Pressure-temperature conditions and significance of Upper Devonian eclogite and amphibolite facies metamorphisms in southern French Massif central

Abstract

The southwestern French Massif central in western Rouergue displays an inverted metamorphic sequence with eclogite and amphibolite facies units forming the top of the nappe stack. They are often grouped into the leptyno-amphibolite complex included, in this area, at the base of the Upper Gneiss Unit. We sampled garnet micaschists and amphibolites to investigate their metamorphic history with isochemical phase diagrams, thermobarometry and U-Pb zircon dating. Our results demonstrate that two different tectono-metamorphic units can be distinguished. The Najac unit consists of biotite-poor phengite-garnet micaschists, a basic-ultrabasic intrusion containing retrogressed eclogites and phengite orthogneisses. Pressure and temperature estimates on micaschists with syn-kinematic garnets yield a prograde with garnet growth starting at 380 degrees C/6-7 kbar, peak pressure at 16 kbar for 570 degrees C, followed by retrogression in the greenschist facies. The age of high pressure metamorphism has been constrained in a recent publication between ca. 383 and 369 Ma. The Laguepie unit comprises garnet-free and garnet-bearing amphibolites with isolated lenses, veins or dykes of leucotonalitic gneiss. Thermobarometry and phase diagram calculation on a garnet amphibolite yield suprasolidus peak P-T conditions at 710 degrees C, 10 kbar followed by retrogression and deformation under greenschist and amphibolite facies conditions. New U-Pb analyses obtained on igneous zircon rims from a leucotonalitic gneiss yield an age of 363 +/- 3 Ma, interpreted as the timing of zircon crystallization after incipient partial melting of the host amphibolite. The eclogitic Najac unit records the subduction of a continental margin during Upper Devonian. It is tentatively correlated to a Middle Allochthon, sandwiched between the Lower Gneiss Unit and the Upper Gneiss Unit. Such an intermediate unit is still poorly defined in the French Massif central but it can be a lateral equivalent of the Groix blueschists in the south Armorican massif. The Uppermost Devonian, amphibolite facies Laguepie unit correlates in terms of P-T-t evolution to the Upper Gneiss Unit in the Western French Massif central. This Late Devonian metamorphism is contemporaneous with active margin magmatism and confirms that the French Massif central belonged to the continental upper plate of an ocean-continent subduction system just before the stacking of Mississippian nappes.