Canadian MineralogistVol. 18, pp. 481-495 (1980)

AMPHIBOLE COMPOSITIONAL TRENDS IN OVERSATURATED ANDUNDERSATURATED ALKALINE PLUTONIC RING.COMPLEXES

ANDRE GIRET. BERNARD BONIN eNo JEAN-MARC LEGERDdpartement de P€trologie'v, Universitd Pierre-et-Marie Curie,4, place lussieu, F75230 Paris Cedex 04

France

ABsrRAcr

Amphiboles from different alkaline ring-com-plexes, representing both silica-oversaturated andsilica-undersaturated petrographic associations, havebeen studied in relation to their host rocks. Tex-tural, optical and chemical evidence emphasizesmajor amphibole compositional trends related tohost-rock chemistry. In rocks whose agpaitic co-efficient is less than 0.9, the Ca + Alt" contentof amphiboles is more than 2.5, whereas it is lessthan 2.5 in agpaitic rocks (NazO*K2O,/ALO" >0.9). This feature is shown firstly by the presenceof solid-solution series from kaersutite to horn-blende or hastingsite with substitutions I Ti €NaAAl. Ti + O <+ Fe3* + OH- and CaAl<= NaSi predominating in silica-undersaturatedrock series and NaAl = n Si in basic and inter-mediate rocks from silica-oversaturated series.Secondly, this feature is shown by tbe presenceof solid-solution series from actinolite or barroisiteto winchite. with CaA|" P Na Si substitution orto katophorite, richterite and then arfvedsonite withbalanced substitutions as n Fes" c> NaFe2+ andCaAlt" <2 Na Si. The first trends are related toearly magmatic stages, and the second to late mag-matic stages. The absence of (Ca + Alb)-richamphiboles in agpaitic rocks and an observed breakbetween (Ca + Alr")-rich amphiboles and(Ca .-1- Ali")-poor amphiboles suggest that (Ca+ Alt')-rich amphibole stability is controlled bymagma alkalinity. One argument is based on thedescription of a reaction, involving hastingsite anda residual liquid, which results in the crystalliza-tion of clinopyroxene and Ti-magnetite, and in theaddition of potential analcime to the residual liquid.This alkalinization of the liquid ultimately pro-duces a peralkaline concentrate,

Keywords: amphiboles, alkaline rocks, anorogenicmagmatism, differentiation processes.

Sotulvrelne

Cette 6tude concerne les amphiboles de diff6-rents complexes annulaires alcalins, aussi biensursatur6s que sous-satur6s en silice, et leurs rela-

Elaboratoire associ6 au CNRS (298) & LaboratoiresScientifiques des T.A.A.F.

tions avec leurs roches h6tes. Des arguments tex-turaux, optiques et chimiques soulignent I'existencede lign6es dans la composition des amphiboles enfonction de la composition chimique des roches.Dans les roches i coefficients d'agpaicit6 inf6rieuri 0.9, le nombre de cations * Alt" des amphi-boles est sup6rieur i 2.5, alors gu'il est inf6rieuri 2.5 dans les roches agpaitiques (NazO*K:O,/Alroi > 0.9). Ce caractere se traduit d'une partpar la pr6sence de solutions solides allant deskaersutites aux hornblendes ou aux hastingsitesavec les substitutions principales suivantes: I Ti<= NaAAl, Ti + O <+ Fe"+ + OH- et Ca Alt'<= Na Si dans les roches sous-satur6es en silice,et substitutions du type Na Al ? n Si dans lesroches basiques et intermddiaires des s6ries sursa-tur6es en silice: et d'autre part la pr6sence desolutions solides allant des actinotes ou des bar-roisites aux winchites selon des substitutions dedeux types: n F""' 32 |rf4Aps2+ et Ca Alt' <2Na Si. Les premidres lign€es sont attribu6es )r desstades magmatiques pr6coces, et les secondes d d:sphases tardives. L'absence d'amphiboles riches en(Ca *- Alt") dans les roches agpaitiques et la dis-continuitd observ6e entre les amphiboles riches en(Ca + Alt') et les amphiboles pauvres en (Ca +AIN) font penser que I'agpaitit6 du magma gou-verne la stabilit6 des amphiboles riches en (Ca rAIr'). A titre d'argument, on donne la descriptiond'une r6action impliquant la hastingsite et un liquider6siduel. et produisant du clinopyroxdne et de latitanomagn6tite ainsi que l'enrichissement en anal-cime potentielle du liquide r6siduel; celui-ci devientalors plus alcalin, ce qui produit en dernier lieuun concentr6 hyperalcalin.

Mots-clds; amphiboles, roches alcalines, magmatis-me anorog6nique, processus de diff6renciation.

INtnopucttoN

Amphiboles are present in many of the rocktypes found in plutonic bodies from alkalinenonorogenic ring-complexes, those defining asilica-saturated to -oversaturated association aswell as those showing silica undersaturation. Inboth cases, the chemical composition of am-phiboles varies considerably from calcic toalkali-rich members according to chemical varia-tions in the differentiating magma and to ther-

481

482 THE CANADIAN MINERALOGIST

TABLE 1. RESULTS OF MAJOR ELEMENT ANALYSES OF ROCKS FROM RING COMPLEXES

1 2 'sr02Tro2

il$:l o o 'Itnoxdoc80xa?oKdt

iz37Ez9-

74.72 75.@ 71.5o.05 t! .21

11.12 12.12 14.94.92 1 .19 .86.72 .82 1.2O.0, .04 .0,.o2 .04 .20.9 t .6 ' 1 .84

4.47 4 .25 4 .5s4 .84 4 .76 4 .O1

tr .o5 .04.4t .51 ,69.15 .05 .1 r

no^ td nd-q6 nn-21

rxou

cBtf scr.t ??.rr8 67.7s 66.77 61.77 17.71

61.7' 61.55 '8.t4 51.41 '1.66 51.rt 69.14r..r.r .8t .98 2.85 .8? .t .27

15,48 15.16 18.02 '15.88 2r.rO 24.01 14.261.U 2.r ' r .?8 1.86 1.56 1.44 2.606.18 4.0t r .65 9.72 1.15 2.r7 1.7 '.11 ' .14 .10 .10 .O7 .05 .O5.96 .72 .1O 2.52 .91 2.1O .12

2.e4 2.O2 '.51 6.12 7.15 10.56 2.t24.r5 1.r1 5.59 4.54 ' . r7 ' . t 1,U4.06 4.41 1.58 2.17 1.9 ' .Q 4.7 '

.4' ,27 .11 .60 .r9 .2O .14

.r5 1.75 .72 .77 1.o7 .50 .17

.09 .r9 .15 .04 .10 .07 .ro99.51 98.99 99.81tm.98 98.01 rxr.29l'O.gO

5rO2T1O2Ar*,fo20,ttroUrocaoNozoxeoPzosP.F:

46.79 57.62 t4.4o 5r.s6 At.r ' 4r.47 41.72 57.202. r5 . r5 t ,71 1 .Ot ' .o2 1 ,27 2 .65 .67

16.86 19.49 r4.ro r9.r8 1r.52 14.82 18.62 1?.80g.g l , .6e 16 .80 6 .48 ' t l .7e 1r .96 g . r2 e .51

.1', l .18 . ' ' t ' , l . ' t5 .14 .21 ,17 .20, .16 .5 , 9 .2 ' 1 .16 ' .9 , t 6 . ' t ' , .12 .506.75 2 .29 12 .46 ' . r ' s .74 9 .46 7 .61 2 .214. r7 6 .40 r .o9 6 .14 t . 'o i .11 5 .o t 5 .14, .66 6 . r8 1 .60 ' .5 r 2 .56 2 .p ' .55 6 .1 t

.72 .21 .84 .54 .76 1.O5 .98 r!t . t1 1 .21 2 .15 2 .41 1 . rS .s t 1 .12 . .77

o l <a aA

TABLE 2. RESULTS OF MJOR ELEMENT ANALYSES OF AMPHIBOLES FROM RING COMPLEXES

LLr L2 Fr P14 DF16 LL14 yAsO

Itn0!r@cso!r2oEaoP.F.

