Download WEST AFRICAN CRATON West Africa, an area roughly the size of

WEST AFRICAN CRATON
West Africa, an area roughly the size of Europe, features a wide variety of
geological terranes* with a complicated geological history. Although the broad
geological framework was established by pioneer workers much earlier in this
century, it has only been in the last few decades that more detailed regional studies
have helped to unravel details of the geological history and establish a general
chronology of events based on fairly extensive radiometric age-dating. Figure 2
outlines the major geological ‘provinces’ of West Africa and the following sections
of this chapter very briefly describe the major features of the region. Wright and
others (1985), Petters (1991), and Dallmeyer and others (1990) have summarized
much of the regional information in several excellent texts. The BRGM report on
the ‘West African Gold Deposits’ (Milesi and others, 1989) also provides a very
good review of the regional geology, as well as considerable detail on various
types of gold deposits in the region.
ARCHEAN
In broad terms, the region features a West African Craton of older Precambrian
rocks surrounded by younger Precambrian and Phanerozoic units. The southern
part of this craton is now generally referred to as the Man Shield. Some French
geoscientists continue to use the term Leo Shield for much of the same area and
restrict the use of Man Shield to the Archean portion of the craton. However, this
has not received wide acceptance and this report will utilize the term Man Shield in
it’s broad context. The oldest Precambrian rocks are Archean in age (>2500 Ma)
age and appear to be limited to a core zone along the coastal region extending from
western Côte d’Ivoire through Liberia, Sierra Leone and into southern Guinea. The
geology of the Archean core has not yet been studied in adequate detail but it
appears to be very similar to major Archean shield areas in many other parts of the
world. Most of the region appears to consist of highly metamorphosed mafic to
felsic gneisses and migmatites representing old basement. Wedged into the
basement rocks are narrow greenstone belts of less metamorphosed supracrustal
rocks with extensive tholeiitic basalts and sequences of metasediments that include
turbidites conglomerates and extensive banded iron formation. Within the
greenstone belts there are some intermediate to felsic volcanic units; late-stage
intrusive complexes (mafic to granitic in composition) are also quite widespread.
Radiometric age-dating has established two widespread thermotectonic events; the
most widespread one is dated at about 2750-2900 Ma and is called the Liberian
event; this event metamorphosed all of the supracrustal rocks and surrounding
basement. Limited data suggest an even earlier Archean event (greater than 3000
Ma), referred to as Leonian but its extent is not well-established as yet. The
Archean units are mainly bounded on the east in Côte d’Ivoire by the very
prominent Sassandra fault, a large N-S regional feature, which generally separates
the Archean units from younger Paleoproterozoic metasediments and metavolcanic
units still further to the east. The northern boundary is also marked by extensive,
shallow dipping faults marking a zone where the same Paleoproterozoic units of
Guinea and northern Côte d’Ivoire have been thrust southwards onto the older
cratonic block. The banded iron formations of the Archean greenstone belts have
been developed on a substantial scale in Liberia and both Sierra Leone and Guinea
have large resources with excellent potential for future development. These belts
contain numerous gold occurrences and some indications of base metals, which
warrant further attention. The Archean cratonic area also hosts quite widespread
diamond-bearing kimberlites of Mesozoic age. Alluvial diamonds from these
intrusions have been mined quite extensively in Guinea and Sierra Leone.
PALEOPROTEROZOIC
The most extensive units of the Man Shield are the Paleoproterozoic
metasediments, metavolcanics and associated intrusive complexes that are exposed
in a more or less continuous X-shaped pattern that covers much of Ghana, Côte
d’Ivoire, Burkina Faso and into southern Mali, northern Guinea and the SW corner
of Niger. Inliers in western Mali and eastern Senegal confirm that Paleoproterozoic
units probably underlie large areas in northern Burkina Faso and Mali where they
are covered by unmetamorphosed Infracambrian-Paleozoic sediments. Virtually all
of the Volta Basin in northern and eastern Ghana will also be underlain by
Paleoproterozoic units and there are also equivalent rocks in NW Africa, which
suggest that much of the western Sahara has the same but buried Paleoproterozoic
cratonic units.
