The geologic record of the exhumed root of the Central African Orogenic Belt in the central Cameroon domain (Mbe - Sassa-Mbersi region)

摘要

We present structural, petrological, geochemical and geochronological data compiled in a new geological map on the high-grade metamorphic rocks exposed in the Mbe - Sassa-Mbersi region located along the Tchollire-Banyo Shear Zone, at the northern edge of the Central Cameroon domain.The region exposes a complex assemblage of metabasic (hornblendite, metagabbro, amphibolite) and metasedimentary (garnet paragneiss, calc-silicate gneiss) rocks alternating with ubiquitous migmatitic intermediate to felsic gneisses. U-Pb ages on detrital zircon in paragneiss point to the contribution of Archean, Paleoproterozoic and Neoproterozoic sources, and suggest a maximum deposition age of 725 +/- 12 Ma for their sedimentary protolith. The protolith of some amphibolites was emplaced during the Neoproterozoic as indicated by U-Pb ages on magmatic zircon at 600 +/- 4 and 599 +/- 6 Ma. Plutonic rocks with I-type chemical signatures were emplaced discontinuously over a period of ca. 80 Ma. The magmatic evolution started with the intrusion of magnesian medium-K tonalites and trondhjemites at 651 +/- 3 Ma, followed by magnesian high-K hornblende-biotite granites and ferroan shoshonitic biotite granites until 599 +/- 2 Ma. Finally, pegmatite dykes were emplaced along dextral shear zones at 573 +/- 2 Ma. The metamorphic rocks show a penetrative shallow-dipping foliation partially to totally transposed in a NE-SW trending steep-dipping foliation bearing a shallow-dipping lineation. These ductile syn-migmatitic fabrics, developed under high-grade metamorphic conditions in the presence of melt, between the metamorphic peak at ca. 1.3 GPa-770 degrees C, and the following isothermal decompression down to ca. 0.8 GPa. In situ U-Pb dating of metamorphic zircon rims from a garnet migmatitic paragneiss yields an age of 582 +/- 4 Ma. Localization of high-grade deformation is indicated by the development of the Tchollire-Banyo Shear Zone, characterized by mutually crosscutting subvertical and shallow-dipping mylonitic to ultramylonitic shear zones associated with sinistral and top-to-the SW kinematic criteria, respectively. These data indicate that the Central Cameroon domain exposed in the Mbe - Sassa-Mbersi region, represents the exhumed mid-to lower part of the former orogenic root of the Pan-African Central Africa Orogenic Belt that has undergone partial melting, lateral flow and intrusion of mafic to felsic calc-alkaline magmas between 650 and 580 Ma. The waning stage of the Pan-African orogeny is marked by the emplacement of syntectonic pegmatitic granite at ca. 575 Ma in subvertical E-W trending retrogressive dextral shear zones.