- Email: [email protected]
The Archaean geology of Sierra Leone
Precambrian Research, 6 (1978) 251--268
251
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
Review THE A R C H A E A N G E O L O G Y OF S I E R R A LEONE
H.R. WILLIAMS
Department of Geology, University of Sierra Leone, Freetown (Sierra Leone) (Received and accepted November 10, i 9 7 7 )
ABSTRACT Williams, H.R., 1978. The Archaean geology of Sierra Leone. Precambrian Res., 6: 251--268. The Archaean of Sierra Leone is divisible into two pene-contemporaneous tectonothermal units, the Liberian granite~greenstone terrain, and the Kasila Group Mobile Belt with its associated klippen, the Marampa Group. The granite--greenstone terrain consists of the Sula Group supracrustals, deformed and metamorphosed over an underlying gneissose granite migmatite Basement, yielding ages of about 2700 Ma. Late kinematic granites are found at the margins of the greenstone belts, and as high-level developmerits of the reactivated Basement. The Kasila Group is a larger scale supracrustal belt which developed into a zone of extreme orogenic activity now forming the southwest margin of the West African Archaean Craton.
INTRODUCTION TO STRATIGRAPHY
Archaean rocks in Sierra Leone, dated as more than 2100 Ma old, form an integral part of the West African Archaean Craton. Two main structural divisions are recognized, the Liberian (Hurley et al., 1971) granite--greenstone terrain, and the pene-contemporaneous Kasila Group Mobile Belt (Fig. 1). Radiometric ages from both divisions show a spread from 2100 Ma to over 3400 Ma (Hedge et al., 1975; tturley et al., 1975). The Liberian granite-greenstone terrain consists of N--S trending foliated and unfoliated "granites" and synformal, dominantly supracrustal amphibolites, greywackes, banded iron formation, magnesian sediments a n d tuffs, and ultramafic and basic intrusives. Most of the terrain is metamorphosed to the epidote--amphibolite facies. The Kasila Group consists of a supracrustal succession oriented NW--SE, but m e t a m o r p h o s e d to the granulite facies and almandine--amphibolite facies. The boundary between the two structural divisions is a zone of high strain and thrusting, with localised reworking of the Liberian structures adjacent to the Kasila Group. The Marampa Group (Allen, 1969; MacFarlane et al., 1974), has a typically Archaean stratigraphy, b u t is neither a greenstone nor a mobile belt.
252
BY GEOLOGICAL ,SURVEY OF
.~..
ICON
\
CO*ST
~IGE~IA
x * KABALA
0?
~ o
^ . . . .
,?"
~'°/[
\,~
°-
~'
/
D
~ \~
O
o, SIERRA
®"o
r
0
,
KENEMA -
'\
"
l)
"%.
/
,
8
~
Of
"
~[~_~I~,,,,TES LEVEL
~
~-.\o \= ,\2 l:.l=A.A~,A~o, LEONE
,.',W
.o,.
I~;W
"
o
S~;K.~T,C
"
,
KAMBUI
~. . . . . . c
j
j .E.E.,
,,].
Fig. l. Arehaean geology of Sierra Leone. Granite ma~ifs; A, I~bala; B, Loma; C, Gbengbe; D, Tingi; E, Kongot~. Supracrustal schist belts of T,iberian age: 1, Loko Hills; 2,
Sankarama; 3, Serekolia; 4, Sula Mountains; 5, Kangari Hills; 6, Nimini Hills; 7, Gori Hills; 8, Kambui Hills; 9, Mano-Moa Granulites (Gola Forests). THE GRANITIC T E R R A I N
Granitic rocks are the dominant c o m p o n e n t of the granite--greenstone division, but have been studied proportionately less than the greenstone belt sequences of the Sula Group. Most reports on the geology of the individual greenstone belts include only a cursory examination of the adjacent granites. A reconnaissance study by MacFarlane et al. (1974), and similar work in
''j
! ___j
253 eastern Sierra Leone by Rollinson (1973), were the first attempts at structural mapping of the granitic terrain. Basement granites The term "basement granites" is one of convenience, since the basement relationship to the Sula Group supracrustals is rarely, if ever, recognisable. The basal quartz--mica schists in the Kambui Schist Belt suggest that the supracrustal sequence developed initially on a granitic terrain, which exhibits a longer structural history than that in the supracrustals (Rollinson, 1974). The difficulty in the recognition of true basement is due to its modification during the Liberian tectono-thermal event, culminating in the formation of distinctly discordant late-kinematic granites, often called "younger granite". Basement granites are syn-kinematic, are not demonstrably younger than the supracrustal belts, and may be described as homogeneous to gneissic granitic (s.1.) migmatites. They have frequently been studied only during reconnaissance surveys, except in the Yengema area, where 400 km2 of detailed mapping indicated a complex structural history (Williams and Williams, 1975). The greater part of the Basement granitic terrain is composed of layers and lenses up to hundreds of metres thick, of pink, grey and white coloured, biotite and hornblende bearing foliated granitic migmatites. These migmatites contain patchy granitic areas consisting of homogenised, parautochthonous Basement, pale in colour and poor in mafic material. These relatively homogeneous bodies are sporadically distributed throughout the Basement, with gradational, intrusive and frequently indistinct contacts with the more foliated host. The migmatites have been described as syn-kinematic granites by most workers (N.W. Wilson, 1965; Andrews~lones, 1966; Marmo, 1971). Marmo describes the granites as becoming more even-textured and homogeneous away from the immediate confines of the supracrustal belts. However, gneissose migmatitic granites also surround parautochthonous syn-kinematic granite massifs such as the Gbengbe Hills (Wilson and Marmo, 1958) (Fig. 2). The syn-kinematic granites are migmatites with quartz-dioritic to granodioritic melanosomes and approximately granitic (s.s.) neosomes. Where the bulk composition is relatively mafic, hornblende is characteristically developed in the neosome, with biotite and hornblende in the melanosome. Nebulous and virtually homogeneous migmatites contain little mafic material, usually biotite. The compositional variation within ~ e term granitic migmatite is largely controlled by the composition, proportion and degree of hybridisation of the included mafic material, and the degree to which the rocks have been subjected to potassium metasomatism and remobilisation. The rocks generally have a high calcium to potassium ratio, causing oligoclase to dominate over microcline, except where late-stage metasomatism has allowed the growth of microcline megacrysts of ovoid and rectangular shape. The smaller, 30 mm long ovoid porphyroblasts frequently grow
254
ALLOCHTHONOU5 LATE- KINEMATIC L E V E L 3-HOOD* GRANITE \ \~"~x
PA RAU TOCHTHONOUS SYN- KINEMATIC GRANITE LEVEL 2
SUPRACRUSTAL GNEENSTONE B~LT GREENSTONE BELT ~ELICTS
x
'
(
-
~
~
/
~.AUREOLE OF 'NTEESE DEFORMATION
"~
LEVEL 2 j
,
+~ ~
~ ~- ~r
soP21g".;~'T~°LS I
,
( ALLOCHTHONOUS LATE- KINEMATIC G~ANI TES
[ I
LEVEL
I
SYN- KI NEMATIC
BASEMENT GIlA Ni TiC MFGMATI TES
Fig.2. Diagrammatic E--W cross-section through N--S trending Liberian granite--greenstone terrain. Vertical exaggeration 10:1; typical length of section I00 krn. (After Anhaeusser et al., 1969; Rollinson, 1973).
mimetically within the foliation, while the larger, rectangular ones, up to 0.1 m long, are more randomly oriented, and widely spaced. Distribution of the ovoid porphyroblasts is restricted to the pink homogeneous portions of the migmatite, while the rectangular ones are sporadically distributed, often in zones paralleling the foliation. Muscovite, like microcline, is a late replacive phase. Allanite, zircon, apatite, magnetite and tourmaline are constant accessories, while epidote is frequently associated with hornblende and sphene. The granites are often hybridised with basal members of the Sula Group on the margins of greenstone belts, and with tectonised remnants of basic dykes. The resulting rocks are usually hornblende-rich quartz--dioritic to syenitic agmatites and nebulites, mesocratic granodiorites, garnetiferous, magnetite- and sillimanite-rich granites (Marmo, 1962). Frequently the contact between the Sula Group and the syn-kinematic granites is modified by late-kinematic granites, but where unmodified, it is gradational, with extensive development of hybrid migmatitic rocks up to 2 km wide.
Amphibolite dykes and relicts in granitic terrain Two major sets of amphibolite dykes and their deformed derivatives may be recognised in many areas within the granitic terrain. An extremely deformed, agmatised, boudinaged, pre-Liberian set oriented parallel or subparallel with the Liberian foliation is cut by a generally discordant, synLiberian set of very variable orientation and composition. The early set of dykes is usually recognisable as lines of separated amphibolite inclusions up to 10 m wide, paralleling the migmatite foliation. They are frequently intruded by their host, which was reactivated during the Liberian tectono-thermal event. These dykes are clearly deformed remnants of a pre- or early Liberian swarm of dolerites.
255
The later, discordant set of dykes has a variable trend which is most clearly seen in zones of low intensity deformation as in east Kono district, where they have intruded along joints in a late-Liberian granite massif, the Tingi Hills. Clearly joint-controlled, they have been mildly deformed and metamorphosed, showing steeply inclined foliations, of variable intensity and trend. Hornblende lineations within the dykes are usually shallow plunging, unlike those in the host. In zones of more intense deformation, near to greenstone belts such as the Nimini Hills (Williams and Williams, 1975), the dykes are again joint-controlled, b u t deformed into buckle folds with vertical axes where the dyke trend is at right angles to the regional gneissose foliation, while those portions paralleling the foliation are relatively unaffected (Fig. 3). The later set of dykes is often intimately associated with contemporaneous granitic material. Composite dykes with basic margins grading inwards to granitic centres, and vice-versa, indicate the pene-contemporaneity of basic and granitic magmas. Only the later set of basic dykes is found in the latekinematic granites.
. ~:,.::"
PLUNGING
. . . . . ..
BUCKLEF O L D S ~ ' ~
J:-i(
~l
( ,
0
AGMATISED EAR~ IY
PPEGM EGMA7)TE ~ANOEb
GRANmCM,GMAT,TE
-°i
..........i-
"
Fig. 3. B a s i c d y k e relationships at Motema, n e a r K o i d u . Discordant sections o f d y k e are sometimes buckled, but concordant sections are not appreciably affected.
