U–Pb SHRIMP-dating of zircon domains from UHP garnet-rich mafic rocks and late pegmatoids in the Rhodope zone (N Greece); evidence for Early Cretaceous crystallization and Late Cretaceous metamorphism

Chemical Geology 184 (2002) 281 – 299 www.elsevier.com/locate/chemgeo

U–Pb SHRIMP-dating of zircon domains from UHP garnet-rich mafic rocks and late pegmatoids in the Rhodope zone (N Greece); evidence for Early Cretaceous crystallization and Late Cretaceous metamorphism Anthi Liati a,*, Dieter Gebauer a, Richard Wysoczanski b a

Institute of Isotope Geology and Mineral Resources, Swiss Federal Institute of Technology (ETH), Sonneggstrasse 5, 8092, Zurich, Switzerland b RSES, Australian National University, Mills Road 0200, ACT Canberra, Australia Received 30 August 2000; accepted 29 August 2001

Abstract U – Pb ion microprobe (SHRIMP) dating of zircon domains assisted by cathodoluminescence imaging was carried out on (1) a garnet-rich mafic rock and (2) a cross-cutting pegmatoid from the area of Kimi, East Rhodope. Quartz exsolution lamellae in cpx of the garnet-rich mafic rock point to a precursor supersilicic cpx implying pressures > 2.5 GPa. This indicates UHP conditions for the studied geotectonic unit and is in line with the (rare) presence of radial cracks around quartz inclusions in garnet, which suggest the former presence of coesite. Bleb-shaped cpx inclusions in garnet may be interpreted as primary inclusions during garnet growth. However, based on their shape and general orientation, the possibility that they represent exsolutions from a precursor majoritic garnet, in an advanced state of recrystallization (coalescing initial needles) remains open and needs further investigation. If the latter is proven true, it implies minimum pressures of 4 GPa. The oscillatory zoned domains of the zircon crystals from the garnet-rich mafic rock yield a weighted mean age at 117.4 F 1.9 Ma (Early Cretaceous) for the magmatic crystallization of the protolith. The zircon domains characterized by clearly fainted oscillatory zoning and/or U-rich embayments of the same zircon crystals are interpreted as metamorphic domains. They yield a weighted mean age of metamorphism at 73.5 F 3.4 Ma (Late Cretaceous). Different degrees of Pb loss resulted in discordant ages scattering between the magmatic and metamorphic age. The euhedral oscillatory zoned zircons of the cross-cutting pegmatoid yield a weighted mean age of 61.9 F 1.9 Ma for the time of emplacement and crystallization of retrograde metamorphic fluids into the middleupper crust. Two scenarios are considered as possible for the interpretation of the SHRIMP-results and the petrological observations: (1) the protolith was an ultra high pressure melt (probably a grt F cpx cumulate) crystallizing in the mantle at Early Cretaceous times, partly exhumed into the lithosphere by mantle upwelling, and then transferred to a subduction zone and metamorphosed under eclogite-facies conditions in the Late Cretaceous; (2) the protolith was a cumulate within a gabbroic melt crystallizing at high crustal levels in the Early Cretaceous and subsequently metamorphosed under UHP conditions in the Late Cretaceous. Retrograde stages of metamorphism (ca. 500 jC, 0.5 GPa) occurred at 61.9 F 1.9 Ma.The petrological findings, which provide evidence for UHP conditions for these rocks of the Kimi Unit, combined with the SHRIMP-results, reveal different conditions of rock formation at different times for the Rhodope zone. The existence of a subduction zone in the Late

*

Corresponding author. Tel.: +41-16321111; fax: +41-16321827. E-mail address: [email protected] (A. Liati).

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Cretaceous shows that the Rhodope was not metamorphosed as a single geotectonic element but consists of different terranes subducted and exhumed at different times. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Rhodope; SHRIMP-dating; UHP-metamorphism; Exsolution textures; Majoritic garnet; Quartz lamellae

1. Introduction High-pressure metamorphic rocks are common in the Alpine system and represent sites of ancient subduction zones. In Greece, the Alpine collisional history between the Eurasian and African plate is recorded in three high-pressure belts (Fig. 1): (a) the ‘external belt of Crete-Peloponnese’ with ca. 20 Ma old HP/LT metamorphic rocks (e.g. Seidel et al., 1982), (b) the ‘median belt of the Attiko – Cycladic zone’ with ca. 45 Ma old HP-rocks (blueschists and

Fig. 1. Sketch map of major tectonic elements in southeastern Europe and northeastern Mediterranean region (from Burchfiel, 1980; Dinter et al., 1995).

eclogites) overprinted in the greenschist-facies ca. 25 Ma ago (e.g. Altherr et al., 1979, 1982; Andriessen et al., 1979; Wijbrans et al., 1990; Bro¨cker et al., 1993) and (c) the central Rhodope terrane with ca. 42 Ma old HP-rocks (eclogites) overprinted ca. 40 Ma ago in the granulite-facies, in the central part of this terrane (Liati and Gebauer, 1999). The Rhodope zone, occurring in northeastern Greece and southern Bulgaria (Fig. 1) has long remained a fairly unexplored area, especially regarding its petrologic and tectonic history, as well as its involvement in the geodynamic evolution of the Alpine orogen. For a long time, it has been regarded as an old continental nucleus, suggested to be Precambrian in age (e.g. Jacobshagen et al., 1978; Horvath and Beckhemer, 1982), which played the role of a ‘Zwischengebirge’ between the Balkan belt to the north and the Dinarides –Hellenides belt to the south. This ‘cratonic’ massif was believed to have suffered a single metamorphic event of medium-pressure type and to have remained generally inactive during the Alpine orogeny. On the other hand, structural indications (e.g. Burchfiel, 1980; Papanikolaou, 1984; Burg et al., 1990), as well as radiometric dating (Liati, 1986; Liati and Kreuzer, 1990; Arnaudov et al., 1990; Gebauer and Liati, 1997; Liati and Gebauer, 1999) proposed that the Rhodope was in fact involved in the Alpine orogeny. Moreover, the discovery of high-pressure (eclogite-facies) relics within pervasively overprinted metabasites, metapelites and gneisses (Liati, 1986; Kolceva et al., 1986; Mposkos, 1989; Liati and Seidel, 1996) implied great depths of burial down to at least 60 km in central Rhodope (Liati and Seidel, 1996) and proved that this domain participated actively in Alpine metamorphic processes. Recently, U –Pb ion microprobe (SHRIMP) dating of zircon domains from metamorphic rocks of Central Rhodope constrained the timing of prograde and retrograde stages (Gebauer and Liati, 1997; Liati and Gebauer, 1999) suggesting that the whole tectono-metamorphic cycle above 300 jC took place in the Eocene and lasted ca. 9 Ma (from ca. 45 to 36

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Ma). This implied average burial- and heating-rates of 15 mm/year and 94 jC/Ma, as well as cooling and exhumation rates of 128 jC/Ma and 7.7 mm/year. In this paper, U – Pb dating of zircon from a metamorphic garnet-rich mafic rock, as well as a cross-cutting pegmatoid (crystallization product of retrograde metamorphic fluids) in eastern Rhodope were undertaken by Sensitive High-Resolution Ion Microprobe (SHRIMP). SHRIMP-dating was assisted by cathodoluminescence (CL) imaging of the zircon crystals dated, in order to distinguish between different types of domains and avoid mixing of ages. The objective of this study was to find out: (a) the age of the protolith of the mafic rock, (b) the age of metamorphism and whether the zone of Rhodope behaved as a single crustal domain subducted in the Eocene or if it consists of different terranes metamorphosed at different times and (c) whether the relatively high rates of exhumation deduced for Central Rhodope apply also for eastern Rhodope. For this purpose, the ion microprobe dating technique accompanied by CL imaging was the most suitable and reliable, mainly because of the complicated formation history (igneous and metamorphic) of the rocks, which affected the growth-and recrystallization patterns of the zircon crystals.

