Potassic mafic rocks with megacrysts from the northwestern Ladoga Lake area (Karelia, Russia): A diversity of mantle sources of potassic rocks in theeast of the Fennoscandian Shield

Minor intrusions of low-evolved potassic mafic rocks with clinopyroxene and mica megacryst (northwest of the Ladoga Lake) were studied to reveal speci fic features of mantle sources of igneous rocks within the Svecofennian blo ck of the Fennoscandian Shield. These rocks are dated at 1753 +/- 130 Ma by a Sm-Nd method and at 1700-1540 Ma by Rb-Sr and K-Ar methods. The medium- to coarse-grained rocks of this area have been classified with shonkinites, plagioclase shonkinites, mafic syenites, and syenites, while the fine-grai ned varieties have been termed as minette and plagioclase minette. The prim ary isotopic composition of studied rocks from the northwestern Ladoga area is characterized by average Sr-87/Sr-86 = 0.7033 and Nd-143/Nd-144 = 0.510 315 and is close to the isotopic composition of mantle sources. A compariso n of geochemical features of potassic mafic and ultramafic rocks from diffe rent blocks in the eastern part of the Fennoscandian Shield indicates that most of them were derived from the enriched mantle of EM I type.