*In this report we use the term ‘terrane’ in the geological sense (for example,
Archean or Birimian terrane) whereas the term ‘terrain’is used in the
geographical or topographical sense (for example, hilly terrain or the terrain of
northern Ghana).*The term ‘Birrimian System’ was first coined by Sir Albert
Kitson in 1918 for a type locality of metasediments and metavolcanics inthe Atewa
Range of the Kibi District of Ghana, which is the headwater area for the Birim
River. The term ‘Birrimian’ later becamewidely used by anglophone and
francophone geologists throughout West Africa. Unfortunately, the name of the
river was later changedto Birim and subsequently an effort was made by geologists
in Ghana to make the minor modification ‘Birimian’ and this is now widely
accepted although many francophone geologists have continued the older usage
‘Birrimian’. Both terms should be considered correct although it is more
appropriate to use the term ‘Birimian’ to reflect the now accepted spelling of the
Birim River where the term wasfirst applied.
From an economic viewpoint, the Paleoproterozoic metasediments, volcanics and
related intrusives are extremely important as they host the vast majority of the
substantial gold resources of the region and, in addition, they host important
manganese deposits. The base metal potential appears less certain although one
significant volcanogenic massive sulphide (Zn) deposit has been outlined in
Burkina Faso (Perkoa) and base metalanomalies in a number of belts in the region
warrant more systematic evaluation.
BIRIMIAN SUPERGROUP
The most important Paleoproterozoic units are extensive sediments and volcanics,
which are generally referred to as the Birimian Supergroup*. The Birimian rocks
are very widely distributed throughout a large part of West Africa. Over the
decades, there has been some controversy on the relative ages and stratigraphic
positioning of the Birimian units. This has partly resulted from efforts to extend
observations from isolated areas to regional generalizations. The problems of
correlation have also been much compounded by generally poor exposures in most
areas. For example, in Ghana for most of the 20th century, the traditional view has
been that the dominantly metasedimentary sequences. (Lower Birimian) are
overlain by the more volcanic-rich sequences (Upper Birimian). Many of the
geologists in the francophone countries observed relationships, which suggested
the opposite. Within the past couple of decades, numerous excellent regional
studies on the structural and stratigraphic relationships in the Birimian have been
carried out by the German Geological Survey group (BGR) in cooperation with the
Ghana Geological Survey (GGS) and more recently in Côte d’Ivoire. Their studies
(Hirdes and others, 1993; Zitzmann, 1997) have concluded that the dominantly
sedimentary sequences, which occur in large basins, are largely coeval with the
metavolcanics,volcaniclastics, and metasediments, which occur in relatively
narrow NE trending belts. The marine sediments in the basins were largely derived
from volcanic terrane in the nearby belts. Results of field mapping and age-dating
in northern Côte d’Ivoire have revealed some interesting variations, which may
have regional implications. Hirdes and others (1996) present quite strong evidence
from fairly precise U-Pb age-dating that indicates there may be two distinct ages
for the Birimian volcanic belts and sedimentary basins in West Africa. The eastern
belts appear to be consistently older (2150-2185 Ma) by about 50 million years
than belts and basins further to the west(approximately 2105 Ma). It has been
suggested that the term ‘Birimian’ be used to refer to the supracrustal rocks (belts
and basins) from the older eastern region, whereas the term ‘Bandamian’ be used
to refer to similar but younger supracrustal units in the western regions. It is
probably a little premature to modify the nomenclature beyond the immediate
confines of the relevant belts and basin in NE Côte d’Ivoire until such time as there
is more corroborative data from the numerous belts and basins further to the west.
