Geochemistry and evolution of Iherzolite-bearing phonolitic lavas from Nigeria, Australia, East Germany and New Zealand

Gwhimw Printed

‘2, Co.\mrxhmuu tn Great

A<,<, Vol. 45. pp

1309 to 1320. 1981

0016.71137

Bittam

81 0x1309-12$02.00.0 Pergrmon PIT\\ LAd

Geochemistry and evolution of Iherzolite-bearing phonolitic lavas from Nigeria, Australia, East Germany and New Zealand ANTHONYJ. IRVING* tunar and Planetary Institute. 3303 NASA

Road I. Houston. TX 77058. U.S.A.

and

RICHARDC. PRICE Department of Geology, La Trobe University. Bundoora. Victoria 3083. Australia (Receiued 6 July 1979; orcepred in revisedform

23 Murch 1981)

Abstract-The major and trace element chemistry of phonolites containing spine1 lherzolite xenoliths from Bokkos (Nigeria), Phonolite Hill (northeastern Australia) and Heldbmg (East Germany) is consistent with an origin by fractional crystallization of basanitic magmas at upper mantle pressures (lo-15 kbar). At Bokkos, spatially associated lavas ranging from hawaiitic nepheline mugearite to nepheline benmoreite can be modeled very well by fractional crystallization of kaersutitic amphibole -t olivine + Fe-T&spine1 + apatite, a crystal extract consistent with experimentally-determined near-liquidus phase relationships for mugearitic liquids. Further fractional crystallization of aluminous clinopyroxene -t- mica + apatite will yield the phonolites. A similar model relating the unusual Iherzolite-bearing mafic nepheline benmoreite from Pigroot (New Zealand) to basanitic lavas of the East Otago province is not supported by major and trace element data. The Pigroot lava is possibly the product of melting of a mantle source region previously enriched in Sr and light rare earth elements, with subsequent minor fractional crystallization of otivine + kaersutite. Dynamic flow crystallization processes operating within conduit systems from mantle pressures are capable of yielding large volumes of evolved phonolitic liquids from primary basanitic liquids, if magma flow rates are appropriate. This mechanism may provide an explanation for the volumetric bias towards salic differentiates in some aikalic provinces.

INTRODUCTION

or transitional (e.g. FINCH and MACDONALD, 1953; Ross et ul., 1954, p. 698; MACDONALD THERE IS considerable evidence that volcanic rocks and KATSURA,1964, p. 99; NICHOLLS and LORENZ, containing xenoliths of Cr-diopside spine1 lherzolite 1973; DUGGAN and WILKINSON,1973; SUTHERLAND, must represent liquids tapped directly from the 1974; Cox and JAMIESON,1974). High pressure inEarth’s mantle with negligible modification by proclusions have also been recorded in other alkaline cesses operative at crustal pressures. FREYand GREEN rocks (notably from eastern Australia and Japan} (1974) and FREYand PRINZ (1978) concluded that Crwhich range in composition from hawaiite and nephediopside lherzolite xenoliths represent chemicallyline hawaiite to mugearite, nepheline mugearite and modified ~ithospheri~ fragments accidentally incornepheline benmoreite (e.g. K~No. 1964: UCHIMIZL’. porated in their enclosing magmas at depths greater 1966; BINNS et ai., 1970; GREENand HIBBERSON, 1970: than 20-30 km (i.e. at pressures greater than 8 kbar). GREEN et ui., 1974: KNUTSON and GREEN, 1975; IRVMany other ultramafic xenoliths rich in aluminous INCAand GREEN. 1976; WASS and IRVING, 1976). pyroxenes, as well as various types of megacrysts, are In this paper we report major and trace element thought to be magmati~ precipitates at pressures of data for several distinctive lherzolite-bearing phonoli10-25 kbar (e.g. IRVING, 1974a, b, c, 1980; KNUTX)N tic rocks and associated lavas from four widely-separand GREEN, 1975). The explosive eruptions which ated localities, which allow some further interpretacarry these dense fragments to the surface allow no tions to be made concerning magma evolution within opportunity for wall-rock assimilation or differenthe upper mantle, particularly the roles of amphibole tiation by crystal settling processes at lower pressures and mica during fractionation. (GREEN,1969, 1970; MAAL~E,1973). The majority of volcanic rocks containing highGEOLOGICAL SETTING pressure xenotiths or megacrysts are alkaline basaltic in character (alkali olivine basalts, basanitcs, olivine nephelinites. olivine melilitites), although a few are tholeiitic

* Present address: Department of Geologicai Sciences, University of Washington. Seattle, WA 98195, U.S.A.

Rocks ranging in chemistry from phonolite to hawaiitic nepheline mugearite are intimately associated in the Bokkos plug, Jos Plateau, Nigeria. This occurrence was described in some detail by WRIGHT(1969),atthough in the

1309

1310

AWHOY’~

J. IKVINGand RI~TIARI) C. PRKI

absence of chemical data he referred to the rock types as trachytes and basalth. One of Wright’s hand-specimens which we examined contains both phonolite and basalt (nepheline hauaiite’!) separated by a narrow hybrid Lone about 1 cm wide. This could be interpreted as an inclusion of one rock type in the other or partly mixed contemporaneous magmas. All rock types contain xenoliths of olivine-rich Cr-diopsidc spine1 lherzolite and the more salic lavaa also contain inclusions of pyroxemte. anorthositc. gabbro. syenite. ilmenitc and sodic feldspar (WRIGH-r, 1969). Other phonolitic and trachytic rocks (apparently lacking high pressure inclusions) occur elsewhere in the same province (GRANT CI trl.. 19721. and are older than most of the associated basltic rocks.

