Zircon U-Pb-Hf Isotopes, Biotite 40 Ar/ 39 Ar Geochronology, and Whole-Rock Geochemistry of the Baogeqi Gabbro in the Northern Alxa, Southernmost Central Asian Orogenic Belt

: The ﬁnal closure time of the Paleo-Asian Ocean and the Permo-Carboniferous tectonic settings in the northern Alxa are very important but controversial tectonic issues. The geochronology and petrogenesis of maﬁc igneous rocks are superior in clarifying regional tectonic settings. Here, we report on zircon U-Pb-Hf isotopes, biotite 40 Ar/ 39 Ar geochronology and whole-rock geochemical data of the hornblende gabbro from the Baogeqi gabbro pluton in the northern Alxa. The LA-ICP MS U-Pb analysis of zircon grains from the hornblende gabbro yield a weighted mean age of 262.7 ± 2.3 Ma (2 σ , MSWD = 0.74), manifesting that the Baogeqi gabbro pluton emplacement was during the late Middle Permian (Capitanian). The 40 Ar/ 39 Ar dating of biotite grains from the hornblende gabbro yields a plateau age of 231.3 ± 1.6 Ma (2 σ , MSWD = 0.55), indicating that the Baogeqi gabbro pluton cooled to below 300 °C in the Triassic. The hornblende gabbro samples are calc-alkaline with metaluminous character, and show enrichment in large ion lithophile elements (e.g., Rb, Ba, Sr, and K) but depletion in Nb, Ta, P, Th, and Ti relative to primitive mantle. Combined with the positive zircon ε Hf(t) values (+4.9–+9.4), we suggest that the magmas formed from the partial melting of depleted mantle were metasomatized by slab-derived ﬂuids. Together with regional geology, these geochemical data suggest that the Baogeqi gabbro pluton was formed in an intracontinental extension setting, further indicating that the Paleo-Asian Ocean in the northern Alxa was closed prior to the late Middle Permian (Capitanian), and this region was in a post-collision extensional setting during the Capitanian-Late Permian. In addition, the Triassic cooling of the gabbro pluton may be a record of the decline of the Capitanian-Late Permian post-collisional extension basin due to the far-ﬁeld effect of subduction-collision during the closure of the Paleo-Tethys Ocean.


