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Article

Mineralogy, Geochemistry and Tectonic Setting of the Raobazhai Ultramafic Complex, North Dabie

by
Fangchao Yan
1,*,
Qing Liu
1,*,
Quanlin Hou
1 and
Miao He
2
1
CAS Key Laboratory of Computational Geodynamics, College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
2
Institute of Earthquake Forecasting, CEA, Beijing 100036, China
*
Authors to whom correspondence should be addressed.
Minerals 2022, 12(3), 286; https://doi.org/10.3390/min12030286
Submission received: 30 January 2022 / Revised: 20 February 2022 / Accepted: 21 February 2022 / Published: 24 February 2022
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Abstract

:
The Raobazhai ultramafic complex is located in the north of the Dabie Mountains and is composed of spinel peridotites accompanied by a few lenticular mafic metamorphic rocks. The spinel peridotites are mostly harzburgite, along with minor dunite and lherzolite. This study reports the petrological, geochemical, and Re-Os isotopic data of spinel and chromite harzburgites from Raobazhai. The major and trace whole-rock geochemistry characteristics indicate that the rocks are remnants of partial melting to different degrees (6–17%). Both mineral and whole-rock geochemistry showed typical abyssal peridotite affinity. Due to the presence of water-bearing minerals, the Sr, Ba, and U were enriched, and the Nb, Zr, and Hf were depleted, which can be attributed to the strong metasomatism of the boninitic melting/fluid in the fore-arc domain. The flat distribution of platinum group elements (PGE) and the decoupling of Pt-Pd were also the result of the fore-arc melting/fluid interaction. The 187Os/188Os ratios (0.1149–0.1266) were generally lower than the recommended value of the primitive mantle and fell within the abyssal peridotite isotope range. This indicated that the Raobazhai harzburgites were likely mantle peridotites with oceanic characteristics that underwent a fore-arc boninitic melting/fluid transformation event.

1. Introduction

The lithospheric mantle is the most important location for material transfer and energy exchange within the shallow Earth. As it is the key link between the shallow crust and the deep mantle, it plays an important role in crust-mantle interactions, crustal growth and differentiation, and mineral resource formation [1,2,3]. To study the nature and evolution of the lithospheric mantle, the most direct natural samples are peridotite from the deep mantle, as the chemical composition of mantle peridotites records information regarding different mantle processes. Orogenic belts contain major outcrops of mantle peridotites and have, therefore, always been the focus of such research [4,5,6]. The detailed mineral compositions and geochemistry of mantle peridotites within orogenic belts provide important information for understanding the melt extraction processes, melt-mantle interactions, and the original tectonic setting where the mantle peridotites formed.
The Dabie orogenic belt is known as the ultra-high pressure (UHP) metamorphism due to continental collision, which also outcrops a series of exposed ultramafic rocks [7,8,9,10,11,12,13]. The Raobazhai area of the North Dabie Complex (NDC) was once interpreted as an island arc type mafic-ultramafic intrusion but drilling work in the last century found that the Raobazhai area was in tectonic contact with the surrounding rock, thus refuting this view [14]. Previous work on the Raobazhai complex has mainly focused on the garnet pyroxenite and peridotites [7,8,9,10,11,12]. Some researchers [13] regard the Raobazhai complex to be oceanic peridotite based on the dual-frequency distribution of spinel and pod-shaped chromitite. These researchers consider the Raobazhai complex to be the harzburgite and dunite mantle residuals of the arc-related environment above an intra-oceanic subduction zone, whereas the lherzolites may represent the weakly depleted upper mantle residual in the mid-ocean ridge (MOR) environment. Meanwhile, the low εNd, high εSr and low Re-Os isotope are considered characteristic of a continental lithospheric or “Alpine” mafic-ultramafic complex [7,10,12,14]. Through a comparison of the geochemical characteristics of the Raobazhai and North China Craton (NCC) mantle peridotite, some scholars believe that the Raobazhai mantle peridotites is a part of the NCC [15]. However, subsequent studies showed that the NCC and the Yangtze Craton have similar Re-Os model ages; therefore, the Re-Os model age has been considered unreliable for judging attribution [16]. The tectonic background of the Raobazhai complex has been a subject of ongoing debate, particularly whether the complex has a continental or oceanic origin.
This study investigated the mineralogy, petrology, and geochemistry of harzburgites from Raobazhai, North Dabie, China, to determine the mantle rock properties. These harzburgites showed different mineral assemblages and geochemical characteristics, which reflect the different transformations due to geological processes in the Raobazhai area. The petrogenesis of this ultramafic body is important to understand the Dabie orogenic belt. Combined with previous studies, the original tectonic environment and formation processes of the Raobazhai mantle peridotites were discussed and elucidated.

