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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 1165°C. 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 (<10%), possibly implyingextrusion and quenching in a deep-sea environment (Jorgensen & Brooks, 1981). Basaltclasts are common in many of the hyaloclastites and lavas. They are often highly corrodedand altered, but fine-grained, doleritic groundmass textures can be seen locally.

The glass-rich lavas of the upper pillow sequence have almost colorless glass and generallycontain <5 vol. % crystalline phases. Euhedral and commonly skeletal olivine phenocrysts(Fig. 2A-C) are invariably present [see Jorgensen & Brooks (1981) and Cameron (1985) fordiscussions of olivine morphology]. Euhedral chromite microphenocrysts are common asinclusions in the olivine (Fig. 2B). Ca-rich pyroxene phenocrysts occur only in the KA lavas,commonly in clusters (Fig. 2D), and may frequently show composite zoned and fibrousinternal fabric and diffuse extinction patterns. Plagioclase is not found in the olivine-bearinglavas, except in the basalt clasts. Feathery, nearly opaque dendrites of pyroxene areabundant in some lavas (Fig. 2F); these either grew on pyroxene phenocrysts (Fig. 2E) andbasalt clasts or nucleated homogeneously. These dendrites texturally resemble thoseproduced experimentally at controlled cooling rates by Schiffman & Lofgren (1982). Thecrystallization order of the upper pillow lavas as inferred from textural relations is: chromite;olivine; Ca-rich pyroxene; (plagioclase). Although not encountered in this study, plagioclasephenocrysts have been reported in relatively evolved upper pillow lavas described byRautenschlein et al. (1985). Flower & Levine (1987) described glassy lavas from the Arakapas

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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 ofintergrown pyroxene and plagioclase are abundant in some lavas (Fig. 3D). The crystalliza-tion order texturally deduced for the lower pillow lavas is: plagioclase; augite, orthopyrox-ene; titanomagnetite (with the pyroxenes appearing almost simultaneously).

PHASE COMPOSITIONS

All mineral and glass analyses were made with an electron microprobe by wavelength-dispersive methods. The standards were natural minerals and glasses, and an alpha-factorcorrection procedure was employed. The electron beam was defocused to minimize sodiumloss. In general, the precision of replicate basalt glass standard analyses (given as onestandard deviation—1 S.D.) is 0-50 wt % for SiO2,0-10-0-20% for A12O3, FeO, MgO, andCaO, and <0-07% for minor elements (Thy, 1991). A total of 43 glasses were analyzedin 34 samples (Table 1); phenocrysts were analyzed only in selected samples (Tables 2-5).Glass analyses by Jorgensen & Brooks (1981), Robinson et a\. (1983), Thy et a\. (1985), andFlower & Levine (1987) are also used in this paper, but there are few pubhshed analyses ofcoexisting glass and minerals for the Troodos extrusive rocks [see Jargensen & Brooks(1981) and Flower & Levine (1987)].

Glass

Glass compositions in Table 1 are averages of 2-8 analyses per thin section or per group ofglass shards. For each glass, values of 1 S.D. for replicate analyses are mostly <0-60 wt.%for SiO2, 0-20% for A12O3, FeO, MgO, and CaO, and 006% for TiO2, MnO, Na2O, K2O,and P2O5. These values compare closely to those listed above for the standard glass,implying that the natural glasses are practically homogeneous. No significant compositionaldifferences were detected between dike rims, pillow rims, lapilli, and hyaloclastites fromsingle localities.

The glasses can be divided into two distinct compositional groups on the basis of theirMgO and SiO2 contents (Fig. 4, Table 1). The group with >4 wt.% MgO consists of basalticandesites and andesites (KA and KY samples); the other group, with <4% MgO, areandesites and dacites (PE and PL samples). These two groups will hereafter be referred to asthe basaltic andesite and the dacitic andesite suites, respectively. They are equivalent to theupper and lower pillow lava sequences, respectively, as defined by Robinson et al. (1983).

Relative to the majority of oceanic basaltic glasses, the glasses of the basaltic andesite suite(calculated anhydrous to 100 wt.%; Figs. 4 and 5) have high SiO2 (52-8-55-6%), low FeO(7-4-8-2%), and low TiO2 (0-4-0-6%). Their Mg/(Mg + Fetoul) values are 0-63-0-65 for the

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TABLE 1

Microprobe analyses of natural glasses, Troodos ophiolite

Sample

KAaKA bKAcKAdKA2KA3KA3hKA3hKA4KA5KA7bKA8bKA8hKA 10bKA lOhKA 11KA 13bKA 13hKA 13hKA 14hKA 14hKA 15KY 1KY 1KY 2KY3b

SiO2

53-2253-2553-7853-84531053-0952-3352-8153-56531653-8254-3153-6654-4252-2653-6453-4454-3153-9953-4154-3452-5051-2551-2651-8151-38

77O2

0-410-380420-40039O40048O40037039039045040038046040040037043041037040051056053053

AI2O3

1511151315-2715-3214-8815-5015-3915-3015-5015-2615-39151615-3115-34151015-0815-0814-8514-8715-2714-8915-6015-2815-28151415-40

FeO*

7-317-317-297-297-697-457-597-637-927-587-807-397-657-597-777-717-557-207-437-227-237-747-517-467-577-65

MnO

013O13015014016012016016015016015013015014016015015017013013014015015014014012

MgO

6-656-746-716-886-606-666-706-686-536-436-576-546-636-786 616-836-747-046-926-477166-757-607-677-697-72

CaO

11-7711-9011-8911-7411-6711-8211-6911-8011-5811-5311-4911-9211-7511-9911-8612-3511-9011-8911-7511-7911-9012-0212-6412-4312-3812-63

Na2O

•45-48•50•54•37•73•69•74•39•47•41•77•72•70•73•73•73•71•70•77

1-691711 751-781 731-72

K20

Oil012009016008008010008005009006012009009010010008OilOilOilOil008006009004008

P2OS

005O040050040030-04004004006003003004003010004004004005005004003005007006003004

Total

96-2196-48971597-3595-9796-8996-1796-64971196-10971197-8397-3998-5396-099803971197-7097-3896-6297-8697O096-8296-7397O697-27

Vol

3-793-522-852654033113-833-362-893-902-892-172-611-473-911-972-892-302-623-382143003183-272-942-73

Mg no.

