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Early Miocene to Quaternary evolution of volcanism and the basin formation in western Anatolia: a review
Journal of Volcanology and Geothermal Research 85 Ž1998. 69–82
Early Miocene to Quaternary evolution of volcanism and the basin formation in western Anatolia: a review Erkan Aydar
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Hacettepe UniÕersity, Department of Geological Engineering, 06532 Beytepe, Ankara, Turkey
Abstract Western Anatolia, largely affected by extensional tectonics, witnessed widespread volcanic activity since the Early Miocene. The volcanic vents of the region are represented by epicontinental calderas, stratovolcanoes and monogenetic vents which are associated with small-scale intrusions as sills and dykes. The volcanic activity began with an explosive character producing a large ignimbritic plateau all over the region, indicating the initiation of the crustal extension event. These rhyolitic magmas are nearly contemporaneous with granitic intrusions in western Anatolia. The ignimbrites, emplaced approximately contemporaneous with alluvial fan and braided river deposits, flowed over the basement rocks prior to extensional basin formation. The lacustrine deposits overlie the ignimbrites. The potassic and ultrapotassic lavas with lamprophyric affinities were emplaced during the Late Miocene–Pliocene. The volcanic activities have continued with alkali basalts during the Quaternary. q 1998 Elsevier Science B.V. All rights reserved. Keywords: ignimbrite; lava; lake; extension
1. Introduction Turkey, situated in the Alpine–Himalayan Orogenic Belt, is mainly located on the Anatolian block and the Eurasian and Arabian plates ŽFig. 1a.. The neo tectonic period in Turkey is characterised by continental collision between the Eurasian and Arabian plates that caused the crustal thickening and shortening in eastern Anatolia ŽDewey et al., 1986.. Consequently, the Anatolian block began to escape toward the west along the ‘North Anatolian Fault’ Ždextral strike-slip. and the ‘East Anatolian Fault’ Žsinistral strike-slip. ŽMcKenzie, 1972.. The escape of the Anatolian block has stopped at the Helen shear zone in the Aegean region and caused N–S )
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extension due to E–W compression ŽSengor ¨ and Yilmaz, 1981.. In addition to the continental collision in the east, local tectonic events have also affected western Anatolia. The Sakarya continent collided to Rodope–Pontide block during the Late Cretaceous Žpre-Maestrichtian.. As a result of this collision, the Bozkir Ophiolitic nappes have moved toward the south. The closure of the Izmir–Ankara ocean, which represents the northern branch of the Neo-Tethys, was followed by the collision of the Anatolid–Taurid platform with the Pontid island arc during the Late Paleocene and caused N–S crustal shortening and intracrustal deformations ŽSengor ¨ and Yilmaz, 1981.. Western Anatolia witnessed intensive magmatic intrusions during the Oligocene–Miocene period ŽBingol ¨ et al., 1982. and widespread volcanic activity in the Early Miocene ŽInnocenti et al., 1982;
0377-0273r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 8 . 0 0 0 5 0 - X
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E. Aydarr Journal of Volcanology and Geothermal Research 85 (1998) 69–82
Fig. 1. Ža. General tectonic outline of Turkey Žmodified after Kocyigit, 1991.. Žb. Regional distribution of the volcanic centers in western Anatolia.
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Keller, 1983.. Crustal extension became the dominant tectonic regime in western Anatolia during the Late Miocene–Pliocene ŽAngelier et al., 1981.. Akcig Ž1988. proposed that the actual west Anatolian crust is 35–40 km thick and has a negative Bouguer anomaly like the ‘Basin and Range Province of USA’. The negative gravity anomalies in the extensional zones indicate the thinning of the crust ŽDarracott et al., 1972.. In his study, Akcig Ž1988. also suggests that while the gravity anomalies in N–S and NE–SW directions are related to the deep-seated mass, E–W and NW–SE directions show shallowly seated mass in the crust and NW–SE directions correspond to the actual active faults.
2. Volcanism of western Anatolia The volcanism of western Anatolia ŽFig. 1b., which developed on the metamorphic rocks of the Menderes massive and the ophiolitic rocks, has been investigated by numerous workers ŽBorsi et al., 1972; Innocenti and Mazzuoli, 1972; Keller and Villari, 1972; Krushensky, 1976; Ercan et al., 1978, 1979, 1980, 1983, 1986; Ozgenc, 1978; Savascin, 1978; Ercan and Gunay, 1981; Gundogdu, 1982; Ercan, 1983; Ercan and Oztunalı, 1983; Yalcin, 1988; Yilmaz, 1989; Savascin and Gulec, 1990; Yilmaz, 1990; Gulec, 1991; Seyitoglu et al., 1992; Savascin and Erler, 1994.. All hypotheses suggested by the previous workers on western Anatolian volcanism and the relations between volcanism and tectonic regime, can be summarised as follows: - Miocene stratigraphy began with the alluvial fan deposits. They laterally pass to fluviatile deposits. The Neogene lacustrine deposits overlie all continental sediments. The tuffs were deposited in the lakes and distinguished as lacustrine tuffs and continental tuffs. - While the compressional regime is characterised by the calc-alkaline volcanism which occurred during the Early–Middle Miocene, the alkaline rock suite appeared during the Middle–Late Miocene in the extensional tectonic context, except the propositions of Seyitoglu et al. Ž1992. that calc-alkaline and alkaline volcanics were erupted together within the extensional regime.
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- Tectonic escape of the Anatolian block toward the west led the transition from compressional to extensional regime. - The origin of the rhyolitic volcanism is related to anatexis ŽKeller and Villari, 1972; Keller, 1983.. - The metasomatism caused the potassic and ultrapotassic lavas generation ŽKeller, 1983.. Numerous elliptical-shaped structures in western Anatolia were detected by LANDSAT and SPOT images where the pyroclastics are dominant in or around most of the structures ŽFig. 2.. The volcanics cover a large area in western Anatolia. Although the Miocene volcanics and basins seem to be related to N–S and NE–SW directions, Pliocene and Quaternary volcanics and basins are related to NW–SE and E–W directions, respectively ŽFig. 2.. These directions are consistent with geophysical trends and can be correlated with radiometric ages ŽTable 1.. It has been longtime believed that the volcanic activities in western Anatolia occurred in nearly E–W-trending basins and that ‘the tuffs’ entered in the Neogene lakes. In this paper, the time and space relationships between volcanic activity and Neogene basin development are investigated in volcanological point of view for different volcanic centers and basins. 2.1. Gordes and Akhisar basins The Gordes basin extends in the NE–SW direction and contains 1000-m-thick fluvio-lacustrine sedimentary covers and volcanic rocks ŽSeyitoglu et al., 1992.. Ercan Ž1983. and Seyitoglu et al. Ž1992., have suggested that the volcanics overlay the lacustrine deposits in the Gordes basin. The volcanism of the region is represented by two different occurrences. While the first one has begun with rhyodacitic ignimbrites successively followed by rhyodacitic and rhyolitic extrusions and protrusions near Gordes town, the second activity has occurred as lava flows near Akhisar. Seyitoglu et al. Ž1992. have dated extrusions and protrusions to 16.5–18.5 My and interpreted them as the initiation ages of volcanism. In addition, they found a leucogranitic intrusion at the basin boundary and gave 24–21 My for the intrusion age. Unfortunately, they did not determine the age of ignimbrites which preceded the lavas. Whole ignimbritic sequence and interstratified sediments outcrop at the left side of Gordes river. Those ignimbrites consist of three units and are well-sep-
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Fig. 2. LANDSAT image of western Anatolia and its interpretation map Žscale: 1r2,000,000..
