U-Pb SHRIMP-II ages of titanite and timing constraints on apatite-nepheline mineralization in the Khibiny and Lovozero alkaline massifs (Kola Peninsula)

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U–Pb SHRIMP-II ages of titanite and timing constraints on apatite–nepheline mineralization in the Khibiny and Lovozero alkaline massifs (Kola Peninsula) N.V. Rodionov a,b,*, E.N. Lepekhina a, A.V. Antonov a, I.N. Kapitonov a, Yu.S. Balashova a, B.V. Belyatsky a, A.A. Arzamastsev b,c, S.A. Sergeev a,b a

A.P. Karpinsky Russian Geological Research Institute, Srednii pr. 74, St. Petersburg, 199106, Russia Institute of Earth Sciences, St. Petersburg State University, Srednii pr. 31, St. Petersburg, 199004, Russia c Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, Makarova nab 2., St. Petersburg, 199034, Russia b

Received 30 April 2017; received in revised form 5 October 2017; accepted 20 October 2017

Abstract Results of this study of titanite samples collected from silicate rocks and apatite–nepheline–(sphene) ores from Paleozoic polyphase alkaline nepheline syenite complexes of the Khibiny and Lovozero massifs revealed the possibility of their in-situ U–Pb dating using sensitive high-resolution ion microprobe SHRIMP-II with an accuracy of 1.0–1.5%, which is comparable with that of U–Pb zircon analysis. Employing different approaches to age determination of the formation of the U–Pb system of titanites, the combined isochrons and mixing lines were plotted from the data obtained from the differentiated complex samples (121 analyses of five Khibiny samples and 52 analyses of one Lovozero sample) and apatite–nepheline ores (120 analyses of five Khibiny samples and 88 analyses of three Lovozero samples). They indicate synchronous crystallization of titanite in silicate rocks throughout the complexes: 374.1 ± 3.7 Ma for the Khibiny massif and 380.9 ± 4.5 Ma for the Lovozero massif, and attest to the later formation of phosphate–rare-metal ores: 371.0 ± 4.2 and 361.4 ± 3.2 Ma, respectively. The relatively delayed ore mineralization specific to the Lovozero massif can be accounted for the significantly lower volumes of magmatic melt and ore fluid involved, different thermal conditions, and the pattern of the investigated mineralization. As such, the obtained U–Pb data from titanite make it possible to limit significantly the time interval (most likely, not exceeding 15–20 Ma) comprising the evolution and activity of the ore-magmatic system of major agpaitic complexes, which is probably associated with plume magmatism. © 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: U–Pb dating; SHRIMP; titanite; sphene; agpaitic syenites; Khibiny; Lovozero; apatite–nepheline ores

Introduction Despite significant recent advancements in analytical chemistry and mass spectrometry allowing isotope analysis of ultralow amounts of substance (10–9 g) with accuracy higher than 1%, absolute dating of formation of alkaline–ultrabasic rocks, carbonatites and related rare-metal mineralization is still problematic in geochronology. This is largely determined by excess partial pressure of the fluid, rather than by specific compositions of parent melts alone, characterized by extreme silica undersaturation and high concentrations of alkalis annihilating zircon, the most recognized mineral geochronometer in rocks (Schaltegger et al., 2015), and bringing down

* Corresponding author. E-mail address: [email protected] (N.V. Rodionov)

concentrations of uranium—a key element in the isotopic system, which is widely used in geology as radiometric dating. The agpaitic melts capability of preserving the dissolved fluid phase throughout the crystallization period (Kogarko, 1977) is manifested in their interaction with crystallizing rock-forming and accessory minerals, contributing thereby to the diffusion of parent and daughter elements of the isotopic systems, up to their complete or partial opening, or re-equilibration. This process results in significant time intervals attributed to the formation and evolution of alkaline massifs. Abundant age-data of various igneous rocks obtained during recent decades for the Khibiny and Lovozero polyphase alkaline massifs as monomineral fractions using both U–Pb, Rb–Sr, Sm–Nd, and Ar–Ar isotopic systems and Rb–Sr, Sm–Nd, and Pb–Pb whole-rock isochrones allow to reduce the time of activity of ore–magmatic systems to the period of 383–362 Ma for Khibiny and 370–359 Ma for Lovozero

1068-7971/$ - see front matter D 201 8, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 8.07.016

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massifs (Arzamastsev and Wu, 2014; Arzamastsev et al., 2013; Kramm and Kogarko, 1994; Zartman and Kogarko, 2014). At that, the closure temperatures used to estimate the age of isotopic systems (Kylander-Clark et al., 2008) differ by more than 300–400 °C: U–Pb systems of perovskite/zircon represents the highest-temperature system comparable to the crystallization temperature of high-temperature (800–900 °C) magmatic melts, while Rb–Sr and Ar–Ar systems of phlogopite has temperature 400–500 °C. Thus far it remains unclear, whether the obtained time interval is associated with actual duration of magmatic activity, sequential introduction of melts of alkaline–ultrabasic series, alkaline syenites, carbonatites and the formation of apatite–rare metal ores, or if it reflects the closure temperature variations specific to a particular isotopic systems. To answer this question, we have tested a number of samples of alkaline rocks and apatite–nepheline ores from both the massifs, using a single approach consisting in in-situ analysis of U–Pb system of titanite, enabling age estimates independent of the isotope systems closure temperatures that can show a good correlation between each other without additional adjustments and corrections. Titanite (sphene) CaTiSiO5, a mineral from the nesosilicate group, which has monoclinic symmetry and occurs as a common accessory mineral in igneous and metamorphic rocks of different formations; it is especially widely developed in alkaline rocks, where it often forms large aggregates to the extent of ore-forming concentrations (Arzamastsev and Arzamastseva, 2013). The uranium and thorium concentrations in titanite typically range from the first gram per ton (10–4% or ppm) up to hundreds of grams (Frost et al., 2000), whereas the Th/U ratio tends to be >1. At this, the effective diffusion of lead in the titanite lattice is realized at relatively high temperatures, providing thereby the U–Pb system closure at 700–750 ºC (Cherniak, 1993, 2006, 2010; Scott and St-Onge, 1995). As such, these specific chemical properties and composition of titanite have prompted its application as a mineral geochronometer in U–Th–Pb dating of different geological events both using isotope-dilution mass spectrometry (IDMS) and local isotope analyses based on secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) (Amelin, 2009; Bonamici et al., 2015; Kennedy et al., 2010; Kohn and Corrie, 2011; Li et al., 2010; Rasmussen et al., 2013; and others). Materials and methods For U–Pb dating, titanite samples were collected from the main rocks and ore varieties in two largest alkaline intrusions on the Kola Peninsula (Fig. 1, Table 1). Samples collected from Khibiny massif represent agpaitic syenites forming the external (khibinites) and internal (foyaite) zone, and rocks adjacent to the apatite–nepheline ore deposits (kalsilite-bearing nepheline syenite, massive urtite, ijolite) and titanite–apatite and apatite–nepheline ores. The Lovozero massif samples were collected from loparite-bearing lujavrite within the differenti-

