Rare-Metal Li–F Granites in the Late Paleozoic, Early Mesozoic, and Late Mesozoic Magmatic Areas of Central Asia

Lithium–fluorine granites were first identified in the geochemical classification of Mongolian granitic rocks as an ore-bearing type associated with Sn, W, Li, and Ta–Nb mineralization and occurring in different regions of Central Asia (Kovalenko et al., 1971b; Tauson, 1977). Previously, this association of rare-metal rocks was defined as ‘apogranites,’ and its postmagmatic metasomatic genesis was widely debated (Beus et al., 1962). Interest in studies of these granitic rocks was revived by the discovery of ongonites, their subvolcanic analogs (Ongon-Khairkhan, Eastern Mongolia), providing strong support for the magmatic genesis of these silicic rocks (Kovalenko et al., 1971a; Kovalenko and Kovalenko, 1976), and rare-metal pegmatites showing a similar mineral composition and geochemical affinity. In connection with this, wide variations in their isotope ages and mineral and geochemical characteristics, as well as petrogenesis of granites enriched in fluorine and rare metals, are of particular interest. In the Central Asian Orogenic Belt (CAOB), Phanerozoic granitic formations occur mainly as mesoabyssal batholiths framed by volcanic–plutonic rocks displaying variations in alkalinity and SiO₂ content, intrusions occurring at medium and shallow depths, and intrusive-dike belts with rare-metal granites forming zonal magmatic areas (Yarmolyuk and Kovalenko, 2003; Yarmolyuk and Kuzmin, 2012).

In Central Asia, Li–F granites occurring in the late Paleozoic (Baikal region), early Mesozoic (Mongolia), and late Mesozoic (Mongolia and Transbaikalia) magmatic areas show a wide age interval (Kovalenko, 1977; Gerel et al., 1999; Kovalenko et al., 1999; Antipin and Perepelov, 2011; Yarmolyuk and Kuzmin, 2012; Antipin et al., 2016; Syritso et al., 2021). All the massifs and small intrusions of rare-metal granites lie within the southern folded framing of the Siberian craton. The composition of all the granitic rocks is not dependent on the composition of the host rocks, which in different regions belong to different types: volcanic–plutonic, metamorphic, and sedimentary. The available data indicate that the Li–F granites become younger from the Baikal region (late Paleozoic) to the Eastern Transbaikalia (late Mesozoic) granitic-rock occurrences. Considering the above spatial and temporal regularities in the distribution of rare-metal granitic rocks over the vast area within Central Asia, we perform a comparative geological, petrological, mineralogical, and geochemical analysis of the origin and evolution of Li–F granites and the associated mineralization. To solve this problem, the article presents new geochemical data on rocks and an analysis of different minerals in granites, to which not enough attention was paid in previous publications. The role of the mantle–crust interaction in the genesis of Li–F granitic rocks is discussed.

Geological mapping of the key zones where different types of granitic rocks occur was conducted using a representative sample of the studied areas. In particular, we studied the rare-metal rocks of the Baikal area, Mongolia, and Transbaikalia, for which there were no previous accurate mineralogical and geochemical data. The results obtained for Li–F granites were compared with the data on the granitic rocks of other geochemical types, generated in different geotectonic settings.

The silicate analyses were performed using a wet-chemistry method by analysts G.A. Pogudina and T.V. Ozhogina and XRF by analyst A.L. Finkel’shtein with inaccuracy of 0.5–5.0%. Alkaline elements were analyzed by flame photometry with the precision of 5–10% by analysts L.V. Altukhova and I.M. Khmelevskaya; trace and rare- earth elements were defined using the ICP MS method by L.A. Chuvashova (σ ± 5–10%) and O.V. Zarubina with inaccuracy of 10–20%. All the analyses were made at the Analytical Center for Collective Use, Irkutsk Science Center, Siberian Branch of the Russian Academy of Sciences (Irkutsk), with the use of equipment installed at the Center for Collective Use, A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences (Irkutsk), and certified reference samples (Geostandards…, 1994). The minerals were analyzed using GEOL Superprobe-733 and J 8200 LF XA (Japan) electron microprobes.

The location of rare-metal Li–F granites in the tectonic structures of Central Asia is shown in Fig. 1. Within the late Paleozoic (PZ₂) and Mesozoic (MZ₁and MZ₂) magmatic areas, rare-metal granites form large differentiated complexes occurring amongst the Caledonian structures (Janchivlan and Baga-Gazriin Chuluu plutons), while small intrusions are usually part of intrusive-dike belts (Abdar–Khoshutula, Urugudei–Utulik, etc.) in the peripheral zones of these areas. We begin the characterization of rare-metal granitic magmatism with reference complexes within the MZ₁ area, since the Li–F granites and related mineralization in Mongolia (Kovalenko et al., 1971b) have been investigated for a long time; therefore, currently they are well studied.

The Janchivlan pluton. The geochemical, geological, and isotope data accumulated by now suggest that Li–F granites comprise rocks with varying geochemical characteristics, resulting from their genetic features. Large multiphase complexes, often containing schlieren pegmatites (stockscheiders) in the apical parts, are represented by typical Li–F granites with Li, Ta, Nb, Sn, and W mineralization. The most representative one in the southern peripheral zone of the MZ₁ area is the multiphase Janchivlan pluton, whose late phases include biotite granites and leucogranites (Qtz + Pl_15–5 + Kfs + Bt) as well as microcline–albite (frequently containing amazonite) granites with a total area of isolated domelike outcrops of about 50 km² (Bural-Khangai, Urtu-Gotszogor, etc.). In the apical part of the Bural-Khangai dome, the amazonite–albite granites are replaced by albite–lepidolite rare-metal granites. Biotite-bearing leucogranites contain topaz, fluorite, monazite, ilmenite, and magnetite as accessory minerals. In the microcline–albite and amazonite– albite granites, the accessory minerals are represented by zinnwaldite, Li-phengite, topaz, fluorite, monazite, zircon, columbite, xenotime, cassiterite, and magnetite. The same minerals are found in albite–lepidolite varieties, which can contain up to 10% topaz and up to 20% lepidolite (Gerel et al., 1999). In the Janchivlan pluton, the Li–F granites of the final phase show enrichment in a number of trace elements (Table 1, Fig. 2). In comparison with early-phase leucogranites, albite–lepidolite varieties demonstrate higher fluorine (by two times), Li (by 4.5 times), Rb (by three times), Be (by two times), Sn (by 8.5 times), Pb (by three times), Ta (by 12 times), and Nb (by three times) contents. At the same time, the K/Rb and Nb/Ta values are significantly lower.

