An integrated zircon geochronological and ... - Springer Link

Contrib Mineral Petrol (2010) 160:719–742 DOI 10.1007/s00410-010-0504-4

ORIGINAL PAPER

An integrated zircon geochronological and geochemical investigation into the Miocene plutonic evolution of the Cyclades, Aegean Sea, Greece: Part 1: Geochronology Robert Bolhar • Uwe Ring • Charlotte M. Allen

Received: 16 March 2009 / Accepted: 23 February 2010 / Published online: 27 March 2010 Ó Springer-Verlag 2010

Abstract We use 369 individual U–Pb zircon ages from 14 granitoid samples collected on five islands in the Cyclades in the Aegean Sea, Greece, for constraining the crystallisation history of I- and S-type plutons above the retreating Hellenic subduction zone. Miocene magmatism in the Cyclades extended over a time span from 17 to 11 Ma. The ages for S-type granites are systematically *2 million years older than those for I-type granites. Considering plutons individually, the zircon data define age spectra ranging from simple and unimodal to complex and multimodal. Seven of the 14 investigated samples yield more than one distinct zircon crystallisation age, with one Itype granodiorite sample from Mykonos Island representing the most complex case with three resolvable age peaks. Two samples from S-type granites on Ikaria appear to have crystallised zircon over 2–3 million years, whereas for the majority of individual samples with multiple zircon age populations the calculated ages deviate by 1–1.5 million

years. We interpret our age data to reflect a protracted history involving initial partial melting at deeper lithospheric levels, followed by crystallisation and cooling at shallower crustal levels. Our study corroborates published research arguing that pluton construction is due to incremental emplacement of multiple magma pulses over a few million years. Assuming that multiple age peaks of our 14 samples can indeed serve to quantify time spans for magmatic emplacement, our data suggest that Aegean plutons were constructed over a few million years. Our tectonic interpretation of the U–Pb ages is that the S-type granites resulted from partial melting and migmatisation of the lower crust, possibly starting at *23 Ma. The I-type granites and associated mafic melts are interpreted to reflect the magmatic arc stage in the Cyclades starting at *15 Ma. Keywords Aegean Sea Cyclades Granitoids Hellenic subduction zone I-type S-type Zircon geochronology

Communicated by C. Ballhaus.

Electronic supplementary material The online version of this article (doi:10.1007/s00410-010-0504-4) contains supplementary material, which is available to authorized users. R. Bolhar School of Earth Sciences, University of Queensland, Brisbane 4072, Australia e-mail: [email protected] U. Ring (&) Department of Geological Sciences, University of Canterbury, Christchurch 8140, New Zealand e-mail: [email protected] C. M. Allen Research School of Earth Sciences, Australian National University, Canberra 2006, Australia

Introduction The Hellenic subduction system is a well-studied example of a retreating subduction zone. Its overriding plate was, and still is, subjected to large-scale extension since *23 Ma (Gautier et al. 1990; Dinter 1998; Ring et al. 2010). The Hellenic subduction system has a fairly limited trench-parallel length (*600–800 km) and such laterally narrow subduction zones typically have rapid retreat rates. A characteristic feature of retreating subduction zones is that the magmatic arc retreats outboard with the subduction zone. Therefore, the age and isotopic characteristics of arcrelated magmatic rocks allows inferring tectonic processes of subduction zone retreat.

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The Cyclades in the central Aegean are currently located in the back-arc of the southward retreating Hellenic subduction zone. The rocks of the Cyclades have been intruded by a variety of granitic and minor mafic rocks and record an important stage of Hellenic subduction zone retreat. The granitoids have a rather large variation in geochemical characteristics and are traditionally distinguished into I- and S-type granites (Altherr et al. 1982, 1988; Pe-Piper et al. 2002). The ages of the magmatic rocks are not well constrained and a rather huge age range from 22 to 10 Ma is currently envisaged (Altherr et al. 1982; Henjes-Kunst et al. 1988). However, recently more precise U–Pb ages by Keay et al. (2001) and Brichau et al. (2007, 2008) suggest a much more restricted age range of *15–12 Ma. Volcanic rocks are only rarely exposed in the Cyclades and their ages range from *12 to 5 Ma (Fytikas et al. 1984; Pe-Piper and Piper 2001; Pe-Piper et al. 2002). The reason for the relatively young ages of the volcanics is that they mainly occur in the hangingwalls of major top-N extensional detachments and thus record a younger, i.e. more southerly, stage of magmatism. Subsequently, they were transported to the north by tens of kilometres by detachment faulting (Avigad and Garfunkel 1991; Buick and Holland 1989; Ring et al. 2001; Brichau et al. 2006). The uncertainty in the age of presumably arc-related plutons has led to disagreement about the role of forearc versus intra/backarc extension in the Cyclades (Avigad et al. 1997; Pe-Piper et al. 2002; Ring and Layer 2003) and most workers envisage that extensional deformation since about 23 Ma occurred entirely in an intra/backarc environment. However, this inference hinges heavily on poorly defined Rb–Sr whole-rock errorchron ages of 22–18 Ma from granites exposed on Ikaria Island by Altherr et al. (1982). In Part I of this paper series, we set out to present U–Pb ages from a variety of granitoid rocks across the Cyclades. This will help to constrain the crystallisation history of plutonic rocks and to better understand the dynamics of continental extension above the retreating Hellenic subduction zone. In particular, we are interested in the relative timing of magmatism and large-scale extensional deformation. In Part II of this paper series, we will then combine geochemical/isotopic and thermal information retrieved at the mineral scale with existing geochronological data to discuss source characteristics and magmatic processes involved in the petrogenetic evolution of the Cycladic plutonic systems.

Geological framework The Hellenides form an arcuate orogen above the subducting Hellenic slab. Traditionally, the geology of the Hellenides is distinguished into several tectonic domains

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Contrib Mineral Petrol (2010) 160:719–742

(Fig. 1), which are characterised by rock type, stratigraphy, tectonometamorphic history and pre-orogenic paleogeography (Du¨rr et al. 1978; Robertson et al. 1991). The Hellenides in the Aegean transect can generally be subdivided from north to south into: (1) the Srednogorie and RhodopeSarkaya blocks, (2) the Vardar-Izmir Oceanic Unit, (3) the Pelagonian-Lycian Block, (4) the partly oceanic Pindos Unit (including the Cycladic Blueschist Unit), (5) the Tripolitza Block, (6) the Ionian Block, and (7) the East Mediterranean Ocean (Du¨rr et al. 1978; Jacobshagen 1986; van Hinsbergen et al. 2005; Jolivet and Brun 2010; Ring et al. 2010). The Cycladic Islands constitute one of the most deeply exhumed parts of the Hellenides and are primarily made up by the Cycladic Blueschist Unit (Du¨rr et al. 1978; Fig. 1). The rock sequence of the Cyclades comprises four main structural units: (1) The Basal Unit as part of the Tripolitza Block, which crops out in windows in the Mt Olympos Massif, as well as on Evia, Tinos and Samos Islands (Avigad and Garfunkel 1991; Ring et al. 1999; Shaked et al. 2000). (2) The Cycladic Blueschist Unit consists of a basement/ cover sequence and the overlying highly attenuated ophiolitic Selçuk Me´lange. Slivers of oceanic crust (gabbro, plagiogranite, basalt) that formed at *80–60 Ma (Keay 1998; Tomaschek et al. 2003) are incorporated in the ophiolitic Selçuk Me´lange. The Cycladic Blueschist Unit experienced eclogite- to blueschist-facies metamorphism under temperature–pressure conditions of 400–550°C and 12–20 kbar (see review in Ring et al. 2010). High-pressure metamorphism commenced at *53 Ma (Wijbrans et al. 1990; Tomaschek et al. 2003; Putlitz et al. 2005; Lagos et al. 2007) in the upper structural levels of the Cycladic Blueschist Unit. The timing of greenschist-facies metamorphism to locally amphibolite-facies metamorphism overprinting of the high-pressure rocks is loosely constrained, with available geochronology indicating a range between[30 Ma and *12 Ma (Keay et al. 2001; Wijbrans and McDougall 1988; Kumerics et al. 2005). (3) The upper unit consists of the weakly to non-metamorphosed composite Cycladic Ophiolite Nappe. (4) Sedimentary basins filled with Miocene and younger sediments (Bo¨ger 1983). The latter two units were not affected by Tertiary high-pressure metamorphism and commonly form the hangingwall of major extensional detachments (Lister et al. 1984; Avigad and Garfunkel 1991; Buick 1991; Lee and Lister 1992). Since about the Oligocene/Miocene boundary, the Aegean region has been the site of pronounced horizontal crustal extension (Lister et al. 1984; Buick 1991). This extensional deformation caused the formation of the Aegean Sea basin (Meulenkamp et al. 1988; Brichau et al. 2008). In the Middle to Late Miocene, the Cyclades became part of the magmatic arc and volcanic rocks and granites formed. The granites either stitch extensional

