Chapter 4. Results
4.2. Synthesis mineral chemistry
The first mineral to crystallize is Fe-Ti oxide and can be recognized as the brightest phase in the SEM images. The crystal size decreases with temperature. The shapes of minerals are both anhedral and euhedral. In the low-pressure experiment, there are five samples chosen for EPMA analysis. At 1274°C, a few tiny Fe-Cr oxides crystallize. The content of Cr2O3 ranges from 10 to 11.5 wt% while TiO2 content just around 1%. The FeOt
content is 61 wt% to 62.5 wt%. At lower temperatures, the FeOt content increases up to 90 wt%. In general the TiO2 content is around 13 wt% to 15 wt% and the crystals belong to thetitanomagnetite series. Some oxide minerals reach 45 wt% 50 wt% TiO2 and are classified as ilmenite.
In the high-pressure experiment, the Fe-Ti oxides are still the first phase, but at lower temperature in comparison with low-pressure experiment. They start to crystallize at 1180°C,
Plag CPx
Fe-Ti oxide
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together with clinopyroxene. The TiO2 content is around 30 wt% to 39 wt%, except some oxides at 1100°C have 7 wt% to 8 wt% TiO2. The FeOt is between 60 wt% and 70 wt%. The oxides belong to different spices of Fe-Ti oxides that include titanomagnetite, titanhematite and ulvospinel (Fig 4.10).
Figure 4-10. The composition of synthesized Fe-Ti oxides are illustrated graphically on the ternary oxide diagram. The vertical distances of the point above FeO-Fe2O3 baseline reflects the amount of titanium in the lattice. Hematite is in a higher state of oxidation than wustite;
hence the horizontal position along the FeO-Fe2O3 axis expresses the degree of oxidation.
The diagram is modified from Buddington and Lindsley (1964).
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Table 4-3.The average composition of the synthesized Fe-Ti oxides at atmospheric pressure
Experiment No. Pz013 Pz008 Pz009 Pz002
Temperature (oC) 1274 1188 1162 1102
No. of analysis 6 10 7 8 2
SiO2 0.66(0.71) 0.12(0.13) 0.06(0.04) 1.39(2.03) 0.54(0.14)
TiO2 1.15(0.14) 11.92(0.31) 13.21(0.40) 4.43(2.23) 43.01(0.85)
Al2O3 6.71(0.53) 1.80(0.11) 1.98(0.11) 1.68(0.27) 1.00(0.52)
Cr2O3 9.48(0.39) 0.56(0.19) 0.35(0.36) 0.35(0.51) 0.01(0.01)
FeO 56.08(1.65) 69.66(0.51) 68.30(1.07) 78.17(3.89) 41.74(0.52)
MnO 0.61(0.13) 0.11(0.03) 0.16(0.02) 0.16(0.06) 0.09(0.02)
MgO 15.40(1.90) 5.23(0.18) 5.93(0.21) 1.99(1.04) 4.53(0.31)
CaO 0.32(0.16) 0.20(0.07) 0.18(0.08) 0.25(0.08) 0.21(0.03)
Na2O 0.05(0.06) 0.02(0.02) 0.01(0.01) 0.11(0.19) 0.09(0.00)
K2O 0.00 0.00 0.00 0.03(0.07) 0.00
Total 90.46(2.96) 89.63(0.81) 90.16(0.67) 88.56(1.39) 91.20(1.33) Numbers of Ions on the basis of 32 (O)
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Table 4-4. The average composition of the synthesized Fe-Ti oxides at high-pressure
Experiment No. PzH5 PzH11 PzH14
Temperature (oC) 1180 1100 1050
Pressure 1.0 0.7 0.7
No. of analysis 10 5 6 2
SiO2 0.14(0.03) 0.82(0.96) 0.51(0.70) 3.36(4.50)
TiO2 29.78(0.26) 7.25(0.24) 34.28(2.30) 53.66(3.93)
Al2O3 2.00(0.14) 8.56(0.18) 1.04(0.07) 1.71(0.21)
Cr2O3 0.17(0.07) 2.51(0.73) 0.01(0.01) 0.00(0.00)
FeO 59.68(0.50) 67.84(1.23) 54.83(2.17) 30.72(3.11)
MnO 0.07(0.04) 0.26(0.05) 0.22(0.03) 0.07(0.02)
MgO 3.74(0.09) 7.21(0.11) 4.32(0.30) 6.16(0.78)
CaO 0.18(0.04) 0.28(0.12) 0.40(0.45) 1.07(1.25)
Na2O 0.02(0.03) 0.06(0.11) 0.03(0.03) 0.09(0.13)
K2O 0.00(0.00) 0.01(0.02) 0.00(0.00) 0.08(0.10)
Total 95.77(0.50) 94.78(1.03) 95.65(0.72) 96.93(0.48)
Numbers of Ions on the basis of 32 (O)
39 4.2.2. Pyroxene
Clinopyroxene is one of the most abundant minerals to crystallize in the experiments.
