Lu–Hf Isotope-Geochemical Zircon Systematics and Genesis of the Neoarchean Alkaline Granites in the Keivy Megablock, Kola Peninsula.

Abstract—Alkaline and subalkaline granites of the Keivy megablock belong to the potassic alkali-calcic ferroan rocks enriched in large-ion lithophile, high-field strength and rare-earth elements. Based on these criteria, they correspond to A2-type granites (Eby, 1992). The granites have elevated Y/Nb (1.9–2.5) and Yb/Ta (3.4–3.7) ratios typical of crustally derived granites. The initial 176Hf/177Hf ratios in the central parts of zircon crystals vary within 0.281004–0.281175 at εHf(T) ranging from –2.89 to 3.79. It is supposed that the underplating of high-temperature mafic melts caused melting of metasomatically altered lower-crustal rocks and formation of palingenetic lower-crustal melts, which during subsequent ascent in the upper crust experienced fractional crystallization to more siliceous subalkaline and alkaline melts. [ABSTRACT FROM AUTHOR]