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Journal of Petrology Volume 42 Number 1 Pages 5-24 2001
© Oxford University Press 2001
Structural Petrology of Plagioclase Peridotites in the West Othris Mountains (Greece): Melt Impregnation in Mantle Lithosphere
VENING MEINESZ SCHOOL OF GEODYNAMICS, UTRECHT UNIVERSITY, PO BOX 80021, 3508 TA UTRECHT, THE NETHERLANDS
Received November 15, 1999; Revised typescript accepted June 28, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONTHE OTHRIS OPHIOLITEDISCUSSIONCONCLUSIONSREFERENCES |
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We present the results of a structural and petrological study of mantle rocks from the strongly dismembered Othris Ophiolite. Part of the mantle section was impregnated with melt, crystallizing plagioclase and clinopyroxene as cumulate phases and refertilizing previously depleted peridotites. Melt impregnation occurred late in the deformation history of the host peridotites. The deformation took place at stresses of 13–26 MPa and at temperatures around 1000–1200°C, at the base of the thermal lithosphere. The melt therefore impregnated relatively cold mantle rocks, implying that the thermal lithosphere reached into the mantle during magmatic activity. We conclude that the Othris Ophiolite represents a spreading environment with a relatively thick lithosphere, such as that near an axial discontinuity or transform fault of a slow-spreading ridge. The proposed magmatic and deformation history of the peridotites is in agreement with episodic magmatism at slow-spreading ridges. We thus conclude that the heterogeneous character of the mantle section of the Othris Ophiolite results from melt impregnation processes. We suggest that the presence of lherzolitic ophiolite types among harzburgitic ophiolite types in the Hellenic–Dinaric chain reflects variable degrees of melt impregnation and refertilization rather than partial melting and melt extraction.
KEY WORDS: lithospheric mantle deformation; melt impregnation; microstructures; Othris Ophiolite; plagioclase peridotites
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONTHE OTHRIS OPHIOLITEDISCUSSIONCONCLUSIONSREFERENCES |
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Studies of ophiolites world-wide have revealed a great structural and petrological diversity. This diversity is commonly attributed to a variety of extensional geodynamic environments in which ophiolites may form. It has been proposed that these environments span the entire Wilson Cycle, from rift basins through ocean ridges to back-arc basins (e.g. Coleman, 1984
; Boudier & Nicolas, 1985; Bonatti & Michael, 1989; Nicolas, 1989). The composition of ophiolitic mantle sections, which are believed to be the residues of partial melting, can give information about the tectono-magmatic environment in which the ophiolites formed. It is generally assumed that harzburgitic mantle sections are formed by a high degree of melt depletion, corresponding to fast-spreading environments. Ophiolites with lherzolitic mantle sections, often referred to as the lherzolite ophiolite type, are thought to correspond to low degrees of melt depletion in slow-spreading or rift settings (Boudier & Nicolas, 1985; Nicolas, 1986a, 1989).
A special kind of lherzolite ophiolite are those with voluminous plagioclase lherzolites, such as the Trinity Ophiolite (Quick, 1981a, 1981b; Jacobsen et al., 1984; Boudier et al., 1989; Cannat & Lécuyer, 1991), the Lanzo Peridotite Massif (Boudier & Nicolas, 1972; Boudier, 1978; Pognante et al., 1985; Bodinier, 1988; Wogelius & Bishop, 1989), and the Othris Ophiolite (Menzies, 1973, 1975, 1976a, 1976b; Hynes, 1974a; Menzies & Allen, 1974; Menzies et al., 1977; Rassios & Konstantopoulou, 1993). The presence of plagioclase in the mantle rocks of these massifs indicates that the lherzolites equilibrated at low pressures (<8 kbar), in the metamorphic plagioclase stability field. If lherzolitic compositions indeed reflect low degrees of melt depletion, then these massifs must have ascended through the mantle without undergoing extensive pressure release melting. In the Voltri Massif it has been shown that lherzolitic mantle rocks were tectonically exhumed from deep mantle levels corresponding to the spinel lherzolite stability field by extensional shear zones (Vissers et al., 1991). The Voltri peridotites followed a largely subsolidus P–T path and the plagioclase in the Voltri peridotites was formed as a result of a subsolidus metamorphic reaction (Vissers et al., 1991; Hoogerduijn Strating et al., 1993).
The mantle section of the Othris Ophiolite (Central Greece), however, is compositionally very heterogeneous, comprising mostly harzburgites and minor lherzolites and dunites. In addition, significant volumes of plagioclase lherzolites containing abundant melt relics occur (e.g. Menzies, 1973). These first-order observations cannot easily be reconciled with a scenario of subsolidus exhumation. According to Menzies (1973) the melt relics in the Othris peridotites represent melt fractions that were not completely extracted and remained in the rock.
In this paper we present the results of a structural, microstructural and petrological study of the mantle section of the Othris Ophiolite. We conclude that the plagioclase lherzolites in Othris are not simply the residue after limited melt extraction, but that they are the product of melt impregnation and refertilization of harzburgitic peridotites. Furthermore, we interpret the structure and petrology of the mantle section in terms of its tectono-magmatic environment of origin.
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THE OTHRIS OPHIOLITE |
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TOPABSTRACTINTRODUCTIONTHE OTHRIS OPHIOLITEDISCUSSIONCONCLUSIONSREFERENCES |
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The dismembered Othris Ophiolite represents a remnant of the Mesozoic Tethyan ocean basin. The ophiolite forms the uppermost tectonic unit of a series of thrust sheets, showing the characteristics of an Upper Triassic–Jurassic passive margin sequence overlying Triassic volcanic rocks associated with rifting (Hynes, 1974b
; Smith et al., 1975). The sheets were emplaced onto the Palaeozoic basement of the Pelagonian Zone in the Late Jurassic–Early Cretaceous and are unconformably overlain by an Upper Cretaceous (Cenomanian to Coniacian) transgressive cover (Hynes et al., 1972; Smith et al., 1975). The entire sequence has later been thrust westward over flysch of Late Cretaceous–Tertiary age (Faupl et al., 1996).
