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, APRIL 2013, 64, 2, 133—140 doi: 10.2478/geoca-2013-0009
Introduction
The origin, evolution and age of the Sithonia Plutonic Com-
plex (SPC) intruding the Circum Rhodope Zone and Serbo-
macedonian Massif have been studied by many researchers
(Soldatos & Sapountzis 1975; Sapountzis et al. 1976, 1979;
De Wet & Miller 1986; Christofides et al. 1990, 1998, 2007;
D’Amico et al. 1990; Perugini et al. 2003; Pipera et al. 2010;
Melfos et al. 2012; Romanidis et al. 2012). The study of the
Sithonia Eocene pluton is of great importance for the clarifi-
cation of the geotectonic evolution of the Sidironero and
Pangeon tectonic units of the Rhodope Massif since the
Serbomacedonian Massif is considered equivalent to the
Pangeon tectonic unit (Christofides et al. 2001). The age of
the SPC was estimated from a whole rock Rb/Sr isochron on
two-mica granodiorite samples which yielded 50.4 ± 0.7 Ma
(Christofides et al. 1990). This age is in accordance with the
U/Pb zircon age of 51.32 ± 0.89 Ma, obtained by Alagna et al.
(2008). Christofides et al. (1990) attributed the low biotite
Rb/Sr ages of some leucogranite samples to a rejuvenation of
the Rb/Sr isotopic system during a tectonic event that took
place at most 29 Myr ago. This rejuvenation was detected due
to the disturbance of biotite ages. Further geochronological
study of the pluton was necessary in order to examine this tec-
tonic (thermal) event that took place and affected the pluton
after the crystallization. The K/Ar dating, as a more “sensi-
tive” dating method on temperature changes, was selected to
work out the discordances of the previous resultant ages. Due
K/Ar mineral geochronology of the northern part of the
Sithonia Plutonic Complex (Chalkidiki, Greece): implications
for its thermal history and geodynamic interpretation
KYRIAKI PIPERA
1
, ANTONIS KORONEOS
1
, TRIANTAFYLLOS SOLDATOS
1
, ZOLTÁN PÉCSKAY
2
and GEORGIOS CHRISTOFIDES
1
1
Department of Mineralogy, Petrology and Economic Geology, School of Geology, Aristotle University of Thessaloniki,
54124 Thessaloniki, Greece; heraia@geo.auth.gr; koroneos@geo.auth.gr; soldatos@geo.auth.gr; christof@geo.auth.gr
2
Institute of Nuclear Research of the Hungarian Academy of Sciences, P.O. Box 51, Bém tér 18/c, H-4001 Debrecen, Hungary;
pecskay@atomki.hu
(Manuscript received November 3, 2010; accepted in revised form October 16, 2012)
Abstract: New K/Ar mineral ages of thirty nine samples (biotite, muscovite, K-feldspar) from the two-mica granodiorite
to granite and leucogranite of the northern part of the Sithonia Plutonic Complex (Chalkidiki, Greece) are given in the
present study. These data along with existing Rb/Sr mica and U/Pb zircon ages are used to investigate the thermal
history of the plutonic complex and shed light on the process that affected it, and caused discordant Rb/Sr and K/Ar
mineral ages. The K/Ar mineral dating yielded ages ranging from 38 to 49 Ma for muscovites, 32 to 47 Ma for biotites
and 37 to 43 Ma for K-feldspars, respectively. The comparison of the K/Ar, Rb/Sr and U/Pb mineral ages and the
closure temperatures of the different isotopic systems for the different minerals indicate a rapid cooling rate for the
Sithonia pluton. The latter supports the hypothesis that the pluton was formed in a post orogenic extensional regime.
Moreover, the K/Ar mineral isochrones indicate that a reheating of the pluton took place before 37 Ma and partially
rejuvenated the K/Ar and Rb/Sr isotopic system of the minerals.
Key words: Tertiary granitoids of Rhodope, Sithonia Plutonic Complex, K/Ar geochronology, thermal evolution.
to the lower closure temperatures of the minerals for the K/Ar
isotopic system in respect to the Rb/Sr system, the former is
more easily affected by any reheating and so it is more possi-
ble to detect any isotopic disturbance. The subject of the
present study is the K/Ar mineral geochronology of the north-
ern part of the Sithonia pluton. The systematic K/Ar study of
the pluton in association with the previous geochronological
studies sheds light on the pluton’s thermal history after its em-
placement and on its affinity with the geotectonic regime.
