Petrochemistry, tectonic evolution and metasomatic mineralisations of Mozambique belt granulites from S Malawi and Tete (Mozambique)

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

161

PETROCHEMISTRY, TECTONIC EVOLUTION AND METASOMATIC MINERALISATIONS OF MOZAMBIQUE BELT GRANULITES FROM S MALAWI AND TETE (MOZAMBIQUE) 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 mineralisations of Mozambique belt granulites from S Malawi and Tete (Mozambique). Precambrian Res., 25: 161--186. The granulites of southern Malawi and Tete (Mozambique) typically comprise prograde two-pyroxene ± garnet e hornblende ± biotite parageneses. High grade metamorphism was imprinted on supracrustal, migmatitic and plutonic rocks at the climax of the Kibaran orogeny (ca. 1100--850 Ma). Amphibolite 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 granulitic rocks is probably volcanic. Analytical data indicates derivation from alkali--olivine to high-alumina basalts. Pelitic and greywacke-like compositions are also present. The granulite facies rock suite additionally comprises intrusive anorthosite and spatially related metamorphosed monzonite, syenite and K-rich charnockitic granite. Toward the latter stages of the Mozambiquian orogenic cycle (800--600 Ma), granulite 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, molybdenite, stibio-niobotantalite and other mineralisations. The mineralising elements were possibly derived from a highly fractionated magma with anorthosite affinity. The evolution of the region is explained b y a plate tectonic model, which envisages subduction 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 ophiolitic relics, bodies of magnesian garnet--olivine ultramafics 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 ily of Precambrian high grade amphibolite facies gneisses and hypersthene-

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bearing lithotypes, akin to granodiorite, diorite, norite, anorthosite, and pyroxenite (Araujo, 1967; Oberholzer, 1968). A comparable lithological association is frequently repeated in the adjacent terranes of southern Malawi, eastern Zambia, Tanzania and Ethiopia which were affected by the PanAfrican 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

[]

2 B [~

Southern Malawl

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: n u m b e r s 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 sedimentary 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 granulites 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 approaching 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, petrological, geochronological and analytical data. FIELD R E L A T I O N S AND P E T R O G R A P H Y

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 orthopyroxene isograde is recorded. West of Blantyre a transition zone between amphibolite and granulite facies terranes is characterised by proxene-gneisses and biotite--hornblende granulites and transgresses obvious sedimentary horizons (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 scattered in the Lilongwe region (Fig. 2b) suggest some tectonic contacts between amphibolite and granulite facies terranes. The orthopyroxene-bearing granulites of 8 Malawi and Tete (Mozambique) are greenish to dark-greenish in colour and resemble typical "charnockites" from elsewhere in the world. This study maintains Btoomfietd's (1968)

165

LEGEND Pyroxene'Brown"granite

Mid- Paleozoic to Recent cover ~ J

Precambrian sedimentary cover Granitoid orthogneiss with scattered basic rocks

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Davidite occurrences ,f

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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, calcsilicate rocks and rare marbles), the latter comprise plutonic, migmatitic or massive rocks. Petrographic investigations indicate that metamorphism generally outlasted deformation and that, in acidic rocks, partial melting processes were

166 p r o b a b l y 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 triplepoint grain boundaries comparable to those in mafic rocks of S Malawi. They are also affected in varying degrees by a combination of ductile, blastomylonitic deformation and microshearing, with polygonisation of older pyroxene and plagioclase. Associated (dipyre} scapolitisation is widespread. The following primary parageneses are typical, and diagnostic of granulite facies metamorphism in the areas considered: I II III IV

opx (1) + cpx + plg + ilm/mt plg + opx + cpx -+ Kfl + qtz plg + qtz + opx + cpx-+ Kfl-+ gar 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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PYROXENE MIGMATITE (5), (6)

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Lithologicalrelationshipsacro~ MalawiamphiboIite--granulitefacieste~anes*

TABLEI

168 GEOCHEMISTRY

Typical granulites of S MaIawi The granulites analysed include melanocratic, mesocratic, leucocratic and garnet-bearing types. Certain lithologies, however, such as khondalite, calcsilicate granulite and marble have been omitted since their metasedimentary TABLE II Average compositions of typical S Malawi granulites Type:

melanocratic

C o l u m n no.

