Zircon Lu-Hf Isotope Systematics and U-Pb Geochron
Zircon Lu-Hf Isotope Systematics and U-Pb Geochron
Zircon Lu-Hf Isotope Systematics and U-Pb Geochron
DOI 10.1007/s00410-017-1346-0
ORIGINAL PAPER
Received: 28 June 2016 / Accepted: 5 March 2017 / Published online: 11 April 2017
© Springer-Verlag Berlin Heidelberg 2017
Abstract The early Mesozoic was a critical era for the ages (TDM1 = 0.65–0.95 Ga). In situ zircon analyses show
geodynamic evolution of the Sakarya Zone as transition that the rocks have variable and positive εHf (t) values
from accretion to collision events in the region. However, (4.6 to 13.5) and single-stage Hf model ages (TDM1 = 0.30
its complex evolutionary history is still debated. To address to 0.65 Ga). Both the geochemical signature and Sr-Nd-
this issue, we present new in situ zircon U–Pb ages and Hf isotopic composition of the gabbroic rocks reveal that
Lu-Hf isotope data, whole-rock Sr-Nd isotopes, and min- the magma of the studied rocks was formed by the partial
eral chemistry and geochemistry data of plutonic rocks to melting of a depleted mantle wedge metasomatized by
better understand the magmatic processes. The Gokcedere slab-derived fluids. The influence of slab fluids is mirrored
pluton is mainly composed of gabbro and gabbroic dior- by their trace-element characteristics. Trace-element mod-
ite. LA-ICP-MS zircon U–Pb dating reveals that the pluton eling suggests that the primary magma was generated by
was emplaced in the early Jurassic (177 Ma). These gab- a low and variable degree of partial melting (~5–15%) of
bros and gabbroic diorites are characterized by relatively a depleted and young lithospheric mantle wedge consist-
low SiO2 content of 47.09 to 57.15 wt% and high Mg# ing of phlogopite- and spinel-bearing lherzolite. Heat to
values varying from 46 to 75. The samples belong to the melt the mantle material was supplied by the ascendance
calc-alkaline series and exhibit a metaluminous I-type of a hot asthenosphere triggered by the roll-back of the
character. Moreover, they are slightly enriched in large Paleo-Tethyan oceanic lithosphere. The rising melts were
ion lithophile elements (Rb, Ba, Th and K) and light rare accompanied by fractional crystallization and encountered
earth elements and depleted in high field strength elements no or minor crustal contamination en route to the surface.
(Nb and Ti). Gabbroic rocks of the pluton have a depleted Taking into account these geochemical data and integrating
Sr-Nd isotopic composition, including low initial 87Sr/86Sr them with regional geological evidence, we propose a slab
ranging from 0.705124 to 0.705599, relatively high εNd (t) roll-back model; this model suggests that the Gokcedere
values varying from 0.1 to 3.5 and single-stage Nd model gabbroic pluton originated in a back-arc extensional envi-
ronment associated with the southward subduction of the
Paleo-Tethyan oceanic lithosphere during the early Jurassic
Communicated by Gordon Moore. period. Such an extensional event led to the opening of the
northern branch of the Neotethys as a back-arc basin. Con-
Electronic supplementary material The online version of this
sequently, we conclude that the gabbroic pluton was related
article (doi:10.1007/s00410-017-1346-0) contains supplementary
material, which is available to authorized users. to intensive extensional tectonic events, which peaked dur-
ing the early Jurassic in response to the roll-back of Paleo-
* Orhan Karsli Tethyan oceanic slab in the final stage of oceanic closure.
okarsli@gmail.com
1
Department of Geological Engineering, Recep Tayyip Keywords Gabbroic pluton · Depleted mantle · Early
Erdogan University, 53000 Rize, Turkey Jurassic pluton · Sakarya Zone-NE Turkey
2
Department of Geological Engineering, Gumushane
University, 29000 Gumushane, Turkey
13
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13
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Study
sp
tion to the Afro-Arabian and N47.50 Sea Scythian Platform
hian
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Eurasian plates [after Okay and Pannonian at form
Odessa
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at
Tuysuz (1999)]. b Detailed geo- Basin r p
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rud Shelf
ja Greater Caucasus
est
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logical map of the Gokcedere
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im
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area exhibiting stratigraphic Rioni Bas
in
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ack Kura
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ean
Sea Basin
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relationships of the calc-alkaline West Black
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gabbroic Gokcedere pluton
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b 2 km
Variscan metamorphic-magmatic basement is uncon- and eclogites of the Karakaya complex (Okay et al. 2002;
formably overlain by post Triassic volcano-sedimentary Okay and Goncuoglu 2004). This period is poorly under-
rocks (Dokuz and Tanyolu 2006; Sen 2007; Kandemir stood due to the rarity of late Triassic intrusive rocks in
and Yilmaz 2009). Late Triassic events in the western the eastern part of the Sakarya Zone (Eyuboglu et al.
part of the Sakarya Zone have been interpreted as associ- 2011; Karsli et al. 2014). Based on the geochemical sig-
ated with a subduction setting, based on the blueschists nature of the intrusive (Dokuz et al. 2006; Ustaomer and
13
Robertson 2010) and volcanic (Sen 2007) rocks, it is sug- Analytical procedure
gested that the late Triassic to the early Jurassic period
was dominated by a continental magmatic arc separated Mineral composition
from the northern margin of Gondwana during the early
Triassic in response to the southward subduction of the To decipher mineralogical features and measure micro
Paleotethyan oceanic slab (Sengor and Yilmaz 1981; chemical compositions, polished thin sections were pre-
Yilmaz et al. 1997; Kocyigit and Altiner 2002; Dokuz pared at the Ludwig Maximillian University, Department
et al. 2006, 2010). This southward subduction of the of Mineralogy, Petrology and Geochemistry, Munich (Ger-
Paleo-Tethyan oceanic slab caused the formation of the many). The micro chemical analyses were performed using
northern branch of the Neotethys Ocean in the southern a Cameca SX-100 electron microprobe equipped with five
part of the Sakarya Zone (Sengor and Yilmaz 1981). wavelength-dispersive spectrometers at Department of
Middle to late Jurassic granitoids and dacites emplaced Mineralogy, Petrology and Geochemistry, Munich (Ger-
within the volcano-sedimentary rocks of the Şenköy For- many). The analytical conditions included accelerating
mation (Dokuz et al. 2006, 2010) together with molasse voltage of 15 kV, a beam current of 20 nA, and a count-
sediments are believed to be the products of an arc-con- ing time of 10 to 30 s. Synthetic and natural oxides and
tinent collision in response to the closure of the Paleo- silicates were used as standard materials. The correction
tethys during the middle Jurassic period and the accretion procedures were based on the CAMECA PAP algorithm
of the Sakarya Zone onto Laurasia in the north (Sengor by Pouchou and Pichoir (1985). The detection limits were
et al. 1980; Sengor and Yilmaz 1981; Yilmaz et al. 1997; generally on the order of 0.1 wt%. The counting time was
Dokuz et al. 2010, 2017a). The late Jurassic to early usually set to 10 s when analyzing for the major elements
Cretaceous period was characterized by platform car- (Si, Al, Fe, Mg, Mn, Ca, Na, K, Cr, and Ti). The analyses
bonates of the Berdiga Formation (Gorur 1997; Tuysuz were performed using a beam diameter of 1 µm, except in
1999). During the late Cretaceous period, the opening of the case of feldspars, when a defocused beam (10 µm) was
the Black Sea in the northern part of the Sakarya Zone used to minimize the alkaline diffusion.
was triggered by the northward subduction of the Neo-
Tethyan oceanic lithosphere beneath the Sakarya Zone LA‑ICP‑MS zircon U–Pb analysis
(Okay et al. 1994; Robinson et al. 1995; Sengor et al.
