From Diorite to Monzogranite: The Early–Middle Jurassic Arc Magmatic Sequence of Xifengshan and Its Constraints on the Southward Subduction of the Mongol–Okhotsk Ocean

Abstract

The subduction polarity of the Mongol–Okhotsk Ocean (MOO) during the Mesozoic remains contentious, with competing models advocating for southward, northward, or bidirectional subduction. The Xifengshan area in the northern Great Xing’an Range, located south of the Mongol–Okhotsk suture, preserves Early–Middle Jurassic calc-alkaline intrusions, which provides important constraints on this debate. We present zircon U–Pb ages, whole-rock geochemistry, and Lu–Hf isotopes for diorite, granodiorite, and monzogranite from this area. Zircon U–Pb dating yields ages of 178–173 Ma, defining a short-lived magmatic pulse. The suite is calc-alkaline, enriched in LILE and depleted in Nb–Ta–Ti, typical of arc magmas. The diorite represents the most mantle-proximal preserved end-member of the system and records substantial mantle input from a slab-modified mantle wedge. Geochemical trends (increasing Rb/Th, decreasing Sr with differentiation) reflect plagioclase-dominated fractional crystallization with minor AFC. Local adakitic-like signatures are better interpreted as differentiation-related effects than as direct evidence for slab melting. Zircon εHf(t) values (+1.62 to +11.55) and T_DM1 ages (363–772 Ma) are greater than the crystallization ages, indicating substantial juvenile input together with the variable involvement of previously accreted crustal components. We suggest that mantle wedge-derived magmas modified by slab-related components triggered the partial melting of the arc crust, whereas subsequent intracrustal differentiation produced the observed intrusive sequence. The continental arc system provides robust evidence for the southeastward subduction of the MOO during the Early–Middle Jurassic, resolving the long-standing polarity controversy.

1. Introduction

The Mongol–Okhotsk Ocean (MOO) was a major Late Paleozoic–Mesozoic oceanic domain that separated the Siberian Craton from the Mongolia–North China continent, and its subduction polarity and closure history remain among the most debated issues in Northeast Asian tectonics. Competing models propose southward/southeastward subduction beneath the Mongolia–North China side, northward subduction toward the Siberian Craton, or bidirectional subduction involving opposite-dipping oceanic slabs [1,2,3,4,5,6]. The timing of final closure is equally disputed, with estimates ranging from Late Triassic to Late Jurassic, and the spatial extent of its tectonic influence on the southern suture margin remains poorly constrained [1,2,3,4,5,6]. These uncertainties hinder a unified understanding of Mesozoic plate interactions and crustal evolution in Northeast Asia.

Northeast Asia lies at the junction of the Central Asian Orogenic Belt (CAOB) and the Paleo-Pacific tectonic domain, making it a natural laboratory for investigating plate interactions and arc–continental crust accretion [7,8]. The MOO separated the Siberian Craton from the Mongolia–North China continent during the Late Paleozoic–Mesozoic, and its evolution profoundly influenced regional tectono-magmatic patterns [5,9,10,11]. Recent studies increasingly suggest that the closure of the MOO was diachronous along strike, with different segments closing at different times and possibly under different kinematic regimes [1,2,3]. Such interpretations are mainly based on the spatial–temporal distribution of arc magmatism [6,12,13,14,15,16,17,18,19,20], sedimentary and detrital zircon records [1,2,3,4], paleomagnetic reconstructions [5,6], and structural relationships along the suture zone [4,9,10,11]. Therefore, magmatic records from different sectors of the southern suture margin are essential for testing whether late-stage MOO evolution was governed by a single subduction polarity or by more complex, segment-dependent processes.

The northern Great Xing’an Range, located at the junction between the Erguna and Xing’an blocks adjacent to the southern Mongol–Okhotsk suture, is critical for evaluating MOO evolution [6,12,13,14,15,16,17,18]. Previous studies suggest that Early–Middle Jurassic (ca. 190–160 Ma) magmatism in the region relates to MOO subduction [8,19,20]. For example, Wang et al. (2024) [20] documented Early Jurassic active continental margin magmatism in the Erguna block related to southward subduction, while Deng et al. (2019) [19] reported Late Jurassic subduction-related ore-forming magmatism in the eastern Great Xing’an Range. However, existing work has focused mainly on the eastern range or central Erguna block, whereas the Xifengshan area lacks systematic petrological and geochemical petrological, geochemical, and isotopic data [18]. Consequently, it remains unclear whether a complete subduction-related magmatic arc developed in Xifengshan, what the magma source characteristics are, and how magmatism was coupled with late-stage MOO evolution.

Thus, despite recent advances in constraining MOO closure [1,2,3], a pronounced spatial gap persists in the late-stage magmatic record along the southern suture margin, particularly in the Xifengshan area, where integrated chronological–geochemical–isotopic evidence remains lacking [18]. This deficiency prevents the rigorous testing of several key questions: (1) whether Early–Middle Jurassic magmatism in Xifengshan records the terminal southward subduction of the MOO or reflects far-field Paleo-Pacific influences; (2) whether slab–mantle interactions can be recognized in the magmatic sequence (e.g., the presence of a mantle-influenced dioritic end-member and its genetic link with the associated felsic rocks); and (3) whether intermediate–felsic intrusions resulted from juvenile arc crust melting, slab melt contributions, or fractional crystallization/AFC processes and whether “adakitic” signatures should be interpreted as source features or differentiation effects [20,21,22]. Addressing these issues is important not only for establishing the magmatic arc framework of Xifengshan but also for understanding how terminal ocean closure, slab-modified mantle input, and juvenile crustal reworking interact during continental arc evolution.

In this study, we present integrated whole-rock geochemistry, zircon U–Pb geochronology, and Lu–Hf isotopic analyses for Early–Middle Jurassic intrusions from the Xifengshan area. Our objectives are to (1) constrain the timing and spatial distribution of magmatism; (2) identify geochemical evidence for slab–mantle interaction and test for the presence of a dioritic end-member; (3) constrain the source characteristics and evolutionary mechanisms of intermediate–felsic rocks and assess the origin of “adakitic” features; and (4) assess whether the Xifengshan intrusive suite records a continental arc response to the terminal southward subduction of the MOO in the context of diachronous closure models [2,3]. These results provide new petrological constraints on Early–Middle Jurassic magmatic evolution along the southern Mongol–Okhotsk suture and advance our understanding of arc–continental crust accretion in Northeast Asia [7,8].

2. Regional Geological Setting

Tectonically, the Xing’an Massif is situated in the eastern segment of the CAOB (Figure 1a). It is bounded to the north by the Erguna block across the Xinlin–Xiguite suture zone (Figure 1b) and to the south by the Songnen block, separated by the Hegenshan–Heihe suture zone [1,23]. Since the Proterozoic, the Xing’an Massif and its surrounding regions have been influenced by the Paleo-Asian Ocean, MOO, and Circum-Pacific tectonic domains, resulting in a composite evolutionary history characterized by multiple tectono-magmatic events [23,24].

