Tuesday, August 26, 2014

Evidence of Oxygenated Marine Bottom Waters at the NeoArchean/PaleoProterozoic Boundary?!?!

Petrology And Geochemistry Of The Wangjiazhuang Banded Iron Formation And Associated Supracrustal Rocks From The Wutai Greenstone Belt In The North China Craton: Implications For Their Origin And Tectonic Setting


Wang et al


The Wutai greenstone belt (WGB) is one of the most extensively studied greenstone belts in China. Together with the Hengshan and Fuping complexes, these three associations compose the central segment of the Trans-North China Orogen (TNCO) in the North China Craton (NCC). The Wangjiazhuang banded iron formation (BIF) is located in the bottom of the Jingangku Formation of the WGB. The associated supracrustal rocks consist of meta-basalts (amphibolites), meta-felsic volcanic rocks (leptynite) and metapelites (mica schist), which have experienced amphibolite-facies metamorphism. Amphibolites are commonly intercalated with the BIF. SIMS zircon U-Pb analyses on amphibolites suggest that the Wangjiazhuang BIF was formed at ca. 2543 ± 4 Ma. Combined with most Neoarchean Algoma-type BIFs in the NCC, these features indicate that a significant tectothermal event of the NCC have occurred at ∼2.5 Ga. Mineral assemblages of the Wangjiazhuang BIF are composed of quartz, magnetite, amphibole, and minor garnet, pyrite and calcite. The precursor deposits of this BIF were likely ferric-oxyhydroxides, fine-grained carbonate oozes, silicate phases rich in Al-Ca-Mg-Fe and amorphous silica. The appearance of garnet and ferro-pargasite, high concentrations of Al2O3, HFSEs, Sc, and positive correlations among Al2O3, TiO2, HFSEs and REE indicate that there was a significant terrigenous input. Even so, the Wangjiazhuang BIF samples display distinctively seawater-like REE + Y profiles, characterized by positive La and Y anomalies and HREE enrichment relative to LREE in PAAS-normalized REE diagrams. Consistently positive Eu anomalies are also observed, which are typically from high-T hydrothermal fluids. In addition, the true negative Ce anomalies recorded in the Wangjiazhuang BIF might indicate the onset of bottom-water oxidation at the Archean-Proterozoic boundary, at least in restricted basins. Amphibolites have geochemical affinity with both MORB- and arc-like components, and the trace element characteristics of leptynites are also consistent with a subduction zone signature. These features suggest that the Wangjiazhuang BIF was deposited in a back-arc basin. Oceanic subduction, coupled with contemporary depleted mantle upwelling related to back-arc basin extension, can account for the typical interaction between these two components.

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