The jadeitites from Syros and Tinos, Cycladic Blueschist Unit, Greece: field observations, mineralogical, geochemical and geochronological characteristics

Abstract This study illustrates the field relationships of jadeitite-bearing block-in-matrix sequences on Syros and Tinos, Cycladic Blueschist Unit, and adds additional U–Pb zircon ages for jadeitites to the geochronological database. The results confirm the importance of Cretaceous (c. 80 Ma) and Eocene (c. 50 Ma) processes in their geological evolution. Interpretations suggesting that the jadeitites were formed by complete metasomatic replacement of a pre-existing rock are not fully supported by field observations. In at least some cases, the formation of jadeitite is likely due to precipitation from Na-Al-Si-rich aqueous fluids, which also caused variable metasomatic alteration of the host rock. Unambiguous age constraints for formation of the Syros and Tinos jadeitites are not available. A relationship to Eocene blueschist facies metamorphism recorded in the associated metamafic rocks seems plausible. However, since high-pressure overprinting of pre-Eocene jadeitite is also conceivable, there is a much larger time window for jadeitite formation, framed by Cretaceous (c. 80–76 Ma) protolith ages of various mélange blocks and the waning stages of blueschist facies metamorphism (c. 40 Ma). Field observations are consistent with the interpretation that the mélange-like occurrences on Syros and Tinos record, to varying extent, multi-stage processes that include detachment of mafic rocks from the subducting plate, local infiltration of Na-Al-Si-rich aqueous fluids, exhumation via a serpentinitic matrix in a subduction channel and reworking of the primary block-in-matrix fabric by sedimentary or tectonic processes during accretionary wedge formation.


Introduction
Jadeitite sensu stricto is a relatively rare rock type that mainly consists of near-endmember jadeite-rich clinopyroxene (>90 vol% pyroxene with on average at least 80 mol% jadeite; Harlow et al. 2015).Such rocks occur as isolated bodies within serpentinite or in serpentinitic mélanges together with high-pressure/low-temperature (HP/LT) metamorphic rocks (e.g.Harlow & Sorensen, 2005;Yui et al. 2010;Tsujimori & Harlow, 2012;Harlow et al. 2015, and references therein).Most jadeitites are interpreted to have formed at P < 2.0 GPa and T < 500 °C (e.g.Harlow et al. 2015), but temperatures >550 °C were also reported for some occurrences (e.g.García-Casco et al. 2009;Schertl et al. 2012;Angiboust et al. 2020Angiboust et al. , 2021)).Petrogenesis of jadeitites is often controversial and associated either with strong metasomatism and replacement of felsic protoliths (R-type) or with precipitation (P-type) from aqueous fluids in veins, or a combination of both processes (e.g.Harlow & Sorensen, 2005;Tsujimori & Harlow, 2012;Harlow et al. 2015, and references therein).In either case, jadeitite formation requires circulation of Na-Al-Si-rich fluids.The fluids for the formation of P-type jadeitites originate from the dehydration of a descending slab in a subduction zone or from crystallization of trondhjemitic melts (e.g.Harlow et al. 2015;Cárdenas-Párraga et al. 2012, 2021).In the case of R-type jadeitites, the most plausible precursors are felsic igneous rocks (e.g.Coleman, 1961;Mori et al. 2011;Compagnoni et al. 2012;Hertwig et al. 2016Hertwig et al. , 2021;;Angiboust et al. 2020Angiboust et al. , 2021)).Understanding the mode of formation is of importance for the evaluation of U-Pb zircon data since the assessment of their geological relevance depends crucially on the petrogenesis of the host rocks (e.g.Fu et al. 2010Fu et al. , 2012)).Furthermore, as jadeitite formation is not restricted to the eclogite and blueschist facies, these rocks may have a different metamorphic age than spatially associated HP/LT rocks (Tsujimori & Harlow, 2012;Harlow et al. 2015), adding further complexities to the interpretation of their geological history.
The focus of this paper is on jadeitites from the Cycladic Blueschist Unit (CBU) of Syros and Tinos and describes their field relations, mineralogy, geochemistry and U-Pb zircon geochronology.Although the importance of Cretaceous ages for the heterogeneous block populations of both islands is well documented (e.g.Keay, 1998;Bröcker & Enders, 1999;Tomaschek et al. 2003;Bröcker & Keasling, 2006;Bulle et al. 2010), uncertainties remain as to when the jadeitites were formed in relation to high-pressure metamorphism and by which process.A particular concern of this study is to improve the geochronological database, which is currently limited to U-Pb zircon ages for only one sample from each of the two islands (Bröcker & Enders, 1999;Bröcker & Keasling, 2006), which may not cover the entire time span of jadeitite formation.Furthermore, the question arises whether there are significant differences in the origin of the jadeitite and metamafic block-bearing sequences on Syros and Tinos.

