Canodian Mineralogist Vol. 23, pp. 431441 (198s) LOW.TEMPERATUREALTERATION OF THEEXTRUSIVE SEOUENCE, TROODOSOPHIOLITE, CVPRUS KATHRYN M. GILLIS AND PAUL T. ROBINSON Departmentof Geologltand Centrefor Marine Geology,Dalhousie University, Hatifax, Nova ScotiaB3H 3Js ABSTRAcT tdt aprdsla formationde Ia cro0teoc6anique. En dehors deszones trbs localis€es d'alt6ration hydrothermale, les laves The lavas of the Troodos ophiolite have been altered du Troodosn'ont pas6t6 sujettes d un m6tamorphisme primarily by low+emperatureinteraction with seawaterand rdpandu;d'ailleurs, en dessous de la zonesup€rieure oxy' by localized hydrothermal upwelling. An uppermost zone dde, on ne trouve aucunevariation systdmatiquedans of highly oxidized lavas that contain abundant calcite and I'intensit6ou le caractdrede I'alt6ration avec la profondeur. smectiteis believedto be due to seafloor weathering.In Lesassemblages demin6raux secondaires ddpendraient des a few sections,where thick layers of umber overlie the vol- variationslocales en perm6abilit6,type d'unit6 de refroi- canic pile, the oxidized zone is absent, suggestingthat the dissement,temp6rature, rapport volumique eaulsolide et crust was sealedoff from circulating seawater.Below the compositioninitiale de la lave. oxidized zone, the rocks are less pervasively altered and (Traduit par la R6daction) locally contain fresh glass.Assemblages of ana'lcime,natro- lite, phillipsite, chabaziteand gmelinite are common in the Mots-cl6s:alt|ration, bassetemp€rature, massif ophioliti- pillow lavas of this sequencewhereas assemblages of clino- que ptilolite, deTroodos, Chypre, cro0te ocfunique, permdabi- mordenite and celadonite are primarily associat- lit6, argiles,zdolites, oxydation. ed with the massiveflows. A basal sequenceof more siliceouslava has abundant secondarysilica. Alteration in thesezones is believedto reflect seawater-rock interaction INTRODUCTION during or shortly after crustal accretion. Outside the very localized zonesof hydrothermal alteration, the Troodos la- Previousstudies, summarized in Honnorez(1981), vashave not beenpervasively metamorphosed, and below have shown that alteration in the upper oceaniccrust the upper oxidized zone there are no systematicvariations is largely the result of seawater-basaltinteraction. in the intensity or characterof alteration with depth. The The distribution and paragenesisof secondaryminer- observed assemblagesof secondary minerals can be ex- als have been shown to dependupon such factors plained by local variations in permeability, type of cool- as lithology, fluid composition, temperatureand age ing unit, temperature, waterlrock ratio and initial lava composition. (Nt et al, 1985,Staudigel et al. 1981,Natland & Ma- honey 1978).The shallow depth of most Deep Sea Keywords: alteration, low-temperature,Troodos ophiolite, Drilling Project holes and the restricted lateral con- Cyprus, ocean crust, permeability, clays, zeolites, oxi- trol over each site have limited our understanding dation. of the processof seawaterpenetration into the crust and the extent of hydrothermal circulation and metamorphismat depth. SouvernE Ophiolites provide an alternative for studying in situ oceaniccrust. Unfortunately, the transportation Les laves du massifophiolitique de Troodos ont princ! and emplacementhistory of many ophiolites have palement 6td altereespar un dchangeavec de I'eau de mer made it difficult to study their original pattern of d bassetempdrature et, localement,par jaillissementde flui- deshydrothermaux. Une zone supdrieurede coul6esforte- alteration. Previousstudents of the Troodos ophio- ment oxyd&s, enrichiesen calcite et smectite,repr6enterait lite concluded that the extrusive sequencehad been une zone de lessivagemarin. Oir d'6paissescouches d'ombre metamorphosed,resulting in the developmentof two recouwent I'empilement volcaniquedans quelquessections, distinct facies interpreted in terms of axis and off- la zone oxyd6eest absente,ce qui indiquerait que la cro0te axis volcanism(Gass & Smewing1973, Smewing el 6tait isol6ede I'eau de mer en circulation. En dessousde al. 1975).In this paper, we show that the extrusive la zone oxyd€e,les rochessont moins complbtementalt6- rocks of the Troodos ophiolite have undergonethe rdeset contiennentm€me, ici et li, du verre frais. Desassem- sametype of low-temperature alteration as that ob- blages d analcime, natrolite, phillipsite, chabasite et servedin the modern oceaniccrusu also, we discuss gmelinite sont courants dans les laves i coussins de cette physical sdquence,et les assemblagesi clinoptilotite, mordeniteet the conditions that control the distribution c€ladonite sont surtout associ6saux coul6esmassives. Une and paragenesisof the secondaryminerals. A more s6quenceinfdrieure de lave plus siliceusecontient beaucoup detailed study of the physicochemicalconditions ac- de silice secondaire. L'alt6ration de ces zones refl6terait tive during the different stagesof alteration is still l'interaction desroches avec de l'eau de mer pendant ou underway. 