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Article

Geochemistry and Mineralogy of Peridotites and Chromitites from Zhaheba Ophiolite Complex, Eastern Junggar, NW China: Implications for the Tectonic Environment and Genesis

by
Zhaolin Wang
1,2,*,
Jiayong Yan
1,2,
Hejun Tang
1,
Yandong Xiao
3,
Zhen Deng
1,
Guixiang Meng
1,2,
Hui Sun
4,
Yaogang Qi
5 and
Lulu Yuan
1
1
Chinese Academy of Geological Sciences, Beijing 100037, China
2
Laboratory of Deep Earth Science and Exploration Technology, Ministry of Natural Resources, Beijing 100094, China
3
Geological Research Academy of Xinjiang, Urimqi 830000, China
4
Hebei Geo-Enviroment Monitoring, Shijiazhuang 050021, China
5
Xining Center of Natural Resource Comprehensive Survey, CGS, Xining 810021, China
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(8), 1074; https://doi.org/10.3390/min13081074
Submission received: 27 June 2023 / Revised: 4 August 2023 / Accepted: 9 August 2023 / Published: 13 August 2023
(This article belongs to the Special Issue Mineralogical and Geochemical Characteristics of Chromitites)
Browse Figures
Review Reports Versions Notes

Abstract

:
The Zhaheba ophiolite is an ocean relic of the Zhaheba-Aermantai oceanic slab, a branch of the early Paleozoic Paleo-Asian Ocean. The peridotites consist mainly of harzburgite, lherzolite and minor dunite, chromitite. This study describes the whole-rock geochemistry and mineral chemistry of the Zhaheba peridotite and chromitite for the purpose of constraining their tectonic environment and genesis. The major oxides and the trace element concentrations of the peridotites are comparable with abyssal peridotite, but fall outside the field of SSZ (suprasubduction zone) peridotite and the fore-arc peridotite. The massive chromites belong to the high-Cr group, with an average Cr# (Cr/(Cr + Al)) atomic ratio) value of chromian spinel of 0.77, whereas the average Mg# value is 0.60. The disseminated chromites give a lower concentration of Cr2O3 (38.96–42.15 wt.%, average 40.35 wt.%) and lower Cr# values (0.50–0.56, average 0.53), but slightly higher contents of MgO (13.23 wt.%) and Mg# (0.61) than the massive chromites. In the diagrams of Cr#-Mg#, NiO-Cr# and TiO2-Cr#, the massive chromites fall in the field of boninite, and the disseminated chromite in the peridotite plot fall in the field of abyssal peridotite and mid-oceanic ridge basalt (MORB). The massive chromitites, with high-Cr, display a boninite affinity, whereas the disseminated chromite plot in the high-Al and abyssal peridotite type field may be generated by the extension of the Zhaheba ocean in the MOR environment then experienced deep subduction and exhumation. The calculated degrees of partial melting for the massive chromites are 21%−22%, and for the disseminated chromites in peridotites the degrees are 17%−18%. The calculated values of fO2 for the massive chromites range from −1.44 to +0.20, and the values for the disseminated chromites range from −0.32 to +0.18. The inferred parental melt composition for massive chromitite falls in the field of boninite in an arc setting, whereas the disseminated chromite in peridotites are in the field of a MORB setting. This indicates that the parental magmas of the former were more refractory than the latter. A two-stage evolution model for the chromites was proposed, in which disseminated chromites were first formed in an MOR environment and then modified by later-stage melts and fluids, and formed massive chromites were formed in an SSZ setting during intra-oceanic subduction.

1. Introduction

Ophiolite complexes are segments of oceanic crust and tectonically exposed by obduction or extruding, where they are used to identify sutures between converging blocks of lithosphere. Ophiolite complexes have a complicated origin and display a variety of tectonic settings, from low degrees of partial melting at MOR settings to higher degrees in arc-related environments [1,2,3,4]. Peridotites, interpreted as the melting residues of upper mantle, record vital information on melted residue, mantle melting, melt extraction processes, as well as the origin, tectonic environment of ophiolites and the formation of economical chromitites they contain [4,5,6,7]. Moreover, it is now widely agreed that the composition of chromites and accessory chromium spinels in mantle peridotites of ophiolites are significant, as they can constrain the physico-chemical conditions, the degree of melting, composition and oxygen fugacity of evolving mafic magmas, as well as the tectonic setting of the magma forming chromitite [1,8,9,10].
A general agreement has emerged that high-Cr chromitites (Cr# > 60) crystallize from boninitic magmas and are widely discovered in fore-arc ophiolites, whereas high-Al chromitites (Cr# < 60) crystallize from less refractory MORB-like tholeiite and form in a sub-ocean-ridge environment [4,11,12,13,14,15,16,17,18,19,20,21]. However, the tectonic settings of ophiolitic chromitites are still under debate; recent research has revealed that high-Al and high-Cr chromitites coexist in the same ophiolite [4,19,20,22,23], and that chromitites with intermediate Cr# (50 < Cr# < 70), relating to subduction initiation, are present in some ophiolites [24,25,26]. Accordingly, the study of geochemistry and mineralogy from peridotites and chromitites could supply valuable clues to better explain the origin and evolution of ophiolites.
The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogenic belts (Figure 1a), and is predominately formed by the forearc accretion of island arcs, accretionary wedges, ophiolites and precambrian microcontinents [27,28,29,30,31]. The Eastern Junggar terrane (EJT) has preserved a complete record of the Palaeozoic magmatism–tectonism and multi-stage accretionary processes in the Paleozoic ophiolite belts of the southern CAOB, making it a unique area for researching the evolution of the CAOB’s southwest segment [32,33,34]. In addition to the Erqis and Kalamaili ophiolite, Zhaheba holds the third ophiolite in the EJT (Figure 1b).
Previous research so far has focused on field relationships, high pressure minerals, petrographic, geochronologic and geochemical data of the Zhaheba ophiolitic complex [35,36,37,38,39,40,41], as well as the mineralogical investigation of chromitites [23]. The Zhaheba-Armantai ophiolite is believed to form in a back-arc basin [42,43], or in an oceanic island setting, and shows SSZ affinities according to the volcanic rocks with alkaline signature [41,44], as well as in MOR [45]. The existence of ultra-high pressure metamorphic rocks, such as garnet–amphibolite, quartz–magnesite and garnet–pyroxenite, as well as retrograde eclogite, illustrates that the Zahheba ophiolite was an ultradeep subduction zone of the oceanic slab [36,37,38]. However, the relationships between subduction and the metallogenic reformation process, as well as the relationship between the boninitic magma and the parental magma of high-Cr chromitites, are usually neglected, especially with regard to the contribution of subduction to the geothermal state, the degree of partial melting and oxygen fugacity, which are the crucial factors for chromitite metallogenesis. Considering that the peridotites and chromitites would reveal the characteristics of parental magmas, and constrain the origin and tectonic evolution of the ophiolitic complex and chromitites, the aim of our work is to review the detailed geochemical and mineralogical composition of peridotites and chromitites of the Zhaheba ophiolite, and to discuss the evolution and tectonic environment of the formation of chromitites in this ophiolite.

