Crystallization Orders and Phase Chemistry of Glassy Lavas from the Pillow Sequences, Troodos Ophiolite, Cyprus

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    Crystallization Orders and Phase Chemistry of Glassy

    Lavas from the Pillow Sequences, Troodos Ophiolite,

    Cyprus

    by P. THY1 AND C. XENOPHONTOS2

    1 Department of Geology, University of Botswana, Private Bag 0022, Gaborone, Botswana2 Geological Survey Department, Nicosia, Cyprus

    (Received 25 November 1988; revised typescript accepted 11 September 1990)

    ABSTRACT

    Three genetically unrelated magma suites are found in the extrusive sequences of the Troodosophiolite, Cyprus. A stratigraphically lower pillow lava suite contains andesite and dacite glasses andshows the crystallization order plagioclase; augite, orthopyroxene; titanomagnetite (with the pyrox-enes appearing almost simultaneously). These lavas can in part be correlated chemically andmineralogically with the sheeted dikes and the upper part of the gabbro complex of the ophiolite. Thesecond magma suite is represented in a stratigTaphically upper extrusive suite and contains basalticandesite and andesite glasses with the crystallizaton order chromite; olivine; Ca-rich pyroxene;plagioclase. This magma suite can be correlated chemically and mineralogically with parts of theophiolitic ultramafic and mafic cumulate sequence, which has the crystallization order olivine; Ca-richpyroxene; orthopyroxene; plagioclase. The third magma suite is represented by basaltic andesite lavasalong the Arakapas fault zone and shows a boninitic crystallization order olivine; orthopyroxene; Ca-rich pyroxene; plagioclase. One-atmosphere, anhydrous phase equilibria experiments on a lava fromthe second suite indicate plagioclase crystallization from 1225 C, pigeonite from 1200cC, and augitefrom 1165C. These experimental data contrast with the crystallization order suggested by the lavasand the associated cumulates. The observed crystallization orders and the presence of magmatic waterin the fresh glasses of all suites are consistent with evolution under relatively high partial waterpressures. In particular, high PH:lO (1-3 kb) can explain the late appearances of plagioclase and Ca-poor pyroxene in the majority of the basaltic andesite lavas as the effects of suppressed crystallizationtemperatures and shifting of cotectic relations. The detailed crystallization orders are probablycontrolled by relatively minor differences in the normative compositions of the parental magmas. Thebasaltic andesite lavas are likely to reach augite saturation before Ca-poor pyroxene saturation,whereas the Arakapas fault zone lavas, which have relatively less normative diopside and more quartz,reached the Ca-poor pyroxene-olivine reaction surface and crystallized Ca-poor pyroxene afterolivine.

    INTRODUCTION

    The plutonic and intrusive rocks of the Troodos ophiolite complex are overlain by a glass-bearing volcanic succession of massive and pillowed flows and minor hyaloclastites andlapilli. This succession has been divided into lower and upper pillow lava sequences, mainlyon the basis of phenocryst assemblages, hydrothermal alteration, and the amounts of dikesand sills (Gass & Smewing, 1973; Smewing et al., 1975). Gass and co-workers suggested thatthe lower sequence formed at a spreading ridge whereas the upper sequence represents off-ridge activity.

    Although the original mapped boundary between the two extrusive sequences has notbeen confirmed in detail by petrographic work (Robinson et al., 1983), fresh volcanic glasses

    [Journal of Petrology. V d 32, Part 2, pp. 403-428, 1991] Oxford Uoivmity Prcu 1991

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  • 404 P. THY AND C. XENOPHONTOS

    collected from them show a two-fold division, with andesite, dacite, and rhyodacitepredominantly in the lower pillow sequence, and basaltic andesite and andesite in the uppersequence (Robinson et al., 1983; Thy et al, 1985). Phenocryst assemblages of the uppersequence consist of chromite, olivine, Ca-rich pyroxene, orthopyroxene, and plagioclase,whereas the lower sequence contains plagioclase, Ca-rich pyroxene, orthopyroxene, andtitanomagnetite. Little information has been available, however, on the chemistry ofcoexisting glass and phenocrysts (Jergensen & Brooks, 1981; Flower & Levine, 1987). Suchinformation is crucial to understanding the origin and evolution of the volcanic sequencesand their relationships to the rest of the ophiolitic sequence. This paper is concerned with thephase compositions of glassy lavas from selected localities in the lava sequences (Fig. 1). Theinvestigated samples cover most, but not all, the geochemical groups of lavas in the extrusivesequences (Cameron, 1985). In this respect, the present investigation complements that ofFlower & Levine (1987).

