In Fig. 3 the relationships between microstructural evolution and mineral growth have been reported. The record of the multi-stage transformations is more complete in eclogites than in serpentinites, ultramafics and schists.
Figure 3. Deformation scheme
Schematic evolution of fabrics and mineral assemblages in eclogites, schists, ultramafics and sepentinites.
Undeformed volumes of TypeIII eclogites preserve the oldest microstructural and mineralogical features, which are brownish Rt-rich cores within Amp and Omp porphyroblasts; Amp Rt-rich cores are rimmed by blue-Amp while the smaller interstitial Omp grains enclose thin aggregates of Rt and oxides in the cores. Brown Amp and interstitial Cpx microstructural site may be interpreted as pre-Alpine remnants according to Compagnoni (Compagnoni, 1977).
D1 is preserved in TypeIII eclogites as a mineral layering defined by alternate Amp-rich and Ep-rich domains. Amp-rich domains are characterised by SPO of AmpI and WmI defining the S1 foliation; GrtI porphyroblasts also occur within these layers and the boundaries between AmpI, WmI and GrtI are rational. In places the cores of GrtI porphyroblasts are rich of Qtz + Amp + Rt inclusions. Ep and Wm grains, showing SPO parallel to the mineral layering, constitute Ep-rich domains. Ep grains are elongate and no undulose extinction occurs within these grains. AmpI are large inequant porphyroblasts characterised by slight undulose extinction. In the matrix Rt occurs as isolate grains or aggregates of grains slightly oriented parallel to the S1 foliation. Qtz is mainly interstitial and is gently oriented parallel to the foliation, showing undulose extinction and deformation lamellae.
The microstuctural relationships allow defining the metamorphic assemblage stable during D1:
AmpI + GrtI + WmI + Rt + Ep ± Qtz
Post-D1a The coronitic growth of Omp (Figs. 3, 4), Grt and Lws, overprinting the pre-existing S1 foliation, characterizes this intermediate stage of the mineral growth. Omp are large grains (up to 3 cm) showing an angle with S1 foliation and displaying inclusions of AmpI, Ep, Rt and Wm, oriented parallel to the external S1 foliation. Omp grains do not show undulose extinction, deformation twins or lamellae. GrtII porphyroblasts also grew in close relations with Omp. Large (up to 3 cm in size) GrtII poikiloblasts include the S1 foliation, marked by SPO of AmpI and Ep; Grt rims also include a few randomly oriented Omp grains.
The microstructural features described above allow relating mineral growth to metamorphic reactions in TypeII eclogites:
AmpI + Ep + GrtI = OmpI + GrtII
WmI (Pg) + Ep = Lws + OmpI
leading us to define the new mineral phases growing during post-D1a as:
OmpI + GrtII + Lws
Lws euhedral porphyroblasts, containing the S1 foliation marked by SPO of AmpI, Ep and Wm, are completely replaced by pseudomorphs of Ky + Ep (Fig. 3, 4). Ky and Ep do not show any preferred orientation; the inferred breakdown reaction of Lws is therefore:
Lws = Ky + Ep
during this stage the stable metamorphic assemblage is defined by:
Omp + Ky + Ep + Rt ± Qtz
Figure 4. Microstructural relationships within eclogites
Microphotographs showing microstructural relationships within eclogites.
a) Ep + Ky replacing Lws in contact with Omp and Grt in Lws-eclogites; Ep and Ky do not show any shape preferred orientation. Plane polarised light.
b) Large Omp porphyroblast overgrows at an high angle S1, underlined by AmpI SPO. Plane polarised light.
c) Pg-Ep aggregate, replacing Lws porphyroblast, is flattened in S2. Crossed polars.
d) Syn-D3 aggregates of Pl, Wm and AmpIII rims AmpI/II; boundaries between Pl and AmpIII are rational surface of either phase. Crossed polars.
e) Syn-D3 micro-fracturing of Omp porphyroblast; an aggregate of AmpIII and Pl fills the micro-fracture neck. Crossed polars.
D2 is characterised by the development of a penetrative foliation, mainly marked by SPO of Omp, AmpII and Rt or fine-grained Rt aggregates. Omp are elongate grains up to 1 cm in size, showing undulose extinction and deformation bands. S2 is marked also by AmpII + Ep + Wm; syn-S2 mineral association is also defined by Grt grains, which show rational boundaries with Omp and AmpII. AmpII show undulose extinction and deformation bands, or occur as small strain-free grains at the rims of the large AmpI porphyroblasts. Ep aggregates mark S2 , while single Ep grains with undulose extinction may occur either parallel or with an angle with respect to S2. Aggregates of Wm + Ep, replacing Lws porphyroblasts, are in general re-oriented and flattened parallel to the S2 foliation (Fig. 3, 4) suggesting that the replacement occurred early in the S2 development. Wm occurring along the S2 foliation is characterized by slight undulose extinction.
These microstructures indicate that the assemblage stable during D2 was:
OmpI + AmpII + Wm + Ep + Rt ± Qtz
and that the disappearing of Ky + Omp assemblages may be explained by the reaction:
Omp + Ky = Amp + Pg + Ep
During D3 microfolds develop. S1 and S2 foliations are gently bent and aggregates of Chl, blue-green AmpIII, Pl and Ep occur (Fig. 4d). In places SPO of AmpIII, Ep and Chl-rich aggregates underline a S3 axial plane foliation. S3 also defines discrete shear bands within eclogites, mainly marked by AmpIII and Ep. AmpIII develops as corona of Omp, AmpI and AmpII or grows within Omp syn-D3 microboudin and microfracture necks associated with Pl (Fig. 4e). Pl also occur as large grains within Pg aggregates. Ttn develops as corona of Rt grains or underlines the S3 foliation.
