Lower Pillow Lavas Research Articles

High-Ca boninitic melt inclusions in lavas of the Troodos ophiolite and a reappraisal of genetic relationships between different lava types

Abstract Boninites in many supra-subduction zone ophiolites are thought to mark the early stage of subduction. The Troodos ophiolite in Cyprus contains a variety of volcanic rocks including the lower pillow lavas and the boninite-bearing upper pillow lavas (UPL). Here we present major, trace element and Pb isotope data for melt inclusions in olivine from the UPL on the northern flank of the Troodos ophiolite. Melt inclusions hosted by high-Mg (Fo = 89.4–93.5) olivine grains have typical high-Ca boninitic compositions with high MgO (11.2–18.4 wt%) and SiO2 (51.2–55.0 wt%) and low TiO2 (0.2–0.6 wt%) contents, in line with the existence of boninites in the northern part of the ophiolite. The melt inclusions have trace element compositions similar to the Troodos boninitic lavas and can be regarded as representing the parental magma of the boninites. The boninitic magmas were derived from high-degrees of partial melting of a refractory source at 1400 °C and 1.5 GPa based on thermobarometers and phase equilibria. The melt inclusions have significantly variable Pb isotopic compositions (208Pb/206Pb = 2.059–2.122; 207Pb/206Pb = 0.828–0.862) and are enriched in fluid-mobile trace elements (e.g., U, Sr, and Pb), indicating that the refractory source was metasomatized by slab fluids. In comparison, recalculation of literature data suggests that the lower pillow lavas were derived from a less refractory and shallower source. We propose a subduction initiation model in which melting of two unique sources was needed to form the chemostratigraphy of lavas in the Troodos ophiolite.

Effects of magmatic volatile influx in mafic VMS hydrothermal systems: Evidence from the Troodos ophiolite, Cyprus

The Troodos ophiolite, Cyprus is the principal on- land analogue for mafic-hosted volcanogenic massive sulfide (VMS) deposits. This study, for the first time, presents sulfur isotope (δ34S) data on a regional scale from VMS deposits and other mineralised zones across the Troodos ophiolite. In combination δ34S, Se/S ratios and trace element chemistry (e.g., Se, Cu and Au) of different hydrothermal sulfides are used to assess variations in magmatic volatile influx and the source of metals and sulfur in ancient hydrothermal systems.Sulfur isotope analyses (n=180) across 19 mineral deposits indicate a variable source of sulfur in the Troodos VMS hydrothermal system, this in turn allows a variable source of metals to be inferred. Pyrite δ34S range from −5.5‰ to +13.2‰ with an average of +4.6‰ (n=160) for all deposits investigated. These δ34S variations cannot only be explained by variable proportions of thermochemical seawater sulfate reduction (δ34S +18 to +19‰) and leaching of primary magmatic sulfur from igneous rocks (δ34S 0-1‰). Consequently, two processes are proposed, explaining the trace metal and δ34S variation across the Troodos ore-forming systems including, i) a variable source of metals in the sheeted dyke complex and ii) the addition of a magmatic volatile-rich phase to the VMS hydrothermal systems.Two distinct lava units exist in the Troodos stratigraphy, namely the upper and lower pillow lavas (UPL and LPL). The more primitive UPL are enriched in Au, Se and Cu relative to As, Sb and Zn that are concentrated in the LPL. Some VMS deposits pre-date the formation of the UPL (e.g., Agrokipia A) suggesting Se, Cu and Au depleted source rocks. Hence, the stratigraphic position of VMS deposits and the ratio of UPL:LPL affinity elements (e.g., As +Zn+Sb vs. Cu+Se+Au) imply a systematic relationship between trace element distribution and stratigraphic depth; this relates to the relative proportion of UPL and LPL affinity lavas within the metal source region.δ34S values <0‰ recorded in some VMS deposits that are less than the Troodos magmatic mean of 0- 1‰ may be related to anhydrite buffering during fluid ascent, microbial sulfate reduction or the direct contribution of magmatically derived sulfur, to the hydrothermal system from an underlying magma chamber via volatile exsolution. We propose that negative δ34S values combined with Se/S × 106 ratios >500 in pyrite suggest the contribution of a magmatic volatile component (e.g., Apliki and Skouriotissa). We demonstrate that the source of metals and sulfur in the Troodos VMS hydrothermal system is affected by regional scale processes related to i) variable source lithologies and, ii) the contribution of a magmatic volatile phase to some Troodos VMS hydrothermal systems.

