The Vourinos Ophiolite, Greece: Cyclic Units of Lineateci Cumulates Overlying Harzburgite Tectonite
EVERETT D. JACKSON U.S. Geological Survey, Menlo Park, California 94025 HARRY W. GREEN II Department of Geology, University of California, Davis, California 95616 ELDRIDGE M. MOORES
ABSTRACT stratiform peridotite-gabbro complex of the and have tectonite fabrics. They are type discussed by Jackson and Thayer strongly foliated, and in places the foliation Re-examination of the Vourinos ophio- (1972). The main portion of the Vourinos cuts directly across contacts between rock lite shows it to be composed of metamor- massif (most of the Moores' [1969] ultra- types. This unit corresponds directly to the phic tectonites, cumulates, plagiogranites, mafic zone), however, consists (as he con- "metamorphic suite" of ophiolites pro- dikes, and lava. The contact between the tended) of highly deformed harzburgites posed by Nicolas and Jackson (1972). Be- tectonites and the cumulates is exposed and and dunites, and it constitutes an alpine cause of the complex folding (Moores, sharp. Beneath the cumulate contact, the peridotite in the terminology of Jackson 1969), there is no way to estimate the rocks have been highly deformed and com- and Thayer (1972). thickness of this unit, but it is at least 60 plexly folded; above that contact, they sim- The presence of relatively undeformed km2 in areal extent, and has a cross- ply have been tilted vertically to expose a cumulates in ophiolite complexes has been sectional thickness of as much as 7 km. The stratiform complex 1,500 m (4,800 ft) noted before (Thayer, 1969a; Davies, 1969, upper surface of the metamorphic suite has thick. The stratiform intrusion is charac- 1971; Juteau, 1970; Moores and Vine, a sharp, gently undulating contact. terized by cyclic units, rich in olivine at the 1971; Bezzi and Piccardo, 1970, 1971; Par- Coarse-grained (average 1.5 cm), rela- base, and rich in feldspar at the top. Some rot, 1969, 1975; Greenbaum, 1972; Al- tively undeformed olivine cumulates lie cumulus diorites are present at the top of lemann and Peters, 1972; Gass and Sinew- cleanly deposited directly on this surface the section which grade into quartz diorites ing, 1973; England and Davies, 1973; with their planar lamination and layering (plagiogranites) with hypautomorphic tex- Peterson and others, 1974; Brown and parallel to the foliation in the tectonites. tures. Lineate lamination characterizes the Bradshaw, 1974). Indeed, the discovery of These cumulates and the rocks above them cumulates and may indicate the direction or the presence of cumulates in ophiolites and correspond to the "magmatic suite" of orientation of the Mesozoic mid-oceanic the number of localities where these cumu- Nicolas and Jackson (1972). The lowest ridge crest with respect to the present posi- lates have been found have multiplied cyclic unit of the cumulus section (see Fig. tion of the complex. A siliceous dike swarm greatly since they were summarized by 1) consists of 90 m (300 ft) of isomodal cuts the upper part of the stratiform com- Jackson as recently as 1971. olivine cumulates that exhibit a great deal plex. The section suggests that in the case of For Vourinos, we have measured a com- of postcumulus overgrowth (see Jackson, Vourinos, a large magmatic chamber posite section of cumulates as much as 1967, for the cumulus terminology and formed at a mid-oceanic ridge crest and 1,565 m (4,800 ft) thick, arranged in at Streckeisen, 1973, for igneous rock names that intrusion was a much more important least 24 cyclic units of the type found in the used in this paper). This lowest layer ap- process than extrusion in the formation of Stillwater, Great Dyke, Muskox, and Bush- pears to be somewhat size graded and is oceanic crust in that area. The reported veld complexes (Jackson, 1970). These succeeded, at a phase contact, by a thin presence of cumulates in many other rocks now stand nearly vertically and are olivine-clinopyroxene cumulate. The lowest ophiolite complexes suggests that these re- gently folded around near-vertical axes. cyclic unit was beheaded and is succeeded lations may obtain generally at most or all The current view that the upper part of by a similar, somewhat thinner olivine spreading ridges. The contact between the ophiolites represents oceanic crust (de cumulate (about 40 m thick), again with an tectonites and the cumulates of the complex Roever, 1956; Brunn, 1960; Hess, 1962, olivine-clinopyroxene cumulate at its top. would not have corresponded with seismic 1965; Gass, 1967; Thayer, 1969b; Cole- The third cyclic unit consists of more than M. Key words: ophiolite, Vourinos, man, 1971), coupled with the wealth of 500 m (1,600 ft) of coarse-grained ( > 1 Greece, tectonites, cumulates, plagiogran- knowledge of cumulus processes already cm) olivine cumulates, but the presence of ites, dike swarm, oceanic crust, ultramafic. gained from studies of layered intrusions three chromite