Bakanasskaya

This approximately circular, 30 km diameter extrusive-intrusive complex has been emplaced into extrusive volcanic and sedimentary formations of late Palaeozoic age. The complex is formed of the rocks of three granitoid suites which, from oldest to youngest, are (1) comendites, comenditic tuffs and peralkaline granites, (2) granosyenites and leucogranites and (3) alaskitic granites. The rocks of the first, peralkaline suite comprise mainly an external ring while within the complex they are present as large fragments within the granitoids of the second suite. The oldest rocks of the peralkaline suite are comendites and comenditic tuffs, which comprise phenocrysts of alkali feldspar and microphenocrysts of aegirine, plus or minus quartz, in a glassy base. These were followed by emplacement of aegirine and arfvedsonite-aegirine granite porphyries, then coarse-grained miarolitic aegirine granites and finally by fine- and medium-grained, massive and miarolitic aegirine granites. The granitoids of the second suite build more than 50% of the Bakanasskaya complex. They differ from the granites of the earliest suite by being generally less alkaline, by the taxitic textures of the mafic minerals, by the abundance of inclusions of earlier rocks and by the abundance of titanite, apatite and fluorite. The rocks comprising the second suite, from oldest to youngest, are biotite-amphibole porphyritic granosyenites and granosyenite porphyries, coarse-grained biotite-amphibole porphyritic granites, fine-grained porphyritic arfvedsonite and biotite granites and dykes of diabase, syenite and granite porphyry. Development of the complex was completed with the intrusion of the alaskites of the third suite which lie along the northwestern margin of the complex.