Introduction
The Central Asian Orogenic Belt (CAOB) is composed of a large number of accretionary complexes, magmatic arcs, arc-related basins, ophiolites, seamounts, and continental fragments, which witnessed the long-term subduction and closure processes of the Paleo-Asian Ocean between the Siberia and East Europe cratons to the north, and the North China and Tarim cratons to the south (Figure 1a) [1][2][3][4][5]. Although a large number of works have been carried out on the CAOB, there is still no consensus on its geological evolution since  [24]; (b) Geological map of the northern Alxa. Modified after [20].
Located at the junction between the North China Craton, Tarim Craton, and Mongolian microcontinents, the northern Alxa area is a part of the CAOB, and is therefore a key area for deciphering the evolution of the Paleo-Asian Ocean [18,23,25]. In terms of the closure time of the Paleo-Asian Ocean in the northern Alxa, there are currently three popular viewpoints: Devonian-Early Carboniferous [19,26]; late Early Permian-early Middle Permian [20,24,26], and latest Permian-Early Triassic [18,27]. Based on the closure time of the Paleo-Asian Ocean, three different perspectives are proposed regarding the Permo-Carboniferous tectonic setting: an intracontinental rift setting [19,25]; a tectonic transitional setting from subduction-collision to intracontinental extension [20,25,27], and a continuous subduction setting [18,28]. These disagreements suggest that more work is needed to clarify the closure time of the Paleo-Asian Ocean and the Permo-Carboniferous tectonic setting in the northern Alxa.
There is a consensus that mafic igneous rocks are superior in clarifying the tectonic settings of key periods due to their direct derivation from partial melting of the mantle [29][30][31]. Therefore, the Baogeqi gabbro pluton in the northern Alxa has become one of the best targets to constrain the closure time of the Paleo-Asian Ocean and the Permo-Carboniferous tectonic setting. In this contribution, we present new data on zircon U-Pb-Hf isotopes, biotite 40 Ar/ 39 Ar geochronology and whole-rock geochemistry of the hornblende gabbro from the Baogeqi gabbro pluton. Our results are used to provide new constraints  [24]; (b) Geological map of the northern Alxa. Modified after [20].
Located at the junction between the North China Craton, Tarim Craton, and Mongolian microcontinents, the northern Alxa area is a part of the CAOB, and is therefore a key area for deciphering the evolution of the Paleo-Asian Ocean [18,23,25]. In terms of the closure time of the Paleo-Asian Ocean in the northern Alxa, there are currently three popular viewpoints: Devonian-Early Carboniferous [19,26]; late Early Permian-early Middle Permian [20,24,26], and latest Permian-Early Triassic [18,27]. Based on the closure time of the Paleo-Asian Ocean, three different perspectives are proposed regarding the Permo-Carboniferous tectonic setting: an intracontinental rift setting [19,25]; a tectonic transitional setting from subduction-collision to intracontinental extension [20,25,27], and a continuous subduction setting [18,28]. These disagreements suggest that more work is needed to clarify the closure time of the Paleo-Asian Ocean and the Permo-Carboniferous tectonic setting in the northern Alxa.
There is a consensus that mafic igneous rocks are superior in clarifying the tectonic settings of key periods due to their direct derivation from partial melting of the mantle [29][30][31]. Therefore, the Baogeqi gabbro pluton in the northern Alxa has become one of the best targets to constrain the closure time of the Paleo-Asian Ocean and the Permo-Carboniferous tectonic setting. In this contribution, we present new data on zircon U-Pb-Hf isotopes, biotite 40 Ar/ 39 Ar geochronology and whole-rock geochemistry of the hornblende gabbro from the Baogeqi gabbro pluton. Our results are used to provide new constraints on the petrogenesis, emplacement, and cooling of the Baogeqi gabbro pluton in the northern Alxa, and shed light on the closure time of the Paleo-Asian Ocean and the Permo-Carboniferous tectonic setting.

Geological Background and Sample Description
The northern Alxa, situated in western Inner Mongolia of China, occupies the middle segment of the southernmost CAOB and connects the Xingmeng orogen to the east and the Beishan-Tianshan orogen to the west [4,32]. Regional geological works in the northern Alxa show that there are two ophiolite belts, i.e., Enger Us Ophiolite Belt in the north and Quagan Qulu Ophiolite Belt in the south [23]. In general, the Enger Us Ophiolite Belt is regarded as the final closure site of the Paleo-Asian Ocean, and the Quagan Qulu Ophiolite Belt is considered to be a suture related to the closure of a back-arc basin [17,23,33].
The Baogeqi gabbro pluton is located in the middle part of the Zongnaishan-Shalazhashan tectonic zone. This pluton, covering an area of approximately 160 km 2 and extending in the EW direction for over 40 km, is the largest mafic intrusion in the Zongnaishan-Shalazhashan tectonic zone ( Figure 2). The western part of the Baogeqi gabbro pluton was emplaced into the Upper Carboniferous to Lower Permian Amushan Formation (Figure 3a,b), while the east part of this pluton is in intrusive contact with later granitoid plutons. The hornblende gabbro samples are located at the western margin of the Baogeqi gabbro pluton. They have moderate-grained texture and are mainly composed of plagioclase and hornblende, with minor pyroxene and biotite (Figure 3c,d).

Geological Background and Sample Description
The northern Alxa, situated in western Inner Mongolia of China, occupies the middle segment of the southernmost CAOB and connects the Xingmeng orogen to the east and the Beishan-Tianshan orogen to the west [4,32]. Regional geological works in the northern Alxa show that there are two ophiolite belts, i.e., Enger Us Ophiolite Belt in the north and Quagan Qulu Ophiolite Belt in the south [23]. In general, the Enger Us Ophiolite Belt is regarded as the final closure site of the Paleo-Asian Ocean, and the Quagan Qulu Ophiolite Belt is considered to be a suture related to the closure of a back-arc basin [17,23,33].
The Baogeqi gabbro pluton is located in the middle part of the Zongnaishan-Shalazhashan tectonic zone. This pluton, covering an area of approximately 160 km 2 and extending in the EW direction for over 40 km, is the largest mafic intrusion in the Zongnaishan-Shalazhashan tectonic zone ( Figure 2). The western part of the Baogeqi gabbro pluton was emplaced into the Upper Carboniferous to Lower Permian Amushan Formation (Figure 3a