An eroded cone or plug of phonolite surrounded bk younger basaltic labas occurs at Phonolite Hill in the late Miocene to Recent McBride Volcanic Province of northeastern Australia (GKIbFIN, 1977; GRll FIN and M(.DoL GALL, 1975: STEPHENSONand GKIF~IN, 1976). The phonoli!e contains xenoliths of Cr-diopaide spine1 lherzolite and altered gabbro (composed of plagioclase. olibine. sodic Tipargasite. apatite and magnetite). Megacrysts of magnetite and altered amphibole are also present. A sample of Crdiopslde IherLolite-bearing nepheline hauaiitr fromnear Rosella Plains Homestead (30 km to the southwest in the same provinccl wah also analyzed in this study.

H~ldhury, Eust Gwntrn~~ The early 14th century castle at Heldburg in the south Thuringia volcanic provmce of East Germany is built in part from Iherrolite-bearing phonolite. According to descriptions by LL~EDL:(.K~(1879). SANnerKC;~K (1888) and PKijS(.HOI.IIT and TH~~R,~(.H (1895a), the hill on which the castle stands is a phonolite plug cut by dikes of ‘limburgate’. Xenoliths of Cr-diopside spine1 lhcrzolite and hypersthene-rich norite occur in the phonolite but apparently have not been observed in the basaltic dikes. Lherzolite xenoliths and augite megacrysts are, however, found in the basamtes m Holrhiuser Waldung several kilometers east of the phonolite plug and in other basalts of the area. Major element analyses of the Heldburg phonolite were given by SANDBER~;ER(1890) and JUNC; (1930). and analyses of the Heldburg and HolzhLuser basanites by PK&.HOLDT and THI:KA~H (1895b). Other phonolitic and trachytic rocks (without high pressure inclusions) are well-represented m the nearby basaltic provinces of West Germany (e.g. FI~u. 1961: Hotbs and WEIIEPOHL, 1968; DIDA and S~HMIN(KL. 197X1.

Lherzolite-bearing phonolitic rocks of unusual composition form a flow dome near Trig L, Pigroot within the Waipiata Volcanics of the East Otago Province. New Lealand (PRICE and COOMBS, 1975: PRICE and WALLACE. 1976). These were first described by WRIC;KT (1966). and further petrographic and chemical data were given by PRICCand GREEN (1972) and PHILPOTTS ef trl. (1972). Lherzolite-bearing basaltic rocks are not closely associated with the Pigroot rock. but do occur elsewhere in the East Otago Province. such as at Black Head 65 km to the south and Siberia Hill 12 km to the north. Rocks ranging in composition from hawaiite to phonolite. but lacking high pressure inclusions. arc also a feature of this province (PRK‘E and TAYLOR, 1973: PRKE and CHAPPELL. 1975). The East Otago phonolitic rocks are of interest because of their similarities to the phonolites from Bokkos, Phonohte Hill and Heldburg, however they also show important differences, as discussed below.

MAJOR ELEMENT DATA New major element analyses and C.I.P.W. norms of rocks from Bokkos, McBride Province and Heldburg are presented in Table 1. and compared with data for several rocks from East Otago whose trace element chemistry is discussed below. The four samples from Bokkos arc all highly nephcline normative. The nomenclature used is based on normative plagioclase composition and Differentiation Index (COOMBS and WILKINSON, 1969) as well as (Na20 + K20)-Si02 variation (see Fig. 6). From hawaiitic nepheline mugearite 774 to mugearitic nepheline bcnmoreite 745 to nepheline benmoreite 744 to phonolite 734. there is a progressive increase in Si. Al. Na and K coupled with a decrease in Ti. total Fe. Mg. Ca and P. There is no obJective way of estimating pre-eruptive Fe201. Fe0 ratios in these rocks. Sample 744 in particular has an anomalously high Fe203 content by comparison with the other Bokkos samples and its Fe20,:Fe0 ratio has been adjusted to a value intermediate between those of samples 745 and 743 in order to remove hematite from the norm. With this adjustment a continuous decrease in the Mg-value (100 Mg,‘Mg + Fe’* 1 from 67.9 to 50.9 is also obserced in the above sequence. The phonolite from Phonolitc Hill is lower in Si but higher in total Fe compared with the Bokkos phonolite. and has a much lower 100 Mg/Mg + ZFe (22.7) than the Rosella Plains nepheline hawaiite (60.1). The Heldburg phonolite is chemically similar to the Bokkos phonolite. and has lower 100 Mg Mg + XFe (47.5 51.8) than the associated basanite dike (62.3). The newly-analyzed Heldburg sample (1007) is acmite-normative. The sample analyzed by JUNG (1930)has higher Ca and lower alkalies than 1007, and contained hematite in its norm before the FeZO,‘FeO ratio was reduced to that of 1007. The unusual Pigroot rock is difficult to classify, since it has very sodic normative plagioclase but low D.I. From its (NazO + K20)/Si02 ratio (see Fig. 6) coupled with its relatively high Mg and total Fe, It may be termed a mafic nepheline benmoreite. It shows some affinities with the Iherzolite-bearing nepheline mugearites of the Newer Basalts Province

BOKKOS PLUG, NIGERIA

Fig. 1. Chondrite-normalized rare earth patterns for the Bokkos mugearitic to phonolitic lavas. Data for nepheline benmoreite 744 omitted for clarity.

D.I.

an/an+ab

H,O

CC

AP

Or Ab An Ne Oi 01 AC Mt I1

C.I.P.W.

