Metamorphism

Abstract

The regional metamorphic pattern within the Ossa-Morena Zone is characterized by an alternation of low and medium-to high-grade belts which retain more or less homogeneous features along the strike (Fig. 1). As already stated by Quesada et al. (this Vol.) it is now unanimously recognized that this metamorphic framework developed during at least two main tectonothermal cycles: Late Precambrian and Hercynian. Nevertheless, major uncertainties still exist concerning which event is to be held responsible for the metamorphic evolution of specific areas; this is mainly due to our still poor knowledge of the relationships pertaining to recrystallization-deformation and the lack of geochronological constraints on these events. Moreover, the few available absolute ages on these (sometimes) highly complex metamorphic rocks are not amenable to straightforward interpretation. The great majority of the data was obtained by the K-Ar or ⁴⁰Ar-³⁹Ar methods; thus, we are dealing with cooling ages, which means that the ages determined for the metamorphic rocks represent the timing when those rocks were brought up to shallower crustal levels and/or to P-T conditions below the closing exchange temperature of their respective isotopic systems. Therefore, what the data is telling us is the ages of the main uplift tectonic events and not necessarily the timing of either their metamorphic recrystallization and/or deformation.