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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

Abstract:

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