Zoned Pyroxenes from Shoshonite Lavas of Lesbos, Greece: Inferences concerning Shoshonite Petrogenesis

Zoned Pyroxenes from Shoshonite Lavas of Lesbos,

Greece: Inferences concerning Shoshonite Petrogenesis

6y GEORGIA PE-PIPER

Department of Geology, Saint Mary's University, Halifax, N.S., B3H 3C3, Canada

{Received 29 May 1983, in revised form 21 September 1983)

(WITH THREE PLATES)

ABSTRACT

Samples from several stratigraphic levels in the Miocene volcanic succession on Lesbos, Greece,contain complexly zoned clinopyroxene and feldspar phenocrysts. Electron microprobe analyses havebeen made of zoned clinopyroxenes and feldspars, and their inclusions; and phenocryst mineralogy iscompared with host-rock chemical composition. The commonest pyroxene zoning sequence is cores ofgreenish salite or augite mantled by diopside which, in turn, may be rimmed by augite, with each typeof pyroxene in optical and chemical discontinuity with the next. None of the pyroxenes has highpressure characteristics. The Ts content of phenocryst cores is correlated with the K-content of thehost rock; and the most diopsidic overgrowths also occur in the most K-rich rocks. Augite rims occuronly in Al-rich rocks.

Most of the features of the pyroxenes can be accounted for by a hypothetical fractionalcrystallization scheme. However, two observations suggest that mixing of small magma batches mayalso be important. The earliest crystallized diopsides that rim salite are rich in Cr2O3, suggestingprecipitation from an unfractionated magma. Anorthoclase (implying advanced crystallization offeldspars) occurs as cores to feldspar phenocrysts and as inclusions in clinopyroxene cores in severalsamples.

I N T R O D U C T I O N

Recent reviews of the mineralogy of orogenic volcanic rocks have suggested that theclinopyroxenes in potash-rich rocks may differ from those in lower-K calc-alkali rocks. Ewart(1982) noted that whereas augite is characteristic of most calc-alkali rocks, there is 'atendency to extend into diopside and salite compositions in the higher K magmas'. Gill (1981)found a general 'increase in Wo content of augite as K-enrichment of the whole rockincreases', and showed that some clinopyroxenes in potash-rich andesites have high A12O3

contents.This paper presents new data on clinopyroxene in Miocene shoshonites from Lesbos,

Greece. Many of these pyroxenes show complex discontinuous zoning, resembling thatdescribed by Huckenholz (1973) in pyroxenes from alkali basalts, and unlike the regularzoning previously described from shoshonites (Joplin et al., 1972; Girod, 1975; Lefevre,1979). The clinopyroxenes include diopsides, augites and salites, many with A12O3 contents inexcess of 3 per cent. The relationship of clinopyroxene mineralogy to the chemicalcomposition of the host rock is examined.

Analytical methods

Mineral analyses are by energy dispersive electron microprobe (Clarke, 1976). Naturalmineral and synthetic oxide standards were used for calibration and the data were reducedusing the EMPADR VII program (Rucklidge & Gasparini, 1969). Whole-rock major-element determinations were by atomic absorption spectroscopy and trace elements byinstrumental neutron activation analysis and X-ray fluorescence (Pe-Piper, 1980ft). Large

IJounul of Petroloiy. Vol. 25. Part 2, pp. 453-4721 19841

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454 G. PE-PIPER

o

O

Ab

.85

O199

Sh IBa

A324 |

L ^A, , K-A

A247

-bA '

PHENOCRYSTS• A l u m i n o u s s a l i t eO S a l i t eA A u g i t e+ D iops ide on ly

50 %SiO 260

FIG. 1. KjO-SiOj plot showing shoshonite suhe nomenclature of Peccerillo & Taylor (1976), K2O and SiO,content of analysed samples, and composition of phenocryst cores. Ab = absarokite, Sh = shoshonite, BA =banakite, K-bA = potash-rich basaltic andesite, K-A = potash rich andesite. K2O content of sample 63 is

uncertain because of alteration.

numbers of petrographic thin sections have been examined, and representative samples rich inpyroxene phenocrysts were selected for microprobe analysis. More than 500 microprobeanalyses of pyroxenes, and 300 of other phases have been made.

Pyroxene atomic formulae have been calculated by dividing Fetot between Fe3+ and Fe2+

so that the sum of the X + Y cations = 2 (Hamm & Vieten, 1971). The components diopside(Di), hedenbergite (Hd), acmite (Ac) and total Ca-Tschermak's molecule (Ts) have beencalculated following Huckenholz (1973). The rock nomenclature of Peccerillo & Taylor(1976) has been used, and is shown in Fig. 1.

Stratigraphic setting

The Miocene volcanic rocks on the island of Lesbos, Greece, comprise a range of basic,intermediate, and acid rocks (Pe-Piper, 1980a). Most of these rocks have high primary potashcontents (Pe-Piper, 19806), and potash-rich basic rocks (absarokite and shoshonite sensustricto) occur at several stratigraphic levels. A chain of andesite stratovolcanoes extendsSW-NE across central Lesbos (Pe-Piper, 1980a), with thick acid pyroclastic rocks andsubsidiary volcanic centres on the eastern and western flanks (Fig. 2). The oldest volcanicrocks are altered absarokites, banakites and K-andesites of the Lower Lava Unit, which onthe flanks of the stratovolcanoes are overlain by agglomerates, and both air-fall andignimbritic tuffs of the Acid Volcanic Unit. The Skoutaros Unit of shoshonites, absarokites,and K-andesites interfingers with the margins of the ignimbrite sheets on the flanks of thestratovolcanoes, whereas it rests directly on the Lower Lava Unit in the central part of thevolcanic chain. It is followed by the Sykaminea Unit consisting of absarokites, K-andesitesand banakites. The Lower Lava, Acid Volcanic, Skoutaros, and Sykaminea Units wereerupted over a period of one to two million years in the mid-Miocene. The younger MytileneUnit comprises absarokites, K-basaltic andesites, and K-andesites, and in places overlieslower Pliocene marls.




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SHOSHONITE LAVAS FROM LESBOS

WEST

ggjMetamorphicbasement

Skoutaros unit

g ^ L o w e r | a v a unit E

ESSykaminea unit

Acid volcanic unit

Mytilene unit

Fio. 2. Schematic east-west cross-section of the volcanic rocks of Lesbos (from Pe-Piper, 1980a) showinglocation of analysed samples. For explanation of units, see text.

