Western Kamchatka

The alkaline rocks of Western Kamchatka occur within early Cretaceous, Palaeogene, and Neogene deposits and Lower Pliocene sedimentary rocks were contact metamorphosed by them. Generally the alkaline rocks form subvolcanic cupolas which vary from 10 m up to 1.5 km in diameter. Dykes and other intrusive bodies have thickness of 0.5 to 200 m and extend along the strike for about 2.5 km. Some small stock-like intrusions of alkaline syenites occupy areas of 1-2 km2. Among these magmatic rocks are trachydolerites, alkali basalts, absarokites, alkaline syenites and crinanites. In most of them there is a small amount (0-10%) of analcime, but in crinanites and analcime basalts the analcime varies between 10 and 15%. The crinanites are holocrystalline, fine-grained, weakly porphyritic rocks composed principally of titanaugite (25-30%), analcime (10-15%), bytownite (40-45%) and olivine (10-15%). A little alkali feldspar (0-5%) and biotite (0-5) are usually present and there is accessory apatite, magnetite and ilmenite. The analcime basalts are similar to the crinanite, but contain more olivine (15-20%) and less pyroxene and plagioclase. The alkaline syenites are leucocratic, medium-grained rocks of alkali feldspar (65-70%), pyroxene (5-10%), biotite (10-15%), analcime (0-15%) and accessory minerals (2-5%).