Analytical methods
In this study, the samples selected for zircon U-Pb-Hf isotopic, biotite 40 Ar/ 39 Ar geochronological, and whole-rock geochemical analyses were fresh and the least affected by post-magmatic weathering. Zircon grains and medium-sized (200-500 μm) biotite grains were separated using conventional heavy liquid and magnetic techniques in the Shangyi Geologic Service Co., Ltd., Langfang, Hebei Province, China. All analyses were performed at the State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, China.

Zircon U-Pb Dating and Hf Isotope Analyses
About 150 zircon grains were randomly selected and mounted in epoxy resin and polished. Cathodoluminescence (CL) images were obtained using a Quanta 400FEG environmental scanning electron microscope equipped with an Oxford energy-dispersive spectroscopy system. In situ LA-ICP-MS zircon U-Pb dating was conducted on an Agilent 7500a ICP-MS instrument equipped with a 193 nm ArF excimer laser and a homogenizing imaging optical system. A fixed spot size of 32 μm with a laser repetition rate of 6 Hz was adopted throughout this study. Standard zircon 91500 and reference material GJ-1 were used for U-Pb isotopic ratio correction. The isotopic ratios were calculated using the GLIT-TER 4.0 program (Macquarie University, Ryde, Australia) and were corrected for both instrumental mass bias, depth-dependent elemental, and isotopic fractionation. Concordia diagrams and weighted mean calculations were made using the Isoplot program (version 4.15) [37]. A detailed analytical procedure of age and trace element determinations of zircons can be found in [38].
In situ zircon Hf isotope analysis was undertaken on a Nu Plasma HR MC-ICP-MS (Nu Instrument Ltd., Wrexham, UK) equipped with a GeoLas 2005 193 nm ArF-excimer

Analytical Methods
In this study, the samples selected for zircon U-Pb-Hf isotopic, biotite 40 Ar/ 39 Ar geochronological, and whole-rock geochemical analyses were fresh and the least affected by post-magmatic weathering. Zircon grains and medium-sized (200-500 µm) biotite grains were separated using conventional heavy liquid and magnetic techniques in the Shangyi Geologic Service Co., Ltd., Langfang, Hebei Province, China. All analyses were performed at the State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, China.

Zircon U-Pb Dating and Hf Isotope Analyses
About 150 zircon grains were randomly selected and mounted in epoxy resin and polished. Cathodoluminescence (CL) images were obtained using a Quanta 400FEG environmental scanning electron microscope equipped with an Oxford energy-dispersive spectroscopy system. In situ LA-ICP-MS zircon U-Pb dating was conducted on an Agilent 7500a ICP-MS instrument equipped with a 193 nm ArF excimer laser and a homogenizing imaging optical system. A fixed spot size of 32 µm with a laser repetition rate of 6 Hz was adopted throughout this study. Standard zircon 91500 and reference material GJ-1 were used for U-Pb isotopic ratio correction. The isotopic ratios were calculated using the GLITTER 4.0 program (Macquarie University, Ryde, Australia) and were corrected for both instrumental mass bias, depth-dependent elemental, and isotopic fractionation. Concordia diagrams and weighted mean calculations were made using the Isoplot program (version 4.15) [37]. A detailed analytical procedure of age and trace element determinations of zircons can be found in [38].
In situ zircon Hf isotope analysis was undertaken on a Nu Plasma HR MC-ICP-MS (Nu Instrument Ltd., Wrexham, UK) equipped with a GeoLas 2005 193 nm ArF-excimer laser-ablation system. During analysis, a laser repetition rate of 10 Hz at 100 mJ was used and spot sizes were 44 µm. Helium was used as carrier gas. Zircon standards 91500 and Mud Tank were treated as quality control every ten unknown samples. The detailed analytical procedure was described by [39].

