Geochronology, Geochemistry, and Tectonic Significance of Early Carboniferous Volcanic Rocks from the Ulanhot Region in the Central Great Xing’an Range

Abstract

The attributes of Late Paleozoic magmatic events are of paramount significance in elucidating the tectonic evolution of the Ulanhot region, which is located in the middle of the Hegenshan–Heihe tectonic belt (HHTB). This study undertook a comprehensive investigation of the petrography, LA–ICP–MS zircon U–Pb dating, whole rock geochemistry, and zircon Hf isotopes of the Early Carboniferous volcanic rocks. The volcanic rocks are predominantly composed of andesite, schist (which protolith is rhyolitic tuff), and rhyolitic tuff. The results of zircon U–Pb dating reveal that the formation ages of volcanic rocks are Early Carboniferous (343–347.4 Ma). Geochemical characteristics indicate that the andesites possess a comparatively elevated concentration of Al₂O₃, alongside diminished levels of MgO and TiO₂, belonging to the high-K calc-alkaline series. The zircon εHf(t) of the andesites range from −13 to 9.4, while the two-stage Hf model ages span from 697 to 1937 Ma. The felsic volcanic rocks have high contents of SiO₂ and Na₂O + K₂O, low contents of MgO and TiO₂, and belong to high-K to normal calc-alkaline series. The zircon εHf(t) values of the felsic volcanic rocks range from −12.8 to 10, while the two-stage Hf model ages span from 693 to 2158 Ma. The Early Carboniferous volcanic rocks exhibit a notable enrichment in large ion lithophile elements (LILEs, such as Rb, K, Ba) and light rare earth elements (LREEs), depletion in high-field-strength elements (HFSEs, including Nb, Ta, Ti, Hf), as well as heavy rare earth elements (HREEs). The distribution patterns of the rare earth elements (REEs) demonstrate a conspicuous right-leaning tendency, accompanied by weak negative Eu anomalies. These characteristics indicate that the andesites represent products of multistage mixing and interaction between crustal and mantle materials in a subduction zone setting. The felsic volcanic rocks originated from the partial melting of crustal materials. Early Carboniferous igneous rocks formed in a volcanic arc setting are characteristic of an active continental margin. The identification of Early Carboniferous arc volcanic rocks in the Central Great Xing’an Range suggests that this region was under the subduction background of the oceanic plate subduction before the collision and amalgamation of the Erguna–Xing’an Block and the Songnen Block in the Early Carboniferous.

1. Introduction

The Central Asian orogenic belt (CAOB), commonly referred to as the Altai orogenic belt, is one of the subduction–accretionary orogenic belts with the longest development history and the most significant crustal accretion and transformation. This region is sandwiched between the Siberian Plate, the North China Plate, and the Tarim Plate [1,2,3,4], which has always been a concern (Figure 1a). The northeast region of China is situated within the eastern expanse of the CAOB, commonly referred to as the Xingmeng Orogenic Belt (XOB) [5,6]. The composition encompasses a series of microlandmasses, subduction–accretionary wedges, island arcs, ophiolites, and magmatic belts. The genesis and evolution of this region were predominantly governed by the tectonic influences of the Paleo–Asian Ocean, the Mongol–Okhotsk Ocean, and the Paleo–Pacific tectonic domain in Phanerozoic [7,8,9,10].

The northeast region of China is located in the eastern part of the CAOB, which can be divided into the Erguna Block, Xing‘an Block, and Songnen Block, progressing from north to south [11,12], of which the Erguna Block and the Xingan Block were bounded by the Xinlin–Xiguitu suture belt and collaged in the Early Paleozoic (500 Ma) [5,12,13], the boundary between the Erguna–Xing’an Block and the Songnen Block was the Hegenshan–Heihe tectonic belt (HHTB) [14,15,16,17]. Currently, there is controversy regarding the extension location and formation period of the HHTB. The southern part of this belt is named after the study of ophiolite in the Hegenshan area [18,19,20], and the northern part of this belt is mainly determined based on the studies of the Duobaoshan island arc, the Xinkailing Group, granitic mylonite, and the Nenjiang Complex [15,21]. However, for the middle section of the HHTB, due to the strong vegetation covering and the relatively little research on magma, structure, and paleogeographic environment, there are still many disputes about the extension location of the HHTB.

Regarding the distribution of paleo-suture zones and the amalgamation history of tectonic blocks in Northeast China, four main academic viewpoints have been proposed. The first suggests that the Hegenshan ophiolite suture zone predominantly extends eastward along the Hegenshan-Baicheng-Jilin-Yanji axis, ultimately being buried beneath Mesozoic-Cenozoic sedimentary strata of the Songliao Basin [6,22]. The second proposes an A-type granite belt stretching from East Junggar through Inner Mongolia to Northeast China, trending along the Solon Mountain-Hegenshan-Jarud Banner-Nenjiang-Heihe line, representing a post-orogenic paleo-sture zone [13,23]. The third viewpoint, based on integrated geophysical and geochemical evidence, suggests the existence of two independent blocks (Erguna–Xing’an and Songnen) during the Early Paleozoic that collided along the Heihe–Nenjiang–Baicheng belt in the Early Carboniferous [24]. The fourth identifies an Early Carboniferous (330–360 Ma) NE-trending magmatic arc extending from the Heihe–Nenjiang–Moguqi zone to southern Mongolia [15,25], based on Late Paleozoic magmatic records. These differing interpretations reflect ongoing research and debate regarding the tectonic evolution of Northeast China.

The formation epoch of the HHTB is primarily regarded as the Late Silurian–Devonian [1,26,27], the Late Devonian–Early Carboniferous [28], the Early Carboniferous [10,17,29,30], the Pre-Permian [31]. Conclusively, owing to the lack of temporal and spatial constraints of petrological evidence in the middle section of the Great Xing’an Range, it has given rise to diverse perceptions of the spatial extension and controversies regarding the formation chronology.

Figure 1. Geological sketch map of the study area. (a) Simplified tectonic framework of Central Asia Orogenic Belt. (after [2]). (b) Simplified tectonic framework of NE China (after [15]). (c) Geological map of the Jalaid Banner area. (d) Geological map of the Ulanhot area.

Figure 1. Geological sketch map of the study area. (a) Simplified tectonic framework of Central Asia Orogenic Belt. (after [2]). (b) Simplified tectonic framework of NE China (after [15]). (c) Geological map of the Jalaid Banner area. (d) Geological map of the Ulanhot area.

The 1:50,000 regional geological survey in the Ulanhot area within the middle section of the Great Xing’an Range has been conducted. The author dissected the formerly designated Early Permian Dashizhai Formation in this region and identified the Early Carboniferous volcanic rocks. This magmatic activity is a crucial subject for the investigation of the HHTB. In light of this, this paper undertakes a systematic examination of the petrography, zircon U-Pb isotopic chronology, and geochemistry of the Early Carboniferous volcanic rocks in the Ulanhot area. In conjunction with the regional geological setting, the magma source, petrogenesis, tectonic setting, and formation mechanism of these volcanic rocks have been studied, furnishing a geological foundation for the collage and evolutionary history of the Erguna–Xing’an Block and the Songnen Block.