2. Geological Setting and Sample Descriptions

The Dabie orogenic belt was formed by the collision between the Sino-Korean Craton and the Yangtze Craton during the Triassic Period, bordered by the Qinling orogenic belt to the west and the Tan-Lu fault zone to the east. The Xiaotian-Mozitan fault, Mituo Hualiangting fault, Shuihu–Wuhe shear zone, and Taihu-Mamiao fault separate the Dabie orogenic belt into five tectonic units. From north to south, these are the Greenschist/subgreenschist belt, High-temperature belt, UHP eclogite belt, High-pressure eclogite belt, Blueschist belt [16].
It is one of the largest of several mafic-ultramafic complexes exposed in the North Dabie Mountains [17,18]. The Raobazhai complex is composed of spinel peridotites, which are predominantly harzburgite with some dunite and lherzolite. These are mixed with a few lenticular mafic metamorphic rocks (e.g., eclogite and granulite). The wall rocks are Neoproterozoic granitic gneisses. Previous studies have shown that the granitic gneisses have also undergone Triassic subduction-collision processes [13] (Figure 1). Core sample data suggest the presence of a structural contact between the Raobazhai peridotites and surrounding gneiss [15].
The samples included spinel and chromite harzburgites (sample information in Table S1). The main mineral assemblages consisted of olivine, orthopyroxene, amphibole, clinopyroxene, and spinel or chromite. All Raobazhai harzburgites suffered serpentinization to various degrees. They displayed fractures, mostly cutting across olivine grain boundaries (Figure 2a). Serpentine minerals occurred along olivine grain boundaries and fractures (Figure 2c,d). Phlogopite and amphibole were widely distributed in all Raobazhai harzburgites (Figure 2d); perfect euhedral habit suggested that they are the product of later geological processes. The spinel harzburgites showed a porphyritic texture (Figure 2b), showing wavy extinction in the olivine grains (Figure 2a). Both clinopyroxene and orthopyroxene are predominantly coarse-grained (approximately 0.5–4.0 mm) with wavy extinction (Figure 2b,c). The chromite harzburgites have a similar mineral assemblage and structure to the spinel harzburgites
Brown spinel is ubiquitous in spinel harzburgite, usually heteromorphic or semi autogenous, and occasionally occurs as a similar “exsolution” phenomenon in the interstratified clinopyroxene and orthopyroxene structures (Figure 2e,f). Two spinel stages can be seen on the BSE images (Figure 2g). Spinel harzburgites show a small amount of pentlandite associated with silicate minerals, but sulfides were rarely found in the chromite harzburgites (Figure 2h).

3. Analytical Methods

The samples were cut into thin sections for petrographic observation and major element mineral analyses. The remaining portions of each sample were ground to powders with sizes of <200 mesh for whole-rock major elements, whole-rock trace elements, highly siderophile elements (HSEs), and Re-Os isotope measurements. All analyses conducted were performed at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS) in Beijing, China, except for the major element composition of the minerals, which was done at the Hefei University of Technology, China.

3.1. Whole-Rock Major Element

Sample powders (0.5 g) were precisely weighed and mixed with 5 g of Li2B4O7 to prepare fused glass disks for whole-rock major elements analysis using an AXIOS Minerals X-ray fluorescence (XRF) spectrometer. Loss on ignition (LOI) was determined by measuring the relative mass lost after heating a 0.5 g sample powder at approximately 1000 °C for 1.5 h. The 95% confidence limits for the XRF analyses were approximately 0.04–0.05% for SiO2 and Al2O3; approximately 0.01–0.02% for Fe2O3, MgO, Na2O, and K2O; and less than 0.01% for TiO2, MnO, CaO, and P2O5, respectively, according to triplicate laboratory measurements using the USGS GSR-1 standard.

3.2. Mineral Major Element

A JEOL JXA-8230 analyzer was used to determine the composition and capture backscattered electron (BSE) images of the main elements using electron microprobe analysis (EMPA). The analysis conditions were 15 kV acceleration voltage of 20 nA beam current, and a counting time of 10–20 s. An electron beam diameter of approximately 3–5 μm was used, except for the analysis of tiny minerals, where a beam size of 3μm was used. Natural minerals were used as standards, and the ZAF program was used for matrix correction.

3.3. Whole-Rock Trace Element

An Agilent 7500a inductively coupled plasma mass spectrometer (ICP-MS) instrument was used to determine the concentrations of trace elements in the whole rock. Before analysis, the sample powder (40 mg) was dissolved in a mixture of HNO3 and HF and heated at 200 °C for 5 days. The solution was then evaporated, refluxed with HNO3, and dissolved in the HNO3-HF acid mixture for 24 h at 150 °C. Then the sample was diluted with an internal standard and 1% HNO3. During this process, the GSR-1 and GSR-3 standards were analyzed, and the accuracy of the data obtained for most elements was greater than 5%.

3.4. HSE and Re–Os Isotope Analyses

The HSE contents and Re-Os isotope ratios were determined via isotope dilution. The detailed procedure has been described in [19]. Approximately 2 g of sample powder and appropriate amounts of a mixed 187Re–190Os spike and a mixed 191Ir–99Ru–194Pt–105Pd spike were digested with 3 mL of purified concentrated HCl and 6 mL of purified concentrated 16 N HNO3 in a Pyrex® borosilicate glass Carius tube. After digestion at approximately 220 °C for approximately 72 h, Os was extracted from the aqua regia solution into CCl4 and then back extracted into HBr, followed by purification via microdistillation [20]. Re, Ir, Ru, Pt, and Pd were separated and purified by ion exchange chromatography using 2 mL resin (AG 1 × 8, 100–200 mesh) [21] for further elution. The Re–Ru, Ir–Pt, and Pd fractions were eluted with 6 N HNO3, 13.5 N HNO3, and 10 N HCl, respectively. The Re fraction was then purified using a small-size anion exchange column with approximately 0.1 mL resin. Each fraction of Re, Ir, Ru, Pt, and Pd was heated to dryness and re-dissolved in 1 mL 0.8 N HNO3 prior to ICP–MS analysis.
Os isotopic compositions were measured using a thermal ionization mass spectrometer (TIMS) and a GV Isoprobe-T mass spectrometer (GV isotopic compositions were measured). Os was loaded onto the platinum filaments during the measurement, and Ba(OH)2 was used as an ion emitter. Os isotopic ratios were corrected for oxygen isotope fractionation using the standard oxygen isotope corrections of Nier and online mass fractionation using 192Os/188Os = 3.08271 and off-line spike corrections. During the analysis, the Johnson–Matthey standard (UMCP) yielded an 187Os/188Os ratio of 0.11378 ± 2 (2σ;σ = relative standard deviation; n = 4), which is consistent with the result (187Os/188Os = 0.113791 ± 15) analyzed by [22]. An international serpentinite standard, UB-N, was used as an external standard and had a measured Os concentration of 3.90 pg and an 187Os/188Os ratio of 0.12690 ± 3 (2SE). The in-run precisions for Os isotopic ratios of all samples were better than ±0.2% which is the 2σ value. The total procedural blanks were approximately 0.93–0.97 pg for Os (n = 2). Re and other HSEs (Ir, Ru, Pt, and Pd) were analyzed at the MC–ICPMS Laboratory using a Thermo Fisher Neptune Plus mass spectrometer. The measured results for the UB-N standard were 3.48 ng for Ir, 7.07 ng for Ru, 7.30 ng for Pt, 5.84 ng for Pd and 0.209 ng for Re. These values are consistent with the recommended values of UB-N from [23] with Ir = 3.38 ng, Ru = 6.30 ng, Pt = 7.42 ng, Pd = 6.11 ng and Re = 0.206 ng. The total procedural blanks for Ir, Ru, Pt, Pd, and Re were 4–10, 18–28, 24–29, 33–37, and 2–4 pg, respectively. The in-run precisions for 185Re/187Re, 191Ir/193Ir, 194Pt/196Pt, 105Pd/106Pd, and 99Ru/101Ru were commonly 0.1–0.3% (2RSD).

