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LITHOS 0
ELSEVIER Lithos 42 (1997) 105-121
Tertiary volcanism of the Galatia province, north-west Central Anatolia, Turkey
Marjorie Wilson a, * , Ayla Tankut b, Nilgiin Guleg b
a Department of Earth Sciences, Leeds Uniuersity, Leeds LS2 9JT, UK b Gmlogical Engineering Department, Middle East Technical University, 06531 Ankara, Turkey
Received 10 February 1997; accepted 25 July 1997
Abstract
Large volumes of trachyandesitic-dacitic lava flows and pyroclastics of Miocene age are associated with small volumes of alkali basalt lava flows in the Galatia volcanic province, northwest Central Anatolia, Turkey. The volcanism postdates continental collision, occurring in a transtensional tectonic setting associated with movement along the North Anatolian Fault zone. Major and trace element (including REE) and Sr-Nd isotope data and K-Ar ages for representative samples of m&c-intermediate volcanic rocks have been obtained from a series of localities within the province. The K-Ar age data indicate that alkali basalts were erupted during two distinct time periods in the Early Miocene (17-19 Ma) and Late Miocene (< 10 Ma). The two groups of basalts are inferred to have been derived from different mantle sources, based on their Sr-Nd isotope and geochemical characteristics. The Late Miocene basalts were derived from a more depleted mantle source than the Early Miocene basalts, which were generated by partial melting of an incompatible element enriched, subduction-mod- ified, mantle source. The depleted source component is inferred to reside within the asthenosphere and has some affinities with the source of HIMU oceanic island basalts. On the basis of a comprehensive major and trace element and Nd-Sr
isotope dataset for the intermediate-acid volcanics and the alkali basalts, it is possible to demonstrate a cogenetic relationship between the alkali basalts and the intermediate volcanics of Early Miocene age, involving fractional crystallisation and assimilation of a heterogeneous upper crustal component. 0 1997 Elsevier Science B.V.
Keywords: Volcanism; Galatia; Turkey; Tertiary; Basalt
1. Introduction ogy and economic potential of the area (Stefanski
The Galatia volcanic province is located within the Pontide tectonic belt (Set-q@ and Yilmaz, 1981)
of northwest Central Anatolia, Turkey (Fig. 1). Pre- vious studies have mainly concentrated on the geol-
amI Lahn, 1940; Stchepinsky, 1942; Erol, 1954;
Rondot, 1956; &giir, 1977; Parlakyiayt and Erler,
1989; Liinel, 1987). They did not provide good constraints for the age of the volcanism and different ages, ranging from Late Cretaceous to Miocene, have been assigned by different authors, many of
* Corresponding author. Tel.: +44-113-2335236; fax: +44-
113-2336619; e-mail: [email protected].
which may be unreliable. Volcanic activity is as- sumed, mainly on the basis of stratigraphic relation-
0024-4937/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved.
HI SOO24-4937(97)00039-X
106 M. Wilson et al./Lithos 42 (1997) 105-121
Fig. 1. Simplified geological map of Turkey showing the location of the main Tertiary-Quatemary volcanic provinces. Abbreviations:
WAV = western Anatolian volcanic province; CAV = Central Anatolian volcanic province; EAV = eastern Anatolian volcanic province;
GV = Galatia volcanic province; NAF = North Anatolian fault; EAF = East Anatolian fault; BSZ = Bitlis suture zone; Ka = Karacadag
volcano.
ships, to have started as early as Late Cretaceous
(Stefanski and Lahn, 1940; Erol, 1954; Keller et al., 1992) or Paleocene (Erk, 1957). Ko$yiayt (1991) published a Maestrichtian K-Ar age for subduction- related intermediate-acid volcanics in the southeast part of the province, which were considered to be
associated with the Campanian-Middle Eocene con- vergent destruction of the Neotetbys ocean. Ach and Wilson (1986) reported Eocene radiometric ages for
calcalkaline andesitic volcanics near Ankara to the south of the Galatia volcanic province. Manetti et al. (1983) proposed that there were two eruptive cycles of intermediate-acid rocks, one of Cretaceous and the
other of Eocene age. Fourquin et al. (1970) proposed an Early Miocene (Burdigalien) age for pyroclastics
from the Giivem area, based upon paleontological constraints for intercalated diatom&es.
Based on recent field mapping, geochemical and K-Ar geochronological studies within the Galatia province (this study; Keller et al., 1992; Toprak et al., 1996) it is now clear that, volumetrically, the majority of the volcanics are Miocene in age. Two distinct eruptive sequences have been recognised. Early Miocene volcanic activity is widespread, domi- nated by intermediate-acid lava flows and associated pyroclastics with relatively minor occurrences of al-
kali basaltic lava flows. Locally, younger, small- volume, alkali basalt lava flows cap the older vol- canic sequence. These young basalts only occur in the eastern part of the province, in the Giivem and Orta areas, and their ages have been suggested to
range from Late Miocene (Akyiirek et al., 1979; Tiirkmenoglu et al., 1991; Keller et al., 1992) to
Quaternary (Erol, 1954; Calgin et al., 1973). The intermediate-acid volcanics were previously
considered to be of continental margin talc-alkaline affinity, related to plate convergence (Tankut et al., 1991; Keller et al., 1992) whereas all of the basal&, regardless of their age, have been described as exten- sion-related alkali basalts (Tankut and Turkmenoglu, 1988; Tankut et al., 1991; Keller et al., 1992). Prior to this study, however, the available geochemical data were inadequate to constrain the tectonic set-
ting. In addition no attempt was made to link the petrogenesis of the mafic and intermediate-acid vol- canics.