cumulates in this section (Wager and Deer, 1939; Hess, 1960; Jack- leads us to believe that at least three cyclic INTRODUCTION son, 1961, 1967), should now allow a great units are present here. The entire lower in- deal of insight into processes presently terval is about 650 m (2,000 ft) thick (A on Moores (1969) divided the Vourinos operating at the near mid-oceanic ridges. Fig. 1). ophiolite into three zones, which he called Stratigraphically above this interval, the ultramafic, transitional, and mafic. We have STRATIGRAPHY character of the cyclic units changes. They recently remapped parts of the upper part become thinner, averaging a little less than of the Vourinos ophiolite; we discovered Most of Moores' (1969) ultramafic zone 33 m (100 ft) thick, somewhat finer that all of Moores' (1969) transition zone in the Vourinos ophiolite consists of com- grained, and better developed. In the inter- and part of his ultramafic and mafic zones plexly interdigitated harzburgite, dunite, val labeled B in Figure 1, each cyclic unit consist of relatively undeformed rocks with and chromitite. These rocks have been begins as an olivine cumulate, but cumulus textures, and that they comprise a folded at least three and perhaps four times, clinopyroxene becomes slightly more
Geological Society of America Bulletin, v. 86, p. 390-398, 6 figs., March 1975, Doc. no. 50315.
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abundant as a postcumulus phase. It then igneous-textured (qtz.) dior. with dikes appears as a cumulus phase at a phase con- tact, and a layer of clinopyroxene-olivine pchc a pcbc q cumulate develops. Some of these appear to interbanded units - 20' thick be mineral graded, and some do not. Fi- nally, olivine disappears as a cumulus phase, pcbc or pbcc 4 14- and clinopyroxene cumulates, some with FAULT- JL postcumulus feldspar, ensue. All of these cbc Pcbc must have been beheaded, since in one chromite Websterite place, a clinopyroxene-plagioclase cumu- late is developed at a phase contact above pcbc the isomodal clinopyroxene cumulate layer. In at least one of these cyclic units, or- 13- bcc_ YteJasiar Lie. thopyroxene joins clinopyroxene as a FAULT- cumulus phase. In the intervals labeled C through G in cpc Figure 1, the trend continues. Olivine .1 cumulate layers become thinner; indeed, in chromite FAULT • cpc higher intervals, cyclic units are bottomed by olivine-clinopyroxene cumulate layers. 12' Clinopyroxene-plagioclase cumulates are present at the tops of all units. In the up- cpccc? to be north. Much has been written about the origin of cyclic units (Jackson, 1961, 1970, 1971; Irvine and Smith, 1967; Irvine, 1970). Most authors agree that these units owe their origin to fractionating batches of magma of gradually changing composition. Most agree that these batches change composi- tion largely by reason of depletion of earlier cumulus phases. The alternative is fraction- unconformity — ation in a hidden chamber, which seems Harzburgite tectonite only to put the problem out of sight. If the majority opinion is correct, then the corol- lary argument is that the entire 1,565-m Figure 1. Columnar section of the stratiform peridotite-gabbro complex in the Vourinos ophiolite. (4,800 ft) section was at one time entirely Section compiled from eight partially overlapping measured sections. liquid and floored by harzburgite tectonite. A. Lower unit, largely olivine cumulates (dunites) resting on the metamorphic suite rocks. The chamber then slowly filled itself with B. Central unit, largely composed of thinner cyclic units of olivine, olivine-clinopyroxene, and magmatic sediments. The general paucity of clinopyroxene cumulates (principally dunites, wehrlites, olivine clinopyroxenites, and clinopyroxen- ites). space-filling postcumulus phases indicates C through G. Upper unit, largely composed of olivine-clinopyroxene, clinopyroxene, and that cooling took a rather long time com- clinopyroxene-plagioclase cumulates, clinopyroxene-orthopyroxene cumulates, and plagioclase- pared with large layered complexes of simi- two-pyroxene cumulates (principally olivine clinopyroxenites, plagioclase-bearing clinopyroxenites, lar depth and thickness (see arguments in gabbros, websterites, and gabbronorites); oc is olivine cumulate; occ is olivine clinopyroxene cumu- Hess, 1960; Jackson, 1961, 1967) and so late; chromite is chromite cumulate; cc is clinopyroxene cumulate; cpc is clinopyroxene plagioclase may indicate a relatively slowly moving cumulate; bcc is orthopyroxene-clinopyroxene cumulate; pcbc is plagioclase-clinopyroxene- ridge (Moores and Jackson, 1974). orthopyroxene cumulte; pchc is plagioclase-clinopyroxene-amphibole cumulate. These are field names, and do not necessarily conform to Jackson's (1967) system of ordering in sequence of decreas- Also, most workers (Jackson, 1961, ing abundance of cumulus phases. Units A, B, and C are from the central part of the complex near 1970, 1971; Irvine and Smith, 