Contrib Mineral Petrol (2010) 160:719–742 Fig. 1 Simplified tectonic map of the Aegean region showing the main tectonic zones above the Hellenic subduction zone. The Mediterranean Ridge represents the modern accretionary wedge that is bounded to the north by a major backthrust system. Line patterns indicate the positions of subduction-related magmaticarc rocks from *38 Ma to the Recent according to Fytikas et al. (1984), Pe-Piper et al. (2002). The migration of the magmatic arc in the overriding plate mimics the retreat of the Hellenic slab. Samples were collected from the islands of Tinos, Mykonos, Naxos, Ikaria and Samos (the three boxes mark positions of maps shown in Fig. 2)

721

Pelagonian-Lycian Block

Miocene / Quarternary basins

Pindos Oceanic Unit (incl. Cycladic Blueschist Unit)

Eocene forearc basins Srednogorie Block

Menderes Block

Rhodope-Sarkaya Block

Tripolitza Block

Vardar-Izmir Oceanic Unit

Ionian Block Anatolian Fault

N or th tro Ae ug ge h a

n

rth No

40°N

Tinos

38°N

Samos Ikaria

Laurium Delos Serifos

Mykonos Naxos Keros

Bodrum Kos

Cycladic Blueschist Unit

Thera

36°N

ed M

~38-23 Ma Magmatic arc

n rra ite ea

~4-15 Ma Volcanic arc

n

He lle

Ridge nic

Accr etion

Subuction

Zo

34°N

ary

Active Volcanic arc

Complex

ne

Africa

100 km 32°N

20°E

22°E

detachments or are syntectonic to them (Avigad and Garfunkel 1991; Lee and Lister 1992; Brichau et al. 2010).

Sample descriptions and previous work We describe samples used for geochronology from the various islands from west to east. For the purpose of this study, the traditional distinction into S-type and I-type granitoid has been simplified based on mineralogy (Chappell and White,

24°E

26°E

28°E

2001): S-type granites mean two-mica (leuco) granites that may also contain garnet; I-type means biotite granitoids that may contain titanite and epidote with a range in compositions from granodiorite to monzogranite/syenogranite. All ages are reported at the two-sigma level. Tinos Island A large (I-type) monzogranite intruded the Cycladic Blueschist Unit on Tinos Island (Fig. 2a). Altherr et al.

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722 Fig. 2 Tectonic maps of Cycladic islands from which samples for zircon geochronology were collected. a Eastern part of Tinos Island with Tinos monzogranite and small S-type granites at its periphery (location of sample T3 is indicated). b Mykonos Island is basically made up by a huge granodiorite intrusion (note locations of samples M1 through M4). c Western part of Naxos Island with huge Naxos granodiorite where samples Na3-Na6 were collected. In contrast to Tinos Island, the S-type granites in Naxos are not cropping out at the periphery of the large I-type pluton and are exposed more than 5 km to the north. d The geology of Ikaria Island is dominated by three granites, the large I-type Raches granite in the west is associated with the much smaller Karkinagrion S-type granite. The S-type Xyolosrtis granite in the central part of Ikaria intruded rocks of the Ikaria nappe. e Map showing westernmost part of Samos Island. Sample Sa7 is from a monzogranitic dike that intruded rocks of the Katavasis high-temperature metamorphic complex. There is no intrusive body exposed on Samos


a

Tinos

b

limit of contact aureole of Tinos granite

+

N

+ +

+

Mykonos

+ + T3 +

+

M1

Kionia

+

Tinos

+

+

+ +

Falatados

c

+

+

2 km

+

+ +

M2

+ M4

M3

+ + N 2 km

Delos

Samos

e

Naxos N

+

Na2

2 km

N 300 m

Na3

+

Naxos

Sa7

+ +

Kallithea

Na4

+

Na5

+ +

Na6

Armenistis

Evdilos

+ +

Ikaria

+

+ +

Agios Kirikos

+ +

+

d

+

+

Ik7

+ N

+

3 km

+ + Ik2

Upper Unit Katavasis Complex

(1988) described rare mafic inclusions associated with the monzogranite but no signs for emplacement of distinct pulses of magma. Evidence for any hydrothermal alteration is lacking (Altherr et al. 1988). At its periphery, small leucogranites intruded the monzogranite as well as the Cycladic Blueschist and Upper unit. The granites have a well-developed contact aureole that formed at *14 Ma (Bro¨cker and Franz 2000). At *12–11 Ma, a number of dacitic dikes intruded the basement of Tinos (Avigad 1998). Altherr et al. (1982) reported an age of *17 Ma for the monzogranite. One of the leucogranites yielded a welldefined U–Pb zircon age of 14.4 ± 0.2 Ma (Keay 1998; Table 1).

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Ik5 Migmatite in CBU

Cycladic Blueschist Unit (CBU) S-type granite

+

I-type granite

Sample locality

Ikaria Nappe

Tripolitza Unit

Extensional detachment

Sample T3 was collected from the monzogranitic intrusion and dated in a previous study by U–Pb zircon geochronology at 14.63 ± 0.22 Ma (Brichau et al. 2007). The sample is mineralogically and geochemically homogeneous, with no evidence for deformation in hand specimen. Mykonos Island Three samples (M1, M2, M3) from the island of Mykonos (Fig. 2b) are granodioritic in composition, one of them (M4) is a monzogranite. The granitoids can be assigned to the I-type category. Altherr et al. (1988) reported the presence of late aplitic dikes and mafic inclusions as well

Foliated granodiorite Foliated granodiorite

M2

M1

Western island

Western island

Western island

Na5

Na6

Na4

Foliated monzogranitic dike

I-type

I-type

I-type

I-type

S-type I-type

I-type

I-type

I-type

I-type

I-type

Qtz, plag, kspar, musc, bio

I-type

S-type

n

N

3

Subordinate 12.36 ± 0.26

7

14.21 ± 0.27

12.12 ± 0.18 Subordinate 11.42 ± 0.18

Dominant


Dominant


13.73 ± 0.29

1.95 2.4

0.11

4

1.05

7 14 1.33

2

7 33 1.7

4

9 41 2.1

Subordinate 14.79 ± 0.26 12 Dominant

0.68 13.33 ± 0.17 15 31 1.66

13.11± 0.15

Subordinate

7 24 1.24

12.98 ± 0.13 18 33 1.7

12.97 ± 0.06 24 32 0.66 12.25 ± 0.18

Dominant

0.43

14.97 ± 0.32 5 5 1.67 12.74 ± 0.11 20 30 1.74

Dominant

Unimodal

Unimodal

Unimodal Unimodal

7

1.31

9 27 1.8

Dominant 1 13.64 ± 0.21

1.49

Dominant 2 14.64 ± 0.23

5

14.59 ± 0.18 13 27 1.43

11.08 ± 0.22 12 27 1.69

13.64 ± 0.22 11 24 1.68

14%

61%

52%

3%

None

3%

6%

None 10%

4%

4%

11%

25%

None

Only rims

Cores ? rims

Cores ? rims

Cores ? rims

Cores ? rims

Only rims

Only rims

Cores ? rims Only rims

Only rims

Only rims

Only rims

Cores ? rims

Cores ? rims

MSWD Inheriteance Analytical approach

14.73 ± 0.22 13 21 1.75

U–Pb age


Dominant

Unimodal

Unimodal

Unimodal

Granitoid type Population

Musc, bio, grt, tourm, S-type

Plag, kspar, qtz, bio

Mineralogy

References for age data: see in text: 2. Geological framework; n = number of zircon analyses selected for age calculation; N = total number of zircon analyses

