This paper reports on the petrography, geochemistry, and geochronology of late Mesozoic volcanic rocks in the Wenkutu area of the northern Great Xing'an Range, North-east China, and discusses the formation age, petrogenesis, and tectonic environment of the Manketouebo, Manitu, Baiyingaolao, and Meiletu formations. Results of Zircon U–Pb dating show that the volcanic rocks in the Wenkutu area were formed during the Early Cretaceous (Manketouebo Formation at 145 ± 1 Ma, Manitu Formation at 141.3 ± 1.7 Ma, Baiyingaolao Formation at 128.7 ± 1.1 Ma, and Meiletu Formation at 129 ± 1 Ma). The Manketouebo, Manitu, and Baiyingaolao formations are dominated by intermediate to acid volcanic rocks that belong to the high-K calc-alkaline series, with Eu/Eu* values of 0.38–0.84.They are enriched in light rare earth elements (LREE) and depleted in heavy rare earth elements (HREE). The Meiletu Formation is dominated by intermediate to basic volcanic rocks, belongs to the high-K calc-alkaline series, has Eu/Eu* values of 0.89–0.97, and is enriched in Ba, K, Th, U, La, Ce, Sr, Nd, and Hf but relatively depleted in Nb, Ta, Zr, P, and Ti, with no obvious fractionation of LREE and HREE. Results indicate that the magma of the Manketouebo, Manitu, and Baiyingaolao formations had a crustal origin, whereas the Meiletu Formation was sourced from the mantle and subsequently contaminated by crustal materials. The late Mesozoic volcanic rocks in the study area were formed during an orogenic stage, and the compressional environment was caused by subduction of the paleo-Pacific Plate below the Asian continental plate.

1 INTRODUCTION

The Great Xing'an Range is located at the suture belt between the Siberian Plate and the North China Plate. This area has experienced large-scale tectonic movements, such as the collisions between the Songnen and Bulieya–Jiamusi massifs, the Xing'an and Eergu'Na massifs, and the Songnen and Xing'an massifs; collision of the Northern China Plate with the Mongolia Xing'an Massif; subduction of the Mongolia Okhotsk Ocean; and oblique subduction of the paleo-Pacific Plate and subduction of the Pacific Plate (She et al., 2012; Zhang et al., 2015; Zhou, Simon, Wilde, Ren, & Zheng, 2011). The volcanic rocks in the area are complex in terms of their tectonic origins and geochemical characteristics and have therefore been the subject of numerous studies (e.g., Ge et al., 2015; Jia, Wei, Gong, & Zhao, 2011; Liu et al., 2011; Ouyang, Mao, Zhou, & Su, 2015; Zheng, Zhang, & Xu, 2015).

There is controversy concerning the ages, geochemical characteristics, and tectonic settings of the volcanic rocks of the Wenkutu area, central northern Great Xing'an Range. Volcanic rocks in the Great Xing'an Range were formed during the Middle Jurassic to Early Cretaceous (Zhang, 2009; Sun et al., 2011; Si et al., 2015; Yin et al., 2006), with a peak during the Late Jurassic to Early Cretaceous (Si et al., 2015; Yin et al., 2006). The main body of Mesozoic volcanic rocks in the northern Great Xing'an Range was formed during the Early Cretaceous, with minor Late Jurassic volcanic rocks being distributed in the western Manzhouli and northern part of the Hailar Basin (Zhang, 2009).

The origin of the Mesozoic volcanic rocks of the Great Xing'an Range is complex and included both mantle and crustal sources. The volcanic rocks in the northern Great Xing'an Range are dominated by alkaline series rocks, whereas the southern area is dominated by calc-alkaline series rocks (Lin et al., 2000). The acid volcanic rocks and sub-alkaline series basalt to andesite in the south-central part of the Great Xing'an Range are the products of magmatic differentiation (Ge, Lin, Sun, Wu, & Li, 2000; Lin et al., 2000; Lin, Ge, Cao, Sun, & Lin, 2003). The basic volcanic rocks and dacitic rocks reflect bimodal magmatism, and the rhyolite was formed by crustal contamination or partial melting of lower crust (Gao, Guo, Fan, Li, & Li, 2005; Guo, Fan, Wang, & Lin, 2001).

There are different views on the tectonic setting and origin of Mesozoic volcanic rocks in the Great Xing'an Range, which are related to the activity of a mantle plume or the relevant board functions (Lin et al., 2000). The formation and evolution of magma in the Great Xing'an Range were related to crustal extension and the ascent of a mantle plume to the base of the crust, rather than being related to subduction of the Pacific Plate (Shao, Liu, Chen, & Han, 2001). Other studies suggest that the volcanic rocks are related to closure of the Mongol–Okhotsk suture zone and subsequent extension of the orogenic belt (Guo et al., 2001; Yin, Zhang, & Du, 2013). Subduction of the paleo-Pacific Plate has also been proposed as a fundamental cause of Mesozoic magmatism in the Great Xing'an Range (Zhang, 2009).

Four regional geological mineral surveys (1:50,000 scale) were undertaken in the Wenkutu area of Inner Mongolia. These surveys also involved extensive petrography, geochemistry, and geochronology study of the late Mesozoic volcanic rocks, and provided an improved understanding of their geodynamic setting and magma sources.

2 GEOLOGICAL BACKGROUND AND PETROLOGIC CHARACTERISTICS

2.1 Geological background

The study area is located at the junction of the Eergu'Na and Xing'an massifs, which is marked by the Xinlin–Toudaoqiao fracture. Mesozoic volcanic rocks make up the central northern part of the Great Xing'an Range (Genhe area), which is one of the key areas for studies of the evolution of the volcanic magmatic belt in the Great Xing'an Range (Zhou et al., 2015; Figure 1a).

Details are in the caption following the image — **Figure 1**
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Geological map of the study area, showing sampling locations. 1 = Quaternary; 2 = Lower Cretaceous Meiletu Formation; 3 = Lower Cretaceous Baiyingaolao Formation; 4 = Lower Cretaceous Manitu Formation; 5 = Lower Cretaceous Manketouebo Formation; 6 = Middle Jurassic Tamulangou Formation; 7 = Early Cretaceous granite porphyry; 8 = Early Cretaceous alkali feldspar granite; 9 = Middle Jurassic alkali feldspar granite; 10 = Early Permian monzonite granite; 11 = Early Permian syengranite; 12 = Early Carboniferous diorite; 13 = Early Carboniferous quartz diorite; 14 = Fault, including fault number; 15 = Sample site; 16 = Sample for age dating [Colour figure can be viewed at wileyonlinelibrary.com]

The study area is dominated by the Manketouebo, Manitu, Baiyingaolao, and Meiletu formations. The Manketouebo Formation, which is dominated by acidic volcanic rock, is exposed mainly to the west of the Wenkutu River and north of the Nemen River, where it conformably overlies the Manitu Formation. The Manitu Formation is composed of intermediate volcanic rock and is mainly exposed at Xiaoguliqina Mountain, where it reaches a maximum thickness of 625 m and underlies the Baiyingaolao Formation across a conformable contact. The Baiyingaolao Formation, which has a maximum thickness of 1,765 m and is dominantly acidic volcanic rock, is mainly exposed to the south of the Nemen River and to the west of the Wenkutu River, near the 1,406 highland. The formation overlies the Meiletu Formation across an angular unconformity. The Meiletu Formation comprises basic volcanic rock (Figure 1b) and is >100 m thick. Its distribution, which is strongly controlled by tectonic structure, is limited in the study area where it is exposed sporadically in the area north of the Nemen River (see more broken jointed belt and cliff face).

The intrusive rocks of the study area, which form the base of the volcanic basin, are mainly exposed towards the margins of the basin. They cover an area of 70 km², accounting for 5% of the whole area. These intrusive rocks are divided into four stages, as follows: early Carboniferous diorite and quartz diorite; early Permian monzonite granite and syenogranite; Middle Jurassic alkali feldspar granite; and Early Cretaceous granite porphyry and alkali feldspar granite.