2003; Kaygusuz et al. 2008; Karsli et al. 2010a, 2012a; Two zircon-bearing gabbroic samples were crushed. The
Aydin 2014). This subduction resulted in a submarine zircons were separated by the hydro separation and elec-
magmatic arc (Okay and Sahinturk 1997; Yilmaz et al. tromagnetic separation techniques and then the zircon
1997; Okay and Tuysuz 1999; Boztug et al. 2004; Altherr grains were selected under a binocular microscope at the
et al. 2008; Boztug and Harlavan 2008; Kaygusuz et al. Key Laboratory of Orogenic Belts and Crustal Evolution,
2008; Cinku et al. 2010; Ustaomer and Robertson 2010; Peking University, Beijing. Cathodoluminescence (CL)
Karsli et al. 2010a, 2012a). The flyschoid sedimentary images were prepared to check the internal structures of
rocks combined with limestone deposited in the south- individual zircon grains and spots were selected for anal-
ern part of the region are indicative of being a fore-arc ysis at the same laboratory. Zircon U–Pb analyses were
system in the south. The early Paleocene plagioleucitites carried out on a Finnigan Neptune MC-ICP-MS and New
in the region have been interpreted as the final products Wave UP213 LA-MC-ICP-MS housed at the Institute of
of the northward subduction (Altherr et al. 2008). The Mineral Resources in the Chinese Academy of Geologi-
Paleocene and early Eocene period in the eastern Sakarya cal Sciences, Beijing, China. Zircon GJ-1 was used as an
Zone were dominated by a continent–continent collision external standard. The analytical procedure and details are
between the Sakarya and the Tauride-Anatolide blocks outlined in Hou et al. (2009). Spot diameters were 25 µm.
in response to the complete closure of the Neo-Tethyan The U, Th, and Pb abundances were measured in reference
Ocean (Okay and Sahinturk 1997; Boztug et al. 2004; to the values in standard zircon M127 (U = 923 × 10−6;
Hisarli 2011; Karsli et al. 2010b, 2011; Topuz et al. 2005; Th = 439 × 10−6; Th/U = 0.475) (Nasdala et al. 2008). The
Rolland et al. 2012). Middle Eocene high-K calc-alkaline ICP-MS Data Cal Software described by Liu et al. (2008)
intrusives have been credited with the orogenic collapse, was used for raw data calculations of the measurements.
following slab-break-related rapid uplift (Boztug et al.
2004, 2006; Karsli et al. 2007, 2012b; Aydincakir and Whole‑rock major and trace‑element analysis
Sen 2013). Additionally, Neogene volcanism was thought
to have formed in a continental extensional setting in the Twenty fresh gabbro and gabbroic diorite samples were
region (Aydin et al. 2008, 2009; Dokuz et al. 2013). collected at the pluton near Gokcedere village (Demirozu-
Bayburt) in northeast Turkey (Fig. 1b) for whole-rock
13
major and trace-element analyses. All the samples were VG354 mass spectrometer. Rb, Sr, Sm, and Nd concentra-
megascopically fresh, undeformed, and unmetamorphosed. tions were measured using the isotopic dilution method.
87
To prepare the rock powders, 1–3 kg of each sample was Sr/86Sr ratios were normalized against 86Sr/88Sr = 0.1194,
initially crushed in a steel crusher and then the samples and 143Nd/144Nd ratios were normalized against
146
were manually fine-powdered in an agate mortar to reduce Nd/144Nd = 0.7219. 87Sr/86Sr ratios were adjusted to the
the grain size to <200 mesh. Analyses of the major oxide NBS-987 Sr standard = 0.710250, and 143Nd/144Nd ratios
and trace elements of the samples were performed at the were adjusted to the La Jolla Nd standard = 0.511860. The
commercial facilities of ACME Laboratories Ltd., Van- precision in concentration analyses by isotopic dilution was
couver (Canada). The amounts of major element oxides ±2% for Rb, ±0.4 to 1% for Sr, and <±0.5% for Sm and
were measured using a Perkin-Elmer Elan 600 ICP-AES Nd, depending on concentration levels; overall precision
(0.2 g of pulp sample by LiBO2 fusion). The detection for Rb/Sr was ±2% and for Sm/Nd was ±0.2 to 0.5%. Pro-
limits are approximately 0.001–0.04%. To measure trace- cedural blanks were Rb = 120 pg, Sr = 200 pg, Sm = 50 pg,
element abundances, 0.2 g of sample powder and 1.5 g of and Nd = 50–100 pg. The detailed analytical procedures
LiBO2 flux were mixed in a graphite crucible and heated to for Sr and Nd isotopic measurements are outlined in Qiao
1050 °C for 15 min in a muffle furnace. The molten sam- (1988).
ple was then dissolved in 100 mL of 5% HNO3 (American
Chemical Society-grade nitric acid in deionized water). The
sample solutions were shaken for 2 h and then an aliquot Results
was poured into a polypropylene test tube and aspirated
into a Perkin-Elmer Elan 600 ICP mass spectrometer. Cali- Brief petrography, mineralogy, and mineral
bration and verification standards, together with reagent composition.
blanks, were added to the sample sequence. The elemental
concentrations of the samples were obtained using BCR-2 Twenty gabbroic samples were collected from the
and BIR-2 (concentrations from USGS) as external stand- Gokcedere pluton with an exposure area of ~20 km2, which
ards. The detection limits ranged from 0.01 to 0.5 ppm for was emplaced in the early Carboniferous Pulur metamor-
most of the trace elements. phics near Gokcedere village, Demirozu-Bayburt area,
northeast Turkey. The sample locations and their latitude
In situ zircon Lu‑Hf isotope analysis and longitude are depicted in Fig. 1b. All the samples were
megascopically fresh, undeformed, and unmetamorphosed.
In situ zircon Lu-Hf isotopic analyses were conducted at The individual pluton showed sharp contacts with early
Institute of Mineral Resources in the Chinese Academy Carboniferous metamorphic basement rocks and is over-
of Geological Sciences, Beijing, China. They were carried lined by the volcano-sedimentary rocks of the Şenköy For-
out on a Neptune MC-ICP-MS in combination with a New mation (Fig. 1b). The gabbroic body was predominantly
Wave UP213 laser ablation inductively coupled plasma medium-grained gabbro, with a more evolved gabbroic
spectrometer (LA-MC-ICP-MS). The analyses were carried diorite (Fig. 2a). These lithologies showed gradational rela-
out on the same spots or domains adjacent to where U–Pb tionships manifested by spatial variation in mineral modal
dating was done. The size of laser-ablated spots was 50 µm abundances. The intrusive samples were fresh and unde-
when the laser repetition rate was 10 Hz. Throughout the formed. No mafic micro granular enclaves were observed
analyses, isobaric interference of 176Lu on 176Hf was cor- within the host rocks, and the samples exhibited no evi-
rected based on measured 175Lu values. 176Yb/172Yb values dence of magma mixing, such as ocellar quartz, spongy
of 0.5887 and the mean βYb value were used to correct for plagioclase, or disequilibrium textures around the other
the interference of 176Yb on 176Hf (Wu et al. 2006). minerals. The samples from the pluton were dark to dark
gray, with a medium-grained texture composed of euhedral
Whole‑rock Sr‑Nd isotope analysis to anhedral plagioclase, quartz, K-feldspar and amphibole,
usually accompanied by ilmenite and zircon (Fig. 2b). Clear
The powdered samples of bulk rocks were dissolved using cumulus textures were lacking in the samples. Plagioclase
acid (HF + HCIO4) in sealed Savillex beakers on a hot plate was observed to occur as large euhedral to subhedral crys-
for 1 week. Separation of Rb, Sr, and light rare earth ele- tals with no exhibited chemical zoning (~2 mm; Fig. 2c, d).
ments (REEs) was done through a cation-exchange column They showed heterogeneous composition ranging from A n8
(packed with Bio-Rad AG5OWx8 resin). Sr and Nd iso- to An74, with Or content rarely exceeding 2 mol% (Sup-
topic analyses were performed at the Institute of Geology plementary Table 1). K-feldspar (Or86−93Ab1−14An0−1),
and Geophysics, Chinese Academy of Sciences, Beijing. which never exceeded 4% in the modal composition,
Isotope analyses were carried out using a multi-collector formed as subhedral to anhedral crystals and contained
13
Fig. 2 a Macroscopic view showing petrographical features of the rocks. c, d BSE views displaying textural relationships of the gab-
pluton (Sample GD19; gabbro). b Microphotograph (under cross broic rocks. Minerals include amphibole, plagioclase, quartz and
polarization) view exhibiting textural relationships of the gabbroic ilmenite. The diameter of 25 coin is 2 cm
small plagioclase, quartz, and amphibole crystals, which Based on the CL images, zircons free of visible inclusions
represent poikilitic textures (Fig. 2c, d). The quartz was were chosen for U–Pb dating analyses; the data is provided
anhedral with irregular cracks and interstitial to the other in Table 1 and shown on a concordia diagram (Fig. 3). A
minerals (Fig. 2c, d). Green to brownish-green amphiboles total of 30 spots from the 28 zircons were measured from
were generally anhedral, with inclusions of Fe-Ti oxide the sample. As shown in Table 1 and Fig. 3, concordant
(Fig. 2c, d). They were generally calcic and characterized analyses yielded weighted mean ages of 176.95 ± 0.49 Ma
by XMg [=Mg/(Mg + Fetot)] = 0.49–0.74 (Supplementary (MSWD =
1.08) from the gabbro samples (GD1) and
Table 1). Ilmenites and magnetite were present as poikilitic 178.41 ± 0.44 Ma (MSWD = 1.10) from the gabbroic dior-
inclusions within mafic silicates, and were of homogene- ite sample (GD23) of the Gokcedere pluton, revealing its
ous composition (Ilm97Hm03; Mt95−75Usp25−05) (Fig. 2b–f; emplacement age.
