- Email: [email protected]
Charnockite genesis and the Proterozoic crust
Precambrian Research, 9 (1979) 303--310
303
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
CHARNOCKITE GENESIS AND THE PROTEROZOIC CRUST
JACQUES MARTIGNOLE Laboratoire de Min6ralogie, Museum d'Histoire Naturelle de Paris/D~partment de G~ologie, Universit~ de Montreal (Canada)
(Received July 31, 1978; revision accepted February 27, 1979)
ABSTRACT Martignole, J., 1979. Charnockitegenesisand the Proterozoic crust. PrecambrianRes., 9: 303--310.
Charnockitic rocks that occur in many Proterozoic high-grade metamorphic terranes have crystallized (or recrystallized) under high-temperature and low water pressure conditions: they usually contain two pyroxenes and mesoperthite as well as orthoclase, plagioclase, quartz and traces of hornblende and biotite. Estimates of physical conditions based on subsolidu~ reactions independant of water pressure show that crystallization occurred in the range 800 to 900 ° C at pressures varying from 7 kbar in the Grenville province to 10 kbar in Africa and in Norway. These physical conditions correspond to a depth of charnockite genesis of about 25--35 km and a gradient of 25 to 30 ° C per km. Charnockite terranes might therefore correspond to regions of crustal thickening as do those which are associated with continental collision processes. If such is the case, their actual distribution might help in understanding Proterozoic plate motion.
INTRODUCTION Like the " a n o r t h o s i t e e v e n t " a t t r i b u t e d b y s o m e a u t h o r s (Herz, 1 9 6 9 ) to r a t h e r e x c e p t i o n a l t h e r m a l condition's at one stage o f the e a r t h ' s h i s t o r y , c h a r n o c k i t e genesis has been a t t r i b u t e d b y S a x e n a ( 1 9 7 7 ) t o an e x c e p t i o n a l ly high g e o t h e r m a l g r a d i e n t o f 7 0 - - 1 0 0 ° C per km. Such c o n d i t i o n s w o u l d have prevailed at various times in the e a r t h ' s h i s t o r y , the last one c o r r e s p o n d ing to the Grenville e v e n t at 110"0 + 2 0 0 Ma. The aim o f this p a p e r is to s h o w t h a t e x t r e m e l y high g e o t h e r m a l gradients are difficult t o reconcile with p e t r o g r a p h i c and chemical d a t a and phase equilibria f r o m the Grenville p r o v i n c e , the A d i r o n d a c k s , Africa, N o r w a y and India. In all o f these areas, c h a r n o c k i t e s c o u l d have f o r m e d u n d e r n o r m a l g e o t h e r m a l gradients, their actual o u t c r o p p i n g at the surface being due to t e c t o n i c r a t h e r t h a n to m e t a m o r p h i c events. MINERAL ASSEMBLAGES IN CHARNOCKITES A l t h o u g h S a x e n a defines c h a r n o c k i t e s as "granites c o n t a i n i n g h y p e r s t e n e " he considers t h e m as m e t a m o r p h i c rocks, recrystallized u n d e r gra-
304
nulite-facies conditions. In fact, true (felsic) charnockites, which are often genetically related to more mafic rocks (refered to as mafic charnockites in this paper) can be generated through three main processes: (1) Water-undersaturated "granitic" magma intruded in a relatively dry ~:rust (high-grade metamorphic basement} at any level, and subsequently recrystallized under high-grade metamorphic conditions; (2) Water-uadersaturated magma intruded in a relatively dry crust during gv'anulite facies metamorphism; (3) In situ dry anatexis during high-grade metamorphism. In any case, as equilibration will take place according to regional physico-,:heroical conditions, mineral equilibria in both the charnockites and the country rocks should reflect these conditions. The preservation of relict igneous textures in some charnockites (ophitic textures in mafic charnockites, exsolved pyroxenes, inverted pigeonite) is best interpreted according to the hypothesis of intrusion during high-grade metamorphism. In this case, one can expect a temperature difference between the hot plutonic mass and the relatively cool surrounding terrane, whereas confining pressures should be the same. The concentric pattern of decreasing temperatures around the Adirondack anorthosite found by Bohlen and Essene (1977) could be reinterpreted in that sense. In estimating P-T conditions for charnockite genesis, Saxena used several equilibrium curves in which water was present as a phase. Most experimental phase equilibrium relations have been studied under dry or H20-saturated conditions. However, reactions in high-grade metamorphic rocks are likely to proceed in the presence of a vapor phase with XH~ O considerably smaller than 1. Fluid inclusions in minerals of high-grade metamorphic rocks (Touret, 1974) have provided a good sample of the vapor phase at various stages of crystallization. In fact, due to differential incorporation of H20 and CO2 in melts, the vapour phase present in hypersolidus assemblages is almost pure CO2. During the last stages of crystallization XH~O increases until saturation of the melt. The a m o u n t of H=O present on reaching the solidus will determine the H20/CO2 ratio of the fluid phase in subsolidus assemblages. The large proportion of mesoperthite in charnockitic rocks excludes the possibility of an H20-saturated system but on the other hand, the fact that most charnockites end their crystallization in the two-feldspar region shows that saturation is attained at least in the last stages of crystallization. As dry melts of charnockite composition would n o t intersect the alkali feldspars critical line (Morse, 1970), one can assume that a finite a m o u n t of H20 is present as a constituent in hypersolidus and probably as a phase in subsolidus charnockitic assemblages. Such conditions are typical of water-deficient, vapor-present systems (Robertson and Wyllie, 1971). According to the experimental work by Morse {1970), the maximum PH~O for hypersolvus assemblages in the Ab--Or--Qz--H20 system is 2.25 kbar. As this pressure will decrease with introduction of anorthite into the system, it can be taken as a maximum PH20 in the charnockite crystallization.
