Comment on "Al-in-Hornblende Thermobarometry and Sr-Nd-O-Pb Isotopic Compositions of the Early Miocene Alaçam Granite in NW Anatolia (Turkey)"

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354
http://journals.tubitak.gov.tr/earth/
Turkish Journal of Earth Sciences
Turkish J Earth Sci
(2013) 22: 354-358
© TÜBİTAK
doi:10.3906/yer-1202-2
Comment on “Al-in-Hornblende ermobarometry and Sr-Nd-O-Pb Isotopic
Compositions of the Early Miocene Alaçam Granite in NW Anatolia (Turkey)”
Sibel TATAR ERKÜL1,*, Fuat ERKÜL2
1Akdeniz University, Department of Geological Engineering, TR-07058, Antalya, Turkey
2Akdeniz University, Vocational School of Technical Sciences, TR-07058, Antalya, Turkey
* Correspondence: statar@akdeniz.edu.tr
Hasözbek et al. (2012, Turkish Journal of Earth Sciences
21, 37-52) published Al-in-hornblende thermobarometry
and new Sr-Nd-O-Pb isotope data and discussed the
emplacement depth and the petrogenesis of the Alaçam
granites. In this paper, they mainly concluded that these
granitoids were emplaced at shallow crustal levels (4.7±1.6
km) and were not deformed in a ductile manner as the
brittle-ductile boundary of an extended continental crust
is much deeper (15-20 km). ey also suggested that the
Sr-Nd-Pb-O isotopic compositions of the Alaçam granite
are consistent with derivation from an older middle crustal
source rather than a mantle source. In our work on the
geological, geochronological, geochemical and isotopic
characteristics of the Alaçamdağ granitoids, together
with other syn-extensional granitoids, we carefully
examined the results of Hasözbek et al. Inconsistencies in
interpretation lead us to comment on some points in this
paper.
1. In their petrography, geochemistry and isotopic
data section, Hasözbek et al. stated that the Alaçamdağ
granites have more or less equigranular, ne to coarse
grained holocrystalline textures. However, our eld and
petrographic observations revealed that the Alaçamdağ
granitoids are not as unique as published and can be
divided into two distinct facies: western (Musalar
granitoids) and eastern (Alaçam granitoids) stocks
(Erkül 2010, 2012; Erkül & Erkül 2010). In Figure 2 of
Hasözbek et al., the western stocks correspond to the
stocks labelled AS-1 and AS-3, while the eastern stock is a
single body extending NW-SE. e western stocks consist
of holocrystalline equigranular granites and granodiorites
with intruding aplitic equivalents while the eastern stocks
are characterised by abundant K-feldspar megacrysts
within the holocrystalline matrix (Erkül 2012). ese two
facies are mineralogically similar to each other and include
large amounts of mac microgranular enclaves (MME),
which are quite important in explaining the petrogenesis
of these granitoids.
e western and eastern stocks contain extensional
ductile shear zones that consist of widespread
ultramylonites and protomylonites, which were not
mentioned by Hasözbek et al. Further information about
these shear zones can be found in Erkül (2010). Erkül
(2010) also reported systematic Ar-Ar biotite cooling ages
from the Alaçamdağ granitoids and associated mylonitic
rocks (e.g., western and eastern stocks) in the Alaçamdağ
region. ese Ar-Ar ages, ranging from 20.5 to 19.5 Ma,
clearly demonstrate that the cooling of eastern stocks
was coeval with the formation of mylonitic rocks in the
shear zones that provide clear evidence for Early Miocene
extensional ductile deformation in the region. erefore,
the ductilely deformed Alaçamdağ granitoids are not
genetically related to an older metagranitoid of the Afyon
Zone or Menderes Massif, as suggested by Hasözbek et al.
Ductile shear zones in the Alaçamdağ granitoids are also
characterised by asymmetric structures in shear bands,
sigma-type quartz and feldspar porphyroclasts, oblique-
grain-shape foliation, asymmetric boudins and mica sh
(Erkül 2010; Erkül & Erkül 2010). ese structures in low-
grade mylonitic rocks can be used as good shear sense
indicators that may provide insights into the development
of the extensional regime in the northern Menderes Massif.
Kinematic analysis of the Simav detachment and associated
low/high-angle shear zones in the northern Menderes
Massif has already been presented in many papers (Işık &
Tekeli 2001; Işık et al. 2004; Seyitoğlu et al. 2004; Purvis &
Robertson 2004, 2005; Ring & Collins 2005; Çemen et al.
2006; omson & Ring 2006; Erkül 2010; Erkül & Erkül
2010). ey provide detailed evidence that the granitoid
rocks and associated basement units underwent low-grade
mylonitic ductile deformation and the overprinting brittle
deformation in the region was due to progressive upli of
footwall rocks in the region, which is a typical exhumation
process in an extended crust (Işık & Tekeli 2001; omson
& Ring 2006; Erkül 2010). ese studies conrm that the
Received: 01.02.2012 Accepted: 08.05.2012 Published Online: 27.02.2013 Printed: 27.03.2013
Research Article

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ERKÜL and ERKÜL / Turkish J Earth Sci
Menderes Massif and associated granitoid intrusions were
locally deformed into low-grade mylonitic rocks due to
extensional detachments and shear zones.
