olivine basalt and trachyandesite peperites formed...
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Journal of Volcanology and Geotherm
Olivine basalt and trachyandesite peperites formed at the
subsurface/surface interface of a semi-arid lake: An example
from the Early Miocene Bigadic basin, western Turkey
Fuat Erkul *, Cahit HelvacV, Hasan Sozbilir
Dokuz Eylul Universitesi, Muhendislik Fakultesi, Jeoloji Muhendisligi Bolumu, 35100 Bornova, Izmir, Turkey
Received 3 June 2004; received in revised form 5 July 2005; accepted 18 July 2005
Abstract
Miocene successions in western Turkey are dominated by lacustrine, fluvial and evaporitic sedimentary deposits. These
deposits include considerable amounts of volcaniclastic detritus derived from numerous NE-trending volcanic centres in
western Turkey as well as in the Bigadic region. Early Miocene syn-depositional NE-trending olivine basalt and trachyandesite
bodies that formed as intrusions and lava flows occur within the Bigadic borate basin. Olivine basalts occur as partly emergent
intrusions, and trachyandesite dykes fed extensive lava flows emplaced in a semi-arid lacustrine environment.
Peperites associated with the olivine basalt and trachyandesites appear to display contrasting textural features, although all
the localities include a large variety of clast morphologies from blocky to fluidal. Fluidal clasts, mainly globular, ameboidal and
pillow-like varieties, are widespread in the peperite domains associated with olivine basalts, apparently due to large-volume
sediment fluidisation. In contrast, fluidal clasts related to trachyandesites are restricted to narrow zones near the margins of the
intrusions and have commonly elongate and polyhedral shapes with digitate margins, rather than globular and equant varieties.
Blocky and fluidal clasts in the olivine basalt peperite display progressive disintegration, suggesting decreasing temperature and
increasing viscosity during fragmentation. Abundance of blocky clasts with respect to fluidal clasts in the trachyandesite
peperite indicates that the fluidal emplacement and low-volume sediment fluidisation in the early stages were immediately
followed by quench fragmentation due to the high viscosity of the magma.
Size, texture and abundance of the blocky and fluidal clasts in the olivine basalt and trachyandesite peperites were mainly
controlled by sediment fluidisation, pulsatory magma injection and magma properties such as composition, viscosity,
vesicularity, and size, abundance and orientation of phenocrysts. Variously combining these contrasting features to varying
degrees may form diverse juvenile clast shapes in peperitic domains.
D 2005 Elsevier B.V. All rights reserved.
Keywords: hypabyssal; peperite; semi-arid lake environment; Early Miocene; Bigadic basin; western Turkey
0377-0273/$ - s
doi:10.1016/j.jvo
* Correspondin
E-mail addre
al Research 149 (2006) 240–262
ee front matter D 2005 Elsevier B.V. All rights reserved.
lgeores.2005.07.016
g author. Tel.: +90 232 3887866; fax: +90 232 3887865.
ss: [email protected] (F. Erkul).
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262 241
1. Introduction
Magma–sediment interaction is common, espe-
cially in arc-related volcanic complexes accompany-
ing continuous sedimentation in a subsiding basin
(e.g., Hanson and Hargrove, 1999; Mueller et al.,
2000; Dadd and Van Wagoner, 2002; Skilling et al.,
2002; Templeton and Hanson, 2003). Sediment+ ig-
neous rock breccia is widely formed at the contacts
between sediments and magma or hot volcaniclastic
deposits. This lithofacies is distinguished as peperite,
referring to a genetic name that describes a rock
formed by mixture of unconsolidated, poorly conso-
lidated or wet sediment and disintegrated magma
(White et al., 2000; Skilling et al., 2002). Recognition
of peperite is useful to establish emplacement mode of
the igneous bodies, palaeoenvironmental reconstruc-
tion and relative comtemporaneity of intruding
magma and sedimentary host rock. Peperite might
also be closely associated with hydrothermal systems
and mineralisation due to magmatic fluid contribu-
tions into a basin and therefore their identification can
be economically important (Delaney, 1982; McPhie
and Orth, 1999).
Although peperite occurrences are common from
submarine environments, subaerial examples are also
known. Peperitic textures are formed by diverse vol-
canic processes in subaerial settings such as pyroclas-
tic flows, phreatomagmatic eruptions and lava flows
(Schmincke, 1967; Branney, 1986; Leat and Thomp-
son, 1988; White, 1991; Rawlings and Sweeney,
1999; Hooten and Ort, 2002; Jerram and Stollhofen,
2002; Lorenz et al., 2002; McClintock and White,
2002; Zimanowski and Buttner, 2002). Controls on
the formation of peperite and influencing factors
based on field and experimental data are well docu-
mented in the literature (Skilling et al., 2002 and
references therein). However, peperite occurrences
identified in lacustrine settings are limited (e.g., Cas
et al., 2001).
In this paper we describe peperites formed in
association with olivine basaltic and trachyandesitic
intrusive rocks emplaced in a semi-arid lacustrine
setting. The lacustrine basin fill hosts the largest
borate reserves in the world (HelvacV, 1995). The
aim of this study is to compare the contrasting
peperitic textures related to coeval syn-depositional
intrusions of different composition. Emphasis is
placed on mode of emplacement of the syn-deposi-
tional magmas, their interaction with lacustrine sedi-
ments, and hydrothermal systems in the Bigadic borate
basin. Field-based work included 1 :25,000 scale geo-
logical mapping with description of the peperitic tex-
tures based on two dimensional outcrop-scale
observations along road sections and small quarries.
Outcrops lack tectonic overprints and major hydro-
thermal alteration, allowing insight into modes of
magma emplacement and related peperite formation.
XRF and ICP-MS analyses were also performed on
representive samples from the syn-depositional intru-
sive and extrusive bodies and plotted onto geochem-
ical discrimination diagrams in order to characterise
their compositions.
2. Regional setting
In western Turkey, the pre-Miocene basement
rocks – Menderes Massif, Sakarya Zone, Lycian
Nappes and the Bornova Flysch Zone, Eocene
BaslamVs Formation and Oligocene detrital rocks –
have undergone extensional deformation since the late
Oligocene, leading to the formation of NE-trending
basins and E–W-trending grabens (Akdeniz, 1980;
Seyitoglu and Scott, 1991, 1992; Okay et al., 1996;
YVlmaz et al., 2000; Bozkurt, 2001a,b; Bozkurt and
Oberhansli, 2001; Ozer et al., 2001; Seyitoglu et al.,
2002; Sozbilir, 2001, 2002a,b; Bozkurt, 2003; Ozer
and Sozbilir, 2003; Beccaletto and Jenny, 2004; Boz-
kurt and Sozbilir, 2004; Erdogan and Gungor, 2004;
Duru et al., 2004; Goncuoglu et al., 2004; Koralay et
al., 2004; Okay and AltVner, 2004; Okay and Goncuo-
glu, 2004; Turhan et al., 2004; Purvis and Robertson,
2004, 2005a,b) (Fig. 1a). The NE-trending basins are
cut by the E–W trending grabens, which likely formed
under an N–S extensional regime existing since the
Tortonian (Sengor and YVlmaz, 1981; Sengor et al.,
1985; Sengor, 1987). The Gordes, Demirci, Selendi
and Usak-Gure basins have formed in NE-trending
grabens that are bounded by oblique-slip faults (Sen-
gor, 1987; Bozkurt, 2003) (Fig. 1b). One of the NE-
trending basins, the Bigadic borate basin, is located on
the Bornova Flysch Zone of Late Cretaceous–Palaeo-
cene age. The basin and its basement are restricted
between the Sakarya Zone to the north and Menderes
Massif to the south.
