Geophysical Journal | 2002 volume 24 6

Geochemical properties of a model of the evolution of magmatic systems and the Earth's crust: a potential source of petrophysical and oregenetic information

© S.E. Shniukov

Application of the wide-spread accessory minerals (WSAM: zircon, apatite, monazite etc.) in the geochemical models of two different types was considered. The first type includes the models of the magmatic systems formation and their further evolution up to magmatic-hydrothermal ore-forming stage. The second type is represented by the models designed for the geochemical evolution of the continental crust-depleted mantle system. Proposed models of the first type are based on a set of equations for trace element behavior during the melt crystallization/partial melting (Rayleigh-Neumann-Ryabchikov-Shaw et al.) and WSAM's solubility equations (Watson-Harrison-Montel) which are mainly used to obtain the model evaluation of the temperature (Tmodel) and fluid regime of the magmatic system from the whole - rock geochemical data. Furthermore, models include some new contributions: 1) specially calibrated VS. 1/T(K) dependence and the equation for the inverse problem solution , , =Y content in coexistent apatite and zircon respectively) which allows to confirm the obtained Tmodel values (key input parameter for most of calculations), 2) equations for the calculation of the fluid/melt distribution coefficient (KF/L=CF/CL; CF, CL - element content in the fluid and melt respectively) and the model element composition of the hydrothermaly altered rocks, 3) procedures for the determination of the initial magma source, estimation of the corresponding degree of partial melting and identification of the comagmatic rock series. These constituents are obligatory for the generalized geochemical model of the magmatic system.

The models of the second type (McCulloch & Bennett, 1994) are based on the estimation of the rate of growth of the continental crust at the expense of the extraction from the depleted mantle. This estimation is a key input parameter to account the evolving trace element compositions of the crust and depleted mantle assuming the partial melting process as a main mechanism for the mantle-to-crust element transfer. The current data about the rates of crustal growth are based on a real distribution of rocks and their Sm-Nd and U-Pb age determinations. Comparable and even more correct results may be derived from single - grain trace element geochemistry of WSAM's large detrital populations that reflect provenance (age, composition and volume content of rocks within the drained area/crustal block). An application of the proposed method to crustal history reconstruction and determination of the significant metallogenic events (kimberlites, alkaline rocks, carbonatites formation) confirms its effectiveness.
Both types of the models are closely connected. Determination of the initial magma source and estimation of the corresponding degree of partial melting in the models of the first type require an information about evolving element content in crust and mantle (input parameter). These data may be derived from the models of the second type. Final geochemical models of the magmatic systems allow to predict the composition of the restite and cumulative rocks within magma sources and magma chambers respectively, as well as their physical properties. Such potentialities of the geochemical models may be regarded as a basis for their integration in various geophysical models.

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