Based on the study of the chemical composition of rock-forming micas and micas from melt inclusions in quartz of a full range of differentiates of Li–F granites of the Orlovka massif in Eastern Transbaikalia, possible mechanisms of the massif formation are discussed. An early stage with a trend of the mica evolution (biotite–Li-containing aluminous annite–Li-rich phengite-muscovite) in rocks, which is manifested in the synchronous Li and F accumulation in the melt, mica from the rock, and melt inclusions (an ongonite trend of the melt evolution), has culminated in the formation of porphyroblast microcline-albite granites with Li-rich phengite-muscovite and snow-ball quartz. It was this porphyroblast granite melt that has undergone the subsequent evolution (crystallization fractioning, repeated manifestation of silicate-salt liquid immiscibility, and post-magmatic metasomatism), which determined the development of the ‘apogranite process’. Melt inclusions in quartz of porphyroblastic microcline-albite granites and later amazonite-bearing rocks contain exclusively lithium-free, high-alumina muscovite. The high Li and F contents in glasses of these melt inclusions obtained after the homogenization experiments indicate the muscovite crystallization in a heterogeneous system from a depleted aluminosilicate melt coexisting with an isolated Li–F-containing hydrosalt phase. The results obtained indicate the convergence of the Li–Fe mica formation mechanism, which makes probable their crystallization both from a fluid-saturated melt (two-mica granites, porphyroblastic microcline-albite granites, amazonite granites of the southwestern flank of the massif and their pegmatitoid bodies) and as a result of metasomatic reworking of a substance (amazonite granites of the main dome) at the late-postmagmatic stage of the Orlovka massif formation.

An Erratum to this paper has been published: https://doi.org/10.1134/S0869591123340015

The paper reports major relations and trends determined in the distributions of trace elements and the Sr and Nd isotope composition of leucogranites of the Turga massif in Eastern Transbaikalia. Three varieties of the leucogranites are distinguished: porphyritic biotite granites, equigranular granites with Li-siderophyllite, and amazonite granites that make up small stocks and dikes. All of the rocks define a single Rb–Sr isochron that corresponds to an age of 134 ± 1 Ma at IR(Sr) = 0.71768 ± 22 and MSWD = 1.2, i.e., all of these rocks crystallized within a narrow age span. The porphyritic biotite granites, which are thought to be parental for the leucogranites, crystallized at lower temperatures (710–740°C) than the younger granites with Li-siderophyllite (810–850°C). Moreover, the granites with Li-siderophyllite have elevated Fe concentrations, which is reflected in that their micas are Fe-rich. The leucogranites are enriched in HFSE (270 ± 59 ppm Zr, 337 ± 93 ppm REE, 72 ± 31 ppm Th, and 16 ± 6 ppm U), which is atypical of amazonite granites in the study area. Considered together with very low Sr and low Ti and P concentrations, these compositional features are distinguishing features of A-type granites. The rare-metal granites of the Turga massif are unique in composition: being similar to amazonite granites in mineralized massifs, these rocks carry accessory minerals typical of peralkaline rocks. They contain LREE fluorides and fluorcarbonates (fluocerite, bastnaesite, and parisite), minerals of the pyrochlore and samarskite groups, and minerals of the isostructural group of REE–Y–Zr–Th–U silicate-phosphates. Our data provide grounds for classifying the rare-metal granites of the Turga massif with an unusual transitional geochemical subtype: peraluminous columbite-bearing amazonite granites of elevated alkalinity with peralkaline REE–Nb–Zr–Th–U mineralization.

The principal possibility of using high-uranium metamict zircon with a high self-irradiation α-dose for U–Pb geochronological studies (ID-TIMS) is demonstrated. The application of the optimal parameters of “chemical abrasion,” accompanied by preliminary high-temperature annealing, allowed us to estimate the age of Li–F granites of the Turga massif (Eastern Transbaikalia) (146 ± 4 Ma and 141 ± 1 Ma).

Zircon from granitoids of the multiphase Turga massif (Eastern Transbaikalia) shows various morphological features and compositions of trace elements. This granitoid massif of the Kukulbey complex is spatially associated with rocks of the Shakhtama monzonitoid complex, whose ages are about 133 and 158 Ma, respectively. Zircon from different granitoid phases of the Kukulbey complex demonstrates a decrease in Hf content and an increase in Th, U, Y, and REE concentrations. The trace element accumulation in zircon and rocks of the main phase of the Turga massif (Li-Li-siderophyllite granites) was accompanied by a significant rise of their crystallization temperature, relatively to the earlier phase. The indicative features of zircon in these granites—the intensive development of specific twinning (up to 30% of all grains) and the presence of contrasting growth zones—confirm the idea of increasing crystallization temperatures and alkalinity of the melt. These features of zircon, together with geochemical composition of rocks, indicate that amazonite granites of the Turga massif belong to the alkaline type of Li–F-bearing granites with a mixed geochemical specialization: on the one hand, Li, Ta, and Nb, which are typical for the ore-bearing (peraluminous rare-metal) Orlovka massif; and on the other hand, Zr, REE, Th, and U, which are more typical for alkaline granites.

