In the Median Segment of south-west Sweden, the Delsjön augen granite-gneiss occurs as elongated, up to 50 km long bands, in some cases associated with the Härsjön supracrustal gneiss. The rock generally contains 25-30% megacrysts of recrystallized microcline aggregates in a matrix of quartz, plagioclase, microcline and biotite. In places, a texture with apparently primary, plagioclase-mantled K-feldspar phenocrysts is observed. The megacrysts are interpreted to be inherited from the primary igneous mode of the rock, which originated as a porphyritic granite. Field relationships of the Delsjön augen granite-gneiss with the surrounding rocks indicate that it is younger than the Härsjön supracrustal gneiss, but probably older than at least most of the non-megacrystic Kallebäck Suite granitoid gneisses which dominate the region. The Kallebäck Suite comprises a wide range of subduction-related mafic to felsic calc-alkaline intrusive rocks in the Median and Western Segments, with an age range from 1630 to 1590 Ma. Geochemical comparisons between the Delsjön augen granite-gneiss and the non-megacrystic Kallebäck Suite granitoids show overlapping characteristics, although the latter rocks show a much larger compositional spread than the uniform Delsjön augen granite-gneiss. Comparing only samples with >65% silica, both groups may be explained as water-unsaturated eutectic melts derived by partial melting of subduction-related mafic source rocks. The K-feldspar megacrysts might be accounted for by a moderate increase in water content before crystallization during magma ascent, which moved the cotectic lines so that dominantly orthoclase crystallized from the melt. The non-megacrystic samples did not experience this hydration process. The Delsjön augen granite-gneiss and the non-megacrystic granitoids have remarkably similar trace element characteristics and both plot as volcanic arc granites. Ion probe U-Pb zircon dating of the Delsjön augen granite-gneiss gives a 1614±5 Ma date. This age is interpreted to date the intrusion, which probably occurred as bodies and dykes along a zone of magma chambers. Local remelting by a gabbro caused a net-veined contact, and formation of secondary 1585±4 Ma zircon rims. Sveconorwegian metamorphic zircon rim formation is dated at 974±22 Ma. The tectonothermal episode of strong deformation of the Delsjön augen granite-gneiss by subsequent non-megacrystic members of the Kallebäck Suite, so well displayed by field features, is not represented in the zircon age record.