The Genesis of Ultramafic Rock Mass on the Northern Slope of Lüliang Mountain in North Qaidam, China

Abstract

The ultramafic rock located on the northern slope of Lüliang Mountain in the northwestern region of North Qaidam Orogen is altered to serpentinite. The occurrence of disseminated chromite within the serpentinite holds significant implications for understanding the petrogenesis of the protolith. This work provides strong evidence of a distinct zonal texture in the chromite found in the ultramafic rock, using petrographic microstructure and electron probe composition analysis. The core of the chromite is characterized by high contents of Cr^#, with enrichment in Fe^3+# (Fe³⁺/(Cr + Al + Fe³⁺)) and depletion in Al₂O₃ and TiO₂. The Cr₂O₃ content ranges from 51.64% to 53.72%, while the Cr^# values range from 0.80 to 0.84. The FeO content varies from 24.9% to 27.8%, while the Fe₂O₃ content ranges from 5.19% to 8.74%. The Al₂O₃ content ranges from 6.70% to 9.20%, and the TiO₂ content is below the detection limit (<0.1%). Furthermore, the rocks exhibit Mg^# values ranging from 0.13 to 0.25 and Fe^3+# values ranging from 0.07 to 0.12. The mineral chemistry of the chromite core in the ultramafic rock suggests it to be from an ophiolite. This ophiolite originated from the fore-arc deficit asthenosphere in a supra-subduction zone. The estimated average crystallization temperature and pressure of the chromite are 1306.02 °C and 3.41 GPa, respectively. These values suggest that the chromite formed at a depth of approximately 110 km, which is comparable to that of the asthenosphere. The chromite grains are surrounded by thick rims composed of Cr-rich magnetite characterized by enrichment in Fe^3+# contents and depletions in Cr₂O₃, Al₂O₃, TiO₂, and Cr^#. The FeO content ranges from 28.25% to 31.15%, while the Fe₂O₃ content ranges from 44.94% to 68.92%. The Cr₂O₃ content ranges from 0.18% to 23.59%, and the Al₂O₃ and TiO₂ contents are below the detection limit (<0.1%). Moreover, the rim of the Cr-rich magnetite exhibits Cr^# values ranging from 0.90 to 1.00, Mg^# values ranging from 0.01 to 0.06, and Fe^3+# values ranging from 0.64 to 1.00, indicating late-stage alteration processes. The LA-ICP-MS zircon U-Pb dating of the ultramafic rock yielded an age of 480.6 ± 2.4 Ma (MSWD = 0.46, n = 18), representing the crystallization age of the ultramafic rock. This evidence suggests that the host rock of chromite is an ultramafic cumulate, which is part of the ophiolite suite. It originated from the fore-arc deficit asthenosphere in a supra-subduction zone during the northward subduction of the North Qaidam Ocean in the Ordovician period. Furthermore, clear evidence of Fe-hydrothermal alteration during the post-uplift-denudation stage is observed.

1. Introduction

The North Qaidam orogenic belt is a prominent example of a continent collision orogenic belt, specifically related to processes of continental subduction, collision, and reversion. This formation materialized through intricate tectonic interplays between the Qaidam Block and the Qilian Block during the Paleozoic era. The limited data that are currently available indicate that early oceanic subduction occurred within this setting [1,2,3,4,5,6,7]. The site has consistently maintained substantial significance in the realm of investigating the exchange of energy and materials between the Earth’s mantle and crust [8,9]. The ultramafic rock located on the north slope of Lüliang Mountain in the northwestern region of North Qaidam represents a distinctive component of the orogenic belt. This rock type plays a crucial role in providing a comprehensive geological record that includes basin creation, development, extinction, and continent–continent collision events. Its significance lies in its capacity to faithfully reconstruct ancient landmass assembly processes and orogenic cycles [10,11,12]. Previous studies have primarily concentrated on conducting comprehensive analyses of mafic–ultramafic rock masses in various regions, such as Yuka, the southern slope of Lüliang Mountain, Luofengpo, and Taipinggou [10,11,13,14,15,16,17].

Spinel-group minerals are common accessory minerals in mafic–ultramafic rocks, and chromite is a major species. Chromite is more stable than other minerals formed from magma and is highly responsive to changes in the environment where it forms [18,19,20,21]. Its extensive spectrum of elemental compositions allows for the reliable determination of the origin and properties of host rocks and mantle rocks, based on the physical and chemical conditions prevailing during their formation. These conditions include temperature, pressure, oxygen fugacity, and the extent of partial melting [18,22,23,24,25,26,27,28,29,30,31,32,33,34]. Numerous studies have demonstrated that only primary chromite, identified through petrographic and chemical composition analyses, holds substantial implications for unraveling and guiding the genesis of rocks [16,18,28,30,35].

The present study focuses on the extensive ultramafic rock mass located on the northern slope of Lüliang Mountain. The extensive serpentinization has posed significant difficulties in analyzing its rock properties and discerning its formation environment [14,15]. Consequently, research in this particular field has been relatively limited over an extended period. The objective of this study is to (1) ascertain the age at which the ultramafic rock was formed, (2) investigate the characteristics of the parent rock, potential mechanisms of formation, the environment in which it formed, and subsequent modification processes associated with the ultramafic rock, and (3) propose a genetic model of the ultramafic rock.

