INTRODUCTION

Rutile is a common accessory mineral that occurs in a wide range of environments and PTX conditions, from ancient deep-seated rocks, such as mantle eclogites and peridotites, to modern sediments and placer deposits, but it is most typical of high-grade metamorphic rocks. In metabasites, rutile is a phase of high pressures and its stability extends to peak parageneses of crustal eclogites and high-pressure granulites. The presence of rutile in high-pressure and high-temperature rocks, along with significant progress in U–Pb geochronology [1] and its use as an independent geothermometer [2] has established its use as a reliable tracer of lithospheric evolution for deep zones of orogenic belts.

Recent studies of high-temperature crustal rocks have noted repeatedly the ability of rutile to retain the U–Pb system state corresponding to the conditions of heating of rock complexes to peak temperatures [3, 4]. Nevertheless, existing estimates of the closure temperature of the U–Pb isotope system in rutile are in the range of ~500–650°C [5‒7], consequently the U–Pb system of rutile should preferably reflect post-peak cooling during exhumation to mid-crustal levels. In turn, rutile in medium- and high-temperature orogenic eclogites from complexes of continental subduction, which often evolve by heating to the conditions of granulite facies during exhumation of rocks, may have biased U–Pb age estimates due to diffusion and partial loss of Pb. This fact, together with systematically low U and Pb contents in rutile of eclogites [7] and the sensitivity of measurement results to correction for non-radiogenic Pb [8], significantly affects the reliability of reconstructed trends in metamorphic evolution.

This study presents new results of ID-TIMS U‒Pb dating and accompanying mineralogical and rare element characteristics of rutile from eclogites of the North Muya block (NMB) of the Baikal–Muya fold belt (BMFB) with different PT trends of exhumation and retrograde hydration [9‒12]. Detailed petrological studies showed that during the Vendian orogeny (~630 Ma; Sm‒Nd age of eclogite and host gneiss [10]; U‒Pb age of zircon from eclogites, [12]), the rocks were subjected to subduction until reaching the conditions of ~1.8–2.7 GPa and ~560–760°C, which reflects the position of eclogites within different crustal fragments. Thus, the metamorphic evolution of eclogites occurred under medium-temperature conditions, which correspond to the range of existing estimates for closure of the U‒Pb system in rutile, while rutile itself is an ideal object for studying the behavior of U and Pb mobility at the prograde, peak, and retrograde stages.

SAMPLES AND METHODS OF STUDY

Seven eclogite samples, comprising individual bodies, boudines, and lenses ranging in size from a few meters to tens of meters and being among metasedimentary rocks and metagranitoids within the 10-km thick belt along the Samokut and Ileir rivers in the NMB (Fig. 1), were used for this study. The eclogites were crushed in jaw and roller crushers to a fraction of <1 mm, after that rutiles were separated from the matrix of rocks in the fraction <0.25 mm (highest Ru-enriched) using magnetic separation and separation in heavy liquids. From 50 to 90 rutile grains from fractions of 160–250 µm of each sample were extracted manually, placed in a one-inch epoxy disk, and polished to expose the central part of the grains.

Fig. 1.
figure 1

(a) Schematic map showing the position of the Baikal–Muya fold belt (BMFB) in the structure of the Central Asian Orogenic Belt relative to the Siberian Craton, (b) simplified structure of the BMFB with the key comprising rock complexes, and (c) the structural scheme of the North Muya Block of high-grade rocks (Cambrian carbonate and younger sediments are not shown).

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The electron probe microanalysis (EPMA) of mineral inclusions in rutile was conducted with a JEOL Superprobe JXA8200 microanalyzer at the Center for Collective Use for Isotope-Geochemical Research, Institute of Geochemistry, Russian Academy of Sciences. The analysis was carried out using five wavelength-dispersive spectrometers with a probe beam of 2 µm at a current of 15 nA and an accelerating voltage of 20 kV. The series of natural and synthetic standards for calibration included albite (Na), pyrope (Al), potassium feldspar (K), diopside (Si, Ca, Mg), olivine (Si, Mg, Fe), garnet (Si, Al, Fe, Mn), rutile (Ti), and chromite (Cr, Fe). The accumulation of the peak and background signal for each element was ten seconds with the analytical error ranging from 0.01 wt % (for minor elements with contents close to the detection limit) to 0.1–0.2 wt %.

