Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry
(IGEM)
N. G. Grinevich
Karelian Geological Survey, Petrozavodsk, Russia
Contents
Abstract
The Burakovka layered pluton of basic and mafic rocks is the largest intrusive
massif in the Baltic Province composed of Si- and Mg-rich boninite-like rocks. The
pluton consists
of two individual bodies, each having its own internal structure, and contacting
each other in their
apical parts, known as the Aganozero and Shalozero-Burakovka bodies. Both bodies
have a similar
rock sequence including five differentiated zones (upward): mafic rocks, pyroxenite,
gabbro norite,
pigeonite gabbro norite, and magnetite gabbro diorite (the latter found only in the
Shalozero-Burakovka
Body). Being generally similar to each other, these bodies differ notably in the
styles of
their cumulate stratigraphy and, to a lesser extent, in composition.
The pluton is distinguished by the presence of markers - singular interlayers of
high-temperature
mafic cumulates emplaced in the sequence of lower-T formations. Their origin is
believed to have been associated with the intrusion of fresh magma portions into
the crystallizing
magma chambers. The same mechanism is believed to have been responsible for a macrorhythmic
pattern found in the southeastern portion of the Shalozero-Burakovka intrusive body.
Using chemical and mineralogical data, it is shown that the bodies discussed were
derived
from similar high-Si and high-Mg magmas, except that the Aganozero Body was emplaced
50
million years later than the Shalozero-Burakovka intrusion: the former was dated
(Sm-Nd isochron)
2372
22 Ma ( e Nd=-3.22
0.13 ),
and the latter, 2433
28 Ma
( e Nd=-3.14
0.14 ).
It is concluded
that the Burakovka Pluton was a long-lived magma center which developed above a local
mantle
plume, the origin of which had been associated with the activity of a megaplume which
had been
responsible for the existence of the Baltic province throughout a period of 200 million
years.
Introduction
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Figure 1a
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Figure 1b
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The Europe-largest Burakovka Early Paleoproterozoic layered pluton of basic and mafic
rocks (Figure 1), also known in literature as the Aganozero-Burakovka and Burakovka-Aganozero
pluton, is located east of Lake Onega in an area almost wholly covered by Quaternary
sediments. It
was discovered in the early 1950s by drilling in the area of a high magnetic anomaly.
Since 1964
extensive geological exploration and geophysical measurements have been made in the
area under
the guidance of V. A. Ganin, N. G. Grinevich, and V. N. Loginov
(Karelian Geological Survey) for
the purpose of the thorough study of the massif and the assessment of its ore potential.
Along with
these field operations, extensive research was done by geologists, petrologists,
and geochemists in
the Leningrad (St. Petersburg) and Moscow universities, in the Leningrad Mining Institute,
and in
many research institutes of the Russian Academy of Sciences, located in Moscow, St. Petersburg,
and Petrozavodsk.
However, in spite of this extensive field work and laboratory research the Burakovka
Pluton
remains to be not enough thoroughly known. The papers available
[Berkovskii et al., 2000;
Chistyakov et al., 2000;
Nikolaev et al., 1995;
Sharkov et al., 1995a, 1995b]
give but a general idea of the structure and composition of the Burakovka Pluton.
This especially
concerns its cumulate stratigraphy and the resultant vagueness of its structure and
relationships
among its different portions. The new data obtained in this study help to revise
many views on the
petrology of the massif, which is the subject of this paper.
Geologic structure of the Burakovka pluton
The Burakovka layered pluton is localized in the southeast of the Baltic Shield in
the
Vodlozero block of the Karelian granite-greenstone province. This block is composed
mainly of
Archean tonalite gneissic granite ("grey gneiss") with an irregular network of greenstone
belts. Some
relict areas of the block include the oldest rocks of the Baltic Shield, namely tonalite
gneiss with a
model Sm-Nd age of ca. 3.54 Ga
[Sergeev et al., 1990]
and fragments of greenstone belts with an age of 3.45 Ga
[Pukhtel et al., 1993].
The Burakovka Pluton has an irregular oval form, curved in map view, and is elongated
in
the NE direction. It is 50 km long, 13-17 km wide, and is as great as 630 km
2 in area. According
to geophysical data, its thickness ranges between 5-7 and 10 km
[Sobolev, 1993].
Earlier [Garbar et al., 1977],
the massif was believed to be one intrusive body, and its present-day structure was
interpreted as the result of the rejuvenation of tectonic activity in Late Proterozoic
time, when the
central and eastern parts of the intrusive massif had been raised stepwise along
combined normal
and strike-slip faults mainly of a roughly N-S strike. As a result of this faulting,
the massif was
broken into three main tectonic blocks: Burakovka, Shalozero, and Aganozero. The
inferred
magnitude of displacement between the latter two blocks was 1-2 km. In the previous
reconstructions of the internal structure of the pluton and its generalized cross-section,
the
Aganozero "block" was interpreted as the lower part of the once united whole massif,
and the
Shalozero and Burakovka blocks, as its middle-upper parts.
At the same time, the geophysical sections (Figure 1) revealed that the Aganozero
"block" was
an independent funnel-shaped body. The Shalozero and Burakovka "blocks" are divided
by a
system of small faults with insignificant vertical displacements and constitute one
body of a lopolith
form. Each of these bodies has its own internal structure and orientation in space:
the Aganozero
block is elongated in a roughly S-N direction, whereas the Shalozero-Burakovka block
strikes NE.
This evidence suggests that the pluton consists of two independent intrusive massifs,
Aganozero and
Shalozero-Burakovka, which contact each other in the region of their upper parts
and occur as one
massif at the level of the pre-Quaternary erosion
[Berkovskii et al., 2000;
Chistyakov et al., 2000].
These new findings called for a more comprehensive investigation of the massif's
structure.
As has been mentioned above, the massif is almost wholly covered by Quaternary sediments,
this
hindering the study of its internal structure. The main evidence of the latter was
provided by the
results of drilling comparatively shallow holes (usually drilled to depths of 200-300 m,
occasionally,
to a depth of 500 m, very few being drilled to depths of 1200-1500 m).
Until recently, the sections
across different parts of the pluton were correlated using mainly geochemical data
[Nikolaev et al., 1995;
Sharkov et al., 1995a, 1995b].
This approach failed to provide accurate results, because it is based
on the methods of mathematical statistics and fails to account for thin layering
and for the real
structure of the rocks (in particular, for relations between
cumulus and intercumulus phases).
Accordingly, where the compositionally close intrusive massifs are correlated, this
approach can
provide a very general pattern, similar not only for the pluton discussed but also
for the other Early
Proterozoic layered intrusive massifs of the Baltic Shield
[Sharkov and Smolkin, 1998].
For this reason, we undertook a special study of the cumulate stratigraphy of the
Aganozero and
Shalozero-Burakovka intrusive bodies, which revealed significant differences between
them. The
representative analyses of their main cumulates are given in Tables 1 and 2.
The best known now is the Aganozero Body (AB) which is composed mainly of ultramafic
rocks. This is a funnel-shaped body with its western side cut off by N-striking faults.
The central
part of the body is composed of gabbroids and has a trough-shaped form elongated
in the northern
direction. According to the geophysical data, the magma feeder of this intrusive
body was located in
its SW part.
The Shalozero-Burakovka Body (SBB) has a U-shaped form with the steeper southern
sides and consists mainly of basic rocks in the pre-Quaternary section, although
the drilling data
suggest its rocks to be generally similar to those of the Aganozero Body. Judging
by the geophysical
and scanty drilling data, the basic rocks are underlain there by a thick unit (more
than half of the
body's total thickness) of ultrabasic rocks. The location of a magma feeder for this
body remains to
be uncertain.
Each of the bodies has its central (layered) series and a marginal zone composed
of the
rocks of the marginal border group. The rocks of the upper marginal zone seem to
be demolished
by erosion, and those of the lower zone have not been reached by drill holes. As
follows from the
drilling data, the contacts between the lateral marginal rocks and the Archean basement
dip toward
the centers of the bodies and are usually steep. The contact surfaces dip at angles
of 60-70o in the
southern surroundings of the Shalozero-Burakovka Body, at 35-40o in its
southern surroundings,
and at 50-60o in the Aganozero Body, the layering in the marginal border
groups being conformable
with the contact surface. As follows from the low-angle layering and trachytoid texture
of the rocks
from the central series ( 30o in the peripheral parts of the intrusions to
10o and less in their central
parts), both bodies have internal structures independent of their contacts.
Comparison of the Sections Across the Aganozero and
Shalozero-Burakovka Bodies
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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As mentioned above, the Burakovka Pluton was believed earlier to be one body. The
composite section of its layered series, based on the correlation of the geological,
petrographic, and geochemical data
[Ganin et al., 1994;
Nikolaev et al., 1995;
Sharkov et al., 1995a, 1995b],
was believed to consist of the following five zones (upward): an zone of ultrabasic
rocks,
a pyroxenite zone, a gabbronorite zone, a zone of pigeonite gabbronorite, and a zone
of magnetite
gabbronorite-diorite, the latter being found only in the Shalozero-Burakovka
Body. The location of
the boundaries between these zones and the correlation of their sections were based
mainly on the petrochemical
classification of rock samples using a cluster analysis. It was demonstrated that
the petrochemical
clusters were generally close to a set of cumulate phases in the layered rocks and
can be used for
stratigraphic purposes with a certain degree of convention, disregarding the possibility
of varying
relations between the cumulus and intercumulus minerals in particular samples. Our
study showed
that generally this structural model was valid for both the Aganozero and Shalozero-Burakovka
bodies (Figure 2),
even though there are substantial differences between them in some details
(Figures 3,
4, and
5).
Ultrabasic zone (UZ) compose the lower units of the layered series in both
bodies. As
indicated by geophysical data, the total thickness of this zone in the Aganozero
Body is about 6000 m,
whereas in the Shalozero-Burakovka Body this zone is as thick as ca. 2 km. At
the pre-Quaternary
erosion level, this zone is most complete in the Aganozero Body, especially in its
southern part. The holes drilled in the SBB penetrated merely the top of this zone.
