The geology, geochemistry and origin of ultrabasic fenites associated with the Pollen Carbonatite (Finnmark, Norway)

Chemical Geology, 40 (1983) 65--95 Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands

65

THE GEOLOGY, GEOCHEMISTRY AND ORIGIN OF ULTRABASIC FENITES ASSOCIATED WITH THE POLLEN CARBONATITE (FINNMARK, NORWAY)

B. ROBINS and M. TYSSELAND

Geological Institute, University of Bergen, 5014 Bergen (Norway) (Received July 26, 1982; accepted for publication December 8, 1982)

ABSTRACT Robins, B. and Tysseland, M., 1983. The geology, geochemistry and origin of ultrabasic fenites associated with the Pollen Carbonatite (Finnmark, Norway). Chem. Geol., 40: 65--95. The syn-Caledonian Pollen Carbonatite was emplaced into gabbroic cumulates, which were subsequently fenitized to ultrabasic rocks containing ferroan pargasite, augite or salite, biotite, apatite and variable amounts of calcite. The metasomatism took place in two stages. The initial fenitization led to the introduction of Ti, Fe ~÷, Fe 3÷, K, Mn, Co, Zn, Zr, Nb, Ba, La, Ce and Nd, and the abstraction of A1, Ni, Cu and Sr. The later carbonatization involved the addition of Ca, C, P, Sr, Y, Ba, La and Nd, removal of Si, A1, Mg, Na, Fe, Ti, Mn, K, V, Cr, Co, Ni and Zn, and the oxidation of Fe 2+ to Fe 3+. Extreme carbonatization resulted in fenites chemically indistinguishable from silicocarbonatitic facies of the magmatic carbonatite. The course of the infiltration metasomatism is deduced to have been controlled by the changing phase relations in a volatile-saturated carbonatite magma.

INTRODUCTION

Alkaline complexes and carbonatites are commonly associated with carbonate-rich rocks formed by replacement. These have been referred to as either metacarbonatites (Verwoerd, 1967; Vartiainen, 1980) or replacement carbonatites (Armbrustmacher, 1979). Replacement of pre-existing rocks appears to have been a process of major importance in the emplacement of both the Fen (Bowen, 1924; Saether, 1958) and Sokli (Vartiainen, 1980) carbonatites. However, the nature of the chemical changes during this special type of fenitization have received little attention and the physicochemical controls on the metasomatic processes are poorly understood. In this contribution to the problem, the geochemical evolution of carbonaterich ultrabasic fenites from gabbroic cumulate parents in contact with a carbonatite sheet is reconstructed. A new physicochemical model which has been erected on the basis of the interpretation of these metasomatic products may find broad application to high-grade fenites elsewhere. 0009-2541/83/$03.00

© 1983 Elsevier Science Publishers B.V.

66 THE GEOLOGY OF THE POLLEN COMPLEX

The Pollen Complex is one of three synorogenic, carbonatite-bearing alkaline complexes within the Caledonian Seiland petrographic province (Heier, 1961; Oosterom, 1963; Sturt and Ramsay, 1965). Their development took place towards the close of an early phase of the Caledonian orogeny with its attendant Barrovian amphibolite facies metamorphism, after irruption of dominantly basic and ultrabasic magmas (Robins and Gardner, 1975); Rb--Sr whole-rock isochron ages and K--Ar dates on nephelines suggest emplacement of the alkaline rocks at 501 -+ 27 Ma (Sturt et al., 1968, 1978).

r-rTw~

.~.o....o,

o o . ~,,

""

A?

,

~z

'i

,,

:' ~ t o \ - . . '

Fig. 1. The location and geology of the Pollen Carbonatite--Fenite Complex, Stjern~by, Norway.

67

The major components of the Pollen Complex are: (1) a strongly deformed complex comprising variably metamorphosed gabbros, syenodiorites and perthositic syenites; (2) a layered mafic complex younger than (1) but still deformed and metamorphosed in the amphibolite facies; (3) fenites derived from (2) ("mafic fenites"); (4) apatite--biotite--amphibole carbonatite (Fig. 1). Syenite pegmatites containing biotite and/or amphibole and biotite-bearing nepheline-albite pegmatites form a volumetrically minor part of the complex. The main carbonatite is a sheet-like body exposed over a strike of 1600 m, and with a width at the surface of up to 200 m. It is situated in a NW- to W-dipping thrust contact between the overlying metagabbro-syenodiorite--perthosite complex and the underlying mafic complex. Its contacts cut the tectonic foliation and relict rhythmic layering in the adjacent rocks, the contact between the mafic fenites and their parents and also a large amphibole--syenite pegmatite which other evidence shows to have been emplaced later than the carbonatite (Fig. 1). If the carbonatite was originally magmatic then this is by no means obvious today. Intense deformation has led to or accentuated the pronounced interbanding of mafic- and calcite-rich carbonatite as well as causing the inclusion, folding, boudinage and partial disintegration o f xenoliths of mafic fenite, later mafic dykes and syenite pegmatite and the lineation of amphibole and apatite (Fig. 2). The contacts of the carbonatite must be considered to be tectonic, the body having been tectonically remobilized and reintruded during deformation. Igneous rocks emplaced into the carbonatite after its deformation have not been recorded.

Fig. 2. Banding and xenoliths of mafic fenite within apatite--biotite--amphibole s~bvite from the Pollen Complex. Hammer shaft is 80 cm in length.

68 The silicate minerals and apatite in the carbonatite have distributions and relationships with inclusions which suggest that they are to a certain extent xenocrysts derived by mechanical assimilation during deformation. Amphiboles, although often subhedral to euhedral in form, are often enriched in zones extending along the foliation from fenite inclusions, while highly irregular, corroded albites are particularly abundant in the vicinity of gneissose pegmatite fragments. The carbonatite sheet itself has a distinct zonation which also may be ascribed to assimilation. Near its lower contact with fenites and mafic cumulates, xenoliths and mafic silicates are abundant. The latter become progressively less prominent towards the upper contact of the carbonatite with perthositic syenite and syenodiorite. Apatite is rarely euhedral in the carbonatite. It usually forms slightly elongated, more or less rounded crystals believed to have been derived either from primary apatite by microboudinage or from the granular apatite characteristic of the fenite inclusions. Both the spheroidal apatite and amphibole are often highly lineated, though their orientation may be irregular even within a single outcrop. The occurrence of euhedral apatite crystals up to 10 cm long and 1 cm in diameter orientated within the foliation suggests, however, that the banding cannot be ascribed solely to deformation. Minor carbonatite intrusions are abundant within the fenites and are also observed within the deformed cumulates. Two types of intrusion have been recognized: (1) relatively little-deformed apatite--biotite--amphibole carbonatite dykes containing subhedral to euhedral amphibole and apatite, the latter occasionally concentrated along the contacts and orientated at a high angle to them; and (2) carbonatite breccias choked with highly contorted and rounded inclusions of gneissose syenite, nepheline--albite pegmatite and/or mafic fenites and characterized by rounded and polished apatite and amphibole and corroded feldspar. These two types of minor intrusion have different age relationships with respect to the alkaline pegmatites. The little-deformed and xenolith-free dykes are cut by variably deformed alkaline pegmatites, while the breccias are developed within the same pegmatites or along their margins (Fig. 3) and may even cross-cut the pegmatites. Evidence of the partial replacement of pegmatites by carbonatitic material is clear even outside the fenitized area and the breccias are interpreted in some cases as due to the subsequent deformation of these bodies. In other cases the breccias are due to the introduction of carbonatite from an external source in a plastic condition, most probably during deformation of the complex. The layered mafic complex resembles the Rognsund Intrusion (Robins, 1982), both texturally and mineralogically. The steeply dipping layers of widely different mineralogical composition range up to a few metres in thickness and are often paralleled by a tectonic foliation. Coarse-grained plagioclase--clinopyroxene--olivine cumulates, occasionally anorthositic, are the most c o m m o n layer type, while cumulates rich in any of olivine,

69

Fig. 3. Carbonatite breccia dyke within the Pollen mafic fenites. The inclusions are of nepheline--albite gneiss and the dyke passes laterally into nepheline--albite pegmatite.

clinopyroxene and magnetite are frequent. Olivine and/or clinopyroxene crescumulates form a significant proportion of the cumulate sequence. Layer boundaries may be ratio, phase or form contacts. They are normally planar and traceable for tens of metres along the strike. Density-graded layers are u n c o m m o n , b u t together with rare erosional contacts they suggest that the cumulate sequence " y o u n g s " southwards. Dykes of picrite and metagabbro up to 1 m wide are c o m m o n within the complex. The majority of these intrusions antedate the syenite and nepheline--albite pegmatites, and some were clearly in a foliated condition when the alkaline pegmatites were emplaced. The layered complex is in sharp contact with more strongly deformed syenodiorites and perthosites, both cut by several generations of basic and ultrabasic dykes, near its contact with the fenites. The alkali feldsparbearing rocks can also be followed into the fenites. The syenodiorites and perthosites are interpreted as forming a large raft within the layered complex, in accordance with the age relationships established elsewhere for these t w o petrographic units (Robins and Gardner, 1975). The mafic fenites, i.e. fenites o f generally mafic parentage, crop o u t in the Pollen Complex over a roughly triangular area of ~ 0.5 km 2. The total area underlain by these rocks is u n d o u b t e d l y larger, since the fenites appear to extend beneath the sea, reappearing on the island of PoUenholm (Fig. 1). The fenites are heterogeneous coarse-grained ultramafic to leucocratic rocks composed mainly of amphibole, clinopyroxene, biotite and apatite, and highly variable amounts of calcite. Feldspar may be present in acces-