Regional geology
The Attico-Cycladic Crystalline Belt (ACCB) represents a major tectonostratigraphic unit of the Hellenides and is mainly exposed in the central Aegean region (Fig. 1a).The ACCB comprises two major tectonic units with different P-T-D-t histories, both consisting of numerous fault-bounded subunits (e.g.Dürr et al. 1978;Dürr, 1986;Forster & Lister, 2005;Ring et al. 2010).The Upper Cycladic Unit includes a heterogeneous sequence of unmetamorphosed Permian to Miocene sediments, Jurassic and undated ophiolitic and metamorphic sole remnants, greenschistfacies rocks with Cretaceous to Palaeogene metamorphic ages as well as Late Cretaceous amphibolite-facies rocks and granitoids (e.g.Altherr et al. 1994;Patzak et al. 1994;Sanchez-Gomez et al. 2002;Kuhlemann et al. 2004;Be´eri-Shlevin et al. 2009;Martha et al. 2016;Lamont et al. 2020a).Evidence for HP/LT metamorphism, which is a key feature in the metamorphic history of the structurally lower sequences, was not reported.The upper units were juxtaposed by low-angle detachments onto the nappe stack of the CBU (e.g.Avigad & Garfunkel, 1989;Brichau et al. 2007;Jolivet & Brun, 2010;Jolivet et al. 2010).
Greenschist-facies rocks at structurally deep levels of Tinos, Evia, Samos, separated from the structurally higher sequences by thrust faults, and regional amphibolite-facies metamorphic rocks at deep levels on Naxos do not record earlier HP/LT conditions or only lower grade blueschist metamorphism than the CBU and are interpreted to be para-allochthonous units (Avigad & Garfunkel, 1989;Ring et al. 1999;Shaked et al. 2000;Lamont et al. 2019).In the case of Tinos, this has not yet been clearly confirmed (Bröcker & Franz, 2005).
A particularly striking feature is an HP/LT mélange, which is best exposed in northern Syros (Kampos mélange; Fig. 1b; e.g.Dixon & Ridley, 1987), but most parts of Syros consist of marbleschist sequences.The significantly greater thickness of the marbles compared to the neighbouring islands of Tinos and Andros probably results from tectonic repetition by thrusting or isoclinal folding (e.g.Dixon & Ridley, 1987;Keiter et al. 2011).
On Tinos Island (Fig. 1c), the metamorphic succession can be subdivided into at least three tectonic subunits, referred to in the local literature as the Akrotiri Unit, the Upper Unit, and the Lower Unit (e.g.Melidonis, 1980;Okrusch & Bröcker, 1990;Katzir et al. 1996).The Akrotiri and Upper subunits (Fig. 1c), which form the uppermost part of the metamorphic sequence, belong to the regional Upper Cycladic Unit and record amphibolite and greenschist-facies P-T conditions (e.g.Patzak et al. 1994;Katzir et al. 1996;Bröcker & Franz, 1998;Zeffren et al. 2005;Lamont et al. 2020a).Field observations as well as lithological and metamorphic similarities indicate a correlative relationship with the two upper subunits on Syros (Bröcker & Enders, 2001;Soukis & Stockli 2013).
The Akrotiri Unit (300-350 m thick) comprises a relatively small occurrence near the main town of the island (Fig. 1c) and mainly consists of an interlayered amphibolite-gneiss sequence

