Petrogenesis of the Early Jurassic–Early Cretaceous Adakite-like Rocks in the Erguna Block, NE China: Implications for the Tectonic Evolution of the Mongol–Okhotsk Ocean

Abstract

The petrogenesis and geodynamic setting of the Mesozoic magmatic rocks in the Erguna Block, NE China remains controversial, especially the relationship between magmatism and the subduction history of the Mongol–Okhotsk oceanic plate. Here we present data for the Early Jurassic–Early Cretaceous adakite-like magmatic rocks from Chaoman Farm in the northeastern part of the Erguna Block. Zircon U-Pb dating reveals that the syenogranites crystallized at around 190–180 Ma, while the monzonites, quartz diorite porphyries, and quartz monzonite porphyries were emplaced at around 147–143 Ma. The syenogranites, monzonites, quartz diorite porphyries, and quartz monzonite porphyries are adakite-like rocks. The syenogranites and quartz monzonite porphyries were produced by the partial melting of a thickened ancient mafic lower continental crust and a thickened juvenile lower crust, respectively. Meanwhile, the monzonites and quartz diorite porphyries were formed as a result of partial melting of the oceanic crust. In conclusion, the occurrence of these Early Jurassic magmatic rocks was closely linked to the process of southward subduction of the Mongol–Okhotsk oceanic plate. On the contrary, the Late Jurassic to early Early Cretaceous magmatism (147–143 Ma) occurred in an extensional environment, and was probably triggered by upwelling of the asthenosphere.

1. Introduction

Early Mesozoic granitoids and late Mesozoic volcanic rocks are widespread in the Erguna Block and its surrounding regions (Figure 1) [1,2,3,4,5,6,7,8,9]. These rocks are linked with the southward subduction of the Mongol–Okhotsk Ocean, which had a strong influence on the Mesozoic tectonic evolution and geodynamic process of the Great Xing’an Range [4,5,10,11]. The southward subduction of the Mongol–Okhotsk oceanic plate not only triggered intense magmatic activity, but also facilitated the formation of porphyry deposits during the Early Mesozoic [4,5]. Therefore, understanding the petrogenesis and geodynamics of Mesozoic magmatism can offer crucial insights into the formation and evolutionary history of the Erguna Block.

Up to now, the petrogenesis and the geodynamic setting of the Mesozoic magmatic rocks in the Erguna Block and the related tectonic transition from the Mongol–Okhotsk Ocean subduction to the continent collision have been intensely debated [2,4,8,9,12,13]. The Mesozoic magmatic suite consists of mafic to intermediate–felsic rocks [2,4,5,8,12]. The mafic rocks have been debated to derive from partial melting of depleted mantle modified by subduction-related fluids [5], or upwelling of the asthenosphere in the oceanic ridge [14]. In contrast, the intermediate–felsic rocks, including granitic and syenitic rocks, are closely related to the Cu–Mo deposits [1,2,4,5,8,9,13,14,15,16,17], and their petrogenesis remains controversial. For example, their sources have been speculated as the depleted and heterogeneous lower crust [5], the oceanic slab melts [8,9,14,18], the thickened mafic lower crust [4,8], the delaminated mafic lower crust [4], or mixing and mingling of lower crustal and upper mantle materials in the crust–mantle transition zone [5,19]. Moreover, the final closure time of the Mongol–Okhotsk Ocean [20,21,22] is controversial, ranging from the Middle Jurassic [2,5,7,11,23], the Late Jurassic–Early Cretaceous [8,9,13,14,24,25,26,27], to the Middle Cretaceous [14,18].

The Erguna Block is situated southeast of the Mongol–Okhotsk suture zone in Chinese territory, and is an ideal locus for elucidating the history of the southward subduction of the Mongol–Okhotsk oceanic plate. In this paper, we present a combined study of zircon U–Pb dating and Hf isotopes, whole-rock major and trace elemental compositions, and Sr–Nd–Pb isotope compositions for newly documented Early Jurassic–Early Cretaceous magmatic rocks from the Chaoman Forest Farm polymetallic exploration area in the northeastern Erguna Block. By integrating available geochemical data, we examine the source-region characteristics and petrogenesis of these rocks, proposing a petrogenetic model linked to different components of the lower crust in the northeastern Erguna Block. The results not only clarify the timing and petrogenesis of the Mesozoic magmatism in the Erguna Block, but also enhance our understanding of the subduction processes of the Mongol–Okhotsk oceanic plate in NE China.

Figure 1. (a) Simplified geological sketch map of the CAOB (the Central Asian Orogenic Belt) showing the main tectonic subdivisions [28]. (b) Tectonic sketch map of NE China [29].

Figure 1. (a) Simplified geological sketch map of the CAOB (the Central Asian Orogenic Belt) showing the main tectonic subdivisions [28]. (b) Tectonic sketch map of NE China [29].

2. Geologic Setting and Petrography

The Great Xing’an Range is situated in the northeast of the Central Asian Orogenic Belt, and consists of the Erguna, Xing’an, and Songnen blocks from west to east (Figure 1) [30,31]. The Erguna Block is bounded by the Mongol–Okhotsk suture zone to the north and the Xinlin–Xiguitu suture zone to the east (Figure 1a) [32]. It is generally accepted that the tectonic evolution history of the Erguna Block was dominated by the subduction of the Paleo-Asian oceanic slab during the Paleozoic and Mongol–Okhotsk oceanic slab during the Mesozoic [3,4,5,6,7,12]. Around 500 Ma, the Erguna Block collided with the Xing’an Block along the Xinlin–Xiguitu suture zone [33,34,35] and later amalgamated with the central Mongolia blocks during the early Paleozoic [36,37,38]. The basement of the Erguna Block consists of the Xinghuadukou, Ergunahe, Luomahu, and Jiageda Groups [39], which include Precambrian metamorphic supracrustal rocks and sporadic Paleoproterozoic and Neoproterozoic intrusions (Figure 1b) [40,41,42]. These are overlain by Paleozoic marine sedimentary rocks, as well as Mesozoic volcanic rocks and terrestrial clastic rocks [12,43].

