Key factors controlling the accumulation of the Fe-Ti oxides in the Hongge layered intrusion in the Emeishan Large Igneous Province, SW China

摘要

The Hongge layered intrusion hosts the largest Fe-Ti-V oxide ore deposit in the central part of the Emeishan Large Igneous Province, SW China. It is divided into Lower Zone (LZ), Middle Zone (MZ) and Upper Zone (UZ) from the bottom to the top. For the LZ, relatively higher Cr (250-3000 ppm) and Ni (50-200 ppm) contents of clinopyroxene, lower εNd_(259Ma) and higher (~(87)Sr/~(86)Sr)_(259Ma) values (- 2.82 to - 0.07 and 0.7057-0.7076, respec-tively) and plenty of hornblende demonstrate a more primitive parental hydrous magma. In contrast, relatively low Cr (<150 ppm) and Ni (<100 ppm) of the clinopyroxene, high εNd_(259Ma) and low (~(87)Sr/~(86)Sr)_(259Ma) values (-0.32 to 0.49 and 0.7058 to 0.7063, respectively) suggest that the MZ rocks were formed from more evolved, Fe-Ti enriched, and weakly contaminated magmas. MELTS calculation indicates that H_2O played a key role in the early crystallization of magnetite and occurrence of abundance of hornblende in the LZ. Whereas, the critical factor for the formation of the massive Fe-Ti oxide layers at the bases of the cyclic units in the MZ is coupling of early crystallization of Fe-Ti oxides from the Fe-Ti highly enriched magma and gravitational resorting and settling of the Fe-Ti oxides. Such magmas were produced by fractional crystallization of olivine and pyroxene in deep-seated magma chamber, and compositional reversals of the cyclic units show repeatedly replenishment of such magmas from deep level. Extensively fractionation during the formation of the LZ and MZ resulted in phosphorus saturation and the formation of the apatite magnetite gabbros in the UZ.