Depositional environment and genesis of the Paleoproterozoic Astingbulake banded iron formations from the Quruqtagh Block, northeast Tarim Craton

Abstract

As common marine chemical sedimentary rocks in sequences of Precambrian cratons worldwide, banded iron formations (BIFs) are key repositories of the early Earth’s geological and environmental evolution. The Tarim Craton is one of the major cratons in China and Central Asia, however, little attention has been paid on the BIFs of the Tarim Craton. Here we present new petrographic, bulk rock geochemical and Sm-Nd isotopic data from the poorly-studied Paleoproterozoic Astingbulake BIF in the Quruqtagh Block, north Tarim. The BIF is hosted in the Plaeoproterozoic Xingditage Group and exhibits typical banded structure with layered magnetite- and quartz-rich bands. The elemental composition of the Astingbulake BIF indicates that it is a chemical sediment with a low terrigenous input, but terrigenous materials do not have significant control on the all geochemical budgets of BIF. The positive Eu anomalies (Eu/Eu* = 1.31) and chondritic Y/Ho ratios of 26.4 to 31.4 indicate a hydrothermal fluid contribution to the BIF. The significant negative εNd(t) values (average of −8.0) suggest significant input of older and isotopically evolved continental flux. Therefore, combined with rare earth elementandyttrium and Nd isotope signatures, a mixture of significant contribution from continental-sourced iron source with minor hydrothermal source was suggested. The Astingbulake BIF shows affinity to Superior–type BIFs and was likely deposited on a continental shelf slope where sediments extended from shelf to deep sea settings and distal from the hydrothermal fluid systems. The absence of negative Ce anomalies, coupled with the positive Eu anomalies, indicate deposition in suboxic to anoxic environments. The deposition of the Astingbulake BIF indicates that ferruginous (anoxic and iron-rich) seawater redox conditions were still established in the Paleoproterozoic Tarim basin.