INTRODUCTION

The intraplate volcanism typically results in fissure eruptions of tholeiitic and alkaline basaltic lavas [1–3], that form plateau basalts, flows, nappes, and shield volcanoes. In some cases the eruptions of tholeiitic lavas alternate with eruptions of alkali basalts, e.g., in the Hawaiian Islands [4, 5]. Trachytes and more differentiated alkali–salic rocks associated with tholeiitic basalts are rare [4, 6–10]. Their origin remains under discussion and is explained by crystal fractionation of alkaline [8, 11] and, lesser, tholeiitic [6, 12] basaltic melts in shallow magma chambers or by processes of mantle–crust interaction [11, 13].

In this work, based on the study of inclusions of mineral-forming media, we revealed the processes leading to the formation of trachytes from the neck that is located on the northeastern slope of Wangtian’e volcano, mainly composed of tholeiitic basalts.

GEOLOGICAL STRUCTURE OF WANGTIAN’E VOLCANO

The Changbaishan area is a large volcanic center in the structure of the Late Cenozoic intraplate province of Central and East Asia, which is mainly composed of products of fissure eruptions of basaltic lavas with increased alkalinity [3]. In comparison with other volcanic complexes of the province, the Changbaishan area is composed of bimodal rock-series from basalts to rhyolites, and basalts belong to tholeiitic and alkaline petrochemical series [10]. The most striking differences concern the structure of two adjacent Changbaishan (China–North Korea) and Wangtian’e (China) volcanoes, which formed with an insignificant time gap for more than the last four million years (Fig. 1). In contrast to the products of the Changbai-shan volcano with strongly differentiated series of alkaline rocks from trachybasalts to comendites and pantellerites [7, 14], the rocks of Wangtian’e volcano include weakly differentiated tholeiitic basaltoids, as well as associated trachytes and rhyolites [7, 9, 10, 15].

Fig. 1.
figure 1

Geological scheme of Wangtian’e and Changbaishan volcanoes of the Changbaishan area [12]. 1, Host rocks; 2, flood basalt of the Changbaishan area, Quanyang Stage (4.50–4.00 Ma); 3, tholeiitic basalts of Wangtian’e shield volcano, Changbai stage (3.82–2.83 Ma); 4, tholeiitic basalts of the cone of Wangtian’e volcano, Wangtian’e stage (2.76–2.67 Ma); 5, dome and necks of Wangtian’e volcano, Hongtoushan stage (2.76–2.69 Ma); 6, alkali basaltoids of Changbaishan shield volcano, Toudao stage (2.77–1.99 Ma); 7, alkali basaltoids of Changbaishan shield volcano, Baishan stage (1.64–1.11 Ma); 8, trachyte, comendite, and pantellerite of the cone of Changbaishan volcano, Baitoushan stage (1.12–0.81 Ma); 9, ignimbrite, pumice, and ash of the caldera of Changbaishan volcano, Bingchang–Baiyufeng–Baiguamiao stage (7854–825 kyr); 10, faults; 11, state boundary; 12, sampling points and sample numbers.

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According to K–Ar dating [10], the evolution of Wangtian’e volcano includes three main episodes: (1) the Changbai stage characterized by fissure eruptions of tholeiitic lavas (3.82 ± 0.13–2.83 ± 0.09 Ma), (2) the Wangtian’e stage with the formation of a cone composed of flows of tholeiitic basaltoids (2.76 ± 0.09–2.67 ± 0.20 Ma), and (3) the Hongtoushan stage corresponding to the origination of trachytic–alkali rhyolitic extrusions on the volcanic slopes, which include necks and a dome (2.76 ± 0.07–2.69 ± 0.07 Ma) (Fig. 1).

PETROCHEMICAL AND GEOCHEMICAL CHARACTERISTIC OF ROCKS

We studied the sections of the northern, southern, and eastern slopes of the volcanic cone of the Wangtian’e stage the necks of the Hongtoushan stage, and the shield volcano of the Changbai stage. In composition, the rocks of Wangtian’e volcano correspond to basalts and trachytes. The basalts, which compose the shield platform and the volcanic cone, are tholeiites with a high content of Fe2O3 (9.6‒15.1 wt %), TiO2 (2.4‒3.6 wt %), and P2O5 (up to 0.7 wt %); a low MgO content (2.4‒4.1 wt %) at SiO2 content of 48.7‒51.2 wt % [10]. The total alkali content varies  from 4.3 to 5.5 wt %. As shown in [10], the tholeiitic basalts of the volcanic shield and cone are porphyric rocks with large plagioclase phenocrysts (An74.3–79.0Ab20.0–25.5Or0.2–1.3). The minerals of the matrix include ferroan olivine (Fo = 43.2–56.4), clinopyroxene (titanaugite, #Mg = 0.64–0.70), ilmenite, titanomagnetite, and fluorapatite [9, 10].