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Michael Bröcker (Patzak et al. 1994).P-T estimates indicate metamorphic pressures of 0.6-0.85GPa and temperatures of c. 490-610 °C (Patzak et al. 1994).K-Ar hornblende and white mica dating of amphibolites and gneisses yielded apparent ages of c. 77-66 Ma and c. 60-53 Ma, respectively (Patzak et al. 1994).The contact to other subunits is not exposed, but judging from general field relationships this gneiss-amphibolite sequence forms the uppermost part of the Tinos metamorphic succession.The position of the Akrotiri rocks within the overall structural architecture of the region is not yet fully understood.The Akrotiri rocks either belong to a specific tectonic subunit on top of the CBU, e.g. the Asteroussia nappe, or are related to the local Tsiknias metamorphic sole (Patzak et al. 1994;Katzir et al. 1996;Lamont et al. 2020a).
Metamorphic conditions of the HP/LT event originally were estimated at ~1.2-2.0GPa and 450-550°C (e.g.Bröcker et al. 1993;Parra et al. 2002).More recent petrological modelling suggests peak P-T conditions of ~2.0-2.6 GPa and 490-570°C (Lamont et al. 2020b).The highest P-T conditions were reported for blueschists and eclogites from the Kionia area, which represents the topmost part of the Lower Unit (Lamont et al. 2020b).For rocks from deeper structural levels, Lamont et al. (2020b) described P-T conditions of ~1.85 GPa and 480-510°C for the HP stage and of ~0.7 GPa and 500-540°C for the retrograde greenschist overprint.According to Parra et al. (2002), overprinting during exhumation includes decompression from 1.5-1.8GPa at 500 °C to 0.9 GPa at 400 °C, a thermal overprint (400-550 °C, ~0.9 GPa) and further decompression from 0.9 GPa at 550-570 °C to 0.2 GPa at 420 °C.White mica dating indicated Eocene white mica ages for HP/LT rocks  and ages between 36 and 21 Ma for greenschist-facies rocks (e.g.Bröcker et al. 1993Bröcker et al. , 2004;;Bröcker & Franz, 1998).More recently, Glodny and Ring (2022) argued that blueschist-facies conditions lasted from at least c. 36 to 33 Ma and reported a c. 22 Ma age for the greenschist-facies overprint.Bulle et al. (2010) and Hinsken et al. (2016) described U-Pb zircon rim data of c. 57-46 Ma and related these ages to the HP/LT stage.
Siliciclastic metasediments also occur as host rocks for mafic and felsic blocks (Fig. 2c-f; Dixon & Ridley, 1987;Keiter et al. 2011), particularly in the upper part of the mélange zone.Metamorphosed mafic blocks in a matrix of blueschists and quartz mica schists have also been reported from rock sequences below the Kampos mélange and its correlative equivalents (Laurent et al. 2016;Kotowski & Behr, 2019), but the focus here is on jadeititebearing occurrences.For detailed field descriptions of the Kampos mélange see Dixon and Ridley (1987) and Gyomlai et al. (2021).
The Kampos block population includes rock fragments with Cretaceous (c.80-75 Ma) U-Pb zircon ages (e.g.Keay, 1998;Tomaschek et al. 2003;Bröcker & Keasling, 2006; Supplementary Table S1) attributed to either igneous crystallization or to metamorphic or hydrothermal processes.Some tectonic fragments yielded a Triassic (c.245-240 Ma) protolith age (Bröcker & Keasling, 2006).Such ages were also reported for similar rocks from structurally lower CBU sequences (Tomaschek et al. 2003), suggesting that tectonic slices originating from disruption of a primary lithostratigraphic unit were also incorporated in the mélange.However, most of the blocks are interpreted as remnants of oceanic lithosphere from a back-arc basin (Seck et al. 1996;Keay, 1998).
The modal and geochemical variability of the metamafic rocks has been linked to the development of their protoliths in different magma chambers (Seck et al. 1996).Protoliths of the coarsegrained metamafic rocks were interpreted to represent gabbros affected to varying degrees by fractional crystallization, while the precursors of finer-grained metamorphosed mafic rocks were associated with ocean floor basalts (e.g.Dixon & Ridley, 1987;Seck et al. 1996).Protolith ages of matrix rocks are poorly constrained.Clastic metasedimentary rocks from the Kampos area indicate a Late Cretaceous MDA (c.95-70 Ma; Keay, 1998;Löwen et al. 2015).
Explanations for the origin of the block-in-matrix sequences on Syros include mechanical mixing by olistostromatic or tectonic processes (e.g.Dixon & Ridley, 1987;Bröcker & Enders, 2001;Bulle et al. 2010) and concepts based on the exposure of mafic and ultramafic rocks on the seafloor in a hyper-extended intra-oceanic setting combined with rheological heterogeneities of subducted rock types (e.g.Kotowski & Behr, 2019, 2022;Gyomlai et al. 2021).

4.b. The Tinos block-in-matrix sequence
Occurrences of metamorphosed mafic blocks in a metasedimentary matrix were recognized in NW-Tinos, near Kionia, SE of Panormos and in the Mavra Gremna area (labelled A, B, C and D in Fig. 1c).However, unlike northern Syros, there is not a clearly defined and mappable mélange subunit.Instead, from the top to the base of the Lower Unit, relatively few isolated blocks of metagabbros, glaucophanites, eclogites, jadeitites and ultramafic rocks (mostly <1-10 m, but up to 300 m) occur at various levels within the marble-schist sequence (Bröcker & Enders, 1999;Bulle et al. 2010).The block-forming process is controversial.Bulle et al. (2010) assumed a meta-olistostromatic origin.In contrast, Lamont et al. (2020a) attributed the blocks to boudinage of mafic lithologies.The block population records HP/LT metamorphic conditions and includes rock types like those in the Kampos mélange (Figs. 3, 4a-c), but their abundance is much smaller, and the matrix consists almost entirely of metasedimentary or metatuffaceous rocks.Especially noteworthy is the existence of jadeitites (Figs.3a, b; 4a-c).Ultramafic rocks are rare (e.g.Lamont et al. 2020b), but some metamafic blocks are surrounded by a thin serpentinite or chlorite schist cover (Bulle et al. 2010), suggesting that these blocks were originally in contact with an ultramafic matrix.Ion probe dating of eclogite, glaucophanite, blackwall and chlorite schist yielded Cretaceous U-Pb zircon ages (c.80 Ma; Bulle et al. 2010; Supplementary Table S1).Similar MDAs have been reported from the enclosing schists, implying that detritus derived from the block-forming mafic lithologies also occurs in the metasediments (Bulle et al. 2010;Hinsken et al. 2016).In various parts of the Lower Unit, meta-conglomerate horizons are associated with the block-in-matrix succession (Fig. 4d-e

