Discovery of Late Permian Adakite in Eastern Central Asian Orogenic Belt: Implications for Tectonic Evolution of Paleo-Asian Ocean

: The Permian to Triassic period represents a pivotal phase in the evolution of the Paleo-Asian Ocean, marked by significant tectonic transitions from subduction, collision, and post-orogenic extension. The timing of closure of the Paleo-Asian Ocean in northeastern China has always been controversial. In this contribution, the petrology, zircon U-Pb geochronology and geochemistry are conducted on granite found in well HFD1, Songliao Basin, eastern part of Central Asian orogenic belt. Zircon U-Pb dating indicates that granite crystallized at 258.9 ± 2.2 Ma, as the product of magmatism occurred in the early Late Permian. The rocks have high SiO 2 , Al 2 O 3 , Na 2 O content, negative Eu anomaly, light enrichment of rare-earth elements, depletion of heavy rare-earth elements, high Sr (448.29–533.11 ppm, average 499.68 ppm), low Yb (0.49–0.59 ppm, average 0.54 ppm), Y (4.23–5.19 ppm, average 4.49 ppm), and high Sr/Y ratios (98–125, average 112) and can be classified as O-type adakite. This is the first discovery of late Paleozoic adakite in the Songliao Basin and the neighboring areas. The geochemistry of adakite indicates derivation by partial melting of MORB-type subducted oceanic crust, indicating that the subduction of the Paleo-Asian Oceanic lithosphere lasted until at least 258.9 Ma.

The eastern segment of the CAOB contains multiple micro-blocks with Precambrian ancient basement, including the Erguna, the Xing'an, the Xilinhot-Songliao, and the Jiamusi micro-blocks, which amalgamated before the latest Paleozoic.These micro-blocks collided with the North China Block at the end of the late Paleozoic, resulting in the closure of the Paleo-Asian Ocean and initiating a post-collisional extensional orogenic stage in northeastern China (Figure 1) [6,[10][11][12]. in the closure of the Paleo-Asian Ocean and initiating a post-collisional extensional orogenic stage in northeastern China (Figure 1) [6,[10][11][12].
Late Permian is a crucial period for the transition from the Paleo-Asian Oceanic tectonic domain to the Circum-Pacific tectonic domain [29,33].The Songliao Basin is a large oil and gas basin in China, but research on its pre-Mesozoic tectonic evolution and deep magma events is scarce, particularly regarding Late Paleozoic magma events [34][35][36].Gao et al. [35] studied granites in the southern part of the Songliao Basin, suggesting that Middle Jurassic magmatic activity formed the majority of the basement granites.Yu et al. [36] studied volcanic rocks in the northern part of the Songliao Basin, confirming multiple magmatic thermal events from the Paleozoic to the Mesozoic, with the Jurassic and Triassic volcanic activities being the most frequent, which might be related to the collision of Northeast blocks and the collision and suturing of the North China Block and Siberia Block at the end of the Permian.However, due to the limited exposure of Figure 1.Tectonic sketch of Eurasia continent (modified from [13]).
Late Permian is a crucial period for the transition from the Paleo-Asian Oceanic tectonic domain to the Circum-Pacific tectonic domain [29,33].The Songliao Basin is a large oil and gas basin in China, but research on its pre-Mesozoic tectonic evolution and deep magma events is scarce, particularly regarding Late Paleozoic magma events [34][35][36].Gao et al. [35] studied granites in the southern part of the Songliao Basin, suggesting that Middle Jurassic magmatic activity formed the majority of the basement granites.Yu et al. [36] studied volcanic rocks in the northern part of the Songliao Basin, confirming multiple magmatic thermal events from the Paleozoic to the Mesozoic, with the Jurassic and Triassic volcanic activities being the most frequent, which might be related to the collision of Northeast blocks and the collision and suturing of the North China Block and Siberia Block at the end of the Permian.However, due to the limited exposure of bedrock in the Songliao Basin and the challenges in obtaining samples and data from deep within the basin, previous research on Late Paleozoic magmatic and tectonic evolution has focused mainly on the peripheral outcrop areas.Studies on the Songliao Basin, particularly isotopic and geochemical studies, are lacking.Therefore, this study aims to investigate intrusive rocks in the HFD1 core from the Songliao Basin to determine their precise ages and discuss their petrogenesis and dynamic background, to understand the connection between the magmatism and evolution of the Paleo-Asian Ocean in the Songliao Basin.[8]).The red star represents the geochronological sample, and the blue circles represent geochemical samples.

