Geochemical Characteristics And Thermal Structure Evolution Of Zijinshan Rock Body In Eastern Ordos Basin

Zijinshan rock body is located in Linxian-Liulin anticline belt of Jinxi flexual fold belt in east Ordos basin, which also belongs to Fenhe alkaline rock belt. The rock body was formed in several stages and seen as pipe and sill. Six rock rings can be seen in ground, which are monzonite, aegirine-augite syenite, malignite, nephiline syenite, phonolite/phonolitic volcanic breccia, and trachyte/trachytic volcanic breccia from the most outer ring to the core.Rock of each stage is of unsaturated or extremely unsaturated SiO₂, rich alkaline, high K, high Fe (some is extremely high), low/poor Ca and low Al. It's also of high REE and enriched LREE. LREE and HREE are strongly distillated, showing enriched REE partioning parten. There is no Eu negative anomaly but of Ce negative anomaly. dO₁₈ ranges from 7.9‰ to 9.2‰, with the average of 8.4‰.According to all aging data, ages of 4 rocks of the former stages is made certain, which are 154Ma, 141Ma, 132Ma and 125Ma. Evolution of the same magma resource in different stages resulted in rocks of each stage. Low (⁸⁷Sr/⁸⁶Sr)o and some other characteristics suggests that the magma came from upper mantle, and enriched mantle partly melted then formed rock body. The melting degree is low, that is about 5%. Rock body formed in plate extensional background and a loose stress state.Six ring belts of the rock body constitute a thermal structure of normal dome type in land form. In magmatic-thermal action depth term, monzonite and the other 3 ring belts belong to intermediate-deep intrusion type, and the else 3 belts belong to superficial/shallow volcanic eruption and hydrothermal action type. Since the beginning of the Mesozoic era (after Triassic period), there have been 3 times of thermal action in Zijinshan area: a superficial/shallow volcanic eruption and hydrothermal action in middle Triassic period, an intermediate-deep intrusion action in late Jurassic to early Cretaceous period and a superficial/shallow volcanic eruption and hydrothermal action in late early Cretaceous period. There maybe two intermediate-deep intrusion thermal action in early Carboniferous period (343Ma) and early early Permian period (287-293Ma), while an event of volcano eruption in middle Triassic period is doubtless.The forming era of Zijinshan intrusive rock body is consistent with the thermal event of northern China plate in the Mesozoic era, which suggests that Jinxi flexual fold belt and northern China plate together undergone the thermal event.