Faults in the study area are mostly normal or strike-slip (trending NE–SW, NNE–SSW, and NW–SE, with fewer trending N–S and E–W) and formed in a tensional tectonic environment; some of the faults are characterized by multistage development. The Xinlin–Toudaoqiao fracture passes through the study area. Based on the results obtained during this work and other studies, it is speculated that the Yiligende River deep fault (F3) and Nemen River deep fault (F19) are two secondary faults associated with the Xinlin–Toudaoqiao fracture. The fault was formed during the early Paleozoic and is deep-seated and shows multiple stages of activity. The Eergu'Na and Xing'an massifs were amalgamated during the early Permian, forming continental collision orogenic granites, with faults controlling the formation and distribution of Paleozoic intrusive rocks. During the Middle Jurassic to Early Cretaceous, tectonic activity reached its peak, and the extension direction was NE–SW to NNE–SSW, which controlled the formation of the Great Xing'an Range volcanic–magmatic intrusive belt. The faults were reactivated during the Cenozoic, deforming Mesozoic volcanic and intrusive rocks. These faults are important ore-controlling and ore-bearing structures (Jiang, Lao, Gong, & Dong, 2016; Liu, Mao, Wu, Wang, & Hu, 2014; Mei, Lv, Tang, Wang, & Zhao, 2015; Ouyang et al., 2014; Yao, Liu, Zhai, Wang, & Xing, 2012). The faults were transpressional during the Paleozoic and mainly tensional after the Mesozoic.

2.2 Petrological characteristics

2.2.1 Manketouebo Formation

The Manketouebo Formation within the study area is dominated by rhyolite and rhyolitic ignimbrite (Figure 2).

The rhyolite is grey, showing rhyolitic and block structure, porphyritic texture, and a felsitic groundmass. The phenocryst content ranges from 20% to 25%, and the mineral assemblage is composed mainly of plagioclase, alkali feldspar, and quartz. Plagioclase phenocrysts make up 5–10% of the rock mass, show a hypautomorphic granular texture, polysynthetic twinning, cracks containing clay, and a grain size of 0.5–3.0 mm. Alkali feldspar phenocrysts make up 5–10% of the rock mass, show a hypautomorphic granular texture, striations, simple twins, alteration to clay, and a grain size of 0.1–2.5 mm. Quartz phenocrysts make up 1–5% of the rock mass, are xenomorphic granular, euhedral, show porphyritic corrosion, and are 0.5–3.0 mm in size. The groundmass displays a felsitic texture with flow banding (Figure 3e) and is composed of felsic minerals with minor magnetite microphenocrysts.

The rhyolitic ignimbrite is grey, with an ignimbrite texture and block structure. The lithics (10–15%) are sub-angular to sub-rounded, consisting of granite, rhyolite, and minor andesite, with sizes of 0.5 to 2.0 mm. Crystals (15–20%) are angular to sub-angular, consisting of plagioclase, alkali feldspar, biotite, and minor quartz, with sizes of 0.2 to 1.5 mm. Plagioclase crystals display polysynthetic twins, speckled texture, and alteration to clay. Alkali feldspar is perthitic and angular and shows striations. Biotite is brown and aligned and shows signs of dolomitization, mostly along cleavage planes. Quartz is angular to sub-angular with euhedral surfaces. The ignimbrite groundmass consists of devitrified felsic minerals with flow banding around crystals (0.5–4.0 mm in size, content of 5–10%). Vitric material is worm- and ribbon-shaped, with sericite development, and shows flow banding around crystals (0.2–2.0 mm in size, content of 1–5%). Volcanic dust filling debris makes up 55–60% of the ignimbrite (Figure 3a,f).

2.2.2 Manitu Formation

The Manitu Formation within the study area consists mainly of basaltic andesite, andesite, and dacite breccia vitric crystal ignimbrite (Figure 2).

The basaltic andesite is characterized by grey and grey-black fresh faces, porphyritic texture, and groundmass with felsitic texture and block structure. The phenocryst content ranges from 15% to 20% and is dominated by plagioclase. Plagioclase is hypautomorphic columnar and prismatic in shape, has polysynthetic twins, zoning, and grain sizes of 0.2–2.5 mm. The groundmass is composed mainly of fine columnar plagioclase crystallites with flow banding and semi-staggered directions, with fine-grained interstitial magnetite and pyroxene.

The andesite is grey and grey purple and shows a porphyritic texture, felsitic groundmass, and block structure. Phenocryst content ranges from 15% to 25%, with an assemblage dominated by plagioclase and biotite. Plagioclase phenocrysts are hypidiomorphic, display polysynthetic twinning, have a grain size of 0.2–2.5 mm, and make up 15–20% of the rock mass. The biotite has a shape-preferred orientation, shows metasomatic alteration to chlorite and iron precipitation, has a grain size of 0.2–2.5 mm, and makes up 1–5% of the rock mass. The groundmass is composed mainly of felsic minerals and minor magnetite (Figure 3g).

Dacite breccia vitric crystal ignimbrite is grey with an ignimbritic texture and block structure. The breccia is sub-angular to sub-rounded, consists of andesite and rhyolite with sericite alteration, contains clasts of 2 to 10 mm in size, and makes up 5–10% of the rock mass. Crystals are angular to sub-angular, consisting of plagioclase, quartz, and minor biotite with sizes of 0.5 to 2.0 mm, and make up 25–30% of the rock mass. Plagioclase crystals (10%–15% of the crystal assemblage) are characterized by polysynthetic twins, speckled texture, bicrystal bending, and alteration to clay. Quartz (5%–10%) is euhedral, and biotite (1%–5%) shows a shape-preferred orientation with chloritization and brown iron staining along cleavage planes. Glass (10%–15%) is brown, worm- and ribbon-shaped with flow banding, and 0.5 to 3.0 mm in size. Magma fragments are elongate at one end or torn at both ends, consisting of plagioclase crystals and devitrified felsic minerals, with flow banding around crystals, comb edge development, 0.2 to 2.0 mm in size, and make up 1–5% of the rock mass. Volcanic dust filling debris makes up 40–50% of the ignimbrite (Figure 3b,h).

2.2.3 Baiyingaolao Formation

The Baiyingaolao Formation within the study area is composed mainly of rhyolite and rhyolitic crystal-rich ignimbrite (Figure 2).

The rhyolite is grey with rhyolitic and block structure, porphyritic texture, and a groundmass with felsitic texture. The phenocryst content ranges from 5% to 10%, and the assemblage is dominated by plagioclase that is hypautomorphic granular with polysynthetic twinning, altered to clay and sericite along cracks, and is 0.5–3.0 mm in size. The groundmass is characterized by flow banding and is composed of felsic minerals with a felsitic texture and alteration to clay (Figure 3c,i).

The rhyolitic crystal-rich ignimbrite is grey, with ignimbritic texture and block structure. Lithic fragments are sub-angular to sub-circular, consisting of granite and rhyolite, with sizes of 0.2 to 1.5 mm and making up 15–20% of the rock mass. Crystals are angular to sub-angular, consisting of plagioclase, biotite, and minor quartz of 0.2 to 1.5 mm in size, and make up 15–20% of the rock mass. Plagioclase show polysynthetic twins, speckled texture, bicrystal bending, and alteration to clay. Biotite shows a shape-preferred orientation and is brown, with petrochemical development mostly along cleavage. Quartz is angular to sub-angular, with euhedral crystal faces. The groundmass (5–10% of the rock) consists of devitrified felsic minerals and shows flow banding around crystals (0.5–4.0 mm in size). Groundmass glass (1–5%) is worm- and ribbon-shaped, with sericite development, shows flow banding around crystals, and is 0.2 to 2.0 mm in grain size. Volcanic dust filling debris makes up 45–55% of the ignimbrite (Figure 3j).

2.2.4 Meiletu Formation

The Meiletu Formation is characterized by basalt and basaltic andesite (Figure 2).

The basalt is grey green and grey black, with block structure and porphyritic texture. It consists mainly of plagioclase (10–15%) with grain size of 0.5–2.0 mm. The groundmass shows an intergranular texture with a framework of plagioclase crystallites that are infilled by pyroxene, magnetite, and minor biotite (Figure 3d,k).

The basaltic andesite is grey purple and grey black, showing a block structure and porphyritic texture. It is composed of plagioclase (10–15%) with grain size of 0.2–2.0 mm. The groundmass has a hyalopilitic texture and is composed of fine columnar plagioclase with interstitial fine magnetite crystals and minor biotite (Figure 3l).

3 SAMPLE PREPARATION AND ANALYTICAL METHODS

3.1 Sample preparation

Geochemical analyses were performed on five samples of the Manketouebo Formation, four of the Manitu Formation, four of the Baiyingaolao Formation, and five of the Meiletu Formation. In addition, subsamples of each sample were collected for age dating and isotopic analysis.

