Clinopyroxene compositional trends in oversaturated ... - Science Direct

JournalofAfrican Earth Sciences, Vol. 3, No. 1/2, pp. 175 183. 1985 Printed in Great Britain

0731 7247/85 $3.0(I +0.00 © 1985 Pergamon Press Ltd.

Clinopyroxene compositional trends in oversaturated and undersaturated alkaline ring complexes B. BONIN Laboratoire de Pdtrographie-Volcanologie, Universite Paris-Sud, F-91405 Orsay Cedex, France

and A. GIRET Laboratoire de P6trologie, E R A 1011, Universit6 Pierre et Marie Curie,4 place Jussieu, F-75230 Paris Cedex 05, et Laboratoires Scientifiques des T . A . A . F . , France

(Received 27 September 1984) Abstract--Clinopyroxenes from different alkaline ring complexes, representing both silica-undersaturated and silica-oversaturated associations, have been studied in relation to their host rocks. Textural, optical and chemical evidence shows that major clinopyroxene compositional trends are related to host rock chemistry. In rocks whose agpaiitic index is less than 0.9, clinopyroxenes are Ca-rich whereas they are either Ca-rich or Na-rich in more peralkaline rocks (Na + K/A1 over 0.9). The Ca levels in calcic clinopyroxenes are controlled by the silica activity in the host rock. Several compositional trends are described according to the nature of the alkaline subseries. The presence of a compositional gap is greatly increased by the silica saturation of the magma. The solid solutions between Ca-clinopyroxenes and Na-clinopyroxenes are controlled by CaMg ,~- CaFe 2+ (diopside-hedenbergite) and CaFe 2+ ~ NaFe 3+ (hedenbergite-aegirine) substitutions. The Fe/Fe + Mg ratios in both clinopyroxenes and host rocks are considered: calcic clinopyroxenes are Mg-enriched with respect to their host rocks (K D > 4.0) whereas sodic clinopyroxenes are richer in Fe than their host rocks. Crystallization of calcic clinopyroxenes is followed by the crystallization of amphiboles, and in peralkaline rocks by the subsequent crystallization of sodic clinopyroxenes. Thus, early calcic clinopyroxenes control the Fe-enrichment in the liquid whereas sodic clinopyroxenes are late and reflect the late-stage differentiation of peralkaline residual liquids.

INTRODUCTION

PYROXENES, and especially clinopyroxenes, together with olivines and plagioclases, play a considerable part in the evolution and differentiation of magmatic series. Calcic and sodic clinopyroxenes are found in most rocks forming anorogenic alkaline ring complexes which may be either silica-saturated to oversaturated or silicaundersaturated. The chemical composition of clinopyroxenes varies considerably from calcic to sodic and may be Mg- or Fe-rich. These variations are related to the physicochemical conditions in the differentiating magma. Numerous studies have dealt with the crystallization of pyroxenes in different magmatic associations, such as Brown (1957), Brown and Vincent (1963) for Skaergaard (Greenland), Yagi (1953), Grapes etal. (1979) for Morotu, Sakhalin, or more generally Le Bas (1962), Carmichael et al. (1974), Lameyre (1975) and Wyllie (1971). Numerous regional studies have been published on the optical and chemical variations in pyroxenes, particularly in agpaiitic alkaline series (review in Mitchell 1980). In the present paper we shall attempt to draw conclusions after comparing pyroxenes from silica-saturated and miaskitic alkaline complexes situated in continental (Niger) and oceanic (Kerguelen Island) environments to other known alkaline series, such as Oslo (Neumann 1976a), Ilimaussaq (Larsen 1976), and Klokken (Parsons 1979). The textural, petrographic and A~:s3:~/2-L

chemical characteristics of the pyroxenes give clear indications of how they control the differentiation of alkaline magmas. Pyroxenes have been named according to the last edition of Deer et al. (1978) and the review of Cameron and Papike, in Prewitt (1980). The general formula is W l _ p ( S , Y)I+pZ206in which the tetrahedral sites are filled by Si, A1 and Fe 3÷. Our chemical data were obtained with an electron microprobe Cameca; we thus were unable to determine ferric iron. The structural formulae of the pyroxenes were recalculated on the basis of four cations (Si + Ti + AI + Fe + Mn + Mg + Ca + Na + K = 4) according to the method advocated by Neumann (1976b). Thus, assuming a perfect stoichiometry, i.e. four cations of six anions, the contents in Fe 2+ and Fe 3+ were obtained by subtraction. Of course, analytical errors are greatly increased (Robinson, in Prewitt 1980) even if this method is commonly used. To avoid such analytical errors, we shall present diagrams in which Fe is not separated into ferrous and ferric, but is presented as Fe total.* THE PYROXENES IN SILiCA-SATURATED TO O V E R S A T U R A T E D SERIES

In these associations, the series consists of basic and intermediate r o c k s (gabbros, monzonites) and acid * Chemical analyses of clinopyroxenes and of host rocks may be obtained by request to the authors.

175

176

B. BONIN and A. GIRET

rocks (syenites, granites). In basic and intermediate rocks, normative quartz or nepheline are found in small proportions (CIPW normative minerals). Acid rocks containing pyroxenes are enriched in modal quartz and trend towards peralkaline types.