PETROGRAPHY AND MINERALOGY

Introduction

Clinopyroxenes have been studied in eleven representative samples. Sample localities andwhole-rock chemistry are shown in Table 1, phenocryst mineralogy is summarized in Table 2,rock nomenclature and K2O/SiO2 ratios are illustrated in Fig. 1, and stratigraphic location is

T A B L E 1

Chemical analyses and CIPW norms of samples examined in this study

Sample

SK),TiO,

A1P,Fzfi,FeOFeO,MnOMgOCaONapKfiffi,HP*Hp-CO,

Total

L125

52-840 87

17-213-523-716 8 80-144-868 473-322-800-400-531020-28

99-27

CIPW Norms0OrAbAnNeDiHy0)MlII

ApC

16-8928-6524-26

—

12-879-501-693-511-690-95—

Skoularos Unit

L24T

59-940-88

17-63——

4-520-082-996-913-453-180-22——

—

99-80

1062188029-1823-21

—

7-604 96—

3-451-670-51—

L32O"

59190 98

18-85——

6-690 0 51 164-783-714-340-33——

—

100-08

7-8225-5931-2921-48

—

7-48—

3-581-860 7 60 1 5

LJ24

52 900-83

16-752 654-456-840 1 4

4 1 38-753-253-210-371130-321-68

100-56

19-4928-2322-21

—

16146 431 533-471-620-88—

£ 7

61130-56

16-574-480-724-750 0 72-065-933-623-500-220-460-320 0 5

99-69

12-4120-9931-061818—

7-714-33—

3 031080-52—

Sykamlnea Unit

L85

49-690-79

13-292-793-956-460 1 37 0 08-402-463-760-702 0 90 734-72

100-50

—

23-9422-4015-18—

19-774-017-773-571-611-75—

53 030-66

1361——

6-800-127-21

10-332-374-290-66——

—

9908

—

25-5518-6813-910-82

26-75—

8-323 1 51-261-5-4

—

L123

58-160-59

14-613-481-70

0 0 84 1 37-873 052-980-211 630-860-55

99-90

9-9818-2226-6817-96

—

16-765-60—

3 1 31-160-50—

L63

54-400-58

14-663-731-554-910 1 04-408-832-752-270 1 01-850-863-89

99-97

8 47144024-9622-47

—

19026-00—

3-241-180-25—

Mylllent Unll

L7I

53010-69

16-441-295-366-520 1 37 0 08 0 43 0 91-760 1 00-670-212-08

99-88

0-2410-7426-9826-61

—

11 2020-71

—

1-931-350-24—

L126

49-050-97

14-752-585-077-390 1 4

10-459 152-562-350-352-420-320-25

100-11

—14-3119-69

23-85—

16-344-54

14-853-691-900-84

—

* These analyses have been made by electron rnicroprobe on glass discs. AD others are by atonuc absorption, with SiO, by netitron activation.




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456 G. PE-PIPER

TABLE2

Summary of the phenocryst assemblages and whole-rock geochemistry of the samples studied

Sample

MineralogyAluminous SalheSiliteAugite coreAugite rimOropstdcEarly augiteOtivineOrthopyroxeneHornWcndeM.ci

L247

EEE

0E

Whole rock geochemistry

S.O,(%)K(%)Rb (p.p.m.)K/RbSr (p.pjn.)La (p.p.m.)Ybm>

59-942-64

95139

103848-8

16

LJ20

EE

EEEO

E

59-193-60

1161557575102-9

L199

E

E

E

E

53033-56

115155855

L126

eEE

0

49051-96

77127511

13-74 9

L324

EEEO

E

52 902-66

97137898

Lai

E

E

O

54 40188

13958

621

L125

EE

O

52-842-32

93125888

39-21-5

Ul

E

E

53 011-46

61120569

LS5

O

O

O

O

49-693 1 2

118132905

3 5 08 1

£ 7

O

O

OOO

61132-90

124117651

3887-5

L123

E

EE

OO0

58-162-48

99125590

E = electron microprobe uialysu (e — microphenocrysts onJy).O •* optical determination.

+ ; + + + \ * x >^ x C a - A u g i t eL'U g i t e T o — -

* + /

/Sa l i t e / A l - s a l i t e

Hd + Ac TsFIG. 3. Selected clinopyroxene analyses plotted on a Di-Ts-(Hd + Ac) ternary diagram, showing nomenclature

used in this study. Symbols indicate clinopyroxene type based on optical appearance.

shown in Fig. 2. Discontinuous zoning in clinopyroxenes is recognized optically, and fiveprincipal types of clinopyroxene are recognized on the basis of both chemical compositionand optical properties. Fig. 3 shows reliable microprobe analyses from samples in whichclinopyroxene type can be determined optically, and is used to define the compositionalranges of the five pyroxene types. For convenience, these types are termed diopside, augite,




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SHOSHONITE LAVAS FROM LESBOS 457

Ca-Aug

FIG. 4. Sketches of clinopyroxene crystals showing location of analyses in Fig. 5.

OPTICAL ZONING—>— continuous-"• - -d iscont inuous

* Dark green• Light green• Colourlesso Mlcrophenocryst

FIG. 5. Representative analyses of zoned clinopyroxenes plotted on a Di-Ts-(Hd + Ac) ternary diagram.Numbers refer to analyses in Fig. 4.

calcic augite, aluminous salite and salite; they correspond closely but not exactly to thenomenclatures of Poldervaart & Hess (1951) based solely on chemical composition.Twenty-nine selected clinopyroxene analyses from the crystals shown in Fig. 4 are used inFig. 5 to illustrate the types of zoning found in clinopyroxene phenocrysts; chemical analysesof thirteen of these selected clinopyroxenes are given in Table 3. Zoning within individualclinopyroxenes is also illustrated on a standard Di—Hd—Fs—En diagram (Fig. 6), but this doesnot show variations in alumina.




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458 G. PE-PIPER

TABLE 3

Representative electron microprobe analyses of clinopyroxenes

SiO2

TiO2

A12O3

FeO t

MnOMgOCaONa,OCr2O3

Total

SiAl"Al*TiF e3+

Fe2+

MnMgCaNaKIV.

zX + Y +DiHd + AcTsMg/(Fe H

Z

Y

X

W

W

-Mg)

1Earlyaugite

51-180-422-509-900-30

15-6519-760-610-28

100-61

1-8840109

0-0120-1420-1630-0090-8590-7800044

1-9932 009

65-521-610-90-74

L247

3Diopside

51-650-292-705-410 0 8

17-7820-710-410-38

99-40

1-893010700100-0080109005700030-9710-8140-029

2-0002-001

80-48-9

10-70-85

4Augite

rim

50-390-642-17

10-860-25

14-7919-210-4O0 0 8

98-79

1-8990-096

001800970-24500080-8310-7760030

1-9952-005

62-128-39-60-71

5Augite

rim

51-960-451-958-650 1 5

16-6219-210-280-11

99-38

1-9270-073001200130-0550-21300050-9190-7630020

20002000

68-923-8

7-30-77

6Salite

49-470-744-868-570-25

13-3922-880-080 0 5

100-58

1-8360-1640049002100790-18700080-7410-9100060

20002001

63-52 0 116-40-74

L320

7Al-salile

44-401-838-57

10-680-10

100323-240-30—

9914

1-6910-30900760-0520-1510-18000030-5690-9480022

2-0002001

48-620-530-90-63

11Diopside

50-370-363015-780 1 1

150922-980 0 30-36

98-35

1-88701130 020001000950-08600040-8420-9220-022

2-0002-001

77-611-211-30-82

(Analyses are located in Fig. 3. Structural formulae calculated using method of Hamm and Vieten, 1971.)