1

weight

99.77

3.38 1.58

4.20 3.30 1.14 0.45 1.95

22.8

67.5

30.7

51.4

0.18 2.9B

0.66

25.30 32.75 9.65 9.49 7.51 6.27

nor1ws:

15.37 26.86 11.38 9.19 12.59 12.98

percent

62.8 48.3

99.92

67.9 59.6

100 Mg/Mg+F$+ 100 Mg/Mg+LFe

49.29

TOTAL

CO2 SO3

H,O-

tl,0+

p205

MgO CaO NaZO K,O

Al,03 Fe& Fe0 MnO

SiO, TiO?

53.51 0.83 17.94 2.33 3.65 0.16 3.45 4.32 5.94 4.28 0.30 2.50 0.48 0.08

Mugearitic nepheline benmoreite 745

Nigeria

1.74 15.69 2.90 6.07 0.15 7.20 6.43 5.18 2.60 0.52 1.40 0.55 0.20

774

Hawaiitic nepheline mugearite

Bokkos,

76.4

14.1

3.68 1.10 0.49 0.50 3.36

25.59 40.57 6.66 10.25 3.93 3.64

67.3 42.8

99.84

55.19 0.58 18.69 3.41 1.75 0.17 2.02 2.88 7.03 4.33 0.21 2.38 0.98 0.22

Nepheline benmoreite 744

84.5

U.I

4.16 0.91 0.39 0.55 1.74

27.66 38.54 0.27 18.32 7.12 0.13

41.5

38.2

2.71 4.24 1.49 0.20 0.82

10.05 21.98 13.61 9.50 13.74 20.80

73.1 60.1

99.48

99.81 50.9 30.9

0.09

46.71 2.23 14.51 5.43 6.11 0.14 9.31 7.08 4.67 1.70 0.68 0.82

0.18 1.29 0.45 0.24

4.68

56.18 0.48 19.23 2.87 1.96 0.16 1.14 2.40 8.55

Phonolite 743

Nepheline hawaiite 141001

McBride Province, Australia

Table

Major

5.36

81.2

39.7

51.2

5.26 2.13 1.37 0.36 5.36

5.73 0.27 0.61 0.32 2.94 6.4

14.48 14.06 14.73 11.14 16.14 14.44

25.47 38.47 2.62 17.21 3.25 0.92

85.9

0.63 0.33 0.20 2.69

25.66 4.53 1.57 3.04

27.42 32.81

5.87 4.20 2.53 0.30 2.63 6.44 6.42 2.13 0.50 1.06

8.6 53.3

65.3 31.9

11.6 82.2

83.2

11.62 3.91 0.01 7.07 2.62 0.28 0.25 0.23 1.35

30.31 41.30

4.3

13.3

99.66

99.72 55.9 46.3

58.56 0.15 17.75 4.25 1.67 0.20 0.14 1.18 8.66 5.13 0.11 1.03 0.32 0.10 0.07

1.61 1.14 0.17 1.46

Kettle, N.Z.

Phonolite' 22553

Mt.

46.70 2.21 13.55 4.05 7.78 0.19 5.52 6.82 6.26 2.79 1.09 2.02 0.61 0.13

12.94 25.49 2.40 14.89 19.08 S.
57.9 48.5

99.20

42.12 3.38 14.38 4.44 8.71 0.22 6.71 11.10 3.86 1.75 0.92 0.54 0.52 0.22 0.27

Mafic nepheline benmoreite& 30424

Pigroot, N.2.

10.34 9.02 16.98 12.57 24.74 8.62

i teL

Nepheline basanite3 30442

Black Head, N.2.

17.91 40.95 5.36 23.34 6.59

67.8 51.8

99.61 83.5 47.5

99.67

99.65

1.22 3.18 9.93 3.03 0.08 1.22 0.24

55.91 0.60 21.58 2.19 0.43

Phonol

71.8 62.3

i te

51.1 22.7

0.16 0.21

0.12 1.16 1.42 10.17 4.64 0.15 2.12 0.57 0.09 0.22

0.98

6.06 8.66 8.01 4.09 2.45 0.58

56.04 0.33 20.61 1.05

44.54 1.12 14.78 3.63

(basanite)'

E. Germany

Phenol 1007

Heldburg,

data

Limburgite

element

97.84

53.48 0.14 19.28 4.42 1.55 0.23 0.91 i .a6 8.30 4.31 0.28 1.34 1.60 0.14

Phonolite x-32

1.

ANTHONYJ.

1312

500

MCBRIDE PROVINCE,

AUSTRALIA

_

0 Phonolite X- 32

.g ,o(y

l

0

5o

i

20-

5

IO5-

and RI(.HARDC. PRK~

N. QUEENSLAND,

200

%

IRVING

Ne

Houoiite 141001

&\

value (55.9) relative to basanites and nepheline hawaiites of the same province. The other East Otago xenolith-free phonolites, exemplified by that from Mt Kettle, are acmite-normative and resemble in many respects those from Bokkos and Heldburg, but have higher Si and much lower Mg and Mg-values.

~+*~oj TRACE ELEMENT

I

I

I

Lo Ce R

I,

I

I

I

I,

I

I

I

+

II

Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 2. Chondrite-normalized rare earth patterns for the Phonolite Hill phonolite and Rosella Plains nepheline from the McBride

Province.

north

Queensland.

of Victoria (IRVING and GREEN, 1976) and the nepheline benmoreite from Mt Mitchell, Queensland (GREEN et al., 1974). Although the Pigroot rock differs in many respects from the Bokkos, Phonolite Hill and Heldburg phonolites (e.g. lower Si. Al, Na. K and higher Ca, Ti, P). it does share with them a low Mg-

Table

2.