In this study we present new major and trace element (including REE) and Sr-Nd isotopic data and K-Ar ages for mafic-intermediate volcanic rocks from a series of localities within the Galatia province. These data provide important constraints for the geodynamic setting of the magmatism.
M. Wilson et al. / Lithos 42 (1997) 105-121 107
2. Tertiary-Quaternary magmatism and the neo- tectonic evolution of Turkey
The neo-tectonic evolution of Turkey has been dominated by the ca’llision of the African and Ara- bian plates with the Eiurasian plate along the Hellenic
arc to the west and the Bitlis-Zagros suture to the east (Sengar and Yilmaz, 1981; Sengijr et al., 1985;
Dewey et al., 1986; Yilmaz, 1993; Oral et al., 1995). This collision gave rise to major crustal shortening and uplift in eastern Anatolia and the formation of the North Anatolian right-lateral and East Anatolian left-lateral major strike-slip faults, along which the wedge-shaped Anatolian block (micro-plate) escaped westwards (Fig. 1; Dewey et al., 1986; Oral et al., 1995). The tectonic. interaction between the Ara-
bian-African and Eurasian plates has resulted in extensive mafic to felsic volcanism in eastern, cen-
tral and western Anatolia, and also along the North Anatolian and East Anatolian strike-slip fault zones
(Fig. 1; Yilmaz, 1990; Pearce et al., 1990; Notsu et al., 1995). The geochemistry of the Tertiary- Quatemary volcanism in western, central and eastern
Anatolia has been reviewed by Innocenti et al. (1982), Pearce et al. (1990), Yilmaz (1990), GiileG (1991) and Notsu et al. (1995).
Continental collision between the Arabian and Eurasian plates in eastern and central Anatolia initi- ated during the Early Miocene and has been associ- ated with talc-alk;aline volcanism from Middle
Miocene to historical times (Pasquare et al., 1988; Yilmaz, 1990; Notau et al., 1995). During the Qua-
ternary large stratovolcanoes, of both alkaline and talc-alkaline affinity, were active along northeast- southwest trends. On the southern (Arabian) side of the plate boundary a large basaltic shield volcano (Karacadag) erupted. alkaline magmas from the Late Miocene until the Quaternary (Notsu et al., 1995).
Western Anatolia differs from the other two re- gions as it is located at the eastern end of the Aegean volcanic arc, resulting from the northward subduc- tion of the African plate. In this area talc-alkaline volcanic activity is generally considered to have
begun in Late Oligocene-Early Miocene times fol- lowed by alkali basaltic volcanism in Late Miocene-Recent times. This change in the style of the volcanism has been attributed to a change in the regional stress field from north-south compression
to north-south extension (Yilmaz, 1990; GilleG,
1991). Yilmaz (1990) proposed that, following conti- nental collision during the late Cretaceous-Early
Eocene, western Turkey was subjected to a north- south compressional regime until the Middle Miocene. As a consequence, the continental crust
was shortened and thickened, possibly to greater than 60 km, resulting in anatexis. Partly penecontempora- neously with these events, intermediate talc-alkaline
volcanic activity initiated. Seyitoglu and Scott (1992) and Seyitoglu et al. (1997) note that whilst the Early Miocene volcanism is of calcalkaline affinity this becomes progressively more alkaline during the Late Miocene-Pliocene. These authors, however, disagree with Yilmaz (1990) about the timing of the cessation of regional compression and consider that the onset
of north-south extension actually occurred prior to the Middle Miocene, during the Late Oligocene-
Early Miocene. They propose that the source of the
Miocene talc-alkaline volcanism was the lower lithospheric mantle modified by earlier subduction and that during the more advanced stages of exten- sion in the Pliocene-Quatemary alkali basalt mag- mas were generated directly from the asthenosphere, once the subduction-modified lithospheric mantle source was exhausted. Within western Turkey the transition from talc-alkaline to alkaline volcanism is associated with a decrease in 87Sr/86Sr at the end of the Middle Miocene (Seyitoglu and Scott, 1992).
This change is attributed to an increased astheno- spheric contribution to the magmatism resulting from
progressive lithospheric thinning.
