New evidences for an early Birimian evolution in the West African Cr
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摘要

The Paleoproterozoic rocks in the Kédougou Kénieba Inlier (KKI) consist of volcanic and volcanosedimentary rocks intruded by granitoids with a wide compositional spectrum. Geochronological data based on single-grain zircon Pb-Pb evaporation, U-Pb single grain, Ar-Ar and K-Ar dating of several plutons point to a long lasting emplacement history in the KKI. The zircon data presented here yielded Paleoproterozoic ages for the emplacement/crystallization of the Badon granodiorite (2213 ± 3 to 2198 ± 2 Ma), tonalitic gneiss from Sandikounda (2194 ± 4 Ma), the Tinkoto pluton (2074 ± 9 Ma) as well as for the felsic host rocks of the Mamakono pluton (2067 ± 12Ma). The Badon granodiorite, formerly regarded to be relatively young and post-tectonic, is substantially older than all other known KKI intrusions. As a consequence, this new age may constrain the ongoing discussion on the existence of an Early Birimian cycle in the West African Craton (WAC). Ar-Ar and K-Ar ages on hornblende (2112 ± 12 and 2118 ± 31 Ma, respectively, in the tonalitic gneiss from Sandikounda) and biotite (2098 ± 26 and 2090 ± 21 Ma, respectively, in the Badon granodiorite) define an age of ca 2090 Ma, which is interpreted to mark the major tectonomagmatic episode in this part of the WAC. This is confirmed by the Paleoproterozoic ages of the Saraya, Tinkoto and Mamakono granite. A hydrothermal event at 2020 Ma followed the emplacement of the granitoids and caused the alteration of the Saraya granite. The new zircon ages, reflecting the magmatic and tectonothermal events in the study area correlate well with various magmatic and metamorphic events elsewhere in West Africa.

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