0619O62206210627O6050614061106090595O602O6O00612O6070614O60306120614063506240615063806090643064706440643

HX•<>Z

anXenzo•oXoz—o

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KY4KY4KY6bKY 6bKY6pKY7hPL 1PL 2PL 2PL 3PL 6PL 8P E 6P E 6P E 9PE9PE 10

52-2451-5951-915213511552-2752-6853-3455-7154-5155-9257-5256-9558-0157-9759115937

0530-550-510-520-530-561-321 291-441-28I 281-311-351-211-231-241-30

14-8115-7415-3815-35151615-7913-7413-3113-5114-3214-5014-0113-6113-8413-4813-591415

7-827-407-847-807-417-71

108211-59103111-239-39

101510569-169-008-759-66

016016016014016015017019021021021018021O20019017018

7-307-377-547-497-707-463073-252-403-222-312-422-591-921-771-67210

12-53121212-6012-6712-5012-517-817-866-708126-666-987116-206-025-946-61

1 661-781-801-791 781-652182-012-623-013-593-573-002-863 512-94317

007O08007O10008007053054021016022019047035022023022

002007009004004005009009012Oi l010015Oil017015017016

971496-8697-90980396-5198-2292-4193-4793-239617941896-4895-9693-9293-5493-8196-92

2-863142101-973-491-787-596-536-773-835-823-524046O86-466193O8

0625O64006320631064906330336033302930338O305029803040272026002540279

O

VI

•<

FeO*Fe 2 + .

••all iron calculated as FeO; vol is estimated volatile content calculated by difference; Mg no. =Mg/(Mg + Fe'°"1), atomic ratio, with all iron asOT]H90ooDOinO•0

X

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TABLE 2

Representative olivine analyses

SiO,FeOMnOMgOCaONiOTotal

SiFeMnMgCaNiTotalFo mol.%

KAcore

403611-640-21

47-75004022

10022

0996O24000041-757O00100043-004

88-0

arim

406112-72O20

46-75O05017

10O50

KA 11sp incl

401512-41019

46-76012023

99-86

Cations calculated to

1-003O263O0041-721O001O0032-997

86-8

09990258O0041-733O003O0053-001

87-0

KA 11rim

4O6013-55021

45-87O08013

10O44

4 oxygens

1-0070281O0041-696O002O0032-993

858

core

39-6713-58021

46-24O02021

99-93

0992O284O0041-723O00100043-008

85-9

KY Irim

405812-27018

46-97012022

10O32

1-002025400041-730O00300042-998

87-2

core, rim = core and rim of phenocryst; sp incl = phenocryst containing spinel inclusions.

TABLE 3

Representative pyroxene analyses

SiOjTiOjA12O3

FeOMnOMgOCaONa,OCr 2 O 3

Total

SiTiAlFeMnMgCaNaCrTotal

En mol.%Fs mol.%Wo moL%Ti/Al

KAcore

52-880132-805-69014

18-7517-96O06080

99-21

1-935O00401210174O0041-0220704O00400233-992

53-89-2

37-0O033

IIrim

53-020093 214-94013

17-072O62O10098

10O16

1-929O00201380150O00409260804O00700283-989

49-28-0

42-8O014

KA bcore

51-900122-734 81013

17-462051012078

98-56

KA acore

53-240061-774-30019

17-902029013031

9819

Cations calculated to 6

1-922O003011901490004096408140-009O0234-007

5O07-7

42-30025

1-967O00200770133OO0609860803000900093-993

51 36-9

41 80026

KAcore

53-440131-565-07010

19-2619-14010028

99-08

oxygens

1-95800040067015500031O510751000700084-005

53-77-9

38-40060

10bcore

52-710223-885-73015

17-9418-88014033

99-98

1-9160006016601740005O972O736O01000093-995

51-79-3

39O0036

clast

51-830264-985O8016

171219-63013069

99-88

1-888O00702140155000509290766000900203-993

5028-4

41-40033

KY 4clast

50870816-145-95012

16-051910013013

99-30

1-866002202660183000408770751000900043-982

48-510041-50083

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GLASSY LAVAS OF TROODOS OPHIOLITE

TABLE 3 (Continued)

411

SiOjTiO2

A12O3

FeOMnOMgOCaONa2OCr2O3

Total

SiTiAlFeMnMgCaNaCrTotal

En mol.%Fs mol.%Wo mol.%Ti/Al

PE6core

51-57053213

11-710-30

14-0019-06024—

99-54

1-943001500950369O01007860769O018

4-004

40819-24O00158

PEcore

50490631 96

1415044

13-5017-62019O02

99-00

1-932O01800880453O014O77007220014O0014-012

39-623-3371O205

9rim

51-37029071

24-75066

19-301-66——

98-74

PE9rim

5216033115

21-96051

21-211 63006—

99-01


1-973000800320795O02111050068

4-003

561404

3-50250

1-968O0090051069300161192O066O004

4-000

611355

3-40176

PLcore

50810541-69

12-84035

141018-31018—

98-82

oxygens

1-9390015007604100011O8020749O013—

4-015

4O920938-20197

6rim

51-000572-46

13-21039

13-7416-85020—

98-42

1-945001601110421001307810689O015—

3-991

41 322-336-40144

PL 8core

5117019O50

24-52073

18-522-92007—

98-62

1-9750006O023079200241-06501210005—

4-010

53-9400

610261

PL 3quench

49-760923-58

1411035

15-5713-78020—

98-27

1-89600260161O45Q0011088405630015—

4-005

46-623-729-70161

core, rim = core and rim of phenocryst; quench = quench phase; clast = fine-grained, groundmass phase of basaltclasts.

TABLE 4

Representative plagioclase analyses

SiO2

AljO3

FeOCaONa2OKjOTotal

SiAlFeCaNaK.TotalAn mol.%Ab mol.%Or mol.%

KY6bclast

47-253114

1-0017-961-79

9914

2-2061-714O03908980162_5-019

84-715-3—

KY 4clast

48-3932-00077

16-732-64005

10O58

2-2181-729O030082202350O035036

77-622-20 3

PE6incl

49-4531-38091

15-67307

10O48

PEcore

49153O90083

14-223-62009

98-81


2-2621-692003507680272—5029

73-826-2

—

2-2801-6900032O7070326O0055-040

68131-405

10rim

49-2431-45

11114-99305005

99-89

oxygens

2-2631-70400430738027200035022

72-926-8O3

PE9micro

540127-85

1O610815-39008

99-20

2-4691-50100410529047800055022

52-347-205

PLcore

49-9831-58067

14-803-34006

10O43

2-2781-6970026072302950O035023

70828-90 3

6rim

49-7532-41041

15O03-45004

101O6

2-2541-7310016072803030O025034

70529-302

clast = fine-grained, groundmass phase of basaltic clast; microrim, core = core and rim of phenocryst

= microlite; incl = inclusion in phenocryst augitc;

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412 P. T H Y A N D C. X E N O P H O N T O S

TABLE 5

Representative spinel analyses

SiO2

TiO2

AI2O3

FeO*MnOMgOCr2O3

Fe2O3

FeOTotal

Al no.Fe'" no.Cr no.Mg no.

