Quartz Porphyries on the Outer Islands in the Gulf of Finland: Volcanic Comagmates of Rapakivi Granites

This paper presents new petrogeochemical data concerning rocks sampled on the Outer Islands in the Gulf of Finland. Earlier, quartz porphyries on Hogland Island were regarded as the standard for volcanic comagmates of rapakivi granites in the Wyborg massif. Sommers Island was found to contain two effusive rock sequences. The “southern” sequence is composed of quartz porphyries whose isotopic age is 1.66 Ga, so they can be regarded as the earliest manifestations of Riphean magmatic activity in the area. The “northern” rock sequence contains, apart from quartz porphyries, also trachybasalts, basaltic andesites, and dacites; the sequence has an isotopic age is 1.59‒1.57 Ga, which is inconsistent with the figures for the adjacent Wyborg massif (1.65‒1.63 Ga). For this reason, in spite of the fact that quartz porphyries and rapakive granites have similar compositions, the volcanics did not result from differentiation of the magma that has produced the granites, but were formed independently of these from a source of the same type which however was in a slightly different structural setting that allowed their free supply onto the ground surface.