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Article

Petrogenesis of Garnet Clinopyroxenite and Associated Dunite in Hujialin, Sulu Orogenic Belt, Eastern China

by
Jianguo Liu
1,
Jian Wang
2,*,
Keiko Hattori
3 and
Zeli Wang
4
1
School of Resources and Environmental Engineering, Shandong University of Technology, Zibo 255000, China
2
College of Earth Sciences, Jilin University, Changchun 130061, China
3
Department of Earth and Environmental Sciences, University of Ottawa, Ottawa, ON K1N 6N5, Canada
4
College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China
*
Author to whom correspondence should be addressed.
Minerals 2022, 12(2), 162; https://doi.org/10.3390/min12020162
Submission received: 16 December 2021 / Revised: 19 January 2022 / Accepted: 25 January 2022 / Published: 28 January 2022
(This article belongs to the Special Issue Petrology, Mineralogy, Geochemistry and Geochronology of Granites)
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Abstract

:
The origin of ultramafic rocks, especially those in suture zones, has been a focus because they are not only important mantle sources of magma, but also provide substantial information on metamorphism and melt/fluid–peridotite interaction. Ultramafic rocks in Hujialin, in the central part of the Sulu orogen, include peridotite and pyroxenite. Although many papers on their origin and tectonic evolution have been published in the past few decades, these questions are still highly debated. Here, we present mineralogy, mineral composition, and bulk-rocks of these ultramafic rocks to evaluate their origin and tectonic evolution. The garnet clinopyroxenite is low in heavy rare-earth elements (HREE, 5.97–10.6 ppm) and has convex spoon-shaped chondrite-normalized REE patterns, suggesting the garnet formed later, and its precursor is clinopyroxenite. It is high in incompatible elements (i.e., Cs, Rb, Ba) and shows negative to positive U, Nb, and Ta anomalies, without pronounced positive Sr or Eu anomalies. Clinopyroxene in garnet clinopyroxenite contains high MgO (Mg# 0.90–0.97). The mineral chemistry and bulk-rock compositions are similar to those of reactive clinopyroxenite, suggesting that it originally formed via peridotite–melt interaction, and that such silicic and calcic melt might derive from the subducted Yangtze continent (YZC). Dunite contains olivine with high Fo (93.0–94.1), low NiO (0.11–0.29 wt.%) and MnO (≤0.1 wt.%), chromite with high Cr# (0.75–0.96), TiO2 (up to 0.88 wt.%), and Na2O (0.01–0.10 wt.%). It has negatively sloped chondrite-normalized REE patterns. Mineral chemistry and bulk rocks suggest dunite likely represent residual ancient lithosperic mantle peridotite beneath the North China Craton (NCC) that was overprinted by aqueous fluids. The lack of prograde and retrograde metamorphic minerals in dunite and irregular shaped mineral inclusions in chromite suggest dunite did not subduct to deep levels. Dunite mingled with garnet clinopyroxenite during exhumation of the latter at shallow depths. These ultramafic rocks, especially hydrated peridotite, may be important sources of Au for the Jiaodong gold province in the NCC.

1. Introduction

The Dabie-Sulu orogenic belt formed from the subduction of Yangtze Craton (YZC) beneath the North China Craton (NCC) in the Triassic in east-central China [1,2,3]. It is one of the largest ultrahigh-pressure (UHP) metamorphic terranes in the world with an exposed area of about 30,000 km2 [4,5]. The terrane is mainly composed of granitic gneisses, with volumetrically minor ultramafic rocks occurring as massifs and lenticular bodies. These ultramafic rocks include garnet peridotite, spinel peridotite, and pyroxenite. They mainly distribute in Weihai, Rongcheng (Chijiadian, Lijiatun, and Macaokuang), Jiangzhuang, Xugou, Rizhao (Suoluoshu and Hujialin), and Yangkou [1,2,6] (Figure 1a).
Ultramafic rocks in Hujialin are surrounded by granitic gneiss, and mainly consist of serpentinized peridotite with minor dunite and clinopyroxenite lenses [7]. The origin of pyroxenite has been a focus not only because pyroxenite subducted into the mantle is an important source for magma, but also because the pyroxenite returned to the surface provides substantial information on metamorphism and melt/fluid–peridotite interaction [8]. Although many papers on the origin of Hujialin clinopyroxenite have been published in the past few decades, the question of whether the clinopyroxenite is a cumulate of magma [7,9,10,11] or a product of melt–peridotite interaction [12] is still highly debated. Besides, given that clinopyroxenite occurs closely associated with dunite somewhere in Hujialin [5], questions such as “What is the relationship between their origins?” and “How did they exhume together ?” require further discussion.
This study presents detailed petrology, mineral composition, and bulk-rock geochemistry of clinopyroxenite and dunite of Hujialin, to evaluate the origin and tectonic evolutions of these two ultramafic rocks.
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Figure 1. (a) A sketch map showing the framework of the Dabie-Sulu orogenic belt and the distribution of orogenic ultramafic rocks (modified after Zheng et al. [13]; Chen et al. [14]). (b) Geological sketch map of the Rizhao area, showing the position of ultramafic rocks in Hujialin (modified after Zhang and Liou [15]). (c) Outcrop map of the Hujialin ultramafic rocks (after Zhang and Liou [15]). Abbreviations: Y-W Fault = Yantai-Wulian Fault; J-X Fault = Jiashan-Xiangshui Fault.
Figure 1. (a) A sketch map showing the framework of the Dabie-Sulu orogenic belt and the distribution of orogenic ultramafic rocks (modified after Zheng et al. [13]; Chen et al. [14]). (b) Geological sketch map of the Rizhao area, showing the position of ultramafic rocks in Hujialin (modified after Zhang and Liou [15]). (c) Outcrop map of the Hujialin ultramafic rocks (after Zhang and Liou [15]). Abbreviations: Y-W Fault = Yantai-Wulian Fault; J-X Fault = Jiashan-Xiangshui Fault.
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2. Geological Setting

