Carboniferous high‐P metamorphism and deformation in the Belledonne Massif (Western Alps)

Abstract

The age and P-T conditions of Variscan high-P(HP) metamorphism in the Palaeozoic basement of the western Alps remain poorly constrained, but is nevertheless crucial to build a consistent tectonic scenario for the southeastern domain of the Variscan Belt. We report here the results of a structural, petrological, thermobarometric, and geochronological investigation of an eclogite-bearing unit exposed in the northeastern part of the Belledonne Massif (France). This unit is mostly composed of metasediments that are locally migmatized and contain decametre-to hectometre-scale lenses of orthogneiss and amphibolites. SIMS U-Pb dating of magmatic zircon cores in two retrogressed ecogites yields ages at 456 ± 4 Ma and 448 ± 6 Ma, which are interpreted to date the emplacement of the magmatic protoliths. The peak pressure stage in the retrogressed eclogites is estimated to be >1.4 GPa at 690-740°C, and was followed by decompression from 1.4 to ca. 1.0 GPa at 700-800°C. By contrast, the investigated migmatitic metasediment does not present any trace of HP metamorphism, but instead preserves prograde evolution from sub-solidus conditions (ca. 0.8-1.1 GPa and 600-700°C) to supra-solidus conditions (1.1-1.4 GPa and 700-780°C). A later stage of retrogression below ca. 0.5-0.8 GPa and 570-610°C is recorded in both lithologies, and is taken to indicate cooling and exhumation to upper crustal levels. Metamorphism was roughly coeval in the retrogressed eclogites and in the migmatitic metasediment. Metamorphic zircon rims yield U-Pb dates scattering between 340 and 310 Ma in both lithologies. In the migmatitic metasediment, a distinct younger age at 306 ± 3 Ma is interpreted to represent late stages of melt crystallization. In the retrogressed eclogites, zircon zoning and chemical composition (Th/U and REE) indicate initial crystallization of the rims during the HP stage followed by protracted growth during decompression to granulitic/amphibolitic conditions. Rutile U-Pb dating in one eclogite sample yields an age of 340 ± 11 Ma similar to the oldest zircon ages and is interpreted to approximate the age of the peak pressure metamorphism. Retrogression in the amphibolite facies is correlated with the development of a penetrative, N30° subvertical mylonitic S2 foliation. Regionally, this deformation occurs in a dextral transpressive corridor interpreted to represent a crustal-scale shear zone active during the mid-late Carboniferous. We therefore wileyonlinelibrary.com/journal/jmg

Two Contrasting Exhumation Scenarios of Deeply Subducted Continental Crust in North Pakistan

“…Based on the above comparison, possibly there exists a hidden fault from Joranar to Saif-ul-Muluk to somewhere near Naran. The eclogite units in Western Himalayan Syntaxis are not coherent and they show distinct temporal and spatial change, indicating the difference of the continental exhumation, which is similar to the whole Himalaya Zhang et al, 2013), the Alps (Jean-Baptiste et al, 2021;Malusà et al, 2015;Schlunegger & Willett, 1999), and the famous continental UHP Dabie-Sulu orogenic belt (Zheng et al, 2009).…”

“…Combined with the differences mentioned above, the multi-slice exhumation mechanism and uplift history can be inferred, which was probably related to the break-off of the Indian lithospheric slab undernearth. Possible scenarios of continental collision and their relation to the mechanisms of exhumation of HP and UHP rocks have already been discussed in previous studies (e.g., Burov et al, 2014;Jean-Baptiste et al, 2021). Due to the rheological complexity of the continental lithosphere, mechanisms of continental convergence are still controversial.…”

A window into an older orogenic cycle: P–T conditions and timing of the pre‐Alpine history of the Dora‐Maira Massif (Western Alps)

“…1. Variscan HP relicts are known in the External Crystalline Massifs (Belledonne and Argentera), where HP metamorphism in eclogites and granulites is dated at 350-330 Ma (Jacob et al, 2021;Rubatto et al, 2010). HP metamorphism in metapelites and eclogites is dated at 350-300 and 390-355 Ma in the northern and middle Adula Nappe, respectively (Liati et al, 2009;Sandmann et al, 2014).…”

Reconstructing the Variscan Terranes in the Alpine Basement: Facts and Arguments for an Alpidic Orocline

“…In the Helvetic zone, the Variscan basement is exposed in the External Crystalline massifs of Argentera, Pelvoux, Grandes Rousses, Belledonne, M t -Blanc, Aiguilles Rouges, and Aar-Gothard (Figures 5 and 6). Due to the weakness of the Alpine overprint, the pre-Permian geology of the External Crystalline massifs has been investigated since a long time (e.g., [4,9,13,36,38,[53][54][55][56][57][58][59]). However, the widespread crustal melting erased the early structures.…”

“…With respect to the early Variscan structures, the southern part of the Belledonne massif appears as one of the key areas. Since it has been recently investigated, only a brief outline is provided below (for details, see [57,59]).…”