U-Pb Zircon Age: Preliminary Data Evaluating the Earth History Recorded by Two Basement Rocks (Granitic Pegmatite and Mica-Schist) in Mamfe Basin (Sw Cameroon, Central Africa)

Abstract

Zircon crystals from two basement rocks (granitic pegmatite and mica-schist) in the Mamfe Basin were studied to provide petrogenetic and geochronologic constraints, therefore were characterized and dated by LA-Q-ICP-MS analytical techniques, after their internal textural features were determined. Zircon crystals with oscillatory zoning from the granitic pegmatite exceed 200 μm are mainly euhedral to prismatic. The U (≤ 169 ppm), Th (≤ 240 ppm), and Th/U (0.6-1.7) are within the range of zircon crystallized in crustal magmatic environment. The 206Pb/238U ages (506 ± 11 to 639 ± 25 Ma) and 207Pb/235U (490 ± 13 to 653 ± 29 Ma) date Early Precambrian (Early Neoproterozoic: Edicaran) to Cambrian event. They are close to the age of rocks crystallized during the Pan-African collision found within the Cameroon Mobile Belt. Zircon crystals from the mica-schist are 200 μm in size, have magmatic (e.g., euhedral prismatic with oscillatory zone) and metamorphic (e.g., form round soccer ball, sub-hedral with overgrowth) features. The U (23-687 ppm), Th (4.0-161 ppm), and Th/U (0.01-1.8) are compatible with values in metamorphic and magmatic zircon crystallized within crustal environment. The highly heterogeneous 206Pb/238U (543 ± 12 - 2019 ± 30 Ma) and 207Pb/235U (526 ± 12 – 2008 ± 18Ma) mainly date Paleoproterozoic, Mesoproterozoic, and Neoproterozoic magmatic and/or metamorphic events.