In both low and high-pressure, they crystallize at ~1180oC. The crystals are much bigger than other minerals in terms of crystal size and often surrounded by small crystals of iron oxides.
Theymostly belong to the augite species. The Mg# [defined as molar 100×Mg/(Mg+Fe)]at low-pressure (Mg#78-80) is moderately higher than those crystallized during the high-pressure experiment (Mg#73-76).
Orthopyroxene always follows clinopyroxene in the crystallization sequence. They crystallize at the same temperature with plagioclase in the low-pressure (1162oC) and before plagioclase in the high-pressure (1100oC). They are subhedral and have elongated shape.The synthetic orthopyroxene from the experiment mostly conform to the enstatite group with Mg#
from 95-93 (low-pressure) and 76-74 (high-pressure).
Figure 4-11. Composition of pyroxene in the Panzhihua intrusion (Pang et al., 2009) and from this study.
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Table 4-5. The average composition of the synthesized clinopyroxene at low-pressure
Experiment No. Pz008 Pz009 Pz002
Temperature (oC) 1188 1162 1102
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Table 4-6. The average composition of the synthesized clinopyroxene at high-pressure
Experiment No. PzH5 PzH11 PzH14
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Table 4-7. The average composition of the synthesized orthopyroxene at atmospheric pressure
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Table 4-8. The average composition of the synthesized orthopyroxene at high-pressure
Experiment No. PzH11
44 4.2.3. Plagioclase
In the low-pressureexperiment, plagioclase is more abundant at lower temperature.
Crystallizing at relatively low temperature, the plagioclase phenocrysts have anhedral shapes and the anorthite content falls from 67 at 1162°C to 41 at 1102°C whereas the orthoclase content increases from 3 to 11 wt%.
Under high-pressure, the plagioclase crystallizes at 1050°C. They are large and euhedral crystals that appear dark under the SEM image and can be easy to misidentify as glass. In comparison to low-pressure condition, the An content of plagioclase in high-pressure is quite low. Except for one sample (An61), the plagioclase range in composition from An40 to An30 whereas, the albite content increases to a maximum of 56.
Comparing the chemical composition of the synthetic plagioclase to data from Pang et al. (2009) from the Panzhihua intrusion shows that the compositions resemble each other in An-Ab content but different in the orthoclase component. The higher orthoclase component is likely related to the enrichment of K within the residual liquid.
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Figure 4-12.Composition of plagioclase in Panzhihua intrusion (Pang et al., 2009) and in this study. Plagioclase (An67-41Ab29-47Or2-17)
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Table 4-9. The average composition of the synthesized plagioclase at atmospheric pressure
Experiment No. Pz009 Pz002
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Table 4-10. The average composition of the synthesized plagioclase at high-pressure
Experiment No. PzH14
48 4.3. Composition of residual glass
The residual melts were quenched into glass. The melt composition ranges from basaltic to silicic. The Hacker diagrams (Fig 4-14) show the variation of SiO2 concentration against FeO, TiO2, MgO, CaO, Al2O3 and Na2O + K2O of glass at different temperature. In both experiments, the SiO2 contents in the residual melt increases with decreasing temperature. The SiO2 content is ~50% at 1300oC and then increases to ~72% at 1102oC in the low-pressure runs whereas the high-pressure glass starts from ~50% at 1240oC and increases to ~61% at 1050oC. This SiO2-enrichment of the melt is directly related to the change of crystal content in the experiment.
