Journal of the Virtual Explorer

The main mineral phases are: epidote (20 - 70%), clinopyroxene (15 - 50%), Mg-chlorite (10 - 20%), garnet (≤ 20%), tremolite (≤ 5%).

S₁ foliation is defined by SPO (Shape Preferred Orientation) of epidote_I and by LPO (Lattice Preferred Orientation) of Mg-chlorite_I and clinopyroxene_I. Epidote_I (locally Fe-epidote) forms euhedral zoned, coarse-grained, and twinned crystals. Garnet_I is enclosed in, and shows rational rims with epidote_I (Fig. 8a). Clinopyroxene_I and minor epidote_I form aggregates parallel to S₁. The margins of clinopyroxene_I show inclusion trails marked by opaque minerals in continuity with, and sub-parallel to the external S₁.

Figure 8. Pre-Alpine and Alpine microstructures in Epidote-bearing rodingites.

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Mineral compositions related to microstructural sites are shown in Figs. 12, 13, and 14 and in Tables 1 and 2. a) D₂ folding deforms Mg-chlorite_I and epidote_I that dynamically recrystallise. Garnet_I is enclosed in epidote_I. b) S₁ foliation, marked by Mg-chlorite_I, garnet_I, and clinopyroxene_I, wraps around an epidote aggregate. Crossed polars. c) clinopyroxene_II in the fractures and along cleavages of a Pre-D₁ clinopyroxene porphyroclast. Crystal-plastic shear deformation is accommodated in clinopyroxene_II, garnet_II, and minor tremolite_I multi-granular aggregate filling fractures. Crossed polars.

S₂ is defined by SPO and/or LPO of clinopyroxene_II, garnet_II, Mg-chlorite_II, epidote_II, and rare tremolite_I. Epidote_II, often forming polygonal aggregates, is also enclosed in Mg-chlorite_II. Veins consisting of pre-D₂ garnet and minor pre-D₂ epidote are folded by D₂ and may be locally transposed into the folded S₁. Mg-chlorite_I forms porphyroclasts within S₂. Titanite_I forms thin trails parallel to S₂ and rarely occurs in clinopyroxene_II layers.

Where S₂ is more developed epidote_II (zoisite and clinozoisite) is in fine-grained crystals forming trails parallel to S₂ with clinopyroxene_II and garnet_II.

Up to 1 cm-sized kinked pre-D₁ clinopyroxene porphyroclasts show SPO either parallel to S₁ or S₂. These porphyroclasts locally have opaque mineral exsolutions and are overgrown by rims of clinopyroxene_I/II at the edges and along fractures. These rims do not contain opaque minerals, are optically continuous with the pre-D₁ clinopyroxene porphyroclasts, and form spikes parallel to S₂ foliation, suggesting that pre-D₁ clinopyroxene porphyroclasts partially recrystallised during D₂. The cores of pre-D₁ clinopyroxene porphyroclasts are locally replaced by Mg-chlorite along cleavages.

Pre-D₁ clinopyroxene porphyroclasts enclose pre-D₁ epidote crystals that also form aggregates wrapped by S₁; the core of these epidote crystals is Fe-rich, as suggested by high birefringence colours (Fig. 8b); epidote strain shadows are filled by Mg-chlorite_I or _II. In more foliated rocks pre-D₁ clinopyroxene porphyroclasts are fractured within S₂, with mineral filling of clinopyroxene_II, garnet_II, and rare tremolite_I (Fig. 8c). The high angle with S₂ and SPO of clinopyroxene_II filling these fractures, suggest that they developed during D₂. These fractures are intersected by other fractures at a high angle with S₂ that are filled by coarse-grained euhedral epidote_II crystals. Fractures cutting S₂ are filled by epidote_III, which is zoisite and clinozoisite where fractures intersect epidote_I or epidote_II and Fe-epidote where fractures intersect garnet, clinopyroxene_II, and Mg-chlorite_II aggregates.

Rare post-D₂ tremolite_II forms randomly oriented crystals overgrowing garnet and epidote layers. Rocks containing tremolite are the poorest in epidote.

