Geochemical Signatures of Igneous Zircon and Apatite: Generation of Archean TTGs in the Barberton Granitoid-Greenstone Terrain, South Africa

Abstract

We performed isotopic and trace element analyses of zircons and apatites from the Inyoni, Batavia, and Badplaas tonalites, trondhjemites and granodiorites series (TTG) in the Barberton Granitoid-Greenstone Terrain (BGGT), South Africa using laser-ablation inductively-coupled plasma-mass spectrometry to investigate Archean tectono-magmatism. The U–Pb ages obtained from these minerals are consistent with previously reported ages ranging from ca. 3.28 to 3.23 Ga. The chondrite-normalized REE patterns of zircons and apatites enabled us to identify grains with primary information on TTG magmatism. Trace element characteristics, such as Sr/Y and Ti concentrations from primary grains, indicated that Inyoni TTG magmas coexisted with eclogite residue, whereas Badplaas and Batavia TTG magmas coexisted with garnet-amphibolite residue during the early formation stages. Furthermore, Lu–Hf isotope data, combined with the U/Yb-Nb/Yb data from zircons, suggested that the tectonic settings for TTG generation involved a subduction-type environment accompanied by crustal reworking. This implies that horizontal plate movement played a significant role in the TTG tectono-magmatism, and the subduction of old and cold plates facilitated slab melting under eclogite facies conditions, leading to TTG generation on the Archean Earth.