中國西南攀枝花火成雜岩之實驗岩石學研究－關於鐵－鈦－釩氧化礦床成因

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(1)e. NATIONAL TAIWAN NORMAL UNIVERSITY DEPARTMENT OF EARTH SCIENCES. MSc. Thesis. An experimental investigation of the Panzhihua igneous complex, SW China-Addressing the genesis of Fe-Ti-V oxide ore deposits. Student: Tran Thi Duyen Supervisor: Dr. J. Gregory Shellnutt. Taipei, July 2016 1.

(2) Acknowledgements There is a saying that “The mediocre teacher tells. The good teacher explains. The superior teacher demonstrates. The great teacher inspires.” My supervisor, Assoc. Prof J. Gregory Shellnutt, is a person who not only transfers knowledge but also conveys inspiration for me. He patiently answers all my questions, remind when I am not working hard and encourage when I have trouble. I would like to express the deepest appreciation to him. Without his guidance and persistent help this thesis would not have been possible. I would like to thank Prof. Teh-Ching Liu for let me do the low and high-pressure experiments in his lab in National Taiwan Normal University. Also, I would like to thank Dr. Yoshiyuki Iizuka and staff members of SEM-EDS and EPMA lab in Institute of Earth Science (Academia Sinica) for their help in geochemical analysis. In addition, I consider it is an honor to receive the help from members of F406 lab (Carol, Wen yu, Robert, Alice, Thuy and Ha) for their kind guidance. It also gives me great pleasure in acknowledging the support of Department of Earth Sciences. A special thanks to both Taiwanese and Vietnamese classmates who have shared experience and overcome difficulties with me in the last three years. I also would like to express my gratitude for the financial support from Society of Economic Geologists (SEG) and Elsevier grant –Goldschmidt 2016. Last but not least, I am indebted to my family and friends who gave me great encouragement during my study.. 1.

(3) Table of contents Acknowledgements ................................................................................................................ 1 Table of contents .................................................................................................................... 2 List of Figures ........................................................................................................................ 4 List of Tables ......................................................................................................................... 6 Abstract .................................................................................................................................. 7 Chapter 1. Introduction .............................................................................................................. 8 1.1. Introduction ..................................................................................................................... 8 1.2. Layered mafic intrusions................................................................................................. 8 1.3. Debated issues ............................................................................................................... 11 1.4. Purpose of this study ..................................................................................................... 14 Chapter 2. Geological Background .......................................................................................... 15 2.1. China’s tectonic framework in the global context ........................................................ 15 2.2. Yangtze Block and the collision to form the South China block .................................. 16 2.3. The Emeishan Large Igneous Province (ELIP) ............................................................ 17 2.4. The layered gabbroic Panzhihua intrusion .................................................................... 19 Chapter 3. Methods .................................................................................................................. 22 3.1. Experimental methods .................................................................................................. 22 3.2. Analytical methods ....................................................................................................... 27 Chapter 4. Results .................................................................................................................... 29 4.1. Crystallization sequence of minerals ............................................................................ 29 4.2. Synthesis mineral chemistry ......................................................................................... 35 4.3. Composition of residual glass ....................................................................................... 48. 2.

(4) Chapter 5. Discussion .............................................................................................................. 53 5.1. Early crystallization of oxide and the occurrence of ore deposit at the base of the Panzhihua intrusion.............................................................................................................. 53 5.2. Ore forming process: Immiscibility vs. Fractional Crystallization ............................... 54 5.3. Composition of residual magma and relationship with surrounding silicic rocks ........ 58 5.4. Comparison among chemical composition of synthesis minerals, MELTS models and mineral from the Panzhihua intrusion .................................................................................. 59 5.5. The effect of thermodynamic parameters on magmatic differentiation........................ 61 Chapter 6. Conclusions ............................................................................................................ 66 References ............................................................................................................................ 67 Appendix .............................................................................................................................. 71. 3.

(5) List of Figures Figure 2-1. Tectonics sketch of China showing the location of major tectonic units (modified after Shellnutt and Pang (2012)) ...................................................................................... 15 Figure 2-2. Distribution of the major Fe-Ti-V oxide deposits in the Panxi region ................. 18 Figure 2-3.Geological map of the Panzhihua intrusion ........................................................... 21 Figure 3-1. The high temperature experiment system ............................................................. 23 Figure 3-2. The piston cylinder device and protocol used for carrying out the high-pressure experiments. ..................................................................................................................... 25 Figure 3-3. Furnace assembly for the high-pressure experiment. ............................................ 26 Figure 3-4. Diagram of furnace assembly for the high-pressure experiment .......................... 26 Figure 4-1. The crystallization sequence of basaltic magma constructed by low-pressure experiment........................................................................................................................ 29 Figure 4-2. BSE image of quenched run products at 1274oC .................................................. 31 Figure 4-3. BSE image of quenched run products at 1122oC .................................................. 31 Figure 4-4. BSE image of quenched run products at 1162oC .................................................. 32 Figure 4-5. BSE image of quenched run products at 1102oC. ................................................. 32 Figure 4-6. The crystallization sequence of basaltic magma constructed by low-pressure experiment........................................................................................................................ 33 Figure 4-7. BSE image of quenched run products at 1180oC, 10 kbar .................................... 34 Figure 4-8. BSE image of quenched run products at 1100oC, 7 kbar ...................................... 34 Figure 4-9. BSE image of quenched run products at 1050oC, 7 kbar ...................................... 35 Figure 4-10. The composition of synthesized Fe-Ti oxides are illustrated graphically on the ternary oxide diagram. ..................................................................................................... 36 Figure 4-11. Composition of pyroxene in the Panzhihua intrusion (Pang et al., 2009) and from this study. ................................................................................................................ 39. 4.

(6) Figure 4-12.Composition of plagioclase in Panzhihua intrusion (Pang et al., 2009) and in this study ................................................................................................................................. 45 Figure 4-13. SiO2 vs.Al2O3, Na2O+K2O, MgO, CaO, FeO (as total iron) and TiO2 for glass from experiment ............................................................................................................... 50 Figure 5-1. FeOt/ (FeOt + MgO) ratio representscrystallization behavior of high-Ti Emeishan basalt in both atmospheric-pressure and high-pressure experiments ............................... 56 Figure 5-2. The silicic compositions from low-pressure experiment (Sample Pz002) compare to Panzhihua rock and immiscible composition .............................................................. 57 Figure 5-3. The cartoons illustrate the progress of crystallization at stages corresponding to different temperature ........................................................................................................ 58 Figure 5-4. MELTS model at different thermodynamic condition compare to experimental data with 0.1 wt% H2O .................................................................................................... 63 Figure 5-5. MELTS model at different thermodynamic condition compare to experimental data at low-pressure with 0.7 wt% H2O ........................................................................... 64 Figure 5-6. MELTS model at different thermodynamic condition compare to experimental data at low-pressure with 1 wt% H2O .............................................................................. 65. 5.