64,55 6r.48 61.50 68.91 65.82 6r.4s 6t,11 .77 .99 .t6 .40 .6' .19

15.2s 15.6',1 1r.90 14.71 't6.1' 1r.57 17.255.25 4.75 2.2A 2.25 1.42 2.75 1.191.90 . r7 2.42 1.51 2.16 1.80 2.09.17 .12 .14 .12 .1t .1t ,08.11 .72 . r4 .12 .1 ' .1 t ,08.r, .15 .60 tr .25 .45 .56.50 .10 .94 t t .40 .84 1 .1 ,

6 .20 6 .21 ' .92 ' .87 6 .01 ' .7 ' 5 ,685.92 6 .01 6 .24 ' .14 5 .42 6 .11 6 .44

.7' .81 .6t .62 ,42 .81 .8'

44.52 47.@ 6.92 44.22 N.74 61,19 68.14, .0a 2.71 1.eo , .29 1.r8 .7t .61

18.54 19.44 17.11 19.76 17.t.t 1s.44 1t.1t6.62 4.1t 4.64 4.14 5.r1 2.92 r . r55.64 ' .44 4.86 ' .8 ' 4.29 .6r r . r5. t 0 . r 9 . 1 t . 11 . 14 . 06 t t

, . 2O r . 2O 2 .55 r . r ? 1 .78 . 58 1 .65

nrJoR EtrlrENr aNAlyaBs 0F fiitp,tl8otrs tton rgB llTnusrcn 0r nALLM-,0-IAtr pENl[6lttt, rERc0ELEr1-1 l1-2 L1't L1'4 t'2'1 L2-2 L2-, Pi-'t 91-2 P11-1 ]I'l6.i 11162 It1/',,'l ll,1,2 tlll.t tlt"! ILlt 5 \no,l vl,8o2 tf26.1

i A;;6-t!n0!go:80ta^o

' A +

42.64 46.09 48,s9 4r,74 46.71 46.44 46.97 4r.98 4s.2r 41.89 4s.y 46.96 11.to a6.41 4?.Ol 48.?o 48.16 49.19 45.b 4t.6a?.9 , zJ -E .aa .26 .94 , 1 . r4 .96 1 .92 .1s .16 .99 1 .66 .06 a .o t 1 .6 t .s6 . i l

' .12 \ . i1 i . t '

1 . r 4 1 . 6 ' . 5 0 . 9 9 r , 1 6 r . 1 9 1 . 2 1 2 . 7 9 . 9 7 1 . 2 1 1 . 6 2 1 . 5 7 . ? 6 1 . 8 0 1 . 5 5 1 . 1 , r 1 . 0 6 . S O a . A Z . a \,o.2o rt.gr r4.s.6. r5.r1 1e.96 22.9j ,2.5r 28.91 t6.69 tj.62 2a.68 l l.!t 22,r8 r2.r4 l l.1e u.re >s.>z'rt. i i zs.z6 26.661 . 9 9 l , r , 1 . 6 6 2 . i o 1 . r t 1 . 5 , 1 . 6 , 1 . 1 2 1 . 6 2 1 . 5 9 1 . 2 1 1 . 2 s 1 . o o j . o 7 . 9 2 t . a a - r . g l . e r i i . r q i . a el .B? 2 ,08 . i6 .2 i io . io ' t .48 1 . r2 5 .21 , 1 .1 , r , . r4 j .5 t , t .9 t 11 .62 z .zo , t . i t6 . r , .q9 t .g l t . tg t t . i l6 .94 4 .59 r .01 i l . zo ' t .64 6 .w 5 .4o , . i6 , .66 1 .69 i . ,o , .e i t .45 , .4o , .o , , .e i a . i2 t ' t .Z t g . tc a . i i4 .6s t . ro B .oo .zs 4 .65 4 .61 4 .48 c .e l l . . te t ,& i .g t t .> i : e . ts s . ie i . r i Z .x t .gz : ' .es 2 ,e i a . i t1 . 6 2 1 . 4 4 1 . r 4 . 2 2 1 . ' t 7 1 . r 4 1 . o o 1 . 2 , . 5 5 1 . 0 9 1 . 4 5 1 . r o . 8 9 i . r o 1 . 1 o 1 . 5 6 . 2 , . 1 7 . 6 t . e i

97.r4 97,69 97.41 96.6t 94.70 9r.86 94.29 97.at 9B.Bt 97.21 9a.t5 9i.52 90.52 98.15 9i.6t 97-5 qq.ri s6.rs qe-oi se-orroJoR E]m mr,ysE; 0F Allpullotrs molt TIE nrnusloN 0r cAuRo-aasTELtcA, oBslca

1-1 1-2 1-, 1-4 1-5 1-6 1-j l-o 2-1 z-2 2-t 2-4 z-5 2_6 2_1 24 e* z-to ,_1 ,_z ,_,) ro?It021120,te0

r80:aote2o

bt -

t1.72 41.61 41,40 44.41 4r.84 44.t4 44.44 42.41 4t.o5 44.11 42.761 . 2 r 1 . 2 1 1 . 0 5 1 . r 5 1 . 4 O 1 . 2 6 1 . 2 9 1 . 4 4 1 . 2 7 1 . 1 1 1 . 1 81.9 t 1 ,6 ' t 1 .58 r .4o 1 .66 t .B9 ' .?8 4 .00 2 .79 2 .92 4 .21

ia .27 ts .66 ,5 , ' t1 t7 . t4 ,6 ,20 ,6 .10 t6 .1o ,6 .11 ,9 .69 t7 .21 t1 . t51 . 4 5 1 . 4 2 1 . 2 1 1 . 2 4 1 . 1 4 1 . 6 2 1 . 6 2 1 . 6 0 1 . 8 9 1 . 1 2 1 . 2 2

.07 .o7 .10 .12 .05 .04 .09 .06 .1 1 .oB .097 . 7 8 ' 1 . 7 9 8 . 2 6 6 . 1 9 1 . 6 6 7 . y ' , t . 2 0 7 . 7 o r . 6 7 r . 8 2 8 . 1 1, .4 r r .19 2 .18 2 ,44 2 .1 r 2 .18 2 .1 r 2 .51 2 .57 t .19 2 .99

.87 .8' .1't .a2 .8' .8' .88 .90 .68 .A .8'1r.22 96.49 9e.r' 91.'t1 9r.r1 97,8o 97.r' 96.9' 94.72 96.4a 99.02

4r.16 41.49 42.61 42.14 4r.t6 46.02 4r.69 41.26 11.9, 4j.n.s0 r . r2 ,91 1 .15 .o1 . r2 .oB i .4s j .69 1 .Az

