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, JUNE 2014, 65, 3, 195—205 doi: 10.2478/geoca-2014-0013
Introduction
Turkey is an East-West oriented peninsula formed by collision
of the Gondwanian and Eurasian continents. The Neotethys
was the ocean between these continents in Mesozoic times
( engör & Y
l
lmaz 1981; Stöcklin 1984; Stampfli 2000;
Robertson et al. 2006). The suture zone that was formed due
to the closure of the Neotethys and the collision of the Laur-
asia and Gondwana continents passes through the north of
Turkey and generally coincides with the east—west trending
I
·
zmir-Ankara-Erzincan ophiolitic belt (Fig. 1a). The zone
starting from north of I
·
zmir is approximately 2000 kilometers
long and continues the Sevan-Akera Zone to the northeast and
Zagros-Neyriz-Oman Zone towards to the southeast (Okay &
Tüysüz 1999; Tekin et al. 2002). The Eldivan Ophiolite taking
place in middle parts of IAESZ, represents the remnants of the
Neotethyan oceanic lithosphere, which rifted in the Upper
Triassic and closed in the Cretaceous—Lower Paleocene
(Bailey & McCallien 1953; Akyürek 1981; Tankut 1984;
Bragin & Tekin 1996; Göncüo˘glu et al. 2003, 2010; Rojay et.
al. 2004; Gökten & Floyd 2006; Çak
l
r 2009; Tekin et al.
2009). It consists of peridotites, gabbroic rocks, isolated dol-
erite dykes, sheeted dykes and large amounts of plagiogran-
ites (e.g. Dilek & Thy 2006; Dangerfield et al. 2011).
Our study focuses on plagiogranites of the Eldivan Ophio-
lite, located at upper levels of a nearly complete ophiolitic
sequence at the central parts of IAESZ. In this paper we doc-
ument geological structure of the Eldivan Ophiolite and pla-
giogranites with detailed investigations on different rock
Geochemistry and origin of plagiogranites from the Eldivan
Ophiolite, Çank
l
r
l
(Central Anatolia, Turkey)
TIJEN ÜNER
1
, ÜNER ÇAKIR
2
, YAVUZ ÖZDEMI
·
R
1
and I
·
REM ARAT
3
1
Department of Geological Engineering, Yuzuncu Yil University, TR-65090 Van, Turkey; tcakici@yyu.edu.tr
2
Department of Geological Engineering, Hacettepe University, TR-06532 Ankara, Turkey
3
Department of Geological Engineering, Dumlupinar University, TR-43050 Kütahya, Turkey
(Manuscript received July 8, 2013; accepted in revised form March 11, 2014)
Abstract: The Eldivan Ophiolite, exposed around Ankara and Çank
l
r
l
cities, is located at the central part of the I
·
zmir-
Ankara-Erzincan Suture Zone (IAESZ). It represents fragments of the Neotethyan Oceanic Lithosphere emplaced to-
wards the south over the Gondwanian continent during the Albian time. It forms nearly complete series by including
tectonites (harzburgites and rare dunites), cumulates (dunites, wherlites, pyroxenites, gabbro and plagiogranites) and
sheeted dykes from bottom to top. Imbricated slices of volcanic-sedimentary series and discontinuous tectonic slices of
ophiolitic metamorphic rocks are located at the base of tectonites. Plagiogranitic rocks of the Eldivan Ophiolite are
mainly exposed at upper levels of cumulates. They are in the form of conformable layers within layered diorites and also
dikes with variable thicknesses. Plagiogranites have granular texture and are mainly composed of quartz and plagio-
clases. The occurrences of chlorite and epidote revealed that these rocks underwent a low grade metamorphism. Eldivan
plagiogranites have high SiO
2
content (70—75 %) and low K
2
O content (0.5—1 %) and display flat patterns of REE with
variable negative Eu anomalies. LREE/HREE ratio of these rocks varies between 0.2—0.99. All members of the Eldivan
rocks have high LILE/HFSE ratios with depletion of Nb, Ti and P similar to subduction related tectonic settings.