Analytical methods
The samples were crushed and the 150—250 µm grain size
was collected. A vibrating table and a Franz Isodynamic
Magnetic separator (model L-1) were used to separate mus-
covite and biotite. K-feldspar was extracted using the same
magnetic separator and the heavy liquid tetrabromoethane
(Br
2
CHCHBr
2
). The separation of the minerals was performed
at the Department of Mineralogy, Petrology and Economic
Geology, Aristotle University of Thessaloniki, Greece. Mus-
covite and biotite chemical analyses were performed on a
JEOL scanning electron microscope at the Department of
Mineralogy, Petrology and Economic Geology, Aristotle Uni-
versity of Thessaloniki, Greece. The operating conditions
were: 20 kV and 20 nA, with a beam diameter < 1 mm.
X-Ray Powder Diffraction analyses (XRPD) were per-
formed on each mineral extract to calculate the proportion of
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mineral and the purity of it. Powder XRPD analyses were ob-
tained on a PHILIPS PW1820/00 X-ray diffractometer of the
Department of Mineralogy, Petrology and Economic Geo-
logy, School of Geology, Aristotle University of Thessaloniki,
carrying a PW1710 microprocessor. The operating condi-
tions for all samples were 35 kV and 25 mA using Ni-filtered
CuK radiation. The 2-theta scanning range was between 3°
and 63° and the scanning speed was 1.2 °/min. Refinements
were done with the PCAPD software and the identification
of the samples was done with the JCPDS-ICDD 2003 data-
base. The purity of biotite and muscovite was calculated
over 98 % and of K-feldspar over 96 %.
The K/Ar dating was performed at the Institute of Nuclear
Research of the Hungarian Academy of Science (ATOMKI),
Debrecen, Hungary following the method described by Balogh
Fig. 1. Geological map of the Sithonia Plutonic Complex (SPC) and its country rocks including the sampling sites (modified map after Christo-
fides et al. 2007). HRM – Hellenic Rhodope Massif, SMM – Serbomacedonian Massif, CRB – Circum-Rhodope Belt, VAZ – Vardar-
Axios Zone.
(1985). An argon extraction line and a mass spectrometer, both
designed and built in the ATOMKI, were used for the Ar mea-
surement. The rock was degassed by high frequency induction
heating and the usual getter materials (Ti sponge, CuO, zeolite
and cold traps) were used for cleaning Ar. The
38
Ar spike was
introduced in the system with a gas pipette before the beginning
of the degassing. The cleaned Ar was directly introduced into
the mass spectrometer. The mass spectrometer was a magnetic
sector type of 150 mm radius and 90 °C deflection and it was
operating in static regime. Recording and evaluation of Ar spec-
trum was controlled by suitable software. For the potassium
content analysis 0.1 g of finely ground sample was digested in
HF with addition of H
2
SO
4
and HClO
4
and finally dissolved in
0.2 M HCl. Potassium was determined by flame photometry
with a Na buffer and Li internal standard.
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Sithonia Peninsula. Despite their modal and textural differ-
ences, both petrographic types are discussed together due to
the similar mineralogy.
The essential mineral constituents are quartz, microcline,
plagioclase, biotite and muscovite. Microcline is slightly per-
thitic and often displaying poikilitic texture enclosing plagio-
clase and mica. The plagioclase is usually subhedral. Discrete
masses of leucogranite have microcline megacrysts which de-
termines a porphyritic texture. The development of myrmekite
among the grains is very frequent. Biotite and muscovite are
euhedral to subhedral developed as individual macro-prismatic
crystals sometimes oriented and elongated and often banded.
The accessory minerals are opaques (mostly ilmenite), apa-
tite, zircon and locally, in the TMG only, epidote.
Mineral chemistry
Soldatos & Sapountzis (1975), Soldatos et al. (1976),
D’Amico et al. (1990) and Christofides et al. (1998) studied
the mineralogy of the SPC rocks. For the needs of the
present study chemical analyses of muscovite and biotite of
each sample analysed by the K/Ar method are presented.
Plagioclase appears in all rock types. It is usually zoned
ranging from An
34
to An
10
in TMG, and An
27
to An
8
in
LG + PLG.
