New isotopic (Rb-Sr,
Sm-Nd) and geochemical (ICP-Ms) data on kimberlites of the different
emplacement phases from the Udachnaya pipe (Yakutia)
Egorov K.N.
Institute of the Earth’s
crust SB RAS, Irkutsk, Russia
egorov@crust.irk.ru
The Udachnaya kimberlite
pipe is one the largest pipe in the world as to reserves and size of the
primary diamond deposits of Yakutia. Its ore structure consists of two
double multi-phase pipes – Udachnaya-Eastern and Udachnaya-Western, four
structurally conjugated satellite “blind”kimberlite bodies and six
kimberlite veins. The Early Triassic dolerite dykes and Cretaceous vein
of K-trachytes are structurally conjugated with occurrences of
kimberlite magmatism (Egorov et al., 1988; 1989). These rocks form a
unified series: dolerites-trachydolerites-trachytes characterized by
gradual change of mineralogo-petrographic and petrochemical
peculiarities from initial to later members. Occurrence of kimberlite
and basic rock magmatism is related to system of disjunctive
dislocations forming the regmatic network: orthogonal (sublatitudinal
and submeridional systems) and diagonal (north-western and north-eastern
systems).
The first stage of the
kimberlite magmatism occurrence in the region of the Udachnaya ore
complex is related to the formation of powerful north-eastern system of
rupture dislocations controlling spatially the majority of prepipe
kimberlite veins and satellite “blind”kimberlite bodies. Kimberlite
veins confined to the north-eastern fault zone consist of carbonatizated
magnophyric kimberlites with variable (5-7%) quantity of phlogopite. The
ground mass of vein kimberlites are characterized by calcite
pseudomorphs after olivine, phlogopite laths, microlites of calcite,
grains of perovskite and apatite.
The highest concentrations
of all elements (LILE, HFSE, and REE) are characteristic of vein
kimberlites in comparison with those of other phases of emplacement
(Fig.). Vein kimberlites are characterized by negative anomalies of U,
Zr, Hf and poorly differentiated gentle slope of the distribution
specter of middle and hard REE. Judging by isotopic composition of
kimberlites (εNd=4.2, 87Sr/86Sr(t)=0.7050) its
mantle source corresponds to the moderately depleted mantle (Tabl.). The
model age of TNd(DM) enrichment of the kimberlite mantle
source is equal to 651 m.y.
The kimberlite veins are
cut by satellite “blind”kimberlite bodies structurally related to
north-eastern, rarely to sublatitudinal system of rupture dislocations.
They are exposed at the depth of 5-20 m from the present-day surface.
The bodies are composed of carbonatizated kimberlite breccia containing
autoliths, fragments of vein kimberlite and xenogenic material.
Massive monticellite and
monticellite-olivine kimberlites composing mainly the peripheral zones
and stock-like bodies (at deep horizons) in the Udachnaya-Eastern and
Udachnaya-Western pipes were emplaced after kimberlite veins and
satellite bodies (Egorov, Bogdanov, 1991; Egorov et al., 1991). These
kimberlite are characterized by obviously zonal olivine with high
Fe(Fe+Mg) content (12-14 mol. %) and high concentration of CaO.
Monticellite exhibits low mg number and high content of Al, Ti and Na.
Unlike the kimberlite of the pre-pipe veins the geochemical composition
of monticellite kimberlite of the early emplacement phases is
characterized by lower values of LILE, HFSE concentration and
particularly hard REE (Fig.). They exhibit the distinct negative
anomalies of Rb, Zr, Hf and Y.
The stock-like and vein
bodies composed of micaceous and mica kimberlites were formed at the
same stage (Egorov et al., 1986). The mica kimberlites with pyroxene
groundmass (Udachnaya-Eastern pipe) and high sphene content (Udachnaya-Western
pipe) are presented in the form of fragments in kimberlite breccias (Egorov
et al., 1991). The age of fragment of the vein unmodified mica
kimberlite from the Udachnaya-Eastern pipe obtained by Rb-Sr method,
corresponds to 352±5 m.y. (Maslovskaya et al., 1982). The mica
kimberlites are characterized by more differentiated character of the
distribution specter of rare incoherent elements (Fig.). They exhibit
sharp positive anomalies of Nb and Ta as well as negative anomalies of
Th and U. Weak positive anomalies of Zr and Hf, and so right with slope
the specter character from Nd to hard REE are the characteristic
property of them (Fig.). According to the obtained isotopic data (εNd=-4.8,
87Sr/86Sr(t)=0.7078) the mantle source of mica
kimberlite corresponds to the enriched mantle of the EM11-type (Tabl.).
The model age of TNd(DM) enrichment of the mantle source of
the mica kimberlite is more ancient and equal to 924 m.y.
It should be noted that
similar negative εNd values for the fragments of vein mica kimberlites
from the Udachnaya pipe are first obtained unlike the all known εNd
isotopic ratios for the kimberlites of the Yuakutian diamondiferous
province as a whole.
In the following stages of
the Udachnaya ore node formation the main volume of kimberlitic material
occurred (according to succession of the phase’s emplacement) in the
form of brecciated ovoid kimberlite, taxite in the Udachnaya-Western
pipe and protoclastic and deuteroporphyric kimberlites, tax
kimberliteite in the Udachnaya-Eastern pipe (Egorov et al., 1991). The
final phases of the brecciated kimberlites emplacement possessed a great
penetration capacity and more frequent reached the upper levels of the
pipes (Egorov, 1985). When rising they actively disintegrated the early
phases of massive porphyric kimberlites. Small in volume post-pipe dyke
and vein formations composed of olivine-monticellite kimberlites occur
among the latest phases of the kimberlite emplacement (Egorov, Bogdanov,
1989; Kornilova et al., 1998). They differ from the early monticellite
variety in composition of olivine, monticellite, spinel and perovskite
of ground mass. Geochemical composition of the monticellite kimberlites
of the late phases of emplacement is characterized by the lowest values
of LILE, HFSE and REE concentrations (Fig.). They exhibit the distinct
negative anomalies of Zr and Hf.
Fig. Distribution specters
of rare incoherent elements in kimberlite vein (spec. Zh-2), in fragment
of mica kimberlite (spec. 78-186 UW), in fragment of monticellite
kimberlite of the early emplacement phase (spec. 218/470-1UE), in
post-pipe veins of kimberlite (spec. 222/407UE, 222/403UE).
Table.
Nd and Sr isotopic composition of the mica kimberlite fragment (spec.
78-186 UW) from the Udachnaya-Western taxite and kimberlite of the vein
(spec. Zh-2) (spec. Zh-2) (2).
№
|
[Sm] |
[Nd] |
147Sm/144Nd |
143Nd/144Nd |
εNd |
TNd(DM)
|
[Rb] |
[Sr] |
87Rb/86Sr |
87Sr/86Sr |
(87Sr/86Sr)t |
ppm |
ppm |
1 |
11.78 |
81.26 |
0.08806 |
0.512390 |
-4.8
|
924 |
132.1 |
858.3 |
0.44557 |
0.71012 |
0.707836 |
2 |
22.50 |
173.7 |
0.0781 |
0.512573 |
4.2 |
651 |
200.7 |
1581.1 |
0.37791 |
0.70696 |
0.705076 |
Note. Sm-Nd and Rb-Sr
isotopic data of the mica kimberlite were obtained at ECI SB RAS
(Irkutsk). Sm-Nd isotopic data of the vein kimberlite were performed at
IGGD RAS (St. Petersburg). Rb-Sr isotopic measurements of the vein
kimberlite were performed at IG KSC RAS (Apatity).
References
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