The trace-element analysis of 105 rutile grains was conducted using secondary ion mass spectrometry (SIMS) with a Cameca-IMS-4f ion microprobe at the Yaroslavl branch of the Physical-Technical Institute, Russian Academy of Sciences (PTI of RAS). The primary beam of \({\text{O}}_{2}^{ - }\) ions was focused to a spot size of ~20–25 μm. Each analysis had three measurement cycles. The content of elements was calculated from the normalized intensities of the 47Ti+ secondary ions using calibration curves based on the analysis of a set of standard glasses. The SRM NIST610 standard glass was used in the analytical session to control the reproducibility of the results, reaching up to 10% for contents higher than 1 ppm and up to 20% for contents in the range of 0.1–1 ppm.

ID-TIMS U–Pb geochronological studies of rutile were conducted at the Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences. The monofractions from the most transparent and homogeneous rutile grains were washed in 0.5 M HNO3 at 70°C and three times in ultra-pure water. Chemical dissolution was carried out in HF at 220°C in steel Teflon bombs at high pressure, then the solutions were divided into two aliquots, and one of them was mixed with a 235U–208Pb tracer. The aliquots were evaporated and converted to bromide form for separation of Pb and U by ion exchange chromatography, and the U fraction was additionally purified in HNO3 on UTEVA resin. The isotopic analysis was carried out using a Triton T1 multicollector thermal ionization mass spectrometer (TIMS) in the static regime. The blank contamination was 25 pg for Pb and 0.5 pg for U. The results were processed in the PbDAT [13] and ISOPLOT [14] software. All errors are presented at the 2σ level.

RESULTS

Rutile in fresh and retrograde eclogites is represented by idiomorphic prismatic grains up to 300 µm in size and by smaller (no greater than 20–30 µm) inclusions in porphyroblastic and medium-grained garnet (Fig. 2). Rutile in the matrix is represented by individual grains or elongated grain segregations and is subjected to replacement by titanite in amphibolized eclogites and more often by ilmenite in simplectite eclogites with a small number of retrograde water-bearing phases. In porphyroblastic eclogites Mu-93-53, Mu-93-90, and Mu-93-93, where garnet cores are rich in water-bearing phase inclusions (epidote, amphibole), rutile inclusions are typical of the outer zones of garnet.

Fig. 2.
figure 2

Petrographic features of the studied eclogites from the North Muya Complex, indicating the main rock-forming, secondary, and accessory phases. Abbreviations used: Grt, garnet; Omp, omphacite; Qz, quartz; Rt, rutile; Cpx, Na–Al-poor clinopyroxene, and Pl, plagioclase. Microphotographs under transmitted light.

