The provenance and internal structure of the Cycladic Blueschist Unit revealed by detrital zircon geochronology, Western Cyclades, Greece

Abstract

This study presents detrital zircon U‐Pb analyses of 23 samples of the Cycladic Blueschist Unit (CBU) from Kea, Kythnos, and Serifos islands, as well as the Lavrion Peninsula of SE Attica. The maximum depositional ages (MDA) and age distributions of detrital zircon U‐Pb dates are used to correlate metasediments between the islands considered herein and infer their provenance. Two distinct detrital zircon U‐Pb age distributions are found in CBU metasediments: “Proterozoic,” comprised of >40% Neoproterozoic zircons with Triassic‐Early Jurassic maximum depositional ages and “Paleozoic,” containing >30% Paleozoic zircons and yielding Late Jurassic‐Cretaceous MDAs. Proterozoic affinity metasediments are rift margin deposits derived from the northern Gondwanan margin. Paleozoic metasediments are flysch sediments most probably sourced from the Internal Hellenides. This metamorphosed flysch forms a distinct marker horizon found in a similar structural position in Lavrion, Kythnos, and Serifos. Based on lithologic correlation, sediment provenance, and MDA estimates, the CBU of Kythnos is correlative to the Lavrion Schists of Attica. On the islands of Serifos and Kythnos and within the Lavrion Schists only young‐on‐old relationships exist between rocks based on MDA estimates.

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

“…Biotite‐bearing granites yielded Triassic crystallization ages in the range of 232‐236 Ma (Figure ), indicative of localized felsic magmatism during the early Mesozoic. This age range is like that of felsic metatuffs that are interbedded with metasedimentary rocks of the overlying CBU and may be associated with early Triassic synextensional bimodal volcanism with U‐Pb ages ranging from ~230 to 240 Ma (Bröcker & Pidgeon, ; Poulaki et al, ; Reischmann, ; Seman et al, ), though the nature of mafic portions of the bimodal volcanism on Naxos is tenuous (Bolhar et al, ). Similar to these results from Ios, a Triassic granitoid (~233 Ma) structurally underlies Carboniferous granitoids on Naxos Island (Reischmann, ).…”

“…Their DZ signatures could be explained by several scenarios: These samples are (1) CBU schists but did not receive any Triassic or younger zircon (i.e., Eurasian‐derived), (2) CB with CBU marbles deposited on top and then folded during subduction processes, and (3) CB that have been thrust imbricated with CBU marbles during subduction processes. Assigning them to the CB rather than the CBU is justified because it is unlikely they would not have received any Triassic zircon since Triassic magmatism is evident on Ios (Poulaki et al, ; this study) and elsewhere in the Cyclades (e.g., Bröcker & Pidgeon, ; Reischmann, ; Seman et al, ). Further, their DZ age signatures are different from the CBU (e.g., Poulaki et al, ; Seman et al, ) and more clearly match the metasedimentary CB (Figures ).…”

“…Assigning them to the CB rather than the CBU is justified because it is unlikely they would not have received any Triassic zircon since Triassic magmatism is evident on Ios (Poulaki et al, ; this study) and elsewhere in the Cyclades (e.g., Bröcker & Pidgeon, ; Reischmann, ; Seman et al, ). Further, their DZ age signatures are different from the CBU (e.g., Poulaki et al, ; Seman et al, ) and more clearly match the metasedimentary CB (Figures ). These relationships are more readily explained by imbricate thrust faults than by tight, isoclinal folds due to the lack of a continuous section (e.g., the lack of Triassic CBU strata).…”

“…Within the Cycladic subduction complex, the metasedimentary CBU mainly consists of alternating marbles, calc‐schists, quartzo‐feldspathic mica schists, meta‐tuffs, mica schists, and metabasites intercalated with metasedimentary layers (e.g., Bröcker, ; Dürr et al, ; Roche et al, ; Schliestedt & Matthews, ). The sedimentary protolith of the CBU was deposited in the Pindos Basin (e.g., Aubuin, ; Bonneau, ; Papanikolaou, ), starting in the earliest Mesozoic until subduction in the Paleocene‐Eocene ~60‐40 Ma (Jolivet & Brun, ; Ring et al, ; Seman et al, ). 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 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, ).…”