The Petrology and Chemical Composition of a Tholeiite Sill on Schaapkraal 68JT, Lydenburg District, Transvaal, South Africa

C. Frick
Department of Geology, University of Pretoria

Summary: A glass-bearing dolerite, which consists of orthopyroxene phenocrysts (γ 1·689, α 1·678, β 1·680, 2Vα 81°, S.G. 3·35) and a glassy mesostasis (n 1·568, S.G. 3·25), is described petrologically. Three new chemical analyses are given. From the petrological and chemical data it can be concluded that the orthopyroxene formed under intratelluric conditions and probably under high pressure. According to the recalculated chemical analysis of the orthopyroxene 0·22 Al3+ is in the Y position and much less Al3+ in the Z position. The chemical analysis of the residual glass is compared with similar residual glasses, and it shows that iron enrichment characterizes the earlier stages of crystallization differentiation, whereas the later stages are marked by an increase in Si4+, K+, and Na+.

Mineralogical Magazine; December 1970 v. 37; no. 292; p. 909-915; DOI: 10.1180/minmag.1970.037.292.06
© 1970, The Mineralogical Society
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