Decoupled Ce-Nd isotopic systematics of the Neoproterozoic Huangling intrusive complex and its geological significance, eastern Three Gorges, South China

Abstract

Cerium is one of multivalent rear earth elements, which can transfer from trivalence to tretavalence at oxidizing environment. This process may cause variable degrees of fractionation of Ce from other trivalent rear earth elements, and thus may provide specific insight into the geological processes associated with marked redoxomorphism. Multiple geochemical tracing of Sr-Nd-Ce isotopes are performed on the felsic and mafic intrusives of the Neoproterozoic (~800 Ma) Huangling complex located at the eastern Three Gorges, South China. The intrusive rocks exclusively show various extents of negative Ce anomalies. On the ε _Ce-ε _Nd plot, most samples from the mafic intrusions scatter within the second quadrant, whereas those from the felsic intrusions within the fourth Quadrant. Both of the two groups exhibit relatively large range of ε _Ce(t) variation but limited ε _Nd(t) range, which cause a deviation from the “crustal array” and reveal a decoupled Nd-Ce isotope correlation. The intermediate-felsic suite have varied Ce/Ce* ratios but broadly proximate ε _Ce(t) values, indicating that their negative Ce anomalies were generated during the magmatism; on the contrary, a positive correlation between ε _Ce(t) and Ce/Ce* is observed for the intermediate-mafic suite, an indication of an origin of post-magmatic alteration or metamorphism for their Ce anomalies. Calculation of model age, the occurrence age of negative Ce anomalies (T _Ce) for the intermediate-mafic samples infers that the alteration events took place >350 Ma. Data showed that negative Ce anomalies of the felsic intrusions may reflect an increase of oxygen fugacity during magma ascending, rather than an inheritance from their source rocks. This explanation implies that the Neoproterozoic magmatism occurred at the continental nucleus of the Yangtze block were developing at a geodynamic context of rapidly regional uplifting.

Keywords

negative Ce anomalies / Ce-Nd isotopic decoupling / alteration / oxidation environment

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