Abstract
Holocrystalline gabbro-anorthosites and anorthosites have been found in the thick interformational veins of the upper portions of the Pliocene-Quaternary Nikolka volcano in Kamchatka. The gabbro-anorthosites contain a marked amount of K-feldspar, and on the basis of their chemistry, they have been assigned to the tephrite-basalts. They are identical to the megaplagiophyre basalts, which are widely distributed in the Central Kamchatka depression. It has been suggested that the magma of subalkaline gabbro-anorthosites occurred in signifcant depths at the base of the crust during emanation differentiation of a saturated tholeiite. Further fractionation of this magma could have led to the formation of autonomous anorthosites and alkaline gabbroids associated with them in natural assemblages. —Authors.
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