Contrasting petrogenesis of Mg–K and Fe–K granitoids and implications for post-collisional magmatism: Case study from the Late-Archean Matok pluton (Pietersburg block, South Africa)
Résumé
This study investigates the origin of the 2.69 Ga-old Matok pluton, emplaced in the Pietersburg block, northern Kaapvaal Craton (South Africa), forthwith after a major tectono-metamorphic event ascribed to continent–continent collision. The Matok pluton consists of diorites, granodiorites and monzogranites. Petrography and whole-rock major- and trace element compositions of the Matok samples are similar to those of post-collisional Fe–K suites, which are very common in Proterozoic terranes. These granitoids are particularly rich in FeOt, TiO2, P2O5, span a wide range of SiO2 contents and display elevated concentrations in K2O, Ba, HFSE and REE, with moderately fractionated REE patterns.
All samples of the Matok pluton have unradionegic Nd isotopic compositions (εNd(2.69 Ga) = − 2.7 to − 4.6), but only a few monzogranite samples derive from reworking of older crust. Crustal contamination of basaltic melt cannot explain either the observed compositions. Most of the suite rather fractionated from a common mafic parent, either by partial melting or crystallization. Geochemical modeling shows that this parent magma or source rock is basaltic in composition, intermediate between calc-alkaline and tholeiitic groups, and enriched in incompatible trace elements with respect to the primitive mantle. It ultimately derives from the involvement of two distinct mantle sources: (1) enriched, sub-continental lithospheric mantle, which was metasomatized by sedimentary material derived from local crust of the Pietersburg block, < 0.3 Ga before the pluton emplacement; and (2) asthenospheric mantle.
This model accounts for the differences between Fe–K suites, such as the Matok pluton, and Mg–K suites, such as sanukitoids, the origin of which only requires metasomatized mantle. We propose that the most appropriate geodynamic setting for the interaction between enriched lithospheric and asthenospheric mantle sources is an episode of “lithospheric reworking” (through slab breakoff, retreat or sub-continental mantle delamination) in response to continental collision. Depending on the relative contribution of the two mantle sources in this environment, the resulting magmas can show a wide range of compositions from Fe–K to Mg–K end-members. From a regional point of view, this conclusion supports that the Kaapvaal Craton and the Central Zone of the Limpopo Belt were amalgamated between 2.80 and 2.75 Ga owing to continent–continent collision.