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Jussi Pokki, University of Helsinki (Finland)
Tom Andersen, University of Oslo (Norway)
Jarmo Kohonen, Geological Survey of Finland (Finland)
Tapani Ramo, University of Helsinki (Finland)
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The Island Suursaari, Gulf of Finland, Russia hosts a unique supracrustal sequence within the Fennoscandian shield. The Svecofennian crystalline basement (c. 1880-1810 Ma) is nonconformably overlain by a ∼30 m thick, slightly NE dipping quartz arenitic conglomerate. The conglomerate is followed by mafic volcanic rocks and supracrustal/subvolcanic 1633±2 Ma quartz-feldspar porphyries. The age of the porphyries corresponds to those of the adjacent mainland rapakivi intrusions. Detrital zircons were studied from a quartz arenitic boulder in the conglomerate and from its sandstone matrix. In preparing the mounts, non-random sampling was used in search of minor age populations and random sampling for the overall age distribution of zircons. The latter was carried out by moving the sample glass plate randomly under the microscope and choosing the grain closest to the cross-hair each time. Grains with 206Pb/204Pb 2000 and central discordance over 10.0% were excluded.
In the quartz arenitic boulder, the youngest 207Pb/206Pb ages at 2σ confidence level are 1688±22 (central discordance 1.6%), 1768±30 (-3.3%), 1811±12 (0.6%) and 1851±24 (-4.9%) Ma. Of 62 random grains, 65% have 207Pb/206Pb ages between 1850 and 1900 Ma and 11% are Archean. Of 76 random grains from the matrix, 7% comply with the age of nearby mainland rapakivi plutons (1.65-1.62 Ga), 38% have 207Pb/206Pb ages between 1850 and 1900 Ma and 11% are Archean. In total, 10 grains from the matrix are of rapakivi age (e.g. 207Pb/206Pb ages 1634±14, 1641±10, 1650±12, 1656±12 Ma; central discordance -1.2-2.4%). The next older 207Pb/206Pb ages are 1696±72 (central discordance -2.0%), 1716±10 Ma (4.0%), 1743±38 (5.8%), and 1841±10 Ma (1.9%).
Field relations suggest that the minimum age of the conglomerate is 1633±2 Ma and the youngest detrital zircon in the matrix sandstone (1634±14 Ma) gives the maximum depositional age for it. The data indicate that the deposition of the conglomerate took place during the emplacement of the rapakivi granites. The conglomerate contains no rapakivi granite clasts or fragments from the overlying quartz-feldspar porphyry; most of the lithic fragments in the conglomerate consist of quartz arenite compositionally resembling the matrix. This, together with the detrital zircon data suggest: (1) local quartz arenite source; (2) exposure of volcanic or plutonic rocks with ages corresponding to the rapakivi granites within the source area; and (3) ultimate provenance dominated by Svecofennian rocks with a minor Archean component. Differences between zircons in the two samples (zircons are less abraded and lighter in colour in the matrix) indicate that the conglomerate matrix is not exclusively derived from the arenites represented by boulders in the conglomerate. Furthermore, no direct Archean source area is necessary, as the Svecofennian metasedimentary rocks are known to contain Archean detrital zircons.
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