KA a

O03027

16-2223-08022

11-4547-9213-9316-4399-51

O3080081O6100545

KA 11

013031

16-8521-75023

12-2347-78

6-2616-1299-91

0319007606060575

KY 1

010039

202122-44025

12-5142-58

7-1715-9999-20

03790-08605350582

KY 1

008043

207321-82028

13-244212

7-5715-0199-46

0385009005250611

PL 8

10825-31

75-280323-94003

44-0035-67

10O10

PE6

O9622102-47

69171512-34—

230248-45

10O85

FeO* = total iron calculated as FeO; Fe2O3 and FeO calculated from spinel stoichiometry; Al no.= Al/(Cr + Al + Fe3+); Fe'" no. =Fe3+/(Cr + Al + Fe3+); Cr no. = Cr/(Cr + Al + Fe3+); Mg no.= Mg/(Mg + Fe2+).

KY glasses and O60-O63 for the KA glasses. Estimated by difference, volatile contents rangefrom 2 to 4 wt.% (average 2-9). Similarly estimated, the volatiles in the Arakapas fault zoneglasses (Flower & Levine, 1987) average 2-2 wt.% (range 1-6-2-6%). These estimates,however, are maximum values because they are based on the assumption that all iron occursas FeO. The basaltic andesite glasses coexist with < 5 vol.% phenocrysts and thuspractically represent the extruded magma.

The glasses of the dacitic andesite suite (PL and PE) contain 57-63 wt.% SiO2. TheirMg/(Mg + FetoUI) values are 0-25-0-34; calculated volatile contents range from 3-0 to7-6 wt.% (average 5-4%). The dacitic andesite glasses are compositionally similar to manysilicic oceanic and island-arc glasses (Figs. 4 and 5). Because the amounts of phenocrysts andmicrolites vary widely, the compositional variations of the glasses in part reflect extensivecrystal fractionation.

Many variation diagrams show a compositional continuum between the two glass suites,but the plot of TiO2 vs. Mg/(Mg + Feloul) (Fig. 6) clearly separates them. The basalticandesite suite shows limited variation, whereas the other suite with low Mg/(Mg + Fetoul)has a large range, partly as a result of two rhyodacitic analyses reported by Robinson et al.(1983). The two analyses that plot at intermediate positions may belong to the basalticandesite suite. The glass analyses from Arakapas fault zone lavas (Flower & Levine, 1987)extend the basaltic andesite suite to 0-25 wt.% TiO2 and to an Mg/(Mg + Feloul) of 0-69. Aspointed out by Thy et al. (1985), the dacitic andesite glasses cannot be related to the primitivebasaltic andesite glasses by crystal fractionation, largely because of inconsistencies betweenthe required fractionating phases and the observed assemblages (i.e., the presence or absenceof plagioclase). Also, the stratigraphic sequence does not support such a simple inter-pretation.

The fresh glasses of the Troodos pillow lavas are consistently high in volatiles, as indicatedby the low totals in Table 1. These volatiles were reported by Kyser et al. (1986) to bedominantly H2O with minor CO2 . Most workers have assumed that the H 2O is magmaticbecause it occurs in optically fresh glass (Robinson et al., 1983; Rautenschlein et al., 1985;

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wt %15 -

wt.%

10 -Oo

5 -

20

toO

15 -

80

75

70

CVJ65O

55

50

45

1 _*-*.

Troodos glasses1

oThit study• Previous studies• Arakapas fault zone

© Mariana trench, IP0D,leg60,holes 460A S 46IA

IA Island arc glasses

4 5 6 7 8MgO Wt %

10 I

- 15

- 10

oa>

- 5

4 5 6MgO

10

FIG. 4. MgO variation diagrams for Troodos glasses and comparisons. All analyses by electron microprobe,recalculated to 100% without H2O, and with all iron as FeO. The Troodos glasses are both from this study(Table 1) and from previous studies (Jergensen & Brooks, 1981; Robinson et al., 1983; Thy el al., 1985). Glasses fromArakapas fault zone lavas are from Flower & Levine (1987). Group I, II, and III glasses are based on the Cameron(1985) definitions. Abyssal glass fields are based on Melson et al. (1977). The calc-alkalic, island-arc field is based ontephra in sediments cored from the Fiji plateau (Jezek, 1975). Mariana trench glasses are from I POD, Leg 60, Sites460A and 461A (Meijer et al., 1981). Boninite glasses are from Sharaskin et al. (1980), Bougault et al. (1981), and

Kuroda et al. (1978).

Thy et al., 1985). On the basis of isotopic data, however, Kyser et al. (1986) found evidencefor post-eruptive exchange with seawater in the basaltic andesite lavas, and estimated thatthey had a magmatic H2O content of only about 1-2 wt.%. Sobolev & Naumov (1985)estimated 1-3 wt.% H2O for a primitive Troodos magma, based on studies of melt inclusionsin phenocrysts. These estimates are all significantly higher than the 05-1 wt.% H2Osuggested by Duncan & Green (1987) for primary Troodos magma.

Olivine

The KY glasses, which have the highest Mg/(Mg + Feloul), coexist with olivine as the onlyphenocryst. The KA glasses coexist with both olivine and Ca-rich pyroxene phenocrysts.The olivine phenocrysts range from Fo8 8 to Fo8 4 (Fig. 7). Slight reverse or normal zoning isevident locally, but most phenocrysts are homogeneous. Chromite inclusions seem to belimited to the olivines with the highest forsterite content (Fo88_87). Flower & Levine (1987)

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FeO

wt.% MgO

FIG. 5. Na2O + K2O —FeO —MgO diagram for Troodos glasses. Details as in Fig. 4; C-T is the calc-alkalic vs.tholeiitic discrimination line (Irvine & Baragar, 1971). Dashed line encloses abyssal glasses.

