Microstructure
The interpretation of the Alpine structural and metamorphic evolution was based on the overprinting relationships indicated by the mesostructural study, linked with the petrographic evidences on the evolutionary sequence of parageneses supporting the microfabric changes. Deformation-metamorphism relationships have been inferred on the basis of the microstructural analysis of 93 thin sections, distributed over an area of ≈10 Km2 (Fig. 2). During successive structural and mineralogical re-equilibration stages careful analysis of the effects of heterogeneous deformation made clear the contemporaneous development, in adjacent rock volumes, of equivalent mineral assemblages, manifest as different textures corresponding respectively with a low (LD = where the granular scale strain is extremely poor and is associated to metamorphic mineral growth localised at the rims of previous grains), medium (MD) and high degree (HD) of fabric evolution, respectively (Fig. 5).
Examples of microfabrics characteristic of LD, MD and HD domains generated during successive deformation stages in metaintrusives and metasediments are shown in Figures 6 and 7, respectively.
Figure 5. Qualitative estimate of the degree of fabric evolution during foliation development, starting from originally foliated or initially isotropic rocks, as proposed by Salvi et al. (2010)
Italic numbers (1-6) of originally foliated rocks refer to the successive stages of crenulation cleavage evolution, up to complete decrenulation, as proposed by Bell and Rubenach (1983). The volume occupied by newly-oriented fabric elements, including the newly-differentiated mineral layering, is used to define the degree of fabric evolution, and to establish conventionally a low, medium or high degree of granular scale deformation (LD, MD and HD, respectively).
Figure 6. Representative microstructures of low, medium and high degree (LD, MD and HD, respectively) of fabric evolution during the successive deformation stages (D1-D5) in the two main types of metagranitoids.
“Grey type” metagranitoids: a) coronitic replacement of igneous Bt and Pl by eclogite-facies mineral as Cpx, Grt, Ph (plane polarized light); b) syn-D2 Omp rimming pre-D3 coronitic Jd – white mica aggregate (BSE image); c) S1+ S2 composite fabric wrapping Aln porphyroclast, associated with syn-D2 Grt and Ph partial re-crystallization (plane polarized light); d) fine-grained white mica growing after Jd aggregates in a poorly deformed domain close to D3 shear zone (crossed polars); e) syn-D1 localized mylonitic shear zone, reactivated during D3 with consequent flattening of Jd aggregates, partially replaced by fine-grained white mica (plane polarized light); f) replacement of syn-D4 Ab and Wm on the previous Jd, Qz and Zo aggregate, along old grain boundaries and microfractures (BSE image); g) very fine-grained re-crystallization of Ph along D5 shear plane (crossed polars); h) dark-Amp, Wm, Adl and Ab after Jd, Qz and Zo aggregates close to a D6 fold hinge (plane polarized light). “Green type” metagranitoids: i/l) incipient (i, plane polarized light) and well developed (l, crossed polars) S1 foliation marked by SPO of Na-Cpx grains; m) syn-D2 coronitic Acm rimming igneous Pl sites (plane polarized light); n) S1+S2 composite fabric associated with recrystallization of Ph, Na-Cpx and Qz (crossed polars); o) syn-D3 Brs rimming compositionally zoned Gln (BSE image); p) S3 foliation marked by Ph and Pg aggregates partially replacing older and coarser Ph and Brs (BSE image); q) Act and Wm filling of D4 fracture across syn-D2 Gln (plane polarized light); r) Na-Cpx stretched in a syn-D5 shear zone; Qz ribbon, new white mica and Chl overgrowing Cpx mark the mylonitic foliation (plane polarized light).
Figure 7. Representative microstructures characterising low, medium and high degree (LD, MD and HD, respectively) of fabric evolution during the successive deformation stages (D1-D6) in the two main types of country rocks.
