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The occurrence of Alpine peridotites may reflect the interaction of two or more lithospheric plates, at least one of which bears continental crust. Bodies called Alpine ultramafic rocks include: (1) hot diapiric masses with thermal aureoles suggesting intrusion into or beneath crustal rocks at high temperatures; (2) mantle slabs, principally of oceanic crust and mantle, such as Tethyan ophiolitic complexes, formed by diapiric upwelling of mantle material at spreading centers, which were subsequently and independently emplaced into the continental margins; (3) disrupted mantle slabs incorporated into melanges; and (4) conformable bodies in regionally metamorphosed terranes representing recrystallization and deformation of occurrences of types 1, 2, or 3. Most mantle slabs possibly are emplaced by abortive subduction of a continental margin. This event is seen in mountain systems as the first deformation of a geosynclinal sequence. The P-T conditions of ultramafic assemblages, contact metamorphism, and regional metamorphism should provide data on the thickness of crust and geothermal gradients at times of mantle upwelling and/or crustal emplacement of ultramafic material.

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