Analytical methods
Mineral compositions were determined with a JEOL 8530F electron microprobe (EMP).Natural and synthetic mineral standards were used for calibration.Operating conditions were a 15 kV accelerating voltage, 5 nA electron beam current and a 5-μm spot size.Counting times were 5 s on the peak and 2 s on the background.
For U-Pb geochronology, zircon crystals were separated from several kilograms of fresh rock by standard routines (jaw crusher, disc mill, Wilfley table, Frantz magnetic separator, methylene iodide heavy liquid and handpicking under stereomicroscope).After preparation of epoxy resin mounts and polishing to expose grain interiors, cathodoluminescence (CL) imaging was applied to reveal the internal zircon structures and to guide spot placement.
Ion microprobe (SHRIMP) dating was carried out at the Centre of Isotopic Research (VSEGEI), St. Petersburg, Russia.Analytical procedures followed standard operating routines and were similar to those reported by Bulle et al. (2010) and Bröcker et al. (2014).Individual spot ages in Supplementary Table S5 are based on the 207 Pb correction method, assuming 206 Pb/ 238 U-207 Pb/ 235U ageconcordance.Most analyses contain very little common Pb and thus are insensitive to the choice of initial isotopic composition.Weighted averages (without error in standard) and intercept ages are quoted as 206 Pb/ 238 U ages with 95% confidence limit and 2σ uncertainty, respectively.The uncorrected data for all samples are presented in Tera-Wasserburg diagrams.Such plots, regressions and weighted mean age calculations are based on Isoplot 4.15 (Ludwig, 2012).
Bulk-rock compositions were analysed by Actlabs, Ancaster, Ontario, applying a lithium metaborate/tetraborate fusion technique followed by acid digestion and analysis by ICP and ICP/MS (4Lithoresearch analytical protocol).

Petrography and mineralogy
Nineteen samples were selected for more detailed petrographic, geochemical and geochronological investigation.Samples from Syros are from the Kampos area (Fig. 1b).Samples from Tinos were collected in various parts of the island (Fig. 1c).GPS coordinates of sampling locations are reported in Supplementary Table S2 (available online at https://doi.org/10.1017/S0016756823000602).Field images are shown in Figs.2-4.Hand specimen images are presented in Fig. 5. Photomicrographs are depicted in Figs. 6 and 7. EMP analyses with the focus on clinopyroxene compositions are summarized in Supplementary Table S3.
Many of the samples examined are not jadeitite sensu stricto but variably albitized or impure jadeitites with lower jadeite contents.Fresh or poorly overprinted jadeitites are massive, non-foliated and granoblastic rocks with mostly light-green colour (Fig. 5).With increasing degree of retrograde overprinting, the colour changes to darker shades of green.Apart from possibly zircon, no igneous relicts or pseudomorphs of such phases were found.
On Tinos, two petrographic and geochemical types of jadeiterich rocks can be distinguished: The first type (sample 4011) corresponds to similar rocks from Syros and occurs as loose block in association with various schists and marble (Figs.3a, b; 4a-c).More strongly overprinted derivatives of such rocks are exposed in the Panormos area both as in-situ block within meta-tuffaceous schists (samples 5535, 5537, 5539; Fig. 4a, b) and as loose boulders (sample 9028; Fig. 4c).Despite severe retrogression, unoriented and mostly fine-grained jadeite (up to Jd >90 mol.%) is still preserved (Fig. 8e; Supplementary Table S3).The second type (sample 1049) is restricted to a small occurrence near Kionia (Fig. 1c) and was originally described as jadeitite (Bröcker & Jadeitites from Syros and Tinos, Greece Enders, 1999) but is more aptly defined as garnet-jadeite granofels, as the mineral composition often contains significant amounts of garnet (Fig. 7g, h).Many hand samples are very similar to eclogite, but the clinopyroxenes are jadeites (Fig. 8f).Most of the original outcrop was destroyed by the construction of a house, but a small part remained and shows the garnet-jadeite granofels in metasedimentary schists (Fig. 3f).
Samples 5171, 5175 and 5176 represent the mafic parts of the compound eclogite-jadeitite block (Fig. 2h; stop 11 in the excursion guide of Dixon & Ridley, 1987; for additional outcrop pictures see Bröcker & Keasling, 2006).The mineral assemblage of the newly dated sample 5175 (Fig. 6g) is dominated by sodium-rich clinopyroxene (>70 vol.%) and relatively little garnet (5-10 vol.%).Epidote, white mica and rutile occur as additional primary phases.Glaucophane, titanite and chlorite are related to a later overprint.Sample 5171 was collected from a different part of the net-veined block and has less garnet than the other two samples but shows cross-cutting omphacitic clinopyroxene veins (Fig. 6 h, i).The compositions of matrix and vein clinopyroxenes partially overlap, but the veins include abundant crystals with lower Jd component (Fig. 8c; Supplementary Table S3).
There are no significant bulk-rock compositional differences between jadeitites from Syros and Tinos (Figs. 9, 10).In MORBnormalized multi-element diagrams, the jadeitites and the garnetjadeite rocks show similar distribution patterns with enrichment in high HFSE and negative troughs for at K, Sr and Ti and flat REE trends (Fig. 10a, c, e).However, based on normalized REE patterns, different groups can be distinguished.The first group is characterized by distribution patterns with distinct negative Eu anomalies (Eu/Eu* = 0.38-0.71;Fig. 10b).Samples of the second group show almost horizontal sinusoidal REE variations (Fig. 10d).Three jadeitites of the third group show enrichments of the LILE and a negative REE slope (Fig. 10f).The REE pattern of sample 5260 differs from all other samples by having a positive Eu anomaly (Eu/Eu* = 1.9;Fig. 10c).