(7) List of Tables Table 3-1.Estimates for the parental magma compositions of the Panzhihua intrusion .......... 22 Table 4-1 Anhydrous melting experiment shows crystallization sequence of high-Ti Emeishan basalt. .............................................................................................................. 30 Table 4-2. High-pressure experiment shows crystallization sequence of high-Ti Emeishan basalt ................................................................................................................................ 33 Table 4-3.The average composition of the synthesized Fe-Ti oxides at atmospheric pressure .......................................................................................................................................... 37 Table 4-4. The average composition of the synthesized Fe-Ti oxides at high-pressure ......... 38 Table 4-5. The average composition of the synthesized clinopyroxene at low-pressure ........ 40 Table 4-6. The average composition of the synthesized clinopyroxene at high-pressure ....... 41 Table 4-7. The average composition of the synthesized orthopyroxene at atmospheric pressure ............................................................................................................................ 42 Table 4-8. The average composition of the synthesized orthopyroxene at high-pressure ....... 43 Table 4-9. The average composition of the synthesized plagioclase at atmospheric pressure 46 Table 4-10. The average composition of the synthesized plagioclase at high-pressure .......... 47 Table 4-11 .The average composition of the synthesized glass at atmospheric pressure ........ 51 Table 4-12. The average composition of the synthesized glass at high-pressure .................... 52. 6.

(8) Abstract The Late Permian Panzhihua layered gabbroic intrusion of SW China hosts one of the largest magmatic Fe-Ti-V oxide deposits within the Emeishan large igneous province and is coeval with a peralkaline granitic pluton. The largest oxide ore body is found at the base of the intrusion, which is unlike other layered intrusions where the Fe-Ti oxide deposits are located in the uppermost portions. This study attempts to model the genesis of the Panzhihua layered intrusion, including the formation of the ore deposit by reconstructing the crystallization sequence of minerals from low and high-pressure experiments. The starting composition used for the experiment is similar to high-Ti Emeishan basalt that resembles the theoretical parental composition of the Panzhihua intrusion. The low-pressure experiments were conducted between 1312oC and 1102oC. The first mineral to crystallize is Cr-rich titanomagnetite at 1274oC. Following Cr-rich titanomagnetite are: Fe-Ti oxides (ilmenite+titanomagnetite); clinopyroxene (Wo39-52En39-47Fs8-16) at 1188oC; plagioclase (An67-41) and orthopyroxene (Mg# = 93-95) at 1162oC. The compositional range of clinopyroxene and plagioclase matches those measured from the rock of the Panzhihua intrusion. The high-pressure experiments occur between 1240oC and 1050oC. Iron-titanium oxide and clinopyroxene (Wo23-48En37-58Fs10-22) appear together as the first phases at 1180oC. The sequence is followed by orthopyroxene at 1100oC and plagioclase (An61-37) at 1050oC. The experiment results indicate that the early crystallization sequence of the parental magma is dominated by Fe-Ti oxide and partially explain why the largest oxide ore deposit of the Panzhihua intrusion is found in the lowermost layers. The low temperature residual glass compositions in both experiments are enriched in SiO2, Al2O3, Na2O and K2O; and depleted in TiO2, FeOt, MgO and CaO. However, minerals crystallize at relatively low temperature in the high-pressure and consequently have less silicic (SiO2 ≈ 61 wt%) residual glass composition than that of the low-pressure experiment (SiO2 ≈ 72 wt%). The similarity between Panzhihua granite and low-pressure residual glass suggests that the Panzhihua intrusion probably formed at shallow depth. Furthermore, the liquid-crystal evolution constructed from the low-pressure experiment show that a parental magma similar to high-Ti Emeishan basalt can produce an early enrichment of oxide minerals and a silicic residual liquid via fractional crystallization. Keywords: Panzhihua, Fe-Ti-V oxide ore deposits, experimental petrology. 7.

(9) Chapter 1. Introduction 1.1. Introduction The Panzhihua intrusion is a part of the Emeishan Large Igneous Province (ELIP), SW China. This intrusion is famous and attractive in terms of both economic value and geological significance. It is a layered gabbroic sill that hosts world-class magmatic Fe–Ti oxides with the reserves up to 1333 million tons (Zhou et al., 2005). In addition, it is considered a typical example of ore-bearing layered intrusion, and therefore, can help to constrain ore-forming mechanisms and complex geochemical processes that occur during magmatic evolution.. 1.2. Layered mafic intrusions Layered mafic intrusions are intrusive bodies which mainly crystallize from basaltic magma. So, they can occur in any tectonic environment where basaltic magma is generated. They are called layered mafic intrusions (LMIs) due to the presence of igneous layering. Layering (or stratification) is a sheet-like feature that can be distinguished by compositional and textural variation. Because of the high temperature and low viscosity, LMIs are ideally natural laboratories to study crystal-liquid fractionation during cooling and crystallization. Some early studies about LMIs focus on big intrusions such as the Bushel Complex of South Africa, the Stillwater Complex of the western United States, and the Skaergård intrusion of eastern Greenland (Hess, 1939; Wager, 1963; Wager and Brown, 1968; Campbell et al., 1983). These studies tried to explain the origin, magmatic processes and layers of layered intrusions. The Precambrian (2.06 Ga) Bushveld Igneous Complex of South Africa is the world’s largest layered intrusion (Eales and Cawthorn, 1996). It is about 300-400 km wide and 9 km thick. The large-scale layering forms the basis for a simple subdivision, including the 8.

(10) Marginal Zone, the Critical Zone, the Main Zone and the Upper Zone (Wager and Brown, 1968). Because of clear stratification with different petrographic composition, the Bushveld Complex considers as stunning example to interpret the cooling and magmatic differentiation processes in a large magma chamber. Layered intrusion may host magmatic ore deposits, containing some of the world's economic concentrations of platinum-group elements (PGE), Fe, Ti, Cu, Ni. Previous studies suggested the ore-forming processes of these deposits including crystal settling, convection, compaction, recharge and magma mixing. 1.2.1. Major types of magmatic Fe-Ti oxide ore in the world Iron-titanium oxide deposits formed as results of accumulation or injection of Fe-Ti rich liquids. Global magmatic Fe–Ti oxide deposits are associated with mafic intrusions or Proterozoic anorthosite complexes and form by the concentration of Fe–Ti oxides in gabbroic or ferrodioritic magma chambers (Lister, 1966). Anorthosite-related ore deposits Anorthosites are large (occasionally enormous) plutonic bodies of nearly pure plagioclase. They are thus as felsic as any granite, but their mineralogy (plagioclase + pyroxene ± olivine) conforms more to mafic rocks (Ashwal and Myers, 1994). The two classic types are Archean and Proterozoic anorthosites (Emslie, 1978). The favored model for anorthosite petrogenesis involves a mantle plume that induces peridotite melting in the spinel– or plagioclase–lherzolite stability field. The resulting aluminous basaltic liquid rises and ponds at the base of the crust. Crystal fractionation produces olivine and pyroxene, which sink, and plagioclase, which floats. The upper plagioclase-rich crystal–liquid mush rises in several pulses to shallower levels, and the dense Fe-rich interstitial liquid is expelled downward, leaving adcumulus masses of anorthosite (Arndt, 2013). Some large anorthosite-. 9.