Microcline occurs as subhedral to anhedral interstitial
crystals and also as megacrysts in some of the leucogranites.
The composition ranges from O
84
to O
96
with the anorthite
component not exceeding 1 %.
Biotite in the TMG and LG is associated with muscovite
and is more Fe-rich compared with the other petrographic
types. In Table 1 biotite analyses of the samples from the
present study are presented.
Muscovite appears in large flakes and lath-shaped crystals,
occurring singly, in clusters or intergrown with biotite and has
characteristics favouring a primary origin. Large flakes resem-
bling those of primary muscovite except for being slightly
coloured and replacing feldspar are suggested to be of post-
magmatic origin. Fine-grained secondary sericite also exists
on plagioclase. On the basis of mode of occurrence and the
TiO
2
content three types of muscovite are recognized (Table 2).
The low-Ti (0.78—1.10 wt. %), moderate-Ti (1.18—1.58 wt. %)
and the high-Ti (1.73—1.95 wt. %) muscovite.
Geochronological results
Dating was carried out on muscovite (Mu) and biotite (Bi)
from all PLG samples, biotite, muscovite and K-feldspar (K-f)
from all TMG samples except samples STH-400 and STH-401
where the muscovite concentration was very low, and on
muscovite, biotite and K-feldspar from all LG samples. Ta-
ble 3 summarizes the K/Ar mineral ages of all 39 analysed
mineral samples.
The resultant ages of STH-170 and STH-400 biotite sam-
ples from TMG, STH-19 and STH-55 biotite samples from
PLG and STH-169 and STH-174 biotite samples from LG
have been subjected to Ar loss as indicated from their low
40
Ar
rad
concentrations ( < 80 %) which causes younger result-
Geological setting
The SPC occupies the greater part of the Sithonia Peninsula
(about 350 km
2
, Fig. 1) that constitutes the middle of the three
peninsulas of Chalkidiki (Macedonia, N Greece). The bigger
part of the Sithonia Peninsula belongs to the Circum Rhodope
Belt while a minor occurrence of the Serbomacedonian Massif
appears on the eastern part as well as a very limited occur-
rence of ophiolites of the Paionia Belt (Vergely 1984).
The intrusion of the SPC pluton caused contact aureole and
affected the regional NW—SE strike of the schistosity and fold
axes of the country rocks. The intrusion itself has been affected
by younger tectonic activity that took place most probably in
the Late Eocene—Oligocene and induced minor shear effects
marked by mica orientation (Sakellariou 1989).
Over most of its outcrop, the SPC pluton reveals a planar
fabric, which varies in intensity, but increases toward the
margins. There is a magmatic foliation in the interior and a
solid-state one in the marginal parts, where the fabric is pla-
nar-linear with the development of a WSW trending stretch-
ing lineation (Tranos et al. 1993).
The SPC consists of two-mica granite to granodiorite
(TMG), leucogranite + porphyritic leucogranite (L + PLG)
including many varieties of textural types, aplite and pegma-
tite (A), biotite granodiorite (BGd), hornblende-biotite grano-
diorite (HBGd), quartz-dioritic and tonalitic enclaves (MME)
and hornblende-biotite granodioritic tonalite (Sapountzis et al.
1976, 1979; Christofides et al. 1990; D’Amico et al. 1990;
Christofides et al. 2007). The LG intrudes the TMG to the
north with sharp contacts and the HBGd to the south with
transitional contacts. The BGd is younger than the HBGd and
intrudes it clearly. Aplites and pegmatites occur all over the
SPC intruding the pluton as well as the country rocks as the
final products of the differentiation process. Tonalitic and
monzonitic enclaves are found dispersed in all types except
the TMG and PLG + LG.
Characteristics of the Sithonia Plutonic Complex
Geochemistry
The geochemistry of the SPC has been studied by Sapountzis
et al. (1976, 1979), De Wet & Miller (1986), Christofides et
al. (1990), D’Amico et al. (1990), Perugini et al. (2003),
Christofides et al. (2007). The chemical composition of the
SPC rocks ranging from tonalites to leucogranites corre-
sponds to a chemical range of 62 % to 77 % SiO
2
.