4. Results

4.1. Mineral Major Element Compositions

4.1.1. Olivine

All analyzed olivine grains from spinel harzburgites have Fo composition of 88.7–91.6 where Fo = 100 × Mg/(Mg + Fe) (see Table S2). The Fo of chromite harzburgites was relatively high (Fo = 91.3–92.4).

4.1.2. Spinel and Chromite

Spinels from spinel harzburgites had Cr# = 0.15–0.46 where Cr# = Cr/(Cr + Al) (see Table S2). Chromite harzburgites had no spinels. There was little difference between the spinels from the same samples, and the relationship between Cr# and spinel particle size was not obvious. The TiO2 content of the spinel was low (0.014–0.077 wt.%), indicating that it was less affected by late geological processes (Figure 3a,b).

4.2. Whole-Rock Major Element Compositions

The major element compositions of the Raobazhai whole-rock samples are shown in Table S3. The samples show different degrees of serpentinization and LOI (1.50–11.61 wt.%). The MgO, Al2O3, and CaO contents of the spinel harzburgites were 35.99–45.21 wt.%, 1.13–3.56 wt.%, and 0.29–2.96 wt.%, respectively. In contrast, the MgO and Al2O3 contents of chromite harzburgites were higher at 46.58–47.18 wt.% and 0.33–0.47 wt.%, while the CaO content was lower at 0.29–0.36 wt.%. The TiO2 content of all the Raobazhai harzburgites was very low (0.02–0.11 wt.%); the overall content of Na2O was low (<0.10 wt.%), except for two samples that had high Na2O (RBZ-17 = 0.17 wt.% and NBZ-10a = 0.24 wt.%), indicating that they were likely affected by metasomatic processes. Al2O3, CaO, and TiO2 showed a good negative correlation with MgO (only NBZ-10a deviated from the trend line) and were similar to the compositional range of the abyssal peridotites (Figure 4). The Raobazhai harzburgites plot paralleled the modeled partial melting trends from [27], however, the Raobazhai samples in this study mostly contained lower CaO at a given MgO (Figure 4).

4.3. Whole-Rock Trace Element Compositions

The trace element contents of the Raobazhai harzburgites are shown in Table S3. Overall, the Raobazhai harzburgites showed chondritic to sub-chondritic trace element abundances. Almost all of them fell within the range of abyssal peridotites, which are relatively enriched in Ba, U, and Sr and depleted in Nb, Zr, and Hf. Negative Y anomalies were observed in some of the samples (Figure 5a). The REE chondrite distribution pattern diagram (Figure 5b) of the spinel harzburgite showed a flat trend. Some samples showed negative Ce anomalies (Ce* =   L a × P r , 0.024–0.650), suggesting that they experienced melt/fluid metasomatism. They also displayed varying degrees of HREE depletion, indicating that the samples had different degrees of partial melting. However, the HREE of sample RBZ-17 was higher than that of the depleted mid ocean ridge basalt (MORB) mantle (DMM), which suggested the process of re-enrichment. A typical DMM has depleted LREEs, and island arc magmatism often leads to the depletion of high field strength elements (HFSEs) such as Nb and Zr. Therefore, two LREE-rich chromite harzburgite samples (RBZ-40b and RBZ-42) were accompanied by negative Nb, Zr, and Hf anomalies, representing metasomatism or other special LREE enrichment processes.

4.4. HSE and Re-Os Isotopic Compositions

The whole-rock HSE and Re-Os isotopic compositions of the 10 Raobazhai harzburgite samples are shown in Table S4. The sample Os concentrations (2.37–5.01 ppb) were generally higher than the primitive mantle Os (3.4 ppb). The concentrations of Ir, Ru, Pt, and Pd in the harzburgite samples were 2.20–4.04 ppb, 4.74–8.16 ppb, 2.19–7.01 ppb, and 0.32–6.43 ppb, respectively. When normalized to the primitive mantle [31], the Ir-group platinum group elements (IPGE) showed a flat distribution pattern for all the samples (Figure 6). The Os/Ir and Ru/Ir (non-normalized ratios are 1.05 to 1.94 (PUM = 1.12 ± 0.09; [31]) and 1.85 to 3.45 (PUM= 2.03 ±0.12; [31]), respectively. Most of the samples also showed a flat Pd group platinum group elements (PPGE) distribution pattern; however, two chromite and one spinel harzburgite showed a right-leaning loss pattern (Figure 6). The non-normalized measured Pt/Ir, Pd/Ir, and Re/Ir were 0.99–2.67, 0.15–2.87, and 0.01–0.08, respectively, which are all lower than the mantle values [31].
The Re and Os contents of the Raobazhai harzburgites are shown in Table S4. The 187Os/188Os ratio was between 0.1149 and 0.1266, which is lower than that of the upper mantle (187Os/188Os = 0.1296; [33]). The 187Re/188Os ratios which were all between 0.022–0.349, all of which are lower than the corresponding values of the upper mantle (187Re/188Os = 0.402; [33]). The correlation between them was indistinct, but there was a strong correlation between 187Os/188Os and Al2O3 (Figure 7). The Re depletion ages (TRD) calculated according to the recommended value of the primitive upper mantle (PUM) varied greatly (min: NBZ-10a = 66 Ma; and max: RBZ-11b = 1778 Ma, Figure 7).