(11) related ore deposits include: the Lac Tio (Canada), Tellnes (Norway) and Damiao (China) intrusions. A range of magmatic processes may be responsible for the formation of these deposits but fractional crystallization is considered to be one of the most important processes. Crystallization of plagioclase within a mafic melt results in the enrichment of Fe and in the residual liquid consequently the dense Fe-Ti rich liquid may sink to the bottom of magma chamber. This is a key factor for the formation of iron oxide deposits. Some additional processes can be considered as immiscibility, magma mixing and compaction (Charlier et al., 2015). Layered intrusion-related deposits Another type of Fe-Ti oxide can be found on the upper sequences of layered intrusions. Perhaps the most well known example is the Bushel Complex where large titaniferous iron ores are located in the Upper Zone (Reynolds, 1985). The ores are associated with gabbronoritic, ferrogabbronoritic and ferrodioritic cumulates and have titanomagnetite as the dominant oxide mineral. It is suggested that the Fe-Ti oxides crystallize at the late stage of magmatic differentiation, after accumulation of mafic minerals as olivine, pyroxene, plagioclase (Cawthorn and Molyneux, 1986). The enrichment of iron oxide from evolved magma at late stage causes the formation of ore bodies and also explains their occurrence at the upper zone of the intrusion. 1.2.2. Special characteristics of the Panzhihua deposits The Panzhihua intrusion hosts large iron oxide deposits with special characteristics that are different than the type of deposits associated with anorothosites or layered maficultramafic intrusions. There are no known anorthosites that are spatial or temporally associated with the Panzhihua intrusion. Instead, the ore deposits are hosted by a rhythmically layered gabbroic intrusion that was emplaced at the same time as the eruption of. 10.

(12) the Emeishan flood basalt. Specifically, the ore bodies are located in the lower part of the intrusion. The location of the ore bodies is interpreted as evidence for the early crystallization of Fe-Ti oxides. It is unusual in comparison with other oxide ore-bearing intrusion where iron oxides crystallize during the late stages of magma evolution and lie on top of cumulate layers of mafic minerals. Pang et al. (2010) suggests the main features that distinguish the Panzhihua intrusion from other layered complexes includes: (1) association with flood basalts of a large igneous province, (2) occurrences of ores in low stratigraphic positions with gabbro as host rocks and (3) presence of granitic rocks surround the intrusion.. 1.3. Debated issues The economic value and geological significance of the Panzhihua intrusion has attracted a lot of attention from the geosciences community and has led to a number of important questions such as: How could a large amount of Fe-Ti oxide concentrate at the base of the intrusion? What is the prominent ore-forming process? Which factors control the crystallization of oxides? Numerous papers about these problems were published (Zhou et al., 2005, Pang et al., 2008, 2009, 2010, 2013; Shellnutt and Jahn, 2010, 2011; Zhou et al., 2013; Shellnutt, 2014). However, the ore forming mechanism and relationship between the Panzhihua and surrounding silicic rocks are still debated issues. 1.3.1. Ore-forming mechanism There are two main mechanisms are proposed for the formation of oxide ores in the Panzhihua intrusion: (1) fractional crystallization which means Fe-Ti oxides are precipitated directly from magmas and (2) silicate immiscibility whereby the parental magma separates into an Fe-rich silicate liquid and Si-Na-rich silicate liquid and the ores crystallize from the Fe-rich end-member.. 11.

(13) Immiscibility Silicate liquid immiscibility is a state in which two liquids with different compositions coexist in equilibrium with each other. Geologically speaking, this is a magmatic process that causes unmixing of magmas into liquids of contrasting compositions. The formation of immiscible liquids in silicate melts has been suggested as one of the possible methods by which natural magmas may produce a variety of rock type (Cassidy and Segnit, 1955). Immiscibility is common in Fe-rich tholeiitic magma, where silicic melt with lower density and viscosity can separate from dense Fe-rich melt. Immiscibility results in enrichment of iron, therefore, are considered as one of ore-forming processes in tholeiitic system. Zhou et al. (2013) develop a two-stage model for the development of magmas and ores in the Panzhihua intrusion. In the first stage, a ferropicritic magma undergoes fractional crystallization, and then segregates into two immiscible liquids, one mafic and the other silicic. The first magma solidifies to form the mafic intrusions and the latter forms the spatially associated granitic intrusions. In the second stage, an immiscible oxide liquid separates from the mafic liquid and accumulates to form the ore deposits. Fractional crystallization Fractional crystallization is a process that is commonly used to explain the genesis of layered mafic-ultramafic intrusion and was advocated by Bowen (1919) after a series of petrological experiment to explain the crystallization sequence, mineral textures and the formation of some igneous rocks. The result from his experiment indicated that if an initial basaltic magma had crystals removed before they could react with the liquid, that the common suite of rocks from basalt to rhyolite could be produced. The most important mechanism for fractional crystallization is gravitational settling, which means the higher density minerals sink down to the bottom while light minerals float on the top of magma chamber. The sinking of crystals may possibly be considered of greater importance in mafic 12.

(14) than in intermediate and silicic rocks on account of greater viscosity of the latter (Bowen, 1915). In the layered gabbroic Panzhihua intrusion, fractional crystallization is suggested playing a leading role in ore formation as well petrogenesis. Shellnutt and Jahn (2010) and Pang et al. (2008) suggested the formation of ores containing essentially Fe-Ti oxides from magnetite gabbro in the Panzhihua intrusion likely involved settling and sorting of dense oxide crystals into stratiform concentrations in the lower portions of the intrusion. They suggest after the emplacement of ferrogabbro parental magma, derived either directly from a mantle plume source or after some differentiation, was injected into the shallow crust and crystallized olivine, pyroxene and plagioclase. Consequently the residual liquid has higher the Fe and Ti contents and thus en masse oxide crystallization occurs creating a stratiform intrusion. 1.3.2. Petrogenetic relationship between gabbro and surrounding rocks In addition to voluminous flood basalts, large igneous provinces (LIPs) may contain a small volume of silicic plutonic rocks. A-type granites are alkaline rocks with high Fe/Mg and K/Na ratios, high content of K2O and low in CaO and Al2O3 contents. They are enriched in incompatible trace elements, including LILE and HFSE, but low in trace elements compatible in mafic silicates (Co, Sc, Cr, Ni) and feldspars (Ba, Sr, Eu) (Loiselle and Wones, 1979). In the Emeishan LIP, syenite and A-type are found in association with flood basalts and ore-bearing mafic intrusion, including Panzhihua and others. The petrogenetic relationship between the layered intrusions and the silicic plutonic rocks is debated. Based on geochronology, Zhong et al. (2009) suggested that Panzhihua A-type granite is part of a ∼253 Ma intrusion and represents a separate magma system from the Panzhihua gabbroic intrusion which is dated ∼260 Ma. However, Shellnutt and Jahn (2010) used geochemical evidence to indicate that there is a genetic link between the Panzhihua layered gabbroic intrusion and surrounding silicic rocks. If the relationship between Panzhihua rocks and surrounding silicic 13.

(15) rocks is elucidated then mechanism for the petrogenesis and metallogenesis of the Panzhihua intrusion will be clearer.. 1.4. Purpose of this study The purpose of this study is to conduct a petrological experiment using material that is compositionally similar to the parental magma of the Panzhihua layered intrusion.Lowpressure (atmospheric) and high-pressure (7-10 kbar) experiments were conducted in order to constrain the magmatic liquid evolution and to determine if magmatic differentiation can explain the formation the oxide ore, host gabbro and neighboring granitic pluton.. 14.

