桌子山中奥陶统克里摩里组下段稀土元素特征与沉积介质分析

REE geochemistry and analysis of sedimentary aqueous media in lower part of Kelimoli Formation, Middle Ordovician, Zhuozishan area

鄂尔多斯盆地西缘北部的内蒙古桌子山地区奥陶纪地层出露完整，沉积类型丰富，是研究鄂尔多斯盆地西缘奥陶纪沉积的关键地区。该区在中奥陶世时位于从浅海向深海的转换部位，达瑞威尔期沉积的克里摩里组由深灰色薄层状石灰岩和灰黑色泥岩组成，是研究鄂尔多斯盆地西缘奥陶纪深水氧化-还原条件的一个较为理想的层位。本文在详细的野外观察基础上，测试克里摩里组薄层细粉晶-泥晶石灰岩和灰黑色泥岩的稀土元素（REY）和其他与氧化-还原条件相关的微量元素，主要包括V、Cr、Ni、Co等，着重分析探讨其形成时的氧化-还原条件。结果表明：1）石灰岩和泥岩稀土总量（REE）分别为28．18×10-6和100．37×10-6，轻、重稀土比值分别为9．60和9．69；2）石灰岩和泥岩稀土元素Y／Ho、（La／Yb）N、（La／Sm）N、（La／Nd）N值分别为29．01、1．35、0．92、0．95和25．72、1．21、0．96、1．02；3）石灰岩无ce负异常或具有微弱的正异常，同时具有明显的Eu正异常，泥岩具有明显的Ce负异常和较弱的Eu正异常；4）石灰岩和泥岩的其他微量元素比值V／Cr、V／（V＋Ni）、Ni／Co分别为1．10、0．62、7．75和3．54、0．89、5．15。依据本文研究结果，结合沉积特征和前人研究成果可以推测，克里摩里组沉积于风暴浪基面之下，远离海岸，但同时又受淡水影响，其中石灰岩形成于氧化环境，泥岩形成于还原环境，同一沉积背景下这种氧化与还原环境的交替是由等深暖流造成的，可称为水动力氧化环境。

Zhuozishan area in Inner Mongolia is a very important area to research sedimentary deposits in Ordovician of western Ordos Basin for its well outcrops and variable sedimentary types. During the Middle Ordovician, the study area is located in the transitional position from shallow to deep sea, especially in Darriwilian stage, Kelimoli Formation which consists of dark gray thin-bedded limestone and grayish black mudstone is an ideal section for research of redox conditions in deep-water from western Ordos Basin in Ordovician. Based on the detail fieldwork, this article focus on the sea water redox conditions while the materials of Kelimoli Formation were deposited using rare earth element and yttrium （REY） and other trace elements related to redox condition such as V, Cr, Ni, and Co in thin-bedded finely crystalline limestone to micrite and grayish black mudstone respectively. The results show following： 1） the rare earth element （REE） concentrations average 28.18×10-6 and 100.37×10-6 in limestone and mudstone respectively, as well as the ratio of LREE and HREE is 9.60 and 9.69; 2） the molar ratios of shale-normalized （PAAS） data of （Y/Ho）N, （La/Yb）N, （La/Sm）N and （La/Nd）N average 29.01, 1.35, 0.92, 0.95 and 25.72, 1.21, 0.96, 1.02 in limestone and mudstone respectively; 3） limestones yielded no Ce anomalies or a little positive anomalies with obvious positive Eu anomalies and the mudstones yielded negative Ce anomalies with a weak positive Eu anomalies which is less than limestone; 4） the ratio of other trace elements including V/Cr, V/（V＋Ni） and Ni/Co is average 1.10, 0.62, 7.75 and 3.54, 0.89, 5.15 in limestone and mudstone respectively. According to these results and the sedimentary characteristics of lower part of Kelimoli Formation and some previous research, it is likely suggesting that： The depositional environment of Kelimoli Formation is a quiet environment under storm wave base which is both apart from seacoast and effected by freshwater, and the limestone deposited in oxic condition while the interlayer mudstone deposited in anoxic condition. This alternation redox conditions may be induced by warm contour current, and we can call this hydrodynamic oxic environment.