Journal of Earth Science  2018, Vol. 29 Issue (5): 1049-1059   PDF    
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Rutile in HP Rocks from the Western Tianshan, China: Mineralogy and Its Economic Implications
Wen Su1, Jilei Li2, Qian Mao1, Jun Gao2, Xin Liu3, Fei Chen1, Xiao-Mei Ge1    
1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
2. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
3. Beijing Institute of Geology for Mineral Resources, Beijing 100012, China
ABSTRACT: Rutile is a common Ti-bearing accessory mineral in high pressure (HP) metamorphic rocks of the western Tianshan. Distribution of rutile in the HP rocks varies from 0.5% in the greenschist to 30% in the rutile-bearing vein. Rutile can be subdivided into three groups based on the variation of trace elements:The first group has an averaged Zr content of 18 ppm-44 ppm and Hf content of 0.8 ppm-2.4 ppm, which correspond to occurrence of rutile from vein; the second group Zr of 59 ppm-63 ppm, Hf of 3.5 ppm-3.7 ppm; and the third group Zr of 150 ppm-160 ppm and Hf of 3.9 ppm, corresponding to rutile in the matrix of blueschist and eclogite, respectively. Rutile has been retrograded to ilmenite and titanite partly or completely, reducing the value of the ore. But rutile of HP rocks in the western Tianshan usually has the lowest content of uranium (< 1 ppm U), which might become an attractive raw material for the Ti industry. Therefore, rutile-bearing HP rocks in the western Tianshan as a mineral resource will be the focus of considerable attention.
KEY WORDS: Ti resource    rutile    HP vein    eclogite    western Tianshan    

0 INTRODUCTION

Titanium is the ninth most abundant element in the Earth's crust and the fourth most abundant metal (e.g., Rudnick and Fountain, 1995; Knittel, 1983; Minkler and Baroch, 1981). Its commercial production commenced in 1948 driven by the demand from the aerocraft industry, and different uses of the titanium dioxide pigment as filler in paper, plastics, rubber industries and as flux in glass manufacture (e.g., Gázquez et al., 2014; Gambogi, 2011). World titanium dioxide production reached 720 million tons in 2015 according to the United States Geological Survey (USGS) (e.g., Gambogi, 2016). It is present in rocks as oxide and silicate minerals and the main titanium-containing minerals are rutile, ilmenite and leucoxene as shown in Table 1 which lists the most common titanium minerals and their chemical compositions (e.g., Meinhold, 2010; Rhee and Sohn, 1990; Whitehead, 1983; Barksdale, 1966). Rutile (TiO2) commonly contains about 95 wt.% TiO2 and is the most titanium-rich mineral. Ilmenite (FeO·TiO2 or TiFeO3) contains 40 wt.%–65 wt.% TiO2, depending on its geological evolution. Leucoxene (Fe2O3·nTiO2) is a natural alteration product of ilmenite, typically containing more than 65 wt.% TiO2. Deposits of titanium can be grouped into a variety of igneous, metamorphic, hydrothermal and sedimentary types (e.g., Force, 1991). At present, the igneous and sedimentary deposits are of great economic importance. However, by 1981, the content of titanium had decreased to less than 1% rutile (e.g., Gambogi, 2011; Minkler and Baroch, 1981). In the next decade, the global demand for TiO2 is expected to continue increasing at an average rate of about 3% annually (Gázquez et al., 2014). Therefore, resources of Ti-minerals are limited in volume and are being rapidly depleted. Recently eclogites have been proved to be a high-quality rutile deposit with a good potential for production in the future (e.g., Meinhold, 2010; Xiao et al., 2006; Korneliussen et al., 1999; McLimans et al., 1999; Korneliussen, 1995; Force, 1991; Korneliussen and Foslie, 1985). The purpose of this article is to describe the occurrence, petrology and formation of rutile in HP rocks and veins from the western Tianshan, China and address their potential use as a Ti resource.

