Partition of trace elements in co-existing biotite, muscovite and potassium feldspar of granitic rocks, northern Portugal

Chemical Geology, 16 (1975) 89--108 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

PARTITION OF TRACE ELEMENTS IN CO-EXISTING BIOTITE, MUSCOVITE AND POTASSIUM FELDSPAR OF GRANITIC ROCKS, NORTHERN PORTUGAL

CARLOS A.R. DE ALBUQUERQUE Department of Geology, Saint Mary's University, Halifax, N.S. (Canada) (Received October 8, 1974; revised and accepted June 18, 1975)

ABSTRACT De Albuquerque, C.A.R., 1975. Partition of trace elements in co-existing biotite, muscovite and potassium feldspar of granitic rocks, northern Portugal. Chem. Geol., 16: 89--108. Chemical analyses are given for Na, K, and trace elements of muscovite and potassium feldspar of granitic rocks. The distribution of trace elements in co-existing minerals suggests that equilibrium was attained and that muscovite is a primary mineral. These observations and the comparison of the trace-element chemistry of megacrysts and groundmass potassium feldspars of porphyritic rock types lead to the conclusion that the megacrysts are in fact phenocrysts which crystallized in equilibrium with the other minerals of the rock. The ranges of values of the distribution coefficients KD(Rb/K) and KD(Cs/K) in mineral pairs confirm other observations on the equilibrium among various minerals. However, application of experimental data on the temperature effect on K D leads to results conflicting with the petrologic observations. The possible influence of other factors on KD is analyzed and among these factors the composition of the feldspar and the solidus-liquidus temperature interval may play a dominant role. INTRODUCTION R e c e n t studies o f t h e p a r t i t i o n o f t r a c e e l e m e n t s in d i f f e r e n t p h a s e s h a v e s h o w n t h a t i m p o r t a n t p e t r o g e n e t i c i n f o r m a t i o n c a n be o b t a i n e d b y applicat i o n o f t h e e x p e r i m e n t a l d a t a or f r o m t h e d e t a i l e d investigations o f n a t u r a l systems. While in m a n y i n s t a n c e s t h e e x p e r i m e n t a l d a t a a n d t h e o r e t i c a l p r e d i c t i o n s are in g o o d a g r e e m e n t w i t h t h e o b s e r v a t i o n s ( Y o d e r et al., 1 9 5 7 ; Piwinskii, 1 9 6 8 a ; C a r r o n a n d Lagache, 1 9 7 1 ) , a p p a r e n t c o n f l i c t s still exist f o r o t h e r s y s t e m s , in p a r t i c u l a r t h o s e n o t y e t t h e s u b j e c t o f e x t e n s i v e investigations. A m o n g these, t h e d i s t r i b u t i o n o f R b ( e x p r e s s e d as t h e R b / K ratios) bet w e e n b i o t i t e a n d p o t a s s i u m f e l d s p a r is t e m p e r a t u r e d e p e n d e n t a c c o r d i n g t o the experimental work of Beswick (1973) and the observations of Dupuy ( 1 9 6 8 a ) f o r volcanic rocks. H o w e v e r , L a n g e et al. ( 1 9 6 6 ) in t h e i r s t u d y o f b i o t i t e - - p o t a s s i u m f e l d s p a r pairs o f a large n u m b e r o f N e w E n g l a n d granitic

