New Data on the Early Riphean Age (U–Pb, Shrimp-II) of Acid and Basic Effusives of the Gulf of Finland (Sommers Island, Russia)

This work presents data on the age of the volcanites of Sommers Island located to the south of the submarine extension of the Vyborg massif, as potential comagmates of rapakivi granites. The U–Pb system of zircon from Early Riphean volcanics is studied to determine their geochronological age using the SHRIMP-II ion microprobe (VSEGEI). For quartz porphyries composing the southern part of Sommers Island, the oldest age determinations of volcanics (1663 ± 8 Ma) were obtained for the Gulf of Finland area for the first time, which can be assigned to the lower parts of the Hogland series of the Early Riphean. In the northern part of the island, there are various rocks: quartz–feldspar porphyries (rhyodacites), andesibasalts, trachybasalts, and granodiorites. All of them underwent modifications under conditions of greenstone metamorphism in contrast to the relatively fresh rocks in the southern part of the island. The trachybasalts (1591 ± 5 Ma) are older than the quartz–feldspar porphyries (rhyodacite, 1578 ± 14 Ma), and a similar modification from older basic rocks to younger acidic ones is typical of all magmatic occurrences of the anorthosite–rapakivi granite formation in the Gulf of Finland region. The “young” ages of the rocks from the northern part of Sommers Island are likely to represent the presence of a younger massif of rapakivi granites than the Vyborg massif in the center of the Gulf of Finland. In this case, the Riphean trough structure traced eastward from Sommers Island is composed not only of rocks of the Hogland series, but part of it correlates with the bottoms of the Pasha graben section and the Priozerskaya Formation.