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Structural Petrology of the Olympus Ultramafic Complex in the Troodos Ophiolite, Cyprus

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Structural Petrology of the Olympus Ultramafic Complex in the Troodos Ophiolite, Cyprus Structural petrology of the Olympus ultramafic complex in the Troodos ophiolite, Cyprus RICHARD P. GEORGE, JR.* U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 ABSTRACT ous, and sedimentary rocks characteristic of a complete ophiolite complex (Penrose Field Conference participants, 1972). This paper The Olympus ultramafic complex is one of three ultramafic presents an analysis of the internal structures and textures of the complexes that crop out at the lowermost stratigraphic levels of the "transition zone" (Jackson and others, 1975) between the mafic Troodos ophiolite, Cyprus. The Olympus ultramafic complex and ultramafic rocks that occur at the stratigraphically deepest comprises two types of ultramafic and related rocks: (1) harzbur- levels of exposure of the Troodos ophiolite. gite tectonite, a residuum of partial fusion, and (2) cumulus The following comparisons of known and inferred properties of chromitite, dunite, wehrlite, pyroxenite, and gabbro, the products oceanic crust and upper mantle with properties of the Troodos of fractional crystallization and magmatic sedimentation of basaltic ophiolite strongly suggest that the exposed portion of the ophiolite magma. The harzburgite tectonite is the basement or "country is part of a single thrust sheet of Cretaceous oceanic lithosphere: (1) rock" intruded by the magma from which the cumulates crystal- Dredge hauls and drill cores from the ocean basins include all lized. Pyrometamorphic textures in the harzburgite (magmatic major rock types found in the ophiolite (Greenbaum, 1972a; Au- pyroxenes in metamorphic peridotite) near its contact with the mento and Loubat, 1971; Scientific Staff, 1974; Engel and Fisher, cumulates perhaps record "contact metamorphism" of the 1975). (2) The ultramafic rocks within the Troodos ophiolite have harzburgite by the magma. compositions, textures, and structures similar in many respects to The stratigraphically lowest cumulates were penetratively de- those of many ultramafic xenoliths in basalts and kimberlites de- formed (and hence are termed "ultramafic metacumulates") during rived from the upper mantle (Wilson, 1959; Gass and Masson- the same event that produced the dominant pyroxene foliation (SJ Smith, 1963; Moores and Vine, 1971; Greenbaum, 1972a). (3) in the underlying harzburgite, whereas the stratigraphically highest Seismic velocity profiles of oceanic crust are in good agreement cumulates were not penetratively deformed. The structural transi- with elastic property profiles of the Troodos ophiolite (Moores and tion between metacumulates and cumulates is gradual and occurs Vine, 1971; Colemen, 1971; Lort and Matthews, 1972; Khan and in olivine cumulates that contain intercumulus clinopyroxene others, 1972; Poster, 1973). (4) The occurrence of a sheeted dike (clinopyroxene-bearing dunites and wehrlites). complex in the Troodos ophiolite (the sheeted intrusive complex) is Two mechanisms can explain the structural transition from consistent with the formation of the complex at a site of continual metacumulates to cumulates: (1) Deformation of the harzburgite crustal extension such as a mid-oceanic ridge or an interarc spread- basement occurred during accumulation of the cumulates (syntec- ing center (Moores and Vine, 1971). tonic magmatic sedimentation); the lowermost (oldest) cumulates If the processes that occurred during the formation of oceanic consequently deformed more than the uppermost (youngest) cumu- lithosphere in Cretaceous time were similar to those occurring dur- lates. (2) The uppermost cumulates, perhaps rich in intercumulus ing the formation of present-day oceanic lithosphere, then the in- liquid (now crudely represented by postcumulus clinopyroxene and terpretations presented here of structures and textures in the plagioclase) at the time of the deformation, may have accommo- Troodos ophiolite may be useful in identifying igenous and tectonic dated the strain by grain boundary sliding ("crystal-mush flow") processes now occurring at mid-oceanic ridges or interarc basins. and consequently left little evidence of solid-state, penetrative de- formation. SETTING If syntectonic and posttectonic magmatic sedimentation, crystal-mush flow, partial fusion of metamorphic peridotite, and The island of Cyprus, situated 70 km south of Turkey and 100 multiple intrusion of magma act simultaneously during the forma- km west of Syria, has four main physiographic provinces that gen- tion of ophiolites, then the resulting field relations, structures, tex- erally trend east-west (see Fig. 1). The Troodos Range lies in the tures, and fabrics will be exceedingly