Microstructural Analysis

We will focus on coronitic microstructures developed over the Mag0 parageneses aiming to reconstruct the microstructural and chemical evolution of igneous microdomain during the Alpine evolution. At macro- and microscopic scale, four igneous microdomains have been distinguished: biotite (BtI), white mica (WmI), plagioclase (PlI), K-feldspar (KfsI). Each microdomain is characterized by the development of peculiar microtextures and mineral assemblages during the Alpine evolution (Figs. 4, 5, 6).

Igneous biotite microdomain (BtI)

BtI occurs as 0.5-5 mm sized single crystals or aggregates of grains of low relief. BtI exhibits red-brown colours with a pleochroism from brown to reddish brown, tabular (001) habit and pseudo-hexagonal outline. Inner part of crystals are darker whereas rims are lighter (Fig. 4). BtI may be in contact with Qtz, PlI and WmI.

The boundaries between BtI and quartz do not show any corona or reaction rims, whereas coronas develop at the boundaries between BtI and PlI (Fig. 4b,d,e,f); BtI-PlI boundaries are delimited by a continuous corona of GrtI (Fig. 4a-f). Toward BtI grains, GrtI corona is associated with grains of about 30-50 µm of white mica (WmII) and pale brown BtII, defining a composite corona; correspondingly, toward PlI microdomains GrtI is associated with aggregates of < 10µm of WmII (Fig. 4b-f). GrtI is cleaner towards PlI microdomain than toward BtI (Fig. 4b, c). This feature is clear at BSE images (Fig. 4e,f) where the darker part of the GrtI corona is always the one towards the PlI microdomains, where it is in contact with WmII; the lighter part of the corona always face toward BtI microdomains associated with large WmII crystals. GrtI growing on the BtI side are also characterized by inclusions of Qtz.

Igneous white mica microdomain (WmI)

WmI occurs as single crystals (Fig. 4g,h) of 0.2–1 mm, showing perfect cleavage (001). Undulose extinction occurs (Fig. 4g,h) and it is often associated with a gentle folding of the WmI grains. WmI crystals are surrounded by thin aggregates of WmII. Where WmI is in contact with PlI, corona of WmII and GrtI develop: they are similar to the ones observed between BtI and PlI (Fig. 4h). WmII close to the WmI crystals have a coarser grain-size than WmII-aggregates at the WmI-PlI microdomain boundaries (Fig. 4g).

Igneous plagioclase microdomain (PlI)

PlI is not preserved but corresponding pseudomorphic assemblages are easily distinguished by their meso- and microscopic shape and mineral association (Fig. 4a,b,d,e). PlI microdomains are several mm in their size and have rectangular shapes; they are constituted by fine-grained aggregates of EpI + WmII + PlII ± GrtI (Fig. 4) that replace the cores of PlI. PlI rims are characterised by coarse grained aggregates of WmII at the PlI-BtI and PlI-WmI boundaries (Fig. 5f). Fine-grained aggregates of EpI + WmII + PlII + Grt occur at the PlI-KfsI microdomain boundaries (Fig. 5b-d).

Igneous k-feldspar microdomain (KfsI)

Several cm-thick KfsI microdomains are preserved (Fig. 3a). KfsI occurs as single crystals characterized by tartan twinning (Fig. 5d), deformation bends and undulose extinction (Fig. 5e). KfsI is fractured and partially replaced by aggregates of PlII (Ab). Coarse-grained (0.5-1 mm) PlII typically replaces KfsI from rims (Fig. 5d) or within fractures and forms aggregates of several mm. PlII may show simple or polysynthetic twinning. Fine-grained (< 50µm) WmII + PlII define corona between KfsI and PlI microdomains. Moreover, several mm to cm sized igneous allanite (AlnI) occurs within coronitic metagranites (Fig. 5g,h). AlnI is characterized by chemical zoning and it is often associated with igneous Ttn (Fig. 5g).

Summary of microstructural analysis

The excellent preservation of the geometry of the igneous microdomain, as well locally mineralogy, allowed definition of the reactions that occurred at specific domain boundaries and reconstruction of the primary igneous assemblage, constituted by: BtI + WmI + Qtz + Kfs + PlI ± Aln ± Ttn. At the boundaries between BtI and PlI microdomains, two coronas develop: 1) BtII + WmII + GrtI; 2) WmII + GrtI. The WmII + GrtI + Ep + Ab aggregates occur between WmI and PlI and KfsI – PlI microdomains and pseudomorphically replace PlI cores. The KfsI cores are partly to completely substituted by Ab from the PlI - Kfs boundaries or along fractures.

Figure 4. Microphotographs


A) Igneous biotite, plagioclase, white mica and quartz microdomains are well distinguishable: igneous plagioclase is replaced by fine-grained aggregates with high relief. The boundaries between ex-plagioclase and BtI microdomains are marked by continuous GrtI + WmII corona. B and C) Details of the GrtI + WmII corona at the boundary BtI-PlI. D, E and F) BSE-SEM image of BtI – PlI microdomain. A continuous corona of garnet defines the boundary between BtI and original igneous plagioclase. WmII grows in association with epidote and albite in plagioclase. Garnet shows clear zoning: the inner part, BtI-facing, is lighter and rich in quartz inclusions; the external part, PlI-facing, is darker and clean. G) WmI surrounded by thin aggregates of WmII. H) BSE-SEM of the WmI microdomain associated with aggregates of WmII + Grt + Ep + Ab.

Figure 5. Microphotographs


A) Plane polarized light image of large K-feldspar site partially substituted by a pale-brown aggregate of albite. B) Image A - crossed polars. C) Detail of B; fine-grained aggregates of WmII associated with fine grained Ab that replace PlI; left side of the microphotograph is occupied by large PlII (Ab) aggregates substituting Kfs megacrystals. D) Kfs megacrystals characterized by tartan twinning partially substituted by aggregates of PlII (Ab) at the rims. Right side of the microphotograph: fine-grained aggregate of PlII+Ep+WmII substituting the PlI microdomains. E) Large crystals of Kfs with undulose extinction and PlII aggregates. F) BSE-SEM image of PlI microdomain completely substituted by aggregates of PlII+WmII+Ep±Qtz and crystals of BtI rimmed by BtII and WmII crystals. G) Igneous allanite in contact with igneous titanite. Aln is characterized by zoning from brown core to pale-brown, colorless rims. H) X-ray map image showing the distribution of Th in the igneous allanite.