Abstract
In the central segment of the Helan Mountain of Ningxia three Proterozoic sedimentary units are underlain by the Huangqikou Granite (K/Ar ages: 1440–1839 Ma). The Proterozoic sequences include, in ascending order, the Huangqikou Group composed of shallow marine clastic deposits, the Wangquankou Group consisting of carbonates and yielding a K/Ar age of 1289 Ma on glauconite from its base. The youngest formation is the Zhengmuguan Formation, which rests unconformably on the underlying beds. Still higher, the Cambrian Suyukou Formation, which contains trilobites, disconformably overlies the terminal Proterozoic to Early Cambrian Zhengmuguan Formation.
The Zhengmuguan Formation can be subdivided into two members. The lower member is composed of diamictite containing carbonate clasts, and the upper one is made up of stratified fine clastic deposits and argillites. The diamictite is mainly subglacial in origin. Intercalations of laminated rocks with dropstones within the diamictite member suggest ice sheet fluctuation and repeated climatic changes. The upper fine clastic and argillaceous rocks with abundant trace fossils were deposited in a relatively deep and stable periglacial basin.
REE data from shales of the upper Zhengmuguan Formation illustrate that they have a high REE abundance (208.7 ppm), a high (La/Yb)N ratio and an obvious negative Eu anomaly. Their REE patterns are quite similar to those of post-Archean shales in western Europe, America and Australia.
Introduction
The Zhengmuguan Formation crops out in the middle segment of the Helan Mountain within a limited area of about 300 km2 (Fig. 8.1). This sequence is named after the type section located in Zhengmuguan (Compiling Group of Stratigraphical Timescale of Ningxia Hui Autonomous Region, 1980).