Petrogenesis of the Baima Fe-Ti-(V) oxide-bearing layered intrusion in the Emeishan large igneous province, SW China

Abstract

Several large layered mafic-ultramafic intrusions of the ~260 Ma Emeishan large igneous province, SW China host important Fe-Ti-(V) oxide deposits. Being the second largest among them, the Baima intrusion is composed of the Lower and Upper Zones. The Lower Zone is characterized by interlayered Fe-Ti oxide ore, troctolite and clinopyroxenite. The Upper Zone consists of subzone A (UZa) of isotropic olivine gabbro and gabbro, and subzone B (UZb) of apatite gabbro and Fe-Ti-P oxide ore. The crystallization order of the minerals is olivine → plagioclase → clinopyroxene → Fe-Ti oxides. Compositional variations of olivine and plagioclase, particularly Sr isotope profile of plagioclase (87Sr/86Sri = 0.70312-0.70510) suggest that the Baima intrusion formed in an open and heterogeneous magma chamber that was periodically recharged by compositionally similar magmas. In each replenishment cycle, the magmas underwent progressive crustal contamination and fractional crystallization.

Plagioclase from the lower part of the UZa through the top part of the UZb contains polycrystalline mineral inclusions. These inclusions consist mostly of clinopyroxene, plagioclase, magnetite, ilmenite and apatite. Daughter minerals have different compositions from primocryst minerals, indicating that the inclusions represent contemporaneous melts when plagioclase crystallized. Homogenized melt inclusions have variable SiO2 (33 to 52 wt.%), TiO2 (0.1 to 12 wt.%), FeOt (5 to 20 wt.%), P2O5 (0.2 to 10 wt.%) and K2O (0 to 2.2 wt.%) contents, suggesting a mixed origin of Fe- and Si-rich silicate liquids. We thus infer that the Baima intrusion formed from fractional crystallization of the Fe-rich melts. The Si-rich counterparts rose upwards and coalesced before they formed fayalite syenite around the Baima intrusion.

Polycrystalline mineral inclusions within cumulus olivine in the Lower Zone are spherical to subspherical and composed dominantly of titanomagnetite and ilmenite with minor apatite, hornblende, phlogopite and pyrrhotite. Titanomagnetite within the inclusions have major and trace element compositions similar to interstitial titanomagnetite. Thus, these inclusions are considered to have formed from early trapped Fe-oxide liquids and that the interstitial Fe-Ti oxides were also crystallized from such liquids. The trapped liquids are estimated to have 82.1 to 59.6 wt.% FeOt, 11.4 to 18.5 wt.% TiO2, 2.69 to 6.12 wt.% Al2O3, 1.40 to 4.47 wt.% MgO, 0.87 to 4.93 wt.% SiO2 and ~1 wt.% volatiles.

The olivine, clinopyroxene and titanomagnetite from the Lower Zone shows disequilibrium Fe isotopic fractionation of Δ56FeCpx-Ol = +0.06 to +0.22‰, Δ56FeMt-Ol = +0.12 to +0.27‰ and Δ56FeMt-Cpx = +0.00 to +0.20‰. This disequilibrium can be best explained by crystallization of silicates and titanomagnetite from two immiscible liquids. Continuous segregation of the Fe-oxide melts affected the Fe isotopic composition of the Fe-rich silicate melts and thus the Fe isotopes of silicates crystallized. The effect of fO2 on the crystallization order of titanomagnetite and ilmenite from the Fe-rich oxide melts gave rise to the disequilibrium Fe isotope fractionation between titanomagnetite and olivine/clinopyroxene. This study indicates that liquid immiscibility is an important process in the differentiation of basaltic magmas and Fe-Ti oxide mineralization.