Keywords: 
• carbonate mounds, pure limestone, sedimentary petrology, geochemistry, Bayan Har Basin

Abstract: The Bayan Har sedimentary basin has been a hotspot and key area in studying QinghaiTibet Plateau geotectonics and Tethys, which is located in the northeast of QinghaiTibet Plateau hinderland. The domination sedimention of Bayan Har basin which has some different sizes of limestone platforms or limestone mounds are composed of Triassic turbidite rocks. In this paper, we study the sedimentary environment of hummocky limestone by the way of sedimentary petrology and geochemistry. 
  Samples of this paper were collected from four isolated carbonate mounds on Long Stone Mountain of Bayan Har Basin, Qinghai province, which was developed in the Permian  Triassic Maerzheng Formation strata with Ammonoid that produced a dark gray thinbedded micritic limestone and grayblack slate. 
  XRD analysis showed that the average calcite content of hummocky limestone from Permian  Triassic strata of Long Stone Mountain of Bayan Har Basin is up to 97%. Major element analysis showed that the CaO average content of hummocky limestone is up to 53.38%, so the CaO content is close to the theoretical value of calcite 56.03%, while Al2O3, Fe2O3, K2O, MgO, MnO, Na2O, P2O5, TiO2 and others content are below 5 ‰. Those indicate that the chemical and minerals composition of carbonate are very pure.
  The shalenormalized REE patterns of 5% HNO3 soluble phase of limestone (carbonate minerals) showed total rare earth elements (ΣREE) were low, negative Ce anomaly , Eu was normal or slightly positive anomalies, Pr /Pr*  was from 1.17 to 1.63. It is consistent with those of Permian limestone, Permian sea water, and modern eastern Pacific sea water and illustrated that it formed in oxidation environment of the open sea.
  The correlation coefficient between total rare earth (ΣREE) and Al, Cu, Zn, K and Ti were from 0.81 to 0.90 and showed a positive correlation, and the correlation coefficient between total rare earth (ΣREE) and Ca, Mg, Mn were from 0 to 0.33 and illustrated there was no obvious relevance or relevance.  It is said that rare earth elements of hummocky limestone mainly related to the background value of sedimentary environment and has no matter with carbonate minerals.
  Hummocky limestone δ13CPDB were from +4.30‰ to +5.93‰ and δ18OPDB were from 13.72‰ to 11.27‰. δ13CPDB is basically consistent with the normal marine carbonate scope and indicating δ13CPDB remained the original sedimentary characteristics of hummocky limestone. But  δ18OPDB was significantly affected by diagenesis and did not represent the original sedimentary features.
  The V / Cr ratios of limestone are very low, *Uau values are only from 0.08 μg/g  to 0.5 μg/g, and the contents of U, V, Mo, Cr, Co, Cu and Zn elements, that were enriched in anoxic environment, are low or absent. All instructed that limestone formed in oxygenrich environment. 
  All above sedimentary rocks and geochemical characteristics show that the Permian  Triassic hummocky limestone might be the product of rapid deposition in relatively oxic environment of open sea.