Precambrian Research, 25 (1984) 161--186 161 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

P E T R O C H E M I S T R Y , T E C T O N I C E V O L U T I O N A N D M E T A S O M A T I C M I N E R A L I S A T I O N S O F M O Z A M B I Q U E B E L T G R A N U L I T E S F R O M S M A L A W I A N D T E T E ( M O Z A M B I Q U E )

MARCO ACHILLE GIACOMO ANDREOLI

Nuclear Physics Research Unit, University of the Witwatersrand, 1 Jan Smuts Avenue, Johannesburg 2001 (South Africa)

ABSTRACT

Andreoli , M.A.G., 1984. Petrochemistry, tectonic evolution and metasomatic mineralisa- tions of Mozambique belt granulites from S Malawi and Tete (Mozambique). Precam- brian Res., 25: 161--186.

The granulites of southern Malawi and Tete (Mozambique) typical ly comprise pro- grade two-pyroxene ± garnet e hornblende ± biot i te parageneses. High grade metamorph- ism was imprinted on supracrustal, migmatit ic and plutonic rocks at the climax of the Kibaran orogeny (ca. 1100--850 Ma). Amphibol i te facies and transitional terranes mainly consist of migmatites, paragneisses and occasional relics of pre-Mozambiquian tonalite, gabbro, dolerite and diabase.

A geochemical investigation of Malawi granulites indicates their marked heterogeneity and a distinct similarity to high grade rocks from elsewhere in Africa. Apart from obvious metasedimentary and metaplutonic types, the bulk chemistry of southern Malawi granu- litic rocks is probably volcanic. Analytical data indicates derivation from alkali--olivine to high-alumina basalts. Pelitic and greywacke-like composit ions are also present. The granulite facies rock suite addit ionally comprises intrusive anorthosite and spatially re- lated metamorphosed monzonite , syenite and K-rich charnockit ic granite.

Toward the lat ter stages of the Mozambiquian orogenic cycle (800--600 Ma), granu- lite facies rocks near Nsanje (S Malawi) and Tete experienced shearing and metasomatism (scapolitisation) owing to a CO,--HC1 rich fluid phase. Near Tete this was responsible for widespread, low grade davidite, molybdeni te , s t ibio-niobotantal i te and other mineralisa- tions. The mineralising elements were possibly derived from a highly fractionated magma with anorthosi te affinity.

The evolution of the region is explained by a plate tectonic model, which envisages subduct ion of oceanic floor at > 1200 Ma, concluded by the collision of the Niassa Craton with an island-arc complex. As a consequence, the region between Tete and SW Malawi mainly comprises migmatised, reworked (Zambesi belt) Archaean granitoids, while typical S Malawi granulites represent the underthrusted island-arc suite. The suture zone is marked by scattered, small ophioli t ic relics, bodies of magnesian garnet--olivine ultra- mafics and a massif of eclogitic garnet--granulite.

INTRODUCTION

I n t h e t y p e a r e a o f t h e M o z a m b i q u e b e l t ( F i g . 1) , t h e r o c k s c o n s i s t p r i m a r - i l y o f P r e c a m b r i a n h i g h g r a d e a m p h i b o l i t e f a c i e s gne i s se s a n d h y p e r s t h e n e -

0301-9268/84/$03.00 © 1984 Elsevier Science Publishers B.V.

162

~5ON

I I I I 25~E ~0 = 35 = 40 •

LEGEND

Mid-Paleozoic to recent cover

I I Orogenic belts

] Archeon nuclei

- -o* j ~ Anorthosite • Cordierits- Garnet Gronulites

• Hypersthene ,' O Garnet- Ca- Pyroxene ,, ~ " Eclogite, garnet-lherzolite

, ~ - Shear Zones

_,.~ ::::::::::::::::::::::::::::::::::::::

++++÷++++ +/ ~ . . - + ~ + + + + + / , I

• . \ +/ ",, ~ +

%% !

I

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\ ~ + + + + . + + +

~-+ + + +DODOMA + + ~ . ~+ + + + + + + + - ~,~'.~, ~ + + + +++÷

+ ~ + + \

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1211 111 11112 211~;12~21;~I;II~)'+++.~HOOEStA~-+++I . ' . ' . ' . ' . ' . ' , ' . ' . ' . ' . ' . ' . ' , - ~ . J c . ' . ' . ' . ' . ' ~ + + ~ + + + + +

:::.:.:.:.:.:.:::.:.:.:.:.:::::::.:.:.:~.;~+ ÷ % ! ! + ++ ++ + + +~ ..-[. : . : .~.:- ' . .- ' .~-:.. ' . ' . ' . . . . .3 + + + + + ÷ + + ) . . . . . . . . . . . . ~ . . . ~ . . . . ~ . : . . . . . . . . . . , , . . + + ~ + + + + + . , . , , . . . . . . , . . . . . . . . ~ . . . , . , ~ , " + + + + ÷ + + + f . . . . . .

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Fig. 1. Main structural units of E Africa (after Andreoli, 1981).

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163

bearing lithotypes, akin to granodiorite, diorite, norite, anorthosite, and pyroxenite (Araujo, 1967; Oberholzer, 1968). A comparable lithological as- sociation is frequently repeated in the adjacent terranes of southern Malawi, eastern Zambia, Tanzania and Ethiopia which were affected by the Pan- African thermotectonic event (Clifford, 1974; Ramsay and Ridgeway, 1977; Figs. 1 and 2a). The Pan-African Mozambique belt is also connected to the Zambezi and Lufilian belts of Zambia, which continue beneath Tertiary to Recent deposits, into the Damara orogenic belt of Namibia (Martin and Porada, 1977). The "vestigeosyncline" character of the Mozambique belt was emphasized by Clifford (1970). He considered the region a thermally and tectonically reactivated basement, either stripped of its sedimentary cover or as one on which no supracrustal sequences had ever been developed. In the same way, Martin and Porada (1977) considered the Zambezi belt as a reconstituted Archaean basement.

