Petrology, Geochemistry, and Tectonic Setting of Plagiogranites of the Chelyuskin Ophiolite Belt

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Abstract
Results of research on the geological, petrochemical, and isotopic-geochronological charac- teristics of plagiogranites from the Chelyuskin ophiolitic belt, on the northern part of East Siberia's Taymyr Peninsula, are presented. Petro-geochemical features and REE distributions for this tonalite-trondhjemite series resemble those of plagiogranites from different ophiolitic complexes. The plagiogranites considered here belong to the low-potassium series of ophiolitic mafics—gabbro, gabbro-dolerite dikes, and basalts. Their spatial relationships; low K2O, Rb, Nb, Ta, U., and Th contents; similar REE patterns; and tonalite and trondhjemite Nd- and Sm-Nd- isotopic ratios typical of mafic rocks confirm the cogenetic nature of these rocks. Zircon U-Pb dating and an Sm-Nd isotopic study suggest a Late Riphean age for the plagiogranites. We regard the 740 ± 38 Ma age as the upper age boundary for the formation of the Chelyuskin ophiolitic belt, and Sm-Nd model ages (850–785 Ma) as its lower boundary. Tonalite-trond-hjemite series could be formed as a result of partial melting of previously formed oceanic crust in a back-arc environment, in association with minor portions of the sediments of the ancient arc.