皖北新元古代硅质灰岩的发现及其地质意义

Discovery of the Neo-Proterozoic siliceous limestone in northern Anhui Province and its geological significance

在皖北新元古代倪园组中发现了硅质灰岩,其主要组分为方解石(90%)和石英(10%),呈白色团块状断续分布于灰质白云岩和灰岩互层的寄主岩石中。在地球化学特征上,以较高的CaO和SiO2为特征,分别达到了85.34%和13.61%,稀土总量较低但轻重稀土分异明显,具有明显的Eu、较弱的Ce的正异常以及较高的Tb/Yb值和较低的Th/Sc值,Ba、(Cu+Pb+Zn)含量较高,且Al/(Al+Fe+Mn)值很低,是海底热液和海水混合作用的产物。结合前人研究成果认为,皖北新元古代所处的大地构造背景可能是与Rodinia超大陆聚合有关的Grenville造山期形成的处于拉张的弧后盆地,而不是前人所认为的对应于Rodinia超大陆裂解期的被动大陆边缘。

Siliceous limestone has been found in the Niyuan Formation formed in Neo-Proterozoic in northern Anhui Province.It is mainly composed of calcite（90%） and quartz（10%）,and distributed as white cumulate mass in the country rock that is composed of lime-dolomite and limestone.The siliceous limestone is characterized by high CaO（85.34%） and SiO2（13.61%） contents.The HREE and LREE are strictly fractionated and the total REE content is low.It is characterized by evident positive Eu and weak positive Ce anomalies,high Tb/Yb ratio and low Th/Sc ratio,high Ba and（Cu＋Pb＋Zn） contents and low Al/（Al＋Fe＋Mn） ratio,indicating that it is precipitated from the mixing of the hydrothermal solution and sea water.In view of previous work,it is considered that the tectonic background in the Neo-Proterozoic of the northern Anhui Province is a young back-arc basin,which was formed during the Grenville orogen in response to the convergence of the Rodinia super continent,rather than the positive continental margin during the breakup of the Rodinia super continent that was presented by previous study.