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ated complex of lujavrite–foyaite–urtites, and from the apatite–titanite mineralization zone of the Suluai site located at the boundary between the eudialytic lujavrite and lujavrite– foyaite–urtite complexes. A preliminary study of individual titanite grains of these samples using scanning electron microscopy (CamScan MX2500S) and energy dispersive microanalysis showed that titanite from Lovozero and Khibiny massifs is for the most part represented by homogeneous grains, while the Lovozero titanite has higher sodium content (up to 4.5 wt.% Na2O) as compared with titanite from Khibiny massif (<1.5 wt.% Na2O). Iron concentrations in the studied titanites of all rock types from both massifs do not exceed 3.5 wt.% (Fig. 2b). A distinctive feature of all titanite specimens from our collection is the absence of appreciable mineral inclusions (Fig. 2a), except for strontium apatite, which has characteristic columnar (locally, evolving into acicular) crystals up to 200 µm and pseudoisometric forms (SrO increases up to 5.5 wt.% in Khibiny massif and up to 8.5 wt.% in Lovozero massif). Besides, the microcomponent contents in the studied titanite specimens differ strikingly. Thus, titanites from Khibiny and Lovozero alkaline rocks have comparable concentrations of high field strength elements (HFSE): Zr = 3000–5000, Nb = 3000–7000, Hf = 60–130, Ta = 200–850 ppm, close contents of Y and rare-earth elements: 400–750 and 13,000–16,000 ppm (the sum total of rare-earth elements—ΣREE), respectively; whereas titanites of the Khibiny apatite–nepheline ores are essentially depleted in all the listed below components: Zr = 2000–2400, Nb = 1400– 1700, Hf = 30–60, Ta = 120–170, Y = 270–300, ΣREE = 3300–5400. In view of trace element abundancies, titanites of the mineralized Lovozero ijolites are indistinguishable from titanite of Lovozero and Khibiny silicate rocks. Fig. 2c represents the above discussed features of the composition of the studied titanites as a normalized rare-earth elements (REE) plot. The analysis of microcomponent abundancies in titanites, including REE, was carried out using the Element-2 inductively coupled plasma mass spectrometer with laser ablation (laser beam diameter: 50–150 µm, pulse frequency range: 3–15 Hz, laser pulse: 200 mJ). The abundancies were calculated with titanium as internal standard, whose concentrations in individual grains were estimated independently using the CamScan MX2500S electron microscope equipped with energy-dispersive spectrometer. The normalized REE distribution shows distinctly discernible difference and affinity between titanites of alkaline silicate rocks and of nepheline– apatite–titanite ores from alkaline massifs, which is associated not only with the composition and enrichment/depletion in elements of the melts during partial melting, but also by crystallization conditions (degree), and, specifically, by the presence of minerals competing in microcomponents uptake (Vuorinen and Halenius, 2005). The relative LREE-depletion, particularly in La and Ce, of titanites of Khibiny and Lovozero silicate rocks is determined by their fractionation from melt predominantly by apatite and perovskite, whose crystallization occurs either prior to or concomitantly with the titanite crystallization (Arzamastsev and Arzamastseva, 2013). The similarity between the normalized REE spectra of titanites of

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Fig. 1. Schematic geological map of Khibiny and Lovozero massifs (compiled by the data from the Murmansk and Central Kola Expedition of the State Company “Sevzapgeologiya”) with locations of the studied collection of titanite samples. 1, carbonatites; 2, pulaskites; 3, 4, foyaites; 5, kalsilite-bearing nepheline syenites (ristschorrites); 6, apatite–nepheline rocks; 7, foidolites; 8, 9, nepheline syenites (khibinites); 10, eudialyte lujavrites; 11, differentiated lujavrite–foyaite–urtite complex; 12, subalkaline volcanites; 13, alkaline–ultrabasic rocks; 14 and 15 rocks of the Precambrian basement. Inset I shows the study area; inset II shows location of Khibiny and Lovozero massifs.

Khibiny silicate alkaline rocks and apatite–nepheline ores, given absolute REE contents are by an order of magnitude lower in the latter, probably reflects their genetic affinity, i.e., their crystallization from the same magmatic source through crystallization of titanite ores from the depleted residual melt (Arzamastsev and Wu, 2014). Whereas normalized REE spectra of titanites in apatite–titanite ores from Lovozero massif can reflect titanite formation by fractionation from enriched ore-forming fluid/melt close in composition to parent melt of alkaline rocks in this massif (Fig. 2c). Alternatively, the amounts and variations of trace elements in the studied titanites of alkaline massifs are consistent with those investigated in (Arzamastsev and Arzamastsev, 2013; Frost et al., 2000) and do not exceed the known characteristics often enriched and typical of titanites of the alkaline complexes and carbonatites (Vuorinen and Halenius, 2005). Recently, the admixture of zirconium into titanite has been commonly used for estimation of mineral crystallization temperature (Cherniak, 2006; Hayden et al., 2008), although experimental results allowed to infer that zirconium abundances depend both on the activity of silicon and titanium in the melt and, in equal measure, on the pressure. However, the paucity of precise data on these parameters add up

to not more than ±40–50 °C to the uncertainty of this estimate (Hayden et al., 2008), whereas thus calculated crystallization temperature of the investigated titanites is within the 770– 820 °C temperature range. On the other hand, numerous results of dating of titanites from metamorphic rocks and assessment of age preservation by their U–Pb system, along with modeling experiments on Pb diffusion in titanite (Cherniak, 2010; Kohn and Corrie, 2011; Scott and St-Onge, 1995; Storey et al., 2007) indicate that the closure temperature of the U–Pb system of titanite is fairly high—770 °C and more. It can thus be expected that the titanite U–Pb dating of alkaline rocks and ores from Khibiny and Lovozero massifs will reflect the time of titanite crystallization from the alkaline melt/ore fluid. SHRIMP-II analysis method for in-situ U–Pb dating of titanite Hand-picked under optical binocular microscope, the titanite grains were mounted in epoxy with reference OLT1 titanite grains (Kennedy et al., 2010). After solidification of the epoxy their surface was polished to about half the thickness of the grains, and gold-coated, to eliminate the excess negative ionic

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N.V. Rodionov et al. / Russian Geology and Geophysics 59 (2018) 962–974 Table 1. U–Th–Pb characteristics of titanite from agpaitic–nepheline syenite whole-rock and ores of the Khibiny and Lovozero complexes Sample No.

