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

Abstract

The Attic–Cycladic Crystalline Belt in the central Aegean region represents a major tectono‐stratigraphic unit of the Hellenides. The essential geological, magmatic and tectono‐metamorphic features are well documented. Unresolved questions concern the time of sediment accumulation and litho‐ and/or tectono‐stratigraphic relationships across the study area. In order to address this issue we have studied siliciclastic metasedimentary rocks from Andros Island, northern Cyclades. The sampling strategy aimed at covering the complete age range recorded by the Andros metamorphic succession. Detrital zircon U–Pb dating of nine samples indicates maximum depositional ages of c. 260 Ma for the topmost part of the metamorphic succession and of c. 160–140 Ma for rock sequences below a prominent serpentinite belt that is interpreted to outline a major tectonic contact. These age constraints are in accordance with interpretations suggesting that the metamorphic rocks of Andros represent different tectonic subunits (Makrotantalon Unit and Lower Unit) that are separated by a thrust fault. Modification of the internal structure of the Lower Unit by tectonic stacking can currently not be ascertained. The new data for the Lower Unit corroborate the importance of Late Jurassic–Early Cretaceous sediment accumulation for the larger study area. In contrast to some of the neighbouring islands, no evidence for transfer of Late Cretaceous (c. 80 Ma) material into the Andros sedimentary environment was found. The most striking feature of the zircon populations of the Lower Unit is a remarkable age cluster at 250–200 Ma that documents the importance of Triassic igneous sediment sources. Copyright © 2015 John Wiley & Sons, Ltd.

Tectono‐magmatic and Stratigraphic Evolution of the Cycladic Basement, Ios Island, Greece

“…However, the exact tectonic setting for the deposition of CBU protoliths remains contentious and centers around the nature of the Pindos Basin (e.g., Bortolotti et al, ; Menant, Jolivet, & Vrielynck, ; Papanikolaou, , ; Stampfli et al, ; Tremblay et al, ; van Hinsbergen, Hafkenscheid, et al, ; van Hinsbergen, Zachariasse, et al, ; van Hinsbergen & Schmid, ). The ages of intercalated volcanic rocks and DZ studies in the western and northern Cyclades demonstrated that CBU deposition spanned from Triassic to Paleogene (Bröcker et al, ; Henjes‐Kunst & Kreuzer, ; Hinsken et al, ; Keay, ; Löwen et al, ; Seman et al, ) and was likely initially accumulated along a rifted continental margin (Seman et al, ). In the southern and central Cyclades, Fu et al () demonstrated that the CBU contains Middle Triassic zircon characterized by rift‐related δO 18 and εHf values.…”

“…The Cycladic Blueschist Unit (CBU), which lies at the heart of the Attic‐Cycladic subduction complex, has been the focus of numerous studies constraining its prograde and retrograde HP‐LT metamorphic evolution and exhumation, especially on the islands of Syros and Sifnos where HP‐LT blueschist facies rocks are excellently preserved alongside younger greenschist to amphibolite facies (e.g., Avigad & Garfunkel, ; Behr et al, ; Dürr et al, ; Grasemann et al, ; Grasemann et al, ; Jolivet et al, ; Laurent et al, ; Philippon et al, ; Soukis & Stockli, ). However, limited detailed work exists about the chronostratigraphy, the depositional history, or the paleogeographic and tectonic evolution of the CBU or Cycladic Basement (CB) protoliths, consequently hampering insights into subduction processes, the juxtaposition of different HP‐LT units, and Aegean exhumation history (e.g., Bröcker et al, ; Bröcker & Pidgeon, ; Fu et al, ; Henjes‐Kunst & Kreuzer, ; Hinsken et al, ; Keay, ; Keay & Lister, ; Löwen et al, ; Poulaki et al, ; Seman et al, ). In particular, little is known about the structurally lowest unit, the CB, which represents a fragment of Adria/Pelagonia continental crust underlying the CBU in the southern Cyclades (Jolivet & Brun, ; Ring et al, ; Stampfli et al, ; Stampfli & Borel, ).…”

New Detrital Zircon Geochronology From the Cycladic Basement (Greece): Implications for the Paleozoic Accretion of Peri‐Gondwanan Terranes to Laurussia

“…Similar maximum deposition ages were considered for the Lower Unit in Andros (Bröcker et al, ). Yet in the Makrotantalon Unit of Andros (Bröcker et al, ; Papanikolaou, ), Permian (ca. 260‐Ma) depositional ages and fossils were defined, although the tectonostratigraphic position of this unit is not very clear.…”

“…The Cover Series section is made of predominantly Mesozoic marine metasediments, typically represented by metapelites in intercalation with metabasic intrusions, Triassic metatuffs, and Triassic-Jurassic marble horizons (the latter are more substantial in the Southern Cyclades), as well as metabasites, eclogites, and metaperidotites that originated from the subducting crust of a Late Mesozoic oceanic basin (e.g., Bröcker et al, 2016;Bröcker & Pidgeon, 2007;Dürr et al, 1978;Fu et al, 2015;Hinsken et al, 2016;Keay, 1998;Melidonis, 1980;Okrusch & Bröcker, 1990;Ring et al, 2010). The lowermost part of the Cover Series of the Lower Unit in Naxos was deposited in the Late Triassic (200-220 Ma; Keay, 1998;Dürr et al, 1978;Papanikolaou, 1978).…”

“…The Cycladic Massif (e.g., Blake et al, ; Bonneau, ; Bröcker et al, , and references therein) represents remnants of the collapsed Alpine orogen, preserved on the Cycladic islands, in the central Aegean Sea (Figure ). Alongside a generally peri‐North African provenance (e.g., Keay & Lister, ) that ranks it as a Cadomian‐type terrane, the Cycladic Cover Series bears a significant Ordovician‐Devonian (450–400 Ma; i.e., Caledonian‐aged) zircon age population (e.g., Bröcker et al, ; Hinsken et al, ; Keay, ; Keay & Lister, ; this study). Such a detrital zircon population is clearly unusual for Cadomian‐type terranes.…”