The Gümüşhane pluton (NE Turkey)

Transcription

LITHOS-02179; No of Pages 19
ARTICLE IN PRESS
Lithos xxx (2010) xxx–xxx
Contents lists available at ScienceDirect
Lithos
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / l i t h o s
Carboniferous high-potassium I-type granitoid magmatism in the Eastern
Pontides: The Gümüşhane pluton (NE Turkey)
Gültekin Topuz a,⁎, Rainer Altherr b, Wolfgang Siebel c, Winfried H. Schwarz b, Thomas Zack d,
Altuğ Hasözbek c, Mathias Barth d, Muharrem Satır c, Cüneyt Şen e
a
İstanbul Teknik Üniversitesi, Avrasya Yerbilimleri Enstitüsü, TR-34469 Maslak, İstanbul, Turkey
Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234-236, D-69120 Heidelberg, Germany
Universität Tübingen, Institut für Geowissenschaften (Geochemie), Wilhelmstrasse 56, D-72074 Tübingen, Germany
d
Institut für Geowissenschaften, Universität Mainz, Johann-Joachim-Becher Weg 21, D-55099 Mainz, Germany
e
Karadeniz Teknik Üniversitesi, Jeoloji Mühendisliği Bölümü, TR-61080 Trabzon, Turkey
b
c
a r t i c l e
i n f o
Article history:
Received 31 March 2009
Accepted 7 January 2010
Available online xxxx
Keywords:
High-K granite
Sr–Nd isotopy
Ar–Ar dating
U–Pb zircon dating
Variscan orogeny
Eastern Pontides
Turkey
a b s t r a c t
The Gümüşhane pluton, a high-K calc-alkaline I-type granodiorite/granite complex, forms an important
component of the pre-Liassic basement of the Eastern Pontides (NE Turkey). In its eastern part, the pluton
shows a compositional zonation ranging from biotite–hornblende granodiorite in the NW through biotite–
hornblende granite to leucogranite/granophyre in the SE. Numerous mafic microgranular enclaves (up to
∼ 40 cm in diameter) suggest the former presence of globules of mafic melt during crystallization.
Emplacement of the pluton occurred during the latest Early Carboniferous, as shown by the 320 ± 4 Ma 40Ar–
39
Ar biotite/hornblende and 324 ± 6 Ma LA-ICP-MS U–Pb zircon ages. In Harker diagrams, samples of the
different rock types exhibit well-defined data trends. With increasing SiO2, the abundances of TiO2, Al2O3,
Fe2Otot
3 , MnO, MgO, CaO, P2O5 and Sc decrease, but those of K2O and Rb increase. However, the variations of
Sr, Ba, (La/Yb)cn, Sr/Y and ∑REEs vs. SiO2 form distinctive groupings, which cannot be explained by a simple
fractional crystallization. Chondrite-normalized (cn) REE patterns of granodiorite/granite samples show
concave-upward shapes with (La/Yb)cn ranging from 5.2 to 12.4 and Eu/Eu* from 0.84 to 0.47, while there is
almost no fractionation of the middle REE relative to the heavy REE. In primitive mantle-normalized element
concentration diagrams, all rocks display marked negative anomalies in Ba, Nb/Ta, Sr, P and Ti, but positive
anomalies in K and Pb. These geochemical features imply a fractionating mineral assemblage of
clinopyroxene, amphibole and plagioclase without significant involvement of garnet. The granophyres are,
on the other hand, characterized by higher K2O/Na2O and Rb/Sr ratios, lower (La/Yb)cn ratios (1.3 to 4.8) and
more pronounced negative anomalies in Ba, Nb/Ta, Sr, Eu, P and Ti. Initial εNd values range from − 3.78 to
− 5.30 and Nd model ages from 1.38 to 163 Ga. The magmas of the granite/granodiorite portion were
probably generated by partial melting of high-potassic amphibolitic rocks, and those of the granophyres by a
relatively felsic micaceous crustal source. The Gümüşhane pluton was emplaced at the wake of the lowpressure–high-temperature metamorphism, and is regarded as a late phase of Hercynian orogeny in the
Eastern Pontides.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
High-K calc-alkaline I-type granitoids form a major constituent of
late orogenic to post-collisional settings (e.g. Küster and Harms, 1998;
Bonin et al., 1998; Altherr et al., 1999, 2000; Altherr and Siebel, 2002;
Bonin, 2004). In these intrusions, the ubiquitous occurrence of mafic
rocks as microgranular enclaves in more felsic hosts suggests a direct
chemical input from the mantle, since extremely high temperatures in
excess of ∼1000 °C must be achieved to generate mafic magmas by
⁎ Corresponding author. Tel.: + 90 212 2856112; fax: + 90 212 2856210.
E-mail address: [email protected] (G. Topuz).
H2O-undersaturated partial melting of mafic crustal source rocks (e.g.
Rapp, 1995; Rapp and Watson, 1995; Patiño Douce and McCarthy,
1998; Sisson et al., 2005). Underplating and injection of mafic magmas
into lower crust are generally regarded as thermal triggers for partial
melting of the lower crust (e.g., Hildreth, 1981; Huppert and Sparks,
1988). High-K calc-alkaline I-type granitoids incorporate variable
mantle- and crust-derived components, implied by large discrepancies between crystallization and Nd model ages as well as inherited
zircon components (e.g. McCulloch and Chappell, 1982; Holden et al.,
1987; Paterson et al., 1992; Williams et al., 1992; Roberts and
Clemens, 1993; Galán et al., 1996; Altherr et al., 2000; Altherr and
Siebel, 2002; Kemp and Hawkesworth, 2005). However, the relative
contributions of different end-members, crust vs. mantle, are difficult
0024-4937/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.lithos.2010.01.003
Please cite this article as: Topuz, G., et al., Carboniferous high-potassium I-type granitoid magmatism in the Eastern Pontides: The
Gümüşhane pluton (NE Turkey), Lithos (2010), doi:10.1016/j.lithos.2010.01.003
ARTICLE IN PRESS
2
G. Topuz et al. / Lithos xxx (2010) xxx–xxx
to quantify. The near ubiquity of negative Eu anomalies and relatively
flat HREE patterns (chondrite-normalized) indicate that magma
generation occurred at relatively shallow depths with plagioclase,
rather than garnet, as a major residual or fractionating phase.
About 40% of the exposed pre-Liassic basement rocks of the Eastern
Pontides (NE Turkey) are made up of granitoids. Despite extensive
exposures, these granitoids have received scant attention (e.g. Çoğulu,
1975; Yılmaz, 1974a,b,c). This study deals with the petrogenesis and
age of a relatively large (∼ 400 km2) high-K calc-alkaline I-type
granitoid, the Gümüşhane pluton. The pluton is composed of biotite–
hornblende granodiorite/granite, leucogranite and granophyre, and
was produced by the partial melting of high-potassic mafic to
relatively felsic rocks during the latest Early Carboniferous, at the
wake of the Hercynian low-P–high-T metamorphism.
2. Geological setting
The Eastern Pontides, NE Turkey, represent a Late Cretaceous
submarine magmatic arc, built on a pre-Liassic composite basement
(e.g. Şengör and Yılmaz, 1981; Yılmaz et al., 1997; Okay and Şahintürk,
1997; Okay and Tüysüz, 1999; Topuz et al., 2004a,b, 2007; Fig. 1). The
pre-Liassic basement comprises (i) Early Carboniferous low-pressure–
high-temperature (low-P–high-T) metamorphic units (Okay, 1996;
Topuz and Altherr, 2004; Topuz et al., 2004a, 2007), (ii) Permo-Triassic
high-P–low-T metamorphic units (e.g. Okay and Göncüoğlu, 2004;
Topuz et al., 2004b), (iii) unmetamorphosed granitoids (Yılmaz 1974a,
b,c; Çoğulu, 1975; Bergougnan, 1987; Bozkuş, 1992) and (iv) PermoCarboniferous molassic sedimentary rocks (Okay and Leven, 1996;
Çapkınoğlu, 2003). In places, distinct basement units may occur in
tectonic contact with each other, and are collectively overlain by Liassic
volcanic and volcanoclastic rocks, indicating that the tectonic juxtaposition occurred before the Jurassic. Although the Eastern Pontides were
involved in Alpine subduction and collision events, the pre-Liassic
basement mostly escaped Alpine regional metamorphism.
The transgressive Liassic volcanics and volcanoclastics comprise
calc-alkaline to tholeiitic, basaltic to andesitic lavas, pyroclastics and
volcanogenic sedimentary rocks. In addition, there are thin discontinuous coal and ammonitico rosso horizons (Bergougnan, 1987;
Bektaş et al., 1999; Arslan et al., 1997; Okay and Şahintürk, 1997; Şen,
2007; Kandemir and Yılmaz, 2009). Rapid lateral facies and thickness
Fig. 1. Geological map of the Gümüşhane area with main pre-Liassic basement units and Liassic to post-Eocene rocks. Inset shows main tectonic units of Turkey. 27 and 28 refer to the
sample locations which lie outside the mapping area, but dated in this study.
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changes and the geochemistry of the volcanics suggest formation in
an extensional arc environment (Görür et al., 1993; Koçyiğit and
Altıner, 2002; Şen, 2007). The Liassic volcanics and volcanoclastics
grade into Upper Jurassic to Lower Cretaceous limestones (Fig. 1).
During Late Cretaceous time, northvergent subduction of Neotethys
along the Izmir–Ankara–Ercinzan (IAE) suture gave rise to submarine
magmatic arc activity in the northern part of the Eastern Pontides,
represented by a more than 2 km thick volcano-sedimentary
sequence with local intrusions of hornblende-biotite granitoids (e.g.
Eğin and Hirst, 1979; Manetti et al., 1983; Çamur et al., 1996; Arslan
et al., 1997; Okay and Şahintürk, 1997; Boztuğ and Harlavan, 2008;
Kaygusuz et al., 2008; Kaygusuz and Aydınçakır, 2009). The southern
zone was in a fore-arc position where flyschoid sedimentary rocks
with limestone olistoliths were deposited. The magmatic front runs
approximately north of Gümüşhane in an E–W direction (Okay and
Şahintürk, 1997). Collision between the Eastern Pontides and the
Southern Taurides occurred in the Paleocene. Post-Cretaceous
magmatic rocks include Paleocene plagioleucitites in the southern
zone (Altherr et al., 2008), Early Eocene “adakitic” granitoids (Topuz
et al., 2005) and Middle to Late Eocene calc-alkaline to tholeiitic,
basaltic to andesitic volcanics as well as crosscutting granitoids
exposed throughout the Eastern Pontides (e.g. Tokel, 1977; Arslan
et al., 1997; Boztuğ and Harlavan, 2008; Karslı et al., 2007; Temizel
and Arslan, 2009). In the Eastern Pontides, the ages of the granitoids
range from Carboniferous to the Late Eocene.
3. The Gümüşhane pluton
The Gümüşhane pluton forms an E–W elongated body, 40 km by
10 km in dimensions (Figs. 1 and 2). It is bounded by the pre-Jurassic
rhyolitic pyroclastics to the south. Farther west along the southern
boundary (outside the mapped area), the leucogranite and granophyres of the Gümüşhane pluton are intrusive into the Carboniferous
Kurtoğlu metamorphic complex (Topuz et al., 2007). However, contact
metamorphic effects are absent in the metamorphics, probably due to
the rather shallow emplacement level of the leucogranites and granophyres. Liassic volcanics and volcanoclastics unconformably overlie the
pluton and the rhyolithic pyroclastics, and also form isolated outcrops
on the pluton. In the north, the Gümüşhane pluton and its cover are
thrust over the Middle Eocene volcanic rocks.