Chronological subdivision of the granitic basement MacFarlane et al. (1974) and Rollinson (1974) have provisionally subdivided the Archaean Granitic Basement into two time-structural-stratigraphic units. The older they called Leonian, and the younger, dominant unit, the Liberian. The subdivision was based on the occurrence of E - W striking foliations in limited areas within the dominant N--S striking Liberian granite--greenstone complex. In the Leonian areas they also concluded that
256 amphibolite dykes were seldom as deformed and rotated into concordancy with the gneissic foliation as in the Liberian terrain (Rollinson, 1973). The erection of a chronologic structural division in such poorly exposed terrain, on the basis of these two criteria, ignores an alternative explanation of the data which agrees with recent radiometric determinations (H. Rollinson, pers. comm.). Deformation intensity in granite--greenstone terrains is variable, being less severe in antiformal areas between the synformal greenstone belts. Variable foliation trends occur within the granitic terrain due to the nature of its multi-stage development, with linear trends modified by production and remobilisation of granitic material causing stellate outcrop patterns in greenstones, and elliptical dome--basin trends in granites (Wilson, 1973). Detailed mapping of the granitic terrain has indicated that E--W foliation trends may be due to the formation of large-scale N--S oriented folds of Liberian age, whose vertically plunging hinge zones form small outcrops of E--W striking rocks, cut by N--S axial planar fabric (Williams and Williams, 1975). Strike directions other than N--S are due to large-scale folds and to local foliation reorientation by shear zones of late-Liberian age, and need n o t be relicts of an earlier, Leonian, deformation. It appears, then, that the Leonian as recognised by E--W striking foliations and barely deformed rocks, is an integral part of the inhomogeneous Liberian event. Recent reconnaissance radiometric age dating mggests that the Leonian and Liberian events are indistinguishable, both being in the range 2700-3000 Ma. Discussion
The rapid and irregular changes in fabric and lithology of the syn-kinematic rocks of the Basement migmatites are partly a function of their position relative to the supracrustal schist belts. Lithologic-structural units are caused by large- and small-scale homogenisation and by reworking of an existing inhomogeneous granitic terrain into which basic dykes were intruded early in the Liberian event. During the deposition of Liberian supracrustals a multiphase structural event was initiated, causing a general alignment of the fabric into a N--S direction, forming fold axes and foliations of vertical attitude. S T R A T I G R A P H Y OF T H E A R C H A E A N
S U P R A C R U S T A L S (KAMBUI S U P E R G R O U P )
These rocks were formerly known as the Kambui Schists, but were renamed in accordance with contemporary nomenclature and called the Kambui Supergroup, which is divided into the Sula, Marampa and Kasila Groups.
257
The Sula Group The characteristic "greenstone" lithology of the granite--greenstone terrain, the Sula Group, was deposited, deformed and m e t a m o r p h o s e d in the Liberian event. It has been subdivided into the Sonfon and Tonkolili Formations (MacFarlane et al., 1974). The equivalents of these formations may be recognised in most of the greenstone belts. Table I shows the relationships and relative stratigraphies of the major supracrustal belts. For comparison, the stratigraphies of the Marampa and Kasila Groups are included. The resemblance of these rocks to those described from other Archaean successions in Africa is striking. However, there are some differences, for example, thick sequences of andesitic and rhyolitic extrusives are absent, while shales and cherts are developed only on a small scale. Sierra Leone contains four major zones of supracrustal greenstone belts (Fig. 1.) The Marampa Group is of much lower metamorphic grade, and is
TABLE I Stratigraphy of individual units within the Kambui Supergroup. Thicknesses are approximate and represent maximum values. Considerable variation in thickness occurs both across schist belts and along separated developments, e.g. Kambui and Nimini Hills. Sonfon Formation
~
Tonkolili Formation
Mano-Moa granulitest ultramafics (1 km), Gori Hills s.b. amphibolites (1 km) Serekolia relicts }
pelites (1 km), quartzites (1 km)
Kambui Hills s.b. Nimini Hills s.b. Sankarama relicts
pelites and greywacke turbidites (2 kin)
~ ultramafics (350 m), l amphibolites (2.5 kin)
Kangari Hills s.b. ~ ultramafics, amphibolites, Sula Mountains s.b. t b.i.f. (1.5 kin)
greywacke turbidites, pelites, conglomerates, b.i.f. (3 kin)
Loko Hills s.b.
amphibolites (200 m)
quartzites, b.i.f. (200 m)
Matoto Formation, basalts, andesites (750 m) basal thrust contact
R o t o k o l o n Formation, pelites, semi-pelites, psammites, b.i.f. matoto formation, basalts, andesites
(75o m) basalt thrust contact basic, gabbroic, anorthositic granuiites (8 kin)
s.b. = schist belts; b.i.f. = banded iron formation.
metasedimentary, granitic granulites (5 km)
258 not infolded into the Basement, while the Kasila Group is a mobile belt development with a similar stratigraphy to that of the greenstone belts {Williams, 1977). The Kambui Schist Belt is the southerly, separated extension of the Nimini Hills Schist Belt, which in turn becomes the Sankarama supracrustal relicts to the north. Similarly, the Mano-Moa Granulites are succeeded northwards by the Gori Hills Schist Belt and the Serekolia Schist Belt relicts. The Sonfon Formation consists predominantly of amphibolites of varying grain sizes and compositions, sometimes pillowed, and either massive or finely layered. Clearly meta-basaltic in origin, some also contain diopside and grossularite, perhaps indicating a spilitic composition, while some are mineralised by sulphides of iron, copper and arsenic. In the southern parts of the Liberian structural domain, the metamorphic grade of the supracrustals and Basement increases from epidote--amphibolite to granulite facies as the Kasila Group Mobile Belt is approached. This increase in grade is reflected in the mineralogy of the supracrustals to one dominated by hypersthene and calcic plagioclase. Elsewhere, the amphibolites contain a blue-green pleochroic hornblende, andesine, sphene and epidote. The base of the Sonfon Formation is always modified by the syn-kinematic granites, or by intrusive complexes of late-kinematic granite. Within the amphibolite sequence there are thin layers of pyritiferous talc schists, talc--chlorite schists, quartzites, magnetite--quartzites and cummingtonite schists. Development of the magnetite--quartzites or banded iron formation is extremely variable, for large thicknesses also occur in the Tonkolili Formation in the Sula Mountains, while only traces occur in the Nimini Hills to the east {Wilson and Marmo, 1958; Rollinson, 1975} {Fig. 4). Intrusive ultramafic rocks occur as concordant bodies within the amphibolites. They exhibit relict igneous mineralogies and textures, and are frequently heavily serpentinised. Chromiferous serpentinites and chromitite layers are found in these ultramafics, chiefly in the Sonfon Formation in the southeast part of the country (Dunham et al., 1958). The Tonkolili Formation is dominantly composed of clastic sediments with thin layers of siliceous and tuffaceous metavolcanics. Overlying the Sonfon Formation, it is rarely found in contact with the syn-kinematic granitic basement, except when all the underlying Sonfon Formation amphibolites have been made over to granite by migmatisation and metasomatism (Andrews-Jones, 1966; Williams, 1976). Greywacke turbidites, quartzites, cordierite-garnet schists and mica schists make up the bulk of the formation. In the Nimini Hills Schist Belt, cordierite schists occur at the base of the Tonkolili Formation, interbanded with amphibolitic tuffs and agglomerates. Polymictic conglomerates containing pebbles of granite, peridotite, serpentinite, amphibolite and quartz, in a matrix of chlorite and quartz, occur as layers up to 10 m thick within the lower parts of the Formation (Wilson and Marmo, 1958}. Siliceous volcanics are not common, though some fine.grained quartzo-feldspathic mica schists may be tuffaceous in origin. The
259
W i
4-
TONKOLI LI FORMATION
GT
;T
GT
3Pc
uo
SONFON FORMATION
// /i f! i! iI l! !/ '1
o
u
+
km
....
2-
+. i I-
o
Fig. 4. Stratigraphic cross-sections at three points along Nimini Hills schist belt, showing variation in lithology and thickness (north to south from left to right). GT, greywacke turbidite; u, ultramafics; a, amphibolites; bif, banded iron formation; cg, cummingtonite--garnet schist;p, pelites; us, ultramafic sill; crosses, younger granite. (After Rollinson, 1975).
"turbidites and quartzites contain way-up structures such as graded bedding, flame-structures and cross-stratification. Evidence from the Gori Hills Schist Belt indicates that the sedimentary facies in the western portion is turbiditic, while that in the east is dominated by quartzites (H. Rollinson, pers. comm.). The stratigraphy of the Sonfon and Tonkolili Formations shows rapid and irregular changes in lithology and thickness. Accurate determinations of thickness are impossible, especially as the duplication and omission due to folding, and proportion of interbanded granite are unknown. However, it is clear from the sequence and character of the lithologies that there is a similarity between the Sula Group and the generalised sequence in Archaean greenstone belts (Anhaeusser et al., 1969).