Sa7

Kallithea: westermost island

Xylosirtis

IK7

Samos

Karkinagrion: southern coast Foliated granite

IK5 Foliated leuocgran

Raches: SW corner of island Undeformed syenogranite

Undeformed granodiorite



IK2

Ikaria

N–W island Western island

Na2 Na3

Leucogranite Undeformed granodiorite

Foliated granodiorite

M3

Naxos

Monzogranite

Undeformed monzogranite

Lithology

M4

Mykonos

T3

Tinos

This study Location

Table 1 Summary of U–Pb zircon ages (Ma, ± 2 standard error) for granitoid from Aegean Islands

Contrib Mineral Petrol (2010) 160:719–742 723

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as signs of widespread hydrothermal alteration in the form of chloritisation of biotite and hornblende and sericitisation of plagioclase. In the southwest part of Mykonos, numerous thin concordant panels of amphibolite-facies schistose wallrock preserve consistent fabric orientations and tectonostratigraphic order. There are also wallrock bodies inside the pluton. Pe-Piper et al. (2002) described a varied suite of gabbro, diorite, tonalite, and granite associated with the main granodiorite intrusion from the satellite island of Delos. The varied suite of magmatic rocks as described by Pe-Piper et al. (2002) suggests incremental emplacement of the pluton. We interpret the wallrock slices to have remained in place as the pluton was intruded in an incremental fashion. U–Pb dating of zircon from a granitoid sample yielded a lower intercept age of 11.1 ± 1.3 Ma consistent with an uranothorite concordant U–Pb age of 11.0 ± 1.1 Ma (Henjes-Kunst et al. 1988). The zircon from granodiorite sample M4 was dated by the U–Pb method in a previous study by Brichau et al. (2008) at 13.51 ± 0.30 Ma. Samples M1 through M3 are undeformed; M4 is weakly deformed, displaying a faint subhorizontal foliation. All samples are devoid of chemical or mineralogical heterogeneities. Naxos Island Naxos exposes a number of small peraluminous S-type granites in the north of the island and one voluminous I-type hornblende-biotite granite in the western part of the island (Andriessen et al. 1979; Altherr et al. 1982; Wijbrans and McDougall 1988; Pe-Piper 2000; Keay et al. 2001; Fig. 3c). The S-type granites intruded during the late stages of high-grade metamorphism and migmatisation (Keay et al. 2001). The hornblende-biotite granite includes microgranular mafic enclaves. The latter are widespread near the margin of the granite (Pe-Piper and Kotopouli 1997). The hornblende-biotite granite comprises medium-grained mesocratic and coarse-grained porphyritic hornblende ± biotite granite varieties. The porphyritic granite is intruded by small bodies of alkalifeldspar granite and also by fine-grained tourmalinebearing leucogranite. The field evidence suggests that the I-type granite has several intrusion stages. Pe-Piper and Kotopouli (1997) showed that late-stage leucogranite differs geochemically from the hornblende-biotite granite and that the two represent separate magma batches. Altherr et al. (1988) noted that granitoids from Naxos and Mykonos share similar geochemical, mineralogical and field characteristics, including evidence for widespread hydrothermal activity.

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Keay et al. (2001) provided four well-constrained U–Pb SHRIMP ages for S-type leuccogranites ranging from 15.4 ± 0.2 to 12.2 ± 0.2 Ma. Discordant zircon fractions from a mafic enclave of the hornblende-biotite I-type granite yielded a U–Pb lower intercept age of 12.6 ± 1.8 Ma and uranothorite from a granite sample gave a nearly concordant U–Pb age of 11.5 ± 1.4 Ma (HenjesKunst et al. 1988). For two I-type granitoids, Keay et al. (2001) reported U–Pb SHRIMP ages ranging from 12.4 ± 0.2 to 11.3 ± 0.4 Ma. Sample Na2 was collected from an S-type leucogranite and samples Na3, Na4, Na5 and Na6 were taken from an I-type hornblende-biotite granite in western Naxos. Ikaria Island Previous workers have distinguished between the large I-type Raches syenogranite occupying the western half of the island and the small peraluminous (S-type) two-mica Xylosirtis granite (Altherr et al. 1982). Close examination in the field shows that the S-type Karkinagrion granite intruded the I-type Raches granite, with both granites cut by tourmaline/garnet-bearing aplitic dikes. Altherr et al. (1982, 1988) demonstrated that both the Xylosirtis and Raches plutons were formed through injection of multiple magma pulses of variable composition. The Raches syenogranite shows petrographic signs of hydrothermal alteration. Altherr et al. (1982) reported Rb–Sr whole-rock ages from the Xylosirtis (18.1 ± 2.2 Ma) and Raches (22.7 ± 0.2 Ma) granite, which are interpreted to represent minimum intrusion ages. Sample Ik2 was collected from the Raches intrusion in the southwestern corner of the island. The sample is an undeformed biotite syenogranite (plag ? kspar ? qtz ? biotite) with I-type affinity. Sample Ik5 belongs to the S-type Karkinagrion granite. The sample has a weakly developed foliation and contains muscovite and biotite, in part also garnet and tourmaline. Sample Ik7 was taken from the Xylosirtis leucogranite (qtz ? plag ? kspar ? white mica ? biotite). Samos Island The Kallithea igneous complex at the western tip of the island of Samos (Fig. 2e) is formed by numerous composite dikes (monzodiorites, monzonites, monzogranites) cutting the Katavasis complex (Ring et al. 1999). The dikes are compositionally variable, comprising diorite, monzodiorite, monzonite, granodiorite and monzogranite and pegmatites (Altherr et al. 1988). Crosscutting relationships point to sequential emplacement of distinct magma pulses, some of which appear genetically unrelated. Net-veined


725

Fig. 3 Cathodoluminescence (CL) images of selected zircons from three granitoid samples from the island of Ikaria (Ik2, Ik5, Ik7). A number of grains show truncated zoning and embayments indicative of resorption. Other characteristic features include the presence of inherited cores and fine oscillatory zoning. Circles indicate the location of trace elemental and U–Pb isotopic analysis. White bars represent 30 lm

structures consisting of spherical microdiorite and surrounding granodiorite and monzogranite were interpreted by Mezger et al. (1985) to reflect coexisting and interacting mafic and felsic magmas. K–Ar dating on a hornblende concentrate from a monzodioritic rock yielded an age of 10.2 ± 0.2 Ma (Mezger et al. 1985). Sample Sa7 was obtained from an I-type monzogranitic dike from the westernmost part of the island of Samos. The sample displays a weak subhorizontal foliation.

Analytical methods Following routine mineral separation, 30–100 zircons from each sample were handpicked and mounted in epoxy resin, polished and examined in detail by cathodoluminescence

(CL). On the basis of CL images, 20–40 grains were selected for analysis by excimer laser ablation (ELA-ICPMS) at the Research School of Earth Sciences, ANU, Australia, following procedures reported by Ballard et al. (2001) and Harris et al. (2004). Laser ablation utilised a pulsed LambdaPhysik LPX 120I UV ArF excimer laser operated at a constant energy of 70 mJ, at 5 Hz (spot diameter of 24 or 32 lm). Ablated material was transferred by a mixed He–Ar gas from a custom-designed sample cell and flow homogeniser to an Agilent 7500 ICP-MS. Data for 18 mass peaks were collected in time-resolved mode with one point per peak. Due to a relatively high 204Hg blank, 204Pb was excluded. Integration times were 40 ms for 206Pb, 207Pb, 208Pb, 232Th, 235U, 238U; 10 ms for 31P, 89 Y, 139La, 140Ce, 147Sm, 153Eu, 163Dy, 175Lu, 177Hf; 5 ms for 29Si, 91Zr and 20 ms for 49Ti. After purging,