3.2 Analytical methods

Zircon U–Pb dating by SHRIMP was undertaken to constrain the ages of the Manketouebo, Manitu, Baiyingaolao, and Meiletu formations. Standard zircon separation procedures were followed, with separation carried out at the Laboratory of Regional Geology and Mineral Resources Investigation Institute of Hebei Province, China. Selected zircons were imaged by cathodoluminescence (CL) at the Beijing Ion Microprobe Centre, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China. SHRIMP U–Pb isotopic analyses were undertaken by SHRIMP II at the Beijing Ion Microprobe Centre. Standard analytical procedures followed those of Jian et al. (2003). Fractionation corrections between elements were carried out using the 91500 zircon standard (Australian Geological Survey). Data processing was undertaken using the SQUID1.02 and ISOPLOT programs (Ludwig, 2003). Common Pb was corrected according to measured ²⁰⁴Pb.

Major and trace element analyses were carried out at the Laboratory of Regional Geology and Mineral Resources Investigation Institute of Hebei Province, China. Major elements were analysed by X-ray fluorescence (XRF), with accuracy better than 5%, and trace elements were analysed by inductively coupled plasma–mass spectrometry (ICP–MS), with accuracy better than 10%.

Sr and Nd isotopic analyses were performed at the Tianjin Institute of Geology and Mineral Resources of China, Bureau of Geology and Mineral Resources. Sr isotope analyses were undertaken using the MAT26 solid isotope mass spectrometer. Mass fractionation was corrected by ⁸⁷Sr/⁸⁶Sr = 0.1194 standardization, using the value for the NBS987 standard of 0.71025 + 0.00008, and that for the national standard GBW04411 of ⁸⁷Sr/⁸⁶Sr = 0.75999 + 0.00004. The whole process is as follows: Rb = 30 × 10⁻¹⁰ and Sr = 1.2 × 10⁻¹⁰. Nd isotope analysis was performed using a Neptune multicollector-inductively coupled plasma–mass spectrometer (MC–ICP–MS). The ¹⁴⁶Nd/¹⁴⁴Nd ratio was standardized using ¹⁴⁶Nd/¹⁴⁴Nd = 0.721900, employing ¹⁴⁶Nd/¹⁴⁴Nd = 0.512643 + 0.000015 (2δ) for the standard sample (BCR-2). The standard value of the instrument is JNdi-¹⁴³Nd/¹⁴⁴Nd = 0.512116 + 0.000008. The whole process is as follows: Sm = 30 × 10⁻¹⁰ and Nd = 1.2 × 10⁻¹⁰ (Liu, Liu, et al., 2016).

4 ANALYTICAL RESULTS

4.1 Zircon U–Pb chronology

A rhyolite sample (MK-1) was collected from the Manketouebo Formation and used for isotopic dating. Zircons separated from sample MK-1 are hypautomorphic columnar with visible cracks, corrosion marks (pits and grooves), and abundant mineral inclusions. In a CL image (Figure 4a), most zircons are bright and have flattened cylindrical crystal shapes. Some zircons display growth zones with oscillatory zoning. The contents of U and Th in 27 measuring sites were 15–4,039 and 67–3,005 ppm, respectively, and Th/U values were 0.40–1.70 (with the exception of one analysis, number 12; Table 1). These results indicate that the zircons have a magmatic origin. Analysed zircons of the Manketouebo Formation rhyolite (27 in total) show corrosion and growth edges (numbers 3, 10, 17, 18, and 21), and zircon analyses 24 and 25 had incomplete crystal shapes with measured ages of 314.3–353.9 Ma. Zircon analyses 4, 9, and 11 also had incomplete crystal shapes, with measured ages of 170.8–173.4 Ma. These ages represent magmatic zircon and are consistent with the two intrusive magmatic events during the early Carboniferous and Middle Jurassic in the study area. The other analyses were in the U–Pb uniform line or within a very small range near the U–Pb line, data point centralization. The ²⁰⁶Pb/²³⁸U ages are 131.4–149.3 Ma, with a weighted mean age of 145 ± 1 Ma (MSWD = 1.6; Figure 3e). These results show that the volcanic rocks of the Manketouebo Formation formed during the Early Cretaceous.

Table 1. Results of SHRIMP zircon U–Pb analyses of late Mesozoic volcanic rocks from the study area