305 .Based upon these considerations it is clear that experimental mineral reactions performed in water-excess conditions can hardly be applied to high-grade metamorphic rocks without corrections for PH20 ( Pload. Although PH:O can be estimated from reactions of the type: annite + oxygen -~ Kspar + magnetite + water (provided fo~ can be estimated), it is probably more convenient to circumvent the problem of fluid composition and to use subsolidus equilibria that involve solid phases only, in order to elucidate the physical conditions of charnockite genesis. PHASE EQUILIBRIA IN CHARNOCKITES OF THE GRENVILLE PROVINCE The P-T conditions of charnockite crystallization (or recrystallization) in the Grenville province have been estimated from several subsolidus equilibrium assemblages in rocks of granitic compositions and in mafic rocks that are supposed to have recrystallized under the same conditions. Charnockites c o m m o n l y contain garnet which normally appears as a late phase growing at the expense of o r t h o p y r o x e n e and plagioclase (Martignole and Schrijver, 1971). Experiments bearing on the first appearance of garnet have been performed by Green (1970). Figure 2 shows the line for the first appearance of garnet in rocks of gabbroic composition (G1) and dioritic composition (G2). The marked compositional dependence of garnet-forming reactions has been shown by Martignole and Schrijver (1973) in the southern part of the Grenville province where rocks with Fe/Mg and Ab/An ratios similar to those of G1 are garnetiferous, whereas rocks with G2 compositions are not. The P-T conditions during garnet growth must therefore lie between lines G1 and G2. Another reaction, well displayed in troctolites associated with charnockites (Martignole and Schrijver, 1971; Martignole, 1974) consists in the disappearance of olivine in the presence of plagioclase: plagioclase + olivine -~ o r t h o p y r o x e n e + clinopyroxene + spinel Curve 3 (Fig. 2) represents the reaction boundary experimentally determined by Emslie (1970) for a composition (ol = Fo~0 ; pl = An73) almost similar to the composition of the troctolites from the southern part of the Grenville province (ol = Fo~4 ; pl = Arts2), whereas curve 4 represents the same reaction with pure forsterite and anorthite (Kushiro and Yoder, 1966). A minimum of 7 kbar seems necessary to cause olivine to react with plagioclase at temperatures accepted for charnockite genesis (750.-900 ° C; Fig. 2). Another reaction which constitutes a potential geobarometer has been proposed by Wood (1977): CaA12Si2Os in plagioclase -* CaAl2SiO6 in clinopyroxene + quartz Experimental reversals have been carried o u t on pure end member reactant
306
and products (Hariya and Kennedy, 1968). This allows extraction of thermodynamic data, but precision is relatively poor, due to the short temperature interval in which the reaction takes place. The following equation can be derived from these data: AG ° = 4 3 7 0 + 3 . 9 4 T - - 0 . 3 4 6 8 P In order to apply this equation to a system where anorthite and Ca-Tschermak occur as solid solution in plagioclase and clinopyroxene, the equilibrium constant K~ is defined as: K 1 = aCpx CaA12 SiO 6
/aplag CaAI 2Si 2 0
=
where aCpx CaAl~SiO6can be taken as equal to the mole fraction of Ca-Tschermak in c l i n o p y r o x e n e (Wood, 1977) and a plag is taken as C aAl~ Si2 O l X Cplag 7 plag [for plagioclase of sodic to intermediate comaAI~Si~O, • CaAl~Si20 , position, activity coefficient = 1.276, Orville, 1972. The mole fraction of Ca-Tschermak was taken as (A1--Na--2Ti)/2 ]. Three sets of data were used (Fig. 1) from which average values of K,
•
a
-i-
Cpx
MADRAS ADI
R ONDA CKS
G R ENVl L L E ® ~ BELLEAU / DESAULNIERS
Cc A I 2 S i 06
• 0,080
and
TOWNSHIPS
MORIN PLUTONIC COMPLEX
0,060
(9
0,040
o
-{-_~ ,-
0,02 0
® o,lo
O,S O
0,50
0,70
PI o Ca A 1 2 S i 2 0 8
Fig. 1. R e l a t i o n b e t w e e n activity o f Ca-Tschermak in c l i n o p y r o x e n e and activity o f ano r t h i t e in plagioclase for various c h a r n o c k i t e localities.
