Petrogenesis of aplites in the Ke'eryin rare metal orefield in the Songpan-Garze Fold Belt, Eastern Tibet: Evidence from mineralogy, geochemistry, geochronology and Hf-Nd isotopes

The Ke'eryin pluton, in the Triassic central Songpan-Garze Fold Belt, Eastern Tibet, China, hosts multiple, broadly orogenic granites but also less well studied aplites. We present mineralogy, geochemistry, geochronology and Hf−Nd isotopic data for Ke'eryin orefield aplites of the Li-mineralized Lijiagou area. Biotite compositions and whole-rock A/CNK values (0.98–1.24) suggest that the Lijiagou aplites are of peraluminous S-type granitic affinity. The aplite samples are characterized by enrichment in LREEs [(La/Yb)N = 11.0–96.3], with strongly negative Eu anomalies (Eu/Eu⁎ = 0.44–0.56). They are generally enriched in Cs, Rb, Th and Hf, but relatively depleted in Ba, Sr, P and Ti. Zircons from two aplite samples yield weighted mean 206Pb/238U ages of 219.6 ± 1.2 Ma and 208.9 ± 1.3 Ma, respectively, the latter sample also yielding a monazite weighted mean 208Pb/232Th age of 207.1 ± 1.3 Ma. The average zircon crystallization temperatures of the two samples are 729 and 829 °C, with low oxygen fugacity (ΔFMQ −2.3 to −2.0). In situ Hf and Nd isotopic analyses on zircon and monazite yield εHf(t) and εNd(t) values of −6.24 to −2.49, and −8.7 to −7.9, respectively. In light of the aforementioned observations, the aplites are interpreted to be produced at a post-collisional orogenic setting, originating from partial melting of ancient metasedimentary rocks in the lower crust. Except for biotites in the Ke'eryin aplite, other minerals and whole-rocks have low concentrations of rare metals, suggesting that the aplites do not have rare metal mineralization potential.