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 deformed (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 deformation. 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

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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 (geologic contacts after Bear, 1963). Circular diagrams are equal-area, lower-hemisphere stereonets of dike attitudes measured by Wil- son (1959) in the sheeted intrusive 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 they are cogenetic (Wilson, 1959; Bear, 1960a, 1960b; Cole- suggested that part of the Olympus ultramafic complex contains man and Peterman, 1975). cumulates. Moores and Vine (1971) confirmed the occurrence of cumulus gabbro and pyroxenite and also corroborated Wilson's Gabbro and Olympus Ultramafic Complex (1959) observations of penetrative deformation of the dunite and the harzburgite. Greenbaum (1972a) documented cumulus textures The gabbro encircles the Olympus ultramafic complex,1 which in the tectonized dunite, as well as in the overlying wehrlite, pyrox- crops out over an area of approximately 60 km2 in the highest part enite, and gabbro. On the basis of mineral parageneses recognized of the Troodos Range. Harzburgite occupies the eastern two-thirds by him, and because these cumulates define a crude stratigraphy of the ultramafic complex (Fig. 3A). The only major unit with that grossly resembles that of large stratiform ultramafic complexes (Fig. 4), Greenbaum (1972b) concluded that these rocks crystallized in a single magma chamber within harzburgitic country rock. 1 The term "Troodos" has been applied in earlier reports to both the Thus, for the purposes of the following discussion, the coarse- ophiolite and its ultramafic complex. To eliminate confusion, the new name grained rocks in the lower part of the Troodos ophiolite — that is, Olympus ultramafic complex is here used to refer to the ultramafic rocks in the Olympus ultramafic complex — are divided into three major the vicinity of and underlying Mount Olympus (Fig. 3). Similar ultramafic categories (from top to bottom; see Fig. 4): (1) undeformed, usually complexes within the Troodos ophiolite occur in the Limassol Forest south- east of Troodos (Pantazis, 1967; A. Panayiotou, 1973, 1976, written and clinopyroxene-bearing igneous rocks with cumulus textures oral communs.) and in the Akamas Peninsula at the western end of Cyprus ("mafic and ultramafic cumulates"); (2) penetratively deformed (Turner, 1971, 1973; Lapierre, 1973; George, 1975). wehrlite, dunite, and chromitite with probable primary cumulus

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Sheeted intrusive complex

Granophyre (plagiogranite)

Gabbro

Ultramafic cumulates

Ultramafic metacumulates

Harzburgite tectonite, with outliers of dunite (D) or plagioclase-bearing wehrlite (W)

Contact

Contact, location approximate Boundary separating Wilson's (1959) zone of abundant chrysotile veinlets (east-central harzburgite) from zone of rare chrysotile veinlets (west harzburgite)

Fault

Inferred fault Regions (Roman numerals) that are discussed in text. Boundaries of regions are gerrymandered as to include maximum number of layering and foliation measurements with uniform or uniformly varied orientations (see Fig. 3C)

Cuts and tailings at Pano Amiandos Asbestos Mine

Location of samples collected for petrofabric analyses

Restricted area (British Army leave camp)

Map modified after George (1975) and Greenbaum and George (in prep.). Geologic contacts after Wilson (1959) and Greenbaum (1972). Meso- scopic structures mapped by George during summers 1972 and 1973.

0.5

Figure 3A. Geologic sketch map of Olympus ultramafic complex. Contacts between ultramafic cumulates and ultramafic metacumulates are arbitrarily chosen to coincide with petrologic contacts between clinopyroxene-rich and clinopyroxene-po'or cumulates mapped by Greenbaum (1972a). Textures diagnostic of cumulates versus metacumulates cannot be recognized in the field at the Olympus ultramafic complex, apparently because the contact between cumulates and metacumulates lies mainly in dunite (in which incip- ient foliation is difficult to recognize). Each region designated by a roman numeral corre- sponds to stereonet plots of mesoscopic data shown in Figure 3C.

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textures and superimposed deformation textures ("ultramafic Sedimentary features that formed in the magma chamber, such as metacumulates"); and (3) penetratively deformed harzburgite with cross-bedding and soft-sediment slump folds or schlieren of olivine possible primary cumulus textures and superimposed deformation metagabbro in anorthositic gabbro, are common locally. Planar textures, or with unknown primary textures, superimposed defor- lamination (igneous foliation) defined by preferred orientation of mation textures, and subsequent partial fusion textures ("harzbur- tabular crystals invariably parallels layering, even in the hinges of gite tectonite"). slump folds. No evidence of solid-state deformation has been observed either at the outcrop or in thin section. The angle of repose MAFIC AND ULTRAMAFIC CUMULATES of an accumulation of crystals at the bottom of a basaltic magma chamber is constrained by the low cohesion of crystals whose sur- Plagioclase is the dominant cumulus phase in the upper part of face tractions are further reduced by the buoying effect of the inter- the section. Clinopyroxene and olivine are the dominant cumulus cumulus liquid. Consequently, attitudes of layering in the gabbro, phases down-section. except near slump folds, probably reflect its orientation relative to the gravitational field at the time of accumulation of the cumulates. Gabbros The attitude of the gabbro layering appears to be independent of the orientation of the gabbro—ultramafic complex contact. "High-level" gabbro is generally characterized by marked changes in grain size and plagioclase/clinopyroxene ratios over Pyroxenite and Wehrlite short distances; layering is rarely well-developed. Layers within high-level gabbro typically are variable in thickness and orientation Clinopyroxene-rich rocks occupy the stratigraphic interval be- over distances of less than 1 m. The high-level gabbro probably is a tween the gabbro and the dunite. The contact relations between the product of late stage, in situ crystallization of the magma that ear- gabbro and the clinopyroxenite and wehrlite are obscured by lier had fractionated to produce the underlying, cumulus "low- numerous local faults, by poor exposures along the western and level" gabbro. southern margins of the Olympus ultramafic complex, and by a Low-level gabbro locally displays well-developed layering with single large, steeply dipping fault zone along the eastern margin. moderately strong preferred orientations (Fig. 3C, regions II and Nevertheless, gradational contacts are preserved locally. III) commonly defined by 1- to 30-mm-thick, plagioclase-rich layers The gabbro grades downward into websterite and clinopyroxe- alternating with clinopyroxene-rich layers. The rock generally has nite (clinopyroxene-orthopyroxene cumulates and clinopyroxene xenomorphic- to hypidiomorphic-granular textures. Most of the cumulates) by rapid increase of clinopyroxene content at the ex- layered gabbro lies within 500 m of the contact between the gabbro pense of plagioclase. Alternatively, the gabbro grades downward and the ultramafic rocks. Near phase contacts between the gabbro into banded wehrlite (alternating layers 2 to 20 cm thick of olivine and ultramafic rocks, layering in the gabbro is typically defined by and clinopyroxene cumulates) and poikilitic plagioclase-bearing bands containing 5% to 20% of cumulus olivine or orthopyroxene. wehrlite (olivine and olivine-clinopyroxene cumulates) by increase The olivine and orthopyroxene are typically surrounded by inter- of olivine content. stitial plagioclase and clinopyroxene grains, which impart sub- Textures and Mesoscopic Structures. Cumulus clinopyroxene poikilitic to poikilitic textures. forms fine- to medium-grained (1 to 6 mm), equant or tabular V ,1 2000 cji i \i J J GC-51' GC-96vr/ si ^ 1800- a> ® E T 1600-

o> « U 1400-

1200

0.5 Km • • • i I j I No vertical exaggeration

Figure 3B. Cross section through western "transition" zone; location shown in Figure 3A. G = gabbro; P = pyroxenite; Wp = poikilitic wehrlite; Dc dinopyroxene-bearing dunite; D = dunite; Hz = harzburgite. Asterisks mark projected localities of samples GC-96 and GC-51.

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Basal Group Axis Sheeted Sequence Intrusive Complex Diabase

a> cu .t: \> iL ,QÜ- eJn I o O c High-Level sg1 Gabbro ^Ql 'Low-Level' Seismic Moho Gabbro j)f_Moores "Banded ~ and Vine (1971) •C Q. Massive Mafic & y-o Pyroxenite CO Ultramafic O oO o o oO o• OI Banded Cumulates "O O a O Poikilitic co » 4 Dunite 1 co p CL S? O .E E OÍ— a> o Oí-"O o Dunite Ultramafic O) o a Meta- & Chromitite cumulates

"Petrologie Moho"

CL E

CTi a> .£>d5 =J c- Harzburgite K o Tectonite o '(Stratigraphie X Basement)

Deepest_level_ of exposure

Serpentinized Peridotite

Figure 3C. Equal-area, lower-hemisphere stereonets of attitudes of / > Seismic Moho compositional layering (S„) or foliation (S„ S ) within regions shown in Fig- \ / i A 2 * I \ M J L _ _ _oLHe.ss_(l96_2) ure 3A. Figure by "north" mark on each stereonet is number of poles plot- \i \ / V and Clague and ted. li 1/ \ I I V \ Straley (1977) • Fresh Peridotite Figure 4. Schematic stratigraphic section of the Troodos ophiolite mod- jf ified after Moores and Vine (1971) and Greenbaum (1972a).

