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

Abstract

The Cycladic Basement (CB) and the overlying Cycladic Blueschist Unit (CBU) are part of the Paleogene Cycladic subduction complex exposed in Miocene metamorphic core complexes in the distended back‐arc of the retreating Hellenic subduction zone of the southern Aegean. While the Cenozoic tectono‐metamorphic evolutions of the CB and the CBU have been the foci of numerous studies, this study presents new laser ablation inductively coupled plasma mass spectrometry bedrock and detrital zircon (DZ) U‐Pb ages that place robust constraints on the presubduction tectonic, magmatic, and paleogeographic evolution of the CB. Zircon U‐Pb ages of crystalline CB are ~306‐330 Ma, demonstrating local plutonism associated with regional voluminous, protracted Carboniferous magmatism related to Paleo‐Tethys subduction. The plutons intruded the CB metasedimentary host‐rock sequence, characterized by distinct Gondwanan DZ U‐Pb provenance, Neoproterozoic to early Paleozoic maximum depositional ages, and synmagmatic, contact metamorphic zircon rims (~300‐330 Ma). DZ U‐Pb dating revealed postmagmatic Permian metasedimentary rocks (~270‐295 Ma) that unconformably overlie the CB and have unimodal DZ spectra that indicate exhumation of the CB prior to Permian deposition within extensional basins, as well as mark the onset of CBU deposition prior to formation of the Pindos rift domain. These U‐Pb results clarify the late Paleozoic‐early Mesozoic evolution of the CB as a peri‐Gondwanan terrane composed of Neoproterozoic and early Paleozoic metasedimentary rocks, intruded by voluminous Carboniferous arc magmatism, and exhumed in the Permian, prior to Triassic rifting and CBU deposition. Additionally, these data provide a chronostratigraphic framework and illuminate subduction‐related juxtaposition within the CB metasedimentary sequence.

Zircon U‐Pb Chronostratigraphy and Provenance of the Cycladic Blueschist Unit and the Nature of the Contact With the Cycladic Basement on Sikinos and Ios Islands, Greece

“…More recently, Zlatkin et al () interpreted Ordovician‐Devonian detrital sources of the metasedimentary CB as part of the Cadomia terrane and located the CB in a more internal position relative to the Pelagonian terrane. In contrast, Flansburg et al () proposed that the CB was emplaced into Cambrian to Neoproterozoic paragneisses and garnet mica‐schists with distinct peri‐Gondwanan affinity and that the CB was exhumed prior to earliest Permo‐Triassic CBU deposition along a rifted continental margin. The CB is mainly exposed on the islands of Paros, Antiparos, Naxos, Ios, Sikinos, Syros, and Samos (Andriessen et al, ; Engels & Reischmann, ; Keay, ; Pe‐Piper & Piper, ; Ring et al, ; Tomaschek et al, ).…”

“…The CB is composed primarily of Carboniferous granitic orthogneiss originally intruded into Silurian and older metasedimentary rocks (Andriessen et al, ; Augier et al, ; Dürr et al, ; Flansburg et al, ; Henjes‐Kunst & Kreuzer, ; van der Maar, ; van der Maar & Jansen, ; Vandenberg & Lister, ; Zlatkin et al, ). The CB has been attributed to the Adria (Apulia) microcontinental block located at the southern margin of Pindos oceanic realm during the Mesozoic (e.g., Bonneau & Kienast, ; Keay & Lister, ).…”

“…When combined with lithostratigraphic and field observations, the MDA constraints allow for the reconstruction of presubduction chronostratigraphy and provenance evolution of the CBU and the CB, which are required to elucidate subduction zone processes (e.g., underplating and deformation associated with subducting sediments atop buoyant continental crustal fragments). In the companion paper, Flansburg et al () present zircon U‐Pb data of magmatic and metasedimentary rocks from the CB on Ios, and the implications of such data for unraveling the local structural architecture and the late Paleozoic‐early Mesozoic tectono‐magmatic evolution of the CB. More specifically the paper reports the crystallization ages of the plutonic CB, the provenance and MDAs of the metasedimentary rocks of the CB, as well as the structural juxtaposition evident within the CB on Ios.…”

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

“…The Basement is built mainly of an orthogneiss core , originally of S‐type granite composition (Henjes‐Kunst & Kreuzer, ) and Variscan crystallization age (Bröcker & Pidgeon, ). Garnet‐mica schists (which were previously analyzed for their detrital zircon content by Keay, , and recently by Flansburg, ) mantle this core and are incorporated within the orthogneiss as a large megaenclave (2 km long) and as small lenses or lenticular bodies. The schist mantle at the uppermost part of the Basement section is enriched in chlorite and occasional calcite, in contrast to the shist horizon, which is associated with the orthogneiss body (Keay, , and references therein).…”

“…Abundant 410 Ma‐aged zircon xenocrysts in the Variscan granites of Ios and Paros suggest that a magmatic source of this age existed in the vicinity of the Cycladic Massif. Finally, the most recent study of Flansburg () reported a felsic gneissic sample (IOS1617), likely representing a tuff or dyke intercalated within the metasedimentary sequence of Ios, which yielded a U‐Pb zircon age of 446.4 ± 2.3 Ma interpreted as the age of crystallization of this rock. This recent finding proves the existence of the late Ordovician magmatism in Ios.…”