Trace

element

data

Bokkos

New analytical data for the rare earth elements (REE) and other trace elements in the Bokkos, McBride Province and Heldburg rocks are given in Table 2. Chondrite-normalized REE patterns for three of the Nigerian lavas are shown in Fig. 1. All the patterns have relatively steep slopes and slight positive europium anomalies. The Bokkos lavas have similar abundances of the heavy REE, but abundances of the light REE increase in the sequence hawaiitic nepheline mugearite --t mugearitic nepheline benmoreite + nepheline benmoreite + phonolite. In this same sequence, abundances of Ba, Cs, Hf, Th, U,

(ppm) McBride

Province

Heldburg

Pigroot

774

745

744

743

141001

X-32

1007

30424

La Ce Nd Sm Eu Tb Yb Lu

64 123 8.2 2.62 0.91 1.58 0.220

90 169

108 173

106 192

96 195

816 2.61 0.87 1.62 0.28

;':2 0:97 1.64 0.26

73 119 36 5.06 1.53 0.72 2.8 0.49

70 113

8.3 2.58 1.02 1.60 0.29

33.4 66 34 6.1 2.01 0.84 1.18 0.172

5173 1.49 0.56 2.08 0.46

13.9 4.36 1.66 1.39 0.189

SC V Cr Ni cu Zn co

10.4 83 249 134 21 103 -

460'

310 I

16 278

13.7

2.70

104 62 16 141

36 31 -

4;9 310

11.8 8 13 125

5.7 61 115 86 16

8:O

32 21 11 167 -

1.81 16 46 38 15 99

Y Hf Zr Nb Ba Sr Rb cs U Th Pb Sn Ta

;91 393 117 795 749 36 1.7 2.3 11.2 6.5 1.04 9.3

27 13.8 832

14.5 25 -

::.2 -

21 11.0

216 815 715

870 -

885 -

?8 205 57 370 884 27

24 14.1 633 124 760 1560 68

?6

214

413

20 17.5 965 236 128 848 36

8.j Zi86

21:7 26i35 27 17140

318

218.6 12

1.20 16

4.3 18

319

20

4.4 21

DATA

553

519 6.1 32.0 16.1 2.11 35

GSFC 136l

188 78.0 14.0 4.24 1.04 0.136

7;8 1600 73.6

414.0 8 9.2

REE, SC, Cr, Co, Hf, Th, Ta by INAA (JSC). V, Ni, Cu. Zn by XRF (La Trobe and ANU), except Ni in 744 and 141001 by INAA (JSC). Y, Zr, Nb, Sr, Rb, Cs, U, Pb, Sn by spark source mass spectrometry (ANU), except for 141001, X-32 by XRF (JSC and ANU, respectively) and Sr, Rb in 30424 by XRF (ANU). Ba by XRF (La Trobe and ANU), except for 744 and 141001 by INAA (JSC). Ni, Cu, Zn, Y. Zr. Nb, Ba. Sr. Rb, U. Pb for X-32 from GRIFFIN (1977). ‘PHILPOTTS c’t trl. (1972); also includes Cd 11.1 ppm, Dy 6.41 ppm, and Er 1.96 ppm. Relatioe data precision: By INAA. La, Ce, Sm, SC (29.), ELI, Hf, Co, Cr (3x), Tb, Yb, Lu, Th (4”,,), Nd. Ba (6”,), Ta (lo’:), Ni (5m15”,,). By XRF’ and SSMS. 5”” or better for most elements.

Lherzolite-bearing

HELDBURG, E.GERMANY and FERNANDO DE NORONHA 0 Phonolite 1007 (Heldburg) l

Phonolite FN 14-3 (Fernando de Noronho)

55

La Ce PT Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 3. C‘hondrite-norm&red rare earth patternof Heldburg phonolite 1007 compared with that for a phonolite from Fernando de Noronha. Brazil (KAY and GAST. 1973).

Sn, Ta and Zn also increase,

whereas there is a regular decrease in abundances of Ni. Cr, Cu. V and SC. The REE patterns for the Phonolite Hill and Heldburg phonolites (Figs 2 and 3) are reasonably similar to each other. They have no significant ELI anomaly but show concave upwards distributions in the La-Eu region. like the patterns for the Bokkos benmoreites and phonolite. However. these phonolites differ from the Bokkos phonolite in having lower absolute light REE abundances and flatter heavy REE distributions. Relatlte to the Rosella Plains nepheline hawaiite (Fig. 2) the Phonolite Hill lava is enriched in both light and heavy REE, enriched in Y, Hf, Rb, Th, Ta (but not Ba). and strongly depleted in SC. Cr and Ni. Data for a phonolite from Fernando de Noronha (KAL and GAST, 1973) are also plotted in Fig. 3. Although this lava apparently lacks high-pressure inclusions it is associated with Iherzolite-bearing olivine nephelinites. and its REE pattern has a very similar shape to those of the Phonolite Hill and Heldburg phonolites. with no Eu anomaly to indicate feldspar fractionation at crustal pressures. The REE patterns for three East Otago rocks are shown in Fig. 4. Our data and those of PHILPOTTS er trl. (1971) for two different samples of the Pigroot matic nepheline benmoreite are very similar. The REE

Table 3.

patterns have the form of two linear segments intersecting in the Eu-Gd region: an unusual feature is the steeper slope for the heavy REE compared with that of the light REE. The Black Head basanite has a linear REE pattern, but is depleted relative to the Pigroot rock in light REE and enriched in heavy REE. The pattern for the Mt Kettle phonolite (which lacks high-pressure xenoliths) contrasts with these patterns in that it is distinctly concave upwards with a large negative Eu anomaly, and is much higher in heavy REE. Relative to the Black Head basanite the Pigroot rock has higher abundances of other large ion lithophile (LIL) elements (Ba, Zr. Nh, Rb, Sr. U. Th, Pb) and lower abundances of the transition metals (V. Cr. Cu). These trends are continued in the East Otago xenolith-free phonolites, except that Ba and Sr are markedly depleted. and the enrichments in Cs and Rb are very great (see PRICE and TAYLOR, 1973). EVALUATION FRACTIONAL

