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Zircon U–Pb–Hf snapshots on the crustal evolution of the Serbo-Macedonian massif: new insights from Ammouliani island (Northern Greece)

Published online by Cambridge University Press:  02 August 2021

Konstantinos Papapavlou Open the ORCID record for Konstantinos Papapavlou [Opens in a new window]
Konstantinos Papapavlou*
Affiliation:
GEOTOP, Université du Québec à Montréal, Montréal, QC, H3C 3P8, Canada
*
Author for correspondence: Konstantinos Papapavlou, Email: constantinepapapavlou@gmail.com
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Abstract

Zircon U–Pb and Lu–Hf isotope microanalysis was conducted in (meta)-igneous units of Ammouliani island to characterize crust-forming and reworking events in the Serbo-Macedonian massif. Zircon grains from an orthogneiss of the Vertiskos unit yielded a U–Pb crystallization age for the igneous precursor at 458.8 ± 11 Ma with dominantly subchondritic ϵHf values indicating reworking of Neoproterozoic basement. A weighted mean ϵHf value of 0.7 ± 2.4 from oscillatory zoned zircon grains of the Ouranoupoli granodiorite indicates juvenile crustal input at 52.1 ± 0.6 Ma. The U–Pb–Hf zircon archive records discrete stages in the crustal evolution of the Serbo-Macedonian massif.

Keywords

crustal evolutionzirconU–PbLu–HfSerbo-Macedonian massifHellenides
Type
Rapid Communication
Information
Geological Magazine , Volume 158 , Issue 11 , November 2021 , pp. 2079 - 2086
Copyright
© The Author(s), 2021. Published by Cambridge University Press

1. Introduction

Peri-Gondwana basement terranes of Cadomian and Avalonian affinity are dispersed along the Alpine–Zagros–Himalayan orogenic edifice (Neubauer, Reference Neubauer2002; Okay et al. Reference Okay, Satir and Shang2008 b; Moghadam et al. Reference Moghadam, Griffin, Li, Santos, Karsli, Stern, Ghorbani, Gain, Murphy and O’Reilly2017; Avigad et al. Reference Avigad, Morag, Abbo and Gerdes2017). One of these terranes, that extends from Serbia to the Chalkidiki peninsula in Northern Greece, is the Serbo-Macedonian massif (SMM; Dimitrijevic, Reference Dimitrijević and Mahel’1974). The SMM is a NW–SE-trending, ribbon-shaped complex of amphibolite- to granulite-facies gneisses and schists, and along with the Rhodope massif towards the east, constitutes the crystalline core of the internal Hellenides (Fig. 1). The crustal evolution and metallogenic endowment of the SMM has been chiefly controlled by igneous and hydrothermal processes that led to the formation of porphyry Cu–Mo–(Au), Cu skarn, Au-rich polymetallic vein, Pb–Zn–Ag–Au carbonate replacement and shear-hosted Cu–Au–Bi–Te deposits (Bristol et al. Reference Bristol, Spry, Voudouris, Melfos, Mathur, Fornadel and Sakellaris2015; Siron et al. Reference Siron, Thompson, Baker, Darling and Dipple2019). Temporal constraints on magmatic and metamorphic petrogenetic processes in the SMM have been placed chiefly using whole-rock Pb–Sr–Nd and mica K–Ar, Rb–Sr and 40Ar–39Ar isotope systematics (Juteau et al. Reference Juteau, Michard and Albarède1986; De Wet et al. Reference De Wet, Miller, Bickle and Chapman1989; Lips et al. Reference Lips, White and Wijbrans2000; Christofides et al. Reference Christofides, Perugini, Koroneos, Soldatos, Poli, Eleftheriadis, Del Moro and Neiva2007) that are prone to metamorphic and metasomatic resetting (e.g. Moorbath et al. Reference Moorbath, Whitehouse and Kamber1997). In this regard, studies that utilize the chronometric, isotope tracer, and/or compositional archive of retentive U-bearing accessory phases remain scarce in this crystalline terrane compared to the Rhodope massif (e.g. Liati et al. Reference Liati, Theye, Fanning, Gebauer and Rayner2016). U–Pb dating combined in some studies with Lu–Hf isotope tracer constraints have been conducted on igneous and detrital zircon grains from this terrane in order to chart in more detail crustal growth patterns of the SMM and Rhodope massif and enhance our understanding about the evolution of the European crust (Himmerkus et al. Reference Himmerkus, Ander, Reischmann, Kostopoulos, Hatcher, Carlson, McBride and Martinez Catalán2007; Peytcheva et al. Reference Peytcheva, Macheva, von Quadt and Zidarov2015; Antić et al. Reference Antić, Peytcheva, von Quadt, Kounov, Trivić, Serafimovski, Tasev, Gerdjikov and Wetzel2016; Abbo et al. Reference Abbo, Avigad and Gerdes2020). In order to investigate further the crustal evolution of the SMM, I present in this contribution new U–Pb and Lu–Hf isotope data from zircon grains of two (meta)-igneous units that crop out in a Greek segment of the SMM that remains underexplored: Ammouliani island in the Mount Athos gulf (Fig. 1). A complete transect, from Palaeozoic basement units that constitute the volumetrically dominant component of the SMM to Palaeogene granitoids (Ouranoupoli granodiorite) with a poorly understood petrogenetic history, crops out on Ammouliani island. Thus, this locality allows the study, at different crustal levels, of geological units associated with the igneous infrastructure of the SMM. Therefore, the main aims of this study are (a) to provide new insights on the petrogenesis of the igneous and meta-igneous units of Ammouliani island and the wider area through the prism of U–Pb and Lu–Hf isotope microanalysis of zircon grains, and (b) to compare and contrast the U–Pb and Lu–Hf zircon isotope data of this contribution with existing isotope data for the SMM in order to advance our fragmentary understanding about the crustal growth and evolution of this terrane.