The tungsten distribution in rocks of the Kukulbei Complex in eastern Transbaikal region results in a high potential of rare-metal peraluminous granites (RPG) for W mineralization and displays a different behavior of W in Li–F and “standard” RPG. These subtypes differ in the behavior of W in melt, spatial localization of mineralization, and the timing of wolframite crystallization relative to the age of the parental granitic rocks. The significant of W concentration is assumed to be due to fractionation of the Li–F melt; however, wolframite mineralization in Li–F enriched granite is not typical in nature. The results of experiments and our calculations of W solubility in granitic melt show that wolframite hardly ever crystallizes directly from melt; it likely migrates in the fluid phase and is then removes from the magma chamber to the host rocks, where secondary concentration takes place in exocontact greisens and quartz–cassiterite–wolframite veins. At the same time, the isotopic age of accessory wolframite (139.5 ± 2.1 Ma) within the Orlovka massif of Li–F granite is close to the formation age of the massif (140.6 ± 2.9 Ma). A different W behavior is recorded in the RPG subtype with a low lithium and fluorine concentration, exemplified by the Spokoininsky massif. There is no significant W gain in the melt. All varieties of wolframite mineralization in the Spokoininsky massif are derived from greisens, veins, and pegmatoids yielding the same crystallization ages (139.5 ± 1.1 Ma), which are 0.9–1.8 Ma later (taking into account the mean-square weighted deviation) than the Spokoininsky granite formation (144.5 ± 1.4 Ma). Perhaps this period corresponds to the time of transition from the magmatic stage to hydrothermal alteration. Comparison of the isotope characteristics (Rb–Sr and Sm–Nd isotope systems) of rocks and the associated ore minerals (wolframite, cassiterite) from all examined deposits shows a depletion in εNd values for ore minerals relative to the rock and the opposite behavior for the intial Sr isotope ratios. This may indicate the specific nature of ore matter, where the effect of the juvenile component is definitely expressed. Our geochronological results show that tantalum and tungsten mineralization took place within a narrow age interval, almost synchronously with the crystallization of associated granites. The coeval development of peraluminous magmatism enriched in lithophile rare elements and volatiles with ore complexes located in different structural settings and separated by a considerable distance from each other (up to 500 km) suggests a regional and deep-seated magma source. Rifting and increased thermal flux from the mantle, manifestations of which have been recorded during this period in the territory, may be a deep-seated process.

Compositional variation (results of electron microprobe analyses and mass-spectrometry analyses) of columbite-group minerals (CGM) from fully differentiated albite–spodumene pegmatites at Kolmozero in the Kola Peninsula is evaluated. Concentric zoning, typical of rare-metal pegmatites, was not observed in the Kolmozero pegmatites. Columbite-group minerals occur in all main parageneses of the pegmatites and form four generations, reflecting the sequence of pegmatite formation. These minerals demonstrate wide variations in the content of major and trace elements. The composition of CGM ranges from columbite-(Fe) to tantalite-(Mn). Fractionation trends were observed in Mn/(Mn + Fe) versus Ta/(Ta + Nb) diagrams and trace-element abundances plotted versus XTa and XMn. The early CGM paragenesis is characterized by homogeneous, oscillatory and progressive oscillatory zoning and corresponds to a primary magmatic type. Late-generation CGM show patchy irregular internal textures replacing earlier regular patterns of zoning. The irregular zoning points to metasomatic replacement processes. For the first time, it is shown that distributions of rare earth elements (REE) in CGM reflect the evolution of a pegmatite-forming system. At Kolmozero, the main trend of REE variation from early to late generations of CGM involves decreasing total REE contents due to a decrease in heavy REE and Y, decreasing negative Eu anomaly and decreasing magnitude of M-shape tetrad effect between Gd and Ho. These changes are accompanied by gradual flattening of the “bird-like” patterns of chondrite-normalized REE distribution. All these features are typical for late differentiates of granitic volatile-rich magma. Late metasomatic tantalite-(Mn) is characterized by sharp changes in its REE distribution pattern: decreasing total REE contents, changing shape of the REE distribution pattern, the absence of Eu anomaly and tetrad effects, and the appearance of a negative Ce anomaly. The textural characteristics and mineral chemistry of CGM indicate that the pegmatite-forming system underwent several stages of evolution. The earliest magmatic stage can be divided into two sub-stages, involving direct crystallization and collective recrystallization, respectively, and was succeeded by a late hydrothermal–metasomatic post-magmatic stage. Variations in chemical composition among the different generations of CGM are explained by the interplay of several processes: fractional crystallization; competitive crystallization of main rock-forming (feldspar, muscovite, spodumene) and accessory (triphylyte–lithiophilite, spessartine, fluorapatite, zircon, microlite) minerals; and evolution of the mineral-forming environment from a melt to a hydrothermal–metasomatic fluid.