307 P r • s s u • e (kbar)
10 8 6
G1
4 2
Temperature
600
700
800
{°C]
900
Fig. 2. Principal P - T curves relevant t o c h a r n o c k i t e genesis K.S. = K y a n i t e - S i l l i m a n i t e inversion curve; black d o t s : P - T c o n d i t i o n s in Grenville m e t a p e l i t e s ; for o t h e r lines see text.
were derived in order n o t to blur the graphical representation. Pyroxene and plagioclase data from Belleau-Desaulniers area and from Grenville township (Philpotts, 1966) give a K~ value of 0.057 (a = 0.018). Data from the Adirondacks (McLelland and Whitney, 1977) give a K1 value of 0.0624 with a standard deviation of 0.008. Pyroxene and plagioclase data from the Morin plutonic complex in the southern part of the Grenville province (Martignole and Nantel, in prep.) give a K~ value of 0.058 (a = 0.008) equivalent to K~ obtained from Belleau-Desaulniers area and Grenville township (Fig. 2). The polygon defined by the intersection of G~--G2 and K~ lines for Grenville (K~ G) and Adirondack (K1A) charnockites delineates approximate physical conditions for charnockite genesis, namely 880 + 20°C and 7 -+ 1 kbar. Olivine-plagioclase reaction is consistent with this P-T range, the composition being intermediate between those of curves 3 and 4. EVIDENCE FROM GRENVILLE METASEDIMENTS
Metapelites associated with charnockites of the Grenville province usually contain sillimanite, garnet and in places cordierite. Sporadic occurence of kyanite (Martignole and Schrijver, 1968) tends to indicate that PT conditions were not far from th~ kyanite inversion curve. Moreover, combination of garnet-cordierite t h e r m o m e t r y with equilibrium curves for the reaction anorthite-*grossular + kyanite + quartz (Wood, 1977) defines a P-T range of 7 + 1 kbar and 680 + 40°C for metamorphic conditions in the metapelites (Nantel and Martignole, 1978). The temperature difference between values obtained from metapelite t h e r m o m e t r y and charnockite t h e r m o m e t r y either reflects a late and more efficient equilibration upon cooling of the aluminous assemblages or, more likely, it corresponds to a real thermal gradient between plutonites and associated supracrustal rocks. However, equilibrium pressures estimated from
308 the anorthite-grossular-kyanite geobarometer are in agreement with those estimated from the anorthite-Ca-Tschermak barometer in charnockites. Therefore, the possible persistance of a temperature difference between plutonites and country rocks casts some d o u b t on the determination of geothermal gradients based on plutonites only. Thus P-T estimates from metapelites in the southern part of the Grenville province are compatible with a gradient slightly lower than 30°C per km. EVIDENCE FROM OUTSIDE THE GRENVILLE PROVINCE There is no d o u b t that temperatures obtained by pyroxene geothermometry on charnockites from various regions in the world fall in the range 750--900°C as stated by Saxena (1977). However, in several localities, pressure estimates from various geobarometers give values up to 10 kbar. For instance, in the Hopen charnockites of Lofoten Island (Norway) the occurrence of ferrosilite-quartz {Ormaasen, 1977) is compatible with a pressure of a b o u t 10 kbar at 800°C. Comparable estimates are reported from the Raftsund mangerite (Griffin et al., 1974). Application of the Ca-Tschermak geobarometer to Madras charnockites (Howie, 1955) also gives pressures in the range 9--10 kbar at 850°C. Finally, simultaneous application of the anorthite-grossular geobarometer, combined with the biotite-garnet geothermometer (Holdaway and Lee, 1977) in metapelites and the ferrosilite-quartz geobarometer (Smith, 1971 ) in associated charnockites of NE Hoggar (Latouche, unpubl, data, pers. comm.) gives a pressure of 9.5--10 kbar at 800°C. Therefore, Lofoten, Madras and Saharan charnockites (Fig. 3) are indicative of a gradient of a b o u t 25°C per km, even lower than the one suggested for the Grenville province and the Adirondacks*. The same conclusion follows from observation on some older (Archean) high-grade metamorphic terranes (see for instance Dickinson and Watson, 1976). Although pressure estimates on charnockites are not yet entirely reliable, it is clear that whenever chemical data on pyroxenes, garnet and plagioclase are available, they allow estimations of physical conditions in the field of intermediate-pressure granulites, in agreement with the persistance of kyanite and other high-pressure assemblages in associated rocks. Refinements of available geobarometers is needed in order to estimate more precisely the magnitude of geotherms in charnockitic terranes. CONCLUSION Rather than areas of abnormal geothermal gradients, charnockite areas *Orthoferrosilite has been reported from the Adirondacks (Jaffe et al., 1975) suggesting higher pressures than those estimated from the Ca-Tsehermak geobarometer.
309 Pressure
I OFOTEN
:kbar)
~ "
CHARNOCKITES J .
.
.
.
.
.
.
C H A R NO CK I T E ~ - T . L r -
6
.....
./"':,
8 /
S
" I-~~MADRAS
.
.I-
/
/ . ~ _ f ~ G R E N V I L LE ~5~""'* METAPELITES
.
.
""
I"
CHARNOCKITES I _
ADIRONDACKS LLE
.........:~3~..~G R ENVI
....
""
4
2
-: o • c/L~ .
.
.
.
.
