塔里木盆地肖尔布拉克剖面下寒武统底部硅质岩微量元素和稀土元素地球化学及其沉积背景

对塔里木盆地北部肖尔布拉克寒武系露头剖面中 8.8m厚的黑色页岩夹硅质岩组合中的硅质岩的系统采样分析表明,该硅质岩具有深源成因特征。其低的Th/U和Rb/Sr比也证明了其沉积时所具有的深部物源和热水注入迹象。硅质岩的稀土元素经北美页岩标准化后的Ce/Ce 从 0.4 2到 0.79,平均 0.5 7。Ce的负异常明显。这些稀土元素的参数特征与加利福尼亚弗朗希斯科杂岩 (FranciscanComplex)中沉积在大洋海底硅质岩的稀土元素特征十分相似。北美页岩标准化后的Eu/Eu 值从 8.0 5下降到 1.0 3。其相对较高的Eu/Eu 值很有可能反映了热水的注入。Eu/Eu 值从剖面底部到顶部的系统降低反映了剖面底部的热水作用最强烈,向上热水作用逐渐减弱。上述一系列的地球化学标志指示该硅质岩应沉积在离洋中脊不远的、具有深源物质/热水注入的远洋盆地背景中。结合区域地质和伴生黑色页岩地球化学的综合分析认为,该套硅质岩的形成与上升洋流的影响有关。上升洋流将形成于大洋盆地背景中的物质带到大陆边缘陆棚环境中发生沉积,造成了沉积在陆棚环境中的硅质岩,在地球化学组成上保留了其大洋盆地背景的特征

Rare Earth and Trace Element Patterns in Bedded-cherts from the Bottom of the Lower Cambrian in the Northern Tarim Basin,Northwest China:Implication for Depositional Environments

Black rock series at the bottom of the Lower Cambrian in the Northern Tarim Basin, China, is com 2posed of black shales interbedded with thin - bedded cherts. Six chert samples were systematically collected from avertical section of 8. 8 meters in depth in Xiaoerbulak, Northern Tarim Basin. The cherts were crushed and ana 2lyzed for trace element and rare earth concentrations. Trace elements such as V, Cu, Zn, U, Pb, Ba, Cd, Ag,Mo, As and Sb are highly enriched, and others such as Rb, Zr, Cs, Hf, Ta, W, Tl, Bi and Th are highly deplet 2ed in the cherts. These trace element patterns suggest that the cherts may be of deep crustal origin. The low ratiosof Th/ U and Rb/ Sr further suggest that the cherts are of earth interior sourcesor received hydrothermal input dur 2ing their deposition. The chondrite - normalized Ce/ Ce3ratio rangesfrom 0. 42 to 0. 83, with an average of 0. 60.North American Shale Composite (NASC) - normalized Ce/ Ce3ratio ranges from 0. 42 to 0. 79, with an averageof 0. 57. Negative Ce anomalies are distinct. ∑ REEs in the cherts generally increase from 10. 50 ppm at the bot 2tom to 35. 97 ppm at the top of the sampled section. NASC - normalized (La/ Lu) _N ratio decreases from 2. 72 atthe bottom to 0. 67 at the top. NASC - normalized (La/ Ce) _N ratio increases from 1. 36 at the bottom to 3. 13 atthe top. These REE patterns are very similar to those for the cherts deposited in the pelagic ocean - basin floor inthe Franciscan Complex exposed at Marin Headlands, California (F - MH chert) (Murray et al.,1991). Chon 2drite - normalized Eu/ Eu3value markedly decreases upward in the section from 5. 54 at the lowermost to 0. 73 atthe top, and NASC - normalized Eu/ Eu3value decreasesfrom 8. 05 to 1. 03. The relatively high Eu/ Eu3ratio forthe chertsfrom the North Tarim Basin is most likely due to a hydrothermal input (e. g., Eu/ Eu3 ～10). The sys 2tematic decrease of Eu/ Eu3ratio from the bottom to the top of the section reflects that the hydrothermal input isthe largest in the lowermost portion of the section and gradually decreases upward. These geochemical characteris 2tics indicate that the cherts from the bottom of the Lower Cambrian in the Northern Tarim Basin should be de 2posited on a pelagic ocean - basin floor in proximity to the mid ocean ridge that provided much interior source/ hy 2drothermal input, but this conclusion is in quite contradiction to that from the sedimentological research. On thebasis of the comprehensive studiesof regional geology and geochemistry of black shales, it is considered that the up 2welling carries the substances formed in the pelagic ocean floor onto shelf to deposit, which results in the pelagicgeochmical characteristics of the bedded cherts deposited on shelf.