I

~a~ses

(2)

mesocratic

2

3

(7)

leucocratic 4

(11)

(10)

5

6

7

8

(6)

(9)

(4)

(2)

Si02

43.28

50.44

59.85

59.41

68.91

70.49

74.31

77.77

Ti02

3.39

1.75

1.05

1 18

0.56

0.47

0.22

0.20

AI203

15.26

14.91

17.11

17 01

14.81

14.22

12.73

11.72

Fe203

2.77

4.21

1.83

3 12

0.80

1.39

0.91

1.41

Fe0

12.45

8.23

4.25

3 38

3.11

2.99

0~97

1.79

Mn0

0.25

0.20

0.09

0 10

0.08

0.08

0.02

0.05

1.15

0.22

0.45

Mg0

6.32

5.80

2.75

1 86

0.73

Ca0

9.61

7.99

5.7

4 15

2.49

3.36

1.76

3.38

Na20

2.45

3.88

4.58

4 37

3.52

4.48

3.52

2.98

K20

0.90

1.09

1.66

4.69

4.47

0.85

4.36

0.36

P205

0.55

0.26

0.26

0.59

0.12

0.11

0.05

0.02

H20

0.05

0.04

0.08

0.07

0.05

0.05

0.05

0.03

H20+

0.29

0.28

0.29

0.24

0.27

0.27

0.32

0.18

97.55

99.08

100.17

99.92

100.91

99.44

100.32

Total

99.5

I: A v e r a g e of cols.

(a) and

2: A v e r a g e of cols.

(c),

(e),(f) and

(g), T a b l e 20

(Loc.cit.).

3: A v e r a g e of cols.

(b),

(c),(g)

and

(j), T a b l e 21

(Loc.cit.).

4: A v e r a g e of cols.

(d),

(e),(g)

and

(h), T a b l e 21

(Loc.cit.).

5: A v e r a g e of cols.

(a),

(d),(f) and

(h), T a b l e 22

(Loc.cit.).

6: A v e r a g e of cols.

(b),

(e) and

7: col. J, Table 22

(Loc.cit.).

8: Col.

(Loc.cit.).

i, T a b l e 22

(b),Table 20

(Andreoli,1981).

(k), 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 study for major, minor and trace elements is presented elsewhere (Andreoli, 1981), as are sampling localities and experimental and analytical procedures. Average compositions of typical granulites are listed in Table II. Melanocratic granulites (SiO2 < 53%) are predominantly undersaturated, with 50--62% normative feldspar of relatively sodic composition (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 compositions (K20/Na20 > 0.7) have a higher concentration of P2Os (m = 0.59%) relative to the more sodic rocks (m = 0.26%). With the exception 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 compositional range of the S Malawi granulites. The broad chemical characteristics of these rocks are better 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 distribution 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.

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

A

Lj

B

~

,/i

5

g

i i0

i , 50

1 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 granulite 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 o r 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 b y a comparable compositional 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 / R b 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 identification of rock types parental to the gneisses and granulites. Petrochemical data m a y 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 investigated. T h e r m o d y n a m i c 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 lithology. 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 leucocratic granulites plot in the field of alkaline olivine basalt and its differentiation 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 Malawi rocks are derived, at least in part, from island-arc volcanics, related ulutonic 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 Chimwadzulu serpentinised peridotite. The associated amphibolites define a tholeiitic 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 Cramphibole and occasionally by corundum or small copper-sulphide mineralisations. Amphibolite also grades in places into an almandine--diopside-hornblende granulite with (greenschist-type) eClogite affinities (Andreoli, 1981). 12

o

I

50

60

7O

S i 0 2 (WT °/o )

Fig. 8. Alkali--silica diagram illustrating the distribution 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 (previously 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--granodioritic orthogneisses comparable to those of the Mwanza area are typical of

174

many Precambrian, mainly Archaean terranes (Tarney, 1976). For this reason, 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 experienced 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 plagioclase and by (hornblende + phlogopite) uralitisation of ultramafic rocks. UREE 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). calcite and diopside implies T ~ 630--700°C and yfluid -~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 maximum 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 present 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 megacrystic diopside nodule in mylonitised granulite was found to include unusually 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 origin, since the scapolitised rocks are (to the author's knowledge) entirely confined 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 deficient in primary volatiles despite their marked alkalic affinity (Nsanje). (2) In the Tete area, thrusting of high grade, older anorthosite and granulite over lower grade U, REE-enriched evaporite. (3) In the Nsanje area, introduction (by subduction?) of evaporitic material into a plutonic complex with mineralogical features indicating crystallisation 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--hartzburgite, etc. which display a subsequent history of progressive unroofing (Andreoli, 1981).

