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Analysis of fabric evolution and metamorphic reaction progress at Lago della Vecchia-Valle d’Irogna, Sesia-Lanzo Zone, Western Alps

ABSTRACT The Lago della Vecchia-Valle d’Irogna rocks are part of the Eclogitic Micaschists Complex (EMC) of the Sesia-Lanzo Zone, western Austroalpine domain. The 1:10,000 scale map includes metaintrusive, minor micaschist, banded gneiss, and metabasic boudins. The multiscale structural analysis reveals successive magmatic and tectono-metamorphic stages: during M0 the metaintrusive protoliths emplaced; D1 took place under eclogite-facies conditions; during D2 stage, a pervasive foliation developed under retrograde blueschist-facies conditions; D3–D4 and D5 structures developed under greenschist-facies conditions; during M6 andesitic dykes intruded. The mapped degree of fabric evolution (FE) and metamorphic transformation (MT) related to D2-foliation shows that the MT was not only controlled by bulk rock and mineral compositions, but also by FE. The development of a pervasive blueschist-facies D2-foliation is in contrast with the eclogitic dominant fabric generally recorded in the EMC. This difference suggests that FE and MT are potentially responsible for km-scale heterogeneities in the tectono-metamorphic record.


Introduction
Within convergent margins, the coupling and uncoupling of lithospheric slices are responsible for the tectonic architecture of metamorphic belts (e.g. Spalla, Gosso, Marotta, Zucali, & Salvi, 2010). The lithospheric slices can be recognized as tectono-metamorphic units once their tectonic and metamorphic history is proved homogeneous over a defined time interval (Spalla, Zucali, di Paola, & Gosso, 2005). Petro-structural mapping and multiscale structural analysis are crucial to characterize the tectono-metamorphic evolution of lithospheric slices and the relationships between degree of fabric evolution (FE) and metamorphic transformation (MT). In particular, the relationship between FE and metamorphic reaction progress with the development of dominant (i.e. pervasive) fabric can be used to investigate the complexity of convergent dynamics (e.g. Gosso et al., 2015). Adjacent rock volumes characterized by the same dominant fabric may record different tectonometamorphic histories. In contrast, adjacent rock volumes showing dominant fabrics developed under different metamorphic conditions may record the same tectono-metamorphic history due to heterogeneous registration of the superposed deformation and metamorphic stages (Spalla et al., 2005). In order to evaluate whether rock volumes record the same tectono-metamorphic history, mapping petro-structural heterogeneities in adjacent rock volumes is crucial (e.g. Delleani, Spalla, Castelli, & Gosso, 2012;Delleani, Spalla, Castelli, & Gosso, 2013;Gosso et al., 2015;Salvi et al., 2010;Zucali, Spalla, & Gosso, 2002). Since deformation heterogeneity commonly affects crystalline basements (e.g. Mørk, 1985;Myers, 1970), the field structural correlation exclusively grounded on geometric criteria is not reliable, because of spatial variations in deformation style . During incremental deformation related to a specific stage, rock volumes may escape deformation (coronite) or may be partially (tectonite) or pervasively (mylonite) deformed (Lardeaux & Spalla, 1990). In coronitic (lowstrain) domains, metamorphic reactions take place without formation of a new oriented fabric. Where the strain is accumulated, tectonitic and mylonitic fabric develop in intermediate and high-strain domains, respectively. In the last two cases metamorphic assemblages define normal planar/linear tectonites or mylonites (Gosso et al., 2015;Salvi et al., 2010;Spalla et al., 2010;Zucali et al., 2002). Therefore structural correlation in metamorphic basements has to be based on a coherent sequence of tectono-metamorphic stages. Heterogeneous partitioning of deformation makes this correlation difficult, but allows the preservation of structures and related metamorphic assemblages that can be used to trace back the tectono-metamorphic history .
In this work, we combined geological mapping and multiscale structural analysis to produce a petro-structural 1:10,000 scale map of FE and MT and investigate the tectono-metamorphic history of the Eclogitic Micaschists Complex (EMC) at Lago della Vecchia-Valle d'Irogna (upper Cervo valley, Biella, Sesia-Lanzo Zone). The study area consists of metaintrusive, minor micaschist, banded gneiss, cataclastic-mylonitic gneiss, and numerous decimeter-sized metabasic boudins and lenses. These rocks preserve Alpine superimposed fabrics that developed under metamorphic conditions varying between eclogite to greenschist facies. The map covers an area of 7.5 km 2 where strain partitioning in metaintrusives led to the preservation of igneous texture and magmatic minerals in coronitic domains wrapped by tectonitic and mylonitic foliations. The presented interactive map shows the Alpine superimposed structures and the associated metamorphic assemblages by means of foliation trajectories indicated with different colors (panel 1 and panel 2 of the Main Map) and the relationships between degree of FE (panel 3 of the Main Map) and degree of MT (panel 4 of the Main Map) for the dominant deformational stage. Mineral abbreviations are as in Kretz (1983) with the exception of Wm (white mica), Amp (amphibole), and Op (opaque minerals).