Field relationships and geochronological data on the Kurtoğlu
metamorphic complex suggest that the emplacement of the Gümüşhane pluton occurred between the amphibolite-facies metamorphism
(337–323 Ma; Topuz et al., 2007) and the Liassic transgression
(∼200–190 Ma; Kandemir and Yılmaz, 2009). Available geochronological data (K–Ar biotite, Rb–Sr whole-rock, and U–Pb zircon) from
the Gümüşhane pluton show a large scatter ranging from 107 to
535 Ma (Çoğulu, 1975; Moore et al., 1980; JICA, 1986; Bergougnan,
1987). In comparison with field relations the K–Ar biotite ages are
generally too young (107 to 162 Ma), and Rb–Sr whole-rock ‘ages’ are
too old (362 to 535 Ma). Only U–Pb zircon ages (298 to 336 Ma) agree
with the field relations.
The western part of the pluton is densely covered by vegetation. The
best and nearly continuous exposures are found along the road from
Gümüşhane to Tekkeköy in the easternmost portion of the pluton (Figs. 1
and 2). The pluton is generally undeformed, but strongly altered and
weathered. The rocks often have a brick red to pink color, except for
strongly chloritised zones that are greenish. For this study, we mapped
the eastern part of the pluton, roughly 180 km2 in area. The pluton shows
a NE–SW trending compositional zonation, whereby granodiorite in the
NW is followed by granite in the central part, and leucogranite and
granophyre in the SE (Fig. 2). No obvious crosscutting relationship
between the granodiorite in the NW to granite in the central part was
discerned. The boundary between the granite and the leucogranite/
granophyres is marked by a zone (∼100–200 m across) with numerous
dikes of leucogranite and granophyres along the road from Gümüşhane
3
to Tekkeköy, and could not be observed elsewhere due to the poor
outcrop relations. The leucogranites occur mostly near the border region,
while the granophyres predominate towards the SE. The rhyolithic
pyroclastics crop out in form of a narrow belt, ∼10 km long and 1–3 km
across (Fig. 2), at the border zone between the granophyres and
Kurtooğlu metamorphics.
Mafic microgranular enclaves, up to 70 cm in diameter, are confined
to the granodiorite/granite part, and locally so abundant that the
outcrops may be enclave-dominated. The contact between the host
granodiorite/granite and the mafic enclaves is mostly sharp and rarely
gradual. Aplitic dikes (up to 70 cm thick) are common within the
granodiorite/granite part.
The leucogranites are characterized by an equigranular medium to
coarse-grained texture and pink to white color. The granophyres are, on
the other hand, fine-grained. At two locations (marked with 12 and 14 in
Fig. 2), the leucogranites/granophyres are heterogeneously intermingled with the fine-grained mafic rocks. Apart from these outcrops,
they are devoid of mafic minerals.
The clasts in the rhyolitic pyroclastics range from 0.5 mm to 45 cm in
length, and are mostly represented by rhyolite to dacite and subordinately by granophyre and micaschist. Fragments of feldspar, quartz and
devitrified glass are common. Similar to the leucogranites and granophyres, the rhyolithic pyroclastics are free of any mafic mineral. Presence
of the granophyre clasts indicates a relatively late emplacement.
The pluton is pierced by a number of intrusive rocks such as
(i) hypabyssal dacitic to rhyolitic rocks, (ii) microdioritic to gabbroic
stocks, and (iii) numerous 50 cm to 10 m thick dikes of basaltic to
rhyolithic composition (Fig. 2). The rhyolithic pyroclastics and the late
intrusive rocks within the pluton are outside the scope of this paper.
4. Analytical techniques
Whole-rock analyses were performed at Acme Analytical Laboratories Ltd. in Vancouver, Canada. Two hundred milligrams of rock
powder were mixed with 1.5 g of LiBO2 flux in a graphite crucible.
Subsequently, the crucible was placed in an oven and heated to
1050 °C for 15 min. The molten samples were dissolved in 5% HNO3
(ACS grade nitric acid diluted in demineralized water). International
reference samples and reagent blanks were added to the sample
sequence. For analyses of major elements and the trace elements Ba,
Nb, Ni, Sr, Sc, Y and Zr, sample solutions were aspirated into an ICP
emission spectrograph (Jarrel Ash AtomComb 975). For the determination of other trace elements including rare earth elements, the
solutions were aspirated into an ICP mass spectrometer (Perkin-Elmer
Elan 6000). Accuracy for major elements is better than 2% and for
trace elements better than 10% relative.
Mineral analyses were carried out at the Institute of Geosciences at
Heidelberg with a CAMECA SX51 electron microprobe equipped with
five wavelength-dispersive spectrometers. Standard operating conditions were 15 kV accelerating voltage, 20 nA beam current and a beam
diameter of ∼1 µm. Counting times were usually 10 s. Feldspars were
analyzed with a defocused beam (10 µm) in order to minimize loss of
alkalis. Natural and synthetic oxide and silicate standards were used
for calibration. The PAP algorithm (Pouchou and Pichoir, 1984, 1985)
was applied to raw data.
Sr and Nd isotope analyses were performed at the Institute of
Geosciences at Tübingen. Rock powders were dissolved in 52% HF for
four days at 140 °C on a hot plate. Digested samples were dried and
redissolved in 6 N HCl, then dried again and redissolved in 2.5 N HCl.
Sr and light rare earth elements were isolated on quartz columns by
conventional ion exchange chromatography with a 5 ml resin bed of
Bio Rad AG 50W-X12, 200–400 mesh. Nd was separated from other
rare earth elements on quartz columns using 1.7 ml Teflon powder
coated with HDEHP, di(2-ethylhexyl) orthophosphoric acid, as cation
exchange medium. All isotopic measurements were made by thermal
ionization mass spectrometry using a Finnigan MAT 262 mass
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4
Fig. 2. Geological map of the eastern part of the Gümüşhane pluton with sample locations and villages.
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spectrometer. Sr was loaded with a Ta-HF activator on pre-conditioned
W filaments and was measured in single-filament mode. Nd was loaded
as phosphate on pre-conditioned Re filaments and measurements were
performed in a Re double filament configuration. The 87Sr/86Sr isotope
ratios were normalized to 86Sr/88Sr =0.1194 and the 143Nd/144Nd isotope
ratios to 146Nd/144Nd =0.7219. The La Jolla Nd-standard yielded a 143Nd/
144
Nd value of 0.511842 ±0.000008 (reference value=0.511850) and
the NBS 987 Sr standard yielded a 87Sr/86Sr value of 0.710257±0.000008
(reference value =0.710248). Total procedural blanks (chemistry and
loading) were 134 pg for Nd and 333 pg for Sr.
40
Ar/39Ar dating was performed at the Institute of Geosciences at
Heidelberg. Biotite and hornblende separates were obtained by
conventional techniques including cracking, sieving, magnetic and
heavy liquid separation, hand picking and ultrasonic cleaning. Irradiation of the Al-wrapped samples within an evacuated and Cd-shielded
quartz tube with ∼1.5 · 1018 fast neutrons/cm2 was performed in FRG-1
reactor of the Nuclear Research Center Geesthacht, Germany. Age and
irradiation gradient monitoring was achieved by an in-house muscovite
standard ‘BMus/2’ (328.5 ± 2.2 Ma, 2σ; Schwarz and Trieloff, 2007a). Ar
was extracted in a resistance-heated Ta furnace in a double-vacuum
system up to 1600 °C. The blank level at 1400 °C was below the mass
spectrometer detection limit of c. 10− 6 nl 40Ar. First, H2O and CO2 from
the extracted gas were frozen by using an acetone–CO2 mixture. For
further cleaning, Zr–Al getters were used. Ar isotope compositions were
measured in a MAT GD-150 gas mass spectrometer (0.38 T permanent
magnet, 180°, 5 cm radius of curvature) with no detectable argon
background. Constants used for age calculations are those recommended by Steiger and Jäger (1977). 40Ar/39Ar age uncertainties
comprise the errors of the 40Ar/39Ar ratios of irradiation monitors. Age
errors of the monitor were included in parentheses. Measured 40Ar/39Ar
data were corrected for irradiation interferences. For comparison of
40
Ar/39Ar ages with U/Pb ages the miscalibration of K and U decay series
must be taken into account, increasing the Ar–Ar ages by c. 1% (e.g.
Schwarz and Trieloff, 2007b). All error assignments of isotope ratios and
age given within this paper are within ± 2σ.
5
For U–Pb analyses through TIMS, zircon fractions consisting of 2–3
morphologically identical grains (19–29 µg) were selected for
analyses. All fractions were washed in 6 N HCl and 6 N HNO3 prior
to dissolution. A mixed 205Pb/235U tracer solution was added to the
grains. Dissolution by vapour digestion was performed in PTFE microcapsules in a Parr acid digestion bomb (Parrish, 1987). The bomb was
placed in a temperature controlled oven at 210 °C for up to one week
in 22 N HF and for one day in 6 N HCl. Separation and purification of U
and Pb were carried out on Teflon columns with a 40 µl bed of AG1-X8
(100–200 mesh) anion exchange media and the separation procedure
was similar to those described by Krogh (1973) and Roddick et al.
(1987). All isotopic measurements were made on a Finnigan MAT 262
mass spectrometer at the Institute of Geosciences at Tübingen. Pb and
U were loaded together with a Si-gel onto a single Re filament and
measured at ∼ 1250 °C and 1350 °C, respectively. Total procedural
blanks were b10 pg for Pb and for U. A factor of 0.9‰ per atomic mass
unit for instrumental mass fractionation was applied to all isotope
analyses, using NBS SRM 981 as reference material. Initial common Pb
remaining after correction for tracer and blank was corrected using
values from the Stacey and Kramers (1975) model. The decay
constants for U are those given in Jaffey et al. (1971). U–Pb age
calculations were done by the PbDat program (Ludwig, 1993) and the
data were plotted using Isoplot/Ex (Ludwig, 2003).
LA-ICP-MS analyses for this study were performed at the Institute
for Geosciences at Mainz utilising a system consisting of a New Wave
213 nm laser coupled to an Agilent 7500ce quadrupole ICP-MS.
Directly before U–Pb dating, samples were cleaned by polishing the
surface with γ-alumina powder to remove carbon coating, then put in
an ultrasonic bath for 5 min with milli-Q water and finally dried
with ethanol-soaked kimwipe. Further cleaning was achieved by
pre-ablating the analysis spot with 5 single laser shots with a beam
diameter of 40 µm. Each analysis consists of background measurement of 40 s followed by 30 s of analysis. U–Pb age data was collected
by ablating zircons with laser beam diameters of 30 µm, a beam
energy density of ca. 3.5 J/cm2 and a repetition rate of 10 Hz. The
Table 1
Selected microprobe analyses of hornblende and biotite from the Gümüşhane I-type granitoid pluton, NE Turkey.
