Keywords: 
• carbon and oxygen isotopes /
• carbonate cements /
• Shangganchaigou Formation /
• Miocene Series /
• Eboliang III tectonic belt

Abstract: Based on petrology, mineralogy and geochemistry, the diagenesis, depositional environment and origin of Miocene clastic rock of Shangganchaigou Formation in Eboliang III tectonic belt on the northern margin of the Qaidam basin were studied. The experimental results were that particles of Miocene clastic rock of Shangganchaigou Formation mainly had point contact or line contact relationships, and had been suffered strong compaction; the clastic rock of Shangganchaigou Formation had high contents of clay minerals, whose average value was 35.03%, and which were dominated by illite, chlorite and mixed layer of illite and montmorillonite; the calcite was the main types of carbonate cements, and the dolomite was minor; the carbon isotope (δ¹³C) of carbonate cements ranged from -6.8‰ to -4.0‰, and the average value was -5.0‰; the oxygen isotope (δ¹⁸O) ranged from -11.1‰ to -5.4‰, and the average value was -8.7‰; Z-value of paleosalinity estimated by δ¹³C and δ¹⁸O ranged from 108.34 to 114.89 and the average value was 112.77; and temperature of carbonate deposited ranged from 43.36℃ to 77.84℃, and the average value was 62.30℃. Above results, combined with the comprehensive analysis of some geochemical elements, such as major elements, trace elements and organic carbon content, suggested that main diagenetic stage of Miocene clastic rock of Shangganchaigou Formation in Eboliang III tectonic belt was in phase B of early diagenetic stage B or phase A of the middle diagenetic stage. The early stage of diagenesis occurred in dry environment, where the water was cold freshwater or brackish water, and to the late stage of diagenesis, the Z-value of paleosalinity of water clearly decreased, and the paleo-environment was dominated by warm and wet climate.