渭北隆起岐山地区蓟县系的浅海相白云岩对鄂尔多斯盆地深层勘探意义重大.在涝川剖面实测及取样的基础上,通过碳氧同位素分析等地球化学分析测试手段,结合蓟县期沉积演化背景与地层展布特征,明确岐山地区蓟县系白云岩地球化学特征,深入探讨蓟县期古环境特征及白云岩成因.研究表明:岐山地区蓟县系岩性以白云岩、硅质白云岩和含燧石条带白云岩为主,发育叠层石,沉积环境为碳酸盐台地潮坪,沉积相类型为潮间带和潮下带,V/Cr、Ni/Co、V/(V+Ni)和Ce/La氧化还原环境判别图显示蓟县系整体形成于弱氧化的浅水沉积环境,古水温数据与Sr/Cu值均反映蓟县系沉积期古水温相对较高,应为干燥炎热的气候背景,Sr/Ba、CaO/(CaO+MgO)判别图表明蓟县系白云岩整体形成于海相咸水环境.初步建立鄂尔多斯盆地蓟县系白云岩成因模式:灰质白云岩、晶粒白云岩以埋藏成因为主,残余结构白云岩以混合水成因为主,溶蚀白云岩多为回流渗透成因.

The shallow marine dolomite of Jixian System in Qishan area of Weibei Uplift is of great significance for the deep exploration in Ordos Basin. Based on the field survey and sampling of Laochuan outcrop, by means of geochemical analysis and testing such as carbon and oxygen isotope analysis, combined with the sedimentary evolution background and stratigraphic distribution characteristics, the geochemical characteristics of the dolomite of Jixian System in Qishan area are clarified, and the characteristics of the paleo-environment and the genesis of the dolomite are deeply discussed. It is shown that: (1) The lithology of Jixian System in Qishan area is mainly dolomite, siliceous dolomite and flint-bearing banded dolomite, which develop stromatolites. According to their structure types, they can be divided into calcareous dolomite, crystalline dolomite, residual structure dolomite, and dissolved dolomite. The sedimentary environment is tidal flat of carbonate platform, and the sedimentary facies are intertidal zone and subtidal zone. (2) The mean value of δCe is 0.64, and Ce is negative anomaly, which indicates that the dolomite of Jixian System is in an oxidized environment during the sedimentary period. Discrimination parameters of redox environment，such as V/Cr, Ni/Co, V/(V + Ni) and Ce/La, indicate that the Jixian System is deposited in shallow water sedimentary environment with weak oxidation. (3) Paleo-water- temperature is in good agreement with values of Sr/Cu, which suggest that regional temperatures were much higher during deposition period of Jixian System, furtherly indicating an arid and scorching climate background. (4) The contents of Sc≤1.53 μg/g, Th≤1.47 μg/g, Hf≤0.47 μg/g, Zr≤17.6 μg/g，which are lower than those of the corresponding elements in the upper crust (Sc=14.9 μg/g, Th=2.3 μg/g, Hf=5.8 μg/g, Zr=240.0 μg/g), the mean value of ΣREE+Y is 12.50 μg/g, which is much smaller than that of terrigenous detritus (100 μg/g). These indicate that the influence of terrigenous detritus is very weak. (5) The mean value of δEu is 0.75, and the negative anomaly is obvious, indicating that the dolomite in the study area has hardly been modified by hydrothermal fluid, the average value of CaO/(CaO + MgO) is 0.58, and the average value of Sr / Ba is 5.43, indicating that the salinity is high, and it is a marine environment. (6) The carbon and oxygen isotope analysis shows that the dolomites of the study area are all formed in marine fluids, environment discriminant graphs of Sr/Ba and CaO/(CaO + MgO) show that the dolomites of Jixian System are formed in marine saltwater. (7) The genetic identification diagram of dolomite shows that the dolomite of Jixian System in Qishan area of Ordos Basin is mainly of burial origin, and there are mixed water origin dolomite and reflux permeability origin dolomite only in some areas. At last, the genetic model of dolomite in Jixian System in Ordos Basin is preliminarily established: the calcareous dolomite and crystalline dolomite are mainly of burial origin, the residual structure dolomite is mainly of mixed water origin, and the dissolution dolomite is mainly of reflux seepage origin.