The mantle rocks of Othris Ophiolite were first studied in detail by Menzies (1973, 1975, 1976a, 1976b), Menzies & Allen (1974) and Menzies et al. (1977), who concluded that the ophiolite formed in a marginal environment at the inception of rifting (Menzies & Allen, 1974). In contrast, Rassios and coworkers concluded that the Othris Ophiolite originated at an ocean ridge, in the vicinity of a northeast-trending transform fault (Rassios & Konstantopoulou, 1993; A. Rassios, personal communication, 1995).
Crustal rocks
Our study has focused on the Fournos Kaïtsa and Katáchloron areas of the Othris Ophiolite (Fig. 1). Detailed structural mapping has confirmed the strongly dismembered character of the ophiolite. The westernmost peridotites of the Katáchloron area are in tectonic contact with an ophiolitic tectonic melange, which tectonically overlies turbidites (‘Late Cretaceous–Tertiary flysch unit’). The melange unit consists of basalts, red cherts and serpentinized peridotite blocks, as well as fragments of amphibolitic meta-gabbros. The amphibolites in the melange strongly resemble amphibolites that occur elsewhere in the Othris Mountains, near the city of Lamia. The Lamia amphibolites have been interpreted as remnants of a sub-ophiolitic metamorphic sole and their age has been established by 40Ar/39Ar dating as 169 ± 4 Ma (Spray et al., 1984).
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The easternmost peridotites of the Fournos Kaïtsa area are in tectonic contact with a unit largely made up of isotropic gabbros, basalts and minor red cherts. This unit is overlain by a limestone–marble unit (‘Late Cretaceous transgressive cover unit’).
Mantle rocks
The peridotites of the study area are transected by steep NE–SW and moderately to gently inclined NW–SE striking serpentinite cataclasites (Fig. 1; see also Fig. 2). Rassios & Konstantopoulou (1993) argued that the fault geometry is that of a stack of NE-vergent imbricate thrusts. The domains between the faults shown in Fig. 1 are more or less coherent. The most striking feature of the peridotites exposed in the study area is their compositional, structural and microstructural variability. An east–west section through the area comprises plagioclase lherzolites and plagioclase harzburgites, spinel lherzolites, harzburgites and amphibole-bearing harzburgites with large dunite bodies. Microstructures range from coarse porphyroclastic to ultramylonitic [nomenclature after Mercier & Nicolas (1975)]. In general, foliations are moderately to steeply inclined. On the basis of structure and composition, we have recognized five lithotectonic domains within the studied mantle section (Fig. 1). From west to east, the domains have the following structural, mineralogical and microstructural characteristics.
Domain I
The westernmost domain, found in the Katáchloron area only, consists of coarse-grained harzburgite tectonites (Fig. 3a) with N–S striking foliations. Large lenses and bodies of dunite (up to 400 m wide) are observed cross-cutting the foliation. Orthopyroxene porphyroclasts in the harzburgites are typically 5–15 mm in diameter. The microstructure of the harzburgites is coarse porphyroclastic; olivine grain sizes range from 0·1 to 3 mm. The harzburgites contain early pargasitic hornblende and post-kinematic tremolitic amphibole.
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Domain II
Towards the east, the harzburgites become mylonitic (Fig. 3b) and locally ultramylonitic. Ultramylonitic bands have a vitreous appearance as a result of extremely fine grain size (10–50 µm). Dunite lenses are transposed into the plane of mylonitic foliation and are strongly reduced in width to
0·1–1 m. The mylonitic peridotites are part of a kilometre-wide N–S trending shear zone.
Domain III
East of the mylonite zone, relatively fine-grained amphibole-free harzburgite tectonites occur in the Katáchloron area (Fig. 3c). Such harzburgites also occur in the Fournos Kaïtsa area (see also Fig. 2). The fine-grained harzburgites often show a clear compositional layering caused by modal variations in orthopyroxene and spinel content. The compositional layering is consistently subparallel to the deformation fabric defined by flattened and stretched orthopyroxene and spinel grains. Foliations and layering generally strike NW–SE. The fine-grained harzburgites contain a few relatively thin (centimetre- to metre-scale) dunite layers, which are locally oblique to the foliation. Remarkably, the rocks of this domain are often almost completely unserpentinized. Orthopyroxene porphyroclasts are generally <5 mm and are surrounded by rims of fine-grained polyphase material (mainly orthopyroxene and olivine). These fine-grained rims are often flattened parallel to the foliation. The orthopyroxene clasts surrounded by fine-grained polyphase rims are embedded into coarser domains of predominantly olivine and minor interstitial orthopyroxene. The microstructure of the harzburgites is fine-grained porphyroclastic (see below).