Taking into account the described microstructural features, the following mineral growth - deformation relationships can be inferred in epidote-bearing rodingites:

- Pre-D₁ mineral relics: Cpx porphyroclasts, ± Ep (Fe-Ep);

- Syn-D₁: Ep_I, Cpx_I, MgChl_I, Grt_I;

- Syn-D₂: Ep_II, Cpx_II, MgChl_II, Ttn_I, ± Grt_II, ± Tr_I;

- Post-D₂: ± Ep_III, ± Tr_II.

Garnet-chlorite-clinopyroxene-bearing rodingites consist of: garnet (20 - 70%), Mg-chlorite (20 - 55%), clinopyroxene (≤ 50%), opaque minerals (≤ 5%), vesuvianite (≤ 10%).

These rocks are heterogeneously deformed and the dominant fabric can be the S₁ foliation, the crenulated S₁ with the newly forming variably differentiated S₂ crenulation cleavage, or the fully decrenulated continuous S₂ foliation.

S₁ is marked by garnet_I, Mg-chlorite_I, clinopyroxene_I, and rare prismatic vesuvianite_I. In samples where S₁ is pervasive, rare vesuvianite porphyroblasts, postdating S₁, enclose fine-grained garnet_I. Up to 1 cm-sized pre-D₁ clinopyroxene porphyroclasts have fractures filled mainly by fine-grained garnet_I and/or clinopyroxene_I showing complex relations: fractures filled by clinopyroxene_I ± epidote_I are intersected and displaced by fractures filled by garnet_I ± Mg-chlorite_I or garnet_I fractures are intersected by clinopyroxene_I fractures (Fig. 9a). Where fractures form a high angle with S₁, the mineral filling may be interpreted as syn-D₁; in other cases the relative chronology of mineral growth filling with respect to the deformation stages is not clear. Pre-D₁ clinopyroxene porphyroclasts are rimmed by garnet_I, which also grew in continuity along the S₁ films. Garnet_I and minor clinopyroxene_I + Mg-chlorite_I also form aggregates, locally parallel to S₁, which could be pseudomorphs after pre-D₁ clinopyroxene porphyroclasts. The aggregates are affected by ductile boudinage with Mg-chlorite_I in the necks. Two sets of conjugate micro-shear zones marked by Mg-chlorite_II and rare vesuvianite_II affect S₁ and kink Mg-chlorite_I.

Figure 9. Pre-Alpine and Alpine microstructures in Garnet-chlorite-clinopyroxene-bearing rodingites.

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Mineral compositions related to microstructural sites are shown in Figs. 13 and 14 and in Tables 1 and 2. a) Pre-D₁ clinopyroxene porphyroclast transected by garnet_I-filled fractures and by clinopyroxene_I-filled fractures. Crossed polars. b) Microfolding and transposition along S₂ of a pre-D₂ garnet-vein; white line highlights vein margins; crossed polars. c) S₁ and the differentiated S₂ marked by garnet_I/II, Mg-chlorite_I/_II, and clinopyroxene_I/II. An uvarovite-rich pre-D₁ garnet porphyroclast is preserved within the crenulated S₁; crossed polars. d) Uvarovite-rich garnet wrapped by S₂ foliation, marked by garnet_II, chlorite_II, and clinopyroxene_II; plane polarised light. e) Uvarovite-rich garnet porphyroclast with Cr-spinel (dark spots) preserved in the core: plane polarised light. f) Folded S₂ defined mainly by Mg-chlorite_II and garnet_II-rich aggregates. Mg-chlorite_III concentrates along the incipient S₃ differentiated cleavage; crossed polars.

Pre-D₂ garnet, clinopyroxene, and minor Mg-chlorite fill pre-D₂ veins, which are parallel to or folded and at a high angle with S₂ (Fig. 9b).

Where S₂ is only incipient, it is a spaced crenulation foliation with folded S₁ relicts within the S₂ films. S₁ and S₂ are marked by SPO of clinopyroxene_I and _II and garnet_I and _II and SPO and LPO of Mg-chlorite_I and _II respectively (Fig. 9c). Some clinopyroxene_II grains show an internal foliation defined by opaque mineral that is slightly folded and in continuity with the external S₂, suggesting that these grains are syn-kinematic to S₂.

