PETROGRAPHICAL PATH VS P-T-TIME PATH OF AN UNUSUAL HB-KY-GT CORONITIC ASSEMBLAGE OF THE METANORITE FROM ARVIEU (FRENCH MASSIF CENTRAL)

A critical problem in polycyclic context is to demonstrate whether a PT path is ascribed to a single thermal event or it results from the superposition of distinct events: Is the "petrographical path" consistent with the real PTt evolution? An eclogitized metanorite from Arvieu (French Massif Central) preserves a spectacular coronitic texture of hornblende, garnet, kyanite, plagioclase and quartz produced at the expense of the primary magmatic assemblage orthopyroxene and plagioclase. This texture can be interpreted as the result of two different PT evolutions with strongly different geodynamic implications. Their validity is evaluated using a quantitative petrogenetic grid and pseudosections constructed with Perple_X-03 in the CFMASH system and suitable geochronological data. The sequence of reactions involving the formation of Hb-Ky-Gt-Q coronitic assemblages are consistent with a simple isobaric cooling at HP (~ 1 GPa) under hydrated conditions. This PT path inferred only using petrographical observations is described as a "petrographical path". However, geochronological constraints (gabbro emplacement at 490 Ma and HP metamorphism at 410 Ma) are inconsistent with the HP continuous evolution. In order to reconcile petrographical observations with geochronological constraints, we propose a discontinuous two stage evolution involving a dramatic change in water activity. (1) Emplacement and cooling of the norite at LP (< 3 kbar) under anhydrous conditions favored the preservation of the magmatic assemblage until LT (350°C). (2) During the hercynian orogeny (~ 410 Ma), the metanorite experienced a prograde HP metamorphism under fluid present conditions. Adding water to the system implies a dramatic change in the grid topology, like restricting the Opx-An assemblage to HT. Therefore, the breakdown of the unstable magmatic assemblage, through apparent retrograde reactions, occurred along the prograde PT path without crossing the equilibrium boundaries of these reactions. Accordingly, the simple continuous "petrographical path" is incorrect, if the "real PTt path" does not cross the equilibrium boundaries of the reactions. The "petrographical path" is then a “fictif path” with no geological meaning.