The FeO and TiO2 have similar behavior in which they decrease with decreasing temperature. Their content strongly depends on the crystallization of titanomagnetite and ilmenite. Before the crystallization of Fe-Ti oxides, the concentration of Fe is >10% and TiO2 is ~3%. However, for low temperature in both experiments, FeO content is from 3-5 wt% and TiO2 is around 1%.
The CaO is quite constant at high-temperature and decrease after the crystallization of pyroxene and plagioclase. The CaO content decreases from 10% to less than 5% at low-pressure and decrease from 10% to 7% at high-low-pressure.
The Al2O3 contents increase in the residual melt until the crystallization of plagioclase.
The abundance of plagioclase at low temperature results in low concentration of aluminum in glass. The lowest content of Al2O3 at low-pressure is around 13 wt% whereas in the high-pressure experiment is can be as low as ~9%
The evolution of total alkali contents (Na2O + K2O) of the glasses shows a general increase with decreasing temperature. The evolution of total alkali content in the melts is
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mainly dependent on the crystal content (or portion) since no mineral phase incorporates these elements in significant amounts (except for Na in plagioclase).
The change in concentration of oxides in residual melt represents the consequence of mineral crystallization. The FeO and TiO2 contents decrease quickly because they are the principal constituents of Fe-Ti oxides that crystallized early during evolution of magma system. The CaO and Al2O3 contents depend on appearance of pyroxene and plagioclase, respectively. Alkali content goes up with decreasing temperature because they do not partitioning to a major mineral phase.
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Figure 4-13. SiO2 vs.Al2O3, Na2O+K2O, MgO, CaO, FeO (as total iron) and TiO2 for glass from experiment. The low temperature residual glass compositions are enriched in SiO2, Al2O3, Na2O, K2O and depleted in TiO2, FeO, MgO and CaO. Syenite and granite data from Shellnutt and Jahn (2010) and Shellnutt and Zhou (2007)
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Table 4-11 .The average composition of the synthesized glass at atmospheric pressure
Experiment No. Pz003 Pz013 Pz008 Pz009 Pz002
Temperature (oC) 1303 1274 1188 1162 1102
No. of analysis 36 32 20 19 24
SiO2 49.09(0.46) 49.22(0.39) 52.21(0.38) 56.67(0.51) 67.20(4.02) TiO2 2.80(0.09) 2.78(0.15) 2.29(0.28) 2.18(0.12) 0.86(0.23) Total 96.32(0.55) 96.17(0.56) 96.10(0.54) 96.98(0.45) 100.04(2..89) CIPW (weight %)
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Table 4-12. The average composition of the synthesized glass at high-pressure
.
Experiment No. PzH1 PzH5 PzH11 PzH14
Temperature (oC) 1240 1180 1100 1050
Pressure (kbar) 10 10 7 7
No. of analysis 10 15 15 4
SiO2 48.47(0.81) 50.49(0.44) 49.59(0.34) 55.68(2.48) TiO2 2.11(0.33) 3.04(0.19) 3.40(0.18) 2.69(1.77) Al2O3 11.41(0.33) 15.26(0.27) 14.37(0.29) 15.88(2.53) Cr2O3 0.03(0.02) 0.01(0.01) 0.01(0.01) 0.01(0.02) FeO 10.61(0.33) 10.27(0.37) 11.29(0.32) 6.21(2.64) MnO 0.15(0.02) 0.14(0.05) 0.13(0.03) 0.11(0.06) MgO 8.75(0.22) 4.49(0.18) 5.56(0.74) 3.44(1.39) CaO 10.01(0.28) 6.81(0.14) 7.64(0.22) 5.94(2.69) Na2O 3.55(0.25) 3.00(0.09) 2.84(0.24) 3.55(0.60) K2O 1.08(0.05) 1.65(0.07) 1.59(0.04) 3.21(1.58) Total 96.17(1.21) 95.17(0.35) 96.42(0.30) 96.72(1.48) CIPW (weight %)
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Chapter 5. Discussion
5.1. Early crystallization of oxide and the occurrence of ore deposit at the base of the Panzhihua intrusion
Some world-class deposits such as the Bushveld Complex host the ore bodies in their upper part. The reason is because iron oxides crystallize at late stage of magmatic differentiation, after the accumulation of silicate minerals like olivine, pyroxene, plagioclase.
