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Pan-African crustal thickening and Palaeoproterozoic eclogite facies
metamorphism in Cameroon
DFG (Sche 265/14)
Position of Cameroon at the transition from the
Congo Craton to the CAFB
(modified after Kusky 2003)
The transition zone between the northern border of the Congo craton into
the E-W striking Pan-African Central African Fold Belt (CAFB) is exposed in
Cameroon. This offers the unique opportunity to study the orogenic
processes and the behaviour of the cratonic border during super-continent
formation in the central part of Gondwana. Moreover, the CAFB is suspected
to be the locus of the suture zone that separates the Congo craton from its
cratonic neighbours. However, ophiolites and eclogites representing remnants
of a former ocean floor have not been identified up to now.
We have reconstructed metamorphic P-T paths and performed in-situ
monazite dating to unravel the tectonic history of Palaeoproterozoic
basement units and Neoproterozoic cover series in the CAFB. In addition,
we analysed major and trace elements of metabasites to find possible
remnants of the oceanic suture.
Geological map of Cameroon with study areas (modified after Toteu et al.,
The Congo craton in Cameroon consists of the Archaean Ntem Complex
and of the Palaeoproterozoic Nyong Complex. It is assumed that both have
not been reworked during the Pan-African orogeny. In contrast, the PP
basement within the CAFB has been overprinted in Pan-African times.
These basement rocks locally display granulite facies mineral assemblages
thought to have formed during a PP metamorphic event. The main part of
the CAFB consists of granites (mostly Pan-African) and Neoproterozoic cover
series metamorphosed from lower greenschist up to granulite facies.
Granulite facies metapelites (Grt-Bt-Ky/Sil) and metabasites
(Grt-Cpx-Qtz±Opx) have been studied from three areas in the PP basement
and from one area in the NP unit. Both units are supposed to have
experienced different metamorphic histories. Surprisingly, metapelitic
garnet from PP and NP units display the same type of well preserved
prograde zoning, indicating a common prograde growth history: Mg/(Mg+Fe)
ratio increases continuously, whereas Ca decreases stepwise from core to
rim. Conventional geothermobarometry and differential thermodynamic
calculations in metapelites in combination with decompression textures in
metabasites points to a clockwise P-T path. It starts at high pressures
(8-10 kbar) in the kyanite stability field and reaches 750-850 °C due
to near isobaric heating prior to uplift.
Well preserved chemical zoning in garnet
Some metabasic rocks of the Palaeoproterozoic Nyong complex
experienced eclogite facies conditions: Omphacitic clinopyroxene (up to
23% jd) contains numerous plagioclase "exsolutions" pointing
to a former significant higher jadeite component during maximum
subduction depth. Thermobarometry gives a minimum pressure of 16 kbar
at 750-800 °C. HREE are 10-19 times chondritic, whereas LREE are
depleted similar to those of MORB (low (La/Sm)N ratios (<1) at variable
Metapelitic monazites from the Palaeproterozoic and the Neoproterozoic
unit show a narrow range in chemical composition. The isochron ages for
both units are in the range of 602 ± 36 to 631 ± 31 Ma, i.e.
all metamorphic monazite grew during the Pan-African orogeny. So far, no
evidence for a Palaeoproterozoic metamorphism has been found, indicating
that the two published zircon ages of 2.1 and 2 Ga of the Palaeoproterozoic
may reflect magmatic formation ages and cannot be correlated with granulite
facies metamorphism. The Pan-African age deduced by chemical monazite
dating is confirmed by U-Pb SHRIMP zircon ages of 572.1 ± 7.9 Ma.
Timing of eclogite facies metamorphism was determined by U-Pb
SHRIMP zircon dating. All zircons from two eclogitic samples yield only
concordant ages of 2093 ± 45 and 2095 ± 70 Ma,
The studied areas of granulite facies Palaeoproterozoic basement and
Neoproterozoic cover series in the CAFB experienced only one metamorphic
event at about 602-631 Ma, due to substantial crustal thickening. The well
preserved stepwise growth zoning of metapelitic garnet indicates a short
duration of this metamorphism.
Retrogressed eclogites with MORB type chemistry occurring near the
NW-edge of the Congo craton indicate the so far unknown site of the suture
zone of Palaoproterozoic times.
Mineral textures in eclogites
Geochemistry of MORB-type eclogites and metagabbros
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