The Erguna Block is characterized by extensive Paleozoic and Mesozoic intrusive and Mesozoic volcanic rocks resulting from multistage oceanic plate subduction and continent–continent collision during the Paleozoic–Mesozoic (Figure 1b) [2,3,4,5,7,13,44]. Mesozoic magmatic rocks are the most widespread throughout the Erguna Block [1,4,8,9,13,33,34]. The Mesozoic intrusive rocks in the Erguna Block comprise various lithologies and are dominated by calc-alkaline I-type granitoids that are accompanied by subordinate intermediate–mafic rocks generated during the Triassic–Early Cretaceous (250–110 Ma) [3,4,5,7,8,9,13]. The Late Jurassic to Early Cretaceous magmatism is characterized by voluminous volcanic and subvolcanic rocks in the Erguna Block [4]. This complex Mesozoic magmatism displays a close temporal, spatial, and genetic relation to the formation of porphyry Cu–Mo deposits and hydrothermal Ag and Pb–Zn deposits in the Erguna Block [45], such as the Badaguan Cu–Mo deposit [46], Taipingchuan Cu–Mo deposit [47], Wunugetushan Cu–Mo deposit [48], Fukeshan Cu–Mo deposit [8], Erentaolegai Ag deposit [49], and Jiawula Ag–Pb-Zn [50].

The Chaoman Forest Farm polymetallic exploration area is situated at the northeastern Erguna Block in the northern Great Xing’an Range, ~50 km southeast of Mohe country (Figure 2). The study area is well covered by dense primordial forest with poor Mesozoic magmatic rock outcrops along the roads (Figure 2), so we took 35 core samples from drillholes ZK0401 and ZK21001 in the study area. The monzonite and quartz diorite porphyry samples were collected from drillhole ZK0401 at a depth of 505 m; syenogranite and quartz monzonite porphyry samples were from drillhole ZK21001 at a depth of 712 m (Figure 2). Rock types, mineral assemblages, crystallization ages, and the main geochemical features are summarized in

Table 1. Lithologies, major mineral assemblages, and geochemical features of the Chaoman Forest Farm Mesozoic magmatic rocks in the Erguna Block.

Sample	Rock Type	Age ± 1σ	Mineral Assemblage	SiO2	MgO	Mg#	εHf (t)	(87Sr/86Sr)i	εNd (t)
ZK21001-53	Syenogranite	190 ± 1.2 Ma	Qtz + Kfs + Pl + Bi	69.31	0.57	34	−4.2~−0.7	0.7085	−8.5
ZK21001-55	Syenogranite		Qtz + Kfs + Pl + Bi	68.71	0.53	32
ZK21001-56	Syenogranite		Qtz + Kfs + Pl + Bi	70.37	0.52	39
ZK21001-57	Syenogranite		Qtz + Kfs + Pl + Bi	68.29	0.55	35
ZK21001-59	Syenogranite	189 ± 0.9 Ma	Qtz + Kfs + Pl + Bi	70.27	0.51	34	−5.3~0.9	0.7086	−8.5
ZK0401-9-2	Monzonite		Pl + Kfs + Amp	56.47	4.15	56
ZK0401-10-1	Monzonite	146 ± 1.5 Ma	Pl + Kfs + Amp	56.53	4.12	55	−0.2~3.7
ZK0401-13-2	Monzonite	144 ± 1.2 Ma	Pl + Kfs + Amp	55.78	3.92	54	−0.1~3.4	0.70658	−3.4
ZK0401-13-3	Monzonite		Pl + Kfs + Amp	55.67	3.84	55
ZK0401-14	Monzonite		Pl + Kfs + Amp	56.44	3.86	54
ZK0401-15-2	Monzonite		Pl + Kfs + Amp	58.96	2.80	52
ZK0401-16-1	Monzonite		Pl + Kfs + Amp	57.14	3.86	55
ZK0401-17	Monzonite	150 ± 1.5 Ma	Pl + Kfs + Amp	57.68	3.87	55	0.4~3.0	0.70656	−3.5
ZK0401-1	Quartz diorite porphyry		Qtz + Pl + Amp	63.90	1.93	52
ZK0401-2	Quartz diorite porphyry		Qtz + Pl + Amp	64.31	1.89	51
ZK0401-3	Quartz diorite porphyry	143 ± 0.9 Ma	Qtz +Pl + Amp	58.24	3.74	56	2.3~5.1	0.70617	−1.1
ZK21001-43	Quartz monzonite porphyry	143 ± 3.7 Ma	Qtz + Pl + Kfs	64.89	1.10	38	0.7~3.5	0.70528	0.0
ZK21001-44	Quartz monzonite porphyry		Qtz + Pl + Kfs	65.19	1.10	38

and representative photographs of the rocks are shown in Figure 3.

The syenogranite is coarse-grained and contains 20–30 vol.% quartz, 15–20 vol.% plagioclase, 30–35 vol.% K-feldspar, and 1–3 vol.% biotite, along with accessory magnetite, zirconm and apatite (Figure 3a,b). The monzonite is massive in structure and contains alkali feldspar (33–35 vol.%), plagioclase (14–21 vol.%), amphibole (20–27 vol.%), biotite (12–18 vol.%), quartz (~3 vol.%), and accessory zircon and apatite (4–5 vol.%; Figure 3c). The quartz diorite porphyry shows porphyritic textures (Figure 3d,e). Phenocrysts of medium- to fine-grained amphibole make up 20 vol.% of the rock and plagioclase phenocrysts make up 15 vol.%; there are a few biotite phenocrysts (Figure 3d,e). Secondary minerals are sericite, quartz, magnetite, chalcopyrite, and pyrite. The quartz monzonite porphyry is porphyritic in texture, and has a microgranular groundmass. Phenocrysts in the quartz monzonite porphyry are mainly medium- to fine-grained stumpy plagioclases and biotite (Figure 3f).

3. Sample Preparation and Analytical Methods

3.1. Zircon U–Pb Dating and Hf Isotopic Analyses

The zircon grains were separated from crushed samples using conventional heavy liquid and magnetic techniques at the Langfang Regional Geological Survey (Langfang, China). Cathodoluminescence (CL) images of analyzed zircon grains were acquired using a FEI NOVA NanoSEM 450 scanning electron microscope outfitted with a Matan Mono CL4 cathodoluminescence system at the Institute of Geology, Chinese Academy of Geological Sciences (CAGS), Beijing, China. The CL image conditions include 2 min scanning time, 15 kV accelerating voltage, and 120 nA beam current. The analysis spots (free from cracks or inclusions) were selected after transmitted light optical microscopy and CL images. The zircon U–Pb analysis used a Neptune laser ablation multiple accepting inductively coupled plasma mass spectrometer (LA–MC-ICP-MS) at the Institute of Mineral Resources, CAGS, Beijing, China. The zircon GJ-1 and the NIST SRM610 were used as external standards for the measurements of the U–Pb isotope ratios and the trace elements, respectively. The spot diameter of the laser ablation pits was 30 μm. Detailed operating conditions for instruments and analytical procedures are the same as described by Hou et al., (2009) [51]. The ICPMSDataCal [52] and Isoplot [53] programs were used for data reduction. Common Pb was corrected following the method outlined by Andersen (2002) [54]. Errors on pooled ages are quoted at the 95% (2σ) confidence level.

