| Popis: |
The ca. 1.88 Ga Sokoman continental margin-type iron formation (IF) was studied by detailed mineral-chemical analyses of drill core to better understand tectonically stable, shallow-shelf marine environments of the late Paleoproterozoic. Suboxic (Fe-oxide-rich, including paragenetically early hematite) and anoxic (Fe-silicate/carbonate-rich) mineral paragenetic pathways operated during IF deposition; post-depositional alteration beyond late diagenesis/metamorphism was negligible. Chemostratigraphic variations in detrital element, rare earth element and yttrium (REE+Y), and redox-sensitive and nutrient-type trace element parameters, evaluated within a sequence-stratigraphic framework, were controlled by a combination of base-level fluctuations, terrigenous input, basin redox stratification, and microbial activity. The REE+Y in the Sokoman IF, like contemporaneous IF localities, exhibit several features reminiscent of modern seawater (LREE-depletion, small negative Ce anomalies, small positive La, Gd, and Y anomalies), but also strong positive Ce anomalies. This study reveals that the redoxcline was very shallow and likely spatially dynamic, producing separate Mn- and Fe-oxide shuttles controlled by different microbes. During Sokoman IF deposition, continental weathering of predominantly felsic Archean rocks under an oxygen-poor atmosphere and locally arid conditions limited the terrestrial supply of redox-sensitive and nutrient-type elements into the ocean, thus restricting the spatial extent of primary productivity to the photic zone of shallow-marine environments. |
