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Our understanding of crust – mantle evolution and geodynamics of the early Earth has been hampered by the scarcity of Hadean to Eoarchean rocks and by the almost ubiquitous isotopic disturbance of the oldest rocks. Here we present in-situ zircon U-Pb ages, O-OH and Lu-Hf isotopes, as well as whole-rock Sr-Nd-Hf-Pb isotopes for the recently identified Eoarchean Aktash gneisses in the Tarim Craton, NW China. The new age data confirm that the Aktash gneisses were formed at least by ∼3.72 Ga and were affected by two metamorphic events at ∼3.56 and ∼2.0 Ga, respectively. Whole-rock isotopic data show that the Sr-Nd-Hf-Pb isotopic systematics have all been variously disturbed by ∼2.0 Ga regional granulite facies metamorphism, resulting in decoupling of Nd-Hf isotopic systematics that is, in some samples, similar to that observed in SW Greenland. The least disturbed sample (16ALT67), however, yielded chondritic to slightly sub-chondritic whole-rock εNd (−0.3) and εHf (−0.9). This is consistent with the in-situ magmatic zircon εHf (weighted mean −0.2 to −1.8) for most samples of the Aktash gneisses, suggesting a mafic source derived from chondritic mantle with a short crustal residence time. Our data thus provide no evidence for the presence of an Eoarchean depleted mantle or a Hadean enriched crust, implying that early Earth crust – mantle differentiation was either limited or balanced by large-scale crustal recycling. In-situ zircon O–OH data show that metamorphism and later alteration tend to decrease the zircon δ18O of the Aktash gneisses, and the weighed mean δ18O of pristine magmatic zircons (6.2–6.3‰) from the least disturbed samples indicate that the mafic source was mildly altered at low temperature by the early hydrosphere, facilitating partial melting and formation of some of the earliest preserved continental crust known on Earth.
1/01/10 → 31/12/10