Certainly this work does confirm that we are likely to see many modifications to
the current broad models as more detailed field studies and improved age-dating
techniques are applied to the vast expanse of Paleoproterozoic units in West
Africa. For purposes of this report, we will continue to use the term ‘Birimian’ for
all of the Paleoproterozoic supracrustal units of the belts and basins in West
Africa.As noted above, the Birimian throughout the region consists of narrow
‘greenstone’ (volcanic) belts, which can be traced for hundreds of kilometres along
strike but are usually only 20-60km wide and they are separated by wider basins of
mainly marine clastic sediments. Along the margins of the basins and belts there
appears to be considerable interbedding of basin sediments and volcaniclastic and
pyroclastic units of the volcanic belts. Thin but laterally extensive chemical
sediments (exhalites), consisting of cherts, fine-grained manganeserich and
graphitic sediments, often mark the transitional zones. The margins of the belts
commonly exhibit faulting on local and regional scales and these structures are
fundamentally important in the development of gold deposits for which the region
is now well-known.However, many of the belts also feature extensive regional
faults cutting across entire belts and the structural evolution of many of the fault
systems is varied and extremely complicated.
Within the volcanic belts, the actual amount of volcanic flows is quite variable.
Most of the older volcanics are mafic in composition and usually display
geochemical characteristics typical of tholeiitic (oceanic) suites. Many of the flows
are massive although pillow structures are preserved in some localities. Younger
intermediate to felsic volcanics of calc-alkaline affinity, including pyroclastics, are
generally subordinate; this appears to be the case in the volcanic belts of Ghana,
although they are a little more widespread in some of the belts further to the west
and in Burkina Faso. The volcanic belts also include substantial coarse
volcaniclastic units, which appear to grade laterally into finer grained, more distal
units towards the basins where they are commonlyinterbedded with basin clastics
(wackes and argillites). Faulting and folding within the belts are very widespread
and it is not possible to accurately determine the total thickness of the volcanics
and related units but they most likely exceed 10,000m. Of course, current models
indicate volcanism was accompanied by extensive erosion into the adjacent basins
so that most of the volcanic chains, which were active for a long period of time,
were substantially denuded so that we are now left with vestiges of the original
volcanic units.
The highly folded, thick sedimentary suites of the basins feature a variety of facies;
argillites and wackes are the most common lithologies and it is common to see
these in thin bedded units with features common to classical flysch sequences. The
extensive isoclinal folding and generally poor exposures make it difficult to
estimate the total thickness of the basin sediments but certainly they would also
appear to be very thick, also probably at least 10,000m.All of the Birimian
sediments and volcanics have been extensively metamorphosed; the most
widespread metamorphic facies appears to be greenschist, although in many areas,
higher temperatures and pressures are indicated by amphibolite facies. Detailed
studies (John and others, 1998) in the southern part of the Ashanti belt indicate that
the current greenschist facies seen in much of the area is probably a retrograde
effect and the peak metamorphism took place under amphibolite facies conditions.
This pattern may well extend to other belts in the region. On a regional basis, it
would generally appear that as you go north from the coastal areas into northern
Ghana, Côte d’Ivoire and Burkina Faso, the general levels of metamorphism
increase. This probably reflects the deeper levels of erosion in the north in
comparison to the southern exposures of the belts and basins. Detailed structural
studies in the region are fairly limited.Studies in the western part of the region
(Milesi and others, 1992) have proposed several separate phases of folding and
fracturing. However, a regional synthesis by Eisenlohr (1989) has concluded that,
although there is onsiderable heterogeneity in the extent and styles of deformation
in many areas, most of the structural elements have common features, which are
compatible with a single, extended and progressive phase of regional deformation
involving substantial NW-SE compression. According to Feybesse and others
(1994), as well as Ledru and others (1994), this extended period of compression
resulted in early stage N-S sinistral (leftlateral) faults of regional extent and later
stage dextral (right lateral) faults oriented approximately NE-SW. A considerable
amount of thrusting with a SE vergence was localized along the margins of the
major volcanic belts.This is well displayed on the NW margin of the Ashanti Belt
in Ghana. The metamorphism, deformation and extensive intrusive activity (see a
subsequent sub-section of this chapter) are all part of the Eburnean thermotectonic
event (or orogenic cycle), which produced mountain belts. The Eburnean (or
Eburnian) orogenic event* (Hirdes and others, 1996) appears to have generally
peaked at about 2100Ma and appears to have affected virtually all of the
Paleoproterozoic units in West Africa. The waning phase of this orogenic cycle
included the widespread development of granitoid intrusions within the highly
deformed, sedimentary basins. These intrusions have been generally dated at
around 2090 Ma and they are generally associated with waning phases of the
Eburnean orogenic cycle.
