| Summary: | 碩士 === 國立臺灣師範大學 === 地球科學系 === 102 === Tcholliré-Banyo Shear Zone (TBSZ), north Cameroon, is a NE-SW trending branch of the Central African Shear Zone (CASZ). The TBSZ situated within the boundaries of three major cratons: the Western African Craton, the Congo Craton, and the Eastern Sahara Block. By conducting geochronology, geochemistry and petrology analyses on ten samples coming from TBSZ and CASZ, this study attempt to decipher the tectonic evolution of this region. The U-Pb zircon ages of gneiss (10CC02C、10CC05), mylonite (10CC03A、10CC03C), and granite (10CC04) revealed two major age groups of (1.85 ~ 2.14 Ga) and (696 ~ 514 Ma). Except for the granite sample, the Th/ U (0.12 ~ 1.23) of the older core of all other samples are all larger than 0.1, which indicate a magmatic origin. The younger rim, however, showed Th/ U (0.0013 ~ 0.0098) < 0.01 indicating a metamorphic origin. The Th/ U are all larger than 0.1 for the granite sample indicating the granite crystallized during the 618 Ma (concordant age) Pan-African Orogeny, and the few older cores are inherited zircons.
In order to decipher the tectonic setting of the magmatic sources, whole rock XRF analysis of major and trace element with Hf isotope analysis of zircons are also conducted. Although the zircon U-Pb geochronology data of the Paleoproterozic core suggested a magmatic origin, the whole rock trace element distributions are far below the normal magmatic rocks. Which pointed out that the gneiss and mylonite samples are metasedimentary rocks, and the age in the core of zircons are inherited. Since the age in the core of zircons corresponds to the Paleoproterozoic magmatic event formed during the Eburnean orogeny by the collision between the South America and West Africa. This could be the source of sediments for these metasedimentary rocks. Although the sedimentary age was uncertain, it is possible that they have the same age as the Mesoproterozoic Yaoundé series in the Lom basins. The wide range of εHf (T) values (－18.5 ~ －11.2; +1.0 ~ +10.5) of the 618 Ma granite sample suggested the formation of this magmatic event is probably due to the Pan-African orogeny with collision between the Eastern Sahara Block with the Congo Craton and West African Craton under an arc environment. Other than magmatic event, the Pan-African orogeny also generated the regional metamorphism of surrounding metasedimentary rocks.
Petrological analysis of the metasedimentary rocks showed retrograde metamorphism from amphibolite facies to greenschist facies after the Pan-African event. 40Ar/39Ar dating of metamorphosed K-feldspar mineral separates of 10CC02C sample showed age ranges from 589 ~ 519 Ma indicating the deformation event of the TBSZ had occurred under the lower greenschist facies during this time.
Key Words:U-Pb Zircon Dating, Tcholliré-Banyo Shear Zone (TBSZ), Tectonic evolution, Pan-African
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