1967; Irvine, Chromion, sections D and E from sections near the Krapa Hills and the Aliakmon River, F from near 1970) conclude that the cyclic units, taken Mikroklisoura, and G from the Aliakmon River. (See map in Moores, 1969, for localities.) together, reveal the order of crystallization Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/3/390/3433715/i0016-7606-86-3-390.pdf by guest on 01 October 2021 392 JACKSON AND OTHERS of the magma. For Vourinos, this order Above the stratiform complex, there are Mohoroviiic discontinuity (Jackson, 1971; would be olivine, chromite, olivine, olivine leucocratic rocks with hypautomorphic Greenbaum, 1972; Gass and Smewing, plus clinopyroxene, clinopyroxene alone (a granular textures (called quartz diorite and 1973; Peterson and others, 1974). Seismic reaction boundary is implied), clinopyrox- dacite by Moores, 1969; trondhjemites by profiles of a section such as the Vourinos ene plus orthopyroxene, clinopyroxene plus Thayer, 1967; or plagjogranites by Thayer, stratiform complex undoubtedly would plagioclase, clinopyroxene plus ortho- 1975, and Coleman and Peterman, 1975), place M at the top of interval A (Fig. 1), pyroxene plus plagioclase, and finally which in the case of Vourinos, appear to be whereas, petrologically, the olivine cumu- clinopyroxene plus plagioclase plus differentiates of the stratiform complex. lates of this interval are magmatic, not hornblende. These rocks and the uppermost cumulates metamorphic rocks. The stratiform rocks of It should be stressed that Figure 1 is a are cut by Na-rich silicic dikes and overlain interval B and perhaps the lower part of in- composite section, compiled from eight by flows of similar composition. No tholei- terval C may correspond to the 7.1- to partly overlapping sections that occur in the itic rocks occur in the area, and if ever pres- 7.8-cm/sec basal layer of Sutton and others area between Chromion and Pondini, ent, have been eroded away. However, the (1971). Variations in the velocities of the through the Krapa Hills to Langhadakia number of faults in the terrain suggests that layer may be related to the amount of post- (see map in Moores, 1969). Although we any original tholeiites could have been cumulus overgrowth on the mafic cumulus can match certain stratigraphie units and faulted out. minerals, as well as to differing amounts of patterns of cyclic units in each of these eight The stratiform section raises some in- serpentinization (Moores and Jackson, sections, we as yet have no idea of the lat- teresting questions about the differences be- 1974). eral continuity of units as a whole. tween the seismic and petrologic We have found two distinctly different generations of chromite in Vourinos; we 43-B-l 59-B-l have documented the existence in ophiolites of thin cumulus chromitites as contended by Thayer (1969a), but we also find that the exploitable deposits we have examined are tectonites and are part of the metamorphic suite of the complex; they therefore appar- ently predate the obviously cumulus chro- mitites. If these latter chromitites originally formed as cumulates, they have now been deformed beyond recognition. Cpx. ^J^ Y=b Cpx. Y=b Opx. ^ X=b TEXTURES AND FABRICS & at Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/3/390/3433715/i0016-7606-86-3-390.pdf by guest on 01 October 2021 VOURINOS OPHIOLITE, GREECE 393 In contrast to the tectonites, the cumu- Figure 3. Pyroxene lineations lates, although now tilted vertically, show from Figure 2 projected into layer- very little sign of deformation. In a few ing planes, S,, in original field pos- itions and then rotated to horizon- places, all phases show weak undulatory tal (S',), the supposed gravitational extinction, and a few (100) kink bands surface at the time of formation. occur in olivine. Crystal shapes, however, Black lenses represent 6 percent are those expected in rocks of cumulus contours for clinopyroxene linea- origin, and the minerals show apposition tions, and diagonally ruled lenses fabrics when rotated to their original gravi- represent 6 percent contours for tational orientation. We emphasize that orthopyroxene. Lighter and textural evidence of deformation does not heavier dotted lines diagrammati- increase with depth in the cumulates; the cally outline the broad general maxima in the original and re- contact with the foliated tectonites is as stored orientations, respectively. sharp with respect to microstructures as Lower hemisphere, equal-area pro- with respect to field structures. We tenta- jections. tively equate the weak, unrecovered defor- mation in the cumulates with the colder, however, that with either interpretation, formed, the lineations of elongate minerals secondary deformation in the tectonites and the fabric shows distinct lineate lamination. for the most part fall within the northwest believe it reflects small internal deforma- In Figure 2b, the intermediate axis is also a or southeast quadrants with respect