2. Geological Setting

The North Qaidam orogenic belt is located in the northeastern margin of the Tibetan plateau, where the Qaidam block and Qilian block intersect within China [36] (Figure 1). This geological unit stretches approximately 700 km in a NW–SE direction and varies in width from 40 to 200 km. Its western boundary is defined by the Altyn Tagh strike-slip fault, while the eastern boundary is demarcated by the Wahongshan–Wenquan fault. The northern boundary is formed by the Lajishan–Zhongqilian southern margin fault, and the southern boundary is characterized by the deep fault of North Qaidam [3,37,38,39,40,41,42,43]. The internal structure of the North Qaidam orogenic belt can be divided into three secondary tectonic units along the Zongwulong–Qinghainanshan fault and Wulan–Yuka fault, from north to south: the Late Paleozoic–Early Mesozoic rift zone of Zongwulongshan, the Quanji block, and the Early Paleozoic suture zone of North Qaidam [44,45,46,47,48]. These units consist of diverse rock assemblages and structural formations that originated during different geological epochs and tectonic settings. They underwent varying degrees of metamorphism and deformation due to subduction–accretion processes involving the Qaidam oceanic basin and arc–continent collisions. The stratigraphic structure within this belt exhibits localized regularity [49]. Dissected ophiolites and ductile shear zones are commonly observed within this geological unit [11,13,14,15,16,17,36,37,38,39,40,41,42,43,44,45].

The study area is located on the northern slope of Lüliang Mountain, within the western segment of the North Qaidam orogenic belt. The exposed strata in this region primarily consist of the Paleoproterozoic Dakendaban Group, Mesoproterozoic Yukahe Group, and Ordovician Tanjianshan Group [50,51], as well as granodiorite (Silurian) [15] and mafic–ultramafic intrusive rocks (Cambrian to Ordovician) [40] (Figure 2). The Dakendaban Group comprises granulite, schist, and quartzite, while the Yukahe Group consists of schist, granulite, eclogite (metamorphic zircons yield an age range of 430 to 495 Ma [52,53,54]), and marble. The Tanjianshan Group is subdivided into a metavolcanic formation and a metaclastic formation. The metavolcanic formation includes metabasalt, metabasaltic andesite, propylite, and schist, while the metaclastic formation is composed of schist, porcelanite, quartzite, and granulite. The granodiorite occurs as lens-shaped bodies enclosed within the Dakendaban Group. There are more than 10 occurrences of mafic–ultramafic intrusive rocks in the study area, with five significant ones extending from north to south: the northern and southern slopes of Lüliang Mountain, Luofengpo, Taipinggou, and Kaipinggou rock bodies. The lithology of the mafic–ultramafic intrusive rocks mainly consists of serpentinite, with the occasional presence of gabbro and amphibolite. These rocks are distributed in a NW–SE trending belt or lens-shaped formations, surrounded by the Dakendaban Group, Yukahe Group, and Tanjianshan Group. The strata and rock formations in the area generally align along a NW–SE tectonic axis, but localized disruptions can be observed. These irregularities are believed to be the result of subsequent regional tectonic events and volcanic activities, leading to alterations in the geological features.

3. Petrography and Field Relations

Due to the extensive freeze–thaw weathering, the rocks primarily occur as fragmented blocks. They have a fine-grained or cryptocrystalline texture with a massive structure. Chromite is dominantly present as veinlet or disseminated aggregates in the serpentinite. The chromite veins have variable thicknesses ranging from 0.5 to 3 cm and length varying from 1 to 15 cm.

Microscopic observations and electron probe results reveal the dominant minerals in the serpentinite to be serpentine, chromite, magnetite, magnesite, and chlorite (Figure 3).

Serpentine makes up the majority, ranging from 70% to 95% of the rock composition. It appears colorless or light gray under plane-polarized light and exhibits foliated or reticulated textures. Chromite content ranges from 0.2% to 5% and displays a range of grain sizes from 0.1 to 3 mm. It appears gray or brown under plane-polarized light with subhedral-to-euhedral textures. Some chromite samples exhibit distinct zonal textures, with the core being darker than the edges.

The magnetite content in the serpentinite is less than 1% and appears as small black particles dispersed along the rims and internal fractures of the serpentine and chromite minerals. The magnesite content is also less than 1% and mainly appears as gray–brown irregular microparticles or locally forming fibrous aggregates within the serpentine. The chlorite content is also less than 1% and appears as elongated strip-like bands distributed within fractures of the serpentine and chromite.

4. Analytical Methods

For this study, we initially performed field investigations and rock microscopy to determine the texture and structure of the ultramafic rock found on the northern slope of Lüliang Mountain in the North Qaidam orogenic area. Next, we conducted LA-ICP-MS zircon U-Pb dating on the ultramafic rock and examined the mineral chemical composition of chromite using electron probe microanalysis (EPMA).

The analysis of major elements in serpentinite was conducted at the Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Nature and Resources, Jilin University, Changchun, China, using an X-ray fluorescence spectrometer (XRF; PW1401/10). The obtained results demonstrated a minimal error rate of less than 5%.

