We conducted a detailed study of a unique metapelite from the ultrahigh-pressure (UHP) Brossasco-Isasca unit (Dora-Maira Massif). It consists of pre-Alpine porphyro-blastic garnet (Grt1), Alpine-age idioblastic garnet (Grt2), phengite, and kyanite. The pre-Alpine garnet contains inclusions of staurolite, kyanite, paragonite, chloritoid, chlorite, quartz, ilmenite, and rutile. The metapelite also includes quartz aggregates after former coesite and fine-grained paragonite aggregates after jadeite. Late chlori-toid and staurolite idioblasts, first reported from a Brossasco-Isasca unit metapelite, are also present in the rock matrix: they grew randomly across the main Alpine UHP foliation defined by phengite. The decompression pressure-temperature ( P - T ) path of the unit was constructed employing a P - T pseudosection calculated for the model MnNKFMASH (MnO-Na 2 O-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O) system in the pressure interval 0.6–1.8 GPa, i.e., below the coesite-quartz transition and within the albite stability field. The absence of albite and biotite, together with the compositions of staurolite, chloritoid, chlorite, and phengite allow us to constrain the 580–590 °C portion of the retrograde P - T path from P = 1.12–0.96 GPa. The results are in good agreement with previously estimated P - T paths, mostly inferred from retrograde assemblages in whiteschist and eclogite of the Brossasco-Isasca unit.

New and recycled intermediate to felsic crust in island arcs and continental margins is generated by magmatism surmounting convergent plate junctions. Paired Pacific-type orogens develop adjacent sites of long-lived subduction of oceanic lithosphere. They consist of (1) an outboard accretionary prism deposited in and continentward from the oceanic trench, the filling of which is derived mainly from (2) an inboard calc-alkaline volcanic-plutonic arc. The trench assemblage includes arc-sourced clastic mélanges, minor but widespread deepwater cherts and carbonates, and tectonically disaggregated ophiolites. These relatively incompetent sections recrystallize under high-pressure (HP) conditions and decouple from the descending oceanic plate at depths of ∼15–50 km. A massive, slightly older to coeval andesitic-granodioritic arc dominates the landward belt where new sialic crust is added from I-type magmas, and preexisting continental materials are recycled as S-type melts; high temperature (HT) characterizes local-regional metamorphism of the wall rocks. In contrast, Alpine-type orogens result from the underflow of an oceanic plate that transports island arcs, microcontinents, and/or continental salients into the subduction zone. Reflecting lithospheric coherence, the sialic crust may be carried down as much as 90–140 km, becomes thermally softened, and decouples from the sinking plate. Metamorphism of deeply subducted parts of Alpine belts ranges from high pressure to ultrahigh pressure (UHP), and is not paired with a HT calc-alkaline arc. In both types of subduction complex, outboard thrust faults dip landward and fold vergence is seaward, reflecting the polarity of underflow (and rollback) of the oceanic lithosphere. Antithetic thrusting typifies some contractional continental realms. Propelled by buoyancy, allochthonous nappes conduct relatively low-density HP and UHP sections to midcrustal levels. At convergent syntaxes (plate-margin cusps) of overthickened arcs, tectonic aneurysms may produce domical uplifts, further exhuming UHP units surfaceward. The regional crustal thickness of an active mountain belt partly reflects climate as well as the main convergent-transform plate processes producing the orogen. The volcanogenic Chilean Cordillera parallels the seaward convergent plate junction and the eastward-sinking Nazca plate. The highest ranges and thickest continental crust (∼70 km) occur in the compressed north-central Andes at ∼25°S, a region of extremely low rain- and snowfall. Aridity and low erosion rates help account for the high-standing calc-alkaline volcanic-plutonic contractional arc and the elevated, internally drained Altiplano downwind. Thus erosional degradation is weakly developed. A very different climate typifies the orogen at ∼45°S, where abundant moisture-laden westerlies bring abundant precipitation to the Chilean margin. The rugged mountain belt is of lower elevation compared with the north-central Andes and is supported by a crust of only moderate thickness (35–40 km). Vigorous erosion supplies voluminous detritus to the offshore Chile-Peru Trench as well as eastward, so a plateau is lacking in the lee of the arc. The Himalayas and Tibetan Plateau, the Sierra Nevada and Colorado Plateau, the Japanese island arc, and New Zealand exhibit somewhat similar relationships between crustal thickness and precipitation-linked erosion.

Abstract In the Western Alps, two tectonic units have unquestionably experienced ultrahigh pressure metamorphism (UHPM): the continental Brossasco-Isasca Unit of the southern Dora-Maira Massif, in which coesite was first reported by Chopin (1984) , and the ocean-derived Lago di Cignana Unit of the Piemonte zone, in which coesite was first reported by Reinecke (1991) . In both units the UHPM recrystallisation, acquired during the early stages of the Alpine orogeny, is largely obliterated by a late Alpine greenschist facies retrogression, more pervasive in the felsic lithologies.