Geology of Nicholson's point granite, Natal Metamorphic Province, South Africa: the chemistry of charnockitic alteration and origin of the granite

Pergamon

PII:

Journal o f African Earth Sciences, Vol. 23, No. 3, pp. 4 6 5 - 4 8 4 , 1996 Copyright o 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain S0899-5362(97)00013-4 0 8 9 9 - 5 3 6 2 / 9 6 $15.O0 + 0 . 0 0

Geology of Nicholson's Point granite, Natal Metamorphic Province, South Africa: the chemistry of charnockitic alteration and origin of the granite G. H. G R A N T H A M ,

1 A. R. ALLEN, 2 D. H. C O R N E L L 3 a n d C. H A R R I S 4 ~Department of Geology, University of Pretoria, Pretoria, 0002, South Africa 2Department of Geology, University College Cork, Cork, Eire 3Geologiska Institutionen, G6teborgs Universitet, $41 296, G6teborg, Sweden 4Department of Geological Sciences, University of Cape Town, Rondebosch, 7700, South Africa

A b s t r a c t - - I n the Port Edward area of southern Kwa-Zulu Natal, South Africa, charnockitic

aureoles up to - 4 m in width are developed adjacent to contacts with Port Edward enderbite and pegmatites intruded into the normally garnetiferous Nicholson's Point granite. Other mineralogical differences between the aureoles and the granite include increased myrmekite and significantly less biotite in the former and the replacement of pyrite by pyrrhotite in the charnockitic rocks. No significant differences in major element chemistry between the garnetbiotite Nicholson's Point granite and charnockitic Nicholson's Point granite are seen, except possibly for higher CaO and TiO 2 in the charnockite. Higher Rb, Th, Nb and Y contents in the garnet-biotite granite suggest that these elements have been locally depleted from garnetbiotite granite during charnockitization. This depletion is considered to be related to the reduction in biotite. Strontium and Ba contents are significantly higher in the charnockite. Generally higher S contents in the charnockite suggest S metasomatism, with S possibly being added from the enderbite. No differences in ~5~80 isotope data are seen between the garnetiferous and hypersthene bearing granite. In the charnockite the LREEs are weakly depleted whereas the HREEs show greater depletion compared to the garnetiferous granite. The depletions in REEs are thought to be related to the breakdown of garnet. Europium is marginally enriched or unchanged in the charnockite relative to the garnetiferous granite. Two-pyroxene thermometry on the Port Edward enderbite suggests that it was intruded at temperatures of -1 000-11 00°C. The replacement of pyrite by pyrrhotite is also consistent with a thermal auroele. Consequently the charnockitlc zones developed around the intrusions of Port Edward enderbite may result from the thermally driven dehydration of biotite. The aureoles developed adjacent to pegmatites are not considered to have resulted from heat but probably by destabilisation of biotite by a low aH20 fluid phase, possibly hypersaline brines. The Nicholson's Point granite has geochemical characteristics typical of within-plate granites, A-type granites and rapakivi granites, however the stable and radiogenic isotope characteristics suggest a significant crustal component in the source. Copyright © 1997 Elsevier Science Ltd. All rights reserved R~sum~--Dans la r~gion de Port Edward dans le sud du Kwa-Zulu Natal en Afrique du Sud, des aureoles charnockitiques pouvant atteindre jusqu'& 4 m de largeur sont d6velopp~es au contact de I'enderbite de Port Edward et des pegmatites intrusives dans le granite g~n6ralement grenatif~re de la Pointe de Nicholson. Des diff6rences du point de vue min~ralogique entre I'aur~ole et le granite se marquent par I'augmentation du nombre de myrm~kites, la diminution significative de la biotite et la substitution de la pyrite par de la pyrrhotine. En ce qui concerne la chimie des 61~ments majeurs, on ne note pas de differences significatives entre les granite grenat et biotite et granite charnockitique de la Pointe de Nicholson, hormis peut-~tre des teneurs plus 61ev~es en CaO et TiO 2 dans la charnockite. Des teneurs plus ~lev6es en Rb, Th, Nb et Y dans le granite ~ grenat et biotite sugg~rent que ces ~l~ments ont ~t~ Iocalement appauvris ~ partir du granite & grenat et biotite Iors de la charnockitisation. Cet appauvrissement est suppos~ 6tre en relation avec la diminution de la quantit~ de biotite. Darts la charnockite, les teneurs en Sr et Ba sont significativement plus ~lev~es. Un m~tasomatisme en S est sugg~r~ par des teneurs g~n~ralement plus 61evSes en S dans la charnockite, le S pouvant

Journal o f African Earth Sciences 4 6 5

G. H. GRANTHAM et al.

provenir de I'enderbite. Au niveau des isotopes 5~80, il n'est pas observe de differences entre les granites & grenat et ceux & hypersthene. Comparees au granite ~ grenat, les terres rares legeres dans la charnockite ne soRt que faiblement appauvries tandis que les terres rares Iourdes le SORtdavantage. Ces appauvrissements en terres rares SORtsupposes 6tre en relation avec la disparition du grenat. Par rapport au granite ~ grenat, I'Eu dans les charnockites n'est enrichi que de fa,con marginale ou reste inchange, La thermometrie des deux pyroxenes de I'enderbite de Port Edward suggere sa mise en place & des temperatures de -1 000-1100°C. La substitution de la pyrite par de la pyrrhotine s'explique egalement par I'existence d'une aureole thermique. Ainsi les zones charnockitiques autour des intrusions d'enderbite de Port Edward pourraient etre le r~sultat de la d6shydratation de la biotite sous I'effet de la chaleur. Les aureoles au contact des pegmatites ne resulteraient pas d'une augmentation de chaleur mais seraient plut6t le r~sultat de la destabilisation de la biotite par une phase fluide aqueuse de basse temperature, ~ventuellement des saumures hypersalines. Le granite de la Pointe de Nicholson possede des caracteristique geochimiques typiques des granites d'interieur de plaques, de granites de type A et de granites rapakivi. Toutefois, les isotopes stables et radiogeniques suggerent une composante crustale significative au niveau de la source. Copyright © 1997 Elsevier Science Ltd. All rights reserved (Received 23 November 1995: revised version received 15 March 1996)

INTRODUCTION Charnockites c o m m o n l y form an integral part of granulite terrains. Numerous models have been p r o p o s e d for t h e g e n e s i s of t h e a n h y d r o u s assemblage of opx + K-feldspar + quartz which is characteristic of c h a r n o c k i t e s e n s u s t r i c t o . These m o d e l s i n c l u d e i g n e o u s c h a r n o c k i t e s (Kilpatrick and Ellis, 1992; Stern and Dawoud, 1991; Wickham, 1988; amongst others), t h e r m a l l y d r i v e n v a p o u r a b s e n t m e l t i n g or dehydration and d e h y d r a t i o n triggered by l o w aH20 fluids (Hansen e t a l . , 1995), particularly CO 2 (Hansen e t a l . , 1 9 8 7 ; Srikantappa e t a l . , 1992 a m o n g s t others) or various c o m b i n a t i o n s of all these factors (Frost and Frost, 1 987; Frost e t a l . , 1989). Numerous studies comparing the c h e m i c a l c o m p o s i t i o n s in r o c k s d i s p l a y i n g transitions from non-charnockitic to charnockitic a s s e m b l a g e s and s i m i l a r l y a m p h i b o l i t e - t o granulite-facies metamorphism h a v e been conducted. Some studies clearly d e m o n s t r a t e significant differences, particularly in K/Rb ratios (Stahle e t al., 1987; N e w t o n and Hansen, 1983; Hansen e t a l . , 1995; Hansen e t a L , 1987; Lamb e t a L , 1986; Petersen, 1 9 8 0 ; a m o n g s t others), w h e r e a s at o t h e r l o c a l i t i e s no s i g n i f i c a n t d i f f e r e n c e s are seen s u g g e s t i n g i s o c h e m i c a l transitions (Andersson e t a L , 1992; Stern and Dawoud, 1991). A n o t h e r i m p o r t a n t aspect of many charnockites is their association w i t h rocks displaying rapakivi t e x t u r e s and/or rocks w i t h c h e m i s t r y similar to rapakivi granites (Hubbard and Whitely, 1978, 1979). This s t u d y presents n e w data from the Natal M e t a m o r p h i c Province (NMP) South A f r i c a , and p r i m a r i l y c o m p a r e s chemical differences b e t w e e n n o n - c h a r n o c k i t i c g a r n e t - b i o t i t e g r a n i t e and c h a r n o c k i t e , and secondly, compares the granite c h e m i s t r y w i t h

