INTRODUCTION

Rare metal granitoids and their volcanic analogs (comendites, pantellerites, ongonites) are a source of a range of economically important metals (Zr, Nb, Ta, Li, Be, etc.), which is attractive for many researchers. By now significant factual material has been accumulated on the geology, mineralogy, geochemistry, and petrology of rare metal granitic magmas [1–5]. The problem of their genesis, however, still has many weakly studied questions including the composition of magmas of rare metal granitoids, the role of volatiles and F in the evolution of melts, and the mechanisms of concentration of trace elements. These questions can be answered on the basis of study of mineral-forming media. The study of melt inclusions provides an exclusive possibility to reconstruct the physicochemical formation conditions of these rocks and to identify the processes leading to the generation of ore-bearing magmas. The aim of these works is to study the composition and formation conditions of comenditic magmas of the Early Mesozoic Adaatsag volcanic association and to reveal mechanisms for the accumulation of rare and rare earth elements.

BRIEF GEOLOGICAL CHARACTERISTICS

The Adaatsag volcanic complex is located in the Kharkhorin Zone of the Early Mesozoic alkaline magmatism, which originated in the western frame of the Khantei Batholith [6–8]. The volcanic complex is conjugated with a graben, which is elongated in the NE direction over 30 km at a width of up to 10 km. The graben tends to the Adaatsag Suture, which records the collision of walls of the Mongol–Okhotsk Ocean in the southern frame of the batholith. The graben is located within the outcrops of the Late Carboniferous–Early Permian mafic and intermediate volcanic rocks. To the southeast, the rocks of the complex make contact with Jurassic rocks along the fault.

The Adaatsag volcanic complex includes a stratified series of volcanic rocks >1000 m thick, which dips to the southeast at an angle of 15°–20°. Its lower part is dominated by members of tuffs and lava breccias of quartz trachyrhyolites, locally, with bodies of trachytes, trachydacites, and comendites. The upper part of the section is composed of thick (up to 50 m) flows of comendites–trachyrhyolites, as well as packages of tuffs and ignimbrites (Fig. 1).

Fig. 1.
figure 1

Schematic structure of the Adaatsag volcanic association, compiled by the authors using data of the 1 : 500 000 geological map of Mongoli, L-48-B. The inset shows the position of the area in the structure of the Early Mesozoic Khantei–Daur zonal igneous area. 1, Meso-Cenozoic sediments; 2, trachyrhyolitic tuff; 3, trachyrhyolite, comendite; 4, Paleozoic rocks; 5, Early Mesozoic leucogranite; 6, faults; 7–12, legend to inset: 7, calc-alkaline granitoids; 8, volcanic and plutonic complexes of subalkaline series; 9, alkaline granite, bimodal volcanic associations; 10, zones with calc-alkaline rocks; 11, zone with rocks of subalkaline series; 12, boundary of the zonal igneous area.

Full size image

An important constituent of the volcanic complex includes dikes of comendites, trachyrhyolites, and alkaline porphyry granites. They are traced along the volcanic graben and form a belt up to 4 km wide. The volcanic rocks, as a rule, are finely crystallized with the formation of aphyric and porphyric varieties. The latter contain K-feldspar phenocrysts and round dark quartz grains up to 2–3 mm in size. The dike rocks vary from glassy to crystallized and contain the same phenocrysts as lavas.

ANALYTICAL METHODS

The contents of major oxides and P in comendites of the Adaatsag pluton were determined using X-ray fluorescence on an Axios mAX spectrometer (PANalytical, Netherlands) at the laboratory of the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences (IGEM RAS, Moscow, Russia, analyst A.I. Yakushev). All trace and rare earth elements were studied using the ICP-MS method at the Laboratory of Nuclear Physical and Mass Spectrometric Analytical Methods, Institute of Microelectronics Technology and High Purity Materials, Russian Academy of Sciences (Chernogolovka, Russia), following [9].