Geochemical modelling indicates that the Eldivan plagiogranites could have been generated by 50—90 % fractional
crystallization and/or 5—25 % partial melting of a hydrous basaltic magma.
Key words: Eldivan Ophiolite, Early Jurassic, plagiogranite, fractional crystallization, partial melting.
groups. A combination of petrographic and whole rock data
was used to interpret the classification, tectonic settings and
origin of Eldivan plagiogranites.
Geological outline and field relationships
The Eldivan Ophiolite, exposed around Ankara and Çank
l
r
l
cities, is located in the central part of the I
·
zmir-Ankara-Erzin-
can Suture Zone (IAESZ). It forms a nearly complete se-
quence by including tectonites (harzburgites and rare dunites),
cumulates (dunites, wherlites, pyroxenites, gabbro and pla-
giogranites) and sheeted dykes from bottom to top. Imbricated
slices of volcanic-sedimentary series and discontinuous tec-
tonic slices of ophiolitic metamorphic rocks are located at the
base of the tectonites. Amphibolites, calcshists and rarely ob-
served micaschists and quartzites are the main metamorphic
rocks. The
40
Ar/
39
Ar ages of these rocks range between
177.08 ± 0.96 Ma and 166.9 ± 1.1 Ma (Early Jurassic) and are
interpreted as the time of an intra-oceanic subduction (Çelik et
al. 2011). A volcanic-sedimentary unit is located at the base of
the tectonites as intercalations of sedimentary (radiolarian
cherts, pelagic limestones and shales) and basic volcanic
rocks. The younger levels of the volcanic-sedimentary units
have been dated as Berriasian—Barremian (Early Cretaceous)
based on the radiolarian fossil content (Tekin et al. 2012).
Therefore the Eldivan Ophiolite is interpreted as a fragment of
the Neotethyan oceanic lithosphere formed during the Late
Triassic and Early Cretaceous time. The emplacement time of
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·
R and ARAT
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the Eldivan Ophiolite over the Gondwanian continent is re-
ported as Albian (Akyürek 1981). Tectonites represent the
major unit of the Eldivan Ophiolite (Fig. 1b) and are com-
posed of harzburgites with regular dunite and pyroxenite
bands, irregular dunite zones and chromite deposits. Folia-
tion, lineation and folds in tectonites are tracers of plastic de-
formation. Gabbro, pyroxenite veinlets and isolated diabase
dykes cut tectonites along these foliation planes. Cumulates
overlying the tectonites have the form of undeformed dunite,
wherlite, pyroxenite and gabbro intercalations (transition
zone). Layered gabbros, flaser gabbros, massive gabbros,
diorites and plagiogranites are distinguishable towards the
top of the cumulates. Plagiogranites are mainly exposed in
the upper parts of cumulates as light coloured tabular levels
and/or pockets within the mafic layered diorites. They are
also observed as dykes cutting gabbro and diorites with vari-
able thicknesses (Fig. 2a). The reported radiometric age of
the plagiogranites in the Eldivan Ophiolites is 179 ± 15 Ma
Fig. 1. a – The delineation of the I
.
zmir-Ankara-Erzincan Suture Zone; b – Geological map of the Eldivan ophiolite (modified from
Akyürek et al. 1979).
Fig. 2. Field photographs of plagiogranites. a – contact of plagiogranites and gabbros; b– diabase dykes in plagiogranites.
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GEOCHEMISTRY AND ORIGIN OF PLAGIOGRANITES FROM THE ELDIVAN OPHIOLITE (ANATOLIA, TURKEY)
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(Early Jurassic) (Dilek & Thy 2006). E-W trending sheeted
dykes with chilled margins cut the massif gabbros, diorites
and plagiogranites (Fig. 2b). Their thicknesses range be-
tween 30 cm—3 m.
Petrography
Plagiogranites are mainly composed of quartz (25—30 %)
and plagioclase (45—55 %), and rarely contain pyroxene
(3—5 %), biotite (2—4 %), hornblende (5—7 %), epidote
(6—8 %) and K-feldspar ( < 1%). They have hypidiomorphic
granular texture with fine to medium sized grains (0.3—1 mm)
and classified as tonalite-trondhjemite (Fig. 3). Moreover,
porphyritic granular texture could also be observed (Fig. 4a)
Fig. 3. Classification of the Eldivan plagiogranites in Q-A-P diagram
(Streckeisen 1973) by using normative mineralogical compositions.