2.1. The Galatia Volcanic Province
The Galatia Volcanic Province (GVP) comprises a number of composite volcanic complexes inti- mately associated with the development of a series of sedimentary basins (Toprak et al., 1996). The
northern margin of the GVP (Fig. 1) is bordered by the North Anatolian fault, and the southern margin is bounded by a continental elastic sedimentary se- quence which interfingers with the volcanics. As noted earlier, two distinct volcanic cycles have been recognised, of Early and Late Miocene age respec- tively. The older cycle is dominated by intermediate-acid lava flows and associated pyroclas- tics with relatively minor occurrences of alkali
Tab
le
1
Rep
rese
nta
tive
m
ajor
an
d tr
ace
elem
ent
anal
yses
of
vo
lcan
ic
rock
s fr
om
the
&.m
iis
(br)
, O
rta
(Or)
, K
aras
ar
(Ka)
, G
iidi
il
(Gii
d),
Gll
vem
(G
ii),
K
yzil
cah
amam
(K
hm
) an
d K
yzik
yayl
a (K
y)
area
s,
Gal
atia
vo
lcan
ic
prov
ince
. L
OI
= l
oss
on i
gnit
ion
. A
ll
sam
ples
ar
e fr
om
the
Ear
ly
Mio
cen
e se
ries
exc
ept
for
alk
ali
basa
lts
Gl1
483,
G
ii48
6,
Gli
487
and
Gli
526,
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hic
h
are
Lat
e M
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ne
Tra
chyd
acit
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Tra
chya
nde
site
s A
lkal
i ba
sal&
6~18
G
lldl
6 G
ii39
5 G
ii49
6 G
ii49
7 G
ii02
G
ll51
4 K
hm
l3
Or2
8 K
a228
K
a262
G
iidl
2 C
id49
0 G
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Gll
511
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hm
23
Kh
m24
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25
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G
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3 G
ii48
6 G
ii48
7 G
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6
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, 66
.26
64.6
1 68
.94
66.1
6 65
.61
69.6
7 64
.38
66.0
1 61
.56
60.5
3 59
.96
62.8
1 58
.28
60.7
2 61
.34
62.6
3 61
.43
61.1
9
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0.60
0.
47
0.44
0.
55
0.52
0.
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0.63
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0.94
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0.78
1.
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0.80
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79
0.72
0.
88
0.89
All
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14.5
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15.6
0 17
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7 16
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17.6
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17.4
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18.6
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0 16
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3.78
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2.84
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2.68
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4.98
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MnO
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0.05
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01
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08
0.01
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09
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0.
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0.17
0.
10
0.10
0.
38
0.03
MgO
1.
50
1.56
0.
39
0.32
1.
80
0.15
1.
33
1.09
0.
58
3.14
2.
37
1.85
0.
62
3.81
3.
23
2.02
1.
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1.15
cao
2.92
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09
2.21
1.
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3.71
3.
93
3.84
3.
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4.43
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5.67
5.
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3.87
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22
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54
4.33
3.
62
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3.
81
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39
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4.69
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3.63
3.
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4.56
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2.93
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1.66
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1.51
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1.
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1.66
1.
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6 16
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6 17
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16.8
5 16
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9.75
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8.89
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8.37
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0.16
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8.01
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6.74
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8.34
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9.67
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9.19
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8.99
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3.65
3.
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4.06
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2.84
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0.97
1.
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1.90
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2.75
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2.30
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0.73
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0.63
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86
0.52
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4.55
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4.02
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99.6
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1.94
2.
22
1.78
99.8
1 10
0.04
10
0.12
Ni
20
27
8 5
31
8 23
36
52
43
53
42
20
77
64
24
59
97
26
37
15
7 17
3 12
8 42
83
96
11
2 97
C
r 33
40
17
5
40
28
26
86
106
63
82
69
18
118
91
32
82
179
126
46
207
190
181
57
85
85
89
177
V
46
49
31
18
51
59
47
64
92
82
81
74
118
94
82
61
81
113
101
65
142
132
126
95
135
136
135
135
SC
4
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3 5
10
1 12
15
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13
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10
6
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9 9
6 13
16
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17
17
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12
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7
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75
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20
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23
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41
42
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SO
39
38
39
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n
36
57
52
69
63
34
63
68
57
63
64
49
115
72
68
65
72
73
68
53
77
77
69
94
61
65
71
64
SI
278
412
466
397
553
339
653
419
739
584
615
462
1261
50
5 52
1 68
4 85
1 86
6 95
8 61
7 96
7 97
3 84
2 91
7 67
3 69
4 66
2 70
3
Rb
184
93
207
144
138
69
151
148
89
54
54
81
136
85
106
127
125
126
178
63
32
35
40
48
42
42
38
41
Ba
372
650
726
843
792
400
719
551
691
359
411
421
1100
57
5 57
1 71
4 74
6 75
3 83
0 42
4 87
2 90
3 64
0 68
5 51
1 51
5 46
2 45
8 T
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33
18
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28
11
35
19
18
11
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37
15
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24
28
28
29
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7 10
9
9 8
10
Zr
323
235
307
401
210
172
274
202
239
205
204
182
409
194
210
248
253
262
275
165
194
201
154
248
160
167
160
180
Nb
26
19
41
51
24
8 41
18
30
21
23
15
67
22
22
32
34
34
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21
29
32
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44
47
45
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20
16
16
24
14
13
18
16
20
19
20
14
28
16
16
18
19
22
20
15
27
28
25
25
23
23
21
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95.6
0 60
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18.6
0 70
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6.77
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2.91
5.
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1.45
1.
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M. Wilson et al./Lithos 42 (1997) 105-121 109
basaltic lava flows, whilst the younger cycle consists of small volume alkali basalt flows capping the older volcanic sequence.
Within the regional plate tectonic framework of northern Turkey, the GVP is assumed to have devel- oped under the infhlence of two main tectonic regimes since the Late Cretaceous.
