Geochemical Characteristics Of The Zijinshan Alkaline Complex In Linxian County, Shanxi Province

The Zijinshan alkaline complex outcrops on the western wing of in the middle part of the Lüliang Mountain, Shanxi Province. Its tectonic location is in the western central North China Craton—the transition zone of Lüliang orogenic belt and Ordos block, and near the Lishi faults zone. In this paper, the author has detailedly and systematically studied on the complex by the method of lithology, geochronology, petrochemistry, and discussed the scientific problems such as time, sources, magmatism evolution and mechanisms of genesis.The Zijinshan alkaline complex is a ring-type alkaline complex, which has experienced muti-petrogenesis stages and epochs, and includes incursionand and eruptive facies. Field geological characteristics indicate that the sequence of irruptive rock units is monzonite, aegirine augite syenite, nepheline-bearing aegirine augite syenite and nepheline syenite. Trachytic breccia and phonolitic breccia are formed at the eruption stages. LA-ICP-MS Zircon U-Pb dating datas confirm that, intrusive magnatism time horizon from monzonite to nepheline syenite was 133.7~116.1Ma, and the deep magmatic thermal effect between 322~282Ma happened in Zijinshan area. All above proves that the deep thermal effect of central and western North China Craton destruction began at Late Carboniferous, Late Paleozoic.The Zijinshan alkaline complex has the chemical characteristics of alkali-rich, Al2O3 -medium, CaO-lean, Fe-rich and Mg-lean, which is belonged to the potassic alkaline-parlkaline series rock. Harker diagrams of major elements indicate that the changes of rock composition have nothing to do with the fractional crystallization, mainly with the origin of the magma. Tectonic discrimination shows that the complex derived from extension environment.Rich in LREE and no significant negative Eu anomalies REE patterns demonstrate that magmatism is characterized by equilibrium crystallization and no dominant crystal fractionation.ΣREE and LREE/HREE gradual reduction from early to late explains that the sources of intrusive rock units are different. Only nepheline syenite has the weak convex REE patterns with Nd as the high point, and it may be related to late hydrothermal effects.Transition elements patterns of the Zijinshan alkaline complex are typical"W"-types; lithophile elements (LILE) such as Ba, Sr, K are riched and high field strength elements (HFSE) such as Nb, Ta ,Ti are defective; trace elements (Sr, Zr, La, Yb, Nb, Th) have poor correlation with SiO2; and Th/Hf-Ta/Hf structural discrimination suggests that the complex is not the fractional crystallization or the partial melting product from a single source. Its source regions may be continental lithosphere mantle, or at least involved lithosphere mantle substances.Monzonite is of the highest initial 87Sr/86Sr ratios (0.70495 ~ 0.70518), and the lowestεNd (t) values (-11.5 ~ -9), and its Sr-Nd values fall in the typical EMⅠ(Ⅰtype enriched mantle) in the Sr-Nd relationship diagrams; compared with monzonite, aegirine augite syenite exhibits a higherεNd (t) values (-7.4 ~ -3.5) and lower 87Sr/86Sr (t) ratios (0.70428 ~ 0.70448), and Sr-Nd values are between EMⅠand DM (depleted mantle); according to similar 87Sr/86Sr(t) ratios (0.70404 ~ 0.70475) andεNd (t) values (-7.8 ~ -2.5) with aegirine augite syenite, we presumed that nepheline-bearing aegirine augite syenite possessed the resemble magma origin with aegirine augite syenite; with the lowest 87Sr/86Sr (t) ratios (0.70412 ~ 0.70432) and the highestεNd (t) values (-5 ~ 0.2), nepheline syenite shows more DM sources. Major and trace elements and Sr-Nd-Pb isotopes geochemistry general characteristics concluded that monzonite magma originated from the lower crust and lithosphere mantle mixing; aegirine augite syenite and nepheline-bearing aegirine augite syenite were from depleted aesthenosphere mantle and partial melt of enriched lithosphere mantle; nepheline syenite also was from depleted aesthenosphere mantle and partial melt of enriched lithosphere mantle, but by comparison, aesthenosphere mantle contributed more in nepheline syenite magma generation.Integrated field geological features, rock major and trace elements, isotopes geochemistry and the magma origin, combined chronology with regional tectonic settings, this paper considered that the northwest subduction and extrusion of ancient Pacific plate, stretching was resulted in central and western North China Craton, and induced alkaline-parlkaline magmatism to occur in Zijinshan area. Magmatism experienced a process from weak to strong. From firstly lithosphere mantle and lower crust partially melted to aesthenosphere mantle larger scale upwelled and mixed with lithosphere mantle, forming a typical alkaline rock—nepheline. The following strong volcanic eruptions even illustrated this point. Zijinshan alkaline complex which generated in late Jurassic to early Cretaceous revealed that the deep magmatism process was consistent with the central and western North China Craton tectonic shift pattern. Late Jurassic to early Cretaceous was also the main stage of North China Craton destructon.