The most important indicators of the differentiation process are decreases in Sr, Ba, and REE concentrations in the rocks of the final phase (Kovalenko, 1977). In the Janchivlan pluton with a total area of about 165 km², a whole-rock Rb–Sr isochron age of 195.3 ± 0.6 Ma at (⁸⁷Sr/⁸⁶Sr)₀ = 0.7063 ± 22 was obtained for all the varieties of Li–F granites, including well-differentiated granites containing topaz and lepidolite (Kovalenko et al., 1999).

The Baga-Gazriin pluton. Along with the Janchivlan, the Baga-Gazriin pluton is particularly representative from the standpoint of the analysis of magma sources and estimation of the ore content. Its rock outcrops occupy an area of about 120 km² amongst the Permian sand–schist strata with interlayers of volcanic rocks. The center of the Baga-Gazriin pluton is mainly composed of coarse- and medium-grained biotite granites of the early phase, which occasionally have a distinct porphyritic texture at the endocontact. Meanwhile, the fine-grained leucogranites of the second phase, containing biotite and, occasionally, topaz, are more abundant in this endocontact zone. The schlieren pegmatites, occasionally containing amazonite, are found amongst the rocks of the late phase. Granitic rocks of both phases, including pegmatites, are crosscut by dikes of biotite greisens with topaz (zwitters), which are especially abundant in the endocontact zone, where they form clusters of dikes of different thicknesses. These are generally vertically oriented dike bodies, which are up to 50 m thick and over 1 km long. In addition to zoned topaz-bearing greisens, the Baga-Gazriin complex contains another type of metasomatic rocks—microclinites, having significantly microcline or microcline–albite–fluorite composition, which commonly occur close to zwitter dikes (Kovalenko, 1977). The microclinite bodies are particularly abundant in the Baga-Gazriin pluton center amongst biotite granites of the early phase. The rocks of the complex comprise rare bodies of quartz–muscovite greisens, which occur mainly in the endocontact zone and are usually superposed on the zwitter bodies.

The Late Triassic–Early Jurassic (average 197 Ma) whole-rock–mica age at initial (⁸⁷Sr/⁸⁶Sr)₀ = 0.7112 ± 11 was previously reported for the Baga-Gazriin complex (Kovalenko et al., 1999). The Late Triassic Ar–Ar age was confirmed for all the major granite varieties in the Baga-Gazriin pluton: 201.0 ± 3.6 Ma for the coarse-, 211.9 ± 4.0 Ma for the medium-, and 209.4 ± 3.2 Ma for the fine-grained leucogranites (Machowiak et al., 2012). The geochronological data obtained by a number of isotope methods constrain the ages of the Baga-Gazriin granites of different phases to the early Mesozoic magmatic cycle in the western part of the Mongol–Okhotsk Belt.

The available chemical data suggest that the Baga-Gazriin granites of the main and late phases are almost identical in their overall chemical composition (Table 1). Enrichment in F, exceeding its percentage abundance in the silicic rocks by 4–5 times, and considerably higher concentrations of this element in zwitters (by 10–20 times) and microclinites (16– 44 times) are a recognized feature of the Baga-Gazriin granites. In granitic rocks of all the phases, F is concentrated in biotite, while in leucogranites, topaz is found along with Li–F micas. In zwitters, F-bearing minerals are zinnwaldite, topaz, and, less commonly, fluorite, which, along with feldspar and quartz, is almost a rock-forming mineral (up to 6%) in the microclinites. At the periphery of the MZ₁ magmatic area, where significant rifting occurred, the early-phase Baga-Gazriin granites show enrichment in a number of trace elements (Li, Rb, Cs, Be, Nb, Ta, Th, and U) and HREE as compared with the average concentrations in the upper continental crust (Table 1, Fig. 2). The rare-metal rocks of the multiphase Janchivlan and Baga-Gazriin plutons exhibit strong Ba, Sr, and Eu negative anomalies, which is a characteristic feature of Li–F granites. The geochemical evolution of the Baga-Gazriin rocks at the postmagmatic stage is expressed as intensive enrichment of greisens and microclinites in lithophile and ore elements (Sn, W, and Zn) with concentrated ore mineralization like cassiterite and wolframite, genetically related to the evolution of the massif. It should be noted that high-field-strength elements (Zr, Hf, Nb, Ta, and Ti), as well as Th and U, do not demonstrate significant differences in granites of different phases and metasomatic rocks of the Baga-Gazriin pluton. Along with that, the differentiation of the investigated rocks tends toward the enrichment of the late-phase rocks in HREE. This process is the most evident in the microclinites, containing REE-concentrating minerals, such as monazite, columbite, and fluorite.

Earlier studies of the Janchivlan and Baga-Gazriin plutons (Kovalenko, 1977; Antipin et al., 2018) indicate that the multiphase plutons are generated in the upper horizons of the continental crust, where deep magma differentiation processes and enrichment of magmatic melts with volatile components take place. This might lead to the generation of metasomatic rocks with varying alkalinity (greisen-zwitters, microclinites, and albitites), producing rare-metal mineralization (Sn, W, Li, Ta, and Nb), at the postmagmatic stage.

