U-Pb age and Sm-Nd isotope composition of enderbites (Northern Part of the Fennoscandian Shield)

Vetrin V. R., Rodionov N. V., Serov P. A.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2021. V. 150. N 6. P. 1-17

https://doi.org/10.31857/S0869605521060095

Language: Russian

Abstract

Hompen gneisses in northern part of the Baltic Shield are represented by enderbites of tonalite-trondhjemite composition with elevateded concentrations of Na₂O, Al₂O₃, Sr and lower concentrations of K, Rb and highly charged and radiogenic elements (Nb, Y, Ti, P, U, Th), and REE with their significant fractionation (La_N/Yb_N = 104). Zircon crystals from Hompen gneisses are divided into 4 generations according to their crystal morphology, internal structure, composition, and age determined by the SHRIMP-II ion microanalyzer. By the generation II of zircon, the age of crystallization of metavolcanic protoliths of Hompen gneisses was determined as 2921 ± 12 Ma, and the age of early granulite metamorphism (by the generation III of zircon) was estimated as 2851 ± 11 Ma. Generation II of zircon crystals contain inclusions of relict grains dating as 2990 ± 4 Ma (the generation I of zircon), they probably detect the age of volcanic rocks of the earlier phase of Hompen gneisses. These grains were captured from rocks of the continental crust. Crystals with the 2778 ± 10 Ma age (generation IV) reveal the age of granodiorite massifs synchronous with the late granulite metamorphism in the region. These crystals were formed in Hompen gneisses probably while they were injected by granitoids of the vein facies in granodiorite massifs. According to the Sm-Nd isotope systematics, the protoliths of Hompen gneisses contained 50–90% of the depleted mantle material, and were probably formed as result of melting of differentaged basic rocks in the lower crust during the underplating of basalts similar to primitive TH-1 tholeiites of the Archean greenstone belts.

Keywords: granitoids of TTG assemblage, U-Pb dating of zircon, Sm-Nd systematic of rocks