Biotite 40 Ar/ 39 Ar Dating
The concentrates of biotite were irradiated at the China Mianyang Research Reactor (CMRR) for 30 h with fluence monitors of ZBH-25 biotite (132.9 ± 1.3 Ma) [40]. The biotite 40 Ar/ 39 Ar dating was conducted on a multi-collector Argus VI noble gas mass spectrometer, linked to a UHV gas extraction/purification line and an ESI MIR10-30 10.6 CO 2 laser system. Mineral sample preparation and 40 Ar/ 39 Ar analytical methods were described in detail by [41]. The 40 Ar/ 39 Ar dating results were calculated and plotted using the ArArCALC software [42]. Decay constant used were those given by [43]. Correction factors for interfering isotopes were calculated from analyses of co-irradiated Ca-glass samples and K-glass samples, and are ( 40 Ar/ 39 Ar) K = 0.00232, ( 39 Ar/ 37 Ar) Ca = 0.000617, and ( 36 Ar/ 37 Ar) Ca = 0.000235.

Whole-Rock Geochemical Analyses
For geochemical analysis, fresh whole-rock samples were crushed in an agate mill to ∼200 mesh. Major element contents were determined using a Rikagu RIX 2100 Xray fluorescence (XRF) spectrometer, with analytical uncertainties lower than 5%. Trace element concentrations were analyzed by using an Agilent 7700a inductively coupled plasma mass spectrometer (ICP-MS), employing United States Geological Survey (USGS) and international rock standards (BHVO-2, AGV-2, BCR-2, and GSP-1). The analytical precision and accuracy for most of the trace elements were better than 5% [38].

Zircon U-Pb Ages and Hf Isotope Compositions
Zircon U-Pb dating results are presented in Table A1 and Figure 4. Zircon grains of the hornblende gabbro from the Baogeqi gabbro pluton are mostly subhedral-euhedral prismatic crystals, ranging in size from 140 to 300 µm with aspect ratios between 1:1 and 3:1. In CL images, they commonly exhibited fine-scale and striped absorption oscillatory zoning ( Figure 4a). Zircon trace element compositions revealed that they have variable Th and U contents of 13-2644 ppm and 58-2688 ppm, respectively, corresponding to Th/U ratios of 0.10-1.49, indicative of a magmatic origin. Twenty-five grains were selected for zircon U-Pb dating using LA-ICP-MS. Two of the analytical spots had a discordance > 10% and were rejected. The other 23 analytical spots were concordant and yielded a weighted mean 206 Pb/ 238 U age of 262.7 ± 2.3 Ma (2σ, MSWD = 0.74) (Figure 4a,b).
Fifteen grains that were previously analyzed by U-Pb methods were also analyzed for Lu-Hf isotopes on the same spot, and the results are presented in Table A2 and  Fifteen grains that were previously analyzed by U-Pb methods were also analyzed for Lu-Hf isotopes on the same spot, and the results are presented in Table A2 and

Biotite 40 Ar/ 39 Ar geochronology
The biotite 40 Ar/ 39 Ar stepwise heating results for the hornblende gabbro from the Baogeqi gabbro pluton are given in Table A3 and Figure 5. Nineteen laser heating stages were carried out for the sample, and stages 8-18 yielded a wide flat age spectrum with plateau age of 231.6 ± 1.6 Ma (2σ), corresponding to 65.14% of the total 39 Ar released and MSWD = 0.55 (Figure 5a). On the inverse 40 Ar/ 39 Ar isochron diagram, the data points defined an isochron with an age of 230.8 ± 6.9 Ma (2σ), corresponding to an initial 40 Ar/ 36 Ar ratio of 300 ± 70 and MSWD= 0.61 (Figure 5b).

Biotite 40 Ar/ 39 Ar Geochronology
The biotite 40 Ar/ 39 Ar stepwise heating results for the hornblende gabbro from the Baogeqi gabbro pluton are given in Table A3 and Figure 5. Nineteen laser heating stages were carried out for the sample, and stages 8-18 yielded a wide flat age spectrum with plateau age of 231.6 ± 1.6 Ma (2σ), corresponding to 65.14% of the total 39 Ar released and MSWD = 0.55 (Figure 5a). On the inverse 40 Ar/ 39 Ar isochron diagram, the data points defined an isochron with an age of 230.8 ± 6.9 Ma (2σ), corresponding to an initial 40 Ar/ 36 Ar ratio of 300 ± 70 and MSWD = 0.61 (Figure 5b).

