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Volume 31 Issue 2
Apr.  2020
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Huang Yang, Deng Hao. Geochemical Characteristics of Zoned Chromites in Peridotites from the Proterozoic Miaowan Ophiolitic Complex, Yangtze Craton: Implications for Element Mobility and Tectonic Setting. Journal of Earth Science, 2020, 31(2): 223-236. doi: 10.1007/s12583-019-1278-x
Citation: Huang Yang, Deng Hao. Geochemical Characteristics of Zoned Chromites in Peridotites from the Proterozoic Miaowan Ophiolitic Complex, Yangtze Craton: Implications for Element Mobility and Tectonic Setting. Journal of Earth Science, 2020, 31(2): 223-236. doi: 10.1007/s12583-019-1278-x

Geochemical Characteristics of Zoned Chromites in Peridotites from the Proterozoic Miaowan Ophiolitic Complex, Yangtze Craton: Implications for Element Mobility and Tectonic Setting

doi: 10.1007/s12583-019-1278-x
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  • The chrome spinel (chromite) in mantle peridotites from ophiolites can shed light on the formation and evolution process of ophiolites. Podiform chromites were found in the Late Proterozoic Miaowan ophiolitic complex (MOC), Yangtze Craton. Due to the metamorphism and intense deformation, most chromite grains in the MOC peridotites show typical chemical zoning (core-rim texture). The values of major and trace elements largely vary from core to rim within chromite grains, indicating that the chromites have undergone strong alteration and element mobility. Major and trace elements in the core parts of chromites are used to infer the tectonic origins and evolution of mantle peridotites in the MOC. The chromites from the MOC peridotites have higher Cr# values and lower Ni and Ga contents with respect to those from Phanerozoic mantle peridotites, indicating a higher degree of depletion. In-situ major and trace elements (e.g., Ga) characteristics of podiform chromites in the MOC show that chromites from both harzburgites and dunites have strong subduction-related signatures, indicating that the MOC has formed in a supra-subduction setting which is consistent with the geological and geochemical data presented in previous studies.
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Geochemical Characteristics of Zoned Chromites in Peridotites from the Proterozoic Miaowan Ophiolitic Complex, Yangtze Craton: Implications for Element Mobility and Tectonic Setting

doi: 10.1007/s12583-019-1278-x
    Corresponding author: Hao Deng

Abstract: The chrome spinel (chromite) in mantle peridotites from ophiolites can shed light on the formation and evolution process of ophiolites. Podiform chromites were found in the Late Proterozoic Miaowan ophiolitic complex (MOC), Yangtze Craton. Due to the metamorphism and intense deformation, most chromite grains in the MOC peridotites show typical chemical zoning (core-rim texture). The values of major and trace elements largely vary from core to rim within chromite grains, indicating that the chromites have undergone strong alteration and element mobility. Major and trace elements in the core parts of chromites are used to infer the tectonic origins and evolution of mantle peridotites in the MOC. The chromites from the MOC peridotites have higher Cr# values and lower Ni and Ga contents with respect to those from Phanerozoic mantle peridotites, indicating a higher degree of depletion. In-situ major and trace elements (e.g., Ga) characteristics of podiform chromites in the MOC show that chromites from both harzburgites and dunites have strong subduction-related signatures, indicating that the MOC has formed in a supra-subduction setting which is consistent with the geological and geochemical data presented in previous studies.

Huang Yang, Deng Hao. Geochemical Characteristics of Zoned Chromites in Peridotites from the Proterozoic Miaowan Ophiolitic Complex, Yangtze Craton: Implications for Element Mobility and Tectonic Setting. Journal of Earth Science, 2020, 31(2): 223-236. doi: 10.1007/s12583-019-1278-x
Citation: Huang Yang, Deng Hao. Geochemical Characteristics of Zoned Chromites in Peridotites from the Proterozoic Miaowan Ophiolitic Complex, Yangtze Craton: Implications for Element Mobility and Tectonic Setting. Journal of Earth Science, 2020, 31(2): 223-236. doi: 10.1007/s12583-019-1278-x
  • Chrome spinel, known as chromite, is a unique mineral that can be generally used to reflect particular tectonic settings since its chemical composition is mainly controlled by mantle melting processes (Mukherjee et al., 2010; Rollinson, 2008; Mondal et al., 2006; Ahmed and Arai, 2002; Kamenetsky, 2001; Stowe, 1994; Dick and Bullen, 1984; Irvine, 1965). For example, high-Cr chromitites (Cr# (100Cr/(Cr+Al)) > 60) are generally proposed to have linkages with arc-related settings (Zhou et al., 1996; Dick and Bullen, 1984), whereas high-Al chromitites (Cr# < 60) are thought to be formed in MORB-related settings (Arai et al., 2011; González-Jiménez et al., 2011; Kamenetsky, 2001; Zhou et al., 1996; Dick and Bullen, 1984). Studies of secular changes of chromite composition in different chromitites and associated peridotites can provide insight into understanding chemical composition changes of the Earth's upper mantle through geological time (Arai and Ahmed, 2018; Stowe, 1994).

