山西临县紫金山岩体地球化学、矿物学特征及岩体成因

摘要

在岩相学和岩石地球化学研究的基础上，结合对主要造岩矿物辉石、长石和霞石的电子探针研究，揭示山西临县紫金山岩体成因。研究表明，紫金山岩体为一套富钾碱性－过碱性岩石。根据各期岩石中矿物组合，可将紫金山杂岩体7期岩石分为两种岩石系列：经典碱性岩浆系列和非经典碱性岩浆系列。经典碱性岩浆系列中均出现似长石（霞石或白榴石）和钾长石，而不出现斜长石，除第三期霓霞钛辉岩外均含霓辉石（钠质辉石）；非经典碱性岩浆系列岩石中均出现透辉石（钙质辉石），而不出现似长石，部分期次岩石中出现斜长石。结合两个岩浆系列中全岩Sr－Nd同位素数据及区域地质资料，推测紫金山岩体成因为：吕梁地区中生代软流圈地幔的上涌及扰动促使来自软流圈、岩石圈地幔和下地壳物质部分熔融产生的岩浆发生多次不同比例的混合，所形成的各期混合岩浆分批上侵依次定位在地壳浅部环境形成7期岩石。因各期岩浆源区中所含壳源物质比例不同，故它们可分为具有不同矿物组合的两个岩石系列。%With pyroxene,feldspar and nepheline as the study objects and by using electron microprobe tech-nology,the authors discussed the petrogenesis for Zijinshan complex on the basis of petrography and petrogeo-chemistry study.The petrochemistry data shows that Zijinshan complex is a potash-rich alkaline-parlkaline rock. According to the mineral assemblages in rocks for different periods,the rocks in seven periods can be divided in two magma series:classical alkaline magma series and non-classical magma alkaline series.Rocks belong to classical alkaline magma series contain feldspathoid (nepheline or leucite ),orthoclase,and aegirine-augite (sodic pyroxene)except rocks at the third stage,and there is no plagioclase.However,rocks belonging to non-classical alkaline magma series contain diopside (calcium pyroxene ),and parts of them contain plagioclase without feldspathoid.Combined with the data of Sr-Nd isotope for whole rock in two magma series and the regional geological materials,it can be speculated that the petrogenesis of Zijinshan complex is as following:continuous asthenospheric upwelling made asthenosphere mantle,lithosphere mantle and lower crust melt and mixed by different proportions for several times,and then each episodic of mixed magmas welled up to the shal-low area of the crust.The proportions of crust-derived material in rocks from different stages are different,and they can be divided into two rock series by their mineral assemblages.