Fig. 4. Microphotographs of plagiogranites in the Eldivan Ophiolite. a – Granular texture of plagiogranites with quartz, plagioclase and
pyroxene minerals; b – Epidote transformation from inner parts of plagioclases; c – Hornblende transformed in part to chlorite; d – Ac-
cessory sphene in plagiogranites. qtz – quartz, pl – plagioclase, px – pyroxene, ep – epidote, hb – hornblende, ch – chlorite.
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in plagiogranites. Plagioclases are found as subhedral miner-
als with medium grain size (0.5—1 mm). Quartz is generally
found as an anhedral mineral and rarely displays recrystalli-
zation textures. Plagioclase-quartz intergrowths (granophy-
ric-micrographic textures) are common and interpreted as
primarily formed textures during eutectic crystallization
(Coleman & Donato 1979). Plagioclase is saussuritized or
sericitized. Most of them are partly transformed to sericite
and epidote minerals (Fig. 4b). Amphibole is partly and/or
completely replaced by actinolite, epidote, and chlorite
(Fig. 4c). Biotite is a primary phase commonly replaced by
chlorite. Most of the original pyroxene is replaced by am-
phibole but some clinopyroxene is still preserved. These al-
terations reveal that hydrothermal alteration possibly
resulting from an ocean floor water circulation has affected
the plagiogranite rocks (Spooner & Fyfe 1973; Lecuyer et al.
1990) or they have undergone weak greenshist metamor-
phism (Coleman & Donato 1979). Accessory minerals are
mainly zircon, apatite and sphene (Fig. 4d).
Whole rock geochemistry
Whole rock major element compositions of 13 plagiogran-
ites (Table 1) were determined by ICP-Emission Spectrome-
try following a lithium metaborate/tetraborate fusion and
dilute nitric acid digestion at ACME (Canada) analytical
laboratories. Trace element contents (Table 2) were deter-
mined in the same laboratory by ICP Mass Spectrometry fol-
lowing a lithium metaborate/tetrabortate fusion and nitric
acid digestion.
In terms of standard chemical classification, using the stable
element ratios such as Zr/TiO
2
diagram (Winchester & Floyd
1977), plagiogranites fall within the rhyolite and dacite
fields (Fig. 5). They have SiO
2
contents of 71.21—74.67 wt. %
and Al
2
O
3
contents of 13.17—15.71 wt. % (Table 1). These
rocks are enriched in Na
2
O (3.51—6.85 wt. %) and depleted
in K
2
O (0.12—0.40 wt. %) with the compositions plotted in
the trondhjemites and tonalities fields of the An-Ab-Or
(O’Connor 1965) diagram (Fig. 6) and oceanic plagiogranites
in the K
2
O vs. SiO
2
diagram (Fig. 7). In terms of trace ele-
ments, plagiogranites display low Rb and Sr concentrations,
and Ba and Nb contents are relatively low for granitoids (Ta-
ble 2). Eldivan plagiogranites display similar features with
the other Tethys ophiolites and plotted within the volcanic
arc and border of the oceanic ridge suites in tectonic discrim-
ination diagrams (Fig. 8).
The chondrite-normalized REE patterns of the Eldivan
plagiogranites are shown with Troodos Oceanic Plagiogran-
ites, Precambrian Saganaga Tonalite and Jabal Turf Conti-
nental Granophyre in Figure 9a. The REE concentrations of
the Eldivan plagiogranites have similar trends with Troodos
oceanic plagiogranite and display flat patterns with variable
negative Eu anomalies. The HREE contents of the Eldivan
rocks are slightly enriched over LREE contents with La/Yb
N
ranging between 0.2—0.99. The presence of the negative Eu
anomaly in most of the Eldivan rocks indicates removal of
plagioclase by fractional crystallization or partial melting of
a rock in which feldspar is retained in the source. Relatively
low content of normative anortite when compared with the
experimentally obtained felsic melts (Koepke et al. 2004)
(Fig. 6) could also be related to plagioclase fractionation.