    Generally, chromites in chromitites and associated peridotites (harzburgites and dunites) can preserve their primary compositions of major elements, so they can be reliable indicators to infer the tectonic origin of chromitite and associated peridotites (e.g., Mondal et al., 2006, references therein). However, some recent studies on major elements of chromites from chromitites and mantle peridotites within both Phanerozoic and Precambrian ophiolites show that major elements of chromites could also have been mobile during post-magmatic alteration and metamorphism (Colás et al., 2014; González-Jiménez et al., 2014). Compared to major elements, trace elements in chromites are more sensitive to formation conditions including pressure, temperature, oxygen fugacity and degree of partial melting and fractional crystallization (González-Jiménez et al., 2014; Dare et al., 2009). Especially, some trace elements (e.g., Ti, Ga, and V) can enter the octahedral site of the crystal structure of the chromite and replace the trivalent elements of Cr, Al, and Fe3+, resulting in general varieties of chromites (Gervilla et al., 2012). Therefore, trace element characteristics in chromites can shed light on petrogenetic settings of chromite as well as its host peridotites (Colás et al., 2014; Dare et al., 2009). In-situ trace element data obtained using LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) on minerals have great advantages in petrogenetic studies since the data are avoided of interference from surrounding mineral phases (Colás et al., 2014; González-Jiménez et al., 2014; Pagé et al., 2012; Dare et al., 2009; Pagé and Barnes, 2009). More and more tudies have focused on trace elements of chromites from Phanerozoic ophiolites in past years (Su et al., 2019; Yu et al., 2019; Colás et al., 2014; Dare et al., 2009; BPagé and arnes, 2009). However, a few researchers paid attention to trace element characteristics of chromites in mantle peridotites from Precambrian ophiolites (Rui et al., 2019; Yu et al., 2019). The Proterozoic Miaowan ophiolitic complex (MOC) is located in the northern margin of Yangtze Craton of southern China (Deng et al., 2017; Jiang et al., 2016; Peng et al., 2010). Previous studies (Deng et al., 2017; Huang et al., 2017; Peng et al., 2012) have revealed that chromites in mantle peridotites of the MOC have similar characteristics to those of podiform chromites from ophiolites worldwide (Thayer, 1964). However, the mantle peridotites in the MOC that are dominated by harzburgites and dunites (Fig. 1) have undergone amphibolite-facies metamorphism and hydrothermal alteration based on field and petrographic observations (Deng et al., 2017; Peng et al., 2012), resulting in that their primary geochemical compositions are hardly preserved and their petrogenetic origin is difficult to be determined. Therefore, the podiform chromites within the mantle peridotites are one of the reliable and indispensable petrogenetic indicators for understanding the origin and evolution of the mantle peridotites in the MOC. Although most of the chromite grains in the MOC have also been altered and display core-rim zoning texture (Huang et al., 2017), in-situ major and trace element analyses on the core can be used to gain new primary information of the chromites that can provide insights into inferring evolution processes of the MOC. Furthermore, a comparison of data obtained on core and rim parts will help understand the alteration process that the mantle peridotites have undergone.

  • The Huangling dome, which is located in the northern part of Yangtze Craton (Huang et al., 2017), consists of the oldest Precambrian basement of this craton, including 3.3–2.6 Ga metamorphosed TTG (tonalite, trondhjemite, granodiorite) gneisses and volcano-sedimentary rocks (Wang et al., 2019; Guo et al., 2014; Chen et al., 2013; Peng et al., 2012; Gao et al., 1999), Paleoproterozoic Shuiyuesi ophiolitic mélange (Han et al., 2017), Mesoproterozoic Miaowan ophiolitic complex (Deng et al., 2017; Peng et al., 2012), and Neoproterozoic Huangling granitoid complex. All of them are unconformably overlain by a Neoproterozoic Nanhua system (Fig. 1; Huang et al., 2017).

    Figure 1.  Geologic map of the Huangling anticline, northern Yangtze Craton, showing the location and regional relations of the Miaowan ophiolitic complex. Modified after Peng et al. (2012).

