Magmatism And Its Dynamic Background Of Zijinshan Alkaline Complex In Linxian County, Shanxi Province

The Zijinshan alkaline complex outcrops on the western wing of in the middle part of the Luliang Mountain, Shanxi Province. Due to its typical alkaline rocktype, the Zijinshan complex has been attracted much attention from geological field. In this paper, the author has detailedly and systematically studied on the complex by the method of structuralgeology, lithology, petrochemistry, isotope geochemistry, combined with the investigation of Lishi fault actions feature and sedimentological alteration on its both sides, and discussed the scientific problems such as sources, mechanisms of genesis, deep dynamic processes and tectonic- magmatism evolutions.The Zijinshan alkaline complex is located to the northwest of Linxian Couty. Its tectonic location is in the Jinxi(Western Shanxi) flexural fold belt, which is in the east margin of Ordos block, and near the Lishi faults zone. Thealkaline complex was formed between 154～91 Ma b.p., corresponds to from thelate Jurassic to early cretaceous of late Mesozoic on geological era. It is outgrowth of the Yanshanian movement.The Zijinshan alkaline complex belong to intermediate-basic rocks, deep-sourced and hypabyssal alkaline complex rocks. It was formed through muti-petrogenesis stages and epochs, and included incursionand and eruptive facies. It is potassic alkaline, attributed to the alkaline-parlkaline series by alkali ratio. The rock assemblage of ring complex is consists of monzonite- aegirine augite syenite- nepheline syenite- breccia(phonolite) series. The mineral association of different type rocks are similar, but their relative content vary greatly. Their main minerals are potassium feldspar. Quartz and anorthose only appear in early intrusive monzonite; Nepheline appear from nepheline-bearing aegirine augite syenite in the medium period of incursion and their content increase progressively. Aegirine-augite and biotite increase with the phases of magmatism, too.The Zijinshan complex has the chemical characteristics of SiO₂-low alkali-rich, K₂O-rich, Fe-rich, Mg-lean, and CaO-medium, Al₂O₃-medium. There is positive correlation between the content of SiO₂ and Al₂O₃ with Na₂O, and there is negative correlation between the content of SiO2 and TFe₂O₃, MgO, with CaO. Their chondrite normalized REE patterns are characterized by high content of rare earth elements, no negative Eu anomalies and rich in LREE. In the primitive mantle-normalized trace element spider diagram, they are enriched in large ion lithophile elements(LILE) such as Cs, Rb, Ba and Th, and depleted in high field strength elements(HFSE) such as Nb, Ta and Ti. Meanwhile,according to isotope geochemistry, they have low value of～(87)Sr/～(86)Sr( 0.704686～0.705198 ) and low value of eNd ( T ) ,δ¹⁸O6.38～14.0SNOW‰. All aboveshow that the complex was derived from enriched mantle in inner continent and the magma formation was mainly controlled by partial melting process.The Zijinshan complex is located in the transition zone between the Shanxi block and Ordos block in Northchina block. Its formation and evolution were closely related to differential uplift and depressed of two block. The tectonic-magmatism might be caused not only by recorded far-field effects produced by different inter-plate convergences, but also by reflected changes in deep-seated thermal tectonic regimes. The explore evolution model of the Zijinshan complex has close connection with its geotectonic setting, the activation of Lishi faults and the uplift of Luliang Mountain.