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Figure 5A. Photomicrograph of phase contact in banded wehrlite (GC- Figure SB. Photomicrograph of harzburgite (GC-35), exemplifying 31). Crossed nicols. Long dimension = 8.3 mm. Contact between dunite grain-boundary relations both in metacumulus dunite and in harzburgite. (olivine adcumulate) on right side and wehrlite (clinophyroxene-olivine Crossed nicols. Long dimension = 6.6 mm. Large dark grains, with irregu- cumulate) on left side is marked by absence or presence of clinopyroxene lar borders, at bottom center and top left are orthopyroxene. Olivine- (grains with (100) parting or multiple twinning, which appear as striations olivine linear boundaries terminating in 120° triple junctions are common; in photograph). Olivine-olivine boundaries are linear and angular. Five a few gendy curved boundaries are present. olivine grains in field of view are kinked, but only one kink-band boundary is evident with thin section in this orientation. Light, left-to-right-trending features are serpentine-filled fractures. Planar and linear lamination (igneous foliation and lineation) and layering are generally poorly developed in massive clinopyrox- (length:width = 2:1) anhedra (Fig. 5A). Postcumulus clinopyroxenite and websterite, except for local size-graded layers or phase ene occurs as large (1 to 2 cm), typically equant oikocrysts that en- layers containing small percentages of cumulus orthopyroxene. close olivine crystals or as small (0.5 to 2 mm) anhedra that assume Postcumulus oikocrysts of clinopyroxene in poikilitic wehrlite the shapes of interstices between olivine and chromite grains (Fig. show no tendency to align in layers. Differential weathering of 6A). The habit typically changes from cumulus to postcumulus cumulus clinopyroxene and olivine enhances the phase layering of with decreasing clinopyroxene concentration. banded wehrlites. Changes in average grain size from one pyroxe- Cumulus olivine grains that are completely enclosed in clinopy- nite layer to the next are common, but size-grading within indi- roxene oikocrysts are generally well-rounded and about half the vidual phase layers is rare. size of olivine grains in clinopyroxene-poor regions — a typical re- Microscopic Structures. Banded wehrlite specimen GC-31 from sorption texture (Jackson, 1961, 1971). In olivine adcumulus the northern part of the Olympus ultramafic complex (location layers in banded wehrlite, boundaries between neighboring olivine shown in Fig. 3) contains both cumulus olivine and cumulus clino- grains are generally straight with slightly rounded corners (Fig. pyroxene. The olivine lattice fabric shows a fairly weakZ-axis par- 5A); there are no deep embayments of one olivine grain into tial girdle parallel to layering and a poorly developed X-axis another. maximum normal to layering (Fig. 7A). Poikilitic wehrlite specimen

Figure 6. Drawings after photomicrographs. White = olivine; where subhedral to euhedral cumulus chromite. Postcumulus dinopyroxene en- heavily serpentinized ("serp"), original grain boundaries are obliterated. doses cumulus chromite in the interstices between olivine grains. The Black = chromite. Pattern of irregular dots = clinopyroxene. Pattern of olivine grain at the right has nearly the same lattice orientation (within 3°) regular dots = plagioclase (altered). A. Chromite- and dinopyroxene- as the olivine at the left. (B) Clinopyroxene aggregate in plagiodase-bearing bearing dunite (GC-28). Long dimension = 8.3 mm. In this olivine- wehrlite (GC-96). Long dimension = 6.6 mm. chromite metacumulate, the original size of settled olivine is outlined by

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Figure 7. Equal-area, lower- hemisphere projections of olivine indicatrix axes in specimens of cumulates and metacumulates. Contours are percentage of axes of given orientation per 1% area of net. Dashed contours = 1%/1% area. Interior solid contours are 2%, 4%, 6%,..., 12%/1% area. (A) Undeformed banded wehrlite (olivine and clinopyroxene-olivine cumulate), GC-31. 100 grains. Great circle shows orientation of layer of dinopyroxenite. (B) Unde- formed poikilitic wehrlite (olivine cumulate), GC-6. 100 grains. (Q Deformed plagioclase-bearing wehrlite (olivine metacumulate?), GC-96. 100 grains. Great circles show orientation of clinopyroxene foliation (Scpx) and plagioclase foliation (Spl). (D) Deformed chromite-bearing dunite (olivine- chromite metacumulate), GC-4S. 98 grains. Great circle shows orientation of limbs of flattened isodinal fold of chromitite (Schr). Asterisk shows orientation of B-axis (fold axis). (E) Deformed pyroxenitic dunite (olivine metacumulate?), GC-40. 100 grains. Great circle shows orientation of flattened clinopyroxene oikocrysts and of foliation (S,) in the enveloping harzburgite.

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GC-6 (an olivine cumulate) from the southwestern part of the wehrlite and clinopyroxene-bearing and clinopyroxene-free dunite complex (Fig. 3A) is texturally isotropic: phase layers and size- (olivine cumulate), clinopyroxene-bearing and clinopyroxene-free graded layers are absent, and neither the partly resorbed olivine chromite-bearing dunite (olivine-chromite cumulate), and chromi- crystals nor the clinopyroxene oikocrysts define an igenous lami- tite (chromite-olivine cumulate). Clinopyroxene is the dominant nation or have a discernible shape fabric. Its olivine lattice fabric is postcumulus phase. Plagioclase and orthopyroxene are present lo- very weak (Fig. 7B). Olivine grains in both specimens contain very cally. small bubbles or inclusions aligned preferentially along (100). The contact between undeformed cumulates and metacumulates These features are similar to those found in continental stratiform is gradational both petrologically and structurally. The petrologic igneous complexes (Jackson, 1961) and in a partially recrystallized transition from wehrlite to clinopyroxene-bearing dunite is Hawaiian metacumulus dunite (Roedder, 1965; Jackson, 1968), primarily by a gradual change in the kind of postcumulus mineral although the olivine lattice fabrics described here are even weaker in the olivine cumulates from clinopyroxene to adcumulus olivine, than those of most olivine cumulates. but the transition is also evident in an increase in the percentage of Plastic deformation has caused kinking or optical straining of cumulus chromite that coprecipitated with cumulus olivine. As the olivine grains in every type of olivine-bearing rock in the Olympus metacumulates are conformable with the undeformed cumulates ultramafic complex except the olivine gabbros. Because the angle of and have primary textures similar to textures that occur in layered external rotation between the olivine lattice in strained and un- intrusions, their origin is attributed to cumulus processes. The strained parts of the crystal is typically only 2° or 3°, and because metacumulates merit separate discussion, however, because most the proportion of grains showing kink bands is generally small, the of their textures have been modified by postcumulus deformation. relatively late event that caused this ubiquitous straining is inferred Dunite, which is commonly more than 50% serpentinized, typi- to have been mild and relatively unimportant (George, 1975). cally contains 95% to 100% olivine, a trace to 5% chromite, and a trace to 2% clinopyroxene. The olivine, whose optical properties