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The studied rutile grains (about 30 grains per sample) from the matrix contain solid-phase inclusions mostly 1–10 µm in size, that are located mainly in central zones of rutile (Fig. 3). The list of observed phases of inclusions is quite extensive and includes titanite, apatite, Ca- and Ca–Na-amphiboles, minerals of the epidote group, rare zircon, and quartz, as well as occasional clinopyroxene, biotite, carbonates (calcite, dolomite), and garnet. More than a half of almost 200 inclusions analyzed are represented by titanite (0.55–2.49 wt % of Al2O3). Amphibole is present in rutile in all samples; however, the quantitative characterization of their crystallochemical formulas is excluded due to the small size of the inclusions and the widespread contamination of Ti analysis from the rutile matrix. Since the nature of metamorphic amphiboles observed as inclusions in garnet porphyroblasts is usually low titanium (mostly below 1 wt % of TiO2) [11], the composition of amphiboles was recalculated on a non-titanium basis, which in most cases correspond to pargasite. The inclusions of minerals from the epidote group discovered in three eclogite samples have \({{X}_{{{\text{F}}{{{\text{e}}}^{{{\text{3 + }}}}}}}}\) in the range from 0.15 to 0.24 and belong to epidotes in most cases. Most mica inclusions were identified in the rutile from eclogite Mu-93-93, where the micas are represented by biotite. Three samples are found to contain single inclusions of carbonate phases, two of which have carbonate represented by ankerite (about 7 wt % of FeO), and in the third sample, carbonate is calcite. Short-prismatic apatite inclusions 1–5 μm in size with 1.56–3.18 wt % Fe are common in rutiles from the Mu-93-71 eclogite and rare in rutiles from the Mu-12-6 and Mu-12-9 symplectite eclogites. Zircon occurs as finely dispersed (<5 μm) isometric grains distributed unevenly in the host rutile. In addition, rutile systematically contains ilmenite lamellae that are most abundant in three samples of the symplectite eclogites. Only five inclusions analyzed in rutile (four clinopyroxenes and one garnet) represent mineral phases that can be considered as high-pressure ones. Clinopyroxene in the rutiles of eclogites from the Ileir River area (Mu-93-90, Mu-93-53, and Mu-93-71) is represented by omphacite with low and high contents of jadeite (XJd = 0.10–0.38) and variable richness in iron #Fe (0.26–0.40), while jadeite-rich clinopyroxenes are similar in composition to such in the matrix. The only garnet inclusion has an almandine–grossular–pyrope composition (Alm52Grs24Prp19Sps4). High-silica inclusions (phengite) were not detected.

Fig. 3.
figure 3

Representative microphotographs of individual rutile grains with characteristic solid inclusions primarily of silicate composition. Photos taken in backscattered electrons (BSE).

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Rutiles systematically contain small amounts of FeO, 0.1–0.3 wt %, while the total content of trace elements usually does not exceed 0.3–0.5%. The classification of rutiles based on their Nb and Cr contents indicates the typical eclogitic (metabasite) nature of rutile (Fig. 4a). Only the composition of high Nb rutile from the Mu-93-93 eclogite corresponds to rutile from metasedimentary rocks. There are two different trends in the rutile enrichment, W‒Ta‒Nb and Cr‒Mo. Zr and Hf show a positive correlation in all seven eclogites (Fig. 4b), while in some cases, the high contents of both elements may indicate the presence of microinclusions of zircon. This fact is consistent with the absence of a Nb‒Zr correlation and a sporadically elevated Zr content in individual rutiles from all eclogite samples (Fig. 4c). The corresponding values of the contents are excluded from further calculations.

Fig. 4.
figure 4

(a–c) Correlation diagrams for the key trace elements in rutile from eclogites and (d) ranges of Zr contents in rutiles from individual samples. The error bars show the highest and lowest contents within the ranges. The shaded areas denote the first and third quartiles, and the segments inside the shaded areas represent the median values.

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The crystallization/recrystallization temperatures were calculated based on the Zr content in rutile, according to [2], assuming parameters of \({{{\text{a}}}_{{{\text{Ti}}{{{\text{O}}}_{{\text{2}}}}}}}\) = 1 and \({{{\text{a}}}_{{{\text{Si}}{{{\text{O}}}_{{\text{2}}}}}}}\) = 0.5 (due to the low abundance of quartz in prograde/peak paragenesis of eclogites), with respect to the observed inclusion phases and conditions of evolution of eclogites [9, 12] for the pressure of 1.5 GPa to compare with the estimated data of garnet–clinopyroxene Fe2+‒Mg geothermometry [10], and the pressure of 2.5 GPa for extrapolation into the range of peak pressures. Rutiles from four eclogites in the Ileir River area have similar Zr contents (Fig. 4d) and yield similar average temperature estimates at 1.5 GPa: 619 ± 12°C (Mu-93-53), 623 ± 21°C (Mu-93-71), 638 ± 27°C (Mu-93-90), and 637 ± 22°C (Mu-93-93) (here and below, the estimates are given at the reproducibility level of ± 1SD and do not contain an error component resulting from instrumental uncertainty). These estimates are to some extent comparable with the results of traditional garnet–clinopyroxene Fe2+‒Mg geothermometry for the same pressure value (680 ± 12°C, 620 ± 26°C, 650 ± 31°C, and 660 ± 30°C, respectively, according to [10]), and deviations may be due to variations in the actual pressure values. Similar and consistent estimates were obtained by calculating for three symplectite eclogites in the Samokut River area: 616 ± 13°C for Mu-12-6, 617 ± 7°C for Mu-12-9, and 614 ± 11°C for Mu-12-10. The temperature estimates obtained for the pressure of 2.5 GPa, corresponding to the peak metamorphism for the case of rutile growth, are systematically higher by ~45°C and range from 659 to 684°C. Fig. 5