(16) Chapter 2. Geological Background 2.1. China’s tectonic framework in the global context China is a tectonically complicated region resulted from amalgamation of numerous continental blocks, which separated from each other by a complex system of faults and orogenic belts. The major terranes in China are Tarim Block, North China Block, South China Block and the Tibetan Plateau.. Figure 2-1. Tectonics sketch of China showing the location of major tectonic units (modified after Shellnutt and Pang (2012)). KL= Kunlun orogenic belt, QDSL = Qinlin-Dabie-Sulu orogenic belt, QLS = Qilinshan orogenic belt, TIMD = Tianshan-Inner Mongolia – Daxinganlin orogenic belt.. 15.

(17) To the North, the Tarim Block and North China Block are separated from the PaleoAsian orogenic belt by the Tianshan-Inner Mongolia-Daxinganling orogenic belt. To the South, these blocks are separated from the Tibetan Plateau and South China Block by the Central Orogenic Belt from west to east: the Kunlun orogenic belt – Qilianshan fold belt – Qinlin – Dabie – Sulu orogenic belt. The South China Block collided with the North China Block along the Paleozoic – Mesozoic Qinling-Dabie orogenic belt. The timing of the collision event is debated and suggested by different authors to be middle to late Mesozoic (Gilder and Courtillot, 1997), early Devonian (Dong et al., 2013) or mostly likely middle to late Triassic (Xu et al., 2014). The Tibetan Plateau consists of at least four different terranes that include: the Lhasa, Qiangtang, Yidun and Songpan-Ganze. These terranes were amalgamated during the subduction of Paleo- to Neo-Tethys since Paleozoic (Metcalfe, 2013).. 2.2. Yangtze Block and the collision to form the South China block The Yangtze Block is a craton in the west of South China Block (Figure 2-1) which is bound by an orogenic belt system: Qinling-Dabie to the North, Jiangnan belt to the east, Red River Fault zone to the south and Xianshuihe Fault to the west. The Yangtze block is mainly composed of Archean to Mesoproterozoic basement include granitic gneisses and metasedimentary rocks. These metamorphic formations are intruded by Neoproterozoic granites of various ages (from 1000 – 800 Ma). The Precambrian basement of Yangtze Block is covered by a thick sequence of Sinian to Permian strata composed of clastic and carbonate sedimentary rocks and meta-volcanic rocks (Zhou et al., 2002). The Yangtze Block,together Cathaysia Block and the Phanerozoic South China Fold Belt form the South China block. The collision of Yangtze and Cathaysia Block to form South China Block created the Jiangnan Orogenic belt (Wang et al., 2007). However the timing of the collision is debated due to disperse tectonics model and geochronological ages in the area. U-Pb zircon dating shown the timing of collision may have been 1000 – 900 Ma 16.

(18) (Li et al., 2002, 2003, 2005; Wang, 2004). Whereas, detrital zircon ages from sedimentary formations in the western part of the Jiangnan orogeny combined with published dates of orogeny-related igneous rocks in this area suggested the timing of collision is between 860 – 800 Ma (Wang et al., 2007).. 2.3. The Emeishan Large Igneous Province (ELIP) The ELIP covers an area of at least 250,000 km2 in southwest China (Yunnan, Sichuan and Guizhou Provinces), and in northwest Vietnam (Chung and Jahn, 1995). It belongs to the western part of the Yangtze Block and the eastern margin of the Tibetan Plateau. The Yangtze Block contains a Proterozoic metamorphic basement, a middle sequence of Paleozoic marine sedimentary strata, and a Mesozoic and Cenozoic terrestrial sedimentary cover (Song et al., 2013). The break up in the western Yangtze during the Early Devonian may have been related to rifting and opening of Paleotethys. The late Permian marks the final subduction of Paleotethys (270-264 Ma) and was followed by the emplacement of the ELIP (264- 257 Ma). It is possible that the closure of the Paleotethys Ocean may be related to the formation of the ELIP (Jian et al., 2009).. 17.

(19) Figure 2-2. Distribution of the major Fe-Ti-V oxide deposits in the Panxi region. Note that the close association of oxide-bearing mafic-ultramafic intrusions and syenitic plutons (modified after Pang et al. (2008) and Zhou et al. (2013)). 18.

(20) The ELIP consists of a succession of predominantly tholeiitic basalt, with minor picritic and rhyolitic lava flows and associated pyroclastic rocks. In addition to lava flows, mafic-ultramafic layered complexes, dykes and sills, syenite and other alkaline intrusions (Atype granites), are part of the ELIP. In the Panxi (Panzhihua-Xichuang) region, some of mafic-ultramafic intrusion host giant Fe-Ti-V oxide and Ni-Cu sulphide deposits (Figure 2-2). The Emeishan volcanic successions unconformably overlie the late Middle Permian carbonate (i.e., the Maokou Formation) and are in turn covered by the uppermost Permian sedimentary rocks in the east and the Late Triassic sedimentary rocks in the central part of the Emeishan LIP (Xu et al., 2007b). The ELIP is considered to be one of the best examples of a mantle plume generated LIP because it contains ultramafic volcanic rocks, a thick sequence of flood basalts, evidence of doming and uplift of the crust and a relatively rapid emplacement duration (Shellnutt, 2014). Xu et al. (2001) distinguished two main magma types in the ELIP that produced the lava flows: high-Ti (Ti/Y>500) and low-Ti (Ti/Y <500) basalt. The high-Ti group (HT) generally has higher Ti/Y (>500) and TiO2 (>3.7 wt%) than the low-Ti ones(LT), which have low Ti/Y (<500) and TiO2 (<2.5 wt%). The petrogenesis of high-Ti and low-Tibasalts have been interpreted in different ways. Chen et al.(2010) and Xiao et al. (2004) said that HT magmas may have formed directly from the melting zone of the mantle plumehead while low-Ti melts were derived from enriched subcontinental mantle lithosphere (SCLM). In another view, Xu et al. (2007a) suggested that low-Ti basalts originated from the plume head, whereas high-Ti basalts from melting of the SCLM and/or plume melts interacting with SCLM.. 2.4. The layered gabbroic Panzhihua intrusion The Panzhihua intrusion is one of the several layered intrusions that host Fe-Ti oxide ores in the central western ELIP. It is a sill-like body that dips 50-60 NW, extends NE-SW 19.

(21) for about 19 km and divided into six blocks that is controlled by faulting system (Zhou et al., 2005). This intrusion concordantly intruded dolomitic limestones of the Late Neoproterozoic Dengying Formation. At the contact of intrusion, carbonate rock of basement have been transformed into marbles and skarns within an up to 300-m-thick contact aureole (Ganino et al., 2008). This Proterozoic basement is intruded by Paleozoic gabbros and granites. Based on differences in internal structure and the extent of oxide mineralization, local geologists previously identified four zones in the intrusion: Marginal Zone (MGZ), Lower Zone (LZ), Middle Zone (MZ) and the Upper Zone (UZ) (Zhou et al., 2005; Pang et al., 2009; Song et al., 2013). Marginal zone is composed of fine-grained hornblende-bearing gabbro and olivine gabbro. The Lower and Middle zones consist of layered melanogabbro and gabbro. The Upper zone consists chiefly of leucogabbro. Most rocks in the body show variable scale rhythmic modal layering in which dark minerals, primarily clinopyroxene, dominate in the lower parts of each layer, and lighter minerals, primarily plagioclase, dominate in the upper parts. The oxide ores occur as layers and lenses within the gabbros and are concentrated in the lower parts of the intrusion (Figure 2-3) (Zhou et al., 2005; Pang et al., 2008, 2013; Shellnutt and Jahn, 2010).. 20.

