Introduction

The Daxingan Mountains’ volcanic rock belt (DMVRB) is in the northern Xing–Meng orogenic belt in the eastern section of the Central Asian Orogenic Belt, China. It is distributed in NE-trending belts, stretching from the Chifeng Linxi–Udan area in the Inner Mongolia Autonomous Region in the south, to the Mohe County area of Heilongjiang Province in the north, with a length of ~ 1200 km and width of up to ~ 400 km. The DMVRB is widely covered by Mesozoic mafic and silicic volcanic rocks, with pre-Mesozoic strata being intermittently exposed. There are many polymetallic deposits in the area, including large and super-large deposits of nonferrous and precious metals, as well as rare earth element deposits (Shao et al. 1997, 1998; Shao and Zhang 1999; Ge et al. 2007a, b; Fan et al. 2003; Meng 2003;Wang et al. 2006; Zhang et al. 2008, 2010; Wu et al. 2011; Şengör et al. 1996; Li et al. 2006; Natal’in and Borukayev 1991; Zhou et al. 2009; Wu et al. 2007, 2011; Zhou et al. 2012; Liu et al. 2004). The geodynamic setting of the DMVRB remains unresolved, with suggestions including a mantle plume origin (Lin et al. 1998; Ge et al. 1999), a basin-and-range structural setting associated with closure of the Mongolia–Okhotsk Ocean and Paleo-Asian Ocean basin (Fan et al. 2003; Meng 2003), the influence of an extensional orogenic structure associated with Paleo-Pacific Plate subduction (Wang et al. 2006; Zhang et al. 2008, 2010, Zhao et al. 1994), and the influence of the Mongolia–Okhotsk and the circum-Pacific tectonic systems (Shao and Zhang 1999; Wu et al. 2011; Liu et al. 2004; Meng et al. 2011;Xu et al. 2009, 2011). The geodynamic setting of the DMVRB belt remains an important unresolved scientific issue.

Rhyolites with high Nb and Ta contents were recently discovered in the Kyihe region of the northern DMVRB, and these are the subject of our study. Such rhyolites have the potential to reveal the tectonic setting of Mesozoic volcanic rocks in the Daxingan Mountains, and the magma sources and origins of large-scale mineralization. The high Nb–Ta rhyolites are widely distributed in the area and have Nb2O5 contents of > 0.03 wt%, which classifies them as an industrial grade Nb–Ta deposit. Research into their petrogenesis is therefore of significance in exploration for similar rhyolites elsewhere in the region.

Regional geological background and rhyolite characteristics

The study area is located in the northern section of the DMVRB, in the northern Xingan Block (Zhou et al. 2016) (Fig. 1), adjacent to the main ridge fault of the Daxingan Mountains. The tectonic evolution of the area is complex and involved formation of a micro-continent with metamorphic basement in the Precambrian, closure of the Paleo-Asian Ocean and amalgamation of several micro-continents during the Paleozoic (Şengör et al. 1993), and superposition and transformation due to Mongolia–Okhotsk Ocean closure and Paleo-Pacific Plate subduction during the middle Cenozoic. The pre-Mesozoic structure has been removed by the Mesozoic tectonism and magmatism. Since the Mesozoic, the regional geological setting has been characterized by magmatic activity controlled by pre-Mesozoic basement faults, which created NE-, NNE-, and NNW-trending structures and magmatism.

Fig. 1
figure 1

Simplified geological map and sampling locations. (1) Quaternary; (2) the Upper Jurassic Manketouebo Group; (3) the Middle Jurassic Tamulangou Group; (4) Cretaceous granite porphyry; (5) rhyolite; and (6) sampling location. F1—the Tayuan-Xiguitu Fault; F2—the Heganshan-Heihe Fault

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The Precambrian rocks in the region form a lithostratigraphic unit composed of metaclastic and volcaniclastic rocks with minor carbonate intercalations, which are mainly distributed in the west and north of the DMVRB. The Xinghua Ferry Group is considered to represent Precambrian metamorphic basement of the Xingan Block (Ge et al. 2007a; Heilongjiang Bureau of Gology and Mineral Resources. Regional Geology of Heilongjiang Province 1993; Miao et al. 2007), including sillimanite–garnet-bearing gneiss, felsic gneiss, amphibolite, and graphite schist (Zhou et al. 2011a, 2016). Paleozoic strata are distributed mainly in the upper Heilongjiang depression in the north of the DMVRB, and exposed intermittently in the west and central–south of the belt. Jurassic–Cretaceous mafic and silicic volcanic and pyroclastic rocks are exposed widely in the study area, including the Tamulangou (J2tm, J2 and tm are abbreviations of the middle Jurassic and Tamulangou, respectively), Manketouebo (J3mk, J3 and mk are abbreviations of the upper Jurassic and Manketouebo, respectively), Manitu (J3mn, mn is the abbreviation of Manitu), Baiyingaolao (K1b, K1 and b are abbreviations of the lower Cretaceous and Baiyingaolao, respectively), and Meiletu (K1ml, ml is the abbreviation of Meiletu) groups. The Tamulangou Group contains volcanic and normal sedimentary rocks, comprising mainly gray–green, amygdaloidal, dense, massive basalt and its breccia, and trachyandesite with black silty mud. The Manketouebo Group comprises mainly silicic volcanic, pyroclastic, and sedimentary rocks. The Manitu Group comprises mainly dacite–dacitic and andesite–andesitic lithic tuff. The upper section of the Baiyingaolao Group comprises silicic volcanics, tuffs, and dacitic tuffs, whereas the lower section contains tuffaceous conglomerate, fine sandstone with silty shale, and small amounts of silicic tuff and volcanic rocks (animal and plant fossils are also found in this group). The Baiyingaolao Group unconformably overlies the intermediate volcanic rocks of the Manitu Group, and is unconformably covered by the Lower Cretaceous mafic–intermediate volcanic rocks of the Meiletu Group. The Meiletu Group comprises mainly a set of fumarolic, amygdaloidal, dense, massive trachyandesite, basalt, and its volcanic eruptions were dominated by lava and some debris flows. The Meiletu Group is distributed mostly as “islands” of higher ground and has a Lower Cretaceous age. In addition to the large-scale Mesozoic volcanic rocks, there are also extensive outcrops of granodiorite, granite porphyry, and other intermediate–silicic intrusive rocks.

