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

Abstract

Upper Cretaceous–Paleocene calc-alkaline dacitic and rhyodacitic lava flows and domes are exposed in the Saraykent (Yozgat) region within Central Anatolian Crystalline Complex (CACC). Saraykent volcanics are generally plagioclase+hornblende+biotite+titanomagnetite±quartz±augite–phyric. Subrounded, vesicular magmatic enclaves, ranging from a few millimeters to a few centimeters in size, are present in the Saraykent dacites. The enclaves are mainly holocrystalline and have similar mineral assemblage to the host dacites. Almost all phases in Saraykent volcanics exhibit varying degrees of disequilibrium features indicative of magma mingling. Plagioclase phenocrysts display sieved and normal type populations, a wide range in rim compositions, and oscillatory zoning. Maximum core to rim compositional change in sieved rhyodacite plagioclases is up to 24 mol% increase in An. Some plagioclase core 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 rhyodacites. Hornblende and biotite phenocrysts have reaction rims indicating disequilibrium crystallization with magma. Both hornblende and biotite crystals show reverse zoning in terms of decreasing Mg/Mg+Fe. Quartz phenocrysts are corroded and embayed. Presence of normally and reversely zoned augites in the same sample, and high-Ti high-Al augites more comparable with basaltic 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 higher Mg/Mg+Fe 2+ ratios than the host dacite biotite crystals and display normal, reverse and oscillatory zoning. Plagioclase core compositions vary between 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 the 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 cores of plagioclases; heterogeneity of plagioclase rims; resorbed and embayed phenocrysts and augite compositions indicating a more mafic magma are enough to support the suggestion that magma mingling was a viable process in the generation of the Saraykent volcanics.