Supplementary Table 1). Zircon was an accessory phase in
all rock types and occurred as prismatic crystals (Fig. 3). Geochemistry
Zircon grains selected from the samples GD1 (gabbro) A complete data set of whole-rock major element analyses
and GD23 (gabbroic diorite) were euhedral to subhedral for the representative samples from the studied gabbroic
and colorless and had mostly prismatic morphologies. The rocks is given in Table 2. Most of the samples have low loss
grains exhibited pyramidal termination and oscillatory zon- on ignition values in a moderate range of 0.90–2.80 wt%,
ing patterns with a length of ~50–350 µm, which suggests which suggests minimal weathering secondary processes
a magmatic origin. Some zircons exhibited a dark-gray and no hydrothermal alteration. Therefore, they can be used
color and had heterogeneous fractured domains (Fig. 3). to document the petrogenesis of the Gokcedere pluton. The
13
0.029 0.0288
184
182
0.0284
180
Pb/238U
Pb/238U
0.028
0.0280 178
206
176
206
0 .0276
0.027 172
174
0.0272
168
Mean=176.95-/+0.49 Mean=178.41-/+0.44
MSWD=1.08 MSWD=1.10
0.026 0.0268
0.0 0.1 0.2 0.3 0 .4 0.12 0.16 0.20 0.24 0 .28
Pb/ U
207 235
Pb/ U
207 235
190
185
186 183
181
182
179
Age (Ma)
Age (Ma)
178
177
174
175
170
Mean = 176.95-/+0.49 [0.28%] 95% conf. 173 Mean = 178.41-/+0.44 [0.25%] 95% conf.
Wtd by data-point errors only Wtd by data-point errors only
MSWD = 1.08, probability = 0.35 MSWD = 1.10, probability = 0.32
166 171
Spots Spots
100 µm 100 µm
Fig. 3 In situ zircon LA-ICP-MS U–Pb age concordia and average age diagram of the Gokcedere pluton. Inset representative CL images of zir-
cons from samples GD1 (gabbro) and GD23 (gabbroic diorite). The scale bar is 100 µm
13
Table 1 LA-ICP-MS U–Pb age data for the zircons of the calc-alkaline Gokcedere gabbroic pluton from the Bayburt area in the eastern Sakarya
Zone
206
Spot Pb (ppm) U (ppm) Pb/238U 1σ err% 207
Pb/235U 1σ err% 207
Pb/206Pb 1σ err%
Sample GD23
1 8 282 0.0280 0.0003 1.01 0.2075 0.0256 12.33 0.0537 0.0068 12.59
2 9 306 0.0282 0.0002 0.68 0.1994 0.0083 4.14 0.0512 0.0021 4.16
3 9 305 0.0277 0.0002 0.70 0.2068 0.0090 4.33 0.0541 0.0023 4.33
4 5 157 0.0283 0.0002 0.65 0.1986 0.0071 3.58 0.0509 0.0019 3.74
5 7 234 0.0280 0.0002 0.73 0.2171 0.0118 5.45 0.0562 0.0031 5.45
6 7 227 0.0283 0.0002 0.67 0.2034 0.0104 5.13 0.0522 0.0026 4.90
7 12 391 0.0283 0.0002 0.62 0.1978 0.0058 2.93 0.0506 0.0015 2.90
8 10 327 0.0279 0.0002 0.66 0.1922 0.0089 4.63 0.0499 0.0023 4.56
9 16 524 0.0282 0.0002 0.63 0.1960 0.0082 4.17 0.0504 0.0021 4.18
10 4 145 0.0281 0.0002 0.72 0.1968 0.0136 6.89 0.0507 0.0034 6.71
11 3 105 0.0283 0.0002 0.62 0.2000 0.0045 2.26 0.0513 0.0012 2.32
12 3 96 0.0281 0.0002 0.72 0.1921 0.0093 4.83 0.0496 0.0024 4.76
13 5 153 0.0278 0.0002 0.85 0.2021 0.0124 6.12 0.0527 0.0032 6.08
14 6 191 0.0280 0.0002 0.61 0.2066 0.0058 2.81 0.0535 0.0015 2.82
15 4 128 0.0279 0.0002 0.66 0.2147 0.0069 3.20 0.0558 0.0017 3.12
16 5 151 0.0279 0.0002 0.70 0.2135 0.0121 5.67 0.0555 0.0031 5.56
17 7 234 0.0279 0.0002 0.77 0.2182 0.0140 6.40 0.0568 0.0037 6.44
18 5 181 0.0283 0.0002 0.67 0.1971 0.0089 4.52 0.0505 0.0022 4.45
19 6 208 0.0278 0.0002 0.60 0.1941 0.0033 1.72 0.0507 0.0009 1.75
20 16 525 0.0279 0.0002 0.69 0.1931 0.0131 6.80 0.0503 0.0035 7.00
21 4 130 0.0281 0.0002 0.63 0.2028 0.0083 4.07 0.0524 0.0021 4.06
22 2 77 0.0285 0.0002 0.71 0.1952 0.0220 11.25 0.0497 0.0058 11.64
23 6 202 0.0281 0.0002 0.76 0.2065 0.0235 11.39 0.0534 0.0061 11.44
24 3 109 0.0277 0.0002 0.84 0.1915 0.0106 5.54 0.0501 0.0027 5.46
25 10 338 0.0284 0.0002 0.64 0.1980 0.0045 2.27 0.0506 0.0011 2.25
26 23 771 0.0281 0.0002 0.71 0.1949 0.0118 6.04 0.0503 0.0030 6.02
27 30 1014 0.0279 0.0002 0.78 0.1992 0.0166 8.33 0.0518 0.0043 8.37
28 5 159 0.0280 0.0002 0.75 0.1947 0.0114 5.85 0.0505 0.0029 5.80
29 7 225 0.0280 0.0002 0.61 0.1930 0.0060 3.10 0.0500 0.0015 3.08
30 7 229 0.0278 0.0003 0.92 0.2091 0.0194 9.27 0.0546 0.0038 6.94
208
Spot Pb/232Th 1σ err% 232
Th/238U 1σ err% 206
Pb/238U 1σ 207
Pb/235U 1σ 207
Pb/206Pb 1σ
(age)
Samp GD23
1 0.0098 0.0003 10.08 0.4010 0.0045 1.13 178 2 191 24 359 284
2 0.0093 0.0001 4.45 0.5207 0.0030 0.57 179 1 185 8 251 96
3 0.0092 0.0001 4.49 0.6241 0.0032 0.52 176 1 191 8 374 97
4 0.0092 0.0001 4.25 0.5651 0.0087 1.54 180 1 184 7 236 86
5 0.0102 0.0002 5.44 0.3756 0.0025 0.67 178 1 199 11 460 121
6 0.0096 0.0001 4.50 0.6011 0.0038 0.63 180 1 188 10 294 112
7 0.0095 0.0003 4.02 0.5738 0.0032 0.56 180 1 183 5 224 67
8 0.0109 0.0002 4.21 0.5341 0.0026 0.49 178 1 179 8 192 106
9 0.0099 0.0002 4.13 0.4877 0.0035 0.71 179 1 182 8 215 97
10 0.0103 0.0003 5.43 0.4689 0.0035 0.75 179 1 182 13 229 155
11 0.0100 0.0003 3.87 0.5959 0.0031 0.52 180 1 185 4 252 53
12 0.0102 0.0001 4.48 0.4905 0.0025 0.51 178 1 178 9 178 111
13 0.0136 0.0001 5.04 0.5608 0.0034 0.60 177 2 187 11 316 138
14 0.0105 0.0004 3.95 0.5753 0.0033 0.57 178 1 191 5 352 64
13
Table 1 (continued)
208
Spot Pb/232Th 1σ err% 232
Th/238U 1σ err% 206
Pb/238U 1σ 207
Pb/235U 1σ 207
Pb/206Pb 1σ
(age)
15 0.0109 0.0001 4.27 0.4672 0.0100 2.14 177 1 197 6 445 69
16 0.0101 0.0003 4.52 0.5015 0.0025 0.49 177 1 196 11 432 124
17 0.0100 0.0003 4.99 0.6314 0.0037 0.58 177 1 200 13 483 142
18 0.0099 0.0003 4.40 0.4920 0.0024 0.49 180 1 183 8 219 103
19 0.0099 0.0004 3.83 0.5710 0.0037 0.64 177 1 180 3 225 40
20 0.0102 0.0003 4.56 0.6551 0.0039 0.59 177 1 179 12 207 162
21 0.0094 0.0003 4.04 0.6236 0.0031 0.49 179 1 187 8 301 92
