The analcime-phyric phonolites (blairmorites) and associated analcime kenytes of the Lupata Gorge, Mocambique

The analcime-phyricphonolites (blairmorites) and associated analcime kenytes of the Lupata Gorge, Mocambique A. R. WOOLLEY & R. F. SYMES

L1THOS

Woolley, A. R. & Symes, R. F. 1976: The analcime-phyfie phonolites Colairmorites) and associated analcime kenytes of the Lupata Gorge, Mocambique~ Lithos 9, 9-15. The petrology and mineralogy of three lavas from the Lupa~a Gorge, Mocambique, containing primary euhedral analcime phenocrysts, up to 1.5 em in diameter, as well as potassium feldspar and nepheline phenocrysts, are described. Electron microprobe analyses of these phases and a whole rock analysis of the blairmorite are given. Reference to published and unpublished experimental work indicates that these rocks must have been generated at water pressures between about 5 and 13 kbars, implying depths of origin of betwe~,en 20 and 50 km. Very rapid transport to tl~e surface and quenching is implied. It is suggested that these indications of the considerable depth of origin of these rocks, taken together with the absence of associated intermediate and basic rocks, lends credence to the hypothesis of D. K. Bailey that the voluminous associations of salic igneous rocks found in parts of the African continenL unaccompanied by associated basic rocks, are explicable in terms of partial melting, und,~r high water pressures, of the lower part of the crust. A. R. Woolley & R. F. Symes, Department o[ Mineralogy, British Museum (,Natural History), Cromwell Road, London S'J'7 5~D, England.

Although analcime is relatively common in some alkaline igneous rocks, only rarely does it form euhedral crystals indicating its probable crystallization from a liquid. Its most spectacular development as a primary phase is probably in blairmorites, which have been described from only two localities: the Crowsnest Formation in Alberta, Canada, and the Lupata Gorge, Mocambique. The type locality is near the town of Blairmore in Alberta, and comprises a sequence of tufts and breccias, with minor flows and dykes (Pearce 1970). Typical Crowsnest blairmorites contain abundant phenocrysts of analcime, together with phenocrysts of titaniferous garnet and aegirine-augite in a matrix of sanidine, analcime, aegirineaugite, and some calcite and zeolite. The analcime phenocrysts are euhedral and measure up to 3 cm in diameter. Along the Lupata Gorge on the Zambezi River, southeast of Tete in Mocambique, a series of volcanic rocks and sediments outcrops, the succession of which has been described by Dixey & Campbell Smith (1929) and Flores (1964). The volcanic rocks incbJde phonolites, kenytes, analcime kenytes, and blairmorites, and were described in some petrographic detail

by Campbell Smith (1929). Five analyses vf these rocks including a blairmorite, are given in Woolley & Garson (1970). Rocks similar to blairmorite were probably also developed in association with the Tundulu carbonatite complex in Malawi, some 180 kilometres northeast of Lupata, for dykes at Tundulu originally described as pseudoleucite phonolites (Dixey et al. 1937) were thought by WooUey & Garson (1970) to be pseudo-analcime phonolites, and were therefore correlated with the Lupata blairmorites. The Dixey-Campbell Smith material from Lupata is housed in the British Museum (Natural History) collection number B.M. 1929, 173 and formed She basis for the present work. Unfortunately, these rocks are not as fresh as could be desired but their undoubted petrological interest and the difficulty of obtaining further specimens would seem to justify their use in this study.

Petrography Three specimens were chosen for detailed work because of their relative freshness. They are:

10

A . R. l ¥ o o l l e y & R. F. S y m e s Table 1. Hand specimen modes of blairmorite (no. 72) and analcime kenytes (nos. 55 and 57).

I ¢m

PHENOCRYSTS

MATRIX VESICLES

Fig. la. Blairmorite showing euhedral analc~me pheno-

crysts (medium to dark grey) and nepheline phenoerysts (light grey) (B.M. 1929, 173(72)).

