Homogenising the upper continental crust: The Si isotope evolution of the crust recorded by ancient glacial diamictites

Twenty-four composite samples of the fine-grained matrix glacial diamictites deposited from Mesoarchaean to Palaeozoic have been analysed for their silicon isotope composition and used establish, first time, long-term secular Si record compositional evolution upper continental crust (UCC). Diamictites with Archaean Palaeoproterozoic Nd model ages show greater heterogeneity than those younger (irrespective depositional age). We attribute anomalously light compositions some presence glacially milled banded iron formation (BIF), substantiated by high content Ge/Si in these samples. infer that relatively heavy signatures (all which ages) are due contribution tonalite-trondhjemite-granodiorites (TTGs), evidenced abundance TTG clasts. By Neoproterozoic (with ranging 2.3 1.8 Ga), diamictite exhibit a range comparable modern UCC. This reduced variability through time is interpreted as reflecting decreasing importance BIF post-Archaean crust. The isotopes offers an independent test models emergence stable cratons onset horizontal mobile-lid tectonism. early UCC was heterogeneous incorporated significant amounts isotopically BIF, but following late stabilisation cratons, coupled oxygenation atmosphere led neoformation diminishing quantities TTGs, became increasingly homogeneous. homogenisation likely occurred via reworking preexisting crust, recorded diamictites.