The Abdar pluton lies on the southwestern continuation of the Abdar–Khoshutula intrusive-dike series, composed of granites from calc-alkaline to alkaline and rare-metal varieties, which are widely distributed in the periphery zone of the MZ₁ magmatic area (Antipin et al., 2019). The Abdar pluton is located within a brachyanticlinal structure amongst the metamorphosed sand–shale rocks of the Devonian Mandal Formation. The granites crop out over an area of about 10 km². Its central dome-shaped part, rimmed by the zone of intermittent medium-grained amazonite–albite granites, is composed of medium-grained biotite-bearing leucogranites. A zoned dike of aplitic and amazonite granites occurs at the eastern endocontact of the pluton. The transitions between the varieties are generally gradual, which may be indicative of their facies relationships. In the Abdar leucogranites, the albite–oligoclase (An_4–18) occurs as tabular segregations in paragenesis with microcline, whose average composition is Ort₇₇Ab₂₃. In comparison with the leucogranites, the amazonite–albite granites contain albite (An_2–7) and microcline with a smaller amount of the albite endmember (Ort_81–85 and Ab_19–15). Typical subordinate minerals in the leucogranites and amazonite–albite granites are magnetite, ilmenite, fluorite, zircon, monazite, and columbite–tantalite. The first age data on the Abdar granites obtained by the K–Ar method on five biotites yielded an average age of 207 Ma (Kovalenko et al., 1971b). This value virtually coincides with the Rb–Sr ages obtained later (209–212 Ma) (Kovalenko et al., 1999).

The mineral and trace-element compositions of minerals from granitic rocks of the studied Abdar–Khoshutula Series indicate their affiliation to different geochemical types and systematic evolution from palingenic calc-alkaline granites (Khoshutula complex) to the Li–F varieties of the Abdar intrusion (Antipin and Odgerel, 2016). As opposed to granitic rocks with ferroan biotite from the Khoshutula pluton, the Abdar rare-metal granites contain lithium micas, which in early leucogranites are represented by mica of protolithionite composition, while in amazonite–albite granites, in addition to protolithionite, it involves zinnwaldite. Lithium micas of the Abdar granites are strongly enriched in tin (440–750 ppm) and the granites of the pluton contain accessory cassiterite. The micas of the Abdar granites demonstrate extremely high concentrations of Nb (553–725 ppm) and Ta (91–107 ppm), which are typical of the Li–F granites from other ore-bearing plutons of Mongolia (Janchivlan, Baga-Gazriin, and others). The obtained mineralogical and geochemical data indicate that the Abdar Li–F granites can be attributed to typical rare-metal rocks, in particular, to Sn- and Ta-bearing granites (Tables 1 and 2).

Based on analysis of the distribution of trace and ore elements, the general trend in the geochemical evolution of the studied granitic rocks can be subdivided into two parts, which correspond to two phases of their formation. At the early phase in the formation of the Khoshutula granitic rocks, which are attributed to the palingenic calc-alkaline series (Tauson, 1977), there is a transition from the phase I granitic rocks to the late granites of additional intrusions. The final phase was responsible for the generation of the dike belt and the Abdar intrusion of rare-metal granites, which are enriched in many trace elements (F, Li, Rb, Sn, W, Ta, Nb, and others) and demonstrate the elevated ore potential (Table 2). The Mt. Tsokh-Uul ongonites are strongly enriched in HREE at a sharp decrease of LREE contents. The value of the Eu negative anomaly in them is similar to that in the ongonites of the Amazonitovaya dike from the Ongon-Khairkhan area of Central Mongolia. It was found that the total REE contents (121.9 ppm) and distribution patterns in the amazonite–albite granites of the Abdar intrusion are almost similar to those in the Mt. Tsokh-Uul ongonites (110.6 ppm), which provides evidence of their affiliation to a single geochemical type of Li–F granites and genetic similarity. As fluorite-bearing rocks, the studied granites and ongonites are enriched in F, Li, Rb, Be, Sn, Pb, Nb, Ta, and HREE and demonstrate lower K/Rb, Nb/Ta, and La/Yb values as compared with those in the palingenic calc-alkaline granitic rocks of the Khoshutula pluton within the studied rock series.

The Abdar Li–F granites within the Abdar–Khoshutula intrusive-dike series, completing the formation of the studied series, are typical crustal rocks with high initial (⁸⁷Sr/⁸⁶Sr)₀ ratios. It is reported that the Abdar intrusion is characterized by ε_Nd(T) (+1.2) and a protolith age of 906 Ma. It can be suggested that the deep-seated mantle magmatism in rifting structures at the peripheries of the MZ₁ area resulted in melting of the crustal protolith and generation of multiple granitic intrusions with simultaneous origin of Li–F granites, controlled by deep magmatic differentiation.

The Henteyn massif. Earlier studies (Antipin et al., 2019) established that within the MZ₁ magmatic area, Li–F granites occur both at the periphery of the magmatic area and in the central part of the giant Daurian–Henteyn batholith, whose southern part in Northern Mongolia is known as the Baga-Henteyn granitic complex (Fig. 1). The Baga-Henteyn pluton lies at the southeastern periphery of the Baga-Henteyn Range and has a three-phase structure (Koval et al., 1978). The first phase is composed of porphyric granodiorite containing a significant number of ovoid-shaped xenoliths of gabbro-diorites. The second phase is biotite and, less often, amphibole–biotite granite containing remnants of the overlying hornfelsed schists and sandstones. The third phase comprises small (up to 2–3 km in diameter) stock-shaped leucogranite bodies. The recent data suggest that the Baga-Henteyn pluton and its riftogenic fringe formed during a close timespan of 230–195 Ma (Yarmolyuk and Kuzmin, 2012).

In close proximity to the Baga-Henteyn pluton in the center of the MZ₁ area, there is the Henteyn massif of Li–F granites. Its northern and southern outcrops occupy an area of about 3.5 km². The coarse- and medium-grained biotite granites of the main phase of the Henteyn massif are cut by sheeted bodies of leucogranite and amazonite–albite granites, which vary transitionally from fine-grained with a flow texture in the endocontact zone to medium- and coarsegrained pegmatitic varieties with schlierenlike textures. The Henteyn granites of the main phase are composed of microcline, oligoclase–albite, quartz, and siderophyllite. In the amazonite granites, micas are of aluminous compositions of the zinnwaldite series, associated with albite and microcline (Ort_96.2Ab_3.8). The secondary minerals in the Henteyn Li–F granites are magnetite, fluorite, and zircon. The two zircons from these granites analyzed at the Geological Institute, Kola Science Center, Russian Academy of Sciences (Apatity), yielded almost identical concordia ages of 200 ± 2 Ma (Antipin et al., 2019). Therefore, the Li–F granites in the central part of the Baga-Henteyn Range originated at the Triassic–Jurassic boundary.