Subduction-zone contributions to axial volcanism in the Oman–U.A.E. ophiolite

Over four decades of research on the Semail ophiolite (Oman–U.A.E.) has greatly in uenced our understanding of processes occurring at fast-spreading ocean ridges. While the well-developed sheeted dike complex and comagmatic lower pillow lavas indicate that the early Semail crust formed at a spreading axis, the precise tectonic setting of this axis—whether true mid-ocean ridge, back-arc or “proto”-arc— is contentious. This is largely because the tectonic implications of the geochemistry of the main axial volcanic unit (Geotimes/V1) are disputed. We bypass this hurdle by focusing on intercalations of primitive lavas that are depleted relative to mid-ocean-ridge basalt and that are deeply intercalated within the early Geotimes axial volcanostratigraphy throughout the northern ophiolite. Our analyses of these intercalations show a clear trace-element in uence from a subducting slab. We interpret the depleted axial melts to have formed by localized, high-degree partial melting assisted by a high-Th/Nb slab uid. These results con rm a deep subduction in uence on the entire axial spreading phase of the world’s largest ophiolite. Considered in the context of later hydrous and boninitic Alley volcanism and of insight from modern tectonic environments, our observations support a proto-arc, subduction-initiation setting for the origin of the Semail ophiolite.

The geology of the northern part of the apliki Cyprus-type ore deposit

The Apliki copper deposit, in the Troodos ophiolite complex of Cyprus, is part of the Skouriotissa mining district. Mining activity at Apliki has resulted in the production of 1,650,000 tons of copper ore. The mineralization is hosted within the sequence of Lower Pillow Lavas of the Troodos ophiolite complex in the western part of the Solea graben. Plagioclase, pyroxenes (augite), magnetite and ilmenite, and rarely olivine are the main magmatic mineral components. The prevailing secondary minerals are celadonite, calcite, analcime and quartz which occur within amygdules or are dispersed within the rock matrix. The mineralized zone is bounded by two N–S striking sub–parallel faults and is, therefore, controlled by local structures which allowed access to the hydrothermal fluids. The basaltic Pillow Lavas at Apliki possess the same low-temperature regional scale alteration as the sequence of Lower Pillow Lavas of the Troodos ophiolite. In the mineralized zone the following facies have been identified: (a) A stockwork zone (b) Veins of amorphous silica (c) Veins of gypsum (d) Oxidized vein of massive mineralization (“red zone”) (e) Oxidation zone The Apliki deposit is associated with cupriferous massive sulphide which has been mined out. The Apliki mineralization, presently exposed in the examined area, is a typical example of stockwork type sulphide mineralization, with more intense presence in the lower levels of the northern wall, where the semi–massive and massive ore are preserved. The stockwork ore is hosted within brecciated chloritized lavas. Weak silicification is observed in the upper parts of the brecciated lavas. Red jasper is spatially associated with the mineralized lavas and occurs as veins or open– space fillings within the mineralized zone. Pyrite, marcasite and chalcopyrite are the predominant ore minerals, whereas bornite, sphalerite, galena, and barite are accessories with quartz as the main gangue. Goethite, hematite, chalcocite, covellite, and Fe–, Cu–, Pb–, Al–, and Ca–sulphates were formed in the supergene environment. The sampling on the northern and southern wall, of the existing opencut, took place along a series of traverses based on a combined trench and rock-chip sampling method so to achieve the best representation of the samples taken. The spatial distribution of the major elements of the ore (Ag, As, Au, Cd, Co, Cu, Fe, Ni, Pb, S, Se, Sb and Zn) in the northern part of the opencut demonstrates an enrichment next to the western fault and at areas where the semi–massive ore is present. Similar work was carried out on the southern part with much lower concentrations, a fact that reflects the limited occurrence of disseminated ore in that region. The spatial distribution of the major elements throughout the Apliki deposit has demonstrated that the western fault may be the one which is the controlling structure, while the eastern one is the boundary for a gradational passage into the Lower Pillow Lavas. Gold grade does not exceed 0.1 g/t, while copper grades ranges between 0.01 to 3.5 wt. % and sulfur between 0.1 to 16 wt. %. Veins of gypsum, with a direction almost parallel to the western fault, and vein–like amorphous silica bodies both occur close to the western border within the mineralized zone. The “red zone”, with an N–S orientation, constitutes probably an oxidized vein of pyrite–chalcopyrite. The mineralogical assemblage of Apliki oxidation zone (iron oxides and hydroxides, cuprite) reflects almost neutral pH conditions, a fact which is the result of insufficient amount of pyrite which produces low pH supergene solutions. The spatial distribution of copper and its content, which is almost the same with the respective one of primary sulphide mineralization, demonstrate the limited mobility of copper that instead of leaching and moving downwards is concentrated to the oxidation zone.