The Marampa Group The Marampa Group is subdivided into a lower, Matoto Formation consisting of pillowed basic lavas, serpentinites and andesites; and an upper, R o t o k o l o n Formation consisting of psammites, pelites and banded iron formation (MacFarlane et al., 1974). Allen (1969) recognised the same lithologies as MacFarlane et al., but did not divide them into formations. The Matoto Formation consists of 750 m of lavas with pillowed horizons up to 25 m thick. Andesites with phenocrysts of pyroxenes are flow banded. Interlayered with mafic lavas are ultramafic rocks, consisting of olivineserpentinites and chloritic rocks. The R o t o k o l o n Formation is mainly metasedimentary and contains pebbly horizons showing cross-laminations, with
260 subsidiary tuffs and andesites. Quartzites in the upper part are frequently rich in haematite, specularite, fuchsite, and are manganiferous. The metasediments consist of sericitic quartz schists, biotite--muscovite schists, garnetiferous schists, quartzites and biotite paragneiss. The volcanics in the lower parts of the Group tend to be chlorite--sericite schists in the east and amphibolites in the west, reflecting the general increase in metamorphic grade from east to west. Granites intrude the paragneisses in the west, forming lit-part-lit gneisses (Pollett, 1951). The basic portions of the Marampa Group pass westwards into the high grade rocks of the Kasila Group. The Kasila Group The Kasila Group consists of a series of basic, granitic, and metasedimentary granulites, flanked by amphibolites. Trending NW-SE, it forms the coastal rim to the West African Archaean Craton {Williams and Williams, 1976). Westward dipping mylonite zones occur within the high grade rocks, and in the eastern, lower grade flank, adjacent to the granite-greenstone terrain. These zones are of Archaean and Pan-African age Much of the Kasila Group is composed of fine to medium grained basic granulites containing minor horizons of quartz--magnetite, quartz--diopside and sillimanite-bearing rocks, representing highly metamorphosed equivalents of banded iron formation, marbles and pelites respectively. Extreme deformation renders reliable estimation of the stratigraphic thickness of lithologies meaningless. The west side of the Group, concealed by Phanerozoic epicontinental deposits (Pollett, 1951), consists of a gradational series from basic granulites to amphibolites and metasedimentary gneisses, while further west, amphibolites form the dominant lithology. Within the basic granulites, along the whole length of the outcrop of the Kasila Group, occur 200 m thick, layered anorthositic and leucogabbroic rocks, representing the metamorphosed equivalents of layered igneous intrusions. The basic and quartzo--feldspathic rocks within the Kasila Group represent a similar stratigraphic sequence to that found in the Sula Group. The chief differences are the occurrence of layered anorthositic rocks and the lack of significant ultramafic lithologies. The leucogabbroic layered complexes contain layers ranging in composition from ultramafic to anorthositic. Sometimes, original igneous textures are l~eserved in zones of low deformation intensity, or where coarse-grained, often pegmatitic, rocks have resisted the intense shear deformation. Major anorthositic bodies with ill
261
contrasts in lithology, metamorphism and structure mitigate against such comparisons, and suggest a similarity with a Limpopo Belt structure, sharing the same relationship with the Liberian granite-greenstone terrain as the Limpopo Belt does with the ghodesian Craton (Mason, 1973). THE LATE-KINEMATIC OR " Y O U N G E R " G R A N I T E S
Granites which are demonstrably younger than the supracrustals, the synkinematic granitic migmatites and the pervasive Liberian deformation are termed "younger" or "late-kinematic" (Marmo, 1971). They are most common around the margins of the Sula Group geenstone belts, especially in the north and east of the country, where the metamorphic grades are lower than in the southeast. Away from schist belts, large massifs of fine gained granite, such as the upper parts of the Tingi Hills, Loma Mountains and the Kabala Hills, are formed of late-kinematic granite, while waall plutons and dykes up to several kilometres in width may be found over much of the granitic terrain (Williams and Williams, 1975). In the southeast of Sierra Leone, late-kinematic material is restricted to small intrusive bodies on the margins of the Kambui schist belt, including some discordant pegmatites and aplites. Wilson (1965) estimates that the volume of late-kinematic granite in the Gola Forests area is a fifteenth of that found in the vicinity of the Sula Mountains schist belt. In contrast, in Northern Sierra Leone, many large massifs and numerous smaller bodies adjacent to greenstone belts have been mapped. The larger bodies appear to have gradational contacts with the surrounding syn-kinematic host, but others appear to overlie the older granites in a fashion which resembles the gani~e tectonic levels of Hunter (1971). The Tingi Hills consist of three such levels (Rollinson, 1973). The lowest consists of normal syn-kinematic basement migmatite, level two is composed of microcline porphyroblastic hornblende granite similar to that seen in the Gbengbe Hills (homogeneous parautochthonous syn-kinematic granite), while level three is situated above this and is true late-kinematic granite, although even this has been mildly deformed during the Liberian event. Cutting the highest level of granite are undeformed dykes of pegmatite and aplite similar to those found cutting late-kinematic granites adjacent to schist belts. The term "granite" accurately describes the general composition of the late-kinematic granites, being more "granitic" (s.s.) than the syn-kinematic granites they intrude. However, there is a large variation in composition within small bodies adjacent to amphibolitic material such as schist belts. Hybridisation with basic material gives rise to hornblende syenites, diorites and granites. In the Kangari Hills, Marmo (1971) recognised compositional variation from hornblende syenites to true granites. Most were microclinerich with strongly sericitised albitic plagioclase. The granites appear to be intrusive, and are themselves intruded by coarse-grained variants rich in magnetite, tourmaline and muscovite, while accessory minerals include
262
cassiterite, columbite, fluorite and molybdenite. The granites rarely contain microcline porphyroblasts, and their mineralogy is dominated by microcline and quartz, with accessory muscovite, epidote and albite. This mineral assemblage suggests a rather lower grade of metamorphism at the time of their intrusion then when the syn-kinematic granites were forming. All the late-kinematic granites are considerably more homogeneous than their antecedents, and are most easily recognised by their lack of tectonised basic relicts. The larger bodies around the schist belts are frequently emplaced in anti~ clinal fold cores, and many partially replace existing supracrustal host, or have gradational contacts with the syn-kinematic granites (Marmo, 1962). In the Nimini Hills, detailed mapping (Williams and Williams, 1975) has shown that several phases of late-kinematic granite are recognisable, forming a continuous spectrum between syn- and late-kinematic granites. The true late-kinematic granites are hardly foliated except in contact with schist belts, where the deformation appears to have been most long lasting and intense. They are cut by tourmaline and hornblende-bearing veins of granite, which are undeformed. The granites also occur as kilometre wide dykes of north-south trend in eastern Sierra Leone, and these are thought to be the level three of Rollinson, equivalent to the Tingi Hills granite. Associated with the younger granites are veins of quartz, graphic granite, pegmatite and aplite. Zoned pegmatite dykes with inward growing albite crystals up to 0.5 m long are characteristically intruded adjacent to the schist belts. The younger granites themselves are cut by amphibolitic to granitic joint-controlled dykes, only mildly deformed, probably during intrusion. These composite dykes are associated with foliated aplites which terminate as shear zones and pegmatites. ARCHAEAN STRUCTURE
The Archaean of Sierra Leone may be divided into two regional structural units, the Liberian granite--greenstone terrain and the Kasila Group Mobile Belt.
Liberian plastic deformation in the granite- greenstone terrain Regionally, the Liberian structural domain consists of syn-kinematic granites and supracrustals deformed and metamorphosed during a period ending 2700 Ma ago. Foliations fan from NNW--SSE in the west, through N--S, to NE--SW in the extreme southeastern part of Sierra Leone. The supracrustal schist belts display a non-linear trend, and each belt consists of an elongate basin structure. Attempts to subdivide the Archaean granitic terrain on the basis of local variations in trend of foliation are probably invalid, since such variation is a complementary part of its tectonic style. Inhomogeneous deformation is common, where synformal greenstone belts are the loci of intense deformation, while the intervening antiformal
263 areas are sites of low intensity deformation characterised by domal structures pierced by successive phases of late-kinematic granites. The Basement granitic migmatites are characterised by a dominantly vertical foliation defined by sporadic gneissose banding, abundant quartzo-feldspathic veins, amphibolitic inclusion trails, microcline porphyroblasts and parallel alignment of matrix constituents. The great majority of foliation measurements record vertical or very steep dips. Lithological layering is sometimes cut by roughly N--S biotite or quartz--feldspar axial planar fabrics developed on or near the hinge of an otherwise indistinguishable mesoscopic fold. These and regional scale folds result in the localised E - W striking foliations seen in the northern and eastern parts of Sierra Leone. Small-scale tight to isoclinal folds with wavelengths and amplitudes in the order of one metre are more often seen in the inhomogeneous granitic rocks, for example, those containing amphibolite inclusion trails. This bias against observing folds in relatively homogeneous granitic rocks is due both to the fact that folds are initiated with difficulty within them, and to their nebulous appearance in the field without the presence of distinct marker horizons. Fold axes in the syn-kinematic granites plunge steeply, and are often vertical, especially near to greenstone belts. On an outcrop scale, fold axes are frequently incongruent or non-linear, suggesting inhomogeneous strain rather than complex refolding mechanisms. The intrusion of basic dykes within the Liberian event does not imply a marked change in the thermal regime during intrusion but a change to high strain rate allowing the formation of relatively brittle structures (Escher et al., 1976).
Deformation in the Kasila Group Mo bile Belt The contrasts in deformation style and trend of the Kasila Group relative to the Liberian granite--greenstone terrain to the east prompted the interpretation that the Kasila Group is a high grade mobile belt, forming the border of the Archaean Craton (Williams and Williams, 1976; Williams, 1977). Characterised by a NNW- SSE to NW--SE strike of the high grade gneisses and granulites, the Kasila Group is clearly discordant to the granite-greenstone terrain structures. The belt contains relatively few minor structures and is typified by straight foliated gneisses with shallow to intermediate dips to the southwest (Mackenzie, 1961; Vallance, 1975).. Fold axes plunge southwest within the foliation. The northeast margin of the belt is poorly exposed and its complexities include both Archaean and subsequent Pan-African events. Granulite facies granulation zones dipping at shallow angles to the southwest represent Archaean thrusting movements towards the northeast. In the south of the country however, the Kasila Group is discordant to, but passes directly into the granite--greenstone terrain (Rollinson, 1974). Elsewhere, the boundary is a thrusted one, involving the interleaving of Kasila Group gneisses with the highly deformed basement of the adjacent granite--greenstone terrain (Allen, 1969).
264
Structure of the Marampa Group The supracrustals of the Marampa Group are not part of a classical greenstone belt structure, neither have they any structural similarity with the adjacent Kasila Group. Stratigraphically, the Marampa Group is similar in some respects to the Sula Group succession, but it is clearly not a greenstone belt. Large refolded recumbent structures, the low grade of metamorphism, the high stratigraphic and structural level, and its lower thrusted contact (Allen, 1969) with the Liberian granitic Basement migmatites suggest that it is a nappe-like structural unit (klippen) derived from the adjacent Kasila Group Mobile Belt. Orogenic activity within the Kasila Group prior to the high grade metamorphism allowed the separation and sliding of high level units of material as nappes, although this is unlikely to be confirmed palinspastically. These nappes, on descending to the adjacent Basement granites which were still undergoing the Liberian tectonothermal event, were intruded by minor quantities of granite and metamorphosed to the greenschist facies. The apparent gradation in stratigraphy and metamorphism between the Marampa and Kasila Groups noticed by Pollett (1951) is clearly that expected on tracing the low-grade nappe structure to its root zone in the Mobile Belt.
Archaean metamorphism The metamorphic grade of the Sula Group and the enclosing syn-kinematic granitic rocks is usually within the lower parts of the amphibolite facies, although small zones of granulite facies rocks occur in the southern half of the Liberian granitic terrain. The greater portion of the Kasila Group is typically granulite facies, but its southwest and northeast flanks are in the almandine--amphibolite facies. The two contrasting structural units are also metamorphic units, and there is a sharp but definite gradation between the two where the original contact is intact. The increase in grade in the granite-greenstone terrain towards the Kasila Group is especially noticeable in the south where the contact is unmodified by later thrusting. Adjacent to the Kasila Group in the south is a zone up to 50 km wide of almandine-amphibolite facies rocks of the granite--greenstone domain, in which granulite facies rocks do occur, usually partially retrogressed. The Mano-Moa granulites are an example of one of these areas which developed synchronously with the much lower grade Kambui schists. Proposals that these variations in metamorphic grade involve complicated age relationships or regional-scale faulting (Wood, 1972) have been discounted by Rollinson (1975) who showed that neither explanation is feasible, or necessary. Williams (1977} has proposed a model to explain the differential metamorphic grades in the Archaean terrain of Africa. In the granite-greenstone terrain, high-pressure mineral assemblages are unknown. Sillimanite, staurolite, cordierite and garnet occur in the highest
265 grades within the metasediments while in the granitic terrain, amphiboles and biotites are the chief metamorphic mafic minerals. Rare orthopyroxenes in dioritic and basic rocks of syn- to late-kinematic age are generally of relict igneous status, and contain high temperature exsolution lamellae. O r t h o p y r o x e n e is,however, found as a metamorphic phase in those parts of the granitic terrain adjacent to the Kasila Group. In the supracrustal sequences the mineral assemblages are typical of those found on the Abukuma Plateau (Miyashiro, 1961). Highest grades occur next to the supracrustal --Basement contact, the lowest, in the central portions of the synclinal schist belts. The Kasila Group, in contrast, is an example of extremely high temperature and pressure metamorphism. Orthopyroxenes are found in all rock compositions, while quartz and aluminous spinels coexist. Kyanite and sillimanite coexist as both syn- and late-kinematic growths. Garnets in the leucogabbros contain over 50% of the pyrope molecule. The Marampa Group is anomalous in that its metamorphic grade is within the greenschist facies, with its grade increasing towards granulite facies as the Kasila Group is approached. This is consistent with the suggestion that the present outcrops of the Marampa Group are nappes and klippen, some with roots in the Kasila Group. Brittle deformation Three major directions of brittle structures were developed within the Late-Archaean, north--south, east-northeast and southeast. The north-south and east-northeast (070 °) structures developed as a set of conjugate faults, with small displacements. The less-common southeast structures developed parallel with the Kasila Group Mobile Belt. The north - s o u t h set is dominant overall because it developed parallel with the Liberian tectonic grain in the granitic terrain. Mylonites have been recorded from many faults, b u t measurements of throw are difficult in such poorly exposed terrain. The senses of displacement are dextral on the north--south faults, and sinistral on the east--northeast, indicating a maximum stress direction from the southwest, in agreement with the thrusting movements associated with the penecontemporaneous Kasila Group Mobile Belt. All brittle structures were reactivated during the Mesozoic, when kimberlitic magmatism occurred in the Cretaceous {Willams and Williams, 1977). DISCUSSION Sierra Leone and the West African Archaean Craton The Sula Group is part of a set of supracrustal bodies which form greenstone belts throughout eastern and central Sierra Leone, Guinea, most of western and central Liberia, and the extreme western parts of Ivory Coast.