2. Geological Background

The EJT is located in the southwest segment of the CAOB, lying between the Erqis and Kalamaili faults. Spatially, the EJT consists of several tectonic units formed by multiple accretionary orogenies, the Dulate arc in the north and the Yemaquan arc in the south, which are separated by Zhaheba–Aermantai ophiolite belt (Figure 1b) [28,29]. The Dulate arc is located along the northeastern side of the Zhaheba–Aermantai ophiolite belt and is mainly composed of Devonian–Carboniferous volcanic rock, including adakite, boninite, high-Mg andesite and Nb-rich basalt [43,46,47,48,49] pyroclastic rocks. The Yemaquan magmatic arc, located between the Zaheba–Armantai and Kalamaili ophiolite belts, is mainly comprised of the Ordovician continental arc, which changed into a continental island arc after the Early Palaeozoic era, and is characterized by Devonian–Carboniferous clastic rocks and sediments and abundant intrusive rocks [50,51].
Zhaheba ophiolite is an ocean relic of the Zhaheba–Aermantai oceanic slab, a branch of the early Paleozoic Paleo-Asian Ocean [33,34]. The ophiolite consists mainly of well-exposed serpentinized peridotites, topped by cumulate gabbro and foliated basalt, followed upward by a clastic sedimentary sequence comprised of chert layers, tuff and tuffaceous sedimentary rocks on top of the previous units—diorite porphyry, diabase dike and interlayered carbonate—cut through above a mafic-ultramafic sequence [41], as well as plagiogranite and anorthosite, which record the slow ocean floor spreading [35,39], but show that typical pillow basalt is lacking (Figure 1c). The ophiolite is overthrust and strongly foliated, being affected by the thrust nappe structure or erupting volcanic rocks in late period [52], which are named the early Devonian Tuoranggekuduke Formation and the middle Devonian Beitashan Formation, situated along the northeast margin of the ophiolite [53]. The Zhaheba ophiolite is thought to be covered by sediments of the Junggar basin and extends to the northwest [54]. Drawing on the results of conodont, radiolarian in chert [32], peridotite Sm-Nd dating [55], sphene U-Pb dating in gabbro, zircon U-Pb dating in layered gabbro [35] and zircon U-Pb dating in cumulate gabbro [40,41], we deduce that Zhaheba ophiolite can be dated to approximately 495~460 Ma, when an initial intra-oceanic subduction started and intra-oceanic arc volcanic rocks emerged [41,48].
Preliminary explorations of chromitites were carried out in the 1960s on the Zhaheba ophiolite; there were no significant discoveries other than several small scales of massive chromitites. Accompanied by the newly discovered Yundukala Cu–Au-Co deposit [56,57,58,59], detailed studies of the mantle peridotites and chromitites in the ophiolite resumed from 2017 [23,41].

3. Geological and Petrographic Characteristics of Peridotites, Chromites and Sample Descriptions

Peridotites, as a major part of ophiolite, are orientated at nearly NNW-SSE and are ~7 km in length and 1–2 km in width. (Figure 1c). Affected by weathering, peridotites are widely distributed as rock fragments and remain hillocks with an outstanding dark green or black color in the ophiolite (Figure 2a). The peridotites are composed of harzburgite, lherzolite and minor dunite, as well as chromitite. The lithofacies zonation of peridotites is not clear, being dominated by harzburgite lithofacies, with a certain quantity of lherzolite. Minor dunite schlieren appears locally as lumps in this lithofacies (Figure 2b–f). The peridotites are strongly deformed and dismembered by gabbros, diorites, dioritic porphyrites, albite granite dike and ophicarbonates (Figure 1c and Figure 2b), forming intrusive contact. Serpentinization, talc, carbonate alteration in the peridotite and four chromites ore spots were discovered in these ultramafic rocks. Although experiencing alteration, bastitic pseudomorphs and crystal relicts of orthopyroxenes, clinopyroxenes, olivines in the peridotites, are still preserved.
To investigate the tectonic environment and genesis of the chromitites and host peridotites, representative samples from the Zhaheba mafic-ultramafic complex, including harzburgites, lherzolites and chromitites, were systematically collected. Polished thin sections made from peridotites and chromites ore samples were chosen and analyzed by electron microscopy.

3.1. Harzburgite

The harzburgite occupies ~70% of the total peridotites mass with gradual contact with lherzolite, dunite. The harzburgite mostly has coarse-grained, granular textures, and chiefly consists of olivine (65%–70%), orthopyroxene (15%~20%) and clinopyroxene (less than 2%), with minor chromian spinel, and magnetite (6%~8%). Most olivine crystals in harzburgite are mostly altered to antigorite and chrysotile. The orthopyroxenes in these harzburgites are highly enstatite and form large (0.5–5 mm), subhedral, typically tabular crystals. Clinopyroxenes sporadically occur as small (0.2–2 mm), anhedral grains. The residual chromian spinels in the harzburgites consist of small (0.5–1 mm), subhedral, interstitial and strongly fractured grains with ferritchromite or magnetite rims.
The alteration minerals include serpentine, talc, small amounts of chlorite and carbonate. Under the microscope, the talcified orthopyroxenes are silver-gray in color and have typical kink-banding, undulatory extinction long cleavage planes, irregular melting shapes and oriented melting-out olivines (Figure 3b,e). During the serpentinization process, granular magnetites were precipitated out.