22 0.0093 0.0003 4.96 0.6468 0.0036 0.55 181 1 181 20 180 271
23 0.0083 0.0003 6.73 0.4899 0.0025 0.51 178 1 191 22 345 259
24 0.0106 0.0003 5.05 0.6410 0.0034 0.53 176 1 178 10 199 127
25 0.0088 0.0003 3.98 0.5487 0.0040 0.73 180 1 183 4 222 52
26 0.0093 0.0004 4.67 0.4687 0.0061 1.30 179 1 181 11 209 140
27 0.0094 0.0003 5.60 0.5286 0.0044 0.83 177 1 184 15 276 192
28 0.0086 0.0003 4.75 0.5303 0.0043 0.81 178 1 181 11 217 134
29 0.0091 0.0003 4.00 0.5438 0.0029 0.54 178 1 179 6 196 72
30 0.0100 0.0003 11.40 0.5627 0.0027 0.48 177 2 193 18 395 156
206
Spot Pb (ppm) U (ppm) Pb/238U 1σ err% 207
Pb/235U 1σ err% 207
Pb/206Pb 1σ err%
Sample GD1
1 3 82 0.0276 0.0002 0.84 0.2054 0.0165 8.04 0.0539 0.0041 7.63
2 2 58 0.0276 0.0002 0.85 0.2159 0.0188 8.72 0.0568 0.0050 8.89
3 2 64 0.0278 0.0002 0.75 0.2050 0.0177 8.64 0.0535 0.0046 8.68
4 6 194 0.0280 0.0002 0.66 0.2138 0.0094 4.41 0.0554 0.0024 4.34
5 1 35 0.0281 0.0003 1.24 0.1953 0.0666 34.07 0.0504 0.0216 42.78
6 3 103 0.0280 0.0002 0.73 0.1923 0.0131 6.81 0.0499 0.0034 6.77
7 2 84 0.0279 0.0002 0.68 0.2049 0.0119 5.81 0.0532 0.0031 5.80
8 3 90 0.0282 0.0002 0.80 0.1969 0.0190 9.65 0.0506 0.0049 9.61
9 2 57 0.0282 0.0002 0.86 0.1986 0.0333 16.78 0.0510 0.0088 17.19
10 1 30 0.0278 0.0004 1.27 0.1915 0.0346 18.04 0.0500 0.0100 19.92
11 1 28 0.0280 0.0007 2.32 0.2046 0.0711 34.73 0.0529 0.0227 42.87
12 1 42 0.0277 0.0003 1.15 0.2667 0.0337 12.64 0.0698 0.0088 12.65
13 4 125 0.0275 0.0002 0.68 0.2058 0.0100 4.87 0.0543 0.0026 4.84
14 1 36 0.0283 0.0004 1.59 0.1954 0.0856 43.81 0.0501 0.0282 56.42
15 7 218 0.0278 0.0002 0.62 0.2040 0.0061 3.01 0.0532 0.0016 2.95
16 2 53 0.0283 0.0003 0.97 0.2023 0.0505 24.97 0.0518 0.0138 26.57
17 7 223 0.0277 0.0002 0.60 0.2066 0.0057 2.74 0.0540 0.0015 2.72
18 1 31 0.0274 0.0004 1.41 0.2589 0.0317 12.24 0.0685 0.0085 12.47
19 6 206 0.0277 0.0002 0.61 0.2082 0.0054 2.61 0.0545 0.0014 2.59
20 1 34 0.0281 0.0003 1.17 0.1961 0.0462 23.57 0.0507 0.0147 29.10
21 1 44 0.0284 0.0003 0.96 0.1973 0.0397 20.12 0.0504 0.0111 22.02
22 3 117 0.0277 0.0002 0.72 0.1992 0.0164 8.24 0.0521 0.0043 8.29
23 6 202 0.0280 0.0002 0.60 0.2333 0.0069 2.94 0.0604 0.0018 2.93
24 4 143 0.0275 0.0002 0.64 0.2308 0.0079 3.42 0.0609 0.0021 3.40
25 3 81 0.0280 0.0002 0.74 0.2292 0.0163 7.13 0.0593 0.0042 7.13
26 2 67 0.0277 0.0002 0.74 0.2156 0.0141 6.53 0.0564 0.0037 6.50
27 3 102 0.0277 0.0002 0.65 0.2105 0.0105 4.97 0.0551 0.0027 4.94
28 3 104 0.0278 0.0002 0.64 0.2404 0.0107 4.45 0.0628 0.0028 4.42
29 2 68 0.0277 0.0002 0.74 0.1911 0.0160 8.38 0.0500 0.0042 8.41
30 3 87 0.0278 0.0002 0.67 0.1919 0.0118 6.12 0.0501 0.0031 6.16
13
Table 1 (continued)
208
Spot Pb/232Th 1σ err% 232
Th/238U 1σ err% 206
Pb/238U 1σ 207
Pb/235U 1σ 207
Pb/206Pb 1σ
(age)
Sample GD1
1 0.0127 0.0003 5.11 0.5231 0.0029 0.56 176 1 190 15 368 172
2 0.0141 0.0001 6.87 0.3504 0.0020 0.58 175 1 198 17 484 196
3 0.0122 0.0001 5.06 0.5214 0.0025 0.47 177 1 189 16 350 196
4 0.0125 0.0001 4.25 0.4099 0.0020 0.48 178 1 197 9 428 97
5 0.0123 0.0002 10.69 0.3688 0.0019 0.51 179 2 181 62 214 991
6 0.0114 0.0001 5.52 0.3946 0.0025 0.64 178 1 179 12 190 158
7 0.0107 0.0003 5.73 0.3329 0.0016 0.49 178 1 189 11 336 131
8 0.0104 0.0002 4.77 0.6131 0.0030 0.49 179 1 182 18 223 222
9 0.0081 0.0002 6.47 0.5663 0.0028 0.49 180 2 184 31 240 396
10 0.0110 0.0003 10.94 0.4064 0.0021 0.51 177 2 178 32 194 463
11 0.0070 0.0003 18.64 0.6425 0.0037 0.57 178 4 189 66 327 973
12 0.0104 0.0001 10.74 0.4577 0.0023 0.50 176 2 240 30 921 260
13 0.0099 0.0001 4.19 0.7902 0.0042 0.53 175 1 190 9 385 109
14 0.0113 0.0004 11.14 0.3813 0.0020 0.53 180 3 181 79 197 1311
15 0.0111 0.0001 3.88 0.6441 0.0041 0.63 177 1 189 6 337 67
16 0.0108 0.0003 5.79 0.6340 0.0044 0.69 180 2 187 47 275 609
17 0.0100 0.0003 3.84 0.8374 0.0043 0.52 176 1 191 5 372 61
18 0.0108 0.0003 13.21 0.4096 0.0022 0.53 174 2 234 29 883 258
19 0.0095 0.0004 3.94 0.6080 0.0083 1.37 176 1 192 5 392 58
20 0.0087 0.0003 11.26 0.4138 0.0021 0.51 178 2 182 43 227 672
21 0.0094 0.0003 6.86 0.5422 0.0028 0.52 180 2 183 37 215 510
22 0.0094 0.0003 5.14 0.5085 0.0039 0.76 176 1 184 15 291 189
23 0.0091 0.0003 3.84 0.7969 0.0039 0.49 178 1 213 6 619 63
24 0.0091 0.0003 4.07 0.7778 0.0037 0.47 175 1 211 7 635 73
25 0.0094 0.0003 4.65 0.6871 0.0033 0.49 178 1 210 15 579 155
26 0.0113 0.0004 5.44 0.5489 0.0094 1.70 176 1 198 13 469 144
27 0.0101 0.0003 4.24 0.6459 0.0031 0.48 176 1 194 10 417 110
28 0.0102 0.0003 4.33 0.6813 0.0035 0.52 177 1 219 10 700 94
29 0.0099 0.0003 5.74 0.4198 0.0020 0.48 176 1 178 15 197 195
30 0.0107 0.0003 4.64 0.4566 0.0022 0.48 177 1 178 11 201 143
pluton shows relatively low S iO2 composition in a range varying from 46 to 75. The transition metals concentrations
of 47.09 to 57.15 wt%. Their K 2O and Na2O contents vary of the samples were lower than those of primitive mafic
from 0.29 to 1.13 and 1.19 to 3.92 wt%, respectively. The magmas (Frey et al. 1978), suggesting the more differenti-
samples form a fairly narrow dispersion in the total alkali ated nature of the former. It is apparent that all the rocks
versus silica diagram (TAS) and plot in the gabbro and gab- generally exhibit positive correlations between SiO2 and A/
broic diorite, which are consistent with the classifications CNK, while they have a negative correlation between A/
performed by observing hand specimens and microscopy CNK and A/NK molar ratio (Fig. 5a, b).