266 Rollinson (1975) defined this area as the West African Archaean Craton, part of the more extensive West African Craton. Collation of data from this region shows that the Sonfon and Tonkolili Formation thin in sympathy from the rim of the Craton, eastwards. In Liberia, the Sonfon Formation ceases to exist, while the overlying Tonkolili Formation is quartzitic. As a whole, the Sula Group thins from 4 km in central Sierra Leone, to a few hundred metres in Liberia. There is no similar recorded variation in the composition or behaviour of the migmatitic Basement rocks, apart from the general decrease in metamorphic grade northwards, away from the Kasila Group. In connection with this variation, large relatively homogeneous, late-kinematic granites, including the massif hills of Kabala, Tingi, Loma and Nimba (Liberia), developed along a line a b o u t 200 km from the Mobile Belt. The Kasila Group is known to exist in southwestern Liberia and in Guinea. In Liberia, where it is discordant to the coastline, it migrates onto the continental shelf where it has been recognised by remote sensing (Behrendt and Wotorson, 1974). Little published data is available, but it appears that the Kasila Group does not change its character along its entire known extent. The Marampa Group is known from Guinea as a source of haematite iron, but it is soon overlain northwards by Phanerozoic formations~
Correlation with adjacent Arehaean shields The Guyana Shield and the West African Craton have many similarities in lithology and tectono-thermal sequence (McConnell, 1969; Hurley et al., 1971). No direct correlation of individual groups of Archaean rocks across the south Atlantic is realistic considering the distances involved, but the major tectonic boundaries separating Archaean, Eburnean and Pan-African units may be traced from one continent to another (Hurley, 1973).
Geochronology The West African and Guyanan Shields are being actively studied by Hedge et al. (1975) and Hurley et al. (1971, 1975). The data shows a division of the Early to Middle Precambrian into three tectono-thermal units. Early Archaean events, possibly igneous crystallisation ages which are recognisable as "relicts", give ages between 3700 and 3100 Ma, using reference isochrons. These early events are found scattered within the Liberian terrain for which numerous isochrons indicate the age of the last major event as 2 8 0 0 - 2 7 0 0 Ma. Cutting the Archaean are fold belts of Early Proterozoic age, giving results of between 2100 and 1800 Ma. On the basis of recent geochronological work, it appears that the early Archaean ages of West Africa are equivalent to the Gurian of Guyana. Similarly, the Liberian and associated Kasila Group are equivalent to the Ima-
267
tacan, while the Eburnean of West Africa is equivalent to the Trans-Amazonian of the Guyana Shield. More precise correlations of units are unrealistic at present.
Mineral potential In c o m m o n with most Archaean terrains, there is considerable interest in mineralisation. In Sierra Leone there is little evidence of large-scale oregrade mineral potential. Reviews of the ore deposits and potential of Sierra Leone (Dixey, 1925; Pollett, 1951; Morel, 1976) have been optimistic, based on the superficial similarity between the Archaean of Sierra Leone and that of highly mineralised areas of the Superior Province or the Rhodesian Craton. The generally modest mineralisation of Sierra Leone is a reflection of the marked differences in stratigraphy and scale in relation to the enormous individual developments of Archaean supracrustals in southern Africa and central Canada. The most major difference is the lack of large quantities of ultramafic and andesitic lavas in the Sula Group, which limits the variety and extent of the volcano-exhalative deposits. The small scale on which ultramafic intrusive activity occurred at the base of the Sula Group has given barely significant nickel and chromium concentrations. At present, the minerals undergoing exploitation from deposits of Archaean origin are bauxite and rutile. Gold, chromite and haematite iron have been mined in the past, b u t have since become uneconomic. ACKNOWLEDGEMENTS
Without access to the unpublished work of the Geological Surveys Department of Sierra Leone, and former members of the Geology Department, University of Sierra Leone, this review would n o t have been possible. Critical discussion and improvements by Hugh Rollinson and R. Williams are gratefully acknowledged. Many of the conclusions presented are the sole responsibility of the author, whose fieldwork has been sponsored by the National Diamond Mining Company of Sierra Leone, the Diamond Corporation of West Africa, and the University of Sierra Leone. REFERENCES Allen, P.M., 1969. Geology of part of an orogenic belt in western Sierra Leone. Geol. Runds., 58: 588--619. Andrews-Jones, D.A., 1966. Geology and mineral resources of the northern Kambui Hills schist belt and adjacent granulites. Geol. Surv. Sierra Leone Bull., 6 : 1 0 0 pp. Anhaeusser, C.R., Mason, R., Viljoen, M.J. and Viljoen, R.P., 1969. A reappraisal of some aspects of Precambrian shield geology. Bull. Geol. Soc. Am., 80: 2175--2200. Behrendt, J.C. and Wotorson, C.S., 1974. Geophysical surveys of Liberia with tectonic and geologic interpretation. U.S. Geol. Surv. Prof. Pap., 8 1 0 : 3 8 pp. Dixey, F., 1925. The geology of Sierra Leone. Q. J. Geol. Soc. Lond., 81: 195--222. Dunham, K.C., Phillips, R., Chalmers, R.A. and Jones, D.A., 1958. The chromiferous ultrabasic rocks of Sierra Leone. Overseas Geol. Min. Res. Bull., 3 : 4 4 pp.
268 Escher, A., Jack, S. and Watterson, J., 1976. Tectonics of the North Atlantic Proterozoic dyke swarm. Philos. Trans. R. Soc. Lond. A, 280: 529--540. Hedge, C.E., Marvin, R.F. and Naeser, C.W., 1975. Age provinces in the Basement rocks of Liberia. J. Res. U.S. Geol. Surv., 3: 425--429. Hunter, D.R., 1971. The granitic rocks of the Precambrian in Swaziland. Inf. Circ. Econ. Geol. Res. Unit, Univ. Witwatersrand, Johannesburg, 6 3 : 2 5 pp. Hurley, P.M., 1973. On the origin of 450 -+200 m.y. orogenic belts. In: D. Tarling and S. Runcorn (Editors), Implications of Continental Drift to the Earth Sciences. Academic Press, London, 1184 pp. Hurley, P.M., Leo, G.W., White, R.H. and Fairbairn, H.W., 1971. Liberian age province (about 2700 m.y. ) and adjacent provinces in Liberia and Sierra Leone. Bull. Geol. Soc. Am., 82: 3483--3490. Hurley, P.M., Fairbairn, H.W. and Gaudette, H.E., 1975. Progress report on the early Archaean rocks in Liberia, Sierra Leone and Guyana. In B.F. Windley (Editor), Early History of the Earth. Wiley, London, pp. 511--521. MacFarlane, A., Crow, M.J., Arthurs, J.W. and Wilkinson, A.F., 1974. The geology and mineral resources of northern Sierra Leone. Inst. Geol. Sci. Overseas Div. Intern. Rep., 34. Mackenzie, D.H., 1961. Geology and mineral resources of the Gbangbama area. Geol. Surv. Sierra Leone Bull., 3 (unpubl.). Marmo, V., 1962. Geology and mineral resources of the Kangari Hills schist belt. Geol. Surv. Sierra Leone Bull., 2 : 1 1 7 pp. Marmo, V., 1971. Granite Petrology and the Granite Problem. Elsevier, Amsterdam, 244 pp. Mason, R., 1973. The Limpopo Belt-southern Africa. Philos. Trans. R. Soc. London A, 273: 463--485. Morel, S.W., 1976. The Geology and Minerals of Sierra Leone. Fourah Bay College Bookshop, Freetown, 21 pp. Pollett, J.D., 1951. The geology and mineral resources of Sierra Leone. Coll. Geol. Min. Res., 5: 3--28. Rollinson, H., 1973. Report on geology of sheets 48, 49, 59, 60. Eastern Kono District. Geol. Surv. Sierra Leone Rep. (unpubl.). Rollinson, H., 1974. Report on geology of sheet 102, south Kambui Hills and surrounding areas. Geol. Surv. Sierra Leone Rep. (unpubl.). Rollinson, H., 1975. Report on geology of sheet 58, Nimini Hills and surrounding areas. Geol. Surv. Sierra Leone Rep. (unpubl.). Vallance, G., 1975. Report on geology of sheets 76, 77, 78, 79, 88, 89, 90. Geol. Surv. Sierra Leone Rep. (unpubl.). Williams, H.R., 1976. Granitisation of basic rocks in the Bjornesund area, near Fiskenaesset, West Greenland. Rapp. Gron. Geol. Unders., 73: 47--54. Williams, H.R., 1977. African Archaean Mobile Belts and granite~greenstone terrain. Nature, 266: 163--63. Williams, H.R. and Williams, R.A., 1975. The geology of the Yengema area, Kono District, Sierra Leone. Report to N.D.M.C., Freetown, Sierra Leone (unpubl.). Williams, H.R. and Williams, R.A., 1976. The Kasila Group, Sierra Leone, an interpretation of new data. Precambrian Res., 3: 505--508. Williams, H.R. and Williams, R.A., 1977. Kimberlites and plate-tectonics in West Africa. Nature. Wilson, J.F., 1973. The Rhodesian Archaean craton - - a n essay in cratonic evolution. Phil. Trans. R. Soc. Lond. A, 273: 389~-411. Wilson, N.W., 1965. The geology and mineral resources of part of the Gola Forests, southern Sierra Leone. Geol. Surv. Sierra Leone Bull., 4 : 1 0 2 pp. Wilson, N.W. and Marmo, V., 1958. Geology, geomorphology and mineral resources of the Sula Mountains. Geol. Surv. Sierra Leone Bull., 1. Wood, R.S., 1972. Early Precambrian Kambui Schist Belt, southern Sierra Leone, and its surrounding basement. Nature, 236: 14.
