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.


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).


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


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.


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.


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.


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


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.


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).



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



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,


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.



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.


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


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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Olivine basalt and trachyandesite peperites formed...

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