic complex and the Foça alkaline volcanics indicate that they were formed proggressively under a single and genetically interrelated tectonic regime.