Timing of Paleozoic Metamorphism in the Jiaobei Terrane, East China: Evidence from Apatite U-Pb Age and Trace Element Composition

: The northward subduction of the Paleo-Tethys oceanic crust in the Paleozoic to Mesozoic is critical for the tectonic evolution of the Qinling-Tongbai-Hong’an-Dabie-Sulu-Imjingang-Gyeonggi orogenic belt. However, the Paleozoic geological fingerprint of the Paleo-Tethys oceanic crust subduction in the Dabie-Sulu orogenic belt remains obscure. In the present study, apatite from the Muniushan monzogranite in the Houkuang area was analyzed to constrain the age of metamorphism in the Jiaobei Terrane and is regarded as the response to the Paleo-Tethys oceanic crust subduction in the Early Permian. Muniushan apatite with obvious negative Eu anomaly is enriched in LREE and depleted in HREE. The chondrite-normalized REE patterns of apatite correspond with I-type granitoids and mafic igneous rocks, implying a magmatic origin. Igneous apatite grains have reset compositional zonation in the cathodoluminescence image. Apatites plotted on a support vector machine apatite classification biplot and Eu/Y-Ce discrimination diagram shows a tendency from the region of “mafic igneous rocks and I-type granitoids” to “low and middle metamorphic”. This evidence consistently suggests that the Muniushan apatite suffered metamorphism at a later stage. The twenty-six apatite grains from the Muniushan monzogranite yield a metamorphic age of 297 ± 8 Ma (by LA-ICP-MS U-Pb), which is different from the Muniushan zircon SHRIMP U-Pb results of 2110 ± 4 Ma, indicating the metamorphism occurred in the Early Permian and reset the U-Pb system of apatite. The Early Permian metamorphism that occurred in the Jiaobei Terrane is synchronous to the subduction of the Paleo-Tethys oceanic crust and is the response to the Paleo-Tethys oceanic crust subduction.


Sample Description
One monzogranite sample was collected from the Muniushan pluton, namely 21HK01 (37 • 19 01 N, 120 • 51 58 E). The sample locations and hand specimen photographs are shown in Figures 2 and 3. Monzogranite intrudes into the Paleoproterozoic Jingshan group (Figure 3a,b). Shear joints are distributed on the surface of Muniushan pluton, which was affected by the mylonitization (Figure 3c,d). The light gray-black monzogranite samples are characterized by a medium-fine granular texture and massive structure (Figure 4a). The main minerals are quartz, plagioclase, and K-feldspar, and the secondary minerals are chlorite and sericite (Figure 4b,c). The quartz from monzogranites is subhedral to anhedral and the particle size distribution ranges from 0.1~0.6 mm (Figure 4b). The plagioclase and K-feldspar have subhedral to anhedral crystals, of which the particle size distribution ranges from 0.2 to 0.5 mm and 0.1 to 0.3 mm (Figure 4b,c). Chloritization and sericitization were observed under the microscope (Figure 4b,c). The apatite is mainly accessory mineral in the monzogranite (Figure 4d).
One monzogranite sample was collected from the Muniushan pluton, namely 21HK01 (37° 19′ 01″ N, 120° 51′ 58″ E). The sample locations and hand specimen photographs are shown in Figures 2 and 3. Monzogranite intrudes into the Paleoproterozoic Jingshan group (Figure 3a,b). Shear joints are distributed on the surface of Muniushan pluton, which was affected by the mylonitization (Figure 3c,d). The light gray-black monzogranite samples are characterized by a medium-fine granular texture and massive structure (Figure 4a). The main minerals are quartz, plagioclase, and Kfeldspar, and the secondary minerals are chlorite and sericite (Figure 4b,c). The quartz from monzogranites is subhedral to anhedral and the particle size distribution ranges from 0.1~0.6 mm (Figure 4b). The plagioclase and K-feldspar have subhedral to anhedral crystals, of which the particle size distribution ranges from 0.2 to 0.5 mm and 0.1 to 0.3 mm (Figure 4b,c). Chloritization and sericitization were observed under the microscope (Figure 4b,c). The apatite is mainly accessory mineral in the monzogranite (Figure 4d).  One monzogranite sample was collected from the Muniushan pluton, namely 21HK01 (37° 19′ 01″ N, 120° 51′ 58″ E). The sample locations and hand specimen photographs are shown in Figures 2 and 3. Monzogranite intrudes into the Paleoproterozoic Jingshan group (Figure 3a,b). Shear joints are distributed on the surface of Muniushan pluton, which was affected by the mylonitization (Figure 3c,d). The light gray-black monzogranite samples are characterized by a medium-fine granular texture and massive structure (Figure 4a). The main minerals are quartz, plagioclase, and Kfeldspar, and the secondary minerals are chlorite and sericite (Figure 4b,c). The quartz from monzogranites is subhedral to anhedral and the particle size distribution ranges from 0.1~0.6 mm (Figure 4b). The plagioclase and K-feldspar have subhedral to anhedral crystals, of which the particle size distribution ranges from 0.2 to 0.5 mm and 0.1 to 0.3 mm (Figure 4b,c). Chloritization and sericitization were observed under the microscope (Figure 4b,c). The apatite is mainly accessory mineral in the monzogranite (Figure 4d).