466 Journal of African Earth Sciences

A - t y p e g r a n i t e s , w i t h i n - p l a t e g r a n i t e s and rapakivi granites. The NMP represents the eastern sector of the - 1 0 0 0 Ma N a m a q u a - N a t a l Belt (Fig. 1). The Margate Terrane (Thomas, 1989) of the NMP is c o m p o s e d of high grade m e t a s e d i m e n t s of p e l i t i c , q u a r t z o f e l d s p a t h i c and c a l c a r e o u s c o m p o s i t i o n s (Leisure Bay Formation) w h i c h were intruded by m a g m a s of granitoid to mafic c o m p o s i t i o n s (Mclver, 1963, 1966; Grantham, 1983; Talbot and Grantham, 1987). The rock u n i t s i n v o l v e d in t h i s s t u d y i n c l u d e t h e N i c h o l s o n ' s Point g r a n i t e , t h e Port E d w a r d enderbite, the mafic to i n t e r m e d i a t e Munster Suite and pegmatite veins. The Nicholson's Point granite f o r m s part of the M a r g a t e Granite Suite (Thomas, 1989) and the Port Edward enderbite part of the Oribi Gorge Suite (Thomas, 1989). In t h i s s t u d y t h e I o c a l i s e d d e v e l o p m e n t of o r t h o p y r o x e n e in the Nicholson's Point granite, adjacent to c o n t a c t s w i t h other meta-igneous r o c k t y p e s and m a r g i n a l t o c r o s s - c u t t i n g p e g m a t i t e s , are d e s c r i b e d t o g e t h e r w i t h associated chemical variations. Three phases of folding and m e t a m o r p h i s m have been recognised in the s t u d y area (Talbot and Grantham, 1987). F~folds are defined by layering in intercalated m e t a p e l i t e s and calcsilicates of the Leisure Bay Formation (Fig. 1) and are isoclinal folds w i t h s o u t h w a r d dipping axial planar foliations and near horizontal fold axes w h i c h plunge e a s t w a r d s or w e s t w a r d . M, is c o r r e l a t e d w i t h F 1 b e c a u s e t h e m i n e r a l assemblages defining the axial planar foliation in t h e m e t a p e l i t e s consist of lenticular garnetiferous n e o s o m e s and c o r d i e r i t e + hypersthene bearing melanosomes. F 1 folds in

Geology o f the Nicholson "s Point granite, Natal Met amor phi c Province, South Africa

u~.*+,

/Richter=weld/

f:~ ¢ + + + + + + /

,,~: +. +. +.+. T T ~

,," 4 - ~ - + - P - v

V~k+++++~

,++,.+t J ~~I

v,,=.v.~-~-~-~-ii

+.11

h, + + + + + + + + + + + + + 4 ) /

~...,~~..

/ /

"

/

/

y P a l m " ':" Beach

.."

,rtobeUo

_.':1 + ++++++ +++++_~J~/"

\--'~.E----w~J'~+ +

31os "

+ + + + + + +_*.-~7~:

kqunster

GLENMORE

/ / O

/

1kin

!

/

I

/

/

/..~

/

Leisure Bay

[~ ~

/ / //

/

Nichoison's Point

~

Drake's Beach

~

Ivy Point

/

~

PORT EDWARD

PORTEDWARD ENDERJBITE

."

PORTOBELLO GRAPfl'TE P.,&£.M BEACH GRANITE ' [~rICHOLSON'SPOINT GRANITE ~m 'UNSTER SUrt-~

~

GLF.J~MORE G~E

~

L£ISUREBAY FORMATION

ORIBI GORGE SUITE YLAJ~GA~ GRIE~I'rE SUITE

300115'E Figure 1. Location map o f the study area s h o w i n g an enlarged geological map o f the l o w e r south coast o f Natal.

the Leisure Bay Formation fold the layering and the S 1 foliation and are typically asymetric northward verging, with near horizontal eastwest orientated plunges. M 2 assemblages are not easily recognised but can be identified by the recognition of garnets containing planar inclusion trails discordant to the foliations which flow around the garnets, indicating that the garnets were pretectonic to D 2. F3 folds have n o r t h - s o u t h o r i e n t a t e d axes w h i c h plunge southward and have an open fold geometry. M 3 assemblages in the Leisure Bay Formation and Munster Suite are identified as consisting of v a r i o u s post-tectonic symplectitic intergrowths, e.g. garnet+quartz after opx + plagioclase, biotite + quartz after garnet/ opx + K-feldspar + water and hornblende + quartz after cpx + plagioclase + orthopyroxene + water.

The foliations in the Nicholson's Point granite, Port Edward enderbite and Munster Suite are typically orientated axial planar to F2 and dip southward at - 4 5 ° (stereonets A, B and C in Fig. 2). Pegmatites intruded into the Nicholson's Point granite, Port Edward enderbite and Munster Suite do not show a foliation but curve along the strike, suggesting that they too may have been deformed by D 2, but did not develop a foliation. Both types of charnockitic aureoles described in this study display planar fabrics indicating that D 2 post-dated the aureole development. Thus the Nicholson's Point granite, Munster Suite and the Port Edward enderbite are regarded to be of post-D1, pre-D 2 age. The spatial distribution of the bodies of Port Edward enderbite in Nicholson's Point granite at Nicholson's Point can be interpreted as defining a fold closure (Fig. 2).

Journal of African Earth Sciences 467

G. H. G R A N T H A M et aL

D D

NICHOLSON'S POINT GRANITE PORT EDWARD ENDERBITE ANORTHOSITE AND UNDIFFERENTIATED ROCKS MUNSTER SUITE APPROXIMATE GARNET/ HYPERSTHENE ISOGRAD DIP AND STRIKE OF FOLIATION PEGMATITE

A

~s

/

GG17.~.

D

B

\

GG94

.I

I !

i"

/

C

f f/

INDIAN

OCEAN

t/~/I

BEACH

~

/

J ~ j

//

i99,/.I,Z} /

~o 0 I

50 I Metres

Figure 2. Geological map o f field relationships at Nicholson "s Point. Stereonet A represents poles to the foliations in the anorthosite (open circles) and poles to the foliations in the surrounding Nicholson's Point granite (dots). Stereonets B and C represent poles to S 2 foliations in the Port Edward enderbite and Nicholson "s Point granite, respectively. Stereonet D represents the poles to lent/cular xenofiths o f Leisure Bay Formation granulites in the Port Edward enderbite. The * in stereonet D represents a ~Tpole to the girdle. Orthopyroxene is found in the granite closest to the Port Edward enderbite. 468 Journal of African Earth Sciences

Geology o f the Nicholson "s Point granite, Natal Metamorphic Province, South Africa

TN

\ \

PORT EDWARD ENDERBITE NICHOLSON'S POINT GRANITE (CHARNOCKITIC) NICHOLSON'S POINT GRANITE (GARNETIFEROUS)

M

MUNSTER SUITE LEISURE BAY FORMATION

sand

ii////f Figure 3. Geological map of field relationships at Drakes Beach.

FIELD A N D AGE RELATIONSHIPS At Nicholson's Point, Drakes Beach and Ivy Point, Natal south coast, the Nichotson's Point granite is in contact with the Port Edward enderbite (Figs 1, 2 and 3) and at Drakes Beach the Nicholson's Point granite is also in contact with the Munster Suite (Fig. 3). No chilled margins are displayed at any of these contacts, however at Drakes Beach the Port Edward enderbite is relatively finer-grained and contains xenoliths of the Munster Suite (Fig. 3). At Nicholson's Point a number of monzonoritic (as defined by Le Maitre, 1989) bodies and t w o b o d i e s of l e u c o n o r i t e and a n o r t h o s i t e composition are enclosed by the Nicholson's Point granite (Fig. 2). The monzonoritic bodies form part of the Port Edward enderbite, being

similar in appearance, mineralogy and chemical c o m p o s i t i o n . T h e y c o n t a i n x e n o l i t h s of metapelitic paragneisses (Leisure Bay Formation), w h i c h p r e d o m i n a n t l y dip s o u t h w a r d but occasional dip northward (Stereonet D, Fig. 2). The northward dipping xenoliths are only seen in one body (Stereonet D, Fig. 2), which is located in the vicinity of the possible fold closure in Fig. 2. The origins of the leuconoritic and anorthositic bodies are not clear, although they display planar fabrics which are discordant with that of the enclosing granite (Fig. 2, stereonet A) suggesting that they may be xenoliths. North of N i c h o l s o n ' s Point, thin ( < 1 0 cm thick) p e g m a t i t e veins c o m p o s e d of quartz and orthoclase intrude the Nicholson's Point granite, the Port Edward enderbite and the Munster Suite

Journalof AfricanEarthSctences=I69

G. H. GRANTHAM et aL

Figure 4. Type 2 chamockitic aureole developed adjacent to a pegmatite intrusion.

