New Nd-Sr isotope-geochemical research of the paleoproterozoic PGE-bearing Monchetundra massif rocks (the Fennoscandian Shield)

Abstract

This paper presents results of Nd-Sr research for mafic rocks from the PGE-bearing Monchetundra massif, the Main Ridge. According to the new isotopic data mafic rocks of the massif different in age are characterized by various isotopic and geochemical characteristics. The oldest rocks of the massif (2520 Ma) show a wide range of εNd(T) values between +0.02 and –2.23 and Mezo- to Paleo-Archean TDM ages. Trachytoid gabbronorites (2500 Ma) have εNd(T) values in the range of –1.70 to +1.42 and TDM ages of 2.7 Ga to 3.5 Ga. The εNd(T) values for massive gabbronorites vary between –3.38 and +2.08, and TDM ages for them are in the range from 2.7 Ga to 3.4 Ga. Segregations of gabbro-pegmatites (2450 Ma) are spatially and genetically associated with massive gabbronorites and have similar isotopic and geochemical characteristics. Gabbros with high positive values εNd(T) of +1.42 to +2.08 as well as TDM close to 2.7 Ga appear to be derived from a depleted mantle source. In general the Nd-Sr isotopic and geochemical data indicate that the main varieties of Monchetundra rocks have formed from mantle source enriched in large ion lithophile elements. The most likely reason of values scatter is evolution of the mantle source during the plume-lithosphere interaction