TARKWAIAN GROUP AND EQUIVALENTS
The Tarkwaian** Group consists of a distinctive sequence of metasediments
occurring within a broad band along the interior of the Ashanti Belt. They host
important paleoplacer gold deposits in the Tarkwa district. Equivalent rock types
occur in quite a few other belts of the region but in relatively restricted areas. As
yet, none of the other bands of Tarkwaian equivalents are known to host major
gold deposits although the potential of the Bui Belt in west-central Ghana is
considered encouraging.
*The term ‘thermotectonic event’ has been widely used to refer to major
metamorphic/tectonic events where it is uncertain if a full orogenic cycle was
completed. It has been mainly used to refer to Pan-African events (approximately
600-450 Ma) in various parts of Africa where the tectonic history was not well
understood. In this report, the term thermotectonic event is used interchangeably
with orogenic event **Early and more recent work in Ghana, has always
separated the Tarkwaian Group from the Birimian units. The latter were
sometimes referred to as the Birimian Series but now generally referred to as the
Birimian Supergroup. As noted by Petters (1991), theTarkwaian Group was
originally included in the definition of the Birimian Supergroup. In this report, we
will continue to separate the two but in view of their close inter-relationship, a
strong case could be made for including the Tarkwaian units as a group within the
Birimian Supergroup, at least within the Ashanti Belt. Formal guidelines used in
establishing stratigraphic nomenclature would probably restrict the usage of
Tarkwaian Group to the Ashanti Belt since it is very unlikely that equivalent types
of units in other belts were ever directly connected and probably formed at quite
different periods of time. Elsewhere they should be referred to as fluvio-deltaic
sedimentsor Tarkwaian equivalents.
The Tarkwaian Group consists of a variety of sandstones (quartzites),
conglomerates and argillites (phyllites). In the type locality at Tarkwa, the
sequence is in the order of 2500m thick, whereas in the Bui belt further to the
north, comparable units are about 9000m thick (see next chapter for more details).
The most extensive and widely reported sedimentological study was in the Tarkwa
district by Sestini (1973) who concluded that the Tarkwaian units were largely
deposited in shallow, non-marine, basins along the flanks of a volcanic chain.
Much of the sediment was transported by fast flowing streams, which discharged
their coarse load onto alluvial fans and in deltas along the margins of the basin and
the fan deposits were progressively re-worked by braided river channels. This
general pattern also fits the model established more recently in the Bui syncline by
Keissling (1997) who recognized several major cycles of sedimentation in two
separate basins, which eventually became linked. In classical terminology, the
Tarkwaian units would be referred to as molasse sediments that mark a rapid
period of erosion and proximal deposition during the late-stage of an orogenic
cycle.
There has always been much controversy on the relationships between the
Tarkwaian and the underlying Birimian units. Many of the early workers in the
region believed the contact to be an angular unconformity. The degree of
metamorphism of the Tarkwaian was believed to be considerably less, and the
styles and timing of folding and faulting between the two groups quite distinct.