LEGEND:

2-A [ : ~ Hercynian and younger orogeny

Pan African belts (600 * 100 m y)

] ~ ] Kibaran belts (1100 [ 200 rn y )

Continental crust stable since 1500 m.y

[] Southern Malawl

2 B [ ~ Mid- Palaeozoic to Recent

Amphibolite facies suite

F ~ Trans,tional rocks

Two-pyroxene granulite

Two pyroxene granulite overprinted on garnet-clinopyroxene granuhte Garnet clinopyroxene granulite

Fig. 2 (A) Generalized map of major structural units of Africa, modified after Clifford (1974). (B) Geology (simplified) of S Malawi: numbers refer to localities m e n t i o n e d in the text (after Andreoli , 1981); (*) Mg-garnet--olivine rocks.

164

Referring to S Malawi, the majority of authors considered the granulite facies rocks to be remnants of older metamorphic events, preserved as relics within corresponding downgraded amphibolites and migmatised rocks (Clifford, 1974). Most of the S Malawi lithologies were considered of sedi- mentary origin (pelites and marls) by the majority of authors (Bloomfield, 1968; Carter and Bennett, 1973). However, Bloomfield (1968), Thatcher and Walter (1968) and others identified occasional volcanic units within the supracrustal sequence. Only recently, Andreoli (1981) confirmed earlier arguments by Morel (1961) and Bloomfield (1968), that the S Malawi granu- lites developed by prograde metamorphism during the Mozambiquian event. He also indicated that the climax of this thermotectonic cycle spanned a period between ca. 1100 and 850 Ma ago and was marked by conditions ap- proaching P + 7--9 Kbars and T -+ 800--950°C over broad areas.

This study specifically deals with the evolution of a region adjacent to the type area for the Mozambique belt. The work presents part of the results of the author's Ph.D. thesis, which relates to an area extending between about 14°30 S and 16030 ' S from the eastern border of S Malawi to the Tete area of Mozambique in the west.

Unless otherwise specified in the text, reference is made to the author's unpublished Ph.D. thesis (Andreoli, 1981) as a source of petrographic, petro- logical, geochronological and analytical data.

FIELD RELATIONS AND PETROGRAPHY

Granulite facies suite

In S Malawi two main complexes of granulite facies rocks occur, one east of Lilongwe in central Malawi and the other in the Blantyre--Zomba region in the south (Fig. 2b). In both areas the prograde nature of the orthopyrox- ene isograde is recorded. West of Blantyre a transition zone between amphi- bolite and granulite facies terranes is characterised by proxene-gneisses and biotite--hornblende granulites and transgresses obvious sedimentary hori- zons (Morel, 1961; Fig. 2b). However, such a zone is apparently absent in the case of a massif of granulites at the SE of Lake Malawi, in the Namwera area (Loc. 1, Fig. 2b). Karoo sediments preserved to the west of the Mwanza fault obscure the relationships between the amphibolite facies rocks of the Tambani area and the high grade granulites and anorthosite of the Tete region (Figs. 2b and 3).

A massif of garnet-granulites and anorthosite with interbanded eclogite near Nsanje (Loc. 2, Fig. 2b) and small lenses of garnet--olivine rocks scat- tered in the Lilongwe region (Fig. 2b) suggest some tectonic contacts be- tween amphibolite and granulite facies terranes.

The orthopyroxene-bearing granulites of 8 Malawi and Tete (Mozambi- que) are greenish to dark-greenish in colour and resemble typical "charnock- ites" from elsewhere in the world. This study maintains Btoomfietd's (1968)

165

LEGEND

Mid- Paleozoic to Recent cover

~ J Precambrian sedimentary cover

Granitoid orthogneiss with scattered basic rocks

Gneisses, Schists, Amphibolites, undifferentiated with structure form-lines

Perthitic (meta-) syenites, and (meta-) granites, charnockites, with some monzonite and ultrabasic rocks

f J r FAULTS

Pyroxene 'Brown"granite

~ ] Granulites, undifferentiated, with "structure form-lines

A ~ Anorthosite and metagabbro with structure form-lines

Scapolitised anorthosite/ granulite

[ - ~ Davidite occurrences

, f / - J INTERNATIONAL BOUNDARY

Fig. 3. Generalized geology of southern Malawi and the adjacent Tete area of northwest Mozambique (after Andreoli, 1981).

subdivision between supracrustal and infracrustal granulites. The former are generally interbanded with more obvious metasediments (meta-pelites, calc- silicate rocks and rare marbles), the latter comprise plutonic, migmatitic or massive rocks.

Petrographic investigations indicate that metamorphism generally out- lasted deformation and that, in acidic rocks, partial melting processes were

166

probably active at some stage. In perthite-rich rocks, the orthopyroxene is interstitial relative to the alkali feldspar. The available samples of granulites from the Tete area are characterised by granoblastic textures, with triple- point grain boundaries comparable to those in mafic rocks of S Malawi. They are also affected in varying degrees by a combination of ductile, blastomy- lonitic deformation and microshearing, with polygonisation of older pyrox- ene and plagioclase. Associated (dipyre} scapolitisation is widespread.

The following primary parageneses are typical, and diagnostic of granulite facies metamorphism in the areas considered:

I opx (1) + cpx + plg + ilm/mt II plg + opx + cpx -+ Kfl + qtz III plg + qtz + opx + cpx-+ Kfl-+ gar IV Kfl + opx + pig + qtz

(1) opx, orthopyroxene; cpx, clinopyroxene; plg, oligoclase-andesine; qtz, quartz; Kfl, perthitic orthoclase; ilm/mt; ilmenite + magnetite; gar, garnet. In addition, hornblende and biotite occur in varying proportions; these may or may not be in equilibrium with the pyroxenes, particularly in areas transitional between the amphibolite and the granulite facies. The garnet-free parageneses are typical of both suites of rocks but paragenesis III(+ garnet) characterises banded rocks with supracrustal affinity.

Electron-microprobe analytical data indicate that the orthopyroxene is generally hypersthene. Bronzite and ferrohypersthene are less common. The clinopyroxene is mainly a Ca-rich augite or, less commonly, a diopside-salite. The pyroxenes from the Tete area granulites differ from those of S Malawi by being markedly more aluminiferous. The hypersthene contains up to 4.8% A1203 (max 2.0% near Zomba--Blantyre) and the coexisting augite contains up to 1.5% TiO~ and 7.7% A1203 (max 0.6% and 3.0%, respectively, near Zomba--Blantyre).

