Rock Magnetism Of The Carbonates In Doushantuo Formation In Yichang, South China

Carbonate rocks distribute widely through geological history. The mineralogy of carbonate rocks and the character of their trace elements and isotopic element are controlled by the carbonate depositional environment. Thus carbonate sediment may well preserve paleoenvironmental changes, including paleotemperature, paleo-precipitation, paleosalinity, paleoproductivity and so on. These changes can also be traced by the character of magnetic composition in the carbonate sediment and thus its magnetism. Therefore, it is potentially valuable to study a carbonate profile by using rock magnetic methods. But because of the complexity of geological processes, various factors may lead to changes of rock magnetic parameters. It is necessary to find out how these factors effect changes of magnetic parameters before deciphering paleoenvironmental changes.20.72 meters carbonate profile was sampled continuously from the upper part of the third member of Doushantuo Formation. 2562 specimens were made for rock magnetic measurements. The profile of magnetic parameters indicates that there exists a special magnetic-enhanced section (SMES) of 11.18~11.65m. Values of mass susceptibility (χ), anhysteretic remanent magnetization susceptibility (χARM) and saturation isothermal remanent magnetization (SIRM) of the SMES are higher than that of the other sections. The magnetic character of the section upon the SMES (the upper section) are different from that of the section below the SMES (the under section). Variations of S-ratio, isothermal remanent magnetization acquision curve and its demagnetic curve suggest that more hard magnetic minerals exist in the upper section than in the under section.Two types of carbonate, find-grained and coarse-grained, can be recognized combined with the investigation in the field and the thin observation. Magnetic measurements show that crystal size of carbonate grains correlates well with values of magnetic parameters such as ARM. Weathering diminishes ARM value of specimens. Thus weathered specimens distribute in the under area in King Plot.X-ray diffraction spectrum shows the carbonate rocks under study mainly contain calcite, ankerite, quartz, feldspar and a little pyrite. In order to get more detailed mineralogical information, several specimens were selected to be dissolved with 10% HCl. Measurements of temperature-dependence susceptibilities (χ-T curves) and results of X-ray diffraction of the acid-dissolved specimens suggested the existence of pyrite. However, these pyrites can not be discovered in the specimens before dissolved from theχ-T curves. It is supposed that diamagnetic minerals, that are carbonate and ankerite, may cover the mineralogical changes of pyrite during heating.