2. Geological Setting and Sample Descriptions

2.1. Geological Setting

The study area is located in the Jalaid Banner–Ulanhot region within the central part of the Great Xing’an Range, and it is positioned in the basin-range transitional area between the Songliao Basin and the Great Xing’an Range. The HHTB forms a vital region for the amalgamation of the Erguna–Xing’an Block and the Songnen Block (Figure 1b). Within the study region, a substantial amount of Mesozoic volcanic rocks and granites, accompanied by a minor quantity of Paleozoic sedimentary strata and Paleozoic granites, are predominant [32]. Regarding the basement of the Xing’an Block, it is conventionally asserted that it encompasses a unified Precambrian metamorphic basement, as exemplified by the “Luomahu Group” and “Zalantun Group”, amongst others [33,34,35]. The preponderance of these has been decomposed into Paleozoic metamorphic rock series. Consequently, a growing multitude of scholars assert that it constitutes a Paleozoic accretionary terrane. The Songnen Block, also known as the Songnen–Zhangguangcai Range Block, essentially encompasses the Songliao Basin, the Lesser Khingan Mountains–Zhangguangcai Range, and a segment of the southern Great Xing’an Range. The strong covering makes it difficult to identify ancient rocks [36,37,38]. In recent years, a multitude of Precambrian geological entities have been successively unearthed in the Longjiang–Ulanhot area situated on the western edge of the Songnen Block [39,40,41]. These data suggest that the Songnen Block ought to possess an Early Precambrian basement.

In the Jalaid Banner, the exposed strata predominantly consist of Mesozoic volcanic-sedimentary formations, with a minor quantity of Early Carboniferous volcanic rocks being locally exposed (Figure 1c). The area of intrusive rocks is comparatively extensive and can be categorized into four phases: the Early Carboniferous, the Late Carboniferous, the Late Jurassic, and the Early Cretaceous. The Late Paleozoic intrusive rocks have been profoundly disrupted by the subsequent magmatic occurrences and overlaid by the Middle Cenozoic volcanoes or sedimentary substances. Regionally, these rock units typically occur as fragmented, discrete blocks exhibiting either fault-bounded contacts with surrounding geological bodies, or preservation within younger magmatic intrusions. The geological outcrops in Ulanhot exhibit a relatively straightforward configuration, with the stratigraphy primarily encompassing Early Carboniferous volcanic rocks, the sedimentary strata of the Middle Permian Zhesi Formation, and Mesozoic volcanic-sedimentary strata. The intrusive rocks are predominantly of Early Permian and Late Jurassic origin.

2.2. Sample Descriptions

In recent years, the author has carried out an elaborate field investigation and sample gathering of the Early Carboniferous volcanic rocks in the Jalaid Banner and Ulanhot regions. The volcanic activities of this era are respectively demarcated in the Jubaotun area in the northwestern part of Jalaid Banner, and in the Lingxia Town and Najin Town in the southern portion of Ulanhot. The rock varieties encompass andesite, schist, and rhyolitic tuff. This work collected representative samples in the above-mentioned areas, along with chronology and geochemical samples. The andesite (WJH-TW7) was collected from the Jubaotun area in the northwest of Jalaid Banner, with the latitude and longitude coordinates of N46°58′48″, E122°3′43″; the schist (D16015) was collected from the northwest of Lingxia Town, with the latitude and longitude coordinates of N45°54′4″, E122°17′42″; the rhyolitic tuff (D17038) was collected from the west of Najin Town, with the latitude and longitude coordinates of N45°47′21″, E122°4′58″.

Andesite (WJH-TW7), grayish-black, porphyritic texture, massive structure (Figure 2a,b). The phenocrysts are plagioclase, with a content of 1%, subhedral tabular, with a particle size of about 0.3–1 mm, first-order gray-white interference color, polysynthetic twins, and individual circle structures can be observed, with a relatively strong surface alteration. The matrix shows an interwoven structure, and the subhedral prismatic plagioclase is oriented. The matrix has been altered strongly, with chloritization and epidotization (Figure 2c).

Schist (D16015), presenting pale gray, exhibits a porphyroblastic texture and a schistose structure (Figure 2d,e). The residual porphyroblast constituents are as follows: Plagioclase, taking an augen shape, with a minor portion in the guise of lenses, minor twinning, mild undulatory extinction, and weak sericitization on the surface. The long axes are slightly oriented, with a grain size ranging from 0.1–0.2 mm and content of approximately 10 to 15%. The matrix component, mica, constitutes roughly 40%, with the components being muscovite and biotite, both manifesting as small and elongated flaky sheets and being distributed in an oriented manner around the residual porphyroblasts. The remainder consists of felsic components, presenting in the form of fine particles or cryptocrystalline. Quartz aggregates are discernible in the form of lenticular and intermittent strips and are all distributed in an oriented manner. The protolith is rhyolitic tuff (Figure 2f).

Rhyolitic tuff (D17038), grayish-white, tuffaceous texture, stratified structure (Figure 2g,h). The rock is predominantly constituted of light brown clay minerals and felsic cryptocrystalline constituents. The crystal fragments primarily consist of angular and subangular feldspar and quartz, with a grain size ranging from 0.03 to 0.1 mm, with a content of approximately 5%. Within the rock, irregularly distributed carbonate minerals are observed in banded and massive forms and fill along the fractures (Figure 2i).

3. Analytical Methods

3.1. Major and Trace Element Analyses

Geochemical analyses were performed on 14 samples at the Northeast China Supervision and Inspection Center of Mineral Resources (Ministry of Natural Resources, Shenyang). Analytical methods included petrographic examination followed by major element analysis using XRF (precision >2%) and trace/REE measurements using ICP-MS (Thermo X Series II). Sample preparation involved removing altered surfaces, crushing to 74 μm with an agate mill, and acid digestion with sequential heating and final dilution to 0.5% HNO₃. Analytical precision ± 5% for elements >1 μg/g, 8% for <10 μg/g, and 10% for transition metals. Detailed analytical methods followed those described by Chen et al. [42]. All results are presented in

Table 1. Major element (wt.%), rare element, and trace element (ppm) composition of volcanic rocks in Ulanhot area.