The high Nb–Ta rhyolites are exposed in southern Kyihe County in the Oroqen Autonomous Banner, to the east of Wulukanggou. The widely exposed strata in the area represent the lower part of the Upper Jurassic Manketouebo Group (Fig. 1), which is distributed in a N–S direction over an area of ~ 2 km2. The fresh rocks are gray with a porphyritic, felsitic, and weakly rhyolitic structure (Fig. 2a). Their phenocryst content is ~ 10%, and the phenocrysts comprise sanidine, quartz, and a small amount of black mica (Fig. 2b). The sanidine (~ 7%) is a high-temperature type with simply twinned crystals (2V = 63°, the term “2V” denotes the optical axial angle of a biaxial crystal), which have a euhedral–subhedral platy form, are 1–2 mm in diameter with micro-cracks, and mostly exhibit a porphyritic or glomeroporphyritic texture (Fig. 2c). The quartz (~ 3%) is euhedral–subhedral, granular or irregular in shape, 1–3 mm in diameter, and is visibly resorbed. Black mica is observed occasionally (< 1%) and is flaky and brown or reddish brown. The rhyolite matrix exhibits a felsitic and microcrystic texture, which is composed of small particles of xenomorphic, granular, or cryptocrystalline quartz and feldspar. Alkali feldspar microcrysts and xenomorphic granular quartz define the rhyolitic structure.

Fig. 2
figure 2

Field photographs a and photomicrographs b of the samples

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Analytical techniques

Major and trace element of rhyolites analyses were conducted at Jilin University, China. For major element analysis, six fresh rock samples were crushed to < 200 mesh in an agate mortar, fused into a glass disk at room temperature and pressure, and analyzed by X-ray fluorescence using an XRF-1500 spectrometer with an accuracy better than ± 1%. For trace element analyses (including rare earth elements; REE), 40-mg samples were dissolved in HF–HNO3, diluted with 1% HNO3, and analyzed using a dual-focus inductively coupled plasma mass spectrometer (ICP–MS) (Finnigan-MAT Element) with an accuracy better than ± 5%.

Zircon crystals were extracted by conventional gravitational and magnetic separation techniques and handpicked under a binocular microscope. The zircons were set in epoxy resin disks, which were then polished to about half thickness. Zircons with good crystal form were chosen for cathodoluminescence (CL) imaging, which were conducted at the Tianjin Institute of Geology and Mineral Resources, using a Gatan Mono CL3+CL system. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb dating was performed at the Laser Plasma Mass Spectrometry Laboratory, China University of Geosciences, using an Agilent 7500a spectrometer with a 193-nm laser system (New Wave UP 193SS). The ablation frequency was 10 Hz, beam diameter 30 μm, pre-ablation time 5 s, and ablation time 45 s. High-purity He was used as a carrier gas. Isotopic ratios were normalized on the basis of zircon standard 91500. Quality control samples included zircon standards TEMORA and Qinghu, and glass standards NIST612 and NIST614. Age calculations used the Isoplot program, and zircon U–Pb isotope ratios were calculated using the Glitter program 5.5. The glass standard NIST610 was used as an external standard and Si was used as an internal standard. Uncertainties on single data points are given at the 1σ level and on weighted means at the 2σ level.

Strontium and Nd isotopic analyses were conducted at the Tianjin Institute of Geology and Mineral Resources. Samples were dissolved in HF–HClO4 (> 12 h at ~ 100 °C) and then purified by solvent extraction using HDEHP. Strontium and Nd contents and isotopic ratios were determined by thermal ionization mass spectrometry (TIMS) using a Triton TIMS system. Procedural blanks were Rb = 5.6 × 10−10 g; Sr = 3.8 × 10−10 g; Sm = 3.0 × 10−11 g; and Nd = 5.4 × 10–11 g. The Nd–Sr isotopic ratios of the BCR-1 Nd and NBS-607 Sr standards obtained were 143Nd/144Nd = 0.512739 ± 0.000005 (2σ) and 87Sr/86Sr = 1.200050 ± 0.000005 (2σ), respectively. Mass fractionation corrections were based on internal standardization to 88Sr/86Sr = 8.37521 and 146Nd/144Nd = 0.7219.

