Quantitative analysis of metamorphic reaction progress along strain gradients in granitoid and basic rocks from Lago della Vecchia, Sesia-Lanzo Zone, Western Alps

The quantification of the actual rock volumes in which metamorphic transformations progress is a key to investigate the complex interactions of mineral chemical and phase changes during heterogeneous deformation development. Recognizing homogeneous metamorphic domains for each re-equilibration stage is the basis for reconstructing the tectono-metamorphic evolution via analysis of structural and petrologic overprints. The Sesia Lanzo Zone (SLZ) belongs to the Austroalpine domain of the Western Alps and is interpreted as a slice of the Adria continental margin involved in the Alpine subduction and collision. The SLZ experienced eclogite facies conditions, followed by a re-equilibration under blueschist conditions compatible with an exhumation accomplished during the subduction of oceanic lithosphere. Afterwards, it was partially affected by a greenschist facies event, associated with the exhumation to the uppermost part of the tertiary nappe stack. The study area (Lago della Vecchia) is located within the central SLZ. To investigate the textural and chemical heterogeneities developed during Alpine deformation and metamorphism partitioning, we used two ArcGis-based tools: (i) Q-XRMA creates polygons that represent mineral phases, quantifies the modal mineral percentage, and the element concentrations of selected mineral phases by calibrating the X-ray maps with mineral chemical analysis used as internal standard; (ii) min-GSD allows the quantitative analysis of the grain size distribution of different phases combing the optical scans, the vectorization of the scans, and the Q-XRMA mineral classification. Here we show the results for a coronitic metagranitoid and mylonitic metabasite samples, with a dominant blueschist imprint and igneous and eclogitic mineral relicts. For both samples we applied both methods to the entire thin-section and to crucial microdomains. Metagranitoid sample displays different D2 coronae of fine-grained mineral associations (Grt+Bt+Wm+Pl) that depend on the igneous microdomains replaced. The analyses focus on the continuous and composite corona of Grt+Wm+Bt+Pl developed between igneous Bt and Pl domains. In particular, Grt displays an Mn-reach core and asymmetrical rim zonation that consists of Mg and Ca enrichment towards Bt and Pl domains, respectively. The metabasite sample analysis is in progress and it focuses on the calibration of garnet, pyroxene, and white mica to quantify the spatial distribution of the successive generation of these phases to be related with chemical variations in relation to D1 and D2 stages. These petrological and chemical results allow more accurate individuation of microdomains in chemical and textural equilibrium for more reliable thermobarometric estimations of D1 and D2 tectono-metamorphic stages and better estimation of degree of metamorphic reaction progress along strain gradients.