义敦褶皱区三叠系复理石的甚低级变质作用研究

The Yidun fold region, the Banyan Har Orogrn and the Songpan-Garze Orogen comprise the Triassic "Qingzang-Dianxi fold belt" in the west of China. This proposal intends to be a continuous study after the NNSFC projects "Very low grade metamorphism of the Triassic flysch in the Songpan-Garze orogen and the Banyan Har orogen" and will focus on the Yidun terrene. The combination of field investigation with laboratory experimental analyses, the theories and methods of low temperature metamorphism will be carried out for this study "very low grade metamorphism of Triassic flysch in the Yidun terrene, Tibetan plateau". Those methods include clay mineralogy, mineral assemblages, international comparable illite crystallinity and chlorite crystallinity, c* domain size, b0 cell-dimensional geobarometry of K-white mica, chlorite geothermometry and evolution of carbonaceous materials as well as Ar-Ar isotopic geochronology. The purposes of this study is to investigate i) the degree of the overprint from diagenesis to metamorphism on the Triassic flysch in the Yidun terrene; ii) the relations between fold axes, faults, strata and boundaries of anchizone and finally iii) to decipher the tectonothermal evolution of the Yidun terrane. It is hopeful also for this proposal to offer useful information for the exploration of oil and gas and for the economic development in the western China.

义敦褶皱区与巴颜喀拉造山带、松潘-甘孜造山带共同构成了我国西部三叠系青藏-滇西褶皱带。把野外地质调查与实验分析相结合，运用现代碎屑岩低温变质理论与方法研究义敦褶皱区三叠系复理石的甚低级变质作用，包括运用黏土矿物学、伊利石结晶度、绿泥石结晶度、层状硅酸盐堆积多型、K云母b0值、碳微区演化、同位素年代学等方法，分析研究复理石变质强度（由IC、CC、多型、矿物组合等表征）的空间变化及其与断裂、褶皱等构造形迹的成因关系，确定变质作用温度变化范围与压力类型，研究区域变质热演化。从而确定造山运动中变质作用的时空范围与强度。研究的完成不仅可解决该区造山过程中存在的变质作用类型之争，完善低温变质作用研究，划分出可与国际对比的变质带；而且可为该区的矿产资源及油气资源的勘探开发提供重要的理论基础。也为西部开发和建设提供基础的地质资料和数据。

根据甚低级变质作用的基本原理，应用伊利石结晶度Kubler指数、绿泥石结晶度Arkai指数、成岩作用变质作用指示矿物、粘土粒级矿物组合和地质温压计研究了义敦褶皱区三叠系复理石所遭受的成岩~甚低级变质作用。全区范围内共采集了326个三叠系复理石样品。由伊利石结晶度Kubler指数的填图，确定出了义敦褶皱区由德格浅变质带、八美浅变质带、理塘近变质带、达江成岩带和宁蒗成岩带组成。达江成岩带是杂多-拉秀成岩带的延续；宁蒗成岩带是青藏高原三叠系复理石最南端的成岩带；德格浅变质带是不冻泉-玉树-甘孜-理塘巨型近变质带中相对高温的区域；八美浅变质带为不冻泉-玉树-甘孜-理塘巨型近变质带中靠近鲜水河断裂带的相对高温的小区域；理塘近变质带是不冻泉-玉树-甘孜-近变质带的延续和扩展。成岩带产出众多的蒙脱石、伊蒙混层、绿蒙混层、高岭石等成岩自生指示矿物；近变质带产出大量的钠云母；浅变质带则产出许多黑云母、阳起石以及石墨， 表明了变质带与变质矿物分布的一致性。由绿泥石地质温度计测量的峰期变质温度为220-320°C，变质压力为低-中压相系（N. New Hampshire low-intermediate pressure type）。.由此，世界上最大的三叠系复理石出露区青藏高原北部~东部三叠系复理石区（约61万km2）一共由11个变质带（区）与3个成岩区构成。即1）丹巴-金川浅变质带；.2）德格浅变质带；3）八美浅变质带；4）壤塘浅变质带；5）不冻泉-玉树-甘孜-理塘巨型近变质带；6）扎陵湖-鄂陵湖近变质带；7）兴海-同德-泽库近变质区；8）久治近变质带；9）黑水近变质带；10）小金近变质带；11）两河口近变质区；12）杂多-达江成岩带；13）若尔盖-达日成岩带（区）；14）宁蒗-九龙-四川盆地成岩带。