Table 1 Titanium-bearing minerals and their chemical compositions
1 GEOLOGICAL BACKGROUND AND TI-BEARING HP ROCK PETROGRAPHY

The western Tianshan high-pressure low-temperature (HP-LT) metamorphic belt in north-western (NW) China extends for ca. 1 500 km (Fig. 1), which connects eastward with the north of Kumux (Gao et al., 1995) and westward with the Kyrgyzstan (e.g., Tagiri et al., 1995; Sobolev et al., 1986) and the Tajikistan Fan-Karategin HP belt (Volkova and Budanov, 1999). The HP-LT metamorphic belt from the NW China mainly consists of blueschist-, eclogite- and greenschist-facies rocks (e.g., John et al., 2008; Gao and Klemd, 2003; Gao et al., 1999). Eclogites occur as boudins, pods, massive blocks or as thin layers interlayered within the blueschist (Li et al., 2016a; Figs. 2a2b). The eclogites have experienced peak metamorphism: T=540–630 ℃, P=1.4–2.1 GPa (e.g., Wei et al., 2003; Klemd et al., 2002). Polycrystalline quartz aggregates and coesite were found as inclusions in garnet of some eclogites, indicating that some rocks underwent ultrahigh-pressure (UHP) peak metamorphism (Lü et al., 2009; Zhang et al., 2005, 2002). Blueschist occurs as small discrete bodies, lenses, bands within greenschist. And sometimes it is interlayered with eclogite and surrounded by blueschist-facies mica schists.

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Figure 1. Local geological map showing the distribution of Ti-bearing rocks in the western Tianshan high-pressure low-temperature metamorphic belt in northwestern China.
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Figure 2. (a) Eclogite, as boudins, is interlayered with the blueschist; (b) eclogite, as thin layers, is interlayered with the blueschist.

Titanium-bearing minerals mainly occur in the blueschist, eclogite, amphibolite and High-pressure (HP) vein or segregation (Figs. 1, 3). Eclogite assemblages were formed by prograde metamorphic reactions (e.g., Liu et al., 2014; Gao and Klemd, 2003). Some omphacite-glaucophane eclogite shows clear foliation demonstrating relatively stronger orientation. The eclogite is medium-grained. It comprises garnet, omphacite, glaucophane, epidote, phengite, paragonite, clinozoisite, quartz, carbonates, rutile, and titanite (cf., Li et al., 2013; Fig. 4a). Accessory minerals include pyrite, apatite, allanite and zircon. Titanium is mainly sinked in rutile, ilmenite and titanite. Rutile occurs mostly as grains in the matrix, a lesser as numerous but tiny inclusions within silicate-minerals, like garnet, omphacite, epidote and glaucophane (Figs. 4b4c). Garnet typically contains inclusions, such as omphacite, amphibole, paragonite, quartz, rutile, carbonate, titanite and apatite (Fig. 4c). Omphacite is idioblastic to xenoblastic and occurs as a matrix mineral together with amphibole and mica. Mica comprises phengite and paragonite. Amphibole porphyroblasts occasionally contain omphacite inclusions. Texturally primary glaucophane was replaced by barroisite along the rims (Su et al., 2009). Local coronitic or symplectic retrogression along late fractures (such as actinolite, epidote, chlorite, calcite, quartz, magnetite, ilmenite) represents a late greenschist-facies overprint.

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Figure 3. Outcrop photograph of rutile in the high pressure rocks. (a) Rutile (Rt) occurs as coarse grain (1.5 cm size) in the blueschist. (b)–(c) A rutile-bearing vein consisting of omphacite (Omp), quartz (Qtz), ankerite (Ank) cuts a blueschist. (d) A rutile-bearing vein consisting of quartz and ankerite presents between blueschist and eclogite. (e)–(h) Rutile-bearing veins/segregations in eclogites. Large rutiles associated with quartz, ankerite, omphacite, and epidote (Epi). (g) Orientated acicular rutile segregation in the center of an eclogite indicating the growth of rutile in a fluid-filled cavity under peak eclogite-facies conditions. (h) The rutiles show plastic deformation. Ap. Apatite.
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Figure 4. Microphotographs showing petrological textures for eclogite of the western Tainshan. (a) The eclogite comprises of garnet (Grt), omphacite, amphibole (Amp), phengite (Phen), paragonite (Pg), clinozoisite, quartz, calcite (Cc), rutile, titanite (Ttn) and albite (Ab) (sample 074A1). (b)–(d) occurrence of rutiles in the eclogite, as grains in the matrix ((b), sample 074e1), as inclusions of the former blueschist facies mineral assemblage in the garnet ((c), sample 074L1)) and omphacite ((d), sample 074e1). Mus. Muscovite; Py. pyrite.