Comparative studies (Antipin et al., 2019) indicate that in the center of the early Mesozoic area, the evolution of calc-alkaline granites of the Baga-Henteyn multiphase pluton was accomplished with the formation of leucogranites. Moreover, there are no considerable variations in the concentrations of lithophile and other elements (Table 2). However, the chemical characteristics of the Henteyn leucogranites are radically different from those of their petrochemical counterparts completing the evolution of the Baga-Henteyn pluton. This is seen in a substantial enrichment in F and more pronounced (five- to tenfold) enrichment of the Henteyn leucogranites and amazonite–albite granites in Li, Rb, Cs, Sn, Ta, Be, Pb, Zn, Hf, and Nb.

A thorough analysis of the new geochemical data demonstrates definite differences in the concentrations of trace elements in the Henteyn granites not only in comparison with those in the Baga-Henteyn granitic rocks in the center of the MZ₁ area but also with those in the rare-metal Li–F granites of its periphery. As opposed to the latter, in the central part of the area (Henteyn intrusion), Li, Ta, Sn, and W are poorly concentrated. At the same time, they are enriched in HFSE (Zr, Hf, Nb, Zn, Th, and U), Pb, and Zn and demonstrate high Nb/Ta (12.1–21.4) and Zr/Hf (18.4–29.4) ratios (Table 2, Fig. 3). The obtained geochemical data indicate that, during the crustal anatexis, followed by magmatic differentiation and the generation of large multiphase plutons (Baga-Henteyn), no intrusions with characteristics of the rare-metal Li–F granites can be formed. The trace-element specialization and enrichment of the Henteyn Li–F granites in HFSE and ore elements evidence no genetic relations between rare-metal granites and palingenic granitic rocks of the calc-alkaline series of the Baga-Henteyn pluton. This implies a possible deep-seated source of these elements (Antipin et al., 2019).

Khamar-Daban, Baikal region. In the Selenga–Vitim structural zone (Khamar-Daban Range), the late Paleozoic intraplate magmatism generated calc-alkaline granitic rocks, subalkaline monzogranites, and Li–F granites and ongonites. Within the Urugudei–Utulik intrusive-dike belt, Li–F granites form multiphase intrusions (Kharagul, Urugudei, and Bitu-Dzhida) with a total area of outcrops of about 10 km² and an age from 311 to 321 Ma as well as associated pegmatite dikes (Antipin and Perepelov, 2011; Antipin et al., 2016). In the studied intrusions, the early-phase biotite granites, usually containing fluorite, are replaced by topazbearing amazonite–albite and, occasionally, pegmatitic granites during the later phases. The later intrusive phases contain Li-micas (zinnwaldite and, occasionally, lepidolite) as well as topaz, columbite–tantalite, cassiterite, monazite, and cyrtolite. The Urugudei–Utulik intrusive-dike complex is associated with ore mineralization represented by veinlet– disseminated zones of the stockwork type and mineralized breccias with fluorite and tourmaline. The late mineralization types are quartz–topaz–cryolite veins with disseminated cassiterite and wolframite.

The evolution of the multiphase intrusions from early biotite granites to late amazonite–albite granites is accompanied by increases in F, Li, Rb, Cs, Sn, Be, Ta, and Pb contents and decreases in the Ba, Sr, Zn, Zr, Th, and U levels (Table 3). The same regularities are common to the subvolcanic rocks being the counterparts of the granites, which supports the genetic assignment of the whole intrusive-dike complex of the Khamar-Daban province (Antipin et al., 2016). The biotite rare-metal granites of the early phases are strongly enriched in LREE in relation to the late-phase amazonite–albite granites, in which LREE contents decrease dramatically (Kharagul and Urugudei intrusions). At the same time, the HREE contents increase, which results in a decreasing La/Yb value from the biotite granites to the amazonite–albite varieties. However, a common geochemical feature of all the rare-metal granites from the Khamar-Daban province is the presence of a strong Eu negative anomaly, indicating a higher degree of differentiation of their primary magmas (Fig. 3b). These processes resulted in intensive accumulation of ore elements in the late phases of the intrusions (Table 3). Along with this, there is a significant difference in REE patterns between the calc-alkaline granitic rocks of the Angara–Vitim batholith and rare-metal Li–F granites of the Khamar-Daban Range, since they represent different zones of the late Paleozoic magmatic area in the Baikal region (Antipin and Perepelov, 2011).

With regard to geochemical characteristics, the Li–F granites of small intrusions of the Khamar-Daban Range are fairly close to the large intrusions of Central Mongolia (Janchivlan and Baga-Gazriin), which formed as multiphase plutons in the upper levels of the crust. The ε_Nd values ranging from –1.2 to –2.7 and Nd model ages older than 1200 Ma were previously obtained for the Urugudei–Kharagul group of late Paleozoic intrusions, indicating that these granite intrusions might have formed by melting of the Precambrian continental crust (Kovalenko et al., 1999). Later, the extensive magma differentiation leading to the enrichment of residual magmas in volatiles resulted in formation of metasomatic rocks of different alkalinity producing rare-metal mineralization (Sn, W, Li, Ta, and Nb). Within the Khamar-Daban Range, the melting of ancient continental crust might have resulted from the heat and fluids derived from the subalkaline (monzonitic) magmas which preceded manifestations of rare-metal granitic magmatism. The geochemical studies suggest that after the completion of palingenesis, the subsequent fluid–magmatic differentiation and, likely, the influx of deep-seated fluids resulted in the enrichment of these granites with lithophile and chalcophile elements, reaching economic concentrations of some ore elements.