(Fig. 4). All the rocks were mostly metaluminous with A/
CNK ratios (molar Al2O3/[CaO + Na2O + K2O]) ranging Trace elements
from 0.64 to 0.97 (Fig. 5a, b) and usually plot in the calc-
alkaline field of the TAS diagram (Fig. 5c). According to The trace-element concentrations are provided in Table 2.
their A/NK and A/CNK ratios, these rocks can be classi- The gabbroic rocks from the pluton had variable con-
fied as I-type affinity (Fig. 5b). These rocks were charac- tent of total REEs. The samples were characterized by
terized by relatively high MgO (2.55–9.43 wt%), A l2O3 low Yb concentrations (0.74 to 4.36 ppm) and slightly
(14.36–23.03 wt%), Fe2O3tot (5.45–11.12 wt%) concentra- positive and negative or no Eu anomalies with a Eu/Eu*
tions and Mg# values [(100×MgO/(MgO + 0.9Fe2O3tot)] range of 0.61 to 1.62, which likely reflects fractionation
13
Table 2 Major oxide (wt%) and trace-element (ppm) compositions of the calc-alkaline Gokcedere gabbroic pluton in the eastern Sakarya Zone
Sample GD1 GD2 GD6 GD7 GD9 GD10 GD12 GD13
Coordinates 37T 566892 E 37T 566744 E 37T 566011 E 37T 565868 E 37T 565752 E 37T 566430 E 37T 567047 E 37T 567640 E
4440228N 4439743N 4439085N 4438358N 4437708N 4437878N 4438589N 4439058N
Rock type gbb gbb gbb gbb gbb gbb gbb dio gbb
13
Table 2 (continued)
Sample GD1 GD2 GD6 GD7 GD9 GD10 GD12 GD13
Coordinates 37T 566892 E 37T 566744 E 37T 566011 E 37T 565868 E 37T 565752 E 37T 566430 E 37T 567047 E 37T 567640 E
4440228N 4439743N 4439085N 4438358N 4437708N 4437878N 4438589N 4439058N
Rock type gbb gbb gbb gbb gbb gbb gbb dio gbb
13
Table 2 (continued)
Sample GD14 GD16 GD17 GD18 GD19 GD21 GD22 GD23
Coordinates 37T 567939 E 37T 568962 E 37T 568567 E 37T 567952 E 37T 568544 E 37T 568756 E 37T 569438 E 37T 570038 E
4439510N 4439713N 4440080N 4440193N 4440997N 4440872N 4440528N 4440368N
Rock type gbb gbb gbb dio gbb dio gbb gbb dio gbb dio gbb dio
13
Table 2 (continued)
Sample GD24 GD28 GD30 GD41
Coordinates 37T 570478 E 37T 570721 E 37T 569486 E 37T 570123 E
4440599N 4441140N 4441576N 4441788N
Rock type gbb dio gbb gbb dio gbb
Rock types: gbb gabbro. gbb.dio gabbroic diorite. LOI: loss on ignition. Mg# is 100xMgO/(MgO + 0.9FeOtot) in molar proportions. Oxides are
given in wt%. trace elements in µg/g (ppm). ASI is the aluminium saturation index [molar A l2O3/(CaO + K2O + Na2O)]. TZr(0C): whole-rock Zr
saturation temperature was estimated using the Zr solubility in granitic melts (Watson and Harrison 1983)
Fig. 4 Rock classification 20
diagram (Middlemost 1994) σ =25 : Gokcedere pluton
for the Gokcedere pluton. σ is 18
a Rittmann index, defined as
(K2O + Na2O)2/(SiO2-43) 16 foid syenite σ =10
14
Na2O+K2O (wt.%)
syenite
12 foid
monzosyenite
foidolite foid
σ=2.5
10
mozo quartz
diyorit monzonite
8 monzo monzonite
foid diorite
6 gabbro monzo
gabbro
tonalite granite
4
diorite granodiorite
2 perido- gabbroic
gabbro gabbro diorite
0
30 40 50 60 70 80 90
SiO2 (wt.%)
or accumulation of plagioclase. Based on the chondrite- and a nearly flat HREE slope with (La/Yb)n varying from
normalized REE patterns of the intrusive rocks, all the 1.39 to 4.84 (Fig. 6a). All the samples exhibited simi-
samples displayed slight LREEs fractionation patterns lar patterns, namely enrichment in large ion lithophile
13
1.2
a S-type a
Al2O3/CaO+Na2O+K2O peraluminous
100
I-type
1
ASI (molar)
Rock/chondrite
metaluminous
0.8
10
0.6
40 45 50 55 60 65 70
SiO2 (wt.%)
10 1
9
b La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Al2O3/Na2O+K2O (molar)
1000
8
b
7 peraluminous
6 100
5 metaluminous
Sample/PM
4
10
3
2
1 peralkaline 1
0.6 0.8 1 1.2
ASI (molar)
Al2O3/CaO+Na2O+K2O 0.1
10 Rb Th Nb Ce Nd Zr Eu Gd Y Yb
c Ba K La Sr Hf Sm Ti Dy Er Lu
8
Fig. 6 a Chondrite values [normalized to values taken from Boyn-
ultra-potassic ton (1984)] of rare-earth element abundance patterns for the selected
K2O (wt.%)
6 series
samples from the studied plutons. b PM-normalized multi-element
variation patterns [normalized to values taken from Sun and McDon-
shoshonitic
4 series high potassic ough (1989)] for the calc-alkaline gabbroic pluton
calc-alkaline series
calc-alkaline
2 series In situ zircon Lu‑Hf isotope systematic
tholeiitic series
0
45 50 55 60 65 70 75
The 19 Hf isotopic spots of 25 zircons from sample GD1
SiO2 (wt.%) and 16 Hf isotopic spots of 20 zircons from sample GD23
of the pluton were analyzed at the same sites as those used
for U–Pb dating. Table 3 exhibits in situ zircon Lu-Hf iso-
Fig. 5 a ASI versus S
iO2, b Al2O3/Na2O + K2O (molar) versus ASI
[after Maniar and Piccoli (1989)], c K2O versus SiO2 [after Peccer- tope data of the gabbroic samples. The zircon U–Pb ages
illo and Taylor (1976)] for the samples from the Gokcedere gabbroic were used for all the zircon grains to recalculate εHf (t) and
samples TDM1 values. The 176Lu/177Hf ratios were close to 0.001,
suggesting no significant accumulation of radiogenic Hf
after zircon crystallization. Based on the CL images of the
elements (LILEs) (Ba, Rb, Th, and K) relative to high zircons, measured 176Hf/177Hf ratios revealed the composi-
field strength elements (HFSEs), with marked negative tion of the Hf isotope system at the time of zircon crystal-
Nb and Ti anomalies and no depletion of Hf and Zr in lization (Wu et al. 2007). The spots are generally plotted
the primitive mantle normalized multiple-element varia- above the CHUR evolutionary line in the diagram of εHf (t)
tion diagrams (Fig. 6b). The positive Eu and Sr anom- versus age (Ma) (Fig. 7). The εHf (t) values of the zircons of
alies of some samples are indicative of some degree of samples GD1 and GD23 from the pluton were variable and
plagioclase accumulation in the generation. The gabbroic positive and in the range of 4.6–13.5, varying over a total
samples of the pluton had low and variable Ni (<20 to range of 8.9 εHf units and an average value of 8.3, which
85 ppm) and Co (14 to 35 ppm) content. corresponds to TDM1 model ages of 0.30 to 0.65 Ga for the
Gokcedere gabbroic pluton.