Fig. 5.
figure 5

207Pb/235U–206Pb/238U diagram for rutile from Mu-93-53 eclogite based on the ID-TIMS data.

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To determine the U–Pb age, rutile aliquotes from six eclogites were analyzed, yielding 206Pb/204Pb ratios ranging from 22 to 88. For geochronometer minerals with a significant non-radiogenic lead component (206Pb/204Pb < 100), uncertainties in the estimation of the isotopic composition of primary lead can lead to significant errors in the resulting age. For minerals with low U/Pb ratios and a small amount of radiogenic lead, the most acceptable method for calculating the age of cogenetic samples with assumed identical protogenic Pb isotopic composition is the use of a three-dimensional linear (total-Pb/U) isochron [15]. Three-dimensional methods do not require knowledge of the isotopic composition of the total (protogenic) lead used for correction, so the age of the “total-Pb/U” isochrone, constructed in the 238U/206Pb–207Pb/206Pb–204Pb/206Pb coordinates is more accurate than the age obtained using the Stacey–Kramers model for protogenic Pb correction. Based on the analysis of the U–Pb geochronology of rutile in the Mu-93-53 eclogite sample (Table 1), which is the richest in U and radiogenic Pb (206Pb/204Pb in the range of 62–88), we obtained a reliable age estimate of 605 ± 2 Ma by the concordia (Fig. 6). Some dates obtained from other rutile samples are less accurate and/or discordant to varying degrees due to low contents of radiogenic Pb. The latter also results in discrepancies of the dates determined from the 206Pb/238U and 207Pb/206Pb ratios. We note that, by the age of intersection with concordia, some of the dates are equivalent within the error to the value obtained for Mu-93-53; for example, the estimated age of 604 ± 13 Ma for Mu-93-90 eclogite.

Table 1. Results of U‒Pb dating for rutile from the Mu-93-53 eclogite in the North Muya Block
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Fig. 6.
figure 6

Calculated curves for the closure temperature of the U–Pb isotopic system in rutile at different cooling rates (according to [19]). The gray area shows the range of the typical grain size of rutile in eclogites. The blue and dark gray rectangles correspond to the range of calculated temperatures of peak equilibrium in eclogites based on the data of garnet–clinopyroxene thermometry [10] and the Zr content in rutile (this work).

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DISCUSSION

Association of mineral inclusions in rutile (primarily titanite, epidote, and amphibole) indicates the predominant growth of rutile during prograde metamorphism due to titanite in the protolith of epidote–amphibolite or amphibolite facies. This is also confirmed by the abundance of water-bearing mineral inclusions in garnet cores of most eclogites studied [10, 12]. The particular conditions of the titanite–rutile transition depend on the chemistry of the rocks, including their calcium, magnesium, and SiO2 contents. For the average composition of basites of the mid-ocean ridge type, it is typical that rutile appears at T = 500–700°C at relatively low pressures of 7–8 kbar [16], whereas the thermobarometry of rutile-bearing epidote–clinozoisite amphibolites indicates a rather wide stability region of the main paragenesis (~500–780°C, 0.7–1.2 GPa) [17]. The formation of rutile due to titanite, for example, according to the reaction An + 2Ttn = Grs + 2Rt + Qz, could therefore be initiated under the conditions of the epidote–amphibolite or amphibolite facies at pressures that are significantly lower than the peak pressures. However, it cannot be excluded that the prograde growth of rutile may have continued until the peak of metamorphism was reached. Therefore, the observed range of Zr contents and the estimated temperatures of rutile crystallization may correspond to the conditions ranging from prograde metamorphism (average estimates of 619–638°C at 1.5 GPa) to peak metamorphism (659–684°C at 2.5 GPa).