Petrography of the two-mica granodiorite to granite and
leucogranites
The TMG body appears more or less homogeneous while
the leucogranite displays textural variations so that several
textural types can be distinguished. In the present paper the
term PLG is applied to the more coarse-grained leucogranite
often displaying porphyritic texture, while the term LG is ap-
plied to the more fine-grained leucogranite that occupies the
eastern part of the pluton traversing the NE coast of the
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ant ages (Wörner et al. 2000; Panter et al. 2006). Regarding
the results of the LG samples, it is obvious that they differ
from those of the other two petrographic types. The ages are
discordant in terms of mineral closure temperature principle
Mu age > Bi age > K-f age. In detail, sample STH-169 K-f ap-
pears older or similar to biotite, taking into account the ana-
lytical error. Concerning Mu, Bi and K-f ages of sample
STH-52 and Mu and K-f ages of sample STH-174 they are
Table 1: Representative analyses of TMG, PLG and LG biotites from the Sithonia Plutonic Complex. Each analysis represents the average
of nine spots (three spots on each of three crystals).
Table 2: Representative analyses of TMG, PLG and LG muscovites. Each analysis represents the average of nine spots (three spots on each
of three crystals).
TMG
PLG
LG
Sample 1 100 162 170 19 37 44 47 55 56 61 52*
174*
174
169*
169
SiO
2
46.70 45.90 46.37 45.50 46.56
46.5 45.83 46.11 45.96 45.37 46.07 45.85 45.84 46.19 46.13 45.82
TiO
2
1.44 1.58 1.42 1.45 1.26 1.21 1.30 1.28 1.18 1.73 1.36 0.78 1.10
1.57 1.05 1.95
Al
2
O
3
31.49 31.21 29.86 31.7 30.18 32.22 31.25 31.08
32.1 31.63 32.33 32.87 30.55
29.76 31.93 32.23
FeOt
3.39 4.89 5.88 4.57 5.43 3.88 5.05 4.69 4.76 4.61 4.34 4.27 6.39 5.42 3.87 3.72
MgO
1.38 1.44 1.40 1.17 1.34 1.06 1.18 1.40 1.03 1.21 0.96 0.93 1.18
1.42 1.22 1.16
Na
2
O
0.58 0.51 0.51 0.72 0.68 0.67 0.48 0.82 0.50 0.54 0.66 0.57 0.90 0.78 0.61 0.60
K
2
O
10.60 10.59 10.74 10.64 10.88 10.90 10.92 10.86 10.79 10.72 10.15 10.72 10.07 10.72 10.72 10.37
Total
95.66 96.17 96.17 95.76
96.34 96.44 96.01 96.24 96.33 95.82 95.85
95.99 95.52 95.86 95.52 95.86
Site allocations (22 O)
Si
6.30 6.21 6.31 6.18 6.32 6.25 6.23 6.26 6.21 6.16 6.20 6.20 6.25 5.27 6.25 6.16
Al
IV
1.70 1.79 1.69 1.82 1.68 1.75 1.77 1.74 1.79 1.84 1.80 1.80 1.75 2.49 1.75 1.84
Z
8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 7.76 8.00 8.00
Al
VI
3.30 3.18 3.10 3.25 3.16 3.36 3.23 3.23 3.31 3.22 3.33 3.43 3.16 1.53 3.34 3.27
Ti
0.15 0.16 0.14 0.15 0.13 0.10 0.13 0.13 0.12 0.18 0.14 0.07 0.11 0.14 0.11 0.20
Fe
2+
0.38 0.55 0.67 0.52 0.62 0.44 0.57 0.53 0.54 0.52 0.49 0.48 0.73 0.52 0.44 0.42
Mg
0.28 0.29 0.28 0.24 0.27 0.21 0.24 0.28 0.17 0.24 0.19 0.19 0.24 0.24 0.25 0.23