Samples
One sample was used for isotopic analyses and seven for major and trace element analyses in this study.The lithology of both the dating samples and the rock geochemical analysis samples is granite.The rocks are light gray in color, exhibit subhedral to anhedral granular textures, and display blocky textures.The intrusive bodies form vein-like intrusions into the Permian Linxi Formation clastic rocks (Figure 2b).

Samples
One sample was used for isotopic analyses and seven for major and trace element analyses in this study.The lithology of both the dating samples and the rock geochemical analysis samples is granite.The rocks are light gray in color, exhibit subhedral to anhedral granular textures, and display blocky textures.The intrusive bodies form vein-like intrusions into the Permian Linxi Formation clastic rocks (Figure 2b).
Microscopic examination of the samples reveals that the rocks are primarily composed of plagioclase, K-feldspar, quartz, biotite, and opaque minerals.Plagioclase exhibits subhedral to euhedral tabular shapes, with visible polysynthetic twinning and albite perthitic twinning.It often displays zoning structures and undergoes slight sericitization and chloritization alteration, with more intense alteration towards the center.The grain size ranges from 0.20 to 2.00 mm, comprising approximately 45% of the sample.K-feldspar mostly exhibits poor euhedral development, appearing as anhedral grains with interstitial distributions.Some display euhedral to subhedral tabular shapes, with visible Carlsbad twinning and slight sericitization and chloritization.It forms micrographic textures with quartz, with particle sizes ranging from 0.06 to 1.25 mm, constituting approximately 30% of the sample.Quartz appears as anhedral grains, with clean surfaces and interstitial distributions.The grain size ranges from 0.03 to 0.30 mm, comprising approximately 15% of the sample.Biotite occurs as elongated or platy shapes, exhibiting brown to light brown pleochroism, parallel extinction, and third-order interference colors.It undergoes chloritization alteration, with particle sizes ranging from 0.06 to 0.25 mm, constituting approximately 5% of the sample.Opaque minerals are black in color, irregular in shape, mostly occurring as interstitial grains, with some appearing as anhedral grains, constituting less than 5% of the sample (Figure 3).ference colors.It undergoes chloritization alteration, with particle sizes ranging from 0.06 to 0.25 mm, constituting approximately 5% of the sample.Opaque minerals are black in color, irregular in shape, mostly occurring as interstitial grains, with some appearing as anhedral grains, constituting less than 5% of the sample (Figure 3).

Zircon Geochronology
Zircon grains were extracted from the samples by the Langfang Chengxin Geological Service Company, China, employing a combined magnetic and heavy liquid separation technique.Subsequently, these grains underwent examination under reflected and

Zircon Geochronology
Zircon grains were extracted from the samples by the Langfang Chengxin Geological Service Company, China, employing a combined magnetic and heavy liquid separation technique.Subsequently, these grains underwent examination under reflected and transmitted light using an optical microscope.Further analysis involved obtaining cathodoluminescence (CL) images utilizing a CL spectrometer (Garton Mono CL 3+ ) (AMETEK, Berwyn, PA, USA) installed on a Quanta 200F scanning electron microscope (SEM) (FEI company, Hillsboro, OR, USA) at Peking University.Operating parameters included an accelerating voltage of 15 kV, a beam current of 120 nA, and a scan time of 2 min.Based on the CL images, distinct domains within the grains were selected for U-Pb analysis.
The U-Pb geochronological analyses of the zircon grains were conducted using a quadrupole ICP-MS (Agilent 7500a) (Agilent Technologies, Santa Clara, CA, USA) equipped with a UP 193 solid-state laser (New Wave™ Research, Fremont, CA, USA) at the Geologic Laboratory Center, China University of Geosciences, Beijing.Laser parameters included a laser energy density of 8.5 J/cm 2 , a repetition rate of 10 Hz, and a beam diameter of 36 µm.Ablation material was transported to the ICP-MS using high-purity He gas at a flux of 0.8 L/min.The 91,500 zircon served as the external standard, while the NIST 610 standard silicate glass was employed for instrument optimization.Additionally, the TEMORA zircon standard from Australia [39] and the Qinghu zircon standard from China [40] were utilized as secondary standards to monitor instrument drift.
Data analysis was performed using the GLITTER software (version 4.4, Macquarie University) to calculate isotopic ratios and element contents.Age calculations and concordia plots were generated using Isoplot 3.0 [41].Common Pb correction was carried out following the Andersen method [42].