13
Whole‑rock Sr‑Nd isotopic compositions 177 Ma based on the U–Pb zircon age technique. The gab-
broic rocks had nearly homogenous and relatively depleted
Table 4 shows the results of five analyses for Sr and Nd isotopic compositions, with (87Sr/86Sr)i of 0.705124 to
isotopic ratios from the Gokcedere gabbroic pluton, Demi- 0.705599 and εNd (t) of 0.1 to 3.5. The single-stage Nd
rozu-Bayburt area, northeast Turkey. The measured isotope model ages (TDM1) relative to a depleted mantle reservoir
ratios of the samples were corrected by an average age of were relatively young, and the values ranged from 0.65 to
Table 3 In situ zircon Hf isotopic data of the calc-alkaline Gokcedere gabbroic pluton from the Bayburt area in the eastern Sakarya Zone
Sample/spot 176Yb/177Hf 176Lu/177Hf 176Hf/177Hf 2σ Age (Ma) εHf(t) fLu/Hf TDM1(Ga)
εHf(0) = ((176Hf/177Hf)S/(176Hf/177Hf)CHUR,0 − 1) × 10,000, fLu/Hf = (176Lu/177Hf)S/(176Lu/177Hf)CHUR − 1
εHf(t) = ((176Hf/177Hf)S − (176Lu/177Hf)S×(eλt − 1))/((176Hf/177Hf)CHUR,0 − (176Lu/177Hf)CHUR×(eλt − 1)) − 1) × 10,000
TDM1(Hf) = 1/l × (1 + ((176Hf/177Hf)S − (176Hf/177Hf)DM)/((176Lu/177Hf)S − (176Lu/177Hf)DM))
(176Hf/177Hf)S measured values; (176Lu/177Hf)CHUR = 0.0332, (176Hf/177Hf)CHUR,0 = 0.282772 (Blichert-Toft and Albarède 1997)
(176Lu/177Hf)DM = 0.0384 ve (176Hf/177Hf)DM = 0.28325 (Griffin et al. 2000); fCC = − 0.548 (composition of continental crust), fDM = 0.16, t = zir-
con crystallization time, λ = 1.865 × 10−11year−1 calculations based on (Söderlund et al. 2004)
13
εHf (t Ma)
sitional ranges of late Carboniferous to early Permian plu-
tons (Karsli et al. 2016), hybrid Harsit pluton (Karsli et al. CHUR
0
2010a), and A-type granites from the Eastern Pontides
(Karsli et al. 2012a) (Fig. 8).
-5
13
10
0.95
0.65
0.68
0.74
0.69
5 50 Ma lower crustal-derived
Sisdagi pluton adakitic rocks, Eastern Pontides
81 Ma A-type
Pirnalli pluton
0
εNd(t) Eocene calc-alkaline
0.1
3.5
2.9
2.3
2.8
Dolek ve Saricicek
plutons
-5
εNd(t )
Everek Hanlari
plagiolositites Late Carboniferous to
2σ m
8
8
7
6
6
-10 Harsit pluton
Nd/144Nd
-15
0.512585
0.512806
0.512784
0.512742
0.512808
143
-20
0.702 0.704 0.706 0.708 0.71
Initial Sr/ Sr
87 86
Sm/144Nd
high-K calc-alkaline Harsit rocks are after Karsli et al. 2012b, Altherr
et al. (2008), and Karsli et al. (2010a), respectively. Eocene Dolek
12
and Saricicek hybrid plutons are after Karsli et al. (2007). The A-type
8
2
5
8
plutons from the eastern Sakarya Zone are from Karsli et al. (2012a)
[Sm] ppm
2010).
0.19
0.14
0.25
0.25
0.13
87
Differentiation processes
[Sr] ppm
12
15
13
and positive εNd (t) values (0.1 to 3.5). The ancient zircon
13
population are expected for the significant contamina- heterogeneity (e.g., Yang et al. 2007; Zhao et al. 2012).
tion during the magma evolution. However, they were not High positive zircon εHf (t) values are indicative of depleted
observed during in situ zircon analysis for the present work. mantle-derived magmas (e.g. Ma et al. 2013). Our samples
Notably, the depletion of Nb and Ti was expected due to had no mafic micro granular enclaves or specific textures,
crustal contamination. Rocks related to crustal contamina- which implies magma mixing and mingling processes in
tion should exhibit enrichment of Zr and Hf. This is not the their generation. The variable and usually positive zircon
case for the Gokcedere gabbroic rocks in the spidergrams εHf (t) values (from 4.6 to 13.5) with young single-stage Hf
(Fig. 6b). Moreover, the gabbroic samples display linear model ages (TDM1 = 0.30 to 0.65 Ga) demonstrate that het-
correlations between the major oxides and S iO2 on Harker erogeneous depleted mantle components were involved in
variation diagrams (not shown), implicating lithological the melting source and the rocks were unlikely to be par-
and geochemical variations of the samples are due to crys- tial melts of a subducting oceanic slab. This is mirrored by
tal fractionation during magma transport and emplacement. the unique signature of the gabbroic rocks, which is incon-
The negative Ba, Sr, and Ti anomalies on the spidergrams sistent with those of experimental melts from infracrus-
of Fig. 6b imply the magmas experienced the fractionation tal rocks such as metabasalts and eclogites with low Mg#
of plagioclase and Fe-Ti oxide, which also supports the (<43) values (Rapp and Watson 1995; Patiño Douce 1999).
above interpretation. Therefore, we propose that the crus- The rocks were similar to the magma derived from par-
tal contamination process may be limited or insignificant in tial melting of a ultramafic mantle, namely the depleted
the generation and that crystal fractionation played a signif- mantle, for which zircon εHf (t) values are usually high
icant role in the slightly evolved signature of the Gokcedere and positive (Griffin et al. 2000; Ma et al. 2013). εHf (t)
gabbroic rocks, which formed in the eastern Sakarya Zone values of an enriched lithospheric mantle and crustal
in response to slab roll-back of the Paleo-Tethyan oceanic materials are expected to be negative (Yang et al. 2006;
lithosphere in the early Jurassic period before the Cimme- Chen et al. 2008), which was not the case for the stud-
rian Orogeny. ied samples. The isotopic compositions of the gabbroic
samples were less radiogenic than those of a depleted
Source nature asthenospheric melt (εNd(t) = +5; Basu et al. 1991). Their
low Nb/La ratios are in complete accord with those of the
The gabbroic samples of the early Jurassic Gokcedere lithospheric mantle (0.3–0.4) rather than OIB-like asthe-
pluton were characterized by relatively high Sr nospheric mantle melts (~>1; Bradshaw and Smith 1994;
(147–222 ppm), high Y (10–39 ppm), and HREE con- Smith et al. 1999). Additionally, the samples had uniform
centrations (Yb = 0.94–4.36 ppm) and low (La/Yb)n and relatively high Zr/Y (2–4) and Nb/Y (0.03–0.16)
(1.39–4.84) values, mimicking geochemical signature com- ratios, suggesting the melting of a depleted lithospheric
parable to normal island arc-derived rocks (Defant and mantle wedge. Moreover, high Sm/Yb (0.85–1.20) and
Drummond 1990). The studied rocks possessed low S iO2 La/Sm (1.60–4.07) values are consistent with the nature
content (47.09–57.15 wt.7%) and high Mg# values (46–75) of a depleted lithospheric mantle composed of spinel-
and relatively high concentrations of compatible elements bearing lherzolite, which was confirmed by the trace-
(Ni = 20–85 ppm; Co = 14–35). These geochemical features element modeling below. These features favor a source
point to a mantle component in their origin. Their moder- of depleted and young subcontinental lithospheric mantle
ately fractionated LREE and HREE patterns, enrichment rather than an enriched mantle or a juvenile mafic lower
in LREEs relative to HREEs, depletion in HFSEs (such crust for the pluton. During the subduction events of the
as Nb and Ti), and general lack of Eu anomalies also indi- Paleo-Tethyan oceanic slab that began in the late Carbon-
cate a subduction signature (e.g., Hawkesworth et al. 1997; iferous period, deep slab dehydration caused a basaltic
Elburg et al. 2002; Cameron et al. 2003). However, all geo- melt. Then, the basaltic melt triggered the partial melting
chemical features of this work reveal that their magmas of a depleted subcontinental lithospheric mantle wedge,
do not represent their primary nature and that they expe- possibly contributing to the generation of the gabbroic
rienced some crystal fractionation. The mantle origin was pluton. The relatively young single-stage Hf (TDM1 = 0.30
confirmed by relatively low (87Sr/86Sr)i values (0.705124 to 0.65 Ga) and Nd (TDM1 = 0.65 to 0.95 Ga) model ages
to 0.705599) and positive εNd (t) values (0.1–3.5), which point to a young subcontinental lithospheric mantle as a
were close to the MORB area in the Fig. 8. Hf isotopic protolith and the lack of involvement of ancient crustal
composition in zircon is an efficient tool to elucidate the materials during partial melting. These suppositions are
nature of magma source and magma mixing processes in accordance with the formation of the late Triassic to
(e.g., Griffin et al. 2002). Accordingly, igneous rocks hav- early Jurassic intrusive Zone. These intrusive rocks refer
ing similar U–Pb zircon ages and varying εHf (t) values are to subduction-related magmatism in the eastern Sakarya
expected to derive from different source magmas or source (Dokuz et al. 2006, 2017a; Karsli et al. 2014).