(i> During the Late Cretaceous-Early Tertiary the area was a site of active plate convergence, related to the closure of the northern branch of the neo-Tethys ocean. During the Campanian, the area formed part of the southeastern margin of the Sakarya continent which was an active continental margin, charac- terised by northward subduction of the neo-Tethyan ocean beneath it (Sengiir and Yilmaz, 1981; Koqyiayt, 1991). Calcalkaline andesitic volcanism generated during this period has been described by Calgin et al. (19731, Tankut (1985), Ach and Wilson (1986) and KoGyiayt ( 1991). The terminal closure of the North Tethyan ocean is generally considered to have occurred during the latest Paleocene-Early Eocene, although convergence is inferred by some authors to have continued until the Middle Miocene (Sengor and Yilmaz, 1981; Yilmaz, 1990).
(ii) During the t erminal stages of collision in the Miocene the area was characterised by a transten-
sional tectonic regime associated with the develop- ment of the North Anatolian transform fault zone, which initiated as a result of the oblique Arabia- Eurasia collision (Ko$yii3yt, 1989).
3. Petrographic characteristics of the Galatia vol- canics
The Early Miocene volcanics are predominantly trachyandesites and dacites with minor rhyolite and alkali basalt. The more evolved members are por- phyritic, containing > 25% phenocrysts. Plagioclase feldspar is an essential constituent and hornblende, orthopyroxene, clinopyroxene and biotite are the common ferromagnesian minerals. Quartz is rare as a phenocryst and shows strongly resorbed outlines when present in andesites, but is abundant in the groundmass of the dacites and rhyolites.
The basalts from both the older and younger series are petrographically indistinguishable. They are holocrystalline with a small proportion of phe- nocrysts. Clinopyroxene and olivine are invariably present both as phenocrysts and in the groundmass. Calcium-poor pyroxene does not occur in any of the
Table 2
K-Ar ages of volcanic rot ks from the Galatia province
Sample No. Locality Sample type
Gii395 Giivem trachyandesite
Gii497 Giivem trachydacite
Gii511 Giivem trachyandesite
Gii486 Giivem alkali basalt
K (%)
4.083
3.63
2.35
2.04
40Ar rad. (Vol) 4oAr rad. (%)
0.3132 70.6
0.3154 68.6
0.2509 63.3
0.2542 66.2
0.1668 34.5
0.1615 30.7
0.0761 31.1
0.0754 33.5
Age (Ma)
19.7 k 0.6
17.8 + 0.50
17.9 + 0.50
9.51 f 0.29
6218
az11
KY~
GiidlO
Giid12
ijzmiis
ijzmiis
Kyzikyayla
Giidiil
Giidiil
trachyandesite
alkali basalt
trachyandesite
alkali basalt
trachyandesite
4.019 0.2813 88.6
0.2845 91.0 18.0 f 0.5
1.36 0.095 1 67.8
0.0987 52.6 18.2 + 0.50
2.26 0.1549 82.3
0.1494 82.5 17.6 + 0.5
2.067 0.1526 70.6
0.1516 71.1 18.8 + 0.6
2.113 0.1401 52.6
0.1426 62.7 16.9 f 0.5
110 M. Wilson et al./Lithos 42 (19971 105-121
basalts. Plagioclase (An,,_,,) is confined to the groundmass.
4. Analytical methods and sample preparation
Samples of mafic and intermediate volcanic rocks, from the Guvem, Karasar, Kyzikyayla, Giidiil, Orta and Kyzilcahamam areas in the Galatia province
(Fig. l), were analysed by X-ray fluorescence spec- trometry for major and trace elements at the Univer- sity of Leeds and by ICP for rare earth elements at
RHUL (Royal Holloway, University of London). Representative analyses are given in Table 1.
4.1. K-Ar dating
K-Ar ages for 9 whole rock samples from Giivem, 6zmiis, Giidul and Kyzykyayla (Table 2) were deter- mined by DC. Rex in the Department of Earth
Sciences, Leeds University. The samples were crushed and sieved and the 2.50-170 pm mesh
fraction collected for analysis. The sieved fraction was washed in N 10% acetic acid to remove any carbonate contamination. Aliquots of this cleaned fraction were used for both potassium and argon determinations. Three separate dissolutions of the sample were conducted for the potassium determina-
tions. The concentrations were measured using a
Table 3 Sr-Nd isotope compositions of volcanic rocks from the Galatia province
Sample Number Rock type 87Sr/86Sr measured Age Ma Rb/Sr 87Sr/86Sr (T) Sm/Nd ‘43Nd/‘44Nd measured
Giivem
Gii395
Gii486
Gii490
Gii496
Gii497
Gii500
Gii502
Gii512
Gii514
ijzmiis
6211
6218
Orta
Or28
Or29
Karasar
Ka228
Ka262
Giidiil
GiidlO
Giidl2
Giiyd16
Kyzikyayla
Ky2
TD 0.706088 19.7 0.444 0.705729
AB 0.703450 9.51 0.073 0.703421
TA 0.705290 20 0.108 0.705201
TD 0.705460 20 0.363 0.705162
TD 0.705770 17.8 0.250 0.705587
TA 0.705310 20 0.168 0.705172
TD 0.705270 20 0.203 0.705103
TA 0.705630 20 0.186 0.705477
TD 0.705620 20 0.231 0.705430
AB 0.705080 18.2 0.036 0.705053
TD 0.705096 18.0 0.662 0.704607
TA 0.705130 20 0.120 0.705031 0.1779 0.512750
AB 0.704580 20 0.048 0.704541 0.1898 0.512800
TA 0.704494 20 0.092 0.704418 0.512768
TA 0.704490 20 0.088 0.704418 0.512755
AB 0.704469 18.8 0.052 0.704429 0.512791
TA 0.705021 16.9 0.175 0.704900 0.512746
TD 0.705178 20 0.226 0.704992 0.512725
TA 0.704534 20 0.102 0.704450 0.512717
0.2194
0.1495
0.1501
0.1657
0.1789
0.2124
0.1655
0.1617
0.1805
0.512605
0.512990
0.512710
0.512740
0.512640
0.512710
0.512830
0.512670
0.512690
0.512760
0.512797
Sr isotope compositions at time T, the age of formation of the rock, have been calculated using the measured K-Ar age of the sample.
