科研之友
微信
新浪微博
Facebook
分享链接
摘要
Banded iron formations (BIFs) are iron- and silica-rich chemical metasedimentary rocks that span much of the Archaean and early Palaeoproterozoic rock record. Given that BIFs precipitated from ambient seawater, they provide an important archive of Earth's ancient marine environments. Within the Slave craton occurs one of the largest and well-preserved Achaean turbidite basins in the world. Interstratified within the greywacke-mudstone turbidites is BIF that is commonly associated with ferruginous sediments and locally with a felsic tuff bed. Field investigations were conducted at four BIF locations from west to east across the Slave craton. These BIFs are composed of micro- and mesobands of quartz and magnetite, with various amounts of metamorphic grunerite, ferro-actinolite, ferrohornblende as well as greenalite and stilpnomelane, the latter two are interpreted as retrograde metamorphic phases. As the BIF was deposited in a deep-water turbidite-dominated basin(s), the chemical component was occasionally diluted by clastic detritus, as illustrated by highly variable A1203 content, and A1203/Ti02 and Cr/Ni that are similar to the clastic sediments. On a layer-by-layer basis, the iron content in the BIF ranges from as low as 4 wt.% and up to 75 wt.% Fe203, whereas the Si02 content ranges from 20 wt.% to as high as 94 wt.%. This, together with a hyperbolic germanium/silicon distribution, suggests that continentally-sourced silica comprised the background precipitation, which was periodically interrupted by pulses of hydrothermally-sourced iron. High-temperature hydrothermal fluids contributed, with an average shale-normalized positive Eu anomaly of 1.92, which indicates input of reduced Eu2* to the ambient seawater. The BIF shows characteristic aqueous fluid-derived rare earths + yttrium fractionation (Pr/Yb < 1) and positive La and Y anomalies. A negative Ce anomaly in parts of the BIF indicates the presence of an oxidizing agent able to fractionate Ce3. to Ce4+. The age and trace element signature of the felsic tuff is similar to arc-type volcanic rocks, potentially linking the BIF and turbidite basin (s) to the Defeat Suite plutons (ca. 2630-2620 Ma). The slow chemical rainout behind BIF deposition presumably took place on the basin plain adjacent to a steep submarine ramp where turbidite currents and slump-generated debris flows, sourced and triggered by the tectonically active arc-terrain, episodically delivered clastic sediment.
- 出版日期2017-5
