Immiscible Fe- and Si-rich silicate melts in plagioclase from the Baima mafic intrusion (SW China): Implications for the origin of bi-modal igneous suites in large igneous provinces

Abstract

The Emeishan large igneous province (ELIP) in SW China is characterized by voluminous high-Ti and low-Ti basalts and spatially associated Fe-Ti oxide-bearing mafic-ultramafic and syenitic/granitic intrusions. The Baima layered mafic intrusion in the central part of the ELIP is surrounded by syenitic and granitic rocks and contains a Lower Zone of interlayered Fe-Ti oxide ores, troctolites and clinopyroxenites and an Upper Zone of isotropic olivine gabbros and gabbros (UZa) and apatite gabbros and Fe-Ti-P oxide ores (UZb). Polycrystalline mineral inclusions, for the first time, were observed in primocryst plagioclase from the basal part of the UZa through to the top of the UZb and consist mostly of clinopyroxene, plagioclase, magnetite, ilmenite and apatite with minor orthopyroxene, sulfide and hornblende. These minerals are commonly anhedral and form irregular shapes. Daughter plagioclase usually crystallizes on the walls of host primocryst plagioclase and has An contents typically 3–6 An% lower than the host plagioclase. Daughter clinopyroxene has similar Mg# but lower TiO2 and Al2O3 contents than primocryst clinopyroxene. These polycrystalline mineral inclusions are considered to crystallize from melts contemporaneous with host plagioclase. The compositional differences between daughter and primocryst minerals can be attributed to equilibrium crystallization in a closed system of the trapped melt inclusions in contrast to fractional crystallization and possible magma replenishment in an open system typical for primo-cumulates of large layered intrusions. Heated and homogenized melt inclusions have variable SiO2 (33–52wt%), CaO (7–20wt%), TiO2 (0.1–12wt%), FeOt (5–20wt%), P2O5 (0.2–10wt%) and K2O (0–2.2wt%). The large ranges of melt compositions are interpreted to result from heterogeneous trapping of different proportions of immiscible Si-rich and Fe-Ti-rich silicate liquids, together with entrapment of various microphenocrysts. The separation of micrometer-scale Si-rich melts at least started from the lower part of the Upper Zone. We thus infer that the Baima high-Ti basaltic magmas evolved into the field of immiscible Fe-rich and Si-rich melts. Prolonged fractional crystallization of olivine, plagioclase and clinopyroxene from the Fe-rich melts formed gabbro and olivine gabbro in the UZa, before the melts became saturated in Fe-Ti oxides and apatite and formed apatite-rich Fe-Ti oxide ores in the UZb. Immiscible Si-rich melts migrated upwards and coalesced to form fayalite syenite around the Baima intrusion. Silicate liquid immiscibility may have played an important role in the petrogenesis of Fe-Ti oxide-rich layered mafic intrusions and syenitic rocks in the ELIP and elsewhere. This mechanism may be common and may explain the traditionally thought bi-modal assemblages of large igneous provinces in intra-continental extensional settings.