湘西—黔东早震旦世大塘坡组锰矿中放射虫的发现及环境意义

The Discovery and Environmental Significance of Radiolaria from Manganese Deposits in the Early Sinian Datangpo Formation in Western Hunan and Eastern Guizhou

摘要:

放射虫是确定沉积环境的生物标志之一。1982年笔者从事扬子区西部震旦系上统硅质岩岩相研究时,首次发现贵州三都—松桃之间的硅质岩中赋存有大量泡沫虫目和罩笼虫目。1987年笔者在研究湘西—黔东大塘坡组锰矿沉积相时,又在贫菱锰矿中发现有少量罩笼虫目与泡沫虫目,这是迄今为止发现放射虫的最老层位。本文以大塘坡组锰矿为例阐述锰质岩内的放射虫特征及其与硅质岩内放射虫的比较。过去有关放射虫的文章,多数着重于放射虫单体的研究,但目前从事岩相古地理研究的地质工作者,常用的工具是显微镜,为此本文结合大塘坡组锰矿中已发现的泡沫虫目和罩笼虫目,详细阐述和对比它们在薄片下和电镜下的特征。

Abstract:

A great deal of Spumellaria and Nassellaria were found by the author (H. Huang) in 1982 in Late Sinian silicalites between Sandu and Songtao, Guizhou and later in 1087, small amounts of them were recognized again by the authors in carbonate manganese ores from the Early Sinian Datangpo Formation in western Hunan and eastern Guizhou, which is the oldest horizon for the occurrence of radiolaria so far. The manganese deposits in the Datangpo Formation which are placed in between the Early Sinian Nantuo Moraine Bed ("Large Moraine") and the Tiesi'ao Formation ("Small Moraine")are widely distributed over the area between Sichuan, Hunan and Guizhou Provinces. The manganese orebodies are stratiform, stratoid and lentieular, trending from NE to SW. The ores may be divided into two types: massive and banded, among which the latter is dominated. The ore beds not only have well-developed horizontal fine beddings but also contain abundant mietoplankton fossils including radiolaria which can be used for .indicating depositional environments. The manganese deposits were formed in a deeper restricted marine basin with a water depth of about 100 m or more. The contents of radiolarian skeletons preserved in manganese ores range generally between 2 and 5 percent. Nassellaria occupies first place and Spumellaria comes second ;conversely, Spumellaria occupies first place and Nassellaria comes second in Mn-bearing siliceous shale (rhodochrosite-poor) in which the contents of radiolarian skeletons may locally reach up to 25 to 30 percent. Nassellaria is mostly 0.09 to 0.13 mm in height and shows a wide range of shapes., lampshade——like, tower-like and umbrella-like, whereas Spumellaria, round to elliptic, generally 0.02 to 0.05mm in diameter. The individuals tend to be smaller as compared with those in the younger Cambrian, Permian, Triassic and Cretaceous siliceous rocks. Both Nassellaria and Spumellaria usually show oriented distribution along the bedding planes and occasionally are enriched to form bands or laminae interbedded with rhodoehrosite bands, thus resulting in the formation of well-defined banded or laminated ores. The ra-diolaria is characterized by well-preserved skeleton, well-defined outline and clear reticulated and acie-ular structures. Part of the skeletons were dissolved and replaced completely or in varying degrees by rhodochrosite and dolomite, but its outline remains retained. The preserved status of radiolarian mor-phology depends mainly upon epidiagenesis.