PYROXENES AND NEPHELINES IN ALKALINE EFFUSIVES OF THE KUSNETZKY ALATAU

摘要

Microprobe analysis was the principal method to study the composition of rock-forming minerals. The maximum range of variety in mineral composition has place in bereshite - the rock containing up to 40 percent of large nepheline fenocrysts. There was revealed the wide variations in composition of pyroxene also: from Fe-poor diopside within cores of fenocrysts, to low magnesian hedenbergite in their rims. Nepheline in these inclusions is rhythmically zonal, and the quantity of kalcilite fractions and excessive silica increases from core towards outer zones, as well as amount of anorthite decreases from inner zone to external rims of fenocrysts. Pyroxenes in nephelinites and tephrites are richer in Fe than in fenocrysts of bereshite. Aluminium minal is replaced in these rocks by hedenbergitic and diopsidic ones. In phonolites fenocrysts cores have the composition similar to later generations of the same minerals in bereshite. Obtained results allow to suppose that, during the bereshite formation, accumulation and crystallization of nepheline from melt was a longlasted process. Meanwhile, the composition of the melt evaluated from mafic to salic, phonolitic one.