Rare felsic rocks in the structure of the volcano are related only to necks and the Hongtoushan extrusive dome. Our collection of felsic rocks includes trachytes with variable compositions: the Na2O + K2O content (with dominant Na2O over K2O) varies from 7.5 to 8.9 wt % at a SiO2 range of 61.5–68.5 wt %. The trachytes have a high Fe2O3 content, which decreases from 9.2 to 5.4 wt % with increasing SiO2 content, as well as a TiO2 content from 1.3 to 0.4 wt % and P2O5 content from 0.50 to 0.06 wt % (Table 1).

Table 1. Chemical (wt %) and trace element (ppm) composition of representative rocks of Wangtian’e volcano
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All studied trachytes of Wangtian’e volcano have similar petrographic characteristics. They are porphyric rocks containing up to 35% plagioclase phenocrysts, olivine (Fo = 20.1‒25.9) and clinopyroxene (hedenbergite) subphenocrysts. The plagioclase phenocrysts have an intermediate composition of (An44.2–49.8Ab45.8–51.0Or3.7–4.8). No zonation occurs in plagioclase grains. The olivine (Fo = 20.1‒25.9) and hedenbergite subphenocrysts often form mineral aggregates with ore minerals, apatite, and Fe-rich silicate glass, which contains up to 25–30 wt % FeO, 4–5 wt % MgO, 1.5–2 wt % CaO, 4 wt % Al2O3, and no more than 0.5 wt % (Na2O + K2O) at a SiO2 content of 42–45 wt % (Table 2). The matrix of trachytes is composed of plagioclase and feldspar microlites, quartz, apatite, ilmenite, titanomagnetite, and pyrrhotite, as well as glass. The glass of the matrix is trachytic with high contents of alkalies (11–12 wt % in total), FeO (up to 3.6 wt %), and TiO2 (up to 0.2 wt %) at an Al2O3 and SiO2 content of 15–16 and 66–68 wt %, respectively.

Table 2. Chemical composition (wt %) of glasses of homogenized melt inclusions and H2O-bearing Fe-rich globules in plagioclase of trachytes of Wangtian’e volcanic neck
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Geochemically, the tholeiitic basalts are weakly differentiated rocks with similar trace element patterns (Table 1, Fig. 2). They have a low REE content with slightly dominant light REEs over heavy REEs: (La/Yb)N = 7‒9. All tholeiitic basalts exhibit a weak positive Eu anomaly (Eu/Eu* = 1‒1.1). They are enriched in Ba (up to 570 ppm) and Pb and are depleted in U (Table 2). Generally, the trace element pattern is similar to that of oceanic island basalts (Fig. 2) except for the higher contents of Ba (up to 570 ppm) and Pb (up to 9 ppm), as well as the lower contents of Th, U, Nb, and Ta (Fig. 2, Table 1).

Fig. 2.
figure 2

Trace and rare earth element patterns of rocks of Wangtian’e volcano. 1, Tholeiitic basalts of the shield and the cone of Wangtian’e volcano; 2, trachytes of Wangtian’e volcano; 3, OIBs. Normalized after primitive mantle and chondrite [16].

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The trachytes of the necks have consistent trace element patterns and, in comparison with basalts, have higher contents of the most incompatible elements (Table 1, Fig. 2). They exhibit a weak negative Eu anomaly (Eu/Eu* = 0.7‒0.9) and a striking Sr minimum. All of them are enriched in REEs with dominant light REEs over heavy REEs ((La/Yb)N = 6‒8).

INCLUSIONS OF MINERAL-FORMING MEDIA IN PHENOCRYSTS OF TRACHYTES

Fluid and Melt Inclusions

Plagioclase of trachytes (sample B-13) contains CO2 fluid inclusions (Fig. 3a). According to the results of Raman spectroscopy [17], the solid phases of the inclusions correspond to carbonate (Fig. 4).

Fig. 3.
figure 3

Fluid inclusions and globules of water-bearing Fe-rich glass in plagioclase of trachytes of Wangtian’e volcanic neck: (a) transmitted light, parallel nicols; (b) Backscattered electron image. 1, H2O-bearing Fe-rich glass; 2, amorphous carbon; 3, CO2 inclusion.

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Fig. 4.
figure 4

Raman spectra of host plagioclase, phases in fluid inclusions, and H2O-bearing Fe-rich globules, as well as glasses of melt inclusions. 1, Host plagioclase; 2, glass of melt inclusion; 3, amorphous carbon in H2O-bearing Fe-rich glass globule; 4, CO2 fluid inclusion with carbonate relics.

