Liquid immiscibility in the Panzhihua mafic layered intrusion: Evidence from melt inclusions in apatite.

The Panzhihua, Hongge and Baima mafic-ultramafic layered intrusions in Panzhihua-Xichang region host large Fe-Ti-V oxide deposits and are parts of the similar to 260Ma Emeishan large igneous province. Two types of melt inclusions can be identified in apatite from the leucogabbro in the middle zone of Panzhihua intrusion in terms of color; dark and light ones. Dark melt inclusions are dark brown or black in color and range in sizes from 10 mu m to 60 mu m. They commonly behave as circular or negative crystal shapes. Daughter minerals within the dark inclusions include clinopyroxene, hornblende, plagioclase and magnetite. Light inclusions are white or slightly green and range in size from 5 mu m to 60 mu m. Daughter minerals in the light inclusions include plagioclase, hornblende, clinopyroxene and minor magnetite and quartz. After being heated to 1080 similar to 1200 degrees C, light inclusions become homogenous glasses. The glasses are Si-rich, with an average composition of 69.7% SiO2, 0.24% TiO2, 14.5% Al2O3, 2.76% FeO, 0. 64% MgO, 5. 14% CaO, 2. 82% Na2O, 2. 26% K2O and 0. 25% P2O5. Dark inclusions are not homogenized after heating. Their averaged compositions that are estimated based on electron probe analysis and mass balance calculation are Fe-rich, with 42. 4% SiO2, 1.43% TiO2, 8. 83% Al2O3 20.5% FeO, 5.46% MgO, 16.3% CaO, 1. 11% Na2O, 0.30% K2O and 1.41% P2O5. Coexistence of Fe-rich and Si-rich melt in the apatite of the leucogabbro unit of the Panzhihua intrusion indicates that liquid immiscibility may have occurred in the evolved magmas. Layering of minerals may be contributed to the liquid immiscibility and gravitational differentiation. In this fashion, Fe-rich melts settled down to form the thick Fe-Ti-V oxide ore body and melagabbro in the lower part of the Panzhihua intrusion, whereas Si-rich melts move upwards to form the felsic vein, lens and leucogabbro in the upper part.