Late Ediacaran post-collisional magmatism in the Guera Massif, South-Central Chad

The Guera Massif of South-Central Chad is composed of collisional and post-collisional intrusive rocks related to the Central African Orogenic Belt. Collisional granites were emplaced at 595-590 Ma but there are a series of post-collisional intrusions that were emplaced at 570-560 Ma. The older granites have zircon U-Pb weighted-mean ages of 569 +/- 6 Ma, 568 +/- 7 Ma, and 568 +/- 6 Ma, whereas the younger granites are 556 +/- 7 Ma and 561 +/- 6 Ma but all ages are within their maximum uncertainty. Both groups of post-collisional granites are peraluminous, ferroan, and alkali calcic to calc-alkalic but dioritic microgranular enclaves were observed within the older granites. The chemical variability of each group can be explained by fractional crystallization and it is likely that their parental magmas were derived by partial melting of the lower crust. The Nd isotopes of the older rocks (epsilon(Nd)(t) = -9.5 to -1.2) are slightly less enriched than the younger rocks (epsilon(Nd)(t) = -13.1 and -9.1). Moreover, the Nd isotopic compositions appear to indicate the existence of two isotopically distinct regions of the Guera Massif with one centred on the Mongo (west) region and the other near the El Hidjer to Zan (east) region. The older group was emplaced during a period of crustal thinning after oblique collision and shearing related to the final suturing of the Congo-Sao Francisco Craton and the Saharan Metacraton. The younger group was likely emplaced after a period of regional WNW-ESE directed crustal shortening. The punctuated nature of deformation/crustal relaxation cycles in the southern Saharan Metacraton suggests that there may have been many small terranes rather than larger cratonic blocks involved in the lithotectonic evolution of the Central African Orogenic Belt north of the Congo-Sao Francisco Craton.

Automated summary

The study of the Guera Massif in South-Central Chad reveals the origins, ages, and chemical characteristics of collisional and post-collisional intrusive rocks. The findings suggest that the formation of these rocks is related to crustal thinning, partial melting, and regional tectonic deformation in the area.