The tectonostratigraphy of the Cycladic Blueschist Unit and new garnet geo/thermochronology techniques




Seman, Spencer Mark

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Due to deformation and recrystallization at high-pressure low-temperature metamorphic conditions, blueschist and eclogite-facies rocks typically lack primary sedimentary features or fossils to constrain depositional age. Without this information, it is difficult to describe how these rocks evolved structurally through time and, therefore, to infer the processes at work during the exhumation. This work defines the tectonostratigraphy of the Cycladic Blueschist Unit (CBU) of Greece using detrital zircon geochronology, to correlate metasediments based on their provenance and estimate maximum depositional age. Two distinct types of metasediments are present within the CBU: those derived from the northern rifted margin of Gondwana and deposited during the Triassic-Early Jurassic, and flysch sourced from the Internal Hellenides deposited during the Late Jurassic-Late Cretaceous. In the Western Cyclades, younger flysch sediments are still found overlying older rift margin sediments. This is a primary depositional relationship at the scale of ~500 meters preserved through the subduction and exhumation process. On Syros Island, composed of a structural section of CBU ~12 km in thickness, I employ detrital zircon geochronology to define repetitions of tectonostratigraphy. Based on repeated Triassic volcanics, Syros is composed of thrust sheets ~3 km in thickness. Next, I use combination of detrital zircon geochronology and (U-Th)/He thermochronology to refine the tectonostratigrapy of southern Attica, specifically Hymittos Mountain and the Lavrion Peninsula. This work shows that two distinct low-angle normal fault systems are present in Attica, one active in the Middle Miocene and the other in the Late Miocene.
Garnet is a common rock-forming mineral in both high-pressure low-temperature metamorphic rocks and magmatic-hydrothermal skarn. I have developed a garnet-based (U-Th)/He thermchronometric technique to constrain the timing of cooling of high-pressure low-temperature metamorphic rocks. This work shows that He diffusion in garnet is sensitive to temperatures between 200-300˚C at geologic timescales and characterizes a series of potential standards of grossular-andradite composition. Finally, in order to better understand the temporal evolution of magmatic-hydrothermal systems, I created a new U-Pb based technique to date the growth of grossular-andradite garnet. I characterize four different potential reference standards and apply to technique to hydrothermal skarn on Serifos Island, Greece.


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