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726

background data were acquired for 20 s followed by 40 s of laser ablation, yielding about 100 mass scans and a penetration depth of *20 lm. Following background substraction and rejection of outlier isotope ratio measurements, corrections were applied to each spot analysis for instrumental mass bias and depth-related elemental fractionation. Depth-related interelement fractionation of Pb, Th, and U were corrected by comparison with standards (TEMORA, NIST610). Measured 207Pb/206Pb, 206Pb/238U, and 208Pb/232Th ratios in the standard zircon and 232Th/238U in the silicate glass standard were averaged over the course of each analytical session and used to calculate correction factors based on accepted values (Black et al. 2003; Pearce et al. 1997). Ages were calculated from 206Pb*/238U ratios (where * indicates radiogenic Pb). Common Pb corrections were applied based on the difference between measured and expected 208Pb/206Pb ratios for the measured 232Th/238U value (Compston et al. 1984). Internal errors for individual analyses were calculated from the observed variation in the corrected isotope ratios for each mass scan over the data interval selected for age calculation. Analyses with resolvable isotopic heterogeneity exhibit internal MSWD (mean squared weighted deviates, calculated using counting statistics errors for each mass scan) values in excess of three and were rejected in most cases from further consideration. Concordance was calculated on the basis of agreement between 207Pb*/235U and 206Pb*/238U ages and discordant grains were discarded (discordancen [15%). Pooled 206Pb*/235U ages and associated errors took into account within session analytical error, external uncertainties in the analysis, and age calibration of the zircon standard (Stern and Amelin 2003). Within session error was estimated using the standard deviation of 206Pb/238U measurements of NIST610 silicate glass. External errors incorporated only the uncertainty in the analysis of TEMORA, here taken as the standard error in the 206Pb/238U measurements, as the reported error in the age is negligible (Black et al. 2003). In addition, a Th disequilibrium correction was applied assuming a Th/U ratio of 3 in the host magma (see Harris et al. 2004). While this correction causes an increase in the 206Pb/238 age of about 0.1 Ma, it was considered necessary for rigorous comparison of zircon Pb/U ages to other dating techniques. During the course of analysis, 45 spot analyses of the reference zircon 91,500 were made as unknowns. All analyses were concordant to near-concordant (C97% concordance), and yielded an error weighted mean 206Pb/238U age of 1,056.4 ± 6.2 Ma (2r; Bolhar et al. 2008), identical within errors to the 206Pb/238U age of 1062.4 ± 0.4 Ma reported by Wiedenbeck et al. (1995). Calculated ages from individual sessions lie within uncertainty of each

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other: 1,055 ± 14 Ma (N = 15), 1,052 ± 16 Ma (12) and 1,059 ± 8 (n = 18) Ma. Thus, the analytical protocol adopted herein allowed the 206Pb/238U age of the reference zircon 91,500 to be determined at levels of accuracy and precision of \2%. The accuracy of the LA-ICPS method for dating young igneous rocks has been demonstrated by repeated analysis of stratigraphically well constrained Miocene porphyritic intrusions from Chile, with zircon 206Pb/238U ages ranging from 6.92 ± 0.07 Ma to 8.46 ± 0.14 Ma (Harris et al. 2004, 2008). The zircon ages were found to conform to cross-cutting relationships as well as other chronometers (e.g. Ar–Ar). Additionally, the suitability of the LA-ICPS method was confirmed by Harris et al. (2004) through comparison with existing SHRIMP age data for the *12 Ma Naxos granodiorite (NX9301: Keay 1998), which is equivalent to Na4 of the present study. Using an identical analytical protocol to the one adopted in the present study, Harris et al. (2004) determined an LA-ICPMS age of 12.31 ± 0.07 Ma, with an MSWD of 0.51 for 25 out of 33 grains analysed. This age estimate is in excellent agreement with the 12.2 ± 0.1 Ma (n = 22) SHRIMP date published by Keay (1998) and the LA-ICPMS dates reported in the present study (13.11 ± 0.15 Ma and 12.25 ± 0.18 Ma). Spot analyses of 91,500 by LA-ICPMS also included HfO2, P, Y, Ti and selected REE (La, Ce, Sm, Eu. Dy, Lu). Analytical errors (expressed as relative standard deviations calculated from the 45 analyses) are ±6% (HfO2), ±11% (P, Y), ±8% (Ti), ±17% (La), ±9% (Ce), ±13% (Sm, Eu), ±12% (Dy) and ±10% (Lu). Trace element abundances of 91,500 as measured in this study (average of 45 analyses) agree well with working values recommended by Wiedenbeck et al. (2003). Ti concentrations were used to calculate zircon crystallisation temperatures, based on the temperature dependant incorporation of Ti4? into crystallising zircon under TiO2-saturated conditions (Watson et al. 2006). Assuming a TiO2 activity of 1 (i.e. rutile is present), Tcrystallisation was determined according to: T[°C] = ((5,080 ± 30)/(6.01 ± 0.03)-(log (Ti))-273 (eq. 7 of Watson et al. 2006). Crystallisation temperatures are reported in ESM 1, see supplementary material. Uncertainty pertinent to calculating Tcrystallisation with the Ti-in-zircon thermometer (‘‘calibration uncertainty’’) has been estimated by Watson et al. (2006) at\10°C (2r) for the temperature range observed in this study. Uncertainty in the order of 10% (relative standard deviation calculated from 45 analyses of standard 91,500: Bolhar et al. 2007) arising from LA-ICMPS analysis of Ti concentrations translates into uncertainties of ca. 10°C. As noted by Ferry and Watson (2007), uncertainties due to activities of TiO2 and SiO2 less than 1 during zircon crystallisation tend to offset one another.


Results

727

that cores were emplacement.

not

assimilated

during

late-stage

Cathodoluminescence (CL) imaging U–Pb zircon analysis Zircon cathodoluminescence (CL) images are reported for three representative granitoid samples from Ikaria Island, namely IK 5, IK7 and IK2 (Fig. 3). IK5 (Karkinagrion granite) Zircons are mostly euhedral, long- to short-prismatic and vary in size (100–350 lm long axis), with aspect ratios of 1:3–1:4. The majority of grains possess inherited domains, which are either homogeneous or faintly zoned (Fig. 3). In the latter cases, core zonation is always truncated by outer zircon material. The interface to the enclosing rim is irregular, suggesting that cores experienced variable degrees of dissolution. Luminescence of the rims is highly variable, while fine oscillatory zoning is a prominent feature. Minor quantities of mineral inclusions are present. The cores are usually surrounded by thick overgrowths, suggestive of entrainment of sedimentary material during early stages of magma evolution. IK7 (Xylosirtis leucogranite) Zircon grains are euhedral, mostly short-prismatic to stubby and vary in length (100–250 lm), with aspect ratios of 1:2–1:3. Inherited cores are common and can be distinguished from the enclosing material by virtue of their brighter luminescence and truncated zoning (Fig. 3). In relation to the size of the cores, rim material is not as thick as in the samples IK5 and IK2, although its general thickness, in combination with evidence for resorption, suggests entrainment of zircons during early stages of magma evolution. The rims show weak oscillatory zoning and are characterised by a lack of luminescence. Commonly, rim material immediately enclosing cores is virtually devoid of luminescence. IK2 (Raches syenogranite) Zircons are euhedral, long- to short-prismatic and range in length from 100 to 450 lm, with aspect ratios of 1:3–1:5. Due to their large sizes a number of grains became fragmented during processing. Under CL, zircons display a very pronounced regular oscillatory zoning, which tends to be less luminescent towards the margins (Fig. 3). Inherited material is not common, but, where preserved, is distinct in luminescence from the enclosing rim and commonly corroded at the core-rim interface. Mineral inclusions are uncommon. Like in IK5, inherited cores are typically surrounded by relatively thick rims, suggesting

