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 Metamorphic and Magmatic Petrology
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14.10.2008		  

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Fluid backflow in subduction zones

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Pan-African crustal thickening and Palaeoproterozoic eclogite facies metamorphism in Cameroon

Staff: D. Loose
V. Schenk
Funding: DFG (Sche 265/14)

Lage Madagascars im East African Orogen
Position of Cameroon at the transition from the
Congo Craton to the CAFB
(modified after Kusky 2003)

INTRODUCTION

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
Geological map of Cameroon with study areas (modified after Toteu et al., 2001

GEOLOGICAL SETTING

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.


GRANULITES

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.


Geological map of Cameroon
Well preserved chemical zoning in garnet

RETROGRESSED ECLOGITES

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 nb/la (0.7-1.4)).


GEOCHRONOLOGY

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, respectively.


CONCLUSION

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.


textures
Mineral textures in eclogites
Geochemistry of MORB-type eclogites and metagabbros


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