A very thick Middle Proterozoic carbonate strata represented by the Wangquankou Formation is distributed in the southwestern Ordos basin.In order to forecast its petroleum exploration prospects

major and trace elements were measured on samples to reveal the rock characteristics

sedimentary environment

tectonic settings and origin of siliceous rocks of the Wangquankou Formation.The Wangquankou Formation mainly consists of dolomites

siliceous dolomites and silicalites

with minor quartz sandstones.Geochemical features of siliceous materials in silicalites and siliceous dolomites clearly show a biochemical origin

with almost no influences from epicontinental clastic input.Discrimination diagrams of redox environment

such as V/Cr

Ni/Co

V/(V+Ni) and Ce/La

indicate that the Wangquankou Formation was deposited in aerobic or transitional dysaerobic shallow water.Paleo-temperature data

in good agreement with Sr/Cu values

suggest that regional temperatures were highest during deposition of the Wangquankou Formation

further indicating an arid and scorching climate background.Sr/Ba and CaO/(CaO+MgO) environment discriminant graphs

Y abnormity and (La/Nd)

pass

ratio show that the carbonate rocks formed in marine saltwater or transitional semi-saltwater environment.A comprehensive analysis of various diagrams

such as La-Th-Sc，Th-Co-Zr/10 and 100Eu/∑REE- LREE/HREE graphs

indicates that carbonate samples from the Wangquankou Formation carry information of active continental margin

continental island arc and middle continent-shallow water sedimentary system.Samples from regions of Long Country Guguan

Tanshan

and Zhuozishan located at the basin margin are all characterized by high REEs

indicating a tectonic background of continental back-arc depression during the Wangquankou Stage.In conclusion

siliceous rocks and carbonate rocks of biochemical origin in this region were deposited in mild-oxidizing

marine-continental transition environment

with the backgrounds of desiccation and torridness with the tectonic settings of epicontinental back-arc depression.