90 rocks did not find any systematic variation of the distribution of that element relative to K in those mineral phases. In this paper a study is presented of the distribution of trace elements among biotite, muscovite and potassium feldspars of the calc-alkali granitic rocks of the Aregos r e ~ n n (northern Portugal) and of its application to the petrogenesis of the rocks. Based on the observations on these mineral pairs from the literature and from this study, a review is made of the factors which may affect some distribution coefficients. The data appear to confirm that distribution coefficients may be used as indicators of equilibrium in rocks (cf. Beswick, 1973). This is complemented by the investigation of the distribution of trace elements in potassium-feldspar megacrysts and potassium feldspar of the groundmass of the porphyritic rocks of the region and its bearing on the origin of the megacrysts. ANALYTICAL METHODS The separation of the ferromagnesian minerals and muscovite was carried o u t by means of a Frantz electromagnetic separator and the concentrates purified in methylene iodide until a purity of better than 99.0--99.5% was obtained. The main impurities of muscovite are small grains of altered plagioclase and composite grains of feldspar--ilmenite. The potassium feldspars of the rocks of the hybrid series were separated from the nonmagnetic fractions of the same powders using bromoform. The megacrysts of potassium feldspar of the porphyritic rocks were hand picked from the rock fragments and cleaned by removal of the grains of other minerals. Those pieces of megacrysts were then crushed prior to purification using electromagnetic separation and bromoform. The same pieces of rock were used for the separation of the potassium feldspar of the groundmass after all the fragments of megacrysts and small megacrysts were removed. The potassium feldspar fractions are therefore representative of the large megacrysts and of the groundmass. The potassium feldspars were then purified by the methods of mineral separation indicated above. The potassium feldspar concentrates have a degree of purity of better than 99.0--99.5%, the principal contaminant being sodic plagioclase. The major elements Na and K were determined b y flame p h o t o m e t r y while the trace-element determinations were carried out by emission spectroscopy using the techniques described earlier (De Albuquerque, 1971, 1973b). DESCRIPTION OF THE ROCKS The minerals studied here were separated from granodiorites, granites and quartz-bearing monzonites of the Hercynian orogenic belt of northern and central Portugal. These rocks have been described (De Albuquerque, 1971, 1973a}, as well as some of their minerals (De Albuquerque, 1973b}, and

91

therefore only a brief summary of the relations between minerals will be given here. Biotite--potassium feldspar pairs were separated from the hornblende-bearing biotite granodiorites, hornblende--biotite monzonites and porphyritic biotite granodiorites while biotite, muscovite and potassium feldspar were separated from the muscovite--biotite granodiorites and granites of the hybrid series and from the porphyritic granites. In addition to the primary muscovite, several granodiorites and granites contain primary sillimanite or andalusite. The porphyritic rocks contain large megacrysts of potassium feldspar which enclose small crystals of plagioclase and biotite while small anhedral grains of potassium feldspar occur in the groundmass of these rocks. The vein granites are medium-grained quartz--potassium feldspar--plagioclase--biotite rocks containing primary muscovite and andalusite. They occur as veins cutting the coarse-grained porphyritic biotite granite to which they show a spatial and, possibly, genetic relationship. The vein granites are petrographically similar to the granites of the hybrid series although differences in their chemistry show that these vein rocks form a separate group. They are also younger in age than both the granites of the hybrid series and the porphyritic granites. DISTRIBUTION OF Na, K, AND TRACE ELEMENTS

Chemical analyses of muscovites and potassium feldspars of the hybrid series and porphyritic rock types are given, respectively, in Tables I--III for Na, K, and trace elements. Element ratios are given in the same tables. The chemical analyses of the co-existing biotites have been published (De Albuquerque, 1973b, tables 1 and 2). The potassium feldspars of the Aregos granitic rocks are microcline-microperthite. It is possible, therefore, that the Na contents (and Na/K ratios) are affected by errors, as the possibility of differential separation of the albiterich fractions during the process of purification of the feldspar cannot be ignored. However, the definition of trends is based on the averages for each rock type in order to minimize such errors. The Na contents of the potassium feldspars of the hornblende--biotite granodiorites are lower than those of the biotite granodiorites and the highest Na/K ratios are those of the felsic granodiorites and granites. This trend has also been observed in the porphyritic types and in rocks from other regions such as granodiorites and granites from Corsica (Carron and Lagache, 1971). The megacrysts of potassium feldspar have slightly higher Na and lower K contents than the potassium feldspar of the groundmass. However, the differences are very small and the ratios Na/K of the megacrysts and of the groundmass potassium feldspars are almost identical, respectively, 0.125 and 0.11 (Table IV). The trace-element contents of the megacrysts of the porphyritic rocks are similar to those of the potassium feldspar of the groundmass (Table IV).