, .08 4 .51 ' .12 ' . ' t5 .20 , r1 . r9 7 ,51 1 . to 6 .eo,5,7O '6.81 '6.rt

".14 41.6 '9.12 4o.r7 '1.75 'O.29 ta.le

1 . 4 1 1 . 1 5 1 . 4 1 ' t . 2 a 2 . 5 2 2 . 6 1 2 . r 9 , 1 . 6 6 . t . 1 4 1 . . t 9.18 .1 ' .Cs .1 ' .o t .o7 .o , 2 . . t7 , .o9 2 .57

7.42 1 .aa 5 .o1 6 .85 . r1 .y .5o 9 .45 9 .67 9 .8o2.71 2 .99 2 . ' , t1 2 . ' , t1 2 .87 4 .01 r .90 2 .1 t 2 .1 t 1 .76

.14 .86 .69 .87 .0, .0, .04 .94 .8, .sj96.20 97.20 94.'t5 94.o' 92.69 9r.O' 9r.61 98.96 98.65 99.1t

tTtr 6t-16 6r. to t4. ta 6.t11 e-(12 F.t t1 r i712-t \17a 61.171 6nZ 61.71t 6147' 74-18 17-11 A9. 1A-61tN2l lo2\]20,

/,nO

480)aOIa 0rao

.2.o2 41,19 4o.2' 4t.r1 46.0' '9.t0 42.06 40.94 40.58 '8.48?i;302 . O 8 2 . 1 4 2 . 1 8 1 . 8 5 1 , 4 t r . 1 O 1 . 4 1 1 . 7 r 1 . 2 t . 1 9 . 4 t7 .81 4 .27 4 .45 t .s t ' .5 , 12 .60 7 .11 8 ,e5 10 .51 1r .OO 9 . t r'5 .57 2r . '15 29 .15 2r .78 2r .94 18 .99 26 .26 26 .82 26 .67 25 .16 ?2 .47

.84 .47 . ro . to .45 ,27 . r5 .41 , r ' . t1 . ro, .20 6 .o9 5 .84 8 .12 8 .8 r 8 .40 r .94 5 .44 4 .86 4 .17 7 .299.89 9 .92 1O. r ' 9 .9 ' 9 .71 11 ,2r 10 .11 10 . r? 10 .8 ' 10 .79 11 . r91 .9 ' 1 .9 ' 1 .79 2 . r5 1 .21 2 .16 1 .14 1 . r9 2 .ao 2 .22 ' r . ro1 .05 1 .19 r .08 .89 .52 1 .28 .81 1 .01 1 .07 .78 .1 't6.r't 96.96 97.5' 9s.r7 97.6e 91.51 96.24 97.02 98.04 9't.t6 97.26

41.95 4r.AB 41.27 4r.21 41.28 46.711 . 4 O . r 8 4 . r B 1 . 1 1 4 . 1 ' 1 . 8 1

10.12 9 .51 11 .84 1C.05 11 . r , t 2 .o t20 .11 21 ,18 l t .@ 11,46 15 .95 t4 .O8

. r9 . t4 ,17 .18 .2 ' . .a48 .41 8 . r i 1 . r .Bo 14 . rs 10 .60 .9 t

r 1 . 4 8 1 1 . 9 0 1 1 . r 0 1 2 . 0 7 1 O . 7 ' ' . 1 62 . 1 2 1 . 4 ' 2 . a A 2 . 1 5 t . r 1 4 . 1 2

.77 .54 .s' .47 .t6 1.2197.t9 97.A9 91.69 97.15 91.9a 97.t1

,8.17 t7.5'6 .7 t r .19

11.96 10.4414,4r 24,1(,

.27 1.0't9.44 5.74

1r .09 10 .4?2.61 2.961:69 1 .71

9? .16 97.86-t, nY 1 nb01

'nao) 71-49 Ta I n2o nJv, Erl o Is4rr E4t, tE-?O 1846r 'le{8,

7A6Ob 7851 78t?.

It0ZLL2OIFeO!ln0l.l80caotraaoKao

) e . t v q v . ) e 1 u . ) > ) 7 . > o 4 v . t > ) o . 2 2 2 t . ) > ) o . ) t 2 6 . t o 4 9 . 1 1 4 ) . y ., .21 5 .59 6 .06 5 . r2 5 . t l , t .40 r .o1 4 .95 r .e6 r .59 1 .22

12.2612.06 '12 .1912.2411.15 9 .11 9 .5 ' S .2e 7 .07 6 .51 7 . r ,16.16 16.11 14.74 14.8t 12.56 2',t.o 21.2' 28.65 29.79 t1.r2 2r..t i

.24 . r0 .29 . ro .12 .84 .77 1 . t 1 .04 1 .59 1 .q9 .8 ' 8 .19 8 .72 9 .44 11 .0O 5 .15 6 . ) ' t .58 1 .00 1 .96 6 .0 i

11 . r ' 1o .aa f .26 11 . r2 10 .88 9 .9 ' 10 .16 6 .14 ? .70 8 .oo 8 .? (2 .6 t r . r ' t 2 .99 r .@ r .29 r .88 r .10 t .18 4 . r ' 1 .62 2 .8 r1 . 5 6 1 . 1 7 1 . 1 2 1 . 1 r 1 . 1 1 1 . r 5 1 . t r . t . 9 5 ' t . 4 4 1 . 4 e . r t

,8.r5 98.e0 91.76 9s.y 96.61 g',t.o1 96.69 96.79 96.80 roo2 9t.?i

t>.11 )8.>o ta.B, ,7.14 40.61 11.94 19.24 19.157.08 5 .55 ' .5o 8 .01 1 .O ' 6 .75 ' .91 ' .A

1r.60 1r.20 11.98 11.56 14.95 16.22 11.12 11.6012.12 12.65 21.65 11.11 22.65 12.r' 16.1t 17.19

.2' . ' t2 .8' .29 .70 .2a ,54 .5112.12 11 .46 5 .61 1r .19 ' .60 r1 .42 9 .47 7 . r11t.49 12.41 10.80 10.4? 1O.7' 11.72 10.19 10.522.60 t r 2 .88 2 .7o , .sO , .U , .11 , .721 . 4 6 1 . r 7 1 . 5 2 1 . 6 1 1 . 4 9 1 . r 1 1 . r 1 1 . r a

98.r' 96.40 97.88 99.18 91.25 9e.10 97.86 ?',t.r2

,7.181.27

ro .@21.17

.55, .21't0.852.141.59

97.9

rnodynamic conditions (Wilkinson 1961, Ernst1962, Stull 1973, Cawthorn1976, Fabribs 1978).At the same time, the role of amphiboles duringmagmatic differentiation may control the com-position of the residual liquid, as has alreadybeen described in some volcanic series (Maury1976, Villemant 1979).

Many studies concerning optical and chemicalvariations in amphiboles have been published,but unfortunately the control exefted by am-phibole in the evolution of alkaline igneousring-complexes is not well known. In this paper,the chemistry of amphiboles from such com-plexes located in both oceanic (Kerguelen,Tahiti) and continental (Corsica, Niger, Rho-desia) environments (see Appendix) is com-pared with host-rock chemistry (Table l), usingnew data as well as data from the petrologicalliterature. Thus the petrographic, textural andchemical characteristics of amphiboles (Table2) enable us to be more specific concerningthe role of the different magma-compositiontrends that were established in relation to theevolution of amphibole composition as magmadifferentiated.