Y
4.10 4.19 4.20 4.15 4.17 4.11 4.18 4.17 4.15 4.17 4.15 4.17 4.24 3.55 4.13
0
Na
0.08 0.08 0.02 0.13 0.03 0.09 0.02 0.04 0.09 0.07 0.14 0.05 0.12 0.10 0.08 0.16
K
1.82 1.83 1.87 1.84 1.89 1.87 1.89 1.88 1.86 1.86 1.74 1.85 1.75 1.55 1.85 1.78
X
1.90 1.91 1.89 1.97 1.92 1.96 1.92 1.91 1.95 1.93 1.89
1.9 1.87 1.65 1.93 1.93
*Low Ti muscovite
TMG PLG
LG
Sample 1 100 162 170 400 401 19 37 44 47 55 56 61 52 169 174
SiO
2
36.75 35.63 35.77 35.97 36.09 35.50 36.37 34.65 35.92 37.09 35.48 34.64 34.93 36.15 35.00 37.46
TiO
2
3.42 3.64 3.44 3.61 3.51 3.73 3.20 3.62 3.00 2.58 3.14 3.19 3.75 3.14 3.43 3.45
Al
2
O
3
16.53 15.88 16.29 16.68 16.41 16.54 15.79 16.45 16.2 15.96 16.18 15.98 15.95 16.34 16.36 16.00
FeOt
21.88 23.44 23.28 22.95 22.87 23.83 22.84 25.23 24.45 22.69 24.77 24.82 24.89 23.18 24.33 22.41
MnO
0.26 0.36 0.22 0.11 0.27 0.82 0.76 0.65 0.45 1.00 0.30 0.42 0.32 0.29 0.67 0.86
MgO
7.99 8.03 7.47 7.74 7.84 6.61 7.94 6.31 6.88 7.76 7.35 7.65 7.01 7.88 6.81 7.08
CaO
0.07 0
0.10 0
0
0
0
0.23 0
0
0.09 0
0
0
0
0
Na
2
O
0
0
0
0
0.11 0.11 0.12 0.11 0
0.10 0
0.24 0.10 0.18 0
0
K
2
O
9.19 9.16 9.28 8.92 8.82 9.12 8.9 8.52 9.01 8.55 8.59 8.69 8.96 9.06 9.42 8.87
Total
96.07 96.16 95.84 95.98 95.92 96.26
95.92 95.78 95.91 95.73 95.88 95.62 95.91
96.21 96.02 96.13
Site allocations (22 O)
Si
5.61 5.5 5.53 5.52 5.54 5.49 5.60 5.42 5.57 5.69 5.50 5.42 5.45 5.55 5.45 5.72
Al
IV
2.39 2.50 2.47 2.48 2.46 2.51 2.40 2.58 2.43 2.31 2.50 2.58 2.55 2.45 2.55 2.28
Z
8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00
Al
VI
0.58 0.39 0.49 0.53 0.51 0.50 0.47 0.45 0.53 0.58 0.46 0.37 0.38 0.53 0.46 0.60
Ti
0.39 0.42 0.40 0.42 0.41 0.43 0.37 0.43 0.35 0.3 0.37 0.37 0.44 0.37 0.40 0.40
Fe
2+
2.79 3.03 3.01 2.95 2.94 3.08 2.94 3.30 3.17 2.92 3.22 3.25 3.25 2.96 3.17 2.86
Mn
0.03 0.05 0.03 0.01 0.04 0.11 0.10 0.09 0.06 0.13 0.04 0.06 0.04 0.04 0.09 0.11
Mg
1.82 1.85 1.72 1.77 1.79 1.52 1.82 1.47 1.59 1.78 1.70 1.78 1.63 1.78 1.58 1.61
Y
5.61 5.73 5.66 5.68 5.69 5.65 5.70 5.74 5.71 5.71 5.78 5.83 5.74 5.68 5.70 5.58
Ca
0.01 0
0.02 0
0
0
0
0.04 0
0
0.01 0
0
0
0
0
Na
0
0
0
0
0.03 0.03 0.04 0.03 0
0.03 0
0.07 0.03 0.03 0
0
K
1.98 1.80 1.83 1.75 1.73 1.8
1.75 1.70 1.78 1.68 1.7 1.74 1.78 1.79 1.87 1.73
X
2.00 1.80 1.85 1.75 1.76 1.83
1.79 1.77 1.78 1.71 1.71 1.81 1.81
1.82 1.87 1.73
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Sample
K
40
Ar
rad
40
Ar
rad
Age
Rock
type
(%)
(ccSTP/g) (%)(r) (Ma)
±
STH-1
Mu
8.26 1.60 10
–5
84.30 49.15 1.51
Bi
7.45 1.30 10
–5
78.60 45.32 1.41
K-f
9.23 1.40 10
–5
89.20 38.56 1.17
STH-100 Mu
8.74 1.65 10
–5
90.60 47.77 1.45
Bi
7.73 1.40 10
–5
86.90 45.84 1.40
K-f 11.76 2.01 10
–5
91.40 43.52 1.32
STH-162 Mu
8.53 1.63 10
–5
89.10 48.62 1.48
Bi
7.50 1.41 10
–5