    The Miaowan ophiolitic complex (MOC) is located in the southern part of the Huangling dome near the Taipingxi and Deng Village areas in Yichang City and present an NWW-strike trending. It mainly consists of mafic-ultramafic rocks, from base to top, including metamorphosed harzburgite and dunite, layered and isotropic gabbro, sheeted diabase dike, plagiogranite, pillow basalt and calc-silicate-bearing siliceous and carbonaceous mylonitic rock (Deng et al., 2017; Jiang et al., 2016; Peng et al., 2012). All rock units within the MOC have undergone intense ductile and brittle deformation with developed foliations and lineations (Deng et al., 2017; Jiang et al., 2016; Peng et al., 2012). The foliations are NWW-trending with a dip nearly vertically to the north. The ultramafic rocks have undergone serpentinization and are mainly exposed in the core of the MOC with a strike length of 13 km and a width of nearly 2 km. Both the north and south sides of ultramafic rocks include foliated meta-mafic rocks and metasedimentary rocks, with an additional width of 2 km (Fig. 2; Huang et al., 2017). All rock units within the MOC were then intruded by the Neoproterozoic Huangling granitic complex on its eastern side, and overlain by the sedimentary strata of Nanhua system on its western side (Fig. 2; Huang et al., 2017).

    Figure 2.  (a) Geological map and (b) cross-section of the Miaowan ophiolitic complex, showing main rock units, structures and locations of samples. Maps modified after Peng et al. (2012) and Jiang et al. (2018).

    Based on studies of petrology, geochemistry, geochronology and tectonic characteristics of rock units within the MOC, Peng et al. (2012) proposed that the MOC has mainly formed during Late Mesoproterozoic to Early Neoproterozoic (1 120–970 Ma) and may have originated in a forearc setting above an intra-oceanic subduction zone. Based on subsequent detailed dating and geochemical studies, Deng et al. (2017) further suggested that the MOC can be divided into two suites including an ophiolitic suite forming at ca. 1 110 Ma that originated in a mid-ocean ridge setting, and a late magmatic suite forming at ca.1 000–970 Ma representing forearc-related magmatic intrusions.

  • Primary geological surveys with ore mining processes that were carried out in the 1970s, have initially reported that there are chromitite ore bodies presenting original contact relationships with dunites and harzburgites in the southern Huangling dome (Fig. 3a; see Huang et al., 2017). However, these original relationships between different rocks are only preserved in a few locations such as the Tianhuasi area. Peng et al. (2012) also reported the occurrence of podiform chromites in peridotites of the MOC and suggested that they are important indicators identifying the Miaowan ophiolite. It can be observed in the field that lenses of disseminated and nodular chromitite are surrounded by dunites that grade into the host harzburgite (Figs. 3b, 3c, Huang et al., 2017), resembling characteristics of typical podiform chromitites in ophiolites worldwide (Kusky et al., 2016; Li et al., 2002). Based on detailed studies of mineral inclusions within the chromites and their major element characteristics, Huang et al. (2017) further proposed that the chromites in the MOC have formed in a forearc tectonic setting during the reaction between boninitic melts and MORB-type harzburgite in a supra-subduction zone (SSZ) mantle wedge.

    Figure 3.  (a) Sketch of chromitite deposit in the MOC (data from Hubei Institute of Geosciences, China), note that the chromite ore bodies are located in harzburgite with dunite envelopes; (b) and (c) sketch and photograph of field relationships between dunite and harzburgite in the Tianhuasi area, these pictures are taken from Huang et al. (2017).

    Chromite usually occurs within ultramafic rocks such as harzburgites and dunites. Detailed petrographic descriptions of serpentinized peridotites that host podiform chromitites are given in Huang et al. (2017). The serpentinized harzburgites in the MOC show grey, black or dark green colors (Fig. 4a) and serpentinized dunites occur as grey, dark black or grey-green colors with lenticular masses in the harzburgites (Fig. 4b). Many of the dunite pods contain disseminated and nodular chromitites. The disseminated chromite grains are small (10–20 μm), whereas the nodules are usually round and range up to 200 μm in diameter. These rock relationships and textures are similar to those within many Phanerozoic ophiolites (Zhou et al., 1996).

    Figure 4.  Photographs of hand specimen, scanned image and BSE images of podiform chromite in MOC peridotites. (a) Hand specimen of disseminated chromite in serpentinized harzburgite; (b) scanned image of a polished chromite sample in serpentinized dunite; (c)–(d) BSE images of zoned chromite; primary core (Chr) with ferrian chromite rim (Fe-Chr) and magnetite patches (Mag); interstitial black materials are serpentine, chlorite and magnesite. Abbreviations: Cpx. Clinopyroxene; Amp. amphibole; Chr. chromite; Fe-Chr. ferrian chromite; Mag. magnetite.