The study area is located at the northeastern tip of the Xing’an Massif (Figure 1b). Stratigraphic units are well exposed and include Paleozoic Silurian, Devonian, and Carboniferous–Permian successions; Mesozoic Jurassic and Cretaceous strata; and Cenozoic Paleogene–Neogene deposits and Quaternary sediments (Figure 1c). The main Paleozoic units relevant to the Jurassic intrusive suite include the Woduhe Formation, which is composed mainly of silty slate and feldspathic lithic sandstone; the Genhe Formation, which consists mainly of siltstone, mudstone, and tuff; the Baoligaomiao Formation, composed of andesite, andesitic tuff, tuffaceous fine sandstone, and silty graywacke; the Longjiang Formation, composed of amphibole andesite, andesite, and dacite; and the Xifengshan volcanic rocks, which are dominated by andesite and andesitic volcaniclastic rocks. These units constitute the principal country rock assemblages of the Jurassic intrusions and provide the geological framework for evaluating magma emplacement and possible wall–rock interactions. Regionally, the structural framework is dominated by NE-trending structures, and both faults and igneous bodies are predominantly distributed along NE-oriented belts.

Igneous rocks are widely developed in the study area and are dominated by Mesozoic intrusions. Of these, Early Jurassic granitic magmatism was the most intense. Intrusive bodies commonly occur as batholiths and extensively intrude in the Silurian and Devonian strata. In contrast, Early Cretaceous intrusions are smaller in outcrop scale, mostly occurring as stocks and mainly intruding Paleozoic strata. The spatial–temporal distribution of these magmatic events is closely related to the evolution of the Mongol–Okhotsk tectonic domain, providing an essential regional framework for discussing the composition of the Early–Middle Jurassic magmatic arc and its constraints on the southward subduction of the MOO.

Mesozoic intrusions in the study area can be divided into two principal magmatic stages at 186–172 Ma and ~118 Ma, corresponding to the Early Jurassic and Early Cretaceous, respectively [1,2,3,4]. Early Jurassic intrusions are widespread and form a discontinuous NE-trending belt. From northwest to southeast, the lithologies comprise quartz diorite, monzogranite, granodiorite, and diorite. These intrusions generally display uniform mineral grain sizes and are dominated by medium- to medium–coarse-grained textures, indicating relatively stable crystallization conditions.

In contrast, Early Cretaceous intrusions are limited in exposure and typically intrude the earlier lithologic units. They are dominated by diorite and are characterized by overall finer grain sizes, with mainly medium- to fine-grained textures, implying pronounced differences in crystallization conditions and magmatic evolution relative to the Early Jurassic magmatic episode.

The Early Jurassic intrusive suite in the Xifengshan area is composed mainly of granodiorite, biotite granodiorite, diorite, and monzogranite. These intrusions are concentrated mainly in the central part of the study area, where they collectively define a NE–NNE-trending belt. They were emplaced into Paleozoic strata, including the Woduhe Formation, Genhe Formation, and Baoligaomiao Formation, and occur as batholiths, stocks, and dike-like bodies. The granodiorite is mainly exposed in the eastern part of the area and commonly contains dioritic enclaves, shows variable degrees of potassic alteration, and is widely cut by intermediate–mafic dikes. Diorite occurs only locally south of the Xinsheng Second Team in the central part of the study area, whereas monzogranite is mainly exposed in the northwestern part, around Xinan Village and the Huapiyao nursery area. Biotite granodiorite is widely distributed in the southwestern, central, and eastern parts of the study area and is locally cut by later granitic to dioritic dikes, indicating multistage magma emplacement and replenishment. These field relationships show that the Early Jurassic intrusions form a spatially coherent arc-related intrusive assemblage and provide an important geological framework for interpreting magma evolution in the Xifengshan area.

3. Analytical Methods

Fresh rock samples of the Early Jurassic granodiorite, biotite granodiorite, diorite, and monzogranite were collected for whole-rock geochemical analyses and zircon separation. Four samples were selected for zircon U–Pb dating and in situ Lu–Hf isotopic analyses (Figure 1). For each dated sample, five subsamples were analyzed for whole-rock geochemistry.

Approximately 20 kg of fresh material was collected for each sample. After washing with water and air-drying, the samples were crushed to 80 mesh. Zircons were separated using standard procedures, including gravity concentration, strong magnetic separation, electromagnetic separation, and alcohol-assisted fine panning. Subsequently, zircon grains were handpicked under a binocular microscope. Zircon separation, mounting, and cathodoluminescence (CL) imaging were conducted at the laboratory of the Hebei Institute of Regional Geological and Mineral Resources Survey.

Zircon U–Pb geochronology was conducted at the Isotope Laboratory of the Tianjin Center, China Geological Survey. In situ U–Pb and Lu–Hf isotopic analyses were performed using a laser ablation multi-collector inductively coupled plasma mass spectrometry system (LA-MC-ICP-MS). The MC-ICP-MS instrument was a Thermo Fisher Neptune (Thermo Fisher Scientific, Waltham, MA, USA). Operating conditions, analytical procedures, and data reduction followed the protocols described by Li et al. (2009) [25].

Whole-rock major and trace elements were analyzed at the laboratory of the Shenyang Institute of Geology and Mineral Resources, Ministry of Natural Resources. Major elements were determined by X-ray fluorescence spectrometry (XRF), with analytical precision of 1%–5%. Trace elements were measured by ICP-MS, with typical analytical uncertainties exceeding 10%.

4. Petrography

The Early Jurassic intrusive suite of the Xifengshan area comprises granodiorite, biotite granodiorite, diorite, and monzogranite. The detailed petrographic characteristics of the representative rock types are described below.

Granodiorite (TK05TW1): The rock is gray to grayish white, with a fine- to medium-grained granitic texture and a massive structure (Figure 2a,b). It is mainly composed of plagioclase (~63%), quartz (~30%), biotite (~7%), and minor alkali feldspar (~5%). Plagioclase is mostly grayish white, with subhedral tabular to broad tabular grains with grain sizes of 1.0–4.0 mm. Quartz occurs as colorless, anhedral granular grains (0.8–2.0 mm). Biotite forms brown, irregular flakes (0.8–1.5 mm). Alkali feldspar is mainly subhedral tabular to anhedral granular, dominated by microcline, with grain sizes of 1–3 mm.