Locally S₁- or S₂-parallel garnet_I/II aggregates wrap pre-D₁ garnet porphyroclasts, which have a green Cr-rich core and colourless rims recrystallised during D₁ or D₂ (Fig. 9d). The Cr-rich core may contain fine-grained Cr-rich spinel (Fig. 9e). When the pre-D₁ garnet aggregates are parallel to S₁ they are dismembered and folded by D₂.

Well-developed S₂ is marked by LPO and SPO of Mg-chlorite_II, garnet_II, clinopyroxene_II, and rare opaque minerals; clinopyroxene_II forms layers parallel to S₂.

Pre-D₁ clinopyroxene porphyroclasts may also show SPO parallel to S₂. In this case garnet_II crystallised in fractures at a low angle with clinopyroxene porphyroclast cleavages and in continuity with S₂. In wider fractures Mg-chlorite_II forms crystals oriented at about 45° with respect to the fracture wall, suggesting wall-parallel shearing.

In D₂ microfolds, Mg-chlorite_I forms aggregates of recrystallised decussate grains or is internally deformed. Mg-chlorite porphyroclasts are overgrown by garnet_II along rims and cleavages and may be related to D₁ or pre-D₁ stages. Euhedral opaque minerals overgrow S₂.

Mg-chlorite_III marks the incipient S₃ crenulation cleavage of locally asymmetric micro-folds (Fig. 9f). Rare Mg-chlorite_III intersects S₂ films at low angle.

The microstructural features described above indicate the following mineral growth - deformation sequence:

- Pre-D₁ mineral phases: uvarovite-rich Grt, Cpx porphyroclasts, ± Cr-rich Spl, ± Mg-Chl

- Syn-D₁ or pre-D₂: Mg-Chl_I, Grt_I, ± Cpx_I, ± Ep_I, ± Ves_I, ± opaque minerals

- Syn-D₂ or post-D₁: Mg-Chl_II, Grt_II, Cpx_II, ± Ves_II, ± opaque minerals

- Syn-D₃: Mg-Chl_III.

Vesuvianite-bearing rodingites consist of: vesuvianite (25 - 55%); Mg-chlorite (10 - 55%); garnet (10 - 30%); clinopyroxene (5 - 20%).

S₂ foliation is marked by fine-grained garnet_II, SPO of zoned vesuvianite_II and clinopyroxene_II, and LPO and SPO of Mg-chlorite_II (Fig. 10a). S₂ is also marked by layers of different grain size and modal proportions of vesuvianite_II, clinopyroxene_II, or garnet_II crystals with rare titanite_I films and minor epidote_I.

Figure 10. Pre-Alpine and Alpine microstructures in vesuvianite-bearing rodingites.

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Mineral compositions related to microstructural sites are shown in Figs. 13, 14, and 15 and in Tables 1 and 2. a) Vesuvianite_II, garnet_II, Mg-chlorite_II, and minor clinopyroxene_II marking S₂; plane polarised light. b) S₁ geometry preserved within S₂ lithons; stages of progressive enrichment of vesuvianite_II, Mg-chlorite_II, and garnet_II are evident by variable thickness of S₂ films; plane polarised light. c) Clinopyroxene porphyroclast wrapped by and aligned along the S₂ foliation, marked by vesuvianite_II, garnet_II, Mg-chlorite_II, and clinopyroxene_II. New clinopyroxene crystallised at the rim and in fractures of the porphyroclast. Vesuvianite marking S₂ is zoned (see also Fig. 15); crossed polars. d) Fractured pre-D₁ clinopyroxene porphyroclast wrapped by S₂; Mg-chlorite_II crystallised mainly in the strain shadows and Mg-chlorite_II and garnet_II filled the fractures. New clinopyroxene_II is concentrated in the layer at the tip of the strain shadows; crossed polars. e) Pre-D₁ vesuvianite enclosed in the core of pre-D₁ garnet; backscattered SEM image. f) Pre-D₁ clinopyroxene porphyroclast replaced by garnet_II and minor clinopyroxene_II that grows along cleavages and fractures; crossed polars. g) Vesuvianite_III overgrowing the S₂ foliation marked by vesuvianite_II, Mg-chlorite_II, and garnet_II; crossed polars.