The ultrabasic
zones have fairly distinct upper contacts which in most holes drilled through both
bodies coincide
with the major chromite horizons (see below). As follows from the drilling data,
all rocks of this zone
in the AB are serpentinized to different extents, at least to a depth of 700-900 m
below the modern
erosion level.
The internal structure of the ultrabasic zones is distinguished by a relatively invariable
mineral
composition of the rocks consisting mainly of olivine (Ol)-chromespinel (Chr) cumulates.
Based on
the ratio between the cumulus and intercumulus phase minerals, two subzones were
distinguished in
the Aganozero Body: the lower dunite subzone (2500-3000 m thick) consisting
of dunite and
olivinite, which are often devoid of intercumulus, and the upper peridotite subzone
(200-400 m
thick) composed mainly of poikilitic peridotite with intercumulus pyroxene and plagioclase
amounting
to 30-40 vol.%. The dunite subzone includes a few indistinct rhythms ranging
between a few tens
and a few hundreds of meters in thickness. The lower parts of the rhythms (75% of
their
thicknesses) are composed of dunite consisting of cumulate olivine (95-98 vol.%)
and a small
amount of cumulate chromite (1-2 vol.%), whereas the upper parts of the rhythms
consist of
interstitial pyroxene and some plagioclase and phlogopite (10-15 vol.%). In
the peridotite subzone,
the amount of poikilitic peridotite increases upward with the total growth of interstitial
pyroxene
(mainly clinopyroxene) up to 20-45 vol.% and plagioclase up to 10 vol.%
with a decline of
cumulate olivine to 50 vol.%. The two-member implicit rhythmic pattern is characteristic
of this zone,
too, except that its dunite interlayers have a minimum thickness here.
The upper sequence of this zone includes several chrome-spinel ore units ranging
between
0.5 and 4.5-7 m in thickness, where the chrome-spinel content is as high as
50-75 vol.%. The
thickest of them (Major Chromitite Horizon, MCH) was traced in the very top of the
zone and often
coincides with the contact of the overlying pyroxenite zone. This unit varies greatly
in thickness and
is lacking in some holes. It has a maximum thickness (5-6 m) in the central
segment of the
Aganozero intrusive body, where it was recorded in all holes drilled through this
segment of the
body, and also traced in the pre-Quaternary erosion surface in the form of occasional
outcrops.
The internal structure of the main chromite unit is characterized by a frequent intercalation
of
peridotite, chromitite, and occasional dunite interlayers. The compositions of its
dark-color minerals
vary toward the growing Mg content. The contents of chrome-spinel in some layers
vary from 10-15
to 60-70%. Grains ranging between the first fractions of mm and 1-2 mm in size
occur as
numerous accumulations among intercumulate pyroxenes and plagioclase, and in some
rare cases as
inclusions in olivine. In the chromite ore proper, chrome-spinels occur as veinlets,
commonly
restricted to contacts between the grains of interstitial pyroxenes or to cracks
in the latter.
A layer combining several closely spaced thin chromitite interlayers, known as the
Yakozero
Unit, was found in the eastern segment of the Aganozero intrusive body in the peridotite
subzone
below the Main Chromite Unit (MCU).
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Figure 6
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Only the uppermost interval of the Ultrabasic zone, composed
of poikilitic peridotite
< 300 m
thick, was investigated in the Shalozero-Burakovka intrusive body. Here, a poorly
expressed
intercumulus-based rhythmicity was observed, with orthopyroxene dominating at the
bases of the
rhythms, and plagioclase and phlogopite, at their tops (Figure 6). The rhythms grow thicker from 5-10
to 30-35 m toward the top of the zone.
Most of the holes showed a chromite ore layer, 5-7 m thick, in the top of the
Ultrabasic
zone (analog of the MCH of the Aganozero intrusive body).
To conclude, the main distinctive feature of the Ultrabasic zone in the SBB is the
predominant development of poikilitic peridotite with the high contents of interstitial
minerals,
dominated by orthopyroxene and plagioclase, with the ubiquitous presence of phlogopite.
Pyroxenite zone (PZ) with a thickness of 190-200 m is exposed in the
pre-Quaternary
erosion surface of the Aganozero intrusive body and occurs merely as fragments in
the
Shalozero-Burakovka intrusive body, where its thickness is as small as 20-80 m.
The upper
contacts of these zones are marked by the ubiquitous occurrence of cumulate plagioclase.
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Figure 7
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In the Aganozero Body, this zone is composed predominantly
of clinopyroxenite,
websterite, and their olivine varieties with individual interlayers and patches of
lherzolite, hartzburgite,
and orthopyroxenite. This zone was studied most thoroughly in the holes drilled in
the central part of
the AB (Figure 7),
where it is distinguished by extremely unstable structures and textures with a great
development of coarse-grained to pegmatoid varieties. Judging by the petrographic
data, the typical
cumulate textures are rare with a wide development of metasomatic granoblastic, serrate
textures.
The most widespread mineral of this zone is inverted pigeonite-augite (Pig-Aug) with
a peculiar
"tweed" solid-solution texture produced by a system of fine orthopyroxene lamellas
intersecting at
right angles. This pigeonite-augite replaced (corroded) the earlier cumulate phases
(olivine, augite,
and orthopyroxene) with the formation of almost monomineral clinopyroxenite. The
clinopyroxene
matrix of the inverted Pig-Aug and the relict cumulate clinopyroxene are almost identical
in
composition (see below). The clinopyroxenite is almost entirely devoid of chrome-spinels
with
magnetite commonly developed instead of them.
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Figure 8
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The data available suggest that the minerals that underwent
replacement there were olivine-chromite,
orthopyroxene, and two-pyroxene cumulates. Some boreholes revealed a peculiar
transition zone, 5-10 m thick, along a contact between the pyroxenite and the
underlying ultrabasic
zone, where the peridotite cumulates are seen to be actively replaced by a clinopyroxenite
aggregate
with the formation of peculiar mottled rocks with relict spots of peridotite composition
(Figure 8).
Another distinctive feature of the rocks from this zone is the presence of numerous
usually round
isolated inclusions, mainly of a quartz-carbonate composition, ranging between fractions
of mm and
2-3 mm in size, usually located inside clinopyroxene (rarely in orthopyroxene)
grains, and also in the
interstices between them. Moreover, the clinopyroxenes with a "tweed" solid-solution
texture were
found to contain quartz-carbonate inclusions, having the form of negative crystals,
the feature typical
of primary inclusions (data provided by I. P. Solovova). These "microamygdaloidal"
clinopyroxenites
are a specific feature of this zone and have not been found in any other parts of
this intrusive body.
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Figure 9
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In contrast to the Aganozero intrusive body, the Pyroxenite
zone of the SBB has a small
thickness and consists of cumulate pyroxenite. As follows from drilling data, it
has a three-member
structure (Figure 9).
Its lower part, 20-50 m thick, is dominated by websterite and olivine websterite
(Aug+Opx
Ol cumulates), whereas its upper part with a maximum
thickness of 25 m is composed
of orthopyroxenite (Opx cumulate) and websterite.
The middle section of the pyroxenite zone includes a peridotite
marker horizon, whose
thickness grows from a few meters in the margins of the intrusion to a few tens of
meters in its
middle (see Figure 9).
The marker usually consists of Ol+Chr cumulates with 70-75 vol.% of olivine
in
its middle and of Ol+Chr+Opx cumulates with 50-55 vol.% of olivine in the top
and base.
Gabbronorite zone (GNZ) is best developed in the central part of the Aganozero
intrusive
body, where it has a maximum thickness (500 m), and was also recorded at the
periphery of the
Shalozero-Burakovka Body. The lower contact of this zone is defined by the stable
development of
cumulate plagioclase, and the upper, by the absence of orthopyroxene and the appearance
of
inverted cumulate pigeonite in the overlying rocks.
Aganozero intrusive body. In the central part of this body the gabbronorite
zone has been
subdivided into two subzones: the lower or banded subzone, 200-250 m thick,
and the upper
subzone having a roughly similar thickness.
The lower (layered) subzone is distinguished by the interlayering of websterite
(Cpx+Opx
Ol
Chr cumulate), orthopyroxenite
(Opx
Ol
Chr), norite (Pl+Opx), gabbronorite
(Pl+Opx+Cpx), gabbro (Pl+Cpx), and gabbro-anorthosite (Pl). Based on the ratios of
the cumulate
minerals in the sequence, we distinguished several rhythms ranging from 20 to 185 m
in thickness
and having indistinct contacts between them. The base of each rhythm is dominated
by Cpx. The
olivine and chrome spinel that are also present there vanish toward the top of the
subzone, where the
dominant mineral in the gabbronorite is orthopyroxene. The tops of the rhythms were
found to be
enriched in sulfides and platinum-group minerals (PGM).
The lower part of this subzone contains a marker horizon, composed of interbedded
poikilitic peridotite and olivine pyroxenite, varying within 3-5 m in thickness
(see Figure 1). Another
thinner marker horizon was found in the top of the sequence. It consists of poikilitic
peridotite,
bronzitite, and websterite containing up to 10-15 vol.% of chrome spinel in
the top. These rock
units are distinguished by the most magnesian compositions of their pyroxenes compared
to the
other rocks of the gabbronorite zone. The olivine from the rocks of the lower marker
horizon occurs
in isometric grains
< 1.5 mm in size, whereas that of the upper marker is represented by very small
(commonly a few fractions of mm), almost round grains. In some sites the marker-horizons
rocks
were found to contain Cu-Ni sulfide.
The upper subzone is composed mainly of gabbronorite including gabbronorite-anorthosite
interlayers. A coarse layering was observed expressed in the alternation of meso-
and leucocratic
gabbronorite varieties and occasional anorthosite layers. The cumulate
minerals are plagioclase
(65-78 vol.%), augite (7-14 vol.%), and orthopyroxene (5-20 vol.%).