Lithology	Andesite (WJH-TW7)					Schist (D16015)				Rhyolitic Tuff (D17038)
Sample No.	-YQ1	-YQ2	-YQ3	-YQ4	-YQ5	-YQ1	-YQ2	-YQ3	-YQ4	-YQ1	-YQ2	-YQ3	-YQ4	-YQ5
SiO2	56.25	60.04	51.69	59.80	60.09	66.2	68.33	71.37	66.80	70.27	68.49	68.67	67.99	67.55
Al2O3	19.47	17.78	21.14	19.39	17.75	13.23	15.46	12.41	13.99	13.77	14.84	14.95	15.31	15.49
Fe2O3	2.74	2.22	3.26	1.92	3.16	1.60	2.66	3.60	3.62	0.96	1.34	1.48	1.40	1.62
FeO	3.95	3.86	5.62	2.34	2.74	2.09	2.02	1.66	2.07	2.38	2.70	2.49	2.43	2.56
CaO	3.66	3.41	4.01	4.91	4.24	4.08	0.54	0.59	1.04	1.54	1.04	0.89	0.94	0.90
MgO	1.41	1.55	1.62	1.35	1.51	1.73	1.31	2.20	2.33	1.45	1.49	1.48	1.33	1.45
Na2O	4.90	4.09	4.61	4.17	3.90	2.83	0.020	0.071	1.07	3.87	3.36	3.38	3.56	3.35
K2O	3.27	3.19	2.92	3.22	2.87	2.09	4.91	3.70	3.63	1.81	2.77	2.90	2.94	3.03
P2O5	0.34	0.31	0.38	0.36	0.33	0.14	0.18	0.13	0.19	0.12	0.11	0.11	0.11	0.12
TiO2	0.99	0.91	1.11	1.05	0.91	0.50	0.66	0.56	0.76	0.52	0.55	0.54	0.56	0.58
MnO	0.07	0.06	0.08	0.04	0.05	0.078	0.029	0.096	0.10	0.10	0.08	0.08	0.08	0.09
LOI	2.54	2.32	3.49	1.59	2.52	5.57	3.32	3.73	3.99	3.24	2.97	2.83	2.97	3.23
SUM	99.59	99.73	99.92	100.12	100.07	100.13	99.44	100.13	99.60	100.04	99.74	99.80	99.61	99.96
La	12.69	11.47	14.13	12.58	19.43	24.4	26.8	22.5	35.8	29.54	30.94	28.53	32.13	31.83
Ce	28.88	26.31	32.04	27.69	41.29	50.4	57.9	46.6	81.2	62.15	66.13	59.42	68.72	68.55
Pr	3.61	3.35	4.17	3.47	4.98	5.85	6.85	5.63	9.24	7.37	7.78	7.14	8.08	8.08
Nd	16.23	15.15	18.78	15.35	22.07	22.7	27.4	23.1	38.0	27.90	29.58	25.77	30.33	30.77
Sm	3.73	3.51	4.30	3.26	4.84	5.02	6.05	5.28	8.94	5.24	6.02	5.47	6.09	6.26
Eu	1.04	0.94	1.15	0.94	1.28	1.23	1.37	1.31	1.88	1.16	1.15	1.09	1.14	1.18
Gd	3.22	2.96	3.58	2.67	3.91	4.20	5.22	4.68	7.62	4.26	4.83	4.61	4.74	5.03
Tb	0.46	0.43	0.51	0.38	0.55	0.70	0.86	0.75	1.26	0.65	0.80	0.75	0.75	0.84
Dy	3.04	2.85	3.28	2.48	3.63	3.99	5.06	4.45	7.45	4.27	5.21	4.91	4.98	5.49
Ho	0.53	0.48	0.55	0.43	0.56	0.84	1.02	0.89	1.50	0.78	0.97	0.94	0.93	1.02
Er	1.54	1.39	1.60	1.24	1.54	2.24	2.87	2.45	4.12	2.42	2.87	2.80	2.85	3.03
Tm	0.21	0.18	0.21	0.16	0.19	0.44	0.54	0.47	0.78	0.36	0.41	0.42	0.41	0.43
Yb	1.59	1.42	1.64	1.22	1.28	2.58	3.33	2.77	4.83	2.59	3.23	3.18	3.05	3.28
Lu	0.20	0.18	0.20	0.15	0.16	0.39	0.48	0.44	0.69	0.33	0.39	0.37	0.37	0.37
Y	15.78	14.68	16.52	12.88	16.61	20.5	25.4	21.8	36.1	22.99	27.42	26.08	26.39	29.38
Ba	685.69	594.79	502.32	1100	765.92	583	771	613	610	358.69	620.00	669.03	727.58	738.48
Cr	25.03	27.19	27.00	27.00	24.06	24.4	46.0	76.9	51.8	13.96	17.18	17.71	16.66	18.88
Ga	19.79	18.74	22.86	19.78	20.63	16.5	22.5	17.5	22.4	15.44	18.19	18.63	18.30	19.52
Nb	5.83	6.02	5.76	6.04	6.95	9.88	12.8	9.21	11.8	10.65	11.75	11.61	12.15	11.94
Rb	77.79	79.08	90.73	71.22	74.79	72.4	168	132	137	66.50	92.76	97.18	98.03	100.27
Sr	335.68	273.76	301.80	369.62	323.54	337	63.6	60.9	139	253.43	243.67	233.35	259.94	252.15
Zr	119.58	112.60	128.44	104.56	126.45	192	214	162	213	210.89	216.52	221.63	223.37	228.54
Li	32.86	31.67	48.47	22.43	28.04	29.1	22.8	55.4	62.0	31.26	31.92	30.91	28.40	32.55
Be	2.20	1.90	2.40	2.01	2.01	1.55	2.40	2.56	2.92	1.85	2.51	2.64	2.68	2.81
Sc	24.51	23.68	28.61	26.46	21.81	9.25	10.9	12.9	19.8	11.48	12.60	12.67	13.03	13.27
Ni	19.23	20.64	23.95	18.50	21.20	17.3	12.5	32.4	23.2	9.03	8.68	8.99	9.53	9.98
U	0.65	0.55	0.62	0.42	0.59	1.68	2.00	1.99	2.46	1.66	1.85	1.76	1.88	2.02
Hf	2.05	1.64	2.10	1.32	1.34	3.31	4.26	4.20	6.50	4.12	4.74	5.01	4.78	4.97
Ta	0.41	0.41	0.28	0.57	0.29	0.59	0.71	0.62	0.99	0.63	0.61	0.61	0.56	0.48
Th	4.79	2.60	2.82	1.60	2.27	5.14	5.46	5.08	7.52	7.53	8.93	9.55	9.30	9.78
ΣREE	76.97	70.62	86.14	72.02	105.71	124.98	145.75	121.32	203.31	149.01	160.29	145.40	164.57	166.17
δEu	0.90	0.87	0.87	0.95	0.87	0.80	0.73	0.79	0.68	0.73	0.63	0.65	0.63	0.62
(La/Yb)N	5.72	5.79	6.18	7.40	10.89	6.78	5.77	5.83	5.32	8.19	6.88	6.44	7.55	6.95

.

3.2. Zircon LA–ICP–MS U-Pb Isotope Dating

Zircon U-Pb dating was conducted using LA–ICP–MS at the Tianjin Institute of Geology and Mineral Resources, employing an Agilent 7500a quadrupole ICP-MS equipped with a New Wave Research UP-193 solid-state laser system (193 nm wavelength). Analytical conditions included a 32 μm laser spot size operating at 10 J/cm² energy density and 8 Hz repetition rate. Zircon 91,500 served as the primary reference material for U-Pb fractionation correction [43], while common lead was corrected using LA–ICP–MS Common Lead Correction (v3.15) based on Andersen’s method [44]. Data processing involved Glitter software (v4.0) for isotopic ratio and elemental concentration calculations, with age determinations and Concordia diagram generation performed using Isoplot/Ex 3.0 [45]. Final results are presented as weighted mean ages (95% confidence level) with 1σ analytical uncertainties on Concordia plots. All data are listed in

Table 2. LA–ICP–MS zircon U-Pb dating data of volcanic rocks from the Ulanhot area.