The petrography and mineralogy of the six rock samples studied in detail are describedbelow, in stratigraphic order from oldest to youngest. Brief mention is also made of theremaining five samples.

Sample L247. K-andesite lava, Skoutaros Unit

This is a porphyritic rock with phenocrysts of clino- and orthopyroxene and plagioclase,microphenocrysts of clinopyroxene, plagioclase and altered olivine, and a groundmass ofpyroxene, plagioclase, K-feldspar, altered olivine and opaque minerals. Aggregates of largeclinopyroxene crystals, some with orthopyroxene, are common.

Most clinopyroxene phenocrysts are optically homogeneous, normally zoned diopside,with a thin augite rim (Fig. 5). Some such diopside to augite phenocrysts surround light greencores of augite. Some crystals show fine oscillatory zoning in both core and margin.

Orthopyroxene phenocrysts are slightly zoned with Mg-rich cores and rim compositions ofEn74 (Table 4). Some are rimmed by, or aggregated with, colourless diopside. Thisaggregation predates the precipitation of the thin outer rim of augite.




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TABLE 3: Continued

SiO2

TiO2

A12O3

FeO t

MnOMgOCaONa2OCr2O3

TotalSiAJ"

Al"TiFe3+

Fe2+

MnMg

CaNaKZX + Y +

DiHd-i-AcTsMg/(Fe -^

7 /

rj

L

Y

X

w

w

l-Mg)

MLI H I P

U20

12Ca-augite

49-231-045-046-930-24

14-3222-160-500-13

99-601-82301770-0430-02901110-1040-0080-7900-8790036—

20002000

67-514-817-70-79

L \J 11 I ii o—) increasing Ts-_ + _ .

35

d e c r e a s i n g Hd

40-/

//

50 V

' /45V

/

\40

18Augite

rim

51010-602-488-540-22

15-3920-690-36—

99-301-8980-1020-0070-01700880-17800070-8530-8250 026—

2-0002001

68-721-110-20-76

/

Oiopside__

L324

19Augite

rim

49180-863-639 1 10-39

14-9120-760-490 0 9

99-421-8310-169001000240165011900120-8270-8280-035—

20002020

• 66-516-616-90-74

Ca-augite^_

s" 1 /-oU j ^ *

\ bl6

50 . ..

23Saiite

48-670-294-50

13-660-26

10-5120-720-77—

99-401-8510-1490053000801360-29900080-5960-8440-057

20002-001

48-736-414-90-58

L199

28Diopside

53050-191064-370-11

18092f-98

0 1 50-74

99-741-9470046—

00050060007400030-9900-8640011—1-9932-007

86-68-84-60-88

A l - s a l i t e

26

A u g i t e

/

\60

—'

29Diopside

50-400-713-086 1 80 0 8

15-7922-69

0 1 10-34

99-401-86901310-00400200-0950-09700030-8730-9020008—

2-0002-002

76-110-81310-82

\70

% Mg

Fio. 6. Representative zoned pyroxene analyses plotted on a Ca-Mg-Fe ternary diagram. Numbers refer toanalyses in Fig. 4.




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SiO,TiO2A1,O,FeO,MnOMgOCaONa2OK2O

Total

/

40-35——

12-880-20

47-09———

100-52

Atomic formulae101SiTiAlFeMnMgCaNaK

40-997——

0-26600041-735———

TABLE4

Typical electron microprobe analyses ofolivine, orthopyroxene, mica

2

3919——

19-350-32

41-88—

—

101-24

40-997——

0-4120-0071-588———

3

38-30——

24-240-47

38-45———

101-46

40-991——

0-52500101-483———

Olivtne

4

38-19——

25-450-50

37-78———

101-52

40-993——

0-55400111-449———

5

40-74——

11120 1 6

47-490-21——

99-98

41-007——

0-23000031-7490006——

6

39-49——

17-400-38

42-560-210 1 7—

100-45

41002——

0-36900081-60900060-008—

7

38-71——

21-630-46

38-700-210 0 7—

99-79

41006——

0-47000101-4990-0060004—

and amphibole

Orthopyroxene

8

56090 1 0118

15060-35

25-621-92——

100-33

62009000300500-45100111-3670-074——

9

55-530-461-04

14-680-39

25-621-980 1 1—

99-79

6200000130-0440-44200121-3750-0760008—

10

55-300-36106

15-350-38

25-301-900-10—

99-74

6200000100-0450-46400121-3630-0740-007—

Mica

11

37-203 1 4

15-529-81—

19-23—

0-728-75

94-54

225-4670-3472-6891-206—

4-211—

0-2051-640

12

41114-82

11-989-03

1805

0-659 1 5

94-79

225-9670-5262-0501096—

3-904

01831-694

Amphibole

13

40-533-50

13-7510-560 0 9

14-3611-762 1 31-28

97-96

235-9590-3872-3831-299001131471-8530-6070-240

460

O

•aro•v-amTO

1-2: Olivine. Phenocryst: core, margin. L71.3: Olivine. Microphenocryst. L71.4: Olivine. Groundmass. L71.

5-7: Olivine. Phenocryst: core, margin. LI26.8—9: Orthopyroxene. Phenocryst: core, margin. L247.

10: Orthopyroxene. Microphenocryst. L247.11: Mica. MicrophenocrysL LI99.12: Mica. Microphenocryst L324.13: Amphibole. Phenocryst L320.




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C • coreR • rim

R. m p C mpc • microphenocryst

247 range of composition— » — zoned compositionooooo sieve structure: ' abrupt contact

oo ooooooooopooooooooooooooo

Ri - 324

— mpc R

C00900

320

50 60 % A n 70 80

FIG. 7. Diagram showing types of complex plagioclase zoning in representative samples.

