内蒙古文圪气镁铁-超镁铁质杂岩体中环带角闪石矿物学特征及成因

内蒙古文圪气镁铁-超镁铁杂岩体主要由辉石岩和角闪石岩组成,普遍发育角闪石环带的角闪石岩位于岩体南部,呈伟晶结构,主要由角闪石(>95%)和磷灰石(3%～5%)组成。选取环带较明显的两个角闪石样品,采用EMPA和LA-ICPMS分别做主量和微量成分剖面,环带角闪石边部为铁韭闪石,核部为韭闪石。从核部到边部,SiO2、MgO、Na2O含量逐渐降低,Al2O3、TiO2、FeOT、K2O和碱金属(Na2O+K2O)逐渐升高。中稀土、重稀土元素和Sc最大值位于核部,最小值位于边部;轻稀土元素、高场强元素、大离子亲石元素及过渡金属元素最大值位于幔边部,最小值位于核部,反映了核部氧逸度较高、温度和压力较低,边部氧逸度较低、温度和压力较高,是两期不同成分、温度、压力和氧逸度岩浆结晶的表现,并且角闪石岩受到后期高氧逸度低温热液的改造。环带角闪石的形成机理与文圪气岩体中铂族元素矿化过程相吻合。文圪气岩体与华北地台北缘中泥盆世镁铁质-超镁铁质岩具有相同的岩石组成、成因和构造背景,环带角闪石具有幔源角闪石特点,核部平均温度大于732℃,边部大于770℃,核、边部温度均小于950℃,结晶压力≤272 MPa,结晶深度≤12.4km。环带角闪石的结晶过程可能与文圪气岩体中铂族元素的成矿过程类似。

Characteristics and petrogenesis of zoned amphiboles in Wengeqi mafic-ultramafic complex,Inner Mongolia

Wengeqi mafic-ultramafic complex is composed of pyroxenite and hornblendite.Hornblendite,enclosed by pyroxenite and biotite pyroxenite,is mainly consisted of amphibole(more than 95%) and apatite(3%-5%),and is characterized by zoned amphibole,pegmatitic texture,and occurred in the southern part of the complex.The major element and trace element contents from the two zoned amphibole sections are analyzed by Electron Probe Microanalysis and LA-ICP-MS respectively.The zoned amphiboles show pargasitic cores rimmed by ferropargasite.From core to rim,zoned amphiboles have progressively lower composition of SiO2,MgO,Na2O and higher Al2O3,TiO2,FeOT,K2O,alkali metals(Na2O+K2O).The maximum composition of Sc,middle and high rare earth elements are situated in the core,and the minimum is in the rim;but the maximum composition of light rare earth elements,LILE,HFSE lies in the rim,and the minimum is located in the core.The core was higher in oxygen fugacity,but lower in temperature and pressure.and the rim is vice versa.The characteristics of main and trace elements above attribute to the crystallization of two magmas with different composition and oxygen fugacity,and with different temperature and pressure.The crystallization temperature of the zoned amphibole is below 950 ℃ with hot rim(mean 770 ℃) and cool core(mean 730 ℃).The zoned amphiboles are crystallized within 13 kilometers under the 272 MPa.The tectonic settings,compositions and petrogenesis of Wengeqi complex are similar to the middle Devonian mafic-ultramafic rocks in the northern margin of North China Block,suggesting that the primary magma of hornblendite was derived from enriched lithospheric mantle.The evolution of zoned amphiboles resembles to that of the mineralization of platinum group elements in Wengeqi.