Journal of Earth Science  2019, Vol. 30 Issue (3): 476-493   PDF    
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Origin of Chromitites in the Songshugou Peridotite Massif, Qinling Orogen (Central China): Mineralogical and Geochemical Evidence
Huichao Rui 1,2, Jiangang Jiao 3, Mingzhe Xia 3, Jingsui Yang 2, Zhaode Xia 3     
1. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;
2. Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
3. School of Earth Science and Resources, Chang'an University, Xi'an 710054, China
ABSTRACT: The Songshugou peridotite massif is located in the north of Shangdan suture zone, North Qinling orogenic belt of Central China. The massif is mainly composed of fine-grained mylonitic dunites, coarse-grained dunites, fine-and coarse-grained harzburgites, and minor clinopyroxenites. The coarsegrained dunites as well as parts of the harzburgites host small-scale chromitites. Chromite grains from various textural types of chromitites and dunites pervasively contain primary and secondary silicate inclusions. Primary inclusions are dominated by monophase olivine, with minor clinopyroxene and a few multiphase mineral assemblages consisting of olivine and clinopyroxene. Secondary inclusions, mainly Cr-chlorite and tremolite, show irregular crystal shapes. Besides, Cr2O3 contents (0.08 wt.%-0.71 wt.%) of primary olivine inclusions are remarkably higher than those of interstitial olivine (< 0.1 wt.%). Chromites in the Songshugou peridotite massif are high-Cr type, with Cr# and Mg# values ranging of 67.5-87.6, and 23.4-41.2, respectively. The Cr-chlorite, formed by reactions between olivine and chromite in the presence of fluid under middle temperature, indicates the Songshugou peridotite massif has undergone alteration/metamorphism process during emplacement. Chromite grains are modified by these processes, resulting in the various degrees of enrichment of Fe2O3, Cr2O3, Zn, Co and Mn, depletion of MgO, Al2O3, Ga, Ti and Ni. Due to low silicate/chromite ratios in the massive ores, chromites from them are slightly influenced by alteration/metamorphism and thus preserve the pristine magmatic compositions. The parental magma calculated based on them has 11.17 wt.%-13.57 wt.% Al2O3 and 0.15 wt.%-0.27 wt.% TiO2, which is similar to the parental melts of high-Cr chromitites from elsewhere and comparable with those of boninites. Combined with informations from previous studies, major and trace elements geochemistry of chromite, as well as the nature of the parental magma, it can be revealed that the Songshugou chromitities formed in a supra-subduction zone environment.
KEY WORDS: peridotite    chromitite    chromite trace element    parental magma    Songshugou    Qinling Orogen    
0 INTRODUCTION

Recording the evolution of oceanic lithosphere, ophiolitic mantle rocks and their hosted chromities carry important information for discussing the geological evolution and history of orogenic belts (e.g., Luo et al., 2018; Lian et al., 2017; Xiong F H et al., 2017a; Xiong Q et al., 2017, 2016; Akbulut et al., 2016; Zanetti et al., 2016; Guo et al., 2015). The tectonic settings of podiform chromitites are closely related with chromites compositions. Generally, high-Al chromitites are considered to form in mid-ocean ridge (MOR) or back-arc basin environments, while high-Cr chromitites likely form in forearc settings (e.g., Xiong et al., 2015; Zhou et al., 2014, 2005). In turn, genesis of podiform chromitites is helpful for understanding the origin of ambient ophiolites. For instance, some ophiolites hosted high-Cr chromitites are interpreted to have derived from MOR, and subsequently modified in SSZ (supra-subduction zone) environments where interactions between hydrous melts with ophilitic mantle rocks contribute to chromitites formation (e.g., Wu et al., 2018; Uysal et al., 2009). Chromite chemistry as well as coexisting silicate minerals compositions could provide clear insights of the geochemical features of those mantle rocks (Wu et al., 2018; Xiong et al., 2017a). Furthermore, chromite major and trace elements can estimate the compositions of the magmas from which they crystallized and reveal the genesis of chromitites (Akbulut et al., 2016; González-Jiménez et al., 2016; Colás et al., 2014; Zhou et al., 2014; Gervilla et al., 2012; Pagé and Barnes, 2009).

The Songshugou peridotite massif is the largest ultramafic block in northern Qinling orogenic belt (Su et al., 2005, Fig. 1). Although it has been well studied by some researchers, its formation processes and tectonic environment are still controversial. Peridotite massif has been interpreted as (1) a cumulate body of ultramafic magma (Wang et al., 2005; Song et al., 1998), (2) fragments of fossil oceanic lithosphere subducted in Paleozoic and formed in the Neoproterozoic (Yu et al., 2017; Dong et al., 2008), (3) a segment of ophiolitic mantle consisted of both residue and cumulate (Lee et al., 2010; Zhang, 1995), (4) an interaction product between mantle peridotite and Mg-rich melt derived from deep mantle (Liu et al., 2014; Su et al., 2005), or (5) highly- refractory peridotite from fore-arc mantle wedge (Cao et al., 2017, 2016), and formed in the Late Mesoproterozoic (Nie et al., 2017). However, the study on chromitites hosted in Songshugou peridotite massif is still poor. By reporting and discussing whole-rock geochemistry of peridotites, and major elements mineral chemistry of chromites, Lee et al. (2010) proposed that chromitites crystalized during the formation of cumulate dunites.

