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
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).