--
- - " .......... " ' D O M A I N
To-o~/~ J- . . . . . . 600
700
SAXENA~1977
Tempe~o:u.e :°~? 800
900
Fig. 3. P-T regimes for the genesis of some charnockites. Dashed lines are geothermal gradients.
might prove to correspond to regions of crustal thickening such as those that can be expected in continental collisions. Indeed, if no underplating is advocated, the presence of rocks emplaced at depth of 25--35 km in areas where the actual crustal thickness is around 30 km is in agreement with a crustal thickness of some 60 km at the time of charnockite genesis. Such crustal thickening which implies basement reactivation is well known from zones of continental collison (Tibet plateau) and has been proposed as a model for high-grade orogenic zones of Grenville type (Dewey and Burke, 1973). Thus, charnockite distribution might help in locating Proterozoic plate junctions. It still remains to be shown whether these conditions of formation are valid for charnockite all over the world, but due to the plutonic nature of these rocks, PT estimates of charnockite genesis can hardly be used to define geothermal gradients without at least a temperature control provided by geothermometry on associated metasediments. ACKNOWLEDGEMENTS
I want to thank J.B. Dawson and K. Schrijver for their critical review of the manuscript. N o t e a d d e d in press
In a recent paper (petrography and origin of granulite-facies rocks in the Western Musgrave block, Central Australia; J. Geol. Soc. Aust., 25 (6) 341 358, 1978) Moore and Goode consider than Australian granulites were metamorphosed under a gradient of 25--30°C per km. REFERENCES Bohlen, S.R. and Essene, E,J., 1977. Feldspar and oxide thermometry of granulites in the Adirondacks Highlands. Contrib. Mineral. Petrol., 6 2 : 1 5 3 - - 1 6 9 .
310 Dewey, J.F. and Burke, K., 1973. Tibetan, Variscan and Precambrian basement reactivation: products of continental collision. J. Geol., 81: 683--691. Dickinson, B.B. and Watson, J., 1976. Variations in crustal level and geothermal gradient during the evolution of the Lewisian complex of Northwest Scotland. Precambrian Res., 3: 363--374. Emslie, R.F., 1970. Upper P--T stability of a natural Olivine-Plagioclase Assemblage. Carnegie Inst. Yearb., 69: 154--155. Green, T.H., 1970. High pressure experimental studies on the mineralogical constitution of the lower crust. Phys. Earth Planet. Interior, 3: 441--450. Griffin, W.L. et al., 1974. General geology, age and chemistry of the Rafsund mangerite intrusion, Lofoten-Vesteralen. Nor. Geol. Unders., 312: 1--30. Hariya, Y. and Kennedy, G.C., 1968. Equilibrium study of anorthite under high pressure and high temperature. Am. J. Sci. 2 6 6 : 1 9 3 - 2 0 3 . Herz, N., 1969. Anorthosite belts, continental drift and the anorthosite event. Science, 164: 944--947. Holdaway, M.J. and Sang Man Lee, 1977. Fe-Mg cordierite stability in high-grade pelitic rocks, based on experimental, theoretical and natural observations. Contrib. Mineral. Petrol., 63: 175--198. Howie, R.A., 1955. The geochemistry of the charnockite series of Madras, India. R. Soc. Edinb., LXII: 725--769. Jaffe, H.J., Robinson, P. and Tracy, R.J., 1975. Orientation of pigeonite exsolution lamellae in metamorphic augite: correlation with composition and calculated optimal phase boundaries. Am. Mineral., 60: 9--28. Kushiro, I. and Yoder, H.S., 1966. Anorthite-forsterite and anorthite-enstatite reactions and their bearing on the basalt-eclogite transformation. J. Petrol., 7: 337--362. Martignole, J., 1974. L'6volution magmatique du complexe de Morin et son apport au probl6me des anorthosites. Contrib. Mineral. Petrol., 4 4 : 1 1 7 - - 1 3 7 . Martignole, J. et Schrijver, K., 1968. D~couverte du disth6ne dans le sud de la province tectonique de Grenville, et signification de ce mineral dans le facies granulite. C.R. Acad. Sci. Paris, 267: 1355--1357. Martignole, J. and Schrijver, K., 1971. Association of (hornblende)-garnet-clinopyroxene "subfacies" of metamorphism and anorthosite masses. Can. J. Earth Sci., 8: 698-704. Martignole, J. and Schrijver, K., 1973. Effect of rock composition on appearance of garnet in anorthosite-charnockite suites. Can. J. Earth Sci., 10: 1132--1139. McLelland, J.M. and Whitney, P.R., 1977. The origin of garnet in the anorthosite-charnockite suite of the Adirondacks. Contrib. Mineral. Petrol., 6 0 : 1 6 1 - - 1 8 1 . Morse, S.A., 1970. Alkali feldspars with water at 5 Kb. J. Petrol., 11: 221--251. Nantel, S. and Martignole, J., 1978. Geothermo-baromJtrie des m~tapelites de la province de Grenville. R.A. Sci. de la Terre, Orsay, 287. Ormaasen, D.E., 1977. Petrology of the Hopen mangerite-charnockite intrusion, Lofoten, north Norway. Lithos, 4: 2 9 1 - 3 1 0 . Orville, P.M., 1972. Plagioclase cation-exchange equilibria with aqueous chlorite solution; results at 700°C and 2000 bars in the presence of quartz. Am. J. Sci., 272: 234--272. Philpotts, A.R., 1966. Origin of the anorthosite-mangerite rocks in southern Quebec. J. Petrol., 7: 1--64. Robertson, J.K. and Wyllie, P.Y., 1971. Rock-water systems, with special reference to the water-deficient region. Am. J. Sci,, 271: 2 5 2 - 2 7 7 . Saxena, S.K., 1977. The charnockite geotherm. Science 198: 614--617. Smith, D., 1971. Stability of the assemblage orthopyroxene-olivine-quartz. Am. J. Sci., 271: 375--382. Touret, J., 1974. Facies granulite et ftuides carboniques. Ann Soc. Geol. Belgique, vol. P. Michot, pp. 267--287. Wood, B.J., 1977. The activities of components in clinopyroxene and garnet solid solutions and their application to rocks. Philos. Trans. R. Soc. Lond., A, 286: 331--342.
