Orogenic eclogites record relative magnitude of deep crustal flow and extent of migmatite-eclogite interaction

Clementine Hamelin, Donna L. Whitney, Françoise Roger, Christian Teyssier

Research output: Contribution to journalArticlepeer-review

Abstract

In exhumed orogens, refractory mafic rocks have the potential to preserve a record of petrogenesis and high-pressure (HP) metamorphism that is commonly obliterated in quartzofeldspathic rocks owing to re-equilibration at high-temperature, low-pressure (LP–HT) conditions. In the Montagne Noire (France) migmatite dome, located in the foreland of the Variscan orogen, eclogite is exposed in both the core and margin of the dome. In this study, we combine in situ U–Pb petrochronology and oxygen-isotope analyses of key eclogite phases to demonstrate that eclogites from the two distinct domains had different protoliths and source regions, traveled relatively variable distances in the deep crust, and differentially interacted with surrounding migmatite prior to exhumation. Dome-margin eclogite zircons are small (∼40 μm) with well-preserved inherited cores and thin (<15 μm) rims, compared to larger (40–120 μm) neo- and recrystallized dome-core zircons with small relict cores and wide (15–30 μm) recrystallized rims. Protolith and HP metamorphism ages were determined using in situ zircon and rutile petrochronology (LASS-ICP-MS). Both eclogites formed in a continental setting; dome margin protolith zircon cores formed at 442.5 ± 3.4 Ma (steep HREE slope, no Eu-anomaly) whereas zircon cores of the dome-core eclogites yielded scattered dates suggesting protolith crystallization between ∼500–400 Ma (steep HREE slope, pronounced Eu-anomaly). Both eclogites experienced HP metamorphism at c. 320–310 Ma in garnet-stable, plagioclase-absent conditions. Most analyzed rutile yielded dates of 307–304 Ma associated with cooling. The record of HP fluid conditions was determined by O-isotope (SIMS) analyses of garnet and zircon. Dome-margin zircon cores and rims have δ18O of ∼8.2–8.5 ‰, indistinguishable within uncertainty, in isotopic equilibrium with isotopically unzoned garnet (δ18O ∼ 8.0–8.2 ‰). In contrast, zircons in dome-core eclogites have systematically lower zircon-core δ18O values compared to their rims and neocrystallized grains, and zircon cores were in equilibrium with major-cation zoned garnet with respect to oxygen. The two dome-core eclogite samples yielded zircon and garnet δ18O values of ∼8.6–9.5 ‰ and ∼ 9.7–10.5 ‰. Based on these results and existing HP fabric data for these eclogites, we propose that (1) gabbro protoliths for the two eclogites were emplaced at different depths in a Cambro-Ordovician continental crustal package; and (2) dome-core eclogites interacted extensively with surrounding gneiss during burial and foreland-vergent crustal flow, whereas the dome-margin eclogite was sourced proximally to the dome-emplacement location and had minimal chemical interaction with surrounding gneiss. At least parts of the Montagne Noire migmatite dome were deeply sourced, but rocks exhumed in the core had a more extensive and protracted history of deep-crustal flow than deep-crustal rocks exhumed at the margin.

Original languageEnglish (US)
Article number106917
JournalLITHOS
Volume434-435
DOIs
StatePublished - Dec 15 2022

Bibliographical note

Funding Information:
Funding for this project comes from NSF grants EAR-1050020 and EAR-1946911 to Whitney and Teyssier, as well as funding from the College of Science and Engineering at the University of Minnesota. Funding for the electron microprobe facility used in this research was provided by NSF grant EAR-1625422. SIMS analyses were carried out at the University of Wisconsin-Madison department of Geology, WiscSIMS is supported by NSF (EAR-2004618) and the University of Wisconsin-Madison. We thank Patrice Rey for collaborative field work to locate and collect the dome-core eclogites, Andrew Kylander-Clark for assistance with LASS-ICP-MS analyses and data processing at the University of Santa Barbara, and Natalie H. Raia as SHRIMP-II zircon data analyst. We also thank John V. Valley and Michael Spicuzza at the University of Madison-Wisconsin, as well as Michelle Gevedon for advice and assistance with sample preparation and Kouki Kitajima for remote analytical session planning, instrument tuning, and SIMS O-isotope data collection. We are grateful for reviews from Chris Clark and Philippe Goncalves, their comments and suggestions helped improve the paper. We would also like to thank Mr. Daniel Daures of Le Teil Farm for his help with the search and collection of eclogite samples from the Cabardès area; Mr. Robert Pistre and the Centre de Recherche du Patrimoine de Rieumontagné (CRPR), as well as the association Les Amis des Sciences de la Nature (ASNAT), for their work archiving the work carried out by M. Demange during his career, including unpublished maps and documents, all of which were an invaluable resource in our work in the Montagne Noire.

Funding Information:
Funding for this project comes from NSF grants EAR-1050020 and EAR-1946911 to Whitney and Teyssier, as well as funding from the College of Science and Engineering at the University of Minnesota. Funding for the electron microprobe facility used in this research was provided by NSF grant EAR-1625422. SIMS analyses were carried out at the University of Wisconsin-Madison department of Geology, WiscSIMS is supported by NSF (EAR-2004618) and the University of Wisconsin-Madison . We thank Patrice Rey for collaborative field work to locate and collect the dome-core eclogites, Andrew Kylander-Clark for assistance with LASS-ICP-MS analyses and data processing at the University of Santa Barbara, and Natalie H. Raia as SHRIMP-II zircon data analyst. We also thank John V. Valley and Michael Spicuzza at the University of Madison-Wisconsin, as well as Michelle Gevedon for advice and assistance with sample preparation and Kouki Kitajima for remote analytical session planning, instrument tuning, and SIMS O-isotope data collection. We are grateful for reviews from Chris Clark and Philippe Goncalves, their comments and suggestions helped improve the paper. We would also like to thank Mr. Daniel Daures of Le Teil Farm for his help with the search and collection of eclogite samples from the Cabardès area; Mr. Robert Pistre and the Centre de Recherche du Patrimoine de Rieumontagné (CRPR), as well as the association Les Amis des Sciences de la Nature (ASNAT), for their work archiving the work carried out by M. Demange during his career, including unpublished maps and documents, all of which were an invaluable resource in our work in the Montagne Noire.

Publisher Copyright:
© 2022 The Authors

Keywords

  • Crustal flow
  • Deep crust
  • Eclogite
  • Fluid-rock interactions
  • Oxygen-isotopes
  • U-Th-Pb petrochronology

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