Petrographical And Geochemical Characteristics of The Zeolitic Akdere Tuff (Demirci, Western Anatolia)

One of the important Neogene basins of Western Anatolia (Turkey) is the NE-SW trending Demirci Basin. The Miocene sequence, consisting of approximately 1000 m thick clastics and volcanics, is unconformably located on the Paleozoic-Menderes Metamorphic Massif (gneiss, schist, marble). The lower part of Miocene sequence contains block–pebble–sand size of clastics. They are continued upward with sandstone, claystone, marl, limestone, mudstone, and shale lithologies laterally transition with pyroclastic rocks at the top. Pyroclastic rocks (Akdere Tuff) are covered by a lacustrine unit consisted of sandstone, mudstone, bituminous shale limestone and tuffite. Akdere tuff, with rhyodacite character, light beige-gray-green color, 10-70 m thickness, is highly zeolitized (heulandite-clinoptilolite). At the bottom of the tuff unit, there are 25-30 m thick, fine-coarse grained, pumiceous, glassy, and glassy-crystalline ash tuffs containing rare lithic fragments. These are overlain by 20-25 m thick glassy dust-ash tuffs. At the top there is a 5 m thick pumicite level with lateral transitions. Zeolitic Akdere tuff petrographically contains small and rare amounts of mineral and lithic fragments in a matrix consisting of glass fragments with concave, sharp straight or curvilinear edges and rarely pumice fragments. Mineral fragments (2-25 %) are feldspars (albite-oligoclase type of plagioclases and K-feldspar) quartz, biotite, muscovite, amphibole, and opaque minerals. According to the X-ray diffraction (XRD) results, the mineralogical composition throughout the samples is 'heulandite-clinoptilolite (hul-cpt) + opal-CT + opal-A + smectite + illite-mica + quartz + feldspar’. The heulandite-clinoptilolite ratio in the samples examined varies between 0-95 percent. The glassy groundmass of the tuffs is almost completely zeolitized. Heulandite-clinoptilolite grains, formed by the transformation of volcanic glass, are in the form of monoclinic plates, shorter than 15 µm according to the results obtained from scanning electrone microscope (SEM) studies. SEM studies also showed that devitrification of volcanic glass formed firstly a gel-like phase before the formation of heulandites-clinoptilolites. Cationic values ​​(Si/Al and Na+K/Ca+Mg ratios) taken from heulandite-clinoptilolite grains by Energy dispersive X-ray spectroscopy (SEM-EDX) gave the composition of both heulandite and clinoptilolite. However, thermal stability results indicated clinoptilolite, and clinoptilolite did not deteriorate at 550 ^oC. Whole-rock chemical values ​​and some chemical parameters of the zeolitic Akdere Tuff are as follows. SiO₂: 68.45-76.50 %, Al₂O₃: 10.10-14.30 %, SİO₂/Al₂O₃: 4.82-7.19, K₂O: 1.34-4.46 %, Na₂O: 0.13-2.92 %, CaO: 0.70-2.92 %, MgO: 0.59-4.85 %, Fe₂O₃: 0.68-1.82 % and (Na₂O+K₂O)/(CaO+MgO): 0.33-5.63. The chemical composition of zeolitized tuffs in the Demirci region is a function of selectivity of zeolite minerals additionally to the primary composition of the volcanism. Tuff samples containing high amounts of hul/cpt have higher values ​​for CaO, H2O, and most trace elements and REEs than the samples containing little or no zeolite.

 

Key words: Clinoptilolite, Demirci, Geochemistry, Turkey, Zeolitization.