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Table 1 Some radiometric ages collected from the published papers Province
Sample
Age
Source
Kutahya Kutahya Kutahya Usak Usak Usak Biga Biga Biga Ayvalik Ayvalik Ayvalik Dikili Dikili Dikili Dikili Ayvacik Bergama Cumaovasi Cumaovasi Karaburun Karaburun Kirka Afyon Afyon Afyon
Rhyodacitic tuff Andesitic tuff Dasitic tuff Rhyolite–Rhyodacite–Andesite Rhyolite–Rhyodacite–Andesite Rhyolite–Rhyodacite–Andesite Andesitic lava and tuff Andesitic lava and tuff Andesitic lava and tuff Rhyodacite Tuff Tuff Lava Lava Lava Lava Ignimbrite Lava–Tuff Dacitic lava–tuff Dacitic lava–tuff Lava–Tuff Lava–Tuff Rhyolitic pumice Trachytic lavas Trachytic lavas Tuff
18.0 " 0.25 18.8 " 0.3 21.0 " 0.4 16.9 " 0.2 18.3 " 0.5 20.8 " 0.5 16.8 " 0.7 19.5 " 0.2 21.5 " 0.7 20.3 " 0.3 19.5 " 0.4 19.8 " 0.3 17.3 " 0.7 18.5 " 0.7 18.1 " 0.3 18.2 " 0.4 17 17 18.2 19.2 19.2 21.3 17 14.75 12.5 8.5
Besang et al., 1977 Besang et al., 1977 Besang et al., 1977 Bingol, ¨ 1977 Bingol, ¨ 1977 Bingol, ¨ 1977 Borsi et al., 1972 Borsi et al., 1972 Borsi et al., 1972 Krushensky, 1976 Benda et al., 1974 Benda et al., 1974 Borsi et al., 1972 Borsi et al., 1972 Benda et al., 1974 Benda et al., 1974 Borsi et al., 1972 Borsi et al., 1972 Borsi et al., 1972 Borsi et al., 1972 Borsi et al., 1972 Borsi et al., 1972 Yalcin, 1988 Besang et al., 1977 Besang et al., 1977 Besang et al., 1977
arated by braided river deposits ŽFig. 3.. The ignimbrites here overlie fluviatile deposits with sharp contacts without any erosional textures on the river deposits. The water contribution features as intensive gas pipes, armoured lapillis, perturbations of the contacts, etc., indicating that the hot contact of the ignimbrites with the lake or the wet sediments during transport or emplacement was not observed. The sequence is ended by the lacustrine deposits, consisting of interstratified clay, limestone and reworked tuff. At the right side of Gordes river, the tuffaceous lacustrine deposits overlie the metamorphic basement and these can be correlated with the top of the left side stratigraphy. The Early Miocene is characterized by the alluvial fan deposits ŽKurtkoy formation. in the region ŽErcan et al., 1978.. The fan deposits, in general, show the transition to braided river deposits in lateral extensions ŽRust, 1980.. The dominant sedimentary occurrence in western Anatolia is the ‘Yenikoy formation’,
which begins at the bottom with the conglomerate, sandstone, mudstone and ends with lacustrine deposits ŽErcan et al., 1978.. The braided river deposits observed in the region correspond to the basal part of the Yenikoy formation, and the top of the formation Žlacustrine deposits. overlies the ignimbrites. The pumice fragments were incorporated in the alluvial fan deposits and were found in an outcrop situated between Gordes and Selendi towns ŽFig. 4a.. As known, the alluvial fan deposits can characterize the faulted basin margins ŽRust, 1980.; in the extensional tectonic context, they characterize the basin collapse and, consequently, the pumice occurrences in the alluvial fan deposits show that the explosive volcanic activities evolved with fan development or preceded it. Finally, the volcanic activities that happened in the vicinity of Gordes town can be summarized as follows, after the field observations. - Following the emplacement of three rhyodacitic ignimbrite units, the caldera collapsed. Seldomly,
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Fig. 3. Stratigraphical columnar section of the left side of Gordes river.
any radiometric ages were available on the ignimbrites, we can suppose that the ignimbrites seem to be emplaced prior to the ring fracture-related lava extrusions and protrusions and they can be syn- or post-leucogranitic intrusions. - Resurgence happened with pushing up the caldera floor and the ophiolitic rocks outcrop by this way as resurgent dome at the center of the caldera ŽAzimdag hill.. - Ring fractures of the caldera have been lately used by the rhyodacitic and rhyolitic magmas. Those lavas have extruded along the ring fractures. They are also observed over the resurgent dome, in the
resurgent dome’s graben. The age of those lavas is dated to 16.5–18.5 My ŽSeyitoglu et al., 1992.. - Youngest volcanic activities of the region are unrelated with the caldera and outcrop around Akhisar. Savascin and Erler Ž1994. described this volcanism and demonstrated that trachybasaltic lavas cut the upper Miocene–Pliocene conglomerates. In this paper, these lavas are interpreted as lamprophyric lavas. 2.2. Demirci–Selendi–SimaÕ basins Ercan and Oztunalı Ž1983. and Ercan et al. Ž1983. investigated the volcanic rocks of the region in petro-
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Fig. 4. Field photographs. Ža. Pumice incorporation in the alluvial fan deposits. Žb. Scarce accretionary lapilli in an ignimbritic unit. Žc. General view of Yagcidag. Žd. Asitepe stratovolcano from the SE.
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logical aspect. They proposed that the Miocene begun with the Kurtkoy formation Žalluvial fan deposits. and continued with the Yenikoy formation which is interstratified with tuffaceous layers. The field works show that the volcanic activities are related to the different volcanic vents Žas epicontinental caldera, stratovolcano, independent fissural lava flows, and dykes.. The first volcanic activities are explosive in character as seen in the Gordes basin. The products are rhyolitic ignimbrites which are emplaced intensively near Yagcidag with eight flow units ŽFig. 5.. The ignimbrites are capped by the lava extrusions over the source area ŽYagcidag. ŽFig. 4c.. The ignimbrites outcrop all around the Yagcidag, but the whole sequence is well-observed in the south and north of the vent. The grain size of plinian pumiceous air-fall deposits show progressive
decrease from Yagcidag to distal parts, indicating that the source area was below Yagcidag. The first unit of the sequence contains abundantly accretionary lapilli ŽFig. 4b.. These accretionary lapilli are enriched on the NE–SW axis of the eruption unit where there is a fissure which was spotted on the satellite image ŽFig. 2.. The ignimbrites are generally fine-grained Žexcept unit 8. and consist of coarse ash and pumices. The lithics are represented by metamorphic basement rock fragments and any volcanogenic lithics have been found in the ignimbrites. On the other hand, the first unit Žwith accretionary lapilli. and the second unit outcrop near Cansa– Duzbagtepe ŽSW of Yagcidag.. The units are wellseparated with 25-cm-thick plinian air-fall deposits. The argillizations decrease from the bottom to the top of the exposure and the top of the second unit is
Fig. 5. Stratigraphical columnar section of Selendi–Yagcidag ignimbrites.