70

sory amounts b u t is generally absent. Amphibole, calcite and apatite are the main constituents of the fenites and can form monomineralic types. Calcite- and apatite-rich fenites generally form irregular areas within amphibole-dominated fenites. However, apatite-rich fenites with amphibole as the other main mineral occur in a distinct zone a few metres wide along part of the eastern fenite--cumulate boundary (Fig. 1). Calcite-rich fenites, essentially identical to intrusive carbonatite, may also form veins up to 50 cm wide within amphibole-dominated fenites. These are characterized by their impersistence, irregular form and gradational contacts with the adjacent fenites. The contact between the fenites and the mafic cumulates is exposed in only two places; it dips steeply and is relatively sharp b u t gradational over 30 cm. In the contact zone the primary mafic minerals are replaced, together with part of the feldspar, by amphibole, clinopyroxene and biotite, while the remaining plagioclase is replaced by scapolite and calcite. Unaltered relicts of the mafic complex are c o m m o n within a few metres of the contact, picritic dykes persisting in recognizable form furthest from the contact of the mafic complex with the fenites. The contact of the fenites in several areas is strongly discordant to the layering in the cumulates, and a relict banding, cross-cut by several generations of fenitized dykes, can in places be detected in the fenites (Fig. 4). However, the calcite-rich fenites may brecciate the more c o m p e t e n t mafic or ultramafic fenites. Bands of amphibole-rich fenites, together with later alkaline pegmatites, have also been boudinaged and folded within the calcite-rich fenites.

Fig. 4. Fenitized mafic cumulates in the Pollen Complex cut by several generations of fenitized mafic dykes.

71 GEOCHEMISTRY The carbonatites

The major- and trace-element geochemistry of the carbonatites within the Pollen Complex reflects their highly variable mineralogical composition. Seventeen samples analysed by X-ray fluorescence spectrometry (Table I) show wide ranges (R) and large coefficients of variation (c) for almost all the elements determined, the variation being largely a function of varying calcite/silicate ratios. This is expressed in the major elements by positive correlations between Ca, Mn and CO~, and negative correlation of these and the remaining elements. The mean major~lement composition of the Pollen Carbonatite differs from that of carbonatites in general only by higher SiO2 and lower CaO, MgO, K20 and CO2 (Table I). However, a strong compositional divergence exists between the mean Pollen Carbonatite and average sedimentary limestones, the former containing significantly more TiO2, FeO t, MgO, Na20 and P2Os. The Pollen carbonatites are, nevertheless, neither alkaline nor peralkaline rocks. They contain no feldspathoids or alkaline mafic minerals, and typically show hy and even Q in their norms. The mean (Na20 +K20)/ A1203 molecular ratio for the analysed carbonatites is well below 1.0 (Table I) and in no single case does it exceed 0.55. r The Pollen carbonatites are also distinguished from sedimentary limestones, and metalimestones from elsewhere on the island of Stjern¢y, by their higher contents of the light REE, Zr and Y (Table I), while they differ little from the metalimestones in Nb. The paucity of the latter element contrasts with many carbonatites and corresponds to the absence of pyrochlore in the Pollen occurrences. The trace elements in the Pollen carbonatites can be separated into two groups on the basis of their mutual variations: Sr, Y, Zr, La, Ce and Nd have significant positive correlations and vary antipathetically with the remaining analysed elements. The REE, Sr, Y and Zr correlate with CaO, CO: and MnO, suggesting their concentration in calcite. The other trace elements appear to be contained within the silicate fractions of the carbonatites. The cumulates

Twenty-five analysed samples from the layered mafic complex are generally ne-normative and have a gabbroic mean composition (Table II). SiO2 and CaO are relatively invariant while MgO, A1203, Fe203 and FeO have large ranges. Elements concentrated in the oxide phases of the cumulates, i.e. TiO2, Fe203, FeO and MnO are negatively correlated with SiO2 and A1203, while MgO is negatively correlated only with A1203. Variations in the proportions of the cumulus minerals are also expressed in the correlation of CaO

72 TABLE I Mean c o m p o s i t i o n (~), standard deviation (s) and s o m e other statistics for carbonatites from the Pollen C o m p l e x (17 analyses)

SiO 2 (wt.%) AI203 TiO 2 Fe203 FeO MgO CaO Na20 K20 MnO P20~ H20 ÷ CO2

24.75 3.92 0.78 3.24 4.86 3.59 32.57 0.68 0.40 0.33 2.83 0.36 22.19

s

c

R

1

2

14.67 3.01 0.83 2.47 3.31 1.84 13.01 0.54 0.47 0.05 1.99 0.14 10.60

0.59 0.77 1.06 0.76 0.68 0.51 0.40 0.79 1.18 0.15 0.71 0.39 0.48

6.82 3.28 1.75 2.55 3.55 2.46 6.45 1.45 1.24 0.44 2.73 0.70 5.89

10.29 3.29 0.73 3.46 3.60 5.79 36.10 0.42 1.36 0.68 2.09 1.44 28.52

6.9 1.7 0.05 0.98 1.3 0.97 47.6 0.08 0.57 0.08 0.16 0.84 38.3

100.50 (Na20 + K20)/AI203 (molecular) 0.36

Ti ( p p m ) V Cr Co Ni Cu Zn Rb Sr Y Zr Nb Ba La Ce Nd

4,638 62 24 18 59 3 38 3 4,397 62 182 23 1,072 314 422 210

3

99.53

0.16

4,828 62 17 18 36 4 24 6 1,829 24 23 34 213 103 125 59

1.04 1.00 0.71 1.00 0.61 1.33 0.63 2.00 0.42 0.39 0.13 1.48 0.20 0.33 0.30 0.28

136 16 8 8 13 4 9 5 78 9 9 10 32 20 22 15

105 102 19 32 588 160 52 7,525 114 461 5 6 0 .1 403 5 , 3 0 0 *5

20 11 0.1 20 4 20 3 527 30 19 0.3 4 < 1 11.5

43 778 25 81 28 417 71 106 52

= arithmetic mean; s = standard deviation; c = c o e f f i c i e n t o f variation (= s/~); R .~ range (= ~ ) . I = average c o m p o s i t i o n o f carbonatites: major e l e m e n t s after Heinrich ( 1 9 6 6 ) ; trace e l e m e n t s after G o l d ( 1 9 6 6 ) ; 2 = average c o m p o s i t i o n o f limes t o n e s ( W e d e p o h l , 1 9 6 9 ) ; trace e l e m e n t s after G o l d ( 1 9 6 3 , 1 9 6 6 ) ; 3 = average tracee l e m e n t c o m p o s i t i o n o f 1 0 m e t a l i m e s t o n e s from Stjern~by (Strand, 1 9 8 1 ) . ,1 Includes Ta. ,2 Total REE.

73 T A B L E II S u m m a r y o f a n a l y t i c a l d a t a for 25 a n a l y s e d c u m u l a t e s f r o m t h e Pollen C o m p l e x a n d 31 f e n i t e s derived f r o m t h e m Cumulates

Fenites s

SiO 2 (wt.%) AI~O 3 TiO2 F%O3 FeO MgO CaO Na20 K20 MnO P205 -H20 + COs

44.36 20.55 0.95 2.80 5.11 8.59 13.90 2.18 0.27 0.16 0.09 0.23 0.01

c 2.05 4.13 1.02 2.10 2.15 3.26 1.36 0.73 0.20 0.03 0.05 0.32 0.04

0.05 0.20 1.08 0.75 0.42 0.38 0.10 0.33 0.74 0.18 0.58 1.39 4.80

R

E 3.10 3.96 1.99 3.01 2.86 3.88 2.43 1.59 0.82 0.32 0.50 0.92 0.45

99.20

s 35.91 10.39 2.54 5.43 10.55 7.51 17.39 1.78 1.18 0.32 1.72 0.59 5.32

c 4.64 2.25 0.58 1.61 2.56 2.21 4.32 0.42 0.50 0.08 2.05 0.20 3.43

0.13 0.22 0.23 0.30 0.24 0.29 0.25 0.51 0.42 0.25 1.19 0.34 0.65

R 4.04 3.41 1.79 2.39 3.33 2.90 3.90 1.56 1.71 0.53 3.46 1.03 3.79

100.63

(Na20 + K20)/AI203 (molecular) Ti ( p p m ) V Cr Ni Cu Zn Rb Sr Y Zr Nb Ba La Ce Nd