Fig. 1. Simplified geological map of the Serbo-Macedonian massif modified from Himmerkus et al. (Reference Himmerkus, Reischmann, Kostopoulos, Robertson and Mountrakis2006). The upper right panel depicts a simplified geological map of Ammouliani island, with sample localities, modified from Kockel et al. (Reference Kockel, Mollat and Antoniadis1978).

2. Geological setting

The SMM comprises four basement units, the Pyrgadikia, Vertiskos, Arnea, and Kerdilion units (Kockel et al. Reference Kockel, Mollat and Antoniadis1978; Kilias et al. Reference Kilias, Falalakis and Mountrakis1999; Himmerkus et al. Reference Himmerkus, Ander, Reischmann, Kostopoulos, Hatcher, Carlson, McBride and Martinez Catalán2007). The Pyrgadikia unit contains small (<1 km2) and scarce exposures of felsic mylonitic orthogneisses and mylonitic quartzites (Himmerkus et al. Reference Himmerkus, Reischmann, Kostopoulos, Robertson and Mountrakis2006). Two samples of felsic orthogneisses from this unit have been dated by the Pb–Pb zircon evaporation technique, giving ages of c. 570 and 587 Ma with whole-rock ϵNd values of −7.59 and −7.64, respectively. U–Pb isotopic dating of detrital zircon grains from the mylonitic quartzite of the Pyrgadikia unit shows a dominant age peak at c. 580 Ma (Himmerkus et al. Reference Himmerkus, Reischmann, Kostopoulos, Robertson and Mountrakis2006; Abbo et al. Reference Abbo, Avigad and Gerdes2020). The Vertiskos unit occupies the central part of the SMM and is composed chiefly of felsic augen and mica gneisses that attained amphibolite-facies conditions (Dixon & Dimitriadis, Reference Dixon, Dimitriadis, Dixon and Robertson1984). Pb–Pb zircon evaporation ages from gneissic samples of this unit have yielded dates, interpreted as protolith ages, of between c. 426 and 443 Ma, but U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) isotope dating suggests a Middle Ordovician, c. 460 Ma age, for this unit (Himmerkus et al. Reference Himmerkus, Ander, Reischmann, Kostopoulos, Hatcher, Carlson, McBride and Martinez Catalán2007; Abbo et al. Reference Abbo, Avigad and Gerdes2020). The LA-ICP-MS U–Pb dating of oscillatory zoned zircon grains from the Arnea unit, a complex of ferroan (A-type) granitoids that intrudes the Vertiskos unit, has yielded a concordia age of 243.6 ± 1.5 Ma (2s), interpreted as the crystallization age of this unit (Poli et al. Reference Poli, Christofidis, Koronaios, Soldatos, Perugini and Langone2009). Recent studies though have reported a U–Pb concordia age of 300 ± 1 Ma from an undeformed granite at the central part of the Arnea Pluton (Abbo et al. Reference Abbo, Avigad and Gerdes2020). The Kerdilion unit occupies the eastern part of the SMM and comprises biotite gneisses, amphibolites, migmatites and marbles that attained amphibolite- to granulite-facies conditions (Dixon & Dimitriadis, Reference Dixon, Dimitriadis, Dixon and Robertson1984). Pb–Pb single zircon evaporation and in situ U–Pb zircon dating studies of igneous zircon grains from gneisses of the Kerdilion unit report protolith ages of c. 145 Ma that intruded a basement unit of c. 300 Ma (Himmerkus et al. Reference Himmerkus, Zachariadis, Reischmann and Kostopoulos2012). The basement units of the SMM are intruded by piercing Eocene granitoids of the Sithonia plutonic complex and Oligo-Miocene porphyritic intrusions that host Cu–Mo–(Au) deposits (Pe-Piper & Piper, Reference Pe-Piper and Piper2002).