Sample No.	Pb	Th	U	Th/U	Isotopic Ratios						Ages (Ma)
	μg/g	μg/g	μg/g		207Pb/206Pb		207Pb/235U		206Pb/238U		207Pb/206Pb		207Pb/235U		206Pb/238U
					Ratio	1σ	Ratio	1σ	Ratio	1σ	Ages	1σ	Ages	1σ	Ages	1σ
Andesite (WJH-TW7)
1	44	384	242	1.59	0.0535	0.0020	0.4027	0.0176	0.0544	0.0015	350	87	344	13	342	9
2	9	51	93	0.55	0.0547	0.0029	0.4030	0.0220	0.0542	0.0015	398	123	344	16	340	9
3	10	61	94	0.64	0.0512	0.0023	0.3688	0.0168	0.0526	0.0013	256	102	319	12	331	8
4	11	64	104	0.61	0.0557	0.0029	0.4228	0.0231	0.0544	0.0011	443	115	358	17	342	7
5	36	203	390	0.52	0.0517	0.0015	0.3804	0.0112	0.0534	0.0011	272	69	327	8	335	7
6	23	141	220	0.64	0.0526	0.0025	0.4012	0.0206	0.0553	0.0019	309	101	342	15	347	11
7	27	142	296	0.48	0.0527	0.0037	0.4091	0.0280	0.0560	0.0014	322	159	348	20	351	9
8	5	93	109	0.85	0.0727	0.0178	0.1873	0.0420	0.0202	0.0005	1006	513	174	36	129	3
9	65	406	595	0.68	0.0527	0.0013	0.4056	0.0098	0.0561	0.0011	317	54	346	7	352	7
10	46	330	264	1.25	0.0558	0.0018	0.4300	0.0143	0.0563	0.0012	443	75	363	10	353	7
11	27	168	259	0.65	0.0537	0.0018	0.4049	0.0136	0.0553	0.0012	367	74	345	10	347	8
12	40	241	366	0.66	0.0550	0.0015	0.4130	0.0130	0.0548	0.0013	409	63	351	9	344	8
13	6	34	59	0.58	0.0561	0.0035	0.4028	0.0219	0.0553	0.0018	454	132	344	16	347	11
14	19	134	134	1.00	0.0617	0.0027	0.4576	0.0206	0.0536	0.0013	663	93	383	14	336	8
15	6	32	68	0.47	0.0564	0.0033	0.4237	0.0238	0.0560	0.0016	478	131	359	17	351	10
16	25	128	184	0.69	0.0548	0.0019	0.4922	0.0168	0.0657	0.0016	467	80	406	11	410	9
17	23	139	197	0.70	0.0546	0.0021	0.4104	0.0160	0.0543	0.0013	398	85	349	11	341	8
18	51	332	440	0.75	0.0536	0.0016	0.3971	0.0130	0.0540	0.0014	354	69	340	9	339	9
19	28	206	164	1.26	0.0550	0.0019	0.4182	0.0162	0.0553	0.0015	409	78	355	12	347	9
20	37	234	288	0.81	0.0557	0.0017	0.4121	0.0151	0.0535	0.0012	443	69	350	11	336	8
Schist (D16015)
1	201	497	323	1.54	0.0528	0.0009	0.3899	0.0073	0.0536	0.0007	320	39	334	5	336	4
2	55	129	115	1.12	0.0519	0.0012	0.3847	0.0102	0.0537	0.0008	280	56	330	8	337	5
3	101	230	251	0.92	0.0535	0.0010	0.3986	0.0087	0.0540	0.0007	350	31	341	6	339	5
4	63	141	127	1.11	0.0522	0.0013	0.3969	0.0112	0.0551	0.0009	300	75	339	8	346	6
5	65	144	181	0.80	0.0527	0.0010	0.4032	0.0110	0.0554	0.0011	317	44	344	8	348	7
6	733	1560	1033	1.51	0.0534	0.0007	0.4544	0.0103	0.0616	0.0012	346	33	380	7	386	7
7	20	41	49	0.85	0.0552	0.0025	0.4248	0.0233	0.0557	0.0014	420	106	359	17	350	8
8	85	185	262	0.71	0.0525	0.0010	0.3948	0.0127	0.0545	0.0014	306	31	338	9	342	9
9	112	224	189	1.19	0.0538	0.0012	0.4425	0.0123	0.0596	0.0014	365	64	372	9	373	8
10	75	163	175	0.93	0.0540	0.0013	0.4022	0.0122	0.0540	0.0013	369	56	343	9	339	8
11	127	283	235	1.20	0.0529	0.0011	0.3952	0.0100	0.0542	0.0012	324	44	338	7	340	7
12	104	224	160	1.40	0.0540	0.0011	0.4205	0.0110	0.0565	0.0014	369	44	356	8	354	8
13	89	148	137	1.08	0.0533	0.0013	0.4588	0.0129	0.0624	0.0013	343	56	383	9	390	8
14	183	73	47	1.58	0.1182	0.0027	5.4438	0.2199	0.3332	0.0093	1929	39	1892	35	1854	45
15	113	257	179	1.43	0.0523	0.0013	0.3871	0.0110	0.0536	0.0010	298	56	332	8	337	6
16	18	36	74	0.49	0.0535	0.0018	0.3923	0.0150	0.0531	0.0012	354	78	336	11	334	7
17	144	338	262	1.29	0.0526	0.0010	0.3825	0.0096	0.0527	0.0010	309	44	329	7	331	6
18	20	26	352	0.08	0.0529	0.0010	0.3817	0.0093	0.0523	0.0010	328	25	328	7	328	6
19	53	111	167	0.66	0.0533	0.0013	0.4134	0.0137	0.0562	0.0015	343	64	351	10	353	9
20	402	865	574	1.51	0.0537	0.0009	0.4198	0.0106	0.0567	0.0013	367	33	356	8	355	8
21	123	278	236	1.18	0.0536	0.0010	0.4061	0.0107	0.0549	0.0011	354	44	346	8	345	7
22	94	190	270	0.70	0.0536	0.0010	0.4239	0.0126	0.0573	0.0013	354	44	359	9	359	8
23	99	220	152	1.45	0.0603	0.0034	0.4492	0.0406	0.0534	0.0019	617	119	377	28	335	12
24	166	362	312	1.16	0.0524	0.0010	0.3979	0.0093	0.0550	0.0010	306	25	340	7	345	6
25	354	781	398	1.96	0.0538	0.0008	0.4230	0.0090	0.0569	0.0010	365	33	358	6	357	6
26	214	497	323	1.54	0.0531	0.0008	0.3958	0.0077	0.0540	0.0008	345	33	339	6	339	5
27	237	495	388	1.28	0.0536	0.0008	0.4298	0.0093	0.0581	0.0011	354	33	363	7	364	7
28	464	1189	339	3.51	0.0547	0.0009	0.4096	0.0095	0.0542	0.0008	467	39	349	7	341	5
29	147	298	318	0.94	0.0534	0.0009	0.4281	0.0152	0.0582	0.0019	343	53	362	11	364	11
30	144	315	257	1.23	0.0528	0.0009	0.4038	0.0092	0.0555	0.0010	320	39	344	7	348	6
Rhyolitic tuff (D17038)
1	154	350	464	0.75	0.0531	0.0008	0.4149	0.0102	0.0567	0.0013	332	33	352	7	355	8
2	146	67	113	0.59	0.0968	0.0013	3.7783	0.0974	0.2829	0.0063	1565	25	1588	21	1606	32
3	176	448	390	1.15	0.0529	0.0009	0.3972	0.0107	0.0544	0.0012	328	25	340	8	341	7
4	64	147	143	1.03	0.0540	0.0013	0.4213	0.0156	0.0566	0.0018	369	50	357	11	355	11
5	641	252	287	0.88	0.1132	0.0012	5.5032	0.1451	0.3523	0.0088	1854	3	1901	23	1946	42
6	61	145	157	0.93	0.0535	0.0013	0.4202	0.0121	0.0569	0.0012	350	56	356	9	357	7
7	61	146	100	1.47	0.0534	0.0014	0.4124	0.0148	0.0560	0.0015	346	61	351	11	351	9
8	82	69	162	0.43	0.0664	0.0011	1.3149	0.0423	0.1435	0.0044	820	33	852	19	865	25
9	104	227	450	0.50	0.0528	0.0009	0.4150	0.0150	0.0569	0.0018	320	53	352	11	357	11
10	39	97	64	1.52	0.0540	0.0021	0.4106	0.0165	0.0552	0.0014	372	83	349	12	346	9
11	126	60	114	0.53	0.0960	0.0016	3.2918	0.1917	0.2481	0.0127	1547	32	1479	45	1429	66
12	91	197	321	0.61	0.0534	0.0011	0.4203	0.0133	0.0571	0.0018	346	44	356	10	358	11
13	89	199	272	0.73	0.0535	0.0010	0.4169	0.0171	0.0565	0.0021	350	31	354	12	354	13
14	497	130	262	0.50	0.1548	0.0019	9.9033	0.3498	0.4637	0.0153	2400	18	2426	33	2456	68
15	61	140	151	0.93	0.0522	0.0013	0.4083	0.0153	0.0567	0.0019	295	69	348	11	356	11
16	349	798	748	1.07	0.0553	0.0009	0.4170	0.0081	0.0547	0.0010	433	39	354	6	343	6
17	146	330	413	0.80	0.0534	0.0009	0.4182	0.0123	0.0568	0.0014	346	53	355	9	356	9
18	112	259	396	0.65	0.0545	0.0009	0.4111	0.0103	0.0547	0.0010	391	136	350	7	344	6
19	71	165	238	0.69	0.0533	0.0012	0.4022	0.0124	0.0547	0.0012	343	64	343	9	343	8
20	53	121	173	0.70	0.0541	0.0014	0.4208	0.0142	0.0564	0.0014	376	42	357	10	354	9
21	58	127	159	0.80	0.0541	0.0012	0.4177	0.0130	0.0559	0.0013	376	50	354	9	351	8
22	151	339	472	0.72	0.0540	0.0010	0.4155	0.0083	0.0558	0.0009	372	31	353	6	350	5
23	99	127	435	0.29	0.0679	0.0044	0.7156	0.1100	0.0732	0.0071	866	136	548	65	455	43
24	174	416	303	1.37	0.0535	0.0008	0.3981	0.0090	0.0540	0.0011	350	33	340	7	339	7
25	70	149	246	0.60	0.0539	0.0010	0.4126	0.0105	0.0555	0.0010	365	58	351	8	348	6
26	473	133	180	0.74	0.1598	0.0019	9.8759	0.3091	0.4479	0.0118	2453	22	2423	29	2386	53
27	87	194	302	0.64	0.0542	0.0009	0.4087	0.0110	0.0547	0.0010	389	39	348	8	343	6
28	172	385	628	0.61	0.0536	0.0008	0.4111	0.0102	0.0556	0.0011	367	39	350	7	349	7
29	36	80	115	0.69	0.0546	0.0016	0.4062	0.0126	0.0540	0.0010	394	67	346	9	339	6
30	39	87	109	0.80	0.0539	0.0018	0.4082	0.0152	0.0549	0.0009	369	72	348	11	344	6