Biotite granodiorite (TK06TW4): The rock is grayish white, with a fine- to medium-grained granitic texture and a massive structure (Figure 2c,d). The principal mineral assemblage comprises plagioclase (~45%), quartz (~30%), alkali feldspar (~15%), and biotite (~10%). Plagioclase occurs as euhedral to subhedral tabular grains with well-developed polysynthetic twinning (0.5–3.0 mm). Quartz is anhedral, granular, and locally shows weak undulose extinction (0.3–0.8 mm). Amphibole is yellow–green and subhedral prismatic (0.4–4.6 mm). Biotite is mostly yellow–brown and locally replaced by chlorite along grain margins (0.3–2.1 mm). Alkali feldspar is anhedral granular microcline (1–3.5 mm). Dioritic enclaves are common in this rock and typically occur as elliptical bodies of variable sizes near the margins of the intrusion, potentially reflecting unmixing/immiscibility during the evolution of the felsic magma [26].

Diorite (TK08TW1): The rock is dark gray to gray–black and shows a fine- to medium-grained granitic texture and a massive structure (Figure 2e,f). It is mainly composed of plagioclase (~52%), amphibole (~40%), and biotite (~5%). Plagioclase occurs as grayish white, subhedral, elongated tabular grains, commonly with pronounced sericitization, with grain sizes of 0.4–2.0 mm. Quartz is colorless, anhedral, and granular (0.3–1.0 mm). Biotite forms yellow–brown flakes with grain sizes of 0.2–5.2 mm. Amphibole occurs as yellow–green prismatic grains (0.6–3.6 mm).

Monzogranite (TK30TW1): The rock is light pink and exhibits a coarse–medium- to medium–fine-grained granitic texture and a massive structure (Figure 2g,h). It is mainly composed of plagioclase (~35%), alkali feldspar (~35%), quartz (~25%), and biotite (~5%). Plagioclase occurs as grayish white subhedral tabular to broad tabular grains (2.0–5.0 mm). Alkali feldspar is light pink, subhedral, and broad tabular to anhedral granular (3.0–8.0 mm). Quartz occurs as colorless, anhedral granular grains (1.5–3.0 mm). Biotite is commonly yellow–brown and occurs as irregular flakes or fine flake aggregates (1.0–2.5 mm).

In summary, the Early Jurassic intrusions in the study area comprise a granodiorite–biotite granodiorite–diorite–monzogranite assemblage. The rocks generally display fine- to medium-grained granitic textures and massive structures, with mineral assemblages dominated by plagioclase, quartz, amphibole, and biotite. Substantial variation in mafic mineral content reflects differences in magma sources and the extent of crystallization differentiation. The widespread occurrence of dioritic enclaves, intermediate–mafic dikes, and multistage dike swarms indicates repeated magma replenishment and interaction during magmatic evolution, consistent with emplacement in an arc-related intrusive system.

5. Zircon U–Pb Results

To constrain the emplacement ages of the intrusions in the Xifengshan area, LA analyzed four representative samples (granodiorite, diorite, and monzogranite) for MC-ICP-MS zircon U–Pb geochronology. Prior to isotopic analyses, CL imaging was conducted to characterize the zircon internal structures. Representative CL images are shown in Figure 3, and the U–Pb analytical results are presented in Table S1 and Figure 4.

Xifengshan Pluton (TK05TW1) is a granodiorite, collected from the eastern side of Xifengshan Township (50°33′52″ N, 126°58′32″ E). Zircons are mostly white, 100–300 μm in size, and occur mainly as short prismatic to subrounded grains, with length/width ratios of 2:1 to 1:1. CL images (Figure 3a) show well-developed, bright oscillatory zoning in most zircons, and some grains display core–mantle structures.

Twenty-five analytical spots yield Th/U ratios of 0.38–0.73. The apparent ²⁰⁶Pb/²³⁸U ages range from 173 to 180 Ma and are tightly clustered on the concordia diagram (Figure 4a). The weighted mean ²⁰⁶Pb/²³⁸U age is 176 ± 1 Ma (MSWD = 0.69). These zircons are interpreted as magmatic in origin, and the age is taken to represent the pluton’s emplacement (crystallization) age, corresponding to the Early Jurassic.

Xinsheng Erdui Pluton (TK06TW1) is a granodiorite, collected from the Xinsheng Erdui area (50°34′43″ N, 126°50′54″ E). Zircons are white, mostly subrounded to short prismatic, with length/width ratios of 2:1 to 1:1 (locally up to 3:1) and grain sizes of 200–400 μm. CL images (Figure 3b) show prominent oscillatory zoning and core–mantle structures.

Twenty-five analytical spots yield Th/U ratios of 0.30–0.66. The ²⁰⁶Pb/²³⁸U apparent ages range from 174 to 182 Ma, showing good coherence on the concordia plot (Figure 4b). The weighted mean ²⁰⁶Pb/²³⁸U age is 178 ± 1 Ma (MSWD = 0.80). The zircons are interpreted to be magmatic, and the age represents the emplacement age of the intrusion, also within the Early Jurassic.

Southern Xinsheng Erdui Pluton (TK08TW1) is a diorite, collected south of Xinsheng Erdui (50°33′52″ N, 126°50′40″ E). Zircons are white and mainly subrounded to short prismatic, with length/width ratios of 1.5:1 to 1:1 (locally up to 2:1) and sizes of 150–350 μm. CL images (Figure 3c) show clear oscillatory zoning and core–mantle structures.

Twenty-five analytical spots yield Th/U ratios of 0.30–0.91. The ²⁰⁶Pb/²³⁸U apparent ages range from 175 to 179 Ma and cluster tightly on the concordia diagram (Figure 4c). The weighted mean ²⁰⁶Pb/²³⁸U age is 177 ± 1 Ma (MSWD = 0.43). These zircons are interpreted as magmatic, and the age is regarded as the emplacement age of the diorite, belonging to the Early Jurassic.

Chengzhongshan Pluton (TK30TW1) is a monzogranite, collected from the western side of Chengzhongshan (50°38′20″ N, 126°54′40″ E). Zircons are white to pale yellowish white, with mostly short to elongate prismatic grains, with length/width ratios of 3:1 to 2:1 (locally up to 4:1) and sizes of 100–200 μm. The CL images (Figure 3d) show dense oscillatory zoning and core–mantle structures.

Thirty analytical spots from sample TK30TW1 yield Th/U ratios of 0.24–0.88. Of these, twenty-four spots define a coherent population with ²⁰⁶Pb/²³⁸U apparent ages of 169–177 Ma and show good consistency on the concordia diagram (Figure 4d). The weighted mean ²⁰⁶Pb/²³⁸U age of this main population is 173 ± 1 Ma (MSWD = 0.98), which is interpreted as the emplacement age of the monzogranite and indicates a Middle Jurassic magmatic event. In addition, six zircons yield a concordant age of 311 ± 3 Ma, significantly older than the host intrusion. These older grains are therefore interpreted as inherited or captured zircons, most likely derived from pre-existing Late Carboniferous magmatic rocks in the region and incorporated during the ascent or emplacement of the Jurassic magma [27]. Overall, the age data indicate that the Chengzhongshan monzogranite was emplaced at 173 ± 1 Ma, whereas the minor ~311 Ma zircon population records limited inheritance from earlier crustal materials rather than the crystallization age of the intrusion.