The inclusions in minerals were first studied in polished sections 0.3 mm thick. The thermometric analysis of melt inclusions was conducted using muffle furnaces and a microthermocamera with visual control (Linkam TS 1500) allowing the high-temperature experiments. The measurement error of the temperature in the muffle furnaces was estimated at ±10°C. For equilibrium between the melt and the host mineral, the exposition time of samples in the muffle furnaces was 30–120 min at a given temperature.

The fluid inclusions were studied in a Linkam THMSG 600 cryocamera, which was cooled by liquid nitrogen to –180°C and calibrated by artificial inclusions with carbon dioxide and aqueous solutions of a certain salinity. The composition of the fluid phase in melt inclusions was determined on a Renishaw Raman spectrometer at the Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences (analyst A.A. Averin).

The chemical compositions of the crystalline inclusions and mineral phases in melt inclusions, as well as the residual and homogeneous glasses of melt inclusions, were studied on a JXA-8200 electron microprobe at the IGEM RAS (major oxides, F, Cl, and S) at an accelerating voltage of 20 kV and a beam current of 10 nA for glasses and 20 nA for mineral inclusions. The content of trace elements and H2O in glasses of melt inclusions were analyzed on a Cameca IMS-3F second-ion mass spectrometer at the Yaroslavl branch of the Institute of Physics and Technology, Russian Academy of Sciences (Yaroslavl, Russia, analyst S.G. Simakin), following [10].

COMPOSITIONAL FEATURES OF COMENDITES

The studied comendites are porphyric rocks with quartz and K-feldspar phenocrysts. The microfelsitic matrix is mostly composed of K–Na feldspar, quartz, alkali amphibole, ilmenite, and titanomagnetite. Zircon is an accessory mineral.

The K–Na feldspar forms large euhedral crystals up to 1 cm in size. It contains (up to) 18 wt % Al2O3, 6.6 wt % Na2O, and 7.5 wt % K2O at a SiO2 content of 66–68 wt %. The alkali amphibole in the matrix corresponds to fluoro-arfvedsonite (F = 2–3 wt %) containing 3 wt % TiO2 and 1.4 wt % CaO. The total alkali content (Na2O + K2O) is 9 wt %. Ilmenite contains 49 wt % TiO2 and 46 wt % FeO. It is noteworthy that the matrix also hosts a mineral with high a Ce, La, and F content, the composition of which was analyzed only semiquantitatively because of the small size of the mineral. A similar mineral was found as a crystalline inclusion in quartz of comendites. Its composition corresponds to that of a REE carbonate (bastnäsite). It contains 33.1 wt % Ce2O3, 25.4 wt % La2O5, 2.1 wt % CaO, and 9.2 wt % F.

In petrochemistry, the comendites belong to felsic alkaline rocks of the K–Na series with an agpaitic coefficient (Ka) of 1.04. The geochemical peculiarities of these rocks include the higher contents of high-field strength elements: Zr (up to 1150 ppm), Nb (up to 44 ppm), Y (up to 98 ppm), and REEs (500–580 ppm in total) and low contents of Ba (28–32 ppm) and Sr (up to 17 ppm). The accumulation of trace and rare earth elements in the studied rocks is accompanied by a striking Eu anomaly that is explained by the significant fractionation of feldspar and thus probably indicates a strong degree of differentiation magmas for these rocks (Fig. 2).

Fig. 2.
figure 2

Trace and rare earth patterns of comendites (1) and homogenized glasses of melt inclusions in quartz of comendites of the Adaatsag volcanic association (2, sample SG-6/4; 3, sample SG-6/6), as well as the glasses of homogenized melt inclusions in quartz of comendites of the Sant complex (4) and pantellerites of the Dzarta-Khuduk complex (5). The average composition of the continental crust (6) is after [19] and the compositions of thte primitive mantle and chrondrite are after [20].