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Figure 1. (a) Geological map showing the major tectonic units in Qinling Orogen (modified after Dong and Santosh, 2016). (b) Regional geologicalmap of the Songshugou area showing the major structural and lithological units (modified after Dong et al., 2008). (c) Enlarged geological map showing the main types of ultramafic rocks in the study area (modified after Su et al., 2005). LLWF. Lingbao-Lushan-Wuyang fault; LLF. Luonan-Luanchuan fault; BSF. Bashan fault; XGF. Xiangfan-Guangji fault; TLF. Tanlu fault.

In this study, we perform systematic investigation of major and trace elements compositions of chromite, major elements compositions of olivine, orthopyroxene and Cr-chlorite in dunites, harzburgites, and chromitites from the Songshugou peridotite massif. Combined with previous published mineral chemical data in literatures, our investigation sheds light on the genesis of chromitites and provides constraints for the nature and origin of the Songshugou peridotite massif.

1 GEOLOGICAL SETTING

The Qinling Orogen is situated in Central China with grand NWW-SEE-trending, which was formed through collision between the North China Craton to the north and the Yangtze Craton to the south. The orogenic belt could be divided into the North and South Qinling belts by the Shangdan suture zone (e.g., Ratschbacher et al., 2003; Zhang et al., 1995). The South Qinling belt is composed mainly of Precambrian basement and overlying Upper Neoproterozoic to Triassic sedimentary rocks. The North Qinling belt is further subdivided into Kuanping Group, Erlangping Group, Qinling Group, Songshugou mafic-ultramafic massif and Danfeng Group, separated by thrust faults or ductile shear zones from north to south (Fig. 1a, more details about the orogenic belt and its evolution see review by Dong and Santosh (2016)).

The Songshugou mafic-ultramafic massif crops out in the north of the Shangdan suture zone as an NW-SE-trending lentoid block. It is separated from Qinling Group by the Jieling ductile shear zone in the north, and from Qinling Group and Fushui Complex intrusion by the Xiaogou shear zone in the south (Fig. 1b). The massif mainly consists of metamorphosed mafic and ultramafic rocks (e.g., amphibolite, garnet amphibolite, garnet pyroxenite, harzburgite, dunite).

The ultramafic massif is about 20 km long, ~2 km wide, and enclosed in mafic rocks bounded by fault and intense ductile shear zone. It chiefly consists of fine-grained mylonitic dunites. Besides, minor coarse-grained dunites, fine- and coarse-grained harzburgites occur as various-sized lentoid blocks randomly scattered in fine-grained mylonitic dunites (Fig. 1c). Coarse-grained dunites as well as parts of the harzburgites host small-scale chromitites (Fig. 2) (containing around 0.2 million tons of chromium).

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Figure 2. Photographs showing (a) disseminated and (b) massive chromitites.

The formation and emplacement age data of the ultramafic massif are still poor and controversial. Metamorphic ages have been recorded in its surrounding mafic rocks due to the contact heating and induced metamorphism by the uplifting ultramafic massif. The metamorphic ages of 480–515 Ma obtained by U-Pb dating of zircons in surrounding mafic rocks have been reported in the literatures (Tang et al., 2016; Yu et al., 2016; Chen et al., 2015; Dong et al., 2011; Li et al., 2009; Liu et al., 2009; Su et al., 2004), which indicate that the ultramafic massif emplaced during the Early Paleozoic (Cao et al., 2017, 2016). Recently, Yu et al. (2017) and Nie et al. (2017) reported that the Songshugou peridotites had Re depletion model ages (TRD) of 446–875 Ma and 1.2–0.8 Ga, respectively.

2 PETROGRAPHY

Based on field and petrographic observations, five typies of lithologies can be identified in the Songshugou ultramafic massif (Fig. 1c, Cao et al., 2016).

2.1 Fine-Grained Mylonitic Dunite

Fine-grained mylonitic dunites are the predominant rock type and make up ~85% of the massif surface area. They are composed of ca. 5% porphyroclastic olivine (0.5–1.5 mm) and matrix of ca. 90% fine-grained olivine (~0.25 mm) with minor amphibole, Cr-chlorite, serpentine, talc and chromite (Fig. 3a).

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Figure 3. Photomicrographs showing the textures of peridotites and chromitites. (a) Fine-grained dunite; (b) euhedral anthophyllite crosscut olivine grains in coarse-grained dunite; (c) euhedral to subhedral and homogeneous chromite grains containing primary olivine inclusions in coarse-grained dunite (BSE); (d) rounded chromite grains with zoned structure in coarse-grained dunite (BSE); (e) subhedral to anhedral chromite grains coexisting with Cr-chlorite are encapsulated in olivine crystals as inclusions in coarse-grained dunite; (f) fine-grained harzburgite; (g) Cr-chlorite occurs in the surrounding area of the chromite in disseminated chromitite; (h) chromite grains contain primary olivine and secondary Cr-chlorite inclusions in disseminated chromitite (BSE). BSE. Backscattered electron image; Ath. anthophyllite; Chr. chromite; Cpx. clinopyroxene; Cr-Chl. Cr-chlorite; Ol. olivine; Opx. orthopyroxene; Tr. tremolite.
2.2 Coarse-Grained Dunite

Coarse-grained dunites crop out as various-sized lentoid blocks in fine grained mylonitic dunites, and occupy up to ~10% of the massif surface area. These rocks show protogranular texture and mainly consist of ca. 95% large olivine grains (0.5–2 mm) and chromite, with minor anthophyllite and Cr-chlorite. Most olivine crystals have rounded shape, and some of them show kink belt and undulatory extinction. Euhedral anthophyllite crystals crosscut olivine grains randomly (Fig. 3b). The chromite grains (0.1–1 mm) are mostly euhedral to subhedral and homogeneous, containing primary and secondary inclusions (Fig. 3c). Some of the chromite grains have rounded shapes and zoned structures (Fig. 3d). Occasionally, a few of subhedral to anhedral chromite grains (~50 μm) coexisting with Cr-chlorite, are encapsulated in olivine crystals as inclusions (Fig. 3e).