The most representative complex of Li–F granites in the Eastern Baikal area is the Bezymyanka intrusion with an outcrop area of about 15 km². The granites cut the Krestovaya quartzite- and gneiss-bearing marble formation. The recent concordant U–Pb and Ar–Ar ages after zircons from leucogranites of the Bezymyanka intrusion were 291.7 ± 3.7 and 291.2 ± 3.6 Ma, respectively (Rampilov, 2013). Most of the intrusion is composed of medium-grained leucogranites and microcline–albite granites, which in its apical part have a fine-grained, often pegmatitic texture and are represented by amazonite–albite varieties. Occasionally, the rocks of the endocontact facies demonstrate a well-developed internal banding expressed by a linear orientation of dark mica. Mica forms an evolutionary trend from ferruginous siderophyllites in the early-phase leucogranites to protolithionites associated with Li-phengite–muscovites in amazonite–albite granites. In addition to fluorite (1–2%) and magnetite, the accessory minerals are allanite, zircon, columbite, monazite, xenotime, cyrtolite, and manganotantalite. The granites of the apical facies contain schlierenlike melanocratic bodies, whose major minerals are amazonite and Li-mica (25–30%). Fluorite and magnetite are less abundant.

As exemplified by the Bezymyanka intrusion, the schlierenlike bodies containing a fluorine-rich (up to 2%) fluid phase being immiscible with silicate melt originate with the progressive accumulation of fluorine in the apical part of the intrusion along with the evolution of residual melts toward substantially albite compositions (Antipin et al., 1997).

The uneven distribution of the immiscible phase occasionally leads to the formation of either schlieren pegmatites or melanocratic schlieren with mica crystallization as well as magnetite and fluorite. Therefore, two parts of the evolutionary trend are formed: sodium (involving magma differentiation) and potassium (separation of the immiscible fluid phase from residual magma). In terms of the composition, ultrarare-metal schlieren exhibit features of subalkaline rocks, and they have high HFSE and REE abundances. Relative to the amazonite-albite varieties, they are enriched in Zr (by 46 times), Hf (by 390 times), and Nb (by 35 times). The process of fluid–magmatic differentiation might have led to the intensive enrichment of the Bezymyanka Li–F granites with Li, Rb, Cs, Sn, Ta, Nb, Zr, and REE (Table 3), and these granites often bear concentrated rare-metal mineralization.

Transbaikalia is a well-recognized rare-metal province with occurrences of rocks belonging to the geochemical type of Li–F granites, which were generated in the late Mesozoic within the Mongol–Okhotsk Belt. The Jurassic Etyka and Orlovka complexes of amazonite–albite granites are the most representative and economically important. These small intrusions are associated with Ta and Li deposits, and their rare-metal granites are enriched dramatically in F, Rb, Cs, Sn, W, and Nb, which is characteristic of Li–F granites. In Transbaikalia, small intrusions of rare-metal Li–F granites cannot be assigned to a specific zonal late Mesozoic magmatic area with a large batholith in the center, which has not yet been distinguished. At a considerable distance from the rare-metal granites of Eastern Transbaikalia, late Mesozoic Li–F granites were discovered in Eastern Mongolia (Barun-Tsogt intrusion, 126.5 Ma; ongonites of the Ongon-Khayerkhan tungsten deposit, 128.3 ± 0.8 Ma), whose occurrences form rare outcrops. The late Mesozoic East Mongolian volcanic area is currently recognized within the huge magmatic province of East Asia (Yarmolyuk et al., 2020).

The Etyka massif lies in the northeastern part of the Kukul’bei Ridge in Eastern Transbaikalia. The Etyka granite massif and the associated rare-metal mineralization are described in detail in (Beskin et al., 1994b). The work presents the mineralogic and geochemical characteristics of rare-metal granites and all the ore formations occurring within the intrusion and its exocontacts. The formation of the Etyka ore–magmatic system is considered in terms of the evolution of rare-metal granitic magmatism and the formation of an ore cluster with different kinds of mineralization. The massif is a two-domed laccolith lying among the Jurassic mudstones, siltstones, sandstones, and conglomerates. Amazonite–albite granites containing lepidolite and topaz predominate in the apical part of the intrusion. With depth, they are replaced by less differentiated microcline–albite granites with amazonite and zinnwaldite, occupying up to 90% of the intrusion. There are a variety of vein formations: finegrained lepidolite–albite–amazonite and lepidolite–albite granites forming dikes and sills. They are associated with pegmatites and dikelike bodies of coarse-grained banded lepidolite–albite–amazonite rocks. In addition to lithium micas (zinnwaldite, cryophyllite, and lepidolite), the amazonite–albite granites contain topaz (up to 2%) and accessory minerals like columbite–tantalite, monazite, zircon, and galena. The Etyka massif has a multiphase structure. All the rock varieties of the intrusion can be attributed to the granites of the Li–F geochemical type. The largest concentrations of fluorine (up to 2300–2600 ppm) and trace elements (Li, Rb, Cs, Be, Ta, Nb, Sn, and W) are typically found in the final-phase granites and veins (Beskin et al., 1994b). A granite-hosted tantalum deposit, whose ores contain the associated Li, Sn, and Nb mineralization, was studied in the apical part of the massif. The main concentrators of tantalum at the deposit are the minerals of the tantalite–columbite group. The age of the Etyka granites is fairly close to the age of rare-metal granites from the Orlovka massif, Eastern Transbaikalia. According to the data from (Kostitsyn et al., 2004), both intrusions have somewhat similar initial strontium isotope ratios (Table 4).

The Orlovka (Khangilai-Shiliin) massif is located in the southwestern part of the Borshchovochnyi Ridge in Eastern Transbaikalia and has three outcrops of granites: Central, Western, and Eastern with a total outcropping area of about 10 km² (Beskin et al., 1994a). The granitic rocks of all the outcrops occur amongst metamorphosed shales and volcanic rocks interpreted as the Proterozoic–Carboniferous formations and cross the Triassic sand–shale strata. The Central outcrop of the massif (8 km²) is composed of biotite granites and leucogranites; the Eastern one is formed mainly by leucogranites and greisens, which contain tungsten ores of the Spokoininskoe deposit. The Western outcrop of the Orlovka massif is composed of microcline–albite granites with Li-micas in the apical part and muscovite in its more eroded parts. The microcline–albite granites are diverse in terms of textural and mineral characteristics. All the rock varieties of the Orlovka massif, including muscovite–zinnwaldite–microcline–albite with amazonite and albite–lepidolite granites, are typical Li–F granites, often forming concentrated mineralization with tantalum niobates and representing the tantalum deposit. A detailed description of ore and accessory minerals in rare-metal granites of the studied massif (columbite, tantalite, microlite, pyrochlore, etc.) is given in (Beskin et al., 1994a). The Rb–Sr whole-rock–mineral isochron dating of all the major granitic varieties ascribed to different phases of the Orlovka massif yielded 143 ± 3 Ma with an initial ⁸⁷Sr/⁸⁶Sr isotopic ratio of 0.706 ± 5 (Table 4) (Kovalenko et al., 1999).