2. Regional geologic framework The Rhodope zone is situated between the Balkan belt to the north and the Dinarides – Hellenides to the south – southwest, at the eastern part of the Alpine orogen (Fig. 1). The Balkan belt is a north to northeast vergent thrust belt bounded by the Moesian platform to the north. The Dinarides and Hellenides constitute a predominantly southwest vergent thrust belt running NW – SE parallel to the Adriatic coast and the southerly lying Ionian coast. The western part of Rhodope is bounded by the Serbomacedonian Massif, a structurally complex domain of predominantly high-grade metamorphic rocks and numerous granitoids. The Serbomacedonian Massif is separated from the Rhodope by a tectonic contact, which is considered as a west-dipping thrust fault of Tertiary age (Kockel and Walther, 1965) but re-interpreted recently as a low angle normal fault formed during middle Miocene to late Pliocene in an extensional regime (Dinter and

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Royden, 1993). Burg et al. (1995) suggest that the Serbomacedonian Massif and the Rhodope should be considered as a ‘single major element of the Tethyan orogenic system’. They both lie east of the Vardar zone, which is commonly interpreted as a mid- to lateMesozoic oceanic suture between Apulia and the southeastern European margin. Both sedimentary and magmatic rocks metamorphosed under eclogite-facies conditions and subsequently overprinted in the amphibolite- and, at least part of them, in the granulite-facies (Liati and Seidel, 1994, 1996 and references therein), constitute the metamorphic part of Rhodope. Abundant post-metamorphic intrusive and extrusive rocks of Oligocene age penetrate and overlie the metamorphic rocks. These include quartzofeldspathic and pelitic gneisses, in places migmatized, amphibolites (some with eclogite relics), marbles, calc-silicate gneisses and ultramafic rocks. A nappe structure characterizes the zone of Rhodope. Burg et al. (1995, 1996) suggest a subdivision for the whole Rhodope into two major thrust domains, the ‘lower terrane’ and the ‘upper terrane’ which represent the crystalline footwall and hanging wall of a crustal-scale duplex. Several intermediate terranes (thrust units) are stacked between the upper and lower terrane. In the Greek part of Rhodope, based on both geological and petrological criteria, Papanikolaou and Panagopoulos (1981) and Mposkos (1989) suggest a subdivision of the metamorphic rocks into an upper tectonic unit (UTU) and a lower tectonic unit (LTU). The UTU is of higher metamorphic grade (upper amphibolite-facies and, at least partly, granulite-facies) and is separated from the LTU (upper greenschist-facies to lower amphibolitefacies) by a major WNW trending thrust plane.

3. Previous geochronological data on the metamorphic rocks Previous geochronological studies on metamorphic rocks of both the Greek and the Bulgarian part yielded widely scattered and relatively inconclusive data: in the Greek part of Central Rhodope, K – Ar data (36 – 50 Ma) on metamorphic minerals (hornblende, biotite, muscovite) were interpreted as Eocene cooling ages for the amphibolite-facies metamorphism, while some much older ages, up to 95 Ma as reflecting significant

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inherited argon in the systems (Liati, 1986; Liati and Kreuzer, 1990). By experience, only the youngest Arages of these high-pressure terranes are the closest to metamorphism (e.g. Gebauer, 1999) or to cooling. This is mainly due to the fact that Ar-data performed on white mica and hornblende from metamorphic rocks with a HP pre-history usually show a tendency to erroneously high ‘ages’, due to excess argon, as substantiated by numerous examples worldwide (see, e.g. Kelley et al., 1994). Thus, a geologically meaningful metamorphic age would be closer to 36 Ma rather than to 50 Ma. A U – Pb monazite age of ca. 55 Ma, interpreted as a peak metamorphic age, is reported from a migmatitic gneiss again of Central Rhodope (Jones et al., 1994). Conventional multigrain U –Pb zircon data scattering between 49 and 58 Ma are reported for ‘migmatic pegmatites’ of the Bulgarian part of Central Rhodope (Arnaudov et al., 1990). Hercynian ages for pre-metamorphic magmatism are reported on the basis of conventional U –Pb zircon dating of an orthogneiss from Thassos island (Wawrzenitz et al., 1992), as well as from some

metagranitoids of the Bulgarian Rhodope (Peytcheva and von Quadt, 1995). U –Pb SHRIMP-data on zircon domains from orthogneisses of Central Rhodope revealed also Hercynian ages for their granitoid protolith (Liati and Gebauer, 1999). For eastern Rhodope (Kimi Unit), where the rocks described in the present paper come from, Wawrzenitz and Mposkos (1997) report a Sm – Nd whole rock – garnet – cpx isochron age of a garnet pyroxenite at 119 F 3.5 Ma, which they interpret as reflecting a Lower Cretaceous HP metamorphic event. For the same area, Mposkos and Wawrzenitz (1995) report a Rb – Sr isochron age from muscovite and feldspar cores of an undeformed cross-cutting ‘pegmatite’ at 65.4 F 0.7 Ma.

4. Field relations The rocks dated by SHRIMP in the frame of this study are from the UTU of East Rhodope, between the villages of Kimi and Smigada (Fig. 2) and belong to

Fig. 2. Generalized geological map of the studied area in East Rhodope, based on Mposkos et al. (1986) and Mposkos and Liati (1993). The sample location for SHRIMP dating is marked by a star.

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the so-called ‘‘Smigada-Kimi Unit’’ (Mposkos et al., 1986). This unit overlies tectonically other metamorphic rocks of the UTU to the west and a large serpentinite body of the LTU to the east. It consists predominantly of gneisses alternating with amphibolites and marbles, as well as ultramafic rocks. It should be noted here that at least part of the UTU

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of East Rhodope should be distinguished from the UTU of Central Rhodope, as a result of the SHRIMPages obtained in this paper (see below). The dated rocks include: (a) a garnet-rich mafic rock (sample RHO22) and (b) a cross-cutting pegmatoid representing the crystallization products of retrograde metamorphic fluids (sample RHO23). The garnet-rich

Table 1 Representative microprobe analyses of cpx and garnet and whole rock composition of the garnet-rich mafic rock dated Anal.