Apatite LA-ICP-MS U-Pb Dating
The separation of apatite grains was carried out at the Langfang Chengxin Geological Service Co., Hebei Province, China. The apatite grains were separated from the Muniushan monzogranite through coarse and fine crushing, panning, magnetic separation, and other methods. Magnetic techniques were used to separate the apatite grains from the clastic grains. In order to investigate the internal structure of the apatite, the relatively complete and transparent apatite crystals were mounted in the epoxy, polished to nearly half-section, and cleaned in an ultrasonic cleaner containing a 5% HNO 3 solution. Transmission, reflection, and CL imaging were also completed at the Langfang Chengxin Geological Service Co., Langfang, China.
The apatite U-Pb analyses were carried out using the LA-ICP-MS at the Yanduzhongshi Geological Analysis Laboratory, Beijing, China. The laser analyses were performed using a Neptune double-focusing multiple-collector ICP-MS attached to a 193 nm NWR193 Ar-F excimer laser ablation system. All apatite analyses were completed with a beam diameter of 37 µm, 10 Hz repetition rate and a 0.25 mJ/pulse power, during a 70 s analysis including 20 s measurement of gas blank, followed by ablation of the sample for approximately 40 s by raster. MAD2 [58], McClure Mt. [59], Otter Lake and Durango were used as the age standards for apatite U-Pb. Data were processed using the Isoplot 3.0 program [21,60,61].

Trace Element Analyses of Apatite
The LA-ICP-MS analyses of trace elements in apatite were carried out at the Isotopic Laboratory, Tianjin Center, China Geological Survey, Tianjin, China, using a 193 nm excimer ArF LA system and an Agilent 7900a ICP-MS. All analyses were conducted with a beam diameter of 29 µm, 8 Hz ablation frequency, and 80 mJ laser energy. In order to ensure the accuracy of the test, the NIST SRM 610 and 612 reference material glasses were used as an external reference material for trace element determinations on apatite. The internal element standard isotope was 43 Ca for apatite trace element analyses. Concordia plots and age calculations were reported at the 2σ uncertainty level and were processed using the ICPMSDataCal 11.8 program [62]. Thirty-three elements such as Ca, P, Si, Al, Fe, Mg, Mn, Sr, Y, and REE of apatite were determined in the experiment.