Domain IV
Both in the Katáchloron and Fournos Kaïtsa (Fig. 2) areas the harzburgites grade into peridotites with more fertile bulk compositions (Fig. 3d–g). Upward and eastward the peridotites first become rich in clinopyroxenite lenses and millimetre-thick veins; eventually they become plagioclase bearing. Locally, clinopyroxene-poor plagioclase peridotites occur. However, no attempt is made to distinguish between plagioclase lherzolites and plagioclase harzburgites. Plagioclase peridotites are often serpentinized and plagioclase is sometimes altered to an assemblage of hydrogrossular and zoisite. Plagioclase peridotites are predominantly fine-grained tectonites, containing orthopyroxene porphyroclasts <5 mm in diameter (Fig. 3d). Within the fine-grained tectonites, however, we locally observe metre- to 100-m-scale domains of coarse-grained tectonites with orthopyroxene porphyroclasts of 5–15 mm (Fig. 3e). In the plagioclase-bearing peridotites plagioclase occurs in millimetre-scale veins and lenses, which are parallel to the lineation defined by stretched orthopyroxene and spinel grains. Plagioclase is commonly associated with clinopyroxene and/or orthopyroxene. Sometimes pargasitic hornblende also occurs in the plagioclase-bearing clusters. Within the coarse-grained plagioclase peridotites plagioclase is often found in tapering aggregates flanking orthopyroxene porphyroclasts (Fig. 3e). Plagioclase peridotites contain a compositional layering caused by modal variations in orthopyroxene or plagioclase content (locally layers have a troctolitic composition). The layering is parallel to a foliation defined by flattened orthopyroxene and spinel. Locally concordant clinopyroxenites and websterites occur, some of which contain plagioclase. Much more common are gabbroic dykes, comprising concordant and discordant olivine gabbros (Fig. 3f) and predominantly discordant coarse-grained pegmatitic olivine gabbros to gabbro–norites (Fig. 3g). Locally, it is observed that discordant gabbro dykes are contiguous with small plagioclase lenses (e.g. Fig. 3g). The change from harzburgites to plagioclase peridotites is associated with a change in orientation of the strikes of foliations from NW–SE to N–S or NE–SW. Microstructures in the plagioclase-bearing peridotites range from fine- to coarse-grained porphyroclastic (see below).
Domain V
In the Fournos Kaïtsa area, plagioclase peridotites with a pervasive serpentine network (Fig. 3h) occur as the most northeasterly mantle domain. They are separated from the less serpentinized plagioclase peridotites of domain IV by a cataclastic fault. The serpentinized peridotites of this domain are coarse-grained tectonites with orthopyroxene porphyroclasts commonly >5 mm. Plagioclase in these rocks is completely altered. In this domain dolerite dykes and bodies are found, in which plagioclase is largely unaltered (unrodingitized). A 100-m-scale isotropic gabbro body was found in this domain, and the published geological map (Institute for Geology and Mineral Exploration, 1962) shows the presence of a 100-m-scale dolerite body, which has been mined for copper. The microstructures of the tectonites of this domain are often difficult to determine as a result of serpentinization, but both coarse- and fine-grained porphyroclastic microstructures have been observed.
Microstructures in plagioclase peridotites
As outlined above, most of the studied plagioclase-bearing peridotites studied are fine-grained tectonites with a fine porphyroclastic microstructure. Their microstructure is characterized by domains of fine-grained olivine (20–200 µm) and orthopyroxene around orthopyroxene porphyroclasts (Figs 4a and b, and 5a) and coarser domains (100 µm–1 mm) consisting of olivine and minor spinel (Figs 4a and 5b). Very locally, the coarser domains contain orthopyroxene with typical interstitial crystal shapes. The larger olivine grains contain irregular closely spaced kinkband boundaries at a high angle to the foliation. Regions between these kinkband boundaries often show undulose extinction. Orthopyroxene occurs as large round porphyroclasts and as smaller interstitial grains predominantly in the fine-grained domains (Fig. 5a).
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Crystal orientation (universal stage) measurements on olivine in relatively coarse olivine bands in a fine-grained tectonite reveal a strong crystallographic preferred orientation (Fig. 6). Olivine a-axes coincide with lineation directions. Olivine b-axes cluster at a high angle to the foliation, with a secondary maximum within the foliation plane. The crystallographic preferred orientation indicates that the dominant deformation mechanism in the tectonites was dislocation creep with [100] (010) and minor [100] (001) as the active slip-system of olivine.
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The fine-grained tectonites enclose areas of coarser-grained tectonites. These have a coarser porphyroclastic microstructure, with olivine porphyroclasts of 0·5–3 mm surrounded by recrystallized grains of 50–200 µm (Fig. 5c and d). The porphyroclasts contain straight kinkband boundaries; these are more widely spaced than kinkband boundaries in fine-grained tectonites.
Clinopyroxenitic and websteritic veins and lenses in the plagioclase peridotites are generally parallel to the foliation. They often show evidence for boudinage, indicating that they were present during the deformation recorded in the peridotites.
Plagioclase occurs as single crystals or in polycrystalline clusters. Plagioclase clusters are often elongate parallel to the spinel and orthopyroxene lineation and foliation in the peridotites (Fig. 5a–e). Plagioclase may be associated with clinopyroxene (Fig. 5c) or orthopyroxene (Figs 5b–d and 7a and c). Some clinopyroxene crystals in plagioclase peridotites exhibit the simple growth twin. Plagioclase is also found in clusters with a poikilitic shape in rare centimetre-thick spinel websterite layers (Fig. 5f) and in the more common centimetre- to decimetre-scale troctolitic layers (Fig. 7a).
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Plagioclase crystals contain much less evidence for intracrystalline deformation than the olivines of the host peridotite. Generally, a magmatic texture is preserved in plagioclase clusters and plagioclase crystals contain relatively wide straight twin lamellae (Fig. 7b and c). Only rarely plagioclase crystals show evidence for crystal plastic deformation; plastically deformed plagioclase crystals exhibit strong undulose extinction, crystal bending, narrow and tapering lamellar twins, and abundant recrystallization to small grains with irregular grain boundaries (Fig. 7d and e).
Locally, the plagioclase peridotites are cut by mylonite zones. In these zones plagioclase crystals occur as clasts in the fine-grained matrix (Fig. 7f), indicating that they formed before the mylonitic deformation. Plagioclase clasts in the mylonites also contain abundant deformation features such as undulose extinction and narrow tapering twins.