U-Pb geochronology
CL images of zircon are shown in Fig. 11 and U-Pb analytical data are reported in Fig. 12 and Supplementary Table S5.For all dated samples, zircon O-Hf isotope data were already reported by Fu et al. (2010Fu et al. ( , 2012)).In all samples, zircon typically has a short prismatic or blocky morphology.Pristine grains or domains show mostly rhythmic zoning or sector-zoned internal patterns, but many crystals have complex cauliflower-like internal structures, indicating partial recrystallization (Fig. 11).Inherited cores or distinct overgrowths were not observed, but some zircon grains of sample 1049 have dark, irregularly shaped, recrystallized domains.
Sample 1078 (Figs. 5a;6a,b) was collected from a loose block in the upper part of the path to Lia beach, just after the gate.The quartz-bearing jadeitite 4030 (Figs. 5b;6c,d) represents the block shown in Fig. 2c, d (stop 15 in the excursion guide of Dixon & Ridley, 1987).Ion probe dating of samples 1078 and 4030 yielded almost identical anchored intercept ages of 80.7 ± 1.7 Ma and 81.7 ± 1.3 Ma, respectively (Fig. 12a, b).
Sample 3148 represents the jadeitite from the eclogite-jadeitite net-veined rock with distinct blackwall alteration zones (stop 11 of Dixon & Ridley, 1987).This sample was dated in a previous study (Bröcker & Keasling, 2006) and is presented here with new data evaluation.The screened data give an intercept age of 79.8 ± 0.4 Ma (Fig. 12c), which only slightly differs in the uncertainty from the original age calculation.From this outcrop, the mafic host rock, represented by sample 5175 (Fig. 6g), was also dated.Eight zircon spot analyses provided an intercept age of 78.0 ± 1.0 Ma (Fig. 12d).
Sample 4011 (Figs.5g, h; 7a-c) was collected in the Mavra Gremna area on Tinos where it occurs as loose boulder together with widely scattered metamorphosed mafic blocks within a marble-schist sequence (Fig. 3a, b).Ten U-Pb spot analyses yielded an intercept age of 82.6 ± 1.6 Ma (Fig. 12e).Sample 1049 (garnet-jadeite granofels) from Kionia was included in this study to check the geological significance of a previously reported TIMS multigrain zircon date (Bröcker & Enders, 1999).U-Pb zircon data define a lower intercept age (not anchored) of 75.0 ± 3.1 Ma and a geologically meaningless upper intercept (Fig. 12f).A recrystallized domain gave the youngest spot age of 52.2 ± 2.1 Ma (spot 10.1; Fig. 11f).The weighted mean 206 Pb/ 238 U age is 78.6 ± 1.9 Ma (spot 10.1 excluded).For all other samples, the weighted average ages are nearly identical to the intercept ages (Supplementary Table S5).