OF HIGH-PRESSURE

CRYSTALLIZATION

From arguments detailed by IRVING and GREEN (1976), none of the nepheline hawaiite to phonolitic lavas can be direct partial melts of peridotitic mantle

EAST OTAGO PROVINCE,

NEW ZEALAND J

+ Phonoliie30424 (Pigroot) 0 PhonoliteGSFC13G (Pigroof) l Basomte 30442 (Black Head1 X Phonohte 22553W. Ke?tle)

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm yb Lu Fig. 4. Chondrite-normalized rare earth patterns for two samples of the Pigroot mafic nepheline benmoreite (Table 2 and PHILPOTTS er ul. 1972), Black Head basanite and Mt Kettle phonolite (PRICF. and TAYLOR, 1973).

OL

AMPH

CPX

MICA

AP

38.2

42.0 3.3 11.8

37.5 3.9 15.6 10.2 18.6

-

;i76 11:3 2.8 1.4

51.4 0.82 4.4 8.0 14.2 18.9 0.91 -

1:1 8.3

71.4

76.0

76.5

23.5 37.3 0.28

ZFe

73.9

MODELS

Approuch

Compositions of phases used in mixing calculations (with Mg/Mg+zFe adjusted for equilibrium with sample 745)

Si02 Ti02 A1 2O3 cFe0 MgO CaO Na20 '(20 PZOS 100 Mg/Mg+

1313

phonolitic lavas

ULVij

5i.8 42.4

29.1 1.5 64.7 2.4 -

-

6.2

AN WOKY J. IRVING and RI(‘HAKI)C. PR~U

1314

MEGACRYST/HOST PUtTITION COEFFICIENTS 20IO-, 5-

La Ce

Nd

SmEu

Tb

Yb Lu

Fig. 5 CrystalSliquid rare earth element partition cients used in calculations (from Table 4).

coeffi-

because their Mg-values (for likely pre-eruptive FezO,/FeO ratios of 10.20) are all below those (66-75) appropriate for equilibrium with mantle olivine (Fo,,~Fo,,,). The intimate spatial association of magmas in the Bokkos plug strongly suggests a genetic relationship among them prior to eruption (and even at high pressure before xenolith entrainment), and we here examine the possibility that such magmas are related via high pressure fractional crystallization processes. Experimental evidence on a nepheline mugearite composition (IRVING and GREEN. 1981) indicates that olivine. kaersutitic amphibole. aluminous calcic clinopyroxene, phlogopite and ulvospinel are likely phases involved in these processes under water-undersaturated conditions at pressures of 10 to 15 kbar. Apatite was also considered, but feldspars were not because the lavas have no characteristic depletion in Eu. Ba. Sr and Cs. and because the mineralogy of the lherzolite xenoliths reflects pressures at or above the limits of feldspar stability in these magma compositions (FREY and GREEN. 1974; IRVING and GREEN. 1976). Fractional crystallization models were evaluated using a similar approach to that used by ZEL~NSKI and FREY (1970). Z~EL~NSKI(1975) and SUN and HANSON (1976) for

Gough Island, Reunion and Ross Island lavas. Major elements were first modeled using a mixing program similar to that of WRIGHT and DOHERTY (1970). The models based on major element fits were then tested using appropriate crystal/liquid trace element partition coefficients to predict trace element abundances for comparison with those observed in the natural rocks. However, our modeling procedures differ from those used previously in two important aspects: (1) major element mineral compositions were based on those of high-pressure experimental phases. and (2) trace element partition coefficients were based largely on data for high-pressure megacrysts and host lavas, and partly on experimental values. The major element compositions of minerals used

in the calculations (except for apatite) were based on near-liquidua those of experimentally-produced phases of The Anakies nepheline mugearite 2102 in presence of 2-5 wt”,, Hz0 (IRVING;and GREEN. 19811 and are given in Table 3. These compositions are very appropriate for modeling the fractionation of the Bokkos lavas because 774 is nearly identical to 2102 in major element composition. For the feromagnesian silicate phases. appropriate Mg Mg + XFe ratio5 (mg) were determined by first fixing a Iratio folliquidus olivine of a given parent liquid usmg Kj;‘“’ of 0.33 for olivine:liquid equilibrium (see IRVIN(i and GRLEN, 1976). The Mg:Mg + XF‘t: ratios of hypothetical amphibole. clinopyroxene and or mica coexisting with olivine and liquid wcrc then estimated using the following empirical relations deduced from expertmental studies (IRVING; and GR1:i.h. I981 and unpublished data): mgamph = 0.968 mg,,,. mg,,,, = 1.028 mg,,,. mg,,,,., = 1.034 mg,,,. In each calculation all six mincral compositions (with appropriate mp values) wcrc used without constraints to test whether given liquids might be inter-related by subtraction of any or all of these minerals. The trace element partition coeflicicnts used arc from several sources, and are given in Fig. 5 and Table 4. Except for olivine. the coetticients for REFi and other lithophile elements are from natural megacryst:host data (F. A. FKI:Y and .A. J. IRVING;. unpublished data. 1981). The amphibole and clinopyroxene partition coefficients for these elements range over a factor of 5 to IO. which probably rctlectz differing physical (T.P) and chemical parameters during megacryst crystallization (see IKVINC;. 1978a). Mica and apatite partition coefficients show smaller ranges. The effects of these variations on the fit of the models are discussed below. The ulviispinel REE partitition coefficients used are for a homogeneous magnetite-ulviispinel megacrqst from 96 Ranch, west Texas (lRV16(;. 1’377). which is similar in ?3FeO and MgO content to the synthetic ulv&pinel. but is lower in Ti02 (I I w,t”,,) and higher in Al20 / (8 wt”,,). Because of the strong effects of temperature on partition coefficients for transition metals (e.g. LINDS~ROM and WE’LL. 1978). experimentallydetermined values at appropriate temperatures I 1100 -12OO’C) and low fo2 were used for clinopyroxene, olivine and spinel. For kaersutitc. biotite and apatite the lack of systematic experimental data for transition metals necessitated the use of megacryst, host values. The megacryst ‘host data should be directly applicable to the pressures considered here. but. since most of the experimental calues were detcrmined at 1 atm. their use here assumes negligible pressure effects (see IRviK*;G,197%).