to tions accompanying tilting of the complex girdle, with a double-maximum at right present north (Fig. 3). The samples we ex- and emplacement in its present position. angles to that of the c axes, as expected in amined were obtained from different struc- The same interpretation of similar deforma- apposition fabrics. tural blocks, and at present, we have no tion in the Stillwater complex was given by Fabric diagrams of both pyroxenes in way of knowing whether the spread in linea- Jackson (1961). sample 59-B-l were prepared in a similar tion directions is primary or the result of In the cumulates, olivine crystals are way. The shape ratios for the clinopyroxene faulting or folding during crustal emplace- nearly equidimensional and show little pre- in this rock showed b: Lbc:c were ment. ferred orientation. Pyroxenes also appear to 0.56:0.65:1.00. Figure 2d shows a b These strong lineations, whatever their show relatively weak fabrics where nearly maximum perpendicular to the layering present directions, presumably arose from equidimensional, but tabular pyroxene and plane much like that of Figure 2a, but less flow during their emplacement, whereas the plagioclase crystals show strong planar pronounced, reflecting the stubbier shape of larger stratiform intrusions of stable shields lamination in the field, and elongate pyrox- the prismatic clinopyroxene of this sample. apparently form as "quiet water" magmatic ene and hornblende crystals appear to have Figure 2f shows a weak girdle distribution sediments (Jackson, 1971). Given the strong lineate laminations in the layering of the long c axes of the clinopyroxene, but kinematic situation near an actively spread- plane. again shows a distinct lineation, this time ing mid-oceanic ridge, at which place we We selected two oriented samples of near S. 45° E. with respect to the trace of suppose the Vourinos stratiform rocks to cumulates and one tectonite for preliminary north in the layering plane. Figure 2e shows have formed, current movement of crystal- petrofabric examination. We wished to test the pattern of the intermediate crystal di- charged magma is not surprising. Symmetry the cumulates for apposition fabrics and to mension to be fairly diffuse. The or- considerations suggest that the principal confirm the strong planar lamination and thopyroxene crystals in sample 59-B-l have flow direction was either parallel or per- suspected lineate lamination observed in stubby, rather short prismatic habits, with pendicular to the ridge axis. Most convec- the field. Sample 43-B-l, an olivine- an a:b:c ratio of 0.87:0.55:1.00, using the tion models would predict hyperbolic flow clinopyroxene cumulate, was selected be- Hess and Phillips (1940) convention. The (Goldsmith and Mason, 1967) perpendicu- cause the clinopyroxenes are especially short X = b axis shows a distinct maximum lar to the ridge, in which case our observed tabular. Their habit proved to be approxi- normal to the layering plane (Fig. 2g), planar and linear laminations predict a mately orthorhombic, with an average whereas both Y = a and Z = c fabrics (Figs. northeast-southwest—trending ridge located b:lbc:c ratio of 0.33:0.78:1.00, equating c 2h and 2i) show girdles. The long Z = c northwest or southeast of the present posi- to unity. The fabric of the clinopyroxene in axes show a strong double maximum cen- tion and orientation of the Vourinos massif. this rock was measured on optic directions, tered near S. 75° E. but distinctly out of the Schilling (1973) and Campsie and others but c and J.be positions were subsequently layering plane. The intermediate Y = a axes (1973a, 1973b), however, have recently obtained by computer rotation (Fig. 2, a, b, display a girdle in the layering plane with a suggested asthenospheric transport and and c). Figure 2a shows the extremely maximum at right angles to that of Z = c. consequent magma mixing along the axis of strong b maximum normal to the layering The asymmetry of the Z = c pattern may be the mid-Atlantic ridge in the vicinity of Ice- plane, reflecting the short b axis that defines another artifact of the small number of land and near the Galapagos Islands. If the planar lamination of the rock. Figure 2c grains measured (50). such a process operated during the forma- shows that the longest crystal axis, c, The field observations and the prelimi- tion of the Vourinos complex, the linea- displays a fabric that could be interpreted nary fabric diagrams both strongly indicate tions may have been parallel to the ridge as a girdle in the layering plane with a max- that rocks of the stratiform intrusion are axis (northwest-southeast relative to the imum approximately perpendicular to the cumulates with shape-controlled apposition present position of the massif). We cannot trace of north on the layering plane, or as fabrics laid down under the influence of deduce the actual Mesozoic ridge orienta- double maxima concentrated at about gravity (Jackson, 1961). The