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95 TABLE IV Average element contents (Na and K in percentages, other elements in ppm) and average element ratios of megacrysts and groundmass potassium feldspars of the porphyritic rocks Element

Megacrysts

Groundmass

Element ratios

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Groundmass

Na K Ga Sr Ba Rb Cs

1.46 11.71 8.7 763 3,270 460 14.5

1.30 12.00 8.8 703 3,020 500 19

Na/K Ba/Sr Ba/K (.104) Ba/Rb Rb/K (-104) Cs/K (-104 ) Rb/Cs

0.125 4.6 278 9.5 39 1.£5 31.5

0.11 4.3 253 8.9 42 1.6 25.5

In fact, the contents of several trace elements such as Ga, Sr, Pb, Ba, and Rb of the megacrysts and groundmass potassium feldspars overlap within the limits of experimental error, while the Cs contents of the megacrysts are lower than those of the groundmass potassium feldspar. Variations of the element contents are reflected in the element ratios (Table IV). The Na/K, Ba/K, Ba/Rb and Rb/Cs ratios are higher and the Rb/K and Cs/K ratios lower in the megacrysts than in the potassium feldspar of the groundmass. The Ba/Sr ratio is almost identical in the two feldspars.

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Plots of distribution of trace elements in the mineral pairs biotite--muscovite, biotite--potassium feldspar and muscovite--potassium feldspar are given, respectively, in Figs.l--3. It is n o t e w o r t h y that relatively high contents of Cr and V are foun~ m the muscovites. Li is also concentrated in biotite as observed in the biotite--muscovite pairs of granitic rocks from Brittany (Carton and Lagache, 1972b) and of metamorphic rocks (Butler, 1960). While Sc is concentrated in muscovite over biotite, Sn has been detected only in muscovite. A general feature observed in the distribution of trace elements among biotite, muscovite and potassium feldspar is that the divalent cations St, Pb, and Ba, which substitute for potassium in the structure of these minerals, are considerably concentrated in potassium feldspar. This appears to be explained by structural considerations (cf. Deer et al., 1962a, b).

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DISTRIBUTION COEFFICIENTS The distribution coefficients for the various mineral pairs have been calculated in the usual manner from the values of the element contents in the mineral phases. For instance, in the distribution coefficient KD B/F (Rb/K) = (Rb/K)biotite/(Rb/K)potassium feldspar, the symbols B, M, and F are representing, respectively, biotite, muscovite and potassium feldspar. The values of the Rb/K ratio in the minerals studied here and from the literature are less than 0.018 and, therefore, the Rb concentrations are in the range of those of the experimental work used for comparisons. Values of various distribution coefficients are given in Table V.

KDB/F(Rb/K) The range of variation of this distribution coefficient for each rock type is narrow (Table V) and average values are similar for comparable rock types. Furthermore, these values are similar to those of the same mineral pair of granitic rocks from other regions such as New England quartz monzonites

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Fig.4. Distribution c o e f f i c i e n t s K D ( R b / K ) b i o t i t e - - p o t a s s i u m feldspar in s o m e granitic rocks, ( R b / K ) • 104. Symbols: • = Aregos; [] = N e w England (after Lange e t al., 1 9 6 6 ) ; ~ = Cape granite (after Kolbe, 1 9 6 6 ) ; 0 = s o u t h e r n California b a t h o l i t h (after Sen et al., 1 9 5 9 ) ; v = Palmer granite (Australia) (after White, 1 9 6 6 ) ; X = Corsica (after Carron and Lagache, 1 9 7 1 ) . F i e l d s o u t l i n e d : solid line = Cape granites; dash-dot line = N e w England granitic rocks; dashed line = Corsica granodiorites and granites.