The Turga massif. In addition to the representative massifs of rare-metal granites, the amazonite–albite Li–F granites of the Turga massif (134 Ma) are of particular interest, because these granites demonstrate mineral and geochemical features that are not characteristic of the Orlovka and Etyka ore-bearing massifs. Together with higher abundances of fluorine, lithium, and a number of trace elements, which is typical of Li–F granites, the Turga granites are characterized by REE–Nb–Zr–Th–U geochemical specialization and are classified as an individual subtype of rare-metal granites (Syritso et al., 2021). The data obtained provide grounds for viewing the Li–F granites of the Turga massif as a geochemical subtype of peraluminous amazonite-bearing granites of elevated alkalinity. Therefore, this scenario allows for a probable effect of the juvenile component on the origin of these granites, and the most likely candidate for the juvenile material is the shoshonite–latite complex in Transbaikalia. In terms of the mineralogic and geochemical characteristics, the Li–F granites of the Turga massif are fairly close to rare-metal granitic rocks of the Bezymyanka massif in the Baikal region and of the Henteyn massif in Mongolia, which can be assigned to a single geochemical type of Li–F granites. The Turga massif is composed of two intrusive complexes of different ages: early-phase monzonitic rocks of the Shakhtama complex, Transbaikalia, and late-phase leucogranites of the Kukul’bei complex, Transbaikalia. The contacts of these rocks are marked by the outcrops of the younger amazonitic rocks occurring as small intrusions and dikes. The main phase of rare-metal granites is composed of potassic feldspar, albite (An_2–4, 25%), quartz, and Li-bearing siderophyllite. Along with the rock-forming minerals, these rocks are characterized by minerals common to alkaline rocks and contain LREE, U, Th, and Nb, including columbite, minerals of the pyrochlore and samarskite groups, fergusonite, cyrtolite, thorite, fluocerite, bastnaesite, and allanite (Ivanova and Syritso, 2019).

The localization of leucogranites and amazonite–albite granites of the Turga massif, as well as their mineral composition, typical of A-type granites, corresponds to the setting of the continental rifts and continental hotspots (Grebennikov, 2014). The comparison of the Turga granites with amazonite–albite granites in ore-bearing massifs of Transbaikalia (Etyka and Orlovka) reveals some similar compositional features of the parental melts and common pathways of differentiation, as shown by significant accumulation of characteristic lithophile trace elements Li, Rb, Cs, F, Ta, Nb, and Sn, which are commonly accumulated in late-phase rare-metal granites. The protolithionite granites of the Turga massif are noted for an increase in the Fe and HFSE concentrations. The amazonite granites of the Turga massif, the derivates of A-type granites, are different from tantalumbearing amazonite granites in the elevated initial Sr isotope ratio (0.717 as compared with 0.706–0.708) and higher crystallization temperatures. The data obtained suggest classifying the rare-metal granites of the Turga massif as a specific geochemical subtype of peraluminous columbite-bearing amazonite granites of elevated alkalinity, which bear Zr– REE–Th–U–Nb mineralization, typical of peralkaline rocks (Syritso et al., 2021).

In Central Asia, several epochs of intraplate magmatism are recognized in the Phanerozoic: early Paleozoic, middle Paleozoic, late Paleozoic–early Mesozoic, and late Mesozoic–Cenozoic. As was shown earlier (Kovalenko et al., 1999; Yarmolyuk et al., 2003; Kuzmin et al., 2011), the intraplate magmatism within the CAOB was controlled by the Central Asian mantle hot field. The geological and geochemical data suggest that the products of the intraplate magmatism are high-alkali rocks: alkali basalts and gabbro, shoshonite–latite series and trachytes, comendites, pantellerites, and rare-metal granites, typical of geodynamic continental rifts and intracontinental regions.

In the CAOB, the studied rare-metal Li–F granites are assigned to different epochs of intraplate magmatism, and they originated within the magmatic areas of different ages (late Paleozoic, early Mesozoic, and late Mesozoic). Moreover, they were generated within a large age interval (321–134 Ma) (Table 4). Large multiphase massifs occur amongst the rocks of the Precambrian microcontinent and the Caledonian basement; small intrusions are located both in the Precambrian block and among magmatic and sedimentary–metamorphic rocks of different compositions and ages. In Central Asia, the Paleozoic and Mesozoic rare-metal Li–F granites, which are parts of the magmatic areas of the corresponding age, are well studied (Kovalenko et al., 1999). During the Phanerozoic, the CAOB experienced intense intraplate magmatism ranging in composition from basites of different alkalinity to alkaline and granitic rocks with different portions of silica representing different geochemical types (Tauson, 1977). Magmatic rocks of different ages making up plutons of palingenic calc-alkaline granites (Baga-Henteyn) in the lower continental crust under the influence of subalkaline and alkali-basalt magmas occur in the western segment of the Mongol–Okhotsk Belt. In the Baikal region and Mongolia, at the peripheries and, occasionally, in the center (Henteyn) of the late Paleozoic and Mesozoic magmatic areas, there are abundant rare-metal Li–F granites forming large strongly differentiated plutons (Janchivlan and Baga-Gazriin) and small intrusions of intrusive-dike belts (Yarmolyuk and Kuzmin, 2012; Antipin and Odgerel, 2016). During the evolution of rare-metal granitic magmatism in the extensional-rifting setting, large differentiated massifs and small intrusions with a series of dikes and pegmatite veins formed in the peripheral zones of the areas. The geochemical evolution of these rocks is expressed as the penetration of ore-bearing solutions into the upper horizons, thus leading to the concentrated rare-metal mineralization. As the geologic and tectonic conditions for the origin of Li–F granites differ in magmatic areas of different ages, the rocks identified within large multiphase differentiated massifs are noticeably dissimilar to the rocks of small intrusions within the intrusive-dike belts.