surprising fea- tion and position with either interpretation N. 65°-75° E. or N. 60°-65° W. Whether ture of the fabrics is the strong lineate lami- of the lineations, however, as we do not this double maximum is significant or an nation in the layering plane, a feature un- know at present the amount of rotation, artifact is not yet known. The small num- common in stratiform intrusions of the other than tilting, involved during and after ber of clinopyroxene grains present in the Stillwater-Bushveld-Great Dyke-Muskox the emplacement of the massif onto the two thin sections examined thus far (84) type (Jackson, 1971). When the layering crust of Greece. makes it possible that the real distribution plane is rotated to horizontal, the presumed Although Moores (1969) clearly de- is a single, broad maximum. It is clear, gravity surface at which the rocks were scribed the tectonitic nature of the Vou- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/3/390/3433715/i0016-7606-86-3-390.pdf by guest on 01 October 2021 394 JACKSON AND OTHERS rinos metamorphic suite rocks, he did not men from the region near the cumulate- layering and foliation form an angle of 25°. publish fabric diagrams. For comparison tectonite contact, because cumulus layering In the field, this specimen (33-1), contained with the cumulus fabrics, we have also and metamorphic foliation, whether for- pyroxene-rich layers alternating with measured the preferred orientation of tuitous or not, are parallel there. Conse- pyroxene-poor layers on a scale of 1 to 5 olivine and orthopyroxene grains in a harz- quently, we chose a very fresh harzburgite cm and showed a distinct foliation defined burgite tectonite from the metamorphic from approximately 3 km below the con- by the shape orientation of pyroxene, part of the complex. One major interest tact which exhibited deformation textures olivine, and spinel grains. Surfaces cut on was to determine if the microfabrics were indistinguishable from those just below the the specimen in the laboratory revealed a related to the layering or to the foliation. cumulate interface and throughout penetrative lineation (defined by orienta- We could not choose an appropriate speci- metamorphic suite rocks, but in which tion and alignment of inequant spinel crys- tals) parallel to the intersection of foliation and layering, and thin sections showed a V-33-1 parallel elongation of olivine and or- thopyroxene crystals. The olivine microfab- rics show a very strong, broad, point max- imum of Z = a parallel to the spinel linea- tion (Fig. 4c) and a well-developed girdle of X = b normal to the lineation (Fig. 4a); the two maxima in the girdle are symmetrically inclined to the foliation plane, but asym- metrically related to the layering. Y = c axes show no recognizable pattern. Or- thopyroxene patterns are similar but weaker, with a Z = c maximum paralleling the spinel lineation (Fig. 4f), an irregular maximum of Y = a normal to the foliation (Fig. 4d), and a diffuse pattern of X = b. These patterns reflect the strong plastic de- formation undergone by these rocks (Raleigh, 1967, 1968; Carter and Ave'Lallemant, 1970; Nicolas and others, 1971, 1973; Green and Radcliffe, 1972a, 1972b; Kohlstedt and Vander Sande, 1973); the dominant glide directions (a in olivine and c in orthopyroxene) have been rotated subparallel to the spinel lineation (the direction of the longest axis of the strain ellipsoid), and the normal to the dominant glide plane in each mineral (b in olivine and a in orthopyroxene) is similarly oriented subnormal to the foliation (the di- rection of the shortest axis of the strain el- lipsoid). The parallelism of the spinel linea- tion with the intersection of foliation and layering is a common observation in peridotite tectonites (Nicolas and others, 1971, 1973; A. Nicolas and F. Boudier, 1973, oral commun.), where it reflects large-scale folding of the layering during deformation. Further work is necessary to confirm or deny this hypothesis for Vou- rinos. MINERAL COMPOSITIONS Preliminary microprobe measurements indicate a range of molar olivine composi- tons from about Mg 91.4 to Mg 93.2 from the metamorphic part of the complex. The cumulus olivine compositions from the lowest part of the stratiform complex (in- terval A of Fig. 1) range from 91.7 to 92.4. The clinopyroxene of sample 43-A-l, a Figure 4. Fabric diagrams of olivine (a-c) and orthopyroxene (d-f) from sample 33-1, a harzburgite clinopyroxene-orthopyroxene-olivine cu- mulate, has a molar composition of Ca tectonite from the metamorphic portion of the Vourinos ophiolite. L = spinel lineation; S 3 = layering plane; S 4 = foliation plane. 