Where K-Ar age data are not available the data have been age-corrected to a maximum age of 20 Ma (values in italics). Rock types: AB = alkali basalt; TA = trachyandesite; TD = trachydacite.
M. Wilson et al./Lithos 42 (1997) 105-121 111
Coming-eel 480 flame photometer incorporating a lithium internal standard. The values quoted in Table 2 are the means of the three dissolutions. Argon was extracted in a glass vacuum system using a 38Ar tracer from an aliquoting system. Special attention was given to the purity of the gas sample before it was analyzed. A two-stage clean-up procedure was used, with stage one incorporating a Ti-sponge fur- nace and a liquid nitrogen trap. Afterwards the puri- fied gases were drawn into a second clean-up section on activated charcoal containing a Ti/Zr sponge furnace. A small aliquot of the gas was then tested on the mass spectrometer for purity before it was analyzed. Argon isotopes were measured on a modi- fied AEI MS10 mass spectrometer fitted with com- puter controlled peak switching. International stan- dards and atmospheric argon ratios are determined on a regular basis. ,4ges were calculated using the decay constants and branching ratio agreed by the UGS Subcommission on geochronolgy (Steiger and Jager, 1977).
4.2. Sr-Nd isotopes
Sr-Nd isotope compositions were measured for 19 samples (Table 3) using N 150 mg of whole rock
0
powder, leached in 6M HCl for 50 min in an ultra- sonic bath to minimise the effects of any post- emplacement alteration, using a technique modified from Gerlach et al. (1987). Sr was measured on a VG 54E Isomass mass spectrometer, and Nd on a VG MM30 mass spectrometer, both at the University of Leeds. Nd isotopic compositions are normalised to 146Nd/144Nd = 0.7219 and Sr isotopic compositions to 86Sr/88 Sr = 0.1194. Values are relative to the following standards: NBS987 = 0.71025; La Jolla = 0.51186.
5. K-Ar ages of the volcanics
The K-Ar ages of the intermediate-acid volcanic rocks (Table 2) range between 16.9 + 0.5 Ma and 19.7 f 0.6 Ma (Early Miocene). Two basalt samples gave ages of 18.2 f 0.5 Ma and 18.8 + 0.6 Ma whilst the basalt sample from the youngest cover basalt sequence in the Guvem area (Gti486) gave a Late Miocene age of 9.51 * 0.29 Ma. Keller et al. (1992) have previously reported K-Ar ages of 10 Ma and 20 Ma respectively for an alkali basalt and an andesite from Orta, consistent with our new data.
-30 40 50 60 70 SiO2
Fig. 2. Total alkalis versus silica (TAS) diagram showing the classification of the Galatia volcanics. After L.e Bas et al. (1992).
112 M. Wilson et al./Lithos 42 (19971 105-121
6. Major and trace element geochemistry
Intermediate-felsic rocks with SiO, contents rang- ing between 56% and 75% are classified as trachyan- desites, trachydacites, dacites and rhyolites in terms of the total alkalis versus silica (TAS) diagram (Fig.
2). The more mat? samples are trachybasalts and basaltic-trachyandesites according to the TAS classi- fication, plotting in the alkali basalt field on a plot of Nb/Y versus SiO, (Fig. 3). Most of the samples plot in the alkaline field in Fig. 2, as discriminated by
Miyashiro (1978). In a plot of K,O versus SiO, (Fig. 4) the samples plot within the medium- and
high-K fields. Samples from the eastern part of the volcanic province (Guvem, Orta and Kyzilcahamam) generally plot in the high-K field whereas those further west (Gzmtis, Gudil and Karasar) have slightly lower K,O contents, plotting in the medium- K field. The exception to this general trend are two samples from Giivem (including Gii502) which plot in the medium-K dacite-rhyolite field and may have a different petrogenesis from the rest of the Giivem
samples. The Ba, K, Rb, Sr, Th, U element contents and
relative Nb, Ti, Yb depletion of the intermediate
composition samples (Fig. 5) are similar to those of active continental margin volcanics. However, their HFSE element contents are relatively higher. The
1.0
NbN
Fig. 3. Nb/Y versus SiO, diagram for the most primitive mtic
samples illustrating their classification as alkali basalts. The shaded
field highlights the Late Miocene ( < 10 Ma) basalt samples from
the Giivem area. After Winchester and Floyd (1977). Symbols as
in Fig. 2.
primitive mantle normalised trace element patterns of intermediate composition samples from Guvem and Orta (Fig. 5) are similar, suggesting that they
may be derived from similar parental magmas. The distinctive negative Nb anomalies are typical of sub- duction-related or postcollisional magmatic rocks. Such Nb anomalies can, however, also be generated by crustal contamination. Sample Gu502, which was
5.0 # n n Gii 395
P 8 4.0 -
o 3.0-
2
2.0 - medium-K _
1.0 -
V.”