5. Discussion

5.1. Partial Melting and Melt/Fluid Metasomatism

The Raobazhai samples with high olivine Fo also had spinel with higher Cr# (Figure 3a), indicating that they had experienced different degrees of partial melting. Most of the spinel composition fell on the olivine-spinel mantle evolution line (OSMA), except for two spinel harzburgites (RBZ-17 and NBZ-10a) with Fo < 90.0, suggesting that the two samples were not simply partial melting residues but had a more complex genesis. The spinel composition of the harzburgites can also be used to estimate the degree of partial melting. The Cr of the spinel will increase through continuous melt extraction because a corresponding relationship is formed between the degree of partial melting and Cr#. Through calculations, Raobazhai spinel harzburgites were shown to have experienced 6–17% partial melting F = 10 · ln (Cr#) + 24 [34], and the spinels and associated olivines of these samples had chemical characteristics that fell within the range of abyssal peridotites. However, the chromites in chromite harzburgites showed a chemical correlation with fore-arc peridotites (Figure 3). A similar estimate for the degree of partial melting was obtained by comparing the main and trace results of the whole rock to previous simulation results (Figure 4 and Figure 5). The Raobazhai harzburgites plot paralleled the modeled partial melting trends from [27] indicating that they must have experienced variable degrees of melting. This is similar to the degree of partial melting experienced by the mantle peridotite of the Kop Mountain ophiolite [35]. The chromite harzburgite illustrated that the degree of partial melting was relatively high (Figure 3, Figure 4 and Figure 5). However, the Raobazhai samples in this study mostly contained lower CaO at a given MgO (Figure 4); petrography showed that the Raobazhai peridotites displayed phlogopite and that amphibole were widely distributed perfect euhedral habit, as well as the characteristic petrographic assemblage of three minerals (clinopyroxene, orthopyroxene, and spinel), which could be ascribed to melt/fluid metasomatism. It is important to note that the two kinds of pyroxene in Figure 2e,f were interstratified and dotted with fine spinel under the microscope. This petrographic feature generally has two interpretations: (1) the garnet-facies peridotite is retrogressed to spinel facies; and (2) it is caused by melt/fluid metasomatism. The former has been previously studied [36]. However, [37] believes that the restored “high Si” garnet composition is questionable and does not agree with the conventional garnet composition. When peridotite is subjected to weak melt/fluid metasomatism, it results in the enrichment of strong incompatible elements (LREEs) and relatively weak incompatible elements (HREEs) [38]. The positive Sr, Ba, and U anomalies indicated that the samples had undergone metasomatism. The positive slope of HREE and the negative irregularity of Y in some samples (e.g., RBZ-17) may suggest that these samples experienced enrichment. The negative Ce anomaly may be related to the redox process of crustal composition fluid/melt metasomatism. Figure 8 shows that the Yb content of sample RBZ-17 was significantly higher than that of FMM, due to the influence of melt infiltration, which is consistent with the REE partition pattern, which is formed by melt/fluid metasomatism of peridotite.
The photomicrographs of the peridotite samples (including the chromite and spinel harzburgites) from the Raobazhai complex showed extensive phlogopite and amphibole mineralization with perfect euhedral habit (Figure 2d), which indicates that these samples experienced melt/fluid metasomatism based on different degrees of partial melting. In addition, the two spinel harzburgites had small Fo values (RBZ-17 = 89.3, NBZ-10a = 88.7) and deviated from the expected Fo range for the abyssal peridotites (Figure 3a,b). The deviation of major elements from the evolutionary trend (Figure 4) and the enrichment of some incompatible elements (such as LREE, Sr, Ba, and U) and depletion of others (such as Nb, Zr, and Hf) are all manifestations of melt/fluid metasomatism. For mantle peridotite, the Cr# increases with an increase in the degree of melting, whereas the TiO2 content is expected to gradually decrease. However, certain samples from the Raobazhai complex displayed an increasing trend in TiO2 values (Figure 9), which may reflect boninitic melt/fluid metasomatism.