The blueschist is mainly composed of garnet, glaucophane, phengite, paragonite, epidote, quartz, rutile, titanite and albite (Figs. 5a5b) with minor clinozoisite, carbonate, apatite, zircon and opaque phases. Garnet contains abundant inclusions, which cover whole matrix mineral assemblage. Relict omphacite inclusion was found in the glaucophane, indicating that blueschist and eclogite underwent an identical metamorphic evolution (Klemd et al., 2002). Paragonite, amphibole and epidote contain inclusions of Ti-bearing minerals (titanite, rutile) (Figs. 5b, 5c, 5d). The rutile and titanite/ilmenite are present in clusters throughout the blueschist samples (Fig. 5a) and are mostly associated with each other (Figs. 5b, 5d).

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Figure 5. Microphotographs showing petrological textures for blueschists and veins of the Atantayi River area in the western Tainshan. (a)–(b) The blueschist comprises of garnet, glaucophane, phengite, paragonite, epidote, quartz, rutile, titanite and albite (sample 075a3). (c)–(d) Rutile occurs as inclusions in the paragonite (c), and epidote ((d), sample 10sw8-2). Ilm. Ilmenite.

The HP veins or segregations occur mainly within blueschists and eclogite (Li et al., 2013; John et al., 2008; Gao et al., 2007; Gao and Klemd, 2001). The size of veins ranges from millimeter×centimeter to decimeter×meter. The HP veins consist of omphacite, quartz, epidote-group minerals, garnet, apatite, carbonates, rutile or titanite, and minor amphibole, phengite with coarse grained, granoblastic heterogranular texture (Figs. 3b3h, 4d). A striking feature of some veins relates up to 10 cm sized rutile grains (Figs. 3b–3h). White mica and glaucophane represent minor phases (< 1.5%). Rutile-bearing vein has high modal proportions of rutile (15%–30%) at the locality (Fig. 3g). In the vein, centimeter-sized rutile crystals are typically randomly oriented, whereas centimeter-sized fibrous omphacite grows perpendicular from the vein wall to the median line (Figs. 3b, 3f; cf. Fig. 2 of Gao et al., 2007). Some centimeter-sized ankerite grains and minor apatite are intergrown with the rutile crystals (Figs. 3f, 3g).

2 TITANIUM-BEARING MINERALOGY AND MICROSTRUCTURES 2.1 Rutile

Rutile is generally the common Ti-phase in the HP rocks of the western Tianshan, and usually, it inhomogenously distributes throughout the HP rocks. Its proportion varies in different metamotphic rocks, ranging from 0.5% in the greenschists to 30% in the rutile-bearing veins (Figs. 3, 5). Equally, rutile has variable grain sizes (0.3 mm–5.0 cm of size in diameter) according to its occurrence (Figs. 3, 5). Figures 3f3h show a single megacrystal of rutile about 5 cm in diameter. There are three types of rutile occurrences. First, rutile occurs mostly as grains or fine-grained xenomorphic clusters (Figs. 3, 5) with brown or puce color occupying the matrix, and in some cases it is elongated and arranged micro-vein along the foliation. Second, rutile displays as oriented, acicular large crystal (centimeter-sized, Figs. 3f–3h) in the irregularly shaped vein. Locally it is concentrated as clusters (Fig. 6a). It is replaced by titanite or ilmenite partly or completely (Figs. 6b6c) during the retrograde metamorphism. The third occurrence is as numerous inclusions within HP minerals like garnet, omphacite, glaucophane, epidote and white mica, and some through garnet grain boundary (Fig. 6d). Rutile is also found as inclusions in pyrite (Fig. 6f), which is replaced by magnetite at the rim (cf. Li et al., 2016b, Fig. 6f). Rutile contains few inclusions of zircon and quartz (Fig. 6f), indicating saturation in zirconium, silicon during formation. In the rutile-bearing veins, rutile is found to generally have a complete (110) cleavage with oxidation-exsolution lamellae of ilmenite and sometimes (011) micro-crack with filling ilmenite (Figs. 6b, 6h6i) as well as lamellae of hematite (Fig. 6h). The micro-crack cuts exsolution lamellae (Fig. 6i), indicating the micro-crack formed later. Rutile-titanite symplectites are common in the blueschists, greenschists and some veins (Figs. 5, 6). It is partly retrograded into ilmenite (Fig. 6e). However, a few rutile occurs as rim or intergrowth with ilmenite as observed in the host eclogite (Fig. 6g) indicating a prograde metamorphism process at the blueschist to eclogite transition (Gao and Klemd, 2001).

