Mineralogical constraint for metamorphic conditions in a shear zone affecting the Archean Ngoulemakong tonalite, Congo craton (Southern Cameroon) and retentivity of U–Pb SHRIMP zircon dates

Abstract

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 (H 2O, CO 2, 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.