Magnesian carbonatite rocks in the Bangong Lake area are due to the crustal carbon cycling under the influence of magma and fluids in the process of Qinghai—Tibet plateau uplift.LA-MC-ICP-MS U-Pb isotope dating of metasomatic zircons from the magnesian carbonatites shows that the weighted average age is 80.3±1.2Ma（MSWD=3.8）. The results explain that metasomatic magnesian carbonatite was formed in Late Cretaceous, and was related to the activities of contemporary acid rock wall in this area. C and O isotope determinations of magnesite and siderite show δ13C V-PDB values ranges from 0.9‰ to 1.6‰, averagely 1.25‰, and δ18O V-SMOW values ranges from 15.9‰ to 21.7‰, averagely 18.65‰, indicatings that the carbon was derived from the marine carbonate rocks in the surrounding rock. The carbon cycle conditions reveal that a lot of fluid as the carrier came from meteoric water, and circulating temperature was evaluated between 300~350℃ from the 100 m to 2.1 km underground. The slack tectonic setting in late Cretaceous contribute to magma upwelling and infiltration of meteoric water. More importantly, it provides an available driver to speed up the emissions of CO2, and carbon circulation in the crust.