Rare pre-D₂ clinopyroxene and Mg-chlorite are oblique to S₂ and locally form aggregates wrapped by S₂. Mg-chlorite_I forms decussate aggregates of re-crystallised grains within S₂ films, evidencing an S₁ relict foliation. S₁ is sporadically preserved between S₂ films as a crenulated foliation (Fig. 10b) mainly marked by Mg-chlorite_I, vesuvianite_I, and garnet_I.

Mm-sized pre-D₁ clinopyroxene porphyroclasts are wrapped by S₂ and may show SPO parallel to S₂ (Figs. 10c, d). Minor epidote may be present as subhedral crystals at the pre-D₁ clinopyroxene porphyroclast rims. Very fine-grained opaque minerals are exsolved from the pre-D₁ clinopyroxene porphyroclast crystal lattice whereas the porphyroclast rims (clinopyroxene_II) do not contain opaque minerals. Strain shadows are filled by vesuvianite_II and Mg-chlorite_II. Cr-rich pre-D₁ garnet porphyroclasts are wrapped by S₂ foliation and contain euhedral to subhedral crystals of pre-D₁ vesuvianite; therefore pre-D₁ garnet and vesuvianite are in contact by rational margins, suggesting textural equilibrium (Fig. 10e).

Garnet and clinopyroxene also grew along rims and fractures of clinopyroxene porphyroclasts (Fig. 10f). In some samples fractures occur in two sets, located at a low and high angle with respect to S₂; fractures in pre-D₁ clinopyroxene porphyroclasts at a high angle with S₂ can be interpreted as syn-D₂. Clinopyroxene_II-bearing fractures cut garnet_II ± Mg-chlorite_II and vesuvianite_II filled fractures.

Vesuvianite_III forms crystals or garbens that overgrow S₂ films (Fig. 10g). Mg-chlorite_III fills veins intersecting S₂. Rare Mg-chlorite_III marks an incipient S₃ axial plane foliation or intersects S₂ at a low angle.

Summarising, the following mineral growth-deformation relationships can be inferred:

- Pre-D₁ or pre-D₂ minerals: Cpx porphyroclasts, ± pre-D₁ Grt, ± pre-D₁ Ves

- syn-D₁ assemblage: Mg-Chl_I, ± Grt_I, ± Ves_I

- syn-D₂ assemblage: Ves_II, Mg-Chl_II, Cpx_II, Grt_II, ± Ttn_I, ± Ep_I

- syn-D₃ minerals: Mg-Chl_III, Ves_III.

The reaction rim of rodingites consists of clinopyroxene and chlorite bearing schists.

S₁ foliation is variably folded and mostly obliterated by the S₂ crenulation cleavage. S₁ is marked by a layering characterised by SPO and LPO of clinopyroxene_I and chlorite_I (Fig. 11a); minor opaque minerals may be scattered along S₁-parallel layers. Clinopyroxene_I may define layers characterised by a remarkably variable grain size. Locally disharmonic folding of chlorite_I- and clinopyroxene_I-layers occurs. S₂ crenulation cleavage is marked by LPO and SPO of clinopyroxene_II, chlorite_II, and amphibole_I (Fig. 11b). Chlorite_III forms rare radial aggregates and randomly oriented amphibole_II overgrew S₁ and S₂ or mimetically replaced clinopyroxene_II.

The above described microstructures suggest the following mineral growth-deformation relationships for the reaction rims:

- D₁: Chl_I, Cpx_I, opaque minerals

- D₂: Chl_II, Cpx_II, Amp_I

- Post-D₂: Amp_II, Chl_III

Figure 11. Alpine microstructures in rodingite reaction rims.

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a) S₁ is marked by a mineral layering of alternating clinopyroxene_I and chlorite_I, and is crenulated during D₂ deformation; plane polarised light. b) The S₁ foliation is crenulated during D₂. Clinopyroxene_I and chlorite_I mark S₁; the incipient differentiation of the S₂ crenulation foliation is highlighted by amphibole_I crystals aligned with S₂; crossed polars.