Zircons from a Pegmatite Cutting Eclogite (Gridino, Belomorian Mobile Belt): U-Pb-O and Trace Element Constraints on Eclogite Metamorphism and Fluid Activity

This report presents new data on U-Pb geochronology, oxygen isotopes, and trace element composition of zircon from a pegmatite vein crosscutting an eclogite boudin on Stolbikha Island, Gridino area, Belomorian mobile belt (BMB). The zircon grains occur as two distinct populations. The predominant population is pegmatitic and shows dark cathodoluminescence (CL); about a third of this population contains inherited cores. The second zircon population is typical of granulite and exhibits a well-defined sectorial (mosaic) zoning in CL. Both the inherited cores and sectorial in CL zircons appear to have been captured from metabasites as xenocrysts during the pegmatite vein formation. A U-Pb age of 1890 ± 2 Ma for the main zircon population is interpreted as the age of the pegmatite injection. This value is close to the age threshold for the BMB eclogites (~1.9 Ga) and unambiguously defines the upper age limit for the eclogite metamorphism. The pegmatite formation is thus related to partial melting events that occurred during the retrograde amphibolite-facies metamorphism shortly after the eclogitization. A U-Pb date of 2743 ± 10 Ma obtained for the sectorial in CL zircons is considered as the age of the granulite-facies metamorphism established previously within the BMB. The values of δ18O in the zircon populations overlap in a broad range, i.e., δ18O in the pegmatitic zircons varies from 6.1‰ to 8.3‰, inherited cores show a generally higher δ18O of 6.7–8.8‰, and in the captured granulitic zircons δ18O is 6.2–7.9‰. As a result of fluid attack during the final stage of the pegmatite vein formation, the composition of the pegmatitic zircons in terms of non-formula elements (REE, Y, Ca, Sr, Ti) has become anomalous, with the content of these elements having been increased by more than tenfold in the alteration zones. Our data provide new constraints on the timing of eclogite metamorphism within the BMB and show that the late-stage pegmatite-related fluids exerted a very pronounced influence on trace element abundances in zircon, yet had no significant impact on the isotopic composition of oxygen.

Thermodynamic modeling of the formation of corundum-bearing rocks within the Belomorian mobile belt using Perple_x software

<p>More than a dozen deposits of corundum-bearing rocks are known within the Belomorian mobile belt (references in Serebryakov, Rusinov, 2004); their genesis remains debatable. Some authors consider corundum-bearing rocks to be normal metamorphic rocks (for example, Lebedev et al., 1974), others suggest the metasomatic genesis of rocks with corundum: 1 – corundum-bearing rocks were formed as a result of high-temperature high-pressure (600 - 700ºC, 7 - 8 kbar) metasomatism which was accompanied by desilification and the introduction of Ca and Na (Serebryakov, Rusinov, 2004); 2 – these rocks are a product of hydrothermal alteration of gneisses by fluids associated with basic intrusions (Bindeman et al., 2014). All these assumptions were made without a detailed physicochemical analysis of the mineral parageneses of corundum-bearing rocks.<br>The Perple_X software package (Connolly, 2005) is discussed in some recent works as an effective tool for the thermodynamic modeling of the open systems (Goncalves et al., 2012, Manning, 2013). Using the Perple_X software package (version Perple_X 6.8.7, updated 04.07.2019) we constructed P-T, T-μ (SiO<sub>2</sub>), and μ(SiO<sub>2</sub>)-μ(Na<sub>2</sub>O) pseudosections for a given chemical composition of kyanite-garnet-biotite gneiss of the Chupa sequence. The hp02ver.dat thermodynamic database was used, the diagram μ(SiO<sub>2</sub>) - μ(Na<sub>2</sub>O) was calculated for P = 8 kbar, T = 650ºC, in the presence of a carbonic-aqueous fluid with X(CO<sub>2</sub>) = 0.3. Selected solid solution models are Ca-Amph(D) for hornblende, Gt(HP) for garnet, St(HP) for staurolite, Bi(HGP) for biotite, feldspar for feldspar, Sp(HP) for spinel.<br>The results show that the majority of corundum-bearing rocks varieties (amphibole-free corundum-bearing rock, amphibole-bearing rock with corundum, altered quartz-free kyanite-garnet-biotite gneiss, kyanite-garnet amphibolite) could be formed by metasomatic alteration of kyanite-garnet-biotite gneisses of the Chupa sequence. This process was characterized by a significant decrease in µ(SiO<sub>2</sub>) and a slight increase in µ(Na<sub>2</sub>O). Our conclusion is partly consistent with the hypothesis that corundum-bearing rocks were formed as a result of metasomatism, which was accompanied by desilification of Ky-Grt-Bt gneisses and the introduction of Na and Ca (Serebryakov, Rusinov, 2004).</p><p>The study was conducted according to the IPGG project 0153-2019-0004.</p><p>Bindeman I.N., Serebryakov N.S., Schmitt A.K. et al. (2014) Field and microanalytical isotopic investigation of ultradepleted in <sup>18</sup>O Paleoproterozoic “Slushball Earth” rocks from Karelia, Russia. Geosphere. V. 10. P. 308-339.</p><p>Connolly J.A.D. (2005) Computation of phase equilibria by linear programming: A tool for geodynamic modeling and its application to subduction zone decarbonation.  Earth and Planetary Science Letters, 236, p. 524–541.</p><p>Goncalves P., Oliot E., Marquer D., Connolly J.A.D. (2012) Role of chemical processes on shear zone formation: an example from the Grimsel metagranodiorite (Aar massif, Central Alps). J. metamorphic Geol., 30, p. 703–722.</p><p>Lebedev V.I., Kalmykova N.A. & Nagaytsev Yu.V. (1976) Corundum-staurolite-hornblende schists of the Belomorskiy complex, International Geology Review, 18:6, 653-662.</p><p>Manning C.E. (2013) Thermodynamic modeling of fluid-rock interaction at conditions of the earth's middle crust to upper mantle. Reviews in Mineralogy & Geochemistry, 76, p. 135-164.</p><p>Serebryakov, N.S., Rusinov, V.L. (2004) High-T high-pressure Ca, Na metasomatism and formation of corundum in the precambrian Belomorian mobile belt. Dokl. Earth Sci. 395, pp. 549–533.</p>