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Melt inclusions were found in hedenbergite and plagioclase of trachytes. They are randomly located in host minerals, have round or ellipsoid shapes, and sizes from 10 to 50 µm. The most representative inclusions more than 20 µm in size were chosen for studies. The inclusions in clinopyroxene are glassy and contain a glass, a rim, and ore minerals (ilmenite, titanomagnetite, and pyrrhotite). The partly recrystallized inclusions in plagioclase are composed of a plagioclase rim and a fine-grained aggregate of clinopyroxene, plagioclase, titanomagnetite, and ilmenite.

The heating experiments with melt inclusions in hedenbergite and plagioclase showed their full homogenization at 1080‒1100°C in hedenbergite and 1050–1060°C in plagioclase (Figs. 5a, 5b).

Fig. 5.
figure 5

Melt inclusions in plagioclase of trachyte (transmitted light, parallel nicols) at temperature of (a) 20°C and (b, c) 1050°C. (1) Glass of melt inclusion; (2) gas bubble; (3) carbonate; (4) CO2.

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Note that the melt inclusions in plagioclase could not always be homogenized. Some of them exploded and, in some cases, the inclusions after quenching contained a glass, a crystal phase, and a gas bubble (Fig. 5c). According to the Raman spectroscopy studies of heated melt inclusions (Fig. 4), they often contain a solid phase with a CO2 bubble after heating experiments (Fig. 4). This solid phase contain CO\(_{3}^{{2 - }}\) ion with a characteristic peak of ~1077 cm–1 (Fig. 4).

The composition of glasses of homogenized melt inclusions in plagioclase correspond to that of the studied trachytes and are characterized by high contents of FeO (up to 6.0 wt %), TiO2 (up to 1.0 wt %), and alkalies (7.0–8.0 wt % in total) at a SiO2 content of 64–67 wt % (Table 2).

Globules, Glasses, and Crystalline Inclusions in Minerals of Trachytes

The hedenbergite and plagioclase phenocrysts of trachytes contain crystalline inclusions, which include apatite, ilmenite, and titanomagnetite. Also such minerals of trachytes hedenbergite, plagioclase, and apatite contain various globules: sulfide (pyrrhotite) and silicate (H2O-bearing Fe-rich glass) (Figs. 3a, 3b, Table 2). The H2O-bearing Fe-rich glass is often intergrown with titanomagnetite and amorphous carbon. In addition, amorphous carbon often forms films on the Fe-rich silicate glass.

The Fe-rich silicate globules, as well as Fe-rich silicate glass, which was found in mineral segregations of the rock matrix, contain (up to) 30 wt % FeO, 5 wt % MgO, 2 wt % CaO, 4 wt % Al2O3, and no more than 0.5 wt % of Na2O + K2O at a SiO2 content of 43–45 wt % (Table 2). Judging from the sum deficit, the Fe-rich glasses contain up to 10–15 wt % H2O.

DISCUSSION

The analysis of petrological–geochemical results of studies of rocks and inclusions of mineral-forming media in phenocrysts of trachytes from Wangtian’e volcano allowed us to identify the following patterns.

Geochemical Studies

The primitive mantle-normalized multielement patterns of the tholeiitic basalts from Wangtian’e volcano are similar to those of the OIB-type basalts (Fig. 2). Wangtian'e tholeiitic basalts differ from OIB in increased concentrations of Ba and Pb, as well as slightly lower contents a number of rare elements such as Th, U, Nb and Ta. These characteristics indicate the involvement of metasomatized lithospheric mantle in the formation of melts [10]. In the series from basalts to trachytes, the latter are enriched in almost all incompatible elements excluding Eu and Sr, that is related to the fractionation of plagioclase. The increase in the Ba and Rb contents upon the transition from basalts to trachytes indicates that no K-feldspar crystallized during the formation of trachytes; thus, Ba and Rb behave similarly to other incompatible elements accumulating in the melt.

To estimate the role of crystallization differentiation during the evolution of melts of Wangtian’e volcano, we used the contents of such incompatible element as Nb whose partition coefficient between crystalline phases and melt is close to zero. It is evident from the variation diagrams (Fig. 6) that the Zr, Ta, and Th (as well as almost all REEs) concentrations of rocks are directly correlated with the Nb content. This trace element behavior indicates the dominant role of crystallization differentiation during the formation of all rocks of Wangtian’e volcano from basalts to trachytes.

Fig. 6.
figure 6

Variation diagrams of concentrations of various trace elements (ppm) relative to the Nb concentration (ppm) in rocks of Wangtian’e volcano. (1) Tholeiitic basalt of Wangtian’e shield and cone, (2) trachyte.

