Quartz Porphyries of the Outer Islands of the Gulf of Finland ‒ Volcanic Comagmates of the Rapakivi Granites

Abstract

The article presents new petrogeochemical data of rocks from the area of the Outer Islands of the Gulf of Finland. Previously, quartz porphyries of Gogland Island were considered as a standard of volcanic comagmates of Rapakivi granite of the Vyborg massif. There are two effusive tolshas on Sommers Island. The “Southern” is composed of quartz porphyries, with an isotopic age of 1.66 Ga old, so they can be considered as the earliest manifestations of Riphean magmatic activity in this area. In addition to quartz porphyries, trachybasalts, andesibasalts, and dacites are present in the “Northern” strata, and its isotopic age is 1.59‒1.57 Ga old, which does not correspond to the values of the nearby Vyborg massif 1.65‒1.63 Ga old. Therefore, despite the pro-ximity of the compositions of quartz porphyry and rapakivi granites, volcanites are not products of differentiation of the magma initial for granites, but were formed independently of them from the same type of source, but in a slightly different structural environment, allowing their free flow to the Earth’s surface.