6. Whole-Rock Geochemical Characteristics

To systematically investigate the whole-rock geochemical characteristics of the Early Jurassic intrusions and to infer their tectonic setting, 20 samples representing different lithologies and localities were collected from the study area. The analytical results for major and trace elements are presented in Table S2.

6.1. Major-Element Characteristics

The Xifengshan Pluton Granodiorite (TK05TW1) is characterized by relatively high content of SiO₂ (63.28%–64.39%), Na₂O (3.71%–3.88%), K₂O (2.30%–2.39%), and Al₂O₃ (15.11%–15.61%), but low TiO₂ (0.69%–0.76%) and P₂O₅ (0.14%–0.16%). Total alkalis (ALK; Na₂O + K₂O) range from 6.02% to 6.21%, and TFeO ranges from 5.12% to 5.79%. The Rittmann index σ = 1.78–1.82, suggesting a calc-alkaline affinity. These samples have Mg# = 44–46, A/NK = 1.74–1.78, and Na₂O/K₂O = 1.56–1.68, indicating a distinct sodic character. On the (Na₂O + K₂O)–SiO₂ diagram (Figure 5a), the samples are plotted in the granodiorite field, and, on the magmatic-series discrimination diagrams, they fall within the calc-alkaline field (Figure 5b). The A/CNK ratios (0.95–1.01) indicate mainly metaluminous compositions (four samples), with one sample being weakly peraluminous (Figure 5c).

The Xinsheng Erdui Pluton Biotite Granodiorite (TK06TW4) displays relatively high SiO₂ (61.75%–62.81%), Al₂O₃ (15.96%–16.24%), and Na₂O (3.82%–3.93%), with comparatively low TiO₂ (0.73%–0.77%) and P₂O₅ (0.20%–0.23%). ALK ranges from 5.95% to 6.10% and TFeO from 5.28% to 5.59%. The Rittmann index σ = 1.84–1.95 indicates a calc-alkaline series. These rocks yield Mg# = 48–49, A/NK = 1.83–1.86, and Na₂O/K₂O = 1.78–1.82, all of which reflect a sodic affinity. On the (Na₂O + K₂O)–SiO₂ diagram (Figure 5a), most samples are plotted in the diorite–granodiorite field, and all fall within the calc-alkaline domain on the series discrimination diagrams (Figure 5b). The A/CNK ratios (0.95–0.97) indicate an overall metaluminous composition (Figure 5c).

The Southern Xinsheng Erdui Pluton Diorite (TK08TW1) is characterized by relatively low SiO₂ (51.74%–53.24%), Na₂O (2.82%–2.96%), and K₂O (1.04%–1.13%), but elevated Al₂O₃ (14.64%–15.07%), MgO (7.37%–7.79%), and TiO₂ (1.51%–1.58%), with P₂O₅ = 0.27%–0.28%. The rocks show Mg# values of 56–57, indicating a pronounced Mg-rich character. The Rittmann index σ = 1.52–1.79 suggests a calc-alkaline affinity. They also show high A/NK ratios of 2.48–2.53 and Na₂O/K₂O ratios of 2.50–2.75, reflecting a strong sodic signature. On the (Na₂O + K₂O)–SiO₂ diagram, the samples fall within the gabbro-diorite field (Figure 5a) and are plotted in the calc-alkaline field on the series discrimination diagrams (Figure 5b). Their A/CNK ratios of 0.85–0.89 indicate metaluminous compositions (Figure 5c).

The Chengzhongshan Pluton Monzogranite (TK30TW1) shows high SiO₂ (69.70%–70.53%), Na₂O (4.58%–4.74%), K₂O (2.30%–2.79%), and Al₂O₃ (15.22%–15.47%), but low MgO (0.50%–0.70%), TiO₂ (0.37%–0.39%), and P₂O₅ (0.14%–0.16%). ALK ranges from 7.04% to 7.44%, whereas TFeO is low (2.62%–2.87%). The Rittmann index σ = 1.84–2.07 indicates a calc-alkaline series (Figure 5a). The samples have Mg# = 25–34, A/NK = 1.43–1.50, and Na₂O/K₂O = 1.65–2.06, suggesting a clear sodic affinity. On the (Na₂O + K₂O)–SiO₂ diagram (Figure 5a), they are plotted within the granite field, and the A/CNK ratios of 1.11–1.14 indicate a peraluminous composition (Figure 5c).

Based on the major-element geochemical characteristics, the Xifengshan granitoid suite shows the following features. (1) SiO₂ content increases systematically with lithological evolution (Figure 5a), from 51.74 wt% in diorite to 70.53 wt% in monzogranite, although intermediate compositions are underrepresented, indicating a genetically linked but incompletely sampled differentiation sequence. (2) The suite as a whole belongs to a calc-alkaline, sodic magma series (Figure 5b). All samples cluster within the calc-alkaline field on the TFeO/MgO–SiO₂ diagram and display Na₂O/K₂O ratios of 1.56–2.75 (average ~1.96), reflecting a pronounced sodic affinity. (3) Aluminum saturation indices show a limited range, with A/CNK = 0.85–1.14, evolving from metaluminous to weakly peraluminous, suggesting a gradual increase in aluminum saturation during magma evolution (Figure 5c). (4) Major elements exhibit systematic covariation trends. With increasing SiO₂, Na₂O, ALK, and Al₂O₃, the content of MgO, total iron (TFe₂O₃), and CaO decreases progressively, indicating pronounced fractional crystallization, accompanied by evolution from a mostly mantle-proximal preserved end-member toward more felsic granitic compositions.

6.2. Trace Elements

Total REE content of granodiorite from the Xifengshan pluton (TK05TW1) (ΣREE) is overall low, ranging within (115.98–128.39) × 10⁻⁶, with an average of 121.36 × 10⁻⁶. The rocks show strong light REE (LREE)–heavy REE (HREE) fractionation, with LREE/HREE ratios of 11.38–12.06 and (La/Yb)_n values of 12.09–13.90. Eu anomalies are insignificant, with δEu = 0.89–0.91, indicating weak negative Eu anomalies.

The ΣREE content of biotite granodiorite from the Xinsheng Erdui pluton (TK06TW4) ranges within (95.34–123.89) × 10⁻⁶ (average 113.45 × 10⁻⁶). LREE–HREE fractionation is moderate, with LREE/HREE = 8.80–10.43 and (La/Yb)_n = 8.11–10.76, showing LREE-enriched patterns. (La/Sm)_n values of 2.77–3.58 indicate pronounced fractionation between LREE and middle REE (MREE), whereas (Gd/Yb)_n values of 1.43–1.55 suggest weak MREE–HREE fractionation. δEu ranges from 0.82 to 0.91, reflecting weak negative Eu anomalies.