Whole-rock Geochemistry
The analytical results of whole-rock major and trace element compositions are listed in  63. The total alkali-silica (TAS) diagram displays the majority of the rocks as gabbro with geochemical traits of calc-alkaline and metaluminous character ( Figure 6).

Whole-Rock Geochemistry
The analytical results of whole-rock major and trace element compositions are listed in Table A4

Whole-rock Geochemistry
The analytical results of whole-rock major and trace element compositions are listed in Table A4   The studied hornblende gabbro samples have low concentrations of Cr (7.47~17.20 ppm) and Ni (6.87~7.25 ppm). In the chondrite-normalized REE diagram (Figure 7a), they display a slight enrichment of light rare earth element (LREE) ([La:Yb] N = 3.52~4.80) and minor negative Eu anomalies (δEu = 0.90~0.91). On the primitive mantle-normalized spider diagram (Figure 7b), all samples are enriched in large ion lithophile elements (LILEs, such as Rb, Ba, Sr, and K) and depleted in high field strength elements (HFSEs, i.e., Nb, Ta, P, Th, and Ti). The studied hornblende gabbro samples have low concentrations of Cr (7.47~17.20 ppm) and Ni (6.87~7.25 ppm). In the chondrite-normalized REE diagram (Figure 7a), they display a slight enrichment of light rare earth element (LREE) ([La:Yb]N=3.52~4.80) and minor negative Eu anomalies (δEu=0.90~0.91). On the primitive mantle-normalized spider diagram (Figure 7b), all samples are enriched in large ion lithophile elements (LILEs, such as Rb, Ba, Sr, and K) and depleted in high field strength elements (HFSEs, i.e., Nb, Ta, P, Th, and Ti).

Interpretation of the Zircon U-Pb and Biotite 40 Ar/ 39 Ar Ages
The LA-ICP-MS U-Pb isotope analyses on zircon grains from the hornblende gabbro sample yielded a weighted mean 206 Pb/ 238 U age of 262.7 ± 2.3 Ma (2σ, MSWD = 0.74), which was identical to the ages of 264 ± 3 Ma and 262 ± 4 Ma previously obtained from the Baogeqi gabbro pluton [34,50]. The CL morphology and internal structures, combined with high Th/U ratios (0.10-1.49) of the zircon grains, were indicative of their magmatic origin. Thus, the 262.7 ± 2.3 Ma (2σ) was interpreted as the crystallization age of these zircon grains, suggesting that the Baogeqi gabbro was emplaced in the late Middle Permian (Capitanian).
In this study, the 40 Ar/ 39 Ar dating of biotite grains from the gabbro sample yielded a plateau age of 231.3 ± 1.6 Ma (2σ). The low MSWD values (0.55) and 65.14% of the total 39 Ar released of the studied sample suggest that this age is reliable. It is worth noting that this 40 Ar/ 39 Ar age was younger than the 262.7 ± 2.3 Ma (2σ) U-Pb zircon crystallization age from the same sample. Previous studies have shown that the closure temperature of biotite in the Ar-Ar system was roughly ~300 °C [51]. Due to the lack of significant superimposed late deformation for the Baogeqi gabbro pluton, this biotite 40 Ar/ 39 Ar age can be interpreted to represent the time of cooling below ~300 °C.

Magma Source and Petrogenesis
The samples in this study are basaltic rocks with low SiO2 contents, which suggested that their primary magmas were originated from the mantle. Zircons from the hornblende gabbro sample of the Baogeqi gabbro pluton have positive εHf(t) values from +4.9 to +9.4 and displayed an evolutionary trend between the depleted mantle and chondrite ( Figure  4c), suggesting that the primary magmas were possibly originated from a relatively depleted mantle source [52,53]. However, in the chondrite-normalized REE diagrams and primitive mantle-normalized trace-element diagrams (Figure 7), the samples show LILE enrichment (e.g. Rb, Ba, Sr, and K) and HFSE depletion (e.g. Nb, Ta, P, Th, and Ti), which are significantly different from depleted mantle-derived magmas. These characteristics