Whole-Rock Geochemistry
Major oxide compositions were determined using a PANalytical Axios Advance (PW4400) X-ray fluorescence (XRF) spectrometer (Malvern Panalytical, Worcestershire, United Kingdom) at the Northeast China Supervision and Inspection Center of Mineral Resources.Loss on ignition was assessed by heating 1 g of powder to 1100 • C for 1 h.The analysis of major oxides was conducted on fused glass with a precision of less than 2%.
Trace element compositions were analyzed using a PerkinElmer SCIEX ELAN 6000 ICP-MS (PerkinElmer, Massachusetts, the USA) at the same facility.For this analysis, 50 mg of each powdered sample was dissolved in a mixture of HNO 3 + HF in a high-pressure Teflon bomb for 48 h at 190 • C [43].Rhodium (Rh) was utilized as an internal standard to calibrate instrumental drift during counting, while the international standard GBPG-1 was employed for quality control.The results of measurements of the GBPG-1 [44] and OU-6 [45] international standards were found to be in agreement with the recommended values.Overall, the precision of all elements was less than 5%.

Zircon U-Pb Results
As observed in the CL image (Figure 4a), the zircons exhibit subhedral to euhedral shapes, with most retaining the original crystal morphology of magmatic crystalline zircons.They are predominantly elongated or short prismatic, with grain sizes mainly concentrated in the range 80-150 µm.The aspect ratio of length to width is mostly 2:1, and zircons display clear magmatic oscillatory zoning.Some zircons show varying degrees of fragmentation, with Th/U ratios ranging from 0.32 to 1.02 (Table 1).These characteristics indicate that the zircons in the samples are of magmatic origin.shapes, with most retaining the original crystal morphology of magmatic crystalline zircons.They are predominantly elongated or short prismatic, with grain sizes mainly concentrated in the range 80-150 µm.The aspect ratio of length to width is mostly 2:1, and zircons display clear magmatic oscillatory zoning.Some zircons show varying degrees of fragmentation, with Th/U ratios ranging from 0.32 to 1.02 (Table 1).These characteristics indicate that the zircons in the samples are of magmatic origin.The analytical results from all 24 analytical spots fall on or near the concordia line, showing a relatively concentrated distribution (Figure 4b).Only one analytical point shows an older age (spot 7, 366 ± 8 Ma), likely representing inherited zircon ages.The 206 Pb/ 238 U weighted mean age of the remaining 23 analysis points is 258.9 ± 2.2 Ma (MSWD = 0.18, n = 23) (Figure 4b).Combining the above analyses, it can be concluded that the age of 258.9 ± 2.2 Ma represents the crystallization age of the magmatic rocks, indicating that the intrusion occurred during the early Late Permian.

Whole-Rock Geochemistry Major Oxides
The results of major and trace element analyses for the granite samples are shown in Table 2.The SiO 2 content of the samples ranges from 70.38 wt.% to 72.23 wt.%, with an average of 71.11 wt.%, indicating acidic composition.On the TAS diagram (Figure 5a), the samples fall within the granite field.The Al 2 O 3 content ranges from 15.38 wt.% to 15.71 wt.%, with an average of 15.54 wt.%.The MgO content varies from 0.77 wt.% to 1.09 wt.%, with an average of 0.92 wt.%.The CaO content ranges from 2.36 wt.% to 2.78 wt.%.The total alkali content ranges from 6.43 wt.% to 7.56 wt.%, with an average of 7.35 wt.%, indicating a distinct enrichment of Na 2 O over K 2 O and a Na 2 O/K 2 O ratio ranging from 1.89 to 2.53.The rocks are classified within the calc-alkaline series (Figure 5b).The average CaO content is 2.54%.The aluminum saturation index A/CNK ranges from 0.97 to 1.14, with an average of 1.02, while the A/NK ratio ranges from 1.38 to 1.69, with an average of 1.46.These data indicate that the rocks are peraluminous and have relatively high alkali content.
The granite samples from the study area exhibit overall light rare-earth element (LREE) enrichment and heavy rare-earth element (HREE) depletion on the standardized distribution diagrams (Figure 6a,b).The content of rare-earth elements (ΣREE) ranges from 64.39 ppm to 69.28 ppm, with an average of 67.13 ppm.The total mass fraction of LREE (ΣLREE) ranges from 60.42 ppm to 65.16 ppm, with an average of 62.98 ppm.The total mass fraction of HREE (ΣHREE) ranges from 3.96 ppm to 4.29 ppm, with an average of 4.14 ppm.The LREE/HREE ratio ranges from 14.92 to 15.78, with an average of 15.20, indicating fractionation between LREE and HREE, with LREE enrichment.The standardized rareearth element distribution curves exhibit similar right-skewed shapes, with (La/Yb) N ranging from 21.07 to 23.76, with an average of 21.29.The δCe values range from 0.90 to 0.94, with an average of 0.92, showing no significant anomalies.The δEu values range from 1.29 to 1.54, with an average of 1.40, indicating a noticeable positive anomaly, suggesting that there was no significant plagioclase crystallization separation during magma crystallization, distinguishing it from the upper continental crust characterized by significant negative Eu anomalies.
15.71 wt.%, with an average of 15.54 wt.%.The MgO content varies from 0.77 wt.% to 1.09 wt.%, with an average of 0.92 wt.%.The CaO content ranges from 2.36 wt.% to 2.78 wt.%.The total alkali content ranges from 6.43 wt.% to 7.56 wt.%, with an average of 7.35 wt.%, indicating a distinct enrichment of Na2O over K2O and a Na2O/K2O ratio ranging from 1.89 to 2.53.The rocks are classified within the calc-alkaline series (Figure 5b).The average CaO content is 2.54%.The aluminum saturation index A/CNK ranges from 0.97 to 1.14, with an average of 1.02, while the A/NK ratio ranges from 1.38 to 1.69, with an average of 1.46.These data indicate that the rocks are peraluminous and have relatively high alkali content.

