The Metamorphic History of Naxos (Central Cyclades, Greece)
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The metamorphic history of Naxos (central Cyclades, Greece) Deciphering the Oligocene and Miocene exhumation events Alexandre Peillod Academic dissertation for the in Geology at Stockholm University to be publicly defended on Monday 4 June 2018 at 10.00 in De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14. Abstract High pressure, low temperature (HP-LT) rocks observed at the surface of the Earth are evidence of past 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. 40Ma 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. 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. Stockholm 2018 http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-155218 ISBN 978-91-7797-240-2 ISBN 978-91-7797-241-9 Department of Geological Sciences Stockholm University, 106 91 Stockholm THE METAMORPHIC HISTORY OF NAXOS (CENTRAL CYCLADES, GREECE) Alexandre Jean Daniel Peillod The metamorphic history of Naxos (central Cyclades, Greece) Deciphering the Oligocene and Miocene exhumation events Alexandre Jean Daniel Peillod ©Alexandre Jean Daniel Peillod, Stockholm University 2018 ISBN print 978-91-7797-240-2 ISBN PDF 978-91-7797-241-9 Front page: "For my friend 'Hard or nuthin' Alexandre Peillod" Efstathios Reppas-Chrysovitsinos Back page: Photo from Etienne Pauthenet Printed in Sweden by Universitetsservice US-AB, Stockholm 2018 Distributor: Department of Geological Sciences, Stockholm University For my grandfather, Jean Peillod A mon grand-père, Jean Peillod “For something to be interesting, one has to stare at it for a long time” Gustave Flaubert “Pour qu’une chose soit intéressante, il suffit de la regarder longtemps” Gustave Flaubert Abstract High pressure, low temperature (HP-LT) rocks observed at the surface of the Earth are evidence of past 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. 1 | P a g e Abstrakt Högtryck och lågtemperatur (HP-LT) bergarter som observeras på jordytan är bevis på tidigare subduktion zoner. För att förstå de tektoniska prosesser som kontrollerar exhumeringen av HP-LT metamorfa bergarter i dessa subduktions zoner behövs en full förståelse av den tryck-temperatur –tids (P–T–t) cykel som bergarten genomgått. I Cykladerna, Grekland hittas den Cykladiska blåskiffer enheten (CBU) som består av både eklogit och blåskiffer facies. Processen som har exhumerat dessa bergarter är omdebaterad. Målet med den här avhandlingen är att förstå hur de eocena HP-LT bergarterna exhumerades i centrala Cykladerna baserat på studier av metamorfa bergarter från öarna Naxos och Syros. I den här avhandlingen har en systematisk geotermobarymetrisk och geokronologisk undersökning gjorts på Naxos för att bättre avgränsa den P–T–t väg som bevarats i bergarten. Data visar att högtryck metamorfism på Naxos skedde under eocen ca. 40 Ma och att HP-LT bergarterna exhumerades under två tektoniska skeden. Den första exhumeringen skedde under oligocen. HP-LT bergarterna exhumerades under konvergens av en extruderingskil. Den högre delen av sekvensen nådde grönskiffer facies förhållanden ca. 32 Ma, däremot så förblev den nedre delen av sekvensen på större djup (likställt med tryck 8–12 kbar). Bergarter från sydöstra Syros visar en liknande eocen/oligocen P–T–t historik som den övre sekvensen på Naxos vilket antyder en gemensam eocen/oligocen metamorf historik för centrala Cylkaderna. Den andra exhumeringen skedde under miocen. Bergarterna exhumerades ytterligare under divergens från ca. 20 till 8 Ma. Den övre delen av sekvensen på Naxos var redan i den spröda skorpan och har därför inga spår av den miocena metamorfismen. Den nedre delen av sekvensen var först isotermiskt exhumerad under högtemperatur förhållanden och därefter snabbt nedkyld. 2 | P a g e Résumé Les roches de haute pression et basse température (HP-BT) qui sont observées à la surface de la Terre sont les témoins des zones de subduction passées. Comprendre les processus tectoniques qui contrôlent l’exhumation des roches métamorphiques de HP-BT dans ces zones de subduction nécessite une bonne compréhension du cycle pression-température-temps (P–T–t) auquel ces roches ont été soumises. L’unité des Schistes Bleus Cycladiques (SBC), dans les Cyclades en Grèce, contient des roches en condition de paragenèse éclogitique et des schistes bleus. Cependant, les processus associés à l’exhumation de ces roches sont discutés. L’objectif principal de cette thèse est de comprendre comment les roches de HP-BT formées à l’Éocène ont été exhumées dans la partie centrale des Cyclades, en étudiant l’histoire métamorphique de l’île de Naxos et de Syros. Dans cette thèse, une étude géothermobarométrique et géochronologique des roches métamorphiques à Naxos a permis de contraindre le trajet P–T–t. Les données indiquent que le métamorphisme de haute pression s’est produit autour de 40 Ma à l’Éocène et que ces roches ont été exhumées en deux évènements tectoniques. La première exhumation s’est effectuée pendant l’Oligocène. Les roches de HP-BT ont été exhumées par prisme d’exhumation en contexte de convergence tectonique. Le sommet de la séquence structurelle atteint les conditions de faciès schistes verts à ~32 Ma, alors que le bas de la séquence structurelle reste à des pressions plus élevées (8–12 kbar). De plus, les roches au Sud Est de Syros ont enregistré une histoire P–T similaire à celle enregistrée au sommet de la séquence structurale à Naxos. Ces résultats suggèrent une histoire Éocène/Oligocène commune pour la partie centrale des Cyclades. La deuxième exhumation s’est effectuée durant le Miocène. Les roches ont été exhumées en contexte extensif de ~20 à 8 Ma. Le sommet de la séquence structurale à Naxos était déjà en condition de la croûte fragile et par conséquent n’a pas enregistré l’histoire Miocène. Le bas de la séquence a été initialement exhumé de façon isotherme à des conditions de haute température, puis a refroidi rapidement. 3 | P a g e List of papers and author contributions This thesis is based on the following papers: I Peillod, A., Ring, U., Glodny, J. and Skelton, A. (2017) 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. Journal of Metamorphic Geology 35, 805–830 II Peillod, A., Ring, U., Skelton, A., Linnros.