西大别大悟地区闪长玢岩岩石地球化学特征及其构造意义

Geochemical Characteristics of Diorite Porphyrite in Dawu Area,Western Dabie and Its Tectonic Significance

基性岩浆能反映地幔源区性质、成因环境和形成演化过程。通过野外地质调查,结合岩石地球化学分析,探讨区域岩浆源区性质、成因及构造环境。全岩地球化学分析结果显示,闪长玢岩样品SiO2含量为49.97%~55.01%,属中-基性岩系列,MgO含量为4.63%~5.49%,Mg^(#)为60.17~90.19,Nb/Ta值为13.06~18.47,Zr/Hf值为40.09~44.05,暗示该岩浆源区可能源自于富集地幔。LREE/HREE值为9.45~13.97,整体表现为较陡右倾型,且亏损高场强元素(Nb、Ta、Hf、Ti)亏损,富集大离子亲石元素Sr,表明岩浆形成过程中可能受到俯冲板片流体交代作用的影响。闪长玢岩脉穿切花岗斑岩脉,花岗斑岩结晶时代为(130.8±1.8)Ma,闪长玢岩的侵位时间可能为早白垩世。在Zr-Ti构造环境判别图解中,闪长玢岩样品落于板内玄武岩区,结合区域构造背景,笔者认为闪长玢岩应形成于造山后伸展-拉张环境。

Basic magmas can reflect the nature of mantle source region,genetic environment and formation and evolution process.Through the detailed field geological survey and rock geochemical analysis,we discuss the nature,genesis and tectonic environment of the magma source area of regional diorite porphyrite.The geochemical analysis of the whole rock shows that the SiO2 content of the diorite porphyrite samples is 49.97%~55.01%,belonging to the medium−basic rock series,the MgO content is 4.63%~5.49%,and the Mg^(#)content is 60.17~90.19,the Nb/Ta value of the sample is 13.06~18.47,and the Zr/Hf value is 40.09~44.05,indicating that the magma source area of the diorite porphyrate may be from the enriched mantle.In addition,The LREE/HREE ratio is 9.45~13.97,and the diorite porphyrite samples show a steep−right diorite,and they are enriched in high field strength elements(Nb,Ta,Hf,Ti)and rich in large ion lithophile elements Sr.It is suggested that porphyrite may have been influenced by fluid metasomatism of subducted plates during the formation process.The field outcrops show that diorite porphyrite dikes intrude into granitic porphyry dikes(forming age is 130.8±1.8 Ma),so the emplacement time of dikes may be early Cretaceous.In the discrimination diagram of Zr−Ti tectonic environment,the diorite porphyrite samples fall in the intraplate basalt area.Combined with the regional tectonic background,we believe that the diorite porphyrite should be formed in the post−orogenic extension environment.