Metamorphism of Alpine Peridotite and Serpentinite

The emplacement of peridotite into continental crust results in changes in its mineralogical, structural, textural, and bulk chemical properties. Accompanying deformation may partially or completely remove from it the evidence of its original stratigraphy and any envelope of contact alteration it might once have possessed. Such rocks have long been known as alpine-type peridotites (Benson 1926) and their field relationships have been well characterized (Wyllie 1967). With advancing metamorphism, many clues as to the original nature of alpine-type peridotites are therefore effaced, but the loss need not be total if the processes involved are understood. To gain this understanding, we need to examine the inter­ relationships between temperature, pressure, stress, mineral transformations and reactions, textures, and diffusion, etc, in a relatively simple chemical system in the earth's crust. It is not the purpose of this review to study the origin of peridotite; melting and fractionation in the upper mantle; upwelling of peridotite at mid-ocean ridges, at fracture zones, or behind island arcs ; the anatomy or significance of ophiolites; or the crystallization of mafic and ultrabasic magma in the crust. The ample literature on these subjects has been well reviewed elsewhere (Wyllie 1967, 1969, 1970, O'Hara 1968, Miyashiro 1975). Further, the emplacement, tectonic environment, and different basic types of alpine peridotite have been extensively discussed in a number of recent papers (e.g. Coleman 1971a, Dewey & Bird 1971, Jackson & Thayer 1972, Moores & MacGregor 1972, Moores 1973). his textbook on metamorphism, Miyashiro was obliged to comment ( 1973, p. 30): In contrast to.other classes of metamorphic rocks, however, no detailed petro­ graphic data are available on the progressive changes to take place [ sic] in this (ultrabasic rock) class. It is hoped that, by gathering and reviewing here some of the recent pertinent literature, this data vacuum may at least partially be filled. This lack of systematic information is in curious contrast to the volume of literature on peridotites exotic to their surroundings-xenoliths in kimberlite, alkali basalt, and nephelinite, alpine cold-slab peridotites, serpentinites in melange zones, and samples dredged from the ocean floor. However, there do exist studies of ultra basic