40 50 60 70 80
SiO,
Fig. 4. Variation of K,O versus SiO, showing the classification of the Galatia volcanics as High- and Medium-K series. After Peccerillo
and Taylor (1976). Numbered samples refer to the analyses in Table 1. Symbols as in Fig. 2.
M. Wilson et al./Lithos 42 (1997) 105-121 113
* __o_
--I+--
__*__
-o- ___@___ __p_
_.*__
GU 490
GU 4%
cdl 497
GU .5Qil
GU 502
ciu 512
GU 514
or28
Rb Ba Th Nb K La Ce Sr P Nd Zr Srn Eu Ti Yb
Fig. 5. Primitive mantle normal&d trace element variation diagram for the intermediate samples from the Galatia province. Normalisation
constants from Sun and McDonough (1989). Data in Table 1.
distinct from the rest of the Givem suite in the K,O-SiO, plot (Fig,. 41, has the lowest normalised trace element abundances, though displaying a broadly similar trace element pattern to the other samples. It also has the largest Nb anomaly.
All the basalt samples, from both older and younger series, have SiO, < 52% and are relatively primitive based on their high Ni (97-173 ppm) and & (107-207 ppm) contents (Table 1). All have
alkaline affinity with total alkalis > 5% and K,O/Na,O ranging between 0.3-0.7. Primitive man- tle normalised trace element patterns (Fig. 6) are similar to those of oceanic island alkali basalts (e.g. St. Helena) from La to Yb. The patterns are, how- ever, clearly enriched in Rb, Ba, Th and K relative to those of oceanic island basalts, which may reflect supra-subduction zone enrichment of their mantle source.
I - St Helena HlMU OlB
Rb Ba Th Nb K La Ce Sr P Nd Zr Sm Eu Ti Yb
Fig. 6. Primitive mantle nonnalised trace element variation diagram for the most primitive basalts from the Galatia province. Shown for
comparison is a typical pattern for a HIMU oceanic island basalt from St. Helena (Sun and McDonough, 1989). Norrnalisation constants
from Sun and McDonough (1989). Dam in Table 1.
114 M. Wilson et aL/Lithos 42 (1997) 105-121
500
400 i
L i 300
m
q 0 n
_ m f .
100 -
Oi 0 20 40 60 80
Nb Fig. 7. Variation of Zr versus Nb for the Galatia volcanics. Shaded field highlights the Late Miocene (i 10 Ma) basalts from Giivem.
Symbols as in Fig. 2.
In a plot of Zr versus Nb (Fig. 7) most of the a wide range of variation in the most silica-rich samples define a coherent trend with the exception samples. This suggests that crustal contamination has of those from the young (< 10 Ma) cover basalt played a significant role in the petrogenesis of the sequence in the Giivem area. A plot of Zr/Nb versus suite. Dacite sample Gii502 has a significantly higher SO, (Fig. 8) indicates that the Zr/Nb ratio increases Zr/Nb ratio than the main trend, consistent with a with progressive magmatic differentiation, exhibiting different petrogenesis from the rest of the samples.
%
t3
25
20
15
10
5
L
Gil 502
m 4% basalts
% Gii 395
ot I 1 40 50 60 70 80
SiO, Fig. 8. Variation of Zr/Nb versus SiO, for the Galatia volcanics. Shaded field highlights the Late Miocene ( < 10 Ma) basalts from Giivem. Symbols as in Fig. 2. Numbered samples refer to the analyses in Table 1.
116 M. Wilson et al./Lithos 42 (1997) 105-121
[IIIIIIIII Tertiary-Quaternary alkaline volcanism -Western Anatolia
m Tertiary cabalkaline volcanism - Western Anatolia
0.5133
2
0.5129
a
z os127
q
0.5125
0.5123 0.7020 0.7040 0.7060 0.7080 0.7100
WrPSr Fig. 11. r43Nd/‘44Nd versus s7Sr/86Sr (T) for the Galatia volcanics. Shown for comparison are the Sr-Nd isotope composition ranges of
volcanics from western Anatolia (Giileq, 1991) and eastern Anatolia (Pearce et al., 1990). T = age of volcanism. Galatia data from Table 3.
Symbols as in Fig. 2. EAR-isotopic composition of the shallow asthenosphere (European Asthenospheric Reservoir) beneath western and
central Europe (&net et al., 1995; Cebrii and Wilson, 1996).
forward modelling. As an illustration, however, in Fig. 10a an AFC curve has been calculated for the assimilation of a hypothetical upper crustal compo- nent (Rb 150 ppm, Nb 20 ppm) by one of the more primitive basalts Ozll (Table 1) concurrent with the fractionation of a 50% plagioclase + 50% amphibole
mineral assemblage. The curve has been calculated for an r value (the ratio of the rate of assimilation to the rate of fractional crystallisation) of 0.4 which is
likely to represent an upper limit. Perfect fractional crystallisation would have an r value of 0 as indi- cated. The model curve suggests that the trace ele- ment variation in the Galatia volcanic series could be explained by variable amounts of crustal assimilation (r ranging from 0 to 0.4) combined with fractional crystallisation. In the context of Fig. lOa, two of the trachydacite samples from Giivem, including Gti502 which was highlighted previously, plot to much higher Rb/Nb ratios and clearly have a different petrogenesis, perhaps involving assimilation of a dif- ferent crustal component. This is highlighted in Fig. lob, a plot of Rb versus SiO,, in which there appear to be diverging differentiation trends from a common basaltic parent.