.

3.3. Zircon Lu-Hf Isotope Dating

Zircon Lu-Hf isotopic analyses were conducted at the Tianjin Institute of Geology and Mineral Resources. In this procedure, a Thermo Fisher Neptune type Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS) and a 193 nm laser ablation system (NEWWAVE193nm FX) with a laser spot size of 5 μm (Thermo Fisher Scientific, Waltham, MA, USA) were employed. The methodologies are based on the techniques presented by Geng Jianzhen et al. [46]. The data are listed in

Table 3. Zircon Hf isotopic data for the volcanic rocks from the Ulanhot area.

Sample No.	Age (Ma)	176Yb 177Hf	176Lu 177Hf	176Hf 177Hf	2σ	εHf(0)	εHf(t)	2σ	TDM1	TDM2	fLu/Hf
Andesite (WJH-TW7)
WJH-TW7-01	342	0.094375	0.003780	0.282476	0.000146	−10.3	−3.7	0.000041	1179	1423	−0.89
WJH-TW7-02	340	0.066819	0.002704	0.282205	0.001034	−19.9	−13.0	0.000045	1540	1937	−0.92
WJH-TW7-03	331	0.048905	0.002450	0.282534	0.000603	−8.3	−1.3	0.000042	1052	1293	−0.93
WJH-TW7-04	342	0.029098	0.001287	0.282467	0.000273	−10.7	−3.4	0.000042	1114	1410	−0.96
WJH-TW7-05	335	0.014210	0.000607	0.282497	0.000379	−9.6	−2.2	0.000028	1052	1342	−0.98
WJH-TW7-06	347	0.019926	0.000864	0.282544	0.000113	−7.9	−0.6	0.000023	993	1253	−0.97
WJH-TW7-07	351	0.024154	0.001043	0.282827	0.000277	2.1	9.4	0.000036	597	697	−0.97
WJH-TW7-11	353	0.068233	0.002800	0.282460	0.000731	−10.9	−4.0	0.000041	1171	1442	−0.92
WJH-TW7-09	352	0.033180	0.001416	0.282549	0.000704	−7.7	−0.5	0.000034	1000	1250	−0.96
WJH-TW7-12	344	0.044328	0.001989	0.282584	0.000061	−6.5	0.6	0.000025	966	1189	−0.94
WJH-TW7-13	347	0.021366	0.000942	0.282712	0.000302	−2.0	5.3	0.000029	759	924	−0.97
WJH-TW7-14	336	0.036488	0.001680	0.282656	0.000394	−4.0	3.2	0.000027	855	1044	−0.95
Schist (D16015)
D16015-03	339.00	0.028159	0.000956	0.282289	0.000012	−17.1	−9.8	0.000015	1358	1967	−0.97
D16015-02	337.00	0.064605	0.002215	0.282670	0.000030	−3.6	3.5	0.000018	851	1129	−0.93
D16015-04	346.00	0.038170	0.001330	0.282485	0.000020	−10.2	−2.9	0.000017	1095	1534	−0.96
D16015-07	350.00	0.042442	0.001431	0.282525	0.000017	−8.7	−1.5	0.000018	1040	1444	−0.96
D16015-08	342.00	0.042795	0.001473	0.282392	0.000011	−13.4	−6.2	0.000014	1230	1743	−0.96
D16015-10	339.00	0.076829	0.002510	0.282858	0.000019	3.0	10.0	0.000021	582	710	−0.92
D16015-16	334.00	0.069594	0.002408	0.282464	0.000021	−10.9	−3.9	0.000016	1158	1597	−0.93
D16015-17	331.00	0.052820	0.001834	0.282331	0.000070	−15.6	−8.5	0.000016	1329	1884	−0.94
D16015-20	355.00	0.058380	0.001933	0.282485	0.000020	−10.2	−3.0	0.000017	1112	1541	−0.94
D16015-24	345.00	0.083758	0.002868	0.282410	0.000042	−12.8	−5.9	0.000018	1251	1722	−0.91
D16015-25	357.00	0.075194	0.002398	0.282211	0.000040	−19.8	−12.8	0.000015	1523	2158	−0.93
Rhyolitic tuff (D17038)
D17038-01	355.00	0.079851	0.002427	0.282668	0.000036	−3.7	3.4	0.000019	860	1135	−0.93
D17038-03	341.00	0.042240	0.001341	0.282433	0.000013	−12.0	−4.7	0.000026	1168	1647	−0.96
D17038-12	358.00	0.054341	0.001757	0.282670	0.000006	−3.6	3.7	0.000017	841	1120	−0.95
D17038-17	356.00	0.080640	0.002583	0.282622	0.000083	−5.3	1.7	0.000021	931	1241	−0.92
D17038-19	343.00	0.068315	0.002118	0.282631	0.000036	−5.0	2.2	0.000018	907	1215	−0.94
D17038-20	354.00	0.048108	0.001558	0.282726	0.000016	−1.6	5.7	0.000018	756	991	−0.95
D17038-25	348.00	0.031863	0.001014	0.282624	0.000010	−5.2	2.2	0.000016	889	1213	−0.97
D17038-27	343.00	0.049414	0.001780	0.282555	0.000021	−7.7	−0.5	0.000018	1008	1381	−0.95
D17038-29	339.00	0.049863	0.001900	0.282734	0.000045	−1.3	5.9	0.000023	752	979	−0.94
D17038-30	344.00	0.067515	0.002092	0.282777	0.000025	0.2	7.3	0.000019	693	884	−0.94

.

4. Analytical Results

4.1. Zircon U–Pb Geochronology

The zircons within the andesite (WJH-TW7) sample exhibit euhedral to subhedral columnar or granular morphologies, with a size predominantly spanning from 70 to 130 μm, and possess distinct oscillatory growth zoning (Figure 3a). The Th/U ratios of the zircons fluctuate between 0.47 and 1.59, with an average of 0.77. These characteristics indicate a magmatic origin. A sum of 20 representative zircons was chosen for examination. Except for point 8, which markedly deviates from the Concordia curve due to evident Pb loss, the remaining test points lie on the Concordia curve. The age of zircon ²⁰⁶Pb/²³⁸U at point 16 is 410 ± 9 Ma, which might be captured zircon. The ages of zircon ²⁰⁶Pb/²³⁸U at the other 18 test points range from 331 Ma to 353 Ma, and the concordant weighted average age is 343.1 ± 3.8 Ma (MSWD = 0.69), corresponding to the middle of Early Carboniferous.