In terms of geochemistry, all the studied rare-metal rocks, as well as the Paleozoic collision-related granitic rocks of the Baikal region and Mongolia, differ in the quantitative abundances and distribution of elements and some indicator ratios (K/Rb and Nb/Ta) characteristic of Li–F granites (Figs. 2–4). The high F and Li contents are distinctive geochemical features of late-phase rare-metal granites and pegmatites of large multiphase intrusions (Janchivlan and Baga-Gazriin) (Table 1). The concentrations of these elements are significantly lower in microcline–albite granites from small intrusions of different ages in the central parts of magmatic areas (Tables 2, 3; e.g., the Henteyn and Turga massifs). The levels of Li and F concentrations in the collision-related Paleozoic granites of the Baikal region (Sharanur complex (Ol’khon Island) and Solzan complex (Khamar-Daban Range)) and the Baga-Henteyn granitic rocks (Figs. 2 and 3) are lower and fairly close to the mean composition of the upper continental crust, which differentiates these rocks from the studied intraplate rare-metal granites. Significant differences were detected in the Rb concentrations and in the K/Rb ratio, which is indicative of the granitic-magma differentiation process between Li–F granites of different ages and different geologic settings (Fig. 4). Compared with palingenic calc-alkali rocks (Baga-Henteyn pluton), the Li–F granites of Central Asia demonstrate elevated Rb concentrations, which are much higher than the clarke in the continental crust. This is particularly common to late-phase amazonite–albite and albite–lepidolite lithium mica-bearing granites, which also demonstrate a decrease in the K/Rb ratio (Fig. 4b). Moreover, their distinctive features are the enrichment in elements, such as Li, Cs, Sn, Be, and Ta, and a decrease in the Nb/Ta ratio, which is evident in multiphase well-differentiated massifs of Li–F granites (Fig. 4c).

In addition to the geochemical evolution of Li–F granites, expressed in the enrichment with F and characteristic lithophile elements (Kovalenko, 1977; Antipin et al., 2016), there are some small intrusions enriched in HFSE, which is typical of high-alkali granites. This is seen in the Henteyn intrusion (Northern Mongolia), lying in the center of the early Mesozoic area, where the amazonite–albite granites are substantially enriched in Zr, Hf, Nb, Th, and U as compared with the early-phase leucogranites (Antipin et al., 2019). A similar geochemical characteristic of Li–F granites is found for the Late Mesozoic Turga massif, Transbaikalia, where the amazonite–albite granites are enriched in the same HFSE, and these granites are classified as a separate subtype (Syritso et al., 2021). Therefore, rocks with different geochemical characteristics are found amongst the rare-metal Li–F granites. Large Mesozoic multiphase massifs and small zonal intrusions occasionally containing pegmatitic schlieren are represented by typical Li–F granites containing Li, Ta, Sn, and W mineralization. Within different provinces of the CAOB, some intrusions of the late Paleozoic magmatic area (Kharagul, Urugudei, and Bezymyanka) demonstrate the intensive enrichment in lithophile elements and REE, Zr, Hf, Nb, and Th, which is typical of high-alkali rocks (Table 3). Such amazonite–albite granites of the Li–F geochemical type, irrespective of their age, can be distinguished as an individual subtype, as was done for the Turga granites in Eastern Transbaikalia. Their geochemical specifics can be explained by the influence of the deep-seated source in the formation of Li–F granites.

Currently, the granitic rocks from the Khangai zonal magmatic area in Mongolia are considered to be thoroughly studied (Yarmolyuk and Kuzmin, 2012; Yarmolyuk et al., 2016). The model of formation of magmatic rocks of different compositions was proposed for the Khangai batholith (270–240 Ma) and its rifting framing. This model suggests different degrees of the crus–mantle interaction in the formation of the magmatic area. It is shown that the mantle-derived materials entrained into the lower levels of the crust and the resultant heating caused crustal melting. The mantle-derived materials might have also interacted with anatectic magmas, giving rise to different types of granitic rocks with varying geochemical and isotope characteristics. The recent model proposed by the authors links the geologic structure of the magmatic area to the activity of a mantle plume.

The early Mesozoic granites were emplaced after the closure of the Mongol–Okhotsk Ocean, which resulted in intense granitic magmatism within Eastern Mongolia and Transbaikalia, producing the Daurian–Henteyn batholith with the Baga-Henteyn pluton located at the center. The emplacement of the early Mesozoic granites is also linked to the influence of the Mongolian mantle plume at the lower levels of the continental crust (Yarmolyuk and Kovalenko, 2003). The batholiths originated in the center of zonal areas owing to collision processes between blocks of the continental crust and framing structures. As a result, large granite plutons referring to the calc-alkaline palingenic granitic rocks formed in lower crustal blocks, and their origin is associated with mantle-derived alkaline and basaltic magmas and related transmagmatic solutions. The Late Triassic–Early Jurassic age was determined using the geological data and U–Pb and Rb–Sr dating of rocks of the major phases (225–195 Ma). The Li–F granite intrusions located both in the center of the MZ₁ area and at its peripheries appear to be slightly younger than the palingenic calc-alkali granitic rocks forming large massifs. The available data suggest that in magmatic areas of varying ages, the Li–F granites become younger from the late Paleozoic (Baikal region) to the late Mesozoic (Eastern Transbaikalia) granitic-rock occurrences (Table 4). The evolution of Phanerozoic intraplate magmatism in Northern Asia is discussed in detail in (Kuzmin et al., 2011), where the movement of Siberia over the hotspot from west to east and the corresponding displacement of the magmatic area hosting Li–F granite occurrences is considered using the paleoreconstructions.

