Ferro-potassic a-type granites and related rocks in NE Poland and S Lithuania: West of the East European Craton

The present study of monzodiorite, quartz monzodiorite, monzonite, quartz syenite, granodiorite, and rapakivi-like granite surrounding the Suwalki, Ketrzyn and Sejny anorthosite, norite and gabbro plutons or forming separate bodies in northern Poland and southern Lithuania, has resulted in the recognition of a specific suite with AMCG affinity. The bodies belonging to the Mazury and Veisiejai complexes are aligned to E-W trending lineaments in southern Lithuania and northern Poland with a continuation through the Baltic Sea into south-central Sweden in the west and into NW Belarus in the east. The rocks of the suite form a continuous series ranging from 45 to 75% SiO2 with no indication of the classical Daly gap between mafic and felsic rocks. The Mazury and Veisiejai complexes chemically belong to the ferro-potassic alkali-calcic type of rocks and are comparable with the AMCG suite, e.g. in Rogaland, southern Norway. The most acidic varieties of the granitoids approach the classic rapakivi granites in their petrogenetic characteristics, elevated contents of incompatible elements and REE, and hence have A-type affinities. The granitoids are assigned to the 'Post-collisional Granite' field. The overall evolution from mafic to acidic rocks is characterized by well-defined trends which most probably represent a liquid line of descent. Fractional crystallization with or without assimilation and hybridization can account for the continuous evolution from mafic to acidic members. The formation of the suite is concentrated in the timespan covering almost 50 Myr from 1.55 to 1.5 Ga. The most felsic rocks yield ages of ca. 1.5 Ga, which are very similar to the Kabehai granites further east in southeastern Lithuania. Cooling and local mylonitization proceeded until 1.4 Ga (hornblende and biotite Ar-40/Ar-39 ages). The Mazury and Veisiejai complexes are new evidence that anorthosites and related rocks can be linked to large lithospheric discontinuities. A thick crust and a Moho offset on the recent EUROBRIDGE geophysical profiles could indicate that a slab of lower crustal rock has been melted under the Suwalki massif to produce the AMCG rocks. Recent structural and kinematic observations indicate predominating transpressional/compressional regimes. (C) 2003 Elsevier Science B.V. All rights reserved.