A total of 30 zircons were examined for the schist (D16015) sample. Amongst them, point 23 demonstrates Pb depletion, and the data deviates from the concordant curve. The remaining 29 points align with the concordant curve. The majority of zircon samples are columnar or granular, with a size spanning from 60 to 120 μm. A significant portion of them exhibits distinct magmatic oscillation rings (Figure 3b). The Th/U ratios of zircons fluctuate between 0.49 and 3.51 (with individual Th/U ratios of 0.08), with an average of 1.21. Certain zircons possess a core-rim structure, with the core being magmatic zircon and the rim being a metamorphic one. Point 14 is situated in the core of the zircon (magmatic zircon); the zircon is round-shaped, the boundary assumes a harbor-like configuration, and a minor metamorphic rim is developed. Its ²⁰⁷Pb/²³⁶U age is 1929.3 ± 39.4 Ma, and the Th/U value is 1.58, representing a magmatic event. Spots 6, 9, and 13 are located in the core of the zircons with a core-rim structure. The ²⁰⁶Pb/²³⁸U ages are 385.6 ± 7.1, 373.3 ± 8.3, and 390 ± 7.8, and the Th/U values range from 1.8 to 1.51, representing a magmatic event. All of the above zircons are captured zircons. The ²⁰⁶Pb/²³⁸U ages of other zircons range from 328 to 364 Ma, and the concordant weighted average age is 343.1 ± 3.7 Ma (MSWD = 2.0), which falls within the middle of Early Carboniferous. In conclusion, the diagenetic age of the rock is 343.1 ± 3.7 Ma, corresponding to the mid-early Carboniferous. After diagenesis, a metamorphic effect is evident, and zircons from the Middle-Late Devonian and Early Paleoproterozoic magmatic events are captured.

In the rhyolitic tuff (D17038) sample, the zircon grains possess a good crystalline form, presenting euhedral to subhedral columnar and granular morphologies, with a size predominantly spanning from 50 to 130 μm (Figure 3c). They exhibit distinct oscillatory zoning, and the Th/U ratios of zircons fluctuate between 0.29 and 1.52, with an average of 0.79. These characteristics indicate a magmatic origin. A total of 30 spots were subjected to analysis, among which spot 23 demonstrates Pb loss and deviates from the concordant curve, whereas the other 29 spots lie on or in proximity to the concordant curve. Six analyzed spots (2, 5, 8, 11, 14, 26) represent captured zircons. Spot 8 yields a ²⁰⁶Pb/²³⁸U age of 865 ± 25 Ma, while the other five spots display ²⁰⁷Pb/²³⁶U ages ranging from 1547 to 2453 Ma. The remaining 23 spots exhibit zircon ²⁰⁶Pb/²³⁸U ages ranging from 339 Ma to 358 Ma, with a concordant weighted average age of 347.4 ± 3.0 Ma (MSWD = 0.64), falling within the Early Carboniferous.

4.2. Major and Trace Element Geochemistry

The SiO₂ content of the andesite samples spans from 51.69%–60.09wt%, attaining a mean value of 57.57wt%, thereby falling within the category of intermediate rocks. The Al₂O₃ content varies between 17.75wt% and 21.14wt%, with an average of 19.11%. The total alkali content (Na₂O + K₂O) ranges from 6.77wt% to 8.17wt%, averaging at 7.43wt%. The TiO₂ content fluctuates from 0.91wt% to 1.5wt%, with a mean value of 0.99%. The MgO content varies from 1.35wt% to 1.62%, with an average of 1.49wt%. The P₂O₅ content ranges from 0.31wt% to 0.38wt%, averaging at 0.34wt%. The Mg# value extends from 26.35 to 38.53, with an average of 32.37. On the Zr/TiO₂-Nb/Y diagram (Figure 4a), the samples are positioned within the andesite/basalt fields and subalkaline basalt, respectively. In the K₂O-SiO₂ diagram (Figure 4b), the rocks pertain mainly to the high-K calc-alkaline series and, secondarily, to the calc-alkaline series. The aluminum saturation index (A/CNK) fluctuates between 1.01 and 1.17, with an average of 1.07, belonging to the weakly peraluminous series. The total rare earth element content (ΣREE) of the andesite is 72.2–15.71 ug/g. The ratio of light rare earth elements (LREEs) to heavy rare earth elements (HREEs) varies from 6.13 to 7.94, with an average of 6.78. The REE distribution pattern is right-skewed (Figure 5a), exhibiting significant fractionation between LREEs and HREEs. The (La/Yb)_N ranges from 5.72 to 1.89, indicating a relative abundance of LREEs and a relative scarcity of HREEs. The weak negative Eu anomaly, with δEu ranging from 0.87 to 0.95, might be associated with the fractional crystallization of plagioclase. In the primitive mantle-normalized trace element spider diagram (Figure 5b), the andesites are enriched in large ion lithophile elements (LILEs) such as Rb, K, and Ba and depleted in elements such as Nb, Ta, Ti, and Hf.

The schist and the rhyolitic tuff exhibit relatively analogous geochemical features. The SiO₂ content spans from 66.2wt% to 71.37wt%, with an average of 68.41wt%, thereby belonging to the felsic rock. The Al₂O₃ content fluctuates between 12.41wt% and 15.49wt%, with an average of 14.38wt%. The total alkali content (Na₂O + K₂O) demonstrates a broad range of variation, ranging from 3.77wt% to 6.5wt%, with an average of 5.48wt%. The TiO₂ content is comparatively low, ranging from 0.5wt% to 0.76wt%, with an average of 0.58wt%. The CaO content varies from 0.54wt% to 1.54wt%, with the CaO content of individual specimens reaching 4.8wt%. The MgO content ranges from 1.31wt% to 2.33wt%, with an average of 1.64wt%. The P₂O₅ content fluctuates between 0.11wt% and 0.19wt%, with an average of 0.13wt%, and the Mg# value varies from 35.92 to 48.7, with an average of 42.89. In the Zr/TiO₂-Nb/Y diagram (Figure 4a), the samples are situated within the range of dacite and rhyolite. In the K₂O-SiO₂ diagram (Figure 4b), the rock pertains to the high-potassium calc-alkaline rock series, while individual points fall within the calc-alkaline series. The content of ΣREE is 121.32–23.31 μg/g, and the ratio of LREEs/HREEs is 6.18–8.52, with an average of 7.2. The REE distribution curve is right-inclined (Figure 5a), and the fractionation between LREEs and HREEs is conspicuous, with (La/Yb)_N being 5.32–8.19. The LREEs are relatively enriched, while the HREEs are relatively depleted, featuring a weak negative Eu anomaly, with δEu ranging from 0.62 to 0.8, which might be associated with the fractional crystallization of plagioclase. In the primitive mantle-normalized trace element spider diagram (Figure 5b), the rock samples are enriched in LILEs such as Rb, K, Ba, etc., and are depleted in elements such as Nb, Ta, Ti, Sr, P, etc.

4.3. Zircon Hf Isotope

In situ micro-area Hf isotope analysis of the Early Carboniferous volcanic rocks in the Ulanhot area was carried out (Table 3, Figure 6). For the andesite (WJH-TW7), the initial ¹⁷⁶Hf/¹⁷⁷Hf ratios of the 12 magmatic zircons, which denote the diagenetic age, fluctuate within the range of 0.282205 to 0.282827. The εHf(t) values span from −13 to 9.4, the one-stage model ages extend from 597 to 1540 Ma, and the two-stage model ages range from 697 to 1937 Ma. Regarding the schist (D16015), the initial ¹⁷⁶Hf/¹⁷⁷Hf ratios of the 11 magmatic zircons representing the diagenetic age vary within the interval of 0.282211 to 0.282858. The εHf(t) values range from −12.8 to 10, the one-stage model ages fluctuate between 582 and 1523 Ma, and the two-stage model ages fall within the range of 710 to 2158 Ma. Concerning the rhyolitic tuff (D17038), the initial ¹⁷⁶Hf/¹⁷⁷Hf ratios of the 12 magmatic zircons indicate the diagenetic age oscillate within the bounds of 0.282433 and 0.282777. The εHf(t) values extend from −4.7 to 7.3, the one-stage model ages fluctuate from 693 to 1168 Ma, and the two-stage model ages lie within the scope of 884 to 1647 Ma.