50 grains of each mineral measured. Contours 1, 3, 6, percent per 1 per- 41.5 Mg 53.0 Fe 5.5; its orthopyroxene cent area. Lower hemisphere, equal-area projections. averages Ca 2.5 Mg 84.2 Fe 13.3; and its Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/3/390/3433715/i0016-7606-86-3-390.pdf by guest on 01 October 2021 VOURINOS OPHIOLITE, GREECE 395 olivine is Mg 83.5. The clinopyroxene of 1971), a somewhat similar feature was dis- composition, time of emplacement, or sample 43-B-l, an olivine clinopryoxene covered in the Vourinos ophiolite (J. F. Par- origin, because samples thus far examined cumulate, has the composition Ca 45.3 Mg rot, 1971, oral commun.). We have are altered to zeolite- and greenschist-facies 49.9 Fe 4.8; the olivine of the rock is too confirmed the existence of an extensive dike assemblages, and hence, chemical serpentinized for analysis. Both these sam- swarm in the upper part of the Vourinos modification is distinctly possible. How- ples were collected from interval B (Fig. 1). ophiolite, underlying an area approxi- ever, the spatial position and chemical Sample 59-B-l, the orthopyroxene- mately 1 km wide and 15 km long. The composition of these dikes suggest that they clinopyroxene cumulate from interval E dikes generally trend northwest and dip represent late-stage volcanism associated (Fig. 1) yields compositions of Ca 2.0 Mg moderately southwest. They typically form with the quartz-diorite bodies located in the 79.0 Fe 19.0 and Ca 45.0 Mg 47.0 Fe 8.0, up to about 50 percent of a given exposure; upper part of the ophiolite. for orthopyroxene and clinopyroxene, re- but in a few places, 100 percent dikes are Comparison of the attitudes of dikes with spectively. It is apparent from these pre- present in dike-on-dike sets similar to those nearby cumulates would afford the oppor- liminary results that the minerals of the described from Cyprus. The country rock in tunity to reconstruct the original attitude of stratiform peridotite-gabbro complex are most of the Vourinos dike swarm is diorite the dikes at the time of their intrusion, in- strongly fractionated in the expected direc- or quartz diorite; but in some places, the asmuch as the layering of the cumulates tion of iron and calcium enrichment upsec- dikes cut the upper part of the stratiform should presumably represent the normal to tion (see Hess, 1960). complex as well. The lower contact of the the gravity field at the time of their forma- The range of olivine compositions in the dike swarm consists of a fairly abrupt tion. Unfortunately, attitudes of cumulates metamorphic suite rocks of the Vourinos change from exposures with abundant near most dike swarm areas are badly scat- ophiolite is virtually identical to that of dikes to ones in which dikes are almost tered owing to faulting and open folding other alpine peridotites of the harzburgite completely absent. Thus although the dikes about steeply dipping axes. However, we subtype (Jackson and Thayer, 1972). In "die out" downward, some clearly predate obtained a fair consistency of cumulus at- particular, they are identical with values the diorite, as the latter contains diabase titudes in the area between Langadhakia given by England and Davies (1973) for xenoliths without chilled border relations. and Mikroklisoura. In this area, when the ultramafic tectonites in Papua, where a The dike swarm, although clearly one of the cumulates are rotated to their original hori- magmatic sequence similar to that at Vou- types discussed by Thayer (1975), does not zontal positions, the dikes strike nearly rinos is also exposed. The olivine, or- conform to all his more generalized obser- east-west and dip steeply south (see Fig. 5). thopyroxene, and clinopyroxene composi- vations. The upper contact of the dike Inasmuch as the dikes cut the upper parts of tions in the cumulates at Vourinos are again swarm is poorly exposed and may possibly the cumulates, they must have been em- almost identical to those from Papua in be faulted; we observed no gradation into placed after much of the oceanic crust had spite of the fact that stratigraphy there ap- pillow lava such as occurs on Cyprus. cooled and moved some distance away pears to be a good deal more irregular and Dike rocks include keratophyre, diabase, from its source at a Mesozoic mid-oceanic lenticular than in Vourinos. fine-grained gabbro, and diorite. Moores ridge. The compositions of quartz diorite and (1969) gave two analyses from this dike Theoretical considerations (Lachen- basalt that lie above or cut the stratiform swarm (his analyses XII and XIII), both bruch, 1973a, 1973b) and observations on part of the Vourinos ophiolite have been characterized by high SiO 2 contents (ap- and near the Mid-Atlantic Ridge (Barrett given by Moores (1969), and we have no proximately 65 and 55 percent, respec- and Aumento, 1970; Aumento and others, further information to add at this time. In tively), again reminiscent of the Sheeted 1971) indicate that normal faults are pres- general, the quartz diorites are charac- Complex of Troodos. In the much larger ent on the flanks of that ridge striking par- terized by rather abundant calcium and collection of dike rocks from our present allel to and dipping toward its axis. These alumina with