Ultra-high-pressure metamorphic rocks in the Western Alps

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Abstract
The focus of this paper is on the UHP rocks of the western Alps and particularly on the southern Dora-Maira massif where demonstrated coherency and a wide range of lithologies preserve UHP mineral species. The coesite-bearing Brossasco-Isasca Unit (BIU) of the southern Dora-Maira massif (DMM) is a coherent fragment of Variscan continental crust involved in the Alpine collisional orogenic belt. Two complexes were recognized: (1) a “polymetamorphic complex,” consisting of paraschist with minor eclogite, marble, and fine-grained phengite-rich gneiss. Some paraschists contain relics of Variscan amphibolite-facies rocks; (2) a “monometamorphic complex,” consisting of orthogneiss with local augen fabric, which contains relics of undeformed metagranitoids and pyrope-bearing whiteschist layers. The primary intrusive relations between “polymetamorphic” and “monometamorphic” complexes are indicated by the local occurrence, all along the main contact, of relics of thermally metamorphosed paraschists. The Variscan age of the granitoid emplacement into the pre-Alpine amphibolite-facies basement is constrained by a 303 ± 1 Ma U/Pb zircon age. Therefore, before the Alpine orogeny the BIU consisted of a Variscan amphibolite-facies basement intruded by late-Variscan granitoids. The peak metamorphic conditions within the BIU are constrained by the occurrence of pyrope + coesite instead of talc + kyanite and of the divariant assemblage jadeite + pyrope + glaucophane and are estimated to be P = 33 ± 3 kbar and T = 750 ± 30° C.

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