Ammouliani island belongs to the SMM and is located between the Sithonia and Mount Athos peninsulas, covering an area of ∼8 km2. The main lithostratigraphic units of Ammouliani island (Fig. 1), based on Kockel et al. (Reference Kockel, Mollat and Antoniadis1978), are (a) a package of quartzofeldspathic and two-mica gneisses that are locally anatectic and are thought to be equivalent to the Vertiskos unit, (b) a package of biotite gneisses, marbles and felsic gneisses that is equivalent to the Kerdilion unit, and (c) a two-mica granodiorite (Ouranoupoli granodiorite) that crops out in the southern part of the island.

3. Sampling and analytical details

3.a. Sample details

Two samples were collected from (meta)-igneous units exposed in the northern and southern parts of Ammouliani island. Sample KP18-1 (40° 20′ 19.4″ N, 23° 55′ 05.6″ E) is a banded biotite-rich orthogneiss of the Vertiskos unit (Fig. 1). The examined outcrop of the Vertiskos unit on Ammouliani island is characterized by the alternation of steeply dipping mica gneisses with massive mesoscale felsic bands. Locally the unit contains synkinematic plagioclase-rich leucosome developed parallel to the gneissic foliation. Sample KP18-3 (40° 19′ 04.2″ N, 23° 55′ 49.7″ E) is an undeformed granodiorite that belongs to the Ouranoupoli granodiorite unit and occupies the southern part of the island. This granodiorite, on Ammouliani island, is commonly undeformed but locally shows penetrative deformation transforming it into an augen gneiss. Sample KP18-1 is characterized by the mineral assemblage Qtz–Pl–Kfs–Bt–Ms–Zr–Ttn ± Ap. Twinned titanite porphyroclasts that define sigmoids are observed in the sample. Myrmekite textures in plagioclase and bands of recrystallized quartz grains are also conspicuous features. Sample KP18-3 is characterized by the mineral assemblage Pl–Qtz–Ep–Bt–Act–Kfs–Zr–Aln ± Ttn ± Ap. The sample contains different plagioclase populations with the conspicuous presence of oscillatory zoned plagioclase megacrysts that host plagioclase grains with polysynthetic twinning. Myrmekite textures are also common in plagioclase. Prismatic, millimetre-scale allanite grains with epidote rims are also observed, with the epidote grains commonly hosting titanite, apatite and biotite inclusions.