13
The primary magmas of the studied gabbroic sam- enriched in LILEs over HFSEs by the metasomatic activ-
ples may contain possible Al-phase types such as garnet ity of fluids derived from the subducted slab or sediments
and spinel during mantle melting at a great depth. Deeper (e.g., Hawkesworth et al. 1997; Elburg et al. 2002; Cam-
sources can be inferred when the melting event occurred in eron et al. 2003). An alternative metasomatism event is
the presence of garnet (i.e., melting of garnet-bearing man- an interaction between the lithospheric mantle and the
tle sources). Such melting resulted in fractionated MREE/ volatile-rich material of the asthenosphere (McKenzie
HREE ratios, and hence negatively inclined the MREE to 1989; Gibson et al. 1995). Such an enrichment is not the
HREE patterns of the produced magmas on normalized case for the geochemical character of the gabbroic pluton.
REE plots. In contrast, spinel-bearing mantles do not result Therefore, we propose that slab-derived fluids or sediment-
in fractionation of the MREE/HREE ratios of the resulting related mantle enrichment played a major role in the gen-
melts. The rocks produced by these ways are characterized eration of the pluton. Accordingly, the negative anomalies
by flatter MREE to HREE patterns. The MREE to HREE of Nb and Ti are typical of rocks formed in a subduction
abundances of the gabbroic samples formed flat patterns setting (Tatsumi and Kogiso 1997), which is consistent
on the chondrite-normalized REE diagram (Fig. 6a). The with the suggested model for the studied pluton. Hence, we
low (Tb/Yb)n values (0.90 to 1.28) of the samples reveal assume that the mantle enrichment occurred during sub-
that they may have been produced in the absence of gar- duction of the Paleo-Tethyan oceanic lithosphere. In such
net in their mantle source (Ge et al. 2002). The melting a case, a large amount of fluid and/or sediment caused the
histories of the gabbroic samples mirrored by the trace- metasomatization of the depleted mantle wedge prior to its
element modeling of Gd/Yb versus La/Yb ratios of the partial melting. The samples exhibited high La/Ta (22–68)
rocks in Fig. 9. In the modeling diagram, primitive man- and low La/Ba (0.02–0.0.06) ratios, similar to those of a
tle (PM; Palme and O’Neil 2004), depleted- and enriched- subcontinental lithosperic mantle modified by subduc-
depleted MORB mantles (D-DMM and E-DMM, respec- tion processes (e.g., Zhou et al. 2005; Wang et al. 2013).
tively; Workmann and Hart 2005), and average E-MORB The narrow range of the Th/Yb (0.27–1.42) and low Th/
(Sun and McDonough 1989) are inserted to demonstrate Ce (0.05–0.33) ratios of the samples reveal that subducted
the mantle array (gray field on Fig. 9). Importantly, the sediment materials may have had little or no joining in the
samples are plotted along the mantle array, indicating that mantle source. Elevation of the values of the samples is
MREE/HREE (Gd/Yb) fractionation did not occur during expected if a sedimentary addition formed, as suggested
their genesis. The samples exhibit a positive trend with a by Taylor et al. (1981) and Woodhead et al. (2001). Rela-
slight variation in their Gd/Yb. This observation suggests tively low Sm/Th (0.78–4.11) and high Th/Y (0.03–0.14)
that the primary magmas of the samples were derived ratios and variable Th/Zr (0.01–0.40), Ba/Th (522–194),
from a shallower mantle source with the absence of garnet. Ba/La (15–47), and Ba/Nb (54–226) ratios of the sam-
The modeling results imply that low and variable portions ples are indicative of subducted slab-derived fluids in the
(~5–15%) of partial melting of a depleted mantle material, source. This is further supported by the generally positive
which consists of spinel-bearing lherzolite, were apparent zircon εHf (t) values of the samples. Beccaluva et al. (2004)
mechanisms for the generation of the samples. Fractional suggested that an important feature of subduction-related
melting curves for both spinel- and garnet-bearing man- mantle metasomatism is the formation of the hydrous
tle mineralogy with PM trace-element composition are mineral phase phlogopite or amphibole (or both). The ele-
depicted in Fig. 9 to support these interpretations. ment compatibility of phlogopite and amphibole can indi-
cate which hydrous phase was formed in the lithospheric
Metasomatism of depleted mantle source mantle source during melting (Furman and Graham 1999;
Yang et al. 2004). The most primitive sample of the pluton,
The depleted isotopic signature of the Gokcedere gab- GD19, had low Ba/Rb (5.0) and Nb/Th (2.0) ratios whereas
broic samples represent the mantle source prior to magma its Rb/Sr ratio (0.05) was relatively high. This suggests that
generation. The samples have depleted isotopic signature, the source should have contained phlogopite rather than
but they show enrichment in their geochemical compo- amphibole. Furthermore, the chondrite-normalized LREE
sitions due to mantle enrichment processes. Indeed, the patterns of the samples were variable and the differences in
samples exhibit slight enrichment of LILEs and LREEs the REE patterns (Fig. 6a) can be attributed potentially to
with negative Nb and Ti anomalies, which are indicative source heterogeneity in the generation. Relative depletion
of subduction-related magmas. These features are com- in MREE is expected in the chondrite-normalized REE pat-
monly attributed to a mantle source. However, how and terns of the samples if amphibole is stable in the source (Ge
when the enriched materials entered the mantle source of et al. 2002). This is not the case for the Gokcedere gabbroic
the Gokcedere gabbros remains debatable. Mantle materi- samples (Fig. 6a). Therefore, this evidence indicates pres-
als in a mantle wedge are expected to have been previously ence of phlogopite in the source mineralogy during their
13
10 et al. 2016; Dokuz et al. 2017b). However, early Juras-
10%
5% 1% sic dynamic events are still debated due to limited data
ies of the intrusive rocks. Therefore, understanding the early
et fac
arn
G 1% Mesozoic evolutional history of the region depends on
10% 5% Spinel facies the abundance of works on the early Jurassic magmatic
Gd / Yb
E-MORB
rocks.
1 PM
D-DMM
E-DMM The calc-alkaline Gokcedere gabbroic pluton with a
metaluminous and I-type signature exhibited enrichment in
LREEs and LILEs and depletion in HREEs and HFSEs in
the chondrite-normalized rare-earth element patterns and
multi-element spidergrams. Such a geochemical fingerprint
0.1 points geochemically to an enriched mantle wedge mate-
0.1 1.0 10 100
La / Yb rial and is also similar to that of an arc magmatic affinity
described by (Sajona et al. (1996) and Stern and Kilian
Fig. 9 The variation diagram of Gd/Yb versus La/Yb for the samples (1996). A few models have been suggested for magma gen-
from the Gokcedere pluton. Also shown are primitive mantle (PM; eration from melting of a geochemically enriched mantle,
Palme and O’Neil 2004), depleted- and enriched-depleted MORB namely (1) the magmatism resulting from the delamination
mantles (D-DMM and E-DMM, respectively; Workmann and Hart of thickened continental crust due to arc- or continent–con-
2005), and average E-MORB (Sun and McDonough 1989) to dem-
onstrate the mantle array (grey field). Mineral and melt modes for tinent collision, (2) melting of an enriched mantle caused
spinel- and garnet facies mantle mineralogy are ol0.53(−0.06) + opx by large-scale extension (Thompson et al. 1989), (3) mag-
0.27(0.28) + cpx0.17(0.67) + sp0.03(0.11) (Kinzler 1997) and ol0.60( matism triggered by flat subduction of an oceanic slab (Li
0.03) + opx0.20(−0.16) + cpx0.10(0.88) + gt0.10(0.09) (Walter 1998), and Li 2007) or related slab window (Sun et al. 2007), and
respectively. Melting curves with increments of 1–10% were calcu-
lated using non-modal fractional melting equation of Shaw (1970). (4) magmatism in response to a back-arc setting related to
The partition coefficients are from Adam and Green (2006) subduction of the Paleo-Tethyan oceanic lithosphere. Sub-
duction is believed to be responsible mechanism for the
magma generation in the late Triassic-early Jurassic, as
generation. We can thus infer that the depleted source mate- proposed by Dokuz et al. (2010) and Karsli et al. (2014).
rial was likely a phlogopite- and spinel-bearing lherzolite. Indeed, collision-related magmatism for the Sakarya Zone
is reported for the late Jurassic period (Dokuz et al. 2017a).
