As part of the Archean greenstone belt, the Qian'an iron deposit is a typical banded iron formation (BIF)‐hosted iron deposit in the North China Craton. The BIF iron orebodies, together with Archean host rocks, have experienced multiphase metamorphism and plastic deformation. Folds, especially large‐scale synclines, played the most important role in the enrichment and preservation of the Qian'an BIF iron deposit. Exploration and mining reveal that orebodies in the deposit are located in the cores of folds. Structural analysis of the BIF ores suggests that rocks and minerals show obvious plasticity and tend to form types II and III folds of the Ramsay fold classification in the early stage of high temperature deformations (granulite facies), whereas type Ib parallel folds and sometimes type Ic folds are widely developed in the late stage of relatively low temperature deformations (e.g., amphibolite facies). Hand specimens of different types of folded iron ores are investigated with respect to their thickening coefficients, dimensional fabrics and lattice‐preferred orientation fabrics, to find out the characteristics and variation of folded layers, and the importance of iron enrichment during formation of different types of folds. Our studies prove that folding in Qian'an ore deposit occurred under different deformation conditions and thus led to various degrees of enrichment of magnetite. Among them, the type III folds have the most important significance to the enrichment of magnetite. Thickening coefficient analysis shows that the thicknesses of the quartz layers remain relatively constant, and those of the magnetite layers vary evidently during folding. It is concluded that the quartz‐rich layers dominate the deformation as competent layers, and magnetite‐rich layers as relatively weak and passive layers. In addition, the EBSD (Electron Back‐Scattered Diffraction) technique is applied to analyze the dimensional and crystallographic fabrics of both quartz and magnetite grains in folded BIF ores. The quartz <0001> and magnetite <001> fabrics of samples from the cores and limbs of the different types of folds show complex orientation patterns related to plastic deformation superposition. It is shown that the EBSD analysis is a very effective technique analyzing dimensional and crystallographic fabrics of deformed iron ores.
Abstract Although long-lived detachment faulting plays an important role in fluid circulation and in accommodating tectonic extension at slow-spreading oceanic ridges, it is still unclear how the fluid-enriched faults contribute to the observed seismic anisotropy in the lower crust. We investigated sheared and altered gabbros along the detachment fault zones from the Xigaze ophiolite in the southern Tibetan Plateau. Results demonstrate that the positive feedback between fluid circulation and shearing, linked by dissolution-precipitation creep of amphibole, resulted in fluid enrichment during strain localization along the fault zones. Based on this shearing-enhanced fluid circulation model, our calculations of the seismic properties show that amphiboles (de)formed by dissolution-precipitation creep along the fault zones largely contribute to the seismic anisotropy (P and S waves) and S-wave delay time in the lower crust at slow-spreading ridges, with the polarization directions of fast shear waves being subparallel to the ridges. The strength of resulting seismic anisotropy is largely a function of crustal thickness, fault zone attitude, and metasomatism intensity. This study provides a novel explanation for the origin of seismic anisotropy in the lower oceanic crust at slow-spreading ridges. The conclusion may also have implications for the origin of seismic anisotropy at fast-spreading ridges where there are high melt supplies.
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