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Figure 6. Microphotographs and back-scattered electron (BSE) images of Ti-bearing minerals. (a) Photomicrograph of rutiles as clusters in eclogite (sample 074e1). (b) BSE image showing rutile-ilmenite intergrowths rimmed by titanite. This type of rutile/ilmenite intergrowth is typical of eclogites that have experienced pervasive early retrogression. The narrow titanite rim indicates retrogression under late retrograded conditions (sample 074E). (c) BSE image of a large rutile grain from a quartz-white mica-omphacite vein in eclogite. Tiny, needle-shaped ilmenite exsolutions occur in the rutile. And rutile has a retrogressive rim of ilmenite and titanite (sample 07RU3). (d) BSE image showing examples of small rutile as inclusions in garnet, and some through garnet grain boundary (sample 075a). (e) Photomicrograph showing rutile contains pyrite inclusions (sample 074E). (f) BSE image showing the rutiles that are occurring near to pyrite. The pyrite contains rutile inclusions, and pyrite replaced by chalcopyrite and magnetite (Mt) in the rim. The rutile contains few inclusions of zircon (Zir) and quartz inclusions (sample 078i). (g) BSE image showing rutile occurs as rim in ilmenite (sample 078i). BSE images showing alteration of rutile to titanite (h) or ilmenite along fine fractures under retrograded conditions (i) (sample 074a). (j) BSE image showing a coarse ilmenite contains abundant inclusions of omphacite, albite, quartz, white mica, apatite, biotite. It is noted that titanite rimmed rutile, and rutile occurs as tiny, needle-shaped and orientational in the ilmenite (sample 075a). (k)–(l) BSE images showing rutile contains carbonate inclusions (sample 07A, 07RU3). Cpy. Chalcopyrite; Ht. hematite.

Rutile from these HP rocks has high contents of FeO (0.5 wt.%–1.23 wt.%) and Cr2O3 (0.03 wt.%–0.2 wt.%). No systematic zoning in major element could be observed in rutiles (Figs. 6b, 6e, 6g, Table 2). But it has shown a weak variety in the trace elements in single grain (Table 3). It is noted that the extent of variations in V, Cr, Zr, Hf, Nb, Ta, Sb, Sn, W among several rutile grains measured within single samples differs depending on occurrence or samples (Table 3, references in Gao et al., 2007). Rutile can be subdivided into three groups based on the variation of trace element: The first group has an average Zr content of 18 ppm–44 ppm, Hf content of 0.8 ppm–2.4 ppm; the second group Zr of 59 ppm–63 ppm, Hf of 3.5 ppm–3.7 ppm; and the third group Zr of 150 ppm–160 ppm, Hf of 3.9 ppm (Table 3, Fig. 7). Three groups correspond to occurrence of rutile as: the first group to rutile in the vein, the second and the third group to rutile in the matrix of blueschist and eclogite, respectively. The rutile as inclusion in the garnet, omphacite, etc., is too small to be analysed by LA-ICPMS. In general, rutile from the eclogites and blueschists contains more Zr, Hf, B and Sn contents, while lower Nb, Ta and V contents than that of the greenschists or the vein (Table 3). But rutile from the pyrite-rich vein contains the most Nb content > 1 400 ppm, rutile as inclusion in the pyrite is 2 065 ppm. Rutile normally has the lowest content of uranium (< 1 ppm U), Sr, Sc (Table 3), which is similar to Norwegian rutile deposits (< 2 ppm in Caledonian eclogite deposits), in the Mg verses Al in the rutile diagram (Fig. 7a), all the rutile samples plot in the mantle sources (Smythe et al., 2008), indicating original mafic-igneous protolith of the HP rocks (Korneliussen et al., 2000). In the Nb versus Cr in the rutile discrimination diagrams (Fig. 7b), the samples are mainly plotted into the metamafic source for rutile, a sample such as 078i plotted into the metapelitic rock (Meinhold et al., 2008; Smythe et al., 2008). However, according to the Nb versus Cr diagram (Triebold et al., 2007), rutile is mainly plotted into the metamafic source, but some are plotted into the metapelitic rock and some lie close to the mixing line of metamafic and metapelitic source. These pieces of evidence indicate that fragments of sediments entered into the subduction channel during subduction, and mixed with magmatic rock blocks, and were later transported to greater depth and experienced high pressure metamorphism (Li et al., 2016a; Liu et al., 2014).

