Megacrystic clinopyroxenes from Victoria (Australia): crystal chemical comparisons of pyroxenes from high and low pressure regimes
Dal Negro, Alberto; Manoli, Stefano; Secco, Luciano; Piccirillo, Enzo Michele
European Journal of Mineralogy Volume 1 Number 1 (1989), p. 105 - 122
published: Mar 21, 1989
ArtNo. ESP147050101013, Price: 29.00 €
Abstract Crystallographic and crystal chemical analysis of a suite of Ca-rich clinopyroxene megacrysts occurring in alkaline rocks from Victoria (Australia) showed the following important features. Cell volumes of clinopyroxene megacrysts are similar to those of clinopyroxenes from spinel peridotite nodules, and are significantly lower than those of phenocryst and microphenocryst clinopyroxenes crystallized at low pressure in alkaline to tholeiitic basalts. The small cell volume of clinopyroxene megacrysts is achieved essentially through a large reduction of Ml and M2 polyhedral volumes. On the contrary, the tetrahedral volume is larger than that of clinopyroxenes from spinel peridotite nodules, but it is comparable with tetrahedral volume of clinopyroxenes of low pressures crystallized from alkaline magmas. A notable feature is that clinopyroxene megacrysts are characterized by Mg/(Mg+Fe2+) ratios distinctly lower than those of clinopyroxenes from spinel peridotite nodules but similar to those of clinopyroxenes of low pressure. Low Mg/(Mg+Fe2+) ratio values, large tetrahedral volumes (i.e. high AlIV content) and small cell volume, are consistent with the clinopyroxene megacrysts having crystallized (from alkaline magmas) at high pressures (e.g. in the spinel peridotite field).