Home – Home – Jornals – Russian Geology and Geophysics 2001 number 6

We have studied the geochemistry of elements in trachybasalts, trachyandesite-basalts, trachyrhyolites, comendites, and alkali and subalkalic granitoids of age J₃-K₁ making up a bimodal volcanoplutonic association in one of the depressions of the Late Mesozoic Transbaikalian rift system (TRS). The petrogeochemical features of the rocks from different regions are compared. Except for some local peculiarities, the parameters of the studied trachybasalts and andesite-basalts may be considered typomorphic for all TRS basaltoids of age J₃-K₁. Using the strontium isotope method, we have established that the upper-crust matter was assimilated by trachybasaltic magma and have estimated its content in the rocks.
The acid volcanic rocks of the West Usugli association are shown to be comagmatic with granitoids of a pre-Tulurian magmatic complex. The source of their substance differed from that of basaltoids, and basic and acid magmas evolved independently of each other. At the same time, the geochemical specificity of the acid members of the association (high concentrations of F, K, Rb, REE, and other lithophile elements in which associated basaltoids are enriched) is related to the effect of fluids from the below-lying trachybasalt layer. The strontium isotope compositions of acid rocks somewhat disagree with isochron models, which confirms the hypothesis that they received Rb at the late stages of their formation.