The results of study of chemical composition, mineral-forming medium, P-T conditions of crystallization, and the age characteristics of subvolcanic felsic rocks that are spatially associated with rare-metal granite massifs in the ore units of Transbaikalia (Sherlovaya Gora, Khangilay, Bukuka, Belukha, and Shumilovka) give grounds for defining cogenetic volcanoplutonic associations. These associations within the studied region consist of rare-metal granites, ongonites, rhyolites, ongorhyolites, and trachyrhyodacites, which have much in common, but also many differences. The common chemical features of these rocks are their peraluminium signature, low mafic index and basicity, as well as enrichment (as compared to crust) in trace lithophile elements (Li, Rb, Nb, Ta, Sn, W, and F), the low contents of Zr, REE, and Sr, and the similar distribution of trace and refractory elements. At the same time, these rocks differ in the proportions of sodium and potassium, levels of concentrations of lithophile trace and refractory elements, REE distribution patterns, P-T regimes of crystallization, and the volatile composition.

By the example of the Orlovka massif of Li-F granites in Eastern Transbaikalia, the major- and trace-element (Li, Be, B, Ta, Nb, W, REE, Y, Zr, and Hf) compositions of the parental melt and the character of its variations during the formation of the differentiated rock series were quantitatively estimated for the first time on the basis of electron and ion microprobe analysis and Raman spectroscopy of rehomogenized glasses of melt inclusions in quartz. It was shown that the composition of the Orlovka melt corresponded to a strongly evolved alumina-saturated granitoid magma (A/CNK = 1.12–1.55) rich in normative albite, poor in normative quartz, and similar to ongonite melts. This magma was strongly enriched in water (up to 9.9 ± 1.1 wt %) and fluorine (up to 2.8 wt %). Most importantly, this massif provided the first evidence for high B2O3 contents in melts (up to 2.09 wt %). The highest contents of trace elements were observed in the melt from pegmatoid bodies in the amazonite granites of the border zone: up to 5077 ppm Li, 6397 ppm Rb, 313 ppm Cs, 62 ppm Ta, 116 ppm Nb, and 62 ppm W. Compared with the daughter rock, the Orlovka melt was depleted at all stages of formation in SiO2 (by up to 6 wt %), Na2O (by up to 2.5 wt %), and, to a smaller extent, in Ti, Fe, Mg, Sr, and Ba, but was enriched in Mn, Rb, F, B, and H2O.

Dikes, stocks and/or sheet flows of felsic volcanic and subvolcanic rocks are typically observed in the vicinity of rare-metal Li-F granite massifs. Their ubiquitous spatial association to rare-metal granites and, often, geochemical affinity to them suggest their certain petrological relation. Compositionally unique ultrapotassic trachydacites enriched in many rare elements were found among these rocks within the Khangilay complex of ore deposits in Eastern Transbaikalia. Melt inclusions in rock-forming quartz were studied to reconstruct the composition and evolution of parent melt. The obtained data demonstrated the existence of a super-potassic peraluminous melt (K2O = 6.12 wt %, Na2O = 1.08 wt %) having elevated contents of rare lithophile elements (730 ppm Rb2O and 900 ppm BaO). The ion-microprobe content of Li is 354.23 ppm at a relatively low F content (up to 0.5 wt %). The residual melt is characterized by the most unusual composition: extremely low contents of mafic components and basicity (< 0.5 wt % femic oxides), a high Al index (A/CNK = 1.53) at comparatively low SiO2 (60 wt %), and high total sodic alkalinity (more than 10 wt % K2O + Na2O; 6.11 wt % Na2O). Such a composition corresponds to ongonite magma. However, the melt contains no F but has a high Cl content (0.34 wt %), which corresponds to the limit Cl saturation of haplogranite melt. SHRIMP-II U-Pb zircon dating showed significant difference between rare metal granites and trachyrhyodacites of the Khangilay complex of ore deposits: 139.9 ± 1.9 Ma and 253.4 ± 2.4 Ma, respectively. The geochemical similarity of these rocks, primarily in terms of abundance of refractory elements, REE distribution patterns, and initial Sr ratio, indicates their derivation from similar protolith.