The multi-element spider diagram of Eldivan plagiogran-
ites normalized to normal mid-ocean ridge basalt (N-MORB)
(Fig. 9b) reveals enrichment of Large Ion Lithophile Ele-
ments (LILE) over High Field Strength Elements (HFSE).
The most striking feature of the diagram is the depletion of
Nb, Ti and P. These negative anomalies are likely to be related
Fig. 5. Plagiogranites plotted in SiO
2
vs. Zr/TiO
2
diagram (Win-
chester & Floyd 1977).
Fig. 6. Feldspar normative An-Ab-Or diagram (after O’Conner
1965) for the Eldivan plagiogranites. The field of experimental melt
compositions for felsic melts produced during the partial melting of
hydrated gabbro was taken from Koepke et al. (2004). To – to-
nalite, Tdh – trondjemite, Gd – granodiorite, Gr – granite.
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Table 1: Major element contents of Eldivan plagiogranites.
Sample SB-1 SB-5 SB-22 SB-6 SB-27 ELD-14B ELD-14F ELD-16 ELD-21 ELD-26 ELD-29 SBK-3B SBK-14B ELD-14e
SiO
2
73.7 73.19 73.52 73.71 73.83 73.83
74.67
72.86 73.89 71.31 71.21 74.26
73.82
48.77
TiO
2
0.72 0.67 0.48 0.59 0.46 0.36
0.25
0.25 0.27 0.84 0.8
0.9
0.51
0.23
Al
2
O
3
13.85 13.25 14.52 13.85 13.08 14.82
13.75
13.34 15.71 13.82 14.69 13.17
14.34
16.26
FeO
2.3 1.14 1.72 1.21 1.41 1.64
1.05
2.22 2.32 1.88 1.6
0.86
1.76
8.41
MnO
0.04 0.02 0.04 0.06 0.07 0
0.05
0.1
0.04 0.08 0.04 0.07
0.04
0.14
MgO
0.38 0.75 0.35 1.96 0.81 0.16
0.85
3
0.39 0.52 0.74 0.48
0.43
9.95
CaO
0.87 5.62 2.67 2.61 3.67 3.93
2.87
3.49 1.51 3.48 4.61 4.09
2.9
12.59
Na
2
O
5.83 4.05 5.37 5.3 6.02 4.29
4.64
3.51 4.56 6.85 4.56 5.03
4.87
1.08
K
2
O
0.2 0.31 0.25 0.17 0.12 0.21
0.4
0.15 0.39 0.14 0.33 0.2
0.35
0.04
P
2
O
5
0.07 0.07 0.08 0.05 0.08 0.04
0.07
0.03 0.05 0.05 0.04 0.04
0.04
0.03
LOI
1.9 1.1 0.8 0.7 0.9 1.3
1.4
0.9
1.1
1.3
0.9
0.8
1.1
2.3
Total
98.63 98.79 98.81 99.52 99.45 99.28
98.69
98.97 99.13 99.31 99.05 99.1
98.76
99.78
Q
36.14 38.36 35.15 33.89 30.77 39.23
39.73
38.82 41.92 24.36 33.78 37.09
37.63
C
2.63 0.00 0.75 0.32 0.00 0.485 0.63
1.13 5.16 0.00 0.00 0.00
0.77
Ab
49.33 34.27 45.44 44.85 50.94 36.30
39.26
29.70 38.58 57.96 38.58 42.56
41.21
An
3.86 17.06 12.72 12.62 8.31 19.24
13.78
17.12 7.16 6.55 18.64 12.77
14.13
Wo
0.00 4.02 0.00 0.00 2.64 0.00
0.00
0.00 0.00 3.37 1.19 2.99
0.00
Il
1.37 1.27 0.91 1.12 0.87 0.68
0.48
0.48 0.51 1.60 1.52 1.71
0.97
Ap
0.17 0.17 0.19 0.12 0.19 0.10
0.17
0.07 0.12 0.12 0.10 0.10
0.10
Bi
1.98 2.81 2.46 1.50 1.12 2.13
3.69
1.36 3.90 1.35 3.07 1.75
3.41
Ho
0.00 1.11 0.00 0.00 4.70 0.00