Zircon Hf isotopic analyses were performed on the same zircon grains as the U–Pb isotope analyses stated above. In situ zircon Hf isotopic analyses were also performed at the Institute of Mineral Resources, CAGS, Beijing, China, using a Neptune MC-LA-ICP-MS with a 193 nm excimer ArF laser ablation system (GeoLas Plus). All zircon grains were analyzed using a single-spot ablation mode with a spot size of 44 μm and a laser repetition rate of 8 Hz. The GJ-1 zircon standard was used for external standardization. The weighted mean ¹⁷⁶Hf/¹⁷⁷Hf isotopic ratio of the GJ-1 zircon grains was 0.282007 ± 0.000011 (2σ, n = 36). Details of the analytical methods are given by Hou et al., (2007) [55]. The measured ¹⁷⁶Lu/¹⁷⁷Hf ratios and the ¹⁷⁶Lu decay constant of 1.867 × 10⁻¹¹ yr⁻¹ [56] were used to calculate initial ¹⁷⁶Hf/¹⁷⁷Hf ratios. The chondritic values of ¹⁷⁶Lu/¹⁷⁷Hf = 0.0336 and ¹⁷⁶Hf/¹⁷⁷Hf = 0.282785 reported by Bouvier et al., (2008) were used for the calculation of the εHf values [57]. The depleted mantle Hf model ages (T_DM) were calculated using the measured ¹⁷⁶Lu/¹⁷⁷Hf ratios based on the assumption that the depleted mantle reservoir has a linear isotopic growth from ¹⁷⁶Hf/¹⁷⁷Hf = 0.279718 at 4.55 Ma to 0.283250 at present, with ¹⁷⁶Lu/¹⁷⁷Hf = 0.0384 [58]. Two-stage model ages (T_DM2) were calculated by assuming that the parental magma was produced from an average continental crust (¹⁷⁶Lu/¹⁷⁷Hf = 0.015) [59].

3.2. Whole-Rock Major and Trace Elemental Analyses

We selected 18 fresh samples for whole-rock geochemical analyses after petrographic examination and removal of the altered/weathered surface. All samples were crushed and powdered to ∼200 mesh in an agate mill. Chemical analyses were performed at the Nuclear Industry Beijing Institute of Geology Research and Testing Research Center. Major element compositions were determined by X-ray fluorescence (MEXRF26) using fused glass disks, with analytical precision and accuracy better than 5%. The trace element concentrations were determined using ICP–MS, which gives a precision better than 10% for most of the elements analyzed.

3.3. Whole-Rock Sr–Nd–Pb Isotopic Analyses

Whole-rock Sr–Nd–Pb isotopic composition was undertaken with a Finnigan MAT-262 mass spectrometer at the Laboratory for Radiogenic Isotope Geochemistry, University of Science and Technology of China, Hefei. For Nd–Sr isotope analyses, Rb–Sr and light rare-earth elements were isolated on quartz columns by conventional ion exchange chromatography with a 5 mL resin bed of AG 50W-X12 (200–400 mesh). Nd and Sm were separated from other rare-earth elements on quartz columns using 1.7 mL Teflon powder as the cation exchange medium. For isotopic measurements, Sr was loaded with a Ta–HF activator on pre-conditioned W-filaments and was measured in single-filament mode. Nd was loaded on Re-filaments and measurements were performed in a double-filament configuration. Measured Sr and Nd isotopic ratios were normalized for mass fractionation using ⁸⁶Sr/⁸⁸Sr = 0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219, respectively. For Pb isotope determinations, 200 mg of the sample material was dissolved in HF–HNO₃ acid at 120 °C for 7 days using Teflon vials. Pb was separated by anion exchange chromatography with diluted HBr acid as the elutant. The precision of ⁸⁷Rb/⁸⁶Sr and ¹⁴⁷Sm/¹⁴⁴Nd ratios is better than 0.5%. The precision of the measured Pb isotopic ratios is better than 0.01%. Analytical precisions are stated as 2σ standard errors. Further details on analytical techniques are given in [60,61].

4. Results

Zircon U–Pb geochronology, major and trace element compositions of the samples, zircon Hf isotope data, and whole-rock Sr–Nd–Pb isotope data are listed in Tables S1–S4, respectively.

4.1. Zircon U–Pb Geochronology

Zircon grains from 12 representative samples were chosen for LA–ICP-MS U–Pb dating. The zircon U–Pb isotopic data are given in Table S1. All the zircons from these intermediate–felsic rocks display well-developed oscillatory growth zoning in the CL images (Figure 4), and have Th/U ratios of 0.23–3.34 (mostly between 0.4 and 1.0) (Table S1), indicative of a magmatic origin [62]. They are euhedral with a long prismatic shape, with average crystal lengths of 80–120 μm and length-to-width ratios from 2:1 to 3:1 (Figure 4). Zircons from the diabase are 80–100 μm long with aspect ratios of 1:1 to 3:1, and show patchy or banded zoning (Figure 4). The U–Pb concordia diagrams are shown in Figure 5.

Sixteen U–Pb spots on 16 zircon grains from sample ZK21001-46, 10 U–Pb spots on 10 zircon grains from sample ZK21001-53 and 18 U–Pb spots on 18 zircon grains from sample ZK21001-59 of the syenogranite yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 181 ± 1.5 Ma (MSWD = 1.08), 190 ± 1.2 Ma (MSWD = 0.0328) and 189 ± 0.94 Ma (MSWD = 0.2), respectively.

Eight U–Pb spots on eight zircon grains from sample ZK0401-10-1, 16 U–Pb spots on 16 zircon grains from the sample ZK0401-13-2 and eight U–Pb spots on eight zircon grains from sample ZK0401-17 of the monzonite yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 147 ± 2.7 Ma (MSWD = 1.90), 147 ± 1.2 Ma (MSWD = 0.63) and 147 ± 1.50 Ma (MSWD = 0.86), respectively.

