Megacrystic clinopyroxenes from Victoria (Australia): crystal chemical comparisons of pyroxenes from high and low pressure regimes.

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 spine I peridotite nodules, and are significantly lower than those of phenocryst and microphenocryst clinopyroxenes crystallized at low pressure in alkaline to tholeiitic basalts. The small celt volume of clinopyroxene megacrysts is achieved essentialty through a large reduction of MI 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 crystaltized from alkaline magmas. A notable feature is that ctinopyroxene megacrysts are characterized by Mg/(Mg+ FeH) ratios distinctly lower than those of ctinopyroxenes from spinel peridotite nodules but similar to those of ctinopyroxenes of low pressure . Low Mg/(Mg+ Fe2+) ratio values, large tetrahedral volumes (i. e. high Apv content) and small cell volume , are consistent with the ctinopyroxene megacrysts having crystallized (from alkaline magmas) at high pressures (e.g. in the spinel peridotite field).