Discussion (2Vy = 87° to 92° ± 2°) indicate a composition of Fo85_92, is fine to medium grained (0.5 to 10 mm), equant to moderately elongate The formation of large stratiform igneous bodies such as the (length:width = 3:1), and subhedral to euhedral (in nearly unde- Skaergaard intrusion (Wager and Deer, 1939), the Great Dyke formed cumulus dunites) or xenomorphic to hypidiomorphic (in (Worst, 1958), the Stillwater complex (Hess, 1960; Jackson, metacumulus dunites). The chromite forms very fine to fine-grained 1961), the Muskox intrusion (Irvine and Smith, 1967), and the (0.1 to 2 mm), equant, euhedral to anhedral crystals that are Bushveld igneous complex (Wager and Brown, 1968) is attributed opaque to very dark reddish brown in thin section. The clinopyrox- to fractional crystallization and gravitational settling of crystals ene is also very fine to fine grained (0.3 to 4 mm), but invariably (magmatic sediments) at the bottom of a magma chamber. The forms irregulary shaped, exsolution-free anhedra that range from higher stratigraphic levels of the Olympus ultramafic complex have equant to highly elongate (length:width = 10:1). several features in common with large stratiform intrusions. The Chromitite typically occurs in small concentrations within dunite Olympus contains the same cumulus phases (olivine, clinopyrox- close to the harzburgite-dunite contact. Chromitite (more than ene, orthopyroxene, and plagioclase) and postcumulus phases (cli- 90% chromite), olivine chromitite (50% to 90% chromite), and nopyroxene, plagioclase). Locally, the Olympus displays cross- chromite-bearing dunite (5% to 50% chromite) generally contain bedding, graded bedding, prominent layering, and planar lamina- only chromite and olivine, but a trace to 5% fine-grained (0.5 to 4 tion. The olivine fabrics of GC-31 (Fig. 7 A) and GC-6 (Fig. 7B) are mm), equant to elongate (length:width = 5:1) anhedral clinopy- consistent with fabrics expected during settling of slightly tabular roxene is present in some of these rocks. Olivine associated with olivine crystals with broad (010) faces (compare Jackson, 1961, most chromite concentrations is completely serpentinized. Figs. 43,44). The original equant and tabular habits of euhedral to subhedral olivine and anhedral clinopyroxene are preserved in Textures banded wehrlite. Adcumulus overgrowth has filled the interstices of massive clinopyroxenite, and postcumulus clinopyroxene has Chromite and clinopyroxene resist alteration during serpentini- partly resorbed the olivine grains in poikilitic wehrlite. zation of ultramafic rocks and thereby preserve the wide variety of The most significant difference between cumulates in the Olym- textures present in the metacumulates. In the terminology of pus ultramafic complex and cumulates in large continental Thayer (1969), these textures include chromite net texture, nodular stratiform intrustions is the absence of extensive lateral continuity chromite (and schlieren with pull-apart texture, where nodules are of layering in the Olympus cumulates. This absence is probably a extensively deformed), orbicular chromite, and interlayered mas- primary feature rather than the result of subsolidus tectonic dis- sive chromitite and dunite. In undeformed rocks that show chro- ruption, for the uppermost cumulates in the Olympus are not mite net texture, a continuous network of small grains of chromite penetratively deformed. outlines "holes" that are occupied by large grains of olivine; thus Moores and Vine (1971) suggested that the sheeted intrusive the "holes" record the size and shape of the original olivine grains. complex formed from multiple intrustions (dike swarms) in a In slightly to moderately deformed chromite-bearing dunite, fine- tectonically active regime. Since the gabbro and the sheeted intru- grained (0.1 to 1 mm) subhedral to anhedral chromite commonly sive complex formed approximately synchronously (Wilson, 1959; ocurrs at olivine grain boundaries, but locally chromite grains are Bear, 1960a, 1960b), the gabbro and related cumulates may also enclosed within olivine crystals. One specimen of deformed dunite have formed from multiple intrusions with limited lateral extent in contains exceptionally large (20 mm), roughly equant olivine grains a tectonically active regime. Corroborating evidence for this that enclose several contiguous 2- to 4-mm "holes" in the chromite hypothesis is discussed below. net and the intervening chromite selvages. Olivine-olivine grain boundaries are generally straight and inter- ULTRAMAFIC METACUMULATES sect at angular or very slightly rounded corners (Fig. 5B), and deep embayments of one olivine grain by another are common. Intersti- The ultramafic metacumulates lie stratigraphically beneath the tial clinopyroxene in some chromite-bearing rocks encloses the undeformed cumulates. The metacumulates include deformed chromite grains and forms chromite-bearing clinopyroxene sel-

vages surrounding the olivine; in parts of the rock with less chromite, clinopyroxene forms chromite-free selvages around the olivine and imparts a sub-poikilitic texture visible in hand specimen. Locally, these selvages have "moth-eaten" borders against olivine and display undulatory extinction. Feldspathic clinopyroxene-bearing dunite and feldspathic wehrlite with distinctive textures crop out within dunite near the harzburgite-dunite contact 600 m west of the summit of Mount Olympus. The rock there contains slightly to moderately elongate, fine-grained xenomorphic olivine and as much as 15% clinopyroxene typically in elongate clusters of tabular grains or as interstitial selvages, approximately 5% altered plagioclase as equant grains within the clinopyroxene clumps (Fig. 6B) or as selvages and oikocrysts enclosing the olivine grains, a trace to 2% subhedral to anhedral chromite, and 0% to 3% orthopyroxene. The clinopyroxene clumps and the plagioclase oikocrysts range in shape from equant to highly flattened.

Mesoscopic Structures

Most of the measurements on layering and foliation in dunite are from outcrops near the dunite-harzburgite contact, where chromite is common. The structural transition from undeformed to deformed cumulates probably occurs over a stratigraphic interval of approximately 500 m but is difficult to locate because it occurs in rocks in which layering and lamination or foliation are difficult to detect, both in hand specimen and in thin section. The most common type of layering within dunite consists of 0.2- (one-grain wide) to 2-mm-wide stringers of equant, euhedral chromite grains in a rock that otherwise contains only minor amounts of fine-grained, disseminated chromite. Monogranular stringers rarely persist for more than 10 cm, and many are less than 1 cm long. Virtually every textural variety of chromitite locally displays compositional layering defined by dunite interlayered with Figure 8. Antiformal, isoclinal fold of interlayered chromitite and du- thin layers of massive chromitite (Fig. 8), with 10- to 20-cm-wide nite. View looking N20°W up the fold axis (plunge = S20°E, 50°). Axial plane of this fold, oriented N34°W, 80°SW, is subparallel to a strong folia- layers with chromite net texture, or with layers containing orbicu- tion in the enveloping harzburgite. Layer thickness normal to bedding is lar chromite. Compositional layering in the dunite is not penetra- nearly constant. tive at outcrop scale and hence does not define a mesoscopic foliation plane (Turner and Weiss, 1963, p. 28). layering is well developed in three of these folds. In the isoclinal Foliation in dunite is defined by olivine or chromite grains flat- folds that lack chromite foliation, the axial planes of the folds are tened or stretched by deformation. Because olivine foliation can be invariably parallel to pyroxene foliation in the enveloping recognized in thin section only, field measurements of foliation are harzburgite (Fig. 8). restricted to areas in which chromite foliation is evident. Foliation Layering and foliation in the dunite are almost invariably parallel typically parallels layering in dunite. Because chromite tends to pull to the layering and foliation, respectively, in nearby parts of the apart along fractures normal to the direction of elongation, greatly harzburgite. The regional trends of structures in the metacumulates flattened chromite grains tend to break up into several approxi- are described below in the section on regional trends in the mately equant subgrains that mimic the appearance of a short harzburgite. chromite stringer. Where this occurs, foliation can be distinguished from layering only by the identification of the irregular, fractured Microscopic Structures: Olivine Fabrics surfaces created in the subgrains during the extension process. Chromite foliation does not parallel chromite layering every- The olivine fabrics of the metacumulates (Figs. 7C, 7D, 7E) are where. Near the interdigitated harzburgite-dunite contact 400 m consistently stronger than those of the undeformed cumulates. The west of Mount Olympus, a region where harzburgite displays two clinopyroxene clumps and the plagioclase grains of feldspathic pyroxene foliations, a layer of dunite parallel to one pyroxene foli- wehrlite specimen GC-96 (Fig. 3A, region VIII; Fig. 6B) are flat- ation (Si, N52°W, 82°SW) contains a concordant chromite stringer; tened. Both minerals define strong, nearly parallel foliations that the foliation defined by the flattened chromite grains in this stringer transect an irregular layer of dunite at high angles. An X-axis par- 2 is parallel to the second pyroxene foliation (S2, N10°W, 81°SW), tial girdle is normal to an 8% maximum of olivine Z-axes lying which transects layering and which evidently is the younger feature. within the clinopyroxene foliation (Fig. 7C). Only six isoclinal folds were observed in chrome-bearing rocks (Fig. 8). The folds are fairly open, unlike the highly flattened folds 2 In order to give a quantitative indication of the strength of preferred characteristic of most isoclinal folds in peridotites (see, for exam- orientation, orientation data are commonly contoured in units of density of data points (usually in percentage of total grains measured) per unit area of ple, Thayer, 1963; Loney and others, 1971; Nicolas and others, the stereonet (usually 1% of the total area). An "8% maximum" indicates a 1972; Zimmerman and Carter, 1973; Nicolas and Boudier, 1975; region in the stereonet in which the highest density of points is 8% of the Loney and Himmelberg, 1976). Axial-plane foliation transecting data per 1% of the stereonet area.