However, clear evidence to support these ideaswas generally lacking, partly
because the contacts are rarely visible and usually are faulted. More recent work
(Kiessling, 1997; Eisenlohr and Hirdes, 1992), which may well apply to most of
the region, confirms that the Tarkwaian/Birimian contacts are almost invariably
sheared. However, they appear to be generally unconformable where the
underlying Birimian units are mainly volcanic, whereas they are essentially
conformable where the underlying units are Birimian sediments. This work also
recognizes that, although the structural styles may differ considerably, they
resulted from the same tectonic regime and that both the Tarkwaian and Birimian
units were affected by the same Eburnean metamorphic event. The metamorphic
assemblages observed in some of the Tarkwaian units of the region indicate
moderate temperatures and fairly high pressures equivalent to burial depths of 10
or more kilometres (Ledru and others, 1994). These conditions were probably
achieved as a result of collisional tectonics in the Eburnean orogeny whereby
Birimian units were thrust over and buried the Tarkwaian sediments to substantial
depths. They were formed in small, fault-controlled (graben) basins developed in
an extensional tectonic setting (Strogen, 1991). In the Ashanti Belt, the extensional
nature of the tectonic setting is demonstrated by the presence of large numbers of
mafic sills, which intruded the Tarkwaian basin prior to folding and metamorphism
(Griffis, 1998).
INTRUSIVES
Throughout the Birimian sedimentary basins and volcanic belts, there are a large
variety of intrusions and two dominant types of occurrences have been recognized
in many parts of the region. Early workers in Ghana distinguished these as ‘Cape
Coast’ and ‘Dixcove’ types. The more massive, ‘Cape Coast’ type batholiths are
dominantly intermediate intrusives (generally K-rich) with biotite being the most
common mafic mineral; typically they display foliation (often gneissic) and are
concentrated in areas where Birimian metasediments are widespread. They are
usually described as concordant and commonly have migmatitic phases on the
margins and display prominent contact metamorphic aureoles. The ‘Dixcove’ type
occurs mainly within the confines of the volcanic belts; they are also generally of
an intermediate composition although more mafic and felsic phases are not
uncommon. They tend to be more Na-rich and in the literature are often referred to
as plagiogranites. Hornblende is usually the dominant mafic mineral and, in many
cases, the intrusives lack strong foliation. They occur in quite small plutons to very
large batholiths, although not as extensive as the major ‘Cape Coast’ complexes.
They are frequently described as discordant intrusives. The relative ages are rarely
established as the two types are not generally seen together but most workers in the
past considered the ‘Cape Coast’ intrusives to be syn-tectonic bodies,whereas the
‘Dixcove’ units were considered to be latetectonic to post-tectonic intrusions.
Elsewhere in the region, similar types have been recognized and sometimes given
type designations from specific localities. In the recent regional mapping program
(BGR/GGS) in SW Ghana, new terminology was proposed for the same broad
types of intrusions. This included ‘basin’ granitoid for the Cape Coast type, which
are confined to the sedimentary basins, and ‘belt’ granitoid for the ‘Dixcove’ type,
which are largely confined to well-defined volcanic belts. This terminology will be
generally followed in this report although, as will be noted in the following
chapter, the designation as belt-or basin-type is not always straightforward.
Considerable radiometric dating is now available for the granitoids in southern
Ghana and contrary to previous assumptions, the belt-type (Dixcove) granitoids
consistently yield dates very close to their Birimian hostrocks, whereas the basintype (Cape Coast) granitoids yield younger ages. The following sub-section
provides a general chronology of geological events based on age-dating and
includes a range of dates for the granitoids.
As noted above, the ‘basin’ and ‘belt’ intrusives are generally of intermediate
composition (granodiorite, tonalite, quartz diorite). The belt intrusives also include
more mafic (diorite) phases and some complexes have more felsic phases (close to
the true granites) but these are not usually very extensive. The basin granitoids also
show considerable variation in composition but with relatively minor mafic phases,
quite often with more prominent felsic phases, which are true granites and often
accompanied by late-stage aplites and pegmatites. The basin granitoids generally
feature lithological and geochemical characteristics (peraluminous) and a tectonic
setting more or less comparable to S-type intrusives, which are believed to have
formed by partial or complete melting in the roots of thick piles of sediments. In
contrast, the belt intrusives have features more associated with I-type intrusives
(metaluminous), which are believed to have had a more direct magmatic origin.
