Mineral Chemistry of Melanite from Calcitic Ijolite, the Oka Carbonatite Complex, Canada： Implications for Multi-Pulse Magma Mixing
摘要
Ti-rich garnet is found within calcitic ijolite from the Oka carbonatite complex in Canada, which is characterized by 58%–73% andradite component（2.12 wt.%–4.18 wt.% TiO2） and classified as melanite. The garnet displays complex zoning and contains abundant high field strength elements（HFSEs） and rare earth elements（REEs）. Three groups（Ⅰ, Ⅱ, Ⅲ） have been identified based on their petrographic nature. Compared to groups Ⅱ and Ⅲ, Group Ⅰ garnet cores contain higher TiO2, Mg O, HFSE, and REE and lower SiO2 abundances. The distinct chemical and petrographic signatures of the investigated garnets cannot be attributed to simple closed system crystallization, but they are consistent with the multi-pulse magma mixing. Combined with previously reported U-Pb ages for apatite from the calcitic ijolite, at least three stages of magma evolution and subsequent mixing have been involved in the generation of calcitic ijolite at Oka. The early-formed melt that generated Group I garnet core was later mixed with at least two small-volume, more evolved melts. The intermediate stage melt formed the remaining garnet along with some pyroxene, calcite, nepheline, and apatite at 127±3.6 Ma. The youngest, most evolved melt generated the majority of pyroxene, calcite, nepheline, and apatite within the calcitic ijolite at 115±3.1 Ma.