Amphibolite facies suite

Rocks of amphibolite facies grade typically characterise the basement suite of central Malawi, the southern region adjacent to Lake Malombe, the Kirk Range and the Mwanza area (Fig. 2b). Among lithologies found in this suite (Bloomfield, 1968) are unusual nepheline- and aegirine-gneisses, the latter being confined to central Malawi. In the region west-northwest of Blantyre, toward Mwanza, it is possible to recognise supracrustal-looking rocks, migmatites and massive orthogneisses structurally similar to some of the granulites. This fact is compatible with the prograde character of the orthopyroxene isograde and suggests that the substantially older gneissic suite, which also includes {meta-) dolerite/diabase (Andreoli, unpublished data) could be the precursor of at least certain types of more reworked lithologies (Table I).

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168

GEOCHEMISTRY

Typical granulites of S MaIawi

The granulites analysed include melanocratic, mesocratic, leucocratic and garnet-bearing types. Certain lithologies, however, such as khondalite, calc- silicate granulite and marble have been omitted since their metasedimentary

TABLE II

Average compositions of typical S Malawi granulites

Type: melanocratic mesocratic leucocratic

Column no. I 2 3 4 5 6 7 8

~a~ses (2) (7) (11) (10) (6) (9) (4) (2)

Si02 43.28 50.44 59.85

Ti02 3.39 1.75 1.05

AI203 15.26 14.91 17.11

Fe203 2.77 4.21 1.83

Fe0 12.45 8.23 4.25

Mn0 0.25 0.20 0.09

Mg0 6.32 5.80 2.75

Ca0 9.61 7.99 5.7

Na20 2.45 3.88 4.58

K20 0.90 1.09 1.66

P205 0.55 0.26 0.26

H20 0.05 0.04 0.08

H20+ 0.29 0.28 0.29

59.41

1 18

17 01

3 12

3 38

0 10

1 86

4 15

4 37

4.69

0.59

0.07

0.24

68.91 70.49 74.31 77.77

0.56 0.47 0.22 0.20

14.81 14.22 12.73 11.72

0.80 1.39 0.91 1.41

3.11 2.99 0~97 1.79

0.08 0.08 0.02 0.05

0.73 1.15 0.22 0.45

2.49 3.36 1.76 3.38

3.52 4.48 3.52 2.98

4.47 0.85 4.36 0.36

0 .12 0.11 0 . 0 5 0 .02

0.05 0.05 0.05 0.03

0.27 0.27 0.32 0.18

Total 97.55 99.08 99.5 100.17 99.92 100.91 99.44 100.32

I: Average of cols. (a) and (b),Table 20 (Andreoli,1981).

2: Average of cols. (c), (e),(f) and (g), Table 20 (Loc.cit.).

3: Average of cols. (b), (c),(g) and (j), Table 21 (Loc.cit.).

4: Average of cols. (d), (e),(g) and (h), Table 21 (Loc.cit.).

5: Average of cols. (a), (d),(f) and (h), Table 22 (Loc.cit.).

6: Average of cols. (b), (e) and (k), Table 22 (Loc.cit.).

7: col. J, Table 22 (Loc.cit.).

8: Col. i, Table 22 (Loc.cit.).

169

origin is generally undisputed. A detailed discussion of more than 70 samples of granulites {also including some transitional rocks) analysed in this s tudy for major, minor and trace elements is presented elsewhere (Andreoli, 1981), as are sampling localities and experimental and analytical procedures. Aver- age composit ions of typical granulites are listed in Table II.

Melanocratic granulites (SiO2 < 53%) are predominantly undersaturated, with 50--62% normative feldspar of relatively sodic composit ion (An28-- An68), and very little K-feldspar. In contrast, mesocratic granulites (SiO2 53--65%) were found to be quartz-, rather than olivine-normative and with a large spread of K-feldspar/sodic plagioclase ratios. In particular, the more potassic composit ions (K20/Na20 > 0.7) have a higher concentrat ion of P2Os (m = 0.59%) relative to the more sodic rocks (m = 0.26%). With the ex- ception of a small number of alkali-depleted leucocratic granulites (col. 8, Table II), most samples of quartz-rich rocks show a normative albite/quartz ratio > 1, while the albite/orthoclase ratio may reach 11. The available data portray the great composit ional range of the S Malawi granulites. The broad chemical characteristics of these rocks are bet ter defined by a K20--Na20-- MgO plot. In this diagram (Fig. 4) the analysed samples define a Y-shaped field, showing bimodal distribution of alkalies in low-Mg compositions. In contrast, Andreoli (1981) observed that a relatively narrow trend is defined by the granulites in an AFM diagram (A = K20 + Na~O; F = total Fe as FeO; M = MgO).

K20

Na20 M90

Fig. 4. Na20--K20--MgO diagram (weight%)of all two-pyroxene granulites (+); and garnet--clinopyroxene granulites (A) listed by Andreoli (1981; tables 20--22).

170

Average concentrations of rubidium, strontium, yttrium, zirconium, and niobium in the granulites are given elsewhere, although the relative distribu- tion of Rb versus K is shown in Fig. 5. This plot indicates a broad trend of decreasing K/Rb ratios from over 700 to < 400, as K varies from 1 to 4%. The S Malawi granulites appear strongly Rb depleted relative to other high grade suites from elsewhere in the world. They do however display a much steeper gradient of decreasing K/Rb ratios with increasing K concentrations.

Commenting on the distribution of Sr versus TiO2, the author (Andreoli, 1981) noted the presence of an irregular trend of increasing Sr in mesocratic types up to a maximum of 600--650 ppm (at 1% TiO:). Sr rapidly decreases in more acidic lithologies (max + 310 ppm). Most granulites contain <200 ppm, 40 ppm and 25 ppm, respectively, of Zr, Y and Nb. In contrast, certain perthite-rich (infracrustal) hypersthene granulites display significant Sr, Ti, Zr, Nb and Y enrichments.