(Fig. 2). Where they intrude the Munster Suite the pegmatites range up to 1 m wide and contain significant magnetite, but they thin progressively southward to - 1 0 cm width in the Nicholson's Point granite. The Nicholson's Point granite is leucocratic and relatively homogeneous, with garnet and biotite as the ferromagnesian phases. However at Nicholson's Point, Drakes Beach and Ivy Point, mineralogical and colour zonations (termed aureoles for convenience) are observed in the granite (Figs 2, 3 and 4). These aureoles are darker than the garnetiferous granite and are characterised by the presence of hypersthene and a low biotite content. Three types of aureoles are recognised. Type 1 aureoles are developed where the Nicholson's Point granite is in contact with the Port Edward enderbite at Drakes Beach and at Nicholson's Point (Figs 2 and 3). Type 2 aureoles are developed adjacent to where the pegmatites intrude the Nicholson's Point granite (Fig. 4). Type 3 aureoles are developed where the Nicholson's Point granite is in contact with the Munster Suite at Drakes Beach (Fig. 3). All these aureoles display the planar fabric typical of S 2. This paper does not consider type 3 aureoles and discusses data derived largely from type 1 aureoles and to a lesser extent type 2 aureoles. The a p p r o x i m a t e position of the garnet/ hypersthene isograd, separating the 'aureoles' from the normal garnetiferous Nicholson's Point granite, is shown in Figs 2 and 3. The widths of

470 Journal of African Earth Sciences

the 'aureoles' vary considerably (Figs 2 and 3) and in the case of the pegmatites range up to 50 cm on either side of the 10 cm wide p e g m a t i t e s , r e p r e s e n t i n g an a u r e o l e / v e i n thickness ratio of 10:1 (Fig. 4). The contacts between the hypersthene and garnetiferous Nicholson's Point granite are relatively sharp for the type 2 aureoles but are sharp to diffuse for the type 1 and 3 aureoles. The sharp contacts are commonly associated with differential weathering w i t h the garnetiferous granite showing more resistance to weathering. Whole rock Rb/Sr dating by Eglington et al. (1986) shows that the ages of the Port Edward enderbite and Nicholson's Point granite overlap, being 987_+19 Ma ( R o = 0 . 7 0 4 5 3 _ + 1 3 ) and 1011 _+ 19 Ma (Ro=0.7063_+6), respectively. A zircon evaporation age of - 1 0 5 5 Ma (Thomas e t a l . , 1995) suggests a minimum age for the Port Edward enderbite, however no zircon-based age data are available for the Nicholson's Point granite. An apophysis of Port Edward enderbite extending into the Nicholson's Point granite at Nicholson's Point (Fig. 2, point A near the top of the map) suggests that the Port Edward enderbite intruded the Nicholson's Point granite.

PETROGRAPHY

The N i c h o l s o n ' s Point granite is t y p i c a l l y composed of slightly porphyritic (3-4 mm) anhedral orthoclase, plagioclase and quartz set

Geology of the Nicholson's Point granite, Natal Metamorphic Province, South Africa

Q

• Gornellferous Chornockltic o Garneliferous

gl'anlle mode granlle mode

granite normative normative

& C h a r n o c k i 1 1 c granite

P

Figure 5. Q-A-P plot showing the modal and normative compositions o f the g a r n e t i f e r o u s and charnockitic Nicholson "s Point granite using the granitoid classification scheme of Le Maitre (1989).

in a granoblastic matrix (average grain size = 1 mm) of quartz, feldspar, biotite, garnet and/ or hypersthene, opaques and accessory zircon and apatite. Modal data for the samples analysed are presented in Table 1. Garnet displays t w o habits, a euhedral to anhedral inclusion-free habit and a type displaying s y m p l e c t i t i c i n t e r g r o w t h w i t h quartz. The inclusion-free type occurs as isolated grains commonly enclosed within feldspar grains. The garnet-quartz symplectites occur marginally to hypersthene, opaques and, rarely, inclusion-free garnet indicating that the symplectitic garnet post-dates the inclusion-free variety. Rocks which contain the inclusion-free garnet do not contain hypersthene. The plagioclase content is variable and is generally greater in the hypersthene bearing granite. Modally, the h y p e r s t h e n e bearing g r a n i t e s overlap the monzo-granite and syeno-granite fields, but the more orthoclase-rich garnetiferous Nicholson's Point granites are confined to the syeno-granite field (Fig. 5). M y r m e k i t e is present in all sections, but is developed to a greater degree in the hypersthene bearing granite. Biotite is greatly reduced in quantity in the charnockitic rocks compared to the garnetiferous granite. The biotite in the garnetiferous granite usually occurs as discrete grains, whereas in the c h a r n o c k i t e it c o m m o n l y mantles opaque minerals and pyroxene. Opaques are most abundant in the hypersthene bearing granite. Polished section, heavy mineral, and magnetic separator studies show that the opaques in the garnetiferous granite consist of pyrite and ilmenite, whereas those in the hypersthene

bearing g r a n i t e c o n s i s t of p y r r h o t i t e and ilmenite. The inclusion-free garnets are interpreted as being primary, whereas the symplectitic garnet is considered to be of M 3 metamorphic age (Talbot and Grantham, 1987). The interpretation of the magmatic origin of the garnet is supported by work by Green (1976), who demonstrated that Fe-rich garnet could crystallize from granitic magmas at pressures of - 7 kb and temperatures -800°C. Harrison (1988) also recognised the crystallization of Mn-rich magmatic garnet developed at relatively low pressures. The distributions of biotite are interpreted to suggest that the biotite in the garnetiferous granite may largely be primary, whereas the biotite in the charnockite appears to represent a later phase of hydration, possibly post D 3.

GEOCHEMISTRY Thirteen samples of hypersthene bearing and garnetiferous Nicholson's Point granite were analysed for major and selected trace elements (Table 2). The analyses were done at the University of Natal, Pietermaritzburg, except the S and Ba values w h i c h were done at the U n i v e r s i t y of Pretoria. All but one of the hypersthene bearing samples analysed were taken from type 1 aureoles with sample NPG (Table 2) representing a type 2 aureole. In addition, rare earth element (REE) analyses were performed on three garnetiferous samples, a garnet separate and two hypersthene bearing samples. All the w h o l e rock analyses are presented in Table 2. Eglington e t al. (1986) describd the Nicholson's Point granite as having w i t h i n plate g r a n i t e c h e m i s t r y using the tectonomagmatic discrimination diagrams of Pearce e t al. (1984). All the samples are characterised by high FeO/ ( F e O + M g O ) , and K = O / ( K 2 0 + N a 2 0 + C a O ) . These characteristics are typical of A-type granites (Eby, 1990) and also rapakivi granites (R~im5 and Haapala, 1995) Normatively both the h y p e r s t h e n e b e a r i n g and g a r n e t i f e r o u s Nicholson's Point granite plot within the granite field (ie monzo-granite; Le Maitre, 1989) of the granitoid classification diagram (Fig. 5). A possible reason for the difference in classification between the observed and normative mineralogy is that the orthoclase in the garnetiferous granite contains a high content of Na in solid solution (i.e. cryptoperthite) supported by microprobe analyses which indicate up to 20 mole percent Ab in the orthoclase. Since Na is calculated as

J o u r n a l o f A f r i c a n Earth Sciences 471

G. H. G R A N T H A M

e t aL

Table 1. Modal data for the Nicholson's Point granite based on approximately 1 0 0 0 points per thin section I<

CHARNOCKITE.

:,

<

GARNET BIOTITE GRANITE.

GG82

GG70

GG76

GG99

GG95

NPG

GG17

GG94

GG19

GG93

GG91

GG98

GG69

qtz

32.1

40.8

27.1

33.1

33.1

29.4

37.5

36.4

39.2

27.3

28.5

25.6

29.0

plag

22.8

27.0

28.1

16.4

13.2

31.2

15.3

10.1

13.7

10.3

15.5

11.8

14,9

kfels

34.8

25.3

36.3

40.4

41.2

36.5

36.4

46.5

37.6

52.6

50.6

51.4

41.9

2.5

1.7

3.3

2.7

2.4

3.8

2.