However, the Panzhihua ore deposits are located at the base of intrusion. The question is why and how a large amount of iron oxide can crystallize early and concentrate at the lower part of Panzhihua intrusion. The results in both low and high-pressure experiments indicate that Fe-Ti oxides crystallize first in the crystallization sequence of mineral. It is noteworthy that at 1274oC, the first oxides minerals crystallize are Cr-rich titanomagnetite (~10 wt% Cr2O3).
“Chromium has high partition coefficient in Cr-Fe-Ti oxides, like magnetite, during fractionation of magma” (McCarthy et al., 1985). Therefore, the appearance of Cr-rich titanomagnetite is clear evidence for the early crystallization of oxides. Moreover, Cr-rich oxide minerals were identified within the Taihe layered intrusion located in the northern part of the Panxi region (Shellnutt et al., 2011). The results support the hypothesis that the saturation and accumulation of Fe-Ti oxides at the early stage of magmatic evolution result in the formation and occurrence of Fe-Ti oxide deposits at the lower part of the intrusion (Pang et al., 2009). However, the reason that triggers the early crystallization is debated. Ganino et al. (2013) suggested “CO2-rich fluids from the carbonates that form the wall rocks of the intrusion play a role in the crystallization of the oxides”. They suggested that “CO2-rich fluids liberated by decarbonatization of carbonate wall rocks increases the oxygen fugacity of the magma. The oxidizing conditions result in the early crystallization of magnetite and leading the formation of the ore deposits”. However, Shellnutt (2014) disputed that “if CO2 is necessary for oxidation then how to explain about the lack of carbonate country rocks in
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other Panxi deposits in the region” (i.e. Baima and Taihe). Howarth and Prevec (2013) suggested that “the early crystallization of Fe-Ti oxides can result from an influx of H2O or H2O-rich magma. H2O will increase in residual liquid during fractional crystallization.
Magma evolving at depth would have increased H2O contents as evolution continued. The late stage magma pulse from depth would have high H2O contents as a result of the concentration of H2O in the residual liquid. The influx of H2O-rich liquid or fluid into the partially crystallized Panzhihua intrusion can account for the occurrence of large scale Fe-Ti oxide ore layers”.
The composition of parental magma which is estimated for the Panzhihua intrusion has high content of TiO2 (~4wt%) (Zhou et al., 2005). “TiO2 plays a role similar to fO2 in which increasing the Fe3+/Fe2+ ratio stabilizesthe crystallization of Fe-bearing oxide” (Fagan, 2014). The experimental results in this study also indicate that a starting composition similar to high-Ti basalts could result in abundant and early crystallization of Fe-Ti oxides via crystal fractionation without adding any special factors like oxygen or water. Therefore it is possible that the high concentration of titanium in the Panzhihua parental magma may be significant for the early and stable crystallization of Fe-Ti oxides. The influx of water or assimilation of carbonate wall rock may be not key factors for the formation of the Panzhihua oxide deposits.
5.2. Ore forming process: Immiscibility vs. Fractional Crystallization
Fractional crystallization and immiscibility are two important processes for the formation of magmaticore deposits. Both of them are suggested as ore-forming mechanism of the Panzhihua intrusion. Zhou et al. (2005) indicated that the Panzhihua ores occur as stratiform layers and in association with magmatic silicates. Therefore, they origin from oxide ore melts which possibly were formed as results of immiscible separation. Xing et al.