  • Twenty-one samples of serpentinized dunites and harzburgites were collected from the Miaowan ophiolitic complex (locations are shown in Fig. 2a). Polished thin sections were first studied by optical microscopy, as well as by field emission scanning electron microscopic (FE-SEM) using the backscattered electron mode (BSE) and energy dispersion spectrometry mode (EDS) prior to electron probe microanalyzer (EMPA) and in-situ trace element analyses. Major element analyses of chromite grains were performed by a JEOL JXA-8100 EPMA equipped with four wavelength-dispersive spectrometers (WDS) at the China University of Geosciences (Wuhan). Detailed analytical methods are given in Huang et al. (2017). Trace elements analyses of chromite grains were performed by LA-ICP-MS at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are described by Zong et al. (2017). Laser sampling was performed using a GeolasPro laser ablation system that consists of a COMPexPro 102 ArF excimer laser (wavelength of 193 nm and maximum energy of 200 mJ) and a MicroLas optical system. An Agilent 7700e ICP-MS instrument was used to acquire ion-signal intensities. Helium was applied as a carrier gas. Argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP. A "wire" signal smoothing device is included in this laser ablation system (Hu et al., 2014).

    The spot size and frequency of the laser were set to 44 µm and 6 Hz, respectively, in this study. Trace elements compositions of chromites were calibrated against various reference materials (BHVO-2G, BCR-2G, and BIR-1G) without using an internal standard (Liu et al., 2008). The preferred values of element concentrations for the USGS reference glasses are from the GeoReM database (http://georem.mpch-mainz.gwdg.de/). Each analysis incorporated a background acquisition of approximately 20-30 s followed by 50 s of data acquisition from the sample. An Excel-based software ICPMSDataCal was used to perform off-line selection and integration of background and analyzed signals, time-drift correction and quantitative calibration for trace element analysis (Liu et al., 2008).

  • Most chromite grains in both serpentinized dunites and harzburgites from the MOC show distinctly dark grey cores surrounded by light grey rims and rarely high-reflectance magnetite in BSE images (Figs. 4c, 4d; Huang et al., 2017). Our previous studies reveal that the core of chromite grains usually contains primary mineral inclusions such as olivine, clinopyroxene, and orthopyroxene, whereas the altered rims have mineral inclusions including chlorite, serpentine, and talc (Huang et al., 2017). This zoned texture in chromite grains is common in ophiolites worldwide, indicating that the cores represent primary chromites and the altered rims represent ferrian chromites (Kapsiotis, 2014; Pal et al., 2014; Derbyshire et al., 2013; Ahmed et al., 2001). More than 200 analytical spots of major elements and 50 spots of trace elements have been obtained on chromite grains in zoned chromites from the MOC peridotites (harzburgites and dunites). In addition, elements (Mg, Fe, Al) distribution maps of chromite grains in the MOC dunite (Fig. 5) and harzburgite (Fig. 6) obtained by EDS also display zoned textures in agreement with BSE images. However, elements distribution maps obtained by EDS cannot accurately distinguish trace element variations (Figs. 5, 6).

    Figure 5.  EDS element mapping (Mg, Fe, Cr, Al, Sc, Ti, V, Mn, Ni, Zn, Ga) of chromite grains in the MOC harzburgite.

    Figure 6.  EDS element mapping (Mg, Fe, Cr, Al, Sc, Ti, V, Mn, Ni, Zn, Ga) of chromite grains in the MOC dunite.

  • All the analyzed chromite cores have low TiO2 values of < 0.35 wt.% and high Cr2O3 contents of > 42 wt.% (Table S1). All chromite cores from serpentinized dunites have high and almost constant Cr# values of 73.5–76.5 (Table S1). However, compared to the chromite cores from serpentinized dunites, the ones from harzburgites show low Cr# values (59.7–71.3) due to their higher concentrations of Al2O3 and MgO (Table S1).

  • As mentioned above, trace elements are more sensitive to alteration process than major elements. In order to reveal the distribution of trace elements in zoned chromite grains from MOC peridotites, in-situ trace element analyses were carried out on their core and rim separately. Sc, Ni and Ti contents in zoned chromites show clear positive relations with increasing Cr# (Fig. 7). Sc ranges from 0.3 ppm to 1.9 ppm in all analyzed cores whereas the rims have higher Sc contents from 2.7 ppm to 12.7 ppm (Table S1). The content of Ni ranges from 375 ppm to 668 ppm in all analyzed cores and is lower than those in the rims with 691 ppm to 2 793 ppm (Table S1). Ti content in chromites cores from all analyzed chromites ranges from 1 000 ppm to 3200 ppm, whereas that in chromites rims ranges from 2 219 ppm to 5 913 ppm (Table S1). Chromite cores from all zoned chromites have various ranges of Mn (2 877 ppm–4 834 ppm), V (520 ppm –986 ppm), and Ga (6.4 ppm–19.3 ppm) than chromite rims that have contents of Mn (2 549 ppm–6 060 ppm), V (525 ppm–1 182 ppm), and Ga (6.9 ppm to 21.5 ppm) (Table S1). In the zoned chromite grains from the harzburgites, Co and Zn show decreasing trends, whereas Sc, Ti, Mn, and Ni show increasing trends from the core to the ferrian chromite rim (Fig. 8; Table S1). Sc, Ti, Co, Ni and Zn in zoned chromites of dunites share the same trends with those of harzburgites, whereas Mn shows an opposite trend that decreases from the core to the rim in dunites with respect to harzburgites (Fig. 8; Table S1). However, there is no clear trend of V and Ga contents between core and rim parts for all chromite samples from both dunites and harzburgites. Furthermore, all chromite grains from different samples show similar distributions (except for Ti, Mn, and Ga) and variations in trace element concentrations from the core to the rim. However, Ti and Mn contents in chromite cores (1 000 ppm–2 179 ppm, 2 877 ppm–3 622 ppm) of the harzburgites are lower than those of dunites (1 973 ppm–3 200 ppm, 3 070 ppm–4 834 ppm), whereas Ga contents (10.7 ppm–19.3 ppm) in chromites of harzburgites are higher than those of dunites (6.4 ppm–13.1 ppm).