AC

The lskou complex, Atr, Niger (Fig. l(a)) This complex (Leger 1980) belongs to the anorogenic province of Niger-Nigeria (Black and Girod 1970, Bowden and Turner 1974). Several petrographic associations have been distinguished: (1) a basic and intermediate association with ieucogabbros, monzogabbros, ferrosyenites, ferroaugitebearing syenites and ferrohedenbergite-bearing syenites; (2) a silica-saturated and aluminous association with monzonites, syenites and biotite-bearing granites; (3) a silica-saturated and peralkaline association with syenites and Na-rich clinopyroxenes-bearing granites. The amphiboles of these rocks have already been described (Giret et al. 1980). In the basic and intermediate associations, clinopyroxenes are always present and make up 5-12% of the rocks (modal composition). Like the plagioclases, they have crystallized somewhat later than olivine, opaques and apatite and they are commonly surrounded by amphiboles (uralitization process of Giret et al. 1980, Fig. l(a)). Basic rocks (leucogabbros, monzogabbros and ferrosyenites) have calcic clinopyroxenes of augite to ferroaugite composition with marked iron-enrichment with respect to calcium which remains roughly constant. With Fe-enrichment, AI and Ti diminish while Mn rises. The clinopyroxenes in ferroaugite syenites and in ferrohedenbergite syenites trend as in the basic rocks, toward extreme enrichment in Fe and a very slight rise in Ti. The major substitutions are of the Mg ~ Fe z+ type. The clinopyroxenes in alkaline syenites are sodic hedenbergites containing up to 25% of the acmite molecule with substitution CaFe 2+ ~--~NaFe 3+. Finally an aluminous biotite-syenite contains a less evolved type of pyroxene, similar to that found in ferroaugite syenites.

Hd

Ac (b)

Di

Hd

Ac

The complexes of Rallier-du-Baty Peninsula, Kerguelen Island (Fig. l(b)) This association of five intersecting centres is situated south-west of the main island. Geological and geochemical data are well known (Nougier 1970, Marot and Zimine 1976, Lameyre et al. 1976, Dosso et al. 1979, Giret 1980, Lameyre et al. 1981). These complexes are essentially made up of alkaline syenites and granites with small quantities of gabbro. The olivine gabbro contains augite surrounded by brown amphibole and biotite, associated with plagioclase and olivine. In alkaline syenites, zoned clinopyroxenes with colourless or pinkish cores and green rims can be found. The cores are made up of augite with major Mg ~ Fe z+ substitution. The rims are commonly more sodic, such as aegirine augites, in which two types of substitution Mg

Di /

,

~

~

~

~

~

,

,

~

\Hd

Fig. 1. Clinopyroxenecompositionalrangein the Di-Hd-Acdiagram. (a) Iskouringcomplex,Air (1) anorthositesand associatedrocks, (2) gabbros and leucogabbros, (3) monzogabbros, (4) ferrosyenites,(5) alkaline syenites.(b) Rallierdu BatyPeninsula,KerguelenIsland (1) gabbro, (2) alkalinesyenite,(3) peralkalinegranite.(c) MontsBallons, KerguelenIsland(1) gabbros, (2) monzonitesand syenites.

Clinopyroxene compositional trends in alkaline ring complexes Fe 2+ and CaFe 2+ ~ NaFe 3+ are found. This suggests a substitutional scheme such as CaMg ~- NaFe 3+ as studied experimentally by Yagi (1966). The granites contain only pure aegirine. The calcic clinopyroxenes in syenites and quartz-syenites crystallized early at the shme time as olivine (fayalite)and the opaques, earlier than the alkaline mesoperthites. The Ca-Na clinopyroxenes, on the other hand, crystallized later or contemporaneously with the sodic-calcic amphiboles and aenigmatite (Giret et al. 1980). Aegirine crystallized much later, together with quartz and sodic amphiboles.

The complex of lle-de-l'Ouest, Kerguelen Archipelago (Fig. 2(a)) This island consists of stratified basalts intruded by gabbro which is cut by several ring-dykes of monzonitic syenites, syenites and alkaline granites. In the basic associations, the pyroxenes are earlier than plagioclase and they commonly mantle olivine. This early crystallization is attested to by the formation of pyroxene accumulates within a plagioclase-rich matrix. The pyroxene composition ranges from augite or salite to ferrosalite with Mg ~ Fe and Ca ~ Fe substitutions. Iron-enrichment is high in quartz-syenites and alkaline granites in which early pyroxenes which crystallized with fayalite (Fo80) are often zoned and have compositions which range from augite to ferrosalite, then to hedenbergite and aegirine-hedenbergite with a slight increase in Na. Therefore, in addition to the above noted substitutions, there must also be a substitution of the type CaFe 2+ ~_ NaFe 3+. Alkaline syenites, on the other hand, contain a second generation of pyroxenes which crystallized in the interstices of alkali feldspar laths. This is aegirine (Na/Na + Ca intermediate between 0.86 and 0.96), associated with sodic amphibole. These show a significant increase in Mn: the Mn/Mn + Fe + Mg ratio in the pyroxenes ranges from 0.013 to 0.029 in basic rocks (SiO2 < 60 wt%) and from 0.029 to 0.095 in differentiated rocks. This evolution suggests that the Ca ~ Mn, Mg ~-- Mn substitutions are a function of silica activity.

177

AI

M9/

\Fe÷Mn

AI

Mg/

" \ Fe+Mn

AI

The complexes of the Jeanne-d'Arc Peninsula and Monts Mamelles, Kerguelen Island These complexes are the oldest known in the archipelago (Giret et al. 1981). They consist mostly of gabbros cut by veins of quartz-syenites. In the Jeanned'Arc Peninsula, pyroxenes make up 35 vol.% of the gabbros and 3-8 vol.% of the syenites. The pyroxenes represent a homogeneous population (augite and salite) and a slight iron-enrichment which is associated with the Mg Fe + Mg magmatic differentiation is less noticeable than in olivines which range from F074 to F059. In the Monts Mammelles, the gabbros and monzogabbros contain up to 25 vol.% of optically non-zoned Fig. 2. Clinopyroxenecompositional range in the AI-Mg-Fe + Mn (a) Ile de l'Ouest, KerguelenIsland. (b) Skaergaard (Brown augite in which the variations of the Fe/Mg ratio is very diagram. and Vincent 1963). (c) Monts Ballons, Kerguelen Island slight. Syenitic dykes contain green fcrroaugite. The (p = pink clinopyroxene, g = green clinopyroxene).

