Luku Alkali Syenite in the Hongge Ore District, SW China: Geochemistry, Geochronology and Metallogenic Significance

Abstract

Luku alkali syenite is distributed throughout the southern end of the Hongge basic‐ultrabasic intrusion in Panzhihua, Sichuan, SW China. Using LA‐ICP‐MS, the alkali syenite intrusion yielded a zircon U‐Pb age of 264.5 ± 1.6 Ma, concordant with the ages of the E'meishan large igneous province (260 Ma). The intrusion displayed silica‐saturated, Al‐adequate and alkali‐rich signatures, with SiO2 62.07%–64.04%, Al2O3 16.26%–16.79% and Na2O + K2O 9.17%–9.91% (averaging 9.31%). The rock falls into the alkaline zone on the SiO2‐A.R. diagram, as well as in the potassium zone on the K2O‐Na2O diagram, indicating a potassium alkaline rock. The rock has a low total REE concentration and showed enrichment in LREE (LREE/HREE = 7.06–7.95), typical negative Eu anomalies (δEu = 0.73–0.80), trace element deficiencies in LILEs (Ba, K, Sr, Zr) and enrichment in HFSEs (Th, U, Nd, Sm, Ta and Nb), displaying crust and mantle element information. Zircons show a strong positive Ce anomaly and negative Eu anomaly, similar to the characteristics of crustal source magmatic zircon, however the (Lu/Gd)N ratio ranges from 1.48 to 3.17, and the (Yb/Sm)N ratio ranges from 38.49 to 77.15, which are similar to the characteristics of mantle‐derived magmatic zircon. In the La/Yb‐δEu correlation diagram, the data plots near the boundary between crust type and crust‐mantle type. From the combined ‘trinity’ spatiotemporal relationship of Indosinian intermediate‐acid alkali intrusive rocks in the Panxi area with E'meishan basalt and basic‐ultrabasic intrusive rocks, the regional tectonic evolution and the partial melting model of the most intraplate magma sources, we believe that the Luku alkali syenite in Sichuan was formed from the partial melting of a crust‐mantle source material, due to underplating of the mantle plume basic magma.