Here, amphiboles are named according tothe recommendations of the I.M.A. (Leake1978). In the general formula Ao-rBzCtsTt"eOzz(OH,F,Cl)r, the tetrahedral sites are filled withSi, Al, then Fe. Our chemical data on amphi-boles have been obtained using a microprob:analyzeri concentrations of OH, F, Cl and Fe'*thus are unknown. Amphibole formulae areestablished on the basis of 13 cations (Si+AhFe'*+Mn+Ti+Mg = 13) according to themethod of Neumann (1976b). When describ-ing amphibole series in terms of composition,we refer to substitutions such as those describedby Fabrids (1978) to derive the most complexformulae from that of tremolite, [nCarMg.SieOu(OH)ri the rslatively large monovalentcations more or less fill the I sites of thestructure. Such substitutions are limited innumber: Fe=Mg, Fe3+=Alt", Na"Al*=ESi,NaRs+=CaRt+, NaaNa+Dta, R3+Alte=R2+Si, Ti=Si, and EaTi::R8+Naa. According tothe standard formula of kaersutite, usually givenon lhe basis of 24(O, OIJ), we can add anothersubstitution: Ti + O'-r:R"* + OH-.

At"tpHtnorEs rN Srlrce-SITURATED To-Ovrnseruneren Rocr Srnres

These petrographic associations contain basicand intermediate rock types (gabbros, monzo-nites) that have minor proportions of normativenephetne or normative quartz (C.I.P.W. nor-

483

mative constituents). They also contain differ-entiated rock types such as syenites and granitesin variable proportions; these are enriched inquartz and usually evolve towards increasingperalkalinity.

The complexes of Rallier-du-Baty peninsula,Kerguelen

This association, found in the southwesternpart of the main island, consists essentially offive overlapping ring-complexes whose petro-graphy and geochronology are well described(Nougier 197,0, Marot & Zimine 1976, Lameyreet al, 1976, Dosso 1977, Dosso et al. 1979, Giret1980). These complexes contain many differentrock types, varying from volumetrically abun-dant syenites and quartz syenites to granites inminor volumes. Amphiboles are the principalferromagnesian minerals, occupying from 2 to5 vol. Vo of the rocks; they crystallized laterthan the feldspars and, in some cases, afterquartz. Their composition is clearly related tolithology. In syenites, they change graduallyfrom katophorite to winchite and then to fer-rorichterite. At the very end of crystallization,ferrorichterite evolves towards alkaline end-members such as arfvedsonite and riebeckitein a probable solid-solution series (Sundius1946). In quartz syenites, the amphiboles arezoned from ferrorichterite in the core to arfved-sonite in the rims. Finally, in the granites, am-phiboles are not zoned, and their compositionlies between end-member riebeckite and end-member arfvedsonite.

For these complexes, the chemical compo-sition of the amphiboles in the syenites andgranites is represented by the evolving serieskatophorite + winchite -> ferrorichterite -->

arfvedsonite, in which the Fe=Mg and NaFe'*3CaFe2+ substitution schemes predominate. Thearfvedsonitic composition of the amphibole ap-pears gradually with silica enrichment of therocks and dtrring a reduction process (FabribsI 978 ) .

The Iskou ring-complex, Air, Niger

This complex forms part of a magmaticanorogenic province known as the Youngergranites of western Africa (Black & Girod1970). General investigations have been re-ported in geochronological (Karche & Vachette1976, 1978) and petrographic papers (Black1965. Black et al. 1967, Bowden & van Bree-men 1972, Moreau et al. 1978). Four groupsare distinguished, all amphibole-bearing: (1) a

AMPHIBOLE COMPOSITIONAL TRENDS IN ALKALINE COMPLEXES

484

basic association of leucogabbros, monzogab-bros and ferrosyenites, (2) a silica-saturated as-sociation of quartz-bearing monzonites and sye-nites, (3) a silica-oversaturated association ofquartz syenites and granites, and (4) ferro-augite-bearing syenites.

In basic and intermediate rock types, theamphiboles usually crystallize after plagioclaseand the pyroxenes but before alkali feldspar.They generally replace the augitic clinopy-roxene in a partly or completely developed ura-lite texture (Fig. la). Their composition evol-ves from kaersutite in gabbros throughhastingsite to ferrohornblende in zoned crystals

THE CANADIAN MINERALOGIST

cc lFIc, 1. Different amphibole textures in rocks from ring complexes. (a)

Amphibole replacing pyroxene and giving uralitic texture' Iskou, Air'(b) Very late amphibole in a peralkaline granite, Cauro-Bastelica, Cor-sica. (c) Early amphiboles in monzosyenite, Montagnes Vertes, Ker-guelen. (d) Coronitic texture of pyroxene f Ti-magnetite, interpretedas an amphibole-liquid reaction product (see text), Montagnes Vertes,Kerguelen. Minerals are: 1 alkali feldspar, 2 plagioclase, 3 nepheline,4 quartz, 5 analcime, 6 amphibole, 7 oxidation corona, 8 pyroxene +Ti-magnetite corona, 9 pyroxene.

in leucogabbros, then to a hastingsitic horn-blende in quartz monzonite and quartz-bearingsyenites. The chemical variations of these calcicamphiboles correspond to a continuous solid-solution series characterized by a decrease inMg/(Mg*Fe) ratio, Ti and Al and an in-crease in Si, with substitutions Tii:Si, MgFFeand AlCar"=NaSi.

In quartz-rich syenites and in granites, am-phiboles are commonly associated with lateaegirine-augite crystals, and have a sodicalciccomposition between the following end-mem-bers: ferrobarroisite, ferrowinchite, ferrorich-terite and katophorite (Fig. 2).

W{,4

i\.\

AMPHIBOLE COMPOSITIONAL TRENDS IN ALKALINE COMPLEXES 485

1972, 1973), consists of two principal typesof granite, an early hypersolvus granite and alater intrusion of subsolvus granite (Bonin &Martin 1974). There are also two distinctive

Iq

+

L

I

I:

i

I

ca) r .ear i ( o .so

-Si + Na+K+

t 1

L 2a 3a 4A 5o 6t r 7

Q e

A l

Frc. 2. Amphiboles from nonorogenic complexes plotted in the diagramCa + AII' versar SifNa-FK: I Tahiti, 2 Courbet peninsula and MontsBallons, Kerguelen, 3 Marangudzi (Borley & Frost 1963), 4 Iskou,Niger, 5 Oslo rift (Neumann 1976a), 6 Rallier-du-Baty peninsula,Kerguelen (Marot & Zimine 1976), 7 Cauro-Bastelica, Corsica, 8 am-phibole end-members (Ts tschermakite, Ha ferro- and magnesiohasting-site, Ed edenite, Ta taramite, Kt katophorite, Ri richterite, Ar arfved-sonite, Hb fe- and Mg-hornblende, Ba barroisite, Tr tremolite, Wiwinchite. Rb riebeckite, Act actinolite).

The Cauro-Bastelica ring-cornplex, Corsica

This complex, of Permian age and intrudingHercynian basement (Bonin 1980, Bonin et al.