Whole-rock

Locality

U–Pb age (Ma) ± 2σ

Mean characteristics

Khibiny massif A-1078

Trachytoid Khibinite, sphene

Upper reaches of the Malaya Belaya R. 381.2±8.2

A-1079

Trachytoid Khibinite, sphene

Upper reaches of the Malaya Belaya R. 367 ± 11

Silicate rocks: Tav = 374.3 ± 9.2 [U] = 1–3 ppm Th/U = 4–11 Pbc = 20–70%

L03-05

Trachytoid foyaite

Kaskasnyuiiok R. valley

367.1 ± 9.9

466/856

K-nepheline syenite

Koashva site

369.8 ± 7.5

301/500

Ijolite

Koashva site

382.0 ± 8.3

50A/83

Apatite urtite

Koashva site

374 ± 9

101A/83

Apatite–nepheline ore

Koashva site

372.2 ± 7.1

1-K

Apatite–nepheline ore

Koashva site

376 ± 10

8-R

Apatite–nepheline ore

Rasvumchorr site

364.8 ± 7.5

22A/81

Titanite–apatite ore

Partomchorr site

376 ± 14

215/817.4

Lujavrite of the differentiated complex

Mt. Alluaiv

379.2 ± 5.3

Silicate rocks: Tav = 379.2 ± 5.3 [U] = 5–11 ppm Th/U = 2–4 Pbc = 46–70%

2015-e1

Apatite–titanite whole-rock

Suluai stream

365 ± 13

2015-e2

Ijolite with titanite and apatite

Suluai stream

365 ± 7.0

2207-K

Ijolite with titanite and apatite

Suluai stream

363.4 ± 6.0

Apatite–titanite ores: Tav = 364.2 ± 4.2 [U] = 4–11 ppm Th/U = 3–7 Pbc = 40–80%

Apatite–nepheline–sphene ores: Tav = 371.4 ± 3.8 [U] = 1–3 ppm Th/U = 4–7 Pbc = 15–60%

Lovozero massif

Note. Tav is weighted average age (Ma), [U] and Th/U are uranium content and the thorium/uranium ratio calculated relative to the reference OLT1 titanite (≈300 ppm), Pbc is percentage (%) of common lead calculated from the 206Pb abundances. Primary U–Th–Pb data are provided in the electronic attachment (https://drive.google.com/file/d/1r-9Eo9SDx2s4oXLEmMGxNCHLnCUDDkM1/view?usp=sharing).

charge produced by the primary high-energy beam of the ion microprobe. Given that titanite as a mineral where distinctive cathodoluminescence is absent, this prevents using CL images for selecting suitable sites for geochronological dating. Therefore, optical images of the studied grains both in the reflected and transmitted light, as well as backscattered electrons (BSE) images, which enable estimations of geochemical heterogeneity of the crystal surface, were taken into account in localizing isotope analysis points. The U/Pb relationships in titanite were measured using high resolution secondary ion microprobe SHRIMP-II at the Center for Isotope Research at Karpinsky Russian Geological Research Institute (VSEGEI), St. Petersburg, employing a method similar to that described in (Kennedy et al., 2010). The primary beam of molecular negatively charged oxygen ions (O−2 ) was adjusted so that the secondary emission intensity was maximal, while the current value was to be 10–15 nA, with the analytical spot (crater) of about 50 microns in diameter. OLT1 titanite whose grains were obtained by disintegration of sphene megacrystal (31 g) from Ca–Ti-skarn (Otter Lake, Quebec, Canada) was used as a reference U–Pb relationship. The measured 206Pb/238U ratios of OLT1 titanite were normalized by 0.1705, which corresponds to the time of titanite crystallization (1015 Ma): 1014.8 ± 2.0 Ma (2σ, n = 6, MSWD = 1.8), and 1016.8 ± 3.8 Ma (MSWD = 3.5, concordance probability: 0.062, n = 87, 4 SHRIMP sessions) (Kennedy et al., 2010). Given that OLT1 titanite is moderately inhomogeneous with respect to the uranium distribution whose

concentrations average between 330 ppm (SHRIMP-II) and 450 ppm (Kennedy et al., 2010), the calculated 238U concentration in the studied titanite samples appears tentative. U contents were calculated by a correlation between the measured 238U16O current intensity and the 40Ca48Ti216O4 reference peak, with the uranium abundancies in the sample subsequently normalized to 300 ppm (the value taken for reference OLT1 titanite). The measured reference CaTi2O+4 current peak intensity representing a function of the matrix chemical composition of unknown mineral and reference standard, is sensitive to changes in electrical conductivity and charge, which results in the increased up to 20% uncertainty of thus calculated uranium and thorium abundancies. When calculating U abundancies (also thorium and radiogenic lead, respectively) in the titanite samples relative to the measured zircon reference sample 91500, the estimated corresponding element abundancies show no more than a two-fold increase comparing to the results obtained with reference OLT1 titanite. Mass spectrometric analysis of OLT1 titanite grains as reference for U–Pb ratios and their concentrations performed in each SHRIMP analytical sessions, showed a slight variance in 206Pb/238U ratio values when the linear regression approach was applied to calibration procedure for correlation ln(Pb/U) vs. ln(UO/U). The measurement error for on average 10 analyses per session was ≤1.5% (2σ), which is comparable to the measurement results for the Temora standard zircon in the conventional U–Th–Pb analyses when the SHRIMP-II zircon dating method was employed. The calculated U concentrations in OLT1 titanite had moderate variance not exceeding 15%

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Fig. 2. Photomicrograph of titanites fraction in the BSE mode (22A/81—Partomchorr site (Khibiny), trachytoid sphene–apatite rock). BSE images and composition analyses (EDX) of representative titanite grains were carried out using a scanning electron microscope CamScan MX2500 (a and b, respectively). Light gray inclusions are strontium apatite (a), variations in the composition of titanites of Lovozero and Khibiny massifs in Fe–Na coordinates; symbols: hollow square—from alkaline rocks of Khibiny massif, black square—from the Khibiny apatite–nepheline ores, hollow circle—from lujavrite of the differentiated complex including lujavrite–foyaites–urtites of Lovozero massif, black circle—of apatite–sphene mineralized ijolite from Lovozero massif (b); and normalized REE patterns of titanites from silicate rocks, and apatite–nepheline ores from Khibiny (1, 2) and Lovozero (3, 4) alkaline massifs (c).