respect to silica, and the study, we are unfortunately unable to give faults would serve as natural conduits for basalts are sodic and silicic. more concrete description of their original the ascent of any available magma still pres- The compositions of minerals in the Vou- rinos ophiolite support the hypothesis that they were produced from an original man- N tle source with a 100x Mg/Mg+Fe ratio of about 89 (compare O'Hara, 1968; and Green, 1971). Such a source rock would have melted to leave a harzburgitic re- siduum with olivines and pyroxenes in the Figure 5. Attitudes of dikes in a range Mg 91-93, and to produce early swarm between Langadhakia and cumulus olivines of about the same com- Mikroklisoura rotated to their position. Both the residual peridotites and orientation when nearby cumu- the early phenocrysts are considerably more lates were horizontal, c = pole to magnesium-rich than the Hawaiian residua cumulus layering in present orien- and phenocrysts (Shaw and Jackson, 1973). tation; D = pole to dike in present orientation; dots = poles to Hence, our data suggest important differ- cumulus layering when mean pole ences between source materials and melting (O) is restored to vertical (X). processes in linear island chains from those Great circles are resultant restored at mid-oceanic ridges, where we suppose dike planes. Lower hemisphere, the Vourinos ophiolite to have formed. equal-area projections. DIKE SWARM Subsequent to the realization of the im- portance of the Sheeted Intrusive Complex on Cyprus (Gass, 1968; Moores and Vine, Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/3/390/3433715/i0016-7606-86-3-390.pdf by guest on 01 October 2021 396 JACKSON AND OTHERS ent in a crustal magma chamber at almost base. The principal difference of the Vouri- tures of its rocks indicate that it formed any stage of its crystallization. If this nos section is that it is the only one report- perpendicular to the gravity field. The mechanism were responsible for the dike edly composed of cyclic units. This is not stratiform rocks are arranged in cyclic units swarm in Vourinos, the Mesozoic spread- surprising, for cyclic units were unrecog- that are rich in olivine near the base (that ing center lay south of the ophiolite. Again, nized in the Stillwater complex for many contain thin chromite cumulates) and we cannot deduce the actual ridge position years before their discovery (Jackson, become first richer in pyroxene, then richer until rotation involved in the emplacement 1961), and although the Muskox cyclic in feldspar, and finally end with amphibole of the Vourinos ophiolite is known. units were described after early examina- among the cumulus phases. These rocks It would also be interesting to compare tion (Irvine and Smith, 1967), cyclic units have shape-oriented fabrics and show con- the directions of lineate lamination in the in the Bushveld and Great Dyke had long siderable lineate lamination. Above the cumulates with dike attitudes. The lamina- been overlooked by many workers (Jack- stratiform complex, there are quartz dior- tions we measured (see Fig. 3) were not in son, 1970; Hughes, 1970). ites with hypautomorphic-granular textures the same structural blocks as the dike A second peculiarity of the Vourinos and some lava flows. The upper part of the swarms, but when both are rotated to the stratiform complex is the strong lineate stratiform complex and the overlying original layering plane of the cumulates, the lamination of its rocks. Again, this feature quartz diorites are cut by swarms of altered lineate lamination lies at angles from 15° to has not been reported from other dikes. 90° to the attitudes of the steeply dipping ophiolite-type stratiform intrusions. We We consider the entire ophiolite to be a Langadhakia-Mikroklisoura dikes (see Fig. hope that it, too, is present but undescribed nearly complete section of Mesozoic 5). However, the lineate laminations of in others, for it may reflect the original oceanic crust and upper mantle formed at a sample 59-B-l, which lies closest to the spreading direction of the ridge at which it mid-oceanic ridge, Miyashiro's (1973) swarm, are nearly at right angles to the was formed, as discussed above. chemical objections to the contrary. If we plane of the rotated dikes. A third exceptional feature of the Vou- are correct, then the ophiolite should shed It is apparent that much information on rinos ophiolite is the exposure of the sharp, light on many of the processes involved in ridge rheology and mechanics is available undisturbed interface between metamor- the formation of oceanic crust and residual at Vourinos, but that it cannot be resolved phic and magmatic suite rocks, with con- mantle at modern mid-oceanic ridges. We without detailed structural mapping with siderable thicknesses of both intact cumu- must emphasize, however, that the follow- particular emphasis on cumulus structures. lates and highly deformed harzburgites ing conclusions