5,661 170 204 426 49 32 7 540 5 47 10 208 8 23 11

0.41 6,120 192 108 151 31 12 4 123 3 20 4 121 11 14 7

1.08 1.13 0.53 0.35 0.63 0.38 0.57 0.23 0.60 0.43 0.40 0.58 1.38 0.61 0.64

154 27 20 23 11 6 4 23 3 8 4 23 7 8 6

13,996 230 178 188 19 133 11 1,156 23 189 67 867 112 163 92

0.08 3,699 70 178 129 42 373 7 750 14 208 37 324 72 105 57

0.26 0.30 1.00 0.69 2.21 2.80 0.64 0.65 0.61 1.10 0.55 0.37 0.64 0.64 0.62

127 18 27 21 14 45 5 53 8 35 13 37 18 22 16

= a r i t h m e t i c m e a n ; s ffi s t a n d a r d d e v i a t i o n ; c = c o e f f i c i e n t o f v a r i a t i o n (= s l ~ ) ; R = range (ffi ~ ) .

and Na20 with SiO2 and A1203. Sympathetic variations of K20 and P2Os may, however, be ascribed to variable amounts of intercumulus material. Compared with the mean ~omposition of the Pollen carbonatites (Table I), the cumulates are richer in SiO2, A1203, MgO and Na20 and poorer in CaO, K20, MnO and P2Os. Certain elements, however, have similar concentrations in the two rock groups, notably TiO2, Fe203 and FeO.

74 The behaviour of the trace elements in the metagabbros is also consistent with varying proportions of cumulus and intercumulus phases. Ti, V, Co and Zn all vary sympathetically and are negatively correlated with Ni and Cr and the residual elements Rb, Y, Zr, Nb, Ba, La, Ce and Nd. Both of the latter groups show positive inter-element correlations. When compared with the carbonatites the cumulates are depleted in all the residual trace elements, with the exception of Rb which on average is relatively enriched in the cumulates.

The mafic fenites The mean of 31 analysed fenites is distinctly ultrabasic with a high tenor of CaO and FeO t (Table II). The fenites are not peralkaline: the mean molecular (Na20 + K20)/Al:O3 ratio is 0.41 and differs little from that characterizing the carbonatites. Relative to the mean cumulate the fenites are depleted in SiOs, A1203 and Na:O and enriched in TiOs, both Fe:O 3 and FeO, CaO, K:O, MnO, PsOs and CO2. MgO differs little between the mean fenite and mean cumulate. CO: varies widely in the fenites and causes corresponding variations in SiO2, but in the opposite direction. The variation exhibited by other oxides, as shown by their ranges and coefficients of variation, differ little in magnitude from the corresponding variations in the cumulates. However, the nature of the variations in the fenites is strikingly different and closely resembles the variations in the carbonatites. As in the latter, the fenites show positive correlations between CaO and COs, but these are joined in behaviour by FesO3 and P:Os. These oxides correlate negatively with all others, whose behaviour largely reflects their fixing in silicates whose mode varies antipathetically with carbonate and apatite. The mean trace-element composition of the fenites diverges strongly from that of the cumulates. The fenites are significantly enriched in V, Co and Rb, and highly enriched in Zn, St, Y, Zr, Nb, Ba and the light REE. The fenites are depleted relative to the cumulates in only Ni and Cu. As in the carbonatites, St, Y, La, Ce and Nd have strong positive inter-element correlations and also correlate with CaO, Fe:O3 and COs. The behaviour of the remaining traces seems to correspond to their concentration in the various silicate components of the fenites: the correlations which exist between Ti, V, Co, Cu, Rb and K:O may be explained by their location in biotite and amphibole. Seven of the analysed fenites have anomalous compositions, and these have n o t been included in the analysis of chemical trends reported below. One of these is an unusually apatite-rich fenite from the eastern fenite-mafic cumulate contact (Fig. 1), while the others, with one exception, are all fenites from the island of Pollenholm. The latter are depleted in MnO, FeO and Fe:O3 relative to the other fenites, and enriched in MgO (Table III). They may represent fenites developed from picritic parents.

75 T A B L E III M e a n c o m p o s i t i o n o f five f e n i t e s f r o m P o l l e n h o l m (wt.%)

~

s

(ppm)

£

s

SiO2 Al~O3 TiO 2 F%O 3 FeO MgO CaO Na20 K~O MnO P205 H20 ÷ CO 2

39.88 12.28 2.82 3.67 7.50 11.24 16.12 1.71 1.02 0.15 0.44 0.50 3.10 100.43

1.36 0.84 0.29 1.14 2.13 2.08 1.68 0.43 0.43 0.017 0.28 0.16 0.47

Ti V Cr Co Ni Cu Zn Rb Sr Y Zr Nb Ba La Ce Nd

16,915 324 114 42 387 91 39 16 430 12 105 28 635 33 58 38

1,725 66 25 10 110 63 12 8 98 2 15 9 290 17 23 11

= arithmetic mean; s = standard deviation.

MINERALOGY

The mineralogy of 5 gabbroic cumulates, 7 fenites and 3 carbonatite samples has been investigated by electron microprobe analysis. The analyses were carried out on an A.R.L. ® SEMQ operated at 15 kV and with a beam current of 8--10 nA, using standard wavelength dispersive techniques (Reed, 1975) and ZAF data reduction (Colby, 1968). A mixture of synthetic oxides, pure metals and simple, well-characterized minerals was employed as standard. The study concentrated on the characterization of phases common to the palaeosome, fenites and the intrusive carbonatite and was, in addition, restricted to silicate minerals. Analytical results are reported in Tables IV--X. The cumulates are dominated by plagioclase, Ca-rich pyroxene and calcic amphibole. Olivine occurs sporadically and is usually accompanied by symplectitic coronas consisting of pyroxenes and spinel due to subsolidus reaction with plagioclase. In most of the analysed specimens olivine appears to have been completely replaced by either pyroxene--spinel symplectite or by granular, Ca-poor pyroxene. Fe--Ti-oxides are found in accessory amounts. The samples investigated vary from slightly deformed and metamorphosed cumulates to strongly foliated and recrystallized amphibolitic metagabbros. The Ca-rich pyroxenes are magnesian salites with a limited compositional variation from Ca4sMg42(Fe,Mn)10 to Ca44Mg42(Fe,Mn)12 (Fig. 5). They

76 T A B L E IV E l e c t r o n m i c r o p r o b e analyses o f Ca-rich p y r o x e n e f r o m t h e c u m u l a t e s S p e c i m e n *l n .2 SiO: Al~O3 TiO2 FeO MnO MgO CaO Na20

1 6

2 7

3 6

4 6

5 8

50.49 6.30 1.01 5.79 0.12 13.72 21.83 0.87

49.70 5.20 0.71 6.27 0.17 13.80 22.09 0.76

49.88 5.54 0.83 7.22 0.17 13.87 21.10 0.72

50.27 5.43 1.13 5.76 0.17 14.67 21.67 0.72

50.38 5.66 0.75 6.62 0.18 13.28 21.48 1.00

100.13

98.71

99.33

99.82

99.35

S t r u c t u r a l f o r m u l a e .3 :

Si Al TM A1VI Ti F e 3÷ Fe 2÷ Mn Mg Ca Na O Ca Mg Fe + M n

1.850 0.150 0.123 0.028 0.033 0.144 0.004 0.749 0.857 0.062 6.000 47.9 41.9 10.1

1.850 0.150 0.078 0.020 0.087 0.108 0.005 0.766 0.881 0.055

1.849 0.151 0.092 0.023 0.065 0.159 0.005 0.766 0.838 0.052

1.845 0.155 0.081 0.031 0.063 0.114 0.005 0.803 0.852 0.051

1.865 0.135 0.112 0.021 0.053 0.152 0.006 0.733 0.852 0.072

47.7 41.4 10.9

45.7 41.8 12.5

44.6 44.6 10.7

47.5 40.8 11.7

1.99 7.83 3.22 0.00 5.48 0.54 38.28 5.41 37.26

2.31 9.15 1.31 0.00 5.17 0.53 38.32 7.95 35.24

3.12 8.06 1.18 0.00 5.12 0.53 40.13 5.69 36.17

2.09 9.34 0.00 1.88 5.30 0.56 36.63 7.60 36.60

Calculated standard molecules:

Ti--Di Ca--Ts CaFe--Ts Jd Ac Johann En Fs Wo

2.78 9.42 0.00 2.86 3.32 0.37 37.67 7.21 36.57

, i A n a l y s e d s p e c i m e n s : 1 = P o 6 4 ; 2 = P o 6 5 ; 3 = P o 6 6 ; 4 = P o 6 7 ; 5 = P o 68. *2n = number of point analyses. ,3 S t r u c t u r a l f o r m u l a e c a l c u l a t e d o n t h e basis o f 4 . 0 c a t i o n s a n d 6.0 oxygens.