No.	²⁰⁶Pb	²³⁸U	²³²Th	²³²Th/²³⁸U	Isotopic atomic ratio				Isotopic age/Ma
No.	(×10⁻⁶)			²³²Th/²³⁸U	²⁰⁷Pb/²⁰⁶Pb	1σ	²⁰⁶Pb/²³⁸U	1σ	²⁰⁷Pb/²⁰⁶Pb	1σ	²⁰⁶Pb/²³⁸U	1σ
Manketouebo Formation rhyolite (MK-1)
1	37.6	1,895	1,002	0.55	0.05219	43.31	0.02309	1.2	294	32	147.2	1.7
2	6.03	299	188	0.65	0.0519	17.72	0.05643	1.3	−125	680	145.1	2.2
3	19.8	407	229	0.58	0.0291	43.92	0.02277	1.6	280	65	353.9	4.4
4	3.12	130	125	0.99	0.0397	36.68	0.02726	1.8	−369	460	173.4	3.0
5	2.98	166	111	0.69	0.0293	49.75	0.02010	1.7	−123	580	128.3	2.1
6	22.5	1,140	644	0.58	0.0475	43.69	0.02289	1.3	74	79	145.9	1.9
7	7.02	385	633	1.70	0.0441	47.56	0.02102	1.6	−107	150	134.1	2.1
8	15.5	769	553	0.74	0.0456	43.03	0.02324	1.3	−26	110	148.1	1.9
9	15.4	653	489	0.77	0.0526	37.11	0.02695	1.3	310	130	171.4	2.2
10	35.2	793	538	0.70	0.05122	19.37	0.05162	1.2	251	32	324.4	3.8
11	3.82	189	128	0.70	0.0555	42.32	0.02363	1.5	432	130	150.5	2.3
12	5.13	279	95	0.35	0.0371	48.76	0.02051	1.7	−546	630	130.9	2.2
13	8.53	425	289	0.70	0.0499	42.79	0.02337	1.4	189	150	148.9	2.0
14	80.6	4,039	3005	0.77	0.0487	43.58	0.02295	1.2	135	61	146.3	1.8
15	45.2	2,365	2348	1.03	0.0479	45.38	0.02203	1.2	95	67	140.5	1.6
16	9.20	428	285	0.69	0.057	44.03	0.02271	1.8	486	680	144.8	2.5
17	29.7	652	455	0.72	0.0531	18.96	0.05274	1.2	332	58	331.3	3.9
18	17.6	389	216	0.57	0.0495	19.11	0.05233	1.3	171	97	328.8	4.1
19	8.18	352	340	1.00	0.0412	37.30	0.02681	1.4	−278	240	170.5	2.3
20	6.68	339	393	1.20	0.0488	44.01	0.02272	1.4	139	110	144.8	2.0
21	5.03	115	67	0.61	0.0500	19.76	0.05061	1.6	197	140	318.3	4.9
22	19.4	1,008	394	0.40	0.0471	44.81	0.02232	1.2	53	76	142.3	1.7
23	4.66	227	205	0.93	0.048	45.2	0.02215	2.2	99	700	141.2	3.1
24	18.4	427	223	0.54	0.0526	20.02	0.04996	1.3	310	77	314.2	3.9
25	16.2	366	295	0.83	0.0500	19.64	0.05090	1.3	195	120	320.1	4.1
26	20.1	1,053	906	0.89	0.0490	44.95	0.02225	1.2	147	55	141.8	1.7
27	3.10	162	262	1.67	0.042	46.2	2.2	2.3	−252	740	138.1	3.0
Manitu Formation dacite breccia vitric crystal ignimbrite (MN-4)
1	13.4	710	628	0.91	0.0465	2.3	0.02182	1.3	48	120	139.1	1.7
2	3.15	172	138	0.83	0.0382	6.7	0.02118	1.6	372	160	135.1	2.2
3	4.98	270	224	0.86	0.0482	4.0	0.02118	1.5	−146	280	135.1	2.0
4	4.14	200	160	0.83	0.0478	8.7	0.02166	2.7	594	650	138.1	3.7
5	4.20	205	186	0.93	0.0526	9.0	0.02008	2.9	−390	130	128.2	3.6
6	3.57	197	163	0.86	0.0464	5.0	0.02086	2.0	47	150	133.1	2.6
7	3.79	210	175	0.86	0.0524	4.3	0.02087	1.7	155	240	133.2	2.2
8	6.65	343	536	1.61	0.0485	3.2	0.02241	1.4	26	130	142.9	1.9
9	3.10	175	141	0.83	0.0520	4.8	0.02038	1.6	198	200	130.1	2.1
10	3.34	172	131	0.79	0.0502	4.6	0.02233	1.6	−32	190	142.4	2.2
11	3.22	177	141	0.82	0.0475	5.1	0.02090	2.0	−95	500	133.3	2.6
12	3.07	157	112	0.74	0.0440	5.4	0.02247	1.8	−134	300	143.2	2.5
13	8.09	183	137	0.78	0.0573	2.7	0.05126	1.4	323	130	322.2	4.5
14	3.39	189	147	0.80	0.0465	9.8	0.02061	1.7	−115	450	131.5	2.3
15	6.40	354	359	1.05	0.0477	3.6	0.02082	1.6	85	180	132.8	2.1
16	3.68	201	156	0.80	0.0567	4.0	0.02097	1.7	−193	410	133.8	2.3
17	4.47	234	211	0.93	0.0542	3.8	0.02200	1.6	10	330	140.3	2.3
18	3.48	196	164	0.87	0.0537	4.3	0.02031	1.7	−395	430	129.6	2.2
19	3.23	175	133	0.79	0.0401	6.4	0.02140	1.8	409	220	136.5	2.4
20	8.87	492	706	1.48	0.0550	3.1	0.02067	1.4	1	250	131.9	1.8
21	11.1	543	634	1.21	0.0585	5.4	0.02212	1.8	−827	900	141.0	2.5
22	5.21	281	456	1.68	0.0541	3.4	0.02138	1.5	93	230	136.4	2.0
23	2.65	151	120	0.82	0.0454	6.0	0.02026	2.4	228	190	129.3	3.0
24	3.65	198	159	0.83	0.0544	4.2	0.02102	1.7	−298	390	134.1	2.2
25	5.03	266	235	0.91	0.0474	5.7	0.02066	2.4	−382	920	131.8	3.1
Baiyingaolao Formation rhyolite (BY-1)
1	9.87	584	476	0.01958	1.4	4.3	51.06	1.4	101	100	125.0	1.7
2	8.29	469	543	0.02048	1.4	3.8	48.83	1.4	−35	93	130.7	1.8
3	3.91	226	311	0.02006	1.6	6.0	49.84	1.6	14	150	128.1	2.0
4	1.54	90	127	0.01985	1.9	11	50.38	1.9	219	250	126.7	2.4
5	11.9	689	830	0.02007	1.4	2.7	49.83	1.4	133	63	128.1	1.7
6	2.62	110	84	0.02781	1.8	4.2	35.95	1.8	415	95	176.8	3.1
7	1.02	56	62	0.02085	2.6	25	48.00	2.6	213	590	133.0	3.4
8	12.3	711	711	0.02005	1.4	2.1	49.87	1.4	10	50	128.0	1.7
9	3.99	230	180	0.02006	1.6	4.6	49.86	1.6	−164	110	128.0	2.0
10	17.1	959	1,073	0.02067	1.4	2.1	48.37	1.4	65	49	131.9	1.8
11	3.36	192	148	0.02037	1.6	3.8	49.09	1.6	181	88	130.0	2.0
12	3.29	183	205	0.02046	1.9	20	48.88	1.9	−280	510	130.5	2.4
13	6.76	393	504	0.01992	1.5	5.3	50.20	1.5	23	130	127.1	1.9
14	3.46	198	231	0.02015	1.7	11	49.64	1.7	−88	260	128.6	2.1
15	7.77	451	372	0.02001	1.4	4.8	49.97	1.4	12	120	127.7	1.8
16	5.92	347	922	0.01990	1.4	2.6	50.24	1.4	296	59	127.1	1.8
17	7.36	430	492	0.01996	1.4	2.5	50.11	1.4	187	58	127.4	1.8
Meiletu Formation basalt (ML-1)
1	2.02	119	139	1.20	0.0758	4.9	0.01926	2.0	601	±290	123.0	2.4
2	3.70	82	59	0.74	0.0639	3.8	0.05224	1.7	495	±120	328.3	5.5
3	21.0	464	450	1.00	0.05538	1.7	0.05199	1.3	44	±180	326.7	4.2
4	10.5	259	144	0.58	0.0557	2.5	0.04678	1.4	191	±130	294.8	4.0
5	56.7	1,373	216	0.16	0.05313	1.1	0.04804	1.2	330	±28	302.5	3.5
6	11.3	136	40	0.31	0.0612	2.3	0.0958	1.5	465	±130	589.9	8.4
7	28.6	1,645	4,020	2.52	0.05007	1.5	0.02022	1.2	135	±52	129.0	1.5
8	66.1	2,239	3,709	1.71	0.05055	1.0	0.03428	1.2	146	±34	217.3	2.5
9	4.26	108	65	0.62	0.0613	3.6	0.04469	1.7	−164	±350	281.8	4.8
10	6.98	291	510	1.81	0.231	4.4	0.02191	4.0	690	±1000	139.7	5.5
11	8.80	398	254	0.66	0.0543	2.7	0.02547	1.5	37	±96	162.1	2.4
12	18.9	1,077	905	0.87	0.0585	1.7	0.02014	1.7	77	±120	128.6	2.1
13	16.0	896	1,086	1.25	0.0736	1.8	0.02030	1.3	510	±140	129.5	1.7
14	15.9	932	1,309	1.45	0.0509	2.0	0.01982	1.3	263	±46	126.5	1.6
15	21.0	1,159	1,572	1.40	0.0727	12	0.02042	1.8	55	±680	130.3	2.3
16	40.2	2,240	4,875	2.25	0.05562	1.2	0.02067	1.4	39	±92	131.9	1.8
17	27.6	1,560	2,193	1.45	0.05114	1.5	0.02052	1.2	169	±57	130.9	1.6
18	29.3	1,722	3,036	1.82	0.05227	1.5	0.01970	1.2	66	±73	125.7	1.5
19	148	1,139	50	0.05	0.1167	0.97	0.1472	1.2	1,484	±42	885.0	9.8
20	14.9	867	953	1.14	0.0544	2.0	0.01980	1.3	−47	±140	126.4	1.6
21	20.6	1,199	1,488	1.28	0.04967	1.8	0.01996	1.3	15	±92	127.4	1.6

A sample of dacite breccia vitric crystal ignimbrite from the Manitu Formation (MN-4) was collected and used for zircon dating. The zircons from this sample are hypautomorphic columnar in shape, broken to sub-angular, with surface corrosion (grooves and pits) and iron staining. In a CL image (Figure 4b), zircons show oscillatory zoning, which is typical of magmatic zircon. The contents of U and Th obtained from 25 analyses are 151–710 and 112–634 ppm, respectively, and Th/U values are 0.74–1.68 (Table 1). These data support a magmatic origin for the analysed zircons. A total of 25 zircons from the Manitu Formation dacite breccia vitric crystal ignimbrite were analysed (analysis 13 shows corrosion and growth edges), with measured ages of 314.3–353.9 Ma. This result is consistent with the early Carboniferous age of intrusive magmatic activity within the study area. The other analyses are in the U–Pb uniform line or within a very small range near the U–Pb line, data point centralization, yielding ²⁰⁶Pb/²³⁸U ages of 128.9–143.7 Ma and a weighted mean age of 141 ± 1.7 Ma (MSWD = 0.66; Figure 3f). These results show that the Manitu Formation is Early Cretaceous in age.

A rhyolite sample from the Baiyingaolao Formation (BY-1) was collected for zircon dating. Zircons from this sample are automorphic to hypautomorphic biconical columnar, with surfaces affected by resorption and corrosion, and have aspect ratios of 1.0–2.5 and grain sizes of 10–150 μm. The zircons show narrow oscillatory zoning (Figure 4c) indicative of a magmatic origin. The contents of U and Th in 17 analyses were 56–959 and 62–1037 ppm, respectively, and Th/U values are 0.79–2.74. These data (Table 1) also indicate a magmatic origin for the analysed zircons. A total of 17 zircons from the Baiyingaolao Formation rhyolite were analysed (analysis 6 shows corrosion and growth edges), with a mean age of 176.8 Ma. This result is consistent with the Middle Jurassic age of intrusive magmatic activity within the study area. The other analyses are in the U–Pb uniform line or within a very small range near the U–Pb line, data point centralization, yielding ²⁰⁶Pb/²³⁸U ages from 129.9 to 113.4 Ma and a weighted mean age of 128.7 ± 1.1 Ma (MSWD = 0.69; Figure 4e). The results show that the Baiyingaolao Formation is Early Cretaceous in age.