However, in view of the fact that the basin sediments were derived from the
volcanic belts, it is not
surprising that there is a similarity in both types of intrusions. Although the vast
majority of Paleoproterozoic plutons throughout the region fall within these broad
categories, there are a few other distinctive types of intermediate to felsic
granitoid, which occur locally in some of the basins and belts. For example, in
northern Ghana, the Bongo-type granitoid (more or less restricted to the Nangodi
belt segment) is a distinctive microcline-rich and hornblende-bearing granitoid,
which is a very latestage intrusive associated with a waning phase of the Eburnean
orogeny. In southern Ghana, the Winneba area also features exposures of
granitoids somewhat similar to the more widespread basin-type (Cape Coast)
intrusive batholiths but with geochemical features indicative of contributions from
older sialic crust. These two types of intrusives will be discussed in more detail in
the following chapter. Although there does not appear to be as much information in
the geological literature on similar occurrences in other parts of West Africa, it can
probably be assumed that the two principal types occur in other belts and basins
further to the west and north. Not to be forgotten are a variety of mafic (mainly
gabbro and diorite) intrusives, which are quite widespread within the volcanic
belts, especially the Ashanti Belt, but largely absent from the adjacent basins.
These intrusives are quite variable; many appear in close association with volcanicrich (mafic tholeiites) areas within the belts and are probably coeval with much of
the volcanic activity. Some quite large mafic complexes occur sporadically,
especially within the Ashanti Belt; these include, for example, the Mpohor diorite
immediately NW of Takoradi. These are similar to descriptions of epidiorites in the
early geological literature of the Ashanti Belt and they probably correlate with the
very extensive mafic sills recently identified by airborne geophysics in the
Tarkwaian units of the Ashanti Belt (Griffis, 1998). Similar types of intrusive have
been identified in other volcanic belts of the region but they do not appear to be as
widespread as in the Ashanti Belt.
RADIOMETRIC AGE-DATING
In the past two decades, many of the fundamental issues relating to the chronology
of geological events in West Africa have been greatly illuminated by fairly
extensive radiometric age-dating. Much of this work has been done by the team of
BRGM scientists of France operating in the francophone countries and the BGR
group of Germany who have carried out extensive regional work in Ghana and
Côte d’Ivoire. It is beyond the scope of this report to review this work in detail but
some general comments are warranted. Ages for the Birimian metasediments and
metavolcanics in Ghana generally fall within the range of 2150-2200 Ma (Davis
and others, 1994). Ages from detrital grains of zircons (Ashanti and Bui belt)
generally fall within the same range and current evidence (Zitzmann and others,
1997) indicates that the Tarkwaian and Birimian units were derived from similar
source areas although depositional activity in Tarkwaian basins may have been
extended to about 2130 Ma. As noted previously, recent work in northern Côte
d’Ivoire (Hirdes and others, 1996) has demonstrated that volcanic belts and basins
in the eastern area (Ghana, eastern Côte d’Ivoire) may be about 50 Ma older than
similar areas further to the west. The age of peak metamorphism and the maximum
stage of deformation, associated with the regional Eburnean orogeny, appear to
closely approximate 2090-2100 Ma (Loh and Hirdes, 1996). This general age may
well apply to the older, eastern belts and basins as well as the younger units further
to the west. Extensive radiometric age-dating of intrusive activity in the region
appears to fall into two distinctive ranges. As noted previously, and contrary to
early views, the smaller discordant belt intrusives generally have ages that fall
mainly in the same range 2150-2200 Ma as the Birimian metasediments and
volcanics (Hirdes and others 1992; Loh and Hirdes 1996). The large basin
intrusives are consistently younger than their belt counterparts and most
radiometric age-dates fall in the range 2080-2125 Ma and appear to correlate well
with the period of peak metamorphism. There is considerably less data on the
relatively minor, late stage post-tectonic, K-rich intrusives such as the Bongo-type
of northern Ghana but the latter has been age-dated at 1968 +/- 49 Ma. Quite
imprecise ages for the Winneba granite, which is most similar in petrological
characteristics to the basin granitoids, have been interpreted to reflect inherited
features from older basement crust (Taylor and others, 1992). Dates are not
available for the relatively late-stage mafic intrusive sills, dikes and possibly
related mafic complexes (i.e., Mpohor) of the Ashanti Belt. Certainly the sills in
the Tarkwaian sequences appear to pre-date the peak Eburnean metamorphism and
they are likely to fall in the 2100-2150 Ma period. It should be emphasized that
although the above general outline of the chronology of geological events probably
applies to much of the Birimian/Tarkwaian terrane, there may be important local
variations. More age-dating and isotopic studies throughout the region will further
clarify regional and local geological events.