The ΣREE content of diorite from the southern Xinsheng Erdui pluton (TK08TW1) is (116.35–120.76) × 10⁻⁶ (average 117.82 × 10⁻⁶). LREE–HREE fractionation is moderate, with LREE/HREE = 6.56–6.90 and (La/Yb)_n = 5.71–6.44, also showing LREE-enriched patterns. (La/Sm)_n values of 2.20–2.49 indicate clear LREE–MREE fractionation, whereas (Gd/Yb)_n values of 1.24–1.30 suggest weak MREE–HREE fractionation. δEu values of 0.78–0.83 indicate weak negative Eu anomalies.

The monzogranite from the Chengzhongshan pluton (TK30TW1) shows the lowest ΣREE content, ranging from 80.53 × 10⁻⁶ to 93.96 × 10⁻⁶ (average 86.95 × 10⁻⁶). LREE–HREE fractionation is pronounced, with LREE/HREE = 10.83–13.66 and (La/Yb)_n = 10.69–16.69. Eu anomalies vary more widely (δEu = 0.84–1.01), and most samples display weak negative Eu anomalies or no significant Eu anomaly, whereas a few samples exhibit slight positive Eu anomalies.

Integrating the REE and trace-element geochemical characteristics, the Early–Middle Jurassic magmatic rocks from the Xifengshan area are characterized by right-inclined chondrite-normalized REE patterns, with the relative enrichment of LREEs and depletion of HREEs (Figure 6b). Primitive mantle-normalized spider diagrams show the systematic enrichment of large-ion lithophile elements (LILE; e.g., Rb, K, Ba, Th, U, and Sr) and the pronounced depletion of high-field-strength elements (HFSEs; e.g., Nb, Ta, and Ti) (Figure 6a), which are typical of subduction-related magmatic rocks [37,38].

Eu anomalies evolve from consistently negative values in the diorite and granodiorite toward weakly negative, or locally slightly positive, anomalies in the monzogranite. This trend suggests that plagioclase-dominated fractional crystallization progressively weakened during magma evolution and that plagioclase retention and/or magma recharge may have occurred during certain stages. The marked Ti depletion is likely related to the fractionation of Fe–Ti oxides (e.g., ilmenite) during magma differentiation.

7. Zircon Lu–Hf Isotopes

In this study, in situ zircon Lu–Hf isotopic analyses were performed on the magmatic suite from the Xifengshan area. The analyzed samples included TK05TW1, TK06TW4, TK08TW1, and TK30TW1. Initial Hf isotopic ratios for zircons were corrected to the initial values using the zircon U–Pb crystallization ages of each sample. Detailed analytical results are provided in Supplementary Table S4.

The zircon Lu–Hf isotopic results indicate that samples TK05TW1, TK06TW4, TK08TW1, and TK30TW1 have broadly similar Hf isotopic compositions. Their initial zircon ¹⁷⁶Hf/¹⁷⁷Hf ratios [(¹⁷⁶Hf/¹⁷⁷Hf)_i] range from 0.282764 to 0.282905 for TK05TW1, 0.282744 to 0.282887 for TK06TW4, 0.282713 to 0.282893 for TK08TW1, and 0.282736 to 0.282993 for TK30TW1. Overall, these values define a narrow range, suggesting a broadly consistent magma source for the suite.

For TK05TW1, the zircon εHf(t = 176 Ma) values range from +3.51 to +8.50, with corresponding single-stage model ages (T_DM1) of 485–687 Ma. For TK06TW4, the zircon εHf(t = 178 Ma) values are +2.88 to +7.91, and the T_DM1 values are 512–711 Ma. For TK08TW1, the zircon εHf(t = 177 Ma) values range from +1.62 to +7.96, with T_DM1 of 522–772 Ma. For TK30TW1, the zircon εHf(t = 173 Ma) values are +2.42 to +11.55, and the T_DM1 values are 363–726 Ma.

Overall, the zircon T_DM1 models show a systematic decrease across the southern Xinsheng Erdui pluton → Xinsheng Erdui pluton → Xifengshan pluton → Chengzhongshan pluton, and, in all samples, the T_DM1 values are significantly greater than the corresponding zircon crystallization ages (Figure 7). In Figure 7 and Tables S3 and S4, the studied samples overlap closely with previously reported Early–Middle Jurassic magmatic rocks from the Erguna–Xing’an massifs (gray dots), exhibiting the common feature of T_DM1 > crystallization age, indicating that the Xifengshan Early–Middle Jurassic intrusions represent an integral component of coeval regional magmatism. The generally positive zircon εHf(t) values indicate substantial juvenile input in the magma source, whereas the older T_DM1 model ages suggest that these juvenile signatures were not derived solely from newly extracted mantle materials but likely involved previously accreted crustal components.

8. Discussion

8.1. Timing of Magmatism

Previous studies have shown that the Xing’an Massif hosts widespread Late Triassic to Early Jurassic magmatic rocks, accompanied by coeval mantle-derived mafic magmatism. These results have substantially improved our understanding of the Mesozoic tectono-magmatic evolution of the Xing’an Massif.

The Early–Middle Jurassic intrusions in the study area constitute an important component of the Mesozoic magmatic activity in the Xing’an Massif. As described above, zircons from the four analyzed samples exhibit clear magmatic characteristics, indicating that their U–Pb ages accurately reflect the crystallization ages of the host intrusions. Specifically, the granodiorite from the Xifengshan pluton (TK05TW1) yields a ²⁰⁶Pb/²³⁸U weighted mean age of 176 ± 1 Ma (n = 25), representing an Early Jurassic crystallization age. The biotite granodiorite from the Xinsheng Erdui pluton (TK06TW4) yields a ²⁰⁶Pb/²³⁸U weighted mean age of 178 ± 1 Ma (n = 25), further indicating Early Jurassic emplacement. The diorite from the southern Xinsheng Erdui pluton (TK08TW1) yields a ²⁰⁶Pb/²³⁸U weighted mean age of 177 ± 1 Ma (n = 25), suggesting Early Jurassic crystallization. The monzogranite from the Chengzhongshan pluton (TK30TW1) yields a ²⁰⁶Pb/²³⁸U weighted mean age of 173 ± 1 Ma (n = 25), representing a Middle Jurassic crystallization age.

These four intrusive lithologies are spatially clustered and temporally closely overlapping. Their systematic progression from dioritic to granitic compositions suggests a close genetic relationship, and they are best interpreted as products of a genetically linked magmatic system with compositional gaps that evolved under a broadly consistent tectonic setting during the Early–Middle Jurassic.