Results

Zircon U–Pb dating

Twenty-one zircons were selected from the rhyolites for LA–ICP–MS U–Pb dating (Table 1). The samples appear pinkish brown in color, and most are hypidiomorphic–euhedral short prismatic crystals, with some being sub-angular crystals. Crystal diameters are 0.1–0.2 mm, with aspect ratios of 1:1 to 2:1. The zircons display magmatic oscillatory zoning (Fig. 3). Zircon total REE content (∑REE) is 655–4926 × 10−6, and the light REE to heavy REE ratio (LREE/HREE) is 0.03–0.19, indicating significant REE fractionation and HREE enrichment (Table 2). The REE partition diagram shows that the whole REE patterns decline to left (Fig. 4a). The zircons display positive Ce anomalies (Ce/Ce* = 2.1–103.4) and negative Eu anomalies (Eu/Eu* = 0.18–0.64), consistent with characteristics of crustal magmatic zircons (Hoskin and Schaltegger 2003). Cerium exists in magma as Ce3+ and Ce4+, and the latter can easily replace Zr4+ and enter the zircon lattice during crystallization, thus fractionating Ce from other REEs. Under high oxygen fugacity (ƒO2) conditions, the higher Ce4+/Ce3+ ratio provides more Ce4+ for the zircon lattice, resulting in strongly positive Ce anomalies (Ballard et al. 2002; Zhao 2010a; Burnham and Berry 2012). The high Nb–Ta rhyolites are therefore considered to have formed in high-ƒO2 conditions. Eu exists mainly as Eu3+ and cannot be fractionated from other trivalent REEs under high-ƒO2 conditions. The negative Eu anomalies are thus inherited from low-Eu magma (Mass et al. 1992).

Table 1 Zircon U–Pb analytical data for rich Nb–Ta rhyolite
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Fig. 3
figure 3

Representative CL images of zircons from the high Nb–Ta rhyolites

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Table 2 Rare earth element composition of zircons LA–ICP–MS
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Fig. 4
figure 4

Zircon chondrite-normalized REE distribution patterns a and LA–ICP–MS zircon U–Pb concordia diagrams b of rich high Nb–Ta rhyolite

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Zircon Th and U contents vary widely, with Th = 315–1669 × 10−6 (average 834 × 10−6) and U = 272–812 × 10−6 (average 515 × 10−6). Th/U ratios of 1.05–2.10 (average 1.50) indicate that the zircons formed by magmatic crystallization. The zircon 206Pb/238U ages vary from 158 to 152 Ma, with the concordia diagrams (Fig. 4b) indicating that the data are concordant and define a weighted-mean age of 155.9 ± 1.0 Ma (MSWD = 0.25; n = 21), which represents an Upper Jurassic age for the rhyolite.

Geochemical characteristics

Major elements

Major element compositions of the high Nb–Ta rhyolites are given in Table 3. Their SiO2 contents (71.9–72.1 wt%) are high, consistent with their silicic nature. The rhyolites have a moderate Al2O3 content (13.8 wt%), with an average aluminum saturation index (A/CNK) of 1.06, indicating a weakly peraluminous rock type. The total alkali (K2O+Na2O) content is relatively high (average 9.2 wt%), with an average K2O/Na2O ratio of 0.93, indicating an Na-rich rhyolite. FeOT (2.5–2.6 wt%), CaO (0.2 wt%), TiO2 (0.1 wt%), and MgO (0.1 wt%) contents are low. In the total alkalis–silica (TAS) diagram (Fig. 5a), data for the rhyolite samples plot as sub-alkaline series rocks. Rittmann index (σ = (Na2O+K2O)2/(SiO2–43); wt%) values of ~ 2.9 wt% indicate that the rocks are of the calc-alkaline series, and in the K2O vs SiO2 diagram (Fig. 5b), the samples plot in the high-K calc-alkaline field. In summary, the high Nb–Ta rhyolites of the Kyihe region are high-Si, low-Ti, peraluminous, high-K calc-alkaline series rocks.

