| 其他摘要 | The Lower Cambrian outcropped in South China has been attracting the attentions of geologists not only for its well-known biota (e.g. the Chengjiang biota), but also for its mineral resources such as phosphorite, barite, rare earths, Ni-Mo-PGE ore deposits etc. Previous studies on the paleontology of the Lower Cambrian in South China have obtained distinguished achievements. Numerous contributions about the Chengjiang biota have been published on the internationally authoritative journals (e.g. Science, Nature) and have a world-wide and significant impact on researches on the evolution of life during early Cambrian. However, studies on geochronology and global correlations for the Lower Cambrian here are still lagging behind. Up to now, there is only one high-precision, reliable U-Pb zircon age (538.2 ± 1.5 Ma) from the K-bentonite at the middle Zhongyicun Member (Bed 5) of the Zhujiaqing Formation at the Meishucun section (Yunnan Province, South China) which is a candidate stratotype for the global Precambrian-Cambrian boundary (Jenkins et al., 2002). As result, the important biota, ore deposits, and globally correlatable carbon isotope excursion occurred in the Lower Cambrian of South China are poorly constrainted by high-precision isotope age. Further, studies on the evolution of life, genesis of ore deposits, and global correlation for the Lower Cambrian in South China have been hampered.
Recently, K-bentonites in the Lower Cambrian in South China have been discovered. These K-bentonites outcropped laterally at many areas such as eastern Yunnan, Maidiping (Emei, Sichuan Province), western and northern Guizhou, Taishanmiao (Yichang, Hubei Province), and Zhangjiajie (Hunan Province). Vertically, they occurred mainly at the middle Zhujiaqing Formation, the bottom of the Shiyantou Formation and their equivalent horizons. The K-bentonites possess significant potential not only for locating the isochronous bed of the stratigraphic framework but also for U-Pb dating. Consequently, systematic investigations carried out on them would possibly improve the laggard status of researches on the Lower Cambrian in South China.
Meanwhile, the widespread phosphorites in the Lower Cambrian over the Yangtze Platform are valuable as an important resource. Any phosphorite bed，on the other hand, represented a unique phosphogenetic event. Thus, correlation study conducted on these phosphorites plays an important role in constructing the stratigraphic framework for the Lower Cambrian in South China.
Moreover, the early Cambrian black shales in South China contain Ni-Mo multi-element-rich layer which extends more than 1600 km along a NE-SW strike belt on the Yangtze Platform (from estern Yunnan to the Zhuji area in Zhejiang Province). It represented an important event of mineralization as well as a significant geochemical layer. Accordingly, correlating this layer regionally and confirming its definitive position at the important sections of the Lower Cambrian in South China are also significant for building up the early Cambrian stratigraphic framework.
The early Cambrian sequences in the Songlin area have increasingly interested geologists for containing Ni-Mo-PGE ore deposits and the Zunyi biota which is equivalent to the Chenjiang biota in eastern Yunnan. The phosphorite, K-bentonite, and the Ni-Mo multi-element-rich layer occurred at the bottom of the Niutitang Formation in the Songlin area, Zunyi are potentially correlatable with those outcropped at the Meishucun section, Yunnan Province. Detailed investigations of mineralogy, geochemistry, and geochronology have been conducted on the K-bentonite occurred at the bottom of the Niutitang Formation in this area in this study. Besides, we correlated the phosphorites, K-bentonite, and Ni-Mo multi-element enriched layer in the Lower Cambrian of the Songlin area to their equivalents in South China. This study aims at correlating the corresponding sequences at the aforementioned two typical areas and constructing the stratigraphic and temporal framework for the Lower Cambrian in South China. The framework would calibrate the age of the important biota, ore deposits, and the carbon isotope excursion better and improve the subdivision and global correlation of the Lower Cambrian in South China. The following conclusions were draw.
(1) The phosphorite in the Zhongyicun Member of the Zhujiaqing Formation and its intercalated K-bentonite (Bed 5) served as the first marker bed for the stratigraphic framework of the Lower Cambrian in South China. This marker bed had been denuded in many profiles on the Yangtze Platform. In comparison with the Neoproterozoic phosphorite in Weng’an, the phosphorite in the Zhujiaqing Formation is characterized by relatively higher REE and Y contents (average 195.45×10-6 and 91.23×10-6, respectively). The K-bentonite of this marker bed is the result of acidic volcanism, and has relatively low Zr (average 196.4×10-6), Nb (average 11.86×10-6) contents and high Zr/Nb ratios (average 16.55). The SHRIMP U-Pb zircon age of the K-bentonite has been revised as 538.2 ± 1.5 Ma (Jenkins et al., 2002).
(2) The phosphorite and its closely overlying K-bentonite occurred at the base of the Niutitang Formation in the Songlin area are equivalent to the phosphorite and the overlying K-bentonite at the bottom of the Shiyantou Formation at the Meishucun section, respectively. They have been considered as the second marker bed for the stratigraphic framework in this study. The phosphorite in this unit can be distinguished from that within the Zhongyicun Member for its particularly high REE (with an average of 636.01×10-6) and Y concentrations (average 262.43×10-6). The K-bentonite of the marker bed has relative higher Zr(with an average of 318.40×10-6), Nb (average 90.68×10-6) and lower Zr/Nb ratios (average 3.61) than that in the Zhujiaqing Formation. It is the result of alkaline volcanism. The SHRIMP U-Pb zircon age of the K-bentonite is 518 ± 5 Ma.
(3) The Ni-Mo multi-element-rich layer acted as the third marker bed for the stratigraphic framework of the Lower Cambrian in South China. Abnormally high Ni and Mo contents (both about 4%) have been found in this layer in Zunyi (Guizhou Province) and northwestern Hunan. However, its accurate position in the Meishucun section is still unclear. We located a ca. 10 cm layer within the Lower Yu’anshan Formation (Bed 13) with highest Ni and Mo content (135×10-6 and 583×10-6, respectively) at this section. It should correspond to the Ni-Mo-rich layer occurring extensively in southern China. Previous works have determined a Re-Os age of 537~541 Ma for the Ni-Mo layer in Zunyi. In our opinion, however, this Re-Os age should not represent the real sedimentary age, according to the stratigraphic sequence and the related SHRIMP U-Pb zircon age.
(4) We have built up a preliminary temporal framework for the Lower Cambrian in South China. It has constrainted the lower limit age of the Ni-Mo-PGE ore deposit, the Chengjiang biota and the oldest trilobite in South China, it also preliminarily constrainted the age of the globally correlatable positive carbon isotope excursion. Combined with the internationally accepted age for Precambrian-Cambrian boundary, it implied that the position of this boundary in China should be placed at the underlying strata of the middle Zhongyicun Member (the K-bentonite at this position has an U-Pb age of 538.2 ± 1.5 Ma). According to the results of paleontology (Zhu et al., 2001; Zhu et al., 2003), the base of the Zhongyicun Member could be a reasonable choice for placement of the boundary. |
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