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Dunite specimen GC-45 (Fig. 7D) from the southern dunite body Significant adcumulus overgrowth must occur to produce a (Fig. 3A) contains an isoclinally folded stringer of fine-grained sub- monomineralic cumulus dunite (Hess, 1960; Jackson, 1961). Ad- hedral chromite. A strong, 12% maxmimum of olivine Z-axes cumulus overgrowth requires that equilibrium be maintained be- parallels the fold axis. Partial girdles of X- and Y-axes normal to tween the main body of magma and magma trapped in the in- the Z-axis maximum display two submaxima of X-axes and two terstices among the primary cumulus phases. To maintain equilib- submaxima of Y-axes at unusually low angles (30° to 60°) to the rium, accumulation of new cumulus crystals must be slow relative chromite layering. to the rate of adcumulus overgrowth, a condition achieved by slow Pyroxenitic dunite specimen GC-40 (Fig. 7E) is an unusual rock cooling of the magma. Since much of the lower stratigraphic levels collected from a 10-m-wide dunitic zone within weakly foliated of the Olympus cumulus and metacumulus sequence consists of harzburgite, (Fig. 3A, region IV). It consists of 5% to 15%, 2-to nearly monomineralic adcumulus dunite, the part of the harzbur- 4-cm-diamater "discs" of fine- to medium-grained orthopyroxe- gite immediately next to the contact with the metacumulates must nite within a matrix of clinopyroxene dunite. The flat sides of the have been subjected to high temperatures ("contact metamor- orthopyroxenite discs are aligned parallel to the foliation of the en- phism") during crystallization of the metacumulates. This conclu- veloping harzburgite. The relatively weak olivine fabric of this rock sion has critical implications for the origin of some of the textures does not seem to be directly related to the foliation plane. in the harzburgite. The outcrop of plagioclase- and clinopyroxene-bearing dunite Discussion and plagioclase-bearing wehrlite from which specimen GC-96 Two lines of evidence strongly suggest that the penetratively de- (Figs. 6B, 7C) and Menzies and Allen's (1974) "residual plagioclase formed rocks described in this section (ultramafic metacumulates) lherzolite" were collected is along the dunite-harzburgite contact. crystallized as cumulates: (1) relict cumulus textures in deformed No demonstrably cumulus rocks occur stratigraphically below chromite-bearing rocks and (2) the field relations between the rocks these feldspathic rocks, although metacumulus dunite occurs strat- described in this section and the undeformed cumulates described igraphically above this outcrop. The feldspathic rocks may therefore in the previous section (mafic and ultramafic cumulates). be genetically related to the harzburgite or the metacumulus dunite With allowances for the textural modifications wrought by or both. penetrative deformation, the chromite net texture described above The plagioclase, clinopyroxene, and orthopyroxene textures ob- is identical to textures in olivine-chromite cumulates of continental served in thin sections of specimen GC-96 and related specimens stratiform complexes (Jackson, 1961, 1970, 1971). Although are magmatic (Menzies and Allen, 1974) and may be either nodular and orbicular chromite do not occur in continental cumulus or partial-melt ("pyrometamorphic" of Pike and stratiform complexes (Thayer, 1960; Greenbaum, 1972a), descrip- Schwarzman, 1977) textures. Whatever their origin, the magmatic tions in the literature indicate that nodular and orbicular chromite pyroxenes3 in these samples display no evidence of being strained, occur in the magmatic sequences of other ophiolites (Bradley and despite the strong olivine fabric (Fig. 7C) and strong clinopyroxene others, 1918; Diller, 1921; Johnston, 1936; Willemse, 1948; and plagioclase foliations. This suggests that pyroxenes and the Thayer, 1960, 1969, 1970; Bilgrami, 1963; Borchert, 1964; Lapin plagioclase were present during deformation, as a liquid phase that and Zhabin, 1965; van der Kaaden, 1970; Bezzi and Piccardo, was contained in interstices whose shapes were controlled by the 1971). deformation. Greenbaum (1972a) mapped several nontectonic contacts be- In strongly deformed peridotites, the X-axes of olivine typically tween the undeformed cumulates and the metacumulates in the are aligned nearly normal to a prominent foliation plane, and the well-exposed transition zone of the Olympus ultramafic complex Z-axes are nearly parallel to a prominent lineation, whether defor- (Fig. 3A). Furthermore, the sequence of assemblages of cumulus mation occurred by syntectonic recrystallization (Ave Lallemant phases recognized by Greenbaum in the metacumulates (from bot- and Carter, 1970) or by plastic deformation (Nicolas and others, tom to top: chromite; chromite + olivine; olivine + chromite; 1971, 1972, 1973). The fabric of GC-96 is in accord with this gen- olivine) is conformable with the cumulus sequence in the unde- eral observation, but the fabrics and textures of GC-45 (Fig. 7D) formed cumulates (continuing upward: olivine; olivine + clinopy- and of GC-40 (Fig. 7E) are not, and they cannot be produced by roxene; olivine + clinopyroxene + plagioclase; clinopyroxene + any of the known flow mechanisms of olivine in a single deforma- plagioclase + orthopyroxene). The two sequences together are tion event: for example, translation gliding on {0&/} [100] can ex- equivalent to one of Irvine's theoretical cumulus sequences (see Ir- plain the fabric of GC-45 (Fig. 7D) if (011) and (012) were the pre- vine, 1970, p. 469, Table LL, sequence 1). The field association of dominant slip planes, but translation gliding cannot explain both the undeformed cumulates and the metacumulates, then, strongly the strength of the fabric and the absence of strong foliation and suggests that they are part of the same cumulus sequence. lineation of olivine and chromite. Perhaps annealing recrystalliza- The cumulus origin of monomineralic dunite in stratiform com- tion has modified both the original textures and fabrics of GC-45 plexes is typically difficult to establish on the sole basis of textural and GC-40. In any event, the fabrics of GC-96, GC-45, and GC-40 criteria. Such dunite typically contains anhedral olivine grains with are consistently stronger than those found in undeformed cumu- both gently curved boundaries and irregular boundaries composed lates (compare with Jackson, 1961, Figs. 43,44) and thus appear to of straight segments, many of which terminate in 120° triple junc- record tectonic modification of the original cumulus fabric. tions; protrusions from some grains deeply embay other grains (see, Structural Transition from Ultramafic Metacumulates to Ultra- for example, Jackson, 1961, Fig. 59). As similar textures occur in mafic Cumulates. The lower stratigraphic levels of the cumulus recrystallized dunite (Ragan, 1963, 1969; Vernon, 1970), these sequence are penetratively deformed, whereas the upper levels are textures are not of themselves conclusive. The deformed not. The structural contact between deformed and undeformed monomineralic dunites of the Olympus ultramafic complex, how- cumulates is gradational. This can be explained either by a gradual ever, occupy the stratigraphic interval between metacumulus chromitite and metacumulus subpoikilitic clinopyroxene-bearing dunites, a relation that leaves little doubt of the ultimate cumulus 3 Calcium metasomatism precludes similar observations on the plagio- origin of the intervening monomineralic dunites. clase in these rocks.