178

B. BONIN and A. GIRFr

pyroxenes of these two massifs both show slight chemical variations in the basic rocks, and Fe- and Mn-enrichment with an Al-depletion in syenitic rocks.

The complex of Klokken (Parsons 1979) The complex of Klokken in the Gardar Province in Greenland has the same age as the well-known agpaiitic complex of Ilimaussaq (Blaxland and Parsons 1975) but it is made up of gabbros and layered syenites. Clinopyroxenes occur in all rocks and cover a wide compositional range from salites to hedenbergites. In gabbros and some massive syenites, schilleritization occurs commonly but is not found in layered series. Clinopyroxenes crystallized later than plagioclase and earlier than alkali felspar. In aplitic syenites, aegirine crystallized late at the rims of amphibole. Clinopyroxenes range from salite to ferrosalite in gabbros and massive syenites, from hedenbergite to sodic-hedenbergite in banded syenites and become aegirinic in late aplitic quartz-bearing syenites. A compositional gap, between sodic hedenbergite and aegirine is filled by sodic-calcic amphiboles, as already established in numerous peralkaline complexes (Stephenson 1972, Ferguson 1977, 1978). Calcic clinopyroxenes appeared earlier than amphiboles while sodic clinopyroxenes came later.

The complexes of the Oslo Graben (Neumann 1976a) The alkaline complexes of the Oslo Graben show a silica-saturated to oversaturated association made up of monzonites (akerites), quartz-bearing syenites (nordmarkites) and peralkline granites (ekerites). This is a minor association compared to the silica-undersaturated association of essexites, monzonites, plagifoyaites and nepheline-syenites. In the silica-saturated association, early calcic clinopyroxenes are rather homogeneous with a limited substitution Mg ~ Fe (augite and salite) in the basic and intermediate rocks. In the acid peralkaline rocks, pyroxenes are either homogeneous (aegirine and aegirine-augite) or heterogeneous with calcic cores and sodic rims showing a Ca (Mg, Fe 2+) ~± NaFe 3+ substitution. The sodic rims of pyroxenes crystallized late at the same time as sodic amphiboles (arfvedsonite).

Conclusion The silica-saturated to oversaturated association of alkaline plutonic ring complexes form two groups of rocks according to the nature of their clinopyroxenes. (1) A group with calcic clinopyroxenes enriched with a limited amount of Na but showing important variations of the Mg/Fe ratio. This group includes basic rocks, intermediate rocks and acid metaluminous rocks. The Mg ,~ Fe substitution is the most important but minor substitutions can occur: the magnesian pyroxenes (salite and augite) of the basic rocks are rich in AI and Ti, and the Fe-bearing clinopyroxenes show high values of Mn.

These trends are rather limited and less spectacular than in agpaiitic series and therefore it is not possible with the Le Bas diagrams (1962) to separate clearly the clinopyroxenes of the basic rocks which belong to alkaline and silica-oversaturated series (Coombs trend of Miyashiro 1978) from those of the basic rocks of subalkaline series. The ternary plot AI-(Fe + Mn)-Mg, however, enables us (Fig. 2) to distinguish alkaline series (Fig. 2(a)) from tholeiitic series (Fig. 2(b)) after data from Skaergaard, Brown 1957; Bushveld, Atkins 1969, Buchanan 1979), the latter showing a low continuous decrease in A1, whereas the saturated alkaline series shows a sudden decrease in AI for most magnesian compositions. (2) A group with sodic clinopyroxenes, near the acmite apex. This group includes acid peralkaline rocks (syenites and granites). The major substitution is CaFe 2+ NaFe 3+, but the aegirinic pyroxenes may also be enriched in Ti (neptunite molecule of Ferguson 1977). These aegirines are late, crystallizing after calcic amphiboles and often after sodic amphiboles (Ferguson 1978, Bonin 1980).

PYROXENES IN SILICA-UNDERSATURATED SERIES

Because of their varied mineralogy and their great petrological interest, these associations have been by far the most studied. The continental undersaturated complexes are now well known (SOrensen 1974) and there are numerous examples of them: in Greenland (Stephenson 1972, Larsen 1976, Nielsen 1979, Jones and Peckett 1980), in Norway (Neumann 1976a, Mitchell 1980), in Brazil (de Barres Gomes et al. 1970), in Zimbabwe (Hossain 1977), in Canada (Mitchell and Piatt 1978, 1979), etc. We here give results from the oceanic complexes of the Kerguelen Island: Monts Ballons and Montagnes Vertes. These complex rocks have abundant normative nepheline but an appreciable alumina content, which places them in the miaskite group.

The complex of Montagnes Vertes, Kerguelen Island This massif which is 800 m in diameter shows a chilled gabbro outer zone and a progressive petrographic evolution towards a nepheline-bearing syenite core. This is a silica-undersaturated aluminous alkaline series in which the existence of nephelinic varieties may be interpreted as due to the destabilization of the hastingsitic amphibole (Giret 1979). Pyroxenes can be found in all the rocks, together with Mg-olivine in the basic rocks and with amphibole in the syenitic differentiated rocks. They are the only mafic minerals in the monzonites. These are salites whose compositions reveal mostly an iron-enrichment connected with differentiation: Mg/Mg + Fe varies from 0.75 to 0.61 and Na/Na + Ca varies from 0.03 to 0.11. In the most differentiated rocks, the salitic pyroxenes become strongly zoned and their rims, as well

Clinopyroxene compositional trends in alkaline ring complexes A¢

as the last crystals, are aegirine-augites with an Na/Na + Ca ratio between 0.34 and 0.49. The nepheline-syenites possess a second generation of salitic pyroxenes, which have been interpreted as products of the reaction of amphiboles with differentiated liquid (Giret 1979).