Emphasis was placed on selecting a wide variety of zircons with differing sizes, morphologies and internal make-up in order to minimise sampling bias. CL images provided the basis for selecting grains free of fractures and visible inclusions. U–Th–Pb isotope and U, Th, Pb elemental data are reported in ESM 1, see supplementary material. Trace element concentrations for Hf, P and REE are also reported as they assisted in detecting mineral inclusions that may impact on the age calculation (Campbell et al. 2006). ‘‘Crystallisation ages’’ of whole rocks using individual zircon U–Pb dates was aided by the following criteria: (1) Zircon analyses were initially screened for apatite inclusions; analyses with La [ 3 ppm and P [ 1,500 ppm were excluded. (2) U–Pb data with \85% concordance and spot mean standard weighted deviations (MSWD) [5 were excluded from age calculation. The spot MSWD represents a statistical criterion to distinguish between observed and expected scatter in the data (Wendt and Carl 1991). For spot MSWD values approaching unity, the assigned or analytical errors constitute the only source of scatter (Ludwig 2003; Campbell et al. 2006). (3) Discrete age populations were identified on the basis of resolvable peaks in density probability diagrams and the mixture modelling procedure devised by Sambridge and Compston (1994). The MSWD for the pooled populations was examined in tandem with the distribution of ages on a cumulative probability plot. If MSWDs 2 (greatly exceeding the theoretical value of 1.0 for a single population), grains with relatively high ages were deemed ‘‘inherited’’ if there were a distinct slope change on the cumulative probability plot. Subsequently, the population MSWD was again examined, and if 2, relatively low ages were treated similarly. In this way, some of the treated populations produce a reasonable single age interpreted to reflect crystallisation. In some cases, a MSWD smaller than 1.5 was not achieved, which means that without significantly more data collection, outlier populations cannot be fully defined and isolated to allow the crystallisation age to be precisely determined. This approach has been adopted in a number of previous studies to exclude zircons that either suffered Pb loss or can be suspected to belong to a population slightly older than ‘‘main crystallisation event’’, i.e. ‘‘subtle inheritance’’ (e.g. Campbell et al. 2006; Harris et al. 2004). It is emphasised that this procedure does not manipulate the age of a zircon population, but rather reduces the error and associated MSWD. (5) Finally, separate zircon age populations were identified, which

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differ by more than their age uncertainty of typically 0.1– 0.3 Ma (2r). Two contrasting datasets were obtained using different approaches: (1) For samples M1, M2, M3, Na3, Na5, Na6 and Sa7, analysis targeted mainly rims/tips in order to achieve maximum precision for the age estimate of the youngest preserved zircon crystallisation event. Only about 2–5 cores were also analysed. (2) For samples M4, T3, Na2, Na4, Ik2, Ik5 and Ik7, both cores and rims of individual zircons were analysed as means of quantifying systematic changes in crystallisation age and composition across grains. This approach also allows assessment of inherited material as a proxy of the magmatic source material as well as identification of evolutionary trends in the studied plutonic systems. In general, U–Pb zircon ages for most analysed samples fall into two major populations, with the exception of granitoid sample M1, where three zircon age populations can be distinguished (Fig. 4). Some granitoids yield an unimodal age distribution, i.e. samples from the islands of Tinos (T3) and Naxos (Na3, Na5, Na6). For Na3 and Na5, the unimodal age spectra probably represent a systematic bias towards the youngest preserved age component as these samples were targeted only for zircon rims/tips. In contrast, in samples Na6 and T3, unimodal age spectra were obtained from both core and rim data and thus are likely to present a true reflection of the geochronological make up of this sample. The majority of granitoids yield complex spectra (Fig. 4) with one (Ik2, Ik5, Ik7, Sa7, M3) or two almost equally dominant peaks (M1, Na4). Since MSWD values associated with calculated dates using all age populations clearly exceeded the analytical precision as dictated by Temora analyses, it is considered a requirement to treat resolvable age populations separately for the purpose of calculating meaningful ages. Figure 4 presents TeraWasserburg type diagrams showing all analytical data corrected for common lead, density probability plots and histograms, the latter distinguishing cores, rims and analyses that were excluded (‘‘deselected’’) from age calculations. U–Pb ages Tinos Four zircon core and nine zircon rim dates (with eight analyses being excluded) were combined to calculate a crystallisation age of 14.73 ± 0.22 Ma (n = 13, MSWD = 1.75) for monzogranite sample T3 (Fig. 4a). In a density probability plot, the data define a unimodal and symmetric distribution, indicative of a single age population. Cores and rims are indistinguishable. The unimodal age distribution fits with the lack of field evidence for multiple

123


intrusion in the I-type monzogranite. The age of 14.73 ± 0.22 Ma for the I-type monzogranite is identical to the U–Pb zircon age of 14.4 ± 0.2 Ma reported by Keay (1998) for a small S-type leucogranite. Mykonos Five rim and six core analyses from monzogranite M4 (seven analyses being excluded) combine to an age of 13.64 ± 0.22 Ma (n = 11, MSWD = 1.68; Fig. 4b). Using a similar dataset from the same sample a statistical identical age of 13.51 ± 0.3 Ma (n = 14) was previously determined by Brichau et al. (2008). The age spectrum in a density probability plot is unimodal and only slightly asymmetric, suggesting one age population. Core and rim ages overlap. A similar scenario unfolds for sample M3, although the unimodal distribution is slightly skewed (Fig. 4c). Twelve rim dates (12 dates were excluded) define an age of 11.08 ± 0.22 Ma (n = 12, MSWD = 1.69). This contrasts with density probability plots for sample M2, where data define an asymmetric, bimodal distribution (Fig. 4d). All dates (nine excluded) combine to an apparent age of 14.26 ± 0.29 Ma (n = 18, MSWD = 7.91). Visual inspection of the density probability distribution, in combination with Sambridge-Compston unmixing modelling, suggests two zircon populations: Two core and sixteen rim dates yield one dominant and one subordinate age: 14.59 ± 0.18 Ma (n = 13, MSWD = 1.43) and 13.43 ± 0.31 Ma (n = 5, MSWD = 1.49). Core and rim ages overlap. In the case of M2, a positive correlation between U content and age for analyses with U [ 1,000 ppm negates Pb loss to account for the younger age (Fig. 5a). The most complex case is presented by sample M1. In a density probability plot, the data define a multimodal, asymmetric spectrum (Fig. 4e). All data (five excluded) combine to produce an apparent age of 14.10 ± 0.4 Ma (n = 21, MSWD = 13.52). Two major peaks are almost identical in extent and reflect two statistically resolvable ages of 13.64 ± 0.21 Ma (n = 9, MSWD = 1.80) and 14.64 ± 0.23 Ma (n = 7, MSWD = 1.31), based on one core and 17 rim data (Sambridge-Compston modelling: 13.42 ± 0.08 Ma and 14.77 ± 0.08 Ma). A minor peak corresponds to a younger, and statistically distinct age of 12.36 ± 0.26 Ma (n = 3, MSWD = 0.43). Like sample M2, zircon data exhibit two intersecting trends in a diagram of U content vs age, with the positively correlated trend extending towards higher values for both U content and age, and a negatively correlated trend towards older ages but lower U contents (Fig. 5a). This relationship is inconsistent with Pb loss in U-rich zircon grains.


729

Fig. 4 Tera-Wasserburg Concordia diagrams showing common Pb corrected (208Pbbased) U–Pb isotopic compositions of zircons from 14 Cycladic granitoids. Cores, rims and zircon analyses excluded from age calculation (‘‘deselected’’) are distinguished in all diagrams, including histograms (right column). Data error bars = 1 standard error level. Uncertainties on the pooled ages are at the 2 standard error level. Only magmatic (i.e. non-inherited) zircons are displayed. Density probability plots using data for all magmatic zircon are also presented. Density probability diagrams were created using ISOPLOT version 3.00

Age data as calculated in this study for the Mykonos monzogranites span a range of 15–11 Ma, and are seen in general agreement with previously reported age constraints of 11.0 ± 1.3 Ma (uranothorite) and 11.1 ± 1.1 Ma

(zircon) (Henjes-Kunst et al. 1988). Brichau et al. (2008) reported an Ar/Ar hornblende age for monzogranite M4 of 12.7 ± 0.6 Ma, consistent with the zircon U–Pb age of 13.64 ± 0.23 Ma from this study.

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Fig. 4 continued

Naxos A total of 67 individual zircon core and rim analyses from four Naxos granodiorites and one leucogranite produce

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precise ages of 14.97 ± 0.32 Ma (Na2, MSWD = 1.67, Fig. 4f), 12.74 ± 0.11 Ma (Na3, MSWD = 1.74, Fig. 4g), 12.97 ± 0.06 Ma (Na5, MSWD = 0.66, Fig. 4h) and 12.98 ± 0.13 Ma

n = 5, n = 20, n = 24, (Na 6,


731

Fig. 4 continued

n = 18, MSWD = 1.7, Fig. 4i). In all four cases, U–Pb dates define distinct symmetric peaks in density probability plots, with no systematic differences between rim and core

data. Twenty-nine analyses were excluded, either based on discordance, elevated P, La contents or to minimise MSDW values and maximise precision on the ages.