Table 2 Electron microprobe analyses of representative Ti-bearing minerals (wt.%) from the HP rocks, western Tianshan
Table 3 Representative trace-element data (ppm) of rutiles from the HP rocks, western Tianshan
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Figure 7. (a) Nb versus Cr discrimination diagram for rutile from different metamorphic lithologies (after Meinhold et al., 2008); (b) Al versus Mg discrimination diagram for rutile derived from crustal and mantle sources (after Smythe et al., 2008).

In comparison to eclogitic rocks, rutile from other types of deposits commonly contains 50 ppm–100 ppm U (Korneliussen et al., 2000), with examples of rutile from the Lindvikkollen rutile-bearing albitite (100 ppm U) and the Odegarden rutilebearing scapolite hornblende rock (61 ppm U) (Korneliussen et al., 2000).

2.2 Ilmenite and Titanite

Most ilmenite and titanite are retrograde products after rutile, whereas some titanite may be prograde relics (Fig. 6j). Backscattered electron images reveal tiny, needle-shaped ilmenite exsolutions occur in the rutile from a quartz-garnet-omphacitemica vein in eclogite (Figs. 6b, 6h6i). A few rutile grains show rim of ilmenite or intergrowth with ilmenite (Fig. 6g). Rutile and ilmenite are altered to titanite during retrogressed process (Fig. 6c). Titanite typically occurs as helicitic porphyroblasts or porphyroblasts, or as fine veins along fine fractures. The former contains relict rutile (Figs. 6c and 6f). A few titanite grains occur as inclusions, displaying replacement by rutile in the rim (Fig. 6j). Concentrations of FeO and Al2O3 in titanite range from 0.3 wt.% to 0.5 wt.% and 1.4 wt.% to 2.0 wt.%, respectively. The TiO2 content of titanite ranges between 37 wt.% and 39 wt.%. The TiO2 content of titanite grains in the greenschists ranges between 34 wt.% and 37 wt.%, whereas higher TiO2 content is related to the titanite rims of rutile in the eclogites or blueschists ranging between 39 wt.% and 41 wt.%. In general, ilmenite contains 53 wt.% TiO2, and some have higher TiO2 content (up to 71 wt.%) (Table 2). The overall MnO content (1.43 wt.%–2.98 wt.%) in ilmenite is higher in the western Tianshan HP belt. The MgO and Cr2O3 contents in ilmenite are low, which are < 0.03 wt.% and < 0.06 wt.%, respectively (Table 2). High Cr2O3 contents in ilmenite have been found in some Ti deposits, such as Norwegian rutile deposits (Sunnfjord region, Egersund Province, etc.) (Korneliussen et al., 2000).