Because of their wide spread in chemical composition and intimate spatial association, the Bokkos lavas were treated in the most detail. Good major element fits were obtained. and are given in Table 5.

-.

Olivine

0.0055 0.0035 -

224 0.042

0.00411

&l;;l

O:O0610

5.0'

1;.;;; 0:28

0.06g6 0.098 0.21 0.26 0.31

0.13412 0.199 0.36 0.51 (0.60) (0.73) 0.76 0.75

0.4413 0.03 0.05 0.004 0.09 0.01

2.0'3 0.6 0.75 1.9

0.05613 0.095 0.22 0.40 0.47 0.55 0.44 0.40

Megacryst/Host

0.1113 0.047 0.24 7.1 0.21 3.2

0.4313 0.06 1.1 0.35

1.8l3 0.04 0.5 1.0 0.39'3 0.038 0.41 0.61 0.61 0.2

0.01213 0.012 (0.013) 0.016 0.030 (0.015) 0.015 (0.015)

Megacryst/Host

0.1413 0.25 0.51 0.80 0.90 0.87 0.57 0.47

Megacryst/Host

Biotite

2.5(2.1"')

_I3 0.94

0.4

0.0413

14.513 15.2 15.1 13.9 13.3 11.0 6.7 5.3

Megacryst/Host

Apatite

0.1313 0.03 0.29

7716 2015 3816

1.015

(0:049)

[;Zi

0.0165 0.0162

(0.0169)

0.017313 (0.0171)

MegacrystlHost

Ti-Spine1

__ ._ _ _

.

-

--

- -

_, - -

__ -

- _

_

--~

-,-._,

-

--

~.

Values in parentheses by interpolation or extrapolation. Drrtr~ .w~~rws: (1) M~KAv and WEILL (1976, 1977). average 1200 and 1240 C, 1 atm: (2) LINDSTROM (1976).II 12 1134 C. 1 atm. (3) MCKAY and W~II I (1976). 1200 C. 1 atm. low fo.: (4) IRVIWG (1978a. Fig. 1). estimate 1150 c‘.1 atm: (5) MCKAY and WELL (1977). 1240 C, 1 atm: (6) GRL,TT~C L er (I/. (1974). LX,, gAn,,Ab, I camp.. 1265 C. 1 atm; (7) LINDSTROM (1976).1150 C. I atm and IRWNG (1978a. Figs 10 and 13); (8) IRVING (1978a, Fig. 15). estimate 1150 C. I atm: (9) IRVING (1978a. Fig. 16). estimate Iow,~;,~: (10) SEITZ (1973). 1150 C. 25 kbar; (11) SHIMIZU (1974). 1100 1200 C. 15-30 kbar: (12) TANAKA and NISHIZAWA (1975).1200 C, 20kbar, run 6: (13) F. A. FREY and A. J. IRVING. unpublished data compilation: (14) KOGARKO (1978). . 1140 C. 1 atm: (15) LINDSTROM (1976).Ti-magnetite. -1120 C. 1 atm,j,,,= 10 ': (16) RINGNOCNI (1970), ulv&spinel. - 1140 C. 1 atm. lowf,,,.

Hf Th Ta Ba Sr Rb

V

1.23

0.372

0:037 (0.045)

Tb Yb Lu

SC Cr Ni

(0.0108)' 0.0110 (0.0117) 0.0125 t;.;;;f)

Experimental

Kaersutite

(D) used in calculations

Clinopyroxene

coefficients

Calcic

partition

Experimental

Crystal/liquid

Experimental

4.

La Ce Nd Sm Eu

Table

ANTHONY

1316 Table

5.

Major element

mixing

J. IK~IKG and RICH~KI) C. PKIU models

for the Bokkos

Parent

Daughter

F(%)

OL(%)

AMPH(%)

CPX(%)

MICA(%)

AP(%)

ULVo(%)

zrp

774 745 744

745 744 743

32.9 14.9 7.8

3.2 -

26.9 7.3 -

2.7 4.2

4.0 3.5

0.8 0.7 0.1

2.0 0.1 -

0.791 0.335 1.29

F is the fraction of parent remaining. x:r’ is the sum of the squares computed parent compositions (for 9 oxides).