South Mathiatis: An unusual volcanogenic sulphide deposit in the Troodos ophiolite of Cyprus

The South Mathiatis deposit in the Troodos ophiolite of Cyprus is located within Lower Pillow Lavas in the central part of the Mitsero (Ayios Epiphanios) graben, and is characterised by elevated Zn contents, and a hydrothermal mineral assemblage in which galena is locally an important component and barite is a common gangue mineral, features which place it apart from other Cyprus-type deposits. The mineralisation is hosted within a gently dipping sheet flow-dominated sequence of lavas with common autobrecciation textures and associated hyaloclastites, and is stratigraphically controlled between lava flows, locally floored by unmineralised hyaloclastites. The environment of extrusion is interpreted as a ponded sequence, similar to the adjacent Agrokipia deposit. The Zn-rich mineralisation appears to be controlled by local structures which allowed access to the hydrothermal fluids, which then migrated laterally along lithologically favourable concordant zones. The sulphide assemblage is dominated by sphalerite together with ubiquitous pyrite; however, both chalcopyrite and galena are locally important; quartz and barite comprise the gangue-mineral assemblage. Red jasper is spatially associated with the sulphide mineralisation and displays abundant filamentous structures similar to forms identified in other VMS environments. Wall rock alteration associated with the mineralisation is weak, and this is reflected in the absence of a magnetic signature, in contrast to typical Cyprus deposits. The Zn mineralisation is also transparent to electrical methods suggesting that other similar deposits may be present in the ophiolite but remain as yet undetected.

Tectonic implications of boninite, arc tholeiite, and MORB magma types in the Josephine Ophiolite, California-Oregon

Abstract The Josephine Ophiolite is a large complete ophiolite flanked by arc complexes, including rifted arc facies, and overlain by volcanopelagic and volcaniclastic sedimentary rocks. The extrusive sequence and sheeted dyke complex record a wide range in magma types and degree of fractionation. The upper part of the extrusive sequence, as well as late dykes in the ophiolite, have mid-ocean ridge basalt (MORB) affinities and include unusual highly fractionated Fe-Ti basalts. The sheeted dyke complex and lower pillow lavas are dominated by transitional island-arc tholeiite (IAT) to MORB, but about 10% consist of low-Ti, high-Mg basalts and andesites. Whole-rock chemistry and Cr-spinel compositions indicate that the low-Ti rocks range from boninite (BON) to primitive arc basalt. The low-Ti samples have trace element characteristics indicating a greater subduction component than the IAT-MORB or MORB samples, as well as derivation from a wide range of sources ranging from depleted to enriched relative to an average N-MORB mantle source. Mixing of low-Ti and MORB magmas may have produced the IAT-MORB magma type that is most characteristic of the ophiolite. Podiform chromites and late magmatic features in the mantle peridotite, described by previous workers, appear to have been formed from the low-Ti magmas. Regional geological relationships and the presence of boninitic magmas suggest that arc rifting and initial sea-floor spreading to form the Josephine Ophiolite occurred in the forearc of a west-facing arc built on edge of the North American plate. Arc magmatism appears to have jumped westward, at which time the Josephine basin became situated in a back-arc setting, analogous to the inferred evolution of the modern Lau back-arc basin. Alternatively, the Josephine Ophiolite may have formed in a setting analogous to the north end of the Tonga Trench or the south end of the North Fiji basin, both sites of modern boninites, where a back-arc spreading centre has propagated across an arc into the forearc. Rift propagation during formation of the Josephine Ophiolite is consistent with the presence of highly fractionated Fe-Ti basalts.