Fig. 1. (a) Location map showing the tectonic setting of Bigadic area in western Turkey (modified from Okay and Siyako, 1993). (b) Geological
map illustrating Miocene NE-trending basins and Cenozoic magmatic rocks in western Turkey. The boundaries between rock units are
simplified from Geological Map of Turkey (1964). Inset shows the location of the represented area.
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262242
Widespread magmatic activity occurred during for-
mation of the NE-trending basins and produced shal-
low-seated granitic intrusions and associated extrusive
rocks (YVlmaz, 1989; Altunkaynak and YVlmaz, 1998;
KaracVk and YVlmaz, 1998). The Miocene successions
in western Turkey are dominated by lacustrine, fluvial
and evaporitic sedimentary deposits (HelvacV and Yag-
murlu, 1995). The sedimentary deposits include con-
siderable amounts of volcaniclastic detritus derived
from numerous NE-trending volcanic centres in wes-
tern Turkey as well as in the Bigadic region (Seyitoglu
and Scott, 1991; Seyitoglu et al., 1992; HelvacV, 1995;
HelvacV et al., 1993; Altunkaynak and YVlmaz, 1998;
Genc, 1998; KaracVk and YVlmaz, 1998; HelvacV and
Alonso, 2000; YVlmaz et al., 2000; Inci, 2002; Boz-
kurt, 2003). Another common feature of the NE-trend-
ing basins is the presence of central volcanoes that cut
or interfinger with basin fill deposits (Seyitoglu and
Scott, 1994; Seyitoglu, 1997; Bozkurt, 2003; Purvis
and Robertson, 2004, 2005a,b). Products of central
volcanism include lava flows, pyroclastic and volca-
nogenic sedimentary rocks. These central volcanoes
are thought to have been controlled either by an
extensional tectonic regime within rift-type basins
(Seyitoglu and Scott, 1994) or by strike-slip structures
within pull-apart type basins (Bozkurt, 2003).
3. Local stratigraphy
3.1. Pre-basin fill units
The pre-basin fill units consist of the Late Cretac-
eous–Palaeocene Bornova Flysch Zone and the Early
Miocene Kocaiskan volcanic unit (Figs. 2 and 3). The
Bornova Flysch Zone is a NE-trending and 50–90 km
wide chaotically deformed zone (Okay et al., 2001)
(Fig. 1a). It comprises flysch-type sedimentary rocks
including sandstone, shale and limestone with large
recrystallised limestone olistoliths and ophiolitic tec-
tonic slices.
The Kocaiskan volcanic unit is characterised by
andesitic rocks that cover more than 800 km2 and
unconformably overlie the Bornova Flysch Zone.
Fig. 2. Geological map illustrating the distribution of volcanic and lacustrine units in the Bigadic borate basin.
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262 243
The unit includes andesitic intrusions, lava flows,
pyroclastic rocks and widespread volcanogenic sedi-
mentary rocks (Erkul et al., in press(a,b)). The ande-
sitic rocks mainly formed under subaerial conditions,
producing a stratovolcano that was subsequently
partly eroded and covered unconformably by the
lacustrine basin fill deposits. For more information
about pre-basin fill units, readers are referred to
Okay and Siyako (1993), HelvacV (1995), Gundogdu
et al. (1996), Okay et al. (1996, 2001), Erkul (2004)
and Erkul et al. (in press(a,b)).
3.2. Basin fill units
The Bigadic borate basin consists of lacustrine
clayey, calcareous and tuffaceous sedimentary rocks
(e.g., lower limestone unit, lower tuff unit, lower
borate unit, upper tuff unit, upper borate unit) that
interfinger with coherent volcanic rocks (e.g.,
SVndVrgV volcanic unit, Golcuk basalt, KayVrlar volca-
nic unit, Sahinkaya volcanic unit) (Fig. 2). Volcanism
accompanied lacustrine sedimentation from approxi-
mately 20 to 18 Ma (Fig. 3). The lacustrine sedimen-
tary rocks and syn-depositional volcanic rocks are
grouped in the Bigadic volcano-sedimentary succes-
sion (BVS) with a maximum thickness reaching up to
700 m. Readers are referred to HelvacV (1995), Gun-
dogdu et al. (1996), Erkul (2004) and Erkul et al. (in
press-b) for detailed descriptions of the rock units
within the BVS.
Lamination, syn-sedimentary slumps and dessica-
tion cracks are common sedimentary structures within
Fig. 3. Simplified stratigraphic column of the Bigadic borate basin. Asterix illustrates K–Ar ages of a rock in millions of years. Numbers in
parentheses indicate age data sources: (1) Erkul et al. (in press(a,b)), (2) Gundogdu et. al. (1989), (3) Krushensky (1976), (4) HelvacV (1995) and
(5) Benda et. al. (1974).
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262244
the sedimentary rocks (HelvacV and Orti, 1998;
HelvacV and Alonso, 2000). The lacustrine sedimen-
tary deposits in the Bigadic borate basin are thought to
have formed in intracontinental shallow lakes such as
playas, perennial interconnected saline and ephemeral
lakes in arid to semi-arid environments (HelvacV and
Firman, 1976; HelvacV, 1995; Palmer and HelvacV,
1995, 1997; Floyd et al., 1998; HelvacV and Orti,
1998).
Felsic volcanic rocks, the SVndVrgV volcanic unit,
cover an area larger than 400 km2 in the eastern and
southern parts of the study area (Fig. 2). They com-
prise widespread lavas, autobreccias, welded and non-
welded ignimbrites and ash-fall deposits.
The lower tuff unit comprises interbedded crys-
tal-rich volcaniclastic sandstone and vitric siltstone;
they are thought to have been emplaced in both
subaerial and lacustrine environments as debris flow
and ash-fall deposits, respectively (Erkul, 2004).