CVJ

O

2.0

1.5-

1.0-

0.5-

0-

Dacjticandesite

Basaltic/;5andesite' - • * \ suite

i0 02 0.4 0.6 0.8 1.0

Mg/(Mg+Fe), cations

FIG. 6. Plot of TiO2 vs. Mg/(Mg + Fe) for Troodos glasses. Details as in Fig. 4. All iron is calculated as Fe J +

Olivine (KA)

n20i°40+-

eo mol.% Fo

Olivine (KY)

ab mol % Fo

Plagioclase (PL.PE)

90 85 80 7'5 7"0 tf5 60

Groundmaii plagloclaM In basoltlc claon (KY)

93 90mol.% An

"eT"9b- si so 75 70 mol% An

FIG. 7. Histograms of analyzed olivine and plagioclase in the Troodos lavas.

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reported that the most magnesian glasses from the basaltic andesite suite coexist withF°88-89 olivine phenocrysts (Flower & Levine, 1987). Olivine is not present in the lavas ofthe dacitic andesite suite.

Pyroxenes

Both suites of lavas contain Ca-rich pyroxene phenocrysts, whereas Ca-poor pyroxeneswere observed only in the dacitic andesite suite (Fig. 8). Pyroxenes in the basaltic andesitesuite (KA samples) are endiopsides, ranging from En48Fs7Wo45 to En J 5Fs1 0Wo3 5 (Fig. 8)with Al <0-15 (based on six oxygens), T i<001 , and Ti/Al of~0-05 (Fig. 9). By contrast, theCa-rich pyroxenes in the dacitic andesite suite are augites, ranging from En41Fs19Wo40 toEn43Fs22Wo35 , with much higher Ti (001-002) and Ti/Al (017). Ca-rich and Ca-poorpyroxenes in the dacitic andesite lavas occur either as composite, coarse-grained inter-growths with Ca-rich pyroxene or as discrete phenocrysts. They appear optically to beorthopyroxene, and their compositions range from En56Fs40Wo4 to En61Fs36Wo3. In lavas

CoMo COFB

OD phenocrysts• quench phase

PES PL lavas

cation % Fe

FIG. 8. Plot of Mg-Ca-Fe for pyroxenes in the Troodos lavas. Representative tie-lines are shown for coexistingaugite and orthopyroxene in the dacitic andesite suite. The outlined field denotes phenocryst compositions of the

basaltic andesite suite.

0.04-

0.03 H

0.02-

0.01 -

4AI=TI

/ Pacific ondesite suite• D phenocryst augite• • quench augite

o opx phenocryst

0.10 020

Basaltic andesite suiteo phenocryst augite• clast augite *

0.10 020 0.30Al (0=6)

FIG. 9. Plot of Ti vs. Al for pyroxenes in the Troodos lavas. The 'clast augites' occur in the groundmass of basalticclasts in the KY samples.

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with few or no Ca-rich pyroxene phenocrysts, the Ca-poor pyroxenes have slightly higher Cacontents (En54Fs4oWo6) and lower Mg/Fe (Table 3). Zoning of major and minor elements ismostly negligible, but some of the Ca-rich pyroxenes show weak normal zoning withincreasing Al toward the rims. Quench pyroxenes in the dacitic andesite glasses, intergrownwith plagioclase (Fig. 3D), range from augite to aluminous subcalcic augites (Figs. 8 and 9).The Ca-poor pyroxenes in the most magnesian lavas of the basaltic andesite suite aredominantly orthopyroxene of composition En86FsuWo2 (Flower & Levine, 1987).

Plagioclase

Plagioclase phenocrysts and microphenocrysts are abundant in the dacitic andesite suite.Microphenocrysts included in Ca-rich pyroxene phenocrysts have the composition An74,whereas grains attached to these phenocrysts are An69_67 (cf. Fig. 3A). Microlites areAn59_48 (Fig. 7). Phenocrysts and microphenocrysts range between An74 and An61, withlocal reverse zoning from cores of An61_ 55 to rims of An73. Highly resorbed, sieve-texturedplagioclase phenocrysts have cores of An77_71 and rims of An7J. All these An values aresignificantly lower than those of cumulus plagioclase in the gabbros of the plutonic complex(Thy et al., 1989). Relatively evolved lavas of the basaltic andesite suite, characterized by aglass phase with 4-5 wt.% MgO, commonly contain plagioclase phenocrysts with80-92 mol.% An (Rautenschlein et al., 1985), but this type of lava was not found in thisstudy. The dacitic andesite lavas contain sparse plagioclase phenocrysts with much morecalcic compositions (An84_80; Fig. 7) than would be expected for equilibrium conditions(An75_73; see below). These phenocrysts are compositionally similar to the groundmass

100

~ 90-

| 8 0 "

8 70"

<J 60-

50-

40-

30-

20-

10-

Boninttes

f\ f f\i \ I i Troodos' \ / / Pônic

' \~f i complex

Basalticandesitesuite

Abyssali v y basalts

100 80 60 40 20 6

FIG. 10. Plot of Cr/(Cr + Al) vt. Mg/(Mg + Fe2+) for chromite microphenocrysts in the Troodos lavas. TheTroodos plutonic complex, boninites, and abyssal basalts are from Dick & Bullcn (1984).

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plagioclase in basalt clasts (An82_79; see below) and to phenocrysts in the relatively evolvedlavas of the basaltic andesite suite.

Spinels

Chromite (Table 5) occurs in the basaltic andesite suite as inclusions in olivinephenocrysts (Fig. 2B) and, more rarely, as microphenocrysts. Its range of 100 x Cr/(Cr +Al)is 58-67, and 100 x Mg/(Mg + Fe2 +) ranges from 53 to 62 (Fig. 10). Except for being richer inCr, Troodos chromites are similar to those found in mid-ocean ridge basalts (Dick & Bullen,1984).

Titanomagnetite forms local phenocrysts in the dacitic andesite suite. Euhedral grainscontain significantly higher ulvospinel component than the embayed, presumably xenocrys-tic, phenocrysts (Table 5).

Basaltic clasts

Highly altered and partially disintegrated clasts of fine-grained, doleritic basalt arecommon to the KY lavas. Despite the extensive alteration of the groundmass in these clasts,the larger mineral grains can sometimes be analyzed, but zoning cannot be detected.Plagioclase compositions range from An8J to An78, with An8 2_7 9 being most common(Fig. 7). Augite grains tend to have slightly higher Fe/Mg and are significantly higher in Tiand Al than augite phenocrysts in the KA lavas (Figs. 8 and 9). However, their Ti/Al values(Fig. 9) are comparable to those of augite phenocrysts in other lavas of the basaltic andesitesuite. These relations suggest that the clasts are autoliths of the host lavas.