Coexistence of two distinct mantle sources during formation of ophiolites: a case study of primitive pillow-lavas from the lowest part of the volcanic section of the Troodos Ophiolite, Cyprus

We present a detailed mineralogical, petrological and melt inclusion study of unusually fresh, primitive olivine + clinopyroxene phyric Lower Pillow Lavas (LPL) found near Analiondas village in the northeastern part of the Troodos ophiolite (Cyprus). Olivine phenocrysts in these primitive LPL show a wide compositional range (Fo(82-92)) and have higher CaO contents than those from the Upper Pillow Lavas (UPL). Cr-spinel inclusions in olivine are significantly less Cr-rich (Cr/Cr + Al = 28-67 mol%) compared to those from the UPL (Cr# = 70-80). These features reflect differences in melt compositions between primitive LPL and the UPL, namely higher CaO and Al2O3 and lower FeO* compared to the UPL at a given MgO. LPL parental melts (in equilibrium with Fo(92)) had similar to 10.5 wt% MgO and crystallization temperatures similar to 1210 degrees C, which are significantly lower than those previously published for the UPL (14-15 wt% MgO and similar to 1300 degrees C for Fo(92)). The fractionation path of LPL parental melts is also different from that of the UPL. It is characterized initially by olivine + clinopyroxene cotectic crystallization joined by plagioclase at similar to 9 wt% MgO, whereas UPL parental melts experienced a substantial interval of olivine-only crystallization. Primitive LPL melts were formed from a mantle source which was more fertile than that of tholeiites from well-developed intra-oceanicarcs, but broadly similar in its fertility to that of Mid-Ocean Ridge Basalt (MORB) and Back Are Basin Basalts (BABE). The higher degrees of melting during formation of the LPL primary melts compared to average MORE were caused by the presence of subduction-related components (H2O). Our new data on the LPL coupled with existing data for the UPL support the existing idea that the LPL and UPL primary melts originated from distinct mantle sources, which cannot be related by progressive source depletion. Temperature differences between these sources (similar to 150 degrees C), their position in the mantle (similar to 10 kbar for the colder LPL source vs 15-18 kbar for the UPL source), and temporal succession of Troodos volcanism, all cannot be reconciled in the framework of existing models of mantle wedge processes, thermal structure and evolution, if a single mantle source is invoked. Possible tectonic settings for the origin of the Troodos ophiolite (forearc regions of intra-oceanic island are, propagation of backarc spreading into are lithosphere) are discussed.

A Strontium Isotopic Investigation of Fluid-Solid Exchange During Hydrothermal Circulation in the Troodos Ophiolite

The structure of sea floor hydrothermal systems and the nature of fluid-solid exchange processes are still poorly constrained despite the significance of the hydrothermal activity to the chemical s t ructure of the ocean crust and oceanic chemistry. Here we report preliminary results of an investigation into the structure of fluid flow and the nature of fluid-solid exchange mechanisms in the upper part of the oceanic crustal sequence preserved in the Troodos ophiolite. The lava sequence on Troodos exhibits a wide range of 90 Ma 87Sr/S6Sr interpreted to imply that fluidsolid exchange was kinetically limited in the upper part of the recharge zone of the high temperature hydrothermal systems (Bickle and Teagle, 1992). The extent of fluid channelling, fluid S7sr/g6sr isotopic composition and possible controls on fluid-solid exchange processes have been investigated by analysing altered basalt, zeolite minerals crystal l ised f rom hydro the rmal fluids and mapping strontium isotopic gradients on centimetre to metre scales. Results from two localities are reported. At the first locality pillows and a single crosscutting dyke from within the lower pillow lavas have been profiled in detail (Figs. 1,2). Samples from the dyke locality (Fig. 1) scatter about a 98-1-13 Ma errorchron. No obvious gradients in STSr/S6Sr are resolved (Fig. 3) despite mineralogical differences between dyke margin and pillow