The crystal-rich sandstone is massive to planar
stratified and made up of quartz, biotite, plagioclase
and hornblende crystals, with pumice and minor
limestone fragments derived from the underlying
sedimentary rocks. The vitric siltstone is distin-
guished by its conchoidal fracture and comprises
biotite fragments up to 0.5 mm and altered volcanic
glass. The upper tuff unit is greenish in colour, and
comprises variable amounts of pumice and lithic
clasts in an ash-grade matrix. Distal parts of the
unit are planar stratified, lack coarse-grained clasts,
and consist entirely of volcanic glass. The upper
tuff unit is interpreted to have formed by pyroclas-
tic flow and fall processes within subaerial and
lacustrine environments (Gundogdu et al., 1996;
Erkul, 2004). Both lower and upper tuff units
locally contain zeolite minerals such as clinoptilolite
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262 245
and heulandite, derived from the alteration of volcanic
glass in an alkaline lake environment (Ataman, 1977;
Gundogdu et al., 1989, 1996; HelvacV et al., 1993;
HelvacV, 1995).
The uppermost volcanic unit in the BVS, the
Sahinkaya volcanic unit, is located in the southern
part of the Bigadic area and represents the latest
volcanism in the basin. The unit consists, from lim-
ited exposures, of basaltic andesite intrusive rocks,
and massive and autobrecciated lava flows inter-
bedded with thin-bedded ash-fall deposits. The BVS
is unconformably overlain by Upper Miocene–Plio-
cene continental deposits. Only volcanic units related
to the formation of peperite are described in detail
below.
a
c
Fig. 4. Representive samples from lavas of the Golcuk basalt and KayVrl
diagrams. (a) TAS variation diagram (after Le Maitre et al., 1989), (b) Zr/
and (c) SiO2 vs. Zr/TiO2 classification diagram (Winchester and Floyd, 19
and sanidine-phyric trachyandesites.
4. Peperite-related volcanic units
Peperite-related volcanic units within the BVS crop
out intermittently along a 35-km-long NE-trending
zone within the lacustrine sedimentary rocks. Two
volcanic units are recognised based on their composi-
tion, the Golcuk basalt and the KayVrlar volcanic unit
(Fig. 4). The Golcuk basalt is alkaline–subalkaline
basalt and basaltic trachyandesite in composition,
while the KayVrlar volcanic unit plots in the andesite
and trachyandesite field (Fig. 4a–c, Table 1). Field
relationships and K–Ar ages from the Golcuk basalt
and KayVrlar volcanic unit indicate that they were
emplaced synchronously (Erkul et al., in press(a,b))
(Fig. 3).
b
ar volcanic unit plotted on the various geochemical discrimination
TiO2 vs. Nb/Y classification diagram (Winchester and Floyd, 1977)
77). See also Section 4.2 for detailed description of the plagioclase-
Table 1
XRF major- and trace-element analyses from Golcuk basalt and KayVrlar volcanic unit
Sample no. F-266 F-267 F-268 F-269 F-272 F-191 F-216 F-248 F-219 F-245 F-251 F-253
Formation Golcuk basalt KayVrlar volcanic unit
Lithology Olivine basalt Plagioclase-phyric trachyandesite Sanidine-phyric trachyandesite
Major oxides (wt.%)a
SiO2 52.20 52.65 49.46 49.55 49.99 60.56 60.72 61.27 61.33 59.96 60.31 59.76
TiO2 1.20 1.23 1.05 1.05 1.04 0.70 0.70 0.61 0.56 0.70 0.73 0.75
Al2O3 16.20 16.75 15.91 15.97 15.87 16.79 16.97 16.44 16.59 16.54 15.24 16.80
Fe2O3* 8.10 7.64 7.87 7.85 7.72 5.57 5.54 4.63 5.11 5.59 5.12 5.82
MnO 0.09 0.09 0.08 0.09 0.11 0.08 0.09 0.06 0.10 0.10 0.04 0.06
MgO 5.08 5.31 5.28 5.38 6.18 4.49 4.14 4.32 3.64 4.05 6.65 5.29
CaO 7.48 7.75 9.30 9.46 9.47 4.65 4.64 4.40 4.56 4.84 4.26 4.34
Na2O 3.27 3.37 2.79 2.84 2.88 2.82 2.87 2.74 3.11 3.04 2.89 2.51
K2O 2.24 2.30 2.10 2.07 2.06 3.80 3.71 4.20 3.49 3.35 3.18 3.99
P2O5 0.50 0.45 0.43 0.41 0.45 0.30 0.29 0.29 0.22 0.29 0.27 0.31
LOI 3.20 2.80 4.90 4.50 3.80 1.63 2.03 2.40 1.60 2.02 2.56 1.90
Total 99.72 100.49 99.35 99.34 99.76 101.39 101.70 101.36 100.31 100.48 101.25 101.53
Trace elements (ppm)b
Ba 1008 1044 1094 1075 1083 1475 1491 1448 1264 1867 1414 1703
Co 25 27 31 28 30 23 29 26 32 30 28 22
Ga 20 20 17 19 17 17 17 17 15 17 18 17
Nb 23 22 16 15 15 16 16 15 13 16 16 16
Rb 61 60 62 61 61 117 148 166 127 126 107 90
Sr 543 595 580 566 593 637 632 570 599 695 624 641
Th 13 13 14 12 12 28 28 28 28 33 22 32
V 157 165 170 167 171 121 121 122 117 143 123 133
Zr 217 229 177 178 177 188 177 175 139 184 163 215
Y 29 31 27 26 28 23 23 24 20 25 20 22
Cu 15 13 20 19 22 14 12 15 17 16 17 19
Pb 6 6 5 5 7 14 12 9 12 9 6 16
Zn 50 49 45 43 40 51 49 50 39 42 59 57
Fe2O3*= total iron, LOI= loss on ignition.a Whole-rock major element analyses were completed at the Mineralogical–Petrographical and Geochemical Research Laboratories (MIPJAL)
of the Geological Engineering of Cumhuriyet University in Sivas, Turkey using a Rigaku E-WDS-3270 X-ray fluorescence spectrometer.
Sample preparation procedures and calibration standarts are presented by Boztug (2000).b The trace element analyses were performed using ICP-MS by ACME Analytical Laboratories Ltd. in Canada and powdered samples were
fused by LiBO2.
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262246
4.1. Golcuk basalt
The Golcuk basalt comprises olivine basalt dykes
and lava domes and is located in a 25-km NE-trending
zone north of Golcuk to Babakoy and Camkoy (Fig.
2). North of Golcuk, an olivine basalt dyke intrudes
the lacustrine sedimentary rocks of the BVS (Fig. 5).
The dyke is up to 100 m thick, dips vertically and
extends 4 km. The eastern contact of the dyke with
massive and laminated limestone is sharp and linear,
whereas the western contact is fault-controlled with
the Kocaiskan volcanic unit. The dyke is highly vesi-
cular. The vesicles are commonly filled by secondary
minerals such as quartz, calcite and zeolites and con-
stitute about 30–40 vol.% of whole-rock.