Mineral chemistry
We obtained chemical analyses of mineral grains using a Cameca SX-50 electron microprobe at the Geology and Geophysics Department of Texas A&M University (USA) with wavelength-dispersive spectrometers equipped with LiF, PET and TAP crystals. Operating conditions comprised an acceleration voltage of 15 kV, a 10 nA beam current, 10 µm beam diameter and counting times of 20–60 s. Sodium was always measured first with a counting time of 20 s. As the primary aim of the chemical analyses was pyroxene geothermometry, relatively clinopyroxene-rich (lherzolitic) samples were selected for chemical analysis. In Table 1 we present average mineral compositions for a plagioclase-bearing peridotite from the Fournos Kaïtsa area (GOF1) and a plagioclase-free peridotite with a lherzolitic composition (96FK14) sampled close (<100 m) to the plagioclase-in boundary in the same area (Fig. 2). Sample GOF1 is mildly serpentinized, whereas 96FK14 is almost unserpentinized. GOF1 contains 5 vol. % Ca-rich (An80–88) plagioclase. Clinopyroxene analyses in GOF1 are all from crystals associated with plagioclase clusters. Notably, they have significantly higher Na, Fe and Ti contents than clinopyroxenes in the plagioclase-free peridotite sample (Table 1). Amphiboles associated with plagioclase have pargasitic hornblende compositions. Orthopyroxene porphyroclasts in both samples are zoned, with Al contents decreasing from core to rim. Orthopyroxene cores sometimes contain oriented grains of spinel, suggesting exsolution from an originally Al-rich orthopyroxene. Orthopyroxenes do not show any obvious Ca zoning. However, cores of orthopyroxene crystals often contain exsolution lamellae of Ca-rich pyroxene. Orthopyroxene crystals in the matrix have compositions overlapping with those of the rims of porphyroclasts.
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The observed variations in Na, Ti and Fe content of clinopyroxenes are also shown in the diagrams of Fig. 8a and b, in which analyses of samples from different domains are plotted (see Figs 1 and 2 for sample localities). Clinopyroxenes associated with plagioclase clusters are chemically distinct, with higher Ti, Na and Fe contents compared with clinopyroxenes from plagioclase-free peridotites. The Ti, Na and Fe contents of clinopyroxenes from one plagioclase-free but lherzolitic coarse tectonite from the Katáchloron area (domain I) partly overlap with those of clinopyroxenes associated with plagioclase.
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The cr-numbers [Cr/(Cr + Al)] of spinels from the study area are very variable between samples, ranging from 15 to 65 (Fig. 8d). Spinels from plagioclase-bearing samples are more Fe rich than spinels from plagioclase-free rocks with similar cr-numbers. It should be noted, however, that the two plagioclase-bearing peridotites analysed are also significantly more serpentinized than the plagioclase-free peridotites.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONTHE OTHRIS OPHIOLITEDISCUSSIONCONCLUSIONSREFERENCES |
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Original geometry of the mantle section
The dismembered nature of the ophiolite and the lack of an unambiguous palaeo-crust–mantle boundary, commonly used as a palaeo-horizontal in ophiolites, pose a major problem in the reconstruction of the original geometry. Yet the following evidence suggests that the easternmost peridotites of domain V represent shallow, near-sea-floor peridotites, whereas the westernmost peridotites of domain I represent deeper levels, possibly the base of the mantle section.
- The serpentinization seen in the most north-easterly (plagioclase) peridotites in the Fournos Kaïtsa area (domain V) is pervasive, whereas serpentinization in the other peridotite domains is limited to shear zones and major tectonic contacts. In addition, the plagioclase peridotites of domain V are cross-cut by unaltered (unrodingitized) dolerites. These observations suggest that serpentinization affecting the domain occurred as a result of sea-floor metasomatism, pre-dating the cessation of magmatic activity, rather than of emplacement-related serpentinization (Coulton et al., 1995). Such an imprint of sea-floor hydrothermal processes implies that hydrothermal circulation penetrated into the mantle and strongly suggests that domain V represents the uppermost part of the mantle section exposed in the area. This interpretation is consistent with the tectonic contact of this domain with magmatic crustal rocks.
- The westernmost peridotites of the Katáchloron area (domain I) contain late, post-kinematic tremolite. They are in contact with a melange unit containing tremolite-bearing amphibolites. We conclude therefore that this domain was part of the base of the ophiolite, which was infiltrated with fluids during intra-oceanic emplacement and formation of the metamorphic sole.
It should be noted that in the present-day geometry the peridotites are probably part of a relatively thin thrust sheet that overlies a melange unit (Smith et al., 1975; Ferrière, 1985). Ferrière (1985) argued that the dismemberment of the entire ophiolitic section had already occurred during or even before the obduction of the Othris Ophiolite. Indeed, Rassios & Konstantopoulou (1993) found that the serpentinite faults that transect the Othris mantle section and often separate mantle domains recognized in our study are sometimes lined with amphibolite facies metamorphic rocks. This observation suggests that the dismemberment took place early in the history of the Othris Ophiolite, when the peridotites were still relatively hot (Rassios & Konstantopoulou, 1993). It is therefore possible that peridotites that originated at different levels in the oceanic mantle were already juxtaposed well before final emplacement of the stack of thrust sheets now forming the Othris Mountains.