3.b. Analytical details

Zircon grains from both samples were separated using conventional techniques (i.e. Wilfley table, heavy liquids, magnetic separation using a Franz isodynamic separator) and the grains from the non-magnetic fraction were annealed at 1000 °C for 48 hours. Selected zircon grains were mounted in epoxy resin and imaged using a Deben Centaurus cathodoluminescence (CL) detector attached to a variable pressure Hitachi S-3400 N scanning electron microscope (SEM) in the facilities of GEOTOP-UQAM (Montreal, QC, CA). The CL images were collected using a voltage of 20 kV and beam current intensity of 140 μA. The U–Pb isotopic data (see online Supplementary Material Table S1) from the texturally characterized zircon grains were collected with a spot size of 25 μm, repetition rate of 5 Hz and fluence of 1–3 J cm−2 using a Photon Machines analyte excimer laser (193 nm) coupled to a single collector Nu AttoM ICP-MS. The 91500 zircon (Wiedenbeck et al. Reference Wiedenbeck, Allé, Corfu, Griffin, Meier, Oberli, von Quadt, Roddick and Spiegel1995) was used as the primary reference material for the correction of inter-element fractionation and instrument drift. The BB9 zircon was used as U–Pb secondary reference material for quality control purposes, yielding a concordia age of 558.1 ± 7.1 Ma (2s), corroborating within uncertainty the published 206Pb–238U date of BB9 zircon at 560 ± 5 Ma (Santos et al. Reference Santos, Lana, Scholz, Buick, Schmitz, Kamo, Gerdes, Corfu, Tapster, Lancaster, Storey, Basei, Tohver, Alkmim, Nalini, Krambrock, Fantini and Wiedenbeck2017). The 204Pb signal (cps) varies from below detection levels up to 0.03 % of the 206Pb signal (cps) and is manifested in the elevated 206Pb/204Pb ratios. Therefore, since the common Pb contribution is negligible, no common Pb correction was applied to the data.

The Lu–Hf isotopic analyses (see online Supplementary Material Table S2) were conducted during two analytical sessions using Faraday cup detectors equipped with amplifiers having 1011 (sample KP18-3) or 1012 Ω (sample KP18-1) resistors. The 171Yb, 173Yb, 174Yb, 175Lu, 176(Yb + Lu + Hf), 177Hf, 178Hf, 179Hf, 180Hf, 181Ta and 182W masses were measured in the first session (sample KP18-3), whereas in the second session (sample KP18-1) the 172Yb mass was measured instead of 171Yb owing to space problems between the Faraday cups and ion counters. The guidelines of Fisher et al. (Reference Fisher, Vervoort and Hanchar2014) were followed to calculate the mass bias corrected contribution of 176Yb and 176Lu on the 176Hf ion beam signal and are described in more detail in Papapavlou et al. (Reference Papapavlou, Strachan, Storey and Bullen2021). The dated intragrain domains were analysed for Lu–Hf using a spot size of 65 μm, repetition rate of 7 Hz (sample KP18-1) or 15 Hz (sample KP18-3) and fluence of 9 J cm−2 using the Photon Machines laser instrument coupled to a Nu AttoM II multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). Secondary reference materials with variable 176Yb/177Hf ratios such as GJ-1, Plešovice and MUN-1 zircon (Morel et al. Reference Morel, Nebel, Nebel-Jacobsen, Miller and Vroon2008; Sláma et al. Reference Sláma, Košler, Condon, Crowley, Gerdes, Hanchar, Horstwood, Morris, Nasdala, Norberg, Schaltegger, Schoene, Tubrett and Whitehouse2008; Fisher et al. Reference Fisher, Hanchar, Samson, Dhuime, Blichert-Toft, Vervoort and Lam2011) were interspersed with the unknowns, corroborating within uncertainty the published 176Hf/177Hf values (online Supplementary Material Table S2). The initial 176Hf/177Hf and ϵHf values were calculated at the crystallization age of sample KP18-3 (Ouranoupoli granodiorite) and using the U–Pb concordia dates from each analysed zircon grain of sample KP18-1 (biotite gneiss). For calculating the ϵHf values and two-stage depleted mantle model ages, the 176Lu decay constant of 1.867 × 10−11 (Söderlund et al. Reference Söderlund, Patchett, Vervoort and Isachsen2004), the Lu–Hf chondritic uniform reservoir (CHUR) parameters of Bouvier et al. (Reference Bouvier, Vervoort and Patchett2008), the depleted mantle reservoir 176Lu/177Hf value of 0.03976 (Vervoort et al. Reference Vervoort, Kemp and Fisher2018) and the bulk continental crust 176Lu/177Hf value of 0.0113 (Rudnick & Gao, Reference Rudnick, Gao, Turekian and Holland2013) were used. The stable 178Hf/177Hf isotope ratio was used to monitor the mass bias corrections and instrument stability yielding a value of 1.46735 ± 60 (1s), corroborating the published value of 1.46735 (Thirlwall & Anczkwiewicz, Reference Thirlwall and Anczkiewicz2004). The analytical errors are quadratic additions of the in-run error and the reproducibility of the secondary reference material and are typically ±2 ϵHf units. The U–Pb and Lu–Hf isotope data were reduced using either Iolite v.3.63 (Paton et al. Reference Paton, Hellstrom, Paul, Woodhead and Hergt2011) or an in-house spreadsheet. The IsoplotR (Vermeesch, Reference Vermeesch2018) and ggplot2 data visualization packages (Wickham, Reference Wickham2016) for the R programming language were used for the plotting of Wetherill-Concordia and contoured bivariate Kernel Density Estimation (KDE) diagrams.

