These volcanic and subvolcanic alkaline rocks comprise mainly alkali basalts, trachybasalts, trachyandesites and phonolites. Representative probe analyses of 53 clinopyroxenes show that in the basic and intermediate rocks these follow the diopside-salite-ferrosalite-hedenbergite fractionation trend which is significant for the undersaturated alkaline rock suites with low silica activity. The Ti and Al content in pyroxenes of alkali basalts increases with crystallization, but the coprecipitation of the other Ti- and Al-bearing phases impoverishes the pyroxenes of trachybasalts and trachyandesites in these elements in the later stages of crystallization. The incorporation of both elements into the pyroxene structure is indicative of non-equilibrium crystallization (e.g. sector zoning). Phonolites have salite and ferrosalite xenocrysts; pyroxenes which crystallized from the phonolitic melt change their composition from hedenbergite to aegirine-hedenbergite; in some of the latter ZrO2 up to 1.9 wt.% may occur. The continuous variation of pyroxene chemistry from diopside to aegirine presents a good example of low-P fractionation in an undersaturated alkaline suite. -R.A.H.
|Number of pages||21|
|Journal||Neues Jahrbuch fur Mineralogie, Abhandlungen|
|Publication status||Published - Jan 1 1987|
ASJC Scopus subject areas
- Geochemistry and Petrology