摘要
巴楚小海子正长岩杂岩体是二叠纪塔里木大火成岩省的重要组成部分。SIMS锆石U-Pb定年显示其形成于279.7±2.0Ma,与本区辉绿岩脉和石英正长斑岩岩脉近于同时侵位。根据矿物学特征,小海子正长岩体可分为铁橄榄石正长岩和角闪正长岩两类。前者主要由碱性长石、铁橄榄石、单斜辉石、角闪石和少量石英、斜长石组成,后者主要由碱性长石、角闪石、黑云母和少量的石英、斜长石组成。小海子正长岩体为铁质、碱性系列,轻稀土相对富集,重稀土亏损,具有明显的Eu正异常,无Nb、Ta负异常,相对低的(87Sr/86Sr)i(0.7033～0.7038)和正的εNd(t)值(+3.1～+3.8),暗示它们来自亏损的地幔源区,没有地壳物质的加入。主微量和同位素地球化学分析,暗示巴楚小海子正长岩的母岩浆为碱性的幔源玄武质岩浆经橄榄石、单斜辉石分离结晶后的残余熔体,并且含有堆晶的碱性长石。这种含有碱性长石堆晶的熔体,在相对还原的条件下结晶,形成铁橄榄石正长岩;在相对氧化的条件下结晶,并经过不同程度斜长石的分离结晶形成角闪正长岩。
Xiaohaizi syenite complex located within Tarim basin is an important part of Permian Tarim Large Igneous Provinces（LIPs）.SIMS zircon U-Pb analysis yields Early Permian age of 279.7±2.0Ma,and coeval with diabase and quartz syenite porphyry dikes in the same area.Based on mineralogical composition,it can be categorized into fayalite syenite and amphibole syenite.The former consists mainly of alkali feldspar,fayalite,clinopyroxene,amphibole with minor quartz and plagioclase,and the latter contains mainly alkali feldspar,amphibole,biotite with minor quartz and plagioclase.Xiaohaizi syenites are ferroan,alkaline series and enriched in LREEs relative to HREEs with pronounced positive Eu anomalies,but no negative Nb and Ta anomalies.Geochemical and Sr-Nd isotopic compositions [（87Sr/86Sr）i=0.7033~0.7038,εNd（t）=＋3.1~＋3.8] indicate that they were derived from a depleted mantle source without crustal assimilation.Major and trace elements and Sr-Nd isotopic compositions suggest that the parental magma of Xiaohaizi syenites were residual melt left by olivine and clinopyroxene fractional crystallization of mantle-derived alkaline basaltic magma.They crystallized as fayalite syenite under relatively reduced condition,and as amphibole syenite under relatively oxidized environment accompanied by variable degree of plagioclase fractionation.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2011年第10期2984-3004,共21页
Acta Petrologica Sinica
基金
国家973项目(2011CB808906)
中国科学院国际合作海外团队项目联合资助
