Geochemistry, Depositional Environment, and Provenance of the Cretaceous Radiolarian Chert in Northeastern Kurdistan, Iraq

Abstract

The radiolarian chert rocks are one of the most important indications of the existence of ophiolites because they are a source of silica supply. These rocks have been studied to refute and reject the idea that ophiolites do not existed. The Si/(Si+Al+Fe+Ca) ratio in the radiolarian chert indicates that the silica is of biogenic origin and adds SiO2 from the shale fraction through diagenetic processes. The high values of Fe2O3 content indicate the hydrothermal effect during precipitation. This effect is controlled by the distance between the sediments and the mid-oceanic ridge or sea floor metamorphism. The phosphorus could be a direct result of volcanic activity associated with ophiolitic rocks, whereas, the negative correlation coefficient of phosphorous with calcium (r = -0.53) supports the idea that the source of phosphorous is volcanic activity that is not related to the carbonate fluorapatite mineral. REEs shows a basic difference as a positive cerium anomaly in section Q2 and a negative cerium anomaly in section Q1. The Al-Fe-Mn diagram shows that all samples fall into the field I non-hydrothermal zone, but in the SiO2 vs Al2O3 diagram the samples of the Q1 section fall into the hydrothermal field, and samples of the Q2 section fall into the non-hydrothermal field. The input materials in Qulqula radiolarian chert come from terrigenous sediments in both sections. On the other hand, the Al/(Al+Fe+Mn) ratio range, (between 0.61 to 0.70), is very close to the average shale composite value, (0.6.19), which may reflect the contribution from continental and non-hydrothermal sediments, while the lower values of this ratio reflect hydrothermal source input. The MnO2 /TiO2 ratios ranged between 0.06 to 2.37 in section Q1 which represents typical characteristics of the deep ocean, trenches, and basaltic plateau sediments.