Evidence on Mixing of Mantle and Crustal Derived Magmas in Bodrum (Mugla) Area Volcanic Rocks, Southwest Turkey

Abstract

The study area is located at Turgutreis-OrtakentYalikavak-Akyarlar areas, the west of Bodrum (Mugla) in the Southwest Turkey where calcalkaline, alkaline and locally shoshonotic volcanism were effective after the end of Middle Miocene. These volcanics has been attributed to continental rifting due to tensional regime (e.g. Ercan et al., 1984). Tensional tectonics in the region affected thickened and partially melted continental crust, and caused the mixing of crustal calcalkalineand mantle derived alkaline magmas. Two volcanic cycles were effective in the Bodrum Peninsula during Middle-Upper Miocene times. First cycle volcanites are tuffagglomerate, high-K calcalkaline andesite-daciterhyodacite-rhyolite suite developed at the end of Middle Miocene. This cycle ceased with shoshonitic latite and trachyandesite, and second cycle developed with shoshonitic basalt, trachybasalt and trachyte as small outcrops and dykes in Upper Miocene (Ercan et al., 1984). The volcanites studied, namely andesite, latite, dacite, rhyodacite and rhyolite (Fig. 1), show generally porphyric and sometimes felsitic, hyalopilitic, microlitic, pilotaxitic textures. Phenoand microphenocrystals are plagioclase, biotite, hornblende, quartz, augite and sanidine. Groundmass is made up generally by feldspar microlites and/or glass, and sometimes alteration products. Plagioclases are subhedral laths and rarely resorbed and fragmented crystals. Some of them commonly display sieve texture with mottled extinction or patchy zoning, and inclusions of pale grown glass, sometimes with an overgrowth rim which may be separated from the cores by a dark grey zone of 'clouded' plagioclase charged with micron-sized melt inclusions. Plagioclases also show discontinuous and sometimes oscillatory zoning. Hornblendes are subhedral to anhedral, but often 'ghost like' crystals, pseudomorphed by opaques, pyroxene, feldspars and quartz or simply rimmed by opaques. Biotites are subhedral to anhedral with common reaction rims or breakdown products, or pseudomorphed by opaques. Breakdown of biotite forms opaques or a granular intergrowth of opx + ilmenite + magnetite + K-feldspar. These reaction products of hornblend and biotite were caused by dehydration above the hornblend and biotite stability limit as result of magma mixing (e.g. Nixon, 1988). The rocks are generally high-K calcalkaline in compos i t ion , having 4 7 % K20 (Fig. 1), 15.8-17.8% AlaO3, 26.3-47.1 Mg-number, low Ni, Cr, Co and high La, Ce contents. Furthermore low Na20/K20, Nb, TiO2 (<1%) and high LILE contents resembles volcanites developed at active continental margins. Geochemical variations show that the rocks are cogenetic and related to each other by fractional crystallisation, assimilation and magma mixing processes during the evolution of crustal magma chambers. Major and trace element variations can be explanied by fractionating phenocrystal phases. Low Y and high La/Y may imply that