251
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
Review THE A R C H A E A N G E O L O G Y OF S I E R R A LEONE
H.R. WILLIAMS
Department of Geology, University of Sierra Leone, Freetown (Sierra Leone) (Received and accepted November 10, i 9 7 7 )
ABSTRACT Williams, H.R., 1978. The Archaean geology of Sierra Leone. Precambrian Res., 6: 251--268. The Archaean of Sierra Leone is divisible into two pene-contemporaneous tectonothermal units, the Liberian granite~greenstone terrain, and the Kasila Group Mobile Belt with its associated klippen, the Marampa Group. The granite--greenstone terrain consists of the Sula Group supracrustals, deformed and metamorphosed over an underlying gneissose granite migmatite Basement, yielding ages of about 2700 Ma. Late kinematic granites are found at the margins of the greenstone belts, and as high-level developmerits of the reactivated Basement. The Kasila Group is a larger scale supracrustal belt which developed into a zone of extreme orogenic activity now forming the southwest margin of the West African Archaean Craton.
INTRODUCTION TO STRATIGRAPHY
Archaean rocks in Sierra Leone, dated as more than 2100 Ma old, form an integral part of the West African Archaean Craton. Two main structural divisions are recognized, the Liberian (Hurley et al., 1971) granite--greenstone terrain, and the pene-contemporaneous Kasila Group Mobile Belt (Fig. 1). Radiometric ages from both divisions show a spread from 2100 Ma to over 3400 Ma (Hedge et al., 1975; tturley et al., 1975). The Liberian granite-greenstone terrain consists of N--S trending foliated and unfoliated "granites" and synformal, dominantly supracrustal amphibolites, greywackes, banded iron formation, magnesian sediments a n d tuffs, and ultramafic and basic intrusives. Most of the terrain is metamorphosed to the epidote--amphibolite facies. The Kasila Group consists of a supracrustal succession oriented NW--SE, but m e t a m o r p h o s e d to the granulite facies and almandine--amphibolite facies. The boundary between the two structural divisions is a zone of high strain and thrusting, with localised reworking of the Liberian structures adjacent to the Kasila Group. The Marampa Group (Allen, 1969; MacFarlane et al., 1974), has a typically Archaean stratigraphy, b u t is neither a greenstone nor a mobile belt.
252
BY GEOLOGICAL ,SURVEY OF
.~..
ICON
\
CO*ST
~IGE~IA
x * KABALA
0?
~ o
^ . . . .
,?"
~'°/[
\,~
°-
~'
/
D
~ \~
O
o, SIERRA
®"o
r
0
,
KENEMA -
'\
"
l)
"%.
/
,
8
~
Of
"
~[~_~I~,,,,TES LEVEL
~
~-.\o \= ,\2 l:.l=A.A~,A~o, LEONE
,.',W
.o,.
I~;W
"
o
S~;K.~T,C
"
,
KAMBUI
~. . . . . . c
j
j .E.E.,
,,].
Fig. l. Arehaean geology of Sierra Leone. Granite ma~ifs; A, I~bala; B, Loma; C, Gbengbe; D, Tingi; E, Kongot~. Supracrustal schist belts of T,iberian age: 1, Loko Hills; 2,
Sankarama; 3, Serekolia; 4, Sula Mountains; 5, Kangari Hills; 6, Nimini Hills; 7, Gori Hills; 8, Kambui Hills; 9, Mano-Moa Granulites (Gola Forests). THE GRANITIC T E R R A I N
Granitic rocks are the dominant c o m p o n e n t of the granite--greenstone division, but have been studied proportionately less than the greenstone belt sequences of the Sula Group. Most reports on the geology of the individual greenstone belts include only a cursory examination of the adjacent granites. A reconnaissance study by MacFarlane et al. (1974), and similar work in
''j
! ___j
253 eastern Sierra Leone by Rollinson (1973), were the first attempts at structural mapping of the granitic terrain. Basement granites The term "basement granites" is one of convenience, since the basement relationship to the Sula Group supracrustals is rarely, if ever, recognisable. The basal quartz--mica schists in the Kambui Schist Belt suggest that the supracrustal sequence developed initially on a granitic terrain, which exhibits a longer structural history than that in the supracrustals (Rollinson, 1974). The difficulty in the recognition of true basement is due to its modification during the Liberian tectono-thermal event, culminating in the formation of distinctly discordant late-kinematic granites, often called "younger granite". Basement granites are syn-kinematic, are not demonstrably younger than the supracrustal belts, and may be described as homogeneous to gneissic granitic (s.1.) migmatites. They have frequently been studied only during reconnaissance surveys, except in the Yengema area, where 400 km2 of detailed mapping indicated a complex structural history (Williams and Williams, 1975). The greater part of the Basement granitic terrain is composed of layers and lenses up to hundreds of metres thick, of pink, grey and white coloured, biotite and hornblende bearing foliated granitic migmatites. These migmatites contain patchy granitic areas consisting of homogenised, parautochthonous Basement, pale in colour and poor in mafic material. These relatively homogeneous bodies are sporadically distributed throughout the Basement, with gradational, intrusive and frequently indistinct contacts with the more foliated host. The migmatites have been described as syn-kinematic granites by most workers (N.W. Wilson, 1965; Andrews~lones, 1966; Marmo, 1971). Marmo describes the granites as becoming more even-textured and homogeneous away from the immediate confines of the supracrustal belts. However, gneissose migmatitic granites also surround parautochthonous syn-kinematic granite massifs such as the Gbengbe Hills (Wilson and Marmo, 1958) (Fig. 2). The syn-kinematic granites are migmatites with quartz-dioritic to granodioritic melanosomes and approximately granitic (s.s.) neosomes. Where the bulk composition is relatively mafic, hornblende is characteristically developed in the neosome, with biotite and hornblende in the melanosome. Nebulous and virtually homogeneous migmatites contain little mafic material, usually biotite. The compositional variation within ~ e term granitic migmatite is largely controlled by the composition, proportion and degree of hybridisation of the included mafic material, and the degree to which the rocks have been subjected to potassium metasomatism and remobilisation. The rocks generally have a high calcium to potassium ratio, causing oligoclase to dominate over microcline, except where late-stage metasomatism has allowed the growth of microcline megacrysts of ovoid and rectangular shape. The smaller, 30 mm long ovoid porphyroblasts frequently grow
254
ALLOCHTHONOU5 LATE- KINEMATIC L E V E L 3-HOOD* GRANITE \ \~"~x
PA RAU TOCHTHONOUS SYN- KINEMATIC GRANITE LEVEL 2
SUPRACRUSTAL GNEENSTONE B~LT GREENSTONE BELT ~ELICTS
x
'
(
-
~
~
/
~.AUREOLE OF 'NTEESE DEFORMATION
"~
LEVEL 2 j
,
+~ ~
~ ~- ~r
soP21g".;~'T~°LS I
,
( ALLOCHTHONOUS LATE- KINEMATIC G~ANI TES
[ I
LEVEL
I
SYN- KI NEMATIC
BASEMENT GIlA Ni TiC MFGMATI TES
Fig.2. Diagrammatic E--W cross-section through N--S trending Liberian granite--greenstone terrain. Vertical exaggeration 10:1; typical length of section I00 krn. (After Anhaeusser et al., 1969; Rollinson, 1973).
mimetically within the foliation, while the larger, rectangular ones, up to 0.1 m long, are more randomly oriented, and widely spaced. Distribution of the ovoid porphyroblasts is restricted to the pink homogeneous portions of the migmatite, while the rectangular ones are sporadically distributed, often in zones paralleling the foliation. Muscovite, like microcline, is a late replacive phase. Allanite, zircon, apatite, magnetite and tourmaline are constant accessories, while epidote is frequently associated with hornblende and sphene. The granites are often hybridised with basal members of the Sula Group on the margins of greenstone belts, and with tectonised remnants of basic dykes. The resulting rocks are usually hornblende-rich quartz--dioritic to syenitic agmatites and nebulites, mesocratic granodiorites, garnetiferous, magnetite- and sillimanite-rich granites (Marmo, 1962). Frequently the contact between the Sula Group and the syn-kinematic granites is modified by late-kinematic granites, but where unmodified, it is gradational, with extensive development of hybrid migmatitic rocks up to 2 km wide.
Amphibolite dykes and relicts in granitic terrain Two major sets of amphibolite dykes and their deformed derivatives may be recognised in many areas within the granitic terrain. An extremely deformed, agmatised, boudinaged, pre-Liberian set oriented parallel or subparallel with the Liberian foliation is cut by a generally discordant, synLiberian set of very variable orientation and composition. The early set of dykes is usually recognisable as lines of separated amphibolite inclusions up to 10 m wide, paralleling the migmatite foliation. They are frequently intruded by their host, which was reactivated during the Liberian tectono-thermal event. These dykes are clearly deformed remnants of a pre- or early Liberian swarm of dolerites.
255
The later, discordant set of dykes has a variable trend which is most clearly seen in zones of low intensity deformation as in east Kono district, where they have intruded along joints in a late-Liberian granite massif, the Tingi Hills. Clearly joint-controlled, they have been mildly deformed and metamorphosed, showing steeply inclined foliations, of variable intensity and trend. Hornblende lineations within the dykes are usually shallow plunging, unlike those in the host. In zones of more intense deformation, near to greenstone belts such as the Nimini Hills (Williams and Williams, 1975), the dykes are again joint-controlled, b u t deformed into buckle folds with vertical axes where the dyke trend is at right angles to the regional gneissose foliation, while those portions paralleling the foliation are relatively unaffected (Fig. 3). The later set of dykes is often intimately associated with contemporaneous granitic material. Composite dykes with basic margins grading inwards to granitic centres, and vice-versa, indicate the pene-contemporaneity of basic and granitic magmas. Only the later set of basic dykes is found in the latekinematic granites.
. ~:,.::"
PLUNGING
. . . . . ..
BUCKLEF O L D S ~ ' ~
J:-i(
~l
( ,
0
AGMATISED EAR~ IY
PPEGM EGMA7)TE ~ANOEb
GRANmCM,GMAT,TE
-°i
..........i-
"
Fig. 3. B a s i c d y k e relationships at Motema, n e a r K o i d u . Discordant sections o f d y k e are sometimes buckled, but concordant sections are not appreciably affected.
