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Coupled replacement and preservation of high pressure rocks caused by infiltration of mixed H2O-CO2 fluids
Stockholm University, Faculty of Science, Department of Geological Sciences.ORCID iD: 0000-0002-3732-7993
Stockholm University, Faculty of Science, Department of Geological Sciences.
Stockholm University, Faculty of Science, Department of Geological Sciences.ORCID iD: 0000-0003-3347-9284
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Eclogites, blueschists and greenschists are found in close proximity to one another along a 1-km coastal section where the Cyclades Blueschist Unit is exposed on SE Syros, Greece. Here we show that the eclogites and blueschists experienced similar metamorphic histories: prograde lawsonite blueschist facies metamorphism at 1.1–1.4 GPa and 380–460 °C followed, at 38–43 Ma, by peak blueschist/eclogite facies metamorphism at 1.5–1.8 GPa and 470–500 °C. We explain co-existence of eclogites and blueschists by compositional variation probably reflecting original (sedimentary) layering. We also show that the greenschists record retrogression at 0.34 ± 0.21 GPa and T = 456 ± 68 °C associated with fluid flow along a shear zone, bearing eclogite facies ultramafic knockers, that cuts across the section. Finally, we show that greenschist facies retrogression ended at (or slightly after) 27 Ma. This implies a period of metamorphic quiescence after eclogite/blueschist facies metamorphism and before greenschist facies retrogression that may have lasted 13–16 million years. We suggest that this reflects an absence of metamorphic fluid flow at that time. Referring to local preservation of blueschists and eclogites along the coastal section, we infer that retrogression only occurred when and where metamorphic fluid flow occurred. Finally, from a tectonic perspective, our findings are consistent with studies showing that 1) the Cyclades Blueschist Unit is a high-pressure nappe stack consisting of belts in which high pressure metamorphism occurred at different times and 2) these rocks were affected by regionally extensive greenschist facies metamorphism during the Oligocene.

National Category
Earth and Related Environmental Sciences
Research subject
Geology
Identifiers
URN: urn:nbn:se:su:diva-155217OAI: oai:DiVA.org:su-155217DiVA, id: diva2:1197773
Available from: 2018-04-13 Created: 2018-04-13 Last updated: 2025-02-07Bibliographically approved
In thesis
1. The metamorphic history of Naxos (central Cyclades, Greece): Deciphering the Oligocene and Miocene exhumation events
Open this publication in new window or tab >>The metamorphic history of Naxos (central Cyclades, Greece): Deciphering the Oligocene and Miocene exhumation events
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

High pressure, low temperature (HP-LT) rocks observed at the surface of the Earth are evidence ofpast subduction zones. Understanding the tectonics processes that control the exhumation of HP-LT metamorphic rocks in these subduction zones requires full comprehension of the pressure-temperature-time (P–T–t) cycle that the rocks experienced. In the Cyclades, Greece, the Cycladic Blueschist Unit (CBU) hosts eclogite and blueschist facies rocks. However, the processes that exhumed them are debated. The overall aim of this thesis is to understand how the Eocene HP-LT rocks were exhumed in the central Cyclades based on a study of the metamorphic history of Naxos Island and nearby Syros Island. In this thesis, I carried out a systematic geothermobarometric and geochronological investigation on Naxos to better constrain the P–T–t paths that are recorded by the rocks. The data indicate that high-P metamorphism on Naxos occurred in the Eocene at c. 40 Ma and the HP-LT rocks were exhumed by two tectonic events. The first exhumation event occurred in the Oligocene. The HP-LT rocks were exhumed in a convergent setting by an extrusion wedge. The top of the sequence reached greenschist facies conditions at c.32 Ma, whereas the bottom of the sequence remained at greater depth (equating to pressures of 8–12 kbar). Additionally rocks from southeastern Syros recorded a similar Eocene/Oligocene P–T–t history to that recorded by the top of the sequence on Naxos, suggesting a common Eocene/Oligocene metamorphic history for the central Cyclades. The second exhumation event occurred in the Miocene. The rocks were further exhumed in an extensional setting from c. 20 to 8 Ma. The top of the sequence on Naxos was already in the brittle crust at that time and therefore did not record this Miocene metamorphism. The bottom of the sequence was first isothermally exhumed at high-T conditions and thereafter cooled rapidly.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University, 2018. p. 54
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper ; 371
Keywords
Cycladic Blueschist Unit, exhumation, fluid flow, geothermobarometry, Hellenide orogen, Rb-Sr dating, subduction-zone metamorphism, Heat flow, Lower crust, extensional domain, Metamorphic core complex
National Category
Earth and Related Environmental Sciences
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-155218 (URN)978-91-7797-240-2 (ISBN)978-91-7797-241-9 (ISBN)
Public defence
2018-06-04, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: Submitted. Paper 4: Manuscript.

Available from: 2018-05-08 Created: 2018-04-13 Last updated: 2025-02-07Bibliographically approved

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Skelton, AlasdairPeillod, AlexandreRing, Uwe

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