羌塘盆地中侏罗统布曲组白云岩有序度与晶胞参数的影响因素及地质意义

Controlling factors and their geological significances of order degrees and unit-cell parameters of dolomites in the Middle Jurassic Buqu Formation in Qiangtang Basin

关键词:  羌塘盆地  布曲组  白云石化作用  白云石有序度  晶胞参数

Key words:  Qiangtang Basin  Buqu Formation  dolomitization  order degree of dolomite  unit-cell parameters

摘要:

研究羌塘盆地中侏罗统布曲组白云岩有序度和晶胞参数的特征、影响因素及地质意义。在前期对白云岩详细分类的基础上，通过X射线粉晶衍射方法进行测试，用jade 6.5软件对测试结果进行处理，获取白云石的最大晶面间距、有序度、晶胞参数等信息，探讨其影响因素及地质意义，结果表明：（1）北羌塘坳陷西南缘唢呐湖地区颗粒白云岩具低—中等的有序度，晶胞参数较为集中，可能是蒸发潟湖环境中发育的颗粒灰岩为高镁方解石，在经历埋藏过程的调整白云石化作用而成；（2）南羌塘坳陷古油藏带布曲组白云岩中，微—粉晶白云岩有序度、晶胞参数分布范围大，为灰岩在准同生阶段快速交代、在埋藏阶段重结晶形成；细晶白云岩有序度较高，晶胞参数呈正相关，发育在稳定的埋藏环境中，白云石晶体在继承先驱灰岩孔隙中自由生长而成；中—粗晶白云岩有序度最高，是在中—深埋藏阶段，经历了足够长久的白云石化作用过程，有限的空间限制了晶体的自由生长；微—粉晶白云岩→细晶白云岩→中—粗晶白云岩的有序度逐渐增高，表明稳定的埋藏过程有利于白云石向有序转化；（3）布曲组白云岩晶胞参数a、c以不同程度向理想白云石左下方偏移，说明布曲组白云岩形成以后经历了强烈的构造挤压，估算压力可达1000MPa，与晚白垩世的构造运动有关，指示南羌塘坳陷布曲组白云岩古油藏是由逆冲推覆运动造成的。

Abstract:

This paper discusses the characteristics, controlling factors and geological significances of the order degrees and unit cell parameters of the Middle Jurassic Buqu Formation dolomites in Qiangtang Basin. Basing on the detailed classification of dolomites in the early stage, the X-ray powder diffraction method is used to test the dolomite, and the Jade 6.5 software is used to process the test results. The maximum crystal plane spacing, order degrees, unit cell parameters and relevant information of dolomite are obtained, and the controlling factors and geological significance are discussed. The results show that: (1) The grain dolomites in Suona Lake area of the north Qiangtang Depression, which are with low to medium order degrees and concentrated unit cell parameters, develops in an evaporation lagoon environment from high magnesium calcites by the adjustment of dolomitization during the burial process; (2) To the dolomites of the Buqu Formation in the South Qiangtang Depression, the micro-powder crystalline dolomites with a large distribution range of order degrees and unit cell parameters are formed by rapid metasomatism of limestone in penesynchronous stage and recrystallization in burial stage; The fine-grained dolomites with a high degree of order and a positive correlation of unit cell parameters develop in a stable burial environment and the dolomite crystals grow freely in the pores of the inherited limestone; The medium-coarser-grained dolomite, whose degree of order is the highest, has experienced a long process of dolomitization in the middle-deep burial stage, and the narrow spaces limit the free growth of crystals. From micro-powder dolomite to fine-grain dolomite to medium-coarser-grain dolomite, the order degree of dolomites increases gradually, indicating that the stable burial process is conducive to the orderly transition of dolomites. (3) The “a” and “c” of unit cell parameters of the Buqu Formation dolomites shifted to the lower left of the ideal dolomite in different degrees, indicating that the Buqu Formation dolomites experienced a strong tectonic compression with an estimated pressure up to 1000 MPa after its formation, which is related to the tectonic movement of the late Cretaceous. It is suggested that the dolomite paleo-oil reservoir of Buqu Formation in southern Qiangtang Depression is caused by the thrust nappe tectonic movement.