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(U-Pb zircon date; Caen-Vachette et al.
1988
), andmigmatites
dated near Boma at 2086
2.4.3.1 The Mboumou-Uganda Craton
The central section of this craton (also referred to as the
Congo-Uganda Block by Tait et al.,
2011
), in northeastern
DRC, is predominantly a classic low-grade granite green-
stone terrain (de Wit and Ashwal
1997
). The oldest TTG
gneisses (c. Nzangi), dated at 3005
64Ma (Rb/Sr whole-rock date; in
Cahen et al.
1984
;Fig.
2.5
). This Paleoproterozoic basement
is locally intruded by younger granites (c. Noqui) dated at
1.0 Ga, and capped by 5-6 km thick basalts and rhyolites of
the Zadinian and Mayumbian Groups, dated between
912
64 Ma (Rb/Sr whole-
rock date, in Cahen et al.
1984
), are intruded by undeformed
tonalites (e.g. Moto and Kilo) dated between 2.7 and 2.8 Ga,
and relatively younger granites dated between 2.4 and
2.6 Ga (Cahen et al.
1984
). These Neoarchean dates are
similar to those of the Kasai and Cuango Cratons. However,
high grade mafic gneisses (Bomu and Monga) that define the
northwestern margin of this craton have been dated at
3.4-3.7 Ga (Cahen et al.
1984
); and in one report the
Boma rocks were speculated to be ca. 4.0 Ga, based on a
Rb/Sr errorchron; Lavreau and Ledent
1975
), which would
make these the oldest mafic gneisses in Africa and some of
the oldest in the world. Clearly new precise geochronology
of these Boma gneisses is sorely needed.
The eastern extremity of the Mboumou-Uganda Craton
contains Neoarchean high-medium grade TTG and para-
gneisses exposed over a wide area of north-central Uganda.
This is supported by more recent (TIMS) U-Pb zircon dates
of 2570
8 Ma (U-Pb zircon dates; Tack
et al.
2001
). These early Neoproterozoic magmatic sequences
are, in turn, unconformably overlain in the eastern part of the
belt by relatively thick (~5 km) carbonate platform and fore-
land metasediments of the West Congolian Group (Frimmel
Topic). Recent U-Pb and Ar/Ar geochronology within these
sequences shows that Pan-African orogenesis occurred during
two main episodes, at ca. 490 Ma and 540 Ma (Moni
´
et al.
2012
). This Early Cambrian (Tardive) phase of deformation is
very similar to that of the Ara¸ua´ Orogen in eastern South
America (e.g. da Silva et al.
2005
).
The deformed Neoproterozoic West Congolian Group
includes in the lower part two formations of diamictites
(the Lower and Upper Mixtites), linked to the Sturtian
(710-750 Ma) and Marinoan (635-650 Ma) glaciations,
respectively. This is only supported, however, with
Sr-isotope dates of 575 Ma from overlying cap carbonates
(Poidevin
2007
). This sequence terminates upward with
molasse-type deposits (the Mpioka Subgroup) unconform-
ably overlain by southward prograding fluvial-delta red
sandstones and conglomerates of the Cambrian Inkisi
Group that might best be interpreted to reflect the final
exhumation and erosion of the West Congolian orogenic
Pan-African mountains (Alvarez et al.
1995
; Tack et al.
2001
; Tait et al.
2011
).
7 Ma and 920
1 Ma obtained from basement
rocks in central Uganda (S. Bowring, in de Wit
2004
, un-
published), and several others between ca. 2550-2650 Ma
(Link et al
2010
;M¨ntt¨ri et al.
2013
). In two other locali-
ties, flanking either side of the Uganda-DRC border, Meso-
archean dates have been recorded (2991 and 3079 Ma,
respectively, M¨ntt¨ri et al.
2013
). The easternmost bound-
ary of this craton is defined by NW-SE trending Neoprotero-
zoic mylonite shear zones (of which the Aswa shear zone is
the best known, and dated around 700-680 Ma; S. Bowring,
in de Wit
2004
, unpublished).
3 Ma and 2616
2.4.3 Northern Central Africa
2.4.3.2 The Oubanguides Belt
ThenorthernmarginoftheCASandthesouthernmarginof
the Central Sahara Shield is marked by the poorly studied,
E-W striking Oubanguides, an orogenic belt that stretches
from Cameroon through the CAR and southernmost South
Sudan to northeast Uganda (Poidevin
1985
;vanSchmus
et al.
2008
; de Wit et al.
2008
; Toteu et al.
1994
,
2001
,
2006
,
and
2014
in preparation). High-grade granitic gneisses and
charnockites are widespread, some of which are Neoarchean
in age (U/Pb zircon dates between 2.56-2.64 Ga;
Toteu et al.
1994
,
2001
,
2006
,and
2014
in preparation) whilst
most others are Neoproterozoic (U/Pb zircon dates; Toteu
et al.
1994
,
2001
,
2006
,and
2014
in preparation). Many
K/Ar and Rb/Sr whole rock dates on paragneisses and
associated batholitic granitoids, and U/Pb dates on their
zircons, cluster in the range of ~800-1200 Ma and between
480-655 Ma. The latter ages relate to dextral strike-slip and
south-directed thrusting of the Oubanguides over the Archean
In northern Central Africa, the vast E-W trending and
S-verging fold-and-thrust belt of the Oubanguides is more
than 3000 km long, extending from Cameroon (where it has
sometimes been called the Central African Fold Belt), and
farther west into northeast Brazil as the Sergipano Belt),
through the CAR, to northeastern DRC and Uganda, where
it is tectonically thrust across the Bouca and Mboumou-
Uganda Cratons (Fig.
2.5
). No diamondiferous kimberlites
have been discovered across this region.
To the North of the Oubanguides lies a region in the
Sahara often referred to as the Sahara Metacraton, but
which in fact as far north as Darfur mostly comprises
Mesoproterozoic basement, with extensive Kibaran grani-
toids (ca. 1000-1100 Ma) deformed during the Pan-African
orogenesis at ca. 625 Ma along the Central Saharan and
Zalingai Fold Belts (CSFB and ZFB, respectively; Fig.
2.2
,
inset A; de Wit et al.
2005
and
2014
, under review).
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