Keywords: 
• chert /
• rare earth and trace elements /
• depositional environment /
• Northern Tarim Basin /

Abstract: 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.