SiO2 TiO2 A2O3 Cr2O3 FeOT MnO MgO CaO Na2O K2O Total

cpx (m)

cpx (ex)

cpx (ex)

cpx (i)

cpx (i)

grt (c)

grt (r)

a-3.1

c-9.3

e-1.20

c-4.10

c-4.14

c-4.12

c-4.13

50.4 0.50 6.82 0.04 5.92 0.08 12.2 22.3 1.42 0.02 99.70

52.0 0.24 5.70 0.05 5.80 0.05 12.7 22.1 1.34 0.02 100.00

51.2 0.53 6.65 – 5.62 0.09 12.3 21.8 1.43 – 99.62

52.0 0.37 4.98 0.01 5.51 0.03 13.3 22.1 1.27 0.01 99.67

52.2 0.40 5.11 0.03 5.30 0.07 13.5 22.1 1.35 – 100.14

39.5 0.07 21.8 0.01 20.2 0.47 7.76 10.28 0.04 0.01 100.14

39.4 0.08 21.5 0.02 20.0 0.46 7.54 10.60 0.03 0.01 99.64

w.r.a wt.% SiO2 TiO2 Al2O3 Fe2O3T MnO MgO CaO Na2O K2O P2O5 LOI Total

43.8 0.85 18.4 15.4 0.28 8.11 12.3 0.51 0.25 0.01 0.34 99.91

Structural formula calculated on the basis of 6 oxygen atoms Si Aliv Alvi Ti Cr Fe3+ Fe2+ Mn Mg Ca Na K Di Hd Jd Opx Ts Ac

1.855 0.145 0.151 0.014 0.001 0.068 0.114 0.002 0.669 0.878 0.102 0.001 63.8 10.9 10.3 1.9 8.3 4.8

12 oxygen atoms 1.909 0.091 0.155 0.007 0.002 0.017 0.161 0.001 0.693 0.868 0.095 0.001 63.7 14.8 9.6 3.5 6.9 1.6

1.884 0.116 0.172 0.015 – 0.017 0.156 0.003 0.674 0.861 0.102 – 61.7 14.3 10.2 3.6 7.9 2.3

1.912 0.088 0.128 0.010 0.001 0.030 0.139 0.001 0.729 0.871 0.091 0.001 66.6 12.7 9.1 3.8 5.2 2.5

1.907 0.093 0.128 0.011 0.001 0.037 0.125 0.002 0.737 0.864 0.095 – 67.0 11.3 9.6 3.9 5.1 3.0

(ppm)

3.008

3.016

1.957 0.004 0.001

1.941 0.005 0.001

1.287 0.030 0.881 0.839 0.006 0.001

1.281 0.030 0.861 0.871 0.005 0.001

Alm 42.3 Sp 1.0 Py 29.1 Gr 27.6

Rb Sr Ba Sc V Cr * Co Ni Cu Zn Zr Nb Y

6 125 72 62 501 300 96 24 72 116 17 13 19

42.1 1.0 28.3 28.6

(m): in matrix; (ex): matrix cpx with quartz and amphibole exsolution lamellae; (i): inclusion in garnet; (c): core; (r): rim; w.r.: whole rock. Cr*: estimated from microprobe analyses of minerals; Di, Hd, Jd, Opx, Ts, Ac: percentage of end members diopside, hedenbergite, jadeite, orthopyroxene, tschermak, acmite in clinopyroxene; Alm, Sp, Py, Gr: percentage of end-members almandine, spessartine, pyrope, grossular in garnet. a Average of four analyses. T Total iron.

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Fig. 3. BSE images of: (A) cpx inclusions (dark grey) in garnet and (B) cpx (dark grey) and quartz (black) inclusions in garnet from the garnetrich mafic rock. Note the rounded shape (bleb-like) especially of the cpx inclusions, indicative of an exsolution origin of cpx (see also text).

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mafic rock occurs, together with amphibolites and small ultramafic lenses, within gneisses. Garnet may constitute up to 90 vol.% of the rock. It commonly forms millimeter- to centimeter-thick layers, more or less altered to amphibole-rich rocks and cross-cut by late plagioclase F amphibole-rich veinlets.

5. Petrology and whole rock chemistry of the dated rocks 5.1. Petrography and mineral chemistry of the garnetrich mafic rock The common mineral assemblage of this rock is: garnet – clinopyroxene – zoisite/epidote – Ca – amphibole – scapolite –plagioclase – quartz –rutile – titanite. Microprobe analyses of minerals for both the garnet-rich mafic rock and the pegmatoid were carried out with a Cameca SX51 (at 15 kV, 20 nA) at the Institute of Mineralogy, University of Heidelberg. BSE images were taken at the same institute. 5.1.1. Garnet It constitutes up to 90% of the rock. It is unzoned and its composition is: Py27 – 33Alm40 – 43Gr25 – 29Sp1 (Table 1). Some garnet crystals have bleb-shaped inclusions of quartz (sometimes in abundance), clinopyroxene, zoisite and, rarely, kyanite (Figs. 3 and 4). It is noted that in the optical microscope, we observe much more numerous inclusions than shown in the BSE picture of Fig. 3. This is because the BSE pictures depict just the surface of the crystals whereas with the optical microscope we can focus also a couple of micrometers below the surface. All inclusions sometimes show a general alignment, which is however obscured by their rounded shape. In rare cases, quartz inclusions with radial cracks were observed (Fig. 4). Finally, numerous rutile inclusions occur in form of oriented needles within garnet. 5.1.2. Clinopyroxene (cpx) It is quite homogeneous, rich in Al and Mg (Mg/ (Mg + Fe) is ca. 0.8) and relatively poor in jadeite component (ca. 10 mol%), due to the bulk rock composition (Table 1). The cpx inclusions in garnet have slightly higher Mg/(Mg + Fe) ratio and lower

Fig. 4. Radial cracks around a quartz (qu) inclusion in garnet, indicating the former possible presence of coesite. The other inclusions are also quartz (the relatively coarser-grained), as well as rutile (abundant rutile needles, not shown in this picture are visible in a slightly different focus plane).

Al contents (Tschermak’s component), but otherwise a very similar composition when compared to the cpx of the matrix. An important feature of the matrix cpx is the presence of oriented quartz lamellae in some crystals (Fig. 5). These are usually 30 – 60 Am long and up to 6 Am wide and their volume percentage in the host cpx varies from grain to grain. Similar to the inclusions in garnet, we observe much more numerous quartz and amphibole lamellae in the cpx with the optical microscope just below the surface, compared to the ones shown in the BSE image of Fig. 5. Most commonly, the quartz lamellae occur together with amphibole lamellae but amphibole lamellae do not always occur together with quartz. The amphibole lamellae are usually longer and wider (up to 120 Am long and up to 10 Am wide) than those of quartz. Both quartz and amphibole lamellae line up in a single crystallographic orientation within the host cpx and have sharp boundaries against each other and against the host cpx (Fig. 5). 5.1.3. Amphibole It occurs in the matrix as a secondary mineral associated with grt + cpx F plag F qu and within cpx in form of lamellae (see above and Fig. 5). The

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Fig. 5. BSE image of quartz (black) and amphibole (dark grey) lamellae within the cpx (outlined dashed) of the garnet-rich mafic rocks, interpreted as exsolutions. Quartz exsolution points to a precursor supersilicic cpx formed under UHP conditions. Detail of some exsolutions is shown at the upper left corner. See also text for details.

composition of matrix amphibole ranges between tschermakitic hornblende, Al-rich ferropargasite, magnesio-hornblende and edenitic hornblende, while the amphibole lamellae are a little richer in Al but otherwise have similar composition (tschermakitic hornblende and magnesio-hornblende).

