Origin of TTG-like rocks from anatexis of ancient lower crust: Geochemical evidence from Neoproterozoic granitoids in South China

Abstract

TTG rocks are commonly assumed to form by partial melting of oceanic crust during slab subduction or anatexis of thickened lower crust. The two models are tested by an integrated study of geochronology and geochemistry for a composite batholith of Neoproterozoic granitoids (mainly TTG-like) from the Yangtze Gorge in South China. Zircon U–Pb dating indicates that all granitoids crystallized in a period of 820 to 800 Ma. Zircon δ18O values are 4.85 to 6.84‰, suggesting limited contributions from supracrustal materials to magma sources. Zircon εHf(t) values for syn-magmatic domains range from −3.6 to −29.7, concordant with whole-rock εNd(t) values of −2.1 to −21.3. Correspondingly, zircon Hf model ages are 2.09 to 3.16 Ga, suggesting their derivation from ancient crust of Paleoproterozoic to Archean ages. TTG-like rocks contain inherited zircon cores that yield two groups of U–Pb ages at Paleoproterozoic (1.8 to 2.0 Ga) and Archean (~2.9 Ga), respectively. They show variable εHf(t) values from −48.4 to −33.4. Because the Neoproterozoic magmatism leads to variable increases of εHf(t) values for some inherited cores, only the lowest εHf(t) values for the inherited cores are assumed to represent the original Hf isotopes in source rocks. The screened results are consistent with those for Archean TTG gneiss and migmatite in this region. In addition, the TTG-like rocks have lower εNd(t) and εHf(t) values, higher Sr/Y and (La/Yb)N ratios and stronger depletion in Nb, Ta and Ti than the other granitoids. Two alternative scenarios are proposed to account for their petrogenesis: (1) anatexis of Archean and Paleoproterozoic mafic crust in the stable depths of garnet and amphibole, respectively; or (2) anatexis of Archean TTG and Paleoproterozoic mafic rocks in amphibole-stable depths. Nevertheless, the second one is preferred because the inherited zircons with Paleoproterozoic to Archean U–Pb ages are preserved in the TTG-like rocks, which is incompatible with high temperatures to generate the primary TTG magmas. Therefore, the Neoproterozoic TTG-like rocks were derived from the anatexis of ancient non-thickened lower crust, with the TTG features inheriting from their source.