8.2. Petrogenesis

The Xifengshan area preserves a coherent Early–Middle Jurassic arc-related intrusive assemblage, including the granodiorite of the Xifengshan pluton, the biotite granodiorite of the Xinsheng Erdui pluton, the diorite of the southern Xinsheng Erdui pluton, and the monzogranite of the Chengzhongshan pluton. These intrusions were emplaced within a narrowly constrained time interval (~173–178 Ma) and display broadly comparable in major-element chemistry, trace-element and REE characteristics, and zircon Lu–Hf isotopic compositions, collectively implying a close petrogenetic affinity and formation within a unified tectono-thermal event.

Integrating geochemical and isotopic evidence, these intrusions are best interpreted as a genetically linked magmatic series with compositional gaps, initiated by mantle-derived mafic parental magma and subsequently modified by variable degrees of AFC [42] during ascent and differentiation. Based on the petrographic and geochemical characteristics, the arc magmatic system can be further divided into three main evolutionary stages.

However, the available whole-rock dataset does not define a fully continuous compositional spectrum. Instead, the samples cluster at ~52%–53% SiO₂ (diorite), ~63%–64% SiO₂ (granodiorite), and ~69%–70% SiO₂ (monzogranite), with intermediate compositions underrepresented. In this study, the term “evolutionary series” therefore refers to a genetically linked magmatic system with compositional gaps, rather than to a continuously sampled SiO₂ array. The coherent major- and trace-element covariation trends, common arc-like geochemical signatures, and broadly similar zircon Lu–Hf isotopic characteristics support derivation from a shared parental system modified by fractional crystallization with variable AFC. The compositional gaps may reflect the limited exposure and sampling of intermediate lithologies, the sequestration of intermediate melts as crystal mush/cumulate components at depth, and/or episodic recharge and melt extraction in open-system arc magma chambers [43,44].

The whole-rock major- and trace-element data collectively indicate that the Early–Middle Jurassic intrusive suite of the Xifengshan area formed in a subduction-related arc setting. The rocks are uniformly calc-alkaline and sodic, show systematic enrichment in LILEs and depletion in HFSEs, and define coherent geochemical variation from Mg-rich diorite to more felsic granodioritic and monzogranitic compositions. Although intermediate lithologies are underrepresented, the major-element covariation trends are consistent with a genetically linked intrusive suite that evolved through progressive magmatic differentiation. These features provide an important geochemical framework for interpreting the Xifengshan rocks as part of an Early–Middle Jurassic continental arc magmatic system related to the subduction of the MOO lithosphere [37,38].

8.2.1. Diorite

The diorite from the southern Xinsheng Erdui pluton (sample TK08TW1) is characterized by relatively low SiO₂ (51.74%–53.24%), relatively high Al₂O₃ (14.64%–15.07%), and elevated MgO (7.37%–7.79%), with Mg# values of 56–57. In addition, it contains relatively high Cr (102–108 ppm) and Ni (39–42 ppm). Overall, it represents the most mafic and most mantle-influenced lithology within the Xifengshan intrusive suite. On the K₂O–SiO₂ diagram, the samples are plotted within the calc-alkaline field.

The diorites display clear subduction-related arc geochemical signatures, including enrichment in LILEs (e.g., Rb, Ba, and Sr) and depletion in HFSEs (e.g., Nb, Ta, and Ti), indicating the involvement of slab-derived components in magma generation. Their relatively flat HREE segments ((Gd/Yb)_N ≈ 1.5–1.7) and low Yb content (~1.6 ppm) suggest residual amphibole rather than garnet in the source, whereas their relatively high Mg#, Cr, and Ni content indicates substantial mantle input. In addition, their moderate Sr/Y ratios (~20–25) and relatively high Y and Yb content do not support derivation as direct slab melts but are more consistent with melts generated from a mantle wedge modified by slab-derived materials.

Taken together, the petrographic, geochemical, and isotopic characteristics suggest that these rocks record interactions between slab-derived components and mantle wedge peridotite in a convergent margin setting. The zircon εHf(t) values range from +1.62 to +7.96, with corresponding T_DM1 model ages of 522–772 Ma, indicating substantial juvenile input in the magma source, together with variable involvement of previously accreted crustal components [7]. These diorites therefore represent the most mantle-proximal preserved end-member of the Xifengshan arc system and likely provided the thermal and material input necessary to trigger the partial melting of the overlying juvenile arc crust and the subsequent generation of more evolved felsic magmas.

8.2.2. Granodiorite

Granodiorites from the Xifengshan and Xinsheng Erdui plutons (samples TK05TW1 and TK06TW4) are characterized by relatively high SiO₂ (63.8%–64.4%) and Al₂O₃ (~15.5%), with Na₂O/K₂O ratios > 1, collectively indicating a sodic calc-alkaline affinity. Relative to the diorite, they display markedly lower MgO (2.1%–2.5%) and Mg# (44–49) and reduced Cr and Ni content, suggesting that the magma underwent significant fractional crystallization of ferromagnesian phases dominated by pyroxene and amphibole during evolution.

Their trace-element characteristics also show enrichment in LILEs and depletion in HFSEs, consistent with formation in a subduction-related tectonic setting. Their moderate Sr content (494–529 ppm) and Sr/Y ratios of ~30–35, together with relatively low Y and Yb concentrations and insignificant Eu anomalies, distinguish them from typical slab melt-derived adakites characterized by high Sr/Y [21].

The zircon εHf(t) values range from +3.51 to +8.50, with T_DM1 model ages of 485–687 Ma, broadly consistent with those of the diorite but slightly younger overall. These isotopic characteristics indicate substantial juvenile input in the magma source, while also suggesting the involvement of previously accreted crustal components. Integrating major-element, trace-element, and isotopic constraints, these granodiorites are more plausibly interpreted as intermediate products genetically linked to a mantle-influenced mafic parental magma, generated through substantial fractional crystallization during ascent, accompanied by limited crustal assimilation [45]. Their Mg# values are higher than those of melts produced solely by the partial melting of mafic sources (commonly < 45), implying continued mantle replenishment and/or magma mixing during evolution [46].

The systematic increase in SiO₂, Na₂O, total alkalis, and Al₂O₃, together with decreasing MgO, TFe₂O₃, and CaO, indicates that magma evolution was strongly influenced by fractional crystallization. In parallel, the REE and trace-element patterns show persistent LREE enrichment, weakly negative Eu anomalies, and marked Nb–Ta–Ti depletion, suggesting that crystal fractionation proceeded within a subduction-related magmatic system that retained a clear arc signature throughout its evolution [37,38]. The transition from Mg-rich diorite to granodiorite and monzogranite is therefore best interpreted as the result of differentiation from a mantle-influenced parental magma, rather than representing unrelated intrusive episodes.