ca. 88 mol%), chlorite and green amphibole. The muscovite shows no zoning and has a Si content between 3.18 and 3.21 atoms p.f.u. In general, the rock shows no deformation features except some slight bending of the plagioclase lamellae. 5.3. Interpretation of the microtextural data

5.1.4. Zoisite – epidote In the matrix, substitution of Al by Fe3+ is higher than in the inclusions: Fe2O3 contents in the matrix range between 6 and 10 wt.% whereas in zoisite inclusions in garnet between 2.5 and 3.6 wt.%. 5.1.5. Scapolite It is Ca-rich (Ca/(Ca + Na) is ca. 0.75). Based on qualitative microprobe analyses, it is rich in S and has no Cl. Scapolite is partly replaced by plagioclase. 5.2. Petrography and mineral chemistry of the pegmatoid This rock consists mainly of plagioclase (oligoclase: An: 26– 28 mol%), quartz and little K-feldspar, muscovite, clinozoisite/epidote (zoisite component:

The presence of oriented quartz lamellae in matrix cpx of the garnet-rich mafic rock has important implications. Quartz lamellae and/or needles have been described in cpx of eclogites and HP granulites in terranes where UHP minerals were also identified (see Liou et al., 1998; Zhang and Liou, 2000 for a summary and review on this topic). They were interpreted as exsolution products of a primary supersilicic cpx (e.g. Gayk et al., 1995; Schma¨dicke and Mu¨ller, 2000). Pressure is the most important factor to stabilize supersilicic cpx but high T is also necessary. Experimental data in the CMAS and NCAS system confirm this observation (e.g. Wood and Henderson, 1978; Gasparik, 1985, 1986). Based on the textural characteristics described above (Section 5.1), the quartz lamellae that occur in matrix cpx crystals of

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the dated garnet-rich mafic rock are interpreted as exsolutions from a precursor Si-richer cpx, thus implying significantly high pressures for this unit of East Rhodope. The precursor Si-rich cpx must have exsolved quartz upon decompression (e.g. Smyth, 1980). As for the amphibole lamellae, these seem to have nucleated in a later stage of decompression at the interface between cpx host and exsolved quartz (compare also Skrotzki et al., 1991). Calcic amphiboles in form of lamellae in cpx have been described from several metamorphic terranes (e.g. in lherzolites at Alpe Arami: Yamaguchi et al., 1978; in garnet granulites at Lake Chatuge, GA: Isaacs et al., 1981) and ascribed variously to late stage alteration, primary epitaxial intergrowth of the two phases and to exsolution of amphibole from cpx. In our case, the textural features indicate rather exsolution. Nevertheless, alteration along the interfaces may also have been the case. The inclusions of cpx in garnet can be, at first sight, interpreted to reflect a certain stage of the prograde path. On the other hand, some textural features like the systematically very rounded shape of the cpx (probably resulting from surface energy reduction) and the general orientation shown by some rounded elongated cpx inclusions allow us to introduce another scenario, which we consider also as possible. This suggests that the cpx derived by exsolution from a precursor majoritic garnet (see also Section 8.1). Exsolution of cpx (and opx) in garnet have been described in garnet peridotites of Western Norway (e.g. Van Roermund et al., 2001 and references therein) or in mantle xenoliths (Collerson et al., 2000). These pyroxene exsolutions most commonly occur in form of very fine oriented needles not resembling our textures. However, in the case of Western Norway, coarse rounded inclusions of opx, and cpx very similar to our bleb-shaped cpx were also described from the same garnets with the oriented needle-like exsolutions (Van Roermund et al., 2001, their Fig. 8) and were interpreted also as exsolution products by the authors. The rounded shape of our cpx inclusions may have resulted by recrystallization and annealing of initially fine cpx needles exsolved by a precursor majoritic garnet and therefore represents the advanced state of garnet transformation. This scenario needs further investigation, which is beyond the scope of the present paper.

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As for the other inclusions (zoisite, quartz and more rarely kyanite), they occur sometimes within the same garnet crystal but in different parts and never in contact to each other indicating that they formed at different times by different reactions. Given the bulk rock composition (Table 1), it is difficult to explain the quartz inclusions, which sometimes are very abundant, as primary. On the other hand, we are not aware of any other case in the literature where such minerals are interpreted as exsolutions from garnet. It is noted that quartz inclusions were rarely observed inside zircon (in the magmatic parts). It can be therefore inferred that quartz may have been entrapped also in garnet during its growth. This must have happened then in the magmatic stage, in which case the rock would be a cumulate. It is obvious that at this state of research there is no straightforward answer possible to these problems and we would leave the two possibilities concerning the inclusions (primary phases or exsolution products) open. 5.4. PT conditions of metamorphism and origin of the garnet-rich mafic rock Regarding the PT conditions in this unit of Rhodope, information so far was provided: (1) by amphibolitized eclogites with cpx-plag symplectites and Naaugite or omphacite (maximum jadeite: ca. 25 mol%), adjacent to the dated rock (Liati and Mposkos, 1990), and (2) by ultramafic rocks nearby (close to the village of Kimi; Fig. 2) for which Mposkos et al. (1994) report metamorphic conditions of 1.35– 1.6 GPa and 750 –775 jC for the high-pressure stage and temperatures below ca. 400 jC for the last stages of overprinting. Finally, Mposkos et al. (2001) mention, in a preliminary report, the presence of polycrystalline quartz (pre-existing coesite) and high-Al titanite in retrogressed eclogites, rutile exsolutions in sodic garnet, as well as ‘carbon cubes and octahedra’ within garnet from metapelites of the Kimi unit. They suggest PT conditions of ca. 3.5 GPa, 700 jC for the eclogites and > 7 GPa for the metapelites of this unit. In the frame of the present work, we applied the grt –cpx geothermometer (Krogh, 2000) to matrix and inclusion pairs and obtained, in both cases, temperatures ranging between 700 and 780 jC. The rock, however, must have seen higher temperatures, if we judge from the Al-content of the cpx as well as from

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the presence of quartz exsolutions in cpx which, besides very high pressures, requires also considerably high temperatures for the stability of the precursor Si-rich cpx. The temperatures obtained by use of the grt –cpx thermometer are therefore equilibration temperatures frozen-in in the rock during cooling. Regarding pressures, we can obtain information from the presence of quartz exsolutions in cpx. Judging from the fact that such quartz exsolutions have been described from UHP terranes (where minimum pressures of 2.5 GPa are reported), a trend to peak pressures on the order of ca. 2.5 GPa or higher is to be expected for this garnet-rich mafic rock of East Rhodope. A rough estimation of the excess SiO2 in the precursor cpx and comparison with experimental studies for cpx in the CMAS or in the NCAS system (e.g. Gasparik, 1985, 1986) is in line with this pressure estimate, as a minimum figure. A more extreme scenario, which leads to even higher pressures is related to the bleb-shaped, sometimes oriented cpx inclusions in garnet. If they do not represent minerals simply entrapped in garnet during its prograde growth but instead exsolutions from a supersilicic (majoritic) garnet precursor, pressures at least > 4GPa would arise, depending on the extent of majoritic substitution (e.g. Gasparik, 1990; Van Roermund and Drury, 1998; Van Roermund et al., 2001). With respect to the origin of the garnet-rich mafic rock, two possibilities are envisaged. (1) The protolith was an ultra high pressure melt (probably a grt F cpx cumulate) which crystallized in the mantle, was partly exhumed, then subducted and metamorphosed under eclogite-facies conditions and finally exhumed on the surface. In that case, the exsolution textures described above would be a result of decompression and cooling of the primary magmatic rock and must have escaped deformation and recrystallization during the later eclogite-facies metamorphic event. Exsolution lamellae of garnet and spinel in cpx of garnet pyroxenites, which occur as layers in garnet – spinel peridotites of the same unit, close to the dated garnet-rich mafic rock (Mposkos et al., 1994) would be in line with this scenario. (2) The protolith was a cumulate within a gabbroic melt crystallizing at high crustal levels and subsequently metamorphosed under UHP conditions. These two scenarios are further discussed in Sections 8 and 9, in the light of the SHRIMP results and the cathodoluminescence patterns of the zircons.

