Monazite EPMA-CHIME dating of Sangmelima granulite and granitoid rocks in the Ntem Complex, Cameroon: Implications for Archean tectono-thermal evolution of NW Congo craton

摘要

Electron microprobe (EPMA) monazite Th-U - Pb ages have been determined from a suite of granulite and granitoids rocks of the Sangmelima granite-greenstone Belt (SGB) of the Ntem Complex in Cameroon or NW Congo Craton. The monazite-bearing rocks include charnockitic gneiss, migmatitic tonalite-trondhjemite-granodiorite (TTG) gneisses, granodiorite, monzogranite and a late pegmatite intrusion. The monazites of Sangmelima can be classified as monazite-Ce. Based on published ages and EPMA Th-U - Pb monazite age data from the SGB, the geodynamic evolution of the NW Congo Craton was refined through its tectonometamorphic and magmatic events. The major charnockitic and TTG magmatism event in Ntem Complex occurred between similar to 2950 and 2850 Ma. This Mesoarchean crystalline basement underwent two major ductile tectonothermal events M-1 - D-1 and M-2 - D-2. (1) The onset of M-1 - D-1 corresponds to the migmatization of TTG at 2843 Ma coeval with the emplacement of syn-kinematic granite dykes (monzogranites) at 2840-2825 Ma. The S-1 regional penetrative foliation (D-1) was progressively generated during the M-1 metamorphic event, which led to the formation of associated hypersthene-garnet-bearing granulitic mineral assemblage that peaked at similar to 2830 Ma. (2) The M-2 - D-2 event between 2760 and 2740 Ma corresponds to the folding of S-1 regional foliation by F-2 folds and development of C-2 shear zones synchronous with the anatexis event that generated the high-K granitoids at 2750 Ma. Late Neoarchean granitic pegmatite/aplite dikes and/or veins emplaced during the brittle deformation D-3 and protracted cooling of the Ntem Complex between similar to 2700 and 2550 Ma. Our new EPMA monazite U-Th - Pb ages from the SGB highlight similar magmatic and tectonothermal events of the adjacent northern Gabonese domain and the Chaillu Massif in Gabon part of the NW Congo craton (ca. 3.0-2.5 Ga).