complex, particularly if sub- south-central part of the island and is underlain primarily by vol- sequent transport and emplacement impose strong metamorphic canic and plutonic rocks of the Troodos ophiolite that crops out and tectonic overprints. One should expect that differences in the over a broadly oval-shaped area more than 90 km long and 30 km relative chronology and intensity of the processes of the formation wide. The volcanic and plutonic rocks form an east-trending elon- of ophiolites make every ophiolite unique. gate, crudely annular pattern in plan, with coarser-grained and generally increasingly mafic rocks nearer the center (Fig. 2). This INTRODUCTION outcrop pattern was probably produced largely by ductile arching The Troodos ophiolite of Cyrpus is a peridotite-gabbro- of originally subhorizontal units about an east-trending anticlino- diabase-basalt complex that contains a suite of metamorphic, igne- rial axis and modified by late uplift of the serpentinized core of the ophiolite along steeply dipping faults (Wilson, 1959; Gass and * Present address: Institute of Geophysics and Planetary Physics, Univer- Masson-Smith, 1963; Moores and Vine, 1971; Vine and Moores, sity of California, Los Angeles, Los Angeles, California 90024. 1972; Greenbaum, 1972a; George, 1975). Geological Society of America Bulletin, v. 89, p. 845-865, 10 figs., June 1978, Doc. no. 80605. 845 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/6/845/3429489/i0016-7606-89-6-845.pdf by guest on 23 September 2021 Figure 1. Geologic sketch map of Cyprus (modified after Bear, 1963). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/6/845/3429489/i0016-7606-89-6-845.pdf by guest on 23 September 2021 STRUCTURAL PETROLOGY OF THE OLYMPUS ULTRAMAFIC COMPLEX, CYPRUS 847 EXPLANATION Upper Pillow Lavas Lower Pillow Lavas WSM Basal Group 1 Diabase Granophyre V///\ Gobbro Ultramaflc rocks 5 ip Km Figure 2. Geologic sketch map of rock units surrounding Olympus ultramafic complex and Limassol Forest complex (geo- logic contacts after Bear, 1963). Circular diagrams are equal-area, lower-hemisphere stereonets of dike attitudes measured by Wil- son (1959) in the sheeted intru- sive complex. Outer (Upper) Part of Ophiolite which the harzburgite shares an unfaulted contact is the dunite. Dunite pods and lenses occur throughout much of the harzburgite. At the periphery of the ophiolite, the upper pillow lavas, inter- They increase in extent in the western part of the area, where they layered umbers, and radiolarian-bearing shales of early Campanian grade into apophyses of large dunite bodies interdigitated with the age (Allen, 1966; Mantis, 1970) unconformably overlie the lower harzburgite. The largest dunite body forms an irregular, 0.5- to pillow lavas (Wilson, 1959; Bear, 1960a; Gass and Smewing, 1-km-wide, 5-km-long strip bordering the western edge of the 1973). Some flows in the lower pillow lavas were fed by dikes that harzburgite. Although the gross lithologic contact between the transect underlying pillow structures of earlier flows. The concen- harzburgite and the largest dunite body runs approximately tration of feeder dikes increases inward in the complex through a N20°E, the major interdigitations of the harzburgite and the dunite unit called the basal group, in which numerous steeply dipping, trend approximately N30°W. Disseminated chromite is common in north-trending dikes nearly obliterate the invaded basaltic host the dunite, but concentrations of chromite in the dunite occur rock, to the diabase, which consists almost entirely of dike swarms mainly near the harzburgite-dunite contact. giving rise to a "sheeted" appearance (Wilson, 1959; Bear, 1960a, The dunite grades into smaller, clinopyroxene-bearing ultramafic 1960b; Gass and Masson-Smith, 1963; Moores and Vine, 1971). bodies to the west. Greenbaum (1972a, 1972b) described nontec- The basal group and the diabase constitute the sheeted intrusive tonic contacts between successively more silicic rock types along complex. Gabbro dikes extend outward into the sheeted intrusive the western edge of the ultramafic complex and constructed a com- complex from the underlying gabbro (Wilson, 1959; Bear, 1960a); posite or idealized stratigraphic section (Fig. 4). the transition between these formations is poorly understood be- Serpentinization is ubiquitous in olivine-rich rocks and is most cause of the common occurrence of the granophyre (consisting of intense in the east-central part of the harzburgite near the asbestos the plagiogranite of Coleman and Peterman, 1975) along the con- mine at Pano Amiandos, which was mapped as a zone of "bastite- tact. The complex interrelations of the lower pillow lavas, the serpentine" by Wilson (1959). sheeted intrusive complex, the gabbro, and the granophyre indicate From Wilson's (1959) petrographic descriptions, Davies (1969) that
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