100

and granites (Lange et al., 1966), the Cape granites (South Africa) (Kolbe, 1966), and the Palmer granite (Australia) (White, 1966) (Fig.4). The highest values of KD are found in the mineral pairs of the mafic granodiorites and monzonites (average KD = 3.9) and, among the porphyritic rock types, in those of the biotite granodiorite (average KD = 4.1). Lower values of KD are observed for felsic granodiorites and granites (average KD = 3.2), the lowest values of this coefficient being found in the mineral pairs of the vein granites (average KD = 1.9) cutting the porphyritic biotite granite. This trend of variation of KD, however, is at variance with the theoretical predictions (McIntire, 1963) and the experimental work (Beswick, 1973) as it is generally assumed that the temperature of crystallization of mafic granodiorites is higher than that of granites and higher temperature should give lower KD values. Data from the literature appear to confirm the observations made on the Aregos rocks. For example, Carron and Lagache (1971) obtained values of KD of 3.4 and 2.8 for, respectively, granodiorites and granites from Corsica, and Lange et al. (1966) reported values of 3.5 and 3.2 for, respectively, quartz monzonites and granites from New England. Possible exceptions to this trend require confirmation as they are based on observations on rocks from different regions, e.g. the Neira granite (Spain) and the Elba granodiorite (Italy) (Dupuy, 1968b) or insufficient number of specimens ( N e ~ England granodiorite, Lange et al., 1966). Although a similar discrepancy, based on the temperatures inferred from the two-feldspar geothermometer, led Carton and Lagache (1971) to postulate slightly higher temperatures of equilibration for granites than for granodiorites of Corsica, the data presented here and f r o m the literature for variations of KD appear to confirm that, in fact, factors other than temperature may affect to a large extent the values of the distribution coefficients in calcalkali granitic rocks. This will be discussed in more detail under "Interpretat-ion".

KD B/M (Rb/K) Little variation is observed for the values of this distribution coefficient in the hybrid series (range 1.7--3.0), the highest values being those of the intermediate granodiorites (Table V). In the porphyritic rocks this coefficient remains almost constant (1.7--1.9) and in the vein granites its values are comparable to those of the other granites. Granitic rocks from other regions have K D values similar to those of the Aregos rocks (Fig.5), mostly in the range 1.7--2.6. The constancy of this distribution coefficient in granitic rocks confirms the observations on the biotite--potassium feldspar pairs on the distribution of the same elements, however, unlike the latter, no correlation can be established with rock type.

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Fig. 5. D i s t r i b u t i o n c o e f f i c i e n t s K D ( R b / K ) b i o t i t e - - m u s c o v i t e in s o m e g r a n i t i c rocks, ( R b / K ) • 104 . Symbols: • = Aregos; ~ = Cape granite ( a f t e r Kolbe, 1 9 6 6 ) ; v = M u r r u m b i d g e e b a t h o l i t h , Australia (after Joyce, 1973). G r a n i t i c r o c k s f r o m F r a n c e ( a f t e r Lagache a n d C a r r o n , 1 9 7 2 ) : x = B r i t t a n y ; 0 = P y r e n e e s ; o = Massif Central.

KD M/F ( R b / K )

The values of this coefficient for most of the Aregos mineral pa~rs vary in a narrow range (1.6--1.9). This includes rocks of both the hybrid series and porphyritic types. It m a y also be noted that the values of KDM/F(Rb/K) and KD B/M ( R b / K ) are almost identical for these rocks. The intermediate granodiorites, however, have lower values (1.1) of KDM/F(Rb/K) as well as the vein granites (1.0 and 0.85). The same mineral pairs from a muscovite granite belonging to the suite of "older granites" and from a muscovite granite, a vein rock cutting the granite of the hybrid series (De Albuquerque, unpublished data, 1968) have identical K D values (1.4). Comparable values of this coefficient are found for the granitic rocks from other regions referred to above. Therefore, the considerations made above for the distribution coefficient KD B/M ( R b / K ) are generally valid for the coefficient KD M/F ( R b / K ) as well.

102

KD(Cs/K) The partition of Cs between potassium feldspar and liquid has been determined experimentally at different temperatures (Eugster, 1955) and for feldspars of different compositions (Lagache, 1969, 1971). However, very few determinations of Cs in mineral pairs are available in the literature (Figs. 6 and 7). In the rocks of the hybrid series, the mineral pairs from the granites have slightly higher values of KDB/F(Cs/K) than those of the granodiorites. The same trend is observed for the mineral pairs of the Aregos porphyritic granodiorites and granites, and of the Corsica granitic rocks (Carton and Lagache,

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Fig.7. D i s t r i b u t i o n c o e f f i c i e n t s K D ( C s / K ) b i o t i t e - - m u s c o v i t e in s o m e granitic rocks, (Cs/K) - 104 . S y m b o l s : e= Aregos; • = Cape granite ( a f t e r Kolbe, 1966). Granitic r o c k s f r o m F r a n c e ( a f t e r Lagache a n d Carron, 1972): a = B r i t t a n y ; v = Massif Central; o = Pyrenees.

