A New UHP-HP Tectono-Metamorphic Architecture for the Southern Dora-Maira Massif Nappe Stack (Western Alps) Based on Petrological and Microstructural Evidence

The southern Dora-Maira Massif, where coesite was first discovered 40 years ago, is among the most studied and better known example of high/ultra-high-pressure (HP/UHP) terranes. Previous to this study, the Polymetamorphic Basement Complex of the southern Dora-Maira Massif has been defined as a nappe stack consisting of three juxtaposed tectono-metamorphic units: the HP San Chiaffredo Unit at the bottom, the UHP Brossasco-Isasca Unit in the middle and the HP Rocca Solei Unit at the top. The origin of UHP metamorphism in the Brossasco-Isasca Unit is still controversial, due to the difficulties in reconciling the abrupt difference between the UHP conditions recorded by the Brossasco-Isasca Unit (i.e., 700 degrees C-730 degrees C, 4.0-4.3 GPa) and the HP conditions (i.e., similar to 500 degrees C-520 degrees C, 2.0-2.2 GPa) registered by the adjacent units. Here, we report new petrologic and microstructural evidence supporting the existence of a previously unrecognised UHP unit in the southern Dora-Maira Massif. Our data demonstrate that the tectonic unit overlying the Brossasco-Isasca Unit (i.e., the former Rocca Solei Unit), so far considered a HP unit, is actually divided in two units, one of which (the lowermost Rocca Solei Unit sensu stricto) experienced UHP conditions and the other (the uppermost Grimbassa Unit) reached HP conditions. The newly defined Rocca Solei Unit experienced UHP metamorphism at significantly different P-T conditions (520 degrees C-550 degrees C, 2.7-2.9 GPa) compared to the underlying Brossasco-Isasca Unit, but along a similar 'cold' T/P ratio (< 200 degrees C/GPa), markedly lower than that defined in the neighbouring Grimbassa Unit and San Chiaffredo Unit (> 230 degrees C/GPa). After more than 30 years of petrologic investigations, the tectono-metamorphic architecture of the southern Dora-Maira Massif is thus redefined, bridging the gap between the UHP Brossasco-Isasca Unit and the adjacent HP units and opening to new scenarios on its HP-UHP architecture. The results of this study have both regional and petrologic implications: (i) Similarities emerge in the structural position, thickness and metamorphic evolution of the new UHP Rocca Solei Unit in the southern Dora-Maira Massif and those of the Chasterain Unit recently discovered in the northern Dora-Maira Massif, suggesting a common architecture throughout the whole Dora-Maira Massif; (ii) the peculiar quartz microstructure in the metagranites described below represents an exceptional documentation of a 'frozen' quartz-to-coesite polymorphic reaction caught in the act and suggests that the availability of fluids was the most crucial factor controlling the progress of the reaction. The metastable persistence of quartz in H2O-undersaturated lithologies makes even more challenging the identification of UHP units that have only slightly exceeded the quartz-coesite transition and justifies why the newly defined UHP Rocca Solei Unit has remained 'hidden' for more than 30 years.