relative to a specified value of 300 ppm. The measured and calculated thorium contents also varied in the same relative range, while Th/U ratio varied from 3 to 3.5. The affinity of titanite for U and Th is known to be significantly less than in the case of such accessory minerals as zircon and monazite (Amelin, 2009; Cherniak, 1993, 2010). Meanwhile, seeing as lead ion Pb2+ can readily enter into the position of bivalent calcium (Ca2+) , since it has a close ion radius (1.26 and 1.04 Å, respectively), titanite is characterized as a rule by the high concentrations of common (nonradiogenic) lead (Frost et al., 2000). The share of common lead thus accounted for 1.3–2.0% for reference OLT1 titanite, whereas a limit of less than 1% for the geochronological standard was established for the U–Pb zircon dating by SHRIMP method. In the OLT1 titanite-based calculations of ages, the measured lead isotopic composition was respectively corrected for the common lead composition and abundances, according to the model for terrestrial lead isotope evolution at the time of titanite for-

mation 1015 Ma: 206Pb/204Pb = 17.0 ± 1, 207Pb/206Pb = 0.910 ± 0.015, 208Pb/206Pb = 2.16 ± 0.1 (Stacey and Kramers, 1975). Prior to each of the in-situ U–Th–Pb analysis, grain surface was subject to ion-rastering for 1 min around the analytical spot. The accumulation of secondary ion pulses for the subsequent calculation of isotopic ratios was carried out on a secondary electron multiplier in a single-collector mode. The registration of isotope ion currents was performed by mass spectrum scanning in the following sequence (in atomic mass units): 40Ca48Ti216O4 (200), 204Pb (204), “background” (204.5), 206Pb (206), 207Pb (207), 208Pb (208), 238U (238), 232 16 Th O (248), 238U16O (254). The selection of the mass spectrometer input/output slit parameters, as well as the adjustment of the secondary ion beam provided a mass resolution of at least 5000 for 0.01 of the UO peak height, which allowed to rule out any isobaric overlaps on the measured masses. For the purpose of statistics

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accumulation and evaluation of analytical errors, each analysis included repeated measurements (5 times per one analysis) for this set of masses, by changing the magnetic field and moving mechanically the collector into the focal plane of the secondary beam. Depending on the assumed age of the sample and amounts of the measured isotopes, the timing of signal accumulation, i.e., pulse counts for each mass stop, was determined and documented individually, while total time of one analysis averaged about 20 minutes. The raw data were processed by SQUID software of version 1.13 (Ludwig, 2000), while the construction of concordia plots for the calculated isotope ratios and age determinations were performed using the ISOPLOT/EX program (Ludwig, 2003, 2009). The errors calculated for concordant ages, as well as those calculated from the “mixing lines” with concordia intercepts, or the weighted average ages, are discussed in the text and listed in Table 1 and the figures either at 2σ level, or for the 95% confidence interval. Given that, unlike zircon, titanite may contain significant amounts of nonradiogenic lead in its composition, both the U–Pb data processing and extracting geochronology information differ greatly from the approaches in mathematical modeling applicable to zirconometry (Levchenkov and Shukolyukov, 1970; Neymark and Levchenkov, 1979; Wendt, 1984). In this case, the assumption that the U–Pb system which has formed at the moment of titanites crystallization was initially concordant and not subject to a postcrystallization disturbance (i.e., partial losses of lead isotopes did not result in discordia), constitutes the primary model hypothesis. The source of nonradiogenic lead which could be either homogeneous or inhomogeneous at the time of titanite crystallization and formation of U–Pb system, however, proved to be uniform for the entire analyzed set of titanite grains. As such, the favorable conditions could be readily realized during crystallization of the studied titanites from Khibiny and Lovozero massifs. On the one hand, Paleozoic magmatism with the formation of alkaline syenite massifs is the most recent manifestation of magmatic activity on the Kola Peninsula, whereas the probability of disturbance of titanites’ isotopic systems under hypergenesis conditions is low. This is evidenced by the good preservation of mineral crystals, showing neither secondary changes nor transformations of grains (SEM data). On the other hand, large magmatic bodies, as Khibiny and Lovozero intrusions, were generated from melts, with their compositions (including isotopic) became homogenized because of the intrachamber mixing and convection processes, while the influence of host rock contamination during the large volumes of alkaline magma intrusions was limited to the contact zone (Arzamastsev et al., 2011). In this case, the formation of the U–Pb system of the array of individual grains of U-containing accessory minerals can be described in terms of the mixing model (Levchenkov and Shukolyukov, 1970; Wendt, 1989; Zheng, 1990). The linear correlations for the data set provided by the mixing model enable verification of the nonradiogenic lead source for its homogeneity, co-crystallization and the U–Pb system closure during the postcrystallization of the analyzed accessories (Storey et al., 2006). The

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absence of such linearity can be explained by many factors, rather than losses (or influx) of radiogenic lead /uranium alone. Seeing as the definition of the linearity “quality” in the data distribution is most critical for appropriate use of the “mixing model”, the mean square of weighted deviation (MSWD) parameter, traditional for linearity evaluation in geochronological constructions, is therefore often complemented by additional statistical criteria: χ-square, maximum similarity, etc. (Ludwig, 1998). In case of low values and high variability of the 206Pb/204Pb ratios, 3D model tend to be preferable, since, besides the age, they allow to estimate variable quantities of nonradiogenic lead whose composition is a priori unknown in a given U–Pb system, and to calculate its composition (Neymark and Levchenkov, 1979; Shukolyukov et al., 1974; Wendt, 1984; Zheng, 1990). Different methods of extraction geochronological information during the U–Pb isotope data processing, including those obtained with the SHRIMP analytical method, are implemented in the Isoplot 3.75 program (Ludwig, 2012); the mathematical processing applied to the calculations is discussed in detail in previous publications (Ireland, 1995, 2004; Ireland and Williams, 2003; Ludwig, 1998, 2003, 2009). Recent research results have shown that the data interpretation procedures applicable to U–Pb data with a high proportion of nonradiogenic lead are largely based on the diversity of methods for estimating numerical age (and assessing the error) (e.g., Aleinikoff et al., 2002, 2004; Cao et al., 2015; Chew et al., 2014; Korh, 2013; Storey et al., 2006; and others), while the advantages of particular methods are fairly thoroughly analyzed in (Ludwig, 1998). We have employed the most common approach, suggesting that the main component of the nonradiogenic lead in the U–Pb isotope system of the studied titanite corresponded to the mid-crustal Pb at the time of titanite crystallization. Then, using the Stacey–Kramers model (Stacey and Kramers, 1975) for determining composition of nonradiogenic component, we calculated titanite ages for all the four groups with application of three basic approaches. When choosing the first and basic method (1) for calculation and interpretation of sample age from the integrated in-situ U–Pb SHRIMP analyses of single titanite grains, as well as their visualization, we opted for constructing 2D version of mixing lines for radiogenic and common (nonradiogenic) components assuming the initial concordance of the U–Pb system of the studied titanite analytical data of which were translated into the Tera–Wasserburg concordia plots (Ludwig, 2003) as measured 207Pb/206Pb and 238U/206Pb ratios, i.e., not corrected for common lead content. Then, based on correlation of the pooled analytical data, the linear trend was calculated, the lower intercept of which with concordia determines the 238 206 U/ Pb dating of the titanite. At this, the isotopic composition (207Pb/206Pb) of the “nonradiogenic lead” component of the U–Pb system of titanite is determined by the calculated linear trend crosscutting the ordinate axis. Therefore, any additional information and affiliated model restrictions for calculating the age of U–Pb isotope system of titanites have been avoided in these constructions.

