Geochemistry and Neodymium Isotopic Composition
of the Late Archean High-Magnesium Granitoids
of the Kostomuksha Block, Western Karelia, Baltic Shield

S. B. Lobach-Zhuchenko, N. S. Guseva, A. V. Kovalenko, and I. N. Krylov

Institute of Precambrian Geology and Geochronology, Russian Academy
of Sciences, nab. Makarova 2, St. Petersburg, 199034 Russia
e-mail: Lobach-Zhuchenko@cards.lanck.net

Received April 4, 2003

Abstract—This paper addresses the geologic setting, composition (major, trace, and rare earth elements), and
neodymium isotopic systematics of Archean high-magnesium granitoids (sanukitoids) and three groups of calc-
alkaline lamprophyres from the Kostomuksha block in Karelia. The Nd(t) values of sanukitoids vary within
a narrow range between –0.7 and +0.6. It was shown that these rocks were derived from a moderately depleted
mantle that had been enriched in light REE about 150 Ma before melting. The results of model calculations for
major and trace element behavior during successive crystallization of the Hbl + Pl + Ap + Mgt + Ort association,
which was joined by Bt, Qtz, and Ksp at lower temperatures, are consistent with the compositional variation of
the Kurgelampi sanukitoid lopolith. This supports the model of in situ fractional crystallization of a melt of
quartz diorite composition (about 61% SiO₂ and mg# ~0.55). The observed incompatible element enrichment
of the sanukitoids was not related to fractionation processes but reflected the existence of an enriched source.
Two possible mechanisms were proposed for the formation of the initial diorite melt of the intrusion: (1) by
direct partial melting of the enriched mantle and (2) formation in an intermediate chamber from a more basic
melt compositionally similar to mafic high-magnesium calc-alkaline lamprophyres, which was in turn derived
by direct partial melting of enriched mantle material.

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