The alkaline polyphase plutons in the NE Fennoscandian Shield, Russia: deep structure and duration of magmatism

Arzamastsev A.A., Arzamastseva L.V., Zhirova A.M.

Geological Institute of the Kola Science Centre RAS, Apatity, Russia

 

The Palaeozoic Kola Alkaline Province comprises giant nepheline syenite plutons, numerous carbonatite intrusions, and dyke swarms. It is a wide-spread opinion that the Khibina and the Lovozero massifs are made up mostly of agpaitic syenites with minor foidolites (Gerasimovsky et al., 1966; Zak et al., 1972), which were formed during a relatively short time span (Kramm and Kogarko, 1994). Geological studies performed during the last two decades give evidence for the presence of carbonatites, peridotites, pyroxenites, melilitic rocks, olivine melteigites, alkaline syenites and zircon-bearing syenites in the Khibina and partly in the Lovozero. The geological position and the volume of these rocks within the massif as well as their genetic relations are still under debate. Additionally, the wide spectrum of rock types that comprise these giant polyphase massifs inspire us to decipher the sequence of the rock formation and to revise the time span of intrusions. The performed studies included geological and geophysical investigations (1), Rb-Sr, U-Pb, Sm-Nd, Ar-Ar isotope dating (2) and petrological modelling (3).

(1) The combined seismic and gravity data were used to create 3D velocity and density models of the Khibina and Lovozero massifs deep structure down to the level of 12 km. The concentrically-zoned structure of the Khibina complex extends throughout the depth range accessible to computation. The western and southern contacts dip inward at angles of 65-70 to a depth of 4 km. The angles diminish to 30 within the depth range of 4 to 6 km but increase again to 50-60 below a depth of 7 km. The eastern contact of the massif, which is close to the carbonatite stock, is almost vertical to the depths of 3-4 km and tends to progressively slope more gently at the depths of 4-5 km. Within the northern sector of the massif, the gravity anomaly indicates the presence of a large zone of alkaline ultramafic rocks. Structural analysis of the central part of the massif indicates that there is a rounded plate-like positive gravity anomaly beneath the foyaite core of the massif. This anomaly is suggested to be caused by remnants of a 2.5 km thick plate of rocks, which characteristics are close to those of foidolites.

The southeastern, southern, and western contacts of the Lovozero massif are almost vertical to a depth of 4 km within the nepheline syenite zone, but develop more gentle slopes at depths below 9-10 km. The northern and northwestern contacts dip at smaller angles, that vary from 50-60 at the surface to 30-40 at depths of 4-5 km, but become nearly vertical from this level to depths of 9-10 km. At depths below 2 km, the Lovozero complex consists of two zones that significantly differ in density: a southeastern zone, which comprise rocks with a density of 2660-2750 kg/m3, and a northeastern zone, which rocks have a density greater than 2800 kg/m3. The northeastern zone probably joins the Kurga alkaline-ultramafic massif at a depth of 7-8 km. The southwestern part of the Lovozero complex seems to be composed of agpaitic syenite to a depth of at least 10 km. This is the most probable location of the magma conduit of the nepheline syenite intrusion. In the central part of the massif, at Seidyavr Lake, we have detected a local negative gravity anomaly which corresponds to a body of zircon-bearing syenite with a density of 2580-2630 kg/m3.

(2) In order to estimate the duration of magmatic activity in the Khibina-Lovozero area, in addition to already published data (Kramm and Kogarko, 1994; ....), we have performed some geochronological studies of the earliest and the youngest rocks.

The volcanogenic pile of the Lovozero massif. Five samples from xenoliths in nepheline syenites, composed of alkaline picrite and alkaline basalt, show a poorly fitting regression line of 87Sr/86Sr vs 87Rb/86Sr that yielded 450+/-50 Ma. High dispersion (ISr from 0.7021 to 0.7037) is suggested to be the result of an isotope resetting due to the influence of a nepheline syenite magma.

The olivine melanephelinite dike which filled concentric faults 5 km south of the Khibina contact. 40Ar/39Ar age determination give the age of 3886 Ma.

The Rb-Sr isochron dating of the olivine melanephelinite from the pipe cutting the Khibina intrusion gives the age estimation 358+/-31 Ma and the initial ratio of ISr=0.70385+/-17 with MSWD = 4.0 (N=7). The Sm-Nd isochron for the same rock gives 362+/-91 Ma (N=6, Initial 143Nd/144Nd = 0.512256+/-0.00062. MSWD = 0.58).

The zircon from the albite-microcline veins in the outer contact of the Lovozero massif. The SHRIMP II U-Pb single grains zircon dating gives the age estimation of 359+/-5 Ma (N-22, MSWD= 0.026).

The zircon from the zircon bearing alkaline syenite in the core of the Lovozero massif. The SHRIMP II U-Pb single grains zircon dating gives the age estimation of 347+/-8 Ma (N-5, MSWD= 1.2).

(3) Taking into account the previous geochronological data (Kramm and Kogarko, 1994; Zaisev et al., 1997; Arzamastsev et al., 1998; 1999; Bayanova, 2004) we suggest the following sequence of magmatic events in the central Kola:

Premagmatic stage. 4276 Ma. Mantle metasomatism, which preceded Paleozoic magmatism (Arzamastsev and Belyatsky, 1999).

Early magmatic stage. 4046 Ma. The collapsed Lovozero caldera originated in the Late Archean tonalite trondhjemite-granodiorite complex; volcanic activity began in the northeastern part of the newly formed structure, the formation of the subalkaline Kurga intrusion.

Main magmatic stage.

388 6 Ma. The formation of concentric faults and the collapsed Khibina caldera originated at the contact between the Late Archean tonalite-trondhjemite-granodiorite complex and the Early Proterozoic Pechenga-Imandra-Varzuga paleoriftogenic complex.

388 - 371 Ma. Ultrabasic-alkaline magmas intruded mainly in the northern part of the Khibina caldera and in the northeastern part of the Lovozero caldera. Formation of olivine pyroxenite, melilitic rocks, olivine melteigite intrusions in the periphery of the calderas.

371 - 362 Ma. Main agpaite syenite series of the Khibina and Lovozero massifs originated. Further collapse of the caldera and development of a layered ijolite-melteigite complex in the central portion of the Khibina caldera. The ijolite-melteigite complex was cut by a series of conical faults, along which phosphate-bearing urtite-juvite-kalsilite syenite intrusion was emplaced. In the central part of the ijolite-melteigite series a new series of conical faults formed and foyaite intrusion occupied the core of the Khibina massif.

367 - 366 Ma. The Khibina pulaskite and carbonatite stock formed.

Late magmatic stage.

363 - 359 Ma. Dike series of olivine melanephelinite, nephelinite, phonolite were formed.

359 5 Ma. Formation of the microcline-albite pegmatoid veins with ilmenite and zircon.

347 8 Ma. Late magmatic processes in the core zone of the Lovozero massif as indicated by the crystallization of zircon bearing alkaline syenites.

This study was financially supported by the RFBR grant 09-05-00224.

 


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