Interpretation of the origin of analcimes with mineralogical, microtextural, and geochemical investigations: a case study from Aktepe region (NE of Kalecik, Ankara, Central Anatolia, Turkey)

Abstract

The presence of analcime minerals in volcanic rocks is a topic requiring attention during classification of rocks. In fact, when transformation from leucite to analcime is present, the original chemistry of the rock will change and this situation will cause falsification in identification of the original character of the rock. As a result, it is important to determine the origin of the analcime in rocks containing analcime. This study focuses on the investigation of the origin of analcimes occurring in alkaline volcanic rocks (tephrite, phonotephrite) of the Aktepe region, located in the northeastern part of Ankara (Central Anatolia, Turkey). These volcanic rocks consist of abundant clinopyroxene, analcime, Fe-Ti oxides, minor olivine pseudomorphs, and scarce microcrystals of feldspar. The analcime phenocrysts are typically euhedral to anhedral (nearly rounded) in shape with up to 1 cm diameter and there are no other hydrous minerals in the samples. Leucite-like crystals are rarely preserved in the cores of larger analcime crystals and twin lamellas observed in them could demonstrate this preservation. Even partially transformed leucites are rare but not impossible to find in nature because of the rapid kinetics of the transformation of leucite to analcime. Examination of microprobe and EDS spectra, corona-type texture observed around the analcime–leucite interface suggest that some leucite crystals did not transform entirely to analcime crystals and a progressive transformation of leucite into analcime occurred locally in the region. Analcime and preserved leucite-like crystal existence could also be seen from the obtained X-ray diffractograms. The fresh fracture surfaces of separated large analcime crystals have microporous texture indicating a characteristic feature of analcimes that formed by an ion-exchange process from leucite. Geochemically, calculations of gains and losses of Na and K contents from the original bulk-rock chemical composition reveal that Na content increase is counterbalanced by a decrease in K content. This could also be strongly explained by an analcimization process and/or progressive transformation of leucite into analcime. It is concluded that analcimes formed secondarily from leucite by an ion-exchange process after cooling of magma via graded infiltration of Na-bearing fluids into the Aktepe ultrapotassic alkaline volcanic rocks.