论文部分内容阅读
华北克拉通古元古代中期构造环境及演化至今尚存有争议,该时期相关岩浆作用的成因研究可为这些问题的解决提供重要证据.本文对华北克拉通中部山西吕梁地区古元古代中期形成的吕梁群近周营组基性火山岩开展综合研究,获得2 180±19 Ma的形成年龄,岩石地球化学低TiO2、高Al2O3和K2O,属钙碱性系列.微量元素富集Rb、Ba、Th、U、Sr等大离子亲石元素,亏损Nb、Ta、Ti和P等高场强元素,富集Pb,具轻稀土略富集、铕弱异常的右倾稀土模式,与典型消减带弧岩浆产物特征一致.同位素组成显示正εNd(t)=1.0～2.6和εHf(t)=1.8～9.2,指示它们来源于亏损地幔,低的Ti/Y (246～ 385)和Ce/Y值(0.99～ 1.77)进一步揭示,地幔源区受到俯冲流体的交代和沉积物的参与,是楔尖晶石二辉橄榄岩与石榴石二辉橄榄岩过渡带略亏损地幔区发生部分熔融,并经历了橄榄石、单斜辉石等矿物分离结晶作用而形成.结合同期埃达克岩—高镁安山岩—富Nb玄武岩—安山岩组合以及弧花岗岩类的出现提出,2 200 Ma华北中部带已演化至活动大陆边缘弧的构造环境.“,”The petrogenesis of related magmatism at Middle Paleoproterozoic is of great importance considering the controversial issue of tectonic setting and evolution of the North China Craton.In this paper,we present comprehensive analyses on the meta-basic volcanic rocks from Jinzhouying Formation of Ltüliang Group in the Lüiliang complex.LA-ICP-MS zircon U-Pb dating on the meta-basic volcanic rock yields the age of 2 180±19 Ma.The meta-basic volcanic rocks are of calc-alkaline series,with low TiO2 and high A12O3,K2O.They are relative enrichment in large ion lithophile elements (Rb,Ba,Th,U,Sr),high in Pb and depletion in high field strength elements (Nb,Ta,Ti,P).REE patterns are of slightly right inclined type with unobvious Eu and Ce anomaly.All above are similar to the typical arc volcanic rocks in the subduction zone.The rocks also show positive εNd(t) =1.0 ～ 2.6 and εHf(t) =1.8 ～ 9.2,which imply that they may derived from the depleted mantle.The low Ti/Y (246～385) and Ce/Y (0.99 ～ 1.77) further reveal that the mantle wedge is commonly modified by addition of materials from fluids and subducted sediments.Thus,the rocks may originated from part melting of depleted mantle in the transition zone between spinel and garnet lherzolites and then exist crystallization fractionation of olivine,clinopyroxene.Combined with the close spatial and temporal association of the suite of adakitic rocks,Nb-enriched basahs,andesites,magnesian andesites and arc-related granitoids,we suggest an 2 200 Ma active continental marginal system in the Trans-North China orogen.