Original paper

New data on pyrochlore- and perovskite-group minerals from the Lovozero alkaline complex, Russia

Chakhmouradian, Anton R.; Mitchell, Roger H.

Abstract

Pyrochlore- and perovskite-group minerals are relatively common accessory constituents of agpaitic murmanite lujavrites at the Lovozero alkaline complex (Russia). These rocks contain euhedral crystals of niobian calcian loparite-(Ce) and, more commonly, ceroan lueshite that occurs as discrete oikocrysts and rims on the loparite-(Ce). The overall compositional range exhibited by these phases is (Na0.44-0.79 REE0.14-0.37 Ca0.02-0.12 Sr0.04-0.09 Th0-0.01) (Nb0.12-0.66 Ti0.33-0.85 Fe0-0.01 Ta0-0.01) O3; it agrees well with the evolutionary trend established previously for perovskite-group minerals from Lovozero. The murmanite lujavrites also contain early-crystallizing uranoan pyrochlore that subsequently underwent alteration to uranpyrochlore (5.0-26.4 wt.% UO2 for both) through interaction with a deuteric fluid. The alteration pattern involves a decrease in Na, Ca and Sr contents from the core outward, increasing ionic deficiency in the A and Y sites, and progressive hydration. The proportion of relatively higher-charged cations does not change or slightly decreases toward the rim. The occurrence of lueshite and U-bearing pyrochlore in the murmanite lujavrites indicates that these rocks crystallized from the most evolved portion of a parental phonolitic magma. Pyrochlore-group minerals also occur in albite-rich, magnesio-arfvedsonite- and aegirine-bearing metasomatic rocks.These parageneses typically contain "silicified" varieties of pyrochlore exhibiting an oscillatory zoning pattern and, in some cases, superimposed secondary zoning. The metasomatic pyrochlore ranges from nearly stoichiometric Na-Ca-rich compositions to cation-deficient strontiopyrochlore (up to 10.3 wt.% SrO) and plumbopyrochlore (up to 37.9 wt.% PbO). The oscillatory zoning involves variations in cation occupancy of the A site (primarily Na, Ca and Sr), and Si content. Elevated levels of Si (up to 16.8 wt.% SiO2) are invariably associated with the zones having the highest cation deficiency and H2O contents. A negative correlation observed between the Si and (Nb+Ti) contents is interpreted to result from changes in pH, a(SiO2), a(Na1+), a(Ca2+), and activities of minor components during the crystal growth. Primary pyrochlore-group minerals from both lujavrites and albitites are characteristically poor in Ta (< 2.6 wt.% ta2O5), and enriched in Sr and light rare-earth elements. Comparative data on pyrochlore-group minerals from other alkaline-rock occurrences are presented.

Keywords

pyrochlorelueshitenepheline syenitealbititeagpaitic rockslovozero complexkola peninsularussia