The Effect of Differentiation Processes in Tholeiite-Basalt Melts on the Rare Elements Distribution

Abstract

Basite magmatism has been manifested repeatedly for a long time in various geodynamic structures within the eastern part of the Siberian platform. In the Middle Paleozoic, it was related to rifting processes, and in the Late Paleozoic-Early Mesozoic – to the initiation and development of trap syneclises. Differences of the geodynamic regime of magma formation are displayed in the material composition of rocks. This report presents a generalizing study of the petro-geochemical features of the tholeiitic basaltic melts formed in different geodynamic settings. The initial magmas composition changes significantly at different stages of the magmatic system development. Using multicomponent analysis, we reviewed the impact of the fractionation processes of the basal melt, which occurred under various PT conditions, on its material composition. Among the trapp bodies there are powerful differentiated intrusions. A significant change of the composition occurs during the intra-chamber differentiation of the melt in a sequence of rock strata varied from the basic magnesian to felsic alkaline rocks. In the resulting series of rocks, the content of rare elements included in the lattice of the early femic phases decreases, as well as the accumulation of almost all incompatible elements. The evolution of melts of normal alkalinity occurs with an increase of REE content and their insignificant separation. Intrusions that have undergone the stage of high-pressure fractionation in the deep transitional chamber are of particular importance. As a result of such differentiation, a peculiar group of rocks is formed in the cross section of the Middle Paleozoic bodies, such as monzonite-porphyries in one case and anorthosite gabbro-dolerites - in the other. . It is established that the monzoitoid type of differentiation is characterized by accumulation of LREE, LILE and elements of the zirconium group Nb, Ta, Hf and Y. Isolation of anorthosite gabbro-dolerites in the cross-section of bodies, as well as an increase in the content of aluminum, calcium, and strontium in them, is an indication of the anorthositic tendency of magmatic melt differentiation/When the basite-tholeiite melt interacts with the mantle reducing fluid in the deep core, its metallization occurs with the release of drop-liquid separations of native iron up to the formation of its large segregations. Intrusions with a large-scale content of native iron are found among the trap formations of Siberia. All this leads to a decrease in the total concentration and activity of iron in the partially metallized silicate matrix, there is a significant increase in the content of MgO and trace elements-Ni, Co, Cr. The Fe-phase-containing intrusives are characterized by minimal LILE, HFSE, and REE contents. Start your abstract here… 250 to 500 words concise and factual abstract is required. The abstract should include the purpose of research, principal results and major conclusions. References should be avoided, if it is essential, only cite the author(s) and year(s) without giving reference list. Prepare your abstract in this file and upload it into the registration web field.