3 FORMATION AND EVOLUTION OF TI-BEARING MINERALS 3.1 Evolution of Ti-Bearing Minerals

The evolution of Ti-bearing minerals is related to metamorphism for the HP rocks. Based on mineral paragenesis, their occurrences and microstructural relationships, three evolutional stages in the HP rocks from the western Tianshan can be recognized: prograde metamorphic evolution, peak eclogite facies conditions and retrograde metamorphic stage of the P-T evolution (Klemd et al., 2002; Gao et al., 1999). The P-T evolution has a major effect on the Ti-bearing mineralization. Rutile is stable in a wide P-T range at high pressure (P > 1.2 GPa), while titanite is confined to low pressure (P < 1.2 GPa) and temperature (T < 700 ℃), and ilmenite occurs at low pressure (P < 1.5 GPa) and high temperature (T > 700 ℃) (Tao et al., 2017). Geochemical data have revealed that protoliths of blueschists and eclogites belong to N-MORB, E-MORB, OIB, gabbro, basic volcanidastic rock with high Fe-Ti contents (Liu et al., 2014; Gao et al., 2007, 1999). The Fe-Ti oxide-bearing metagabbroic rocks have undergone eclogite-facies metamorphism during collision of the Yili and the Tarim blocks along the southwestern margin of the Altaids during the Late Carboniferous (Liu et al., 2014; Gao et al., 2011; Li et al., 2011; Su et al., 2010). Pyrite contains not only rutile or ilmenite inclusions (Fig. 6f) but also omphacite, garnet, lawsonite. It intergrowth with rutile, and magnetite occurs in the rim of pyrite (Fig. 6f). Rutile occurs in the rim of ilmenite (Fig. 6g). It is notable that ilmenite contains abundant inclusions of omphacite, albite, quartz, white mica, apatite, biotite, and rutile occurs as tiny, needle-shaped and orientational in ilmenite (Fig. 6j). It indicates that rutile and pyrite have formed from Fe-Ti oxidebearing protoliths during peak eclogite facies conditions

$ \begin{array}{*{20}{l}} {{\rm{FeTi}}{{\rm{O}}_3}\left({{\rm{ilmenite}}} \right) + 2{\rm{S}} \to {\rm{Fe}}{{\rm{S}}_2}\left({{\rm{pyrite}}} \right) + {\rm{Ti}}{{\rm{O}}_2}\left({{\rm{rutile}}} \right) + {\rm{O}}}\\ {3{\rm{FeTi}}{{\rm{O}}_3}\left({{\rm{ilmenite}}} \right) + {\rm{O}} \to {\rm{F}}{{\rm{e}}_{\rm{3}}}{{\rm{O}}_{\rm{4}}}\left({{\rm{magnetite}}} \right){\rm{ + Ti}}{{\rm{O}}_{\rm{2}}}\left({{\rm{rutile}}} \right)} \end{array} $

Saturation of Fe-Ti oxides can occur in evolved basalts in all tectonic settings (e.g., Yang et al., 1998; Hooper and Hawkesworth, 1993; Frey et al., 1990; Leeman et al., 1976), therefore a characteristic occurrence of rutile in most eclogites is as clusters or aggregates of rutile (Figs. 56), which maybe the pseudomorph of early Fe-Ti oxides in the protolith (Korneliussen et al., 2000). Rutile in the veins/segregations occurs as crystals up to several centimeters in size (Figs.56) and U-Pb data of rutile from eclogite, blueschist and HP veins obtained ca. 318 Ma (Li et al., 2011), indicating that the rutile formed at peak metamorphism. It implies that rutile (Figs. 56) is the stable Ti mineral during eclogitisation. The eclogitisation was associated with fluid activity or dehydration and metasomatic processes, leading to transformation of blueschist into eclogite, formation of typical garnet-omphacite eclogite, and a variety of metasomatic, rutile-bearing, quartz-omphacite-garnet-mica rocks and quartz-omphacite veins (Gao et al., 2007, 1999). Rutile has begun to form from Fe-Ti oxide-bearing protoliths, whereas FeO is strongly fractionated into silicates, such as entering into garnet. Rutile contains titanite inclusions (Figs. 6b, 6e, 6i) may indicate titanite is another major phase in the lower metamorphism, and then transformed into rutile by high pressure metamorphism from blueschist to eclogite.