Mapping method and analytical techniques
The mapping strategy used here required mesoscale and microscale structural analysis to constrain metamorphic assemblages marking different fabrics and crystallized at different times (Passchier, Myers, & Kroner, 1990;Spalla, Siletto, di Paola, & Gosso, 2000;Zucali et al., 2002). Mesoscale structural analysis aims at recognizing strain gradients, continuously followed in the field, that are classified in terms of coronitic, tectonitic, and mylonitic domains ( Figure 2 This analysis is crucial to understand the superposition of tectonic structures that are represented on a structural map by means of form surface trajectories. According to this method, rock types have been represented on an 'outcrop' map (panel 1 of the Main Map) in which surfacing rocks are separated by cover drift and for any outcrop the syn-D2 degree of FE is represented. In the 'interpretative' map (panel 2 of the Main Map), cover drift is not reported and lithological boundaries are interpreted on the basis of structural data. The two maps, derived from an original mapping at 1:5000 scale, are presented at 1:10,000 scale and show the outcrop contours, lithology, and structural elements that were georeferenced and stored in a Geographic Information System (GIS) geo-database. The topography shape file was acquired from the Geo-Portal of the Piemonte Region (http://www.geoportale.piemonte.it/ cms). The orientations of structural elements, such as axial plane foliations, fold axial planes, and axes, are reported on the outcrop map with different symbols to indicate their relative chronology. Two cross-sections and the interpretative map overlay on a digital elevation model (DEM; panel 5 of the Main Map), which is available from the SINAnet (Isprambiente) website (http:// www.sinanet.isprambiente.it/it/sia-ispra/download-mais/ dem20/view), have been performed to show the 3Dgeometry of the mapped area. The orientation data are represented on equal area lower hemisphere Schmidt diagrams and grouped according to their relative chronology. The contouring of domains that show homogeneous fabric development is facilitated by integrating meso-with microscale structural analysis to estimate the FE and MT in volume percentage (Figure 2(B); Gosso et al., 2015;Salvi et al., 2010). The estimate of the FE is based on the degree of grain-scale reorganization of the dominant fabrics. The successive stages of crenulation cleavage development up to complete transposition (Bell & Rubenach, 1983;Salvi et al., 2010) were used as a guide. Multiscale deformation partitioning in metaintrusive for the D2 deformational stage is reported as an example in Figure 3. The degree of FE and degree of metamorphic reaction progress are estimated for each deformational stage and rock type. Therefore, for each deformational stage the results are presented in Table 1 and consist of: . Modal bulk: rock types distinguished based on modal mineral composition and CIPW normative analysis;  Compagnoni et al., 1977;Passchier, Urai, Van Loon, & Williams, 1981;Spalla & Zulbati, 2003).
The approach used in this study permitted us to define the pervasiveness of a single deformation-metamorphic stage. Two 1:10,000 thematic maps represent the correlation of FE and MT for D2 stage (panel 3 and panel 4 of the Main Map).

Field data
The mapped area consists of metaintrusives and their country rocks (Zucali, 2011), both characterized by a penetrative foliation marked by Qtz, Wm, Ep, and Grt that wraps metabasite boudins and lenses and micaschist lenses. Based on microstructural analysis, metamorphic assemblages were reconstructed ( Table 2). The microstructural and petrographic observations indicate that the pervasive fabric developed at retrograde blueschistfacies conditions and structural relicts of eclogite-facies assemblages are preserved only in the metabasite. Greenschist-facies mineral assemblages are restricted to the main shear zones. Detailed microstructural features of these rock types for each successive deformational stage are displayed in Table 2. coexistence of contrasted metamorphic texture due to the deformation partitioning at meso-and micro-scales (modified after Lardeaux & Spalla, 1990). (B) Semi-quantitative estimate of FE during progressive foliation development, starting from originally foliated or isotropic rocks (Bell & Rubenach, 1983;Gosso et al., 2015;Salvi et al., 2010). Dn: successive deformation stage.