9.a. Zircon geochronology and bulk-rock compositions
One aim of this study was to expand the small geochronological dataset for jadeitites from the Cyclades and to resolve ambiguities of existing multigrain TIMS zircon data.In the case of Syros, it was easy to select additional samples as virtually all jadeitites of the Kampos area are zircon-rich.The situation is more difficult on Tinos, where jadeitites are much rarer and only occasionally contain zircon.
The newly dated samples from both islands yielded U-Pb intercept ages of c. 83-80 Ma and almost identical weighted average 206 Pb/ 238 U ages (Fig. 12a-c, e; Supplementary Table S5).The metamafic host rock and cross-cutting jadeitite of a brecciated eclogite-jadeitite block yielded U-Pb ages of 78.0 ± 1.0 Ma and 79.8 ± 0.4 Ma, respectively, which overlap within analytical uncertainty (Fig. 12c, d; see also Bröcker & Keasling, 2006).Taken together, these data document that there is only a single age group of jadeitites on Syros and Tinos.1594 Michael Bröcker Based on normalized REE patterns, different groups of jadeitites and jadeite-rich rocks can be distinguished (Fig. 10b,  d, f).This variability in composition may be related to derivation from different plagiogranitic protoliths, such as described from the Oman and Troodos ophiolites (e.g.Rollinson, 2009;Freund et al. 2014), to metasomatic alteration of a single protolith type by fluids of variable composition, to direct precipitation from different jadeitite-forming fluids, to differences in modal abundance of accessory phases or to changes in REE signatures during retrograde processes.At this stage, this aspect cannot be further clarified because, among others things, no geochemical data are available for potential felsic protoliths.A special case is the in-situ occurrence of garnet-jadeite granofels near Kionia (Figs. 1c,3f).This rock has a very Zr-rich rock bulk-rock composition (c.1700-2000 ppm) and corresponding zircon abundance (Fig. 7g, h), which is also typical for a very small occurrence of loose eclogite fragments (Zr: c. 3500-5000 ppm) located a few hundred metres away at the foot of the same hill (Bröcker & Enders, 1999, 2001).
The bulk-rock composition of the garnet-jadeite samples can be clearly distinguished from the Tinos jadeitites (samples 4011 and 5539), but their REE patterns are very similar to those of Syros samples 3148 and 1077 (Fig. 10).Oscillatory-zoned jadeite (Fig. 7i) and the similarity of trace element and rare earth distribution patterns to those of a sample representing fracture-filling jadeitite indicate original crystallization in an open system and a P-type formation mode.However, in contrast to the Kampos mélange, which locally hosts jadeitites in metasedimentary schists (Fig. 2c), there is no evidence here of subsequent tectonic or sedimentary incorporation of foreign material into the host rock, such as remnants of a serpentinized matrix or blackwall alteration around the granofels as a result of initial contact with ultramafic matrix.Thus, this outcrop could represent a boudin of an extrusive or intrusive meta-igneous layer or vein, as postulated by Lamont et al. (2020b) for the formation of metamafic blocks in the Tinos schist sequences.However, it should be emphasized that there is no general evidence for a boudinage origin of the block-in-matrix occurrences on Tinos.Isolated metamafic blocks (meta-gabbro, eclogite, glaucophanite) and jadeitites occur sporadically in schists.There are no lithological horizons of such rocks that are continuously or discontinuously exposed over longer distances.In addition, blackwall alteration and altered ultramafic schists around blocks have also occasionally been described from the Tinos metamafic blocks (Bulle et al. 2010), suggesting incorporation of exotic material by tectonic or sedimentary reworking.In any case, the Kionia garnet-jadeite rocks and eclogites are unusual and rare lithologies that are not known anywhere else in the CBU.
In a previous study, multigrain U-Pb zircon TIMS dates of c. 63-61 Ma were reported for sample 1049 from the Kionia in-situ outcrop (Bröcker & Enders, 1999).This apparent age is not confirmed by ion probe dating of zircons from the same sample, which instead yielded a U-Pb intercept age of 75 ± 3.1 Ma and a single spot age of c. 50 Ma for a recrystallized domain (Fig. 11; Supplementary Table S5).Similar Cretaceous (78.2 ± 1.4 Ma) and Eocene (ca.57-54 Ma) U-Pb ages were reported for a Kionia eclogite (Bulle et al. 2010).