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not affected by this alteration. Features, indicating the interaction of hot ignimbritic flows with the lake water, were not observed in the region and this can be interpreted as the development of the lake after the ignimbrites. Asitepe, a small stratovolcano ŽFig. 4d., is situated approximately 15 km from Yagcidag toward the west. The ignimbrites outcrop in the valley floor and are overlain by sub-aphanitic and porphyritic lava flows which are well-observed between Ahatlar and Delidemirci villages. The lava flows contain cognate inclusions in decimetrical size. The block and ash flows outcrop on a wide area at the southern flank. Debris avalanche deposits have also been found at the WNW flanks and the summit of volcano represents a horseshoe-shaped depression. The micabearing aphanitic lavas complete the volcanic sequence of the volcano. The hydrothermal activities affected the volcanics and colored most of the volcanics in green. The secondary chlorite phenocrysts have been found in the aphanitic lavas. The Kurtkoy formation Žalluvial fan deposits. outcrops in the vicinity of Ahatlar village ŽE of Asitepe., at 840 m of altitude and consists of large metamorphic blocks
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and contains also a big block Ž) 10 m. of ignimbrite. On the other hand, an elliptical-shaped caldera morphology is situated in the north of Kula, detected by satellite imagery. This caldera is 12 = 8 km in diameter ŽFig. 2.. The lava flows near Kula have come from the caldera boundary. Volcanic activity of the region continued with black, aphanitic lava flows and dyke emplacements. Those lamprophyric lavas contain micaceous peridotite nodules in the body, observed near Selendi. The nodules are in centimetrical size and are composed of olivineq phlogopite. Lamprophyric lavas have also been observed in the vicinity of Simav by Ercan et al. Ž1984a,b., and named as ‘Payamtepe Volcanics’ consisting of trachytes and lamproitic trachytes. ‘Karaboldere Volcanics’ of Ercan et al. Ž1984a., from the same region, which are felsic in character, have preceded the lamprophyric lavas generation. The dykes are scarcely found in the region and they exhibit silicified aspects. Sulphurs, hematites and sometimes, gypsum occurrences, are observed together with the silicified dykes.
Fig. 6. Stratigraphical columnar section of Koroglu Caldera-Seydiler Ignimbrites.
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2.3. Kirka basin Numerous economical geology works have been carried out on Kirka basin for its boron mineralizations. Yalcin Ž1988. has distinguished two tuff facies as lacustrine tuffs and continental tuffs in Kirka basin and supposed that the lacustrine tuffs have been deposited in the lake environment. The radiometric ages ŽKrAr. of the pumices and the basaltic lavas of the basin are 17 My and 9 My, respectively ŽYalcin, 1988.. In fact, the tuffs of Yalcin Ž1988. are ignimbrites, which are partly reworked around Kirka, and are derived from ‘Koroglu Caldera’ situated at the south of the basin, 40 km N NE of Afyon city ŽAydar et al., 1994, 1996.. Keller and Villari Ž1972. have petrologically investigated those ignimbrites without making some approaches to the physical aspect of the volcanism, and suggested that anatexis was the origin of this rhyolitic magma. Koroglu caldera is a result of the emission of ‘Seydiler Ignimbrites’ ŽFig. 6., emplaced at least in two sequences as Lower Seydiler ŽLS. and Upper Seydiler ŽUS.. Koroglu caldera is 13 = 18 km in diameter with a
resurgent dome in the center and produced LAR ŽLow Aspect Ratio. type ignimbrites, transported 50 km away from the source area ŽAydar et al., 1994.. The ignimbrites show the argillization type alteration, decreasing from the bottom to the top of deposits around Kirka basin and in the vicinity of the source area. The transitions from ignimbrites to reworked tuffs have been observed without any features indicating that the ignimbrites have been deposited in the lake environment Že.g., secondary created degassing pipes, oxidation, surges, and armoured lapillis.. The collapse of Koroglu caldera happened with the rhyolitic ignimbrite eruption following a NE–SW-trending fracture. The concerned fracture was used by a small river and locally caused intensive oxidations, numerous degassing pipes and armoured lapillis occurrences on US-U1 ŽUpper Seydiler-Unit 1.. At the distal part of this contact zone were found conglomerate deposits of the river. The structural analysis of the caldera shows that tectonic activities have also developed after the caldera collapse ŽFig. 7.. Two main trending for the linear alignments in the region were distinguished as
Fig. 7. Structural map of the Koroglu caldera. Heavy lines correspond to the ring fractures.
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NE–SW and NW–SE and the latter seems to be the youngest occurrence. A NW–SE-trending fissure was used by a lamprophyric magma. The lamprophyre concerned, consisting of leuciteq phlogopiteq Crdiopsideq olivineq feldspars, was extruded in the
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lacustrine deposits. The lavas generation have also developed along the ring fractures of the caldera with trachyandesitic flows and trachybasaltic dykes. The dykes are composed of olivineq phlogopiteq Cr-diopsideq salite q K-feldsparq Ba-feldspar Žhy-
Fig. 8. Geological map of the Afyon stratovolcano.
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alophane.. The lava flows contain scarcely cognate inclusions which are trachybasaltic andesites in composition. 2.4. Afyon region Keller and Villari Ž1972. and Keller Ž1983. have petrologically investigated the volcanism of the Afyon region, and defined the alkaline character for the volcanics. Keller Ž1983. has supposed that the volcanism was dominantly potassic and ultrapotassic in character, due to the mantle metasomatism caused by lithospheric subduction from the Hellenic trench. This volcanism has been dated by Besang et al. Ž1977. and the ages were ranged between 8.5–14.5 My. Aydar and Bayhan Ž1995. have supposed that the volcanism of Afyon was related to a stratovolcano which built up on the lacustrine deposits ŽYenikoy formation. and any volcanogenic fragment was encountered in these deposits. On the other hand, Harut Ž1995. has described the stratigraphically Pliocene-aged ‘Koprulu formation’, which contains volcanogenic layers in carbonated lacustrine deposits, overlying the ‘Yenikoy formation’ NNW of Afyon. The relation between these lacustrine formations is obscure and the unique difference is the presence of the volcanogenic sediments in the ‘Koprulu formation’. Lamprophyric lava flows were interstratified in the ‘Koprulu formation’ ŽHarut, 1995.. The lake development in the region seems to have occurred between 17 My Žage of Seydiler Ignimbrites at the north of Afyon. and 14.5 My Žknown age of the oldest product of Afyon stratovolcano.. The stratovolcano of Afyon ŽFig. 8., built up by trachybasaltic and trachyandesitic lavas flow, and then has witnessed a summit caldera collapse following ignimbrite eruptions. The megasanidine Žup to 5 cm. bearing trachytic lava domes and flows, associated block and ash flows constitute the post-caldera volcanic activities. The black, aphanitic lamprophyric lava flows and phlogopite-bearing dyke intrusions complete the volcanic sequence. 2.5. Kula area The known youngest volcanic activities of western Anatolia occurred in the vicinity of Kula, represented by basaltic cinder cones, maars and lava
flows. The main period of the volcanism is between 1.1 My and 0.01 My ŽErcan et al., 1980.. There are at least three volcanic phases. The first one is dominantly characterised by lava flows derived from the ring fracture of a caldera which was detected by SPOT image. The other basaltic phases are represented by cinder cones, maars and fissure-related lava flows. This basaltic volcanism developed along a graben fault which aligned E–W. The common properties of the second and third volcanic phases are the presence of ultramafic xenoliths, such as gabbro, lherzolithe, hornblendite and pyroxenite, within the basaltic ejectas. 3. Discussion and conclusion The main tectonic trends of the region are N–S, NE–SW, NW–SE and E–W. Volcanism data show that N–S trends are the oldest accidents in the region. Afterwards, NE–SW directional accidents were developed during the Early–Middle Miocene, followed by NW–SE trends after the Middle Miocene period. These observations are compatible with the geophysical data. Melts derived from subduction of the African plate along the Hellenic Trench have caused the anatexis of the lower crust, and seem to be a reason for the crustal thinning. The rhyolitic magmas were erupted, as ignimbrites, approximately in the same time in whole western Anatolia, dominantly between 17 and 21 My. The anatexitic granitic intrusions preceded the ignimbrites and were emplaced between 20 and 24 My ŽBingol ¨ et al., 1982.. With the beginning of the anatexis process, the crust has begun to thin and to extend. The alluvial fan deposits which occurred during the basin formation, contain pumices and ignimbritic blocks which indicate the penecontemporaneous development of the explosive volcanic activities. The ignimbrites of the different regions of western Anatolia, erupted between 17 and 21 My and the lakes have developed after the emplacement of the ignimbrites. The explosive activities follow the NE–SW directional fissures which is the main direction of the Miocene basins. The distinction of the lacustrine tuffs and continental tuffs has to be regarded with suspicion and has to be avoided. Instead of this distinction, it would be better to use ‘ignimbrite’ and ‘reworked tuff’ terms.
