PLIOCENE VOLCANISM IN THE GÖLCÜK AREA, ISPARTA/WESTERN
TAURIDES
N. Özgür, A. Pekdeger, H.-J. Schneider,
lnstitut für Geologie, Geophysik und Geoinformatik der Freien Universität Berlin,
Wichernstr. 16, D-1OOO Berlin 33, Germany
and A. Bilgin
Akdeniz Üniversitesi, Isparta Mühendislik Fakültesi, Isparta/Turkey
Abstract
The Gölcük area represents a post-tectonic Pliocene volcanic field upon a Mesozoic paleorift in the WTaurides and consists of sedimentary' and volcanic rocks. The volcanic rocks of an alkaline sequence
are tephriphonolite (stage I), pyroclastic series (stage II), and trachyandesite together with trachyte
(stage III) indicating a sialic origin. The volcanic rocks consist primarily of pyroxene, hornblende,
sphene, biotite, fluorite, fluorapatite, and glassy ground mass as main F-carrier. This F-bearing
groundmass is an essential reason for the high F--contents of the drinking water of the Isparta area.
Introduction
The Gölcük area is situated SW of Isparta (Pekdeger et al., this volume) and represents the
post tectonic Neogene volcanism upon a Mesozoic paleorift of the W- Taurides (Poisson et al.
1984), the so called Isparta angle between Lycian and Hadim-HoyranBeysehir nappes. The
tectonic structures of this region result from the main Alpine orogenic phases of the HellenoTauric belt (Kissel et al. 1989). An extensive volcanic activity took place in central Anatolia
from Paleocene through Quaternary (Innocenti et al. 1975; Ercan 1986). The volcanics of the
Gölcük area belong to a Pliocene sequence. The aim-of this paper is to elucidate the evolution
of volcanic activity and to determine the F contents in various mineral phases, because the
fresh water regime in this area is characterized by unusually high fluorine contents.
A research scheme has been carried out in 1988/1989, divided into two main fields: (i)
petrochemical and geochemical investigations based on detailed geological mapping (this
paper) and (ii) comprehensive hydrogeological and hydrogeochemical investigations
(Pekdeger et al.. this volume).
Geologic setting
As allochthonous, the Triassic through Upper Cretaceous Akdag-limestone and the Upper
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Cretaceous to Lower Tertiary volcano-sedimentary series constitute the basement rocks. They
are transgressively overlain by marine clastic series of Eocene and conglomerates of
Oligocene age. The volcanic sequence of an alkaline character (Pekdeger et al. 1991) is of
Lower Pliocene age (4.07 ±0.20 to 4.70 ±0.50 Ma by K-Ar method; Lefevre et al. 1983).
The extrusions include tephriphonolite, trachyandesite, and trachyte (Pekdeger et al. 1991).
According to the same authors, the pyroclastic series consists of friable tuff (150 m),
ignimbrite (20 m), and pumice tuff (10m) dominating the recent landscape. Trachyandesites
and trachytes occur as different extrusions varying in size and shape of vents, dikes and
volcanic domes.
Generally, the volcanic rocks consist primarily of varying constituents of K (Na)-sanidine,
oligoclase, biotite, pyroxene, hornblende, pyroxenitic xenoliths, glassy ground mass, and
minor quantities of apatite, fluorite, and sphene commonly. Moreover, the tephriphonolite
contain augite and nepheline additionally.
Petrochemistry of volcanic rocks
During the geological mapping of the Gölcük area 217 rock samples have been collected
(Pekdeger et al. 1991). Optical (thin sections) and chemical analyses (XRF, AAS, NAA)
confirmed the petrographic character of the various lavas according to which they can be
divided into tephriphonolite, trachyandesite, and trachyte (Fig. 1). Furthermore, the pyroclastic series indicate a composition between trachyandesite and trachyte as demon strated
in Fig. 1.
Fig. 1: Discrimination of the Gölcuk lava flows (∆)and pyroclastics (*) according to the classification scheme of
Le Maitre (1984). Sl :trachybasalt; S2: benmoreite (Na) orshoshonite (K).
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The appearance of the volcanic rocks limited in time and place as well as their petrographic
variation allow to suppose the varying stages of magmatic differentiation processes.
According to the Harker diagrams (Pekdeger et al. 1991), the major element oxides TiO2,
Fe203 (as total FeO), MnO, MgO, and CaO, Ba and Sr exhibit a negative correlation against
Si02 as expected. On the other hand, the other major element oxides Al203' Na20, and K20
distinguish against Si02 by a positive correlation. This assumption establishes that the
volcanic activity is caused by a magmatic differentiation obviously.
