Sm-Nd and U-Pb isotope behavior of REE-rich accessory minerals in pegmatite during overprinted metamorphic and hydrothermal events: Evidence from the Paleoproterozoic rare-earth pegmatite in the lesser Qinling district of China

Sm-Nd and U-Pb isotope systems are widely used to dating various magmatism and tracing geological processes, but their behaviors during post-magmatic metamorphic and/or hydrothermal disturbances remain unclear. Here, we investigated the trace elements, U-Pb and Sm-Nd isotope systems of accessory minerals (allanite, titanite, and apatite) from the Huangjiagou Paleoproterozoic rare-earth pegmatite, to determine the timing of magmatism and also the overprinted metamorphic and hydrothermal events, with a particular emphasis on the isotopic behavior of these accessory minerals under metamorphic event and fluid-rock reaction. Two-period accessory minerals are identified: Group I allanite, crystallized in the magmatic period but was modified by high-temperature metamorphism and hydrothermal metasomatism; and Group II allanite, titanite and apatite, which newly precipitated in the late hydrothermal event. In-situ U-Pb dating of the Group I allanite (type Ia) yielded a magmatic age of ca. 1.82 Ga, whereas the type Ib allanite record the high-temperature anhydrous thermal metamorphism at ca. 1.75 Ga. Group II allanite and titanite yielded U-Pb ages of ca. 131 Ma, indicating a Mesozoic hydrothermal metasomatism overprinting, due to the emplacement of neighboring granites. Group I allanite exhibits different zones and inconsistent U-Pb ages, but they exhibit an identical Sm-Nd isochron age (∼1.83 Ga) broadly consistent with the pegmatite age, indicating immobility nature of the Sm-Nd system. Group II titanite and allanite exhibit a Sm-Nd isochron age of ∼130 Ma with initial ε Nd (t) values of −27.5 to −29.8, consistent with the whole-rock ε Nd (t = 130 Ma) values of granitic rocks, implying a total redistribution of the REEs. While, Group II apatite and coexisting allanite did not produce geologic meaningful Sm-Nd ages with a wider initial ε Nd (t) range of −15.8 to –23.7, indicating the fluid has incompletely equilibrated Sm-Nd isotopic system due to various fluid-rock reaction. Our study provides a typical example to illustrate the robustness of the Sm-Nd isotopic system of magmatic allanite under the disturbance of later metamorphism and metasomatism, and shows the different degree equilibration of Sm-Nd isotopes in hydrothermal systems under various fluid-rock reactions.