热水沉积区黑色页岩稀土元素特征及其地质意义——以贵州中部和东部地区下寒武统牛蹄塘组页岩为例

Characteristics and geological implications of rare earth elements in black shale in hydrothermal sedimentation areas: A case study from the Lower Cambrian Niutitang Fm shale in central and eastern Guizhou

为了弄清热水沉积与优质烃源岩发育两者之间的关系,以贵州中部和东部地区下寒武统牛蹄塘组页岩为例,通过对该区5个剖面(毕节市联兴村、贵阳市温水村、遵义市中南村、黔东南羊跳村、黔东南九门村)22个样品的主量元素和稀土元素进行测试分析,研究热水沉积作用影响下的稀土元素沉积特征,并利用稀土元素特征来判别热水活动强度及黑色页岩的沉积成因,进而结合有机碳含量探讨了热水沉积作用与优质页岩之间的关系。研究结果表明:(1)牛蹄塘组底部沉积受陆源输入、海相沉积和热水活动的共同影响,轻、重稀土元素在纵向上分异明显,底部重稀土元素相对富集,向上轻稀土元素相对富集,仅在热水活动最强的羊跳村剖面表现出稀土元素总量极低的特征;(2)在羊跳村剖面和中南村剖面附近存在2个热水活动中心,研究区由西向东受热水影响逐渐增大,热水沉积区的范围具有向九门村剖面东南方向延伸的趋势;(3)正常海相沉积和低温热水沉积的黑色页岩中δEu/Al(铕异常值/铝)、δY/Al(钇异常值/铝)值较低,随热水沉积影响强度增大而增大;(4)δEu>1的烃源岩的有机碳含量普遍较高,δEu/Al和δY/Al介于5~10时,低温热水沉积作用有利于有机质的富集和保存,对优质烃源岩的形成具有增益作用。

In this paper, the Lower Cambrian Niutitang Fm shale in Guizhou was taken as an example to figure out the relationship between hydrothermal sedimentation and high-quality source rock development. Firstly, 22 samples taken from 5 sections in Guizhou（i.e.Lianxing-cun section in Bijie, Wenshui-cun section in Guiyang, Zhongnan-cun section in Zunyi, and Yangtiao-cun section and Jiumen-cun section in SE Guizhou） were tested for major elements and rare earth elements（REEs）, in order to identify the characteristics of REEs under the effect of hydrothermal sedimentation. Then, the hydrothermal activity intensity and the sedimentary genesis of black shale were distinguished according to the REE characteristics. Finally, combined with total organic carbon（TOC） content, the relationship between hydrothermal sedimentation and high-quality source rocks was discussed. The following results were obtained. First, the deposition at the bottom of the Niutitang Fm is jointly controlled by terrestrial input, marine deposition and hydrothermal activity. The light and heavy REEs are obviously differential in the vertical direction. The heavy REEs are enriched at the bottom, while the light ones are enriched at the top. Extremely low REE content is only observed in the Yangtiao-cun section with the strongest hydrothermal activity. Second, there are two hydrothermal activity centers near the Yangtiao-cun and Zhongnan-cun sections, respectively. The influential intensity of hydrothermal activity increases from the west to the east in the study area, and the coverage of hydrothermal sedimentation intends to extend southeastwards to the Jiumen section. Third, δEu/Al（europium anomaly/aluminum） and δY/Al（yttrium anomaly/aluminum） are lower in the black shale formed in normal marine sedimentation and low-temperature hydrothermal sedimentation, and increase with the hydrothermal sedimentation intensity. And fourth, TOC of source rocks with δEu greater than 1 is higher. When δEu/Al and δY/Al range between 5 and 10, the low-temperature hydrothermal sedimentation is favorable for the enrichment and preservation of organic matters, and plays a positive role in the formation of high-quality source rocks.