鲁西隐伏含铁岩系——前寒武纪济宁岩群地球化学特征及沉积环境

Geochemical Characteristics and Sedimentary Environment of Precambrian Jining Group in Western Shandong Province

济宁岩群是分布于山东省鲁西南地区的隐伏浅变质地层,埋深大于900m,由变火山碎屑岩组、含铁岩组和千枚岩组组成,是一套海相火山-沉积岩系。岩石中SiO2、Al2O3、Fe2O3、K2O等亲陆元素含量高,岩石化学的成熟度指数不高,反映其源岩是鲁西TTG岩系;m值(4.75～31.86)和MnO/TiO2(0.05～0.2)值低,指示了海陆过渡性近岸浅海陆架沉积环境。富集轻稀土、重稀土平坦、铕异常不明显;微量元素K、Rb、Ba富集,贫Nb、Ti和P、Cr。Sr/Ba值(主要为0.16～0.77)小,指示沉积时的盐度较低;V/(V+Ni)值较高(>0.46),形成于还原环境。岩石的δ13CV-PDB=-4.7‰～-15‰,δ18OV-PDB=-12.1‰～-18.8‰,δ30SiNBS-28=-0.8‰～-1.2‰,数值较低,显示了热水沉积特征。济宁岩群是受华北克拉通1.8Ga大陆裂解事件影响形成的裂谷型热水沉积。

The Jining Group is a set of hidden low-grade metamorphic marine volcano-sedimentary strata distributed in southwestern Shandong Province buried under more than 900 m.This paper studies the lithological and geochemical characteristics and formation environment based on study of the drilling cores.The research is of importance for discussing regional tectonic evolution,analyzing the origin of iron deposit and guiding further exploration.The Jining Group is mainly composed of sericite phyllite,chlorite sericite phyllite,calcite sericite phyllite,tuffaceous quartz sericite phyllite,calcite magnetite quartzite,crystallite marble and phyllitic metamorphic argilliferous sandstone.From the bottom to the top it can be divided into meta-pyroclastic formation,iron-bearing formation and phyllite formation.The high SiO2 contents（37.25%~73.18%） of the rocks reflect a source rich in silicon.High contents of terrestrial components（Al2O3=14.75%~19.84%,Fe2O3=0.84%~3.23%,K2O=2.51%~5.2%） and low chemical petrologic maturity index [Al2O3/（K2O＋Na2O）=2.32~3.69] of the rocks indicate that its source rocks are the TTG rocks in the western area of Shandong Province.The low m[m=（100×MgO/Al2O3）] values（4.75~31.86） and MnO/TiO2 values（0.05~0.2） of the rocks show a transitional off-shore neritic shelf sedimentary environment.The rocks are relatively enriched in LREE with minor or nil Eu anomalies.The rocks are enriched in large ion lithophile elements like K,Rb and Ba and depleted in high field strength elements such as Nb,Ti and P,Cr.Low Sr/Ba values（0.16~0.77 mainly） of the rocks suggest a low salinity environment of sedimentation.High V/（V＋Ni）（0.46） values of the rocks hint a reducing environment.The lower C and O isotope values of the rocks（δ13CV-PDB=-4.7‰ ^-15‰,δ18OV-PDB=-12.1‰ ^-18.8 ‰,δ30SiNBS-28=-0.8‰^-1.2‰） are obviously different from those of the normal marine carbonate,indicating a hydrothermal origin.The Jining Group was formed in hydrothermal basin caused by the North China craton rifting ca.1.8 Ga ago.