La–Ce and Sm–Nd isotopic systematics of early Proterozoic leucogranite with tetrad REE pattern

Abstract

We report La–Ce and Sm–Nd isotopic data for leucogranites having tetrad rare earth element (REE) patterns at Imweon area in the northeastern Yeongnam Massif, South Korea. The leucogranites are strongly peraluminous (A/CNK = 1.18–1.79) S-type granite generated by the partial melting of metasediments, which consist of quartz, K-feldspar, plagioclase, muscovite, biotite, garnet and sillimanite. The leucogranites have three types of chondrite-normalized REE patterns: one showing a tetrad REE pattern of M-type with a large negative Eu anomaly, another with slightly V-shaped REE pattern with a positive Eu anomaly, and third with an REE pattern of crustal type with an LREE-enriched and HREE-depleted pattern. U–Pb zircon ages and, Sm–Nd and Rb–Sr whole-rock ages from the leucogranites are 1853 ± 15 Ma − 1859 ± 14 Ma (2 σ), 1833 ± 220 Ma (2 σ) with ε Nd(1.83 Ga) = −6.6 ± 5.5 (2 σ) and 1740 ± 71 Ma (2 σ) with initial 87Sr/ 86Sr = 0.755 ± 0.050 (2 σ), respectively. These values suggest that the leucogranites were emplaced from magma formed by partial melting of the crustal material. In the ε Ce– ε Nd diagram, the present Ce and Nd isotopic compositions of the leucogranites with a tetrad REE pattern are plotted in the positive ε Nd and ε Ce field whereas those of the leucogranites without tetrad REE pattern are in the negative ε Nd and positive ε Ce field. The initial Ce isotope values of the leucogranites with a tetrad REE pattern are plotted in a range of −2.31 < ε Ce(1.85 Ga) < + 3.3. The initial Ce isotopic values of the leucogranites without a tetrad REE pattern are extremely high, + 3.3 < ε Ce(1.85 Ga) < + 21.8. The initial ε Nd(1.85 Ga) values of the leucogranites are similar in the range of −8.63 < ε Nd(1.85 Ga) < −4.19 regardless of the existence of a tetrad REE pattern, suggesting that Ce isotopes of the source materials should be more heterogeneous than Nd isotopes. Our La–Ce, Sm–Nd, and Rb–Sr isotopic data indicate that the tetrad REE pattern was formed by a specific geochemical reaction during granitic magma emplacement rather than by metamorphism, alteration, and weathering.