6.6

2.6

3.4

2.7

1.6

4.3

3,

1.2

2.3

2.7

3.9

1.1

2.7

5.

bt

1.2

1.4

0.9

0.4

0.8

0.5

opx

5.9

3.5

2.5

4.9

5.9

2.0

gt myra N.D.=

0.2

1.8

4.5

4.5

5.3

ND

not d e t e r m i n e d

plagioclase in the norm, the rock will thus contain a higher proportion of normative plagioclase than is reflected in the mode. Harker v a r i a t i o n d i a g r a m s for t h e major e l e m e n t s reveal no s i g n i f i c a n t d i f f e r e n c e s b e t w e e n the t w o g r a n i t e v a r i e t i e s , e x c e p t possibly in the diagrams for CaO and TiO 2 (Fig. 6). In both these plots the CaO and TiO 2 are possibly slightly enriched in the charnockite. The slightly higher CaO and TiO 2 might suggest partial assimilation of the more mafic Port Edward enderbite and M u n s t e r Suite by the Nicholson's Point granite because the charnockitic granite is generally associated with bodies of these rock. M c l v e r ( 1 9 6 3 , 1966) interpreted the field relationships in this manner. Study of the geochemistry of the Port Edward enderbite and the M u n s t e r Suite reveal that potentially they could be sources for CaO and TiO 2, as they have relatively high contents of these elements (Grantham, 1983). However, if assimilation of the Port Edward enderbite or the Munster Suite by the Nicholson's Point granite had occurred it would also be expected to have caused higher P205, FeO and MgO contents in the aureole, as well as higher MgO/(MgO + FeO) since the potential contaminants (Port Edward e n d e r b i t e and M u n s t e r Suite) r e f l e c t these differences. No such differences are seen and c o n s e q u e n t l y the assimilation model is not supported by the data (see later discussion of the trace elements). The MgO/(MgO + FeO) ratio has been shown to be important in determining whether garnet or hypersthene will be present in a rock (Martignole and Schrijver, 1973; Green and R i n g w o o d , 1 9 6 7 ) . G a r n e t i f e r o u s rocks generally have lower values for this parameter. In addition, Martignole and Schrijver (1973)

4 72 Journal of A frican Earth Science5

showed that the normative A b / ( A b + A n ) ratio also influences g a r n e t - h y p e r s t h e n e relations (albeit to a lesser degree), in that rocks with high normative A b / ( A b + A n ) ratios (-0.8-1.0) contain hypersthene instead of garnet. However, no significant differences in these parameters exist between the garnetiferous and hypersthene bearing granites from Nicholson's Point. The most striking feature of trace element distributions within the Nicholson's Point granite is the coherence of the garnetiferous granite as a group compared to the hypersthene bearing varieties, which exhibit variable trace element contents. Variation diagrams of Rb versus Sr, Th versus Zr and Y versus Nb (Fig. 7) reveal significant differences between the garnetiferous and charnockitic granites. The Rb content is lower in the hypersthene bearing granite such that K/Rb values for the charnockitic granites range b e t w e e n 3 0 0 - 5 0 0 , whereas those for the garnetiferous granite vary b e t w e e n 2 0 0 - 3 0 0 . In the Rb versus Sr (Fig. 7) plot it may be seen that the charnockite has a wider range and generally higher c o n t e n t of Sr and therefore l o w e r Rb/Sr values t h a n the g a r n e t i f e r o u s granite. Barium contents are also significantly higher in the charnockite. The plot of Th versus Zr reveals that the hypersthene bearing granites are so depleted ~n Th that no Th was detected in some of the samples (detection limits of 0 . 1 p p m + 3 % ) . In the Y versus Nb plot it may be seen that the charnockitic granite is also depleted in Y and Nb (Fig. 7). Barium contents are s i g n i f i c a n t l y higher in the c h a r n o c k i t e samples. There is a higher concentration of S in the charnockitic rocks (mean S content = 295 ppm; standard d e v i a t i o n = 1 1 6 ) t h a n in t h e

Geology of the Nicholson "s Point granite, Natal Metamorphic Province, South Africa

Z

1.8

m

o

TiO:

(charn)

1.6 []

1.4

[]

CaO(charn) EB

~1.2

T i O 2 (Grt) []

0.8

CaO(Grt)

0.6

~mm~

0.4 0.2

I

70

71

i

I

72

m~) i

I

I

73

74

75

SiO 2 (wt%)

Figure 6. Harker variation diagrams for CaO and TiO 2 in the Nicholson "s Point granite. The charnockite samples are shown as filled symbols and the g a r n e t biotite granite as open symbols.

garnetiferous rocks (mean S content = 194 ppm; standard deviation = 44) (Fig. 7, Table 2). There are no significant differences in the values of 8~80 between the garnetiferous granite and charnockite (Fig. 7, Table 2). Depletions of Rb, Th and Y, together with Pb, Cs and U in granulite grade metamorphic rocks, have been noted by various authors and are considered characteristic of granulite-facies metamorphism (Heier, 1973; Lambert and Heier, 1967; Fyfe, 1973; Tarney e t al., 1972; Collerson and Fryer, 1979). These large ionic radius elements are believed to be depleted by fluids derived either from metamorphic dehydration, mantle outgassing or melting. Because of their large ionic radii, these elements are partitioned into the volatile or melt phases, since they are not easily incorporated into the lattices of minerals which form at high metamorphic grades. The depletion of Rb, Th, Y and Nb in the hypersthene bearing granite is considered to have been facilitated by volatiles or fluids of a secondary origin derived from the dehydration of hydrous minerals (biotite in the garnetiferous granite) and fluids from the intruding pegmatites. Depletion of Rb in the charnockite appears to be related to the significantly lower biotite content of this rock type. Partition coefficients (K~) for Rb in biotite are of the order of 3.5 compared to 0.38 for orthoclase (Henderson, 1982, p93). Thus the breakdown reaction of biotite + quartz-+hypersthene + orthoclase (Luth, 1967) will result in the release of Rb into the fluid phase. The depletion of Nb may also be related to the breakdown of biotite in that the K~ for Nb in biotite is of the order of 3 in acid rocks, whereas the K~ for Nb in hypersthene is 0.15. (Pearce and Norry, 1979). Furthermore, whereas the K~ for Y in garnet is -2, it is - 0 . 2 in hypersthene (Pearce

and Norry, 1 9 7 9 ) and t h e r e f o r e w i t h the breakdown of garnet to orthopyroxene, Y would be preferentially partitioned into the fluid phase. Although the Kd's published by Pearce and Norry (1979) are derived from volcanic phenocryst/ matrix pairs and thus may not be strictly applicable in a metamorphic environment, the difference in magnitude is considered to be significant. It is thus postulated that the formation of charnockite by the breakdown of garnet and biotite to hypersthene resulted in the depletion of Rb, Th, Nb and Y. The increased Sr and Ba contents in the charnockitic rocks may suggest the introduction of these mobile elements from the intruding Port Edward enderbite, and possibly the pegmatite, since the Port Edward enderbite contains anomalously high Sr and Ba contents (200-1900 ppm and 4 8 3 - 2 2 9 7 ppm, respectively; Grantham, 1983). At Nicholson's Point the apparent increase in S in the charnockite also suggests S metasomatism. This process has resulted in a relative increase in the Fe-content of the sulphide phase with pyrite being replaced by pyrrhotite. Hansen e t a L (1987) describe two charnockitep r o d u c i n g r e a c t i o n s , one i n v o l v i n g the breakdown of calcic amphibole, biotite and quartz and the other involving the breakdown of biotite and quartz + garnet. The reaction at Nicholson's Point, like the latter reaction of Hansen e t al. (1987); involves garnet, biotite and quartz. With their second charnockitisation reaction, Hansen e t al. (1987) recognise small increases in SiO 2, Na20 and possibly slight losses of Fe203 and MgO. Hansen e t a/. (1987) also recognise depletions in Rb and Y, but state that no consistent loss of CaO was recorded. The charnockite producing reaction (involving biotite and garnet) recognised by Hansen e t al. is: Bt + 0.99Grt + 2.37Qtz = 0.21An + 0.89Kfs + 2.53Opx +0.3111m + (H2 +O2). (1) Significantly this reaction produces ilmenite with the Ti presumably being derived largely from b i o t i t e . In N i c h o l s o n ' s Point granite, the charnockitic aureoles are characterised by the presence of pyrrhotite and ilmenite. A possible reaction to explain the various mineral and chemical changes recognised in the aureoles is shown in equation (2). The lack of variation in the 8180 values of the Nicholson's Point granite might suggest that large volumes of external fluids have not played a significant role in the generation of the charnockitic aureoles.

