PYROXENES OF ALKALINE-ULTRABASIC ROCKS FROM NORTH-WESTERN PART OF THE UKRAINIAN SHIELD

Abstract

In alkaline-ultrabasic rocks from North-Western part of the Ukrainian Shield pyroxenes are predominantly presented by diopsides with various contents of Cr2O3 (up to 2.4%, chrome diopside), Na2O and Al2O3 (to 7.6%). But correlation between contents of Al2O3 and Na2O generally is absent except two samples. Natrium goes into composition of acmite, cosmochlore or jadeite minals. In pyroxenes with high Al2O3 aluminum goes into tetrahedral positions. Pyroxenes (include aegirines) with increase TiO2 content (to 1.5%) rarely occur. Aegirine and intermediate between latter and diopside aegirine-diopsides have limited development. They could crystallize in at least eroded dike rocks that consist of melteigite porphyres with high agpaicity ((Na+K)/Al ≈1). Majority of studied pyroxenes crystallized from magnesian (to 22% MgO) primitive alkaline-ultrabasic melts in hypabyssal intrusions. It is supposed that limited amount of intermediate aegirine-diopsides or a gap in series diopside-aegirine is caused by the crystallization of alkaline and subalkaline amphiboles.