5.2. Effect of Melt/Fluid Metasomatism on HSE

5.2.1. PGE and Re

PGE and Re are HSEs that have extremely high sulfide/silicate partition coefficients. It is generally believed that the HSE content in the mantle is mainly controlled by sulfide stability. At a low degree of partial melting, sulfides or metal alloys preferentially remain in the residual phase; the concentration of HSEs will increase, but there is no obvious change in its model. When a larger degree of partial melting (>15–20%) occurs, PPGEs (i.e., Pt and Pd) show incompatibility, whereas IPGEs (i.e., Os, Ir, and Ru) still show compatibility [40]. The petrology (Figure 2h) of sulfides and the PGE distribution pattern (Figure 6) in Raobazhai harzburgites also reflected this characteristic. Spinel harzburgites from the Raobazhai complex usually contained some sulfide minerals (mainly pentlandite) that are stable at a lower degree of partial melting. In contrast, no obvious sulfides were found in the chromite harzburgite, which may reflect a higher degree of partial melting. Most spinel harzburgites have flat PGE distribution patterns, however, a few of the spinel harzburgites with high degrees of partial melting (such as sample RBZ-11b) and the chromite harzburgites have PPGE depletion patterns. All peridotite samples except RBZ-40b had flat IPGE distribution patterns, which can be explained by PUM batch melting, assuming equilibrium between monosulfide solid solution (MSS) and sulfide melt exists. In this mode, IPGEs migrate proportionally with the increase in the degree of melting, so that the sulfides in the melt and residue are always in equilibrium. The IPGE fractionation of sample RBZ-40b may be related to the chromite harzburgite sample because Ru is more compatible with chromite than other IPGEs under mantle conditions (Figure 10a). It may also be related to other rich IPGE alloys because, for mantle peridotites with a higher degree of partial melting, the proportion of IPGEs affected by the alloy will increase. The Re/Ir ratio of the Raobazhai harzburgites was lower than that of the partial melting model, which indicated that melt extraction could effectively remove incompatible Re (Figure 10b). The high Re content of the individual samples may imply that the introduction of sulfides led to the enrichment of Re. The Pd/Ir ratio is a measure of the PPGE/IPGE fractionation and can be used to indicate melt depletion and metasomatism e.g., [41,42]. Typical primitive mantle residues were not subjected to strong secondary overprinting and, overall, showed a strong positive correlation between the Pd/Ir ratio and silicate melt extraction index (such as Al2O3) [41]. The Pd/Ir of the Raobazhai harzburgites was higher than that of the melting model, which may be due to the metasomatism of the Pd-rich Cu-Ni sulfide melt to the Pd-depleted residual mantle (Figure 10c), which is similar to the Yarlung-Zangbo ophiolite [43,44] in Tibet and Lesvos [45]. The Raobazhai chromite harzburgites (samples RBZ-40b and RBZ 42) showed obvious Pt-Pd differentiation with a higher Pt/Pd ratio (Figure 10d). It is difficult to explain the fractionation of Pt and Pd simply by the separation and precipitation of sulfides, because they had similar partition coefficients. High sulfur fugacity and multi stage partial melting are the two most likely conditions for forming of Pt-rich phases that result from Pt enrichment [46,47]. However, high sulfur fugacity is unlikely to cause the Pt enrichment of Raobazhai chromite harzburgites. However, the enrichment of Pt can be explained by the formation of Pt-Ir-Os alloys, which usually begin to form at low degrees of partial melting; it has been proven that monosulfide can dissolve refractory Pt-Ir-Os alloys [47] when sulfur fugacity approaches the Fe-FeS buffer zone during partial melting of mantle peridotites. However, Pd is more likely to enter the Cu-rich sulfide melt and eventually be removed along with the silicate melt. The increase in the degree of partial melting leads to the continuous consumption of base metal sulfides, leading to a continuous decrease in sulfur fugacity. At a high degree of partial melting, the continuous decrease in sulfur fugacity leads to the complete replacement of the Pt-Ir-Os alloy with refractory monosulfide and the Os-Ir alloy [48]. The geochemical characteristics of Raobazhai chromite harzburgites, such as high MgO content, olivine Fo, spinel Cr# values, and low HREE content, which indicates a very high degree of partial melting. Therefore, Pt-Ir-Os alloys cannot be stable in harzburgite, which does not explain the Pt enrichment characteristics. Therefore, the enrichment of Pt in Raobazhai chromite harzburgites likely resulted from a high degree of partial melting caused by multiple melt extraction events. During the multistage partial melting process, refractory Pt-Fe alloys can be formed and retained in the residual mantle, which can lead to the fractionation of Pt and other platinum group elements, which is consistent with the strong fractionation of PPGEs and IPGEs [46].

5.2.2. Re-Os Isotope System

Unlike lithophile elements (such as Sm-Nd and Rb-Sr), the Re-Os isotope system has strong resistance to the late metasomatism process [51]. The 187Os/188Os ratios of the Raobazhai harzburgites are lower than the recommended values of the upper mantle (187O/188Os = 0.1296 ±0.0008; [33]). The 187Re/188Os ratios vary considerably, all of which are lower than the corresponding values of the upper mantle, and the correlation between them is not apparent, indicating that they are likely the result of a variety of geological processes. The content of Re in the sample was also lower than that of the primitive mantle (approximately 0.35 ppb, [31]).
The age of TRD varies greatly, reaching the maximum of 1778 Ma. However, this does not mean that the Raobazhai harzburgites are ancient continental lithospheric mantles. The inherent weakness of whole-rock Re-Os is that they produce a mixed model age without geological significance. For example, sulfides as high as 2.66 Ga are observed in the Ligurian ophiolite [52]. There are two main types of sulfides in the mantle. One is sulfides wrapped in silicate minerals, dominated by pentlandite and chalcopyrite. The other is often metasomatic base-metal sulfides (BMSs) in intergranular positions or as inclusions in clinopyroxenes. A single closed BMS may even be 0.5–1.5 Ga earlier than peridotites [53]. The ancient depleted components recorded in abyssal peridotites and ophiolites are usually interpreted as: (1) the infusible region preserved in the asthenosphere mantle (the heterogeneity of the mantle); or (2) the continental lithosphere mantle which “ran aground” in the asthenosphere [35], The peridotites of St Peter-Paul and Sal Island in the Atlantic Ocean were formed according to the latter model. The Raobazhai harzburgites may be more inclined to the former for the following reasons: the Cr# and Mg# of spinel in the St Peter-Paul peridotites were relatively low (Figure 3b), and the spinel compositions of the Raobazhai harzburgites were closer to the category of abyssal peridotites, which have higher spinel Cr# and Mg#. The second is the apparent correlation between 187Os/188Os and Al2O3 in orogenic peridotites, as with the study of abyssal peridotites, it has been found that there is also such a correlation in the ocean [28]. In addition, previous studies on the effects of subducted plates on Os isotopes have shown that high 187Os/188Os ratios usually occur in mantle wedges with low Os content (<1 ppb), because it is easier for Os isotopes in oxidized plate fluids to eliminate the isotopic characteristics of the surrounding mantle, thus superposing plate-derived Os isotope signals [54]. However, there was no such correlation between Os content and 187Os/188Os in the Raobazhai peridotites, and the Raobazhai Os/Ir ratios were higher than that of the primitive mantle. This strongly suggests that the age of the ancient, depleted components in Raobazhai harzburgites belongs to the inherent attributes of the sample, which is more consistent with the first explanation. Studies have shown that it is common for melt/fluid metasomatism to add radioactive Os to the subducted mantle, but the change of Os isotopes caused by this process is usually very small, and the linear trend of Al2O3 and Os isotopes observed also opposes the effect of molten fluid metasomatism on Os isotopic composition [16,22,55]. Although it is common for melt/fluid metasomatism to add radioactive Os to subducted mantle peridotite, the change in Os isotopes is minimal, and the linear trend of Al2O3 and Os isotopes of Raobazhai harzburgite also showed that metasomatism of the molten fluid had little effect on the Os isotopic composition (Figure 7).
Isotope research in the last 20 years on abyssal peridotite has shown a high degree of heterogeneity in the asthenosphere at different scales. The use of Re-Os and Lu-Hf isotopes to identify ancient mantle components (Re depletion model ages of more than 2 billion years) in abyssal peridotites, indicating that they experienced very ancient events of partial melting and melt extraction is arguably the most remarkable achievement. These ancient melting events occurred much earlier than the current oceanic expansion (less than 200 Ma); therefore, the ancient mantle in the asthenosphere is regarded as a component recycled into the mid-ocean ridge. However, at present, there is no restriction on the origin of ancient mantle components in the asthenosphere. The main viewpoints include the following: (1) the ancient oceanic mantle [56] is recycled into the asthenosphere by subduction; or (2) the continental lithospheric mantle [57], is recycled into the asthenosphere by delamination or thermal erosion of the mantle plume; and (3) the ancient island arc mantle wedge [58] is subducted into the asthenosphere. By summarizing the mantle peridotite in the abyssal peridotite and Neo-Tethys ophiolite, it was found that they are highly similar as both mantles display mantle heterogeneity [35]. From the perspective of this study, this heterogeneity may have a larger scale of similarity.

