## 6 Discussion
### Origin of the #50 Phlogopite Zone
#### 6.1.3 Origin of #50 Low-Cr/Cr-Poor Phlogoptite Outer Rims  
The low-Cr/Cr-poor zones of the #50 phlogoptite are typically observed as the outer rims of crystals, which constitute a significant portion of the phlogoptite zone in the region. In the process of magmatic emplacement, the magmatic composition undergoes continuous evolution due to magmatic fractionation and assimilation, as well as contamination by crustal wall rocks. The low-Cr/Cr-poor outer rims are rich in Al, Ba, Ti, F, Zr and Nb and depleted in Mg, Cr, Sr and Ni; this pattern is consistent with the composition of groundmass phlogoptite grains crystallized directly from the host-kimberlite magma [4; 21; 23; 26; 63]. Cr-spinel inclusions occur frequently within the zone, suggesting that the majority of the zone was formed after Cr-spinel fractionation, which aligns with the low chromium content characteristics exhibited by the zone [23; 67]. However, spinel inclusions also appear in the high-Ti-Cr zone (Figure 4). Therefore, differentiation via Cr-spinel crystallization may not be the sole cause of the presence of low-Cr/Cr-poor outer rims. Due to the significant variation in the composition of the low-Cr/Cr-poor rims, particularly characterized by the extreme depletion of Cr and enrichment of Ti, the progressive decrease in V/Sc ratios is combined with the decreasing Ni levels in the low-Cr/Cr-poor phlogoptite (Figure 7) to document an increase in the oxygen fugacity ratio (fO\({}_{2}\)) [68]; this increase in fO\({}_{2}\) could be one of the factors contributing to the influx of Ti from residual magma into the zone in significant quantities [69]. Furthermore, considering the resemblance of the high-Ti-Cr zone to magmatic mica in polymictic preccias and mica overgrowths found in other mantle xenoliths, it is plausible that the high-Ti-Cr zone crystallized within the mantle environment and might have been influenced by intermittent injections of early kimberlite magma. Phlogoptite is considered a magmatic mineral in kimberlite magmas, and the crystallization process may begin in the lithospheric mantle and continue into the crust [9; 23]. The conditions for phlogoptite crystallization are limited to pressures of <20 kbar and are most conducive to formation at ~10 kbar, i.e., in the crust [70]. The low-Cr/Cr-poor zone, as the outermost zone of phlogoptite, and its similarity to groundmass phlogoptite in composition suggest that it was formed in a near-crustal environment [9]. Based on the distinct crystallization environments, these minerals may not have originated from the same batch of melted pulses [9].  
Furthermore, a partial evolutionary trend toward the composition of biotite can be observed in the low-Cr/Cr-poor outer rims of the #50 phlogoptite (Figure 8). Al-rich biotite megacrysts (type I biotite) surrounded by late groundmass mica were found for the first time in mica kimberlites in South Africa (Swartruggens, Saltpeerpan and Monteleo) and Canada (Upper Canada Mine, Ontario) [71]. Phlogoptite-related biotite megacrysts have also been found in the group II kimberlite New Elands, Sover Mine, Bellsbank, South Africa [26]. The occurrence of closely related phlogoptite-biotite megacrysts has also been documented in the Mengyin kimberlite. It is speculated that the appearance of these megacrysts is related to metasomatic metamorphism of the wall rock during the late magmatic transformation or ascent, and such megacrysts may have formed through the reaction of an H\({}_{2}\)O-CO\({}_{2}\)-rich fluid (or melt) in the late magmatic period with nearby iron-rich wall rock or crustal xenoliths [69].  
Figure 7: (**a-l**) Trace element covariation diagrams for phlogoptite grains from the southern Liaoning kimberlite.