Chronological subdivision of the granitic basement MacFarlane et al. (1974) and Rollinson (1974) have provisionally subdivided the Archaean Granitic Basement into two time-structural-stratigraphic units. The older they called Leonian, and the younger, dominant unit, the Liberian. The subdivision was based on the occurrence of E - W striking foliations in limited areas within the dominant N--S striking Liberian granite--greenstone complex. In the Leonian areas they also concluded that
256 amphibolite dykes were seldom as deformed and rotated into concordancy with the gneissic foliation as in the Liberian terrain (Rollinson, 1973). The erection of a chronologic structural division in such poorly exposed terrain, on the basis of these two criteria, ignores an alternative explanation of the data which agrees with recent radiometric determinations (H. Rollinson, pers. comm.). Deformation intensity in granite--greenstone terrains is variable, being less severe in antiformal areas between the synformal greenstone belts. Variable foliation trends occur within the granitic terrain due to the nature of its multi-stage development, with linear trends modified by production and remobilisation of granitic material causing stellate outcrop patterns in greenstones, and elliptical dome--basin trends in granites (Wilson, 1973). Detailed mapping of the granitic terrain has indicated that E--W foliation trends may be due to the formation of large-scale N--S oriented folds of Liberian age, whose vertically plunging hinge zones form small outcrops of E--W striking rocks, cut by N--S axial planar fabric (Williams and Williams, 1975). Strike directions other than N--S are due to large-scale folds and to local foliation reorientation by shear zones of late-Liberian age, and need n o t be relicts of an earlier, Leonian, deformation. It appears, then, that the Leonian as recognised by E--W striking foliations and barely deformed rocks, is an integral part of the inhomogeneous Liberian event. Recent reconnaissance radiometric age dating mggests that the Leonian and Liberian events are indistinguishable, both being in the range 2700-3000 Ma. Discussion
The rapid and irregular changes in fabric and lithology of the syn-kinematic rocks of the Basement migmatites are partly a function of their position relative to the supracrustal schist belts. Lithologic-structural units are caused by large- and small-scale homogenisation and by reworking of an existing inhomogeneous granitic terrain into which basic dykes were intruded early in the Liberian event. During the deposition of Liberian supracrustals a multiphase structural event was initiated, causing a general alignment of the fabric into a N--S direction, forming fold axes and foliations of vertical attitude. S T R A T I G R A P H Y OF T H E A R C H A E A N
S U P R A C R U S T A L S (KAMBUI S U P E R G R O U P )
These rocks were formerly known as the Kambui Schists, but were renamed in accordance with contemporary nomenclature and called the Kambui Supergroup, which is divided into the Sula, Marampa and Kasila Groups.
257
The Sula Group The characteristic "greenstone" lithology of the granite--greenstone terrain, the Sula Group, was deposited, deformed and m e t a m o r p h o s e d in the Liberian event. It has been subdivided into the Sonfon and Tonkolili Formations (MacFarlane et al., 1974). The equivalents of these formations may be recognised in most of the greenstone belts. Table I shows the relationships and relative stratigraphies of the major supracrustal belts. For comparison, the stratigraphies of the Marampa and Kasila Groups are included. The resemblance of these rocks to those described from other Archaean successions in Africa is striking. However, there are some differences, for example, thick sequences of andesitic and rhyolitic extrusives are absent, while shales and cherts are developed only on a small scale. Sierra Leone contains four major zones of supracrustal greenstone belts (Fig. 1.) The Marampa Group is of much lower metamorphic grade, and is
TABLE I Stratigraphy of individual units within the Kambui Supergroup. Thicknesses are approximate and represent maximum values. Considerable variation in thickness occurs both across schist belts and along separated developments, e.g. Kambui and Nimini Hills. Sonfon Formation
~
Tonkolili Formation
Mano-Moa granulitest ultramafics (1 km), Gori Hills s.b. amphibolites (1 km) Serekolia relicts }
pelites (1 km), quartzites (1 km)
Kambui Hills s.b. Nimini Hills s.b. Sankarama relicts
pelites and greywacke turbidites (2 kin)
~ ultramafics (350 m), l amphibolites (2.5 kin)
Kangari Hills s.b. ~ ultramafics, amphibolites, Sula Mountains s.b. t b.i.f. (1.5 kin)
greywacke turbidites, pelites, conglomerates, b.i.f. (3 kin)
Loko Hills s.b.
amphibolites (200 m)
quartzites, b.i.f. (200 m)
Matoto Formation, basalts, andesites (750 m) basal thrust contact
R o t o k o l o n Formation, pelites, semi-pelites, psammites, b.i.f. matoto formation, basalts, andesites
(75o m) basalt thrust contact basic, gabbroic, anorthositic granuiites (8 kin)
s.b. = schist belts; b.i.f. = banded iron formation.
metasedimentary, granitic granulites (5 km)
258 not infolded into the Basement, while the Kasila Group is a mobile belt development with a similar stratigraphy to that of the greenstone belts {Williams, 1977). The Kambui Schist Belt is the southerly, separated extension of the Nimini Hills Schist Belt, which in turn becomes the Sankarama supracrustal relicts to the north. Similarly, the Mano-Moa Granulites are succeeded northwards by the Gori Hills Schist Belt and the Serekolia Schist Belt relicts. The Sonfon Formation consists predominantly of amphibolites of varying grain sizes and compositions, sometimes pillowed, and either massive or finely layered. Clearly meta-basaltic in origin, some also contain diopside and grossularite, perhaps indicating a spilitic composition, while some are mineralised by sulphides of iron, copper and arsenic. In the southern parts of the Liberian structural domain, the metamorphic grade of the supracrustals and Basement increases from epidote--amphibolite to granulite facies as the Kasila Group Mobile Belt is approached. This increase in grade is reflected in the mineralogy of the supracrustals to one dominated by hypersthene and calcic plagioclase. Elsewhere, the amphibolites contain a blue-green pleochroic hornblende, andesine, sphene and epidote. The base of the Sonfon Formation is always modified by the syn-kinematic granites, or by intrusive complexes of late-kinematic granite. Within the amphibolite sequence there are thin layers of pyritiferous talc schists, talc--chlorite schists, quartzites, magnetite--quartzites and cummingtonite schists. Development of the magnetite--quartzites or banded iron formation is extremely variable, for large thicknesses also occur in the Tonkolili Formation in the Sula Mountains, while only traces occur in the Nimini Hills to the east {Wilson and Marmo, 1958; Rollinson, 1975} {Fig. 4). Intrusive ultramafic rocks occur as concordant bodies within the amphibolites. They exhibit relict igneous mineralogies and textures, and are frequently heavily serpentinised. Chromiferous serpentinites and chromitite layers are found in these ultramafics, chiefly in the Sonfon Formation in the southeast part of the country (Dunham et al., 1958). The Tonkolili Formation is dominantly composed of clastic sediments with thin layers of siliceous and tuffaceous metavolcanics. Overlying the Sonfon Formation, it is rarely found in contact with the syn-kinematic granitic basement, except when all the underlying Sonfon Formation amphibolites have been made over to granite by migmatisation and metasomatism (Andrews-Jones, 1966; Williams, 1976). Greywacke turbidites, quartzites, cordierite-garnet schists and mica schists make up the bulk of the formation. In the Nimini Hills Schist Belt, cordierite schists occur at the base of the Tonkolili Formation, interbanded with amphibolitic tuffs and agglomerates. Polymictic conglomerates containing pebbles of granite, peridotite, serpentinite, amphibolite and quartz, in a matrix of chlorite and quartz, occur as layers up to 10 m thick within the lower parts of the Formation (Wilson and Marmo, 1958}. Siliceous volcanics are not common, though some fine.grained quartzo-feldspathic mica schists may be tuffaceous in origin. The
259
W i
4-
TONKOLI LI FORMATION
GT
;T
GT
3Pc
uo
SONFON FORMATION
// /i f! i! iI l! !/ '1
o
u
+
km
....
2-
+. i I-
o
Fig. 4. Stratigraphic cross-sections at three points along Nimini Hills schist belt, showing variation in lithology and thickness (north to south from left to right). GT, greywacke turbidite; u, ultramafics; a, amphibolites; bif, banded iron formation; cg, cummingtonite--garnet schist;p, pelites; us, ultramafic sill; crosses, younger granite. (After Rollinson, 1975).
"turbidites and quartzites contain way-up structures such as graded bedding, flame-structures and cross-stratification. Evidence from the Gori Hills Schist Belt indicates that the sedimentary facies in the western portion is turbiditic, while that in the east is dominated by quartzites (H. Rollinson, pers. comm.). The stratigraphy of the Sonfon and Tonkolili Formations shows rapid and irregular changes in lithology and thickness. Accurate determinations of thickness are impossible, especially as the duplication and omission due to folding, and proportion of interbanded granite are unknown. However, it is clear from the sequence and character of the lithologies that there is a similarity between the Sula Group and the generalised sequence in Archaean greenstone belts (Anhaeusser et al., 1969).
