Source composition, fractional crystallization and magma mixing processes in the 3.48–3.43 Ga Tsawela tonalite suite (Ancient Gneiss Complex, Swaziland) – Implications for Palaeoarchaean geodynamics

摘要: Abstract The ca. 3480–3430 Ma Tsawela Gneiss (TG) is a well-preserved coarse-grained diorite to tonalite suite within the Ancient Complex of Swaziland, eastern Kaapvaal craton. These gneisses are texturally and compositionally distinct from hosting oldest components 3200–3660 Ma TTG-type Ngwane Gneisses (NG). Major trace elements, in combination with whole-rock hafnium-neodymium isotopic data, were analyzed TG three samples 3450 Ma grey NG constrain sources magmatic processes. High-field-strength element data (HFSE) combined U-Pb SHRIMP II ages Hf-in-zircon for key their petrogenesis. In contrast widespread view that Archaean crust mainly composed TTG igneous suites formed juvenile sources, geochemical compositions indicate these rocks represent calc-alkaline plutonic which possibly by magma mixing processes involving juvenile, mantle-derived tholeiitic melts as well partial older gneiss. Alternatively, may fractional crystallization hydrous intermediate magma. results field evidence relatively uniform homogeneous composition. Our geochronological show intruded during time span at least 50 Ma without any significant compositional change source. predominant influence mafic magma, assimilation-fractional-crystallization (AFC) can be excluded major modelling. proposed here suggest efficient approximately equal amounts magmas those derived basement supported largely Hf-Nd samples. We propose melting occurred lower consisted >3.50 Ga was triggered plume-related underplated intraplated sills and/or laccoliths. This model end member lacking negative Nb anomaly implying subduction not involved formation TG.