79.40 47.62 1.47
K-f 11.19 1.87 10
–5
94.10 42.39 1.28
STH-170 Mu
8.73 1.60 10
–5
86.00 46.42 1.42
Bi
7.40 1.16 10
–5
59.40 39.83 1.31
K-f
9.87 1.50 10
–5
93.10 38.70 1.17
STH-400 Bi
7.41 1.03 10
–5
57.00 35.25 1.20
K-f
9.16 1.33 10
–5
89.70 36.96 1.12
STH-401 Bi
7.52 1.38 10
–5
87.10 46.47 1.42
TM
G
K-f
9.71 1.55 10
–5
88.10 40.63 1.24
STH-19
Mu
8.59 1.48 10
–5
85.70 43.81 1.34
Bi
7.41 1.01 10
–5
70.60 34.69 1.11
STH-37
Mu
8.85 1.54 10
–5
85.40 44.29 1.36
Bi
6.04 8.63 10
–5
80.40 36.38 1.13
STH-44
Mu
8.50 1.47 10
–5
87.20 43.87 1.34
Bi
7.42 1.19 10
–5
86.90 40.64 1.24
STH-47
Mu
8.53 1.45 10
–5
86.40 43.17 1.32
Bi
7.60 1.17 10
–5
83.60 39.33 1.21
STH-55
Mu
8.87 1.51 10
–5
86.70 43.28 1.32
Bi
8.00 1.01 10
–5
58.60 32.19 1.09
STH-56
Mu
9.12 1.60 10
–5
87.70 44.59 1.36
Bi
7.10 1.20 10
–5
82.50 42.91 1.32
STH-61
Mu
8.78 1.53 10
–5
87.50 44.32 1.35
PLG
Bi
7.54 1.20 10
–5
88.10 40.51 1.23
STH-52
Mu
8.87 1.34 10
–5
76.00 38.53 1.21
Bi
7.14 1.14 10
–5
74.40 40.69 1.27
K-f 11.70 1.83 10
–5
87.90 39.86 1.22
STH-169 Mu
8.74 1.61 10
–5
81.40 46.66 1.44
Bi
7.92 1.16 10
–5
68.00 37.14 1.19
K-f 11.42 1.79 10
–5
92.50 39.93 1.21
STH-174 Mu
8.08 1.29 10
–5
79.00 40.75 1.26
Bi
6.78 3.96 10
–5
29.60 14.96 0.75
LG
K-f 10.83 1.65 10
–5
86.50 38.84 1.19
almost identical in each sample. TMG muscovite and biotite
ages are in general terms older than those of PLG (Figs. 2
and 3). Thereby, the cooling of TMG took place first, fol-
lowed by PLG, which is in accordance with the crystalliza-
tion sequence given by Christofides et al. (2007).
The muscovite ages of TMG obtained from the present study
are similar to each other, ranging from 46.42 to 49.15 Ma.
Moreover, they are similar to the Rb/Sr muscovite ages
(Fig. 4) and to the emplacement age (Christofides et al. 1990).
All the minerals not subjected to Ar loss were processed by
the
K%/
40
Ar
rad
isochron method. The software Isoplot/Ex 3.66
(Kenneth 2008) was used to extract the regression lines.
Excluding the muscovites and biotites of the LG samples
due to the discordances of their ages and the Ar loss of some
Table 3: K/Ar mineral ages of the Sithonia TMG, PLG and LG.
Mu – muscovite, Bi – biotite, K-f – K-feldspar.
Fig. 2. TMG and PLG K/Ar biotite ages.
Fig. 3. TMG and PLG K/Ar muscovite ages.
Fig. 4. K/Ar (present study) and Rb/Sr ages of Christofides et al.
(1990).
minerals, the regression lines of the TMG and PLG samples
gave remarkable results. All the regression lines obtained for
the TMG and PLG muscovites constitute isochrones with
MSWD = 0.61 and MSWD = 0.40 respectively (Figs. 5 and 6).
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Fig. 6.
40
K/
40
Ar isochron calculated for the TMG muscovites.
The analytical error is 2 (68 % confidence level) and the
MSWD = 0.40.
Fig. 5.