Hornblende
Biotite
Sample 1
1
28
28
93
93
267
267
1E
93E
93E
1
1
28
28
28
28
267
267
Rock
type
BHG
BHG
BHG
BHG
BHG
BHG
BHGD
BHGD
ME
ME
ME
BHG
BHG
BHG
BHG
BHG
BHG
BHGD
BHGD
SiO2
TiO2
Al2O3
Cr2O3
FeOtot
MnO
MgO
CaO
Na2O
K2O
Total
42.43
1.28
8.31
0.01
23.58
0.85
7.58
10.40
1.47
0.94
96.86
43.05
1.46
8.04
0.00
22.80
0.53
8.21
10.48
1.42
0.98
96.96
42.85
1.26
8.11
0.00
26.26
0.72
5.45
10.33
1.76
0.92
97.65
41.89
2.00
8.59
0.03
26.78
0.60
4.60
10.18
2.02
0.77
97.36
44.50
0.96
7.61
0.04
20.45
0.75
9.46
10.94
1.34
0.81
96.87
45.67
1.61
7.26
0.04
18.25
0.55
10.95
11.00
1.38
0.82
97.54
49.87
0.38
4.66
0.06
16.11
0.60
13.29
11.12
0.88
0.32
97.29
47.78
0.90
6.18
0.00
16.65
0.46
12.32
10.95
1.12
0.60
96.95
43.04
1.28
7.91
0.00
24.32
1.12
6.53
10.72
1.68
0.94
97.54
44.50
0.96
7.61
0.04
20.45
0.75
9.46
10.94
1.34
0.81
96.87
45.67
1.61
7.26
0.04
18.25
0.55
10.95
11.00
1.38
0.82
97.54
35.16
3.44
13.31
0.00
26.76
0.34
8.19
0.04
0.11
8.12
95.46
35.38
3.57
13.16
0.01
25.97
0.22
8.20
0.02
0.12
8.78
95.43
34.70
3.74
12.98
0.00
28.45
0.48
5.71
0.00
0.06
9.00
95.12
34.27
4.35
13.72
0.00
28.21
0.45
4.96
0.00
0.06
8.94
94.96
34.90
3.70
13.32
0.02
28.46
0.48
5.39
0.01
0.06
9.02
95.35
33.91
3.25
13.54
0.01
28.67
0.49
5.38
0.02
0.09
8.72
94.08
35.90
3.11
13.40
0.00
21.53
0.30
11.57
0.02
0.10
8.44
94.36
36.59
3.40
13.56
0.02
21.28
0.28
11.26
0.00
0.16
9.16
95.70
Cations on the basis of 23 oxygens and 15 cations excluding Na
Si
6.624
6.684
6.751
6.633
6.826
6.895
AlIV
1.376
1.316
1.249
1.367
1.174
1.105
AlVI
0.153
0.156
0.257
0.239
0.202
0.187
Ti
0.150
0.170
0.149
0.239
0.111
0.183
3+
Fe
0.288
0.198
0.000
0.000
0.188
0.000
Cr
0.001
0.000
0.000
0.004
0.005
0.005
Fe2+
2.790
2.763
3.460
3.554
2.435
2.304
Mn
0.112
0.070
0.096
0.081
0.097
0.070
Mg
1.764
1.900
1.280
1.088
2.163
2.465
Ca
1.740
1.743
1.744
1.731
1.798
1.779
Na
0.445
0.427
0.537
0.621
0.399
0.404
K
0.187
0.194
0.185
0.156
0.159
0.158
Total
15.632 15.622 15.708 15.712 15.557 15.555
XMg
0.364
0.391
0.270
0.234
0.452
0.517
and K for hornblende, and 11 oxygens for biotite
7.377
7.149
6.737
6.826
6.895
2.778
0.623
0.851
1.263
1.174
1.105
1.222
0.189
0.239
0.196
0.202
0.187
0.017
0.042
0.101
0.151
0.111
0.183
0.204
0.029
0.000
0.068
0.188
0.000
0.000
0.007
0.000
0.000
0.005
0.000
0.000
1.964
2.083
3.115
2.435
2.304
1.768
0.075
0.058
0.148
0.097
0.070
0.022
2.931
2.748
1.524
2.163
2.465
0.965
1.762
1.755
1.798
1.798
1.779
0.003
0.252
0.325
0.510
0.399
0.404
0.016
0.060
0.115
0.188
0.159
0.158
0.819
15.313 15.425 15.698 15.557 15.555
7.814
0.595
0.569
0.329
0.470
0.517
0.353
2.795
1.205
0.020
0.212
0.000
0.001
1.716
0.015
0.966
0.002
0.018
0.884
7.834
0.360
2.797
1.203
0.030
0.227
0.000
0.000
1.918
0.033
0.687
0.000
0.010
0.926
7.831
0.264
2.763
1.237
0.066
0.264
0.000
0.000
1.902
0.031
0.596
0.000
0.010
0.920
7.789
0.239
2.802
1.198
0.062
0.223
0.000
0.001
1.911
0.033
0.646
0.001
0.009
0.924
7.810
0.253
2.769
1.231
0.072
0.200
0.000
0.001
1.957
0.034
0.654
0.002
0.014
0.908
7.842
0.250
2.797
1.203
0.028
0.182
0.000
0.000
1.403
0.020
1.344
0.001
0.015
0.839
7.832
0.490
2.813
1.187
0.041
0.197
0.000
0.001
1.368
0.018
1.291
0.000
0.024
0.898
7.838
0.486
ARTICLE IN PRESS
6
Table 2
Whole-rock analyses of selected samples from the Gümüşhane I-type granitoid pluton.
Sample
1
2
3
7
17
19
93
109
181
267
268
307
Rock type
BHG
BHG
BHG
BHG
BHG
BHG
BHG
BHGD
BHGD
BHGD
BHGD
BHGD
SiO2
TiO2
Al2O3
Fe2Otot
3
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Total
Sc
Ni
Co
V
Cu
Zn
Cs
Rb
Ba
U
Th
Pb
Sr
Nb
Ta
Zr
Hf
Y
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Mg#
ASI
(La/Yb)cn
Eu/Eu*
71.32
0.28
14.2
2.66
0.03
0.79
1.76
3.39
4.12
0.07
1.30
99.92
5
0.6
3.8
44
0.7
21
2.4
149
706
3.0
16.3
6.4
165
7.1
0.8
144
4.4
19.0
34.6
61.2
5.61
19.3
3.10
0.79
2.91
0.46
2.76
0.57
1.62
0.30
2.00
0.29
37.0
1.07
11.66
0.80
70.59
0.28
14.53
2.62
0.04
0.86
1.61
3.48
4.22
0.07
1.60
99.90
6
0.6
0.9
38
26.2
33
2.0
144
767
2.1
15.5
8.8
166
7.7
0.8
143
4.2
20.0
39.3
70.1
6.57
22.5
3.90
0.88
2.81
0.53
3.06
0.60
1.79
0.33
2.13
0.29
39.4
1.10
12.44
0.81
68.45
0.34
14.67
3.09
0.05
0.71
2.03
3.48
3.62
0.09
3.40
99.92
7
1.2
4.9
43
0.9
35
3.7
147
681
3.3
14.7
5.2
152
8.4
0.8
161
5.0
22.0
30.7
57.3
5.68
20.5
3.80
0.83
3.13
0.60
3.42
0.70
1.99
0.36
2.42
0.32
31.3
1.10
8.55
0.74
72.10
0.20
13.97
2.62
0.05
0.52
1.58
3.48
4.25
0.05
1.10
99.87
5
0.4
3.4
22
1.6
25
2.4
185
617
8.2
32.1
5.0
173
12.2
1.6
165
5.3
31.2
39.0
74.2
7.85
26.6
5.03
0.74
4.56
0.82
4.76
1.02
3.27
0.56
3.82
0.60
28.2
1.06
6.88
0.47
68.67
0.37
15.34
3.70
0.07
1.36
2.91
3.55
3.15
0.07
0.60
99.83
8
2.2
6.2
82
5.8
28
2.6
147
769
3.7
16.1
5.9
298
8.5
1.0
173
5.0
20.4
30.2
56.1
5.96
21.1
3.68
0.97
3.42
0.58
3.26
0.69
2.14
0.36
2.37
0.38
42.1
1.05
8.59
0.84
70.86
0.25
13.25
2.70
0.06
0.65
1.18
4.29
3.42
0.06
3.20
99.93
7
2.3
3.8
19
1.9
29
6.1
116
602
2.6
13.8
2.8
97
7.5
0.8
150
4.8
28.2
32.7
63.3
6.53
26.3
5.50
1.02
4.83
0.68
4.37
0.85
2.28
0.40
2.70
0.35
32.3
1.03
8.17
0.61
67.39
0.33
15.67
3.69
0.06
1.35
2.65
3.60
3.63
0.08
1.40
99.83
9
3.7
8.0
67
3.4
27
2.5
155
764
3.4
13.4
4.8
269
8.0
0.9
140
4.1
21.3
19.4
38.0
4.46
16.5
3.44
0.89
3.26
0.58
3.45
0.70
2.19
0.36
2.34
0.38
42.0
1.07
5.59
0.81
62.90
0.47
15.48
5.44
0.09
3.10
4.78
2.93
3.00
0.08
1.50
99.81
15
11.0
16.4
122
9.6
34
3.9
123
627
3.2
12.9
2.1
234
7.5
0.8
122
3.4
18.9
22.3
44.3
4.87
17.5
3.35
0.83
3.21
0.55
3.18
0.65
2.01
0.31
2.07
0.32
53.0
0.92
7.26
0.77
65.88
0.50
15.16
5.01
0.06
2.94
1.99
3.05
2.91
0.10
2.20
99.80
13
6.4
12.3
100
2.6
52
3.3
116
604
4.2
18.1
2.8
196
8.3
0.9
202
5.7
22.0
26.9
45.3
5.62
20.4
3.89
0.85
3.77
0.63
3.49
0.74
2.13
0.35
2.26
0.36
53.7
1.29
8.02
0.68
64.81
0.47
15.08
4.92
0.08
2.67
3.96
2.87
3.15
0.10
1.70
99.80
15
9.1
13.6
105
4.7
30
4.2
115
547
2.9
11.7
1.9
190
7.4
0.8
101
3.1
19.2
16.6
33.1
4.07
16.4
3.36
0.74
3.30
0.48
3.44
0.69
2.1
0.33
2.16
0.33
51.8
0.98
5.18
0.68
64.01
0.48
15.47
5.26
0.09
2.95
4.28
2.91
3.09
0.10
1.20
99.83
15
10.9
14.5
110
6.5
30
3.2
119
548
3.0
13.0
3.7
218
6.9
0.8
128
3.6
17.3
24.5
45.6
4.81
16.9
3.24
0.76
3.18
0.51
2.89
0.61
1.86
0.30
2.00
0.31
52.6
0.97
8.26
0.72
66.27
0.47
15.1
4.87
0.09
2.13
2.77
3.20
3.42
0.10
1.40
99.85
13
4.4
11.8
88
2.0
30
2.0
113
593
2.2
11.1
5.5
256
7.7
0.8
120
3.3
19.9
20.0
40.0
4.59
17.1
3.42
0.79
3.45
0.58
3.39
0.70
2.17
0.35
2.21
0.34
46.4
1.08
6.10
0.70
Rock types: BHG = biotite–hornblende granite, BHGD = biotite–hornblende granodiorite, ME = mafic enclave, A = aplite, LG = leucogranite, G = granophyre; Mg# = 100 × MgO/
(MgO + FeOtot) in molar porportions; ASI = aluminium saturation index = molar Al2O3/(CaO + Na2O + K2O); (La/Yb)cn = chondrite-normalized La/Yb ratio; oxides are given in wt.%,
1E elements in
17E
93E
268E
307E
310E
3B
10B
H2
10C
11C
328
335
trace
ppm.
ME
ME
ME
ME
ME
ME
A
aerosol produced during ablation was carried to the ICP-MS in a mixed
Ar–He carrier gas at a flow rate of 1.3 l/minute. Isotopes were
measured in time-resolved mode. For U–Pb dating, dwell times for
each isotope for each mass scan are 10 ms for 232Th and 238U, 30 ms
for 201Hg, 204Hg + Pb and 206Pb and 50 ms for 207Pb and 208Pb. Th and
U concentrations, 206Pb/204Pb ratios, as well as 207Pb/235U, 206Pb/238U
and 208Pb/232Th ages are calculated offline from time-resolved raw
counts provided by the ICP-MS. Here, the zircon standard PL
(Plesovice; Sláma et al., 2008) was used as a primary standard to
correct for laser-induced as well as ICP-induced fractionation by
integrating the exact same time segments for each sample and
standard zircon (Jackson et al., 2004). Accuracy of 207Pb/235U, 206Pb/
238
U and 208Pb/232Th ages is currently given as 1.5%, based on regular
analysis of the zircon standards GJ-1 (Jackson et al., 2004; also used to
calculate U and Th concentrations), 91,500 (Wiedenbeck et al., 1995)
and Mud Tank (Black and Gulson, 1978).