D\Hd

The complex of Monts Ballons, Kerguelen Island (Figs l(c) and 2(c)) This is the coarse-grained apex of a volcano-plutonic complex only partly uncovered by erosion. The part of this elliptic massif which can be seen is 0.6 km in diameter and 0.3 km high. It is made up of three concentric ring-shaped intrusions of olivine-gabbro, diorite, micromonzonite which is very rich in amphiboles, and finally a ring-dyke of nephelinesyenite. Each intrusion presents petrographic variations, which confer on the whole complex the appearance of a continuous petrographic series ranging from gabbro to nepheline-syenite (Giret and Lamayre 1980). Pyroxenes are to be found in all the rocks and their zoning increases with differentiation. These pyroxenes are M-rich and their compositions range from salite in gabbros, to ferrosalite ~n monzonites, then to aegirinic augite in nepheline-synites. In connection with the Feand Na-enrichment, there is a marked depletion in alumina whose amounts range from 5.42 wt% in gabbros to 1.22 wt% in nepheline-syenites.

The complexes of the Oslo Graben (Neumann 1976a) The Oslo undersaturated association constitute the first and the most important intrusive phase. It is made up of olivine-bearing diorites (sOrkedalites), monzonites (kjels&sites, larvikites), foyaites (lardalites) and nepheline-syenites (hedrumites). The miaskitic undersaturated rocks contain a range of clinopyroxenes from salite to ferroaugite with limited substitutions Mg ~ Fe and Ca ~ Fe. The peralkaline undersaturated rocks (i.e. agpaiitic) contain sodic clinopyroxenes with cores that are more or less calcic (Na/Na + Ca = 0.1) and rims that are more sodic (Na/Na + Ca reaches 0.6) with a very constant Fe/Mg ratio, that is a sign of the CaFe 2+ ~ NaFe 3+ substitution. The appearance of sodic pyroxenes is directly linked to the chemical composition of the magma: sodic pyroxenes appear only in rocks with an agpaiitic index (Na + K/A1) greater than 1.(J without any chemical gap between the pyroxene cores of the agpaiitic rocks and the calcic pyroxenes of the miaskitic rocks (Fig. 4).

Conclusions (Fig. 3) Alkaline-undersaturated rocks contain pyroxenes which are more varied than those encountered in alkaline-saturated rocks. Indeed, miaskitic associations contain calcic and sodic pyroxenes identical to those found in peralkaline silica-saturated series. Agpaiitic associations contain pyroxenes of several types (Mitchell 1980):

179

AC

AC

Hd= Fe.Mn i Na+K, AC: Na*KI / '

L

Pi I. 3. Clinopyroxene compositional range from different alkaline series in tie D i - I I - A c diagram (compiled from various sources). (I) Agpaiitic subseries, acmitic trends, (1') agpaiitic subseries, acmite hedenbergitic trend, (2) miaskitic subseries, (3) peralkaline silicasaturated subseries, (4) metaluminous silica-saturated subseries, (5) tholeiitic continental series (Skaergaard, Bushveld).

(i) AI-Na diopsides rarely to be found as cores in lamprophyre pyroxenes; (ii) Ti-AI salites in nephelinic basic rocks; (iii) an evolution of the acmitic hedenbergite type in agpaiites s.s., ijolites and silico-carbonatites, an evolution which is very close to that of the miaskites; (iv) an evolution of the acmitic type, to be found in the urtites and carbonatites with a CaMg ~ NaFe 3+ substitution, that is along the Di-Ac join. Agpaiitic associations have pyroxene compositions covering the whole of the D i - H d - A c triangle with the exception of the diopside apex. Miaskitic associations show a more limited compositional range since there is no acmitic trend. Late sodic pyroxenes often reveal particular chemical compositions, with large amounts of Ti (neptunite molecule NAT of Ferguson 1977, Grapes

Na-,.K/AI Is. Kerguelen,T A.A.F 12,

O~(~o--0,0

--0

\ " " Oslo

|O

¢

2 3

i" 1

Na/ Na÷Ca Px

Fig. 4. Relations between the acmite contents in clinopyroxenes (expressed by Na/Na + Cae~) and the agpaiitic indexes of the host rocks (Na + K/AIR) in Kerguelen Island alkaline complexes. (1) gabbros, (2) peralkaline silica-saturated syenites, (3) metaluminous silicaundersaturated syenites and monzonites, (4)peralkaline granites. Dashed area = data from Oslo region (Neumann 1976a) shown for comparison.

180

B. BONIN and A. GIRET

et al. 1979, Nielsen 1979), of Zr (NAZ molecule of Jones

and Peckett 1980, Larsen 1976), associated with aenigmatites and arfvedsonites. Late substitutions are evidence of reducing conditions: 2Fe 3+ ~ Fe2+Ti4+ and 2Fe 3+ ~ Fe2+Zr4+, whereas the evolution from calcic clinopyroxenes to sodic clinopyroxenes shows an oxidation process. Low oxygen fugacity at the end of the crystallization of alkaline magmas (Bailey 1969) implies low contents in water, the competition between pyroxenes, amphiboles and aenigmatites (Ferguson 1978, Parsons 1979, Grapes et al. 1979)could justify the rapid changes in the oxidation state of iron in these minerals.

