Journal
JOURNAL OF AFRICAN EARTH SCIENCES
Volume 58, Issue 1, Pages 67-80Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.jafrearsci.2010.01.009
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- Institut des Sciences de la Terre d'Orleans (France)
- AUF
- PDR OSREM04 BRGM
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In the Ngoulemakong region of the Ntem unit (South Cameroon), tonalite crops out as intrusions of various sizes cross-cutting the charnockite suite. Both of these granitoids are affected by NE-SW and WNW-ESE sinistral and dextral shear zones. Tonalite in the WNW-ESE shear zone are deformed and shows metamorphic assemblages represented by quartz-microcline-biotite-garnet-plagioclase-scapolite-fluoro-apatite and chlorite-sulfides-epidote-muscovite-quartz-calcite not recorded by the undeformed rocks outside it. These mineralogical assemblages provide evidence of decreasing pressure-temperature conditions from granulite-amphibolite-facies in the moderate deformed part to greenschist-facies in the central part of the shear plane. The higher fluid (H2O, CO2, S. F, Cl, K, and Na) activities and high-grade recrystallizations recorded in the shear zone favour a Pb loss hypothesis in the zircons. U-Pb SHRIMP zircon dating yield an emplacement age of 2865 +/- 4 Ma for the tonalite but does not permit the determination. of the age of the high-grade event responsible for the Pb loss. These results provide evidence of the retentivity of U-Pb zircon dates under high grade conditions. (C) 2010 Elsevier Ltd. All rights reserved.
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