5.3. Tectonic Setting

The Raobazhai mafic-ultramafic complex was considered: (1) part of an island arc mafic-ultramafic intrusive body [59]; (2) a fragments of ophiolitic mélange [8,60,61]; (3) a remnants of the subcontinental lithospheric mantle [7,10,15,16] rebalanced under granulite-amphibolite facies.
In the last century, it was found that the Raobazhai complex is a cold intrusive rock sheet, so it is clear that the first conclusion is not applicable [14]. There is some controversy between the latter two. The authors of [16] claimed that there was no basis for excluding ophiolites without typical volcanic or crustal units. With the study of current abyssal peridotites, slow and ultra-slow ocean expansion were considered the main form of ocean expansion. According to this model, there might have been a lack of mafic magma and sheet dikes. Most studies have suggested that the Raobazhai harzburgites are continental lithospheric mantle, which is mainly based on the correlation between the Re and Os isotope and the Al2O3 contents of the whole-rock composition. At the same time, according to the older age of the substitute isochron (approximately 1.8–2.0 Ga), Raobazhai harzburgites have been considered remnants of continental mantle peridotites [15,16]. Combined with the zircon study of [15] and the similarity with the Ce anomalies of the Xinyang mantle xenoliths in North China, [16] it was believed that the Raobazhai peridotite originated from the lithospheric mantle of the NCC. However, in a subduction zone environment, our study supports that the negative Ce anomaly originated from the influence of a subducted plate. A negative Ce anomaly was not common in the Raobazhai peridotite (Figure 5). The zircon itself was the origin of the chemical signal for metasomatism; through the comparison of mineral chemistry, main trace element composition of the whole-rock and PGE value with the abyssal peridotite, we found that the Raobazhai peridotite had oceanic affinity no matter the spinel composition or the distribution pattern of major and trace elements and PGE.
According to the covariant diagrams of spinel Cr# and oxygen fugacity (logfO2 (FMQ)), when Cr# increased, oxygen fugacity increased accordingly, which is consistent with the trend of interaction between the oceanic ridge and supra-subduction zone magma (Figure 11). The above evidence indicated that the harzburgites in Raobazhai may have been the result of the transformation of oceanic mantle peridotites by fore-arc boninitic magma. Unfortunately, we did not have strong evidence to constrain the old TRD of Os isotopes and whether the correlation between 187Os/188Os and Al2O3 is a “pseudo correlation” of two terminal mixing.
The oceanic core complex (OCC) belongs to a seafloor expansion model of poor magma or non-magma. Most of it formed in the medial corner of the slow- or ultra-slow spreading mid-ocean ridge and originated in areas where magma supply is insufficient [64]. Through the REE and Ni-Co-Sc diagram of the Raobazhai garnet pyroxene and peridotites, it is believed that there is a “melt-residual” relationship between garnet pyroxenes and peridotites [11]. If this hypothesis is true, the corresponding “melt” of peridotites is minimal (Figure 1b), which may coincide with the lower degree of partial melting mentioned in our study, which coincides with the slow and ultra-slow expansion of mid-ocean ridges. Detachment of the detachment fault often exposes the OCC, and the ductile deformation zone is not necessarily the origin of the metamorphic floor when located in the orogenic belt. Therefore, the wave extinction of some minerals (Ol, Opx, and Cpx) is likely the origin of the detachment fault during the initial formation of the mid-ocean ridge [65]. The Raobazhai harzburgites, overall, showed traces of the ductile deformation of minerals (Figure 2a–c), which may reflect the formation of oceanic detachment faults. With the closure of the ocean basin, the ductile structure of the oceanic core complex and its large-scale detachment were eventually inherited and maintained in the ophiolite of the orogenic belt.
The Raobazhai harzburgites experienced a low-to-moderate partial melting process, but some marks of melt/fluid metasomatism were preserved at the same time, resulting in a high Na content of the whole-rock, enrichment of trace elements such as Sr, Ba, U, and Pt, and depletion of Nb, Zr, and Hf, indicating that the Raobazhai harzburgites experienced fore-arc boninitic melt/fluid transformation after the formation of the OCC. Based on previous geochemical evidence and studies, we propose a model to reconstruct the possible formation model of the Raobazhai complex (Figure 9).
An ophiolite is an oceanic fragment that has undergone tectonic emplacement and an orogenic process of emplacement and uplift to the surface. The Raobazhai harzburgites are oceanic peridotites that experienced low to moderate partial melting and were affected by fore-arc action. There are three possible ways of influencing the fore-arc domain. As shown at ① in Figure 12, when the MOR-type ophiolite floor is underplated, a duplex will be formed at the depths of the subducted channel, and some fragments of downward plates will be involved in the duplex, thus, creating a structural mass of “block-in-matrix“. Owing to the subduction dehydration of the descending plate, there is a plate fluid and/or boninitic magma metasomatism duplex. Owing to the extrusion of the subducted channel (Figure 12, ②), when the two oceanic crusts converged, an accretionary wedge was formed at the convergent plate boundary. The rock fragments (MORB and oceanic mantle) in the upper and lower walls may have been mixed with accretive wedges. The island arc basalts (IABs) intrude into metasomatism mantle fragments of the accretionary wedge. As the IAB may impact the entire fore-arc domain, a similar metasomatism will be formed if the continental core complex (CMCC) occurs where the ocean meets the continent (Figure 12, ③).