For quartz diorite porphyry sample ZK0401-3, the ²⁰⁶Pb/²³⁸U ages from 20 analyses ranged from 144 to 143 Ma (Table S1), yielding a weighted mean ²⁰⁶Pb/²³⁸U age of 143 ± 0.9 Ma (MSWD = 0.10; Figure 5g), which represents the crystallization age of the quartz diorite porphyry. For quartz monzonite porphyry sample ZK21001-53, the ²⁰⁶Pb/²³⁸U ages from five analyses ranged from 140 to 150 Ma (Table S1), yielding a weighted mean ²⁰⁶Pb/²³⁸U age of 143 ± 3.7 Ma (MSWD = 1.3) (Figure 5h), which represents the crystallization age of the quartz diorite porphyry.

In summary, these zircon U–Pb ages indicate that the magmatic rocks from the Chaoman Forest Farm polymetallic exploration area were emplaced in two stages, during the Early Jurassic, and the Late Jurassic–early Early Cretaceous, similar to magmatic rocks (~144–193 Ma) associated with the Fukeshan deposit near the Chaoman Forest Farm (Figure 2) [8].

4.2. Whole-Rock Major and Trace Elemental Compositions

Whole-rock major and trace elemental compositions of the magmatic rocks from the Chaoman Forest Farm are listed in Table S2. On a total alkalis (Na₂O + K₂O) versus SiO₂ (TAS) diagram (Figure 6a) [63], most samples are classified as monzogabbro, monzodiorite, monzonite, quartz monzonite, or with some plotting as granodiorite or granite (Figure 6a). According to the K2O versus SiO2 (Figure 6b) [64] diagrams, the intermediate–felsic rocks belong to the high-K calc-alkaline series. In addition, the syenogranites are peraluminous rocks with A/CNK [molar Al2O3/(CaO + K2O + Na2O)] ratios of 1.0–1.2 (Figure 6c and Table S2) [65], while the monzonites, quartz diorite porphyries and quartz monzonite porphyries are metaluminous with A/CNK ratios of 0.8–1.0 (Figure 6c). The intermediate–felsic rocks are transitional I-type granite to A-type granite on the (Na2O + K2O) versus 10,000 Ga/Al diagrams (Figure 6d). In addition, syenogranites, monzonites, quartz diorite porphyries, and quartz monzonite porphyries have relatively low heavy rare-earth element (HREE) (Yb = 0.57–1.84 ppm) and Y concentrations (9.40–19.0 ppm), as well as high Sr contents (345–1168 ppm) and Sr/Y ratios (27.3–111). Therefore, these rocks have an adakitic character according to the definition of Defant and Drummond (1990, 1993) [66,67], which is also revealed by the use of the Sr/Y versus Y and (La/Yb)_N versus (Yb)_N discrimination diagrams (Figure 7). In the PRIMA-normalized spidergram (Figure 8a), monzonite and quartz diorite porphyry show strong enrichments in large ion lithophile elements (LILEs) and pronounced negative anomalies in Nb, Ta, and Ti. Furthermore, they display slightly negative Eu anomalies and have inclined chondrite-normalized rare-earth element (REE) patterns (Figure 8b). The syenogranite and quartz monzonite porphyry samples have low total rare trace elements (REEs) with slightly negative Eu anomalies, and show slight LREE enrichment (Figure 8c). On the primitive mantle-normalized trace element spidergram (Figure 8d), the samples exhibit notable negative Nb, Ta, and Ti, along with positive Sr anomalies.

4.3. In Situ Zircon Lu–Hf Isotopes

The Table S3 lists the in situ Lu–Hf isotopic data for the representative zircon grains from the nine samples.

A total of 20 Hf isotopic analyses on the zircon grains from two samples of the syenogranite yielded initial ¹⁷⁶Hf/¹⁷⁷Hf ratios ranging from 0.282517 to 0.282692 and gave ε_Hf(t) values of –5.3 to 0.9 and T_DMC Hf crustal model ages of 1.8–1.3 Ga (Figure 9a). Zircon grains from the three samples of monzonite yielded initial ¹⁷⁶Hf/¹⁷⁷Hf ratios ranging from 0.282687 to 0.282799 with consistent ε_Hf(t) values of –0.2 to 3.7 (n = 30) and T_DMC Hf model ages of 1.4 to 1.09 Ga (Figure 9a). Ten Hf isotopic spot analyses on the zircon grains from the quartz diorite porphyry sample yielded initial ¹⁷⁶Hf/¹⁷⁷Hf ratios ranging from 0.282764 to 0.282842 with corresponding ε_Hf(t) values of 2.3 to 5.1 and T_DMC Hf model ages of 1199–984 Ma (Figure 9a). Four Hf isotopic spot analyses on the zircon grains from sample of the quartz monzonite porphyry yielded a narrow range of ε_Hf(t) values of 0.7 to 3.5 and T_DMC Hf model ages of 1326 to 1107 Ma (Figure 9a).

4.4. Whole-Rock Sr–Nd–Pb Isotopes

Rb–Sr, Sm–Nd, and Pb isotopic data of the magmatic rocks from the Chaoman Forest Farm are given in Table S4. All rocks show a limited variation in their radiogenic Sr, Nd, and Pb isotope composition when calculated based on the crystallization ages presented above.

The syenogranites have high (⁸⁷Sr/⁸⁶Sr)_i (0.7063–0.7086) and low ε_Nd(t) (−1.7 to −8.5) (Figure 9b) and show slightly lower radiogenic Pb isotopic compositions, with (²⁰⁶Pb/²⁰⁴Pb)_i = 18.292–18.467, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.622–15.642 and (²⁰⁸Pb/²⁰⁴Pb)_i = 38.388–38.532 (Figure 10). The monzonites, quartz diorite porphyries, and quartz monzonite porphyries have low (⁸⁷Sr/⁸⁶Sr)_i (0.7053–0.7066) and high ε_Nd(t) (−3.5 to 0.5) (Figure 9b) and show slightly less radiogenic Pb isotopic compositions, with (²⁰⁶Pb/²⁰⁴Pb)_i = 18.406–18.486, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.587–15.624 and (²⁰⁸Pb/²⁰⁴Pb)_i = 38.344–38.482, respectively (Figure 10). The initial ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios plot above the Northern Hemisphere Reference Line (NHRL) (Figure 10) [79].