Table 3 Contents of major elements (wt%) and trace elements (×10−6) of high Nb–Ta of rhyolite
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Fig. 5
figure 5

Plots of SiO2 vs (SiO2+K2O) (a) and plots of SiO2 vs K2O (b) for high Nb–Ta rhyolite

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Trace elements

The rhyolite REE content is high with ΣREE = 326–344 × 10−6 (Table 3; Fig. 6a), ΣLREE = 284–305 × 10−6, ΣHREE = 38.3–39.6 × 10−6, and LREE/HREE = 7.4–8.0. (La/Yb)N ratios (3.8–4.5) indicate that there was significant fractionation between LREE and HREE, with LREE enrichment, while (La/Sm)N ratios of 4.8–5.5 also indicate significant LREE fractionation. The ΣHREE content is about ten times that of standard chondrite. (Gd/Yb)N ratios (1.26–1.34) and relatively flat HREE patterns (Fig. 6a), with no obvious fractionation, indicate that a significant amount of garnet was incorporated in the source magma. The greater YbN content (28.9 × 10−6) than HoN (24.1 × 10−6) indicates a slight loss of middle REEs, suggesting residual amphibole was present in the magma source. The rhyolites exhibit an obvious positive Ce anomaly (Ce/Ce* = 2.44–2.67), indicating that the rocks were formed in a strongly oxidizing environment with melting and enrichment of Ce-bearing minerals. The negative Eu anomalies (Eu/Eu* ~ 0.06) are due to residual Ca-rich plagioclase in the source, which could also explain the low Ca, Sr, and Ba contents (alteration such as sericitization or albitization was not observed in the rhyolite, and so reactions between volatile fluids containing F and Cl were not involved). Enrichment in Nb, Ta, Zr, Hf, Rb, Th, and Ce and depletion in Sr, Ba, P, and Ti are evident in the trace elements data (Table 3) and spider diagrams (Fig. 6b). High Th/U (4.94–5.12) and Rb/Sr (12.0–13.5) ratios indicate that the source rocks were associated with continental crustal materials and/or residual minerals such as plagioclase, rutile, and apatite during melting. Rhyolite Zr contents are in the range of 804–892 × 10−6, whereas compatible elements such as Co (1.4–2.0 × 10−6), Ni (1.6–2.0 × 10−6), and V (2.1–2.3 × 10−6) have low contents, indicating that Zr-rich minerals were incorporated during melting, whereas Fe- and Mg-rich minerals such as pyroxene remained as residue minerals in the source. Sr and Yb contents of 18.5–20.6 × 10−6 and 7.0–7.3 × 10−6, respectively, indicate that the rock is a low-Sr/high-Yb (Zhang et al. 2006) rhyolite.

Fig. 6
figure 6

Chondrite-normalized REE patterns (a) and primitive mantle-normalized spidergram (b) of high Nb–Ta rhyolite (normalization values after Sun and Mc Donogh, 1989). Mesozoic post-orogenic igneous rocks in Northeast China (from Wu et al., 1997). Mesozoic igneous rocks in northeastern Inner Mongolia (from Zhou et al., 2003)

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Sr and Nd isotopic compositions

Properties of magma sources can sometimes be inferred from Sr and Nd isotopic compositions, and these results are given in Table 4. Rhyolite Rb/Sr ratios are relatively high (12.0–13.5), whereas Sm/Nd ratios are low (0.28). (87Sr/86Sr)i ratios and εSr(t) values are in the ranges of 0.70300–0.71160 and − 18.21 to 102.94. The variability of these data is indicated by four of the five samples having positive εSr(t) values and one a negative value, whereas three samples have (87Sr/86Sr)i ratios in the range 0.7040–0.7060 and two have ratios of > 0.7060. (143Nd/144Nd)i ratios and εNd(t) values are in the ranges of 0.512537–0.512539 and 1.9–2.0, respectively, similar to those of granites in continental crust in the Xing–Meng orogenic belt (Mcdonough and Sun 1995; Hong et al. 2000, 2003), indicating that the magma was derived from weakly or undepleted mantle (Shao et al. 1998; Shao and Zhang 1999). The relative deviation of the Sm/Nd ratio from the chondritic value, fSm/Nd (fSm/Nd = (147Sm/144Nd –(147Sm/144Nd)CHUR)/(147Sm/144Nd)CHUR) is in the range of 0.09 and 0.5 for the rhyolites, and slightly different from the average crustal value (0.118) indicating differentiation of147Sm and 144Nd during melting and metamorphism. This may distort model ages, although fractionation of Sm and Nd in the source is not obvious. As such, the two-stage Nd isotopic model age (T2DM) may better represent the age of the source materials remaining in the crust. The T2DM age of the rhyolites is 789–785 Ma, and older than granites in the Central Asian Orogenic Belt (700–600 Ma) (Hong et al. 2000, 2003; Guo et al. 2014). This relatively old formation age suggests that there was a crustal growth event during the Neoproterozoic, with crustal materials originating from depleted mantle or partial melting of juvenile crust. In a (87Sr/86Sr)i vs (143Nd/144Nd)i diagram (Fig. 7) (Zindler and Hart 1986), data for the samples plot in the upper region of the fourth quadrant, with three plotting in the depleted mantle field and two in the enriched mantle field, indicating that partial melting of juvenile crust may have been the main source of the high Nb–Ta rhyolitic magma.