6. Conclusions

The Raobazhai harzburgites from the NDC were similar to present abyssal peridotites, whether in the whole-rock major, trace or PGE and Re-Os isotopes, mineral major, and suffered a certain degree of boninitic melt/fluid metasomatism, indicating that the Raobazhai harzburgites are oceanic mantle peridotites in the fore-arc domain.
(1)
Our geochemical study of Raobazhai spinel harzburgites and chromite harzburgites showed that the former had experienced 6–17% partial melting and the latter partial melting degree experienced higher. Some samples are rich in Na and enriched in elements such as Sr, Ba, and U and while depleted in Nb, Zr, and Hf. This indicates that the Raobazhai harzburgites experienced low to moderate partial melting and then boninitic melt/fluid metasomatism. The normalized PGE pattern of the mantle showed that most spinel harzburgites had a flat pattern. In contrast, the chromite harzburgites and some spinel harzburgites had a right-dipping PPGE pattern, indicating that the PPGE was carried out of the complex by the melt under a high degree of partial melting. Detailed analysis showed that Re/Ir was significantly lower than that of the partial melting model, indicating that the partial melting process can effectively remove incompatible Re, and that the Raobazhai harzburgites had also experienced Pd-rich Cu-Ni sulfide melt metasomatism. Combined with previous studies and ductile deformation in petrography, it was considered that the Raobazhai harzburgites are OCCs with oceanic affinity. The enrichment in Na, Sr, Ba, and U and the negative anomaly of trace elements Nb, Zr, and Hf suggest that Pt–Pd differentiation in chromite harzburgites may be influenced by the fore-arc process.
(2)
We propose three possible explanations for the above characteristics: (a) the abyssal type mantle peridotites were metasomatized during subduction by plate fluids and glassy magma; (b) before the ocean-continent collision, IAB metasomatism occurred in the mantle peridotite located in the accretionary wedge; (c) the CMCC in the fore-arc was metasomatic using IAB.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min12030286/s1, Table S1: Modal compositions of the Raobazhai Peridotites; Table S2: Mineral Major Element Compositions of the Raobazhai Peridotites; Table S3: Bulk-Rock Major and Trace Element Compositions of the Raobazhai Peridotites; Table S4: Highly Siderophile Element Abundances and Re-Os Isotopic Compositions of the Raobazhai Peridotites.

Author Contributions

Conceptualization, F.Y. and Q.L.; Data curation, Q.L.; Formal analysis, F.Y.; Funding acquisition, Q.H.; Investigation, F.Y., Q.L., Q.H. and M.H.; Methodology, M.H.; Writing—original draft, F.Y. Writing—review and editing, Q.L. and M.H.; All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Key R&D Program of China (2016YFC0600401), the National Natural Science Foundation of China (41273046), and the Fundamental Research Funds for the Central Universities (E1E40407X2).

Data Availability Statement

The data presented in this study are available in supplementary material.