TABLE 5

Inclusions in zoned phenocrysts

Sample Phenocryst

Labradorite cores Early augite Sallte Dlopside

L324 — AnI3 — San, PhlogL247 Cpx, Opx, Qz Opx, ?An43 — AnM, ?O1, KFL32O Anorth Ap — Opx, QzL63 — — — QzLI 99 — — (Mg,Fe)CO3 ApL123 — Anorth — —

Many plagioclase phenocrysts (Fig. 7) have normally zoned more sodic cores (An34 toAnJ0) surrounded by zones with sieve structures and then more calcic rims (An37_6,). Themost calcic compositions (An39_61) are found on some phenocryst margins and in thegroundmass; these compositions probably co-precipitated with the groundmass sanidine (Fig.8). Inclusions of augite in the more sodic plagioclase cores and of plagioclase (An6)) in augite(Table 5) indicate some co-precipitation of these two phases. Plagioclase cores also containinclusions of orthopyroxene, quartz and opaque minerals.

Sample L320. Banakite lava, Skoutaros Unit

This is a porphyritic rock with clinopyroxene, plagioclase and altered hornblendephenocrysts. The same minerals, together with K-feldspar, opaque minerals and alteredolivine, occur as microphenocrysts. The groundmass minerals are too fine to identify.




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462 G. PE-PIPER

( - - ^ E n v e l o p e s o( p lag loc laseana lyses

« + Ind iv idua l p h e n o c r y s t a n a l y s e s ,samples 324. 123

e e I n d i v i d u a l m l c r o p h e n o c r y s t org r o u n d m a s s a n a l y s e s , samples 324, 123

o O the r m l c r o p h e n o c r y s t ana l yses

A n o r t h o c l a s e ana lyseso r i m s• I n c l u s i o n s

Fio. 8. Feldspar analyses plotted on a Ab-An-Or ternary diagram. Tie-lines indicate coexisting phases.

Most ciinopyroxene phenocrysts are colourless diopsides similar to those in L247,containing inclusions of quartz and orthopyroxene (Table 5). There are also large, dark greenaluminous salite crystals with a narrow colourless diopside rim (Fig. 5). The aluminous salitesare euhedral, with irregular, anhedral, embayed, lighter-coloured salite inclusions. Anothertype of ciinopyroxene phenocryst comprises euhedral calcic augite surrounding severalrounded crystals of diopside which probably represent corroded aggregates. There are alsooccasional large corroded augite phenocrysts with apatite inclusions (Table 5). Mostplagioclase feldspars (Fig. 7) have labradorite (AnJ6_j9) cores with rims of andesine (An49). Afew have cores with sieve texture of either andesine (An30) or anorthoclase (Fig. 9). Theandesine cores contain anorthoclase inclusions.

Sample L324. Shoshonitic stock, Skoutaros Unit

This is a holocrystalline rock with ciinopyroxene, plagioclase and altered olivinephenocrysts, and microphenocrysts and a groundmass of the same minerals plus K-feldspar,anhedral phlogopite mica (Table 4) and opaque minerals. Aggregates of ciinopyroxene, mica,and K-feldspar phenocrysts occur. Some plagioclase phenocrysts (Fig. 7) have calcic cores(An80_86), a zone with sieve textures or fine banding (An67_78) and then usually more sodicrims (Ansl_M). Microphenocryst compositions range from An42 to An38; groundmassplagioclase is similar or more sodic. In some other phenocrysts, there is only normal zoningwith a maximum range An7g to An31. Others show reverse zoning; the cores are sodic withsieve structure (An43) and the rims are calcic (An70).

Three types of ciinopyroxene phenocryst are recognized (Fig. 5):(1) those with colourless diopside cores and greenish augite or calcic augite overgrowths




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zoned rims

dlopslde

cores orcorrodedphenocrysts

L247

SKOUTAROS

L320 L324 L125

I * . * *"Aug

An.,

12CaAug

t V

to-P 21-22CaAug

8-19Aug

\J\An.

D-BDi

Aug

Aug

7eAl-Sa

6 9-CSa Aug

20Aug

SYKAMINEA

L123 L199

Aug

AnS4

I A n rCaAug

An80-66

MYTILENE

L63 L126

An,66-81

Anortrf

Aug

232B-29

Oi

An80-81

Ana-65

wSa

Sa •Aug

Fio. 9. Summary of zoning observed in clinopyroxene and plagioclase phenocrysts. lines join phases observed inindividual zoned crystals. 'Aug indicates augite zoned with decreasing Hd; all other pyroxenes are zoned with

increasing Ts. Numbers 1 to 29 refer to analyses in Figs. 4 and 5. A = aggregation, C = corrosion.

with fine oscillatory zoning near the rim. The cores are cracked and embayed, with sanidineand phlogopite in the interstices.

(2) greenish augite phenocrysts and microphenocrysts that are very similar to the greenishovergrowths of type 1;

(3) large embayed crystals of almost uniform augitic composition containing inclusions ofbytownite and opaque minerals (Table 5).

Sample L125, from a shoshonite dyke in the Skoutaros Unit, is petrographically andmineralogically similar to sample L324. Clinopyroxene phenocrysts are diopsides with augiteovergrowths.

Sample L123. K-andesite lava, Sykaminea Unit

This sample is a porphyritic rock with phenocrysts and microphenocrysts of plagioclase,K-feldspar, clinopyroxene, orthopyroxene, quartz, altered biotite, hornblende, and very rareapatite. The groundmass is made of colourless glass (commonly devitrified to spherulites)with feldspar, pyroxene, zircon, iron oxide and some biotite.

Complexly zoned plagioclase phenocrysts contain dusty cores of either anorthoclase orbytownite (~An81), overgrown by labradorite (An81 to An66), with outermost rims havingcompositions between An79 and An49. Microphenocrysts and groundmass have compositionsbetween An62 and An69. Clear, subhedral, optically unzoned phenocrysts and mic-rophenocrysts of sodic andesine (An34-An41) are also found.

Simple clinopyroxene phenocrysts are normally zoned diopsides, often in aggregates; someare rimmed with augite. One augite rim has a composition similar to salite. Complexphenocrysts have greenish or colourless augite cores with diopside or calcic augite rims. Oneis zoned from an augite core to calcic augite to augite again.

Sample L7, also from the Sykaminea Unit, is petrologically and mineralogically similar.

Sample L199. Absarokite dyke, Sykaminea Unit

This is a vesicular rock with clinopyroxene phenocrysts and microphenocrysts ofclinopyroxene, altered olivine, plagioclase, K-feldspar, and phlogopite (Table 4). Thegroundmass consists of plagioclase, K-feldspar, clinopyroxene and opaque materials. The




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plagioclase is labradorite (An61_65). Textural relations suggest that the labradorite andK-feldspar have co-precipitated (Fig. 8).

