Study of the nature of magmatic complexes using NRE ratio values

Volodkova T.V.

Yu. A. Kosygin Institute of Tectonics and Geophysics, Khabarovsk, Russia

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The major part of ore regions of Priamurye is covered by aerogeophysical survey (aeromagnetic prospecting, aerogammaspectrometry) scale of 1:50000-1:10000, using complex aerogeophysical station devices SKATЂ77 an STK-11. Measurement accuracy of the natural radioactive elements (NRE) content attains to the following values: for uranium - (0.1-0.15)*10-4%,–for thorium – (0.55-0.65)*10-4%, for potassium – (0.05-0.10)%. Magmatic complexes are characterized by average background U/Th, K/Th, U/K ratio values; as compared to NRE contents, these are less dependent on survey errors and are commensurable with the results obtained by ground survey. The confidence level values (±σ) attain to: U/Th – 0.1; K/Th – 0.1; U/K – 0.25.

In terms of isotopic geology, uranium, thorium and potassium are a part of a group of extremely incompatible elements-indicators of magmatic processes. Unaltered rocks, formed due to processes of crystallization differentiation in closed magma chambers are characterized by constant NRE ratios irrespective of the composition. They are close in age, being related to the main stages of a single magmatic cycle, which is comparable with the time of existence of magma chamber. Hydrothermally-metasomatically altered rocks (degree of alteration of more than 10-20 %) are distinguished by the anomalous NRE ratios. In the open magma chambers conditions for crystallization vary from phase to phase due to deep fluid inflow, therefore, the average NRE ratio values will vary regularly for magmatic rocks. The background NRE ratio values vary sharply for magmatic complexes due to affecting hydrothermal-metasomatic processes, mantle metasomatism and geodynamic environments. It has been elaborated methodology taking into consideration the influence of hydrothermal-metasomatic processes on NRE ratio characteristics, such data are eliminated from sampling when making statistical reports. The mantle metasomatism and juvenile fluidization are typical of plume magmatism; intrusion of exclusively subalkali and alkali rocks is associated with it. In Priamurye magmatic rocks of plume nature are associated with hot spot areas and the Maya-Selemdzha plume which are distinguished from geological-geophysical data. The NRE ratio characteristics for subalkali rocks are weakly anomalous (weak fluid inflow); these may reach “hurricane” values for alkali rocks (the Ingili, Arbarastakh massifs).

Characteristics of average NRE ratio values for normal granitoids in different geodynamic environments.