486 THE CANADIAN MINERALOGIST

chemical trendb represented by whole-rock com-positions: (1) aluminous, essentially anorthite-or corundum-normative granitic rocks, €.g.,the biotite-bearing granites crystallizing as bothhypersolvus and subsolvus types; these may con-tain amphibole-rich xenoliths; (2) an alkalinetrend that clearly becomes peralkaline, repre-sented by the amphibole- and fayalite-bearinghypersolvus granites.

In the amphibole-rich enclaves that are in-terpreted 10 result from the mixing of majorvolumes of acid magma with minor volumesof basic magma, the amphiboles are of ferro-hornblendic composition, lying between hasting-site and barroisite end-members. They may be-long to a solid-solution series, with substitutionsof the following types: NaAl'"=nSi and CaFe'*+NaFe3+, possibly induced by oxidation(Fabrids 1978). In hypersolvus amphibole- andfayalite-bearing granites, most of the amphi-bole crystals have a deep green color (whichprobably reflects variations between ferroede-nite dnd ferrobarroisite compositions) with bluerims of a Na-grunerite, suggesting an outwardincrease in iron and a decrease in alkalis. Suchgrunerites are associated with fluorite and an-nitic micas in an interstitial position or in therock cavities: this association reveals a latestage of crystallization. Some of the hypersolvusgranites contain a blue amphibole as the onlyferromagnesian phase; its composition lies be-tween barroisite and winchite. Such substitu-tions (Fe3+Alt"+Fe'?+Si) may be indicative ofa reduction process.

We note that in the Cauro-Bastelica complex,all the amphiboles are iron-rich and aluminum-poor. In granites, sodicalcic amphiboles are al-ways late in appearance; they commonly crystal-lize after quartz and perthitic feldspars (Fig.lb). In contrast, calcic amphiboles of ferro-hornblendic composition occur only in the basic,amphibole-rich inclusions.

The anorogenic complexes of the Oslo region

Continental alkaline ring-complexes from theOslo rift (Barth 1944, Oftedahl 1946, Saether1962, Dietrich et al. 1965, Heier & Compston1969, Jacobson & Raade 1975, Neumann 1976a)are characterized by two main magmatic pulses:(l) a silica-undersaiurated association whoseamphiboles will be described in the next section,and (2) a silica-saturated to -oversaturatedassociation of monzonites, nordmarkites andgranites. In these, amphiboles constitute twogroups depending on rock type. In the alkalinerocks (quartz monzonites and quartz syenites),

magnesian hornblendes show a silica increasethat is related to the activity of silica in themagma (Neumann 1976a).In the peralkalinerocks, amphiboles belong to a solid-solutionseries from subcalcic Na-edenite to magnesioarf-vedsonite. with CaAlt"::NaSi the dominant sub-stitution-scheme; these amphiboles show a de-crease of Ti, Mg, Ca and Al from core torim in zoned crystals.

Conclusions

In the silica-oversaturated suites of alkalineplutonic ring-complexes, we can distinguish twogroups of rocks on the basis of their amphi-boles: (l) Basic and intermediate rock typesin which amphibole compositions show a de-crease in Ca*Alt" without any increase in theSi+Na+K parameter (Fig. 2). This trend gen-erally evolves from hastingsite to hornblende(Iskou, Oslo), which implies a solid-solutionseries where the principal substitution can besummarized as NaaAlr"=nSi. Independently ofthis series, the substitution scheme Fe=Mg leadsto magnesio- or ferroamphibole types. (2) Dif-ferentiated rocks, such as peralkaline syenitesand granites, with amphiboles evolving frombarroisite to katophorite, to winchite, to richte-rite and lastly, to arfvedsonite. These solid-solution series issue from balanced substitutionssuch as CaAlt"=NaSi and nFes*=Na"Fe'*,with a filling of the I sites by alkalis (essen-tially Na) under reducing conditions.

The descriptions made above emphasize thatSi-enrichment is the main characteristic of am-phiboles in both basic and differentiated silica-oversatured alkaline rocks. In order to com-pare the evolution of rock composition withamphibole chemistry, we have used the vonWolff diagram of normative constituents, de-scribed by Johannsen (1939); the diagramenables one to plot minerals and rocks in thesame figure (Fig. 3). L represents feldsparsand M, silica-saturated ferromagnesian min-erals: the line LM is therefore the silica-satura-tion line separating an upper silica-saturatedfield from a lower silica-undersaturated field.This diagram shows the progressive silica over-saturation of rocks and the simultaneous Si in.crease in amphiboles (e.g., Cauro-Bastelica,Rallier-du-Baty, Iskou, Oslo). Note that anoverall iron enrichment in conjunction with anAl and Ca decrease is also associated with thisfeature; in the von Wolff diagram, this is shownby amphibole compositions drawing nearerand nearer to M and rock compositions goingfurther and further from L.

AMPHIBoLES IN SILIcA-UNDERsATURATEDRocr Ssmns

These rock associations contain basic to dif-ferentiated types in which the nepheline-nor-mative character is always present and some-times important. These rocks may be separatedinto two groups: one is weakly silica-under-saturated and evolves towards aluminous dif-ferentiates, whereas the other is strongly silica-undersaturated and usually evolves along aperalkaline trend.

Silica-undersaturated rocks lrom the Olso rins-complexes

In the Oslo region, silica-undersaturated rocks(essexites, plagiofoyaites and nepheline-bearingsyenites) were formed during the first mainregional magmatic phase. In these rocks, am-phiboles may be automorphic or pseudomor-phic after clinopyroxene; amphiboles from thetwo groups are very similar in composition.Neumann (1976a) notes only the Fe,+=Mg

48'7

and CaAlr"=NaTi schemes of substitution inthese titaniferous hornblendes.

Plutonic rocks from Tahiti

The magmatic history of Tahiti has beenthe subject of many investigations (Dana 1886,Lacroix 1904, 1910, 1927, Williams 1933, De-neufbourg 1964, Krummenacher & Noetzlin1966, McBirney & Aoki 1968, Nitechy-Novot-ny 1975). These authors established the ex-istence of two trends diverging from similargabbroic compositions. Both trends are silica-undersaturated, but the first is peralkaline andthe second is aluminous. These chemical trendsmirror closely the volcanic evolution of Tahiti,where there are both trachytic and phonoliticfractionation sequences. Amphiboles are com-mon in each petrographic unit (except in cumu-lates such as melanocratic gabbros and olivine-bearing pyroxenites), but they are not abun-dant in transitional rocks between essexites andnepheline-bearing syenites. Their pleochroismis reddish brown to dark brown, and they pre-

AMPHIBOLE COMPOSITIONAI. TRENDS .IN ALKATINE COMPL,BXES

oRocKs * AI 'PHIBOLES

Ftc. 3. Amphiboles and their host rocks plotted on the normative von Wolff diagrams, e quartz, Lfeldspars, M mafic minerals, Ne nepheline. Some significant tielines linking amphiboles ani their hostrocks have been drawn to aid interpretation.

- - ? t E a g n a t l

r E N G U E I E NR A L L ' E B D U B A I Y

K E N G U E L E NA A L L O N - P e n . C O U n B E l

488 THE CANADIAN MINERALOGIST

sent oxidized rims in differentiated members.All are subsilicic and titanium-rich; the numbrof cations ranges between 5.53 and 5.98 for Siand between 0.55 and 0.76 for Ti, on the basisof thirteen cations in the Tr" and Ct sites. Theirchemical composition changes from kaersutitecrystallizing in gabbros to ferrokaersutite ap-pearing in the undersaturated syenites. Thisevolution means an increase in Mn, Fe,Ca and Al with a corresponding decrease inTi and Mg, according to substitutions such as(Mn,Fe)'z+=Mg and CaAlt"=NaSi (or NaTi).