Interpretation of the Zircon U-Pb and Biotite 40 Ar/ 39 Ar Ages
The LA-ICP-MS U-Pb isotope analyses on zircon grains from the hornblende gabbro sample yielded a weighted mean 206 Pb/ 238 U age of 262.7 ± 2.3 Ma (2σ, MSWD = 0.74), which was identical to the ages of 264 ± 3 Ma and 262 ± 4 Ma previously obtained from the Baogeqi gabbro pluton [34,50]. The CL morphology and internal structures, combined with high Th/U ratios (0.10-1.49) of the zircon grains, were indicative of their magmatic origin. Thus, the 262.7 ± 2.3 Ma (2σ) was interpreted as the crystallization age of these zircon grains, suggesting that the Baogeqi gabbro was emplaced in the late Middle Permian (Capitanian).
In this study, the 40 Ar/ 39 Ar dating of biotite grains from the gabbro sample yielded a plateau age of 231.3 ± 1.6 Ma (2σ). The low MSWD values (0.55) and 65.14% of the total 39 Ar released of the studied sample suggest that this age is reliable. It is worth noting that this 40 Ar/ 39 Ar age was younger than the 262.7 ± 2.3 Ma (2σ) U-Pb zircon crystallization age from the same sample. Previous studies have shown that the closure temperature of biotite in the Ar-Ar system was roughly~300 • C [51]. Due to the lack of significant superimposed late deformation for the Baogeqi gabbro pluton, this biotite 40 Ar/ 39 Ar age can be interpreted to represent the time of cooling below~300 • C.

Magma Source and Petrogenesis
The samples in this study are basaltic rocks with low SiO 2 contents, which suggested that their primary magmas were originated from the mantle. Zircons from the hornblende gabbro sample of the Baogeqi gabbro pluton have positive εHf(t) values from +4.9 to +9.4 and displayed an evolutionary trend between the depleted mantle and chondrite (Figure 4c), suggesting that the primary magmas were possibly originated from a relatively depleted mantle source [52,53]. However, in the chondrite-normalized REE diagrams and primitive mantle-normalized trace-element diagrams (Figure 7), the samples show LILE enrichment (e.g., Rb, Ba, Sr, and K) and HFSE depletion (e.g., Nb, Ta, P, Th, and Ti), which are significantly different from depleted mantle-derived magmas. These characteristics indicate that the source of the magma was an enriched mantle or a depleted mantle influenced by crustal contamination. In addition, the samples have single-stage zircon Hf model ages (794-529 Ma) older than their crystallization ages (262.7 ± 2.3 Ma, 2σ), which also indicated that the source of the magma was enriched mantle or was influenced by crustal contamination [53].
It is generally accepted that crustal materials are rich in Zr and Hf, but depleted in Nb and Ta. Therefore, extensive crustal contamination might produce significant negative Nb and Ta anomalies, accompanied by positive Zr and Hf anomalies [54]. The studied hornblende gabbro samples were characterized by slight negative Nb and Ta anomalies and no obvious Zr and Hf anomalies, which probably implied negligible crustal contamination. Moreover, mantle-derived magmas unusually had low Lu/Yb ratios (0.14~0.15), while continental crust had relatively high Lu/Yb ratios (0.16~0.18) [49]. The studied hornblende gabbro samples had Lu/Yb ratios in the range of 0.14~0.15, which also suggested insignificant crustal contamination. In addition, the positive in situ zircon εHf (t) values (+4.9-+9.4) and absence of xenolith of these hornblende gabbro samples further ruled out the possibility of notable crustal contamination.
Generally, the fluids derived from dehydration of the altered oceanic crust or subducted sediment and melts of the oceanic slab or subducted sediment are usually enriched in LILEs and depleted in HFSEs [55]. Thus, the geochemical features of the studied hornblende gabbro samples suggested that the parent magma was modified by subductionrelated fluids or melts. In the La/Nb vs. La/Ba diagram (Figure 8a), the studied hornblende gabbro samples show low La/Ba ratios with high La/Nb values, which indicated a tendency for subduction-modified lithospheric mantle. Furthermore, the studied samples plot near E-MORE but deviate from the MORB-OIB evolution line in the Nb/Yb vs. Th/Yb diagram (Figure 8b), which also suggested the influence of subduction components. Trace element ratios can be used to differentiate between fluid or melt-related enrichments during subduction [56]. In the Nb/Zr vs. Th/Zr and Nb/Zr vs. Ce/Y diagrams (Figure 8c,d), the studied samples show a tendency for fluid-related enrichment, which indicated that the magma source was likely modified by subduction-related fluids.
The hornblende gabbro samples of the Baogeqi gabbro pluton had lower Mg#, Cr, and Ni contents than the primary magma of basaltic melts (Mg# > 73; Cr > 1000 ppm, and Ni > 400 ppm), which probably indicated fractionation of olivine and pyroxene during the magma evolution [57]. In addition, the studied samples showed no obvious negative anomaly of Eu and Sr, which implied insignificant plagioclase fractionation.
Collectively, the hornblende gabbro of the Baogeqi gabbro pluton may be derived from the partial melting of depleted mantle metasomatized by slab-derived fluids, and subsequently experienced significant fractionation of olivine and pyroxene, but insignificant crustal contamination during magma evolution.