    Figure 7.  Plots of Cr# vs. Sc, Ni, Zn, Co, Ti and Fe2O3 of chromites in dunite and harzburgite from the Miaowan ophiolitic complex, Yangtze Craton. MORB and BON fields are from Pagé and Barnes (2009).

    Figure 8.  Variations of major elements (obtained by EPMA) and in-situ trace elements (obtained by LA-ICP-MS) in zoned chromite grains from the MOC peridotites. (a) Zoned chromite grain in MOC dunite; (b) zoned chromite grain in MOC harzburgite.

  • It has been widely accepted that the mafic and ultramafic rocks in ophiolites usually have undergone extensive alteration, which may change chromite composition hosted by dunites and harzburgites (Kapsiotis, 2014; Pal et al., 2014; Derbyshire et al., 2013; Ahmed et al., 2001). Furthermore, most of the Precambrian ophiolitic chromites would show zoned textures because of metamorphism and alteration, such as Pan African ophiolite (Ahmed et al., 2001), Outokumpu ophiolite (Vuollo et al., 1995), and Archean Zunhua podiform chromite in the North China Craton (Kusky et al., 2004). This zoned texture is also very common in Phanerozoic chromites including the Shetland ophiolite complex (Derbyshire et al., 2013), Manipur ophiolite (Pal et al., 2014) and Oman ophiolite (Rollinson et al., 2012). Previous studies show that rock units in the MOC have undergone seafloor hydrothermal alteration (Wang et al., 2012), intensive deformation and amphibolite-facies metamorphism (Deng et al., 2017; Peng et al., 2012). The chromite grains in the MOC peridotites show clear zoning texture, which means they have undergone alteration processes such as serpentinization and metamorphism. This characteristic is in accordance with their compositional variations in major elements and alteration minerals such as serpentine, talc, chlorite in host dunites and harzburgites (Huang et al., 2017). Therefore, it is necessary to evaluate the effect of alteration and metamorphism process on the modification of major and trace element compositions of chromite in the MOC peridotites.

    The values of major and trace elements largely vary from core to rim within chromite grains in the MOC peridotites. For major elements, primary cores have higher contents of Al2O3, MgO and Cr2O3, lower contents of Fe2O3 and FeO than those of altered rims (Fig. 8; Table S1). As a result, Cr# values (=100Cr/(Cr+Al)) of the zoned chromites increase sharply from cores to altered rims (Table S1). The cores have lower Sc, Ti, and Ni, higher Co, and Zn contents than those of altered rims (Fig. 8; Table S1). These lines of evidence indicate that the chromite rims have undergone strong alteration and element mobility and their geochemical compositions cannot represent primary chemical compositions of the chromites. Therefore, discussions of tectonic origins of Miaowan harzburgites and dunites below are based on geochemical compositions including major and trace elements from the cores of chromite grains.

