Fabric evolution vs reaction progress
The study of inter-relationships between the degree of planar fabric evolution and of metamorphic changes was respectively supported by structural maps of foliation trajectories (visualizing the history of superposed granular deformation imprints, Fig. 2), and by maps of degree of fabric evolution and metamorphic transformation domains (Fig. 8 and 9); the deformation history was derived from the structural study of the Mt. Mucrone metagranitoids (Delleani et al., submitted), in which the chronology of superposed foliations (S1 to S5) and fold systems (D1 to D6) was inferred on the basis of overprinting criteria, aided by cross-controls of corresponding metamorphic assemblages.
Figure 8. Maps of the degree of fabric evolution (D1 and D2) and metamorphic transformation (M2) for D1 and D2 stages.
Syn-D1 mineral assemblages always constitute more than 70% of the rock volume. For this reason the map of the degree of metamorphic transformation is show only for D2 stage (M2). D1 HD domains are mainly localised in micaschists, leucocratic metagranitoids and in the eastern portion of the paragneisses. In metagranitoids syn-D1 HD domains mainly occur close to the boundaries with paragneisses (eastern portion of Mt. Mucrone southern slope) and micaschists (north of Mt. Mucrone).
Figure 9. Maps of the degree of fabric evolution (D3 and D4) and metamorphic transformation (M4) for D3 and D4 stages.
D3 mineral assemblages always constitute less than 10% of the rock volume in LD domains and no more than 60% in HD. For this reason the map of the degree of metamorphic transformation is show only for D4 stage (M4).
Individuation of homogeneous fabric domains was facilitated by integration of meso-structural information with microstructural analysis, from samples selected in accord with the deformation sequence. The degree of fabric evolution, indicating the volume percentage of new planar fabric, has been reconstructed together with the associated degree of syn-tectonic metamorphic transformations, indicating the modal amount of mineral assemblages expressed in percentage.
As already experienced by Salvi et al. (2010), the degree of grain-scale reorganization of the dominant fabric has been used as a guide to estimate the fabric evolution referring to the successive stages of crenulation cleavage development, as proposed by Bell & Rubenach (1983), from crenulation up to a complete transposition (see also Passchier & Trouw 2005). The scheme proposed by Salvi et al. (2010), elaborated starting from originally foliated or isotropic igneous fabric, has been adopted as a reference to discriminate between low-, medium- and high-deformation degrees (Fig. 5). This reference perfectly fits with the examined case of Mt. Mucrone, where, during Permian times, granitoids emplaced in polydeformed metapelites sharing since then the polyphase Alpine structural and metamorphic evolution.
Microstructural analysis showed that the discriminating intervals well fit with those proposed by Salvi et al. (2010) in which the low degree (LD) of deformation corresponds to the early development stage of the new fabric (volume of newly oriented fabric ranges from 0 to 20%) and includes an incipient crenulation or the appearance of a weak, non-persistent new foliation in country metapelites, and of no strain or a weak foliation in meta-intrusives, respectively; in other words this corresponds to the fabrics referred to as coronitic replacement.
The medium degree of deformation (MD) corresponds to a successive evolution up to the differentiation of a new foliation, which can reach the stage of a differentiated crenulation cleavage or a pervasive foliation (volume of newly differentiated fabric ranging from 20 to 60%), in originally foliated or isotropic rocks, respectively; this corresponds to the fabrics described above as tectonitic. The high degree of deformation (HD) coincides with the progressive obliteration of relicts of the earlier fabric in metapelites and metaintrusives and to development of a new continuous foliation of mylonitic type (volume of newly oriented fabric can rise up to 100%), thus corresponding to fabrics above described as mylonitic.
The estimate of volume percentage occupied by Alpine mineral assemblages syn-tectonic with successive deformation stages varies from 5 to 100%. Generally the higher is the degree of fabric evolution, the more pervasive is the mineral growth of the related syn-tectonic assemblages. The assemblages developed during D1, under eclogite-facies conditions, represent an exception: they occupy a volume ≥ 90% in the mapped area, irrespectively of the degree of fabric evolution.
Disregarding D1 stage, LD is generally characterised by a volume of synkinematic metamorphic products ranging from 0 to 20% in metasediments (= originally foliated rocks). In metaintrusives (the only originally isotropic rocks), the relationships between degree of fabric evolution and metamorphic transformation is directly proportional, with a special case occurring during D4 in the leucocratic metagranitoids, outcropping at the south-western margin of the map (Fig. 9), where the volume occupied by the Ph and Na-Cpx breakdown products rises up to 70%. MD domains show a volume of syn-tectonic metamorphic products varying from 20 to 70% both in originally foliated and in isotropic rocks. Finally, always with the anomaly of syn-D1 assemblages, in HD domains the mineral–chemical re-equilibration can reach up to 80% of the volume during D2 and never exceed 50% during D3: this latter represents the maximal degree of syn-D3 mineral replacement, which is generally < 10% in LD and MD syn-D3 domains.
Rock composition seems to exert an additional control on reaction progress, as can be inferred from Fig. 8 and 9, where the different modal amount of white mica and Na-Cpx influences the development of syn-D2 and syn-D4 metamorphic assemblages that are more pervasive in micaschists and leucocratic metagranitoids, respectively. In addition to original mineral rock composition and fabric evolution, the thermal regime can significantly influence the degree of metamorphic transformation where the fabric evolution remains below the HD stage: for the same degree of fabric evolution, the metamorphic reaction progress is more evolved during D2 (eclogite facies) than during D3 (blueschist facies).
The microstructural analysis results highlight that the two processes, degree of deformation and metamorphic transformation, do not necessarily develop at the same rate, but above the transition to the HD, the mechanical and mineral–chemical transformations of the rocks increase proportionally, in agreement with the observations of Salvi et al. (2010), Spalla et al. (2005) and Zucali et al. (2002b).