To the west of Cukurdere, lava domes are of small-
volume and display circular and elliptical outlines in a
plan view. Each lava dome covers an area up to 1 km2.
The inner part of the domes is massive and poorly
vesiculated, but vesicularity increases toward the mar-
ginal contacts. The lava domes rarely contain shale
and quartzite clasts up to 5 cm long derived from the
underlying basement rocks. Upper contacts of the lava
domes are concordantly overlain by massive and
Fig. 5. Geological map of the Golcuk area illustrating peperite outcrops associated with the Golcuk basalt. See Fig. 2 for location of the map.
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262 247
laminated clayey limestones while lower contacts are
intrusive (Fig. 6a,c,e). Olivine basalt domes display at
the upper contacts typical flow foliation, and contain
sub-horizontal elongate vesicles (Fig. 6b). The intru-
sions at the lowermost part of the lava domes are
characterised by a peperitic contact zone and up to
10-cm long flow-aligned elongate vesicles that are
subvertical and more or less parallel to the contact
with sedimentary host rock. The contact zone between
the intrusive lower part and the lava domes is char-
acterised by a breccia that comprises fluidal clasts
surrounded by banded carbonate and silica as vug-
infill (Fig. 6d). The olivine basalt is composed of
phenocrysts of olivine, rare plagioclase and augite in
an intersertal groundmass of plagioclase microlites
and argillised volcanic glass. The olivine phenocysts
are usually subhedral to euhedral and are commonly
altered to iddingsite and serpentine.
b c
a
d e
Fig. 6. (a) Geological cross-section showing contact relationships of olivine basalt dome with lacustrine sedimentary rocks. See Fig. 5 for
location of the cross-section. (b) Subhorizontal flow foliation and elongate vesiculation at the upper contact of the olivine basalt dome. Head of
hammer is 16 cm long. (c) Clayey limestone on the olivine basalt dome displaying planar and laminated stratification. (d) Vug-infill quartz and
carbonate mineralisations around fluidal and vesicular olivine basalt clasts. Pen is 14 cm long. (e) Peperitic contact zone at the lowermost
contact of the olivine basalt with massive limestone as a host rock. Pillow- and lobe-shaped olivine basalt bodies (dark colours) intrude the
lacustrine sedimentary rocks (light colours) and contain irregular cracks filled by host rock (black arrow). Some pillow-shaped lobes are
connected with fluidal necks (white arrow). The man is 1.78 m tall.
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262248
4.2. KayVrlar volcanic unit
The KayVrlar volcanic unit is located in the north-
east of the study area, covering about 20 km2 around
KayVrlar and Dogancam districts (Fig. 2). The unit
comprises trachyandesitic dykes and associated mas-
sive and autobrecciated lava flows. The dykes that fed
extensive lava flows intrude massive limestone and
are characterised by unidirectional subvertical flow
foliations and phenocryst orientations. The dykes
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262 249
trend N408E in direction, have a uniform width of
about 100 m and extend about 1.5 km in the Dogan-
cam area (Fig. 7). Subsidiary dykes, up to 1 m wide,
and elongate bodies with fluidal contacts, also occur
perpendicular to the NE-trending main dykes. Tra-
chyandesitic lava flows form a lens-shaped body at
least 6 km long that is intercalated with lacustrine
sedimentary rocks. The maximum thickness of the
lava flows is up to 400 m in the KayVrlar area. Sub-
horizontal flow banding and subvertical polyhedral
columnar jointing are major diagnostic features of
the lava flows at outcrop-scale. Autobrecciated lava
flows form lens-shaped beds separating the volcani-
clastic sandstones and overlying coherent lava flows.
They are characterised by angular, partly in situ frag-
mented and monomictic clasts up to 20 cm long
embedded in a lava–fragment matrix. The lower con-
tact of the lava flows and autobreccias is only exposed
in the Dogancam area where they conformably overlie
crystal-rich volcaniclastic sandstones (Fig. 8a). The
upper lava contact is sharply overlain by stratified
Fig. 7. Geological map of the Dogancam area illustrating peperite outcrops
the map.
silica-rich limestone containing abundant chert
bands and nodules (Fig. 8b).
There are two contrasting trachyandesite composi-
tions: plagioclase-phyric and sanidine-phyric. Tra-
chyandesitic dykes and lava flows contain abundant
ellipsoidal microcrystalline enclaves. The plagio-
clase–phyric trachyandesite is exposed in the Dogan-
cam area, forming dykes and associated flows. It is
commonly porphyritic with phenocrysts of euhedral to
subhedral plagioclase, biotite, hornblende, clinopyr-
oxene and minor quartz, apatite and opaque phases
within hyalopilitic matrix formed by small microlites
and volcanic glass. Phenocrysts make up to 50 vol.%
of whole-rock. Microcrystalline enclaves display typi-
cal ophitic texture and consist of abundant plagio-
clase, clinopyroxene and hornblende. The sanidine-
phyric trachyandesites occur as flows that are widely
exposed in the southeastern part of KayVrlar. They
differ from the plagioclase-phyric trachyandesite in
containing up to 2–3 cm long sanidine and abundant
corroded quartz phenocyrsts. Mafic enclaves are oli-
associated with the KayVrlar volcanic unit. See Fig. 2 for location of
b c
a
Fig. 8. (a) Geological cross-section showing upper and lower contact relationships of trachyandesitic flows. See Fig. 7 for location of the cross-
section. (b) Sharp and concordant upper contact (broken white line) of massive trachyandesite lava flows (tl) overlain by planar stratified silicic
limestones with abundant chert bands and nodules (sl). Field of view is ~200 m across. (c) Sharp lower contact of autobrecciated trachyandesites
(ab) underlain by volcaniclastic sandstones (vs). Hammer is 33 cm long.
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262250
vine basalt, showing similar mineralogic composition
to the Golcuk basalt.
5. Field evidence for magma/wet sediment
interaction
5.1. Peperitic textures associated with Golcuk basalt
Peperitic textures are only exposed at the lower-
most contact of the olivine basalt domes with the
massive limestone along a 30-m-wide zone, but are
also associated with intrusions that fed the lava domes
(Fig. 6). Olivine basalt clasts in the peperite domain
are monomictic and have mixed morphologies such as
blocky, fluidal and globular (Busby-Spera and White,
1987; Doyle, 2000; Skilling et al., 2002). They are
variable in size, ranging from a millimetre to deci-
metres long, and are closely packed or dispersed
within micritic limestone. Although micritic limestone
as host sedimentary rock is massive in the peperite
domain, it commonly displays planar and laminated
stratification away from the intrusive contacts.
The dome-feeding intrusive bodies are surrounded
by subsidiary lobe-shaped intrusions that are con-
nected to the main body or isolated in the sedimentary
host rock. Chilled margins are found along fluidal
margins of the intrusions, lobes and clasts in the
peperite domain. The lobes are subhorizontally
oriented, enclosed by peperitic breccia and display
folds that partially enclose massive limestone (Fig.