Morphology and Geochronology of Apatite
The 21HK01 apatite was dated by LA-ICP-MS, and the morphology and plots of data are shown in Table 1

Trace Elements of Apatites
The trace element composition and plot of apatite are shown in Table 2 and Figure 6. The total LREE (ΣLREE) ranges from 650 to 1591 ppm, and the total HREE (ΣHREE) ranges from 231 to 855 ppm. The Eu anomalies are >0.62 (Eu/Eu* = 0.62-1.02). Apatite is enriched in LREE and depleted in HREE. The contents of Sr and Y from the Muniushan

Trace Elements of Apatites
The trace element composition and plot of apatite are shown in Table 2

Trace Elements of Apatites
The trace element composition and plot of apatite are shown in Table 2

Origins of Apatite
Apatites in this study are subhedral to anhedral and show pronounced LREE enrichment and HREE depletion on their chondrite-normalized REE patterns (Figures 5a and 6). As seen in the CL images, apatite grains show a changed but homogeneous compositional zonation (with light-dark zoning) (Figure 5a) and are interpreted to be of primary magmatic origin [64,65]. The "coupled REE pattern" of Muniushan apatite also shows the similar curve of apatite in mafic igneous rocks and I-type granitoids previously published, supporting the view of magmatic origin ( Figure 6) [65][66][67][68][69][70][71]. In addition, the support vector machine apatite classification biplot and Eu/Y-Ce discrimination diagram show that Muniushan apatite has a trend toward the low-and medium-grade metamorphic and metasomatic fields that is interpreted as the influence of metamorphism at a later stage (Figure 7) [3,24,72,73]. The apatite only appears in Muniushan monzogranite, which implies that the apatite has a genetic relationship with the monzogranite. Previous studies of the geochronological data of Muniushan monzogranite show emplacement ages of ca. 2.1 Ga and metamorphic ages of ca. 1.85 Ga [74][75][76][77]. The apatite yields a U-Pb Tera-Wasserburg concordia plot with lower intercept age of 297 ± 8 Ma, which is significantly different from the zircon SHRIMP U-Pb geochronology results of the Muniushan monzogranite cluster at 2110 ± 4 Ma [77]. This may be explained by the closure temperature, at which the rock is formed and cools to a temperature where the loss path of daughter isotopes is negligible compared to the accumulation amount after radioactive timing begins [73,78]. The temperature of medium-grade metamorphism is above 550 • C, which is often higher than the apatite U-Pb closure temperature of 350-550 • C, and is easily to lead the isotopic resetting [3,[79][80][81][82]. Therefore, we present Muniushan apatite with a magmatic origin that is affected by the metamorphism later. loss path of daughter isotopes is negligible compared to the accumulation amount after radioactive timing begins [73,78]. The temperature of medium-grade metamorphism is above 550 °C, which is often higher than the apatite U-Pb closure temperature of 350-550 °C, and is easily to lead the isotopic resetting [3,[79][80][81][82]. Therefore, we present Muniushan apatite with a magmatic origin that is affected by the metamorphism later.  [83,84] (b). Abbreviations: ALK-alkali-rich igneous rocks; HM-partial melts/leucosomes/high-grade metamorphic rocks; IM-mafic I-type granitoids and mafic igneous rocks; LM-low-and mediumgrade metamorphic and metasomatic rocks; S-S-type granitoids and high aluminum saturation index (ASI) 'felsic' I-types; UM-ultramafic rocks including carbonatites, lherzolites, and pyroxenites.

Implications of the Timing of Paleozoic Metamorphism in the Jiaobei Terrane
The metamorphism of Muniushan apatite may be caused by the hydrothermal fluid from the magmatic intrusion event that occurred in the Early Permian. The apatite U-Pb age of 297 ± 8 Ma from the monzogranite samples represents the age of the magma intrusion. However, no magmatic hydrothermal events have been reported in the late Paleozoic in the Jiaobei Terrane so far. Geological fingerprints of the Paleozoic magmatic intrusion are also lacking in the Jiaobei Terrane to prove this [33][34][35]45,48]. Therefore, we suggest that the metamorphic age of Muniushan apatite may be explained by the northward subduction of the Paleo-Tethys oceanic crust. In the Carboniferous to early Mesozoic, the Paleo-Tethys oceanic crust subducted northward, and the Qinling-Tongbai-Hong'an-Dabie-Sulu (east-central China)-Imjingang-Gyeonggi (central Korea) orogenic belt was influenced by the subduction [85][86][87][88][89][90][91][92][93]. Thus, we propose that metamorphism of Muniushan apatite may be affected by the northward subduction of the Paleo-Tethys oceanic crust and is the response of the Jiaobei Terrane to the subduction event. Apatite's U-Pb age of 297 ± 8 Ma represents the age of the Early Permian subduction.