1 cm

i

fig. lb. Analcime kenyte showing euhedral nepheline

phenocrysts (pale to medium grey) and feldspars (dark grey). Small analcime phenocrysts lie amongst the dark ~atrix and are difficult to distinguish in a black and white print (B.M. 1929, 173(55)).

Iml=ml~m

1 cm

J:ig. It. Analcime kenyte showing euhedrai analcime Iwith thin, white alteration rims), nepheline (white), i~nd feldspar (medium to dark grey, cracked) pheno~::rysts (B.M. 1929, 173(55)).

Blairmodte (Dixey number Analcime kenyte (Dixey number Analcime kenyte (Dixey number

Analcime Nepheline Feldspar Magnetite

72

55

57

25.4 14.4 58.5 1.7

2.9 2.7 13.5 11.2 10.5 12.5 0.5 7 2 . 6 73.6 -

B.M. 1929, 173(72) N 724) B.M. 1929, 173(55) N 703) B.M. 1929, 173(57) N 705)

The three rocks are described petrographically by Dixey & Campbell Smith (1929). Modal analyses were made on the hand specimens with the results given in Table 1. Their striking appearance is apparent from Fig. 1. The analcimes of the blairmorite are euhedral and up to 1.5 cm in diameter; they often show multiple twin lamellae in thin section, similar to those of leucite, and commonly have narrow reaction rims. The euhedral nepheline phenocrysts are completely altered and are nearly isotropic in section, suggesting possible partial replacement by analcime. The rest of the rock, which in hand specimen would be referred to as groundmass, comprises subhedral blades of perfectly fresh ootassic feldspar similar in size to the phenocrysts. Although the euhedral nature of the analcimes and nephelines indicates that they crystallized before the feldspar, the coarseness of the latter suggests that it developed shortly after the analcime and feldspar phenocrysts. The feldspar poikilitically encloses myriads of minute crystals of an opaque mineral and small altered nephelines; aegirine and apatite are also present. In the analc~,me kenyte no. 55 the reaction rims around the analcimes are much broader than in the blairmorite, with sometimes only a small remnant of fresh analcime remaining in the centre. Both nepheline and feldspar phenocrysts are densely turbid, with occasJional clearer patches in the feldspars. Small phenocrysts of magnetite and apatitc are also present, whilst calcffe and chlorite fill |he vesicles. Analcime kenyte no. 57 is fresher than no.

Analcime-phyric phonolites

11

Table 2. Electron microprobe analyses of analcime, nepheline and feldspar. 1

2

3

4

5

6

7

8

SiO. AI~O~ CaO Na,O K20

53.5 22.3 0.00 12.8 0.31

54.9 22.2 0.02 12.7 0.15

53.9 22.00 0.00 11.4 2.2

53.6 22.6 0.00 12.8 0.13

64.0 17.9 0.03 2.1 13.4

62.1 20.0 0.46 4.6 9.4

64.5 19.6 0.37 50 8.9

44.9 31.S 0.10 !6.0 5.7

Total

88.91

89.97

89.50

89.13

97.43

96.56

98.37

98.50

Numbers o/ions on the basis o] 6 (0) ]or analcimes, and 32 (0) ]or ]eldspars and nephelines Si AI Ca Na

2.02 0.99 0.94

2.04 0.97 0.001 0.92

2.03 0.98 0.87

2,01 1.00 0.93

12.03 3.96 0.01 0.77

11.65 4.43 0.09 1.67

11.83 4.23 0.07 1.79

8.70 7.25 0.21 6.01

K

0.01

0.01

0.11

0.01

3.22

2.25

2.07

1.40

Qz Ne Ks

32.1 66.7 1.2

33.8 65.6 0.6

31.2 60.6 8.2

32.8 66.7 0.5

44.1 9.5 46.4

43.4 22.7 33.9

44.7 24. l 31.2

7.2 73.3 19.5

1. 2. 3. 4. 5. 6. 7. 8.