The diffusion of Zr in rutile is a significantly slower process compared to the diffusion for Pb and some other cations and cations with variable valency (Co, Fe, Mn, Sc) [18]. According to the estimated data [19], at the peak temperature of metamorphism in the range of ~650°C, for rutile with a grain radius <80 μm, reequilibration of the initial levels of Zr in the entire volume of the grains can occur during exhumation accompanied by cooling slower than ~10°C/Ma. According to the currently prevailing models of exhumation of ultrahigh pressure rocks, cooling and decompression of rocks to the mid-crustal levels occur much faster (<10 Ma). However, some continental high-pressure complexes are characterized by the limited temperature growth associated with the lithosphere thickening during the collision stage. Based on the data of classical thermobarometry and PT modeling [10–12], the peak temperature values for the eclogite-bearing NMB complex could reach ~750–770°C [9, 11, 12]), which facilitates the post-growth diffusion of Zr. As a result, the content of immobile Zr in rutile of individual samples (Fig. 4d) may vary due to its only partial mobilization, which is consistent with the substantial variations in more active elements such as Cr, Fe, and Mn. The proximity of the temperatures obtained for seven samples in this case may indicate a common process of prograde metamorphism of rocks.

Compared to the diffusion of highly charged elements, the diffusion speed for Pb in the rutile structure is approximately an order of magnitude higher, and therefore, the Pb diffusion process is much more sensitive to the temperature regime than the Zr diffusion [5]. Importantly, the determined age (~605 Ma) is significantly younger than the age of high-pressure metamorphism (630 Ma) [9, 12], which implies a long history of cooling for the eclogite-bearing lithosphere of the complex below the closure temperature in the rutile U–Pb system [5–7]. The long-term Late Cambrian thermal evolution of the lithosphere in the northeastern BMFB, followed by the formation of granitoid batholiths and acidic extrusive rocks, associated with orogenic collapse and post-collision extension, at least until ~590–570 Ma, suggests the potential role of long-term cooling [20]. The closure temperatures for rutile with a grain radius of 25–80 µm, calculated based on the classical model [19] using the corrected diffusion coefficients [5], can vary within the range of up to ~50°C for the common rate of rock cooling. For example, for rutile with the above grain radius, at cooling rates of 1 and 10°C/Ma, the closure temperature values range within ~540–590°C and ~590–650°C, respectively (Fig. 6a), which corresponds to the range of empirically established values of the closure temperature in the rutile U–Pb isotopic system [1, 6, 7].

The consistency of the garnet–pyroxene and Zr–rutile thermometry data for at least four out of seven eclogites indicates the absence of any significant heating of the rocks after rutile growth; consequently, rutile must have grown at a temperature even higher than the closure temperature of the U–Pb system with its subsequent conservation during cooling of the rocks. Based on the size of the grains studied and the geothermometry data (Fig. 6), it is highly unlikely that, during the exhumation of eclogitized rocks, cooling occurred faster than ~10°C/Ma, and the maximum cooling rates are estimated at ~5–6°C/Ma. This cooling rate, together with the estimated age (605 Ma), allows us to limit the possible closure temperature of the U–Pb isotopic system for rutile of the selected sizes to ~500–525°C. Such a slow cooling rate is typical of granulite–gneiss complexes [1, 6, 7], as well as of the second (intracrustal) stage of exhumation of high-pressure complexes, at which exhumation is controlled by erosion and/or tectonic denudation.