1972a). The differences in the ranges of variation are more marked than for KD(Rb/K). Less variation in the values of KD B/M (Cs/K) is observed n o t only in the Aregos rocks b u t in this mineral pair from other regions (Fig.7). The values of the distribution coefficient KDM/F(Cs/K) are more variable. While the granodiorites of the hybrid series show low values (1.15--1.17) and that of the granite is relatively high (6.3), in the porphyritic granites this coefficient is in the range 4.0 to 9.0 and the vein granites show the low values of 2.1 and 2.8. A value of KD = 1.75 was calculated from the data of Carron and Lagache (1972b) for this mineral pair from a granite from Brittany and mineral pairs of pegmatites from South Africa have KD values of 2.25 and 5.8 (after Kolbe, 1966). As the data on the Cs distribution in minerals of granitic rocks are limited, the conclusions drawn from the observed K D values are only tentative. It is apparent, however, that the values of KD (Cs/K) have wider ranges of variation than K D (Rb/K), with the possible exception of K D (Cs/K) in the biotitemuscovite pairs. The variations of the values of KD(Cs/K ) for rocks from

104

various regions confirm several observations made for KD (Rb/K) as to the possible influence of factors other than temperature on the values of these coefficients. Further investigations of the variations of KD(Cs/K) in mineral pairs of naturally occurring rocks and experimental systems are necessary, however, for a better understanding of the influence of those factors and the possible effect of the crystalllochemical differences between Cs and K. INTERPRETATION OF K D VALUES

The conclusions drawn from the study of the variations of the distribution coefficients for R b / K and, to a lesser extent, Cs/K, of the mineral pairs investigated by comparison with experimental data are at variance with the petrologic observations and the experimental data on the physical conditions of crystallization ot magmas of granitic composition (cf. Winkler, 1967; Piwinskii, 1968b). In particular, the application of the experimental data on the variation of the distribution coefficient KD (Rb/K) leads to the conclusion that the temperature of equilibration of mafic granodiorites is lower than that of granites. Therefore, this possibility and the influence of factors other than temperature which may affect to a large extent the values of the distribution coefficients of the elements Rb, Cs, and K in granitic rocks need further investigation. The considerations below are based on the assumption that the minerals have n o t been affected by late- or post-magmatic alteration, which can be expected as there are no signs of such p h e n o m e n a in the rocks. Also, it has been seen above that the range of R b / K values (0.018) is low and therefore the Rb solutions can be treated as dilute solutions conforming to ideal-solution behaviour {cf. Iiyama, 1968; Beswick, 1973). The constancy of KD values obtained for mineral pairs of comparable rocks from various regions warrants the conclusion that some form of equilibrium must be attained. However, kinetic disequilibrium constitutes a distinct possibility and will be discussed below. The influence of pressure appears to be sr.lall as deduced theoretically (McIntire, 1963) an~ observed experimentally (Eugster, 1955). Reequilibration at low temperature does n o t appear to be the explanation for the variations observed. Although Hall (1967) pointed o u t that feldspars are affected by unmixing at low temperature, this process does n o t appear to alter the trace-element distributions as inferred b y Rhodes (1969). The potassium feldspars of granitic rocks have variable Na contents, which are related to the composition of the coexisting plagioclase. More-mafic rocks will normally have potassium feldspar richer in K than that of the felsic types. Lagache (1971) defined a curve for the distribution of Cs in feldspars with variable Na contents which shows that the m a x i m u m values of KD are reached when the feldspar composition is Ors0Abs0 and that KD decreases progressively towards both end members. Therefore, in the Aregos rocks and in other ex~,-,p]~o, ~ ~ - f f e c t of the feldspar composition would tend to offset variations