The geochemistry and petrogenesis of volcanics and sheeted dikes from the Hatay (Kizildag) Ophiolite, southern Turkey: Possible formation with the Troodos Ophiolite, Cyprus, along fore-arc spreading centers

Geochemical and petrological investigations of the Hatay (Kizildag) Ophiolite, southern Turkey, indicate that the volcanics and sheeted dikes compositionally range from island-arc tholeiites and basaltic andesites to transitional boninitictype lavas formed within a supra-subduction zone environment. Geochemical modeling indicates that the compositional trends within both the sheeted dike and pillow basalt sections of Hatay cannot be related through fractional crystallization alone but require multiple parental magmas of differing compositions within each unit. The more refractory liquids (higher MgO, Ni and Cr coupled with lower concentrations of Ti, Zr, Y and REE) formed at lower pressures (shallower depths) through greater degrees of partial melting of a more depleted mantle source relative to less-refractory magmas which formed at higher pressures and possibly lower degrees of melting. The Hatay volcanics and dikes span the compositional range of Lower Pillow Lavas (LPLs) and Upper Pillow Lavas (UPLs) from the Troodos Complex, Cyprus, suggesting, along with other evidence, that the two ophiolites may be petrogenetically and tectonically related. Formation of the Hatay and Troodos ophiolites was possibly associated with extension within a fore-arc environment following compression and detachment along an intra-oceanic ridge system and development of a subduction zone. The range of parental liquid compositions for volcanics and dikes may reflect variable mixing of geochemically diverse melt increments generated within a polybaric melting column in the mantle wedge. The melting column may have additionally received contributions of LREE-enriched melts from deeper, more juvenile (fertile) sources. Formation of parental liquids through variable mixing of melt increments appears to be common to both the Hatay and Troodos ophiolites.

Crustal accretion and tectonic setting of the Troodos ophiolite, Cyprus

The Troodos ophiolite is composed of two distinct magmatic suites. An early, relatively high-TiO 2 basaltic suite constitutes the upper gabbros, the sheeted dike complex, and the andesitic to rhyodacitic lower pillow lavas. A late, low-TiO 2 basaltic andesite suite constitutes the lower ultramafic to gabbroic cumulates and the upper pillow lavas. Parental magmas to the early suite originated as partial melts from hydrous, relatively undepleted, peridotite diapirs rising in a mantle wedge in response to dehydration of a subducting crust. The late basaltic andesite suite is highly depleted in incompatible elements and show similarities with “second-stage” or refractory basalts from oceanic spreading centers. The Troodos basaltic andesites and associated cumulates may thus have formed in response to continued adiabatic upwelling as second-stage melts from the hydrous, water-undersaturated peridotite diapirs from which the earlier spreading related magmas were extracted. The basaltic andesites formed in a relatively slow-spreading or non-spreading tectonic environment and dominantly crystallized in magma chambers underplating the Troodos ophiolite. The extrusion of the basaltic andesites of the upper pillow lavas occurred simultaneously with still active evolving volcanic centers belonging to the earlier magmatic suite. An alternative possibility would be that the basaltic andesites separated from renewed diapirs rising as a consequence of re-initiation of subduction and liberation of fluids from the subducting slab into the overlying depleted peridotite. The existence of LREE and LIL enriched lavas among the upper pillow lavas support this possibility. The apparent conflict between a spreading related geology and the arc petrology and geochemistry of the Troodos ophiolite may be an artifact of lack of detailed studies of the geology and chemistry of oceanic and back-arc rifting environments. However, it is proposed that the available data for the Troodos ophiolite are consistent with its formation in a short-lived spreading basin formed by splitting of an immature, submarine arc.

Episodic magma chambers and amagmatic extension in the Josephine ophiolite

Geochemical and structural aspects of the Josephine ophiolite indicate that it formed by alternating cycles of magmatic and structural (amagmatic) extension. Episodic magma chambers are indicated by the presence of very primitive lavas in the lower pillow lavas which would have mixed into a magma chamber. Fe-Ti basalts that occur in the uppermost extrusive rocks and plagiogranites that occur along the sheeted-dike/gabbro contact represent highly fractionated magmas produced during the last stages of crystallization of a magma chamber. During periods of amagmatic extension, breccias accumulated and the entire crustal sequence was tilted >50°, probably by faulting above an upper-mantle detachment. More than 1000 m of uplift and down-dropping between magmatic cycles is suggested by the presence of out-of-place screens in the sheeted-dike complex.

