Igneous petrology, zircon geochronology and geochemistry of multiply emplaced granitoid bodies from the Palaeoproterozoic Usagaran domain in central Tanzania

This study reports igneous petrology, zircon ages and geochemistry for Palaeoproterozoic I-type granitic, granodioritic, Qtz-monzonitic, Qtz-monzodioritic and monzodioritic rocks from the Usagaran domain of southwestern Tanzania. These rocks can be subdivided into three groups according to their age, T-t evolution and geochemistry. The oldest group is made up of amphibolite-facies calcic-alkalic gneisses, ranging in age between ∼1984 and ∼1910 Ma, which have been divided into two subgroups regarding their T-t evolution. The first subgroup shows a constant increase in T, and the second subgroup first shows a decrease in T, followed by an increase in T. The second group consists of alkalic to calcic-alkalic greenschist- and/or amphibolite-facies gneisses that vary in age between ∼1895 and ∼1863 Ma. This group could also be subdivided regarding its T-t evolution. The first subgroup experienced an increase in T, followed by a decrease in T, whereas the second subgroup shows a constant decrease in T-t conditions. The third group consists of slightly deformed gneisses ranging in age between ∼1847 and ∼1817 Ma. The T-t evolution of these rocks is indicative of a decrease in T, followed by an increase in T.

We interpret the different T-t paths, coupled with the emplacement of several granitoid bodies over a time period of ∼170 Ma, as a history of multiple injection of granitoid bodies into Archaean and early Palaeoproterozoic crustal material as indicated by zircon xenocrysts. This is also suggestive of a continuation of the Archaean Tanzania craton into the lower crust of the Usagaran domain. We explain the geochemical differences as well as the various T-t paths as an interaction between granitoid melts with primary arc basaltic magma signatures originating by partial melting from a hydrated mantle wedge above a downgoing slab. The interaction between these primary magmas with more differentiated magmas can produce various combinations of assimilation, differentiation, fractionation, crystallization and magma mixing. We conclude that large parts of the Usagaran domain formed during magmatism at different times and at different locations. The granitoid precursors are interpreted to have formed in a continental arc environment during the continent-wide Eburnian crust-forming event.