The Oldest Grey Gneisses and Tonalite-Trondhjemite Granodiorites in the Fennoscandian Shield: ID-TIMS and SHRIMP Data

Genesis of the oldest continental crust retains a marked trace in the Earth’s evolution over its 4.5 Ga history. Despite ample isotope data on the role of the continental crust in the Earth’s evolution, there has been much debate on the origin of grey gneisses and tonalite-trondhjemite-granodiorites (TTG). Precise U-Pb (ID-TIMS) and SHRIMP data on single zircon for paragneisses and TTG (3158.2 ± 8.2 Ma) have indicated the Central-Kola and Belomorian (White Sea) megablocks of the Fennoscandian Shield to be 3.16 Ga and 3.70 Ga, respectively. The newly obtained ages of zircon from these megablocks indicate the origin of the discrete continental crust to be 3.16 and 3.70 Ga. It is close to the Nordsim zircon data on the Siurua TTG (Finland), which are 3.45 and 3.73 Ga in the core. The new summarized data on the Earth’s oldest rocks (basement and continental crust) indicate the younger age of the rocks in the Fennoscandian Shield as compared to those in Australia (Kronendonk et al., 2019).


Introduction
The oldest 35 pieces of the continental crust mostly composed of tonalit-

Materials and Methods
Zircons have been separated from gneiss samples with the weight of about 30 kg. The U-Pb (ID-TIMS) dating of single grains has been carried out using the method described by Krogh [12] with an artificial 205 Pb spike; the results are shown in Table 1. Following this method, the samples have been dissolved in strong (48%) hydrofluoric acid at a temperature of 205˚C -210˚C during 1 to 10 days. To dissolve the fluorides, the samples have been reacted with 3.1 N HCl at a temperature of 130˚C for 8 to 10 hours. To determine the isotope composition of lead and to measure the concentrations of lead and uranium, the sample has been divided into two aliquots in 3.1 N HCl, and a mixed Pb + U tracer has been added. Pb and U have been separated on an AG 1 × 8, 200 -400 mesh anion exchanger in Teflon columns. The laboratory blank for the whole analysis is 0.1 -0.08 ng for Pb and 0.01 -0.04 ng for U. All isotopic determinations for zircons have been made on a Finnigan MAT-262 mass spectrometer; the Pb isotopic composition has been analyzed on a secondary-ion multiplier in an ion-counting mode. The measurements of the Pb isotopic composition are accurate to 0.025% when calibrated against the NBS SRM-981 standards. The U and Pb concentrations have been measured in a single-filament mode with added H 3 PO 4 and silica gel using the method described by Scharer and Gower [13] and Scharer et al. [14]. Pb and U concentrations have been measured within the temperature ranges of 1350 -1450 and 1450˚C -1550˚C, respectively. All the isotopic ratios are corrected for mass discrimination during the static processing of replicate analyses of the SRM-981 standard (0.12% + 0.04% per a.m.u.). The errors in the U-Pb ratios are calculated during the statistical treatment of replicate analyses of the IGFM-87 standard and are assumed equal to 0.5%. If the actual analytical errors are higher, they are reported in the table of isotopic data. Isochrons and sample points have been calculated using the Squid and Isoplot programs [15] [16].
Age values have been calculated with conventional decay constants for U [17], all errors are reported at a 2-sigma level. Corrections for common Pb are made according to Stacey and Kramers [18]. Besides, corrections are made for the composition of Pb separated from syngenetic plagioclase or microcline if the admixture of common Pb is >10% of the overall Pb concentration and the The zircon fraction from the same gneisses has been analyzed using the SHRIMP method described by Williams et al. [19] at the A.P. Karpinsky Russian Geological Research Institute (VSEGEI); the results are given in Table 2 and Figure 6(b). All geochemical REE and trace element analyses of the whole rock have been made at the Institute of Geology and Mineralogy, Siberian Branch RAS, Novosibirsk, using the method described by Panteeva et al. [20].
Plagioclase occurs as colorless isometric and usually not twinned grains. Some of them are slightly sericitized. Quartz appears in small irregular grains (0.2 -0.6 mm) and also forms large lenses up to 3.5 mm in length (Figure 3(b)). Biotite is found in tabular or flaky 0.05 -0.90 mm grains uniformly scattered throughout the rock. Biotite may also intergrow with garnet ( Figure 3(a)). Garnet occurs as large porphyroblasts (2 -3 mm) with irregular outlines and encloses quartz and biotite inclusions. Some of the garnet cracks are filled with fine-grained mica. Sillimanite forms colorless elongated prismatic grains with a length of up to 0.6 mm; fine-grained fibrolites are less common (Figure 3(d)). Kyanite grains include quartz, which has an elongated prismatic (0.3 -1.1 mm) shape with irregular outlines. Staurolite is represented by sporadic elongated prismatic grains with a length of up to 0.5 mm (Figure 3(c)).

