4-1 Texture of minerals
The Fig. 4-1(a) shows that first crystallized mineral, iron-titanium oxide, encloses glass. The Fig. 4-1(b) shows that pyroxene encloses both glass and iron-titanium oxide.
They are similar to the textures of Fe-Ti oxides in the thin-section of the rocks of the Panzhihua intrusion described by Pang et al. (2008). In Fig. 4-1(c), it shows the magnetite inclusions in a clinopyroxene grain enclosed in Fe-Ti oxide ore.
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Fig. 4-1. (a) The BEI of run BM-027 shows that iron-titanium oxide encloses glass.
(b) The BEI of run BM-007 shows that pyroxene encloses glass and iron-titanium oxide. (c) The magnetite inclusions in a clinopyroxene grain enclosed in iron-titanium oxide ore (Cpx = clinopyroxene, Opa = opaque oxides) (Pang et al., 2008).
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4-2 Mineral and residual melt composition
Figures 3-7 and 3-19 show synthesized pyroxene composition of 1 GPa and 1 atm and the data of natural pyroxene from the BIC. Figures 3-8 and 3-20 show the synthesized plagioclase composition of this study compared with natural plagioclase in BIC. The results of the 1 atm are compositionally more similar to the natural rock data than the results from the 1 GPa.
The Ba and Sr contents of the residual melts at atmospheric pressure increased from the highest temperature to the lowest temperature (Fig 4-2b). The chemical variability follows the path outlined by Shellnutt et al. (2009) for the BIC in which crystallization of mafic silicate and Fe-Ti oxide minerals crystallize from the lower portion to upper portion (Fig 4-2a). As iron-titanium oxide and pyroxene crystallized, strontium and barium both increase in the residual liquid. As plagioclase crystallizes, strontium decreases and barium increases. Finally, as the residual liquid becomes more silicic alkali feldspar should crystallize and therefore both should decrease.
The residual glass compositions at 1 GPa and 1 atm of this study are plotted in Fig.
4-3 to be compared with the natural rocks of BIC (Shellnutt et al., 2009, 2010) and Baima layered series (Chen, 1990). The „ultramafic series‟ consist of Fe-Ti oxides, olivine, clinopyroxene and spinel which have the highest densities among the minerals
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in intrusion. It can be considered as the lowest portion of the product from the fractional crystallization of the ultramafic series, upper mafic series, and light color mafic series (Chen, 1990). It is suggested that the light color mafic series (LCMS) is the product from the fractional crystallization of the previous phase. The plotting of the compositions of the residual melts of this study fit in the middle portion of the liquid line of descent. The differentiation trend ends at Baima syenodiorite and syenites (Fig.
4-3). As the silica of the residual liquid increases, the MgO, tFeO and TiO2 of the melts decrease; while the K2O of the melts increases. The CaO and Al2O3 of the melts increase from the initial stage to the middle stage by crystallizing Fe-Ti oxide and pyroxenes and decrease toward the final stage by crystallizing plagioclase through the fractionation.
The results of MELTS modeling by Shellnutt et al. (2009) show that under pressure conditions equal to ~1000 bars (~0.1 GPa), a starting temperature of 1200 °C, final temperature of 800 °C and oxygen fugacity of FMQ +1, they were able to model bulk rock compositions similar to the BIC syenite at ~950 °C. The mineral assemblage of quartz, ulvospinel, ilmenite and fayalite (QUILF) can provide information on temperature, pressure, oxygen fugacity, and the activities of SiO2 and TiO2 at which the phases were in equilibrium (Anderson et al., 1993). Frost et al. (1988) suggested that the QUILF equilibrium can be applied to place much tighter constraints on temperature and
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oxygen fugacity than those determined from the oxide alone. The application of QUILF on the ilmenite-magnetite pairs of the lowest temperature runs indicates the relative oxidation state was FMQ ＋0.85 at atmospheric pressure and FMQ ＋1.6 at 1 GPa.
The experimental results of this study and those of previous studies are similar (Fig.
4-4).
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Fig. 4-2. The barium versus strontium contents of the residual melts at atmospheric pressure compared with natural rock data proposed by Shellnutt et al. (2009). (a): The barium versus strontium contents of the natural rock data of Baima (red line), Taihe (blue line), and Panzhihua (green line), from lower portion to upper portion. (b): The barium versus strontium contents of the residual melts of this study from the highest temperature to the lowest temperature.