9a). Olivine basalt lobes are amygdaloidal up to 2 m
long and a few decimetres thick. Vesicles are spherical
or ellipsoidal up to 1 cm long and infills commonly
dc
e f
a b
Fig. 9. Peperitic textures related to olivine basalts. (a) Polished slab from the longitudal section of an olivine basalt lobe (b) showing folded flow
pattern (broken line) with enclosed massive limestone (s). The lobe contains irregular, chilled (arrow) and brecciated margin (p), and hairline
cracks (solid lines). (b) Dispersed tapered and platy lava clasts (b) with digitate margins within micritic limestone (ls). Clasts are highly vesicular
and vesicles are filled by micritic limestone near the margin. Note also incomplete brecciation of the tapered clast (arrow). (c) Globular clasts
with irregular margins surrounded by non-stratified micritic limestone. (d) Amoeboidal-shaped margins of vesicular olivine basalt clasts in
micritic limestone. White circles indicate the micritic limestone-filled vesicles near the contact. Note progressive disintegration of the clasts
along irregular cracks (arrow), grading to dispersed peperite. Hairline cracks may partially cut the fluidal clasts and form incomplete brecciation.
Disintegrated clasts show both irregular and curviplanar margins. (e) Pillow-like lobe cut by hairline cracks parallel and perpendicular to margin.
The lobe is gradually disintegrated from margin to host sedimentary rock and is surrounded by variably shaped, rotated and in situ fragmented
clasts. (f) Vug-infill quartz and carbonate mineralisations within peperitic domain. In situ fragmented clasts are also surrounded by carbonate
and silica (black arrow). b: basalt clast, v: vugs, q: colloform–crustiform veining. Note also the fluidally shaped margins of juvenile clasts (white
arrow).
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262 251
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262252
have a quartz core and carbonate rim. Some lobes
were fragmented in situ and display jig-saw fit texture.
The in situ fragmented clasts are variably shaped, but
are predominantly platy and tapered with digitate
margins. Platy and tapered clasts, reaching up to 40
cm long, are usually sparsely packed within micritic
limestone. They locally grade into elongate clasts a
few centimetres long derived from the margin (Fig.
9b). The orientation of the platy clasts, as well as
hairline cracks filled by limestone, is more or less
parallel to the margin of the lobes. Globular clasts
are up to 3 cm in diametre and partly interconnected.
Each clast displays fluidally shaped sharp contact
with sedimentary host rock (Fig. 9c). Some clasts
have spongy margins and are amoeboidal in shape;
these are commonly intersected by hairline cracks
with micritic limestone infill. Hairline cracks that
irregularly intersect highly vesicular clasts partially
form typical jig-saw fit textures. Amoeboid-shaped
olivine basalts are progressively disintegrated into
smaller, from a millimetre to 5 cm long, dispersed
clasts displaying both fluidal and curviplanar margins
(Fig. 9d). Vesicles at the marginal parts of the amoe-
boid-shaped clasts are filled with micritic limestone
as a host rock.
Pillow-like lobes are up to 40 cm in diametre, and
are enclosed within peperite domains (Fig. 9e). They
have low vesicularity in the cores, increasing toward
the margins. Hairline cracks, which are commonly
filled by micritic limestone and quartz, occur parallel
or perpendicular to the margins. Progressive disinte-
gration toward the margin is common, generating
platy and tapered clasts up to 20 cm long around the
pillow-like lobes. Peperite breccias have also formed
near the lobe cores due to penetration of sediment
along fine irregular cracks.
Fig. 10. Peperitic textures related to plagioclase-phyric trachyandesites of
clast (arrow) detached from subsidiary dyke (td) intersecting intensely s
sediment invasion along a fracture within the subsidiary dyke. (b) Mesoblo
are enveloped by a few millimetres thick chalcedony rim. Note also fl
curviplanar margins of the in situ fragmented blocky clasts (white arrow).
(td) displaying irregular contact relationships. (d) Close-up of the conta
consisting of angular juvenile clasts with hairline cracks and partial jig-sa
dyke and sedimentary host rock, showing stratification and syn-sedimenta
slump structures within peperite domain. Host sedimentary rock surroundin
limestone (ml), showing syn-sedimentary deformation. Note also stratificat
filled vugs surrounded by in situ fragmented polyhedral clasts. Central par
Brecciated limestone with greyish silica matrix, partly enclosing blocky c
Stockwork quartz and carbonate veins and vug-
infills within peperitic contact zones, locally exposed
in the southern part of Dastepe, characterise hydro-
thermal mineralization. Quartz and carbonate miner-
alisation is well-developed within the interconnected
vugs that are surrounded by fluidally shaped olivine
basalt clasts (Figs. 6d and 9f). Chalcedony and opa-
line silica are usually colloform–crustiform banded,
and partly crystalline. The mineralisation only occurs
within the peperitic zone and terminates at the contact
between the olivine basalt and overlying limestone at
the uppermost part.
5.2. Peperitic textures associated with KayVrlar
volcanic unit
Peperitic textures around Dogancam area are only
exposed along NE-trending plagioclase-phyric dykes
(Fig. 7). A closely packed sediment–trachyandesite
breccia comprises mainly blocky and minor fluidal
clasts. It is closely associated with subsidiary dykes
emplaced in a perpendicular direction to the NE-
trending dykes, and is interpreted as peperite breccia.
The 10-m-wide contact zone is also characterised by
quartz veining and intense silicification of the sedi-
mentary host rock, which is locally brecciated.
Subsidiary dykes exhibit both curviplanar and flui-
dal contacts with massive and silicified limestone host
rock. Dykes with curviplanar margins were emplaced
as subvertical bodies and contain irregular cracks a
few centimetres wide filled by silicified limestone;
they are enclosed by a breccia that contains blocky,
angular, platy and tapered clasts. Clast size is com-
monly bimodal, ranging from a millimetre to a few
centimetres and up to 10–30 cm long. Blocky and
tapered clasts are usually aligned parallel to the mar-
the KayVrlar volcanic unit. (a) Partially in situ fragmented, tapered
ilicified and brecciated limestone (sl). Broken white line indicates
cky clasts partly displaying in situ fragmentation. Mesoblocky clasts
uidal margins of the mesoblocky clasts (black arrow) and planar
(c) Assimilated limestone block (ls) within main trachyandesite dyke
ct between the trachyandesite and the assimilated limestone block
w fit texture. (e) Polished slab near the contact between subsidiary
ry deformation such as clast rotation (rc), pipelike channels (pl) and
g the rotated juvenile clasts is composed of silicified (s) and micritic
ion of silicified and micritic limestone layers. (f) Silicified limestone-
t of the limestone is more intensely silicified than that of margin. (g)
lasts with jig-saw fit texture.
a
c
e g
f
b
d
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262 253
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262254
gin of subsidiary dykes, and are disintegrated into
smaller angular clasts forming jig-saw fit textures
(Fig. 10a). Phenocrysts within the subsidiary dykes
are also oriented parallel to the margins. Blocky clasts
are commonly polyhedral with planar–curviplanar and
digitate margins, and rarely connected to the dykes by
fluidal necks (Fig. 10b). No chilled margins were
recognised on the in situ fragmented blocky clasts.