Crystallization Age
In this study, U-Pb zircon dating analysis was conducted on granite samples from well HFD1 in the northern Songliao Basin.The sampled rocks are preserved and unmetamorphosed, with zircons displaying clear magmatic oscillatory zoning.Addition-  [52,53]).Discrimination diagrams of high silicon and low silicon adakites (c-f) (base map after [54]).[48,49].LOI: loss ion ignition.The data marked with "*" are from [46,47].

Crystallization Age
In this study, U-Pb zircon dating analysis was conducted on granite samples from well HFD1 in the northern Songliao Basin.The sampled rocks are preserved and unmetamorphosed, with zircons displaying clear magmatic oscillatory zoning.Additionally, the geochemical characteristics of the rocks confirm their magmatic origin.All 23 data points obtained from the samples fall on the U-Pb concordia line, representing the age of magmatic intrusion and crystallization (Figure 4b).Therefore, the age of the granite in the study area is determined to be 258.9± 2.2 Ma (MSWD = 0.18, n = 23), indicating an early Late Permian intrusive event.
Considering the above studies, the formation of adakites can be summarized into three main types: (1) partial melting of subducted oceanic slab; (2) basaltic intrusion and partial melting of mantle or lower crust ultramafic and mafic rocks; (3) crystallization under highpressure conditions in the initial arc magma of island arc settings [67,68].The classification of adakites indicates that O-type adakites and High-SiO 2 adakites share similar chemical characteristics, both representing Na-enriched oceanic crust-derived adakitic rocks, consistent with the original definition of adakites, referred to as "typical adakites" [54].
The granite in the northern part of the Songliao Basin has high SiO 2 and Al 2 O 3 contents, characteristic of Na-rich and K-poor features.The high Na 2 O/K 2 O ratio suggests a possible origin from low-K tholeiitic primary magmas, often formed in island arc or active continental margin environments, mainly related to the melting of oceanic crust.The rock has high Sr content but low Yb and Y, displaying typical geochemical characteristics of adakites.Additionally, the samples exhibit high Sr/Y ratios (98-125, average 112) and La/Yb ratios (average 21.3), consistent with typical features of adakite.Thus, the granite studied here is considered adakitic rock (Figure 7a,b).
From the harker diagrams, it can be observed that the adakite samples in this study were mostly located in the area of subducted oceanic crust melting and thickened lower crust melting (Figure 8).However, with relative Na enrichment and K depletion, an average SiO 2 content of 71.11 wt.%, low MgO content, low Th content (4.73-6.29 ppm), and low Th/Ce ratios, the samples exhibit LREE enrichment, HREE depletion, and lack of negative Eu anomalies, indicating a distinct difference from C-type adakites formed by partial melting of mafic rocks in the lower crust.C-type adakites are characterized by significant K enrichment.In the Cr-Ni diagram, the samples display low Cr and Ni contents, reflecting characteristics of slab melting (Figure 9a), while also exhibiting characteristics of O-type adakite.
The intrusive rocks studied in this research exhibit highly consistent patterns in their rare-earth elements and trace elements, showing pronounced light-heavy rare-earth fractionation, which distinguishes them from island arc calc-alkaline series rocks [68].Analysis using La-(La/Yb) N and Sm-La/Sm diagrams similarly reveals significant partial melting features in the studied adakitic rocks, without significant crystal fractionation, indicating they are not products of primary basaltic magma separation (Figure 9b,c).Empirical diagrams, including K-Rb, Sr-(CaO + Na 2 O), K/Rb-SiO 2 /MgO, and Cr/Ni-TiO 2 , for highsilica and low-silica adakitic rock classification (Figure 7c-f), consistently categorize the rock samples as high-silica adakitic rocks.Based on these analyses, we conclude that the granite studied here represents typical adakitic rock, exhibiting characteristics of O-type adakitic and SiO 2 -rich adakite rocks.
The granite in the northern part of the Songliao Basin has high SiO2 and Al2O3 contents, characteristic of Na-rich and K-poor features.The high Na2O/K2O ratio suggests a possible origin from low-K tholeiitic primary magmas, often formed in island arc or active continental margin environments, mainly related to the melting of oceanic crust.The rock has high Sr content but low Yb and Y, displaying typical geochemical characteristics of adakites.Additionally, the samples exhibit high Sr/Y ratios (98-125, average 112) and La/Yb ratios (average 21.3), consistent with typical features of adakite.Thus, the granite studied here is considered adakitic rock (Figure 7a,b).
From the harker diagrams, it can be observed that the adakite samples in this study were mostly located in the area of subducted oceanic crust melting and thickened lower crust melting (Figure 8).However, with relative Na enrichment and K depletion, an average SiO2 content of 71.11 wt.%, low MgO content, low Th content (4.73-6.29 ppm), and low Th/Ce ratios, the samples exhibit LREE enrichment, HREE depletion, and lack of negative Eu anomalies, indicating a distinct difference from C-type adakites formed by partial melting of mafic rocks in the lower crust.C-type adakites are characterized by significant K enrichment.In the Cr-Ni diagram, the samples display low Cr and Ni contents, reflecting characteristics of slab melting (Figure 9a), while also exhibiting characteristics of O-type adakite.The intrusive rocks studied in this research exhibit highly consistent patterns in their rare-earth elements and trace elements, showing pronounced light-heavy rare-earth fractionation, which distinguishes them from island arc calc-alkaline series rocks [68].Analysis using La-(La/Yb)N and Sm-La/Sm diagrams similarly reveals significant partial melting features in the studied adakitic rocks, without significant crystal fractionation, indicating they are not products of primary basaltic magma separation (Figure 9b,c).Empirical diagrams, including K-Rb, Sr-(CaO + Na2O), K/Rb-SiO2/MgO, and Cr/Ni-TiO2, for high-silica and low-silica adakitic rock classification (Figure 7c-f), consistently categorize the rock samples as high-silica adakitic rocks.Based on these analyses, we conclude that the granite studied here represents typical adakitic rock, exhibiting characteristics of O-type adakitic and SiO2-rich adakite rocks.

