Porja Guba, White Sea: new mineralogical, isotopic and geochemical data
Lokhov K.I.*,**, Kornakov A.S.*, Presnyakov S.L.**, Kapitonov
I.N.**, Bogomolov E.S.**
*Saint-Petersburg state university, Saint-Petersburg, Russia
Karpinsky Russian Geological Research institute, Saint-Petersburg,
In the area of Porya Guba in Kandalaksha bay of the White Sea several
dikes of micaceous lamprophyres occur. Their similarity to lamproites in
mineralogical and bulk rock composition was shown by (Proskuriakov,
Uvadiev, 1992; Nikitina et al., 1999). L.P. Nikitina and coworkers
(1999) obtained Rb-Sr isochron age of 1719±8 Ma for these rocks that was
not supported by Sm-Nd data. In the same area, there are a few dikes of
nepheline picrites presumably Devonian in age. The intersections of
dikes of these 2 kinds are absent. The lamprophyre dikes are cut by
sulfide-bearing carbonate veins, well-known for their silver
mineralization, with the age of 802 Ma (Lokhov et al., 2010). We have
carried out a study of mineralogical and bulk rock compositions of the
lamprophyres, U-Pb dating of zircons separated from these rocks using
SIMS SHRIMP II and a study of Lu-Hf isotope system using LA-MCICPMS
Finnigan Neptune/DUV-193 following the method of (Lokhov et al., 2009).
In addition to data presented by (Proskuriakov, Uvadiev, 1992; Nikitina
et al., 1999; and others), it is necessary to note that the lamprophyres
range from rocks with phenocrysts of diopside (Di) and phlogopite (Phl)
and calcite-Di-Phl ground-mass to those having Phl phenocrysts and
K-feldspar, K-richterite, Sr-apatite and Phl in the ground-mass. Olivine
and leucite are absent. Among 22 dikes we have studied, the rocks with
abundant calcite in the ground-mass predominate. One of the bodies is a
carbonatite dyke that intruded a previously existing lamprophyre dike.
Phlogopite both in phenocrysts and in the ground-mass is zoned. Although
the compositional variations are not wide (10-12 wt.% Al2O3,
2-4 wt.% TiO2), general tendency is towards
tetraferriphlogopite but an increase in Al2O3 and
BaO contents is observed in some cases. Rhythmic zoning and xenogenic
cores are frequently present. Diopside is poor in Ti, Na and Al (up to 2
wt.% Al2O3); it has aegirine-augite overgrowths.
Potassic richterite is poor in Ti (0.3-1.3 wt.% TiO2).
Potassic feldspar (Kfs) is present as orthoclase and microcline; it
contains up to 3 wt.% FeO and 13 wt.% BaO. Several dikes contain albite
along with Kfs. Its appearance is probably due to secondary alteration
as well as hematite and Ba-rich feldspar exsolution and the transition
of orthoclase into microcline. Accessory minerals are rutile, monazite
and zircon. The latter is the mineral of ground-mass that crystallized
before calcite and Kfs but later than apatite and Phl. This explains
irregular shape of zircon crystals and appearance of edges and faces
when zircon contacts with calcite and Kfs. The oscillatory zoning is
seen in transmitted light and in BSE but not seen in CL because of low
luminescence which is due to high Th (440-3880 ppm) and U (330-1380 ppm)
contents and high concentration of related lattice defects. Zircon is
characterized by high Th/U ratio (0.7-3.8). Most grains are fractured;
thin alteration zones occur along cracks.
The study of isotopic composition of U and Pb in 10 zircon grains has
shown that 8 of them are reversely discordant. This is probably due to
zircon alteration and enhanced ion yields from a radiogenic labile Pb
and/or the big difference in composition between the analyzed zircon and
the standard zircon. Most grains have 207Pb/206Pb
age in the range of 1760-1830 Ma. A normally-discordant grain is much
younger. Intercepts of discordia and concordia give age values of
1738±22 and 827±150 Ma that correspond to intrusion of the dikes and to
the superimposed alteration respectively. The last value is close to the
time of silver-bearing veins formation. Intrusion of alkaline picrites
and/or calcite veins could produce the observed alteration of minerals
in lamprophyres and the partial loss of radiogenic Pb in zircon. The Lu-Hf
isotopic system has shown that zircons have low 177Lu/176Hf
<0.001 and contain excess radiogenic Hf. The initial isotopic
composition varies broadly:
from -7 to +42 that is not possible in case of single magmatic zircon
generation. No correlation between 177Hf/176Hf and
177Lu/176Hf is observed. The radiogenic Hf hence
is not related to inclusions of minerals having high 177Lu/176Hf
ratios, for example, apatite, garnet and carbonates. This suggests that
a component containing radiogenic Hf was captured by zircon when it
crystallized or reacted with a fluid during late-magmatic stage. Hf-Nd
correlation diagram (Fig. 1) demonstrates heterogeneous distribution of
excess radiogenic Hf in zircon. Same situation was described in a
carbonate-silicate metamorphic rock (Lokhov et al., 2009). Partial
recrystallization or replacement of minerals with high Lu/Hf in a
deep-seated source is necessary to produce melts or fluids having an
excess of radiogenic Hf. Garnet from Phl-eclogites that are present as
xenoliths in these dikes is a probable source of radiogenic Hf.
Phlogopite in eclogites replaces garnet giving rise to a fluid with
radiogenic Hf. Isotopic data for lamprophyres: εNd(T) = -8 and (87Sr/86Sr)I
= 0.7027 support our interpretation.
Fig. 1. Hf-Nd correlation diagram. Dotted grey lines demonstrate a
terrestrial array – a correlation band for magmatic rocks, ellipses
present fields of kimberlites of first (1), second (2) and transitional
(3) type and lamproites and minettes (4)
(Davies et al., 2006; Nowell et al., 2004).
Despite the obvious similarity to lamproites, the presence of calcite
and Na-rich feldspar excludes the possibility to use the term
“lamproite” for these rocks. The term “minette” is probably best.
Minettes from Churchill province, Canada (Peterson et al., 2002),
including diamond-bearing bodies, are compositional analogues of Porja
Guba lamprophyres. These authors as well as L.P. Nikitina and coworkers
(1999) relate the origin of the minettes to postcollisional melting of a
mixed crustal-mantle source. A further study of magmatic evolution of
the minettes is necessary to understand their origin. The complex zoning
of minerals and the presence of a carbonatite in this suite point to
possible interaction of melts of different origin.
This study was performed within the scope of Scientific Research Work
funded by federal budget № 126.96.36.1991 and № 188.8.131.521.
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