Paragneisses: a) randomly oriented growth of Omp and Grt in an Amp-Omp-Grt-rich layer (BSE image); b) mm-spaced S1 foliation mainly marked by Ph SPO wrapping pre-Alpine Grt porphyroclasts (plane polarized light); c) atoll Grt in D2 LD domains, enclosing Ph, Pg and Gln (BSE image); d) D2 folding of S1 with bending of Grt- and white mica-rich layers; the latter is only partially re-crystallized (crossed polars); e) Brs partially replacing syn-D2 Gln in Amp-Omp-Grt-rich layer along mm-thick syn-D3 shear zone (BSE image); f) D4 microfolding of syn-D1 Ph with partial re-crystallization of Ph in new white mica and Ttn re-crystallisation (plane polarized light); g) Chl overgrew Grt and Gln in a MD domain close to D5 shear zone (plane polarized light); h) Ab and new white mica aggregates developed along syn-D5 shear plane (crossed polars). Micaschists: i) S1 foliation marked by Jd and Ph, whereas the timing of Pg growth is ambiguous and may postdate S1 (BSE image); l) coronitic growth of syn-D2 Gln enclosing a large amount of fine-grained Grt, Ph and Qz marking S1 (plane polarized light); m) S2 foliation marked by Ph and Omp SPO and by elongated Qz-domains with polygonal structure (crossed polars); n) S3 shear plane marked by Ph, with the formation of Ttn rims on Rt (plane polarized light); o) syn-D4 coronitic replacement of Gln by Ab, Wm, Ep and Act aggregates (crossed polars); p) D4 folding of syn-D1 Ph (crossed polars); q) very fine-grained aggregate of Ab and Wm developed on Na-Cpx during D5 (plane polarized light); r) Chl, Wm and Fe-oxides formation along D5 shear plane (plane polarized light).
Pre-Alpine relicts (pre-D1)
Relicts of the pre-Alpine mineral associations are widespread both in Mt. Mucrone metagranitoids and their country-rock. In metagranitoids, several igneous mineral relicts have been preserved in LD and MD domains. They consist of: i) medium- to fine-grained brownish Bt (Fig. 6a), constituting up to 20% of the modal composition of grey-type metagranitoids; ii) medium-grained Aln (Fig. 6c) and Kfs (< 5% in volume) and rare, very fine-grained Ap and Zrn. Skeletal-shaped reddish Grt cores are pre-Alpine mineral relicts in metagranitoid country rocks (Fig. 7b), well preserved in low D1 strain domains of paragneisses, and occasionally occurring in HD D1 domains. These Grt have numerous inclusions of fine-grained Qz, Ap and Rt. Medium-grained, pale-violet Aln relicts occur also in paragneisses and zoisitites as in metagranitoids, whereas coarse-grained pinkish Mc occur in porphyric layers of the paragneisses.
As already pointed out, in poorly deformed domains pre-Alpine igneous relicts are preserved in metagranitoids. However, most of these rock volumes are replaced by Alpine high pressure minerals. Poorly deformed metagranitoids consist of grey-type metagranitoids, metaaplites and metapegmatoids, in which the eclogite-facies minerals Jd, Ph, Grt and Zo replace as coronas more than 70% of the igneous assemblage (Fig. 6a). Pl is replaced by a fine-grained Jd-Qz-Zo aggregate, where colourless, randomly oriented and anhedral Jd, up to 3mm-sized encloses bubbly Qz and randomly-oriented acicular Zo; rare fine-grained Ph occurs within the symplectitic sites of plagioclase. Kfs is replaced by fine-grained and randomly oriented Ph crystals, and brownish Bt is only partially overgrown by fine- to medium-grained Ph. Coronas formed by trails of fine- to medium-grained Grt mark the margins between igneous Bt and Pl.
In paragneisses syn-D1 LD domains are characterised by randomly oriented growth of Omp and Grt in an Amp-Omp-Grt-rich layers (Fig. 7a).