In the Shalozero-Burakovka intrusive body, the Gabbronorite zone has a thickness
of
< 100 m and is composed mainly of gabbronorite with interlayers of anorthosite,
norite, and gabbro
and resembles the upper gabbronorite subzone from the Aganozero Body. Orthopyroxene
is seen to
dominate in the total content of pyroxene in the rocks upward. The amount of cumulate
plagioclase
varies within 55-70 vol.%. The interstitial minerals are quartz, magnetite,
and apatite, as well as
some poor pentlandite-pyrite-chalcopyrite sulfide mineralization.
To sum up, the distinctive features of the Gabbronorite zone in the SBB are (1) its
small
thickness, (2) the absence of a different cumulate layered unit in the lower
part of the zone (that is,
the absence of any analog of this subzone of the Aganozero gabbronorite zone), (3) the
absence of
any distinct rhythmic zoning, (4) the absence of peridotite markers, and (5) a
relative enrichment in
sulfides.
Pigeonite Gabbronorite Zone (PGZ) was observed mainly in the Shalozero-Burakovka
intrusive body and is poorly developed in the central part of the Aganozero body.
This zone is composed of gabbronorite with reversed pigeonite (Pig) and pigeonite-augite
(plagioclase-two pyroxene and less common plagioclase cumulates). The northeastern
segment of
the Shalozero-Burakovka intrusive body showed a poorly expressed layering produced
by the
alternation of the layers of meso- and leucocratic gabbronorite varieties ranging
between 1.5 and 10 m
in thickness. Almost all of the studied samples were found to contain interstitial
quartz, biotite, and
K-feldspar in the amounts of
< 5 vol.%, and also magnetite and apatite. The pyroxenes are often
amphybolized. The gabbronorite is often seen to be cut by plagiomicrocline granite
bodies ranging
between 0.5 and 50 m in thickness (see below).
Several boreholes revealed a plagioclase harzburgite marker horizon (Ol+Opx+Chr
cumulus) varying from 0.6 to 2.2 m in thickness in the gabbronorite of this
zone's top.
As to the central segment of the Aganozero Body, only the tops of the borehole sections
were found to contain gabbronorite with the cumulate grains of inverted pigeonite,
which could be
attributed to the pigeonite gabbronorite zone.
Magnetite Gabbronorite-Diorite Zone (MGDZ) is the uppermost zone in the sequence
of the layered-series rocks in the Shalozero-Burakovka intrusive body and is absent
in the
Aganozero body. The cumulus consists of inverted pigeonite-augite (10-29 vol.%)
and pigeonite
(9-22 vol.%), titanomagnetite (3-12 vol.%), and plagioclase (oligoclase-andesine)
(42-75 vol.%);
interstitial K-feldspar and quartz, as well as apatite, were found occasionally in
the top of the zone.
The rocks of this zone show a vague rhythmic pattern produced by the alternation
of
melanocratic and mesocratic rocks. The former were found to contain as much as 10-12%
cumulate magnetite and as much as 51% pyroxene. The content of magnetite declines
to 1-5% and
that of pyroxene, to 20-35%, in the latter. Anorthosite interlayers were recorded
in the tops of the
rhythms.
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Figure 10
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Geologic section of Hole 67. This is the deepest hole
of those drilled in the Shalozero-Burakovka
Body. It crossed its eastern segment in the direct vicinity of the marginal-series
rocks
(see Figure 1). The rocks penetrated by this hole are generally comparable with
the pluton's rocks
elsewhere, yet show some specific features, the principal of them being the occurrence
of two thick
layers of Ol+Chr cumulates at the bases of two megarhythms with pigeonite-bearing
gabbronorite in
the top (Figure 10).
Both megarhythms show a cumulate succession typical of the Burakovka Pluton
(and common for most of the Early Paleoproterozoic layered intrusions of the Baltic
Shield):
Ol+Chr
Ol+Chr+Opx
Opx+Chr
Opx
Opx+Cpx
Opx+Cpx+Pl
Pig+Pig-Aug+Pl.
In spite of the similar structure of the megarhythms, our study revealed that olivine
and
orthopyroxene in the peridotites of the lower megarhythm differ by their higher Mg
contents (Fo86-87 and
En
87, respectively) from the similar rocks in the upper megarhythm
(Fo84-85 and En81-84 )
(see Figure 10).
This fact suggests that here we deal with two independent megarhythms instead of
the doubling of the
sequence along a fault.
Concluding the description of the cumulate stratigraphy of the Burakovka Pluton,
it should
be mentioned that both intrusive bodies show the following succession of the main
typomorphic
cumulates: Ol
Chr (Ultrabasic zone)
Opx
Cpx
Ol
Chr (Pyroxene zone)
Opx + Cpx + Pl
Ol (Gabbronorite zone)
Pig + Pig-Aug + Pl
(Pigeonite gabbronorite zone)
Pig + Pig-Aug + Pl + Mag (Magnetite gabbronorite
zone
which is absent in the Aganozero body, where its
seems to have been eroded). In this respect the Burakovka Pluton does not show any
differences
from most of the Early Paleoproterozoic layered intrusions of the Baltic Shield,
and also from the
classical massifs, such as Bushveld (South Africa) and Stillwater (North America).
The Marginal Border Group is represented in both intrusive bodies by a group
of contact
rocks which usually occur as a band
< 200-400 m wide along the periphery of the massif. Three
zones can be distinguished in the sequence of the marginal rocks: a zone of direct
contact, an outer
(banded) zone, and an inner zone.
In the southern surroundings of the Shalozero-Burakovka intrusive body, the zone
of direct contact is composed of microgabbronorite enriched in accessory sulfides
of a pentlandite-pyrite-chalcopyrite
assemblage and in titanomagnetite (totaling 5-10%) and ranging between 0.4 and 0.9 m
in thickness. In the Aganozero Body, the contact gabbroids are represented by amphibolized
quartz-bearing norite and gabbronorite, roughly 120 m thick (Hole 196), sometimes
porphyritic,
containing large (up to 2 mm long) plagioclase (An
44-63 ) crystals, often normally zoned, in a fine- and
medium-grained plagioclase-pyroxene groundmass with small ( < 1 mm) veinlets and fine-grained
patches of irregular form. The rock includes single sulfide and magnetite grains
everywhere. There
are occasional pyroxene and anorthosite interlayers and also fragments of a quartz-feldspar
rock,
which can be interpreted as the xenoliths of the host tonalite granite gneiss. A
similar sequence was
observed in the northeastern surroundings of the Shalozero-Burakovka intrusive body.
The outer (banded) zone, 10 to 210 m thick, occurs in the surroundings
of the Shalozero-Burakovka
intrusive body. It is composed mainly of fine- to medium-grained usually leucocratic
gabbronorite (clinopyroxene Wo47En38Fs15,
orthopyroxene Wo5En65Fs30,
plagioclase An47 ) with interlayers of plagioclase websterite where ortho- and
clinopyroxene grains
are enclosed in plagioclase oikocrysts. The outer zone is mostly absent in the Aganozero
body, where the
near-contact gabbroids are replaced abruptly by the peridotites of the inner zone.
The exception is Hole
177 drilled in the south of this body, where the rocks of the outer zone occur as
a 20-meter layer of
medium-grained plagioclase websterite which is changed by contact gabbroid closer
to the contact.
The inner zone, 20 to 170 m thick, is present in both bodies and consists
of chrome-spinel-bearing
(1-5%) poikilitic serpentinized peridotite with occasional interlayers of plagioclase
websterite in the Aganozero intrusive body and with those of websterite and less
common peridotite
in the Shalozero-Burakovka Body. The data available suggest that this difference
in the structure of
the internal zones depends on the type of the layered-series rocks, with which they
are in contact.
For instance, in the AB, the inner zone contacts the ultramafic rocks, whereas in
the SBB, it contacts
the mafic rocks in the west and the ultramafic rocks of the layered series in the
east. The olivine from
the peridotite samples collected in Hole 28a showed a magnesian number which was
found to be the
lowest for the rocks of the pluton (Fo52 ), the ortho- and clinopyroxenes yielded
En78.5 and
En46.6, respectively, and the interstitial plagioclase, An34-38.
In the inner zone, the contents
of SiO2, TiO2, Al2O3, FeO,
Na2O, and K2O grow toward the contact with a decreasing MgO
content caused by the enrichment of the marginal peridotite in interstitial plagioclase.
Therefore, the differences recorded in the structure of the marginal series can be
associated
with the composition of the adjacent rocks of the layered series. The most complete
sequence of the
rocks was observed in the Shalozero-Burakovka Body where we recorded a transition
from the
near-contact fine-granular gabbronorite via fine- to coarse-granular gabbronorite
to plagioclase
websterite and further, via its olivine variety, to the rocks of the layered series.
Cryptic Layering
The compositions of minerals were determined throughout the sequences of the layered
series of the Aganozero and Shalozero-Burakovka bodies. Measurements were made at
IGEM
RAN using an MS-46 Cameca microprobe with the experimental conditions of 15 kV
and 50 mA.
The time of measuring each element was 70 s.
|
Figure 11
|
|
Figure 12
|
The rocks of the layered series in both intrusive bodies of the Burakovka Pluton
are
characterized by cryptic layering expressed in the regular compositional variations
of the major
rock-forming minerals (olivine, pyroxene, and plagioclase) from the bottom to the
top of the body.
Our study revealed obvious variations in the compositions of the mineral phases against
the general
trend of the declining Mg content in the olivines and pyroxenes, and the Si content
in the plagioclases
(Tables 3,
4,
5,
6,
7,
8,
9, and
10,
Figures 11 and
12)
Olivine is the main cumulate mineral in the ultrabasic zones and in the overlying
peridotite
marker horizons. In a 200-meter section examined in the top of the ultrabasic zone
in the
Shalozero-Burakovka Body, the Mg content of olivine declines up the section
from Fo88 to Fo84 and increases again to 86-89% Fo in the Main Chromite Horizon, with
the NiO content in the
olivine varying between 0.45 and 0.55% and tending to decline up the zone. In the
peridotite marker
horizon of the Pyroxenite zone, olivine is more ferrous (Fo79-80 ) with the NiO content varying
between 0.37 and 0.43%. Therefore the olivine composition varies from Fo88
to Fo79.
