4.2. Geochemical and mineralogical composition of oolitic grainstones
Our materials come from a series of thin- to medium-bedded oolitic grainstones with total thickness of 7.2 m (157.2-164.4 m) near the top of the Menqu section (Fig. 3). These oolitic grainstones of the Nieniexiongla Formation consist mainly of ooids with a few bioclasts and quartz grains. Some grains are surrounded by bladed to dogtooth cement (variously shaped terminations of crystals). The intergranular space is filled by mosaic crystals which may indicate the influence of meteoric water during diagenetic process (Javanbakht, Wanas, Jafarian, Shahsavan, & Sahraeyan, 2018). Additionally, the color, size, shape and type of ooids from the bottom to the top change gradually through the Nieniexiongla Formation. Oolitic grainstones closed to the bottom are mainly composed of gray colored ooids with diameters ranging from 300 to 500 μm. Diverse nuclei including peloids, quartz grains, bioclasts and preexisting ooids are enclosed by cortices exhibiting indistinct micritic to radial-fibrous structure. Part of the ooids have both micritic and radial-fibrous cortex, which suggests change in sea-water conditions. About 70% of the ooids are broken, compounded and regenerated ooids occurred too, part of these ooids have abraded outline (Fig. 4a). In contrast, oolitic grainstones closed to the top are mainly composed of red colored ooids. Red ooids are 200 to 400 μm in size. The characteristics of their nuclei and cortices are similar with gray ooids. Most ooids are accompanied by circumgranular cement rims. Much fewer broken red ooids were observed than gray ooids (Fig. 4b).
The EPMA backscattered electron images show that the material distribution within ooids is inhomogeneous (Fig. 5), and the EPMA examination reveals the element composition of different minerals (Table 1) in oolitic grainstones. Ooids are mainly composed of calcite (gray color in BSE images, Fig. 5). Mineral displaying average contents of 57.56 wt% CaO (59.58 wt% in total) supposes to be calcite crystal. Minor oxides in calcite are 0.49 wt% MgO, 0.37 wt% SiO2 and 0.91 wt% FeO, which may come from adjacent quartz grains and ferruginous minerals. The second abundant mineral among ooids is quartz which exhibits dark-gray color in BSE images. Particles contain an average chemical composition of 99.16 wt% SiO2 and 0.26 wt% CaO belong to quartz particles which occur as the nuclei or in the cortex of some ooids. There are some ferruginous mineral scattered randomly in ooids (white color in Fig. 5). These particles have significantly high contents in FeO (80.61 wt% with 89.00 wt% in total). The minor oxide concentrations in this mineral are 3.53 wt% SiO2 and 3.46 wt% CaO. From the base (Fig. 5a) to the top (Fig. 5b) of a single oolitic grainstone bed, the rock color becomes more pinkish as the concentration of ferruginous mineral increases.