5.5. PT conditions of formation of the pegmatoid As described above, the pegmatoid is cross-cutting the schistosity of the surrounding mafic rocks and is in general undeformed. It is considered to have crystallized from retrograde metamorphic fluids during final stages of metamorphism, at high crustal levels. Based on petrological criteria from nearby ultramafic rocks in the same unit, Mposkos et al. (1994) report temperatures < 400 jC for late metamorphic stages. Taking into account the anorthite content of the plagioclase (An26 – 28), we consider temperatures of ca. 500 jC for the initial formation of this rock as a good approximation. It is still possible that later fluids further affected the rock at lower temperatures as indicated by alteration of the plagioclase and CL characteristics of the zircon (see below, Chap. 7). Regarding pressures, based on the Si content of the muscovite (Massonne and Schreyer, 1987) in these rocks (see Section 5.2), minimum pressures of ca. 0.5 GPa result for temperatures of ca. 500 jC. 5.6. Whole-rock chemistry Major and trace element chemistry has been determined in Zurich by XRF. Bulk rock analyses of the dated garnet-rich mafic rock reveal a quite peculiar composition (Table 1), which also poses some difficulties concerning the nomenclature of this rock. According to the SiO2 content, the rock is an ultrabasite but the concentration of other elements (Mg, Cr, Ni) is too low, which is in contradiction with this term. On the other hand, the major and trace element chemistry would generally agree with that of a basic rock, i.e. a gabbro (except for the low SiO2). Thus, taking into account the mineralogical composition in combination with the whole rock chemistry, we call this rock ‘garnet-rich mafic rock’ as a descriptive term. Given the layering observed in this rock, we consider it to be rather a grt – cpx cumulate. It is noted that the protoliths of amphibolitized eclogites in the broad area are characterized as MORB (Mposkos et al., 1988). However, an e-Nd value of 2.5 from a nearby garnet pyroxenite (recalculated from Wawrzenitz and Mposkos, 1997 for 119 Ma) argues against a MOR-origin for at least this rock. This implies that this garnet pyroxenite and the

A. Liati et al. / Chemical Geology 184 (2002) 281–299

protoliths of the MORB amphibolitized eclogites are not co-genetic.

6. Dating techniques and data evaluation 6.1. SHRIMP II The data listed were obtained on SHRIMP II at the Geological Survey of Canada in Ottawa, as well as on SHRIMP II at the Australian National University in Canberra, following the standard operating techniques. Depending on the size of distinct types of zircon domains, spot sizes between about 8 and 20 mm were chosen. For data collection, seven scans through the critical mass range were made. 6.2. Cathodoluminescence (CL) All CL- and secondary electron (SE) pictures were photographed from a split screen on a CamScan CS 4 scanning electron microscope (SEM) at ETH in Zu¨rich operating at 13 kV. The same sample mount was used later for both the CL-SE studies and SHRIMP-dating. The SEM is equipped with an ellipsoidal mirror that is located close to the sample within the vacuum chamber and that can be adjusted by electro-motors. The sample can thus be located in one focal point while the second focal point lies outside the sample chamber. Here, the CL-light enters a highly sensitive photo multiplier through a quartz glass-vacuum window and a light channel. The signal of the photo multiplier is then used to produce the CL-picture via a videoamplifier. The SE-pictures were produced simultaneously with the CL-pictures using a different detector. In general, strong CL means high amounts of minor and trace elements, weak CL means low amounts of minor and trace elements, including U. Thus, the Ucontents can be qualitatively predicted via CL. 6.3. Data evaluation For the calculation of the Pb/U ratios, the data were corrected for common Pb using the 207Pb correction method, following the standard procedures of Compston et al. (1992) and Williams and Claesson (1987). Since this correction is based on the assumption of concordance, the data are graphically presented on

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Tera – Wasserburg (TW) diagrams (Tera and Wasserburg, 1972), where 207Pb/206Pb, uncorrected for common lead, is plotted on the y-axis. This diagram allows a fast estimation of the amount of common lead of the projected analyses. For the data obtained with the SHRIMP II in Ottawa, the amount of common Pb was calculated by using the recommended 207Pb/206Pb value of 0.887 (see also Stern, 1997). For the data obtained at the ANU, the amount of common Pb was calculated using the isotope composition of common Pb obtained from the model of Cumming and Richards (1975) since most analyzed spots plot in the Tera – Wasserburg diagram along a mixing line closer to this isotopic composition (207Pb/206Pb = 0.835). Individual data points in the TW diagrams are plotted with two r errors. The mean ages were calculated both as normal mean and weighted mean. Although in both cases the resulting ages are almost identical, the weighted mean is preferred here because it is more representative of the analytically best measurements. Weighted mean ages in the text and figures are given at the 95% confidence level. For single analyses (Table 2), the one r error is given.

7. CL-patterns and morphology of zircons The zircons of the garnet-rich mafic rock are typically 100 – 300 Am long, prismatic and euhedral, as is the case for melt precipitated crystals. Some of them show clear oscillatory zoning in CL (Fig. 6), in others fainted or ghost oscillatory zoning is observed and finally some of them show areas with more homogeneous CL patterns (Fig. 7). The large crystal size and oscillatory zoning indicate that the protoliths of these rocks were not basaltic, in which case comagmatic zircon crystals would not have time to crystallize or, at the best, would have been very tiny because of very rapid crystallization of the basaltic melt. This interpretation is in line also with other observations (petrography, whole-rock chemistry; see Section 5). SHRIMP analyses from the oscillatory domains provide the protolith (magmatic crystallization) ages (see below). Inside the magmatic domains CL-dark (U-richer) embayments are sometimes observed (Fig. 7B). As discussed below, domains

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Table 2 U, Th, Pb SHRIMP data for zircons from the garnet-rich mafic rock (RHO22; analyses 1 – 24) and the pegmatoid (RHO23; analyses 25 – 34) of East Rhodope Sample

U (ppm)

Th (ppm)

Th/U

rad. Pb (ppm)

f 206Pb (%)

207 Pb/206Pb (1r) (uncorrected)

238

U/206Pb (1r) (uncorrected)

206

Pb/238U (1r)

age (Ma) Pb/238U (1r)

