Lomam

This occurrence is represented by several stocks the largest of which has an area of 1.5 km2. The outcrops of alkaline rocks are isolated and are known as Lomam's stocks. According to new observations (N.V. Vladykin, pers. comm.) the alkaline rocks form a circular body around a central massif of calc-alkaline granite and syenite that is called 'Bilibin'. The alkaline rocks cut the Cambrian and lower Jurassic country rocks and include micaceous peridotite, missourite, shonkinite and fergusite. The main rock- forming minerals are olivine of variable composition, diopside, biotite and phlogopite, leucite and pseudoleucite and K-feldspar; amphibole (richterite-tremolite) is found in shonkinites and fergusites; kalsilite, and much more rarely nepheline, are found in altered leucite; apatite, titanite, chromite and magnetite are present as accessory minerals. The amount of apatite reaches a maximum in some melanocratic rocks in which P2O5 values of more than 2% occur. The micaceous peridotites consist of olivine, clinopyroxene and phlogopite in equal amounts; inclusions of leucite about 0.05 mm in diameter are included in the pyroxene. Olivine-rich xenoliths or lenses are present in the micaceous peridotites, the olivine having a higher Mg content than that of the peridotite olivine. The proportion of mafic minerals decreases in the missourites and, apart from leucite inclusions in pyroxene, up to 30% of leucite occupies the area between the mafic minerals. Primary melt inclusions have been found in olivine. Shonkinites and fergusites, that grade into melanocratic pseudoleucite syenites, have a similar suite of minerals but in different proportions. The shonkinites are rather variable in composition consisting of pseudoleucite with feldspar + kalsilite + muscovite in the proportions 6:2:1. The pseudoleucite and unaltered leucite crystals have well defined crystal shapes and contain circular inclusions of K-feldspar + nepheline. The pyroxene is diopside, olivine varies from Fo92 in micaceous peridotite to Fo65-63 in fergusite; phlogopite has 22-24% MgO in the micaceous peridotites and biotite 17-18% in the shonkinites and fergusites. From their whole rock and mineral compositions phlogopite verite and phlogopite missourite could be the plutonic facies of lamproites.