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

: 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), TiO 2 (up to 0.88 wt.%), and Na 2 O (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.


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/highpressure (LT/HP) blueschist-facies zone in the south consisting of quartzite schist, orthoand 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 ultrahighpressure 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].

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) 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.

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), Cr 2 O 3 (Cr), MnO (Mn), NiO (Ni), and NaCl (Cl). Raw data were corrected using a ZAF program. Fe 3+ and Fe 2+ contents of chromite were calculated based on stoichiometry of AB 2 O 4 .
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 HNO 3 . 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 HNO 3 , 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% HNO 3 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%.

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* Al VI /(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 (SiO 2 52.  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.  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].

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 SiO 2 (28.  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). 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].  . 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].  [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].

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 Al 2 O 3 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 Figures 9 and 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 + SiO 2 (melt 1 ) = orthopyroxene (+melt 2 ), and (2) orthopyroxene + Ca (melt 3 ) = clinopyroxene (+melt 4 ), 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.
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 (Figures 6 and 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 TiO 2 (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.

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 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).

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. 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).

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].