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The lamprophyric magmas, emplaced during the Late Miocene–Pliocene, are the result of the diapiric rises of the asthenosphere. The asthenospheric diapirs were probably generated after the ignimbritic sequence of western Anatolia. The lamprophyric magmas ascent from the deep mantle has taken upper mantle fragments into the body Žthe nodules. and interact with the crustal material in the magma chambers. They are found in the subvolcanic depth as dykes or generated lava flows at the surface. Most of the volcanics, cited in the literature as trachyte, andesite, trachyandesite, trachybasalt, lamproitic trachyte, lamproite, and shoshonitic basalt, are in fact lamprophyres. The youngest volcanics of western Anatolia are interpreted as typical rift volcanism by Ercan and Oztunalı Ž1983.. Acknowledgements I am thankful to Professor P.M. Vincent ŽBlaise Pascal University, Clermont Ferrand, France., who contributed greatly to the manuscript. I also appreciate criticisms, comments and language corrections of the manuscript by Dr. A. Ciner, Hacettepe University, Ankara, Turkey and MSc. H. Stevens. References Akcig, Z., 1988. Investigation of the structural problems of western Anatolia by the gravity data. Turk. Geol. Bull. 31, 63–70, in Turkish. Angelier, J., Dumont, J.F., Karamanderesi, H., Poisson, A., Simsek, S., Uysal, S., 1981. Analyses of fault mechanism and expansion of southwestern Anatolia since the Late Miocene. Tectonophysics 75, T1–T9. Aydar, E., Bayhan, H., Erkan, Y., 1994. Koroglu Caldera, Midwest Anatolia, Turkiye. Abstract, IAVCEI, Int. Volcanol. Congress, Ankara, Turkiye. Aydar, E., Bayhan, H., 1995. Le volcanisme alcalin d’Afyon. Anatolie de l’ouest orientale, Turquie: approche volcanologique et petrologique. Bull. Section Volcanol., Soc. ´ Geol. Fr. 36, 1–5. Aydar, E., Bayhan, H., Zimitoglu, O., 1996. Investigation of volcanological and petrological evolution of Afyon stratovolcano. Hacettepe Univ. Earth Sci. 18, 87–107, in Turkish. Benda, L., Innocenti, F., Mazzuoli, R., Radicati, F., Steffens, P., 1974. Stratigraphic and radiometric data of the Neogene in northwest Turkey. Z. Deutsch. Geol. Ges. 125, 183–193. Besang, C., Eckhardt, F.J., Harre, W., Kreuzer, H., Muller, P.,
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1977. Radiometrische Altersbestimmungen an neogenen Eruptivegesteinen der Turkei. Geol. Jahrb. B 25, 3–36. Bingol, ¨ E., 1977. The geology of Muratdagi on the petrology of the main rock units. Bull. Geol. Soc. Turk. 20 Ž2., 13–66. Bingol, ¨ E., Delaloye, M., Ataman, G., 1982. Granitic intrusions in western Anatolia: a contribution to the geodynamic study of this area. Eclogae Geol. Helv. 75, 437–446. Borsi, S., Ferrara, G., Innocenti, F., Mazzuoli, R., 1972. Geochronology and petrology of recent volcanics in the eastern Aegean sea ŽWest Anatolia and Lesvos Island.. Bull. Volcanol. 36, 473–496. Darracott, B.W., Fairhead, J.D., Girdler, R.W., 1972. Gravity and magnetic surveys in northern Tanzania and southern Kenya. Tectonophysics 15, 131–141. Dewey, J.F., Hempton, M.R., Kidd, W.S.F., Saroglu, F., Sengor, ¨ A.M.C., 1986. Shortening of continental lithosphere: the neotectonics of Eastern Anatolia—a young collision zone. In: Coward, M.P., Ries, A.C. ŽEds.., Collision Tectonics. Geol. Soc. London, Spec. Publ. 29, 91–105. Ercan, T., Dincel, A., Metin, S., Turkecan, A., Gunay, E., 1978. Geology of the Neogene basins in the vicinity of Usak. Bull. M.T.A. 21, 97–106, in Turkish. Ercan, T., Dincel, A., Gunay, E., 1979. Petrology of Usak volcanics and plate tectonic significance in Aegean region. Bull. Geol. Soc. Turk. 22, 185–198. Ercan, T., Gunay, E., 1981. Tertiar volcanism in the vicinity of Soke and its regional extent. Bull. Geomorphol. ŽTurkey. 10, 117–137, in Turkish. Ercan, T., 1983. Petrology of Gordes volcanics ŽManisa, Turkey. and its genetical interpretations. Bull. Geol. Soc. Turk. 26, 41–48, in Turkish. Ercan, T., Oztunalı, O., 1983. Cenozoic volcanism around Demirci–Selendi ŽManisa,Turkey. and its genetical interpretations. H.U. Earth Sci. 10, 1–15, in Turkish. Ercan, T., Gunay, E., Bas, H., 1983. Petrology of Denizli volcanics ŽTurkey. and its tectonic implications. Bull. Geol. Soc. Turk. 26, 153–160, in Turkish. Ercan, T., Gunay, E., Dincel, A., Turkecan, A., 1980. Geology and petrology of the volcanics of Kula–Selendi area. M.T.A. Report, No. 6801, in Turkish. Ercan, T., Gunay, E., Savascın, M.Y., 1984a. Regional interpretation of the Cenozoic volcanism around Simav. Bull. M.T.A. 97r98, 86–101, in Turkish. Ercan, T., Turkecan, A., Akyurek, B., Gunay, E., Cevikbas, A., Ates, M., Can, B., Erkan, M., Ozkirisci, C., 1984b. Geology of Dikili–Bergama–Candarli ŽIzmir. area and petrology of magmatic rocks. Bull. Turk. Geol. Eng. 20, 47–60, in Turkish. Ercan, T., Satir, M., Turkecan, A., Akyurek, B., Cevikbas, A., Gunay, E., Ates, M., Can, B., 1986. Geology of Ayvalik area and petrology of the volcanic rocks. Bull. Turk. Geol. Eng. 27, 19–30, in Turkish. Gulec, N., 1991. Crust–mantle interaction in western Turkey: implications from Sr and Nd isotope geochemistry of Tertiary and Quaternary volcanics. Geol. Mag. 128, 417–435. Gundogdu, M.N.G., 1982. Geological, mineralogical and geochemical investigations of Bigadic sedimentary basin. PhD thesis, Hacettepe University, 386 pp.
