1 Introduction
Many mafic-ultramafic layered intrusions are considered to be the frozen remnants of dynamic magma chambers that record the processes of in situ crystallization of silicate, oxide and other minerals (e.g. sulphide and phosphate minerals). The concept of crystal settling and sorting within magma bodies was given by the seminal study by Wager and Deer's classic 1939 memoir on the Skaergaard Intrusion in East Greenland.
From that point on their study is recognized as a major influence on the differentiation of basaltic magmas. Layered intrusions are very important hosts of base and precious metals as many contain significant reserves of platinum-group elements (PGEs), base metal sulphides, chromite, magnetite, and ilmenite. For example, meter-thick layers of pegmatitic pyroxenite in the Bushveld and Stillwater are major world repositories of platinum-group elements, i.e. Pt, Pd, Rh, Ir, Ru, and Os (c.f. Eales and Cawthorn, 1996).
Layered intrusions are considered to represent the simple concept of a magma chamber undergoing differentiation as a result of early formed crystals settling out of a magma and accumulating in layers on the floor of the chamber. This classical view has since been discarded by most petrologists in favor of models involving in situ crystallization, in which magma chambers are thought to have the general form of a central mass of nearly crystal-free magma that gradually loses heat and crystallizes inwards from its
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margins.
The Emeishan large igneous province (ELIP) located in southwest China consists of voluminous mafic, ultramafic and silicic volcanic rocks, and layered mafic-ultramafic intrusions some of which are spatially associated with felsic plutonic rocks. A number of the mafic-ultramafic intrusions host economic to sub-economic Ni-Cu-(PGE) sulphide deposits whereas others host world class Fe-Ti-V oxide deposits. The layered intrusions can be divided into two groups based on their compositions and stratigraphy.
One group of deposits at Hongge and Xinjie may originate from a more primitive parental magma (i.e. ultramafic) but subjected to open system magmatic processes (i.e.
loss of residual liquid, significant crustal contamination). The other group consisting of the Baima, Panzhihua and Taihe deposits are likely derived from more evolved parental magmas (i.e. basaltic) and were comparatively less influenced by the open system magmatic processes (Shellnutt, 2014; Zhong et al., 2005). The layered gabbroic intrusions are all spatially and temporally associated with peralkaline granitic plutons suggesting there may be a petrogenetic link between the two rock types.
The mechanism for the formation of the Panzhihua, Baima and Taihe gabbro-granitoid-ore complexes, like many aspects of the ELIP, is a highly debated issue. For example, some consider the gabbro-ore to be exclusive from the formation of the neighboring granitic rocks; whereas the others suggest that they are part of the same
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intrusion or they are a rare example of large scale silicate liquid immiscibility (Zhong et al., 2011; Shellnutt et al., 2011, Liu et al., 2014). The major features (e.g. composition, stratigraphy and age) of these three gabbroic intrusions are similar enough to suggest that they likely underwent similar processes of formation (Zhong et al., 2005).
Undoubtedly each intrusion had some unique developments that are more of a function of initial parental magma composition and location of emplacement. There are two different models on the formation of Fe-Ti-V oxide ore in the Baima intrusion. One model suggests fractional crystallization of a basaltic parental magma led to the early crystallization of Fe-Ti oxide minerals (Shellnutt et al., 2009) whereas the second model suggests silicate immiscibility at the early stage of the evolution of a ferro-basaltic magma (Zhou et al., 2013).
In this study, origin of the Baima gabbro-granite-ore complex was investigated by experimental petrological methods. A starting composition similar to Emeishan basalt (i.e. high-Ti basalt) with a similar composition as the theorized parental magma of the Baima igneous complex (Xu et al., 2001) was melted at atmospheric pressure and at high pressure conditions in order to determine the crystallization order, chemical evolution of the residual liquid composition and if such a single parental magma can produce a liquid composition similar to the spatially associated silicic pluton.
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1-1 Geological background
1-1-1 Geology of SW China (South China Block and Songpan Ganze terrane)
The Yangtze Block consists of Mesoproterozoic granitic gneisses and metasedimentry rocks which have been intruded by Neoproterozoic (~800 Ma) granites (Li, 1999; Zhou et al., 2002b). The granites are overlain by a series of marine and terrestrial strata from the Late Neoproterozoic (~600 Ma) to the Early Permian. The eastern part of the Tibetan Plateau, known as the Songpan–Ganze terrane, is composed primarily of late Triassic–early Jurassic marine sediments (Bruguier et al., 1997).
The Emeishan large igneous province (ELIP) is located in southwestern China, along the western margin of the Yangtze Block to the east and the eastern most part of the Tibetan Plateau to the west (Fig. 1-1). It covers an area of ~0.3x106 km2 of SW China and northern Vietnam (Ali et al., 2005; Shellnutt, 2014). The ELIP is not particularly large LIPs (c.f. Siberian Traps and Ontong-Java Plateau) but it contains a number of economic magmatic Cu-Ni-(PGE)-bearing sulfide deposits and mafic-ultramafic layered intrusions that host giant Fe-Ti-V deposits (Fig. 1-2) (Zhong et al., 2002; Zhou et al., 2005; Shellnutt, 2014).