8.2.3. Monzogranite

The monzogranite from the Chengzhongshan pluton (TK30TW1) exhibits the highest SiO₂ (69.7%–70.5%) and K₂O (~2.7%) and the lowest MgO (0.5–0.7%), Mg# (25–34), and Cr–Ni content, representing the most felsic end-member of the magmatic system. A pronounced negative Eu anomaly indicates strong plagioclase-dominated fractional crystallization.

Its zircon εHf(t) values vary widely (+2.42 to +11.55), and the T_DM1 model ages can be as young as ~363 Ma, distinctly younger than those of the other intrusions. These isotopic features suggest a dominantly juvenile source signature but also indicate greater source heterogeneity and the variable involvement of previously accreted crustal components during magma generation and evolution [42]. Although the monzogranite displays clear intracrustal differentiation signatures, its overall positive εHf(t) values indicate that juvenile accreted materials remained important in the source, rather than the magma being derived predominantly from an old, stabilized continental basement.

This implies the more substantial assimilation of juvenile crustal materials during its formation, consistent with an intensified AFC process. Although the monzogranite displays clear intracrustal differentiation signatures, its overall positive εHf(t) values still suggest that the ultimate source was dominated by juvenile accreted crust rather than old, stabilized continental crust.

The REE and trace-element data are fully compatible with the major-element variation trends and support a coherent differentiation sequence with compositional gaps from Mg-rich diorite to more felsic granodioritic and monzogranitic rocks. Collectively, these geochemical features suggest that magma evolution was jointly controlled by mantle-derived input, slab-related modification of the source, and subsequent differentiation processes within an arc-related magmatic system.

Overall, the Xifengshan arc records a genetically linked but incompletely preserved differentiation sequence, initiated by mantle-influenced mafic magma and evolving from a diorite end-member through sodic granodiorite to monzogranite. This sequence reflects progressively enhanced magma–crust interaction under a Middle Jurassic subduction regime.

8.3. Tectonic Setting and Petrogenetic Mechanisms of the Xifengshan Arc System

The geochemical characteristics further suggest that the Xifengshan intrusive suite was generated through variable contributions from a slab-modified mantle source, crust–mantle interaction, and subsequent magma differentiation. In particular, the Mg-rich diorite preserves the clearest evidence for mantle input, whereas the more evolved granodioritic and monzogranitic rocks record progressive intracrustal differentiation and limited assimilation. In this context, the geochemical transition across the suite reflects the combined effects of mantle-derived recharge, partial melting of the juvenile arc crust, and fractional crystallization/AFC processes during the emplacement and evolution of the arc-related magmatic system.

The Early–Middle Jurassic (~173–178 Ma) magmatic event in the Xifengshan area occurred in the northeastern part of the Xing’an Massif, immediately south of the Mongol–Okhotsk suture. Its spatiotemporal position is broadly consistent with the coeval convergence between the Siberian Craton and the Mongolia–North China blocks, as indicated by regional paleomagnetic constraints [2,47]. Considering the ongoing debate regarding subduction polarity, the short-lived, coherent suite of calc-alkaline intrusions developed on the southern side of the suture was used to evaluate constraints on the southeastward subduction of the MOO lithosphere.

The geochemical characteristics of the Xifengshan intrusive suite are consistent with generation in an active continental margin setting. Their magma sources likely involved variable contributions from a slab-modified mantle wedge, juvenile crustal materials, and crust–mantle interaction, providing an important basis for evaluating the evolution of the arc magmatic system in response to the southward subduction of the MOO lithosphere.

On the other hand, Figure 8 shows that Th/La is relatively high under low Sm/La conditions and that the data display a degree of dispersion toward an upper-crustal end-member within a limited range [48] (Figure 8). Rather than treating this feature as direct evidence for the large-scale addition of sediment melts, a more robust interpretation is that, upon a baseline dominated by a juvenile arc crust, the magmatic system was overprinted by limited inputs from subducted sediments and/or variable degrees of AFC during magma ascent and residence, thus producing a subtle shift toward an upper-crustal component [49,50]. In addition, the combination of low Lu/Hf and relatively high Th/La and Th/Yb in Figure 8c,d suggests a contribution that is more consistent with a continental crustal signature than with oceanic crust contamination [8,18]. Consequently, from a source-tracing perspective, Figure 8 constrains the Xifengshan magmatic system to be dominated by the remelting of the juvenile arc crust/lower crust, with only minor superposed contributions from subducted sediments and/or crustal contamination. This source-level inference provides a prerequisite for interpreting the system as a continental arc on the overriding plate, which is geometrically compatible with the southeastward subduction polarity of the MOO lithosphere.

Mafic magmatic enclaves are widespread in the diorites, indicating the involvement of mantle-derived magma and magma mixing during their formation [52,53]. In subduction systems, hydrous mafic magmas not only provide heat but can also deliver volatiles that depress the solidus of the lower crust and enhance the melting efficiency, thereby triggering the partial melting of the pre-existing arc crust/lower crust [54,55,56,57]. In the study area, the dioritic end-member is characterized by relatively low SiO₂ (>52%), high MgO (>5%), and the enrichment of compatible elements such as Cr and Ni, and it shows a geochemical affinity to Mg-rich andesitic rocks in subduction settings [58,59,60]. Thus, it represents the most mantle-proximal end-member of the system and plausibly acted as a “trigger” by supplying both thermal energy and volatiles to the crustal melting system.

With respect to process-based criteria, the samples define coherent differentiation arrays on the SiO₂–P₂O₅ and SiO₂–Zr diagrams (Figure 9c,d), which are more consistent with an evolutionary sequence generated within a single magmatic system than with the simple mixing of multiple end-members [43]. In addition, the decrease in Zr at high SiO₂ (>65%) (Figure 9b) reflects the onset of zircon saturation, recording intensive late-stage differentiation [61]. Eu–Rb modeling (Figure 9a) further supports melt extraction from a crystal framework and allows comparison with the evolutionary processes documented for high-silica granites from Southern Tibet and the PRB [44,62]. Therefore, Figure 9 collectively demonstrates—through three complementary lines of evidence (differentiation trajectories, zircon saturation, and melt extraction)—that the Xifengshan magmatic sequence represents a system that was initiated by mantle input and subsequently evolved continuously at the arc crust scale. This system’s behavior is consistent with a continental arc scenario involving coupling between a subduction-metasomatized mantle wedge and the overlying arc crust, thus providing process-level support for the southeastward subduction polarity of the MOO lithosphere.

The systematic covariation of Rb, Sr, and Th with SiO₂ provides direct constraints on the differentiation path. Sr decreases with increasing SiO₂, indicating the efficient removal of plagioclase (and potentially associated accessory phases), whereas Rb and Th increase rapidly with SiO₂, consistent with the enrichment of strongly incompatible elements in residual melts during crystal fractionation [55,66]. This coupled response—Sr depletion accompanied by incompatible element enrichment—is a diagnostic feature of differentiation dominated by strong fractional crystallization.