Table 4 Sr–Nd isotopic results of high Nb–Ta rhyolite
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Fig. 7
figure 7

Diagram of εNd(t) vs (87Sr/86Sr) for source discrimination

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Age and tectonic setting

Rhyolite age

Mesozoic volcanic rocks of the Daxingan Mountains include, from older to younger, the Tamulangou, Manketouebo, Manitu, Baiyingaolao, and Meiletu groups. The high Nb–Ta rhyolite of the Kyihe area is located in strata within the Manketouebo Group which, in turn, is located within the lower–middle layers of the DMVRB. The Manketouebo Group comprises volcanic strata containing silicic volcanic, pyroclastic, and epiclastic rocks. These volcanic rocks have been considered as being the first section of the Lower Cretaceous Guanghua Group (Qu 1997), although collectively they are considered to be within the Manketouebo Group (Wu 1996). Recent zircon LA–ICP–MS U–Pb dating indicates that the Manketouebo Group can be divided into two age groups of 143–132 Ma (Zhang et al. 2014; Ji 2017; Du et al. 2017; Zhou and Jiang 2017; Wang et al. 2017) and 160–151 Ma (Yang et al. 2012; Zhang 2009; Chen et al. 2012; Li et al. 2016), with the former belonging to the Baiyingaolao Group and the latter being the eruptive ages of the volcanic rocks of the Manketouebo Group. The mean zircon U–Pb age of the Kyihe rhyolites has been determined in the present study to be 156 ± 1 Ma (MSWD = 0.25; n = 21), consistent with the volcanic rocks of the Manketouebo Group (160–151 Ma). Field characteristics of the rhyolites also match those of the Manketouebo Group, which is conformably overlain by andesite of the Manitu Group in the north of the study area. The high Nb–Ta rhyolite is thus considered to be the product of the Manketouebo volcanic eruption cycle during the Upper Jurassic.

Tectonic setting

Previous studies have indicated that the Songliao Basin and the area to the east of it were influenced by the Mesozoic Pacific rim tectonic system, whereas areas to the west of the basin and on the northern margin of the North China Block (including the Xingan Block) (Zhao et al. 1994; Xu et al. 2013) were influenced by the Mongolia–Okhotsk Ocean tectonic system. However, the subduction direction and closure time of the Mongolia–Okhotsk Ocean are unresolved. Some studies indicate that the ocean was closed in the Triassic (Maruyama et al. 1997), whereas others suggest that this took place in the Upper Jurassic–Lower Cretaceous (Cogné et al. 2010; Metelkin et al. 2010; Pei et al. 2011). Some studies have suggested that the Mongolia–Okhotsk oceanic lithosphere was subducted northward under the Siberian Plate during the late Permian (Meng 2003; Voo et al. 1999; Zorin 1999), whereas others have suggested that the formation of Triassic porphyry Cu–Mo deposits and granites in the Ergun Block was related to southward subduction (Wu et al. 2011; Jiang et al. 2010; Chen et al. 2010; Zeng et al. 2014).

Permian–Lower Jurassic sedimentary strata are not evident in the DMVRB, which would have been subjected to uplift and erosion, although in the Middle Jurassic, a series of faults and depressions developed from north to south in the Daxingan Mountains, including the Mohe basins, middle Daxingan Mountains’ fault zone, and Longjiang, Hailar, and Tuquan basins (HBGMRRGHP 1993; Zhao et al. 2014). The Emuerhe Group, in the upper Heilongjiang depression, formed in the Middle–Upper Jurassic, whereas sediments of the Xiufeng Group were deposited as late as the Middle Jurassic (Zhao et al. 2014). Both normal and listric faults developed during extensional activity in the south part of the Mohe basins, which has the characteristics of a fault basin, indicating that the formation age was earlier than the Middle Jurassic (Shi et al. 2014). The Wanbao Group, which comprises coarse-grained sediments of terrestrial fluvial facies, developed between the Daxingan Mountains’ fault, Longjiang fault basin, and Tuquan basin and has a zircon age for intercalated volcanic rocks of 165.2 ± 1.7 Ma (Zhang et al. 2017), indicating that they formed at the end of the Middle Jurassic (HBGMRRGHP 1993; Inner Mongolia Bureau of Geology and Mineral Resources. Regional geology of Inner Mongolia 1991; Bao et al. 2011; Wang et al. 2013a). Furthermore, the rifting period of the Hailar fault basin coincided with the eruption of the Middle Jurassic Tamulangou Group (Zhao 2010b). It can therefore be concluded that the Daxingan Mountains’ crust had been thickening until the end of the Lower Jurassic, with the region having been in an extensional tectonic setting since the beginning of the Middle Jurassic. The Mesozoic occurrence of volcanic activity in the Daxingan Mountains indicates that crustal evolution involved mantle-derived magma in an extensional tectonic setting within the continental lithosphere, but was not associated with magmatic activity in the subduction system of the active continental margin (Shao and Zhang 1999).