The Sulu orogen is displaced northward from the Dabie orogen by the sinistral Tan-Lu fault for about 500 km. It is bounded by the Yantai-Wulian to the north, and Jiashan-Xiangshui fault to the south (Figure 1a). Three main fault-bounded metamorphic zones are classified, with (I) low-temperature/low-pressure (LT/LP) greenschist-facies zone in the north composed of Neoproterozoic igneous rocks and Neoproterozoic-pre-Triassic sedimentary rocks; (II) mid-temperature/ultrahigh-pressure (MT/UHP) eclogite-facies zone in the central dominated by ortho- and paragneiss with layers and blocks of eclogite, amphibolite, marble, and sporadically distributed ultramafic bodies; and (III) low-temperature/high-pressure (LT/HP) blueschist-facies zone in the south consisting of quartzite schist, ortho- and paragneiss, marble, and blueschist [16,17,18,19]. They are unconformably overlain by Jurassic siliciclastic and Cretaceous volcanoclastic rocks and intruded by post-orogenic Mesozoic granites [19].
The rocks in UHP zones retain evidence of UHP metamorphism, such as coesite inclusions in minerals of garnet, omphacite, jadeite, kyanite, and epidote from both eclogite and metasedimentary rock in Donghai area [16]; the presence of exsolved clinopyroxene, rutile, and apatite in garnet from Yangkou eclogite [20], and coesite and other ultrahigh-pressure mineral inclusions in zircon from eclogite and gneiss core samples of the Chinese Continental Scientific Drilling Site (CCSD) [19,21,22]. This indicates that both supracrustal and basement rocks underwent UHP metamorphism at subarc depths more than 100 km. U-Pb zircon dating of garnet peridotites [23], UHP gneisses [24], and eclogites [25] shows that they were subjected to UHP metamorphism at 245–220 Ma. The peak metamorphic conditions were considered to be 750–850 °C and 3.4–4.0 GPa [19].
The Hujialin ultramafic complex is situated in Rizhao City and located in the middle of the Sulu orogen. It trends NNW-SSE for about 6 km long and is cut by a NE-SW-trending fault (Figure 1b). The ultramafic complex is in fault contact with the UHP gneiss. It mainly consists of serpentinized peridotite that contains minor discontinuous lenses of garnet clinopyroxenite and dunite (Figure 1c). Petrological studies, such as mineral exsolution texture in clinopyroxene, mineral assemblage, and associated P-T calculation, indicate that the Hujialin garnet clinopyroxenite underwent UHP metamorphism [5,15,26]. Zircons discovered from the ultramafic complex yielded U-Pb ages of 215–226 Ma, consistent with the time of initial exhumation of deeply subducted continental crust in the Dabie-Sulu orogenic belt [27,28,29]. Some spinel peridotites show no evidence of UHP metamorphism and may have undergone different paths [2,11].

3. Materials and Methods

3.1. Sample Description

We examined nine clinopyroxenite samples and six dunite samples. They were collected in the northern lenses of the Hujialin complex in a stone pit (Figure 1c), with eight clinopyroxenite samples at 35°12′24″ N, 119°15′6″ E; one clinopyroxenite (HJ-19) at 35°12′23″ N, 119°15′8″ E; and dunite at 35°12′35″ N, 119°15′2″ E. The Hujialin complex is not very large, and the pyroxenite and dunite are mainly exposed in the northern part of the complex based on our field observation. Therefore, the samples that we collected may represent the whole set of outcrops of pyroxenite-dunite.
Clinopyroxenite is divided into omphacite-bearing (type-1, samples of HJ-1, HJ-6) and omphacite-free (type-2) varieties. Type-2 clinopyroxenite is subdivided into garnet-rich (type-2A, samples of HJ-8, HJ-16) and garnet-poor (type-2B, samples of HJ-2, HJ-9, HJ-10, HJ-12, HJ-19) varieties. Type-1 and type-2A clinopyroxenite are partly altered and show porphyroblastic texture, in which coarse-grained garnet (20–50 vol.%, 1.5–5 mm in size) occurs in a fine-grained matrix (0.4 mm on average) that is composed of garnet (~10 vol.%), clinopyroxene with minor alteration minerals (amphibole, chlorite, and epidote), and Fe-oxides (magnetite and ilmenite) (Figure 2a,b). Clinopyroxene, including omphacite (euhedral to anhedral, 35–65 vol.%) occurs as inclusions in garnet and isolated grains (Figure 2c). Clinopyroxene shows an interlocking texture with fine-grained garnet and has irregular boundaries. It is euhedral to subhedral in sample of HJ-8, and sometimes shows 120° triple junction (Figure 2d). In sample of HJ-16, clinopyroxene grains are elongated and show preferred orientation (Figure 2e). Fe-oxide intergrowths in type-1 clinopyroxenite (commonly <0.2 mm in size, <2 vol.%) mainly occur in interstitial spaces of other minerals and are enclosed in clinopyroxene and occasionally in garnet.
Type-2B clinopyroxenite is moderately altered and shows equigranular texture, except for HJ-9, which shows porphyroblastic texture. Garnet-poor clinopyroxenite contains clinopyroxene (euhedral to anhedral, 0.5 mm on average, >90 vol.%), garnet (<1 mm in size, ~5 vol.%), alteration minerals (amphibole, chlorite, and epidote), Fe-oxides (magnetite, and ilmenite), and titanite (Figure 2f). Clinopyroxene occurs as inclusions in garnet and isolated grains in matrix. In HJ-9, coarse-grained clinopyroxene (~1 mm in size) is commonly surrounded by fine-grained clinopyroxene (~0.1 mm). Garnet commonly contains clinopyroxene inclusions and occasionally contains amphibole and Fe-oxides (Figure 2g). It is mostly altered and rimmed by amphibole, epidote, and chlorite (Figure 2f). Similar to garnet-rich clinopyroxenite, Fe-oxides and titanite intergrowths (commonly <0.2 mm in size, <2 vol.%) mainly occur in interstitial spaces of other minerals (Figure 2h,i). A grain (irregular in shape and 0.4 mm in size) showing chromite and ilmenite intergrowth was discovered in interstitial space of clinopyroxene in sample HJ-10.
Dunite (samples of HJ-20, HJ-23, HJ-25, HJ-27, HJ-28, and HJ-29) is slightly to moderately altered along cracks and boundaries of olivine into serpentine, amphibole, and chlorite (Figure 2j–l). It shows a granular mosaic structure, and mainly consists of olivine (~0.6 mm in size, >95 vol.%) and chromite (<3 vol.%). No sulfide minerals are found. Some olivine and chromite grains are mylonitized and show preferred orientation (Figure 2j). Chromite shows uneven size (<0.1 mm to 2 mm) and varying color (dark red to opaque). It occurs in interstitial spaces of olivine grains. Several chromite grains form aggregates.

