山西临县紫金山碱性火山机构岩体岩浆演化——岩相学及岩石矿物学的证据

Petrogenesis of the alkaline complex from Lixian,Shanxi Province:Petrography and mineralogical evidences

目的通过分析紫金山碱性火山机构岩体的岩相学及岩石矿物学特征来研究其岩浆的演化关系。方法在野外工作的基础上,进行室内显微镜观察分析。结果紫金山碱性杂岩体是华北地台西部、鄂尔多斯地块东缘,活动期在早白垩纪的一个点式火山机构。岩相学上主要表现为由深成相岩瘤或包体、超浅成相、火山颈相和少量熔岩组成。岩性上主要发育碱性系列和偏碱性系列两个系列岩石类型。碱性系列与偏碱性系列展现出不同的矿物组合。结论紫金山碱性杂岩体所有岩石类型原始岩浆可能来自两个旋回性质不同地幔条件的部分熔融,派生出8次分期岩浆活动。第一旋回两种性质的原始岩浆在上侵运移过程中有有限混合现象发生。混合的结果使得二长岩中次透辉石中霓石分子的含量增高,显微镜下成绿色,角闪正长岩的正长石斑晶中出现霓石嵌晶,应该为粗安岩的喷出相岩石演化成了粗面质火山岩。碱性系列霞辉正长岩中霓辉石结晶成了次透辉石,使得形成岩石的碱度下降。第二旋回原始岩浆上行过程中有霓霞钛辉岩、正霞正长岩、白霞正长岩的分离结晶作用发生。

Aim Based on field geology,petrography and previous study on the Zijinshan alkaline complex to propose that the Cretaceous Zijinshan alkaline complex occurred in the eastern margin of the Ordos basin.Methods The alkaline complex was mainly consisted of plutonic facies typhoon or enclaves,hypabyssal facies（batholith,circular and radiant dykes around the collapse volcanic crater）,lava neck facies and minor lavas（flood and eruptive facies）,and it mainly contains alkaline and meta-alkaline series,the alkaline rocks are consisted of leucite＋caliofilite＋iiwaarite＋alkaline feldspar,the corresponding dykes are consisted by aegirine＋hornblende＋kalifeldspath＋matrix or kalifeldspath＋matrix.The meta alkaline rocks are mainly consisted of plagioclase＋hornblende＋baicalite＋biotite＋orthoclase＋quartz or nepheline＋baicalite＋acmite＋orthoclase＋hornblende＋biotite,the corresponding dykes are composed of hornblende＋plagioclase＋orthoclase＋matrix or orthoclase＋matrix.Results Then it can be inferred that the parental magma of the Zijinshan alkaline may be derived from two different mantle source and can be divided into eight phase volcanic activity.The alkaline magma may be derived from partial melting of CO2-dearing lherzolite.The two types of magmas from the first circle produce limited magma mixing during their ascent.Conclusion The magmas from the second circle experience fractional crystallization of sussexite＋juvite.Magma mixing elevated the aegirine molecule in the baicalite.Aegirine inclusion in the orthoclase crystals indicate the syenite evolved into trachydacite.Acmite evolved into baicalite and decreased the alkaline content in the magma.The Si,Al contents is significantly affected by fraction crystallization of the alkaline magma.