DEFORMATION AND METAMORPHISM RELATIONSHIPS IN ACID AND FEMIC PROTHOLITHS OF THE AUSTROALINE CONTINENTAL CRUST SUBDUCTED AND EXHUMED IN THERMAL REGIME SEVERELY DEPRESSED

Intrusives of pre-Alpine age and their high grade country rocks, subducted-exhumed in Alpine times, has been studied with petro-structural methods, applied after quality mapping. In this subduction environment, focus was oriented on interaction of deformation and metamorphic transformation progress, in areas offering a mosaic of different tectonometamorphic steps. Such apparently incoherent distribution of structural imprints, representing various discrete states of the tectonic sequence heterogeneously frozen in adjacent space, has been recomposed into a coherent progression of deformation events by means of foliation trajectory mapping. Such work guaranteed a petrographic analysis of the mineral support of sequentially ordered planar fabrics, used for thermobarometrical estimates of the deformative and metamorphic evolution. This evaluation has been performed in acid and femic protoliths intruded in the continental crust of the Sesia-Lanzio Zone in the internal Western Alps

Map of the strain and reaction-rate partitioning in the Mombarone-Mt. Mucrone area (Sesia Lanzo Zone, Italian Western Alps)

The Mt. Mucrone \u2013 Mt. Mars \u2013 Mombarone area includes several Permian intrusive bodies and metasediments preserving granulitic to amphibolitic pre-Alpine metamorphism widely re-equilibrated under eclogite-facies conditions during the Cretaceous. They are now part of the Eclogitic Micaschists Complex of the Sesia-Lanzo Zone in the Alpine HP metamorphic belt of the Western Alps. During the field structural analysis, seven groups of ductile structures were recognized as having evolved as follows: the D1 \u2013 D3 fabrics took place under eclogite-, D4 under blueschist- and D5 to D7 under greenschist-facies conditions, respectively. Combined structural and petrologic studies generated maps that correlate the degree of fabric evolution and metamorphic transformation at map scale and allow to conclude that, over a critical threshold of fabric reworking, the accomplishment of metamorphic transformations increase proportionally with the degree of fabric evolution. Such map-scale counterpart of outcrop-size deformation zones makes strain-partitioning a critical factor controlling reaction progress

Multiscale structural analysis in the subducted continental crust of the internal Sesia-Lanzo Zone (Monte Mucrone, Western Alps)

Intrusives of Permian age and their high-grade country rocks, subducted and exhumed in Alpine time, have been studied with petro-structural high resolution mapping. In this subduction environment of the Sesia-Lanzo Zone of the internal Western Alps, focus was oriented on interaction mechanisms between deformation and progress of metamorphic transformations, in an area offering a mosaic of different tectonometamorphic evolutionary steps. Such apparently incoherent distribution of structural imprints, representing various discrete states of the tectonic sequence heterogeneously frozen in adjacent spaces, has been previously recomposed into a coherent progression of deformation events by means of foliation trajectory mapping. This work guaranteed a petrographic analysis of the mineralogical support of sequentially ordered planar fabrics. Six deformation episodes (D1 to D6) were recognised, as superposed folds, foliations and ductile shear zones systems, evidently related to the Alpine tectonic history. Petrologic estimates on time-related microstructures well manifest that eclogite facies metamorphic conditions assisted D1 and D2 deformation stages, the blueschist-facies re-equilibration was contemporaneous with D3 and the greenschist-facies re-equilibration with D4. Recognition of numerous assemblages and textures of pre-Alpine protoliths supported validity of the individuation of the earliest, more cryptic tectonic environment of the widespread eclogitisation that predates D2. The coherent correlation between structural and metamorphic mineral growth histories on a wide area served as a base for further refinement and revealed that full granular scale diffusion of newly-forming parageneses is generally related to attainment of a threshold of intensity in the development of the new planar fabrics, as similarly established in another subducted-exhumed metamorphic complex of the internal Central Alps

A new petro-structural map of the Monte Mucrone metagranitoids (Sesia-Lanzo Zone, Western Alps)

The Mt. Mucrone metagranitoid is an extensively investigated intrusive Permian body, located in the Eclogitic Micaschists Complex of the Sesia-Lanzo Zone, within the high-pressure metamorphic belt of the Western Alps formed during the Alpine plate convergence. The structure of the northwestern sector of Mt. Mucrone, including both the metagranitoids and country rocks, has been mapped, although an integrated structural and petrological analysis is still lacking in its southwestern sector, a topic investigated in this contribution. During the field structural analysis, six groups of ductile structures were recognized to have evolved as follows: the D1 and D2 fabrics took place under eclogite-facies, D3 under blueschist-facies and D4 to D6 under greenschist-facies conditions, respectively. Foliation trajectories revealing the chronology of the superposed structures are represented in the analytical (drift and solid) and interpretative (solid) maps, both at a 1:10,000 scale, and a panel of structural cross-sections allows the 3D representation of the poly-deformed lithostratigraphy. The related metamorphism indicates the changes within the subduction-collision tectonic frame