3.2. Analytical Methods

Mineral compositions were determined using a JEOL JXA-8230 microprobe by the wavelength dispersive method at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, China. The peak counting time was 10 s for each element. Analytical conditions were 15 kV accelerating voltage, 20 nA beam current, and 5 μm beam spot. The standards were jadite (Si, Al, Na), forsterite (Mg), rutile (Ti), topaz (F), potash feldspar (K), wollastonite (Ca), hematite (Fe), Cr2O3 (Cr), MnO (Mn), NiO (Ni), and NaCl (Cl). Raw data were corrected using a ZAF program. Fe3+ and Fe2+ contents of chromite were calculated based on stoichiometry of AB2O4.
Samples were crushed to powders of 2 μm in agate mortars for bulk-rock major and minor elements analysis in Yanduzhongshi Geological Analysis Laboratories Ltd., Beijing, China. Bulk-rock major elements were analyzed by an X-ray fluorescent spectrometer (XRF-1800). Bulk-rock powder was added to a flux of lithium metaborate, mixed well, and fused in a furnace at 1150 °C. A flat glass disk was prepared, and then the major elements were analyzed by XRF. Relative standard deviations for major element oxides are within 1%.
Minor elements were analyzed by inductively coupled mass spectrometry (ICP-MS, M90). The sample preparation procedure was similar to that described by Li et al. [12]. Sample powder was heated in an oven at 105 °C for 12 h, and 50 mg of powder was weighed and placed into Teflon bombs with a mixture of HF and HNO3. The Teflon bomb was put in a stainless-steel pressure jacket and heated to 190 °C for 24 h. After cooling, the Teflon bomb was opened and placed on a hotplate at 140 °C and evaporated to incipient dryness. Then, 1 mL HNO3, 1 mL MQ water, and 1 mL internal standard solution of 1 ppm In was added. The Teflon bomb was resealed and put in the oven at 190 °C for >12 h. The final solution was transferred to a polyethylene bottle and diluted to 100 g by 2% HNO3 for ICP analysis. Standards of AGV-2, BHVO-2, BCR-2, BGM-2, and GSR-2 were used to monitor the analytical quality. Relative standard deviations for most minor elements are within 5%.