Isotope ID-TIMS and SHRIMP Data
Gneisses were sampled from the vast outcrops of the Central-Kola megablock to determine the age of a submeridional basite dyke, which cuts the complex of high-alumina paragneisses (Figure 1(b) and Figure 2).
According to the IUGS TAS plot, gneisses comply with the dacite field of tholeitic series (Table 3; Figure 4). The gneiss samples are rich in LREEs and poor in HREEs (Table 4; Figure 5), which conforms to typical TTG patterns according to Moyen and Martin [21].  (Table 1) from the oldest population.
Hand-picked grains have magmatic oscillatory zoning and a diagnostic core with older ages.
All SHRIMP zircon ages for the high-alumina gneisses with different peaks and intervals of origin at 2794 -2763 Ma are similar to the ID-TIMS data of amphibolite metamorphism (2753 ± 3 Ma). The data in the range of 2924 -2886 Ma seem to reflect the low-granulite facies metamorphism. One point coinciding with the concordant age of 3695 ± 5 Ma represents the oldest age of the studied gneisses ( Figure 6(b) and Figure 7(a)). Coeval SIMS and LA-ICP-MS isotope measurements have been carried out for the Finnish part of the Fennoscandian Shield (Figure 7(b)). In result, the age of 3.45 Ga has been obtained for the rims and the age of 3.73 Ga has been obtained for the core parts of the zircon from      Table 3 and Table 4.
the Siurua TTG complex [7]. The U-Pb and Sm-Nd data [24] (Figure 7(b)) are found close to the U-Pb (ID-TIMS) and SHRIMP zircon data on the Central-Kola megablock.

Discussion
The new U-Pb (ID-TIMS) data on single zircons from paragneisses of the Central-Kola megablock show the age of 3.17 Ga. The cores of these zircons have the age of 3695 ± 5 Ma (SHRIMP-II) and ca. 100 Ma older ages as compared to paragneisses of the Central-Kola megablock, according to Myskova et al. [6] (Figure   7(a)). The age of the amphibolite facies metamorphism has been estimated at 2753 ± 3 Ma. The Voche-Lambina geological site lies at the boundary between the Belomorian mobile block and the Central-Kola domain [5]. New Neoarchean U-Pb data on single zircons of the Voche-Lambina TTG have yielded the age of 3158.2 ± 8.2 Ma. The zircon has low U and Pb concentration and the low U/Th ratio of 0.2. REE plots for grey gneisses show high fractionation of La/Yb > 30 riched in LREE and poor in heavy Yb (<0.6 ppm). The precise (ID-TIMS) single zircon age of the amphibolite facies metamorphism has been estimated at 2704.3 ± 5.9 Ma. Model Sm-Nd WR data indicate the protolith ages of 3.4 to 3.2 Ga, positive εNd of +1.29 to +3.3 and ISr of 0.702 [5].
Thus, the new data on single zircon grains from TTG and paragneisses of the Central-Kola megablock imply a long-term and discrete evolution of the continental crust in the Fennoscandian Shield (3.17 -3.73 Ga). Noteworthy, the oldest part of the Hadean component is well-preserved in zircons that were subject to high-pressure metamorphism in Finland and granulite facies metamorphism [25] in Siberia (Aldan Shield, Russia). The latter has been dated using the ID-TIMS Figure 7. SHRIMP-II and ID-TIMS data (see Table 1) on zircon grains from Archean rocks of East Fennoscandia (a) and U-Pb and Sm-Nd ages of rocks from Finland (b). The red line indicates the primary rock based on results for 990 samples (data on 740 of them are published). The green line indicates the age distribution of detrital zircons in Paleoproterozoic metasediments based on 1936 U-Pb analyses made by SIMS (ca. 1000 published) and LA-MC-ICPMS (unpublished). Image of ca. 3.5 Ga zircon from the oldest rock in Siurua (Finland) with an older core of ca. 3.73 Ga [7]. method; the single zircon age has been estimated at 3.94 Ga.

Conclusion
For the first time, zircons from the Central-Kola megablock have been dated using the SHRIMP method. As a result, the oldest age of 3.7 Ga has been obtained. Taking into account the age of Siurua TTG, Finland (3.73 Ga), these results suggest the presence of older (Hadean) rocks with zircon ages of >4.0 Ga. It will also allow specifying the age of the continental crust, which provides the basement necessary for the formation of the regional deposits, such as the Neoarchean BIF in the Olenegorsk ore area, Paleoproterozoic PGE-Cu-Ni and PGE-Cr-Ti deposits (Monchegorsk, Fedorovo-Pana and Imandra ore areas) and Cu-Ni deposits (Pechenga, as well as large-scale Proterozoic apatite-nepheline and phosphorite deposits (Khibiny, Lovozero, Kovdor, etc.).