5. Discussion

5.1. The Episodic Magmatism in the Erguna Block during the Mesozoic

Our age data of this study, together with literature zircon U–Pb age data, reveal that the Mesozoic magmatism in the Erguna Block is dominated by granitoids and rhyolites with minor mafic rocks and can be subdivided into five episodes: Early-Middle Triassic (249–237 Ma), Late Triassic (229–201 Ma), Early-Middle Jurassic (199–171 Ma), Late Jurassic (155–145 Ma), and Early Cretaceous (145–119 Ma) [1,3,4,5,7,8,9,12,13,82]. The Early-Middle Triassic (249–237 Ma) intrusive rocks form a suite of gabbro–diorites, diorites, quartz diorites, granodiorites, monzogranites, and syenogranites [1,3]. The Late Triassic (229–201 Ma) intrusive rocks consist of a suite of granodiorites, syenogranites, and gabbro–diorites, and are widespread in the Erguna Block [2,5,83,84]. The Early-Middle Jurassic (199–171 Ma) intrusive rocks in the Erguna Block consist of monzogranites, syenogranites, quartz porphyries, and granite porphyries [1,2,83,85,86] and show close relationships with porphyry Cu–Mo deposits, such as the Taipingchuan [74] and Wunugetushan porphyry Cu–Mo deposit [19,75]. The Late Jurassic to Early Cretaceous intrusive rocks contain syenogranites, monzonites and quartz monzonites, diorite porphyries, quartz monzonite porphyries and granodiorite porphyries, and have close relationships with Cu–Mo mineralization, such as Fukeshan [8] and Xiaokele [9]. The Early Cretaceous (145–119 Ma) magmatic rocks are characterized by large-scale volcanic rocks, and minor granitoids in the Erguna Block [4,7,12,23].

In this study, we have obtained new zircon U–Pb ages from magmatic rocks in the northeastern Erguna Block, which include ~190 Ma and ~136 Ma to ~140 Ma. The magmatic rocks from the Chaoman Forest Farm were produced during two periods of activity: (1) Early Jurassic (190–180 Ma), when abundant granitoids were emplaced, including syenogranites; and (2) Late Jurassic to early Early Cretaceous (147–143 Ma), when the monzonite, quartz diorite porphyry and quartz monzonite porphyry were emplaced.

According to the definition of adakite [66,87], adakite-like rocks are those with SiO₂ ≥ 56 wt. %, Al₂O₃ ≥ 15 wt. %, Sr/Y ≥ 20, Y ≤ 18 ppm and Yb ≤ 1.9 ppm. In this study, the Early Jurassic syenogranites and Late Jurassic to early Early Cretaceous monzonites, quartz diorite porphyries, and quartz monzonite porphyries belong to the adakite-like group.

5.2. Origin of the Adakite-like Rocks

As previously described, the Early Jurassic syenogranites and Late Jurassic to early Early Cretaceous monzonite, quartz diorite porphyry and quartz monzonite porphyry have high SiO₂ contents (up to 70.37 wt.%), Sr (345–1168 ppm) and Sr/Y (27–111) ratios, low Y (9.40–19.0 ppm) and Yb (0.57–1.84 ppm) concentrations and slightly negative Eu anomalies (Figure 7 and Figure 8) (Table S2), typical of adakites [66]. As initially defined by Defant and Drummond (1990), adakites originated from the partial melting of a young subducted oceanic crust exhibiting distinctive geochemical signatures, including high Sr and low Y and Yb contents, resulting in high Sr/Y ratios [66]. In addition, later studies have proposed different models to interpret the genesis of adakitic rocks, including (1) magma mixing between felsic and basaltic magmas [88], (2) partial melting of thickened or underplated lower crust [89,90,91] or delaminated continental crust [92], and (3) fractional crystallization (FC) or crustal assimilation and fractional crystallization (AFC) from basaltic parent magmas [87,93,94].

We propose that the studied Chaoman Forest Farm Late Jurassic–early Early Cretaceous monzonites and quartz diorite porphyries probably formed by partial melting of subducting oceanic crust based on the following reasons: (1) The Chaoman Forest Farm monzonites and quartz diorite porphyries have relatively low K₂O/Na₂O (0.43–0.97) and high CaO/Al₂O₃ (0.21–0.36) values, which are similar to those observed in adakitic rocks formed from partial melting of subducted oceanic crust (with K₂O/Na₂O < 0.71 and CaO/Al₂O₃ > 0.2) [95], just like the Late Jurassic slab-derived adakites from the Fukeshan deposit in its neighboring areas (Figure 6) [8]. Furthermore, the monzonites and quartz diorite porphyries mostly plot in the field of adakitic rocks derived from melting of subducting oceanic crust in the Ni, Cr, Mg^# and MgO discrimination diagrams (Figure 11a–e). The high MgO and Mg^# values may indicate the interaction of a slab-derived adakitic melt and a mantle-derived magma, which is also supported by the presence of the contemporaneous mafic rocks [96,97].

The SiO₂ versus Mg^# diagram (Figure 11b) illustrates that mantle AFC modelling [102] suggests that the adakitic melts probably mixed with a minor fraction (<10%) of mantle-derived magma [103]; (2) The HREE and Y contents, and the Sr/Y, (La/Yb)_N and (Dy/Yb)_N ratios of the monzonites and quartz diorite porphyries are generally consistent with those of the Late Jurassic adakites derived from subducted oceanic crust in the Erguna Block. Their comparatively low (La/Yb)_N ratios could also be attributed to the interaction with a mantle-derived melt. Furthermore, their low Th and Th/La values are comparable to those of typical MORB and the Cretaceous slab-derived adakites in the southern Lhasa terrane (Figure 11f), indicating that their formations are closely associated with the subducted basaltic oceanic crust (Figure 11f) [104,105]; and (3) The monzonites and quartz diorite porphyries display relatively low (⁸⁷Sr/⁸⁶Sr)_i (0.7062–0.7066) and slightly enriched to depleted ε_Hf(t) values (–0.2 to 5.1) and ε_Nd(t) values (–3.49 to –1.13), similar to the Early Jurassic oceanic slab-derived adakitic rocks associated with the Taipingchuan porphyry Cu–Mo deposit, as well as the Late Jurassic to Early Cretaceous slab-derived adakitic rocks linked to the Fukeshan porphyry Cu–Mo deposit (Figure 9) [8,74]. Furthermore, most of the ε_Hf(t) values plot between the “new continental crust” evolutionary line and chondrite line [78] (Figure 9a), implying a significant involvement of newly formed juvenile crustal material. The weakly negative ε_Hf(t) and ε_Nd(t) values indicate that the monzonites and quartz diorite porphyries were derived from an oceanic slab with minor contamination of old continental crustal materials [8,74].