~: o

A ""A~ '--44t o , ~ ' / /

A B g

L j ~ , / i i i , 1 5 i0 50 i00

Rb ppm

Fig. 5. Distribution of K and Rb in melanocratic (•); mesocratic (•); and leucocratic (•) orthopyroxene-bearing granulites listed by Andreoli (1981; tables 24--26), in relation to trends of: (A), island arc igneous suites (Jak~s and White, 1970); (B), lower crust granu- lite and charnockite suites (Lewis and Spooner, 1973); and (C), high-level igenous rocks (Shaw, 1968). Stars (~) are: (1), average of five rhyolites (Groome and Hall, 1974); and (2), Saipan dacite (Taylor et al., 1969).

Typical amphibolite facies and transitional rocks or s Malawi

Most chemical data relating to rocks from amphibolite facies terranes of southern and central Malawi have been summarized by Bloomfield (1968). These rocks have a distinctive corundum-normative character and higher K20/Na20 ratios relative to the granulites. This is shown in Fig. 6, where a number of points fall in the Na20-depleted region. The same figure indicates

171

that the transitional rocks usually plot in the granulite field and that they are characterised by a comparable composit ional spread. Only a few amphibolite facies and transitional rocks were analysed for trace elements. The available data shows a distinct enrichment of these rocks in Rb relative to K, with K/Rb ratios in the 400--200 range.

K20

{9 • •

Na20 MgO

Fig. 6. Na20--K~O--MgO diagram (weight%) of all amphibolite facies gneisses (e); and amphibolite to granulite facies transitional rocks (~) listed by Andreoli (1981) in tables 27 and 28, in relationship to the field (shaded) of granulites in Fig. 4.

DISCUSSION

Petrogenesis of the granulites

A major problem in high-grade metamorphic complexes is the identifica- t ion of rock types parental to the gneisses and granulites. Petrochemical data may provide clues to the nature of the original rocks only if processes such as partial melting and neosome/palaeosome separation (McCarthy, 1976) or metasomatism (Robinson and Leake, 1975) did not change the chemistry of the lithologies appreciably.

The presence of migmatitic granulites, of late crystallisate texture in acidic rocks, and the generally high thermal conditions (+800°C), strongly suggest that allochemical processes might be expected to have affected the suite in- vestigated.

Thermodynamic calculations and petrographic observations led the author

172

to conclude, however, that the detailed study of a large number of S Malawi rock analyses may provide reliable clues to the nature of the original lithol- ogy. The affinity o f the granulites to the chemistry of igneous rocks is shown by a Na20--K20--MgO diagram (Fig. 7). In an AFM diagram the granulites follow the general field of the calc-alkaline igneous rocks. The distribution of alkalis and silica amongst the granulites defines a distribution pattern in which the melanocratic granulites, part of the mesocratic, and a few leuco- cratic granulites plot in the field of alkaline olivine basalt and its differentia- tion products. However, a large proportion of mesocratic and leucocratic rocks plot in or near the calc-alkaline field of the high-alumina basalt and consanguineous magmas (Fig. 8).

K20

Na20 Mg0 Fig. 7. K20--Na~O--MgO diagram illustrating the distribution of the southern Malawi granulites (shaded area; see Fig. 4) in relation to sedimentary and igneous fields given by de La Roche (1966). Dots (e) 1--10 are average plots of: (1), alkali gabbro; (2), Ural spilite; (3), spilite; (4), and (5), andesite; (6), Fiji (low-K) rhyodacite; (7), New Zealand quartz-keratophyre; (8), monzonite; (9), alkali syenite; (t0), calc-alkali granite. Data sources are given by Andreoli (1981).

From major element data it is then very likely that the majority of S Ma- lawi rocks are derived, at least in part, from island-arc volcanics, related ulu- tonic rocks and sediments akin to greywacke.

This interpretation is supported by the occasional occurrence of: quartz-- Mn--pyroxene--spessartine rock in locality 11 (Fig. 2b); nepheline gneisses, e.g., near Ncheu (Bloomfield, 1968; Fig. 2b); aegirine gneisses/granulites in central Malawi (Carter and Bennett, I973). These rocks probably repre-

173

sent metamorphosed manganese chert, analcime-bearing ruffs or ashes (Bloomfield, 1968) and peralkali-rhyolite, respectively.

Relic ophiolitic sequences may be preserved in the Likudzi river and Chimwadzulu areas (Kirk Range, Loc. 9 and 10, Fig. 2b). Bloomfield (1968) noted the alpino-type (hartzburgitic) nature (MgO/FeO ~ 9.5) of the Chim- wadzulu serpentinised peridotite. The associated amphibolites define a tho- leiitic trend in an AFM diagram. Preliminary data for the Likudzi area (Andreoli, unpublished data) show a strict association of garnetiferous quartzite (ferruginous metachert), with zoisite-metagabbro, flaser-gabbro, amphibolite and serpentinised peridotite (see Table II); Bloomfield and Garson, 1965). Amphibolite is frequently characterised by a bright green Cr- amphibole and occasionally by corundum or small copper-sulphide mineral- isations. Amphibolite also grades in places into an almandine--diopside-- hornblende granulite with (greenschist-type) eClogite affinities (Andreoli, 1981).

12

o

I 7O 50 60

S i 0 2 (WT °/o )

Fig. 8. Alkali--silica diagram illustrating the distr ibution of the S Malawi granulites (shaded area after Andreoli , 1981) in relation to fields of (I), alkali-olivine basalt; (II), high-alumina basalt; and (III), tholeiite (Kuno, 1968). The arrow defines the trend of pelitic rocks from Japanese Palaeozoic geosynclines.

In contrast to these island arc and ophiolite suites marked by a deficiency of K, the group of perthite-rich granulites represents a later intrusive event (Bloomfield, 1968). This study indicates that the plutonic K-rich rocks (pre- viously interpreted as late-kinematic metasomatites) are spatially associated with anorthosite. This association is strongly reminiscent of the mangerite-- charnockite and anorthosite suites of other high-grade Proterozoic mobile belts elsewhere in the world (Bridgwater and Windley, 1973; Emslie, 1978).