Tectonic Implications
The Permo-Carboniferous tectonic setting in the northern Alxa has been debated with various models including an intracontinental rift setting, a subduction setting or a tectonic transitional setting from subduction-collision to intracontinental extension [18][19][20][21]. Although our data from the hornblende gabbro samples of the Baogeqi gabbro pluton had some subduction imprints, they showed relatively high TiO2 and Nb content, which was markedly different from those of volcanic arc mafic rocks in subduction settings [61]. In the Zr vs. Zr/Y and Nb×2-Zr/4-Y diagrams (Figure 9), the studied samples all fall in the field of within-plate setting, which implied a continental rift setting for the Baogeqi gabbro. In addition, the Baogeqi gabbro and synchronous extensive granites together formed a bimodal plutonic suite in the northern Alxa [34], which also supported an extensional setting during the late Middle Permian times. This interpretation is also supported by the regional geology. Magmatism, sedimentology, and provenance studies showed that the Paleo-Asian Ocean in the northern Alxa had been closed at the end of the Early Permian, and the late Middle Permian had entered a post-collisional extension stage [20,25,27,34]. Therefore, the Paleo-Asian Ocean in the northern Alxa was closed prior to the late Middle Permian (Capitanian), and this region stepped into a post-collisional extension basin evolutionary stage during the Capitanian-Late Permian.

Tectonic Implications
The Permo-Carboniferous tectonic setting in the northern Alxa has been debated with various models including an intracontinental rift setting, a subduction setting or a tectonic transitional setting from subduction-collision to intracontinental extension [18][19][20][21]. Although our data from the hornblende gabbro samples of the Baogeqi gabbro pluton had some subduction imprints, they showed relatively high TiO 2 and Nb content, which was markedly different from those of volcanic arc mafic rocks in subduction settings [61]. In the Zr vs. Zr/Y and Nb×2-Zr/4-Y diagrams (Figure 9), the studied samples all fall in the field of within-plate setting, which implied a continental rift setting for the Baogeqi gabbro. In addition, the Baogeqi gabbro and synchronous extensive granites together formed a bimodal plutonic suite in the northern Alxa [34], which also supported an extensional setting during the late Middle Permian times. This interpretation is also supported by the regional geology. Magmatism, sedimentology, and provenance studies showed that the Paleo-Asian Ocean in the northern Alxa had been closed at the end of the Early Permian, and the late Middle Permian had entered a post-collisional extension stage [20,25,27,34]. Therefore, the Paleo-Asian Ocean in the northern Alxa was closed prior to the late Middle Permian (Capitanian), and this region stepped into a post-collisional extension basin evolutionary stage during the Capitanian-Late Permian. The biotite 40 Ar/ 39 Ar age of the hornblende gabbro from the Baogeqi gabbro pluton with 231.3 ± 1.6 Ma (2σ) suggested that there was a cooling and exhumation event in the northern Alxa during Triassic times. In fact, the Triassic cooling and exhumation event was also documented in the adjacent area of the Baogeqi gabbro based on 40 Ar/ 39 Ar dating, apatite U-Pb, and fission track thermochronology [64,65]. The stratigraphic contact relationship of the northern Alxa also suggested that this region experienced a cooling and exhumation event during the Triassic times, characterized by high-angle unconformity between the strongly deformed Paleozoic strata and gently deformed Mesozoic strata. From the viewpoint of regional tectonics, the Paleo-Tethys Ocean in the southern North China-Tarim craton was closed during the Triassic [32,[66][67][68][69][70]. Therefore, the Triassic cooling and exhumation event in the northern Alxa may be a record of the decline of the Capitanian-Late Permian post-collisional extension basin due to the far-field effect of subduction-collision during the closure of the Paleo-Tethys Ocean.