4. Results

4.a. Zircon cathodoluminescence electron beam imaging (SEM/CL)

The zircon grains of sample KP18-1 are euhedral to subhedral with aspect ratios of 1:1 to 3:1. They exhibit bright cores in CL images with homogeneous, oscillatory or sector zoning and are commonly overgrown by oscillatory zoned or homogeneous domains with grey or dark grey CL emittance (Fig. 2). Rarely, dark cores overgrown by domains of higher luminosity are also recorded. Resorption of the oscillatory zoned cores by the darker in CL overgrowths is also occasionally observed. The zircon grains of sample KP18-3 are euhedral with aspect ratios of 1:1 to 3:1. The core domains are dark in CL images and have a possibly metamictic origin or exhibit homogeneous grey emittance in CL and are overgrown by oscillatory zoned mantle and rim domains (Fig. 2).

Fig. 2. Representative cathodoluminescence images of zircon grains from sample KP18-3 (upper panel) and KP18-1 (lower panel). The yellow circle represents the spot location for U–Pb analysis and the red for Lu–Hf analysis.

4.b. U–Pb and Lu–Hf isotope microanalysis (LA-MC-ICP-MS)

4.b.1. Sample KP18-1 (Vertiskos unit)

The U–Pb isotopic analyses of the zircon cores show a cluster in concordia space between 400 and 500 Ma, with nine analyses on zircon grains of possible xenocrystic origin yielding concordia dates between c. 500 and 700 Ma (Fig. 3a). Specifically, the analyses from grains of the 400–500 Ma age population with Th/U values that vary between 0.1 and 0.77 yield an upper intercept U–Pb concordia age of 458.8 ± 11 Ma (2s, n = 45, MSWD = 0.8) that is interpreted as the crystallization age of the igneous precursor to the Ammouliani gneiss (Fig. 3b). The lower intercept of the discordia chord overlaps within uncertainty with the origin of the concordia diagram and possibly manifests recent Pb loss. Notably, 14 analyses in the 400–500 Ma age cluster, with Th/U ratios <0.1, yield a concordia age of 451.7 ± 4.4 Ma (2s, n = 14, MSWD = 2.1) that overlaps within uncertainty with the crystallization age of the orthogneiss. Concordant U–Pb dates in the population of xenocrystic zircon grains range from 513.5 ± 6.8 Ma to 688.9 ± 5.8 Ma with a cluster of three dates at c. 600 Ma. In total, 87 % of the Lu–Hf isotope analyses on the 400–500 Ma age population have subchondritic ϵHf values and overall vary from −6.2 to 2.3 ϵHf units (Fig. 4). The 2s external reproducibility of Lu–Hf analyses on the 400–500 Ma age population is 4.1 ϵHf units (n = 39). Seven Lu–Hf analyses in zircon grains of the 500–700 Ma age population show a spread in ϵHf values from −10.6 to 4.1. The analysed zircon grains yield two-stage depleted mantle model ages (TDM) that vary from 1.07 to 1.52 Ga. The crustal residence times, which represent the delay between the extraction of a juvenile magma from the depleted mantle reservoir and the crystallization of the magmatic lithology (Lancaster et al. Reference Lancaster, Storey, Hawkesworth and Dhuime2011), were calculated as the difference between the U–Pb concordia date of each analysis and the TDM and vary between 630 and 1077 Ma.