Finally, no chemical data are yet available for the tonalitic, granitoid rocks described in the Mwanza--Blantyre area (Fig. 3). Tonalitic--granodio- ritic orthogneisses comparable to those of the Mwanza area are typical of

174

many Precambrian, mainly Archaean terranes (Tarney, 1976). For this rea- son, the Mwanza orthogneisses are perhaps genetically related to the adjacent Niassa craton in NW Mozambique (Figs. 1 and 3).

Scapolitisation and metallogenesis

The Tete area (Mozambique) The granulite facies anorthosite--metagabbro rock suite of Tete experi-

enced widespread, locally intense, secondary scapolite-metasomatism over an area of at least 800 km 2 (Fig. 3; Davidson and Bennett, 1950).

The movement of elements was associated with arrays of shear planes/ zones with widths from a few tens of microns to over 10 m; and 1600 m in length. Metasomatism is normally manifested by scapolitisation of plagio- clase and by (hornblende + phlogopite) uralitisation of ultramafic rocks. U- REE mineralisations were locally developed within the calcite + scapolite, albitite and diopsidite (+ tourmaline, phlogopite, apatite) gangues of the shear zones. The main ore mineral is davidite, frequently accompanied by ilmenite, magnetite, rutile, chalcopyrite, molybdenite, niobiotantalite, etc. (Davidson and Bennett, 1950; Coelho, 1969).

Relatively high grade P--T conditions prevailed during the mineralisation event (caused by H20-deficient, CO2--Cl-rich + P, B, F fluids) because:

1. A bronzite--phlogopite--hornblende--scapolite (dipyre) paragenesis was observed in one of the samples investigated. The bronzite coexisting with scapolite is chemically similar to a relic granulite facies orthopyroxene.

2. Metasomatic Mg-calcite reacts with quartz to yield diopside (but not tremolite nor forsterite) in pegmatoid pyroxenite bodies (Davidson and Bennett, 1950).

yfluid calcite and diopside implies T ~ 630--700°C and -~co~ > 95mo1%, if Pfluid 5 kbars (Winkler, 1974), in the CO:--H:O system. 3. If the kyanite observed in cataclastic plagioclase has developed during

the davidite-forming event, P 1> 6 kbars (Richardson et al., 1968). 4. In the mineralised shear zones (often entirely composed of metasomatic

carbonate) davidite may attain pegmatite-like dimensions (>~ 30 cm maxi- mum dimension; Davidson and Bennett, 1950).

The Nsanje area (S Malawi) The Tete scapolite-bearing metasomatites present a number of affinities

with rocks found near Nsanje in S Malawi (Loc. 2, Fig. 2b). In this region a suite of ariegitic eclogite, garnet---elinopyroxene granulite and oligoclase-- andesine anort.hosite are occasionally characterised by Na-scapolite:

(1) intergrown with megacrystic diopside (_+ plagioclase, hornblende) as nodules in blastomylonitic, retrogressed granulites,

(2) as occasional relic augens (~ 3 cm across) in the same rocks which pre- sent the above diopside-scapolite nodules,

(3) in small (< 1 mm), scattered grains either in equilibrium with plagio-

175

clase, pyroxene and garnet; or forming with hornblende kelyphitic rims at the expense of garnet; or (granoblastic) in equilibrium with secondary plagioclase and hornblende. Neither davidite nor albitite are known in the Nsanje area; however a mega- crystic diopside nodule in mylonitised granulite was found to include unusu- ally abundant inclusions of Ce-sphene, metamictic allanite and Cu-bearing pyrite. Furthermore, the mylonites are locally enriched in apatite and coarse poikiloblastic zircon with biotite inclusions (unpublished data, Andreoli).

Origin of scapolite The CO2--Cl rich fluids of Tete and Nsanje are probably of magmatic ori-

gin, since the scapolitised rocks are (to the author's knowledge) entirely con- fined within the anorthositic, metaplutonic suites of Tete and Nsanje. CO2-- HC1 (±HF, P, H20 etc.)-enriched magmatic volatiles (Bailey, 1982; Darzi and Winchester, 1982) could generate scapolite from the breakdown of primary plagioclase according to reactions like:

(1) 8 NaA1Si308 + 2 HC1 -+ 2Na4[A13 Si9 024] C1 + A12SiOs+ 5 SiO2 + H20 albite fluid marialite kyanite fluid

(2) 4 CaA12Si208 + CO2 -+ Ca4[A16 Si6 024] CO3 + A12SiOs + SiO2 anorthite fluid meionite

The phases (A1, Si oxides) produced by the above reactions were identified as follows: corundum or kyanite in kataclastic, relic plagioclase; alumina in paragasitic amphibolite and mica; silica in the quartz lodes occupying near Tete scapolitised shear zones (Davidson and Bennett, 1950).

A (indirect) mantle origin for these volatiles is supported by the affinity between the diopside (+ hornblende, phlogopite, Ti-REE phases, scapolite) pegmatoids of Tete and Nsanje, and the MARID-type parageneses described in metasomatised mantle nodules by Jones et al. (1982).

The alternative model of meta-evaporite involvement for the scapolite rocks (Appleyard and Williams, 1980) is less attractive because it requires the following exceptional "adohoc" circumstances:

(1) Addition of evaporitic material only to metaplutonic complexes de- ficient in primary volatiles despite their marked alkalic affinity (Nsanje).

(2) In the Tete area, thrusting of high grade, older anorthosite and granu- lite over lower grade U, REE-enriched evaporite.

(3) In the Nsanje area, introduction (by subduction?) of evaporitic ma- terial into a plutonic complex with mineralogical features indicating crystal- lisation near the base of the crust (Andreoli, unpublished data).

Scapolitisation and related metasomatic processes affected near Nsanje garnet-granulite, ariegitic eclogite (P ~- 13 kbars; T ~- 900°C), spinel--hartz- burgite, etc. which display a subsequent history of progressive unroofing (Andreoli, 1981).

176

e Uplift and erosxon C8. 8 5 0 - 6 5 0 m . y .

