Geology and geochronology of the Archean plutonic rocks in the northeast Democratic Republic of Congo

Here we report the first regional-scale study of Archean plutonic rocks from 50,000 km(2) of the northeast Democratic Republic of Congo (DRC). We include 50 new U-Pb zircon Sensitive High-Resolution Ion MicroProbe (SHRIMP) ages supported by petrographic and whole rock geochemical data from a further similar to 400 samples, and 39 additional U-Pb zircon ages reported by Kabete et al. (submitted) and Bird (2016). Felsic-intermediate magmatism across the northeast DRC occurred from similar to 3200 to 2530 Ma. Plutonism before similar to 2670 Ma is restricted to the West Nile Gneiss in eastern and northern-most DRC, whereas plutonism across the remainder of the northeast DRC initiated between 2670 and 2640 Ma. Two Neoproterozoic granite plutons similar to 1000-950 Ma are also present within the West Nile Gneiss. Magmatism between similar to 2670 and 2530 Ma marks the principal period of crustal growth in the northeast DRC. Felsic-intermediate plutonism between similar to 2670 and 2600 Ma included two suites of magnesian, calc-alkaline rocks, (1) a sodic-calcic suite with high Sr/Y ratios (>40) comparable with tonalite-trondhjemite-granodiorite (TTG) suites in other Archean terranes, and (2) a calcalkaline to high-K suite of granitoids with low Sr/Y ratios (<40) and lower Na and Al contents, comparable with low-Ca granites in the Yilgarn Craton. In contrast, most granitoids emplaced between similar to 2600 and 2530 Ma have ferroan, high-K calc-alkaline compositions, and are enriched in high field strength (HFSE) and rare Earth elements (REE). Although the HFSE-elevated granitoids are comparable with Closepet-type, and mafic granites in other Neoarchean terranes, their volume within the northeast DRC appears unprecedented. Before similar to 2600 Ma partial melting of hydrous metabasalts at depths > 40 km is inferred to have produced the high Sr/Y magnesian granitoids, whereas the low Sr/Y magnesian granitoids were derived from meltingassimilation-storage-hybridization (MASH) processes in the overlying low-mid crust during the same period. Trans-lithospheric extension after similar to 2600 Ma, enabled asthenospheric mantle to interact with the base of the crust, resulting in high-temperature melting that produced the HFSE-elevated granitoids. Inherited Meso- and Paleoarchean zircons are almost completely restricted to the area within 30-40 km of the West Nile Gneiss, implying most of the northeast DRC is underlain by juvenile Neoarchean crust.

Automated summary

This study presents a regional-scale analysis of Archean plutonic rocks in the northeast Democratic Republic of Congo, including new U-Pb zircon ages and petrographic data. Felsic-intermediate magmatism in the region occurred from about 3200 to 2530 Ma, with two distinct rock series identified during the period. The presence of high field strength and rare Earth elements in the granitoids suggest unique geological processes in the area.