(22) Figure 2-3.Geological map of the Panzhihua intrusion (modified after Zhou et al. (2005)). 21.

(23) Chapter 3. Methods 3.1. Experimental methods 3.1.1 Starting material composition selection The similarity in the U-Pb zircon age (263 ± 3 Ma) (Zhou et al., 2005) and Sm-Nd isotope (Song et al., 2013) suggested that there is a genetic link between the Panzhihua intrusion and the Emeishan flood basalts. The Panzhihua and other ore-bearing intrusion within the ELIP are located in the inner zone where is characterized by high-Ti basalt and suggests that there may be a petrogenetic association between the parental magma of the Panzhihua intrusion and high-Ti Emeishan basalts (Zhou et al., 2008). Although it was generally accepted that the Panzhihua parental magma resembles high-Ti Emeishan basalt, different parental composition were suggested. The difference between the parental magma compositions is mostly in the SiO2 (42-49 wt%) and the TiO2 (3-5 wt%) contents. In this study, we use the sample GS03-003 (Shellnutt and Jahn, 2010) with relatively high SiO2 content (48.03 wt%) and low TiO2 content (2.87 wt%). This composition was expected to generate a range of composition from mafic to silicic via fractional crystallization. Table 3-1.Estimates for the parental magma compositions of the Panzhihua intrusion Intrusion Panzhihua1 Panzhihua2 Panzhihua3 Panzhihua4 SiO2 (wt%) 48.03 42.6 49.18 45.83 TiO2 2.87 3.99 2.94 4.85 Al2O3 11.26 15.8 11.53 15.62 t Fe2O3 12.37 12.67 FeOt 15.6 FeO 11.36 Fe2O3 2.23 MnO 0.19 0.19 0.23 MgO 8.54 5.99 8.74 7.18 CaO 10.29 11.9 10.54 7.52 Na2O 2.23 2.45 2.28 3.26 K2O 1.09 0.31 1.12 1.41 1 2 3 4 Sample GS03-003, Zhou et al. (2005), Shellnutt and Jahn (2011), Pang et al. (2008) 22.

(24) 3.1.2. Experimental setting and procedure At atmospheric pressure In the low-pressure experiment, about 1.0g rock powder was loaded into platinum envelopes. The experiments were made by suspending the Pt envelopes with sample in a 1atm vertical quenching furnace. The thermal gradient in the uniform zone of the furnace is less than 1oC per 10cm. The run products were quenched by water in the end of the run by arcing the fine platinum wire with high voltage through the coarse platinum wire in the top. All temperatures were measured using a R-type thermocouple (Pt-Pt87Rh13 thermocouple). The precision of the temperature measurement is ± 1ºC (Liu et al., 1997).. B A. Figure 3-1. The high temperature experiment system. (A) Schematic diagram on the arrangement of the high temperature experiment in the quenching furnace at atmospheric pressure (modified after Liu et al. (1997)). (B) The high temperature tube furnace (Department of Earth Sciences, National Taiwan Normal University). 23.

(25) At high-pressure (7-10 kbar) The high-pressure experiments were also conducted at Department of Earth Sciences, National Taiwan Normal University. For this experiment, the runs were carried out using a Quickpress 3.0 piston cylinder apparatus, Depths of the Earth Company. Pressure is generated by pumping a hydraulic ram to force a piston into a pressure plate that consists of concentric layers of hardened-steel around a tungsten carbide core. The operating machine generates a pressure range between 5 and 25 kbar (~75km depth) and a maximum temperature of 2200oC. Au75-Pd25 capsules were used as sample containers. The temperature for the high-pressure experiments were measured with a C-type thermocouple (W5Re95W26Re74). The cooling water was circulated through a recirculation chiller and the temperature of the water is stable at 17oC.. 24.

(26) D C. B Figure 3-2. The piston cylinder device and protocol used for carrying out the high-pressure experiments (A) The parts of the pressure cell and thermocouple for a furnace asembly in high-pressure experiment (B) The capsule is placed inside a pre-formed cylinder (sleeve) made from pressed powdered NaCl that acts as a pressure-transmitting medium. (C) The sample assembly is then placed inside the central bore of the pressure plate. The tungsten carbide piston is inserted from the top. Thermocouple wires inserted from the A. bottom and passing through the lower pressure support block to make contact with the sample capsule were used to measure the temperature. (D) Pressure is applied by hand pumping the oil-driven hydraulic ram to raise the pressure plate. 25.

(27) Figure 3-3. Furnace assembly for the high-pressure experiment.. Figure 3-4. Diagram of furnace assembly for the high-pressure experiment. 26.

(28) 3.2. Analytical methods Experimental charges were mounted in epoxy and polished in longitudinal section. Phases in the run products were first identified microscopically in reflected light. Characteristic relief, reflectivity, and crystal habit were used for phase identification, along with electron micro-probe analysis and back-scattered electron imaging in questionable cases. Mineral identification and quantitative chemical analyses were made at the Institute of Earth Sciences, Academia Sinica in Taipei. The preliminary geochemical analyses were carried out using a JEOL JSM-6360LV Scanning electron microscope equipped with an INCA-300 energy dispersive X-ray spectrometer (SEM-EDS analysis). The analysis was carried out under an acceleration voltage of 15kV and 0.2 – 0.3 nA in the primary electron beam current under low-vacuum conditions (25 Pa). The Electron Probe Micro Analysis (EPMA) was made after SEM-EDS for quantitative analysis. The operated beam condition utilized was 15 kV, 10 nA, and 2 µm defocused beam for the acceleration voltage, beam current and beam size, respectively. Analysis points were carefully selected within the secondary and the back-scattered electron images. The quantitative data were corrected as oxides with standard calibration by the Phirho-z (PR-ZAF) method, which is a matrix correction with factors of atomic number (Z), absorption (A) and fluorescence (F) and depth distribution function (ρx), which represents the X-ray intensity per unit mass depth (ρz) (Philibert and Tixier, 1968; Reed, 1993). Synthetic and natural chemical-known materials were used as standards: wollatonite for Si and Ca, rutile for Ti, corundum for Al, chrome-oxide for Cr, fayalite for Fe, tephroite for Mn, pyrope for Mg, albite for Na, and adularia for K. The relative standard deviations of analysis for all 10-elements are less than 1.0%. The duration time of analysis for each element was 20 seconds, 10 seconds on the peak and 5 seconds each on the upper and lower side of the baseline. Grains of minerals in the quenched products chosen for analysis were usually larger 27.

(29) than 10 µm diameter and the diameter of analyzed glass pools was usually larger than 30 µm (Liaw et al., 2006).. 28.

