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, šnumerous 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).š
(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 65-70œ to a depth of 4 km. The angles diminish
to 30œ within the depth range of 4 to 6 km but increase again to 50-60œ 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 50-60œ at the surface to 30-40œ 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.š
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 388±6 Ma. šš
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; Zaisevš et al., 1997; Arzamastsev et al., 1998; 1999;
Bayanova, 2004) we suggest the following sequence of magmatic events in the
central Kola:
Premagmatic stage. 427±6 Ma. Mantle
metasomatism, which preceded Paleozoic magmatism (Arzamastsev and Belyatsky,
1999).
Early magmatic
stage. 404±6
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.
š388 ± 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.
š388 - 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.š
This study was financially supported by the RFBR grant
09-05-00224.