A basalt sample from the Meiletu Formation (ML-1) was collected for zircon dating. Zircons from this sample are automorphic to hypautomorphic biconical columnar and stubby, with aspect ratios of 1.0–2.5 and grain sizes of 10–130 μm. Most zircons have flattened cylindrical shapes and are bright in CL images (Figure 4d), and some zircons display intact growth zones with oscillatory zoning. The 21 analyses yielded U and Th contents of 82–2,240 and 40–4,875 ppm, respectively, and Th/U values of 0.58–2.52 (with the exception of analyses 5, 6, and 9; Table 1). A total of 21 zircons from the Meiletu Formation basalt were analysed for age dating, with analyses 4, 5, and 9 showing corrosion and growth edges, and analyses 2 and 3 showing incomplete crystal shapes with measured ages of 281.8–328.3 Ma. This result is consistent with the Early Triassic age of intrusive magmatic activity within the study area. Analysis 11 yielded a measured age of 162.1 Ma, which is consistent with the Middle Jurassic igneous activity in the study area. Analyses 6 and 19 (zircons which displayed corrosion phenomena and growth edges) yielded measured ages of 589.9 and 885.0 Ma, respectively, suggesting that these represent xenocrystic zircons inherited from metamorphic host rocks. Excluding xenocrystic zircons, the other analyses lie along or close to the U–Pb uniform line, yielding ²⁰⁶Pb/²³⁸U ages of 121.2–137.3 Ma and a weighted mean age of 129 ± 1 Ma (MSWD = 1.3; Figure 3h). The results show that the Meiletu Formation rocks formed during the Early Cretaceous.

4.2 Petrogeochemistry

4.2.1 Major element geochemistry

Rocks of the Manketouebo Formation are characterized by 68.95–77.87 wt.% SiO₂, 12.17–14.61 wt.% Al₂O₃, 1.37–3.83 wt.% TFeO, 0.15–0.50 wt.% CaO, 0.19–0.41 wt.% MgO (with the exception of 1.20 wt.% in sample MK-3), 0.10–0.59 wt.% TiO₂, and Mg^# of 21.06–38.53. Samples belong to the calc-alkaline series with high Si and low Mg, with total alkali (Na₂O + K₂O) contents of 7.07–8.59 wt.% (average of 8.01 wt.%), Na₂O/K₂O values of 0.35–0.95 (Na₂O < K₂O), and Rittman index values of 0.41–0.63 (Table 2). The samples plot within the rhyolite region on a TAS diagram (Figure 5). All samples are part of the high-K calc-alkaline series on a K₂O–SiO₂ diagram (Figure 6a). All samples are peraluminous with Α/CNK values of 1.60–1.69 and Α/NK values of 1.64–1.73 (Figure 6b). Gottignies index (τ) values are 20.51–89.70 (i.e., >10), which indicates that the volcanic rocks in this formation are orogenic volcanic rocks (Qu, Chang, PeiRF, & MeiYX, 2011).

Table 2. Geochemical data of late Mesozoic volcanic rocks from the study area

Sample no.	MN-1	MN-3	MN-4	MN-2	MK-4	MK-3	MK-5	MK-1	MK-2	ML-3	ML-1	ML-2	ML-4	ML-5	BY-4	BY-2	BY-3	BY-1
Rock type	Basaltic	Andesite	Dacite	Andesite	Rhyolite				Rhyolitic	Basalt			Basaltic andesite		Rhyolite
Stratigraphic code	Manitu Formation (K₁mn)				Manketouebo Formation (K₁mk)					Meiletu Formation (K₁m)					Baiyingaolao Formation (K₁b)
SiO₂	50.33	54.47	68.53	61.24	71.67	68.95	77.87	74.45	74.13	49.59	51.29	50.38	53.60	55.81	72.86	73.91	76.19	74.35
TiO₂	1.48	2.01	0.55	0.85	0.29	0.59	0.10	0.20	0.18	1.60	1.42	1.95	1.29	1.31	0.26	0.27	0.19	0.20
A₂O₃	16.75	16.21	15.15	16.46	14.61	14.24	12.17	13.44	13.74	15.21	16.34	16.67	16.67	17.79	14.24	13.64	12.46	13.85
TFe₂O₃	10.09	9.00	4.54	9.33	2.42	3.83	1.37	3.00	1.95	9.44	13.43	14.58	12.31	7.90	1.76	1.66	1.34	2.89
MnO	0.17	0.14	0.08	0.13	0.19	0.18	0.10	0.05	0.14	0.28	0.18	0.15	0.15	0.18	0.14	0.16	0.09	0.05
MgO	4.34	3.07	0.80	2.27	0.38	1.20	0.19	0.41	0.26	7.79	4.48	4.03	4.88	2.37	0.24	0.15	0.16	0.42
CaO	4.08	3.28	1.54	4.43	0.26	0.50	0.15	0.22	0.21	8.81	8.29	7.90	6.30	3.80	0.29	0.20	0.17	0.06
Na₂O	5.50	4.45	4.47	3.46	3.45	2.14	3.20	3.12	3.88	2.81	4.17	3.98	3.90	5.27	4.85	3.71	2.41	4.48
K₂O	1.85	2.67	4.41	3.52	5.14	6.09	3.87	5.08	4.10	1.87	1.82	1.81	2.32	2.23	4.17	5.31	5.99	3.81
P₂O₅	0.78	1.14	0.11	0.27	0.05	0.14	0.01	0.03	0.08	0.52	0.58	0.64	0.63	0.49	0.05	0.03	0.02	0.05
LOI	4.33	3.18	0.60	0.83	1.42	1.90	0.94	0.69	1.15	1.79	1.62	1.01	1.44	2.55	0.98	0.81	0.84	0.56
Total	95.37	96.44	100.19	101.97	98.46	97.86	99.03	99.98	98.67	97.92	102.01	102.09	102.05	97.15	98.86	99.04	99.02	100.16
Mg^#	46.25	40.56	26.06	32.70	23.90	38.53	21.72	21.35	21.06	62.28	40.03	35.59	44.23	37.51	21.43	15.31	19.28	22.48
σ	2.01	1.24	0.70	0.77	0.60	0.63	0.41	0.52	0.51	1.42	1.45	1.57	1.17	1.17	0.60	0.58	0.51	0.53
τ	7.60	5.85	19.31	15.36	38.48	20.51	89.70	52.28	54.78	7.75	8.54	6.50	9.87	9.56	36.12	36.78	52.89	47.20
A/NK	2.28	2.28	1.71	2.36	1.70	1.73	1.72	1.64	1.72	3.25	2.73	2.88	2.68	2.37	1.58	1.51	1.48	1.67
A/CNK	1.47	1.56	1.45	1.44	1.65	1.63	1.69	1.60	1.68	1.13	1.14	1.22	1.33	1.57	1.53	1.48	1.45	1.66
La	58.97	74.18	44.88	36.82	34.92	40.92	10.29	21.91	24.02	37.81	42.83	36.97	48.72	36.89	22.55	40.81	17.11	17.31
Ce	131.40	155.50	95.44	79.02	80.16	83.67	27.88	54.34	46.04	81.19	94.12	83.37	100.40	76.69	58.99	67.15	31.71	58.69
Pr	17.99	19.28	11.68	9.69	8.09	10.42	1.81	4.71	5.39	10.96	12.22	11.11	12.25	10.15	5.19	8.28	3.42	4.80
Nd	74.69	72.09	43.36	36.67	28.77	38.99	4.93	15.25	17.74	44.19	47.74	44.91	46.81	39.93	17.43	27.27	10.60	16.89
Sm	15.19	11.81	7.98	6.84	5.37	7.13	0.83	2.70	2.96	8.58	8.90	8.90	8.48	7.45	3.13	4.45	1.71	3.27
Eu	3.76	2.78	1.76	1.64	0.85	1.49	0.11	0.42	0.60	2.40	2.55	2.50	2.30	2.18	0.65	0.80	0.34	0.46
Gd	11.33	8.67	6.31	5.57	4.58	5.79	0.95	2.32	2.48	7.05	7.17	7.51	7.01	5.92	2.87	3.73	1.46	2.54
Tb	1.56	1.04	1.00	0.84	0.77	0.91	0.19	0.37	0.38	1.08	1.09	1.26	1.06	0.89	0.47	0.59	0.22	0.45
Dy	7.21	4.36	5.17	4.28	4.61	5.09	1.46	2.19	2.20	5.67	5.33	6.31	5.22	4.52	2.78	3.30	1.34	2.64
Ho	1.17	0.66	0.94	0.83	0.89	0.94	0.32	0.48	0.43	1.00	0.98	1.19	1.00	0.80	0.54	0.61	0.27	0.59
Er	3.05	1.64	2.81	2.43	2.78	2.79	1.09	1.58	1.28	2.60	2.91	3.63	3.02	2.13	1.61	1.84	0.83	1.88
Tm	0.40	0.19	0.51	0.43	0.46	0.45	0.21	0.29	0.22	0.36	0.50	0.66	0.54	0.31	0.26	0.30	0.15	0.36
Yb	2.61	1.23	3.17	2.53	3.28	3.16	1.61	2.02	1.63	2.22	2.93	3.71	3.02	2.01	1.85	2.17	1.09	2.23
Lu	0.54	0.45	0.48	0.37	0.84	0.83	0.41	0.31	0.34	0.36	0.44	0.59	0.46	0.49	0.48	0.59	0.25	0.34
Y	31.66	17.30	26.41	35.44	25.84	27.56	11.19	35.00	13.01	26.19	27.05	32.12	27.04	21.40	15.99	18.84	8.11	42.72
∑REE	361.53	371.18	251.89	223.40	202.21	230.14	63.28	143.89	118.72	231.66	256.76	244.75	267.33	211.76	134.79	180.73	78.61	155.14
δEu	0.84	0.80	0.73	0.79	0.51	0.69	0.38	0.50	0.66	0.92	0.95	0.91	0.89	0.97	0.65	0.58	0.64	0.47
(La/Yb)_N	15.23	40.66	9.56	9.79	7.18	8.73	4.31	7.31	9.94	11.48	9.85	6.71	10.87	12.37	8.22	12.68	10.58	5.23
Cs	1.27	4.32	7.93	7.41	9.03	7.24	4.48	7.52	3.48	2.56	1.41	5.15	4.57	9.97	3.18	3.33	3.71	5.21
Rb	46.10	81.00	213.72	151.85	214.40	192.60	165.90	190.51	108.10	34.60	26.05	36.82	43.67	62.70	93.70	198.60	223.80	130.80
Sr	767.90	987.80	342.38	713.56	173.50	262.30	42.40	106.21	159.50	789.40	1400.00	1100.00	988.94	1237.00	77.60	109.20	128.50	93.46
Ba	797.20	1356.00	857.99	742.12	619.30	1159.00	65.00	680.33	1095.00	736.60	628.50	563.02	1100.00	958.70	879.80	1187.00	739.60	466.86
Nb	11.41	23.56	13.94	10.06	20.77	16.44	14.08	14.61	10.16	11.99	14.09	17.09	19.32	10.34	10.17	7.45	7.49	13.89
Zr	284.40	408.70	338.87	130.22	284.80	287.40	100.30	55.20	134.70	294.40	10.00	16.41	135.50	218.90	210.20	209.00	166.80	165.49
Hf	7.19	11.26	11.10	8.20	8.68	7.65	4.64	13.18	4.80	9.61	6.62	7.62	7.50	6.53	6.83	5.92	5.08	18.21
Th	6.60	5.68	15.43	15.60	22.93	15.75	22.05	11.45	10.65	4.55	4.17	3.51	4.03	5.88	10.59	22.47	15.54	8.02
U	1.48	1.12	7.90	6.23	5.15	5.06	2.16	6.41	1.77	0.77	1.13	1.03	1.19	1.49	2.65	5.79	2.30	3.00
Co	31.90	24.00	4.79	10.00	2.10	3.50	1.50	1.65	1.30	28.40	27.85	32.08	26.10	21.00	1.00	1.40	1.30	1.72
Ni	101.50	36.80	4.20	7.70	3.80	5.10	6.50	2.68	3.30	110.30	76.52	78.98	85.55	20.30	6.00	4.50	2.80	1.87
Rb/Sr	0.06	0.08	0.62	0.21	1.24	0.73	3.91	1.79	0.68	0.04	0.02	0.03	0.04	0.05	1.21	1.82	1.74	1.40