The Marampa Group The Marampa Group is subdivided into a lower, Matoto Formation consisting of pillowed basic lavas, serpentinites and andesites; and an upper, R o t o k o l o n Formation consisting of psammites, pelites and banded iron formation (MacFarlane et al., 1974). Allen (1969) recognised the same lithologies as MacFarlane et al., but did not divide them into formations. The Matoto Formation consists of 750 m of lavas with pillowed horizons up to 25 m thick. Andesites with phenocrysts of pyroxenes are flow banded. Interlayered with mafic lavas are ultramafic rocks, consisting of olivineserpentinites and chloritic rocks. The R o t o k o l o n Formation is mainly metasedimentary and contains pebbly horizons showing cross-laminations, with
260 subsidiary tuffs and andesites. Quartzites in the upper part are frequently rich in haematite, specularite, fuchsite, and are manganiferous. The metasediments consist of sericitic quartz schists, biotite--muscovite schists, garnetiferous schists, quartzites and biotite paragneiss. The volcanics in the lower parts of the Group tend to be chlorite--sericite schists in the east and amphibolites in the west, reflecting the general increase in metamorphic grade from east to west. Granites intrude the paragneisses in the west, forming lit-part-lit gneisses (Pollett, 1951). The basic portions of the Marampa Group pass westwards into the high grade rocks of the Kasila Group. The Kasila Group The Kasila Group consists of a series of basic, granitic, and metasedimentary granulites, flanked by amphibolites. Trending NW-SE, it forms the coastal rim to the West African Archaean Craton {Williams and Williams, 1976). Westward dipping mylonite zones occur within the high grade rocks, and in the eastern, lower grade flank, adjacent to the granite-greenstone terrain. These zones are of Archaean and Pan-African age Much of the Kasila Group is composed of fine to medium grained basic granulites containing minor horizons of quartz--magnetite, quartz--diopside and sillimanite-bearing rocks, representing highly metamorphosed equivalents of banded iron formation, marbles and pelites respectively. Extreme deformation renders reliable estimation of the stratigraphic thickness of lithologies meaningless. The west side of the Group, concealed by Phanerozoic epicontinental deposits (Pollett, 1951), consists of a gradational series from basic granulites to amphibolites and metasedimentary gneisses, while further west, amphibolites form the dominant lithology. Within the basic granulites, along the whole length of the outcrop of the Kasila Group, occur 200 m thick, layered anorthositic and leucogabbroic rocks, representing the metamorphosed equivalents of layered igneous intrusions. The basic and quartzo--feldspathic rocks within the Kasila Group represent a similar stratigraphic sequence to that found in the Sula Group. The chief differences are the occurrence of layered anorthositic rocks and the lack of significant ultramafic lithologies. The leucogabbroic layered complexes contain layers ranging in composition from ultramafic to anorthositic. Sometimes, original igneous textures are l~eserved in zones of low deformation intensity, or where coarse-grained, often pegmatitic, rocks have resisted the intense shear deformation. Major anorthositic bodies with ill
261
contrasts in lithology, metamorphism and structure mitigate against such comparisons, and suggest a similarity with a Limpopo Belt structure, sharing the same relationship with the Liberian granite-greenstone terrain as the Limpopo Belt does with the ghodesian Craton (Mason, 1973). THE LATE-KINEMATIC OR " Y O U N G E R " G R A N I T E S
Granites which are demonstrably younger than the supracrustals, the synkinematic granitic migmatites and the pervasive Liberian deformation are termed "younger" or "late-kinematic" (Marmo, 1971). They are most common around the margins of the Sula Group geenstone belts, especially in the north and east of the country, where the metamorphic grades are lower than in the southeast. Away from schist belts, large massifs of fine gained granite, such as the upper parts of the Tingi Hills, Loma Mountains and the Kabala Hills, are formed of late-kinematic granite, while waall plutons and dykes up to several kilometres in width may be found over much of the granitic terrain (Williams and Williams, 1975). In the southeast of Sierra Leone, late-kinematic material is restricted to small intrusive bodies on the margins of the Kambui schist belt, including some discordant pegmatites and aplites. Wilson (1965) estimates that the volume of late-kinematic granite in the Gola Forests area is a fifteenth of that found in the vicinity of the Sula Mountains schist belt. In contrast, in Northern Sierra Leone, many large massifs and numerous smaller bodies adjacent to greenstone belts have been mapped. The larger bodies appear to have gradational contacts with the surrounding syn-kinematic host, but others appear to overlie the older granites in a fashion which resembles the gani~e tectonic levels of Hunter (1971). The Tingi Hills consist of three such levels (Rollinson, 1973). The lowest consists of normal syn-kinematic basement migmatite, level two is composed of microcline porphyroblastic hornblende granite similar to that seen in the Gbengbe Hills (homogeneous parautochthonous syn-kinematic granite), while level three is situated above this and is true late-kinematic granite, although even this has been mildly deformed during the Liberian event. Cutting the highest level of granite are undeformed dykes of pegmatite and aplite similar to those found cutting late-kinematic granites adjacent to schist belts. The term "granite" accurately describes the general composition of the late-kinematic granites, being more "granitic" (s.s.) than the syn-kinematic granites they intrude. However, there is a large variation in composition within small bodies adjacent to amphibolitic material such as schist belts. Hybridisation with basic material gives rise to hornblende syenites, diorites and granites. In the Kangari Hills, Marmo (1971) recognised compositional variation from hornblende syenites to true granites. Most were microclinerich with strongly sericitised albitic plagioclase. The granites appear to be intrusive, and are themselves intruded by coarse-grained variants rich in magnetite, tourmaline and muscovite, while accessory minerals include
262
cassiterite, columbite, fluorite and molybdenite. The granites rarely contain microcline porphyroblasts, and their mineralogy is dominated by microcline and quartz, with accessory muscovite, epidote and albite. This mineral assemblage suggests a rather lower grade of metamorphism at the time of their intrusion then when the syn-kinematic granites were forming. All the late-kinematic granites are considerably more homogeneous than their antecedents, and are most easily recognised by their lack of tectonised basic relicts. The larger bodies around the schist belts are frequently emplaced in anti~ clinal fold cores, and many partially replace existing supracrustal host, or have gradational contacts with the syn-kinematic granites (Marmo, 1962). In the Nimini Hills, detailed mapping (Williams and Williams, 1975) has shown that several phases of late-kinematic granite are recognisable, forming a continuous spectrum between syn- and late-kinematic granites. The true late-kinematic granites are hardly foliated except in contact with schist belts, where the deformation appears to have been most long lasting and intense. They are cut by tourmaline and hornblende-bearing veins of granite, which are undeformed. The granites also occur as kilometre wide dykes of north-south trend in eastern Sierra Leone, and these are thought to be the level three of Rollinson, equivalent to the Tingi Hills granite. Associated with the younger granites are veins of quartz, graphic granite, pegmatite and aplite. Zoned pegmatite dykes with inward growing albite crystals up to 0.5 m long are characteristically intruded adjacent to the schist belts. The younger granites themselves are cut by amphibolitic to granitic joint-controlled dykes, only mildly deformed, probably during intrusion. These composite dykes are associated with foliated aplites which terminate as shear zones and pegmatites. ARCHAEAN STRUCTURE
The Archaean of Sierra Leone may be divided into two regional structural units, the Liberian granite--greenstone terrain and the Kasila Group Mobile Belt.
Liberian plastic deformation in the granite- greenstone terrain Regionally, the Liberian structural domain consists of syn-kinematic granites and supracrustals deformed and metamorphosed during a period ending 2700 Ma ago. Foliations fan from NNW--SSE in the west, through N--S, to NE--SW in the extreme southeastern part of Sierra Leone. The supracrustal schist belts display a non-linear trend, and each belt consists of an elongate basin structure. Attempts to subdivide the Archaean granitic terrain on the basis of local variations in trend of foliation are probably invalid, since such variation is a complementary part of its tectonic style. Inhomogeneous deformation is common, where synformal greenstone belts are the loci of intense deformation, while the intervening antiformal
263 areas are sites of low intensity deformation characterised by domal structures pierced by successive phases of late-kinematic granites. The Basement granitic migmatites are characterised by a dominantly vertical foliation defined by sporadic gneissose banding, abundant quartzo-feldspathic veins, amphibolitic inclusion trails, microcline porphyroblasts and parallel alignment of matrix constituents. The great majority of foliation measurements record vertical or very steep dips. Lithological layering is sometimes cut by roughly N--S biotite or quartz--feldspar axial planar fabrics developed on or near the hinge of an otherwise indistinguishable mesoscopic fold. These and regional scale folds result in the localised E - W striking foliations seen in the northern and eastern parts of Sierra Leone. Small-scale tight to isoclinal folds with wavelengths and amplitudes in the order of one metre are more often seen in the inhomogeneous granitic rocks, for example, those containing amphibolite inclusion trails. This bias against observing folds in relatively homogeneous granitic rocks is due both to the fact that folds are initiated with difficulty within them, and to their nebulous appearance in the field without the presence of distinct marker horizons. Fold axes in the syn-kinematic granites plunge steeply, and are often vertical, especially near to greenstone belts. On an outcrop scale, fold axes are frequently incongruent or non-linear, suggesting inhomogeneous strain rather than complex refolding mechanisms. The intrusion of basic dykes within the Liberian event does not imply a marked change in the thermal regime during intrusion but a change to high strain rate allowing the formation of relatively brittle structures (Escher et al., 1976).
Deformation in the Kasila Group Mo bile Belt The contrasts in deformation style and trend of the Kasila Group relative to the Liberian granite--greenstone terrain to the east prompted the interpretation that the Kasila Group is a high grade mobile belt, forming the border of the Archaean Craton (Williams and Williams, 1976; Williams, 1977). Characterised by a NNW- SSE to NW--SE strike of the high grade gneisses and granulites, the Kasila Group is clearly discordant to the granite-greenstone terrain structures. The belt contains relatively few minor structures and is typified by straight foliated gneisses with shallow to intermediate dips to the southwest (Mackenzie, 1961; Vallance, 1975).. Fold axes plunge southwest within the foliation. The northeast margin of the belt is poorly exposed and its complexities include both Archaean and subsequent Pan-African events. Granulite facies granulation zones dipping at shallow angles to the southwest represent Archaean thrusting movements towards the northeast. In the south of the country however, the Kasila Group is discordant to, but passes directly into the granite--greenstone terrain (Rollinson, 1974). Elsewhere, the boundary is a thrusted one, involving the interleaving of Kasila Group gneisses with the highly deformed basement of the adjacent granite--greenstone terrain (Allen, 1969).
264
Structure of the Marampa Group The supracrustals of the Marampa Group are not part of a classical greenstone belt structure, neither have they any structural similarity with the adjacent Kasila Group. Stratigraphically, the Marampa Group is similar in some respects to the Sula Group succession, but it is clearly not a greenstone belt. Large refolded recumbent structures, the low grade of metamorphism, the high stratigraphic and structural level, and its lower thrusted contact (Allen, 1969) with the Liberian granitic Basement migmatites suggest that it is a nappe-like structural unit (klippen) derived from the adjacent Kasila Group Mobile Belt. Orogenic activity within the Kasila Group prior to the high grade metamorphism allowed the separation and sliding of high level units of material as nappes, although this is unlikely to be confirmed palinspastically. These nappes, on descending to the adjacent Basement granites which were still undergoing the Liberian tectonothermal event, were intruded by minor quantities of granite and metamorphosed to the greenschist facies. The apparent gradation in stratigraphy and metamorphism between the Marampa and Kasila Groups noticed by Pollett (1951) is clearly that expected on tracing the low-grade nappe structure to its root zone in the Mobile Belt.
Archaean metamorphism The metamorphic grade of the Sula Group and the enclosing syn-kinematic granitic rocks is usually within the lower parts of the amphibolite facies, although small zones of granulite facies rocks occur in the southern half of the Liberian granitic terrain. The greater portion of the Kasila Group is typically granulite facies, but its southwest and northeast flanks are in the almandine--amphibolite facies. The two contrasting structural units are also metamorphic units, and there is a sharp but definite gradation between the two where the original contact is intact. The increase in grade in the granite-greenstone terrain towards the Kasila Group is especially noticeable in the south where the contact is unmodified by later thrusting. Adjacent to the Kasila Group in the south is a zone up to 50 km wide of almandine-amphibolite facies rocks of the granite--greenstone domain, in which granulite facies rocks do occur, usually partially retrogressed. The Mano-Moa granulites are an example of one of these areas which developed synchronously with the much lower grade Kambui schists. Proposals that these variations in metamorphic grade involve complicated age relationships or regional-scale faulting (Wood, 1972) have been discounted by Rollinson (1975) who showed that neither explanation is feasible, or necessary. Williams (1977} has proposed a model to explain the differential metamorphic grades in the Archaean terrain of Africa. In the granite-greenstone terrain, high-pressure mineral assemblages are unknown. Sillimanite, staurolite, cordierite and garnet occur in the highest
265 grades within the metasediments while in the granitic terrain, amphiboles and biotites are the chief metamorphic mafic minerals. Rare orthopyroxenes in dioritic and basic rocks of syn- to late-kinematic age are generally of relict igneous status, and contain high temperature exsolution lamellae. O r t h o p y r o x e n e is,however, found as a metamorphic phase in those parts of the granitic terrain adjacent to the Kasila Group. In the supracrustal sequences the mineral assemblages are typical of those found on the Abukuma Plateau (Miyashiro, 1961). Highest grades occur next to the supracrustal --Basement contact, the lowest, in the central portions of the synclinal schist belts. The Kasila Group, in contrast, is an example of extremely high temperature and pressure metamorphism. Orthopyroxenes are found in all rock compositions, while quartz and aluminous spinels coexist. Kyanite and sillimanite coexist as both syn- and late-kinematic growths. Garnets in the leucogabbros contain over 50% of the pyrope molecule. The Marampa Group is anomalous in that its metamorphic grade is within the greenschist facies, with its grade increasing towards granulite facies as the Kasila Group is approached. This is consistent with the suggestion that the present outcrops of the Marampa Group are nappes and klippen, some with roots in the Kasila Group. Brittle deformation Three major directions of brittle structures were developed within the Late-Archaean, north--south, east-northeast and southeast. The north-south and east-northeast (070 °) structures developed as a set of conjugate faults, with small displacements. The less-common southeast structures developed parallel with the Kasila Group Mobile Belt. The north - s o u t h set is dominant overall because it developed parallel with the Liberian tectonic grain in the granitic terrain. Mylonites have been recorded from many faults, b u t measurements of throw are difficult in such poorly exposed terrain. The senses of displacement are dextral on the north--south faults, and sinistral on the east--northeast, indicating a maximum stress direction from the southwest, in agreement with the thrusting movements associated with the penecontemporaneous Kasila Group Mobile Belt. All brittle structures were reactivated during the Mesozoic, when kimberlitic magmatism occurred in the Cretaceous {Willams and Williams, 1977). DISCUSSION Sierra Leone and the West African Archaean Craton The Sula Group is part of a set of supracrustal bodies which form greenstone belts throughout eastern and central Sierra Leone, Guinea, most of western and central Liberia, and the extreme western parts of Ivory Coast.