RELATIONS BETWEEN CLINOPYROXENES AND ROCK COMPOSITIONS

Clinopyroxenes show variations in their chemical compositions according to the rocks which contain them and the magmatic association to which they belong. Mg Fe and Ca ~ Na substitutions play an important part in magma evolutions. The variations of the compositions of clinopyroxenes are complex and are found not only in the Na/Na + Ca, Mg/Fe ratios, but also in the amounts of A1, Ti, Mn, etc. It is difficult to account for these variations with one diagram only. For example, the Le Bas diagrams based on old analyses should be reexamined as a number of pyroxenes of normal alkaline rocks that were analysed with electron microprobes, and are situated in the field of non-alkaline rocks (Gibb 1973). Recent attempts have been made by Maury et al. (1982) and their diagrams are particularly helpful, especially for basic rocks without any associated late-stage rocks. Overlap between subalkaline rocks pyroxenes and alkaline rocks pyroxenes appears in numerous diagrams, particularly in the Di-Hd-Ac triangle (Fig. 3). The AI-(Fe + Mn)-Mg triangle (Fig. 2(a)-(c)) is better adapted and provides a more distinct discrimination: the tholeiitic rocks (Fig. 2(b) after Skaergaard and Bushveld) show uniformly low content in Al, the alkaline rocks show early relatively aluminous pyroxenes whose AI contents decrease rapidly and then remain uniformly low. The differences in shape of the curves at the beginning of the differentiation makes discrimination easy. All the clinopyroxenes of alkaline rocks can be properly represented in the Mg-(Fe + Mn-Na-K)-(Na + K) triangle (i.e. the Di-Hd-Ac triangle) as other components are found in very small quantities in their structural formulae (Cameron and Papike 1981). In this diagram, the different associations are placed in this way (Fig. 3): (i) Agpaiitic series cover the whole Di-Hd-Ac triangle with the exception of an empty space around the diopside apex. Several trends can be distinguished (Mitchell 1980): a Di-Ac acmitic trend for feldspathoidic series and carbonatites, a Di-Hd-Ac acmitic hedenbergitic trend for agpaiitic series, the transformation from calcic clinopyroxenes to sodic clinopyroxenes being more or less rapid (Stephenson 1972). The Ilimaussaq

example (Larsen 1976) is an extreme case with a Di-Hd, then Hd-Ac evolution showing a succession of Mg Fe 2+, then CaFe 2+ ~ NaFe 3+ substitutions under very low fO 2 conditions. (ii) Miaskitic series cover a smaller surface along Di-Hd and Hd-Ac joins. A number of miaskitic complexes show trends similar to the one in Ilimaussaq (Montagnes vertes, Monts Ballons). (iii) Peralkaline saturated series cover the same surface as miaskitic series (see evolution of Rallier-du-Baty as compared to the one in Monts Ballons, Fig. l(b) and l(c)). (iv) Metaluminous saturated series are more restricted along the Di-Hd join. The end of the evolution shows sodic hedenbergites, not aegirirines. (v) Finally, in tholeiitic series, only calcic clinopyroxenes are found and the acmite molecule in the structural formula is always very low. Therefore we note that as one proceeds from the most undersaturated series to the tholeiitic series, there is an increase in the empty space from the diopside apex towards the acmite field with the increase of SiO2. This phenomenon can also be seen in the Poldervaart and Hess triangle through a lowering of the Ca levels from nephelinic series to tholeiitic series (Larsen 1976): silica activity rather than total alkalinity is responsible for peralkaline saturated series having clinopyroxenes which are close to those of tholeiitic series. The presence of acmite molecule in clinopyroxenes is therefore not only connected with the variations of oxygen fugacity. There is no compositional gap between calcic and sodic clinopyroxenes in spite of the local presence of a large empty gap which varies according to regional massifs. This is now a well-established fact in silica-saturated series (Ferguson 1978) as well as in undersaturated series (Yagi 1966). The appearance of sodic clinopyroxenes is directly connected with the agpaiitic index (Na + K/A1) (Fig. 4). In peralkaline series, whether silica-saturated or undersaturated (Rallier-du-Baty, Oslo), sodic clinopyroxenes follow calcic clinopyroxenes for an AI greater than 1.0 (Neumann, 1976a), thus justifying the Shand classification and the CIPW definitions. On the other hand, in metaluminous series (Iskou, Montagnes Vertes, Monts Ballons), sodic clinopyroxenes appear for a lower AI around 0.85. This critical value of 0.85 can be compared to the 0.9 value found for the appearance of sodic amphiboles (Giret et al. 1980): calcic amphiboles exist in rocks with an AI less than 0.9, whereas sodic-calcic amphiboles characterize rocks with an AI greater than 0.9. Finally, the sodic clinopyroxenes of the miarolitic cavities in the syenites of Ile-de-l'Ouest show the characteristics of the late clinopyroxenes of highly differentiated agpaiitic rocks; therefore, it is likely that they crystallized from liquids or fluids with very peralkaline compositions. The role of clinopyroxenes in the differentiation of magmatic series (Wager and Brown 1967) and the shapes of the curves in the AFM diagram have often been stressed (Irvine and Baragar 1971, Barker 1978). There

Clinopyroxene compositional trends in alkaline ring complexes

181

R

(a) .9

.7 mm

k

~ v

t

•

---QO

•

mm

..... •

i :..Z

•

~o

,sou

Bush

/

3

i Px

Px

5

.~

.~9

3

~---

--"

A---

/ -

.7

.9

R

j, (C)

.5

9~

•

(d)

i

•

.7

.5

•

1

•

2

.5

.3

,3~ /

Rallier-du-Baty,

T.A.A.F. Px

.;

S

7

;