105

due to temperature. The experimental work of Piwinskii (1968b) and Piwinskii and Wyllie (1968) on the melting of tonalites, granodiorites and granites has shown that the temperature interval between the solidus and the liquidus (at pressures of the order of 2--3 kbar) is much larger for tonalites than for granites. Among the minerals of these rocks, biotite melts at relatively high temperature, whereas potassium feldspar completely melts at temperatures near the solidus. If this picture is true for the crystallization of the same magmas it would lead to the existence of a "changing equilibrium" solid-liquid or kinetic disequilibrium. If this is postulated, the temperature of equilibrium biotite--melt is higher (and therefore Rb/K should be higher) than the temperature of equilibrium for potassium feldspar-melt leading to lower Rb/K in this mineral. This would also tend to offset the temperature effect. These observations apply to the trends observed for KDB/F(Rb/K)and possibly KDB/F(Cs/K),although the variations are larger for the latter coefficient. Trends cannot yet be defined for KD B/M (Rb/K) or KD M/F (Rb/ K ) and this may, at least in part, be due to the narrow range of composition, and possibly temperature of crystallization, of the muscovite-bearing igneous rocks. Although no conclusions can be drawn for KD(Cs/K) owing to the limited data available, it is possible that these coefficients will permit further definitions of the conditions of crystallization of rocks. PETROLOGICAL APPLICATIONS

A metasomatic origin for the muscovites or potassium feldspars of the granitic rocks studied here is not confirmed by the geochemical evidence. In particular, the trace-element patterns of these minerals with high contents of Cr and V in muscovite and the normal contents for minerals of igneous rocks of elements which are strongly enriched in metasomatic solutions such as Rb and Cs (cf. review by Taylor, 1965) are not consistent with that hypothesis. The marked differences in the trace-element patterns of muscovite and potassium feldspar and the constancy of KD values of this mineral pair provide evidence against metasomatic replacement. The similarity of trace-element patterns (and element ratios) of megacrysts and groundmass potassium feldspars and the observations on the origin of this mineral confirm that the megacrysts of these granitic rocks are true phenocrysts. The trace-element pattern of the megacrysts of potassium feldspar in one enclave is different from those of the megacrysts of the surrounding granite. This observation and that element pattern (high Sr and Ba contents and low Cs content) do not lend support to the hypothesis that megacrysts of potassium feldspar in enclaves are of metasomatic origin. The partition of elements among biotite, muscovite and potassium feldspar and the values of the distribution coefficients indicate that these minerals crystallized in equilibrium. This also appears to be valid for other calc-alkali granitic rocks. However, the occurrence of primary muscovite in magmatic

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rocks places several limitations on the conditions of crystallization of the magmas. It may be n o t e d that sillimanite and andalusite are also primary minerals in the Aregos granitic rocks (De Albuquerque, 1971, 1973b) and, therefore, the P--T conditions of crystallization of these magmas can be defined fairly accurately b y comparison with experimental data. In the porphyritic granites, muscovite precedes potassium feldspar in the order of crystallization. This can be explained b y a marked decrease in pressure while the temperature decreases slightly during crystallization of the magmas (cf. Harris et al., 1970). These conclusions are drawn from the comparisons of the upper stability curves for muscovite, with a positive dT/dP slope, and possible melting curves for granite, which s h o w very little variation of T for marked changes in P in the pressure range 2--5 kbar (cf. Piwinskii, 1968b; Harris et al., 1970). It may be noted that a similar explanation for the formation of megacrysts of potassium feldspar was offered by Oen (1960) on geological grounds. As wet granitic magmas (and abundant pegmatites are associated with the granites of central and northern Portugal) are unlikely to rise high in the crust (Harris et al., 1970) the estimates of the P--T conditions of crystallization of the same magmas (P = 2--3 kbar, T = 680°C) place the source of such magmas within the continental crust. Low-pressure metamorphism has been recognized in the area just north of the region studied here (Brink, 1960) and partial melting of the metasedimentary rocks of the orogenic belt would occur at relatively high crustal levels under high geothermal gradients. CONCLUSIONS