Therefore, we can rule collisional magmatism out based on
Implications for geodynamic processes the available data. We can also omit the large-scale lith-
and tectonic evolution ospheric extension as an alternative model due to the rarity
of late Triassic to early Jurassic magmatism in the region.
The Gokcedere gabbroic pluton formed in the eastern Similarly, wide temporal and spatial distribution of mag-
Sakarya Zone, where Carboniferous to late Triassic plu- matism are expected if flat subduction and related slab win-
tonic rocks are characterized by enriched mantle- and dow models generate gabbroic rock (Li and Li 2007; Sun
crustal-derived Sr-Nd isotopic compositions (Topuz et al. 2007). There is no evidence of flat subduction- and
et al. 2010; Dokuz 2011; Kaygusuz et al. 2012; Karsli slab window-related magmatic activity in the area. Subse-
et al. 2016). This indicates the occurrence of an enriched quently, we propose a back-arc setting related to subduc-
subcontinental lithospheric mantle and lower crustal tion of the Paleo-Tethyan oceanic lithosphere because the
material beneath the Sakarya Zone, northeast Turkey. sporadic nature of the intrusive rocks agrees well with the
However, the Gokcedere gabbroic samples have a rela- derivation from partial melting of an enriched lithospheric
tively depleted Sr-Nd isotopic signature. Changes in the mantle beneath the Gondwana. This view is reinforced
isotopic compositions of the resultant magmas represent by trace-element variation diagrams (Pearce et al. 1984;
changes in tectonic processes (Chu et al. 2011). High- Pearce 1996), which exhibit arc evolutionary trends. We
precision LA-ICP-MS in situ zircon U–Pb techniques attempt to use a plot of Rb versus Y + Nb, where the sam-
yielded weighted mean ages between 176.95 ± 0.49 Ma ples cluster into the volcanic arc granitoid (VAG) field
(MSWD = 1.08) and 178.41 ± 0.44 Ma (MSWD = 1.10) (Fig. 10a) and the samples plot within the field of a pre-
for the Gokcedere pluton, revealing its emplacement age. plate collision phase on the R1-R2 diagram (Batchelor and
This age corresponds with the early Jurassic period in the Bowden 1985), supplying tectonic discrimination for the
region. Early Carboniferous to early Permian tectonother- granitoid plutons (Fig. 10b). This trend agrees with the
mal events are well known in the Sakarya Zone (Topuz clustering of the samples in the VAG field in Fig. 10a. In
et al. 2010; Dokuz 2011; Kaygusuz et al. 2012; Karsli this model, the roll-back of the subducted slab is thought to
13
have provided upwelling of a hot asthenosphere, triggering are in accordance with the observations that the plutons
partial melting of the mantle wedge (Fig. 11). were likely emplaced in an extensional setting during the
The early to middle Carboniferous period is referred to late stage of the southward subduction of Paleo-Tethyan
arc-continent collision and post-collision events, and many oceanic slab.
researchers have reached a consensus on this area (Topuz In summary, the tectonic affiliation and petrogenetic
et al. 2010; Dokuz 2011; Kaygusuz et al. 2012; Dokuz nature of the Gokcedere gabbroic pluton point to an ori-
et al. 2017b). However, subduction of the Paleo-Tethyan gin in a late phase of a continental magmatic arc setting
Oceanic lithosphere beneath the Gondwana has been sug- in response to southward subduction of the Paleo-Tethyan
gested as starting start from the late Carboniferous to early oceanic lithosphere beneath the Gondwana during early
Permian period (Karsli et al. 2016). We thus believe that the Jurassic period. In the case of the southward subduc-
the extensional events provided the emplacement of the tion model, it was thought that the northern margin of the
pluton and may have no relationship with collision and Paleo-Tethyan Ocean was passive. During its subduction,
post-collisional phases. Paleo-Tethyan oceanic subduc- the dip angle of the subducted slab increased. Continental
tion polarity in the region is still debated, and two models extension was induced by a roll-back of the slab as conse-
have been proposed. The southward subduction (Sengor quence of dipping and then the continental lithosphere was
and Yilmaz 1981; Sengor et al. 1984; Yilmaz et al. 1997; thinned by extension, as shown in Fig. 11. The back-arc
Dokuz et al. 2010) and northward subduction models (Rob- lithospheric extension caused upwelling of the hot asthe-
ertson and Dixon 1984; Robinson et al. 1995; Ustaomer nosphere, which transferred heat energy into the man-
and Robertson 2010) have been proposed to explain the late tle wedge, resulting in the partial melting of the depleted
Paleozoic to early Mesozoic tectonomagmatic events in the mantle wedge, which was metasomatized by fluids released
region. According to the northward subduction model, the by the subducted slab, formed beneath the continent. The
Pontides are accepted as a continental margin bordering the parental basic melt may have been formed in this stage and
Paleo-Tethyan Ocean on the north. In contrast, in the south- the depleted mantle-derived basic magma subsequently
ward subduction model, the Pontides are referred to as the evolved via crystal fractionation with no or minor crus-
northern margin of Gondwana throughout the early Triassic tal contamination during its ascent into the crust. Conse-
to early Jurassic period. In this model, the Paleo-Tethyan quently, we conclude that the eastern part of the Sakarya
oceanic lithosphere was southwardly subducted beneath the Zone was favored by final phase of southward subduction
Gondwana until the middle Jurassic period, and the Cim- of the Paleo-Tethyan oceanic lithosphere beneath the Gond-
merian continent, including the eastern Sakarya Zone, was wana throughout the early Jurassic period.
separated from the Gondwana due to the subduction event
in the Triassic. These events resulted in the opening of an
intra-continental back-arc basin. Closure of the Paleo-Teth- Conclusions
yan Ocean led to an amalgamation of the eastern Sakarya
Zone with the Laurasia during the middle Jurassic period New in situ zircon U–Pb ages and Hf isotopes, whole-rock
(Sengor 1979; Sengor and Yilmaz 1981; Sengor et al. geochemical data, and Sr-Nd isotopes of a scarce volume
1980, 1984; Yilmaz et al. 1997; Dokuz et al. 2010). Recent gabbroic intrusion from the eastern Sakarya Zone (Bayburt
paleobiogeographical reconstructions, based on early Juras- area, NE-Turkey) provide constraints on the source, magma
sic brachiopod fauna from the Sakarya Zone, indicate that processes, and geodynamic events and reveal the following
the Cimmerian micro continent was far from the Gond- results in this work.
wana and near the Eurasian margin of the Tethys (Vörös
and Kandemir 2011). The position of the Cimmerian micro 1. New in situ zircon U–Pb ages show igneous crystalli-
continent clearly supports the southward subduction model zation ages ranging from 176 to 178 Ma, correspond-
in the early Jurassic. This geodynamic model is also con- ing with the early Jurassic period. We interpret this
sistent with the generation of the early to middle Jurassic range as the likely emplacement age for the Gokcedere
magmatism in the Yusufeli region in the eastern Sakarya gabbroic pluton.
Zone reported by Dokuz et al. (2010). What is more, the 2. The early Jurassic gabbroic pluton has a metalumi-
presence of Hardisi and Catalcesme Formations deposited nous geochemical character and belongs to a slightly
in a marginal basin in the south of the area is also evidence evolved I-type series. Its geochemical fingerprints and
for its southward subduction. In this case, the southward whole-rock Sr-Nd and in situ zircon Hf isotope compo-
subduction of the Paleo-Tethyan oceanic slab is more con- sitions represent a depleted mantle source. Thus, a gab-
sistent with the generation of the early Jurassic Gokcedere broic pluton with a narrow range of rocks types may
gabbroic pluton, based on its unique isotopical and geo- have been produced by partial melting of an isotopi-
chemical signatures mentioned above. All of these results cally depleted mantle wedge, which was subsequently
13
POG WPG
100 of the unique composition of the calc-alkaline gab-
broic pluton. In such a dynamic system, upwelling of
VAG a hot asthenosphere triggered by roll-back of the slab
10
ORG in a subduction setting caused partial melting of the
depleted mantle wedge.
3. The subduction of the Paleo-Tethyan oceanic litho-
1
1 10 100 1000 10000 sphere beneath the Gondwana continent commenced
Y+Nb (ppm)
in the late Carboniferous to early Permian period. The
b 2000 roll-back events were a consequence of the late phase
of subduction of the Paleo-Tethyan oceanic lithosphere
Mantle fractionation
formed during the late Triassic to early Jurassic peri-
1500 ods. The integration of geological data with the new
results of this study emphasizes that the final phase
R2=6Ca+2Mg+Al
13
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