40
K/
40
Ar isochron calculated for the PLG muscovites.
The analytical error is 2 (68 % confidence level) and the
MSWD = 0.61.
Fig. 7.
40
K/
40
Ar isochron calculated for the TMG K-feldspars.
The analytical error is 2 (68 % confidence level) and the
MSWD = 0.051.
Fig. 8.
40
K/
40
Ar isochron calculated for the LG K-feldspars.
The analytical error is 2 (68 % confidence level) and the
MSWD = 0.30.
Fig. 9. Calculated trend line for the cooling rate of the TMG.
It must be stressed here that in the TMG isochron calculation,
sample STH-170 was not included due to its younger age and
its petrographic differences relative to the other TMG sam-
ples. The regression lines of TMG and PLG biotites do not
constitute isochrons (MSWD = 2.8 and MSWD = 3.8 respec-
tively) even though it seems that the biotites have not been
subjected to Ar loss (Table 3). The regression lines of TMG
and LG K-feldspars are isochrons with MSWD = 0.051
(Fig. 7) and 0.30 (Fig. 8) respectively.
A cooling rate line for the TMG could be calculated based
on the results of the present study, the Rb/Sr results of Christo-
fides et al. (1990) and the zircon age of Alagna et al. (2008). In
Fig. 9 the cooling trend line for the TMG samples is depicted.
An average cooling rate of 51.8 ± 7.4 °C/Ma was calculated.
Discussion
Muscovite K/Ar results of all TMG and PLG samples gave
undisturbed ages and isochron regression lines. PLG biotite
and K-feldspar results do not form isochrons indicating a
disturbance on the K/Ar isotopic system of these minerals.
TMG biotite results do not form an isochron, but the TMG
K-feldspars form a very good isochron.
The results of Christofides et al. (1990) were used in the
present study for comparison between the two isotopic sys-
tems. Taking into account that the closure temperature of mus-
covite for the K/Ar isotopic system is lower (375 ± 25 °C;
Jäger & Hunziker 1979; Harrison et al. 1985) than the clo-
sure temperature of the Rb/Sr system (500 ± 50 °C; Jäger &
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Hunziker 1979), and the K/Ar system of our samples re-
mained undisturbed, there is still more reason to expect the
same for the Rb/Sr system. The Rb/Sr TMG muscovite re-
sults of Christofides et al. (1990) used for the calculation
yielded an isochron MSWD = 0.34 confirming the K/Ar iso-
chron results. There are no Rb/Sr results for the PLG in order
to carry out the same analysis.
Biotites of TMG and PLG show different behaviour than
muscovites. Some samples from both types indicate Ar loss.
For the rest of the samples regression lines were calculated
but there was no K/Ar isochron formation either for the
TMG or the PLG biotites. The Rb/Sr isotopic data for TMG
were also used to calculate regression lines for biotites and,
similarly, they do not form isochrons.
The behaviour of biotites could only be interpreted as the
result of a thermal rejuvenation which reached the closure
temperature of biotite for both K/Ar (310 ± 30 °C; Jäger &
Hunziker 1979; Harrison et al. 1985) and Rb/Sr systems
(300 ± 50 °C; Jäger & Hunziker 1979) and disturbed the K/Ar
and Rb/Sr isotopic systems of biotites. Such rejuvenation
should be attributed to a mild reheating event because the iso-
topic systems did not completely reset (Hayatsu & Carmichael
1970). A strong thermal event would have caused resetting
of the isotopic systems and the regression lines would be iso-
chrons indicating the age of the thermal event.
The K-feldspars of TMG and PLG indicate contrasting be-
haviour. TMG K-feldspars form isochrons but PLG K-feld-
spars do not. This probably means that the thermal reheating
mentioned above was sufficient to rejuvenate the K-feld-
spars of TMG having a closure temperature of 150 ± 25 °C
(Lovera et al. 2002) but not those of PLG.
LG resultant ages are discordant and indicate different be-
haviour from the TMG and PLG samples. The muscovite re-
gression line does not form an isochron and this is probably
due to the two muscovite generations. The presence of the
post-magmatic low Ti muscovite (Table 2) probably indi-
cates another event that affected the LG. The muscovite ages
in STH-52 and STH-174 are almost the same as the biotite
and K-feldspar ages, respectively. The biotite ages are either
not reliable (STH-174) or discordant in terms of isotopic clo-
sure (STH-52 and STH-174). The K-feldspar results seem to
be unaffected forming isochrons and this is a very different
behaviour compared to the other two minerals. The event af-
fected the LG should be one of different nature than the re-
heating event affected the other two petrographic types.