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The second (2) method for age calculation included the “anchored”, i.e., fixed mixing line, which is to be an intercept point with a priori determined coordinates. As is the case with in-situ geochronology, the 207Pb/206Pb value is specified at the point of the mixing line crossing the ordinate axis, determining thereby parameters of the nonradiogenic Pb component in the U–Pb system of the studied mineral. This value is derived either from the isotopic composition of a syngenetic mineral with a high Pb/U ratio (e.g., plagioclase, galenite, etc.), or from the model estimates of tentative isotopic composition, for example, according to the Stacey–Kramers model of the evolution of the isotopic composition of the Earth’s lead for the age obtained from previous calculations. In our case it was impossible to use a cogenetic mineral with high Pb/U ratio to estimate a nonradiogenic component, inasmuch as the studied collection of titanites was not supplemented with rock-forming and other accessory minerals, and we therefore used the model parameters. Thus, the isotopic composition of common lead dated 360–370 Ma after (Stacey and Kramers, 1975) corresponds to a value of 0.86 for the 207 Pb/206Pb ratio. It should be emphasized that the model parameters of lead composition are essentially inert throughout the Phanerozoic (207Pb/206Pb = 0.87 ± 0.05) proving therefore the exact time of the system formation to be of minor importance for determining model Pb isotopic composition, since this additional fixed point affects neither calculation of linear correlation, nor determination of the lower concordia intercepts. However, it significantly reduces the relative variance (MSWD). The coincidence or proximity of the ages calculated with these two methods is critical, since it corroborates the correctness of the obtained estimate of titanite age. In addition, the use of mixing lines with a fixed point for the nonradiogenic component composition can significantly reduce the error in age determinations, particularly, if variations of the measured isotope ratios are limited to a narrow interval. Only a small part of analyses grains of titanite with relatively high U (10–50 ppm) allows to determine 206Pb*/238U age using the correction procedure for the Pb isotopic composition (Ludwig, 2003) by the measured 204Pb isotope. For most low-U titanites (the first units of ppm and less) and in particular for the studied Paleozoic samples, the correction by the measured 207Pb isotope appears more appropriate, given the assumption on the concordance between ages calculated from two independent radioactive decay systems 238 U → 206Pb and 235U → 207Pb (Williams, 1998). Graphically, this procedure is differentiated from the previous calculation method by projecting the measured isotopic composition of each titanite data-point onto the concordia line by a line passing through this point, and the nonradiogenic component composition (e.g., 207Pb/206Pb = 0.87 ± 0.05 according to the Stacey–Kramers model). Both the errors and the resulting ages will show the greater “scatter” from the value calculated by the second method (2), the farther from the concordia line will be the points of the measured compositions. It stand to reason that the weighted average age calculated from the pooled concordant ages will differ from the result obtained by the second method (Algorithms used by

SQUID-2 (Ludwig, 2009) for mathematical algorithms applicable to the calculations). Besides, this calculation method is the most sensitive to the disturbance of the initial model conditions in all of the three methods applied, which means that in case of the disturbed initial concordance, one titanite grain from the analyzed set is sufficient for the statistical parameters of the calculations to be worse dramatically (i.e., both MSWD and error of the weighted average age estimate are high), and, conversely, given the model conditions for all of the studied titanite grains are fulfilled, the calculated parameters of this method give the lowest errors, MSWD and a high probability of similarity. Specifically, this calculation method (3) based on weighted average age was used for comparing four groups of titanite from statistically representative samples of silicate rocks and apatite ores of Khibiny and Lovozero agpaitic massifs. However, all of these calculation methods and age estimation procedures were used to interpret the U–Pb data on each of the four groups of titanites identified in Khibiny and Lovozero silicate rocks and apatite ores, and the results were compared with estimates obtained using 3D mixing models in those cases where the excess scatter of the analytical points could not be minimized by constructing linear correlations in two-dimensional model. All dated titanite samples from Paleozoic nepheline syenites are a priori younger than the reference OLT1 titanite and are characterized by a significantly lower U and Th contents and have therefore an extremely low content of radiogenic lead, with the proportion of common (nonradiogenic) lead naturally being significantly higher. Obtaining correct geochronological data, given such critical geochemical parameters of the U–Pb system of titanite samples, required additional mass spectrometric measurements, application of the sophisticated calculation methods, and geochronological interpretations of the obtained isotopic results discussed above. However, this study involving a total of 380 in-situ U–Th–Pb SHRIMP-II analyses (20–30 analyses for each sample) for single titanite grains of silicate rocks and apatite–nepheline ores of the agpaitic massifs not only enabled estimation of the age of mineral crystallization but also allowed to extract information about the time of formation of the massifs and ore mineralization.