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732


Fig. 4 continued

In contrast, U–Pb analyses for zircon cores and rims for Na4 produce a broad and asymmetric peak (Fig. 4j), which is interpreted to reflect two discrete age populations (Sambridge-Compston modelling: 13.32 ± 0.72 Ma, 12.4 ± 0.28 Ma). Accordingly, two statistically distinguishable ages were calculated: 13.11 ± 0.15 Ma (n = 7, MSWD = 0.68) and 12.25 ± 0.18 Ma (n = 7, MSWD = 1.24). In the latter case, lead loss by diffusion in radiation-damaged zircon may explain the slightly younger age. To test this possibility, U–Pb ages for Na4 and other Naxos samples were plotted against U content. The reason being that Pb loss, and hence a shift towards apparently younger dates, is most likely to occur in zircons with high (i.e. [1,000 ppm) U contents (e.g. Keay et al. 2001). A number of analyses plot at elevated U contents and at lower U–Pb dates, defining a broad negative correlation (Fig. 5b). These four youngest U–Pb zircon dates were excluded from age calculations because of expected Pb loss. The range in ages from the present study agrees with U–Pb zircon ages of 11.30 ± 0.4–13.20 ± 0.2 Ma by Keay et al. (2001). Keay et al. (2001) also identified two distinct zircon age populations within each of their three analysed I-type granite samples, totalling six statistically distinguishable ‘‘crystallisation ages’’ for the Naxos I-type granitoid. This phenomenon was explained in terms of distinct periods of crystallisation, possibly during initial partial melting and later crystallisation during cooling (Keay et al. 2001).

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Ikaria All three investigated Ikaria granitoids yield multimodal and asymmetric age spectra, which are skewed towards older ages. For the I-type Raches granite sample Ik2 (Fig. 4k), a dominant (13.33 ± 0.17 Ma, n = 15, MSWD = 1.66, Fig. 4k) and subordinate age (14.79 ± 0.26 Ma, n = 12, MSWD = 1.95) were calculated using 10 core and 17 rim analyses (all data combine to an age of 13.82 ± 0.33 Ma, n = 27, MSWD = 7.97; Sambridge-Compston unmixing modelling: 13.7 ± 0.2 Ma, 14.9 ± 0.3 Ma). The histogram highlights that the older age is largely derived from core data, while the younger age is based on rim analyses. In view of a general negative trend in U content versus age space, the younger age may be due to Pb loss (Fig. 5c). Sample Ik5 from the S-type Karkinagrion granite likewise yields two statistically distinct ages of 13.73 ± 0.29 Ma (n = 9, MSWD = 2.1, Fig. 4l) and 16.67 ± 0.64 Ma (n = 4, MSWD = 2.4; Sambridge-Compston unmixing modelling: 13.73 ± 0.17 Ma; 16.74 ± 0.48 Ma; all data: 14.07 ± 0.61 Ma, n = 19, MSWD = 12.11), with six analyses being excluded. Both ages are derived almost exclusively from rim data. Again, U content and age are negatively correlated (Fig. 5c). This may hint at Pb loss in younger zircons, although both high and low U contents are present in rims, in contrast to sample Ik2 with rims that tend to have higher U contents.


10 4

104

a

M1

b

Na4

Na2 1000

1000

Na3

U(ppm)

Na6

U (ppm)

Fig. 5 Diagrams showing U concentrations of zircons as a function of their respective U– Pb dates. Negatively correlated arrays may hint at Pb loss (in U rich zircons) to account for anomalously young dates

733

M2 M4

M3

Na5

Mykonos

Naxos 100

100

10 4

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c

Ik2

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104

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Ikaria 10 10

100 11

12

13

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15

16 17 18 19 20

Age (Ma)

Sample Ik7 from the S-type Xylosirtis granite is characterised by one dominant and one minor age peak corresponding to ages of 14.21 ± 0.27 Ma (n = 7, MSWD = 1.7) and 16.58 ± 0.21 Ma (n = 2, MSWD = 0.11; Fig. 4m). All data combine to an age of 14.53 ± 0.67 Ma (n = 10, MSWD = 7.82). Sambridge-Compston unmixing calculations suggest ages at 14.2 ± 0.19 Ma and 16.6 ± 0.6 Ma. Only rims were analysed, with one data point being excluded. The possibility of Pb loss in radiationdamaged zircons is indicated by negatively correlated U contents and ages (Fig. 5c). However, a counterargument is presented by the fact that the younger age population in all three Ikaria granitoids is the more dominant one in terms of number of zircons analysed. Also Pb loss is expected to cause a gradational range of apparent ages starting from the crystallisation ages, rather than distinct peaks in density probability plots. Hence, when all considerations are combined, Pb loss is considered unlikely for all three Ikaria granitoids. Even our oldest U–Pb zircon age from the I-type Raches granite of 14.78 ± 0.32 Ma is distinctly younger than the age of 22.7 ± 0.2 Ma reported by Altherr et al. (1982). Likewise, our U–Pb ages for the S-type Xylosirtis granite are younger than the age of 18.2 ± 2.2 Ma reported by Altherr et al. (1982). However, the oldest, weakly constrained, age peak at 16.67 ± 0.64 Ma overlaps within 2r errors with the Altherr et al. (1982) age. Samos Two core and nine rim analyses were used for the monzodioritic sample Sa7 (Fig. 4n). The data define a broad and asymmetric peak, which is skewed towards younger ages. All data combine to an age estimate of

10

11

12

13

14

15

16

17 18 19 20

Age (Ma)

11.86 ± 0.24 Ma (n = 11, MSWD = 4.59). SambridgeCompston unmixing calculations suggest two populations at 11.24 ± 0.27 Ma and 12.07 ± 0.16 Ma. These observations are clearly indicative of two age populations corresponding to 11.42 ± 0.18 Ma (n = 4, MSWD = 1.05) and 12.12 ± 0.18 Ma (n = 7, MSWD = 1.33). The U–Pb ages reveal a broad negative correlation with U content (Fig. 5d) suggesting Pb loss. The younger date is older than the published K–Ar ages obtained from hornblende and biotite of 10.2 ± 0.2 Ma from monzodiorite (Mezger et al. 1985). Tentatively, the *12 Ma U–Pb zircon age is taken as the better approximation to the final crystallisation and emplacement of the granodiorite intrusion on Samos because of its predominance in the density probability plot.

Discussion A record of protracted crystallisation histories Combining U–Pb ages of all the studied intrusive rocks in Fig. 6a illustrates that Miocene magmatism in the Cyclades extended over a time span from *17 to *11 Ma. Magmatic activity appears episodic as reflected in distinct age clusters around 16.5, 15, 13, 12 and 11 Ma (Fig. 6b) based on 22 individual ages. There is no clear geographic trend in the crystallisation ages. Furthermore, compiled U–Pb zircon data reveal that the crystallisation ages for S-type granites in the Cyclades are systematically about 0.5–2 million years older than those for I-type granites. However, in Tinos and western Ikaria where I- and S-type granites occur in contact with each other the ages for both granite types are not distinguishable.

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Considering within-pluton variability zircon data define age spectra ranging from simple and unimodal (e.g. Na3 in Fig. 4g) to complex and multimodal (e.g. Na4 in Fig. 4j). Seven out of 14 investigated granitoid samples yield more than one distinct zircon crystallisation age, with sample M1 representing the most complex case with three resolvable age peaks. Two (S-type) samples from Ikaria (Ik5, Ik7) appear to have crystallised zircon over 2–3 million years, whereas for the majority of samples with multiple zircon age populations the calculated ages deviate by 1–1.5 million years (e.g. Na6). Protracted (and episodic) pluton construction is further indicated by systematic age differences for individual zircon grains (Fig. 6c). After excluding inherited ages (shown in the inset diagram), 26 rim-core pairs (out of 34 shown) display ages that deviate from one another by more than can be attributed to analytical uncertainty, which is typically \0.3 million years (2r). Eleven cores formed approximately 2 million years earlier than the respective rims, while some cores appear to be substantially older, by up to 4 million years (e.g. one corerim pair from M3). The microtextural context of core-rim age variability is shown for the Ikaria granitoids in Fig. 3. A puzzling observation is that some rims appear older than their respective cores (Fig. 6c). In principle, this may be explained by the fact that LA-ICPMS analysis produces an ablation pit of ca. 20 mm depth, making it possible to transect cores (or rims) that are not visible microscopically. As recommended by Campbell et al. (2006), care was exerted to minimise such effects by using trace and U/Pb depth profile monitoring for each grain during data processing. Hence, apparent ‘age reversals’ remain difficult to explain but may be due to diffusive transport of radiogenic Pb from core to rim. Zircon age data in this study provide a record of protracted and episodic crystallisation histories for pluton