206

0.0554 F 0.0024 0.0586 F 0.0028 0.0646 F 0.0034 0.0542 F 0.0054 0.1741 F 0.0144 0.3901 F 0.0287 0.2702 F 0.0144 0.0596 F 0.0033 0.0633 F 0.0050

54.2 F 1.4 53.3 F 1.2 53.8 F 1.5 53.4 F 1.9 49.5 F 2.2 34.8 F 2.2 44.3 F 2.3 58.7 F 1.5 55.7 F 1.7

0.0183 F 0.0005 0.0185 F 0.0004 0.0182 F 0.0005 0.0186 F 0.0007 0.0172 F 0.0008 0.0170 F 0.0015 0.0166 F 0.0010 0.0168 F 0.0004 0.0176 F 0.0005

116.8 F 2.9 118.2 F 2.7 116.3 F 3.2 118.9 F 4.3 109.7 F 5.1 108.9 F 9.2 106.3 F 6.1 107.3 F 2.8 112.6 F 3.4 WM: 117.4 F 1.9

Metamorphic zircon domains (homogeneous CL: anal. 10 – 12 and dark embayments: anal. 13 – 16) 10 A RHO22-4.1 90 45 0.50 11 O RHO22-4.2 25 5 0.20 12 O RHO22-8.7 105 30 0.29 13 O RHO22-8.2 895 20 0.02 14 O RHO22-8.5 1200 15 0.01 15 O RHO22-8.4 1230 10 0.007 16 O RHO22-8.1 1175 6 0.005

0.1771 F 0.0064 0.3705 F 0.0399 0.5189 F 0.0257 0.1106 F 0.0043 0.0549 F 0.0029 0.0809 F 0.0027 0.1057 F 0.0045

76.9 F 2.0 56.0 F 6.5 40.5 F 2.1 83.9 F 1.9 87.2 F 2.7 80.3 F 1.7 75.7 F 2.1

0.0109 F 0.0003 0.0110 F 0.0015 0.0108 F 0.0010 0.0110 F 0.0003 0.0114 F 0.0004 0.0119 F 0.0003 0.0123 F 0.0004

69.7 F 1.9 70.4 F 9.8 69.5 F 6.1 70.7 F 1.7 72.9 F 2.2 76.6 F 1.6 78.8 F 2.2 WM: 73.5 F 3.4

Zircon domains showing ghost oscillatory zoning (lead loss; 17 – 22) and mixed domains (magmatic and metamorphic; 23, 24) 17 O RHO22-4.3 55 40 0.7 1 43.6 18 O RHO22-4.4 60 30 0.5 1 38.9 19 O RHO22-2.2 75 10 0.16 1 38.8 20 O RHO22-2.3 155 15 0.1 2 34.8 21 O RHO22-2.4 55 20 0.35 1 20.0 22 O RHO22-2.5 55 40 0.7 1 25.0 23 O RHO22-8.3 1130 15 0.015 14 4.2 24 O RHO22-8.6 1575 15 0.008 22 3.4

0.4138 F 0.0236 0.3741 F 0.0315 0.3739 F 0.0185 0.3397 F 0.0226 0.2158 F 0.0201 0.2574 F 0.0188 0.0826 F 0.0043 0.0762 F 0.0028

44.4 F 2.5 43.2 F 2.5 40.8 F 2.8 46.0 F 2.3 60.8 F 4.4 55.0 F 4.6 70.3 F 2.0 62.5 F 1.3

0.0127 F 0.0010 0.0142 F 0.0012 0.0150 F 0.0012 0.0142 F 0.0009 0.0132 F 0.0010 0.0137 F 0.0012 0.0136 F 0.0004 0.0156 F 0.0003

81.3 F 6.1 90.6 F 7.6 95.8 F 7.4 90.8 F 5.8 84.2 F 6.6 87.4 F 7.7 87.2 F 2.5 98.9 F 2.1

Late, undeformed pegmatoid 25 A RHO23-2.1 26 A RHO23-2.2 27 O RHO23-2.3 28 O RHO23-4.4 29 A RHO23-8.1 30 31 32 33 34

A O A A O

RHO23-8.2b RHO23-4.5b RHO23-4.1b RHO23-4.2b RHO23-8.3b

480 375 275 445 2065 190 10 5735 5 8

10 20 11 13 33 0.04 0.4 745 0.2 0.02

1 <1 1 9 12 13 13

16 39 56 7.5 <1 4.0 6.9

0.02 0.05 0.04 0.03 0.02

4 3 2 4 19

4.4 6.2 3.2 2.3 <1

0.0819 F 0.0021 0.0962 F 0.0034 0.0855 F 0.0038 0.0780 F 0.0026 0.0489 F 0.0012

102.0 F 2.8 95.0 F 2.4 102.0 F 3.1 102.6 F 3.4 100.9 F 2.4

0.0094 F 0.0003 0.0100 F 0.0003 0.0095 F 0.0003 0.0095 F 0.0003 0.0100 F 0.0002

0.0002 0.04 0.13 0.04 0.002

1 <1 64 <1 <1

2.9 37.9 <1 80.5 56.5

0.0772 F 0.0038 0.3729 F 0.0426 0.0508 F 0.0041 0.6835 F 0.0834 0.5272 F 0.1151

110.2 F 3.3 69.5 F 7.0 83.3 F 6.8 28.7 F 3.3 38.9 F 6.7

0.0087 F 0.0003 0.0089 F 0.0012 0.0120 F 0.0010 0.0068 F 0.0040 0.0112 F 0.0041

60.0 F 1.6 63.3 F 1.6 60.9 F 1.8 61.1 F 2.1 63.5 F 1.5 WM:61.9 F 1.9 56.0 F 1.7 57.3 F 7.4 76.6 F 6.3 43.6 F 24.1 71.7 F 26.0

WM: weighted mean; the error on the weighted mean is given at the 95% confidence level. A: samples run at ANU, Canberra; O: samples run at GSC, Ottawa. a b

Analyses 5 – 9 (in italics) are from—more or less—fainted oscillatory domains and are not considered in the weighted mean age calculations (see text).

Analyses in italics are not considered in the weighted mean age calculation: 30 and 31 are from outermost white rims (lead loss); 32 is from a very U-rich domain and 33, 34 from very U-poor domains (see text). 206Pb: percentage of Pb that is common Pb.

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Garnet-rich mafic rock Magmatic zircon domains with clear oscillatory zoning (anal. 1 – 4) and with slightly fainted oscillatory zoning (lead loss; anal. 5 – 9) 1 A RHO22-1.1 115 100 0.89 2 <1 2 A RHO22-1.2 135 140 1.03 3 1.3 3 A RHO22-1.3 115 110 0.99 2 2.0 4 A RHO22-5.1 125 110 0.87 2 <1 85 75 0.87 2 15 5 O RHO22-4.5a 60 45 0.70 1 41 6 O RHO22-4.7a 170 100 0.57 3 27 7 O RHO22-8.8a 155 155 1.0 3 1.4 8 A RHO22-2.1a 60 10 0.18 1 1.9 9 A RHO22-6.1a

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diagrams (Figs. 9, 10 and 11). The weighted mean ages and the errors (given at the 95% confidence level) are also summarized in Table 2, at the end of the listing of the individual data for each type of zircon domain. 8.1. Garnet-rich mafic rock Nine spots were analysed from the magmatic, oscillatory zoned zircon domains of the garnet-rich mafic rock. Four of these are from zircon domains

Fig. 6. Cathodoluminescence (CL) pictures (on the left) and secondary electron (SE) pictures (on the right) and spot ages of zircon crystals from the garnet-rich mafic rock (samples RHO22-5 and RHO22-1) showing clear oscillatory, euhedral growth zoning, typical for crystals precipitated from a melt.

with clearly homogeneous CL, as well as such with dark embayments are interpreted to have resulted during metamorphism. The cross-cutting pegmatoid contains euhedral, oscillatory zoned zircons showing sometimes very U-poor rims, very bright in CL (Fig. 8A). Formation of these outermost rims is probably due to postcrystallization lead loss caused by interacting late fluids at low temperatures ( < 500 jC).

