Browsing by Subject "Cyclades"
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Item Novel (U-Th)/He thermochronometric constraints on serpentinized ultramafic rocks(2017-08) Cooperdock, Emily Jane; Stockli, Daniel F.; Ketcham, Richard; Barnes, Jaime; Lavier, Luc; Müntener, Othmar; Klein, FriederSerpentinization, the hydration of peridotite, is a widespread process that impacts rheological properties along plate boundaries, fluid-mobile element cycling and biogeochemical processes. While numerous studies exist on the petrology, structure and geochemistry of serpentinites, geochronological analysis has been elusive due to the lack minerals with well-established dating techniques. With technological advances in radiogenic isotope analysis in the past decade, it is now possible to address the timing of serpentinization by dating the growth of magnetite that forms as a direct result of the breakdown of primary peridotite phases in the presence of water. This study develops and applies a (U-Th)/He-based method that is tailored to analyze magnetite that grow during the alteration of ultramafic rocks. The technique includes procedures to screen for grain quality in opaque phases with X-Ray Computed Tomography, physical air abrasion to remove the alpha implantation/ejection zone in the outer grain boundary, and analytical procedures to measure low parent nuclide concentrations typical of these lithologies. The method is then modified to test the applicability to date magmatic spinel, a primary mineral phase in peridotite, to date peridotite exhumation by tectonic and volcanic processes. The magnetite (U-Th)/He technique is applied to serpentinites within an exhumed subduction complex in Syros, Greece, uplifted serpentinites within the Malenco Ultramafic Unit, Italian Alps, and to carbonated serpentinite in Wadi Fins, Oman. The results of these studies provide evidence for episodic or continuous magnetite growth over millions of years, which implies that magnetite formation can continue to occur in serpentinites under changing metamorphic conditions. These studies also show that multiple generations of magnetite growth can be distinguished based on magnetite grain size and trace element chemistry, and that hydrothermal magnetite can record changing chemical conditions and act as an important host phase of U and Th in serpentinites. Overall, the magnetite (U-Th)/He chronometer can be used to distinguish between multiple episodes of fluid-rock alteration, which has implications for the cooling history and geochemical exchanges in serpentinized peridotite. This technique, coupled with spinel (U-Th)/He, has broad applicability to investigate the timescales of mantle exhumation and alteration of ultramafic rocks in a variety of tectonic settings.Item Pre-Cenozoic tectono-metamorphic evolution of the Cycladic Basement, Ios Island, Greece(2018-08-08) Flansburg, Megan Elysia; Stockli, Daniel F.The Cycladic Basement is a HP-LT unit exposed in the highly distended back-arc region of the retreating Hellenic subduction zone of the southern Aegean. The HP-LT Cycladic Blueschist Unit (CBU) and the Cycladic Basement are parts of the Paleogene Cycladic subduction complex and are exposed in Miocene metamorphic core complexes in the southern Cyclades, such as Ios, Naxos, and Paros islands. While the Paleogene tectono-metamorphic evolution of the Cycladic Basement and the CBU have been the foci of numerous studies, robust constraints of the Cycladic Basement’s pre-subduction magmatic, tectonic, and paleogeographic evolution have been largely lacking. This study presents new bedrock and detrital zircon U-Pb age data to elucidate the pre-subduction tectonic, magmatic, and paleogeographic evolution of the Cycladic Basement. Zircon U-Pb dating of granitoids from the crystalline core of Ios yielded an age of ~306-330 Ma, demonstrating voluminous and protracted Carboniferous magmatism related to Paleo-Tethys subduction and emplaced into Peri-Gondwanan crust. The plutons were emplaced into paragneisses and garnet mica schists of the Carapace metasedimentary host-rock sequence, characterized by distinct Gondwanan detrital zircon U-Pb (DZ) provenance, Neoproterozoic to lower Paleozoic maximum depositional ages, and syn-magmatic zircon rims (~300-330 Ma). DZ U-Pb dating also revealed a distinct sequence of post-plutonic Permian metasedimentary rocks (~270-295 Ma), previously lumped with the pre-plutonic Carapace or CBU, that disconformably overlies the Basement complex. These DZ ages, coupled with early Permian apatite U-Pb cooling ages, indicate rapid cooling and exhumation of the Cycladic Basement prior to the deposition of these Permian siliciclastic sediments within extensional basins. These Transitional Rocks mark the onset of deposition of the Permo-Triassic to Late Cretaceous CBU within the Pindos rift domain. In summary, these new U-Pb results illuminate the pre-Mesozoic evolution of the Cycladic Basement as a peri-Gondwanan terrane composed of Neoproterozoic and lower Paleozoic metasedimentary rocks, intruded by voluminous Carboniferous arc magmatism, and rapidly exhumed in the Permian, prior to Permo-Triassic rifting and CBU deposition. These data provide a critical puzzle piece in the pre-Mesozoic reconstruction of the Cyclades and for the understanding of early Mesozoic rifting and Paleogene subduction processes.Item Zircon U-Pb chronostratigraphy and provenance of the Cycladic Basement and CBU on Sikinos and Ios Islands, Greece(2018-08) Poulaki, Eirini Maria; Stockli, Daniel F.Sikinos and Ios Islands, located in the Southern Cyclades of the Aegean, are part of a Cenozoic metamorphic core complex system that exposes subduction-related rocks in the highly-extended back-arc region of the Hellenic subduction zone. The HP-LT metamorphic units exposed on the Islands are the Mesozoic metasedimentary rocks of the Cycladic Blueschist Unit (CBU) and Paleozoic ortho- and paragneisses of the Crystalline Basement Unit. While these units appear to share a common subduction metamorphic history, the magmatic and stratigraphic evolution of these two units and their relationship remain poorly understood. In particular, the nature of the contact between the CBU and Basement has been variably described as either an extensional shear zone or a subduction-related thrust that was potentially reactivated as an extensional top-to-the-north detachment. This study employed zircon U-Pb geochronometry on 59 samples to constrain (1) the crystallization ages of the Cycladic Basement on Sikinos Island, (2) the Maximum Deposition Ages (MDA) and the detrital provenance of the metasedimentary units. These new data allow to establish of a (chrono-) stratigraphic framework for the CBU and Cycladic Basement, elucidate the nature of the contact, and shed light on the subduction processes and pre-subduction paleogeography. The results reveal that (1) the Basement is composed of early Paleozoic metasedimentary rocks intruded by Carboniferous granites, (2) the Cycladic Basement is unconformably overlain by Permian and Mesozoic metasedimentary rocks of the CBU, (3) the CBU stratigraphy is clarified and its deposition spans from Permo-Triassic to Late Cretaceous times. This chronostratigraphic framework is also supported by the detrital zircon provenance U-Pb record that provides an additional stratigraphic fingerprint. These relationships in southern Sikinos demonstrate a nearly continuous stratigraphic record from the Cycladic Basement into the CBU and argue for a depositional contact and against both a subduction or extension-related structural juxtaposition of the CBU and the Cycladic Basement. On the other hand, the same stratigraphic constraints exhibit clear old-over-young relationships in NE Sikinos and Ios Islands, supporting structural repetition by thrust imbrication of the CBU slivers as a result of underplating during subduction in the Paleogene.