Plagioclase peridotites: products of melt impregnation
The occurrence of plagioclase in small lenses and dykes, often in association with clinopyroxene, suggests a melt origin (Menzies, 1973; Nicolas, 1986b; Girardeau & Mercier, 1992; Rampone et al., 1997). Menzies (1973) interpreted these melt features as melt segregations, i.e. the products of in situ partial melting of the host rock and incomplete extraction. Plagioclase lenses in the Trinity Ophiolite, which are very similar to those in Othris, were also interpreted as melt segregations by Quick (1981a). The following considerations lead us to favour an alternative interpretation in which plagioclase and clinopyroxene represent the cumulate phases of a basaltic melt that impregnated harzburgites, locally turning them into lherzolites:
- in the plagioclase peridotites the compositional layering and lineation, defined by elongate plagioclase-bearing lenses, is always parallel to the olivine and orthopyroxene foliation and spinel lineation. This is confirmed by the measurements of crystallographic fabrics, which show that olivine slip-planes and slip-directions are subparallel to the plagioclase layering and lineation (Fig. 6). However, plagioclase crystals are generally much less deformed than the other minerals in the host peridotite. Moreover, gabbro dykes that are contiguous with plagioclase lenses cross-cut foliations. We conclude therefore that plagioclase crystallized late in the deformation history recorded in the plagioclase peridotites. The following microstructural features indicate that this deformation, and thus plagioclase crystallization, occurred at relatively cold (subsolidus) conditions. Most of the plagioclase peridotites are relatively fine-grained tectonites with typical ‘lithospheric’ microstructures (Ceuleneer et al., 1988; Nicolas, 1989; Ildefonse et al., 1998). The fine grain size of the tectonites and the substructure of olivine grains indicate deformation conditions at relatively high stresses and at temperatures at which recovery is not very efficient. The dynamically recrystallized olivine grain size in the plagioclase-bearing tectonites in Othris is estimated to be <0·5 mm, probably as small as 0·2 mm. According to the olivine piezometer of Van der Wal et al. (1993), this recrystallized grain size corresponds to deviatoric stresses of 13–26 MPa. These stresses give minimum deformation temperatures of 970–1030°C to maintain geological strain rates >10-13 s-1, using the dry peridotite dislocation creep flow law of Chopra & Patterson (1984). It should be noted that this flow law determined by Chopra & Patterson corresponds to that of relatively strong peridotites; the temperatures of deformation may be lower if wet or melt-weakened rheologies apply. Estimates for deformation temperatures for similar ‘lithospheric’ tectonites range typically from 1200 to 800°C (e.g. Ceuleneer et al., 1988; Suhr, 1993; Ildefonse et al., 1998). We conclude therefore that during the last stages of deformation the ambient temperature of the host mantle rocks was below 1200°C—possibly as low as 1000°C—i.e. well below the dry peridotite solidus, when plagioclase and clinopyroxene crystallized. If the melt was produced in situ and was not extracted from the host peridotite then the melt should have become saturated in plagioclase when the temperature of the host peridotite fell below 1200–1250°C (Bender et al., 1978; Elthon & Scarfe, 1984; Kelemen & Aharonov, 1998). If this had been the case then plagioclase should have been deformed during subsequent subsolidus deformation. The microstructural observation that plagioclase is generally undeformed shows that plagioclase simply could not have been present during most of the lithospheric deformation recorded in the Othris peridotites. It is therefore unlikely that the plagioclase clusters represent the products of in situ generated melts.
- Magmatic segregations formed by in situ partial melting often show depletion haloes, i.e. zones depleted in pyroxenes surrounding plagioclase clusters (Nicolas, 1986b). Nicolas (1986b, 1989) cited the presence of depletion haloes as one of the key criteria to distinguish in situ partial melting. In the Othris peridotites depletion haloes are extremely rare. In fact, we often observe the opposite, i.e. plagioclase clusters surrounded by ortho- and clinopyroxenes.
- Plagioclase peridotites contain up to 10 vol. % plagioclase. Estimates for the amount of melt that may be stable in partially molten peridotites before permeability is attained range from <1% (e.g. Von Bargen & Waff, 1986; McKenzie, 1989) to 3% at most (Faul, 1997). It is therefore unlikely that the peridotites could have partially melted to produce melt volumes of 10% or more without the melt being extracted. It seems more plausible that plagioclase represents a cumulate phase of a fractionating melt impregnating peridotites.
Melt composition
On the basis of the above we interpret the plagioclase in the plagioclase peridotites as the product of low-pressure crystallization from a melt that impregnated peridotites of the Othris mantle section. Clinopyroxenes associated with the plagioclase clusters are chemically and texturally distinct and were probably derived from the same melt. The high mg-numbers, i.e. Mg/(Mg + Fe), of these clinopyroxenes show that the aggregates of melt-derived minerals do not represent frozen-in melt fractions, but rather cumulates deposited during fractional crystallization. The high anorthite contents of the plagioclase suggest that the melt had a high CaO/Na2O ratio. The presence of orthopyroxene rims at the contact between olivine and melt-derived plagioclase clusters and the frequent occurrence of interstitial orthopyroxene in the Othris peridotites suggests that the melt may also have been relatively silica rich. Compositions of cumulate plagioclase and clinopyroxene plot at the most depleted end of the field of mid-ocean ridge cumulate minerals (Fig. 9). They partly overlap with compositions of cumulate minerals from Ocean Drilling Program (ODP) site 334 at the Mid-Atlantic Ridge, albeit at higher clinopyroxene mg-numbers (Ross & Elthon, 1993). The high mg-number of the clinopyroxenes may indicate that the melt from which they crystallized had reacted with peridotites on its way up.
Based on the compositions of the cumulate minerals we conclude that the impregnating melt probably had a depleted to ultra-depleted melt composition, i.e. the composition of a melt which is produced by low-pressure partial melting of a refractory source peridotite (Bloomer et al., 1989; Natland, 1989; Ross & Elthon, 1993; Sobolev & Shimizu, 1993). Melting of a refractory source requires a high melting temperature or the presence of water in the source. Alternatively, a depleted melt composition may also be produced by reaction of small melt fractions with a refractory peridotite. It should be noted that we cannot rule out that the melt composition was that of a magnesian andesite (boninite), leaving open the possibility that the mantle section of the Othris Ophiolite has the imprint of supra-subduction zone processes.