Note. Mg^# = 100{[ω(MgO)/40ω(MgO)/40 + 0.8998ω(TFe₂O₃)/72]}; δ = 2[ω(Na₂O) + ω(K₂O)ω(SiO₂) − 43]; τ = [ω(Al₂O₃) − ω(Na₂O)]/ω(TiO₂); τ is the Gottignies index. The unit of the mass fraction of major elements is wt.%; the unit of the mass fraction of trace elements is ppm (from Si et al., 2015).

Rocks of the Manitu Formation contain 50.33–68.53 wt.% SiO₂, 15.15–16.75 wt.% Al₂O₃, 4.54–10.09 wt.% TFeO, 1.54–4.43 wt.% CaO, 0.80–4.34 wt.% MgO, 0.55–2.01 wt.% TiO₂, and Mg^# of 26.06–46.25. Samples belong to the calc-alkaline series, being characterized by low Si and high Mg, total alkali (Na₂O + K₂O) contents of 6.98–8.88 wt.% (average of 7.58 wt.%), Na₂O/K₂O values of 0.98–2.97 (Na₂O > K₂O), and Rittman index values of 0.70–2.01 (Table 2). The samples are classified as trachyte, trachyandesite, and basaltic trachyandesite (Figure 5). Samples fall into the high-K calc-alkaline series (Figure 6a) with Α/CNK values of 1.44–1.56 and Α/NK values of 1.71–2.36. All samples are peraluminous (Figure 6b), with Gottignies index (τ) values of 5.85–19.3. These data indicate that the volcanic rocks in this formation were formed within orogenic to non-orogenic environments.

The Baiyingaolao Formation contains 72.86–76.19 wt.% SiO₂, 12.46–14.24 wt.% Al₂O₃, 1.34–2.89 wt.% TFeO, 0.06–0.29 wt.% CaO, 0.15–0.42 wt.% MgO, 0.19–0.27 wt.% TiO₂, and Mg^# of 15.31–22.48. Analysed samples belong to the calc-alkaline series, being characterized by high Si and low Mg, with total alkali (Na₂O + K₂O) contents of 8.29–9.02 wt.% (average of 8.68 wt.%), Na₂O/K₂O values of 0.40–1.18 (Na₂O < K₂O), and Rittman index values of 0.51–0.60 (Table 2). The samples are classified as rhyolite (Figure 5), are within the high-K calc-alkaline series (Figure 6a; Α/CNK values of 1.45–1.66 and Α/NK values of 1.48–1.67), and are peraluminous (Figure 6b). Gottignies index (τ) values are 36.12–52.89, indicating that the volcanic rocks in this formation are orogenic.

The Meiletu Formation is characterized by 49.59–55.81 wt.% SiO₂, 15.21–17.79 wt.% Al₂O₃, 7.90–14.58 wt.% TFeO, 3.80–8.81 wt.% Cao, 2.37–7.79 wt.% MgO, 1.29–1.95 wt.% TiO₂, and Mg^# of 35.59–62.28. The rock is characterized by low Si and high Mg, total alkali (Na₂O + K₂O) contents of 4.68–7.50 wt.% (average of 6.04 wt.%), Na₂O/K₂O values of 1.50–2.36 (Na₂O > K₂O), and Rittman index values of 1.17–1.57. Samples belong to the calc-alkaline series (Table 2) and are classified as basalt, trachybasalt, and trachyandesite (Figure 5). All samples are part of the high-K calc-alkaline series (Figure 6a) with Α/CNK values of 1.13–1.57 and Α/NK values of 2.37–3.25 and are peraluminous (Figure 6b). Gottignies index (τ) values are 6.50–9.87, which indicates that the volcanic rocks in this formation were formed in non-orogenic environments.

4.2.2 Trace elements

The Manketouebo Formation is characterized by total rare earth element (ΣREE) contents of 63.28–230.14 ppm (average of 151.65 ppm). Eu/Eu* values are 0.38–0.69, showing a pronounced negative Eu anomaly. The (La/Yb)_N values are 4.31–9.94. Fractionation of REE is significant in the samples analysed, which are enriched in LREE and depleted in HREE. The degree of REE fractionation is correlated with the magnitude of Eu/Eu*. In the chondrite-normalized REE diagram (Figure 7a), the trend shows a significant increase towards the LREE. The primitive-mantle-normalized trace element spider diagram (Figure 7b) shows enrichment in large-ion lithophile elements (LILE; Rb, Ba, K, Th, and U) and depletion in high-field strength elements (HFSE; Nb, Sr, and Ti; Table 2).

Samples of the Manitu Formation are characterized by ΣREE contents of 223.10–371.18 ppm (average of 302.00 ppm), Eu/Eu* values of 0.73–0.84, weak negative Eu anomalies, and (La/Yb)_N values of 9.56–40.66. Fractionation of REE is not significant, and the degree of REE fractionation is not directly correlated to Eu/Eu*. In the chondrite-normalized REE diagram (Figure 7c), LREE are slightly enriched with a gentle positive trend towards LREE. The primitive-mantle-normalized trace element spider diagram (Figure 7d) shows enrichment in LILE (Rb, Ba, K, Th, and U) and depletion in HFSE (Nb, Ta, P, and Ti; Table 2).