Results and discussion Age of rocks and apatite–nepheline ores of Khibiny massif Titanite from igneous rocks of the layered complex, represented by samples from the differentiated melteigite– ijolite complex in southwestern part of the massif (samples A-1078, A-1079), foidolite from southeastern part (sample 301/500) and calcite-bearing nepheline syenites (rischorrite) from the SE of the massif and the central foyaite intrusion (L03-05, 466/856), exhibit similar isotope-geochemical characteristics: U concentration not exceeding 2–3 ppm, Th in the range from 10 to 20 ppm, rarely rising to 30–40, Th/U ratio

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varies between 4 and 11, and the proportion of common (nonradiogenic) lead—from the first percent to 60, and in some cases up to 70% (Table 1). At this, the 206Pb radiogenic isotope content is quite low, varying from 0.1 to 0.3 ppm. Geochemical characteristics of these titanites are essentially close: Zr = 2300–4500, Nb = 2700–5600, Hf = 60–130, Ta = 200–700, and Y = 400–600 ppm, with the sum of rare-earth elements up to 14,000–16,000 ppm, (La/Yb)n = 50–100 and weak Ce-anomaly and a distinct Eu-anomaly: Ce/Ce* = 1.2– 1.3, Eu/Eu* = 0.7–1.05 (Fig. 2c). The ages calculated from the U–Pb (SHRIMP-II) analysis of individual titanite samples vary within a narrow range of 367–382 Ma with an error of ±7–11 Ma (Table 1), which suggests synchronous development of the U–Pb system of the studied titanites of the Khibiny igneous rocks. To verify this assumption, we calculated the U–Pb age for the entire set of 121 analyses of five samples of magmatic silicate rocks, based solely on the analytical data dispersion (calculation method 1), obtaining thereby fairly accurate estimate: 379.6 ± 5.8 Ma with the mean square of weighted deviations (MSWD) at the level of 1.06, which was determined by a good linear stretch of the figurative points of analysis along the trend, comprising almost entirely the interval between the lower and upper concordia intercepts (Fig. 3a). Out of all the analyses conducted, two involved U contents less than 1 ppm, allowing to determine fairly accurately the upper point of the calculated trend intersecting with the ordinate (0.870 ± 0.013), which is different from the accepted 207Pb/206Pb value of 0.86 for the age corresponding to 360–380 Ma (according to the Stacey–Kramers model), probably due to a more radiogenic lead-composition of the host crustal rocks. In calculations of the fixed mixing line (method 2), the determined age had a lower total error: 375.9 ± 3.5 Ma, being actually slightly younger and showing higher MSWD values (1.1). The calculation of the 206Pb*/238U weighted average age with correction for the content and composition of common lead by the measured 207Pb (calculation method 3) gives the value of 374.1 ± 3.7 Ma (MSWD = 1.04, probability = 0.36 except for two analyses with minimum U content), which is further accepted as the most accurate estimate of the age of titanite crystallization in the igneous rocks of Khibiny alkaline agpaitic syenites massif. A remarkable linearity of the plotted analytical points of this array and the value of data stretch along the trend are obviously determined by the formation of U–Pb system of these titanites by mixing only two components (radiogenic and nonradiogenic). This is also evidenced by the calculation of the linear regression of the 3D model, with the concordia intercept corresponding to the age of 379 ± 12 Ma at MSWD = 4.0 and “zero” compliance probability, by the calculated composition of the nonradiogenic component with 206Pb/204Pb = 18.6 ± 1.2 and 207Pb/204Pb = 16.11 ± 0.95 (207Pb/206Pb = 0.8661), and by the Stacey–Kramers evolution curve for the Pb isotopic composition intersected by the regression line at the age corresponding to 942 Ma and µ = 12. The planar regression (Ludwig, 2009; Wendt, 1989) no solution for 3D model and specific analytical dataset. In case the fixed mixing line (calculation method 2) for all 121 analyses with the

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nonradiogenic component parameters determined from a 3D model, i.e., 207Pb/206Pb = 0.8661, is applied to age calculation, the age corresponding to the concordia intercept will be 378.1 ± 3.5 at MSWD = 1.05 and probability = 0.33. The values for the mixing line parameters are comparable to those obtained by the calculation method 3, which can be viewed as an extra argument in favor of the correctness of the obtained age estimate. Both calculation and plotting of mixing lines for titanites of each sample representing apatite–nepheline ore of Khibiny massif have revealed that the titanite ages vary in the range from 365 to 376 Ma, regardless of the samples locality and ore type (Table 1). The geochemical characteristics of the studied samples of titanite grains provide strong evidence of their affinity described as: slightly varying HFSE content: Zr = 2000–2400, Nb = 1400–1700, Hf = 30–60, Ta = 120–170 ppm, practically unchanged Y abundance from sample to sample, remaining within 270–300 ppm, and REE sum within 3300–5400 ppm with a sufficiently flattened normalized REE pattern (Fig. 2c): (La/Yb)n = 35–64; the absence of Ce-anomaly and the presence of negative Eu-anomaly: Ce/ Ce* = 1.01–1.07, Eu/Eu* = 0.84–0.96; similarly to the U–Pb system characteristics, U content is consistently low: 1–2 ppm (in individual grains up to 3, and sporadically up to 4 ppm), Th/U is within 5–7; the proportion of common lead varying widely, but usually significantly less than 60% (Table 1). This suggests a single origin of the studied titanites in ores, which is indirectly corroborated by a correlation between all the measured U–Pb ratios in 120 analyses on a unified Tera–Wasserburg linear trend 238U/206Pb–207Pb/206Pb (Fig. 3b). Despite the relatively high errors of the measured 238 206 U/ Pb ratio due to the low U content in these titanites, all the analytical data in Fig. 3b form a compact pool, which determines the isochrone crossing the concordia intercept corresponding to the age 368 ± 10 Ma at MSWD = 1.8 (“zero” probability). The calculation of the weighted average age for 206 Pb*/238U with the measured Pb isotopic composition corrected for the common lead by the measured isotope 207Pb content results the same value within error, but significantly lower total error of 371.1 ± 4.2 Ma (95% confidence interval, MSWD = 1.7) for a total of 120 analyses. It should be noted that calculation of parameters of the mixing line with a nonradiogenic component having an isotopic ratio of 207 Pb/206Pb equal to 0.86 (calculation method 2), yields almost identical age estimate: 374.6 ± 4.2 Ma with a slightly higher MSWD = 1.8 increasing MSWD values in all the mentioned variants of age calculations, may indicate a more complex composition (multicomponent mixing) of the U–Pb system of titanites of the Khibiny apatite–nepheline ore. As is the case with the above, the application of the age calculations is most effective in 3D space: 238U/206Pb–207Pb/206Pb–204Pb/206Pb (Wendt, 1989; Ludwig, 1998). However, the linear regression calculations from 120 analyses in 3D space resulted in a solution identical to the 2D space: 368 ± 10 Ma with MSWD equal to 1.7, probability = 0, with the Pb isotopic composition of the nonradiogenic component equal to: 206Pb/204Pb = 14.5 ± 1.1 and 207Pb/204Pb = 11.96 ± 0.59

Fig. 3. Tera–Wasserburg concordia diagrams, not common lead- corrected for the studied titanite of Khibiny alkaline massif. a, Titanite from the polyphase differentiated ijolite-urtite complex: T = 379.6 ± 5.8 Ma, MSWD = 1.06, n = 121; b, titanite from apatite–nepheline ore varieties: T = 368 ± 10, MSWD = 1.8, n = 120. The size of error ellipses of figurative points on the diagrams corresponds to the 2σ value. The represented typical grains from the studied titanite samples are 200–400 µ m in size, observed by transmitted optic microscopy.