77 TABLE V Electron m i c r o p r o b e analyses of plagioclase f r o m the cumulates Specimen .1

1

2

3

4

5

n .2

9

6

6

6

7

50.14 31.96 0.04 0.05 0.01 0.00 14.39 3.10 0.03

47.45 33.19 0.03 0.04 0.00 0.00 15.83 2.23 0.05

47.90 32.45 0.05 0.05 0.03 0.00 15.85 2.37 0.04

47.00 33.88 0.00 0.04 0.00 0.00 16.79 1.87 0.02

50.18 31.67 0.00 0.07 0.03 0.00 13.99 3.31 0.01

99.72

98.82

98.74

99.60

99.26

SiO 2 Al203 TiO 2 FeO MnO MgO CaO Na20 K20

Structural formulae ,3. Si A1 Ti Fe Mn Mg Ca Na K O

9.147 6.877 0.005 0.008 0.001 0.000 2.816 1.096 0.007 32.000

8.785 7.241 0.004 0.005 0.000 0.000 3.140 0.803 0.011

8.879 7.088 0.007 0.008 0.004 0.000 3.149 0.851 0.008

8.655 7.352 0.000 0.005 0.000 0.000 3.313 0.665 0.005

9.189 6.840 0.000 0.011 0.005 0.000 2.748 1.172 0.001

Si + A1 Ca + Na + K

16.024 3.919

16.026 3.954

15.967 4.008

16.007 3.983

16.029 3.921

An%

71.9

79.5

78.6

83.2

70.1

,i For sample Nos. see Table IV. ,2 n = n u m b e r of point analyses. ,3 Structural formulae calculated o n the basis of 32.0 oxygen ions.

are relatively rich in A1203 as compared with pyroxenes from low-pressure cumulates of tholeiitic affinity though not significantly different in TiO2 concentrations. In terms of calculated standard end members the pyroxenes are rich in Ca-Tschermak's molecule and contain more acmite than jadeite (Table IV). The pyroxenes are compositionaUy similar to those from the lowermost cumulates of the nearby Rognsund intrusion which are interpreted as fractionates of a critically undersaturated basalt (Robins, 1982). The plagioclases are bytownites with mean compositions varying from An83 to AnT0 (Table V). In individual samples, however, the plagioclase varies considerably in composition; both normal and reverse zonation has been noted.

78 TABLE VI Electron microprobe analyses of amphibole from the cumulates Specimen .1 n .2 SiO~ A1203 TiO: FeO MnO MgO CaO Na20 K20

Structural

formulae

1 6

2 6

3 6

4 6

5 6

41.47 16.24 2.50 9.39 0.10 13.85 11.23 3.03 0.48

40.61 16.50 2.70 10.33 0.13 12.48 11.49 2.47 1.10

41.02 15.86 2.86 10.70 0.10 12.34 11.43 2.75 0.73

40.92 16.59 1.92 8.46 0.07 14.02 11.37 2.86 0.85

41.09 16.33 2.06 11.10 0.16 12.84 11.15 2.90 0.55

98.29

97.80

97.79

97.06

98.18

.3 :

Si A1TM A1V1 Ti Fe 3 Fe: Mn Mg Ca Na K O

5.920 2.080 0.652 0.268 0.534 0.588 0.012 2.947 1.717 0.838 0.086 23.000

Ca Mg Fe + Mn

29.6 50.8 19.6

5.903 2.097 0.729 0.295 0.301 0.955 0.016 2.704 1.789 0.695 0.204 31.0 46.9 22.1

5.971 2.029 0.693 0.314 0.233 1.070 0.012 2.678 1.782 0.776 0.135 30.9 46.4 22.8

5.913 2.087 0.739 0.209 0.453 0.570 0.009 3.021 1.760 0.800 0.157 30.3 52.0 17.8

5.906 2.094 0,672 0.223 0.632 0.703 0.020 2.750 1.717 0.809 0.100 29.5 47.2 23.3

*~ For sample Nos. see Table IV. ,2 n = number of point analyses. *3Structural formulae calculated on the basis of 13 cations (excluding Ca + Na + K) and 23.0 oxygen ions.

A m p h i b o l e s are p a r g a s i t e s ( F i g . 6) s i g n i f i c a n t l y m o r e i r o n - r i c h t h a n t h e a s s o c i a t e d C a - r i c h p y r o x e n e s {Fig. 5). TiO2, N a 2 0 a n d K 2 0 are all s t r o n g l y fractionated into amphibole relative to the pyroxene. Tie-lines between c o e x i s t i n g p y r o x e n e s a n d a m p h i b o l e s i n t h e a n a l y s e d c u m u l a t e s (Fig. 5) do n o t intersect a n d suggest e q u i l i b r i u m Mg--Fe d i s t r i b u t i o n b e t w e e n these phases. T h e a n a l y s e d o l i v i n e ( T a b l e V I I ) h a s a c o m p o s i t i o n o f Fo71 a n d is p a r t l y r e p l a c e d b y g r a n u l a r b r o n z i t e (En74). B r o n z i t e ( E n v l ) i n a n o t h e r s p e c i m e n also a p p e a r s t o f o r m p s e u d o m o r p h s a f t e r o l i v i n e .

79 Ca

Mg

50/

Fe.Mn

'~1~ CPX ~!,&~"...'~

ca 4

I', ','~

\ \\\\v

,,,,f

o Cumulates • Fenites x Carbonatites

~,/~AM

20/-

/ /

/ /

11 /I

40/

/

/

i /

/

III / Mg

/

/

~

o~x ~ ~

v

10

20

Z.0

30

i

~_~_~ _

50 60 70 Fe*Mn Fig. 5. Ca--Mg--(Fe + Mn) relationships of analysed biopyriboles from the Pollen mafic cumulates, fenites and carbonatite.

(No*K) A > 0.50 ; Fe3 < A [ vt 1,00

Mg~Ng,Fe2

P0rgositic Pargasite Hornblende 8

o o

0.50

Ferroon gerroon Pargosite Porgasitic Hornblende ,x

o Cumulates • Fenites x Carbonatites

•

FerroFerro- Pargasite Porggsitic Hornblende 0.00•.50

I

625

6.00

I

5.75

Si

Fig. 6. Compositional relationships of analysed calcic amphiboles from the Pollen Complex. Amphibole classification after Leake (1978).

80 TABLE VII Electron microprobe analyses of olivine and Ca-poor pyroxene from the cumulates Specimen *~ n .2 SiO~ A1203 TiO 2 FeO MnO MgO CaO Na20

4

4

2

4

5

3

37.12 0.01 0.01 26.11 0.36 36.25 0.02 0.01

52.52 3.76 0.09 15.93 0.39 27.10 0.54 0.02

50.86 4.39 0.08 17.92 0.34 25.59 0.44 0.03

99.89

100.35

99.67

S t r u c t u r a l formulae*3 :

Si A1 Ti Fe Mn Mg Ca Na O Ca Mg Fe + Mn

0.987 0.001 0.000 0.580 0.008 1.436 0.001 0.001 4.000 0.0 70.9 29.1

1.893 0.160 0.003 0.480 0.012 1.456 0.021 0.002 6.000 1.1 74.0 25.0

1.866 0.190 0.002 0.550 0.011 1.400 0.018 0.002 0.9 70.8 28.4

,1 For sample Nos. see Table IV. ,2 n = number of point analyses. , 3 Structural formulae calculated on the basis of 4.0 oxygen ions (olivine) and 6.0 oxygen ions (Ca-poor pyroxene). The c a r b o n a t i t e c o n t a i n s calcite, which EDS m i c r o p r o b e analyses s h o w t o c o n t a i n in t h e o r d e r o f 1--2 wt.% FeO and ~ 1 wt.% SrO, apatite, calcic a m p h i b o l e a n d either Ca-rich p y r o x e n e or biotite or b o t h . Feldspar o c c u r s in limited a m o u n t s as highly-strained irregular crystals, p r o b a b l y x e n o crysts, a n d is usually m a n t l e d b y c o r o n a s c o n t a i n i n g a m p h i b o l e with biotite a n d / o r Ca-rich p y r o x e n e . The a m p h i b o l e s are t i t a n i a n - - f e r r o a n pargasite or s o d i a n - - t i t a n i a n - ferroan pargasitic h o r n b l e n d e (Fig. 6). C o m p a r e d with the a m p h i b o l e s in the c u m u l a t e s , t h e y have l o w e r Mg/(Mg + Fe2+), c o n t a i n less A1203 (Fig. 7) a n d CaO (Fig. 5), a n d m o r e N a 2 0 + K 2 0 (Fig. 7) and TiO2. The c l i n o p y r o x e n e s are sodian ferroaugites. As is the case with the carb o n a t i t e a m p h i b o l e s , the c l i n o p y r o x e n e s are less calcic and c o n t a i n signific a n t l y less A1203 a n d m o r e N a 2 0 t h a n t h e Ca-rich p y r o x e n e in the c u m u l a t e s

81 20

®

7 A mph ibole

A[203

wt%

6

oo~e 15

5

%.