Kerguelen silica-undersaturated. rocks

In the main island of the Kerguelen archi-pelago, three complexes of this type have beenstudied: the Monts Ballons in the Cenfial Prov-ince of the island and the Montagnes Vertesand Les Mamelles in the eastern peninsula,called Courbet Province. The Monts Ballonsring-complex consists of four successive in-trusions emplaced concentrically. A central bio-tite-bearing gabbro is followed successively bymonzodiorite with syenitic variants, then by mi-cromonzonites and syenitic variants containingnumerous angular to round enclaves of basicand amphibolitic xenoliths and amphibole xeno-crysts; finally, a discontinuous peripheral ring-dyke of nepheline syenite partially encloses thisplutonic body (Giret 1979a). Amphiboles arefound in most of the rocks in this complexexcept in the olivine-rich gabbros. They arecommonly clustered together in cumulate tex-tures that constitute most of the enclaves irnthe micromonzonite, an indication of their earlycrystallization. In these rocks. which closelyresemble the plutonic rocks of Tahiti, amphi-boles are also very similar, commonly Ti-rich(0.41<Ti<0.80) and subsil icic (5.35<Si<6.16). Their composition ranges from kaersutiteto a composition intermediate between ferro-kaersutite, taramite and hastingsite, owing tosubstitutions already described for amphibolesfrom Tahiti.

The Montagnes Venes complex is a smallplutonic body 0.8 km across, where an outerzone of chilled or coarse grained gabbro gradesimperceptibly through nepheline-bearing mon-zonites into an inner core of nepheline-bearingsyenites (Giret 1978, Giret & Lameyre 1980).Amphiboles, whose early crystallization can bededuced from their common inclusion in pla-gioclases (Fig. lc), are rare in the gabbros, butthey can occupy up to 14 vol. /6 in monzonitesand syenites. Their evolution is emphasized bya zonation from brown cores to green rims,evidence of a transition from kaersutite to

ferrokaersutite to taramite through substitutionssuch as Ti=R3+ Na, Mg=Fe and CaAlb=NaSi.

In the Montagnes Vertes complex, the generalaluminum-enrichment trend expressed by therock association gabbro-monzonite-nepheline-bearing syenite diverges towards nepheline sye-nites at the very end of the differentiationsequence. This feature has been related to thebreakdown of calcic and sodicalcic amphibolesunder magmatic conditions (Giret 1979b).Actually, an early amphibole-liquid reaction(Fig. ld) is observed in some syenites. The re-action diffen considerably from the well-knownoxidation under P(HzO) decrease and a cor-relative increase in temperature or under P(O:)increase commonly encountered in volcanicrocks (Coombs & Wilkinson 1969, Merrill &Wyllie 1975). This breakdown that reflectsoxidation conditions is also observed in thenepheline-bearing syenites from MontagnesVertes, and it happens earlier than the crystal-lization of alkali feldspar, since it concerns onlyamphibole crystals that are not included in pla-gioclases (Fig. 1c). The suspected reaction inthe other case involves both brown and greenamphiboles and leads to clinopyroxene and Ti-magnetite in coronitic textures. The followingequation has been established: amphibole *0.2 Na* from liquid 'l --> 2 clinopyroxenes fTi-magnetite + liquid 2. In this reaction theresidual liquid (2) is enriched with 0.2 K* and1.3 moles of potential analcime. The amphiboleis zoned from hastingsite in cores to ferrohas-tingsite in rims, with a general cation contentof Sie.rrAl,.zeFer.soMgr.grMno.r,Car.ruNa,.,sKn.trTi6.r,for O': 23.The pyroxene is an aegirine augite(cation content Sit.orAln.r"Feo.rnMgo.nMno.o,Cao.soNao.orTio.or for O : 6), and Ti-magnetite hasthe following composition: MnrTiO4 0.25, Fe,TiO{ 0.59, Fe'O, 0.39. In this reaction, the re-sidual liquid becomes poorer in silica and richerin alkalis by incorporating 1.3 moles of anal-cime. Consequently, a peralkaline petrographictrend appears, expressed by an increase in modalfeldspathoid minerals (from 1 to 8, but in rarecases, up to 13 vol. %) and by the crystalliza-tion of sodic pyroxene. Although the crystal-lization of analcime occurs late [this mineralis not stable at magmatic temp'eratures exceptat very high P(HzO): Wilkinson 1962, 1965'1968. Roux & Hamilton 19767, we note thatthe reverse reaction has been obtained experi-mentally from an olivine- and analcime-bearingdolerite under thermodynamic conditions thatare realistic, considering the setting of the Mon-tagnes Vertes pluton lP(HrO) : 1 kbar,7oo'c<T< l02o"cl.

AMPHIBOLE COMPOSI'TIONAL TRENDS IN ALKALINE COMPLEXES 489

In the second complex of the Courbet penin-sula, [rs Mamelles, located. near MontagnesVertes, gabbros are abundant, and are inter-sected by a minor volume of syenite and quartzsyenite. Arguing on the basis of whole-rockcompositions and petrographic data, Giret &Lameyre (1980) offer the following explana-tion: the silica oversaturation of rocks is pro-duced by amphibole and biotite fractionationfrom an alkaline magma from which evolvedthe gabbros. The existence of angular biotite-bearing amphibolitic inclusions in the neighbor-ing basaltic dykes and the scarcity of biotite(less than 2.5 vol. %) and amphiboles (lessthan 1 vol. %) in gabbros and quartz-bearingsyenites and quartz syenites support such athesis. Amphiboles are partly or completelydestroyed by an oxidation process, and analyzedrelics belonging to the magnesiohastingsite typeshave been found in amphibole solid-solutionseries in the Montagnes Vertes.

Conclusions

With regards to amphiboles. the silica-under-

saturated rocks differ from the silica-oversatu-rated ones. In undersaturated seriesn amphibolespresent some definite characteristics: ( 1) thepresence of Ti-rich amphiboles such as kaer-sutite in basic and intermediate rock types, (2)chemical composition evolving from kaersutiteto hastingsite by way of Ti * O=Fe3* + OH*or flTi=Na"Al substitutions, and from hasting-site to hastingsitic hornblende and to taramitewith predominant CaAI"=NaSi substitution(Fig. 2), (3) absence of alkali amphiboles andof Na-rich sodicalcic amphiboles even in thealkaline rock types, and (4) absence of anysilica enrichment in amphibole compositions(Fig. 3): all amphiboles are to be found in avery limited area of the von Wolff diagram.