Journal o f African Earth Sciences 473

G. H. GRANTHAM et aL

2 K + + H S + (K2Fe5Ti)(AISi)O2o(OH)4 + Fe3AI2Si3012 + FeS2 + 8SiO 2 --~ 4KAISi308 + 5FeSiO 3 + FeTiO 3 + 3FeS + H20 (2) 2K - + HS + titaniferous biotite + garnet + pyrite + quartz-~ orthoclase + orthopyroxene + ilmenite + pyrrhotite + water

Table

2. M a j o r e l e m e n t , t r a c e e l e m e n t a n d o x y g e n

isotope data from orthopyr0xene

bearing and

garnetiferous granite

I<

CHARNOCKITE

>

<

G A R N E T BIOTITE G R A N I T E

GG82

GG70

GG76

GG99

GG95

NPG

GG17

GG94

GG19

GG93

GG91 GG98 GG69

SiO 2

72,08

74.31

72.15

72.17

72,63

73.21

73.38

73.63

71.86

73.95

70.35

AI20 ~

14.15

Fe203* 0.34

72.45

72.61

13.57

14.26

13.93

13.73

13.55

13.66

13.90

14.31

13.54

14.81

12.76

14.0

0.23

0.26

0.28

0.28

0.65

0.27

0.28

0.31

0.34

0.31

0.3

0.26

FeO

2.78

1.84

2.07

2.28

2.23

1.66

2.22

2.29

2.49

2.03

2.73

2.47

2.13

MnO

0.09

0.03

0.03

0.02

0.02

0.02

0.03

0.03

0.03

0.03

0.03

0.04

0.04

MgO

0.52

0.55

0.61

0.45

0.4

0.31

0.4

0.45

0.46

0.32

0,54

0.47

0.48

CaO

1.84

1.93

1.92

1.75

1.61

1.58

1.50

1.46

1.46

1.40

1.80

1,58

1.42

Na20

2.78

2.59

2.61

2.80

2.61

2.32

2.89

2.64

2.68

2.77

2.64

2.71

2.81

K20

5.58

4.96

5.53

5.88

5.90

6.13

5.53

5.36

5.62

5.53

6.19

5.66

5.45

TiO 2

0.47

0.51

0.56

0.41

0.37

0.35

0.34

0.35

0.38

0.34

0.45

0.40

0.39

P20~

0.12

0.05

0.11

0.16

0.13

0.12

0.12

0.12

0.13

0.11

0.14

0.13

0.12

Total

100.75 100.57 100.11 100.13 99.91

99.90

100.34 100.51 99.73

100.26 99.99

98.97

99.73

Rb

104

120

127

185

153

189

228

220

223

180

173

201

190

Sr

208

554

363

177

169

139

149

151

176

157

209

167

203

Th

nd

nd

nd

6.9

12.5

10.6

13.5

12.5

14.1

11.7

17.3

14.8

12.7

Zr

324

344

308

261

262

198

241

243

263

229

3()0

274

276

Nb

9.7

10.2

10.4

15.1

10.9

10

16.5

17

16.9

14.4

16.7

16.2

16.6

Y

19.6

12.9

17

44.5

23.7

37

70

67

60

44

48

57

60

Ba

660

1136

1096

517

457

826

395

412

515

438

590

ND

477

Ga

22

21

21

23

22

23

23

23

23

22

21

23

21

S

509

195

313

251

199

301

158

162

N.D.

241

241

144

219

8180

11.6

10.6

10.4

10.7

11.1

10.9

ND

10.8

10.5

12.2

10.5

10.8

10.8

La

59.5

45.6

53.5

Ce

95.2

78.3

102.0

70.1

78.32

126.01

160-

Pr

10.5

11.0

14.5

16.15

18.36

Nd

46.7

42.0

54.2

64.29

73.15

Sm

10.2

8.5

12.0

9.5

10.9

Eu

2.0

1.81

1.51

1.84

19 5

Gd

7.84

6.85

9.6

9.18

10.8

Dy

6.61

6.3

10.44

7.72

9.85

Ho

1.22

1.41

2.36

1.6

2.07

Er

3.52

4.25

7.12

4.67

6.16

Yb

2.45

3.99

6.41

4.15

5.42

*indicates Fe203content after Le Maitre (1 976). The REE analyses were conducted at the University of Stellenbosch. N.D. = not determined. * *represents values interpolated from adjacent values.

474 Journal or African Earth Sciences

Geology of the Nicholson's Point granite, Natal Metamorphic Province, South Africa

T=

[]

[]

[] D D

E Z

I

I

I I

I

I I 0

(uJdd) A

E

I I

~0

I

I

==

=

,

I

,~

I

I

~

0

I

==

o

(uudd) JZ

C5~

0

o

-

(~

~

E ~. ~L

~

o z, v '

v

<

o

>-

c~

EJD []

[]

[]

•

•

E

[] I

i T-

(uJdd) qN 0

~

Journal of African Earth Sciences 475

G. H. G R A N T H A M et al.

1000 - m

'

i

GG99

I

A

E Q.

!

Q.

NPG

"

G) imm

GG17

I,,.

"r0.

100

+

0 .C

tO

I

GG93

I

z/'\

0

n~

GG98

m

10

La Ce Pr Nd Sm Eu

d

Dy He Er

Yb

Figure 8. Chondrite normalised rare earth e l e m e n t distributions for five samples o f the Nicholson "s Point granite. The charnockite samples are s h o w n as filled symbols and the g a r n e t biotite granite as open symbols.

10 -DA

E O.

GG99

L_

NPG

(9 6~ > <

D

.~_

]E

B-

T

I

..~

m

L

O

i

.L_

--

+

La Ce Pr N d S m l~u Gd

.

.

Dy Ho Er

÷

Yb

Figure 9. Rare earth element distributions for the charnockite samples normalised by the average REE content

the three samples of garnedferous granite,

q76

Journal

of

African Earth Sciences

of

Geology of the Nicholson "s Point granite, Natal Metamorphic Province, South Africa

REE chemistry Six samples have been analysed for the REEs using an ICP m e t h o d at the U n i v e r s i t y of Stellenbosch. The samples include five whole rock analyses and an analysis of an inclusionfree garnet separate from sample GG17. Three samples were taken from the garnetiferous granite (GG17, GG98 and GG93), a type 1 aureole (GG99) and a type 2 aureole (sample NPG). The whole rock REE data are presented in a chondrite normalised figure (Fig. 8). The whole rock REE analyses are presented in Table 2. Normalising values for the chondrite were taken from Evensen e t al. (1978). A notable difference between the garnetiferous and charnockitic rocks is that the hypersthene bearing granites have slightly lower HREE contents and would thus appear to be slightly depleted in the HREEs. The garnet bearing samples and charnockitic sample NPG have flat, r e l a t i v e l y u n d e p l e t e d HREE d i s t r i b u t i o n s , whereas the charnockitic sample GG99 shows greater HREE reduction (Fig. 8). All five samples have negative Eu anomalies, although the anomalies in the hypersthene bearing granite are less p r o n o u n c e d t h a n t h o s e f r o m t h e garnetiferous granite. The reduced negative anomaly in the charnockites is more a function of their depleted HREEs than lower Eu contents in the garnetiferous granite. In fact it may be seen that Eu values in all the samples are similar (Table 2) with the charnockite sample GG99 having marginally higher contents. From the chondrite normalised profiles in Fig. 8 it may also be seen that the LREE distributions of the samples are similar. In Fig. 9, the REE distributions for the two charnockitic samples normalised by the average values for the three garnetiferous samples are shown. From Fig. 9 it may be seen that the charnockitic samples are comparatively slightly depleted in the LREEs, s h o w slight or no enrichment of Eu, with sample GG99 showing strong depletions of the HREEs, whereas sample NPG shows a lesser degree of depletion of the HREEs. The flat HREE distributions in the garnet bearing samples may support the interpretation of Mclver (1963, 1966) and Grantham (1983) that the inclusion-free garnet was a stable primary igneous phase in the Nicholson's Point granite. Pride and Muecke (1980) have shown that the HREEs are strongly partitioned into garnet relative to magma, which is supported by the REE data for garnet separated from sample GG17 (Table 3). If the garnet is assumed to be