Tectonic settings are distinguished in the (Rb/30) vs Hf vs (3Ta) and Rb vs (Y+Nb) diagrams (Fig. 8a, b), where data for the Kyihe rhyolite samples plot in the intraplate granite field. A1/A2 tectonic settings are distinguished in the Nb vs Y vs Ce diagram (Fig. 8c), where data for the samples plot in the A1 field, indicating that the rocks were formed in an intraplate extensional tectonic setting associated with rifting, mantle plumes, and hot spots, rather than in an extensional setting developed by post-collisional or post-orogenic collapse of continental crust. Recent studies have indicated that volcanic rocks of the Upper Jurassic Manitu and Lower Cretaceous Baiyingaolao groups were also formed in an intraplate extensional tectonic setting (Sun et al. 2011; Gou et al. 2010). The study area had thus been in an intraplate extensional tectonic setting during the Upper Jurassic–Lower Cretaceous, with the Mongolia–Okhotsk Ocean having been closed, or oceanic lithosphere subducted northward under the Siberian Plate, during the Upper Triassic–Lower Jurassic. The resulting extension of the Xingan Block in the southern Mongolia–Okhotsk Ocean caused emplacement of the A-type granites (widely exposed in the study area) and large-scale eruption of high-K calc-alkaline magmas.

Fig. 8
figure 8

Diagrams for discriminating tectonic setting of Hf-Rb/30-Ta × 3 (a), Y+Nb-Rb (b), and Y-Nb-Ce (c) for high Nb–Ta rhyolite

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Petrogenesis and magma sources

Petrogenesis

Major element compositions of the Kyihe high Nb–Ta rhyolites are characterized by high SiO2 and K contents and low Ca and Mg contents, while trace element compositions are characterized by enrichment in Nb, Ta, Zr, Hf, Rb, Th, U, and Ce and depletion in Sr, Ba, Ti, and P. Their REE patterns exhibit a “V” shape with obvious negative Eu anomalies. Ga × 104/Al > 2.6, the rhyolite thus has a composition similar to A-type granite (Zhang et al. 2012; Whalen et al. 1987). A-type granites are formed under alkali-rich, anhydrous, and anorogenic conditions by the melting of source rocks under low-pressure and high-temperature conditions (Loiselle and Wones 1979; Zhang et al. 2007). A hydrous and high viscosity A-type granitic magma has a similar density to that of rock-forming minerals and does not undergo crystallization differentiation. The lack of cumulates associated with the rhyolite, and synchronous intermediate–mafic igneous rocks in the study area, further indicates that there was no crystallization differentiation of granitic magma (Zhang et al. 2007, 2012; Reid et al. 1993), with the chemical composition also indicating that there was no mixing of mafic and silicic magmas. Magmatic processes are distinguished by the La/Sm vs La diagram (Fig. 9), where data for the samples plot on an oblique line as a result of partial melting. It is concluded that the geochemical and Sr–Nd isotopic compositions indicate that the rhyolitic magma originated from partial melting of crustal materials.

Fig. 9
figure 9

Discrimination diagram of La-La/Sm for high Nb–Ta rhyolite

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Magma source

Major and trace element compositions, Ce and Eu anomalies, and Sr–Nd isotopic compositions provide indications of the nature of the rhyolite magma source, as discussed in the following sections.

Nb, Ta, and Zr

The most distinctive trace element characteristics of the rocks are their high Nb (183–188 × 10−6), Ta (10–11 × 10−6), and Zr (804–892 × 10−6) contents, which provide important clues concerning properties of the magma source.

Nb, Ta, and Zr contents increase as the volcanic lithology changes from mafic to silicic and tholeiitic to alkaline (Liu 1987). Ocean island basalt (OIB) formed from alkaline basaltic magma has the highest Nb (48 × 10−6) and Ta (2.7 × 10−6) contents (Zhao et al. 2008; Sun et al. 1989) of mantle materials, with Nb and Ta being hosted primarily in Ca- and Ti-rich minerals such as rutile, titanite, perovskite, Fe–Ti oxides, and allanite, and secondarily in Fe- and Mg-rich minerals such as pyroxene, hornblende, and biotite (Liu 1987). The low Ca, Ti, Fe, and Mg contents of the rhyolite thus indicate that its source is independent of such minerals, with no genetic connection to alkaline basaltic rocks.

The Zr contents of the Kyihe rhyolites are in the range of 813–892 × 10−6, whereas that of high Nb–Ta sodium amphibole rhyolite in the Aliwula region (in the southern segment of the Daxingan Mountains) is 8528–15,560 × 10−6 (Wang et al. 2013b) indicating large-scale Zr enrichment in the source area. The Zr ionic radius is similar to those of Nb and Ta, and so the latter can replace Zr in the crystal lattice, forming naegite rich in Nb, Ta, and REEs, with zircon becoming the main carrier of Nb and Ta. However, as an accessory mineral, the zircon content of primary magmatic rocks is very low. Its origin may therefore be more related to coastal or continental clastic rocks.

During weathering, placer sediments in riverbed and coastal environments tend to become enriched in Nb, Ta, and Zr, with ~ 24% of Nb, Ta, and Zr mining involving placer deposits (Liu 1987; Mou 1999). We speculate that coastal or continental clastic rocks rich in Nb, Ta, and Zr are present in the underlying continental crustal rocks, and these may have participated in the formation of the high Nb–Ta rhyolite.