LG
G
G
G
G
G
5. Petrography and mineral compositions
The granodiorite/granite is medium to coarse-grained and consists
of plagioclase, orthoclase, quartz, biotite, hornblende, allanite,
magnetite, zircon and apatite. Plagioclase (An35-10) is subhedral to
euhedral and normally zoned. In many samples it is transformed to
sericite, partly also to epidote. Orthoclase [XK = K/(K + Na + Ca) =
0.96–0.82] is generally xenomorphic and rarely microperthitic. Quartz
locally forms large grains but also fills the interstitial spaces left
behind from the early crystallizing plagioclase and mafic minerals.
Biotite is characterized by small intra-sample compositional variations with XMg [=MgO/(MgO + FeOtot)] varying by no more than about
0.05, while inter-sample variations are much larger (XMg = 0.23 to
0.50; Table 1). Ti contents are relatively low (0.18 to 0.30 cations per
11 oxygens). In most samples, biotite is strongly chloritised and/or may
be partially replaced by prehnite and/or pumpellyite. Quite often, the
ARTICLE IN PRESS
7
Table 2 (continued)
1E
17E
93E
268E
307E
310E
3B
10B
H2
10C
11C
328
335
ME
ME
ME
ME
ME
ME
A
LG
G
G
G
G
G
54.30
1.01
16.92
10.96
0.17
3.11
3.41
4.98
2.40
0.17
2.50
99.93
28
2.2
15.3
180
1.8
85
4.6
163
560
7.0
14.1
5.7
147
43.3
4.1
256
8.5
98.6
10.5
27.7
4.26
22.4
8.20
0.79
10.31
2.09
13.51
2.87
8.86
1.60
10.63
1.55
36.0
1.00
0.67
0.26
57.60
0.76
16.81
8.04
0.18
3.68
4.93
4.53
1.76
0.15
1.40
99.83
25
7.0
14.1
180
7.4
49
2.9
115
301
5.0
16.4
7.5
200
11.7
1.3
154
4.9
36.3
20.8
48.5
6.42
26.3
5.61
0.97
5.55
1.01
5.89
1.31
4.04
0.66
4.31
0.70
47.5
0.92
3.25
0.53
52.90
0.71
16.39
9.07
0.28
5.08
6.70
4.44
1.98
0.09
2.10
99.78
31
3.6
20.3
208
10.3
34
2.9
104
392
6.0
8.3
9.2
158
10.0
1.3
100
3.5
44.1
25.4
64.8
8.34
31.2
6.95
1.25
6.56
1.21
7.39
1.56
4.88
0.83
5.63
0.91
52.6
0.76
3.04
0.57
54.91
0.68
16.64
8.17
0.15
5.06
6.05
3.36
2.54
0.11
2.10
99.78
28
12.0
24.6
206
44.2
34
6.0
127
427
3.5
12.9
8.4
229
7.6
0.9
95
2.8
20.7
17.9
38.4
4.54
16.8
3.68
0.84
3.59
0.64
3.69
0.81
2.34
0.38
2.48
0.39
55.1
0.86
4.87
0.71
53.92
0.64
17.31
8.24
0.22
4.6
6.66
3.89
2.14
0.10
2.10
99.8
26
6.5
22.5
184
7.5
29
2.8
87
376
1.9
8.4
7.4
253
7.6
0.7
90
2.6
27.8
19.9
49.2
6.26
24.0
4.89
1.27
4.62
0.83
4.8
1.02
3.04
0.50
3.31
0.53
52.5
0.83
4.05
0.82
59.37
0.51
15.34
6.67
0.13
4.85
5.42
2.98
2.82
0.08
1.60
99.8
23
24.6
22.0
144
20.6
28
3.0
97
546
2.3
10.5
3.1
211
6.4
0.6
121
3.3
23.8
19.4
40.9
4.95
18.6
3.94
0.93
3.93
0.71
3.99
0.87
2.64
0.41
2.65
0.41
59.0
0.86
4.94
0.72
77.24
0.02
12.50
0.69
0.01
0.04
0.40
3.48
4.85
0.01
0.80
100.04
1
0.2
b 0.5
6
1.8
11
2.4
226
104
9.2
36.8
17.9
20
13.7
4.8
80
5.4
57.5
10.7
25.4
3.11
12.7
4.10
0.12
5.12
1.08
7.16
1.67
5.16
0.95
7.42
1.07
10.3
1.07
0.97
0.08
76.17
0.09
13.06
0.88
0.02
0.04
0.33
3.41
5.21
0.01
0.80
100.02
7
0.3
0.6
b5
1.7
14
1.1
124
60
1.8
9.6
2.9
18
9.7
0.6
208
6.4
18.6
37.7
83.2
9.01
32.5
5.90
0.20
3.67
0.56
2.87
0.56
1.60
0.27
1.87
0.25
8.3
1.10
13.58
0.13
78.57
0.03
12.04
0.66
0.02
0.06
0.18
3.36
4.31
0.01
0.70
99.95
4
1.2
1.1
b8
1.1
19
2.5
146
413
2.8
10.2
24.4
43
7.3
0.8
75
2.9
27.8
17.5
38.7
5.16
20.8
5.16
0.48
4.95
0.83
4.44
0.92
2.77
0.35
2.50
0.36
15.2
1.14
4.71
0.29
78.31
0.05
12.19
0.28
0.01
0.12
0.48
2.98
4.25
0.01
1.20
99.88
4
0.4
b 0.5
b5
1.8
3
1.2
147
308
3.6
6.9
10.1
19
10.3
1.2
87
4.4
45.2
19.6
43.1
5.04
20.4
5.30
0.36
5.18
0.93
5.8
1.40
4.09
0.68
4.80
0.62
45.9
1.17
2.75
0.21
77.31
0.06
12.35
1.14
0.02
0.16
0.46
3.19
4.20
0.01
1.10
100.00
5
2.1
b 0.5
b5
4.4
28
1.3
150
309
3.1
8.3
12.2
26
12.0
1.2
93
4.0
47.7
19.2
42.2
5.00
22.6
5.80
0.40
6.24
1.19
7.02
1.45
4.13
0.71
4.59
0.59
21.7
1.16
2.82
0.20
77.02
0.07
13.07
0.31
0.01
0.04
0.26
2.96
4.77
b0.01
1.40
99.96
4
1.4
0.6
b5
0.5
21
3.3
207
338
2.1
16.6
5.1
39
14.4
1.4
92
3.7
28.7
12.8
26.7
4.21
19.8
4.77
0.32
5.18
0.98
5.88
1.19
3.46
0.57
3.74
0.53
20.3
1.24
2.30
0.20
76.56
b0.01
12.39
0.90
0.01
0.03
0.36
3.55
4.91
b0.01
1.30
99.95
1
b0.1
b0.5
11
2.0
27
1.2
191
254
4.4
15.9
4.3
18
14.1
1.1
94
5.3
45.8
8.3
26.8
3.96
16.4
5.08
0.30
6.22
1.00
7.34
1.52
4.46
0.69
4.40
0.66
6.2
1.05
1.27
0.16
host biotite sheets are deformed around secondary prehnite/pumpellyite grains. Primary inclusions in biotite are magnetite, apatite and
zircon.
Clinopyroxene is only present in some samples of the granodiorite
and in some mafic enclaves. It has the composition Wo39-47En44-40Fs13-17.
Hornblende frequently contains inclusions of biotite (chlorite), apatite,
magnetite, zircon and clinopyroxene (the latter only in granodiorite).
Representative hornblende analyses are given in Table 1. Overall, the
contents of Al2O3 and K2O vary from 5.4 to 8.7 wt.% and from 0.3 to
1.5 wt.%, respectively. Individual hornblende crystals are often zoned,
whereby the brownish cores show higher Ti, Al, Na and K contents than
the greenish rims. Hornblende may also be chloritised.
Mafic microgranular enclaves consist of hornblende (sometimes
overgrowing earlier clinopyroxene), biotite, magnetite, plagioclase,
zircon and apatite. In some enclaves, small amounts of quartz and/or
orthoclase are also present. Alteration products are similar to those of
the granite. Aplitic dikes are characterized by a fine-grained equigranular texture.
The leucogranite is relatively coarse-grained. The K-feldspars
display XK values of 0.87–0.97, and the plagioclases An24-0. Sporadic
biotite grains are totally altered. Secondary phases include sericite,
chlorite, calcite and Fe-oxides. Granophyres show micrographic and/
or spherulitic textures (intergrowth of K-feldspar + quartz), suggesting rapid cooling. Phenocrysts of quartz and plagioclase have
subidiomorphic corroded outlines, and locally form strained crystals.
Plagioclase in the granophyres is represented by albitic composition
(An0-3), and K-feldspar by XK of 0.94–0.99.
6. Bulk-rock chemistry
Selected chemical analyses of representative samples from the
Gümüşhane pluton are given in Table 2 and Harker plots are
ARTICLE IN PRESS
8
presented in Fig. 3. SiO2 contents vary from about 62 to 72 wt.% in the
granodiorite/granite and from 76 to 79 wt.% in the leucogranite,
granophyre and aplite. The microgranular mafic enclaves have SiO2
contents between 53 and 60 wt.%. The aluminium saturation index
[ASI = molar Al2O3/(CaO + Na2O + K2O)] increases with SiO2 from
about 0.76 to 1.10 in the granodiorite/granite, and from 1.05 to 1.24 in
the leucogranite, granophyre and aplite (Fig. 3). Elevated ASI values
are shown by the altered samples. The rocks belong to the high-K
series (Peccerillo and Taylor 1976; Fig. 3). With increasing SiO2, the
abundances of TiO2, Al2O3, Fe2Otot
3 , MnO, MgO, CaO, P2O5 and Sc
decrease, but those of K2O and Rb increase (Fig. 3). Sr and Ba first
increase and then decrease. Among the microgranular mafic enclaves,
sample 1E is an exception in having higher ASI, TiO2, Fe2Otot
3 , Rb, Y and
lower Eu/Eu* than the other enclaves (Fig. 3). The K2O/Na2O ratios
range from 0.39 to 0.95 in mafic microgranular enclave, from 0.80 to
1.22 in the granodiorite/granite, and from 1.28 to 1.61 in the
leucogranite, granophyre and aplite. Total rare earth element contents
range from 80 to 180 ppm.
On primitive mantle-normalized element concentration diagrams
(Fig. 4), all rock types display marked negative anomalies in Ba, Nb/Ta,
Sr, P and Ti, but positive anomalies in K and Pb, indicating
fractionation of plagioclase, hornblende, biotite, pyroxene, apatite
and magnetite. The magnitude of the negative anomalies is more
pronounced in the leucogranite, granophyre and aplite.
Chondrite-normalized (cn) rare earth element (REE) patterns of the
granite and granodiorite samples (Fig. 5) are generally characterized by
concave-upward shapes [(La/Yb)cn = 5.2–12.4] and variable negative Eu
anomalies [Eu/Eu* = Eucn/(Smcn · Gdcn)0.5] of 0.47–0.84 (Table 2). There
is almost no fractionation of the middle relative to the heavy REE. Most
microgranular mafic enclaves have low (La/Yb)cn (3.04–4.94) and
significant negative Eu anomalies (Eu/Eu* = 0.82–0.53). Sample 1E is an
exception with (La/Yb)cn = 0.67 and Eu/Eu*= 0.26 (Table 2; Fig. 5). This
sample is also characterized by relatively high contents of Nb and Ta, Zr
and Hf as well as of HREE (Figs. 4 and 5). The leucogranite shows LREE/
HREE enrichment similar to the granodiorite/granite, but has more
pronounced negative Eu anomaly (Fig. 5). The aplitic dike (sample 3B)
Fig. 3. Selected Harker variation diagrams for the different rock types from the Gümüşhane pluton. 1E and 181 are sample numbers. BHG: biotite–hornblende granite; BHGD: biotite–
hornblende granodiorite; ME: mafic enclave, A: aplite, LG: leucogranite, G: granophyre.
