Lovozero

The Lovozero alkaline complex was first described by Ramsay (1887), his preliminary work laying the foundations for an expedition which between 1890 and 1899 made a detailed study of this complex and the adjacent Khibina intrusion (locality 17). Ramsay thought that the two intrusions were parts of a single complex of which Lovozero comprised the upper portion. He described for the first time the layered structures and was aware of the similar structures in occurrences in Greenland. He considered that the layering was generated by successive intrusion. Work then ceased until resumed by Fersman under whom teams of petrologists and mineralogists in the 1920s undertook systematic research. This and subsequent research, up to 1966, is briefly detailed in the Introduction to the book by Vlasov et al. (1966). Lovozero is a notably rich source of minerals and many species have been recorded for the first time from this complex. The Lovozero complex is located in the central part of the Kola Peninsula. It consists of a mountainous upland with steep precipitous slopes and flat plateau-like summits, elevated almost a kilometre above the surrounding hilly plain. It is located between the two large lakes of Umbozero and Lovozero with Lake Seydozero nestling in the central part of the complex. The massif is rectangular in plan and has an area of 650 km2. Lovozero lies in a northwesterly striking tectonic zone within which a sunken, east-west-trending belt of Palaeozoic rocks has been preserved. Within the complex are included areas of steeply dipping granite-gneisses and rare patches of highly aluminous rocks and ferruginous quartzite of Proterozoic age, all of which have a northwesterly strike. The roof of the intrusion is younger than these formations and consists of the Lovozero effusive-sedimentary formation of middle Palaeozoic age. The rocks of the Lovozero formation occur as isolated xenoliths within the nepheline syenites of the massif. The Lovozero extrusive rocks comprise picrite porphyries, augite porphyries, essexite porphyries and a suite of alkaline rhomb porphyries, porphyritic phonolites and pseudoleucite porphyries with the more alkaline varieties becoming more predominant with time. The age of these formations is similar to that of the intrusive rocks, which is confirmed by the presence of a late Devonian flora in sedimentary rocks interbedded with the volcanics. The Lovozero intrusive complex is emplaced in the Archaean granite gneiss sequence and has the form of a laccolith with a broad base. According to the geophysical work of Shablinsky (1963) the alkaline rocks can be traced to a depth of more than 7 km, but their lower limit could not be determined. Geophysical investigations have also shown that the intrusion consists of two structural units. The upper part comprises a layered intrusion about 2 km thick and covering 20x30 km. The lower stock-like part has been displaced towards the east relative to the centre of the layered body, measures 12x16 km in plan and extends to a depth of more than 5 km. The dip of the contacts of the intrusion with the surrounding rocks is steep and close to vertical. The lower part of the intrusion, according to the same geophysical investigations, has a concentrically zoned structure. The density of the rocks of successive zones increases from the centre towards the periphery, but the density of the rocks of the lower stock-like part of the complex remains close to the values known for the main varieties of nepheline syenites of the intrusion. The upper layered part of the alkaline intrusion has in large measure been exposed by erosion and comprises a complex single-phase formation. The geological structure of the complex and the sequence of formation of the rocks have been interpreted in different ways by different investigators. The following table summarises the principal ideas on the general igneous relationships within the complex. The ideas developed by us on the structure of the massif, and presented here, are closest to the system of Eliseev et al. (1937). The rocks of the Lovozero complex comprise four assemblages, formed in four intrusive phases. The rocks of phase I occupy only about 5% of the total volume, but it is possible that the role of these rocks increases with depth. The rocks of phase II comprise the main area (77%) and those of phase III a lesser amount (18%). Rare dykes of phase IV are volumetrically insignificant (0.01%). The oldest intrusive formations (Phase I) are even-grained nepheline syenites, nepheline-nosean syenites, poikilitic nosean syenites and metamorphosed nepheline syenites. Amongst this assemblage of rocks there are suites of hypabyssal character (nepheline-syenite porphyries). The rocks of Phase I, occurring in their original position, are located in the marginal