Geochemistry and LA-ICP-MS zircon U-Pb dating for trachyte porphyry of Hongshan intrusion in the southern Taihang Mountains

Based on detailed field investigation, the authors studied the petrology, geochemistry and geochronology of trachyte porphyry in Hongshan intrusion. The SiO₂ and Al₂O₃ of the rock range from 63.57% to 67.85% and 16.75% to 18.77%, 65.19% and 18.21% on average respectively. Rittmann index(σ) ranges 5.6~8.2, the K₂O/Na₂O ratio ranges 1.00~1.74, with an average of 1.24. A/CNK varies from 1.00 to 1.06. These data suggest that it belongs to alkaline syenite rock. The REE content is between 33.75×10^-6 and 144.30×10^-6, with(La/Yb)_N ratios of 9.18~33.28 andδEu of 0.99~1.16. The intrusion is remarkably characterized by enriched LREE and depleted HREE. The chondrite-normalized REE patterns show weak positive Eu anomaly. In the primitive mantle nor-malized trace element spider diagrams, the samples show strong enrichment of LILEs and relative depletion of HFSEs. In addition, tra-chyte porphyry's LA-ICP-MS zircon U-Pb dating indicates that the trachyte porphyry were emplaced at ca.128.6±1.0Ma. The zir-con ε_Hf(t) values(-16.0~-11.4) yielded zircon Hf crustal modal ages of 1258~1432Ma. Based on the data obtained, the authors hold that Hongshan volcanic rocks originated from partial melting of an EMI-type mantle source, contaminated by LCC. Trachyte por-phyry formed in the same time as the Hongshan intrusion in the Mesozoic Cretaceous. Magmatic activities took place in the transfor-mation period of the collision environment, causing the partial melting of the mantle and producing the trachyte porphyry. This might have been one of the important ways of lithospheric mantle thinning.