2.3 Fine- and Coarse-Grained Harzburgite

Harzburgites make up ~2% of the surface area, occurring as lentoid blocks in various sizes in the fine grained mylonitic dunites. Similar to the dunites, they could be subdivided into fine- and coarse-grained harzburgites.

Fine-grained harzburgites are composed of matrix of olivine (~90%) and porphyroclastic orthopyroxene (Opx, ~25%) with minor tremolite and chromite. The porphyroclastic Opx crystals are prismatic and 2–10 mm in diameter, showing clear preferred orientation and a set of parallel bent cleavages, which suggest plastic deformation (e.g., Yu et al., 2017). Usually, the Opx grains are surrounded by fine-grained olivine and have irregular curving boundaries (Fig. 3f). Euhedral tremolite crystals crosscut olivine and Opx grains randomly. The morphology of chromite crystals is analogous to those in the coarse-grained dunites. Besides, Cao et al. (2016) observed some chromite crystals showing patchy compositional patterns in these rocks.

Coarse-grained harzburgites consist of olivine (~50%) and orthopyroxene with minor tremolite and chromite. The modal contents of Opx are much higher and vary largely than those in fine-grained harzburgites, and which can occupy up to ~80% in some samples (e.g., Su et al., 2005). Opx grains are subhedral to anhedral with regular boundaries, and 5–30 mm in diameter, exhibiting poikilitic and impregnate texture. Anhedral to subhedral chromite grains (~1.0 mm) are interstitial, or enclosed in large Opx crystals.

2.4 Clinopyroxenite

Clinopyroxenites occur as dykes (a few meters wide) in fine-grained dunites and exhibit a mosaic texture, consisting of clinopyroxene (Cpx, ~90%), olivine, tremolite, chlorite, serpentine, chromite with minor base metal sulfide. The large Cpx grains (~3 mm) occur as megacrysts in the fine-grained matrix. Generally, the Cpx megacrysts are euhedral to subhedral, and dense parallel lamellae ferrite exsolutions are observed within.

2.5 Chromitite

Chromitites occur in the coarse-grained dunites and harzburgites with sharp contact, forming irregular layers ranging from a few centimeters to a few meters wide (Fig. 2, Lee et al., 2010). Massive and disseminated chromitites are primary structural types of chromitites. The massive chromitites consist of chromite (~90%), olivine (~15%) and minor Cr-chlorite. Chromite crystals are anhedral to subhedral and uniform in size (~2 mm). Disseminated chromitites are also composed of olivine (~70%), chromite (~35%) with minor Cr-chlorite (Fig. 3g). Chromite crystals are subhedral to euhedral with smaller sizes (~1 mm), unevenly distributed in olivine matrix. Cr-chlorite is rose pink under microscope, always surrounding the chromite or enclosed in the chromite grains, and its mode abundance is highly variable (up to ~5% in some samples). Inclusions in chromite are observed normally, including primary olivine, pyroxene, and secondary Cr-chlorite and tremolite (Fig. 3h).

3 ANALYTICAL METHODS

Major elements compositions of olivine, orthopyroxene and Cr-chlorite were analyzed using a JEOL JXA-8100 electron probe micro-analyzer (EPMA) at the Key Laboratory of Western China Mineral Resources and Geological Engineering, Ministry of Education, Xi'an, China. The analytical conditions were 15 kV accelerating voltage, a 10 nA beam current, 1 μm beam spot and 20 seconds counting time for each element. Major and trace element compositions of chromite were analyzed using Teledyne Cetac Technologies Analyte Excite laser- ablation system connected to a Agilent Technologies 7700x quadrupole ICP-MS at Nanjing FocuMS Technology Co. Ltd. The 193 nm ArF excimer laser was focused on chromite surface with fluence of 6.0 J/cm2, and ablation protocol employed a spot diameter of 40 μm at 7 Hz frequency for 40 seconds. United States Geological Survey basaltic glasses (BIR-1G, BHVO-2G, BCR-2G and GSE-1G) were used as external calibration standards. Raw data reduction was performed by ICPMSDataCal software using 100%-normalization strategy without applying internal standard (Liu et al., 2008).

4 RESULTS 4.1 Olivine

Representative microprobe analyses data of olivine from coarse-grained dunites, coarse- and fine-grained harzburgites, and chromitites are listed in Table 1. Fo values [100×Mg2+/ (Mg2++Fe2+)] of olivine display a limited range (90.1–92.9) with a slight increase from coarse-grained harzburgites, to fine-grained harzburgites and coarsed-grained dunites, except those from one dunite sample SSD-11 which has a considerable lower Fo values of 89.0–89.8 (Fig. 4a). NiO contents have a wider variation ranging from 0.07 wt.% to 0.35 wt.%, exhibiting roughly an increase with increasing of Fo values. CaO contents of olivine generally below 0.05 wt.%. Besides, anomalously high Cr2O3 contents up to 0.71 wt.% were obtained from olivine inclusions in chromite (Fig. 4b), which have similar Fo values (90.9–92.4) and NiO contents (0.19 wt.%–0.30 wt.%) with interstitial olivine.

