Discussion
The coronitic metagranite of the Lago della Vecchia is characterized by the preservation of igneous textures but a partial to total replacement of primary minerals. The Alpine evolution produced reactions and corona textures by the exchange of chemical elements, occurring at different times, between adjacent minerals.
Igneous biotite is partly preserved, white mica and quartz grains record re-equilibration but relicts are still preserved; plagioclase and K-feldspar are nearly completely replaced by Alpine parageneses. The mineralogy, grain-size and thickness of the Alpine reaction rims and corona depend on the nature of the reacting domains, chemistry and position in the rock body of the reacting microdomains.
The microstructural study presented here show the development of a series of textures:
Biotite microdomain at contact with plagioclase – two coronas are observed: 1) BtII + WmII + Alm-rich GrtI; 2) WmII + Grs-rich Grt coronas;
White mica microdomain with plagioclase: a single WmII + Grt + Ep + Ab corona develops;
K-feldspar microdomain with plagioclase: a single Ab + WmII ± Ep ± Grt corona develops;
Plagioclase core microdomain: Ab + WmII (fine-grained) + Ep ± Grt;
K-feldspar core microdomain: large igneous grains are partly to completely replaced by Ab from the plagioclase-K-feldspar boundaries or along fractures.
These microstructures and chemical patterns display peculiar characters elsewhere described for similar rocks, with some differences.
From the Sesia-Lanzo Zone the example of the Mucrone and the Mars metagranites and metagranodiorites are well known (Oberhansli, 1985; Koons et al. 1987; Zucali et al., 2002; Bruno and Rubbo, 2006). They are characterized by microstructural relicts of the igneous assemblages, now completely replaced by the eclogite facies Alpine mineral assemblages, except for local biotite and amphibole (Rubbo et al., 1999; Zucali et al. 2002b).
Despite the microstructural similarities, the presented microstructures differ from those of the Monte Mucrone-Mars for the metamorphic assemblages produced within original igneous plagioclase microdomain and, in particular, the absence of jadeite in the rocks of the Lago della Vecchia. The plagioclase microdomain in the Mucrone metagranite is replaced by aggregates of Jd + Grt + Wm + Qtz while at the Lago della Vecchia, Zo + Wm + Grt + Qtz + Ab aggregates replace the plagioclase. Microstructures and mineral assemblages within the other igneous domains are similar as well the compositions. Moreover, similar garnet corona have been described for the eclogite facies Alpine metamorphism from metapelites of the Monte Rosa nappe, Western Italian Alps (Keller et al., 2004; 2006). The microstructures and chemical variations are described as enhanced by mass transfer through short-circuit diffusion, where nanometer wide channels occur as direct links between reaction fronts, providing fast diffusion preferred paths. Chemical profiles of the garnet corona between BtI and PlI domains suggest diffusive mass transfer for the growth of garnet during the high pressure metamorphic event as inferred by Keller et al. (2006) and Bruno and Rubbo (2006).
The garnet growth may have started with an homogenous composition (Alm-rich), produced until Pl was stable; when PlI released Ca the Grs content consequently increased, starting from the grain boundary with PlI and Alm decreased (Fig. 10). Grs content increase stopped when Zo became stable. At this point the Ab should have become unstable as the reaction curve Ab = Jd + Qtz was crossed, but this does not seem to have occurred in this rocks. However, as suggested by Konrad-Schmolke (2005), the complex evolution of garnet growth and the limited element supply during the growth evolution may mask the thermobarometric history.
The calculated composition of the plagioclase core (Table 2; Plagioclase microdomain) differs from an igneous oligoclase composition (e.g. Na0.8Ca0.2Al1.2Si2.8O8); the reconstructed Pl composition is enriched in K2O (2.50 wt%), MgO (1.15 wt%), FeO (2.01 wt%), Fe2O3 (1.95 wt%) and H2O (e.g. Zo and Wm) and it is depleted in Na2O (< 4.00 wt%).
Following the combination of microstructural criteria and chemical observations it is possible to describe unbalanced qualitative reactions, involving the igneous mineral chemistry and adding other chemical species to account for diffusion, most likely enhanced by the H2O produced by the progressive break-down of biotite at high pressure (Fig. 11):
BtI → BtII + WmII + Alm-rich GrtI
BtI + Ca2+ (PlI) → BtII + WmII + Grs-rich GrtI
WmI + Ca2+ (PlI) → WmII + Grt
PlI + H2O + K+ + Ti4+ + Mg2+ +Fe2+ → Ab + WmII + Ep ± Grt
PlI rim + H2O + K+ + Ti4+ + Mg2+ +Fe2+ → WmII + Ab ± Ep ± GrtI
PlI core + H2O + K+ → WmII + Ep + Ab + GrtI
K-feldspar + Na+ → Ab + K+
Thermobarometrical estimates of the Alpine evolution (Fig. 11) point to a blueschists facies conditions re-equilibration at P = 12.5kbar and T = 450-550°C, where the pressure are limited due to the absence of jadeite or omphacite in the assemblage.
Conversely, the mesoscopic observations (Figs. 2, 3) and the field continuity with the rest of the Eclogitic Micaschists Complex, where the stable associations developed under eclogite facies conditions at P > 20kbar and T = 500-550°C (Tropper and Essene, 2002; Zucali 2002a, b; Zucali and Spalla, 2011), suggest consideration of similar P-T conditions for these rocks.
In support of this hypothesis, garnet growth modelling for the Monte Mucrone (Bruno and Rubbo, 2006) predicts peak conditions of P > 16kbar and T = 550-600°C; qualitative comparison of chemical evolutions of the garnet corona from the Lago della Vecchia with those described for the Monte Mucrone and their similar size (30-70 µm) allows a similar growth model and, consequently, inference of similar P-T conditions of growth for the GrtI corona, in agreement with mesoscopic observations (Fig. 2). But there is still a discrepancy regarding the absence of Na-pyroxene and the widespread presence of Ab within Pl and Kfs domains, since albite should be replaced by high pressure assemblages as jadeite + quartz, while attaining eclogite facies conditions.
Future analyses should aim at defining the P-T conditions reached by surrounding deformed metagranites to discriminate between the two possible interpretations: i) the coronitic metagranites only recorded part of the prograde path while the surrounding rocks recorded peak conditions (e.g. Arenas and Martinez Catalán, 2002); ii) the Lago della Vecchia metagranites suffered lower pressure conditions compared to the southern part of the same Eclogitic Micaschists Complex (e.g. Monte Mucrone-Monte Mars area).
The P-T estimates for the igneous stage provide T = 700-730°C, obtained from the igneous BtI compositions. No other constraints are available for this pre-Alpine stage.
Figure 11. P-T-t path of the Lago della Vecchia metagranite
P-T-t path of the Lago della Vecchia metagranite using microstructural analysis and thermobarometrical estimates from this work and P-T estimates from Zucali et al. (2002a) for comparison with similar evolution of the high strain rock volumes within the same Eclogitic Micaschists Complex and reference to pre-Alpine igneous crystallization P-T conditions.