3.2. Lherzolite

Lherzolite occurs as ribbons in the harzburgites, mostly NNW-SSE trending and 50–500 m long. Lherzolite is a yellowish-brown, medium-grained rock composed chiefly of ~60% olivine (mostly altered to antigorite and chrysotile), 8%–20% orthopyroxene, 8%–15% clinopyroxene and with minor chromian spinel and magnetite.
Olivine grains are usually less than 0.3 mm in diameter, with subeuhedral columnar crystals and a typical mesh texture (Figure 3c). The orthopyroxenes mostly form large (0.5–5 mm), generally euhedral to subhedral, tabular crystals, which display extensive crystalflexing, undulatory extinction and exsolution lamellae of neogenic clinopyroxene (Figure 3a,g). Clinopyroxene grains are mainly diopsides, with small (0.5–2 mm), anhedral grains and a typical structure of melted residual (Figure 3d,f,h).

3.3. Dunite

The dunites are only scattered as schlieren in the northwestern part (Figure 1c). These rocks primarily consist of olivine pseudomorphs and minor chromites and magnitites. Most olivines in the dunite are altered to antigorite and lizardite with mesh textures due to serpentinization. However, dunites with unaltered olivine cores or whole grains can be found with subeuhedral-euhedral crystals and variable grain sizes (from 0.3 to 0.5 mm).

3.4. Chromitites

Small chromitite pods are scattered in the carbonated dunite of the Zhaheba ophiolite. The chromitites crop out discontinuously, and individual bodies are generally 1–3 m long and 0.2–1 m thick. The chromitites mainly show brecciated, massive, nodular and irregular structures, cemented by carbonated dunite (Figure 2g,h). The massive ores contain up to 80 vol% chromite. The chromite grains in the massive chromitite show polygonal or angular crystal appearances and pull-apart fractures, cataclastic textures. In contrast, disseminated chromian spinels in harzburgites and lherzolites are typically small (0.2–0.6 mm), subhedral to amoeboidal and have a melting corrosion structure, altered to ferritchromite (Fe-chr) along their cracks or edges.

4. Analytical Techniques

4.1. Whole-Rock Major, Trace, and Rare-Earth Elements

The representative samples from harzburgite and lherzolite were washed and trimmed to remove weathered surfaces, then powdered in an agate mill to approximately 200 mesh for major, trace and rare-earth elements analyses. Major elements were determined at the Beijing GeoAnalysis Technology Co., Ltd. using X-ray fluorescence (XRF-1800; SHIMADZU, Kyoto, Japan) on fused glass. Trace elements, including rare earth elements (REE), were determined by inductively coupled plasma mass spectrometry (ICP-MS, 7500; Agilent) at Beijing Createch Testing Technology Co., Ltd. on samples after acid digestion in Teflon bombs. Loss on ignition was measured after heating to 1000 °C for 3 h in a muffle furnace. The analytical precision was within ±2% for oxides with >0.5 wt.% and within ±5% for other oxides > 0.1 wt.%. Sample powders were accurately weighed (~40 mg) into Teflon bombs and dissolved using a 1:1 mixture of HF and HNO3 and heated for 48 h at 190 °C. The solution was evaporated to dryness, redissolved using concentrated HNO3 and evaporated at 150 °C, to dispel any fluorides. Samples were diluted to approximately 80 g for analysis after dissolution in 30% HNO3 overnight. An internal standard solution containing Rh was used to monitor signal drift during the analyses. Results from USGS standards indicated the uncertainty for most elements was ±5%.

4.2. Major Element Analyses for Minerals

The major element compositions of minerals (pyroxenes, chrome spinels) in silicates and oxides was conducted by wavelength-dispersive X-ray analysis using a JEOL electron-probe micro analyzer (EPMA) JXA-8230 at the Institute of Mineral Resources, Chinese Academy of Geological Sciences. The analyses were carried out using an acceleration voltage of 15 kV, 20 nA for the beam current and a 5 μm beam diameter, and the counting time for major elements was between 20 and 40 s and 40 and 60 s for minor elements. SPI mineral standards (USA) were used for calibration. The precision for all elements analyzed exceeded 98.5%. The Cr- and Mg-numbers (Cr# and Mg#) of the chromian spinel were the Cr/(Cr + Al) and Mg/(Mg + Fe2+) atomic ratios, respectively. We assumed all Fe in silicates was ferrous.

5. Analytical Results

5.1. Whole-Rock Geochemistry

5.1.1. Major Geochemistry

Supplement Sheet S1 lists the analytical data of major, trace and REE for peridotites from the Zhaheba ophiolite. All the analyzed samples are variably serpentinized to varying degrees (LOI = 11.87–13.24 wt.%). All the peridotites display minor variations, ranging from 39.56 to 42.05 wt.% in SiO2, from 0.47 to 1.98 wt.% in Al2O3 and from 0.02 to 0.04 wt.% in TiO2. However, they are rich in MgO (36.88–39.36 wt.%). The peridotites show relatively linear decreases in CaO, Al2O3 and SiO2, with increasing MgO (Figure 4a–c). The content of total Fe2O3 (Fe2O3T) ranges from 4.11 to 8.43 wt.%. In addition, the peridotite samples are alkali-poor, with negligible K2O and Na2O (Supplement Sheet S1). For the lherzolites, the samples are characterized by higher CaO (0.81–2.00 wt.%) and Al2O3 (1.05–1.98 wt.%) compositions than the harzburgites (CaO: 0.03–0.71 wt.%; Al2O3: 0.47–1.51 wt.%) (Figure 4a,b). The low CaO content in harzburgites is consistent with the scarcity of clinopyroxene, whereas the lherzolites with relatively high CaO content are in accordance with the presence of clinopyroxene. The major oxide features of the lherzolites and harzburgite in the Zhaheba ophiolite are comparable with abyssal peridotite, but higher than the SSZ peridotite [5,60] (Figure 4a–c) and lower than the fore-arc peridotite (FAP) [5,61] (Figure 4d).