The zircon grains were carefully selected from two serpentinites (WKLN1 and WKLN2) at the Hongxin Geological Survey Technical Service Corp. in Langfang, Hebei Province, China, following stringent selection criteria. They were separated by conventional heavy liquid and magnetic techniques combining with hand-picking under a binocular microscope. They were mounted in epoxy resin and polished to expose the centers of the grains on the polished surface and then to be given cathodoluminescence (CL) images. The zircon U-Pb dating analysis was performed at MA-LAB Co., Ltd., Xi’an, China, using LA-ICP-MS analysis techniques. Laser sampling in this study was conducted using a New Wave NWR213 laser ablation system. The ion-signal intensities were acquired using a Thermo Fisher iCAP RQ ICP-MS instrument. Helium was used as the carrier gas, while argon was utilized as the make-up gas, mixed with the carrier gas via a T-connector before entering the ICP. Each analysis consisted of a background acquisition of approximately 10 s (gas blank), followed by 40 s of data acquisition from the sample. For data processing, off-line selection, and integration of background and analyte signals, time-drift correction, and quantitative calibration for trace element analyses and U-Pb dating were performed using Iolite 4 [55]. The zircon reference 91500 [56] was used as an external standard for U-Pb dating, while zircon references GJ-1 [57] and Plešovice [58] were used as monitors. Time-dependent drifts of U-Th-Pb isotopic ratios were corrected using interpolation with time. The uncertainty of the preferred values for the external standard 91500 was propagated to the final results of the samples. Concordia diagrams and weighted mean calculations were performed using Isoplot/Ex-ver 3 [59]. The trace element compositions of zircons were calibrated against reference materials (NIST610), with Si used as internal standardization. The preferred values of element concentrations for the NIST reference glasses are available on the GeoReM database.

The EPMA (electron probe microanalysis) analysis of the chromite samples was conducted at Wuhan Sample Solution Analytical Technology Co., ltd., Wuhan, China. The experiment was performed in the EPMA laboratory using a JXA-8230 instrument (JEOL, Tokyo, Japan) operating at 15 KV and a current of 2.00 × 10⁻⁸ A. The beam spot size was set to 1 μm, and data calibration utilized the ZAF correction method developed by JEOL in Japan. Data processing involved software applications such as AX (Version 2.0) and Geokitpro (Version 20231201). Based on the stoichiometry of spinel (AB₂O₄), a dedicated software program calculated the total number of cations, distinguishing between Fe²⁺ and Fe³⁺. Mineral abbreviations used in this study adhere to Shen Qihan’s nomenclature [60].

5. Results

5.1. Major Element Characteristics of the Serpentinite

The serpentinite samples collected from the study area exhibit varying compositions of major elements (

Table 1. The major elements data of serpentinite and recalculated normative values (wt.%).

Sample	230901A 001-026	230901A 001-027	230901A 001-029	230901A 001-030	230901A 001-031	230901A 001-033	230901A 001-034	230901A 001-035	230901A 001-036	230901A 001-037	230901A 001-038
SiO2	38.0	39.8	40.8	39.5	42.14	44.1	41.8	39.8	37.6	37.2	38.2
TiO2	0.09	0.02	0.04	0.02	0.06	0.16	0.28	0.02	0.02	0.02	0.02
Al2O3	2.32	0.33	0.94	1.07	1.97	2.66	2.31	0.47	0.44	0.34	0.35
Fe2O3	8.33	6.28	6.00	6.43	4.91	3.87	5.01	6.42	5.44	5.38	5.82
FeO	0.54	0.49	2.23	1.81	4.23	3.01	3.74	1.02	0.95	1.48	1.10
MnO	0.14	0.05	0.08	0.13	0.11	0.17	0.17	0.04	0.09	0.08	0.06
MgO	39.1	39.4	36.8	38.5	34.4	31.8	33.3	39.7	39.9	40.1	40.1
CaO	0.22	0.40	0.57	0.21	1.18	4.37	2.84	0.08	0.10	0.07	0.06
Na2O	0.06	0.05	0.05	0.05	0.07	0.26	0.15	0.13	0.13	0.21	0.12
K2O	0.09	0.00	0.00	0.01	0.03	0.04	0.02	0.01	0.01	0.00	0.00
P2O5	0.02	0.01	0.03	0.01	0.03	0.08	0.33	0.01	0.01	0.01	0.01
LOI	10.9	12.7	12.	12.13	10.7	9.22	9.87	12.2	15.2	15.1	14.0
Total	99.75	99.61	99.86	99.87	99.80	99.74	99.77	99.88	99.88	99.99	99.83

Note: the coordinates of samples (230901A001-026, 230901A001-027, 230901A001-029, 230901A001-030, and 230901A001-031) is E94°58′00″ and N37°58′30″; the coordinates of samples (230901A001-033, 230901A001-034, 230901A001-035, 230901A001-036, 230901A001-037, and 230901A001-38) is E94°58′00″ and N37°58′30″.