Petrogenesis
The O-type adakitic rocks, enriched in Na, are primarily associated with subduction processes of tectonic plates, partly related to the partial melting of low-K mantle plume basalts or thickened lower crust with oceanic crust characteristics [71].From the original mantle trace element spider diagram (Figure 6b), the granite is enriched in large ion lithophile elements and depleted in high field strength elements, with relative enrichments in Th, Sr, Zr, and U, resembling features of adakitic rocks formed by subducted oceanic crust melting [72][73][74][75].The ΣREE of the samples is relatively low, showing sig-

Petrogenesis
The O-type adakitic rocks, enriched in Na, are primarily associated with subduction processes of tectonic plates, partly related to the partial melting of low-K mantle plume basalts or thickened lower crust with oceanic crust characteristics [71].From the original mantle trace element spider diagram (Figure 6b), the granite is enriched in large ion lithophile elements and depleted in high field strength elements, with relative enrichments in Th, Sr, Zr, and U, resembling features of adakitic rocks formed by subducted oceanic crust melting [72][73][74][75].The ΣREE of the samples is relatively low, showing significant fractionation with enrichment in LREE and depletion in HREE, along with parallel distribution curves, reflecting characteristics of magmatic evolution from a single source.The presence of a significant positive Eu anomaly indicates minimal or absent strong or multistage crystallization differentiation processes, consistent with features reflected in Sm-La/Sm and La-(La/Yb)N diagrams (Figure 9b,c), further confirming the granite as an O-type adakitic rock.
According to Zhang et al. [71], Na-enriched adakitic rocks have three main genetic types: partial melting of subducted plate MORB, composition of oceanic crust, and lower crustal intrusion of low-K basalt.As previously described, the geochemical attributes of the rocks suggest that the Na enrichment is attributed to partial melting of subducted plate MORB, indicating that the granite was generated from the melting of subducted plate MORB during the late stage of oceanic subduction.Based on adakitic rock discrimination diagrams such as SiO 2 -Yb and SiO 2 -TiO 2 (Figure 8), the samples mostly fall within the adakitic rock zone formed by subducted oceanic crust melting, further supporting the model of subducted oceanic crustal plate melting.
Experimental studies indicate that the formation of O-type adakitic rocks requires high pressure.Rapp et al. [76] reported the results of basaltic melting experiments at different pressure stages ranging from 0.8 to 3.2 GPa.With increasing pressure, the residual phases also change systematically.Peacock et al. [77] suggested that adakitic rock formation occurs within a narrow pressure range (1.8-2.3GPa, equivalent to depths of 60-70 km), with residual phases consisting of garnet + clinopyroxene + amphibole.Therefore, O-type adakitic rocks are typically interpreted as being formed by partial melting of subducted oceanic crust at depths of 75-85 km (corresponding to the garnet amphibolite-eclogite transition zone) [52,77,78].
In the Y-Sr/Y diagram (Figure 7a), all adakitic rock sample points are located between the garnet amphibolite and garnet amphibolite-garnet clinopyroxenite evolution lines, indicating that the source minerals of the adakitic rocks (granite) in this study are related to garnet and clinopyroxene.This reveals that the original rocks of the studied adakitic rocks should be garnet amphibolite or amphibolite metamorphic rock series [79], and Zhang and Jiao.[63] also pointed out that the solid solution in equilibrium with garnet and eclogite is a fundamental attribute of adakitic rocks.The granite has Y/Yb ratios of 7.94-8.79and (Ho/Yb) N ratios of 0.97-1.05,with right-skewed HREE curves, indicating the presence of roughly equal proportions of garnet and clinopyroxene in the residual phase, with depleted Y, Yb, and HREE, suggesting the stable presence of garnet during partial melting and unstable presence of clinopyroxene in the source region, and almost no or little plagioclase in the residual minerals, indicating magma crystallization [52,71].Therefore, it is inferred that the Late Permian adakitic rocks originated from oceanic crustal plates undergoing garnet amphibolite metamorphism during late Paleozoic subduction to the lower crust.As the oceanic crustal plate subducted deeper into the subduction zone, partial melting occurred in the garnet amphibolite metamorphic zone, generating adakitic magmas, which then ascended and intruded to form the adakitic rocks.

