中国东部塘沽地区沙河街早期细粒沉积岩中方沸石记录的地质作用

Analcime can be indirectly or directly brought into sedimentary rocks through volcano eruption, weathering, or hydrothermal activity. The study on its origin, hence, can enable the geological processes traceable which provide sediments in the formation of sedimentary rocks. However, sedimentary analcime problem appears when it comes to fine-grained sedimentary rocks. A set of analcime-rich fine-grained sedimentary rocks (dolomite series and mudstone series) have been drilled in the Lower Shahejie Formation of the Tangu area, Eastern China, and different modes of occurrence of analcime make its origin complicated. In our previous study, we proposed that weathering, hydrothermal fluid migration and hydrothermal sedimentation are recorded by those analcimes. To further verify our hypothesis and answer sedimentary analcime problem from a more comprehensive perspective, we are going to finish fluid inclusion analysis, and improve the accuracy of mineralogy and geochemical analysis, which is used to identify the dominant geological process of different types of analcime by analyzing vertical continuity of sediment supply, local thermal anomalies, vertical stability of sediment source evolution as well as difference of sediment sources. The success of this study will deepen our understanding of parts of different geological processes played in the formation of fine-grained sedimentary rocks, Including material output range, time period and mutual relationship, and provide a scientific basis for utilizing analcime in fine-grained sedimentary rocks to recognize different geological processes.

方沸石可经由火山喷发、母岩风化及热水活动间接或直接地产出于沉积岩之中，明确其成因则能够识别岩石形成中提供沉积物输入的地质作用，但细粒沉积岩的这类研究中存在“沉积方沸石问题”。中国东部塘沽地区沙河街早期发育一套富含方沸石的细粒沉积岩（白云岩类+泥岩类），岩石中两大类五小类产状的方沸石反映着其复杂的成因。我们在前期研究中已提出这些方沸石分别记录了母岩风化、热水运移以及热水沉积几类作用。为进一步验证以上假说及从更全面的角度回答沉积方沸石问题，本研究拟补充完善流体包裹体分析以及提高矿物学和地球化学分析精度，从物质供给连续性、局部热异常响应、物质来源差异及物质来源演化稳定性四个角度约束及建立起各产状方沸石与地质作用的关联。项目的顺利实施，有助于深化理解不同类型地质作用在细粒沉积岩形成中的物质输送范围、持续时间以及相互关联，为利用细粒沉积岩中方沸石识别岩石形成期的地质作用供科学依据。

中国东部塘沽地区沙河街早期形成的细粒沉积岩（白云岩类+泥岩类）中赋存着基质型及充填型两大类方沸石，五种子类型的产出反映了多样地质作用的发生。前期研究中我们已提出了不同类别的方沸石分别记录了母岩风化、热水运移以及热水沉积几类地质作用的成因假说。为对上述假说形成验证，本项目开展了相似矿物学特征方沸石纵向分布连续性分析、高温方沸石温-盐属性差异及局部热异常分析、各产状方沸石物质来源属性差异性及纵向演化稳定性分析三项重点研究内容。研究中发现：方沸石在纵向上的产出不具备连续性，全岩微量元素指标、微量元素图解及方沸石计算锶同位素的异常等共同证明了沙河街下部沉积物中存在着物源混合的现象，而计算锶同位素指示泥晶基质型方沸石（纹层富集型/纹层贫乏型及均匀分散型）的形成与幔源物质的稳定输送密切相关，配合新发现的泥晶小球、斑块聚集型方沸石与泥晶基质型方沸石矿物形貌及元素组成的相似性，本次研究更新提出泥晶基质型方沸石为基性火山喷发的沉积记录。粉晶基质型（粉晶纹层型）及粉-细晶充填型（微裂缝充填及溶孔充填）方沸石的流体包裹体学高温属性叠合稀土元素配分模式中的正铕异常证实了热水活动的发生。稀土元素、锶同位素、钕同位素解释结果间的不匹配暗示这两种类型的物质来源均具备壳-幔混合特征。结合粉晶纹层富集型与粉-细晶充填型方沸石的矿物学相似性、残留石膏矿物的发现及不同产状伴生黄铁矿硫同位素的相似性，更新提出粉晶纹层型方沸石与粉-细晶溶孔充填型方沸石均为热水交代作用的活动记录，而粉晶微裂缝充填型方沸石则主要记录了热水运移作用。本研究首次提供了热水方沸石的全面矿物学及地球化学特征，探索及论证了火山喷发在咸水湖中的形成沉积记录，对利用细粒沉积岩中不同类别方沸石回溯地质事件及地质过程以及依据相关地质作用进一步查明类似沉积的空间分布范围提供了科学依据。