Rr,lertoNsHtp BrrwBr,N Cotvtpostt:loN onANarHrsoI-Es AND oF Rocxs

Von Wolff noimative diagrams (Fig. 3) ex-press the close relationship already known(Cawthorn 1976) between silica content of am-phiboles and silica activity in the magma con-stituting the silica-saturated and oversaturated

O ^ ^ |

i l

r lI. ^ :

a a I

r ^ lf_" r^

" i " " i

O 1'E r 'l r

t

I

t

l - _ b

t " d f ^ "t E g o I oI u o' ! ^

I oUool o at 0 ai i "oF

@0.9

II

' - - - { - r - - f : t@^ep----- - - r : ' i l *#* * . -

$:"^ t^

Ftc. 4, Linear plots relating compositional variations in amphiboles and their host rocks. (a) Relationshipbetween Na f K/Al in amphiboles and agpaitic coefficient of their host rocks. (b) Relationship be-tween Ca + Alt" in amphiboles and agpaitic coefficient of their host rocks. (c) Relationship betweenSi -| Na -1- K in amphiboles and agpaitic coefficient in their host rocks.

E+ gz 8+ €6 E

Na + Kl.At( tock )

490 THE CANADIAN MINERALOGIST

associations (Corsica, Rallier-du-Baty peninsula,Iskou, Oslo). Such a relationship has beenexplained in terms of the solubility of Al inthe complex amphiboles by Neumann (L976a),who suggested a reaction Caa(Fe,Mg)aAlSirAlOp(OH), * CaALSizOs -+ Ca:(Mg,Fe)gAl,SiuAlrOr,(OH), + Ca(Fe,Mg)SizO6 + SiOr,showing that silica goes into amphibole andaluminum into plagioclase when the activityof silica in the magma increases, as reflected inour petrographic and chemical data. For thesilica-undersaturated rock series, analogousdiagrams sho_w that there is no obvious changein amphiboles clustered below line LM andconfined to a limited field (Oslo, Tahiti, MontsBallons, Montagnes Vertes, Les Mamelles). Inthese series, howevero chemical analyses ofrocks and amphiboles suggest a close relation-ship between Ca and Mg contents in rocks andamphiboles with significant correlation coeffi-cients (those for Mg and Ca are, respectively,+0.70 and *0.96 in the Tahiti rock asso-ciation and *0.96 and *0.81 in the MontagnesVertes petrographic association).

Other diagrams that take into account theagpaitic ratio of rocks (Fig. ) suggest theexistence of two principal amphibole trends. InFigure 4a, this ratio in rocks has been plottedin terms of the same ratio in amphiboles, and

thus two distinct amphibole populations canbe found. To the first group belong amphibolesfound in rocks whose agpaitic ratio is less than0.9; the common characteristic of these am-phiboles is a very low (Na'O+KrO)/AloO,ratio (less than 0.1). To the second groupbelong amphiboles from rocks whose agpaiticratios exceed O.9; unlike the preceeding am-phiboles, these have an agpaitic ratio evolvingfrom 0.O5 to 45.0. Such bimodality can bespecified in the next diagrams (Figs. 4b, 4c),which clearly separate amphiboles from ag-paitic and nonagpaitic rocks in regard to theirchemical parameters. Thus, the first group ofamphiboles has a Ca + Alt content greaterthan 2.5 and a Si + Na + K content less than orequal to 8, whereas the same parameters areless than 2.5 and generally more than 8, re-spectively, in the second group.

The absence of (Ca + Af")-rich amphibolesin agpaitic rocks suggests the breakdown ofsuch amphiboles when differentiated liquids be-come peralkaline, which occurs as P(HaO) de-creases or P(Or) increases (Ferguson 1978)in most observed examples. In the same way,the absence of (Ca * Alt")-poor amphiboleswhere the agpaitic ratio of rocks is less than0.9 indicates that crystallization of silica-richamphiboles occurs only in alkaline or peralka-

q+G

LATE MAGMATIC

O STAGES

70 Si+Na+K

Frc. 5. Schematic representation of the different amphibole compositionaltrends in alkaline ring-complexes.

I Sd-undera a tu ra te d to oksV s i -ounoeatura ted , roaks

t

II

9

b r e a k d o w o o f C ar i c h

EARLY MAGMATIC

STAGES

It u - I - o - ,

i bo l+*ru"2o * Kzo ..

- r ro"- ( in rocks)

s u g g e s E & A 1amph

AMPHTBOLE COMPOSITIONAL TRENDS IN ALKALINE. COMPLEXES 4gl

line liquids, i.e., in very differentiated magmas. sitional trends and the chemistry of their hostThis, added to the previous descriptions (Fig. rocks. Substitutions in amphibolei are controlledL), shows that such silicic and alkali-rich am- essentially by the silica activity of the magma,phiboles crystallize only rq very late magmatic Whereas the Ca + Alt" content of an amphibolestages and even during hydrothermal stages, depends upon the agpaitic ratio in the rock.since their field of stability is controlled by in tne

"m*o" rock series, we suggest the

both P"outu." and chemical composition of the "*irt""o

oi ;;"p that sepaiates the-iCa +magma (Ernst 1962, Charles 1975, Ferguson Ap)_rich, (Si T- Na * k;-poo, group of1978)'

, -_ -L- L-_!- -r -,^_, amphiboles from the (Ca * Alt)-poo-r, (Si +More precisely, and -on the basis of petro-- N;';-it:.i"t -group.

The first group develops,ciXplf,^

t^":9:il^"19 _chemical data, one mav

""trv, ""it-inri'"uiri"t than plalioclases, ar ainfer the existence of different amphibole com- ;;;"d" ;tug;.

- Th" second develops late,positional trends (Fie. 5). With several ex- il;ffilt iui!i',t"" alkali feldspars, some-amples, different compositional evolutionary ;;:;l;, tilun quurt" in a late magmatic stage

ll]i* 9'"^^T::,:::9ttshe{ in amlhiboles bv ;; il;; iio'r.-ir,"r,nur stases, according to3""#:^:r_^"":1i'i:,'1L'li1*j:,T:::^lL,jT F;,g;;'(;ffirj. such evidence emphasizess|l tca-saturateoancoversaturatedassoclat lonsor a,Karne nng-complexes, the preaominani lT,,'::..1,1*1t^1i:I 3:1?11t"I:3: i:l*.*substiturion in icu l- Ark)-rich amphibores is ::1fl;:'ilH[j,"il":#'#,"11'Isiffi"fl (iffi;.NaAlb+ESi, which differs from thq silica-un- ;:":::":_-:":::', .dersaturated associations, in which we note

" ::-::"jli:t'^:lll:'i?9tT-lli:-11Trcar-and

tex-

Ti decrease and caAlb+NaSi as the predomi] ::i11,3,:-::-tgest the existence of two solid-

nant substirution scheme. In (ca + Al'")-poi ytTl.)ri1lT; 11" first being silica-poor (6.00

amphiboles, which are only encountered in ti. S !i S 7'01) and the second silica-rich (6'76

silica-saturated and oversaturated rock series ii < si < 8'@), as shown in Figure 6'

this study, the substitution CaAl'":+NaSi pro- Thus, the role of amphiboles in the magmaticduces a main trend from whish the flFel*+ Process can be specified. According to GunnNaFe2+ substitution gives a bifurcating trend !lllZ),,C_21vthorn (1976), Maury (1976) andillustrated by the barroisite-to-arfvddsonite Helz (1976), the early fractionation of Ca-richevolution. In this schematic representation, and Sipoor amphiboles induces an increaseFe=Mg substitution occurs independently of of Si in the magma, which contributes to theany trend. production of silica-rich differentiated liquids

(i.e., those of granitic composition) when mag-

DrscussroN mas are silica-oversaturated, and permits theevolution to quartz-bearing rocks from silica-