02

Pyroxene

o0~

.%

o Cumulates

4

°~

10

® o

• Fenites x Carbonatites

3 I

i

0', '016 '0B

l

02

I

0.4

'

016

i

o'.8

No20.K20 wt % °o

~2

o~%

o.2

I

I

I

o'.6

I

o'B

FeO/M.gO +FeO

o.2

t

I

o16

i

o18

FeO/MgO+ FeO

Fig. 7. Variation of AI20 , and Na20 + K20 with FeO/(MgO + FeO) for Ca-rich pyroxenes and calcic amphiboles from the Pollen Complex.

(Fig. 7). This is reflected in the calculated standard end members by lower amounts of Ca-Tschermak's molecule a n d higher jadeite and acmite. The difference between the Mg/(Mg + Fe) ratios in coexisting pyroxenes and amphiboles is also larger in the carbonatites than in the cumulates. Tielines for these phases in the carbonatite are almost parallel to the Ca-Fe+Mn side in the Ca--Mg--Fe+Mn triangle, while the equivalent tie-lines for the cumulates are much steeper (Fig. 5). The biotites analysed in two samples (Table X) have higher Mg/(Mg + Fe) ratios than the coexisting amphiboles, but a biotite coexisting with pyroxene has a lower Mg/(Mg + Fe) ratio than the latter (Fig. 5). The fenites are dominated by amphibole and calcite. These may be accompanied by Ca-rich pyroxene and biotite. Apatite is usually present in accessory amounts, but locally forms a major part of the mode. Feldspar can occur in limited amounts as relicts rimmed by amphibole. The amphibole is usually a sodian--titanian--ferroan pargasite compositionally similar to, but more magnesian than, the carbonatite amphibole (Fig. 6). The amphibole appears to define a trend of increasing Fe/(Fe + Mg) (Fig. 5) accompanied by decreasing A1203 and increasing Na20 + K20 (Fig. 7). Amphiboles from two of the three analysed carbonatite samples lie on the Fe-rich extension of this trend. The pyroxenes are sodian augite and sodian salites (Fig. 5) varying from Ca43Mg34(Fe,Mn)23 to Ca46Mg2s(Fe,Mn)26. As for the amphibole, the fenite pyroxenes seem to extend along a short Fe-enrichment trend terminating at the compositions of the carbonatite pyroxene (Fig. 5). Generally, the fenite pyroxenes contain slightly lower concentrations of Na20 and higher

97,06

98.39

26.5 29.5 44.0

Ca Mg Fe + Mn

27.9 26.9 45.2

27.4 30.3 42.3

6.376 1.624 0.393 0.346 0,000 2.430 0.047 1.722 1.608 1.062 0.288

98.59

41.98 11.27 3.03 19,14 0.37 7.83 9.88 3.61 1.48

3 5

25.6 32.8 41.6

6.118 1.882 0.316 0.341 0.548 1.837 0.043 1.914 1.496 1.085 0.259

99.15

41.03 12.51 3.04 19.13 0.35 8.61 9.36 3.76 1.36

4 6

26.3 28.2 45.5

6.195 1.805 0.421 0.345 0.190 2.377 0.049 1.617 1.509 1.207 0.276

97.46

40.24 12.27 2.98 19.94 0.37 7.05 9.15 4.05 1.41

5 6

27.4 30,8 41.8

6.113 1.887 0.339 0.377 0.275 2.141 0.051 1.817 1.612 1.000 0.294

98.89

40.51 12.52 3.32 19.15 0,39 8.08 9.97 3.42 1.53

6 6

24.7 35.0 40,3

6.093 1.907 0.356 0.290 0.772 1.504 0.056 2.021 1.432 1.097 0.236

99.28

41.35 13.03 2.62 18.47 0.45 9.20 9.07 3.84 1.25

7 3

26.7 26.4 46.9

6.299 1.701 0.340 0.370 0.064 2.636 0.045 1.545 1.562 1.132 0.299

97.92

40.80 11.22 3.19 20.91 0.35 6.71 9.44 3.78 2.52

8 8

29.6 30.8 39.6

6.137 1.863 0.487 0.321 0.000 2.305 0.043 1,830 1.755 0.896 0.356

97.28

39.87 12.96 2.78 17.91 0.33 7.98 10.64 3.00 1.81

9 4

Carbonatite

26.6 260 47,4

6.293 1.707 0.224 0.378 0.130 2.670 0.041 1.557 1,592 1.108 0.304

98.69

40.93 10.66 3.27 21.78 0.32 6.79 9.67 3.72 1.55

10 4

*~Analysed s p e c i m e n s : 1 = P o 14: coarse-gr, g r a n u l a r b i . - a p . - c a l . - c p x - a m p h , f e n i t e ; 2 = Po 16: coa~se-gr, ap.-cal.-bi.e p x - a m p h , f e n i t e with p o l k . a m p h . ; 3 = P o 18: as P o 14; 4 = Po 19: as Po 14; 5 = P o 20: v . c o a r s e , g r . b i. - o x . - a p . -c a l . a m p h . f e n i t e c o n t a i n i n g r e l i c t feld. w i t h a m p h . r e a c t i o n r ims ; 6 = Po 22: as P o 14; 7 = P o 74: as P o 14; 6 = P o 3 7 : coarse-gr, b i . - a p . - e p x - a m p h , e a r b o n a t i t e w . f e l d , x e n o e r y s t s w i t h b i . - a m p h , r e a c t i o n rims; 9 - P o 3 9 : coarse-gr, ap.-bi.a m p h . c a r b o n a t i t e w. highly s t r a i n e d reid. x e n o e r y s t s ; 1 0 = Po 77: coarse-gr, b i . - a p . - a m p h - e p x c a r b o n a t i t e w. feld. x e n o c r y s t s w i t h b i . - a m p h . - c p x r e a c t i o n rims. *: n = n u m b e r o f p o i n t analyses. . 3 S t r u c t u r a l f o r m u l a e c a l c u l a t e d on th e basis o f 13 c a t i o n s ( e x c l u d i n g Ca + N a + K) a n d 2 3 . 0 o x y g e n ions.

6.192 1.809 0.338 0.401 0.228 2.269 0.055 1.709 1.536 1.079 0.289 23.000

Si AI I v AI v t Ti Fe 3 Fe: Mn Mg Ca Na K O

6.196 1.804 0.222 0.410 0.000 2.681 0.055 1.629 1.691 1.058 0.332

39.54 10.97 3.48 20.47 0.41 6.98 10.07 3,48 1.66

2 4

40.73 11.98 3.50 19.63 0.42 7.55 9.43 3.66 1.49

i 6

Fenite

S t r u c t u r a l f o r m u l a e *~ :

SiO 2 AI20.~ TiO 2 FeO MnO MgO CaO Na:O K:O

Specimen* ~ n .2

E l e c t r o n m i c r o p r o b e analyses of a m p h i b o l e f r o m f e n i t e s a n d c a r b o n a t i t e s

T A B L E VIII

Oo

83

T A B L E IX E l e c t r o n m i c r o p r o b e analyses of p y r o x e n e s from fenites and c a r b o n a t i t e s

S p e c i m e n *~ n .2 SiO~ AI203 TiO~ FeO MnO MgO CaO Na~O

Fenite 1 7

2 5

3 7

4 9

6 6

Carbonatite 8 10 8 7

7 6

52.31 4.46 0.68 12.72 0.35 8.15 17.10 3.82

50.53 4.08 0.86 13.27 0.43 8.04 18.72 3.44

53.45 3.74 0.54 12.47 0.39 8.55 18.92 3.22

52.06 4.64 0.95 12.36 0.37 8.85 17.74 3.51

51.36 4.56 0.77 12.44 0.36 8.82 18.58 3.25

52.15 4.27 0.42 11.91 0.48 10.04 18.02 3.18

52.13 3.65 0.75 13.98 0.35 7.86 17.92 3.66

51.19 3.98 0.76 15.08 0.39 7.51 18.36 3.40

100.19

99.37

101.29

100.50

100.14

100.47

100.30

100.69

Structural formulae*3: Si A1TM Al v l Ti Fe 3 Fe 2 Mn Mg Ca Na O Ca Mg Fe + Mn

1.946 0.054 0.142 0.019 0.150 0.245 0.011 0.452 0.705 0.275 6.000 45.1 28.9 26.0

1.904 0.096 0.085 0.024 0.213 0.205 0.014 0.452 0.756 0.251 46.1 27.6 26.3

1.974 0.026 0.137 0.015 0.089 0.296 0.012 0.471 0.749 0.231

1.930 0.070 0.133 0.026 0.137 0.247 0.012 0.489 0.705 0.252

1.913 0.087 0.114 0.022 0.165 0.223 0.011 0.490 0.742 0.235

1.926 0.074 0.112 0.012 0.166 0.201 0.015 0.553 0.713 0.228

1.949 0.051 0.110 0.021 0.165 0.272 0.011 0.438 0.718 0.265

1.916 0.084 0.092 0.021 0.196 0.276 0.012 0.419 0.736 0.247

46.3 29.1 24.6

44.3 30.8 24.9

45.5 30.0 24.5

43.2 33.5 23.3

44.8 27.3 27.9

44.9 25.6 29.5

1.29 0.00 13.76 8.89 1.22 23.56 14.84 36.44

2.65 1.72 11.57 13.66 1.16 24.45 12.33 32.46

2.16 4.38 6.99 16.49 1.14 24.48 11.13 33.24

1.17 5.08 6.14 16.63 1.50 27.63 10.07 31.78

2.11 0.91 10.07 16.46 1.11 21.90 13.62 33.82

2.14 4.11 5.10 19.58 1.24 20.95 13.82 33.08

Calculated standard molecules: Ti--Di Ca--Ts Jd Ac Johann En Fs Wo

1.90 1.63 12.53 15.02 1.10 22.59 12.27 32.95

2.44 4.75 3.78 21.35 1.37 22.58 10.23 33.51

,1 For d e s c r i p t i o n of analysed specimens, see Table VIII. ,2 n = n u m b e r of p o i n t analyses. ,3 S t r u c t u r a l f o r m u l a e calculated on the basis of 4.0 cations and 6.0 oxygens.

