Mineral compositional constraints on petrogenesis and oxide ore genesis of the late Permian Panzhihua layered gabbroic intrusion, SW China

Abstract

A systematic study of mineral compositions has been carried out on stratigraphic sections through the Panzhihua intrusion, a layered gabbroic sill of late Permian age that hosts significant magmatic Fe–Ti–V oxide deposits in the Emeishan large igneous province, SW China. The intrusion, which is ∼ 19 km long and ∼ 2 km thick, is divided into a Marginal zone (MGZ), Lower zone (LZ), Middle zone (sub-zones MZa and MZb) and Upper zone (UZ) from the base upwards. Compositions of the most primitive cumulus olivine (Fo 81) and plagioclase (An 73) found in the intrusion suggest that the parental magma was similar to the Mg-rich end-member of the coeval high-Ti basalts in the region. Two reversals in terms of mineral compositions are found in the intrusion, one in the lower MZa and the other at the transition between the MZb and the UZ. The first reversal is marked by a gradual increase in the An content of plagioclase (∼ An 60 to ∼ An 70) over a ∼ 100 m vertical interval. The other is characterized by abrupt changes in the compositions of olivine (∼ Fo 28 to ∼ Fo 70), clinopyroxene (Mg# = ∼ 51 to 75) and plagioclase (∼ An 24 to ∼ An 51) over a < 20 m vertical interval. These suggest that the Panzhihua intrusion opened at least twice for recharge of more primitive magmas. Strong depletion of incompatible trace elements, such as Zr, in the cumulate sequence without corresponding enrichment in any border series is suggested to be evidence for significant loss of residual liquid at a late-stage during solidification of the intrusion. Compared to other well-studied layered intrusions, the Panzhihua intrusion contains Fe–Ti oxides crystallized at a high temperature, consistent with evidence obtained in earlier studies. Early appearance of liquidus Fe–Ti oxides in the Panzhihua magma is most likely a combined effect of a rather high degree of initial oxidation and elevated volatile fugacity.