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 ultra-mafic -mafic layered intrusions.Modelling is consistent with numerous experimental studies on ferro-basaltic 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.

http://www.sciencedirect.com/science/article/pii/S1674987113000376

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.