TECTONIC MODELS
For decades it was generally assumed that much of the Birimian terrane of West
Africa is underlain by older (Archean) continental crust and most models on the
tectonic development of the Birimian / Tarkwaian / Eburnean units adopted this
concept. The basis for this was largely due to assumptions concerning the highgrade Dahomeyan metamorphic units present in southeastern Ghana and the
widespread migmatites in northern Ghana, Burkina Faso and Côte d’Ivoire. These
were believed to be equivalent to the Archean cratonic areas exposed in large parts
of western Côte d’Ivoire, Liberia and Sierra Leone. However, quite extensive age-
dating has yet to reveal any consistent indications of old crustal material. In fact,
most of the areas believed to be Archean (i.e., the extensive migmatites of northern
Ghana and neighbouring areas) have age-dates and geochemical characteristics that
are similar to Eburnean basin granitoid complexes. The Dahomeyan metamorphic
units yield much younger age-dates, which correspond to Pan-African (550-600
Ma) events. These dates may represent a re-setting of the radiometric clocks but
few other geochemical features clearly point to older (Archean) crustal material.
However, as noted earlier in this chapter, the geochemical characteristics of the
Winneba granitoids suggest some possible inheritance from Archean crustal
sources. The geochemical and isotopic data collected in many Birimian areas of
West Africa indicate that much of the early tholeiitic volcanism and coeval
intrusive activity (belt granitoids) have features of a juvenile magmatic origin.
There is very little evidence that any of these areas were affected by any
underlying older crust, which probably would have influenced ascending magma.
For now, the issue is not fully resolved but the preponderance of current data does
not support the presence of any substantial amount of Archean crust in the region
now dominated by Birimian-age sediments, volcanics and related intrusions.
However, the presence of highly attenuated, old sialic crust cannot yet be totally
excluded; the attenuated zone may not have been thick enough to have
significantly effected the geochemistry of Birimian magmas from an underlying
mantle zone. The growing set of data on major, minor, and trace element
geochemistry has resulted in new tectonic / structural models to explain the
Birimian / Eburnean geological province. Earlier ideas on an initial stage of
intracontinental rifting have been largely abandoned in favour of the development
of a series of tholeiitic volcanic chains within an oceanic crustal environment
(Boher and others, 1992). The tholeiitic volcanics were followed by calc-alkaline
volcanism probably related to subduction beneath island arcs within an intraoceanic environment. New ocean was created and probably consumed along
numerous parallel volcanic chains across a large area. The oceanic basins were
closed by converging Archean cratonic blocks, which resulted in the accretion of
Paleoproterozoic volcanic chains onto the margins of the older continental crustal
blocks and by thick sequences of basin sediments extensively deformed and
pushed onto, and in places, over the margins of volcanic belts. The
deformational/tectonic regime was very complex with extensive movements on
many major fault systems of regional extent. The peak period of deformation,
metamorphism and development of widespread anatectic granite intrusive at the
base of very thick, deformed sedimentary piles climaxed at about 2090-2100 Ma
when a very large area of new continental Paleoproterozoic crust was created
between older crustal blocks to the east and west. The region must have comprised
a vast series of mountain ranges formed by thrusting of thick sequences of
Birimian metasediments over many of the volcanic belts and Tarkwaian basins,
which were deeply buried but later exhumed by extensive erosion. Although a
great amount of new information has been synthesized into a general regional
model, there remains much detail to be clarified. It is hoped that the laudable
efforts over the past two decades on detailed field mapping, structural studies,
radiometric age-dating and geochemical studies will be sustained.