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

1
Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, St. 199034 Petersburg, Russia
2
Saint-Petersburg Mining University, 199106 St. Petersburg, Russia
3
Saint-Petersburg State University, 199034 St. Petersburg, Russia
4
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
5
Institute of Geology Karelian Research Centre, Russian Academy of Sciences, 185910 Petrozavodsk, Russia
6
Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russia
*
Author to whom correspondence should be addressed.
Geosciences 2020, 10(5), 197; https://doi.org/10.3390/geosciences10050197
Received: 15 April 2020 / Revised: 15 May 2020 / Accepted: 19 May 2020 / Published: 21 May 2020
(This article belongs to the Special Issue Distribution of REE and Trace Elements for Different Types of Zircons)
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. View Full-Text
Keywords: zircon; trace elements; rare earth elements; U-Pb zircon geochronology; eclogites; pegmatites; Gridino; Belomorian mobile belt zircon; trace elements; rare earth elements; U-Pb zircon geochronology; eclogites; pegmatites; Gridino; Belomorian mobile belt
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    Doi: 10.5281/zenodo.3822116
    Link: https://zenodo.org/deposit/3822116
    Description: Table S1: Zircon U-Pb-O SIMS data (sample 101d), Table S2: Trace element composition of zircons (sample 101d).
MDPI and ACS Style

Skublov, S.G.; Berezin, A.V.; Li, X.-H.; Li, Q.-L.; Salimgaraeva, L.I.; Travin, V.V.; Rezvukhin, D.I. Zircons from a Pegmatite Cutting Eclogite (Gridino, Belomorian Mobile Belt): U-Pb-O and Trace Element Constraints on Eclogite Metamorphism and Fluid Activity. Geosciences 2020, 10, 197.

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