176 e

Uplift

Thrust, s h e a r i n g and m i g r a t i o n of r e s i d u a l

÷ +

+

*

+

~

+

+

+

÷

*

\

÷

+

~

~

~ ~

'-'

H:igh- p . . . . . . . chthonous

bodle s

m.y.

C o o l i n g and d o w n g r a d i n g of g r a n u l i t e s

fluids

÷ ÷ J

÷

C8. 8 5 0 - 6 5 0

and e r o s x o n

'

÷

'

+

+

''"

+

+

÷

+

Lithospheric

+++++÷+++~

mantle

~

I s o s t a t i c r e a d j u s t e m e n t of crust-lithosphere boundary

d

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

ns

D e l a m l n a t l o n and slnkl g of l i t h o s p h e r i c m a n t l e

C

Ophiolitic r e m n a n t s Flysch-like

÷

n e r p l a t l n g by asthenospheric partial melts

+

+

++÷

÷

÷++ ÷ + ÷ + + ÷ + * +

÷ + + + .......

I

÷

÷÷÷:+ +

n

ca.110o

deposits

c

÷ + . ÷~+

.

U

+

÷

+

÷

+ + * ÷ ÷ ÷ ÷ ÷ ÷ ÷ + + + * + + ÷ ÷ + * +

S

waning

H y p e r sthene

isogr ade

b

¢ a . 1 2 0 0 m.y.? thrusting

÷+÷

++***+*÷÷÷

*

++

÷

m.y.

M i g m a t ires

+

+

+

+

~

+

+

+

÷

+

of o p h i o l i t e s ; ~

÷

vvvvvvvvvv

+

÷

+

+

÷

+

+

+

+

÷

+

+

, Incipient inversion of s u b d u c t i o n

a

MId-proterozoic

WEST

S Mal&awi i s l a n d - a r c oceanic

Nle%es.Cr)ton

/

.

.

floor o p h i o l i t e s

.

ASTHENOSPH~RE

.

Greywackes

~¢~* " ~ ¢ ¢

EAST Shelf sediments

Lurlo ? ¢ , e t o n

177

S Malawi model

Available petrological and geochronological data are not sufficient to precisely 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 island 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. During 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 perhaps 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 between the blocks (Weber and Ahrendt, 1983). Available petrographic and analytical data suggest that in S Malawi waterdeficient 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, contamination 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 crystallising mafic and acidic igneous suites. These fluids were enriched in many incompatible elements, especially U and REE. Thrusting also allowed the rapid uplift of deep seated rocks (garnet--olivine ultramafics and ariegitic garnetFig. 9. Simplified and schematic sections showing five stages (a--e) in the suggested evolution of the Mozambique belt in S Malawi; see text for explanation.

~

I

'~ ''i

ZONE

Peak of metamorphism -- closure of Rb-Sr isotopes in granulites and transitional rocks

Cooling: concordia age of zircons from supracrustal rocks/granulites Cooling: concordia age of zircons from ancient migmatised pyroxene-orthogneiss

Early activity of Mwanza fault zone? 3

I Resetting ? of P b - ~ zircon ages m Tambani i nepheline gneisses :~ J

Cooling, uplift and resetting of K-At mmeraJ ages in amphibolite fac[es rocks

Mwanza fauit 3

MWANZA TRANSITION

~

i

i

I

Resetting and dowrigrading of granu~ites I thrusting of ultramafic rocks?

!

NIASSA CRATON

Deposition of pelites, semipelites and limestones

Emplacement of do~erite and diabase dykes?