Acknowledgments

We thank of Xue D.S., Li W.J. and Chu Z.Y., of the Institute of Geology and Geophysics, Chinese Academy of Sciences, China, for assistance in major elements, trace elements, platinum group elements and Re-Os isotope analysis. Shi Y.H. and Wang J. of Hefei University of Technology guided the authors in electronic microprobe analysis. Thanks to Chen Y.C. for his guidance and help to the author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Simplified geological maps of (a) Dabie complex and (b) the Raobazhai complex. After [15,16].
Figure 1. Simplified geological maps of (a) Dabie complex and (b) the Raobazhai complex. After [15,16].
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Figure 2. Photomicrographs and BSE images of the Raobazhai harzburgite samples. (a): olivine (Ol) with wavy extinction in phenocrystal orthopyroxene (Opx); (b): phenocrystal Opx with wavy extinction; (c): phenocrystal clinopyroxene (Cpx) with wavy extinction; (d): amphibole (Amp) and phlogopite (Phl) are commonly developed with perfect euhedral habit; (e,f): (RBZ-17) clinopyroxene and orthopyroxene occur interstratified, and spinel (Sp) occurs within the interstratified minerals as exsolution grains; (g): two stages of spinel are present; (h): pentlandite (Pn), a sulfide phase, is seen associated with spinel).
Figure 2. Photomicrographs and BSE images of the Raobazhai harzburgite samples. (a): olivine (Ol) with wavy extinction in phenocrystal orthopyroxene (Opx); (b): phenocrystal Opx with wavy extinction; (c): phenocrystal clinopyroxene (Cpx) with wavy extinction; (d): amphibole (Amp) and phlogopite (Phl) are commonly developed with perfect euhedral habit; (e,f): (RBZ-17) clinopyroxene and orthopyroxene occur interstratified, and spinel (Sp) occurs within the interstratified minerals as exsolution grains; (g): two stages of spinel are present; (h): pentlandite (Pn), a sulfide phase, is seen associated with spinel).
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Figure 3. (a) Spinel Cr# versus olivine Fo values, and (b) spinel Mg# values for the Raobazhai harzburgites. The residual abyssal peridotite data is from [24], and fore-arc peridotites is from [25,26].
Figure 3. (a) Spinel Cr# versus olivine Fo values, and (b) spinel Mg# values for the Raobazhai harzburgites. The residual abyssal peridotite data is from [24], and fore-arc peridotites is from [25,26].
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Figure 4. Compositions of major elements (wt.%) of harzburgites from the Raobazhai complex. The polybaric melting curve at 25–10kbr from [27] and abyssal peridotite data from [28,29]).
Figure 4. Compositions of major elements (wt.%) of harzburgites from the Raobazhai complex. The polybaric melting curve at 25–10kbr from [27] and abyssal peridotite data from [28,29]).
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Figure 5. (a) Whole-rock primitive mantle (PM)-normalized trace element patterns and (b) chondrite normalized REE patterns from the Raobazhai harzburgites. The DMM and partial melting pattern diagrams are from [24]; and the abyssal peridotite data is from [29,30]).
Figure 5. (a) Whole-rock primitive mantle (PM)-normalized trace element patterns and (b) chondrite normalized REE patterns from the Raobazhai harzburgites. The DMM and partial melting pattern diagrams are from [24]; and the abyssal peridotite data is from [29,30]).
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Figure 6. Diagram of the primitive mantle (PM)-normalized highly siderophile element (HSE) patterns of the Raobazhai harzburgites (PM data is from [31]; abyssal peridotites is from [32]).
Figure 6. Diagram of the primitive mantle (PM)-normalized highly siderophile element (HSE) patterns of the Raobazhai harzburgites (PM data is from [31]; abyssal peridotites is from [32]).
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Figure 7. Re-Os isotope values from the Raobazhai harzburgites. The PUM recommended value comes from [28] and the abyssal peridotite isotopes are plotted according to [33]).
Figure 7. Re-Os isotope values from the Raobazhai harzburgites. The PUM recommended value comes from [28] and the abyssal peridotite isotopes are plotted according to [33]).
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Figure 8. Diagram of whole-rock Ti-Yb covariation of harzburgites in Raobazhai. The partial melting trend follows that of [26]).
Figure 8. Diagram of whole-rock Ti-Yb covariation of harzburgites in Raobazhai. The partial melting trend follows that of [26]).
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Figure 9. Diagram of TiO2 and Cr# content from spinel minerals from the Raobazhai complex [39], porosity 0%.
Figure 9. Diagram of TiO2 and Cr# content from spinel minerals from the Raobazhai complex [39], porosity 0%.
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Figure 10. Diagrams of correlation between whole-rock Ru/Ir (a), Re/Ir (b), Pd/Ir (c), Pt/Pd (d) (inhomogeneous) and Al2O3 (wt.%) of Raobazhai peridotites. The solid line in Figure (a–c) are from [49], that the composition of the remaining peridotite after partial melting of sulfide solid solution (MSS)-sulfide melt is balanced. The dotted line in (b) represents the overall distribution of moderately incompatible Re. The composition of PUM comes from [30]. Range of chondritic Ru/Ir, Re/Ir, Pd/Ir are from [50]. Data of Dazhuqu and Lesvos are from [43,45].
Figure 10. Diagrams of correlation between whole-rock Ru/Ir (a), Re/Ir (b), Pd/Ir (c), Pt/Pd (d) (inhomogeneous) and Al2O3 (wt.%) of Raobazhai peridotites. The solid line in Figure (a–c) are from [49], that the composition of the remaining peridotite after partial melting of sulfide solid solution (MSS)-sulfide melt is balanced. The dotted line in (b) represents the overall distribution of moderately incompatible Re. The composition of PUM comes from [30]. Range of chondritic Ru/Ir, Re/Ir, Pd/Ir are from [50]. Data of Dazhuqu and Lesvos are from [43,45].
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Figure 11. Diagram of Cr# and logfO2 (FMQ) of the spinel harzburgite from Raobazhai [27]. For the oxygen fugacity calculation method refer to [62]. The fields of abyssal (mid-ocean ridge) peridotites, continental (orogenic) peridotites and oceanic arc peridotites are taken from [25,63].
Figure 11. Diagram of Cr# and logfO2 (FMQ) of the spinel harzburgite from Raobazhai [27]. For the oxygen fugacity calculation method refer to [62]. The fields of abyssal (mid-ocean ridge) peridotites, continental (orogenic) peridotites and oceanic arc peridotites are taken from [25,63].
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Figure 12. Tectonic model illustrating the possible environment of Raobazhai ultramafic complex (modified after [45]). OOST: out-of-sequence thrust; IAB: island arc basalt.
Figure 12. Tectonic model illustrating the possible environment of Raobazhai ultramafic complex (modified after [45]). OOST: out-of-sequence thrust; IAB: island arc basalt.
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Yan, F.; Liu, Q.; Hou, Q.; He, M. Mineralogy, Geochemistry and Tectonic Setting of the Raobazhai Ultramafic Complex, North Dabie. Minerals 2022, 12, 286. https://doi.org/10.3390/min12030286

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Yan F, Liu Q, Hou Q, He M. Mineralogy, Geochemistry and Tectonic Setting of the Raobazhai Ultramafic Complex, North Dabie. Minerals. 2022; 12(3):286. https://doi.org/10.3390/min12030286

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Yan, Fangchao, Qing Liu, Quanlin Hou, and Miao He. 2022. "Mineralogy, Geochemistry and Tectonic Setting of the Raobazhai Ultramafic Complex, North Dabie" Minerals 12, no. 3: 286. https://doi.org/10.3390/min12030286

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