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(207Pb/206Pb = 0.8248). Note that the planar regression, as in the above case, has no solution. Recalculation of the fixed mixing line with account of the nonradiogenic component composition derived from the 3D calculation of the previously discussed linear regression can significantly reduce the error in determining the concordia– mixing line intercept corresponding to 368 ± 4.2 Ma, but the spread of analytical points relative to this line remaining nevertheless significant: MSWD = 1.8 (probability = 0). As already mentioned, the excess scatter of analytical points for these titanites is determined by the extremely low U contents (up to 1 ppm), which is impossible to countervail neither by constructing linear correlations in two and three-dimensional space, nor by planar correlations. Anyway, when one analysis with the lowest U is removed from the calculations, the weighted average age calculated with respect to 206Pb*/238U provides the most steady-state estimate of 370.0 ± 3.8 Ma with MSWD = 1.4 and with probability of accordance slightly different from zero (0.005). Given that this estimate is consistent with all variants of age calculations within the error limits, it has the best statistical parameters and is therefore taken by us for the crystallization age of this titanite. When compared, the obtained average age estimates for titanite of the Khibiny silicate rocks and phosphate ores: 374.1 ± 3.7 and 370 ± 3.8 Ma, respectively, attest to their subsynchronous character, and given the principal affinity between the geochemical parameters of titanite (the observed variations, as was mentioned above, are determined by the co-crystallization of apatite and titanite in the ores), this may indicate a genetic relationship between the apatite–nepheline mineralization in Khibiny and agpaitic melts, which supports the previous inference (Arzamastsev and Wu, 2014). Age of lujavrite and titanite mineralization in Lovozero massif The analyzed dark brown, large (up to 350 µm and more) fragments of titanite grains and crystals from lujavrite of the differentiated complex in the area of Alluaiv Mountain are characterized by high U (from 5 to 12 ppm) and Th (40 ppm) contents, fairly constant Th/U ratio (2.2–3.2) and significantly larger vs. the studied titanite of Khibiny massif, proportion of nonradiogenuic lead, which varied in the range of 50–70%, while the radiogenic 206Pb content did not not exceed 1 ppm (Table 1). The homogeneity of the titanite composition was reflected in its U–Pb isotopic system (Fig. 4b) where the array of figurative points for 52 analyses is set out as a compact group in the middle of the U–Pb plot considerably distanced from the concordia, providing thereby major uncertainties both for plotting and age interpretation. The observed analytical dispersion of U–Pb ratios allows calculating and constructing a linear trend corresponding to the age of 383 ± 13 Ma and MSWD equal to 1.1 (calculation method 1), whereas the plotted mixing line under the assumption of the component mixing with 207Pb/206Pb equal to 0.86 (calculation method 2) has determined a significantly younger age (359.8 ± 3.6 Ma) and elevated MSWD value (1.3).

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The calculation of the weighted average 206Pb*/238U age when the isotopic composition is corrected for measured 207Pb isotope (calculation method 3) for all 52 analyses yielded the age estimate at 378.3 ± 5.7 Ma at higher MSWD (1.6) and lower probability = 0.003. If 5 analyses with maximal individual errors are excluded from the calculations, the age determined from 47 analyses will be at 380.7 ± 4.5 Ma (1.2%, 2σ) with the best statistical parameters: MSWD = 1.13 and probability = 0.25, which can be interpreted as the time of formation of the isotopic system and crystallization of titanite prompted by the lujavrite intrusion of the differentiated complex of Lovozero massif. The attempt to calculate the linear regression for this set of analytical data in 3D space resulted in age estimate at 380 ± 17 Ma and MSWD = 1.7 (probability = 0), with the Pb composition of the nonradiogenic component 206Pb/204Pb = 17.67 ± 0.74 and 207Pb/ 204 Pb = 15.78 ± 0.41 (207Pb/206Pb = 0.8930), crossing the Stacey–Kramers model curve at the age point corresponding to 1022 Ma and µ = 10.81, which in terms of its statistics is by far worse than the calculated correlations for the 2D model. However, the calculation of the mixing line with nonradiogenic component having identical characteristics (207Pb/ 206 Pb = 0.8930) allows to obtain the age estimate at 379.4 ± 3.9 Ma (95% confidence interval) at MSWD = 0.89 and probability = 0.69, which approximates the calculation results of the third method, and was therefore taken for the best estimate of the age of this set of titanites. Whilst, in comparison with the other three groups of analyses, the isotopic composition of this set is more homogeneous, given that it is represented by titanite grains of one sample, which actually ruled out the planar regression calculations for the 3D model. The integrated mixing lines (calculation methods 2 and 3) were calculated and the U–Pb dates were determined from in total 88 measurements for three titanite samples: 2015-e1, 2015-e2 and 2207-K of apatite–sphene mineralized ijolites from the eastern part of the massif (Fig. 1). Fig. 4a shows figurative points for the performed analyses of three titanite samples which form a single linear trend in the Tera–Wasserburg diagram, allowing to calculate 238U/206Pb age at the concordia intercept with the following statistical parameters: plot-derived MSWD equals 1.6 (with only analytical data dispersion taken into account), the error is ± 8.7 Ma (2.4%, 2σ). Analysis of the geochronological dating results obtained by different approaches provided the comparisons between: 366.1 ± 8.7 and 361.5 ± 3.4 Ma (at 207Pb/206Pb = 0.86 for common lead captured during crystallization; MSWD = 1.6, probability = 0.13), and enabled us to more accurately represent the weighted average 206Pb*/238U age of 361.4 ± 3.2 Ma (0.9%, 2σ, MSWD = 1.2, probability = 0.13) and interpret it as the time of titanite crystallization from the REE-enriched ore fluid and the formation of mineralized ijolite at Suluai site of the Lovozero massif of agpaitic nepheline syenites. The linear regression calculation for the 3D model determines the timing of the U–Pb system formation in titanites from this group: 364.7 ± 8.6 Ma at MSWD = 1.6 (probability = 0) and parameters: 206Pb/204Pb = 17.59 ± 0.50 and 207Pb/204Pb = 15.25 ± 0.32 (207Pb/206Pb = 0.86697) of the nonradiogenic

Fig. 4. Tera–Wasserburg concordia diagrams, not common lead-corrected for the studied titanite of Lovozero alkaline massif. a, Titanite from lujavrites of the differentiated complex: T = 383 ± 13 Ma, MSWD = 1.1, n = 52; b, titanite from apatite–sphene mineralized ijolite: T = 366.1 ± 8.7 Ma, MSWD = 1.6, n = 88. The size of error ellipses of figurative points on the diagrams corresponds to the 2σ value. The represented typical grains from the studied titanite samples are 250–350 µ m in size, observed by transmitted light microscopy.

