Conclusive remarks
Multiscale structural analyses performed in a portion of the subducted-exhumed Alpine continental crust evidenced that the accomplishment of syn-tectonic polyphase metamorphic transformations was controlled not only by bulk and mineral compositions, but was also variably influenced by fabric evolution, as shown by the maps of degree of fabric evolution and metamorphic transformation. This result agrees with the conclusions obtained in other portions of the Alpine continental crust, variably reworked in the Mesozoic-Tertiary subduction (Roda & Zucali, 2008; Salvi et al., 2010; Spalla et al., 2000; Spalla et al., 2005; Zucali et al., 2002b). The heterogeneity of Alpine deformation assisted the preservation of volumes dominated by relicts of igneous textures, even where magmatic minerals were widely replaced by Alpine eclogitic assemblages. The detailed structural and petrographic mapping allowed reconstruction of the tectonic and metamorphic evolutionary steps of the Permian granitoids of the Mt. Mucrone southern slope, back to the nearly undeformed igneous textural relicts.
The correlation between degree of fabric evolution and reaction progress in different lithotypes has shown that the differences of mineral assemblages and textures of the protoliths influenced the reaction accomplishment when the degree of fabric evolution remains lower than HD, as discussed in the section “Fabric evolution vs reaction progress”. Maps of volumetric estimates of mineral transformation, compared with those showing the diffusion of granular scale deformation throughout the rock indicate, as already proposed by Salvi et al. (2010), that the HD stage represents a threshold after which deformation and metamorphism effects proportionally increase up to the total replacement of pre-existing minerals where new fabrics evolved up to the stage of a continuous foliation. These observations call attention on the role of strain energy in catalysing metamorphic reactions (Hobbs et al., 2010).
In addition the evaluation of metamorphic reaction progress for the same degree of fabric evolution, during successive deformation stages, suggests that also the metamorphic environment exerts an influence as indicated by the more widespread development of eclogite-facies assemblages (syn-D2) with respect to the blueschist-facies (syn-D3) or greenschist-facies ones (syn-D4).
Finally, strain gradients may induce variations in lithostratigraphy as in the case of: i) transformation of the grey-type into green-type metagranitoids, as a consequence of deformation and mineral replacement intensity during syn-eclogitic stages, or ii) the transformation of paragneisses into micaschists localized along syn-D1 HD domains. Caution is therefore suggested in the use of lithostratigraphy as an independent key to contour tectonic units in polydeformed and polymetamorphic terrains, without the support of the multiscale structural and petrographic analysis.
Structural and metamorphic evolution point to the development of D1 and D2 stages under PT conditions of the Qz-eclogite facies not far from the boundary with the Coe stability field, of D3 stage under blueschist-facies conditions, and of D4-D6 stages under greenschist-facies conditions (Fig. 15).
Peak conditions attained during D2 (T = 480°-580° C and P = 2.3 – 2.7 GPa) were accomplished after a P and T prograde path during which the Mt. Mucrone metagranitoids and their country rocks recorded D1 deformation at T = 430° - 530°C and P = 1.9 ± 0.3 GPa. The retrograde path is marked by a transition to T = 470° - 560°C and P = 1.4 ± 0.4 GPa during D3, and successively re-equilibrated under greenschist-facies conditions up to P ≤ 0.4 GPa and T ≤ 400° C (syn-D5 re-equilibration). This structural and metamorphic history predates the emplacement of Oligocene dykes, indicating that these rocks were exhumed to greenschist-facies conditions before Tertiary magmatic activity, in agreement with the exhumation time suggested for adjacent areas of SLZ, also on the basis of the structural relationships with Biella and Traversella intrusive stocks (Zanoni, 2010; Zanoni et al., 2008; Zanoni et al., 2010; Zucali, 2002; Zucali et al., 2002a). The age of deformation history recorded under eclogite-facies conditions can be estimated between 90 and 65 Ma according to U/Pb determinations on Aln and Zrn (Cenki-Tok et al., 2011; Rubatto et al., 1999). The inferred PTdt evolution is in good agreement with that deduced by Zucali et al. (2002) for the Mt. Mucrone-Mt. Mars area (Fig. 15), but minimal P climax conditions attain higher values of 2.3 GPa and P is constrained at maximum values of 2.7 GPa by the Coe stability field.
To conclude, D1 and D2 stages are characterised by a P/T ratio lower than that of cold subduction zones (Cloos, 1993), whereas D3 falls in this thermal state of about 6°C/Km, indicating that this part of the structural and metamorphic history has been recorded in a scenario of active subduction of a cold and old oceanic plate. Post-D3 exhumation took place under a thermal state comprised between those corresponding to warm subduction zones and plate interior (Cloos, 1993), therefore compatible with continental collision.