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Study of Inclusions of Mineral-Forming Media

According to obtained Raman spectra CO2 inclusions in plagioclase, contain CO\(_{3}^{{2 - }}\) ions (Fig. 4). It indicates the presence of carbonate in the system. The presence of ferrocarbonates in tholeiitic basalts of Wangtian’e volcano, that were crystallized from a ferrocarbonate melt immiscible with the silicate one, was previously described by us [18]. The ferrocarbonate melt formed as a result of the reaction of ascending basaltic magmas with the host marbles [18]. The carbonate relics in trachytes are most likely inherited from the parental basaltic melt, that contained an immiscible ferrocarbonate liquid. It is likely that the ferrocarbonate melt coexisted with the basaltic magma was decomposed in the shallow magma chamber according to the following reaction with the formation of magnetite, carbon, and CO2 [19]:

$$6{\text{FeC}}{{{\text{O}}}_{{\text{3}}}}{\text{L}} = {\text{F}}{{{\text{e}}}_{{\text{3}}}}{{{\text{O}}}_{4}}\left( {{\text{Mt}}} \right) + {\text{C}} + 5{\text{C}}{{{\text{O}}}_{2}},$$
(1)

where FeCO3L is the ferrocarbonate melt and Mt is magnetite.

In addition, the process of decomposition of the ferrocarbonate melt is evidenced by the presence of magnetite and amorphous carbon films on Fe-rich silicate globules associated with CO2 fluid inclusions. (Figs. 3a, 4). Moreover, melt inclusions in plagioclase of trachytes in some cases contain carbonate relics, amorphous carbon, and CO2 after heating experiments (Fig. 5c), that also is an evidence of the partial decomposition of carbonate.

Fe-rich silicate glasses found in minerals of trachytes (Fig. 3, Table 2) have a similar composition to those that were previously found as globules in melt inclusions in plagioclase and in the matrix of basalts from Wangtian’e shield volcano [9]. The high H2O content (10–5 wt %) in Fe-rich silicate glasses found in minerals of trachytes is comparable with that of H2O-bearing Fe-rich glasses in tholeiitic basalts of Wangtian’e volcano [9]. As was shown in [9], the presence of interstitial Fe-rich and Si-rich glasses in a matrix, as well as in melt inclusions of plagioclase of basalts, indicates the silicate–silicate liquid immiscibility at the final stages of crystallization of these rocks. The separation into two immiscible silicate liquids occurs when the FeO concentrations of melts reach 15–18 wt % during the Fenner-type crystal fractionation of ferrobasaltic magmas for both closed-system magmatic differentiation (Skaergaard and Sept Iles intrusions) and open-system one (trap provinces of Siberia and India, volcanic complexes El Laco, Wangtian’e, etc.) [2, 9, 20]. In particular, this mechanism occurred upon the formation of shield basalts of Wangtian’e volcano. Judging from the above, the trachytic melts that form during crystallization differentiation of basaltic magmas, inherited products of both silicate–carbonate and silicate–silicate immiscibility.

The formation of trachytes with these characteristics is considered in the framework of the following model. The basaltic melt intruded into a shallow magma chamber together with small portions of the ferrocarbonate melt, formed upon the interaction between silicate melt and crustal carbonates. The Fenner-type crystal fractionation of the basaltic melt produces two immiscible silicate liquids: Si- and Fe-rich. Part of this magma erupted on the surface and formed the shield basalts of Wangtian’e volcano [9]. The residual melt in the magmatic chamber continued to differentiate changing the character of the trend as a result of crystallization of magnetite and Fe-bearing minerals (clinopyroxene and fayalite). The crystallization differentiation led to the formation of a trachytic melt in the shallow magma chamber. This melt inherited immiscible ferrocarbonate and hydrous Fe-rich silicate liquids from the parental basaltic magma. The ascent of the hybrid melt led to the decomposition of the ferrocarbonate liquid on magnetite, carbon, and CO2 under decreasing temperature and pressure.

CONCLUSIONS

(1) It has been identified from the melt inclusion study that hedenbergite and plagioclase of trachytes of Wangtian’e volcano crystallized from a trachyitic melt in the subsurface magmatic chamber at temperatures of 1080–1100 and 1050–1060°C, respectively. After heating experiments, the melt inclusions in plagioclase contained CO2, amorphous carbon, and CO\(_{3}^{{2 - }}\) ions. Water-bearing Fe-rich globules and CO2 inclusions with carbonate relics were found in phenocrysts of trachytes.

(2) The results of geochemical studies of Wangtian’e volcano show that the entire range of the studied volcanic rocks from tholeiitic basalts to trachytes formed with a dominant role of crystal fractionation process.

(3) It has been shown that the trachytic melt formied upon crystal fractionation inherited immiscible ferrocarbonate and water-bearing Fe-rich liquids from the parental basaltic magma. Ascending to the surface, this hybrid melt was decomposed on magnetite, carbon, and CO2 at decreasing temperature and pressure.