5.1.2. Trace and Rare Earth Elements

In terms of trace elements, peridotites contain enriched levels of large-ion lithophile elements (LILE) (such as Cs); high field strength elements (HFSE) (U, Zr, Hf); and Er, relative to primitive mantle-normalized trace element patterns. Significantly negative anomalies in Nb, Ba were also observed. The rare earth element (REE) contents are relatively low, from 0.27 to 12.9 ppm (on average, 2.93 ppm). The chondrite normalized REE patterns are slightly flat to heavy rare earth element (HREE) enriched.
Figure 5 compares the trace element compositions of the Zhaheba peridotites with those of abyssal and SSZ-type peridotites. Elements Y, Sc, Yb and Lu, which are all incompatible during partial melting, also show slightly inverse correlations with MgO (Figure 5a,b). The trace element concentrations of the harzburgites and lherzolites are comparable to those of the abyssal peridotite and very unlike the SSZ-type peridotites and the fore-arc peridotite (Figure 5a–d).

5.2. Mineral Chemistry

The microprobe analysis data for chromian spinels, olivines, clinopyroxenes and orthopyroxenes in the Zhaheba chromitites and peridotites are listed in Supplement Sheet S2.

5.2.1. Clinopyroxenes

Clinopyroxenes occur mostly in the lherzolites and harzburgites. Except for Z19-25-9-1 and ZHB-363-5-2, all the clinopyroxenes are diopside, with En values of 41.68–53.47; Wo values of 36.28–50.56; and Mg# (100 × Mg/(Mg + Fe2+)) ranging from 88.41 to 95.24. Both clinopyroxenes have similar concentrations of Cr in the lherzolite and harzburgite (0.06–1.44 wt.% Cr2O3), but the clinopyroxenes in the harzburgite samples have lower Al2O3 (0.10 to 4.08 wt.%, average 2.69 wt.%); TiO2 (0.01 to 0.10 wt.%, average 0.05 wt.%); and MgO (15.55 to 16.28 wt.%, average 15.83 wt.%) and higher CaO (22.87 to 24.92 wt.%, average 23.80 wt.%); and Cr# (16.32 to 26.92, average 20.64). The clinopyroxenes in the harzburgites have a higher melting point than those in the lherzolites (Figure 6a,c). On the diagrams of Cr2O3 vs. Al2O3, Cr2O3 vs. Mg# and Mg# vs. Al2O3 (Figure 6a–c), all the clinopyroxenes in the harzburgite and most clinopyroxenes in the lherzolites plot in the field of abyssal peridotite (ABP), which means that the lherzolites and harzburgites are formed in ABP.

5.2.2. Orthopyroxene

Most of the orthopyroxenes in harzburgites and lherzolites are replaced by bastite or represented by bastitic pseudomorphs. Only eight representative fresh orthopyroxene grains in harzburgites were selected for analyses, and the data are listed in Supplement Sheet S2. All of the orthopyroxenes are enstatite, with En values of 84.99–86.03 and Mg# (100 × Mg/(Mg + Fe2+)) ranging from 90.21 to 91.42. The Cr2O3 contents of the orthopyroxenes are low (0.67–0.88 wt.% Cr2O3), the Al2O3 contents range from 2.41 to 4.09 wt.%, the MgO contents range from 29.92 to 31.84 wt.% and the CaO contents are typically <0.3 wt.%. Fe2O3 and the TiO2 contents of the orthopyroxenes are overall negligible. The orthopyroxene grain plots of Mg# vs. Al2O3 and Cr2O3 vs. Al2O3 fall within the field of abyssal peridotites (ABP) (Figure 7a–b).

5.2.3. Chromite

All the chromian spinels in the massive chromites show higher Cr, Mn contents and Cr# values, but lower Al concentrations than the disseminated chromites in the harzburgites. The chromian spinels in the sample of ZHB-b56 from massive chromites show a lower content of Cr2O3 (52.62–55.58 wt.%, average 53.60 wt.%), but a higher content of TiO2 (0.43–0.45 wt.%, average 0.44 wt.%); Al2O3 (11.83–14.45 wt.%, average 12.79 wt.%); MgO (12.69–13.54 wt.%, average 12.90 wt.%); and Mg# (62.99–63.64, average 63.36) than those of ZHB-b40 and ZHB-b58. The massive chromites belong to the high-Cr group, with the average Cr# value of the chromian spinel being 0.77, whereas the average Mg# value is 0.60.
Disseminated chromites are common in the harzburgites, but rarely exceed 5 modal%. The Cr2O3 (38.96–42.15 wt.%, average 40.35 wt.%) and Cr# values (0.50–0.56, average 0.53) of disseminated chromites in the harzburgites are lower, and Al2O3 (22.59–26.04 wt.%, average 24.10 wt.%) are higher than those of massive chromites. The average MgO (13.23 wt.%) and Mg# (0.61) were slightly higher than that of the massive chromite, while the FeO contents are similar to those of the massive chromite.
In the Cr2O3-Al2O3 diagram, the samples of massive chromites fall into the podiform chromite in ophiolite and the high-Cr chromite of Luobusha, but far from the stratiform chromite, while the samples of disseminated chromites mostly plot in the field of high-Al chromite of Sartohay (Figure 8a). In the diagram of Cr#-Mg# and TiO2-Cr#, the massive chromites fall in the field of boninite (BON) and the disseminated chromites in the peridotite plot in the field of abyssal peridotite and MORB (Figure 8b,c). In the NiO-Cr# diagram (Figure 8d), the sample of massive chromites are approached the field of BON, while the disseminated chromites mostly plot in the field of abyssal peridotite. The TiO2-Al2O3 variation diagram (Figure 8e) shows that the massive chromites mostly fall within the island arc series field, and the disseminated chromites are approached the field of MORB. The Fe3+#[= 100 × Fe3+/(Cr + Al + Fe3+)] of chromian spinels are low (less than 7.8) in both the massive chromites and disseminated chromites (Figure 8f).