Plagioclase-out or plagioclase-in?
The boundary between plagioclase-free and plagioclase-bearing peridotites is everywhere oriented at a high angle to the foliation in the tectonites (Figs 1 and 2). Quick (1981b) argued that the boundary between plagioclase-free and plagioclase-bearing peridotites in the Trinity Ophiolite is a plagioclase-out boundary. The Trinity peridotites contain large dunite bodies, which were probably melt conduits. Plagioclase was removed from the peridotite margins of these dunite by a reaction with transient melts (Quick, 1981b). However, in Trinity the transition from dunite, via harzburgite, to plagioclase lherzolite occurs over a distance ranging from 15 cm to a few metres at most (Quick, 1981b) and is therefore of a much smaller scale than the transition in Othris. In Othris, the transition from harzburgites to plagioclase lherzolites occurs over a distance of a few hundred metres at least. It seems unlikely that the harzburgite domain could represent a melt conduit, which must have been colossal in size (kilometre scale), and that plagioclase was removed from the conduit margins by melt–wall-rock reaction over a distance of hundreds of metres. The plagioclase-free harzburgites adjacent to the plagioclase peridotites contain some small dunite bands, but these are generally deformed and transposed into the foliation. Therefore, these dunites pre-date or are coeval with the deformation recorded in the harzburgites and cannot have been associated with a melt flow responsible for plagioclase removal taking place after deformation. Large cross-cutting dunite bodies are found only in domain I, separated from the rest of the Othris peridotites by the large late-stage mylonitic shear zone of domain II.
Instead, we suggest that that the plagioclase peridotites represent a zone of melt accumulation and impregnation and that the boundary between plagioclase-free and plagioclase-bearing peridotites is in fact a plagioclase-in boundary. The zone of melt impregnation was probably not restricted to the plagioclase peridotites alone. Plagioclase-free peridotites adjacent to the plagioclase peridotites are enriched in clinopyroxene, which sometimes occurs in veins. Furthermore, most harzburgites contain interstitial orthopyroxene, which may also have been derived from an impregnating melt by fractional crystallization or by melt–rock reaction. From Fig. 2 it seems that there may even be a layered structure with an upward transition from peridotites with interstitial orthopyroxene (harzburgites of domain III) to peridotites with interstitial clinopyroxene ± orthopyroxene to peridotites with interstitial plagioclase ± clinopyroxene ± orthopyroxene (plagioclase peridotites of domain IV and V). This implies that the boundaries have some petrological significance, i.e. that they represent the boundaries in P–T space at which the impregnating melt became saturated in orthopyroxene, clinopyroxene and plagioclase.
Most interstitial orthopyroxene and clinopyroxene in websterite and clinopyroxenite veins and lenses generally shows evidence for plastic deformation whereas aggregates of plagioclase ± clinopyroxene ± orthopyroxene are generally undeformed. This implies that plagioclase occurred late at the liquidus of the impregnating melt, after orthopyroxene and clinopyroxene. This is not in agreement with crystallization experiments on basaltic melts, which show that plagioclase crystallizes before pyroxene (Bender et al., 1978; Elthon & Scarfe, 1984; Grove et al., 1992; Kinzler & Grove, 1992). This ‘delay’ of the crystallization of plagioclase might be explained by the fact that the Othris peridotites had refractory compositions before impregnation and were therefore undersaturated with respect to plagioclase. Harzburgites can potentially ‘resorb’ some plagioclase components precipitating early from the impregnating melt in the available mineral structures, mainly spinel and clinopyroxene (Suhr & Robinson, 1994). Plagioclase then only starts to crystallize if the host peridotite is sufficiently refertilized. Furthermore, melt percolating through a harzburgite by porous flow can re-equilibrate with the host rock. As a result of the reaction of the melt with the host harzburgite plagioclase may disappear from the liquidus (Kelemen, 1990; Suhr & Robinson, 1994). In such a scenario plagioclase crystallizes as soon the melt ceases to equilibrate with the host peridotite, for instance if the melt fraction becomes very large or if the melt percolation is arrested. This means that plagioclase crystallizes from a melt only in a zone of melt accumulation.
It should be emphasized that we cannot determine the vertical thickness of the zone of plagioclase peridotites in Othris. Our mapping shows that the plagioclase peridotites form the highest levels of the Othris peridotite massif and that the zone of plagioclase peridotites may be only a few hundred metres in thickness. We therefore do not need to invoke that the impregnating melts travelled over large distances in relatively cold (<1200°C) rocks. The Othris plagioclase peridotites probably represent the level in the mantle where the host peridotites became too cold for further porous flow and where the melts started to ‘freeze-out’, crystallizing orthopyroxene, clinopyroxene and finally plagioclase. From the mineral analyses it is clear that these minerals represent cumulate phases, implying that the remaining melt must have escaped. It is not unlikely that the remaining melt escaped by hydro-fracturing through dykes, for which the discordant gabbro–norite dykes are possible candidates.