In addition, carbonate is found as inclusions in garnets, indicating that decomposition of ankerite liberating CO2 into the fluid during eclogitization (Gao et al., 2007). Inclusions of carbonate in rutile (Figs. 6k, 6l) indicate CO2-rich fluid exists during formation of rutiles. While CO2 add into H2O-rich fluids that reduces solubility of rutile (Ayers and Watson, 1993), and causes the precipitation of rutile (e.g., Gao et al., 2007; Molina et al., 2004).

$ \begin{array}{*{20}{l}} {{\rm{2Glaucophane + 6Titanite + Ankerite}} \to 4{\rm{Jadeite + }}}\\ {{\rm{7Hedenbergite + 6Rutile + 2}}{{\rm{H}}_{\rm{2}}}{\rm{O + 2C}}{{\rm{O}}_{\rm{2}}}} \end{array} $

Rapp et al. (2010) observed that F-rich fluid unusually enhances the solubility of rutile (e.g., Van Baalen, 1993; Bright and Readey, 1987). Fluorine-rich apatites precipitate in the fluid that leads to poor-F fluid, and decrease the solubility of TiO2, and result in rutile deposition from the fluid (Rapp et al., 2010; John et al., 2008; Gao et al., 2007; Malaspina et al., 2006). It is thus reasonable to observe the apatite paragenesis with rutile in the vein or the segregation (Figs. 3f3g).

During retrograde metamorphism, fractures and shear zones opened for an influx of fluids which triggered post-peak blueschist facies and epidote-amphibolites facies retrograde reactions in which rutile altered to ilmenite. Figures 6h6i show ilmenite or titanite replacing rutile along cracks within rutile during epidoteamphibolite facies. The latest retrograde metamorphic stage, rutile was retrograded into ilmenite or titanite.

3.2 Rutile Thermometry

We calculated rutile thermometer of the three rutile groups using the Zr in rutile thermometer of Zack et al. (2004), obtaining the first group with Zr content of 18 ppm–44 ppm, the second with 59 ppm–63 ppm, third with 150 ppm–160 ppm corresponding to temperatures of ca. 401–475, ca. 520, 638 ℃, respectively. The second and the third groups are in accordance with the peak eclogite metamorphic results (e.g., Wei et al., 2003; Klemd et al., 2002). Others are lower than the temperature of the peak metamorphic condition. Considering their occurrence from HP veins, it is interpreted the rutiles of the first group to reflect Zr contents of early retrograde metamorphism, but still HP metamorphic stage. Most likely, these grains with lower Zr contents have been found in the veins may be the cause of metamorphic fluids in the diffusion and early retrograde metamorphic recrystallization (e.g., Meinhold, 2010; Watson et al., 2006; Zack et al., 2004). Therefore, it may be result of differences of Zr loss by retrograde metamorphism and fluid effects due to the rutile in different locations, its occurrence, gain size, local environmental differences (Cherniak et al., 2007; Watson et al., 2006). Maybe rutile as inclusions in mineral or core in rutile is likely to avoid diffusion and fluid retention effect and records the highest temperature record.

4 CONCLUSIONS

(1) Ti-bearing minerals of the HP-LT metamorphic belt from western Tianshan are mainly rutile, titanite and ilmenite. The rutile among them is mainly Ti-bearing minerals, others are retrogressive products of rutile.

(2) Three types of rutile occur in the HP rocks from western Tianshan: occurs as inclusions in the HP minerals; grains or fine-grained xenomorphic clusters in the matrix; is elongate deformation and arranged micro-vein along the foliation in the location.

(3) The rutiles of the HP-LT metamorphic belt usually contain the lowest content of uranium. It might be a qualified raw material for the Ti-mine industry.

In a word, rutile-bearing HP rocks in the HP-LT metamorphic belt from the western Tianshan as a mineral resource will be the focus of considerable attention for the future requirements of the Ti-mine industry.

ACKNOWLEDGMENTS

This paper is dedicated to the celebration of Prof. Zhendong You's 90th birthday. This study was funded by the National Natural Science Foundation of China (Nos. 41672061, 41472059) and the State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. Professor Ming Zhang is thanked for the correction in the English expression. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0848-7.


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