266 Rollinson (1975) defined this area as the West African Archaean Craton, part of the more extensive West African Craton. Collation of data from this region shows that the Sonfon and Tonkolili Formation thin in sympathy from the rim of the Craton, eastwards. In Liberia, the Sonfon Formation ceases to exist, while the overlying Tonkolili Formation is quartzitic. As a whole, the Sula Group thins from 4 km in central Sierra Leone, to a few hundred metres in Liberia. There is no similar recorded variation in the composition or behaviour of the migmatitic Basement rocks, apart from the general decrease in metamorphic grade northwards, away from the Kasila Group. In connection with this variation, large relatively homogeneous, late-kinematic granites, including the massif hills of Kabala, Tingi, Loma and Nimba (Liberia), developed along a line a b o u t 200 km from the Mobile Belt. The Kasila Group is known to exist in southwestern Liberia and in Guinea. In Liberia, where it is discordant to the coastline, it migrates onto the continental shelf where it has been recognised by remote sensing (Behrendt and Wotorson, 1974). Little published data is available, but it appears that the Kasila Group does not change its character along its entire known extent. The Marampa Group is known from Guinea as a source of haematite iron, but it is soon overlain northwards by Phanerozoic formations~
Correlation with adjacent Arehaean shields The Guyana Shield and the West African Craton have many similarities in lithology and tectono-thermal sequence (McConnell, 1969; Hurley et al., 1971). No direct correlation of individual groups of Archaean rocks across the south Atlantic is realistic considering the distances involved, but the major tectonic boundaries separating Archaean, Eburnean and Pan-African units may be traced from one continent to another (Hurley, 1973).
Geochronology The West African and Guyanan Shields are being actively studied by Hedge et al. (1975) and Hurley et al. (1971, 1975). The data shows a division of the Early to Middle Precambrian into three tectono-thermal units. Early Archaean events, possibly igneous crystallisation ages which are recognisable as "relicts", give ages between 3700 and 3100 Ma, using reference isochrons. These early events are found scattered within the Liberian terrain for which numerous isochrons indicate the age of the last major event as 2 8 0 0 - 2 7 0 0 Ma. Cutting the Archaean are fold belts of Early Proterozoic age, giving results of between 2100 and 1800 Ma. On the basis of recent geochronological work, it appears that the early Archaean ages of West Africa are equivalent to the Gurian of Guyana. Similarly, the Liberian and associated Kasila Group are equivalent to the Ima-
267
tacan, while the Eburnean of West Africa is equivalent to the Trans-Amazonian of the Guyana Shield. More precise correlations of units are unrealistic at present.
Mineral potential In c o m m o n with most Archaean terrains, there is considerable interest in mineralisation. In Sierra Leone there is little evidence of large-scale oregrade mineral potential. Reviews of the ore deposits and potential of Sierra Leone (Dixey, 1925; Pollett, 1951; Morel, 1976) have been optimistic, based on the superficial similarity between the Archaean of Sierra Leone and that of highly mineralised areas of the Superior Province or the Rhodesian Craton. The generally modest mineralisation of Sierra Leone is a reflection of the marked differences in stratigraphy and scale in relation to the enormous individual developments of Archaean supracrustals in southern Africa and central Canada. The most major difference is the lack of large quantities of ultramafic and andesitic lavas in the Sula Group, which limits the variety and extent of the volcano-exhalative deposits. The small scale on which ultramafic intrusive activity occurred at the base of the Sula Group has given barely significant nickel and chromium concentrations. At present, the minerals undergoing exploitation from deposits of Archaean origin are bauxite and rutile. Gold, chromite and haematite iron have been mined in the past, b u t have since become uneconomic. ACKNOWLEDGEMENTS
Without access to the unpublished work of the Geological Surveys Department of Sierra Leone, and former members of the Geology Department, University of Sierra Leone, this review would n o t have been possible. Critical discussion and improvements by Hugh Rollinson and R. Williams are gratefully acknowledged. Many of the conclusions presented are the sole responsibility of the author, whose fieldwork has been sponsored by the National Diamond Mining Company of Sierra Leone, the Diamond Corporation of West Africa, and the University of Sierra Leone. REFERENCES Allen, P.M., 1969. Geology of part of an orogenic belt in western Sierra Leone. Geol. Runds., 58: 588--619. Andrews-Jones, D.A., 1966. Geology and mineral resources of the northern Kambui Hills schist belt and adjacent granulites. Geol. Surv. Sierra Leone Bull., 6 : 1 0 0 pp. Anhaeusser, C.R., Mason, R., Viljoen, M.J. and Viljoen, R.P., 1969. A reappraisal of some aspects of Precambrian shield geology. Bull. Geol. Soc. Am., 80: 2175--2200. Behrendt, J.C. and Wotorson, C.S., 1974. Geophysical surveys of Liberia with tectonic and geologic interpretation. U.S. Geol. Surv. Prof. Pap., 8 1 0 : 3 8 pp. Dixey, F., 1925. The geology of Sierra Leone. Q. J. Geol. Soc. Lond., 81: 195--222. Dunham, K.C., Phillips, R., Chalmers, R.A. and Jones, D.A., 1958. The chromiferous ultrabasic rocks of Sierra Leone. Overseas Geol. Min. Res. Bull., 3 : 4 4 pp.
268 Escher, A., Jack, S. and Watterson, J., 1976. Tectonics of the North Atlantic Proterozoic dyke swarm. Philos. Trans. R. Soc. Lond. A, 280: 529--540. Hedge, C.E., Marvin, R.F. and Naeser, C.W., 1975. Age provinces in the Basement rocks of Liberia. J. Res. U.S. Geol. Surv., 3: 425--429. Hunter, D.R., 1971. The granitic rocks of the Precambrian in Swaziland. Inf. Circ. Econ. Geol. Res. Unit, Univ. Witwatersrand, Johannesburg, 6 3 : 2 5 pp. Hurley, P.M., 1973. On the origin of 450 -+200 m.y. orogenic belts. In: D. Tarling and S. Runcorn (Editors), Implications of Continental Drift to the Earth Sciences. Academic Press, London, 1184 pp. Hurley, P.M., Leo, G.W., White, R.H. and Fairbairn, H.W., 1971. Liberian age province (about 2700 m.y. ) and adjacent provinces in Liberia and Sierra Leone. Bull. Geol. Soc. Am., 82: 3483--3490. Hurley, P.M., Fairbairn, H.W. and Gaudette, H.E., 1975. Progress report on the early Archaean rocks in Liberia, Sierra Leone and Guyana. In B.F. Windley (Editor), Early History of the Earth. Wiley, London, pp. 511--521. MacFarlane, A., Crow, M.J., Arthurs, J.W. and Wilkinson, A.F., 1974. The geology and mineral resources of northern Sierra Leone. Inst. Geol. Sci. Overseas Div. Intern. Rep., 34. Mackenzie, D.H., 1961. Geology and mineral resources of the Gbangbama area. Geol. Surv. Sierra Leone Bull., 3 (unpubl.). Marmo, V., 1962. Geology and mineral resources of the Kangari Hills schist belt. Geol. Surv. Sierra Leone Bull., 2 : 1 1 7 pp. Marmo, V., 1971. Granite Petrology and the Granite Problem. Elsevier, Amsterdam, 244 pp. Mason, R., 1973. The Limpopo Belt-southern Africa. Philos. Trans. R. Soc. London A, 273: 463--485. Morel, S.W., 1976. The Geology and Minerals of Sierra Leone. Fourah Bay College Bookshop, Freetown, 21 pp. Pollett, J.D., 1951. The geology and mineral resources of Sierra Leone. Coll. Geol. Min. Res., 5: 3--28. Rollinson, H., 1973. Report on geology of sheets 48, 49, 59, 60. Eastern Kono District. Geol. Surv. Sierra Leone Rep. (unpubl.). Rollinson, H., 1974. Report on geology of sheet 102, south Kambui Hills and surrounding areas. Geol. Surv. Sierra Leone Rep. (unpubl.). Rollinson, H., 1975. Report on geology of sheet 58, Nimini Hills and surrounding areas. Geol. Surv. Sierra Leone Rep. (unpubl.). Vallance, G., 1975. Report on geology of sheets 76, 77, 78, 79, 88, 89, 90. Geol. Surv. Sierra Leone Rep. (unpubl.). Williams, H.R., 1976. Granitisation of basic rocks in the Bjornesund area, near Fiskenaesset, West Greenland. Rapp. Gron. Geol. Unders., 73: 47--54. Williams, H.R., 1977. African Archaean Mobile Belts and granite~greenstone terrain. Nature, 266: 163--63. Williams, H.R. and Williams, R.A., 1975. The geology of the Yengema area, Kono District, Sierra Leone. Report to N.D.M.C., Freetown, Sierra Leone (unpubl.). Williams, H.R. and Williams, R.A., 1976. The Kasila Group, Sierra Leone, an interpretation of new data. Precambrian Res., 3: 505--508. Williams, H.R. and Williams, R.A., 1977. Kimberlites and plate-tectonics in West Africa. Nature. Wilson, J.F., 1973. The Rhodesian Archaean craton - - a n essay in cratonic evolution. Phil. Trans. R. Soc. Lond. A, 273: 389~-411. Wilson, N.W., 1965. The geology and mineral resources of part of the Gola Forests, southern Sierra Leone. Geol. Surv. Sierra Leone Bull., 4 : 1 0 2 pp. Wilson, N.W. and Marmo, V., 1958. Geology, geomorphology and mineral resources of the Sula Mountains. Geol. Surv. Sierra Leone Bull., 1. Wood, R.S., 1972. Early Precambrian Kambui Schist Belt, southern Sierra Leone, and its surrounding basement. Nature, 236: 14.