Age and origin of a Palaeozoic nepheline syenite from northern Shanxi Province, China: U-Pb zircon age and whole-rock geochemical and Sr-Nd isotopic constraints

摘要

Geochronological, geochemical, and whole-rock Sr-Nd isotopic analyses were performed on a suite of Palaeozoic nepheline syenites from Zijinshan to characterize their ages and petrogenesis. Laser ablation inductively coupled plasma-mass spectrometry U-Pb zircon analyses yield consistent ages of 525.7 ± 2.8 million years for a sample (HYK01). These intrusive rocks belong to the foid syenite magma series in terms of K_2O + Na_2O contents (14.3-15.2 wt.%) and to the shoshonitic series based on their high K_2O contents (5.42-5.61 wt.%). The nepheline syenites are further characterized by high light rare earth element contents [(La/Yb)_N - 29.1-36.1]; show modest negative Eu anomalies (dSEu = 0.5-0.6) and positive anomalies in Rb, Th, U, Pb, Zr, and Hf;;are depleted in Ba and high field strength elements (P and Ti). In addition, all the nepheline syenites in this study display relatively low radiogenic Sr (~(87)Sr/~(86)Sr)_i (0.7042-0.7043) and positive ε_(Nd) (t) (0.7-0.8). These results suggest that the nepheline syenites were derived from depleted continental crust. The parent magmas likely experienced fractional crystallization of plagioclase, Ti-bearing oxides (e.g. rutile, ilmenite, and titanite), apatite, and zircon during ascent, with negligible crustal contamination before final emplacement at a high crustal level.