(30) Chapter 4. Results 4.1. Crystallization sequence of minerals 4.1.1 Atmospheric pressure experiment In the low-pressure experiment, thirteen runs were performed over a range of temperatures from 1102 to 1312°C at atmospheric pressure to locate the liquidus temperature, the solidus temperature, and the melting interval of the basaltic melt (Table 4-1). Cr-rich titanomagnetite is the first phase to crystallize at 1289°C (average temperature between glass phase and the first mineral crystallize). Therefore, the liquidus temperature of the basaltic melt was determined to be 1289°C. Lowering the temperature result in successive crystallization of Fe-Ti oxides (titanomagnetite + ilmenite) at 1263°C (average temperature between the first Cr-rich oxide and the first Fe-Ti oxide crystallize), clinopyroxene at 1191°C (average temperature between the first oxide and the first clinopyroxene crystallize) and finally is the appearance of plagioclase and orthopyroxene at 1167°C (average temperature between the first clinopyroxene and the first plagioclase crystallize). Figures from 4-2 to 4-9 show SEM images of minerals crystallize at different tempereature. Figure. 4-1.. sequence. of. The. crystallization. basaltic. constructed. by. experiment.. Gl:. magma. low-pressure glass,. Plag:. plagioclase, CPx: clinopyroxene, OPx: orthopyroxene. 29.

(31) Table 4-1 Anhydrous melting experiment shows crystallization sequence of high-Ti Emeishan basalt. CPx: clinopyroxene; Gl: glass; OPx: orthopyroxene; Plag: plagioclase No. Samples. Temperature o. Duration. ( C). ( hr : min ). Mineral phases. 1. Pz-001. 1312. 6:00. Gl. 2. Pz-003. 1303. 6:00. Gl. 3. Pz-013. 1274. 7:30. Gl+ Fe-Ti oxide. 4. Pz-007. 1252. 19:00. Gl+ Fe-Ti oxide. 5. Pz-004. 1201. 7:00. Gl+Fe-Ti oxide. 6. Pz-012. 1194. 15:00. Gl+Fe-Ti oxide. 7. Pz-008. 1188. 23:45. Gl+Fe-Ti oxide+CPx. 8. Pz-006. 1171. 19:00. Gl+Fe-Ti oxide+CPx. 9. Pz-009. 1162. 25:25. Gl+ Fe-Ti oxide+CPx+OPx+Plag. 10. Pz-005. 1151. 9:00. Gl+ Fe-Ti oxide+CPx. 11. Pz-010. 1122. 23:00. Gl+ Fe-Ti oxide+CPx. 12. Pz-011. 1114. 25:15. Gl+ Fe-Ti oxide+CPx+Plag. 13. Pz-002. 1102. 65:25. Gl+ Fe-Ti oxide+CPx+Plag. 30.

(32) Glass. Cr-rich Fe-Ti oxide. Figure 4-2. BSE image of quenched run products at 1274oC. At this temperature, Cr-rich titanomagnetite are tiny crystals within the glass (low-pressure experiment).. Figure 4-3. BSE image of quenched run products at 1122oC showing the abundance of Fe-Ti oxide (bright phases) and CPx (gray phases) on the dark ground of glass, low-pressure experiment.. 31.

(33) CPx. OPx Fe-Ti oxide. Figure 4-4. BSE image of quenched run products at 1162oC, low-pressure experiment. All crystals are subhedral to anhedral. CPx: clinopyroxene, OPx: orthopyroxene. Plag Fe-Ti oxide CPx Gl. Figure 4-5. BSE image of quenched run products at 1102oC, low-pressure experiment. Fe-Ti oxides are small (<10μm), surround phenoscrysts of Pyroxene and plagioclase. Gl: glass, Plag: plagioclase, CPx: clinopyroxene.. 32.

(34) 4.1.2. High-pressure experiment Four runs were conducted at pressures from 7 to 10 kbar, the duration is from 6 to 12 hours (Table 4-2). The experiment results show that Fe-Ti oxides and clinopyroxene crystallize first at 1210oC (average temperature between glass phase and the first mineral crystallize). Orthopyroxene were found at 1140oC (average temperature between the first mineral and the first orthopyroxene crystallize) and finally is crystallization of plagioclase at 1075oC (average temperature between the first orthopyroxene and the first plagioclase crystallize). Table 4-2. High-pressure experiment shows crystallization sequence of high-Ti Emeishan basalt. CPx: clinopyroxene; Gl: glass; OPx: orthopyroxene; Plag: plagioclase No. Samples. Temperature. Pressure. Duration. (oC). (kbar). (hr : min ). Mineral phases. 1. PzH1. 1240. 10. 6:00. Gl. 2. PzH5. 1180. 10. 8:00. Gl + Fe-Ti oxide + CPx. 3. PzH11. 1100. 7. 7:00. Gl + Fe-Ti oxide + CPx + OPx. 4. PzH14. 1050. 7. 12:00. Gl+ Fe-Ti oxide + CPx + OPx + Plag. Figure 4-6. The crystallization sequence of basaltic magma constructed. by. low-pressure. experiment.. Gl:. glass,. Plag:. plagioclase, CPx: clinopyroxene, OPx: orthopyroxene. 33.

(35) CPx. Fe-Ti oxide. Figure 4-7. BSE image of quenched run products at 1180oC, 10 kbar. Fe-Ti oxide and CPx crystallize as the first phases. CPx: clinopyroxene. Fe-Ti oxide. Gl OPx CPx. Figure 4-8. BSE image of quenched run products at 1100oC, 7 kbar. Gl: glass, Plag: plagioclase, CPx: clinopyroxene, OPx: orthopyoxene. 34.

(36) CPx Fe-Ti oxide Plag. Figure 4-9. BSE image of quenched run products at 1050oC, 7 kbar. Plag: plagioclase, CPx: clinopyroxene. 4.2. Synthesis mineral chemistry 4.2.1 Fe-Ti oxides 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,. 35.

(37) 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).. 36.

(38) Table 4-3.The average composition of the synthesized Fe-Ti oxides at atmospheric pressure Experiment No. Temperature (oC) No. of analysis SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total. Si Ti Al Cr Fe (ii) Mn Mg Ni Ca Na K. Pz013 1274 6 0.66(0.71) 1.15(0.14) 6.71(0.53) 9.48(0.39) 56.08(1.65) 0.61(0.13) 15.40(1.90) 0.32(0.16) 0.05(0.06) 0.00 90.46(2.96). 0.22 0.28 2.62 2.48 15.49 0.17 7.56 0.00 0.11 0.03 0.00. Pz008 1188 10 0.12(0.13) 11.92(0.31) 1.80(0.11) 0.56(0.19) 69.66(0.51) 0.11(0.03) 5.23(0.18) 0.20(0.07) 0.02(0.02) 0.00 89.63(0.81). Pz009 1162 7 0.06(0.04) 13.21(0.40) 1.98(0.11) 0.35(0.36) 68.30(1.07) 0.16(0.02) 5.93(0.21) 0.18(0.08) 0.01(0.01) 0.00 90.16(0.67). Numbers of Ions on the basis of 32 (O) 0.04 0.02 3.24 3.53 0.77 0.83 0.16 0.10 21.09 20.27 0.03 0.05 2.82 3.13 0.00 0.00 0.08 0.07 0.01 0.01 0.00 0.00. 37. Pz002 1102 8 1.39(2.03) 4.43(2.23) 1.68(0.27) 0.35(0.51) 78.17(3.89) 0.16(0.06) 1.99(1.04) 0.25(0.08) 0.11(0.19) 0.03(0.07) 88.56(1.39). 2 0.54(0.14) 43.01(0.85) 1.00(0.52) 0.01(0.01) 41.74(0.52) 0.09(0.02) 4.53(0.31) 0.21(0.03) 0.09(0.00) 0.00 91.20(1.33). 0.52 1.30 0.77 0.11 25.69 0.05 1.15 0.00 0.10 0.08 0.02. 0.16 9.45 0.34 0.00 10.20 0.02 1.97 0.00 0.07 0.05 0.00.