Full size image

STUDY OF INCLUSIONS IN MINERALS

Melt Inclusions

Melt inclusions are found in quartz of two comendite samples SG-6/6 and SG-6/4, which were taken in different parts of the section of the volcanic sequence. The melt inclusions 30–90 µm in size exhibit a negative crystal morphology (Fig. 3). The melt inclusions in phenocrysts from both samples are similar and consist of glass, a vapor bubble, and daughter minerals (fluorite, mica, K-feldspar). Fluorite is observed as skeletal crystals and contains a minor amount of Ce2O3. Mica corresponds to polylithionite with 60 wt % SiO2, 11.3 wt % Al2O3, 3.4 wt % FeO, 11.6 wt % K2O, and 10 wt % F. The sum deficit is 7 wt %, on average. The ion microprobe studies revealed the high Li content.

Fig. 3.
figure 3

Melt inclusions in quartz of comendites: a, c, d, sample SG-6/4; b, sample SG-6/6. transmitted light, without analyzer. Gl, glass; g, gas bubble; Fl, fluorite; Pln, polylithionite.

Full size image

The residual (unheated) glasses of melt inclusions in all studied samples exhibit extremely high Na2O + K2O concentrations (11–12 wt %) at the contents of SiO2 of 67–70 wt %, Al2O3 of up to 13.5 wt %, and FeO of 3.3–3.6 wt %. The F concentrations is 0.4–0.7 wt %. The H2O content of glasses of melt inclusions reaches 2.3 wt %. According to the SiO2–(Na2O + K2O) classification diagram, the studied residual glasses of melt inclusions correspond to rhyolites in composition [11].

The thermometric experiments with melt inclusions in quartz of comendites (samples SG-6/6 and SG-6/4) showed that they homogenized in the temperature range of 880–930°C. In some inclusions, no full homogenization was achieved and the gas bubble was present up to 1200°C.

The composition of the gas phase was studied using Raman spectroscopy. The Raman spectra exhibit two evident broad peaks (Fig. 4): in areas of 3580 and 1630 cm–1 related to O–H Raman scattering in H2O molecules [12]. The spectrum of the fluid phase contains intense lines with wave numbers of 354, 586, 4126, 4143, 4156, and 4161 cm–1 typical of H2 [12]. The Raman characteristics thus indicate that H2O and H2 are the dominant gas components.

Fig. 4.
figure 4

Raman spectra of the gas phase of melt inclusions. 1, H2O peaks; 2, H2 peaks; 3, spectrum of glass in inclusion; 4, spectrum of quartz containing melt inclusion.

Full size image

Fluid Inclusions

The primary fluid inclusions were identified in quartz phenocrysts. The finding of fluid and melt inclusions in different growth zones of quartz grains is unlikely to be a result of their synchronous entrapment. The fluid inclusions are confined to the marginal parts of the phenocrysts, their size varies from 20 to 30 µm, and they consist of an aqueous fluid and a gas bubble.

The cryometric studies showed that the liquid phase freezes in the temperature range from –36 to –37°C. The eutectic temperatures vary from –21.5 to –22.1°C. The ice melting temperatures are –2.7 to –2.6°C corresponding to the fluid salinity of 4.5–4.3 wt % NaCl-equiv. The homogenization temperatures of fluid inclusions are 101–103°C. The calculated fluid density is 0.96 g/cm3.

Note that the eutectic temperatures are similar to those of the H2O–NaCl and H2O–KF binary systems. Taking into account the high content of F and alkalis in glasses of homogenized melt inclusions, as well as the presence of fluorite as a daughter phase of these inclusions, it can be suggested that the fluid phase is KF. The KF content of the fluid is 4.0–4.1 wt %.