). The SiO₂ content ranges from 37.23% to 44.14%, TiO₂ from 0.02% to 0.28%, Al₂O₃ from 0.33% to 2.66%, Fe₂O₃ from 3.87% to 8.33%, FeO from 0.49% to 4.23%, MnO from 0.04% to 0.17%, MgO from 31.76% to 40.09%, CaO from 0.06% to 4.37%, Na₂O from 0.05% to 0.26%, K₂O 0% to 0.09%, and P₂O₅ from 0.01% to 0.33%. Considering the rock assemblage structure and microstructural characteristics of serpentinite, as well as the tectonic setting of the study area, it can be inferred that the protolith of the serpentine corresponds to ultramafic cumulates (refer to Figure 4). These findings are consistent with observations from adjacent areas such as the southern slope of Lüliang Mountain, Luofengpo, Taipinggou, and Kaipinggou [11,15,16,17].

5.2. LA-ICP-MS Zircon U-Pb Dating

A total of 200 zircons were hand-picked from two samples of serpentinite (WKLN1: E94°58′00″, N37°58′30″; KKLN2: E94°58′00″, N37°58′30″). However, some zircons showed clear growth edges along their periphery. For LA-ICP-MS zircon U-Pb isotope dating, eighteen well-preserved zircons were selected, which exhibited intact morphology without cracks or inclusions and featured an internal oscillating ring structure (refer to Figure 5). These zircons were predominantly characterized by high concentrations of Th and U, with Th/U ratios exceeding 0.1, indicating their magmatic origin [61]. The measurement sites primarily focused on the oscillation zone within the zircon crystals, which provided a more accurate representation of the ages of magma crystallization.

Eighteen spots were carried out on magmatic zircons. These data exhibit excellent agreement within the error range. Additionally, all of the data fall around the concordant curve (

Table 2. LA-ICP-MS zircon U-Pb isotope dating data.

Point Number	Pb *	Th	U	Th/U	Isotope Ratio						Ages (Ma)
	ppm	ppm	ppm		207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ	207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ
WKLN1-02	65	396	592	0.668	0.05712	0.0011	0.6075	0.01176	0.07713	0.00087	496	24	482	7	479	5
WKLN1-03	56	272	522	0.52	0.05676	0.00105	0.60711	0.01123	0.07757	0.00087	482	22	482	7	482	5
WKLN1-04	23	17	277	0.062	0.05693	0.00213	0.60115	0.02212	0.07658	0.00104	489	57	478	14	476	6
WKLN1-05	76	167	848	0.197	0.05727	0.00103	0.60729	0.01102	0.0769	0.00086	502	21	482	7	478	5
WKLN1-06	43	74	445	0.166	0.05613	0.00109	0.60309	0.01177	0.07793	0.00088	458	24	479	7	484	5
WKLN1-09	97	141	1008	0.14	0.05723	0.00091	0.61004	0.00979	0.07731	0.00085	500	18	484	6	480	5
WKLN1-14	124	401	1221	0.328	0.05744	0.00078	0.61636	0.00856	0.07782	0.00084	508	14	488	5	483	5
WKLN1-15	97	150	1001	0.15	0.05908	0.00112	0.62059	0.01182	0.07618	0.00086	570	23	490	7	473	5
WKLN1-19	28	28	302	0.094	0.05709	0.00127	0.60986	0.01352	0.07748	0.0009	495	29	483	9	481	5
WKLN1-20	53	26	531	0.049	0.05742	0.00094	0.61743	0.01023	0.07797	0.00086	508	18	488	6	484	5
WKLN1-21	50	189	505	0.375	0.05709	0.00099	0.60863	0.01064	0.07732	0.00086	495	20	483	7	480	5
WKLN1-22	80	122	860	0.141	0.05706	0.00086	0.60901	0.00935	0.07741	0.00085	494	16	483	6	481	5
WKLN1-23	28	14	312	0.045	0.05684	0.00173	0.61184	0.0183	0.07806	0.00098	485	44	485	12	485	6
WKLN1-25	55	101	603	0.167	0.05877	0.00113	0.62163	0.01199	0.07671	0.00087	559	23	491	8	476	5
WKLN1-27	38	73	430	0.169	0.057	0.00153	0.60527	0.01608	0.07701	0.00094	492	37	481	10	478	6
WKLN1-28	49	105	527	0.198	0.05693	0.00106	0.61209	0.01145	0.07797	0.00088	489	22	485	7	484	5
WKLN1-29	41	32	454	0.07	0.05625	0.00107	0.60692	0.01157	0.07824	0.00088	462	23	482	7	486	5
WKLN1-30	22	16	253	0.064	0.05612	0.00278	0.59841	0.02901	0.07733	0.00121	457	80	476	18	480	7

Note: Pb * = 0.241 × ²⁰⁶Pb + 0.221 × ²⁰⁷Pb + 0.524 × ²⁰⁸Pb.

) (Figure 6). The weighted average age of ²⁰⁶Pb/²³⁸U is calculated to be 480.6 ± 2.4 Ma (MSWD = 0.46, n = 18). Considering the geological setting and the Th and U content, it can be inferred that the ultramafic rock on the north slope of Lüliang Mountain was formed approximately 480.6 ± 2.4 Ma ago. This formation coincides with the northward subduction stage of the North Qaidam Ocean during the Ordovician period, which is consistent with previous findings from studies conducted on the south slope of Lüliang Mountain, Taipinggou, and Luofengpo ultramafic rocks [11,15].

5.3. Mineral Chemistry of Chromite

According to petrographic characteristics and EPMA data (

Table 3. EPMA data of the core of chromite (wt.%).