(39) Table 4-4. The average composition of the synthesized Fe-Ti oxides at high-pressure Experiment No. Temperature (oC) Pressure No. of analysis SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total Si Ti Al Cr Fe (ii) Mn Mg Ni Ca Na K. PzH5 PzH11 PzH14 1180 1100 1050 1.0 0.7 0.7 10 5 6 2 0.14(0.03) 0.82(0.96) 0.51(0.70) 3.36(4.50) 29.78(0.26) 7.25(0.24) 34.28(2.30) 53.66(3.93) 2.00(0.14) 8.56(0.18) 1.04(0.07) 1.71(0.21) 0.17(0.07) 2.51(0.73) 0.01(0.01) 0.00(0.00) 59.68(0.50) 67.84(1.23) 54.83(2.17) 30.72(3.11) 0.07(0.04) 0.26(0.05) 0.22(0.03) 0.07(0.02) 3.74(0.09) 7.21(0.11) 4.32(0.30) 6.16(0.78) 0.18(0.04) 0.28(0.12) 0.40(0.45) 1.07(1.25) 0.02(0.03) 0.06(0.11) 0.03(0.03) 0.09(0.13) 0.00(0.00) 0.01(0.02) 0.00(0.00) 0.08(0.10) 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) 0.04 0.23 0.15 0.84 6.85 2.59 7.68 10.20 0.72 2.83 0.37 0.51 0.04 0.56 0.00 0.00 15.27 17.83 13.68 6.50 0.01 0.06 0.05 0.02 1.70 3.22 1.92 2.31 0.00 0.00 0.00 0.00 0.05 0.00 0.13 0.29 0.01 0.03 0.02 0.04 0.00 0.00 0.00 0.03. 38.

(40) 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 highpressure 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.. 39.

(41) Table 4-5. The average composition of the synthesized clinopyroxene at low-pressure Experiment No. Temperature (oC) No. of analysis SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total. Si Al Al Ti Cr Fe (ii) Mn Mg Ni Ca Na K En (Mg) Fs (Fe) Wo (Ca). Pz008 1188 12 51.24(1.85) 0.62(0.46) 2.05(1.30) 0.13(0.18) 7.11(0.64) 0.18(0.08) 15.96(0.93) 20.77(1.17) 0.29(0.09) 0.00(0.01) 98.35(0.48). Pz009 1162 13 52.51(0.65) 0.27(0.22) 1.09(0.56) 0.13(0.22) 7.47(1.13) 0.24(0.10) 14.71(1.14) 22.12(0.93) 0.22(0.05) 0.00(0.01) 98.73(0.62). Numbers of Ions on the basis of 6 (O) 1.93 1.97 2.00 2.00 0.07 0.03 0.02 0.02 0.02 0.01 0.00 0.00 0.22 0.23 0.01 0.01 2.02 2.00 0.89 0.82 0.00 0.00 0.84 0.89 0.02 0.02 0.00 0.00 45.76 42.27 11.43 12.05 42.80 45.68. 40. Pz002 1102 11 52.68(0.49) 0.33(0.23) 1.22(0.66) 0.24(0.28) 7.11(1.40) 0.24(0.12) 15.78(1.36) 20.69(1.45) 0.32(0.17) 0.04(0.05) 98.66(0.49). 1.97 0.03 0.02 0.01 0.01 0.22 0.01 0.88 0.00 0.83 0.02 0.00 45.57 11.52 42.92. 2.00. 2.00.

(42) Table 4-6. The average composition of the synthesized clinopyroxene at high-pressure Experiment No. Temperature (oC) Pressure No. of analysis SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total Si Al Al Ti Cr Fe (ii) Mn Mg Ni Ca Na K En (Mg) Fs (Fe) Wo (Ca). PzH5 PzH11 1180 1100 1.0 0.7 15 9 49.60(1.58) 48.96(0.74) 0.95(0.25) 1.37(0.37) 4.58(1.74) 5.95(1.89) 0.22(0.24) 0.08(0.06) 9.44(1.64) 10.22(0.62) 0.20(0.05) 0.21(0.06) 16.53(1.16) 15.17(2.16) 17.03(2.04) 15.52(2.95) 0.62(0.21) 0.75(0.34) 0.01(0.02) 0.19(0.28) 99.18(0.58) 98.43(0.79) Numbers of Ions on the basis of 6 (O) 1.85 1.84 2.00 2.00 0.14 0.15 0.05 0.10 0.02 0.03 0.00 0.00 0.29 0.32 0.00 0.00 2.04 2.02 0.92 0.85 0.00 0.00 0.68 0.62 0.04 0.05 0.00 0.00 48.50 47.20 15.60 18.10 35.90 34.70. 41. PzH14 1050 0.7 9 51.67(0.73) 0.35(0.30) 1.19(0.51) 0.11(0.24) 8.92(1.72) 0.33(0.13) 14.27(1.07) 21.79(1.11) 0.22(0.05) 0.02(0.02) 98.88(0.44) 1.95 0.04 0.00 0.01 0.00 0.28 0.01 0.80 0.00 0.88 0.01 0.00 40.80 14.03 44.80. 2.00. 2.02.

(43) Table 4-7. The average composition of the synthesized orthopyroxene at atmospheric pressure Experiment No. Temperature (oC) No. of analysis SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total. Pz009 1162 9 55.56(0.86) 0.45(0.12) 2.68(0.74) 0.02(0.02) 3.66(0.71) 0.27(0.03) 34.24(0.99) 2.19(0.32) 0.12(0.09) 0.04(0.06) 99.25(0.37). Numbers of Ions on the basis of 6 (O) Si 1.92 2.00 Al 0.08 Al 0.03 Ti 0.01 Cr 0.00 Fe (ii) 0.11 Mn 0.01 2.02 Mg 1.77 Ni 0.00 Ca 0.08 Na 0.01 K 0.00 Mg# 94. 42.

(44) Table 4-8. The average composition of the synthesized orthopyroxene at high-pressure Experiment No. PzH11 Temperature (oC) 1100 Pressure 0.7 No. of analysis 2 SiO2 51.19(0.23) TiO2 1.09(0.52) Al2O3 5.98(2.27) Cr2O3 0.05(0.05) FeO 13.30(1.28) MnO 0.22(0.05) MgO 22.76(4.01) CaO 3.88(1.34) Na2O 0.58(0.57) K2O 0.26(0.32) Total 99.31(0.15) Numbers of Ions on the basis of 6 (O) Si 1.86 2.00 Al 0.13 Al 0.12 Ti 0.03 Cr 0.00 Fe (ii) 0.40 Mn 0.00 2.00 Mg 1.23 Ni 0.00 Ca 0.15 Na 0.04 K 0.01 Mg# 75. 43.

