张北汉诺坝地区碳酸岩岩石地球化学特征及成因研究

发布时间：2018-04-10 20:13

本文选题：碳酸岩 + 地球化学特征　；参考：《石家庄经济学院》2015年硕士论文

【摘要】：前人对张北地区碳酸岩的研究主要集中在二道边碳酸岩筒,本次对张北汉诺坝地区野外地质调查发现了黑脑包一带有碳酸岩脉产出,规模十几米至一百多米,与碱性玄武岩及其中的地幔橄榄岩包体密切共生。本文通过岩相学、矿物学、岩石地球化学分析结果研究张北地区碳酸岩岩石成因。研究表明张北二道边碳酸岩筒和黑脑包碳酸岩脉的碳酸岩均为含CaO较高的方解石碳酸岩,主要矿物为方解石,次要矿物为镁橄榄石、斜方辉石、单斜辉石及铬尖晶石。局部脉状碳酸岩可能与包含的幔源包体进行交代,而使MgO的含量增加。研究区碳酸岩普遍具有结晶柱状结构,说明碳酸岩为火成成因。微量稀土元素地球化学特征与甘肃礼县新生代喷发碳酸岩特征相似。碳酸岩及方解石的Sr元素与Ta、Zr、Hf元素含量特征与火成成因碳酸岩的特征一致。笔者推测张北汉诺坝地区碳酸岩是富CO_2的硅酸岩岩浆液态不混溶作用形成,硅酸盐熔体先喷出地表形成碱性玄武岩,受后期热应力影响产生大量裂隙,碳酸盐熔体随后灌入先固结的碱性玄武岩中,形成黑脑包脉状碳酸岩,呈脉状穿插于玄武岩和地幔橄榄岩中。当分溶的碳酸盐熔体具备较大压力时就会冲破上覆地层,产生爆破式喷发,形成二道边的爆破碳酸岩筒。富CO_2的硅酸岩岩浆演化过程中斜长石由于结晶分异作用晶出,造成岩浆低Eu特征,碳酸盐熔体与硅酸盐熔体溶离后继承了低Eu的特征,可能是张北汉诺坝地区碳酸岩Eu负异常的原因。推测方解石单矿物稀土元素Eu正异常是由岩浆喷发活动造成物质不均一性导致的。
[Abstract]:Previous studies of carbonatite in Zhangbei area were mainly focused on the two sides of the carbonatite tube. This field geological survey of Hannouba area in Zhangbei has found that carbonatite veins are produced in the area of Hannoba in Zhangbei, with a scale of 10 to more than 100 meters.It is closely associated with alkaline basalt and mantle peridotite xenoliths.In this paper, the petrogenesis of carbonatite in Zhangbei area is studied by the results of petrographic, mineralogical and petrogeochemical analysis.The results show that the carbonatite in both the Carbonate tube of Erdaobian in Zhangbei and the carbonatite in the Kuronaobao vein are calcite carbonate rocks with high CaO content. The main minerals are calcite, the secondary minerals are mafic olivine, clinopyroxene, clinopyroxene and chromium-spinel.The local vein carbonate may be metasomorphic with the contained mantle xenoliths, thus increasing the content of MgO.Carbonatite in the study area generally has crystalline columnar structure, indicating that carbonatite is the origin of pyrogeny.The geochemical characteristics of trace rare earth elements are similar to those of Cenozoic eruptive carbonatite in Lixian County Gansu Province.The contents of Sr element and Tazrzru HF element in carbonatite and calcite are consistent with those of pyrogenic carbonatite.The authors speculate that carbonatite in Hanooba area of Zhangbei is formed by liquid immiscible process of CO_2 rich silicate magma, and silicate melt first ejects from the surface to form alkaline basalt, and a large number of cracks are produced under the influence of late thermal stress.Carbonate melts are then poured into preconsolidated alkaline basalts to form black-brain encrusted carbonatites interlaced in basalts and mantle peridotite.When the dissolved carbonate melt has a high pressure, it will break through the overlying strata and produce a blasting eruption, forming a blasting carbonatite tube with two sides.In the process of magma evolution of silicalite rich in CO_2, plagioclase is characterized by crystallization differentiation, which results in low EU characteristics of magma, and inherits the character of low EU after dissolution of carbonate melt and silicate melt.It may be the cause of negative EU anomaly of carbonatite in Hannoba area, Zhangbei.It is inferred that the positive anomaly of EU in calcite monmineral is caused by the material heterogeneity caused by magmatic eruption.
【学位授予单位】：石家庄经济学院
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：P588.245

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