As the rare-metal granites were generated over vast territories within magmatic areas of different ages, the geodynamic conditions of their formation are of particular importance. All the studied Li–F granites of the late Paleozoic, early Mesozoic, and late Mesozoic ages, as well as palingenic granites of the Baga-Henteyn pluton and typical collision-related early Paleozoic granites within Ol’khon Island (Sharanur complex) and the Khamar-Daban terrane (Solzan massif), are shown in the geotectonic discrimination diagrams of Pearce (Fig. 5). On these diagrams, almost all the Li–F granites, irrespective of their age, fall within the field of within-plate granites (WPG). Only some granitic rocks of small intrusions (Turga, Abdar, Kharagul, and Urugudei) are plotted close to the boundary between WPG and volcanic-arc granites (VAG) (Fig. 5). At the same time, the collision-related Paleozoic granitic rocks of Ol’khon Island and granites of the Baga-Henteyn pluton, located at the center of the early Mesozoic magmatic area, fall within the fields of syncollision granites (syn-COLG) and VAG + syn-COLG.

This may point to the major role of collision processes preceding the intraplate magmatism, which resulted in the origin of multiphase massifs and small intrusions of Li–F granites. As was shown in (Kuzmin et al., 2011), the intraplate magmatic activity in large areas of Central Asia had sharply decreased by 190 Ma, and the late Paleozoic–early Mesozoic epoch of intraplate magmatism had been over.

In Central Asia, there are a number of occurrences of Li–F granites which formed in the intraplate geodynamic setting (Baikal region, Transbaikalia, and Mongolia), when they are significantly different in geochemical characteristics from the early Paleozoic granitic rocks formed by melting of the upper continental crust and show broad (⁸⁷Sr/⁸⁶Sr)₀ variations (Table 4). Low values of the initial ratio (⁸⁷Sr/⁸⁶Sr)₀ (no more than 0.707) were obtained for many massifs of rare-metal Li–F granites in Eastern Transbaikalia (Kostitsyn et al., 2004). Currently, the mechanism generating rare-metal magmas is related to the mantle–crust interaction, when CO₂–H₂O–F-bearing mantle-derived fluids affect the granulitic-facies rocks in the lower parts of the continental crust with involvement of crustal components in the supposed magma source (Kovalenko et al., 1999; Cuney and Barbey, 2014; Antipin et al., 2016). Granite formation and the associated mineralization might result from the interaction of the juvenile fluids with the crustal material followed by differentiation of granitic magmas. These processes are particularly evident at the peripheries of the magmatic areas related to rifting structures and bearing the rare-metal mineralization. This points to prospects of batholith rift framing in terms of the genetic relations between rare-metal mineralization and magmatism. The granitic rocks from the center of the Mesozoic area (Henteyn intrusion) are slightly enriched in lithophile elements but show stronger enrichment in HFSE (Zr, Hf, Nb, Zn, Th, and U), thus confirming the classification of these rocks as an individual subtype of Li–F granites with a possible juvenile source of accumulation of these elements. We have to note the specific features of intraplate magmatic processes on the southern framing of the Siberian craton related to the effect of mantle plumes on the continental crust, leading to the formation of large zonal magmatic areas of different ages which host intrusions of ore-bearing Li–F granites. Rising mantle-derived alkaline-basaltic and alkaline melts might have concentrated in the lower continental crust and caused its anatexis. The rocks of multiphase massifs and Li–F granitic intrusions in the magmatic areas of different ages demonstrate broad compositional variations, serve as an important indicator of the mantle–crust interaction and differentiation of granitic magmas, and might highlight the nature of zonal areas during the evolution of intraplate magmatism. The studied rare-metal complexes of Southern Siberia and Mongolia might be indicators of the evolution of ore–magmatic systems bearing rare-metal mineralization.

Regarding the geologic setting, age, and mineral–geochemical characteristics, Li–F granites in Central Asia formed in the late Paleozoic, early Mesozoic, and late Mesozoic areas of granitic magmatism within a wide age interval (321–134 Ma). In the magmatic areas, Li–F granites are typically located in the peripheral zones, which underwent rifting magmatism, responsible for the formation of rocks belonging to series displaying variations in alkalinity and SiO₂ content and occurring as bimodal, basaltic, and granitic volcanic–plutonic complexes. Occasionally, Li–F granites are found in the center of magmatic areas (Henteyn massif, Mongolia, MZ₁).

Within the CAOB, Li–F granites enriched in F, Li, Sn, W, and Ta make up relatively large multiphase massifs (Janchivlan and Baga-Gazriin) and small intrusions (Etyka, Orlovka, and Abdar) bearing the concentrated mineralization at the late magmatic and postmagmatic stages. As exemplified by large massifs, the multiphase plutons typically originate in the upper levels of the crust, where the extensive magma differentiation can lead to the enrichment of residual magmas with volatiles, resulting in formation of metasomatic rocks of varying alkalinity, which bear rare-metal mineralization (Sn, W, Li, Ta, and Nb).

The late Paleozoic and Mesozoic small intrusions (Kharagul, Urugudei, Bezymyanka, Henteyn, and Turga) are represented by high-alkali Li–F granites which are enriched in Zr, Nb, Hf, Th, U, and REE and are different from typical ore-bearing granites in geochemical and mineralogic characteristics. Following (Syritso et al., 2021), it is reasonable to classify these granites as an individual subtype of rare-metal Li–F granites. Irrespective of geochemical characteristics, the studied Li–F granites of Central Asia differ drastically from typical collision-related granitic rocks formed by melting of the upper continental crust.

Lithium–fluorine granites are the intraplate formations that demonstrate wide (⁸⁷Sr/⁸⁶Sr)₀ variations. The role of the mantle–crust interaction is considered in the genesis of Li–F granites with different geochemical characteristics. In the geochemical evolution of Li–F granites, an important role is played by deep-seated fluids containing trace elements and differentiation of granitic magma in the crustal intermediate chambers, which is favorable for forming rare-metal mineralization.

The study was carried out as part of a government assignment (topic ID 0284-2021-0007) and was supported by grants 19-05-00172, 20-55-44002 Mong_a, and 18-55-91049 Mong_omi from the Russian Foundation for Basic Research.