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Figure 4. Diagrams of (a) Fo vs. NiO (wt.%) and (b) Fo vs. Cr2O3 (wt.%) of olivine for the different lithologies (fields after Pagé et al., 2008). ABP. Abyssal peridotites; FAP. fore-arc peridotites. The melting trend from Ozawa (1994).
Table 1 Representative electron microprobe analyses of olivines (wt.%) in the peridotites and chromitites
4.2 Orthopyroxene

Microprobe analyses data of orthopyroxene from coarse- and fine-grained harzburgites are listed in Table 2. Opx is compositionally enstatite with En [100×Mg2+/(Mg2++Fe2++ Ca2+)] values of 90.6–94.8, and has minor Al2O3 (0.08 wt.%–1.20 wt.%), CaO (0.13 wt.%–0.72 wt.%) and Cr2O3 (0.06 wt.%–0.61 wt.%). The Al2O3 and Cr2O3 contents of Opx in the fine-grained harzburgites are a little higher than those of Opx in the coarse-grained harzburgites (Figs. 5a and 5b). High Mg# values [100×Mg2+/(Mg2++Fe2+): 90.7–92.9] of Opx indicate harzburgites are highly refractory.

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Figure 5. Diagrams of (a) Al2O3 (wt.%) vs. Mg# and (b) Al2O3 vs. Cr2O3 (wt.%) of orthopyroxene in the fine- and coarse-grained harzburgites (fields after Pagé et al., 2008). ABP. Abyssal peridotites; FAP. fore-arc peridotites. The melting trend from Smith and Elthon (1988) and fractionation trend is from Varfalvy et al.(1997, 1996). Legends are the same as in Fig. 4.
Table 2 Representative electron microprobe analyses of orthopyroxenes (wt.%) in the harzburgites
4.3 Cr-Chlorite

Representative microprobe analyses data of Cr-chlorite from peridotites and chromitites are listed in Table 3. Cr-chlorite showing a narrow compositional variation, has relatively high MgO (32.04 wt.%–34.01 wt.%), medium SiO2 (31.24 wt.%–33.60 wt.%) and Al2O3 (11.67 wt.%–16.28 wt.%), low FeO (2.11 wt.%–3.22 wt.%), Cr2O3 (2.46–4.47) and NiO (0.07 wt.%–0.21 wt.%), and high Mg# values (94.9–96.5). Compared with coexisting chromite, Cr-chlorite has slight higher NiO contents (0.07 wt.%–0.21 wt.%), which may be due to the nickel will transfer from chromite into silicate minerals like chlorite during alteration/metamorphism process (Gervilla et al., 2012). According to the traditional classification based on the silicon and Fe/(Fe+Mg) values, these Cr-chlorites belong to clinochlore and penninite (Fig. 6).

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Figure 6. Classification of chlorite in the Songshugou peridotite massif based on diagram proposed by Hey (1954).
Table 3 Representative electron microprobe analyses of chlorites (wt.%) in the peridotites and chromitites
4.4 Chromite 4.4.1 Major elements (Al, Cr, Mg, Fe)

Major and trace elements compositions of chromite in the Songshugou peridotite massif and related chromitites are listed in Table 4. Collecting the major elements compositions of chromite reported by several researchers in literatures (e.g., Nie et al., 2017; Yu et al., 2017; Cao et al., 2016), we have plotted a summary of the chromite data in Fig. 7. All our new data approximately fall within the spread of compositions reported previously, characterized by high Cr# values (67.5–87.6), low Mg# values (23.4–41.2), and most of them roughly scatter in the edge of chromite compositional field of bonitites (Fig. 7a). The Cr# values of chromite increase and the Mg# values decrease continually from fine-grained harzburgites, coarse-grained harzburgites, to coarse-grained dunites. The cores of zoned chromite from coarse-grained dunites and chromite from massive chromitites show lowest Cr# and highest Mg# values, respectively. Chromite compositions from literatures show a wide variation of Fe# values [100×Fe3+/(Fe3++Cr+Al)], while the data in this study display a limited scope ranging of 2.4–16.6 (Fig. 7b). Due to their high-Cr and Ti-poor nature, most of the measured chromite compositions distribute in the overlap region of podiform chromitite and boninite field in the diagram of TiO2 versus Cr# (Fig. 8).

