Amphibole geochemistry of the Baima layered intrusion, SW China: Implications for the evolution of interstitial liquid and the origin of Fe-Ti oxide ores

Abstract

The Baima layered intrusion hosting a giant Fe-Ti oxide ore deposit in the Panxi region (SW China) is mainly composed of troctolite and olivine gabbro in the lower zone (LZ) and isotropic gabbro, olivine gabbro and apatite olivine gabbro in the upper zone (UZ). Amphibole, as a late-stage crystalized phase, occur throughout the Baima intrusion in several forms and can be used to trace the evolution of interstitial melt. The brown amphibole commonly occurs as rims on Fe-Ti oxides, interstitial granular grains, and inclusions in olivine and clinopyroxene. They are abundant in the Fe-Ti oxide ore layers. The different occurrences have similar major and trace element compositions (1.60–4.71 wt% TiO2, 68–160 ppm REE; 2.35–5.25 wt% TiO2, 55–159 ppm REE; and 5.24–5.33 wt% TiO2, 158–177 ppm REE, respectively). They crystallized in the magmatic stage (948–1033 °C, 954–1041 °C, and 983–1015 °C, respectively). The olive amphibole commonly occurs as granular grains at the base of the UZ. These crystals have 0.04–1.17 wt% TiO2 and 24–80 ppm REE, which are lower than those of the brown amphibole, and they crystallized at 634–755 °C. The green amphibole commonly occurs as granular grains, with <1.59 wt% TiO2, 5–144 ppm REE, and a formation temperature of 649–814 °C.The higher REE contents of the brown amphibole compared to the olive amphibole suggest that immiscibility occurred in the interstitial melt at the late magmatic stage of the Baima intrusion. The absence of negative Ti anomalies in the brown amphibole and their textural relationship indicate that the Fe-Ti oxides crystallized at almost the same time as the brown amphibole. Both the brown amphibole and the Fe-Ti oxides crystallized from an interstitial, immiscible Fe-rich liquid and then formed the Fe-Ti oxide ores. The evolved Si-rich melt migrated upward and mixed with the upper melts, which ended the olivine crystallization at the base of the UZ. The olive amphibole crystallized from the mixed melts. The green amphibole exhibits various REE patterns, suggesting secondary alteration.