7. Sr-Nd isotopes
Fig. 11 shows the variation of 143Nd/‘44Nd ver- sus s7Sr/86Sr for these samples compared to fields of published data for Tertiary-Quaternary volcanic
rocks from Western and Eastern Anatolia. The initial s7Sr/86Sr ratios of both intermediate and basaltic samples from the Early Miocene series are clearly
elevated with respect to that of the Late Miocene cover basalt from Giivem (Gii486), plotting close to Bulk Earth values at the least radiogenic end of the Western Anatolian talc-alkaline field. The Late Miocene Giivem basalt has a much more depleted isotopic composition with 87Sr/86Sr of 0.70342 and 143Nd/144Nd of 0.51297 (Table 3; Fig. 11). The highly evolved dacite samples from Gi.ivem (Gii395 and Gii502) have distinctly different Sr-Nd isotope compositions, confirming the differences in their pet- rogenesis suggested previously on the basis of their trace element compositions.
The variation of 143Nd/144Nd against SiO, (Fig.
12) demonstrates that the majority of the intermedi- ate samples could have been derived from a parental basalt similar to Or29 by a process of combined
M. Wilson et al./Lithos 42 (1997) 105-121 117
0.51290
0.51280 . . . . . . . . . . . . ...)
0.51270
0.51260 45 50 55 60 65 70
SiO, Fig. 12. Variation of ‘43Nd/‘44Nd versus SiO, for the Galatia
volcanics. The AFC arrow indicates the predicted trend of varia-
tion caused by high level a,kmilation and fractional crystallisation
(AFC) processes. The FC arrow indicates the trend for closed
system fractional crystallisation (FC). Note that the Late Miocene
basalt Gii486 is distinct from the rest of the samples with a much
higher 143Nd/144Nd ratio. Symbols as in Fig. 2.
assimilation and fractional crystallisation (AFC). On the basis of this figure Gtivem dacite CXi502 could have been derived by almost closed system fractional crystallisation from ,a parental alkali basalt. Its Sr isotope composition is, however, more radiogenic than that of Or29 (Fig. 11) confirming the suggestion made previously, on the basis of its Zr/Nb ratio, that
AFC processes must have been involved in its petro- genesis but with a different crustal contaminant to the rest of the series. To generate the near horizontal
AFC trajectory this crustal component must have a high Sr/Nd ratio and a relatively high 87Sr/86Sr ratio compared to the dominant crustal contaminant.
8. Discussion
Based on the K-Ar age determinations obtained as part of this study, we conclude that the main phase of volcanism in the Galatia province occurred during the Early Miocene (17-19 Ma). This phase of activity was characterised by the eruption of a coge- netic volcanic series; ranging in composition from alkali basalt (trachybasalt) to trachyandesite and tra-
chydacite with rare dacite and rhyolite. AFC pro- cesses, involving assimilation of an upper crustal component are important in the petrogenesis of the intermediate-acid magmas. Following a major hiatus in volcanic activity, eruption of a cover sequence of alkali basalts began in the Late Miocene ( < 10 Ma).
Although both Early Miocene and Late Miocene
basalts have similar major and trace element charac- teristics, comparable in many respect to typical within
plate alkali basalts, the older basalts clearly have more radiogenic Nd-Sr isotope compositions sug-
gesting that they may have been derived from a more enriched mantle source region. Elevated concentra- tions of Rb, Ba, Th and K in the Early Miocene basalts suggest that their mantle source may have been metasomatically enriched by fluid/melt infil-
tration above a Late Mesozoic-Early Tertiary sub- duction zone. The Nd-Sr isotope composition of the single specimen analysed from the Late Miocene cover basalt sequence suggests the involvement of a more depleted mantle source component in its petro-
genesis. This component is isotopically similar to
that of the HIMU-like component (Fig. 11; European Asthenospheric Reservoir) inferred to be present in the shallow asthenospheric mantle throughout west- em and central Europe (Granet et al., 1995; Cebria and Wilson, 1996). Whilst it is possible that the Sr-Nd isotopic differences between the Early and Late Miocene basalts could be explained by crustal
contamination, this seems unlikely on the basis of AFC modelling. Consequently the Late Miocene basalts may provide the best indication of the geo-
chemical characteristics of the asthenospheric mantle beneath the Galatia province.
A plot of Th/Y versus Nb/Y (Fig. 13) provides some useful constraints concerning the different source components involved in the petrogenesis of the Galatia magmas. Samples from the main Early Miocene series define a coherent trend, with a Th/Nb ratio close to 1.0, which may be attributed to the combined effects of crustal assimilation and frac- tional crystallisation. The parental mat% magmas to this series clearly fall within the field of potassic and ultrapotassic mafic lavas from western Turkey, which are inferred by Seyitoglu et al. (1997) to be of lithospheric origin. The Late Miocene basalts (e.g. Gu486) have much lower Th/Y and Th/Nb ratios, plotting closer to the MORB-OIB array. The Orta
118 M. Wilson et at. / Litlaos 42 (1997) 105-121
1.00
*
a
0.10
NbN Fig. 13. Variation of Th/Y versus Nb/Y for the Galatia volcanics. Shown for comparison are the compositional ranges of oceanic basalts (MORB + OIB), K-rich lavas from western Turkey (diagonal shaded field) and melilitites from the Rhinegraben area of Germany (diamonds in the OIB field). Data sources: this study; Seyitoglu et al. (1997); Wilson et al. (1995). Numbered samples refer to the analyses in Table 1. The open arrow represents the observed AFC trend in the Early Miocene series. The filled arrow represents the effects of subduction (fluid) enrichment of the mantle source. Dashed lines are contours of fixed Th/Nb ratio. Symbols as in Fig. 2. EAR = European Asthenospheric Reservoir.