431 Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/23/3/431/3420067/431.pdf by guest on 28 September 2021 432 THE CANADIAN MINERALOGIST Recentstudies of the Troodos Massif of Cyprus Robinson et al. (1983)have outlined two geochem- have confirmed that it is a fragment of oceanic ical suites for the northern flank of the Troodos lithosphere,as suggestedby Moores & Vine (1971), ophiolite on the basisof glasscompositions: 1) a low- although it is now believed to have originated in a er andesite- dacite - rhyodaciteassemblage of arc subduction-zone environment rather than at a mid- tholeiite affinity and 2) an upper picritic basalt - an- ocean ridge (Moores et al. 1984, Robinson el a/. desitic basalt assemblagewith a depletedarc tholeiite 1983,Schmincke et al:1983, Mcculloch & Cameron affinity. The stratigraphic level of this new geochem- 1983). The excellent exposure and relatively un- ical boundary is variable within the extrusive se- deformed nature of this complex provide a unique quencein relation to the old UPL/LPL boundary opportunity to study the processesof alteration (Frg. l). A third suiteof highly depletedbasalts hav- through a completesection of oceaniclithosphere. ing "boninitic" affinities has beenrecognized on the Early workers divided the extrusive sequenceinto southern flank of Troodos (J.M. Mehegan, pers. the Upper Pillow Lavas (UPL) and Lower Pillow La- comm. 1984). vas (LPL) on the basisof color, abundanceof oli- Detailed studiesof stratigraphically controlled sec- vine phenocrysts,assemblages of secondaryminerals tions betweenthe villagesof Malounda and Margi and the location of the sulfide orebodies@ear 1960, (Fie. 2) have shown that the extrusive sequencehas Gass1960). Gass & Smewing(1973) divided the se- not beenpervasively metamorphosed. Fresh volcanic quenceinlo an axis sequence,consisting of the Lower glassis preservedsporadically throughout the extru- Pillow Lavas and the SheetedDykes, and a youn- sive sequence,efiending downward to the top of the ger, off-axis sequenc€,represented by the Upper Pil- Basal Group (9090sheeted dykes). Furthermore, the low Lavas, based on a supposed metamorphic distribution of the secondaryminerals within the ex- boundary within the complex. trusive sequenceindicates that thereis neithera sys- MALod,JNDA KAMBIA MARGI HIGH MsO- HIGH SiOt ;.'..j.t - -tptUPL - SUTTE eac THOLEIITE SU'TE HIGH MgO. H|GH SiO,' UPL SUITE LPL ARC THOLEIITE SUITE s o c upperZone o ffi Middte zone HIAH MgO- % HIGH SiO2 SU'IE Lowerzone ARC ffi THOLEIITE SUITE FIo. l. Schematicrepresentation of the geology of the study areas,outlining the strati- graphicposition of the old Upper Pillow Lavallower Pillow Lava boundaryand the new geochemicalsuites, Upper zone: aphyric to olivine-phyric basalts and pi- crites;middle zone:aphyric to slightly olivine-clinopyroxene-phyricbasalt to an- desitic basalt; lower zone: aphyric to slightly clinopyroxene-plagioclase-phyric andesites- dacites - rhyodacites. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/23/3/431/3420067/431.pdf by guest on 28 September 2021 LOW.TEMPERATURE ALTERATION IN THE TROODOS OPHIOLITE. CYPRUS Ftc. 2. Geological map of the,northeastern portion of the extrusivesequence. The Malounda, Kambia and Margi study are:Nare outlined. Note that the old Upper Pillow Lavallower Pfuow Lava boundary is rtro*n. r'or a cffiarison' of this old boundary with the new geochemicalboundary for each study area, seeFigure l. tematic increase in the intensity of alteration with the basisof color, type of cooling unit and mineral- depth nor a metamorphic boundary within the lavas. ogy (Mehegan& Robinson 1984,Schmincke et ol. Localized zones of high-temperature hydrothermal 1983). These units have been grouped into three alteration underlie the rhassive-sulfideorebodies IIn- major stratigraphic sequencesreferred to as the up- ternational Crustal Research Drilling Croup per, middle and lower zones. Although the same (ICRDG) 19841,but theseare spatially restrictedand generalsequence is found throughout the area, these have little overall effect on the lavas. The presence zonesvary greatly in thickness, lithology and later- of an oxidized zone in the upper 20 - ZO0m of the al extent. extrusivesequence and the relationshipsamong the The basal 500 - 600 m of the extrusive sequence ass€mblages of secondaryminerals, type of cooling contains aphyric to sparsely clinopyroxene- and unit and host Iithology both suggestthat alteration plagioclase-phyricmassive and sheetflows, pillow