Implications of the Timing of Paleozoic Metamorphism in the Jiaobei Terrane
The metamorphism of Muniushan apatite may be caused by the hydrothermal fluid from the magmatic intrusion event that occurred in the Early Permian. The apatite U-Pb age of 297 ± 8 Ma from the monzogranite samples represents the age of the magma intrusion. However, no magmatic hydrothermal events have been reported in the late Paleozoic in the Jiaobei Terrane so far. Geological fingerprints of the Paleozoic magmatic intrusion are also lacking in the Jiaobei Terrane to prove this [33][34][35]45,48]. Therefore, we suggest that the metamorphic age of Muniushan apatite may be explained by the northward subduction of the Paleo-Tethys oceanic crust. In the Carboniferous to early Mesozoic, the Paleo-Tethys oceanic crust subducted northward, and the Qinling-Tongbai-Hong'an-Dabie-Sulu (eastcentral China)-Imjingang-Gyeonggi (central Korea) orogenic belt was influenced by the subduction [85][86][87][88][89][90][91][92][93]. Thus, we propose that metamorphism of Muniushan apatite may be affected by the northward subduction of the Paleo-Tethys oceanic crust and is the response of the Jiaobei Terrane to the subduction event. Apatite's U-Pb age of 297 ± 8 Ma represents the age of the Early Permian subduction.
Previous studies along the Qinling-Tongbai-Hong'an-Dabie-Sulu collisional belt show that the Hong'an-Dabie-Sulu collisional belt was influenced by the subduction of the Paleo-Tethys oceanic crust in the Permo-Triassic [2,17,88,94,95]. The Hong'an Block situated in the west of the Dabie-Sulu orogenic belt forms the transition between zones exposing low-pressure and ultra-high-pressure rocks and is a coherent part of the Triassic Dabie-Sulu HP-UHP Terranes [88,89]. The subduction of the Paleo-Tethys oceanic crust possibly starts at ca. 320~300 Ma, as suggested by the late Paleozoic eclogite facies metamorphism in the Huwan shear zone in the Hong'an Block [2]. Published Nd isotope studies for eclogites in the Huwan shear zone have shown that eclogites have high Nd (t) values of −1.9 to 5.8, indicating the protoliths were derived from oceanic basalts [96]. Previous zircon Hf isotope analyses from the eclogite in the Huwan shear zone exhibit a large range in εHf (t) values from −4.9 to 12.4 and young T DM age of 516 ± 48 Ma and are interpreted as the incorporation of a small quantity of older crustal materials during the protolith formation [1]. All these studies, therefore, argue for an eclogite connection of the Late Silurian. Metamorphic zircons from the eclogite in the Huwan shear zone are characterized by relatively flat REE patterns, low 176 Lu/ 177 Hf ratios (0.000004-0.000075) and slightly negative Eu anomalies (Eu/Eu * = 0.31-1.02) [2]. The early crystallization of plagioclase would reduce the Eu contents in residual melts and then influence their concentrations in zircon crystallizing from such magma [64]. Thus, it is considered that zircon was formed under eclogite facies conditions, in the presence of garnet and absence of feldspar [2]. Published 40 Ar- 39 Ar, U-Pb, Rb-Sr, and Sm-Nd ages to date the eclogite facies metamorphism in the Hong'an Block span from ca. 420-220 Ma [1,13,14,[97][98][99]. According to the in situ zircon U-Pb and mineral Lu-Hf isochron methods, the Carboniferous metamorphic ages were documented from the same samples in several outcrops from the Huwan shear zone [94,95,97,98]. The Carboniferous zircon U-Pb ages range from 315 ± 17 to 301 ± 13 Ma with a weighted mean of 309 ± 3 Ma, which is taken as the best estimated age of the eclogite facies metamorphism, starting in the Huwan shear zone [2,14]. In addition, previous simultaneous in situ trace element and Lu-Hf and U-Pb isotope analyses of zircon grains from the Xiongdian and Sujiahe