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Figure 7. Mg# [100×Mg/(Mg+Fe2+)] vs. Cr# [100×Cr/(Cr+Al)] and Fe# [100×Fe3+/(Fe3++Cr+Al)] of chromite in the Songshugou peridotite massif (fields after Akbulut et al., 2016).
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Figure 8. Cr# vs. TiO2 of chromites in the Songshugou peridotite massif (fields after Pagé and Barnes, 2009). Legends are the same as in Fig. 7.
Table 4 Major-element oxides (wt.%) and trace-element compositions (ppm) of chromites in the peridotites and chromitites by LA-ICP-MS analysis
4.4.2 Minor and trace elements (Sc, Ti, V, Mn, Co, Ni, Zn, Ga)

Chromites in chromitites have relatively higher Ti and Ni contents, lower Zn and Co contents than those in peridotites (Figs. 9a9d). The Sc, V, Mn and Ga contents of chromites in peridotites show similar ranges with those in chromitites (Figs. 9e9h). To facilitate a comparison of minor and trace elements (Sc, Ti, V, Mn, Co, Ni, Zn, Ga) and major oxides (Al2O3, MgO, FeOT, Cr2O3), the average chromite compositions data from different textural types were normalized to the compositions of chromite from MORB, and plotted following the elements order suggested by Pagé and Barnes (2009) (Figs. 10a and 10b). Both of the chromites in the Songshugou peridotites and chromitites show enrichment in Zn, Co Mn and FeOT, and depletion in Al2O3, Ga, Ti, Ni and MgO, and either slight enrichment or depletion in V and Sc relative to the MORB chromite. While, relative to the chromite from TMO boninites and BON boninites, they show enrichment in Zn, Co, Mn and FeOT, depletion in Ni, MgO and Sc, and either enrichment or depletion in Al2O3, Ga, Ti and V. Overall, chromite compositions of the Songshugou chromitites are generally consistent with the chromite field of the Thetford Mines ophiolite (TMO) chromitites (Canada), with slightly negative Sc, MgO, and positive Mn and FeOT anomalies (Fig. 10a). Whereas chromite data from peridotites show distinct negative anomalies of Ti, Ni, MgO and Sc, and strong positive anomalies of Zn, Co and Mn (Fig. 10b).

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Figure 9. Plots of Cr# vs. Ti, Ni, Zn, Co, Sc, V, Mn and Ga in chromites of the Songshugou peridotite massif. Fields of chromite from high-Cr chromitites, high-Al chromitites and layered intrusions are from González-Jiménez et al. (2017). Legends are the same as in Fig. 7.
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Figure 10. Multi-element patterns of chromites in the Songshugou peridotite massif, normalized to compositions of chromite from MORB (Pagé and Barnes, 2009). Data for BON boninites, TMO boninites, and TMO chromitites from Pagé and Barnes (2009), Luobusha chromitites from Zhou et al. (2014), Zedang harzburgite from Xiong et al. (2017), Byrapur and Bhaktarhalli stratiform chromitites from Mukherjee et al. (2015). Legends are the same as in Fig. 7.
5 DISCUSSION 5.1 Origin of Mineral Inclusions 5.1.1 Silicate inclusions in chromite grains

In podiform chromitite deposits, chromite commonly hosts mineral inclusions. Most of them are silicate minerals (e.g., olivine, orthopyroxene, clinopyroxene, amphibole, phlogopite and chlorite), base-metal sulfides (pentlandite, pyrrhotite, and chalcopyrite), and platinum-group minerals (Os-Ir alloy and laurite) (e.g., Liu et al., 2017; Akbulut et al., 2016; González-Jiménez et al., 2015; Merlini et al., 2011). Unusual minerals are also reported, such as diamond, coesite, zircon and kyanite (Lian et al., 2017; Xiong et al., 2017b; Griffin et al., 2016; Yang et al., 2014, 2007; Zhou et al., 2014), and their origins are in discussed (Ballhaus et al., 2017; Yang et al., 2015). There are three genesis modes of the primary silicate inclusions: (1) entrapment of pre-existing silicates during chromite crystallization (e.g., Zhou et al., 2014), (2) entrapment of anhydrous liquidus silicates interacting with fluid/melts during chromite crystallization (e.g., McElduff and Stumpfl, 1991), or (3) entrapment of differentiated melts within liquidus chromite and then crystallized silicates during cooling (Xiong et al., 2017 and references therein). Secondary inclusions are generally believed to form during alteration/metamorphism process (Colás et al., 2014; Gervilla et al., 2012).

Chromite grains in different textural types of the chromitites and dunites from Songshugou ubiquitously contain silicate inclusions. Common primary monophase inclusions are olivine grains, followed by minor clinopyroxene and phlogopite. Furthermore, a few multiphase mineral assemblages inclusions consisted of olivine and clinopyroxene are observed occasionally. Rounded shapes and differentiated structural of multiple phase mineral assemblages suggest that these inclusions are crystallization products of trapped liquids during crystallization of the chromite, and closely related with chromite formation. However, the genetic relation between monophase minerals (mostly olivine) and chromite is uncertain. They may be entrapment of reaction relics in harzburgire or lherzburgite, or crystallization products of trapped liquids. Secondary inclusions are mainly Cr-chlorite and tremolite grains showing irregular crystal shapes, which are reaction products between olivine and chromite in the presence of fluid (see below).