The primary origin of muscovite in these granitic rocks is confirmed by the geochemical evidence and the distribution of the trace elements between biotite and muscovite suggests that they crystallized in equilibrium. The distribution of Ba, Rb and Cs also appears to reflect equilibrium between muscovite and potassium feldspar. These considerations are probably valid for granitic rocks from several other regions as well. The potassium-feldspar megacrysts of the porphyritic rock types have trace-element contents and element ratios similar to those of the groundmass potassium feldspars with the possible exception of Cs. The Ba/K, Ba/Rb, and Rb/Cs ratios are slightly higher in the megacrysts of potassium feldspar. These observations confirm that, in these rocks, the megacrysts of potassium feldspar are, in fact, phenocrysts. The distribution of elements between potassium feldspar and biotite or muscovite suggests that equilibrium was attained during crystallization. The values of the distribution coefficients KD (Rb/K) for biotite--potassium feldspar and biotite--muscovite pairs of rocks of granitic composition from several regions are similar. This probably proves that in such rocks these minerals crystallized in equilibrium and that the physical conditions of crystallization are also comparable.

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The temperature effect on KD values deduced from theoretical considerations and experimental data is n o t confirmed for many of these rocks, in particular, when comparisons are made of the more mafic with felsic rock types. It is possible that the relatively small variation of KD values due to the temperature factor is overshadowed b y other factors such as composition and structure of the potassium feldspar and the solidus--liquidus temperature interval. These observations cast doubts on the use of extrapolations to the lowtemperature minerals of plutonic rocks of granitic to tonalitic composition. However, further work on the quantitative influence of different factors will permit the definition of the effects of parameters during the crystallization of magmas. The KD(Cs/K) values in the same mineral pairs show a larger range of variation than KD (Rb/K), although the considerations made above for the latter appear to be valid for (Cs/K) as well. These observations further demonstrate the feasibility of using trace-element partitioning as a criterion of equilibrium in granitic rocks and that geochemical evidence can be used to define the paragenesis of minerals such as muscovite and potassium feldspar, which places limitations on the P--T conditions of crystallization of the rocks. REFERENCES Beswiek, A.E., 1973. An experimental study of alkali metal distributions in feldspars and micas. Geochim. Cosmochim. Acta, 37: 183--208. Brink, A.H., 1960. Petrology and ore geology of the Vila Real--Sabrosa--Vila Pouca de Aguiar region, Northern Portugal. Comun. Serv. Geol. Port., 43: 1--143. Butler, B.C.M., 1960. The Moine Series of the Ardnamurchan District of Scotland. Ph.D. Dissertation, University of Cambridge, Cambridge (unpublished). Carron, J.-P. and Lagache, M., 1971. La distribution des ~l~ments alcalins Li, Na, K, Rb dans les min~raux essentie!s des granites et granodiorites du sud de la Corse. Bull. Soc. Ft. Min~rat. Cristallogr., 94: 70--80. Carron, J.-P. and Lagache, M., 1972a. Etude par activation neutronique du partage du c~sium entre les min~raux potassiques des roches granitiques de Corse. Bull. Soc. Fr. Mineral. Cristallogr., 95: 161--162. Carton, J.-P. and Lagache, M., 1972b. Etude du partage des ~l~ments alcalins Na, K, Li, Rb et Cs entre les min~raux de quelques roches granitiques de France. 24e Congr. G~!ol. Int., Sect., 10: 60--66. De Albuquerque, C.A.R., 1971. Petrochemistry of a series of granitic rocks from northern Portugal. Bull. Geol. Soc. Am., 82: 2783--2798. De Albuquerque, C.A.R., 1973a. The origin of enclaves in granitic rocks from northern Portugal. Geol. Soc. S. Afr., Spec. Publ., 3: 479--493. De Albuquerque, C.A.R., 1973b. Geochemistry of biotites from granitic rocks, northern Portugal. Geochim. Cosmochim. Acta, 37: 1779--1802. Deer, W.A., Howie, R.A. and Zussman, J., 1962a. Rock-forming Minerals, 3. Sheet Silicates. Longmans, London, 284 pp. Deer, W.A., Howie, R.A. and Zussman, J., 1962b. Rock-forming Minerals, 4. Framework Silicates. Longmans, London, 448 pp. Dupuy, C., 1968a. Rubidium et caesium dans biotite, sanidine et verre des ignimbrites de Toscane (Italie). Chem. Geol., 3: 281--291.