Fig. 3. Wetherill-Concordia and weighted mean diagrams that depict: (a) U–Pb analyses from zircon grains of sample KP18-1 (Vertiskos gneiss); (b) U–Pb analyses from zircons of the c. 400–500 Ma age population in sample KP18-1; (c) U–Pb analyses from zircon grains of sample KP18-3 (Ouranoupoli granodiorite); (d) weighted mean ϵHf value and analyses from zircon grains of sample KP18-3.

Fig. 4. Contoured Kernel Density Estimation diagram with compilation of Lu–Hf data from detrital and igneous zircon data presented in Abbo et al. (Reference Abbo, Avigad and Gerdes2020) and this study. CHUR – chondritic uniform reservoir; DM – depleted mantle.

4.b.2. Sample KP18-3 (Ouranoupoli granodiorite)

The U–Pb isotopic analysis of zircon grains with oscillatory zoning or homogeneous grey response in CL and Th/U values from 0.1 to 0.3 yielded a concordia date of 52.1 ± 0.6 Ma (2s, n = 66, MSWD = 1.9) that is interpreted as the crystallization age of the granodiorite (Fig. 3c). At the precision levels of this study, age differences between the different CL domains were not resolved. Thirty-eight Lu–Hf isotope analyses of oscillatory zoned zircon grains show clustered ϵHf values, with ϵHf varying from −1.4 to 3, resulting in a near-chondritic, weighted mean ϵHf value of 0.7 ± 2.4 (Fig. 3d; 2s, n = 38, MSWD = 1.5). The external reproducibility (2s) of the analyses is 2.4 ϵHf units and the TDM ages range from 750 to 980 Ma, resulting in crustal residence times that vary from 695 to 929 Ma.

5. Discussion and conclusions

The upper intercept zircon crystallization age of 458.8 ± 11 Ma (2s) for the protolith of the examined orthogneiss is 20–30 Ma older than the single zircon thermal ionization mass spectrometry (TIMS) Pb evaporation ages from orthogneisses of the same unit in the Vertiskos mountains area (Himmerkus et al. Reference Himmerkus, Reischmann and Kostopoulos2009). This age gap raises the question of whether the Vertiskos unit comprises different gneissic subunits that represent discrete igneous events or that the evaporation ages still contain Pb loss. Three U–Pb concordia dates at 468 ± 2 Ma, 466 ± 2 Ma and 455 ± 2 Ma from granitic and biotite gneisses of this unit (Abbo et al. Reference Abbo, Avigad and Gerdes2020) lend credence to a Middle Ordovician and not Silurian age for the emplacement of this unit. In addition, Middle Ordovician ages have also been reported from orthogneisses in different eastern Mediterranean terranes of Peri-Gondwanan origin and have been characterized as Avalonian, Cadomian or Carpathian affinity (Okay et al. Reference Okay, Bozkurt, Satir, Yiǧitbaş, Crowley and Shang2008 a; Balintoni & Balica, Reference Balintoni and Balica2013; Bonev et al. Reference Bonev, Ovtcharova-Schaltegger, Moritz, Marchev and Ulianov2013; Antić et al. Reference Antić, Peytcheva, von Quadt, Kounov, Trivić, Serafimovski, Tasev, Gerdjikov and Wetzel2016).