Depth at which impregnation occurred
We attempted to constrain the ambient pressure and temperature conditions during melt impregnation by applying geothermobarometry. Unfortunately, we found that all minerals have (partially) re-equilibrated during cooling. Model temperatures obtained on the rims of mineral grains all lie in the range 700–900°C. Some cores of orthopyroxene porphyroclasts give model temperature plateaux of 1000–1100°C. Lack of equilibrium between pyroxenes precludes the use of geobarometers. However, the complete absence of metamorphic plagioclase, even in very fine-grained domains and small mylonite zones in clinopyroxene-rich peridotites adjacent to plagioclase peridotites in the Fournos Kaïtsa area, suggests that the plagioclase-free peridotites equilibrated in the metamorphic spinel peridotite facies. The transition of the spinel peridotite to plagioclase peridotite facies, the univariant equilibrium 2Forsterite + Anorthite = Spinel + Diopside + Enstatite, is dependent not only on pressure and temperature, but also on the composition of the host peridotite. We have taken the most ‘depleted’ end-member activities for olivine (aOlFo = 0·81), orthopyroxene (aOpxEn = 0·75), clinopyroxene (aCpxDi = 0·79) and spinel (aSpSp = 0·31) in the plagioclase-free sample 96FK14 (Table 1) very close to the plagioclase-in boundary mapped in the field (Fig. 2), to determine the univariant equilibrium using the Thermocalc v.2.4 program (Powell & Holland, 1988; Holland & Powell, 1990). This yields a pressure of 3 kbar at 1000°C to 4 kbar at 1200°C for the metamorphic spinel to plagioclase boundary. It follows that plagioclase-free peridotites in the Fournos Kaïtsa area equilibrated at pressures exceeding these values. As these peridotites are in structural continuity with plagioclase peridotites, the plagioclase peridotites cannot have been equilibrated at (much) lower pressures. We thus infer a minimum pressure for the magmatic plagioclase-in boundary in the Fournos Kaïtsa area of 3–4 kbar. It should be noted that more ‘fertile’ end-member activities move the phase equilibrium to higher pressures.
Rift, slow- or fast-spreading ridge?
As plagioclase and at least some clinopyroxene in the plagioclase peridotites have crystallized from an impregnating melt, the original host rocks were probably much more depleted in composition. We estimate that before melt impregnation the host rocks must have had harzburgitic, yet clinopyroxene-bearing, modal compositions, comparable with the compositions of adjacent plagioclase-free harzburgites. This indicates that the plagioclase peridotites had been subject to significant melting and melt extraction before melt impregnation. The overall harzburgitic composition of the mantle section, as well as the presence in the Othris Mountains of all the lithological elements of a true ophiolite sequence (Menzies, 1973; Hynes, 1974b), i.e. oceanic sediments, pillow lavas, dolerites, gabbros, mantle rocks and fragments of a metamorphic sole thrust, suggests that the Othris Ophiolite formed at an ocean ridge, rather than in a rift setting.
Most of the studied harzburgites contain clinopyroxene porphyroclasts, whose formation clearly pre-dates deformation and melt impregnation. Moreover, peridotites occur that have more lherzolitic compositions, containing >5 vol. % clinopyroxene porphyroclasts. Their preservation indicates that early melt extraction in the Othris peridotites was limited and locally left >5% clinopyroxene in the residue. Menzies (1975) reported lherzolites from Othris with near-chondritic rare earth element abundances, also compatible with low degrees of melt extraction. We conclude that the bulk compositions of the peridotites are similar to those of peridotites from the slow-spreading Mid-Atlantic Ridge (MAR). MAR peridotites are generally clinopyroxene-rich harzburgites or lherzolites, in contrast to peridotites from the fast-spreading East Pacific Rise (EPR), which have only low modal clinopyroxene contents (e.g. Michael & Bonatti, 1985; Constantin et al., 1995).
On the basis of our study of the plagioclase peridotites in Othris, we infer a three-stage scenario for their formation: (1) partial melting and melt extraction leaving a predominantly cpx harzburgite residue; (2) deformation at ‘lithospheric’ mantle temperatures (1000–1200°C) and stresses of 13–26 MPa; (3) impregnation with magmas with a depleted composition during and after the last stages of deformation. Such a scenario is in good agreement with the episodic nature of spreading and magmatism postulated for slow-spreading ridges (Karson et al., 1987; Cannat, 1993; Cannat & Casey, 1995; Tartarotti et al., 1995). Stage (1) probably represents a (waning) magmatic stage, followed by an amagmatic stage (stage 2) during which extension (spreading) is accommodated by deformation of the lithosphere at the ridge axis, followed by the start of a new stage of magmatism (stage 3).
The extensive hydrothermal alteration and the presence of dolerite dykes in the uppermost plagioclase peridotites, as well as the absence of large volumes of oceanic crustal rocks in the area (Menzies & Allen, 1974) point to a thin magmatic crust. This is also in accordance with limited magmatism and possibly tectonic denudation of mantle rocks during amagmatic spreading at slow-spreading ridges (Karson et al., 1987; Cannat, 1993, 1996; Cannat & Casey, 1995).
Segment-centre or transform environment?
We have inferred that melt impregnation of the Othris peridotites occurred at temperatures below 1200°C—possibly as low as 1000°C. These conditions correspond to the base of the thermal lithosphere. In our definition the lower boundary of the thermal lithosphere corresponds to the transition between the adiabatic interior and the conductively cooled thermal boundary layer of the Earth. In this definition, the base of the thermal lithosphere has a temperature of 1250–1300°C. Our observations suggest that the thermal lithosphere may have been relatively thick during magmatic activity at the Othris ridge, reaching well into the mantle. The lower boundary of the impregnated horizon was probably located at a pressure of at least 3–4 kbar, implying that the thermal lithosphere was at least 9–12 km thick at or close to the ridge axis.
A relatively cold mantle structure with a thick thermal lithosphere is expected at slow-spreading ridges, where cooling by conductive heat loss to the surface plays an important role (Cannat, 1993, 1996). Transform fault environments may have a particularly thick thermal lithosphere as a result of the ‘cold-wall effect’ (Nicolas & Dupuy, 1984; Nicolas, 1986b, 1989; Cannat, 1996). Magmatic impregnation is not uncommon in peridotites from transform regions of modern slow-spreading oceans such as the Mid-Atlantic (e.g. Dick, 1989; Bonatti et al., 1992; Cannat et al., 1992; Cannat & Casey, 1995; Tartarotti et al., 1995). However, melt-impregnated peridotites have also been recovered from transform regions of fast-spreading ridges such as the EPR (e.g. Cannat et al., 1990; Constantin et al., 1995; Constantin, 1999). Ghose et al. (1996) argued that trapping of magma in a cold lithospheric mantle root may be required to fully explain variations in crustal thickness in the Kane transform area (MAR). Melt-impregnated, plagioclase-bearing peridotites have also been found in an inferred ophiolitic transform fault in New Caledonia (Prinzhofer & Nicolas, 1980; Nicolas & Dupuy, 1984). We thus conclude that it is most likely that the Othris Ophiolite originated in a (near-)transform environment of a slow-spreading ridge (Fig. 10).
