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.