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Fig. 4-3. The residual glass composition of this study compared with natural rock in BIC by Shellnutt et al. (2009, 2010) and Baima layered series selected chemical compositions by Chen (1990). (a):Silica versus magnesium oxide, (b): Silica versus calcium oxide.
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(continued) (c): Silica versus aluminum oxide contents, (d): Silica versus total iron oxide.
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(continued) (e): Silica versus titanium dioxide, (f): Silica versus potassium oxide.
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Fig. 4-4. Estimated oxygen fugacity and temperature of the Baima (black circles) (Shellnutt and Iizuka, 2011) and this study (red and blue circles). GC = graphite–CO2 equilibrium; FMQ = fayalite–magnetite–quartz buffer; HD Il = hedenbergite–ilmenite; TMQ = titanite–magnetite–quartz; HM = hematite–magnetite buffer. Rd = reduction of magma by the addition of carbon;
Ox = oxidation of magma by addition of oxygen or hydrogen loss; Fr = closed-system fractionation trend of layered basaltic magmas.
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4-3 Implication of oxide ore deposit genesis
With the magnification of 250 at SEM image, five different areas of each run product was chosen to estimate the percentage of the mineral phases. The average proportions of the mineral phases at some temperature are listed in Tables 4-1 and 4-2.
The results of the Table 4-1, and Table 4-2 show that the initial crystallized minerals are Fe-Ti oxide and pyroxene. The figure 4-5 shows that stratigraphic section of Panzhihua, Baima, and Taihe intrusions, all of them are hosted oxide ores in lower portion. The concentration of oxide minerals in the lower parts of the intrusions is probably a consequence of early oxide crystallization as demonstrated in this study.
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Table 4-1. Percentages of the phases in the run products at 1 atm of this study.
Sample Temp. (oC) Gl (%) Fe-Ti oxide (%) Px (%) Pl (%)
BM-023 1310 100 0 0 0
BM-005 1300 99 1 0 0
BM-026 1180 89 8 3 0
BM-027 1159 75 8 10 7
BM-007 1125 46 12 22 20
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Table 4-2. Percentages of the phases in the run products at 1 GPa of this study.
Sample Temp. (oC) Gl (%) Fe-Ti oxide (%) Px (%) Pl (%)
BM-H20 1240 100 0 0 0
BM-H16 1140 86 3 11 0
BM-H09 1120 47 6 32 15
BM-H05 1000 26 8 42 25
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Fig. 4-5. Generalized stratigraphic section, shows the general petrographic variations in the Panzhihua, Baima, and Taihe intrusions (modified from She et al., 2014; Zhong et al., 2014).
93 Fe-Ti-rich melts immiscible separated from silicate magmas. Liu et al. (2014) suggested that the S, P, F and H2O contents of the Baima parental magma may have triggered the immiscible separation of Fe-Ti-rich melt at an early stage of magma differentiation, which may explain the occurrence of massive or net-textured Fe-Ti oxide ores in the lower or lower-middle parts of the layered intrusions in the Panxi region. Their model also explains the formation of the Fe-Ti oxide deposit. However, experimental work by Waston (1976) showed that nearly all incompatible elements (e.g., Zr, Hf, Ti, Nb, Ta, and REEs) partition into the Fe-rich end-member. It is clear that the BIC syenites contain most of the incompatible elements (Shellnutt et al., 2009), which is the opposite of what would be expected if the syenitic unit was the Si-rich immiscible end member.
Therefore, it is unlikely that the BIC gabbroic unit and syenitic unit formed by silicate immiscibility. Shellnutt et al. (2009), used MELTS to calculate the mineral compositions from the layered gabbroic unit that were fractionated to produce the
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bimodal compositions that match those of the syenitic and gabbroic units of the BIC.
Their result shows that major and trace element modeling are consistent with each other.
Consequently they speculate that the modeling supports fractional crystallization as the principle process responsible for the formation of all units of the BIC. Our experiment results shows that (1) Fe-Ti oxide will occur early stage, (2) residual melt becomes more silisic and (3) its composition is similar to Baima enclaves. Therefore, it is suggested that the fractional crystallization is the main mechanism to form the Baima igneous complex.
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