ARTICLE IN PRESS
9
Fig. 3 (continued).
Fig. 4. Primitive mantle-normalized element abundance patterns of granitoid rocks from the Gümüşhane pluton. For normalizing values and sequence of elements, see Sun and
McDonough (1989). All samples are characterized by negative anomalies of Ba, Nb–Ta, Sr, P and Ti, and positive anomalies of K and Pb.
ARTICLE IN PRESS
10
Fig. 5. Chondrite-normalized rare earth element diagrams. Normalizing values were taken from Palme and Jones (2005).
has nearly flat REE pattern with pronounced Eu anomaly (Fig. 5). REE
patterns of the granophyres vary between that of the leucogranite and
the aplitic dike.
7. Radiometric dating
To constrain the emplacement age of the Gümüşhane pluton,
stepwise 40Ar/39Ar dating on biotite and hornblende separates from
four samples (7, 17, 27, and 28) as well as conventional and in-situ
LA-ICP-MS U–Pb dating on zircons from samples 7 and 28 were
carried out. Samples 7 and 17 are from the eastern end and samples 27
and 28 from the central part of the pluton (Figs. 1 and 2).
The samples 7 and 17 are medium-grained granite to granodiorite,
comprising K-feldspar, plagioclase, quartz, biotite, hornblende, and
minor magnetite, zircon and apatite. Biotite is locally chloritised and
replaced by pumpellyite, albite and hematite. Hornblende contains
primary inclusions of magnetite, zircon and apatite, and is replaced by
epidote, prehnite, ±pumpellyite and chlorite along the fractures. The
samples 27 and 28 contain the same mineral assemblage as samples 7
and 17 but are less altered.
7.1.
40
Ar/39Ar dating
The samples (7 and 17) yielded inconsistent and highly divergening biotite and hornblende ages, ranging from 73 to 304 Ma. The
inconsistency in biotite and hornblende ages is ascribed to a partial or
total resetting due to a later hydrothermal overprint. The relatively
fresh samples (27 and 28), on the other hand, gave consistent biotite
and hornblende ages within the range of error (∼317 Ma). For the
sake of brevity only the age spectra of the latter samples (27 and 28)
are discussed below, and given in Table 3 and Fig. 6.
Biotites from samples 27 and 28 yielded integrated age values of
316.8 ± 3.6 and 316.0 ± 3.9 Ma, respectively. The spectra are slightly
hump-shaped (Fig. 6a and c), probably a feature of Ar redistribution
during irradiation, when 39Ar produced from 39 K is recoiled into a less
retentive K-poor phase (such as chlorite). This leads to younger
apparent ages at the beginning of step heating, when the recoiled 39Ar
is released. During the degassing of the main phase (biotite), the same
amount of 39Ar which was released at lower temperatures is missing
at higher temperatures, leading to higher apparent ages. In principle,
the integrated ages of those biotites are the geological significant
cooling ages. The Ca/K ratios increase at the last incremental steps,
most probably caused by Ca-rich inclusions (apatite and/or hornblende) in the biotites.
The spectra of 27-Hbl and 28-Hbl are relatively flat, while the Ca/K
spectra show increasing values with temperature (Fig. 6b and d). Both
spectra are slightly hump-shaped (27-Hbl more than 28-Hbl) at the
beginning of the gas release, which can be explained by the presence of
a small portion of biotite in the dated separate. At lower temperatures,
biotite would degas first, leading to the lower Ca/K ratios. The Ca/K
ratios will increase at higher temperatures when biotite is nearly
degassed and hornblende is the main degassing phase. In any case, the
total ages of 316.5 ± 4.3 Ma for 28-Hbl and of 316.0 ± 3.9 Ma (2σ) for
28-Bt are identical. Thus, even a small contamination of biotite in the
hornblende separate would not change the apparent age.
The spectrum of 27-Hbl is partially degassed, yielding a total gas
age of 309.2 ± 5.3 Ma (Fig. 6b). In principle, this may be due to partial
thermal degassing of the hornblende. However, this is unlikely
ARTICLE IN PRESS
11
Table 3
Ar–Ar data for biotite and hornblende separates from the Gümüşhane pluton.
40
Ar*
(%)
37
27 Bt (J = 52.93 ± 0.12 (0.41)) * 10− 4)
1
400
0.037 ± 0.081
2
500
1.342 ± 0.081
3
580
4.668 ± 0.078
4
640
9.723 ± 0.079
5
680
9.571 ± 0.063
6
720
8.066 ± 0.037
7
770
10.007 ± 0.058
8
810
8.105 ± 0.079
9
850
5.471 ± 0.084
10
900
8.871 ± 0.046
11
950
15.186 ± 0.078
12
990
22.74 ± 0.14
13
1030
15.141 ± 0.086
14
1060
3.119 ± 0.060
15
1150
1.27 ± 0.11
16
1400
0.503 ± 0.075
Total
123.83 ± 0.33
0.44 ± 0.96
24.2 ± 1.5
74.4 ± 1.2
94.11 ± 0.78
97.11 ± 0.64
98.10 ± 0.54
91.63 ± 0.56
82.85 ± 0.82
97.2 ± 1.5
97.11 ± 0.54
99.14 ± 0.64
99.54 ± 0.82
98.85 ± 0.60
92.0 ± 2.0
72.3 ± 6.0
68 ± 10
86.28 ± 0.24
0.162 ± 0.051
0.147 ± 0.006
0.029 ± 0.002
0.032 ± 0.007
0.013 ± 0.001
0.006 ± 0.001
0.014 ± 0.001
0.057 ± 0.002
0.034 ± 0.002
0.034 ± 0.001
0.032 ± 0.001
0.028 ± 0.001
0.066 ± 0.003
0.323 ± 0.013
1.103 ± 0.044
1.265 ± 0.070
0.061 ± 0.003
27 Hbl (J = 53.12 ± 0.34 (0.51)) * 10− 4)
1
600
0.061 ± 0.037
2
700
0.283 ± 0.027
3
800
0.277 ± 0.031
4
900
4.095 ± 0.059
5
950
3.149 ± 0.021
6
1000
1.230 ± 0.035
7
1070
6.096 ± 0.058
8
1300
0.015 ± 0.032
Total
15.20 ± 0.12
1.39 ± 0.84
12.5 ± 1.2
37.2 ± 4.2
90.6 ± 1.3
94.71 ± 0.66
88.2 ± 2.3
92.18 ± 0.90
11 ± 23
64.85 ± 0.50
28 Bt (J = 53.13 ± 0.26 (0.47)) * 10− 4)
1
400
0.093 ± 0.067
2
500
1.491 ± 0.016
3
580
6.203 ± 0.091
4
640
8.695 ± 0.060
5
680
8.27 ± 0.11
6
720
12.011 ± 0.066
7
760
7.893 ± 0.059
8
820
10.247 ± 0.092
9
870
7.401 ± 0.030
10
920
23.37 ± 0.10
11
950
18.668 ± 0.066
12
980
12.181 ± 0.083
13
1010
4.721 ± 0.055
14
1150
1.289 ± 0.042
15
1400
0.950 ± 0.064
Total
123.48 ± 0.28
28 Hbl (J = 53.41 ± 0.23 (0.46)) * 10− 4)
1
600
0.137 ± 0.018
2
700
0.5320 ± 0.0050
3
780
0.356 ± 0.011
4
840
0.8009 ± 0.0065
5
880
2.346 ± 0.014
6
920
3.924 ± 0.011
7
960
1.189 ± 0.013
8
1000
1.567 ± 0.015
9
1030
3.301 ± 0.020
10
1060
1.581 ± 0.022
11
1100
0.0627 ± 0.0092
12
1400
0.0319 ± 0.0068
Total
15.837 ± 0.053
Step
T
(°C)
40
Ar*
(10− 6 cm3 STP/g)
Ar/39Ar
40
Ar*/39Ar
39
Ar
(%)
Age
(Ma)
10 ± 21
9.48 ± 0.57
35.53 ± 0.60
36.84 ± 0.30
36.89 ± 0.27
37.27 ± 0.23
37.36 ± 0.22
40.15 ± 0.42
39.98 ± 0.62
38.62 ± 0.26
37.70 ± 0.22
37.15 ± 0.32
36.86 ± 0.23
36.74 ± 0.75
36.6 ± 3.1
32.2 ± 4.9
36.27 ± 0.35
0.11
4.15
3.85
7.73
7.60
6.34
7.85
5.91
4.01
6.73
11.80
17.93
12.03
2.49
1.02
0.46
100.00
89 ± 188 (188)
88.3 ± 5.2 (5.2)
310.9 ± 4.8 (5.3)
321.4 ± 2.5 (3.3)
321.8 ± 2.3 (3.1)
324.9 ± 2.0 (2.9)
325.5 ± 1.9 (2.9)
347.6 ± 4.9 (5.4)
346.3 ± 4.9 (5.4)
335.6 ± 2.2 (3.1)
328.2 ± 1.9 (2.9)
323.9 ± 2.6 (3.4)
321.6 ± 1.9 (2.9)
320.6 ± 6.0 (6.4)
319 ± 25 (25)
284 ± 40 (40)
316.8 ± 2.9 (3.6)
2.16 ± 0.14
1.217 ± 0.069
3.13 ± 0.18
4.74 ± 0.19
4.33 ± 0.17
4.47 ± 0.18
5.08 ± 0.20
0±0
4.62 ± 0.22
13.2 ± 8.1
27.9 ± 2.8
30.2 ± 3.6
35.37 ± 0.51
35.74 ± 0.27
34.22 ± 0.97
36.32 ± 0.36
28 ± 61
35.20 ± 0.62
1.06
2.34
2.12
26.80
20.39
8.32
38.85
0.12
100.00
123 ± 72 (72)
250 ± 24 (24)
269 ± 30 (30)
310.7 ± 4.5 (5.0)
313.6 ± 2.8 (3.5)
301.3 ± 8.1 (8.3)
318.3 ± 3.4 (4.0)
254 ± 512 (512)
309.2 ± 5.3 (5.7)
1.7 ± 1.2
38.92 ± 0.42
86.2 ± 1.3
95.86 ± 0.68
99.1 ± 1.4
98.21 ± 0.62
94.37 ± 0.72
93.69 ± 0.86
98.72 ± 0.44
99.57 ± 0.52
99.58 ± 0.36
99.81 ± 0.76
97.5 ± 1.1
88.6 ± 2.9
63.7 ± 4.3
91.34 ± 0.22
0.241 ± 0.028
0.109 ± 0.005
0.031 ± 0.002
0.009 ± 0.001
0.000 ± 0.000
0.014 ± 0.001
0.033 ± 0.001
0.058 ± 0.003
0.025 ± 0.001
0.026 ± 0.001
0.036 ± 0.001
0.043 ± 0.002
0.133 ± 0.005
1.235 ± 0.050
0.668 ± 0.032
0.054 ± 0.002
15 ± 11
12.35 ± 0.16
34.92 ± 0.54
36.16 ± 0.25
36.85 ± 0.52
36.76 ± 0.22
37.10 ± 0.31
38.31 ± 0.36
38.11 ± 0.19
37.21 ± 0.19
36.86 ± 0.14
37.01 ± 0.32
36.81 ± 0.43
35.8 ± 1.2
33.3 ± 2.3
36.03 ± 0.36
0.18
3.52
5.18
7.02
6.55
9.54
6.21
7.81
5.67
18.33
14.78
9.61
3.74
1.05
0.83
100.00
142 ± 99 (99)
114.7 ± 1.5 (1.7)
307.1 ± 4.6 (5.0)
317.1 ± 2.5 (3.3)
322.6 ± 4.4 (4.9)
321.8 ± 2.3 (3.2)
324.6 ± 2.9 (3.6)
334.2 ± 3.3 (4.0)
332.6 ± 2.2 (3.1)
325.5 ± 2.2 (3.1)
322.7 ± 1.9 (2.9)
323.8 ± 3.0 (3.7)
322.3 ± 3.8 (4.4)
314 ± 10 (10)
294 ± 18 (18)
316.0 ± 2.3 (3.9)
5.84 ± 0.76
41.83 ± 0.40
73.7 ± 2.2
90.51 ± 0.90
96.26 ± 0.60
98.42 ± 0.34
96.3 ± 1.1
99.35 ± 0.98
98.01 ± 0.60
95.2 ± 1.4
70 ± 10
14.9 ± 3.2
80.97 ± 0.28
2.557 ± 0.102
1.571 ± 0.062
1.919 ± 0.076
3.02 ± 0.12
3.93 ± 0.16
4.03 ± 0.16
4.09 ± 0.16
4.40 ± 0.17
4.67 ± 0.19
4.63 ± 0.18
5.91 ± 0.26
6.46 ± 0.58
4.04 ± 0.18
19.6 ± 2.6
30.80 ± 0.31
34.4 ± 1.0
37.38 ± 0.41
36.82 ± 0.23
36.34 ± 0.12
35.88 ± 0.48
36.58 ± 0.36
36.40 ± 0.24
36.25 ± 0.52
32.3 ± 4.8
35.2 ± 8.1
35.90 ± 0.44
1.58
3.92
2.35
4.86
14.44
24.48
7.51
9.77
20.56
9.89
0.44
0.21
100.00
179 ± 22 (22)
274.8 ± 2.8 (3.3)
304.4 ± 8.5 (8.7)
328.4 ± 3.6 (4.2)
323.9 ± 2.3 (3.1)
320.0 ± 1.6 (2.7)
316.3 ± 4.1 (4.6)
322.0 ± 3.2 (3.9)
320.5 ± 2.3 (3.1)
319.3 ± 4.4 (4.9)
287 ± 39 (39)
311 ± 65 (65)
316.5 ± 3.8 (4.3)
Uncertainties are ±2σ. Errors in parentheses comprise age error and uncertainty of standard. The in-house standard ‘Bmus/2’ (Bärhalde muscovite) was used; age is 328.5 ± 2.2 Ma (2σ).