4. Results

4.1. Mineral Chemistry

4.1.1. Clinopyroxene and Garnet in Clinopyroxenite

Clinopyroxene is diopside and omphacite in type-1 clinopyroxenite (Supplementary Materials Table S1, Figure 3). Omphacite inclusions contain slightly higher Jd content (=100* AlVI/(Na + Ca)) of 29–30 mol% than isolated ones (26–28 mol%), both of which are similar to the composition of omphacite from eclogite in Sulu orogen [31,32] (Figure 3). The composition of diopsidic clinopyroxene is not listed in Supplementary Materials Table S1 because of its slightly low total content (SiO2 52.9 wt.%, TiO2 0.12 wt.%, Al2O3 2.57 wt.%, Cr2O3 0.04 wt.%, FeO 2.96 wt.%, MnO < 0.01 wt.%, MgO 13.9 wt.%, CaO 23.8 wt.%, Na2O 0.33 wt.%, K2O < 0.01 wt.%, with total content 96.6 wt.%).
Diopside in type-2 clinopyroxenite contains high MgO (15.7–17.6 wt.%) with Mg# (=Mg2+/(Mg2+ + Fetotal)) 0.90–0.97, SiO2 (53.1–56.3 wt.%), low TiO2 (<0.25 wt.%), Na2O (0.38–0.94 wt.%), and Al2O3 (0.72–3.13 wt.%). Diopside inclusions have identical composition with isolated grains (Supplementary Materials Table S1). The composition is close to that of diopside in mantle peridotite of the North China Craton (Figure 4). The diopside studied here shows little exsolved phases and is likely close to the composition at the time of crystallization.
Garnet shows variable proportions of pyrope, grossular, and almandine components. It is represented by Prp19–25Alm22–26Gro48–57, Prp28–38Alm21–24Gro41–49, and Prp28–42Alm20–30Gro34–47Spe0–1 (Prp: pyrope; Alm: almandine; Gro: grossular; Spe: spessartine) in type-1, type-2A, and type-2B clinopyroxenite, respectively, similar to the B-type and C-type garnet defined by Coleman et al. [33] (Supplementary Materials Table S2, Figure 5). The Si content of garnet is generally >6 apfu, suggesting the Hujialin garnet clinopyroxenite underwent UHP metamorphism as documented by Zhang and Liou [15].
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Figure 3. Classification diagrams for clinopyroxene [34]. Jd (Jadeite) = 100 × AlVI/(Na + Ca), Aeg (Aegirine) = 100 × (Na − AlVI)/(Na + Ca), WEF (wollastonite + enstatite + ferrosilite) = 100 − Jd − Ae.
Figure 3. Classification diagrams for clinopyroxene [34]. Jd (Jadeite) = 100 × AlVI/(Na + Ca), Aeg (Aegirine) = 100 × (Na − AlVI)/(Na + Ca), WEF (wollastonite + enstatite + ferrosilite) = 100 − Jd − Ae.
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Figure 4. Major element oxides variation diagrams of clinopyroxene in Hujialin type-2 clinopyroxenite (a) CaO; (b) Na2O; (c) Al2O3; (d) TiO2. Data sources: cumulate clinopyroxene worldwide is based on Liu and Ying [35] and references therein; field of clinopyroxene phenocrysts of host lavas from the North China Craton (NCC) is from Xu et al. [36], and Liu and Ying [35]; mantle peridotite from the NCC is from the Zheng et al. [37,38], Ying et al. [39], and Xu et al. [36,40].
Figure 4. Major element oxides variation diagrams of clinopyroxene in Hujialin type-2 clinopyroxenite (a) CaO; (b) Na2O; (c) Al2O3; (d) TiO2. Data sources: cumulate clinopyroxene worldwide is based on Liu and Ying [35] and references therein; field of clinopyroxene phenocrysts of host lavas from the North China Craton (NCC) is from Xu et al. [36], and Liu and Ying [35]; mantle peridotite from the NCC is from the Zheng et al. [37,38], Ying et al. [39], and Xu et al. [36,40].
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Figure 5. Composition of garnet. Three types of garnet are after Coleman et al. [33].
Figure 5. Composition of garnet. Three types of garnet are after Coleman et al. [33].
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4.1.2. Olivine and Chromite in Dunite

Olivine in dunite has consistently high Fo (Fo = 100 × Mg/[Mg + Fe]; 93.0–94.1), and low NiO (0.11–0.29 wt.%) and MnO (≤0.1 wt.%) (Supplementary Materials Table S3). The Fo for studied samples is comparable to that of mantle olivine and ancient sub-continental lithospheric mantle (SCLM), but the NiO and MnO are both lower than those of mantle olivine, ancient SCLM, and Cenozoic SCLM (Figure 6).
Chromite contains high Cr# (=Cr/[Cr + Al]; 0.75–0.96), relatively high TiO2 (up to 0.88 wt.%) and Na2O (0.01–0.10 wt.%), and low XFe3+ (=Fe3+/[Fe3+ + Al + Cr]; ≤0.18) (Supplementary Materials Table S4). It is not homogeneous in composition within individual grains, with higher Cr, Mg, and Al and lower Fe in cores (Cr2O3 52.3–60.0 wt.%, MgO 4.98–8.40 wt.%, Al2O3 5.52–12.5 wt.%, FeO 20.8–30.0 wt.%) than in rims (Cr2O3 12.6–53.8 wt.%, MgO 1.20–3.83 wt.%, Al2O3 0.01–2.15 wt.%, FeO 36.2–70.3 wt.%) (Figure 7), similar to altered chromite in forearc mantle serpentinites of the Rio San Juan complex reported by Saumur and Hattori [41]. On a ternary diagram of Fe3+-Cr-Al, and binary Cr#-Mg and Al2O3-TiO2 diagrams, chromite plots into the fields of Himalayas and forearc peridotite (Figure 8).
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Figure 6. Compositional variations of olivine. (a) NiO vs. Fo. (b) MnO vs. Fo. The mantle olivine array is based on Hattori et al. [42]. Partial melting trend is from Ozawa [43]. Trend of cumulate olivine is from Ozawa [43] and Nakamura [44]. Fields of Ancient and Cenozoic subcontinental lithospheric mantle (SCLM) are from Xie et al. [2] and references therein.
Figure 6. Compositional variations of olivine. (a) NiO vs. Fo. (b) MnO vs. Fo. The mantle olivine array is based on Hattori et al. [42]. Partial melting trend is from Ozawa [43]. Trend of cumulate olivine is from Ozawa [43] and Nakamura [44]. Fields of Ancient and Cenozoic subcontinental lithospheric mantle (SCLM) are from Xie et al. [2] and references therein.
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4.1.3. Other Minerals