Many clinopyroxene phenocrysts consist of colourless diopside; microphenocrysts andgroundmass pyroxene have similar compositions. Some phenocrysts have green salite coresthat are often rounded and embayed, with rims of colourless diopside.

Sample L85, from a nearby locality, is petrographically and mineralogically similar.

Sample L63. K-basaltic andesite lava, Mytilene Unit

This lava has phenocrysts and microphenocrysts of clinopyroxene, plagioclase (An66-An72), and calcite pseudomorphs after olivine. In addition to diopside phenocrysts, it containsrare dark green salite as cores to diopside. It also contains embayed quartz crystals,sometimes rimmed with colourless diopside prisms. The groundmass contains K-feldspar.This sample is altered and the whole-rock chemical analysis is suspect (Fig. 1).

Sample L71 is a K-basaltic andesite lava from the Mytilene Unit and has diopside andolivine phenocrysts.

Sample LI26 is an absarokite lava from the Mytilene Unit with large olivine crystals andrare augite in a matrix of abundant K-feldspar, plagioclase (An39-An63), diopside, Fe-Tioxides, mica and smaller olivine crystals.

PYROXENESTypes of pyroxene

The chemical composition of each of the five distinct types of clinopyroxene issummarized in a diopside-acmite plus hedenbergite-Tschermak's molecule ternary diagram(Figs. 3 and 5). Selected samples are also plotted on a wollastonite-enstatite-ferrosilite ternarydiagram (Fig. 6). Selected chemical analyses are given in Table 3.

PLATE 1. Green salite rimmed by colourless diopside. L199, x 100, x-polars.




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S H O S H O N I T E LAVAS F R O M LESBOS 488

PLATE 2. Zoned and embayed diopside with cracks and voids filled with colourless sanidine, rimmed by calcicaugite with oscillatory zoning. L324, x 100, x-polars.

PLATE 3. Diopside phenocrysts. Lower crystal shows mosaic texture due to aggregation of phenocrysts duringcrystal growth. L32O, x 100, x-polars.




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(1) Light green salite, occurring as cores to phenocrysts (analyses 6 and 23 in Table 3; alsoanalyses 24, 26, 27). The salites typically contain high FeOt, ranging from 8 to over 14per cent. A12O3 is between 4-5 and 8 per cent and Na2O from 0-7 to 0-9 per cent in mostsamples. TiO2 is variable, but less than 0-8 per cent. Cr2O3 is low.

(2) Dark green aluminous salite, again as cores to phenocrysts, but rimming light greensalite (analysis 7 in Table 3; also analysis 8). The aluminous salite contains around 8-5 percent A12O3, 8-5—11 per cent FeOt, 0-6-1-8 per cent TiO2, and relatively low Na2O (0-3 percent).

(3) Diopside as phenocrysts or overgrowths on augite, salite or aluminous salite cores,frequently aggregated or corroded (analyses 3, 11, 28, 29 in Table 3; also analyses 13, 14,25). Typical diopsides contain 1-5 per cent A12O3, 4—6 per cent FeOt, low Na2O (<0-2 percent) and TiO2, and a strong zoning of Cr2O3 from as much as 1-5 per cent in cores to lessthan detection limits at the rims.

(4a) Colourless or light green augite, occurring as corroded phenocrysts, and occasionallyas cores to diopside (analysis 1 in Table 3; also analyses 2, 9, 10, 20) (early augite). (4b)Colourless or light green augite occurring as microphenocrysts, and as rims on diopside,chemically similar to the corroded augite phenocrysts (analyses 4, 5, 18, 19 in Table 3; alsoanalyses 16, 17). Both augites have 2-6 per cent A12O3, 8-11 per cent FeOt, and relativelyhigh TiO2 (generally 0-6-0-9 per cent). Both Na2O and Cr2O3 are very low.

(5) Colourless calcic augite rimming aggregates of diopside (analysis 12 in Table 3; alsoanalysis 22). It has a chemical composition similar to the augites, except that the Ca/(Mg +Fe) ratio is higher.

The salite cores are usually embayed, suggesting corrosion. Diopside cores are corroded insamples L32O and L324, and in the latter they also are cracked. Chemically similar diopsidesthat surround salites are neither corroded nor cracked. There are no gradual compositionalchanges from one clinopyroxene type to another. Where a phenocryst consists of two or moreclinopyroxene types, there is a compositional gap between them.

Each of these clinopyroxene types exhibits gradual normal zoning with enrichment in theTschermak's component from core to rim, with an increase in Ti, Al, Fe and Na (Figs. 5, 6).The earliest diopsides are Cr-rich, but Cr-content drops rapidly towards the margin ofdiopside crystals. Augite or calcic augite that rims diopside shows two types of zoning:normal zoning by enrichment in Tschermak's component, and zoning by depletion in the Hdcomponent, with corresponding enrichment in Di (Figs. 5 and 6). In addition, some samplesshow oscillatory zoning.

Chemical interpretation of the pyroxenes

Although the complex zoning of the Lesbos pyroxenes is unusual, their chemicalcomposition lies within the range recognised for K-rich calc-alkali rocks and shoshonites(Gill, 1981; Ewart, 1982). For example, similar salites are described by Pe (1973) and Lefevre(1979); diopsides by Joplin et al. (1972) and Pe (1974); and augites by Girod (1975) andFairbrothers et a/..(1978).

The discriminant function diagrams of LeBas (1962) and Nisbet & Pearce (1977) suggestthat the diopside, calcic augite and augite have calc-alkaline character, whereas the aluminoussalite is alkaline. However, the aluminous salite is unlike most pyroxenes described fromalkali rocks in its low Na and Ti contents (Fig. 10), which are lower than in the otherwisesimilar 'fassaitic augites' of Huckenholz (1973).

Wass (1979) suggested that the atomic composition ratios (Ti + Al^/Si, Al^/Al* andTiO2/(Mg/Mg + Fe) (Fig. 10) may be useful as clinopyroxene geobarometers. Because apartition of Fe2+ and Fe3+ within Fetot must be assumed, estimates of these ratios are




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3-|

2-

1-

8»

T

©

^

23

* *

0.5 0.6 0.7Mg

0.8/ (Mg + Fe)

0.9

FIG. 10. Hot of TiO2 vs. Mg/(Mg + Fe^J for pyroxenes in Figs. 4 and 5. H1-H4 are the four pyroxene types insample 136 of Huckenholz (1973). Fel01 assumes all iron as FeO.

imprecise. Nevertheless, only the aluminous salites show any tendency towards high pressurecharacteristics, but this tendency is slight (e.g. the Al^/Al'* ratio of 1:4 is within the rangefound for quench pyroxenes). Since the aluminous salites form rims to salite lacking highpressure characteristics, it is concluded that none of the pyroxenes present crystallized underhigh-pressure (mantle) conditions.

