MESOZOIC BASALT VOLCANISM AND ZEOLITE MIN ERALIZATION OF WESTERN TRANSBAIKALIA (BY EXAMPLE OF THE MARGINTUI VOLCANIC FIELD)

The Margintui basalt field is one of the greatest (more than 1000 km(2 )) volcanic fields, whose formation was involved with the Mesozoic con tinental rifting. It comprises SiO2-subsaturated mainly Ne-normative o livine basalts with a subordinated amount of Hy-normative basalts. A c haracteristic feature of the Margintui volcanic field is intense zeoli tization of basalts and a great diversity of zeolite minerals such as stilbite, heulandite, chabasite, mesolite, natrolite, analcime, and ot hers. The zeolitization leads to decreased SiO2 and FeO contents and i ncreased Fe2O3 with variable CaO, Na2O and K2O. Widespread Ca, Na and Ca-Na zeolites are correlated with country rock composition. Irregular distribution of zeolite mineralization within the basalt field, its c onfinement to zones of paleovolcanic vents with intense fumarole-solfa taric processes support the low-temperature nature of zeolite minerali zation. Zeolites mostly occur in the form of amygdules, thus determini ng their morphological type as amygdaloidal.