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component. The Stacey–Kramers curve intersects with this linear regression at a point with age corresponding to 24 Ma and µ = 8.34. Geochemical characteristics of the studied titanites from the whole-rock of the differentiated complex and mineralized ijolites are inferred to be close due to: the HFSE contents: Zr = 3050 and 5000, Nb = 6700 and 6000, Hf = 114 and 129, Ta = 850 and 600 ppm, respectively, Y contents: 750 and 550, and the REE abundances: 12,400 and 13,000 ppm (Fig. 2c), as well as parameters of the U–Th–Pb system of these titanites include U contents: 5 and 10 ppm, the Th/U ratio: within 2–4 and 3–7, respectively, and the common lead proportion: 40 and 70% (Table 1). All of these indicate a possible proximity of the titanites origin, associated with a single source of substance such as magmatic system or melt–fluid. However, the time interval spanning 20 Ma, significantly exceeding the analytical error in determining ages, between the formation of titanite of the silicate rocks of the complex and apatite–titanite ores, as well as the spartial separation of the sampling points (10–15 km from the west to the east of the massif) and the uncertainty of the origin of mineralized ijolite, do not allow us to speak about a direct relationship between these geological events. Nevertheless, it is highly possible that the magmatic chamber of Lovozero massif was active for a appreciable period of time (at least within 20 Ma) and its hydrothermal-metasomatic activation appears to have proceeded repeatedly, beginning from the formation of the main complex of agpaitic rocks (379.4 ± 4 Ma) and through the time of formation of later titanite (sphene) ores and mineralized ijolites (361 ± 3 Ma) and microcline-albite veins with ilmenite and zircon (359 ± 5 Ma) (Arzamastsev et al., 2007). Our sample collection was lacking the titanite from rocks of the alkaline–ultrabasic phase of Khibiny and Lovozero massifs, which, in itself, represents the earliest magmatic derivative of magmatic chamber of agpaitic massifs and therefore its age determination is beyond the scope of this research. However, the time span of the formation of alkaline nepheline syenites of Lovozero massif is almost entirely overlapped with the known age estimates for these rocks: 379 ± 4 and 383 ± 7 Ma, respectively (Arzamastsev and Wu, 2014). This bears fairly clear evidence of the relative shortterm stage of the formation from magma of Paleozoic intrusions of Lovozero and Khibiny massifs, with the intruding thereby alkaline–ultrabasic melts and the major volumes of agpaitic nepheline syenites within the time span of 5–7 Ma. At this, most of the melts of the differentiated complex and nepheline syenites, accompanied by the formation of phosphate ores and carbonatites of Khibiny massif, was formed at the later stage of activity dated 374–370 Ma. Note that in this study, the dating of titanites of magmatic rocks and apatite– rare-metal ores of Lovozero and Khibiny massifs are generally consistent with the previous ages obtained with various geochronological methods (Arzamastsev and Wu, 2014; Arzamastsev et al., 2007; Kogarko et al., 2010; Kramm and Kogarko, 1994; Kramm et al., 1993; Zartman and Kogarko, 2014) and range between 380 and 360 Ma, indicating the

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ore–magmatic system activity of these massifs over a time span of 20 million years.

Conclusions The U–Pb system of titanite (sphene), being resistant in the conditions of fluid-saturated alkaline–ultrabasic melts allows in-situ U–Pb isotope dating of Devonian polyphase massifs (Kola Peninsula), despite low U concentrations (1–10 ppm), using high resolution secondary ion microprobe SHRIMP-II with an error of less than 1.5% (2σ). The nonradiogenic component in the U–Pb system of titanites of silicate rocks and phosphate ores of Khibiny and Lovozero alkaline syenite massifs can reach 60–70% or more, while the isotopic composition (207Pb/206Pb) of this component varies within the range from 0.824 to 0.893, sometimes differing significantly from the model values (Stacey–Kramers curve). Higher ratios characterize the Lovozero titanites component whose higher radiogenic Pb compositions are typical of titanites of silicate rocks. The age of the agpaitic nepheline–syenite intrusion of Khibiny massif, according to U–Pb dating of titanite, is 374.1 ± 3.7 Ma, which is found almost coeval with apatite– nepheline ore mineralization (370.0 ± 3.8 Ma), while the cristallization time of the intrusion of the differentiated complex of lujavrites–foyaites–urtites of Lovozero massif is estimated at 379.4 ± 3.9 Ma, and the onset of the ore mineralization is dated 361.4 ± 3.2 Ma. The obtained estimates of titanite age allowed to significantly constrain the time interval of the evolution of major agpaitic complexes of Lovozero and Khibiny, which probably did not exceed a time span of 20 Ma, however the activity of the ore-magmatic system may have had a pulsation pattern. The work was supported by the Federal State Agency on Mineral Resources “Rosnedra” within the framework of the state-commissioned contract No. K.41.2014.014 and by St. Petersburg State University (grant 3.38.224.2015). The authors would like to express their gratitude to the SHRIMP-laboratory and Dr. Allen Kennedy, its representative, in person (Curtin University, Perth, Australia) for the provided reference OLT1 titanite, without which we would not be able to obtain such accurate and reliable age estimates.

References Aleinikoff, J.N., Wintsch, R.P., Fanning, C.M., Dorias, M.J., 2002. U–Pb geochronology zircon and polygenetic titanite from the Glastobury Complex, Connecticut, USA: an integrated SEM, EMPA, TIMS, and SHRIMP study. Chem. Geol. 188, 125–147. Aleinikoff, J.N., Horton, J.W., Drake, A.A., Wintsch, R.P., Fanning, C.M., Yi, K., 2004. Deciphering multiple Mesoproterozoic and Paleozoic events recorded in zircon and titanite from the Baltomore Gneiss, Maryland: SEM imaging, SHRIMP U–Pb geochronology, and EMP analysis. GSA Memoirs 197, 411–434. Amelin, Yu.V., 2009. Sm–Nd and U–Pb systematics of single titanite grains. Chem. Geol. 261(1), 53–61. Arzamastsev, A.A., Arzamastseva, L.V., 2013. Geochemical indicators of the ultrabasic-alkaline series of Paleozoic massifs of the Baltic shield. Petrologiya 21 (3), 277–308.

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Editorial responsibility: A.E. Izokh