The Variscan metamorphism in the Pyrenees is dominantly of the low-pressure-high-temperature (LP-HT) type. The relics of an earlier, Barrovian-type, metamorphism that could be related to orogenic crustal thickening are unclear and insufficiently constrained. A microstructural and petrological study of micaschists underlying an Ordovician augen orthogneiss in the core of the Canigou massif (Eastern Pyrenees, France) reveal the presence of two syntectonic metamorphic stages characterized by the crystallization of staurolite (M1) and andalusite (M2), respectively. Garnet is stable during the two metamorphic stages with a period of resorption between M1 and Accepted Article This article is protected by copyright. All rights reserved. M2. The metamorphic assemblages M1 and M2 record similar peak temperatures of 580 °C at different pressure conditions of 5.5 kbar and 3 kbar, respectively. Using chemical zoning of garnet and calculated P-T pseudosections, a prograde P-T path is constrained with a peak pressure at ~6.5 kbar and 550 °C. This P-T path, syntectonic with respect to the first foliation S1, corresponds to a cold gradient (of ~9 °C/km), suggestive of crustal thickening. Resorption of garnet between M1 and M2 can be interpreted either in terms of a simple clockwise P-T path or a polymetamorphic two stage evolution. We argue in favour of the latter, where the medium-pressure (Barrovian) metamorphism is followed by a period of significant erosion and crustal thinning leading to decompression and cooling. Subsequent advection of heat, probably from the mantle, lead to a new increase in temperature, coeval with the development of the main regional fabric S2. LA-ICP-MS U-Th-Pb dating of monazite yields a well-defined date at c. 300 Ma. Petrological evidence indicates that monazite crystallization took place close to the M1 peak-pressure conditions. However, the similarity between this age and that of the extensive magmatic event well documented in the eastern Pyrenees suggests that it probably corresponds to the age of monazite recrystallization during the M2 LP-HT event.

The P–T evolution of a mafic eclogite sample from the Haut-Allier was studied in order to constrain the dynamic of the Variscan subduction in the eastern French Massif Central. Three successive metamorphic stages M1, M2 and M3, are characterized by assemblages comprising garnet1-omphacite-kyanite, garnet2-plagioclase, and amphibole-plagioclase, respectively, and define a clockwise P–T path. These events occurred at the conditions of eclogite (M1; ∼ 20 kbar, 650 °C to ∼ 22.5 kbar, 850 °C), high-pressure granulite (M2; 19.5 kbar and 875 °C) and high-temperature amphibolite facies (M3; < 9 kbar, 750–850 °C), respectively. Pseudosection modelling of garnet growth zoning and mineralogy of the inclusions reveal a prograde M1 stage, first dominated by burial and then by near isobaric heating. Subsequent garnet1 resorption, prior to a renewed growth of garnet2 is interpreted in terms of a decompression during M2. High-pressure partial melting is predicted for both the M1 temperature peak and M2. M3 testifies to further strong decompression associated with limited cooling. The preservation of garnet growth zoning indicates the short-lived character of the temperature increase, decompression and cooling cycle. We argue that such P–T evolution is compatible with the juxtaposition of the asthenosphere against the subducted crust prior to exhumation driven by slab rollback.

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<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>

Two eclogite samples from the Haut-Allier record a prograde evolution from $20 kbar, 650 C to 750 C, 22-23 kbar followed by heating up to 850-875 C and partial melting. Incipient decompression in high-pressure granulite facies conditions (19.5 kbar, 875 C) was followed by exhumation to high-temperature amphibolite facies conditions (<9 kbar, 750-850 C). Following a detailed geochemical, petrological, and geochronological investigation using trace-element data and laser ablation inductively coupled plasma mass spectrometry U-Pb dating of zircon, apatite, and rutile, the eclogites reveal an Ordovician (c. 490 Ma) rifting event followed by Devonian (c. 370-360 Ma) subduction and Carboniferous (c. 350 Ma) exhumation in this part of the French Massif Central. The previously proposed Silurian age for the subduction, which strongly influenced many tectonic models, is definitively rejected. In the light of other geological data from the French Massif Central, including the lithological and geochemical zoning of calc-alkaline Devonian volcanism, we propose a southward polarity of the subduction and question the very existence of the so-called Massif Central Ocean. Furthermore, we infer that following subduction, the eclogites were relaminated to the upper plate and exhumed at the rear of the magmatic arc pointing to similarities with the geodynamics of the Bohemian Massif. The petrochronological record of zircon is particularly complex. Metamorphic zircon with clear eclogitic rare-earth elements patterns (no Eu anomaly and flat heavy rare-earth elements) and inclusions (garnet, rutile, and omphacite)shows concordant apparent ages that spread from c. 380 down to c. 310 Ma. This apparent age pattern strongly contrasts with the well-defined age of apatite and rutile of c. 350 Ma. Apparent zircon ages younger than 350 Ma unequivocally testify that zircon can recrystallize outside the conditions of the eclogite facies, which resets the U-Pb while preserving an apparent eclogitic signature. Local fractures filled by analcite, thomsonite, plagioclase, and biotite testify to late interaction of the eclogites with alkaline fluids at relatively low temperatures. This interaction, possibly at c. 310 Ma or later, could lead to the recrystallization of zircon while leaving apatite unaffected.