2. In the mineral chemistry section, systematic sample
locations chosen for Al-in hornblende thermobarometry
evaluations were neither shown in the gure nor indicated
as geographic coordinates. is also fails to explain the
argument that the emplacement depth of the Alaçamdağ
granitoids increases from east to west.
3. In the discussion section, Hasözbek et al. argue that
the Alaçamdağ granitoids, together with other Aegean and
NW Anatolian granitoids, were emplaced at shallow crustal
levels. ey reported estimated emplacement depths
averaging 4.7±1.6 km for the Alaçamdağ granitoids and
denied the presence of extensional ductile deformation (e.g.,
detachment faults and shear zones) as the ductile-brittle
transition zone occurs at deeper levels (about 15-20 km).
Although the emplacement depth of each stock forming the
Alaçamdağ granitoids is not clear due to missing location
data, ther estimated average emplacement depth conrms
the shallow emplacement of syn-extensional granitoids in
the northern Menderes Massif (Akay 2009; Erkül 2010,
2012). Increasing emplacement depth of granitoids from
east to west in the Alaçamdağ region is also consistent
with previous assumptions (Erkül 2010). However, the
absence or presence of ductile deformation based on depth
parameters alone appears unlikely as low-grade mylonite
formation can be controlled by many other factors as well
as depth. Other factors include lithology (e.g., contrasting
behaviours of minerals), temperature, deviatoric stress,
uid content, uid pressure and uid compositions (Lister
& Davis 1989; Blenkinsop 2002; Passchier & Trouw 2005;
Tro uw et al. 2010 and references therein). e temperature
range for low-grade mylonites is widely accepted as
occurring between 250 and 500 °C (Trouw et al. 2010),
and each mineral has a dierent behaviour at constant
temperature. For instance, thermodynamic estimations in
low-grade mylonitic rocks suggest that plastic deformation
in quartz and mica usually occurs at temperatures greater
than 200 °C and plastic deformation of feldspars is widely
accepted to begin at about 450 °C. Amphiboles, common
mac minerals, begin to deform plastically above 500 °C
(Blenkinsop 2002). However, quartz can deform ductilely
at e.g. 300 oC while feldspars behave in a brittle manner at
the same temperatures. erefore, variation in behaviour
of dierent minerals means that no unique depth or
temperature can be proposed for brittle-ductile transitions.
In the Alaçamdağ region, the mylonitised eastern stocks
include retrograde mineral assemblages dened by an
alteration of biotite to chlorite. is alteration process
suggests that the western stocks were heated at temperatures
above 250 °C. Local skarn mineralisation along the contact
of the Alaçamdağ granitoids with host rocks also indicate
that the uid-related parameters mentioned above can
be other controlling factors during the formation of low-
grade mylonitic rocks in the Alaçamdağ region. Adjacent
metamorphic core complexes (e.g., Kazdağ, Rhodope and
Cycladic Core Complexes), even the footwall of central
Menderes Massif, was also intruded by shallow-seated,
syn-extensional granitoids emplaced on the footwall of
a detachment or cut by shear zones; therefore shallow
emplacement of syn-extensional granitoids is a common
event in the extended continental crust of the Aegean
region.
4. In the isotopic compositions of the Alaçam granite
section, authors indicate that the Miocene granitoids
in northwestern Turkey have mainly peraluminous and
minor metaluminous characters. However, this is not
correct, as Eocene to Middle Miocene granitoid rocks
have I-type, mostly metaluminous and a slightly to mildly
peraluminous character (Aydoğan et al. 2008; Karacık et al.
2008; Boztuğ et al. 2009; Erkül & Erkül 2010; Erkül 2012).
eir A/CNK values and mineralogical composition is
characterised by the presence of hornblende and biotite
as the main mac phases and the absence of sillimanite
and garnets as restite minerals, which is compatible with a
metaluminous rather than peraluminous character.
5. In the section “isotopic compositions of the
Alaçam granite”, Hasözbek et al. cited that Aldanmaz
et al. (2000), Dilek & Altunkaynak (2007, 2009) and
Aydoğan et al. (2008) claimed a slab break-o model for
the origin of the Miocene granitoids. However, Dilek &
Altunkaynak (2007, 2009) only suggested this model for
Eocene granitoids in north-western Turkey. Lithospheric
delamination is a widely accepted model for the origin
of Miocene magmatism that has been proposed in many
papers (Aldanmaz et al. 2000; Köprübaşı & Aldanmaz
2004; Dilek & Altunkaynak 2007, 2009; Ersoy et al. 2010,
2012). It is claimed that the Miocene granitoids were
derived from hybrid magmas formed by mixing of crust
and mantle (Aydoğan et al. 2008; Boztuğ et al. 2009; Dilek
& Altunkaynak 2009, 2010; Öner et al. 2010; Erkül & Erkül
2010; Erkül 2012).