Metaintrusive
The intrusive protoliths consist principally of monzogranite that occurs as meter-to decameter-sized outcrops and minor meter-sized granodioritic lenses within the monzogranite. The partitioning of the Alpine deformation is responsible for the preservation of igneous textures and minerals in meter-sized coronitic lenses and the development of tectonitic and mylonitic fabrics in the metaintrusives.

Banded gneiss
Banded gneiss has intermediate composition between metaintrusive and metapelite and consists of finegrained rocks. These rocks show a centimeter-to meter-thick layering of leucocratic Wm-poor and mafic Wm-Ep rich layers. The layering seems to be related to the degree of FE; the layering thickness decreases with the increase of the finite strain, as it occurs near Irogna Inferiore. Banded gneiss mostly outcrops between mylonitic and tectonitic metaintrusive and metapelitic lenses (see the Main Map).

Metabasite
Metabasite occurs as meter-to decameter-sized lenses and as centimeter-sized boudins within metaintrusive, banded gneiss, and metapelite. The metabasite is characterized by a foliation marked by SPO of Cpx, Grt, and Wm, which locally is overprinted by the successive structural and metamorphic re-equilibrations.

Gneiss
Gneiss outcrops as fine-grained and meter-to decameter-tick rocks displaying a mylonitic contact with all the previous rock types. Gneiss is characterized by cataclastic-mylonitic fabric. Chl + Amp + Qtz + Op constitute the mylonitic millimeter-spaced foliation that transposed and obliterated all previous structures and lithologies.

Structural analysis
The panels 1 and 2 of the Main Map show the location of the different strain domains for each rock type. In particular, they show coronitic domains wrapped by a network of superposed tectonitic foliations and mylonitic shear zones related to successive deformational stages. The identified mineral assemblages (Table 2) linked to the successive deformational stages (Table 3) allow definition of the tectono-metamorphic history. Two groups of magmatic (M0, M6) structures and five groups of superposed tectonic structures (D1-D5) are defined. Numbers associated with magmatic and tectonic structures refer to relative chronology. M0 stage consists of undeformed, most probably pre-Alpine, igneous intrusion (Zucali, 2011). D1-D5 structures consist of fold systems, foliations, and shear zones that are comparable, in terms of their structural features and relative chronology, to the Alpine structures described by Zucali et al. (2002) and Delleani et al. (2012) in the nearby area of M. Mucrone. M6 stage is represented by andesite dyke intrusion that is quite common in the area and dated at the Oligocene (e.g. Kapferer, Mercolli, Berger, Ovtcharova, & Fügenschuh, 2012). Table 3 is a schematic representation of deformation timing and related pre-Alpine and Alpine mesostructures developed in metaintrusive, banded gneiss, and metapelite. The orientations of fabric elements are summarized in the structural data section of the Main Map. The relationships between the superimposed fabrics and mineral assemblages ( Table 2) recognized at the multiscale have been used to reconstruct the following structural history.
M0. Isotropic textures are preserved in meter-to decameter-sized coronitic metaintrusive domains. Coronitic metaintrusive (at Rifugio Lago della Vecchia and west of P.ta Canaggio) preserves Bt 0 (Figure 4(A)), Kfs 0 , euhedral to subeuhedral Pl 0 grains, and interstitial Qtz. In the tectonitic and mylonitic domains, the magmatic textures are strongly deformed and the amount of igneous mineral relicts is lower than in the coronitic metaintrusives. Primary magmatic contacts between granitoid and country rocks were not found. Metaintrusives are separated by metapelite by progressively high-strain mylonitic domains that consist of banded gneiss and/or mylonitic metaintrusives themselves (east of Lago del Giaspret, SE of Rifugio della Vecchia, and ESE of Irogna). Aplite dykes intersect igneous fabric and mineral relicts in metaintrusive coronitic domains and are overprinted by Alpine foliations in tectonitic domains. In this last case, the igneous contact between aplite and metaintrusive is intersected at high angle by the Alpine foliations (Figure 4(B)).
D2. During D2 stage a pervasive S2 foliation developed and previous structures, such as igneous structures, lithological surfaces, and S1 foliation were parallelized and often transposed into S2. S2 is characterized by 15-50°dip and dip azimuth varying from 90°to 320°, with cluster at a dip azimuth of 125°and dip of 42°. In the micaschists S2, also occurs as axial plane foliation of isoclinal to tight folds (SW-shore of Lago della Vecchia, east of Lago del Giaspret, and north of Colle d'Irogna; see the panel 2 of the Main Map).
Metaintrusives record the maximum variability of syn-D2 fabric types (coronite to mylonite). In tectonitic-mylonitic domains, the S2-foliation wraps around Table 3. Schematic representation of mesostructures developed in metaintrusive, banded gneiss, and metapelite during the pre-Alpine and Alpine evolution. coronitic domains and elongated metabasic boudins (e.g. toward SE of Lago del Giaspret; see the panel 2 of the Main Map) and overprints aplitic dykes. D2 is also responsible for the layering of the banded gneiss being parallel to the S2 foliation (Figure 4(E)).
D3. This deformational stage mainly produced open folds characterized by sub-vertical SE dipping axial planes that affected all lithological boundaries. Generally, D3 folding is not associated with the development of a new pervasive foliation, however a differentiated S3 axial plane foliation is developed locally in metaintrusive and banded gneiss (at the top of Valle d'Irogna and between the Rifugio Lago della Vecchia and C.le della Vecchia; see the panel 2 of the Main Map), within 1-2 cm thick localized shear zones at high angle with the S2 foliation (Figure 4(F)). D5. These structures are related to the development of ductile-brittle meter-to kilometer-long and meterthick NE-SW trending shear zones. Within these shear zones, mylonitc fabrics are marked by greenschist-facies minerals (see the panel 1 and 2 within the Main Map). These shear zones ubiquitously affect the contacts between the metaintrusive and banded gneiss (see the panel 2 and panel 5 within the Main Map).
trend WNW-ESE. They crosscut all rock types and foliations (several outcrops between the Lago del Giaspret and C.le Irogna; NW of Olmo, along the Torrente Irogna; see the panel 1 of the Main Map). Only the relationship with the D5 shear zones was not observed.