of igneous origin and crystallized from a magma of composition the same as GG17, then pseudopartition coefficients may be calculated by dividing the REE content in the garnet by the whole rock content. The coefficients calculated in this manner are shown in Table 3, with average values for garnet from Henderson (1982, p93) shown for comparison. From Table 3 it may be seen that the values are almost identical and that the garnet contains significant amounts of HREEs. Considering the high HREE content in the garnet and that garnet comprises only - 3 vol% of the garnetiferous granite (Table 1), it can be concluded that almost all the HREEs in the garnetiferous granite resided in the garnet. This may support the interpretation that the euhedral garnet is of magmatic origin and was not derived by an earlier in s i t u metamorphic breakdown of biotite, because if biotite had crystallized as the primary igneous phase it would not have accomodated the HREE to the same extent that garnet did. Nash and Crecraft (1985) and Mahood and Hildreth (1983) have shown that the LREEs are strongly fractionated into b i o t i t e (K~'s of - 5 to - 2 for La to Sm, respectively) and to a lesser degree for the HREEs (K~'s o f - 2 t o - 1 for Gd to Yb). If garnet replaced biotite metamorphically, it would require a high mobility of the HREEs from a source other than the garnet to provide the secondary garnet with the same HREE content. The d e p l e t e d HREE d i s t r i b u t i o n s of the charnockitic Nicholson's Point granite relative to the garnet bearing granite suggests that, associated with the transformation of garnetiferous to hypersthene bearing granite, there has been a loss of HREEs. The REE pattern for the garnet separate indicates a high HREE content for the garnet (Table 3). The breakdown may possibly be compared to a situation where a granite with a composition similar to the a v e r a g e of the 3 g a r n e t i f e r o u s s a m p l e s fractionates garnet only and therefore depletes the magma. Figure 10 shows a modelled REE pattern for granite with the REE composition the same as the average of the three garnet bearing samples (GG17, GG93 and GG98) from which 2% garnet has been removed by equilibrium crystallization using the above-derived partition coefficients and assuming that the remaining 98% fractionate had partition coefficients of 1. From Fig. 10 it may be seen that the HREE depletions may be modelled with a degree of success in this manner, but that this model does not explain the LREE depletions. Lemarchand e t al. (1987) present REE Kd's for biotite and show

J o u r n a l o f A f r i c a n Earth Sciences 4 7 7

1000 []

20 V

40 0

60 7<

100

80 100

10

i

i

t

I

e

I

E

I

I

La Ce Pr Nd Sm Eu Gd

t

t

t

Dy Ho Er

I

Yb

Figure 10. Fractionation model showing the effect o f garnet fractionation on the breakdown o f garnet from a composit/on similar to N/cholson "s Point granite. The values o f 20 to I 0 0 in the legend reflect the amount o f melt remaining.

10

-BB-

GG991GG98

0.1

I

I

I

t m

t

I

I

La Ce Pr Nd Sm Eu Gd

t

~

~

:

Dy Ho Er

!

I

Yb

Figure 11. The REE content o f sample GG99 (charnockite) normalised by sample GG98 (nearby garnet biotite gramteL

Geology o f the Nicholson "s Point granite, Natal Metamorphic Province, South Africa

Di 5 kb mole %

~

~

~

-- ~

--500 o_ --

I

4. ! I 25

100 /

w vv

En

25

,~0

Figure 12. Pyroxene compositions from the Port Edward enderbite plotted on the pyroxene quadrilateral for 5 kb from Lindsley (1983J.

#

o : $

#

Qtz 20 7O / X Ab/An= ® PH20=Road (Wtnkler 1974)\ \/ \ /

Ab/An = ® PH2O=Road / (1"utile and Bowen (1956) 60/ o Ab/An=2.9 PH20=Road / (Wlnkler 1974) / -- Ab/An= = I~120=0 / ( 1 ~ , 1969) 50/ $ Ab/An= aH20=0.3 / Ebadi & Johannes ( 1 9 9 1 ) /

40 /

ao/ ~/

/

4+

X Gametiferous Granite '\

"~n \ o~ •

A Average Nicholson's Point Granite

40

zO2 5O

o.~- _ ~

',//

lo_.~ J

!5

x ,

'\ Ab

50

40

30

20

70 Or

Figure 13. The normative composition o f Nicholson "s Point granite shown in comparison with experimental data for varying H20 contents, melt compositions and fithostatic pressures. The sources o f the various data are shown in the figure.

Journal of African

Earth Sciences 479

G. H. GRANTHAM et aL

that they vary from values < 1 to > 1. Thus it is possible that the breakdown of biotite has also contributed to REE depletion. Hansen e t al. (1987) compared the REE chemistry of pairs of samples taken in close proximity to one another. In the present study, samples GG99 (charnockitic) and GG98 (garnet bearing) may be viewed as such a pair having been taken from within a few metres of each other (see Fig. 2 for sample localities). In Fig. 1 1 the REE contents of the charnockite sample GG99 normalised by sample GG98 (garnetiferous granite) are shown and it may be seen that both the LREEs and HREEs are depleted, whereas the Eu content is virtually unchanged. Factors which have been shown to influence Eu are: i) the feldspar varieties present at the time of crystallization (Schnetzler and Philpotts, 1970); and ii) the oxygen fugacity (Philpotts, 1 970; Drake, 1975; Drake and Weill, 1975). Only relatively minor and subtle differences exist between the feldspars of the garnetiferous and hypersthene bearing granites with relative proportions varying little. The hypersthene bearing g r a n i t e a p p e a r s to have h i g h e r proportions of plagloclase and m y r m e k i t e , . although very little difference is obvious in the major element chemistry apart from possibly slightly higher CaO and TiO 2 contents in the charnockite. Slightly higher Eu contents in the charnockite may be related to similar higher Sr contents in the charnockite since the chemical behaviour of Sr and Eu are very similar (Drake,

1975). The relative enrichment of Ba, Sr and, to a lesser extent, Eu and Ca in the charnockitic aureoles suggests the introduction of Ca, Eu and Sr into the granite from the Port Edward enderbite. The Port Edward enderbite has higher contents of these elements (Grantham, 1983) and thus would be a potential source for these elements. The type 2 aureole adjacent to the pegmatite s h o w s similar variations, but is unrelated to the Port Edward enderbite. Whilst the p e g m a t i t e s may be e x p e c t e d to have reasonably high Ba, Eu and Sr contents, they are unlikely sources for Ca. The LILE chemistry of the charnockite, notably Rb and Sr variations, has implications for the radiogenic isotope studies of the Nicholson's Point granite. This study demonstrates significant differences between the Rb/Sr contents in the charnockitised Nicholson's Point granite and the garnetiferous Nicholson's Point granite, the differences resulting from dehydration of biotite and possibly the open system introduction of Sr and a loss of Rb. The REE mobility likewise has similar implications for Sm-Nd dating techniques. Black (1988) has questioned the postulated immunity from resetting of the Sm-Nd isotopic system and has suggested that recrystallization provides a means of mobilizing the REEs. The REE mobility in the Nicholson's Point granite, r e s u l t i n g f r o m the r e a c t i o n of g a r n e t to orthopyroxene and p o s s i b l y b i o t i t e to orthopyroxene, would have detrimental implications for Sm-Nd dating in the Margate Terrane of the NMP.

Table 3. REE data for the garnet separate from GG17 (column 1) with pseudo-partition coefficients calculated from separate/whole rock data from GG17 compared with average Kd data from Henderson (1 982, p92)

GT (ppm)

Kd gt GG17

Kd gt (Henderson,1982)

La

19.2

0.36

0.39

Ce

39.2

0.38

0.62

Nd

28.8

0.53

0.63

Sm

17.6

1.47

2.2

Eu

0.35

0.7

0.23

Gd

73.8

7.69

7.7

Dy

303

29.02

29

Ho

74.9

31.7

28

Er

257

36.09

43

Yb

254

39.62

43

4 8 0 Journal o f A f r i c a n Earth Sciences

Geology

of

the

Nicholson

"s P o i n t

granite,

Natal

Metamorphic

Province,

Africa

.... y ,

t999 1000 A

South

types

1000

E

"E lb.

CL EL v

100

10

1 1000

10(;00

lC0000

1000000

Ga*1000/AI

v~ ,

, , ~ ,,,,I

....... 10 Y + Nb

I 100 (ppm)

........

ORG., 1000 1999

Figure 14. Zr versus Ga*IOOOO/AI for the garnet biotite Nicholson's Point granite showing the A-type nature after Whalen et al. (1987).

Figure 15. Rb versus Nb + Y after Pearce et al. (1984) for the Nicholson's Point granite. The charnockite samples are shown as filled symbols and the garnet biotite granite as open symbols. The figure shows that the gametiferous granites plot in the within plate granite field, whereas the charnockitic rocks plot within the volcanic arc granite field.

Origin of the aureoles

dehydration reactions similar to that described above. A fluid inclusion s t u d y currently in progress will explore this possibility.

