Geochemical and Sr-Nd isotopic study of alkaline syenites in Liangtun-Kuangdonggou, Liaoning Province, China: Evidence for enriched mantle before 1.86 Ga and implications

Abstract

The 1.86 Ga Liangtun-Kuangdonggou complex (LKC) is one of the oldest alkaline syenite bodies so far discovered in China. This syenite suite has elevated contents of total alkali (K2O+Na2O), with an average of 10.50%, and a mean Rittmann Index (σ) of 6.48. The intrusions have slightly higher concentrations of K2O than those of Na2O on a weight percent basis, indicating the rocks belong to potassium-rich alkaline syenite series. Total rare-earth element concentrations (ΣREE) of the rocks are relatively high, ranging from 324×10−6 to 1314×10−6, with a mean value of 666×10−6. The REE patterns are subparallel and rightward steep with (La/Yb)N>33, showing mild negative to positive Eu anomalies (δEu: 0.63–1.15). All samples exhibit strong LILE and LREE enrichments and TNT (Nb, Ta, Ti) and P depletions in multi-element spidergrams. On theɛSr(t)-ɛNd(t) correlation diagram, most analytical data points plot within the enriched mantle field with low (87Sr/86Sr)i ratios (0.7045–0.7051) and negativeɛNd(t) values (−3.72–−3.97), falling among those kimberlites from Fuxian County, Liaoning Provinve, from Mengyin County, Shandong Province and the II-type kimberlites from South Africa. These characteristics imply that the LKC-rocks may have the same source as the above-mentioned kimberlites, i.e., they have close connections to the materials derived from enriched mantle reservoirs, further revealing that the upper mantle beneath the northeastern part of the North China Plate had been highly enriched before 1.86 Ga. Geodynamically, the LKC-rocks were formed in a within-plate environment with close genetic connections to rift-related alkaline magma activities possibly controlled by ancient mantle plumes.