parts of the complex and exposed at low levels at the foot of the mountains. Outcrops in a small sector in the northwestern part of the massif indicate that the contact of the rocks of Phase I with Phase II dips towards the centre of the complex at an angle of 20-30°. It is probable that the rocks of the first phase underlie the rocks of intrusive Phase II, but at what depth and to what extent is unknown. Within the nepheline-nosean syenites cross-cutting dykes, including pegmatites, of intrusive Phase II have been identified. Abundant xenoliths of rocks of intrusive Phase I are found throughout the complex in rocks of later phases. In individual xenoliths the varieties of nepheline syenite of intrusive Phase I have gradual transitions between them but the in situ rocks of Phase I are mainly nepheline-nosean and poikilitic nosean syenites. The second intrusive phase comprises a strongly differentiated complex of urtite, foyaite and lujavrite. Outcrops of these rocks are found on the slopes of the mountains. The geometry of this suite of rocks has not been finally established, although it has been studied, as a result of drilling, to a depth of 2 km, and probably has a circular form. Phase II consists of a layered sequence with, as seen in vertical sections, a regular alternation of layers of urtite, juvite, foyaite and aegirine and amphibole lujavrite which range in thickness from a few centimetres to hundreds of metres. The stratigraphical order of the rock types is the same in the various parts of the massif, and the inward dips of the layers are at low angles and vary little between the margins and the centre of the complex. In the marginal parts, bordered by older formations, the layering as a rule disappears and the facies of the rocks close to the outer contact consists of mesocratic, coarse-grained, sometimes pegmatitic foyaites. Among the lujavrites there are circular bodies of poikilitic sodalite syenite. In the lower part of the urtite-foyaite-lujavrite suite an eruptive breccia is sometimes developed which contains xenoliths of rocks of intrusive Phase I. The individual layers of nepheline syenite in this suite are enriched in xenoliths of Devonian extrusive rocks and of intrusive Phase I rocks for distances of several kilometrtes. A well-known eruptive breccia of altered tuffaceous rocks occurs in a thin horizon of apatite urtites, which can be traced around the entire perimeter of the suite. The rocks of intrusive Phase III comprise a suite of eudialyte lujavrites which cut, and overlie, the upper part of the rocks of Phase II. The plane of contact between rocks of Phases II and III dips towards the centre of the complex with the angle of dip increasing from the margins towards the centre. In general the suite of eudialyte lujavrites has the form of a complex funnel-shaped body (ethmolith). The rocks of Phase III form the summits of the mountains of the Lovozero Massif, and thethickness of this suite reaches 450 m, but, because of erosion, decreases from northwest to southeast. Near the contact with the rocks of the second intrusive phase the eudialyte lujavrites contain an eruptive breccia of these rocks with xenoliths measuring from a few metres up to 2 km across. Moreover, in the eudialyte lujavrites there are also xenoliths of Phase I rocks with porphyritic textures. The rocks of intrusive Phase III include leucocratic, mesocratic and melanocratic eudialyte lujavrites, eudialyte foyaite and urtite and a coarser layering is developed than in the rocks of the second intrusive phase. On the boundary with the rocks of Phase II there are ubiquitously layered bodies of porphyritic lujavrite, which are late derivatives of Phase III. Individual dykes of porphyritic lujavrite, up to several kilometres long, cut the rocks of Phases I and II. Poikilitic sodalite syenite and tawite (sodalitite with some aegirine and minor nepheline, alkali feldspar and eudialyte) are found in the form of equidimensional, sharply defined bodies amongst the eudialyte lujavrites of Phase III. The rocks of intrusive Phase IV consist of rare dykes of alkaline lamprophyres (monchiquite, fourchite, tinguaite, etc) which cut all the older alkaline rocks, and also the surrounding granite gneisses. The principal rock-forming and characteristic accessory minerals of the Lovozero rocks are given in the accompanying table. A general account of the complex, which concentrates on the pegmatites, mineralogy and geochemistry of individual elements, is that of Vlasov et al. (1959) of which there has been an English translation (Vlasov et al., 1966). Gerasimovsky et al. (1966) have also described the geology, petrology and geochemistry of Lovozero and this is also available in English (Gerasimovsky et al., 1968). Khomyakov (1987) has described unstable and water soluble minerals.