84 TABLE X E l e c t r o n m i c r o p r o b e analyses of b i o t i t e f r o m fenites a n d c a r b o n a t i t e s Fenite S p e c i m e n .1 n *~ SiO~ Al:O 3 TiO2 FeO MnO MgO CaO Na20 K:O

Carbonatite

2 4

5 6

6 5

8 3

9 6

32.86 13.09 6.70 23.71 0.24 7.88 0.07 0.34 8.22

34.42 14.22 5.71 22.56 0.24 8.83 0.00 0.39 8.96

34.96 15.20 4.48 21.55 0.22 10.96 0.12 0.46 8.97

34.16 12.99 5.65 22.91 0.15 8.73 0.07 0.45 8.82

34.31 14.13 6.31 17.97 0.19 11.89 0.01 0.28 9.44

93.11

95.33

96.92

93.93

94.53

Structural formulae*3 : Si A1TM A1VI Ti Fe Mn Mg Ca Na K O

5.302 2.489 0.000 0.812 3.199 0.033 1.896 0.013 0.107 1.692 22.000

Ca Mg Fe + Mn

0.2 36.9 62.9

5.374 2.617 0.000 0.670 2.943 0.032 2.005 0.000 0.117 1.786 0.0 40.8 59.2

5.328 2.672 0.057 0.514 2.745 0.029 2.490 0.020 0.136 1.744 0.4 47.1 52.5

5.437 2.436 0.000 0.677 3.050 0.021 2.071 0.012 0.139 1.791 0.2 40.2 59.6

5.304 2.575 0.000 0.734 2.324 0.025 2.738 0.003 0.084 1.862 0.1 53.8 46.1

,1 F o r d e s c r i p t i o n o f analysed specimens, see T a b l e VIII. *: n = n u m b e r o f p o i n t analyses. ,3 S t r u c t u r a l f o r m u l a e calculated o n t h e basis of 22.0 o x y g e n ions.

concentrations of A1203 than the carbonatite pyroxenes (Fig. 7). However, in minor elements the fenite pyroxenes are distinctly different from the Ca-rich pyroxenes from the mafic cumulates (Fig. 7). Tie-lines connecting coexisting pyroxenes and amphiboles in the fenites are very nearly parallel and do not intersect, suggesting complete Mg--Fe equilibrium between these phases. Both pyroxenes and amphiboles contain significantly less Ca than in the cumulates (Fig. 5). The biotite in three samples (Table X) vary considerably in the Mg/(Mg + Fe) ratio (Fig. 5), but are generally similar in composition to the carbonatite biotites.

85

THE CHEMICALEVOLUTIONOF THE FENITES

Method Several methods have been proposed for the recalculation of wt.% analyses in order to detect real chemical changes during metasomatic processes (see discussion in Appleyard, 1980). The general metasomatic equation derived by Gresens (1967) requires few assumptions as to volume change or choice of immobile element(s) but cannot be applied to the Pollen fenites owing to the heterogeneity of their cumulate parents. The recalculation employed is essentially that proposed by Barth (1948) and subsequently modified by McKie (1966). Analyses have been recast to 100 wt.% free of H20-, and recalculated as cations in a standard cell of 100 (O). The basic assumption is that a framework of oxygen anions remained intact during fenitization, while cations were exchanged between rock and the fenitizing fluids. Vertiainen and Woolley (1976) argue that carbonate-rich fenites should be compared with their parents on a carbonate-free basis. They presume that carbonates are simply added and result in an increase in volume. However, in the Pollen Complex the carbonate replacement of syenite pegmatites without change in their widths suggests constant-volume metasomatism. The recalculation employed here assumes that only C and not CO2, COl- or other carbon--oxygen ion was added. This assumption is supported to a certain degree by the relatively constant number of cations in the standard cells for both the cumulates and their fenitized equivalents, and also by the unsystematic relationship which exists between C and the cationic contents of the standard cells.

Choice of variation diagram In a progressive contact metasomatic aureole developed in relatively homogeneous country rocks, an independent index of fenitization is available in the decreasing distance to the source of the metasomatizing fluids {Robins and Tysseland, 1979). Metasomatism of increasing grade can also show an unsystematic spatial relationship with the source of the fluids, due to migration of fluids along structurally defined channels. In such cases it is the usual practice to plot variation diagrams against an element which varies systematically in a positive or negative direction with increasing alteration. The most commonly used are Si (or SiO2) and Na (or Na20), since these often appear respectively to decrease and increase with fenitization. Fenites have been reported, however, in which one of these elements shows "anomalous" behaviour. Appropriate indices of fenitization therefore need to be selected for each example of fenitization. In the present study variation diagrams with all possible combinations of ordinate and abscissa were plotted by computer for the standard cells

86 and trace-element data in ppm. This revealed that systematic behaviour is exhibited by a large number of elements against C as the fenitization index. Chemical trends

Least-squares linear regression of the standard cells of the fenites, employing C as the "independent" variable reveals consistent chemical trends corresponding to the process of carbonatization. Si exhibits a regular, linear decrease with increasing C (Fig. 8). The slope of the regression line is indistinguishable from --1.0, suggesting the direct replacement of Si4÷ by C4÷. A1 also decreases with increasing C, but the trend is much more dispersed, as shown by the lower coefficient of correlation (--0.71). Both Mg and Na show rapid depletion with C, while Ti, Mn and K show lesser degrees of change and more scattered variations. The fall in Fe 2÷ is only partly offset by an increase in Fe 3÷, decrease in the content of Fe t being accompanied by oxidation. Apart from Fe 3÷, the only major elements showing sympathetic variation with C are Ca and P, reflecting the dependence of apatite enrichment on the degree of carbonatization. The variations in major cations agree with the field evidence of the disappearance of feldspar and olivine during the initial stages of fenitization of the cumulates, and the "flooding" of the fenites by calcite and apatite. The transition elements V, Cr, Co and Ni all decrease with increasing C, and reach very low concentrations in the calcite-rich fenites. Zn also decreases in concentration, but to a much more limited extent. The large-ion lithophile elements Sr, Y, Ba, La, Ce and Nd all show linear positive correlations with C (Fig. 8), while Nb shows a rather surprising decrease with increase in C. According to Robins and Tysseland (1979) the light REE are very sensitive indicators of the progress of fenitization. In the Pollen fenites these correlate with C in every case to better than +0.90. The net result of extreme carbonatization is a fenite chemically indistinguishable from the more silico-carbonatitic facies of the intrusive carbonatite (Fig. 9). Chemical variations in the carbonatites are for some elements continued in the trends defined by fenites showing varying degrees of carbonatization. For certain other elements, for instance Fe 3÷ and P (Fig. 9), variation trends for fenites and carbonatites converge on the more carbonate-rich fenites and silicate-rich carbonatites. However, projection of the linear trends in the data for the fenites towards the cumulates, and comparison of C-poor fenites and cumulates reveals significant differences for the following elements: Al, Ti, Fe, K, Mn, Co, Ni, Cu, Zn, Zr, Nb, Ba, La, Ce and Nd. Compared with unaltered cumulates, C-poor fenites are highly depleted in A1, and enriched in Ti, Fe 2÷, Fe 3÷, K and Mn (Fig. 9). Of the trace elements, Ni, Cu and Sr are depleted in the low-C fenites relative to the cumulates, while Co, Zn, Zr, Nb, Ba, La, Ce and Nd are enriched. These differences in composition are believed to be due to a phase of fenitization earlier than the carbonatization which n o w dominates the appearance

87 10 Cations/100(O)

•C aI t i o• n s / 1 0 0 ( 0 )

NI

0 0 l

I

•

0 Q QI

I

0

15

:I

A

l C : t i o n : / l ~ 0(O)

5

Cations/100(0)

®

75

65

•

55

•

/*5

i 12

Cations/100101

175- Cations/100(O) I

3 15 25

•

Q

ae

125

O

2 I0

15 • ppm

112 •

75

L

15" Cations/100(O)

•

63 ,= 54

45

•

05

36

ppm

3000 Ppm I'~

240C

~

~

~

•

I~.

