班公湖镁质碳酸岩体是青藏高原隆升过程中,在岩浆和流体作用下,含碳物质壳内循环作用的产物.本文利用LA-MC-ICP-MS测试技术对镁质碳酸岩样品中的交代型锆石进行了U-Pb同位素测年分析,其加权平均年龄为80.3±1.2Ma(MSWD=3.8),说明班公湖地区交代型镁质碳酸岩形成于晚白垩世,与这一地区同期的中酸性岩浆活动有关.岩体中菱镁矿和菱铁矿δ13 CV-PDB在0.9‰～1.6‰之间,平均为1.25‰；δ18OV-SMOW值在15.9‰～21.7‰,平均为18.65‰,指示碳来自于围岩中的海相碳酸盐岩.碳循环条件研究认为,大量的流体作为碳循环的载体,来自于大气降水,循环温度可能在300～350℃之间,循环范围从地表以下100 m到2.1 km左右.班公湖岛弧带白垩纪晚期伸展松弛的构造背景为下部岩浆上涌、天水下渗循环、加快CO2气体排放、加速碳的壳内循环提供了重要的驱动条件.%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. 2 Ma (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 0 isotope determinations of magnesite and siderite show δ13CV.PDB values ranges from 0. 9%e to 1.6%e, averagely 1. 25%c, and δ18 OV.SMOW values ranges from 15. 9%o to 21.7%e, averagely 18. 65%c, 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. lkm 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.
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