Chlorite, epidote, amphibole, and titanite occur as retrograde minerals in clinopyroxenite. Chlorite, amphibole, and serpentine occur as secondary minerals in dunite. Chlorite in different types of samples shows different composition (Supplementary Materials Table S5). It is clinochlore in type-2A clinopyroxenite with moderate SiO2 (28.8–30.6 wt.%), Al2O3 (18.4–19.6 wt.%), FeO (1.91–5.19 wt.%), and MgO (29.8–31.4 wt.%). Chlorites are clinochlore and sheridanite varieties in type-2B clinopyroxenite with relatively low SiO2 (26.8–29.1 wt.%) and MgO (24.8–29.8 wt.%), and high Al2O3 (19.8–21.2 wt.%) and FeO (5.67–11.17 wt.%). Chlorite in dunite contains the highest SiO2 (35.2 wt.%) and MgO (33.3 wt.%), while the lowest Al2O3 (11.2 wt.%) and FeO (1.2 wt.%) belong to pennine variety.
Epidote in type-1 clinopyroxenite shows slightly lower SiO2 (36.4–37.9 wt.%), Al2O3 (23.7–24.5 wt.%), and Cr2O3 (0.01–0.05 wt.%), and higher FeO (6.76–8.17 wt.%) relative to epidote in type-2B clinopyroxenite (SiO2 37.4–38.4wt.%, Al2O3 24.0–25.3 wt.%, Cr2O3 0.22–0.44 wt.%, FeO 5.21–7.21 wt.%). The XFe (=Fe3+/(Fe3+ + Al + Cr + Mn)) of epidote in type-1 and type-2B clinopyroxenite is 0.06–0.13 and 0–0.08, and belongs to clinozoisite-epidote and zoisite, respectively, based on the classification of Enami et al. [49] (Supplementary Materials Table S5).
Amphibole is pargasite in clinopyroxenite and tremolite in dunite based on the classification described by Hawthorne et al. [50]. Pargasite contains SiO2 (43.9–45.4 wt.%), Al2O3 (12.5–14.5 wt.%), FeO (2.91–8.50 wt.%), MgO (14.6–18.4 wt.%), CaO (10.5–12.5 wt.%), Na2O (1.51–3.05 wt.%), and K2O (0.47–1.80 wt.%). Tremolite contains higher SiO2 (58.8 wt.%), MgO (24.4 wt.%), and CaO (13.0 wt.%), and lower Al2O3 (0.21 wt.%), FeO (0.88 wt.%), Na2O (0.09 wt.%), and K2O (0.02 wt.%) (Supplementary Materials Table S6).
Serpentine in dunite shows two varieties. One contains SiO2 (41.2–43.8 wt.%), MgO (39.1–43.1 wt.%), Al2O3 (≤0.13 wt.%), and CaO (≤0.01 wt.%). The other type contains relatively high SiO2 (59.3 wt.%), low MgO (29.5 wt.%), and comparable Al2O3 (0.14 wt.%) and CaO (0.04 wt.%) (Supplementary Materials Table S6).

4.2. Bulk-Rock Compositions

4.2.1. Major Elements

Type-1 clinopyroxenite contains lower SiO2 (44.9–45.9 wt.%), MgO (9.38–9.74 wt.%), and CaO (17.6–17.9 wt.%), and higher Al2O3 (13.2–13.9 wt.%), Na2O (1.52–2.07 wt.%), and K2O (0.10–0.12 wt.%) relative to type-2 clinopyroxenite (SiO2 45.9–47.5 wt.%, MgO 12.6–14.4 wt.%, CaO 18.6–20.1 wt.%, Al2O3 4.84–9.28 wt.%, Na2O 0.46–0.63 wt.%, K2O 0.04–0.10 wt.%) (Supplementary Materials Table S7). The compositions of them are slightly different from those of eclogite and garnet pyroxenite metamorphosed from cumulative pyroxenite (Figure 9).
Dunite contains similar contents of SiO2 (38.2–39.8 wt.%), Fe2O3 (7.72–8.20 wt.%), and MnO (0.11–0.17 wt.%), higher MgO (46.6–48.8 wt.%) and Na2O (0.02–0.04 wt.%), and lower CaO (0.05–0.35 wt.%) and TiO2 (~0.01 wt.%) relative to that reported by Xie et al. [2].

4.2.2. Minor Elements

Clinopyroxenite samples have similar convex-upward primitive mantle-normalized trace element patterns (Figure 10), with high concentrations of incompatible elements (i.e., Cs, Rb, Ba), negative to positive U, Nb, Ta anomalies; consistently negative Th and Zr anomalies; and positive Hf anomalies. Type-1 clinopyroxenite has no obvious Eu anomaly, negative to without Sr anomalies, and higher concentrations of REE (77.0–82.1 ppm), while type-2 clinopyroxenite shows negative Eu anomaly, negative to positive Sr anomalies, and relatively lower concentrations of REE (25.7–36.6 ppm) (Supplementary Materials Table S7, Figure 10). They have similar convex spoon-shaped chondrite-normalized REE patterns, showing enrichment from La to Nd, and depletion from Nd to Lu (Figure 10).
Dunite samples have low concentrations of trace elements as a whole (0.55–1.39 ppm) and display negatively sloped primitive mantle-normalized trace element patterns (Supplementary Materials Table S7, Figure 10). They show positive Ba, U, Ta, Pb, and Hf anomalies; negative Rb, Th, Nb, Zr anomalies, slightly positive Sr anomaly; and negative Eu anomaly (Figure 10). They display negatively sloped chondrite-normalized REE patterns and show depletion from La to Tm and flat to slight enrichment from Tm to Lu. The characteristics of dunite are similar to those of Archean cratonic mantle peridotite beneath the North China Craton (NCC) [40] and Suoluoshu serpentinite [3] (Figure 10).

