Mineral compositional and textural evidence for magma mingling in the Saraykent volcanics

Upper Cretaceous-Paleocene calc-alkaline dacitic and rhyodacitic lava flows and domes are exposed in the Saraykent (Yozgat) region within Central Anat olian Crystalline Complex (CACC). Saraykent volcanics are generally plagioc lase + hornblende + biotite + titanomagnetite +/- quartz +/- augite-phyric, Subrounded, vesicular magmatic enclaves, ranging from a few millimeters to a few centimeters in size, an present in the Saraykent dacites. The enclav es are mainly holocrystalline and have similar mineral assemblage to the ho st dacites. Almost all phases in Saraykent volcanics exhibit varying degrees of disequi librium features indicative of magma mingling. Plagioclase phenocrysts disp lay sieved and normal type populations, a wide range in rim compositions, a nd oscillatory zoning, Maximum core to rim compositional change in sieved r hyodacite plagioclases is up to 24 mol% increase in An. Some plagioclase co re compositions are unusually Ca-rich. They range from An(27) to An(75) for the Saraykent dacites and from An(30) to An(66) for the Saraykent rhyodaci tes. Hornblende and biotite phenocrysts have reaction rims indicating diseq uilibrium crystallization with magma. Both hornblende and biotite crystals show reverse zoning in terms of decreasing Mg/Mg + Fe. Quartz phenocrysts a re corroded and embayed. Presence of normally and reversely zoned augites i n the same sample, and high-Ti high-Al augites more comparable with basalti c compositions, are other lines of evidence for disequilibrium. The enclave hornblendes are both edenitic and pargasitic, the latter being more typical of mafic compositions. The enclave biotite crystals have highe r Mg/Mg + Fe2+ ratios than the host dacite biotite crystals and display nor mal, reverse and oscillatory zoning. Plagioclase core compositions vary bet ween An(44) and An(55) in the enclaves. Although the development of sieve texture in plagioclase and reaction rims on mafic minerals may be attributed to decompression, this cannot explain t he observed compositional change and normal type of plagioclase in the same sample. Similarly, a cognate (i.e. fractionation) origin for the enclaves can be suggested due to their mineralogical similarity to the host dacite, However, other criteria such as oscillatory zoning and unusually calcic cor es of plagioclases; heterogeneity of plagioclase rims; resorbed and embayed phenocrysts and augite compositions indicating a more mafic magma are enou gh to support the suggestion that magma mingling was a viable process in th e generation of the Saraykent volcanics. (C) 2001 Elsevier Science B.V. All rights reserved.