Uranium-lead isotopic ages of plagiogranites from the Troodos ophiolite, Cyprus, and their tectonic significance

Zircon populations from two plagiogranites in the plutonic section of the Troodos ophiolite yield U-Pb dates between 90.3 ±0.7 and 92.4 ±0.7 Ma. Zircons from one of the two samples yield U-Pb dates of 90.3 ±0.7 and 92.1 ±0.7 Ma. This difference is greater than our analytical error, and the zircons are therefore considered to be slightly discordant. Two fractions from another sample are internally and externally concordant and have an average date of 91.6 ±1.4 Ma. This date is adopted as the age of plagiogranite crystallization. The plutonic section of the Troodos ophiolite is complex in that several generations of intrusion can be identified. The plagiogranites occur at the top of the intrusive section and represent fractionated residua of magma chambers related to the lower pillow lavas. They are possibly slightly older than small ultramafic layered complexes that represent the final intrusive episode within the Troodos ophiolite. There is good agreement between our results and the ages of radiolaria which occur in umbers intercalated with the upper pillow lavas of Troodos (Blome and Irwin, 1985). This shows that the 75 to 85 Ma K-Ar dates on the lavas and sheeted dikes (Desmet et al., 1978; Delaloye and Desmet, 1979; Delaloye et al., 1980; Staudigel et al., 1986) represent age resetting unrelated to ophiolite formation. The Troodos U-Pb zircon ages are slightly lower than U-Pb zircon ages from the Samail ophiolite in Oman, which vary between 93.5 and 97.9 Ma and average 95 Ma (Tilton et al., 1981). The small difference in age between the Troodos and the Samail ophiolites and the geochemical similarities of basalts from both complexes to island-arc volcanic rocks imply that a 3000-km axis of rift-related island arcs formed between 91 and 98 Ma. Emplacement of the ophiolites followed during the early Maastrichtian.

A COMPARISON OF IP TRANSIENT SHAPES OVER DISSEMINATED AND MASSIVE SULFIDE SHEETS IN THE LOWER PILLOW LAVAS OF CYPRUS*

AbstractA number of time‐domain IP traverses were carried out across two parallel mineralized sheets in the Lower Pillow Lavas, near Mitsero, Cyprus with Huntec Mark III equipment using the pole‐dipole array. In one sheet the mineralization was disseminated (2%S), and in the other it was massive (30%S). The transients were recorded at separation n= 2 at a number of points to give the complete shape of the curves. The normalized time integrals were anomalous over the two sheets, but were not significantly different; the highest values being observed over the disseminated sheet. Both sheets were also associated with high electromagnetic components of the decay curve. The chargeability and resistivity values obtained over the disseminated body were considerably higher. The metal factor was also of value in discriminating between massive ore, disseminated mineralization, and barren rock. The values of P2 and P3 for the two bodies were also compared (P2 and P3 are defined by image where M1 to M4 are the amplitudes of the decay curve at 55, 130, 280 and 580 ms respectively). For the massive ore, P was inversely related to M, but for the disseminated ore P was independent of M. Four simple parameters from the decay curves show that indices of curve shape offer the best prospect of grade discrimination.

Major oxide geochemistry and origin of the Troodos ophiolitic complex

New major element data from the layered and doleritic (static) gabbros, the plagiogranites, the sheeted complex and the volcanic series of the Troodos ophiolite complex clearly show that : the plutonic rocks are indiscutably the products of fractional crystallization and that the volcanics form continuous and smooth trends on oxide variation diagrams. This continuity in trend is also apparent in the case of the sheeted complex and the lower part of the volcanic series (Lower Pillow Lavas) when Miyashiro's diagrams are used. The role of metamorphism and related phenomena such as calcitisation, albitisation, adularisation and silicification is emphasised. It is proved once more that, despite ocean floor metamorphism, the Troodos ophiolite was derived from tholeiitic magma with calc-alkaline affinities. Furthermore both geochemical and field evidence indicate that the plutonic, sheeted complex and volcanic rocks are consanguinal.