(2014) used immiscible model to explain the presence of apatite-rich middle zone above the Fe-Ti oxide layers. They suggested that “the Panzhihua intrusion formed due to
high-55
temperature liquid immiscibility in an evolved ferrobasaltic parental magma. Phosphorus tends to be partitioned into the Fe-rich melt during the separation and was enriched after crystallization of Fe-Ti oxides. The residual P-rich melt may have mixed with the Si-rich melt due to intensive convection, forming a P-and Si-rich melt in the upper part of the chamber”. Evidence of immiscibility is also found within apatite inclusions from the Panzhihua intrusion (Wang et al., 2013). Figure 5-1 shows the concentration of SiO2 vs. TiO2, FeO, MgO, Al2O3, Na2O, K2O and P2O5 of experimental data, Panzhihua rocks and immiscible composition from Wang et al. (2013). The immiscible composition doesn’t match very well with the data from Panzhihua rocks. For examples, the silicic rocks of the Panzhihua intrusion have lower CaO and higher K2O concentration than that in immiscibility.
The reason for the difference is the immiscibility has no partition of CaO into pyroxene and plagioclase as well no enrichment of K2O in residual magma during fractionation. It indicates that the Panzhihua silicic rocks maybe not results of immiscibility. In addition, it is difficult to distinguish immiscible liquids from liquids trapped during the growth of crystals. The crystallization of late stage minerals such as apatite has the possibility to trap liquids witheither mafic or felsic composition. Even if immiscibility occurs, it is often in small scale at late stage of magmatic evolution (Philpotts, 1982). Consequently, the role of silicate immiscibility as major process for the genesis of the Panzhihua intrusion, including the formation of ore deposit is questionable. Whereas, many other studies use fractional crystallization to explain the formation of ore bodies, it is generally accepted that the Panzhihua magma chamber is an open system. The injection of Fe-rich magma combines with the crystallization of silicate minerals as olivine, pyroxene, plagioclase result in the formation of Fe-Ti oxides (Pang et al., 2008, 2009, 2010, 2013; Shellnutt, 2014; Shellnutt and Jahn, 2010, 2011; Song et al., 2013). The experimental results show the crystallization of mafic minerals in the order of: Fe-Ti oxides, pyroxene, plagioclase, results in the enrichment
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of SiO2, Al2O3, Na2O and K2O; and depletion of TiO2, FeO, MgO and CaO in residual magma. This trend can also be seen in the composition of minerals as at lower temperatures, the clinopyroxene and plagioclase have higher SiO2 contents.
The function (FeOt)/(FeOt + MgO) rock is a measure of fractionation: low values signify primitive basaltic melts and high values more evolved melts. In this study, the high value of FeO/MgO corresponds to highly evolved magma at low temperature. From 1274oC to 1162oC the FeO/MgO ratio decrease strongly because the crystallization of Fe-Ti oxides.
The FeO/MgO increases from 0.48 (1162oC) to 0.56 (1102oC) (Figure 5-2).
In general, the crystallization sequence and composition of minerals as well the evolution of residual magma from the experiment shows typical characteristics of fractional crystallization. The results of the this study suggests that the formation of oxide ore deposit as well layered gabbro of the Panzhihua intrusion are due to fractional crystallization and not silicate liquid immiscibility.
Figure 5-1. FeOt/ (FeOt + MgO) ratio representscrystallization behavior of high-Ti Emeishan basalt in both atmospheric-pressure and high-pressure experiments
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Figure 5-2. The silicic compositions from low-pressure experiment (Sample Pz002) compare to Panzhihua rock and immiscible composition. The data of the Panzhihua rock is from Shellnutt and Zhou (2007), Shellnutt and Jahn (2010). The immiscible composition is from Wang et al. (2013) and Hou and Veksler (2015)
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Figure 5-3. The cartoons illustrate the progress of crystallization at stages corresponding to different temperature. The percentages of remaining melts decrease with lower temperature, approach the solidus at 1102oC.