8. Results of SHRIMP-dating, interpretation and correlation with the petrology The SHRIMP-data for the analysed zircons are given in Table 2 and plotted on Tera – Wasserburg

Fig. 7. Cathodoluminescence (CL) pictures (on the left) and secondary electron (SE) pictures (on the right) and spot ages of zircon crystals from the garnet-rich mafic rock (samples RHO22-2 and RHO22-8). Opposite to zircons in Fig. 6, here oscillatory zoning is fainted in both (A) and (B). Discordant ages between the magmatic (117.4 F 1.9 Ma) and the metamorphic (73.5 F 3.4 Ma) age from these domains are interpreted to be due to various degrees of lead loss. In (B) spot 7 (in a homogeneous CL domain), as well as spots 1, 2, 4 and 5 (in a U-rich embayment, dark in CL) yield the metamorphic age. Spots 3 and 6 lie on two different domains and yield mixed ages (see also text).

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domains (see above). Similar to some zircon samples from the Dora Maira Massif, these U-rich embayments yield the age of UHP-metamorphism (Gebauer et al., 1997). On a TW diagram, seven out of the fifteen analyses (numbers 10 –16 of Table 2) plot along a mixing line with common Pb and 238U/206Pb as end members (Fig. 10; filled circles). The other eight data points (numbers 17 –24 of Table 2) plot on the left side of the mixing line. Their individual 206 Pb/238U ‘ages’ are scattering between 81.3 F 6.1

Fig. 8. Cathodoluminescence (CL) pictures (on the left) and secondary electron (SE) pictures (on the right) and spot ages of zircon crystals from the crosscutting pegmatoid (samples RHO23-8 and RHO23-2). Note bright rims in (A) which may reflect lead loss due to an interaction with fluids after the crystallization of the pegmatoid.

with very clear, well preserved oscillatory zoning (Fig. 6) and yield all consistent ages around 117 Ma (Table 2, analyses 1 – 4). The other six spots which lie in domains showing slightly fainted oscillatory zoning (Fig. 7) all yield younger, very probably discordant ages between 112.6 F 3.4 and 106.3 F 6.1 Ma (analyses 5– 9 of Table 2). This is probably due to lead loss during later metamorphism (see below). On a TW diagram all nine analysed spots plot along a mixing line with common Pb and 238 206 U/ Pb radiogenic as end members (Fig. 9). For the mean age calculation we took into consideration only the four analyses from the clear oscillatory zoned domains (filled circles in Fig. 9). They yield a weighted mean age at 117.4 F 1.9 Ma which is interpreted to correspond to the crystallization age of the protolith. Fifteen spots were analysed from parts of the zircon crystals, which are homogeneous in CL (e.g. spot 7 in Fig. 7B) or show U-rich embayments (e.g. spots 1, 2, 4 and 5 in Fig. 7B) inside the magmatic

Fig. 9. Tera – Wasserburg (TW) diagrams with data obtained from magmatic domains of zircons from the garnet-rich mafic rock. Filled circles: data from the clear oscillatory domains of the zircon crystals (e.g. Fig. 6) considered in the weighted mean age calculation at 117.4 F 1.9 Ma; open circles: data from domains with slightly fainted oscillatory zoning (e.g. spot 1 of Fig. 7A or spot 8 of Fig. 7B) plotting on the right side of the mixing line due to lead loss (see also text). (B) is an enlargement of the dashed rectangle in (A). WM: weighted mean. Error bars are 2r errors.

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in the Upper Cretaceous (73.5 F 3.4 Ma). If the second scenario applies, the protolith of the garnetrich mafic rock crystallized at shallow depths from a gabbroic melt, as a cumulate, in the Lower Cretaceous (117.4 F 1.9 Ma) and was subsequently metamorphosed under ultra high pressure conditions in the Upper Cretaceous (73.5 F 3.4 Ma). Critical UHP minerals (like coesite or diamond) were not found as inclusions in the zircons studied so far. Our observations from the cathodoluminescence study of the zircons in this mafic rock reveal the following: by

Fig. 10. Tera – Wasserburg (TW) diagram with data obtained from metamorphic domains of zircons from the garnet-rich mafic rock. Filled circles: data from domains with homogeneous CL (e.g. spot 7 of Fig. 7B) and U-rich embayments considered in the weighted mean age calculation at 73.5 F 3.4 Ma; open circles: data from fainted oscillatory zoned domains (e.g. spots 3 or 5 of Fig. 7A) reflecting lead loss and from two neigbouring domains different domains in age reflecting mixing ages (e.g. spots 3 or 6 of Fig. 7B). The latter plot on the left side of the mixing line due to an older lead component from the time of magmatic crystallization at 117.4 F 1.9 Ma (see also text). WM: weighted mean. Error bars are 2r errors.

and 98.8 F 2.1 Ma, that is between the magmatic age at 117.4 F 1.9 and the younger metamorphic age (see below), thus reflecting different amounts of lead loss of the originally 117.4 F 1.9 Ma old zircons. Although some of these data points which are plotted on the left of the mixing line still fall on the mixing line within limits of error, we did not consider them in the mean age calculation because in CL they are lying partly on oscillatory zoned domains (e.g. spot 3 of Fig. 7B). The weighted mean age of seven analyses from zircon domains showing U-rich embayments or clearly homogeneous CL yielded 73.5 F 3.4 Ma, which we interpret to correspond to the time of metamorphism. The SHRIMP results and CL-information of zircon would agree with both scenarios mentioned under Section 5.3 about the origin and evolution of the garnet-rich mafic rock. In the first case, the protolith would have been a magmatic rock crystallized as a cumulate under ultra high pressures in the mantle, 117.4 F 1.9 Ma ago, then brought to shallow depths and subsequently metamorphosed under eclogite-facies conditions (1.35 –1.6 GPa, 700 –780 jC)

Fig. 11. Tera – Wasserburg (TW) diagrams with data obtained from the zircons of the crosscutting pegmatoid. Filled circles: data from the oscillatory zoned domains considered in the weighted mean age calculation at 61.9 F 1.9 Ma; open circles: data from bright rims yielding younger ages (e.g. at 56.0 F 1.7 Ma in Fig. 8A) reflecting lead loss probably due to a later interaction with fluids. (B) is an enlargement of the dashed rectangle in (A). WM: weighted mean. Error bars are 2r errors.