9.b. Relationship between zircon and jadeitite formation
In the central Aegean region, jadeitites occur on the islands of Syros, Tinos and Andros (Fig. 1a).In terms of bulk-rock composition, the age complexity of the zircon population and the protolith age, the jadeitite from Andros differs significantly from similar rocks on the neighbouring islands (Höhn et al. 2022).Particularly striking is the more complex zircon population of the only dated sample, which is characterized by Jurassic overgrowths (163.1 ± 3.9 Ma and 174.3 ± 2.0 Ma) on Middle Proterozoic (c.1126 Ma and c. 1421 Ma) and Permian (c.273 Ma and c. 281 Ma) grains (Bulle et al. 2010), indicating an R-type origin.It is very likely that the Andros occurrence belongs to another tectonic subunit within the nappe stack of the CBU (Höhn et al. 2022).
The jadeitites from Syros and Tinos are different.These rocks yielded Late Cretaceous U-Pb zircon ages (c.80 Ma; Fig. 12; Supplementary Table S5), as did the zircons from spatially associated metamafic mélange rocks, which are interpreted as protolith ages.Similar observations in the Rio San Juan Complex, Dominican Republic, led to the conclusion that the jadeitite protoliths were part of the oceanic crust before subduction (Hertwig et al. 2021).However, since the samples studied here are from a mélange, it is possible that rocks of the same age, formed in different ways and in different places, were mixed later in the subduction channel or during tectonic or sedimentary reworking.
The significance of zircon in jadeitites (inherited or newly formed?) is often controversial, and the different interpretations proposed for the jadeitites of Syros and Tinos are a good example of such controversies.Early studies interpreted the U-Pb ages of jadeitites and related blackwall zones as indication for Cretaceous metamorphic or hydrothermal subduction zone processes (Bröcker & Enders,1999;Bröcker & Keasling, 2006).This explanation was challenged in later studies, which instead argued for an igneous zircon origin, based on mantle-like oxygen isotope ratios and initial epsilon hafnium values of zircons (Fu et al. 2010(Fu et al. , 2012)), similar interpreted protolith ages of associated eclogites and  Michael Bröcker meta-gabbros and a re-evaluation of presumed high-pressure mineral inclusions as pseudo-inclusions (Tomaschek et al. 2003;Fu et al. 2010).These arguments led to the conclusion that the jadeitites were formed by complete metasomatic replacement of a pre-existing meta-igneous rock during Eocene HP/LT metamorphism recorded in other CBU rocks (Fu et al. 2010(Fu et al. , 2012)).However, the petrogenesis of the studied jadeitites does not seem to be related to the formation of blackwalls at the contacts between blocks and ultramafic matrix, as some jadeitite blocks, like other lithologies, also show such reaction rinds.Nevertheless, uncertainties remain regarding the petrogenesis and the timing of jadeitite formation.Eocene HP/LT metamorphism is evident, but the jadeitites may also reveal earlier subduction-related processes under different P-T conditions.There is a much larger time window for jadeitite formation, bracketed by Cretaceous zircon protolith ages (c.80 Ma) of various mélange blocks, and white mica ages for the waning stages of blueschist facies metamorphism (c.40 Ma).
As discussed below, the conclusion that the jadeitites were formed by metasomatic replacement of a pre-existing rock rather than precipitation from aqueous fluids, with the zircons representing relicts of the igneous protolith (Fu et al. 2010(Fu et al. , 2012)), is not entirely convincing.Most jadeitites are found as isolated blocks, completely detached from the original site of formation.Only in one case is the original relationship documented by a brecciated eclogite-jadeitite block broken into two larger pieces that show the net-veined block interior very well (Fig. 2h).This structure documents either the forceful injection of felsic melts into already lithified parts of a mafic protolith or the precipitation of jadeitite from vein fluids in fractures.However, this observation does not necessarily mean that the jadeitite formed during the blueschist-facies overprint of pre-existing eclogite (Tsujimori & Harlow, 2012).The age of jadeitite formation may be older than both the eclogite-and blueschist-facies P-T stages recorded in other mélange rocks.
The lack of a resolvable age difference between host rock (78.0 ± 1.0 Ma) and cross-cutting jadeitite (79.8 ± 0.4 Ma; Fig. 12c,  d) could be due to the following explanations:  There is currently no fully convincing answer to the question of which of these alternatives apply.The distinction between hydrothermal, igneous or metamorphic zircon is often difficult and ambiguous.Many of the criteria commonly used as distinguishing features, such as Th/U ratios, trace element and REE signatures (e.g.Flores et al. 2013), do not provide clear indications of the mode of formation (e.g.Schaltegger, 2007;Bulle et al. 2010;Zhong et al. 2018).Nevertheless, in the present case, a misinterpretation of the O-Hf zircon characteristics is not very likely.The relatively small compositional range of average δ 18 O (4.7 % to 5.5 %) and initial ε Hf (t) values (þ10 to þ24) is consistent with an igneous origin and indicates zircon crystallization from melts that were produced from a depleted mantle source (Fu et al. 2010(Fu et al. , 2012)).However, zircons in jadeitite may be xenocrysts that have been entrained by melt or aqueous fluids.The presence of such zircons in aqueous solutions does not seem to be a particularly realistic hypothesis, but such scenarios have been proposed both for zircon in jadeitites (Meng et al. 2016; see also Yui & Fukuyama, 2015) and for zircon in quartz veins cross-cutting eclogite (Sheng et al. 2012).Formation of jadeitite requires hydrothermal fluids that induce metasomatism or direct open-system crystallization in fractures.Field observations indicate that at least some of the studied jadeitites belong to the P-type.Interestingly, oscillatory-zoned clinopyroxenes of some Syros omphacitites suggest a similar origin (see Shigeno et al. 2012 for a similar example from western Kyushu, Japan).So far, neither the claim that all jadeitites of Syros are metasomatized rocks nor the age of jadeitite formation has been conclusively proven.Both aspects require further investigation.Such studies should also take a closer look at the geochemistry of meta-plagiogranitic dykes and melt injections in meta-gabbros as potential precursors of R-type jadeitites, as a direct lineage may be established here.In this context, it is interesting to note that Hertwig et al. (2021) reported that jadeite rocks from the Rio San Juan Complex, Dominican Republic, could have been derived from plagiogranites by isochemical replacement or metasomatic desilication without extensive chemical exchange.