because biotite separate from the same sample yielded a thermally
undegassed spectrum. Partial alteration of hornblende (e.g. chloritization) leading to Ar degassing is more probable. The seventh step
of the degassing spectrum of 27-Hbl corresponds to an age value of
318.3 ± 4.0 Ma, which agrees well with the total age of 27-Bt (316.8 ±
3.6 Ma).
Summarizing, two pairs of biotite and hornblende (samples 27 and
28) yield the same age of ∼317 Ma. This age could be regarded as the
cooling age of the Gümüşhane pluton below ∼570–470 °C, corresponding to the closure temperature for Ar diffusion in hornblende
(Harrison et al., 1985; Grove and Harrison, 1996; Dahl, 1996, and
references therein).
ARTICLE IN PRESS
12
Fig. 6.
40
Ar–39Ar incremental spectra of biotites and hornblendes from the Gümüşhane pluton.
7.2. Conventional U–Pb zircon dating
Conventional U–Pb zircon dating was performed on samples 7 and 28.
Zircon grains from both samples are mostly fine-grained (63–125 µm),
subhedral to euhedral and have aspect ratios of about 1 to 3 (Fig. 7).
Inclusions of apatite and internal fractures are common. SEM imaging
revealed prism-parallel oscillatory and occasionally sector zoning,
suggesting magmatic growth. No inherited cores are identified. Many
Fig. 7. Back-scattered electron images of typical zircons from the Gümüşhane pluton.
ARTICLE IN PRESS
13
grains are corroded and may display altered domains. Six and five zircon
fractions were analyzed from samples 7 and 28, respectively. Analytical
results are listed in Table 4 and plotted on concordia diagrams (Fig. 8a
and b).
All the fractions gave discordant U–Pb ages, indicating Pb loss. The
zircon fractions from sample 7 define a discordia (Fig. 8a) with
intercept ages of 313 ± 24 Ma and 135 ± 72 Ma (2σ). If a forced
regression through 0 Ma is regarded, the discordia yields an upperintercept of 301 ± 15 Ma (MSWD = 2.3). Likewise, zircon fractions
from sample 28 gave discordant U–Pb ages and have 207Pb/206Pb ages
of 340 to 342 Ma (Fig. 8b). A forced regression through 0 Ma leads to
341 ± 5 Ma (2σ). All these data suggest that the U–Pb system in zircon
has been opened after igneous crystallization. As cited above, both
samples were overprinted by a late subgreenschist-facies hydrothermal metamorphism. Most probably, this metamorphic event was
responsible for the corrosion of zircon grains.
In general, zircon has a high U–Pb isotopic closure temperature
and is regarded as resistant against Pb loss even during hightemperature metamorphism (Lee et al., 1997; Cherniak and Watson,
2000). However, several studies have shown that zircons may yield
discordant U–Pb ages from low-grade metamorphic and/or weathered
rocks (e.g. Gebauer and Grünenfelder, 1976; Hann et al., 2003). Pb loss at
low temperatures is ascribed to radiation damage mediated by fluid
access (Mattinson, 2000). Hence, we consider that the Pb loss occurred by the interplay of radiation damage and fluid access during the
subgreenschist-facies hydrothermal event, rather than by volume
diffusion. In the view of the inferred magmatic origin of sub- to euhedral
zircons with oscillatory and sector zoning, the upper-intercept model
ages should be regarded as times approximating roughly the time of the
granite emplacement.
Fig. 8. Conventional U–Pb data on zircons from the Gümüşhane pluton.
7.3. LA-ICP-MS U–Pb zircon dating
Zircon grains from samples 7 and 28 were analyzed for their U–Pb
isotope compositions by LA-ICP-MS, whereby only the uncorroded inner
parts of the grains were analyzed (see Table 5). 27 zircons of sample 7
and 15 zircons of sample 28 were analyzed, of which only 5 analysis
were excluded from age calculation (marked by an asterisk in Table 5)
due to irregular behavior of the ablation signal (e.g., sudden change of
207
Pb/206Pb ratios). These samples are also characterized by their
elevated 204Pb signal, suggesting that Pb-rich phases (e.g., apatite) or
epoxy was ablated.
The grains have a wide range of U contents (113–3160 ppm) and
relatively high Th/U ratios between 0.26 and 0.78, typical for magmatic
zircons (Teipel et al., 2004). Unlike the conventional whole-grain U–Pb
zircon data, all analyzed zircon cores were nearly concordant, yielding a
weighted mean age of 329 ± 6 Ma for sample 28 and of 319 ± 5 Ma (2σ)
for sample 7 (Fig. 9). These results suggest that the discordancy of
conventional U–Pb dates is most probably due to the altered zircon
domains which were not analyzed by LA-ICP-MS. The ages for the
samples 7 and 28 determined by LA-ICP-MS are not resolvable within
Table 4
U–Pb isotope dilution data for zircons (samples 7 and 28) from the Gümüşhane pluton.
#
Weight
(mg)a
206
Pb/204Pbb
Ua
(ppm)
Pba
(ppm)
206
Pb*/208Pb*
Isotopic ratiosc
206
Pb*/238U
Model ages
(Ma)
207
Pb*/235U
207
Pb*/206Pb*
206
Pb/238U
207
Pb*/235U
207
Pb/206Pb
Sample 7
1
0.021
2
0.026
3
0.010
4
0.020
5
0.013
6
0.008
888
2190
467
2747
1724
336
297
292
1040
760
494
1017
14
14
39
37
21
42
7.91
7.53
8.04
9.69
7.04
6.39
0.04720 ± 31
0.04696 ± 34
0.03784 ± 34
0.04947 ± 32
0.04102 ± 21
0.03080 ± 30
0.34152 ± 47
0.33831 ± 40
0.27340 ± 87
0.35990 ± 37
0.29423 ± 19
0.28538 ± 71
0.05247 ± 62
0.05224 ± 49
0.05239 ± 16
0.05275 ± 41
0.05201 ± 19
0.05199 ± 12
297
295
239
311
259
251
298
295
245
312
261
254
305
296
302
318
285
285
Sample 28
1
0.019
2
0.020
3
0.029
4
0.029
5
0.027
1148
1157
1375
1590
1456
579
1098
851
638
740
23
43
31
30
32
7.41
7.53
7.24
6.67
7.25
0.03741 ± 23
0.03602 ± 21
0.03360 ± 19
0.04320 ± 25
0.04110 ± 22
0.2748 ± 28
0.2645 ± 25
0.2469 ± 19
0.3175 ± 32
0.3020 ± 20
0.05328 ± 45
0.05325 ± 42
0.05329 ± 29
0.05329 ± 29
0.05330 ± 21
237
228
213
273
260
247
238
224
280
268
341
340
341
341
342
* = radiogenic lead, errors are 2σm.
a
Weight and concentration error better than 20%.
b
Measured ratio corrected for mass discrimination and spike contribution.
c
Corrected for blank Pb, U and initial Pb (model of Stacey and Kramers, 1975).
ARTICLE IN PRESS
14
Table 5
LA-ICP-MS U–Th–Pb isotopic data and calculated ages for zircons from the Gümüşhane pluton.
