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塔里木盆地西北缘蓬莱坝剖面中寒武统发育大量的硅质岩,一般顺层分布,以结核状、条带状、不规则团块状产出,部分可见纹层状构造,形成于同生-准同生期。硅质岩δ18O分布范围25.25‰~26.196‰(SMOW),指示成岩温度范围52.59~57.87℃,高于海水温度;微量元素Ba/Sr值和U/Th值都大于1,前者分布为4.67~22.52之间,平均为14.52,后者为2.91~9.21之间,平均值为6.28,反映了热水的注入。硅质岩REE配分曲线特征指示成岩时同时受到热液及海水的影响;Ce异常值(Ceanom)可以判断沉积时的氧化-还原环境,Ceanom大于-0.1,Th/U比值小于2,表明形成于还原环境。此外,北美页岩标准化后的Ce/Ce*比值0.939~1.156,平均值1.036;(La/Ce)N比值为0.936~1.311,平均值为1.124,反映其构造背景为大陆边缘环境。笔者认为,该区在中寒武统曾经发生过一期比较强烈的构造运动,并引发了海底热液活动,形成了上述硅质岩记录。
The middle Cambrian of the Penglaiba section in the northwestern margin of the Tarim Basin is characterized by a large number of siliceous rocks developed in the order of bedding, with nodular, banded and irregular clumps. Partially visible laminar structures are formed in the same genera - Quasi syngenetic period. The δ18O of the chert is from 25.25 ‰ to 26.196 ‰ (SMOW), indicating that the diagenetic temperature ranged from 52.59 to 57.87 ℃, higher than the seawater temperature. The trace elements Ba / Sr and U / Th were both above 1 and the former were from 4.67 to 22.52 Between the average of 14.52, the latter between 2.91 ~ 9.21, with an average of 6.28, reflecting the injection of hot water. The characteristics of REE distribution curves indicate that diagenesis is affected by both hydrothermal and seawater. The Ceanom can determine the oxidation-reduction environment during deposition. The Ceanom is greater than -0.1 and the Th / U ratio is less than 2, Restore the environment. In addition, the Ce / Ce * ratios of North American shale normalized from 0.939 to 1.156 with an average of 1.036, and the ratio of (La / Ce) N from 0.936 to 1.311 with an average of 1.124 reflect the tectonic setting of the continental margin. In my opinion, there was a relatively intense tectonic movement in the middle Cambrian during the middle Cambrian, and led to the seabed hydrothermal activity, forming the record of silicalite.