Tectonic Implications
Adakitic rocks have been regarded as a special rock type with significant tectonic implications since their proposal [52], and were believed to be products of plate melting in a subduction context.The genesis of adakitic rocks represents the tectonic environment in which they formed, providing unequivocal evidence for crustal subduction and plate convergence processes.The geochemical characteristics and genesis of the granite in this study indicate that the rock was formed by partial melting of MORB-type subducted oceanic crust.Combined with previous research [8,9,11,80], it is generally believed that the Paleo-Asian Ocean closed from the Late Permian to the Early-Middle Triassic, indicating that the adakitic rocks formed during the Early Late Permian were derived from melting of subducted slabs at depth, implying that in the Early Late Permian, the Paleo-Asian Ocean in the study area had not closed and was likely in the late stage of subduction.
Combined with data from adjacent areas, this study provides a comprehensive constraint on the tectonic evolution of the Paleo-Asian Ocean.The Paleo-Asian Ocean has experienced the process from subduction, collision to post-closure crustal extension in the study area.Each process has different rock records.The author studied Triassic granites developed in the Horqin Righ Middle Banner and Ulanhot areas of Inner Mongolia (Figure 10a; [46,47]), and conducted a comprehensive discussion on the evolutionary process of the Paleo-Asian Ocean.The age of the granites in Horqin Right Middle Banner is 242.6 Ma, indicating their emplacement during the Early-Middle Triassic.The geochemical characteristics suggest they are S-type granites (Figure 6a,b), indicating their formation in a regionally compressed tectonic background and thickened crust caused by collisional orogenic activity, recording the collisional amalgamation time of the Paleo-Asian Ocean in this area (Figure 10b) [47].Reports on Triassic magmatic events in the Linxi area in the southern part of the study area indicate the development of intrusions at Linxi and Shuangjing, with ages of 237.2 ± 2.7 Ma and 229.2 ± 4.1 Ma, respectively.These granites are likely derived from thickened crust in orogenic belts and re-melting of relatively old continental margins, suggesting the beginning of the collisional amalgamation of the Paleo-Asian Ocean in the mid-Permian and its end in the mid-Triassic [11].In the Ulanhot region, there are occurrences of biotite monzogranite with a crystallization age of 214. 4 Ma and an emplacement age in the middle-late Triassic [46].These rocks exhibit strong negative Eu anomalies and significant Sr depletion, characteristic of low-Sr, high-Yb granite (Figure 6a,b).This granite is interpreted as an A2-type post-orogenic granite formed in a post-orogenic extensional tectonic setting (Figure 10b), indicating the conclusion of collision during the Middle Triassic and the onset of post-orogenic extension in the late Triassic [46,47].Ge et al. [81] reported Triassic granites in the Ulanhot region (235-225 Ma), associating them with lithospheric extension after the closure of the Paleo-Asian Ocean.Li et al. [11] proposed that magmatism in the northern margin of the North China Block and adjacent areas occurred in two stages during the Early-Mid and Late Triassic, with late Triassic intrusive rocks around 220 Ma reflecting regional extensional tectonics related to late-stage orogenic evolution, signifying the termination of orogenesis and the beginning of a new tectonic evolution stage.