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Compositional variability among Hellenic–Dinaric ophiolites
Several workers have emphasized the compositional variation amongst mantle sections of ophiolites of the Hellenic–Dinaric chain, the ophiolite belt running from central Greece to northern Bosnia (e.g. Nicolas & Jackson, 1972; Pamic’, 1983; Smith & Spray, 1984; Smith, 1993). For instance, the Vourinos Ophiolite in Greece (Jackson et al., 1975; Ross et al., 1980; Smith, 1993) and the ophiolites of the Inner Dinaric belt such as the Kukës Ophiolite in Albania (Hoxha & Boullier, 1995) have strongly depleted mantle sections and well-developed lower-crustal sequences. In contrast, ophiolites of the Central Dinaric ophiolite bear resemblance to the Othris Ophiolite, comprising mixed mantle sections in which harzburgites, lherzolites, and plagioclase-bearing peridotites are exposed (Pamic’, 1983). Notably, Boudier, Nicolas and coworkers (Boudier et al., 1999; Nicolas et al., 1999) also concluded that the heterogeneous character of the Dinaric Mirdita Ophiolite in Albania results from melt impregnation of the uppermost part of the mantle section.
We suggest that the compositional variability among the Hellenic–Dinaric ophiolites does not require that the pertinent realm of the eastern Tethys ocean consisted of numerous small oceanic basins (Nicolas & Jackson, 1972), nor does it require a large Jurassic left-lateral strike-slip fault juxtaposing ophiolites from different oceanic domains (Smith & Spray, 1984). The episodic nature of magmatism associated with slow sea-floor spreading inferred in this study renders an alternative explanation. We hypothesize that Vourinos and Kukës were formed during magmatic stages of slow sea-floor spreading, during which the thickness of the thermal lithosphere at the ridge axis did not exceed the thickness of the magmatic crust. Consequently, the mantle sections of these ophiolites principally record melt extraction and near- to hypersolidus (‘asthenospheric’) deformation conditions. In contrast, the Othris mantle section has recorded a history of episodic magmatism and lithospheric deformation in a near-transform environment of a slow-spreading ridge.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONTHE OTHRIS OPHIOLITEDISCUSSIONCONCLUSIONSREFERENCES |
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- Plagioclase peridotites in Othris are the product of impregnation of harzburgites with a fractionating melt, which crystallized plagioclase and clinopyroxene and possibly orthopyroxene. The melt probably had a depleted composition, i.e. it was formed by low-pressure partial melting of a refractory source peridotite, or alternatively by a reaction of a melt with such a refractory peridotite. The first-order compositional variability of the peridotites of the Othris mantle section is thus the result of melt impregnation rather than variable degrees of melt depletion.
- The dominant rock type in the Othris mantle section is clinopyroxene-bearing harzburgite. We estimate that the plagioclase peridotites also had clinopyroxene-bearing harzburgitic modal compositions before melt impregnation. Relatively clinopyroxene-rich harzburgite is the dominant peridotite type recovered from the slow-spreading MAR, whereas clinopyroxene-poor harzburgites are typical for the fast-spreading EPR. We conclude therefore that it is most likely that the Othris Ophiolite formed at a slow-spreading ocean ridge.
- Plagioclase peridotites have recorded a multi-stage history of melt depletion, deformation and melt impregnation. This scenario is in good agreement with the episodic nature of magmatism and spreading at slow-spreading ridges.
- The melt has impregnated mantle rocks deforming at temperatures of 1000–1200°C and stresses of 13–26 MPa. These conditions correspond to those at the base of the conductively cooled boundary layer (the thermal lithosphere). We conclude therefore that the thermal lithosphere reached into the mantle during magmatic activity. Such a cold mantle structure points to a segment-end, near-transform environment of an ocean ridge.
- The compositional variability amongst the mantle sections of the Hellenic–Dinaric ophiolites may be a reflection of the episodic nature of magmatism and deformation at slow-spreading ridges. Ophiolites with thick crustal sections and strongly depleted mantle sections recording ‘asthenospheric’ deformation are probably formed during magmatic stages of sea-floor spreading. In contrast, Othris-like ophiolites with thin crustal sections and melt-impregnated mantle sections recording ‘lithospheric’ deformation probably principally reflect amagmatic spreading during which the thermal lithosphere reaches well into the mantle.
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ACKNOWLEDGEMENTS |
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We gratefully acknowledge support in the field and access to unpublished data from Anna Rassios. Allan G. Smith is thanked for giving directions to the outcrop of amphibolites of the Othris metamorphic sole near Lamia. We thank Ray Guillemette for his help with the microprobe analyses at Texas A&M University. We also wish to thank Bregje Hulscher for a thorough review of the draft version of the manuscript, and Martin Menzies, Françoise Boudier and an anonymous reviewer for a thorough and critical review of the submitted manuscript. This work is supported by NWO-PIONIER subsidy No. 030-75-346.
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FOOTNOTES |
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*Corresponding author. Present address: Department of Geology, University of Leicester, Leicester LE1 7RH, UK. Telephone: +44-116-252-5355. Fax: +44-116-252-3918. E-mail: ahd3{at}le.ac.uk
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