  • The chromite belongs to the spinel group mineral with an ideal chemical formula of AB2O4. There are two positions for ions to occupy. Position A is for major elements of Mg, Fe2+ and trace elements of Zn, Co, Mn, Ni, whereas position B is for major elements of Cr, Al, Fe3+ and trace elements of V, Sc, Ga, Ti (Table 1). During the hydrothermal alteration and metamorphism process, major and trace elements within chromites would be modified and form core-rim textures. Based on analyses of differences of major element contents between the core and the rim of chromite grains, the alteration product can be expressed as (Fe2+, Mg) (Cr, Fe3+, Al)2O4 due to replacement of Mg2+ and Al3+ by Fe2+ and Fe3+ respectively in the spinel structure during alteration processes. The production is known as ferrian chromite (formerly called 'ferritchromite' or 'ferritchromit', Derbyshire et al., 2013; Gervilla et al., 2012; Mukherjee et al., 2010; González-Jiménez et al., 2009). Moreover, with the increasing degree of the replacement, the ferrian chromite rims can totally transform into magnetite that forms the outer rims of chromite grains. Formation of ferrian chromite has been thought to be related to serpentinization processes (Mukherjee et al., 2010; Pagé and Barnes, 2009; Burkhard, 1993) or as a consequence of metamorphism mostly in greenschist- to lower-amphibolite-facies (Singh and Singh, 2013; Grieco et al., 2012; González-Jiménez et al., 2009; Merlini et al., 2009; Mellini et al., 2005; Proenza et al., 2004). In addition, compositional modification of the chromite grains also depends on the size of grains and the deformation degree of host peridotites. In most cases, the compositions of smaller grains are easier to be modified compared to larger grains. Like major elements, trace elements are diverse in different parts of chromite grains and various rock types. Because of the high detection limit, the contents of trace elements cannot be obtained by EDS and EMPA. However, in-situ LA-ICP-MS analyses on different parts of chromite grains can shed light on the changes of trace elements. As mentioned above, during the formation of the core-rim texture of chromite grains both in dunites and harzburgites of the MOC, positive correlations of Sc, Ti and Ni and negative correlations of Co versus Cr# values (Fig. 7) suggest these trace elements have significant mobility during alteration. From the EMPA and in-situ ICP-MS data, we can see that Co and Zn behave similarly to Mg in position A and they can be substituted by Fe2+ and Ni during alteration, whereas Cr and Al behave similarly in position B and they can be substituted by Fe3+ with Ti and Sc (Table 1). The substitution process can occur since these elements have similar ionic radii (Table 1) resulting in the exchange of elements in different parts during the alteration process.

    Mineral IMA classification Formula Position Major element Trace element
    Element Ionic radius ( ) Element Ionic radius ( )
    Chromite Spinel subgroup AB2O4 A(IV) Mg2+ 0.57 Zn2+ 0.6
    Co2+ 0.58
    Fe2+ 0.63 Ni2+ 0.55
    Mn2+ 0.66
    B(VI) Cr3+ 0.615 V3+ 0.64
    Al3+ 0.535 Ga3+ 0.62
    Fe3+ 0.645 Sc3+ 0.745
    Ti4+ 0.605
    Note: 1 Å=0.1 nm; IMA: International Mineral Association. Ionic radius data from Shannon (1976).

    Table 1.  Major and trace elements distribution in the chromite structure (modified from Colás et al., 2014)

  • Podiform chromite typically has a wide range of major elements composition in mantle peridotites with different mechanisms of formation and different tectonic settings (Ahmed et al., 2012). Therefore, many discriminant diagrams are proposed for chromites based on major elements contents (Arai et al., 2011). Compared with major elements, trace elements are more sensitive to the formation environment of chromite and associated peridotites. There are many studies on the trace elements of primary chromite composition in mantle peridotites all over the world. However, most studies are related to well-preserved Phanerozoic ophiolites (Su et al., 2019; Xiong et al., 2017; Zhou et al., 2014). Compared to Phanerozoic mantle peridotites, Precambrian mantle peridotites are thought to have higher degrees of partial melting based on their high Cr# values (Huang et al., 2017; Zhou et al., 1996). After comparing average values of Cr# in chromites through geological time, Arai and Ahmed (2018) suggested that Archean mantle peridotites are more depleted than Proterozoic mantle peridotites that were in-turn more depleted than Phanerozoic mantle peridotites. However, the differences of major elements in chromites are not remarkable between Phanerozoic and Precambrian podiform chromites (Huang et al., 2017; Ahmed et al., 2001). In this study, we have summarized reported trace element data of chromite grains in peridotites from Phanerozoic ophiolites including Luobusa, Purang, and Zedang to compare to those from the Proterozoic MOC. The statistical data and analyses are given in Fig. 9 and Table 2, respectively. It can be observed that Ni and Ga contents in the MOC chromites are lower than those of Phanerozoic chromites, whereas Ti, Mn, Co, and Zn are generally higher than those of Phanerozoic chromites. However, the values of V in the MOC chromites are in the ranges of the Phanerozoic chromite grains (Fig. 9; Table 2). Nickel values in chromites can be used to infer the degrees of depletion of residual mantle peridotites (Zhou et al., 2014; Paktunc and Cabri, 1995). With increasing degrees of depletion of the host rocks, Cr# value of chromite in these rocks increases (Paktunc and Cabri, 1995), the value of Ni decreases. Gallium behaves similarly during partial melting of mantle peridotites due to the similar partition coefficients (DNi=1.3-11; DGa=1.83-3.75; https://earthref.org).