Rock-Forming Components and Trace Elements in Melt Inclusions

The glasses of homogenized melt inclusions in quartz of comendites are characterized by a rhyolitic composition (SiO2 = 71–75 wt %) and a high A/CNK value (1.1–1.2) and alkalinity (Na2O + K2O = 9.5–11.8 wt %) at almost similar Na2O and K2O contents. The agpaitic coefficient is 1.11–1.51. All analyses of glasses of melt inclusions exhibit a deficit of the major oxide sum varying from 1.5 to 4.5 wt % (Table 1). Ion microprobe studies show the presence of a significant H2O content of 1.6 to 3.7 wt %. The high F content (0.9–1.2 wt %) is an important feature of the composition of glasses.

Table 1. Content of major oxides (wt %), volatiles (wt %), and trace (ppm) elements in rocks and melt inclusions in quartz of comendites of the Adaatsag bimodal association
Full size table

Study of the trace element composition of glasses of homogenized melt inclusions in quartz of all studied samples and rocks showed that they have a similar trace element pattern (Fig. 2) and are enriched in most trace and rare earth elements in comparison with the average composition of the continental crust. For example, the Zr, Rb, Nb, Y, and Th contents in glasses of melt inclusions are 900–1568, 114–240, 40–70, 72–112, and 21–40 ppm, respectively. The strong Ba, Sr, and Eu minima on spider-diagrams indicate significant fractionation of feldspars upon crystallization of rocks. The largest difference in the composition of glasses of melt inclusions is related to the Li content: maximum 62 ppm in comendites in contrast to 266–633 and 503–1566 ppm in glasses of melt inclusions for samples SG-6/6 and SG-6/4, respectively.

The normalized REE pattern of both the glasses of melt inclusions and rocks is identical (Fig. 2). There is a general enrichment of melts and rocks in REEs with a weak predominance of light REEs over heavy REEs. The total REE content of glasses of melt inclusions is 207–546 ppm at a (La/Yb)N value of 2–4. Note the higher concentrations of light REE in rocks compared to melt inclusions.. This is probably related to the crystallization of REE minerals during evolution of comendite magma, which is supported by the finding of crystalline bastnäsite inclusions in quartz of comendites, as well as the presence of this mineral in the rock matrix.

DISCUSSION

The results of melt and fluid inclusion studies characterize the composition and formation conditions of comenditic magmas of the Adaatsag bimodal association. The phenocrysts of these rocks crystallized at a temperature of 880–930°C from water-saturated (up to 3.7 wt % H2O) rare metal melts with high Li, Zr, and F and higher Rb, Nb, Y, and Th contents.

The comparison of our data with the H2O content of the melt and the results of thermodynamic modeling of the correlation between the H2O solubility and the temperature and pressure in felsic silicate melts [13] allowed the estimation of a minimum pressure of crystallization of comenditic magmas. The H2O content of homogeneous glasses of melt inclusions in quartz of comendites at a temperature of 880°C reaches 3.7 wt % corresponding to 1000 bar. Judging from these data, it can be concluded that quartz crystallized from the comenditic melt at a depth of ~3.5 km. Note that the use of fluid inclusions for the estimation of the crystallization pressure is poorly substantiated due to the absence of convincing evidence of syngenetic melt and fluid inclusions.

Water and hydrogen were identified in the gas phase of some inclusions in quartz of comendites from sample SG-6/4 using Raman spectroscopy. The finding of H2 in the gas phase is a rare event registered only in melt inclusions of Cr-bearing diopside of the Inagli deposit in Yakutia [14] and chkalovite from alkaline rocks of the Gardar province in Greenland [15].

The presence of gaseous H2 in inclusions could be explained by the decomposition of H2O during separating of volatiles from the magmatic melt. It is shown in a review of the composition of volcanic gases [16] that the highest temperature fumarolic gases contain a few percentage points of H2 controlled by a QFM buffer. The presence of H2 in the inclusions probably indicates the entrapment of a vapor phase separated from the degassing melt. The calculated H2 content for a temperature of 900°C and a pressure of 1000 bar is 1 mol % [16].

