The most salient characteristic of the detrital zircon U-Pb data is the preponderance of ages between 500-650 Ma in all samples from the forearc exposures in Peloponnese, Kythera, and western Crete, and the volcanic arc in Santorini and Milos (Fig. 6, 7). This general characteristic suggests that the original provenance of Hellenic forearc Aegean detrital zircons is Pan-African. The youngest detrital zircon grains provide an estimate of the maximum age of deposition of the PQU protolith. Taken in aggregate, the range of minimum ages and major peak ages of the Hellenic detrital zircon grains from Peloponnese, Kythera, Crete, and Santorini HP-LT rocks suggests that they share a common age of deposition. Milos and Santorini islands are located in the Recent volcanic arc and share the same basement. Blueschist facies rocks from Milos yield additional post-Variscan detrital zircon grains, while maintaining the older zircon age populations and similar Late Miocene zircon fission-track ages with the PQU samples from the forearc (Marsellos et al., in review). It is, therefore, possible that the Milos bluschist rocks are not metamorphosed PQU, even though they share the same tectonothermal history. An exotic rock unit could have been structurally emplaced within the PQU during the Late Miocene Hellenic subduction. Overall, the detrital and inherited zircon U/Pb ages from all of the South Aegean metamorphic basement samples are similar suggesting that all of the rocks correlated with the PQU originated from the same sources and deposited at about the same time.

Fault-bounded lenses of Potamos gneiss (Petrocheilos, 1966) occur within schists of the PQU on Kythera and locally along the entire Cretan-Peloponnese ridge. U-Pb zircon age population from the Potamos gneiss (Xypolias et al., 2006) are similar to those from the PQU with the exception of the additional Variscan-aged and Late Cretaceous zircons. The presence of the younger zircons suggest that the gneisses have a different provenance and younger maximum depositional age than the PQU. This suggests that the Potamos gneisses are an exotic unit that was structurally emplaced within the PQU after Cretaceous time. These results, however, do not reveal if the gneisses were emplaced into the PQU basement during the Variscan, integrated during Hellenic subduction (Late Eocene-Early Miocene) or during exhumation of the Hellenic HP-LT rocks (Middle Miocene-Late Miocene).

Mica-schist of the PQU show arc-parallel extensional fabric and zircon fission track cooling ages of 10-13 Ma related to exhumation during expansion of the arc during subduction roll-back (Marsellos & Kidd, 2008). The Potamos gneisses show mostly arc-normal extensional structures and younger zircon fission-track cooling ages (ca. 9 Ma) (Marsellos et al., 2010). This suggests that the tectonic juxtaposition of the Potamos gneiss and PQU took place during the exhumation after mica-schists experienced the localized arc-parallel extension (Marsellos & Kidd, 2008). The allochonous nature of the gneiss and younger cooling ages suggests that tectonic emplacement occurred within about a million years of arc expansion (Marsellos et al., 2010).

The detrital zircon age populations from the PQU and intercalated Variscan units constrain the origins of the basement and Series rocks of the South Aegean forearc and volcanic arc. Four Precambrian crustal development cycles at 2700-2500, 2200-1900, 1200-900, and 800-550 Ma were highlighted by Gebauer (1993) as characteristic of the European Variscides. The periods of crustal development suggests derivation of the Variscanides from West Gondwana (Gebauer, 1993). Zircon age populations of the metamorphic rocks of the South Aegean, as well as detrital age populations in Central and North Aegean units (Keay & Lister, 2002; Meinhold et al., 2008) are similar to the Variscan tectonic cycles. The Aegean fore arc, arc and back arc detrital zircons (this study and Keay & Lister, 2002) lack Mesoproterozoic zircons, which would be expected from Laurasian (Williams and Claesson, 1987) and East Gondwana sources. The abundance of 550-650 Ma zircons along with populations consistent with the Variscan, and lack of Mesproterozoic zircons, suggests sources from North and West Africa, Arabia, and the Menderes Massif of western Turkey (Kroner and Sengor, 1990; Cahen et al., 1984; Reischmann et al., 1991; Ring et al., 1999; Keay & Lister, 2002).

The four Paleoarchean zircons are from the upper plate sandstone on Kythera, which did not experience the Miocene thermal event in the lower plate (Marsellos et al., 2010b). Paleoarchean zircons are absent in the Miocene metasedimentary rocks from Crete, Kythera, Peloponnese, Milos and Santorini, possibly because the metamorphism recrystallized or broke down any ancient zircons with substantial radiation damage. The presence of these ancient zircons may also provide constraints to the origin of the pre-Alpine basement. It is most likely that the Paleoarchean zircons were derived from the Kalahari craton in South Africa (e.g., Kranendonk et al., 2009) or another Gondwanan Archean craton (e.g., Pilbara or Yilgarn of Western Australia, Smithies et al., 1999; Smithies et al., 2001; Bagas et al., 2008), either as first or polycyclic detritus.

Of the current hypotheses for the origin of the pre-Alpine basement of the External Hellenides – derived by rifting of Apulia from the northern margin of Gondwana (Robertson et al., 1991, 1996), the western termination of the Cimmerian super-terrane (Stampfli & Mosar, 1999; Stampfli & Borel, 2002), and Permo-Triassic rifting of Cimmerian fragments coeval with southward subduction of the Palaeotethyan ocean beneath northern Gonwana (Sengor et al., 1984) – the Permo-Triassic rift model of Sengor et al. (1984) is most consistent with southern Gondwanan sources.