Origin of ultramafic rocks, their tectonic setting and history: A contribution to the discussion of the paper “the origin of ultramafic and ultrabasic rocks” by P.J. Wyllie

Abstract

A subdivision of the orogenic peridotites into ophiolitic (or truly Alpine-type) and “root-zone” peridotites is proposed, and their differences in tectonic setting and metamorphic facies are discussed. Some current hypotheses for the origin and mode of emplacement of the orogenic peridotites are briefly reviewed. It is shown next how various microstructural approaches, such as interpretative petrotectonic studies and the less commonly employed comparative petrofabric analysis of peridotites and their country-rocks, may be used to unravel the igneous, metamorphic and tectonic histories of ultramafic rocks in general. Criteria are established by which igneous flow fabrics, tectonic deformation fabrics and piezocrystalline fabrics of olivine may be distinguished. Special attention is paid to the piezocrystalline shape and lattice fabrics of olivine, deduced by Hartman and Den Tex (1964) through application of W.B. Kamb's theory of thermodynamic equilibrium orientation to the orthorhombic crystal lattice of olivine. The petrofabric habit of olivine and its role in shaping foliations or schistosities in metamorphic peridotites is emphasized. In the second part brief summaries are presented of the fabric and metamorphic data concerning three types of peridotite: (1) a “root-zone” peridotite, (2) an ophiolitic peridotite, and (3) some Iherzolitic nodules from alkali-olivine basalts, while their fabric and metamorphic histories are compared and contrasted. It is shown that they are all more of less complex metamorphic tectonites, having suffered several solid-state deformations and metamorphic events that are largely lacking in their host rocks, thus indicating that they were emplaced in the predominantly solid state. The garnetiferous “root-zone” peridotite of Alpe Arami (Swiss Pennine Alps) has a principal γ-olivine fabric perpendicular to the primary layering and first axial plane schistosity. It developed under eclogite- and granulite facies conditions, and is indicative of piezocrystallization in the absence of a liquid capable of dissolving olivine. After its solid emplacement it has also suffered Alpine tectonization and amphibolite facies rétrogradation. The ophiolitic lherzolite of the French Pyrenees has a principal α-olivine fabric perpendicular to the primary layering and first axial plane schistosity. This is also suggestive of piezocrystallization but in the presence of a liquid capable of dissolving olivine. The Iherzolite was then emplaced, aided by explosive gas release but, except for some fracture cleavages, it was not affected by penetrative Alpine deformation or metamorphism. The Iherzolitic nodules from Dreiser Weiher (F.R.Germany) possess a simple fabric, somewhat similar to that of the Pyrenean Iherzolite, whereas those from the Plateau de Velay in France have a much more complex fabric that may be derived from a “Dreiser Weiher fabric” by solid-state deformation and recrystallization of olivine and orthopyroxene crystals. It is concluded that solid derivation from upper mantle material is the most likely origin for the three types of peridotites, and that their impoverishment in eclogitic or basaltic constituents by partial fusion is improbable on account of their fabric and metamorphic history.