OCEAN/SIALIC

AND GREYWACKE

? FLOOR

ISLAND ARC VOLCANICS

Overridden plate

Cooling: closure of Rb-Sr isotopes in gnelsses from transitional area Uplift of garnet-granulite -- eclogite t , Emplacement of charnockitic granite t Emplacement of anorthosite Peak of metamorphism-closure of Rb-Sr isotopes in granulite

t

',

BELT

G R A N U L I T E S U I T E OF S M A L A W I

MOZAMBIQUE

Cooling, uplift and emplacement of lake Malawi I granite stocks a n d dykes

I

N I A S S A C R A T O N C O L L I D E S W I T H I S L A N D A R C C O M P L E X OF S M A L A W i

Overriding ptate?

Metasomatism and davidite development? Shearing of anorthosite? Mafic dykes emplacement? Emplacement of anorthosite and peak of granuSte facies metamorphism?

Resetting of davidite - 2

Resetting of davidite~

- - G R A N U L I T E S U I T E OF TETE

BELT

Modified after Andreoli, 1981; (1); Cahen, 1957; (2)~ Darnley et al, 1961; (3): Cooper and Bloomfield. 1961

t

ANORTHOSITE

=

ZAMBEZI

Sequence of events in the pre-Karoo history of S Malawi and Tete (Mozambique)

TABLE III

ARCHAEAN

MID-EARLY PROTEROZOIC

-- 1200

1100--

-!000

-900

-- 800

700

- 500

-- 400

AGE (m.y.)

(3O

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" isochrons 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 ~ .

-:, • ",.~.:~J:"

•,..'.'.~::.

..,.-~ .'::..'7: : :::::::::

:i:,t::--71.ti.ii~}.:7.1 i :%;::.::: .".."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

Kafue-Hook, zambia

Saldanha Bay S Africa

450 - 500

ca.520

NW TanzaniaE Zaire

Upington, S Africa

Nababeep, S Africa

850 - 950

1020 - 1080

1100

1100

!~50

ca.

ca.

ca.

SE Madagascar

S Natal, S Africa

Tete, Mozambique

600 - 800 ?

PROTEROZOIC

Swakopmund, Na/~ibia

Area,State

ThtU,(Sn,P,F, Mo,Cu)

U

Cu

N b,Ta,Be,(Fe, B,REE,Wo,Cu, U,Mn,Li,Bi

Sn,Wo,(Ta,Co ~)--

Fe,Ti,U,REE, (Mo,Cu)

Th,U,(Mo)

Cu,Fe,As,Ag,Au, Bi~Sb~ZntCo~Ni, Pb

~

Mineralisation

Diopside (+ calcite * phlogoplte-÷ scapoli~e) pyroxenite Tn granulites associated to anorthosite and K-rlch(charnockitic) met~granite.

High U background in rapakivl granitoids7 and introduction of U in older pegBatlte and schists in mlqmatitic (granulltlc) terrane.

Cu-sulphides in norite consanguineous to anorthosite I in granulite facies terrane.

Albitised, silicified K-richpeg~%atites intruding (scapolitised) charnockite and granulite related to rapakivi.

Sn-bearlnglate-kinematic K-rich pegmatite/granlte; related veins a n d contact schists,occasionally intruded by quartz-norite/ hypersthene granophyre.

Scapolite(+calcite)/alhite (lcalcite)Tdiopsidite metasomatised anorthosite in granulite facies terrane.

Albitised (÷ scapolite calcite) al~ali-granite kalialaskite

(Scapolitised)syeniteqranite,rapakivi, kalialasklte

Pan African/ Kibaran overprint on Archaean?

Kibaran

Kibaran

Kibaran

Kibaran

KibaranPan African

Cape Province granitesPan-African

Pan African

Pan African

Orogeny

including K-rich granites

Setting

suite,

K-rich pegmatitic granlte,rapakivl in places

related to igneous rocks of the anorthosite

468 ! 8

PHANEROZOIC

Age(m.y.)

Mineralisations

1971; Toens et al. I

1983.

Caen-Vachette, 1970; Kieft,1967;Roubalt, 1958; Boulanger,1959; de la Roche,1963; Besaire,1966.

Hart, 1983; K e r ~ 1982, personal Coramunlcation.

Mclver et al.,

Von Backstr~m,1964; Lipson,1980;Lipson and McCarthy,1977jNicolaysen. 1982, personal con~nunication

Stockley and Williams, 1938; Cahen,1970, Pelletier,1964;Klerkx,1983, personal c o ~ u n i c a t i o n .