In essence, the derivation of the nepheline benmoreites from hawaiitic nepheline mugearite 774 involves removal of dominantly kaersutitic amphibole with minor olivine, apatite and Ti-spinel. whereas the further fractionation to phonolite requires subtraction of chnopyroxene and mica instead. These changes in the nature of the crystalline extract are more clearly illustrated on an alkaliessilica diagram. In Fig. 6, the compositions of a number of Iherzolite-bearing lavas are plotted. together with the compositions of various high pressure experimental and megacryst phases. It is apparent that the lineage basanite - nepheline benmoreite must be controlled by low-Si02 phases such as Tiamphibole and olivine. The bend in the fractionation trend from nepheline benmoreite to phonolite requires control by a more SiOz-rich phase such as calcic clinopyroxene. The role of kaersutitic amphibole + olivinc in the evolution of nepheline mugearite liquids is consistent with the experimental results of IKVING and GREEN (1981). They found that The Anakies nepheline mugearite 2102 is simultaneously saturated with these two phases at 14 i 1 kbar for a bulk H20 content of 4.5 + 0.5 wt”,,. In the absence of experimental data for nepheline benmoreite and phonolite liquids at high pressures. it is difficult to find independent support for the change to a clinopyroxene + mica crystalline assemblage. However, the enhanced Hz0 contents in these very evolved liquids may well stabilize mica instead of amphibole as a near-liquidus phase. Orthopyroxene is not likely to be stable in phonolitic liquids since it was never encountered in experiments on the less evolved nepheline mugearite 2102 (IRVING and GREEN, 1981). In order to test the Bokkos fractionation model using trace element data. we have assumed that the fractional crystallization process can be approximated by a series of equilibrium crystallization steps. This is a good assumption because of the fairly even spread of evolved lava compositions. Bulk partition coefficients were calculated using the same procedure as ZIELINSKI (1975). Predicted

and

observed

trace

element

abundances

REE the model predicts abundances within IO”,, of those observed, with very few exceptions. Abundances of Th. Ta and Hf are also predicted well, especially for 744 and 743. Calculated Ba and. where available. Sr abundances are within 20”,, of those observed. but are conare

lavas

compared

in Table

6 and

Fig.

7. For

of residuals

between

observed

and

sistently too high. The estimated bulk partition coethcients for Ba and Sr are mostly in the range 0.5 3. but whether the values are greater than or less than unity depends critically on individual values of D”’ for apatite. D”” for mica, and to a lesser extent II”.’ and Ds’ for amphibole. all of which are not only poorly constrained by available natural data but which probably have strong temperature dependcnces. The degree of fit for the four transition metals considered ranges from better than lo”,, to more than 40”,,. even though there is a consistent decrease in the predicted abundances with increasing fractionation Most of the uncertainty for these elements probably results from the strong dependence of transition metal partition coefficients on temperature (and bulk composition). Attempts were made to take this into account for olivine. clinopyroaenc and spine1 (SW Table 4). but the absence of experimental data for

0

36

40

44 46 Wt% Si02

52

56

60

Fig. 6. Alkalies vs silica plot of Ntgerian, Heldburg. East Otago and eastern Australian Iherzolite-bearing lavas (vola tile-free. with Fe20,;Fe0 ratios adjusted to 0.15). Dashed line is boundary between mildly-undersaturated and strongly-undersaturated alkalic lavas from SAGGI.KSON and WILLIAMS (1964). H = Heldburg basanite (PK~WHOLI~I and THURATH, lX95b). J = Heldburg phonolite (Jrl~(,. 1930). M = Mt Mitchell nepheline henmoreite (GKE~N 1’1
Lherzolite-bearing Table

6.

Comparison of predicted and observed abundances for Bokkos lavas

774 + 745 Predicted Observed

element

744 -f 743 Predicted Observed

745+744 Predicted Observed

82

90

!I7

151

169

180

Sm

8.3

Eu Tb

2.61

8.3 2.58 1.02 1 .60

a. 3 :?. 55 1.03 I. 71

0.29

,I. 31

‘27 105 36

31 36

4.8

3.1

16 2.32

25.0 18

25.5 18

27.5 13

27.0

774 690

870

743

885

14.5

15.4

16.2

Lu

0.33 1.83 0.27

Ni Cr V SC

70 110 49

8.8

62 104 40 6.1

Th

16.2

21.6

Ta

12

16

Sr

875

715

Ba Rb Hf

950 50 11.7

815 91 13.8

the Austruliun,

East

Germany

108 173 8.6 2.61 0.87 1 .62 0.28

116

106

185

192

9.0

8.2 2.62

2.74 0.91 i. 71

0.97

1.64 0.26

0.30 20

21 32 16 2.78

36

21

32 15.4

Ti-amphibole and mica prevented a rigorous appraisal of temperature effects for those phases. By analogy with data for olivine and clinopyroxene, partition coefficients for other ferromagnesian silicates shculd also increase with decreasing temperature (i.e. increasing fractionation), which would have the effect of improving most of the fits in Table 6. Variations in oxygen fugacity during crystallization would also affect the partitioning of Cr and V, however the oxygen fugacity during fractionation within the mantle may be buffered at relatively low values. In addition. changes in the composition of the spine1 phase (from Ti-rich magnetite to Ti-poor magnetite) as fractionation proceeded could lead to large changes in the partitioning behavior of transition metals. Given these various sources of uncertainty. we consider that the fractionation models in Table 5 for the Bokkos lavas are adequately confirmed by the trace element data. Models for

trace

La Ce

Yb

Zealand

1317

phonolitic lavas

und

New

laous

We have not attempted detailed modeling for the McBride Province and Heldburg lavas because of the lack of samples at an intermediate stage of evolution (see Fig. 6). However, the strong analogies with the Bokkos lavas in both major and trace element abundances suggest very similar fractionation schemes leading from basanite and nepheline hawaiite liquids to phonolite liquids in all of these provinces. In contrast, there is no such simple scheme for relating the Pigroot mafic nepheline benmoreite to the basanite lavas of the East Otago Province. From Fig. 6 it appears that the Pigroot lava (30424) might be derived from a liquid similar to the Black Head basanite (30442) by removal of kaersutitic amphibole, and indeed kaersutite xenocrysts are present in the Pigroot rock (PRICE and GREEN. 1972). However. we

could not obtain a successful major element match between these lavas using various combinations of olivine, kaersutite, pyroxenes and Ti-spinel. A number of unusual chemical features of the Pigroot rock suggest that more complicated processes were involved in its origin: (1) Although it has more ‘evolved’ alkalies and Si02 contents than the East Otago basanites. it has comparable P,05 and normative olivine contents. and a lower A1203 content. (2) It is extremely enriched in Sr (1600 ppm) compared with other East Otago lavas. but has a comparably low *‘Sr%r ratio of 0.70297 (PRICEand COMPSTON,1973). (3) Its unusual REE pattern cannot be simply related to the essentially linear or concave upwards patterns of the other East Otago lavas (PRICE and TAYLOR. 1973) by considering subtraction of common phases and using typical REE partition coefficients. We conclude that a mixing or contamination process was involved in producing this unusual lava composition. One possibility is contamination of a Iherzolite-bearing basanite melt by materiai rich in Sr and light REE with an overall concave downwards REE EIOKKOS MODEL