basalt sample Or29 plots in a similar position. As its age has not been determined by K-Ar dating it is possible that this sample could also be of Late Miocene age. The group of unusual high-silica rocks (e.g. Gii502), previously inferred to have a different paragenesis from the other rhyo-dacitic samples, are also distinct in terms of their Th/Y and Nb/Y ratios.
Shown for comparison in Fig. 13 are the composi- tions of melilitites from the Rhinegraben area of Ge~any, considered by Wilson et al. (1995) to represent partial melts of the thermal boundary layer at the base of the European lithosphere. These sam- ples are used to define the trace element character- istics of the European As~enosphe~c Reservoir (EAR). If the EAR also extends beneath western Turkey then the trace element characteristics of the Late Miocene alkali basalts from Galatia may indi- cate partial conta~nation with lithospheric partial melts. This conclusion is, however, somewhat specu- lative.
On the basis of the experimental studies of Hirose and Kushiro (1993) and Baker et al. (1995), the Al,O, contents (16-17 wt%, Table 1) of the most primitive mafic magmas from both Early and Late
Miocene series suggest that they segregated from their mantle source at minimum pressures of ca. 18-20 kb, equivalent to depths of 60-70 km. This is consistent with the REE patterns of these samples (Fig. 9b) which suggest equilibration within the spine1 peridotite stability field.
An important problem concerns the relations~p between the Miocene magmatism and lithospheric extension. There are clearly conflicting views as to the timing of the onset of extension in western Turkey. Seyitoglu et al. (1997) consider that exten- sional tectonics prevailed throughout the Miocene (Fig. 14) whereas previous workers (e.g. Yilmaz, 1990; Giileq, 1991) consider that the region was under compression until the end of the Middle Miocene. Does the hiatus in magmatic activity from 17 to 10 Ma in the Galatia province correspond to a change in the prevailing tectonic regime from Early Miocene compression to Late Miocene extension? Alternatively does the evidence for changing magma source regions between the Early Miocene and Late Miocene reflect a process of progressive thinning of the lithosphere in a transtensional tectonic setting, with enriched lithospheric mantle source regions be- coming rapidly depleted by partial melting during
M. Wilson et al. / Lithos 42 (1997) 105-121 119
I
3: 81
asthenosphen
Ql EN wl
I
: subduction- modified
: lithospheric mantle
Temper&we w
Temperature -
Fig. 14. Age of volcanism in relation to the age of onset of extensional tectonics in the Galatia province and the changing nature of the
mantle source. I, timing of the onset of extension in western Turkey after Yilmaz (1990) and Giilq (1991). II, timing of the onset of
extension in western Turkey after Seyitoglu et al. (1997). a Schematic volatile-present mantle solidus; b schematic anhydrous mantle
solidus; c composite mantle solidus (volatile enriched lithosphere; anhydrous asthenosphere).
the initial stages of extension? In the latter scenario the Middle Miocene hiatus in magmatic activity may
reflect a period when the amount of extension was insufficient to generate partial melting of the as- thenosphere and the lithosphere was already too refractory to melt. A further question concerns how decompression melting actually occurs in transten- sional tectonic settings. This may be sporadic and
may be intimately associated with stress release in major earthquakes along the splay of faults associ- ated with the north Anatolian fault zone. This could explain why only small volumes of magma were generated in the Late Miocene.
Our preferred mechanism for magma generation involves progressive lithospheric thinning throughout
the Miocene. Both phases of magmatism are clearly postcollisional, generated in a transtensional tectonic setting associated with movement along the North Anatolian fault zone. Extensional tectonics in Galatia date back to at least the Early Miocene, providing an important constraint for the timing of continental collision. The Ear1.y Miocene volcanism was the most voluminous, s,ourced from a volatile-enriched,
low melting point region in the base of the continen- tal lithosphere (Fig. 14). Once this source was ex- hausted subsequent partial melting of the essentially
anhydrous asthenosphere could only occur if it lo- cally underwent rapid adiabatic decompression to depths of ca. 60 km.
Acknowledgements
This study was financially supported by Grant No. YBAG-0059 from the Scientific and Technical Research Council of Turkey (TUBITAK). We would like to thank the following: Vedat Toprak and Yil- maz Savaqin for discussions concerning the relation-
ship between magmatism and extensional tectonics in the Galatia province; Orhan Akiman for assistance with field sampling; Dave Rex for the K-Ar age
determinations; Frank Buckley for sample prepara- tion for ICP analysis; Nick Walsh for REE analysis by ICP at RHUL; Alan Gray for assistance with the XRF analyses. A constructive review by Csaba Sz- ab6 improved the final version of this manuscript.
120 M. Wilson et al. / Lithos 42 (1997) 105-121
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