Analysis
Th
U
Th/U
206
Pb/204Pb
207
Pb/235U
Sample 7
130
131
132
133
134
135
136
137
138
139
145
146
147
148
149*
150*
151*
152
153
154
155
156
157
158*
159
160
161
586
306
243
417
386
173
337
843
368
150
715
385
273
925
113
291
222
407
387
604
361
907
469
1264
370
353
389
1066
898
757
1159
1031
579
882
2588
1033
570
1764
883
819
3160
340
729
541
1062
782
1218
968
2057
1047
2529
991
1020
1498
0.55
0.34
0.32
0.36
0.37
0.30
0.38
0.33
0.36
0.26
0.41
0.44
0.33
0.29
0.33
0.40
0.41
0.38
0.50
0.50
0.37
0.44
0.45
0.50
0.37
0.35
0.26
N 2045
2399
N 1377
N 2284
N 1952
639
N 1806
4001
N 1876
N 1153
N 3314
N 1782
N 1455
2895
38
71
108
N 2105
1079
N 2315
N 1817
N 3617
N 1927
190
N 1708
N 2103
N 2844
0.353(19)
0.363(18)
0.371(17)
0.380(18)
0.356(17)
0.362(25)
0.368(19)
0.349(17)
0.356(18)
0.343(22)
0.375(18)
0.385(29)
0.357(20)
0.378(17)
5.792(614)
2.909(261)
1.585(155)
0.364(19)
0.371(19)
0.392(19)
0.360(19)
0.374(17)
0.367(19)
0.923(116)
0.371(20)
0.373(19)
0.373(20)
Sample 28
231
232
233
234
235
236*
237
238
239
240*
241
242
243
244
245
1095
885
791
566
524
181
205
474
239
655
229
506
377
229
425
1406
1171
1198
774
842
491
381
1669
571
973
491
1011
678
500
777
0.78
0.76
0.66
0.73
0.62
0.37
0.54
0.28
0.42
0.67
0.47
0.50
0.56
0.46
0.55
N 3054
1163
3841
875
N 1783
228
N 788
N 3772
N 1020
678
N 1048
N 2395
N 1459
N 1154
N 1678
0.416(18)
0.477(29)
0.391(19)
0.372(16)
0.386(16)
0.926(132)
0.383(19)
0.380(35)
0.356(59)
0.664(40)
0.374(18)
0.381(20)
0.380(17)
0.378(22)
0.366(22)
206
Pb/238U
Rho
207
Pb/206Pb
208
Pb/232Th
208
Pb/232Th
207
Pb/235U
206
Pb/238U
0.0495(21)
0.0509(20)
0.0519(17)
0.0511(16)
0.0508(18)
0.0507(26)
0.0505(20)
0.0446(16)
0.0502(19)
0.0494(26)
0.0506(18)
0.0507(34)
0.0511(22)
0.0523(18)
0.0944(85)
0.0730(34)
0.0601(35)
0.0512(20)
0.0519(20)
0.0517(18)
0.0505(20)
0.0510(18)
0.0501(22)
0.0506(52)
0.0509(21)
0.0499(21)
0.0506(23)
0.35
0.27
0.21
0.25
0.26
0.29
0.25
0.31
0.29
0.31
0.26
0.38
0.27
0.26
0.76
0.26
0.39
0.34
0.23
0.19
0.27
0.31
0.28
0.38
0.30
0.30
0.37
0.0517(28)
0.0517(28)
0.0519(27)
0.0539(27)
0.0507(26)
0.0518(38)
0.0528(30)
0.0567(29)
0.0514(27)
0.0504(34)
0.0538(28)
0.0550(44)
0.0506(31)
0.0524(26)
0.4447(309)
0.2892(259)
0.1911(177)
0.0516(28)
0.0519(30)
0.0550(30)
0.0517(29)
0.0532(26)
0.0531(30)
0.1323(170)
0.0529(30)
0.0542(30)
0.0535(30)
0.0152(09)
0.0153(08)
0.0154(08)
0.0159(08)
0.0156(08)
0.0159(11)
0.0156(08)
0.0144(10)
0.0156(08)
0.0159(10)
0.0157(08)
0.0160(11)
0.0166(10)
0.0167(09)
0.2731(301)
0.1179(107)
0.0639(62)
0.0156(08)
0.0162(10)
0.0162(10)
0.0158(09)
0.0159(08)
0.0154(09)
0.0338(44)
0.0158(09)
0.0154(09)
0.0158(09)
304 ± 18
307 ± 16
309 ± 15
318 ± 15
313 ± 16
318 ± 22
313 ± 17
288 ± 20
314 ± 16
319 ± 21
314 ± 16
321 ± 22
332 ± 19
335 ± 17
4881 ± 483
2253 ± 194
1252 ± 119
313 ± 16
325 ± 19
326 ± 20
318 ± 18
318 ± 16
310 ± 17
672 ± 86
317 ± 18
309 ± 17
318 ± 17
307 ± 14
314 ± 14
321 ± 13
327 ± 13
309 ± 13
314 ± 19
318 ± 14
304 ± 13
310 ± 14
300 ± 16
323 ± 13
331 ± 21
310 ± 15
326 ± 13
1945 ± 96
1384 ± 70
964 ± 63
316 ± 14
321 ± 15
336 ± 14
312 ± 14
323 ± 13
318 ± 14
664 ± 63
321 ± 15
322 ± 14
322 ± 15
311 ± 13
320 ± 12
326 ± 11
321 ± 10
320 ± 11
319 ± 16
318 ± 12
281 ± 10
316 ± 12
311 ± 16
318 ± 11
319 ± 21
322 ± 13
328 ± 11
582 ± 50
454 ± 20
376 ± 22
322 ± 13
326 ± 12
325 ± 11
318 ± 12
321 ± 11
315 ± 13
318 ± 32
320 ± 13
314 ± 13
318 ± 14
0.0524(17)
0.0534(17)
0.0528(22)
0.0518(17)
0.0534(16)
0.0539(44)
0.0521(15)
0.0522(45)
0.0456(71)
0.0554(20)
0.0519(18)
0.0529(24)
0.0519(18)
0.0519(23)
0.0516(26)
0.61
0.49
0.55
0.63
0.64
0.30
0.54
0.49
0.55
0.52
0.52
0.55
0.68
0.54
0.52
0.0575(20)
0.0647(34)
0.0537(24)
0.0521(18)
0.0525(17)
0.1247(177)
0.0534(23)
0.0528(47)
0.0566(86)
0.0869(45)
0.0522(22)
0.0523(24)
0.0530(17)
0.0529(27)
0.0515(28)
0.0172(06)
0.0191(08)
0.0171(07)
0.0168(06)
0.0171(06)
0.0428(57)
0.0167(06)
0.0167(14)
0.0157(26)
0.0247(12)
0.0167(07)
0.0169(07)
0.0161(06)
0.0161(07)
0.0166(09)
344 ± 11
382 ± 15
342 ± 14
336 ± 12
343 ± 12
847 ± 110
335 ± 12
336 ± 29
315 ± 52
494 ± 24
335 ± 13
339 ± 13
324 ± 12
322 ± 14
332 ± 17
353 ± 13
396 ± 20
335 ± 14
321 ± 12
332 ± 12
666 ± 72
330 ± 14
327 ± 26
309 ± 45
517 ± 25
322 ± 13
328 ± 15
327 ± 13
326 ± 16
317 ± 16
329 ± 11
335 ± 11
332 ± 14
326 ± 10
335 ± 10
338 ± 27
328 ± 9
328 ± 28
288 ± 44
348 ± 12
326 ± 11
332 ± 14
326 ± 11
326 ± 14
324 ± 16
U and Th concentrations are estimated from sensitivity factors calculated from GJ zircon (the Mainz crystal has 322 ppm U and 10.7 ppm Th). 204Hg interferences on 204Pb are
subtracted using a 201Hg/204Hg ratio of 1.918. 235U is calculated from 238U using a 238U/235U ratio of 137.88. * = Analysis not used for age calculation. Rho = error correlation defined
as the quotient of the propagated errors of the 206Pb/238U, 207Pb/235U and 207Pb/206Pb ratios. Uncertainties in parantheses are given for the last two digits and correspond to 1σ.
the given accuracy and we therefore conclude that the emplacement age
for the Gümüşhane pluton can be given as 324 ± 6 Ma.
9. Discussion
9.1. Intrusion age of the Gümüşhane pluton
8. Nd–Sr isotopes
40
Bulk-rock Nd and Sr isotope data are listed in Table 6. Initial Sr
and Nd isotope compositions are based on 324 Ma. Initial εNd values
[εNd(I)] of the granodiorite/granite and of the mafic microgranular
enclaves range from − 3.78 to −5.30 and initial 87Sr/86Sr isotope
ratios [87Sr/86Sr(I)] vary from 0.70212 to 0.70726 (Fig. 10). There is no
correlation between εNd(I) and 87Sr/86Sr(I). Leucogranites, granophyres and aplites display wide-ranging 87Sr/86Sr(I) values (0.68878–
0.70956). Extremely low 87Sr/86Sr(I) values (e.g. 0.68878–0.70212)
are shown by the highly altered samples. These relationships suggest
that the Sr isotopic budget was more severely influenced by
hydrothermal events than that of Nd. The Nd model ages, calculated
relative to depleted-mantle after the model of Liew and Hofmann
(1988), range from 1.38 to 1.63 Ga. The granophyres tend to have
slightly lower εNd(I) values (−5.15 to −6.84) and higher Nd model
ages (1.50–1.63 Ga).
Ar–39Ar ages of hornblende and biotite from samples 27 and
28 indicate a minimum intrusion age of ∼320 ± 4 Ma (adjusted for
the K decay constant miscalibration, see Analytical techniques) for
the Gümüşhane pluton. This minimum age constraint is indistinguishable from the LA-ICP-MS U–Pb zircon data of 324 ± 6 Ma (2σ)
within the range of analytical uncertainty. Thus, we infer that the
Gümüşhane pluton was emplaced during the latest Early Carboniferous (Serpukhovian, Davydov et al., 2004). Identical hornblende and
biotite 40Ar–39Ar ages (318–322 Ma) and rock types were reported for
the Köse granitoid, located 5 km to the SE of the Gümüşhane pluton
(Fig. 1; Dokuz et al., 2009). Both the Gümüşhane and Köse plutons can
be regarded as part of the same large intrusion. The 40Ar–39Ar cooling
ages of biotites and muscovites from mica schists of the Kurtoğlu
metamorphic complex into which the Gümüşhane pluton intruded
are ∼ 323 Ma (∼326 Ma given K decay miscalibration; Topuz et al.,
2007), slightly older than the 40Ar–39Ar biotite and hornblende ages
ARTICLE IN PRESS
Fig. 10. Variation of initial
15
87
Sr/86Sr and εNd values in the Gümüşhane pluton.
from the Gümüşhane pluton. This implies that the cooling of the
Gümüşhane pluton and Kurtoğlu metamorphics occurred diachronously, and intrusion of the granite pluton postdates the peak of lowP–high-T metamorphism.
One question is the timing of the hydrothermal event which led to
a partial, sometimes nearly complete resetting of 40Ar–39Ar biotite
and hornblende ages, a disturbance of Sr isotopic system and Pb loss in
the marginal domains of zircon grains. As a general approximation,
this hydrothermal event is either as old as the youngest Ar–Ar age
(74 Ma, Late Cretaceous) or even younger. The 40Ar–39Ar spectra of
the biotite separate from the sample 17 (not shown in Fig. 6) yielded a
well-defined plateau age, ∼ 74.4 ± 1.3 Ma with more than 85% 39Ar
released. Hence, we suggest that the later thermal event occurred at
∼74.4 Ma before present, and are most probably related to Late
Cretaceous igneous activity.
9.2. Magma genesis and intra-pluton evolution
Fig. 9. Concordia diagrams of LA-ICP-MS U–Pb zircon ages from the Gümüşhane pluton. Error
ellipses are given at the 2σ level. Stippled data point from sample 7 has not been considered
in concordia age calculation. Concordia ages calculated by Isoplot 3.50 (Ludwig, 2003).
Based on the linear variation of TiO2, Al2O3, Fe2Otot
3 , MgO, CaO, K2O,
P2O5, Sc, ASI vs. SiO2 (Fig. 3), the felsic rock types (leucogranite and
Table 6
Sr–Nd isotopic data for whole-rock samples from the Gümüşhane I-type granitoid pluton.
Sample
Rb
(ppm)
Sr
(ppm)
Sm
(ppm)
Biotite–hornblende granite/granodiorite (BHG,
1 BHG
143.0
152.3
3.51
2 BHG
139.4
155.1
3.63
3 BHG
148.1
138.9
4.61
7 BHG
189.0
88.2
5.70
19 BHG
117.5
95.8
5.73
93 BHG
154.8
269.2
3.44
307 BHGD
113.2
255.8
3.42
Nd
(ppm)
87
Rb/86Sr
87
Sr/86Sr
87
Sr/86Sr(I)
147
Sm/144Nd
143
Nd/144Nd
143
Nd/144Nd(I)
εNd(I)
TDM
(Ga)
BHGD)
22.9
24.0
30.2
42.5
31.1
16.5
17.1
2.7194
2.6030
3.0889
6.2124
3.5543
1.6650
1.2810
0.718344
0.717824
0.720940
0.730772
0.721924
0.715278
0.713172
(10)
(08)
(10)
(09)
(10)
(10)
(09)
0.70580
0.70582
0.70670
0.70212
0.70553
0.70760
0.70726
0.0933
0.0917
0.0927
0.0815
0.1120
0.1266
0.1214
0.512189
0.512198
0.512224
0.512196
0.512187
0.512260
0.512251
(10)
(08)
(10)
(10)
(08)
(07)
(07)
0.511991
0.512003
0.512027
0.512023
0.511949
0.511991
0.511993
− 4.48
− 4.24
− 3.78
− 3.86
− 5.30
− 4.48
− 4.44
1.43
1.41
1.38
1.38
1.50
1.43
1.43
Mafic microgranular enclave (MME)
1E
161.7
142.2
8.03
93E
104.1
157.6
6.95
307E
86.6
252.6
4.89
21.8
31.2
24.0
3.2941
1.9126
0.9923
0.720499 (07)
0.715728 (10)
0.711622 (10)
0.70531
0.70691
0.70705
0.2235
0.1353
0.1237
0.512447 (11)
0.512303 (05)
0.512271 (10)
0.511973
0.512016
0.512009
− 4.84
− 4.00
− 4.14
1.46
1.39
1.41
Aplite (A)
3B
231.6
Granophyre (G)
H2
146.3
10C
153.1
11C
143.3
335
190.6
18.82
4.97
15.42
36.1118
0.855340 (18)
0.68878
0.1957
0.512403 (12)
0.511988
− 4.55
1.44
42.8
20.5
23.5
18.2
5.16
5.67
5.73
5.08
20.8
25.4
19.8
16.4
9.9359
21.7495
17.7467
30.7064
0.755381
0.794839
0.781072
0.844916
0.70956
0.69454
0.69923
0.70332
0.1506
0.1357
0.1757
0.1881
0.512190
0.512245
0.512267
0.512351
0.511870
0.511957
0.511894
0.511952
− 6.84
− 5.15
− 6.37
− 5.25
1.63
1.49
1.59
1.50
(10)
(10)
(10)
(10)
(07)
(07)
(08)
(07)
Uncertainties for the 87Sr/86Sr and 143Nd/144Nd ratios are 2σm errors in the last two digits (in parantheses). εNd(I) values (at 324 Ma) are calculated relative to CHUR with presentday values of 143Nd/144Nd = 0.512638 and 147Sm/144Nd = 0.1967 (Jacobsen and Wasserburg, 1980). Nd model ages (TDM) are calculated with a depleted-mantle reservoir and
present-day values of 143Nd/144Nd = 0.513151 and 147Sm/144Nd = 0.219 (Liew and Hofmann, 1988).