Point Number	WKLTZ1- Q2-001	WKLTZ1- Q2-002	WKLTZ1- Q2-003	WKLTZ1- Q2-004	WKLTZ2- Q2-001	WKLTZ2- Q2-002	WKLTZ2- Q2-003	WKLTZ2- Q2-004
CaO	0.00	0.01	0.04	0.01	0.03	0.00	0.00	0.00
TiO2	0.17	0.11	0.15	0.17	0.15	0.14	0.12	0.12
Na2O	0.02	0.06	0.08	0.04	0.02	0.03	0.03	0.02
MgO	2.81	2.83	2.66	2.23	4.63	4.54	4.24	4.04
Cr2O3	52.19	52.02	51.64	51.81	53.72	53.50	53.65	53.16
MnO	1.39	1.47	1.54	1.65	1.38	1.48	1.55	2.09
Fe2O3	7.72	8.41	8.64	8.74	5.46	5.45	5.19	5.71
FeO	26.94	26.92	27.17	27.80	24.90	24.95	25.27	24.99
SiO2	0.02	0.03	0.01	0.00	0.03	0.02	0.02	0.00
Al2O3	6.91	6.82	6.96	6.70	9.04	9.20	9.16	9.05
K2O	0.00	0.01	0.00	0.00	0.00	0.00	0.00	0.00
Total	98.17	98.67	98.90	99.15	99.35	99.32	99.23	99.18
Oxygen	4	4	4	4	4	4	4	4
Ca	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Ti	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Na	0.00	0.00	0.01	0.00	0.00	0.00	0.00	0.00
Mg	0.15	0.15	0.14	0.12	0.24	0.24	0.22	0.21
Cr	1.49	1.47	1.46	1.47	1.48	1.47	1.48	1.47
Mn	0.04	0.04	0.05	0.05	0.04	0.04	0.05	0.06
Fe3+	0.21	0.23	0.23	0.24	0.14	0.14	0.14	0.15
Fe2+	0.81	0.81	0.81	0.83	0.72	0.73	0.74	0.73
Si	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Al	0.29	0.29	0.29	0.28	0.37	0.38	0.38	0.37
K	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
sum	3	3	3	3	3	3	3	3
Cr#	0.84	0.84	0.83	0.84	0.80	0.80	0.80	0.80
Mg#	0.16	0.16	0.15	0.13	0.25	0.25	0.23	0.22
Fe3+#	0.11	0.11	0.12	0.12	0.07	0.07	0.07	0.08
Fe2+#	0.84	0.84	0.85	0.87	0.75	0.75	0.77	0.78

Note: Cr^# = Cr/(Cr + Al); Mg^# = Mg/(Mg + Fe²⁺); Fe^3+# = Fe³⁺/(Cr + Al + Fe³⁺); Fe^2+# = Fe²⁺/(Mg + Fe²⁺); (after reference [21]); Fe²⁺ is calculated from the structure formula of chromian spinel using AX software (Version 2.0), the same as below.

and

Table 4. EPMA data of the rim of chromite (wt.%).

Point Number	WKLTZ1- Q2-005	WKLTZ1- Q2-006	WKLTZ1- Q2-007	WKLTZ1- Q2-008	WKLTZ2- Q2-005	WKLTZ2- Q2-006	WKLTZ2- Q2-007	WKLTZ2- Q2-008
CaO	0.00	0.00	0.01	0.00	0.03	0.02	0.02	0.04
TiO2	0.22	0.14	0.07	0.06	0.40	0.21	0.07	0.02
Na2O	0.00	0.00	0.02	0.02	0.03	0.03	0.05	0.00
MgO	0.19	0.18	0.13	0.11	0.97	0.57	0.33	0.15
Cr2O3	11.13	6.42	4.27	1.99	23.59	9.66	2.90	0.18
MnO	0.52	0.19	0.04	0.03	2.23	0.87	0.13	0.11
Fe2O3	57.30	62.88	64.28	67.07	44.94	59.40	66.61	68.92
FeO	30.72	31.14	30.92	31.15	28.25	29.81	30.74	30.86
SiO2	0.06	0.03	0.05	0.06	0.06	0.01	0.00	0.04
Al2O3	0.02	0.00	0.00	0.03	0.02	0.03	0.00	0.01
K2O	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Total	100.17	100.98	99.79	100.52	100.52	100.62	100.83	100.34
Oxygen	4	4	4	4	4	4	4	4
Ca	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Ti	0.01	0.00	0.00	0.00	0.01	0.01	0.00	0.00
Na	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Mg	0.01	0.01	0.01	0.01	0.05	0.03	0.02	0.01
Cr	0.34	0.19	0.13	0.06	0.70	0.29	0.09	0.01
Mn	0.02	0.01	0.00	0.00	0.07	0.03	0.00	0.00
Fe3+	1.65	1.80	1.86	1.93	1.27	1.69	1.91	1.99
Fe2+	0.98	0.99	0.99	1.00	0.89	0.94	0.98	0.99
Si	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Al	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
K	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
sum	3	3	3	3	3	3	3	3
Cr#	1.00	1.00	1.00	0.98	1.00	1.00	1.00	0.90
Mg#	0.01	0.01	0.01	0.01	0.06	0.03	0.02	0.01
Fe3+#	0.83	0.90	0.93	0.97	0.64	0.85	0.96	1.00
Fe2+#	0.99	0.99	0.99	0.99	0.94	0.97	0.98	0.99

