Lithotypes and mesostructures
The area is characterized by a penetrative mineralogical foliation marked by SPO of eclogite facies minerals, varying as a function of the bulk rock chemistry. Lithotype distribution and mesostructures are reported in Figure 2. Mineral abbreviations after Kretz (1983).
Figure 2. Simplified geological-structural map of the Lago della Vecchia area. Original field survey at scale 1:5.000.
Lithotypes
Metagranites are divided into two groups, distinguished on the basis of mesoscopic fabrics: 1) Coronitic metagranites and 2) deformed metagranites; minor lithotypes are metapelites, glaucophanites, eclogites and dykes, further distinguished on the basis of their mineral assemblages.
The deformed metagranites constitute the most common lithotype in the area (Fig. 2); they are characterized by a well-developed foliation marked by shape preferred orientation (SPO) of white mica and quartz (Fig. 2). Omphacite and glaucophane are also characterized by SPO generally parallel to the main foliation. Eclogites occur as decametre to metre-thick boudins within deformed metagranites (Fig. 2). Glaucophanite bodies may reach ten of meters of width and hundred of meters of length. The SPO of glaucophane marks the lineation within glaucophanites whereas the SPO of omphacite defines the lineation in eclogites.
Coronitic (i.e. undeformed) metagranites have been found in two localities within phengitic-mica + omphacite-bearing gneisses as meter-size lenses (Fig. 2). Coronitic metagranite is constitute by quartz (30-40%) + plagioclase (30-35%) + biotite (15-25%) + K-feldspar (5-10%) ± white mica (< 5%). K-feldspar, biotite and plagioclase are euhedral to subhedral while quartz is anhedral being interstitial among other phases. The aggregates of coronitic metagranites are inequigranular, polygonal to interlobate where K-feldspar defines up to 1.5 cm crystals (Fig. 3a); plagioclase, biotite, white mica and quartz size varies from < 2 mm to 5 mm.
Geological evolution and mesostructures
Geological map of figure 2 shows the distribution of main structures; the area is characterized by a penetrative mineralogical foliation marked by SPO of eclogite facies minerals, varying as a function of the bulk rock chemistry.
As metre-scale relict volumes, undeformed igneous textures are still preserved, wrapped by the eclogitic foliations. Magmatic structures are abbreviated Mag while deformational D, both with subscripts that refers to the relative chronology, from older (e.g. Mag0) to younger (D5).
On the basis of the mesostructural overprinting relations the following superimposed groups of structures have been divided:
Isotropic textures are preserved in coronitic metagranites (Mag0) (Fig. 3a). They are characterized by preserved biotite, K-feldspar and plagioclase euhedral-subhedral grains and interstitial quartz. Biotite preserves a dark red color and its size ranges from 0.5 cm to 2 cm. K-feldspar may occur as white to pink up to 2.5 cm grains. Plagioclase has a white to greenish colour and sizes from 1 to 2 cm.
During D1 and D2 a mm-thick foliation develops within deformed metagranites (S1); S1 is marked by SPO of white-mica, quartz and omphacite; cm-thick omphacite grains also define S1 (Fig. 3b,c,d). Glaucophane SPO marks the S1 foliation within glaucophane-bearing deformed metagranitoids together with Wm aggregates. During D1 boudinage occurs, producing meter-scale boudins of eclogites and glaucophanites. Within boudins the S1 foliation is marked by SPO of omphacite, glaucophane and white mica.
During D2, S1 is folded (Fig. 3c) and an axial planar foliation S2 formed. S2 is marked by the SPO of white mica, omphacite, glaucophane and quartz in deformed metagranites; elongated boudins of eclogites and glaucophanites also occur parallel to S2 (Fig. 3e,g). Within the boudins S1 shows an high angle with respect to external S2 and lithological boundaries.
D3 is associated with open folding at metre-scale with steep axial plane E-W dipping. D3 folding is generally associated with discrete ductile-brittle shear zones and a localised crenulation cleavage marked by white mica ± chlorite.
During Mag4 the intrusion of andesitic dykes occurs in the whole area. Andesitic dykes are 50cm to 1-2m thick and generally crosscut all lithotypes and foliations. These dykes have been dated across the Sesia-Lanzo Zone at about 29-33 Ma (Scheuring et al. 1973; Kapferer et al. 2009) (Fig. 3h).
D5 is associated with fracturing and faulting that involve all lithotypes.
Figure 3. Mesostructural features of the Lago della Vecchia metagranite and associated rocks
A) coronitc metagranite preserving igneous textures and mineralogy. Cm-sized K-feldspar crystals and mm-sized biotite are visible in the photograph. Pale-brown aggregates corresponds to plagioclase crystals replaced by aggregates of epidote and white mica. B) Localised shear zone occurring within the metagranite body along lithological contact with leucocratic granite. S-C foliations develop in few centimeters within the undeformed granite. C) Within deformed metagranites the foliation marked by shape preferred orientation of white mica, omphacite and quartz is locally folded. D) Dark, centimeter-sized stretched enclaves in mylonitic metagranite. The mylonitic foliation in the metagranite is marked by shape preferred orientation of quartz, white mica and omphacite, while it is marked by omphacite and glaucophane in the enclaves. E) Omphacite, garnet, white mica epidote eclogite. F) Garnet-bearing leucocratic metagranite. Cm-sized garnet show mm-sized corona of glaucophane; glaucophane mm-sized crystals form aggregates slightly oriented parallel to the foliation. G) Metre-sized eclogite boudins within deformed metagranite. H) Andesitic dykes crosscutting foliated metagranite.