5. Discussion

5.1. Origin of Hujialin Garnet Clinopyroxenite and Nature of Metasomatic Agent

Although there is a compositional overlap between eclogite and garnet pyroxenite in Ca, Fe, Al, and Mg contents, the two are not identical in origin [51,57]. Eclogite is linked to the subducted oceanic slab and has a basaltic or gabbroic precursor, while garnet pyroxenite is a cumulate of mantle-derived melt or a product of melt–peridotite interaction [36,42,51,54]. Type-1 garnet clinopyroxenite contains omphacite, high Al2O3 content (13.2–13.9 wt.%), and low MgO content (9.38–9.74 wt.%) relative to type-2 clinopyroxenite, and itseems to show affinity with eclogite (Figure 9); however, its trace-element features argue against this possibility. Pronounced positive Sr and Eu anomalies and flat HREE patterns are striking features of eclogite, due to its plagioclase-bearing protolith [51,57], while type-1 garnet clinopyroxenite shows no evident positive Eu or Sr anomalies and has negatively sloped HREE patterns (Figure 10). In addition, if type-1 garnet clinopyroxenite is derived from metamorphism of solid-state recycled oceanic slab, its major compositions and normalized REE patterns should be similar to those of MORB [57] (Figure 10). As illustrated in Figure 9 and Figure 10, the high CaO content and REE patterns of type-1 garnet clinopyroxenite, together with type-2 garnet clinopyroxenite, do not resemble those of MORB and metamorphic recycled oceanic slab, indicating the Hujialin garnet clinopyroxenite is different from eclogite in origin.
The garnet clinopyroxenite samples have low concentrations of HREE (5.97–10.6 ppm, Supplementary Materials Table S7) and convex spoon-shaped chondrite-normalized REE patterns, indicating that garnet formed later during deep subduction, and the prograde metamorphic process formed in a closed system because garnet preferentially incorporate HREE [42]. If garnet was an initial phase or there was melt/fluid injected during the metamorphic process, the garnet clinopyroxenite samples would contain high concentrations of HREE [42]. This interpretation is also in agreement with the common occurrence of clinopyroxene and ilmenite inclusions in garnet. Therefore, the precursor of Hujialin garnet clinopyroxenite is clinopyroxenite.
As previously mentioned, there are two possible origins for clinopyroxenite: cumulate of a mantle-derived melt [42,51] and interactive product between melt and peridotite [36,54].
The Hujialin clinopyroxenite shows negatively sloped REE patterns and may be a cumulate from a melt. However, the clinopyroxene contains high MgO with Mg# up to 0.90–0.97 (Supplementary Materials Table S1), which is distinct from that of clinopyroxene phenocrysts of host lavas worldwide (Figure 4), but similar to that of clinopyroxene of mantle peridotite from the North China Craton (Figure 4). Furthermore, Hujialin clinopyroxenite contains high CaO and the major compositions are different from those of garnet pyroxenite metamorphosed from cumulate pyroxenite (Figure 9). The REE patterns of Hujialin garnet clinopyroxenite are also different from those of cumulate pyroxenite, but similar to those of clinopyroxene of reactive origin, because cumulate pyroxenite has almost flat LREE and HREE patterns (Figure 10). Therefore, we propose the protolith of Hujialin garnet clinopyroxenite is not a cumulate of a mantle-derived melt. Instead, it is a product of melt–peridotite interaction, which is also supported by the relic olivine grains with rounded and eroded shapes included in clinopyroxene [27].
The findings above raise the question of the nature of the metamorphic melt. The replacement of olivine by clinopyroxene indicates the metamorphic agent is a silicic and calcic melt, and the transformation from olivine to clinopyroxene may have involved two reactions: (1) olivine + SiO2 (melt1) = orthopyroxene (+melt2), and (2) orthopyroxene + Ca (melt3) = clinopyroxene (+melt4), similar to the formation of websterite xenoliths from the Feixian basalts in the eastern NCC [36]. Petrographic observations of calcite in matrix, dolomite in porphyroblastic garnet [12], and high CaO content of bulk-rock composition (Figure 9) support the involvement of a carbonatitic melt. Such a silicic and calcic melt may derive from the Yangtze continent during its subduction beneath the North China Craton. Therefore, the studied samples were primarily formed from peridotite-silicic and calcic melt interaction, which were later transformed to garnet clinopyroxenite during deep subduction of the Yangtze continent.

5.2. Origin of Dunite

There are three possible origins for dunite: (1) cumulate of a mafic melt [58], (2) reactive product of peridotite and silicon-unsaturated mafic melt [59], and (3) residue after partial melting [42].
Dunite contains relatively high concentrations of Cs, Ba, U, Pb, and LREE, and low concentrations of HREE, and therefore maybe of cumulate origin. Cumulate dunite containing high-Fo olivine and high-Cr# chromite was documented by Arai [60] and Wang et al. [61]. However, the Fo in olivine is uniform, and the relationships between NiO vs. Fo and MnO vs. Fo in olivine do not support a cumulate origin (Figure 6). Chromite inclusions in olivine are common in cumulate dunite, but this is not the case for the studied dunite. Therefore, we discount this possibility.
Dunite formed from peridotite-melt interaction commonly contains relics of orthopyroxene [62,63], and shows variable and low Fo in olivine [64]. For the studied dunite, it contains olivine with consistently high Fo, and there areno relics of orthopyroxene. In addition, dunite is large, voluminous, and free of the evidence for melt percolation in the field. Therefore, dunite is likely a residue after partial melting, which is consistent with olivine with high Fo (93.0–94.1) and chromite with high Cr# (0.75–0.96).
It is worth noting that the studied dunite shows negatively sloped chondrite-normalized REE patterns (Figure 6 and Figure 10), similar to those of Suoluoshu serpentinite and serpentine [3]. Considering olivine in residual mantle has low concentrations of REEs, we attribute such negatively sloped REE patterns of the dunite to serpentinization. Serpentinization is also responsible for the low NiO (0.11–0.29 wt.%) and MnO (≤0.1 wt.%) contents in olivine, as well as high TiO2 (up to 0.88 wt.%) in chromite, because these elements are mobile during this alteration [3]. Taken together, dunite is a residue after partial melting and overprinted by aqueous fluids metasomatism.