[]

200 e Q

15(

1~00

10(

,200

•e ee

Number

of c a r b o n

ions per

e

100 oxygens

Fig. 8. Variation diagrams for selected elements in the Pollen fenites illustrating geochemical changes during carbonatization.

of the metasomatized cumulates. The chemical differences are similar to those which arose during the fenitization of gabbroic rocks on Seiland adjacent to litchfieldite pegmatites (Robins and Tysseland, 1979), especially with respect to the depletion of A1 and the concentration of Ti, Fe and K. The Seiland fenites also demonstrate the introduction of appreciable C.

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Fig. 9. Comparison of the geochemical variations in standard cells for the Pollen magmatic carbonatite (triangles), fenites (filled circles) and their parental gabbroic cumulates (solid arrows show the variation exhibited by the main grouping of samples and dashed arrows the remaining range).

89

It is not suggested, however, that the initial stage of alteration of the Pollen fenites was necessarily petrogenetically related to a nepheline syenite magma, though this may possibly have been the case. The principal conclusion is that the metasomatic transformation of the gabbroic cumulates of the Pollen Complex must have taken place in two stages: first by fenitization and subsequently by carbonatization. Chemical changes resulting from fenitization were enhanced, unaffected or were reversed during carbonatization. Transitions from enrichment to depletion are exhibited by Ti, Fe 2+, K, Mn, Co, Zn and Nb (Fig. 9). Some elements enriched during the initial stages of alteration could remain immobile during subsequent metasomatism: this was clearly the case for Zr. Other elements were apparently relatively little affected by the earlier phase but were very mobile during the later: Si, Mg, Ca, Na and P are examples of this type of behaviour. Finally, one element, St, shows evidence of an early depletion being followed by enrichment. PETROGENESIS OF THE FENITES

General physical considerations Fenite is the term used for contact metasomatic rocks developed in association with alkaline or carbonatitic intrusions, whatever the composition of the original country rocks, the extent of the induced chemical changes or the detailed nature of the processes involved (Br~bgger, 1921; yon Eckermann, 1948). A variety of fenites can be generated by a single metasomatic cycle (Verwoerd, 1966), but fenites in many alkaline complexes lie within several superimposed metasomatic aureoles and are thus polygenetic. The interpretation of the metasomatic history in polygenetic fenites can be extraordinarily difficult, and identification of the sources of the activity may be ambiguous (see discussions in Heinrich, 1966; Le Bas, 1977). Despite some attempts to attribute fenitization to diffusion, towards as well as away from magmas acting only as sources of thermal and chemical gradients (Woolley, 1969; Vartiainen and Woolley, 1976; Rubie, 1982), the most plausible mechanism by which extensive contact metasomatism is effected is by infiltration of the wall rocks (Korzhinsky, 1970} by a very low viscosity phase emanating from a cooling magma. The evolution of the gas or fluid phase and the pressure gradient which drives it outwards may arise by the phenomenon of second boiling (Morey, 1922); the magmas which gave rise to fenitization were clearly volatile-rich even at the onset of crystallization. Alternatively, evolution of a fluid phase, enriched in either CO2 or H20 may be due to the upward motion, and hence decompression, of volatile-saturated magmas at varying depths (Mysen, 1975). The semipermeable character of the wall rocks required for infiltration (Goranson, 1937) is realized by movement of the fenitizing fluid as a film along grain boundaries and through microscopic pores in rocks of low permeability.

90 The transport of heat by a fluid migrating from a magma into the surrounding rocks is likely to be much more efficient than by diffusion alone, and contact effects where fenitization is active are normally orders of magnitude greater than those due to ordinary thermal metamorphism. For this reason fenitic aureoles can attain substantial widths around even narrow dyke intrusions (Robins and Tysseland, 1979). In addition, steeper thermal gradients can exist during infiltration metasomatism than thermal metamorphism alone (Vartiainen and Woolley, 1976). As a result of changes in the physical state and composition of the fenitizing medium as it migrates outwards, the fenitization that takes place at low temperature is not necessarily typical of the precursors of higher-grade fenites.

The Pollen fenites The rapid transition between unaffected mafic cumulates and thoroughly altered rocks lacking any systematic chemical or mineralogical zonation is good evidence of an infiltration origin for the Pollen fenites. The overlapping chemical compositions of the fenites and the carbonatites strongly suggests that the latter were the source of the metasomatism: no other possible source is found within the complex at the present level of erosion. Some of the relationships within the complex could conceivably suggest an alternative, more extreme, model such as that proposed by Oosterom (1963), and also by Heier (1961) for the nearby Lillebukt Complex, in which the main carbonatite represents the final product of fenitization of the mafic rocks; the fenitization being related to an underlying intrusion, or some unspecified cause. However, the presence of s~bvite dykes in both the fenites and in the sun'ounding unaltered mafic rocks demonstrates that carbonatite magmas were emplaced during the development of the complex. An intrusive carbonate-rich magma is therefore accepted as the cause of the fenitization. The two phases of alteration deduced from the geochemistry of the Pollen fenites, viz. fenitization and subsequent carbonatization, are both attributed to a single metasomatic cycle occurring simultaneously with the crystallization of a single batch of carbonatite magma. The changes in the chemical evolution of the fenites are ascribed to continuous and/or abrupt changes in the activities of components in a highly reactive fluid released from the carbonatite. According to Korzhinsky (1970), the usual trend during infiltration metasomatism is towards simpler mineral assemblages, the number of stable phases at any time being determined by the inert components of the primary paragenesis. Many fenites show, however, the mineralogical and chemical convergence with their associated intrusive that is most decidedly exhibited by the Pollen fenites (WooUey, 1969; Tanner and Tobisch, 1972; Robins, 1974; Le Bas, 1977; Kj~bsnes, 1980). In mineralogically simple palaeosomes, such as quartzites, the number of phases in-

91 creases during fenitization (Woolley et al., 1972; Siemiatkowska and Martin, 1975) although it should be added that not all of the phases developed in such rocks need have been in mutual equilibrium. Although the interpretation of fenite aureoles is complicated by the presence of thermal gradients and dynamic re-equilibration of the metasomatic fluids, the equilibrium mineral phases developed in fenites of the highest grade, those whose temperature closely approaches that of the magmatic source of the metasomatism and which the active fluids first encounter, should be related to those crystallizing from the magma itself. The fluids emanating from a magma which is in equilibrium with either a single crystalline phase or several phases will also be in equilibrium with the same phase or phases. Under isothermal conditions and when equilibrium is achieved, the phases developed in the host-rocks will ideally be chemically identical to those precipitating from the magma, though possibly fewer in number. In the Pollen fenites as a whole, the composition of the minerals developed during fenitization have been shown to be closely similar to those in the associated carbonatite, and in restricted areas there are fewer phases in the fenites than in the carbonatite. The coarse grain size of the Pollen fenites, the limited range in mineral chemistry and the lack of textural evidence of replacement of the main constituent minerals also suggest that they approached metasomatic equilibrium. The sequence of chemical changes in the fenites is believed to be a result of changing phase relationships in a cooling carbonatite magma. The initial fenitization, involving addition of Ti, Fe 2÷, Fe 3÷ and K, and depletion in Al with apparently no significant desilicification, may be explained by the growth of amphibole, biotite and minor amounts of pyroxene at the expense of the primary mafic minerals and some of the plagioclase. The same phases as developed in the country rocks during fenitization are believed to have been simultaneously precipitated from the intrusive carbonatite magma. Later, these mafic silicates were joined on the liquidus by calcite, presumably due to a progressive decrease in silica activity in the fractionating carbonatite magma. The increase in the activity of carbonates and lowering of silica activity in the coexisting fluid phase then resulted in progressive desilicification of the fenites and their gradual replacement by calcite. Carbonatization was accompanied by the introduction of phosphorus, and the oxidation of Fe claimed by von Eckermann (1948) to be due to CO2. The degree of oxidation was insufficient for the stabilization of the alkali pyroxenes and amphiboles which are so commonly developed in fenites, especially those generated from leucocratic palaeosomes. There may be several reasons for this: the bulk chemistry of the fenites was not conducive to the crystallization of alkali mafic phases; the buffering capacity of the palaeosome was too large for equilibrium to be attained; oxygen fugacity in the fenitizing fluid was too low; the temperatures within the metasomatic aureole were exceptionally high and similar to that of the

92

source of the fenitizing fluids. The absence of Fe3÷-rich pyroxenes and amphiboles within the intrusive carbonatite suggests a relatively low oxygen fugacity in the fenitizing fluid it evolved. The similar mineral chemistries in the fenites and in the intrusive carbonatite further suggest a close approach to thermal and chemical equilibrium with the metasomatizing magma. Pre-existing Ca was fixed as calcite and apatite, and further calcite was introduced. The partial-to-complete replacement of the alkaline pegmatites, which were emplaced into both the carbonatites and the fenites, by calcite, apatite and minor biotite or amphibole suggests that the waning stages of carbonatization were prolonged, and not necessarily directly related to the crystallization of carbonatite magmas. SIGNIFICANCE OF THE POLLEN FENITES F O R CARBONATITE GENESIS