6. Discussion

6.1. Parental Magmas Characteristics of Chromitites and the Degrees of Partial Melting

The chromian spinels can preserve information about the parental melts and are usually used to calculate the parental magma composition in the chromitites [1,2,4,9,17,73,78,79,80,81,82,83,84], in which the Al2O3 contents of the melt can be computed through the experimental results of spinel-liquid equilibrium at 1 bar from the following formula:
(Al2O3 melt) = 4.1386 ln (Al2O3 spinel) + 2.2828
The FeO/MgO ratio of melts can also be calculated with the [79] equation,
Ln(FeO/MgO)spinel = 0.47−1.07Al# spinel + 0.64 Fe3+# + Ln(FeO/MgO)
where FeO and MgO are in wt.%. The TiO2 contents of the melt are obtained from [9], with the formulation of
high-Al ((TiO2 melt) = 0.708ln (TiO2 spinel) + 1.6436) and high-Cr ((TiO2 melt) = 1.0897(TiO2 spinel) + 0.0892)
Based on the above formula, the calculated values of the Al2O3, TiO2 and FeO/MgO ratios of the parental melt are listed in Supplement Sheet S3. The Al2O3 and TiO2 concentrations and FeO/MgO values of the parental melt from massive chromites range from 9.08 to 12.05 wt.% (average 10.67 wt.%); 0.21 to 0.59 wt.% (average 0.35 wt.%); and 0.81 to 1.12 wt.% (average 0.92 wt.%), respectively, whereas those from the disseminated chromites have a range of 14.49–15.37 wt.% (average 14.88 wt.%); 0.30–0.72 wt.% (average 0.49 wt.%); and 0.97–1.33 wt.% (average 1.09 wt.%), respectively.
In the diagrams of melt (Al2O3) vs. Al2O3, melt (TiO2) vs. TiO2 and melt (FeO/MgO) vs. FeO/MgO, all the disseminated chromites samples fall in the MORB field, while the massive chromites counterparts fall in the ARC field (Figure 9a–c). In the diagrams of melt (FeO/MgO) vs. Al2O3, the disseminated chromites plot in the field of MORB, while the massive chromites fall the overlap between the MORB magmas and boninite (Figure 9d). The calculated component of parental melts of the disseminated chromites in peridotite are similar to MORB, while the parental melts of massive chromites are characterized by a boninitic affinity [72].
The degree of partial melting (F) of original peridotites was determined by the equation from [84], where F = 10ln (Cr#) + 24.
The degrees of partial melting of the parental melt are given in Supplement Sheet S3. The calculated degree of melting of the massive chromites gives the range from 21% to 22%. In contrast, the disseminated chromites in peridotites display low degrees (17%–18%) of partial melting, which is also consistent with the appearance of clinopyroxene, and corresponds to a ~17% depleted mantle composition in a MORB setting [9]. Abyssal peridotites generally comprise lherzolites and clinopyroxene-rich harzburgites, formed by MORB-type melt extraction as a result of 5%–15% partial melting of fertile mantle under dry conditions [1,85,86]. Due to the lower degrees of partial melting of abyssal peridotites, the whole rock compositions of abyssal peridotites are more enriched in incompatible elements compared to the SSZ-type peridotites [5,8,86]. Clinopyroxene is consumed the most rapidly, but can also remain for a long period during the partial melting of peridotite. The peridotite will have a range of modal clinopyroxene from ~15% in a fertile (unmelted) peridotite to 0% after ~25% melting [5]. The abundance of clinopyroxenes in the investigated peridotites and their relative high CaO contents in lherzolites (generally >1 wt.%; Figure 4a) indicate a low degree of partial melting. The progressive depletion of peridotites from the lherzolites to harzburgites provides—in both the Al2O3, CaO and Lu, Y, Yb and Sc contents—evidence for increasing degrees of melt extraction during partial melting (Figure 4a,b and Figure 5a–d). Therefore, significant geochemical variations for the investigated peridotites are largely the result of various factors, but low degrees of partial melting and dunites, the product of high degree of partial melting, are rare in the Zhaheba ophiolite.
Minerals 13 01074 g009
Figure 9. Chromite-melt relationships for (a) Al2O3, (b) TiO2 and (c) FeO/MgO. The fields of ARC and MORB are from [87]. (d) FeO/MgO vs. Al2O3 in the Zhaheba chromitites. Tectonic compositional fields are from [72]. Symbols are as in Figure 8.
Figure 9. Chromite-melt relationships for (a) Al2O3, (b) TiO2 and (c) FeO/MgO. The fields of ARC and MORB are from [87]. (d) FeO/MgO vs. Al2O3 in the Zhaheba chromitites. Tectonic compositional fields are from [72]. Symbols are as in Figure 8.
Minerals 13 01074 g009

6.2. Geothermal State and Oxygen Fugagity

The temperature (T) and oxidation state (oxygen fugacity, fO2) calculations of the upper mantle minerals are very important for revealing the possible deep processes for the evolution of the mantle minerals. Coexisting olivine–spinel exchange thermometers from [78,88,89,90] can be applied to estimate the temperature and oxygen fugacity of the magma system from which the peridotites crystallize. Temperatures of equilibration can be calculated at an arbitrary pressure of 1.5 GPa using the Fe–Mg exchange thermometer of Ballhaus et al. (1991), and the fO2 is presented in Log units relative to the QFM (quartz- fayalite-magnetite) buffer at 1.5 GPa, where Δlog fO2 QFM refers to the deviation from the QFM line. Supplement Sheets S4 and S5 list the temperature of formation and oxygen fugacity for the Zhaheba chromitite.
The calculated temperature values for the massive chromites range between 862 and 1045 °C (975 °C on average), while the values for the disseminated chromites in peridotites are from 752 to 858 °C (816 °C on average). The calculated temperature values may not be those of magma systems for chromites in peridotites, as peridotite may preserve the lowest equilibrium temperatures during cooling [91], or the equilibrium temperatures may represent reequilibration during the postmagmatic stage [92].
The oxygen fugacity conditions influence the behavior of Fe, Cr and the value of fO2 estimated principally depends on the proportion of Fe3+ of the spinel [88,93]. The calculated values of fO2 for the massive chromites and the disseminated chromites are from −1.44 to +0.20 (−0.50 on average) and −0.32 to +0.18 (−0.07 on average), respectively. The former, with high Cr#, are mostly scattered above the line of the MORB and SSZ dunite and along the MOR mantle-BON magma interaction trend, while the latter plot below the line of the MORB and SSZ harzburgite (Figure 10a). In the diagrams of ΔlogfO2 (FMQ) versus Fe3+/Fetot, all the samples are fall in the field of abyssal peridotite (Figure 10b).