5.3. Tectonic Evolutions of Hujialin Garnet Clinopyroxenite and Dunite

The tectonic evolution of Hujialin garnet clinopyroxenite has been discussed by many other researchers [5,10,11,15,26,28,65]. They share a common view that Hujialin garnet clinopyroxenite underwent HP-UHP metamorphism during continental collision and subsequently exhumed to the surface with granitic gneisses, but the detailed tectonic evolution of the garnet clinopyroxenite is controversial. Based on our data, we propose the Hujialin garnet clinopyroxenite formed through at least three stages (Figure 11): (1) Clinopyroxenite was formed by interaction of peridotite with a silicic and calcic melt derived from subducted Yangtze continental crust during continental subduction. (2) Clinopyroxenite formed at shallow depths was incorporated into subduction channel and metamorphosed to garnet clinopyroxenite during deep subduction. The P-T conditions of peak metamorphism of the Hujialin garnet clinopyroxenite were estimated at P ≥ 5.0 GPa, T ≥ 750 °C [10,26], and 4.5 ± 0.5 GPa, 800 ± 50 °C [5] based on the Mg–Fe exchange of coexisting garnet and clinopyroxene. The conditions are equivalent to a depth of about 150 km. (3) Exhumation to surface occurred. During this stage, retrograded minerals such as amphibole, chlorite, and epidote formed around the garnet at shallow crustal levels (Figure 2c,f,g).
Dunite consists of chromite and olivine, and occurs with garnet clinopyroxenite, suggesting two possible paths for it. One possibility is that dunite underwent a deep subduction process. It was incorporated into the subduction channel, and exhumed to the surface later with the garnet clinopyroxenite. The lack of garnet in dunite is likely attributed to the low content of Al in the bulk rock composition, similar to the dunite in the subduction complex of the northern Dominican Republic [42]. However, we discount this possibility, because serpentine is unstable under HP to UHP conditions [3], and will breakdown, forming secondary forsterite, enstatite, or talc [66]. This is not the case for the studied samples. The dunite is slightly to moderately serpentinized along olivine boundaries and cracks, without secondary forsterite and enstatite. In addition, mineral inclusions in chromite are irregular in shape (Figure 7) and different from those formed under HP to UHP conditions, which tend to be needle-like [67]. Therefore, we propose that dunite was not subjected to deep subduction. Instead, dunite mingled with the garnet clinopyroxenite during the exhumation process of the latter at relatively shallow depths (Figure 11).

6. Conclusions

Mineral chemistry and bulk-rock composition suggest that the precursor of Hujialin garnet clinopyroxenite is clinopyroxenite that formed from the interaction of peridotite with a Si- and Ca- rich melt during subduction of the Yangtze continent beneath the NCC. Dunite is a residue that was overprinted by aqueous fluids.
Clinopyroxenite that formed at shallow depths was incorporated into a subduction channel and transformed to garnet clinopyroxenite during deep subduction. The lack of prograde and retrograde metamorphic minerals in dunite, together with the irregularly shaped mineral inclusions in chromite, indicates that dunite did not subduct to deep levels. Most likely, dunite mingled with garnet clinopyroxenite during exhumation of the latter at relatively shallow depths. These ultramafic rocks, especially hydrated peridotite, may have supplied Au to the Jiaodong Au province [68].

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min12020162/s1, Table S1: Representative composition of clinopyroxene in Hujialin garnet clinopyroxenite (wt.%); Table S2: Representative composition of garnet in Hujialin garnet clinopyroxenite (wt.%); Table S3: Repre-sentative composition of olivine in Hujialin dunite (wt.%); Table S4: Representative composition of chromite in Hujialin dunite (wt.%); Table S5: Representative composition of chlorite and epidote in Hujialin clinopyroxenite and dunite (wt.%); Table S6: Representative composition of amphibole and serpentine in Hujialin clinopyroxenite and dunite (wt.%); Table S7: Bulk rock major and minor element compositions of garnet clinopyroxenite and dunite at Hujialin.

Author Contributions

Conceptualization, J.L.; methodology, J.L.; validation, J.W. and K.H.; formalanalysis and data curation, J.L. and J.W.; project administration, J.W. and J.L.; writing—original draft, J.L.; field work, J.W. and Z.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially co-supported by the National Natural Science Foundation of China (No. 41472051) to J. Wang and the grant from the Natural Science Foundation of Shandong Province (No. ZR2021QD068) to J.G. Liu.

Data Availability Statement

Not applicable.

