西藏措勤盆地白垩系白云岩地球化学特征及其成因分析

Geochemical characteristics and genetic analysis of the Cretaceous dolomite in the Cuoqin Basin,Qinghai-Tibet Plateau.

通过岩石学、阴极发光、同位素和岩石地球化学等方法对青藏高原措勤盆地郎山组白云岩进行了成因研究。研究表明本区白云岩岩石类型主要为中-细晶白云岩和灰质白云岩,阴极发光主体呈棕红-暗棕红色光,并且具有高的有序度和低的 CaCO_3摩尔含量。地球化学分析表明岩石 U 含量(1.5～45.2×10^(-6))和 U/Th(0.89～68)明显高于一般的海相碳酸盐;高的^(87Sr)/^(86)Sr(0.710092～0.711719)同位素比值,表明在成岩过程中 Sr 的加入。白云岩δ^(18)O 值为-15.13‰～-6.94‰,平均-8.7%,δ^(13)C 为-0.56‰～2.53‰,平均1.7‰,反映出白云岩具有较高的盐度指数 Z 值(118.0～128.0)和成岩温度(50.77～95.95℃),表明形成于超成浓缩海水环境并具较高的成岩温度。综合分析认为本区白云岩属于埋藏成因,并在后期由于淡水加入发生去白云化作用。

The types and genetic mechanism of the Cretaceous dolomite in the Cuoqin Basin are investigated by means of petrological analysis, cathodoluminescence microscopic observation, X-rays identification, carbon and oxygen isotope geochemistry and trace element analysis. Dolostones in study areas mainly consist of fine or medium dolomites and calcite dolomites with euhedral or subhedral crystals. Dolostones mainly show red-brown and dark brown, the degree of order is high and the molar volume of CaCO3 is low. The content of U （ 1.5 -45.2 × 10^-6） and U/Th （0.89 -68） are much higher than normal carbonates, moreover the high ^87Sr/^86Sr （0. 710092 - 0.711719） indicates the input of Sr during the formation of dolostones. The δ^13 C is -0.56‰ - 2.53‰ （PDB） and averages 1.7‰; δ^18O is - 15. 13 ～ -6.94‰ （PDB） and averages -8.7‰. The higher salinity index（118.0 - 128.0） and diagenetic temperature （50. 77 -95.95℃ ）illuminate dolostones were formed under the concentrated salinity and high temperature condition. Various data show that these dolostones were formed under deep burial conditions, and dedolomitization occurred in late stage for the input of freshwater.