The margins of the blocky clasts are usually digitate,
with millimetre-scale salients into the host rock. Hair-
line cracks filled by host rock are also common in the
blocky clasts. Some of the blocky and tapered clasts
are rimmed by 3–4 mm thick chalcedony. Platy clasts
locally display random orientation, suggesting clast
rotation.
The NE-trending intrusive body appears to contain
large limestone blocks assimilated by the trachyande-
site. Limestone blocks may reach up to 2 m in dia-
metre, including altered trachyandesite clasts up to 10
cm long (Fig. 10c). We infer that trachyandesite
magma was injected along fractures developed in
massive limestone. The contacts between trachyande-
site and assimilated limestone are irregular, and in
places are lined with angular clasts displaying jig-
saw fit texture (Fig. 10d).
Limestone as a host rock in the peperite domain is
composed entirely of micritic calcite and is distin-
guished by white to creamy colours. It contains sili-
cified and brecciated horizons near the intrusive
contact. Although it is commonly massive, it may
locally display stratification perpendicular to the
dips of the dykes near their contacts, where blocky
clasts are abundant. Stratified limestone is interbedded
with silicified and clast-dominated horizons, which
display syn-sedimentary deformation such as folding,
load casts and 3 cm long pipe-like structures locally
intersecting stratification (Fig. 10e).
Silicification of the sedimentary host rock, quartz
veins and breccia zones is common, but is mainly
restricted to the peperitic contact zone in the Dogan-
cam area. Silicification usually occurs around brec-
cia zones and blocky peperites. The in situ
fragmented blocky clasts are enclosed by silicified
limestone, locally as vug-infill displaying intense
silicification parallel to margin of clasts in the
core and less silicified at the margin of the tra-
chyandesite clast (Fig. 10f). Intensely silicified
parts of massive limestone contain a monomictic
breccia zone with angular limestone clasts up to a
few centimetres long. Breccia matrix surrounding
the blocky clasts is made up entirely of angular
silicified limestone clasts enveloped by dark
coloured chalcedony (Fig. 10g). Limestone clasts
in the breccia zone are polyhedral and rarely in
situ fragmented. Colloform and crustiform quartz
veining is widespread in intrusions, lava flows and
contact zone. Stockwork-type quartz veining is also
exposed in sanidine-phyric lava flows in the north
of KayVrlar area. Quartz veins a few centimetres
thick consist of colloform–crustiform banded opaline
and chalcedony.
6. Discussion
Extensive lacustrine successions and accompany-
ing volcanism in the Early Miocene NE-trending
Bigadic borate basin provided appropriate conditions
to form peperite. Syn-depositional volcanism is
mainly represented by intercalations of felsic pyro-
clastic rocks with lacustrine sedimentary rocks. In this
study, we have also described basaltic and intermedi-
ate magmas with peperitic margins within the semi-
arid lacustrine succession in the Bigadic borate basin.
The major characteristics of the peperites are: (1)
fluidal and blocky clasts enclosed by non-stratified
host sediment (cf. Goto and McPhie, 1996), (2)
chilled margins around fluidal clasts indicating con-
tact with wet sediment or water (cf. Cas and Wright,
1987; McPhie et al., 1993), (3) in situ fragmented
clasts forming jig-saw fit texture, (4) common irregu-
lar margins of magma clasts within the host sediment
(cf. Doyle, 2000), and (5) secondary stratification of a
mixture of juvenile clasts and host sediment due to
sediment fluidisation. Development of peperite clearly
indicates that the host sediment was wet and uncon-
solidated at the time of intrusion.
Although the host sediment is the same as the
peperite domains in the KayVrlar and Golcuk areas,
syn-depositional olivine basalt and trachyandesite
intrusions appear to have contrasting peperitic textures
which may reflect difference in: (1) magma composi-
tion and viscosity (cf. Dadd and Van Wagoner, 2002),
(2) vesicle and volatile content (cf. Doyle, 2000;
Gifkins et al., 2002), and (3) size, proportion and
alignment of crystals (cf. Hanson and Hargrove,
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262 255
1999; Doyle, 2000). Olivine basalts are of small-
volume, highly vesicular and contain a small amount
of phenocrysts, whereas trachyandesites are volumi-
nous, poorly to non-vesiculated and contain abundant
phenocrysts with local preferred orientation. The dif-
ferent combinations of such contrasting intrusion
properties are associated with diverse juvenile clast
shapes in peperitic domains.
6.1. Emplacement mode of peperite-related
syn-depositional volcanic units
The sharp and concordant upper contacts of the
peperite-related volcanic units with lacustrine sedimen-
tary rocks suggest that the Golcuk basalt and KayVrlar
volcanic unit formed domes and lava flows emplaced in
a subaerial environment. Hyaloclastite facies are not
associated with either volcanic unit, suggesting that
water level was more or less at, or immediately
below, the sediment level at the time of magma empla-
cement. Lacustrine sedimentation resumed after empla-
cement of the syn-depositional volcanic rocks, and the
limestone, now silicified, transgressively covered the
volcanic ridges in response to ongoing subsidence of
the basin. Emplacement modes of olivine basalts and
trachyandesites also differ. Olivine basalts occur as
small-volume emergent lava domes. Outer margins
and the lowermost parts of the domes are intrusive,
and lava flows show very limited extent on the sedi-
mentary rocks. Trachyandesite dykes up to 100 m wide
developed, in contrast, along fracture zones and fed
extensive and thick lava flows emplaced in a subaerial
setting.
Peperitic textures associated with the olivine
basalts appear to have formed at the base of lavas
along the ground surface. High vesicularity of the
olivine basalt intrusions also suggests low confining
pressure at shallow level (Goto and McPhie, 1996).
The peperitic textures associated with trachyandesite
dykes appear to have formed entirely in subsurface
conditions. Absence of peperitic textures or hyalo-
clastites at the lower contact between the trachyande-
sitic flows and the volcaniclastic sandstones clearly
indicates that the sediment surface was dry after
sedimentation of volcaniclastic layers that blanketed
the limey mud and water level was probably below
the sediment level at the time of lava flow emplace-
ment. This is consistent with semi-arid climate, shal-
low-water and ephemeral lake environments
previously inferred for the basin (HelvacV, 1995;
HelvacV and Orti, 1998).