In the Ultrabasic zone of the Aganozero Body, the Mg content of olivine declines
upward
from 90 to 86% Fo, increasing again to Fo88 in the MCH. The relict olivine grains in the
pigeonite-augite clinopyroxenite from the Pyroxenite zone showed a comparatively
constant composition
(Fo82-83 ). At the base of the Gabbronorite zone the olivine gabbronorite
contains olivine with 80%
Fo, its composition being similar to that of olivine in the lower peridotite marker
horizon (Fo80-81 ). In
the upper marker, olivine is more ferroan (Fo76 ).
Therefore the Mg content of olivine varies from
Fo90 to Fo76 up the section of the Aganozero Body.
Almost all of the olivine grains analyzed in the Burakovka massif are devoid of silica
and Ca
oxide, the olivine always containing isomorphous NiO (0.3 to 0.56 wt.%).
|
Figure 13
|
Low-Ca pyroxene is represented by orthopyroxene and
pigeonite in the rocks of the
Burakovka Pluton. In terms of the content of a wollastonite end member (4-6%) the
pyroxenes
occupy a boundary position with the pyroxenes of a pigeonite series (Figure 13). In
the ultrabasic
zones and peridotite marker horizons, Opx is a usual constituent of the intercumulus
and is
represented mainly by cumulate crystals in the overlying rocks. The orthopyroxene
compositions are
presented in Tables 5 and 6.
In the Ultrabasic zone of the Shalozero-Burakovka Body, the Mg content of
the
orthopyroxenes declines upward along a 200-meter section from En86
to En83, increasing again to
85-86% En in the main chromite unit. In the Pyroxenite zone, orthopyroxene has a
Wo4En78Fs18 composition in the websterite below the marker and is more ferroan
(Wo5-6En73-75Fs21-22 ) above it.
In the gabbronorite interlayer of the Lower Pyroxenite horizon orthopyroxen has a
Wo3En71Fs26. In the
peridotite marker horizon, Opx contains 4-5% Wo with 77-78% En. Orthopyroxene has
a
composition of Wo4En66Fs30 in the Gabbronorite zone.
The pyroxene from the Pigeonite gabbronorite zone is represented by pigeonite (Pig)
and
pigeonite-augite (Pig-Aug). Orthopyroxene (Wo1.5-5En51-54Fs43-44 )
occurs as lamellas in the inverted
Pig-Aug and as a matrix in the inverted Pig with a normal zoning: Wo1-3En54-58Fs41-44
in the cores
and Wo2En46-54Fs44-52
in the margins. Therefore the lamellas in Pig-Aug and the matrix
in the
inverted pigeonite have almost the same composition. In the upper marker horizon,
the
orthopyroxene composition (Wo4En79Fs17 )
is close to that of the marker in the Pyroxenite zone. The
composition of the orthopyroxene matrix of the inverted pigeonite from the magnetite
gabbronorite
zone varies upward from Wo2-5En54-56Fs40-44
to Wo4En38Fs59. Therefore the total variation in the Opx
composition, including the exsolution texture compositions of the inverted Pig and
Pig-Aug upward
the layered series in the Shalozero-Burakovka Body, is Wo4En86Fs10
to Wo4En38Fs59.
In the Aganozero Body, Opx appears later than Cpx in the form of single interstitial
grains in
the rocks of the Ultrabasic zone. Orthopyroxene occurs as a cumulate mineral above
this zone. In
the Pyroxenite zone the quantity of this mineral is also subordinate to that of Cpx
which occurs as
scarce cumulate crystals, often highly corroded by the adjacent pigeonite-augite
grains. The
orthopyroxene has a Wo3En76Fs21
composition. In the lower intervals of the Pyroxenite zone, the
pyroxenite contains, along with inverted metasomatic pigeonite-augite, inverted pigeonite
showing
exsolution textures in the form of coarse lamellas parallel to (001) and small augite
grains of irregular
form. The composition of the orthopyroxene matrix is Wo2En80Fs18,
and that of the augite lamellas,
Wo45-47En45-47Fs8.
The cumulate Opx has a composition of Wo3En76Fs21
in the lower part of the Gabbronorite
zone, and Wo4En62Fs34 in the upper.
There are orthopyroxenite interbeds where orthopyroxene
has
a composition of Wo3En65Fs32.
The interstitial Opx in the peridotite marker horizons is more
magnesian: W4En80Fs16 in the lower unit,
and Wo3.5En75Fs21.5 in the upper.
Some holes drilled in the central part of the Aganozero Body intersected gabbronorite
containing inverted pigeonite with an orthopyroxene matrix having a composition of
Wo2En63Fs35,
this pigeonite being the most ferrous one in the AB.
To conclude, the widest Opx variation range in the layered series of the Aganozero
Body is
Wo3En80Fs17 to Wo2En63Fs35.
High-Ca pyroxenes are represented by augite and
pigeonite-augite, the latter being usually
inverted. Some samples were found to contain uninverted pyroxene grains. The clinopyroxene
compositions are presented in Tables 7 and 8 and displayed in Figure 13. In the Shalozero-Burakovka Body
Cpx is present throughout the section of study. In the Ultrabasic zone, augite is
among the main intercumulus phases, and its En content declines upward from En51 to
En44 with
some small variations throughout the section. In the main chromite unit, augite has
a more magnesian
composition, similar to bronzite. Single Pig-Aug grains (Wo38En51-55Fs7-11 )
were found in the
ultrabasic zone. In the Pyroxenite zone, augite is substantially more ferroan,
Wo46-48En43-45Fs9, in the
pyroxenite below the peridotite marker horizon - Wo47En40Fs12
and in the gabbronorite interlayer - Wo47En40Fs12;
the composition of intercumulus augite is Wo42-46En46-49Fs9-10
in the peridotite, and Wo47-48En41-42Fs10-12 in the upper
pyroxenite layer. Augite has a composition of Wo46En40Fs14
in the Gabbronorite zone.
In the zone of Pigeonite gabbronorite, the composition of the augite matrix in the
inverted
pigeonite-augite varies from Wo45-46En34-36Fs18-19
in the cores of the grains to Wo43-45En35-36Fs20-21 in
their rims, the composition of the inverted pigeonite lamellas being Wo44-47En40Fs16-19.
In the
Magnetite gabbrodiorite zone, the iron content of the augite matrix in the inverted
Pig-Aug grows
steadily up the section from Wo46-47En35-36Fs17-18
to Wo16En28Fs26. Some samples (e.g., 47/83.9)
were found to contain pigeonite-augite with a composition of Wo33En30Fs38.
Thus, the total variation
of the augite composition across the layered series was found to be Wo43En51Fs6
in the ultrabasic
zone to Wo46En28Fs26 in the zone of Magnetite gabbrodiorite.
In the Aganozero Body, Cpx is also among the main rock-forming minerals.
It occurs as
angular interstitial grains in the dunite of the ultrabasic zone, and as large oiko-crystals
in the
peridotite from the top of the zone. The Cpx composition in the ultrabasic zone corresponds
to
augite with a low iron content, minimal for the marginal rocks, which generally grows
up the section
from 4.5 to 7.8% Fs.
In the Cr-bearing peridotite of the Main Chromitite Horizon, clinopyroxene
is slightly more
magnesian(Wo43-46En47-50Fs6-7 ) and shows an elevated Cr2O3
content (up to 1.0-1.2 wt.%).
Clinopyroxene is more ferroan in the Pyroxenite zone. The clinopyroxene matrix in
the
inverted Pig-Aug from the metasomatic clinopyroxene and websterite has
a Wo45-47En44-47Fs8-9 composition.
The primary composition of pigeonite-augite, obtained
with a defocused beam, was
found to be Wo36.2En52.4Fs11.4,
which is generally close to the Wo33En53Fs14
composition found for
the undecomposed portion of one of the grains. The cumulate augite was found to be
Wo47En44Fs9.
In the wehrlites (Sample 16/159) augite is more magnesian - Wo45-47En44-46Fs8-9.
Analyses of individual pyroxene grains did nor reveal any zoning, but showed some
heterogeneity expressed in the compositional variations (mainly in terms of a Fe/Mg
ratio) in some
regions of the grains. All clinopyroxenes from the ultramafic part of the sequence
were found to
contain 0.12-0.8% Cr2O3 and
< 0.08% NiO.
Above the Pyroxenite zone, augite occurs mainly as a cumulate mineral in a plagioclase-two
pyroxene assemblage. In the Gabbronorite zone, Cpx is more ferroan
(Wo45-46En41-44Fs10-13 ). Its
Cr2O3 content is not higher than 0.1% and that of NiO, 0.09%, the most common
values being 0-0.06%
and 0.05%, respectively. In the peridotite marker horizons, the interstitial clinopyroxene
has
a composition of Wo44En47Fs10,
this composition being most magnesian for the Gabbronorite zone.
In the Pig-bearing gabbronorite from the top of the layered series, clinopyroxene
has the highest iron
content and a Wo45-46En40Fs14-15 composition.
To conclude, viewed along the section of the layered series in the Aganozero Body,
the
clinopyroxene composition varies from Wo47En48.5Fs4.5 at the bottom
to Wo45-46En40Fs14-15 in the
upper pigeonite-bearing gabbronorite.
|
Figure 14
|
Plagioclase is widely developed in the intercumulus
of the ultrabasic and pyroxenite zones
and is the main cumulate phase in the overlying zones. Variations in its composition
are plotted in
Figure 14
and listed in Tables 9 and 10.
In the Ultrabasic zone of the Shalozero-Burakovka Body, plagioclase is highly chloritized.