Thrust, shearing and ~ Cooling and downgrading migration of residual fluids

of granulites

÷ ÷ ÷ J \ ~

+ + * + ÷ + + + ÷ * ÷ + ~ ~ ~ ' ÷ ' + + + + ÷ + +++++÷+++~

' - ' ''" Lithospheric mantle

H:igh- p ....... chthonous bodle s ~

Isostatic readjustement of crust-lithosphere boundary

d C a . 1 0 0 0 - 9 0 0 m,y.

ns

Delamlnatlon and slnkl g n erplatlng by of lithospheric mantle asthenospheric partial melts

C Ophiolitic remnants ca.110o m . y .

Flysch-like deposits Migmat ires

+ ++÷ ÷++ ÷ ÷ ÷ + + + + * ÷ ÷ ÷ ÷ ÷ + ÷ + ÷ + . + ÷ ÷ ÷ ÷ ÷ + + + * ÷ + + + + ÷ + * + + ÷~+ . + + ÷ ÷ + * + ....... ÷÷÷:+

I n c U S waning

Hyper sthene isogr ade

b ¢a .1200 m.y.?

thrusting of ophiolites

;

÷+÷ ++***+*÷÷÷ ~ + + ~ ÷ + + ÷ + +

* ÷ ++ + + + + + ÷ + vvvvvvvvvv + ÷ + + ÷ + +

, Incipient inversion of subduction

a MId-pro te rozo ic

WEST EAST S Mal&awi island-arc

oceanic floor ophiolites Greywackes Shelf sediments

N l e % e s . C r ) t o n / . . . . ~¢~* " ~ ¢ ¢ Lurlo ? ¢ ,e ton

ASTHENOSPH~RE

177

S Malawi model

Available petrological and geochronological data are not sufficient to pre- cisely define the geological evolution of the S Malawi--Mozambique region. Figure 9a--e depicts, however, what is at present the most reliable model based on two main hypotheses: first, island arc--continent collision and, later, crystallisation of "gabbro"-oligoclasite/andesinite suite near the base of the granulitic crust (e.g., Nsanje).

Figure 9a: the low Sr87/Sr 86 initial ratio of granulites and gneisses (0.7027--0.7048; Andreoli, 1981) supports their development from an is- land arc overlying subducted, mid-Proterozoic oceanic crust. This oceanic basin perhaps formed by initial rifting of the Lurio and Niassa cratons, if Piper's (1982) hypothesis of a Proterozoic supercontinent is accepted. Dur- ing this early rifting episode (not shown), certain anorthosite massifs were possibly emplaced at shallow level (Emslie, 1978; Morse, 1982).

Figure 9b: change in the direction of subduction (required by geometry relationships) and island arc-continent collisions have Phanerozoic analogues (McKenzie, 1969; Templeman-Kluit, 1979).

Figure 9c: subduction of the S Malawi island arc by the Niassa craton to form a granulite terrane is consistent with a recent model by Newton and Perkins (1982). Underthrusting of the island arc (basalt + andesite) was per- haps favoured by its overall higher density relative to the Niassa craton (mainly granitoids). Contributing factor was perhaps also the presence of soft trench sediments along the thrust plane which reduced the friction be- tween the blocks (Weber and Ahrendt, 1983).

Available petrographic and analytical data suggest that in S Malawi water- deficient igneous rocks were preferentially upgraded to granulites relative to the more hydrous metasedimentary suites.

Models by Richardson and England (1979) suggested that granulite (rather than eclogite-) metamorphism resulted from a combination of high heat flows in the subducted island arc, and reduced thickness of the overriding plate.

Figure 9d: lithosphere delamination and crustal underplating by mantle partial melts are modelled after KrSner (1982). Crystallisation heat and escaping residual fluids probably triggered lower crust anatexis, contamina- tion of the mafic underplated melts, and emplacement of late-kinematic monzonite--syenite--K-rich granite/charnockite plutons at higher crustal levels (Emslie, 1978; Newton et al., 1980).

Figure 9e: isostatic uplift was marked by the development of important shear zones. These allowed the escape of volatiles released from the crystal- lising mafic and acidic igneous suites. These fluids were enriched in many in- compatible elements, especially U and REE. Thrusting also allowed the rapid uplift of deep seated rocks (garnet--olivine ultramafics and ariegitic garnet-

Fig. 9. Simplified and schematic sections showing five stages (a--e) in the suggested evolu- tion of the Mozambique belt in S Malawi; see text for explanation.

TA

BL

E I

II

(3O

Seq

uen

ce o

f ev

ents

in

the

pre-

Kar

oo h

isto

ry o

f S

Mal

awi

and

Tet

e (M

ozam

biq

ue)

i ZA

MB

EZI

BEL

T I

MO

ZAM

BIQ

UE

BEL

T =

i A

NO

RT

HO

SIT

E

--

GR

AN

UL

ITE

S

UIT

E O

F T

ET

E

~ M

WA

NZ

A

TR

AN

SIT

ION

Z

ON

E

~ G

RA

NU

LIT

E

SU

ITE

O

F S

MA

LA

WI

I I

t R

eset

ting

of d

avid

ite~

Mw

anza

fau

it 3

Res

ettin

g of

dav

idite

-2

Met

asom

atis

m

and

davi

dite

dev

elop

men

t?

She

arin

g of

ano

rtho

site

? M

afic

dyk

es e

mpl

acem

ent?

E

mpl

acem

ent

of a

nort

hosi

te a

nd p

eak

of

gran

uSte

fac

ies

met

amor

phis

m?

Ove

rrid

ing

ptat

e? '~

Coo

ling,

upl

ift a

nd r

eset

ting

of K

-At

mm

eraJ

age

s '

in a

mph

ibol

ite f

ac[e

s ro

cks

'i I

Res

ettin

g ?

of P

b-~

zir

con

ages

m T

amba

ni

i ne

phel

ine

gnei

sses

:~

J

Ear

ly a

ctiv

ity o

f M

wan

za f

ault

zone

? 3

Coo

ling:

con

cord

ia a

ge o

f zi

rcon

s fr

om s

upra

- cr

usta

l ro

cks/

gran

ulite

s C

oolin

g: c

onco

rdia

age

of

zirc

ons

from

anc

ient

m

igm

atis

ed p

yrox

ene-

orth

ogne

iss

Pea

k of

met

amor

phis

m

-- c

losu

re o

f R

b-S

r is

otop

es i

n gr

anul

ites

and

tran

sitio

nal

rock

s

Coo

ling,

upl

ift a

nd e

mpl

acem

ent

of l

ake

Mal

awi

I gra

nite

sto

cks

and

dyke

s

!