The Nb/Ta ratio is also of significance as a tracer of magma sources. Increasing fractionation during magmatic evolution generally leads to increasing Ta content and decreasing Nb/Ta ratios (Liu 1987; Zhao et al. 2008), with silicic rocks therefore having the lowest ratios (Liu 1987). However, in alkaline magmas, the Nb/Ta ratio increases during evolution. Measured Nb/Ta ratios of the rhyolites are in the range of 17.65–17.99, similar to that of primitive mantle (Nb/Ta = 17.5) (Zhang et al. 2010) and Precambrian alkaline volcanic rocks of the North China Craton (Nb/Ta = 15.6 ± 2.9 to 18.8 ± 1.2) (Liu et al. 2005), and obviously higher than that of continental crust (Nb/Ta = 11) (Mclennan 1985).The rhyolite is therefore not a highly fractionated granite. Its source rocks have an affinity with original alkaline mantle-derived materials, and likely originated from crust composed of mafic mantle-derived rocks (alkaline basalt) and terrigenous clastic rocks rich in zircon (and hence Nb and Ta).

Ce and Eu anomalies

The positive Ce anomalies indicate high ƒO2 and high Ce contents in the magma during petrogenesis. Igneous rocks generally have either no Ce anomaly or a weak negative one; ferromanganese nodules of colloidal origin have obvious positive anomalies, mid-ocean ridge around 400 km sediments have obvious negative anomalies (Ce/Ce* = 0.29), marine basin sediments have moderate negative anomalies (Ce/Ce* = 0.55), and continental margin clastic rocks have weakly negative or positive anomalies (0.90–1.30) (Murray et al. 1990; Zhao 1997). The Fe and Mn contents of the high Nb–Ta rhyolites are low, indicating no genetic connection between the positive Ce anomalies and marine ferromanganese nodules. Their petrogenesis is therefore not related to subduction of oceanic crust or deep-sea sediments.

The high valence and small ionic radius of Ce4+ result in its preferential adsorption by sediments, and Ce is therefore relatively enriched in terrigenous sediments while other (trivalent) REEs tend to remain in solution (Murray et al. 1990). Strongly positive Ce anomalies therefore imply that the source rock was a product of partial melting of terrigenous clastic rocks. They may also be related to the enrichment and melting of zircon in the source region. The zircon trace element diagram (Fig. 3) demonstrates that Ce and HREEs can be enriched in zircons, resulting in strongly positive Ce anomalies. The positive Ce anomalies and high HREE contents of the rhyolites may therefore have been inherited from zircons, indicating that zircon-rich, terrigenous clastic rocks participated in source magma evolution.

The weathering of sedimentary clastic rocks in the source region would have resulted in negative Eu anomalies (Cullers et al. 1975) because Eu2+ is relatively soluble as compared with other REEs. Europium would be removed by chemical weathering in an open system, leaving Eu-poor sediments. Significant negative Eu anomalies may thus be caused by Eu depletion of the source rock and plagioclase in the source residue.

Major elements

Aluminous A-type granites are normally formed by the partial melting of lower crust rich in Al (Patino Douce 1997; Rajesh 2000). However, the high Nb–Ta rhyolites are weakly peraluminous, with Al-rich minerals in the source region, such as plagioclase and hornblende, having preferentially assimilated Al. The source rock of the rhyolites thus has an affinity with Al-rich ancient crust. In the mineral characterization diagrams (Na2O+K2O+MgO+TiO2) vs (Na2O+K2O)/(FeOT+MgO+TiO2) and (Al2O3+FeOT+MgO+TiO2) vs Al2O3/(FeOT+MgO+TiO2) (Fig. 10a, b) (Patino Douce 1999), data for the samples plot in the metamorphic greywacke and basic argillaceous rock fields.

Fig. 10
figure 10

Na2O+K2O+FeOT+MgO+TiO2−(Na2O+K2O)/FeOT+MgO+TiO2(a) and Al2O3+FeOT+MgO+TiO2−Al2O3/FeOT+MgO+TiO2(b) diagrams of high Nb–Ta rhyolite (After Patino Dounce, 1999)

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Sr–Nd isotopic compositions

The (87Sr/86Sr)i ratios of the rhyolites (0.70300–0.71160) vary greatly, and εSr(t) values (− 18.21 to + 102.94) are mostly positive. The (143Nd/144Nd)i ratios are in the range of 0.512537–0.512539, and εNd(t) values are positive (1.9–2.0). These factors indicate that the source was likely to have been a product of partial melting of ancient crust composed of pre-existing mantle-derived materials.

Physico-chemical conditions of rhyolite formation

The negative Eu anomalies and low Ca, Fe, Sr, and Mg contents of the rhyolites are consistent with melting of residual phases of hydrous tholeiite at temperatures of 650–800 °C and pressures of < 0.8 GPa. The residual phases are mainly Ca-rich plagioclase + amphibole ± orthopyroxene ± ilmenite (with no garnet) (Wu et al. 2002). Granite formed in this process is of the low-Sr/high-Yb type (Zhang et al. 2006). The rhyolite is thus a low-Sr/high-Yb-type granite, probably formed at very low pressures (< 0.5 GPa) at depths of < 30 km (Zhang et al. 2006). It is generally considered that A-type granite is low-ƒO2 granite (Loiselle and Wones 1979; Martin 2006), although it may be generated under oxidizing conditions (Anderson and Morrison 2005; Dall’Agnol and Oliveira 2007). The positive Ce anomalies of the rhyolites indicate that they were formed in high-ƒO2 conditions.