Alpine microstructures (D1-D6)
Tectonitic to mylonitic syn-D1 foliations develop in both metaintrusive and metasedimentary rocks. In “green type” metagranitoids, metaaplites and metapegmatoids, cm- to 10-cm-spaced discontinuous S1 foliation is marked by syn-D1 Omp/Jd, Ph, Gln, Zo SPO and lenticular Qz-rich domains (Figs. 6i and 6l). The Omp/Jd forms euhedral coarse-grained crystals: in “green type” metagranitoids and metapegmatoids Cpx is mostly pale green Omp, or more rarely Jd; in metaaplites only colourless Jd occurs. Medium- to coarse-grained Ph crystals enclose Qz and Rt with rational grain boundaries. The grain size of Ph and Cpx is coarser in metapegmatoids and “green type” metagranitoids. Grt trails are aligned parallel to the S1 foliation, and are formed by colourless or pale pink crystals with very fine-grained inclusions of Ap, Qz, Ph and Rt. Syn-D1 Grt are mainly fine-grained and with larger dimensions in highly strained domains (up to 5mm in diameter). Larger-sizes (up to 5cm) occur in Grt near the boundary between “green type” metagranitoids and zoisitites. In metaquarzdiorite, the 10- to 20 cm-spaced discontinuous S1 foliation is only locally preserved after D2 and D3 reworking: Omp, Ph, Zo SPO, elongated lenticular Qz domains and Grt trails mark S1. Omp has an internal foliation (Si) parallel and continuous with S1 and marked by SPO of fine-grained Ph and Zo and by small size Grt and Rt rows. In leucocratic metagranitoids and porphyric gneisses, scarce syn-D1 minerals are locally preserved within MD syn-D2 domains: coarse-grained Ph and rare fine-grained crystals of violet Gln and Zo with SPO parallel to S1. In porphyric gneisses, S1 is mainly underlined by coarse-grained Ph, Gln and Zo SPO, as in leucocratic metagranitoids, and wraps around Mc porphyroclasts showing recrystallized rims.
In syn-D1 HD domains the S1 foliation is generally continuous and has almost the same characteristics and mineralogical support in both micaschists and paragneisses, but in micaschists is only preserved within S2 microlithons. S1 is marked by SPO of medium-grained Ph, Gln Omp/Jd, Zo, by Grt strings and by mm- to cm-thick Qz lenses (Fig. 7i). Cpx generally is Omp, and Jd appeared only once. Ph is medium- to coarse-grained with inclusions of round-shaped Qz and euhedral Grt and Rt. Locally paragneisses are massive, with randomly oriented Omp, Gln, Zo and Ph occurring together with Grt and Qz in different modal amount within alternating layers. In glaucophanites S1 foliation is mainly preserved as Si in coarse-grained Omp and Grt and is marked by the alignment of Gln, Ph, Zo and accessory Rt. Omp and Grt porphyroblasts are interpreted as syn-tectonic with late D1, as suggested by gentle bending of their Si, and as pre-tectonic with respect to D2, as suggested by the relationships with S2, wrapping these porphyroclasts. In zoisitites the S1 foliation is mainly preserved as Si within Grt megablasts (up to 12 cm in diameter) and rare relicts occur in the S2 microlithons within the matrix. S1 spaced foliation is marked by elongated lenticular Qz-domains and by SPO of fine-grained Czo, Ph, Gln and Omp.
During D2, normal foliations textures (S2) were imprinted exclusively in leucocratic metagranitoids, micaschists, eclogites, zoisitites and glaucophanites. Where D2 axial-planes intersect at a low angle, the S1 foliation acted as a microshear plane during D2, developing an S1-S2 composite fabric (of S-C type), accompanied by the growth of new fine-grained Ph crystals aligned in the foliations (Fig. 6n). In leucocratic metagranitoids relict S2 is marked by SPO of fine- to medium-grained Ph and Gln; Gln also defines L2. In micaschist the continuous S2 foliation is marked by SPO of Ph, Pg, Gln, Omp/Jd and Zo/Czo, by Grt trails and by Qz elongated lenses (Fig. 7m). Euhedral fine-grained Ph and Pg form new-grains from syn-D1 Ph or grew independently with long margins parallel to S2; light pink Grt rims syn-D1 Grt in microlithons, or define S2 with trails of euhedral fine-grained new grains. Fine-grained violet Gln elongated parallel to S2 may preserve pre-D2 pale cores, and up to 1cm long grains mark D2 axes (Fig. 7l). Omp/Jd and Zo/Czo can occur in rims of mm-thickness on the earlier Cpx and Zo grains.