Under normal conditions, the gas bubble of the inclusion can retain an H2 pressure of 2.5 kbar calculated by the equation of the ideal gas state. It is likely that this significant H2 content can be preserved in melt inclusions and can be revealed by Raman spectroscopy. Thus, the comeditic magmas crystallized from the fluid-saturated melts at temperatures of 880–930°C and a pressure of 1000 bar at a depth of ~3.5 km accompanied by degassing.

The analysis of the trace element composition of glasses of melt inclusions indicates the following principles of the behavior of trace elements of melts. The plots (Fig. 5) show the variations of some trace and rare earth elements in glasses of homogenized melt inclusions and rocks relative to Zr, which was chosen as the differentiation index. Evident correlations among the Zr, Nb, Y, Dy, and Yb contents in the entire range of compositions of melts and rocks indicate their accumulation upon magmatic evolution. Similar trends are typical of other elements (Rb, Ta, Hf, Pb, U, Sm, Gd, Er). The absence of correlation between Zr and light REEs, as was noted above, is related to the fractionation of REE minerals at the early stages of crystallization of the magmatic melt. The presence of common trends between melts and rocks thus indicate the major role of crystallization degassing melt of many trace elements in felsic alkaline melts of the Adaatsag volcanic complex.

Fig. 5.
figure 5

Variations of the content (ppm) of trace elements relative to Zr in rocks (1–3) and glasses of melt inclusions in quartz of comendites (4, 5) of the Adaatsag volcanic association. 1, Comendite, sample SG-6/4; 2, comendite, sample SG-6/6; 3, rhyolite; 4, 5, glasses of homogenized melt inclusions in quartz of comendite; 4, sample SG-6/4; 5, sample SG-6/6.

Full size image

Our previous important results concern the composition and evolution of magmas, which are involved in the formation of rocks of the Dzarta-Khuduk and Sant bimodal magmatic associations within the Kharkhorin Rift Zone [17, 18]. It should be emphasized that these complexes and the Adaatsag complex have similar composition of their rocks. It was shown that strongly differentiated rare metal silicate alkaline melts with high Li, Zr, Rb, Y, Hf, Th, U, and REEs are responsible for the formation of a series of alkaline salic rocks. The thermometric studies of melt inclusions in phenocrysts in pantellerites of the Dzarta-Khuduk bimodal association allowed us to determine the phenomena of the immiscibility of silicate and Li-bearing fluoride melts at a temperature of 800°C, which was an argument in favor of the possible separation of rare metal salt melts from alkaline magmas at the final stages of differentiation.

The comparison of the mineral composition of unheated melt inclusions of the Agaatsag, Dzarta-Khuduk, and Sant bimodal associations showed their definite similarity. They are characterized by the presence of daughter fluorite and polylithionite, which reflects the Li and F specificity of melts. The glasses of homogenized melt inclusions in the phenocrysts of rocks of all volcanic complexes are geochemically similar including the levels of contents and the character of the distribution of trace and rare earth elements (Fig. 2). An important feature is related to the presence of the maximum Li content in the trace element pattern excluding some melt inclusions from the phenocrysts of comendites of the Sant association. Taking into account that the glasses of the latter contain extremely low amounts of Li, F, and H2O, they are considered residual silicate magmas after the separation of the salt (fluoride) melt.

CONCLUSIONS

The study of the nature of comendite melts of the Adaatsag bimodal association revealed similarities with the evolution of coeval (Early Mesozoic) compositionally similar rocks of bimodal associations of the Kharkhorin Rift Zone. This gives reason to propose similar formechanism of formation for them, involving the accumulation of many rare and rare-earth elements, as well as volatile components (F, H2O) in the process of crystallization differentiation. Subsequently, a salt melt rich in Li, F and H2O could be separated from such comendite magmas. The detection of fluoride aqueous inclusions in quartz allows us to suppose the further evolution of the salt melt leading to the appearance of a concentrated aqueous fluid and the possible participation of the latter in metasomatic processes.