Although some diopsides (e.g. analyses 25, 28) resemble diopsides from peridotitexenoliths (Huckenholz, 1973; Wass, 1979) in their high SiO2 and Cr2O3 content, they differ intheir low A12O3 content. Furthermore, these chrome diopsides rapidly zone to almostchrome-free crystals.

FELDSPARS

Like the clinopyroxenes, the plagioclase phenocrysts show discontinuous zoning, withnormally zoned cores separated from normally zoned rims by zones with sieve texture (Fig.7). In most samples, phenocryst rims and microphenocrysts are in the range AnJ7-An62 and,like the clinopyroxenes, may show oscillatory zoning. The exception is L320, wherephenocryst rims are An49 overgrowing cores of An3i-An39; in this sample, the finalclinopyroxene is a calcic augite. In many samples, sodic labradorite phenocrysts(AnJ(rAii37) are common, with zoning outwards to the more sodic compositions. These maybe separated by a zone with sieve structure from the An j7-An62 rims. In L324, sodiclabradorite phenocrysts have cores of An43. In L247, diopside contains sodic labradoriteinclusions. Phenocrysts with bytownite cores ( A n ^ A n ^ ) are found in samples L324 andL123.




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The plagioclase that co-precipitated with a particular pyroxene can be inferred bycomparing zoning in the two minerals, and using evidence from inclusions. Bytownite is foundonly in samples with early augites, and occurs as inclusions in the early augite in L324.Bytownite is overgrown discontinuously by sodic labradorite, and early augites areovergrown discontinuously by diopside. Furthermore, in L247, diopside contains sodiclabradorite inclusions. This suggests that sodic labradorite and diopside co-precipitated.Augite rims on diopside are associated with AnJ9_61 rims on plagioclase; calcic augite rimswith An49 or anorthoclase. Samples with salite cores rimmed by diopside contain only simplyzoned sodic labradorite: since this plagioclase is normally associated with diopside, the salitemay have crystallized before any feldspar.

Potash feldspars are present in the groundmass of all samples. In L324, sanidine fillscracks in aggregated diopside crystals. Anorthoclase (Fig. 8) occurs in the cores ofplagioclase phenocrysts in both L320 and L123. Potash feldspar microphenocrysts have alarge sodium content (An33_47) in all samples where crystals are large enough to analyse (Fig.8).

The relationship between plagioclase and alkali feldspar composition is illustrated in Fig. 8.The tie-lines between coexisting plagioclase and sanidine are similar to those found byRahman & MacKenzie (1969) from trachytes at the Island of Ischia, Italy. The trend ofsanidine compositions in a single rock is similar to that found by Nicholls & Carmichael(1969) from shoshonites from the Absaroka volcanic field. The final stages of crystallizationof the feldspars in most of the Lesbos rocks are thus in no way unusual. In L125, however,anorthoclase which rims plagioclase phenocrysts has unusual compositions which lie close tothe common composition of plagioclase and alkali feldspars postulated by Carmichael et al.(1974, fig. 5-3). Similar anorthoclase occurs in the cores of plagioclase in LI23 and L320,suggesting that a batch of magma may have almost totally crystallized and was thensubsequently disrupted and incorporated as xenocrysts in a later magma.

RELATIONSHIP OF MINERALOGY TO WHOLE ROCK CHEMISTRY

There is a close correlation between clinopyroxene mineralogy and large ion lithophileelement (LILE) enrichment in the studied shoshonites. Salites or aluminous salites occur inrocks with high potash (Fig. 1) and other LILE (Table 2). Where analyses are available, theserocks show intermediate REE depletion (shown by high Yb/Tb ratio in Table 2). Rocks withdiopside only are least potassic, while those with early augite phenocrysts are intermediate inpotash content (Fig. 1).

A simple rank correlation test between the CaTs content of salite or early augite andcertain chemical species shows good correlation with K {r\ = 0-64, p < 0-01). There is apoorer correlation with other LIL elements such as Rb (rj = 0-29, p < 0-1), and Sr (r^ =0-21, p < 0-02), which probably reflects only the covariance of these elements with K.

Furthermore the most Mg-rich diopsides occur in the most potassic rocks: LeBas (1962)observed a similar trend in basalts. The exception to this trend is sample L32O, which is theonly analysed sample to contain hornblende, and has low Mg content in diopside. Theremaining samples show a rank correlation coefficient r\ = 0-81 (p < 0-1). Augite or calcicaugite rims are almost restricted to rock samples with more than 16 per cent A12O3 (x

2 — 2-23,p - 0-1). Thus, there appears to be a systematic and probably genetic relationship betweenthe occurrence of salites and aluminous salites and the enrichment of shoshonitic magma inK2O.

Neither soda nor titania content show any systematic variation with mineralogy (Tables 1,3). No systematic variations in clinopyroxene composition are seen between rocks that are




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quartz normative and those that are olivine normative or between those with modal quartzand orthopyroxene and those with modal olivine. Neither is there any systematic variationbetween Fe/Mg ratio in the rock and in clinopyroxenes (contrast Gill, 1981, p. 174).

It therefore appears that the amount of potash and alumina in the melt exerted animportant influence on the type of pyroxene that has crystallized. Unfortunately, there are fewdata applicable to crystallization of aluminous pyroxenes from melts with a large orthoclaseor anorthite component (Hoover & Irvine, 1978).

HISTORY OF CRYSTALLIZATION

Despite the diverse ages and stratigraphic settings of the various samples, the same generalcrystallization trends are seen in most samples (Fig. 9).

(a) Cores of some clinopyroxene phenocrysts consist of salite, and exceptionally aluminoussalite (L32O), with rather variable compositions, showing a normal zoning with Fe-enrichment. Salite occurs only in K-rich rock samples and appears to have crystallized beforeany feldspar. The crystallization of salite is followed by resorption in some samples, and thendiopside has crystallized in most samples.

(b) Diopside has crystallized in most samples, and in some it mantles salite cores.Early-formed crystals of diopside are Cr-rich. Labradorite co-precipitated with the diopside,which also contains inclusions of phlogopite, K-feldspar, and quartz.

(c) In some samples, diopside is aggregated, which may suggest that it was a cumulusphase.