Conclusion
The analysis carried out on these metamorphic rocks show that the progress of MTs was controlled by bulk rock, mineral compositions, but it was mainly affected by FE. The maps of degree of FE and MT support this conclusion showing that FE (panel 3 of the Main Map) and MT (panel 4 of the Main Map) are strictly correlated regardless of rock types. Moreover, the dominant tectono-metamorphic imprint is related to the D2 stage that is characterized by the highest degree of FE and MT.
Our observation agrees with results obtained in other portions of the subducted continental crust of the Western and Central Alps. For instance, in the nearby area of M. Mucrone the correlation between FE and MT shows that the dominant tectono-metamorphic imprint is related to high degree of FE and MT (Delleani et al., 2012;Delleani et al., 2013;Zucali et al., 2002). Similar results have been also obtained in the Mont Morion complex of the Dent Blanche nappe (Roda & Zucali, 2008) and in the Languard-Tonale unit in the central Austroalpine domain . In this last example, the application of this method results in a 3D model of the relationships between FE and MT and the estimation of the volumes affected by textural reworking during polyphase pre-Alpine and Alpine deformations.
In the area mapped in this work, the D2 event is more pervasive with respect to D1 and D3-D4 and S2 foliation displays the highest degree of FE and MT. In all rock types, blueschist-facies mineral assemblages (Table 3) define S2. This finding is in contrast with the nearby metaintrusive of M. Mucrone, where eclogitic mineral assemblages mark the pervasive fabric (Delleani et al., 2012;Delleani et al., 2013;Zucali et al., 2002). Thus, different FE and MT would have affected the retrograde blueschist re-equilibration and potentially generated km-scale heterogeneities within the EMC.

Software
The geological map, thematic maps, cross-sections, and 3D-model were drawn using Esri ArcMap 10.2.2, Adobe Illustrator CS6, and Move 2016.2. The Main Map was built using Adobe Illustrator CS6 and Adobe Acrobat X Pro. Photos and diagrams were edited using Adobe Illustrator CS6. Stereoplots were produced with Stereonet 9 (Allmendinger, 2016).