    FIELD RELATIONS

    The volcanic sequences of the Troodos ophiolite are overlain by pelagic sediments andgrade downward into a sheeted dike complex rooted in gabbro (Moores & Vine, 1971; Allen,1975; Gass, 1980). Beneath the dike complex are a succession of coarse-grained, gabbroicand ultramafic cumulates that rest conformably on, and in places intrude, mantle harzbur-gite (Greenbaum, 1972; Allen, 1975). The volcanic sequences comprise massive and pillowedlava flows, high-level dikes, and extrusive breccias related to individual volcanic centers(Schmincke et al., 1983). Volcanic glass occurs in chilled pillows, flow and dike margins, andin lapilli. The lapilli are often fragmented and bedded in a matrix of variably palagonitizedhyaloclastite. The amount of pyroclastic material is generally low, and sheeted or pillowedflows predominate. Some of the upper pillowed flows are picritic in composition (Gass, 1958;Searle & Vokes, 1969). The exposed lavas, dikes, cumulates, and mantle sequence areconsidered by most workers to represent cogenetic suites.

    Troodos OphiolitePi HOW lavas

    HUD Sheeted dike complex

    Gabbros

    Ultramofic cumulates

    Harzburgites

    FIG. 1. Geological sketch map of the Troodos ophiolite, simplified from Gass (1980). [[Note the location of theArakapas fault zone (Simoiuan & Gass, 1978).] The areas where lavas were sampled for this study are: KA,

    Kalavasos dam site; K.Y, Kythreotis quarry, PE, Pedhieos valley, and PL, Pleristerka quarry.

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  • GLASSY LAVAS OF TROODOS OPHIOLITE 405

    Glassy lavas, hyaloclastites, and lapilli of the upper pillow lava sequence were collected inthe Kalavasos dam site area (KA) and the Kythreotis quarry (KY) (Fig. 1). The KY sites arelocated to the north of the Arakapas fault zone, and the KA sites are to the south. The lowerpillow lavas were sampled along the Pedhieos river valley (PE) and in the nearby Pleristerkaquarry (PL). Lavas erupted along the Arakapas fault zone were not sampled during thisstudy.

    PETROGRAPHY OF THE GLASSY LAVAS

    The lavas typically contain phenocrysts, microphenocrysts, and microlites in a matrix ofisotropic glass (Figs. 2 and 3). Although the glass shows various degrees of palagonitization

    FIG. 2. Photomicrographs of glassy lavas from the Troodos basaltic andesite suite. (A) Basaltic andesite glass witholivine (ol) and Ca-rich pyroxene (cpx) phenocrysts. (B) Olivine phenocryst with inclusions of chromite (ch). (C)Skeletal olivine phenocryst in fresh glass (gl). (D) Cluster of Ca-rich pyroxene phenocrysts. (E) Pyroxene dendritesrimming a Ca-rich pyroxene phenocryst. (F) Olivine phenocryst in basaltic andesite glass fringed by almost opaque,

    feathery pyroxene dendrites. Subcalcic pyroxene laths are also present as a relatively late crystallizing phase.

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  • 406 P. THY A N D C. X E N O P H O N T O S

    FIG. 3. Photomicrographs of glassy lavas from the Troodos dacitic andesite suite. (A) Dacitic andesite glass with aCa-rich pyroxene (cpx) phenocryst containing inclusions of plagioclase (pi). (B) Orthopyroxene (opx) micro-phenocryst. (C) Plagioclase and pyroxene (px) microphenocrysts in glass (gl) containing abundant plagioclase

    microlites. (D) Cluster of plagioclase and subcalcic pyroxene microlites.

    and alteration to clay minerals, fresh isotropic remnants sufficient for electron microprobeanalysis can commonly be found. The hyaloclastites typically contain fresh angular glassfragments and lapilli. The vesicle content is relatively low (

  • GLASSY LAVAS OF TROODOS OPHIOLITE 407

    fault zone containing olivine, orthopyroxene, and rare pigeonite phenocrysts; they suggestedthe crystallization order: chromite; olivine; orthopyroxene; clinopyroxene; plagioclase. Theoccurrence of pigeonite in the Troodos basaltic andesite lavas has also been noted byDuncan & Green (1987).

    Fresh isotropic glass of the lower pillow lavas is brownish and always contains plagioclasephenocrysts (Fig. 3). Orthopyroxene and Ca-rich pyroxene microphenocrysts appear insmall amounts in many samples. Total phenocryst content is generally < 10 vol.%, but somelavas contain large amounts (30-50 vol.%) of microphenocrysts and microlites (Fig. 3C).Plagioclase phenocrysts locally show resorption features and overgrowth rims. Pyroxenephenocrysts are euhedral and in places occur in bow-tie intergrowths with plagioclase laths(Fig. 3C). Inclusions of plagioclase microphenocrysts can be found in Ca-rich pyroxenephenocrysts (Fig. 3 A). Orthopyroxene is the only mafic mineral in some of the more evolvedlower pillow lavas (Fig. 3B). Magnetite phenocrysts occur sporadically and microlites ofintergrow