The formation of P-rich Fe–Ti oxide ore layers in the Taihe layered intrusion, SW China: Implications for magma-plumbing system process

Abstract

The Taihe intrusion is one of the layered intrusions that host giant Fe–Ti oxide deposit in the central part of the Emeishan Large Igneous Province, SW China. Different from the other layered intrusions, the Taihe intrusion has a massive Fe–Ti oxide ore layer at the top of the Lower Zone (LZ), particularly, the rocks of both Middle and Upper Zones contain as much as 5–12% apatite, and the most important Fe–Ti oxide ore layers (apatite magnetite clinopyroxenite) occur at the bases of cyclic units in the Middle Zone (MZ), rather than in the Lower Zone (LZ). High Cr and Ni concentrations of clinopyroxene (65–263ppm and 66–83ppm, respectively) and high An contents of plagioclase (79–80) of the LZ rocks indicate their more primitive parental magmas. The clinopyroxene and plagioclase of the MZ rocks are characterized by low Cr and Ni contents (1–21ppm and 0.4–10ppm, respectively) and low An (51–67), indicating more evolved parental magma. The Fe–Ti oxide ore layers in the MZ are not only high in Fe and Ti, but also enriched in P2O5 and rare earth elements, indicating an unusual Fe–Ti–P-rich parental magma. It is speculated that the MZ represents a later intrusion of Fe–Ti–P-rich magmas that were produced by the Fe–Ti enriched magma from a deep-seated magma chamber mixing with the extensively evolved P-rich magma in a middle level magma chamber. Early crystallization and cumulation of Fe–Ti oxide and apatite together with clinopyroxene from the Fe–Ti–P-rich magmas from the middle level magma chamber resulted in the formation of Fe–Ti oxide ore layers in the MZ. Fe–Ti–P-rich magma and apatite magnetite clinopyroxenites occur in the base of cyclic units, whereas apatite gabbros are presented at the upper parts of each cyclic unit. The Upper Zone (UZ) is the product of the crystallization of highly differentiated magma, consists chiefly of apatite gabbro and minor intercalated apatite clinopyroxenite.