| 其他摘要 | 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. The LZ is characterized by containing as high as 5-15 modal% hornblendes, the MZ is defined by occurrence of massive Fe-Ti oxide ore layers, and the UZ is marked by appearance of abundant euhedral apatite. For the LZ, relatively higher Cr (250-3000 ppm) and Ni (50-200 ppm) contents of clinopyroxene demonstrate a more primitive parental magma which was produced by ~49.1% fractional crystallization of olivine and pyroxenes in deep-seated magma chamber. Meanwhile, the lower εNd259Ma (-2.82 to -0.07), higher (87Sr/86Sr)259Ma (0.7057-0.7076) values and plenty of hornblende in the LZ indicate that the parental magma assimilated ~15% footwall meta-sandstone and introduced ~1.5 wt% external water when it intruded into the shallow magma chamber. H2O played a key role in the early crystallization of magnetite, absence of plagioclase and occurrence of abundant hornblende in the LZ. However, the percentage of early crystallized magnetite is only 9.6%, which is not enough to form massive ore. In contrast, relatively low Cr (<150 ppm) and Ni (<100 ppm) contents of the clinopyroxene, high CaO content of the olivine, as well as high εNd259Ma (-0.32 to 0.49) and low (87Sr/86Sr)259Ma (0.7058 to 0.7063) values suggest that the MZ rocks were formed from more evolved, Fe-Ti enriched, and weakly contaminated magmas. MELTS calculations indicate that such magmas experienced ~56.6% fractional crystallization of silicate minerals in deep-seated magma chamber, and the early crystallized magnetites are up to 19.2% in the shallow magma chamber. Moreover, compositional reversals of the cyclic units show repeatedly replenishment of such magmas, which supports for the bulk crystallization of magnetite. Due to gravitational resorting and settling, abundant magnetite settled down and concentrated in the bottom of each cycle unit of the MZ, and then the massive ore layers formed. The lowest Cr and Ni contents in the clinopyroxene (Cr<10 ppm，Ni<3 ppm) of the UZ demonstrate that the parental magma of the UZ experienced the most extensive fractional crystallization. The high εNd259Ma (-0.98 to -0.5) and low (Th/Yb)PM (1.96-4.73) values demonstrate that the parental magma of the UZ has experienced weak crustal contamination. Extensively fractionation during the formation of the LZ and MZ resulted in phosphorus saturation of the residual magma. When the parental magma of the UZ mixed with such P2O5-saturated magma, apatite crystallized and cumulated along with silicates and Fe-Ti oxides to form the apatite magnetite gabbros in the UZ. The formation of the hornblende and phlogopite in the Hongge intrusion is associated with mantle-derived magma. The calculations using electron microprobe data indicate that the hornblende crystallized at the temperature of 1000~1100°C, the pressure of lower than 2.2 kbar and the oxygen fugacity of NNO-0.55 to NNO+0.73. Combined with the MELTS calculation, we estimated that the Fe-Ti oxide ore layers of the Hongge intrusion crystallized at the temperature of 1100~1165°C and the oxygen fugacity of higher than NNO+0.73. The ratios of Fe3+/(Fe3+/Fe2+) of the hornblende as well as Fe3+/Fe2+ and Mt/(Mt+Ilm) of the whole rock decrease upwards in each cycle unit in the LZ and MZ of the Hongge intrusion, whereas, V2O3 contents of the magnetite increase upwards, indicating that oxygen fugacity decreases upwards during fractional crystallization of magnetite. However, the whole-rock Fe3+/Fe2+ and Mt/(Mt+Ilm) ratios increase as the magnetite V2O3 contents decrease upwards in the cycle unit IX of the UZ, which is opposite to those in the LZ and MZ, indicating that oxygen fugacity increased upwards in the cycle unit IX which is probably due to the P2O5 enriched parental magma of the UZ. |
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