中國四川白馬火成雜岩之實驗岩石學研究與鐵-鈦-釩-氧化物礦床成因隱示

Abstract

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The Baima igneous complex (BIC) consists of a cumulate layered gabbroic unit, a thick Fe-Ti oxide ore zone and an isotropic peralkaline quartz syenite. The BIC contains 1150 Mt of Fe, 44.8 Mt of Ti and 2.85 Mt of V and is one of at least five world class orthomagmatic oxide deposits in the Emeishan large igneous province. The formation of the oxide deposit of the BIC is debated. There are two different models on the formation of Fe-Ti-V oxide ore in the Baima intrusion. One model suggests fractional crystallization of a basaltic parental magma led to the early crystallization of Fe-Ti oxide minerals whereas the second model suggests silicate immiscibility during the early stages of the evolution of a ferro-basaltic magma. The purpose of this study is to determine if a parental magma similar to high-Ti Emeishan basalt can produce all three rock-types observed in the BIC by experimental petrology at atmospheric pressure and mid-crustal pressure (i.e. 1 GPa). The experimental results at atmospheric pressure show that the liquidus temperature and solidus temperature of the basaltic melt are estimated to be 1303 oC and 1120 oC. The crystallization sequence is determined as: titanomagnetite, plagioclase (An65) and pyroxene (Wo43-47En32-45Fs11-23). The residual glass composition, represented by the quenched glass, evolves from lower SiO2 (SiO2 = ~45 wt %) values to higher SiO2 values (SiO2 = ~60 wt %) with corresponding decrease in Ti, Fe, Mg, Ca and increase of Na and K. The experimental results at 1 GPa show that the liquidus temperature is ~1220 oC, whereas the solidus temperature is estimated to be 980 oC. The crystallization sequence of the basaltic melt at 1 GPa is determined to be: titanomagnetite, pyroxene (Wo37-46En33-38Fs18-25) and plagioclase (An36). The most evolved glass compositions in the low and high pressure experiments are similar to the enclaves of the Baima syenitic unit but only the low pressure experiments could reproduce the mineral compositions observed in the Baima gabbroic unit. Thus, the liquidus mineral is iron-titanium oxide which is consistent with the observation of basal oxide-ore formation in the gabbroic unit. The low pressure results of this study indicate that early crystallization of Fe-Ti oxides will occur assuming a geologically reasonable starting material and that the residual liquid is silicic. The direct implication is that the oxide deposits and spatially associated granitic rocks formed together by crystallization from a basaltic parental magma. Furthermore, it is suggested that crystallization of a typical high-Ti basaltic parental magma can produce world-class giant magmatic oxide ore deposits and that the occurrence of some alkaline granites may be an indicator of their presence.