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a

18 17

dominant age subordinate age least pronounced age

15

13

S-type

S-type

14

11

Tinos

12 Mykonos

Naxos

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16

Age (Ma)

Ikaria

Ik7

Sa7

Ik2

Ik5

Na2

Na3

Na4

Na6

Na5

M4

M2

M3

West

M1

10

T3

Fig. 6 a Compilation of zircon age (and associated errors) data for c individual Cycladic granitoid samples. While zircon crystallisation ages for the majority of granitoids overlap, the analytical errors for ages allow identification of multiple resolvable events even within individual granitoids. Crystallisation events in spatially separated plutons occur concurrently, with some events spanning periods of up to 3 million years, significantly beyond analytical error (typically \0.3 million years, 2r). b Density probability plot highlighting the complex crystallisation history for plutonic magmatism in the Cyclades, with prominent peaks at 16.5, 14.8, 13.5, 13, 12 and 11 Ma. This observations rules out simple and instantaneous crystallisation in the course of cooling; data (underlined and vertical) for Iand S-granitoids from Naxos are also included (Keay et al. 2001). c Binary diagram of U/Pb ages for magmatic (including those that were excluded from age calculation) cores and rims for individual zircons. A considerable number of core-rim age pairs plot below the equiline (slope of unity), suggesting that cores crystallised significantly earlier than rims, well beyond analytical uncertainty (\0.3 million years). Error bars are shown as 1r. Inset diagram shows all age data, including inherited grains. Note the high proportion of inherited material in IK5 and IK7


East

b

c

crystallisation in the Cyclades, both for individual intrusions and on a regional scale. Several petrogenetic processes may account for the observed zircon age spectra, and are discussed below in the context of available field observations as well as zircon geochronological and geochemical data. Figure 7 (right column) displays diagrams of Th/U ratios and U–Pb crystallisation ages, with each of the identified age populations being highlighted by a grey bar. The range in Th/U zircon ratios from zero to unity reflects the extent of igneous differentiation with higher ratios signifying crystallisation from more mafic melts (Amelin 1998). An extended magmatic differentiation


history is indicated by the wide range in crystallisation temperatures from 900 to 600°C, probably starting with incipient zircon saturation in the melt and ending just before complete solidification. The variability in Th/U (and other trace element ratios including Zr/Hf), zircon crystallisation temperatures as well as U concentrations, ranging over almost two orders of magnitude (Fig. 7), strongly suggests that whole rocks should be viewed as an assembly of crystals that formed over potentially large periods of time under varying compositional and physical conditions. We discuss three feasible petrogenetic scenarios for explaining the zircon age spectra. (1) Progressive igneous differentiation and simultaneous cooling: This is the simplest scenario, whereby zircon age and compositional data are expected to follow a simple trend from older, more mafic melt compositions registering higher crystallisation temperatures towards younger and more evolved compositions at correspondingly lower temperatures. This systematic relationship would especially apply to intra-grain variability from core to rim. Several granitoids from the Cyclades seem to conform to this prediction: M2, Ik5, Na4 and Ik2. Strictly speaking, though, only Ik2 provides zircon data with a systematic evolution from core to rim. This suggests that other processes may have operated in tandem with simple igneous differentiation. (2) Emplacement of multiple melt-mush batches: Recent studies have addressed pluton construction utilising U–Pb zircon geochronology (Paquette et al. 2003; Miller et al. 2007; Michel et al. 2008; Miller, 2008; Walker et al. 2007; Bolhar et al. 2008). These studies demonstrate that pluton construction should not be viewed as near instantaneous emplacement of large volumes of magma (i.e. the classic concept of a magma chamber), but rather as incremental emplacement of multiple magma pulses over periods of time (e.g. Glazner et al. 2004). The latter approach can explain common features of intrusions, such as concentric compositional zoning, internal magmatic contacts and a general increase in zircon ages towards the rims of individual plutons (Matzel et al. 2006; Coleman et al. 2004). Amalgamation of separate melt batches would most probably manifest itself in age clusters, with zircon compositions and crystallisation temperatures not following a simple evolutionary trend towards evolved melt compositions at cooler temperatures. Intra-grain variability would be erratic, and rims can have either less or more evolved compositions than cores. Several granitod plutons appear likely candidates based on field evidence of compositionally variable magma intrusions within the pluton (Naxos, Mykonos, Ikaria, Samos). In some cases where multiple zircon age populations are present, zircon compositional data form semilinear trends when plotted against age or temperature (e.g. M2, IK2, IK5, Na4). Hence, it appears that the geochemical and thermal evolution was controlled by differentiation that

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was punctuated by repeated addition of compositionally varied melt batches. Similar conclusions were reached by Keay et al. (2001) who found that three highly fractionated hornblende-biotite granites from the I-type Naxos granodiorite body crystallised episodically between 12 and 11 Ma. This observation was attributed to a protracted history involving initial partial melting at deeper crustal levels, followed by crystallisation and cooling at progressively shallower crustal levels. Older age peaks may represent material that had formed during an earlier phase of magmatism, thus predating the main event of assembly and crystallisation. Granitoid sample M1 deserves special attention: two cores are younger than half of all analysed rims. Campbell et al. (2006) made a similar observation for granitic rocks from Chile, which contained zircon with rims slightly older than cores. The authors state ‘‘it is common to find inherited zircon that are only slightly older than the crystallisation age of the intrusion’’. Their preferred explanation is that younger intrusions intrude into older ones at crustal levels controlled by their relative density. As a consequence, preexisting intrusions were probably partially reworked, giving rise to the co-existence of multiple generations of zircon. (3) Reheating and remobilisation by injection of mafic magmas (e.g. Annen and Sparks 2002): Older zircons become remobilised or recycled at some later point by remelting and disaggregation of partly to wholly solidified host rock (e.g. Charlier et al. 2005). The required heat may have been provided by recharge with mafic, possibly CO2rich melts (Wark et al. 2007), in addition to ‘‘background’’ heat released from continuous crystallisation. Following Hildreth (presentation at Penrose Conference on ‘‘Longevity and Dynamics of Rhyolite Magma Systems’’, 2001) and subsequent studies (Charlier et al. 2005; Bacon and Lowenstern 2005; Miller and Wooden 2004), zircons of this type may be referred to as ‘‘antecrysts’’. This process has the potential to disrupt a simple evolutionary trend profoundly as hotter and more primitive material is added repeatedly in the course of igneous differentiation. A proportion of the rims is expected to register higher temperatures and more mafic melt compositions than cores. Likely candidates for this scenario include the monzogranites from Samos (Sa7) and Tinos (T3). In the case of T3, cores show a total overlap with rims with respect to Th/U and Zr/Hf ratios as well as crystallisation ages and temperatures, whereas Sa7 lacks a sufficient number of supporting core analyses. Both granitods yield zircon data that lack a consistent covariability for compositional, thermal and geochronological data, ruling out simple igneous differentiation. Finally, the overlap in zircon age spectra between various Aegean granitoids suggests that the Miocene plutons, albeit geographically separated by several tens of kilometres, once formed parts of one or several interrelated

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736 Fig. 7 Th/U ratios vs. U–Pb zircon dates (left column) and temperatures (Ti-in-zircon) vs. Zr/Hf ratios (left column) for all Cycladic granitoids examined for zircon chemistry and geochronology. Each of the identified age populations are highlighted by a grey bar. Cores and rims are represented by filled und open circles, respectively