Fluorine and rare earth elements
The volcanic rocks have high F-contents in a range of 33 to 3200 ppm and a background
value of 1000 ppm (Pekdeger et al. 1991; Fig. 2) which can be attributed to different Fbearing
mineral phases. The two samples of the pyroxenitic xenoliths with their unusually high Fcontents are, due to their rare occurrence, without relevance for hydro geochemical questions.
In general, there is a close correlation between F and P20S in the volcanic rocks (Pekdeger et
al. 1991; Fig. 3), which could indicate the geochemical role of fluorapatite as one of the
important F-carriers. The diagram displays a depletion of F from the basic towards the acidic
rocks which is contradictory to a common magmatic differentiation. It might be interpreted by
an increasing discharge of F during fumarolic stages subsequent to the volcanic eruptions
indicating a degassing of an open system. This suggestion is corroborated by the relatively
low F-contents of the pyroclastic series.
Fig.2: Range and bacground values of fluorine in various rocks of the Gölcük area.
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Fig. 3: Close correlation between F and P205 in various volcanic rocks.
The volcanic rocks are distinguished by extremely high REE contents with a maximum about
0.15 % of ΣREE which can be derived from the occurrence of F-apatite (Pekdeger et al. 1991;
Fig. 4). The remarkable similarity of the REE patterns in all samples suggests a common
petrogenic origin for the whole rock spectrum of the volcanic sequence (Capaldi et al. 1972).
Pliocene volcanism
The volcanic activity in the Gölcük area might be temporally divided into three stages (Fig.
5): (I) As a result of the development of the graben systems, the basic tephriphonolite intruded
into the space of the recent Gölcük Lake accompanied by local eruptions. Subsequently, the
lavas of the tephriphonolite have been eroded intensively. The second stage (II) is
characterized by strong volcanic explosions around the center of the recent Gölcük caldera.
Products of this stage are great masses of friable tuffs, ignimbrites, and pumice tuffs
dominating the recent landscape.
The coarse-grained pyroclastic components, like bombs and lapillis, consist of sedimentary
country rocks, basic xenoliths, and tephriphonolite. Trachyandesite and trachyte are missing,
pointing to a younger stage of the volcanic activity.
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After the explosive removal of a great part of the material out of the magma chamber the
surface collapsed forming the Gö1cük caldera. As a late stage (III) isolated extrusions of
trachyandesites and trachytes have taken place at various localities in the center and the
surrounding area of the caldera. They are preserved as vents, dikes, and volcanic domes
proving that the trachytes are always the latest volcanic event.
Fig. 4: REE distribution in the volcanic rocks of the Gölcük area.
Discussion
The volcanic activity in central Anatolia seems to have developed in relation to the plate
tectonic evolution of the region. As a result of formation of the plate tectonic event which is
related to the Afro-Arabian plate being subducted under the Euro-Asiatic plate (Dewey et al.
1973), the volcanism began in central Anatolia at least as early Paleocene continuing up to
Quaternary (Innocenti et al. 1975, 1982; Ercan 1986).
The volcanic activity in the Gölcük area with its Lower Pliocene age and an alkaline character
represents one of the youngest events in central Anatolia. The volcanic rocks can be attributed
to a sialic source (Fig. 6; Gottini 1969). Therefore, they might have originated at least from
the continental crust. The unambiguously origin of volcanic rocks and their pyroxenitic
xenoliths as deep-seated rocks is debatable, it will be reported by detailed investigations of
87
Sr/86Sr and 143Nd/144Nd in a separate paper only.
The development of the volcanism can be temporally divided into three stages: (I)
tephriphonolites, (II) pyroclastic series, and (III) trachyandesites and trachytes. In accordance
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with this succession the petrochemical plots (Figs. 1, 3, 6) discriminate a distinct grouping
which indicates a magmatic differentiation suite.
Fig.5: Schematic record of the volcanic stages in the Gölcük area. Not to scale.
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Fig. 6: Origin of the volcanic rocks in the Gölcük area according to the classification scheme of Gottini (1969).
The volcanic rocks of the Gölcük area are distinguished from the sedimentary rocks by their
high F-contents which can be attributed to different mineral phases as determined in thin
sections. A remarkable relative depletion of fluorine from the basic towards the acidic rocks
indicates a trend to increasing degassing during the different stages. In this connection
important portions of F might be discharged during fumarolic activity. The exceptionally high
REE contents establish that the main part of the F-contents of the volcanic rocks might be
controlled by the occurrence of fluorapatite, because there is remarkable correlation between
F and P205.
Furthermore, the small variability of the REE patterns indicates a close petrogenic relationship of the different volcanic stages. They are differentiation products of a common
magma chamber.
Acknowledgements
The financial support of the project (contract no. Pe 362/3-1) by Deutsche
Forschungsgemeinschaft is gratefully acknowledged. The authors wish to thank the university
of Akdeniz, Isparta, and MTA, Ankara, for kind cooperation.
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