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Harut, B., 1995. Mineralogical, petrographical and geochemical study of Erkmen Volcanics ŽNW of Afyon, Turkey.. MS thesis, Hacettepe University, 78 pp. Innocenti, F., Mazzuoli, R., 1972. Petrology of the Izmir– Karaburun volcanic area. Bull. Volcanol. 36, 83–103. Innocenti, F., Manetti, P., Mazzuoli, R., Pasquare, G., Villari, L., 1982. Anatolia and north-western Iran. In: Thorpe, R.S. ŽEd.., Andesites and Related Rocks. Wiley, pp. 327–349. Keller, J., 1983. Potassic lavas in the orogenic volcanism of the Mediterranean area. J. Volcanol. Geotherm. Res. 18, 321–335. Keller, J., Villari, L., 1972. Rhyolitic ignimbrites in the region of Afyon ŽCentral Anatolia.. Bull. Volcanol. 36, 342–358. Kocyigit, A., 1991. An example of an accretionary fore-arc basin from northern Central Anatolia and its implications for the history of subduction of Neo-Tethys in Turkey. Geol. Soc. Am. Bull. 103, 22–36. Krushensky, R.D., 1976. Neogene calc-alkaline extrusive and intrusive rocks of the Karalar–Yesiller area. Bull. Volcanol. McKenzie, D.P., 1972. Active tectonics of the Mediterranean region. Geophys. J. R. Astron. Soc. 30, 109–185. Ozgenc, I., 1978. Age relationships between two generations of acid volcanic extrusions in Cumaovasi ŽIzmir, Turkey. area. Bull. Geol. Soc. Turk. 21, 31–34, in Turkish. Rust, R.B., 1980. Facies models 2: coarse alluvial deposits. In: Walker, R.G. ŽEd.., Geoscience Canada, Reprint series 1, 211 p.
Savascin, M.Y., 1978. Mineralogical and geochemical investigation of the Neogene volcanics around Foca–Urla Žwestern Turkey. and genetical interpretations. State thesis, Ege University, 65 p., in Turkish. Savascin, M.Y., Erler, A., 1994. Neogene–Quaternary magmatics and related ore deposits of Western Anatolia. Int. Volcanol. Cong. Excursion Guide, Spec. publ. No. 4. IAVCEI-94, Ankara. Savascin, M.Y., Gulec, N., 1990. Neogene volcanism of western Anatolia. IESCA Publ. No. 3 Ž78.. Seyitoglu, G., Scott, B.C., Rundle, C.C., 1992. Timing of Cenozoic extensional tectonics in west Turkey. J. Geol. Soc. London 149, 533–538. Sengor, ¨ A.M.C., Yilmaz, Y., 1981. Tethyan evolution of Turkey: a plate tectonic approach. Tectonophysics 75, 181–241. Yalcin, H., 1988. Mineralogical, petrographical and geochemical investigation of volcanosedimentary occurrences in Kirka ŽEskisehir, Turkey. area. PhD thesis, Hacettepe University, 209 p., in Turkish. Yilmaz, Y., 1989. An approach to the origin of young volcanic rocks of Western Turkey. In: Sengor, ¨ A.M.C. ŽEd.., Tectonic Evolution of the Tethyan Region. Kluwer Academic Publishers, pp. 159–189. Yilmaz, Y., 1990. Comparison of young volcanic associations of western and eastern Anatolia formed under a compressional regime: a review. J. Volcanol. Geotherm. Res. 44, 69–87.
Early Miocene to Quaternary evolution of volcanism and the basin formation in western Anatolia: a review Erkan Aydar
)
Hacettepe UniÕersity, Department of Geological Engineering, 06532 Beytepe, Ankara, Turkey
Abstract Western Anatolia, largely affected by extensional tectonics, witnessed widespread volcanic activity since the Early Miocene. The volcanic vents of the region are represented by epicontinental calderas, stratovolcanoes and monogenetic vents which are associated with small-scale intrusions as sills and dykes. The volcanic activity began with an explosive character producing a large ignimbritic plateau all over the region, indicating the initiation of the crustal extension event. These rhyolitic magmas are nearly contemporaneous with granitic intrusions in western Anatolia. The ignimbrites, emplaced approximately contemporaneous with alluvial fan and braided river deposits, flowed over the basement rocks prior to extensional basin formation. The lacustrine deposits overlie the ignimbrites. The potassic and ultrapotassic lavas with lamprophyric affinities were emplaced during the Late Miocene–Pliocene. The volcanic activities have continued with alkali basalts during the Quaternary. q 1998 Elsevier Science B.V. All rights reserved. Keywords: ignimbrite; lava; lake; extension
1. Introduction Turkey, situated in the Alpine–Himalayan Orogenic Belt, is mainly located on the Anatolian block and the Eurasian and Arabian plates ŽFig. 1a.. The neo tectonic period in Turkey is characterised by continental collision between the Eurasian and Arabian plates that caused the crustal thickening and shortening in eastern Anatolia ŽDewey et al., 1986.. Consequently, the Anatolian block began to escape toward the west along the ‘North Anatolian Fault’ Ždextral strike-slip. and the ‘East Anatolian Fault’ Žsinistral strike-slip. ŽMcKenzie, 1972.. The escape of the Anatolian block has stopped at the Helen shear zone in the Aegean region and caused N–S )
E-mail: [email protected]
extension due to E–W compression ŽSengor ¨ and Yilmaz, 1981.. In addition to the continental collision in the east, local tectonic events have also affected western Anatolia. The Sakarya continent collided to Rodope–Pontide block during the Late Cretaceous Žpre-Maestrichtian.. As a result of this collision, the Bozkir Ophiolitic nappes have moved toward the south. The closure of the Izmir–Ankara ocean, which represents the northern branch of the Neo-Tethys, was followed by the collision of the Anatolid–Taurid platform with the Pontid island arc during the Late Paleocene and caused N–S crustal shortening and intracrustal deformations ŽSengor ¨ and Yilmaz, 1981.. Western Anatolia witnessed intensive magmatic intrusions during the Oligocene–Miocene period ŽBingol ¨ et al., 1982. and widespread volcanic activity in the Early Miocene ŽInnocenti et al., 1982;
0377-0273r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 8 . 0 0 0 5 0 - X
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Fig. 1. Ža. General tectonic outline of Turkey Žmodified after Kocyigit, 1991.. Žb. Regional distribution of the volcanic centers in western Anatolia.
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Keller, 1983.. Crustal extension became the dominant tectonic regime in western Anatolia during the Late Miocene–Pliocene ŽAngelier et al., 1981.. Akcig Ž1988. proposed that the actual west Anatolian crust is 35–40 km thick and has a negative Bouguer anomaly like the ‘Basin and Range Province of USA’. The negative gravity anomalies in the extensional zones indicate the thinning of the crust ŽDarracott et al., 1972.. In his study, Akcig Ž1988. also suggests that while the gravity anomalies in N–S and NE–SW directions are related to the deep-seated mass, E–W and NW–SE directions show shallowly seated mass in the crust and NW–SE directions correspond to the actual active faults.
