Two contrasted P-T-time paths of coronitic metanorites of the French Massif Central: are reaction textures reliable guides to metamorphic histories?

摘要

Metanorites from two eclogitized metagabbros of the Hercynian French Massif Central preserve coronitic textures of hornblende, garnet, quartz and/or kyanite produced at the expense of the primary magmatic assemblage orthopyroxene and plagioclase. Using a petrogenetic grid in the CFMASH system, two possible P-T evolutions for the origin of the coronas are evaluated. The sequence of reactions involving the formation of Hbl (-Ky) +/- Grt and Qtz coronitic assemblages is consistent with an isobaric cooling at high pressure (c. 1-2 GPa) under hydrated conditions. However, this P-T path, inferred by using only petrographical observations, is inconsistent with the geochronological constraints: emplacement of the gabbro at 490 Ma and high-pressure metamorphism at 410 Ma. In order to reconcile petrographical observations with geochronological constraints, we propose a discontinuous two-stage evolution involving a change in water activity with time. (1) Emplacement and cooling of the norite at low pressure under anhydrous conditions, at 490 Ma. (2) During the Hercynian orogeny, the norite experienced an increase in pressure and temperature under fluid-present conditions. Adding water to the system implies a dramatic change in the petrogenetic grid topology, restricting the orthopyroxene-plagioclase assemblage only to high temperatures. Therefore, the breakdown of the unstable magmatic assemblage, through apparent retrograde reactions, occurred along the prograde P-T path which never crossed the equilibrium boundaries of these reactions.