Notably, the monzonites show higher Sr/Y and (La/Yb)_N ratios than the quartz diorite porphyries (Figure 9 and Figure 10). The batch melting model shown on the (La/Yb)_N versus (Yb)_N diagram indicates that the monzonites could have been derived from ~20% melting of a presumed 10% garnet-bearing amphibolite source, and that the quartz diorite porphyries was probably derived from the partial melting of a 3% garnet-bearing amphibolite source (Figure 7). In summary, the Chaoman Forest Farm monzonites and quartz diorite porphyries were likely generated by partial melting of subducting oceanic crust. As these slab-derived melts ascended, they mixed with mantle-derived magma, resulting in increased MgO and compatible elements contents, followed by minor crustal contamination.

The adakitic Early Jurassic syenogranites and Early Cretaceous quartz monzonite porphyries have high SiO₂ and K₂O and low A/NK ratios, MgO (0.51–1.10 wt.%) and Mg^# (32–39) values similar to those of experimentally-derived metabasaltic and eclogitic melts at high pressures (1.0–4.0 GPa) and adakitic rocks originating from a thickened lower crust (Figure 6b,c and Figure 11a,b). Melting experiments indicate that primary melts from basaltic rocks exhibit low MgO and Mg^# values [106], akin to those observed in adakitic rocks formed through partial melting of thickened lower crust [107]. In the Cr versus SiO₂, Ni versus SiO₂, and Ni versus Cr diagrams (Figure 11c–e), all syenogranites and quartz monzonite porphyries plot within the adakitic rocks field, indicating derivation through partial melting of the thickened lower crust, consistent with their very low concentrations of compatible elements (Cr = 1.89–2.91 ppm, Ni = 1.47–2.68 ppm) (Figure 11e). Therefore, we suggest that the Early Jurassic syenogranites and Early Cretaceous quartz monzonite porphyries were probably derived by partial melting of the thickened lower crust.

Prior research has suggested that adakites derived from the lower crust originate from eclogite or garnet amphibolite source rocks [91,108,109]. Nevertheless, garnet exhibits significance enrichment in HREE, whereas the amphibole is relatively enriched in MREE [110]. The concave-upward Dy–Ho–Er–Tm-depleted patterns (Figure 8d) observed in the syenogranites and quartz monzonite porphyries suggest that their formation involved partial melting of the basaltic lower crust under variable water fugacities, producing a residue in garnet and amphibole [11,69]. The syenogranites have higher La/Sm and Sm/Yb ratios than the quartz monzonite porphyries, suggesting that the syenogranites originated from a thickened lower crust with either an LREE-enriched source or a garnet-bearing, strongly metasomatized source typical of continental arc settings [96]. According to the (La/Yb)_N vs. (Yb)_N diagram derived from batch-melting modelling (Figure 7), the syenogranites may have formed through less than 20% melting of a presumed source composed of 15% garnet-bearing amphibolite. In contrast, the quartz monzonite porphyry samples show 10%–30% partial melting of a 5% garnet-bearing amphibolite source (Figure 7).

In addition, the syenogranites show markedly distinct initial Sr–Nd–Pb-Hf isotopic compositions from the quartz monzonite porphyries (Figure 9 and Figure 10), there is a disparity in their geochemical characters indicating different magma sources. The syenogranites have lower ε_Hf(t) (−5.3 to 0.9), ε_Nd(t) (−8.46 to −8.45) values, high (⁸⁷Sr/⁸⁶Sr)_i (0.7085–0.7086) and slightly low radiogenic Pb isotopic compositions with (²⁰⁶Pb/²⁰⁴Pb)_i = 18.247–18.292 relative to those of the quartz monzonite porphyries (Figure 9 and Figure 10). All syenogranite samples plot in the evolutionary zone of the Paleoproterozoic–Mesoproterozoic crust below the chondrite evolutionary line (Figure 9a), implying that the syenogranites were probably formed by reworking of Paleoproterozoic–Mesoproterozoic mafic lower crust. However, the quartz monzonite porphyries have high Hf isotopic compositions close to those of the monzonite and quartz diorite porphyries (Figure 9a), indicating that they were the mixing products between crustal and mantle-derived melts rather than a single Mesoproterozoic juvenile mafic lower crust.

Therefore, we conclude that the Early Jurassic syenogranites resulted from the partial melting of a thickened Paleoproterozoic–Mesoproterozoic mafic lower rocks (15% garnet-bearing amphibolite). The Early Cretaceous quartz monzonite porphyries were probably derived by partial melting of a thickened Mesoproterozoic juvenile mafic lower crust (5% garnet-bearing amphibolite) with variable contributions of mantle-derived magma.

5.3. Spatial-Temporal Variations of Nd and Hf Isotopes of the Mesozoic Magmatic Rocks in the Erguna Block

The integration of isotopic data from the Mesozoic magmatic rocks in the Erguna Block reveals distinctive geochemical characteristics in zircon ε_Hf(t) and whole-rock ε_Nd(t) values across various locations [5,19,74,75,82]. The zircon ε_Hf(t) and whole-rock ε_Nd(t) values of the Mesozoic granitoids in the Erguna Block change with longitude and latitude, specifically. The lower continental crust shows a progressive increase in the incorporation of ancient crustal material from southwest to northeast [5,82]. For example, the Early Jurassic granitoids from the western part of the Erguna Block have higher zircon ε_Hf(t) and whole-rock ε_Nd(t) values than those of the rocks in the eastern part of the Erguna Block [5,19,74,75,82]. Examining the correlation between zircon ages, T_DM2 ages, and the variation in ε_Hf(t) values across latitudinal gradients, Sun et al., 2017 suggested that the Early-Middle Triassic granitoids in the northeast of the Erguna Block were derived from Mesoproterozoic source rocks; the Late Jurassic granitoids in the central Erguna Block were derived from late Mesoproterozoic source rocks; and the Late Jurassic and Early Cretaceous granitoids in the southwest Erguna Block were derived from Neoproterozoic source rocks [82]. Notably, most of the Early Jurassic felsic rocks in the southwest part of the Erguna Block have higher ε_Hf(t) values than those of contemporaneous mafic rocks in the Erguna Block (Figure 12) [5]. Thus, the mantle-derived melt additions are not a cause of the relatively high zircon ε_Hf(t) values of the Early Jurassic felsic rocks in the southwest. This implies that the spatial variation of isotopic compositions of the Mesozoic rocks is a response to a heterogeneity of the lower continental crust of the Erguna Block during the Mesozoic [5,82]. This interpretation is supported by the presence of outcrops of Neoproterozoic and Paleoproterozoic granitoids in the northeast of the Erguna Block, which have obvious negative ε_Hf(t) values (−3.9 to −8.5) with corresponding Archaeozoic to Paleoproterozoic Hf model ages, indicating that the Erguna Block is underlain by a Paleoproterozoic and even an Archaeozoic basement [42,83,111,112,113,114].