0.00
0.00 0.00 3.67 1.75 0.14
0.00
Sample
SB-1 SB-5 SB-22 SB-6 SB-27
ELD-14B ELD-14F ELD-16 ELD-21 ELD-26 ELD-29 SBK-3B SBK-14B ELD-14e
Sc
11.0 13.00 11.00 12.00 16.00 12.00 10.00 21.00 15.00 15.00 18.00 14.00 11.00 46
Ba
24.0 23.00 14.00 27.00 24.00 16.00 48.00 62.00 72.00 56.00 49.00 31.00 19.40 41
Co
18.9 26.30 28.00 23.40 15.00 29.90 29.80 27.20 20.40 23.00 14.00 16.10 12.90 50.5
Ga
12.70 13.80 12.70 12.00 14.60 12.30 13.90 13.50 13.30 13.10 12.30 12.60 3.80 11.9
Hf
2.40 2.80 2.10 2.80 2.20 3.00 2.20 2.90 2.70 2.73 3.30 3.10 2.81 0.3
Nb
1.20 1.10 1.20 1.30 2.40 1.30 1.70 1.90 1.30 2.70 2.40 1.30 2.30 0.2
Rb
3.80 1.20 3.50 2.10 1.10 1.30 4.90 3.10 4.70 2.00 4.30 1.80 4.50 1.7
Sr
108.50 108.00 88.00 93.10 144.50 74.90 124.10 117.30 110.70 99.00 128.00 79.60 90.20 84.8
Ta
0.30 0.40 0.50 0.50 0.20 0.40 0.40 0.20 0.40 0.14 0.16 0.30 0.70
nd
Th
0.40 0.50 0.60 0.70 0.40 0.50 0.20 0.00 0.50 0.40 0.60 0.60 0.30
nd
U
0.20 0.30 0.40 0.30 0.20 0.30 0.20 0.00 0.20 0.20 0.40 0.30 0.60
nd
V
7.00 35.00 6.00 <8
23.00 4.00 11.00 104.00 23.00 29.00 17.00 <8
320.10 230
W
178.4 246.30 237.30 330.10 157.50 284.60 255.90 115.20 260.80 185.80 194.40 224.40 99.90 35.9
Zr
66.80 65.90 34.10 76.50 55.00 78.80 63.80 24.70 83.60 84.00 96.00 80.80 29.00 7.7
Y
21.60 32.20 39.20 27.90 22.00 33.30 18.50 16.90 20.60 19.40 22.60 34.20 32.30 5.7
La
0.60 2.50 6.90 2.80 1.80 3.70 1.60 1.00 1.80 6.10 4.80 2.70 6.80 0.3
Ce
2.00 7.10 11.60 7.50 6.20 9.60 5.40 3.00 5.00 4.60 5.41 8.30 9.09 1
Pr
0.38 1.25 2.07 1.22 0.93 1.66 0.89 0.49 0.73 0.51 0.62 1.35 1.50 0.16
Nd
1.90 7.10 10.80 6.60 5.60 8.20 4.70 2.70 3.60 5.78 10.60 8.60 2.31 1
Sm
0.83 2.52 3.43 2.58 2.09 3.10 1.86 1.13 1.03 2.14 3.36 3.07 0.78 0.45
Eu
0.25 0.61 0.69 0.79 1.03 0.67 0.60 0.47 0.42 0.73 1.20 0.90 0.64 0.22
Gd
1.24 3.90 5.76 3.75 3.08 4.34 2.41 1.66 1.22 3.82 4.28 4.77 1.77 0.74
Tb
0.27 0.81 1.55 0.77 0.60 0.86 0.52 0.35 0.23 0.57 1.27 0.97 1.46 0.15
Dy
1.70 4.86 7.39 4.95 3.96 5.10 2.93 2.29 1.18 3.83 8.72 6.12 4.12 0.98
Ho
0.38 1.14 1.66 1.09 0.84 1.21 0.62 0.51 0.29 0.84 1.92 1.44 2.31 0.22
Er
1.27 3.34 6.30 3.23 2.52 3.68 1.74 1.61 0.96 2.42 5.85 4.47 3.55 0.68
Tm
0.21 0.54 1.00 0.51 0.40 0.61 0.30 0.24 0.16 0.37 0.68 0.68 0.62 0.1
Yb
1.60 3.51 4.63 3.55 2.46 3.85 2.01 1.63 1.70 2.26 4.21 4.17 4.56 0.7
Lu
0.26 0.55 0.93 0.56 0.40 0.61 0.30 0.25 0.17 0.35 0.86 0.70 0.20 0.11
Mo
0.30 0.30 0.20 0.20 0.20 0.30 0.30 0.00 0.30 0.30 0.30 0.20 25.30 0
Cu
19.60 4.30 0.90 0.30 16.80 0.80 1.40 3.00 1.00 18.10 21.60 93.10 0.50 39.5