Hasözbek et al. suggest in their Figure 9 that the
Alaçamdağ granitoid samples plot in the eld corresponding
to middle crust composition, which is dierent from those
of the Central Aegean granitoid samples (e.g., Ikaria and
Tinos granitoids). However, this gure does not show any
eld dening middle crustal compositions. Hasözbek et
al. (2011) had already suggested an upper crustal origin
for the same Alaçamdağ granitoid samples, based on
normalising values of Rudnick & Gao (2003). Finally,
the origin of the Alaçamdağ granitoids explained in this
paper clearly contradicts the suggestions of Hasözbek et
al. (2011).
Hasözbek et al. argued that the Alaçamdağ granitoids
were derived from an older crustal source (e.g., Menderes
Massif or Afyon Zone) based on Sr-Nd-O-Pb isotopic data.
However, our recent research indicates the presence of a
mantle contribution into the crustal components during
the formation of the Alaçamdağ granitoids (Erkül & Erkül

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ERKÜL and ERKÜL / Turkish J Earth Sci
2012). Compiled 87Sr/86Sr and δ18O data from the Aegean
granitoids reveal that the Alaçamdağ granitoids have δ18O
values between 8 and 10.5‰ and therefore plot on the
mixed eld, corresponding to mixed magmas (Whalen
et al. 1996) (Figure). MMEs bear critical mineralogical
and geochemical information that may highlight the
petrogenesis of the Alaçamdağ granitoids. Oligocene and
Miocene granitoids in western Turkey have abundant
MMEs up to metres across that are circular to ovoid (Erkül
2012). e MMEs are monzonitic, monzodioritic and
dioritic in composition and their sharp contacts with host
rock are commonly attributed to the undercooling and
mingling of hybrid mac microgranular globules formed
by the mixture of mac and felsic magmas (Perugini et al.
2004). On a microscopic scale, disequibilirium textures
(spongy cellular plagioclase, antirapakivi mantling, blade
shaped biotite and acicular apatite) suggest chemical,
thermal and mechanical equilibrium conditions
(Eichelberger 1980; Barbarin & Didier 1991, Hibbard 1991,
1995; Boztuğ et al. 2009; Erkül & Erkül 2010, 2012; Erkül
2012). Lower SiO2 contents than the host rock, and higher
MgO and Mg numbers of MMEs requires the presence
of a mac component, rather than pure crustal material.
A hybrid origin for the granitoids in western Turkey is
not a new idea and has been suggested by many authors
(Aydoğan et al. 2008; Akay 2009; Boztuğ et al. 2009; Dilek
& Altunkaynak 2009; Erkül & Erkül 2010; Erkül 2012).
Geological, mineralogical and geochemical features of the
MMEs appear to have been neglected by Hasözbek et al.
in revealing the petrogenesis of the Alaçamdağ granitoids.
Hasözbek et al. also support an older purely crustal
source with U-Pb ages of 500-550 Ma obtained from
inherited zircon grains in the Alaçamdağ granitoids.
However, U-Pb dating from inherited zircon grains
requires more systematic study to reveal the protolith of the
granitoids. e granitoid rocks were generated by partial
melting with crustal contamination, crystal fractionation
and magma mixing processes that aect primary melts.
erefore, older ages from inherited zircon grains
may also derive from various processes such as crustal
contamination by host meta-sedimentary or igneous rocks
and by partial melting of source protoliths at deeper crustal
levels. e limited number of U-Pb ages (e.g., 500-550 Ma)
from the Alaçamdağ granitoids is insucient to support
an old crustal protolith for the Alaçamdağ granitoids.
In conclusion, the Alaçamdağ granitoids are not as
unique as suggested in the paper by Hasözbek et al. and
they include rather complex lithological and structural
features that need careful examination to highlight their
emplacement mode and to evaluate petrogenetic models.
To relate the emplacement depth of granitoids with
brittle and ductile deformation conditions may lead to
erroneous assumptions, due to various parameters that
must be taken into account. Mineralogical, geochemical
and isotopic features of the MMEs, which were omitted by
Hasözbek et al., appear to have a crucial importance in the
understanding of the magmatic origin of the Alaçamdağ
granitoids. erefore, the older crustal origin for the
Alaçamdağ granitoids suggested by Hasözbek et al. must
be considered with caution.
granodioritic and low-silica granite
s
Supracrustal Continental Crust
high-silica granites
monzonitic and monzogranitic
granitic and granodioritic
Mantle
Mixed
Menderes granitoids
Cycladic granitoids
Lamprophyric-monzonitic dykes
Alaçamdağ granitoids
Laurium, Keros, Serifos,
Mykonos, Delos, Naxos)Tinos,
Ikaria
Kos,
Bodrum,
Samos
0.700 0.705 0.710 0.715 0.720
Sr/ Sr
87 86
δ
18
O
9
10
11
12
8
7
6
Figure. Comparison of the 87Sr/86Sr versus oxygen isotopic composition of the
Aegean and Northwestern Anatolian (NW) granitoids and lamprophyric rocks.
Oxygen and 87Sr/86Sr data are taken from Altherr et al. (1998), Altherr & Siebel
(2002), Hasözbek et al. (2012) and Erkül (2012). Mantle, mixed and supracrustal
rock values are from Whalen et al. (1996).

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