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Diagenetic process of bedded chert

层状燧石成岩过程

基本信息

批准号：
62540591
负责人：
SAITO Yasuji
金额：
$ 0.9万
依托单位：
National Science Museum,
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1987
资助国家：
日本
起止时间：
1987 至 1988
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-62540591/
关键词：
chert diagenesis radiolaria 珪質海綿骨針続成珪質生物起源海水晶出起源

项目摘要

Bedded cherts of Japanese Paleozoic and Mesozoic systems mainly consist of radiolarian remains and siliceous sponge spicules with subordinate claymaterial of aeolian origin and Fe-Mn minerals precipitated from sea water. By using siliceous skeletons as a starting material, experimental investi-gation under hydrothermal condition (hydrostatic pressure of 100 to 300 kg/cm2, about 100 to 300゜C )were carried out, in particular with relation to textural change of siliceous sediments during diagenesis. The results after 130 or more hours shows that interspaces among siliceous skeletons were filled with secondary preci-pitated silica, while some skeletons were dissolved. A dissolution-redeposition process is involved in the reaction, suggesting that neoformation of matrix silica and cementation progress. The experiments treated under high-temperature condition, corresponding to those of contact metamorphism, show that all siliceous skeletons of amorphous silica were trans-formed into equigranular aggregates of quartz by recrystallization. Evidences from texture of the bedded chert and the experimental products support the following diagentic process: highly porous siliceous sediments (amorphous silica) compaction and change of intersti-cial water due to silica solution decrease of porosity due to precipitation of silica from the intersticial water as increas of burial depth and temperature (opal CT) hard and compact chert completely filled with secondarily redeposited silica (cryptocrystalline quartz). The transformation from opal CT to quartz is probably a solid-state reaction because original sekeleton shapes are preserved even in quartz chert. The bedded cherts of orogenic belts were formed under deep-sea floor conditon and after diagenesis were accret-ed in subduction zones.

日本古生代和中生代系统的层状硅质岩主要由放射虫遗迹和硅质海绵针状物组成，并伴有风成粘土和海水中沉淀的铁、锰矿物。以硅质骨架为原料，在水热条件下(静水压力100~300 kg/cm~2，约100~300゜C)进行了实验研究，重点研究了硅质沉积物在成岩作用过程中的结构变化。130小时以上的结果表明，硅质骨架之间的空隙被二次预置的二氧化硅填充，部分骨架被溶解。反应过程为溶解-再沉积过程，提示基质二氧化硅的生成和胶结过程。与接触变质相对应的高温条件下处理的实验表明，无定形二氧化硅的所有硅质骨架都通过重结晶转变为石英的等粒状集合体。层状硅质岩和实验产物的结构证据支持如下成岩过程：高孔硅质沉积物(无定形二氧化硅)压实和硅溶液引起的间隙水变化，间隙水中二氧化硅的析出使孔隙度随埋深和温度的增加而减少(蛋白石CT)，坚硬而致密的硅质岩完全充填二次再沉积的二氧化硅(隐晶石英)。从蛋白石CT到石英的转变可能是一种固相反应，因为即使在石英硅质岩中也保留了原始的骨架形状。造山带的层状硅质岩是在深海海底条件下形成的，成岩作用后在俯冲带中聚集。