白垩纪大洋红层的致色机制及成因研究

The Colouration Mechanism and Genetic Study of Cretaceous Oceanic Red Beds

为了查明白垩纪大洋红层的致色矿物及其赋存状态,并探讨白垩纪大洋红层可能的制约因素及成因模式,本文以中国西藏床得剖面红色页岩、意大利Vispi Quarry剖面红色灰岩以及北大西洋ODP1049C孔12X岩芯段Aptian-Albian期高频旋回红色泥灰岩为研究对象,对各剖面(或岩芯)中的红色和非红色样品分别进行了X射线衍射和漫反射光谱测试,同时对配制的含赤铁矿的标样进行了同样的测试。测试结果表明,无论是红色页岩、红色灰岩,还是红色泥灰岩,赤铁矿都是主要的致色矿物,其中西藏床得剖面红色页岩由结晶较好的碎屑状赤铁矿和结晶较差的细分散状的赤铁矿共同致色。在意大利Vispi Quarry剖面红色泥岩中,结晶程度相近的赤铁矿是唯一的致色铁氧化物,而在ODP1049C孔红色泥灰岩中,结晶差的赤铁矿和针铁矿的出现是泥灰岩呈红色的矿物学根源。赤铁矿的形成主要受铁的来源、沉积时的氧化还原条件以及成岩作用的影响,这些因素也成为制约红层形成的关键因素。本课题组在多年研究的基础上,结合前人研究成果进一步从矿物学的角度深化了大洋红层的成因模式。

To identify the colouration minerals and their forms of occurrence in Cretaceous Oceanic Red Beds(CORBs) and to better understand the constraining factors and genetic models of CORBs, the red and non-red specimens collected from the CORBs, including both of the Chuangde section in Tibet, the Vispi Quarry section in Italy and the ODP Hole 1049 C Core from the North Atlantic, were measured using X-ray diffraction(XRD) and diffuse reflectance spectroscopy(DRS). The calibration specimens composed of hematite('kidney ore' or 'specularite') and calcite were also measured using the diffuse reflectance spectroscopy. The results showed that hematite is the principal mineral responsible for colouration of these red beds, although the lithologies of the three sections(or core samples) were different. The red colour of shales in the Chuangde section is caused by two forms of hematite : a detrital one and a fine poorly crystallized one. Hematite with similar crystallinity is responsible for the red colour of limestone from the Vispi Quarry in Italy. As for the loosely cemented sediments from ODP Hole 1049 C Core in the North Atlantic, poorly crystallized hematite and goethite are responsible for the colour change of sediments from non-red to red. The occurrence of hematite is affected by the supply of iron ions, the redox state and the diagenesis, which also are the decisive factors for the occurrence of CORBs. Based on analyses made in this study and on summaries of previously produced studies, a further genetic model of CORBs is proposed from a mineralogical perspective.