华北北缘大同地区三叠纪煌斑岩铂族元素成因研究

Genetic study upon the platinum-group element geochemistry of the Triassic lamprophyres in the Datong area(Shanxi Province), northern margin of the North China Craton

华北克拉通北缘中段大同地区发育呈岩墙形式产出的煌斑岩,该煌斑岩为超钾质碱性岩,极度低硅低钠高镁高钙高钾,且富H_2O和CO_2等流体组分。本文利用镍锍试金-电感耦合等离子体质谱法测定了这套煌斑岩的铂族元素组成,获得了其形成及演化过程的一些信息。大同煌斑岩铂族元素含量极低,∑PGE=2.14×10^(-9)~4.50×10^(-9),原始地幔标准化铂族元素配分型式为左倾型,亏损Ir亚组(IPGE),富集Pd亚组(PPGE),(Pd/Ir)_N=3.48~10.43。低铂族元素含量、高Cu/Ni比值(48101~90926)和高(Pd/Ir)_N比值指示大同煌斑岩形成于地幔低程度部分熔融(1%~13%),熔体形成时仍有部分硫化物残留于源区。岩浆演化过程中,未发生明显的结晶分异作用和地壳混染作用,硫未达到饱和-过饱和,未发生硫化物熔离作用。计算获得铂族元素在原始地幔与煌斑岩熔体间的分配系数分别为D_(Os)=35~124,D_(Ir)=13~35,D_(Ru)=34~91,D_(Rh)=5.8~10,D_(Pt)=3.1~9.7,D_(Pd)=2.2~4.5;PPGE较IPGE明显更不相容。与峨眉山大火成岩省玄武岩对比研究发现,岩浆岩中的铂族元素含量主要受源区地幔部分熔融程度和演化过程中硫化物熔离作用共同影响;而铂族元素之间的分异受源区部分熔融程度和岩浆分离结晶作用共同影响。

Abundant lamprophyre dykes occur near the northern margin of the North China Craton,in Datong area,Shanxi Province.The lamprophyres are ultrapotassic alkaline mantle-derived melts,characterized by low contents of SiO2,Na2O,and high contents of MgO,CaO,K2O,and high fluid compositions （H2O and CO2）.Using Nickel Sulfide Fire Assay-ICPMS method,we have acquired the platinum-group element （PGE） concentrations of the lamprophyres,and obtained some information on their formation and evolution processes.The lamprophyres have extremely low total PGE contents ranging from 2.14 × 10-9 to 4.50 × 10-9.They show left-leaning primitive mantle-normalized PGE patterns with depletion in Iridium group （IPGE） and enrichment in Palladium group （PPGE）,with （Pd/Ir） N =3.48 ～ 10.43.Their low PGE contents,high Cu/Ni ratios （48101 ～ 90926） and high （Pd/Ir） N ratios reveal that the lamprophyres were originated by low degree of partial melting （1% ～ 13%） of the mantle source,with sulfides remain in the mantle during partial melting.Sulfide segregation could not happen during the evolution of the lamprophyres,due to the fact that neither significant fractional crystallization nor crustal contamination has been involved in the formation of the lamprophyres.We determined the PGE distribution coefficients for the lamprophyres,with Dos =35 ～ 124,DIr =13 ～ 35,DRu =34 ～ 91,DRh =5.8 ～ 10,DPt =3.1 ～ 9.7 and Dpd =2.2 ～4.5,which clearly indicates that the PPGE are more incompatible than IPGE.Based on the comparative studies of the lamprophyres and the Emeishan continental flood basalts,we find that the PGE contents in igneous rocks are related with the degrees of partial melting in the mantle source and the removal of sulfides before their emplacement;whereas the fractionation of IPGE and PPGE are mainly controlled by the degrees of partial melting and fractional crystallization processes together.