2. Volcanism of western Anatolia The volcanism of western Anatolia ŽFig. 1b., which developed on the metamorphic rocks of the Menderes massive and the ophiolitic rocks, has been investigated by numerous workers ŽBorsi et al., 1972; Innocenti and Mazzuoli, 1972; Keller and Villari, 1972; Krushensky, 1976; Ercan et al., 1978, 1979, 1980, 1983, 1986; Ozgenc, 1978; Savascin, 1978; Ercan and Gunay, 1981; Gundogdu, 1982; Ercan, 1983; Ercan and Oztunalı, 1983; Yalcin, 1988; Yilmaz, 1989; Savascin and Gulec, 1990; Yilmaz, 1990; Gulec, 1991; Seyitoglu et al., 1992; Savascin and Erler, 1994.. All hypotheses suggested by the previous workers on western Anatolian volcanism and the relations between volcanism and tectonic regime, can be summarised as follows: - Miocene stratigraphy began with the alluvial fan deposits. They laterally pass to fluviatile deposits. The Neogene lacustrine deposits overlie all continental sediments. The tuffs were deposited in the lakes and distinguished as lacustrine tuffs and continental tuffs. - While the compressional regime is characterised by the calc-alkaline volcanism which occurred during the Early–Middle Miocene, the alkaline rock suite appeared during the Middle–Late Miocene in the extensional tectonic context, except the propositions of Seyitoglu et al. Ž1992. that calc-alkaline and alkaline volcanics were erupted together within the extensional regime.
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- Tectonic escape of the Anatolian block toward the west led the transition from compressional to extensional regime. - The origin of the rhyolitic volcanism is related to anatexis ŽKeller and Villari, 1972; Keller, 1983.. - The metasomatism caused the potassic and ultrapotassic lavas generation ŽKeller, 1983.. Numerous elliptical-shaped structures in western Anatolia were detected by LANDSAT and SPOT images where the pyroclastics are dominant in or around most of the structures ŽFig. 2.. The volcanics cover a large area in western Anatolia. Although the Miocene volcanics and basins seem to be related to N–S and NE–SW directions, Pliocene and Quaternary volcanics and basins are related to NW–SE and E–W directions, respectively ŽFig. 2.. These directions are consistent with geophysical trends and can be correlated with radiometric ages ŽTable 1.. It has been longtime believed that the volcanic activities in western Anatolia occurred in nearly E–W-trending basins and that ‘the tuffs’ entered in the Neogene lakes. In this paper, the time and space relationships between volcanic activity and Neogene basin development are investigated in volcanological point of view for different volcanic centers and basins. 2.1. Gordes and Akhisar basins The Gordes basin extends in the NE–SW direction and contains 1000-m-thick fluvio-lacustrine sedimentary covers and volcanic rocks ŽSeyitoglu et al., 1992.. Ercan Ž1983. and Seyitoglu et al. Ž1992., have suggested that the volcanics overlay the lacustrine deposits in the Gordes basin. The volcanism of the region is represented by two different occurrences. While the first one has begun with rhyodacitic ignimbrites successively followed by rhyodacitic and rhyolitic extrusions and protrusions near Gordes town, the second activity has occurred as lava flows near Akhisar. Seyitoglu et al. Ž1992. have dated extrusions and protrusions to 16.5–18.5 My and interpreted them as the initiation ages of volcanism. In addition, they found a leucogranitic intrusion at the basin boundary and gave 24–21 My for the intrusion age. Unfortunately, they did not determine the age of ignimbrites which preceded the lavas. Whole ignimbritic sequence and interstratified sediments outcrop at the left side of Gordes river. Those ignimbrites consist of three units and are well-sep-
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Fig. 2. LANDSAT image of western Anatolia and its interpretation map Žscale: 1r2,000,000..
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Table 1 Some radiometric ages collected from the published papers Province
Sample
Age
Source
Kutahya Kutahya Kutahya Usak Usak Usak Biga Biga Biga Ayvalik Ayvalik Ayvalik Dikili Dikili Dikili Dikili Ayvacik Bergama Cumaovasi Cumaovasi Karaburun Karaburun Kirka Afyon Afyon Afyon
Rhyodacitic tuff Andesitic tuff Dasitic tuff Rhyolite–Rhyodacite–Andesite Rhyolite–Rhyodacite–Andesite Rhyolite–Rhyodacite–Andesite Andesitic lava and tuff Andesitic lava and tuff Andesitic lava and tuff Rhyodacite Tuff Tuff Lava Lava Lava Lava Ignimbrite Lava–Tuff Dacitic lava–tuff Dacitic lava–tuff Lava–Tuff Lava–Tuff Rhyolitic pumice Trachytic lavas Trachytic lavas Tuff
18.0 " 0.25 18.8 " 0.3 21.0 " 0.4 16.9 " 0.2 18.3 " 0.5 20.8 " 0.5 16.8 " 0.7 19.5 " 0.2 21.5 " 0.7 20.3 " 0.3 19.5 " 0.4 19.8 " 0.3 17.3 " 0.7 18.5 " 0.7 18.1 " 0.3 18.2 " 0.4 17 17 18.2 19.2 19.2 21.3 17 14.75 12.5 8.5
Besang et al., 1977 Besang et al., 1977 Besang et al., 1977 Bingol, ¨ 1977 Bingol, ¨ 1977 Bingol, ¨ 1977 Borsi et al., 1972 Borsi et al., 1972 Borsi et al., 1972 Krushensky, 1976 Benda et al., 1974 Benda et al., 1974 Borsi et al., 1972 Borsi et al., 1972 Benda et al., 1974 Benda et al., 1974 Borsi et al., 1972 Borsi et al., 1972 Borsi et al., 1972 Borsi et al., 1972 Borsi et al., 1972 Borsi et al., 1972 Yalcin, 1988 Besang et al., 1977 Besang et al., 1977 Besang et al., 1977
arated by braided river deposits ŽFig. 3.. The ignimbrites here overlie fluviatile deposits with sharp contacts without any erosional textures on the river deposits. The water contribution features as intensive gas pipes, armoured lapillis, perturbations of the contacts, etc., indicating that the hot contact of the ignimbrites with the lake or the wet sediments during transport or emplacement was not observed. The sequence is ended by the lacustrine deposits, consisting of interstratified clay, limestone and reworked tuff. At the right side of Gordes river, the tuffaceous lacustrine deposits overlie the metamorphic basement and these can be correlated with the top of the left side stratigraphy. The Early Miocene is characterized by the alluvial fan deposits ŽKurtkoy formation. in the region ŽErcan et al., 1978.. The fan deposits, in general, show the transition to braided river deposits in lateral extensions ŽRust, 1980.. The dominant sedimentary occurrence in western Anatolia is the ‘Yenikoy formation’,
which begins at the bottom with the conglomerate, sandstone, mudstone and ends with lacustrine deposits ŽErcan et al., 1978.. The braided river deposits observed in the region correspond to the basal part of the Yenikoy formation, and the top of the formation Žlacustrine deposits. overlies the ignimbrites. The pumice fragments were incorporated in the alluvial fan deposits and were found in an outcrop situated between Gordes and Selendi towns ŽFig. 4a.. As known, the alluvial fan deposits can characterize the faulted basin margins ŽRust, 1980.; in the extensional tectonic context, they characterize the basin collapse and, consequently, the pumice occurrences in the alluvial fan deposits show that the explosive volcanic activities evolved with fan development or preceded it. Finally, the volcanic activities that happened in the vicinity of Gordes town can be summarized as follows, after the field observations. - Following the emplacement of three rhyodacitic ignimbrite units, the caldera collapsed. Seldomly,
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Fig. 3. Stratigraphical columnar section of the left side of Gordes river.