Hydrothermal alteration of the basaltic lavas of the Troodos Ophiolite Complex associated with the formation of the massive sulphide deposits

Summary The Cyprus massive sulphide deposits are restricted to and constitute an integral part of the pillow lava succession of the Troodos Ophiolite Complex. Extensive geophysical, geological and geochemical data collected in the last 20 years suggest that this complex represents an uplifted fragment of oceanic lithosphere and underlying upper mantle, which evolved in the Mesozoic at divergent plate boundaries. The massive sulphide deposits consist of the basaltophilic elements S, Fe, Cu and, less commonly, Zn. They have been formed at the original sea water—Lower Pillow Lavas or Basal Group interface and are characterized by distinct vertical zoning comprising, in downward succession, a zone of massive ore, a zone of sulphide with silica and the stockwork zone. The latter consists of veins and disseminations of pyrite in brecciated and/or pillow lavas, which have undergone extensive silicification, chloritization and argillization. The veins represent fillings of fractures in the lavas by sulphides, whereas the disseminated pyrite is believed to have been essentially produced by sulphurization of the titanomagnetite of the basaltic lavas and it often contains inclusions of euhedral leucoxene. Part of the disseminated pyrite formed from the iron present in the glassy mesostasis and the ferromagnesian minerals of the lavas. Detailed optical, chemical and X-ray diffraction studies indicate that the sulphide ore-forming solution produced extensive mineralogical and chemical changes in the stockwork lavas. These are distinctly different from the mineralogical and chemical changes in the unmineralized lavas produced by regional hydrothermal metamorphism. Mineralogical changes in the stockwork lavas include the complete pseudomorphism of the primary minerals plagioclase and pyroxene by quartz, the sulphurization of magnetite and the alteration of the glassy mesostasis to a mixture of chalcedony, chlorite, illite and, less commonly, kaolinite. The chemical changes in the stockwork lavas involved the dramatic decrease of Ca, Na and, to a lesser extent, Al and K, and considerable increase of S and Fe. Magnesium decreases considerably in the intensely mineralized lavas, whereas it increases in the weakly mineralized lavas. The stockwork lavas are invariably intensely silicified, but there was no appreciable removal or introduction of silica in these lavas. Silicification of these lavas was probably produced by the selective leaching of the bases from the primary minerals and the glass, leaving behind residual silica in the form of quartz and chalcedony. In the stockwork lavas Cu, Zn and Co increased, whereas Ni and Mn decreased in comparison with the unmineralized lavas. Field, chemical and mineralogical relationships suggest that the stockwork zones represent the roots of the massive ores and the channels through which the ore-bearing fluids have ascended to form the massive ores and the sea water-pillow lava interface. The persistent presence of illite and absence of montmorillonite as a hydrothermal alteration product in the stockwork zone, combined with experimental data and studies of the active geothermal areas, suggest that the temperature of the ore-forming fluids was 250–260°C and the pH 6.6–6.8. Recent detailed stable isotope studies suggest that these fluids were mostly heated marine waters, which leached the ore metals from the underlying Sheeted Complex.

Evidence of co-genesis of the Troodos Lavas, Cyprus

SummaryPetrological and geochemical features of recently discovered basalts of the Troodos Massif, Cyprus — a dyke in the Basal Group and altered pillow lavas in the Lower Pillow Lavas — suggest that all the lavas are part of a single volcanic suite.

Origin of the troodos and other ophiolites: A reply to hynes

Geologic and tectonic evidence on the origin of the Troodos ophiolitic complex is ambiguous, being compatible with its creation in an island arc or a continent as well as in a mid-oceanic ridge. However, there is decisive petrologic and chemical evidence against its origin in a mid-oceanic ridge (and in a marginal sea). A large proportion (about half) of volcanic rocks in the sheeted complex and lower pillow lavas of Troodos have SiO 2 > 52.5% and FeO*/MgO > 2.0 in contrast to mid-oceanic ridge volcanics which are almost entirely basic (SiO 2 < 52.5%) with FeO*/MgO < 2.0. (FeO* means total iron as FeO.) A considerable proportion of volcanic rocks in Troodos belong to the calc-alkalic series. These facts indicate that the Troodos massif was formed probably in an island arc or a continental region. In order to rebut the above chemical evidence, Hynes resorts to an assumption of thorough compositional change of the Troodos volcanic rocks by metasomatism. However, this assumption is not plausible.