Using experimental data one can infer the undersaturated magmas. The persistence andconsiderable importance of physical conditions breakdown of calcic and sodicalcic amphiboles,of crystallizalion li.e., T, P(total), P(H,O), as observed in syenites from the Montagnesl(Or)l controlling amphibole compositions Vertes intrusion, produce a sodium enrichment(Boyd 1959, Ernst 1962, kake 1965, Gilbert in the residual liquid that generates a limited1966, Phillips & Rowbotham 1968, Huebner & peralkaline trend.Papike l970a, l970b, Wones 1970, Holloway& Burnham 1972, Charles 1975, 1977, Helz AcrNowLEpcEMENTs1973, 1976, Merrill & Wyllie 1975, Yagi et al.1975, Mitchell & Platt 1978). In the same We are indebted to Drs. P. Bowden (Uni-way but using petrographic and chemical data, versity of St. Andrews) and R. F. Martin (Mc-Cawthorn (1976) demonstrated that physical Gill University) for their aid in preparing theconditions govern the partition coefficients of manuscript and to Professor J. Lameyre (Labo-certain elements between amphiboles and mag- ratoire de P6trologie, Universit6 Pierre-et-ma. However, it is suggested that magma chem- Marie Curie) for his useful comments. Researchistry is the most critical factor (Neumann facilities were obtained from C.N.R.S. through1976a, Ferguson 1978). its grant to L.A. 298 (Laboratoires Scientifi-

Although one should consider both physical ques des T.A.A.F.), which covered field workand chemical parameters together and not sep- by A. Marot, S. Zimine and A. Giret in thearate their specific roles, based on limited Kerguelen Islands. We thank authorities of theexperimental evidence, this study establishes Niger Republic, in particular M.M. Boukarclose relationships between amphibole compo- (ONAREM) and the University of Niamey.

492 THE CANADIAN MINERALOGIST

T'1 . 3 4 N a "

0 . 6 7 N a ,

{\F

)ar-rrr-r)Ar'rPHrBoLEs

f "oo,"-"o,-",")

arunxrsous

cALC lc

ATPHIBOLES'Yl.Oo(rua +K)6

8.00 s i 7.0 ls i 6.76si 6.00 sr

Ftc. 6. The two groups of amphiboles in the threedimensional diagram Nan, (Na * K)o, Si (Leake1978) .

The translation from French to English hasbeen very kindly done by Mrs. C. Gauthier,teacher of English in the Lyc6e Fremin (Bondy,France).

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&(Ca +Na)r) 1 .3 t

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Coovnl D.S. & WntrtNsoN, J.F.G. (1969): Lin-cages and fractionation trends in undersaturatedvolcanic rocks from the East Otago volcanicprovince (New Zealand) and related rocks. J.Petrology 10, 440-501.

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Received February 1980, revised ,nanuscript ac-cepted July 1980.

AppENDtx: Loclnot AND DETERMTNATIoN oFArrapHrsoLEs AND Hosr RocKs (NEw ANer-vsls)

Kergaelen island, Rallier-du-Baty peninsula

Rocks: LL-l Syenite from tle second intrusionof the southern centre; L-2 syenite from the firstintrusion in the southern centre; P-l syenite fromthe third intrusion in the southern centre; P-14granite from the fourth intrusion in the southerncentre; DF16 syenite from the sixth intrusion inthe southern centre; LLl4 syenite from the Ansesyenite intrusion, fayalite-bearing nordmarkite, andVA80 syenite of the middle centre.

Amphiboles: LLl6-1 katophorite, LLl6-2 ferro-richterite. LLl6-3 arfvedsonite, LLl64 ferroacti-nofite, L2-l & L2-2 same crystal of katophorite,L2-3 ferrorichterite, Pl-l & 2 katophorite, Pt4-larfvedsonite, DFl6-l & 2 same crystal of winchitein the core and rims of arfvedsonite, LL14-l rich-terite around a fayalite crystal, LLl4-2,3 & 4 samecrystal with ferrorichterite in the core and mediumzone and with arfvedsonite in the rims. LLL4-5ferroactinolite, VA80-1 amphibole rimming augite,VA80-2 actinolite, W28-1 actinolite.

Kerguelen. island, Courbet peninsula

Rocks: 77-35 gabbro, MV-l monzogabbro' 77-60 rich kaersutite-bearing monzonite, 7749 mon-zogabbro, 77-20 syenite, MV-2 dyke of apliticmicrosyenite, 77-lo3c quartz-poor granite dyke.

49s

Amphiboles: 77-35 ferrokaersutite, MV-l fer-rokaersutite, 77.60-L & 2 ferrokaerxttite, 7749kaersutite, 77--20-l & 2 hastingsite and Mg-hasting-site in the same crystal, MV.2-1, 2 & 3 samecrystal of hastingsitic hornblende in the core, grad-ing to taramite in the cenlre and rim, 103c Mg-hastingsite.

Kerguelen island, Monts Ballons, Province duCertlre

Rocks: 78-3tc micromonzonite, 78-47 nephe-line-bearing syenite, 78-20 amphibole cumulate inmonzogabbro, 78-48 nepheline-bearing syenite, 78-60b monzogabbro, 78-51 gabbro, 78-52 monzo'gabbron 78-64 nepheline-bearing syenite.

Amphiboles: 78-3lc subsilicic kaersutite, 7E-47-1 & 2 same crystal of brown kaersutite in thecore and green ferrokaersutite rims, 78-20 sub-silicic kaersutite, 78-48-1 subsilicic kaersutite, 78-48-2 kaersutite, 78-60b kaersutite' 78-51 tara-mite, 78-52-1 hastingsite ferrokaersutite, 7U58--2kaersutite. 78-64 ferrokaersutite.

Niger, Iskou

Rocks: GBH quartz monzonite, SGM quartzsyenite, 77-138 quartz syenite, 67-:18 monzo-gabbro, 66--77 leucogabbro, 6I--77 leucogabbro,17-77 alkaline quartz syenite, 83-:76 monzogabbro,78J7 leucogabbro, 75-78 monzogabbro.

Amphiboles: GBH ferrohornblende, SGM fer-rotschermakitic hornblende' 138-:77 hastingsitichornblende, 66J7-l & 2 ferrotschermakitic horn-blende. 67-18 ferroedenitic hornblende, 83J6ferrohornblende, 74--78 FeMg-pargasite, 78,77-lhasti n gsitic hornblende, 7 8.7 7 -2 hastingsite, 7 8.7 7 -3

ferrohornblende, 62.77-3 kaersutite, 61.774 ede'nitic hornblende, 74--78 FeMg-pargasite, l7--77 fer-robarroisite.

C<trsica, Cauro-Bastelica

Rocks: 1 blue soda-grunerite-bearing granite, 2ferroedenite-" ferrobarroisite- and fayalite-bearinggranite, 3 hydrid xenoliths resulting from a mixingof magmas in the biotite-bearing granite.

Amphiboles: l-l ferroedenite, 1-3 to 8 ferro-barroiiite, 2-2 & 3 same crystal of ferrobarroisitewith a ferroedenitic core, 2-1 ferrobarroisite, 2-4, 6& 7 ferrobarroisite, 2-5 ferroedenite, 2-8 to 10sodic grunerite, 3-l to 3 Fes+-hornblende.

AMP}TIBOLE COMPOSITIONAL TRENDS IN ALKALINE COMPLEXES