.9

7

9

Px

,/~

(e)

Y :

;

o

0

3 Px

.3

S

5

7

.9

Fig. 5. Relations between FeT/Fex + Mg ratios in clinopyroxenes (Px) and FeT/FeT + Mg ratios in host rocks (R). (a) Tholeiitic continental series: Sk, Skaergaard (Brown and Vincent 1963) (full symbols); Bush, Bushveld (Atkins 1969) (empty symbols). Square = layered gabbros (the FeT/FeT + Mg ratios in host rocks is averaged, according to Wager and Brown, 1969). Triangle = ferrodiorites, ferrogabbros and syenodiorites. Diamond = melagranophyres and transitional granophyres. (b) Iskou ring complex, Air; symbols as in Fig. l(a). (c) Rallier-du-Baty Peninsula, Kerguelen Island; symbols as in Fig. l(b). (d) Monts Ballons, Kerguelen Island; symbols as in Fig. l(c). (e) Oslo area (1) olivine diorite, (2) monzonites, (3) alkaline syenites, (4) ekerites (peralkaline granites), (5) plagifoyaites, (6) nepheline syenites. (Data compiled from Neumann 1976a.)

182

B. BONIN and A. GIREI

is often a tacit understanding about the fact that clinopyroxenes which crystallized in one magma showed Fe/Mg relations that evolve in the same way as the magma Fe/Mg relations. As microprobe analyses are presented with FeO x, we have chosen a comparison between various magmatic series in the FeX/Fe x + Mg (cpx) vs. FeV/Fe x + Mg (WR) diagram (Fig. 5), which does not enable us to take into account late oxidation. Magmatic series are represented by their plutonic varieties which are not true liquids but result from the crystallization of liquids and the accumulation processes. For each rock, a ratio is established: KD = Mg/Fe x (cpx)/Mg/Fe T (WR). Tholeiitic series (Fig. 5(a)), as exemplified by Skaergaard and Bushveid massifs, show an identical trend with KD maxima of 2.0: rocks with K D = 1.0 are basal cumulates, rocks with intermediate KD between 1.0 and 2.0 are the cumulates of the layered series and rocks with KD = 2.0 are the most evolved (ferrodiorites, granophyres) and result from liquids. Alkaline series (Fig. 5(b)-(e)) systematically show KDS that are greater than 2.0 for calcic clinopyroxenes and KDS that can be less than 1.0 for sodic clinopyroxenes. The Kerguelen complexes (Fig. 5(c) and (d)) show the highest KD but there is no evidence of instability of the calcic clinopyroxenes in the rocks. The great variations of KD in zoned pyroxenes show an Fe 2+enrichment in the process of crystallization and/or the presence of acmite molecule. The l:'eX/FeT + Mg (cpx) vs. FeT/Fe T + Mg (WR) diagram therefore emphasizes the important role played not only by cumulate clinopyroxenes but also magmatic clinopyroxenes in the evolution of magmatic liquids; it also shows the equilibrium of late pyroxenes with residual liquids. The evolution of KD in the magmatic series is also to be noticed: this has not yet been taken into account experimentally in spite of some recent attempts (Cawthorn et al. 1973, Gamble and Taylor 1980). The fact that calcic clinopyroxenes are systematically more magnesian than the liquids has been explained by the influence of early crystallization of metallic oxides in metaluminous lavas (Carmichael 1967), of aenigmatite in peraikaline lavas (Nicholls and Carmichael 1979). But this simple scheme does not account for the enrichment in iron of evolved liquids nor the Mg --, Fe zonation from the core of the edge of zoned pyroxenes.

Turnock 1980). Petrological and chemical studies have also depicted the part played by the composition of magmas (Neumann 1976a, Ferguson 1978) and the presence of amphiboles (Ferguson 1978, Giret et al. 1980). Physical and chemical parameters are linked but on account of limited experimental data at our disposal, it is difficult to evaluate their specific roles. Nevertheless this study of clinopyroxenes from continental and oceanic alkaline complexes establishes the relations between the compositions of the clinopyroxenes and their host rocks. Substitutions in the pyroxenes are mainly controlled by the evolution of the Fe/Mg ratio in the magma whereas the Na contents are related to the silica activity of the magma and its agpaiitic index. For alkaline rock series, we suggest a two-stage model. Early magmatic calcic clinopyroxenes crystallized at the same time as or later than olivine and oxides, but earlier than plagioclase. Late sodic clinopyroxenes are often later than alkali feldspars and sometimes in association with quartz around sodic amphiboles (Bonin 1980; Ferguson 1978) in a late or postmagmatic phase. The two periods of crystallization of clinopyroxenes are separated by a period when calcic amphiboles crystallized either from pre-existing pyroxenes (uralitization) or straight from the liquid (Giret et al. 1980). Thus the petrological role of clinopyroxenes can be described more clearly. Early calcic relatively magnesian clinopyroxenes control the Fe/Mg ratio in the liquid whereas their Ca content reveals the silica activity of the magma on which they have no influence. Contemporary calcic amphiboles (kaersutite, hastingsite) induce an Si increase in the magma (Giret et al. 1980). The destruction of these amphiboles in silica-undersaturated magmas results in the crystallization of reactional calcic pyroxenes and the production of liquids that are more silica-undersaturated and peralkaline (Giret 1979). Late sodic clinopyroxenes show through their chemistry the composition of the residual liquid. They can appear only for an agpaiitic index greater than 0.9. In very peralkaline liquids, aegirines crystallize later than sodic amphiboles (arfvedsonite) with which they have ambiguous textural relations. The chemical compositions of aegirines then shows processes of concentration of the hygromagmatophile elements in the residual liquids. Both "active" and "passive" roles are played by clinopyroxenes, like amphiboles, and it is worth comparing them.

