Neoproterozoic hydrothermally altered zircons reveal ca. 750 Ma post-magmatic overprint and elemental behaviors in the western Yangtze block, South China

Abstract

Geochemical criteria of hydrothermal zircons and behaviors of immobile elements in hydrothermal processes lack systematic investigation. The Neoproterozoic western Yangtze block is featured by long-lasting magmatism, diverse metal deposits and multiple hydrothermal events. Here we report zircon U-Pb-Hf isotope data of two granitoids in this region, to decode the impacts of hydrothermal alteration on zircons. The Shimian monzogranite markedly suffered subsequent alterations, and contains both ca. 807 Ma magmatic zircons and ca. 750 Ma hydrothermally altered zircons. The latter zircon population has diffuse border, spongy texture and mineral inclusions, and exhibits flat LREE patterns and insignificant Ce anomalies (Ce/Ce* = 0.89–3.53). Importantly, their age slightly postdates ca. 764 Ma Kangding tonalite in the periphery, indicating post-magmatic modification and a newfound hydrothermal event in the Neoproterozoic western Yangtze block. All the zircon categories yield ε_Hf(t) values of 2.9–9.6, implying their dominant derivation from juvenile crust. Specifically, ca. 750 Ma hydrothermally altered zircons show superchondritic Y/Ho ratios of 33.6, low logfO₂ values of –17.2 and ΔFMQ of –2.9, indicating fluorine-rich and low oxygen fugacity signatures. When compared with other geochemical criteria, e.g., Th/U and (Sm/La)_N ratios, LREE-I values (LREE-I = Dy/Nd + Dy/Sm) are more reliable for distinguishing hydrothermally altered zircons (LREE < 30) from magmatic ones (LREE > 30). Our integrated data further indicate that LREEs, Th, U and P of zircons are highly mobile in fluorine-rich hydrothermal fluids, and these elements should be discreetly employed for geochemical tracing. In contrast, HREEs, Ti, Nb, Ta, Y, Hf and Hf isotope compositions are hardly modified.