Rare earth elements

Rare earth element (REE) concentrations, determined for hand-picked glasses byinstrumental neutron activation methods, are illustrated in Fig. 11 as chondrite-normalizedpatterns. The PL and PE glasses of the dacitic andesite suite can evidently be related bycrystal fractionation because of their similar REE patterns. The basaltic andesite suiteglasses have depleted patterns, mostly below seven times chondrite abundances. The moreprimitive of these (KY) show strong depletion in the light REE and cannot yield the almostflat pattern of the KA glasses by crystal fractionation. It therefore seems likely that thesedistinct patterns signify different sources. The relative enrichment of the light REE in the KAglasses may be related to source metasomatism (Cameron et al., 1983; McCulloch &Cameron, 1983). The KY glasses accordingly belong to the light REE depleted 'group I'lavas of Cameron (1985), whereas the relatively light REE enriched KA glasses belong toCameron's 'group IF lavas. The Cameron (1985) 'group III' lavas were not sampled in thisstudy but are represented among the glasses collected from the Arakapas fault zone byFlower & Levine (1987) (Fig. 11). The Arakapas fault zone glasses and lavas characteristic-ally show strong REE depletion, coupled with an enrichment in La and Ce (McCulloch &Cameron, 1983; Flower & Levine, 1987; Taylor & Nesbitt, 1988).

PHENOCRYST EQUILIBRIA AND CRYSTALLIZATION ORDERS

Fe/Mg distribution

The /C£e/M| distribution coefficient [K£e/M| for coexisting mineral (xl) and glass (liq) isdefined as (Arf,eOA'{fq

8O)/(A'J|gOAr{;|o)] for olivine and glass in the basaltic andesite suite

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La Ce Yb Lu

FIG. 11. Chondrite-normalized rare-earth element patterns for selected Troodos glasses. Samples PE and PL arefrom the dacitic andcsite suite; K.Y and KA are from the basaltic andesite suite. Numbers in parentheses are thenumbers of analyses used for calculating each average. Pattern AFZ represents glass from the Arakapas fault zone(Flower & Levine, 1987, table 5, analysis AM-9). Chondrite normalizing values are from Evensen et al. (1978).

averages 030, essentially the value (030 ±003) obtained experimentally (Roeder & Emslie,1970). This value is based on the assumption that Fe3+/(Fe3+ + Fe2+) = O14 in the glass.

With all iron as Fe2+, values of K£e/Mf for the Ca-rich pyroxenes and their host glassesaverage 025, which may be high compared with the range commonly observed in mid-oceanridge basalts (020-025; Perfit & Fomari, 1983; Stakes et al., 1984) and under experimentalconditions (Grove & Bryan, 1983; Baker & Eggler, 1987). The ferric iron contents of the Ca-rich pyroxenes can be estimated by balancing charge deficiencies and charge excesses.Substitutions of Na on M2 sites and Al on tetrahedral sites can be charge-balanced bysubstituting AL, Cr, Ti, and Fe3+ on octahedral sites (Papike et al., 1974). On this basis, theanalyses of Ca-rich pyroxene are evenly clustered around Fe3+ =0 (Fig. 12), implying thatFe3 + is not systematically present. If it further is assumed that Fe3+/(Fe3 + + Fe2+)=O14inthe glass, the KD values are significantly higher (029).

Charge-balance calculations for augite from the dacitic andesite suite (Fig. 12) suggest thatferric iron is present in amounts such that Fe3+/(Fe3+ + Fe2+)=O06 (range 002-009). Ifthis value is assumed and that Fe3+/(Fe3+ +Fe2+)=O14 in the glass, then KêlM* averages0-26, not much different from the values (~O23) observed both in mid-ocean ridge basaltsand in experiments (Grove & Bryan, 1983).

The orthopyroxene phenocrysts from the dacitic andesite suite yield an averagecrystal-glass K£e/M* value of 029 (range 0 26-030), significantly higher than the average ofO23 obtained for pigeonite in ferrobasaltic mid-ocean ridge lavas by Perfit & Fornari (1983)and the experimental value of 019 determined for orthopyroxene by Baker & Eggler (1987).

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0.00' 1 I • I I0.00 0 0 5 0.10 Q.I5 0.20 0.25

(0=6)FIG. 12. Charge balance of pyroxenes in the Na + AliT vs. Al'' + 2Ti + Cr diagram (Papike etal^ 1974). Symbols as in

Fig. 9.

Because of the lack of Fe 3 + estimates for the Ca-poor pyroxenes, these values werecalculated on the assumption that all iron is Fe 2 + in both the minerals and the glass.Although orthopyroxene was not found in the basaltic andesite lavas in the present study,Malpas & Langdon (1984), Cameron (1985), and Flower & Levine (1987) have all describedit as a phenocryst phase in Troodos lavas. The analyses reported by Flower & Levine (1987)yield an average Mg/(Mg + Fe) of 0-88 and Kê/Ml values of 0-34-0-41. The latter values areprobably too high to reflect equlibrium and suggest that the orthopyroxenes equilibratedwith liquids richer in iron than the glasses analyzed by Flower & Levine (1987). However, theMg/(Mg + Fe) of the orthopyroxenes is typical of mantle values measured for the Troodoscomplex (0-88-0-90; Allen, 1975), so it would appear that they may have crystallized from, orequilibrated with, primary liquids.

Minor element relations of the pyroxenes

The partition coefficients (D) [£> is defined as (wL% element in mineral)/(wt.% element inliquid)] for Na, Ti, Cr, and Al between Ca-rich pyroxene and coexisting glass can reflect boththe glass composition and crystal growth rates. For the basaltic andesite suite, an averageDNmi° (cpx/liq) value is 0-08; D™2 is 0-37 (range 0-15-0-69); and D*'203 is 0-17 (range0-10-0-24). The coefficients for TiO2 are higher than experimentally predicted for pyroxenesfrom mid-ocean ridge basalts (0-25-0-27; Grove & Bryan, 1983; Nielsen & Dungan, 1983)

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420 P. THY ANDC. XENOPHONTOS

whereas those for A12O3 are in better agreement (0-17-0-22). The incorporation of'excess'TiO2 into the Ca-rich pyroxenes (estimated values would be 010 wt.%, in contrast to theobserved concentrations of 0-06-0-30 wt.%) is difficult to explain in terms of crystal growthrate in that the higher TiO2 is not accompanied by higher A12O3 as is normally observed incooling-rate experiments (Coish & Taylor, 1979; Grove & Bence, 1979; Gamble & Taylor,1980). Even though the natural D values for TiO2 and A12O3 are significantly higher than theexperimentally determined values, they are still within the ranges observed for many naturalmid-ocean ridge basalts (e.g., Perfit & Fornari, 1983). However, the high Ti and Al in the Ca-rich pyroxenes of the fine-gTained basaltic clasts (Fig. 9) probably record higher growthrates.

