Hydration vs. oxidation: Modelling implications for Fe–Ti oxide crystallisation in mafic intrusions, with specific reference to the Panzhihua intrusion, SW China

Recent work on the Panzhihua intrusion has produced two separate models for the crystallisation of the intrusion: (1) low-Ti, high CaO and low H2O (0.5 wt.%) parent magma (equivalent to Emeishan low-Ti basalt) at FMQ; and (2) high-Ti, low CaO and higher H2O (>1.5 wt.%) parent magma (equivalent to Emeishan high-Ti basalt) at FMQ + 1.5. Modelling of these parent magma compositions produces significantly different results.
We present here detailed f(O2) and H2O modelling for average compositions of both Emeishan high-Ti and low-Ti ferrobasalts in order to constrain the effects on crystallisation sequences for Emeishan ultramafic–mafic layered intrusions. Modelling is consistent with numerous experimental studies on ferrobasaltic magmas from other localities (e.g. Skaergaard intrusion). Modelling is compared with the geology of the Panzhihua intrusion in order to constrain the crystallisation of the gabbroic rocks and the Fe–Ti oxides ore layers. We suggest that the gabbroic rocks at the Panzhihua intrusion can be best explained by crystallisation from a parent magma similar to that of the high-Ti Emeishan basalt at moderate H2O contents (0.5–1 wt.%) but at the lower end of TiO2 content for typical high-Ti basalts (2.5 wt.% TiO2). Distinct silicate disequilibrium textures in the Fe–Ti oxide ore layers suggest that an influx of H2O may be responsible for changing the crystallisation path. An increase in H2O during crystallisation of gabbroic rocks will result in the depression of silicate liquidus temperatures and resultant disequilibrium with the liquid. Continued cooling of the magma with high H2O then results in precipitation of Mt–Uv alone.
The H2O content of parent magmas for mafic layered intrusions associated with the ELIP is an important variable. H2O alters the crystallisation sequence of the basaltic magmas so that at high H2O and f(O2) Mt–Uv crystallises earlier than plagioclase and clinopyroxene. Furthermore, the addition of H2O to an anhydrous magma can explain silicate disequilibrium texture observed in the Fe–Ti oxide ore layers.