6.2. Formation of peperitic textures associated with
olivine basalts
Peperitic textures associated with the Golcuk basalt
occur along contacts of the partly emergent intrusions
— similarly to those shown by White et al. (2000) —
with sedimentary host rock. Irregular and fluidal
clasts, as well as olivine basalt lobes, are thought to
be closely associated with more mafic magma com-
positions and lower viscosity which may allow easier
penetration of magma into the host sediment (Goto
and McPhie, 1996; Dadd and Van Wagoner, 2002).
The olivine basalt magmas probably had a high vola-
tile content, low silica abundance (49.46–52.65 wt.%)
and relatively low viscosity during formation of the
fluidally shaped clasts and lobes. The presence of
amoeboid-shaped clasts in the peperite domain also
indicates dismembering of ductile, low-viscosity, and
hence relatively hot, magma (cf. Squire and McPhie,
2002). Vesiculation has also partially controlled the
formation of spongy margins in the fluidal clasts
where the vesicles are filled by host sediment directly
at the contact. The vesicle walls locally forming the
margins of the fluidal clasts also indicate that the
vesiculation occurred during fluidal emplacement
and limey host was drawn into vesicles as magmatic
gas in the vesicles cooled and condensed.
Formation of blocky clasts followed the fluidal
stage as indicated by in situ fragmentation of clasts
with fluidal margins (Fig. 9d). Gradual disintegration
of fluidal clasts along hairline cracks and combination
of both blocky and fluidal clasts strongly suggest that
the blocky clast formation was closely associated with
gradually decreasing temperature, increasing viscosity
and resulting quench fragmentation (Goto andMcPhie,
1996; Dadd and Van Wagoner, 2002) (Fig. 11a).
Fluidisation of the limey host was also another
important factor for the generation of fluidal clasts
during the early stages of olivine basalt emplacement.
The main evidence for sediment fluidisation is the
locally non-stratified nature of the calcareous sedi-
mentary rocks which are dominated by laminated
limestone away from peperite domains in the succes-
sion (cf. Kokelaar, 1982; Kano, 1989, 1991; McPhie,
Fig. 11. A summary diagram for the formation of peperitic textures associated with (a) Golcuk basalt and (b) KayVrlar volcanic unit. See text for
explanation.
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262256
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262 257
1993; Goto and McPhie, 1996; Hanson and Hargrove,
1999; Dadd and Van Wagoner, 2002). Hairline cracks
intersecting fluidal clasts and sediment-filled vesicles
near the outer surfaces of clasts are other diagnostic
features of sediment fluidisation (Brooks et al., 1982;
Boulter, 1993; Goto and McPhie, 1996; Dadd and Van
Wagoner, 2002).
Large blocky clasts, which are mainly tapered and
platy varieties, are widely dispersed and rotated, and
are found together with irregular fluidal clasts. Incom-
plete in situ brecciation of the sparsely packed blocky
clasts suggests dispersion and rotation of the clasts
after quench fragmentation (Fig. 9b). Formation of
dispersed and rotated clasts has been mainly attributed
to steam explosions (Kokelaar, 1982; Busby-Spera
and White, 1987; Hanson and Hargrove, 1999), but
no corroborative field evidence for steam explosions
was recorded in the peperite domain. Dispersed platy
and tapered clasts within the pore water-rich limey
host may instead here have been emplaced during
forceful emplacement of the new magma pulses that
disrupted the early formed peperite (Kokelaar, 1986).
The presence of both fluidal and blocky clasts in the
olivine basalt peperite can also be evidence for fluc-
tuating magma rheology and thus perhaps multiple
intrusive pulses (cf. Goto and McPhie, 1996; Squire
and McPhie, 2002). Fluidal and irregular contacts can
be generated by hotter pulses, with later pulses impos-
ing stress on parts that have begun to cool and soli-
dify, causing formation of clasts having mixed
morphologies (Goto and McPhie, 1996).
6.3. Formation of peperitic textures associated with
trachyandesites
Fluidal contacts of clasts and dykes are limited, and
their paucity is not only related to magma rheology (i.e.
high viscosity), but also local and low-volume fluidisa-
tion of the host sediment as evidenced by centimetre-
scale pipe-like structures in the peperite close to the
intrusive contact (Kokelaar, 1982; Busby-Spera and
White, 1987). Laminated beds and slump structures
found together with pipe-like structures are probably
related to sediment fluidisation during initial intrusion
of magma (Fig. 11b). The low-volume fluidisation
resulted in sediment-filling fractures within dykes and
millimetre-scale chilled margins at the irregular contact
between magma and host rock. Sediment fluidisation
continued through the early stages of quench fragmen-
tation, which is indicated by nearly in-place clast rota-
tion near intrusive contacts.
Mesoblocky clasts in the peperite domain are
inferred to have formed during low-volume sediment
fluidisation. Digitate margins and partly fluidal con-
tacts with non-stratified host sediment are diagnostic
features of a fluidal emplacement. Mesoblocky clasts
with digitate margins are commonly rimmed by a
chalcedony with a uniform thickness of a few milli-
metres. Insulation of a vapour film at magma–wet
sediment interface probably occurred during forma-
tion of mesoblocky clasts and the chalcedonic rim
defines preserved open spaces after removal of the
vapour film. Open spaces as zones of weakness at
magma–sediment interface were probably filled by
hydrothermal solutions during quench fragmentation.
Physical experiments also demonstrate that a vapour
film layer acting as insulating barrier may promote
passive quenching (Wohletz, 2002) and it was pre-
sumably preserved through initial stages of quench
fragmentation. In this case, chalcedonic rim around
juvenile clasts is a field evidence for vapour film
development during fluidal emplacement of magma.
Fluidal emplacement was immediately followed by
rapid cooling of magma forming jig-saw fit textures.
Blocky clast formation and in situ fragmentation are
thought to be controlled simply by quench fragmenta-
tion (Brooks et al., 1982; Kokelaar, 1982; Hanson and
Hargrove, 1999; Doyle, 2000; Hooten and Ort, 2002).
Quench fragmentation is inferred to have occurred
due to the relatively high viscosity and decreasing
temperature of the intruding magma. Mesoblocky
clasts were also disintegrated by quench fragmenta-
tion into variable-sized blocky clasts displaying both
planar–curviplanar and digitate margins (Fig. 10b).
Fractures formed during cooling were filled by injec-
tion of mixture of host sediment with pore water and
magmatic fluids during, or immediately after, quench-
ing (cf. Brooks et al., 1982; Brooks, 1995; Kokelaar,
1982). Contribution of magmatic fluids into host sedi-
ment in the peperite domain is indicated by a hydro-
thermal breccia that comprises intensely silificified
limestone and colloidal silica enclosing blocky clasts
with jig-saw fit textures (Fig. 10g). Monomictic nat-
ure of the breccia suggests that partly consolidated
limey hosts were breached and filled open spaces
around rigid and quenched juvenile clasts.
