Petrogenetic link between metasedimentary rocks, migmatites and granitoids in the Variscan basement of the Pontgibaud area, French Massif Central - implications for the crustal structure

2025

Article référencé dans le Web of Science WOS:001531720700001

do Carmo C. P., Vanderhaeghe O., Laurent O., Berger J., Bellanger M., Leisen Mathieu, Chmeleff J.

BSGF - Earth Sciences Bulletin, 2025, 196, p. 13 [32 p.] ISSN 0037-9409

The architecture of the crust in the French Massif Central (FMC) is described as a nappe stack composed, from top to bottom, of the Upper Gneiss Unit (UGU), the Lower Gneiss Unit (LGU), and the Para-Autochthonous Unit (PAU), which are intruded by Carboniferous granitic plutons. In the Pontgibaud region, the Upper Gneiss Unit/Lower Gneiss Unit (UGU/LGU) attribution and the relationship between metamorphic and magmatic rocks are uncertain. This hinders both our understanding of the geodynamic processes that led to the construction of the Variscan orogenic belt, and a precise assessment of the deep geothermal potential of this high-heat flow area. Our field investigations document a metamorphic gradient with, from South to North, micaschists and paragneiss with a gently, dominantly southward-dipping foliation grading into migmatites that display a steeper foliation. A maximum late Ediacaran deposition age of sedimentary protoliths is determined from LA-ICPMS U-Pb dates of the youngest detrital zircon populations that range from 588.9 +/- 7.1 to 571.8 +/- 8.2 Ma. Equilibrium phase diagrams calculated for selected samples document a metamorphic gradient from 2.3- 5.3 kbar, and 570 degrees-660 degrees C, characterizing a mid- to low pressure-high temperature event dated from 357.6 +/- 1.5 to 342.2 +/- 4.6 Ma by LA-ICP-MS U-Pb analyses of metamorphic monazite. The contact between the metasedimentary rocks and migmatites is crosscut by peraluminous, cordierite-biotite-bearing Claveix and Gelles granitic plutons dated at 338.0 +/- 7.3 Ma and 323.3 +/- 9.7 Ma, respectively by U-Pb on zircon. The distribution of U-Pb dates and Lu-Hf isotopic compositions of detrital zircon in metamorphic rocks; the absence of HP mineral relics; the age of HT metamorphism of ca. 357-345 Ma; and the lack of a major mylonitic tectonic contact, which would typify the base of UGU, collectively support that the exposed crust of the Pontgibaud area corresponds to LGU metamorphic rocks. Moreover, the mineralogy, bulk chemistry and zircon Lu-Hf isotopic composition of the granites and migmatites are consistent with an origin by partial melting of LGU metasedimentary rocks. These data show that the Pontgibaud area exposes a section along a Carboniferous MP-LP/HT metamorphic gradient from micaschists to migmatites. Genesis and extraction of a large volume of granitic magma require higher temperatures (>800 degrees C) that were not reached by the exposed rocks. Accordingly, we propose that the local mid- and lower crustal levels are made of rocks with a similar protolith as those exposed at the surface, but affected by higher metamorphic grade and magma extraction.

Géologie et formations superficielles [064]

FRANCE ; MASSIF CENTRAL

Fonds IRD [F B010094366]

fdi:010094366