Combining the regional research findings with our results and previous publications [46,47], we infer that the O-type adakite (granite) formed in the early Late Permian (approximately 258.9 Ma) represents the final stage of subduction of the Paleo-Asian Oceanic slab, formed by partial melting of the subducted Paleo-Asian Ocean crustal slab In the Ulanhot region, there are occurrences of biotite monzogranite with a crystallization age of 214. 4 Ma and an emplacement age in the middle-late Triassic [46].These rocks exhibit strong negative Eu anomalies and significant Sr depletion, characteristic of low-Sr, high-Yb granite (Figure 6a,b).This granite is interpreted as an A2-type post-orogenic granite formed in a post-orogenic extensional tectonic setting (Figure 10b), indicating the conclusion of collision during the Middle Triassic and the onset of post-orogenic extension in the late Triassic [46,47].Ge et al. [81] reported Triassic granites in the Ulanhot region (235-225 Ma), associating them with lithospheric extension after the closure of the Paleo-Asian Ocean.Li et al. [11] proposed that magmatism in the northern margin of the North China Block and adjacent areas occurred in two stages during the Early-Mid and Late Triassic, with late Triassic intrusive rocks around 220 Ma reflecting regional extensional tectonics related to late-stage orogenic evolution, signifying the termination of orogenesis and the beginning of a new tectonic evolution stage.
Combining the regional research findings with our results and previous publications [46,47], we infer that the O-type adakite (granite) formed in the early Late Permian (approximately 258.9 Ma) represents the final stage of subduction of the Paleo-Asian Oceanic slab, formed by partial melting of the subducted Paleo-Asian Ocean crustal slab at specific depths (under high-pressure conditions).This indicates that the Paleo-Asian Ocean had not closed by the early Late Permian and represents the terminal stage of subduction (Figure 11).

1.
The U-Pb age of adakite is 258.9 ± 2.2 Ma, representing the timing of emplacement and crystallization, indicating the magmatism in the early Late Permian.

2.
The adakite is characteristic of typical O-type adakite.The adakite is enriched in light rare-earth elements with a relatively consistent parallel flat pattern of heavy rare-earth elements, reflecting characteristics of magmatic evolution from a single source and slab melting.

3.
We propose that the adakite is derived from the melting of subducted MORB-type ocean crust during the subduction of the Paleo-Asian Oceanic slab.This indicates that in the early Late Permian in northeastern China, the Paleo-Asian Ocean had not closed, and oceanic subduction continued until 258.9 ± 2.2 Ma.In the early Late Permian (258.9Ma), the Paleo-Asian Ocean was still in the subduction phase, forming typical O-type adakite under high-pressure conditions.

Figure 2 .
Figure 2. (a) Geological map of the Songliao Basin and adjacent region.(b) Borehole HFD1 comprehensive histogram (modified from [8]).The red star represents the geochronological sample, and the blue circles represent geochemical samples.

Figure 2 .
Figure 2. (a) Geological map of the Songliao Basin and adjacent region.(b) Borehole HFD1 comprehensive histogram (modified from [8]).The red star represents the geochronological sample, and the blue circles represent geochemical samples.

Figure 3 .
Figure 3. (a) Core photographs and (b,c) microscopic photographs of the granite (2088.3TWS).Pl refers to Plagioclase, Kfs refers to Orthoclase, Qz refers to Quartz, Bt refers to Biotite.

Figure 3 .
Figure 3. (a) Core photographs and (b,c) microscopic photographs of the granite (2088.3TWS).Pl refers to Plagioclase, Kfs refers to Orthoclase, Qz refers to Quartz, Bt refers to Biotite.

