Abstract

The Olympus ultramafic complex is one of three ultramafic complexes that crop out at the lowermost stratigraphic levels of the Troodos ophiolite, Cyprus. The Olympus ultramafic complex comprises two types of ultramafic and related rocks: (1) harzburgite tectonite, a residuum of partial fusion, and (2) cumulus chromitite, dunite, wehrlite, pyroxenite, and gabbro, the products of fractional crystallization and magmatic sedimentation of basaltic magma. The harzburgite tectonite is the basement or “country rock” intruded by the magma from which the cumulates crystallized. Pyrometamorphic textures in the harzburgite (magmatic pyroxenes in metamorphic peridotite) near its contact with the cumulates perhaps record “contact metamorphism” of the harzburgite by the magma.

The stratigraphically lowest cumulates were penetratively deformed (and hence are termed “ultramafic metacumulates”) during the same event that produced the dominant pyroxene foliation (S1) in the underlying harzburgite, whereas the stratigraphically highest cumulates were not penetratively deformed. The structural transition between metacumulates and cumulates is gradual and occurs in olivine cumulates that contain intercumulus clinopyroxene (clinopyroxene-bearing dunites and wehrlites).

Two mechanisms can explain the structural transition from metacumulates to cumulates: (1) Deformation of the harzburgite basement occurred during accumulation of the cumulates (syntectonic magmatic sedimentation); the lowermost (oldest) cumulates consequently deformed more than the uppermost (youngest) cumulates. (2) The uppermost cumulates, perhaps rich in intercumulus liquid (now crudely represented by postcumulus clinopyroxene and plagioclase) at the time of the deformation, may have accommodated the strain by grain boundary sliding (“crystal-mush flow”) and consequently left little evidence of solid-state, penetrative deformation.

If syntectonic and posttectonic magmatic sedimentation, crystal-mush flow, partial fusion of metamorphic peridotite, and multiple intrusion of magma act simultaneously during the formation of ophiolites, then the resulting field relations, structures, textures, and fabrics will be exceedingly complex, particularly if subsequent transport and emplacement impose strong metamorphic and tectonic overprints. One should expect that differences in the relative chronology and intensity of the processes of the formation of ophiolites make every ophiolite unique.

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