Deep crustal source of gneiss dome revealed by eclogite in migmatite (Montagne Noire, French Massif Central)

Abstract

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.

Late Variscan (315 Ma) subduction or deceptive zircon REE patterns and U–Pb dates from migmatite‐hosted eclogites? (Montagne Noire, France)

“…The temperature estimates, both from the pseudosections (~700-750°C) and the Ti-in-zircon thermometry (~670-780°C), are compatible with the results of the Zr-in-rutile thermometer of Tomkins et al (2007) obtained by Whitney et al (2015;~670-780°C). On the other hand, the markedly lower pressure inferred by Whitney et al (2015), Whitney et al (2020) can be related to the use of an inappropriate chemical system (not including Fe 3+ and H 2 O) and/or outdated mixing models (especially for clinopyroxene). In particular, considering an H 2 O-absent system results in overestimating the stability of anhydrous assemblages towards low temperatures and pressures, and a generally unrealistic topology (e.g.…”

Extreme metamorphism and metamorphic facies series at convergent plate boundaries: Implications for supercontinent dynamics

“…The highest thermobaric ratio of 518 °C/GPa was obtained for eclogite inclusions hosted by migmatites in the cores of gneiss domes at Montagne Noire in the French Massif Central (Whitney et al, 2015). However, phase equilibrium calculations yielded metamorphic conditions of ~680 °C and 1.6-1.7 GPa for the dome-margin eclogite (Whitney et al, 2020). This results in metamorphic thermobaric ratios of 400-425 °C/GPa, corresponding to metamorphic thermal gradients of 12.0-12.8 °C/km.…”

“…This results in such occurrences where HP to UHP eclogites and HP granulite-facies rocks are hosted by felsic gneisses with domical structures in collisional orogens, such as the Western Gneiss Region in Norway (Cuthbert et al, 2000;Walsh and Hacker, 2004), the European Variscides (e.g., O'Brien and Carswell, 1993;Stipska et al, 2008;Gaggero et al, 2009;Whitney et al, 2015), the North Qaidam in western China (Mattinson et al, 2006), the Dabie-Sulu in eastern China (Wang et al, 2011;Groppo et al, 2015;Ji et al, 2017), the Asian Himalayas (Groppo et al, 2007;Cottle et al, 2009;Corrie et al, 2010), and the Woodlark Rift in Papua New Guinea (Baldwin et al, 2008;Little et al, 2011). Host gneisses typically do not record eclogite-facies HP to UHP metamorphic conditions, leading to long-standing controversy about the relationship between eclogite inclusions and host gneiss (Eskola, 1921;Lappin and Smith, 1978;Brueckner, 2018;Whitney et al, 2020). Geochronological studies indicate that high-grade metamorphic rocks in collisional orogens may experience two to three episodes of polymetamorphism at contrasting thermal gradients.…”

Ultrahigh-temperature granulite-facies metamorphism and exhumation of deep crust in a migmatite dome during late- to post-orogenic collapse and extension in the central Adirondack Highlands (New York, USA)

“…These could be separate domes or, alternately, could represent a double dome structure like Montagne Noire in the French Massif Central or the Naxos dome (Cyclades Islands, Greece) where two sub-domes are separated by a near-vertical high-strain zone (Fig. 2; Rey et al, 2011;Whitney et al, 2020). If the latter, it will be important to locate and document a high-strain zone between Ledge Mountain and the Marcy massif as well as the domal foliation that would necessarily surround this south-central region of the Highlands (Figs.…”