change upsection in rheologic properties of the cumulates or by constitutes as much as 5% of the harzburgite, in which it occurs as syntectonic magmatic sedimentation. small, exsolution lamellae—free, xenomorphic grains spatially as- The rheologic properties of a polyphase body are controlled sociated with orthopyroxene and spinel. dominantly by the flow properties of either the most abundant phase or the most ductile phase. Olivine is the most abundant phase Textures in the metacumulates (dunite, clinopyroxene-bearing dunite, and Olivine textures in the harzburgite are similar to those in the wehrlite) and lowermost undeformed cumultes (wehrlite) and is metacumulus dunite. Both linear and gently curved boundaries be- also the most ductile phase in these rocks (S. H. Kirby, 1975, oral tween olivine grains are common and typically occur together in commun.; Carter, 1976); hence, the subsolidus flow properties of the same specimen of harzburgite, but sharply curved boundaries the deformed and the undeformed wehrlites probably were similar definining deep embayments of one olivine crystal into another are and were only slightly affected by the gradual increase upsection in rare. The size of olivine grains varies slightly about an average of 3 the clinopyroxene content of the wehrlite. At temperatures above to 4 mm. However, the abundance of linear boundaries terminating the solidus, however, the most ductile phase in these igneous bodies in 120° junctions between adjacent olivine grains of similar optic was the intercumulus melt, not the olivine. orientation and the absence of a strong olivine foliation despite the The textures of the clinopyroxene-bearing dunite and poikilitic strong olivine optical fabric in these rocks (see below) suggest that wehrlite indicate that the intercumulus melt was rich in clinopyrox- the olivines are subgrains recrystallized from an earlier generation ene. The hypersolidus rheologic properties of the wehrlite were of coarser olivine grains. The similarity of olivine textures in the strongly controlled by the proportion of melt present, now roughly harzburgite and the dunite is believed to result from metamorphic represented by the percentage of postcumulus clinopyroxene. Seem- (tectonic) overprinting on earlier textures and does not, in itself, ingly undeformed cumulates, which now contain moderate suggest a common igneous origin of the two rock types. amounts of clinopyroxene, may have undergone large strains The spinel textures, fabrics, and composition in the harzburgite through "crystal-mush" or polygenic flow (Thayer and Jackson, differ significantly from those in the dunite. In harzburgite, the 1972), during which strain was accommodated by the intercumulus spinel is dark reddish brown [10 R 2/4; notation from Geol. Soc. liquid. The clinopyroxene-poor metacumulates, which contained America's (1970) Rock-color chart] in thin section, has ragged little intercumulus liquid during deformation, accommodated the edges, is commonly "wormy" or amoeboid and elongate, and strain primarily by solid flow, as suggested by the olivine textures defines a foliation. The spinel in the harzburgite has a higher Mg2+/ and fabrics. (Mg24" + Fe2+) ratio and a lower Cr3+/(Cr3+ + Al3+ + Fe3+) ratio The process of syntectonic magmatic sedimentation [synchro- than the chromite in the dunite (Greenbaum, 1972a, 1977; Allen, nous deposition and deformation of magmatic sediments (cumu- 1975), as is commonly observed in alpine-type peridotites (Thayer, lates)] may be compared to deposition of clastic sediments in basins 1946; Irvine, 1967). These differences in composition in members undergoing penecontemporaneous deformation (see, for example, of the same solid-solution series are probably not great enough to Krumbein and Sloss, 1963; Bird and Dewey, 1970): if folding of cause large differences in subsolidus deformation behavior of spinel the stratigraphic basement occurs continuously during the accumu- and chromite. Consequently, the difference between the spinel tex- lation of sediments, nowhere in the resulting stratigraphic column tures and the chromite textures must be attributed either to an early will there be a pronounced angular unconformity. Yet the oldest deformation or to a partial fusion event that affected the harzbur- sedimentary rocks will be infolded with the basement, and the gite but not the dunite. youngest sediments will be relatively undeformed. Most orthopyroxene occurs in large (5 to 15 mm), elongate Crystal-mush flow and syntectonic sedimentation probably are (length:width s 5:1) aggregates uniformly distributed throughout complementary processes during fractional crystallization of mag- the harzburgite (Fig. 9A). These aggregates consist predominantly mas in tectonically active regimes such as might be expected at of fine-grained (2 to 4 mm), relatively equant (up to 2:1 elongation) constructive plate margins. The relative contributions of these two xenomorphic orthopyroxene grains, some of which are irregularly processes to the petrogenesis of cumulates and metacumulates in sutured with mutually embaying relations; other grains are joined the Olympus ultramafic complex will probably be recognized only together along disrupted kink-band boundaries. Orthopyroxene by careful stratigraphic control on subtle changes in textures and grains embay and are embayed by olivine. fabrics. The orthopyroxene commonly contains lamellar and blebby ex- solutions of diopside that grade into discrete grains of diopside. HARZBURGITE TECTONITE These diopside grains have no cleavage, no growth twins, and no The harzburgite lies stratigraphically beneath the ultramafic exsolution lamellae; they have the yellow-green color typical of metacumulates. It consists predominantly of olivine and subordi- diopside in depleted lherzolites (J.-Cl. Mercier, 1974, oral com- nate orthopyroxene. Clinopyroxene, although present in small mun.; George, 1975). amounts, is a critical petrogenetic indicator. Four specimens of harzburgite collected within 500 m of the The harzburgite, which is commonly more than 50% serpen- harzburgite dunite contact contain a second, younger generation of tinized, consists of (1) 70% to 85% fine- to medium-grained (0.5 to both orthopyroxene (Figs. 9B, 9 C) and clinopyroxene (Fig. 9D) 8 mm), equant to moderately elongate (length:width = 3:1), that display several features characteristic of magmatic origin.

inclusion-free, xenomorphic olivine whose optical properties (2Vy These pyroxenes constitute less than 5% of the rock (10% to 20% = 87° to 89° ± 2°) indicate a composition of Fos^; (2) 15% to of the pyroxenes). They typically occupy cuspoid interstices be- 30% fine- to medium-grained (0.5 to 10 mm), equant to elongate tween olivine grains (Menzies and Allen, 1974) and may have as-

(length:width = 5:1), xenomorphic orthopyroxene whose 2Vy pect ratios (length-to-width ratios) as large as 10:1; yet they display ranges from 78° ± 2°, indicating a composition of En89_92, and neither optical strain nor strong preferred orientation of their long which contains fine exsolution lamellae of diopside parallel to axes. The clinopyroxene grains contain growth twins and cleavage (100); and (3) a trace to 2%; fine-grained (0.1 to 1 mm), equant to and have the distinctly gray to black color commonly found in moderately elongate (length:width = 3:1), xenomorphic spinel. igneous clinopyroxenes (J.-Cl. Mercier, 1974, oral commun.; Clinopyroxene, typically present in only trace amounts, locally George, 1975).

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Mesoscopic Structures pyroxene lineations are generally not evident in the field. In addition, preferred orientation of orthopyroxene cleavage traces is gen- The oldest mesoscopic structure recognized in the harzburgite is erally absent, an observation consistent with the weak crystallo-

compositional layering (S0), probably pyrometamorphic in origin graphic preferred orientation of orthopyroxene in these rocks. (Menzies and Allen, 1974), defined by discontinuous or disrupted Several types of dikes, too few in number and too small to show layers of different orthopyroxene concentrations. Typically, layers on Figure 3A, transect both layering and foliation in the harzbur- of orthopyroxenite one grain thick occur between pyroxene-poor gite. Coarse-grained gabbroic and pyroxenitic dikes 1 to 30 cm harzburgite or dunite layers 1 to 4 cm thick in otherwise massive thick are common. In one outcrop, 2- to 20-cm-wide dunite dikes harzburgite. Layered zones commonly constitute only 20% to 30% transect interdigitated harzburgite and chromite-bearing dunite of an outcrop of the harzburgite and hence do not define a penetra- and appear to be similar to those described from the peridotite at tive foliation at outcrop scale (Turner and Weiss, 1963, p. 28). Burro Mountain, California (Loney and others, 1971). Regardless Folded layers in the harzburgite are either rare or difficult to detect: of composition, most dikes have irregular trends, as though follow- unequivocal evidence of folding has been observed in only two ing pre-existing fractures in the harzburgite. places. Regional Trends. Two cross-cutting pyroxene foliations have Foliation in harzburgite is defined by preferred orientation either been observed in only six outcrops, three within a single area 200 of slightly to moderately elongate orthopyroxene crystals or aggre- to 400 m west of the Mount Olympus summit and two within a gates or of elongate spinels. It is commonly more evident in layered small part of region VI (Fig. 3 A). Because the appearance of the two harzburgite than in massive harzburgite. In most of the outcrops of foliations in hand specimen is similar in all regards but orientation, foliated harzburgite, only one foliation plane is visible, and it typi- classification of foliation measurements into S, (northwest-

cally parallels layering. In only seven of the outcrops observed does trending) and S2 (north- or northeast-trending) is based solely on foliation transect layering, typically at angles of less than 15°. orientation. Because the orthopyroxene grains and aggregates tend to form Si pervades the western and southern margins of the harzburgite oblate rather than prolate ellipsoids in well-foliated harzburgite, (Fig. 3A: regions IV, VIII, IX, and X) and is thought to represent

Figure 9. Drawings after photomicrographs. Patterns as in Figure 6; ruled pattern = orthopyroxene with lines drawn parallel to trace of (100). (A) Typical polycrystalline aggregate of orthopyroxene in haizburgite (GC-13). Long dimension = 3.3 mm. Aggregate is slighdy concave down. When thin section is rotated counterclockwise, extinction sweeps through the aggregate from left to right. Small circle within orthopyroxene at top center of drawing is exsolved bleb of diopside. (B) Interstitial orthopyroxene in harzburgite (GC-51). Long dimension = 3.3 mm. (Q Unusual orthopyroxene in harzburgite (GC-51). "Holly-leaf," subpoikilitic habit of orthopyroxene perhaps resulted from overgrowth of first-generation, "metamorphic" orthopyroxene by second-generation, magmatic orthopyroxene. (D) Magmatic dinopyroxene in harzburgite (GC-13). Long dimension = 2.15 mm. "Fish-shaped" dinopyroxene is 0.8 mm long and 0.1 mm wide. The bulbous "head" of the fish (left) contains a growth twin. Olivine grains surrounding dinopyroxene all have approximately the same lattice orientation.