Acknowledgments

We thank Zhenyu Chen (Institute of Mineral Resources, Chinese Academy of Geological Sciences) for helping with EPMA analysis. We also thank to the anonymous reviewers who significantly improved the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 2. Photomicrographs of garnet pyroxenite and dunite at Hujialin in the Sulu orogen, showing mineral assemblages and characteristic textures. (a,b) Type-1 and type-2A clinopyroxenite show porphyroblastic texture. (c) Clinopyroxene occurs as inclusions in garnet and isolated grains. (d) Euhedral to anhedral clinopyroxene. (e) Clinopyroxene grains are elongated and show preferred orientation in some samples. (f) Garnet-poor clinopyroxenite composed of clinopyroxene, garnet, alteration minerals, Fe-oxides, and titanite. (g) Clinopyroxene, minor amphibole, and Fe-oxides are included in garnet. (h,i) Fe-oxides and titanite intergrowths occur in interstital spaces of other minerals. (j–l) Dunite consists of olivine and chromite. Some chromite grains show preferred orientation. Amp = amphibole; Chl = chlorite; Chr = chromite; Cpx = clinopyroxene; Ep = epidote; Grt = garnet; Ilm = ilmenite; Mag = magnitite; Ol = olivine; Srp = serpentine; Ttn = titanite. Mineral abbreviations after Whitney and Evans [30].
Figure 2. Photomicrographs of garnet pyroxenite and dunite at Hujialin in the Sulu orogen, showing mineral assemblages and characteristic textures. (a,b) Type-1 and type-2A clinopyroxenite show porphyroblastic texture. (c) Clinopyroxene occurs as inclusions in garnet and isolated grains. (d) Euhedral to anhedral clinopyroxene. (e) Clinopyroxene grains are elongated and show preferred orientation in some samples. (f) Garnet-poor clinopyroxenite composed of clinopyroxene, garnet, alteration minerals, Fe-oxides, and titanite. (g) Clinopyroxene, minor amphibole, and Fe-oxides are included in garnet. (h,i) Fe-oxides and titanite intergrowths occur in interstital spaces of other minerals. (j–l) Dunite consists of olivine and chromite. Some chromite grains show preferred orientation. Amp = amphibole; Chl = chlorite; Chr = chromite; Cpx = clinopyroxene; Ep = epidote; Grt = garnet; Ilm = ilmenite; Mag = magnitite; Ol = olivine; Srp = serpentine; Ttn = titanite. Mineral abbreviations after Whitney and Evans [30].
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Figure 7. Back-scattered electron image of chromite in dunite (a) and distribution diagrams of elements for Cr (b), Fe (c), and Mg (d) in chromite.
Figure 7. Back-scattered electron image of chromite in dunite (a) and distribution diagrams of elements for Cr (b), Fe (c), and Mg (d) in chromite.
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Figure 8. Plots of Cr/(Cr + Al) vs. Mg/(Mg + Fe2+) (a) and Cr-Al-Fe3+ (b) for chromite in dunite of Hujialin. Data sources: forearc peridotite [45], Mariana fore-arc peridotite [46], Himalayan forearc mantle peridotite [47], and sub-arc peridotite (SAP) [48]. Fields of ancient and Cenozoic SCLM beneath the North China Craton, as well as CCSD-PP1 peridotite, are based on Xie et al. [2] and references therein.
Figure 8. Plots of Cr/(Cr + Al) vs. Mg/(Mg + Fe2+) (a) and Cr-Al-Fe3+ (b) for chromite in dunite of Hujialin. Data sources: forearc peridotite [45], Mariana fore-arc peridotite [46], Himalayan forearc mantle peridotite [47], and sub-arc peridotite (SAP) [48]. Fields of ancient and Cenozoic SCLM beneath the North China Craton, as well as CCSD-PP1 peridotite, are based on Xie et al. [2] and references therein.
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Figure 9. Ca-Fe-Al-Mg variation in eclogite and garnet pyroxenite metamorphosed from cumulate pyroxenite [51]. Average composition of MORB is based on White and Klein [52].
Figure 9. Ca-Fe-Al-Mg variation in eclogite and garnet pyroxenite metamorphosed from cumulate pyroxenite [51]. Average composition of MORB is based on White and Klein [52].
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Figure 10. Primitive mantle-normalized trace elements and chondrite normalized REE for Hujialin pyroxenite and dunite in Sulu orogen. Archean cratonic mantle beneath the North China Craton [40], Suoluoshu serpentinite [3], global average MORB [52], cumulate pyroxenite [53], reactive clinopyroxene [36], and metamorphic recycled oceanic crust [54,55] are listed for comparison. Data of primitive mantle and chondrite are from McDonough and Sun [56].
Figure 10. Primitive mantle-normalized trace elements and chondrite normalized REE for Hujialin pyroxenite and dunite in Sulu orogen. Archean cratonic mantle beneath the North China Craton [40], Suoluoshu serpentinite [3], global average MORB [52], cumulate pyroxenite [53], reactive clinopyroxene [36], and metamorphic recycled oceanic crust [54,55] are listed for comparison. Data of primitive mantle and chondrite are from McDonough and Sun [56].
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Figure 11. Formation and evolution model for the garnet clinopyroxenite and dunite in Hujialin, Sulu orogen. (a) Formation of the serpentinized dunite and clinopyroxenite. (b) Clinopyroxenite was incorporated into subduction channel and metamorphosed to garnet clinopyroxenite during deep subduction. It mingled with dunite during its exhumation at shallow depths.
Figure 11. Formation and evolution model for the garnet clinopyroxenite and dunite in Hujialin, Sulu orogen. (a) Formation of the serpentinized dunite and clinopyroxenite. (b) Clinopyroxenite was incorporated into subduction channel and metamorphosed to garnet clinopyroxenite during deep subduction. It mingled with dunite during its exhumation at shallow depths.
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Liu, J.; Wang, J.; Hattori, K.; Wang, Z. Petrogenesis of Garnet Clinopyroxenite and Associated Dunite in Hujialin, Sulu Orogenic Belt, Eastern China. Minerals 2022, 12, 162. https://doi.org/10.3390/min12020162

AMA Style

Liu J, Wang J, Hattori K, Wang Z. Petrogenesis of Garnet Clinopyroxenite and Associated Dunite in Hujialin, Sulu Orogenic Belt, Eastern China. Minerals. 2022; 12(2):162. https://doi.org/10.3390/min12020162

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Liu, Jianguo, Jian Wang, Keiko Hattori, and Zeli Wang. 2022. "Petrogenesis of Garnet Clinopyroxenite and Associated Dunite in Hujialin, Sulu Orogenic Belt, Eastern China" Minerals 12, no. 2: 162. https://doi.org/10.3390/min12020162

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