(d) In some samples, all of which are in the Skoutaros Unit, diopside is rimmed by augite orcalcic augite, zoned with increasing Ts component outwards. It may be finally rimmed byaugite zoned with decreasing Hd component. Augite co-precipitated with labradorite(An59_6l); calcic augite with andesine (An49) or anorthoclase.

(e) Some samples also contain optically unzoned corroded phenocrysts of augite, and inLI23 and L247 similar augites form cores to diopside. In many samples with these earlyaugites, the diopside crystals also have augite rims and none of the diopsides has a high Crcontent. Augite phenocrysts occur in rocks with intermediate K contents.

PETROGENESIS

Most authors would agree that the petrogenesis of orogenic volcanic rocks, including theshoshonites, involves a variety of processes which leave their particular evidence in the finalrocks. The enrichment of shoshonites in potash is probably the result of mantle processes(e.g. Gill, 1981). The complex zoning of the clinopyroxene phenocrysts is apparently alow-pressure phenomenon.

The composition of clinopyroxenes crystallizing from a melt is a function of temperature,pressure, melt composition (particularly SiO2 and Fe3+ activities) (Barberi et al., 1971;Robinson, 1980), cooling rate (Papike et al., 1976; Huebner & Turnock, 1980), oxygenfugacity (Kushiro, 1973) and co-existing minerals. Discontinuous zoning accompanied bydisequilibrium features probably results from magma mixing (Sakuyama, 1978), althoughreverse zoning may also result from a pressure decrease (Ewart et al., 1975),/O2 increase(Luhr & Carmichael, 1980) or the onset of hornblende precipitation (Helz, 1973; Ferguson,1978). Normal zoning probably reflects isobaric fractionation, or isothermal ascent ofhydrous magma, or any combination of the two.

Variation in chemical composition between salites and diopsides (particularly in Al andNa) may be the result of crystallization at different pressures, although no phases suggesting> 10 kb pressure have been observed. However, the relationships between the chemical




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composition of clinopyroxene and of the host-rock suggest that many of the variations inclinopyroxene mineralogy are principally the result of fractional crystallization from melts ofslightly differing compositions.

Experimental investigations of pyroxene crystallization generally have not distinguishedbetween different types of Ca-rich clinopyroxenes (Huebner, 1980, pp. 213-215).Furthermore, there have been few investigations of crystallization from potash- andalumina-rich melts (Hoover & Irvine, 1978). As a result, it is difficult to interpret the Lesbospyroxenes in the light of experimentally determined phase relationships.

A hypothetical fractional crystallization sequence can be postulated from our observationsto account for most of the observed variations in clinopyroxene mineralogy. The firstclinopyroxene phase to crystallize appears to depend on the potash content of the magma. InK-rich magma, the first clinopyroxene is salite; with the appearance of labradorite at theliquidus, aluminium activity decreases and diopside begins to crystallize. In less potash-richsamples, augite and bytownite crystallize first; diopside again crystallizes with the appearanceof a particular phenocryst phase, perhaps hornblende or phlogopite. Augite or calcic augiterims occur only in rocks with high Al content, with calcic augite related to precipitation ofanorthoclase. Both show complex, often oscillatory, zoning similar to that described by Ewart(1976). Although this model elegantly explains the observed mineralogy, it must beemphasized that it is entirely hypothetical.

However, the disequilibrium features and discontinuous zoning in the clinopyroxenephenocrysts might suggest that magma mixing has also been a significant process. Furtherevidence of mixing is seen in the distribution of anorthoclase, which is common in many ofthe rock samples, and represents final crystallization of feldspar from a magma. It occurs notonly interstitially in the groundmass and mantling phenocrysts, but also as cores to complexfeldspar phenocrysts in L123 and L32O.

Another relevant observation is that the earliest diopside phases mantling salite are oftenvery rich in Cr, which then declines rapidly. Cr is rapidly removed during early stages offractional crystallization by crystallization of pyroxenes and iron oxides (Campbell & Borley,1974): its abundance thus suggests that the 'diopside parent magma' was essentiallyunfractionated.

In most rocks there is evidence for the mixing of two magma batches. Batch I precipitatedeither augite or salite and aluminous salite. These phenocrysts were picked up andincorporated in batch II which precipitated diopside. In one rock (L320), there may havebeen mixing with two batches, one with early augite and one with salite phenocrysts.

It is unclear whether these magma batches were quite distinct in character, or whethermixing took place at the margins of an essentially continuous magma conduit. If there werelarge-scale mixing of magma batches of very different composition, it would be difficult toaccount for the correlation of mineralogy with host-rock geochemistry, or the systematicdiscontinuous zoning sequences. There is no evidence of mixing of an alkali magma with acalc-alkali magma as suggested by Prostka (1973) to produce shoshonites, and the low TiO2

contents argue against such a hypothesis.Since similar evidence of mixing occurs at several stratigraphic levels, it is unlikely that the

mixing is a chance process. More probably, the magma batches were derived from a commonsource. The REE spectra from Lesbos suggested to Pe-Piper (1980) that shoshonites andassociated calc-alkali rocks were derived from partial melting of a rising LILE-enrichedmantle diapir. Small amounts of partial melting would result in K-enriched magma (the"salite parent magma"), while further melting would result in lower K-content and highercontents of compatible elements such as Cr (the "diopside parent magma")- Such a modelcannot be further evaluated without better information on fractional crystallization processesin K- and Al-rich shoshonitic magmas.




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CONCLUSIONS

The Miocene shoshonite suite of Lesbos, Greece, includes many rocks with complexlyzoned clinopyroxene and feldspar phenocrysts. None of the pyroxenes appears to be mantlexenocrysts; all appear to be the product of low-pressure fractional crystallization.

There are good correlations between pyroxene composition and whole-rock composition.The Ts component of augite or salite cores is correlated with whole-rock potash content; themost diopsidic compositions mantling these cores occur in the most potash-rich rocks; andaugite rims occur only in alumina-rich rocks.

There are insufficient experimental data on the composition of Ca-rich pyroxenescrystallizing from anorthite or orthoclase-rich melts. However, many of the observedmineralogical features can be accounted for by a hypothetical fractional crystallizationscheme. In potash-rich magma, salite crystallizes first; with the appearance of labradorite atthe liquidus, diopside begins to crystallize. In less potassic magmas, augite and bytownitecrystallize first; diopside again crystallizes next, perhaps with the appearance of hornblendeor biotite at the liquidus.

There are several indications that magma mixing is in part responsible for the observedmineralogy. Anorthoclase (implying complete crystallization of feldspars) occurs in cores ofboth feldspar and clinopyroxene phenocrysts. The earliest crystallized diopsides that rimsalite are rich in Cr2O3, suggesting crystallization from an unfractionated magma.