Analcime from blairmorite. B.M. 1929, 173(72). Average of four analyses. Analcime from analcime kenyte. B.M. 1929, 173(55). Average of four analyses. Potassium-rich area in analcime from analcime kenyte. B.M. 1929, 173(55). Analcime from analcime kenyte, B.M. 1929, 173(57). Average of two analyses. Feldspar from blairmorite. B.M. 1929, 173(72). Best of five analyses. Feldspar from analcimo kenyte. B.M. 1929, 173(55), Average of two analyses. Feldspar from analcimo kenyte. B.M. 1929, 173(57). Average of two analyses. Nepheline from analcime kenyte. B.M. 1929, 173(57). Best of three analyses, The analcimes are calculated to 6 rather than 7 oxygens to allow for the Jack of data for water. Analysts: R. F. Symes and J. C. Bevan. The analyses were made using a Cambridge Instruments Geoscan at an accelerating voltage of 15KV and a current of 0.5× 10-T amps. Analysed silicates and oxides were used as standards. Results were corrected after the method outlined in Sweatman & Long (1969) using the B.M.-I.C,-N.P.L. computer programme. Wherever possible, at least ten point-counts on each grain analysed were taken, as a check for homogeneity. The analyses are generally expected to be accurate to _+2°/c of the amount present for the major elements, but the relative errors on the minor constituents are probably grealer than this figure. Two separate analyses were made of feldspar no. 55 (analysis 6), arid both were stoichiometrically balanced but gave similar low corrected totals.

55. N e p h e l i n e a n d feldspar p h e n o c r y s t s exceeding a c e n t i m e t r e in length a r e p r e s e n t ; the nepheline is s o m e w h a t tarbid, whilst t h e feldspar is fresh. T h e analcirnes h a v e e i t h e r b r o a d reaction r i m s (Fig. lc) o r a r e c o m p l e t e l y altered to masses of a finely fibrous, subr a d i a t i n g m i n e r a l . Since m a n y of t h e a n a l c i m e p h e n o c r y s t s a n d p s e u d e m o r p h s a r e so small t h e y will h a v e b e e n included w i t h t h e m a t r i x in t h e m o d e ( T a b l e 1)° S t o u t e u h e d r a l crystals of a colourless c l i n o p y r o x e n e are plentiful, and m i c r o p h e n o c r y s t s of m a g n e t i t e a n d a p a t i t e are also present.

Mineralogy A n a l y s e s of suitably fresh a n a l c i m e , n e p h e l i n e , a n d feldspar p h e n o c r y s t s h a v e been m a d e by e l e c t r o n m i c r o p r o b e ( T a b l e 2). T h e n e p h e l i n e p h e n o c r y s t s in the b l a i r m o r i t e p r o v e d to be t o o a l t e r e d for r e p r e s e n t a t i v e analysis., whilst t h e f e l d s p a r analyses r e p o r t e d f r o m this rock w e r e m a d e on t e e r a t h e r coarse, b u t fresh, f e l d s p a r s w h i c h com, titute the b u l k of the m a t r i x . I n r o c k no. 55 also t h e n e p h e l i n e s are too a l t e r e d to justify analysis, b u t f r o m r o c k no. 57 good a n a l y s e s of all t h r e e phases were obtained. T h e