Furthermore, based on the statistics of zircon ε_Hf(t) and whole-rock ε_Nd(t) values of the Mesozoic magmatic rocks in the Erguna Block, these magmatic rocks have formed from the end of the Permian at least to the end of the Mesozoic and also show temporal variation in Nd-Hf isotopic composition (Figure 12). In terms of Nd–Hf isotopic compositions (Figure 12), it is found that their zircon ε_Hf(t) and whole-rock ε_Nd(t) values gradually increase with a decrease in their ages (Figure 12). As discussed above, the Early Jurassic granitoids of the Chaoman Forest Farm show negative whole-rock ε_Nd(t) and zircon ε_Hf(t) values, suggesting their predominant derivation from mature or recycled continental crust materials. In contrast, the Late Jurassic to Early Cretaceous granitoids from the Chaoman Forest Farm mainly show negative to near-zero ε_Nd(t) and positive ε_Hf(t) values (Figure 12), implying that they were probably derived from the juvenile crust. The relationship between Nd-Hf isotopic data and the timing of magmatic rock formation indicates a shift in magma sources from ancient crustal melting to juvenile crust during the Mesozoic [82,114]. The temporal variations observed are equal in scale to spatial geochemical trends persistent throughout the Mesozoic across the Erguna Block and such spatial–temporal variations of isotopic compositions are a common feature of magmatic rocks generated in continental magmatic arcs with different basement components [91,115,116].

Figure 12. Diagrams of whole-rock ε_Nd(t) (a) and zircon ε_Hf(t) (b) against U–Pb ages of the Chaoman Forest Farm Mesozoic magmatic rocks. Data sources: Mesozoic mafic rocks in the Erguna Block [5,19,111]; Mesozoic intermediate–felsic rocks in the Erguna Block [2,4,5,8,10,13,15,16,17,74,75,117,118,119,120,121].

Figure 12. Diagrams of whole-rock ε_Nd(t) (a) and zircon ε_Hf(t) (b) against U–Pb ages of the Chaoman Forest Farm Mesozoic magmatic rocks. Data sources: Mesozoic mafic rocks in the Erguna Block [5,19,111]; Mesozoic intermediate–felsic rocks in the Erguna Block [2,4,5,8,10,13,15,16,17,74,75,117,118,119,120,121].

It is widely accepted that the Mongol–Okhotsk Oceanic plate was subducted southward beneath the Erguna Block during the Late Permian to Early Jurassic [3,4,5,82,114]. Consequently, the north margin of the Erguna Block formed a continental magmatic arc and the Late Permian to Early Jurassic juvenile mafic rocks underplated into the lower crust of the magmatic arc. As a result, the Late Jurassic and Early Cretaceous granitoids in the west derived from the juvenile mafic lower crust of the Permian–Jurassic magmatic arc have relatively high zircon ε_Hf(t) and whole-rock ε_Nd(t) values, whereas the Early Jurassic granitoids in the east derived from the ancient mafic rocks in the lower crust of the Erguna Block have relatively low zircon ε_Hf(t) and whole-rock ε_Nd(t) values (Figure 12b). Overall, the Mesozoic granitoids in the Erguna Block were derived from the source rocks with different ages in different parts of the lower continental crust due to crustal accretion and reworking process [5,82,114]. The findings suggest a gradual replacement of the ancient lower crust in the Erguna Block by mantle-derived juvenile materials from the Early Jurassic to Early Cretaceous, with the underplating of arc basaltic magmas playing a pivotal role in vertical crustal growth.

5.4. Tectonic Setting of Mesozoic Magmatism in the Erguna Block

5.4.1. Early Jurassic Active Continental Margin

After the final closure of the Paleo-Asian Ocean during the end of the Permian to beginning of the Early Triassic [1,86,122,123], the tectonic evolution of NE China was characterized predominantly by the Paleo-Asian and Paleo-Pacific tectonic regimes during the Mesozoic [3,5,13]. It has previously been thought that the Mongol–Okhotsk Oceanic plate was subducted southward beneath the Erguna Block between the Late Permian and Early Jurassic, as supported by the occurrence of calc-alkaline volcanic belts and granitoid intrusions along the southern margins of the Mongol–Okhotsk orogenic belt [1,3,5,45,85,86,124]. The northern margin of the Erguna Block is generally considered as an Andean-type continental arc during the early Mesozoic, i.e., Late Triassic-Early Jurassic magmatism in the Erguna Block was associated with the southward subduction of the Mongol–Okhotsk Ocean [3,6,13]. The existence of subduction-related porphyry Cu–Mo deposits [16] (e.g., Wunugetushan, ~180 Ma) indicates an active continental margin setting for the Erguna Block. In addition, calc-alkaline geochemical characteristics, enrichment in LILE, and depletion in HFSE also clearly point to a subduction-related origin for the Early Jurassic magmas [5,8,13].

The Early Jurassic syenogranite samples are characterized by significant enrichment in LILE and LREE elements, along with depletion in HFSE and HREE elements. They fall within the volcanic arc field (Figure 13), showcasing geochemical features akin to subduction-related magmas [125]. This indicates that the Mongol–Okhotsk Oceanic plate was undergoing persistent southwards subduction during the Early Jurassic (Figure 14a) [8]. The Early Jurassic syenogranites in the Chaoman Forest Farm are adakitic and formed from primary magmas generated by the partial melting of a thickened lower crust. Earlier research has demonstrated that during the Early Jurassic, the Erguna Block underwent crustal shortening and thickening [11,86]. The compressional active continental margin setting during the Early Jurassic might be triggered by a low subduction angle of the Mongol–Okhotsk Oceanic plate, similar to an Andean-type active continental margin [126].