6.3. Possible Tectonic Enviroments of the Zhaheba Peridotites and Chromitites

Abyssal peridotites are melting residues of variable degrees of mantle melting resulting from decompression and melt extraction processes beneath the mid-ocean spreading centers [6,60,84,86]. The major oxides features of the lherzolites and harzburgite in the Zhaheba ophiolite are consistent with abyssal peridotite, but differ from the SSZ peridotite [5,60,94] and the fore-arc peridotite [5,61] (Figure 4a–d). The contents of disseminated chromites in Zhaheba lherzolite plot in the high-Al and abyssal peridotite type fields and may be generated by the extension of the Zhaheba ocean in the MOR environment [23]. However, the discovery of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks (i.e., lherzolite, garnet–pyroxenite, quartz–magnesite, garnet–amphibolite and retrograde eclogite) [36,37,38] suggests that the Zhaheba peridotites have experienced deep subduction and exhumtion. Additionally, boninite, adakite, high-TiO2 and low-TiO2 basalts are identified in the Zhaheba ophiolites and their related Dulate arc. Boninite in particular represents the primary magma composition derived from partial melting of refractory mantle peridotite fluxed by a subduction slab-derived fluid component [48]. Boninite generally occurs in a fore-arc tectonic setting and represents a style of magmatism that is interpreted to reflect the melting of highly refractory mantle source regions that have been variably metasomatized by subducted slab fluids. Boninite is a high-magnesian andesite depleted in high field strength elements and somewhat enriched in large-ion-lithophile elements relative to mid-oceanic ridge basalts [95].
The clinopyroxenes and orthopyroxenes in harzburgites and lherzolites are scattered in ABP and out of FAP (Figure 6a–c and Figure 7a,b) and all the disseminated chromites in the peridotite plot in the field of ABP or MORB, whereas the massive chromites fall in the fielid of ARC or BON (Figure 8b–f), suggesting that the parental magmas for massive chromites are more refractory than those of disseminated chromites. Hence, we infer that the peridotites and chromitites formed in different settings. The Zhaheba peridotites would have initially originated in the MOR, and could then be trapped by SSZ melts just before the closure of oceans, as a consequence of intra-oceanic subduction [23,41].

6.4. Genesis of the Zhaheba Peridotites and Chromitites

During the mantle convection, mantle peridotites were uplifted and formed a new oceanic crust by low degree partial melt at a MORB setting. Clinopyroxenes would dissolve and release Cr to produce harzburgites and dunites. With continuous spreading, asthenospheric upwelling and decompressional melting would accelerate the production of MORB magmas, but no major accessory chromites and chromitites would be expected, as the low degrees of partial melting and absence of fluids in mid-oceanic ridges do not favor chromite crystallization to form large podiform bodies [17].
At approximately 460–420 Ma, an intra-oceanic subduction developed, such that the Zhaheba peridotite was trapped in the mantle wedge above the subduction zone [41,43]. Dehydration of the subducted slab would cause greenschist, amphibolite and even ecologite facies metamorphism, and slab breakoff took place because of these relatively dense metamorphic rocks, particularly eclogites [36,37,38]. Slab breakoff, fluid release and melting of the subducted slab and overlying mantle wedge generated hydrous boninitic melts [17,48,96,97]. Boninitic magma has been identified as being oversaturated with chromian spinels with high-Cr signature [94], which were an essential component for the formation of the chromitites in Zhaheba. The interaction of boninitic magma with the surrounding mantle peridotites can lead to dissolution of pyroxene and precipitation of olivine [17,19,20,98,99]. There is a general consensus that high-Cr podiform chromitites may have generally crystallized from an interaction between the boninitic magma and refractory peridotite, whereas high-Al chromitites may be derived from MORB-like tholeiitic magmas [2,9,17,98,100,101,102,103]. Hence, the high-Cr massive chromitites in the Zhaheba complex suggest a boninite affinity and present a distinct evolutionary tendency with disseminated chromites in peridotites [104,105]. The calculated compositions of parental melts for massive chromites have affinities with boninite in an arc setting, whereas the disseminated chromites in peridotites show compositions in a MORB setting, suggesting that the parental magmas for massive chromites were more refractory than those of disseminated chromites and a two-stage evolution from a MOR to SSZ setting.

7. Conclusions

(1) The Zhaheba peridotites are composed of harzburgite, lherzolite and minor dunite, chromitite. The major oxides and trace element concentrations of the lherzolites and harzburgite are consistent with abyssal peridotite, but differ from the SSZ peridotite and the fore-arc peridotite. The Zhaheba peridotites and chromitites formed in a different setting. The peridotites would have initially originated in the MOR then been trapped by SSZ melts just before the closure of oceans as a consequence of intra-oceanic subduction. The high-Cr massive chromitites suggest a boninite affinity and present a distinct evolution tendency with disseminated chromites in peridotites. The contents of disseminated chromites plot in the high-Al and abyssal peridotite type fields, and may be generated by the extension of the Zhaheba ocean in a MOR environment then experienced deep subduction and exhumation.
(2) The investigated Zhaheba peridotites have low degrees of partial melting. the calculated degrees of partial melting of the massive chromites are from 21% to 22% and the disseminated chromites in peridotites are from 17% to 18%.
(3) The calculated temperature values of magma system for the massive chromites and the disseminated chromites in peridotites range between 862 and 1045 °C (975 °C on average) and from 752 to 858 °C (816 °C on average), respectively. The calculated values of fO2 for the massive chromites and the disseminated chromites are from −1.44 to +0.20 (−0.50 on average) and −0.32 to +0.18 (−0.07 on average), respectively. The former are mostly scattered above the line of the MORB and SSZ dunite and along the MOR mantle-BON magma interaction trend, while the latter plot below the line of the MORB and SSZ harzburgite.
(4) The calculated compositions of parental melts for massive chromites have affinities with boninite in an arc setting, whereas the disseminated chromites in peridotites show compositions in a MORB setting, suggesting that the parental magmas for massive chromites were more refractory than those of the disseminated chromites. A two-stage evolution model for the chromites was proposed: disseminated chromites first formed in an MOR environment and were then modified by later-stage melts and fluids, and formed massive chromites in an SSZ setting during intra-oceanic subduction.