The Baiyingaolao Formation is characterized by ΣREE contents of 78.61–180.73 ppm (average of 137.32 ppm), Eu/Eu* values of 0.47–0.65, negative Eu anomalies, and (La/Yb)_N values of 5.23–12.68. REE fractionation is significant, showing enrichment in LREE and depletion in HREE, and the degree of fractionation is clearly related to the intensity of Eu/Eu*. In the chondrite-normalized REE diagram (Figure 7e), the curve shows an obvious positive trend towards LREE. The primitive-mantle-normalized trace element spider diagram (Figure 7f) shows enrichment in LILE (Rb, Ba, K, Th, and U) and depletion in HFSE (Nb, Ta, Sr, and Ti; Table 2).

The Meiletu Formation is characterized by ΣREE contents of 211.76–267.33 ppm (average of 242.45 ppm), Eu/Eu* values of 0.89–0.97, weak negative Eu anomalies, and (La/Yb)_N values of 6.71–12.37. Fractionation of REE is not obvious, and the degree of fractionation is not directly related to the intensity of Eu/Eu*. In the chondrite-normalized REE diagram (Figure 7g), the REE generally shows similar standardized trends, with LREE being slightly enriched. The primitive-mantle-normalized trace element spider diagram (Figure 7h) shows enrichment in LILE (Ba, K, Th, U, La, Ce, Sr, Nd, and Hf), depletion in HFSE (Nb, Ta, Zr, P, and Ti), and high contents of compatible elements such as V, Cr, Co, and Ni (Table 2).

4.3 Isotopes

The initial Sr–Nd isotope values of late Mesozoic volcanic rocks in the Wenkutu area show the following characteristics: the test Sr isotope value (⁸⁷Sr/⁸⁶Sr) of the Manketouebo Formation rhyolite is 0.715478, the I_Sr value is 0.705626, the ε_Nd value is 4.55157, and the two-stage Nd model age (T_DM2) is 839.6 Ma. The test Sr isotope value (⁸⁷Sr/⁸⁶Sr) of Manitu Formation andesite is 0.705953, the I_Sr value is 0.704912, the ε_Nd value is 2.27365, and the two-stage Nd model age (T_DM2) is 879.6 Ma. The test Sr isotope value (⁸⁷Sr/⁸⁶Sr) of Baiyingaolao Formation rhyolite is 0.712022, the I_Sr value is 0.705651, the ε_Nd value is 4.5627, and the two-stage Nd model age (T_DM2) is 685.6 Ma. The test Sr isotope value (⁸⁷Sr/⁸⁶Sr) of Meiletu Formation basalt is 0.704611, the I_Sr value is 0.704513, the ε_Nd value is 4.37638, and the two-stage Nd model age (T_DM2) is 710.3 Ma (Table 3). These volcanic rocks therefore have weakly to moderately radiogenic origins (0.704513–0.705626), with positive ε_Nd(t) values and young T_DM2 model ages. It is suggested that newly formed crustal materials played important roles in the formation of these rocks (Fan, Guo, Gao, & Li, 2008; Ho et al., 2013).

Table 3. Isotope characteristics of late Mesozoic volcanic rocks in the study area

Sample	Stratigraphic code	⁸⁷Rb/⁸⁶Sr	⁸⁷Sr/⁸⁶Sr	2σ	I_Sr	¹⁴⁷Sm/¹⁴⁴Nd	¹⁴³Nd /¹⁴⁴Nd	2σ
MK-1	K₁mk	4.7801	0.715478	0.0000013	0.705626	0.1005	0.512706	0.000009
MN-2	K₁mn	0.5423	0.705953	0.00009	0.704912	0.1124	0.51268	0.000005
BY-1	K₁b	3.4827	0.712022	0.00007	0.705651	0.1185	0.512806	0.000006
ML-1	K₁m	0.0537	0.704611	0.00004	0.704513	0.1111	0.512790	0.000012

Sample	Stratigraphic code	T(Ma)	εSr	εNd	I_Nd	T_DM	T_DM2
MK-1	K₁mk	145	18.371	4.55157	0.512684	713.6	839.6
MN-2	K₁mn	135.1	8.0691	2.27365	0.5125806	832.6	879.6
BY-1	K₁b	128.7	18.466	4.5627	0.512706	689.1	685.6
ML-1	K₁m	129	2.3029	4.37638	0.5126963	664.0	710.3

5 DISCUSSION

5.1 Formation age

Previous geochronological studies of the Mesozoic volcanic rocks of the Great Xing'an Range have yielded a wide range of ages. Most volcanic rocks of the Great Xing'an Range yield ages of 111–162 Ma (Sun et al., 2011), mainly 122–173 Ma, which can be divided into three peaks: Late Jurassic (160 Ma), early Early Cretaceous (140 Ma), and late Early Cretaceous (125 Ma; Si et al., 2015). The main body of Mesozoic volcanic rocks in the northern Great Xing'an Range was formed during the Early Cretaceous, and only a small amount of Late Jurassic volcanic rocks occur in the western Manzhouli and the northern part of the Hailar Basin (Zhang, 2009). The Mesozoic volcanic rocks of the Great Xing'an Range are divided into four stages: 174–186, 155–166, 140–145, and 120–130 Ma (Wang, Xu, Wang, & Meng, 2012). Significant igneous activity within the northern area of the Great Xing'an Range occurred during the Late Jurassic to Early Cretaceous and can be divided into three cycles of magmatic evolution, as follows: (a) the Tamulangou–Jixiangfeng cycle during the Late Jurassic; (b) the Shangkuli cycle; and (c) the Yilikede cycle during the Early Cretaceous (Yin et al., 2006). Results show that the main body of the Great Xing'an Range is younger than 140 Ma (Early Cretaceous), whereas the Late Jurassic volcanic rocks are distributed in the western part of the Great Xing'an Range (Zhang et al., 2007). Volcanic eruptions in the area commenced during the Late Jurassic, with most volcanic activity occurring later during the Early Cretaceous. Ying, Zhou, Zhang, and Wang (2010) suggested that an increase in mantle temperature during the Cretaceous provided the heat source for volcanic activity during this period.

Previous 1:250,000 regional geological mineral surveys in the Xiaowuerqihanlinchang area of Inner Mongolia indicated that the Manketouebo, Manitu, and Baiyingaolao formations are Late Jurassic in age and that the Meiletu Formation is Early Cretaceous in age. Four 1:50,000 regional geological mineral surveys in the Wenkutu area of Inner Mongolia indicated that the Manketouebo and Manitu formations are Late Jurassic in age, whereas the Baiyingaolao and Meiletu formations are of Early Cretaceous age. These studies identified growth zoning in magmatic zircon and U–Pb dating results that yielded high Th/U values (indicative of a magmatic origin), meaning that the results are likely to reliably represent the age of each formation (Liu, Yang, et al., 2016). The dating results were as follows: Manketouebo Formation, 145 ± 1 Ma; Manitu Formation, 141.3 ± 1.7 Ma; Baiyingaolao Formation, 128.7 ± 1.1 Ma; and Meiletu Formation, 129 ± 1 Ma. These ages are consistent with the timing of Early Cretaceous volcanic activity in the Great Xing'an Range.

5.2 Characteristics of the magma source

The origin of the magma that produced the late Mesozoic volcanic rocks in the middle part of the northern Great Xing'an Range has been the focus of numerous studies and is extensively debated. One area of debate is concerning the origin of the voluminous acidic and intermediate to basic volcanic rocks; i.e., whether these were the product of cognate magmatic evolution or were derived from different sources (Ge et al., 1999, 2000; Lin et al., 1998; Lin, Ge, Sun, & Wu, 1999; Fan, Guo, Wang, & Lin, 2003; Chen et al., 2006). The second area of debate focuses on whether the volcanic rocks were sourced from the crust or mantle (Ge et al., 2000; Lin et al., 1998, 1999).

The Mesozoic volcanic rocks in the central southern area of the Great Xing'an Range are enriched in major elements, LILE and HFSE, and depleted in Nb, Ta, and Fe. These geochemical characteristics are indicative of an enriched mantle source region. Following the closure of the paleo-Asian Ocean, the ongoing descent of its oceanic plate (and overlying sediment) and the dehydration associated with this descent may have been the main factors leading to enrichment of the mantle source region (Lu et al., 2004). The formation of the acid volcanic rocks in this area is most likely a result of post-orogenic lithospheric extension. Differentiation of mantle-derived mafic parental magmas can result in a series of rocks from basic to acid at depth. Underplating can provide heat sources for volcanic activity, leading to the recycling of crustal material (Gao et al., 2005). The felsic volcanic rocks in the area are characterized by high initial values of Nd of Early Cretaceous from the crust (Guo, Fan, Li, Gao, & Miao, 2009). By comparison, the Mesozoic volcanic rocks in the Longjiang Basin reflect magma derived from the crust and are a product of crustal partial melting during closure of the Mongolia Okhotsk Ocean (Li, Gao, Bian, Chen, & Ding, 2013).

