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 SKAT€77 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.

The U/K ratio can be used as a geodynamic criterion. Purely subduction (Andean) amphibole-bearing biotite granitoids as well as those transitional to collisional (early collisional) can be referred to I-type (U/K≥ 0.75). Taking into consideration the U/K ratio value, one can refer calcareous plagiogranites, the Kurile island arc granites to I-type. The category of collisional granitoids of S-type (U/K=1.30-1.75) includes syncollisional and late collisional granitoids; we can distinguish post-collisional and proper intraplate ones among the magmatic rocks of á-type. The post-collisional granitoids are characterized by U/K=0.75-1.30 values; as for intraplate rocks with increased alkalinity, associated with plume effect, U/K≤0.75 values are typical. Taking into consideration the U/K ratio the island-arc granitoids (the Kuriles), namely: quartz diorites, diorites, calc-alkali gabbroids are referred to í-type. From geophysical data, the Kurile subduction calcareous granitoids are associated with the crustal magma sources, while those of í-type –with the mantle ones being formed due to the Kurile mantle diapir (plume) (Volodkova, 2008). Granitoids formed due to the mantle sources, the intraplate continental (A-type) and island-arc (M-type) ones, are not differentiated from the U/K ratio criterion; for the granitoids of both categories increased alkalinity is typical and the U/K ratio values are significantly lower as compared to characteristics of depleted mantle. By all appearances, both categories of granitoids are formed due to the mantle metasomatism processes.

Geodynamic types of magmatic rocks are determined by the processes occurring within the lithosphere and asthenosphere; magmas of plume nature reflect deeper physical-chemical conditions. The formation of EM I, EM II, HIMU mantle reservoirs is related to the plume influence (Weaver, 1991); EM I and HIMU reservoirs are contrastingly distinguished, while EM II one is close to the crustal parameters according to NRE ratios. It is in consistency with the well-known point of view on the formation of EM II from the continental crust due to recycling.

Taking into account the NRE ratio values, the magmatic complexes with increased alkalinity are to either extent formed due to the following sources (Volodkova, 2008):

1.      EM I source (U/K <0.75);

2.      HIMU source (U/K >1.75);

3.      Rocks with intermediate characteristics of NRE ratio of 1.25<U/K<1. 75 more rarely occur; they are formed based on substrate with ingreased basicity, recycled by deep fluids;

4.      Probably, EM I and HIMU sources characterize two mantle levels of different depth;

5.      Geodynamic types of magmatic rocks are related to the plume processes and can be formed as follows: I-type –due to HIMU mantle source, A-type – due to EM I mantle source.

References

1.      Volodkova T.V. Characteristics of alkali rocks of Priamurye from aerogammaspectrometry data // Alkali magmatism of the Earth.š St. Petersburg, 23-26 May, 2008. (in Russian). P. 27.

2.      Volodkova T.V. Granites and Earths Evolution: geodynamic Position, Petrogenesis and Ore Content of Granitoid Batholiths. Ulan-ude, 26-29 August 2008. (in Russian). P. 68-71.

3.      šBarry L. Weaver. The origin of ocean island basalt end-member compositions trace element and isotopic constraints // Earth and Planetary Science Letters. 1991. Vol. 104. P.381-397.