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An Eocene/Oligocene blueschist-/greenschist facies P-T loop from the Cycladic Blueschist Unit on Naxos Island, Greece: Deformation-related re-equilibration vs. thermal relaxation
Stockholm University, Faculty of Science, Department of Geological Sciences.
Stockholm University, Faculty of Science, Department of Geological Sciences.
Stockholm University, Faculty of Science, Department of Geological Sciences.
Number of Authors: 42017 (English)In: Journal of Metamorphic Geology, ISSN 0263-4929, E-ISSN 1525-1314, Vol. 35, no 7, p. 805-830Article in journal (Refereed) Published
Abstract [en]

Geothermobarometric and geochronological work indicates a complete Eocene/early Oligocene blueschist/greenschist facies metamorphic cycle of the Cycladic Blueschist Unit on Naxos Island in the Aegean Sea region. Using the average pressure-temperature (P-T) method of thermocalc coupled with detailed textural work, we separate an early blueschist facies event at 576 +/- 16 to 619 +/- 32 degrees C and 15.5 +/- 0.5 to 16.3 +/- 0.9kbar from a subsequent greenschist facies overprint at 384 +/- 30 degrees C and 3.8 +/- 1.1kbar. Multi-mineral Rb-Sr isochron dating yields crystallization ages for near peak-pressure blueschist facies assemblages between 40.5 +/- 1.0 and 38.3 +/- 0.5Ma. The greenschist facies overprint commonly did not result in complete resetting of age signatures. Maximum ages for the end of greenschist facies reworking, obtained from disequilibrium patterns, cluster near c. 32Ma, with one sample showing rejuvenation at c. 27Ma. We conclude that the high-P rocks from south Naxos were exhumed to upper mid-crustal levels in the late Eocene and early Oligocene at rates of 7.4 +/- 4.6km/Ma, completing a full blueschist-/greenschist facies metamorphic cycle soon after subduction within c. 8Ma. The greenschist facies overprint of the blueschist facies rocks from south Naxos resulted from rapid exhumation and associated deformation/fluid-controlled metamorphic re-equilibration, and is unrelated to the strong high-T metamorphism associated with the Miocene formation of the Naxos migmatite dome. It follows that the Miocene thermal overprint had no impact on rock textures or Sr isotopic signatures, and that the rocks of south Naxos underwent three metamorphic events, one more than hitherto envisaged.

Place, publisher, year, edition, pages
2017. Vol. 35, no 7, p. 805-830
Keywords [en]
Cycladic Blueschist Unit, exhumation, fluid flow, geothermobarometry, Hellenide orogen, Rb-Sr dating, subduction-zone metamorphism
National Category
Earth and Related Environmental Sciences
Research subject
Geology
Identifiers
URN: urn:nbn:se:su:diva-147040DOI: 10.1111/jmg.12256ISI: 000407401500005OAI: oai:DiVA.org:su-147040DiVA, id: diva2:1143092
Available from: 2017-09-20 Created: 2017-09-20 Last updated: 2018-04-13Bibliographically 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: 2018-06-21Bibliographically approved

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