DISCUSSION AND CONCLUSIONS

Acknowledgements Wc arc indebted to Professor .I. Lamayre and Dr. R. Black, Laboratoire de Petrologic. Universite Pierre et Marie ('uric, for their useful comments and advice. Research facilities were obtained from C.N.R.S. through its grant to I..A. 298 and from Laboratoires Scientitiques des T . A . A . F . for field work. Translation from French to English has been very kindly done by Mrs. CI Gauthier, teacher in English in the Lyccc Frcmm at Bondy, France: and very carefully reviewed by Dr. R. Black.

Clinopyroxenes are found in all alkaline series, with the exception of differentiated rocks from peraluminous biotite-bearing series. They are good mineralogical tracers both of the chemical evolution of magmas (silica activity, agpaiicity, iron-enrichment) and of the physical conditions of emplacement (T, Pmtal, fO2). Experimental data have documented the part played by the physical conditions of crystallization (Yagi 1966, Nolan 1969, Bailey 1969, Lindsley and Munoz 1969, Gilbert 1969, Gamble and Taylor 1980, Huebner and

REFERENCES Atkins, F. B. 1969. Pyroxenes of the Bushveld intrusion, South Africa. J. Petrol. 10,222-249. Bailey, D. K. 1969. The stability of acmite in the presence of HeO. A m . J. Sci. 267, Schairer vol., 1-16.

