Heavy silicon and oxygen isotope signatures of TTGs formed in distinct tectonic settings

Abstract

Tonalite–trondhjemite–granodiorite suites (TTGs) are key components of Archean continental crust and are therefore crucial to understanding the evolution of Archean Earth. However, the tectonic settings of their formation remain unresolved. Here we present new in situ zircon Si–O–Hf isotopes and quartz Si–O isotopes from 2.95 to 2.54 Ga TTGs of the Jiaodong terrane in the North China Craton. Through first-principles calculations, the corresponding TTG melt δ30Simelt and δ18Omelt values are determined as –0.03 ‰ to 0.06 ‰ and 7.12 ‰ to 7.60 ‰, respectively, indicating heavier isotope signatures than those of mantle-derived juvenile rocks. The 2.95–2.93 Ga and 2.56–2.54 Ga TTGs have depleted Hf isotopes [εHf(t) = +3.7 to +4.9, and +2.5 to +5.5, respectively] and higher (La/Yb)N ratios compared with the 2.75–2.71 Ga TTGs, which have relatively less depleted Hf isotopes [εHf(t) = +0.9 to +2.0] and lower (La/Yb)N ratios. Although the heavy Si–O isotope compositions indicate the involvement of supracrustal materials in the melting sources of all studied TTGs, the variable zircon Hf isotopes and (La/Yb)N ratios suggest that these rocks formed in distinct tectonic settings. The 2.95–2.93 Ga and 2.56–2.54 Ga TTGs are inferred to have been generated in a subduction setting involving the melting of juvenile oceanic crust under high-pressure conditions, whereas the 2.75–2.71 Ga TTGs are inferred to have been derived from the melting of buried ancient oceanic crust under a mantle-plume-induced lower-pressure environment. We conclude that the heavy Si–O isotope signatures of Archean TTGs may not be directly related to plate-tectonic processes.