A strong positive correlation for the pyroxenes of the basaltic andesite suite between Allv

and Alvi, corrected for Ti and Cr substitutions, suggests a dominantly Ca-Tschermaksubstitution (Alvi-Allv), coupled with Cr-Aliv, and minor Ti-2Allv substitutions. Thesesubstitutions are essentially similar to those observed in pyroxenes of mid-ocean ridgebasalts (Schweitzer et al., 1979; Basaltic Volcanism Study Project, 1981), but an importantdifference is the dominance of Cr over Ti in the Troodos pyroxenes.

For the Ca-rich pyroxenes of the dacitic andesite suite, average D values for Na2O, TiO2,and A12O3 are 0-07,0-44, and 014, respectively. The minor element substitutions can largelybe accommodated by Ti-2Aliv and Alvl-Aliv exchanges. Subcalcic pyroxene microlites showhigh Ti and Al (Fig. 9), probably as a quenching effect (Grove & Bence, 1979; Gamble &Taylor, 1980), but the substitution mechanism appears to be identical to that indicated forthe phenocrysts.

Expected and observed phenocryst assemblages

Crystallization temperatures for the basaltic andesite suite, calculated from the glasscompositions (Nielsen & Dungan, 1983), are 119O-1175°C for the appearance of Ca-richpyroxene and 1165-1130°C for the appearance of olivine. The Roeder et al. (1979)formulation of the olivine-spinel thermometer yields equilibration temperatures of1065-920 CC, but they probably in part represent subsolidus relations. The high crystalliza-tion temperature of Ca-rich pyroxene relative to olivine is questionable in view of itsapparent late crystallization.

The dacitic andesites contain phenocryst plagioclases with an average composition ofAn74. According to the Kudo & Weill (1970) and Drake (1976) plagioclase-melt geothermo-meters these plagioclases probably crystallized at about 1190-1185 °C. The observedcompositions for phenocryst plagioclase (An73_75) are within the range of An6 7_7 6

estimated on the basis of the Nielsen & Dungan (1983) liquid geothermometer. Calcicplagioclase compositions of up to An64 (Fig. 7) may be xenocrystic or may representcrystallization under higher water pressures (e.g., Arculus & Wills, 1980; Thy, 1987a). Thepyroxene equilibrium temperatures for the dacitic andesite suite, determined by the Nielsen& Dungan liquid geothermometers, are 1100-1040°C for the Ca-rich pyroxene and1100-1065 CC for the Ca-poor pyroxene (pigeonite according to their model). Estimatedtemperatures of 1100-1000°C were obtained by the Lindsley & Andersen (1983) two-pyroxene geothermometer. These estimates are consistent with the petrographic crystalliza-tion order.

One-atmosphere melting experiments on a basaltic andesite glass (Table 6) showed thatplagioclase crystallized from 1225 °C, followed by pigeonite from 1200°C, and Ca-richpyroxene from 1165 °C. This order is in contrast to the observed natural order because of theearly appearance of plagioclase and pigeonite, and the absence of olivine (Table 6). Similarexperiments on a dacitic andesite lava showed plagioclase crystallization from 1170°C, and

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T A B L E 6

Crystallization orders {temperatures in parentheses)

Basaltic andesite suite

Petrographic Chromite; olivine; Ca-rich pyroxene; plagioclaseobservationsMelting Plagioclase (1225°Q; pigeonite (1200oQ augite (1165°C)experiments*

Arkapas fault zone lavas

Petrographic Chromite; olivine; orthopyroxene; clinopyroxene; plagioclaseobservationsf

Melting Olivine (135O°Q; pigeonite (122O°CX subcalcic augite (1210°Q;

experiments^ plagioclase (1190 °C)

Dacitic andesite suite

Petrographic Plagioclase; augite, orthopyroxene; titanomagnetiteobservationsMelting Plagioclase (1170°Q; pigeonite, quartz, titanomagnetite (1100°Qexperiments§

* One-atmosphere melting experiments on KA b glass (Table 1). The experiments were done in avertical quench furnace with a CO-CO2 gas mixture at the fayalite-magnetite-quartz oxygen bufferand using a Pt-wire loop.

t Petrographic studies of natural lavas (~ 10 wt.% MgO; Flower & Levine, 1987).I One-atmosphere, anhydrous melting experiments on a model primary magma (15 wt.% MgO)

(Duncan & Green, 1987).§ One-atmosphere, anhydrous melting experiments on PE 9 glass (Table 1). Experimental

technique as for melting experiments on KA b glass.

near coprecipitation of plagioclase, pigeonite, quartz, and titanomagnetite from 1100°C.This crystallization order deviates slightly from the natural order by the absence of augiteand the early appearance of quartz (Table 6).

The expected phenocryst assemblages can also be evaluated on the normative triangulardiagrams olivine-diopside-quartz (Fig. 13) and olivine-plagioclase-quartz (Fig. 14). Onthese diagrams, the Troodos glasses define a trend parallel to the anhydrous, low-pressure(four-phase cotectic) liquid line of descent for oceanic tholeiites experimentally defined byWalker et al. (1979). Glasses of the dacitic andesite suite are slightly displaced towardsplagioclase. Their relative location, nevertheless, is consistent with the effect of elevatedwater activity on the cotectic determined for Mount St. Helens dacites by Merzbacher &Eggler (1984).

The glasses of the basaltic andesite suite (KY and KA) are displaced toward both highernormative plagioclase and higher diopside. Although their normative variation suggestsevolution by coprecipitation of first augite, olivine, and plagioclase, and then two pyroxenesand plagioclase (Figs. 13 and 14), this possibility cannot be supported because plagioclasedoes not occur in the lavas. Furthermore, the majority of these glasses plot where Ca-poorpyroxene should crystallize, but this phase is absent as a phenocryst. The glasses from theArakapas fault zone (AFZ), which locally coexist with orthopyroxene, plot with the lowestnormative diopside and plagioclase.

