DERIVED GRANITOIDS AND THEIR LOWER CRUSTAL RESIDUE: NUMERICAL SIMULATION OF THE COLLISION-RELATED PLIOCENE TYRNYAUZ GRANITES, GREATER CAUCASUS

Abstract

The synthesis of petrological, geochemical, and isotopic data on granitoids from the Tyrnyauz area (El'dzhurtinskii Massif) and the critical analysis of the current petrologic-tectonic systematics of granitoids indicates that the Tyrnyauz granites affiliate with the collisional geochemical type. They were derived from a crustal protolith of the subducted Transcaucasian Plate (at lower crustal depths of 30 km) during a temperature increase coupled with the tectonic thickening of the crust under metamorphic granulite-facies conditions (T≈ 1000°C, P ≈ 8 kbar). Numerical simulation for these parameters indicates that the removal of the partial granite melt to upper crustal levels was accompanied by the basification of the lower crust. An independent test of this model was accomplished with the use of well-known models for the P-T evolution of collisional systems, according to which the temperature at depths of 30-40 km attains 900°C for 20-30 Ma due to the thermal relaxation of the obducted plates. This is consistent with the historical-geological and petrological conditions of the granite under Caucasian collision conditions. It is demonstrated that, in the vertical crustal section of the Caucasian collisional system (1) granite was derived at a depth of 30 km and forced upward; (2) it was concentrated in the decollement zone at depths of 10-15 km and gave rise to a 10-km-thick lower velocity layer (waveguide); and (3) some portions of the melt intruded the overlying folded complex, to which the Tyrnyauz granite belongs. The ascent of the granite melt and the complimentary basification of the residue are supposed to be indicative, in this situation, of the segregation of the continental crust into an upper, predominately granitic, and lower, mafic granulite, parts.