The status of the Makrotantalon Unit (Andros, Greece) within the structural framework of the Attic-Cycladic Crystalline Belt

Abstract

This study focuses on the status of the Makrotantalon Unit (Andros, Greece) within the framework of the Cycladic nappe stack. We document unambiguous evidence that this unit has experienced blueschist-facies metamorphism and identify previously unknown lawsonite ± pumpellyite assemblages in glaucophane-free metasediments. The position of the presumed tectonic contact at the base of this unit is vague, but roughly outlined by serpentinites. Only a single outcrop displays a weak angular unconformity with cohesive cataclasites in the footwall. Rb-Sr geochronology was carried out on 11 samples representing various rock types collected within or close to inferred or visible fault zones. Owing to a lack of initial isotopic equilibration and/or subsequent disturbance of the Rb-Sr isotope systematics, isochron relationships are poorly developed or non-existing. In NW Andros, direct dating of distinct displacement events has not been possible, but a lower age limit of ß 40 Ma for final thrusting is constrained by the new data. Sporadically preserved Cretaceous ages either indicate regional differences in the P-T-d history or a different duration of metamorphic overprinting, which failed to completely eliminate inherited ages. The detachment on the NE coast records a later stage of the structural evolution and accommodates extension-related deformation. Apparent ages of ß 29-25 Ma for samples from this location are interpreted to constrain the time of a significant deformation increment. On a regional scale, the Makrotantalon Unit can be correlated with the South Evia Blueschist Belt, but assignment to a specific subunit is as yet unconfirmed.

New structural and U–Pb zircon data from Anafi crystalline basement (Cyclades, Greece): constraints on the evolution of a Late Cretaceous magmatic arc in the Internal Hellenides

“…Rocks of the Upper Unit are exposed on the islands of Anafi (Reinecke et al 1982), Donousa (Altherr et al 1980), Gramvonisi (Dürr 1985), Ikaria (Altherr et al 1994), Makra (Thorbecke 1987), Nikouria (Dürr et al 1978b), Syros (Bonneau et al 1980) and Tinos (Patzak et al 1994). Rocks of the Makrotantalon Unit in northern Andros, previously correlated with the Upper Unit (Bröcker and Franz 2006), show evidence for HP-LT metamorphism (Huyskens and Bröcker 2014). In turn, the Upper Unit escaped Eocene HP-LT metamorphism that affected the Cycladic Blueschist Unit underneath.…”

U–Pb dating of detrital zircons from Andros, Greece: constraints for the time of sediment accumulation in the northern part of the Cycladic blueschist belt

“…The metamorphic rocks in the northern part of Andros are considered to represent a distinct tectonic unit (=MU; Papanikolaou, , , ; Bröcker and Franz, ; Huyskens and Bröcker, ). This notion is further substantiated by detrital zircon geochronology.…”

“…The underlying clastic sequences of the LU are characterized by Triassic or younger depositional ages. The new zircon age data is consistent with previous interpretations suggesting an older‐on‐younger structural juxtaposition, indicating that the tectonic contact separating the MU from the LU is a thrust fault (Papanikolaou, ; Bröcker and Pidgeon, ; Huyskens and Bröcker, ).…”

“…Mehl et al ., ). Eocene ages ( c. 40 Ma; Rb–Sr internal isochrons) recorded in samples collected close to the presumed fault zone provide a lower time limit for tectonic juxtaposition (Huyskens and Bröcker, ).…”

“…80–70 Ma) and Miocene ( c. 24–21 Ma) times (Bröcker and Franz, ). Field observations (Papanikolaou, ) and newly found occurrences of blue amphibole in Makrotantalon schists suggest that the topmost rocks on Andros represent a distinct tectonic slice belonging to the lower main unit of the Cyclades that has experienced HP/LT metamorphism and widespread, but incomplete greenschist‐facies overprinting (Huyskens and Bröcker, ). The exact position of the inferred thrust fault at the base of the MU is difficult to localize, but is roughly marked by serpentinites (Papanikolaou, ; Huyskens and Bröcker, ; cf.…”