12 A. R. Woolley & R. F. Symes rewards a silica-rich composition. The Lupata and Crowsnest analcimes lie close to ~he A •~Lhermal trough of Kim & Burley. One of the analysed analcime crystals in ~ock no. 55 (Table 2, no. 3) contains 2.20% by weight K20, which contrasts with the averages for the other analcimes of 0.15% K20. Peters et al. (1966) estimated that only some 2% by weight of K20 can substitute in analcime, whilst Fudali (1963) found that only 1.7% K20 Ne Ks could be substituted. In view of these data, the Fig. 2. Plot of the analcime, feldspar, and nepheline high potassium Lupata analcime was investicompositions from the Lupata rocks, given in Table, 2, gated further and elemental scanning traces in l~art of the system quartz-nepheline-kalsilite (norindicated the potassium to be concentrated in mative w; percent). Circles, analcime; ringed circle, irregular patches scattered throughout the potassium-rich area in analcime of rock 55; triangles, feldspar; :~tars, nepheline; M, nepheline composition of crystal. These patches appear to be similar in Morozewicz (1930); solid squares joined by dashed tie composition to the altered analcime rims, line are the analcime and phenocrystal feldspar comalthough analysis of the rims could only be positions from a bla~rmorite of the Crowsnest formaiion (Pearce 1970, Tables 1 and 3); open square, comsemi-quantitative because of their complexly position of matrix feldspar from the blaitmorite of *he intergrown nature. X-ray powder photographs Crowsnest volcanics (Pearce 1970, Table 1); crosses, of samples of the altered ri[ms in both analcime blairmorite whole rocks of which L is f~rom Lulrata kenytes showed them to consist of a mixture and C from Crowsnest; ringed cross, thermal trough of the analcime stability field (Kim & Burley 1971). of analcime and orthoclase, and it is therefore thought likely that the high potassium analcime in fact includes a proportion of subdata presented in Table 2 represent either microscopic orthoclase. It is probable that some averages of between two and four analyses, of the high potassium analcimes reported in wihere the compositional ranges were small, or, the literature consist of similar analcime--orthoin the case of the feldspar of rock rio. 72 and clase :mixtures. thee nepheline of rock no. 57, the best analyses, The analysed nepheline from kenyte no. 57 chosen on a basis of stoichiometry, frc~m five (Table 2, no. 8) is chemically similar to the and three available analyses respectively. formula proposed by Morozewicz (1930), and The analcime analyses from all ~three rocks, therefore falls within the Morozewicz-Buerger with the exception of one which is high in convergence field defined by Tilley (1954). This potassium, lie a little to the silica-rich side field is characteristic of nephelines from rocks of the ideal stoichiometdc composition of containing low-temperature feldspars, particuNa[AISi206]. H20 (Fig. 2), assuming that the larly nepheline syenites and nepheline gneisses, balance in the totals is mainly water. The anal- whereas nephelines from volcanic rocks show a cime of the Crowsnest blairmorite, also plotted greater range of compositions which vary ~cin Fig. 2 is closely comparable in composition. cording to the chemical environment (Tflley The Lupata and Blairmor,~; analcimes are 19:54). The position of the Lupata nepheline in .rather richer in silica than most of those: the Morozewicz-Buerger field may be coincianalysed from igneous rocks, in which there is dental and determined by host rock composiusually some substitution of NaAl for Si (Wil.. tion, or it may represent adjustment to a lowkinson 1965). temperature composition, consistent with that The experimental work of Kim & Budey of the co-existing analcime which lies near tlhe (1971) has shown that liquidus analcime thermal trough of the analcime stability fieM changes from a composition near that of natro- as determined by Ki:a & Burley (1971). lite towards a thermal trough of the analcime The feld,~par phenocry.,4s of the analcim~.~ stability field close to the ideal formula, indi- kenytes have compositions of Or60 and Ors~, cated in Fig. 2. Similarly, Wilkinson (1965) Eas but the bllairmorite feldspar (Orsa) is notably shown for the Square Top intrusion that the mere potassic. Pearce (1970) found that thz compositional trend of liquidus analcime with Crowsnest blairmorite feldspar phenocrysts differentiation was from a point near natrolite have compositions of Ors0, whilst the ground-

Analcime-phyric phonolites Table 3. Analyses of blairmorites from the Lupata

Gorge, Mocambique, and for comparison~ from the Crowsnest Formation, Alberta, Canada. norms SiO~ TiO~ AlsO,, F%O 8