As a result, we were able to examine merely two of the single grains: one of labradorite
Or3Ab51An46, the other of oligoclase-andesine
Or3Ab66An30. In the MCH, the interstitial plagioclase
is represented by even more silicic oligoclase Or1Ab84An15.
Intercumulate plagioclase has a
composition of Or2-3Ab42-43An55
in the Pyroxenite zone below the marker horizon and is represented
by labradorite Or2Ab44An53-54 and scarce oligoclase
(Or2-4Ab71An25-27 ) above it. Plagioclase has a
labradorite composition (Or4Ab45An51 ) in the rocks of the marker horizon.
Plagioclase is a cumulate mineral in the rocks above the Pyroxenite zone. In the
Gabbronorite zone it is almost wholly unzoned and has a composition of
Or2-4Ab46-48 An50-52.
Plagioclase is slightly normally zoned in the Pigeonite gabbronorite zone, where
its composition
varies rather widely (Or2-3Ab46-58An40-51 ).
However, because this zone was poorly studied, we did
not find any systematics in these variations. Some plagioclase grains showed inverse
zoning, from
An42 in the core to An59
in the margin. We believe this pattern to have been caused by the
roiling of
the settling crystals by fresh magma portions
[Higgins et al., 1997].
In the Magnetite gabbrodiorite zone the composition of plagioclase varies from andesine
Or3-4Ab55-57An39-42 to
Or2Ab66An32. The total variation of the cumulate plagioclase across the layered
series was found to be An50 to An32
(without taking into account the interstitial phases of the Ultrabasic
and Pyroxenite zones).
In the Ultrabasic zone of the Aganozero Body, the composition of interstitial
plagioclase
varies from Or1Ab47An52 to
Or2Ab54An44. In the Pyroxenite zone, the composition of the interstitial
plagioclase is variable too (Or1-3Ab38-48An50-60 ).
It is only in the Gabbronorite zone that plagioclase
occurs as the main cumulate phase, growing more silicic up the sequence, from
Or1Ab42An57 to
Or3Ab35An42. Some grains are normally zoned with the An component declining by
3-4% from the
cores to the margins of the grains. Reversed zoning is less common. The plagioclase
composition is
Or0.5-5.5Ab50-63An39-49 in the peridotite of the markers.
To sum up, our study of the cryptic layering in the Aganozero
and Shalozero-Burakovka
bodies of the Burakovka Pluton revealed the following variations in the mineral compositions
up the
sequences of the layered rocks (see Figures 11 and 12): in the latter, Fo88 declines to Fo79 in olivine,
En86 to En38 in low-Ca pyroxene, En51 to En28 in augite,
and An50 to An32 in plagioclase; in the
former, Fo88.5 declines to Fo76 in olivine,
En80 to En63 in low-Ca pyroxene,
En48.5 to En40 in
clinopyroxene, and An60 to An39 in plagioclase.
Comparing these data, one can see that the olivine
and pyroxenes have similar compositions in the ultramafic rocks of the layered sequences
in both
bodies. In the Shalozero-Burakovka Body, the clinopyroxene is more ferroan toward
the top of the
sequence, this possibly being accounted for by the more complete sequence of a gabbroid
portion in
its layered series.
Chrome-spinels are typomorphic accessory minerals
in the rocks of the ultrabasic and
pyroxenite zones and in the peridotites of the peridotite marker horizons. Their
compositions are
presented in Tables 11 and 12.
|
Figure 15
|
Accessory chrome spinelids are represented in the ultramafic zones mainly by
subferrialuminochromite and are distinguished by high Mg and Cr contents, which grow
up the zones
(Figure 15).
Chrome-spinel from the ultrabasic zone of the Shalozero-Burakovka Body is
distinguished by high vanadium, nickel, and zinc contents.
Chrome-spinel is represented by subferrialuminochromite also in the chromite ore
units of
both bodies, ore varieties with higher Mg and Cr contents (subferrichromite) occurring
also in the
Aganozero Body. The chrome-spinels found in the Yakozero Unit are distinguished by
Cr2O3 as
high as 56.81 wt.% (the maximum value for the Burakovka Pluton).
A special study of chromites in the Main Chromite Horizon in the Aganozero Body revealed
the presence of numerous fine Ru-Os sulfide crystals ranked with a laurite-erlichmanite
group
[Smirnova and Dmitrienko, 1994].
Neither accessory chromite nor chromitites from the Shalozero-Burakovka Body have
been studied in this respect.
Chrome-spinel grains were found to account for 3-5 to 20-30% in the peridotite marker
horizons of the Gabbronorite zone of the AB and in the Pyroxenite zone of the SBB.
The chrome-spinel
was identified as subalumoferrichromate or subferrialumochromite. Magnetite was found
in
rims around some grains and is also developed in the cracks of the latter. In some
places ilmenite
lamellas were found. The chrome-spinel was found to contain less Cr and more Fe.
|
Figure 16
|
To sum up, the chrome-spinels tend to be lower in Cr up the
section of the layered series in
both bodies of the Burakovka Pluton with Cr
Fe
+3 +Al being the leading isomorphism for
chrome-spinels in the course of their formation (Figure 16). The chrome-spinels of the ore units
in both
bodies are distinguished by the higher Mg and Cr contents compared with the accessory
chromite in
the dunites and peridotites of the ultrabasic zones, where more aluminiferous varieties
are
developed, and also by the higher Mg content of the coexisting dark-colored silicate
minerals. This
suggests the higher temperature of the formation of the ore units, which was reported
to be 1300-1200o C
[Nikolaev, 1997].
Summarizing the results of studying cryptic layering in the Burakovka Pluton, we
can state
that the character and variation range of its main typomorphic minerals is similar
to those in the other
large layered plutons, such as Bushveld (South Africa), the Great Dike in Zimbabwe,
and others.
For instance, in the Great Dike, the Mg content of olivine varies up the sequence
from Fo94 to Fo85,
and the compositions of cumulate clino- and orthopyroxene vary from
En51 to En38 and from En92 to En58, respectively
[Layered Intrusions, 1996].
In the west of the Bushveld Massif the composition of
olivine varies from Fo89 to Fo83, and of orthopyroxene from
En89 to En30 [Layered Intrusions, 1996].
In the Stillwater massif the total changes of the composition of olivine vary through
the section from Fo91 to Fo81, that of clinopyroxene,
from En52 to En42, and that of orthopyroxene,
from En83 to En58 [Wager and Brown, 1970].
However, in contrast to other intrusive massifs, the
Burakovka Pluton is distinguished by its more silicic plagioclase, the composition
of which varies up
the layered rock sequence from An60
in the Aganozero Body to An32 in the Shalozero-Burakovka
Body.
Vein Rocks
Vein rocks are especially widely developed in the Shalozero -Burakovka Body, where
they
occur as compositionally various veins ranging from a few meters to a few dozens
of meters in
thickness, their dominating rocks being microgabbronorite, gabbronorite-pegmatite,
granophyre, and
potassic granite. Vein rocks are scarce in the Aganozero intrusion.
The vein-shaped bodies of microgabbronorite seem to have been feeders that channeled
fresh magma portions into the crystallizing intrusive chambers, while the gabbronorite-pegmatite
bodies were local accumulations of residual magma in the cumulate rocks.
Of particular interest are vein granites. These rocks are widely developed
in the
northeastern part of the Shalozero-Burakovka Body, where they occur mainly in the
Pigeonite
Gabbronorite Zone. The granites are gray or light gray, massive, mainly fine- and
medium-grained
rocks which occur as cross-cutting veins ranging from 0.5 cm to a few dozens
of meters in
thickness. They vary in composition from biotite leucogranodiorite to K-granite.
The granite veins
usually have distinct, rectilinear or angular contacts with the host gabbronorite
and occasionally
include small fragments of the latter. Neither chill zones nor any low-temperature
alterations of the
host rocks were observed. The isotopic and geochemical characteristics of the granites
[Bogina et al., 2000]
preclude their formation by way of the basement rock melting and suggest the same
source of origin, common for the granites and their host mafic rocks. The geological,
petrologic, and
isotopic-geochemical data available suggest that the granites originated synchronously
with the
gabbroids in situ within the zone of Pigeonite gabbronorite. It is believed
that these granites were
derived from the residual interstitial silicic melt that was retained in the almost
solidified cumulates of
this zone. Where the process of contraction was operating, this magma must have been
sucked into
shrinkage cracks under the resulting vacuum effect.
The Shalozero-Burakovka Body also includes widely developed late gabbroid dikes which
are believed to be the late Paleoproterozoic formations of the Pudozhgora submeridional
dike belt.
Geochemistry of the Burakovka Rocks
To study the compositions of the rocks typical of the Burakovka Pluton samples were
collected from the zones identified in the Aganozero and Shalozero-Burakovka bodies,
which had
been least subject to secondary alteration, and also a few samples from the rocks
of the Marginal
Border Group.
Methods of Geochemical Study. The bulk compositions of the rocks were determined
using a conventional chemical analysis in the Central Chemical Laboratory of the
Institute of Geology
of Ore Deposits, Petrography, Mineralogy and Geochemistry (IGEM), Russian Academy
of
Sciences. The contents of trace elements were determined by X-ray fluorescence analysis.
Several
methods were used to determine REE contents. Some REE were determined by neutron
activation
analysis, with the radiochemical separation of "noise" elements, in the Geological
Institute, Russian
Academy of Sciences, using a method proposed by
Lyapunov et al. [1980].