303
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
CHARNOCKITE GENESIS AND THE PROTEROZOIC CRUST
JACQUES MARTIGNOLE Laboratoire de Min6ralogie, Museum d'Histoire Naturelle de Paris/D~partment de G~ologie, Universit~ de Montreal (Canada)
(Received July 31, 1978; revision accepted February 27, 1979)
ABSTRACT Martignole, J., 1979. Charnockitegenesisand the Proterozoic crust. PrecambrianRes., 9: 303--310.
Charnockitic rocks that occur in many Proterozoic high-grade metamorphic terranes have crystallized (or recrystallized) under high-temperature and low water pressure conditions: they usually contain two pyroxenes and mesoperthite as well as orthoclase, plagioclase, quartz and traces of hornblende and biotite. Estimates of physical conditions based on subsolidu~ reactions independant of water pressure show that crystallization occurred in the range 800 to 900 ° C at pressures varying from 7 kbar in the Grenville province to 10 kbar in Africa and in Norway. These physical conditions correspond to a depth of charnockite genesis of about 25--35 km and a gradient of 25 to 30 ° C per km. Charnockite terranes might therefore correspond to regions of crustal thickening as do those which are associated with continental collision processes. If such is the case, their actual distribution might help in understanding Proterozoic plate motion.
INTRODUCTION Like the " a n o r t h o s i t e e v e n t " a t t r i b u t e d b y s o m e a u t h o r s (Herz, 1 9 6 9 ) to r a t h e r e x c e p t i o n a l t h e r m a l condition's at one stage o f the e a r t h ' s h i s t o r y , c h a r n o c k i t e genesis has been a t t r i b u t e d b y S a x e n a ( 1 9 7 7 ) t o an e x c e p t i o n a l ly high g e o t h e r m a l g r a d i e n t o f 7 0 - - 1 0 0 ° C per km. Such c o n d i t i o n s w o u l d have prevailed at various times in the e a r t h ' s h i s t o r y , the last one c o r r e s p o n d ing to the Grenville e v e n t at 110"0 + 2 0 0 Ma. The aim o f this p a p e r is to s h o w t h a t e x t r e m e l y high g e o t h e r m a l gradients are difficult t o reconcile with p e t r o g r a p h i c and chemical d a t a and phase equilibria f r o m the Grenville p r o v i n c e , the A d i r o n d a c k s , Africa, N o r w a y and India. In all o f these areas, c h a r n o c k i t e s c o u l d have f o r m e d u n d e r n o r m a l g e o t h e r m a l gradients, their actual o u t c r o p p i n g at the surface being due to t e c t o n i c r a t h e r t h a n to m e t a m o r p h i c events. MINERAL ASSEMBLAGES IN CHARNOCKITES A l t h o u g h S a x e n a defines c h a r n o c k i t e s as "granites c o n t a i n i n g h y p e r s t e n e " he considers t h e m as m e t a m o r p h i c rocks, recrystallized u n d e r gra-
304
nulite-facies conditions. In fact, true (felsic) charnockites, which are often genetically related to more mafic rocks (refered to as mafic charnockites in this paper) can be generated through three main processes: (1) Water-undersaturated "granitic" magma intruded in a relatively dry ~:rust (high-grade metamorphic basement} at any level, and subsequently recrystallized under high-grade metamorphic conditions; (2) Water-uadersaturated magma intruded in a relatively dry crust during gv'anulite facies metamorphism; (3) In situ dry anatexis during high-grade metamorphism. In any case, as equilibration will take place according to regional physico-,:heroical conditions, mineral equilibria in both the charnockites and the country rocks should reflect these conditions. The preservation of relict igneous textures in some charnockites (ophitic textures in mafic charnockites, exsolved pyroxenes, inverted pigeonite) is best interpreted according to the hypothesis of intrusion during high-grade metamorphism. In this case, one can expect a temperature difference between the hot plutonic mass and the relatively cool surrounding terrane, whereas confining pressures should be the same. The concentric pattern of decreasing temperatures around the Adirondack anorthosite found by Bohlen and Essene (1977) could be reinterpreted in that sense. In estimating P-T conditions for charnockite genesis, Saxena used several equilibrium curves in which water was present as a phase. Most experimental phase equilibrium relations have been studied under dry or H20-saturated conditions. However, reactions in high-grade metamorphic rocks are likely to proceed in the presence of a vapor phase with XH~ O considerably smaller than 1. Fluid inclusions in minerals of high-grade metamorphic rocks (Touret, 1974) have provided a good sample of the vapor phase at various stages of crystallization. In fact, due to differential incorporation of H20 and CO2 in melts, the vapour phase present in hypersolidus assemblages is almost pure CO2. During the last stages of crystallization XH~O increases until saturation of the melt. The a m o u n t of H=O present on reaching the solidus will determine the H20/CO2 ratio of the fluid phase in subsolidus assemblages. The large proportion of mesoperthite in charnockitic rocks excludes the possibility of an H20-saturated system but on the other hand, the fact that most charnockites end their crystallization in the two-feldspar region shows that saturation is attained at least in the last stages of crystallization. As dry melts of charnockite composition would n o t intersect the alkali feldspars critical line (Morse, 1970), one can assume that a finite a m o u n t of H20 is present as a constituent in hypersolidus and probably as a phase in subsolidus charnockitic assemblages. Such conditions are typical of water-deficient, vapor-present systems (Robertson and Wyllie, 1971). According to the experimental work by Morse {1970), the maximum PH~O for hypersolvus assemblages in the Ab--Or--Qz--H20 system is 2.25 kbar. As this pressure will decrease with introduction of anorthite into the system, it can be taken as a maximum PH20 in the charnockite crystallization.