Magma source

The Kyihe high Nb–Ta rhyolite T2DM age of 789–785 Ma suggests that the source rock was Neoproterozoic crustal basement, corresponding to Neoproterozoic Rodinia tectono-magmatic evolution (Zhou et al. 2016; Zeng et al. 2014) and the formation of khondalite in the Xingan Block (Zhou et al. 2011a; Zeng et al. 2014).

The metamorphic basement of the Xingan Block is represented by the Xinghua Ferry Group, which is widely exposed in the area between Xinghua–Central Xinlin–Jiagedaqi in the Huma area (northern Xingan Block) and the Zhalantun area (southern Xingan Block). Its main rock types include sillimanite–garnet-bearing gneiss, felsic gneiss, amphibolite, and graphite schist (HBGMRRGH 1993; Miao et al. 2007; Zhou et al. 2011a; Wu et al. 2012), and the protoliths of which can be divided into metamorphic plutonic rocks and metamorphic supracrustal rocks. The metamorphic supracrustal rock is a typical volcanic–sedimentary formation, including metamorphosed mafic volcanic and sedimentary rocks, such as metamorphosed clastic rocks (Biao et al. 1999; Sun et al. 2002; Liu 2007) with ages of 970–660 Ma (Luo et al. 1997; Wu et al. 2012; Miao et al. 2003; Zhou et al. 2011a, b). The rock complex comprises Al-rich schist–gneisses and amphibolites. The source rocks of the Al-rich schist–gneisses are mudstone and sandstone with negative Eu anomalies (Eu/Eu* = 0.32–0.79) and positive Ce anomalies (Ce/Ce* = 0.90–1.30), indicating an unstable continental margin sedimentary environment. The source rock of the amphibolites was weakly alkaline basalt (Zhou et al. 2016; Dong and Zhou 2012). The formation age and geochemical characteristics of metamorphic crustal rocks of the Xinghua Ferry Group, which form the basement of the Xingan Block, correspond to the characteristics of the source rocks of the rhyolites.

Petrogenesis

In the Upper Jurassic (155 Ma), the northern segment of the Daxingan Mountains was in an intraplate extensional tectonic setting in which the thinned crust was underplated by mantle-derived basaltic magma. This resulted in the partial melting of crustal rocks of the basement Xinghua Ferry Group as the pressure decreased and temperature increased. The volcanic–sedimentary formation of Xinghua Ferry Group supracrustal rocks included metamorphosed mafic volcanic and terrigenous clastic rocks. It is speculated that the latter was rich in zircon, and hence Nb and Ta. Zircon and garnet were melting phases under shallow, low-pressure (< 30 km and < 0.8 GPa), and high ƒO2 conditions during the melting of supracrustal rocks in which the rhyolitic magma was formed, whereas a residual mineralogy of Ca-rich plagioclase, amphibole, and orthopyroxene remained in the source.

Conclusions

(1) Zircons in the high Nb–Ta rhyolite display magmatic oscillatory zoning and have the same REE compositions as crustal magmatic zircons. A U–Pb age of 156 ± 1 Ma indicates that the rhyolites formed during the Upper Jurassic.

(2) The rhyolites are characterized by high SiO2 and alkali contents; low Fe, Ca, and Mg contents; and a weakly peraluminous nature, indicating that they are high-K calc-alkaline rocks. Their trace element contents indicate enrichment in Nb, Ta, Zr, Hf, Ce, and Rb and depletion in Sr, Eu, Ba, P, Ti, Co, and Ni. The rhyolites are thus low-Sr/high-Yb rhyolites.

(3) The T2DM Nd model ages of the rhyolites range from 789 to 785 Ma, indicating a crustal growth event during the Neoproterozoic, with crustal material originating from the depleted mantle or partial melting of juvenile crust. The εSr(t) values of the rocks (− 18.21 to + 102.94) are variable and mostly positive, and εNd(t) values are positive (1.9–2.0), indicating that the source comprised a mixture of crustal and mantle-derived materials.

(4) The rhyolite source rocks were metamorphosed supracrustal rocks comprising metamorphosed mafic volcanic and terrigenous clastic rocks of the Xinghua Ferry Group, which forms the basement of the Xingan Block.

(5) The high Nb–Ta rhyolite was formed in a tectonic setting which developed after the closure of the Mongolia–Okhotsk Ocean, or during the northward subduction of oceanic lithosphere under the Siberian Plate. Basaltic magma underplating provided the heat necessary for partial melting of metamorphic supracrustal rocks of the Xinghua Ferry Group. During partial melting of the source rocks, zircon and garnet were involved in melting, and plagioclase + amphibole + pyroxene were residual phases, due to the shallow depth, high temperature, and high oxygen fugacity melting conditions.