The convergence of the analytical results was estimated using a BCR-1 standard. Most
of the samples
were analyzed by ICP-MS in the IGEM Central Laboratory. Analyses were made using
a
quadrupole Plasma Quad PQ2+Turbo mass spectrometer, a VG Instruments product. The
correct
use of the technique was checked using certified standard solutions and an AGV international
standard sample.
|
Figure 17
|
|
Figure 18
|
Results. The representative analyses of the rocks are presented in Tables 1 and 2. Many
rocks were marked by the combination of high Mg, Cr, and Ni contents with high SiO
2 concentrations and low contents of Ti, alkalies, HREE, Nb, Y, etc. Satisfactory
correlation trends
were observed for some of the major elements with MgO: negative for the alkali sums
and Al
2 O
3 (Figure 17)
and positive for Ni (Figure 18).
|
Figure 19
|
|
Figure 20
|
Figure 19 shows the compositions of the Burakovka rocks, normalized
to mid-oceanic
tholeiites (MORB). Compared with the latter, all rocks of the pluton are enriched
in lithophyle
elements (Rb, Ba, Sr), and the gabbroids are also enriched in LREE with the lower
contents of
HREE and Ti, the contents of the latter being significant only in the gabbronorite
containing cumulate
titanomagnetite from the Shalozero-Burakovka body. The curve of the distribution
of REE and trace
elements in the rocks of the pluton (Figure 20), normalized to the primitive mantle composition
after
Hofmann [1988],
showed the enrichment of the rocks in Rb, Sr, and Ba with the relatively low
contents of metallic elements such as Ni, V, and Cr. The contents of the latter were
found to be
significantly higher in the rocks of the ore-bearing chromitite layers.
In spite of some insignificant differences, all rocks of
the pluton showed a generally similar
REE distribution trend. The total REE concentrations grow normally from the mafic
cumulates at the
bottoms of both bodies to the gabbronorite in their tops (Table 13). In the case of the Shalozero-Burakovka
body the REE content grows from 10 ppm in poikilitic peridotite to 72.8 ppm
in
magnetite gabbronorite, this variation range being 2.5 to 20.6 ppm in the Aganozero
Body. The
rocks also show a successive enrichment in LREE: the Ce/Yb ratio grows from 13.7
to 49.2 ppm in
the SBB and from 1.5 to 4.5 in the AB.
|
Figure 21
|
The results of our study revealed a significant enrichment in REE and LREE of the
Shalozero-Burakovka rocks relative to the same rocks in the Aganozero body (Figure 21), and
also
the high REE contents in the peridotite of the markers relative to the rocks of the
Ultrabasic Zone,
this suggesting the enrichment of their parental magmas which are interpreted as
later intrusions.
The REE distribution patterns in both bodies of the pluton are similar to the patterns
observed in similar layered intrusions in the Baltic and other shields, and also
in the high-Mg basalts
of the same age from the Vetreny Belt. All rocks studied in the Burakovka Pluton
yielded similar
REE distribution patterns, characterized by the growth of REE concentrations up the
rock sequence
with the growing LREE contribution suggesting the accumulation of the latter in the
residual melt
during the directional solidification of the intrusions. The higher REE enrichments
of the SBB rocks
relative to the same rocks of the Aganozero Body suggest the relative enrichment
of the primary
magma (or magmas) parental for the former.
|
Figure 22
|
To sum up, the geochemical features characteristic of the
rocks from both bodies of the
pluton suggest that they were produced by the intrusion of siliceous high-Mg (boninite-like)
magma.
This is supported by the composition of the gabbronorite from the contact the SBB
(Sample
28a/218), which is similar to the B1-type boninite-like parental magma of the Bushveld
Complex
[Sharpe and Hulbert, 1985] (Figure 22).
Its REE distribution pattern is close to that of the chilled endocontact
rocks from the other layered intrusions of the Baltic Shield, combined into one large
igneous
province of a siliceous high-magnesian rock series (SHMS) (see below). Magmas of
this type
constitute a specific feature of igneous activity during the Early Proterozoic, and
the rocks they
produced are abundant in all Precambrian shields.
[Bogatikov et al., 2000;
Sharkov et al., 1997].
Isotopic Studies
The isotopic studies of our rock samples were carried out
in the Institute of Precambrian
Geology and Geochemistry, Russian Academy of Sciences, St-Petersburg. For this purpose
we
collected samples along the sections across the layered series of both bodies (see
Figure 2),
which
contain all major rock types, least subject to secondary alteration. The methods
of Sm-Nd and Rb-Sr
studies are described in
[Amelin and Semenov, 1996],
and the results are summarized in Table 14.
|
Figure 23
|
Sm-Nd isochrons were plotted for individual minerals (Pl, Cpx, and Opx) and whole
rocks
(Figure 23)
using two rock samples collected from the tops of the layered series in both bodies
(gabbronorite-anorthosite from the Aganozero Body and magnetite gabbronorite from
the SBB).
The isochron slope for the Aganozero sample yielded an age of 2372
22 Ma ( e Nd = -3.22
0.13 ),
and the Sm-Nd isochron age for all Aganozero samples, including the bulk samples
and
monomineral fractions, was found to be 2374
29 Ma ( e Nd = -3.03
0.19 ), this value fitting the value
given by the mineral isochron within analytical error. The sample from the Shalozero-Burakovka
Body yielded an
Sm-Nd isochron age of 2433
28 Ma ( e Nd = -3.14
0.14 ), which coincides, within error, with the
age of 2449
1.1 Ma found earlier for the gabbro from the
Shalozero-Burakovka Body by the U-Pb method using zircon
[Amelin and Semenov, 1996].
|
Figure 24
|
At the same time the isotopic ratio values in the rocks of both intrusive bodies
are fairly
close (see Table 14).
Also close are the model ages of the rocks in these bodies: 2935 to 3065 Ma
(Shalozero) and 3007 to 3033 Ma (Aganozero). This suggests that large amounts
of the crustal
rocks of Archean age were involved in melting to produce basic magma which crystallized
as the
tops of the intrusions, as follows from the model proposed earlier for the origin
of magma parental
for the rocks of the siliceous high-Mg series
[Sharkov et al., 1997].
At the same time the rocks of
the Shalozero Body showed a wider variation of isotope characteristics, the evidence
suggesting
some differences in the compositions of the melting rocks which contributed to the
formation of the
parental magmas for the bodies concerned. The same is indicated by Pb isotopes (Table 15,
Figure 24).
However, in spite of minor differences, all of our isotopic and geochemical data
suggest the
same type of parental magmas for the intrusive bodies discussed.
The intrusion of each body was apparently accompanied by the injections of new magma
portions into the solidifying chambers, the evidence of this being the emplacement
of peridotite
marker horizons (see below). These new magma injections are characterized by lower
87Sr/86Sr(T)
ratios (0.7019 for Shalozero and 0.7032 for Aganozero) and by higher
e Nd (T) values ( - 1.32 for
Shalozero and
- 2.35 for Aganozero). The pigeonite-augite clinopyroxenite from the Aganozero
body (Sample 273/5.4) showed an abnormally high
87Sr/86Sr(T) value (0.7060) and a low
eNd (T)
value ( - 3.58). On this basis we interpret it as a product of subsolidus metasomatism. This
isotopy
seems to suggest that crustal fluids were involved in the process of its formation.
Discussion
Individual Features of the Structure and Composition of
the Burakovka Pluton
Our study revealed that along with the material and structural characteristics common
to the
other large Early Paleoproterozoic layered intrusions of the Baltic Shield, the Burakovka
Pluton has
its own particular features which call for a special discussion. The most important
among them are
differences between the rock sequences, compositions, and ages of its individual
bodies, as well as
the origin of the marker horizons and the structure of the southeastern segment of
the Shalozero-Burakovka Body (Hole 67).
Origin of the marker horizons. As has been mentioned above, the specific
feature of the
pyroxenite, gabbronorite, pigeonite gabbronorite, and magnetite gabbronorite zones
in both bodies
is the occurrence of thin-bedded ultrabasic-rock marker horizons. The emplacement
of these rock
units among the cumulates of much lower temperatures suggests the injections of fresh
magma
portions into the solidifying magma chamber. This new magma had a higher density
and spread over
the chamber floor displacing the old, more evolved magma upward. As a result, the
new magma
was immediately involved in crystallization and later - in convection, as it lost
its density, the
composition of magma in the intrusive chamber was equalized, and the crystallization
trend returned
almost to its original state.
The fact that fresh magma was periodically injected into the solidifying magma chambers
is
proved by the findings of cumulate plagioclase crystals with a reversed zoning, the
fact emphasized
above. This kind of zoning could hardly be produced during magma crystallization
in a stagnant
zone, but was quite feasible where the settling crystals were stirred up by a fresh,
hotter magma.
Above, we called the attention of the reader to the unusually great thickness of
the ultramafic
zone in the Aganozero body (as high as six kilometers). It is absolutely impossible
to envision a
magma that might have produced such a great thickness of olivine-chromite cumulates
in the case of
the single-act filling of an intrusive chamber. The presence of a poorly expressed
rhythmic layering in
this zone can be interpreted as the indication that its formation might have been
associated with the
multiple additions of fresh ultramafic melt into the crystallizing magma chamber.
Inasmuch as the
newly formed cumulates are almost indistinguishable from the pre-existing cumulates
here, in contrast
to the gabbroid portion of the intrusion, they built up the ultramafic rock sequence
producing an
impression of the single-act formation of the zone. In this case the old evolved
magma, enriched in
the products of the magma crystallizing differentiation, must have been displaced
continuously
upward to form the upper, basic portion of the body, as the supply of fresh magma
declined in
connection with the attenuation of magma generation in the mantle.
So far, we cannot offer an unambiguous interpretation of the rocks crossed by
Hole 67.
There are three possible versions: (1) the lower megarhythm consists of the
rocks of the Marginal
Border Group, (2) the megarhythms are characteristic of the entire Shalozero
intrusive body, but are
not expressed as obviously in the other parts of the body, and (3) this structure
is characteristic only
of this segment of the body.