Conclusions
(1) The hornblende gabbro from the Baogeqi gabbro pluton in the northern Alxa yielded a LA-ICP MS zircon U-Pb age of 262.7 ± 2.3 Ma (2σ), manifesting that the Baogeqi gabbro pluton shaping was during the late Middle Permian (Capitanian). The 40 Ar/ 39 Ar dating of biotite grains from the Baogeqi gabbro pluton yielded a plateau age of 231.3 ± 1.6 Ma (2σ), which suggested that the pluton cooled to below 300 °C in the Triassic.
(2) The Baogeqi gabbro pluton formed as a result of the partial melting of depleted mantle metasomatized by slab-derived fluids, and subsequently experienced significant fractionation of olivine and pyroxene, but insignificant crustal contamination during magma evolution.
(3) The Baogeqi gabbro pluton intruded in an intracontinental extension setting, which implied that the Paleo-Asian Ocean in the northern Alxa was closed prior to the late Middle Permian (Capitanian), and this region stepped into a post-collisional extension basin evolutionary stage during the Capitanian-Late Permian. The Triassic cooling of the gabbro pluton may be a record of the decline of the Capitanian-Late Permian postcollisional extension basin due to the far-field effect of subduction-collision during the closure of the Paleo-Tethys Ocean.  The biotite 40 Ar/ 39 Ar age of the hornblende gabbro from the Baogeqi gabbro pluton with 231.3 ± 1.6 Ma (2σ) suggested that there was a cooling and exhumation event in the northern Alxa during Triassic times. In fact, the Triassic cooling and exhumation event was also documented in the adjacent area of the Baogeqi gabbro based on 40 Ar/ 39 Ar dating, apatite U-Pb, and fission track thermochronology [64,65]. The stratigraphic contact relationship of the northern Alxa also suggested that this region experienced a cooling and exhumation event during the Triassic times, characterized by high-angle unconformity between the strongly deformed Paleozoic strata and gently deformed Mesozoic strata. From the viewpoint of regional tectonics, the Paleo-Tethys Ocean in the southern North China-Tarim craton was closed during the Triassic [32,[66][67][68][69][70]. Therefore, the Triassic cooling and exhumation event in the northern Alxa may be a record of the decline of the Capitanian-Late Permian post-collisional extension basin due to the far-field effect of subduction-collision during the closure of the Paleo-Tethys Ocean.

Conclusions
(1) The hornblende gabbro from the Baogeqi gabbro pluton in the northern Alxa yielded a LA-ICP MS zircon U-Pb age of 262.7 ± 2.3 Ma (2σ), manifesting that the Baogeqi gabbro pluton shaping was during the late Middle Permian (Capitanian). The 40 Ar/ 39 Ar dating of biotite grains from the Baogeqi gabbro pluton yielded a plateau age of 231.3 ± 1.6 Ma (2σ), which suggested that the pluton cooled to below 300 • C in the Triassic.
(2) The Baogeqi gabbro pluton formed as a result of the partial melting of depleted mantle metasomatized by slab-derived fluids, and subsequently experienced significant fractionation of olivine and pyroxene, but insignificant crustal contamination during magma evolution.
(3) The Baogeqi gabbro pluton intruded in an intracontinental extension setting, which implied that the Paleo-Asian Ocean in the northern Alxa was closed prior to the late Middle Permian (Capitanian), and this region stepped into a post-collisional extension basin evolutionary stage during the Capitanian-Late Permian. The Triassic cooling of the gabbro pluton may be a record of the decline of the Capitanian-Late Permian post-collisional extension basin due to the far-field effect of subduction-collision during the closure of the Paleo-Tethys Ocean.