The volcanic sequences range from 1.0 to 5.0 km in thickness in the western half of ELIP and 0.2 to 2.6 km in the eastern half and consist mostly of flood basalts with picrites in the lower half and basaltic-andesites and silicic volcanic rocks in the upper
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half. The volcanic rocks erupted at equatorial latitude of eastern Pangaea within one normal-polarity cycle suggesting rapid emplacement (Shellnutt, 2014).
The age of the ELIP is geologically constrained as the Late Permian having an emplacement age of ~260 Ma as determined by high precision zircon U–Pb geochronological analysis of the layered, mafic–ultramafic intrusions and felsic plutons (Zhou et al., 2002a, 2005, 2006; Shellnutt and Zhou, 2007; Shellnutt et al., 2012).
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Fig. 1-1. The Emeishan large igneous province is located in southwest China (modified from Shellnutt and Pang (2012)) (BIC: Baima igneous complex).
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Fig. 1-2. The distribution of the volcanic rocks of the Emeishan large igneous province (green area) and the Ti-V oxide and Ni-Cu-PGE sulphide deposits (from Shellnutt (2014)).
Fig. 1-2
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The Emeishan basalts can be classified into two major magma types (Xu et al.,
2001; Zhou et al., 2008). They are: 1) a low-Ti (LT) type that exhibits low Ti/Y (<500), εNd(t) (−4.8 to +1.4), 87Sr/86Sr (0.7046 to 0.7064), Mg# (0.52 to 0.64) and 2) a high-Ti sensitive indicator of crustal contamination. The oxide-bearing intrusions have very low
(Th/Yb)PM ratios and high (Nb/Th)PM ratios, high εNd values (−0.1 to +4.6), low
87Sr/86Sr ratios (0.7039 to 0.7054) and were probably derived from a parental magma
more similar to the high-Ti basalt (Zhou et al., 2008). The high-Ti basaltic rocks were considered as the parental magmas for the Fe–Ti oxide bearing layered intrusions of the Panxi region, whereas the Ni–Cu sulphide bearing intrusions belong to the low-Ti series (Zhou et al., 2008). The trace element characteristics suggest that high-Ti basalts experienced extensive crystal fractionation from parental magmas either in magma chambers or en route to the surface (Xu et al., 2001).
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1-1-2 Layered gabbroic intrusions of the Panzhihua-Xichang region
The ELIP is an important host of sulphide and oxide deposits many of which are located between the cities of Panzhihua and Xichang (Panxi) in southern Sichuan. In addition to numerous orthomagmatic deposits the ELIP contains a diverse array of within-plate A-type granitic rocks (Fig. 1-3). A-type granites are considered to silicic rocks from an anorogenic tectonic setting that is not associated with regional metamorphism or convergent plate tectonics. They are typically of peralkaline and contain sodic amphibole or pyroxene (Blatt et al., 2006). The deposits can be divided into two groups based on their compositions and stratigraphy. One group of the deposits at Hongge and Xinjie may have originated from more primitive parental magmas but were certainly subject to open system magmatic processes (i.e. recharge, assimilation).
Another group is the Panzhihua, Baima and Taihe deposits likely originated from more evolved parental magmas (i.e. basaltic) and were comparatively less influenced by open system magmatic process. Furthermore, the Panzhihua, Baima and Taihe gabbros are petrogenetically related to isotropic peralkaline quartz-bearing granitoids whereas the others do not appear to be so.
The Fe-Ti-V oxide deposits of the ELIP are a substantial economic resource.
Collectively the Panzhihua, Baima and Taihe deposits contain a total known metal
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reserve of ~4980 Mt of Fe, ~480 Mt of Ti, and ~14 Mt of V (Zhong et al., 2005) (Table 1-1).
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Fig. 1-3. Geological map of the Panzhihua – Xi Chang region (Panxi) showing the distribution of Late Permian granitic rocks (modified from Shellnutt et al., 2012).
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Table 1-1. The main reserve of iron-titanium-vanadium deposits in the Panxi region (Zhong et al., 2005).
Intrusion Fe (Mt)* Ti (Mt) V(Mt)
Panzhihua 2050 237 6.01
Taihe 1780 200 5.18
Baima 1150 44.8 2.85
Total 4980 481.8 14.04
*Mt: million tons.
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1-1-3 Baima igneous complex
This research is focused on the Baima igneous complex (BIC), which is located in the central part of the Panxi region, ~100 km northeast of Panzhihua, and consists of a gabbroic unit, a syenitic unit and oxide ore deposit (Fig. 1-4) (Chen, 1990; Yang et al., 1997). The layered gabbroic unit lies to the east of the syenitic unit, dipping ~25° to the west. The two units cover an area of similar size, but their respective volumes are unknown.