(45) 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 highpressure 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.. 44.

(46) Figure 4-12.Composition of plagioclase in Panzhihua intrusion (Pang et al., 2009) and in this study. Plagioclase (An67-41Ab29-47Or2-17). 45.

(47) Table 4-9. The average composition of the synthesized plagioclase at atmospheric pressure Experiment No. Temperature (oC) No. of analysis SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total. Mol. %. Pz009 1162 11 53.07(1.01) 0.22(0.19) 27.09(2.47) 0.00(0.01) 1.24(0.73) 0.03(0.03) 1.05(2.19) 11.51(0.93) 3.94(0.48) 0.41(0.11) 98.56(0.69). Pz002 1102 12 55.79(2.42) 0.33(0.33) 24.75(2.44) 0.00(0.01) 1.84(1.67) 0.02(0.02) 0.79(0.67) 9.08(1.28) 4.36(0.42) 1.12(0.51) 98.08(0.94). Numbers of Ions on the basis of 8 (O) Si 2.45 2.58 Ti 0.01 0.01 Al 1.47 1.35 Cr 0.00 0.00 Fe (ii) 0.05 0.07 Mn 0.00 0.00 Mg 0.07 0.05 Ni 0.00 0.00 Ca 0.57 0.45 Na 0.35 0.39 K 0.02 0.07 An (Ca) 60.20 49.48 Ab (Na) 37.24 43.05 Or (K) 2.57 7.46. 46.

(48) Table 4-10. The average composition of the synthesized plagioclase at high-pressure. Mol. %. Experiment No. PzH14 Temperature (oC) 1050 Pressure 0.7 No. of analysis 23 SiO2 57.13(2.05) TiO2 0.26(0.15) Al2O3 24.50(2.34) Cr2O3 0.01(0.01) FeO 1.73(0.79) MnO 0.02(0.02) MgO 0.42(0.25) CaO 7.31(1.73) Na2O 5.22(0.52) K2O 1.98(0.80) Total 98.59(0.71) Numbers of Ions on the basis of 8 (O) Si 2.62 Ti 0.00 Al 1.32 Cr 0.00 Fe (ii) 0.06 Mn 0.00 Mg 0.02 Ni 0.00 Ca 0.35 Na 0.46 K 0.11 An (Ca) 38.05 Ab (Na) 49.40 Or (K) 12.51. 47.

(49) 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 lowpressure and decrease from 10% to 7% at high-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 highpressure 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 48.

(50) 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.. 49.

(51) 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). 50.

(52) Table 4-11 .The average composition of the synthesized glass at atmospheric pressure Experiment No. Temperature (oC) No. of analysis SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total. Pz003 1303 36 49.09(0.46) 2.80(0.09) 11.38(0.17) 0.02(0.02) 10.94(0.26) 0.13(0.04) 8.58(0.14) 10.07(0.12) 2.26(0.06) 1.05(0.03) 96.32(0.55). Pz013 1274 32 49.22(0.39) 2.78(0.15) 11.31(0.14) 0.02(0.02) 10.88(0.35) 0.16(0.04) 8.55(0.16) 10.01(0.16) 2.22(0.08) 1.02(0.03) 96.17(0.56). Pz008 1188 20 52.21(0.38) 2.29(0.28) 12.68(0.23) 0.01(0.01) 7.43(0.22) 0.14(0.04) 7.71(0.10) 9.82(0.13) 2.59(0.07) 1.22(0.04) 96.10(0.54). CIPW (weight %) Quartz Plagioclase Orthoclase Nepheline Diopside Hypersthene Olivine Ilmenite Magnetite. 0.00 38.25 6.44 0.00 27.03 18.42 1.60 5.51 2.74. 0.00 38.04 6.26 0.00 26.73 20.71 0.05 5.49 2.73. 3.19 42.90 7.51 0.00 24.51 15.51 0.00 4.52 1.87. 51. Pz009 Pz002 1162 1102 19 24 56.67(0.51) 67.20(4.02) 2.18(0.12) 0.86(0.23) 14.11(0.35) 16.11(2.38) 0.01(0.02) 0.01(0.02) 5.37(0.18) 2.94(0.59) 0.14(0.04) 0.08(0.06) 5.69(0.37) 1.79(0.79) 7.86(0.42) 4.54(1.34) 3.02(0.11) 3.14(0.53) 1.92(0.09) 3.33(0.54) 96.98(0.45) 100.04(2..89). 9.01 46.18 11.70 0.00 16.25 11.25 0.00 4.27 1.33. 22.45 46.57 19.68 0.00 2.06 6.77 0.00 0.82 0.46.

(53) Table 4-12. The average composition of the synthesized glass at high-pressure Experiment No. Temperature (oC) Pressure (kbar) No. of analysis SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total CIPW (weight %) Quartz Plagioclase Orthoclase Nepheline Diopside Hypersthene Olivine Ilmenite Magnetite. PzH1 1240 10 10 48.47(0.81) 2.11(0.33) 11.41(0.33) 0.03(0.02) 10.61(0.33) 0.15(0.02) 8.75(0.22) 10.01(0.28) 3.55(0.25) 1.08(0.05) 96.17(1.21). PzH5 1180 10 15 50.49(0.44) 3.04(0.19) 15.26(0.27) 0.01(0.01) 10.27(0.37) 0.14(0.05) 4.49(0.18) 6.81(0.14) 3.00(0.09) 1.65(0.07) 95.17(0.35). PzH11 1100 7 15 49.59(0.34) 3.40(0.18) 14.37(0.29) 0.01(0.01) 11.29(0.32) 0.13(0.03) 5.56(0.74) 7.64(0.22) 2.84(0.24) 1.59(0.04) 96.42(0.30). PzH14 1050 7 4 55.68(2.48) 2.69(1.77) 15.88(2.53) 0.01(0.02) 6.21(2.64) 0.11(0.06) 3.44(1.39) 5.94(2.69) 3.55(0.60) 3.21(1.58) 96.72(1.48). 0.00 34.87 6.62 4.80 32.06 0.00. 1.30 51.18 10.28 0.00 9.20 21.96. 0.00 47.46 9.75 0.00 13.87 18.81. 7.27 39.81 28.48 0.00 11.59 9.40. 17.44 4.18 0.00. 0.00 6.08 0.00. 3.38 6.70 0.00. 0.00 3.44 0.00. .. 52.

(54) 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 53.

(55) 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 high54.

(56) 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. 55.

(57) 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 56.

(58) 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). 57.

(59) 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.. 58.

(60) 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.. 59.

(61) 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 well with these rocks. The FeOt content from experiment is higher than Panzhihua silicic rocks probably because of oxidation state. Another difference is concentration of CaO. In the experiment, volatile content is not controlled in the experiment nor the MELTs model consequently, apatite did not crystallize and thus the CaO content is higher in the silicic rocks.. 60.

中國西南攀枝花火成雜岩之實驗岩石學研究－關於鐵－鈦－釩氧化 礦床成因

中國西南攀枝花火成雜岩之實驗岩石學研究－關於鐵－鈦－釩氧化礦床成因