These microstructural relationships may be used to suggest the stable assemblage during D3:
AmpIII + Chl + Pl + Ep + Ttn
due to the interactions of several metamorphic transformations such as
Omp + Qtz + H2O = Ab + AmpIII
Omp + H2O = AmpIII + Ep
Cpx + Rt + Wm = Ab + Chl + Ttn +Qtz
Pg = Ab + Qtz + H2O
D1 is characterized by the S1 foliation marked by SPO of Amp + Ep + Rt. Grt occur both as porphyroblasts (Fig. 5) and aggregates of small grains. Grt-rich layers are parallel to S1 . AmpI are large inequant grains, showing undulose extinction and deformation bands. Large AmpI individuals are partially rimmed by an aggregate of smaller strain-free grains (AmpII). AmpII also show SPO parallel to S1 . Ep grains show slight undulose extinction and their SPO defines S1 : where Ep occurs at an angle with respect to S1 , it shows undulose extinction and deformation bands. Rt occurs as isolated grains with SPO slightly parallel to S1 . Grt is associated to AmpI and Ep within S1 foliation.
Figure 5. Microstructural relationships within ultramafites
Microphotographs showing microstructural relationships within ultramafites (a-b), serpentinites (c-d), eclogitic metabasics (e).
a, b) Di porphyroblasts rimmed by Amp aggregates. Plane polarised light (a) and crossed polars (b).
c) SPO of Chl aggregates, within serpentinites, defining the S2 foliation. Plane polarised light.
d) Cpx porphyroblast wrapped by Srp+Chl+Carb aggregates. Plane polarised light.
e) S2 foliation marked by AmpI and Chl SPO in Eclogitic metabasics. Crossed polars.
The microstructural relationships allow defining the stable assemblage during D1:
Amp + Grt + Ep + Rt ± Wm ± Qtz
During post-D1 stage the coronitic growth of large inequant Omp grains occurs. Omp porphyroblasts contain an internal foliation marked by SPO of AmpI and Rt aggregates parallel to S1 . Grt is also included within Omp porphyroblasts (up to 1 cm in size). Omp lies at a high angle with respect to the S1 foliation and it is only slightly affected by undulose extinction.
The S2 foliation is rare within hornblende-rich metabasics; it is defined by AmpII SPO associated with Ep and Omp. Here Omp shows an intense undulose extinction, deformation bands and, in places, sub-grains. AmpII mainly consist of aggregates of small grains, occurring at the AmpI rims or defining the newly formed foliation. Wm may also occur as single grains showing SPO parallel to S2 and AmpII.
These microstructural relationships allow defining the stable assemblages during post-D1 and D2 :
Amp + Grt + Ep + Omp + Rt ± Wm ± Qtz
During D3 coronitic transformation widely occur: Chl partially replaces Grt; microboudinaged Omp and Amp are partially substituted by aggregates of Pl + green Amp; Ttn rims Rt. Where S1 and S2 are bent Ep occurs within micro-hinges. Micro-shear planes, marked by SPO of Pl + Wm + Chl + Ep crosscut the pre-existing S1 and S2 foliations.
The assemblage inferred to be stable during D3 is:
Green Amp + Pl + Ttn + Wm + Chl + Ep ± Qtz
Pre-D1: The oldest features preserved are brownish Rt-rich cores within Amp porphyroblasts. These relict cores are preserved only in small less deformed volumes of schists.
D1 is recorded as large inequant AmpI porphyroblasts showing undulose extinction, deformation bands and a shape fabric at an angle (up to 20 degrees) with respect to the dominant S2 foliation.
The penetrative S2 foliation is marked by SPO of AmpII + Mg-Chl + Ep + Rt ± Omp . AmpII are strain-free grains characterized by blue-violet pleochroism. Aggregates of Mg-Chl define the S2 foliation. The boundaries among Amp, Chl and Carbonates are straight. Omp grains are wrapped by AmpII aggregates within Amp-rich domains; Omp shows undulose extinction and deformation bands. Aggregates of carbonates also occur within the S2 foliation.
The inferred stable assemblage during D2 is:
Amp + Mg-Chl + Ep + Rt ± Carb ± Wm ± Omp
D3 is characterised by a gentle folding of S2; Fe-Chl, Pl and Ttn occur within microfold-hinges.
Ultramafics are mainly constituted by fine-grained Amp and Cpx, associated with Chl, Rt, carbonates and scarce Tlc and Srp. Brownish cores of Chl and Amp characterise the relict microstructures in ultramafics. Ultramafics do not record D1 microstructures.
The S2 penetrative foliation is defined by alternate layers of Chl + Tlc + Srp and Amp + Cpx. Amp and Chl show SPO parallel to the S2 foliation. Chl has a Fe-Mg-rich core while rims are colourless or with pale-green pleochroism. Cpx (Di) also occur within Amp-rich domains. Amp includes Chl grains without any preferred orientation. Carbonates are interstitial with respect to Amp and Chl. Grain boundaries between Amp, Chl and carbonates are rational.
Serpentinites display a penetrative foliation (S2) marked by alternate Amp + Cpx + Chl + Carb (layerI) and Srp + Carb + Chl + opaque layers (layerII). Amp and Cpx within layerI show undulose extinction and subgrains but no SPO parallel to the S2 foliation. Chl within layerI shows a penetrative SPO parallel to S2 and undulose extinction. Within layerII S2 is defined by SPO of Chl, Carb and opaque trails; in layerII Srp and Cpx show SPO mainly parallel to S2 .
The stable assemblage in ultramafics during D2 is:
Amp + Cpx + Chl + Tlc + Srp
while in serpentinites it is defined by:
Srp + Chl + Carb + opaque ± Amp ± Cpx