Pb
0.40 0.10 0.30 0.20 0.40 0.20 0.50 0.40 0.30 0.20 0.40 0.10 0.70 0.2
Zn
28.00 3.00 23.00 6.00 30.00 0.00 33.00 24.00 27.00 18.00 36.00 20.00 1.00 11
Ni
0.50 1.60 0.30 0.30 2.70 0.10 0.20 6.80 0.60 2.70 1.30 1.10 1.20 6.4
Table 2: Trace element content of Eldivan plagiogranites.
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Fig. 7. Semi logarithmic K
2
O—SiO
2
diagram of Eldivan plagiogranites (Coleman
1977). Continental rock types are also shown for comparison.
Fig. 8. Tectonic environment diagrams of the Eldivan plagiogran-
ites; Rb versus Y + Nb diagram (after Pearce et al. 1984). Data
source: Troodos – Aldiss (1978); Semail – Alabaster et al. (1982);
Antalya – Cocherie (1978) and Magganas (2007). ORG – ocean
ridge granite, VAG – volcanic arc granite, WPG – within plate
granite, syn-COLG – syn-collisional granite.
to hornblend, Fe-Ti oxides and apatite fractionation, respec-
tively. The high ratio of LILE/HFSE and especially pro-
nounced depletion of Nb are characteristics of arc-related
ophiolitic plagiogranites.
Classification of Eldivan plagiogranites as identical rocks
in Figures 3, 5, 6 and also well organised REE-MORB norm-
alised patterns with a few exceptions suggest that most of the
elements have not been mobilized significantly, although
these rocks have experienced isochemical alteration.
Discussion
The IAESZ is a major tectonic boundary in
northern Turkey, which (e.g. Rojay 2013) sepa-
rates the Pontides, to the north, from the Ana-
tolide—Tauride and the K
l
r ehir blocks to the
south (e.g. engör & Y
l
lmaz 1981; Dilek &
Moores 1990; Okay & Tüysüz 1999; Rojay
2013; Çelik et al. 2013). Çelik et al. (2013) state
that it has a critical position between the Juras-
sic—Lower Cretaceous Neotethyan ophiolites of
the Balkans and those in Armenia and Iran. The
Eldivan Ophiolite, part of the Ankara mélange
within the center of the IAESZ is composed of
various types of igneous rocks and displays
N-MORB, OIB and supra—subduction magmatic
affinities (e.g. Dilek et al. 2007; Dangerfield et
al. 2011; Çelik et al. 2013). The negative Nb
anomalies shown in Figure 9 are diagnostic for
arc-related petrogenesis in the source of the El-
divan plagiogranites. It is consistent with the
earlier geochemical and tectonic models and
suggests that the plagiogranites from the Eldivan
Ophiolite are likely to have evolved in a supra—
subduction zone environment (e.g. Tankut et al.
1998; Dilek et al. 2007; Dangerfield et al. 2011; Çelik et al.
