Sr-Ba-Rb systematization of the megacrysts of alkali feldspar from the basaltoids of Central Asia

Variations in the Sr, Ba, and Rb contents in the megacrysts of anorthoclase and sanidine from the alkali basaltic rocks of Central Asia are described. Two series of these megacrysts have been revealed: low-Ba (Vitim volcanic field, Bartoi volcanoes, Taryat depression, Tokinsk Stanovik) and high-Ba (Udokan plateau, Minusinsk depression). The Ba/Rb ratio varies from 4-35 in the low-Ba series to 35-300 in the high-Ba one. The low-Ba series is close in geochemistry to host basalt, which reflects the peculiar composition of magmatic sources in these regions. It is shown that alkali feldspars might have crystallized from residual magmas after the crystallization of megacryst association - clinopyroxene, ilmenite (titanomagnetite), and biotite. It is supposed that the high-Ba feldspar megacrysts resulted from the basaltic-magma-crust interaction, which led to changes in their isotopic and trace-element characteristics, and is the product of deep differentiation.