Olivine inclusions in chromite have remarkably higher Cr2O3 contents (0.08 wt.%–0.71 wt.%) relative to interstitial olivine (usually below 0.1 wt.%). This kind of chromium-rich olivine inclusions were reported by some researchers. For examples, the Cr2O3 contents of olivine inclusions in chromite from Troodos chromitites (Cyprus), Luobusha chromitites, and Kudi chromitites are up to 1.25 wt.%, 1.28 wt.%, and 1.49 wt.%, respectively (Liu et al., 2017; Liang et al., 2014; Merlini et al., 2011). There are several approaches that may contribute to the formation of chromium-rich olivine. The first method is diffusion of Cr from chromite to olivine. However, most of the Cr diffusion experiments focused on its transfer out of their olivine rather than its transfer into olivine, and the difference between out- diffusion versus in-diffusion is poorly understood (Jollands et al., 2018). If the Cr content results from diffusion, this mechanism may be accompanied by other cations such as Mg and Ni (Fe-Mg exchange between chromite and olivine is common under subsolidus conditions, e.g., Xiao et al., 2016), and thus generate olivine inclusions with remarkable higher Fo, and even NiO contents than interstitial ones (e.g., Xiong et al., 2015). The second method is inheritance of Cr from its precursor phase. Liang et al. (2014) investigated Luobusha chromitites and suggested that Cr2+ entry in olivine lattice in ultra-high pressure and reduced state, and the chromium-rich olivine may originate from mantle transition zone or lower mantle and their precursor phase might be wadsleyite or ringwoodite. When those deep mantle materials rise up to shallow crust, the olivine will eliminate Cr2+ as chromite in oxidizing condition. In case of the olivine is included in chromite, it may act as shields to prevent producing exsolution, and subsequently form chromium-rich olivine inclusions. Electron backscattered diffraction analysis confirmed that olivine preserves the crystallographic preferred orientation of wadsleyite, and transformation of wadsleyite to olivine is accompanied by exsolution of diopside/coesite in chromite grains from Luobusha (Satsukawa et al., 2015). The third approach is that previously depleted mantle harzburgite reacts with the melt containing Cr, and produces olivine solid solution added with Cr (Ren et al., 2008).

In the Songshugou peridotite massif, no diopside/coesite exsolution or other ultra-high pressure evidence has been observed in our study or reported in literatures, so it is unsuitable to infer that Cr-rich olivine transform from its high-pressure polymorph. Besides, Cr-rich olivine inclusions show similar range of Fo and NiO contents with interstitial olivine grains (Fig. 4a), which is different from those olivine inclusions undergone a certain degree of diffusions (e.g., Xiong et al., 2015). Therefore, Cr-rich olivine inclusions may be interaction products between peridotite and Cr-bearing melt, which is consistent with the conclusion that parental magma of chromitites is boninitic melt (see below).

5.1.2 Chromite inclusions in olivine grains

Interstitial olivine include rare rounded chromite grains generally surrounded by Cr-chlorite, with sizes varying from 5 to 20 μm. Cr-chlorite coexisting with chromite inclusions may form by same mechanism as those coexisting with interstitial chromite. Flake-like chromite inclusions in olivine are reported in Luobusha ophiolite. They are interpreted as exsolution of chromium-rich olivine during recycle process from deep to shallow mantle section (Liang et al., 2014). However, rounded morphological characteristics, and semi-orientation distribution of the chromite inclusions in olivine of the Songshugou peridotites suggest that they may represent almost simultaneous precipitation of olivine and chromite. Clinopyroxene and orthopyroxene normally contain more chromium content than olivine in mantle peridotite. When migrating melt translates pyroxene into olivine, chromium will be released into melt (Zhou et al., 2014, 1996; Kelemen et al., 1997). Temporary chromite saturation in melt may result in almost simultaneous precipitation of olivine and chromite.

5.2 Effects of Metamorphism and Alteration 5.2.1 Origin of Cr-chlorite

Some researchers found that ferrian chromite always coexists with Cr-chlorite (González-Jiménez et al., 2016; Colás et al., 2014; Gervilla et al., 2012; Merlini et al., 2009). Merlini et al. (2009) and Gervilla et al. (2012) detailedly discussed formation mechanisms of them and proposed two different models.

(1) During the prograde metamorphic evolution above 300 ℃, chromite reacts with antigorite in the presence of fluid to form chlorite and ferrian chromite (chromite+antigorite+H2O+ O2→Fe2+-rich chromite+chlorite, Merlini et al., 2009).

(2) During retrograde metamorphic evolution (eclogite- facies to amphibolite-facies) accompanied with fluid infiltration, chromite reacts with olivine to produce chlorite, Cr- and Fe2+-rich residual chromite (ferrous chromite) at ~700 to ~450 ℃ (chromite+olivine+H2O+SiO2 aq.→Fe2+-rich chromite+chlorite+ brucite, González-Jiménez et al., 2016; Colás et al., 2014; Gervilla et al., 2012).

In our peridotite samples, the Cr-chlorite usually occurs in the contact zone of olivine and chromite, and surrounds the chromite grains. In chromitites samples, part of the olivine grains have almost totally been replaced by Cr-chlorite and occurs as relics (Fig. 3g). In addition, no antigorite has been observed around Cr-chlorite and chromite grains. These phenomena suggest Cr-chlorite in peridotites and chromitites from Songshugou is formed by reaction of chromite and olivine in the presence of fluid.

