Vein granites of the Burakovskii layered Massif, Southern Karelia

First data are presented on vein granophyric granites from Europe's largest Early Paleoproterozoic Burakovskii layered mafic-ultramafic pluton, Southe rn Karelia. The pluton consists of two individual bodies: Aganozero and Sha lozero-Burakovskii. Granites were found only in the latter They are confine d mainly to the Pigeonite Gabbronorite Zone and practically do not occur in other zones. The veins show abrupt contacts without chill zones. The grani tes range from leucogranodiorite to potassium leucogranite. Their texture i s strongly variable from hypidiomorphic to granophyric, In terms of geologi c and compositional features, these rocks are classified with transgressive granites of layered intrusions. The vein granites of the Burakovskii Massi f belong to the calc-alkaline series and continue the trend of mafic rocks from the intrusion with a gap at SiO2 = 59-65 wt %. The epsilon(Nd) values of the granites vary slightly, from -2.4 to -3.7, and are very close to tho se of the pyroxenites and pigeonite gabbronorites of the Shalozero-Burakovs kii intrusion (from -1.4 to -3.0), but differ sharply from those of the enc losing Archean rocks (from -5.7 to -10.7). These facts are inconsistent wit h granite generation through basement melting and suggest a single source f or the granites and mafic rocks. Based on the negative epsilon(Nd) values, this source was contaminated by crustal material. The geologic, petrologica l, and isotope geochemical data indicate that the vein granites were coeval to grabbroids and formed in situ within the Pigeonite Gabbronorite Zone en riched in quartz-feldspar intercumulus material. It is suggested that the g ranites were formed from residual intergranular acid melt, which preserved in almost completely solidified cumulates of this zone. The contraction led to vacuum extraction of the melt into shrinkage fractures. In the largest fractures, the separated granite melt experienced subsequent differentiatio n up to hyperacid varieties. The independence of contraction phenomena and melt evolution in the intergranular space could be responsible for the comp ositional gap between mafic and acid rocks.