In orogens worldwide and throughout geologic time, large volumes of deep continental crust have been exhumed in domal structures. Extension-driven ascent of bodies of deep, hot crust is a very efficient mechanism for rapid heat and mass transfer from deep to shallow crustal levels and is therefore an important mechanism in the evolution of continents. The dominant rock type in exhumed domes is quartzofeldspathic gneiss (typically migmatitic) that does not record its former high-pressure (HP) conditions in its equilibrium mineral assemblage; rather, it records the conditions of emplacement and cooling in the mid/shallow crust. Mafic rocks included in gneiss may, however, contain a fragmentary record of a HP history, and are evidence that their host rocks were also deeply sourced. An excellent example of exhumed deep crust that retains a partial HP record is in the Montagne Noire dome, French Massif Central, which contains well-preserved eclogite (garnet+omphacite+rutile+quartz) in migmatite in two locations: one in the dome core and the other at the dome margin. Both eclogites record P ~ 1.5 ± 0.2 GPa at T ~ 700 ± 20°C, but differ from each other in whole-rock and mineral composition, deformation features (shape and crystallographic preferred orientation, CPO), extent of record of prograde metamorphism in garnet and zircon, and degree of preservation of inherited zircon. Rim ages of zircon in both eclogites overlap with the oldest crystallization ages of host gneiss at c. 310 Ma, interpreted based on zircon rare earth element abundance in eclogite zircon as the age of HP metamorphism. Dome-margin eclogite zircon retains a widespread record of protolith age (c. 470–450 Ma, the same as host gneiss protolith age), whereas dome-core eclogite zircon has more scarce preservation of inherited zircon. Possible explanations for differences in the two eclogites relate to differences in the protolith mafic magma composition and history and/or the duration of metamorphic heating and extent of interaction with aqueous fluid, affecting zircon crystallization. Differences in HP deformation fabrics may relate to the position of the eclogite facies rocks relative to zones of transpression and transtension at an early stage of dome development. Regardless of differences, both eclogites experienced HP metamorphism and deformation in the deep crust at c. 310 Ma and were exhumed by lithospheric extension—with their host migmatite—near the end of the Variscan orogeny. The deep crust in this region was rapidly exhumed from ~50 to <10 km, where it equilibrated under low-P/high-T conditions, leaving a sparse but compelling record of the deep origin of most of the crust now exposed in the dome.
Bibliographical noteFunding Information:
We acknowledge funding from the National Science Foundation (EAR-1050020) to Teyssier and Whitney, and research support 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. The X-Ray Computed Tomography facility was funded by a UMN Infrastructure Investment Initiative grant. EBSD analyses were carried out in the Characterization Facility of the College of Science and Engineering, University of Minnesota, which receives partial support from NSF through the MRSEC programme. We thank Andrew Kylander-Clark for assistance with LA-ICPMS analyses at UC-Santa Barbara, C. Mark Fanning for assistance with SHRIMP analyses at the Australian National University, Ashley Steiner and Charles Knaack for assistance with XRF and ICP-MS analyses at WSU, and Besim Dragovic and Philippe Goncalves for advice about phase diagram calculations. We are grateful to reviewers Chris Mattinson and Pavel Pitra for their thorough comments that helped us improve the paper. Finally, we thank Mr. Daniel Daures of Le Teil Farm for helping us search for eclogite in the Cabard?s area, we thank Mr. Robert Pistre and the Centre de Recherche du Patrimoine de Rieumontagn? (CRPR), as well as the association Les Amis des Sciences de la Nature (ASNAT), for having preserved, digitized, and made available a number of Professor M. Demange's unpublished maps and documents that proved critical in our quest for the Montagne Noire eclogites.
© 2020 John Wiley & Sons Ltd
- Montagne Noire
- deep crust
- gneiss dome