Using petrochronology to re-investigate the age of the HP metamorphism in the French Massif Central

Abstract

<p>The P–T–t evolution of eclogite samples from a locality of the French Massif Central where a Silurian age for the high-pressure metamorphism is commonly accepted is reinvestigated. Petrology combined with LA-ICP-MS U-Pb dating and trace-element analysis in zircon and apatite discard the Silurian age and rather reveal an Ordovician (c. 490 Ma) rifting, a Devonian (c. 370 to 360 Ma) subduction and a Carboniferous (c. 350 Ma) exhumation in this part of the French Massif Central.</p> <p>The petrological study using pseudosection document a prograde evolution in the eclogite facies marked by an increase of pressure above 20 kbar associated with a strong temperature increase from 650 to 850 °C. Peak-temperature and incipient decompression to the high-pressure granulite facies (19-20 kbar and 875°C) were accompanied by partial melting of the eclogite. Further decompression resulted in partial equilibration in the high-temperature amphibolite facies (<9 kbar, 750-850°C). Local fractures filled by analcite and thomsonite testify to late interaction with alkaline fluids. Metamorphic zircon with eclogitic REE patterns (no Eu anomaly, flat HREE) and inclusions (garnet, rutile and probably omphacite) shows concordant apparent ages that spread from c. 370 down to c. 310 Ma. A c. 350 Ma age of apatite attributed to cooling following decompression from the eclogite facies indicates that zircons younger than 350 Ma, were rejuvenated but preserved an apparent eclogitic signature. It is suggested that interaction with alkaline fluids at low temperatures would lead to the recrystallisation of zircon while leaving apatite unaffected.</p> <p>Comparison with available P–T–t data from eclogites in Western Europe shows that Devono-Carboniferous high-temperature eclogites are also recognized in the Saxo-Thuringian and Moldanubian zones of the Bohemian Massif suggesting they belonged to the same subducting bloc. Devono-Carboniferous trench/arc and arc/back-arc relationships recognized in the Bohemian Massif and the French Massif Central respectively point to a southward subduction in both areas. This comparison challenges the historical interpretation of a northward subduction in France and brings an overall more coherent picture of the Variscan belt.</p>

Short-lived active margin magmatism preceding Variscan collision in the Western French Massif Central

“…Extension, sedimentation magmatism related to a back-arc or arc setting locally forming incomplete and dismembered "ophiolites" (Figs. 1 and 10) or basaltic lava flows interbedded with Devonian sediments are also preserved in the Armorican massif (but the igneous products are not dated; Thieblemont and Cabanis, 1994;Ducassou et al, 2011), the Eastern French Massif Central (371-357 Ma, uncertainties Bertrand et al (2001), Pin and Paquette (2002) and this study; age of Brévenne magmatism is from Pin and Paquette (1997), recent ages U-Pb zircon and Sm-Nd/Lu-Hf garnet ages obtained on eclogites are from Lotout et al (2018Lotout et al ( , 2020) and de Hoÿm de Marien et al (2023), UGU anatexis as dated by U-Th-Pb monazite chemical age is from Melleton et al (2009) and the curve indicating the relative igneous activity is from Chopin et al (2023) traced with data compiled by Vanderhaeghe et al (2020). (b) Tentative snapshot of the geodynamic situation around 360 Ma for the Western French Massif Central, with emphasis on diorites-tonalites, eclogites and migmatitic rocks belonging to the UGU (see text for explanations).…”

“…It is interesting to note that reinvestigations of ID-TIMS zircon eo-variscan ages obtained by Paquette et al (1995) on southern FMC eclogites with the in-situ LA-ICP-MS method (Paquette et al, 2017) did not reproduced the eo-variscan Early Devonian ages. Moreover, recent geochronological investigations on FMC eclogites (including new investigations on previously dated occurrences) involving in situ U-Pb dating, Lu-Hf and Sm-Nd garnet dating (Lotout et al, 2018(Lotout et al, , 2020Benmammar, 2021;de Hoÿm de Marien et al, 2023) all yielded Middle Devonian to Lowermost Carboniferous ages (Fig. 10).…”

“…This belt formed after Cambro-Ordovician rifting of the Northern Gondwanan margin and opening of (an) oceanic domain(s), an event is coeval with the formation of large volume of predominantly felsic magmatic rocks now forming Furongian-Ordovician orthogneiss in metamorphic units. The Devonian convergence, accommodated by subduction between Laurussia and Gondwana, is evidenced by numerous eclogites whose age have been re-evaluated recently at Middle Devonian to Lowermost Carboniferous times (Lotout et al, 2018(Lotout et al, , 2020Benmammar, 2021;de Hoÿm de Marien et al, 2023). Final collision of continental blocks occurred during the Lower Carboniferous and triggered a massive phase of syn-to late-orogenic, dominantly silicic magmatism derived from crustal and mantle sources and lasting from about 360 Ma to 300 Ma (Vanderhaeghe et al, 2020).…”

“…One of the key arguments to favor/support a particular model is the age of the high pressure event(s). Recent dating performed on high pressure rocks (see Paquette et al, 2017;Lotout et al, 2018Lotout et al, , 2020de Hoÿm de Marien et al, 2023) suggest that eo-variscan 430-390 Ma ages obtained in the past for the HP metamorphism in the French Massif Central (see Faure et al, 2009) are artifacts and that most if not all the eclogites and related HP units could be related to a single Upper Devonian-Lowermost Carboniferous high pressure event as it is generally recognized in the Armorican massif (Bosse et al, 2000(Bosse et al, , 2005Ballèvre et al, 2014). There is less ambiguity on the age of supra-subduction magmatism spanning from 379 to 355 Ma for calc-alkaline plutons (Bernard-Griffith et al, 1985;Cuney et al, 1993;Bertrand et al, 2001;Pin and Paquette, 2002;Fig.…”

Deciphering the nature and age of the protoliths and peakP−Tconditions in retrogressed mafic eclogites from the Maures-Tannneron Massif (SE France) and implications for the southern European Variscides

“…However, considering the spread of U-Pb data between the two populations, it is also possible that all these analyses do not define a statistically valid age but result from a natural spread of dates, in addition to analytical uncertainties, which is actually reported for metamorphic zircons in Variscan eclogites (e.g. Schmädicke et al, 2018;De Hoÿm de Marien, 2019;Pitra et al, 2022).…”