Journal of the Virtual Explorer

Bt_I occurs as 0.5-5 mm sized single crystals or aggregates of grains of low relief. Bt_I 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). Bt_I may be in contact with Qtz, Pl_I and Wm_I.

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

Wm_I 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 Wm_I grains. Wm_I crystals are surrounded by thin aggregates of Wm_II. Where Wm_I is in contact with Pl_I, corona of Wm_II and Grt_I develop: they are similar to the ones observed between Bt_I and Pl_I (Fig. 4h). Wm_II close to the Wm_I crystals have a coarser grain-size than Wm_II-aggregates at the Wm_I-Pl_I microdomain boundaries (Fig. 4g).

Pl_I is not preserved but corresponding pseudomorphic assemblages are easily distinguished by their meso- and microscopic shape and mineral association (Fig. 4a,b,d,e). Pl_I microdomains are several mm in their size and have rectangular shapes; they are constituted by fine-grained aggregates of Ep_I + Wm_II + Pl_II ± Grt_I (Fig. 4) that replace the cores of Pl_I. Pl_I rims are characterised by coarse grained aggregates of Wm_II at the Pl_I-Bt_I and Pl_I-Wm_I boundaries (Fig. 5f). Fine-grained aggregates of Ep_I + Wm_II + Pl_II + Grt occur at the Pl_I-Kfs_I microdomain boundaries (Fig. 5b-d).

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

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: Bt_I + Wm_I + Qtz + Kfs + Pl_I ± Aln ± Ttn. At the boundaries between Bt_I and Pl_I microdomains, two coronas develop: 1) Bt_II + Wm_II + Grt_I; 2) Wm_II + Grt_I. The Wm_II + Grt_I + Ep + Ab aggregates occur between Wm_I and Pl_I and Kfs_I – Pl_I microdomains and pseudomorphically replace Pl_I cores. The Kfs_I cores are partly to completely substituted by Ab from the Pl_I - Kfs boundaries or along fractures.

Figure 4. Microphotographs

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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 Bt_I microdomains are marked by continuous Grt_I + Wm_II corona. B and C) Details of the Grt_I + Wm_II corona at the boundary Bt_I-Pl_I. D, E and F) BSE-SEM image of Bt_I – Pl_I microdomain. A continuous corona of garnet defines the boundary between Bt_I and original igneous plagioclase. Wm_II grows in association with epidote and albite in plagioclase. Garnet shows clear zoning: the inner part, Bt_I-facing, is lighter and rich in quartz inclusions; the external part, Pl_I-facing, is darker and clean. G) Wm_I surrounded by thin aggregates of Wm_II. H) BSE-SEM of the Wm_I microdomain associated with aggregates of Wm_II + Grt + Ep + Ab.

Figure 5. Microphotographs

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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 Wm_II associated with fine grained Ab that replace Pl_I; left side of the microphotograph is occupied by large Pl_II (Ab) aggregates substituting Kfs megacrystals. D) Kfs megacrystals characterized by tartan twinning partially substituted by aggregates of Pl_II (Ab) at the rims. Right side of the microphotograph: fine-grained aggregate of Pl_II+Ep+Wm_II substituting the Pl_I microdomains. E) Large crystals of Kfs with undulose extinction and Pl_II aggregates. F) BSE-SEM image of Pl_I microdomain completely substituted by aggregates of Pl_II+Wm_II+Ep±Qtz and crystals of Bt_I rimmed by Bt_II and Wm_II 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.