Clinopyroxene compositional trends in alkaline ring complexes

183

Barker, D. S. 1978. Magmatic trends on alkali-iron-magnesium dia- Hossain, M. T. 1977. Pyroxenes from the gabbros of the Marangudzi ring-complex, Rhodesia. Miner. Mag. 41,111-119. grams. A m . Miner. 63,531-534. de Barres Gomes, C., Moro, S. L. and Dutra, C. V. 1970. Pyroxenes Huebner, J. S. and Turnock, A. C. 1980. The melting relations at 1 bar of pyroxenes composed largely of Ca-, Mg-, and Fe-bearing compofrom the alkaline rocks of Itapirapua, Silo Paulo, Brazil. A m . Miner. nents. A m . Miner. 65,225-271. 55,224-230. Black, R. and Girod, M. 1970. Late palaeozoic to recent igneous Irvine, T. N. and Baragar, W. R. A. 1971. A guide to the chemical classification of the common volcanic rocks. Can. J. Earth Sci. 8, activity in West Africa and its relationship to basement structure. In: 523-548. African Magmatism and Tectonics (Edited by Clifford, T. N. and Jones, A. P. and Peckett, A. 1980. Zirconium-bearing aegirines from Gass, I. G.), pp. 185-210. Oliver and Boyd, Edinburgh. Motzfeldt, South Greenland. Contr. Miner. Petrol. 75,251-255. Blaxland, A. B. and Parsons, I. 1975. Age and origin of the Klokken gabbro-syenite intrusion, South Greenland: Rb-Sr study. Bull. Lameyre. J. 1975. Roches et MinOraux. Doin, Paris. Lameyre, J., Marot, A., Zimine, A., Cantagrel, J. M., Dosso, L. and geol. Soc. Denmark 24, 27-32. Bonin, B. 1980. Les complexes acides anorog6niques continentaux: Vidak Ph. 1976. Chronological evolution of the Kerguelen Islands I'exemple de la Corse. ThOse Doct. d'Etat, Universit6 Paris VI, syenite-granite ring-complex. Nature, Lond. 263, 3(16-307. Lameyre, J. et al. 1981. Etude g6ologique du complexe plutonique de 756 pp. Bowden, P. and Turner, D. C. 1974. Peralkaline and associated la Peninsule Rallier du Baty, Iles Kerguelen. Bull. C . N . F . R . A . 49, ring-complexes in the Nigeria-Niger province, West Africa. In: The Larsen, L. M. 1976. Clinopyroxenes and coexisting mafic minerals Alkaline Rocks (Edited by SCrensen, H.), pp. 3,30-351. John Wiley, from the alkaline llimaussaq intrusion, South Greenland. J. Petrol. London. 17,258-290. Brown, G. M. 1957. Pyroxenes from the early and middle stages of Le Bas, M. J. 1962. The role of aluminium in igneous clinopyroxenes fractionation of the Skaergaard intrusion, East Greenland. Miner. with relation to their parentage. A m . J. Sci. 260,267-288. Mag. 31, 511-543. Leger, J. M. 1980. Evolution pdtrologique des magmas basiques et Brown, G. M. and Vincent, E. A. 1963. Pyroxenes from the late stages alcalins darts le complexe anorog6nique d'Iskou (Air, Niger). ThOse of fractionation of the Skaergaard intrusion, East Greenland. J. Doer. 3 ° cycle, Universit6 P. et M. Curie, Paris VI, 218 pp. Petrol. 4, 175-197. Linsley, D. H. and Munoz, J. L. 1969, Subsolidus relations along the Buchanan, D. L. 1979. A combined transmission electron microscope join hedenbergite-ferrisilite. A m . J. Sci. 267, Schairer vol., 295-324. and electron microprobe study of Bushveld pyroxenes from the Marot, A. and Zimine, S. 1976. Les complexes annulaires de sydnites Bethal area. J. Petrol. 20,327-354. et granites alcalins dans la pdninsula Rallier du Baty, lies Kerguelen Cameron, M. and Papike, J. J. 1981. Structural and chemical (TAAF). ThOse Doct. 3 ° cycle, Universit6 P. et M, Curie, paris VI, variations in pyroxenes. A m . Miner. 66, 1-50. 420 pp. Carmichael, 1. S. E. 1967. The iron-titanium oxides of volcanic salic Mitchell, R. H. 1980.Pyroxenes of the Fen alkaline complex, Norway. rocks and their associated ferromagnesian silicates. Contr. Miner. A m . Miner. 65, 45-54. Petrol, 14, 36-64. Mitchell, R. H. and Platt, R. G. 1978. Mafic mineralogy of ferroaugite Carmichael, I. S. E., Turner, F. J. and Verhoogen, J. V. 1974. Igneous syenite from the Coldwell complex, Ontario, Canada. J. Petrol. 19, Petrology. McGraw Hill, New York. 627-651. Cawthorn, R. G., Ford, C. E., Biggar, G. M., Bravo, M. S. and Mitchell, R. H. and Platt, R. G. 1979. Nepheline-bearing rocks from Clarke, D. B. 1973. Determination of the liquid composition in the Poohbah Lake complex, Ontario: malignites and malignites. experimental samples: discrepancies between microprobe analysis Contr. Miner. Petrol. 69,255-264. and other methods. E. P.S.L. 21, 1-5. Miyashiro, A. 1978. Nature of alkalic volcanic rocks series. Contr. Deer, W. A., Howie, R. A. and Zussmann, J. 1978. Rock-forming Miner. Petrol. 66, 91-104. minerals, Vol. 2A. In: The chain silicates. Longmans, London. Neumann, E. R. 1976a. Compositional relations among pyroxenes, Dosso, L., Vidal, P., Cantagrel, J. M., Lameyre, J., Marot, A. and amphiboles and other mafic phases in the Oslo Region plutonic Zimine, S. 1979. Kerguelen continental fragment or oceanic island?: rocks. Lithos 9, 85-109. Petrology and isotopic geochemistry evidence. E.P.S.L. 43, 46-60. Neumann, E. R. 1976b. Two refinements for the calculation of Ferguson, A. K. 1977. The natural occurrence of aegyrine-neptunite structural formulae for pyroxenes and amphiboles. Norsk. Geol. solid solution, Contr. Miner. Petrol. 60,247-253. Tidsskr. 56, 1-6. Ferguson, A. K. 1978. The crystallization of pyroxenes and Nicholts, J. and Carmichael, I. S. E. 1969. Peralkaline acid liquids: a amphiboles in some alkaline rocks and the presence of a pyroxene petrological study. Contr. Miner. Petrol. 20,268-294. compositional gap. Contr. Miner. Petrol. 67, 11-15. Nielsen, T. F. D. 1979. The occurrence and formation of Ti-aegyrines Gamble, R. P. and Taylor, L. A. 1980. Crystal/liquid partitioning in in peralkaline syenites. Contr. Miner. Petrol. 69,235-244. augite: effects of cooling rate. E . P . S . L . 47, 21--33. Nolan, J. 1969. Physical properties of synthetic and natural pyroxenes Gibb, F, G. F. 1973. The zoned clinopyroxenes of the Shiant Isles Sill, in the system diopside-hedenbergite-acmite. Scotland. J. Petrol. 14,203-230. Nougier, J. 1970. Contribution i l'dtude geologique et g6oGilbert, M. C. 1969. High-pressure stability of acmite. A m . J. Sci. 267, morphologique des lies Kerguelen (Terres Australes et Schairer vol., 241-244. Antarctiques Frangaises). Bull. C . N . F . R . A . 27. Giret, A. 1979. Gen6se de roches feldspathoidiques par la d6stabilisa- Parsons, I. 1979. The Klokken gabbro-syenite complex, South Greention des amphiboles: massif des Montagnes Vertes, Kerguelen land: cryptic variation and origin of inversely graded layering. J. (T.A.A.F.). ('.r. Acad. Sci. Paris 289,379-382. Petrol. 20,653-694. Giret, A. 1980. Carte geologique au 1/50 000 de la Peninsule Rallier du Prewitt, C. T. 1980. Pyroxenes. Min. Soc. Amer., Reviews in Baty. Bull. C . N . F . R . A . 45. Mineralogy n" 7. Giret, A. and Lameyre, J. 1980. Mise en place et 6volution mag- S0rensen, H. 1974. The Alkaline Rocks. Elsevier, Amsterdam. matique des complexes plutonique de la cald6r de Courbet, Ile Stephenson, D. Alkali clinopyroxenes from nepheline syenites of the Kerguelen (T.A.A.F.). Bull. Soc. gOol. Fr. 22,437-446. South Q 6roq centre, South Greenland. Lithos 5,187-201. Giret, A., Bonin, B. and Leger, J. M. 1980. Amphibole compositional Wager, L. R. and Brown, G. M. 1969. Layered Igneous Rocks. Oliver trends in oversaturated and undersaturated alkaline plutonic ringand Boyd, London. complexes. Can. Miner. 18,481-495. Wyllie, P. J. 1971. The Dynamic Earth. John Wiley, New York. Giret, A., Cantagrel, J. M. and Nougier, J. 1981. Nouvelles donnees Yagi, K. 1953. Petrochemical studies on the alkalic rocks of the sur les complexes volcano-plutoniques des Iles Kerguelen. TAAF. Morotu district, Sakhalin. Bull. geol. Soc. A m . 64,769-810. C.r. Acad. Sci., Paris293, 191 194. Yagi, K. 1966. The system acmite-diopside and its bearing on the Grapes, R., Yagi, K. and Okumura, K. 1979 Aenigmatite, sodic stability of natural pyroxenes of the acmite-hedenbergite-diopside pyroxene, arfvedsonite and associated minerals in syenites from series. A m . Miner. 51,976-1000. Morotu, Sakhalin. Contr. Miner. Petrol. 69, 97-103.