Bartoiskoe Volcanic Field

The Bartoiskoe volcanic field is one of the numerous volcanic fields of the Baikal rift system. The maximum volcanic activity took place in the Miocene and during the Pliocene and Holocene the intensity of volcanic activity decreased markedly. The magmatic and tectonic activity were not synchronous in so far as the rate of the sinking of the bottom of the rifts and the growth of a dome were slight in the Oligocene to early Pliocene, that is during the period of maximum volcanic activity. However, from the Pliocene up to the present the growth of rift depressions and domes has sharply increased, although volcanic activity has stopped. The lavas extend beyond the rifts, and lava fields of different ages occupy three geomorphological levels in the present relief. The following groups, based on geomorphology, can be distinguished: (1) ‘summit’ moderately alkaline olivine basalts. The overall thickness of the lava flows is about 400-500 m and they have an age of 27 Ma; (2) ‘valley’ lava flows of hawaiite are widespread: they have ages of about 3 Ma; (3) the youngest ‘bed’ lava flows and small cinder-pumice volcanic cones have an age have an age of 1.4 Ma. The volcanic rocks of the last stage are basanites or olivine tephrites, and sometimes contain leucite. In the cinder-pumice cones xenoliths of spinel lherzolite and augite and anorthoclase megacrysts are found, as are more rarely Ti-biotite megacrysts. The volcanic rocks change in composition between the first and third stages, notably in the increase in content of the alkalis and the elements Zr, Nb, RE, Sr, Pb, etc., while the degree of silica undersaturation increases. In terms of mineral composition the proportion of modal alkali feldspar and normative nepheline in hawaiites (second stage) increases (5-8%), modal leucite appears and the content of normative nepheline in the basanites of the concluding stage of volcanic activity increases to between 10 and 20%. The composition of the lavas leads to the conclusion that they are differentiated products of mantle melts, and that there was an increasing degree of the metasomatic transformation of the mantle source concomitant with a decrease of the extent of melting with time. Geophysical data indicate that the lithosphere in the region of Bartoiskoe volcanoes had a thickness only 60-75 km and amongst the xenoliths both hydrous and ‘dry’ peridotites are present in approximately equal numbers. The isotopic composition of Sr and Nd in the mantle xenoliths seems to indicate that the mantle in this region was heterogeneous. Detailed data on the compositions of the rocks can be found in papers by Kononova et al. (1986, 1987, 1988 and 1993).