2013). A recent study of Çelik et al. 2013 proposed that the
Eldivan Ophiolite makes the bridge between the discontinu-
ous outcrops of Upper Jurassic Ophiolite of the Hellenide—Di-
narides to the West and those of Armenia and Iran to the East.
In this part we discuss the plagiogranite formation in the
Eldivan Ophiolite from a gabroic parent which has a subduc-
tion related magmatic affinity.
Plagiogranite petrogenesis
The term plagiogranite is used for leucocratic rocks contain-
ing mainly quartz, plagioclases and rarely ferromagnesian min-
erals (Coleman & Peterman 1975; Coleman & Donato 1979;
Amri et al. 1996; Rao et al. 2004; Kour & Mehta 2005). Several
models have been put forward to explain the plagiogranite for-
mation. Koepke et al. (2007) summarized the generally accep-
ted models. The first one involves late stage differentiation of
low-K tholeiitic MORB magmas (Coleman & Peterman 1975;
Coleman & Donato 1979; Pallister & Knight 1981; Floyd et al.
2000). The second model assumes hydrous partial melting of
gabbro or similar melts in a MOR setting (Gerlach et al. 1981;
Spulber & Rutherford 1983; Floyd et al. 2000). Liquid immisci-
bility in an evolved MORB liquid has been suggested as third
scenario for the plagiogranite generation (Phillpotts 1976;
Dixon & Rutherford 1979; Floyd et al. 2000). In our subsequent
discussion we assess the viability of the first two processes in
explaining the major element and REE data of plagiogranites.
Fractional crystallization
Differentiation of MORB or low K-tholeiite type parental
melt has been proposed to explain the origin of plagiogran-
ites in several ophiolitic complexes. In order to see if the
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Fig. 9. Chondrite normalized (a) and N-MORB normalized (b) spider diagrams of Eldivan Plagiogranites. Data sources: Continental gra-
nophyre samples – Coles (1974); oceanic plagiogranite – Kay & Senechal (1976); Saganata tonalite – Arth & Hanson (1972).
Eldivan plagiogranites originate from the late stage differen-
tiation product of a gabbroic/basaltic magma, we used
MELTS code of Ghiorso & Sack (1995). MELTS allows the
modelling of liquid lines of descent of silicate magmas as a
function of P, T, f O
2
and H
2
O. In our modelling we used a
tholeiitic gabbroic sample with a MgO content of 9.95 wt. %
(Sample Eld 14e, Table 1) which is the most mafic member
of the Eldivan Ophiolite. The crystallization temperature in-
terval is 1000—800 °C and f O
2
value is chosen as QFM + 1 .
The modelling performed under hydrous and anhydrous con-
ditions. The modelled dry and hydrous differentiation trends
and Eldivan plagiogranites are shown in Figure 10. The best
match between modelled and observed differentiation trends
was obtained for P = 1 kbar, H
2
O = 1 wt. %. In all major ele-
ment contents plagiogranites seem to be a result of fraction-
ation of a gabbroic end member under hydrous conditions. On
this basis, namely that some plagiogranites could be generated
by fractional crystallization of gabbros, a further attempt was
done by using the normalized REE patterns of the Eldivan
gabbro and Eldivan plagiogranites and the equation of modal
Rayleigh fractionation. The model is carried out for a hydrous
gabbro “parent” with a mineral composition and proportions
of 0.1
olivine
+ 0.5
plagioclase
+ 0.3
clinopyroxene
+ 0.1
hornblende
(Sample
Eld 14e)
.
As seen in Figure 11, the modelled patterns com-
prise 50—90 % fractionation of a tholeiitic gabbro and fall
within the plagiogranite envelope with the development of a
small Eu anomaly.
Partial melting
Numerous experimental studies on the dehydration melt-
ing of amphibolites or basaltic melts have been performed
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·
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Fig. 10. FeO
T
, CaO, Al
2
O
3
and MgO vs. SiO
2
for Eldivan plagio-
granites and MELTS (Ghiorso & Sack 1995) fractional crystalli-
zation modelling curves. Crystallization temperature interval is
1000—800 °C and f O
2
value is chosen as QFM + 1. Calculation pa-
rameters; 1 kbar hydrous (1 % H
2
O) (black continuous curve),
1 kbar anhydrous (0 % H
2
O) (black dashed curve).