Basalt geochemistry used to investigate past tectonic environments on Cyprus

Study of the geochemical fingerprints of four geologically distinct suites of volcanic rocks on Cyprus are used to sketch a tectonic history of the island. Lavas from the Mamonia complex resemble alkalic within-plate basalts; lower pillow lavas and diabases of the Troodos Massif have features both of ocean-floor and island-arc tholeiites and could have been erupted in an interarc basin; the upper pillow lavas of the Troodos Massif resemble primitive tholeiitic basalts from island arcs; lavas from the Kyrenia range resemble transitional to alkalic within-plate basalts. The low TiO 2 concentrations from the Troodos Massif may indicate a slow spreading rate. The Sr concentrations in the upper pillow lavas indicate an eruption at a maximum distance of 80 km above a Benioff zone. The results suggest formation of the Troodos Massif in the Campanian by spreading in an interarc basin followed by eruption of island-arc tholeiites. Obduction of continental material and ocean islands may have taken place in the Maestrichtian and Middle Miocene.

The Troodos ophiolitic complex was probably formed in an island arc

The Troodos ophiolitic complex in Cyprus has been widely regarded as a fragment of oceanic crust that was created in a mid-oceanic ridge. However, about one-third of the analyzed rocks of the lower pillow lavas and sheeted complex in it follows a calc-alkalic trend. This strongly suggests that the massif was created as a basaltic volcano in an island arc with a relatively thin oceanic-type crust rather than in a mid-oceanic ridge. Other chemical features and the structure of the massif are also consistent with an island arc origin. Some other ophiolitic complexes also appear to have been formed in island arcs.

Seismic Velocities Measured in Rocks of the Troodos Igneous Complex

Summary The Troodos igneous complex of Cyprus is an ophiolite body composed of ultramafics, gabbros, dolerites and pillow lavas. Seismic velocities measured in these rocks were expected to be similar to- those in the layers under the deep ocean. Seismic experiments made in the field in individual rock types revealed only low velocities typical of deep sea layer 2. We suggest that the low velocities are due to high porosity in the shattered rocks. Geophysical field work was carried out during March 1971, in order to measure the seismic velocities at outcrop of the rocks of the Troodos igneous complex. The aim of the experiment was to compare the velocities obtained with those of the oceanic crust already established by marine seismic refraction work. Financial support for the expedition came from I.B.M. (U.K.) Ltd grants to the University of Cambridge. A preliminary account of the results has already appeared (Matthews etal. 1971). The Troodos Massif is believed to be a fragment of Mesozoic oceanic crust which has been thrust up on land during the northerly movement of the African plate towards Europe (Gass & Masson-Smith 1963; Gass 1968). The Massif occupies the central and south-eastern part of the island (Fig. 1); the ultramafics form the domed central circular outcrop around the summit of Mt Olympus at 6403ft, and are pierced by a serpentinite diapir. The rocks belong to an igneous suite characteristic of ophiolite complexes, and comparable petrologically with rocks dredged from the deep ocean floor (Moores & Vine 1971). The plutonic members range from dunites, harzburgites and pyroxenites to olivine gabbros, gabbros and quartz diorite and are ringed by the Sheeted Complex which is a swarm of steeply dipping altered basic dykes, without intervening screens, possibly formed by sea floor spreading. The Pillow Lavas occupy the outer margin of the oval outcrop pattern, and are divided into Upper Pillow Lavas with few dykes and Lower Pillow Lavas with abundant basic dykes, separated by an unconformity recognizable in only a few places. Moores and Vine classify the rocks into 3 units: Pillow Lavas, Sheeted Complex and Plutonic Complex, and these are used for the purposes of the following discussion rather than the more complicated scheme of the Cyprus Geological Survey. They suggest that a correlation may be made with rocks of the oceanic crust and upper mantle. Experimental work was carried out after careful site selection, using the geological maps of the Survey memoirs (see Appendix), and the detailed field knowledge of Drs I. G. Gass and F. J. Vine. With three exceptions all the lines were shot using the