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experience, most zircons that have seen HP metamorphism at temperatures exceeding ca. 650 jC develop a homogeneous recrystallization rim, 10 – 20 m thick, around the primary magmatic domain. This is not the case with the zircons from this rock. Metamorphic ages were obtained mainly from a Urich embayment (Fig. 7B) and from areas of the zircons with clearly fainted oscillatory zoning. It is worth mentioning that in the Dora Maira massif the zircon domains that yielded the age of UHP metamorphism show also U-rich, randomly distributed metamorphic domains sometimes occurring around coesite, which was introduced in zircon along cracks (Gebauer et al., 1997). Hence, in this respect, the CL characteristics of the zircons would favour rather the second scenario but the first one remains undoubtedly under consideration mainly for petrological reasons. 8.2. Cross-cutting pegmatoid Ten spots were analysed from the zircon crystals of the pegmatoids. Five of them plot clearly along a mixing line in the TW diagram of Fig. 11B. They yield a weighted mean age of 61.9 F 1.9 Ma, which is interpreted to correspond to the time of emplacement and crystallization of these fluids into the upper crust at ambient temperatures of ca. 500 jC, at a depth of ca. 15 km. Two analyzed spots (open circles in Fig. 11) are from outermost rims of the zircons, which are very bright in CL (e.g. Fig. 8A) and yield younger ‘ages’ of 56.0 F 1.7 and 57.3 F 7.4 Ma. They probably reflect lead loss due to the latest interaction with fluids, probably at temperatures lower then 500 jC. Although one of them falls on the mixing line within error limits, it was not considered in the mean age calculation because of the difference in CL characteristics of the outermost rim. Another two analyzed spots (numbers 33, 34 of Table 2) have extremely low U contents (5 and 8 ppm). They yield ages with extremely large errors and therefore were also neglected in the calculations. Finally, another spot (number 32 of Table 2) was not taken into consideration as it yields a too high ‘age’ of 76.6 F 6.3 Ma very probably because of its very high U content (5722 ppm). In SHRIMP-dating it is commonly the case that analyses with high U contents (usually ca. >2000 ppm) may be erroneous and should be

neglected in the age calculation (see also Mc Laren et al., 1994; Williams et al., 1996).

9. Implications and conclusions The identification of quartz exsolution lamellae in clinopyroxene from the garnet-rich mafic rock dated in the frame of this study implies ultra high pressures (>2.5 GPa) for this unit of East Rhodope. The option that this rock has seen even higher pressures on the order of > 4 GPa (if the inclusions of cpx within garnet are interpreted as exsolution products of a precursor majoritic garnet) remains open and needs further investigation. Regarding the interpretation of the SHRIMPresults, there are two possibilities depending on the scenario for the origin and evolution of the garnetrich mafic rock. (1) The protolith of this rock formed magmatically in the course of asthenosphering mantle upwelling under UHP conditions in Lower Cretaceous times (117.4 F 1.9 Ma). It then may have risen as part of the convecting mantle into the lithosphere and subsequently transferred into a subduction zone. Exsolution phenomena and other textural and/or mineral chemical resetting features described under Section 5 must have taken place by pressure and temperature drop during prolonged ascent into the lithosphere. Subduction to depths of 40 – 48 km (eclogite-facies) occurred at 73.5 F 3.4 Ma. (2) The garnet-rich mafic rock had a low-pressure protolith, which probably was a cumulate rock crystallized from a gabbroic melt at 117.4 F 1.9 Ma and subsequently metamorphosed at UHP conditions at 73.5 F 3.4 Ma. It is noted that an age of 119 F 3.5 Ma obtained by Sm – Nd dating (whole rock, garnet, cpx) of a garnet – pyroxenite layer within garnet – spinel metaperidotite also from the Kimi unit is interpreted by Wawrzenitz and Mposkos (1997) to reflect the eclogite-facies HP metamorphic event. This interpretation is not supported by our SHRIMP-results, which point to a metamorphic age at ca. 73 Ma (metamorphic domains of zircons). The 119 F 3.5 Ma Sm –Nd age, however, if meaningful and not an artifact of, e.g. disequilibrium, is in good agreement with our SHRIMP-age of 117.4 F 1.9 Ma, which we obtained from the magmatic domains of the zircons. This

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would be in line with the first scenario of UHP crystallization in the mantle. Moreover, we have some evidence of Upper Jurassic/Lower Cretaceous rift-related magmatic underplating in the continental crust in Central Rhodope supported by SHRIMPdata at ca. 150 and 135– 140 Ma recorded in both basic and acid lithologies (Gebauer and Liati, 1997; Liati and Gebauer, 2001). This may be correlated with the mantle upwelling process involved in the first scenario. The late stages of the 73.5 F 3.4 Ma metamorphic event occurred at ca. 61.9 F 1.9 Ma (age of late pegmatoid fluids). The ca. 62 Ma age of the end of metamorphism is in line with the presence of Maestrichtian– Paleocene sediments discordantly overlying the crystalline basement in the area of Krumovgrad in the Bulgarian part of East Rhodope (Goranov and Athanasov, 1992), ca. 20 km north of the location of the rocks dated by SHRIMP in the frame of this study. The similarity in age between late stages of metamorphism and overlying sediments argues for relatively rapid exhumation processes. Average (minimum) exhumation rates calculated on the basis of the age determined by SHRIMP for the garnet-rich mafic rock and the pegmatoid, as well as the approximate P conditions of UHP metamorphism ( Pmin = 2.5 GPa) and of the pegmatoid formation ( P = 0.5 GPa) are 5.2 km/Ma (or 5.2 mm/year). This is in line with high exhumation rates inferred from our SHRIMP and the paleontological data (see above) but still lower than the 7.7 km/Ma reported for Central Rhodope (Liati and Gebauer, 1999). However, if UHP conditions have been really above 4 GPa, exhumation rates are significantly higher ( > 9 km/Ma). The exhumation rate is lower (2.2 – 2.8 km/ Ma) in the case of the first scenario, for which we assume an UHP protolith for the garnet-rich mafic rock and eclogite-facies metamorphism ( P = 13.5 – 16 kbar) at 73.5 F 3.4 Ma. Considering the available geochronological data for metamorphism in the Rhodope zone, we observe that in East Rhodope metamorphism is significantly older (ca. 74 Ma) than in the central part (ca. 42 Ma). Therefore the Rhodope was not metamorphosed as a single geotectonic element but rather consisted of different fragments subducted and exhumed at different times. Another implication of the SHRIMP-results is that the upper tectonic unit itself cannot be consid-

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ered as a uniform element and has to be subdivided at least into two sub-units.

Acknowledgements We greatly appreciate the help of R.A. Stern (GSA, Ottawa) during various stages of SHRIMP data production and evaluation. We also thank W. Wittwer for the tedious work involved in separating zircons. The help of Petraki Meyer (Heidelberg) during microprobe and SEM work is much appreciated. Thanks also to Florian Schwandner and Volker Dietrich (ETH, Zurich), for their help with the whole rock analyses. Review by M. Tho¨ni (Vienna), and an anonymous reviewer of an earlier version of the manuscript helped improving the paper. Useful comments and constructive suggestions by Ch. Chopin (Paris) are greatly appreciated. This work was supported by a grant of the Swiss National Science Foundation (20-52662.97). RR

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