Field observations show that numerous and voluminous peg-
matites intrude the LG. The frequency and intensity of this
intrusion is not observable in the other petrographic types of
SPC. Strachan et al. (1996), considers that the intrusion of
numerous pegmatites could result in disturbance of the isoto-
pic systems in terms not only of temperature but also of
chemistry. Based on the Strachan et al. (1996) interpretation
and on field observations, we suggest that LG muscovite and
biotite isotopic systems were affected from the intrusion of
the numerous pegmatites. The LG K-feldspar samples, prob-
ably affected from the thermal event previously discussed
for TMG and PLG, rejuvenated from this pegmatitic event
resulting in the homogenization of their isotopic system, and
therefore gave a good isochron plot.
A recent fluid inclusions study (Melfos et al. 2012) of peg-
matitic quartz from all over the SPC rocks gave remarkable re-
sults supporting the previous consideration. The fluid
inclusions study, revealed the absence of primary fluid inclu-
sions and the presence only of secondary or pseudo-secondary
inclusions (probably due to a tectonic event). The homogeni-
zation temperatures vary between 270 and 310 °C with a peak
at 290 °C that is very close to the closing temperature for K/Ar
system in biotite. The only exception is the STH-5 sample (a
pegmatite intruding LG) showing a second peak of homogeni-
zation temperatures at 230 °C. Thus, homogenization temper-
atures are in very good agreement with the results of the
present study.
Regarding the age of the reheating, probably of tectonic
origin as indicated from the fluid inclusions study, the
younger resultant age of STH-400 K-feldspar (about 37 Ma)
could be the upper limit for this thermal event because it is
the younger reliable age measured. For the local event that
disturbed the LG little can be said because of the unknown
nature of this event. Probably it was imminent or younger
than the reheating that affected also TMG and PLG.
The disturbance in mineral ages due to the mild thermal
event did not result in important differences in ages, so the
calculation of the cooling rate is more or less reliable if it is
considered as a minimum due to the slightly younger K-feld-
spar ages.
The average cooling rate of the TMG as calculated above
(average 51.8 ± 7.4 °C/Ma) is very high. Fig. 9 shows that the
cooling rate of TMG did not remain stable during the whole
cooling history of the pluton. In general, the rapid cooling of
plutons is attributed to extensional setting. According to Kilias
et al. (2002) the extensional collapse in Rhodope Massif took
place behind the orogenic arc in the back arc area during the
Eocene/Oligocene. The geotectonic setting described by the
latter workers interprets the rapid cooling of TMG calculated.
Conclusions
Biotites from the northern part of the Sithonia Plutonic Com-
plex have been subjected to Rb/Sr and K/Ar isotopic distur-
bance as obtained from the discordances of their ages and the
isochron plot results. Muscovite remained undisturbed resulting
in good agreement with the existing dating of the pluton and the
good isochron plots. K-feldspars of two-mica granodiorite
(TMG) were rejuvenated from a reheating event which did
not affect the K-feldspars of porphyritic leucogranite. A mild
thermal event close to the closure temperature of biotite for
the Rb/Sr and K/Ar closure temperature is considered to
have affected the pluton isotopically. On the other hand, in a
small area where the leucogranite (LG) outcrops, the volumi-
nous pegmatitic intrusions seem to have affected locally the
LG intrusion. This activity should be younger or imminent with
the thermal event, which was probably of tectonic origin ac-
cording to recent fluid inclusion studies. The age of the thermal
event has a maximum of 37 Ma and the cooling rate that was
calculated for the TMG is very high. The rapid cooling of the
pluton is attributed to the extensional collapse of the Rhodope
Massif during the Eocene/Oligocene when the SPC intruded.
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Acknowledgments: The authors would deeply like to thank
Prof. Eleftheriades G. for his valuable assistance on mineral
separation, Lecturer Papadopoulou L. for her valuable assis-
tance during the microprobe analyses and the PhD student
Theodosoglou E. for her help in mineral separation. Critical
reviews by Peter Marchev, Igor Petrík and one anonymous
reviewer helped as improve the paper.
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