Organic geochemistry and mineralogy of mudstones affected by contact metamorphism

为了研究接触变质作用对有机成熟度和沉积岩矿物的热效应，测定了青森县大洼町、南洼津加鲁两组中新世泥岩的有机碳含量（Co）、可见干酪根组成、镜质体反射率（Ro）、统计热蚀变指数（stTAI）和x射线衍射。从沉积岩有机质含量、组成和成熟度来看，接触变质作用的有机质成熟程度随着离岩脉距离的减小而加深。在距离沙白石岩脉1.5 m范围内的样品中，Co和非晶干酪根的含量随距离岩脉的减小而增加。这一现象可以解释为高温高压下干酪根产生的液体成分与残留的干酪根重新组合形成焦状物质。从Ro - stTAI图中可以看出，接触变质作用与成岩作用导致的有机质成熟途径不同。这两种途径之间的差异归因于加热时间和温度尺度的巨大差距，以及Ro和stTAI对加热时间和温度的敏感性。在未受接触变质作用影响的情况下，两系有机质成熟度相当，而石英矿物分带则表明萨白石系比森山系成岩作用进展更大。上新世黑石断裂和三井井断裂运动后，萨白石地区比森山地区深330 m。断裂运动后有机质成熟得到了固定，石英矿物分带从断裂运动前就开始维持。

The content of organic carbon (Co), the composition of visual kerogen, vitrinite reflectance (Ro), statistical Thermal Alteration Index (stTAI) and X-ray diffraction in two series of Miocene mudtones near the dykes at Owani-machi, Minami-Tsugaru, Aomori, were measured to investigate the thermal effect of contact metamorphism on organic maturation and minerals in sedimentary rocks. The organic maturation by contact metamorphism progressed with decreasing distance from the dykes with regard to the content, composition and maturity of organic matter in sedimentary rocks. In the samples within 1.5 m from the dyke at Sabaishi, the contents of Co and amorphous kerogen increase with decreasing distance from the dyke. This phenomenon may be explained by the idea that liquid constituent generated from the kerogen under high temperature and high pressure was recombined the residual kerogen to form the coke-like material. It is recognized in a diagram of Ro against stTAI that the pathway of organic maturation caused by contact metamorphism is different from that by diagenesis. This difference between these two pathways attributes to the large gap of heating time and temperature scales, and of sensitivity of Ro and stTAI to heating time and temperature. Without suffering effect of contact metamorphism, organic maturation indicates equivalent between two series, while silica mineral zoning indicates that the Sabaishi series has more highly progressed diagenesis than the Moriyama series. Kuroishi and Mitsumenai faults made the Sabaishi area 330 m deeper than the Moriyama area just after the movements of the faults, Pliocene epoch. It is concluded that organic maturation was fixed after the fault movements, and that silica mineral zoning has maintained from the time before the fault movements.