305 .Based upon these considerations it is clear that experimental mineral reactions performed in water-excess conditions can hardly be applied to high-grade metamorphic rocks without corrections for PH20 ( Pload. Although PH:O can be estimated from reactions of the type: annite + oxygen -~ Kspar + magnetite + water (provided fo~ can be estimated), it is probably more convenient to circumvent the problem of fluid composition and to use subsolidus equilibria that involve solid phases only, in order to elucidate the physical conditions of charnockite genesis. PHASE EQUILIBRIA IN CHARNOCKITES OF THE GRENVILLE PROVINCE The P-T conditions of charnockite crystallization (or recrystallization) in the Grenville province have been estimated from several subsolidus equilibrium assemblages in rocks of granitic compositions and in mafic rocks that are supposed to have recrystallized under the same conditions. Charnockites c o m m o n l y contain garnet which normally appears as a late phase growing at the expense of o r t h o p y r o x e n e and plagioclase (Martignole and Schrijver, 1971). Experiments bearing on the first appearance of garnet have been performed by Green (1970). Figure 2 shows the line for the first appearance of garnet in rocks of gabbroic composition (G1) and dioritic composition (G2). The marked compositional dependence of garnet-forming reactions has been shown by Martignole and Schrijver (1973) in the southern part of the Grenville province where rocks with Fe/Mg and Ab/An ratios similar to those of G1 are garnetiferous, whereas rocks with G2 compositions are not. The P-T conditions during garnet growth must therefore lie between lines G1 and G2. Another reaction, well displayed in troctolites associated with charnockites (Martignole and Schrijver, 1971; Martignole, 1974) consists in the disappearance of olivine in the presence of plagioclase: plagioclase + olivine -~ o r t h o p y r o x e n e + clinopyroxene + spinel Curve 3 (Fig. 2) represents the reaction boundary experimentally determined by Emslie (1970) for a composition (ol = Fo~0 ; pl = An73) almost similar to the composition of the troctolites from the southern part of the Grenville province (ol = Fo~4 ; pl = Arts2), whereas curve 4 represents the same reaction with pure forsterite and anorthite (Kushiro and Yoder, 1966). A minimum of 7 kbar seems necessary to cause olivine to react with plagioclase at temperatures accepted for charnockite genesis (750.-900 ° C; Fig. 2). Another reaction which constitutes a potential geobarometer has been proposed by Wood (1977): CaA12Si2Os in plagioclase -* CaAl2SiO6 in clinopyroxene + quartz Experimental reversals have been carried o u t on pure end member reactant
306
and products (Hariya and Kennedy, 1968). This allows extraction of thermodynamic data, but precision is relatively poor, due to the short temperature interval in which the reaction takes place. The following equation can be derived from these data: AG ° = 4 3 7 0 + 3 . 9 4 T - - 0 . 3 4 6 8 P In order to apply this equation to a system where anorthite and Ca-Tschermak occur as solid solution in plagioclase and clinopyroxene, the equilibrium constant K~ is defined as: K 1 = aCpx CaA12 SiO 6
/aplag CaAI 2Si 2 0
=
where aCpx CaAl~SiO6can be taken as equal to the mole fraction of Ca-Tschermak in c l i n o p y r o x e n e (Wood, 1977) and a plag is taken as C aAl~ Si2 O l X Cplag 7 plag [for plagioclase of sodic to intermediate comaAI~Si~O, • CaAl~Si20 , position, activity coefficient = 1.276, Orville, 1972. The mole fraction of Ca-Tschermak was taken as (A1--Na--2Ti)/2 ]. Three sets of data were used (Fig. 1) from which average values of K,
•
a
-i-
Cpx
MADRAS ADI
R ONDA CKS
G R ENVl L L E ® ~ BELLEAU / DESAULNIERS
Cc A I 2 S i 06
• 0,080
and
TOWNSHIPS
MORIN PLUTONIC COMPLEX
0,060
(9
0,040
o
-{-_~ ,-
0,02 0
® o,lo
O,S O
0,50
0,70
PI o Ca A 1 2 S i 2 0 8
Fig. 1. R e l a t i o n b e t w e e n activity o f Ca-Tschermak in c l i n o p y r o x e n e and activity o f ano r t h i t e in plagioclase for various c h a r n o c k i t e localities.