the major ductile deformation event to affect the Olympus ultra- wide range of olivine-orthopyroxene ratios, probably for two

mafic rocks. Attitudes of layering (S0) in these regions show only reasons: (1) Coprecipitation of uniform proportions of cumulus slightly more scatter than attitudes of S, (Fig. 3C), a relation consis- olivine and orthopyroxene is limited by their reaction relation at tent with the paucity of folds. Mesoscopic layering approximately low confining pressures or high water pressures (Bowen, 1914; parallels the macroscopic interdigitations of the dunite-harzburgite Jackson, 1961; Kushiro and others, 1968). (2) Trapping of uniform contact and maintains a remarkably uniform orientation despite percentages of intercumulus liquid (giving rise to postcumulus or- the swing in gross orientation of the contact from northwest- thopyroxene) within an olivine cumulate for large stratigraphic in- trending in the south to north-northeast-trending in the northwest. tervals is unlikely. Several apparent anomalies occur in this pattern. The large scat- In contrast, the harzburgite in the Olympus ultramafic complex ter in orientations of chrysotile veinlets (Wilson, 1959, p. 113) and comprises two-thirds of the exposed ultramafic rocks in the com- pyroxene layering in the east-central area (region XI) of the plex (Fig. 3). The olivine/orthopyroxene ratio has a narrow range harzburgite probably indicates rigid-body rotation of many small (Menzies and Allen, 1974). Most of the harzburgite has no igneous blocks during the intense serpentinization characteristic of this re- textures, despite its weakly foliated or massive character. The gion. The apparent northeast trend of the dunite bodies mapped by harzburgite spinel is distinctly different from nearby cumulus Wilson (1959) in the central part of the harzburgite (region VII) chromite (Greenbaum, 1972a). Menzies and Allen (1974) observed perhaps reflects the tendency of talus to slide down the steepest that the harzburgite is chemically homogeneous: it does not exhibit slopes (generally east- to northeast-facing in this area) rather than the cryptic mineralogic or bulk variation in Mg/(Mg + Fe) common showing the structural grain of this poorly exposed area. Meso- in cumulus peridotites. scopic structures in this region generally strike west to northwest. Nicolas and Jackson (1972) suggested that harzburgite tectonite Wilson (1959, p. 113; 1976, written commun.) noted, however, in harzburgite-subtype ophiolites is the refractory residuum derived that rare examples of statistically aligned chrysotile veinlets are by extraction of a basaltic magma from a peridotite (Green and concordant with similarly poorly exposed, northeast-trending du- Ringwood, 1967; Ringwood, 1969). Greenbaum (1972b) showed nite bodies in region XI. that the harzburgite has the bulk composition expected for the re- In the northern dunite body (region V) and in the northern por- siduum of the partial fusion that produced the parental magma of tion of the harzburgite (region VI), layering and a prominent folia- the basalt, diabase, gabbro, and ultramafic cumulates of the

tion (S2) strike to the north and northeast and dip steeply to the Troodos ophiolite. Similarly, Menzies and Allen (1974) and Allen west. In two outcrops, S2 overprints an older foliation whose at- (1975) concluded that the harzburgite is a residuum of partial fu- titude parallels that of Si in other areas. Poorly developed S2 occurs sion of upper-mantle peridotite. I agree with their findings and in a few outcrops in regions VIII, IX, and X. Presumably, the event therefore interpret the harzburgite as the country rock upon and

that produced S2 was mild throughout most of the complex but (or) in which the metacumulates were deposited. strong enough to transpose layering to S2 orientation and to oblit- The strong foliation defined by former intercumulus melt (now erate Si in regions V and VI. Whether the second-generation mag- represented by nearly strain-free pyroxenes and plagioclase) in matic pyroxenes discussed above are related to the second- some rocks (for example, GC-96) within 20 m of the contact be- generation foliation (S2) is not clear, but their lack of internal de- tween the dunite and the harzburgite suggests that locally tempera- formation suggests that they at least crystallized or recrystallized tures were above the olivine-orthopyroxene-clinopyroxene- after the major (Si) deformation event. spinel-plagioclase solidus during (as well as after) the co- deformation of the harzburgite and the dunite. Since magmatic PETROFABRICS pyroxene textures in the harzburgite appear to occur only within 500 m of the main harzburgite-dunite contact, I propose that they The olivine fabrics of all harzburgite specimens measured are are contact features attributable to either (1) local maintenance of typified by a weak to moderately strong maximum of Z = [100] solidus or hypersolidus temperatures in the harzburgite near the axes subparallel to pyroxene foliation and by a poorly to moder- magma chamber where the parent magma of the cumulates ately developed partial girdle of X = [010] axes nearly normal both coalesced (and ultimately crystallized) during the major partial- to the Z-axis maximum and to the foliation (Figs. 10A, 10C, 10E, melting event in the harzburgite or (2) local re-heating of the previ- 10G, 101). The strength of the olivine fabrics contrasts markedly ously depleted harzburgite country rock to solidus or hypersolidus with the weakness of olivine foliation and lineation. Or- temperatures during the intrusion of the parent magma of the thopyroxene fabrics characteristically show a broad Z-axis girdle metacumulates ("contact metamorphism" of the harzburgite by the subparallel to pyroxene foliation (Figs. 10B, 10D, 10F, 10H, 10J) parent magma of the metacumulates), perhaps derived from partial and are weaker than the fabrics of coexisting olivine. Deviations melting of peridotite at greater depths. Thus, some partial-melt fea- from this pattern in specimens GC-35 and GC-32 (Figs. 10C, 10D, tures in the residuum may be totally unrelated to the petrogenesis 101, 10J) can be related to partial and complete overprinting, re- of the original parent magma and instead may be related to intru-

spectively, of Si by S2 and are suggestive of two periods of defor- sion and crystallization of the magma in the harzburgite country mation. rock. Whatever the source of the melt, if it crystallizes in the metamorphic peridotite (harzburgite), textures that mimic cumulus Petrogenesis textures may develop (see Pike and Schwarzman, 1977). Mechanisms of Flow during the "Major Event." The uniform The occurrence of the harzburgite beneath the metacumulus du- orientation of Si throughout the Olympus ultramafic complex, the nite indicates either that the harzburgite is a stratigraphically lower similarity of olivine and orthopyroxene fabrics from diverse parts member of the metacumulates [as proposed by Parrot (1973) for of the harzburgite, and the similarity of olivine textures throughout harzburgite of the Kizil Dag ophiolite in Turkey] or that it is part of deformed portions of the ultramafic complex suggest that the same the basement ("country rock") intruded by the magma from which deformation mechanism acted on most of the ultramafic rocks to the metacumulates crystallized (Greenbaum, 1972b). Harzburgite produce approximately the same orientation of finite strain. Only of known cumulus origin in continental stratiform intrusions gen- three currently known mechanisms can plausibly explain the types erally makes up only a small percentage of the cumulates and has a of deformation textures and fabrics observed in the olivine: (1)

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 X = b Y = c Z=o

Figure 10. Equal-area, lower- hemisphere projections of olivine (A, C, E, G, I) and orthopyroxene (B, D, F, H, J) indicatrix axes in five specimens of harzburgite. Great aides show orientations of orthopyroxene foliations S, and/or S2. Contours are percentage of axes of given orientation per 1% area of net. Dashed contours = 1%/1% area. Interior (solid) contours are 2%, 4%, 6%,..12%/1% area.

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 10. (Continued). (A) Olivine (100 grains) and (B) orthopyroxene (100 grains) in GC-49. (Q Olivine (150 grains) and (D) orthopyroxene (100 grains) in GC-35. (E) Olivine (100 grains) and (F) orthopyroxene( 102 grains) inGC-13. (G) Olivine (147 grains) and (H) orthopyroxene (99 grains) in GC-51. (I) Olivine (100 grains) and (J) orthopyroxene( 100 grains) inGC-32. Asterisk shows orientation of trace of S, on a weathered surface.