1

2

49.3 1.7

54.04 0.20

20.0 5.5

FeO MnO MgO CaO Na~O

0.2

KgO H20 +

4.7 5.5

CO:e

1.2

H~OPaO6 F

O- F To~.al

1

2

c or

2.5 27.8

13A

18.86

ab

35.9

51.9

3.30 0.76

an ne

2.9 12.1

0.9 16.6

-

0.25

0.08

di

-

3.8

0.3 2.5 6.9

0.70 2.32 9.77

wo ol mt hm

0.5 -

0.3 2.1

5.5

1.8

il

0.9 1.2 0.7 2.7 5.5 1.3

0.4 -

1.3 0.30

0.10 99.75 0.04 99.71

2.26 ~

~7"00 0.80

-

ru ap cc H~O+ H20-

1.8

} 7.0

100.09

1. Blairmorite. Between Gorufa and Sungo, Lupata Gorge, Mocambique. Analyst: A. A. Moss. B.M. 1929, 173(72). 2. Blairmorite (MacKenzie 1914, p. 23, anal. 1). Crowsnest Formation, Alberta, Canada. Analyst: M. F. Conner.

mass feldspar is Orss. The Lupata and Crowsnest blairmori~es are thus closely comparable in feldspar compositions, and are characterised by the co-existence of a highly potassic feldspar with analcime, indicated by the tie lines in Fig. 2. The trend of increasing potassium in the Crowsnest blairmorit¢ feldspars shown b y the phenocryst,~; and groundmass feldspar compositions, is confirmed by an increase in potassium from core to rim in zoned feldspars (Pearce 1970).

Petrochemistry In Table 3 an analysis is given of the Lupata blairmorite, with an analysis of the Crowsnest blairmorite for comparison (MacKenzie 1914). The two analyses are very sim;.lar, the only noteworthy difference being the rather more potassic nature of the Lupata rock. Further, although the Crowsnest rock is rather more sodic than normal, both lit: within the typical compo-

13

sitional range for nepheline syenites. This indicates, of course, that the unusual mineralogy of these rocks is a function of P - T conditions and not a reflection of an atypical chemical composition. Both rocks are plotted in Fig. 2, and the Lupata blairmorite lies close to the tie line of its constituent analcime and feldspar. Such a plot would suggest that the altered nepheline in the rock, which could not be analysed, had been replaced by a mixture of analcime and potassic feldspar. However, it is possible that the ferric to ferrous iron ratio as determined is too high, perhaps because of weathering; this would decrease the degree ,)f undersaturation, expressed in the norm. Although a high ferric to ferrous iron ratio is usual in such rocks it is thought likely that there has been some oxidation, and the rock should plot a little nearer to the Ne-Ks join in Fig. 2. The leucocratic minerals in the analysed Crowsnest blairmorite are described as analcime, feldspar, and nepheline by MacKenzie (1914) but Pearce (1970), in a more detailed study, did not detect nepheline. In Fig. 2 the position of the rock above its analcime-feldspar tie line, defined by the data of Pearce (1970), suggests that this rock also has been somewhat weathered, and it would also preclude the presence of nepheline. The scarcity of material together with heterogeneity caused by the relatively large size of the phenocrysts prevented analyses being made of the Lupata analcimc kenytes.

Discussion A number of papers have been published on experimental work in the system NaAISiO 4NaAISiaOs-H20, notably by Peters et al. (1966), Boettcher & Wyllie (1969), Kim & Burley (1971), and Hamilton (1972), in which liquidus analcime has been encountered, and which give some idea of the P - T conditions necessary for the crystallization of this mineral. In natural rocks, however, potassium is a very significant component so that the work of Morse (1968) and Roux & Hamilton (in press) on the sodium-rich portion of the system NaAlSiaO 8KAISiaO8-NaA1SiO4-KAISiO4 at high water pressures is the most relevant to the present study. Although there are some differences in the