6\

*OO +
\ \

so0

‘\

,:I

\

% 20g c.f~ IO-

‘\

\ + Ne Mugeorite 774 . Ne Benmoreile 745 0 745 Model

5I

I

I

La Ce R

‘_

“--G

I I I I I I I I II I I Nd Pm Sm Eu Gd Tb Dy Ha Er Tm Yb Lu

Fig. 7. Comparison of calculated and observed patterns for Bokkos nepheline benmorette

rare earth 745.

1318

ANTHONY

J.

IRVING

and RI~AKD

pattern. Apatite associated with alkalic lavas possesses these features (see Table 4 and Fig. 5). but the amount of apatite contamination would be limited by the minimal enrichment in Pz05 and relative depletion in CaO (unless there were subsequent removal of amphibole or clinopyroxene). Direct melting of a mantle source region which had pre\iously been contaminated by metasomatic fluids rich in Sr and light REE might produce an unusual basanitic liquid which. upon minor high-pressure fractionation of olivine and kaersutite. might yield the Pigroot IaLa composition. Whatever the processes involved. it seems likely from the relatively primitive X‘Sr;H”Sr ratio of this rock and the presence of lherzolite xenoliths. that they operated within the mantle. CONCLUSIONS From the observations and model calculations above we conclude that fractional crystallization processes at mantle pressures have produced at least some phonolitic liquids which remained essentially unmodified during subsequent ascent to the surface. Classically. phonolites and trachytes have been interpreted as products of low-pressure (-c 5 kbar) fractional crystallization of alkalic basaltic magmas within the crust. although the high volumetric abundance of evolved lavas in some provinces (e.g. East Africa) has been a perplexing problem (e.g. BAILEY. 1964: GILL. 1973: GOLES. 1976). which has led some to advocate a more important role for mantle processes. Admittedly, evolved lavas like those of this study iacking evidence for rrny crustal processes in their genesis are rare. and more commonly there is good evidence for removal or addition of feldspar within the crust. This is apparent from the Eu. Sr and Ba anomalies characteristic of evolved alkalic lavas from East Otago (PRICE and TAYLOR. 1973). Comoro Islands (FLOWER. 1971, 1973), Principe (FITTON and HUGHES. 1977). Reunion (ZIELINSKI. 1975). Ross Island (SUN and HANSON, 1976). West Germany (HERRMANN. 1968). and East Africa (BAKER ~lt ul.. 1977: WEAVERet (I/.. 1972). Strontium isotope data for some phonolitic and trachytic lavas have been interpreted as suggesting crustal contamination (e.g. HOEFS and WEDEPOHL, 1968; PRICE and TAYL.OR,1973). however ROCK (1976) concluded that the great majority of evolved alkalic liquids (including nepheline syenites and other plutonic equivalents) are derived from mantle liquids with negligible contamination. Mechanisms proposed for fractional crystallization of magmas have typically involved gravity-controlled crystal settling (or non-settling in the case of feldspars) within bodies of magma enclosed by ‘chambers’ of more or less spherical form. An alternative model involving continuous plating of crystals from ascending magmas onto conduit walls was suggested by IRVING (1978b, 1980). This mechanism predicts that polybaric fractionation will be the expected process affecting ascending magmas. with the extent of frac-

C. PRI(.~

tionation depending on magma flow rate. Thus. even where evolved alkalic lavas show evidence of feldspar fractionation at relatively low pressures. it is impossible to rule out prior fractionation of other phases at higher pressures as suggested for the Iherlolite-bearing lavas studied in this paper. Dynamic fractional crystallization (or How crystalliratinn) within conduit4 v.111 produw Laying volumes tloa

of evolved

rate.

In

appropriate. continuously

liquids

principle.

an ascending zoned

from

as a function provided column more

fin\\

of

magma

rates

Mere

of magma

could

primitive

to

bc

more extruded

evolved with decreasing depth. Magmas from such a conduit system could lx prcdominantl! rvol\ed varieties with little or no e\~dcnce of theildeep-seated parental liquids. For \alic magma biases to bo produced by such ;I mechanism it is nccessarq that tectonic or other factora can control the ROM rates of ascending magmas at appropriate values for sufficient

periods

of time.

.4~~krlor~luilyrt,Ir~lli,\ We are fratcflll t0 1.. FRlX’lI. J. B WRI(;HT, T. J. GRIFFIX. S. MAI THFS and K. HIIIX for supplying samples. and to B. W. CHAIJIJILI for carrying 0111 three of the XRF analyses. R. W. JOIIUSOS. R. J AR(‘III.IK H.-l’. S~HMINC.KE and F. A. FRY> provided helpful comments. Most of the work reported here v,as done while the first author was a Visiting Research Sclcnrist at the Lunarand Planetary Institute. which i\ operated by the Universities Space Research Association under Contract No NSR 09-051-001 with the National Aeronautics and Space Administration. This paper constllutes Lunar and Planetary Institute Contribution No. 392.

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phonolitic

iavas

1319

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