Clinopyroxenes chemically similar to those in Lesbos occur in other K-rich calc-alkalineand shoshonitic rocks, supporting the interpretation that the occurrence of both diopside andsalite is related to chemical composition of the melt.

The characteristics of these pyroxenes are thus probably best explained by fractionalcrystallization models; but more experimental data are needed to make a rigorousinterpretation. The complex zoning has not been described from other shoshonites; thisfeature may result, partly or entirely, from magma mixing processes.

ACKNOWLEDGEMENTS

Microprobe analyses were made at Dalhousie University. I wish to thank R. M. MacKayfor his assistance in the microprobe laboratory. I also thank Dr. H. G. Huckenholz for reviewof an early draft of this manuscript; Dr. D. B. Clarke for a discussion and the anonymousreferees for their suggestions. Laboratory work was partly supported by an NSERCoperating grant.

REFERENCESBarberi, F., Bizouard, H., & Varet, J., 1971. Nature of the clinopyroxene and iron enrichment in alkalic and

transitional basaltic magmas. Contr. Miner. Petrol. 33, 93-107.Campbell, I. H., & Borley, G. D., 1974. The geochemistry of pyroxenes from the Lower Layered Series of the

Jimberlana Intrusion, Western Australia. Ibid. 47, 281-97.Carmichael, I. S. E., Turner, F. J., & Verhoogen, J., 1974. Igneous Petrology. McGraw-Hill.Clarke, D. B., 1976. Petrological applications of microbeam techniques. In: Smith D. G. W., (ed.) Short Course in

Microbeam Techniques. Mineralogical Association of Canada.Ewart, A., 1976. A petrological study of the younger Tongan andesites and dacites and the olivine tholeiites of

Niua Fo'ou Island, S.W. Pacific. Contr. Miner. Petrol. 58, 1-21.1982. The mineralogy and petrology of Tertiary-Recent orogenic volcanic rocks: with special reference to

the andesitic-basaltic compositional range. In: Thorpe, R. S. (ed.) Andesites: Orogenic Andesites andRelated Rocks, London: Allen & Unwin, 25-87.Hildreth, W., & Carmichael, I. S. E., 1975. Quaternary acid magma in New Zealand. Contr. Miner. Petrol.

51, 1-27.Fairbrothers, G. E., Carr, M. J., & Mayfield, D. G., 1978. Temporal magmatic variation at Boqueron Volcano, El

Salvador. Ibid. 67, 1-9.Ferguson, A. K., 1978. The crystallization of pyroxenes and amphiboles in some alkaline rocks and the presence

of a pyroxene compositional gap. Ibid. 67, 11-15.Gill, J., 1981. Orogenic Andesites and Plate Tectonics. Springer-Verlag.




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Girod, M., 1975. Donnees petrologiques sur une shoshonite de Stromboli (lies Eoliennes). Petrologie, 10, 189-96.Hamm, H. M., & Vieten, K., 1971. Zur Berechnung der Kristallchemischen Formel und des Fe^-Gehaltes von

Klinopyroxen aus FJektronstra-Mikroanalysen. Neves Jb. Mineral. Mh. 7, 310-14.Helz, R. T., 1973. Phase relations of basalts in their melting range at Pnja = 5 kb as a function of oxygen fugacity.

J. Petrology, 17, 139-93.Hoover, J. D., & Irvine, T. N., 1978. Liquidus relations and Mg-Fe partitioning on part of the system

MgjSiO^FejSiO.-Ca MgSijCy-CaFeSij-KAlSijCVSiO,. Yb. CarnegieInstn. Wash. 77, 774-84.Huckenholz, H. G., 1973. The origin of fassaitic augite in the alkali basalt suite of the Hocheifel area, Western

Germany. Contr. Miner. Petrol. 40, 315-26.Huebner, J. S., 1980. Pyroxene phase equilibria at low pressure. In: Prewitt, C. T. (ed.) Pyroxenes. Reviews in

Mineralogy!, 213-88.& Turnock, A. C , 1980. The melting relations at 1 bar of pyroxenes composed largely of Ca-, Mg- and Fe-

bearing components. Am. Miner. 65, 225-71.Joplin, G. A., Kiss, E., Ware, N. G., & Widdowson, J. R-, 1972. Some chemical data on members of the

shoshonite association. Miner. Mag. 38, 936—45.Keller, J., 1974. Petrology of some volcanic series of the Aeolian arc, southern Tyrrhenian Sea: calc-alkaline and

shoshonitic associations. Contr. Miner. Petrol. 46, 29-47.Kushiro, I., 1973. Crystallization of pyroxenes in Apollo 15 mare basalts. Yb. Carnegie Instn. Wash. 72, 647-50.Le Bas, M. J., 1962. The role of alumina in igneous clinopyroxenes with relation to their parentage. Am. J. Sci.

260, 267-88.Lefevre, C , 1979. Un exemple de volcanisme de marge active dans les Andes du Perou (sud) du Miocene a

l'actuel. These D.is Scl. Nat., Universite des Sciences et Techniques du Languedoc.Luhr, J. F., & Carmichael, I. S. E., 1980. The Colina volcanic complex, Mexico. Contr. Miner. Petrol. 71, 343-72.Nicholls, J., & Carmichael, I. S. E., 1969. A commentary on the absarokite-shoshonite-banakite series of

Wyoming, U.S.A. Schweiz. miner.petrogr. Mitt. 49, 47-64.Nisbet, E. G., & Pearce, J. A., 1977. Clinopyroxene compositions in mafic lavas in different tectonic settings.

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mineralogy, and petrology. Rev. Geophys. Space Phys. 14, 475—450.Pe, G. G., 1973. Petrology and geochemistry of volcanic rocks of Aegina, Greece. Bull. Volcan. 37,491-514.

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19806. Geochemistry of Miocene shoshonites, Lesbos, Greece. Contr. Miner. Petrol. 72, 387-96.Peccerillo, A., & Taylor, S. R., 1976. Geochemistry of Eocene calc-alkaline volcanic rocks from the Kastamonu

Area, Northern Turkey. Ibid. 58, 63-81.Poldervaart, A., & Hess, H. H., 1951. Pyroxenes in the crystallization of basaltic magma. / . Geol. 59, 472-89.Prostka, H. J., 1973. Hybrid origin of the absarokite-shoshonite-banakite series, Absaroka volcanic field,

Wyoming. Bull. geol. Soc. Am. 84, 697-702.Rahman, S., & Mackenzie, W. S., 1969. The crystallization of ternary feldspars: a study from natural rocks. Am.

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Zoned Pyroxenes from Shoshonite Lavas of Lesbos, Greece: Inferences concerning Shoshonite Petrogenesis