9.c. Different origins for the jadeitite-bearing sequences on Syros and Tinos?
A characteristic feature of the Kampos mélange on Syros is the abundance of blocks within a relatively well-defined and mappable horizon, the presence of both serpentinitic and clastic metasedimentary host rocks and metasomatic reaction rinds at contacts between blocks and ultramafic matrix.On Tinos, only few blocks   Bröcker and Enders (2001).N-MORB normalization values are from Sun and McDonough (1989).Chondrite normalization values are from McDonough and Sun (1995).
occur in metasedimentary and meta-tuffaceous sequences, serpentinite is rare and blackwall rinds around blocks have either never existed or are only sporadically preserved (Bulle et al. 2010).
The coincidence of the U-Pb zircon ages of meta-igneous blocks and the MDAs of metasediments led to the interpretation  Michael Bröcker that both blocks and sedimentary detritus of similar age originated from the same source and were mixed during gravity-driven transport (Bulle et al. 2010).This explanation was questioned by Lamont et al. (2020b) and Kotowski et al. (2022).In the case of Tinos, Lamont et al. (2020b) argued that the sporadic blocks in a mainly metasedimentary matrix differ significantly from the Kampos mélange on Syros and suggested a primary sedimentaryvolcaniclastic origin, with the eclogites representing boudinaged mafic intrusions or lava flows rather than olistoliths.Lamont et al.
(2020b) compared the field situation on Tinos with the Chroussa subunit on Syros, which also comprises block-bearing coherent sequences (Keiter et al. 2011;Laurent et al. 2016) and described a lithological difference between the blocks on Tinos, interpreted as boudins of mafic layers, and the more diverse rock types of the mélange on Syros, interpreted to represent fragments of subducted oceanic crust.Field observations are not fully consistent with this interpretation.Boudinage at all scales is a common feature on Tinos; however, the same rock types are found in the block population as in the Kampos melange, including meta-gabbros, eclogites and jadeitites (Figs. 3,, but in much smaller numbers.Some of these blocks are surrounded by thin ultramafic or chloritic selvages and blackwall zones (Bulle et al. 2010).This suggests that these rocks were originally in contact with a serpentinite matrix before being redeposited into the present host rocks.This is similar to parts of the Kampos area where blocks with a thin ultramafic cover

Figure 1 .
Figure 1.(Colour online) (a) Geographic overview of the larger study area.(b, c) Simplified geological maps of Syros (modified after Keiter et al. 2004) and Tinos (modified after Melidonis, 1980).Red rectangle in (b) outlines Kampos area.Red rectangles in (c) show areas where blocks occur more frequently in the schist sequences.A = NW Tinos; B = Panormos; C = Mavra Gremna; D = Kionia.

Figure 5 .
Figure 5. (Colour online) Hand specimen pictures of jadeitites from Syros and Tinos.

Figure 7 .
Figure 7. (Colour online) Thin section images of samples from various locations on Tinos jadeitites and garnet-jadeite granofels from Tinos.(a, b, c) Jadeitite 4011, Mavra Gremna area.(d) Overprinted jadeitite 5535.(e, f) Strongly overprinted jadeitite 9028, Panormos area.(g, h, i) Garnet-jadeite granofels 1049, Kionia area.Red rectangle in (g) outlines area shown as close-up in (h).Red arrows in (h) point to zircon grains in and around garnet: Ab, albite; Bt, biotite; Chl, chlorite; Cpx, clinopyroxene; Ep, epidote; Grt, garnet; Pg, paragonite; Ph, phengite; Zrn, zircon. 1596 (a) The igneous origin of both rock types and the intrusive relationship are correctly interpreted.The zircons of both rock types were formed during the same magmatic event before subduction and indicate the protolith age.Both rock types were later metasomatically altered by hydrothermal fluids, with the felsic rock showing pervasive jadeititization, while the mafic host rock was much less affected.Apart
Ta Ce Pr Nd Hf Eu Gd Dy Y Tm Lu Cs Ba U Nb La Pb Sr Zr Sm Ti Tb Ho Er Yb

Figure 11 .
Figure 11.Cathodoluminescence images of representative zircons of U-Pb-dated samples from Syros and Tinos with spot identification numbers and 206 Pb/ 238 U ages (1σ).White lines for scale are 200 μm in length. 1600

Figure 12 .
Figure 12.Tera-Wasserburg diagrams of U-Pb-dated samples from Syros and Tinos.Data point error ellipses indicate 1σ uncertainties.Black dashed lines in (a, b, d, e) indicate mixing trend with a common Pb composition anchored at 207 Pb/ 206 Pb values of 0.841 (t = 78-83 Ma; Stacey & Kramers, 1975) and in (c) of 0.9618 (Broken Hill gold).Dashed line in (f) indicates an unanchored mixing trend.
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 10.(Colour online) Normalized trace element and REE patterns for jadeitites and eclogites from Syros and Tinos, including data reported by