', R

eset

ting

and

dow

rigr

adin

g of

gra

nu~i

tes

I thr

ustin

g of

ultr

amaf

ic r

ocks

?

t cl

osur

e of

Rb-

Sr

isot

opes

in

Coo

ling:

gn

elss

es

from

tra

nsiti

onal

are

a U

plift

of

garn

et-g

ranu

lite

-- e

clog

ite

t , E

mpl

acem

ent

gran

ite

of c

harn

ocki

tic

t E

mpl

acem

ent

of a

nort

hosi

te

Pea

k of

met

amor

phis

m-c

losu

re o

f Rb-

Sr i

soto

pes

in g

ranu

lite

Ove

rrid

den

plat

e

NIA

SS

A C

RA

TO

N

CO

LL

IDE

S

WIT

H

ISL

AN

D

AR

C C

OM

PL

EX

O

F S

MA

LA

Wi

Em

plac

emen

t of

do~

erite

and

dia

base

dyk

es?

ISL

AN

D

AR

C V

OL

CA

NIC

S

AN

D G

RE

YW

AC

KE

D

epos

ition

of

pelit

es,

sem

ipel

ites

and

limes

tone

s

NIA

SS

A C

RA

TO

N

OC

EA

N/S

IAL

IC

? F

LO

OR

AGE

(m.y

.)

-- 4

00

- 50

0

700

-- 8

00

-90

0

-!0

00

1100

--

-- 1

200

MID

-EA

RLY

PR

OTE

RO

ZOIC

ARC

HAE

AN

Mod

ified

aft

er A

ndre

oli,

1981

; (1

); C

ahen

, 19

57;

(2)~

Dar

nley

et

al,

1961

; (3)

: C

oope

r an

d B

loom

field

. 19

61

179

granulite) near the western margins of the granulite facies terranes toward the cratonic foreland (Fig. 2b). These rocks define a crude swinging belt which may mark a suture zone between reworked Archaean rocks (Zambezi belt) and the accreted island arc complexes (Mozambique belt granulites).

The sequence of events depicted in Fig. 9 (a--e) is summarized in Table III.

CONCLUSIONS

The plate tectonic model proposed in the previous section is compatible with the results of recent investigations in other Pan-African terranes. Among others, Shackleton et al. (1980) and Vearncombe (1983) described Late Precambrian ophiolite suites in the Mozambique belt terranes of Sudan, Egypt and Kenia (S, E, and K; Fig. 10). In Zambia, volcanics younger than 1300 Ma (Cahen, 1970) were extruded at the base of the Lufilian sequence (Z, Fig. 10). Vrana et al. (1975) attributed to this stratigraphic position the ultramafic rocks and mafic volcanics metamorphosed to eclogite which occur south of the Mwembeshi shear zone in southern Zambia (Fig. 1). In the Zambezi Province of Mozambique (M, Fig. 10) reworked granitic gneisses and possible meta-ophiolite suites marked by ca. 1000 Ma "Kibaran" iso- chrons are described by Sacchi (1984).

In the Kibaran terranes of S Africa, Matthews (1972) described remnants of ophiolitic sequences obducted on Archaean basement in northern Natal

n ~ / ' ~ . : ~ ~ed/ment s ~i&. ~6 ~ .

-:, • ..,.-~ :i:,t::--71.ti.ii~}.:7.1 i

",.~.:~J:" .'::..'7: : :%;::.::: .".."t ' : t o ~ • ,..'.'.~::. : : : : : : : : : ========================== K

, .

Fig. 10. Sketch map of Africa, Saudi Arabia and S America restored to pre-drift positions (after Shackleton, 1976) showing: (shaded areas), terranes yielding K--Ar ages ~ 1000 Ma; (dots), distribution of possible Mid-Late Proterozoic ophiolites; (dashes), juvenile crust < 1800 Ma in S Africa (see text). Letters are localities referred in the text; square represents the area studied.

TableW

Mineralisations

related to igneous rocks of the anorthosite suite, including K-rich granites

00

O

Age(m.y.)

Area,State

Mineralisation

Setting

Orogeny

Reference

PHANEROZOIC

468 !

8 Swakopmund,

~ K-rich pegmatitic

Pan African

Na/~ibia

granlte,rapakivl

in

places

450 -

500

Kafue-Hook,

Cu,Fe,As,Ag,Au,

(Scapolitised)syenite-

Pan African

zambia

Bi~Sb~ZntCo~Ni,

qranite,rapakivi,

Pb

kalialasklte

ca.520

Saldanha Bay

Th,U,(Mo)

Albitised

(÷ scapolite

Cape Province

S Africa

calcite)

al~ali-granite

granites-

kalialaskite

Pan-African

PROTEROZOIC

600 -

800 ?

Tete, Mozambique

Fe,Ti,U,REE,

Scapolite(+calcite)/alhite

Kibaran-

(Mo,Cu)

(lcalcite)Tdiopsidite

Pan African

850 -

950

NW Tanzania-

Sn,Wo,(Ta,Co

E Zaire

~)--

1020 -

1080

Upington,

N b,Ta,Be,(Fe,

S Africa

B,REE,Wo,Cu,

U,Mn,Li,Bi

ca. 1100

Nababeep,

Cu

S Africa

ca. 1100

S Natal,

U S

Africa

ca.

!~50

SE Madagascar

ThtU,(Sn,P,F,

Mo,Cu)

metasomatised anorthosite

in granulite facies

terrane.

Sn-bearlnglate-kinematic

Kibaran

K-rich pegmatite/granlte;

related veins and contact

schists,occasionally

intruded by quartz-norite/

hypersthene granophyre.

Albitised,

silicified

Kibaran

K-richpeg~%atites

intruding

(scapolitised)

charnockite

and granulite related to

rapakivi.

Cu-sulphides in norite

Kibaran

consanguineous

to

anorthosite I in granulite

facies terrane.