, Figure 3), chromite exhibits a distinct zonal texture, with a black core and gray rim; the composition of the two parts is evidently distinct. The composition is relatively uniform in the core and rim of the chromite. In the core of the chromite, the composition ranges for various elements are as follows: 51.64% to 53.72% for Cr₂O₃, 6.70% to 9.20% for Al₂O₃, 5.19% to 8.74% for Fe₂O₃, 24.9% to 27.8% for FeO, 2.23% to 4.63% for MgO, and 0.11% to 0.17% for TiO₂. The calculated values for Fe^3+# range from 0.07 to 0.12, Fe^2+# range from 0.75 to 0.87, and Cr^# range from 0.80 to 0.84. As to the rim, Cr₂O₃ content ranges from 0.18% to 23.59%, Al₂O₃ from 0% to 0.03%, and Fe₂O₃ from 44.94% to 68.92%. FeO ranges from 28.25% to 31.15%, MgO from 0.11% to 0.97%, and TiO₂ from 0.02% to 0.40%. The calculated values for Fe^3+# range from 0.64 to 1.00, Fe^2+# range from 0.94 to 0.99, and Cr^# range from 0.90 to 1.00.

Based on the spinel group classification [62] and EPMA data (Table 3 and Table 4), the core’s composition closely resembles that of the chromite end-member (FeCr₂O₄), with small amounts of spinel (MgAl₂O₄) and magnetite (FeFe₂O₄) components. On the other hand, the rim of chromite exhibits a chemical composition resembling that of Cr-rich magnetite, suggesting that the samples had experienced hydrothermal alteration in an oxidizing environment. The Al-Cr-Fe³⁺ trivalent cation diagram shows the degree of the metamorphism of chromite (Figure 7) and the samples of Cr-rich magnetite from rim fall within the genetic region of the lower amphibolite facies. This indicates that these samples experienced hydrothermal metamorphism at temperatures ranging from 500 °C to 600 °C. Additionally, in the transition from the core to the rim, there is a significant increase in Fe^3+# and Fe³⁺ content, a substantial decrease in Mg^# (indicating a decrease in the magnesium component), and a slight increase in Cr^# and Fe^2+#. The TiO₂ content remains relatively stable throughout this transition.

6. Discussion

6.1. Characteristics of Protolith

Based on microscopic observation and mineral chemistry, this study proposes that the core of chromite preserves original components from the secondary alteration. EPMA data confirm that the core of the chromite exhibits a high degree of stability and uniformity, with no discernible variations between particles, indicating equilibrium within each particle [64,65]. The analysis and calculations related to the protolith discussed in this study are based on data derived from the core of the chromite.

Regarding the characteristics of the core of chromite, its TiO₂ content is significantly low, ranging from 0.11% to 0.17%. This aligns with the features observed in ophiolites, where TiO₂ content is typically below 0.3% [66,67]. In the structural discrimination diagram of the Cr-Al-(Fe³⁺ + 2Ti) relationship (refer to Figure 8), the Al, Ti, and Fe³⁺ contents are noticeably depleted, while the Cr content is significantly enriched. The majority of samples predominantly fall within the ophiolite region, with some overlapping into the stratified complex area. Considering the tectonic location and microstructure, it can be inferred that the core of chromite exhibits characteristics typical of ophiolitic formations, with the host rocks (ultramafic cumulate) identified as ophiolitic members.

6.2. Tectonic Environment

The spinel in ultramafic rock mass formed in different tectonic environments has different TiO₂, Al₂O₃, and Fe²⁺/Fe³⁺ values, which is clearly reflected in Figure 9 [34] and Figure 10 [68]. The Al₂O₃ content of spinel formed in island arc environment ranges from 1% to 18%, while the TiO₂ content is less than 2%. The Al₂O₃ content of spinel in MORB ranges from 16% to 47%, and the TiO₂ content from 0.07% to 2%. The Al₂O₃ content of spinel in SSZ type peridotite ranges from 5% to 39% and TiO₂ less than 0.4%, and Fe²⁺/Fe³⁺ content ranges from 1.5% to 25%. The Al₂O₃ content of spinel in MORB type peridotite is more than 21%, TiO₂ is less than 0.6%, and Fe²⁺/Fe³⁺ ranges from 1.5% to 22%. The content of Al₂O₃ ranges from 1% to 42%, and the content of Fe²⁺/Fe³⁺ ranges from 0.3% to 3% in the volcanogenic spinel.

In the tectonic discrimination diagram of the Al₂O₃–TiO₂ relationship (Figure 9), the cores of the chromite in our samples predominantly fall within an overlapping region among two areas: the supra-subduction-zone peridotite area and arc-high-Ti area. This suggests that the samples exhibit characteristics that can be associated with these tectonic settings. On the other hand, in the structural discrimination diagram of the Al₂O₃-Fe²⁺/Fe³⁺ relationship (Figure 10), the core of the chromite primarily occurs within the supra-subduction-zone peridotite area and volcanic spinel area. This indicates that the samples share similarities with the chromite found in these tectonic environments.