ARTICLE IN PRESS
16
granophyre) can be interpreted as fractionation product of more
mafic rock types. Nonetheless, the distinct rock types of the
Gümüşhane pluton (granite/granodiorite, mafic enclave and granophyre) form distinctive groupings on the bi-variate plots of Sr, Ba,
(La/Yb)cn, Sr/Y and ∑REEs vs. SiO2 (Figs. 3 and 11), which can be best
accounted for by the partial melting of distinct sources, rather than
a simple fractionation history. Below we discuss the role of the
fractional crystallization for each rock suite separately.
The chemical variation within the granite and granodiorite
suggests fractionation of plagioclase and hornblende, accompanied
by accessory phases, such as magnetite and apatite. Plagioclase
fractionation causes negative anomalies in Sr and Eu, fractionation of
hornblende and magnetite may be responsible for the negative
anomaly in Ti, and the negative anomaly in P is most probably the
result of apatite fractionation (Fig. 4a–b). Such a fractionation trend is
also suggested by the variation in REE patterns (Fig. 5). In particular,
fractionation of hornblende should increase La/Yb values of the
residual melts, and in more evolved systems the residual melts should
develop chondrite-normalized REE patterns with a concave-upward
shape (e.g. Romick et al., 1992), as shown by the Gümüşhane
granodiorite/granite samples (Fig. 5). The Sr/Y ratios of the granite/
granodiorites are correlated with the SiO2 (Fig. 11), suggesting that
the ratio is modified by the fractionation. Since chondrite-normalized
REE patterns show almost no fractionation of the middle to heavy REE
and Sr/Y ratios in the least fractionated granodiorite sample are low
(≤15), garnet was not involved in the genesis of the magmas (Figs. 5
and 11). Low Rb/Sr ratios (0.44–1.20) rule out an origin from a mafic
magma by extensive fractional crystallization. On the other hand,
relatively high MgO contents preclude metasedimentary or felsic
igneous rocks as source. Partial melting of mafic sources seems
necessary to explain the major and trace element features. Experimental works have shown that high-K calc-alkaline granitic magmas
Fig. 11. (a) Sr/Y vs. Y (b) Sr/Y vs. SiO2.
can only derive from the melting of high-K calc-alkaline mafic
metamorphic rocks in the crust (e.g. Helz, 1976; Sisson et al., 2005).
The round microgranular mafic enclaves prove the coexistence of
dispersed basaltic to andesitic magma from the mantle. Such a rock
association is common to almost all I-type plutons world-wide, in
particular to the composite plutons, and is often explained by a
complex process involving mafic mantle-derived melts and granitic
crustal melts generated by partial melting of mafic crustal rocks (e.g.
Altherr et al., 1999, 2000; Kemp and Hawkesworth, 2005; Sisson et al.,
2005; Ratajeski et al., 2005). In case of the Gümüşhane pluton, mafic
microgranular enclaves display REE patterns and enriched Nd–Sr
isotopic signatures similar to those of the host granite and
granodiorite. Intensive mass transfer between mafic globules and
felsic host magma seems to be the rule rather than the exception (e.g.
Holden et al., 1991; Elburg, 1996; Galán et al., 1996; Petford et al.,
1996) and this is also suggested by experimental evidence (van der
Laan and Wyllie, 1993; Lesher, 1994).
The leucogranite (sample 10B) displays an evolved composition,
most probably by fractionation of K-feldspar and plagioclase leading to
significantly lower contents of Rb, Sr and Ba and lower Eu/Eu* (0.13).
It represents either a later magma batch or the most differentiated
products of the granite/granodiorite.
The granophyres have geochemical features distinct from the granite/
granodiorite (higher SiO2, K2O/Na2O and Rb/Sr ratios, and more
pronounced negative anomalies in Ba, Nb–Ta, Sr–Eu and Ti). Besides,
they display slightly more negative εNd values (−5.15 to −6.84), and
higher Nd model ages (1.49–1.63 Ga). In log–log diagrams of Ba and Rb
vs. Sr, the granophyres define linear trends (Fig. 12). This linear trend can
be explained by the combined fractionation of plagioclase + biotite (in
ratio of 4 to 1) or plagioclase-K-feldspar (in ratio of 2 to 1). As the Kfeldspar is a late crystallizing phase, fractionation of plagioclase + biotite
Fig. 12. Variation of (a) Rb vs. Sr, and (b) Ba vs. Sr. Fractionation vectors were calculated
according to the partition coefficients listed in Rollinson (1993). Tick marks in the
vectors indicate the percentage of the fractionating mineral in 10% intervals.
ARTICLE IN PRESS
seems probable. The Sr/Y ratios are not correlated to SiO2 (Fig. 11),
suggesting that the ratio is not modified by fractionation. Systematic
decrease of (La/Yb)cn ratios in the REE patterns (Fig. 5) require separation
of minerals such allanite or monazite, which preferentially incorporate
light rare earth elements relative to the heavier ones. The geochemical
features (high Rb/Sr ratios and contents of K2O and SiO2) are consistent with derivation from a felsic micaceous crustal source (cf. van de
Flierdt et al., 2003; Jung et al., 2009).
9.3. Geodynamic implications
Late Paleozoic paleogeographic reconstructions consistently indicate
a large westerly narrowing oceanic embayment between the two
supercontinents, the Gondwana in the south and the Laurussia in the
north, in the present-day Eastern Mediterranean realm (e.g. Smith et al.,
1981; Scotese, 1984; Stampfli and Borel, 2002). The oceanic domain
between these supercontinents is known as Paleotethys. The location
of the Eastern Pontides during late Paleozoic time is contentious:
(i) Şengör and Yılmaz (1981) and Şengör (1990) maintain that the
Eastern Pontides formed part of the active northern margin of the
southern continent (Gondwana). (ii) Okay et al. (2006) and Moix et al.
(2007), on the other hand, suggest that the Eastern Pontides were
located at the southern margin of Laurussia.
Carboniferous low-P–high-T metamorphism and I- and S-type
granitoid magmatism are widespread throughout the Sakarya zone
and in the Caucasus, which forms the eastward extension of the
Eastern Pontides (e.g. Hanel et al., 1992; Okay et al., 2002; Somin et al.,
2006; Nzegge et al., 2006; Dokuz et al., 2009; Treloar et al., 2009). So
far, no Carboniferous metamorphism and magmatism as well as
Permo-Triassic accretionary complexes have been documented from
the south of the Izmir–Ankara–Erzincan suture (e.g. in the Anatolide–
Tauride block). The Anatolide–Tauride block has a Neo-Proterozoic
crystalline basement overlain by different sedimentary successions
ranging from Mid-Cambrian to Miocene in age. The basement and
parts of the overlying successions were strongly deformed and partly
metamorphosed during the Alpine Orogeny (Okay et al., 2006, and the
references therein). Striking differences in the stratigraphy and in the
type and age of the basement rock associations suggest that the
Sakarya Zone and Anatolide–Tauride block formed distinct entities
during the Late Paleozoic–Early Mesozoic (Carboniferous to Late
Triassic). Hence, the Eastern Pontides along with the Caucasus were
probably part of the Laurusssia during the Carboniferous.
Latest Early to Late Carboniferous accretionary complexes and high
pressure metamorphics (blueschists and low-T eclogites) are not
known in the Eastern Pontides and Caucasus which would indicate
the presence of a coeval subduction zone during the emplacement of
the Gümüşhane pluton. Therefore, low-P–high-T metamorphism (i.e.
in the Kurtoğlu and Pulur regions; Topuz et al., 2004a, 2007) and
granitic plutons such as those of Gümüşhane and Köse in the Eastern
Pontides can be regarded as a late phase of this orogeny, similar to the
Hercynian chain in Central Europe (e.g. Finger et al. 1997).
The large amount of Early Carboniferous granitoids in the Eastern
Pontides (Gümüşhane pluton ∼400 km2, Köse pluton ∼250 km2) is
probably a reflection of the special thermal circumstances in concert
with the presence of fertile lithologies in the lower crust. In a postcollisional setting, delamination of the sub-continental lithosphere
might have occurred, leading to underplating of mafic rocks. These
underplated magmas may have provided the heat necessary for
melting of the existent mafic to relatively felsic lower crustal rocks.
10. Conclusions
The latest Early Carboniferous Gümüşhane I-type pluton forms a
significant constituent of the pre-Liassic basement of the Eastern
Pontides. This composite intrusion comprises high-K granodiorite,
granite, leucogranite and granophyre. The emplacement age of the
17
pluton is constrained to ∼324 ± 6 Ma by LA-ICP-MS U–Pb dating of core
domains of zircon grains. In addition, 40Ar–39Ar dating of hornblende
and biotite from the least overprinted samples yielded ages around
∼320 Ma within error limits of the U–Pb age. The Gümüşhane pluton
was emplaced a few million years after the low-P–high-T metamorphism
of the surrounding rocks.
The granite/granodiorite members of the high-K Gümüşhane
pluton were probably generated by melting of mafic crustal rocks,
induced by the injection of basaltic magmas from the upper mantle, as
suggested by numerous mafic microgranular enclaves. However, the
granophyres probably represent partial melts of a relatively felsic
micaceous source. Overall, the chemical compositions of the pluton
portions suggest melting at relatively shallow crustal levels, leaving a
plagioclase–hornblende-bearing restite without significant amounts
of garnet.
The Gümüşhane pluton together with the other Early to Late Carboniferous plutonic rocks in the Sakarya Zone and Caucasus represent a late
phase of the Hercynian orogeny in the Eastern Mediterranean.
Acknowledgements
This research was supported by the grant no. 104Y284 from the
Turkish Research Foundation (TÜBİTAK) and the TÜBA GEBIP
program. Ilona Fin and Oliver Wienand are thanked for preparing
hundreds of high-quality (polished) thin sections. Assistance of HansPeter Meyer during the EMPA and SEM work, of Gürsel Sunal and
Taylan Sancar during mineral separation and of Şener Ceryan during
field work are gratefully acknowledged. We also thank Aral I. Okay,
Kent Ratajeski, Yücel Yılmaz and Nelson Eby for their constructive comments which considerably improved the quality of the manuscript.
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The Gümüşhane pluton (NE Turkey)

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