High U

background in

Kibaran

rapakivl granitoids7

and introduction of U

in older pegBatlte and

schists in mlqmatitic

(granulltlc) terrane.

Diopside

(+ calcite *

phlogoplte-÷ scapoli~e)

pyroxenite Tn granulites

associated to anorthosite

and K-rlch(charnockitic)

met~granite.

Pan African/

Kibaran

overprint on

Archaean?

Nash, 1971; Toens et al. I

1979.

Cikin and Drysdall,

1971;Phillips,19~ ;

Brandt,1955,

Schoch(1982,personal

communication)

Schoch

and Burger,1977.

This paper.

Stockley and Williams,

1938; Cahen,1970,

Pelletier,1964;Klerkx,1983,

personal co~unication.

Von Backstr~m,1964;

Lipson,1980;Lipson

and

McCarthy,1977jNicolaysen.

1982, personal con~nunication

Mclver et al., 1983.

Hart, 1

983; Ker~

1982,

personal Coramunlcation.

Caen-Vachette,

1970;

Kieft,1967;Roubalt,

1958; Boulanger,1959;

de la Roche,1963;

Besaire,1966.

980 - 1090

Bancrof,Ont.,,

U,Th,REE,(Cu,Mo

Syenitic(÷ calcite, fluo-

Grenville

Canada

~,F}

rite, apatite)Degmatites

related to qrenvillian

granite-syenite suite; at

places intruding

anorthosite.

ca. I040

Pikes Peak,

Be,Ta,Nb,Y,Ce

Pegmatites related to

Colorado

Co.,USA

Mo,F,Ba,Zn,B

batholith of (anorthosite-

Front Range

syenite-)potassic granite

suite.

1094 - 1200

Adirondacks,N.Y.,

F ee(Cu,Mo,U,Th,

Replacement(?)ores in

Grenville

USA

F)

grenvillian paragneisses

within region of

charnockitic plutonism.

1450 + 20

Wheeler Basin,

U,Cu,Mo

Uraninite disseminated

Correlative

Col.,USA

in biotite-gneiss and

Of (1700m.y.)

migmatite associated to

Idaho Springs

K-rich pegmatite

Form.

1700

Koresten Complex,

Sn

Rapakivi granite

n.a.

Ukraine,USSR

-anorthosite complex.

1730 ?

Radium Hill,

~,REE,Fe,Ti,

Phl~gopite lodes in

n.a.

S Australia

sheared soda granite,

possibly related to

K-rich granite, in

Archaean? granulites

1730

Duobblon,Sweden

U(Fe,Ti,Mn,

Stratabound mineralisation

Carelian

MO)

in altered K-rich

ignimbrite related to

rapakivi

2000 + 100

Kodar Complex

Sn,Ta,Nb,F,

Rapakivi granite, in

Junction of

W Aldan,USSR

REE,Au?

anorthosite bearing

Aldan Shield

area.

and Dzhugdzhur-

Stanovoy belt

ARCHAEAN

> 2350

ca. 3070

Elliot Lake,

U

K-rich pegmatitic

Pre-Huronian

Ont.,Canada

granite is postulated

source of uraniferous

conglomerate.

S Swaziland-

Sn(REE,Ta,Nb)

Potassic pegmatites of

Barberton

Barberton area

K-rich Hood granite

greenstone

(RSA)

overlying migmatites and

belt

occasional charnockitic

quartz-diorite.

I Underlined element indicates its economic exploitation in the past or at-present (when known)

Bede~, Ig82;Rimsaite,

1982.

Barker et a1.,1975;

Gross and Heinrich,

1966.

Prucha,1956;Narten and

McKeOwen, 1952;Crump and

Beutner,1968.

Young and Hauff, 1975.

Bridgwater and Windley,

1973;Mitskevich,1963.

Parkin and Glasson,

1994;Joplin,1957.

Smellie,1982;Bridgwater

and Windley~1973.

Sviridenko,1975.

Robinson and Spooner,

~982.

Viljoen and Viljoen,

1969;

Hunter,1959.

00

182

(N, Fig. 10). More recently, Barton and Burger (1983) argued for absence of Archaean crust and for possible continental accretion from oceanic-type mantle (at 1800--1700 Ma and -+ 1300 Ma) within the Namaqualand--Natal mobile belt (dashed line, Fig. 10). The chemical affinities between the S Malawi high grade rocks and ~240 analyses of African granulites quoted by Clifford (1974) support a more general validity of the model presented in Fig. 9.

In addition, the results of an extensive survey (Table IV) of the mineralisa- tions associated with anorthosite and the spatially associated K-rich granite suites support the validity of the mineralisation model proposed for Tete.

Finally, if the model of S Malawi (Table III) is correct, the Mozambiquian orogenic cycle spans the time sequence of two major orogenic events, the Kibaran and Pan-African in southern Africa.

ACKNOWLEDGEMENTS

During the preparation of this work I have benefitted from the discussions with Professors T.N. Clifford, L.O. Nicolaysen, D. Groves and numerous other colleagues and friends. I thank in particular R. Hart for the editing of the manuscript, and G. Cawthorn, G. Davies, D. Piper, R. Sacchi and G. Martinotti for contributing to the ideas exposed.

The University of the Witwatersrand is thanked for financially supporting my research and the preparation of this article.

I am grateful to the Malawi Geological Survey for the permission granted to me to carry out field work and for the logistic support.

I am indebted to Mr. G. Hutchinson, for electron-microprobe data and to Dr. A.J. Burger and Dr. D.C. Rex for unpublished age determinations.

I thank Miss D.D. Mthembu for patient typing and Mrs. A. Saiet for the draughting of figures.

REFERENCES

Andreoli, M.A.G., 1981. The amphibolite and the granulite facies rocks of Southern Malawi. Ph.D. Thesis, Univ. Witwatersrand, S. Aft. (unpubl.).

Appleyard, E.C. and Williams, S.E., 1981. Metasomatic effects in the Faraday meta- gabbro, Bancroft, Ontario, Canada. TMPM Tschermaks Mineral. Petr. Mitt, 28: 81-- 97.

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