5.4.2. Middle Jurassic–Early Early Cretaceous Extension

In the Badaguan area of the northern Euguna Block, the syenogranite with an age of ~155 Ma has strongly negative Eu anomalies and high alkaline contents, suggesting its affinity to A-type granite [4]. The volcanic rocks of the Tamulangou Formation in the Manzhouli area of the central Euguna Block (Figure 1), such as basaltic trachyandesites, trachyandesites, and trachytes, dated between 158–166 Ma (with a peak age of 162 Ma), exhibit geochemical characteristics transitioning from alkaline to sub-alkaline compositions [12,128]. Thus, the Erguna Block could have undergone a transition from compression to extension during the late Middle Jurassic. This process can also be demonstrated by the developmental history of the Mohe Basin. During the Jurassic, against the backdrop of the gradual west-to-east subduction of the Mongol–Okhotsk Ocean, the Mohe Basin underwent four distinct stages: initial basin formation, compression and subsequent uplift, formation of volcanic rift basins, and subsequent uplift and contraction. The western part of the basin displays thrust-over structures, while the central and eastern portions exhibit characteristics of extensional rifting [129,130]. A new paleomagnetic study suggests that the Mongol–Okhotsk Ocean was still ~3000 km wide in its eastern part at ca. 155 Ma [25], so the extensional setting is unlikely to have resulted from a post-collisional setting. Instead, a more plausible explanation is that it was triggered by the rollback of the subducted Mongol–Okhotsk Oceanic plate.

However, the geodynamic mechanism related to the late Late Jurassic–early Early Cretaceous (155–143 Ma) magmatism in the Euguna Block has been the subject of intense debate. One of the most controversial issues regarding the tectonic evolution is the timing of the final closure of the Mongol–Okhotsk Ocean. Although some authors have suggested that the Mongol–Okhotsk Ocean closed during the Middle Jurassic, to the northwest of the Erguna Block [2,11,22,131,132,133,134,135], a larger number of studies have argued that the southwards subduction of the Mongol–Okhotsk Oceanic plate lasted until the Late Jurassic–Early Cretaceous on the basis of the Late Jurassic–Early Cretaceous subduction-related igneous rocks associated with the newly discovered Fukeshan porphyry Cu–Mo deposit and paleomagnetic data [24,25]. In addition, Yang et al., (2015) suggested that the rapid closure of the Mongol–Okhotsk Ocean during the latest Jurassic–earliest Cretaceous led to the development of a large fold-and-thrust belt in northern China and Mongolia [26]. Guo et al., (2017b) reviewed the stratigraphic and sedimentological characteristics for the Mohe–Upper Amur Basin and proposed that the closure of the eastern Mongol–Okhotsk Ocean should have occurred in the latest Jurassic–early Cretaceous [119].

In this study, we explore these questions by analyzing the associations of magmatic rocks and investigating spatial and temporal variations in the late Mesozoic magmatism within the Erguna Block. The Early Jurassic to late Early Cretaceous magmatic rocks are composed of a series of basic to acid rocks with different geochemical characteristics. As discussed above, the late Late Jurassic to early Early Cretaceous monzonites, quartz diorite porphyries, and quartz monzonite porphyries in the Erguna Block are adakite-like rocks. The monzonites and quartz diorite porphyries were formed by partial melting of the subducting oceanic crust, and the quartz monzonite porphyries were formed by partial melting of a thickened lower crust. In addition, the trace element distribution patterns of the studied samples exhibit enriched LREE, LILE, and fluid mobile elements and depleted HFSE (Nb-Ta-Ti), as observed in subduction-related arc magmatic rocks [136,137]. In both Nb versus Y and Rb versus (Y + Nb) diagrams [127], the felsic samples plot within the volcanic arc field (Figure 13). All these features suggest that the tectonic setting of the Late Jurassic to early Early Cretaceous magmatic rocks might belong to an active continental margin, related to the subduction of the Mongol–Okhotsk Oceanic plate.

Experimental results and thermal modeling indicate that partial melting of the subducted oceanic crust only occurs primarily in hot subduction zone settings where temperatures range between 800 and 1000 °C at depths of 70–80 km [97,138,139], including mid-ocean ridge subduction [104,140] and oceanic slab roll-back regions [69,105]. Thus, a simple normal-angle (cold) subduction model cannot easily explain shallow slab-melting (i.e., oceanic slab-derived adakite-like rocks such as the studied monzonites and quartz diorite porphyries). Deng et al., (2019a) attributed the late Late Jurassic adakite-like magmas to the subduction of a mid-ocean ridge [8]. If this model were correct, mid-ocean ridge subduction would produce a high temperature and relatively H₂O-poor magmas above the slab window. This is inconsistent with requirement of the amphibole crystallization as noted above. Here, we suggest that asthenospheric upwelling—probably a result of slab roll-back—provided the required high thermal regime necessary for partial melting of the subducting Mongol–Okhotsk Oceanic crust and thus was responsible for the formation of the Chaoman oceanic slab-derived monzonites and quartz diorite porphyries (Figure 14b). The slab roll-back would induce the invasion of deep asthenosphere into the mantle wedge and the development of an extensional environment in the overlying lithosphere mantle, which was further verified by the presence of the simultaneously A-type granites [4]. The high thermal anomaly resulted in the partial melting of subducted oceanic crust, forming the adakitic monzonites and quartz diorite porphyries (Figure 14b). Furthermore, the thickening and reworking of the ancient crust of the Erguna Block, formed the coexisting thickened lower crust-derived adakitic quartz monzonite porphyries (Figure 14b). Thus, the Erguna Block may still be an active continental margin attributed to the southward subduction of the Mongol–Okhotsk Oceanic plate before ca. 143 Ma.

6. Conclusions

(1) The syenogranites have consistent ages of ca. 190 Ma and were emplaced in the Early Jurassic. Intrusions of the monzonites, quartz diorite porphyries, and quartz monzonite porphyries occurred during the Late Jurassic to early Early Cretaceous at ca. 147–143 Ma. These rocks exhibit typical adakitic geochemical characteristics.

(2) The syenogranites and quartz monzonite porphyries originated from the thickened ancient mafic lower continental crust and thickened juvenile lower crust, respectively. In contrast, the monzonites and quartz diorite porphyries potentially formed because of melting associated with an oceanic slab.

(3) There is a heterogeneity of the lower continental crust in the Erguna Block during the Mesozoic and during the Early Jurassic to Early Cretaceous, and a gradual replacement of the ancient lower crust by mantle-derived juvenile materials occurred in the Erguna Block. The Erguna Block may still be an active continental margin attributed to the southward subduction of the Mongol–Okhotsk Oceanic plate before ca. 143 Ma.

(4) The Early Jurassic magmatic rocks were formed in an active continental margin setting related to the southward subduction of the Mongol–Okhotsk oceanic plate. The Late Jurassic to early Early Cretaceous magmatism in the Erguna Block occurred in an extensional environment that was probably triggered by the asthenosphere upwelling as a result of the rollback of the subducted Mongol–Okhotsk Oceanic plate, whereas the late Early Cretaceous magmatism occurred in an extensional environment that was related to the upwelling of asthenospheric material as a result of delamination of the previously thickened continental lithosphere.