" Magmatisme, Métamorphisme et minéralisations associées  (3MA)", Agadir, Mai 5-7, 2005

ARE REACTION TEXTURES RELIABLE GUIDES TO TRACE PTt PATHS ?

NICOLLET Christian (1), and GONCALVES Philippe (2)
1 Laboratoire Magmas et Volcans, OPGC - UBP - CNRS, 5, rue Kessler, 63 038 Clermont-Ferrand cedex, France (c.nicollet@opgc.univ-bpclermont.fr)
2 Département de Géosciences, Université de Franche-Comté, 16, route de Gray, 25 030 Besançon cedex, France

Assemblages in coronitic and reactional textures are commonly used to characterize a portion of PT(t) path. However, caution must be used in their interpretation in terms of PT path and tectonic mechanisms. Three examples are used to point out that metamorphic reactions are likely produced by a great variety of PT paths .

1 – Rubie(1990) emphasized: "the timing of mineral reactions will not necessarily depend on when equilibrium boundaries are crossed in P-T space, but rather on when fluid becomes available at reaction sites". This assumption is well documented by the mafic rocks from Beforona ( Madagascar ), where cm-size veins consisting of cpx-gt +/-hbl, bt and Mg-Fe carbonates occur in anhydrous garnet-free granulites (opx-cpx-pl-q). The local development of hydrated HP assemblages in the veins is not related to variations of P and/or T, but, in contrast, to a local fluid ( H 2 O + CO 2 ) influx.

2 – A reaction boundary could never be crossed : Eclogitized metanorites from the French Massif Central preserve coronitic textures of hornblende, garnet, kyanite, plagioclase and quartz produced at the expense of the primary magmatic assemblage orthopyroxene and plagioclase. This texture is interpreted as the result of a two stage PT evolutions model (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. 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 .

3 – A portion of PTt path completely fictive : in the Andriamena formation ( Madagascar ), very high temperature granulites show complex mineral associations with 3 generations of opx, at least 2 of sapph, sill, grt and cordierite. Geochronological constraints show that a portion of the retrograde PTt path inferred from petrographical observations is not representative of the actual PTt path. This portion is an artifact with no geological meaning resulting from the equilibration of refractory peak assemblages at lower PT conditions.