are preliminary, based We are currently mapping much of the area above and below, respectively. principally on field observations, and that between Chromion and Taxiarchis in the much more laboratory work and map com- hope of ultimately understanding the de- pilation remains to be done. Nevertheless, tails of these processes. SUMMARY AND CONCLUSIONS the following preliminary points can be raised. COMPARISON WITH OTHER The Vourinos ophiolite consists of a 1. A large magmatic chamber formed at OPHIOLITE SECTIONS lower metamorphic suite of highly de- the ridge crest beneath a thin capping of formed harzburgites, dunites, and chromi- basalts; it persisted at some distance from Moores and Jackson (1974) have re- tites. This unit is in sharp contact with the ridge crest and fractionated as it cooled. cently compiled data on thicknesses and gently folded but otherwise little-deformed 2. A much greater volume of magma rock types of the upper parts of 11 ophio- cumulates that compose a stratiform was intruded at the ridge than was ex- lites. Part of their Figure 4 is reproduced peridotite-gabbro complex about 1,500 m truded. here as Figure 6. Nearly all sections contain (4,800 ft) thick. The magmatic layering is 3. The magmatic sediments were depos- mafic and ultramafic cumulates near the now approximately vertical, but the tex- ited cleanly on harzburgite tectonite, which was probably the residuum left after partial melting of the upper mantle. PA 4. Currents in the magmatic chamber gave rise to cumulates with strong lineate lamination. These currents may have flowed parallel or perpendicular to the ridge axis. 5. The magnesian nature of the re- B I siduum and of the earliest cumulates is con- sistent with an original pyrolitic source r5 CM material. V 6. The seismic M discontinuity would BC e PI d e not correspond to the contact between the Km g-d d d metamorphic and magmatic suite rocks but g-d g would lie about 650 m (2,000 ft) above it. g "cPg*" The central part of the stratiform complex t t ivv-i may correspond to the seismic basal layer determined in parts of oceanic crust, and y the upper part of the complex, along with Figure 6. Crustal sections of selected ophiolite complexes, e = extrusives, generally mafic pillow the intrusive diorites, may correspond with lava; d = dolerite; g = gabbro; t = mixed mafic-ultramafic or "transition" zone; u = ultramafic rocks. the seismic oceanic layer. Symbols for ophiolite complexes are BI — Bay of Islands; CM — Canyon Mountain; BC — Betts Cove; E — Elba; V — Vourinos; T — Troodos; KD — Kizil Dagh; S — Semail, Oman; O — Othris; 7. The amount of postcumulus over- PI — Pindos; PA — Papua (after Moores and Jackson, 1974). The Canyon Mountain section is after growth in the Vourinos cumulates suggests Coleman (1971) and is subject to serious revision as a result of new but unpublished work (T. P. that the complex was formed at a relatively Thayer, 1974, written commun.). slowly spreading ridge. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/3/390/3433715/i0016-7606-86-3-390.pdf by guest on 01 October 2021 VOURINOS OPHIOLITE, GREECE 397 8. A swarm of dikes is present that High-temperature flow of dunite and Irvine, T. N., 1970, Crystallization sequences in originally dipped steeply but not perpen- peridotite: Geol. Soc. America Bull., v. 81, the Muskox intrusion and other layered in- dicularly-with respect to the horizontal pos- p. 2181-2702. trusions: Geol. Soc. South Africa Spec. Pub. ition of the cumulates. These may have Coleman, R. G., 1971, Plate tectonic emplace- 1, p. 441^76. ment of upper mantle peridotites along con- been parallel to and north of the Mesozoic Irvine, T. N., and Smith, C. H., 1967, The tinental edges: Jour. Geophys. Research, v. ultramafic rocks of the Muskox intrusion, ridge crest and probably acted as feeders for 76, p. 1212-1222. Northwest Territories, Canada, in Wyllie, late fractionated magmas. Coleman, R. G., and Peterman, Z. E., 1975, P. J., ed., Ultramafic and related rocks: Oceanic plagiogranites: Geol. Soc. America New York, John Wiley &c Sons, Inc., p. Bull, (in press). 38-49. ACKNOWLEDGMENTS Davies, H. L., 1969, Peridotite-gabbro-basalt Jackson, E. D., 1961, Primary textures and min- complex in eastern Papua [Ph.D. thesis]: eral associations in the ultramafic zone of We thank E. Schwartzman and D. Bideau Stanford, Calif., Stanford Univ., 88 p. the Stillwater Complex, Montana: U.S. for measurement of microfabrics, R. Witt- 1971, Peridotite-gabbro-basalt complex in Geol. 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P., 1967, Chemical and structural re- MANUSCRIPT RECEIVED BY THE SOCIETY MARCH olivine: Jour. Geophys. Research, v. 73, p. lations of ultramafic and feldspathic rocks 27, 1974 5391-5403. in alpine intrusive complexes, in Wyllie, P. REVISED MANUSCRIPT RECEIVED JUNE 27, 1974 Printed in U.S.A. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/3/390/3433715/i0016-7606-86-3-390.pdf by guest on 01 October 2021