5.3. Composition of residual magma and relationship with surrounding silicic rocks
The composition of the residual glass is due to the crystallization of Fe-Ti oxides, mafic silicate minerals and plagioclase. The evolved composition is very silicic with SiO2 content range from ~60 wt% up to ~72 wt%. It is interesting that this composition is similar to the composition of the neighbouring peralkaline A-type granite. Therefore it is possible that a basaltic magma, via fractional crystallization, could generate the Fe-Ti oxide deposits and differentiate into syenodiorite, granite and trachyte. Fractional crystallization is an important process which causes the heterogeneity of the Earth crust. Keller et al. (2015) indicated that “fractional crystallization, rather than crustal melting, is predominantly responsible for the production of intermediate and felsic magmas, emphasizing the role of mafic cumulates as a residue of crustal differentiation”. Theresults from this study support evidence to emphasis the role of fractional crystallization in producing variety of rock type, especially A-type granite and in support of the cogenetic origin of the Panzhihua layered intrusion and the spatially and temporally associated silicic plutons.
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5.4. Comparison among chemical composition of synthesis minerals, MELTS models and mineral from the Panzhihua intrusion
5.4.1 MELTS models
MELTS is a software package developed to model relations between different phases of igneous system (mineral, rock and liquid) during melting and crystallization (Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998; Ghiorso et al., 2002; Asimow et al., 2004; Smith and Asimow, 2005). The software can be applied to model different magmatic processes including partial melting, equilibrium crystallization, fractional crystallization and assimilation. The condition ranges of magmatic systems that MELTS can compute are 500 – 2000oC and 0 – 2 GPa which corresponding to the depth of 0 ~ 65 km.
MELTS predicts the chemical change of minerals and liquid based on the change of thermodynamics parameters: temperature, pressure and volume of the system. For each change of state (P-T-X conditions) of the system, MELTS find a set of phases that result minimum Gibbs free energy. Therefore, it predicts the minerals that will crystallize in a magma chamber and their composition at each stage of pressure-temperature conditions.
MELTS also tracks the composition of the remaining liquid during melting or fractionation.
These calculations are calibrated based on experiments so the results are predictive rather than an explicit representation of the magmatic processes.
MELTS calculates based on an input bulk composition data of a parental magma from the user. Thermodynamic conditions including temperature (T), pressure (P), oxygen fugacity (fO2) and volatile content of magmas (e.g. H2O, CO2) are starting parameters for each time MELTS runs. Results of the calculations show the predict evolution of each phases through magmatic processes.
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The MELTS program was used in this study to test how a composition was similar to high-Ti basalt evolved via fractional crystallization at a wide range of temperature in both low and high-pressure. The bulk composition used as input for MELTS model is similar to the starting composition of the experiment. The anhydrous condition with initial temperature is 1300oC, fO2 = FMQ-1 and pressure of 1kbar and 10 kbar were chosen as input parameter for the model. The model with 1300oC, fO2 = FMQ – 1, pressure = 0.7 kbar is from Shellnutt and Jahn (2010).
5.4.2 Comparison among chemical composition of synthesis minerals, MELTS models and Panzhihua mineral composition.
The MELTS models shows that a starting composition similar to high-Ti Emeishan basalts could produce a range of composition, from mafic to silicic via fractional crystallization. Different thermodynamic conditions result in different temperature of mineral crystallization. In comparision with low-pressure experimental data and real data, at higher pressure condition, oxide and olinopyroxene crystallize earlier, concentration of alumium and alkalis are higher while MgO and CaO content is lower. High fO2 in the model result in higher TiO2 content in comparing to experimental result. The results from these models and eperimetal data suggest that the Panzhihua magma chamber may have been emplaced at the shallow depth with relatively low fO2 (FMQ -1 to FMQ + 0).
The composition of residual magma is also compared to the syenite and granite of the Panzhihua intrusion. It is interesting that the low-pressure glass composition matches quite
The composition of residual magma is also compared to the syenite and granite of the Panzhihua intrusion. It is interesting that the low-pressure glass composition matches quite