Yllymakh

This mixed volcanic and intrusive complex of central type has an area of about 40 km2. It is circular and has a zonal structure which is complicated by a complex pattern of faults. Extrusive formations play a prominent role and are represented by pseudoleucite phonolites and peralkaline trachyte and their tuffs, which occupy zones in the western and eastern parts of the complex. The volcanics often contain small xenoliths and nodules of rocks of ultrabasic composition. The intrusive rocks cut through the volcanics and the earliest group includes malignites, melanocratic syenites and monzonites. The shonkinites and malignites form arcuate bodies and small stocks in the marginal parts and near to the complex. The malignites contain xenoliths of wehrlite and pyroxenite in which the olivine is more magnesian (Fo74) than in the malignite (Fo48). A later intrusive phase is represented by nepheline and pseudoleucite syenites which form large fault-bounded blocks in the interior of the complex as well as lens-like bodies of nepheline-bearing pegmatites. Both small xenoliths and large blocks (1.5-2 m) of feldspathized volcanic rocks are found in them. Throughout the complex the syenites are intensively sericitized and near late granites they are albitized. Dykes of pseudoleucite tinguaite occur within the volcanics and alkaline gabbroids. Alkaline leucocratic syenites (pulaskites) are younger than the nepheline syenites and contain xenoliths of melanocratic syenite, malignite, monzonite and phonolite. A central stock consists of aegirine granite and granosyenite, which cuts across the alkaline leucocratic syenites. Dykes of granite porphyry and granosyenite aplite are spatially associated with the granitoids. Intense metasomatic alteration of many rock types is manifest, but particularly in the volcanics, including replacement of dark minerals by aegirine, zeolitization and sericitization of feldspathoids and microclinization and albitization. In different syenites and in malignites postmagmatic changes have led to extensive recrystallization and the transformation of orthoclase into microcline and its replacement by albite. Albite-rich metasomatic rocks that were formed in linear zones among the nepheline syenites contain agpaitic accessory minerals including lamprophyllite, eudialyte, rosenbushite, loparite, lomonosovite, rinkolite and labuntsovite. Chemical analyses of rocks will be found in Orlova (1990) and Bilibin (1947); Chepurov (1973) describes melt inclusions in pseudoleucite.