The AlIV compositions of chlorite can be used as geothermometers to interpret its formation temperature. This method is widely applied in deposits associated with hydrothermal action in crustal scale (e.g., Shabani, 2009; Klein and Koppe, 2000), and some times, in ultramafic rocks and associated podiform chromitites in mantle scale (González-Jiménez et al., 2016). The geothermometers formulas are compiled as follows

$\begin{array}{l} {\rm{A}}{{\rm{l}}^{{\rm{IV}}}} = 4.71 \times {10^{ - 3}}{\mathit{T}} - 8.26 \times {10^{ - 2}}\\ \end{array} $ (1)
$\begin{array}{l} ({\rm{Cathelineau\;and\;Nieva}}, 1985)\\ T = 106 \times {\rm{A}}{{\rm{l}}_{{\rm{corrected}}}}^{{\rm{IV}}} + 18\\ \end{array} $ (2)
$\begin{array}{l} {\rm{A}}{{\rm{l}}_{{\rm{ corrected}}}}^{{\rm{IV}}} = {\rm{A}}{{\rm{l}}_{{\rm{ sample}}}}{^{{\rm{IV}}}{ + 0.7/({\rm{Fe}} + {\rm{Mg}})}}\\ \left({{\rm{Kranidiotis\;and\;MacLean}}, 1987} \right) \end{array} $ (3)
$\begin{array}{l} {\rm{A}}{{\rm{l}}_{{\rm{corrected}}}}^{{\rm{IV}}} = {\rm{A}}{{\rm{l}}_{{\rm{sample}}}}^{{\rm{IV}}} - 0.88\left[ {{\rm{Fe/}}\left( {{\rm{Fe + Mg}}} \right) - 0.34} \right]\\ \left( {{\rm{Zang\;and\;Fyfe,1995}}} \right) \end{array} $ (4)
$\begin{array}{l} T = - 61.92 + 321.98{\rm{A}}{{\rm{l}}^{{\rm{IV}}}}\\ \left({{\rm{Cathelineau}}, 1988} \right) \end{array} $ (5)

Using above formulas to chlorite of the Songshugou peridotite massif constrains the ranges of formation temperatures: [Eq. (1)]=179–225 ℃, [Eqs. (2) & (3)]=181–228 ℃, [Eqs. (2) & (4)]=207–254 ℃, [Eq. (5)]=182–253 ℃. Nevertheless, these ranges of temperatures are considerably lower than those of retrograde metamorphic temperature of surrounding metamafic rocks (825–400 ℃, from granulite facies to amphibolite facies) estimated by Liu et al. (1995) and Tang et al. (2016).

5.2.2 Modification of chromite compositions

Compared to olivine and pyroxene, chromite has high resistance to alteration, which helps it preserve more original magmatic features and become a reliable petrogenetic indicator in ultramafic rocks (Mukherjee et al., 2015; Evans et al., 2013; Kamenetsky et al., 2001; Maurel and Maurel, 1982; Irvine, 1967). However, recent investigation of coupled study of Fe and Mg isotopes in chromite and olivine, confirms that Fe-Mg diffusion between chromite and olivine plays an important role in controlling their compositions (Xiao et al., 2016). Modeling calculation attests that 3% exchange of Fe in olivine could decrease Mg# of chromite from 80 down to 33 in dunite with chromite less than 10%, while the chromite in massive chromitites with rare olivine has not been affected by this process (Xiao et al., 2016). Moreover, modification for compositions and structures of chromite in different intensions during alteration and metamorphism has been noticed and discussed (Colás et al., 2014; Gervilla et al., 2012; Merlini et al., 2009; Burkhard, 1993). Thus, using chromite as dependable petrogenetic index mineral must be based on its pristine magmatic compositions.

Although most olivine grains of the Songshugou peridotites and chromitites are fresh, the commonly distributed amphibole (mostly anthophyllite+minor tremolite), coexisting Cr-chlorite with olivine, and zoned chromite grains demonstrate that these rocks have undergone alteration/metamorphism process.

Generally, modified chromite has significantly higher FeO and Cr2O3, lower Al2O3 and MgO than fresh ones (Colás et al., 2014; Zhou et al., 2014; Gervilla et al., 2012; Merlini et al., 2009; Barnes, 2000). Whereas, the TiO2 contents could increase (Graham et al., 1996) or decrease (González-Jiménez et al., 2015; Colás et al., 2014) due to the different chemical and physical conditions. The interaction between olivine and chromite produces Cr-chlorite and secondary chromite in the Songshugou peridotites and chromitites. This reaction diminishes the volume of the secondary chromite, and decreases the number of octahedral sites which is available for Ti and Ga (Colás et al., 2014; Gervilla et al., 2012). Chromite grains in the dunites and harzburgites display similar features of strong enrichment in Zn, Co and Mn, and moderate depletion in Ga, Ti and Ni (Fig. 10b). This phenomenon results from that most tetrahedral sites, which can be replaced by Mn, Zn and Co, are still occupied by Fe2+ at low Fe3+/(Fe3++Fe2+) ratio (< 0.3) (Sack and Ghiorso, 1991), whereas octahedral sites occupied by Cr and Fe3+, which hinders the entry of Ga, Ti and Ni in chromite lattice (Gervilla et al., 2012). However, the high contents of Ti and slight enrichment of Zn, Co and Mn (Fig. 10a) and lower Fe# values (Fig. 7b) of chromites in massive chromitites suggest the modification of their compositions is weak relative to those in peridotites. Which maybe resulted from the low ratios of silicate/chromite in massive ore limits the alteration/metamorphism intension (e.g., González-Jiménez et al., 2015). Therefore, chromite grains in massive chromitites are more suitable to be treated as indicators to discuss their petrogenesis and tectonic setting.

5.3 Parental Melts of the Songshugou Chromitites

The compositional relationships between the chromite and its parental magma have been well studied (Zaccarini et al., 2011 and references therein). In this article, on account of high- Cr nature of the Songshugou chromitites, the Al2O3 contents, TiO2 contents and FeO/MgO ratios of parental melts crystallizing the chromitites are calculated using the empirical formulas suggested by Zaccarini et al. (2011), Kamenetsky et al. (2001) and Maurel and Maurel (1982), respectively.