307 P r • s s u • e (kbar)
10 8 6
G1
4 2
Temperature
600
700
800
{°C]
900
Fig. 2. Principal P - T curves relevant t o c h a r n o c k i t e genesis K.S. = K y a n i t e - S i l l i m a n i t e inversion curve; black d o t s : P - T c o n d i t i o n s in Grenville m e t a p e l i t e s ; for o t h e r lines see text.
were derived in order n o t to blur the graphical representation. Pyroxene and plagioclase data from Belleau-Desaulniers area and from Grenville township (Philpotts, 1966) give a K~ value of 0.057 (a = 0.018). Data from the Adirondacks (McLelland and Whitney, 1977) give a K1 value of 0.0624 with a standard deviation of 0.008. Pyroxene and plagioclase data from the Morin plutonic complex in the southern part of the Grenville province (Martignole and Nantel, in prep.) give a K~ value of 0.058 (a = 0.008) equivalent to K~ obtained from Belleau-Desaulniers area and Grenville township (Fig. 2). The polygon defined by the intersection of G~--G2 and K~ lines for Grenville (K~ G) and Adirondack (K1A) charnockites delineates approximate physical conditions for charnockite genesis, namely 880 + 20°C and 7 -+ 1 kbar. Olivine-plagioclase reaction is consistent with this P-T range, the composition being intermediate between those of curves 3 and 4. EVIDENCE FROM GRENVILLE METASEDIMENTS
Metapelites associated with charnockites of the Grenville province usually contain sillimanite, garnet and in places cordierite. Sporadic occurence of kyanite (Martignole and Schrijver, 1968) tends to indicate that PT conditions were not far from th~ kyanite inversion curve. Moreover, combination of garnet-cordierite t h e r m o m e t r y with equilibrium curves for the reaction anorthite-*grossular + kyanite + quartz (Wood, 1977) defines a P-T range of 7 + 1 kbar and 680 + 40°C for metamorphic conditions in the metapelites (Nantel and Martignole, 1978). The temperature difference between values obtained from metapelite t h e r m o m e t r y and charnockite t h e r m o m e t r y either reflects a late and more efficient equilibration upon cooling of the aluminous assemblages or, more likely, it corresponds to a real thermal gradient between plutonites and associated supracrustal rocks. However, equilibrium pressures estimated from
308 the anorthite-grossular-kyanite geobarometer are in agreement with those estimated from the anorthite-Ca-Tschermak barometer in charnockites. Therefore, the possible persistance of a temperature difference between plutonites and country rocks casts some d o u b t on the determination of geothermal gradients based on plutonites only. Thus P-T estimates from metapelites in the southern part of the Grenville province are compatible with a gradient slightly lower than 30°C per km. EVIDENCE FROM OUTSIDE THE GRENVILLE PROVINCE There is no d o u b t that temperatures obtained by pyroxene geothermometry on charnockites from various regions in the world fall in the range 750--900°C as stated by Saxena (1977). However, in several localities, pressure estimates from various geobarometers give values up to 10 kbar. For instance, in the Hopen charnockites of Lofoten Island (Norway) the occurrence of ferrosilite-quartz {Ormaasen, 1977) is compatible with a pressure of a b o u t 10 kbar at 800°C. Comparable estimates are reported from the Raftsund mangerite (Griffin et al., 1974). Application of the Ca-Tschermak geobarometer to Madras charnockites (Howie, 1955) also gives pressures in the range 9--10 kbar at 850°C. Finally, simultaneous application of the anorthite-grossular geobarometer, combined with the biotite-garnet geothermometer (Holdaway and Lee, 1977) in metapelites and the ferrosilite-quartz geobarometer (Smith, 1971 ) in associated charnockites of NE Hoggar (Latouche, unpubl, data, pers. comm.) gives a pressure of 9.5--10 kbar at 800°C. Therefore, Lofoten, Madras and Saharan charnockites (Fig. 3) are indicative of a gradient of a b o u t 25°C per km, even lower than the one suggested for the Grenville province and the Adirondacks*. The same conclusion follows from observation on some older (Archean) high-grade metamorphic terranes (see for instance Dickinson and Watson, 1976). Although pressure estimates on charnockites are not yet entirely reliable, it is clear that whenever chemical data on pyroxenes, garnet and plagioclase are available, they allow estimations of physical conditions in the field of intermediate-pressure granulites, in agreement with the persistance of kyanite and other high-pressure assemblages in associated rocks. Refinements of available geobarometers is needed in order to estimate more precisely the magnitude of geotherms in charnockitic terranes. CONCLUSION Rather than areas of abnormal geothermal gradients, charnockite areas *Orthoferrosilite has been reported from the Adirondacks (Jaffe et al., 1975) suggesting higher pressures than those estimated from the Ca-Tsehermak geobarometer.
309 Pressure
I OFOTEN
:kbar)
~ "
CHARNOCKITES J .
.
.
.
.
.
.
C H A R NO CK I T E ~ - T . L r -
6
.....
./"':,
8 /
S
" I-~~MADRAS
.
.I-
/
/ . ~ _ f ~ G R E N V I L LE ~5~""'* METAPELITES
.
.
""
I"
CHARNOCKITES I _
ADIRONDACKS LLE
.........:~3~..~G R ENVI
....
""
4
2
-: o • c/L~ .
.
.
.
.
--
- - " .......... " ' D O M A I N
To-o~/~ J- . . . . . . 600
700
SAXENA~1977
Tempe~o:u.e :°~? 800
900
Fig. 3. P-T regimes for the genesis of some charnockites. Dashed lines are geothermal gradients.
