Sm-Nd, Rb-Sr AND Pb-Pb ISOTOPE SYSTEMATICS OF BASITES-ULTRABASITES IN GREMIAKHA-VYRMES MASSIF: IMPLICATION FOR THE ROLE OF CRUST-MANTLE INTERACTION WHILE THE MAGMA GENERATION AND TI-ORE-FORMING PROCESS

摘要

Sm, Nd, Rb and Sr contents, Nd and Sr isotope composition have been determined for basic and ultrabasic rocks of Gremiakha-Vyrmes massif. The Sm-Nd isochrone are 1926+/-74 Ma (MSWD=1.2; #epsilon#_(Nd)=+0.8) coincides with the Pb-Pb one of apatite (T=1911+/-87 Ma; MSWD=9.0; #mu#=9.0) having average value ~(87)Sr/~(86)Sr=0.7044 and reflects the time of magma crystallization. The Sr isotope compositions: initial one, calculated for the whole-rock samples, and the one directly measured in apatite from various basic and ultrabasic rocks, testify to different degree of the mantle melt contamination by the crust substance. Variations of #epsilon#_(Nd) and I_(Sr) show obviously some repeated intrusions of melts with different initial isotopic characteristics. In general, two kinds of the mantle melt contamination could be distinguished: by frame rock in situ and by the way of interaction with crust material in the mantle. Value of ~(87)Sr/~(86)Sr=0.7019+/-2 directly measured in a carbonate vein within basic-ultrabasic complex corresponds to Sr composition of depleted mantle and, thus, permits to assume an evolution of the melt with achievement of the carbonatite formation stage. It could be supposed also that magma generation and the following Ti-bearing ore forming process in Gremiakha-Vyrmes have been caused by the crust-mantle interaction.