Calculated parental magmas compositions based on chromite compositions from massive chromitites contain 11.17 wt.%– 13.57 wt.% Al2O3, 0.15 wt.%–0.27 wt.% TiO2. The calculated values fall into the boninitic field in the diagram of Al2O3(melt) versus TiO2(melt) (Fig. 11a), which indicates a supra- subduction origin of the Songshugou chromitites (e.g., González-Jiménez et al., 2014; Pagé and Barnes, 2009). In addition, multiphase mineral assemblages inclusions consisted of olivine and clinopyroxene hosted in chromites of the Songshugou chromitites demonstrated that the trapped melts have high MgO content. However, the calculated FeO/MgO ratios (2.34–2.82) in parental melts are higher than those in boninite melts (Fig. 11b), which might have resulted from subsolidus reequilibration.

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Figure 11. Diagrams of (a) TiO2 vs. Al2O3 (fields after Akbulut et al., 2016) and (b) FeO/MgO vs. Al2O3 in the parental magma (fields after Barnes and Roeder, 2001). Legends are the same as in Fig. 7.
5.4 Tectonic Setting

Supra-subduction zone (SSZ) setting is the most widely accepted tectonic environment for the formation of podiform chromitites (e.g., Xiong et al., 2017a; Akbulut at al., 2016; González-Jiménez et al., 2016, 2015; Malpas et al., 1997). Recently, Xiong et al. (2015) and Uysal et al. (2009) proposed an integrated model in which the formation of chromitites originally is beneath mid-ocean spreading centers, and followed by subduction to the asthenosphere mantle, subsequently uprises back to the surface by mantle convection.

Mineral chemistry combined with whole-rock major elements, trace elements and PGEs geochemistry suggested the Songshugou peridotites were sourced from the mantle wedge above a subduction zone (Cao et al., 2017, 2016; Nie et al., 2017). From passive oceanic margins to subduction-related active margins, the Al2O3 contents of orthopyroxene decrease, while the Cr# values of chromite increase due to the increase degree of depletion of peridotites (Bonatti and Michael, 1989). The data obtained from harzburgites in the Songshugou distribute in the field of subduction margins (Fig. 12a). TiO2 contents of chromites are treated as vital indicator of the tectonic setting of chromite formation (e.g., Arai and Matsukage, 1998). The composition data in this study plot in the region of supra-subduction zone (SSZ) peridotites in the diagram of TiO2 versus Al2O3 (Fig. 12b), clustering along the compositional array of arc-related volcanic rock chromite (ARC) field. And our calculated compositions of parental magma fall into the boninitic field (Fig. 11), suggesting an arc-related affinity and a supra-subduction origin (González-Jiménez et al., 2014; Pagé and Barnes, 2009) for the Songshugou chromitites. In addition, comparison of the normalized multi- element patterns of chromite grains in the study area with others from well constrained tectonic settings could also provide clue to speculate the tectonic setting (e.g., González-Jiménez et al., 2015; Zhou et al., 2014). The normalized multi-element patterns of chromites in the Songshugou chromitites show clear comparability with high-Cr type chromites in the TMO chromitites (Fig. 10a), which are generally considered to form in fore-arc environment (Pagé and Barnes, 2009). Therefore, combined with information of previous studies, major and trace elements geochemistry of chromite, as well as the nature of parental magma, it can be revealed that the Songshugou chromitities were formed in a SSZ environment.

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Figure 12. (a) Cr# of chromite vs. Al2O3 of orthopyroxene in harzburgites (fields after Bonatti and Michael, 1989); (b) Al2O3 vs. TiO2 of chromite in the Songshugou chromitites (fields after Kamenetsky et al., 2001). MORB. Mid-oceanic ridge basalt; BON. boninite; LIP. large igneous province basalt; OIB. ocean island basalt; ARC. arc-related volcanic rock. Legends are the same as in Fig. 7.
6 CONCLUSIONS

In this study, we report major and trace elements compositions of chromite, major elements compositions of olivine, orthopyroxene and Cr-chlorite of peridotites and chromitites from Songshugou peridotite massif. Based on these data, the following conclusions can be drawn.

(1) Chromite grains in different textural types of chromitites and dunites from Songshugou ubiquitously contain primary and secondary silicate inclusions. Primary inclusions are dominated by monophase olivine, with minor clinopyroxene and a few multiphase mineral assemblages consisting of olivine and clinopyroxene. Secondary inclusions are mainly Cr-chlorite and tremolite grains.

(2) The Songshugou peridotite massif has undergone alteration/metamorphism process, which results in formation of Cr-chlorite and enrichment of Fe2O3, Cr2O3, Zn, Co and Mn, depletion in MgO, Al2O3, Ga, Ti and Ni in chromites.

(3) The Songshugou peridotite massif represents a segment of oceanic mantle. Its hosted chromitites are high-Cr type, and the formation of them is closedly related with boninite melt derived from a supra-subduction zone environment.

ACKNOWLEDGMENTS

This research was financially supported by the National Natural Science Foundation of China (No. 41672064) and the International Geoscience Programme "Diamonds and Recycled Mantle" (No. IGCP-649). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1227-8.


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