Whole-rock Sm–Nd isotope analyses of the Vertiskos unit have yielded ϵNd values of −4.61 and −6.46 (Himmerkus et al. Reference Himmerkus, Ander, Reischmann, Kostopoulos, Hatcher, Carlson, McBride and Martinez Catalán2007), which correspond to ϵHf values of −5.9 and −8.8 following the relationship of the Hf–Nd terrestrial array (Vervoort et al. Reference Vervoort, Plank and Prytulak2011), in agreement with the dominantly subchondritic ϵHf values in the zircon grains of the Ammouliani orthogneiss. The presence of xenocrystic grains with concordia ages from c. 500 to 700 Ma and the older than 1.2 Ga Hf TDM ages in the Ammouliani orthogneiss indicate the reworking of Neoproterozoic basement with Cadomian affinity, as has been reported also in other U–Pb–Hf studies of basement terranes in the eastern Mediterranean (Zlatkin et al. Reference Zlatkin, Avigad and Gerdes2018; Abbo et al. Reference Abbo, Avigad and Gerdes2020). To further investigate the latter premise, some interesting points arise from compiling existing Lu–Hf data from igneous and detrital zircon grains of the SMM (Abbo et al. Reference Abbo, Avigad and Gerdes2020) with those of the present study in a contoured bivariate KDE diagram (Fig. 4). Firstly, the ϵHf values from the Ammouliani orthogneiss fall in a subchondritic ϵHf cluster of the KDE map at c. 460 Ma. The interpreted xenocrystic zircon grains of the 500–700 Ma age population show also chiefly subchondritic ϵHf values, with one analysis overlapping with the suprachondritic ϵHf cluster at c. 580 Ma, which is probably related to the Pyrgadikia terrane (Abbo et al. Reference Abbo, Avigad and Gerdes2020). Interestingly, the negative whole-rock ϵNd values reported for the Pyrgadikia gneisses (Himmerkus et al. Reference Himmerkus, Ander, Reischmann, Kostopoulos, Hatcher, Carlson, McBride and Martinez Catalán2007) contrast with the dominantly juvenile zircon Hf isotope record from this unit and necessitates further isotope investigation. Lu–Hf analyses that overlap with a near-chondritic ϵHf cluster at c. 300 Ma, which most probably represents grains derived chiefly from the Arnea granite (Abbo et al. Reference Abbo, Avigad and Gerdes2020) or the Permo-Carboniferous basement of the Kerdilion unit (Himmerkus et al. Reference Himmerkus, Zachariadis, Reischmann and Kostopoulos2012) and the Rhodope massif (Liati & Gebauer, Reference Liati and Gebauer1999), are missing from the Ammouliani U–Pb–Hf dataset. However, Lu–Hf data from the Ammouliani granodiorite, which show a weighted mean ϵHf value of 0.7 ± 2.4 (2s), overlap with a near-chondritic cluster that extends to an array of positive ϵHf values between c. 50 and 65 Ma (Fig. 4).