This paper.

Schoch(1982,personal communication) Schoch and Burger,1977.

Cikin and Drysdall, 1971;Phillips,19~ ; Brandt,1955,

Nash, 1979.

Reference

00 O

Bancrof,Ont.,,

Adirondacks,N.Y., USA

Wheeler Basin,

1094 - 1200

1450 + 20

Radium Hill, S Australia

Duobblon,Sweden

Kodar Complex W Aldan,USSR

1730 ?

1730

2000 + 100

Potassic pegmatites of K-rich Hood granite overlying migmatites and occasional charnockitic quartz-diorite.

Sn(REE,Ta,Nb)

S SwazilandBarberton area (RSA)

ca. 3070

I Underlined element indicates its economic exploitation in the past or at-present

K-rich pegmatitic granite is postulated source of uraniferous conglomerate.

U

Rapakivi granite, in anorthosite bearing area.

(when known)

Barberton greenstone belt

Pre-Huronian

Junction of Aldan Shield and DzhugdzhurStanovoy belt

Carelian

Stratabound mineralisation in altered K-rich ignimbrite related to rapakivi

n.a.

n.a.

Correlative Of (1700m.y.) Idaho Springs Form.

Grenville

Colorado Front Range

Grenville

Phl~gopite lodes in sheared soda granite, possibly related to K-rich granite, in Archaean? granulites

Rapakivi granite -anorthosite complex.

Uraninite disseminated in biotite-gneiss and migmatite associated to K-rich pegmatite

Replacement(?)ores in grenvillian paragneisses within region of charnockitic plutonism.

Pegmatites related to batholith of (anorthositesyenite-)potassic granite suite.

Syenitic(÷ calcite, fluorite, apatite)Degmatites related to qrenvillian granite-syenite suite; at places intruding anorthosite.

Elliot Lake, Ont.,Canada

Sn,Ta,Nb,F, REE,Au?

U(Fe,Ti,Mn, MO)

~,REE,Fe,Ti,

Sn

U,Cu,Mo

F ee(Cu,Mo,U,Th, F)

Be,Ta,Nb,Y,Ce Mo,F,Ba,Zn,B

U,Th,REE,(Cu,Mo ~,F}

> 2350

ARCHAEAN

Koresten Complex, Ukraine,USSR

1700

Col.,USA

Pikes Peak, Co.,USA

Canada

ca. I040

980 - 1090

1975.

Viljoen and V i l j o e n , 1969; Hunter,1959.

Robinson and Spooner, ~982.

Sviridenko,1975.

Smellie,1982;Bridgwater and Windley~1973.

Parkin and Glasson, 1994;Joplin,1957.

Bridgwater and Windley, 1973;Mitskevich,1963.

Young and Hauff,

Prucha,1956;Narten and McKeOwen, 1952;Crump and Beutner,1968.

Barker et a1.,1975; Gross and Heinrich, 1966.

B e d e ~ , Ig82;Rimsaite, 1982.

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 ~ 2 4 0 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 mineralisations 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 metagabbro, Bancroft, Ontario, Canada. TMPM Tschermaks Mineral. Petr. Mitt, 28: 81-97. Araujo, J.R.F., 1967. The Mozambique belt in the Baru~ area, Manica and Sofala District, Mozambique, with special reference to the petrology, stratigraphy and metamorphism. Univ. Leeds Inst. Aft. Geol., Ann. Rep., 11: 19. Bailey, O.K., 1982. Mantle metasomatlsrn-continuing chemical change within the Earth. Nature, 296: 525--530. Barker, F., Wones, D.R., Sharp, W.N. and Desbrough, G.A., 1975. The Pikes Peak batholith, Colorado Front Range, and a model for the origin of the gabbro--anorthosite-syenite--potauic granite suite~ Precambrian Res., 2: 97--160. Barton, E.S. and Burger, A.J., 1983. ReconJ~!~nce isotopic investigations in the Namaqua Mobile Belt and implications for Proterozoic crustal evolution, Upington geotraverse. Spec. Publ. Geol. Soc. S. Afr., 10 (in press).

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