Based on our research findings, the rocks in the study area are identified as ultramafic cumulates. Furthermore, no volcanic rocks have been detected in this region, thus ruling out the possibility that the source of the protolith originated from volcanic spinel areas. The tectonic identification diagram of the Cr-Al-Fe³⁺ and 2Ti relationship (Figure 8) illustrates that the core of the chromite is predominantly located within the (fore-arc) peridotite region. It is hypothesized that the formation environment of the core of the chromite in this area corresponds to a supra-subduction zone (SSZ), demonstrating characteristics of (fore-arc) peridotite. These characteristics can be attributed to the complex tectonic cycles prevalent in this region. The observed findings align with the regional tectonic evolution stage of ultramafic rocks, as determined by LA-ICP-MS zircon U-Pb dating, during the northward subduction of the North Qaidam Ocean. The Cr–TiO₂ relationship diagram, employed to assess mantle depletion (Figure 11), clearly indicates that the core of the chromite is exclusively found within the region of depleted peridotites, which aligns with the characteristics of the major elements data of serpentinite (enrichment in Fe₂O₃, FeO, and MgO and depletion in CaO, Al₂O₃, and TiO₂.). This suggests that the ultramafic cumulate, serving as the host rock, was initially formed under conditions of mantle depletion.

6.3. Physical and Chemical Conditions of Protolith

The core of the chromite represents the primary chromite, offering valuable information about the physical and chemical conditions during its formation and providing insights into the protolith origin. Consequently, EPMA data obtained from the core of the chromite are utilized to examine the pressure, temperature, and other relevant physical and chemical conditions prevailing during protolith formation.

The formula employed for calculating the temperature of chromite is as follows (after reference [70]):

$$\mathrm{T}=(4250{\mathrm{Y}}_{\mathrm{Cr}}^{\mathrm{Sp}}+1343)/\mathrm{In}{\mathrm{K}}_{\mathrm{Cr}}^0+1.825{\mathrm{Y}}_{\mathrm{Cr}}^{\mathrm{Sp}}+0.571, \mathrm{In}{\mathrm{K}}_{\mathrm{Cr}}^0=0.34+1.06({\mathrm{Y}}_{\mathrm{Cr}}^{\mathrm{Sp}}{)}^2$$

(1)

The formula is applicable under the conditions of T = 1500 K, where T represents the absolute temperature measured in Kelvin. ${\mathrm{Y}}_{\mathrm{C}\mathrm{r}}^{\mathrm{S}\mathrm{p}}$ denotes the mole fraction of chromium (Cr) within the trivalent element of chromite, specifically calculated as ${\mathrm{Y}}_{\mathrm{C}\mathrm{r}}^{\mathrm{S}\mathrm{p}}$ = Cr/(Cr + Al + Fe³⁺).

The temperature calculations in this study utilize data from eight chromite samples obtained from the ultramafic rocks located on the northern slope of Lüliang Mountain (Table 4). The results indicate that the crystallization temperature of the core of the chromite ranges from 1303 °C to 1308 °C with an average value of 1306 °C.

The formula employed for calculating the pressure of chromite is as follows (after reference [71]):

$$\mathrm{P}={\mathrm{P}}^0+27.9({\mathrm{X}}_{\mathrm{Cr}}^{\mathrm{Sp}}+{\mathrm{X}}_{\mathrm{Fe}}^{\mathrm{Sp}});$$

(2)

The unit of P and P⁰ is 10⁸ Pa, and, in this study, P⁰ is chosen as 18.7. ${\mathrm{X}}_{\mathrm{C}\mathrm{r}}^{\mathrm{S}\mathrm{p}}$ and ${\mathrm{X}}_{\mathrm{F}\mathrm{e}}^{\mathrm{S}\mathrm{p}}$ are the mole fractions of Cr³⁺ and Fe³⁺ in chromite, respectively.

The pressure calculations in this study are based on data obtained from eight chromite samples collected from the studied ultramafic rocks (refer to

Table 5. Physical and chemical conditions of the chromite based on EPMA data (wt.%).

Point Number	The Crystallization Temperature of the Chromite			The Crystallization Pressure of the Chromite
	${\mathbf{Y}}_{\mathbf{C}\mathbf{r}}^{\mathbf{S}\mathbf{p}}$	Ink	T (°C)	${\mathbf{X}}_{\mathbf{C}\mathbf{r}}^{\mathbf{S}\mathbf{p}}$	${\mathbf{X}}_{\mathbf{F}\mathbf{e}}^{\mathbf{S}\mathbf{p}}$	P (Gpa)
WKLTZ1-Q2-001	0.747	0.932	1303.37	0.495	0.070	3.45
WKLTZ1-Q2-002	0.741	0.922	1305.77	0.491	0.076	3.45
WKLTZ1-Q2-003	0.735	0.913	1308.07	0.487	0.078	3.44
WKLTZ1-Q2-004	0.739	0.919	1306.63	0.490	0.079	3.46
WKLTZ2-Q2-001	0.742	0.924	1305.37	0.492	0.048	3.38
WKLTZ2-Q2-002	0.739	0.919	1306.59	0.490	0.047	3.37
WKLTZ2-Q2-003	0.743	0.925	1305.15	0.493	0.045	3.37
WKLTZ2-Q2-004	0.737	0.916	1307.20	0.490	0.050	3.38
Range	0.735–0.747	0.913–0.932	1303.37–1308.07	0.487–0.495	0.045–0.079	3.37–3.46
Average	0.740	0.921	1306.02	0.491	0.062	3.41