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737

Fig. 7 continued

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magmatic systems. A close genetic relationship is further indicated by the similarity in whole rock (Altherr and Siebel 2002) and mineral chemistry and isotopic composition as well as thermal characteristics (see Part II). Inheritance: a window into source regions

a

0.02

relative probability

Inherited age spectra offer valuable insights into crustal source regions and the extent of wall rock assimilation. A total of 64 analyses yielded U–Pb ages in excess of 20 Ma. These dates are interpreted as inherited cores, in alignment with observations from CL imaging. S-type granitoids from Ikaria contain the highest proportion of inherited grains (Ik7: 61%, Ik5: 52%), while inheritance was noticeably smaller in I-type granitoids: 25% (M4), 14% (Sa7), 11% (M3), 10% (Na3), 6% (Na5), 4% (M1, M2), Ik2 (3%) and 3% (Na6). Two I-type granitods (T3, Na4) and one S-type leucogranite (Na2) were devoid of any detectable inheritance. The inherited ages are graphically displayed in density probability plots and one histogram (Fig. 8). The ages

0.015

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Inherited ages (>20 Ma) 0.03

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Fig. 8 a Probability density diagram showing age distribution of magmatic and inherited zircons from Cycladic granitoids Note distinct age peaks at 20–45, 60, 120 and *300 Ma. Inset diagram represent a close-up with ages ranging from 20 to 100 Ma. The higher resolution highlights the presence of distinct age peaks in this age range. b Histogram of same age data used in (a) with bin widths of 50 million years, distinguishing granitoid samples which were examined for zircon geochronology. Text inside diagram summarises the preponderance of zircon ages according to island

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range from 17 Ma (defined as the upper limit of plutonic activity) to *1,900 Ma. From ca. 375 Ma onwards only sporadic peaks are present, with 15 inherited ages exceeding 500 Ma. The preponderance of younger ages may be attributed to the better preservation potential of younger zircons due to higher U contents and longer times of radiation damage. Several peaks can be recognised at 320–280, 120, 60 and 45–20 Ma (Fig. 8a). Closer inspection of the younger age range reveals four peaks at 45, 35 and 23 Ma (inset diagram in Fig. 8a). We first discuss the latter three peaks followed by a discussion of the various older age peaks from the youngest to the oldest. The broad peak at *45 Ma probably correlates with the regional blueschist to eclogite facies metamorphic event in the Cyclades. This high-pressure event commenced at about 53 Ma (Tomaschek et al. 2003; Ring and Layer 2003; Forster and Lister 2005) and lasted until about C30 Ma in the deep structural levels of the Cycladic Blueschist Unit (Wijbrans et al. 1990). As the deeper levels of the Cycladic Blueschist Unit experienced highpressure metamorphism, higher structural unit was already exhumed and was overprinted by medium-pressure greenschist-facies metamorphism (Ring et al. 2010). The age peak at 35 Ma might either reflect the highpressure event in the deeper units, the medium-pressure overprint in the higher structural units or a combination of the two. In the early Miocene, another medium-pressure metamorphic event reached upper amphibolite-facies conditions in the central Cyclades on Naxos and Paros islands at C20.7 Ma (Keay et al. 2001). The zircon age peak at 23 Ma might well reflect the initial stages of high-temperature metamorphism in the central Cyclades. However, we realise that this inference is rather speculative. The peak at *60 Ma, as well as the little humps at *70 and *80 Ma (inset of Fig. 8a) is not well understood. They may correlate with formation of oceanic crust in the Selcuk Me´lange of the Pindos Ocean, as already inferred by Keay (1998). The geological significance of the *120 Ma age peak remains speculative, but may be related to one of two Cretaceous metamorphic events documented in upper units of the Cyclades consisting of Mesozoic sedimentary rocks and ophiolites (e.g. Bro¨cker and Franz 1998). A prominent peak can be identified at 320-280 Ma, largely based on zircon analyses from granitoids from Mykonos (M1, M2, M3, M4 and Ik5). This peak can be matched with a magmatic event at *320–300 Ma (Ring et al. 1999; Engel and Reischmann 1997) that occurred shortly after a high-temperature metamorphic event (Perraki et al. 2006). Our and Keay et al.’s dataset have mismatching peaks at *375 and 400–450 Ma, respectively. The tectonic significance of these age peaks is very poorly understood.


On the basis of predominant zircon inheritance ages in the range of 650–550 Ma, Keay et al. (2001) concluded that large portions of basement and overlying material constituting the Cyclades were derived from the northern margin of Gondwana (presently North Africa). A similar geochronologic fingerprint is presented by inherited U–Pb ages for zircons in this study, with one subtle peak at *550 Ma (Fig. 8a, b). In addition, a small zircon population with ages between 900 and 700 Ma may point towards material contribution from the Arabian-Nubian shield as part of the West African Craton (Reischmann et al. 1991), as has been already inferred by Keay et al. (2001).

Tectonic implications A conspicuous feature of the I-type granites in the Cyclades is the occurrence of mafic melts associated with these granitoids (Altherr et al. 1982, 1988). Altherr and Siebel (2002) showed that lamprophyric dikes display chemical and isotopic signatures that are compatible with an origin from either a mantle source previously enriched by slabderived hydrous fluids/melts or a new K-rich basaltic underplate. Melting of the lower crust to produce the mafic melts can be ruled out because temperatures of [1,000°C are needed for producing mafic melts by partial melting of parts of the lower crust (e.g. Petford and Gallagher 2001; Best 2003). There is no indication throughout the Cyclades that metamorphic temperatures in the Early to Middle Miocene exceeded 1,000°C. In parts of Naxos and Paros Islands, the Early to Middle Miocene lower crust is exposed and reported temperatures reached up to *700°C (Buick and Holland 1989). Therefore, we consider it much more likely that the mafic melts represent partial melting of the upper mantle in a subduction-related magmatic-arc setting. If so, the I-type granites associated with the mafic melts most probably represent the magmatic arc stage, which our age data bracket between 15 and 11 Ma. An important finding of our geochronologic study is that the S-type granites are systematically older than the I-type granites. The S-type granites most probably result from partial melting of the lower crust in the early and middle Miocene. Keay et al. (2001) already showed that the migmatites on Naxos formed between C20.7–16.8 Ma. As speculated previously, we attribute the inherited 23 Ma zircon age peak to initial migmatisation in the lower crust. It is interesting to note that the age of C20.7–23 Ma corresponds to the age for the onset of large-scale extension that started simultaneously across the entire Aegean Sea region at that time (see review in Ring et al. 2010). Large-scale extensional deformation is commonly triggered by a drastic change in the constitutional behaviour of

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the continental lithosphere (Dewey 1988). Our discussion of the U–Pb zircon ages for the Naxos migmatites of Keay et al. (2001) along with our zircon age peak at 23 Ma may suggest that migmatisation of the lower crust caused the change in the constitutional behaviour of the lithosphere and may have triggered or helped triggering the onset of Aegean-wide extension. Ring et al. (2010) proposed that the draping of the Hellenic slab over the 660-km discontinuity associated with a period of enhanced rollback caused *300 km of rapid and pervasive extension across the entire Aegean starting at about 23 Ma. The ages of the S-type granites of *17 Ma in the Cyclades would then represent an advanced stage of migmatisation of the lower crust. The magmatic arc as represented by the I-type granitoids with ages B*15 Ma would have moved through the Cyclades at a later stage. In other words, the magmatic arc reached the Cyclades at a stage when severe lithospheric extension was already under way for *8 million years. We concur with Altherr and Siebel (2002) that extensional deformation facilitated pathways for magma transport through the crust. Several studies have shown the intimate relationships between extension and intrusion of granitoids in the Aegean (Lee and Lister 1992; Kumerics et al. 2005; Grasemann and Petrakakis 2007; Brichau et al. 2010). However, we strongly emphasise that granitoid magmatism was not the trigger for large-scale extensional deformation but occurred at a stage when extension was already well underway. If it was accepted that the I-type granitoids and associated mafic melts represent a subduction-related magmatic arc, then it follows that the onset of extension in the Cyclades occurred earlier than the establishment of the magmatic arc and thus in a forearc position. Acknowledgments Mike Palin is thanked for his expertise and technical support during ICPMS analysis, for making available geochronology software and for numerous thoughtful discussions on igneous petrology and zircon geochronology. Thoughtful comments by two anonymous reviewers helped to improve the quality of the paper. Thorsten Nagel is thanked for his detail and constructive criticism, and Chris Ballhaus for his editorial assistance. RB acknowledges financial support through a postdoctoral fellowship at the University of Canterbury. UR acknowledges funding through grant E5345 of the Brian Mason technical trust.

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