Introduction The Lovozero alkaline complex in the Kola Alkaline Province (approximately 67º47’ N, 34º45’ E) is one of the most interesting of the family of sodic nepheline-syenite intrusions owing to its colossal size and remarkable degree of differentiation. The complex consists of several intrusive series emplaced in the Archean metamorphic rocks during the Devonian. The major series include: (1) porphyritic alkali and nepheline syenites; (2) poikilitic feldspathoid syenites; (3) a differentiated series consisting of alternating urtite, foyaite and lujavrite units; (4) eudialyte lujavrites; (5) murmanite lujavrites; and (6) alkaline lamprophyre dikes (Bussen & Sakharov, 1972). The mode of occurrence and textural features of series (5) are significantly different from typical lujavrites, and more consistent with their clas- sification as phonolites (authors’ unpubl. study). However, to avoid confusion, we shall refer to these rocks as murmanite lujavrites, because they are known as such to most researchers familiar with the geology of Lovozero. Late-stage metasomatic (albite, aegirine-albite and amphi- bole-albite) rocks are very common at the margins of the complex, where they develop at the expense of earlier-crys- tallized intrusive suites and metamorphic wall rocks. The mineralogy of the Lovozero complex is described by Vlasov et al. (1966) and Semenov (1972). However, these two studies, as well as the recently published monographs of Khomyakov (1995) and Pekov (2000) focus primarily on alkaline pegmatites and associated hydrothermal parage- neses; these rocks, although mineralogically unique, are volumetrically minor and very scarce (particularly, their well-differentiated types). Very few studies on the miner- alogy and petrology of the major intrusive series have been undertaken since the publication of Bussen & Sakharov’s (1972) work, and our knowledge of these rocks is utterly inadequate. Wet-chemical analyses of major minerals from Eur. J. Mineral. 2002, 14, 821-836 New data on pyrochlore- and perovskite-group minerals from the Lovozero alkaline complex, Russia ANTON R. CHAKHMOURADIAN 1)* and ROGER H. MITCHELL 2) 1) Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 2) Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada P7B 5E1 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 (Na 0.44-0.79 REE 0.14-0.37 Ca 0.02-0.12 Sr 0.04-0.09 Th 0-0.01 ) (Nb 0.12-0.66 Ti 0.33-0.85 Fe 0-0.01 Ta 0-0.01 ) O 3 ; 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.% UO 2 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 compo- sitions 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.% SiO 2 ) are invariably associated with the zones having the highest cation deficiency and H 2 O contents. A negative correlation observed between the Si and (Nb+Ti) contents is interpreted to result from changes in pH, a(SiO 2 ), a(Na 1+ ), a(Ca 2+ ), 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.% Ta 2 O 5 ), and enriched in Sr and light rare-earth elements. Comparative data on pyrochlore-group minerals from other alkaline-rock occurrences are presented. Key-words: pyrochlore, lueshite, nepheline syenite, albitite, agpaitic rocks, Lovozero complex, Kola Peninsula, Russia. 0935-1221/02/0014-0821 $ 7.20 © 2002 E. Schweizerbart’sche Verlagsbuchhandlung. D-70176 Stuttgart DOI: 10.1127/0935-1221/2002/0014-0821 *e-mail address of the corresponding author: CHAKHMOU@ms.umanitoba.ca