Microstructural constraints for the formation of net-textured Fe-Ti oxide ores of the Baima layered intrusion in the Emeishan large igneous province, SW China

Abstract

The mafic–ultramafic layered intrusions in the Emeishan large igneous province (LIP) host significant amounts of Fe, Ti and V resources. The Fe-Ti oxide ore layers hosted in these layered intrusions are variable in thickness and it is enigmatic how the large amounts of Fe-Ti oxides are accumulated to form the Fe-Ti oxide ore layers. The Fe-Ti oxide ore layers are mainly composed of net-textured ores, however, the processes to form the net-textured ores have not yet been well constrained due to lack of diagnostic microstructural evidence of Fe-Ti oxides in the ores. Here we use the electron back-scattered diffraction (EBSD) technique to characterize the microstructures of Fe-Ti oxides in the net-textured ores of the Baima layered intrusion in the Emeishan LIP, e.g., crystallographic orientation and neighbor-/random-pair misorientation, and to assess the formation model of net-textured ores. The net-textured ores are composed of silicate crystals (olivine, and minor plagioclase and clinopyroxene) isolated by interconnected matrix composed of Fe-Ti oxides (titanomagnetite and ilmenite) and minor sulfides. It is likely that the Fe-Ti oxides simultaneously crystallized from a common Fe–Ti–(P)-rich melt, but postdated olivine and other cumulus minerals. The compaction induced by the gravity of overlying cumulus minerals only caused weak intragrain deformation of Fe-Ti oxides so that the Fe-Ti oxides did not experience intense dynamic recrystallization. Instead, the relatively high temperature and water-rich condition facilitated the operation of static recrystallization, resulting in the straight or smoothly curved grain boundaries of Fe-Ti oxides. The neighbor-pair misorientation patterns of Fe-Ti oxides show the preference for low-angle misorientation, supporting the idea that adjacent crystallized Fe-Ti oxides were rotated to coincident orientation in a melt. This study provides new microstructural evidence for the presence of immiscible Fe-rich melt from which the Fe-Ti oxides crystallized to form the net-textured ores.