). The results indicate that the crystallization pressure of the core of the chromite ranges from 3.37 GPa to 3.46 GPa, with an average value of 3.41 GPa. Considering a static rock pressure estimate of 31 MPa/km, it can be inferred that the ultramafic rock formed at a depth approximately equal to 110 km. It is widely accepted that the top of the asthenosphere maintains temperatures ranging from 1280 to 1350 °C and pressures around 3.0 GPa [72], which closely aligns with the calculated results presented in this study. Therefore, it can be concluded that the chromite hosted within the investigated ultramafic rocks originated from the top of the asthenosphere.

6.4. Petrogenesis

Previous research has determined that peridotite in orogenic belts originates from three distinct sources: mantle peridotite within subduction zones, ultramafic cumulate complexes within subduction zones, and subducted metamorphic mantle peridotite [73]. These rocks are formed in different tectonic settings, reflecting diverse tectonic evolution [14]. The ultramafic rock found on the northern slope of Lüliang Mountain was formed during the northward subduction stage (480.6 ± 2.4 Ma) of the North Qaidam Ocean. It represents a typical ultramafic cumulate complex within a subduction zone and originated in a supra-subduction zone (SSZ) setting. The rock exhibits characteristics that are indicative of both mid-ocean ridge (MORB) and pre-arc peridotite.

Therefore, we propose that the ultramafic rock was generated within the asthenosphere with significant depletion under a fore-arc setting, during the initial phase of the northward subduction of the North Qaidam Ocean in the Early Paleozoic period (Figure 12). Simultaneously, the rock underwent significant metasomatism due to the fluid derived from the subducting oceanic plate [74]. During the arc–continent collision stage in the Early Silurian, the rock experienced crustal scraping due to the return of the subducted plate, subsequently undergoing alteration by ferriferous hydrothermal fluids and serpentinization, as a result of crustal uplift and denudation.

The ultramafic rock represents the largest chromite-bearing rock with the highest chromite grade (Cr₂O₃ values ranging from 13% to 43.33% [11,15]) within the North Qaidam orogenic belt. A comprehensive analysis of its properties, genesis, formation environment, and subsequent transformation processes not only provides new insights into the tectonic evolution of the North Qaidam orogenic belt but also offers valuable guidance for further exploration of chromite deposits in this region. Consequently, in future exploration phases in this region, greater attention should be given on the ultramafic cumulate and ultramafic rock containing abundant chromite that was formed during the subduction stage of the North Qaidam Ocean. In particular, the recent discovery of a significant number of chromium-bearing mineralization messages in ultramafic cumulate (diagenetic age 480–544 Ma) [75] within the Shaliuhe area, located in the eastern part of the North Qaidam orogenic belt, provides further evidence supporting the potential for chromite enrichment in this environment characterized by ultramafic rock.

Figure 12. Schematic model for formation of chromite and ultramafic cumulate (modified after reference [76]). (a) Schematic map of tectonic setting for formation of chromite and ultramafic cumulate. (b) Schematic map of mechanism for formation of chromite and ultramafic cumulate.

Figure 12. Schematic model for formation of chromite and ultramafic cumulate (modified after reference [76]). (a) Schematic map of tectonic setting for formation of chromite and ultramafic cumulate. (b) Schematic map of mechanism for formation of chromite and ultramafic cumulate.

7. Conclusions

• The LA-ICP-MS analysis of zircon U-Pb dating on the serpentinite on the northern slope of Lüliang Mountain in the North Qaidam orogenic belt yields an age of 480.6 ± 2.4 Ma. These data indicate that the rock was formed during the northward subduction stage of the North Qaidam Ocean in the Ordovician period.
• The chromite within the ultramafic rock exhibits a distinct zonal texture. The core’s composition closely resembles that of the chromite end-member (FeCr₂O₄), with small amounts of spinel (MgAl₂O₄) and magnetite (FeFe₂O₄) components, representing the original chromite. It has high contents of Cr₂O₃ and Cr^#. It is enriched in Fe^3+# and depleted in Al₂O₃ and TiO₂. The rim consists of Cr-rich magnetite exhibiting high Fe^3+# contents while being depleted in Cr₂O₃, Al₂O₃, TiO₂, and Cr^#, indicating hydrothermal alteration.
• The ultramafic rock is classified as an ultramafic cumulate and is part of the ophiolite suite. It originated from the fore-arc deficit asthenosphere in a supra-subduction zone. Clear evidence of Fe-hydrothermal alteration is observed during the later uplift–denudation stage. The crystallization temperature of the ultramafic rock ranges from 1303 °C to 1308 °C, with an average value of 1306 °C, while the crystallization pressure ranges from 3.37 GPa to 3.46 GPa, with an average value of 3.41 GPa.
• In the North Qaidam orogenic belt, ultramafic cumulates containing abundant chromite, formed during the subduction stage of the North Qaidam Ocean, have significant potential for chromite-ore formation. Therefore, future ore prospecting efforts in the region should focus on these rock types.