Assessment of the conditions of granitoid systems formation using the thermodynamic modeling method (Aldan Shield, Far East)
A new universal method is suggested and tested involving thermodynamic modeling of parameters of granitoid systems formation based on data on the chemical composition and relation of minerals in the study rock sample. The method is the solution of the reverse problem of convex programming, and is based on solving for the minimum of Gibbs free energy in the physical-chemical system for the multitude of defined limitations. Capabilities of the this method are demonstrated when assessing conditions of formation (P-T and fluid regime) of different types of Late Mesozoic granitoids of four large massifs along strike of the Stanovoy Range (Aldan shield southern framing). These granitoids vary in alkalinity (calc-alkaline and subalkaline), melanocratic character (diorites to leicogranites), and genesis (magmatic and palingenetic-metasomatic). The simulated conditions of formation of the study granitoids generally coincide with the estimates based on geological data and also on the mineralogical geobarometers and geothermometers, and characterize different facies of the depth of these assemblages - from hyperabyssal (<10-12 km) to abyssal (>20 km). These data essentially supplement the available calculation diagrams (performed for the first time) at the expense of amphibole-free varieties. Generally, the simulated temperature values of the formation of magmatic granitoids lie in the range of crystallization of the bulk of the rocks, and those of water-saturated metasomatic granitoids are virtually similar to the temperatures in all geothermometers given the most actual values of the oxygen potential. This result illustrates the rightfulness of the suggested methods and the possibilities of its wide use in practice. The prospects of the suggested method for any regional and global geodynamic constructions are obvious, as this makes it possible to assess and compare the depth (and also the temperature and parameters of fluid regime) of formation of different geologic assemblages: metamorphic, metasomatic, and magmatic. The contemporary derivation of such information is essential for assessment of the ore potential of the region. Accordingly, further investigations along this line are necessary, incidentally, both in the field of further automatization of calculations and in application of the methods for not only granitoids at other sites but also for gabbroids.