This near-chondritic to juvenile array represents the Hf isotope signature of plutons possibly consanguineous with the Sithonia plutonic complex and the younger Ouranoupoli granodiorite. Regarding the latter, the U–Pb concordia age of 52.1 ± 0.6 Ma from the Ouranoupoli granodiorite indicates that biotite and muscovite 40Ar−39Ar plateau ages at 44 ± 1.1 Ma and 47 ± 0.7 Ma (De Wet et al. Reference De Wet, Miller, Bickle and Chapman1989) do not represent igneous crystallization ages. The U–Pb isotopic data from the 52.1 ± 0.6 Ma Ouranoupoli granodiorite and granites of the c. 65 Ma Sithonia plutonic complex (Abbo et al. Reference Abbo, Avigad and Gerdes2020) demonstrate that these magmatic events are not coeval as has been inferred by previous studies using isotope systems with volatile radiogenic daughters that are susceptible to alteration (De Wet et al. Reference De Wet, Miller, Bickle and Chapman1989). A U–Pb uranothorite date of 53.6 ± 6.2 Ma and Pb–Pb titanite isochron date of 51 ± 16 Ma, interpreted as the emplacement ages of the Ierissos granite (Frei, Reference Frei1996), may indicate a co-genetic relationship between the latter and the Ouranoupoli granodiorite. The near-chondritic to juvenile Hf isotope signature in zircon grains of the Ouranoupoli and Sithonia plutonic complex granitoids implicates either the partial melting of a chondritic to juvenile source or extreme differentiation of mantle-derived melts with assimilation of pre-existing crust. In this regard, the main petrogenetic mechanisms proposed for the formation of two-mica granites and evolved leucogranites in the Sithonia plutonic complex is the partial melting of lower crust amphibolites, by lamprophyric underplates that derived from a metasomatized mantle wedge (Christofides et al. Reference Christofides, Perugini, Koroneos, Soldatos, Poli, Eleftheriadis, Del Moro and Neiva2007), and mixing of the mantle-derived and anatectic batches of melt (Perugini et al. Reference Perugini, Poli, Christofides, Eleftheriadis, Koroneos and Soldatos2004). Thus, this amphibolitic/mafic lower crust reservoir could represent the parental source that after partial melting inherited the radiogenic, slightly suprachondritic, Hf isotopic signature to the zircon grains of the Ouranoupoli granitoid. Interestingly, the proposed petrogenetic setting is similar to that proposed for the petrogenesis of adakite/trondhjemite–tonalite–granodiorite (TTG) igneous suites based on experimental and geochemical grounds (Qian & Hermann, Reference Qian and Hermann2013; Marchev et al. Reference Marchev, Georgiev, Raicheva, Peytcheva, von Quadt, Ovtcharova and Bonev2013). However, weak petrogenetic links, using a phase equilibria modelling approach, still remain between the presence of leucosomes observed in the Kerdillion and Vertiskos units and the potential of melt connectivity with the Sithonia granitoids. Moreover, petrographic constraints from these lower crust amphibolites are missing, and the Lu/Hf fractionation to produce time-integrated radiogenic Hf isotopic compositions necessitates the presence of restitic garnet in this type of reservoir (Vervoort et al. Reference Vervoort, Patchett, Albarède, Blichert-Toft, Rudnick and Downes2000). On another note, juvenile zircon Hf isotope signals are broadly expected by numerical geodynamic models in extensional tectonic settings (Kohanpour et al. Reference Kohanpour, Kirkland, Gorczyk, Occhipinti, Lindsay, Mole and Le Vaillant2019). Thus, the emplacement of juvenile Sithonia granitoids could manifest an extensional pulse, potentially concomitant with strike-slip shearing (Pe-Piper & Piper, Reference Pe-Piper, Piper, Dilek and Pavlides2006), but the latter premise needs to be further supported by structural data from bounding syn-magmatic shear zones.

Acknowledgements

KP thanks J. H. F. L Davies for comments on an early version of the manuscript and A. Poirier for the analytical assistance. Insightful comments by Avishai Abbo and an anonymous reviewer are greatly appreciated.

Supplementary material

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Figure 0

Fig. 1. Simplified geological map of the Serbo-Macedonian massif modified from Himmerkus et al. (2006). The upper right panel depicts a simplified geological map of Ammouliani island, with sample localities, modified from Kockel et al. (1978).

Figure 1

Fig. 2. Representative cathodoluminescence images of zircon grains from sample KP18-3 (upper panel) and KP18-1 (lower panel). The yellow circle represents the spot location for U–Pb analysis and the red for Lu–Hf analysis.

Figure 2

Fig. 3. Wetherill-Concordia and weighted mean diagrams that depict: (a) U–Pb analyses from zircon grains of sample KP18-1 (Vertiskos gneiss); (b) U–Pb analyses from zircons of the c. 400–500 Ma age population in sample KP18-1; (c) U–Pb analyses from zircon grains of sample KP18-3 (Ouranoupoli granodiorite); (d) weighted mean ϵHf value and analyses from zircon grains of sample KP18-3.

Figure 3

Fig. 4. Contoured Kernel Density Estimation diagram with compilation of Lu–Hf data from detrital and igneous zircon data presented in Abbo et al. (2020) and this study. CHUR – chondritic uniform reservoir; DM – depleted mantle.

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