T w o p y r o x e n e t h e r m o m e t r y after Lindsley (1981) on the intruding Port Edward enderbite yields temperatures of between 550°C and 1050°C (Fig. 12). The lower values are from grain rims, whereas the high values are from grain cores. The lower values are interpeted as cooling closure temperatures, whereas the core temperatures are interpreted as crystallisation temperatures. The high temperatures for the Port Edward enderbite suggests that the type 1 charnockitic aureoles in the Nicholson's Point granite a s s o c i a t e d w i t h the Port Edward enderbite could be i n t e r p r e t e d as thermal aureoles. In addition, the higher S content in the aureole indicates S metasomatism. The formation of pyrrhotite in the aureoles is also consistent with higher temperatures because pyrite breaks down incongruently at - 7 3 0 ° C to pyrrhotite + S 2 at low pressures (Barton and Skinner, 1967). At higher pressures ( - 6 kb) this reaction occurs at higher temperatures of the order of > 8 0 0 ° C (Toulmin and Barton, 1964). The type 2 charnockitic aureoles developed adjacent to the pegmatite dykes are unlikely to have resulted from increased temperatures, particularly when the aureole/pegmatite width ratio is taken into consideration. The nature of the type 2 aureoles suggest the involvement of an as yet unidentified anhydrous fluid, which would have destabilised the hydrous biotite causing the same reaction of biotite + quartz--> orthopyroxene + K-feldspar + H20, which was temperature driven in the type 1 aureoles. A possible cause for the type 2 aureoles is provided by Hansen e t al. (1995) and Aranovich and Newton (1995), who suggest that the H20 can be significantly reduced in brine solutions such that they may be the driving force behind

Origin of Nicholson's Point granite A comparison of the normative feldspar and quartz composition of garnetiferous Nicholson's Point granite with minimum melt data show that the N i c h o l s o n ' s Point granite samples plot between the locus of samples with aH20
J o u r n a l o f A f r i c a n Earth Sciences 4 8 t

G. H. GRANTHAM et al.

Nb

//

\

island-arc complexes collided with and were accreted onto the Kaapvaal Craton approximately - 1 2 0 0 Ma ago. The Nicholson's Point granite would then have been derived by partial melting of such material post-D1/M ~ between 1200 and 1055 Ma ago. \

/ Y

Ga*3

Figure 16. Nb- Y-3Ga diagram for the Nicholson "s Point granite after Eby (1992) showing the fields for the A 2 and A ~granite types. The chamockite samples are shown as filled symbols and the garnet biotite granite as open symbols.

Nicholson's Point granite shows characteristics typical of the A 2 granites of Eby (1992) (Fig. 16), A-type granites (Eby, 1990) and rapakivi granites (R~m6 and Hapaala, 1995) in general. The 87Sr/86Sr ratio of the Nicholson's Point granite is 0.7063 + 6 (Eglington et al., 1986), consistent with the involvement of older crustal material in its source. The 8180 values for the charnockitic and garnetiferous granites are relatively enriched showing values mostly between 10.4 and 11.5 ppm. Javoy and Weiss (1987) showed that anorogenic A-type granites have 8180 values in whole rock samples which vary between - - 3 to - + 9 with most samples falling between - + 2 to + 6 . The e n r i c h e d 5180 v a l u e s in the Nicholson's Point granite are therefore clearly higher than those ascribed to the anorogenic granites studied by Javoy and Weiss (1987). O'Neill e t a l . (1 977) showed that S-type granitic suites with significant crustal components in their sources have 8180 values > 1 0 . The relatively enriched 8180 values in the Nicholson's Point granite would also support a degree of crustal i n v o v e m e n t in the s o u r c e region of the Nicholson's Point granite and might be expected from a source similar to that described for the A 2 t y p e g r a n i t e s , w h i c h are s o u r c e d in continental crust or under-plated crust that has been through a cycle of island-arc accretion as has been suggested for the NMP by Jacobs and Thomas (1994). These above described characteristics are consistent with the general setting of the NMP suggested by various workers (Jacobs and Thomas, 1994; Eglington et al., 1989) who have suggested that it represents an accretionary terrane where calc-alkaline

482 Journalof African Earth Sciences

ACKNOWLEDGEMENTS A major portion of this study formed part of an MSc thesis by G. H. G at the University of Natal, Pietermaritzburg. The assistance of the staff of the Department of Geology, University of Natal is gratefully acknowledged as well as financial assistance from the University of Natal in the form of research grants (A.R.A.) and a graduate assistantship (G.H.G.). The costs of probe work were defrayed against a research grant from the University of Pretoria which is also gratefully acknowledged. Robert C. Newton is thanked for a particularly constructive review. This paper is a contribution to IGCP projects 348 and 368.

REFERENCES Andersson, U. B., Larsson, L. and Wikstr0m, A. 1992. Charnockites, pyroxene granulites, and garnet-cordierite gneisses at a boundary between Early, Sveeofennian rocks and Sm~land-V&rmland granitoids, Karlskoga, southern Sweden. Geologiska FSreningens I Stockholm FSrhandlingar 114, 1-15. Aranovich, L. Ya. and Newton, R. C. 1995. Experimental determination of H20 activity in concentrated aqueous sodium chloride solutions at high P and T. Geological Society America Abstracts Programs 27, A-155. Barton, P. and Skinner, B. J. 1 9 6 7 . Sulfide mineral stabilities. In: Geochemistry o f Hydrothermal Ore Deposits (Edited by Barnes, H. L.) p p 2 3 6 - 3 3 3 . Holt, Rinehart and Winston, Inc., New York. Black, L. P. 1988. Isotopic resetting of U-Pb zircon and Rb-Sr and Sm-Nd whole-rock systems in Enderby Land, Antarctica: implications for the interpretation of isotopic data from p o l y m e t a m o r p h i c and m u l t i p l y d e f o r m e d terrains. Precambrian Research 38, 355-366. Collerson, K. D. and Fryer, B. J. 1979. The role of fluids in the formation of and subsequent development of early continental crust. Contributions Mineralogy Petrology 67, 151-167. Drake, M. J. 1975. The oxidation state of europium as an indicator of oxygen fugacity. Geochimica Cosmochimica Acta 39, 55-64. Drake, M. J. and Weill, D. F. 1975. Partition of Sr, Ba, Ca, Y, Eu2+ , Eu 3+ and other REE between plagioclase feldspar and magmatic liquid: an experimental study. Geochimica Cosmochimica Acta 59, 689-712. Ebadi, A. and Johannes, W. 1991. Beginning of melting and composition of first melts in the system Qz-Ab-OrH20-CO 2. Contributions Mineralogy Petrology 106, 286295. Eby, G. N. 1990. The A-type granitoids: A review of their occurrence and chemical characteristics and speculations on their petrogenesis. Lithos 26, 115-134. Eby, G. N. 1992. Chemical subdivision of the A-type granitoids: P e t r o g e n e t i c and t e c t o n i c i m p l i c a t i o n s . Geology 20, 641-644.

Geology of the Nicholson's Point granite, Natal Metamorphic Province, South Africa