eclogite in the Hong'an Block identify that the eclogite facies metamorphism occurred persistently in the Late Carboniferous to Early Permian period at ca. 306 to 283 Ma [90,91,100]. Taken together, these dates provide unambiguous evidence for the Carboniferous eclogite facies metamorphism. Previously, it has been suggested that the oceanic and continental subductions were a continuous process in the Hong'an orogenic belt, and the subduction of Paleo-Tethys oceanic crust began prior to ca. 315 Ma, reaching peak HP eclogite facies metamorphism at ca. 310 Ma, and starting exhumation at ca. 260 Ma [95]. Thus, the eclogite facies metamorphism in the Carboniferous in the Huwan shear zone is regarded as the response to the Paleo-Tethys oceanic crust subduction. Similar evidence was also found in the Ogcheon belt in South Korea [4,15,16,101]. The Ogcheon belt (south of the Gyeonggi belt), located in the east of the Dabie-Sulu orogenic belt, is often seen as the eastern continuation of the Dabie-Sulu orogenic belt [4,92]. According to the Pb-Pb whole-rock ages of slate and uraninite electron-microprobe Th-U total Pb chemical ages, the Ogcheon belt underwent the Early Permian (290-280 Ma) regional metamorphic event that was synchronous to the Paleozoic Paleo-Tethys oceanic crust subduction [4,15,16,101]. This study supplied the Early Permian metamorphic ages that are consistent with the Hong'an Block and Ogcheon belt and are recorded by the Muniushan apatite in the Jiaobei Terrane.
The late Paleozoic has been recognized as a quiet period for the Jiaobei Terrane due to the lack of Paleozoic geological fingerprints that record relevant geological information. However, detrital zircon of the Early Permian age (280 Ma) with metamorphic genetic characteristics was found in the Linsishan Formation in the Jiaolai Basin, indicating the Jiaobei Terrane may have experienced the metamorphism during the Early Permian at ca. 280 Ma [102]. The stratum of the Linsishan Formation consists of conglomerate and microconglomerate, which have the properties of nearby depositions, implying the source of metamorphic zircon is Jiaobei Terrane [103,104]. The Muniushan apatite in this study yields the Early Permian metamorphic age (297 ± 8 Ma) that is compatible with the metamorphic zircon (280 Ma) and thus is interpreted as the occurrence of the Early Permian metamorphism in the Jiaobei Terrane. The Jiaobei Terrane was in a stable uplift and denudation stage in the late Paleozoic. Here, we argue that the Early Permian metamorphism in the Jiaobei Terrane may be caused by the northward subduction of the Paleo-Tethys oceanic crust, and the geochronological result of the Hong'an Block and Ogcheon belts is an important response to the Paleo-Tethys oceanic crust subduction, which supports this point. In summary, this research proves the occurrence of the Early Permian metamorphism in the Jiaobei Terrane, which is synchronous to the northward subduction of the Paleo-Tethys oceanic crust and may be the response to the subduction.

1.
The origin of the Muniushan apatite is magmatic, the compositional characteristics being similar to the apatite in mafic igneous rocks and I-type granitoids previously published. Apatite was formed simultaneously with the Paleoproterozoic Muniushan monzogranite but suffered metamorphism later.

2.
The combination of the composition and U-Pb ages of apatite can be used to constrain the younger metamorphic event that occurred in the Jiaobei Terrane in the Early Permian. Uranium-Pb geochronology of the Muniushan apatite gives a metamorphic age of 297 ± 8 Ma, consistent with the metamorphic ages in the Hong'an Block and Ogcheon belt which represent the response of the Jiaobei Terrane to the Paleo-Tethys oceanic crust subduction in the Early Permian.