Synchronous BIF, HT metamorphism and granitic plutonism at ca. 2.8 Ga in Gabon
Résumé
New field and geochronological data obtained in the course of a mapping project financed by the European Union (Sysmin Funds) and by the Gabonese Republic enable precise chronological constraints to be placed on the timing and context of BIF deposition in Gabon. Field and petrographic observations allow the BIF-hosting greenstone belts to be related to a single lithostratigraphic unit: the "Belinga Group" (Chevallier et al., 2002; Thiéblemont et al., 2009; Thiéblemont, 2009). This group was affected by a generalized Neoarchean tectono-metamorphic event which culminated at 2746 ± 7 Ma in Northeastern Gabon. Dating of detrital zircons from the BIF and related coarse-grained detrital rocks from 2918 ± 7 Ma to 2868 ± 9 Ma implies that the deposition and further deformation of the Belinga Group took place in the interval 2868 - 2750 Ma, close to the Mesoarchean - Neoarchean boundary. The Mesoarchean in Gabon was a period of intense plutonic activity, with a wide range of granitoids dated between ca. 3 Ga and 2.8 Ga. Plutonism started with mainly intermediate rocks of dioritic to granodioritic composition and ended with the emplacement of pink granitoids of monzogranitic to syenogranitic composition. Precise geochronological data show that these granites were emplaced between 2870 and 2810 Ma. In addition to granitic plutonism and BIF deposition, the existence of vast charnockitic domains within the Mesoarchean basement attests for a period of intense heating of the recently accreted crust around the Mesoarchean - Neoarchean boundary. Repeated geochronological measurements (U/Pb on zircon, U-Th-Pb on monazite) on a syn- to late-charnockitic leucogranite from Northeastern Gabon enable this HT metamorphism to be precisely dated around 2820 Ma (~± 10 Ma). This implies that the emplacement of BIF around the Mesoarchean - Neoarchean boundary was correlative with an intense heat transfer from the mantle to the crust which preceded a major orogenic event around 2.75 Ga.