Geology and diagenesis of a zeolitic Foça tuff unit deposited in a Miocene phreatomagmatic lacustrine environment (western Anatolia)

Turkish Journal of Earth Sciences http://journals.tubitak.gov.tr/earth/ Research Article Turkish J Earth Sci (2013) 22: 611-631 © TÜBİTAK doi:10.3906/yer-1203-11 Geology and diagenesis of a zeolitic Foça tuff unit deposited in a Miocene phreatomagmatic lacustrine environment (western Anatolia) Mustafa ALBAYRAK, Abdullah Mete ÖZGÜNER* Mineral Research and Exploration General Directorate, Ankara, Turkey Received: 19.03.2012 Accepted: 31.10.2012 Published Online: 13.06.2013 Printed: 12.07.2013 Abstract: This study explains the relations between the depositional environment of a zeolitic tuff unit and its diagenesis. It gives new ideas of the juvenile phreatomagmatic origin of the zeolitic unit with its bentonitic cap rock, and finds that the diagenetic alteration of the volcanic ash deposition in a hot hydrologic system is penecontemporaneous and not epigenetic. A massive, fine-grained zeolitic unit has a sharp contact with the underlying shallow subaqueous rhyolitic dome intrusions and their surrounding volcanoclastic ejecta. Juvenile emanations from the basal subaqueous intrusions activated thorough zeolitic diagenesis within the overlying rhyolitic tuff deposits extending as far as the periphery of the underlying intrusions. The bentonitic cap rock suggests that the diagenesis diminished vertically with the weakened phreatomagmatic activity up to the overlying limestone. The lack of sedimentary evaporite minerals and scarcity of boron-bearing authigenic K-feldspar indicate a nonsaline-alkaline depositional and diagenetic environment during the zeolitic transformation. Geochemical data from the zeolitic tuff samples indicated that the main diagenetic factors were hydrolysis of the glassy tuff in an open hydrologic system, under high heat flow rates and one of several scales of ion transfer. Zeolitisation developed with a significant loss of alkaline elements and iron oxide, which were compensated for by an important gain in the alkaline earth elements and absorption of strontium. The rhyolitic glass was altered by hydrolysis to form smectite and clinoptilolite, resulting in the release of excess silica that was not removed from the system but was changed in crystal form to opal-CT. Key words: Clinoptilolite, hydrolysis, opal-CT, peperites, pyroclastic flow, rhyolitic ash, subaqueous intrusion 1. Introduction The study area is situated north of the city of İzmir and the Güzelhisar River, between the gulfs of Aliağa and Çandarlı on the Aegean coast of western Turkey (Figure 1). Neogene volcano-sedimentary basins in western Anatolia contain evaporites and diagenetically related important industrial minerals resources, such as borates, gypsum (+celestite), industrial clays, and zeolites, within subaqueously deposited calc-alkaline volcanoclastic sediments (Kumbasar et al. 1985; Helvacı et al. 1988; Yalçın 1988; Gündoğdu et al. 1989; Helvacı et al. 1993; Gündoğdu et al. 1996; Uz et al. 1996; Whateley et al. 1996). The zeolitic tuff unit, with its bentonitic cap rock in the study area, has similar subaqueously deposited calc-alkaline volcanic characteristics. Some other publications directly related to local zeolite occurrences in western Anatolia (e.g., Esenli 1986; Türkbileği 1988; Sirkecioğlu et al. 1990; Esenli & Özpeker 1993; Özpınar et al. 1999; Köktürk et al. 2000; Albayrak 2008) are mostly concerned with diagenetic, mineralogical, geochemical, and technological properties of zeolites, but their relations to the properties of the depositional environments have mostly been neglected or * Correspondence: not examined in detail. Most of them have a hydrothermal and epigenetic origin. The aim of this manuscript is to describe the geological properties of the depositional environment of the Foça zeolitic unit and their relations to its diagenesis. The origin of the Foça zeolitic unit, with its bentonitic cap rock, was a penecontemporaneous diagenetic alteration of subaqueous volcanic ash fall deposits in a hot juvenile phreatomagmatic environment, and had a vertical zonation with the overlying bentonitic cap rock. The subaqueous-subaerial properties and calcalkaline–alkaline evolution of Neogene volcanism in the Foça region, immediately south-west of the study area, were examined by Akay & Erdoğan (2001, 2004). They showed that mid-Miocene rhyolitic domes intruded directly into a subaqueous environment and formed hyaloclastic blankets in the region. Clinoptilolite-heulandite and opal-CT are the most abundant diagenetic minerals within rhyolite tuffs. According to Resing & Sansone (1999), the zeolitisation process started with clay rim formation around volcanic glass shards in weak acidic fluids, originating from 611 ALBAYRAK and ÖZGÜNER / Turkish J Earth Sci N BLACK SEA İstanbul AEGEAN SEA Ankara TURKEY İzmir Antalya 300 km 0 MEDITERRANEAN Location map of the study area. D Altınova İ K KOZAK PLUTON İ L İ Bergama G yunt. R A Dikili B E N yunt. B E R G A M A G R A B E N N Zeytindağ Çandarlı Ça G nda ul rlı f ? 0 AEG EAN Aliağa Gulf lst ? lst Şakran Yuntdağ Volcano yunt. yunt. ? lst lst rhy. Dumanlıdağ Volcano Lacustrine limestone rhy Foça rhyolites yunt. Yuntdağ volcanics ? ? lst Meso. Mesozoic basement ? rhy. Foça Quaternary deposits yunt. Aliağa Miocene SEA Pınarcık lst rhy. 10 km ? Menemen Central volcanic vents Meso. yunt. ? Yamanlar Volcano ? ? Çiçekli İZMİR Inferred weakness zones in basement Probable direction of extentional forces Study area of zeolite Centres of settlement Figure 1. Distribution of the volcanic outcrops and main volcanic vents around the Aliağa-Pınarcık zeolite deposits. The regional weakness zones in the basement were inferred on the basis of the alignment of volcanic vents and dyke trends (partly after Akay and Erdoğan, 2004). 612 ALBAYRAK and ÖZGÜNER / Turkish J Earth Sci magmatic volatiles in the subaqueous environment, followed by hydrolysation of soluble cations, leading to a weakly basic environment for zeolitisation. Lander & Hay (1993), Ghiara et al. (1999), and Snelling et al. (2008) explained that clinoptilolite formed in situ from volcanic glass in rhyolitic tuffs and that excess silica was not removed from the system, but crystallised as opal CT. Hay (1963), Boles & Coombs (1975), Rice et al. (1992), Leggo et al. (2001), and Cocheme et al. (2003) further explained that the clinoptilolite, cristobalite, and possibly amorphous silica were pseudomorphic after rhyolitic glass shards, and that in such an environment smectites developed from the glass, forming a clay rim around the outline of the shards. 2. Materials and methods 2.1. Field methods Stratigraphical sections were determined in different parts of the volcano- sedimentary sequence. Detailed geological mapping of the study area was undertaken, concentrating on the origins of the volcano-sedimentary bodies, each time correcting it when different origins and mutual relations were discove (...truncated)