Inferences on the Mesozoic evolution of the North Aegean from the isotopic record of the Chalkidiki block

Abstract

International audienceThe Chalkidiki block is a major domain in the North Aegean that, contrary to other domains in the region, largely escaped thermal perturbations during Tertiary extension. As a result, the Chalkidiki block is an ideal candidate to glean information related to the timing of Mesozoic thermal events using appropriate geochronological techniques. We have undertaken a laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) study (U-Th-Pb on monazites and U-Pb on zircons) coupled with 40Ar/39Ar dating on nine samples from various structural levels within the thrust system of the Chalkidiki block. The eastern, and structurally lower part of the system revealed a complete isotopic reset of Carboniferous – Early Triassic monazites coeval with partial monazite destruction, REE-mobilisation and formation of apatite-allanite-epidote coronas at ~ 132 Ma, a reaction that is commonly observed in amphibolite-facies rocks. These coronas formed after crystallisation of garnet (i.e., at T > 580 °C) and, in all probability, either close to the peak-temperature conditions (~ 620 °C) on a prograde path or during retrogression between the peak-temperature and the low-temperature boundary of the amphibolite facies. Cooling of these rocks and arrival at mid-crustal levels occurred at 95–100 Ma. By contrast, the western, and structurally uppermost part of the system went through the same event by 120–125 Ma. Further structural considerations with respect to medium-temperature geochronology data imply that syn-metamorphic thrusting must have ceased by early Late Cretaceous. We emphasize that, with the sole exception of the Chalkidiki block, no pre-45 Ma medium-temperature geochronology data are preserved in other North Aegean domains, a feature that is clearly related to the extension-induced thermal perturbation of the region during the Tertiary

Polycyclic alpine orogeny in the Rhodope metamorphic complex: the record in migmatites from the Nestos shear zone (N. Greece)

“…Based on their results, Kirchenbaur et al (2012) suggested that the two HP events at ∼42 and ∼51 Ma inferred by Liati and Gebauer (1999) and Liati (2005) probably reflect the same subduction event. Several authors have expressed doubts on the interpretation of the above ages as reflecting HP conditions, at least for those implying a subduction setting in the RMC as late as 42-51 Ma (Barr et al, 1999;Krohe and Mposkos, 2002;Turpaud and Reischmann, 2010;Burg, 2011;Moulas et al, 2013; Kydonakis et al, 2016). Although not the main topic of this paper, we will come back to this issue in the discussion (see Sect.…”

“…The latter point is consistent with an interpretation of the ∼115 to ∼70 Ma "geochronological gap" of the RMC as reflecting a period of tectonic quiescence. In addition, Kydonakis et al (2016) suggest that the high-temperature conditions that prevailed in the RMC during the Cenozoic have erased its earlier medium-temperature geochronological record (Cretaceous 40 Ar/ 39 Ar ages), which is now preserved in Chalkidiki only. A pre-Cenozoic 40 Ar/ 39 Ar record may indeed have been erased in the tectonic units of the RMC that display U-Th-Pb evidence of the mid-Mesozoic metamorphic loop (e.g., the SU, see Sect.…”

“…1), Kydonakis et al (2016) have recently reported 40 Ar/ 39 Ar white mica ages between ∼95 and ∼125 Ma and, from a garnet-kyanite micaschist, one U-Th-Pb monazite age of ∼130 Ma. Combining these ages with petrological and structural data, Kydonakis et al (2016) infer that the Chalkidiki region and the RMC shared a common tectonometamorphic evolution during at least part of the Mesozoic, as long suspected (e.g., Ricou et al, 1998;Kilias et al, 1999), and that thrusting in Chalkidiki ceased before ∼120-125 Ma. The latter point is consistent with an interpretation of the ∼115 to ∼70 Ma "geochronological gap" of the RMC as reflecting a period of tectonic quiescence.…”

“…Cenozoic widespread magmatism and high-temperature metamorphism, attested by numerous geochronological data throughout the RMC, is now interpreted as resulting from a process of mantle delamination initiated at ∼40 Ma (Yanev et al, 1998;Burg, 2011) or ∼50-56 Ma (Jolivet and Brun, 2010;Marchev et al, 2013). Following Burg (2011), mantle delamination may have triggered crustal-scale "core complex"-type extension and, importantly, may explain the development of a major tectonometamorphic event during the Cenozoic, long after the midMesozoic orogeny, without the requirement of a new orogenic period (see also Kydonakis et al, 2016). Alternatively, mantle delamination may have occurred in the immediate aftermath of an early Cenozoic orogeny (Nagel et al, 2011;Froitzheim et al, 2014).…”

“…Since the late 1990s, numerous scenarios have been proposed for the tectonic evolution of the RMC during Alpine times. A primary difference between these scenarios is whether there has been a single protracted orogenic cycle since around the midMesozoic (Ricou et al, 1998;Barr et al, 1999;Papanikolaou and Kranis, 2004;Turpaud and Reischmann, 2010;Krenn et al, 2010;Burg, 2011;Kydonakis et al, 2016) or whether Alpine orogeny involved distinct episodes of subduction and crustal accretion (Liati, 2005;Mposkos et al, 2010;Liati et al, 2011Liati et al, , 2016Nagel et al, 2011;Kirchenbaur et al, 2012;Froitzheim et al, 2014;Wawrzenitz et al, 2015). The difference between the two views mainly stems from different interpretations of the available geochronological data (e.g., Burg, 2011;Liati et al, 2011).…”