S2 in eclogite is confined to 20cm-thick rims at the margins of boudinaged layers and is defined by SPO of fine- to medium-grained Omp and Ph, Pg and Grt rows. In D2 LD domains coarse-grained pale-green pre-D2 Omp grains with Grt, Ph and Rt inclusions have random orientations and colourless Grt occur also as medium grains with Rt and Ph inclusions. Oriented syn-D2 Omp developed from the re-crystallization of pre-D2 grains; thin coronas of rose Grt rims pre-D2 Grt. In zoisitites up to the 50% of the rock volume is replaced by new minerals during D2. Czo, Ph and Gln define S2, together with lens-shaped Qz-aggregates. In glaucophanites up to the 70% of the rock volume is replace by syn-D2 minerals and few Omp and Grt coarse-grained porphyroclasts occur. S2 foliation is supported by the alignment of Gln, Ph and Czo: fine-grained new grains of Gln replace pre-D2 crystals. Qz shows deformation bands, subgrains and polygonal new grains indicating syn-D2 dynamic recrystallization in LD domains of successive deformation stages in all rocks.
In D2 LD domains coronas of Omp, Grt, Gln, Ph and Czo rimming syn-D1 minerals never exceed 30% of the whole rock volume. Syn-D2 rims are mainly of < 1mm-thickness, regardless of the presence of an early oriented fabric. Within these rims inclusions are very rare, only locally very-fine grained Qz, Rt and Ap occurs. In metagranitoids and metapegmatoids fine-grained green-yellowish Acm coronas develop around Bt and Pl sites at the contact with Qz (Fig. 6m) and locally colourless Jd is partially replaced and rimmed by greenish Omp (Fig. 6b), randomly oriented and as very fine-grained needle-like crystals or very thin rims.
In cm- to m-thick D3 shear zones mylonitic texture developed in all rocks and are accompanied by the growth of new minerals replacing up to 50% of the previous mineral assemblages. In paragneisses poorly deformed during D1 the D3 deformation is localised in Qz- and Grt-bearing layers, thinned during this stage. In micaschists, zoisitites, glaucophanites and paragneisses, with a penetrative S1, D3 deformation is diffused, with no signs of concentration in shear zones, with a minor modal amount (<20%) of syn-D3 minerals. S3 is accompanied by grain size reduction and is marked by fine- to very fine-grained Qz elogated grains, SPO of Ph, Brs, Czo and by Ttn strings (Figs. 6d, 6e, 6o, 7e and 7p). In metaquartzdiorite fine-grained oriented Mg-Chl develops parallel to S3 or in pressure shadows of Omp porphyroclasts, very thin rims of Aug and Grt develop on pre-D3 Omp and Grt crystals and Ph + Pg aggregates replacing the earlier white mica grains are individuated at the electron microscope (Fig. 6p).
Generally, during D4, poor new minerals growth is observed and do not exceed the 20% of the whole rock volume in all lithologic types, with the exception of Ph-rich micaschists and leucocratic metagranitoids in which syn-tectonic mineral transformations could replace up to the 60% of the previous assemblages. Diffused fine-grained granoblastic Qz aggregates are associated with this deformation stage. Mineral transformations are mainly characterised by growth of fine- to medium-grained aggregates of Ab, new white mica, Ep and Act replacing older Amp (Figs. 6q and 7o), Na-Cpx (Fig. 6f) and Ph (Figs. 7f and 7p); within porphyric gneisses and micaschists Adl also occurs in these aggregates. Fe-Chl partially replaces Grt along boundaries and micro-fractures; green Bt partially overgrowing Ph and Grt may occur in paragneisses and micaschists.
During D5 mm- to cm-thick shear zones develop in all lithotypes and the new fabric (S5) is marked by fine- to very fine-grained Qz, Ab, Wm, Act, green Bt, Ep, Ttn and Fe-oxides grain alignments (Figs. 6g, 6r, 7h and 7r). The same mineral association may occur in < 0.5 mm-thick veins with syntaxial filling. Aggregates of Ab, white mica, Ep and Act replace older Amp and Na-Cpx (Fig. 7q) and Fe-Chl partially replaces Grt along boundaries and micro-fractures (Fig. 7g), as already described for D4 microstructures. Transformations associated with D5 are localized along shear planes and never exceed 5% of the whole rock volume.
Mineral transformations are poor or absent within microstructures related to D6; they are a faint and variably spaced disjunctive axial plane foliation (fracture cleavage-type). Partial replacement of pre-D3 Jd aggregates by very fine-grained Ab, Wm, brown-Amp and Adl occurs locally in metagranitoids along the microfracture sets (Fig. 6h).