Supplementary Materials

Supplementary data to this article can be found online at https://www.mdpi.com/article/10.3390/min13081074/s1. Supplement Sheet S1: Major (wt%) and trace element (ppm) abundances in whole rocks of Zhaheba mafic-ultramafic complexes.

Author Contributions

Conceptualization, Z.W.; Software, H.S.; Investigation, H.T., Y.X. and Z.D.; Writing—review & editing, L.Y.; Visualization, Y.Q.; Supervision, G.M.; Project administration, J.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Chinese Geological Survey: DD20230335.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Simplified tectonic map of the CAOB. (b) is outlined (modified after [29]). (b) Schematic tectonic map of the Altay orogenic belt and EJT showing the major tectonic units and faults (modified after [28]). (c) Geological map of the Zhaheba ophiolite (modified after [23]).
Figure 1. (a) Simplified tectonic map of the CAOB. (b) is outlined (modified after [29]). (b) Schematic tectonic map of the Altay orogenic belt and EJT showing the major tectonic units and faults (modified after [28]). (c) Geological map of the Zhaheba ophiolite (modified after [23]).
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Figure 2. Field photographs showing the various lithologies in the Zhaheba ophiolite. (a) Harzburgites are infiltrated by ophicarbonate; (b) harzburgites are intruded by an albite granite dike; (c) the transition zone between harzburgites and cumulate structural amphibolites; (d,e) serpentinized dunites; (f) lherzolite with fresh clinopyroxenes; (g) brecciated and massive chromitites (ZHB-b40) in carbonated dunite; (h) massive chromitites, cemented by carbonated dunite (ZHB-b56).
Figure 2. Field photographs showing the various lithologies in the Zhaheba ophiolite. (a) Harzburgites are infiltrated by ophicarbonate; (b) harzburgites are intruded by an albite granite dike; (c) the transition zone between harzburgites and cumulate structural amphibolites; (d,e) serpentinized dunites; (f) lherzolite with fresh clinopyroxenes; (g) brecciated and massive chromitites (ZHB-b40) in carbonated dunite; (h) massive chromitites, cemented by carbonated dunite (ZHB-b56).
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Figure 3. Micrographs of peridotites from the Zhaheba ophiolite (under cross-polarized light). (a) Serpentinized lherzolite with neogenic olivine, clinopyroxene (Z19-25); (b) euhedral Cr-spinel embedded within serpentinized harzburgite (ZHB-b11); (c) serpentinized lherzolite with fresh olivine(Z19-25); (d) lherzolite with relicts of orthopyroxene, clinopyroxene and olivine, neogenic olivine (Ol2) (Z19-26); (e) serpentinized harzburgite with relicts of orthopyroxene, clinopyroxene and olivine (Z19-31); (f) Cr-spinel (Spl) and relicts of clinopyroxene embedded within serpentinized lherzolite (D363); (g) lherzolite with melted residual orthopyroxene, exsolusion lamellae of neogenic clinopyroxene in the bastite (D363); (h) structure of melted residuals in lherzolite, plane-polar (D363). Opx: orthopyroxene; Cpx: clinopyroxene; Ol: olivine; Bas: bastite; Cr: Cr spinel; Serp: serpentine; Ol2: new neogenic olivine; Cpx2: new neogenic clinopyroxene; Mag: magnetite.
Figure 3. Micrographs of peridotites from the Zhaheba ophiolite (under cross-polarized light). (a) Serpentinized lherzolite with neogenic olivine, clinopyroxene (Z19-25); (b) euhedral Cr-spinel embedded within serpentinized harzburgite (ZHB-b11); (c) serpentinized lherzolite with fresh olivine(Z19-25); (d) lherzolite with relicts of orthopyroxene, clinopyroxene and olivine, neogenic olivine (Ol2) (Z19-26); (e) serpentinized harzburgite with relicts of orthopyroxene, clinopyroxene and olivine (Z19-31); (f) Cr-spinel (Spl) and relicts of clinopyroxene embedded within serpentinized lherzolite (D363); (g) lherzolite with melted residual orthopyroxene, exsolusion lamellae of neogenic clinopyroxene in the bastite (D363); (h) structure of melted residuals in lherzolite, plane-polar (D363). Opx: orthopyroxene; Cpx: clinopyroxene; Ol: olivine; Bas: bastite; Cr: Cr spinel; Serp: serpentine; Ol2: new neogenic olivine; Cpx2: new neogenic clinopyroxene; Mag: magnetite.
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Figure 4. Harker diagrams showing the MgO plotted against CaO, Al2O3 and SiO2 contents of the Zhaheba peridotite (a–c). Abyssal and SSZ peridotite fields are from [5,60], respectively. Whole-rock MgO and Al2O3 contents normalized to SiO2 contents from the Zhaheba peridotite (d). The fore-arc peridotite data are from [5,61], and the peridotite depletion trend is from [62]. The blue star shows the Primitive mantle (PM) values, which are from [63]. Abyssal peridotite data are from [60].
Figure 4. Harker diagrams showing the MgO plotted against CaO, Al2O3 and SiO2 contents of the Zhaheba peridotite (a–c). Abyssal and SSZ peridotite fields are from [5,60], respectively. Whole-rock MgO and Al2O3 contents normalized to SiO2 contents from the Zhaheba peridotite (d). The fore-arc peridotite data are from [5,61], and the peridotite depletion trend is from [62]. The blue star shows the Primitive mantle (PM) values, which are from [63]. Abyssal peridotite data are from [60].
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Figure 5. Variation diagrams of MgO versus elements of Y, Sc, Yb and Lu (a–d) in the Zhaheba peridotite. SSZ peridotite and Abyssal fields are from [5] and [60], respectively. The blue stars show the residual compositions of Primitive Mantle, which are from [63].
Figure 5. Variation diagrams of MgO versus elements of Y, Sc, Yb and Lu (a–d) in the Zhaheba peridotite. SSZ peridotite and Abyssal fields are from [5] and [60], respectively. The blue stars show the residual compositions of Primitive Mantle, which are from [63].