14 A. R. Woolley & R. F. Symes location of the field of Frimary analcim, e in the proposed liquidus diagrams of Morse (1968, Fig. 21) and Roux & Hamilton (in press, Fig. 3), determined at 5 kbar and 6 kbar Pro0, respectively, the isothermal sections given iv. the range 600-665°C are very similar. These indicate a triangular field in which analchne+ nepheline + potassium-rich feldspar co-exist, and a more restricted field corresponding to the assemblage analcime + potassium-rich feldspar; tie lines for these assemblages are closely comparable with those obt,~ined from natural rocks (Fig. 2). The experimental work indicates the highly potassic nature of the feldspar in many assemblages, and the potassic composition of the nepheline where it co-exists with feldspar. This agrees with the mineralogy of the blairmorites and analcime kenytes. Although Roux & Hamilton (in press) did not determi~Le the composition of the analcime produced during their runs they conclude, from the geometry of their diagrams, that it must by very low in potassium, certainly containing less than 5% KAlSizO6, which again corresponds wiith the evidence from the Crowsnest and Lupata rocks. Kim & Burley (1971), studying the system Ab-Ne-H20 up to 15 kbar, found liquidus analcime only within the range 575-657:'C and 5.15-12.5 kbar. Unpubli.~hed preliminary work by Roux & Hamilton indic:ates that the blairmorite assemblage is stable over a small temperature range close to 600°C and at pre:ssures from a little above the invariant point of Kim & Burley at 5.15 kbars to about 13 kba~,~, where jadeite becomes stable (D. L. Hamilton, pers. comm. 1975). These pressures correspond to a minimum depth for the formation of the blairmorite assemblage of about 20 km and a range of 20-50 km, corresponding to the lower crust or upper part of the upper mantle. As Pearce (1970) has pointed out, these depths r~-,';~.~ that the blairmorites must have been transferred to the surface very rapidly indeed and then quenched. The Crowsnest blairmodte suite consists principally of taffs which, Pearce suggests, indicates that volcanism was highly explosive, as would be e~pected in a situation in which the magma had travelled rapidly up from depth. Although there are tufts in the Lupata succession, the blairmorites themselves are lava:s, but the field geology of this area is ~oo poorly known for any conclusions to be drawn regarding the nature of their ejection. The Lupata Series lies in the Zambezi Valley

in an area where the Zambezi Rift structure joins the southern end of the main East African Rift. The tectonics of this situation would seem to accord well with the concept that the Lupata analcime-bearing rocks were brought to the surface, very rapidly, if it is accepted that rift tectonics are dominated by tensional faulting, probably resulting from regional 'arching'. The Lupata Series consists of phonolites, kenytes, and blairmorites, with no basic or intermediate rocks in the succession (Dixey & Campbell Smith 1929). Similarly, the Chilwa Province of southern Malawi, a short distance to the north east which is probably co-magmatic with the Lupata volcanics (Woolley & Garson 1970), comprises large plutons and associated dyke suites of saturated and undersaturated salic rocks and carbonatites, with a singular lack of basic and intermediate rocks. This situation is similar to that prevailing along much of the Gregory Rift in Kenya, where huge volumes of phonolites and trachytes are not directly associated with basic or intermediate rocks (Williams 1971). The Crowsnest volcanics, apart from the blairmorites, consist principally of tufts of trachytic composition, and there are no basic rocks to which the parentage of the blairmorites and trachytes might be attributed by crystal-liquid differentiation. Bailey (1964, 1974) has explained the association on the African continent of large volumes of volatile-rich salic alkaline rocks by suggesting upwarping of the continental plate along the sites of the rifts, thus reducing the pressure in the lower crust and concentrating fugitive components from the underlying mantle, leading to partial melting of the lower crust. Wright (1969), on the evidence of olivine nodules in trachytes in Nigeria, has suggested that trachytes can be generated directly at subcrustal levels, so explaining the dearth of basic rocks in some continental volcanic associations. The experimental evidence indicates that rocks containing liquidus ~malcimc can only be produced at high water pressures, and cannot have been formed at high levels in the crust. This cons~tderation, together with the lack of associated basic rocks, and the tectonic setting of the African blairmorites, would seem to support the hypothesis that these rocks were generated directly by partial melting under high water pressures to,wards the base of the crust or within the upper mantle.