Geochemical Features and Geological Processes Timescale of the Achaean TTG Complexes of the Ingozero Massif and the Pechenga Frame (NE Baltic Shield)

This article provides a geological review and results of the structural, metamorphic, and geochronological studies of the Pechenga frame outcrops located in the NW part of the Central-Kola terrain and the Ingozero massif outcrops situated in the northeastern part of the Belomorian mobile belt of the Kola Region (NW Baltic Shield). As a result of the work, the deformation scales and ages of the geological processes at the Neo-Archaean–Paleoproterozoic stage of the area’s development were compiled, and the reference rocks were dated. The petrochemical and geochemical characteristics of the Ingozero rocks are similar to those of tonalite–trondhjemite–granodiorite (TTG) complexes established on other Archaean shields. The isotope U–Pb dating of individual zircon grains from the biotite gneisses provided the oldest age for magmatic protolith of the Ingozero gneisses, which is 3149 ± 46 Ma. Sm–Nd model ages showed that the gneisses protolite initial melt formed at 3.1–2.8 Ga. Ages of metamorphic processes were determined by using isotope U–Pb dating ID TIMS (isotope dilution thermal ionization mass spectrometry): Biotite gneisses—2697 ± 9 Ma; amphibole–biotite gneisses—2725 ± 2 Ma and 2667 ± 7 Ma; and biotite–amphibole gneisses 2727 ± 5 Ma. Ages of granitoids, which cut the deformed gneisses, are 2615 ± 8 Ma and 2549 ± 31 Ma for plagiogranites and pegmatoid veins in gneisses, respectively. The following age sequence of geological processes was established by using U–Pb zircon dating: 2.8 Ga—The time of the garnet–biotite gneiss metamorphism; 2722 ± 9 Ma—The granodiorite crystallization time; 2636 ± 41 Ma—The aplite emplacement age and 2620 ± 16 Ma—The age of pegmatites origin, which marked final stages of the Archaean evolution; 2587 ± 5 Ma—The age of gabbros emplacement and 2507 ± 7 Ma—The age of gabbros metamorphism; 2522–2503 Ma—The origin time of the iron quartzite interpreted as the age of gabbros and biotite gneiss metamorphism.

First data on yatulian (2.1 Ga) metamorphism in the Belomorian mobile belt

The Sm-Nd isotope ages of apoamphibolite eclogites and eclogite-like garnet-clinopyroxene crystalline schists collected in the Chupa and Engozero segments of the Belomorian mobile belt are estimated. Eclogites and eclogite-like garnet-clinopyroxene rocks demonstrated nearly the same isotope age (2119±170 and 2191±39 Ma, respectively) on the same pair of minerals (garnet-clinopyroxene). The age obtained (about 2.1 Ga) is the oldest indicator of the onset of Paleoproterozoic metamorphism in the Belomorian mobile belt.