any radiometric ages were available on the ignimbrites, we can suppose that the ignimbrites seem to be emplaced prior to the ring fracture-related lava extrusions and protrusions and they can be syn- or post-leucogranitic intrusions. - Resurgence happened with pushing up the caldera floor and the ophiolitic rocks outcrop by this way as resurgent dome at the center of the caldera ŽAzimdag hill.. - Ring fractures of the caldera have been lately used by the rhyodacitic and rhyolitic magmas. Those lavas have extruded along the ring fractures. They are also observed over the resurgent dome, in the
resurgent dome’s graben. The age of those lavas is dated to 16.5–18.5 My ŽSeyitoglu et al., 1992.. - Youngest volcanic activities of the region are unrelated with the caldera and outcrop around Akhisar. Savascin and Erler Ž1994. described this volcanism and demonstrated that trachybasaltic lavas cut the upper Miocene–Pliocene conglomerates. In this paper, these lavas are interpreted as lamprophyric lavas. 2.2. Demirci–Selendi–SimaÕ basins Ercan and Oztunalı Ž1983. and Ercan et al. Ž1983. investigated the volcanic rocks of the region in petro-
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Fig. 4. Field photographs. Ža. Pumice incorporation in the alluvial fan deposits. Žb. Scarce accretionary lapilli in an ignimbritic unit. Žc. General view of Yagcidag. Žd. Asitepe stratovolcano from the SE.
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logical aspect. They proposed that the Miocene begun with the Kurtkoy formation Žalluvial fan deposits. and continued with the Yenikoy formation which is interstratified with tuffaceous layers. The field works show that the volcanic activities are related to the different volcanic vents Žas epicontinental caldera, stratovolcano, independent fissural lava flows, and dykes.. The first volcanic activities are explosive in character as seen in the Gordes basin. The products are rhyolitic ignimbrites which are emplaced intensively near Yagcidag with eight flow units ŽFig. 5.. The ignimbrites are capped by the lava extrusions over the source area ŽYagcidag. ŽFig. 4c.. The ignimbrites outcrop all around the Yagcidag, but the whole sequence is well-observed in the south and north of the vent. The grain size of plinian pumiceous air-fall deposits show progressive
decrease from Yagcidag to distal parts, indicating that the source area was below Yagcidag. The first unit of the sequence contains abundantly accretionary lapilli ŽFig. 4b.. These accretionary lapilli are enriched on the NE–SW axis of the eruption unit where there is a fissure which was spotted on the satellite image ŽFig. 2.. The ignimbrites are generally fine-grained Žexcept unit 8. and consist of coarse ash and pumices. The lithics are represented by metamorphic basement rock fragments and any volcanogenic lithics have been found in the ignimbrites. On the other hand, the first unit Žwith accretionary lapilli. and the second unit outcrop near Cansa– Duzbagtepe ŽSW of Yagcidag.. The units are wellseparated with 25-cm-thick plinian air-fall deposits. The argillizations decrease from the bottom to the top of the exposure and the top of the second unit is
Fig. 5. Stratigraphical columnar section of Selendi–Yagcidag ignimbrites.
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not affected by this alteration. Features, indicating the interaction of hot ignimbritic flows with the lake water, were not observed in the region and this can be interpreted as the development of the lake after the ignimbrites. Asitepe, a small stratovolcano ŽFig. 4d., is situated approximately 15 km from Yagcidag toward the west. The ignimbrites outcrop in the valley floor and are overlain by sub-aphanitic and porphyritic lava flows which are well-observed between Ahatlar and Delidemirci villages. The lava flows contain cognate inclusions in decimetrical size. The block and ash flows outcrop on a wide area at the southern flank. Debris avalanche deposits have also been found at the WNW flanks and the summit of volcano represents a horseshoe-shaped depression. The micabearing aphanitic lavas complete the volcanic sequence of the volcano. The hydrothermal activities affected the volcanics and colored most of the volcanics in green. The secondary chlorite phenocrysts have been found in the aphanitic lavas. The Kurtkoy formation Žalluvial fan deposits. outcrops in the vicinity of Ahatlar village ŽE of Asitepe., at 840 m of altitude and consists of large metamorphic blocks
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and contains also a big block Ž) 10 m. of ignimbrite. On the other hand, an elliptical-shaped caldera morphology is situated in the north of Kula, detected by satellite imagery. This caldera is 12 = 8 km in diameter ŽFig. 2.. The lava flows near Kula have come from the caldera boundary. Volcanic activity of the region continued with black, aphanitic lava flows and dyke emplacements. Those lamprophyric lavas contain micaceous peridotite nodules in the body, observed near Selendi. The nodules are in centimetrical size and are composed of olivineq phlogopite. Lamprophyric lavas have also been observed in the vicinity of Simav by Ercan et al. Ž1984a,b., and named as ‘Payamtepe Volcanics’ consisting of trachytes and lamproitic trachytes. ‘Karaboldere Volcanics’ of Ercan et al. Ž1984a., from the same region, which are felsic in character, have preceded the lamprophyric lavas generation. The dykes are scarcely found in the region and they exhibit silicified aspects. Sulphurs, hematites and sometimes, gypsum occurrences, are observed together with the silicified dykes.
Fig. 6. Stratigraphical columnar section of Koroglu Caldera-Seydiler Ignimbrites.
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2.3. Kirka basin Numerous economical geology works have been carried out on Kirka basin for its boron mineralizations. Yalcin Ž1988. has distinguished two tuff facies as lacustrine tuffs and continental tuffs in Kirka basin and supposed that the lacustrine tuffs have been deposited in the lake environment. The radiometric ages ŽKrAr. of the pumices and the basaltic lavas of the basin are 17 My and 9 My, respectively ŽYalcin, 1988.. In fact, the tuffs of Yalcin Ž1988. are ignimbrites, which are partly reworked around Kirka, and are derived from ‘Koroglu Caldera’ situated at the south of the basin, 40 km N NE of Afyon city ŽAydar et al., 1994, 1996.. Keller and Villari Ž1972. have petrologically investigated those ignimbrites without making some approaches to the physical aspect of the volcanism, and suggested that anatexis was the origin of this rhyolitic magma. Koroglu caldera is a result of the emission of ‘Seydiler Ignimbrites’ ŽFig. 6., emplaced at least in two sequences as Lower Seydiler ŽLS. and Upper Seydiler ŽUS.. Koroglu caldera is 13 = 18 km in diameter with a
resurgent dome in the center and produced LAR ŽLow Aspect Ratio. type ignimbrites, transported 50 km away from the source area ŽAydar et al., 1994.. The ignimbrites show the argillization type alteration, decreasing from the bottom to the top of deposits around Kirka basin and in the vicinity of the source area. The transitions from ignimbrites to reworked tuffs have been observed without any features indicating that the ignimbrites have been deposited in the lake environment Že.g., secondary created degassing pipes, oxidation, surges, and armoured lapillis.. The collapse of Koroglu caldera happened with the rhyolitic ignimbrite eruption following a NE–SW-trending fracture. The concerned fracture was used by a small river and locally caused intensive oxidations, numerous degassing pipes and armoured lapillis occurrences on US-U1 ŽUpper Seydiler-Unit 1.. At the distal part of this contact zone were found conglomerate deposits of the river. The structural analysis of the caldera shows that tectonic activities have also developed after the caldera collapse ŽFig. 7.. Two main trending for the linear alignments in the region were distinguished as
Fig. 7. Structural map of the Koroglu caldera. Heavy lines correspond to the ring fractures.