    Elements Lithology Precambrian Phanerozoic
    MOC Luobusa ophiolite Purang ophiolite Zedang ophiolite
    Ti Harzburgite 1 000–2 179 417–881 254–403 421–837
    Dunite 1 973–3 200 1 258–1 811 487–886 756–1 266
    V Harzburgite 586–986 790–1 191 570–1 405 952–1 468
    Dunite 520–938 1 058–1 297 720–1 282 930–2 016
    Mn Harzburgite 2 877–3 622 1 019–3 837 629–1 704 1 298–1 508
    Dunite 3 070–4 834 2 993–3 141 1 186–1 529 1 577–1 881
    Co Harzburgite 529–582 271–519 184–414 347–392
    Dunite 387–631 613–651 271–349 353–377
    Ni Harzburgite 415–668 743–1 748 241–1 453 605–972
    Dunite 375–588 525–855 302–1 446 830–1 207
    Zn Harzburgite 1 806–3 983 975–4 381 470–1 759 809–1 927
    Dunite 1 216–5 475 1 229–1 400 978–1 633 1 322–2 102
    Ga Harzburgite 7.7–21.8 20.1–46.2 15.8–39 32.5–41
    Dunite 6.4–13.1 14.8–34.9 11.6–36.9 32.2–37

    Table 2.  Representative trace elements (ppm) in harzburgites and dunites from Precambrian and Phanerozoic ophiolites and ophiolitic complex

    Figure 9.  Comparison of trace elements (Ti, V, Mn, Co, Ni, Zn, Ga) in primary chromites from mantle peridotites (harzburgites and dunites) of Miaowan ophiolitic complex (this study), Zedang ophiolite (Xiong et al., 2017), Luobusa ophiolite and Purang ophiolite (Su et al., 2019).

    Therefore, it is suggested that the lower contents of Ni and Ga in the chromites from the Proterozoic Miaowan peridotites than those of Phanerozoic mantle peridotites indicate their higher degree of depletion (Arai and Ahmed, 2018). Gahlan and Arai (2007) proposed that the Zn, Mn, and Co would be enriched in chromites due to decomposition and transformation of olivine into serpentine during serpentinization. The higher contents of Zn, Mn, and Co in Precambrian MOC chromites may indicate that the Precambrian MOC has undergone stronger serpentinization than Phanerozoic ophiolites, consistent with the field observation (Deng et al., 2017; Peng et al., 2012).

    Collectively, the MOC chromites and the Phanerozoic chromites show significant contrasting trace elements values. We propose that these differences may be related to different degrees of partial melting between the Precambrian era and the Phanerozoic era since Precambrian peridotites generally underwent high degrees of partial melting and/or melt-rock reaction than Phanerozoic mantle peridotites.

