Journal of the Virtual Explorer

Protolith ages define two main groups: Palaeozoic and Jurassic-Cretaceous (Table 1).

The older group is Carboniferous to Permian, from ca. 300 to ca. 250 Ma. Zircon cores of two eclogitic gabbros are 245.6 ± 3.9 [Liati, 2005] and 255.8 ± 2.1 Ma [Liati et al., 2011]. Cores of monazites between 265 and 295 Ma [Bosse et al., 2009] may also witness this magmatic event.

Two gabbros of the Kroumovitsa unit are an exception showing protolith ages >500 Ma and metamorphic rims of 350-300 Ma [Carrigan et al., 2003]. They fall in a time bracket identified also in the Upper Terrane [e.g. Himmerkus et al., 2006], which raises the question as to whether this imbricate is a "Serbo-Macedonian" thrust sheet.

Concordant zircons from orthogneisses indicate Late Jurassic-Early Cretaceous intrusions from ca. 160 to ca. 130 Ma (Table 1). Forty zircon grains dated between 121 and 159 Ma [Bosse et al., 2009] may represent this magmatic event. The 117 Ma oscillatory zircon domain of a garnet-mafic rock (Table 1) may also reflect the protolith age [Liati et al., 2011].

Ages obtained in western, central and eastern Rhodope are similar, which fits a first-order process that goes beyond regional variations. The UHP metamorphism had probably started during the Early Jurassic [older than 170 Ma, Reischmann and Kostopoulos, 2002; Bauer et al., 2007]. The Sm-Nd method applied to a 1.5-1.6 GPa pyroxenite from Eastern Rhodope yielded 119 ± 3.5 Ma, interpreted as the age of the HP metamorphism [Wawrzenitz and Mposkos, 1997]. U-Pb SHRIMP analyses on oscillatory zoned zircon of a garnet-bearing basic rock from Central Rhodope yielded an equivalent, weighted mean age (117.4 ± 1.9 Ma), which was considered as dating the protolith [Liati et al., 2002] but may very well date early metamorphic conditions (no clear core). Metamorphic, zircon rims from a garnet-kyanite paragneiss for which high-pressure conditions have been suggested are dated at 148.8 ± 2.2 Ma, complemented by ages of 147.2 ± 4.7 Ma (paragneiss) and 143.4 ± 3.3 Ma (strongly amphibolitized eclogite) zircon domain ages [Liati, 2005].

Further constraints are provided, in Central Rhodope, by oscillatory zircon domains at 73.5 ± 3.4 Ma interpreted to reflect HP/UHP metamorphism [Liati, 2005] but possibly dating regional, amphibolite-facies metamorphism. This weighted mean age for metamorphism is further supported by the 61.9 ± 1.9 Ma pegmatite vein that intruded these rocks [Liati et al., 2002] and the 65-63 Ma trondhjemitic veins cutting amphibolitized eclogites in eastern Rhodope [Baziotis et al., 2007] where an additional Rb-Sr isochron age of 65.4 ± 0.7 Ma is given for another cross-cutting pegmatite [Liati et al., 2002]. In eastern Rhodope, two syn- to post-deformation granites were dated at ca. 70 Ma [Marchev et al., 2006]. In western Rhodope, zircon rims of a Palaeozoic gabbro are 51.0 ± 1.0 Ma [Liati, 2005]. These dated veins provide solid evidence that amphibolite-facies deformation was waning by ca. 50 Ma. The number of ages distributed with no obvious hiatus between this upper bound and ca 170 Ma (Tables 2 and 3) reflects a protracted residence under evolving eclogite-, granulite- and amphibolite-facies conditions.

42 Ma zircons in a retrogressed eclogite were interpreted as dating the eclogite-facies event [Liati and Gebauer, 1999] while zircon rims between ca. 40 to ca. 38 Ma in migmatites containing the eclogites and in an adjacent amphibolite have been interpreted as dating the regional, amphibolite-facies metamorphism [Liati, 2005]. These ages (Table 4) are within the range of the numerous K-Ar and ⁴⁰Ar/³⁹Ar amphibole and mica ages between 50 and 35 Ma reported for many rocks of the intermediate units [Liati and Kreuzer, 1990; Kaiser-Rohrmeier et al., 2004] and with the Eocene age of zircons from discordant leucosomes [Ovtcharova et al., 2002; Peytcheva et al., 2004]. They are also coincident with the many cooling ages measured within this time span all over the Rhodope Metamorphic Complex (Tables 4, 6 and 7) and the ca. 35 Ma age of hydrothermal deposits and volcanic rocks in Eastern Rhodope [Márton et al., 2010]. Since K-Ar and ⁴⁰Ar/³⁹Ar are reportedly low temperature systems, one cannot exclude that the 42-38 zircons also registered retrogression down to hydrothermally influenced metamorphic conditions of ca. 300°C - 0.3 GPa [Liati and Seidel, 1996]. As such, the many Tertiary ages may simply record cooling from amphibolite to greenschist-facies. Zircon and apatite fission-track ages overlapping K-Ar and ⁴⁰Ar/³⁹Ar dates (Table 4) demonstrate very fast cooling in Oligocene times [Wüthrich, 2009; Márton et al., 2010].