5. Discussion

5.1. The Geochronological Significance of Volcanic Rocks from Ulanhot Area

This study undertakes zircon U-Pb isotope analyses on the volcanic rocks of the erstwhile mapped Early Permian Dashizhai Formation in the Jalaid Banner and Ulanhot regions. The results reveal that the zircon U-Pb ages of the diagenetic ages of volcanic rock are 343.1 ± 3.8 Ma, 343.1 ± 3.7 Ma, and 347.4 ± 3.0 Ma, respectively, which belong to the Early Carboniferous. The Dashizhai Formation is primarily disseminated to the north of the Xar Moron tectonic belt and to the south of the HHTB. The rock assemblage predominantly consists of a suite of intermediate–basic to felsic volcanic rocks and sedimentary rocks, forming in the Early Permian [52]. Owing to the minor strata with volcanic rocks attributed to the Permian in the central and southern sectors of the Great Xing’an Range, the volcanic rocks exposed during this era are frequently incorporated into the Dashizhai Formation, engendering numerous complications and misunderstandings in subsequent applications [53]. Predecessors have carried out a large number of chronological studies on the volcanic strata of the Dashizhai Formation in different regions. However, the volcanic rocks show a large age span, mainly concentrated in the Early Permian, Early Silurian, and Late Carboniferous [52,53,54,55]. The Dashizhai Formation is of great significance for the study of the geological history, the division of the tectonic framework, and the exploration of mineral resources in this area. Therefore, we propose that the Dashizhai Formation, which was mainly divided according to petrography in the past, should be disintegrated to redefine the temporal and spatial distribution of volcanic-sedimentary strata.

Currently, numerous reports have been presented on the Early Carboniferous intrusive rocks in the Central Great Xing’an Range; however, the reports concerning the Early Carboniferous volcanic rocks are comparatively scarce. Wang et al. [56] recognized the Early Carboniferous intermediate–basic intrusive rocks associated with subduction in the Jalaid Banner area. Zhao et al. [14,29] respectively identified the volcanic and intrusive rocks related to subduction in the Hadayang Town, Wuerqihan area, and Yinhe area. Combined with the identification of Early Carboniferous volcanic rocks in this study, it indicates that the magmatic activity in the Early Carboniferous is very frequent, and most of them have the characteristics of arc magmatic rocks, which can form a magmatic arc [15].

5.2. Petrogenesis and Tectonic Setting

The lithogeochemical data indicate that the andesites in the Jalaid Banner region possess a relatively moderate SiO₂ content, low MgO content, Mg#, and compatible element Cr (with an average of 26.6 ug/g), Ni (with an average of 2.7 ug/g). The above data significantly deviate from the composition range of the original mantle magma [57], indicating that its genesis has no direct correlation with the primary mantle melt. The key elemental ratios (Nb/Ta = 10.6–23.97) exhibit transitional characteristics between mid-ocean ridge basalt (MORB, Nb/Ta ≈ 17.55) and continental crust (Nb/Ta ≈11–12) [58], and the Th/Ta ratio of the andesites varies from 2.81 to 11.68, surpassing the Th/Ta ratio of the primitive mantle (which is 3.4) [58], combining with the high P₂O₅ contents (0.31%–0.38%), showing the characteristics of crust–mantle admixture.

The Hf isotope composition of zircon further confirms this mechanism: the εHf(t) values exhibit a bimodal distribution (−13–9.4), negative end-member (T_DM2 = 1410–1937 Ma), and age of Precambrian fragments within the Xingmeng Orogenic Belt (e.g., 1.8 Ga–2.5 Ga magmatic activity in the Longjiang–Ulanhot area) [39,40,41] were of consistent age, indicating that the recycled continental crust material carried by subducting plates or the detachment of micro landmasses were involved in partial melting [59]. The positive end-member (T_DM2 = 697–1253 Ma) reflects the juvenile crust of the Phanerozoic era (possibly due to the contribution of sub arc crustal melting or plate melting) [7]. High Ba/Th (143~687), low Th/Nb (0.26~0.82), and significant Nb, Ti negative anomalies (Figure 5b) are consistent with the typical characteristics of plate dehydration releasing Ba-rich fluids to metasomatize mantle wedges [60,61].

The flat HREE patterns (Gd/Yb ≈ 2–3) indicate magma generation within the spinel stability field (depth < 80 km), as spinel exhibits weak fractionation of HREEs, resulting in relatively flat HREE distributions in the melt [62]. The elevated Dy/Yb ratios (1.9–2.8) typically suggest the coexistence of garnet and spinel in the source region (transitional zone at depths of ~60–80 km) [63]. The negative Eu anomalies (δEu = 0.87–0.95) in the rocks and the negative P₂O₅-SiO₂ correlation indicate that plagioclase and apatite fractional crystallization dominated the late-stage magmatic evolution [64]. The subdued negative Eu anomalies may result from limited plagioclase fractionation or redistribution of Eu by coexisting minerals (e.g., apatite, K-feldspar). In the Sr/Y-Y diagram (Figure 7a) and the (La/Yb)_N-Yb_N diagram (Figure 7b), the samples are situated within the domain of conventional island arc rocks, and both the Sr/Y and Y magnitudes are rather low, signifying a mixing source [65]. In the trace element spider diagram, Nb and Ti exhibit conspicuous negative anomalies, which can be construed as typical indicators of magmatic activity in the subduction zone [60].

In summary, the Early Carboniferous andesites from the Ulanhot region were not derived from direct melting of the primitive mantle but rather represent products of multistage mixing and interaction between crustal and mantle materials in a subduction zone setting [66]. The magmatic source involved both components from fluid-metasomatized mantle wedge melting and contributions from melts derived from ancient continental crust (e.g., delaminated microcontinental fragments or recycled sediments) as well as Phanerozoic juvenile crust. During magma ascent and evolution, these melts further underwent late-stage differentiation dominated by plagioclase and apatite fractional crystallization. These integrated evidence not only reveals the complex crust–mantle dynamics during the Early Carboniferous in the Xing’an-Mongolia Orogenic Belt but also provides critical geochemical constraints for understanding the magmatic-tectonic evolution during the final closure stage of the Paleo–Asian Ocean domain.

Felsic volcanic rocks possess a relatively elevated SiO₂ content (with an average of 68.41wt%), a considerable Na₂O + K₂O content (averaging 5.48wt%), a relatively low MgO content (averaging 1.64wt%), and a rather low TiO₂ content (averaging 0.58wt%), exhibiting the same characteristics as the continental crust [67]. Furthermore, these rocks are enriched in LREEs and LILEs (such as Rb, K, Ba) yet depleted in HREE and HFSEs (such as Nb, Ta, Ti, Sr, P), implying that they are derived from the crust. The rocks predominantly consist of calc-alkaline and high-potassium calc-alkaline rocks (Figure 4b), exhibiting either I-type or S-type granites. The felsic volcanic rocks possess relatively elevated Zr + Nb + Ce + Y values and (Na₂O + K₂O)/CaO values and diminished TFeO/MgO values. In the rock type discrimination diagrams, they all lie within the I-type and S-type granites areas (Figure 8). Nevertheless, the typical aluminous minerals of S-type granite, such as corundum, tourmaline, and garnet, are not discernible in the mineral composition. The P₂O₅ content of the felsic volcanic rocks is comparably low, spanning from 0.11% to 0.19%, whereas the P₂O₅ content of S-type granites typically exceeds 0.2% [68,69]. Therefore, the felsic volcanic rocks in the study area are I-type granites rather than S-type granites.