DISCUSSION

The basaltic andesite suite

The expected low-pressure, early crystallization of plagioclase and Ca-poor pyroxene forthe lavas of the basaltic andesite suite is inconsistent with the observed late appearance of

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Cfi

Basaltic andesitesuite (KY.KA)Arakapas faultzone lavasDacitic andesitesuite (PL.PE)

FIG. 13. Normative oh'vine (ol)-diopside (di}-quartz (q) projection from plagiodase for the Troodos glasses. Dataare from Table 1 and Flower & Levine (1987). The 1-atm, liquid line of descent for mid-ocean ridge tholeiitic basalts(MORB, 1 atm) is from Walker et al. (1979); the MORB glass field is from Melson et al. (1977). The fields labeledaugite and opx mark the compositions of augite and orthopyroxene phenocrysts found in the Troodos lavas. Theprojection is based on a CIPW molecular norm with iron recalculated as Fe2+/(Fe2+ + Fe3+)=O86, following the

procedure outlined by Presnall et al. (1979).

Basaltic andesitesuite (KY.KA)Arakapas faultzone lavasDacitic andesitesuite (PL.PE)

FIG. 14. Normative oUvine (ol)-plagioclase (pl)-quartz (q) projection from diopsidc for the Troodos glasses. Detailsas in Fig. 13. The effect of water pressure on the position of the plagiodase-augite-orthopyroxene cotectic is based

on Merzbacher & Eggler (1984).

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these phases in the natural lavas (Table 6). It would seem, therefore, that the observedphenocryst assemblages did not crystallize under low-pressure, anhydrous conditions. Apossible alternative is that the magma evolved at somewhat elevated H2O pressures (Thy,1984; Thy et al, 1985). Increasing PHlO would depress the crystallization of plagioclase(Kudo & Weill, 1970) and increase its anorthite content (Arculus & Wills, 1980). The lattereffect has been used to explain the anomalously calcic plagioclases of the ophioliticcumulates and in the lava phenocrysts (e.g., Thy, 1987a; Thy et al, 1989). The relatively highmagmatic water content of the glasses also indicates high water activity during crystalliza-tion (Cameron et al, 1980; Robinson et al, 1983; Cameron, 1985; Sobolev & Naumov, 1985;Thy et al, 1985; Thy, 1987a). Cameron (1985) estimated that group II lavas contained up to 3wt% H2O, and Walker & Cameron (1983) suggested an H2O activity of 0-3 to account forthe amphibole in the groundmass of some of the lavas. A confining pressure on the Troodosmagma chambers of 1-3 kb would be a reasonable upper pressure limit, which can beinferred from the depth to layer 2 of the ophiolite.

Relatively high water pressure or activity would shift the liquidus boundary for olivineand Ca-poor pyroxene toward quartz (Kushiro, 1972; Warner, 1973). This shift wouldexplain why, contrary to the prediction from Figs. 13 and 14, Ca-poor pyroxene rarelyoccurs as a phenocryst phase in the basaltic andesites, and predicts its occurrence only in thelavas with relatively high normative quartz (i.e., the KA and AFZ samples). These sameglasses also show marked enrichment in the light REE (Fig. 11), a feature that has beenattributed to metasomatic modifications of their source region before their melting andextraction (cf. McCulloch & Cameron, 1983). It is therefore suggested that the liquids thatreflect metasomatism are also the most likely to have crystallized early Ca-poor pyroxene.On the other hand, the AFZ glasses, which may plot close to, if not within, an eventualhydrous low-Ca pyroxene liquidus volume (Figs. 13 and 14) can reach saturation witholivine and/or Ca-poor pyroxene before reaching augite or plagioclase saturation. Accord-ing to Cameron (1985), these glasses can be referred to as 'boninitic'. Unfortunately, neitherthe anhydrous nor the hydrous liquid line of descent for the Troodos lavas has beendetermined experimentally.

The overall compositional trend of the Troodos glasses suggests strong plagioclasecontrol, but this feature conflicts with the observation that plagioclase crystallizes late. Twoexplanations appear possible. One is that the plagioclase control was exerted at highpressure (> 3 kb). The other is that the compositional trend reflects source inhomogeneitiesor changing melting conditions. The first possibility is not supported either by the REEvariations, which suggest that the glasses represent genetically unrelated groups (Fig. 11), orby the absence of phenocrystic plagioclase. The second possibility is apparently consistentwith pseudoinvariant partial melting of lherzolitic mantle source under changing conditionsof PHlO (Thy, 1984; Thy et al, 1985). The relatively low pressures (< 10 kb) required areconsistent with the pressures advocated for the formation of the Troodos basaltic andesitemagmas by Duncan & Green (1980,1987). It is therefore suggested that the Troodos basalticandesite glasses represent a continuum of mantle-derived compositions modified by low-pressure fractional crystallization of olivine and pyroxenes.

Estimates based on available primocryst and mantle data suggest a primary liquid with atleast 13-14 wt% MgO and point to a significant amount of olivine fractionation beforeeruption (cf. Duncan & Green, 1980; Malpas & Langdon, 1984; Sobolev et al, 1986).Duncan & Green (1987) melted a model primary Troodos magma (15 wt.% MgO) based ona basaltic andesite of the type found along the Arkapas fault zone. They found that theanhydrous, low-pressure crystallization order was: olivine (135O°Q; pigeonite (1220°C);subcalcic augite (1210 °C); plagioclase (1190°C). They observed that orthopyroxene took the

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place of pigeonite at ~ 5 kb and suggested that the presence in the lavas of microphenocrystswith cores of pigeonite rimmed by subcalcic augite coexisting with orthopyroxene reflectedcrystallization at a pressure of ~ 5 kb. They noted also that the addition of 1-2 wt.% H2Owould depress the crystallization of Ca-poor pyroxene to temperatures at which ortho-pyroxene would crystallize instead of pigeonite. On this basis, they inferred that theminimum amount of water allowed would be < 1 wt.%. The observation by Grove & Juster(1987) that oxygen fugacity can control the crystallization of orthopyroxene vs. pigeoniteprovides an alternative possibility for the interpretation of the textures described by Duncan& Green (1987). Nevertheless, the similarities between the experimental, low-pressurecrystallization order obtained by Duncan & Green (1987) and that observed in the rocks(Table 6) appear to support the suggestion of a relatively lower water content for the AFZlavas [or the type HI lavas of Cameron (1985)]. This would be consistent with the highlyrefractory nature of these lavas (~ 15 wt.% MgO and 0-20% TiO2).