Analcime-ph yric phonolites Acknowledgements. - We are particularly indebted to Dr. D. L. Hamilton for making available to us unpublished results and for suggesting improvements to the paper. Dr. D. R. C. Kempe and Dr. A. C. Bishop criticaUy read the manuscript for which we are most grateful, and Mr. J. G. Francis took and interpreted the X-ray powder photographs.

References Bailey, D. K. 1964: Crustal warping - a possible tectonic control of alkaline magmatism. J. Geophys. Res. 69, 1103-1111. Bailey, D. K. 1974: Melting in the deep crust. Pp. 436442 in S6rensen, H. (Ed.), The Alkaline Rocks. Wiley, New York. Boettcher, A. L. & Wyllie, P. J. 1969: Phase relationships in the system NaAISiO,-SiO~H~O to 35 kilobars pressure. Am. 1. Sci. 267, 875-909. Dixey, F. & Campbell Smith, W. 1929: The rocks of the Lupata gorge and the north side of the lower Zambezi. Geol. Mag. 66, 241-259. Dixey, F., Campbell Smith, W. & Bisset, C. B. 1937 (revised 1955): The Chilwa series of Southern Nyasaland. Bull. geol. Sure. Nyasaland 5. Flores, G. 1964: On the age of the Lupata rocks, lower Zambezi River, Mozambique. Trans. geol. Soc. S. Afr. 67, 111-118. Fudali, R. F. 1963: Experimental studies bearing on the origin of ~.~udoleucite and associated problems of alkalie rock systems. Bull. GeoL Soc. Am. 74, 1101-1125. Hamilton, D. L. 1972: Progress in experimental petrology. Second progress report of research supported by N.E.R.C. at Edinburgh and Manchester Universities, 1969-1971. N.E.R.C., Series D, No. 2, 24-27 Kim, Ki-Tae & Burley, B. J. 1971: Phase equilibria in the system NaAISiaOs--NaAISiO,-H20 with special emphasis on the stability of analcite. Can. I. Earth Sci. 8, 311-337.

15

MacKenzie, J. D. 1914: The Crowsnest volcanics. Can. Geol. Surv. Mus. Bull. 4, geological series No. 20. Morozewicz, J. 1930: Der Mariupolit und seine Blutsverwandten. Tschermaks miner, petrvgr. Mitt. 40, 335--436. Morse, S. A. 1968: Syenites. Carnegie Institution Year Book: Annual Report of the Geophysical Laboratory. 67, 112-120. Pearce, T. H. 1970: The analcite-bearing volcanic rocks of the Crowsnest formation, Alberta. Can. 1. Earth Sci. 7, 46-66. Peters, T. J., Luth, W. C. &Tuttle, O. F. 1966: The melting of analcite solid solutions in the system NaAISiO~NaAISisOs-H~O. A m . Mineral. 51, 736'753.

Roux, J. & Hamilton, D. L. Primary igneous analcite an experimental study (in press). Sweatman, T. R. & Long, J. V. P. 1969: Quantitative electron-probe microanalysis of rock-form~ng minerals. I. Petrol. I0, 332-379. Tilley, C. E. 1954: Nepheline-alkali feldspar parageneses. A m. J. Sci. 252, 65-75. Wilkinson, J. F. G. 1965: Some feldspars, nephelines and analcimes from the Square Top intrusion, Nundle, N.S.W.I. Petrol. 6, 420--444~ Williams, L. A. J. 1971: The volcanic.~ of the Gregory Rift Valley, East Africa. Bull. Volcanol. 34, 439-465. Woolley, A. R. & Garson, M. S. 1970: Petrochemical and tectonic relationship of the Ma]awi carbonatitealkaline province and the Lupata-Lebombo volcanics, pp. 237-262 in Clifford, T. N. & Gass, I. G. (Eds.), African Magmatism and Tectonics. Oliver & Boyd, Edinburgh. Wright, J. B. 1969: Olivine nodules and related inclusions in trachyte from the Jos Plateau, Nigeria. Min. Mag. 37, 370-374. Accepted for publication August 1975 Printed January 1976