Version (1) does not agree with the higher-temperature rocks of the lower megarhythm
where cumulate olivine is much more magnesian than that in the rocks of the Marginal
Border Group
(see above). Version (2) is also improbable, because the composition of the dark-colored
silicates
from the ultramafic rocks of the lower megarhythm is similar to that of the ultramafic
rocks from the
Ultrabasic Zone, and that from the upper megarhythm is more ferrous, similar to the
composition of
minerals from the peridotite marker horizon. Apparently, the more suitable version
is (3) which
suggests that this macrorhythmic pattern is a specific feature of the SE segment
of the Shalozero
body.
|
Figure 25
|
The presence of the two megarhythms that produced this rock
sequence can be related to
the intrusion of the new large portion of fresh magma into the partially solidified
intrusive chamber
(Figure 25),
which might have resulted in the formation of the second (upper) peridotite layer
with a
thickness of
> 200 m. The potential analogs of the Upper megarhythm peridotite in the other
parts of
the Shalozero body are the peridotite layers in the Pyroxenite Zone, ranging between
a few meters
and 25-30 m in thickness and violating the general crystallization trend (Holes
174, 335, 338, and
341). Apparently, the macrorhythm was associated with the origin of a new magma channel
in the
SE part of the pluton.
This macrorhythmic pattern is not a unique feature of layered intrusions. One of
the
examples is the Zlatogorsky layered intrusion in Northern Kazakhstan, where there
are two
macrorhythms with thick dunite layers at the base, which are replaced, via the zones
of rhythmic
ultrabasic and basic cumulate interbedding, by norite and gabbronorite layers in
the top
[Sharkov, 1980].
Similarities and differences in the cumulate stratigraphy and composition of
the Aganozero and Shalozero-Burakovka rocks. The results of our study revealed
the following
specific features in the structure and composition of these two intrusive bodies.
Both bodies are
characterized by the high degree of layering, and their sections reflect the same
main sequence of
cumulate parageneses, this providing a basis for distinguishing similar zones in
them. However, the
more detailed study of the structure and composition of each zone revealed substantial
differences
between them. One example is that the Ultrabasic zone of the Shalozero-Burakovka
Body contains
only poikilitic peridotite, whose intercumulate phases are dominated by plagioclase
with ubiquitous
phlogopite. Conversely, the upper peridotite subzone of the ultrabasic zone in the
Aganozero body
includes dunite layers, with almost wholly absent plagioclase and scarce phlogopite.
Differences are even more substantial in the pyroxenite zones. In the Shalozero-Burakovka
Body this zone is as thick as 80 m and consists mainly of websterite, orthopyroxenite,
and their
olivine varieties. In addition, its middle portion includes a peridotite layer interpreted
as a marker
horizon for the Shalozero Body. The Pyroxenite zone in the Aganozero Body is ca.
200 m thick and
is dominated by clinopyroxenite with a noncumulus texture. The main mineral composing
up to 90%
of the rock volume is inverted pigeonite-augite whose grains are seen to intensively
corrode the
single grains of cumulate augite and orthopyroxene. In the lower part of the zone,
a clinopyroxene
aggregate replaces the peridotite minerals with merely single relict areas remaining
intact. All rocks
of the Pyroxenite zone in the Aganozero Body contain numerous small inclusions mainly
of a quartz-carbonate material.
The Gabbronorite zone of the Shalozero-Burakovka Body also has a reduced thickness
and
does not include a banded subzone represented by interlayered rocks of different
compositions and
accounting for up to half of the thickness of the Gabbronorite zone. We failed to
correlate the
structures of the overlying zones. In the Aganozero body, the Pigeonite gabbronorite
zone is very
thin, and the rocks of the Magnetite gabbrodiorite zone are absent. Moreover, in
contrast to the
Aganozero body, the pyroxene in the SBB is dominated by orthopyroxene, and there
are intersticial
quartz and orthoclase grains (2-3 vol.%) over the whole sequence of the Pyroxenite
and overlying
zones.
The comparison of the compositions of cumulate silicate minerals revealed their substantial
similarity in the similar portions of the Aganozero and Shalozero-Burakovka bodies,
this reflecting a
similarity between the physicochemical conditions of magma crystallization during
the emplacement
of these intrusions.
To sum up, our analysis of the structure and composition of the Aganozero and Shalozero
bodies shows that with the general sequence of cumulate mineral assemblages, common
for both
bodies, they show a number of significant differences. Our data suggest that the
Aganozero and
Shalozero-Burakovka intrusive bodies were derived from similar parental magmas, which,
nevertheless, were slightly different in composition.
Age relations between the Aganozero and Shalozero-Burakovka intrusions. These
two
intrusive bodies are separated by a large fault, and there are two different views
as to their age
relations: (1) both bodies were intruded at the same time, and differences in
their structure were
caused by the tectonic activity that occurred during the emplacement of the pluton
[Tevelev and Grokhovskaya, 1999];
(2) the two bodies of the Burakovka Pluton have different ages, the SBB
being younger than the AB
[Berkovskii et al., 2000].
However, as a result of our study we found
that the Aganozero Body was intruded 50 million years later than the Shalozero-Burakovka
intrusion
(see above).
To sum up, our study proved the Early Paleoproterozoic Burakovka Pluton to consist
of
two independent intrusive bodies, AB and SBB, which were emplaced with an interval
of ca. 50
million years, the pluton itself being a long-lived magmatic center. The intrusions
were derived from
similar-type but slightly different parental magmas of a silicic high-Mg series with
similar isotopic and
geochemical characteristics, this being responsible for the absence of principal
differences in the
physico-chemical trends of their crystallization. Both bodies were emplaced in the
course of the
regular replenishment of the solidifying intrusions by new portions of fresh magma
from a mantle
magma-formation source and also from intermediate crustal magma chambers rising through
the
crust by the mechanism of zone melting (see above).
This situation does not seem to be unique for the Baltic Shield. According to recent
evidence
[Sharkov et al., 2001;
Smolkin et al., 2001],
the Monchegorsk Complex in the Kola Peninsula,
ranking second in size in the region concerned, also consists of two intrusive bodies:
the
Monchegorsk Ni-bearing pluton of ultrabasic and basic rocks and the Monche-Chuna-Volchie-Tundra
gabbroid massif, which were emplaced with a period of 30-50 million years between
them
[Sharkov et al., 2001;
Smolkin et al., 2001].
Burakovka Pluton as an Example of the Early Proterozoic
Layered Intrusions of the Eastern Baltic
Shield
|
Figure 26
|
The eastern part of the Baltic Shield is one of the largest
regions in the world with many
Early Proterozoic layered intrusions of basic and ultrabasic rocks: more than 12
very large massifs
were found there (Figure 26). All of them had been derived from magmas of the
siliceous high-Mg
series (SHMS) and have a similar structure and a close composition, although many
of them differ in
structural details and cumulate stratigraphy
[Alapieti et al., 1990;
Sharkov and Smolkin, 1998].
These intrusions are usually located at the peripheries of Paleoproterozoic sedimentary-volcanic
structures, often plunging under their bases. Since the lower portions of these structures
are usually
composed of siliceous high-Mg volcanics, these intrusive massifs seem to have been
intermediate
magma sources for these structures. In some rare cases, for example, in the Burakovka
Pluton,
volcanic rocks occur outside of the intrusive massifs, apparently because their complementary
volcanics were eroded.
With a very similar assemblage of cumulates, almost identical for most of the intrusive
bodies, the latter differ in the development of their particular rocks. Usually,
the massifs are
dominated by basic cumulates, such as gabbronorite or gabbronorite-anorthosite, although
there are
some exceptions. For instance, it has been shown above that in the Aganozero Body
ultrabasics
dominate over basic rocks, the olivine-spinel cumulates having an unprecedented thickness
of 6 km.
This thickness is not higher than 2 km in the SBB, being a few hundred meters
in most of the similar
massifs. The same is true of the other cumulates; some bodies are dominated by pyroxene
(usually
orthopyroxene) cumulates, others, by plagioclase-pyroxene cumulates, still others,
by plagioclase
cumulates, and so on. Commonly found are various violations of the cumulate stratigraphy,
associated with the repeated injections of new portions of fresh magma into the crystallizing
intrusive chambers
[Sharkov and Smolkin, 1998].
Apparently, differences in the composition of the layered intrusions were associated
with the
particular functioning mechanisms of individual magma systems, which were controlled
by the flow
rate of magma ascending from its source and intermediate chambers, by particular
geomechanic
conditions in the surrounding region, and also by the composition of the Archean
crust in a given
area, different under the granite-greenstone regions and under the granulite belts.
A good illustration for this is the Burakovka Pluton. The differences between its
Aganozero
and Shalozero bodies, revealed in this study, seem to have stemmed from the different
contributions
of the components which, as follows from
[Amelin and Semenov, 1996],
participated in the generation of this pluton's magma. This is in good agreement
with our data on the long
life of the Burakovka magma source. Differences in the rock compositions of the bodies
can be explained by
the crustal contamination of magma during its generation, by the different lithologies
of these rocks,
by differences in the source depths of the parental magma derived from the same mantle
superplume, as has been demonstrated by
Abaouchami and Hofmann [1998]
using the Pb isotope ratios of the Hawaiian magmas. It appears that all of these
factors were valid in our case.
Layered Intrusions as Members of the Large Baltic Igneous
Rock Province of a Siliceous High-Mg
Series (SHMS)
As has been shown above, during the Early Proterozoic, roughly 2.5 to 2.3 billion
years
ago, the eastern part of the Baltic Shield was a large igneous rock province of a
siliceous high-Mg
series, similar in its geological position to Phanerozoic trap basalt regions, but
greatly different from
them by the composition of its magmas, typical of subduction environments
[Sharkov et al., 1997, 2000].
The distinctive features of the rocks of this series are the high contents of SiO2, MgO, and
Cr, the medium contents of Ni, Co, Cu, and V, and the low contents of Ti, alkalies,
and Nb. The
predominant rock is basalt, the associated basaltic andesites and andesites being
often characterized
by an elevated Mg content. As far as the recent igneous rocks are concerned, these
features are
characteristic only of island-arc rocks, especially of the boninite series. These
rocks resemble
boninites and modern island-arc tholeiites in terms of their contents of major, trace,
and rare-earth
elements with their typical Nb and Ti minima and also in terms of their high
Al2O3/TiO2 ratios.