Dacitic andesite suite

The experimental crystallization order for the dacitic andesite glass shows only slightdeviation from the observed crystallization order (Table 6). The normative projection(Fig. 14) shows a systematic shift toward plagioclase. Such a shift is consistent with thepredicted effect of water on the multisaturated cotectic (Fig. 14; Spulber & Rutherford, 1983;Merzbacher & Eggler, 1984). The glasses of the dacitic andesite suite are multisaturated inplagioclase, orthopyroxene, and augite, and this shift may, therefore, reflect a relatively highwater activity compared with the anhydrous liquid line of descent for mid-ocean ridgebasalts.

Correlation with the cumulate sequences and sheeted dike complex

According to Allen (1975), the cumulates of the Troodos ophiolite have the crystallizationorder: chromite; olivine; augite; orthopyroxene; plagioclase. However, based on Ti/Alrelations of cumulus augites, the cumulates can be shown to be composed of two unrelatedsuites (Thy, 1987a, b; Thy et al., 1989). The lower, coarse-grained, mafic to ultramaficcumulates contain augites with very low Ti/Al (~009) and show the crystallization orderfound by Allen (1975). The upper, finer-grained cumulates contain augites with significantlyhigher Ti/Al ratios (~O16) and are gabbroic, composed of plagioclase, augite, orthopyrox-ene, and occasionally titanomagnetite. In particular, the lack of plagioclase in the primitivebasaltic andesites and the composition of the augites suggest that parental magmas to thelower cumulate sequence should be sought among the basaltic andesite extrusives.

In an extensive study of the Troodos sheeted dike complex, Baragar et al. (1987) observedthat plagioclase and augite phenocrysts were present in most dikes whereas titanomagnetiteoccurs only in dikes with < 5 wt% MgO and pseudomorphed olivine phenocrysts only insome dikes with > 5 wt.% MgO. Thy et al. (1989) described plagioclase- and augite-bearingdike rocks with a groundmass of plagioclase, augite, orthopyroxene, and titanomagnetite.These latter rocks contain augites with high Ti (Ti/Al~O17). Chemical and mineralogicalevidence suggests a cogenetic sequence of plagioclase- and augite-bearing dike rocks rangingin composition from basalt to rhyolite (Baragar et al., 1987). The phenocryst assemblageobserved in the dacitic andesite suite and the composition of the augites is consistent withthe assemblages observed in the sheeted dikes, but the latter extend to more basalticcompositions than represented by the dacitic andesite glasses.

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Although the upper cumulates appear mineralogically to conform with the phenocrystassemblage observed in the dacitic andesite suite, this correlation is ambiguous becauseplagioclase-bearing cumulates also occur in the uppermost part of the lower cumulatesequence (Thy et al., 1989). Less ambiguous correlation, however, can be made by taking theaugite compositions into consideration. The cumulus augites of the upper gabbros andphenocrysts of dacitic andesites have the same Ti/Al ratio, and reveal a strong chemicalrelationship between the upper cumulates, the dacitic andesite suite, and the sheeted dikecomplex (Thy, 1987a, b; Thy et al., 1989).

Field mapping of the plutonic complex has defined two intrusive suites (Murton, 1986;Benn & Laurent, 1987; Malpas et al, 1987). Benn & Laurent (1987) and Malpas et al. (1987)observed an early suite comprising dunite, olivine pyroxenite, and gabbro. This suite isintruded by a later suite composed of dunite, poikilitic wehrlite, plagioclase wehrlite, andgabbro. Benn & Laurent (1987) showed that the late suite is poorer in Ti and richer in Ni andCr than the early suite. It would appear, therefore, that the early suite might correlate withthe basaltic andesite lavas examined in this paper, and the late suite with the lavas along theArakapas fault zone. Based on published chemical data, it appears that none of theseplutonic suites can be correlated with the dacitic andesite suite and the sheeted dike complex(Thy, in press).

CONCLUSIONS

The basaltic andesite and andesite lavas of the upper pillow sequence contain chromite,olivine, and augite phenocrysts, and plagioclase appears only during groundmass crystal-lization. By contrast, geothermometers as well as melting experiments suggest earlyappearance of plagioclase and Ca-poor pyroxene. This is in contrast to the observed lateappearance of plagioclase and the absence of orthopyroxene in most of these lavas.Nevertheless, major and minor element partitioning between glass and phenocrystsindicates that these are essentially in equilibrium. It is suggested that the discrepancybetween the observed and expected crystallization orders for the basaltic andesite lavas canbe resolved by assuming relatively high water activity, consistent with the high water contentof fresh glasses. High water pressure would suppress the crystallization of plagioclase andcause a shift in the olivine-orthopyroxene reaction surface toward silica, explaining whyorthopyroxene is not present as a phenocryst phase.

The lower pillow lavas of the Troodos ophiolite contain andesitic to dacitic glasses thatcrystallized plagioclase, augite, orthopyroxene, and titanomagnetite. Application ofmineral-melt and mineral-mineral geothermometers and consideration of distributioncoefficients indicate equilibrium relations and suggest early crystallization of plagioclase,followed by the more or less simultaneous appearance of augite and orthopyroxene. Thissequence is consistent with petrographic observations and suggests that evolution of thelower pillow lavas was controlled by relatively low-pressure phase equilibria. The high watercontent of fresh glasses and the systematic shift in the plagioclase-augite-orthopyroxenecotectic toward normative plagioclase suggest high water activity during crystallization ofthese lavas. Both groups of magmas probably evolved under relatively high water activitiesand low confining pressures (<3 kb).

ACKNOWLEDGEMENTS

The first author acknowledges grants from the Nordic Council, the Danish NaturalScience Research Council (DNSRQ, the NATO Science Fellowship Programme, and the

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Carlsberg Foundation. Part of the work was done while the senior author held a SeniorResearch Fellowship from Aarhus University, Denmark, and subsequently a NationalResearch Council-NASA Research Associateship at Johnson Space Center's experimentalpetrology laboratory. Microprobe analyses were carried out at the Nordic VolcanologicalInstitute, University of Iceland. INA analyses were provided by L. H. Christensen,Fors0gsanlaeg Riso, with support from DNSRC Comments on the manuscript from J. S.Beard, T. N. Irvine, D. A. Morrison, E. M. Moores, M. J. Rutherford, and J. R. Wilson werevery helpful.

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