LA-ICP-MS zircon U-Pb dating and geochemical characteristics of the Jiaogou syenite in western Henan Province and its geological implications

The Zhifang-Huangzhuang(ZH)area in western Henan Province has the largest outcrop of Indosinian alkaline intrusives in eastern Qinling orogenic belt.In this paper, the LA-ICP-MS zircon U-Pb ages as well as major elements and trace elements of the Jiaogou syenite were tested in order to obtain new constraint on formation time, rock source of partial melting and tectonic process of syenites in the ZH area.The U-Pb dating sample JG1701 is aegirine-augite syenite, and zircon age spectrum is composed of 7 age groups(~2.6 Ga, ~2.5 Ga, ~2.4 Ga, ~2.3 Ga, ~2.1 Ga, ~1.9 Ga and~243 Ma).The weighted mean age of 6 zircon spots form the last group is 243.2±4.8 Ma, which suggests that it was formed in Middle Triassic.The Jiaogou syenite is ultrapotassic rocks and belongs to alkaline-peralkaline series with higher Na₂O+K₂O content(11.50%~14.01%), alkalinity ratios(3.92~9.17)and K₂O/Na₂O ratios(3.78~10.25).The total rare earth elements(REEs)and(La/Yb)_N ratios of the Jiaogou syenite are 176×10^-6~315×10^-6 and 9.51~20.43, which shows the characteristics of LREE enrichment and HREE depletion without obvious Eu negative anomaly.They are enriched in large ion lithophile elements and depleted in high field strength elements, exhibiting negative anomalies of Th, Nb, Ta, P and Ti and positive anomalies of Ba, K, Zr and Hf in spider diagram.The source rock of the Jiaogou syenite was situated in the garnet-spinel transition zone at the depth of about 60~80 km, and its residual phases included phlogopite, Ti-enriched minerals such as rutile and ilmenite, garnet and spinel.The Jiaogou syenite is the mixed product of mantle-derived K-enriched magmas/fluids and crust-derived magmas/fluids, and it was controlled by regional lithosphere delamination in Middle Triassic.