The dichotomy which exists in the compositions of extrusive and intrusive carbonatites has led to considerable discussion on the nature, derivation and evolution of carbonatite magmas (Watkinson and Wyllie, 1971; Cooper et al., 1975; Koster van Groos, 1975; Le Bas, 1977; Gittins and McKie, 1980). Many authors have suggested that alkali carbonate liquids, possibly generated by liquation of carbonated nephelinitic or phonolitic magmas, subsequently evolve into Ca-rich carbonatite magmas by loss of alkalis to the wall rocks (Deans et al., 1972; Cooper et al., 1975; Gittins et al., 1975; Le Bas, 1977; Verwoerd, 1978; Freestone and Hamilton, 1980; Woolley, 1982). The alkali carbonatite lavas of Oldoinyo Lengai are Nadominated (Dawson, 1962, 1966; Gittins and McKie, 1980}. Von Eckermann (1948), however, proposed that the carbonatite magma emplaced in the Aln¢ Complex was initially potassic. Differences in the composition of fenites developed around carbonatite complexes may have been due to variable magma composition with respect to Na and K (Dawson, 1964; Woolley, 1982) and depth of emplacement may also be an important factor (Deans et al., 1972; Le Bas, 1977). The Pollen fenites were enriched in K during the early phase of metasomatism, and during carbonatization Na was removed. The style of fenitization, therefore, suggests a relatively potassic carbonatite magma which was able to generate a fenitizing phase with a K/(K+Na) ratio sufficient to induce fixing of K in its metasomatic aureole. The field relationships and mineralogy of the Pollen Carbonatite suggest that the composition of the carbonatite magma was extensively modified by interaction with the gabbroic rocks which it intruded. The mineralogical and chemical convergence of the country rocks and the carbonatite would appear to be the result of two distinct but interrelated processes: reaction with, and assimilation of, the mafic cumulates by the carbonatite magma; infiltration metasomatism of the cumulates by a transient fluid phase in equilibrium with the carbonatite magma. Although the chemical changes induced in fenites cannot at present be extrapolated to give information on the detailed composition of the causative magma, they clearly provide

93 a priori evidence o f an earlier a b u n d a n c e o f alkalis in t h e m a g m a s generating s~bvites (Gittins et al., 1 9 7 5 ) as well as the residual n a t u r e o f these r o c k s {Dawson, 1 9 6 4 ) . ACKNOWLEDGEMENTS

B. R o b i n s was in r e c e i p t o f a grant f r o m the N o r w e g i a n Research C o u n c i l f o r Science a n d t h e H u m a n i t i e s ( N . A . V . F . , p r o j e c t No. D . 4 8 . 2 2 - 1 8 ) d u r i n g t h e c o u r s e o f this w o r k . E. Irgens c o n s t r u c t e d t h e line drawings. This p a p e r is a c o n t r i b u t i o n t o I.G.C.P. P r o j e c t 27 - - T h e C a l e d o n i d e Orogen.

REFERENCES

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94 Heier, K.S., 1961. Layered gabbro, hornblendite, carbonatite and nepheline syenite on Stjern~y, north Norway. Nor. Geol. Tidsskr., 41: 109--155. Heinrich, E.Wm., 1966. The Geology of Carbonatites. Rand McNally, Chicago, Ill. Kj~snes, K., 1980. Fenitization of layered mafic rocks in the Lillebukt alkaline complex, Stjern6y, Norway. Lithos, 13 : 215 (abstract). Korzhinsky, D.S., 1970. Theory of Metasomatic Zoning. Oxford University Press, London. Koster van Groos, A.F., 1975. The effect of high CO 2 pressures on alkaline rocks and its bearing on the formation of alkalic ultrabasic rocks and the associated carbonatites. Am. J. Sci., 275: 163--185. Leake, B.E., 1978. Nomenclature of amphiboles. Am. Mineral., 63: 1023--1052. Le Bas, M.J., 1977. Carbonatite--Nephelinite Volcanism. Wiley--Interscience, New York, N.Y. McKie, D., 1966. Fenitization. In: O.F. Tuttle and J. Gittins (Editors), Carbonatites, Wiley--Interscience, New York, N.Y., pp. 261--294. Morey, G.W., 1922. The development of pressure in magmas as a result of crystallization. Wash. Acad. Sci. J., 12: 219--230. Mysen, B.O., 1975. Solubility of volatiles in silicate melts at high pressure and temperature: the role of carbon dioxide and water in feldspar, pyroxene and feldspathoid melts. Carnegie Inst. Washington, Yearb., 74: 454--468. Oosterom, M.G., 1963. The ultramafites and layered gabbro sequences in the granulite facies rocks on Stjern~y, Finnmark, Norway. Leidse Geol. Meded., 2 8 : 1 7 9 2 9 6 . Reed, S.J.B., 1975. Electron Microprobe Analysis. Cambridge University Press, London. Robins, B., 1974. Synorogenic alkaline pyroxenite dykes on Seiland, northern Norway. Nor. Geol. Tidsskr., 54: 257--268. Robins, B., 1982. The geology and petrology of the Rognsund intrusion, West Finnmark, Northern Norway. Nor. Geol. Unders., 371: 1--55. Robins, B. and Gardner, P.M., 1975. The magmatic evolution of the Seiland province and Caledonian plate boundaries in northern Norway. Earth Planet. Sci. Lett., 26: 167--178. Robins, B. and Tysseland, M., 1979. Fenitization of some mafic igneous rocks in the Seiland province, northern Norway. Nor. Geol. Tidsskr., 59: 1--23. Rubie, D.C., 1982. Mass transfer and volume change during alkali metasomatism at Kisingiri, western Kenya. Lithos, 15: 99--109. Saether, E., 1958. The alkaline rock province of the Fen area in southern Norway. K. Nor. Vidensk. Selsk., 1 , 1 5 0 pp. Siemiatkowska, K.M. and Martin, R.F., 1975. Fenitization of Mississagi quartzite, Sudbury area, Ontario. Geol. Soc. Am. Bull., 86: 1109--1122. Strand, T., 1981. Lillebukt alkaline kompleks: Karbonatittens mineralogi og petrokjemi. Thesis, University of Bergen, Bergen (unpublished). Sturt, B.A. and Ramsay, D.M., 1965. The alkaline complex of the Breivikbotn area, S~r~y, Northern Norway. Nor. Geol. Unders., 231: 5--142. Sturt, B.A., Miller, J.A. and Fitch, F.J., 1968. The age of alkaline rocks from West Finmark, northern Norway, and their bearing on the dating of the Caledonian orogeny. Nor. Geol. Tidskr., 47: 255--283. Sturt, B.A., Pringle, I. and Ramsay, D.M., 1978. The Finnmarkian phase of the Caledonian orogeny. J. Geol. Soc. London, 135: 597---610. Tanner, P.W.G. and Tobisch, O.T., 1972. Sodic and ultra-sodic rocks of metasomatic origin from part of the Moine nappe. Scot. J. Geol., 8: 151--178. Vartiainen, H., 1980. The petrography, mineralogy and petrochemistry of the Sokli carbonatite massif, northern Finland. Geol. Surv. Finl. Bull:, 3 1 3 , 1 2 6 pp. Vartiainen, H. and Woolley, A.R., 1976. The petrography, mineralogy and chemistry of the fenites of the Sokli carbonatite intrusion, Finland. Geol. Surv. Finl. Bull., 2 8 0 : 8 7 pp.

95 Verwoerd, W.J., 1966. Fenitization of basic igneous rocks. In: O.F. Tuttle and J. Gittins (Editors), Carbonatites, Wiley--Interscience, New York, N.Y., pp. 295--308. Verwoerd, W.J., 1967. The carbonatites of South Africa and South West Africa. S. Aft. Geol. Surv. Handb., 6 : 4 5 2 pp. Verwoerd, W.J., 1978. Liquid immiscibility and the carbonatite--ijolite relationship: preliminary data on the join NaFeS+Si20,--CaCO3 and related compositions. Carnegie Inst. Washington, Yearb., 77: 767--774. yon Eckermann, H., 1948. The alkaline district of Aln~b Island. Sver. Geol. Unders., Ser. C, 36, 176 pp. Watkinson, D.H. and Wyllie, P.J., 1971. Experimental study of the composition join NaAISiO4--CaCO3--H20 and the genesis of alkali rock--carbonatite complexes. J. Petrol., 12: 357--378. Wedepohl, K.K. (Editor), 1969. Handbook of Geochemistry. Springer, Berlin. Woolley, A.R., 1969. S o m e aspects of fenitization with particular reference to Chilwa Island and Kangankunde, Malawi. Bull. Br. Mus. Nat. Hist. (Mineral.), 2: 191--319. Woolley, A.R., 1982. A discussion of carbonatite evolution and nomenclature, and the generation of sodic and potassic fenites.Mineral. Mag., 46: 13--17. Woolley, A.R., Symes, R.F. and Elliott, C.J., 1972. Metasomatized (fenitized) quartzites from the Borralan complex, Scotland. Mineral. Mag., 38: 819--836.