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NE–SW and NW–SE and the latter seems to be the youngest occurrence. A NW–SE-trending fissure was used by a lamprophyric magma. The lamprophyre concerned, consisting of leuciteq phlogopiteq Crdiopsideq olivineq feldspars, was extruded in the
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lacustrine deposits. The lavas generation have also developed along the ring fractures of the caldera with trachyandesitic flows and trachybasaltic dykes. The dykes are composed of olivineq phlogopiteq Cr-diopsideq salite q K-feldsparq Ba-feldspar Žhy-
Fig. 8. Geological map of the Afyon stratovolcano.
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alophane.. The lava flows contain scarcely cognate inclusions which are trachybasaltic andesites in composition. 2.4. Afyon region Keller and Villari Ž1972. and Keller Ž1983. have petrologically investigated the volcanism of the Afyon region, and defined the alkaline character for the volcanics. Keller Ž1983. has supposed that the volcanism was dominantly potassic and ultrapotassic in character, due to the mantle metasomatism caused by lithospheric subduction from the Hellenic trench. This volcanism has been dated by Besang et al. Ž1977. and the ages were ranged between 8.5–14.5 My. Aydar and Bayhan Ž1995. have supposed that the volcanism of Afyon was related to a stratovolcano which built up on the lacustrine deposits ŽYenikoy formation. and any volcanogenic fragment was encountered in these deposits. On the other hand, Harut Ž1995. has described the stratigraphically Pliocene-aged ‘Koprulu formation’, which contains volcanogenic layers in carbonated lacustrine deposits, overlying the ‘Yenikoy formation’ NNW of Afyon. The relation between these lacustrine formations is obscure and the unique difference is the presence of the volcanogenic sediments in the ‘Koprulu formation’. Lamprophyric lava flows were interstratified in the ‘Koprulu formation’ ŽHarut, 1995.. The lake development in the region seems to have occurred between 17 My Žage of Seydiler Ignimbrites at the north of Afyon. and 14.5 My Žknown age of the oldest product of Afyon stratovolcano.. The stratovolcano of Afyon ŽFig. 8., built up by trachybasaltic and trachyandesitic lavas flow, and then has witnessed a summit caldera collapse following ignimbrite eruptions. The megasanidine Žup to 5 cm. bearing trachytic lava domes and flows, associated block and ash flows constitute the post-caldera volcanic activities. The black, aphanitic lamprophyric lava flows and phlogopite-bearing dyke intrusions complete the volcanic sequence. 2.5. Kula area The known youngest volcanic activities of western Anatolia occurred in the vicinity of Kula, represented by basaltic cinder cones, maars and lava
flows. The main period of the volcanism is between 1.1 My and 0.01 My ŽErcan et al., 1980.. There are at least three volcanic phases. The first one is dominantly characterised by lava flows derived from the ring fracture of a caldera which was detected by SPOT image. The other basaltic phases are represented by cinder cones, maars and fissure-related lava flows. This basaltic volcanism developed along a graben fault which aligned E–W. The common properties of the second and third volcanic phases are the presence of ultramafic xenoliths, such as gabbro, lherzolithe, hornblendite and pyroxenite, within the basaltic ejectas. 3. Discussion and conclusion The main tectonic trends of the region are N–S, NE–SW, NW–SE and E–W. Volcanism data show that N–S trends are the oldest accidents in the region. Afterwards, NE–SW directional accidents were developed during the Early–Middle Miocene, followed by NW–SE trends after the Middle Miocene period. These observations are compatible with the geophysical data. Melts derived from subduction of the African plate along the Hellenic Trench have caused the anatexis of the lower crust, and seem to be a reason for the crustal thinning. The rhyolitic magmas were erupted, as ignimbrites, approximately in the same time in whole western Anatolia, dominantly between 17 and 21 My. The anatexitic granitic intrusions preceded the ignimbrites and were emplaced between 20 and 24 My ŽBingol ¨ et al., 1982.. With the beginning of the anatexis process, the crust has begun to thin and to extend. The alluvial fan deposits which occurred during the basin formation, contain pumices and ignimbritic blocks which indicate the penecontemporaneous development of the explosive volcanic activities. The ignimbrites of the different regions of western Anatolia, erupted between 17 and 21 My and the lakes have developed after the emplacement of the ignimbrites. The explosive activities follow the NE–SW directional fissures which is the main direction of the Miocene basins. The distinction of the lacustrine tuffs and continental tuffs has to be regarded with suspicion and has to be avoided. Instead of this distinction, it would be better to use ‘ignimbrite’ and ‘reworked tuff’ terms.
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The lamprophyric magmas, emplaced during the Late Miocene–Pliocene, are the result of the diapiric rises of the asthenosphere. The asthenospheric diapirs were probably generated after the ignimbritic sequence of western Anatolia. The lamprophyric magmas ascent from the deep mantle has taken upper mantle fragments into the body Žthe nodules. and interact with the crustal material in the magma chambers. They are found in the subvolcanic depth as dykes or generated lava flows at the surface. Most of the volcanics, cited in the literature as trachyte, andesite, trachyandesite, trachybasalt, lamproitic trachyte, lamproite, and shoshonitic basalt, are in fact lamprophyres. The youngest volcanics of western Anatolia are interpreted as typical rift volcanism by Ercan and Oztunalı Ž1983.. Acknowledgements I am thankful to Professor P.M. Vincent ŽBlaise Pascal University, Clermont Ferrand, France., who contributed greatly to the manuscript. I also appreciate criticisms, comments and language corrections of the manuscript by Dr. A. Ciner, Hacettepe University, Ankara, Turkey and MSc. H. Stevens. References Akcig, Z., 1988. Investigation of the structural problems of western Anatolia by the gravity data. Turk. Geol. Bull. 31, 63–70, in Turkish. Angelier, J., Dumont, J.F., Karamanderesi, H., Poisson, A., Simsek, S., Uysal, S., 1981. Analyses of fault mechanism and expansion of southwestern Anatolia since the Late Miocene. Tectonophysics 75, T1–T9. Aydar, E., Bayhan, H., Erkan, Y., 1994. Koroglu Caldera, Midwest Anatolia, Turkiye. Abstract, IAVCEI, Int. Volcanol. Congress, Ankara, Turkiye. Aydar, E., Bayhan, H., 1995. Le volcanisme alcalin d’Afyon. Anatolie de l’ouest orientale, Turquie: approche volcanologique et petrologique. Bull. Section Volcanol., Soc. ´ Geol. Fr. 36, 1–5. Aydar, E., Bayhan, H., Zimitoglu, O., 1996. Investigation of volcanological and petrological evolution of Afyon stratovolcano. Hacettepe Univ. Earth Sci. 18, 87–107, in Turkish. Benda, L., Innocenti, F., Mazzuoli, R., Radicati, F., Steffens, P., 1974. Stratigraphic and radiometric data of the Neogene in northwest Turkey. Z. Deutsch. Geol. Ges. 125, 183–193. Besang, C., Eckhardt, F.J., Harre, W., Kreuzer, H., Muller, P.,
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