Correspondingly, the stepwise covariation among Rb, Th, and Sr (Figure 10), in combination with an increase in (La-N/Yb-N) with progressive differentiation (Figure 6), suggests that—within a framework dominated by fractional crystallization—the magmatic system was further overprinted by variable degrees of AFC, thus producing scatter along the differentiation trajectories [49,57]. The locally elevated Sr/Y and related “adakitic” features observed in the Xifengshan magmatic sequence should, therefore, be preferentially interpreted in terms of magmatic evolution processes, rather than invoking direct slab melting.

The classical “adakite” concept emphasizes control of Sr/Y by the melting of a young slab and/or high-pressure residual minerals (e.g., garnet) in the source region [21]. However, an increasing body of work has demonstrated that high Sr/Y and high La/Yb “adakitic signatures” are not unique to slab melts. They can also be generated within arc lower crust/arc–crustal magma chambers through hydrous fractional crystallization and crystal melt separation processes. The preferential partitioning of Y and HREEs into amphiboles (potentially accompanied by further Y-REE removal by accessory phases such as titanite and apatite) can drive Y (and HREEs) to decrease more rapidly than Sr—while Sr is already declining due to plagioclase crystallization—thus amplifying Sr/Y in the residual melt as a process-driven signal [67,68]. Accordingly, the relatively high Sr/Y values in this area are better explained as differentiation effects developed during the intense evolution of mantle-influenced arc magmas in mid- to shallow-crustal reservoirs, involving amphibole ± accessory-phase fractionation and superposed AFC/crystal melt segregation, rather than as evidence for a high-pressure garnet-bearing source or direct slab melting.

By integrating source tracing (Figure 8), system-scale differentiation and melt behavior (Figure 10), and covariation constraints on fractional crystallization/AFC (Figure 6 and Figure 10), a process model for the Xifengshan area can be proposed as follows. Slab-derived inputs (dehydration fluids with a minor sediment component) metasomatized the mantle wedge and promoted the generation of hydrous mafic magmas (Figure 11①) [53,69]. These magmas, represented by a dioritic end-member, which is the most mantle-proximal preserved lithology in the suite, delivered heat and volatiles into the lower crust and induced the partial melting of the arc crust/lower crust (Figure 11②) [55,57]. Although the dioritic end-member constitutes only a small proportion of the exposed rocks (<5%) at the present emplacement level and no typical ultramafic–mafic cumulates are observed in the field, its role as a trigger and recharge component does not require the preservation of large-scale cumulate bodies. Instead, it is expressed mainly through the thermal and volatile input that promoted crustal melting and subsequent evolution [62,70].

Subsequently, during crustal storage and evolution, gabbro–dioritic magmas progressively evolved toward granodioritic to monzogranitic melts through fractional crystallization accompanied by AFC (Figure 11③–④). This pathway plausibly accounts for the volumetrically dominant granodiorite–monzogranite assemblage within the batholithic system. Thus, Figure 11 integrates “juvenile arc crust dominance (Figure 8)–mantle triggering and system differentiation (Figure 10)–fractionation/AFC trajectories (Figure 6 and Figure 10)” into a coherent continental arc evolutionary record with preserved compositional gaps along the southern margin of the suture. This record is temporally consistent with regional convergence at ~173–178 Ma [2,47] and is geometrically compatible with expectations that an overriding-plate continental arc should develop south of the suture under the southeastward subduction polarity of the MOO lithosphere. Accordingly, it provides a more internally consistent constraint on the southeastward subduction of the MOO slab and the associated magmatic response in the northern segment of the Greater Xing’an Range.

In summary, the Early–Middle Jurassic (~173–178 Ma) calc-alkaline intrusive suite sequence of the Xifengshan area lies south of the Mongol–Okhotsk suture and records a temporally focused continental arc magmatic pulse, offering a critical test of the subduction polarity. The diorite is characterized by relatively high MgO–Mg#, elevated Cr–Ni, and arc-like enrichment in LILEs coupled with strong Nb–Ta–Ti depletion, indicating substantial mantle input and source modification by slab-derived components within an overriding-plate continental arc setting. Zircon εHf(t) values predominantly indicate substantial juvenile input in the magma source, whereas the corresponding T_DM1 model ages suggest that these juvenile signatures were at least partly carried by previously accreted crustal components, rather than by exclusively newly extracted mantle materials. Taken together, the Hf isotopic data argue against the dominant remelting of an old, stabilized basement and are more consistent with a source dominated by juvenile accreted materials south of the suture. Notably, locally “adakitic” Sr/Y signatures covary with strong differentiation trends (Sr decreases whereas Rb–Th increase with rising SiO₂) and are, therefore, more plausibly interpreted as process effects rather than signals of slab melting or garnet residues. Combined with its spatiotemporal context and the source process constraints, the magmatism in the Xifengshan area supports the southeastward subduction of the MOO lithosphere during the Early–Middle Jurassic and places the arc front along the northern margin of the Xing’an Massif.

9. Conclusions

The zircon U–Pb ages of the Xifengshan calc-alkaline intrusive suite, including biotite granodiorite (178 ± 1 Ma), diorite (177 ± 1 Ma), granodiorite (176 ± 1 Ma), and monzogranite (173 ± 1 Ma), indicate a short-lived Early–Middle Jurassic (~172–178 Ma) magmatic pulse. Although intermediate lithologies are underrepresented, these rocks constitute a genetically linked intrusive assemblage with compositional gaps, extending from dioritic to felsic granitic compositions.

The whole-rock geochemistry is characterized by calc-alkaline affinity, enrichment in LILEs (e.g., Rb, Th, and K), and depletion in HFSEs (e.g., Nb, Ta, and Ti), consistent with a subduction-related continental arc setting. The diorite (TK08TW1) is characterized by relatively high MgO–Mg#, elevated Cr–Ni content, and arc-like trace-element distributions, indicating substantial mantle input from a slab-modified mantle wedge rather than direct slab melting.

Systematic increases in Rb and Th and decreases in Sr with increasing SiO₂ indicate that magma evolution was dominated by plagioclase-controlled fractional crystallization, with additional contributions from amphibole and accessory-phase fractionation and variable AFC. The locally elevated Sr/Y values are therefore better interpreted as differentiation-related effects rather than as direct evidence for slab melting.

The zircon εHf(t) values (+1.62 to +11.55) indicate substantial juvenile input in the magma source, whereas the T_DM1 model ages (363–772 Ma) suggest the variable involvement of previously accreted crustal components rather than an exclusively juvenile end-member. Taken together, the Xifengshan intrusive suite records a short-lived continental arc magmatic event involving mantle wedge input, remelting of the juvenile arc crust, and intracrustal differentiation and is consistent with the southeastward subduction of the Mongol–Okhotsk Ocean during the Early–Middle Jurassic.