Appreciable differences between them were found only in the isotopic characteristics
of the SHMS
rocks: the
e Nd (T) value varies from
- 1.0 in the ultramafics to
- 3.1 in the gabbroids. A similar pattern
was found in the other similar layered intrusions: in Bushveld in South Africa
[Schiffries and Rye, 1989]
and in Stillwater in North America
[Lambert et al., 1994].
The unusual geologic position of the origin of these magmas
and the specific features of their
geochemistry and isotopy suggest a principally different origin of the SHMS magma,
as compared to
the similar Phanerozoic magmas. We believe that the origin of the magmas discussed
was associated
with the extensive contamination of the parental high-T mantle magma with the Archean
crustal
material as it rose toward the Earth's surface. The leading mechanism seems to have
been the
"floating" of the high-T magma through the crustal rock sequence by melting the top
by way of zone refinement
[Sharkov et al., 1997].
The layered intrusions seem to have acted as intermediate
collectors where these melts accumulated (Figure 26).
|
Figure 27
|
In this connection it seems reasonable to discuss the character
of tectonic processes that
were operating in the Baltic Shield during the Early Paleoproterozoic. As one can
see in Figure 27,
the
major tectonic elements that existed in the region at that time were the Kola and
Karelian rigid
cratons with the Laplandia-Umba granulate belt (LUGB) between them. The latter was
a region of
subsidence with a large sedimentary basin forming on its surface. Mobile belts were
developing
along the contacts between the granulite belt and the cratons being the zones of
low-angle
tectonic flow; the Belomorian mobile belt representing the best preserved zone. Fragments
of
a similar belt were found in the Tersky-Lotta Belt extending along the northeastern
periphery of the
Laplandia-Umba granulite belt. These major tectonic elements produce a regional structural
and
metamorphic zoning which is characterized by a slow growth in the intensity of deformation
and
metamorphism toward the granulite belt
[Sharkov et al., 2000].
In the Kola and Karelian cratons, igneous activity manifested itself in the formation
of large
layered intrusions, gabbronorite dike swarms, and volcanic sedimentary belts restricted
to the linear
graben-shaped structures. Here, the SHMS volcanics are represented by a great variety
of rocks
ranging from low-Ti picrite and basalt to andesite, dacite, and rhyolite, the dominating
rocks being
basalts. These volcanogenic-sedimentary belts had been continental rifts that developed
mainly in
subaerial environments.
The intermediate mobile belts of the Belomorian type were distinguished by a dispersed
intrusive magmatism which resulted in the emplacement of numerous small synkinematic
intrusions of
basic and ultrabasic rocks of the same types found in the layered intrusions, except
that here
individual intrusions are composed of one rock variety, namely, peridotite, gabbronorite,
or
anorthosite (e. g., Belomorian drusitic complex). Highly subordinate are enderbite-charnockite
massifs. The granulite belt does not contain any SHMS rocks, the main igneous rock
types being
crustal enderbite and charnockite.
To sum up, in contrast to the Phanerozoic foldbelts, the distinctive feature of the
Early
Paleoproterozoic tectono-magmatic activity in the Baltic Shield was the coexistence
of three main
types of structural domains:
(1) large regions of rising and extension, accompanied by intensive mantle magmatism
and
active erosion (Kola and Karelian cratons); magmatism was of within-plate type; there
is no
evidence of any processes that operated at active plate boundaries;
(2) a region of compression and subsidence between them marked by abundant crustal
enderbite-charnockite magmatism (Laplandia-Umba granulite belt), where excess of
crustal
(including sedimentary) material accumulated at the expense of erosion and denudation
in adjacent
areas;
(3) peculiar belts of the low-angle tectonic flowage of crustal material from
regions of
extension toward granulite belt, such as the Belomorian belt, which originated in
the extension
environment and had a mixed character of magmatism.
|
Figure 28
|
Of these structural provinces, only type (1) is the closest
to the Phanerozoic type, because it
resembles the regions of continental rifting and trap basalt emplacement, the origin
of which is
believed to be associated with mantle plumes. Our analysis of the geologic processes
that occurred
in the Baltic Shield during Early Paleoproterozoic time enables us to describe the
geological situation
of that time in terms of plate tectonics. In accordance with our model (Figure 28), the
dominant
mechanism of tectonic activity in the Early Precambrian, and also in the Phanerozoic,
was the ascent
of superplumes with the formation of large extension zones (cratons) above their
spreading heads. However, in
contrast to the Phanerozoic, the spreading process developed at depths of 150-200 km
[Girnis and Ryabchikov, 1988]
and, hence, was not accompanied by the breaking of the sialic crust, the
formation of the oceanic lithosphere, or the origin of compensation-type structures,
such as
subduction zones. Instead of this activity, there arose extension and subsidence
zones, typical of the
Early Precambrian and complementary to the extension zones: moderate-pressure granulite
belts
surrounded by the intermediate zones of tectonic flow, similar to the Belomorian
Belt
[Sharkov et al., 2000].
Apparently, this situation resembled the pattern that arose in the experiments of
H. Ramberg [1981]
in his simulation of the vertical rise of diapirs through a more viscous matrix,
where
the material of the latter flowed into the intermediate space between them.
Of interest is the point of the initial size of the Early Paleoproterozoic province
of boninite-like
magmatism, the Baltic province, discussed above, being its part. Similar rocks are
traceable in
the basement of the Russian Platform, in Scotland, Greenland, in the Canadian Shield
(Matatchewan
and Herst dike swarms and volcanic rocks at the base of the Huron basalt plateau)
and in the
Wyoming Craton
[Heaman, 1997].
According to Heaman's paleomagnetic and stratigraphic
reconstructions, these cratons were initially parts of the Laurentia-Baltia supercontinent
and
separated in the Late Proterozoic. The initial size of this igneous province was
supposed to be 2500 km
long and at least 1500 km wide, the size comparable with the largest Phanerozoic
trap
provinces.
As mentioned above, this type of igneous activity developed
widely in the east of the Baltic
Shield throughout the Early Paleoproterozoic, this suggesting the long existence
(ca. 200 million
years) of a superplume
[Sharkov et al., 1997].
A distinctive feature of geologic processes at that
time was the localization of the centers of tectono-magmatic activity in the same
geologic structures
(Pechenga, Imandra-Varzuga, Vetrennyi Belt, etc.) with closely spaced periods (spaced
10-50
million years apart) of volcanic and intrusive activities (see Figure 27). Our
evidence on the long-lasting
activity of the Burakovka igneous center are in good agreement with this statement.
Although there
are some geochemical and isotopic differences between the individual bodies, the
physico-chemical
trends of their crystallization did not have any principal differences, as would
be expected.
Differences in the lithology can be explained either by differences in the degree
of crustal rock
contamination in the course of magma generation, or by differences between the depths
of the
parental magma in the same superplume.
Judging by geochemical data and the mineral composition of the highest-temperature
cumulates including high-Mg olivines and pyroxenes, as well as chromites, the primary
mantle
magma was produced by the melting of a highly depleted ultramafic material. According
to the
isotopic and geochemical data available, as the newly produced magma ascended toward
the
Earth's surface, it assimilated the material of the ancient lower crust, getting
enriched in Si, Al, and
Ca, this explaining the predominance of basic rocks in the intrusions. The above
evidence suggests
that the composition of the early Paleoproterozoic mantle plumes was drastically
different from that
of the Phanerozoic plumes depleted in ultrabasic material. The primary melts were
very hot (ca. 1600o C
[Girnis and Ryabchikov, 1988]),
this promoting the assimilation of crustal material, the
process uncharacteristic of the Phanerozoic intraplate magmatism.
The emergence of local plumes responsible for the origin of long-lived magmatic centers,
marked by large layered intrusions, was obviously associated with local rises on
the superplume
surface, where the mantle material was melted at the expense of decompression, and
where the
magmas of the siliceous high-Mg series were produced.
Conclusions
1. The Burakovka layered pluton of mafic and ultramafic rocks is one of the
largest layered
massifs in the Baltic igneous province of siliceous high-Mg rocks. It consists of
two independent
bodies (Aganozero and Shalozero-Burakovka), each having its own internal structure,
which contact
each other in their tops.
2. The two bodies of the pluton have a similar rock sequence consisting of five
differentiated
zones: ultramafics, pyroxenite, gabbronorite, pigeonite gabbronorite, and magnetite
gabbronorite
(the latter was found only in the Shalozero-Burakovka Body). However, with this general
structural
similarity, these bodies differ markedly in the character of their cumulate stratigraphies
and, to a
lesser extent, in their rock compositions.
3. A distinctive feature of the pluton is the presence of markers: single interlayers
of high-temperature
ultrabasic cumulates in the sequence of lower-temperature formations. Their origin
is
believed to be associated with the replenishment of fresh magma portions into the
crystallizing
magma chambers. The same mechanism explains the macrorhythmical pattern recorded
in the SE
segment of the Shalozero-Burakovka body.
4. As follows from the geochemical, mineralogic, and isotopic-chronological
data, the two
bodies were produced by the compositionally siliceous high-Mg magmas, except that
the Aganozero
Body was intruded 50 million years later than the Shalozero-Burakovka Body, the Sm-Nd
isochron
age of the former being 2372
22 Ma ( e Nd = -3.22
0.13 ) and that of the latter,
2433
28 Ma ( e Nd = -3.14
0.14 ).
5. The Burakovka Pluton seems to have been a long-lived igneous center which
evolved
above a local mantle plume. The origin of the latter is believed to have been associated
with the
activity of a superplume which provided for the existence of the Baltic igneous province
for ca. 200
million years.
Acknowledgments
We are grateful to Prof. A. F. Grachev for the useful discussion of this
paper.
This work was supported by the Russian Foundation for Basic Research (grants 01-05-64673
and 01-05-06069),
by the Russian Federal Program for Supporting the Leading Scientific
Schools (grant 00-15-98560), and by the Program "Platinum of Russia".
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