The late Mesozoic intermediate to acidic volcanic rocks in the middle part of the northern Great Xing'an Range are characterized by relatively high Al₂O₃ and K₂O contents, a large range of P₂O₅ and MgO contents, and low Mg^# of 15–30. The REE generally have similar standardized curves to each other, with high LREE enrichment and strongly negative Eu anomalies. The trace elements are characterized by enrichments in Rb, Ba, K, and Th and depletions in Nb and Ti. The average value of Rb/Sr is 1.20, which is close to the crustal ratio (0.15) but higher than that of the primitive mantle (0.03), E-MORB (0.033), and OIB (0.047; Sun & McDonough, 1989). The average value of ⁸⁷Sr/⁸⁶Sr is 0.711151 (i.e., >0.705). These geochemical characteristics are similar to those of crust-derived rock, suggesting that the volcanic rocks were sourced from partial melting of the crust.

The late Mesozoic intermediate to basic volcanic rocks in the middle part of the northern Great Xing'an Range are characterized by relative high Al₂O₃, CaO, TiO₂, and TFeO, low K₂O, relatively constant P₂O₅ and MgO, and high Mg^# of 35.59–62.28. The REE generally show similar patterns to each other, with LREE being slightly enriched, little differentiation between LREE and HREE, and weak negative Eu anomalies. Trace elements are characterized by enrichments in Ba, K, Th, U, La, Ce, Sr, Nd, and Hf and depletions in Nb, Ta, Zr, P, and Ti. The characteristics of Th and U are the most obvious among the large ion parent elements, and most rocks show positive K anomalies. The average value of Rb/Sr is 0.0325, which is intermediate between values of the primitive mantle (0.03) and E-MORB (0.033), and significantly lower than the values for crust (0.15) and OIB (0.047; Sun & McDonough, 1989). The average value of ⁸⁷Sr/⁸⁶Sr is 0.704611 (i.e., <0.705). In summary, the geochemical characteristics of the late Mesozoic intermediate to basic volcanic rocks in the middle part of the northern Great Xing'an Range are consistent with both crustal and mantle source rocks.

5.3 Tectonic setting

The study area is located at the juncture between the Eergu'Na and Xingan blocks and has a complex tectonic history. Four models have been proposed to explain the tectonic history of the Mesozoic volcanic rocks in the Great Xing'an Range region.

The first model states that Mesozoic volcanic rocks in the Great Xing'an Range are related to activity of a mantle plume or intraplate interaction. The formation of the Great Xing'an Range was related to both the Paleo-Asia and Pacific tectonic regimes and the geochemical processes of the deep mantle. One of the most important factors controlling the formation of these Mesozoic volcanic rocks was the deep subduction of the cold Paleo-Asia Plate, which caused the rise of the mantle plume (Lin et al., 2000, 1999; Lin et al., 1998; Ge et al., 1999, 2000).

The second model states that the formation and evolution of the Mesozoic volcanic rocks occurred within an intraplate extensional environment, related to underplating, and the mechanism of magma formation was related to intraplate upwelling of the asthenosphere. Magmatic activity was related to shearing deformation of the upper lithosphere, which in turn was related to the relative shear movement of the oceanic plate (Shao et al., 2001; Shao, Zang, & Mou, 1994). Mesozoic volcanic rocks in the middle south section of the Great Xing'an Range formed in an intraplate extensional tectonic environment.

The third model states that the Mesozoic volcanic rocks in the Great Xing'an Range formed during closure of the Mongolia–Okhotsk Ocean, which resulted in stretching of the orogenic belt (Guo et al., 2001; Fan et al., 2003; Meng, 2003; Wang, Liu, Wang, & Song, 2002; Li, Liu, Xu, Li, & Zhang, 2015; Tang et al., 2014; Wang, Zhou, Zhang, Ying, & Zhang, 2006;Ying et al., 2010). Primary magma was derived from partial melting of crustal rocks in the Chaihe area of the middle Great Xing'an Range. Volcanic rocks of the Manitu Formation were formed in the Mongolia Okhotsk suture zone after the cessation of lithospheric extension (Si et al., 2015), after which the southern part of the Great Xing'an Range was no longer strongly influenced by the Pacific tectonic system (Li et al., 2016).

The fourth model states that subduction of the paleo-Pacific Plate was the primary cause of magmatism within China during the Mesozoic (Jiang & Quan, 1988; Zhao, Yang, Fu, Yang, & Yang, 1989; Zhang et al., 2008; Uyeda & Miyashiro, 1974; Hilde et al., 1977; Sengor & Natal'in, 1996). During the Jurassic to Early Cretaceous, the temporal and spatial distribution of volcanic rocks in the Great Xing'an Range indicate that their formation was controlled by subduction of the Pacific Plate (Zhang et al., 2009; Zhang et al., 2010). Recent studies show that the Heilongjiang Group distributed on the Jiamusi block is characterized by the collage accretion complex, and its formation age is about 190 Ma, indicating that during the early Jurassic period, the paleo-Pacific Plate had subducted to the edge of the Asian continent (Wu et al., 2007).

Based on the results of previous studies and the data presented here, we conclude that the volcanic rocks in the middle part of the northern Great Xing'an Range formed as a result of subduction of the paleo-Pacific Plate under the Asian continental plate. This conclusion is supported by the following lines of evidence. (a) Late Mesozoic volcanic rocks within the study area are distributed NE–SW, parallel the continental margin. (b) The geochemical characteristics of the Manketouebo and Baiyingaolao formations (Gotini index (τ) = 20.51–89.70; i.e., >10) indicate that these volcanic rocks are related to an orogenic belt environment. The Manitu Formation (Gotini index (τ) = 5.85–19.31) was related to either orogenic or non-orogenic environments, and the Meiletu Formation (Gottignies index (τ) = 6.50–9.87) was related to a non-orogenic environment. (c) In terms of Nd and Sr isotopic characteristics, the Manketouebo and Baiyingaolao formations are comparable with the Xing Meng orogenic belt, the Manitu Formation is related to the junction between the Xing Meng orogenic belt and the mantle zone, and the Meiletu Formation is related to the mantle (Figure 8). (d) The volcanic rocks in the Wenkutu area are 120–150 Ma, which is consistent with the oblique subduction stage of the paleo-Pacific Plate. (e) The volcanic rocks in the Wenkutu area, Great Xing'an Range, China, are gradually changing new from the west to the east.

6 CONCLUSIONS

Results of our petrography, petrogeochemical, and chronological study of the late Mesozoic volcanic rocks in the Wenkutu area (middle part of the northern Great Xing'an Range) yield the following conclusions.

The intermediate to acid volcanic rocks of the Manketouebo, Manitul, and Baiyingaolao formations are characterized by high Rb/Sr, low Mg#, and strong negative Eu anomalies, whereas the intermediate to basic volcanic rocks of the Meiletu Formation have low Rb/Sr, high Mg#, and weak negative Eu anomalies. These results show that their magma sources were different, with the intermediate to acid magma being derived from the crust and the intermediate to basic magma being sourced from the mantle and contaminated by crustal material.
The results of SHRIMP zircon U–Pb dating show that late Mesozoic volcanic eruptions occurred in the study area during the Early Cretaceous.
The volcanic rocks in the middle part of the northern Great Xing'an Range are related to subduction of the paleo-Pacific Plate below the Asian continental plate. The Manketouebo and Baiyingaolao formations were formed in an orogenic environment, whereas orogenic activity was weak during the formation of the Manitu Formation, and the Meiletu Formation was related to a post-orogenic intracontinental rift environment. These results show that the region underwent a transition from compression to extension during the late Mesozoic.

ACKNOWLEDGEMENTS

Huan-li Zhao (The Fifth Geological Exploration Institute of Heilongjiang Province) is thanked for guidance during fieldwork. Professor Ke-yong Wang, Jin Liu, and Jian-feng Li (Jilin University) provided valuable input during the writing of this manuscript.

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Citing Literature

Volume54, Issue3

May/June 2019

Pages 1343-1360

Geochronology and geochemistry of late Mesozoic volcanic rocks in the Wenkutu area, Great Xing'an Range, China

Abstract

1 INTRODUCTION