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262258
Platy and tapered clasts formed during early stages
of brecciation, and are generally oriented along the
intrusion directions (Fig. 10a); this may be explained
by a preferred alignment of phenocrysts that formed
zones of weakness between crystals and groundmass.
Digitate margins of the blocky and tapered clasts may
also be evidence for internal heterogeneities, such as
the size and abundance of phenocrysts (Doyle, 2000).
6.4. Hydrothermal activity in the peperite domains
Magmatic fluid contribution is seen by the abun-
dant quartz-carbonate veins and hydrothermal breccias
within host sediment and magma bodies in the Golcuk
and KayVrlar areas. Hydrothermal activity is mainly
restricted to peperitic domains and locally lava flows.
The timing of magmatic fluid contributions to the
olivine basalt and trachyandesite peperite domains
appears to differ. Olivine basalt peperites show no
positive evidence for hydrothermal activity before or
at the time of magma emplacement. Hydrothermal
mineralization in the Golcuk area is characterised by
vug-infill quartz and carbonate mineralisations around
fluidal clasts (Figs. 6c and 9f). Formation of these
large vugs may be attributed to the presence of steam
generated from heating of the pore fluids (cf. Squire
and McPhie, 2002). Steam-generated large cavities are
inferred to have filled with hydrothermal fluids after
peperite formation. However, termination of hydro-
thermal veins at the upper contact of olivine basalt
domes strongly suggests that magma emplacement
was immediately followed by hydrothermal fluid cir-
culation in the peperite domain. In this case, steam-
generated vugs in the peperite formation led to a
highly permeable environment where hydrothermal
fluids can circulate in the contact zone.
In the KayVrlar area, silicification and hydrothermal
brecciation indicate spatial and temporal relationships
between magma emplacement and hydrothermal
activity. Considerable amounts of magmatic fluids
appear to have fluxed the host sediment. This is
evidenced by alternation of silicic and micritic limey
layers and their syn-sedimentary deformation at the
time of peperite formation (Fig. 10e). Moreover, no
silica-rich horizons related to hydrothermal systems
were defined within the lacustrine sedimentary rocks
that predate the magma emplacement, implying that
the magmatic fluid contribution was localised within
the peperite domain and began during, or immediately
before, peperite formation. Hydrothermal activity con-
tinued after peperite formation, and resulted in accu-
mulation of a large amount of silica within the basin
and formation of thick siliceous sediments on the
uppermost part of the volcanic pile after volcanism.
Hydrothermal activity after peperite formation pro-
duced intense quartz stockwork veining within the
subaerial trachyandesitic lava flows.
7. Conclusion
Olivine basalt and trachyandesite peperites formed
at the contacts between coherent magma bodies and
lacustrine limestones in the Bigadic borate basin have
been described on the basis of clast shape, peperite
textures and emplacement conditions. Olivine basalt
and trachyandesite magmas display contrasting empla-
cement modes. Olivine basalts occur as partly emer-
gent intrusions emplaced in a semi-arid, ephemeral
lacustrine, environment. Trachyandesite dykes, in con-
trast, formed along narrow fracture zones and fed
extensive lava flows emplaced in a wholly subaerial,
post-lacustrine, setting. Water level of the lake was
below the sediment level at the time of magma empla-
cement and the sediment surface was too compacted
and/or dry for lavas to form basal peperites. The fol-
lowing subsidence of the basin due to crustal extension
during the Early Miocene in western Turkey resulted in
the deposition of lacustrine sediments over the volca-
nic ridges in the Golcuk and KayVrlar areas.
Peperite associated with the olivine basalt and
trachyandesites includes a large variety of clast
morphologies from blocky to fluidal. Fluidal clasts
are widespread in the olivine basalt peperite domain.
Fluidal clasts related to trachyandesites are, in con-
trast, restricted to a narrow zone near the margins of
the intrusions and have commonly elongate and poly-
hedral shapes with digitate margins, rather than glob-
ular and equant varieties. Blocky and fluidal clasts in
the olivine basalt display stepwise fragmentation, sug-
gesting in turn decreasing temperature and increasing
viscosity during fragmentation. Sediment fluidisation
during fluidal clast generation played an important
role in sustaining vapour films at the magma–sedi-
ment interface and hence suppressing steam explo-
sions. Abundance of blocky clasts with respect to
F. Erkul et al. / Journal of Volcanology and Geothermal Research 149 (2006) 240–262 259
fluidal clasts in the trachyandesite peperite indicates
that the fluidal emplacement and low-volume sedi-
ment fluidisation in the early stages were immediately
followed by quench fragmentation due to the high
viscosity of the magma. Mesoblocky clast formation
is also inferred to have been associated with fluidal
emplacement during initial intrusion of trachyandesite
magma.
Size, texture and abundance of the blocky and
fluidal clasts in the olivine basalt and trachyandesite
peperites were mainly controlled by sediment fluidi-
sation, pulsatory magma injection and magma proper-
ties such as composition, viscosity, vesicularity, and
the size, abundance and orientation of phenocrysts.
Different combinations of these magma properties
produced diverse juvenile clast shapes in peperitic
domains.
In Golcuk area, olivine basalt emplacement was
immediately followed by hydrothermal fluid circula-
tion in the peperite domain, reflecting a highly perme-
able environment in the contact zone. In the KayVrlar
area, hydrothermal systems were spatially and tempo-
rally associated with magma emplacement and result-
ing trachyandesite peperite formation. Hydrothermal
activity was active during or immediately before
peperite formation and continued throughout the
emplacement of trachyandesite magma. This resulted
in accumulation of a large amount of silica within the
Bigadic borate basin.
A model for the formation of economic borate
deposits proposed by HelvacV (1995) and HelvacV
and Alonso (2000) suggests that the borate minerali-
sation is closely associated with intrabasinal faults
feeding hydrothermal fluids into the basin. The hydro-
thermal activity associated with syn-depositional vol-
canism in the Golcuk and KayVrlar areas is the first
field evidence of such mineralisation within the wes-
tern Anatolian borate basins. Therefore, investigation
of the borate contribution related to these hydrother-
mal systems is a target of further work for under-
standing the mineralising potential of magmatic
fluids in the region.
Acknowledgements
This study is a result of two different research
projects supported by Dokuz Eylul University (Project
no: 0922.20.01.36) and by the Scientific and Techni-
cal Research Council of Turkey (TUBITAK, Project
no: YDABCAG-100Y044). This paper is also part of
Ph.D. study of the first author. Durmus Boztug and
Sibel Tatar are acknowledged for providing major
element analyses. The authors wish to thank autho-
rities of the Eti Maden Company in Bigadic for their
logistic support. YalcVn Ersoy and Okmen Sumer are
also appreciated for their field assistance. The manu-
script was greatly improved through reviews by refer-
ees Kelsie Dadd and Jim White.
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