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plastic deformation (translation gliding) followed by annealing re- might explain the unusual texture of metacumulus dunite specimen crystallization, (2) syntectonie recrystallization, and (3) flow of a GC-28, in which the grain size of the olivine exceeds the size in- "crystal mush." The first two involve solid-state flow in which inferred from the chromite-net "holes" for the original cumulus ternal strain energy and surface energy provide the driving force for olivine grains. the recrystallization process. In the third, grain boundary sliding in The small percentage of former interstitial melt in the harz- the presence of interstitial melt accommodates the strain. burgite precludes crystal-mush flow as the dominant material Since plastic deformation can produce strong fabrics only transport mechanism during the late stages of depletion of the through large strains (Raleigh, 1963), a necessary condition for harzburgite. The possibility that crystal-mush flow was important identification of plastic flow followed by annealing recrystallization during earlier stages of partial melting when interstitial melt might as the dominant flow mechanism in peridotites with strong fabrics have been more abundant cannot be discounted; indeed, such flow is the recognition of large strains. Extensive plastic flow of olivine might physically aid migration and coalescence of interstitial melt has been documented in many other ultramafic complexes ("filter pressing") to form large magma chambers. (Raleigh, 1963, 1965; Davies, 1969; Nicolas and others, 1971; Other Structures. Faulting and serpentinization are late events Zimmerman and Carter, 1973). Microscopic features characteristic in the development of the complex. Serpentinite veins crosscut and of strong plastic deformation in these complexes include olivine offset both recovered and unrecovered kink-band boundaries; grains with well-developed kink bands with large angles (10° to faulting has rotated large blocks of foliated harzburgite in the 30°) of external rotation and large aspect ratios, recognizable even vicinity of the Pano Amiandos asbestos mines; and faulting along in recrystallized mosaics derived from the parent porphyroclasts. the eastern edge of the harzburgite has brought the harzburgite in Olivine fabrics are typically asymmetric to the foliation withZ-axis contact with the gabbro. Since the highest degree of alteration and maxima inclined to foliation at angles to 30°. Orthopyroxene hydration of both the harzburgite and the gabbro occur together in grains in such rocks are highly elongate nearly parallel to their slip the region near Pano Amiandos, much of the hydration and ser- plane, which is inclined at a small angle to a strong foliation. These pentinization of the harzburgite probably took place during or after features are generally absent from the harzburgite — even the the late-stage faulting event. Similarly, Margaritz and Taylor specimens that display the strongest olivine fabrics. (1974) interpreted oxygen and hydrogen-isotope ratios of the Macroscopic and mesoscopic evidence of large strains in the Olympus ultramafic complex serpentinites as indicating that most harzburgite is somewhat equivocal. The interdigitations along the of the serpentinization of the complex occurred at low tempera- dunite-harzburgite contact may have resulted from (1) large-scale tures, probably late in the emplacement history of the complex. isoclinal folding (Greenbaum, 1972a, 1972b), in which case total strain along this contact was large; (2) tectonic transposition of ORIGIN OF THE TROODOS OPMOLITE feeder dikes (filled with either cumulus or refractory olivine and chromite) connected to the magma chamber, in which case total The large volumes of basalt, diabase, and cumulates in ophiolites strain might have been small; or (3) gravitational sinking of rela- imply that extrusion and intrusion of magmas are essential proces- tively dense chromite-rich pods of the dunite into the underlying ses in their formation. Davies (1969) concluded that the country hot, weak harzburgite (Dickey, 1975), in which case strain would rock for the intrusions from which the cumulates of the Papuan have been large only along the margins of the pods. The rarity of ophiolite crystallized was previously consolidated cumulates. isoclinal folds in the complex could mean that total strain was so Moores and Vine (1971) proposed that the dikes of the sheeted in- great throughout most of the complex that most isoclinal folds trusive complex in the Troodos ophiolite formed by multiple intru- were attenuated or flattened beyond recognition. But the peculiar sions of basaltic magma into other, newly solidified dikes and pil- open geometry of the few folds observed supports the alternative low lavas — that is, older dikes were the country rock for newer interpretation: total strain was so small that folds developed only in dikes. More important, they proposed that the dikes were intruded local moderately strained zones. along a zone of continual crustal extension, such as might be ex- Experimental syntectonic recrystallization of olivine can produce pected at a mid-oceanic ridge. strong fabrics, even if total strain is only 10% to 20% (Ave Lalle- Greenbaum (1972a, 1972b) combined the concepts of Davies mant and Carter, 1970). It is not yet known whether syntectonic (1969) and Moores and Vine (1971) to explain the formation of the recrystallization alone can cause significant net material transport Olympus cumulates. He hypothesized that the cumulates formed in (large strains), or whether it is primarily a reorienting recovery an extensional regime at a mid-oceanic ridge in a "steady-state" mechanism that accompanies an unrecognized material-transport magma chamber in which the magma was replenished continually mechanism: high-temperature recovery processes such as polygoni- by new influxes. Parental peridotite continually moved upward by zation and grain-boundary migration commonly accompany syn- solid-state flow in the mantle beneath the spreading center. During tectonic recrystallization (Carter, 1976). Moreover, textures pro- this flow, partial melting of the peridotite produced tholeiitic or duced by syntectonic recrystallization of olivine may be indistin- picritic magmas and left a refractory harzburgitic residuum. Some guishable from those produced by annealing recrystallization of of the magma extruded directly onto the sea floor and produced pil- plastically deformed olivine (George, 1975). low lavas; some intruded the lowest pillows as dike swarms parallel Olivine textures in the experimental charges that were deformed to the ridge axis; the remaining magma cooled in magma chambers at the highest temperatures and lowest strain rates (Ave Lallemant above the harzburgitic residuum and beneath the lavas. Near the and Carter, 1970) are characterized by slightly flattened, polygonal ridge axis, this remaining magma continuously crystallized to form grains that define a weak to moderate foliation normal to the the early cumulus phases of chromite and olivine directly onto the

maximum principal compressive stress cr1 (George, 1975). X-axes harzburgitic country rock. The harzburgite, the cumulus chromitite form a maximum parallel to cr, (Ave Lallemant and Carter, 1970) and dunite, and the partially consolidated magma that overlay

and normal to a Z-axis maximum parallel to cr3 (Ave Lallemant, them were displaced progressively outward by continuous upwel- 1975). The fabrics and textures of the harzburgite are similar to ling of more peridotite. At progressively greater distances from the those produced experimentally during syntectonic recrystallization, ridge axis, then, magma that had been passively carried outward with allowances for a limited time for grain growth in the experi- was cooler and more fractionated, since early crystallizing phases mental apparatus. In addition, grain-boundary migration of olivine had previously settled from this more distal magma when it had