(e.g. Spulber & Rutherford 1983; Beard & Lofgren 1991;
Wyllie & Wolf 1993). These studies reveal that partial melt-
ing of basic rocks could produce silicic melts. Firstly, Koepke
et al. (2004) performed hydrous partial melting experiments
at low pressures under slightly oxidizing conditions on dif-
ferent oceanic gabbros. The result of this experimental study
indicates that plagiogranites can be generated by low degree
partial melting of oceanic gabbros. To test the hypothesis that
plagiogranites may form by partial melting of gabbroic source
rocks under hydrous conditions, modal equilibrium batch par-
tial melting equations (Shaw 1970) were applied to the REE
data of the Eldivan Ophiolites. A tholeiitic gabbroic sample
(Sample Eld 14e, Table 2) was taken as a mafic end member
due to its primitive nature for the modelling. Mineral/liquid
partition coefficients for REE are taken from Rollingson
(1993). The partial melting model of gabbro and Eldivan
plagiogranites are given in Figure 12. The results of the
model reveal that 5, 10, 15 % partial melting of the gabbroic
source can reproduce some of the Eldivan plagiogranites, al-
though the model cannot generate the plagiogranites with
higher REE content.
In summary, both fractional crystallization and partial
melting of gabbroic ‘parent material’ could provide the ini-
tial generation of plagiogranitic melts. However, the former
process is more likely to produce most of the range of pla-
giogranite compositions observed.
Conclusions
The I
·
zmir-Ankara-Erzincan suture zone in northern Tur-
key is a remnant of the I
·
zmir-Ankara-Erzincan Ocean branch
of the Neotethys that formed during collision of the K
l
r ehir
block and Tauride-Anatolide platforms. This suture zone
consists of ophiolitic material and forms the Ankara mélange
in its central parts.
The Eldivan Ophiolite is a part of Ankara mélange repre-
senting remnants of Neotethyan oceanic lithosphere which
rifted during Late Triassic. It consists of peridotites, gabbroic
rocks, dolerite dykes, sheeted dykes and a large amount of
plagiogranites. The plagiogranites of the Eldivan Ophiolite
have the form of conformable layers within the layered dior-
ites and also dykes with variable thicknesses. They have gran-
ular texture and display traces of low grade metamorphism in
their mineralogical content. Tectonic discrimination diagrams
and high LILE/HFSE ratios with pronounced Nb depletion in-
dicate the presence of subduction component related to island
arc activity in the source region of the Eldivan plagiogranites.
In terms of their origin, plagiogranitic melts could have been
generated by either fractional crystallization under moderately
hydrated (H
2
O = 1 wt. %) and QFM + 1 (quartz-fayalite-magne-
tite) conditions at 1 kbar pressure with 50—90 % fractionation
or 5—25 % partial melting of a gabbroic material. However,
the fractional crystallization model is more likely to produce
most of the range of observed plagiogranite compositions.
Acknowledgments: This study was financially supported by
Yüzüncü Y
l
l University Scientific Research Foundation
(BAP, Project No. 2008-FBE-D006). We would like to
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Fig. 12. Model REE patterns of plagiogranite generation by modal equilibrium batch melting of a gabbro parent (Sample Eld 14e, Table 2)
with a mineral assemblage of 0.1
olivine
+ 0.5
plagioclase
+ 0.3
clinopyroxene
+ 0.1
hornblende
. Mineral/liquid partition coefficients for REE are from
Rollingson (1993).
Fig. 11. Model REE patterns of plagiogranite generation by fractionation of a gabbro parent (Sample Eld 14e, Table 1) with a mineral
assemblage of 0.1
olivine
+ 0.5
plagioclase
+ 0.3
clinopyroxene
+ 0.1
hornblende
. Distribution coefficients are from Rollingson (1993).
thank Eva Chorvátová and two anonymous reviewers for ex-
tremely helpful and constructive comments.
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