Eglington, B. M., Harmer, R. E. and Kerr, A. 1 9 8 6 . P e t r o g r a p h i c , Rb-Sr i s o t o p e and g e o c h e m i c a l characteristics of intrusive granitoids from the Port Edward Port Shepstone area, Natal. Transactions Geological Society South Africa 89, 199-214. Eglington, B. M., Harmer, R. E. and Kerr, A. 1989. Isotope and Geochemical Constraints on Proterozoic Crustal Evolution in south-eastern Africa. Precambrian Research 45, 159-174. Evensen, N. M., Hamilton, P. J. and Onions, R. K. 1978. Rare e a r t h a b u n d a n c e s in c h o n d r i t i c m e t e o r i t e s . Geochimica Cosmochimica Acta 42, 1199-1212. Frost, B. R. and Frost, C. D. 1987. CO 2, melts and granulite metamorphism. Nature 327, 5 0 3 - 5 0 6 Frost, B. R., Frost, C. D. and T o u r e t , J. L. R. 1 9 8 9 . M a g m a s as a c o u r c e of heat and fluids in g r a n u l i t e m e t a m o r p h i s m . In: F l u i d M o v e m e n t s - E l e m e n t transport and the c o m p o s i t i o n o f the Deep Crust ( E d i t e d by B r i d g e w a t e r , D.) p p 1 - 1 8 . Kluwer A c a d e m i c Publishers. Fyfe, W. S. 1973. The granulite facies, partial melting and the Archean crust. Philosophical Transactions Royal Society London 273A, 4 5 7 - 4 6 1 . Grantham, G. H. 1983. The tectonic, metamorphic and intrusive history of t h e Natal Mobile belt b e t w e e n Glenmore and Port Edward, Natal. M.Sc. thesis 243p. University of Natal, Pietermaritzburg. Green, D. H. and Ringwood, A. E. 1967. An experimental investigation of the gabbro to eclogite transformation and its p e t r o l o g i c a l i m p l i c a t i o n s . Geochimica Cosmochimica Acta 32, 767-833. Green. T. H. 1976. Experimental generation of cordieriteor g a r n e t - b e a r i n g g r a m t i c l i q u i d s f r o m a p e l i t i c composition. Geology 3, 85-88. Hansen, E. C., Janardhan, A. S., Newton, R. C., Prame, W. K. B. N. and Ravindra Kumar, G. R. 1987. Arrested charnockite formation in southern India and Sri Lanka. Contributions Mineralogy Petrology 96, 225-244. Hansen, E. C., N e w t o n , R. C., Janardhan, A. S. and Lindenburg, S. 1995. Differentiation of late Archean Crust in the eastern Dharwar Craton, Krishnagiri-Salem Area, South India. Journal Geology 103, 629-651. Harrison, T. N. 1988. Magmatic garnets in the Cairngorm granite. Mineralogical Magazine 52, 6 5 9 - 6 6 7 Heier, K. S. 1973. Geochemistry of granulite facies rocks and problems of their origin. Philosophical Transactions Royal Society London 273A, 4 2 9 - 4 4 2 . Henderson, P. 1 9 8 2 . Inorganic g e o c h e m i s t r y . 3 5 3 p . Pergamon, Oxford. Hubbard, F. H. and Whitely, J. E. 1978. Rapakivi granite, anorthosite and charnockitic plutonism. Nature 271, 439-440. Hubbard, F. H. and Whitely, J. E. 1979. REE in charnockite and associated rocks, S o u t h w e s t Sweden. Lithos 12, 1-11. Jacobs, J. and Thomas, R. J. 1994. Oblique collision at about 1.1Ga along the southern Margin of the Kaapvaal continent, south-east Africa. Geologische Runschau 83, 322-333. Javoy, M. and Weis, D. 1987. Oxygen isotopic composition of alkaline anorogenic granites as a clue to their origin: the problem of crustal origin. Earth Planetary Science Letters 84, 4 1 5 - 4 2 2 . Kilpatrick, J. A. and Ellis, D. J. 1992. C-type magmas: igneous charnockites and their extrusive equivalents. Transactions Royal Society Edinburgh: Earth Sciences 83, 155-164. Lamb, R. C., Smalley, P. C. and Field, D. 1986. P-T conditions for the Arendal granlites, southern Norway: implications for the roles of P, T and CO 2 in deep crustal LILE-depletion. Journal Metamorphic Geology 4, 143-160. -

Lambert, I. B. and Heier, K. S. 1967. The vertical distribution of uranium, thorium and potassium in the continental crust. Geochimica Cosmochimica Acta 31, 377-390. Le Maitre, R. W. 1976. Some problems in the projection of chemical data into mineralogical classifications. Contributions Mineralogy Petrology 56, 181-189. Le Maitre, R. W. 1989. A classification of Igneous Rocks and Glossary of Terms. Blackwell Scientific Publications. Oxford. Lemarchand, F., Villemant, B. and Calas, G. 1987. Trace element d i s t r i b u t i o n c o e f f i c i e n t s in alkaline ser~es. Geochimica Cosmochimica Acta 51, 1071-1081. Lindsley, D. H. 1983. Pyroxene thermometry. American Mineralogist 68, 4 7 7 - 4 9 3 . Luth, W. C. 1967. Studies in the system KAISiO4-Mg2SiO 4SiO2-H20 I: Inferred phase relations and p e t r o l o g y applications. Journal Petrology 8, 372-416. Luth, W. C. 1969. The system NaAISi3Os-SiO 2 and KAISi308SiO 2 to 20kb and the relationship between H20 content, pH20 and Ptotal in granitic rocks. American Journal of Science 267, 325-341. Mahood, G. and Hidreth, W. 1983. Large partition coefficients for trace elements in high silica rhyolites. Geochimica Cosmochimica Acta 47, 11-30. Martignole, J. and Schrijver, K. 1973. Effect of rock composition on appearence of garnet in Anorthosite Charnockite Suites. Canadian Journal Earth Sciences 1O, 1132-1139. Mclver, J. R. 1963. A contribution to the Precambrian Geology of southern Natal. Ph.D. thesis 203p. University of the Witwatersrand, Johannesburg, South Africa. Mclver, J. R. 1966. Orthopyroxene-bearing granitic rock from southern Natal. Transactions Geological Society South Africa 66, 99-117. Nash, W. P. and Crecraft, H. R. 1985. Partition coefficients for trace e l e m e n t s in silicic m a g m a s . Geochimica Cosmochima Acta 49, 2 3 0 9 - 2 3 2 2 . Newton, R. C. and Hansen, E. C. 1983. The origin of Proterozoic and late Archean charnockites - evidence from field relations and experimental petrology. Geological Society America Memoir 161, 167-177. O'Neill, J. R., Shaw, S. E. and Flood, R. H. 1977. Oxygen and hydrogen isotope compositions as indicators of granite gneiss in the New England Batholith, Australia. Contributions Mineralogy Petrology 62, 313-328. Pearce, J. A. and Norry, M. J. 1979. Petrogenetic implications of Ti, Zr, Y and Nb variations in volcanic rocks. Contributions Mineralogy Petrology 69, 33-47. Pearce, J. A., Harris, N. B. W. and Tindle, A. G. 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal Petrology 25, 956-983. Petersen, J. S. 1980. The zoned Kleivan granite - an end member of the anorthosite suite in southwest Norway. Lithos 13, 79-95. Philpotts, J. A. 1970. Redox estimation from a calculation of Eu 2+ and Eu 3÷ concentrations in natural phases. Earth Planetary Science Letters 9, 257-268. Pride, C. and Muecke, G. K. 1980. Rare earth element distributions among coexisting granulite facies minerals, S c o u r i a n C o m p l e x , N.W. S c o t l a n d . C o n t r i b u t i o n s Mineralogy Petrology 73, 403-412. R~m6, O. T. and Haapala, I. 1995. One hundred years of Rapakivi Granite. Mineralogy Petrology 52, 129-185. Schnetzler, C. C. and Philpotts, J. A. 1970. Partition coefficients of rare-earth elements b e t w e e n igneous matrix material and rock-forming mineral phenocrysts, I1. Geochimica Cosmochimica Acta 34, 331-340. Srikantappa, C., Raith, M., and Touret, J. L. R. 1992. Synmetamorphic High-Density Carbonic Fluids in the lower crust: Evidence from the Nilgiri Granulites, southern India. Journal Petrology 33, 733-760.

J o u r n a l o f A f r i c a n Earth Sciences 4 8 3

G. H. GRANTHAM e[ al.

Stahle, H. J., Raith, M., Hoernes, S. and Delfs, A. 1987. Element mobility during incipient granulite formation at Kabbaldurga, southern India. Journal Petrology 28, 803834. Stern, R. J. and Dawoud, A. S. 1991. Late Precambrian (740Ma) Charnockite, Enderbite, and Granite from Jebel Moya, Sudan: A link b e t w e e n the Mozambique Belt and the Arabian-Nubian Shield? Journal Geology 99, 648-659. Talbot, C. J. and Grantham, G. H. 1987. The Proterozoic deformation of deep crustal 'sills' along the south coast Natal. South African Journal Geology 90, 520-538. Tarney, J., Skinner, A. C. and Sheraton, J. W. 1972. A geochemical comparison of major Archean gneiss units from Northwest Scotland and east Greenland. Procedings of the X X l V International Geological Congress, Montreal 1, 162-174. Thomas, R. J. 1989. A tale of t w o tectonic terranes. South African Journal Geology 92, 306-321,

4 8 4 Journal o f African Earth Sciences

Thomas, R. J., Walraven, F. J. and Eglington, B. M. 1995. Zircon evaporation geochronology of granitoids from southern Natal. Geocongress "95 abstracts, RAU, Johannesburg 1, 418-421. Toulmin, P. and Barton, P. B. 1964. A thermodynamic study of pyrite and pyrrhotite. Geochimica Cosmochimica Acta 28, 1019-1037. Tuttle, O. F. and Bowen, N. C. 1958. The origin of granite in the light of e x p e r i m e n t a l studies in the system NaAISi3Os-KAISi3Os-SiO 2 -H20. G e o l o g i c a l S o c i e t y America Memoir 74, 153p. Whalen, J. B., Currie, K. L. and Chappel, B. W. 1987. Atype-granites: geochemical characteristics, discrimination and petrogenesis. Contributions Mineralogy Petrology 95, 407-419. Wickham, S. M. 1988. Evolution of the lower crust. Nature 333, 119-120. Winkler, H. G. F. 1974. Petrogenesis of Metamorphic Rocks. 334p. Springer Verlag, Amsterdam.