Yang et al. (1997) suggested that the REE and Sr isotope of BIC data show that the parental magma from which the Baima igneous complex crystallized is a basaltic magma. The U–Pb SHRIMP zircon ages of 258 ± 4 Ma and 259 ± 5 Ma of the syenitic unit are within error of the 261 ± 2 Ma of the gabbroic unit (Shellnutt et al., 2009). It is thought that the layered intrusive rocks and the syenites were comagmatic (Yang et al., 1997). The gabbroic unit can be divided into four lithologic zones from the bottom to the top: 1. a lower cumulate zone; 2. an oxide ore zone; 3. an olivine gabbro zone; and 4.
an upper gabbro zone (Chen, 1990) (Fig. 1-5). The characteristics of mineralogy, petrography, petrochemistry and trace element geochemistry indicate that fractional crystallization of the intrusion began at the bottom and finished at the top (Chen, 1990).
The rocks consists of coarse grained cumulate olivine, plagioclase, clinopyroxene and
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interstitial Fe–Ti oxide minerals with minor amounts of sulphide minerals, spinel, and apatite (Shellnutt and Pang, 2012). The ore deposits are hosted within the lower portions of layered cumulate gabbroic intrusions (Shellnutt et al., 2009). The syenitic unit is structurally above the gabbroic unit and contains abundant ellipsoidal mafic enclaves varying in size from a few centimeters to 10s of centimeters in length (Shellnutt et al., 2010). The syenitic unit is granular except for a few small bands of finer grained textures which appear to be more siliceous in composition. The syenites are coarse grained and consist of perthitic alkali feldspar, amphibole, quartz, aegirine and accessory (≤ 5 vol %) apatite, titanite, zircon, plagioclase, fluorite, biotite, ilmenite, magnetite and pyrite (Shellnutt and Iizuka, 2011). The BIC was subsequently intruded by two metaluminous syenitic plutons and numerous alkaline mafic dykes (Shellnutt and Zhou, 2007; Shellnutt and Iizuka, 2011; Shellnutt et al., 2012).
1-1-4 Mafic dykes
There are abundant felsic and mafic plutonic rocks in Panxi region. Intruding these plutons are narrow (generally ≤ 5 m wide) mafic dykes. The dykes differ considerably
from other mafic intrusions of the Panxi region because they have diabasic textures, while the other mafic intrusions are granular and have cumulate textures. The dykes consist primarily of plagioclase (50%), clinopyroxene (35%), biotite (5–10%), and Fe–Ti oxides (5–10%), with minor olivine (< 5%), apatite (< 5%) and sulphides
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(Shellnutt et al., 2008).
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Fig. 1-4. The geological map of Baima igneous complex, including syenite (blue), gabbro (red) and oxide ore deposit (black) (modified from Shellnutt and Pang, 2012).
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Fig. 1-5. Stratigraphy of Baima layered intrusion showing the subdivisions of the Layered Series (modified from Chen, 1990; Shellnutt and Pang, 2012).
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1-1-5 Origin of the Baima Fe-Ti-V oxide models
The formation of Fe-Ti-V oxide is considered to form by either silicate liquid immiscibility or fractional crystallization (Liu, 2014; Shellnutt et al., 2009; Zhou et al., 2008). Under suitable conditions (e.g. lower pressure, alkalic composition) two-silicate liquid immiscibility will produce two end-members, Fe-rich and Si-rich members, of approximately equal proportions (Roedder, 1979). The composition of the Fe-rich end-member may resemble the bulk composition of a mafic cumulate rock whereas the Si-rich end-member will be similar to some granitic rocks. Liu et al. (2014) suggested that plagioclase and olivine began to crystallize during cooling of the Baima parental magma. Olivine sinks to the floor of the magma chamber whereas the plagioclase floats upward. The residual magma becomes denser and more enriched in Fe and Ti. The dense Fe-Ti-rich melt droplets with low viscosity tended to sink to the bottom of the chamber where they infiltrated downwards through the unconsolidated crystal pile and filled in the interstitial spaces between the silicate minerals, forming the net-textured and disseminated ores in the bottom side. The immiscible Fe-Ti-rich melts were trapped in the growing silicate crystals and crystallized titanomagnetite, ilmenite, apatite, phlogopite, hornblende and pyrrhotite. On the other hand, Shellnutt et al. (2009, 2011) suggested that a mafic magma was injected into the crust and crystallized olivine, plagioclase and clinopyroxene. During fractionation, the relative oxidation state (i.e.
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O2) increased and led to en masse crystallization of oxide minerals. The removal of
plagioclase from the parental magma likely caused a „plagioclase effect‟ which resulted
in the residual magma to become peralkaline. Continued fractionation and decreasing
O2 produced an ilmenite dominated upper gabbro near the boundary with the syenitic
unit.
1-2 Purpose
The formation of the oxide ore deposits is debated, the three main problems are as follows: (1) formation of the oxide ore deposits by silicate-liquid immiscibility or fractional crystallization, (2) composition of parental magma, and (3) relationship between A-type granitoids and ore-bearing gabbroic intrusions. Therefore, the purpose of this study is to determine if a parental magma similar to high-Ti Emeishan basalt can produce all three rock-types observed in the BIC by experimental petrology. If so, the results will have significant implications for the genesis of orthomagmatic oxide ore deposits and the formation of A-type granites.
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