been closer to the ridge axis. The data presented in this paper and derlying ultramafic cumulates. The deep-water sediments inter- summarized below support a significant implication of Green- layered with the pillow lavas indicate that the Troodos ophiolite baum's model: magmas at spreading centers crystallize in dynamic, formed at a site well removed from continental terranes (Elderfield tectonically active regimes. and others, 1972). The penetrative deformation was probably related to processes associated with spreading-center tectonics, not to Structural Evolution of the Olympus Ultramafic Complex subsequent transport or to continental emplacement tectonics. With the possible exception of mild plastic deformation of The Olympus ultramafic complex is divisible into two structural olivine by (0kl) [100] slip (George, 1975), there are no penetrative faciès: (1) penetratively deformed ultramafic rocks (harzburgite structures in the Olympus ultramafic complex that can be related to and metacululates) and (2) undeformed ultramafic and mafic cumu- emplacement of the Troodos ophiolite onto Cyprus. This suggests lates and late differentiates. The ultramafic complex is also divisible that emplacement was a nonpenetrative and presumably low- into two petrogenetic units, but they do not coincide with the struc- temperature event. tural divisions (Fig. 3B): (1) refractory residuum (harzburgite) and (2) a magmatic sequence comprising metacumulates and cumulates Implications for Other OphioEtes (chromitite, dunite, wehrlite, pyroxenite, and low-level gabbro) and late differentiates (high-level gabbro and granophyric rocks). Relatively undeformed igneous rocks and strongly deformed, Several lines of evidence suggest that penetrative deformation of metamorphic peridotite are characteristic of ophiolite complexes the harzburgite and the metacumulates occurred at temperatures at (Penrose Field Conference participants, 1972; Nicolas and least above the basalt solidus and probably above the peridotite sol- Jackson, 1972; Thayer and Jackson, 1972). The contact between idus. The comparable strength of olivine fabrics in the harzburgite the magmatic rocks and the metamorphic peridotite (the harzburg- and the metacumulus dunite, the parallelism of S, in the harz- ite) of the Olympus ultramafic complex is obscure. In retrospect, burgite to S! in the dunite, and the similar attitude of S[ and the the difficulty in locating the contact is easy to understand. Each of macroscopic interdigitations of the dunite-harzburgite contact the five processes of (1) posttectonic magmatic sedimentation, (2) suggest that the oldest recognizable deformation to affect the syntectonic magmatic sedimentation, (3) crystal-mush flow, (4) harzburgite affected the dunite to a comparable extent. Thus, the S, partial fusion of residual metamorphic peridotite, and (5) multiple deformation probably did not start much before the onset of intrusion of magma is by itself difficult to recognize. And if all five chromite and olivine crystallization from the magma and probably processes art simultaneously during the formation of an ophiolite continued after complete solidification (adcumulus overgrowth) of complex, the resulting field relations, structures, textures, and fab- the metacumulus dunite (olivine adcumulate). The temperature in rics will be exceedingly complex, particularly if subsequent trans- the metacumulus dunite must have remained high, even during post- port and emplacement impose a strong metamorphic and tectonic solidification deformation, because the temperature of the overly- overprint. ing undeformed cumulates had to remain high during their syntec- The relative chronology and importance of the five processes tonic magmatic sedimentation and (or) crystal-mush flow. Penetra- may strongly control the variety of internal structures, textures, tive deformation of the harzburgite and metacumulates apparently and fabrics found in ophiolites. For example, Nicolas and Jackson was complete by the time that the undeformed cumulates had so- (1972) stated that there is generally a sharp division between the lidified, the interstitial magmatic pyroxenes in the harzburgite had magmatic rocks and the metamorphic peridotites. In the Vourinos crystallized, and the gabbroic, pyroxenitic, and dunitic dikes had Ophiolite Complex, Greece, a sharp contact does indeed occur be- formed. tween relatively undeformed cumulus rocks and strongly deformed, The absence of passive markers with known original geometry metamorphic peridotite (Jackson and others, 1975). Nicolas (in makes difficult an estimate of total strain in the harzburgite and Nicolas and Jackson, 1972), however, anticipated the discovery of thereby eliminates a critical clue to the flow mechanisms. Micro- metamorphic rocks ultimately of cumulus origin within metamor- scopic textures and fabrics and mesoscopic structures seem to indi- phic peridotite of several complexes (for example, Red Mountain, cate that the harzburgite underwent fairly small amounts of strain: California; Canyon Mountain, Oregon). If Thayer (1970) was cor- plastic flow of the olivine apparently was neither the dominant rect in proposing that podiform chromitite deposits are prima facie crystal-orienting mechanism nor the dominant flow mechanism. In evidence of cumulus processes, then the Vourinos Complex con- the specimens of harzburgite with magmatic pyroxenes, the per- tains at least two distinct generations of cumulates. The older, pre- centage of interstitial melt present during deformation was proba- tectonic generation (the Skoumtsa and Xerolivado podiform bly insufficient to allow grain boundary sliding (crystal-mush flow) chromitites and associated dunites) was complexly folded with the to be the dominant flow mechanism. The only known flow mechan- metamorphic peridotite (Moores, 1969). The younger, posttectonic ism that can explain most of the textures, fabrics, and structures generation was deposited cleanly on top of the deformed peridotite observed in the harzburgite is syntectonic recrystallization of and is undeformed (Jackson and others, 1975). These two genera- olivine, a high-temperature flow mechanism (Avé Lallemant and tions of cumulates are end-members of cumulates formed by syn- Carter, 1970). tectonic magmatic sedimentation, as at the Olympus ultramafic As concluded here, penetrative deformation of the tectonites complex. Hopson and others (1975) described field relations indi- probably occurred within an interval of time comparable to that cating that four of these same five processes (partial fusion of needed to crystallize the metacumulates. Gass and Smewing (1973) metamorphic peridotite being the exception) occurred during the concluded that the thermal gradient through the lower pillow lavas evolution of the Point Sal, California, ophiolite. Apparently, re- and the sheeted intrusive complex during metamorphism to zeolite mobilization of mushes of cumulus crystals was more intense in the and greenschist fades soon after crystallization was steep, perhaps Point Sal ophiolite than in the Olympus ultramafic complex. as steep as 150°C/km. Even though so steep a thermal gradient im- Students of other ophiolites have found it difficult to find a well- plies that these formations were good thermal insulators, estimates defined contact between the cumulates and the metamorphic for their combined stratigraphie thickness [4 to 5 km by Vine and peridotite (Allemann and Peters, 1972; Engin, 1972; Blake and Moores (1972) and 3 to 4 km by Coleman (1971)] suggest that Landis, 1973; Himmelberg and Loney, 1973; Southwick, 1974; these formations were too thin to prevent rapid cooling of the un- Goullaud, 1975). From the evidence, I believe the contact between

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metamorphic peridotite and cumulates in most harzburgite-sub Jour. Sci., ser. 4, v. 38, p. 207-264. type ophiolites to be obscure for the same reasons that it is obscure Bradley, W. W., Huguenin, E., Logan, C. A., Tucker, W. B., and Waring, C. A., 1918, Manganese and chromium in California: Calif. State in the Olympus ultramafic complex. I propose that the five proces- Mining Bur. Bull., v. 76, 248 p. ses that controlled the structural and petrologic evolution of the Carter, N. L., 1976, Steady state flow of rocks: Rev. Geophysics and Space Olympus ultramafic complex in an extensional regime all played a Physics, v. 14, p. 301-360. role to a greater or lesser degree in the evolution of most Clague, D. A., and Straley, P. F., 1977, Petrologic nature of the oceanic Moho: Geotimes, v. 5, p. 133-136. harzburgite-subtype ophiolite complexes. Coleman, R. G., 1971, Plate tectonic emplacement of upper mantle peridotites along continental edges: Jour. Geophys. Research, v. 76, ACKNOWLEDGMENTS p. 1212-1222. Coleman, R. G., and Peterman, Z. E., 1975, Oceanic plagiogranite: Jour. Geophys. Research, v. 80, p. 1099-1108. Research for this study was conducted as part of a Ph.D. disser- Davies, H. L., 1969, Peridotite-gabbro-basalt complex in eastern Papua: An tation at the State University of New York at Stony Brook. overthrust plate of oceanic mantle and crust [Ph.D. thesis]: Stanford, Additional literature research and writing of this paper was com- Calif., Stanford Univ., 88 p. pleted as part of the project "Fabrics of Deformed Ultramafic Dickey, J. S., Jr., 1975, A hypothesis of origin for podiform chromite de- Rocks" during my tenure as a National Research Council Postdoc- posits: Geochim. et Cosmochim. Acta, v. 39, p. 1061-1074. Diller, J. S., 1921, Chromite in the Klamath Mountains, California and toral Research Associate at the U.S. Geological Survey. Financial Oregon: U.S. Geol. Survey Bull., v. 725-A, p. 1-35. support was provided by National Science Foundation Grant GA- Elderfield, H., Gass, I. G., Hammond, A., and Bear, L. M., 1972, The origin 31569 awarded to N. L. Carter and by U.S. Geological Survey of ferromanganese sediments associated with the Troodos Massif of Project Number 9940-01487 awarded to E. D. Jackson. The Hel- Cyprus: Sedimentology, v. 19, p. 1-19. lenic Mining Corporation kindly provided housing and transpor- Engel, C. G., and Fisher, R. L., 1975, Granitic to ultramafic rock complexes of the Indian Ocean Ridge system, western Indian Ocean: Geol. Soc. tation during my fieldwork. I am grateful to C. Xenophontos, A. America Bull., v. 86, p. 1553-1578. Panayiotou, and M. Mantis of the Cyprus Geological Survey for Engin, T., 1972, Petrology of the ultramafic rocks and brief geology of the help with logistics. I thank E. D. Jackson, R. G. Coleman, R. A. Andizlik-Zimparalik area, Fethiye, southwest Turkey: Mineral Re- Loney, D. Greenbaum, H. G. Ave Lallemant, and C. A. Hopson for search Exploration Inst. Turkey Bull., foreign edition, no. 78, p. 1-18. their helpful suggestions on earlier drafts of this paper: J.-CI. Gass, I. G., and Masson-Smith, D., 1963, The geology and gravity anomalies of the Troodos massif, Cyrpus: Royal Soc. London Philos. Mercier, for instructive discussions and for use of his stereonet con- Trans., ser. A, v. 255, p. 417-467. touring program DENSITY; and Julie Guenther and Barbara Gass, I. G., and Smewing, J. D., 1973, Intrusion, extrusion, and Nielsen of the U.C.L.A. staff for technical help. I especially thank metamorphism at constructive margins: Evidence from the Troodos N. L. Carter and E. D. Jackson for their guidance, encouragement, massif, Cyprus: Nature, v. 242, p. 26-29. and support. Geological Society of America, 1970, Rock-color chart: Boulder, Colo., Geol. Soc. America. George, R. 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