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Figure 6. Clinopyroxene composition in the lherzolites and harzburgites of the Zhaheba peridotite. (a) Cr2O3 (wt.%) versus Al2O3 diagram; (b) Mg# versus Cr2O3 (wt.%) diagram; (c) Al2O3 (wt.%) versus Mg# diagram. The melting trend is from [64], the fields of ABP and FAP are from [65].
Figure 6. Clinopyroxene composition in the lherzolites and harzburgites of the Zhaheba peridotite. (a) Cr2O3 (wt.%) versus Al2O3 diagram; (b) Mg# versus Cr2O3 (wt.%) diagram; (c) Al2O3 (wt.%) versus Mg# diagram. The melting trend is from [64], the fields of ABP and FAP are from [65].
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Figure 7. Compositional range of orthopyroxene in the lherzolites of the Zhaheba peridotite. (a) Al2O3 (wt.%) versus Mg# diagram; (b) Al2O3 (wt.%) versus Cr2O3 (wt.%) diagram. ABP and FAP are from [65]. The melting trend is from [66]. The fractionation trend is from [67,68].
Figure 7. Compositional range of orthopyroxene in the lherzolites of the Zhaheba peridotite. (a) Al2O3 (wt.%) versus Mg# diagram; (b) Al2O3 (wt.%) versus Cr2O3 (wt.%) diagram. ABP and FAP are from [65]. The melting trend is from [66]. The fractionation trend is from [67,68].
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Figure 8. Chemical characteristics of chromian spinel in the Zhaheba chromitites, showing the possible tectonic setting. (a) Al2O3 versus Cr2O3 (wt.%) diagram of spinels in peridotite and massive chromite. Compositional fields of stratiform and podiform chromitites are from [69]; compositional fields of Sartohay are from [17] and the compositional fields of Luobusha are from [70]; (b) compositional ranges of chromian spinel in different lithologies are from [8]; (c) the compositional relationship between the Cr# of spinel and TiO2 content in the Zhaheba chromitites. The fields of abyssal peridotite are from [1,12], boninite are from [71]; (d) NiO (wt.%) versus Cr# diagram of chromian spinel. The fields of ABP, FAP and BON are from [65]; (e) Al2O3 versus TiO2 (wt.%). Fields of the island arc series, MORB and back-arc basin basalt (BABB) are after [72], the Alaskan-type complex is after [73,74,75], the Bushveld complex is after [76] and the SED podiform chromitite is from [77]; (f) relationships between Fe3+# and TiO2 content of chromian spinels.
Figure 8. Chemical characteristics of chromian spinel in the Zhaheba chromitites, showing the possible tectonic setting. (a) Al2O3 versus Cr2O3 (wt.%) diagram of spinels in peridotite and massive chromite. Compositional fields of stratiform and podiform chromitites are from [69]; compositional fields of Sartohay are from [17] and the compositional fields of Luobusha are from [70]; (b) compositional ranges of chromian spinel in different lithologies are from [8]; (c) the compositional relationship between the Cr# of spinel and TiO2 content in the Zhaheba chromitites. The fields of abyssal peridotite are from [1,12], boninite are from [71]; (d) NiO (wt.%) versus Cr# diagram of chromian spinel. The fields of ABP, FAP and BON are from [65]; (e) Al2O3 versus TiO2 (wt.%). Fields of the island arc series, MORB and back-arc basin basalt (BABB) are after [72], the Alaskan-type complex is after [73,74,75], the Bushveld complex is after [76] and the SED podiform chromitite is from [77]; (f) relationships between Fe3+# and TiO2 content of chromian spinels.
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Figure 10. Mantle oxidation state of the Zhaheba chromitites, as represented by ΔlogfO2 (FMQ) versus Cr# of spinels (a) andΔlogfO2 (FMQ) versus Fe3+/Fetot (b). The discrimination boundaries for dunites (solid line), harzburgites (dashed line) and lava fields are after [90]. MORB hz = mid-ocean ridge harzburgite, SSZ hz = supra-subduction zone harzburgite, MORB dun = mid-ocean ridge dunite, SSZ dun = supra-subduction zone dunite, IAT = island arc tholeiite, BON = boninite. The fields of ABP and FAP are from [65]. Symbols are as in Figure 8.
Figure 10. Mantle oxidation state of the Zhaheba chromitites, as represented by ΔlogfO2 (FMQ) versus Cr# of spinels (a) andΔlogfO2 (FMQ) versus Fe3+/Fetot (b). The discrimination boundaries for dunites (solid line), harzburgites (dashed line) and lava fields are after [90]. MORB hz = mid-ocean ridge harzburgite, SSZ hz = supra-subduction zone harzburgite, MORB dun = mid-ocean ridge dunite, SSZ dun = supra-subduction zone dunite, IAT = island arc tholeiite, BON = boninite. The fields of ABP and FAP are from [65]. Symbols are as in Figure 8.
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Wang, Z.; Yan, J.; Tang, H.; Xiao, Y.; Deng, Z.; Meng, G.; Sun, H.; Qi, Y.; Yuan, L. Geochemistry and Mineralogy of Peridotites and Chromitites from Zhaheba Ophiolite Complex, Eastern Junggar, NW China: Implications for the Tectonic Environment and Genesis. Minerals 2023, 13, 1074. https://doi.org/10.3390/min13081074

AMA Style

Wang Z, Yan J, Tang H, Xiao Y, Deng Z, Meng G, Sun H, Qi Y, Yuan L. Geochemistry and Mineralogy of Peridotites and Chromitites from Zhaheba Ophiolite Complex, Eastern Junggar, NW China: Implications for the Tectonic Environment and Genesis. Minerals. 2023; 13(8):1074. https://doi.org/10.3390/min13081074

Chicago/Turabian Style

Wang, Zhaolin, Jiayong Yan, Hejun Tang, Yandong Xiao, Zhen Deng, Guixiang Meng, Hui Sun, Yaogang Qi, and Lulu Yuan. 2023. "Geochemistry and Mineralogy of Peridotites and Chromitites from Zhaheba Ophiolite Complex, Eastern Junggar, NW China: Implications for the Tectonic Environment and Genesis" Minerals 13, no. 8: 1074. https://doi.org/10.3390/min13081074

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