The felsic volcanic rocks within the study area possess a comparatively elevated A/CNK ratio, yet their Zr/Hf ratio ranges from 32.77 to 58.1, falling within the spectrum of highly fractionated granites (25–55) [70], suggesting their affiliation with highly fractionated granites. The extant research on the magmatic process of fractionated granites intimates that the cause for the variance in the geochemical characteristics of this sort of fractionated granite is the fractional crystallization of feldspar throughout the evolution of the granite magma, which results in a reduction of the CaO content within the rocks [71]. The εHf(t) values of the two felsic volcanic rock samples fluctuate within the ranges of −12.8 to 10 and −4.7 to 7.3, respectively. Concurrently, the two-stage model ages span from 582 to 1523 Ma and 884 to 1647 Ma, exhibiting a considerable variation spectrum. This implies that the magma originates from the partial melting of the freshly accreted crust and Middle Proterozoic crustal substances. Furthermore, the felsic volcanic rocks possess a relatively low Sr/Y ratio and a relatively elevated Y content, featuring classic island arc rock characteristics (Figure 7). This indicates that the magma could not have been generated in the stable region of the garnet mineral phase (≥10 kPa). Commonly, within this region, adakitic magma is formed as a result of the partial melting of the oceanic subduction slab or of the thickened lower crust [65,72,73]. The felsic volcanic rocks in the study area might have been formed in the crustal portion above the stable region of the garnet mineral phase. In summary, the magma of the felsic volcanic rocks in the study area derives from the partial melting of shallower crustal materials.

Figure 8. (a) (Na₂O+K₂O)/CaO versus (Zr + Nb + Ce + Y) diagram [74], (b) TFeO/MgO versus (Zr + Nb + Ce + Y) diagram [74] for the volcanic rocks from the Ulanhot area. A: A-type granites; FG: Fractionated granites; OGT: unfractionated granites.

Figure 8. (a) (Na₂O+K₂O)/CaO versus (Zr + Nb + Ce + Y) diagram [74], (b) TFeO/MgO versus (Zr + Nb + Ce + Y) diagram [74] for the volcanic rocks from the Ulanhot area. A: A-type granites; FG: Fractionated granites; OGT: unfractionated granites.

The volcanic rocks within the study area primarily encompass andesite, schist (with the protolith being rhyolitic tuff), and rhyolitic tuff. Certain rocks exhibit pronounced schistosity and carbonatization. Generally, high-field-strength incompatible elements such as Th, Hf, Ta, and Nb are not readily influenced by the subsequent metamorphism and can be employed to discern the tectonic milieu of volcanic rocks. The volcanic rocks exhibit a relatively low TiO₂ content (with an average of 0.73%), a comparatively high Al₂O₃ content (with an average of 16.7%) as well as K₂O content (with an average of 3.9%). They are enriched in LILEs such as Rb, K, and Ba, while being depleted in HFSEs like Nb, Ta, Ti, and Hf, which suggest that the protoliths of these rocks possess the characteristics of arc volcanic rocks and form in an island arc or active continental margin environment. The comparatively elevated La/Nb ratio (ranging from 1.91 to 3.3) further accentuates the features of subduction-related arc volcanic rocks. In the Hf/3-Th-Nb/16 diagram, all the andesite samples are encompassed within the island arc calc-alkaline basalts area (Figure 9a). In the Th/Yb-Nb/Yb diagram, all the projections are positioned in the continental arc region (Figure 9b). In the Rb-(Y + Nb) diagram, all the felsic volcanic rock samples are located within the volcanic arc area (Figure 9c). In conclusion, we contend that the Early Carboniferous volcanic rocks in the Ulanhot area are the products of subduction in an active continental margin.

5.3. Geological Significance

The HHTB is the final collage position between the Erguna–Xing’an Block and the Songnen Block [8,15,20,23,78]. As previously stated, there remain substantial controversies concerning its precise extensional position and era. The Early Carboniferous volcanic rocks under investigation in this study are situated in the Ulanhot region within the middle portion of the HHTB. The zircon U-Pb age spans from 343 to 347 Ma, corresponding to the middle Early Carboniferous. The tectonic setting pertains to the active continental margin, and its genesis is intimately associated with the evolution of the HHTB. In recent years, a considerable number of magmatic occurrences with a subduction tectonic setting have been documented in the vicinity of the study area. For instance, Wang et al. [56] discerned the Early Carboniferous gabbros and diorites in the Jalaid Banner region, which were engendered in an active continental margin setting associated with subduction, analogous to an island arc tectonic setting. Yang et al. [79] disclosed Early Carboniferous high-Mg andesites in the Xing’an Reservoir area of Ulanhot beneath a subduction zone, intimating the presence of a suite of Early Carboniferous magmatic arcs trending toward the northeast in the Jalaid Banner–Ulanhot area. Regionally, there are also numerous reports of Early Carboniferous magmatic rocks. For example, Early Carboniferous gabbro associated with subduction in the Moguqi area [15], I-type and differentiated I-type granite formed by island arc magmatic activities in the Late Devonian–Early Carboniferous subduction setting in the Zalantun area [9], andesite showing the characteristics of intraplate ocean island basalt of the Late Devonian Daminshan Formation and intermediate-felsic volcanic rocks with the features of arc volcanic rocks in the Zalantun area [80], as well as Late Devonian–Early Carboniferous I-type granite exposed in Yargenchu in the southwestern Zalantun [81]. These magmatic activities are presumably linked to the HHTB, suggesting that a continuous subduction process transpired between the Erguna–Xing’an Block and the Songnen Block during the Late Devonian–Early Carboniferous. The age of the Hegenshan ophiolite in the southern part of the HHTB has been found by some scholars to be no later than the Early Carboniferous [82,83]. The age of the Duobaoshan ophiolitic mélange, remnants of the Nenjiang ophiolitic mélange, and arc magmatic rocks in the northern section of Great Xing’an Range is primarily the Late Devonian–Early Carboniferous [14,84]. Consequently, the aforementioned evidence suggests that a continuous subduction process transpired between the Erguna–Xing’an Block and the Songnen Block throughout the Late Devonian to Early Carboniferous period, accompanied by intense arc magmatism. The suture zone stretches from the Heihe–Nenjiang–Zalantun region to the Jalaid Banner–Ulanhot area. The discovery of S-type granite in the middle Early Carboniferous (334 Ma) in the Jalaid Banner area [10] and the identification of magmatic activities in the late Early Carboniferous (322 Ma) within the late-orogenic to post-orogenic extensional tectonic setting in the Quanshenglinchang area [85] suggests that the HHTB was already undergoing collisional orogeny during the middle and late Early Carboniferous. In conclusion, the Early Carboniferous volcanic rocks in the Ulanhot region are likely to have originated in a subduction setting before the collision between the Erguna–Xing’an Block and the Songnen Block. The HHTB might be spatially distributed along the Heihe–Nenjiang–Moguqi–Jalaid Banner–Ulanhot–Hegenshan–Erenhot zone.

6. Conclusions

(1)

The LA–ICP–MS zircon U-Pb dating results show that the crystallization ages of the andesite, the schist, and the rhyolitic tuff originally assigned to the Dashizhai Formation in the Ulanhot area are 343.1 ± 3.8 Ma, 343.1 ± 3.7 Ma, and 347.4 ± 3.0 Ma, respectively, and their formation period is the Early Carboniferous.

(2)

In the study area, the Early Carboniferous andesites are the products of the amalgamation of crustal and mantle substances in the vicinity of the subduction zone, and a certain degree of crystal fractionation has occurred. The felsic volcanic rocks originate from the partial melting of shallower crustal materials. Both are formed in the volcanic arc environment of the active continental margin.

(3)

The central Greater Xing’an Range gave rise to an Early Carboniferous magmatic arc, which emerged within the subduction setting before the collision and amalgamation of the Erguna–Xing’an Block and the Songnen Block.