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Dupuy, C., 1968b. Le rubidium dans la biotite et le feldspath potassique de la granod~orite du Monte Capanne (Italie), et du granite de Neira (Espagne). C.R. Acad. Sci. Paris, 266: 2223--2226. Eugster, H.P., 1955. The cesium--potassium equilibrium in the system sanidine--water. Carnegie Inst. Washington, Yearb., 54: 112--113. Hall, A., 1967. The distribution of some major and trace elements in feldspars from the Rosses and Ardara granite complexes, Donegal, Ireland. Geochim. Cosmochim. Acta, 31 : 837--847. Harris, P.G., Kennedy, W.Q. and Scarf e, C.M., 1970. Volcanism versus plutonism - - the effect of chemical composition. In: G. Newall and N. Rast (Editors), Mechanism of Igneous Intrusion, Liverpool Geological Society, Liverpool, pp.187--200. Iiyama, J.T., 1968. Etude exp~rimentale de la distribution des ~l~ments en trace entre feldspath potassique et plagioclase coexistants. Distribution de Rb, Cs, Sr et Ba ~ 600°C. Bull. Soc. Fr. Mineral. Cristallogr., 91: 130--140. Joyce, A.S., 1973. Chemistry of the minerals of the granitic Murrumbidgee batholith, Australian Capital Territory. Chem. Geol., 11: 271--296. Kolbe, P., 1966. Geochemical investigation of the Cape granite, South-Western Cape Province, South Africa. Trans. Geol. Soc. S. Afr., 69: 161--199. Lagache, M., 1969. Etude exp~rimentale de la r~partition des "~l~ments-traces" sodium et c~sium, entre la leucite, l'orthose et des solutions hydrothermales ~ 600°C. C.R. Acad. Sci. Paris, 268: 1241--1243. Lagache, M., 1971. Etude exp~rimentale de la r~partition du c~sium entre les feldspaths sodipotassiques et des solutions hydrothermales a 700°C, 1000 bars. C.R. Acad. Sci. Paris, 272: 1328--1330. Lagache, M. and Carron, J.-P., 1972. La distribution des ~l~ments alcalins entre les biotites et les muscovites des roches granitiques. C.R. Acad. Sci. Paris, 275: Set. D: 157-159. Lange, I.M., Reynolds, R.C. and Lyons, J.B., 1966. K]Rb ratios in coexisting K-feldspars and biotites from some New England granites and metasediments. Chem. Geol., 1 : 317--322. McIntire, W.L., 1963. Trace element partition coefficients - - a review of theory and applications to geology. Geochim. Cosmochim. Acta, 27: 1209--1264. Oen Ing Soen, 1960. The intrusion mechanism of the late-Hercynian, post-tectonic granite plutons of Northern Portugal. Geol. Mijnbouw, 39: 257--296. Piwinskii, A.J., 1968a. Studies of batholithic feldspars: Sierra Nevada, California. Contrib. Mineral. Petrol., 17: 204--223. Piwinskii, A.J., 1968b. Experimental studies of igneous rock series, Central Sierra Nevada batholith, California. J. Geol., 76: 548--570. Piwinskii, A.J. and Wyllie, P.J., 1968. Experimental studies of igneous rock series: A zoned pluton in the Wallowa batholith, Oregon. J. Geol., 76: 205--234. Rhodes, J.M., 1969. On the chemistry of potassium feldspars in granitic rocks. Chem. Geol., 4: 373--392. Sen, N., Nockolds, S.R. and Allen, R., 1959. Trace elements in minerals from rocks of the Southern California batholith. Geochim. Cosmochim. Acta, 16: 58--78. Taylor, S.R., 1965. The application of trace element data to problems in petrology. Phys. Chem. Earth, 6: 133--213. White, A.J.R., 1966. Genesis of migmatites from the Palmer region of South Australia. Chem. Geol., 1: 165--200. Winkler, H.G.F., 1967. Petrogenesis of Metamorphic Rocks. Springer-Verlag, Berlin, 237 pp. Yoder, H.S., Stewart, D.B. and Smith, J.R., 1957. Ternary feldspars. Carnegie Inst. Wash., Yearb., 56: 206--214.