  • Previous studies proposed that the origin of the MOC mantle peridotites was related to a supra-subduction zone environment, based on petrology, whole-rock geochemistry and high-Cr chromite (Deng et al., 2017; Huang et al., 2017; Peng et al., 2012). The harzburgites in the MOC are characterized by LREE-depleted chondrite-normalized REE patterns, whereas the dunites in the MOC show typical U-shaped REE patterns (Deng et al., 2017). Accordingly, Deng et al. (2017) and Huang et al. (2017) proposed that the harzburgites initially were the depleted residuum of partial melting of MORB-source mantle and then influenced by subduction-related melts, and the dunites were formed from the reaction between the harzburgites and the subduction-related melts when the MOC was trapped with the harzburgites as a part of the mantle wedge above a subduction zone. In Fig. 10, selected major and trace elements of chromites from the MOC are plotted together with high-Cr chromites from the ophiolites at Thetford mines ophiolite in Canada (Pagé and Barnes, 2009), Ouen Island in New Caledonia (González-Jiménez et al., 2011), Indo-Myanmar ophiolite in northeastern India (Maibam et al., 2017) and Zedang ophiolite in China (Xiong et al., 2017). All data have been normalized to the chromite composition from MORB in Pagé and Barnes (2009). The trace-element contents obtained in the cores of both dunites and harzburgites from the MOC show a similar trend (Fig. 10) which is different from MORB. Most of these elements are overlapped with those of chromites from SSZ-type ophiolites in Canada, New Caledonia, Cuba, India, and China. Studies of major elements in primary chromites play an important role in determining the tectonic origins and inferring their parental magmas (Su et al., 2019; Xiong et al., 2015; Zhou et al., 2014). Previous studies show that the major element compositions of the chromite cores in the MOC peridotites either plot close to or within the field of ophiolitic podiform chromite on different diagrams that confirm their ophiolitic features (see Fig. 7 in Huang et al., 2017). In this study, most of the primary chromite cores from dunites plot in the field of BON (boninite), whereas those from harzburgites fall in or between the field of MORB and BON on the discrimination diagram of Fe2O3-Cr# (Fig. 8f). The result is consistent with the results of previous studies about major element compositions of chromites in MOC (discrimination diagrams of TiO2-Cr#, TiO2-Al2O3 in Huang et al., 2017), indicating the dunites were products of reaction between the primary harzburgites and the subduction-related boninitic melts and the primary harzburgites were subsequently influenced by the boninitic melts as well. The values of trace elements in chromites can also be significant indicators for the evolution of MOC peridotites. There are several valuable studies of trace element compositions of ophiolitic chromites in recent years (Colás et al., 2014; Zhou et al., 2014; Dare et al., 2009). However, there are few applications of these data to understand the formation and evolution of chromite and associated peridotites in Precambrian ophiolite (Rui et al., 2019; Yu et al., 2019). Dare et al. (2009) discussed the application of Fe3+, Ga and Ti as tectonic setting indicators of podiform chromites in chromitites and associated peridotites. By using these values, chromite-bearing mantle peridotites from different origins can be distinguished. Due to the similar ionic radius (Table 1), Gallium can behave similarly to Fe3+ (Dare et al., 2009). In addition, Gallium also has similar behaviors to Al3+, since they are in Group III of the Periodic Table (Goodman, 1972). Therefore, Gallium can occupy the position B of chromite that can provide the most effective fingerprints of chromite (Dare et al., 2009). In addition, Gallium has a low diffusivity in silicate minerals that hardly transport during the re-equilibration process (Scowen et al., 1991), leading to that Gallium is an ideal element for tectonic discrimination of chromite. In Fig. 11, Ga and Fe3+ from primary chromite cores in harzburgites and dunites of MOC are used to determine their origins. All of the primary chromite cores in harzburgites of the MOC plot in the field of SSZ harzburgites with some of them in the field of typical harzburgite ranges. In addition, all primary chromite cores in dunites of the MOC display subduction-related affinities and most of them clustered in the typical dunite ranges. Collectively, the trace element characteristics of the chromites within harzburgites and dunites in the MOC are consistent with the proposal of Deng et al. (2017) and Huang et al. (2017), suggesting that the formation of the harzburgites and dunites in the MOC were associated with a subduction-related environment, indicating the MOC have formed in a subduction zone process. It is noted that major elements of chromites from the harzburgites show mixed MORB-type and SSZ-related signatures whereas trace elements of chromites from the harzburgites only display SSZ-related signatures. We interpret that this difference resulting from different sensitivity of major and trace elements in melt-rock interaction during the emplacement of MOC, considering that the trace elements are more sensitive and are easier to be mobile than major elements. Therefore, the mobility of trace elements in the chromites should be taken into account when inferring the tectonic origin and evolution of chromite and associated peridotites in future related studies. This study suggests that it is better to understand the origin and evolution of chromites in mantle peridotites using both major and trace element compositions.

    Figure 10.  Selected MORB-normalized trace elements and major elements of the MOC chromites that are compared to the chromites at Thetford mines ophiolite in Canada (Pagé and Barnes, 2009), Ouen Island in New Caledonia (González-Jiménez et al., 2011), Indo-Myanmar ophiolite in North-eastern India (Maibam et al., 2017) and Zedang ophiolite in South-western China (Xiong et al., 2017), the MORB compositions are taken from Pagé and Barnes (2009).

    Figure 11.  Plot of Ga vs. Fe3# of chromite cores in Miaowan harzburgite (a) and dunite (b) chromite, the shaded fields (MOR-light grey field; SSZ-dark grey field) are the typical composition of chromite in mantle peridotites Dare et al. (2009).

  • (1) Podiform chromites in the Late Proterozoic MOC show distinct core-rim textures that can be easily observed under a microscope and SEM. These zoned textures resulted from the re-distribution of major and trace elements in chromite grains. Only geochemical characteristics of the chromite cores from mantle peridotites can be used in inferring the tectonic origins and evolution of mantle peridotites in the MOC.

    (2) Detailed in-situ analyses show that Sc, Ti and Ni contents have positive correlations, whereas Co content displays a negative correlation with Cr# value. Trace elements in chromites from Precambrian and Phanerozoic mantle peridotites (harzburgites and dunites) show significant differences. This phenomenon is interpreted to be that the Precambrian podiform chromites are more depleted than Phanerozoic ones.

    (3) In-situ major and trace element characteristics of podiform chromite in harzburgites and dunites of MOC show that both harzburgites and dunites have strong signatures of a SSZ environment, indicating that their formations were related to a subduction zone process.

  • This study was supported by the National Natural Science Foundation of China (Nos. 41802240, 41902036) and the Fundamental Research Funds of the Central Universities granted by China University of Geosciences (Wuhan) (No. 007-G132354 1792) to Hao Deng. This research was also supported by an opening fund from the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan) (No. GRMR20 1607). We are grateful to Profs. Songbai Peng, Timothy M. Kusky, and Lu Wang for field investigations and sample collections. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1278-x.

    Electronic Supplementary Material: Supplementary material (Table S1) is available in the online version of this article at https://doi.org/10.1007/s12583-019-1278-x.

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