might prove to correspond to regions of crustal thickening such as those that can be expected in continental collisions. Indeed, if no underplating is advocated, the presence of rocks emplaced at depth of 25--35 km in areas where the actual crustal thickness is around 30 km is in agreement with a crustal thickness of some 60 km at the time of charnockite genesis. Such crustal thickening which implies basement reactivation is well known from zones of continental collison (Tibet plateau) and has been proposed as a model for high-grade orogenic zones of Grenville type (Dewey and Burke, 1973). Thus, charnockite distribution might help in locating Proterozoic plate junctions. It still remains to be shown whether these conditions of formation are valid for charnockite all over the world, but due to the plutonic nature of these rocks, PT estimates of charnockite genesis can hardly be used to define geothermal gradients without at least a temperature control provided by geothermometry on associated metasediments. ACKNOWLEDGEMENTS
I want to thank J.B. Dawson and K. Schrijver for their critical review of the manuscript. N o t e a d d e d in press
In a recent paper (petrography and origin of granulite-facies rocks in the Western Musgrave block, Central Australia; J. Geol. Soc. Aust., 25 (6) 341 358, 1978) Moore and Goode consider than Australian granulites were metamorphosed under a gradient of 25--30°C per km. REFERENCES Bohlen, S.R. and Essene, E,J., 1977. Feldspar and oxide thermometry of granulites in the Adirondacks Highlands. Contrib. Mineral. Petrol., 6 2 : 1 5 3 - - 1 6 9 .
310 Dewey, J.F. and Burke, K., 1973. Tibetan, Variscan and Precambrian basement reactivation: products of continental collision. J. Geol., 81: 683--691. Dickinson, B.B. and Watson, J., 1976. Variations in crustal level and geothermal gradient during the evolution of the Lewisian complex of Northwest Scotland. Precambrian Res., 3: 363--374. Emslie, R.F., 1970. Upper P--T stability of a natural Olivine-Plagioclase Assemblage. Carnegie Inst. Yearb., 69: 154--155. Green, T.H., 1970. High pressure experimental studies on the mineralogical constitution of the lower crust. Phys. Earth Planet. Interior, 3: 441--450. Griffin, W.L. et al., 1974. General geology, age and chemistry of the Rafsund mangerite intrusion, Lofoten-Vesteralen. Nor. Geol. Unders., 312: 1--30. Hariya, Y. and Kennedy, G.C., 1968. Equilibrium study of anorthite under high pressure and high temperature. Am. J. Sci. 2 6 6 : 1 9 3 - 2 0 3 . Herz, N., 1969. Anorthosite belts, continental drift and the anorthosite event. Science, 164: 944--947. Holdaway, M.J. and Sang Man Lee, 1977. Fe-Mg cordierite stability in high-grade pelitic rocks, based on experimental, theoretical and natural observations. Contrib. Mineral. Petrol., 63: 175--198. Howie, R.A., 1955. The geochemistry of the charnockite series of Madras, India. R. Soc. Edinb., LXII: 725--769. Jaffe, H.J., Robinson, P. and Tracy, R.J., 1975. Orientation of pigeonite exsolution lamellae in metamorphic augite: correlation with composition and calculated optimal phase boundaries. Am. Mineral., 60: 9--28. Kushiro, I. and Yoder, H.S., 1966. Anorthite-forsterite and anorthite-enstatite reactions and their bearing on the basalt-eclogite transformation. J. Petrol., 7: 337--362. Martignole, J., 1974. L'6volution magmatique du complexe de Morin et son apport au probl6me des anorthosites. Contrib. Mineral. Petrol., 4 4 : 1 1 7 - - 1 3 7 . Martignole, J. et Schrijver, K., 1968. D~couverte du disth6ne dans le sud de la province tectonique de Grenville, et signification de ce mineral dans le facies granulite. C.R. Acad. Sci. Paris, 267: 1355--1357. Martignole, J. and Schrijver, K., 1971. Association of (hornblende)-garnet-clinopyroxene "subfacies" of metamorphism and anorthosite masses. Can. J. Earth Sci., 8: 698-704. Martignole, J. and Schrijver, K., 1973. Effect of rock composition on appearance of garnet in anorthosite-charnockite suites. Can. J. Earth Sci., 10: 1132--1139. McLelland, J.M. and Whitney, P.R., 1977. The origin of garnet in the anorthosite-charnockite suite of the Adirondacks. Contrib. Mineral. Petrol., 6 0 : 1 6 1 - - 1 8 1 . Morse, S.A., 1970. Alkali feldspars with water at 5 Kb. J. Petrol., 11: 221--251. Nantel, S. and Martignole, J., 1978. Geothermo-baromJtrie des m~tapelites de la province de Grenville. R.A. Sci. de la Terre, Orsay, 287. Ormaasen, D.E., 1977. Petrology of the Hopen mangerite-charnockite intrusion, Lofoten, north Norway. Lithos, 4: 2 9 1 - 3 1 0 . Orville, P.M., 1972. Plagioclase cation-exchange equilibria with aqueous chlorite solution; results at 700°C and 2000 bars in the presence of quartz. Am. J. Sci., 272: 234--272. Philpotts, A.R., 1966. Origin of the anorthosite-mangerite rocks in southern Quebec. J. Petrol., 7: 1--64. Robertson, J.K. and Wyllie, P.Y., 1971. Rock-water systems, with special reference to the water-deficient region. Am. J. Sci,, 271: 2 5 2 - 2 7 7 . Saxena, S.K., 1977. The charnockite geotherm. Science 198: 614--617. Smith, D., 1971. Stability of the assemblage orthopyroxene-olivine-quartz. Am. J. Sci., 271: 375--382. Touret, J., 1974. Facies granulite et ftuides carboniques. Ann Soc. Geol. Belgique, vol. P. Michot, pp. 267--287. Wood, B.J., 1977. The activities of components in clinopyroxene and garnet solid solutions and their application to rocks. Philos. Trans. R. Soc. Lond., A, 286: 331--342.