Figure 4 .
Figure 4. (a) CL images and 206 Pb/ 238 U ages of the granite.(b) Zircon U-Pb concordia diagram of the granite.Red circles indicate analytical spots for dating.Figure 4. (a) CL images and 206 Pb/ 238 U ages of the granite.(b) Zircon U-Pb concordia diagram of the granite.Red circles indicate analytical spots for dating.

Figure 4 .
Figure 4. (a) CL images and 206 Pb/ 238 U ages of the granite.(b) Zircon U-Pb concordia diagram of the granite.Red circles indicate analytical spots for dating.Figure 4. (a) CL images and 206 Pb/ 238 U ages of the granite.(b) Zircon U-Pb concordia diagram of the granite.Red circles indicate analytical spots for dating.

Figure 5 .
Figure 5. TAS (a) and K2O-SiO2 (b) diagrams of the granite.The granite samples from the study area exhibit overall light rare-earth element (LREE) enrichment and heavy rare-earth element (HREE) depletion on the standardized distribution diagrams (Figure 6a,b).The content of rare-earth elements (ΣREE) ranges from 64.39 ppm to 69.28 ppm, with an average of 67.13 ppm.The total mass fraction of LREE (ΣLREE) ranges from 60.42 ppm to 65.16 ppm, with an average of 62.98 ppm.The total mass fraction of HREE (ΣHREE) ranges from 3.96 ppm to 4.29 ppm, with an average of 4.14 ppm.The LREE/HREE ratio ranges from 14.92 to 15.78, with an average of 15.20, indicating fractionation between LREE and HREE, with LREE enrichment.The standardized rare-earth element distribution curves exhibit similar right-skewed shapes, with (La/Yb)N ranging from 21.07 to 23.76, with an average of 21.29.The δCe values range from 0.90 to 0.94, with an average of 0.92, showing no significant anomalies.The δEu values

Figure 6 .
Figure 6.Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element spidergrams of granite (b).The samples represented by colored legends are from this study, while those represented by gray legends are from previously published samples [46,47].

Figure 6 . 19 Figure 7 .
Figure 6.Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element spidergrams of granite (b).The samples represented by colored legends are from this study, while those represented by gray legends are from previously published samples [46,47].On the normalized trace element diagrams (Figure 6a,b), the granite exhibits significant enrichment in large ion lithophile elements (LILEs) such as Rb, Pb, Sr, and Ba, while showing depletion in high field strength elements (HFSEs) including Nb, Ta, Ti, Zr, and Ce.The Sr mass fraction in the granite ranges from 448.29 ppm to 533.11 ppm, with an average of 499.68 ppm, exceeding 400 ppm.The Yb mass fraction ranges from 0.49 ppm to 0.59 ppm, with an average of 0.54 ppm, below 2 ppm, while Y ranges from 4.23 ppm to 5.19 ppm, with an average of 4.49 ppm.These characteristics classify the rock as high-Sr, low-Yb granite, indicative of formation under significant pressure [50,51].On the Sr/Y-Y and (La/Yb) N -Yb N diagrams (Figure 7a,b), all samples points fall within the adakitic rock field.Minerals 2024, 14, x FOR PEER REVIEW 11 of 19
Minerals 2024, 14, x FOR PEER REVIEW 15 of 19 sional amalgamation of the Paleo-Asian Ocean in the mid-Permian and its end in the mid-Triassic [11].

19 Figure 11 .
Figure 11.Tectonic evolution model of the study area during Late Permian.

1 .
The U-Pb age of adakite is 258.9 ± 2.2 Ma, representing the timing of emplacement and crystallization, indicating the magmatism in the early Late Permian.2. The adakite is characteristic of typical O-type adakite.The adakite is enriched in light rare-earth elements with a relatively consistent parallel flat pattern of heavy rare-earth elements, reflecting characteristics of magmatic evolution from a single source and slab melting.3. We propose that the adakite is derived from the melting of subducted MORB-type ocean crust during the subduction of the Paleo-Asian Oceanic slab.This indicates that in the early Late Permian in northeastern China, the Paleo-Asian Ocean had not closed, and oceanic subduction continued until 258.9 ± 2.2 Ma.In the early Late Permian (258.9Ma), the Paleo-Asian Ocean was still in the subduction phase, forming typical O-type adakite under high-pressure conditions.

Figure 11 .
Figure 11.Tectonic evolution model of the study area during Late Permian.

Table 1 .
The U-Pb isotopic composition of the granite.

Table 2 .
Major and trace elemental compositions of magmatic rocks in northeastern China (wt.% for major oxides and ppm for trace elements).

Table 2 .
Major and trace elemental compositions of magmatic rocks in northeastern China (wt.% for major oxides and ppm for trace elements).